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Electrochemical water splitting is a clean and sustainable process for hydrogen production on a large scale as the electrical power required can be obtained from various renewable energy resources. The key challenge in electrochemical water splitting process is to develop low‐cost electrocatalysts with high catalytic activity for the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. OER is the most important half‐reaction involved in water splitting, which has been extensively studied since the last century and a large amount of electrocatalysts including noble and non‐noble metal‐based materials have been developed. Among them, transition metal borides and borates (TMBs)‐based compounds with various structures have attracted increasing attention owing to their excellent OER performance. In recent years, many efforts have been devoted to exploring the OER mechanism of TMBs and to improving the OER activity and stability of TMBs. In this review, recent research progress made in TMBs as efficient electrocatalysts for OER is summarized. The chemical properties, synthetic methodologies, catalytic performance evaluation, and improvement strategy of TMBs as OER electrocatalysts are discussed. The electrochemistry fundamentals of OER are first introduced in brief, followed by a summary of the preparation and performance of TMBs‐based OER electrocatalysts. Finally, current challenges and future directions for TMBs‐based OER electrocatalysts are discussed. [ABSTRACT FROM AUTHOR]

Electrochemical water splitting has proven to be the most promising way to generate hydrogen. However, the high cost and low efficiency of the catalysts for cathode and anode in water splitting device remain the most knotty obstacle that needs to be stepped over. The optimal electrocatalysts can lower the energy costs required for water splitting via the reduced overpotential. Therefore, developing efficient and nonprecious electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) is highly critical and challenging. Cobalt (Co)-based electrocatalysts have been widely applied as electrocatalysts for HER and OER in water splitting. In this review, the mechanisms of HER and OER, was first illustrated, the preparation and electrochemical performance of various Co-based electrocatalysts, including cobalt oxides, hydroxides, sulfides, phosphides, nitrides and selenides for HER or OER are then summarized. Finally, the potential challenges and the future perspective of Co-based electrocatalysts are proposed. Image 1 [ABSTRACT FROM AUTHOR]

Catalytic systems based on nanospheres usually show satisfied catalytic activities due to their huge specific surface areas for active sites immobilization. However, their catalytic activities are usually restricted by the absence of active sites inside the nanospheres and the inadequate communications between the interior and exterior of the nanospheres. Therefore, it can be anticipated that great improvement will be achieved if a porous three-dimensional catalytic system can be designed, through which the activity sites can be distribute interior and exterior. Herein, polydopamine (PDA) NPs with uniform sizes of 300 nm are prepared. Then, the Ru nanoclusters (Ru NCs) are uniformly generated inside and on the surface of the PDA NPs to afford the bi-nanospheres (Ru-PDA NPs). After annealing, the porous carbonized Ru-PDA NPs (Ru-cPDA-750 NPs) with electroactive nitrogen atoms are finally obtained, exhibiting small overpotential (42 mV at the current density of 10 mA cm−2), low value of Tafel slope (35 mV decade−1) and robust stability for hydrogen evolution reaction (HER), which are comparable to that of Pt/C catalyst. Moreover, the study on the effect of Ru NCs amount on electrocatalytic ability reveals that Ru-cPDA NPs loaded with 0.08 wt% of Ru NCs exhibit the best HER catalytic performance compared with the Ru-cPDA NPs loaded with other amount of Ru NCs (0.04–0.16 wt%). Polydopamine nanoparticles (PDA NPs) were obtained by self-polymerization of dopamine and used as the porous framework to load nanoclusters (Ru NCs) either inside or outside for all-around generation of hydrogen. The catalytic system exhibited low overpotential, low value of Tafel slope, robust stability, and low cost for hydrogen evolution reaction, which are comparable to that of Pt/C catalyst. Image 1 • Spherical PDA NPs with controlled sizes from 50 to 500 nm are successfully prepared. • Ru NCs are generated both inside and outside the porous PDA NPs with electroactive N. • Both the size of Ru-cPDA NPs and the loading amount of Ru NCs can be well controlled. • The Ru-cPDA NPs show excellent HER activity and lower cost compared to that of Pt/C. [ABSTRACT FROM AUTHOR]

Catalysts of multiphase nickel sulfide coupling with CeO 2 (NiS x /CeO 2 /NF) are fabricated through one step hydrothermal and calcination process. The as-obtained NiS x /CeO 2 /NF exhibits excellent activity towards overall water splitting reaction with a low potential of 1.53 V for 10 mA cm−2 in 1 M KOH. [Display omitted] Constructing nanostructured electrocatalysts with heterointerface and finetuning their electronic properties are essential for high-efficient overall water splitting. Here, we prepared a well-designed nano-flower-like multiphase and hybrid material of NiS/NiS 2 /CeO 2 /NF (NiS x /CeO 2 /NF) with rich heterointerfaces and abundant active sites through solvothermal reaction and post-annealing treatment. The as-fabricated NiS x /CeO 2 /NF exhibits exceptional catalytic performance for OER and HER. Specifically, in 1 M KOH solution, it requires the low overpotentials of 326 and 92 mV to achieve the current density of 200 and 10 mA cm−2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. More satisfactorily, when NiS x /CeO 2 /NF is used as the bifunctional catalyst, a low voltage of only 1.53 V is required to achieve a current density of 10 mA cm−2 for overall water splitting. The excellent catalytic performance should be attributed to its special heterogeneous structure and the synergy effect between NiS x and CeO 2. This work emphasizes the important significance of constructing efficient bifunctional electrocatalysts by reasonably designing heterostructures and multiphase components for overall water splitting. [ABSTRACT FROM AUTHOR]

[Display omitted] Designing heterojunction catalysts with energy effects at the interface, particularly combining the surface structure advantages of super-hydrophilic interfaces with the high activity advantages of bimetal synergistic optimisation, is the key to developing economical and efficient industrial electrocatalytic water-splitting catalysts. In this study, a coupled nanoflower-like NiFe(OH) x /(Ni, Fe)Se heterostructure catalyst supported on Ni foam (NF) (NFSe@NFOH/NF) was designed and successfully prepared using hydrothermal and electrodeposition strategies. Owing to the electron interaction at the heterogeneous amorphous (NFOH)/crystalline (NFSe) interface and the bimetallic synergistic effect of Ni and Fe, the prepared NFSe@NFOH/NF exhibited excellent and stable oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalytic properties, with low overpotentials of 214/276 mV at 100 mA⋅cm−2 and 262/340 mV at 500 mA⋅cm−2. The assembled water electrolyser comprising NFSe@NFOH/NF || NFSe@NFOH/NF needed only small voltages of 1.73 and 1.85 V to yield current densities of 100 and 500 mA⋅cm−2, respectively. This study offers an innovative design idea for the rational adoption of interface engineering and amorphous–crystalline engineering techniques to construct catalysts with excellent catalytic activity and stability for electrocatalytic overall water splitting (EOWS) at a high current density, which further facilitates the advancement of sustainable energy technology in the future. [ABSTRACT FROM AUTHOR]

Ru@Fe-Ni(OH) 2 /NF catalyst fabricated through Fe-doped and interfacial Ru nanoclusters co-engineering of Ni(OH) 2 nanosheets directly grafted on Ni foam exhibits excellent activity towards oxygen evolution reaction (OER) in alkaline media with an overpotential of 266.4 mV to achieving a current density of 50 mA cm−2 in 1 M KOH. Our results show that we have successfully developed an efficient OER catalyst for green and efficient electrocatalytic hydrolysis to produce H 2 and O 2 , providing a promising method for clean H 2 production. [Display omitted] The creation of inexpensive, high-performance catalysts to reduce the overpotential of the oxygen evolution reaction (OER) process is critical for the electrolysis of water for hydrogen production. Therefore, we applied a one-step hydrothermal method using cation exchange reaction (CER) to prepare Fe-doped and interfacial Ru nanoclusters co-engineered Ni(OH) 2 nanosheets directly grafted on Ni foam (Ru@Fe-Ni(OH) 2 /NF) for OER process. Results of electrochemical tests reveal that Ru@Fe-Ni(OH) 2 /NF has excellent OER activity, and its overpotential (η) is only 266.4 mV when the current density is 50 mA cm−2 in 1 M KOH solution, even lower than that of commercial OER catalyst RuO 2 (355 mV). The Tafel slope also decreases from 133.8 mV dec−1 for pristine Ni(OH) 2 /NF material to 24.1 mV dec−1 for Ru@Fe-Ni(OH) 2 /NF, indicating the higher charge transfer rates and fastest kinetics for water oxidation. At an overpotential of 300 mV the optimal turnover frequency (TOF) of 0.062 s−1 for Ru@Fe-Ni(OH) 2 /NF is achieved compared to that of Ni(OH) 2 /NF (0.014 s−1, NN), demonstrating the fast reaction kinetics of the as-prepared electrocatcalyst. After 24 h stability test, the catalytic activity of Ru@Fe-Ni(OH) 2 /NF was only attenuated by 2 %, showing excellent OER stability and durability. Our results show that we have successfully developed an efficient OER catalyst for green and efficient electrocatalytic hydrolysis to produce H 2 and O 2 , providing a promising method for clean H 2 production. [ABSTRACT FROM AUTHOR]

[Display omitted] • This study realizes rational construction of the hierarchical wing-like iron/molybdenum oxide heterojunction on nickel foam (FMO/NF). • Hierarchical porous structure composed of the nanoparticle-micron sheets facilitates electrolyte/catalyst contact and bubble release. • The synergistic effect of Fe and Mo combined with the interfacial effect of the heterojunction optimizing structural configurations. • FMO/NF exhibits distinguished performance for overall water splitting in alkaline solution. Designing heterojunction catalysts with high-energy interfacial effects, especially combining the geometrical advantages of hierarchical micro-nano structures with the advantages of bi- or multi-metal syergistically optimised electronic coordination environments, is crucial for achieving efficient and stable water splitting. In this study, a simple one-step hydrothermal method was used to construct a hierarchical wing-like iron/molybdenum oxide heterojunction with a porous structure on nickel foam (FMO/NF). The synergistic effect of Fe, Mo, and the heterostructures can enrich structural defects, overcome the disadvantages of the individual components, and improve material performance by optimising the structural configurations and electronic properties and exploiting the electronic interactions that occur between interfaces composed of different phases. In addition, owing to the high porosity of the hierarchical micro-nano structure and abundant active sites, the wing-like FMO/NF was utilised as an efficient bifunctional catalyst for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), presenting low overpotentials of 278.06 and 263.72 mV, respectively, at a current density of 100 mA·cm−2 in 1 mol/L KOH. Furthermore, assembling FMO/NF as both the anode and cathode (FMO/NF || FMO/NF) required a cell voltage of 1.87 V to drive 100 mA·cm−2 in 1 mol/L KOH, and it proceeded continuously for 110 h with negligible cell voltage decay. This work provides a rational synthetic route for the preparation of innovative double transition metal-based micro-nano hierarchical heterostructured electrocatalysts with a synergistic effect and further advances the development of energy-conversion technology. [ABSTRACT FROM AUTHOR]

[Display omitted] Exploration of highly efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts is of crucial importance for the development of water splitting. In recent years, Cu-based materials have been widely concerned in OER due to their non-toxicity, rich reserves and highest reversibility. Meanwhile, Cu x O nanorods is easy to be prepared in industry. Herein, we report a fast preparation method to construct an integrated Cu x O@CoNi-LDH electrocatalyst with a unique 1D nanowire-2D nanosheet hierarchical core–shell structure by electrodepositing CoNi-LDH nanosheet arrays directly onto Cu x O nanorods (Cu x O were in situ-created on the Cu foam) at a large deposition voltage of −3.0 V vs SCE. The unique heterogeneous core–shell nanostructure, large deposition amount of CoNi-LDH and the synergistic effects between Cu x O core and CoNi-LDH shell can provide abundant active sites, rich open-channels and reduced charge transfer resistance (R ct) for effective oxygen release and facile electron transport. Consequently, the optimized Cu x O@CoNi-LDH/CF exhibits a low overpotential of 207 mV in 1 M KOH solution at the current density of 10 mV cm−2 and a small Tafel slope of 50.1 mV dec−1. After 60 h of long-term stability test, the catalytic performance is only slightly weakened. This work demonstrates a new approach to design the high-performance LDH and Cu-related OER catalysts by constructing a unique hierarchical core–shell nanostructure. [ABSTRACT FROM AUTHOR]

• Mo 3 S 4 /Co 1-x S nanosheets are synthesized by a simple one-step hydrothermal method. • Mo 3 S 4 /Co 1-x S@CF-8 has high hydrophilicity. • The bimetallic sulfide compound has high catalytic activity. • Mo 3 S 4 /Co 1-x S@CF-8 can achieve efficient overall water splitting. Hydrogen generation through electrochemical water decomposition is a promising method to address the global energy crisis. Herein, we report the synthesis of a series of flower-like Mo 3 S 4 /Co 1-x S composites on Co foil (Mo 3 S 4 /Co 1-x S@CF) as high-performance electrochemical water-splitting catalysts in an alkaline environment. The flower-like array structure of Mo 3 S 4 /Co 1-x S@CF not only increases the electrochemically active surface area of ​​the catalyst, but also facilitates the release of bubbles generated, resulting in enhanced catalytic activity. For the hydrogen evolution reaction, the Mo 3 S 4 /Co 1-x S@CF electrode exhibits good stability and excellent catalytic activity in 1.0 M KOH (η 10 = 105 mV), 1.0 M PBS (η 10 = 92 mV) and 0.5 M H 2 SO 4 (η 10 = 68 mV) solutions. For the oxygen evolution reaction, the electrode displays excellent stability and catalytic activity in 1.0 M KOH solution (η 10 = 215 mV). When used for overall water splitting in 1.0 M KOH solution, Mo 3 S 4 /Co 1-x S@CF achieves a current density of 10 mA cm−2 at a low potential of 1.58 V and maintains it stably for 40 h. This study presents a simple method for preparing transition metal-based bimetallic composite catalysts for efficient hydrogen production. [ABSTRACT FROM AUTHOR]

Considering the synergistic effect of transition metal phosphides and 3D structure substrates, herein, we directly deposited well-defined hierarchical radial Ni x P nanospheres on Ni foam via a facile, one-step co-electrochemical method to function as highly active flexible electrodes for both hydrogen evolution reaction and supercapacitor. Impressively, the bifunctional Ni x P/NF electrode requires an overpotential of only 63 mV to achieve a current density of 10 mA cm −2 and demonstrates a low Tafel slop of 55 mV dec −1 in 1 M KOH. Moreover, the electrochemical supercapacitor based on this hierarchical Ni x P/NF electrode also shows a superior specific capacitance of 1684 mF cm −2 (382.7 F/g) at a current density of 2 mA cm −2 and an outstanding retention ratio of 110.9% of its original capacitance value after 4000 cycles in 6 M KOH. Hopefully, excellent electrochemical performances demonstrate that this Ni x P/NF material could be a promising candidate for future applications in both hydrogen evolution reactions and supercapacitors. [ABSTRACT FROM AUTHOR]

Modifying electrocatalysts' structures and regulating their reaction kinetics is essential to improve the performance of electrocatalysts. Here, we report the Fe 3 O 4 /NiFe layered double hydroxides/Fe 3 O 4 (Fe 3 O 4 /NiFe LDH / Fe 3 O 4) with a sandwich structure by a one-step hydrothermal process. The interface synergistic effect between Fe 3 O 4 and NiFe LDH together with abundant electroactive sites encapsulated inside the Fe 3 O 4 oxide layer can stabilize important charge transfer intermediates and promote the reaction kinetics, which could result in remarkable electrochemical performance. As a result, the resulting Fe 3 O 4 /NiFe LDH/Fe 3 O 4 exhibits the lowest overpotential requirements of η 10 = 134 mV and η 50 = 267 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH, respectively. A two-electrode electrolyzer assembled with Fe 3 O 4 /NiFe LDH/Fe 3 O 4 as both cathode and anode require a low cell voltage of 1.648 V to achieve the current density response of 10 mA cm−2 toward overall water splitting. Furthermore, the Fe 3 O 4 /NiFe LDH/Fe 3 O 4 also displays excellent industrial catalytic performance by simulating the industrial electrolysis for OER, requiring only 353 mV at the large current density of 1000 mA cm−2. This work puts a deep insight into the role of the confined space of transition metal-based catalysts in electrocatalysis. • Fe 3 O 4 /NiFe LDH/Fe 3 O 4 is fabricated via a one-pot hydrothermal method. • Fe 3 O 4 /NiFe LDH/Fe 3 O 4 shows prominent electrocatalytic properties and good stability. • Fe 3 O 4 /NiFe LDH/Fe 3 O 4 achieves efficient water oxidation at a large current density. [ABSTRACT FROM AUTHOR]

• Cu/CuO@Co/Co 3 O 4 samples were synthesized by a simple three-step reaction process. • Cu/CuO@Co/Co 3 O 4 -300 exhibits remarkable HER with low Tafel slopes and durable performances. • This research provides a new method for designing efficient non-precious catalysts for hydrogen evolution reaction. [Display omitted] The hydrogen evolution reaction (HER) is significant to the development of next-generation electrochemical energy devices. The exploration for a non-noble metal catalyst for energy conversion to replace rare and expensive noble metals is fully necessary. Herein, a novel hierarchical Cu and Co-decorated CuO@Co 3 O 4 nanorod-thorns structure is generated on the Cu foam (denoted as Cu/CuO@Co/Co 3 O 4) and utilized as an efficient electrocatalyst for hydrogen evolution reaction (HER) in 1.0 M KOH electrolyte. The Cu/CuO@Co/Co 3 O 4 -300 electrode displays low overpotentials for HER (82 mV) electrocatalytic performance at the current density of 10 mA cm−2 and low Tafel slopes of 48 mV dec-1 are also achieved. Impressively, Cu/CuO@Co/Co 3 O 4 -300 can consecutively generate H 2 with of its initial performance after 35 h in 1.0 M KOH electrolyte. Meanwhile, the electrochemical performance of Cu/CuO@Co/Co 3 O 4 -300 is superior or considerable to most of reported on Cu-based water splitting electrocatalysts. As evidenced, reduction of oxides to metal for electrocatalysts can remarkably raise the catalytic activities for hydrogen evolution reaction. In a word, this work affords a new thought to design efficient non-precious catalysts for hydrogen evolution reaction. [ABSTRACT FROM AUTHOR]

• Ti mesh-supported heterostructured CoO/Co 3 O 4 nanowire array is fabricated. • CoO/Co 3 O 4 heterojunctions provide abundant active sites while array architecture enhances stability to boost charge transfer. • Heterostructured CoO/Co 3 O 4 NA/Ti acts as efficient catalyst for HER, demonstrating superior activity and stability. • CoO/Co 3 O 4 NA/Ti also shows high activity and stability for HER in neutral conditions. Heterostructured catalysts CoO/Co 3 O 4 consist of abundant interfaces between CoO and Co 3 O 4 have demonstrated extraordinary electrocatalytic performance toward HER. [Display omitted] Heterostructured CoO/Co 3 O 4 nanowire array on Titanium mesh (CoO/Co 3 O 4 NA/Ti) is fabricated via a facile controllable calcination treatment of the Co 3 O 4 precursor in a reducing atmosphere (H 2 /Ar) and applied as efficient electrocatalyst for hydrogen evolution reaction (HER). This electrode could catalyze the HER in 1.0 M KOH electrolyte with initial overpotential as low as 2.7 mV and need overpotential of 108 mV to approach 10 mA cm−2. Remarkably, it also shows high catalytical activity and long-time durability for HER both in phosphate buffer saline (PBS) and NaCl solution. This work may inspire the design of other high-performance three dimensional-nanostuctured (3D-nanostuctured) HER catalysts with abundant heterojunction interfaces. [ABSTRACT FROM AUTHOR]

• Fe-doped Ni 3 S 2 nanorods are synthesized by a simple one-step hydrothermal method. • Application of bimetallic composite foam (FeNi 3) avoid the use of iron salt reagents. • Fe doping significantly improves the catalytic performance of Ni 3 S 2. • Fe-Ni 3 S 2 @FeNi 3 -8 can achieve efficient overall water splitting and urea electrolysis. The development of high-performance and inexpensive bifunctional electrocatalysts to replace precious metal catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) remains a thorny issue in the field of electrochemical water splitting. Here, a three-dimensional rods array catalysts of Ni 3 S 2 doped with trace amount of Fe on foam FeNi 3 (Fe-Ni 3 S 2 @FeNi 3) are synthesized. It is found experimentally that Fe-Ni 3 S 2 @FeNi 3 has excellent catalytic activity and corrosion resistance in both acidic, neutral and alkaline solutions. In 1.0 M KOH solution, only small overpotentials of 105 mV and 213 mV are required to achieve the current densities of 10 mA cm−2 for HER and OER respectively. In addition, Fe-Ni 3 S 2 @FeNi 3 -8 also exhibits excellent catalytic activity in 0.5 M H 2 SO 4 solution (η 10 = 48 mV) and 1.0 M PBS solution (η 10 = 83 mV) for HER. In 1.0 M KOH and 0.33 M urea solution, Fe-Ni 3 S 2 @FeNi 3 -8 requires only 1.40 V to provide the current density of 10 mA cm−2 for urea oxidation reaction (UOR). As evidenced, doping Fe element into catalyst can significantly increase the catalytic activities for HER and OER. This work demonstrates a new and facile approach to prepare novel bimetallic transition metal catalysts as inexpensive alternatives to precious metal catalysts for efficient hydrogen production. [ABSTRACT FROM AUTHOR]