Green Hydrogen for Energy Transition: A Critical Perspective

Abstract

Green hydrogen (GH₂) is emerging as a key driver of global energy transition, offering a sustainable pathway to decarbonize energy systems and achieve climate objectives. This review critically examines the state of GH₂ research production technologies and their integration into renewable energy systems, supported by a bibliometric analysis of the recent literature. Produced via electrolysis powered by renewable energy, GH₂ shows significant potential to decarbonize industries, enhance grid stability, and support the Power-to-X paradigm, which interlinks electricity, heating, transportation, and industrial applications. However, widespread adoption faces challenges, including high production costs, infrastructure constraints, and the need for robust regulatory frameworks. Addressing these barriers requires advancements in electrolyzer efficiency, scalable fuel cell technologies, and efficient storage solutions. Sector-coupled smart grids incorporating hydrogen demonstrate the potential to integrate GH₂ into energy systems, enhancing renewable energy utilization and ensuring system reliability. Economic analyses predict that GH₂ can achieve cost parity with fossil fuels by 2030 and will play a foundational role in low-carbon energy systems by 2050. Its ability to convert surplus renewable electricity into clean energy carriers positions it as a cornerstone for decarbonizing energy-intensive sectors, such as industry, transportation, and heating. This review underscores the transformative potential of GH₂ in creating a sustainable energy future. By addressing technical, economic, and policy challenges and through coordinated efforts in innovation and infrastructure development, GH₂ can accelerate the transition to carbon-neutral energy systems and contribute to achieving global climate goals.

1. Introduction

Green hydrogen (GH₂) production from renewable energy sources is increasingly being recognized as a cornerstone of global decarbonization efforts. A critical challenge lies in scaling electrolyzer technologies to meet the growing demand while achieving cost competitiveness. The successful integration of GH₂ into energy systems relies heavily on supportive policy frameworks that address economic and infrastructure barriers. Innovation in materials science and system design is vital for fostering hydrogen markets and driving global adoption. Advancements in electrolyzer technology have underscored the potential of cutting-edge materials and manufacturing processes to significantly enhance production efficiency while reducing energy loss. These innovations are pivotal for realizing scalable and cost-effective solutions, particularly in regions where renewable energy resources support GH₂ production. Continued research and development efforts are fundamental for creating robust modular electrolyzer systems capable of operating under diverse conditions, ensuring their suitability for a range of energy applications [1].

The coupling of renewable energy sources (RESs) with GH₂ production is an area of active research and development, driven by the need for sustainable energy solutions and the decarbonization of various industrial sectors. The current state of scientific research in this field is very active and constantly evolving, focusing on various innovative strategies and technologies. A recent study carried out an in-depth bibliometric review of GH₂ production technologies, recording an exponential increment in scientific products since 2017 and demonstrating a strong increase in the interest of the scientific community toward this research area [2]. The purpose of the present work is to review the most prominent and highly cited research papers, as well as recent reviews and foundational studies, mainly published in journals focused on energy systems and hydrogen technology. In the remainder of this brief introduction, we describe some important properties of hydrogen as a fuel and its effectiveness in this role. The environmental impacts of GH₂ production are predominantly linked to electricity and water usage, making regional strategies vital for optimizing renewable feedstock and minimizing lifecycle emissions [3]. In Section 2, we summarize the main findings regarding its production processes, environmental impact, and economic characteristics. Section 3 delves into green production processes in detail. In Section 4, we deepen the GH₂ storage methods and possible integration with sector-coupled smart grids. Section 5 discusses the conversion of electricity into various forms of energy carriers, such as hydrogen or chemicals, while in Section 6, the role of hydrogen conversion processes will be analyzed in detail. In the last Section 7 and Section 8, the key challenges, research directions, and analysis of strategies addressed to reduce the operating costs of GH₂ will be discussed in detail.

Hydrogen itself is not a new energy technology. The concept of using hydrogen as an energy source dates back to the 19th century, and hydrogen fuel cells were developed more than 150 years ago. However, its applications as an energy carrier and its potential role in a sustainable energy system have gained significant attention in recent years [4]. This renewed interest is driven by the search for clean energy alternatives to fossil fuels, especially in the context of climate change and the need to reduce greenhouse gas (GHG) emissions. Hydrogen possesses a high energy density, particularly in terms of energy per unit mass, due to its chemical and physical properties. The key reasons why hydrogen is known for its high energy density are as follows:

1.1. High Energy Content per Unit Mass

Hydrogen is an atomic and molecular species. Hydrogen is the simplest and lightest element in the universe, consisting of only one proton and one electron. In its atomic form, an unpaired electron makes it highly reactive and readily forms bonds to achieve a stable configuration. It is also an important intermediate in radical and plasma reactions, forms the basis of proton transfer mechanisms, and acts as a reactant in fuel cells [5]. Under standard conditions, its diatomic form is a gas with low reactivity at room temperature, which requires activation energy or a catalyst for reactions. Its high energy-to-weight ratio makes it a valuable fuel. Specifically, hydrogen has an energy content of about 120 MJ/kg or −286 kJ/mol (lower heating value, LHV), which represents the energy released when molecular hydrogen is combusted and water vapor remains in the gaseous state. This amount of energy is much higher than that of conventional fuels like gasoline (~46 MJ/kg) and natural gas (~50 MJ/kg) [6].

Bonding energy and electrochemical context. Molecular hydrogen splitting requires a significant amount of energy, which is equal to 436 kJ/mol. In galvanic cells [7], it acts as a fuel, splitting into protons and electrons during anodic reactions: H₂ → 2H⁺ + 2e⁻. This process occurs after a preliminary stage involving hydrogen adsorption on platinum in the fuel cell: H₂ → 2H_ads. Conversely, it can be produced at the cathode of electrolytic cells according to the semi-reaction: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Understanding the dual role of hydrogen as an atomic and molecular species and the complexities of electrode reactions underpins progress in energy systems and chemical technologies.

1.2. Energy per Unit Volume (Consideration of Challenges)

Low Density as a Gas. While hydrogen has an exceptionally high energy density per unit mass, its energy density per unit volume is relatively low when in its gaseous state under standard conditions (0 °C and 1 atm of pressure). This discrepancy arises from hydrogen’s inherently low density as a gas, which results in it occupying significantly more space compared to other fuels.

Compression and Liquefaction. To increase the volumetric energy density, hydrogen can be compressed to high pressures (e.g., 700 bar), thus achieving a volumetric energy density of approximately 5.6 MJ/L [8], or liquefied by cooling it to extremely low temperatures (−253 °C). In these forms, hydrogen’s volumetric energy density increases, making it more practical for storage and transportation in applications like fuel cells for vehicles. Despite these enhancements, even liquid hydrogen’s volumetric energy density (8 MJ/L) is lower than that of conventional fuels like gasoline (32 MJ/L). However, hydrogen’s superior mass-based energy density often compensates for this limitation, particularly in applications where lightweight energy carriers are advantageous, such as in aerospace or portable energy systems.

1.3. Fuel Cell Efficiency

Electrochemical Conversion. In fuel cells, hydrogen is used to generate electricity through an electrochemical reaction rather than combustion. This process is more efficient than burning traditional fossil fuels because it avoids the thermal energy losses typical of combustion engines [9,10]. The efficiency of fuel cells can reach 60% or higher [11,12], further enhancing the practical energy density of hydrogen when used in such applications.

1.4. Potential for High Energy Release

Combustion Energy. When hydrogen is burned in the presence of oxygen, it produces water and releases a large amount of energy. This reaction is highly exothermic, meaning that it releases heat energy, which solidifies hydrogen’s status as a high energy fuel.

Chemical Reactivity. Hydrogen’s chemical reactivity with oxygen is integral to its high energy release. The bond energies involved in the formation of water molecules result in the liberation of substantial amounts of energy, which can be harnessed in various forms including heat, electricity, and mechanical work.

Hydrogen can be produced from various sources, including water (through electrolysis), natural gas (through Steam Methane Reforming, SMR), and biomass. It can be used in fuel cells to generate electricity, in internal combustion engines, or as feedstock in various industrial processes.

2. The Chromatic Scale of Hydrogen Production Technologies

There are several routes to produce hydrogen, and the method is conventionally defined by the corresponding color code. Hydrogen produced from coal is termed “black”, while production from lignite is referred to as “brown”. If it is obtained from methane through SMR, it is called “gray”. When the Capture and Carbon Storage (CCS) of CO₂ is planned downstream, one refers to “blue” hydrogen. Alternatively, hydrogen can be obtained by the electrolysis of water, a highly energyvorous process. If the electricity required for the electrolysis process is entirely from renewable sources, hydrogen is called “green”, while it is “purple” (or “pink”) if the electricity is derived from nuclear energy. Finally, the hydrogen produced by methane pyrolysis is identified by the “turquoise” color. The most mature technology for producing hydrogen is coal gasification (with an efficiency of 75–80%), which is used mainly in the chemical industry to obtain ammonia. With 20 kg of CO₂ per kg of hydrogen (black or brown) produced, this method is characterized by the highest intensity of GHG emissions. However, GHG emissions can be reduced by up to 90% by combining coal gasification with CCS [13], although there are still several unsolved drawbacks, such as economic inadequacy, high energy requirements, and long-term storage risks [14,15].

This makes it one of the lowest-carbon options for producing blue hydrogen, whose cost ranges between $1.63 to $9.24 per kg H₂ depending on infrastructure material cost, local policies, and coal quality, which may mainly affect production efficiency [16]. When natural gas is used as the feedstock for hydrogen production, the most commonly employed methods are SMR, Auto-Thermal Reforming (ATR), and methane pyrolysis. SMR directly emits 9 kg of CO₂ per kg of hydrogen produced. Additional GHG emissions (1.9–5.2 kg CO₂/kg hydrogen) must also be accounted for, depending on production facilities and the logistics of natural gas production and transportation. ATR technology, which allows higher CO₂ capture rates than SMR (≥95%), relies on O₂ instead of steam, which requires electricity (rather than methane) as fuel input. A portion of the world’s ammonia and methanol production currently uses ATR technology, even without CCS. Finally, the pyrolysis process of methane produces turquoise hydrogen and high-purity carbon as a by-product, which in turn can serve as raw material for various production processes. However, factors such as the low technology readiness level, the input of methane per unit of hydrogen produced (twice compared to SMR), and the need to reach high temperatures make this process difficult to pursue at the moment [17]. Figure 1 shows the correlation between the different hydrogen production methods and their associated carbon footprints.

The diagram employs a horizontal bar chart where numerical values serve as illustrative examples rather than measurements of the carbon footprint, but are used to provide a visual representation of the relative carbon impact of each hydrogen production method. Specifically, “0” (Green H₂) represents near-zero emissions, as hydrogen is produced using renewable energy; “10” (Purple H₂) indicates low emissions, since nuclear energy powers the electrolysis process; “30” (Turquoise H₂) signifies lower emissions, as CH₄ pyrolysis generates solid carbon instead of CO₂; “50” (Blue H₂) corresponds to moderate emissions, involving natural gas with carbon capture and storage (CCS); “80” (Gray H₂) associated with high emissions, for hydrogen produced via SMR using natural gas; “100” (Black/Brown H₂) represents highest emissions, as hydrogen is derived from coal or lignite, which are highly carbon-intensive. The comparative

Table 1. CO₂ emissions, direct and process water usage, energy requirements, and costs of hydrogen production methods [22,23,24,25].

Hydrogen Type	CO2 Emissions (kg CO2/kg H2)	Direct H2O Usage (L/kg H2) 1	Process H2O Usage (L/kg H2) 2	Energy Requirement (kWh/kg H2)	Cost ($/kg H2)
Black/Brown	19–20	2–4	150–200	30–40	1–2
Gray	9–10	4	100–150	40–50	1–2
Blue	1–2	4	100–150	40–50	1.5–3
Turquoise	0 (solid C)	2	50	20–30	2–3
Purple	0	9	250–300	50–55	3–5
Green	0	9	100–200	50–55	4–6

¹ Represents the water chemically required for splitting to produce H₂ (e.g., via electrolysis or methane reforming). ² Includes cooling, steam generation, and other auxiliary processes involved in H₂ production technologies.

summarizes the CO₂ emissions, direct and process water usage, energy consumption, and costs for each hydrogen production technology.

3. Current Methods of Production of Green Hydrogen

Currently, the production of GH₂ is achieved by various methods, each of which has its own pros and cons, and the choice of method often depends on specific requirements, such as cost, efficiency, scalability, and the availability of RES [26]. GH₂ is produced by splitting water into hydrogen and oxygen using renewable energy sources, such as wind, solar, or hydroelectric power [27,28,29,30,31]. GH₂ production is energy-intensive, with electrolysis requiring 50–60 kWh of renewable electricity per kilogram of hydrogen, which represents a significant energy input. High-temperature methods like SOEC can reduce energy use by incorporating waste heat, achieving electrical-to-hydrogen conversion efficiencies of ~80–85%, the highest among electrolysis methods. Concerning water usage, producing 1 kg of hydrogen requires 9 L of demineralized water, which must be carefully managed in water-scarce regions. Alternative sources like seawater (via desalination) and wastewater are being explored, but these add cost and complexity, as will be discussed in more detail in Section 7. While alkaline electrolysis and PEM systems are suitable for coupling with solar and wind energy, providing a path to scalable GH₂ production, SOEC and photocatalytic splitting represent future advances with the potential for greater efficiency and sustainability, although they require significant investment in R&D [32]. Finally, emerging techniques, such as MECs and photobiological splitting, offer promising alternatives, especially in waste valorization and solar-driven processes. The best-known methods for producing GH₂ are as follows.

3.1. Electrolysis

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Proton Exchange Membrane (PEM) Electrolysis. This method uses a solid polymer electrolyte that conducts protons, separates hydrogen and oxygen, and insulates the electrodes. The main advantages include high efficiency (~70–80%), quick response times, and compact design. Hydrogen purity is >99.99%, which is suitable for fuel cells. Challenges include the use of expensive materials such as platinum (Pt) and iridium (Ir), with Pt loading at 0.05–0.3 mg/cm². The energy consumption ranges from 50 to 60 kWh per kg of hydrogen, with operating temperatures between 50 °C and 80 °C. Under optimal conditions, this allows for a lifespan of over 5 × 10⁴ operational hours [33,34,35,36,37].

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Alkaline Water Electrolysis (AWE). In this process, water is split using an alkaline electrolyte (usually potassium hydroxide) between the two electrodes. The advantages are that it is a mature technology with lower costs than PEM and can operate for long periods. However, AWE has a lower efficiency (~60–70%), slower response times, and requires a larger physical footprint. Hydrogen purity is ~99.5%, making it less suitable for applications requiring ultra-high purity. Energy use is ~55–65 kWh per kg of hydrogen, with current densities of 0.2–0.4 A/cm² and a lifespan of 6 × 10⁴–9 × 10⁴ h. Details on the AWE working principle, together with recent developments in used electrocatalysts can be found in refs. [29,38].

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Solid Oxide Electrolysis Cell (SOEC). This method uses a solid ceramic electrolyte and operates at high temperatures (typically 700–1000 °C). Advantages: Higher efficiency (~80–85) due to the use of waste heat, which reduces electricity requirements to as low as ~40–50 kWh per kg of hydrogen. Several papers report studies showcasing how SOEC technology can efficiently produce hydrogen with minimal energy loss, often by utilizing waste heat from industrial processes to enhance the overall efficiency [39,40,41].

Table 2. Comparative overview of electrolysis processes for hydrogen production [29,42].

Electrolysis Type	Faradaic Efficiency (%)	Hydrogen Purity (%)	Cost ($/kg H2)	Key Advantages	Key Limitations
Proton Exchange Membrane (PEM)	85–95	99.99	3–7	high purity, compact design	expensive catalysts (Pt, Ir)
Alkaline Water Electrolysis (AWE)	95–99	99.5	2.5–6.5	mature, low-cost materials	larger footprint, slower response
Solid Oxide Electrolysis (SOEC)	90–97	99.9	2.7–6	high efficiency uses heat energy	high operating temperature (>700 °C)
Anion Exchange Membrane (AEM)	90–96	99.8	3–7.5	potential for low-cost catalysts	less mature technology

lists the Faradaic efficiencies, costs per kilogram of hydrogen, and purities obtained for the electrolysis processes currently used to produce hydrogen. AWE achieves high Faradaic efficiency at lower costs but compromises responsiveness compared to PEM, which in turn leads to purity, which is crucial for fuel cells and energy applications. SOE can achieve cost efficiency through integration but remains in early-stage development.

Maturity vs. Innovation: AWE is commercially mature, while AEM presents a promising low-cost alternative, but requires further research.

3.2. Advanced Membranes for Electrolysis

PEM [26,27] and anion exchange membranes (AEM) [41] are used in electrolysis to produce hydrogen from water. AEM achieves 65–75% efficiency, serving as a promising low-cost alternative by avoiding noble metal catalysts. Composite membranes integrate organic and inorganic materials to enhance their performance. Membranes with high ionic conductivities and chemical stabilities enhance the efficiency of hydrogen production and storage. Improvements in these membranes directly translate to better performance in electrochemical hydrogen storage systems. These materials and methods represent some of the most promising advancements in hydrogen storage from an electrochemical viewpoint. These are integral to improving the efficiency, capacity, and durability of hydrogen storage systems, particularly in applications related to fuel cells and renewable energy storage.

Upgrading or modifying existing electricity grids to efficiently accommodate the transportation and distribution of GH₂ involves several technical, infrastructural, and operational changes. The integration of GH₂ into the grid requires significant adaptations to ensure that the entire system can handle the production, storage, and delivery of hydrogen, along with traditional electricity [42]. Below are the key upgrades and modifications needed:

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Dynamic load management: Electrolyzers can be designed to operate flexibly, ramping up hydrogen production when there is excess renewable energy and scaling back when energy is scarce. This helps absorb surplus energy and prevent curtailment, effectively acting as a controllable load that can stabilize the grid.

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Demand Response: Electrolyzers can participate in demand response programs, where they reduce or increase power consumption in response to grid signals, thereby helping maintain grid stability.

PEM and SOEC offer high efficiency but have significant material and operational costs. AEM presents an emerging alternative with low-cost potential but is less proven for large-scale use. Composite membranes integrate the best organic and inorganic materials for enhanced performance.

Table 3. Summary of advanced membranes for electrolysis technology [43,44,45].

Membrane Type	Material	Conductivity (S/cm)	Durability (hours)	Operating Temp (°C)	Cost ($/m2)	Notes
PEM	perfluorosulfonic acid (PFSA)	0.1–0.2	>20,000	60–80	800–1200	High efficiency, expensive materials
AWE	alkaline polymer	0.05–0.15	10,000–15,000	30–60	150–300	Low-cost catalysts, less mature tech.
SOE	Yttria-stabilized zirconia	0.01–0.05	>30,000	700–900	1000–1500	High temperature, excellent durability
composite membranes	hybrid organic-inorganic	0.1–0.15	~15,000	50–100	500–800	Improved thermal stability and mechanical support

summarizes the main features of the membranes, such as materials, conductivity, durability, operating temperature, and costs.

3.3. Photolysis

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Photoelectrochemical (PEC) Water Splitting [46,47,48]. This method involves using sunlight directly to split water molecules in a photoelectrochemical cell. The principal advantage is the direct use of solar energy, potentially achieving solar-to-hydrogen (STH) efficiencies of 2–5%, which is far below the commercial viability target of >15%. Challenges include low efficiency, high cost, and stability of photoelectrodes.

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Photocatalytic Water Splitting [49,50,51,52]. This technique uses semiconductor materials that absorb sunlight and generate electrons to split water into hydrogen and oxygen. A very recent review by Kafadi et al. [53] discusses advancements in utilizing two-dimensional transition metal carbides, nitrides, or carbonitrides, also known as MXene-based materials, for hydrogen production through photocatalysis. MXenes exhibit exceptional electrical conductivity, tunable surface chemistry, and structural flexibility, making them excellent candidates for photocatalytic applications. Indeed, composites such as Ti₃C₂ with other semiconductors like Zn₂In₂S₅ or g-C₃N₄ significantly enhance photocatalytic performance, achieving hydrogen evolution rates (HER) of ~50–100 µmol/h under visible light. These materials improve charge separation, electron transport, and visible-light absorption, leading to higher hydrogen evolution rates under specific conditions. This review highlights various preparation methods, such as hydrothermal and hydrofluoric acid (HF) etching techniques, which influence the physiochemical properties of MXenes and their photocatalytic efficiency. In addition, replacing noble metals like platinum with cost-effective MXene-based materials (e.g., RuNi composites) is proposed to reduce production costs while maintaining high catalytic activity. This research emphasizes the need for further optimization of MXene-based heterostructures and scalable, sustainable production techniques to achieve practical solar-to-hydrogen conversion efficiencies.

The main characteristics required for membranes in solar spectrum utilization are summarized as follows:

• Broad solar spectrum absorption

Membranes and their associated materials must capture a wide range of the solar spectrum, including visible and near-infrared regions. A bandgap of 1.6–2.2 eV is optimal for light absorption and energy conversion efficiency [54,55].

• Charge separation and transport

The integration of heterojunctions or multi-layer nanostructures reduces the recombination of photogenerated carriers, improving charge separation. High electron mobility materials such as titanium dioxide (TiO₂) and doped nanomaterials ensure effective charge transport [56].

• Photocatalysis efficiency

Advanced membranes must exhibit high photocatalytic activity for water splitting. Promising options include carbon nitride (g-C₃N₄), bismuth vanadate (BiVO₄), and metal-doped oxides [57,58,59].

• Selective permeability and gas separation

Membranes need to manage hydrogen and oxygen diffusion effectively, preventing gas crossover. Proton-conductive materials like Nafion^® are frequently employed [60].

• Cost-effectiveness and scalability

Affordable precious materials are essential for large-scale production. However, the avoidance of rare earth metals reduces costs and environmental impacts [61,62].

3.4. Biological Processes

• Microbial Electrolysis Cells (MECs).

Bacteria consume organic matter and produce electrons, which split water molecules. Hydrogen yields are ~0.4–0.7 m³ H₂ per m³ reactor volume per day under lab conditions. The efficiency is typically <30%, and scalability remains a significant challenge. Sustainability is a key advantage, as MECs can use waste products as feedstock.

A comprehensive review by Abd-Elrahman et al. [63] examines the impact of variables such as nanomaterials, substrates, and operational conditions on hydrogen production rates in MECs. It also discusses the challenges and future directions for improving biohydrogen production efficiency. Another study carried out by Dubrovin et al. [64] explores the use of encapsulated bacterial anodes to enhance hydrogen production in MECs, addressing challenges like contamination by non-electrogenic bacteria and operational stresses. The encapsulated system demonstrated improved hydrogen evolution rates compared to traditional setups. In general, the main advantage of MECs is the opportunity to use waste products as feedstock, making the process sustainable and environmentally friendly. Challenges: MECs are still in the experimental stage, with low hydrogen production rates and scalability issues.

• Photobiological Water Splitting.

This method uses microorganisms, such as algae, that naturally produce hydrogen under certain conditions [65]. The study by Chen et al. [66] highlights the use of [FeFe]-hydrogenases (<100 µmol H₂/L/h) in algae for efficient hydrogen production. These enzymes are optimized within the cellular framework to catalyze the formation of molecular hydrogen under specific conditions, such as low-oxygen environments. By engineering robust systems, researchers have achieved prolonged hydrogen production, demonstrating the potential for sustained hydrogen output in green algae systems over extended periods. This study introduces the concept of “liquid sunshine” applications, where engineered algae systems can be deployed for scalable industrial hydrogen production. This idea leverages solar energy, water, and biocatalysis to produce GH₂ in a sustainable and eco-friendly manner, thereby offering a promising alternative to fossil fuel-derived hydrogen.

3.5. Thermodynamical Water Splitting

This method involves using high temperatures (typically over 500 °C) from concentrated solar power or nuclear reactors to drive a series of chemical reactions that ultimately split water [67]. A two-step thermochemical water-splitting process that utilizes redox materials, such as ferrites and cerium-based oxides, has been recently reviewed by Oudejans et al. [68]. The authors discuss reactor designs and highlight advancements in material efficiency and stability to improve hydrogen production. Efficiencies are ~40–50%, with the potential to exceed 60% using advanced materials like ferrite and cerium-based oxides. Future directions will focus on lowering the reduction temperatures and enhancing the cyclability of the cycles as described, e.g., in the study provided by Yamamoto et al. [69], who present a breakthrough in room temperature thermochemical water splitting using a mechanocatalytic approach. High-purity hydrogen was produced in compact, low-power reactors using metals and metal oxides in water. This process offers the potential for scalable and energy-efficient hydrogen generation.

Figure 2 summarizes the efficiency percentage (as a vertical bar chart), defined as the proportion of input energy converted into usable hydrogen, and the global adoption ratio (depicted as a line plot), expressed as the percentage of the total hydrogen production for each method. Additionally, the figure includes the Technology Readiness Level (TRL), indicating the maturity of each technology on a scale from 1 (basic principles) to 9 (fully deployed). In particular, SMR and coal gasification are fully mature, with a TRL of 9 (high). PEM and AWE, which are widely used in industrial hydrogen production and GH₂ projects, have a TRL of 8–9 (high). SOEC, progressing with industrial demonstrations, has a TRL of 6–7 (moderate). Thermochemical methods and other processes (e.g., partial oxidation of oil or biomass) demonstrated in small-scale pilot plants have a TRL of 5–6 (moderate). Finally, biological processes and photolysis, constrained by scalability and material stability, remain in early-stage research, with a TRL of 3–5 (low).

In summary, it can be seen that although SMR is affected by a moderate efficiency of 65% due to inherent energy losses during methane reformation, it is still the most widely used method for producing hydrogen (48% of total production) worldwide. On the contrary, SOEC is the most efficient among electrolysis methods (85%) but is in the early stage of adoption for industrial and high-temperature applications. Indeed, electrolysis is currently the most advanced and widely used method, particularly PEM and alkaline electrolysis, for GH₂ production. Photolysis and biological processes are promising, but are still in the research and development stages. Thermochemical water splitting offers moderate efficiency, but faces challenges in terms of material durability and process complexity.

4. Green Hydrogen Storage Methods and Grid Integration

Hydrogen-Incorporated Sector-Coupled Smart Grids (HISCSG) represent an advanced, integrated approach to energy management and distribution that combines the capabilities of smart grids with hydrogen energy systems. The key features of smart grids include automation, real-time data monitoring, self-healing capabilities, and enhanced reliability. This concept focuses on the integration of various energy sectors—such as electricity, heating, cooling, and transportation—into a cohesive, efficient, and flexible energy network. The smart grid uses the concept of sector coupling to improve energy efficiency and facilitate the transition to fully decarbonized energy systems [74]. Thus, by connecting electricity, heating, transportation, and industrial processes, sector coupling enables the efficient use of renewable energy and supports the decarbonization of the entire energy system. In this context, hydrogen plays a crucial role as an energy carrier that can store and transport energy across sectors. Produced using surplus renewable electricity, GH₂ can be stored for long periods, making it an asset for balancing the supply and demand in the grid. This integration allows different energy sectors to be connected, maximizing the use of renewable sources and reducing dependence on fossil fuels.

In an HISCSG, the production of GH₂ is achieved through Power-to-Hydrogen (PtH) technologies [75,76], where renewable electricity powers electrolyzers that produce hydrogen. The GH₂ produced can serve multiple purposes, as follows:

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Direct use in industry: As feedstock for various industrial processes, particularly in sectors that are difficult to decarbonize, such as steel production and chemical manufacturing.

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Conversion to Fuels: Through processes like Hydrogen-to-Fuel (HtF), hydrogen can be converted into synthetic fuels, which can be used in transportation or stored for later use.

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Energy Storage: Hydrogen can be stored in large quantities over long periods, providing a reliable means to balance the grid and manage fluctuations in the renewable energy supply.

4.1. Green Hydrogen Storage

Storage plays a vital role in the Power-to-X (PtX) framework, particularly when using hydrogen as an energy storage solution. When renewable energy sources, such as solar and wind, generate more electricity than needed, excess energy can be used to produce hydrogen. This hydrogen can then be stored and later converted back into electricity using fuel cells or turbines during periods of low renewable energy production, such as at night or during calm weather. This ability to store and release energy as needed helps stabilize the grid and compensates for the intermittent nature of renewable energy sources. According to widely recognized approaches based on current knowledge and existing research in the field [77,78], hydrogen can be stored through key technologies, including the following:

4.1.1. Large-Scale Storage

Large-scale underground hydrogen storage is often performed in salt caverns, which provide a natural, secure, and low-cost storage solution. Research has focused on improving the integrity and monitoring of these caverns to prevent leaks and ensure their long-term stability [79,80]. Mehr et al. [81] reviewed hydrogen storage technologies, emphasizing salt caverns for large-scale applications, and described operational efficiencies and the role of salt caverns in energy security. Gianni et al. [82] evaluated the economic feasibility of salt caverns for underground hydrogen storage. They also discussed safety measures and energy storage strategies. Zhu et al. [83] focused on geographic and geological factors influencing the selection of salt caverns for hydrogen storage. Design considerations for long-term safety and cost efficiency are also provided.

4.1.2. Electrochemical-Based Storage Methods

Based on established research and knowledge in the field, here are some key materials and methods that are considered highly effective from an electrochemical perspective [84].

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Metal hydrides. These compounds, such as magnesium hydride (MgH₂), are formed when hydrogen gas reacts with metals. They can store hydrogen in solid form [85], which can be released by applying heat. Metal hydrides allow for reversible hydrogen storage and release, making them highly efficient for electrochemical applications like fuel cells. They offer a high volumetric hydrogen density and can undergo multiple cycles without significant degradation.

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Carbon-based materials. Materials such as carbon nanotubes (CNTs), graphene, and activated carbons (ACs) possess exceptionally high surface areas, allowing them to adsorb hydrogen effectively. These materials are lightweight, exhibit good conductivity, and are ideal for electrochemical cells, where rapid hydrogen uptake and release are required. For example, Reza et al. [86] discussed graphene, CNTS, and ACs for hydrogen storage, highlighting their roles in modern energy storage and electrochemical cell applications. Sdanghi et al. [87] analyzed hydrogen storage in microporous carbon materials, including ACs, CNTs, and nanofibers, and evaluated the adsorption properties and structural characteristics influencing hydrogen storage capacity. Attia et al. [88] have recently reviewed the integration of metal nanoparticles on porous ACs and CNTs for hydrogen storage. Moreover, recent advances in electrochemical hydrogen storage using graphene and its derivatives have been illustrated by Kopak [89], who also evaluated the impact of material modifications on the overall efficiency. Finally, Boateng et al. [90] explored graphene and CNT modifications for improved energy and hydrogen storage and highlighted functionalization techniques for enhanced adsorption properties.

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Metal-Organic Framework (MOFs). MOFs are crystalline materials with extremely high surface areas and tunable pore sizes, enabling hydrogen storage through the physisorption process. They provide high hydrogen storage capacities at low pressures, and their properties can be tailored for specific electrochemical applications to enhance the efficiency of hydrogen storage and release in fuel cells. Hydrogen adsorption in MOFs primarily occurs through two mechanisms: physisorption and chemisorption. Understanding the distinctions between these mechanisms is crucial for optimizing hydrogen storage performance. The former involves weak van der Waals interactions between hydrogen molecules and the MOF surface. These interactions are typically characterized by low enthalpies of adsorption, ranging from 4 to 7 kJ/mol, which means that hydrogen is adsorbed without forming strong chemical bonds. As a result, physisorption is generally reversible and occurs at near-ambient temperatures [91]. In contrast, chemisorption involves the formation of stronger chemical bonds between hydrogen molecules and the MOF, leading to higher enthalpies of adsorption. This process is often irreversible under standard conditions and can alter the electronic structure of the MOF [92]. However, in the context of MOFs, hydrogen storage predominantly occurs via physisorption due to the typically low binding energies involved. Factors influencing hydrogen adsorption in MOFs are: (a) surface area and pore volume, that is, higher surface areas and larger pore volumes in MOFs provide more adsorption sites for hydrogen, enhancing storage capacity [93]; (b) pore size and geometry, i.e., the size and shape of the pores influence the accessibility and diffusion of hydrogen molecules within the MOF structure [94]; and (c) introducing functional groups or metal sites within the MOF can increase the binding affinity for hydrogen, potentially enhancing storage capacity [95]. However, this must be balanced to avoid overly strong interactions that could lead to chemisorption.

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Chemical hydrogen storage. This method involves storing hydrogen in chemical compounds such as ammonia, methanol, or formic acid, which can release hydrogen through catalytic reactions. These compounds allow for stable and safe storage under ambient conditions. Furthermore, the electrochemical oxidation of these compounds in fuel cells directly produces electricity, which is a compact and efficient energy source. For example, Mishra et al. [96] analyzed in their review the application of formic acid and ammonia as hydrogen carriers along with the development of high-efficiency catalysts for hydrogen release, whereas Meng et al. [97] proposed an innovative electrochemical synthesis of formamide, which could be integrated with hydrogen storage and release technologies.

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Electrochemical Hydrogen Compression (EHC). EHC involves using electrochemical reactions to compress hydrogen gas to high pressure, which can then be stored in tanks [98,99]. Compared to mechanical compression, this method is more efficient and has fewer moving parts. In detail, this process utilizes electrochemical reactions facilitated by PEM technology. Hydrogen is oxidized at the anode, passes through the PEM as protons, and is reduced to molecular hydrogen at the cathode under increased pressure. This technology offers advantages in terms of efficiency, scalability, and integration into renewable energy systems. Its ability to provide pure hydrogen at high pressures makes it suitable for various applications, including energy storage and fueling stations [98,99,100]. Indeed, EHC is particularly useful for small-scale, distributed hydrogen storage systems. For example, Zängler et al. [101] have recently proposed the first tubular EHC design, which enhances scalability and operational efficiency. This study highlights the compactness and integration capabilities of this design. In another work, Sdanghi et al. [102] discuss water management solutions for EHC devices to improve system performance at pressures up to 10 MPa. The study integrates drying systems downstream of the compressor. Nordio et al. [103] focus on the electrochemical behavior of hydrogen compressors when dealing with different gas mixtures, analyzing recovery, purity, and electric consumption. Zou et al. [104] examine the electrochemical performance of hydrogen compressors in diverse gas mixtures, analyzing their recovery, purity, and electric consumption. The above references collectively provide a thorough understanding of the EHC technology, covering its theoretical foundations, design innovations, and practical applications.

The flow diagram in Figure 3 illustrates the key points of the hydrogen storage technologies discussed above. The three options, namely, salt cavern storage, electrochemical hydrogen storage (EHS), and electrochemical hydrogen compression (EHC), use hydrogen gas from upstream sources of hydrogen production (e.g., by electrolysis or SMR). These hydrogen storage strategies are further subdivided into a variety of outputs. To sum up, EHS allows hydrogen to be stored in solid-state materials such as metal hydrides (e.g., MgH₂), metal-organic frameworks (MOFs), or carbon-based materials, whereas EHC uses PEM electrolysis principles and is sent to storage tanks, fueling stations, and energy conversion (fuel cells). In the other mechanism, hydrogen is injected into underground salt caverns, which act as natural storage reservoirs. As a final remark, both EHS and EHC involve electrochemical processes for handling hydrogen, but they serve different purposes and rely on distinct mechanisms.

4.2. Green Hydrogen Integrated into Sector-Coupled Smart Grids

GH₂ produced using renewable energy sources like wind and solar is gaining traction as a key element in the transition to cleaner energy systems. The ability to store GH₂ allows for

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Grid balancing. Excess renewable energy stored as GH₂ can be integrated with smart grid technologies that monitor and predict renewable energy generation and grid demand in real-time. This allows for the optimized operation of electrolyzers, matching hydrogen production with renewable availability and grid needs.

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Power-to-Gas (PtG). Surplus electricity can be used to produce hydrogen via electrolysis, which can then be injected into a natural gas grid (blended with natural gas) or used in industrial processes to provide a large-scale energy storage solution.

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Energy security. Stored hydrogen provides a buffer against supply disruptions and ensures a continuous energy supply.

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Sector flexibility. Hydrogen storage enables its use across different sectors—such as in Hydrogen-to-Mobility (HtM) for transportation or Hydrogen-to-Chemicals (HtC) for industrial applications-depending on demand.

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Advanced forecasting and control systems: Implementing AI and machine learning algorithms to predict renewable energy generation and grid demand can help in the optimal scheduling of hydrogen production, ensuring that it complements the grid’s needs.

Incorporating GH₂ into sector-coupled smart grids is an innovative approach that links different energy sectors—such as electricity, heating, and transportation—to optimize the overall efficiency, reliability, and sustainability of the energy system [105]. This concept is increasingly recognized as a critical strategy for achieving deep decarbonization and enhancing the flexibility and resilience of energy systems [74]. The strategy of sector coupling in smart grids, energy management, the electricity market, and hydrogen economics has been recently reviewed by Hayati et al. [106].

The benefits of sector coupling are numerous and transformative: (a) Increased efficiency. By linking different sectors, sector coupling maximizes the use of renewable energy across the entire energy system, minimizing waste, and improving overall efficiency. (b) Enhanced flexibility. Sector coupling enhances system flexibility by enabling the use of different energy carriers (e.g., electricity, hydrogen, and heat) in response to fluctuating supply and demand conditions. (c) Reduced carbon emissions. Integrating sectors allows for the widespread use of renewable energy, significantly reducing GHG emissions across multiple sectors, including those that are traditionally difficult to decarbonize. (d) Resilience and reliability. A sector-coupled smart grid is more resilient and reliable, as it can draw on diverse energy sources and storage options, ensuring a continuous energy supply even in the face of disruptions.

The following are some key examples illustrating integration across energy sectors.

4.2.1. Electricity

At the heart of sector coupling is the electricity grid, which connects renewable energy sources, e.g., wind, solar, and hydro, to various end-use sectors [107]. The term “hydro” refers to hydroelectric power, which is a renewable energy source generated by harnessing the potential energy of flowing or falling water to drive turbines connected to electric generators. The goal is to use electricity generated from renewables as the primary energy source across different sectors [108].

4.2.2. Heating/Cooling

The integration of the heating and cooling sectors with the electricity grid allows the use of renewable electricity in electric heat pumps, district heating systems, and cooling technologies. This reduces reliance on fossil fuels for heating and cooling, while enhancing overall energy efficiency [109,110].

4.2.3. Transportation

The transportation sector is linked to the electricity grid through the electrification of vehicles—such as electric cars, buses, and trains—and the use of GH₂ in fuel cell vehicles (FCVs). This shift reduces emissions and enables the use of renewable energy for mobility [111].

4.2.4. Industry

In industry, sector coupling involves using electricity and GH₂ to power industrial processes that traditionally rely on fossil fuels. This includes electric furnaces, electrolyzers for hydrogen production, and other electrification measures for processes like steelmaking, chemical production, and cement manufacturing [112,113].

4.2.5. Decentralization and Digitalization

Sector coupling is further enhanced by integrating Decentralized Energy Resources (DERs), such as distributed generation (e.g., rooftop solar), energy storage systems (e.g., batteries), and demand response capabilities [114,115]. These resources enable the greater flexibility and responsiveness of the energy system. Furthermore, the coordination of different energy sectors relies heavily on Smart Grids and Digitalization (SGD). Smart grids use advanced sensors, communication networks, and data analytics to monitor and control energy flows in real-time. This allows for efficient balancing of supply and demand across sectors, optimization of energy usage, and seamless integration of variable RESs [116].

4.2.6. Energy Storage and Flexibility

Sector coupling often involves the use of energy storage technologies (ESTs) to store excess electricity generated from renewable sources [117,118]. This stored energy can be used later to power heating systems, charge electric vehicles, or convert it into hydrogen for industrial use. Flexibility is further enhanced through demand response programs, where consumers adjust their energy consumption based on real-time price signals or grid requirements. This helps balance the grid, particularly when integrating high levels of renewable energy [119,120]. Combining hydrogen storage with renewable energy sources and smart grid technologies allows for better management of energy flows. This integration can optimize the timing of hydrogen production, storage, and usage, reducing losses and improving the overall system efficiency [121].

4.3. Integration of Green Hydrogen in Various Industrial Sectors

GH₂ has the potential to revolutionize various industrial processes by providing a clean and versatile energy source that contributes to sector coupling and energy system integration. The integration of GH₂ into various sectors, such as heavy industries, petrochemicals, glass manufacturing, cement and chemical production, transportation, and agri-food, has the potential to contribute significantly to decarbonization, reduce reliance on fossil fuels, and mitigate climate change [122,123]. It is possible to classify the different types of industrial processes according to the role played by GH₂ as an energy carrier (GH₂ is used as a fuel or heat source to replace fossil fuels in energy-intensive processes) or as a feedstock (GH₂ is used as a chemical reactant in industrial processes rather than a direct energy source).

4.3.1. GH₂ as an Energy Carrier

• Cement production

Cement production is a carbon-intensive process, with emissions arising from both the energy consumed and the chemical reactions involved in producing the clinker. GH₂ serves as a viable alternative fuel for high-temperature kilns required in cement manufacturing, thereby reducing reliance on fossil fuels and lowering GHG emissions [124]. The integration of GH₂ into cement production bridges the building materials industry with the renewable energy sector, fostering greater energy system integration and contributing to the decarbonization of industrial processes. One relevant example of the use of GH₂ in industrial cement production is the HYIELD collaborative project [125] funded by the European Commission under the Horizon 2020 program. HYIELD involves multiple partners across Europe, including industry leaders, academic institutions, and research organizations. Some notable partners include Cemex (cement industry), Hydrogen Europe, Siemens Energy (technology provider), and the Fraunhofer Institute (research and development). The HYIELD project is a significant step toward achieving a low-carbon industrial future, with a particular focus on industries like cement, steel, and chemicals that have traditionally been difficult to decarbonize. By utilizing advanced hydrogen technologies, such as SOEC [39,40,41], this project aims to demonstrate that GH₂ can be both a sustainable and cost-effective energy source for industrial applications. The HYIELD project is expected to play a significant role in Europe’s broader Green Deal and Fit for 55 initiatives that aim to cut greenhouse gas emissions by 55% by 2030.

• Glass manufacturing

High-Temperature Processes. Glass manufacturing requires high temperatures, which are often achieved through the combustion of fossil fuels. GH₂ can be used as a clean-burning fuel in glass furnaces, helping decarbonize the industry, as detailed in several reviews [122,123,126,127,128]. This application promotes energy integration by enabling the glass industry to utilize renewable energy through hydrogen, linking the manufacturing sector with the energy sector.

• Textile and pulp and paper industries

Both the textile and pulp and paper industries require significant amounts of heat for drying and bleaching processes. GH₂ can replace fossil fuels in these applications, contributing to lower carbon emissions [129,130,131]. The use of GH₂ in these industries links them to renewable energy, promoting greater energy system integration and sustainability.

• Aluminum production

Aluminum production involves the reduction of alumina (aluminum oxide) using a high-temperature process. GH₂ can be used as a reducing agent, replacing carbon-intensive methods and reducing overall emissions [132,133]. The integration of GH₂ into aluminum production links the mining and metals sector with the renewable energy sector, contributing to a more sustainable energy system.

• Transportation and mobility

By using GH₂, Fuel Cell Vehicles (FCVs) help decarbonize the transportation sector, reducing GHG emissions and improving air quality in urban areas. Relevant examples, such as Toyota Mirai and Hyundai Nexo, are among the leading automakers developing hydrogen FCVs. These vehicles use GH₂ to generate electricity in a fuel cell, powering electric motors [134,135]. The only emission from these vehicles is water vapor, making them a clean alternative to traditional internal combustion engine vehicles [136]. Hydrogen-powered buses, which have been adopted by cities like London, Tokyo, and Los Angeles, are a notable example of hydrogen’s application in the modern transportation sector. A recent study evaluates the environmental and economic feasibility of transitioning to hydrogen-powered public transport in Trentino Alto Adige, an Alpine region in Northern Italy, compared to battery-electric and hybrid buses [137]. These buses utilize hydrogen fuel cells to power electric drivetrains, offering a zero-emission alternative to diesel buses. The adoption of hydrogen buses in public transportation systems reduces the carbon footprint of urban transit and supports the transition toward cleaner and more sustainable public transport solutions. In their study, Miraftabzadeh et al. [137] reported that adopting battery-electric vehicles (BEVs) could reduce global emissions by 68% when using a well-to-wheel (WTW) approach, which is a comprehensive methodology for assessing the total environmental impact and energy efficiency of a fuel or energy source [138]. This result serves as an indirect indication of hydrogen’s potential to similarly achieve significant reductions in CO₂ emissions. Furthermore, hydrogen use in rail transport is a well-established technological reality today. For example, the Alstom Coradia iLint is the world’s first hydrogen-powered train, developed by Alstom [139]. It operates on routes in Germany using GH₂ to power its fuel cells, producing only water and steam as by-products. Replacing a single diesel regional train with a hydrogen-powered Coradia iLint is estimated to reduce the annual CO₂ emissions equivalent to those produced by 400 cars. Additionally, the regional train company LNVG reports that the project saves 1.6 million liters of diesel fuel annually [140]. Hydrogen trains are particularly valuable for routes that are not electrified, offering a zero-emission alternative to diesel trains and contributing to the decarbonization of the rail sector.

4.3.2. GH₂ as a Feedstock

• Green steel production

The traditional method of steel production, which relies on coke (a derivative of coal) for iron ore reduction, is highly carbon-intensive. GH₂ can be used for direct reduction to convert iron ore into iron, generating water instead of carbon dioxide as a by-product. This process, known as Hydrogen Direct Reduction (HDR), significantly lowers emissions in steel production.

The integration of GH₂ into steel production not only defossilizes the sector, but also links it with the renewable energy sector because the hydrogen used can be produced from excess renewable electricity [141]. We can point out, for example, the HYBRIT (Hydrogen Breakthrough Ironmaking Technology) project, which is a joint venture between SSAB, LKAB, and Vattenfall in Sweden [142]. It aims to replace coal with GH₂ in the steelmaking process using hydrogen to directly reduce iron ore into steel. This project has the potential to eliminate nearly all CO₂ emissions from steel production, making a significant contribution to decarbonizing the steel industry, which is one of the largest industrial sources of carbon emissions.

• Green ammonia production

Ammonia production, primarily for fertilizers, is traditionally based on the Haber-Bosch process, which requires hydrogen derived from natural gas (SMR) [143]. Replacing this with GH₂ produced via electrolysis can significantly reduce the carbon footprint of ammonia production. By producing ammonia using GH₂, the chemical industry becomes interconnected with the renewable energy sector [144]. Additionally, ammonia itself can be used as a carbon-free fuel or as a hydrogen carrier, further integrating different energy sectors. An example is provided by Yara and Ørsted green ammonia plants [145]. Yara, a leading fertilizer company, and Ørsted, a renewable energy company, are collaborating on a project in Norway to produce green ammonia using GH₂. Hydrogen is produced via electrolysis, which is powered by renewable energy. Green ammonia can be used both as a fertilizer and as a carbon-free fuel, reducing the carbon footprint of agriculture and potentially the shipping industry, which can use ammonia as a marine fuel [146].

• Chemical production

Methanol and Synthetic Fuels. GH₂ can be combined with captured CO₂ to produce methanol and other synthetic fuels, which can be used in a variety of industrial processes or as transportation fuels [85,147,148,149]. This process helps reduce emissions and creates use for CO₂, which would otherwise contribute to global warming. This application connects the chemical industry with renewable energy, carbon capture, and utilization (CCU) technologies, integrating multiple sectors into a cohesive, low-carbon energy system.

• Refining and petrochemicals

Refineries use large amounts of hydrogen for hydrocracking and desulfurization processes. Typically, this hydrogen is produced from natural gas. Switching to GH₂ can help refineries significantly reduce their carbon emissions [150]. Integrating GH₂ into refineries links the petrochemical industry with renewable energy sources. This also opens pathways for producing sustainable fuels, such as synthetic fuels derived from GH₂ and captured CO₂ [151]. This concept is being implemented in the Shell Rheineland Refinery in Germany, which integrates GH₂ into its operations as part of the REFHYNE project [152]. GH₂ is used in refinery processes such as hydrocracking and desulfurization, which are essential for producing cleaner fuels. By replacing gray hydrogen (produced from natural gas) with GH₂, the refinery reduces its carbon emissions, contributing to the decarbonization of the petrochemical sector.

4.3.3. The Benefits of Green Hydrogen in the Agri-Food Industry

Thanks to our involvement in research areas focused on the study of technological processes of interest in the agri-food sector [153,154,155], here we want to direct our analysis on how GH₂ can be beneficial for the industrial sector of food and beverage. The agri-food industry can fall into both categories depending on how GH₂ is utilized. Many agri-food processes, such as drying, pasteurization, refrigeration, and processing, require significant energy inputs. GH₂ can be used as a clean energy source to replace fossil fuels. In addition, hydrogen is a critical input for producing ammonia, the base for nitrogen fertilizers, which are essential in agriculture. The food and beverage industry is present in every country and often plays a significant role in their economies. Therefore, it can significantly benefit from using GH₂ as an energy vector in its production plants. GH₂ offers several advantages that align with the industry’s goals of reducing carbon emissions, enhancing sustainability, and improving operational efficiency. Below is an overview of how GH₂ can be beneficial.

• Decarbonization of energy-intensive processes

The food and beverage industry relies on heat-intensive processes like pasteurization, sterilization, drying, and cooking, traditionally powered by fossil fuels. GH₂ offers a clean alternative that reduces carbon emissions. Additionally, GH₂ can be converted into electricity via fuel cells, ensuring sustainable energy for production facilities, especially in off-grid areas or regions dependent on fossil-based grids.

• Energy storage and flexibility

GH₂ supports the food and beverage industry by addressing seasonal production shifts and grid independence. It can be produced and stored during low electricity demand or surplus renewable energy, and then used to meet peak energy needs, ensuring continuous operations. Additionally, GH₂ reduces reliance on the electricity grid, enhancing energy security and protecting against power outages or instability.

• Sustainable packaging and logistics

Hydrogen-powered transport in the food and beverage industry reduces logistics emissions by using hydrogen-fueled trucks, forklifts, and material handling equipment. Additionally, hydrogen fuel cells power refrigeration units in cold chain logistics, ensuring optimal storage conditions for perishable goods while replacing diesel generators.

• Water recycling and waste management

The agri-food industrial sector is both energy-intensive and water-intensive, making it an excellent candidate for adopting hydrogen-based systems that integrate water recovery. The production of GH₂ through electrolysis generates water as a by-product when hydrogen is used in fuel cells. This water can be captured and reused in the plant, promoting sustainable water management practices that are particularly valuable in water-intensive industries. As a hypothetical case study, a food processing plant producing packaged fruits and vegetables using GH₂ for its energy would need 1 ton/day of hydrogen demand, corresponding to the theoretic water recovery of 9 m³/day calculated from the hydrogen-to-water conversion rate (9 kg H₂O/kg H₂). For medium-sized agri-food plants with typical water consumption in the range of 20–50 m³/day, mainly used to clean equipment, cool machinery, and wash raw material, this would give rise to a reduction in total water withdrawal by approximately 18–45%, demonstrating significant environmental and economic benefits. Of course, the actual percentage reduction would depend on the facility’s specific water usage profile. The factors influencing water recovery are as follows: (a) fuel cell efficiency, i.e., water recovery depends on how efficiently the fuel cell operates and captures the water vapor produced during the reaction; (b) water capture system, i.e., effective systems to condense and collect the water vapor are necessary for optimal recovery; and (c) scale of operations, i.e., large-scale hydrogen production results in proportionally higher water recovery.

Waste to hydrogen. Organic waste generated by food and beverage plants can be converted into biogas, which can then be reformed into hydrogen. This process provides a renewable hydrogen source while addressing waste management challenges and fostering a circular economy.

• Reducing carbon footprint

Sustainability goals. The food and beverage industry is under increasing pressure to reduce its carbon footprint and meet sustainability goals. By incorporating GH₂ into energy use, companies can significantly reduce their GHG emissions, contributing to a more sustainable and eco-friendly production process. Brand image and consumer demand. As consumers become more environmentally conscious, there is growing demand for sustainably produced goods. Adopting GH₂ can enhance brand reputation and attract eco-conscious consumers.

• Regulatory compliance and future-proofing

Meeting Emissions Regulations. As governments worldwide introduce stricter emission regulations, adopting GH₂ can help the food and beverage industry comply with these rules and avoid potential penalties. Future-proofing operations. Investing in GH₂ technology prepares companies for rising carbon prices, fossil fuel supply risks, and evolving environmental regulations.

• Research and innovation opportunities

R&D in hydrogen applications. The industry can lead in developing new GH₂ applications, such as hydrogen-powered food processing equipment or innovative packaging solutions, reducing its overall carbon footprint.

Collaboration with energy providers. Partnering with energy companies and research institutions to develop tailored hydrogen solutions can drive innovation and operational efficiency. Overall, integrating GH₂ into food and beverage production processes connects this industrial sector with renewable energy systems, reduces fossil fuel dependence, and enhances flexibility and sustainability across energy systems.

4.3.4. Key Factors Influencing Water Footprint

Unlike conventional systems that rely on fossil fuels with a relatively lower water requirement, GH₂ derived from electrolysis adds significant use to water unless alternative sources are used (e.g., treated wastewater and seawater). Retrofitting industrial processes to use hydrogen often initially involves higher water use, although lifecycle water use can decrease if renewable water sources are integrated. Therefore, adopting alternative water sources, like seawater desalination and wastewater recycling, can mitigate water footprint increases, making GH₂ more sustainable.

Table 4. Water footprint comparison table for the production processes of gasoline, aluminum cement, and steel. (Data obtained from refs. [156,157,158]).

Product	Without GH2 (Liters per kg or MJ)	With GH2 (Liters per kg or MJ)
Gasoline	~1.5 L/MJ	~11 L/MJ (including water for GH2)
Aluminum	~3000 L/kg (electricity-intensive)	~3500 L/kg (if powered with GH2)
Cement	~100 L/kg (mineral processing and cooling)	~150 L/kg (for high-temp GH2 processes)
Steel	~140 L/kg (blast furnace processes)	~200 L/kg (if GH2 replaces coke/coal)

showcases a comparative analysis of the water footprint of several industrial products with and without GH₂. The water footprint measures the water volume used directly and indirectly in the production of a product. For fuels and materials like gasoline, aluminum, cement, and steel, incorporating GH₂ impacts the water footprint, especially during its production phase.

4.4. Illustration of Some of the Best-Known Examples of GH₂ Integrated into Sector-Coupled Smart Grids

• H2RES Project (Denmark)

The H2RES project [159], located near Copenhagen, Denmark, is a pioneering initiative that integrates wind power with hydrogen production. The project uses offshore wind turbines to produce GH₂ via electrolysis, which is then used in transportation (e.g., hydrogen FCVs) and potentially in other industrial applications. The project links the power generation sector with the transportation sector, showcasing the potential of hydrogen to serve as a storage medium for renewable energy and as a fuel for heavy-duty vehicles.

• HEAVENN Project (Northern Netherlands)

The HEAVENN project (H₂ Energy Applications in Valley Environments for Northern Netherlands) [160] is one of Europe’s largest GH₂ projects. It integrates various renewable energy sources, including wind and solar energy, to produce GH₂, which is then used across multiple sectors. According to the sector coupling strategy, HEAVENN combines the electricity, gas, heating, and mobility sectors in a smart grid, demonstrating the versatility of hydrogen in balancing supply and demand across different energy systems, thus promoting the integration of RESs with industrial and residential energy use.

• NortH2 project (Netherlands)

The NortH2 project in the Netherlands [161] aims to produce GH₂ using electrolyzers powered by offshore wind energy and distribute it across various sectors, including industry and transportation. NortH2 integrates the electricity, industrial, and transportation sectors, showcasing the potential of GH₂ to enable a fully decarbonized energy system with plans to produce up to 1 million tons of hydrogen per year by 2040.

• REFHYNE Project (Germany)

Located in Germany, the REFHYNE project (Clean Refinery Hydrogen for Europe) [152], is a large-scale GH₂ production facility integrated into a refinery. It uses renewable electricity to produce hydrogen, which is then used in the refining process and other applications. This project highlights the coupling of the power sector with industrial processes, showing how GH₂ can decarbonize energy-intensive industries.

• Westküste 100 Project (Germany)

The Westküste 100 project [162] is a German initiative that links wind energy with hydrogen production, which is then used in various industrial applications and as a feedstock for synthetic fuels. This project connects renewable energy production, hydrogen storage, industrial applications, and synthetic fuel production, creating a comprehensive sector-coupled smart grid, thereby contributing to Germany’s energy transition.

• HyDeploy Project (United Kingdom)

The HyDeploy project [163] in the UK is focused on blending GH₂ with natural gas in the existing gas grid. This initiative aims to reduce carbon emissions from heating while utilizing the existing infrastructure. Through a sustainable strategy of sector coupling, HyDeploy integrates the gas and heating sectors with renewable hydrogen production, highlighting the potential of hydrogen to decarbonize heating systems.

• HyNet North West Project (United Kingdom)

HyNet North West is a major UK project [164] that aims to decarbonize the North West of England and North Wales by integrating hydrogen production with renewable wind and solar energy. Then, GH_2, produced via electrolysis and blue hydrogen (from natural gas with CCS), is used in industry, heating, and transportation. HyNet North West is an example of a hybrid approach that uses both green and blue hydrogen to accelerate decarbonization while developing infrastructure for future GH₂ expansion.

• HydrOm Hub Project (Oman)

Oman’s Green Hydrogen Hub [165] aims to become one of the largest hydrogen production facilities in the world. The project will use solar and wind power to produce GH₂, which will be both exported and utilized domestically across various sectors. It establishes a connection between renewable energy generation, export markets, and local applications in transportation and industry, thereby supporting a diversified energy economy. This initiative underscores the potential for countries rich in renewable resources to emerge as leading exporters of GH₂, significantly contributing to global decarbonization efforts.

• Haru Oni Project (Chile)

The Haru Oni project [166] in Chile’s Patagonia region is an initiative by Siemens Energy, Porsche, and other partners to produce GH₂ using wind energy. This hydrogen is then converted into synthetic fuels for use in vehicles. The hydrogen is combined with captured CO₂ to produce synthetic fuels like e-methanol and e-gasoline. Thus, the project integrates renewable electricity generation with the production of green fuels, effectively bridging the gap between the electricity and transportation sectors. Haru Oni exemplifies the potential of utilizing GH₂ not only for direct energy applications but also as a critical component in the production of carbon-neutral fuels for transportation. This is particularly valuable for sectors like aviation, where electrification remains a significant challenge.

• ACWA Power Neom Project (Saudi Arabia)

The Neom project, led by ACWA Power [167], aims to build one of the world’s largest GH₂ production facilities in Saudi Arabia powered by solar and wind energy. The GH₂ produced by large-scale electrolyzers can be used for domestic consumption and export, particularly as ammonia for use in global energy markets. Neom represents a key example of how large-scale renewable energy projects can be integrated with GH₂ production to supply global markets with clean energy, positioning Saudi Arabia as the leader in an emerging hydrogen economy.

These projects highlight the versatility and potential of GH₂ in fostering interconnected, efficient, and sustainable energy systems. The integration of GH₂ with renewable energy sources supports the decarbonization of various sectors, including transportation, industry, heating, and synthetic fuel production. As these projects expand and technology advances occur, GH₂ is expected to play a key role in the global transition to a low-carbon economy.

4.5. Benefits of Optimized Integration and System Efficiency

The main benefits of optimized hydrogen integration into smart grids are, e.g., the possibility to perform advanced data analysis and control and to design systems with high resilience and reliability.

Advanced Data Analytics and Control. The smart grid can leverage advanced data analytics, machine learning, and real-time monitoring to optimize the flow of energy and hydrogen across sectors. This ensures that energy is produced, stored, and consumed in the most efficient way possible, reducing losses and improving the overall system efficiency.

Resilience and Reliability. Integrating hydrogen into a smart grid enhances the system resilience. Hydrogen fuel cells can provide backup power in the case of outages, and the ability to store large amounts of energy as hydrogen improves the grid’s reliability during periods of high demand or low renewable energy generation.

Several papers provide comprehensive insights into the current state of research on integrating GH₂ production with renewable energy sources and their role in stabilizing smart grids. They are key resources for understanding technological, economic, and system integration challenges and opportunities in this field. Kharel et al. [168] explore the role of hydrogen as a storage solution for renewable energy and its integration into the energy grid. They discuss the economic and technical aspects of using hydrogen for long-term energy storage to mitigate the intermittency of renewable sources. Nguyen et al. [169] review the challenges of integrating intermittent renewable energies into hydrogen production systems.

Saadat et al. [80] discuss the potential of underground storage for stabilizing GH₂ production from intermittent sources. Park et al. [170] analyze the economic aspects of integrating green and blue hydrogen with ammonia production. Gharibvand et al. [171] address the role of microgrids in enhancing the flexibility of renewable energy integration with hydrogen production. As has been recalled above, Hayati et al. [106] discuss how incorporating hydrogen into smart grids can address challenges like intermittency, energy storage limitations, and grid integration issues. This reference provides insights into methods that can be utilized to effectively promote and integrate hydrogen within sector-coupled smart grids, thereby enhancing the overall efficiency and stability of energy systems.

5. Power-to-X Technologies: Current Methodologies for Efficient and Long-Lasting Hydrogen Storage

As the world increasingly adopts renewable energy, the challenge of integrating these intermittent energy sources into the grid becomes more pronounced. This is where the concept of PtX within sector-coupled smart grids comes into play, providing a versatile solution to the energy transition challenge.

PtX refers to the conversion of electricity—primarily generated from renewable sources such as wind, solar, and hydro or from non-renewable sources requiring CCS—into various forms of energy carriers or chemicals, such as hydrogen, synthetic fuels, heat, and chemicals [172,173,174]. Within this framework, GH₂ produced through electrolysis can be stored and used across multiple applications, thus linking the electricity sector to energy-intensive sectors, such as transportation, industry, and heating. The main components and processes involved in PtX are as follows:

5.1. Power-to-Heat (PtH)

Excess renewable electricity can be converted into thermal energy for residential, commercial, and industrial heating applications [175,176]. This is typically achieved using electric heat pumps or direct resistive heating systems [177]. PtH plays a critical role in balancing the electricity grid by absorbing excess electricity during periods of overproduction, thus supporting grid stability and preventing energy waste. It also helps decarbonize the heating sector by replacing fossil fuel-based heating with renewable energy-driven solutions.

5.2. Power-to-Gas (PtG), Power-to-Hydrogen (PtH₂)

Power-to-gas technologies convert excess renewable energy into gas, typically hydrogen, via electrolysis (PtH₂). This GH₂ can be used directly in industrial processes, blended into natural gas grids, or converted into synthetic fuels, e.g., methane via methanation (reaction of hydrogen with CO₂) and methanol for use in various sectors. The hydrogen produced through the PtH₂ process can also serve as a form of energy storage. It can be stored and later converted back into electricity using fuel cells or hydrogen turbines during periods of high demand or when renewable generation is low or fluctuates. PtG technologies have been gaining attention because of their ability to integrate electricity and gas networks, providing a pathway for decarbonizing industries reliant on natural gas. In the European Union, PtG is considered a crucial method for reducing dependence on fossil fuels and increasing energy security, especially in light of recent geopolitical disruptions affecting gas supply. These systems also present opportunities for sector coupling, allowing stored hydrogen or methane to be used in power generation, industrial processes, and transportation fuels, thereby enhancing the grid flexibility and stability [178,179].

5.3. Power-to-Liquids (PtL)

This involves the production of liquid fuels, such as synthetic diesel or jet fuel, from renewable electricity. Often, the Fischer-Tropsch synthesis process is used, where hydrogen produced from electrolysis is combined with captured CO₂ to produce hydrocarbons. These fuels can be used in sectors where electrification is challenging, such as aviation and heavy-duty transport [180]. PtL also offers a potential pathway to achieving carbon neutrality by leveraging existing infrastructure and minimizing disruptions to industries reliant on liquid fuels.

5.4. Power-to-Chemicals (PtC)

Owing to the Haber—Bosch process, hydrogen produced through electrolysis can be combined with nitrogen to create ammonia, which is a critical component for fertilizers, household cleaning, and other industrial products. Along with crude oil refining, ammonia production represents the largest use of hydrogen globally [146,181]. Another important chemical obtained from hydrogen produced through electrolysis is methanol. This compound is synthesized using syngas (H₂ + CO) in a reactor in the presence of a metallic catalyst [182,183]. Alternatively, methanol can be produced directly through the reaction of hydrogen with carbon dioxide [184]. This process offers the opportunity to convert atmospheric CO₂ into a liquid fuel, which can serve as a direct substitute for fossil fuels in transportation, as shown by the reaction 3H₂ + CO₂ → CH₃OH + H₂O [185].

5.5. Power-to-Mobility (PtM)

These technologies harness the potential for renewable energy conversion to support sustainable transportation, reduce carbon emissions, and minimize reliance on fossil fuels by creating sustainable mobility solutions [186]. In addition to the previously discussed application of hydrogen as a precursor of synthetic fuels for motor vehicles, PtM technology includes the direct use of renewable electricity in battery-electric vehicles (BEV), where electricity powers the electric motors [187]. This approach leverages advancements in battery technology to store and utilize renewable energy efficiently.

The scheme illustrated in Figure 4 outlines the general framework of PtX technologies in sector-coupled smart grids. It demonstrates how energy flows from power generation sources, subdivided into RESs such as wind, solar, and hydro, as well as fossil energy sources like natural gas and coal, to energy conversion technologies (PtX). The diagram also illustrates several processes of hydrogen conversion, such as H₂-to-Fuel (H₂tF), H₂-to-Industry (H₂tI), and H₂-to-Chemical (H₂tC). In addition to power generation sources and PtX technologies, key components include storage systems and end-use sectors.

The diagram reveals that the storage systems encompass electrical storage (batteries), gas storage (H₂ tanks), thermal storage (hot water tanks), and chemical storage (ammonia, methanol). End-use sectors are defined as electricity (power distribution for residential, commercial, and industrial use); transportation (hydrogen can be used as a fuel in FCVs or as a component of synthetic fuels, enabling the decarbonization of road transport, shipping, and aviation); heating/cooling (hydrogen can be injected into natural gas grids to decarbonize heating systems or used directly in hydrogen boilers and combined heat and power (CHP) systems); and industry (hydrogen can be used as a feedstock in various industrial processes, such as steel production, ammonia synthesis and refining, replacing fossil fuels and reducing carbon emissions). The diagram comprehensively covers the flow of hydrogen not only as a fuel for mobility but also as a critical input for industrial processes and chemical production. Finally, the smart grid infrastructure incorporates advanced technologies and systems that improve the efficiency, reliability, and sustainability of electricity distribution and consumption. These include communication systems, sensors, Internet of Things (IoT) devices, grid management, and control systems.

5.6. Advantages of PtH₂ in Producing Green Hydrogen

Several aspects of PtH₂ technology emphasize its positive impact on the building of smart grid infrastructure. Indeed, grid balance and flexibility are guaranteed by PtH₂, which helps manage the variability of power output associated with the intermittent nature of RESs by absorbing excess electricity during periods of high generation and low demand and converting it into hydrogen for later use. Moreover, by converting renewable electricity into green hydrogen, PtH₂ contributes to the decarbonization of hard-to-abate sectors that are difficult to electrify directly, such as the steel industry, transportation, and heating, thereby reducing GHG emissions and supporting global climate goals [188,189].

From the point of view of economic and energy efficiency, PtH₂ creates economic value by utilizing surplus renewable electricity that might otherwise be curtailed (wasted), turning it into a valuable energy carrier. It should be considered that although the overall efficiency of the conversion processes (electricity-to-hydrogen and back-to-electricity or other uses) may be lower than that of direct electricity use, the ability to store and transport energy as hydrogen provides a flexible and versatile energy solution [190].

5.7. Power-to-Hydrogen-to-Power (PtH₂tP)

The PtH₂tP framework has been increasingly recognized for its potential to address renewable energy intermittency and enhance grid stability. Hydrogen serves as an energy storage medium by converting surplus renewable electricity into storable hydrogen through electrolysis, which can later be re-electrified using fuel cells or turbines to meet fluctuating energy demands. This capability positions hydrogen as a vital component in renewable grid integration and decarbonization strategies [191]. Recent advancements in solid oxide fuel cells (SOFC) and other electrolyzer technologies have improved the efficiency and economic feasibility of PtH₂tP systems; however, several challenges persist. These include low round-trip efficiency and high cost of infrastructure development. However, further research and development are necessary to overcome these technical and economic barriers; in addition, policy frameworks and financial incentives must be aligned with technological advancements to foster hydrogen’s widespread adoption as a viable energy carrier in global energy markets. Hydrogen holds significant promise in enabling sustainable and resilient energy systems, and addressing the current limitations of PtH₂tP systems is critical to achieving their full potential.

6. Role of H₂ Conversion Processes: H₂-to-Fuel and H₂-to-Gas Technologies

“H₂-to-Fuel” and “H₂-to-Gas” are applications that refer to the use of hydrogen in different ways within energy systems [192]. Both of these applications play crucial roles in the context of energy transition, particularly in integrating renewable energy, decarbonizing various sectors, and enhancing energy storage and distribution.

6.1. Hydrogen-to-Fuel (H₂tF)

H₂tF refers to the use of hydrogen as a fuel for generating electricity. In this application, hydrogen is converted into electrical energy through various technologies, most notably:

6.1.1. Fuel Cells

Hydrogen can be used in fuel cells to generate electricity. In a fuel cell, hydrogen reacts with oxygen (typically from air) to produce electricity, water, and heat according to the overall chemical reaction: 2H₂ + O₂ → 2H₂O + electricity + heat. The main components of a fuel cell are [193] (a) anode (the negative side where hydrogen gas is supplied), (b) cathode (the positive side where oxygen from air is supplied), (c) a proton-conducting membrane (in PEMFCs) that allows only protons to pass through while blocking electrons, (d) thin layers of Pt-based material on the electrodes to trigger the electrochemical reactions, and (e) grooved plates that distribute hydrogen and oxygen gases uniformly over the electrode surfaces. This process is highly efficient and does not emit no carbon dioxide, making it an environmentally friendly option for power generation. Applications include stationary power generation (for home heating, businesses, or grid support) as well as transportation (e.g., FCVs powered by hydrogen), offering a clean alternative to traditional gasoline or diesel engines [19,194,195,196].

6.1.2. Combustion in Gas Turbines

Hydrogen can be burned in modified gas turbines to generate electricity. This method is applicable in existing power plants with adjustments to account for hydrogen’s unique combustion characteristics compared to natural gas. It is particularly suitable for large-scale power generation, where hydrogen can complement or replace natural gas [197,198,199]. H₂tF is particularly relevant for (a) grid balancing. It provides a way to store excess renewable energy (e.g., from wind or solar) by converting it into hydrogen, which can later be used to generate electricity when demand is high, and (b) decarbonizing the hard-to-abate industrial sectors. It offers a pathway to reduce carbon emissions in electricity generation, especially in regions heavily reliant on fossil fuels.

6.2. Hydrogen-to-Gas (H₂tG)

H₂tG refers to the injection or blending of hydrogen into natural gas networks [200] or the conversion of hydrogen into synthetic natural gas (SNG), which can be used in existing natural gas infrastructure [201,202]. SNG is one of the commodities that can be produced from coal-derived syngas through the methanation process. H₂tG can be implemented in several ways. For example, hydrogen can be mixed with natural gas and used in existing pipelines, appliances, and power plants [203,204,205,206]. This reduces the carbon intensity of the natural gas used in heating, power generation, and industrial processes. Typically, hydrogen can be blended at low percentages (up to 20% by volume) without significant modifications to the existing gas infrastructure.

H₂tG is particularly relevant for

Decarbonizing the gas sector by blending or substituting natural gas with hydrogen or SNG reduces the carbon emissions associated with gas use in heating, cooking, and industrial processes.

Energy storage. It allows for large-scale, long-term storage of renewable energy in the form of hydrogen or methane, which can be distributed through the existing natural gas infrastructure.

7. Key Challenges and Critical Issues of Green Hydrogen-Integrated Smart Grids

The numerous projects currently implemented worldwide and many more to be realized in the near future serve as compelling evidence that GH₂-integrated smart grids are at the forefront of energy transition. These systems present a viable pathway for decarbonizing various sectors by coupling renewable energy sources with hydrogen production and utilization. However, despite their vast potential, these systems encounter notable challenges and gaps that must be addressed to fully realize their promise. Below is an overview of the key challenges, gaps, future prospects, and potential advancements in these systems, accompanied by the suggested directions for research.

7.1. Key Challenges

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Water scarcity and competition. The significant water demand required to produce hydrogen through electrolysis can compete with drinking water and agriculture in areas facing water scarcity [207,208,209].

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Incorporation of seawater. The use of seawater offers a viable alternative to freshwater sources. However, seawater electrolysis requires advanced desalination processes or corrosion-resistant materials to handle impurities, which can increase costs. Moreover, although seawater utilization reduces competition for freshwater, it introduces new challenges, such as the energy intensity of desalination and handling of brine waste [210,211,212,213].

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High production costs. The cost of producing GH₂ through electrolysis is currently higher than that of conventional production methods of gray and blue hydrogen (e.g., SMR). This cost disparity is a major barrier to widespread adoption [214].

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Energy efficiency. The process of converting electricity to hydrogen and then using hydrogen in various applications (e.g., reconversion to electricity and fuel for vehicles) entails energy losses at each stage, thereby reducing the overall system efficiency [196,215,216].

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Infrastructure development. The existing infrastructure for hydrogen production, storage, transportation, and distribution remains underdeveloped. Establishing and retrofitting the necessary infrastructure requires substantial investment and coordination [217].

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Grid integration and stability. Integrating GH₂ into existing smart grids introduces challenges related to grid stability [218,219], particularly when dealing with variable renewable energy sources like wind and solar [220,221]. Balancing supply and demand in real-time while accommodating hydrogen production adds complexity to grid management.

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Policy and regulatory barriers. The lack of standardized policies and regulations governing the production, storage, and use of hydrogen hinders the development and scaling of hydrogen-integrated smart grids [218,222].

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Public acceptance and safety. The public perception of hydrogen safety, particularly regarding its storage and transportation, poses a significant challenge. Addressing safety concerns and raising awareness of the benefits of hydrogen is crucial.

7.2. Gaps

Technology maturity. Many GH₂ technologies, particularly large-scale electrolysis and hydrogen storage, are still in the early stages of development and require further refinement to become commercially viable (see Figure 2) [223].

Data and modeling. Comprehensive data and models to assess the behavior of GH₂ systems at scale are currently lacking, particularly regarding grid interaction and long-term stability.

Interoperability. Ensuring that different components of a hydrogen-integrated smart grid (e.g., electrolyzers, fuel cells, and storage systems) function seamlessly remains a significant challenge requiring targeted solutions.

Economic viability. Robust economic models that account for the entire value chain of GH₂ production, storage, distribution, and end use are currently limited, making it difficult to assess the true economic potential of these systems.

7.3. Future Prospects and Potential Advancements

Cost reduction through innovation. Advances in electrolysis technologies, such as the development of more efficient catalysts and electrolyzers, can significantly reduce the cost of GH₂ production. Additionally, economies of scale and increased investments in renewable energy sources are expected to further reduce costs.

Hybrid systems. Combining hydrogen production with other energy storage technologies (e.g., batteries) and renewable energy sources could lead to hybrid systems that enhance both efficiency and reliability. These systems can provide continuous power by switching between different energy sources depending on availability, thereby enhancing grid reliability.

Advanced materials. Research on new materials for hydrogen storage (e.g., metal hydrides) and transportation (e.g., hydrogen pipelines with advanced coatings) could contribute to enhanced safety and cost reduction.

Digitalization and AI. The integration of digital technologies, such as AI and machine learning, offers opportunities to optimize smart grid management by forecasting energy demand, enhancing grid stability, and improving the efficiency of hydrogen production and utilization.

Sector integration. Expanding the integration of GH₂ into additional sectors beyond electricity and transportation—such as heavy industry, agriculture, and residential heating—could unlock new value streams and increase system resilience (fuel cells for vehicles) [224,225] or industry [226] (as feedstock), creating a more interconnected energy system where energy can flow between sectors based on demand and supply, further stabilizing the grid.

7.4. Research Directions

Alternative Water Sources. GH₂ production through water electrolysis can utilize alternative water sources like seawater, treated wastewater, and other non-potable water. This approach addresses the challenge of competing with freshwater for agriculture and human needs. Specifically, the use of seawater requires advanced desalination processes (e.g., reverse osmosis or multi-effect distillation) to produce deionized water. The development of corrosion-resistant electrodes and catalysts for direct seawater electrolysis is currently underway. The economic implications are due to the increase in capital costs associated with desalination and the reduction in costs through large-scale offshore hydrogen plants that integrate renewable energy and desalination. The application of wastewater requires pre-treatment processes (e.g., filtration and disinfection) to remove contaminants and organics, together with the adaptation of electrolyzers to handle residual impurities. Advantages include the utilization of existing water treatment infrastructure, reducing costs in urban settings, and promoting a circular economy and sustainability by repurposing waste streams. Finally, the use of brackish water requires less intensive desalination compared to seawater, making it a feasible option in inland regions, although electrolyzers must tolerate low levels of salinity and impurities. Economic implications include lower desalination costs compared to seawater and the viability of distributed hydrogen production systems in water-scarce areas.

Techno-economic analysis. Comprehensive techno-economic assessments should be conducted to evaluate the feasibility and scalability of GH₂-integrated smart grids considering the entire value chain.

Energy systems modeling. Advanced models to simulate the interaction between hydrogen production and smart grids should be developed, focusing on grid stability, energy losses, and efficiency.

Policy research. The impact of different policy frameworks on the adoption of hydrogen-integrated smart grids should be investigated. Research should focus on identifying the best practices for policy design and regulatory harmonization.

Safety and public perception. The safety aspects of hydrogen systems, particularly in relation to storage and transportation, should be explored. Research should also focus on public education and outreach to address misconceptions and build trust in hydrogen technology.

Materials science. In order to enhance efficiency, reduce costs, and improve safety, new materials for electrolysis, hydrogen storage, and transportation are in high demand. For example, the high operating temperatures of SOEC technology lead to rapid decomposition of materials, and this implies that the technology is still in the early stages of development (TRL 6–7).

Interdisciplinary research. Collaboration between engineers, economists, policymakers, and social scientists should be fostered to address the multifaceted challenges of integrating GH₂ into smart grids.

Pilot projects and demonstrations. Large-scale pilot projects and demonstrations should be supported to test the viability of GH₂-integrated smart grids under real-world conditions, providing valuable data and insights for future development.

8. Analysis of Possible Strategies to Reduce the Operating Costs of Green Hydrogen

Reducing the costs associated with producing, storing, and distributing GH₂ is a critical challenge that must be addressed to make it a viable alternative to traditional fossil fuels. Based on current scientific and technological knowledge, several predictions can be made regarding the timescales and methods by which these costs may be reduced.

8.1. Cost Reduction in Green Hydrogen Production

• Advancements in electrolysis technology.

The current electrolysis methodology, particularly using a proton exchange membrane (PEM) and Alkaline Electrolyzers, is the primary method for producing GH₂ [35,227,228]. However, the costs are currently high due to the relatively low efficiency and high capital costs of electrolyzers [229]. The reduction in the production costs of GH₂ can be realized in two successive phases. In the short-term period (2024–2030), incremental improvements in electrolyzer efficiency and scaling up of manufacturing processes are expected to drive down costs. Governments and private companies are already investing heavily in R&D to improve catalyst materials, increase stack lifetimes, and reduce the energy required for electrolysis. By 2030, the cost of electrolyzers could decrease by 50% or more, making GH₂ more competitive [230]. Further breakthroughs in electrolyzer technology, such as the development of solid oxide electrolyzers and more efficient PEM systems, could lead to significant cost reductions. These advancements, combined with the anticipated decline in renewable energy costs, could bring GH₂ production costs down to $1–2 per kilogram in the long-term period (2030–2050), which is competitive with fossil-fuel-based hydrogen [228].

• Scaling up renewable energy integration.

The current cost of renewable energy is decreasing, but the integration of large-scale renewables with hydrogen production needs to be optimized to ensure a continuous, low-cost energy supply for electrolysis. It is expected that in the short-term period (2024–2030), increasing the deployment of renewable energy sources, particularly solar and wind, will reduce the cost of electricity, which is the largest cost component of GH₂ production [231,232,233,234]. Coupling electrolysis directly with renewable energy projects (e.g., solar farms and offshore wind) will further reduce costs. Over a long-term period (2030–2050), innovations in grid management, energy storage, and smart grid technology will optimize the use of renewable energy for hydrogen production, minimizing downtime, and maximizing efficiency. As renewable energy becomes more abundant and cheaper, the overall cost of producing GH₂ will decrease further.

8.2. Cost Reduction in Hydrogen Storage

• Advancements in hydrogen storage materials

Storing hydrogen, especially in compressed or liquefied forms, is costly due to the need for high-pressure tanks, cryogenic conditions, and the low volumetric energy density of hydrogen [235,236,237]. In the near short-term future (2024–2030), improvements in compression technologies and the development of more efficient storage materials, such as metal hydrides, are expected to reduce storage costs. Research on advanced carbon fiber tanks and other lightweight materials will also contribute to cost reductions [238]. In the long term (2030–2050), new storage methods, such as solid-state hydrogen storage or chemical carriers (e.g., ammonia and liquid organic hydrogen carriers) [239], could revolutionize how hydrogen is stored and transported, significantly lowering costs. These technologies can also enhance safety and reduce energy loss during storage.

• Development of hydrogen hubs and pipelines

Currently, developing hydrogen infrastructure, including pipelines and storage facilities, is capital-intensive, limiting the scalability of GH₂. In the short term (2024–2030), the creation of hydrogen hubs, where production, storage, and distribution are centralized, can reduce costs through economies of scale. The repurposing of existing natural gas pipelines for hydrogen transport is also expected to reduce infrastructure costs [240]. In the long term (2030–2050), the development of dedicated hydrogen pipelines and large-scale underground storage facilities (e.g., salt caverns) will decrease the cost of hydrogen distribution and storage, making it more economical to transport hydrogen over long distances [241,242,243].

8.3. Cost Reduction in Hydrogen Distribution

• Optimization of hydrogen transport

Currently, transporting hydrogen, especially over long distances, is expensive due to the need for specialized infrastructure and the low density of hydrogen. In the near future (2024–2030), advances in hydrogen compression and liquefaction [237], as well as the development of more efficient hydrogen carriers (e.g., ammonia) [239], will reduce transport costs. The establishment of regional hydrogen networks can also optimize distribution and reduce costs by minimizing transport distances. In the long term (2030–2050), the widespread adoption of hydrogen carriers and the development of global hydrogen shipping routes akin to the current liquefied natural gas (LNG) infrastructure could drastically reduce the cost of hydrogen transport. Technological innovations in hydrogen liquefaction, including magnetic cooling [244,245] and other advanced methods, will further reduce energy losses and costs.

8.4. Policy and Market Mechanisms

• Government subsidies and incentives

Government policies and subsidies are crucial for reducing the cost of GH₂ by supporting the research, development, and scaling up of hydrogen technologies. In the near future (2024–2030), continued government support, including subsidies, tax incentives, and carbon pricing, is expected to reduce the cost gap between GH₂ and fossil fuels. National and international hydrogen strategies will focus on creating demand, supporting infrastructure development, and fostering innovation. In the long term (2030–2050), as GH₂ technologies mature and scale, market mechanisms such as carbon pricing will make GH₂ increasingly competitive with fossil fuels. Eventually, GH₂ could become cost-competitive without subsidies, driven by market demand and the global shift toward decarbonization [246,247,248].

8.5. Other Factors

8.5.1. Electrolysis Efficiency

• High energy input.

Electrolysis, the process of splitting water into hydrogen and oxygen using electricity, requires a significant amount of energy [31,249]. Current commercial electrolyzers typically operate with efficiencies of around 60–70%, meaning that a substantial portion of the energy input is lost as heat. The efficiency can be affected by the type of electrolyzer used (such as alkaline, proton exchange membrane water electrolysis PEMWE [28,29,33], or solid oxide), operating conditions, and purity of the water used [250].

• Material limitations.

The catalysts used in electrolyzers, often made from expensive and rare materials like platinum or iridium, are not only costly but also contribute to energy losses. Additionally, there is a need for improved membranes that can operate at higher efficiencies and lower costs.

8.5.2. Compression and Storage Losses

• Energy-intensive compression.

Hydrogen gas needs to be compressed to high pressures (typically around 350–700 bar) for storage and transportation, which is an energy-intensive process. Compressing hydrogen can consume roughly 10–20% of the energy content of the hydrogen itself.

• Storage challenges.

Storing hydrogen efficiently is a challenge due to its low energy density by volume. Whether stored as compressed gas, liquid hydrogen, or in solid-state form (e.g., metal hydrides [251,252] or hydrogen hydrates [253]), each method involves energy loss and safety concerns [254]. Liquid hydrogen storage, for example, requires cryogenic temperatures (−253 °C), leading to energy losses during liquefaction and boil-off during storage. Moreover, storage systems may have losses through insulation or leakage, and storage technologies (like high-pressure tanks or cryogenic systems) are still being optimized for better efficiency.

8.5.3. Fuel Cell Inefficiencies

• Conversion losses.

Fuel cells convert hydrogen back into electricity; however, this process is not 100% efficient. Proton exchange membrane (PEM) for fuel cells [255], which are commonly used for their quick start-up times and low operating temperatures, typically have efficiencies of around 40–60% under optimal conditions, with additional losses during energy conversion. The rest of the energy is lost as heat. Moreover, heat management and maintaining optimal operating conditions can also lead to energy losses.

• Durability and degradation.

Over time, fuel cells degrade, leading to a loss of performance and efficiency. This degradation can be caused by catalyst poisoning, membrane thinning, or other material issues.

8.5.4. Systemic Challenges

• Thermodynamic limits:

Some energy conversion processes are subject to thermodynamic limits dictated by the laws of thermodynamics, which inherently restrict efficiency. Each conversion step has inherent efficiency barriers that cannot be overcome.

• Intermittence of renewable energy.

GH₂ is primarily produced using renewable energy sources like wind and solar. The intermittent nature of these energy sources can lead to inefficiencies in hydrogen production when there is a mismatch between energy supply and demand.

• Integration and scalability.

Integrating hydrogen production, storage, and usage into existing energy systems on a scale is complex and can introduce additional inefficiencies. The lack of infrastructure development for transport and distribution, such as pipelines and refueling stations [256,257], adds complexity and energy losses.

8.5.5. Economic Factors

• High costs.

The inefficiencies in these processes make GH₂ more expensive compared to conventional fossil fuels. The high energy input required, coupled with the cost of materials and infrastructure, makes the overall process less economically viable without significant subsidies or advances in technology. Many of these processes are still under development, and ongoing research may lead to breakthroughs that will significantly improve their efficiency in the future [258].

8.5.6. Scale and Integration Challenges

Economies of scale can improve efficiency as more systems and technologies are deployed; however, achieving this scale can take time and requires substantial investment and integration into existing energy systems.

9. Conclusions

The role of hydrogen in sustainable energy transition is increasingly recognized as pivotal, offering a pathway to decarbonize multiple sectors while enhancing the flexibility and resilience of energy systems. Defining hydrogen demand remains a complex challenge, as it requires balancing technological readiness, infrastructure capacity, supportive policy environments, and dynamic market conditions. Successfully addressing these factors is essential for integrating hydrogen into global energy systems and realizing its transformative potential. Achieving this integration demands significant advancements in materials science, engineering, and system optimization. Innovations in electrolyzer efficiency, durable and scalable fuel cell technologies, and effective storage solutions are critical for addressing the current inefficiencies in the hydrogen supply chain, making green hydrogen economically competitive and environmentally sustainable.

Within the framework of sector coupling, green hydrogen production and utilization are integral to the Power-to-X paradigm, which aims to interconnect electricity, heating, transportation, and industrial applications. This interconnection maximizes the utility of renewable energy sources, enhancing grid stability and resilience while contributing to climate goals. Hydrogen-incorporated sector-coupled smart grids (HISCSGs) exemplify a forward-looking approach to energy system integration, leveraging hydrogen’s versatility as an energy carrier to optimize energy storage, improve system efficiency, and drive decarbonization.

However, the transition to hydrogen-integrated systems presents several challenges. Substantial investment is required to develop and modernize infrastructure, including production facilities, advanced storage solutions, and digitalized smart grids. Regulatory and policy frameworks must evolve to incentivize sector coupling and renewable energy integration while fostering innovation. Continued technological advancements in electrolysis, fuel cells, and grid management systems are essential for achieving scalable and cost-effective solutions. Addressing these challenges requires strategic actions, such as infrastructure investment, research and innovation support, harmonized regulations, and public engagement to build trust and societal acceptance of hydrogen technologies.

The economic potential of green hydrogen is significant, with production and distribution costs projected to decline as technological advancements and economies of scale drive efficiency improvements. By 2030, green hydrogen is expected to compete with traditional fossil fuels, and by 2050, it is likely to become the cornerstone of a global low-carbon energy system. This transition will enable the decarbonization of energy-intensive sectors, such as industry, transportation, and heating, while enhancing the resilience of interconnected energy systems.

Green hydrogen’s ability to convert surplus renewable electricity into a versatile and clean energy carrier further strengthens its role in a sustainable energy future. It can be re-electrified for power generation (H₂-to-Power) or integrated into natural gas systems (H₂-to-Gas) to decarbonize and stabilize the gas sector. Such sector-coupled approaches enhance the flexibility and reliability of smart grids, ensuring that renewable energy resources are utilized to their fullest potential.

Ultimately, green hydrogen offers a transformative solution for achieving a decarbonized, sustainable energy landscape. By addressing existing challenges through coordinated efforts in technology, policy, and infrastructure, the integration of hydrogen into energy systems will play a critical role in meeting climate goals, advancing renewable energy utilization, and accelerating the global transition to a carbon-neutral economy.