Cutting-edge techniques in low-temperature electrochemical water splitting: advancements in hydrogen production

4.1.1 The concept

The decoupled water electrolysis concept was proposed to overcome several disadvantages of classical cell electrolyzers, where the HER and OER happened simultaneously (Figure 3a), possibly lead to H₂/O₂ crossover, even if an ostensibly gas impermeable membrane (or diaphragm). ²¹ Thus, scenario is particularly high probable at low current densities or under high gas pressure electrolysis. ⁵⁰ This problem could touch the H₂ purity, in addition to the risk of creating of an extremely explosive or oxidative environment due to the coexistence H₂, O₂ and electrocatalysts under favorable electrocatalytic conditions, which pose safety concerns, degrade the electrolyzer and shorten its operation lifespan. ^{4 , 51 , 52} These issues call for alternative electrolyzer designs, not only avoiding the crossover of H₂/O₂ gases but also allowing suppleness in controlling electrolysis products.

Figure 3:

Comparison of traditional ‘coupled’ electrolysis and decoupled electrolysis for water splitting: (a) Conventional water electrolysis in acidic conditions. (b) Decoupled oxygen generation and mediator reduction. (c) Decoupled mediator reoxidation and hydrogen evolution. In panels (b) and (c), the circle denotes the oxidized form of the decoupling agent “medaitor”, while the diamond represents the reduced form of this decoupling agent. From. ⁵⁴

Figure 3b and c presents schematic illustrations of a decoupled water electrolyzer. In this system, during the HER at the cathode, a redox mediator (dissolved in the electrolyte) undergoes oxidation at the anode instead of the OER. This oxidized redox mediator is then reduced back to its initial state at the same electrode, while the OER is coupled to another working electrode. Through the alternation of these steps, hydrogen and oxygen are generated sequentially. This method allows for the temporal and/or spatial separation of HER and OER, facilitated by redox mediators. ²¹ Essentially, the redox mediator serves as a reversible electron and/or proton donor/acceptor. ²¹ When the redox mediator can simultaneously accept or donate both protons and electrons, it can act as a pH buffer during electrolysis and is referred to as an electron coupled-proton buffer, ECPB. ⁵³

Mediators can be categorized into three main types: soluble, dispersed, and solid-state decoupling agents. ⁵⁴ An example of a dissolved mediator is the polyoxometalate Li₆[P₂W₁₈O₆₂], which has the remarkable capacity to reversibly accept up to 18 electrons in water media. Transition metal salts, such as those involving the V(III)/V(II) and Ce(III)/Ce(IV) redox couples, ⁵⁵ as well as complexes like Fe(CN)₆³⁻/Fe(CN)₆⁴⁻, ⁵⁶ are also commonly used in this category. Additionally, organic systems like hydroquinone sulfonate and anthraquinone-2,7-disulfonic acid serve as dissolved mediators. ^{57 , 58} Dispersed mediators, on the other hand, involve particulate systems dispersed in a solution containing a redox couple. For instance, Zhao et al. ⁵⁹ utilized BiVO₄ particles dispersed in an aqueous medium comprising the Fe(III)/Fe(II) redox pair. Upon solar light radiation, BiVO₄ particles oxidize water to generate O₂ while reducing Fe(III) to Fe(II) at the same time. Solid-state mediators, exemplified by NiOOH/Ni(OH)₂, ⁶⁰ are integrated with electrodes and operate without the need for solubilization. These solid mediators facilitate electron transfer reactions directly at the electrode surface. The choice of mediator phase significantly influences the physical components and operation of the system. Indeed, soluble redox mediator systems can operate under static or flow conditions, as the mediator can be pumped between chambers. Conversely, in systems with solid-state mediators, the decoupling agent is typically integrated with an electrode that can be moved between cells to interact with either the HER or OER. Detailed analysis of key studies in this area will be discussed further.

The success of redox mediator-assisted decoupled water electrolysis hinges largely on the electrochemical properties of the redox mediator. Several crucial criteria must be considered when selecting suitable candidates for redox mediators: ⁶¹

1. High solubility in aqueous media (of soluble mediators),

2. Rapid and reversible electron transfer between the redox mediator and the inert electrode, ensuring that the pH of the reaction medium remains unchanged throughout both the OER and HER steps.

3. Well-positioned redox potential between the onset HER and OER potentials,

4. Excellent durability over a large number of redox cycles,

5. Economical composition and synthesis.

4.1.2 Decoupling water splitting systems

Several innovative strategies for nonconventional water splitting have been summarized by You and Sun, ¹⁸ wherein H₂ production from water splitting is integrated with other oxidation reactions of greater utility than O₂ evolution. Additionally, Liu et al. ⁶¹ Paul and Symes, ⁵⁴ Wallace and Symes ⁶² have provided valuable mini-reviews on decoupled water splitting agents, highlighting recent progress in the field. More recently, McHugh et al. ²¹ reported a comprehensive review article focusing on redox mediators for water splitting. Table 4 provides an overview of various decoupling agents along with their key performance metrics, demonstrating their effectiveness in electrochemical water splitting. Readers interested in more detailed information are encouraged to refer to the provided references for further insights.

Symes and Cronin achieved the initial practical implementation of decoupled oxygen and hydrogen evolution through electrolytic water splitting in 2013. ⁵³ They employed the polyoxometalate phosphomolybdic acid (H₃PMo₁₂O₄₀) to separate the OER from the HER under acidic conditions, as described by the following equations:

Therefore, during the oxidation of water to produce O₂, protons, and electrons, the polyoxometalate underwent reduction, forming H₅PMo₁₂O₄₀. Upon subsequent reoxidation, H₅PMo₁₂O₄₀ was converted back to H₃PMo₁₂O₄₀, releasing protons and electrons which were utilized to generate hydrogen at the cathode. Subsequently, the same researchers established a range of mediators (organic redox), such as 1,4-hydroquinone derivatives and anthraquinone-2,7-disulfonic acid. ^{57 , 58} Additionally, Symes’s and Cronin’s groups investigated other inorganic polyoxometalate complexes, including phosphotungstic acid (PTA) and silicotungstic acid (STA), as alternative redox mediators with more negative redox potentials than the HER onset potentials of the catalysts. ^{63 , 64} Unlike previous redox mediator systems, which required two electrochemically driven steps, PTA and STA mediators are initially reduced (electrochemically) and accept protons during the OER. In the subsequent non-electrochemical step, the reduced PTA or STA evolve hydrogen spontaneously when making contact with Pt, Ni₂P, Ni₅P4, or Mo₂C catalysts without additional energy input, regenerating the oxidized PTA or STA, as the HER onset potentials (on those catalysts) are more positive than the redox potentials of the reduced from of STA and PTA. ^{62 , 63} This approach also accomplished decoupled HER and OER.

After Symes’s and Cronin’s works, various types of decoupled systems/setups were proposed. ²¹ In 2014, Amstutz et al. ⁵⁵ introduced a novel vanadium- and cerium-based anolyte system for decoupled electrolysis. The adopted device configuration (Figure 4a) allowed for both conventional operation as a redox flow battery and the conversion of energy to hydrogen, offering large flexibility for renewable energy usage. They utilized a V(III) salt in sulfuric acid, in which V(III) is reduced to V(II) on a graphite felt electrode:

Figure 4:

Schematic representation of a sample of experimental setups utilized by researchers for decoupled water electrolysis. From. ⁵⁴

The position of the V(III)/V(II) redox couple (−0.26 V vs SHE) is more negative than the proton reduction couple, allowing for spontaneous hydrogen generation when the V(II) solution is exposed to hydrogen evolution catalysts. The HER was achieved over an Mo₂C catalyst with a Faradaic yield of up to 96 %. At the anode, the oxidation of an acidic solution of Ce(III) to Ce(IV) took place:

The position of Ce(III)/Ce(IV) redox under these circumstances is more anodic than that for water oxidation. This means that Ce(IV) can oxidize water to generate oxygen, becoming reduced back to Ce(III) in the process. This catalytic water oxidation reaction was achieved over RuO₂ nanoparticles, although the Faradaic yield for the OER was somewhat less than optimal (78 % ± 8 %) due to a side reaction involving degradation of the RuO₂ catalyst by Ce(IV). The authors suggested transitioning to an all-vanadium system to mitigate this issue, where the reduction of V(V) to V(IV) would mediate the OER.

Ho et al. ⁵⁵ introduced a similar combined chemical/electrochemical system for hydrogen production (Figure 4b). A photoelectrochemical cell was employed to reduce V(III) to V(II) while performing OER at the anode. The V(II) solution was directed to a flask containing a Mo₂C catalyst for on-demand H₂ evolution. A bipolar membrane maintained a significant pH differential between the anode and cathode compartments, crucial for the system’s operation. The dual electrolyte system comprised an anodic compartment with a Ni mesh oxygen evolution electrode in KOH concentrated solution and a cathodic compartment with a Pt foil electrode in sulfuric acid. At the Ni anode, OH⁻ was oxidized, producing electrons, water, and oxygen. These electrons reduced V(III) ions to V(II), and the charged V(II) solution was transferred to a detached reactor for hydrogen production. The study investigated hydrogen production efficiency at various depths of charge of the vanadium solution, showing Faradaic efficiencies ranging from 83 % to 87 %. Additionally, the system demonstrated hydrogen generation under elevated pressures, with efficiencies approaching 60 % at pressures of up to ten atmospheres. Furthermore, the authors showcased the system’s direct powering by renewable energy through coupling it to a photovoltaic module. Hydrogen was generated with a Faradaic efficiency of 80 %, under solar irradiation, achieving a solar-to-hydrogen energy conversion efficiency of 5.80 %.

Goodwin and Walsh ⁵⁶ presented a novel method to spatially decouple the OER/HER using the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple and a closed bipolar electrode. In their setup (Figure 4b), they utilized carbon cloths connected by a wire as the closed bipolar electrode, bridging between two separate two-compartment cells. Each cell contained a solution of K₃Fe(CN)₆/K₄Fe(CN)₆ in 0.1 m KOH, with one end of the closed bipolar electrode submerged in each solution. The device operated by facilitating water oxidation at a Pt electrode coupled with the reduction of Fe(CN)₆³⁻ at one end of the closed bipolar electrode in one cell. In the other cell, oxidation of Fe(CN)₆⁴⁻ occurred at the opposite end of the closed bipolar electrode, accompanied by hydrogen evolution at the second platinum electrode.

Li et al. ⁵⁰ investigated the use of ferrocene derivatives, specifically (ferrocenylmethyl)trimethylammonium chloride (FcNCl), as a decoupling agent to separate the HER and OER in a two-compartment H-cell setup (Figure 4d). One compartment contained a carbon counter electrode in a solution of FcNCl, while the other compartment housed electrodes for the HER and OER. The system enabled temporally separated HER and OER to take place, demonstrating stability over 20 redox cycles without significant pH variation. Furthermore, the study highlighted the potential for sunlight-driven hydrogen production, achieving currents of approximately 20 mA/cm² for the HER using a photovoltaic module under solar irradiation.

Zhao et al. ⁵⁹ proposed a more direct decoupled solar-to-hydrogen system, where sunlight activates BiVO₄ particles dispersed in an aqueous solution containing the Fe(III)/Fe(II) redox couple. Under solar irradiation, BiVO₄ particles oxidize water to produce oxygen while simultaneously reducing Fe(III) to Fe(II). The Fe(II) ions can then be re-oxidized to Fe(III) in an electrolytic cell, generating hydrogen at the cathode with 100 % Faradaic efficiency. Notably, this process operates at cell potentials significantly lower than conventional methods. Although conceptually similar to previous solar-to-hydrogen systems, Zhao et al.’s work achieved remarkable solar-to-hydrogen efficiencies of around 2 %, a substantial improvement over earlier studies.

Chen et al. ⁶⁰ proposed a novel approach to decoupled systems where the electrode served as the redox mediator. In their single-compartment setup, a nickel hydroxide (Ni(OH)₂) electrode underwent oxidation at the cathode, facilitating the HER. The electrode could then be regenerated through reduction coordinated with the OER at the anode. Similarly, Landman et al. ^{65 , 66} proposed a similar system but implemented it in two separate chambers using photoelectrochemistry. Ma et al. ⁶⁷ also proposed a comparable system utilizing polytriphenylamine (PTPAn) as the electrode material. They demonstrated that this system achieved near quantitative Faradaic efficiencies for H₂ and O₂ production, with a photovoltaic-driven setup exhibiting an excellent overall solar-to-hydrogen conversion efficiency of 5.4 %. Additionally, Dotan et al. ⁶⁸ presented a decoupled system involving Ni(OH)₂, where the oxidation of Ni(OH)₂ was coordinated with H₂ production, and the electrode was regenerated through introducing a heated solution to the electrochemical cell.

These examples showcase the potential of decoupled water splitting systems, although they currently lack the capability to generate electricity in one of their decoupled cycles, hindering their function as a dual-use H₂ generation/electricity storage system. An example of such a system described by Bienvenu ⁶⁹ involved the reduction of Zn²⁺ ions to Zn⁰ at the cathode during OER at the anode. Upon polarity switching, the Zn⁰ electrode underwent spontaneous oxidation, resulting in the generation of both electricity and hydrogen at the cathode.

In conclusion, decoupled water electrolysis systems represent a promising solution to mitigate the challenges associated with traditional electrolyzers, offering efficient hydrogen production while avoiding gas crossover and ensuring safety. These systems, facilitated by various redox mediators and innovative configurations, demonstrate significant potential for advancing clean hydrogen generation technologies. However, further research is needed to address limitations and enhance the scalability and dual-use capabilities of decoupled systems for broader applications in renewable energy and beyond.

4.2.1 Reagent-sacrificing type

Hydrazine and ammonia electrooxidation are typical examples of this category. ²³ This system involves the anodic conversion of hydrazine and ammonia, both commercially valuable substrates, into non valuable products like N₂ (Eqs. (19) and (20)).

Crucially, the thermodynamic potential of these reactions, i.e., −0.38 V for reaction (19), is markedly lower than that of the OER (1.23 V), resulting in a significant decrease in the overall cell voltage needed for water splitting. ²³ In many cases, hydrazine oxidation reaction (HzOR) enabled the attainment of a current density of up to 500 mA/cm² at a cell voltage of 1 V without generating O₂. Additionally ^{75 , 82 , 83} , 10 mA/cm² could be achieved at a microvoltage level in the presence of hydrazine. ²³ Bifunctional electrocatalysts have been developed to facilitate the HER coupled with hydrazine oxidation, further enhancing the efficiency of hydrogen generation. the addition of 0.02 M hydrazine to a 1 M KOH solution reduced the required voltage for reaching 15 mA/cm² from 1.97 V to 0.63 V, utilizing the bifunctional Ni@Pd–Ni alloy nanosheets as electrodes. ⁸⁴ Sun et al. ⁸⁵ reported that CoS₂/TiM nanoarrays and Ni₂P/NF catalysts enable efficient hydrogen evolution at relatively low cell voltages, with the former achieving hydrogen evolution with only a cell voltage of 0.81 V to achieve 100 mA/cm² (pure water splitting required 1.89 V to achieve this current density). Furthermore, Ni₂P/NF catalysts enable H₂ production with 1.0 V at 500 mA/cm², exhibiting remarkable durability and highly Faradaic efficiency for H₂ generation. ⁸² Moreover, Tang et al. ⁸⁶ introduced a porous nitrogen-doped carbon material encapsulating cobalt/LaCoO_x hybrid nanoparticles (Co/LaCoO_x@N–C) as a bifunctional catalyst for both HER and HzOR. This catalyst demonstrated remarkable electrochemical activity in HzOR, achieving 69.2 mA/cm² at only 0.3 V. Moreover, the development of direct hydrazine fuel cells (DHFCs) offers a promising avenue for sustainable hydrogen generation. In these systems, hydrazine serves as the feedstock, leveraging its high energy density and utilization rate. ^{87 , 88}

Durable bifunctional electrocatalysts like P,W co-doped Co₃N nanowire on NF (PW–Co₃N NWA/NF) demonstrate remarkable electrochemical performance in hydrazine splitting, facilitating efficient hydrogen production with minimal energy consumption. ⁸⁷ PW-Co₃N NWA/NF achieves hydrogen evolution with an exceptionally low cell voltage of 28 mV to reach 10 mA/cm², outperforming overall water splitting. ⁸⁷ The oxidation of hydrazine can sustain a current density of 200 mA/cm² over an overpotential of 607 mV, underscoring the enhanced energy efficiency of hydrazine oxidation coupled with HER for H₂ production. ⁸⁷ Furthermore, the integration of direct hydrazine fuel cells with overall hydrazine-splitting units (using Fe-doped CoS₂ nanosheets as electrocatalysts for both the HER and HzOR) in a self-powered system demonstrates the feasibility of efficient hydrogen evolution without external power input, achieving an H₂ evolution rate of 9.95 mmol/h, a 98 % Faradaic efficiency, and a 20-h stability (electrode:3D nanoporous NixCo1-xSe nanorod array). ⁸³

Similar to hydrazine oxidation, the oxidation of ammonia (Eq. (20)) offers a thermodynamically more favorable alternative to OER. Ammonia, being more ubiquitous and easily accessible compared to hydrazine, holds greater promise as a replacement for OER. ²³ While Pt has traditionally been recognized as the most effective catalyst for ammonia oxidation, ^{23 , 89} its high cost and scarcity have spurred the exploration of non-noble metal-based alternatives. Mei et al. ⁹⁰ introduced a catalyst consisting of N-doped NiZnCu layered double hydroxides (LDH) with reduced graphene oxide on nickel foam (N-LDH/rGO). In a two-electrode system utilizing the prepared NiZnCu LDH as both anode and cathode, 50 mA/cm² was achieved at 0.769 V, surpassing the noble-metal system where Pt/C served as the cathode and IrO₂ served as the anode by 0.170 V. Furthermore, this two-electrode system maintained the current density for at least 18 h. Further studies reported in the literature highlight the advantageous use of ammonia in advancing HER using various synthesized electrocatalysts for the cathode material, such as Ni(OH)₂–Cu₂O, N–NiZnCu LDH, Ni₅₀Cu₅₀/C, among others. ^{91 , 92 , 93}

Overall, despite the added expense associated with sacrificial reagents, their utilization ensures stable water electrolysis at high current densities without O₂ production, rendering them an optimal choice for hybrid water electrolysis systems.

4.2.2 Pollutant-degrading type

Sacrificial reagents tend to increase the overall system cost compared to the value of the starting substrate (such as hydrazine or ammonia), despite the potential reduction in cell voltage. To address this challenge, these substrates could be substituted with environmental pollutants, offering opportunities for oxidative degradation (remediation) while also advancing HER. When integrated with water electrolysis, this process of anode oxidation, resembling waste treatment, is referred to as the pollutant-degrading type. ²³

A typical example is Urea (CO(NH₂)₂), commonly found in industrial wastewater and domestic sewage, although organic dyes are also viable alternatives. Combining the urea oxidation reaction (UOR) with water electrolysis can achieve ecological preservation and clean energy production by efficiently generating H₂ and purifying urea-rich wastewater or urine (Gnana Kumar et al., 2020). The UOR (Eq. (21)) coupled with HER (Eq. (6)) in alkaline electrolysis results in the overall reaction depicted in Eq. (22): ⁹⁴

Reaction (21) has undergone extensive examination across various cathode and anode materials. The overall cell potential required to attain 10 mA/cm² (V₁₀) fluctuates depending on the specific anode/cathode pairing and the concentration of urea. This variability is demonstrated below:

1. V₁₀ = 1.41 V for MnO₂/CoP_x couple using 1.0 M KOH with 0.5 M CO(NH₂)₂, ⁹⁵

2. V₁₀ = 1.38 V for Ni₃Se₄/CoP_x couple using 1.0 M KOH with 0.1 M CO(NH₂)₂, ⁹⁶

3. V₁₀ = 1.305 V for N–NiZnCu LDH/N–NiZnCu LDH couple using 1.0 M KOH with 0.3 M CO(NH₂)₂, ⁹⁰

4. V₁₀ = 1.4 V for CoP/C/CoP/C couple using 1.0 M KOH with 0.1 M CO(NH₂)₂, ⁹⁷

5. V₁₀ = 0.89 V for N–CoP/N–CoP couple using 0.5 M H₂SO₄, 0.96 M FeSO₄/0.74 M Fe₃(SO4)₂. ⁸⁰

In all of these systems, successful degradation of urea has been accomplished. Research into the mechanism and kinetics of UOR on Ni-based electrocatalysts in an alkaline medium indicates that urea oxidation takes place subsequent to the formation of NiOOH under electrooxidation conditions. ²² NiOOH then interacts with urea molecules, replenishing active sites for subsequent adsorption/desorption of intermediates. ²²

In the other hand, Zhang et al. ⁹⁸ explored the combination of HER with the decomposition of methylene blue, a textile dye. They utilized FeNiP nano-sheets coated with a carbon shell as the cathode catalyst. The oxidation was observed at approximately 1.2 V across different concentrations of methylene blue. Due to the excellent protection and conductivity supplied by the carbon shell, the catalyst exhibited an overpotential of 104 mV to achieve a current density of 10 mA/cm², which remained stable even after 1,000 cycles of CV scans. Additionally, under a constant current density of 1 mA/cm², entire degradation of methylene blue could be achieved in 53 min, where the corresponding potential remained steady for over 2,500 s.

In conclusion, considering the ongoing escalation of environmental pollution, the coupling of HER with pollutant degradation undeniably holds promising prospects for the future. However, since real-world pollutants are much more complex, further research efforts should be directed towards realizing real hybrid electrolysis under practical scenarios.

4.2.3 Value-added type

Table 5 provides an overview of recent electrooxidation of organic (EOO) reactions considered value-added. Briefly, small alcohols undergo anodic partial oxidation, yielding aldehydes/ketones in a 2e⁻ process or carboxylic acids in a 4e⁻ process, while aldehydes can be oxidized to carboxylic acids. Oxidative coupling of two alcohols enables ester production, exemplified by ethyl acetate from ethanol. Ring-opening reactions, like adipic acid production from cyclohexanone/cyclohexanol, are also feasible. ⁹⁹ Larger biomass-derived compounds, such as 5-hydroxymethylfurfural (HMF) or glycerol, yield various products based on reaction conditions and catalysts. Glycol oxidation, for instance, yields multiple commercially relevant products, including formic acid, lactic acid, and carbon dioxide. ^{100 , 101} Alkenes enable the synthesis of epoxides, diols, and higher oxidized species, while amines can be oxidized to nitriles via dehydrogenation or coupled to form diazo compounds. Some detailed analyses oof the results were discussed in the subsequent sections.

Hydrogen sulfide (H₂S), a notorious environmental contaminant, can be recovered as elemental sulfur, which has diverse applications like pesticide manufacturing and lithium-sulfur batteries. ⁷⁹ Integrating the sulfide oxidation reaction (SOR) with the HER in hybrid water electrolysis systems offers a more favorable theoretical voltage (Figure 5) and faster reaction kinetics than traditional water electrolysis, leading to reduced energy consumption. ¹⁰²

Besides elemental sulfur, SOR can yield sulfites, thiosulfates, and sulfates. However, sulfur’s insolubility water leads to deposition on the anode, forming a high-resistivity passivating layer (∼10¹⁵ Ω/m). ⁷⁹ While highly alkaline electrolytes may mitigate this, a more effective approach involves preventing sulfur deposition through interface engineering. Zhang et al. ¹⁰³ introduced a “self-cleaning” concept, developing a hydrophobic NiS₂ catalyst that resists sulfur species deposition, enabling prolonged desulfurization (100 h at 20 mA/cm²). Zhang et al. ¹⁰⁴ proposed “chainmail catalysts”, constructing a graphene-encapsulated CoNi alloy catalyst (CoNi@NGs) preventing direct metal-sulfur contact. This catalyst catalyzed sulfide oxidation via graphene shell-induced electron transfer, exhibiting high activity over 500 h (30 mA/cm²). Various electrocatalysts were also developed by various researchers, showing variable stabilities (e.g., Ni–Co–C: 64 h at 100 mA/cm², ¹⁰² CoS₂@C/MXene: 180 h at 30 mA/cm², ¹⁰⁵ WS₂: 14 h at 100 mA/cm^{2 106} ).

Crude glycerol, a by-product constituting 10 wt% of the biodiesel production process, poses a significant oversupply issue, with global annual production surpassing 5 billion liters. ⁷⁹ Anodic oxidation within hybrid water electrolysis systems not only converts glycerol into valued chemicals but also concurrently generates hydrogen at the cathode, offering notable carbon-neutral advantages. ¹⁰⁷ Glycerol’s susceptibility to nucleophilic attack, owing to its three adjacent hydroxyl groups, results in a spectrum of oxidation products ranging from C₁ to C₃, including formic acid, lactic acid, glycolic acid, and dihydroxyacetone. Oxidizing glycerol to formic acid is favored due to its potential to minimize byproduct formation. The theoretical oxidation potential for glycerol to formic acid (Eq. 24, 0.69 V) is significantly lower than that of the OER (Figure 5). However, efficient catalysts are crucial for this conversion, with nickel compounds garnering considerable attention. ⁷⁹

Li et al. ¹⁰⁸ introduced a carbon fiber cloth-supported NiMoN nanoplate catalyst, requiring only 1.36 V voltage in a hybrid water electrolysis system at 10 mA/cm² and achieving a remarkable Faradaic efficiency of 95 % for formic acid production. Zhu et al. ¹⁰⁹ proposed a Ni₃N/Co₃N heterogeneous nanowire catalyst, achieving 1 A/cm² at a voltage of 2.01 V and realizing an impressive Faradaic efficiency of 94.6 % toward formate production. Tran et al. ¹¹⁰ explored the performance of glycerol oxidation under various crystalline MnO₂ catalysts, i.e., α-, β-, and γ-MnO₂. The three phases showed selectivity exceeding 90 % for the C₃ product, yet γ-MnO₂ demonstrated superior stability and current density compared to the other phases.

The cellulosic waste conversion into individual components, such as glucose, furfural, oligosaccharides, and 5-hydroxymethylfurfural (HMF), through hydrolysis processes holds promise for waste valorization. ¹¹¹ HMF serves as a crucial platform chemical facilitating the subsequent conversion of cellulose waste into several valuable products, including 2,5-diformylfuran (DFF), 5-formyl-2-furancarboxylic acid (FFCA), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), and 2,5-furandicarboxylic acid (FDCA). ¹¹² The theoretical voltage necessary for the oxidation of HMF to FDCA is merely 0.30 V, as shown in Eq. (25), significantly lower than conventional water electrolysis (1.23 V), enabling resource recovery and hydrogen production with low energy consumption.

Grabowski et al. achieved a 71 % yield of FDCA through the HMF electrochemical oxidation in a 1.0 M NaOH solution using nickel oxide/hydroxide catalysts. ¹¹³ Nickel-based catalysts have demonstrated efficacy in oxidizing HMF through an electrochemical-chemical coupling mechanism. ¹¹³

In conclusion, the integration of the HER with anodic electrooxidation of organics significantly reduces the cell voltage required for water splitting, leading to improved hydrogen production. Despite these advantages and recent advancements, this coupling technology is in its nascent stage, and its transformation efficiency remains inadequate. Therefore, the development of high-performance electrodes, precise identification of active sites, and strategic construction of the electrolyzer are crucial for future industrial applications.

4.2.4 Perspectives

The integration of the HER with anodic electrooxidation of organics significantly reduces the cell voltage required for water splitting, leading to improved hydrogen production. Despite these advantages and recent advancements, this coupling technology is in its nascent stage, and its transformation efficiency remains inadequate. To advance hybrid water electrolysis further, several critical aspects must be addressed to realize commercial and large-scale applicationss: ^{22 , 79}

1. Accelerating catalyst discovery: Expediting the discovery of high-performance catalysts is imperative. For this regard, utilizing DFT calculations and machine learning can streamline catalyst design and enhance our comprehension of the structure-performance relationship. This approach holds potential for developing catalysts with superior activity and selectivity.

2. Employing advanced characterization Techniques: Integration of advanced in situ techniques and analytical methods is essential for comprehending the dynamic behavior of catalysts during electrochemical process. Techniques such as X-ray absorption spectroscopy and Raman spectroscopy facilitate real-time monitoring of reaction intermediates and catalyst structure evolution. This deeper understanding can inform the design of high-performance catalysts for efficient hybrid water splitting.

3. Developing bifunctional catalysts: The development of bifunctional catalysts based on earth-abundant elements is crucial for cost-effective and practical applications of hybrid water splitting. These catalysts can simultaneously facilitate multiple oxidation reactions, including biomass and pollutant oxidation, while efficiently harvesting hydrogen with minimal energy consumption. They offer a simplified and economically viable solution for large-scale hydrogen production.

4. Advancing membrane-based systems: Implementation of membrane-based systems is indispensable for practical HWE, enabling the efficient separation of anode and cathode products. Selecting membranes based on the applied electrolyte ensures optimal product separation and overall system performance. These membrane-based systems are pivotal for realizing large-scale and commercial application of HWE technology.

5. Integrating with solar systems: Integrating electrolyzers with solar systems represents a key strategy for further reducing energy costs in hybrid water electrolysis. By leveraging solar energy, the electrolysis process becomes more sustainable and economically viable for large-scale implementation.

4.4.1 Reactor configurations

Diverse reactor configurations and materials have been explored under different operational conditions, with single-chambered and two-chambered setups being the most prevalent (Table 6). ²⁵ External voltage necessary for the process can be sourced from conventional power supplies or renewable energy, with solar energy standing out as the most promising, cleanest, efficient, and cost-effective option. When the process is conducted under solar illumination, it is referred to as solar-assisted MECs. ¹¹⁷

Initial research on MECs focused on H-type two-chambered configurations, typically constructed from glass or various plastics and separated by a membrane (Figure 7b). ^{125 , 126 , 127} Commonly used membranes include proton exchange membrane (PEM), anion-exchange membrane (AEM), cation exchange membrane (CEM), and bipolar membranes. ¹¹⁹ In these systems, microorganisms in the anodic chamber, known as exoelectrogens, oxidize organic matter to release electrons. These electrons flow through an external circuit to the cathodic chamber, where hydrogen gas is produced through reduction reactions. The membrane facilitates proton transfer while maintaining gas separation to preserve hydrogen purity. Two-chambered MECs typically achieve around 90 % substrate degradation in the anodic chamber and hydrogen production efficiency ranging from 60 % to 75 %. For instance, an H-type MEC fed with acetate generated 1.19 mmol of hydrogen at 0.540 V, ¹²⁵ while a solar-assisted dual-chambered MEC exhibited faster hydrogen production with a Pt-coated electrode at 0.7 V. ¹²⁸ Besides, higher H₂ levels of up to 50 m³/m³/d with acetate under certain conditions, have been recorded in dual-chambered MECs (Table 6). Nevertheless, the operation of dual-chambered MECs is prone to energy losses attributed to factors like Ohmic loss, activation loss, and concentration loss. ¹²⁶ Furthermore, the membrane itself contributes to an overpotential. Rozendal et al. ¹²⁹ found that concentration energy loss could reach 0.38 V due to differences in concentration and pH gradient between the two chambers. However, hydrogen production using dual-chambered MECs may achieve higher levels, up to 50 m³/m³/d with acetate under certain conditions. The calculated cathodic hydrogen recovery and energy input for different cell designs, substrates, and operating voltages are summarized in Table 6.

In contrast, single-chambered MECs lack a membrane between the anode and cathode (Figure 7a), making them compact and cost-effective. These systems offer lower internal resistance and overpotential due to the absence of a membrane. Stacked systems with anodes surrounding the cathode further minimize resistance. ^{130 , 131} However, a major challenge with single-chambered MECs is methane interference from methanogens, compromising hydrogen purity. ^{129 , 132} Studies comparing mixed culture and pure culture in single-chambered MECs observed methane co-production in mixed cultures, negatively impacting hydrogen production rates. ^{132 , 133} Nevertheless, some studies have demonstrated high hydrogen recovery and production rates in single-chamber systems without membranes. ¹³⁴ Studies on single-chambered MECs have reported hydrogen production rates up to 3.12 m³/m³/d under different operating conditions and feed compositions (Table 6). Despite achieving higher production rates, single-chambered MECs exhibit lower purity due to methane presence. ^{134 , 135} Suppression of methanogens in MEC reactors could enhance hydrogen purity, further improving bio-hydrogen quality. ¹³⁵

4.4.2 Electrode materials

The effectiveness of MECs technology relies on the selection of suitable and cost-effective electrode materials, with carbon-based materials being the most common choices. ¹³⁶ These electrodes could enable microbial organisms (MOs) to form biofilms, facilitating crucial electron transfer from the anode to the cathode for hydrogen production. ^{136 , 137} Anode materials must meet specific criteria to support biofilm formation and efficient electron transfer. Graphite-based materials like granular graphite beds, granules, graphite felt, and brushes have shown high performance and efficiency due to their porosity and surface area. ²⁵ However, maintaining surface area while preventing clogging and biofouling poses a challenge. ¹³⁷ Various electrode configurations, such as porous and plate types, offer advantages in conductivity, cost, and stability, but increasing their surface area for improved HER presents difficulties. ¹³⁸ A list of adopted electrode materials used for MECs is shown in Table 7 and Figure 8. Further details are given in subsequent sections, with additional information can been retrieved in Ref. ^{25 , 117 , 119 , 123 , 139 , 140 .}

Figure 8:

Various conventional and composite electrode types: a) Graphite rods, b) graphite felt, c) carbon cloth, d) carbon fiber brush, e) carbon paper, f) platinized wet-proof carbon cloth, g) stainless steel mesh, h) nickel mesh, i) carbon nanotubes, j) bioanode or biocathode, k) graphene. Adopted from. ²⁵

4.4.3 Microbial populations: exoelectrogens

Extensive research on exoelectrogens utilized in MECs has been comprehensively reviewed by Kadier et al., ¹³⁹ Liu et al. ¹⁴⁰ and Parkhey and Gupta. ¹²³ Among the exoelectrogenic bacteria for hydrogen production, Shewanella oneidensis and Shewanella putrefaciens from the Gammaproteobacteria family, and Geobacter sulfurreducens from the Deltaproteobacteria family are frequently reported. ¹³⁹ Additionally, bacteria from other genetic groups such as Ochrobactrum and Rhodopseudomonas from Alphaproteobacteria, Arcobacter from Epsilonproteobacteria, Rhodoferax from Betaproteobacteria, Clostridium from Firmicutes, Propionibacterium from Actinobacteria, and Geothrix from Acidobacteria have also shown anodic respiratory capabilities. ¹³⁹

These anode-respiring bacteria (ARB) employ one of three mechanisms for extracellular electron transfer: ^{140 , 157} (i) a direct contact, where exoelectrogens transfer electrons through their outer membrane directly to the anode-contacting region, ¹⁵⁷ (ii) utilizing soluble electron shuttles such as phenazines (e.g., S. oneidensis MR 1), quinones (e.g., S. putrefaciens), and flavins (e.g., Shewanella), ^{158 , 159 , 160} and (iii) via nanowires or connecting appendages, which is commonly observed in Shewanella and Geobacter. ¹⁶¹ Geobacter species, in particular, have been extensively studied for electron transfer in bioelectrochemical systems, and their dominance in MEC reactors has been well demonstrated. ^{162 , 163} The analysis of anodic microbial communities revealed that MEC reactors with predominant populations of the Geobacteraceae family displayed higher power densities than those with varied bacterial communities. ^{162 , 163}

While pure cultures offer a controlled system for MECs, ¹³⁹ most MEC reactors utilize mixed consortia of bacteria for H₂ generation. ¹²³ Mixed cultures sourced from wastewaters and sludge present advantages over monocultures, as they are more robust to environmental changes and exhibit higher current densities, leading to enhanced coulombic efficiencies in MEC reactors. ^{164 , 165 , 166 , 167} However, a major drawback of mixed cultures from wastewaters is the potential for cross-contamination with methanogens, which utilize hydrogen gas to produce methane, thereby reducing overall hydrogen yields. ¹⁶⁸ Three methods have been proposed to inhibit the growth of methanogens: ¹⁶⁹ the first one is adjusting the environmental pH by utilizing a medium solution (pH 5.8) with phosphate buffer. The second is aerating the cathode for 15 min when methane content in the headspace exceeds 5 %. The last one is subjecting the anodes from MFCs to a 15-min boiling process before incorporating them into MECs.

4.4.4 Substrates

Substrates significantly impact hydrogen production in MECs (Figure 9), affecting yield based on their composition, concentration, and type. Various substances, such as acetic acid, lactic acid, butyric acid, glucose, glycerol, cellulose, starch, phenol, methanol, milk, and different types of wastewaters, can feed an MEC for hydrogen production. ^{25 , 123 , 125 , 126 , 140 , 170 , 171} These substrates are typically categorized into three groups: non-fermentable, fermentable, and various types of wastewaters. ²⁵ Different substrates used in MECs are summarized Tables 6 and 7.

Figure 9:

Hydrogen gas production rate of microbial electrolysis cells (MECs) depending on the substrate types. From. ¹¹⁷

Non-fermentable substrates, such as acetic acid, butyric acid, lactic acid, glucose, and propionic acid, are well-suited for MECs. ^{119 , 168} Among these, acetate stands out as the most extensively studied feed material. ¹³⁹ Acetate exhibits promising results, with two-chamber MECs capable of achieving a theoretical hydrogen recovery of 4 mol H₂/mol at an external applied voltage of 1 V. Moreover, single-chamber MECs fed with acetate demonstrate higher hydrogen production quantities due to reduced resistance and lower pH gradients within the system. ^{172 , 173}

Fermentable organics, such as lignocellulosic biomass, stand out as highly viable feedstock for MECs due to their extensive study and widespread acceptance. ¹⁷⁴ Among these, polymeric substrates like hemicellulose, cellulose, and aromatic polymers, including lignin, exhibit remarkable potential for HER. ²⁵ Despite their cost-effectiveness and renewability, these substrates require conversion into monosaccharides or low molecular weight compounds to optimize hydrogen yield. For addressing this issue, pre-treatment methods are often employed to remove lignin from lignocellulosic biomass, thereby enhancing its digestibility within MECs. ^{25 , 171} A notable approach involves the combined use of dark fermentation and electro-hydrogenesis, which has shown promising results in dealing with recalcitrant lignocellulosic materials, leading to increased yields and improved rates of hydrogen production. ^{175 , 176} Comparative analyses have further underscored the advantages of winery wastewater over domestic wastewater as a feedstock for MECs, indicating higher outputs. ¹⁶⁵ Effective processing of complex waste substrates involves hydrolysis and fermentation to break down complex macromolecules into simpler compounds, facilitating efficient hydrogen production. ^{25 , 176}

Wastewater is abundant in contaminants and nutrients that require treatment. Additionally, wastewater treatment plants generate large amounts of waste activated sludge (WAS), posing a significant disposal challenge. Both wastewater and WAS can serve as valuable feedstocks for hydrogen production in microbial electrolysis cells (MECs). ^{170 , 177} MECs provide the dual benefit of contaminant removal and energy generation. ^{170 , 177} Various types of wastewater, including those rich in cellulose and glucose, as well as domestic and industrial wastewater have been considered in MECs. ^{165 , 170 , 177 , 178} Therefore, the integration of MECs into wastewater treatment infrastructure represents a step towards a circular economy, where waste products are transformed into valuable resources. By utilizing either WAS or wastewater itself as feedstocks for hydrogen production, MECs contribute to resource recovery and energy independence goals. ¹⁷⁷

4.4.5 Challenges and future aspects

In summary, biological hydrogen production in MECs is gaining prominence due to its sustainable nature and low voltage requirements. However, there’s a need for improved performance and cost-effectiveness, driving the exploration of new building materials and optimized designs for MECs. ²⁵ To enhance efficiency, reducing overpotential and associated losses is crucial. Strategies such as electrode optimization, membrane enhancements, and the use of high surface area electrodes show promise. ^{119 , 139} Studies have demonstrated significant overpotential reduction by introducing CO₂ to the cathodic chamber and employing novel catalysts like Ni₂P nanoparticles coated electrodes. ¹ Additionally, integrating photocatalysts and exploring bio-cathodes offer avenues for further improvement. ¹¹⁷ Membrane development is also essential, with research focusing on cost-efficient alternatives to Nafion membranes and strategies to combat biofouling.

On a larger scale, pilot studies have highlighted challenges related to substrate utilization and membrane durability. ^{178 , 179} While phosphate buffer solution remains a popular choice for catholytes, further research into electrolytes is warranted. Understanding the role of exoelectrogens like Shewanella and Geobacter in biofilm formation and electron conduction is critical for optimizing MEC performance. ^{119 , 137} Techno-economic assessments and life cycle analyses are essential for evaluating MEC viability. ¹⁸⁰

Future efforts should focus on scaling up MECs, utilizing real wastewater, and integrating MECs with other processes like anaerobic digestion for enhanced performance. While a succent review is provided here, a comprehensive overview of MEC technology and its challenges, further research is needed in electrolytes, efficient design, hydrogen storage, and materials to realize successful pilot-scale implementation. Additional reviews addressing these domains would be valuable for advancing MEC technology.

4.5.1 Ultrasonic field

Ultrasonic irradiation stands as a widely embraced technique among researchers aiming to facilitate the gas bubbles detachment from electrodes. This method capitalizes on the phenomenon of acoustic cavitation, where tiny microbubbles form, expand, and abruptly collapse, inducing micro-agitation and intense microturbulence, known as acoustic streaming, in the vicinity of the electrode surface. ¹⁸¹ Essentially, ultrasonic vibrations cause the medium to oscillate back and forth, leading to a steady streaming away from the piezo location, as illustrated in Figure 10a. This oscillatory flow triggers an acoustic streaming effect, as depicted in Figure 11a, characterized by notable micro-agitation near the electrode surface and consequential mass transfer effects, fostering intense mixing. Moreover, the interaction between sound waves and gas bubbles gives rise to bubble streamers in an acoustic field, as shown in Figure 10b. ¹⁸¹ These oscillating gas bubble streamers, driven by primary and secondary Bjerknes forces, propel towards pressure antinodes at high speeds. This microstreaming effect engenders a plethora of physical forces, pivotal in various applications, including expediting the detachment of gas bubbles from the electrode. Among the significant forces generated during acoustic cavitation are microjets (Figure 10c) and shockwaves (Figure 10d). ¹⁸¹ Asymmetric collapse of cavitation bubbles, particularly near a boundary, forms high-speed liquid jets capable of pitting metal surfaces, whereas symmetrical collapse leads to the generation of high-intensity shock waves. Collectively, these ultrasound-induced actions in the solution effectively cleanse the electrode surface and augment the mass transfer of electrolytes from the solution to the electrode, thereby reducing overpotential through decreased solution and gas resistance. A 2019 review by Islam et al. ¹⁸² investigated the potential of power ultrasound to overcome limitations in electrochemical water splitting for hydrogen production. Their findings suggest ultrasound offers several advantages. Firstly, it can clean and activate electrode surfaces, leading to enhanced performance. Secondly, ultrasound promotes increased ions movement within the electrolyte and near the electrode surface, boosting mass transfer and facilitating the reaction. Thirdly, it effectively removes gas bubbles from both the electrolyte and the electrode surface, improving overall efficiency. Islam et al. also observed an increase in electrolytic efficiency (up to 15–20 %) attributed to a combination of factors like higher ion concentration and bubble detachment at the electrode surface, as shown in Figure 11. Photographic visualization of hydrogen bubbles on Pt wire electrode (Figure 12) clearly shown the cleaning effect of sonication.

Figure 10:

Physical effects of acoustic cavitation, (a) acoustic streaming (oscillator is located at the bottom and liquid surface is at the top), (b) microstreamers, (c) microjet and (d) shockwaves generated by ultrasound and acoustic cavitation. Form. ¹⁸¹

Figure 11:

Ultrasonication effect on (a) cell voltage (b) efficiency (ε) and (c) specific energy for hydrogen production (USA: Ultrasound-assisted). From. ¹⁸²

Figure 12:

Hydrogen production on Pt wire before and during sonication (US). From. ¹⁸²

In the context of water sono-electrolysis, several studies have been done. Li et al. ¹⁸³ investigated the effect of ultrasonic fields on water electrolysis in alkaline solutions. They observed a significant decrease in cell voltage with the application of an ultrasonic field, particularly at higher current densities and lower electrolyte concentrations. At a constant current density, the reduction in cell voltage under an ultrasonic field decreased with increasing NaOH concentration. The efficiency of H₂ production increased by 5–18 % at higher current densities under an ultrasonic field, with energy savings of up to 10–25 %. Hung et al. ¹⁸⁴ studied the combined effect of an ultrasonic field and PVDF-g-G1 BM synthesis membrane for H₂ production. They found that H₂ production efficiency was higher in electrolytic cells with an ultrasonic field compared to those without. Energy saving in H₂ generation using PVDF-g-G1 BM membrane was 15.0–20.0 % and 8.0–12.0 % for cells operated with and without an US field, respectively. Lin and Hourng ¹⁸⁵ investigated the impact of a 133 kHz-ultrasonic field with power ranging from 225 to 900 W, in conjunction with varied input voltages, electrolyte concentrations, and electrode gaps. They found that applying a 2.0 V potential along with the US field improved both concentration impedances and activity impedances, while also accelerating the release of hydrogen bubbles. Notably, ultrasonic power, electrolyte concentration, and electrode gap emerged as pivotal factors influencing water electrolysis performance. Under normal temperature conditions, with a 2 mm electrode gap, 4 V potential, and 40 wt% electrolyte concentration, the difference in current density between electrolysis with and without 225 W sonic power was 240 mA/cm². Accounting for the power required for the ultrasonic wave, this resulted in a power saving of 3.5 kW and an economic power efficiency of 15 %. Additionally, they observed a 20 % increase in electrolysis efficiency with a 675 W ultrasonic power, in 10 wt% KOH, with a 5 mm electrode gap, and a specific input power.

Merabet and Kerboua ¹⁸⁶ conducted a thorough investigation into alkaline water electrolysis, examining the impact of continuous and pulsed ultrasound under various operational parameters. They found that integrating continuous ultrasound under optimal conditions of KOH electrolyte and nickel foam electrodes led to a 7.78 % increase in cell current, corresponding to 64 % decrease in bubbles resistance. Interestingly, the effect of sonication was negligible at 45 °C (the optimal temperature), while room temperature showed better operation performance of the sono-electrolyzer in compared to silent conditions. Continuous mode sonication consistently outperformed pulsed mode, both kinetically and energetically. In a subsequent study, Merabet and Kerboua ¹⁸⁷ introduced a membraneless sono-electrolyzer powered by photovoltaics (PV), utilizing alkaline electrolysis and indirect pulsed and continuous 60 kHz sonication. The results showed that indirect continuous sonication achieved the lowest electrode coverage percentage at 37 %, compared to 82 % without sonication and 58.3 % with pulsed ultrasonication. Furthermore, the bubble resistance decreased significantly from 569.81 mΩ without sonication to 132.54 mΩ with continuous sonication and 273.42 mΩ with pulsed sonication. Indirect continuous sonication evidenced the most effective mode for bubbles subtraction, resulting in a 76 % reduction in bubble resistance compared to no sonication and a 52 % reduction compared to pulsed sonication.

However, it should be noted that prolonged use of sonication must be approached with caution, as it can cause electrode degradation due to the asymmetric collapse of bubbles near solid surfaces, resulting in micro-jets with speeds of up to 100 m/s. ¹⁸¹ While low-frequency sonication (20–100 kHz) is beneficial for physical processes like cleaning and degassing, high-frequency sonication (above 100 kHz) may lead to electrolyte degradation, especially with organic electrolytes or ion liquids. The chemical effects of sonication in aqueous solutions are commonly used for the degradation of organic and inorganic contaminants. ^{188 , 189} Therefore, in water electrolysis for hydrogen production, frequency selection should not exceed 100 kHz. It is advisable to use low frequency with higher power to avoid any chemical effects of ultrasound. Additionally, while the power required to generate the ultrasonic field was found to be lower than the power saved from the enhanced efficiency, ¹⁸³ there’s a suggestion that employing an ultrasonic field in water electrolysis might not be economically viable for hydrogen production due to the cost associated with the system required to generate the ultrasonic field. ¹⁸⁵ This includes expenses related to transducers and utilities for measuring the acoustic pressure (e.g., hydrophones). This economic challenge could become more significant as operations scale up to industrial levels.

4.5.2 Magnetic field

Studies examining the impact of magnetic fields on electrolysis efficiency showed predominantly positive outcomes. ^{190 , 191 , 192 , 193 , 194} The enhancement effect of magnetic fields on electrochemical reactions primarily stems from magnetohydrodynamic convection induced by the Lorenz force, ^{195 , 196} which improved bubble detachment. Iida et al. ¹⁹⁷ applied a strong magnetic field of 5 T to water electrolysis, resulting in significant reductions in cell voltage, particularly notable in alkaline solutions and at high current densities. Lin et al. ¹⁹⁸ observed that this effect becomes more pronounced with decreased electrode spacing. Additionally, materials exhibiting ferromagnetism, like nickel, outperform paramagnetic (e.g., platinum) and diamagnetic (e.g., graphite) materials when subjected to a magnetic field. ¹⁹⁸ For instance, using a nickel electrode with a 2 mm inter-electrode distance and a 4 V cell voltage led to a 14.60 % increase in current density. ¹⁹⁸ Iida et al. ¹⁹⁷ demonstrated improved water electrolysis efficiency through a magnetic field’s ability to reduce electrode overpotential [1817]. This effect was observed across various electrolyte conditions, including acidic media (0.05 M H₂SO₄) and alkaline solutions (4.46 M and 0.36 M KOH). Notably, the reduction in OER overpotential was greater compared to the hydrogen evolution reaction HER overpotential under the magnetic field. This difference was attributed to the varying sizes of gas bubbles produced during each process. The researchers proposed that magnetohydrodynamic (MHD) convection plays a crucial role. MHD convection influences bubble detachment from the electrode surface, leading to a significant decrease in the void fraction and the area covered by gas bubbles. This ultimately enhances the overall electrolysis efficiency. Matsushima et al., ¹⁹⁹ through measuring supersaturation in the electrode-electrolyte interface and the mass transfer coefficient of dissolved H₂ via the current interrupter method, concluded that a magnetic field promotes the mass transfer of dissolved H₂, releasing supersaturation near the electrode. In another study, Matsushima et al. ²⁰⁰ found that the average bubble layer thickness (δ) on both cathode and anode was reduced by a magnetic field. Wang et al. ²⁰¹ expanded on the explanation of magnetic field effects, suggesting that the rotational force applied to H₂O molecules due to the interacting charge and induced charge of the Lorentz force from the induced magnetic field contributed to increased efficiency. They also proposed that the induced magnetic field diminished friction caused by hydrogen bonding. Consequently, ohmic voltage drops and reaction overpotential are noticeably reduced in the presence of a magnetic field, resulting in lower cell voltage for water electrolysis. Furthermore, it was found that the inclusion of a magnetic field allowed for a reduction in electrolyte concentration, ^{192 , 193 , 194 , 198} which would be valuable in lowering the overall electrolyte corrosive aspect, thus reducing operating costs associated with electrode corrosion.

4.5.3 Super gravity field

The slow detachment of bubbles in the electrolytic cell, especially at high cell voltages, is a significant issue in water electrolysis. Wang et al. ¹⁹⁶ provide an insightful analysis of how a high-gravity field influences bubble detachment. Bubble movement within the cell is primarily controlled by interphase buoyancy, Δρg, where Δρ is the phase density difference and g is the applied acceleration. ¹⁹⁶ Buoyancy acts as the driving force for bubble disengagement, causing bubbles to move in the direction of buoyancy during electrolysis. Applying an external force in alignment with the buoyancy direction can significantly enhance bubble disengagement. Consequently, in the context of water electrolysis, this approach can diminish bubble coverage on the electrode surface and reduce bubble dispersion in the electrolyte, particularly in the presence of a super gravity field. As a result, both reaction overpotential and ohmic voltage drop are mitigated, leading to a decrease in cell voltage and energy consumption.

Figure 13a depicts a captured photograph of the electrode surface during hydrogen bubble evolution under normal gravity conditions. ²⁰² The image reveals numerous bubbles, each approximately 200 μm in diameter, adhering to the electrode surface. In contrast, Figure 13b illustrates the same process under a super gravity field, where only a few bubbles are observed, and their movement is barely discernible. To gain further insight into this phenomenon, let’s consider the scenario inverted and operate under microgravity conditions (with gravitational forces weaker than those on Earth). In such conditions, there is a higher intensity of larger bubble coverage on the electrodes, as demonstrated in Figure 13c and 13d. This scenario presents several challenges, including bubble adherence to the electrode and membrane, the accumulation of high gas volume fractions in the electrolyte, and the need to increase electrode spacing to facilitate bubble removal. ²⁰³ These factors contribute to increased overpotential and process costs.

Figure 13:

The electrolytic formation of hydrogen and oxygen, (a,b) photographs of cathode surface during hydrogen bubble evolution process at 0.05 A/cm² under (a) normal gravity condition (G = 1) and (b) super gravity field (G = 1 01) in 0.5 M H₂SO₄ aqueous solution. (c,d) gas (H₂, O₂) bubbles evolution in alkaline electrolyte under (c) terrestrial gravity and (d) microgravity conditions, at 8 s after starting water electrolysis using KOH solution. (e,f) Bubbles layer formed in the vicinity of electrodes’ surfaces in the absence (e) and under super gravity field (f). [(a, b): From Wang et al. ²⁰² (c,d): From Matsushima et al. ³⁰⁵ (e,f): From Wang et al. ¹⁹⁶

Therefore, the enhanced buoyancy force and reduced bubble volume under the super gravity field facilitate the rapid separation of bubbles from the electrolytic cell. ²⁰⁴ Consequently, the bubble layer thicknesses (δ_HG and δ_OG) near both the cathode and anode under the super gravity field (Figure 13f) are expected to be smaller than those (δ_H and δ_O) under normal gravity conditions (Figure 13e). The reduction in bubble layer thickness plays a crucial role in reducing electrolyte resistance and the ohmic voltage drop, primarily due to the diminished dispersion of bubbles within the electrolyte when exposed to the super gravity field. Importantly, the impact of the super gravity field on reducing cell voltage is more pronounced with higher current densities for water electrolysis, as indicated by the relationship between bubble layer thickness (δ) and its effect on cell voltage reduction. ²⁰⁴

Cheng et al. ²⁰⁵ were the first who demonstrate the effectiveness of a super gravity field in intensifying the water electrolysis process. They achieved a reduction in cell voltage of approximately 0.70 V at 3.0 kA/m² under 190 g (a relative acceleration) compared to normal gravity environments. Similar findings were replicated by Wang et al. ^{202 , 204} The relationship between the cell voltage of water electrolysis under a super gravity field (U_G) and the gravity coefficient (G) was expressed using the general formula: ²⁰⁴

where, U₁​ represents the cell voltage under normal gravity conditions (G = 1), D signifies the rate of change of cell voltage with the logarithm of G. The maximum energy saving percentage reaches approximately 17 %. ²⁰⁴ Cheng et al. ²⁰⁵ estimated that the potential energy savings could reach up to 30 kW in a 100 kA electrolytic cell, whereas it was only 2 kW for the super gravity field. The reduction in cell voltage was primarily attributed to a decrease in ohmic voltage drop compared to the reduction in reaction overpotential, particularly at higher current densities and gravity accelerations. ²⁰⁴

Lao et al. ²⁰³ also reported the increase of electro-hydrogen yield in a centrifugal acceleration field. At normal conditions (7.7 M KOH solution, 348 K), significant reductions in cell voltage were observed at an acceleration of approximately 16 g (∼500 rpm of rotational speed). The rotary electrolyzer achieved cell voltages about 0.25–0.5 V lower than equivalent static cells under similar conditions, depending on the current density. These voltages were comparable to those of fully developed pressurized cells in industrial settings, even without an effective electrode catalytic coating. Furthermore, at a higher acceleration of 41 g, the rotary cell sustained a current density of up to 13.5 kA/m² without experiencing gas bubble blinding of the membranes and electrodes. This study demonstrates the potential for intensification compared to typical commercial systems with current densities of about 5 kA/m².

4.6.1 Challenges of seawater electrolysis

The complexity of seawater poses a significant challenge for seawater electrolysis, primarily due to the diverse range of components present, including microorganisms, sediment, and various ion species. Sediment and microorganisms, which primarily impact the cleanliness of seawater electrolyzers, can be addressed through filtration pretreatment. However, seawater ionic species (i.e., Cl⁻, Br⁻, SO₄²⁻, HCO₃⁻, CO₃²⁻, F⁻, Mg²⁺, Na⁺, Ca²⁺, Cu²⁺, K⁺, Sr²⁺, and Cd²⁺) cannot be easily filtered out, these ions can lead to veritable issues at both the cathode and anode during electrolysis. Therefore, the influence of these species must be carefully considered in seawater electrolysis processes.

At the cathode, the significant challenge for HER arises from the dramatic rise in local pH near the cathode surface as the electrolysis current increases. This pH increase can lead to the formation of precipitates such as Ca(OH)₂ and Mg(OH)₂ from seawater cations, consequently blocking HER reactive sites. ⁴⁴ Currently, two potential solutions are being explored: ²⁸ (i) the addition of pH buffers to stabilize pH fluctuations in the seawater electrolysis system, and (ii) the design of suitable seawater electrolyzers to facilitate the separation of sediment from the cathode surface. Furthermore, certain metal ions dissolved in seawater, such as Cu²⁺, Cd²⁺, and Pb²⁺, have a tendency to deposit on the cathode surface at specific potentials. These deposits not only compete with HER but also significantly reduce electrode durability. ²⁷ Therefore, catalyst designs should aim to suppress these undesirable electrochemical processes to achieve high selectivity for HER in seawater electrolysis.

At the anode, the OER is notably influenced by chemically active anions found in seawater. Specifically, chloride (Cl⁻) chemistry is expected to compete with the anodic OER due to its standard redox potential range of 0.9–1.6 V. ²⁶ Pourbaix diagram incorporating both the OER and chloride chemistry is shown in Figure 14. This ^{29 , 206} diagram reveals that the chlorine evolution process (ClER) occurs at low pH (<3) (Eq. (35)), while hypochlorite production occurs at high pH (>7.5) (Eq. 336), representing the primary competing reactions for OER at the anode. Consequently, Cl⁻ ions in seawater compete with the OER reaction, and the resulting Cl₂ (oxidant) can lead to severe corrosion of the seawater electrolyzer.

Figure 14:

The pourbaix diagram for the OER and the chloride chemistry in aqueous saline electrolyte. 0.5 M NaCl dissolved in this aqueous solution and no other chlorine sources, including the H2O/O2 and the Cl-/cl₂/HOCl/ClO- redox couples. This figure depicts the potential-pH region where OER and chloride oxidation reactions may occur thermodynamically. The green line represents the thermodynamic equilibrium between H2O and O2. When the electrode potential is more positive than the green line, the OER process becomes thermodynamically possible. The red line shows the competing acidic oxidation of chloride to free gaseous Cl2. The black and purple lines mark the beginning of the oxidation of chloride to HOCl or OCl–. Form. ²⁰⁶

Although thermodynamics favor the OER, both ClER and hypochlorite production (Eqs. (35) and (36)) are two-electron reactions that are kinetically more favorable compared to the four-electron OER. Interestingly, when the electrolyte becomes alkaline, the product of Cl⁻ oxidation transitions from HOCl to OCl⁻, and the thermodynamic overpotential maintains a consistent gap of 480 mV with OER (Figure 14a). Therefore, utilizing a high-performance OER catalyst under alkaline conditions and limiting the overpotential below 480 mV theoretically allows for the complete prevention of Cl⁻ oxidation. ²⁷ This principle, known as the ‘alkaline design criteria’ for seawater electrolysis, has been elucidated by Tong et al. ²⁰⁶ and Dionigi et al. ²⁹

4.6.2 Electrocatalysts for seawater splitting

Seawater electrolysis presents a big challenge due to the presence of cationic metals and abundant corrosive chloride anions, necessitating robust and efficient electrocatalysts, particularly for the anode. ⁴⁴ Figure 15 compares the performance of various tested catalysts under diverse testing conditions, including acidic, neutral, basic, simulated seawater, and natural seawater electrolytes. ²⁸ Among the tested OER catalysts, in addition to those adopted in pure water, noble metals such as Ir and Ru, as well as transition metal oxides, hydroxides, nitrides, phosphorus-doped complexes, and layered double hydroxides (LDHs), have demonstrated high activity. ²⁸ However, when considering only current density and overpotential, the performance differences among various OER catalysts are minimal (Figure 15), as only a portion of the currents is utilized for OER, with additional energy consumed by the oxidation of Cl⁻. Actual seawater electrolysis is even more complex, with many catalysts exhibiting excellent performance in simulated seawater but suffering from performance losses in natural saltwater due to the presence of numerous impurity ions. ⁴⁴ The ClER reaction of Cl⁻ competes with OER, while the deposition of heavy metal ions in seawater affects the catalytic performance of the cathode. ²⁷ Subsequently, we will analyze numerous OER and HER electrocatalysts for seawater electrolysis.

Figure 15:

Comparison of the activity of various HER and OER catalysts tested in acidic, neutral, and basic media. Different solutions (natural seawater, simulated seawater or buffered seawater) are distinguished by different color patches. The term overpotential (η) is used to define the overpotential at a certain current density (e.g., 5, 20 and 100 mA/cm², labeled η₅, η₂₀ and η₁₀₀). If there is no special description, it means that the current density is 10 mA/cm². From. ²⁸

Guided by DFT computation, Hsu et al. ²⁰⁷ has engineered an MHCM-z-BCC (MHCM: Transition metal hexacyanometallate; z-BCC: ZIF-67 on the basic cobalt carbonate surface) electrode, demonstrating remarkable selectivity towards the OER in seawater. While Pt^OER//Pt^HER and IrO₂^OER//Pt^HER electrolyzers produced 50 and 22 µM of Cl₂, respectively, after 12 h in buffered seawater (pH ∼7), the MHCM-z-BCC^OER//NiMoS^HER electrolyzer exhibited no detectable Cl₂ even after 100 h, indicating minimal CIER. Additionally, using the NiMoN@NiFeN^OER//NiMoN^HER electrolyzer in simulated seawater (1 M KOH with 0.5 M NaCl), only H₂ and O₂ gases were detected with a molar ratio close to 2:1, and a Faradaic efficiency of approximately 97.80 %, ²⁰⁸ demonstrating high selectivity for OER on the anode. Moreover, CoFe LDH displayed a Faradaic efficiency of ∼94.0 % at an overpotential (η) of 560 mV. ²⁰⁹ Nevertheless, deviations in Faradaic efficiency were observed for both O₂ and H₂ evolution during OER and HER, respectively. NiCoP exhibited approximately 96.5 % Faradaic efficiency for about 60 min, ²¹⁰ while Ni₅P₄@Ni^{2+ δ}O_δ(OH)_2-δ displayed nearly 93.0 % Faradaic efficiency with minimal side of CIER reactions during electrolysis (the H₂ yield increased linearly). ²¹¹ Similarly, Mn–NiO–Ni showed nearly 100 % Faradaic efficiency within 1 h, but deviations occurred after 1 h, with approximately 70.0 % Faradaic efficiency observed after 7 h, suggesting the presence of side reactions consuming about 30 % of the total charge. ²¹² Furthermore, NiRuIr–Graphene exhibited nearly 100 % Faradaic efficiency for 2 h during HER in seawater, ²¹³ indicating minimal side reactions.

In more detail, Kuang et al. ²¹⁴ demonstrates the effectiveness of dual-layered anode fabrication and subsequent activation in combating chloride corrosion for long-term stability in seawater electrolysis. For instance, their research shows that the NiFe-NiSx-Ni^#OER//Ni–NiO–Cr₂O₃^HER electrolyzer, equipped with an activated NiFe-NiSx-Ni anode, maintains negligible decay even at high current densities. Specifically, the activated NiFe-NiSx-Ni anode exhibits a low overpotential of ∼300 mV at 400 mA/cm² in 1 M KOH with 0.5 M NaCl, significantly lower than the typical of ∼480 mV (threshold η). This reduced overpotential indicates its superior resistance to hypochlorite formation, a critical factor for ensuring selective oxygen evolution. Moreover, while the activated NiFe-NiSx-Ni anode remains stable for over 500 h, its counterpart, the activated NiSx-Ni anode, deteriorates rapidly in harsh electrolytes, such as 1 M KOH with 2 M NaCl. These findings underscore the importance of anode activation in achieving high corrosion resistance and OER activity, paving the way for efficient seawater electrolysis.

Yu et al. research ²⁰⁸ highlights the benefits of employing a metal nitride anode with a core–shell structure, particularly with a NiFe-based shell, for enhancing OER in seawater electrolysis. This configuration enables the in situ formation of thin amorphous layers during OER, improving corrosion resistance against chloride ions. Additionally, the core–shell design allows for electronic structure tuning, increased exposure of active sites, enhanced charge transfer, and improved gas evolution, collectively boosting OER efficiency in seawater. Their study demonstrates that the NiMoN@NiFeN^OER//NiMoN^HER electrolyzer maintains negligible decay even after 100 h of operation at 100 mA cm² in. Furthermore, studies by Ding et al. ²¹⁵ demonstrate that cellular stainless steel, characterized by high conductivity and tensile strength, facilitates efficient OER performance by exposing ample active sites and supporting effective charge transfer. Their research shows negligible decay over 110 h at 40 mA/cm². Similarly, the nitrogen-rich, nanostructured Mo₅N₆, atomically thin, as observed by Jin et al., ²¹⁶ enhances HER due to its modified electronic structure and increased active site exposure. Moreover, Lu et al. ²¹² find that manganese-doped nickel oxide/nickel heterostructures and nanoarray-structured NiCoP further optimize HER efficiency by facilitating charge transfer and gas evolution processes. Their study reports a significantly high activity with η₁₀ of −170 mV in seawater, with reasonable stability maintained for 14 h at η₁₀ of −140 mV. These findings underscore the importance of material design and structure in achieving high-performance electrocatalysis for seawater electrolysis.

Lv et al. ²¹⁰ discovered that nanoarray structured NiCoP demonstrates high activity with η₁₀ of approximately −287 mV in seawater, indicating its effectiveness in the HER. Moreover, it exhibits reasonable stability with η of about −290 mV for 20 h, suggesting its reliability for prolonged operation. Similarly, Huang et al. ²¹¹ reveals that the nanostructured Ni₅P₄ with a thin amorphous Ni₅P₄@Ni^{2+ δ}O_δ(OH)_2-δ layer displays significantly high activity, with η₁₀ of approximately −144 mV in seawater. Furthermore, it exhibits negligible decay even at various current densities over 40 h, indicating its remarkable stability. Sarno et al. ²¹³ investigated the performance of the nanostructured NiRuIr alloy anchored on graphene, revealing its substantial stability with approximately 90 % retention over 200 h in seawater. Additionally, Zhou et al. ²¹⁷ on nanostructured T@Td-ReS₂ with Re vacancies highlights enhanced HER performance in seawater. Interestingly, they observed that while the highest conductivity of the electrolyte is observed at approximately 45 °C, the highest hydrogen production rate occurs at about 35 °C. This discrepancy was attributed to the synergy between conductivity and dielectric constant, ultimately leading to the highest hydrogen production rate.

Zheng et al. ²¹⁸ found that the PtMo alloy exhibits substantial activity with η₁₀ of about −254.6 mV. Moreover, it maintains excellent stability, with 91.13 % retention at η of −800 mV for 173 h. Zhang et al. ²¹⁹ compared the HER of nickel (Ni) alloys, including NiCo and NiCu, in seawater. Fabricated on pretreated Ti substrates via the electrodeposition method, NiCu displayed a nanostructure with a feather-like morphology, while NiCo exhibited a loose structure. The nanostructure of these alloys exposed abundant active sites, contributing to their improved HER performance. Specifically, in filtered seawater, NiCo exhibited an overpotential (η) of approximately −1,000 mV at a current density of 111 mA/cm², with negligible decay observed over ∼10 h. Similarly, NiCu showed an overpotential of approximately −1,000 mV at a current density of 88 mA/cm², with reasonable stability maintained over ∼10 h. Niu et al. ²²⁰ conducted a comparative study on the HER activity of RuCoMox alloys with RuCo alloy in filtered seawater. These alloys were deposited on pretreated titanium foil using cyclic voltammetry deposition. The RuCo alloy features nanoparticles morphology with an average size of approximately 50 nm, while the RuCoMo₇ alloy exhibits a compact structure with cracks. In filtered seawater, RuCo demonstrates higher HER activity against RuCoMo, RuCoMo₂, RuCoMo₃, RuCoMo₅, and RuCoMo₇. Specifically, RuCo exhibits an overpotential η₁₀ of approximately −387 mV, with 70 % retention at an overpotential of −1,200 mV over 12 h. Meanwhile, RuCoMox shows an η₁₀ of approximately −550 mV, with negligible decay at an overpotential of −1,200 mV over 12 h. Li et al. ²²¹ compared the HER performance of various PtRu-based alloys with Pt in seawater. The alloys, including PtRuMo, PtRuCo, PtRuNi, PtRuFe, and PtRuCr, were deposited on Ti mesh using cyclic voltammetry. These alloys exhibited a face-centered-cubic structure similar to Pt, with changes in crystal constants indicating alloying effects. Among the alloys, PtRuMo showed a nanostructure and the lowest charge transfer resistance, indicating enhanced charge transfer. The HER activity followed this order: PtRuMo > PtRuNi > PtRuCo > PtRuFe > PtRuCr > PtRu > Pt. PtRuMo exhibited the highest activity in seawater, with a low η₁₀ of −196 mV and excellent stability, retaining 97.9 % efficiency for 172 h. PtRuNi, PtRuCo, PtRuFe, and PtRuCr also showed high activity with reasonable stability in seawater.

The bamboo-like multi-walled carbon nanotubes incorporating carbon-coated cobalt nanoparticles and nitrogen dopants produces a synergistic effect that enhances the performance of the HER. Research by Gao et al. ²²² illustrates the remarkable activity and stability of Co/N–C for HER in seawater with a pH around 7. Co/N–C exhibits significantly higher activity in pH-buffered seawater compared to natural seawater, suggesting that neutralizing the seawater pH enhances HER activity. In pH-buffered seawater, Co/N–C achieves an η₁₀ of around −250 mV, maintaining reasonable stability for 7 h with η₁₀ of approximately −270 mV.

4.6.3 Seawater electrolyzers

Currently, two primary processes exist for producing green hydrogen through seawater electrolysis, with power supplied by renewable sources such as wind, solar, or tidal energy: one-step and two-step processes. ²⁸ The one-step technique involves direct electrolysis of seawater to produce hydrogen, utilizing electrolyzers designed to handle seawater as a feedstock. In contrast, the two-step process first involves desalinating seawater to produce ultra-pure water, which is then electrolyzed to generate hydrogen. Advocates of the two-step approach argue that direct seawater splitting research lacks cost-effectiveness. ²²³ They contend that, considering the efficiency and costs associated with reverse osmosis technology, the expenses for initial seawater purification are significantly lower than those for seawater electrolysis. ^{223 , 224} Therefore, they propose focusing on the two-step method as a more economically viable option for green hydrogen production from seawater. ²²⁴

Seawater electrolyzers, on the other hand, are categorized according to their electrolyte type, with three notable technologies: ²⁸ Direct Seawater Electrolyzer (DWE), Proton Exchange Membrane Water Electrolyzer (PEMWE), and Anion Exchange Membrane Water Electrolyzer (AEMWE). The characteristics of each type are summarized in Table 8.

Briefly, DWE utilizes seawater as its electrolyte and requires specific electrode materials for corrosion resistance and energy efficiency. It involves a voltage of approximately 4.0 V to achieve 10 mA/cm². The total investment cost exceeds 6,000$/kW, with estimated annual operation and maintenance (O&M) costs of 4.0–5.0 % of the investment cost. ²⁸ PEMWE, on the other hand, relies on high-purity water and a solid polymer electrolyte (e.g., Nafion membrane). It is sensitive to impurities, with magnesium and calcium ions potentially impacting performance via precipitates deposition on the cathode. Investment costs range from 600 to 1,300 €/kW, with estimated annual O&M costs of 3.0–5.0 % of the investment cost. ²⁸ Finally, AEMWE operates in an alkaline environment, allowing for the use of non-precious metal catalysts and less expensive materials. However, the technology is still under development, particularly regarding anionic membranes. ²⁸ Anion exchange membranes are crucial to prevent unwanted migration of anions, such as chloride ions, during seawater electrolysis.

4.6.4 Future directions for seawater electrolysis

In summary, despite significant recent efforts in seawater electrolysis, ^{26 , 27 , 28 , 29} there remains ample room for improvement in achieving high-performance hydrogen production. Seawater electrolysis presents unique challenges due to its complex composition compared to freshwater electrolysis. The catalytic activity and stability of reported catalysts often fall short of practical application requirements due to insufficiently low overpotentials. Moreover, most studies utilize artificial saline water rather than real seawater. To enhance seawater electrolysis performance, several key directions for future research are proposed: ^{26 , 44}

1. Conducting combined experimental and theoretical analyses to elucidate reaction pathways and active sites of catalysts: Multimetallic compounds and heterostructured catalysts show promise, but their complexity makes understanding reaction pathways and active sites challenging. Systematic research combining experimental data with theoretical analyses can guide the design of materials with desired structures and properties.

2. Employing in situ characterization methods to identify true active sites: Many electrocatalysts undergo surface changes during seawater electrolysis, potentially altering their active sites. In situ characterization techniques, such as Raman, XAS, and Fourier transform infrared spectroscopy, are essential for tracking changes in intermediates during catalytic reactions and guiding the design of high-efficiency catalysts.

3. Developing electrocatalysts with high activity and stability in seawater: Selectivity to hydrogen and oxygen evolution reactions in the presence of multiple cations and chloride anions is crucial. Modulating the electronic structure of active sites through alloying, heteroelement doping, vacancy engineering, and interface engineering can improve catalytic performance. Integration of different active materials into hybrid catalysts is also promising.

4. Designing advanced reactors specific to seawater electrolysis: Beyond catalysts, reactor design plays a critical role. Asymmetric reactor designs, with alkaline water in the anode chamber and seawater in the cathode chamber, offer advantages in Cl⁻ diffusion and anode catalyst protection, crucial for seawater electrolysis.