Earth-Abundant Transition-Metal-Based Bifunctional Electrocatalysts for Overall Water Splitting in Alkaline Media

The depletion of fossil fuels has accelerated the search for clean, sustainable, scalable, and environmentally friendly alternative energy sources. Hydrogen is a potential energy carrier because of its advantageous properties, and the electrolysis of water is considered as an efficient method for its industrial production. However, the high-energy conversion efficiency of electrochemical water splitting requires cost-effective and highly active electrocatalysts. Therefore, researchers have aimed to develop high-performance electrode materials based on non-precious and abundant transition metals for conversion devices. Moreover, to further reduce the cost and complexity in real-world applications, bifunctional catalysts that can be simultaneously active on both the anodic (i.e., oxygen evolution reaction, OER) and cathodic (i.e., hydrogen evolution reaction, HER) sides are economically and technically desirable. This Minireview focuses on the recent progress in transition-metal-based materials as bifunctional electrocatalysts, including several promising strategies to promote electrocatalytic activities for overall water splitting in alkaline media, such as chemical doping, defect (vacancy) engineering, phase engineering, facet engineering, and structure engineering. Finally, the potential for further developments in rational electrode materials design is also discussed.     

Designing Self‐Supported Metal‐Organic Framework Derived Catalysts for Electrochemical Water Splitting

To achieve efficient water splitting, it is essential to develop catalysts with high electrochemical performance, enhanced durability and tunable properties. Most of the transition metal‐based catalysts employed for the water splitting have been fabricated on the solid‐electrode support by using binder, which decreases the activity and durability of the catalyst system. In this respect, self‐supported metal organic framework (MOF) derived catalysts have been introduced with enhanced catalytic activity and mechanical stability for the electrochemical water splitting. The self‐supported MOF derived catalysts exhibit improved electronic conductivity, high electrochemical surface area, enhanced mechanical stability and strong catalyst‐support interaction. Moreover, these catalysts possess highly porous and hollow structure with designed morphology and multi‐metallic composition. Recently, a tremendous effort has been provided to explore this newly growing field and new dimensions and directions have been achieved. Looking at this point, we have described here the basic principles of catalyst design from self‐supported MOF, structural and interface engineering by controlling the electronic structure of the catalysts to improve the water splitting activity. In addition, the challenges and difficulties associated with this field have been pointed out and addressed for the future progress in this field.     

Hollow Nanotube Ru/Cu2+1O Supported on Copper Foam as a Bifunctional Catalyst for Overall Water Splitting

Hydrogen energy is considered as one of the ideal clean energies for solving the energy shortage and environmental issues, and developing highly efficient electrocatalysts for overall water splitting to produce hydrogen is still a huge challenge. Herein, for the first time, Ru-doped Cu₂₊₁O vertically arranged nanotube arrays in situ grown on Cu foam (Ru/Cu₂₊₁O NT/CuF) are reported and further investigated for their catalytic properties for overall water splitting. The Ru/Cu₂₊₁O NT/CuF presents ultrahigh catalytic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline conditions, and it exhibits a small overpotential of 32 mV at 10 mA cm⁻² in the HER, and only needs 210 mV overpotential to achieve a current density of 10 mA cm⁻² in the OER. Importantly, the alkaline electrolyzer using Ru/Cu₂₊₁O NT/CuF as a bifunctional electrocatalyst only needs 1.53 V voltage to deliver a current density of 10 mA cm⁻², which is much lower than the benchmark of IrO₂(+)/Pt(−) counterpart (1.64 V at 10 mA cm⁻²). The excellent performance of the Ru/Cu₂₊₁O NT/CuF catalyst is attributed to its high conductive substrate and special Ru-doped nanotube structure, which provides a high electrochemical active surface area and 3D gas diffusion channel.     

Fe/Co‐based Bimetallic MOF‐derived Co3Fe7@NCNTFs Bifunctional Electrocatalyst for High‐Efficiency Overall Water Splitting

Electrocatalytic water splitting to produce hydrogen and oxygen is regarded as one of the most promising methods to generate clean and sustainable energy for replacing fossil fuels. However, the design and development of an efficient bifunctional catalyst for simultaneous generation of hydrogen and oxygen remains extremely challenging yet is critical for the practical implementation of water electrolysis. Here, we report a facile method to fabricate novel N‐doped carbon nanotube frameworks (NCNTFs) by the pyrolysis of a bimetallic metal organic framework (MIL‐88‐Fe/Co). The resultant electrocatalyst, Co₃Fe₇@NCNTFs, exhibits excellent oxygen evolution reaction (OER) activity, achieving 10 mA/cm² at a low overpotential of just 264 mV in 1 M KOH solution, and 197 mV for the hydrogen evolution reaction. The high electrocatalytic activity arises from the synergistic effect between the chemistry of the Co₃Fe₇ and the NCNTs coupled to the novel framework structure. The remarkable electrocatalytic performance of our bifunctional electrocatalyst provides a promising pathway to high‐performance overall water splitting and electrochemical energy devices.     

Ligand‐Assisted Co‐Assembly Approach toward Mesoporous Hybrid Catalysts of Transition‐Metal Oxides and Noble Metals: Photochemical Water Splitting

A bottom‐up synthetic approach was developed for the preparation of mesoporous transition‐metal‐oxide/noble‐metal hybrid catalysts through ligand‐assisted co‐assembly of amphiphilic block‐copolymer micelles and polymer‐tethered noble‐metal nanoparticles (NPs). The synthetic approach offers a general and straightforward method to precisely tune the sizes and loadings of noble‐metal NPs in metal oxides. This system thus provides a solid platform to clearly understand the role of noble‐metal NPs in photochemical water splitting. The presence of trace amounts of metal NPs (≈0.1 wt %) can enhance the photocatalytic activity for water splitting up to a factor of four. The findings can conceivably be applied to other semiconductors/noble‐metal catalysts, which may stand out as a new methodology to build highly efficient solar energy conversion systems.     

Molybdenum Carbide‐Embedded Multichannel Hollow Carbon Nanofibers as Bifunctional Catalysts for Water Splitting

With the environmental pollution and non‐renewable fossil fuels, it is imperative to develop eco‐friendly, renewable, and highly efficient electrocatalysts for sustainable energy. Herein, a simple electrospinning process used to synthesis Mo₂C‐embedded multichannel hollow carbon nanofibers (Mo₂C‐MCNFs) and followed by the pyrolysis process. As prepared lotus root‐like nanoarchitecture could offer rich porosity and facilitate the electrolyte infiltration, the Mo₂C‐MCNFs delivered favourable catalytic activity for HER and OER. The resultant catalysts exhibit low overpotentials of 114 mV and 320 mV at a current density of 10 mA cm^?2 for HER and OER, respectively. Furthermore, using the Mo₂C‐MCNFs catalysts as a bifunctional electrode toward overall water splitting, which only needs a small cell voltage of 1.68 V to afford a current density of 10 mA cm^?2 in the home‐made alkaline electrolyzer. This interesting work presents a simple and effective strategy to further fabricating tunable nanostructures for energy‐related applications.     

Hierarchical NiCo2O4 Hollow Microcuboids as Bifunctional Electrocatalysts for Overall Water‐Splitting

Bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte may improve the efficiency of overall water splitting. Nickel cobaltite (NiCo₂O₄) has been considered a promising electrode material for the OER. However, NiCo₂O₄ that can be used as an electrocatalyst in HER has not been studied yet. Herein, we report self‐assembled hierarchical NiCo₂O₄ hollow microcuboids for overall water splitting including both the HER and OER reactions. The NiCo₂O₄ electrode shows excellent activity toward overall water splitting, with 10 mA cm^?2 water‐splitting current reached by applying just 1.65 V and 20 mA cm^?2 by applying just 1.74 V across the two electrodes. The synthesis of NiCo₂O₄ microflowers confirms the importance of structural features for high‐performance overall water splitting.     

Constructing RuCoOx/NC Nanosheets with Low Crystallinity within ZIF-9 as Bifunctional Catalysts for Highly Efficient Overall Water Splitting

Electrocatalysts play a pivotal role in accelerating the sluggish electrochemical water splitting reaction. Herein, a Ru−Co oxides and carbon nitrides hybrid (RuCoO_x/NC) electrocatalyst was constructed by employing ZIF-9 to disperse Ru precursor and deliberately regulating the calcination temperature. The moderate calcination temperature results in the RuCoO_x nanocomposites with small particle size and low crystallinity as well as the co-existence of multi-valence metal compounds, thus boosting the amount and species of active sites. Moreover, the strong interactions between Co and Ru species induce the electron transfer from Co to Ru, thus enhancing the adsorption of anion intermediates on the electron-deficient Co species and the proton capturing capacity of electron-sufficient Ru species. As a result, the optimized RuCoO_x/NC-350 catalyst behaved good electrocatalytic activities with 73 and 210 mV overpotential to achieve 10 mA cm⁻² for HER and OER, respectively. Remarkably, it showed good durability by holding at 100 mA cm⁻² for 100 h in HER and 50 mA cm⁻² for 24 h in OER with small activity decline. This study may shed new light on the rational construction of highly efficient Ru-based catalysts for electrochemical water splitting.     

Surface‐Polarity‐Induced Spatial Charge Separation Boosts Photocatalytic Overall Water Splitting on GaN Nanorod Arrays

Photocatalytic overall water splitting has been recognized as a promising approach to convert solar energy into hydrogen. However, most of the photocatalysts suffer from low efficiencies mainly because of poor charge separation. Herein, taking a model semiconductor gallium nitride (GaN) as an example, we uncovered that photogenerated electrons and holes can be spatially separated to the nonpolar and polar surfaces of GaN nanorod arrays, which is presumably ascribed to the different surface band bending induced by the surface polarity. The photogenerated charge separation efficiency of GaN can be enhanced significantly from about 8 % to more than 80 % via co‐exposing polar and nonpolar surfaces. Furthermore, spatially assembling reduction and oxidation cocatalysts on the nonpolar and polar surfaces remarkably boosts photocatalytic overall water splitting, with the quantum efficiency increased from 0.9 % for the film photocatalyst to 6.9 % for the nanorod arrays photocatalyst.     

Preparation of Hierarchical Cube‐on‐plate Metal Phosphides as Bifunctional Electrocatalysts for Overall Water Splitting

To rationally design efficient and cost‐effective electrocatalysts, a simple but efficient strategy has been developed to directly anchor prussian blue analogue (PBA) nanocubes on cobalt hydroxide nanoplates (PBA@Co(OH)₂) via the in‐situ interfacial precipitation process. Subsequently, the thermal treatment in the presence of sodium hydrogen phosphite enabled the successful transition into metal phosphides with the hierarchical cube‐on‐plate structure. When used as electrocatalytsts, the obtained bimetal phosphides exhibited good bifunctional electrocatalytic activities for hydrogen and oxygen evolution reactions with good long‐term stability. Thus, an enhanced performance for overall water splitting can be achieved, which could be ascribed to the hierarchical structure and favorable composition of as‐prepared bimetal phosphide for rapid electron and mass transfer. The present study demonstrates a favorable approach to modulate the composition and structure of metal phosphide for enhancing the electrocatalytic ability toward water splitting.     

Bifunctional Electrocatalysts for Overall Water Splitting from an Iron/Nickel‐Based Bimetallic Metal–Organic Framework/Dicyandiamide Composite

Pyrolysis of a bimetallic metal–organic framework (MIL‐88‐Fe/Ni)‐dicyandiamide composite yield a Fe and Ni containing carbonaceous material, which is an efficient bifunctional electrocatalyst for overall water splitting. FeNi₃ and NiFe₂O₄ are found as metallic and metal oxide compounds closely embedded in an N‐doped carbon–carbon nanotube matrix. This hybrid catalyst (Fe‐Ni@NC‐CNTs) significantly promotes the charge transfer efficiency and restrains the corrosion of the metallic catalysts, which is shown in a high OER and HER activity with an overpotential of 274 and 202 mV, respectively at 10 mA cm^?2 in alkaline solution. When this bifunctional catalyst was further used for H₂ and O₂ production in an electrochemical water‐splitting unit, it can operate in ambient conditions with a competitive gas production rate of 1.15 and 0.57 μL s^?1 for hydrogen and oxygen, respectively, showing its potential for practical applications.     

钴铁双金属氧化物多孔纳米棒的制备及其电解水析氧性能

通过简单的钴铁前躯体热分解法制备了系列一维Co_1-xFe_xO_y（0≤x≤1）多孔纳米材料,并在1 mol·L^-1 KOH溶液中研究了其电解水析氧催化性能。研究发现不同Fe掺杂量对材料的结构与电解水析氧催化性能有较大的影响,其中16%（n/n）Fe掺杂量的Co_1-xFe_xO_y具有最优的析氧催化性能。在10 mA·cm^-2电流密度下其析氧过电位为345 mV,塔菲尔斜率为54 mV·dec^-1,并表现出优异的析氧稳定性能。廉价、高效的Co_1-xFe_xO_y多孔纳米棒材料有望成为优良的析氧催化剂用于电解水制氢。     

钴铁双金属氧化物多孔纳米棒的制备及其电解水析氧性能

通过简单的钴铁前躯体热分解法制备了系列一维Co_(1-x)Fe_xO_y(0≤x≤1)多孔纳米材料,并在1 mol·L~(-1) KOH溶液中研究了其电解水析氧催化性能。研究发现不同Fe掺杂量对材料的结构与电解水析氧催化性能有较大的影响,其中16%(n/n)Fe掺杂量的Co_(1-x)Fe_xO_y具有最优的析氧催化性能。在10 m A·cm~(-2)电流密度下其析氧过电位为345 mV,塔菲尔斜率为54 mV·dec~(-1),并表现出优异的析氧稳定性能。廉价、高效的Co_(1-x)Fe_xO_y多孔纳米棒材料有望成为优良的析氧催化剂用于电解水制氢。     

Two-Dimensional NiIr@N-Doped Carbon Nanocomposites Supported on Ni Foam for Electrocatalytic Overall Water Splitting

Electrochemical water splitting can provide a promising avenue for sustainable hydrogen production. Highly efficient electrocatalysts toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are extremely important for the practical application of water splitting technology. Herein, a one-step annealing strategy is reported for the fabrication of a metal–organic framework-derived bifunctional self-supported electrocatalyst, which is composed of two-dimensional N-doped carbon-wrapped Ir-doped Ni nanoparticle composites supported on Ni foam (NiIr@N-C/NF). The resultant NiIr@N-C/NF displays excellent electrocatalytic performance in 1.0 m KOH, with low overpotentials of 32 mV at 10 mA cm⁻² for the HER and 329 mV at 50 mA cm⁻² for the OER. Particularly, the HER-OER bifunctional NiIr@N-C/NF needs only 1.50 V to yield 10 mA cm⁻² for overall water splitting.     

A Molecular Tetrahedral Cobalt–Seleno-Based Complex as an Efficient Electrocatalyst for Water Splitting

The cobalt–seleno-based coordination complex, [Co{(SePⁱPr₂)₂N}₂], is reported with respect to its catalytic activity in oxygen evolution and hydrogen evolution reactions (OER and HER, respectively) in alkaline solutions. An overpotential of 320 and 630 mV was required to achieve 10 mA cm⁻² for OER and HER, respectively. The overpotential for OER of this CoSe₄-containing complex is one of the lowest that has been observed until now for molecular cobalt(II) systems, under the reported conditions. In addition, this cobalt–seleno-based complex exhibits a high mass activity (14.15 A g⁻¹) and a much higher turn-over frequency (TOF) value (0.032 s⁻¹) at an overpotential of 300 mV. These observations confirm analogous ones already reported in the literature pertaining to the potential of molecular cobalt–seleno systems as efficient OER electrocatalysts.     

Efficient Visible‐Light‐Driven Z‐Scheme Overall Water Splitting Using a MgTa2O6−xNy /TaON Heterostructure Photocatalyst for H2 Evolution

An (oxy)nitride‐based heterostructure for powdered Z‐scheme overall water splitting is presented. Compared with the single MgTa₂O_6?xN_y or TaON photocatalyst, a MgTa₂O_6?xN_y /TaON heterostructure fabricated by a simple one‐pot nitridation route was demonstrated to effectively suppress the recombination of carriers by efficient spatial charge separation and decreased defect density. By employing Pt‐loaded MgTa₂O_6?xN_y /TaON as a H₂‐evolving photocatalyst, a Z‐scheme overall water splitting system with an apparent quantum efficiency (AQE) of 6.8 % at 420 nm was constructed (PtO_x‐WO₃ and IO₃^?/I^? pairs were used as an O₂‐evolving photocatalyst and a redox mediator, respectively), the activity of which is circa 7 or 360 times of that using Pt‐TaON or Pt‐MgTa₂O_6?xN_y as a H₂‐evolving photocatalyst, respectively. To the best of our knowledge, this is the highest AQE among the powdered Z‐scheme overall water splitting systems ever reported.     

Organic Photocathode Supported by Copper Nanosheets Array for Overall Water Splitting

The Z-scheme overall solar water splitting is a mimic of natural photosynthesis to convert solar energy into chemical energy. Since the energy levels of most organic semiconductors match well with the hydrogen evolution potential, they have great application prospects as photocathodes in Z-scheme photoelectrochemical systems. However, due to the weak light absorption and difficult carrier separation, the photocurrent density and onset potential of organic photocathodes are still low. To solve these problems, we introduced a copper nanosheets array (Cu NSA) framework under organic layers to increase the surface reaction sites, improve the light absorption and enhance the distribution range of built-in electric field simultaneously. As a result, the photocurrent density and onset potential of poly(3-hexylthiophene) : [6,6]-phenyl-C₆₁-butyric acid (P3HT : PCBM) photocathode were enhanced significantly. The onset potential increased by 50 mV to 0.65 V vs. RHE, and the photocurrent density reached −1 mA cm⁻² at 0 V vs. RHE, which was 18 times that of the sample without Cu NSA. The optimized photocathode was connected with titanium dioxide nanorods array photoanode in a tandem manner to realize the spontaneous overall water splitting. Without bias and co-catalyst, the photocurrent density was maintained at 110 μA cm⁻² and the solar-to-fuel conversion efficiency was 0.14 % in neutral solution. These results provide a feasible method for optimizing the performance of organic photocathodes.