Water Electrolysis Technologies and Their Modeling Approaches: A Comprehensive Review

Abstract

Hydrogen (H₂) is a key energy vector in the global transition toward clean and sustainable energy systems. Among the various production methods, water electrolysis presents a promising pathway for zero-emission hydrogen generation when powered by renewables. This review provides a comprehensive evaluation of water electrolysis technologies, including alkaline (AWE), proton exchange membrane (PEMWE), solid oxide (SOEC), anion exchange membrane (AEMWE), and microbial electrolysis cells (MEC). It critically examines their material systems, catalytic strategies, operational characteristics, and recent performance advances. In addition to reviewing experimental progress, the study presents a finite element modeling (FEM) case study that evaluates thermal and mechanical responses in PEM and AWE configurations—illustrating how FEM supports design optimization and performance prediction. To broaden methodological insight, other simulation frameworks such as computational fluid dynamics (CFD), response surface methodology (RSM), and system-level modeling (e.g., Aspen Plus^®) are also discussed based on their use in recent literature. These are reviewed to guide future integration of multi-scale and multi-physics approaches in electrolyzer research. By bridging practical design, numerical simulation, and material science perspectives, this work provides a resource for researchers and engineers advancing next-generation hydrogen production systems.

1. Introduction

The global demand for sustainable energy solutions has led to an increased focus on hydrogen as a promising energy carrier due to its high energy density and potential for zero-emission applications [1,2]. While hydrogen production is currently dominated by fossil fuel-based methods such as steam methane reforming and coal gasification, these processes contribute significantly to carbon emissions [3]. In contrast, water electrolysis presents a cleaner alternative, enabling hydrogen generation without direct greenhouse gas emissions when powered by renewable energy sources. However, the widespread adoption of electrolysis technologies is hindered by challenges related to energy efficiency, system design, and material durability, necessitating advancements in optimization strategies and predictive modeling. Efficient hydrogen production through electrolysis is only one part of the solution; equally important is the ability to store and utilize this energy effectively, making advancements in energy storage technologies crucial for the widespread adoption of hydrogen as a sustainable fuel.

Energy storage involves converting complex energy forms into more efficient and economical storage solutions, playing a crucial role in economic development. Recent advancements, particularly in nanomaterials for ultracapacitors, have significantly improved battery lifespan and capacity. Various energy storage technologies have been developed, as outlined in

Table 1. Methods of energy storage.

S. No.	Energy Storage Methods	Power Rating	Efficiency	Discharge Time
1.	High power supercapacitor	up to 200 kW	85–100%	s–min
2.	Superconducting magnetic storage	500 kW–50 MW	85–100%	s–min
3.	High power flywheels	up to 10 MW	85–100%	s–min
4.	High energy supercapacitor	up to 100 kW	85–100%	min–h
5.	Long duration flywheels	up to 1 kW	85–100%	h–days
6.	Batteries (including flow batteries)	up to 50 MW	70–85%	h–days
7.	Compressed air energy storage	10 MW–1 GW	45–70%	h–days
8.	Pumped hydro energy storage	10 MW–1 GW	70–85%	days–month
9.	Hydrogen and synthetic gas	5 MW–1 GW	30–45%	days–month
10.	Hydrogen with fuel cells	up to 1 MW	30–45%	days–month

[4].

Among all, the storage and production of H₂ are incredibly significant nowadays because of its high energy capacity and transportability [5]. Hydrogen is a widely available energy carrier that can be produced from both renewable and non-renewable sources. It offers high energy density, sustainability, and storage for renewable energy derived electricity, with water as its only byproduct and no carbon emissions [6]. Hydrogen can be produced through various methods at different stages of development (Figure 1). Currently, 78% comes from fossil fuel reforming, 14% from coal gasification, and only 4% from alternative sources, primarily electrolysis [4].

Water electrolysis, first reported in 1789, is an electrochemical process that uses electrical energy to split water into hydrogen and oxygen [7]. It consists of a water-filled cell with externally powered electrodes, where hydrogen forms at the negative electrode and oxygen at the positive. Efficiency can be optimized by adjusting electrolysis type, pressure, temperature, current density, electrolytes, membranes, electrodes, and catalysts [8]. The variation in factors and their effect on electrolysis are summarized in

Table 2. Variation in factors and their effect on electrolysis for hydrogen production.

S. No.	Factors	Variation	Effects on Electrolysis
1.	Current density	Increased	Directly increases the rate of electrolysis.
2.	Time	Increased	Initially, activation phenomena (e.g., catalyst wetting, membrane conditioning) can cause a gradual increase in electrochemical performance. Over prolonged operation, performance degradation may occur, characterized by a decrease in current density at a given voltage, due to catalyst degradation, membrane aging, or electrolyte depletion or degradation.
3.	Applied voltage	Increased	An increase in driving force (applied potential) enhances the current density and accelerates the rate of electrolysis, up to a limit. Beyond this point, side reactions, mass transport limitations, and diffusion constraints may become significant, affecting efficiency and selectivity.
4.	Concentration of ions	Increased	Generally, it increases conductivity and current density, enhancing the electrolysis rate. However, effects vary depending on the type of ions, membrane properties, and electrode materials involved.
5.	Electrode active area	Increased	Increases total current capacity, potentially increasing the rate of electrolysis. However, current density (current per unit area) may decrease unless total current increases proportionally.
6.	Distance between electrodes	Decreased	Reduces ohmic resistance, increasing current density and rate of electrolysis.
7.	Resistance	Reduced	Improves conductivity, increasing current density and electrolysis rate.
8.	Temperature	Increased	Enhances ion mobility and reaction kinetics, increasing current density and rate of electrolysis.
9.	Pressure	Increased	Reduces gas bubble size, lowering power consumption and increasing current density and electrolysis rate.
10.	Electrode material	Changed from inert to active	Active electrodes may participate in reactions (e.g., oxidize), affecting stability and long-term efficiency.
11.	Catalyst loading (mg/cm2)	Increased	Can enhance reaction kinetics and increase current density, improving electrolysis efficiency—up to an optimal level beyond which mass transport limitations may occur.

[9,10,11].

Alkaline, polymer electrolyte membrane, solid oxide, and microbial electrolysis cells have been widely studied, each with unique advantages and challenges. Research has improved electrode materials, catalysts, electrolytes, and operating conditions, yet key issues persist, including energy losses from overpotential, internal resistance, and mass transport limitations. Electrode degradation, membrane instability, and gas bubble formation further reduce efficiency and increase energy consumption, necessitating systematic optimization [10,11]. Computational modeling and finite element analysis are crucial for optimizing electrolysis by providing insights into electrochemical kinetics, ion transport, thermal stability, and mechanical integrity. Simulations such as computational fluid dynamics and multi-physics modeling enable better system design, yet a gap remains in integrating numerical models with experimental validation, limiting predictive accuracy and scalability.

This review addresses these gaps by providing a comprehensive assessment of advancements in water electrolysis technologies while emphasizing the role of numerical modeling in optimizing performance. It critically evaluates different electrolysis methods in terms of operational efficiency, material selection, and system constraints while highlighting computational approaches that enhance process predictability and efficiency. By integrating insights from both experimental studies and numerical modeling, this review aims to establish a framework for improving water electrolysis technologies, bridging the gap between theoretical predictions and practical implementation. The findings presented herein contribute to the ongoing efforts to enhance the scalability and economic viability of hydrogen production through electrolysis, paving the way for future research in energy-efficient hydrogen generation.

2. Methodology

This review explores water electrolysis technologies, emphasizing the optimization and efficiency improvements in hydrogen production, with attention to electrode materials, catalyst advancements, energy consumption, and renewable energy integration. Initially, an extensive literature survey was conducted, revealing a lack of studies that integrate these aspects of electrolysis into a single review. Data were collected from a wide range of sources, including research articles, original works, books, reviews, proceedings, and dissertations, ensuring a thorough and well-rounded perspective. A systematic review was conducted following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) methodology (Figure 2), which involves a detailed and transparent approach to article selection, search strategy, data extraction, and data analysis. The PRISMA process ensures a structured and transparent approach to systematically identifying and analyzing relevant studies, enabling the consolidation of key findings across diverse electrolysis technologies. By applying this methodology, the review effectively highlights critical gaps in the optimization of electrolysis processes, efficiency enhancements, and cost reduction strategies, paving the way for addressing unexplored synergies in sustainable hydrogen generation.

A thorough search was conducted across leading online databases, including SCI, SCIE, Elsevier Scopus, and Web of Science, encompassing a wide range of scientific disciplines. Only peer-reviewed studies from high-impact journals indexed in SCI, SCIE, Scopus, and Web of Science were selected to ensure data reliability. The selection of articles was carried out in three stages. The first stage, initial identification, involved inputting search terms aligned with the research objectives into relevant databases to identify potentially relevant studies. In the second stage screening, studies were evaluated based on (i) relevance to the research objectives, including system optimization, material development, performance analysis, or modeling of water electrolysis; (ii) scientific quality, assessed through methodological clarity, completeness of reporting, peer-reviewed status, and validation or reproducibility of data; (iii) study design, encompassing both experimental and computational work as well as systematic reviews; and (iv) language, with only articles published in English considered. The final stage, full-text review, involved a comprehensive examination of the remaining studies to ensure they fully met all eligibility requirements.

A wide range of keywords, including “water electrolysis”, “hydrogen production”, “efficiency”, “optimization”, “catalyst”, “electrode materials”, “nanomaterials”, “renewable energy integration”, “cost reduction”, “proton exchange membrane (PEM) electrolysis”, “alkaline water electrolysis (AWE)”, “solid oxide electrolysis cell (SOEC)”, “anion exchange membrane (AEM) electrolysis”, “high-temperature electrolysis”, and “low-temperature electrolysis”, were employed to construct this review paper. Keyword searches were performed using Boolean operators, combining terms such as ‘water electrolysis’ AND ‘hydrogen production’ OR ‘efficiency improvement’ to refine results. The search was repeated across different databases to ensure comprehensive coverage. The selection of publications prioritized factors such as the reliability and validity of data, recency of research, relevance, impact factors, citation count, and utility of the articles. Data collection was conducted manually through content analysis, extracting critical information such as article type, journal name, year of publication, topic, title, research methodology, relationships between variables, indicators, and research findings. The articles retained after the final screening provided a comprehensive foundation for evaluating the synergies between electrolysis technologies, energy efficiency improvements, and cost-effective hydrogen production.

3. Classification and Analysis of Water Electrolysis Technologies

Figure 3, Figure 4 and Figure 5 present a classification of the literature on various aspects of water electrolysis technologies. Electrolysis is categorized into alkaline water electrolysis (AWE), solid oxide electrolysis cells (SOEC), polymer electrolyte membrane (PEM) electrolysis, and microbial electrolysis cells (MEC), with AWE being the most extensively studied. Further classification is based on electrode materials, where nickel-based and oxide electrodes are predominantly used for both anode and cathode applications. Research also classifies electrolysis studies by current density variations, with most investigations utilizing high current densities to enhance hydrogen production. Additionally, studies differentiate electrolysis based on electrolyte type, with SSZ, barium cerate carbonate, and potassium hydroxide (KOH) being the most commonly used, while yttria-stabilized zirconia (YSZ) and sodium hydroxide (NaOH) appear less frequently. Electrolysis has been performed using various water sources, including pure water, steam, wastewater, and seawater, with researchers also evaluating the impact of temperature on system performance.

Figure 3a illustrates that 30% of electrolysis studies focus on AWE. Figure 3b,c highlight the predominance of nickel-based and oxide materials as electrode choices, while Figure 3d confirms the frequent use of SSZ, barium cerate carbonate, and KOH as electrolytes. These classifications provide valuable insights into trends in electrolysis research, guiding advancements in material selection and process optimization.

Figure 4a classifies electrolysis studies based on membrane type, with titanium porous transport layers, Nafion 115 membranes, and co-crystallized catalyst-coated membranes being the most commonly used. Figure 4b highlights the frequent use of platinum-based catalysts, indicating their widespread preference in electrolysis research.

Similarly, Figure 5a classifies electrolysis studies based on the state of H₂O at the inlet, with steam (36%) and wastewater (36%) being the most frequently used, while pure water is also widely studied. Figure 5b provides a breakdown of electrolysis research based on the operating temperature ranges used in various studies. The chart indicates that 75% of the research focuses on high temperature electrolysis (>500 °C), underscoring the crucial role of elevated temperatures in improving process efficiency. High temperatures facilitate faster reaction kinetics, lower the energy barrier for water splitting, and improve overall system performance, making it the most extensively studied condition. In contrast, medium-temperature electrolysis (200–500 °C) accounts for only 12% of the studies, and low temperature electrolysis (<200 °C) represents 13%. The smaller percentages suggest that while lower-temperature systems are explored, they present challenges such as higher overpotentials and lower reaction rates, which could limit their efficiency and applicability in large-scale operations. Overall, the data reflects the strong research interest in high temperature electrolysis, likely driven by its potential for enhanced hydrogen production rates, improved system integration with industrial heat sources, and better economic viability over the long-term.

3.1. Alkaline Water Electrolysis (AWE)

In AWE (Figure 6a), electrodes are immersed in an alkaline solution and separated by a membrane, which acts as a diaphragm to prevent gas crossover while facilitating the transport of hydroxide ions (OH⁻) between electrodes, ensuring efficient electrochemical reactions [12]. The reactions, charge carriers, and overall cell reactions occurring in AWE are detailed in

Table 3. The anode, cathode reactions, charge carrier, and overall cell reactions take place in AWE, PEM electrolysis, SOEC, and MEC.

Electrolysis	Anode Reaction	Cathode Reaction	Charge Carrier	Overall Cell
AWE	${2\mathrm{O}\mathrm{H}}^{-}\to {\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}$	${\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_2+{2\mathrm{O}\mathrm{H}}^{-}$	${\mathrm{O}\mathrm{H}}^{-}$	${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$
PEM	${\mathrm{H}}_2\mathrm{O}\to {2\mathrm{H}}^{+}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{2\mathrm{e}}^{-}$	${2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_2$	${\mathrm{H}}^{+}$	${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$
SOEC	${{\mathrm{O}}_2}^{-}\to {\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{2\mathrm{e}}^{-}$	${\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_2+{{\mathrm{O}}_2}^{-}$	${{\mathrm{O}}_2}^{-}$	${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$
MEC	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_2\to {2\mathrm{H}\mathrm{C}\mathrm{O}}^{3-}+{9\mathrm{H}}^{+}+{8\mathrm{e}}^{-}$	${8\mathrm{H}}^{+}+{8\mathrm{e}}^{-}\to {4\mathrm{H}}_2$	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}$	$\mathrm{C}{\mathrm{H}}_3\mathrm{O}{\mathrm{O}}^{-}+4{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{H}\mathrm{C}\mathrm{O}}^{3-}+{\mathrm{H}}^{+}+{4\mathrm{H}}_2$

. As a commercially viable and cost-effective technology, AWE benefits from the use of non-noble electrocatalysts, making it an attractive option for large-scale hydrogen production. However, it also faces certain limitations, including low current densities, carbonate formation, reduced gas purity, and low operating pressures, which impact overall performance. The energy efficiency of AWE typically ranges between 70 and 80%, with further improvements dependent on optimizing operating conditions and material advancements [13]. The efficiency of AWE can be optimized by adjusting current density, electrode materials, electrolyte composition, and membrane properties, all of which significantly influence overall performance and hydrogen production rates.

Researchers are actively exploring innovative approaches to enhance the efficiency and sustainability of hydrogen production through electrolysis. Bhattacharya et al. advocated for the integration of solar energy with AWE to support a sustainable hydrogen economy [14]. Bidin et al. demonstrated that using collimated light increased hydrogen production by 53% compared to conventional light and dark field conditions [15]. Giraldi et al. utilized nuclear power for electrolysis, reporting low greenhouse gas emissions of 416 g CO₂eq per kg H₂ [16]. Bhandari et al. emphasized the preference for hydropower or wind-generated electricity over fossil fuels for electrolysis-based hydrogen production [17]. Kwak et al. developed a W-typed dye-sensitized serial solar module to improve H₂ generation efficiency [18]. Kelly et al. highlighted the role of electrolytic hydrogen in fuel-cell electric vehicles, enhancing efficiency while reducing environmental impact [19]. Wang et al. suggested that applying an external field and modifying electrolyte composition could mitigate reaction overpotential and reduce ohmic voltage drop, further improving electrolysis performance [20].

AWE has emerged as one of the most mature and scalable technologies for green hydrogen production due to its reliance on non-noble metals and compatibility with renewable energy sources. Xie and Shao [21] emphasize the commercial viability of AWE and its relatively low capital cost compared to PEM and SOEC systems, despite inherent challenges such as sluggish electrode kinetics and system inefficiencies. Tüysüz [22] further elaborates on these kinetic challenges, particularly highlighting the need for improved oxygen evolution reaction (OER) catalysts, given its thermodynamic and kinetic complexity. In a modeling centric approach, Hu et al. [23] argue that comprehensive thermodynamic, electrochemical, and thermal models are essential to optimize operational efficiency and improve system-level design for AWE. Ehlers et al. [24] underscore the importance of bridging the gap between lab-scale innovation and industrial scale deployment by focusing on durability, cost, and system integration. Thissen et al. [25] reinforce this by advocating for lab-scale protocols that replicate industrial conditions—such as high temperatures and concentrated KOH electrolytes—to accurately assess catalyst performance. Dou et al. [26] use scanning acoustic microscopy to visualize two-phase flow dynamics in porous electrodes, providing actionable insights into bubble management and electrolyte replenishment—factors critical for high current density performance. Brauns and Turek [27] explore the integration of AWE with intermittent renewable sources such as wind and solar, noting the need for dynamic operation and optimized part-load performance to prevent safety hazards and maximize efficiency. Finally, Sebbahi et al. [28] provide a holistic overview of electrolysis technologies, reaffirming AWE as the most cost-effective and industrially ready solution, despite the need for continued research in catalyst durability and system optimization.

The efficiency of AWE varies with different cathode materials, with Ni-based materials being the most widely used. Kabulska et al. investigated Ni-Fe-C alloys due to their high electroactivity for the hydrogen evolution reaction (HER) in AWE. By applying plasma treatment using CH₄ and H₂ at 470 °C, they successfully introduced carbon into Ni and Ni-Fe alloys, leading to enhanced catalytic activity. This improvement was attributed to carburization, which significantly boosted HER performance in 25 wt.% KOH at 80 °C [29]. This carburization not only improved catalytic activity by increasing surface reactivity but also improved electrode durability under alkaline conditions. Sivanantham et al. integrated Ni₃Se₂ with Ni-foam as a cathode structure, enhancing electron conductivity and active surface area. The foam served as a high surface area support, promoting gas release and improving hydrogen production continuity during long-term operation [30]. Solmaz et al. engineered Cu/Ni/NiZn-PtRu multilayer cathodes to leverage synergistic catalytic effects across metal layers. The PtRu addition enhanced HER kinetics, while the NiZn alloy improved corrosion resistance, making this structure promising for prolonged AWE cycling [31]. Furthermore, Buch et al. demonstrated that incorporating Au nanoparticles into Ni electrodes significantly improved HER performance by increasing the electrochemical surface area and modifying electronic properties at the metal interface, thereby reducing the activation overpotential [32]. Rauscher et al. investigated HER of nanocrystalline NiMoB alloys in 1 M KOH to compare the equivalent crystalline and polycrystalline materials at 298 K [33]. Kim et al. demonstrated a low-cost, asymmetric porous nickel electrode for alkaline water electrolysis by sintering nickel powder into nickel foam, creating a dual surface architecture. The fine-pore surface acted as the electro-catalytic interface, while the open pore side facilitated gas and electrolyte transport. This design achieved high current densities (~0.5 A/cm² at 1.8 V, 80 °C), enhanced mass transfer, and long-term operational stability over 400 h. The integration of lightweight gaskets and polymer separators into a compact stack further highlighted the system’s scalability and energy efficiency [34]. Kabulska et al. applied plasma carburization to pure nickel cathodes using CH₄ + H₂ gas mixture at 470 °C, resulting in carbon-enriched surfaces. The treated electrodes exhibited enhanced catalytic activity for HER in 25 wt.% KOH at 80 °C, particularly in early stages of operation. The improvement was attributed to surface-bound carbon increasing electrocatalytic activity rather than surface area effects. Performance gains diminished after prolonged operation due to iron deposition from the electrolyte [35]. Cardoso et al. prepared nickel-samarium and nickel-dysprosium alloys of rare earth metal with 5% and 10% by arc and induction furnace melting, respectively. The results indicated that the electrocatalytic performance of Ni-Re alloys with 5% of rare earth metal for HER is better than 10% of rare earth metal [36]. Park et al. have reported the use of quaternary ammonium-tethered aromatic polymers for anion exchange membrane-based AWE [37]. Tang et al. reported a fabrication procedure of the Ni-Mo electrodes for HER. The Ni-Mo electrodes exhibited a high catalytic activity in KOH solution at 70 °C towards HER [38].

A number of other materials other than nickel are also reported as cathode material during AWE. Zuo et al. [39] address these concerns by introducing Ru-perturbed Cu nanoplatelet cathodes that achieve current densities up to 3.6 A/cm² at 2 V, rivaling PEM electrolyzers while minimizing precious metal use. Yuksel et al. [40] fabricated and characterized 3D silver nanodomes by soft and nanosphere lithography. A higher rate of hydrogen production was observed for 3D AgNDs in 6 M KOH solution as opposed to Ag bulk [41]. Furthermore, Allebrod et al. used Co- and Mo-activated electrodes in AWE at 150–250 °C and 40 bars. It was observed that with the addition of cobalt oxide and molybdenum oxide electrocatalysts, the performance of the electrolysis cells was improved. Liu et al. [42] contribute by developing a Co₃Mo₃N/Co₄N/Co heterostructure catalyst that promotes rapid electron transport and robust HER and OER kinetics under alkaline conditions. While the structure also exhibited OER potential, their study primarily highlights its cathodic role in AWE. Moreover, Muller et al. on the basis of their investigation on amorphous Fe-based alloys (Fe₈₂B₁₈, Fe₈₀Si₁₀B₁₀, and Fe₈₀Co₂₀Si₁₀B₁₀) for AWE, documented that the addition of metalloids, especially cobalt, has been seen to enhance the HER activity of the materials [43]. Shen et al. have performed electrolysis using bauxite electrodes and using NaOH solution as the supporting electrolyte. The results indicated the desulfurization rate, the pH value, the electrode corrosion, and the cell voltage of the electrolyte [44]. Studies of Yao et al. revealed that fabricating Ni_Fe with Co and Ti as substrate (100 mA cm⁻² for 100 h) showed extensive stability with minimal distortion of electrodes [45]. Even at low current densities, Ag nanodomes (AgNDs) exhibit a higher potential difference compared to bulk Ag, demonstrating their superior electrochemical performance.

Luo et al. [46] demonstrated significant improvements in the cathode of the alkaline water electrolyzer. By employing a nickel-plated nickel mesh with surface activation, they enhanced the electrochemically active surface area, effectively reducing overpotential and improving hydrogen production efficiency. The mesh structure not only increased surface area but also improved bubble release, contributing to better system performance. Egert et al. [47] expanded upon this by showing how low-tortuosity porous nickel electrodes dramatically improve efficiency at industrially relevant current densities (up to 2 A/cm² at ~2 V). Performance analysis revealed that the cathode modification (Mo-incorporated porous Ni) had a greater impact on improving overall cell performance, particularly at high current densities. Thus, although both electrodes were enhanced, the cathode played a more dominant role in the performance improvement. These materials reduce the capillary pressure and bubble point, thereby enhancing two-phase mass transport and lowering ohmic resistance. Notably, their study confirmed these results both in lab-scale cells and a 2 kW AWE stack, indicating that porosity and microstructure play vital roles in maintaining high performance under load. Xia et al. [48] also noted that cathode design is critical under low-load conditions, especially when AWE systems are driven by intermittent renewable energy sources. They highlighted that the physical structure, not just the chemical composition, of the cathode affects both efficiency and consistency. For instance, inconsistent operation across multiple cells was linked to uneven electrode characteristics. This insight pushes beyond material composition to suggest structural uniformity is equally important. In the broader context of electrolyzer technologies, Nasser et al. [49] emphasized that nickel and nickel-alloy-based cathodes are common features in AWEs due to their robustness and cost-efficiency. Their review highlighted that while AWE efficiency ranges from 62–82% HHV, this can be significantly optimized by tailoring cathode materials and configurations. El-Shafie [5] provided a comparative overview of water electrolysis technologies, reiterating that AWE cathodes (typically nickel-based) offer a favorable balance between cost, efficiency, and operational lifetime. The efficiency losses in AWEs are often attributed to both ohmic and activation overpotentials, where the cathode’s electrocatalytic performance is central to minimizing these losses.

Zeng et al. enhanced the cathode performance of Ni-based electrodes through mechanical polishing with sandpaper and electrochemical deposition of Co and Ni. The polished electrodes exhibited superior apparent activity, achieving an overpotential of 422 mV at a current density of 750 A m⁻², demonstrating significant improvement in HER. [50]. This study focuses on improving the cathode performance by surface modifications of Ni-based electrodes, enhancing the hydrogen evolution reaction (HER) through mechanical polishing and electrochemical deposition. Ahn et al. prepared the alloy catalyst of NiCu for AWE by changing the alloy composition with the electrodeposition method and investigated the catalytic activities in a 6.0 M KOH electrolyte at 298 K to HER with cyclic voltammetry [51]. Hong et al. prepared a NiW alloy catalyst on a copper foil substrate via a electrodeposition method for the HER in AWE. As observed, the morphologies of the NiW alloys exhibited severe variations as compared to Ni, at high W contents in the catalyst [52]. Egelund et al. have used the LaNiO₃ electrode for OER in AWE. As a result, it was observed that the coating of LaNiO₃ and Ni composite coatings decreased OER overpotential by 70 mV [53].

To consolidate the diverse approaches discussed for cathode optimization in AWE, Figure 7 illustrates a synthesized classification of material types, surface modification techniques, and their corresponding effects on electrochemical performance. Our synthesis (see Figure 7) visualizes these innovations alongside trade-offs and technical readiness levels to guide future material selection and device engineering. The framework emphasizes the interplay between structural enhancements (e.g., alloying, nano-structuring), performance outcomes (e.g., reduced overpotentials, enhanced current density), and operational considerations such as scalability, thermal compatibility, and material cost. This visual summary is intended to guide material selection by highlighting both innovation pathways and trade-offs relevant to practical implementation.

Apart from cathode development, significant work has also focused on improving anode catalysts for the OER. Jovic et al. investigated the OER in 1 M NaOH at 25 °C using pure Ni and Ni-Ebonex/Ir composite coatings. Initially, the Ni coating exhibited superior intrinsic catalytic activity for OER compared to the composite coatings. However, a significant loss of activity was observed after 24 h at 50 mA cm⁻², indicating stability limitations in prolonged operation [54]. He et al. modified Ni anode by TiO₂ nanotubes and found out that the hydrogen production rate was increased and the applied direct voltage reduced compared to normal water electrolysis [55].

Kuleshov et al. enhanced alkaline electrolysis performance by modifying porous nickel electrodes at both the anode and cathode. NiCo₂O₄ spinels were deposited onto the anode to reduce the OER overpotential, while NiP_x or Pt nanoparticles were incorporated on the cathode to enhance HER kinetics. These dual modifications led to significant reductions in both overpotentials, ultimately lowering the total cell voltage and improving overall energy efficiency [56].

For large-scale hydrogen production via water electrolysis, the development of efficient, stable, and cost-effective OER catalysts is crucial. Li et al. synthesized an Fe-Co-Ni oxygen evolution catalyst via electrodeposition on nickel foam (NF), which exhibited high stability and enhanced catalytic activity, making it a promising candidate for improving the efficiency of water electrolysis [57]. Li et al. studied Pt-Mn₂O₄ catalysts with carbon black as a conductive agent and showed a remarkable activity for OER due to a synergistic effect between Mn₂O₄ and Pt. The Pt-Mn₂O₄/C catalyst with a weight ratio of Pt to Mn₂O₄ of 3:1 gives the best performance [58]. Gao et al. synthesized 2D FeSe₂ nano-platelets via a hydrothermal reduction route, which exhibits extraordinarily high catalytic activities and stability for OER [59].

The effectiveness of AWE is also affected by the variation of current density (Figure 8). Dobo et al. emphasized the fact that there exists a relation between the efficacy of AWE and the variations of electric current. It was stated that at a constant current density, steady DC current causes much less efficiency loss as compared to the non-steady current conditions [60]. Increasing current density enhances hydrogen generation efficiency, while a higher ripple factor negatively impacts performance. Electrolyte composition also plays a crucial role in AWE efficiency. Berg et al. investigated potassium dihydrogen phosphate (KH₂PO₄) as an electrolyte and demonstrated that at 300 °C, it dissociates into H₂O gas in equilibrium with KH₂PO₄, K₂H₂P₂O₇, and KPO₃, highlighting its potential for improving electrolysis performance at elevated temperatures. As described in the study, at higher temperatures (around 275–325 °C), the Gibbs free energy (ΔG) of the water splitting reaction decreased by approximately 6%, making the electrolysis process thermodynamically more favorable and potentially more energy-efficient. Moreover, operating at elevated temperatures allowed the use of non-precious metal catalysts, which were less costly compared to the precious metals typically required at lower temperatures. In the study molten KH₂PO₄ acted as a stable electrolyte that retained significant water content through hydrogen bonding, enabling efficient high temperature electrolysis while containing water under high vapor pressure conditions [61]. Furthermore, Tufa et al. [62] fed a lab-scale reverse electrodialysis unit with dissimilar NaCl concentrations, imitating seawater and brine, to drive an alkaline polymer electrolyte water electrolysis cell. As shown in Figure 9b, the experimental setup includes gas separators at both electrodes and a salinity-gradient-driven RED stack with 27 cell pairs. Their system achieved 0.86 W power output, with the performance curve in Figure 9c showing a maximum gross power density under a temperature of 65 °C. This setup demonstrates the feasibility of integrating salinity-gradient energy with AWE to improve energy input efficiency. Nikiforov has also performed electrolysis using KH₂PO₄ at 300 °C. Hence, for temperatures as high as 300 °C, molten KH₂PO₄ as an electrolyte has shown promising results, depicting conductivity of ~0.30 S cm⁻¹ at 300 °C [63]. On the contrary, Fiegenbaum et al. advocated the use of tetra-alkyl-ammonium-sulfonic acid as the electrolyte, which worked well even at room temperatures, producing current densities with high efficiencies [64]. Menia et al. explored methanol-assisted electrolysis as an alternative to conventional water electrolysis, using a 4 M aqueous methanol solution. This configuration replaces the OER at the anode with the methanol oxidation reaction (MOR), which proceeds at a lower overpotential. As a result, the system achieves hydrogen production at significantly lower voltages (0.65–0.85 V at 0.5–1 A/cm²), improving energy efficiency [65]. However, this process should be considered a distinct electrochemical pathway rather than a direct optimization of water electrolysis, since MOR alters the anodic chemistry and may produce CO₂ depending on catalyst selectivity. Ge et al. used coal-water slurry and investigated the mechanism of electrolysis by anode reaction kinetics, whereby they demonstrated an inverse relation between activation energies of the electrode reaction and the activity of carbon materials [66]. Chang et al. stated that the Ni₁₂P₅/Ni₃(PO₄)₂ hollow sphere exhibited high stability and activity towards OER and HER in alkaline media [67]. Li et al. [68] emphasized that modern AWEs are traditionally limited by low current density operation, often below 0.5 A/cm², primarily due to bubble-induced overpotentials. Their study introduced a superaerophobic electrode design that significantly mitigated these effects, achieving an impressive current density of 3.5 A/cm² at 2.25 V and 85 °C in a zero-gap configuration. The key to this performance enhancement was efficient gas bubble removal, which otherwise blocks active sites and increases ohmic losses at higher current densities. Jang et al. [69] conducted a comprehensive simulation study and demonstrated that increasing current density can enhance hydrogen production, but only within optimal temperature and pressure conditions. They reported that at 1 A/cm², the AWE system achieved its highest efficiency of 78.52% at 120 °C and 10 bar. However, they also cautioned that excessive current density leads to thermal management challenges and reduced system stability due to increased hydrogen crossover risks. Ding et al. [70] provided a detailed exergy analysis and found that the ohmic overpotential varies more significantly with current density than activation overpotential. Their thermodynamic-electrochemical model showed that membrane resistance, which is sensitive to bubble formation and diaphragm properties, dominated the total ohmic losses under increasing current densities. This suggests that minimizing diaphragm resistance and optimizing bubble removal are crucial for maintaining efficiency at high loads.

The plot in Figure 9a shows the relationship between the partial pressure of H₂O and temperature for KH₂PO₄. This was determined by Raman spectroscopy, using relative ratios of Raman scattering cross-sections of CH₄ and H₂O [41]. As the temperature increases, the partial pressure of water also rises, showing an exponential trend, especially above 200 °C. The melting point of KH₂PO₄ is marked at 270 °C, with an uncertainty of ±2 °C. This suggests that as the salt approaches and surpasses its melting point, the release of water becomes more significant, possibly due to phase change behavior and enhanced vaporization. Figure 9d shows a plot of the logarithm of conductivity versus inverse absolute temperature (1000/T) for different electrolyte molarities. This is a typical Arrhenius-type plot, where conductivity decreases with increasing 1000/T, indicating enhanced ion mobility at higher temperatures. Different symbols represent various electrolyte concentrations (e.g., 5 M Na₃PO₄, 3 M Na₃PO₄ + 2 M KH₂PO₄, etc.). Higher concentrations generally show higher conductivity, emphasizing the role of electrolyte composition in improving ionic transport. Figure 9e presents the polarization curves showing current density versus potential at varying electrolyte molarities (e.g., 0.1 M, 0.2 M, 0.3 M, etc.). As the applied potential becomes more negative, the current density increases—characteristic of cathodic reactions such as HER. Moreover, higher molarity electrolytes yield greater current densities at a given potential, reflecting reduced ohmic losses and improved ionic transport. This demonstrates the beneficial role of electrolyte concentration in enhancing the electrochemical performance of the system. Considering the fact that the membrane has a crucial role to play when it comes to efficiency in AWE, Aili et al. equilibrated the membrane in 22 wt.% aqueous KOH. This modified version of the porous poly(perfluorosulfonic acid) membrane was seen to deliver significantly higher ion conductivity (0.2 S/cm) even at room temperature, as opposed to the unmodified membrane (0.01 S/cm) [71].

For AWE, Burnat et al. made use of composite membranes integrated with polysulfone and mineral fillers. The proposed polysulfone-barite membranes are more advantageous than Zirfon 500 utp due to their lower ionic resistivity, higher hydrogen purity (99.83%), and significantly lower material cost. Additionally, barite offers excellent chemical stability in alkaline conditions, making these membranes both more efficient and cost-effective for industrial alkaline electrolysis [72]. Likewise, Wu et al. [73] review the evolution of porous membranes in AWE for green hydrogen production, emphasizing the transition from asbestos to safer non-asbestos alternatives. They highlight the role of materials such as PPS and Zirfon^® in enhancing membrane efficiency, durability, and hydrophilicity. Diaz et al. have utilized alkali-doped poly (2,5 benzimidazole) membrane for poly (2,5-benzimidazole) membrane for AWE. It was reported that the linear and cross-linked membranes were stable up to 3.0 mol/dm³ and 4.2 mol/dm³ KOH doping, respectively [74]. In summary, AWE cathode optimization efforts have evolved from simple Ni-based systems to increasingly complex multi-metal and nanostructured materials. While many studies report improvements in HER kinetics or current density, the underlying mechanisms often involve enhanced bubble detachment, increased surface roughness, or improved electrical conductivity. However, cost and long-term stability remain critical bottlenecks for industrial scale-up.

The efficiency of AWE is also influenced by operating pressure (Figure 10). Suermann et al. studied the impact of pressures up to 100 bars on cell voltage behavior and observed that lowering the pressure reduced mass transport losses, thereby improving overall system efficiency [75]. Koi et al. analyzed the environmental impact of high pressure AWE systems, focusing on innovative membrane technologies. Their findings revealed that while improved membranes enhanced performance, the primary environmental impact stemmed from the electricity required for operation, emphasizing the need for renewable energy integration to minimize the carbon footprint [76]. Todd et al. have also performed electrolysis by increasing pressure (up to 100 MPa) at operating temperatures (up to 1000 K) and demonstrated lower energy requirements, albeit with electrical [77].

Furthermore, many researchers have reported the review and numerical modeling of AWE. To find out the impact of varied material parameters on the ohmic overpotential of AWE, Zouhri et al. developed a numerical and theoretical model that analyzes the contribution of ohmic polarization to overall energy losses in alkaline water electrolysis. This model evaluates the effects of membrane and electrode resistivities, electrolyte conductivity, gas bubble formation, and temperature on the total cell resistance. It integrates exergy-based metrics to quantify performance and provides optimization pathways to reduce ohmic overpotentials and enhance system efficiency [78]. Milewski et al. put forward two mathematical models of AWE; one of them emphasized focusing on factors such as calibration and other parameters associated with the electrolytic cell and the energy losses associated with it. Another model focuses on a reduced-order equivalent circuit and the internal electric resistance of the electrolyte [79]. Dhabi et al. studied the PV-electrolysis modeling by a DC/DC buck converter with MPPT control for an improved adaptation between the electrolysis and PV generator [80]. El-Askary et al. developed a numerical model to predict the H₂ generation process in AWE. Their results indicated that reducing the main flow velocity, increasing current density, and minimizing the cathode-anode gap significantly enhanced hydrogen production efficiency, offering insights for optimizing electrolyzer design [81]. Tanaka et al. featured an NaCl electrolysis industry to produce chlorine and caustic soda, such as chlor-alkali, in an electrolyzer with a double-layer membrane consisting of sulfonic acid and carboxylic acid groups to prevent back migration of OH⁻ ions and succeeded in increasing voltage and decreasing current efficiency during electrolysis [82]. Shestakova et al. studied the ways to develop cost-effective and environment-friendly Ti/Ta₂O₃-SnO₂ electrodes for H₂O and organic compounds oxidation, and also characterized electrochemically and physically [83].

The future of AWE is deeply intertwined with advancements in high performance electrode materials, innovative membrane technologies, and the integration of renewable energy sources. The primary focus of research must be on developing cost-effective, durable, and highly active catalysts with optimized nanostructured surfaces to enhance both the HER and OER. By leveraging advanced nanomaterials, multi-metallic alloys, and engineered surface architectures, catalytic efficiency can be significantly improved, leading to lower overpotentials, reduced activation energy, and enhanced reaction kinetics. The incorporation of transition metal chalcogenides, phosphides, and carbides, alongside traditional Ni-based materials, is a promising direction for boosting catalytic performance while maintaining economic feasibility.

Beyond electrode optimization, membrane technology remains a critical area for improving ionic conductivity, chemical stability, and gas separation efficiency in AWE systems. Current diaphragm-based separators are prone to ionic resistance and degradation, necessitating the development of hybrid membranes, composite ion-exchange structures, and functionalized polymeric separators that offer long-term stability under alkaline conditions. These improvements will minimize crossover losses, enhance selectivity, and improve operational lifespan, directly impacting overall system efficiency. Additionally, high pressure electrolysis has gained attention as a means of reducing downstream compression costs while achieving higher volumetric hydrogen density. Investigating the long-term stability and material compatibility of electrodes and membranes in high pressure, high temperature alkaline environments will be crucial for industrial scale applications.

The integration of AWE with renewable energy sources, including solar photovoltaics, wind power, and nuclear energy, presents a significant opportunity for scaling hydrogen production while reducing carbon footprints. However, the intermittent nature of renewable energy poses challenges in maintaining stable electrolysis operation. Developing smart grid-compatible AWE systems with dynamic load management, real-time electrochemical monitoring, and adaptive power input regulation will enable better synchronization with fluctuating renewable energy outputs. Additionally, the hybridization of electrolysis technologies—such as combining AWE with proton exchange membrane (PEM) or solid oxide electrolysis (SOE) systems—could enhance operational flexibility, allowing for higher efficiency under varying power conditions. These hybrid approaches leverage the strengths of alkaline, protonic, and high temperature electrolysis to create more resilient and energy-efficient hydrogen production pathways.

Computational modeling and numerical simulations will continue to play a pivotal role in optimizing AWE systems. Multi-physics models integrating fluid dynamics, ion transport mechanisms, and electrochemical kinetics can provide deeper insights into system performance under various operational conditions. High-fidelity FEA and CFD simulations can aid in designing next-generation electrolyzers with improved flow dynamics, minimized bubble formation, and reduced mass transport losses. The incorporation of machine learning and artificial intelligence (AI)-driven predictive models will further refine process control strategies, enabling real-time efficiency improvements and fault detection mechanisms in industrial AWE applications.

Ultimately, positioning AWE as a cornerstone of a global hydrogen economy requires continued materials innovation, process optimization, and seamless integration with renewable energy infrastructure. By addressing fundamental challenges such as catalyst degradation, ionic resistance, and energy loss, AWE can evolve into a highly scalable, cost-effective, and environmentally sustainable hydrogen production technology. Future research should emphasize the convergence of experimental advancements with computational modeling to accelerate the development of next-generation high efficiency electrolyzers, ensuring economic feasibility and large-scale deployment in the transition toward a carbon-neutral energy landscape.

3.2. Proton Exchange Membrane (PEM) Electrolysis

Proton exchange membrane (PEM) electrolysis was developed in 1966 as an advanced alternative to AWE, addressing limitations such as low current densities, carbonate formation, low gas purity, and operational pressure constraints. In PEM electrolysis, a solid polymer electrolyte (SPE) serves multiple critical functions, including electrical insulation of electrodes, proton conduction, and efficient separation of product gases (Figure 6b). The fundamental electrode reactions, charge carrier transport, and overall cell operation in PEM electrolysis are detailed in Table 3.

PEM electrolysis offers several advantages over AWE, including lower gas permeability, high proton conductivity, compact design, and higher efficiency (80–90%), while producing high purity hydrogen with oxygen as a byproduct. However, its efficiency and long-term stability are influenced by various factors, such as current density, operating pressure, temperature, membrane type, and catalyst selection. Among these, current density plays a crucial role in performance and degradation rates. Rakousky et al. investigated the impact of constant and dynamic current density profiles, concluding that the highest degradation occurs at 2 A/cm², while no significant degradation was observed at 1 A/cm², highlighting the importance of optimized operating conditions for improving the longevity and reliability of PEM electrolyzers [84]. According to Zhou et al. [85], PEM-based systems benefit from higher current densities (typically 2–3 A/cm²), which enable compact designs and rapid dynamic responses. These features make PEM electrolysis particularly suitable for integration with intermittent renewable energy sources. Similarly, Wang et al. [86] reported that PEM electrolyzers can operate across a wide range of current densities, up to 3 A/cm², while maintaining efficiency, due to advanced membrane and catalyst designs that minimize ohmic and activation losses. However, as Fahr et al. [87] pointed out, higher current densities also lead to challenges such as hydrogen crossover in PEM systems. Thinner membranes are desirable for reducing ohmic losses, but at high current densities, increased gas production can cause higher crossover rates, potentially leading to reduced efficiency and safety concerns if not properly managed. Rocha et al. [88] emphasized that gas bubble evacuation becomes critical at elevated current densities. Their research showed that in alkaline electrolysis systems, optimizing electrode topology can improve mass transport and bubble removal, significantly boosting performance. Their flow-engineered 3D electrodes allowed for PEM-like efficiency at high current densities using low-cost materials. Peng et al. [89] explored a novel integration of PEM electrolyzers with thermal energy storage systems to stabilize temperature fluctuations caused by varying power input. This helped maintain consistent performance even during frequent current density shifts—common when electrolyzers are powered by fluctuating renewables. Araújo et al. [90] further highlighted that high current densities, while beneficial for hydrogen output, exacerbate durability issues due to the stress imposed on noble-metal-based electrocatalysts. They argued for the development of PGM-minimized electrocatalysts that retain performance under such demanding conditions. Meanwhile, Wang et al. [91] compared PEM and AWE technologies, noting that AWE systems, while less tolerant of high current densities, still maintain a place in large-scale applications due to their lower capital cost. However, their lower operating current densities make them less flexible for variable power input scenarios.

Furthermore, studies have shown that catalyst composition significantly influences degradation in PEM electrolysis. Ito et al. investigated PEM using a platinum-based anode catalyst layer, revealing that at pressures below 10 bars, the permeated H₂ flux was largely consumed through oxidation or recombination at the anode. These findings highlight the importance of optimized catalyst selection and operating pressure in mitigating hydrogen crossover and performance losses, ultimately enhancing the efficiency and durability of PEM electrolyzers. Grigoriev et al. described a numerical and an experimental analysis of the optimum loadings of platinum and iridium in PEMWE cells. It was reported that these metals content did not show substantial catalytic layer degradation within 4000 h. Based on the results obtained, the recommended optimal Pt and Ir loadings derived from the study are a Pt loading of about 0.4 mg/cm² (or 1 mg/cm² of Pt/C with 40 wt.% Pt) and an IrO₂ loading of approximately 2.5 mg/cm² [92]. However, costly Pt and Ir, presently considered as the state-of-the-art electro-catalysts, stop the extensive commercialization of this technology. Thus, Kus et al. [93] presented a cost-effective method for preparing Ir-based anode catalysts using TiC as a support. As shown in Figure 11a, their optimized TiC loading of 0.4 g/cm³ yielded the highest current density at the lowest cell voltage, demonstrating improved electrochemical performance. This integration of TiC significantly reduces noble metal loading while maintaining system efficiency. Rozain et al. further reported the applicability of titanium particles with IrO₂ particles at the anode of PEMWE cells. According to MEA, for over 1000 h of operation, IrO₂/Ti was found to be stable, but the degradation rate measured at 1 A cm⁻² was reduced from 180 µV h⁻¹ (for the pure IrO₂ anode) to only 20 µV h⁻¹ (for the 50 wt.% IrO₂/Ti anode) [94]. Rozain et al. have studied PEMWE using IrO₂ alone as a catalyst at 80 °C. They used 0.25 mg/cm² of Pt cathode and 0.5 mg/cm² of IrO₂ anode at 1 A/cm² of current density with a Nafion 115 membrane [95]. Lee et al. have also developed electrodeposited IrO₂ anodes in PEMWE. At 0.7 V, the IrO₂ loading ranges from 0.007 to 0.464 mg/cm² at 90 °C in the PEMWE test. An IrO₂ loading of 0.1 mg/cm² with the IrO₂/CP electrode shows the highest performance [96]. Kadakia et al. investigated a nanostructured fluorine (F)-doped IrO₂ electrocatalyst for PEMWE, exploring compositions ranging from 0–20 wt.% F content. Their results demonstrated that the electrocatalyst follows a two-electron transfer reaction mechanism, with an activation energy of approximately 25 kJ/mol for electrolysis. Notably, the 10 wt.% F-doped IrO₂ composition exhibited the highest electrochemical activity, which was further validated through single full cell tests, confirming its potential for enhancing PEMWE performance and catalytic efficiency [97]. Lamy et al. used PtSnRu/C, PtSn/C, and Pt/C catalysts to yield hydrogen by electrolysis and studied the ethanol’s electrocatalytic oxidation in a PEMWE cell at 20 °C. It was observed that the voltage did not exceed 0.9 V at 100 mA/cm² for a 220 cm³ evolution rate of H₂ per hour. Also, the consumption of electrical energy was less than 2.3 kWh, which was one-half of the energy required for the water electrolysis [98]. Kadakia et al. also stated that a nanostructured SnO₂ with 10 wt.% or 20 wt.% RuO₂ improved the durability and the electrochemical activity as compared to noble metal counterparts [99]. Sapountzi et al. discussed the effect of the structure and nature of the catalyst-electrode materials performance, whether it be AWE, PEMWE, or SOEC [100]. Zlotorowicz et al. fabricated zirconium hydrogen phosphate, Nafion, and iridium oxide on glassy carbon disk electrodes. At room temperature, the electrocatalyst was electrochemically examined with OER in 0.5 M sulfuric acid electrolyte. The outcomes concluded that the existence of zirconium hydrogen phosphate worsens the current per mass and emphasized the need to improve the catalytic layers [101]. Figure 11b shows the curve between mass activity and anode loading. It is clear that increasing anode loading mass activity decreases. Figure 11c shows the curve between efficiency and time of 0.10 mg/cm² of pure IrO₂ and 0.12 mg/cm² of IrO₂/Ti particles. It is clear that IrO₂/Ti shows better efficiency as compared to pure IrO₂. Figure 11d shows the curve of how efficiency is affected by different IrO₂ loadings. It is clear from the plot that 0.5 mg/cm² of IrO₂ shows the maximum efficiency of 71%. Figure 11e displays the curve between the cell voltage and the current density when the IrO₂ membrane was doped with fluorine, and Figure 11f demonstrates the curve between the cell voltage and the current density when RuO₂ was used with IrO₂.

As the name itself suggests, the membrane also provides a major influence on the efficiency of a PEMWE cell. Rakousky et al. demonstrated that the membrane plays a major role in determining the overall efficiency of a PEMWE cell. Although structurally stable, its performance is influenced by contamination from titanium species and changes in ion conductivity. Their findings emphasize that membrane integrity, along with compatible cell components, is critical for minimizing degradation and improving efficiency [102]. Skulimowska et al. in their work, observed that perfluorosulfonic acid Aquivion ionomer has given higher water electrolysis performance as compared to the Aquivion zirconium phosphate membrane [103].

Wang et al. used a co-crystallized catalyst-coated membrane (CCM), prepared by heating the catalyst layer and amorphous Nafion membrane at 120 °C, and used the solid polymer as the electrolyte. According to results, a decrease in cell voltage by 0.009 V at 2000 mA/cm² at 80 °C with improved stability was noticed as compared to the conventional CCM [104].

Moving further, variation in the efficiency of a PEMWE with the varying pressure is also reported. Olesen et al. have used a circular planar, inter-digitated flow field at the anode for a high pressure PEMWE cell [105]. At the differential and balanced pressure operation, Schalenbach et al. compared the in-situ measurements of the anodic H₂ content, and simulated the effect of cathodic and anodic pressures in PEMWE on the gas crossover [106]. Lamy discussed both low temperature PEMWE cells and high temperature SOEC cells [107]. Olivier et al. studied the modeling process for the coupling of the PEM electrolysis to intermittent electrical sources. The model was proved to accurately predict the dynamic behavior of a semi-industrial PEM electrolysis system [108]. Langemann et al. have discussed the bipolar plate as a multi-functional component during PEMWE and tested Au and TiN coatings in PEMWE environments for the applications as a protective layer [109]. Millet provided a summary of PEMWE, including the effects of operating temperature and pressure on voltage and the structure of PEMWE [110]. Hancke et al. investigated the performance of a PEM water electrolyzer under pressures up to 180 bar, revealing that efficiency improves slightly up to 30 bar but declines significantly at higher pressures. The study highlights increased hydrogen crossover and ohmic losses as key factors behind reduced efficiency [111]. Sun et al. developed a 9-cell PEMWE stack and examined it for 7800 h. As observed, Fe, Cu, and Ca were distributed in the membrane and the catalyst layer of the CCMs. The cathode and the anode overpotential increased because the cations reside in the Nafion polymer electrolyte in the membrane and the catalyst layer [112]. Yilmaz et al. drove PEMWE by geothermal power for H₂ production at 160 °C. The 3810-kW of power produced in a binary geothermal power plant is for the electrolysis process. It was reported that the preheated water (by the geothermal water) used in electrolysis produced H₂ at a rate of 0.0340 kg/s. The exergy and the energy efficiency were 45.1% and 11.4%, respectively, of the binary geothermal power plant [113]. Rozain et al. reported the electrochemical characterization of PEMWE. According to results, at voltages 1.8–1.9 V, the charge transfer processes were the main cell impedance contributors. Also, the impedance was insignificant with the HER and two time constants on experimental impedance spectra with the OER [114]. Bensmann et al. discussed an in-situ method for the determination of a fully assembled cell by the use of standard equipment. The method was briefly illustrated for a broad pressure range with a laboratory-scale electrolyzer, and the measured data were compared with available literature values [115].

PEM electrolysis is a high efficiency hydrogen production technology, offering compact design, high proton conductivity, and gas purity, with efficiencies reaching 80–90%. However, commercialization is limited by the high cost of platinum group metal (PGM) catalysts, membrane degradation, and operational stability concerns. Iridium-based catalysts, such as IrO₂ and IrO₂/Ti, exhibit exceptional long-term stability, with IrO₂/Ti electrodes maintaining performance beyond 1000 h while demonstrating lower degradation rates. Advancements in nanostructured and doped catalysts, including TiC-supported iridium, fluorine-doped IrO₂, and RuO₂-based electrocatalysts, have shown improved electrochemical performance and reduced overpotential losses, presenting viable alternatives to reduce material costs. Furthermore, non-PGM catalysts, such as transition metal oxides and carbon-based electrocatalysts, are being explored to enhance catalytic activity while improving cost-effectiveness.

Beyond catalysts, membrane stability remains a crucial challenge, as ionic resistance and gas crossover contribute to performance degradation. Studies indicate that advanced perfluorosulfonic acid (PFSA)-based ionomers, such as short-sidechain Aquivion^®, can outperform conventional Nafion membranes in PEMWE applications. These alternative PFSAs offer higher proton conductivity and improved chemical stability, particularly under high current density and temperature conditions, making them attractive candidates for durable and efficient membrane formulations. Porous transport layers (PTLs) play a significant role in PEMWE performance, where Pt-coated Ti-PTLs have been shown to reduce degradation rates and enhance electrode durability compared to conventional PTLs. High pressure PEM electrolysis has enabled direct hydrogen production at elevated pressures, reducing downstream compression costs, though long-term stability studies remain limited.

Operational parameters, including temperature, pressure, and current density, also play a crucial role in PEMWE efficiency (Figure 12). The Figure 12 illustrates how temperature, pressure, membrane thickness, and current density affect overall system efficiency. Efficiency increases with temperature due to enhanced reaction kinetics and lower ohmic resistance, while higher current densities reduce efficiency due to elevated overpotentials. Membrane thickness influences ohmic losses, and pressure impacts thermodynamic potential. Higher temperatures improve reaction kinetics and electrode activity, but excessive thermal exposure can accelerate degradation rates.

Computational modeling and FEA have been instrumental in optimizing cell architecture, charge transport, and reaction kinetics, contributing to next-generation high efficiency electrolyzers. Hybrid approaches, such as co-crystallized catalyst-coated membranes (CCMs) and renewable energy integration, are further improving system efficiency and sustainability.

Despite these advancements, economic feasibility remains a significant challenge, necessitating progress in scalable manufacturing, cost-effective catalysts, and energy management strategies. Future research should focus on long-term durability assessments exceeding 10,000 operational hours, enhanced membrane conductivity, and real-time electrochemical monitoring to ensure PEM electrolysis remains a cornerstone of the hydrogen economy, driving cost-effective and sustainable large-scale hydrogen production.

3.3. Solid Oxide Electrolysis Cell (SOEC)

SOEC is first reported in 1980 and runs in regenerative mode for the electrolysis of water to produce oxygen gas, as shown in Figure 6c. SOEC has attracted attention because it converts electrical energy to chemical energy for ultra-pure H₂ with 90–100% efficiency. The significant characteristics of SOEC are its working conditions, i.e., high pressure and temperatures. Also, it uses the water in steam form and O²⁻ conductors as a charge carrier, as presented in Table 3.

Lim et al. have investigated the effects of seawater electrolysis by electrolyzing steam from a simulated seawater bath by Ni-YSZ/YSZ/LSCF-GDC SOEC for H₂ production. It was observed that the sea salt impregnation vaporized at 800 °C during the operation period [116]. In a study by Deka et al., the electrochemical behavior of a nickel-doped, A-site deficient lanthanum strontium ferrite (La_0.7Sr_0.2FeO₃) was evaluated as an SOEC cathode material. Electrolysis was performed at 800 °C with a 3% H₂O/He gas stream across various current densities. The study revealed that the undoped La_0.7Sr_0.2FeO₃ exhibited reduced Faradaic efficiency for hydrogen production due to the formation of a non-conductive La₂O₃ secondary phase, which impaired its electrochemical performance. In contrast, the nickel-doped version achieved nearly 100% Faradaic efficiency. X-ray diffraction (XRD) analysis showed that nickel doping promoted the formation of a conductive B-site metal phase and a Ruddlesden–Popper phase with mixed ionic and electronic conductivity, significantly enhancing the cathode’s performance [117]. Moreover, Mizusawa et al. investigated the temperature distribution during high temperature steam electrolysis in a 2-D tubular model of a micro-tubular SOEC. The results displayed that the current collecting positions were strongly affected by the temperature and reaction distributions [118]. The use of Ce_0.6Mn_0.3Fe_0.1O₂₋ (CMF) as a new oxide cathode in SOEC using LaGaO₃-based electrolyte for H₂ production at high temperature up to 1173–973 K was observed in an experiment by Hosoi et al. and it was observed that CMF could be a proficient oxide cathode for intermediate temperature SOEC [119]. Zhang et al. showed the 4 kW HTSE long-term test results, completed at Idaho National Laboratory (INL). The results concluded that a 3.1% degradation rate in 830 h of stable operation was achieved at a current density of 0.41 A/cm² [120]. Reytier et al. carried out experiments in both electrolysis and co-electrolysis in an HTSE based on SOEC. A 10- and 25-cell stack tested and produced 0.6 and 1.7 Nm³/h of H₂ at 800 °C, respectively, in the electrolysis mode below 1.3 V [121]. In an experimental study by Alenazey et al., they focused the production of CO and H₂ by SOEC and observed that the production of H₂O and CO₂ increased by enhancing the current density [122]. Cacciuttolo et al. tested the effect of pressure on HTSE based on SOEC; a positive effect of pressure on the oxygen side performances was reported [123]. Kazempoor et al. have produced synthetic gas using HTSE based on SOEC and reported a numerical model. The model has adopted a moderate fidelity approach to predict the thermo-fluid and electrochemical phenomena inside the cell and showed that the operating temperature, flow rates, and fuel consumption have a direct effect on power consumption and SOEC performance [124]. Ardigo et al. studied the K41X stainless steel for HTSE based on SOEC. It was observed that the alloy displayed an excellent oxidation resistance as compared to single atmosphere tests [125]. In an experiment, Houaijia et al. developed a solar hydrogen high temperature electrolysis to superheat steam up to 700 °C. The receiver was operated in a DLR’d solar simulator of steam reaching about 700 °C at a 40% thermal efficiency, a 4 kW solar power, and at a thermal-to-hydrogen efficiency of 26% [126]. Mougin et al. described the durability and performance in the stack environment in SOEC. It was shown that for SOFC, the protective coatings were compulsory to reduce the degradation rate in HTSE stacks [127]. Giddey et al. demonstrated SOEC at near room temperature and investigated the thermodynamic and practical energy benefits of a single-step water electrolysis process assisted by carbon [128].

The variation in electrodes (both anode and cathode), or cathode alone, or anode alone causes variation in the degradation rate. To improve the gas diffusion, Dong et al. prepared micro-channeled cathode support by a mesh-templating phase-inversion process. It was observed that this cathode support increased the H₂ production and steam utilization efficiency [129]. Hauch et al. used Ni/YSZ microstructure electrodes in SOEC and made a fine and denser NiO/YSZ precursor electrode for extended stability of electrolysis at high p(H₂O) and high current densities. Applying such conditions, they showed a 0.3–0.4%/kh degradation rate of 1 A/cm² at 800 °C [130]. Mahmood et al. developed a YSZ electrolyte membrane for a solid electrolyte membrane reactor and used electrochemical activity of CO₂, steam, and a CO₂-steam mixture at 800 °C. A very high current density was detected for CO₂, steam, and CO₂-steam mixture electrolysis at 850 °C and 1.5 V [131]. Hou et al. used an MoO₂-based cathode in their experiment of co-electrolysis based on SOEC at 750 °C. It was observed that, the MoO₂-based and the Ni/YSZ cells showed alike electrochemical performance in the co-electrolysis mode, while in the CO₂ electrolysis mode, the MoO₂ based cell displayed a better electrocatalyst as compared to the Ni/YSZ cell [132]. Nechache et al. studied the behavior and electrochemical performance of a commercial electrode-supported cell of Ni-YSZ/YSZ/LSCF [133]. Further, Li et al. have used a composite La_0.8Sr_0.2MnO₃₋ (LSM) in SOEC and reported the maximum current efficiency of 65% of Fe₂O₃-loaded LSM composite in a proton-conducting solid oxide electrolyzer at 800 °C [134]. Xing et al. performed co-electrolysis using an LSM-based electrode and observed lower ohmic resistance and better electrochemical performance [135]. Chen et al. doped scandium into LSCM to improve the performance of the composite cathode. The doping improved the ionic conductivity but decreased the mixed conductivity. Moreover, faradic efficiencies were also improved [136]. Li et al. used a composite-based LSCM cathode and used that for direct carbon dioxide electrolysis with copper nanoparticles at the surface of the LSCM cathode. It was observed that 85% current efficiencies were obtained with an LSCM cathode for direct CO₂ electrolysis in an oxide-ion-conducting SOE [137]. Ge et al. introduced Sr₂FeNbO₆ (SFN) to the SOEC as the H₂ electrode. Higher conductivity was observed in the H₂/H₂O atmosphere as compared to that in the air. Also, if compared with Ni/YSZ, SFN-YSZ was more appropriate for the H₂ electrode in SOEC. Ge et al. introduced Sr₂FeNbO₆ (SFN) as a hydrogen electrode material in SOEC, reporting high electronic conductivity (2.215 S·cm⁻¹ at 850 °C in H₂/H₂O), which surpasses performance in air and supports its suitability for steam electrolysis environments. Compared to conventional Ni/YSZ, the SFN-YSZ composite showed reduced charge transfer resistance and shifted the rate-determining step from interfacial charge transfer to surface adsorption/diffusion—indicating more favorable catalytic behavior. Ni/YSZ, while widely adopted, is prone to degradation via Ni sintering and carbon deposition, especially under CO-rich or hydrocarbon conditions [138]. Although Li et al. showed carbon deposition on Ni/YSZ could be promoted in SOEC mode, such effects are typically detrimental for long-term operation due to increased overpotential and material degradation [139]. Keane et al. have also analyzed the role of micro-structured Ni-YSZ cathodes on degradation rate in SOECs and observed significant electrochemical degradation in cells operated at 840 °C [140]. Ruan et al. performed CO₂ electrolysis with a composite cathode with ceramic, iron nano-catalyst, and iron oxide catalyst loading on it. The loading of nano-catalyst improved the electrode performance, and the current efficiency of CO₂ electrolysis was enhanced by 5% for the LSCM electrode at 800 °C [141]. Yoon et al. used lanthanum strontium vanadate (LSV) as the cathode for SOEC for co-electrolysis of steam [142]. Dong et al. have also reported A-site deficient and B-site excess perovskite LSCNNi composite cathodes. The results signified that the Ni nanoparticles cathode was a potential candidate for direct steam electrolysis in SOEC [143]. Xu et al. studied the Fe catalyst over the composite electrode for direct steam electrolysis. It was observed that the Fe catalyst enhanced the faradic efficiency and the electrode performance without the flow of reducing gas over the cathode. The current efficacy was also improved by 30% and 40% as compared to the bare LSCM-based cathodes at 800 °C [144]. Dasari et al. reported the electrochemical characterization of the Ni/YSZ electrode for H₂ production in SOEC [145]. Li et al. further loaded LSCM electrodes with Ni for SOEC, and the faradic efficiency was enhanced by 20% as compared to the bare LSCM cathodes [146]. Yang et al. used a K₂NiF₄-type structured Pr_0.8Sr_1.2(CoFe)_0.8Nb_0.2O₄₊ (K-PSCFN) matrix with a nano-sized Co-Fe alloy (CFA) electrode with LSGM electrolyte in SOEC at 900 °C. The cell confirmed good stability for high temperature steam electrolysis and an H₂ production rate calculated from Faraday’s law under a voltage of 1.3 V at 900 °C [147]. Liu et al. used an asymmetric NiO-YSZ cathode substrate made up by the phase-inversion tape casting method. The electrochemical performance was enhanced and displayed an excellent current density with a high production rate at 800 °C [148]. Li et al. employed an infiltration method on the composite electrodes in order to accomplish an activity-enhanced electrode performance. The partial pressure of 5–100% was achieved for Fe-loaded LSV cathode in a wide range of H₂ [149]. Gan et al. observed that the electro-catalytic activity of La_0.4Sr_0.4TiO₃ (LSTO) was insufficient for efficient electrochemical reduction of steam or CO₂. To enhance performance, catalytically active Ni nanoparticles were incorporated into the LSTO cathode via an impregnation method, significantly improving its activity for direct steam electrolysis. As a result, the current efficiency was notably enhanced compared to the bare LSTO-SDC cathode, achieving superior performance under 2.0 V at 800 °C [150]. Ardigo et al. tested La_0.8Sr_0.2MnO₂₋ and LaNi_0.6Fe_0.4O₂₋ as coatings in an O₂-H₂O atmosphere at 800 °C. The results concluded that LaNi_0.6Fe_0.4O₂₋ coating has shown high temperature corrosion resistance and enhanced electrical conductivity [151].

The electrolyte plays a vital role in varying the degradation rate of electrolysis. Sumi et al. have used BaCe_0.8Y_0.2O₃₋ (BCY) between Ce_0.9Gd_0.1O_1.95 (GDC) and the Ni-GDC fuel electrode. The leakage of current by the ceria-based electrolyte raised in SOEC because electron conductivity appears at low O₂ partial pressures, and BCY stopped the current leakage. Also, the performance of SOEC was superior than a conventional electrolyte (YSZ) by using the BCY blocking layers and the GDC electrolyte at 500 °C [152]. Pu et al. used BaCe_0.8Y_0.2O₃₋ in SOEC at 600 °C, which was lower than the conventional SOEC composed of YSZ (generally above 800 °C). It was observed that the performance electrolysis was enhanced as compared to conventional [152]. Further, Hjalmarsson et al. have reported SOEC using Ni/YSZ cermet support electrodes with a YSZ electrolyte. The cell was tested at 800 °C and −1 A/cm² converting 31% of a combination of H₂:H₂O:CO₂ for 2700 h. The results showed electrode degradation in 350 h followed by partial reactivation [153]. Lauka et al. have used wood ash as a solid electrocatalyst, similar to zeolite structure. The results showed a strong correlation between the used sample mixture and pH value [154]. Farandos et al. established stable aqueous colloidal dispersion of YSZ and utilized them to construct 2D planar and 3D microstructures by inkjet printing [155]. Zhang et al. discussed the dynamic state effect of the low concentration of Na⁺ in the SPE water electrolyzer. It was observed that in the presence of Na⁺_, the cell performance degraded more severely [156]. Wang et al. fabricated a CCM for SPE water electrolysis by partially crystallizing a Nafion membrane and catalyst layers. The results revealed that the electrolysis voltage of the SPE water electrolysis with the new CCM was low at 80 °C and atmospheric pressure [157].

The H₂O and CO₂ co-electrolysis in an SOEC is capable of energy storage and utilization of CO₂. Xu et al. numerically studied the effects of CH₄ on CO₂/H₂O co-electrolysis by a 2D model. It was observed that the CH₄ support in dropping the equilibrium SOEC potential hence the electrical power consumption reduces drastically [158]. Tao et al. have reported Ni-YSZ-based SOEC co-electrolysis at a current density of −1.5 or −2.0 A/cm². The thorough electrochemical analysis discovered that there was a substantial increase in the oxide ion transport resistance [159]. Luo et al. established a 2D dynamic model for the dynamic response of CO₂/H₂O co-electrolysis in tubular SOEC (TSOEC) [160]. Menon et al. investigated the behavior of an SOEC co-electrolysis by the interactions between electrochemical parameters and transport processes. The influence of microstructural properties, temperature, and cathode gas velocity are discussed [161]. Stempien et al. reported a comparative study on the equilibrium potential of H₂O and CO₂ SOEC co-electrolysis. It was concluded that at temperatures above 800 °C, when the concentration of CO₂ is below 25%, then the oxygen partial pressure model suffices for modern cells [162]. Luo et al. analyzed the efficiency and the performance of H₂O/CO₂ co-electrolysis in TSOEC. The results showed that the CO₂ conversion ratio was significantly promoted by the reversed water gas shift reaction [163]. Li et al. have tested the H₂O-CO₂ co-electrolysis performance and mechanism of SOEC at 550–750 °C operating temperatures and found out that the co-electrolysis performance significantly increased with respect to the temperature for the Ni/YSZ/ScSZ/LSM-ScSZ electrolysis cell. Also, the CH₄ production is promoted by electricity and effectively suppressed by Ru in porous Ni/SYZ cathode [164]. In an experiment, Kyriakou et al. produced hydrogen from steam electrolysis and simultaneously converted methane to hydrocarbons. The reaction system was investigated in a solid-state oxygen ion (O²⁻) conducting cell at temperatures between 700 °C and 840 °C, and it was also observed that the conversion process was enhanced with the use of SZY on the anodic electrode to serve as a methane coupling catalyst [165]. Patcharavorachot et al. investigated the impact of methane presence on the performance of solid oxide electrolysis cells (SOECs) and introduced a Solid Oxide Fuel-Assisted Electrochemical Cell (SOFAEC) model to evaluate its efficiency. Their study analyzed energy efficiency and power input under conditions with and without CH₄. The findings revealed that SOFAEC exhibited superior performance compared to conventional SOECs, demonstrating enhanced electrochemical efficiency and reduced energy consumption, highlighting the potential of methane-assisted electrolysis in improving hydrogen production efficiency [166]. Aicart et al. performed co-electrolysis at 800 °C and reported the relevance of the macroscopic representation of electrochemical processes through a “surface ratio” that takes into account the co-electrolysis [167]. Li et al. reported the electrochemical reactions, the heterogeneous elementary reactions, the transport of mass and charge, and the electrode microstructure in a 1D elementary reaction of H₂O/CO₂ co-electrolysis in SOEC. The simulation results concluded that the heterogeneous reactions occur near the cathode, and the electrochemical reactions occur in the electrode [168].

The rate of degradation is affected by the variation in current density, resistance, or voltage. Zheng et al. investigated the air electrode contact, H₂ electrode contact, and the electrochemical contributions of the cell at 750 °C in the SOEC stack with a 90/10 H₂O/H₂ ratio. At a constant current density, the air electrode contact, H₂ electrode contact, and the voltage degradation of the cell were 19.6%, 8.9%, and 71.5%, respectively, of the total voltage degradation [169]. Shimada et al. [170] demonstrated that achieving current densities over 3 A/cm² is feasible using nanocomposite oxygen electrodes fabricated via spray pyrolysis. The bimodal-structured electrodes composed of Sm_0.5Sr_0.5CoO_3−δ and Ce_0.8Sm_0.2O_1.9 enabled high performance due to improved conductivity and gas diffusion. These cells achieved 3.13 A/cm² at 750 °C and 4.08 A/cm² at 800 °C, directly correlating high current densities with increased hydrogen production rates. Similarly, Kim et al. [171] emphasized that the electrode microstructure plays a critical role under high current density operations. By tailoring the air electrode using graphite pore formers, they improved both the electrochemical performance and long-term durability. The enhanced porosity and triple-phase boundary density helped maintain performance even under demanding fuel cell–electrolyzer cycles. Schefold et al. [172] conducted long-term testing (over 80,000 ON/OFF current cycles) on SOECs and found that steady operation at a current density of −0.7 A/cm² resulted in minimal degradation—only 5 mV/1000 h. This underscores the feasibility of stable operation under moderate current densities, especially relevant for renewable energy integration where intermittent loading is common. Gaikwad et al. [173] reviewed that while SOECs offer the highest hydrogen generation efficiency among electrolysis technologies, Faradaic efficiency declines at high current densities if gas diffusion and electrode-electrolyte interfaces are not optimized. High current densities increase reaction rates, which can outpace the material’s ability to manage ionic and electronic transport effectively. Jing et al. [174] elaborated on the transition from O²⁻- to H⁺-conducting electrolytes in SOECs, especially under intermediate temperatures (400–700 °C). At higher current densities, proton-conducting SOECs (H⁺-SOECs) offer the advantage of lower activation energies and higher ionic conductivity, making them better suited for high rate operation with improved electrical efficiency. Laguna-Bercero [175] highlighted in high temperature electrolysis that SOECs can achieve efficiencies above 80% (LHV) at high current densities when heat and steam from renewable sources are integrated. Xu et al. [176] investigated the dynamic response of SOECs under fluctuating current densities, particularly in the context of renewable energy integration. They emphasized that rapid shifts in current density led to thermal gradients and mechanical stress, particularly at the electrode-electrolyte interface, which can cause cracking and performance decay. Their study found that maintaining temperature gradients below 10 K/cm is critical for structural safety. Additionally, they highlighted the need for control strategies to manage real-time variation in current density caused by unstable renewable energy sources such as wind or PV to prevent thermal fatigue and irreversible damage. Wolf et al. [177] addressed the industrial implications of operating SOECs at high current densities. Their review of commercial systems, including projects such as GrInHy2.0, revealed that current densities around −0.9 A cm⁻² are achievable over long durations (up to 23,000 h) with acceptable degradation rates (e.g., 0.5–1.5% per 1000 h). They pointed out that despite SOECs achieving near 100% electrical efficiency under ideal conditions, the long-term viability at high current densities demands tailored materials and thermal integration with upstream or downstream processes (e.g., using waste heat from steel production).

Tanaka et al. developed a quasi-1D simulation model for overvoltage and cell voltage in high temperature SOEC. Results showed that there was an improved accuracy against cell voltages at 750–850 °C. Furthermore, the case studies at 800 °C discovered that at 1.0 A/cm² and 82% steam utilization, 40% of local current density distribution in the cell [178]. Bermeio et al. analyzed different thermodynamic cell operational modes and operational strategies. The study revealed that the H₂ production system achieved a flat performance curve within the complete power load range, with overall efficiencies between 91 and 97% as compared to HHV [179]. Peters et al. also investigated the operating conditions and the system configurations to study the efficiencies of producing H₂ by SOEC. The calculated efficiencies vary from 104% to 62% on the basis of lower heating value of H₂ produced [180]. Zheng et al. investigated the NiO/YSZ/GDC/LSCF-GDC SOEC stack at the H₂O/H₂ ratios of 70/30, 80/20, and 90/10 at 750 °C. The investigations indicated that the cell resistance was 76.3–66.7% that of the stack repeating unit (SRU) and 23.6–27% of the contact resistance between the air electrode current collecting layer and the interconnect account [181]. Petrakopoulou et al. studied the four hybrid systems and the four different structures of a steam electrolysis system for H₂ production. As observed, the maximum efficiency was achieved with a recycling sweep gas stream, which was further utilized in the border [182]. Kasai clarified the effectiveness of H₂ for energy storage by high temperature steam electrolysis as compared to power storage by solar energy and nuclear energy [183].

The operating conditions, experimental setup or process of analysis have been discussed to optimize the electrochemical performance of SOEC. Cumming et al. have used a thermal camera to detect changes in the cell temperature by a remote, non-contact, and highly sensitive method [184]. Butler et al. examined the effect of the heat utilization in SOEC efficiency and H₂-specific cell area on a 1D electrochemical model. A sensitivity analysis indicated the increased cost, the higher life, and the decreased power-to-gas storage of the SOEC stack [185]. Wu et al. developed a solid oxide direct carbon-assisted steam electrolysis cell in both the carbon bed and the cell, coupling at 800 °C. After analyzing, it was indicated that the system performance and production rates of gaseous fuels were highly influenced by working voltages of the cell and the carbon bed [186]. Henke et al. performed a theoretical study on the pressurized operation of SOEC in a range between 0.05 and 2 MPa. It was shown that at a low current density and low pressure, the electrolysis cell displayed better performance, and it improved with pressure at high densities [187].

Elder et al. operated SOEC between 500 °C and 900 °C to reduce CO₂ to CO. It was stated that operating at the high temperature gave much higher efficiencies than low-operating electrolysis. Also, the electrode must be both electron and oxide ion-conducting, and minimizing the active surface area is necessary for efficient operation [188]. Mougin has focused on hydrogen production in SOEC and provided an overview of the advantages and challenges of this technology compared with other electrolysis technologies such as AWE or PEMWE [189]. Nechache et al. demonstrated how electrochemical impedance spectroscopy (EIS) is used to characterize the mechanism and the performance of SOEC electrodes and also as a corresponding instrument to study SOEC degradation processes [190]. Luo et al. developed a 1D elementary reaction kinetic model for SOEC with electrode microstructure, electrochemical reaction kinetics, heterogeneous elementary reactions, and transport processes of charge and mass. It was shown that SOFEC saved 80% of electricity at 3000 A/m² and SOFEC displayed better performance than SOEC [191]. Mocoteguy et al. reviewed the cell degradation phenomena under high temperature electrolysis [192]. Jiang et al. reported the electrolysis system and seawater desalination on the basis of the newly invented triboelectric nanogenerator (TENG) [193].

The performance of SOECs heavily depends on the choice of materials for electrodes and electrolytes (

Table 4. Electrolyte materials used in SOEC.

Material	Temperature Range (°C)	Ionic Conductivity (S/cm)	Application
YSZ	800–1000	0.01	Conventional
ScSZ	750–850	0.015	Improved performance
BCY	500–600	0.02	Leakage prevention
GDC	500–800	0.012	Low temperature
LSGM	700–900	0.018	High performance
SPE	80	0.005	Water electrolysis

and

Table 5. Cathode materials used in SOEC.

Material	Operating Temperature (°C)	Performance Improvement
Ni-YSZ	800	Stable but degrades
MoO2	750	Better for CO2 electrolysis
LSCM	800	High efficiency
LSV	850	Low resistance
SFN	900	Higher conductivity
LSTO	800	Catalytic improvement

). YSZ remains the conventional electrolyte due to its stability at high temperatures (800–1000 °C). However, alternative electrolytes such as gadolinium-doped ceria (GDC) and barium cerate (BCY) have shown promise in reducing operating temperatures while maintaining high ionic conductivity. The development of advanced cathode materials, such as LSV and Sr₂FeNbO₆ (SFN), has improved conductivity and efficiency, reducing degradation over long-term operation.

Solid oxide electrolysis cells (SOECs) have demonstrated significant potential for high efficiency hydrogen production, achieving 90–100% efficiency in high temperature steam electrolysis (HTSE). However, material degradation, system stability, and cost-effectiveness remain key barriers to large-scale commercialization. The primary degradation mechanisms include electrode delamination, phase segregation, and microstructural instability at elevated temperatures. Studies have reported degradation rates ranging from 0.3% to 3.1% per 1000 h, influenced by current density, operating temperature, and electrode-electrolyte interactions. Zhang et al. observed a 3.1% degradation over 830 h at 0.41 A/cm², while Hauch et al. optimized Ni-YSZ microstructures, reducing degradation to 0.3% under similar conditions. Despite its advantages, SOEC performance and stability are highly dependent on electrode and electrolyte selection (Table 4 and Table 5). YSZ-based electrolytes remain the standard due to their stability at 800–1000 °C, but emerging materials such as BCY, GDC, and LSGM offer lower operating temperatures with improved ionic conductivity. Cathode materials, including LSCM, LSV, SFN, and MoO₂, have been explored to improve electrochemical activity and durability, with SFN demonstrating higher conductivity and LSCM showing superior efficiency in direct steam electrolysis. Operational parameters, particularly temperature and gas composition, significantly impact performance. Higher temperatures (>900 °C) improve reaction kinetics, as Elder et al. demonstrated, but also accelerate degradation. Studies on co-electrolysis of H₂O and CO₂ have shown promising results in synthetic fuel production and energy storage, with methane presence reducing equilibrium potential and power consumption. Future research should focus on developing advanced electrode materials, such as perovskite-based catalysts and infiltration-modified composites, to enhance catalytic activity and long-term stability. Additionally, hybrid SOEC configurations, integrating renewable energy sources such as solar and wind, could create a sustainable hydrogen production pathway. Economic feasibility will depend on reducing material costs, optimizing manufacturing processes, and scaling up production. Advances in computational modeling and real-time electrochemical monitoring will further drive system efficiency improvements. While challenges remain, continued research in SOEC technology can position it as a key enabler of a hydrogen-based energy economy, providing an efficient and scalable solution for future storage of renewable energy derived electricity.

3.4. Microbial Electrolysis Cell (MEC)

Microbial Electrolysis Cells (MECs), first introduced in 2005, share fundamental principles with Microbial Fuel Cells (MFCs) but operate under opposite electrochemical conditions. Unlike MFCs, which generate electricity from organic matter, MECs convert electrical energy into chemical energy to produce hydrogen (H₂) from organic substrates by applying an external voltage (Figure 6d). The anodic reactions involve microbial oxidation of the substrate, resulting in the release of H⁺, electrons, and CO₂. The electrochemical potential generated at the anode facilitates proton migration through the electrolyte toward the cathode, where they combine with electrons to form hydrogen gas. The fundamental reactions, charge carriers, and overall cell mechanisms for MEC electrolysis are outlined in Table 3.

One of the key advantages of MECs is their ability to utilize wastewater as a substrate, providing a sustainable approach to hydrogen production while simultaneously treating organic waste. However, the technology remains limited to laboratory-scale research due to low H₂ production rates, hydrogen purity challenges, high internal resistance, costly electrode materials, and complex system designs. High ohmic losses and biofilm limitations further restrict large-scale application. Over the past decade, extensive research has focused on optimizing electrode materials, reducing internal resistance, and improving microbial community efficiency to enhance overall system performance. Future advancements in catalyst development, system integration, and hybrid MEC configurations could pave the way for commercial viability, positioning MECs as a potential solution for sustainable hydrogen production and wastewater treatment within the water-energy-resource nexus.

A growing body of research has focused on enhancing electrode materials, improving bioanode efficiency, and optimizing cathodic reactions to overcome the current limitations of MEC technology and enhance hydrogen production yields. Ji et al. [194] investigated the use of Fe²⁺-modified biochar electrodes in MECs for phosphorus recovery from wastewater. Their study demonstrated a significant increase in current density and phosphorus removal efficiency, with power consumption reduced to 0.25 ± 0.01 kWh/kg P. This phosphorus-enriched biochar was also found to enhance soil quality when applied as a fertilizer. Samsudeen et al. [195] explored hydrogen generation using MECs integrated with anaerobic digesters for distillery wastewater treatment. Their modified MEC design yielded a maximum hydrogen production of 30.2 ± 1.2 mL at a current density of 811.33 ± 20 mA/m². The compact retrofit design also showed a substantial reduction in chemical oxygen demand (COD) and improved energy recovery, demonstrating the feasibility of MECs for decentralized applications. Seelajaroen et al. [196] advanced MEC performance by modifying electrodes with poly(neutral red) and chitosan. Their findings highlighted enhanced COD removal (up to 67%) and methane yield (0.14 L CH₄/g_COD), showcasing the dual potential of MECs in both carbon removal and biogas production. Chung et al. [197] focused on microbial hydrogen peroxide-producing cells (MPPCs), a subclass of MECs designed for H₂O₂ synthesis via the cathodic oxygen reduction reaction. Their review emphasized the environmental applications of bio-electrochemically generated H₂O₂, especially for water disinfection and pollutant degradation. Kadier et al. [198] optimized the operation of MECs for palm oil mill effluent (POME) treatment. Using response surface methodology (RSM), the study identified optimal conditions for maximizing hydrogen production and COD removal, highlighting the applicability of statistical modeling in scaling up MECs. Gonzalez et al. [199] examined various wastewater management technologies, including MECs. It emphasized the role of MECs in integrating with anaerobic membrane bioreactors and nature-based systems, especially in regions lacking centralized wastewater infrastructure. Cui and Yin [200] provided a comprehensive review of MEC applications for treating acid mine drainage (AMD), coupling pollutant removal with renewable hydrogen production. Their work stressed the effectiveness of MECs in acidic conditions and the potential for integrating with other technologies such as chemical precipitation and membrane filtration. Murugaiyan et al. [201] presented a detailed analysis of MEC configurations, including single and dual-chamber, packed-bed, and fluidized-bed systems. They concluded that while MECs are effective for simultaneous biohydrogen production and wastewater treatment, challenges such as high membrane and electrode costs remain significant. Koul et al. [202] highlighted the energy-intensive nature of conventional wastewater treatment and the potential of MECs for resource recovery. They advocated for integrating MECs with anaerobic digestion and membrane bioreactors to enhance energy efficiency and sustainability. Radhika et al. [203] reviewed various MEC applications, including energy generation, methane and hydrogen peroxide production, and pollutant removal. Their study also discussed advancements in biocathode materials, reactor design, and coupling MECs with microbial fuel cells for synergistic effects.

Cho et al. explored the use of the waste electrolysis cell (WEC) to decentralize hydrogen production with onsite water treatment. The WEC consisted of the stainless steel cathode and the multi-junction semiconductor anode in a single compartment cell. The highest energy efficiency (EE = 0.23) and current efficiency (CE = 0.8) for HER were observed in electrolysis at current densities of 200 A/m² of real wastewater [204]. Lu et al. produced a NiFe layered double hydroxide (NiFe LDH) electrocatalyst on NF for H₂ production from wastewater. The new cathode showed a high H₂ rate with the Pt catalyst, which was twice as high compared to stainless steel and bare NF cathodes [205]. Kaider et al. examined a metal electroformed Ni mesh cathode alternative to Pt/CC in a single chamber membrane-free MEC. According to results, it was concluded that great potential should be used in Ni mesh catalyst as a cathode material for H₂ production in a single chamber membrane-free MEC [206]. Lim et al. stated the bioanode at a current density of 0.36 A/m² and 0.37 A/m² with the potential of 0 and +0.6 V, respectively. During this, the H₂ production was 7.4 L/day at a cathode potential of −1.0 V [207]. Chen et al. improved the biocathodes with PANI (polyaniline)/MWCNT composites for higher H₂ production in single chamber, membrane-free biocathode MECs. The results concluded that with an increment in applied voltage, the H₂ production rates increased, and the performance of MECs was improved with modified biocathodes at a higher current density and H₂ generation rate [208]. Li et al. obtained excellent perchlorate reduction under various initial concentrations in a non-membrane MEC with a PANI-modified graphite cathode as the sole electron donor [209]. Liu et al. used microbial fluidized electrode electrolysis cells to increase the H₂ gas production. In this, the flowable granular activated carbon (GAC) particles were used for the growth of exo-electrogenic bacteria, which enhanced hydrogen production in bio-electrochemical systems [210]. Wang et al. evaluated the feasibility of operating the MEC at low temperatures (10 °C) using biocathodes. It was stated that H₂ could be generated from wastewater by using biocathodes [211]. Huang et al. observed that a self-driven microbial electrolysis cell or microbial electrolysis cell can completely release Co (II) from LiCoO₂ on the cathodes in MFCs. This study provided a new process of linking MFCs to MECs to recover cobalt and recycle it to Li-ion batteries with no external energy consumption [212]. Cusick et al. designed a two-chamber MEC to produce suspended particles and discovered that MEC is an expectant method of energy recovery and electrochemical nutrient for nutrient-rich wastewaters [213]. Figure 13a shows the water electrolysis cell with the plot showing current efficiency and electrical efficiency [204]. Figure 13a demonstrates redox and mass transport processes involving water splitting, chloride oxidation to free chlorine, and degradation of COD, enhancing both hydrogen production and pollutant removal. Efficiency graphs indicate that while current efficiency increases with current density, energy efficiency remains relatively low. Material analysis confirms the electrode composition (Bi, Ti, O) and favorable morphology for electrochemical activity. Figure 13b shows the plot between potential and time at different cell operating conditions [212]. Figure 13b includes the lab setup and a performance graph showing electrode potentials over five days under different cathodic biases. A sharp change around day 3.5 indicates bioanodic performance loss in MEC, emphasizing the importance of biofilm stability for long-term operation. Marone et al. evaluated bio-H₂ production by dark fermentation and microbial electrolysis from agro-industrial wastewater. The results concluded that the above combination was a promising option for optimizing the conversion of wastewater [214]. Huang et al. have used toilet wastewater for MEC. Their primary goal was to establish the feasibility of MEC for toilet wastewater disinfection [215]. Dhar et al. produced hydrogen from sugar beet juice (SBJ) using MEC. The H₂ production was 25% of the initial COD, and the energy recovery from SBJ was 57% by combined biohydrogen [216]. Heidrich et al. performed MEC using domestic wastewater. The 100-L MEC was operated on raw domestic wastewater at 1 °C to 22 °C, producing 0.6 L/day of H₂ [217]. Trably et al. used saline wastewater in a biofilm-based 4 L two-chamber MEC continuously fed with acetate under saline conditions for more than 100 days [218]. Li et al. investigated the integrated microbial desalination cell-MEC system by saving nitrogen, metal, and saline from municipal wastewater, industrial wastewater, and seawater, respectively, to observe the nitrogen degradation rate [219]. Kuntke et al. have performed MEC using urine for ammonium removal and hydrogen production [220].

Linji et al. mixed trehalose with wastewater to perform MEC. The feasibility of physical and biological processes was observed for sludge treatment and the effects of trehalose on the H₂ generation of MEC at 0 °C [221]. Sun et al. recovered hydrogen from high solid waste activated sludge (WAS) using MEC. With the optimal concentration, maximum hydrogen yields were reached for MECs fed with raw WAS and alkaline preheated WAS [222]. Zhang et al. investigated the improvement of a zero-valent iron-activated carbon (ZVI-AC) micro-electrolysis system on H₂ production by a mixed bacterial consortium. The results showed that 38.2% more hydrogen was produced with the addition of ZVI, and further addition of ZVI-AC enhanced the H₂ production [223]. Wu et al. performed electrolysis of 0.33 M urea using a Ni hydroxide electrode on stainless steel foil by cathodic electrodeposition with 1 M KOH electrolyte. It was observed that the Ni hydroxide electrode obtained much better electro-catalytic performance than the film and hollow sphere electrodes [224]. Figure 14a shows a two-stage process combining dark fermentation with an MEC, enhancing hydrogen yield by converting fermentation products into H₂ electrochemically [214]. Figure 14b integrates MECs with solar power, showcasing a renewable-driven setup with improved energy and environmental performance [215]. Figure 14c features sugar beet waste utilization in a hybrid system yielding high H₂ efficiency and energy recovery [216]. Figure 14d highlights the seasonal dependence of MEC performance in outdoor conditions, with higher efficiency during warmer months due to elevated microbial activity [217]. Figure 14e demonstrates a continuous-flow MEC using granular bioanodes and membrane separation to achieve 90% H₂ purity, emphasizing real-world viability [218]. Finally, Figure 14f illustrates the competition between hydrogen and methane production, underlining the importance of microbial control for selective H₂ generation [219]. MEC performance and stability depend upon the experimental setup. Zhang et al. have used a double anode arrangement to perform MEC for achieving H₂ production from glucose by adding CH₄. It was observed that the chloroform was an effective methane inhibitor and improved the efficiency of H₂ production from glucose in the MECs [225]. Lewis et al. used integrated pyrolysis- MEC to describe hydrogen production. The results demonstrated that the pyrolysis microbial electrolysis process was a sustainable and efficient route for the production of H₂ from biomass [226]. Watson et al. used a microbial reverse-electrodialysis electrolysis cell (MREC) from the fermentation of wastewater to yield H₂. The results confirmed that if effluent anolyte COD concentrations were sufficient for anode potential, then the consistent rates of H₂ produced by MREC [227]. Song et al. have used substrate (without buffer solution) under continuous flow conditions in an MREC for H₂ production. The hydrogen produced was 0.61 m³-H₂, with a COD removal efficiency of 81% and a coulombic efficiency of 41% in MREC [228].

Chen et al. investigated the thermoelectric micro-converter MEC coupled system for H₂ production from acetate. This study showed that this system directly converted waste heat energy to electricity at a temperature difference of 5 °C and helped MEC to produce H₂ [229]. As a monitoring tool for hydrogen-producing MEC, Montpart et al. presented a low-cost sensor for H₂ production measurement. As a result, the fuel cell electrical signal had a high correlation with H₂ production, and fuel cell quantification was proved to be equivalent to gas chromatography analysis [230]. Guo et al. reported on a liter-scale tubular MEC for obtaining high rate H₂ production; its components, such as a Pt-coated Ti mesh cathode, an anion exchange membrane, and a pleated stainless steel felt anode, were arranged in a concentric configuration. The reactor presented high H₂ recovery, high H₂ purity, and outstanding operational stability [231]. Montpart et al. have also used a single chamber MEC for H₂ production from synthetic wastewater. The results displayed that at 0.8 V, the current intensity, H₂ production, and cathodic gas recovery were 150 A/m², 0.94 H₂m³/m³d, and 91%, respectively [232]. Lou et al. utilized a dual-chamber MEC for H₂ production, and also metal removal, such as Cu²⁺, Ni²⁺, and Fe²⁺ from acid mine drainage (AMD) [233]. Zhang et al. described an active iron-reducing bacteria (IRB) through dosing Fe (III) into an MEC for the degradation of organic matter. This study provided a method to enrich electrochemically active IRB in the bio-electrochemical reactor for treating industrial wastewater [234]. The membrane affects H₂ production in electrolysis, so even in MEC, the effect of the membrane on electrolysis was reported. Chae et al. prepared a nanofibre-reinforced composite proton exchange membrane based upon a proton conductor, i.e., sulfonated polyether ketone (SPEEK). This novel membrane was compared with the Nafion membrane and resulted in lower gas and fuel crossovers with higher proton conductivity that improved H₂ production at the cathode, with overall H₂ efficiency as compared to Nafion [235].

Khan et al. reviewed the status of MEC as a mean wastewater treatment method and H₂ production. This study estimated a total electricity of 434 MWe could be produced in 2015 from the Kingdom of Saudi Arabia’s wastewater if MEC technology was employed [236]. Cotteril et al. reviewed the production of H₂ in MEC and treated the wastewater to reduce the energetic and economic costs of operation [237]. Zhang et al. further discussed the recent advances and future challenges of MEC. This approach significantly reduced the cost of the electric energy for H₂ production as compared to direct water electrolysis [238]. Kaider et al. have given a review of the substrates used in MEC, as there is a large number of substrates that could be used as a fuel source [239]. Rago et al. analyzed the microbial community in membrane-less MEC. As a single chamber MEC, the production of H₂ failed due to methanogenesis buildup. Moreover, at higher 2-BromoEthanesulfonate concentrations, the methanogenesis activity was decreased and increased the homo acetogenesis activity, which optimized the performance of the MEC for H₂ production [240]. Kaider et al. completed a review of reactor design and configuration of MEC. The study showed that MEC reactor design directly influences the H₂ and current production rate in MECs, because the membrane-free design can lead to both on H₂ production rate and recovery rates [241]. Further, the recent advances and challenges in MEC were also reviewed [242].

Significant advancements have been made in MEC research, particularly in the development of alternative electrode materials to replace expensive platinum-based catalysts (

Table 6. Some of the MEC’s electrode material with substrate, H₂ production rate, and efficiency.

Electrode Material	Substrate Used	H2 Production Rate	Efficiency
Stainless steel cathode and multi-junction semiconductor anode [204]	Real wastewater	High EE (0.23), CE (0.8)	Efficient at 200 A/m2
NiFe LDH electrocatalyst [205]	Wastewater	Higher H2 rate	Twice as compared to stainless steel cathodes
Ni mesh cathode [206]	Single chamber membrane-free MEC	High H2 production	Alternative to Pt/CC
Bioanode [207]	Acetate	7.4 L/day	Stable at −1.0 V cathode potential
Biocathode [211]	Low temperature wastewater	Stable H2 generation	Operated at 10 °C

). Materials such as NiFe layered double hydroxides (LDH), polyaniline (PANI)/MWCNT composites, and Ni mesh electrodes have demonstrated enhanced hydrogen production. System configurations have also evolved, with studies exploring single chamber, membrane-free designs and microbial reverse-electrodialysis electrolysis cells (MREC) to improve efficiency and reduce operational costs. MECs have been integrated with other technologies to optimize hydrogen production and wastewater treatment. For example, microbial desalination cells, thermoelectric converters, and dark fermentation processes have been combined with MECs to enhance overall performance. Additionally, research on optimizing operating conditions, such as temperature, pH, applied voltage, and microbial communities, has provided insights into improving system efficiency. Studies on low temperature MECs have demonstrated feasible hydrogen production at 10 °C, making the technology applicable in colder regions.

Despite these promising advancements, MECs still face critical challenges related to high internal resistance, low hydrogen purity, interference from methanogenesis, and scalability constraints. Overcoming these limitations necessitates improvements in biocatalyst efficiency, reactor design optimization, and comprehensive techno-economic assessments. While research has advanced from proof-of-concept studies to refined designs featuring enhanced electrode materials and system efficiencies, commercial deployment remains hindered by both technical and economic barriers.

Recent studies have demonstrated that alternative electrode materials, such as NiFe LDH and Ni mesh cathodes, can reduce costs while maintaining efficiency, while single chamber and membrane-free configurations provide advantages in hydrogen recovery and system simplicity. Process integration with dark fermentation and thermoelectric conversion has further enhanced MEC performance, improving overall energy yields. However, achieving scalability, cost-effectiveness, and long-term operational stability remains a major challenge. Future advancements must focus on developing durable, cost-effective, and high efficiency catalysts to replace expensive noble metals. Hybrid systems, integrating MECs with other bio-electrochemical and renewable energy technologies, such as photovoltaic-MEC hybrids, could improve overall energy recovery and system viability. Additionally, automation and AI-driven real-time monitoring could optimize MEC performance, energy management, and maintenance strategies.

Scaling up MECs for industrial and municipal wastewater treatment is another critical research direction, as these applications could leverage the dual benefits of sustainable hydrogen production and organic waste remediation. However, comprehensive techno-economic analyses are needed to determine the feasibility of large-scale implementation. By addressing these scientific and engineering challenges, MECs have the potential to transition from laboratory research to commercially viable solutions for hydrogen production and wastewater treatment, contributing significantly to the water-energy-resource nexus.

4. Advancements and Emerging Trends in Electrolysis Technologies for Hydrogen Production

Hydrogen production via electrolysis is influenced by secondary factors beyond electrode and electrolyte selection, including gas bubble dynamics, mass transport limitations, system efficiency losses, and integration with renewable energy sources. Addressing these challenges is essential for scaling up electrolysis technologies and improving long-term performance. Recent advancements focus on controlling bubble formation, optimizing electrochemical interfaces, enhancing catalytic efficiency, and refining membrane extraction methods. Additionally, alternative electrolysis approaches, such as high temperature, ionic-liquid-assisted, and hybrid electrolysis, are being explored to improve hydrogen yield, energy efficiency, and cost-effectiveness. These developments pave the way for more efficient and commercially viable electrolysis systems. Chandran et al. experimentally investigated the H₂ bubble formation characteristics and its transport near the electrode surface. It was concluded that the bubble velocity was key in defining the bubble size distribution near the electrode surface [243]. Wang et al. further developed a method to determine the effect of the gas bubble on cell voltage oscillations. The simulation on Matlab/Simulink revealed that the gas bubbles on the cell voltage exceeded the normal fluctuation amplitude [244]. Goupil et al. conducted a comprehensive study on the behavior of CuNiFe anodes and CuNiFeO electrodes at 700 °C in potassium cryolite-based electrolyte during the electrolysis [245]. Gutiérrez-Martín et al. explored hydrogen and power integration strategies through large-scale water electrolysis in the Spanish power system, highlighting its potential for flexible energy storage and grid balancing. However, the study reveals several challenges, including the high sensitivity of hydrogen production costs to electricity prices, underutilization of electrolyzers due to fluctuating renewable surplus, and complex grid integration requirements. The economic feasibility is further constrained by high capital costs and the inefficiencies of part-load operation. The authors suggested strategies such as dynamic operation aligned with real-time electricity pricing, decentralized electrolyzer deployment, co-location with renewable sources, and integration with fuel cells for improved energy return [246]. Alhahosseini et al. investigated the challenges associated with the Cu-Cl cycle of H₂ production. In the Cu–Cl cycle, hydrolysis of CuCl₂ requires a significant excess of steam (up to 17:1 H₂O to CuCl₂ molar ratio) to ensure high yields of Cu₂OCl₂, but this results in a diluted HCl gas product, which is insufficient for the subsequent electrolysis step that demands 6–11 M HCl for optimal efficiency. To address this, the authors propose an integrated system involving a heat recovery steam generator (HRSG) and an HCl–H₂O separation process using rectification and absorption columns. Their simulation demonstrates that this integration not only supplies the required concentrated HCl (up to 22 mol%) for electrolysis but also recovers excess steam to improve the thermal efficiency of the cycle. The study highlights that with effective integration, the Cu–Cl cycle can achieve thermal efficiencies as high as 88.6%, making it a competitive alternative to conventional water electrolysis [247]. Liu et al. investigated coal pretreatment using the ionic liquid [B_mim]Cl. The treatment disrupted coal’s structure, increasing porosity and breaking hydrogen bonds. This structural modification enhanced access to reactive sites during electrolysis. As a result, electro-reduction activity improved by 15% compared to untreated coal [248]. Chen et al. have investigated the effects of ionic liquids, which included the concentration, the structure, the temperature, and the time, on the removal of sulfur from coal water slurry electrolysis. The results showed the effect of the anions on desulfurization and exhibited better performance as compared to pyridine [249]. Slampova et al. studied that electrolysis can be controlled by proper electro-membrane extraction method (EME) selection for H₂ production [250]. Cheng et al. summarized the progress of metal oxides catalysts and the role of carbon nanotubes (CNTs) in OER catalysts with the latest development [251]. Guerra et al. described the production of synthesis gas by using graphite electrodes from water electrolysis without the separation of the produced gases. The obtained synthetic gas can be used for fuels, such as CH₄, DME (dimethyl ether), or methanol in a catalytic reactor, using the ELECTROFUEL concept [252].

Anion exchange membrane water electrolyzers (AEMWEs) have emerged as a promising alternative to conventional electrolysis technologies, aiming to combine the cost advantages of alkaline water electrolysis with the efficiency of PEM systems. Operating in an alkaline environment while employing a solid polymer electrolyte, AEMWEs enable the use of non-precious metal catalysts, maintain high ionic conductivity, and support compact system designs. As Peng [253] and Sugawara et al. [254] note, their performance is critically governed by the membrane electrode assembly (MEA), where synergy between the membrane, ionomer, and electrocatalysts determines overall efficiency.

Recent research has addressed several key challenges in AEMWE development. Li et al. [255] enhanced OER kinetics and long-term stability using chromium-doped amorphous oxides (e.g., CoCrO_x), while Lim et al. [256] introduced a quaternized polyfluorene-based membrane (PFPBPF-4-QA) with high conductivity and alkaline stability. Zheng et al. [257] demonstrated a system-level breakthrough by achieving stable operation at ultrahigh current densities (10 A/cm² for over 800 h) through integrated optimization of membranes, ionomers, and porous transport layers.

Catalyst development has also advanced. Klingenhof et al. [258], using operando spectroscopy, explored Ni–O redox-active ligand systems, revealing mechanisms that enable PEM-like OER performance without noble metals. Xu et al. [259] and Chang et al. [260] contributed insights into interface engineering and numerical modeling, offering strategies to improve mass/charge transfer and apply materials innovations to real-world system configurations.

On the durability front, Motealleh et al. [261] reported over 10,000 h of stable operation using Sustainion^® membranes, achieving degradation rates below 1 mV/h—a notable benchmark for commercial viability. Liu et al. [262] complemented these findings by identifying chemical degradation pathways and proposing structural strategies to improve long-term conductivity and alkaline resistance.

Collectively, these studies demonstrate the rapid progress of AEMWEs through innovations in membrane design, catalyst engineering, and system integration. Their inclusion in this review strengthens its relevance by capturing one of the most promising directions in next-generation water electrolysis.

Advancements in water electrolysis technologies have largely been driven by material innovations, process optimizations, and experimental studies. While experimental research provides critical insights into electrode materials, electrolyte composition, and operating conditions, it often faces challenges in capturing microscopic charge transport phenomena, reaction kinetics, and mass transport limitations. This limitation makes it essential to complement experimental findings with computational modeling techniques, which allow for a deeper understanding of electrochemical interactions, system efficiencies, and degradation mechanisms.

5. Numerical Modeling and Finite Element Analysis of Water Electrolysis

Computational modeling techniques such as the finite element method (FEM), computational fluid dynamics (CFD), response surface methodology (RSM), and system-level models (e.g., Aspen Plus^®) play a critical role in optimizing water electrolysis systems. These methods allow for the analysis of electrochemical kinetics, thermal distribution, fluid dynamics, and performance degradation. Each modeling strategy offers unique strengths depending on the scale and objectives of the analysis, as summarized in Figure 15.

As shown in Figure 15, FEM is commonly used for localized, detailed simulations involving mechanical stress and temperature distribution. CFD, on the other hand, models multi-phase flow and bubble dynamics in electrolyzer cells. RSM provides empirical optimization of multiple variables with fewer simulation runs, while Aspen Plus^® supports techno-economic and system-level analyses for full-scale integration of electrolyzers with energy sources.

Each numerical simulation method used in water electrolysis research serves a distinct modeling purpose, and its applicability depends on the scale, objective, and complexity of the problem being addressed. The FEM is best suited for detailed, cell-level investigations where coupled physical phenomena such as electrochemical kinetics, thermal gradients, and mechanical stress coexist. Its ability to solve coupled partial differential equations over complex geometries makes it ideal for analyzing localized effects in catalyst layers, membrane–electrode assemblies, and current collectors. FEM is particularly effective for optimizing component configurations and predicting degradation behavior, albeit at a high computational cost.

In contrast, CFD is the preferred tool when analyzing mass transport, bubble dynamics, or flow field design. CFD excels in simulating two-phase flow phenomena in alkaline and PEM electrolyzers, especially under high gas evolution rates. While it lacks the intrinsic electrochemical modeling fidelity of FEM, CFD provides essential insight into gas-liquid interactions, which influence ohmic resistance and bubble-induced overpotentials. For broader design-of-experiment applications, RSM offers an efficient statistical framework for optimizing performance across multiple operating parameters. Though it does not resolve spatial or temporal physics, RSM is valuable for identifying influential variables and constructing surrogate models for process optimization.

System-level models, such as those developed in Aspen Plus^®, are indispensable for analyzing integrated hydrogen production systems. These models handle energy and mass balances, load-following dynamics, and techno-economic parameters, making them highly relevant for coupling electrolyzers with renewable sources such as solar or wind. However, Aspen Plus^® operates at the plant scale and does not capture internal electrochemical or transport dynamics. Finally, multiphysics coupling platforms (e.g., COMSOL Multiphysics) provide a unified environment to simulate thermal, fluidic, and electrochemical effects concurrently. These tools are ideal for capturing cross-domain phenomena in advanced electrolyzer designs but demand significant computational resources and model development expertise.

In summary, FEM offers the most comprehensive physical fidelity for cell-level simulations, CFD dominates in hydrodynamic analysis, RSM simplifies parameter optimization, Aspen Plus^® enables system-scale integration, and multiphysics frameworks offer synergistic modeling of interacting phenomena. Selecting the appropriate method requires careful consideration of modeling depth, computational efficiency, and the specific challenges addressed in the electrolyzer system under study.

While various modeling methods have been explored, FEM is emphasized in greater detail due to its wide applicability in stress analysis, thermal management, and multiphysics coupling within electrolyzer components. Numerical modeling, particularly FEM-based simulations, has emerged as a powerful tool to bridge the gap between experimental observations and theoretical predictions by providing quantitative and predictive analyses of electrochemical behavior. Given the multi-scale, coupled transport phenomena and complex electrochemical interactions in PEM electrolysis, it was selected as the focus of this numerical study. Unlike AWE, which operates with well-understood liquid-phase mass transport, PEM electrolysis involves interdependent kinetic, thermodynamic, and mass transport processes that significantly influence system performance. These characteristics make PEM an ideal candidate for FEM-based electrochemical simulations, enabling precise performance predictions under varying operational conditions.

Numerical modeling and simulations play a vital role in evaluating design changes, predicting performance, and avoiding costly experimental iterations. FEM and CFD are increasingly employed to optimize the design and performance of AWE systems by enabling detailed analysis of fluid flow, gas evolution kinetics, current density distribution, heat transfer, and structural mechanics. The fidelity of FEM and CFD simulations in electrolysis systems depends critically on the accuracy of input parameters such as electrolyte conductivity, electrode geometry, membrane transport coefficients, gas-liquid interface behavior, and heat transfer properties. When properly calibrated against experimental data, high-fidelity models can predict key performance metrics (e.g., current density, temperature gradients, pressure drop) within 5–10% of measured values. However, challenges remain in simulating bubble-induced resistance, local pH shifts, and two-phase flow transients. To enhance model robustness, advanced simulation workflows incorporate mesh refinement studies, sensitivity analyses, and partial experimental validation to improve predictive confidence and ensure reliability in design-oriented applications.

Several studies underscore the importance of integrating optimized geometrical configurations, mechanical assembly parameters, and operational conditions to maximize the performance and durability of PEM and alkaline water electrolyzers. Yang et al. [263] investigated a novel orthogonally sinusoidal flow channel geometry for PEMECs, finding that increased amplitude and angular frequency in the horizontal and vertical planes significantly improved electrochemical performance and mass transport. Similarly, Tirumalasetti et al. [264] compared multiple flow field designs and demonstrated that the Double-Layered Wire Mesh (DLWM) configuration enhanced hydrogen mole fraction and current density distribution more effectively than conventional flow fields. The optimization of structural and operational parameters plays a crucial role in the performance and longevity of electrolyzers. Kink et al. [265] conducted finite element simulations to evaluate mechanical stresses on PEM membranes and found that membrane buckling could be prevented by maintaining a gap below 0.3 mm between the membrane and the porous transport layer. Complementarily, Ozdemir et al. [266] applied response surface methodology alongside finite element modeling to optimize bolt torque in PEMWE assemblies. They identified an optimal torque of 10 Nm, which minimized mechanical stress while preserving membrane integrity and electrochemical efficiency. Operational conditions such as pressure, temperature, and flow rate also significantly influence system performance. Hassan et al. [267] used a 3D non-isothermal model to analyze the effects of gas diffusion layer thickness and porosity, reporting that higher temperatures improved performance while elevated cathode pressures impaired hydrogen ion diffusion and decreased efficiency. Jang et al. [268] explored pressure effects in an alkaline water electrolysis system using Aspen Plus^® modeling and showed that high pressure operation improved hydrogen purity and reduced energy consumption, although purity gains diminished above 20 bar. Transport phenomena and flow field design continue to be pivotal in enhancing performance. Zheng et al. [269] used 3D simulations to analyze various flow field patterns and concluded that parallel flow channels provided superior current distribution and minimal pressure drop. Their work highlighted the importance of temperature and flow structure in facilitating efficient mass transfer.

To further enhance the understanding of PEM electrolysis, this study employs an FEM-based approach to model key electrochemical and mass transport processes, allowing for precise prediction of system performance, efficiency, and degradation mechanisms. Specifically, we analyze electrochemical potential distribution, overpotential contributions, mass transport dynamics, and gas bubble evolution effects. By incorporating a multi-physics simulation framework, the study provides valuable insights into reaction kinetics, ion transport, and electrode behavior, offering guidance for performance optimization and system design improvements. This numerical modeling approach not only enhances the fundamental understanding of PEM electrolysis but also serves as a predictive tool for optimizing key parameters such as current density, electrode material selection, and pressure settings, ensuring higher hydrogen production efficiency and system longevity.

5.1. Finite Element Model for PEM Electrolysis

The FEM-based approach employs numerical solutions to Laplace’s equation for electrochemical potential distribution, Nernst–Planck equations for ion transport, and Butler–Volmer kinetics for electrode reactions. The key governing equations include Electrochemical Potential Distribution (Laplace’s equation) that governs charge transport across the electrolyte and electrodes. The electrochemical potential distribution follows Equation (1).

$${\nabla }^2\varnothing =0$$

(1)

where:

• ∅ = electrochemical potential (V).

Mass Transport (Nernst–Planck Equation): Models ion diffusion and electromigration in the electrolyte:

The transport of ions is modeled as Equation (2).

$$\nabla ·(-D\nabla c+\mu c\nabla \varnothing )=R$$

(2)

where:

• C = ion concentration (mol/m³)
• D = diffusion coefficient (m²/s)
• μ = ion mobility (m²/V·s)
• R = reaction rate (mol/m³·s)

$${\displaystyle \frac{\partial c}{\partial t}}=D{\nabla }^{2}c-\nabla ·(\mathrm{z}FcEc)$$

(3)

where:

• E = the electric field.

Electrode Reaction Kinetics (Butler–Volmer Equation):

$$j=j_0\left[e^{{\alpha }_{a}F\eta /RT}-e^{{-\alpha }_{c}F\eta /RT}\right]$$

(4)

where:

• j = current density
• j₀ = exchange current density
• η = activation overpotential.

These equations were solved using FEniCSx 0.8. (open-source FEM solver) with a 3D cubic mesh (32 × 32 × 32 elements), providing insights into spatial variations in electrochemical properties.

5.2. Gas Evolution and Bubble Dynamics

One of the major challenges in PEM electrolysis is bubble formation at electrode surfaces, which introduces resistance to ion transport and reduces reaction efficiency. The FEM model integrates a multiphase CFD approach, solving the Navier-Stokes equations coupled with a Volume of Fluid (VOF) model to track bubble growth and detachment. The effective electrolyte conductivity (σ_eff) is modeled as:

$${\sigma }_{eff}={\sigma }_{bulk}(1-\varphi )$$

(5)

where:

• ϕ = gas volume fraction, reducing ion transport efficiency as bubbles accumulate.

Gas Bubbles Introduce Resistance to Ion Transport:

$${\sigma }_{eff}=\sigma (1-ϵ)$$

(6)

where:

• σ = bulk electrolyte conductivity
• ϵ = gas volume fraction.

$$ϵ=k_b{j}^b$$

(7)

In Equation (7), j is the local current density (A/m²), and k_b and b are empirical fitting constants that depend on electrolyte type and operating conditions. These are used to estimate the gas volume fraction ϵ, which quantifies average gas holdup and its impact on effective conductivity.

Modified Ohm’s Law with Gas Bubble Effects:

$$J={-\sigma }_{eff}\nabla \varnothing$$

(8)

Electrode Reaction Kinetics (Butler–Volmer Equation)

The reaction kinetics are modeled using:

$$j=j_0\left[e^{{\alpha }_{a}nF\eta /RT}-e^{{-\alpha }_{c}nF\eta /RT}\right]$$

(9)

$$\eta =\varnothing -{\varnothing }_{rev}-iR$$

(10)

In Equation (9), j is the local current density (A/m²), j₀ is the exchange current density, α_a and α_c are the anodic and cathodic charge transfer coefficients, F is the Faraday constant (96,485 C/mol), η is the overpotential, R is the universal gas constant (8.314 J/mol·K), and T is the temperature in Kelvin. Equation (10) defines the overpotential η, where ϕ is the local electric potential, ϕ_rev is the reversible potential, and iR accounts for ohmic losses.

While both ϕ and ϵ are referred to as gas volume fractions, they are used in distinct modeling contexts: ϕ represents the local, spatially resolved gas fraction typically used in FEA/CFD-based simulation, whereas ϵ is a bulk or averaged gas holdup used in empirical conductivity correction models.

5.3. 3D Finite Element Method Implementation

To enhance the accuracy of the electrochemical and mass transport simulation, a 3D FE model is implemented using the FEniCS open-source FEM solver. The inclusion of 3D modeling allows the study of spatial variations in electrochemical potential, current density distribution, and reaction kinetics that are not captured in a simplified 2D approach.

5.3.1. Computational Domain and Discretization

A cubic computational domain is used, representing a unit segment of the PEM electrolyzer. The domain is meshed into 32 × 32 × 32 finite elements, ensuring sufficient resolution for electrochemical and mass transport processes. The mesh is refined near the electrode-electrolyte interface to improve accuracy in regions of high reaction activity. This resolution was selected based on a mesh independence test to ensure numerical stability and computational efficiency. The model represents a simplified unit domain of a PEM water electrolysis cell under steady-state conditions, assuming uniform material properties and isotropic conductivity. The mesh is sufficiently fine to resolve key electrochemical gradients—particularly near the catalyst–membrane interface—without introducing significant error in current density or potential profiles. Regions of high reaction intensity were verified to exhibit smooth potential distribution, supporting the mesh adequacy for the intended resolution of local ohmic and activation losses.

5.3.2. Boundary Conditions in 3D Simulation

• Anode (x = 0): Fixed potential of 1.23 V (standard water splitting potential)
• Cathode (x = L): Grounded at 0 V
• Other Boundaries (y and z directions): Insulated, with zero normal flux (Neumann conditions)
• Electrode Reaction Interface: Butler–Volmer boundary conditions applied at electrode surfaces to model the charge transfer reaction kinetics.

Although the anode (x = 0) and cathode (x = L) were set to 1.23 V and 0 V, respectively, for solving Laplace’s equation, the model does not neglect cell overpotential. Instead, the activation, ohmic, and concentration overpotentials are separately computed and superimposed to simulate realistic cell performance. These include contributions from charge transfer kinetics (via the Butler–Volmer equation), electrolyte conductivity losses, and mass transport limitations—ensuring the overall electrochemical losses are comprehensively captured.

5.3.3. Numerical Solution Strategy

The weak form of the governing equations is derived and solved using Galerkin FEM discretization. The nonlinear system is solved iteratively using the Newton-Raphson method. The direct linear solver (UMFPACK) is employed for high-precision numerical solutions. A grid independence study is conducted to ensure numerical accuracy. The assumptions and parameters for the PEM electrolyzer are given in

Table 7. Assumptions and parameters for PEM electrolyzer.

Parameter	Value	Units
Temperature	60 °C	°C
Exchange current density (i0)	10−3 A/cm2	A/cm2
Electrolyte conductivity (σ)	0.1 S/cm	S/cm
Cell resistance (R)	0.2 Ω·cm2	Ω·cm2
Transfer coefficient (α)	0.5	-
Number of electrons (n)	2	-
Faraday’s constant (F)	96,485	C/mol

.

The values used in Table 7 represent commonly reported baseline parameters for PEM electrolyzer modeling under standard operating conditions (e.g., 60 °C, 1 atm, acidic media). These values were selected based on consistency with multiple simulation studies and general consensus in the literature. Where necessary, parameters were adjusted to match experimental calibration and ensure numerical stability in the FEM implementation.

5.3.4. Results from 3D FEM Implementation

The 3D model captures non-uniformity in electrochemical potential across the domain, providing a realistic assessment of performance variations within the PEM electrolyzer. Higher current density is observed near the electrode edges, aligning with experimental studies. The impact of bubble formation on localized resistance and mass transport limitations is better visualized in 3D. This detailed 3D FEM implementation significantly enhances model fidelity, allowing a more accurate representation of PEM electrolysis under real-world conditions.

The electrochemical potential distribution within the electrolyte, as shown in Figure 16, provides critical insight into the charge transport behavior across the PEM electrolyzer. This figure is obtained by solving Laplace’s equation for electrochemical potential, considering the applied boundary conditions at the electrodes. The anode is set at 1.23 V, corresponding to the standard water splitting potential, while the cathode is grounded at 0 V. The numerical results illustrate a smooth voltage gradient across the electrolyte domain, confirming uniform charge transport under steady-state conditions.

In the 2D model, the potential distribution follows a predictable linear decline, with minor variations due to localized resistance variations at the electrode-electrolyte interface. The 3D model, however, reveals more complex spatial variations, particularly at the electrode edges, where non-uniform current density distributions arise. This behavior is attributed to the geometric effects of the electrode structure and the influence of localized reaction kinetics, which are more accurately captured in the 3D FEM model. The voltage gradient confirms that ohmic losses within the electrolyte are the primary contributors to internal resistance, a conclusion that aligns well with experimental studies. These results validate the numerical approach’s accuracy in simulating realistic charge transport behavior, making it a reliable tool for optimizing electrode configurations and electrolyte properties in practical PEM electrolyzers.

The breakdown of overpotential contributions, depicted in Figure 17, illustrates the role of activation, ohmic, and concentration overpotentials in governing overall electrolyzer efficiency. This figure is generated by computing individual overpotential contributions based on the Butler–Volmer equation (activation), Ohm’s law (ohmic), and the Nernst–Planck equation (concentration effects). The numerical results closely align with experimental polarization data, reinforcing the accuracy of the model.

The activation overpotential, which dominates at low current densities, arises due to the sluggish OER kinetics at the anode. This overpotential follows an exponential dependence on current density, as dictated by the charge transfer kinetics in the Butler–Volmer equation. As the current density increases, the ohmic overpotential becomes the dominant factor, exhibiting a linear dependence on current density, in agreement with Ohm’s law. This behavior is well captured by the model, with deviations of less than 2% from experimental results [270,271], confirming the accuracy of the electrolyte conductivity and cell resistance assumptions. At very high current densities, the concentration overpotential becomes noticeable, attributed to mass transport limitations in the electrolyte. This effect is modelled using the Nernst–Planck transport equation, capturing the impact of ion depletion near electrode surfaces. The slight deviations in concentration overpotential (~5%) compared to experimental results [270,271] may be due to bubble formation effects, which introduce additional transport resistance. Overall, the agreement between the numerical and experimental overpotential trends confirms that the model provides an accurate representation of electrochemical losses in PEM water electrolysis.

The final gas fraction distribution, illustrated in Figure 18, highlights the impact of bubble formation on mass transport and electrolyte conductivity. The gas fraction distribution is obtained using a multiphase CFD approach, which tracks the growth and detachment of hydrogen and oxygen bubbles within the electrolyte. These bubbles form as a result of the electrochemical splitting of water, and their accumulation affects local conductivity and ion transport dynamics. The simulation considers gas-phase transport equations, incorporating bubble nucleation rates, growth mechanisms, and detachment criteria to predict the spatial distribution of gas fractions. The results indicate that gas fraction is highest near the electrode surfaces, where gas evolution occurs. As bubbles grow and detach, they introduce localized regions of reduced conductivity, increasing electrolyte resistance and affecting overall cell performance. The model captures the spatial variation of bubble-induced transport resistance, showing that regions with higher gas fraction exhibit higher ohmic losses.

In 3D simulations (Figure 19), gas accumulation effects are more pronounced, particularly near the electrode edges and flow channels, where bubble detachment rates vary due to local hydrodynamic conditions. The final gas fraction distribution aligns well with experimental observations, showing that bubble-induced resistance must be accounted for in practical PEM electrolyzer designs. The results suggest that strategic electrolyte flow control and electrode surface modifications can mitigate the negative impact of gas accumulation, optimizing system efficiency. The model provides an accurate predictive tool for evaluating gas evolution effects in large-scale electrolyzer systems, demonstrating its potential for performance enhancement and design optimization.

The comprehensive analysis of electrochemical potential distribution, overpotential contributions, and gas fraction effects provides crucial insights into the internal mechanisms governing PEM water electrolysis. The observed trends in voltage gradients, activation losses, and bubble-induced resistance directly influence overall cell performance and dictate key operational parameters. Building on these findings, the discussion now extends to the polarization behavior of the electrolyzer, which serves as a macroscopic performance indicator by relating cell voltage to current density. Furthermore, the hydrogen production rate—a fundamental measure of electrolyzer efficiency—is examined in the context of Faraday’s Law and reaction kinetics. Finally, to assess system performance holistically, the efficiency vs. current density relationship is explored, identifying operational regimes where the electrolyzer achieves optimal energy utilization. Together, these evaluations provide a detailed framework for understanding both the internal electrochemical dynamics and the practical performance characteristics of the system.

The polarization curve represents the fundamental relationship between current density and cell voltage, serving as a critical performance indicator for PEM water electrolysis. This curve is derived by solving the coupled electrochemical potential equation (Laplace’s equation) and reaction kinetics (Butler–Volmer equation) under varying current densities. The numerical results reveal a characteristic increase in cell voltage with increasing current density, attributed to three primary overpotential contributions: activation, ohmic, and concentration overpotentials. At low current densities, the activation overpotential dominates due to the inherent sluggishness of the OER at the anode. This effect is well captured by the Butler–Volmer equation, which accounts for the exponential dependence of the charge transfer kinetics on the applied overpotential. As the current density increases, the ohmic overpotential becomes the major contributor to voltage rise, governed by Ohm’s law and dependent on the electrolyte conductivity and electrode resistance. The model accurately captures the linear increase of ohmic losses, validating its agreement with experimental data. At very high current densities, concentration overpotentials become noticeable, primarily due to mass transport limitations in the electrolyte. This results from the depletion of reactant ion concentrations near the electrode surfaces, which the model incorporates by solving the Nernst–Planck equation for ion transport.

The polarization curve (Figure 20a) illustrates the relationship between cell voltage and current density, serving as a critical indicator of electrolyzer performance. The breakdown of overpotential contributions across different current densities further validates the model’s accuracy. Additionally, overpotentials contribute to the difference between the thermodynamic and actual cell voltages. The activation overpotential follows an exponential trend (Figure 20b), while the ohmic overpotential scales linearly, and the concentration overpotential remains minimal but rises at higher current densities. A comparison with experimental studies shows that the activation overpotential deviates by ~5%, likely due to approximations in exchange current density and reaction kinetics. Meanwhile, the ohmic overpotential matches experimental values within <2% deviation, demonstrating the reliability of the conductivity and resistance assumptions.

The hydrogen production rate is another critical performance parameter, derived using Faraday’s Law, which establishes a direct relationship between current density and hydrogen generation rate. The model accurately predicts that hydrogen production increases linearly with current density, consistent with electrochemical principles (Figure 21a). The total hydrogen production rate aligns within 2% of experimental data, further reinforcing the model’s predictive capability. Deviations may arise due to gas bubble formation effects, which introduce localized resistance and impact mass transport, partially accounted for in the model through porosity corrections in electrolyte conductivity. The model has been extended to include efficiency and energy consumption analysis (Figure 21b,c). Efficiency decreases as current density increases due to higher ohmic and activation losses. Peak efficiency occurs at low current densities but comes at the cost of lower hydrogen production rates. Efficiency is calculated as the ratio of the thermodynamic voltage (1.23 V) to the actual cell voltage. Energy consumption per mol of hydrogen increases with current density due to higher overpotentials. This highlights the trade-off between production rate and energy efficiency.

This

Table 8. The key comparisons and insights from the model versus experimental data. (Experimental results refer to those reported in [270,271], which offer detailed electrochemical performance metrics for PEM water electrolyzers under conditions analogous to those used in our simulations. These datasets serve as the validation benchmark for evaluating the model’s predictive accuracy).

Performance Parameter	Our Model’s Prediction	Experimental Data	Deviation and Remarks
Cell voltage at 1 A/cm2	~1.8 V	~1.85 V [272]	0.05 V (~2.7%) deviation, indicating good agreement.
Total overpotential at 1 A/cm2	~0.57 V	~0.6 V [271]	0.03 V (~5%) deviation, suggesting reasonable accuracy.
Activation overpotential	Follows exponential trend	Shows similar trend in experiments	~5% deviation due to approximations in exchange current density and reaction kinetics.
Ohmic overpotential	Matches experimental values closely	Deviates by <2%	Reliable conductivity and resistance assumptions.
Concentration overpotential	Minimal but rises at higher current densities	Similar behavior was observed in experiments	The model captures the trend effectively.
Hydrogen production rate	Increases linearly with current density	Aligns within 2% of experimental data	Slight deviation due to gas bubble formation and localized resistance.
Efficiency at 1 A/cm2	~68%	~66% [272]	2% deviation, demonstrating close alignment with experimental observations.
Efficiency trend	Decreases with increasing current density	The same trend was observed in experiments	Consistent with electrochemical principles.
Energy consumption vs. current density	Increases due to higher overpotentials	Experimental results confirm trend	The trade-off between production rate and efficiency is captured well.
Discrepancies and limitations	Due to simplifying assumptions and parameter variability	Experimental data affected by real-world complexities	Refinements are needed to capture temperature variations, catalyst effects, and dynamic conditions.

effectively summarizes the key comparisons and insights from the model versus experimental data, highlighting accuracy, deviations, and possible improvements. The comparisons above demonstrate that our FE model aligns well with experimental data, with deviations within acceptable ranges. These discrepancies can be attributed to factors such as simplifying assumptions in which the model assumes uniform temperature and neglects certain dynamic effects, which may not fully capture real-world complexities. Another is the parameter variability in which differences in membrane properties, catalyst loadings, and cell configurations between the model and experimental setups can lead to variations.

Our extended FEM model (Figure 22) effectively predicts PEM electrolyzer performance metrics, closely matching experimental work (EW) from reputable studies (EW(A) [272], EW(B) [271], and EW(C) [270]). The minor deviations observed are within acceptable limits and can be addressed in future work by incorporating more detailed physical phenomena and refining model parameters.

Although the absolute deviation values between the simulation and experimental results remain small, a slight increasing trend in deviation with rising current density is observed, particularly in Figure 22b. This behavior can be attributed to several factors: (i) nonlinearities become more prominent at higher current densities due to mass transport limitations and local heating effects that are only partially captured in the current model framework; (ii) membrane dehydration may occur under high load, affecting ohmic resistance, which the model presently treats as constant; (iii) gas accumulation effects near the electrode surface can locally increase overpotentials and are not explicitly resolved in the simulation; and (iv) minor numerical smoothing due to mesh resolution may underrepresent sharp gradients that develop at higher currents. These sources of deviation, while modest, offer insight into physical behaviors not fully captured by the current computational model and highlight areas for potential refinement in future simulations. At lower current densities, the deviations observed in Figure 22c,d can be attributed to underutilization of the electroactive area, slow activation of the electrode surface, and increased influence of membrane resistance and contact losses. These factors lead to divergence from the idealized model predictions. While such trends are well understood in electrochemical theory, direct experimental parallels for the specific datasets shown here are limited. Nevertheless, our extended FEM model aligns closely with experimental benchmarks [270,271], with average predictive error remaining within ±5% across the entire current density range.

In addition to conventional modeling techniques such as FEM, CFD, and system-level tools, recent studies have begun incorporating emerging computational approaches to address complex electrolysis challenges. Machine learning (ML) algorithms—including artificial neural networks, random forest models, and support vector regression—have shown promise in accelerating performance prediction, parameter tuning, and degradation forecasting. Meanwhile, dynamic control-oriented models, often implemented in platforms such as MATLAB/Simulink or Modelica, are being developed to simulate transient behavior, renewable coupling, and real-time control strategies. Multiscale modeling frameworks, which link electrochemical reaction kinetics with macroscopic transport phenomena, are also gaining traction for their ability to capture spatial and temporal variability across electrode and system domains. While these methods are still maturing, their integration into future electrolysis design and optimization workflows holds considerable potential.

6. Concluding Remarks

This review presents a comprehensive summary of advancements in electrolysis technologies for hydrogen production, focusing on material innovations, process optimizations, numerical modeling, and system integration. Electrolysis powered by renewable energy sources such as solar, wind, nuclear, and hydropower provides a sustainable pathway for large-scale hydrogen production. However, commercialization still faces challenges, including catalyst degradation, high energy consumption, operational stability, and economic constraints.

Significant progress has been made in electrode materials and electrocatalysts to enhance HER. Ni-based and modified Ni-alloy electrodes, as well as Mo, La, TiO₂, Co, and metalloids combined with Ni, have improved catalytic efficiency and system longevity in AWE. In SOECs, perovskite composite electrodes such as CMF, Ni/YSZ, LSCM, SFFN/YSZ, and LSTO-SDC have demonstrated promising performance, particularly in co-electrolysis applications. Protective coatings, blocking layers, and reaction enhancements such as the reverse water gas shift (RWGS) reaction have further improved stability and conversion efficiencies. For PEM electrolysis, Pt and Ir catalysts remain dominant due to their high durability, with IrO₂/Ti mitigating hydrogen degradation and F-doped IrO₂ electrocatalysts enhancing electrochemical activity. Membrane optimization has also played a crucial role, where perfluorosulfonic acid ionomers outperform zirconium phosphate-modified membranes, and porous poly(perfluorosulfonic acid) membranes enhance ion conductivity over conventional membranes.

For MECs, advances in biocathode modifications, including PANI/MWCNT composites and Ni mesh cathodes, have improved biofilm formation and catalytic efficiency. Additionally, methane inhibition strategies and thermoelectric integration have shown potential in increasing MEC-driven hydrogen production efficiency. Alternative electrolytes, such as KH₂PO₄ for pressurized electrolysis, tetra-alkyl-ammonium-sulfonic acid for low temperature applications, and aqueous methanol for reduced operating voltages, provide new pathways for improving system efficiency and adaptability.

Beyond experimental advancements, numerical modeling and FEM-based simulations have proven to be powerful tools for understanding electrochemical kinetics, charge transport, and system optimization. PEM electrolysis, in particular, has benefited from multi-physics simulations, enabling predictive analyses of electrode behavior, current density effects, and efficiency improvements. These models provide valuable insights into performance optimization, guiding the design of next-generation electrolyzers.

Despite these advancements, future research should focus on pressure optimization, electrode durability, membrane improvements, and system integration with renewable energy sources to enhance hydrogen production efficiency and economic feasibility. Addressing scalability challenges, reducing operating temperatures, and refining reactor designs will be key to making electrolysis technologies commercially viable. Hybrid approaches, such as solar-assisted electrolysis, hybrid bio-electrochemical systems, and thermoelectric energy recovery, can further enhance efficiency and sustainability. With continued research and innovation, electrolysis has the potential to become a cornerstone technology for the global hydrogen economy, supporting large-scale storage for renewable energy derived electricity and long-term sustainability. Future research should focus on overcoming key technological bottlenecks such as catalyst degradation, membrane instability, and high system energy demand. Advanced materials for electrodes and membranes, pressure and temperature optimization strategies, and innovative cell designs will be critical to improving efficiency and durability. Additionally, integrating PEM and AWE systems with renewable energy sources, developing hybrid configurations, and leveraging digital tools such as AI-based modeling and real-time control systems offer promising pathways for scalable, flexible hydrogen production. These directions are vital to bridge the gap between laboratory performance and commercial deployment.