Bifunctional Nickel Phosphide Nanocatalysts Supported on Carbon Fiber Paper for Highly Efficient and Stable Overall Water Splitting

Self‐supported electrodes comprising carbon fiber paper (CP) integrated with bifunctional nickel phosphide (Ni‐P) electrocatalysts are fabricated by electrodeposition of Ni on functionalized CP, followed by a convenient one‐step phosphorization treatment in phosphorus vapor at 500 °C. The as‐fabricated CP@Ni‐P electrode exhibits excellent electrocatalytic performance toward hydrogen evolution in both acidic and alkaline solutions, with only small overpotentials of 162 and 250 mV, respectively, attaining a cathodic current density of 100 mA cm^?2. Furthermore, the CP@Ni‐P electrode also exhibits superior catalytic performance toward oxygen evolution reaction (OER). An exceptionally high OER current of 50.4 mA cm^?2 is achieved at an overpotential of 0.3 V in 1.0 m KOH. The electrode can sustain 10 mA cm^?2 for 180 h with only negligible degradation, showing outstanding durability. Detailed microstructural and compositional studies reveal that upon OER in alkaline solution the surface Ni‐P is transformed to NiO covered with a thin Ni(OH)_x layer, forming a Ni‐P/NiO/Ni(OH)_x heterojunction, which presumably enhances the electrocatalytic performance for OER. Given the well‐defined bifunctionality, a full alkaline electrolyzer is constructed using two identical CP@Ni‐P electrodes as cathode and anode, respectively, which can realize overall water splitting with efficiency as high as 91.0% at 10 mA cm^?2 for 100 h.     

Efficient and Durable 3D Self‐Supported Nitrogen‐Doped Carbon‐Coupled Nickel/Cobalt Phosphide Electrodes: Stoichiometric Ratio Regulated Phase‐ and Morphology‐Dependent Overall Water Splitting Performance

The construction of a novel 3D self‐supported integrated Ni_xCo_2?xP@NC (0 < x < 2) nanowall array (NA) on Ni foam (NF) electrode constituting highly dispersed Ni_xCo_2?xP nanoparticles, nanorods, nanocapsules, and nanodendrites embedded in N‐doped carbon (NC) NA grown on NF is reported. Benefiting from the collective effects of special morphological and structural design and electronic structure engineering, the Ni_xCo_2?xP@NC NA/NF electrodes exhibit superior electrocatalytic performance for water splitting with an excellent stability in a wide pH range. The optimal NiCoP@NC NA/NF electrode exhibits the best hydrogen evolution reaction (HER) activity in acidic solution so far, attaining a current density of 10 mA cm^?2 at an overpotential of 34 mV. Moreover, the electrode manifests remarkable performances toward both HER and oxygen evolution reaction in alkaline medium with only small overpotentials of 37 mV at 10 mA cm^?2 and 305 mV at 50 mA cm^?2, respectively. Most importantly, when coupling with the NiCoP@NC NA/NF electrode for overall water splitting, an alkali electrolyzer delivers a current density of 20 mA cm^?2 at a very low cell voltage of ≈1.56 V. In addition, the NiCoP@NC NA/NF electrode has outstanding long‐term durability at j = 10 mA cm^?2 with a negligible degradation in current density over 22 h in both acidic and alkaline media.     

Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting

A 3D hierarchical porous catalyst architecture based on earth abundant metals Ni, Fe, and Co has been fabricated through a facile hydrothermal and electrodeposition method for efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The electrode is comprised of three levels of porous structures including the bottom supermacroporous Ni foam (≈500 μm) substrate, the intermediate layer of vertically aligned macroporous NiCo₂O₄ nanoflakes (≈500 nm), and the topmost NiFe(oxy)hydroxide mesoporous nanosheets (≈5 nm). This hierarchical architecture is binder‐free and beneficial for exposing catalytic active sites, enhancing mass transport and accelerating dissipation of gases generated during water electrolysis. Serving as an anode catalyst, the designed hierarchical electrode displays excellent OER catalytic activity with an overpotential of 340 mV to achieve a high current density of 1200 mA cm^?2. Serving as a cathode catalyst, the catalyst exhibits excellent performance toward HER with a moderate overpotential of 105 mV to deliver a current density of 10 mA cm^?2. Serving as both anode and cathode catalysts in a two‐electrode water electrolysis system, the designed electrode only requires a potential of 1.67 V to deliver a current density of 10 mA cm^?2 and exhibits excellent durability in prolonged bulk alkaline water electrolysis.     

Hierarchical Porous Ni3S4 with Enriched High‐Valence Ni Sites as a Robust Electrocatalyst for Efficient Oxygen Evolution Reaction

Electrochemical water splitting is a common way to produce hydrogen gas, but the sluggish kinetics of the oxygen evolution reaction (OER) significantly limits the overall energy conversion efficiency of water splitting. In this work, a highly active and stable, meso–macro hierarchical porous Ni₃S₄ architecture, enriched in Ni³⁺ is designed as an advanced electrocatalyst for OER. The obtained Ni₃S₄ architectures exhibit a relatively low overpotential of 257 mV at 10 mA cm^?2 and 300 mV at 50 mA cm^?2. Additionally, this Ni₃S₄ catalyst has excellent long‐term stability (no degradation after 300 h at 50 mA cm^?2). The outstanding OER performance is due to the high concentration of Ni³⁺ and the meso–macro hierarchical porous structure. The presence of Ni³⁺ enhances the chemisorption of OH^?, which facilitates electron transfer to the surface during OER. The hierarchical porosity increases the number of exposed active sites, and facilitates mass transport. A water‐splitting electrolyzer using the prepared Ni₃S₄ as the anode catalyst and Pt/C as the cathode catalyst achieves a low cell voltage of 1.51 V at 10 mA cm^?2. Therefore, this work provides a new strategy for the rational design of highly active OER electrocatalysts with high valence Ni³⁺ and hierarchical porous architectures.     

A High‐Performance Binary Ni–Co Hydroxide‐based Water Oxidation Electrode with Three‐Dimensional Coaxial Nanotube Array Structure

Developing nanostructured Ni and Co oxides with a small overpotential and fast kinetics of the oxygen evolution reaction (OER) have drawn considerable attention recently because their theoretically high efficiency, high abundance, low cost, and environmental benignity in comparison with precious metal oxides, such as RuO₂ and IrO₂. However, how to increase the specific activity area and improve their poor intrinsic conductivity is still challenging, which significantly limits the overall OER rate and largely prevent their utilization. Thus, developing effective OER electrocatalysts with abundant active sites and high electrical conductivity still remains urgent. In this work, a scrupulous design of OER electrode with a unique sandwich‐like coaxial structure of the three‐dimensional Ni@[Ni^(2+/3+)Co₂(OH)_6–7]_x nanotube arrays (3D NNCNTAs) is reported. A Ni nanotube array with open end is homogeneous coated with Ni and Co co‐hydroxide nanosheets ([Ni^(2+/3+)Co₂(OH)_6–7]_x) and is employed as multifunctional interlayer to provide a large surface area and fast electron transport and support the outermost [Ni^(2+/3+)Co₂(OH)_6–7]_x layer. The remarkable features of high surface area, enhanced electron transport, and synergistic effects have greatly assured excellent OER activity with a small overpotential of 0.46 V at the current density of 10 mA cm^?2 and high stability.     

A Cobalt‐Based Amorphous Bifunctional Electrocatalysts for Water‐Splitting Evolved from a Single‐Source Lazulite Cobalt Phosphate

Over the years, cobalt phosphates (amorphous or crystalline) have been projected as one of the most significant and promising classes of nonprecious catalysts and studied exclusively for the electrocatalytic and photocatalytic oxygen evolution reaction (OER). However, their successful utilization of hydrogen evolution reaction (HER) and the reaction of overall water‐splitting is rather unexplored. Herein, presented is a crystalline cobalt phosphate, Co₃(OH)₂(HPO₄)₂, structurally related to the mineral lazulite, as an efficient precatalyst for OER, HER, and water electrolysis in alkaline media. During both electrochemical OER and HER, the Co₃(OH)₂(HPO₄)₂ nanostructure was completely transformed in situ into porous amorphous CoO_x (OH) films that mediate efficient OER and HER with extremely low overpotentials of only 182 and 87 mV, respectively, at a current density of 10 mA cm^?2. When assemble as anode and cathode in a two‐electrode alkaline electrolyzer, unceasing durability over 10 days is achieved with a final cell voltage of 1.54 V, which is superior to the recently reported effective bifunctional electrocatalysts. The strategy to achieve more active sites for oxygen and hydrogen generation via in situ oxidation or reduction from a well‐defined inorganic material provides an opportunity to develop cost‐effective and efficient electrocatalysts for renewable energy technologies.     

Boosting H2 Generation Coupled with Selective Oxidation of Methanol into Value‐Added Chemical over Cobalt Hydroxide@Hydroxysulfide Nanosheets Electrocatalysts

The sluggish kinetics of oxygen evolution reaction (OER) is the main bottleneck for the electrocatalytic water splitting to produce hydrogen (H₂), and the by‐product is worthless O₂. Therefore, designing a thermodynamically favorable oxidation reaction to replace OER and coupling with value‐added product generation on the anode is of significance for boosting H₂ generation under low electrolysis voltage. Herein, cobalt hydroxide@hydroxysulfide nanosheets on carbon paper (Co(OH)₂@HOS/CP) are synthesized as bifunctional electrocatalysts to facilitate H₂ production and convert methanol to valuable formate simultaneously. Benefiting from the influences/changes on the composition, surface properties, electronic structure, and chemistry of Co(OH)₂, the as‐obtained electrodes exhibit very high selectivity for methanol to value‐added formate oxidation (MFO) and boost electrocatalytic performance with low overpotential of 155 mV for MFO and 148 mV for hydrogen evolution reaction at a current density of 10 mA cm^?2. Furthermore, the integrated two‐electrode electrolyzer drives 10 mA cm^?2 at a cell voltage of 1.497 V with united 100% Faradaic efficiency for anodic and cathodic reaction and continuous 20 h of operation without obvious decay. The electrocatalytic hydrogen production with the assistance of alternative oxidation by the robust electrocatalyst can be further used to realize the upgrading of other organic molecules with less energy consumption.     

3D Self‐Supported Fe‐Doped Ni2P Nanosheet Arrays as Bifunctional Catalysts for Overall Water Splitting

The development of highly efficient bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial for improving the efficiency of overall water splitting, but still remains challenging issue. Herein, 3D self‐supported Fe‐doped Ni₂P nanosheet arrays are synthesized on Ni foam by hydrothermal method followed by in situ phosphorization, which serve as bifunctional electrocatalysts for overall water splitting. The as‐synthesized (Ni_0.33Fe_0.67)₂P with moderate Fe doping shows an outstanding OER performance, which only requires an overpotential of ≈230 mV to reach 50 mA cm^?2 and is more efficient than the other Fe incorporated Ni₂P electrodes. In addition, the (Ni_0.33Fe_0.67)₂P exhibits excellent activity toward HER with a small overpotential of ≈214 mV to reach 50 mA cm^?2. Furthermore, an alkaline electrolyzer is measured using (Ni_0.33Fe_0.67)₂P electrodes as cathode and anode, respectively, which requires cell voltage of 1.49 V to reach 10 mA cm^?2 as well as shows excellent stability with good nanoarray construction. Such good performance is attributed to the high intrinsic activity and superaerophobic surface property.     

Promoting Active Sites in Core–Shell Nanowire Array as Mott–Schottky Electrocatalysts for Efficient and Stable Overall Water Splitting

Developing earth‐abundant, active, and robust electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remains a vital challenge for efficient conversion of sustainable energy sources. Herein, metal–semiconductor hybrids are reported with metallic nanoalloys on various defective oxide nanowire arrays (Cu/CuO_x, Co/CoO_x, and CuCo/CuCoO_x) as typical Mott–Schottky electrocatalysts. To build the highway of continuous electron transport between metals and semiconductors, nitrogen‐doped carbon (NC) has been implanted on metal–semiconductor nanowire array as core–shell conductive architecture. As expected, NC/CuCo/CuCoO_x nanowires arrays, as integrated Mott–Schottky electrocatalysts, present an overpotential of 112 mV at 10 mA cm^?2 and a low Tafel slope of 55 mV dec^?1 for HER, simultaneously delivering an overpotential of 190 mV at 10 mA cm^?2 for OER. Most importantly, NC/CuCo/CuCoO_x architectures, as both the anode and the cathode for overall water splitting, exhibit a current density of 10 mA cm^?2 at a cell voltage of 1.53 V with excellent stability due to high conductivity, large active surface area, abundant active sites, and the continuous electron transport from prominent synergetic effect among metal, semiconductor, and nitrogen‐doped carbon. This work represents an avenue to design and develop efficient and stable Mott–Schottky bifunctional electrocatalysts for promising energy conversion.     

Metal‐Ion (Fe,V, Co,and Ni)‐Doped MnO2 Ultrathin Nanosheets Supported on Carbon Fiber Paper for the Oxygen Evolution Reaction

Manganese dioxides (MnO₂) are considered one of the most attractive materials as an oxygen evolution reaction (OER) electrode due to its low cost, natural abundance, easy synthesis, and environmental friendliness. Here, metal‐ion (Fe, V, Co, and Ni)‐doped MnO₂ ultrathin nanosheets electrodeposited on carbon fiber paper (CFP) are fabricated using a facile anodic co‐electrodeposition method. A high density of nanoclusters is observed on the surface of the carbon fibers consisting of doped MnO₂ ultrathin nanosheets with an approximate thickness of 5 nm. It is confirmed that the metal ions (Fe, V, Co, and Ni) are doped into MnO₂, improving the conductivity of MnO₂. The doped MnO₂ ultrathin nanosheet/CFP and the IrO₂/CFP composite electrodes for OER achieve a low overpotential of 390 and 245 mV to reach 10 mA cm^?2 in 1 m KOH, respectively. The potential of the doped composite electrode for long‐term OER at a constant current density of 20 mA cm^?2 is much lower than that of the pure MnO₂ composite electrode.     

Metal (Ni,Co)‐Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) along with hydrogen evolution reaction (HER) have been considered critical processes for electrochemical energy conversion and storage through metal‐air battery, fuel cell, and water electrolyzer technologies. Here, a new class of multifunctional electrocatalysts consisting of dominant metallic Ni or Co with small fraction of their oxides anchored onto nitrogen‐doped reduced graphene oxide (rGO) including Co‐CoO/N‐rGO and Ni‐NiO/N‐rGO are prepared via a pyrolysis of graphene oxide and cobalt or nickel salts. Ni‐NiO/N‐rGO shows the higher electrocatalytic activity for the OER in 0.1 m KOH with a low overpotential of 0.24 V at a current density of 10 mA cm^?2, which is superior to that of the commercial IrO₂. In addition, it exhibits remarkable activity for the HER, demonstrating a low overpotential of 0.16 V at a current density of 20 mA cm^?2 in 1.0 m KOH. Apart from similar HER activity to the Ni‐based catalyst, Co‐CoO/N‐rGO displays the higher activity for the ORR, comparable to Pt/C in zinc‐air batteries. This work provides a new avenue for the development of multifunctional electrocatalysts with optimal catalytic activity by varying transition metals (Ni or Co) for these highly demanded electrochemical energy technologies.     

Amorphous Cobalt–Iron Hydroxide Nanosheet Electrocatalyst for Efficient Electrochemical and Photo‐Electrochemical Oxygen Evolution

Finding efficient electrocatalysts for oxygen evolution reaction (OER) that can be effectively integrated with semiconductors is significantly challenging for solar‐driven photo‐electrochemical (PEC) water splitting. Herein, amorphous cobalt–iron hydroxide (CoFe? H) nanosheets are synthesized by facile electrodeposition as an efficient catalyst for both electrochemical and PEC water oxidation. As a result of the high electrochemically active surface area and the amorphous nature, the optimized amorphous CoFe? H nanosheets exhibit superior OER catalytic activity in alkaline environment with a small overpotential (280 mV) to achieve significant oxygen evolution (j = 10 mA cm^?2) and a low Tafel slope (28 mV dec^?1). Furthermore, CoFe? H nanosheets are simply integrated with BiVO₄ semiconductor to construct CoFe? H/BiVO₄ photoanodes that exhibit a significantly enhanced photocurrent density of 2.48 mA cm^?2 (at 1.23 V vs reversible hydrogen electrode (RHE)) and a much lower onset potential of 0.23 V (vs RHE) for PEC‐OER. Careful electrochemical and optical studies reveal that the improved OER kinetics and high‐quality interface at the CoFe? H/BiVO₄ junction, as well as the excellent optical transparency of CoFe? H nanosheets, contribute to the high PEC performance. This study establishes amorphous CoFe? H nanosheets as a highly competitive candidate for electrochemical and PEC water oxidation and provides general guidelines for designing efficient PEC systems.     

High‐Performance Metal‐Free Nanosheets Array Electrocatalyst for Oxygen Evolution Reaction in Acid

Development of low cost electrocatalysts with outstanding catalytic activity and stability for oxygen evolution reaction (OER) in acid is a major challenge to produce hydrogen energy from water splitting. Herein, a novel metal‐free electrocatalyst consisting of a oxygen‐functionalized electrochemically exfoliated graphene (OEEG) nanosheets array is reported. Benefitting from a vertically aligned arrays structure and introducing oxygen functional groups, the metal‐free OEEG nanosheets array exhibits superior electrocatalytic activity and stability toward OER with a low overpotential of 334 mV at 10 mA cm^?2 in acidic electrolyte. Such a high OER performance is thus far the best among all previously reported metal‐free carbon‐based materials, and even superior to commercial Ir/C catalysts (420 mV at 10 mA cm^?2) in acid. Characterization results and electrochemical measurements identify the COOH species in the OEEG acting as active sites for acidic OER, which is further supported by atomic‐scale scanning transmission electron microscopy imaging and electron energy‐loss spectroscopy. Density functional theory calculations reveal that the reaction pathway of dual sites that is mixed by zigzag and armchair edges (COOH‐zig‐corner) is better than the pathway of single site.     

Overall Water Splitting Catalyzed Efficiently by an Ultrathin Nanosheet‐Built,Hollow Ni3S2‐Based Electrocatalyst

Making highly efficient catalysts for an overall ?water splitting reaction is vitally important to bring solar/electrical‐to‐hydrogen energy conversion processes into reality. Herein, the synthesis of ultrathin nanosheet‐based, hollow MoO_x/Ni₃S₂ composite microsphere catalysts on nickel foam, using ammonium molybdate as a precursor and the triblock copolymer pluronic P123 as a structure‐directing agent is reported. It is also shown that the resulting materials can serve as bifunctional, non‐noble metal electrocatalysts with high activity and stability for the hydrogen evolution reaction (HER) as well as the oxygen evolution reaction (OER). Thanks to their unique structural features, the materials give an impressive water‐splitting current density of 10 mA cm^?2 at ≈1.45 V with remarkable durability for >100 h when used as catalysts both at the cathode and the anode sides of an alkaline electrolyzer. This performance for an overall water splitting reaction is better than even those obtained with an electrolyzer consisting of noble metal‐based Pt/C and IrO_x/C catalytic couple—the benchmark catalysts for HER and OER, respectively.     

Electro‐Oxidation of Ni42 Steel: A Highly Active Bifunctional Electrocatalyst

Janus type water‐splitting catalysts have attracted highest attention as a tool of choice for solar to fuel conversion. AISI Ni42 steel is upon harsh anodization converted into a bifunctional electrocatalyst. Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are highly efficiently and steadfast catalyzed at pH 7, 13, 14, 14.6 (OER) and at pH 0, 1, 13, 14, 14.6 (HER), respectively. The current density taken from long‐term OER measurements in pH 7 buffer solution upon the electro‐activated steel at 491 mV overpotential (η) is around four times higher (4 mA cm^?2) in comparison with recently developed OER electrocatalysts. The very strong voltage–current behavior of the catalyst shown in OER polarization experiments at both pH 7 and at pH 13 are even superior to those known for IrO₂‐RuO₂. No degradation of the catalyst is detected even when conditions close to standard industrial operations are applied to the catalyst. A stable Ni‐, Fe‐oxide based passivating layer sufficiently protects the bare metal for further oxidation. Quantitative charge to oxygen (OER) and charge to hydrogen (HER) conversion are confirmed. High‐resolution XPS spectra show that most likely γ?NiO(OH) and FeO(OH) are the catalytic active OER and NiO is the catalytic active HER species.     

Boosting Activity on Co4N Porous Nanosheet by Coupling CeO2 for Efficient Electrochemical Overall Water Splitting at High Current Densities

Developing highly active nonprecious electrocatalysts with superior durability for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial to improve the efficiency of overall water splitting but remains challenging. Here, a novel superhydrophilic Co₄N‐CeO₂ hybrid nanosheet array is synthesized on a graphite plate (Co₄N‐CeO₂/GP) by an anion intercalation enhanced electrodeposition method, followed by high‐temperature nitridation. Doping CeO₂ into Co₄N can favor dissociation of H₂O and adsorption of hydrogen, reduce the energy barrier of intermediate reactions of OER, and improve the compositional stability, thereby dramatically boosting the HER performance while simultaneously inducing enhanced OER activity. Furthermore, the superhydrophilic self‐supported electrode with Co₄N‐CeO₂ in situ grown on the conductive substrate expedites electron conduction between substrate and catalyst, promotes the bubble release from electrode timely and impedes catalyst shedding, ensuring a high efficiency and stable working state. Consequently, the Co₄N‐CeO₂/GP electrode shows exceptionally low overpotentials of 24 and 239 mV at 10 mA cm^?2 for HER and OER, respectively. An alkaline electrolyzer by using Co₄N‐CeO₂/GP as both the cathode and anode requires a cell voltage of 1.507 V to drive 10 mA cm^?2, outperforming the Pt/C||RuO₂ electrolyzer (1.540 V@10 mA cm^?2). More significantly, the electrolyzer has extraordinary long‐term durability at a large current density of 500 mA cm^?2 for 50 h, revealing its potential in large‐scale applications.     

Etching‐Doping Sedimentation Equilibrium Strategy: Accelerating Kinetics on Hollow Rh‐Doped CoFe‐Layered Double Hydroxides for Water Splitting

Exploring highly active and inexpensive bifunctional electrocatalysts for water‐splitting is considered to be one of the prerequisites for developing hydrogen energy technology. Here, an efficient simultaneous etching‐doping sedimentation equilibrium (EDSE) strategy is proposed to design and prepare hollow Rh‐doped CoFe‐layered double hydroxides for overall water splitting. The elaborate electrocatalyst with optimized composition and typical hollow structure accelerates the electrochemical reactions, which can achieve a current density of 10 mA cm^?2 at an overpotential of 28 mV (600 mA cm^?2 at 188 mV) for hydrogen evolution reaction (HER) and 100 mA cm^?2 at 245 mV for oxygen evolution reaction (OER). The cell voltage for overall water splitting of the electrolyzer assembled by this electrocatalyst is only 1.46 V, a value far lower than that of commercial electrolyzer constructed by Pt/C and RuO₂ and most reported bifunctional electrocatalysts. Furthermore, the existence of Fe vacancies introduced by Rh doping and the typical hollow structure are demonstrated to optimize the entire HER and OER processes. EDSE associates doping with template‐directed hollow structures and paves a new avenue for developing bifunctional electrocatalysts for overall water splitting. It is also believed to be practical in other catalysis fields as well.     

Engineering an Earth‐Abundant Element‐Based Bifunctional Electrocatalyst for Highly Efficient and Durable Overall Water Splitting

The simultaneous and efficient evolution of hydrogen and oxygen with earth‐abundant, highly active, and robust bifunctional electrocatalysts is a significant concern in water splitting. Herein, non‐noble metal‐based Ni–Co–S bifunctional catalysts with tunable stoichiometry and morphology are realized. The engineering of electronic structure and subsequent morphological design synergistically contributes to significantly elevated electrocatalytic performance. Stable overpotentials (η₁₀) of 243 mV (vs reversible hydrogen electrode) for oxygen evolution reaction (OER) and 80 mV for hydrogen evolution reaction (HER), as well as Tafel slopes of 54.9 mV dec^?1 for OER and 58.5 mV dec^?1 for HER, are demonstrated. In addition, density functional theory calculations are performed to determine the optimal electronic structure via the electron density differences to verify the enhanced OER activity is related to the Co top site on the (110) surface. Moreover, the tandem bifunctional NiCo₂S₄ exhibit a required voltage of 1.58 V (J = 10 mA cm^?2) for simultaneous OER and HER, and no obvious performance decay is observed after 72 h. When integrated with a GaAs solar cell, the resulting photoassisted water splitting electrolyzer shows a certified solar‐to‐hydrogen efficiency of up to 18.01%, further demonstrating the feasibility of engineering protocols and the promising potential of bifunctional NiCo₂S₄ for large‐scale overall water splitting.     

New Iron‐Cobalt Oxide Catalysts Promoting BiVO4 Films for Photoelectrochemical Water Splitting

Owing to the sluggish kinetics for water oxidation, severe surface charge recombination is a major energy loss that hinders efficient photoelectrochemical (PEC) water splitting. Herein, a simple process is developed for preparing a new type of low‐cost iron‐cobalt oxide (FeCoO_x) as an efficient co‐catalyst to suppress the surface charge recombination on bismuth vanadate (BiVO₄) photoanodes. The new FeCoO_x/BiVO₄ photoanode exhibits a high photocurrent density of 4.82 mA cm^?2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which corresponds to >100% increase compared to that of the pristine BiVO₄ photoanode. The photoanode also demonstrates a high charge separation efficiency of ≈90% with excellent stability of over 10 h, indicating the excellent catalytic performance of FeCoO_x in the PEC process. Density functional theory calculations and experimental studies reveal that the incorporation of Fe into CoO_x generates abundant oxygen vacancies and forms a p‐n heterojunction with BiVO₄, which effectively promotes the hole transport/trapping from the BiVO₄ photocatalyst and reduces the overpotential for oxygen evolution reaction (OER), resulting in remarkably increased photocurrent densities and durability. This work demonstrates a feasible process for depositing cheap FeCoO_x as an excellent OER cocatalyst on photoanodes for PEC water splitting.     

A Self‐Standing High‐Performance Hydrogen Evolution Electrode with Nanostructured NiCo2O4/CuS Heterostructures

An efficient self‐standing 3D hydrogen evolution cathode has been developed by coating nickel cobaltite (NiCo₂O₄)/CuS nanowire heterostructures on a carbon fiber paper (CFP). The obtained CFP/NiCo₂O₄/CuS electrode shows exceptional hydrogen evolution reaction (HER) performance and excellent durability in acidic conditions. Remarkably, as an integrated 3D hydrogen‐evolving cathode operating in acidic electrolytes, CFP/NiCo₂O₄/CuS maintains its activity more than 50 h and exhibits an onset overpotential of 31.1 mV, an exchange current density of 0.246 mA cm^?2, and a Tafel slope of 41 mV dec^?1. Compared to other non‐Pt electrocatalysts reported to date, CFP/NiCo₂O₄/CuS exhibits the highest HER activity and can be used in HER to produce H₂ with nearly quantitative faradaic yield in acidic aqueous media with stable activity. Furthermore, by using CFP/NiCo₂O₄/CuS as a self‐standing electrode in a water electrolyzer, a current density of 18 mA cm^?2 can be achieved at a voltage of 1.5 V which can be driven by a single‐cell battery. This strategy provides an effective, durable, and non‐Pt electrode for water splitting and hydrogen generation.