A Comprehensive Review of Bimetallic Nanoparticle–Graphene Oxide and Bimetallic Nanoparticle–Metal–Organic Framework Nanocomposites as Photo-, Electro-, and Photoelectrocatalysts for Hydrogen Evolution Reaction

Abstract

This review extensively discusses current developments in bimetallic nanoparticle–GO and bimetallic nanoparticle–MOF nanocomposites as potential catalysts for HER, along with their different synthesis methodologies, structural characteristics, and catalytic mechanisms. The photoelectrocatalytic performance of these catalysts was also compared based on parameters such as Tafel slope, current density, onset potential, turnover frequency, hydrogen yield, activation energy, stability, and durability. The review shows that the commonly used metal alloys in the bimetallic nanoparticle–GO-based catalysts for HERs include Pt-based alloys (e.g., PtNi, PtCo, PtCu, PtAu, PtSn), Pd-based alloys (e.g., PdAu, PdAg, PdPt) or other combinations, such as AuNi, AuRu, etc., while the most used electrolyte sources are H₂SO₄ and KOH. For the bimetallic nanoparticle MOF-based catalysts, Pt-based alloys (e.g., PtNi, PtCu), Pd-based alloys (e.g., PdAg, PdCu, PdCr), and Ni-based alloys (e.g., NiMo, NiTi, NiAg, NiCo) took the lead, with KOH being the most frequently used electrolyte source. Lastly, the review addresses challenges and prospects, highlighting opportunities for further optimization and technological integration of the catalysts as promising alternative photo/electrocatalysts for future hydrogen production and storage.

1. Introduction

The continued use of fossil fuels is exacerbating the global energy crisis and environmental pollution, which are becoming increasingly serious issues [1]. It is concerning to see that the world is facing a growing energy crisis and environmental pollution, largely driven by the continued use of fossil fuels. This is a major challenge that requires urgent attention and action to ensure a sustainable future for all [2]. The potential of converting hydrogen gas into energy is a promising solution to address the current crisis [3]. Hydrogen is the lightest and most abundant element in the universe, constituting about 75% of its elemental mass. While hydrogen is highly abundant in its diatomic molecular form (H₂) in the atmosphere, it is also present in various chemical compounds. The use of hydrogen offers a pathway to reduce dependence on conventional fossil fuels, contributing to efforts to mitigate climate change and promote sustainability. According to Air Products South Africa, the price of gray hydrogen in South Africa was about 300 ZAR/kg (21.17 USD) in February 2021 [4]. The Japanese target price for imported blue/green hydrogen is set at 52.20 ZAR/kg (3.68 USD). This has been deemed achievable for South African hydrogen producers. Furthermore, a joint European Union and South African investigation into power fuels and green hydrogen found that a long-term price of 26.50 ZAR/kg for exported South African green hydrogen is possible [5].

Hydrogen’s high energy density, light molecular structure, carbon-free nature, and nontoxic properties position it as a highly favorable and versatile option for the future energy economy. When it is combined with oxygen through either thermal or electrochemical combustion, energy and water are created. However, when hydrogen is burned in the air, there is a possibility of generating nitric oxide air pollutants, but this can be controlled [6]. Hydrogen has been identified as a promising candidate for a clean and almost limitless energy system due to its versatility and potential for production from various sources [7]. Hydrogen is abundant in water, and the process of electrolysis, which separates hydrogen and oxygen in water using electricity, can provide a sustainable and renewable source of hydrogen.

In recent years, the use of bimetallic nanoparticle–GO and bimetallic nanoparticle–MOFs as photo/electrocatalysts for HERs has garnered greater interest. However, previous reviews only concentrated on GO-based or MOF-based photo/electrocatalysts for HERs, while comprehensive details covering these fast-growing developments is still missing. As a result, an up-to-date summary of these fast-growing fields is highly desirable for a better understanding of the structures, properties, synthesis methods, catalytic mechanisms, and strategic guidelines to develop better bimetallic nanoparticle–GO and bimetallic nanoparticle–MOF-based photo/electrocatalysts for HERs. Hence, this comprehensive review focuses on recent developments in bimetallic nanoparticle–GO-based and bimetallic nanoparticle–MOF-based photo/electrocatalysts for HERs. Herein, we summarize the latest advances in the design and architecture of bimetallic nanoparticle–GO-based and bimetallic nanoparticle–MOF-based photo/electrocatalysts for HERs and highlighted novel strategies to improve the photo/electrocatalytic activities. Finally, the existing challenges and future potentials of these materials are also covered.

1.1. Production of Hydrogen

Hydrogen is a versatile fuel that can be produced using various methods. Thermochemical processes involve using heat and chemical reactions to release hydrogen from organic materials like fossil fuels, biomass, or water. Electrolysis and solar energy can also be used to split water into hydrogen and oxygen. Finally, microorganisms like bacteria and algae can produce hydrogen through biological processes, which we will discuss further.

1.1.1. Fossil Fuels

(1)

Steam Methane Reforming (SMR)

Steam methane reforming (SMR) is a widely used industrial process for the production of hydrogen gas (H₂) from natural gas. It is a key method for large-scale hydrogen production and is commonly employed in various industries, including petrochemicals, ammonia production, and refineries [3]. The process involves the reaction between methane (CH₄) and steam (H₂O), producing hydrogen and carbon monoxide as primary products. The reaction is endothermic, requiring a significant amount of heat for the conversion [3,8]. Equation (1) is the chemical equation for steam methane reforming.

CH₄ + H₂O → CO + 3H₂

(1)

This reaction is typically carried out at high temperatures (700–1100 °C) and moderate pressures, with a catalyst often used to enhance the reaction rate [3,9]. Commonly employed catalysts include nickel (Ni) or other supported metal catalysts. In principle, methane and steam are mixed and injected into a reformer. The endothermic reaction takes place in the reformer, where methane reacts with steam to produce hydrogen and carbon monoxide. The reaction absorbs heat, requiring external energy input. A catalyst, typically based on nickel, is employed to enhance the reaction rate and improve the efficiency of the process. The heat required for the endothermic reaction is often supplied by burning a portion of the produced hydrogen in an external combustion chamber. The hot flue gas from this combustion is then passed through a heat exchanger to provide the necessary heat for the reforming reaction. The product gas stream, consisting of hydrogen, carbon monoxide, unreacted methane, and other by-products, is then processed to separate and purify the hydrogen [9].

SMR is known for its efficiency and is economically viable on a large scale. However, it produces carbon monoxide as a by-product, and additional steps are often required to further purify the hydrogen for certain applications, especially in industries such as fuel cells, where low levels of impurities are critical. Researchers and engineers continue to explore methods to improve the efficiency of SMR and reduce its environmental impact through technologies like carbon capture and utilization [8,9].

(2)

Partial Oxidation

Partial oxidation (POX) is a thermochemical process in which a hydrocarbon fuel, often methane or natural gas, is partially oxidized with a limited supply of oxygen or air to produce synthesis gas (syngas). Syngas is a mixture of hydrogen (H₂) and carbon monoxide (CO), and it is a versatile intermediate for the production of various chemicals and fuels. Partial oxidation is an important industrial process with applications in the production of hydrogen, ammonia, and liquid fuels [10].

The process typically involves the reaction of a hydrocarbon with oxygen or air. For example, in the case of methane, the partial oxidation reaction is represented by Equation (2).

CH₄ + ¹/₂O₂ → CO + 2H₂

(2)

The partial oxidation process consists of both exothermic and endothermic reactions. The initial partial oxidation step is exothermic, but subsequent reactions are endothermic, requiring additional heat input [11]. Partial oxidation is commonly used for the production of hydrogen, a key industrial feedstock. The produced syngas can be further processed to separate and purify hydrogen. Syngas generated through partial oxidation is a precursor for ammonia production, a critical component in the fertilizer industry. Syngas can be converted into liquid fuels through Fischer–Tropsch synthesis or other conversion processes. Various catalysts, such as nickel-based catalysts, are employed to enhance the reaction rates and improve the efficiency of the partial oxidation process [11,12]. Partial oxidation can occur in different reactor configurations, including fixed-bed reactors, fluidized-bed reactors, and autothermal reformers. One challenge in partial oxidation is the potential for carbon formation (coking) on the catalyst, which can reduce the catalyst’s activity and life span.

Achieving optimal operating conditions, including temperature, pressure, and oxygen-to-fuel ratios, is crucial for maximizing the efficiency of partial oxidation. Efforts are underway to address the environmental impact of partial oxidation by incorporating carbon capture and utilization technologies to mitigate CO₂ emissions [10,11,12]. Partial oxidation continues to be an area of research and development, with ongoing efforts to improve efficiency, reduce environmental impact, and expand its applications in the transition towards sustainable energy and chemical production.

1.1.2. Biomass

Biomass is a term that encompasses a wide range of organic materials derived from plants and animals. It is a renewable and sustainable resource that can be converted into various forms of energy and biobased products. It is possible to obtain hydrogen from biomass through a pyrolysis–gasification process. Biomass sources that are commonly used include wood, crop residues, and other organic materials from agriculture. Certain crops, such as switchgrass and miscanthus, are grown specifically for energy production. To prepare biomass for this process, a mixture of biomass and water is heated to high temperatures and pressure in a reactor. This causes the biomass to decompose and partially oxidize, resulting in a gas mixture that contains hydrogen, methane, CO₂, CO, and nitrogen. Mineral matter is then extracted from the bottom of the reactor. The gas mixture is then sent to a high-temperature shift reactor to increase the hydrogen content [13,14].

Biomass utilization faces challenges, especially in developed countries, primarily due to economic factors, land use concerns, and competing uses for biomass resources [15]. Biomass production, harvesting, and conversion technologies can be expensive. The economic viability of biomass utilization often depends on factors such as government incentives, subsidies, and the cost competitiveness of alternative energy sources [15,16,17]. However, in developed countries, there is often intense competition for land, which is a limited and valuable resource. Land is used for various purposes, including agriculture, urban development, and conservation. Allocating large areas for biomass cultivation may be challenging in such contexts. The competition between using land for food production or for growing biomass crops (such as bioenergy crops) has led to debates about the ethical and practical implications of dedicating arable land to energy production [17,18]. Despite the challenges, biomass remains a significant contributor to the renewable energy portfolio, and ongoing research aims to address these challenges while expanding the role of biomass in the transition to a more sustainable and low-carbon energy future [19,20].

1.1.3. Thermochemical Water-Splitting Cycle

A thermochemical water-splitting cycle is a set of chemical reactions that use heat energy to decompose water (H₂O) into hydrogen (H₂) and oxygen (O₂) [21]. Unlike traditional electrolysis, which uses electrical energy to split water molecules, thermochemical water-splitting cycles rely on high-temperature chemical reactions to achieve the same result [22]. These cycles are often designed to be performed in a closed-loop system, where the reactants are recycled, making the overall process more sustainable. In this process, a high-temperature endothermic chemical reaction is employed to split water into hydrogen and oxygen. One of the commonly considered reactions is the thermochemical decomposition of water using metal oxides [23]. After the water-splitting reaction, hydrogen and oxygen are separated using various techniques, such as condensation, membrane separation, or other physical processes [24,25,26]. The separated hydrogen is collected for subsequent use as a clean and renewable fuel. The products of the water-splitting reaction, i.e., metal oxides or other compounds used in the process, are regenerated for reuse in the next cycle [27]. This regeneration step is often carried out through an exothermic reaction or another process. In some thermochemical cycles, an exothermic reaction is employed to release heat and assist in the regeneration of the reactants. This step may help improve the overall efficiency of the cycle.

The advantage of thermochemical water-splitting cycles lies in their potential to harness high-temperature heat sources, such as concentrated solar energy or advanced nuclear reactors, to drive the water-splitting process [28]. These cycles offer a pathway to produce hydrogen sustainably without directly relying on electricity. Different thermochemical cycles exist, each with its specific set of reactions and conditions. Some well-known examples include the sulfur–iodine cycle, the copper–chlorine cycle, and cerium-based cycles [29].

1.1.4. Photolysis

Photolysis is a chemical process in which a substance is broken down into simpler components through the absorption of light. This phenomenon is particularly relevant in environmental science, atmospheric chemistry, and biological processes [12]. This process involves the breaking of chemical bonds in a molecule or compound upon absorption of photons (light energy). Photolysis often involves the dissociation of molecules into smaller fragments. For instance, the photodissociation of nitrogen dioxide (NO₂) in the atmosphere can lead to the formation of nitrogen monoxide (NO) and oxygen atoms. Photolysis is a key component of photocatalytic processes where semiconductors, such as titanium dioxide, are employed to initiate chemical reactions under light exposure [14]. Researchers have explored various methods of photoconversion, including photobiological systems, photochemical assemblies, and photoelectrochemical cells. In principle, photobiological systems utilize living organisms, such as algae or cyanobacteria, to harness solar energy through photosynthesis and produce hydrogen. In these systems, certain microorganisms can undergo biological water splitting, releasing oxygen and producing hydrogen as a by-product. Photochemical assemblies, on the other hand, involve the use of synthetic molecules or complexes that mimic natural photosynthetic processes to generate hydrogen. This method uses light-absorbing molecules to capture solar energy and transfer it to catalysts, which drive the water-splitting reaction to produce hydrogen [12]. However, it is worth noting that the enzyme hydrogenase is very sensitive to oxygen, which inhibits its activity and prevents it from producing hydrogen [12].

1.1.5. Electrolysis

Electrochemical and photochemical water splitting are indeed regarded as promising approaches for energy storage and conversion, offering potential means to address the energy crisis and environmental concerns. Both methods involve the decomposition of water into hydrogen and oxygen through the application of external energy, either in the form of electricity or light [30,31]. The occurring half-reactions at the electrodes are given in Equations (3) and (4), while the overall chemical reaction of a water electrolysis process is given by Equation (5).

Anode:  H₂O → 2H⁺ + O₂ + 2e⁻

(3)

This reaction involves the oxidation of water molecules at the anode, leading to the release of protons (H⁺), oxygen gas (O₂), and electrons (e⁻).

Cathode: 2H⁺ + 2e⁻ → H₂

(4)

This reaction represents the reduction of protons (H⁺) at the cathode, resulting in the formation of molecular hydrogen (H₂).

Overall:  H₂O → H₂ + ¹/₂O₂

(5)

The overall chemical reaction combines the anode and cathode half-reactions. It represents the electrolysis of water, producing hydrogen gas (H₂) at the cathode and oxygen gas (O₂) at the anode. The stoichiometry of the overall reaction is crucial for balancing the number of electrons transferred in both half-reactions, ensuring that charge is conserved in the electrochemical cell. In this case, the number of electrons released in the anode half-reaction is equal to the number of electrons consumed in the cathode half-reaction. The use of water electrolysis, however, is often more expensive than hydrocarbon reforming, a widely used technology for hydrogen production, especially through processes like steam methane reforming (SMR). Furthermore, the hydrogen evolution reaction (HER) in water electrolysis may require a large overpotential to initiate the reaction, affecting the overall energy efficiency. Moreover, water electrolysis systems may suffer from stability problems over long-term operation, impacting their reliability and leading to shutdowns [32].

Photovoltaic Electrolysis

Photovoltaic electrolysis is a process that combines photovoltaic (PV) technology with electrolysis to produce hydrogen from water using sunlight [33]. This method harnesses solar energy to drive the electrochemical reaction that splits water into hydrogen and oxygen. The photovoltaic cells and the electrolyzer are integrated into a system, allowing for a direct and efficient conversion of solar energy into hydrogen. The overall process involves two main components: photovoltaic cells and an electrolyzer [34]. Photovoltaic cells, commonly known as solar cells or solar panels, convert sunlight into electricity [35]. When sunlight hits the photovoltaic cells, it excites electrons, generating an electric current. The electrolyzer then uses the electric current generated by the photovoltaic cells to split water into hydrogen and oxygen. The hydrogen produced during the electrolysis process is collected and can be stored for later use as a clean and renewable energy source. Simultaneously, oxygen is released as a by-product and can either be released into the atmosphere or captured for other applications. Despite its advantages, photovoltaic electrolysis faces challenges related to efficiency, cost, and the intermittent nature of sunlight. Some researchers are actively working on improving the efficiency of both photovoltaic cells and electrolyzers to make this technology more competitive and practical for widespread use in the transition to a sustainable energy future [36,37].

Hydrogen Evolution Reaction (HER)

The hydrogen evolution reaction (HER) is a key process in water electrolysis, where hydrogen gas is produced by the reduction of protons (H⁺) at the cathode. The HER is a multistep electrochemical reaction that involves the conversion of protons and electrons into hydrogen molecules [38]. The kinetics of the HER can be influenced by various factors, and one aspect of importance is the desorption of hydrogen molecules from the cathode surface. The desorption step is crucial in determining the rate at which hydrogen gas is released from the cathode into the electrolyte [38,39]. Amongst other factors is the choice of catalyst at the cathode, which significantly influences the reaction kinetics. Efficient catalysts, such as platinum and other transition metal-based catalysts, can enhance the rate of the HER. Catalysts play a central role in minimizing overpotential. They facilitate the reaction by providing an alternative reaction pathway with lower energy barriers. Noble metals like platinum are effective catalysts for the HER, but their high cost and scarcity have led to extensive research into alternative, more cost-effective materials [40]. It is also important to note that finding a balance in the strength of the catalyst’s interaction with hydrogen atoms is crucial. Indeed, the interaction strength between the catalyst and hydrogen species is a critical factor influencing the efficiency of the HER. If the catalyst’s surface has too weak a bonding strength with hydrogen atoms, it may struggle to efficiently adsorb the reactant (protons) to initiate the HER [41,42].

Therefore, protons may not be effectively held at the catalyst surface, leading to slower reaction kinetics and a higher overpotential requirement to drive the reaction. The HER mechanisms in acidic and alkaline media proceed through a series of three elementary reaction steps, involving both electrochemical and chemical processes. The HER typically involves a series of three elementary reaction steps, incorporating both electrochemical and chemical processes. These steps are often referred to as the Volmer, Heyrovsky, and Tafel reactions [40,41,42]. The three elementary reaction steps in acidic and alkaline medium are given in Equations (6)–(11).

HER mechanisms in acidic solution are as follows.

Volmer reaction (electrochemical discharge):

H⁺ + e⁻ → H_ad

(6)

Heyrovsky reaction (electrochemical desorption):

H_ad + H⁺ + e⁻ → H₂

(7)

Tafel reaction (catalytic desorption):

H_ad + H_ad → H₂

(8)

HER mechanisms in alkaline solution are as follows.

Volmer reaction (electrochemical discharge):

H₂O + e⁻ → OH⁻ + H_ad

(9)

Heyrovsky reaction (electrochemical desorption):

H_ad + H₂O + e⁻ → OH⁻ + H₂

(10)

Tafel reaction (catalytic desorption):

Had + Had → H₂

(11)

In both cases, the Volmer step involves the reduction of either a proton (H⁺) in acidic media or a water molecule (H₂O) in alkaline media to produce a surface-adsorbed hydrogen atom (H). In an acidic medium (Equation (6)), a proton (H⁺) from the acidic solution is reduced by gaining an electron (e⁻), leading to the formation of a surface-adsorbed hydrogen atom (HH). In a basic medium (Equation (8)), the water molecule (H₂O) is reduced by gaining an electron (e⁻), leading to the formation of a hydroxide ion (OH⁻) and a surface-adsorbed hydrogen atom (HH). The Heyrovsky step consists of the reaction of a proton or hydroxide ion (OH⁻) with the surface-adsorbed hydrogen atom, resulting in the formation of molecular hydrogen gas (H₂) and the regeneration of an electron (e⁻). Finally, the Tafel step involves the combination of two surface-adsorbed hydrogen atoms to produce molecular hydrogen gas (H₂) [38]. The HER involves a series of steps, including the adsorption and desorption of hydrogen atoms on the surface of the catalyst. A suitable catalyst for HERs should exhibit a good balance between these two steps [38,41,42]. In the adsorption step, hydrogen atoms (protons) from the electrolyte are adsorbed onto the catalyst surface. This step is critical because it initiates the reaction by providing reactive sites for subsequent electrochemical processes. In the desorption step, the adsorbed hydrogen atoms combine to form molecular hydrogen gas (H₂) and are released from the catalyst surface. This step is equally vital, as it completes the reaction cycle, allowing the catalyst to be ready for the next round of adsorption [41,42].

A good balance between adsorption and desorption is essential for efficient catalysis. If adsorption is too strong, it might hinder the timely release of hydrogen, slowing down the overall reaction kinetics. On the other hand, if desorption is too rapid, the catalyst may not effectively capture enough hydrogen atoms to sustain the reaction. The balance directly influences the overpotential required for the HER. Overpotential is the additional voltage needed to drive a reaction beyond its thermodynamic potential. Balancing adsorption and desorption helps minimize overpotential, making the HER more energy-efficient. A suitable catalyst should maintain stability over multiple reaction cycles [41,43,44,45,46]. An imbalance between adsorption and desorption can lead to catalyst degradation over time, impacting its long-term performance [44]. An efficient electrocatalyst should be thermally stable to withstand operating conditions, especially in processes like water electrolysis, where high temperatures may be involved [47,48]. Furthermore, electrocatalysts should possess mechanical stability to withstand stresses and mechanical forces that may occur during their synthesis, handling, and utilization. Moreover, cost-effectiveness is a critical factor for the widespread adoption of hydrogen production technologies. Low-cost catalysts are essential for making the HER economically competitive [44]. The volcano plot (Figure 1) is a widely used graphical representation in electrocatalysis, illustrating the correlation between the HER activity of catalysts and their adsorption properties. In this plot, the exchange current density (related to the catalytic activity) is typically plotted against the Gibbs free energy of adsorbed atomic hydrogen on the catalyst. PGMs, especially Pt, exhibit high catalytic activity for the HER. They can efficiently facilitate both the adsorption and desorption of hydrogen atoms, crucial steps in the overall reaction [43,47,49]. The adsorption of hydrogen atoms on PGM surfaces is energetically favorable, contributing to the overall efficiency of the HER. This favorable adsorption is reflected in relatively low Gibbs free energy of adsorbed atomic hydrogen (ΔG_H*). PGMs generally require low overpotentials to drive the HER, making them effective at reducing the energy input needed for hydrogen production. These PGMs are known for their stability and durability under harsh electrochemical conditions. They can withstand the corrosive environment associated with water electrolysis. However, the limited abundance of certain PGMs raises concerns about the sustainability of widespread use in a growing hydrogen economy [47,49]. While PGMs such as Pt, Pd, Ir, and Rh exhibit excellent catalytic activity, the high cost and limited abundance of these materials pose challenges for their widespread adoption, especially in large-scale applications. The important goal is indeed to replace PGM-based electrocatalysts with materials that are not only cost-effective but also maintain high catalytic activity. This pursuit is driven by the need for sustainable and economically viable solutions for various electrochemical processes, including the HER [50,51]. One of the challenges associated with the use of non-platinum active metals, such as Fe (iron), Ni (nickel), Mo (molybdenum), or Co (cobalt), is their susceptibility to corrosion and passivation under reaction conditions. This presents a significant obstacle to their long-term stability and performance in electrocatalytic processes [41,52,53]. Although challenges exist, the unique properties and versatility of carbon-based materials and MOFs make them promising candidates for advancing electrocatalytic technologies. Ongoing research aims to refine these materials and optimize their performance for practical applications in hydrogen production and other electrochemical processes [54,55].

To convert the chemical energy stored in hydrogen back into electrical energy, hydrogen fuel cell (HFC) technology is a highly promising and efficient method. Hydrogen fuel cells electrochemically convert hydrogen and oxygen into water, producing electricity in the process [57,58]. Hydrogen fuel cells are recognized for their environmental friendliness and high energy efficiency compared to traditional combustion technologies. The principle of HFC consists of the anode and cathode electrodes, which are separated by a membrane/electrolyte (Figure 2). The anode is the electrode where hydrogen is supplied. At the anode, hydrogen molecules (H₂) are split into protons (H⁺) and electrons (e⁻) through a process called electrochemical oxidation. The cathode is the electrode where oxygen (usually from the air) is supplied. Oxygen molecules (O₂) combine with protons and electrons from the anode to form water (H₂O) through electrochemical reduction. The membrane selectively allows protons to pass through while blocking electrons. This ensures that the electrons must flow through an external circuit to reach the cathode, creating an electric current [59].

At the cathode, oxygen gas (O₂) from the air is supplied. The protons that have passed through the PEM combine with oxygen and electrons from the external circuit to form water. The oxidation process at the anode of a hydrogen fuel cell involves the electrocatalytic separation of hydrogen molecules into protons and electrons. Therefore, the electrocatalyst, often platinum, facilitates the separation of hydrogen molecules into protons (H⁺) and electrons (e⁻) [1,59,60]. The efficiency of hydrogen fuel cells, combined with their ability to produce electricity without emitting pollutants, makes them a promising technology for various applications, including transportation, stationary power generation, and portable devices. As advancements in fuel cell technology continue, the potential for widespread adoption and integration into clean energy systems becomes increasingly viable. Addressing both the production and storage challenges in hydrogen technology is key to creating a sustainable and scalable hydrogen economy. Collaboration between governments, industries, and research institutions is essential for driving innovation and overcoming these hurdles. Various technologies and processes are being employed to address the challenges related to the production of hydrogen.

Electrocatalytic HERs

Electrocatalytic HERs are hydrogen evolution reactions (HERs) driven by electrocatalysis. Electrocatalysis involves the use of a catalyst to facilitate and accelerate electrochemical reactions [30,31]. In the context of HERs, the electrocatalyst enhances the rate of the reduction of protons (H⁺) to produce hydrogen gas (H₂) at the electrode surface. The electrocatalyst is a material that promotes the electrochemical reaction without being consumed in the process [31,32]. Commonly used electrocatalysts for HERs include platinum (Pt), palladium (Pd), nickel (Ni), and other transition metal-based materials. These catalysts provide active sites for the adsorption and subsequent reduction of protons. The electrocatalyst is typically deposited on an electrode surface, which can be made of various conductive materials, such as carbon or metal substrates. The electrode serves as the interface where the electrochemical reactions take place. The primary electrochemical reaction involved in HERs is the reduction of protons to form hydrogen gas. Electrons required for the reduction reaction are usually supplied by an external power source, such as a battery or a solar cell. The electrocatalyst facilitates the transfer of electrons from the external circuit to the protons at the electrode surface. The net result of electrocatalytic HERs is the electrochemical reduction of protons to produce hydrogen gas.

The electrocatalytic HER is an important process in the development of electrochemical devices, such as water electrolyzers and fuel cells, for the production and utilization of hydrogen as a clean energy carrier. Researchers are actively working to discover and design efficient and cost-effective electrocatalysts to improve the overall performance and sustainability of electrocatalytic HER processes.

Photocatalytic HERs

Photocatalytic HERs are driven by photocatalysis. In this context, photocatalysis involves the use of a semiconductor material to absorb light energy and promote chemical reactions. The photocatalytic HER is a sustainable and environmentally friendly approach for producing hydrogen gas as a clean energy source. The photocatalytic process typically employs semiconductor materials such as titanium dioxide (TiO₂), zinc oxide (ZnO), or other metal oxides. These materials can absorb light energy and generate electron–hole pairs. When the semiconductor material is exposed to light, electrons in the valence band are excited to the conduction band, leaving behind positively charged holes in the valence band. This separation of charge creates electron–hole pairs. The electrons from the conduction band can then participate in reduction reactions. In the context of the photocatalytic HER, the reduction reaction involves the conversion of protons (H⁺) into hydrogen gas (H₂). Simultaneously, the holes in the valence band can participate in an oxidation reaction. However, in the context of HERs, the oxidation reaction usually involves the oxidation of water to produce oxygen gas. Overall, the net result is the production of hydrogen gas through the reduction of protons using the energy provided by light.

The photocatalytic HER has garnered interest as a potential method for renewable and sustainable hydrogen production. It harnesses solar energy to drive the chemical reaction, and if optimized, it could contribute to the development of efficient and clean energy systems. Researchers continue to explore and improve the efficiency of photocatalytic materials and processes for hydrogen production.

Photoelectrocatalytic HERs

The development of efficient processes for harnessing solar energy to directly generate hydrogen through water splitting is indeed a significant and promising approach in the pursuit of clean and renewable energy. Water electrolysis, a means of producing hydrogen, plays a central role in this context, particularly for energy storage when the required energy is sourced from renewable resources like solar power [17]. A photovoltaic (PV) cell, also known as a solar cell, can be coupled with an electrolysis cell to harness solar energy and produce hydrogen and oxygen through the process of water splitting [20]. The evaluation of photocatalysts, especially for photoelectrochemical (PEC) water splitting and the HER, involves a variety of criteria to assess their performance [20,52,61]. Photocatalytic water splitting is a complex process involving a series of photophysical and electrocatalytic steps. The overall reaction splits water into hydrogen and oxygen using light energy, generating electrons and electron holes in the conduction and valence bands, respectively (Figure 3) [43]. Electrons in the conduction band and holes in the valence band are spatially separated. This spatial separation prevents the recombination of electron–hole pairs, a process that can reduce the efficiency of the photocatalytic reaction. The photocatalyst facilitates the absorption of light, generating electron–hole pairs. Water molecules are adsorbed at the sites where electron–hole pairs are present, and these water molecules undergo oxidation to release protons and oxygen gas. The generated protons are then reduced to produce hydrogen gas, completing the water-splitting process [17,20,52,61].

The success and efficiency of this technology heavily depend on identifying and developing suitable materials, both for efficient photon absorption and effective electrocatalysis. The photocatalyst needs to efficiently absorb photons from the entire solar spectrum, especially in the visible-light and near-infrared regions. This ensures maximum utilization of solar energy for the water-splitting process. The catalyst material plays a crucial role in facilitating the electrochemical reactions, such as the reduction of protons to form hydrogen gas. Catalysts should provide active sites for these reactions and exhibit high efficiency. Hence, researchers work towards overcoming challenges related to material stability, charge-carrier dynamics, and catalytic activity to improve the overall efficiency of PEC systems.

2. Graphene Oxide (GO)

Graphene oxide (GO), which is also known as graphitic acid, has gained the attention of numerous research groups [62,63,64,65,66]. GO is a component in composite materials with photochemical, electric, or adsorptive properties, as demonstrated by Li et al. and Petit et al., and it is a layered material formed by the oxidation of graphite [62,63]. The graphene-derived sheets in graphite oxide (GO sheets) are heavily oxygenated compared to pristine graphite [63]. The introduction of these oxygen functional groups significantly alters the properties of graphene, transforming it into graphene oxide. Graphene oxide is hydrophilic and has increased chemical reactivity compared to pristine graphene [64]. This makes graphene oxide suitable for various applications, including the development of composites, sensors, and biomedical materials. The oxygen-containing groups enhance the dispersibility of graphene oxide in aqueous solutions and provide functional handles for further chemical modification [67,68,69,70]. The oxidation of graphite to graphene oxide is often achieved through processes like the Hummers method or the Brodie method, which involve the use of strong oxidizing agents to introduce oxygen functionalities [71]. Understanding the specific functional groups present on the surface of graphene oxide is crucial for tailoring its properties to suit specific applications. The oxidation of graphite allows the incorporation of oxygen atoms on the basal planes and edges of graphene layers. These oxygen functional groups identified so far on the surface of GO include epoxide, keto, and hydroxylic groups on the basal planes and carboxylic groups on the edges [72,73].

The direct incorporation of oxygen atoms into the graphene layers is an important phenomenon observed in various studies involving the oxidation of graphene. This incorporation occurs during the functionalization or oxidation processes, where oxygen-containing groups are introduced directly into the graphene lattice. Unlike the functional groups mentioned earlier (epoxide, keto, hydroxylic, and carboxylic groups), which are attached to the edges or basal planes, the direct incorporation involves oxygen atoms being embedded within the carbon lattice of graphene [74,75]. The oxygen functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and epoxide groups, create polar sites on the graphene oxide surface, making it readily interact with water molecules These groups contain polar O-H bonds, contributing to the hydrophilic character of GO. Furthermore, the oxygen atoms in epoxide groups also contribute to the polar nature of GO. Moreover, the groups contain polar O-H bonds, contributing to the hydrophilic character of GO. Understanding the hydrophilic properties of graphene oxide is crucial for tailoring its applications and interactions with other materials. This property has opened up diverse avenues for the use of GO in fields ranging from materials science to biomedicine, highlighting its versatility and potential impact in various technological applications, including sensors, fuel cells, batteries, transparent conductive films, capacitors, batteries, and high-frequency circuits [71,76,77].

2.1. GO Structure

The preparation of GO is mostly conducted using the Hummers method, making it a widely adopted approach for the large-scale production of GO. In this method, graphite powder is oxidized using strong oxidizing agents, with potassium permanganate (KMnO₄) being a common choice, in the presence of concentrated sulfuric acid (H₂SO₄). This step initiates the oxidation of graphite and introduces oxygen-containing functional groups onto the graphene layers, leading to the formation of graphene oxide (GO) [78]. The oxidation process disrupts the two-dimensional pi-conjugation of the stacked graphene sheets, resulting in a complex structure with nanoscale graphitic sp2 domains, surrounded by disordered, highly oxidized sp3 domains, and featuring defects such as carbon vacancies [79]. The derivatization involves the attachment of various oxygen-containing functional groups, and the distribution of these groups on the GO sheets can be crucial for determining the material’s properties and potential applications. The specific functional groups mentioned include carboxylic acid at the edges and phenol, hydroxyl, and epoxide groups primarily at the basal plane, as shown in Figure 4b [80].

After the synthesis of GO, centrifugation is often used to separate the GO sheets from the reaction mixture, and then the GO can be reduced to obtain reduced graphene oxide (rGO) [81] through thermal or chemical methods (Figure 4a) [82]. Achieving complete removal of all oxygen-containing groups from graphene oxide (GO) and obtaining pristine graphene is challenging. The extent of reduction depends on various factors, with the choice of reducing agent being a critical determinant. Different reducing agents exhibit varying reducing abilities or capacities, and researchers often choose them based on the desired level of reduction and specific application requirements.

2.2. Synthesis of GO

The Hummers method, developed by Richard E. Smalley and Richard F. Curl at Rice University and later modified by Walter S. Hummers and Richard E. Offeman in 1958, is one of the most common and widely used methods for synthesizing GO. This method is widely employed for its simplicity and scalability in producing GO from graphite [63,69]. The Hummers method for synthesizing GO offers several advantages over previously used techniques, contributing to its widespread adoption in research and industrial applications. Firstly, this method, with its modifications, allows for the completion of the reaction within a few hours. Shorter reaction times contribute to efficiency and enable researchers to produce graphene oxide more quickly. This is beneficial for both laboratory-scale experiments and large-scale production. Secondly, substituting potassium permanganate (KMnO₄) for potassium chlorate (KClO₃) eliminates the formation of explosive chlorine dioxide (ClO₂). This modification significantly improves the safety of the reaction. Chlorine dioxide is a hazardous gas, and its elimination enhances the overall safety of the synthesis process. Finally, substituting sodium nitrate (NaNO₃) for fuming nitric acid (HNO₃) eliminates the formation of acid fog. Acid fog, consisting of nitric acid vapor, can be corrosive, hazardous, and challenging to handle. Using sodium nitrate improves safety by avoiding the formation of acid fog during the reaction (Figure 5) [83].

In pristine graphite, individual graphene layers are stacked on top of each other, and these layers are held together by strong van der Waals forces. The oxidation of graphite involves the introduction of oxygen-containing functional groups, such as epoxide, hydroxyl, and carboxyl groups, onto the graphene layers. This process disrupts the regular packing of graphene layers [84]. After the oxidation of graphite to form graphite oxide, the resulting material, often referred to as graphene oxide, can be exfoliated directly in several polar solvents (N-methylpyrrolidone, tetrahydrofuran, dimethylformamide, ethylene glycol, and water). This exfoliation process is crucial for obtaining individual graphene oxide sheets or small stacks with enhanced dispersibility. The choice of solvent can influence the stability, reactivity, and properties of the resulting graphene oxide dispersion. Each solvent has its own set of advantages and considerations, and the choice depends on the specific requirements of the intended application.

The selection of a suitable solvent is crucial for achieving homogeneous dispersions of graphene oxide, and researchers often optimize the solvent system based on the desired characteristics of the final material [84]. These solvents are commonly used not only for exfoliation but also for subsequent processing steps in the fabrication of graphene oxide-based materials, such as films, coatings, and composites [85]. Due to the nonstoichiometric and amorphous nature of GO, individual sheets may exhibit variations in the types and density of oxygen functional groups, as well as in the level of reduction if subsequent reduction steps are applied [68]. Due to these functional groups, graphene oxide is hydrophilic and can be dissolved in water by sonication or stirring [78,86]. The presence of oxygen-containing functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and epoxy (C-O-C) groups, renders graphene oxide hydrophilic. This hydrophilicity enables graphene oxide to interact with water, making it readily dispersible in aqueous solutions. The adaptability and tunability of graphene oxide properties, coupled with its ease of functionalization, make it a versatile material for a wide range of applications. Ongoing research continues to explore new ways to harness the unique properties of graphene oxide for advancements in materials science, sensing technologies, environmental remediation, and energy storage.

2.3. GO-Based Materials for HERs

Nanostructured and carbonaceous materials have shown great potential for enhanced electrochemical performance. Recently, electrocatalysts based on GO materials have gained significant attention for their potential in hydrogen evolution reactions under both acidic and alkaline conditions. This has been demonstrated in various studies [87,88,89,90]. For example, the Tafel slopes for Ni metal and graphene oxide (GN) were found to be 85, 64, and 68 mV.dec⁻¹ for Ni-to-GO ratios of 2.0 (GN2), 6.0 (GN6), and 8.0 wt.% (GN8), respectively (Figure 6 and Table 1). However, these values did not correspond to any simple kinetic model (Volmer, Heyrovsky, or Tafel reaction), indicating complex mechanisms for hydrogen evolution on the GN hybrids [87]. Additionally, the Tafel slope for GN2 was higher, while the slope for GN6 was close to that for GN8, possibly due to the difference in the interaction between the carbon-vacancy defects and Ni nanoparticles (NPs) and the difference in their sandwich structures [87,91]. Furthermore, Sun [51] demonstrated that the GGNR@MoS₂ hybrid exhibited good electrochemical performance. HER performance, with a low onset potential of −105 mV, a small Tafel slope of 49 mV.dec⁻¹, and a large current density (10.0 mA.cm⁻² at η = 183 mV), makes this composite a promising and highly efficient electrocatalyst for hydrogen evolution reaction. The NFO–RGO catalyst exhibited greater HER activity than bare NFO and other reported catalysts, such as sulfides, carbides, phosphides, and bimetals of molybdenum and iron–cobalt, with a low onset overpotential of 5 mV (vs. reference hydrogen electrode (RHE)), high cathodic current density, low Tafel slope of 58 mV.dec⁻¹, low charge transfer resistance, and turnover frequency of 0.48 s⁻¹ [88]. Another study showed HER activity of nanocomposites (CFG and NFG) that were tested for hydrogen evolution at an applied potential of −1.2 to 0.8 V in acidic electrolyte [89]. It revealed good exchange current density of 47.9 and 41.2 mA.cm⁻² at overpotential of 248.3 and 259 mV and Tafel slopes of 116.6 and 121.4 mV.dec⁻¹ for CFG and NFG nanocomposites, respectively [89].

As observed from Table 1, some electrochemists have focused on GO or rGO due to their impressive conductivity, lightweight nature, strong mechanical properties, high surface area, and good chemical stability [64,72,78]. However, these materials have some obstacles in their electrochemical applications due to their insulating properties, which are related to the presence of oxygen functionalities. To overcome this issue, GO has been conjugated with metal particles, metal oxide, conductive polymers, and biopolymers [92,93,94]. As mentioned above, small metal particles have been acknowledged as a good mediator in the fabrication of electrochemical applications due to their biocompatibility, fast electron transfer rate, and excellent conductivity. Therefore, the decoration of small metal nanoparticles and/or organometallic compounds on the surface of GO sheets is proposed, not only to further resolve the insulation properties problem of GO but also to significantly increase the electrochemical activity.

Table 1. Comparison of the properties of various GO electrocatalysts toward HERs.

Material	H2 Source in Electrolyte	Tafel Slope mV.dec−1	Current Density (io) mA.cm−2	Ref.
GGNR@MoS2	0.5 M H2SO4	49	10.0	[51]
NFO–RGO	0.5 M H2SO4	58	25.2 × 10−2	[88]
CFG	0.5 M H2SO4	116.6	47.9	[89]
NFG	0.5 M H2SO4	121.4	41.2	[89]
rGO–Au48Pd52	0.5 M H2SO4	149	-	[90]
Pd NPs–GO	0.5 M H2SO4	-	5.2	[95]

Table 1. Comparison of the properties of various GO electrocatalysts toward HERs.

Material	H2 Source in Electrolyte	Tafel Slope mV.dec−1	Current Density (io) mA.cm−2	Ref.
GGNR@MoS2	0.5 M H2SO4	49	10.0	[51]
NFO–RGO	0.5 M H2SO4	58	25.2 × 10−2	[88]
CFG	0.5 M H2SO4	116.6	47.9	[89]
NFG	0.5 M H2SO4	121.4	41.2	[89]
rGO–Au48Pd52	0.5 M H2SO4	149	-	[90]
Pd NPs–GO	0.5 M H2SO4	-	5.2	[95]

2.4. Bimetallic Nanoparticle–GO-Based Materials for HERs

Bimetallic nanoparticle–GO-based materials represent a promising class of catalysts for the HER, crucial for hydrogen production in renewable energy systems. Numerous researchers have utilized these catalysts for HERs owing to their outstanding HER performance.

Darabdhara and his team [96] electrochemically deposited AuPd NPs onto rGO nanosheets, employing ascorbic acid as a green reducing agent. The electrocatalytic performance of the fabricated nanocomposites (Au NPs–rGO, Pd NPs–rGO, and AuPd NPs–rGO) was evaluated for the efficient generation of hydrogen in a deaerated aqueous solution of 0.5 M H₂SO₄. This assessment was carried out through the utilization of polarization and impedance measurements. Based on experimental observations, it has been determined that the catalysts under investigation demonstrate rapid kinetics for hydrogen evolution. The onset potentials for Au NPs–rGO, Pd NPs–rGO, and AuPd NPs–rGO were recorded as −17, −7.2, and −0.8 mV vs. RHE, respectively. Furthermore, the electrochemical measurements reveal Tafel slopes of 39.2, 33.7, and 29.0 mV.dec⁻¹ for Au NPs–rGO, Pd NPs–rGO, and AuPd NPs–rGO, respectively. Additionally, exchange current densities of 0.09, 0.11, and 0.47 mA.cm⁻² were recorded for the catalysts. The fabricated materials exhibit sustained high performance even after undergoing 5000 sweep cycles, with a concomitant enhancement in their activity following this aging process. The observed results indicate that the fabricated electrocatalysts, specifically AuPd NPs–rGO, exhibit great potential as contenders alongside other noble metal catalysts in the context of hydrogen evolution. They approach the performance of the commercially used Pt–C catalyst, as evidenced by similar onset potential (0.0 mV), Tafel slope (31 mV.dec⁻¹), and exchange current density (0.78 mA.cm⁻²). The high HER activity exhibited by AuPd NPs–rGO can be attributed to the plentiful presence of active catalytic sites, the enhanced electrochemically accessible surface area, and the notably improved electrochemical conductivity.

A similar work on immobilizing AuPd NPs on rGO nanosheets was published by Al-Nayili and Albdiry [97]. The sol-immobilization method was employed to synthesize monometallic Au and Pd catalysts, as well as bimetallic AuxPdy (with mole ratios of Au:Pd of 3:1, 1:1, and 1:3), supported on rGO. it was observed that the bimetallic AuxPdy–rGO catalyst displayed superior catalytic activity and selectivity compared to the monometallic Au–rGO and Pd–rGO catalysts. This was observed in the generation of hydrogen through the decomposition of formic acid in water at a temperature of 50 °C. Au₁Pd₃–rGO had the highest initial turnover frequency of 1050 h⁻¹ and apparent activation energy of 38.16 kJ.mol⁻¹ of the three bimetallic alloys tested. The enhanced activity of the bimetallic catalyst can be attributed to the synergistic impact of AuPd nanostructures and the uniform distribution of increased reaction sites on the rGO support. The reusability and recovery assessment of the bimetallic AuxPdy–rGO catalyst, conducted for a minimum of five cycles, exhibited a negligible decline of 7% in its initial activity.

Another study related to the PdAu catalyst was conducted by Rakocevic and his research group [98]. They immobilized PdAu nanoparticles on rGO to study the hydrogen evolution reaction. The HER activity of the PdAu–rGO electrode exhibited a remarkable increase, displaying an onset potential of approximately −0.01 V, nearly equivalent to the equilibrium potential for HERs. The activity of the hydrogen evolution reaction exhibited stability throughout a 3 h testing period, demonstrating a low Tafel slope of approximately −46 mV.dec⁻¹ following extended hydrogen evolution at a constant potential. A steady-state current density of −22.3 mA.cm⁻² was observed over a duration of 7 min, alongside a hydrogen yield of 12.4 × 10⁻¹⁰ mol. A lower Tafel slope than this was obtained by Darabdhara in a similar study.

In a separate study, Song and his research team [99] synthesized PdAg@ZrO₂–C–rGO nanocomposites for electrocatalytic HERs. The immobilization of ultrafine PdAg nanoparticles on zirconia–porous carbon–reduced graphene oxide (ZrO₂–C–rGO) nanocomposites was accomplished using MOF–GO. Due to the synergistic interplay between metal nanoparticles (NPs) and the supporting material, the PdAg@ZrO₂–C–rGO demonstrated remarkable electrocatalytic prowess. It showcased a high turnover frequency of 4500 h⁻¹ at 333 K, accompanied by 100% selectivity towards H₂ and an activation energy (Ea) of 50.1 kJ.mol⁻¹. These exceptional attributes enable efficient dehydrogenation of formic acid (FA). The absence of an apparent decline in catalytic efficiency throughout five cycles is of significance, suggesting that the PdAg@ZrO2–C–rGO catalyst that was prepared exhibits remarkable stability for the dehydrogenation of FA.

In a similar work, Mallikarjuna and co-workers [100] fabricated PdAg nanoparticles employing an environmentally friendly method involving stevia extract and subsequently deposited them onto rGO. The PdAg–rGO photocatalyst demonstrated an impressive photocatalytic performance in the context of hydrogen production, yielding 5802 µmol.g⁻¹ at an ideal dosage of 5 mg. The remarkable photocatalytic performance of PdAg–rGO arises primarily from the presence of PdAg, which functions as an electron sink, impeding the recombination rate and offering a greater number of catalytically active sites to facilitate efficient H₂ evolution. The reason for the excellent catalytic activity of PdAg–rGO is due to its elevated surface area, reduced bandgap energy, and diminished electron–hole recombination rate.

Darabdhara and colleagues [101] electrochemically synthesized AuNi NPs and rGO employing a facile solution chemistry approach, and subsequently employed it as an electrocatalyst for the hydrogen evolution reactions, investigating its performance through polarization and impedance studies. The electrocatalytic activity of the AuNi NP–rGO nanocomposite was observed to be remarkably enhanced in aqueous solutions containing sulfuric acid. This enhancement was observed to be directly proportional to the increase in the Ni content within the AuNi NP–rGO nanocomposite. In the context of the hydrogen evolution reaction (HER), the electrocatalyst with the highest nickel (Ni) content demonstrated exceptional performance, as evidenced by an onset overpotential that approached zero, compared to the reversible hydrogen electrode. Additionally, it exhibited a low overpotential of merely 37 mV at a current density of 10 mA.cm⁻². Furthermore, an impressively low Tafel slope of 33 mV.dec⁻¹ and a remarkably high exchange current density of 0.6 mA.cm⁻² were observed, exhibiting a striking resemblance to the characteristics typically exhibited by commercial Pt–C catalysts. The superior catalytic activity of the AuNi NP–rGO catalyst is primarily attributed to the synergistic effect between Ni and Au in the loaded AuNi NPs.

In a recent electrochemical investigation on the utilization of bimetallic nanoparticles supported on graphene oxide for photocatalytic H₂ generation, Gonuguntla and his team [102] electrochemically synthesized a ZnCd zeolite imidazolate framework (ZIF) and effectively incorporated it on GO through an electrochemical solvothermal approach (Figure 7). The ZnCd-ZIF8–GO catalyst demonstrated remarkable performance in terms of photocatalytic hydrogen generation, achieving an efficiency of 2.4 μmol.g⁻¹.h⁻¹ and an apparent quantum yield (AQY) of 1.56% over a period of 4 h. The optical and photoelectrochemical investigations of the photocatalysts revealed the proximity of the catalytic active sites of GO in relation to water splitting and the ZnCd metal center. This was accompanied by their chemical bonding and stabilization, leading to enhanced efficiency in separating electron–hole pairs and facilitating charge transfer across the nanopores. The real-time investigations revealed that the robust π − π interactions between ZnCd–ZIF8 and GO exhibited remarkable efficacy in terms of photoinduced charge separation, thereby showcasing their facile electron transfer across the hybrid photocatalyst, ultimately resulting in significantly improved water splitting.

Balčiūnaitė and his research group [103] unveiled an innovative platform for hydrogen production. The utilization of highly active Pt, along with low-cost transition metals, such as Co, Ni, and Cu, in bimetallic alloy nanoparticles was employed in conjunction with an efficient carbon-based electrocatalyst support—reduced graphene oxide (rGO). NiPt–rGO, CoPt–rGO, and CuPt–rGO were fabricated and their electrochemical prowess for HERs was scrutinized using linear sweep voltammetry (LSV) analyses in an 8 M potassium hydroxide solution. The Tafel slope b and exchange current density j₀ values acquired for the hydrogen evolution at temperatures ranging from 298 K to 338 K were as follows: Pt–rGO, b of 110–128 mV.dec⁻¹ and j₀ of 0.39–1.43 mA.cm⁻²; CoPt–rGO, b of 75–109 mV.dec⁻¹ and j of 0.17–0.96 mA.cm⁻²; NiPt–rGO, b of 87–100 mV.dec⁻¹ and j₀ of 0.07–0.35 mA.cm⁻²; CuPt–rGO, b of 82–107 mV.dec⁻¹ and j₀ of 0.09–0.48 mA.cm⁻². The Pt–rGO electrocatalysts exhibited enhanced current densities, surpassing those of the monometallic Pt–rGO counterpart, while also demonstrating significantly reduced Tafel slopes. The augmentation of temperature up to 338 K resulted in a significant enhancement in hydrogen evolution reaction (HER) current densities across all electrocatalysts. The HER activation energies observed in the rGO-supported electrocatalysts were found to vary from 27.5 to 36.7 kJ.mol⁻¹, as determined by the corresponding Arrhenius analysis. The onset potentials of the electrodes at 338 K were Pt–rGO (59 mV), CoPt–rGO (68 mV), NiPt–rGO (76 mV), and CuPt–rGO (67 mV).

Yang and his colleagues [104] prepared a HER catalyst by incorporating PtPd alloy nanoparticles on rGO. The developed catalyst (PtPd–rGO) demonstrated excellent electrocatalytic activity for HERs, yielding a lower onset potential (Eonset, −34 mV), smaller overpotential (57 mV) at 10 mA.cm⁻², and Tafel slope (36 mV.dec⁻¹). In a separate investigation carried out by Rakocevic et al. [105], the researchers applied a layer of PtAu nanoparticles onto reduced graphene oxide. The resulting catalyst, PtAu–rGO, exhibited a notably elevated level of catalytic activity for the hydrogen evolution reaction (HER) in a solution of sulfuric acid. The PtAu–rGO electrode exhibited remarkable HER activity, with an initial potential in proximity to the equilibrium potential for HERs and a Tafel slope of −38 mV.dec⁻¹, indicating a faster rate of reaction. This was electrochemically confirmed through impedance spectroscopy. An onset potential value of −0.005 V at a current density of −10 mA.cm⁻² was recorded for PtAu–rGO. The chronoamperometric measurement was conducted over a duration of 40 min to assess the hydrogen evolution process. The PtAu–rGO catalyst exhibited commendable stability and durability at a consistent potential. The Tafel slope obtained in the study (PtAu–rGO) was relatively similar to that observed by Yang using PtPd–rGO.

By employing a facial hydrothermal approach, Zhang and co-workers [106] successfully synthesized an HER photocatalyst through the integration of CuCo nanoparticles onto reduced graphene oxide (rGO). The catalyst that was developed exhibited remarkable electrocatalytic prowess for the process of HERs, resulting in H₂ evolution rates of 365.7 μmol.g⁻¹.h⁻¹ and a superior photocurrent value of 36.7 μA.cm⁻². The exceptional photocatalytic activity of CuCo–rGO for HERs was attributed to the involvement of rGO as an electron acceptor in the photogenerated electron transfer process. This facilitates the separation of electron–hole pairs and enhances the overall photocatalytic activity. The fabricated photocatalyst exhibited exceptional stability and repeatability, showcasing its electrochemical prowess.

Khalid and his team [107] synthesized a simple HER catalyst by incorporating gold (Au) and ruthenium (Ru) nanoparticles on rGO. The catalyst’s intrinsic HER activity was enhanced by the strong electronic coupling interaction between Au NPs, Ru NPs, and RGO, resulting in faster reaction kinetics and a high turnover frequency. This was supported by both experimental and theoretical findings, highlighting the influence of highly dispersed AuRu bimetallic nanoparticles on rGO sheets. The developed RuAu–rGO catalyst exhibited remarkable HER activity, displaying an overpotential of 56 mV at a current density of 10 mA.cm⁻² in a 1 M KOH solution. The AuRu–rGO catalyst demonstrated exceptional stability in the practical process of water splitting, surpassing the Pt–C–IrO₂ system, which served as the benchmark. Moreover, the catalyst was utilized as a cathode in conjunction with a zinc (Zn) anode within an aqueous Zn–CO₂ system for the production of hydrogen. It exhibited a favorable increase in the onset potential of the HER by 0.24 V compared to HERs in a conventional three-electrode testing setup, thereby expanding its potential applications.

In another study conducted by Du et al. [108], Pt–Ni alloy nanoparticles were coated on reduced graphene oxide via a one-step chemical reduction approach. The developed catalyst PtNi–rGO displays a remarkably electrocatalytic HER performance in alkaline solution, yielding an overpotential of −82 mV at a current density of 10 mA.cm⁻² with a Tafel slope of 56 mV.dec⁻¹. An insignificant current loss with an unchanged onset potential of the catalyst indicated its excellent stability.

Two different bimetallic nanoparticles (PtSn and PtCu) were synthesized and successfully incorporated into graphene by Huang and his research group [109], and their photocatalytic HER performances were subsequently investigated in 1.0 mmol.L⁻¹ eosin Y under visible light irradiation. A remarkably high hydrogen evolution rate (HER) was observed for both PtSn–G and PtCu–G catalysts. PtSn–G exhibited a hydrogen production of 1137.9 μmol in 6 h, corresponding to a hydrogen evolution rate of 189.3 mmol.h⁻¹. On the other hand, PtCu–G generated 1007.6 μmol of hydrogen in 6 h, with a hydrogen production rate of 167.9 mmol.h⁻¹. The synthesized photocatalysts were subjected to evaluation of their apparent quantum efficiency (AQE) under visible light irradiation. It was found that PtSn–G exhibited an AQE of 12.46%, while PtCu–G demonstrated an AQE of 11.06%. The noteworthy observation is that the incorporation of Cu or Sn into Pt–G greatly amplifies the efficiency of charge separation induced by light and facilitates the reduction of protons to molecular hydrogen, surpassing the performance of Pt–G alone. A summary of these studies on bimetallic nanoparticles-based graphene oxide for photo/electrocatalytic HERs is given in

Table 2. Bimetallic nanoparticle-based graphene oxide for photo/electrocatalytic HERs.

Catalyst	H2 Source in Electrolyte	Tafel Slope mV.dec−1	Current Density mA.cm−2	Ref.
AuPd–rGO	0.5 M H2SO4	29.0	0.47	[96]
PdAu–rGO	0.5 M H2SO4	46	22.3	[98]
AuNi–rGO	0.5 M H2SO4	33	10	[101]
CoPt–rGO	8 M KOH	109	0.96	[103]
NiPt–rGO	8 M KOH	100	0.35	[103]
CuPt–rGO	8 M KOH	107	0.48	[103]
PtPd–rGO	0.5 M KOH + 0.5 M glycerol	36	10	[104]
PtAu–rGO	0.5 M H2SO4	38	10	[105]
RuAu–rGO	1 M KOH	113	10	[107]
PtNi–rGO	1 M KOH	56	10	[108]

.

3. Metal–Organic Frameworks (MOFs)

Metal–organic frameworks (MOFs) are inorganic compounds composed of metal ions (inorganic nodes) connected by organic ligands (organic linkers). These organic ligands typically contain oxygen and nitrogen atoms, forming coordination bonds with the metal ions. The combination of metal ions and organic ligands leads to the creation of a porous and highly tunable framework [110]. Their tunable pore structures and well-defined chemical properties make them promising potential multifunctional porous shell materials [111]. During the early development of MOFs, there was no universally accepted standard nomenclature, resulting in the use of various terms to describe these hybrid materials [54,110]. Examples include porous coordination polymers [112] and networks [113], microporous coordination polymers [114], zeolite-like MOFs [115], and isoreticular MOFs [112,114,116]. MOFs consist of two primary building units: inorganic units and organic units. These units come together to form a porous and crystalline framework. The inorganic and organic building units in MOFs are connected through coordination bonds. Coordination bonds form between metal ions or metal clusters (inorganic units) and organic ligands (organic units). The inorganic units in MOFs are typically metal ions or metal clusters. These metal species act as nodes in the framework, providing coordination sites for the attachment of organic ligands. The organic units in MOFs are referred to as linkers or bridging ligands. These ligands are organic molecules with coordination sites (atoms) that can form bonds with the metal ions or clusters. Organic ligands in MOFs are commonly di-, tri-, or tetradentate, indicating the number of coordination sites available on the ligand molecule. The higher the dentate nature, the more coordination sites are available for bonding with metal ions [115,117,118]. Organic ligands used in MOFs include carboxylates (carboxylic acid-derived ligands), phosphonates, sulfonates, and various heterocyclic compounds. Carboxylates are particularly common and contribute to the carboxylate-based frameworks of many MOFs. One of the defining features of MOFs is their adjustable and tunable pore size. The modular nature of MOF synthesis allows researchers to select specific inorganic nodes and organic linkers, influencing the size and shape of the resulting pores [93]. The ability to adjust pore size is crucial for accommodating molecules of various sizes and shapes. This versatility makes MOFs suitable for a range of applications, including gas storage, separation, and catalysis. For example, certain MOFs can selectively adsorb specific gases or separate molecules based on size, contributing to advancements in gas storage and purification technologies. MOFs are also known for their exceptional porosity, which is directly related to their high surface area [93,94]. The porous structure allows for a significant surface area per unit volume. The high surface area is advantageous for applications such as gas adsorption, where the large, exposed surface can interact with and adsorb gas molecules [94]. This property is also valuable in catalysis, where the increased surface area provides more active sites for chemical reactions. The adjustable pore size and high surface area of MOFs have attracted attention from scientists in various fields, including physicists, biologists, and environmental engineers [119].

3.1. Structure of MOFs

MOF materials are formed through a controlled assembly process where metal nodes and organic ligands come together to create a three-dimensional framework. This assembly is driven by coordination bonds between the metal nodes and the coordinating sites on the organic ligands [120]. The metal nodes in MOFs are typically derived from metal nitrates or chlorides. These metal precursors provide the metal ions that serve as the inorganic building blocks of the MOF structure [86,96]. Understanding the specific metal precursors and organic ligands chosen for MOF synthesis allows researchers to tailor the properties of the resulting MOF material. The design flexibility in selecting these components contributes to the vast diversity and tunability of MOFs for various applications.

3.1.1. Organic Ligands

Organic ligands are well known for their intricate features, especially in the context of coordination chemistry. They contain specific functional groups that can coordinate with metal ions, forming stable coordination bonds [110,114,120]. The complex nature of organic ligands allows for the design of ligands with varying sizes, shapes, and functionalities [89]. This versatility is crucial for tailoring the properties of MOFs for specific applications. Organic ligands play a central role in MOF formation by establishing coordination bonds with metal ions. These bonds contribute to the connectivity and stability of the MOF structure [86,89,96]. The organic ligands in MOFs often exist in anionic states. This is important for their ability to interact with and form bonds with metal ions, which are typically positively charged. The anionic nature of organic ligands facilitates the coordination process, allowing them to effectively coordinate with metal ions and contribute to the overall structure of MOFs. The most commonly used organic ligands in the synthesis of MOFs are benzene 1,3,5-tricarboxylic acid (H₃btc), benzene 1,4-dicarboxylic acid (H₂bdc), 4,5-imidazole dicarboxylic acid, and pyrazine-2-carboxylic acid (Figure 8) [112]. H₃btc is a tricarboxylic acid with three carboxylate groups attached to a benzene ring. It provides multiple coordination sites for metal ions, facilitating the formation of strong coordination bonds. H₂bdc is a dicarboxylic acid with two carboxylate groups attached to a benzene ring. Due to its simplicity and coordinating ability, H₂bdc is widely used in MOF synthesis. Also, 4,5-imidazole dicarboxylic acid incorporates both imidazole moieties and dicarboxylic acid groups. The presence of imidazole groups adds heterocyclic functionality to the ligand, influencing the properties of the resulting MOF. Finally, pyrazine-2-carboxylic acid is a ligand that contains a pyrazine ring and a carboxylic acid group. The inclusion of a heterocyclic pyrazine ring provides unique structural and electronic characteristics to the ligand [87,88,89,90,92,93,94,95,96].

These organic ligands are chosen for their coordinating abilities and their impact on the resulting MOF structure. The selection of a specific ligand can influence the size and geometry of the pores in the MOF, as well as its overall stability and functionality. Researchers often explore a variety of ligands to achieve the desired properties for specific applications, ranging from gas storage to catalysis. The systematic exploration of different ligands and metal nodes allows for the creation of a vast library of MOFs with diverse structures, opening up possibilities for tailoring these materials to meet specific needs in areas such as gas separation, sensing, and drug delivery.

3.1.2. Metal Sites

The metal ions or clusters within MOFs serve as coordination sites, where organic ligands coordinate to form the framework. These metal coordination sites play a pivotal role in the adsorption of molecules, particularly gases and small molecules [121]. Metal sites in MOFs often act as Lewis acids, capable of accepting electron pairs from coordinated organic ligands. This Lewis acid–base interaction facilitates the activation of organic substrates bound to the metal sites [121,122]. The activated organic substrates can undergo catalytic transformations, ranging from small-molecule activation to more complex organic reactions. The metal sites play a crucial role in catalyzing these reactions within the MOF framework. Different metal ions exhibit varying Lewis acidity, redox activity, and coordination geometries [55]. This diversity allows researchers to design MOFs with a wide range of catalytic activities tailored for specific transformations. Transition metals are known for their ability to exhibit multiple oxidation states, meaning they can lose different numbers of electrons in chemical reactions. The variability in oxidation states contributes to the diverse chemistry of transition metals. It allows them to form compounds with different stoichiometries and coordination environments. The electron configuration of copper in its zero-valent state is [Ar]³d¹⁰4s¹. This configuration reflects the filling of the 3d subshell with nine electrons, leaving one unpaired electron in the 4s orbital. Copper can exhibit oxidation states of +1 and +2, among others. The electron configuration changes with the oxidation state, influencing the coordination geometry around the copper center. In some cases, transition metal complexes may exhibit distorted geometries due to the Jahn–Teller effect, especially when the metal center has an odd number of electrons [32,107]. Copper (II) complexes often adopt geometries such as square planar or octahedral, depending on the nature of the ligands and the coordination number. These coordination geometries of transition metal centers in catalysts play a crucial role in facilitating electron transfer, especially in processes like the HER [32]. Efficient electron transfer is crucial for the overall catalytic activity. The faster the electrons can move within the catalyst, the more efficient the catalytic process becomes.

3.1.3. Secondary Building Units (SBUs)

SBUs serve as the foundational building blocks in the construction of MOFs. The way SBUs assemble and connect forms the basis for the overall architecture and topology of the MOF. The arrangement and connectivity of SBUs play a decisive role in determining the final topology of the MOF structure. Different SBUs can lead to distinct MOF topologies [121,122,123]. The use of carefully selected multidentate linkers under specific conditions can lead to the aggregation and locking of metal ions at defined positions, resulting in the formation of secondary building units (SBUs). Multidentate linkers are organic ligands with multiple coordination sites, allowing them to bind to metal ions at multiple points simultaneously. The presence of multiple coordination sites on these linkers provides versatility in forming complex structures with metal ions [122,124]. Organic ligands, also known as organic linkers, play a crucial role in connecting and bridging the SBUs. The use of rigid organic ligands ensures that the connectivity between SBUs is well defined and does not introduce excessive flexibility in the MOF structure. The rigidity of the organic linkers contributes to the overall structural stability of the MOF. Rigid linkers help maintain the spatial arrangement of SBUs, preventing structural collapse. The coordination bonds formed between the metal nodes (from SBUs) and the organic linkers provide a stable framework. The rigidity of the linkers helps in maintaining the integrity of these coordinated bonds [125]. Varying the metal-to-ligand ratio can impact the coordination environment of metal ions within the SBUs, affecting the geometry. Furthermore, the choice of solvent during the synthesis of MOFs can influence the coordination and assembly of SBUs. Thus, different solvents may promote specific interactions between metal ions and ligands, influencing the geometry and stability of SBUs [121,126].

3.1.4. Pores in MOFs

MOFs are known for their high porosity, characterized by the presence of voids or empty spaces within their structures. These pores are formed during the synthesis of MOFs and can be further manipulated or expanded through post-synthetic treatments [127]. During the synthesis or activation process, MOFs often incorporate guest molecules within their pores. The removal of these guest molecules creates voids or pores within the MOF structure. MOFs can exhibit structural flexibility, allowing them to adopt wide-open structures with spacious internal voids. The design and choice of organic linkers and metal nodes allow researchers to tune the pore sizes in MOFs, creating structures with varying diameters. These internal diameters of up to 4.8 nm indicate a substantial free space within the MOF structure [100]. The large free space in MOFs makes them attractive for gas storage applications, including the storage of molecules like hydrogen [100,103]. MOFs have been extensively studied for their potential in hydrogen storage. The porous structure provides an environment for hydrogen molecules to be adsorbed and stored. The primary objective of designing an ideal pore size is to achieve optimal interaction of the absorbate gas (H₂) with the potential surface of all the surrounding walls. This design consideration ensures that the available surface area within the pores is effectively utilized for gas adsorption. The kinetic diameter of H₂ is specified as 0.289 nm. The pore size is intentionally chosen to be close to the kinetic diameter of H₂, promoting a stronger interaction between H₂ molecules and the framework of the porous adsorbent material. NU-100 is cited as an example of a porous adsorbent that contains micropores with sizes less than 2 nm. Despite having micropores, NU-100 demonstrates a significant storage capacity for H₂, reaching 8 wt.% [128]. This example illustrates that even with micropores, the specific design allows for efficient storage of H₂ molecules. Conversely, MOF-5 has micropores with a size of 0.77 nm, while NU-100 has micropores of (0.77 nm) and stores close to 7 wt.% [124]. While larger pores may introduce challenges in maintaining permanent porosity, they can offer advantages in specific applications, such as enhanced performance in hydrogen storage, where the benefits of larger pores outweigh the potential drawbacks [124].

3.1.5. Intrinsic Properties/Features of MOFs

MOFs possess special features that favor their choice as excellent materials for HERs. These features include light-harvesting capability, large surface area and active sites, tailorable porosity and surface chemistry, efficient charge separation and transport, tunable band structure, chemical stability under light, synergy between metal ions and organic ligands, and cost-effectiveness [129].

i.

Light-harvesting capability

Certain MOFs contain photoactive metal centers or organic ligands incorporated in their structures, enabling them to absorb light across a broad range of wavelengths, including visible and near-infrared regions [130]. This light-harvesting capability allows MOFs to serve as photocatalysts for driving the HER under illumination [131].

ii.

Large surface area and active sites

MOFs typically exhibit large surface areas due to their porous structures, providing abundant active sites for the HER. This large surface area enhances the accessibility of reactants to the catalytic sites, leading to improved catalytic activity under illumination [132]. Additionally, MOFs offer the possibility of incorporating cocatalysts or functional groups within their structures to further enhance catalytic activity and promote hydrogen evolution [129].

iii.

Tunable band structure

The electronic properties of MOFs can be tailored by selecting specific metal ions and organic ligands, enabling the optimization of the band structure for efficient charge transfer processes during the HER. This tunability enables the design of MOFs with enhanced photocatalytic activity and improved efficiency for hydrogen production [133,134].

iv.

Efficient charge separation and transport

MOFs offer ordered structures with well-defined pores, facilitating the separation and transport of photogenerated charge carriers (electrons and holes). This efficient charge separation prevents rapid recombination of charge carriers, enhancing the probability of charge transfer to the catalytic sites responsible for the HER [129,135].

v.

Chemical stability under illumination

Many MOFs exhibit excellent stability under illumination, ensuring prolonged photoelectrocatalytic performance for the HER. This stability is crucial for maintaining catalytic activity over extended periods, particularly in solar-driven hydrogen production systems, where continuous light exposure is required [136,137].

vi.

Synergy between metal ions and organic ligands

The interaction between metal ions and organic ligands in MOFs can lead to synergistic effects that enhance their photoelectrocatalytic properties. This synergy may result in improved light absorption, charge separation, or catalytic activity for the HER compared to either component alone [129,138].

vii.

Tailorable porosity and surface chemistry

MOFs allow for precise control over their porosity and surface chemistry, which can be tailored to optimize reactant diffusion, adsorption, and desorption kinetics during the HER. This tunability enables the design of MOFs with customized properties to meet specific requirements for efficient photoelectrocatalytic hydrogen production [139,140].

3.2. Synthesis of MOFs

MOFs can be prepared using various methods, including microwave-assisted synthesis, mechanochemical synthesis, electrochemical route synthesis, and solvo/hydrothermal synthesis. However, liquid-phase synthesis is the most widely used method, where solutions of metal salts and ligands are mixed in a reaction vial [141].

3.2.1. Microwave-Assisted Synthesis

In this method, microwave irradiation is used to accelerate the chemical reactions involved in MOF synthesis. Microwave heating can lead to faster reaction rates and more controlled synthesis compared to traditional methods. This method involves the use of electromagnetic waves, specifically microwaves, to drive and accelerate chemical reactions during the synthesis of materials, including MOF. Microwaves are a form of electromagnetic radiation with frequencies between radio waves and infrared radiation [142,143]. Microwaves interact primarily with mobile electric charges within a material. In solutions, polar solvent molecules with electric dipoles can interact with microwaves. Charged ions in a solution can also be influenced by the electromagnetic field of microwaves. In solid materials, the interaction can occur with mobile electrons or ions [144,145]. Application of an appropriate frequency of microwaves results in the inducement of collisions between molecules. These collisions result in an increase in the kinetic energy of the molecules involved. Moreover, the elevated kinetic energy translates into a rise in temperature within the system.

Microwave-assisted synthesis (MAS) provides many advantages that have shown direct impact on the catalytic properties of MOFs. Microwave-assisted synthesis allows rapid and uniform heating of the reaction mixture, which in turn promotes faster nucleation and crystal growth, leading to shorter reaction times. With this, well-defined structures with regulated morphologies can be produced within a short period. Equally, the uniform and rapid heating also helps to reduce side reactions and impurities during MOF synthesis. This results in the formation of MOF crystals with higher purity and crystallinity, which can contribute to improved catalytic activity and stability of the MOF [146,147]. Another benefit of MAS is the ability to precisely regulate reaction parameters like temperature and pressure, which can affect the size, shape, and distribution of MOF particles. Adjusting these parameters during MAS can produce MOF materials with well-dispersed active sites and tailored particle sizes, which are essential for catalytic applications [147]. The use of MAS for MOF preparation also improves the porosity and surface area of the MOF. The fast and uniform heating facilitates the removal of solvents and guest molecules from the MOF pores, producing an MOF with increased accessible surface areas and pore volumes. These properties are advantageous for catalytic applications, as they increase the number of active sites and promote the mass transport of reactants and products. Finally, the optimization of metal–ligand coordination is made possible by the precise control over reaction conditions provided by MAS [148,149,150]. This can improve the stability and catalytic activity of the resulting MOF materials. All these advantages contribute to the production of highly efficient MOF catalysts for HERs.

Additionally, different metals (III) provide diversity in MOF properties, including potential variations in structure, porosity, and reactivity. These include Fe (iron), Al (aluminum), Cr (chromium), V (vanadium), and Ce (cerium), among others. The use of microwave-assisted synthesis is highlighted not only for its general mechanism in raising temperature through efficient energy transfer but also for its successful application in the preparation of metal (III) carboxylate-based MOFs. This method showcases its versatility and effectiveness in the synthesis of various MOF structures with different metal centers [144,145,151].

3.2.2. Mechanochemical Synthesis

Mechanochemical synthesis involves chemical reactions induced by mechanical forces such as grinding or milling. It is the mechanical breakage of intramolecular bonds followed by a chemical transformation. This method has been utilized in synthetic chemistry for a considerable period. The synthesis of porous MOFs using mechanochemistry was first reported in 2006. Mechanochemistry has proven to be a valuable method for the preparation of porous MOFs, offering an alternative approach to traditional synthesis methods [152,153].

Several improvements to the catalytic features of MOFs have been made by employing mechanochemical synthesis. This method can promote increased reactivity between metal ions and organic ligands due to the mechanical forces applied during milling or grinding. This enhanced reactivity can lead to faster formation of MOF crystals with well-defined structures and controlled compositions, which is important for achieving high catalytic activity. This approach also allows precise control over the size, shape, and morphology of MOF particles by adjusting milling parameters such as milling time, rotation speed, and ball-to-powder ratio. Fine-tuning these parameters allows for the production of MOF materials with optimized particle sizes and morphologies, which can significantly influence their catalytic performance [154,155,156]. Incorporating guest molecules (catalytic species or templates) into the MOF structure is facilitated during the milling process. The mechanical forces applied during milling can promote the diffusion of guest molecules into the MOF pores, yielding composite materials with improved catalytic properties. Added to these benefits is the ability to induce structural rearrangements and defects in the MOF framework, thereby producing MOF materials with increased accessible surface areas and pore volumes [157,158,159]. This gives the MOF more active sites and improves their catalytic performance. Mechanochemical synthesis has been successfully utilized for different types of MOFs, such as MOF-74 [160], MOF-5 [161], Fe-MIL-88A [162], HKUST-1 (Cu₃(BTC)₂, BTC = 1,3,5-benzene tricarboxylate) and MOF-14 (Cu₃(BTB)₂, BTB = 4,4′,4″-benzene tribenzoate) [163].

3.2.3. Electrochemical Synthesis

The electrochemical synthesis of MOFs, introduced in 2005, aimed to exclude certain anions during the synthesis process, addressing concerns related to large-scale production. This approach reflects a commitment to optimizing the synthesis of MOFs, ensuring high quality, and minimizing the presence of undesired components in the final products [164]. Anion exclusion contributes to the improvement in MOF quality and purity by preventing the incorporation of unwanted anions that may have adverse effects on the properties of the MOF. This suggests a focus on optimizing the synthesis process to control the composition and characteristics of the resulting MOFs. Electrochemical methods provide a means of selectively controlling the synthesis conditions, including the exclusion of specific ions or anions. Instead of using preformed metal salts, electrochemical synthesis involves continuously introducing metal ions into the reaction medium. The metal ions are introduced through anodic dissolution, a process in which metal atoms dissolve into the solution due to oxidation at the anode [120,164]. As a trade-off, the electrochemical reduction in protic solvents results in the formation of molecular hydrogen (H₂). This is a common occurrence in certain electrochemical reactions, particularly those involving the reduction of protons (H⁺) to produce H₂ gas [165]. The choice of protic solvents is effective in preventing the undesired deposition of metal on the cathode during electrochemical synthesis. Protic solvents act as a protective medium, inhibiting the reduction of metal ions at the cathode. The use of protic solvents in electrochemical synthesis is a strategic choice to prevent metal deposition on the cathode. However, the generation of hydrogen gas (H₂) as a by-product needs to be considered and managed in the overall design and scale-up of electrochemical synthesis processes. Thus, electrochemical synthesis allows for precise control over reaction conditions, leading to enhanced reproducibility and purity [165].

Many significant improvements to the catalytic features of the MOF have been recorded through electrochemical route synthesis. These include the provision of precise control over composition and structure, enabling in situ modification and functionalization, enhancing morphological control and nanostructuring, facilitating direct integration with electrodes, and tuning redox properties [166]. Because of these characteristics, MOFs produced electrochemically are very attractive catalysts for a variety of heterogeneous and electrocatalytic applications. A variety of MOF materials have been prepared using this method, such as Cu₂(MTCP), Cu₃(BTPA)₂, and Cu₃(TPTC)₂ MOF films [167], MOF-5 [168], MIL-100(Fe), Cu–BTC MOF HKUST-1 [169], Ni-MOFs [170], etc.

3.2.4. Solvo/Hydrothermal Synthesis

The solvo/hydrothermal method is a crystallization process wherein compounds are synthesized based on their solubility in water under autogenous pressure. The autoclave serves as the vessel for the controlled crystallization process, providing an environment conducive to the formation of crystalline compounds [128]. The autoclave used in the solvo/hydrothermal method is typically made of thick-walled steel cylinders. This construction is chosen for its ability to withstand high temperatures and pressures for extended periods, ensuring the safe and durable execution of the crystallization process [110,171]. The autoclave provides a controlled environment for the synthesis process, ensuring specific temperature and pressure conditions. The sturdy design contributes to the reproducibility of experiments conducted using the solvo/hydrothermal method. The autoclave design incorporates features that prioritize safety when handling high temperatures and pressures. The thick-walled steel construction helps prevent leaks and ensures the containment of the reaction within the autoclave. Moreover, the autoclave is designed to have a prolonged life span, enabling its repeated use in experiments without compromising its structural integrity.

The samples are initially placed within Teflon bags. Once the samples are inside the Teflon bags, the bags are sealed. The sealed Teflon bags with the samples are then placed in a Teflon cup. The Teflon cup is closed with a cap on the top. The Teflon cup containing sealed Teflon bags and samples is placed into the autoclave. The autoclave is tightly closed, ensuring a sealed environment. The sealed autoclave containing the Teflon cup with samples is placed within an oven. The oven is set to achieve the high temperatures necessary for the solvo/hydrothermal method [145]. This approach helps contain the reaction and provides a controlled environment for sample synthesis.

Solvo/hydrothermal synthesis offers several benefits for strengthening the catalytic features of MOFs by providing controlled crystallinity, improving purity and homogeneity [172,173], tailoring porosity and surface area, enabling encapsulation of catalytic species, and guaranteeing structural stability. MOFs that are solvo/hydrothermally produced are excellent catalysts for a range of catalytic uses, including hydrogen production [174]. Testifying to the advantages of this method, a simple one-pot solvo/hydrothermal approach was employed by Shasha and his research team to produce a Co₃O₄@Co MOF under high alkaline conditions. The synthesized prepared materials displayed excellent electrocatalytic performance, such as larger surface area, unique, structure and high alkali stability [175]. Many other researchers have also used this method to prepare various forms of MOF materials such as a chitosan composite of an iron MOF (CS/MOF-235) [176], ZIF-8 [177], etc.

3.2.5. Epitaxial Growth Method

In epitaxial growth, a thin film of an MOF grows over a crystalline substrate surface within which the MOF crystallizes in a way that matches the substrate’s crystal lattice. Nucleation and growth of the MOF crystals take place on the substrate, producing an ordered and well-oriented thin film [178]. The following is a succinct explanation of the mechanism underlying the MOF synthesis process via epitaxial growth:

i.

Choice of Substrate

Generally, a well-defined substrate with a known crystal structure is needed for epitaxial growth. Single-crystal surfaces, such as those of metal oxides (such as TiO₂ and ZnO) or other inorganic materials with smooth, ordered surfaces are common substrates.

ii.

Preparation of Substrate

The substrate surface is cleaned and functionalized to facilitate nucleation, adhesion, and growth of the MOF crystals. This could entail procedures like chemical etching, functionalization using linker molecules, or seed layer deposition.

iii.

Introduction of Precursor Solution

A precursor solution comprising metal ions or clusters and organic ligands (linkers) is prepared and deposited onto the substrate surface by vapor deposition, drop casting, or spin coating. The composition and structure of the MOF to be synthesized dictate the choice of precursors.

iv.

Adsorption and Nucleation

The precursor molecules adsorb onto the substrate surface, forming nucleation sites. The nucleation process starts when the precursor molecules undergo coordination chemistry, forming metal–ligand bonds and initiating the formation of MOF nuclei.

v.

Epitaxial Growth

As MOF nuclei form on the substrate surface, subsequent layers grow epitaxially, meaning they align with the crystal lattice of the underlying substrate. This alignment is caused by the structural compatibility between the substrate and MOF, resulting in the ordered growth of MOF crystals.

vi.

Controlled Growth Conditions

The kinetics of nucleation and growth of MOF crystals are regulated under a controlled temperature, pressure, and concentration to produce a tailored size, shape, and oriented MOF crystals.

vii.

Post-Synthesis Treatment

The resulting MOF thin films are subjected to washing, drying, or annealing to remove any residual solvent molecules and improve the film’s crystallinity and stability,

viii.

Characterization and Optimization

Throughout the epitaxial growth process, characterization techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and spectroscopic methods are used to monitor the morphology, structure, and quality of the synthesized MOF films.

Numerous advantages have been attached to MOFs produced by this method, such as highly oriented and controllable growth orientation, controllable thickness based on the quantity of material deposited, smoothness and homogeneity, and ease of scaling up in challenging environments [179,180]. By carefully controlling the substrate surface and growth conditions, epitaxial growth offers a precise and scalable method for synthesizing MOF films with tailored properties for applications such as sensing, catalysis, and gas separation [181]. Moreover, the use of a crystalline substrate provides a stable template for MOF growth, yielding high-quality thin films with well-defined crystalline structures and surface morphologies. Epitaxial growth can be designed to accommodate numerous substrate materials and properties, such as composition, surface chemistry, and crystal structure [178]. MOFS such as NH₂–MIL-125(Ti) [182], DUT-52@MIL-88B, DUT-52@MIL-88C, DUT-52@MIL-88B@MIL-88C [183], MIL-96 [184], etc., have been prepared via epitaxial growth.

3.2.6. In Situ Growth Method

In situ growth involves the synthesis of MOFs directly on a substrate surface under ambient conditions, without the need for preformed crystals or templates. The MOF synthesis occurs entirely on the substrate, resulting in the formation of a continuous thin film. This method allows for the control of the MOF formation process in real time and often enables the integration of MOF materials into specific devices or applications [185]. The process typically proceeds as follows.

i.

Substrate Preparation

Similarly to epitaxial growth, the substrate surface is prepared to promote nucleation and adhesion of the MOF crystals. Surface functionalization techniques may be employed to modify the substrate properties and enhance MOF growth.

ii.

Deposition of Reactants

The precursor components necessary for MOF synthesis, including metal ions or clusters and organic ligands, are introduced onto the substrate surface either sequentially or simultaneously. This can be achieved through techniques such as solution deposition, vapor-phase deposition, or molecular beam epitaxy.

iii.

MOF Formation and Growth

The precursor molecules react and self-assemble on the substrate surface, driven by coordination chemistry and intermolecular interactions. This results in the nucleation and growth of MOF crystals directly on the substrate, forming a continuous thin film.

iv.

Film Post-Treatment

After MOF growth, the thin film may undergo post-synthesis treatments such as washing, drying, or annealing to remove any residual reactants or solvent molecules and improve film quality.

In situ growth offers a simple and versatile method for synthesizing MOF thin films directly on various substrate surfaces without requiring complex procedures or templates. This method can also be applied to a wide range of substrate materials, including metals, metal oxides, polymers, and ceramics, making it suitable for diverse applications [186]. Again, the simplicity of this approach makes it amenable to large-scale production, offering potential cost savings compared to more complex synthesis methods. Overall, the in situ growth method allows for the direct synthesis of MOF materials in a controlled manner, offering flexibility in tailoring the properties of the resulting materials for various applications in gas storage, catalysis, sensing, and more [187]. Applications of the method for MOF production are numerous in the literature, such as Ti₃C₂@MIL–NH₂ [188], MIL-101(Fe) [189], NH₂–MIL-101@CA, NH₂–MIL-88@CA [190], MIL-100@NiMn-LDH [191], TiO₂/NH₂–MIL-125(Ti) [192], etc.

3.3. MOF-Based Materials as HER Electrocatalysts

There are two half-reactions that collectively represent the electrochemical water-splitting process, where water is split into hydrogen and oxygen using electrical energy. In the HER, water molecules are reduced by gaining electrons to produce hydrogen gas and hydroxide ions. This reduction half-reaction takes place at the cathode. In the OER, water molecules are oxidized by losing electrons to produce oxygen gas, protons (H⁺), and electrons. This oxidation half-reaction occurs at the anode. Water splitting involves breaking strong chemical bonds, requiring significant activation energy. Therefore, the pursuit of high-performance, cost-effective electrocatalysts with precise structures is crucial for advancing the efficiency and feasibility of this technology for sustainable hydrogen production [11,31,193]. The use of homogeneous catalysts in low concentrations is indeed a notable strategy in catalysis, often resulting in high activity in terms of turnover frequency (TOF). However, homogeneous catalysts may be sensitive to high temperatures, and when exposed to reactive species or harsh reaction conditions, chemical degradation/low stability tends to result [112,194,195]. Therefore, the use of MOF-derived (metal–organic framework-derived) inorganic materials represents a promising approach that aims to combine the advantages of both conventional heterogeneous and homogeneous catalysts, while also addressing some of the challenges associated with these catalytic systems [94,196]. In this regard, Sandra Loera-Serna et al. demonstrated the electrochemical behavior of a Cu–MOF, confirming that this material has high catalytic activity [197]. Furthermore, Lin et al., constructed metal–organic framework-derived cobalt diselenide (MOF–CoSe₂) with CoSe₂ nanoparticles anchored into nitrogen-doped (N-doped) graphitic carbon through in situ selenization of Co-based MOFs [198]. It was detected that the MOF–CoSe₂ delivered excellent HER performance with an onset potential of approximately 150 mV and a high current density of 80 mA.cm⁻² at about −0.33 V (vs. RHE), and behaved better than bare CoSe₂. The Tafel slope of the MOF–CoSe₂ was 42 mV.dec⁻¹, which is much lower than that of bare CoSe₂—72 mV.dec⁻¹. Moreover, Ramohlola and co-workers showed the merits of combining MOF material with conductive polymers to produce a highly active material (Tafel plots presented in Figure 9) [199,200].

Table 3. Comparison between the characteristics of various MOF electrocatalysts towards HERs.

Material	H2 Source in Electrolyte	Tafel Slope (mV.dec−1)	Current Density (mA.m−2)	Ref.
MOF–CoSe2	0.5 M H2SO4	42	0.080	[55]
Pd@CuPc–MOF	0.3 M H2SO4	176.9	8.900	[55]
MOF–PANI	0.3 M H2SO4	199.3	7.943	[199]
MOF–5wt.% PABA	0.3 M H2SO4	153.5	50.12	[200]
MOF–3wt.% PABA	0.3 M H2SO4	166.7	31.62	[200]
PABA–MOF	0.3 M H2SO4	130.5	35.48	[201]
Cu–MOF–8 wt.% GO	0.5 M H2SO4	-	−300	[202]

presents the Tafel parameters of MOF–polyaniline (PANI), MOF–3 wt% poly(3-aminobenzoic acid) (PABA), PABA–MOF and MOF–5 wt% PABA composites with high exchange current densities of 7.943 [199,200], 31.62, [201] 35.48 [201], and 50.12 A.m⁻² [200] and Tafel slopes of 199.3, 166.7 [200,201], 130.5 [200], and 153.5 mV.dec⁻¹ [201], respectively.

The integration of a Cu–MOF with GO is suggested to enhance electron transfer. This improvement in electron transfer is likely to lead to more efficient catalytic activity for the HER. The mechanism behind this enhancement may involve the synergistic effects of the unique properties of the Cu–MOF and GO [202]. It was also found that the GO content affected the HER activity of the nanocomposite catalysts. The optimized GO content was about 8 wt%. The HER current density of the (GO 8 wt%) Cu–MOF (Table 3) was high, up to −30 mA.cm⁻² at an overpotential of −2.0 V in N₂-saturated 0.5 M H₂SO₄, whereas the overpotential of 30 wt% Pt was −0.06 V at a current density of −30 mA.cm⁻² [202]. The electrochemical hydrogen evolution reaction performance of the Pd@CuPc–MOF and Tafel analysis were evaluated by Monama et al. [55]. The Tafel slope of this composite was found to be 176.9 mV.dec⁻¹ and the transfer coefficient 0.67, with an exchange current density of 8.9 mA.m⁻² (Table 3) The HER results revealed that the Pd@CuPc–MOF composite has better catalytic characteristics, such as high catalytic activity and lower onset potential compared to MOF. More importantly, they reported the significant enhancement of HER performance at ambient temperature for the composite with Pd content to be ascribed to the hydrogen spillover mechanism in such a system.

3.4. Bimetallic Nanoparticle–MOF-Based Materials for HERs

Researchers have taken advantage of the combine unique properties of bimetallic nanoparticles with high surface area, tunable porosity, and chemical versatility of MOFs to develop catalysts with enhanced catalytic activity and stability for HERs. The details are given herein.

Through the utilization of a facile wet-reduction technique, Han and colleagues [203] effectively fabricated a catalyst for the hydrogen evolution reaction (HER) by integrating ultrafine PdAg nanoparticles onto a NH₂-functionalized metal–organic framework (MOF): MIL-101(Cr). Due to the presence of the amine group, the size of the PdAg nanoparticles obtained was regulated to 2.2 nm. These nanoparticles exhibited a uniform distribution on the surface of NH₂–MIL-101(Cr). The electrocatalytic performance of the PdAg NP–NH₂–MIL-101(Cr) catalyst was found to be highly impressive for the decomposition of formic acid under mild conditions. This led to a gas (H₂ + CO₂) yield of 144 mL in 4.87 min at a temperature of 323 K with a TOF of 1475 h⁻¹. The remarkable electrocatalytic performance of PdAg NPs–NH₂–MIL-101(Cr) for the hydrogen evolution reaction can be ascribed to the nanoscale dimensions and excellent distribution of the PdAg NPs. Furthermore, the presence of the amine group in NH₂–MIL-101(Cr) support enhances the process of O-H bond dissociation in formic acid, thereby enhancing the kinetics of formic acid decomposition.

Mandegarzad et al. [204] employed both hydrothermal (HT) and electrochemical (EC) approaches to immobilize CuPd bimetallic nanoparticles on a nanoporous carbon composite (NPCC) obtained from MOF-199. Afterward, the electrocatalytic performance of both CuPd–NPCC–EC–GCE and CuPd–NPCC–HT–GCE electrodes were investigated by LSV in 0.5 M H₂SO₄ solution at 5 mVs⁻¹. The CuPd–NPCC–EC system was proposed to follow a Volmer–Heyrovsky mechanism, as indicated by the α value of 1.66, which closely approximates 1.5. The CuPd–NPCC–EC–GCE exhibited a higher exchange current density of 0.056 mA.cm⁻² and a higher Tafel slope of 28.2 mV.dec⁻¹ than the CuPd–NPCC–HT–GCE, which had an exchange current density of 0.03 mA.cm⁻² and a Tafel slope of 12.6 mV.dec⁻¹. Furthermore, the HER onset potential was shifted towards a more positive value (−0.13V vs. NHE) when utilizing CuPd–NPCC–EC–GCE. The enhanced catalytic activity of CuPd–NPCC–EC compared to CuPd–NPCC–HT can be attributed to the increased surface area of Cu–NPCC–EC, the synergistic effect of the bimetallic composite resulting from the reduction of the d-band center of Pd by the Cu component, and the improved conductivity of CuPd–NPCC–EC due to the presence of high electron transfer sites and the high conductivity of the electrochemically synthesized MOF-199.

Zhen and co-workers [205] fabricated NiMo alloy nanoparticles anchored in MIL-101 via a double-solvent approach. Significantly, the NiMo@MIL-101 photocatalyst, after being sensitized by eosin Y during visible-light irradiation, demonstrated a remarkable photocatalytic performance (740.2 μmol.h⁻¹ for HER), Tafel slope (76 mV.dec⁻¹), high stability, high apparent quantum efficiency (75.7%) under 520 nm illumination at pH 7, lower overpotential (−0.51 V), and longer fluorescence lifetime (1.57 ns). The obtained results exhibited a noteworthy Tafel slope (76 mV.dec⁻¹) for the NiMo@MIL-101 catalyst. This value is significantly lower than that for MIL-101, Ni@MIL-101, and Mo@MIL-101. These findings imply that the production of H₂ occurs through the Volmer−Heyrovsky mechanism, thereby reinforcing the exceptional catalytic performance of NiMo@MIL-101 in the HER process.

In other research conducted by Chang et al. [206], a new photocatalyst, NiTi–NH₂–MIL-125, was produced via a one-pot approach. The catalyst that was developed exhibited a significantly high level of photocatalytic performance for the HER. Specifically, it achieved a hydrogen evolution rate of 699 μmol.g⁻¹.h⁻¹ when exposed to visible light. This rate is approximately nine times that observed for the original Ti–MOFs and Ti–NH₂–MIL-125 materials. The experimental procedure entailed the utilization of a 0.5 mL volume of triethanolamine (TEOA) as a hole scavenger. The heightened photocatalytic performance observed in NiTi–NH₂–MIL-125 can be attributed to the incorporation of Ni²⁺, which facilitates the efficient separation of photogenerated charge carriers. Furthermore, the synergistic effect resulting from the presence of the two metals enhanced electron enrichment and led to an increased production of hydrogen.

In other research conducted by Chen et al. [207], a stepwise method was employed to incorporate AgNi alloy nanoparticles onto nitrogen-doped carbon nanostrips derived from MOFs. The developed catalyst, AgNi–NC, exhibited a significantly efficient electrocatalytic performance in 1 M KOH solution, yielding a low overpotential of 103 mV at a current density of 10 mA.cm⁻² with a Tafel slope of 126.19 mV.dec⁻¹ and a high retention rate of 90.9% after 10 h. An insignificant current loss with an unchanged onset potential of the catalyst indicates its excellent stability. The remarkable HER characteristics exhibited by AgNi–NC can be ascribed to the synergistic interplay among the hierarchical carbon materials, partial nitrogen doping, and the abundant presence of AgNi alloy nanoparticles.

Antil et al. [208] were able to effectively synthesize a new photocatalyst for the hydrogen evolution reaction (HER) by combining NiCoP nanoparticles with a ZnCo MOF. This process is visually represented in Figure 10. The synthesized catalyst, NiCoP@ZnCo–MOF, demonstrated exceptional electrocatalytic activity for the hydrogen evolution reaction (HER) under visible light irradiation. This was evidenced by the high H₂ evolution rate of 8583.4 g⁻¹.h⁻¹ and an apparent quantum yield (AQY) of 20.1% at a wavelength of 590 nm. Moreover, the catalyst maintained its performance over an extended period of 18 h during repeated cycling for H₂ production, without experiencing any degradation. The high photocatalytic performance of NiCoP@ZnCo–MOF for the HER can be attributed to the effective separation and transfer of charges at the n − n heterojunction, as well as the rapid dynamics of the H₂ evolution reaction facilitated by the reduction of activation energy through the presence of the NiCoP catalyst.

In another work undertaken by Yang and his research team [209], the researchers incorporated RuCu nanoparticles onto a BTC metal–organic framework (MOF) to augment the charge transfer capacity of the BTC MOF for the hydrogen evolution reaction (HER) in a 1 M KOH solution. The electrocatalyst was synthesized through the direct pyrolysis of the metal–organic framework Cu–BTC, which had been previously exchanged with Ru. The synthesized catalyst, designated Ru-Cu@C, exhibited remarkable performance in the hydrogen evolution reaction (HER) with a significantly low overpotential of 20 mV at a current density of 10 mA.cm⁻². It also demonstrated an exceptionally low Tafel slope of 37 mV.dec⁻¹, indicating efficient electrochemical kinetics. Additionally, the catalyst showed a small charge transfer resistance of 16.8 Ω, indicating rapid electron transfer. Moreover, it possessed a large electrochemically active surface area and exhibited excellent stability over time.

Gao et al. [210] developed ultrafine and well-dispersed CrPd nanoparticles on NH₂–MIL-101 by a straightforward sequential impregnation–reduction method. Due to the bimetallic synergistic effect of Cr and Pd and the strong metal-support interaction between the CrPd nanoparticles and MIL-101–NH₂ support, the CrPd–MIL-101–NH₂ catalyst demonstrated remarkable catalytic activity for the production of H₂ from formic acid. It achieved a TOF of 2009 mol H₂ mol Pd⁻¹ h⁻¹ at 323 K, surpassing the performance of other noble-metal heterogeneous catalysts reported for the HER under comparable conditions.

Nadeem and his team [211] succeeded in immobilizing three different bimetallic nanoparticles (PtNi, PtCu, and PtEr) onto N-doped porous carbon (PCN920) obtained through pyrolysis of a metal–organic framework. The synthesized electrocatalysts, PtNi@PCN920, PtCu@PCN920, and PtEr@PCN920, were employed for both electrocatalytic hydrogen evolution reaction and oxygen evolution reaction via LSV in 1 M KOH solution. PtNi@PCN920 showed a Tafel slope value of 82 mV.dec⁻¹ which is close and comparable with the standard 20% Pt/C (74 mV.dec⁻¹). The Tafel slope for PtNi@PCN920 was lower than PtEr@PCN920 (88 mVdec⁻¹) and PtCu@PCN920 (195 mV.dec⁻¹), as depicted in Figure 11. The catalyst’s overpotentials at a current density of –10 mA.cm⁻² were −42, −79, and −127 mV for PtNi@PCN920, PtEr@PCN920, and PtCu@PCN920, respectively. Similarly, the electrocatalysts PtNi@PCN920, PtEr@PCN920, and PtCu@PCN920 exhibited charge transfer resistances (R_ct) of 88 Ω, 106 Ω, and 225 Ω, respectively. Notably, PtNi@PCN920 demonstrated the most efficient electron transfer, as indicated by the lowest R_ct value of 88 Ω. The double-layer capacitance (CdI) values for the PtNi@PCN920, PtEr@PCN920, and PtCu@PCN920 catalysts were determined to be 5.5 mF.cm⁻², 5 mF.cm⁻², and 4.5 mF.cm⁻², respectively. The Cdl value of PtNi@PCN920 was found to be greater than that of PtEr@PCN920 and PtCu@PCN920, indicating a higher surface area for PtNi@PCN920 compared to the other catalysts. The catalysts exhibited a high degree of stability at 0.5 V for 9 h. A summary of these studies on bimetallic nanoparticle-based MOFs for photo/electrocatalytic HERs is given in

Table 4. Bimetallic nanoparticle–MOF-based photo/electrocatalysts for HERs.

Material	Electrolyte	Tafel Slope mV.dec−1	Current Density mA.cm−2	H2 Yield/Production Rate	TOF h−1	Ref.
PdAg–NH2–MIL-101(Cr)	Formic acid	-	-	144 mL/4.87 min	1475	[203]
CuPd–NPCC–EC	0.5 M H2SO4	28.2	0.03	-	-	[204]
NiMo@MIL-101	-	76	-	740.2 μmol.h−1	-	[205]
NiTi–NH2–MIL-125	-	-	-	699 μmol.g−1.h−1	-	[206]
AgNi–NC	1 M KOH	126.2	10	-	-	[207]
NiCoP@ZnCo–MOF	-	-	-	8583.4 μmol.g−1.h−1	-	[208]
RuCu@C	1 M KOH	37	10	-	-	[209]
CrPd/MIL-101–NH2	Formic acid	-	-	225 mL/7.5 min	2009	[210]
PtNi@PCN920	1 M KOH	82	10	-	-	[211]
PtEr@PCN920	1 M KOH	88	10	-	-	[211]
PtCu@PCN920	1 M KOH	195	10	-	-	[211]
Ag–AgMOF	-	-	-	1025 μmol.h−1.g−1	-	[212]

.

As illustrated in Table 2, the investigation of GO-based photo/electrocatalysts has been extensively conducted in acidic and alkaline media for hydrogen production, owing to their electron-rich properties and the enrichment of functional groups. However, the electrochemical applications of these materials have exhibited certain limitations due to their insulating characteristics resulting from oxygen functionalities present on the surface of the GO. Nevertheless, it has been noted that adding metal nanoparticles, metal sulfides, and various types of MOFs to graphene improves performance for HERs.

The synthetic, structural, and electrochemical characteristics of MOF materials, including HKUST-1, MILs, and ZIF MOFs, are given in Table 4. Due to their exceptional stability and distinctive semiconductor characteristics, MOF-based materials exhibit remarkable efficacy in the conversion of water/sulfuric acid into hydrogen energy. Nevertheless, the majority of these MOFs exhibit limited solubility in aqueous solutions, thereby hindering their widespread application in water electrolysis. Therefore, effective techniques to improve MOFs’ water stability can be used, such as adding hydrophobic substituents to the ligands. Regarding structural integrity, both MOFs and MOF composites retain their original structures even after being modified with nanoparticles or other guest molecules. Nonetheless, in order to increase the electrocatalytic performance of MOF-based materials for water-splitting systems, some novel approaches and/or challenges still require further study.

In recent times, MOFs have emerged rapidly established as electrocatalysts for HERs due to their extraordinary structures. MOFs are preferred as electrocatalysts because their pore structures and functions are modifiable and can be designed; therefore, it is straightforward to design and fabricate the active sites for HERs in these substances throughout the synthesis procedure. Nevertheless, most of the fabricated MOFs exhibit inadequate electronic conductivity, resulting in low efficiency of electron transfer and consequently hampered catalytic performance. MOF catalysts are still beset with numerous scientific and technical challenges that must be resolved prior to achieving commercial viability. Approaches such as fabricating electrocatalysts with a core–shell configuration, modifying electrocatalysts with amines, preparing materials with homojunction or heterojunction structures, preparing materials with hollow structures, and preparing surfaces rich in boron are effective strategies to improve the performance, stability, and longevity of HERs. It has been observed that the integration of MOFs with GO can significantly enhance electron transfer, which in turn has a substantial impact. Additionally, it is crucial to investigate the HER mechanisms, as well as the transfer and diffusion properties of GO/MOF-based electrocatalysts, as potential alternative routes for hydrogen production and storage technologies to meet growing energy demands.

It is apparent that considerable effort has been devoted to HERs, which have garnered international interest in the realm of efficient photo/electrocatalyst materials due to the pressing need to address the substantial demands for clean and sustainable energy, as well as the potential environmental risks associated with fossil fuels, which have rendered them unreliable.

3.5. Influence of Bimetallic Particle Size on Catalytic Performance

There is no doubt that bimetallic nanoparticles serve as efficient catalysts for HERs, but the concern here is the influence of the particle size on their electrocatalytic performance in HERs. Generally, the electronic or geometric structures of metals are altered when the particle size of metal nanoparticles is tuned. With the particle size decreasing, the proportion of the low-coordination surface sites (corners or edges) increases, which leads to enhanced catalytic activity [213].

Bimetallic nanoparticles with smaller particles usually exhibit a higher surface area-to-volume ratio compared to larger particles. This increased surface area offers more active sites for the adsorption and dissociation of hydrogen species, leading to improved catalytic activity [214]. The size of metal particles can influence their electronic structure. For bimetallic nanoparticles, the electronic properties of one metal can be altered by the presence of another metal. Changes in electronic structure can affect the adsorption energies of reaction intermediates and subsequently alter the reaction kinetics. Tuning the size of bimetallic particles allows for control over these electronic effects [215,216]. Moreover, small bimetallic particles may exhibit higher surface strain and more defects compared to larger particles. These surface defects can serve as active sites for hydrogen adsorption and facilitate the HER process [217]. Smaller particles tend to have higher surface energy and are more prone to agglomeration or sintering under reaction conditions. Controlling the particle size can help maintain the stability of the catalyst during prolonged operation [218,219]. The diffusion of reactants and products to and from the catalyst surface can be influenced by particle size. Mass transport is also facilitated by smaller particles owing to shorter diffusion pathways, thereby improving the reaction kinetics [220]. In bimetallic systems, the ratio of the two metals can affect the catalyst’s performance. By controlling the particle size, one can potentially optimize the distribution and interaction of the two metals on the nanoparticle surface, thereby enhancing the overall catalytic activity and selectivity [221].

Ma and co-workers [222] emphasized the effects of bimetallic particle size on electrocatalytic HER activity by synthesizing platinum nanoparticles of different particle sizes modified on CNTs via a photoreduction approach. The results showed that the 1.5 nm Pt catalyst demonstrated much higher HER performance than its counterparts in all pH solutions, and its mass activity was even 23–36 times that of Pt/C. The high HER performance of the 1.5 nm Pt catalyst was due to the strong metal–support interaction (SMSI) between Pt and the CNT matrix and the higher ratio of face sites to edge sites, which is meaningful for the design of efficient electrocatalysts for renewable energy application [222]. This effect can be seen clearly in Figure 12a–c. As the size of Pt nanoparticles decreases, the overpotentials required to reach a current density of 10 mA.cm⁻² (η₁₀) become lower. Moreover, the 1.5 nm Pt catalyst offered the smallest onset potential: approximately 1.3 mV versus RHE. The η₁₀ of the 1.5 nm Pt catalyst is only 13.0 mV, which is much smaller than those of 3 nm Pt (33.7 mV) and 6 nm Pt (38.7 mV) catalysts and even 6 mV lower than that of Pt/C (19.5 mV), showcasing its superior electrocatalytic HER activity, owing to its smaller particles. The stability of catalysts in acidic, neutral, and alkaline solutions after 10,000 cycles was also investigated, as depicted in Figure 12d–f, respectively. The η₁₀ of the 1.5 nm Pt catalyst reduced by 10, 6.0 and 21.6 mV, respectively, comparable to the 10.2, 22.3 and 8.1 mV of Pt/C, displaying its excellent HER stability in all media. Again, the 1.5 nm Pt catalyst offers the lowest Tafel value of 16.7 mV.dec⁻¹ (Figure 12g–i), which is even lower than that of Pt/C (19.6 mV.dec⁻¹), suggesting much faster HER kinetics of the former. In addition, the exchange current density (J₀) of the 1.5 nm Pt catalyst was higher than those of the 3 nm and 6 nm Pt catalysts in all pH electrolytes, implying the abundant electrocatalytic active sites of the former, leading to its excellent HER performance. A similar thread was observed for electrochemical double-layer capacitance (C_dl), where the 1.5 nm Pt catalyst had C_dl values of 5.25, 3.51, and 3.95 mF.cm⁻² in acidic, neutral, and alkaline solutions, respectively, much higher than those of its counterparts. Yan and his research group [223] also investigated the size effect of Pt nanoparticles supported on S-doped carbon material on HERs and found that ~1.5 nm Pt clusters had better HER activity than Pt single atoms, indicating that electron transfer played an important role in HERs.

In general, optimizing the size of bimetallic particles in catalysts for the HER requires a delicate balance between maximizing active site availability, maintaining stability, controlling electronic effects, and facilitating mass transport. It is important to know that when particle size decreases, the catalyst’s structural characteristics change, which affects the HER performance. Adjusting these parameters through precise synthesis and characterization techniques is important for fabricating efficient catalysts for hydrogen production.

Again, the efficacy of catalysts in the HER can be strongly impacted by the interactions between various metals and the metal–support interface. Strong metal–support interaction (SMSI) plays an important role in the catalytic process [224,225]. Factors such as the composition, morphology, doping, and surface modification of the support, as well as changes in the size or composition of the nanoparticles, can affect the SMSI, thereby affecting the catalytic performance [226]. The strong metal–support interaction (SMSI) between the metal and carrier causes the charge transfer and mass transport from the support to the metal [213]. Due to the unique pore structure of MOFs and the high surface area of GO and MOFs, bimetallic nanoparticles can be effectively anchored homogeneously.

The synergistic effects arise from the cooperative interactions between the metals and the support (GO or MOF), giving the catalyst unique combined properties, leading to an enhanced HER performance compared to monometallic or unsupported catalysts [224,227]. This interaction can also modulate the electronic structure of the catalyst by affecting the adsorption energies of reaction intermediates and subsequently influence the reaction kinetics of the HER [226].

The stability and durability of catalysts can also be improved through this interaction. Strong interactions can anchor the bimetallic nanoparticles onto the support, preventing their agglomeration or leaching during the reaction. This ensures the long-term stability of the catalyst under harsh reaction conditions, leading to sustained catalytic activity [226]. The interaction between bimetallic nanoparticles and the metal–support interface influences the dispersion of nanoparticles on the support surface. Proper dispersion is crucial for maximizing the exposure of active sites, thereby enhancing the catalytic activity. Additionally, the interaction can affect the accessibility of active sites to reactants, facilitating efficient HER performance [228]. The interaction between bimetallic nanoparticles and the metal–support interface creates a unique chemical environment at the catalyst surface that promotes specific reaction pathways in the HER, leading to improved selectivity and efficiency of the catalyst. The interaction can promote mass transport phenomena and charge transfer processes at the catalyst surface. These effects can modulate the kinetics of the HER and affect the availability of active sites for hydrogen evolution [229,230].

The interaction between bimetallic nanoparticles and the metal–support interface must be optimized to produce effective catalysts for the HER. Acquiring the appropriate interactions and qualities that result in improved catalytic performance entails carefully choosing the bimetallic components and the support material in addition to adjusting the synthesis conditions.

4. Conclusions and Future Prospects

In this review, we have thoroughly examined the state-of-the-art research on bimetallic nanoparticles supported on GO or MOF for photocatalytic and photoelectrocatalytic hydrogen production. The study provides insights into the complexities of their synthesis, structural properties, catalytic mechanisms, sacrificial electrolytes employed, and existing challenges. The integration of bimetallic nanoparticles with GO or MOF has proven to be a dynamic and promising avenue in the quest for efficient and sustainable hydrogen generation.

The synthesis methodologies discussed, ranging from microwave-assisted synthesis, mechanochemical synthesis, electrochemical route synthesis, solvo/hydrothermal synthesis, in situ growth method, to epitaxial growth method, underscore the versatility in tailoring the structural and compositional attributes of these hybrid materials. The intricate interplay between the bimetallic nanoparticle, GO, and MOF architecture has been elucidated, emphasizing the importance of optimizing these parameters to achieve superior catalytic activity and stability.

Despite the promising advancements, this review also highlights the existing challenges in hydrogen energy, such as hydrogen storage and production and the mitigation thereof, which can be accomplished through the fabrication of graphene-based metal–organic framework nanocomposites. The reaction mechanisms of HERs, the transfer and diffusion properties of reactants and products, and the effects of electrolytes on the catalytic performance of GO/MOF-based photoelectrocatalysts were identified in this review as alternative routes to study the hydrogen storage and production behavior of these materials. However, issues related to catalyst stability over prolonged operation periods, the development of cost-effective and scalable synthesis routes, and potential toxicity concerns associated with specific bimetallic compositions demand focused attention. Addressing these challenges will require interdisciplinary collaboration, combining expertise in materials science, chemistry, and engineering to propel the field forward.

In essence, this review acts as a guide for future research on the next frontier of photocatalytic and photoelectrocatalytic hydrogen production using bimetallic nanoparticle–GO/MOF systems. Through continued exploration, innovation, and collaboration, these hybrid materials hold the key to unlocking the full potential of renewable energy technologies, propelling us towards an environmentally friendly and carbon-reduced era.