Recent Progress in Decoupled H 2 and O 2 Production from Electrolytic Water Splitting

Layered double hydroxide modified bismuth vanadate as an efficient photoanode for enhancing photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has attracted significant interest as a promising approach for producing clean and sustainable hydrogen fuel. An efficient photoanode is critical for enhancing PEC water splitting. Bismuth vanadate (BiVO4) is a widely recognized photoanode for PEC applications due to its visible light absorption, suitable valence band position for water oxidation, and outstanding potential for modifications. Nevertheless, sluggish water oxidation rates, severe charge recombination, limited hole diffusion length, and inadequate electron transport properties restrict the PEC performance of BiVO4. To surmount these constraints, incorporating layered double hydroxides (LDHs) onto BiVO4 photoanodes has emerged as a promising approach for enhancing the performance. Herein, the latest advancements in employing LDHs to decorate BiVO4 photoanodes for enhancing PEC water splitting have been thoroughly studied and outlined. Initially, the fundamental principles of PEC water splitting and the roles of LDHs are summarized. Secondly, it covers the development of different composite structures, including BiVO4 combined with bimetallic and trimetallic LDHs, as well as other BiVO4-based composites such as BiVO4/metal oxide, metal sulfide, and various charge transport layers integrated with LDHs. Additionally, LDH composites incorporating materials like graphene, carbon dots, quantum dots, single-atom catalysts, and techniques for surface engineering and LDH exfoliation with BiVO4 are discussed. The research analyzes the design principles of these composites, with a specific focus on how LDHs enhance the performance of BiVO4 by increasing the efficiency and stability through synergistic effects. Finally, challenges and perspectives in future research toward developing efficient and stable BiVO4/LDHs photoelectrodes for PEC water splitting are described.

Decoupling H2 and O2 Release in Particulate Photocatalytic Overall Water Splitting Using a Reversible O2 Binder

H2 and O2 evolutions occur simultaneously for conventional particulate photocatalytic overall water splitting (PPOWS), leading to a significant backward reaction and the formation of an explosive H2/O2 gas mixture. This is an issue that must be addressed prior to industrialization of PPOWS. Here, a convenient, cost-effective, and scalable concept is introduced to uncouple hydrogen and oxygen production for PPOWS. Based on this idea, a three-component photocatalyst, Co(5 %)-HPCN/(rGO/Pt), is constructed, consisting of a photoresponsive chip (HPCN), a H2 evolution cocatalyst (rGO/Pt), and a cobalt complex capable of reversibly binding O2 (Co), to achieve the decoupling of PPOWS under alternating UV and visible light irradiations. The asynchronous O2 and H2 evolution strategy have considerable flexibility regarding the photocatalyst structure and light sources suitable for PPOWS.

In situ electrosynthesis of quinone-based redox-active molecules coupling with high-purity hydrogen production

Clean hydrogen production via conventional water splitting involves sluggish anodic oxygen evolution, which can be replaced with more valuable electrosynthesis reactions. Here, we propose one novel strategy for coupling in situ organic electrosynthesis with high-purity hydrogen production. A benzoquinone-derivative disodium 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron)-o1 and a naphthoquinone-derivative 2,6,8-trismethylaminemethylene-3,5-dihydroxy-1,4-naphthoquinone (TANQ) were in situ electrosynthesized and directly used in a flow battery without any further purification treatment. Constant, simultaneous production of TANQ and hydrogen was demonstrated for 61 hours, while stable charge-discharge capacities were retained for 1000 cycles. The work provided a new avenue for achieving in situ redox-active molecule synthesis and high-purity hydrogen.

Ni─Co─O─S Derived Catalysts on Hierarchical N-doped Carbon Supports with Strong Interfacial Interactions for Improved Hybrid Water Splitting Performance

Simultaneously improving electrochemical activity and stability is a long-term goal for water splitting. Herein, hierarchical N-doped carbon nanotubes on carbon nanowires derived from PPy are grown on carbon cloth, serving as a support for NiCo oxides/sulfides. The hierarchical electrodes annealed in N2 or H2/N2 display improved intrinsic activity and stability for hydrogen evolution reaction (HER) and glucose oxidation reaction. Compared with Pt/C||Ir/C in alkaline media, the glucose electrolysis assembled with electrodes exhibits a cell voltage of 1.38 V at 10 mA cm-2, durability for >12 h at 50 mA cm-2, and resistance to glucose/gluconic acid poisoning. In addition, electrocatalysts can also be applied in ethanol oxidation reactions. Systematic characterizations reveal the strong interactions between NiCo and N-doped carbon support-induced partial charge transfer at the interface and regulate the local electronic structure of active sites. Density functional theory calculations demonstrate that the synergistic effect between N-doped carbon supports, metallic NiCo, and NiCo oxides/sulfides optimize the adsorption energy of H2O and the H* free energy for HER. The energy barrier of the dehydrogenation of glucose effectively decreased. This work will attract attention to the role of metal-support interactions in enhancing the intrinsic activity and stability of electrocatalysts.

Plasma-induced, N-doped, and reduced graphene oxide-incorporated NiCo-layered double hydroxide nanowires as a high-capacity redox mediator for sustainable decoupled water electrolysis

Although the recent emergence of decoupled water electrolysis prevents typical H2/O2 mixing, the further development of decoupled water electrolysis has been confined by the lack of reliable redox mediator (RM) electrodes to support sustainable H2 production. As energy storage electrodes, layered double hydroxides (LDHs) possess inherently poor conductivity/stability, which can be improved by growing LDHs on graphene substrates in situ. The proper modification of the graphene surface structure can improve the electron transport and energy storage capacity of composite electrodes, while current methods are usually cumbersome and require high temperatures/chemical reagents. Therefore, in this study, dip coating was adopted to grow graphene oxide (GO) on nickel foam (NF). Then, the GO was reduced using nonthermal plasma (NTP) to reduced GO (rGO) in situ while simultaneously implementing N doping to obtain plasma-assisted N-doped rGO on NF (PNrGO/NF). The uniform conductive substrate ensured the subsequent growth of less-aggregated NiCo-LDH nanowires, which improved the conductivity and energy storage capacity (5.93 C/cm2 at 5 mA/cm2) of the NiCo-LDH@PNrGO/NF. For the decoupled system, the composite RM electrode exhibited a high buffering capacity for 1300 s during the decoupled H2/O2 evolution, and in the conventional coupled system, the necessary input voltage of 1.67 V was separated into two lower ones, 1.42/0.33 V for H2/O2 evolutions, respectively. Simultaneously, the RM possessed outstanding redox reversibility and structural stability during long-term cycling. This work could offer a feasible strategy for using NTP to synthesize excellent RM electrodes for application to decoupled water electrolysis.

Decoupled supercapacitive electrolyzer for membrane-free water splitting

Green hydrogen production via water splitting is vital for decarbonization of hard-to-abate industries. Its integration with renewable energy sources remains to be a challenge, due to the susceptibility to hazardous gas mixture during electrolysis. Here, we report a hybrid membrane-free cell based on earth-abundant materials for decoupled hydrogen production in either acidic or alkaline medium. The design combines the electrocatalytic reactions of an electrolyzer with a capacitive storage mechanism, leading to spatial/temporal separation of hydrogen and oxygen gases. An energy efficiency of 69% lower heating value (48 kWh/kg) at 10 mA/cm2 (5 cm-by-5 cm cell) was achieved using cobalt-iron phosphide bifunctional catalyst with 99% faradaic efficiency at 100 mA/cm2. Stable operation over 20 hours in alkaline medium shows no apparent electrode degradation. Moreover, the cell voltage breakdown reveals that substantial improvements can be achieved by tunning the activity of the bifunctional catalyst and improving the electrodes conductivity. The cell design offers increased flexibility and robustness for hydrogen production.

Recent Progress in Modular Electrochemical Synthesis of Hydrogen and High-Value-added Chemicals based on Solid Redox Mediator

Electrochemical synthesis of H2 and high-value-added chemicals is an efficient and cost-effective approach that can be powered using renewable electricity. Compared to a conventional electrochemical production system, the modular electrochemical production system (MEPS) based on a solid redox mediator (SRM) can separate the anodic and cathodic reactions in time and space. The MEPS can avoid the use of membranes and formation of useless products, as well as eliminate the mutual dependence of production rates at anode and cathode. The SRM can temporarily store or release electrons and ions to pair with cathodic and anodic reactions, respectively, in MEPS. Designing of SRMs with large charge capacity and good cyclability is of great significance for constructing a high-performance MEPS. This work summarizes the design principles, recent advances in MEPS based on SRM, and application in redox flow cells. Moreover, structure design strategies as well as in situ characterization techniques and theoretical calculations for SRM is also proposed. It is expected to promote the vigorous development of MEPS based on SRM. Finally, the challenges and perspectives of MEPS based on SRM are discussed.

High buffering capacity cobalt-doped nickel hydroxide electrode as redox mediator for flexible hydrogen evolution by two-step water electrolysis

Two-step water electrolysis has been proposed to tackle the ticklish H2/O2 mixture problems in conventional alkaline water electrolysis recently. However, low buffering capacity of pure nickel hydroxide electrode as redox mediator limited practical application of two-step water electrolysis system. A high-capacity redox mediator (RM) is urgently needed to permit consecutive operation of two-step cycles and high-efficiency hydrogen evolution. Consequently, a high mass-loading cobalt-doped nickel hydroxide/active carbon cloth (NiCo-LDH/ACC) RM is synthesized via a facile electrochemical method. The proper Co doping can apparently enhance the conductivity and simultaneously remain the high-capacity of the electrode. Density functional theory results further confirms more negative values in redox potential of NiCo-LDH/ACC than Ni(OH)2/ACC on account of the charge redistribution induced by Co doping, which can prevent the parasitic O2 evolution on RM electrode during decoupled H2 evolution step. As a result, the NiCo-LDH/ACC combined the superiorities of high-capacity Ni(OH)2/ACC and high-conductivity Co(OH)2/ACC, and the NiCo-LDH/ACC with 4:1 ratio of Ni to Co presented a large specific capacitance of 33.52F/cm2 for reversible charge-discharge and high buffering capacity with two-step H2/O2 evolution duration of 1740 s at 10 mA/cm2. The necessary input voltage (2.00 V) of the whole water electrolysis was broken into two smaller ones, 1.41 and 0.38 V, for H2 and O2 production, respectively. NiCo-LDH/ACC provided a favorable electrode material for the practical application of two-step water electrolysis system.

Cable-Car Electrocatalysis to Drive Fully Decoupled Water Splitting

The increasing demand for clean energy conversion and storage has increased interest in hydrogen production via electrolytic water splitting. However, the simultaneous production of hydrogen and oxygen in this process poses a challenge in extracting pure hydrogen without using ionic conducting membranes. Researchers have developed various innovative designs to overcome this issue, but continuous water splitting in separated tanks remains a desirable approach. This study presents a novel, continuous roll-to-roll process that enables fully decoupled hydrogen evaluation reaction (HER) and oxygen evolution reaction (OER) in two separate electrolyte tanks. The system utilizes specially designed "cable-car" electrodes (CCE) that cycle between the HER and OER tanks, resulting in continuous hydrogen production with a purity of over 99.9% and Coulombic efficiency of 98% for prolonged periods. This membrane-free water splitting system offers promising prospects for scaled-up industrial-scale green hydrogen production, as it reduces the cost and complexity of the system, and allows for the use of renewable energy sources to power the electrolysis process, thus reducing the carbon footprint of hydrogen production.

pH-Universal Decoupled Water Electrolysis Enabled by Electrocatalytic Hydrogen Gas Capacitive Chemistry

In conventional water electrolysis (CWE), the H₂ and O₂ evolution reactions (HER/OER) are tightly coupled, making the generated H₂ and O₂ difficult to separate, thus resulting in complex separation technology and potential safety issues. Previous efforts on the design of decoupled water electrolysis mainly concentrated on multi-electrode or multi-cell configurations; however, these strategies have the limitation of involving complicated operations. Here, we propose and demonstrate a pH-universal, two-electrode capacitive decoupled water electrolyzer (referred to as all-pH-CDWE) in a single-cell configuration by utilizing a low-cost capacitive electrode and a bifunctional HER/OER electrode to separate H₂ and O₂ generation for decoupling water electrolysis. In the all-pH-CDWE, high-purity H₂ and O₂ generation alternately occur at the electrocatalytic gas electrode only by reversing the current polarity. The designed all-pH-CDWE can maintain a continuous round-trip water electrolysis for over 800 consecutive cycles with an electrolyte utilization ratio of nearly 100%. As compared to CWE, the all-pH-CDWE achieves energy efficiencies of 94% in acidic electrolytes and 97% in alkaline electrolytes at a current density of 5 mA cm^-2. Further, the designed all-pH-CDWE can be scaled up to a capacity of 720 C in a high current of 1 A for each cycle with a stable HER average voltage of 0.99 V. This work provides a new strategy toward the mass production of H₂ in a facilely rechargeable process with high efficiency, good robustness, and large-scale applications.

Decoupled Artificial Photosynthesis

Natural photosynthesis (NP) generates oxygen and carbohydrates from water and CO₂ utilizing solar energy to nourish lives and balance CO₂ levels. Following nature, artificial photosynthesis (AP), typically, overall water or CO₂ splitting, produces fuels and chemicals from renewable energy. However, hydrogen evolution or CO₂ reduction is inherently coupled with kinetically sluggish water oxidation, lowering efficiencies and raising safety concerns. Decoupled systems have thus emerged. In this review, we elaborate how decoupled artificial photosynthesis (DAP) evolves from NP and AP and unveil their distinct photoelectrochemical mechanisms in energy capture, transduction and conversion. Advances of AP and DAP are summarized in terms of photochemical (PC), photoelectrochemical (PEC), and photovoltaic-electrochemical (PV-EC) catalysis based on material and device design. The energy transduction process of DAP is emphasized. Challenges and perspectives on future researches are also presented.

Aluminum Cation Doping in Ruddlesden-Popper Sr2TiO4 Enables High-Performance Photocatalytic Hydrogen Evolution

Hydrogen (H2) is regarded as a promising and renewable energy carrier to achieve a sustainable future. Among the various H2 production routes, photocatalytic water splitting has received particular interest; it strongly relies on the optical and structural properties of photocatalysts such as their sunlight absorption capabilities, carrier transport properties, and amount of oxygen vacancy. Perovskite oxides have been widely investigated as photocatalysts for photocatalytic water splitting to produce H2 because of their distinct optical properties, tunable band gaps and excellent compositional/structural flexibility. Herein, an aluminum cation (Al3+) doping strategy is developed to enhance the photocatalytic performance of Ruddlesden-Popper (RP) Sr2TiO4 perovskite oxides for photocatalytic H2 production. After optimizing the Al3+ substitution concentration, Sr2Ti0.9Al0.1O4 exhibits a superior H2 evolution rate of 331 μmol h−1 g−1, which is ~3 times better than that of Sr2TiO4 under full-range light illumination, due to its enhanced light harvesting capabilities, facilitated charge transfer, and tailored band structure. This work presents a simple and useful Al3+ cation doping strategy to boost the photocatalytic performance of RP-phase perovskites for solar water splitting.

Co(OH)2 Nanoflowers Decorated α-NiMoO4 Nanowires as a Bifunctional Electrocatalyst for Efficient Overall Water Splitting

The development of bifunctional electrocatalysts with high catalytic activity and cyclic stability is an effective method for electrocatalytic water splitting. Herein, a promising hydroxide/oxide Co(OH)2/α-NiMoO4 NWs/CC heterostructure with nanoflowers decorating the nanowires was fabricated on a carbon cloth (CC) substrate via hydrothermal and calcination methods. In contrast to one-dimensional nanomaterials, the interfaces of Co(OH)2 nanoflowers and α-NiMoO4 nanowires on CC provide more active sites for electrocatalytic reactions; therefore, they exhibit obviously enhanced electrocatalytic activities in overall water splitting. Specifically, the Co(OH)2/α-NiMoO4 NWs/CC electrodes exhibit an overpotential of 183.01 mV for hydrogen evolution reaction (HER) and of 170.26 mV for oxygen evolution reactions (OER) at the current density of 10 mA cm−2 in 1.0 M KOH. Moreover, the electrocatalytic oxygen evolution reaction (OER) activity of the Co(OH)2/α-NiMoO4 NWs/CC electrocatalyst was enhanced after long-term stability tests.

Co, Mn co-doped Fe9S11@Ni9S8 supported on nickel foam as a high efficiency electrocatalyst for the oxygen evolution reaction and urea oxidation reaction

The Earth's fossil resources will be exhausted soon, so it is urgent to find clean and efficient new energy for replacing fossil resources. Hydrogen energy is gradually attracting the attention of the public and electrolysis of water is considered to be one of the important means of hydrogen production because of its simplicity and convenience. In this paper, a hydrothermal method for the synthesis of a Co and Mn co-doped bimetallic sulfide Fe₉S₁₁@Ni₉S₈ electrocatalyst is proposed for the first time. The prepared Co-Mn-Fe₉S₁₁@Ni₉S₈/NF electrocatalyst exhibits excellent electrocatalytic activity for the oxygen evolution reaction (OER) and urea oxidation reaction (UOR). It can provide a current density of 10 mA cm^-2 with only 193 mV overpotential for the OER and a current density of 10 mA cm^-2 with only 1.33 V potential for the UOR, which are far superior to those of most reported electrocatalysts. What is noteworthy is that the unique nanoflower structure of Co-Mn-Fe₉S₁₁@Ni₉S₈/NF increases the specific surface area of the material and the introduction of Co and Mn ions promotes the formation of high valence state Ni and Fe and enhances the charge transfer rate. The density functional theory (DFT) calculation shows that the in situ generated Co-Mn-Fe-NiOOH material derived from Co-Mn-Fe₉S₁₁@Ni₉S₈ exhibits the best water adsorption energy and the best electrical conductivity, thus improving the catalytic performance of the material. This work provided a new idea for the development of bimetallic cation doped electrocatalysts with high efficiency and low cost.

Avoiding Pitfalls in Comparison of Activity and Selectivity of Solid Catalysts for Electrochemical HMF Oxidation

Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) offers a renewable approach to produce the value-added platform chemical 2,5-furandicarboxylic acid (FDCA). The key for the economic viability of this approach is to develop active and selective electrocatalysts. Nevertheless, a reliable catalyst evaluation protocol is still missing, leading to elusive conclusions on criteria for a high-performing catalyst. Herein, we demonstrate that besides the catalyst identity, secondary parameters such as materials of conductive substrates for the working electrode, concentration of the supporting electrolyte, and electrolyzer configurations have profound impact on the catalyst performance and thus need to be optimized before assessing the true activity of a catalyst. Moreover, we highlight the importance of those secondary parameters in suppressing side reactions, which has long been overlooked. The protocol is validated by evaluating the performance of free-standing Cu-foam, and CuCoO modified with NaPO2 H2 and Ni, which were immobilized on boron-doped diamond (BDD) electrodes. Recommended practices and figure of merits in carefully evaluating the catalyst performance are proposed.

Stable Tetrasubstituted Quinone Redox Reservoir for Enhancing Decoupled Hydrogen and Oxygen Evolution

Redox reservoirs (RRs) may be used to decouple the two half-reactions of water electrolysis, enabling spatial and temporal separation of hydrogen and oxygen evolution. Organic RRs are appealing candidates for this application; however, their instability limits their utility. Here, we show that a tetrathioether-substituted quinone, tetramercaptopropanesulfonate quinone (TMQ), exhibits significantly enhanced stability relative to anthraquinone-2,7-disulfonate (AQDS), the most effective organic RR reported previously. The enhanced stability, confirmed by symmetric flow battery experiments under relevant conditions, enables stable electrochemical production of H₂ and O₂ in a continuous flow electrolysis cell. The reduced RR, tetramercaptopropanesulfonate hydroquinone (TMHQ), is not susceptible to decomposition, while the oxidized state, TMQ, undergoes slow decomposition, evident only after sustained operation (>60 h). Analysis of the byproducts provides that basis for a decomposition mechanism, establishing a foundation for the design of new organic RRs with even better performance.

Coaxial Ni-S@N-Doped Carbon Nanofibers Derived Hierarchical Electrodes for Efficient H2 Production via Urea Electrolysis

Electrochemical water splitting into hydrogen is a promising strategy for hydrogen production powered by solar energy. However, the cell voltage of an electrolyzer is still too high for practical application, which is mainly limited by the sluggish oxygen evolution reaction process. To this end, hybrid water electrolyzers have drawn tremendous attention. Herein, coaxial Ni/Ni₃S₂@N-doped nanofibers are directly grown on nickel foam (NF), which is highly active for hydrogen evolution reaction. Meanwhile, the Ni₃S₂@N-doped nanofibers on NF prepared in an Ar atmosphere display superior urea oxidation reaction performance to previously reported catalysts. The cell voltage is about 1.50 V in urea electrolysis to deliver a current density of 20 mA cm^-2, lower than that of a traditional water electrolyzer (1.82 V). The current density is around 77% relative to its initial value of 20 mA cm^-2 after 20 h, superior to Pt/C|Ir/C-based urea electrolysis (14%). It is found that the synergistic effect between metallic Ni and Ni₃S₂, as well as the interfacial effect between metal centers and N-doped carbon, favors the initial dissociation of H₂O and the adsorption/desorption of H* with thermal neutral Gibbs free energy. Meanwhile, the in-situ generated NiOOH on the outer surface of Ni₃S₂ possessed lower electrochemical activation energy for urea decomposition. Meanwhile, the abundant oxygen vacancies in electrodes could expose more active sites for the adsorption of intermediates, including H* and OOH*. It is also found that the hierarchical nanostructure of densely packed nanowires provides ideal electronic and ionic transport paths for fast electrocatalytic kinetics. The present work indicated that the modulation of compositions and hierarchical nanostructure is effective to prepare efficient catalysts for H₂ production via urea electrolysis.

Decoupled Redox Catalytic Hydrogen Production with a Robust Electrolyte-Borne Electron and Proton Carrier

Electrolytic water splitting is an effective approach for H₂ mass production. A conventional water electrolyzer concurrently generates H₂ and O₂ in neighboring electrode compartments separated by a membrane, which brings about compromised purity, energy efficiency, and system durability. On the basis of distinct redox electrochemistry, here, we report a system that enables the decoupling of both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) from the electrodes to two spatially separated catalyst bed reactors in alkaline solutions. Through a pair of close-loop electrochemical-chemical cycles, the system operates upon 7,8-dihydroxy-2-phenazinesulfonic acid (DHPS) and ferricyanide-mediated HER and OER, respectively, on Pt/Ni(OH)₂ and NiFe(OH)₂ catalysts. Near unity faradaic efficiency and sustained production of hydrogen has been demonstrated at a current density up to 100 mA/cm². The superior reaction kinetics, particularly the HER reaction mechanism of DHPS as a robust electrolyte-borne electron and proton carriers, were scrutinized both computationally and experimentally. We anticipate the system demonstrated here would provide an intriguing alternative to the conventional water electrolytic hydrogen production.

Oxygen Vacancy-rich Ni/NiO@NC Nanosheets with Schottky Heterointerface for Efficient Urea Oxidation Reaction

H₂ production via electrocatalytic water splitting is greatly hindered by the sluggish oxygen evolution reaction (OER). The urea oxidation reaction (UOR) draws specific attention not only because of its lower theoretical voltage of 0.37 V compared with OER (1.23 V), but also for treating sewage water. Herein, Ni/NiO nanosheets with an ultrathin N-doped C layer containing a Schottky Ni and NiO heterointerface is constructed. Because of the self-driven charge redistribution at the heterointerface, janus charge domains are successfully created to drive the cleavage of urea molecules. Meanwhile, the synergistic effect between N-doped C and Ni/NiO restrains the deactivation of active sites in alkaline solution. The catalyst displays 1.35 V for UOR at 10 mA/cm² , 0.27 V lower than that of OER. The final potential increase is only 2 mV after long-term stability test of 12 h for UOR, much smaller than the uncoated sample (38 mV). The present work shows that C-coated transition metal nanomaterials with oxygen vacancies and a Schottky heterointerface are promising candidates for simultaneously boosting UOR with both high activity and long-term stability.

Water Splitting: From Electrode to Green Energy System

Hydrogen (H2) production is a latent feasibility of renewable clean energy. The industrial H2 production is obtained from reforming of natural gas, which consumes a large amount of nonrenewable energy and simultaneously produces greenhouse gas carbon dioxide. Electrochemical water splitting is a promising approach for the H2 production, which is sustainable and pollution-free. Therefore, developing efficient and economic technologies for electrochemical water splitting has been an important goal for researchers around the world. The utilization of green energy systems to reduce overall energy consumption is more important for H2 production. Harvesting and converting energy from the environment by different green energy systems for water splitting can efficiently decrease the external power consumption. A variety of green energy systems for efficient producing H2, such as two-electrode electrolysis of water, water splitting driven by photoelectrode devices, solar cells, thermoelectric devices, triboelectric nanogenerator, pyroelectric device or electrochemical water-gas shift device, have been developed recently. In this review, some notable progress made in the different green energy cells for water splitting is discussed in detail. We hoped this review can guide people to pay more attention to the development of green energy system to generate pollution-free H2 energy, which will realize the whole process of H2 production with low cost, pollution-free and energy sustainability conversion.

Nickel Phosphide Electrocatalysts for Hydrogen Evolution Reaction

The production of hydrogen through electrochemical water splitting driven by clean energy becomes a sustainable route for utilization of hydrogen energy, while an efficient hydrogen evolution reaction (HER) electrocatalyst is required to achieve a high energy conversion efficiency. Nickel phosphides have been widely explored for electrocatalytic HER due to their unique electronic properties, efficient electrocatalytic performance, and a superior anti-corrosion feature. However, the HER activities of nickel phosphide electrocatalysts are still low for practical applications in electrolyzers, and further studies are necessary. Therefore, at the current stage, a specific comprehensive review is necessary to focus on the progresses of the nickel phosphide electrocatalysts. This review focuses on the developments of preparation approaches of nickel phosphides for HER, including a mechanism of HER, properties of nickel phosphides, and preparation and electrocatalytic HER performances of nickel phosphides. The progresses of the preparation and HER activities of the nickel phosphide electrocatalysts are mainly discussed by classification of the preparation method. The comparative surveys of their HER activities are made in terms of experimental metrics of overpotential at a certain current density and Tafel slope together with the preparation method. The remaining challenges and perspectives of the future development of nickel phosphide electrocatalysts for HER are also proposed.