Nitrogen and molybdenum co-doped CoP nanohoneycombs on 3D nitrogen-doped porous graphene as enhanced electrocatalyst for oxygen evolution reaction

The four-electron transfer process involved in anodic oxygen evolution reaction (OER) of electrocatalytic water splitting causes the sluggish kinetics and significantly limiting the efficiency of energy conversion. It's urgent to explore low-cost, efficient and stable electrocatalysts for OER. In the work, we design the nitrogen and molybdenum co-doped CoP with nanohoneycombs structure on three-dimensional (3D) nitrogen-doped porous graphene (N/Mo–CoP@NPG) as an efficient OER electrocatalyst. The N/Mo–CoP@NPG delivers the current density of 10 mA cm⁻² at a low overpotential value of 201 mV in 1.0 M KOH, meanwhile the electrocatalytic activity shows no obvious degradation after 50 h. The NPG substrate provides plentiful ligaments for growth of N/Mo–CoP nanohoneycombs and 3D network for rapid electronic transfer. Additionally, doping N and Mo atoms into CoP synergistically modifies the micromorphology and electronic structure, benefiting the electrocatalytic ability. This work offers a promising strategy to improve the electrocatalytic activity of transition metal phosphides.     

Meritorious spatially on hierarchically Co3O4/MoS2 phase nanocomposite synergistically a high-efficient electrocatalyst for hydrogen evolution reaction performance: Recent advances & future perspectives

Hydrogen is zero-emission fuel production for a clean environment as alternative effective the energy source is still moreover, an effective challenge in near future due to the lack of efficient and inexpensive catalysts. An efficient electrocatalysts structure having logical design which holds a paramount significance for the hydrogen evolution reaction (HER) but rarely noble metal Pt-like activity achieved by the transition metal oxides electrocatalysts based on oxides matured and cooperative with coupling metal oxides could be considered as a desired substitute electrocatalysts to change Pt/C based nano composite materials. Herein, un-noble the metal oxides of hetero structure consisting of Co₃O₄/MoS₂ based-electrocatalysts nanocomposite material. The desirable out-comes show that Co₃O₄/MoS₂ composite material providing extraordinary efficient HER kinetics activity in different experimental designs. The Co₃O₄/MoS₂ based electro-catalyst increases the best activity of HER kinetics performance especially measured in 1 M KOH solution condition and offers an influential interfacing engineering strategy at very minute over potential of 348 mV evaluated and small Tafel slopes 46 mV/dec for HER performance. This work elucidates interest for efficient electrocatalysts for a broader range of scalable applications in the development of renewable energies, the functional materials such as solar cells, lithium sulphur-batteries and energy chemistry advancing.     

Electrochemical fabrication of Ni–Mo nanostars with Pt-like catalytic activity for both electrochemical hydrogen and oxygen evolution reactions

Synthesis of electrocatalysts with excellent performance for hydrogen and oxygen evolution are the main challenges for production of hydrogen by electrochemical water splitting method. Here, Ni–Mo nanostars were created by electrochemical deposition process at different morphologies and their electrocatalytic behavior was studied for hydrogen and oxygen evolution reactions in 1.0 M KOH solution. Increased electrochemically active surface area due to the nanostars formation, improved intrinsic electrocatalytic activity, increased surface wettability, as well as being binder-free during electrode production, resulted in excellent electrocatalytic behavior. For optimized condition, 60 mV and 225 mV overpotential are needed for generating the current density of 10 mA.cm-² in HER and OER process respectively in the alkaline medium. The lower slope of the electrode compared to the other electrodes also indicated that the kinetics of HER on the surface of the electrode was better. Also, there was very little change in the potential during the stability test, indicating the excellent electrocatalytic stability of the synthesized electrode. The present study introduces a rational, cost-effective and binder-free method for the synthesis of high performance electrocatalysts.     

Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition

NiFe layered double hydroxides (LDHs) is considered to be one of the LDHs electrocatalyst materials with the best electrocatalytic oxygen evolution properties. However, its poor conductivity and inherently poor electrocatalytic activity are considered to be the limiting factors inhibiting the electrocatalytic properties for oxygen evolution reaction (OER). The amorphous NiFeAl-LDHs electrocatalysts were prepared by electrodeposition with nickel foam as the support, and the D-NiFeAl-LDHs electrocatalyst with defect sites was then obtained by alkali etching. The mechanism of catalysts with defect sites in OER was analyzed. The ingenious defects can selectively accelerate the adsorption of OH⁻, thus enhancing the electrochemical activity. The D-NiFeAl-LDHs electrocatalyst had higher OER electrocatalytic activity than NiFe-LDHs electrocatalyst: its accelerated OER kinetics were mainly due to the introduction of iron and nickel defects in NiFeAl-LDHs nanosheets, which effectively adjusted the surface electronic structure and improved OER electrocatalytic performance. There was only a low overpotential of 262 mV with the current density of 10 mA cm⁻², and the Tafel slope was as low as 41.67 mV dec⁻¹. The OER electrocatalytic performance of D-NiFeAl-LDHs was even better than those of most of the reported NiFe-LDHs electrocatalysts.     

Design of Ni/NiO–TiO2/rGO nanocomposites on carbon cloth conductors via PECVD for electrocatalytic water splitting

The facile synthesis and design of noble metal-free efficient catalysts to accelerate the sluggish kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is still a big challenge for electrolytic water splitting. In that context, the preparation of efficient catalysts with superior catalytic activity from cheap raw materials on a large scale is crucial. Briefly, Ni/NiO/TiO₂/rGO is designed using the environmental-friendly and easily up-scalable PECVD technique. This trinary composite presents significance in regulating the crystalline structure, composition and electronic properties towards superior HER and OER activity in acidic solution as bifunctional electrocatalysts for efficient water splitting. Together with the promising long-term stability and durability, Ni/NiO/TiO₂/rGO displays excellent electrocatalytic activity towards HER with η₁₀ of 130 mv vs RHE and a Tafel slope of 40 mV/dec.     

双金属Ni-Co基析氧反应电催化剂研究进展

  目的  能源消耗的持续增长和化石燃料燃烧带来的环保和能源安全问题已经引起世界各国的广泛关注。因此,发展清洁能源生产技术已成为世界范围内的主要研究重点。氢能具有无污染、比能密度高、资源丰富等特点,是最具潜力的传统化石燃料替代品之一。电催化分解水被认为是最有希望的制氢方法,但阳极上的析氧反应动力学缓慢,能量转换效率低,是大规模制氢的主要瓶颈。与稀有和昂贵的贵金属催化剂相比,镍-钴（Ni-Co）基电催化剂由于具有可调的电子结构、高导电性和低成本优势,有望在碱性溶液中实现卓越的OER活性和耐久性。  方法  文章总结并讨论了在OER中Ni-Co基电催化剂的最新研究发展。重点讨论了Ni-Co基电催化剂的设计和合成,以及在OER过程中提高其电催化性能的研究策略。  结果  为了代替钌、铱等贵金属催化剂,研究者们对Ni-Co基非贵金属催化剂进行了大量研究。包括氧化物、氢氧化物、合金、氮化物、硫化物、磷化物等在内的多种Ni-Co基催化剂通过化学结构的调控,从阳极角度提高了电催化制氢的活性。但这些催化剂又分别面临不同的缺陷,有待进一步研究克服。  结论  开发具有高OER活性的非贵金属催化剂是降低电解水制氢成本,促进氢能产业发展的重要途径。虽然仍有一些技术问题尚未解决限制了Ni-Co基催化剂替代贵金属催化剂,但作为重要的贵金属催化剂替代品,Ni-Co基催化剂的研究为新型催化剂的开发提供了重要选择。     

Surface plasmons activate the oxygen evolution reaction over nickel hydroxide electrocatalysts

Improving the intrinsic poor conductivity and unsatisfactory electrocatalytic performance of the transition metal based electrocatalysts is highly desirable yet challenging in the research domain of water electrolysis. Herein, we report a novel electrocatalyst by decorating silver (Ag) nanoparticles (NPs) onto Ni(OH)₂ (NSs) towards electrocatalyzing oxygen evolution reaction (OER). The presence of Ag NPs can effectively promote the charge transfer because of the improved conductivity. More importantly, surface plasmons are generated in the Ni(OH)₂/Ag composite under the light irradiation, providing hot carriers which can not only accelerate the reaction kinetics by lowering the activation energy and charge transfer resistance, but also participate in the electrocatalytic reaction to further boost the activity. With the assistance of surface plasmon, the OER performance of Ni(OH)₂/Ag is significantly enhanced compared with pristine Ni(OH)₂ counterpart. This work offers a strategy to design electrocatalysts and demonstrates the importance of surface plasmon to the enhancement of OER activity.     

Enhanced oxygen evolution reaction kinetics through biochar-based nickel-iron phosphides nanocages in water electrolysis for hydrogen production

Highly-efficient and stable non-noble metal electrocatalysts for overcoming the sluggish kinetics of oxygen evolution reaction (OER) is urgent for water electrolysis. Biomass-derived biochar has been considered as promising carbon material because of its advantages such as low-cost, renewable, simple preparation, rich structure, and easy to obtain heteroatom by in-situ doping. Herein, Ni₂P–Fe₂P bimetallic phosphide spherical nanocages encapsulated in N/P-doped pine needles biochar is prepared via a simple two-step pyrolysis method. Benefiting from the maximum synergistic effects of bimetallic phosphide and biochar, high conductivity of biochar encapsulation, highly exposed active sites of Ni₂P–Fe₂P spherical nanocages, rapid mass transfer in porous channels with large specific surface area, and the promotion in adsorption of reaction intermediates by high-level heteroatom doping, the (Ni_0.75Fe_0.25)₂P@NP/C demonstrates excellent OER activity with an overpotential of 250 mV and a Tafel slope of 48 mV/dec at 10 mA/cm² in 1 M KOH. Also it exhibits a long-term durability in 10 h electrolysis and its activity even improves during the electrocatalytic process. The present work provides a favorable strategy for the inexpensive synthesis of biochar-based transition metal electrocatalysts toward OER, and improves the water electrolysis for hydrogen production.     

CoP-anchored high N-doped carbon@graphene sheet as bifunctional electrocatalyst for efficient overall water splitting

Electrocatalytic water splitting, as an ideal technology in renewable energy applications, suffers from high electrical energy consumption due to the slow kinetics of HER and OER reactions. Therefore, it is urgent to design efficient bifunctional catalysts to improve the reaction kinetics. Herein, a self-supported electrode, anchoring CoP nanoparticles on N-doped carbon/graphene (NC-G) and chemically growing on Ni foam as a whole electrode (denoted as NC-G-CoP/NF) displays promising electrocatalytic performance in 1.0 M KOH electrolyte, with a low overpotentials of 68 mV at 10 mA cm^?2 for HER and 255 mV at 50 mA cm^?2 for OER. This bifunctional electrocatalyst only needs 1.435 V to generate 10 mA cm^?2 for overall water splitting. The outstanding electrocatalytic performance is ascribed to the following factors: i) inherent nature of transition metal phosphides, ii) abundant and high dispersion N active sites in NC-G, iii) strong interaction between the NC-G and CoP nanoparticles, and iv) rapid electron transfer between the catalytic centers and Nickle foam. This provides a new perspective to design efficient electrocatalysts for electrocatalytic water splitting.     

Role of macrocyclic salen-type Schiff base ligands in one-dimensional Co(II) complexes for superior activities toward oxygen reduction/evolution reactions

Developing high activities, stable, non-precious metal based bi-functional electrocatalysts oxygen evolution/reduction reactions (OER/ORR) in rechargeable metal-air batteries and regenerative fuel cell technologies is essential for future energy conversion and storage. In this work, the potential of utilizing the synthesized one-dimensional transition metal salen-type complexes (TM-SCs) as bi-functional electrocatalysts of ORR and OER is systematically explored by computational screening approach. The results demonstrate that different types of macrocyclic ligands, including N₂O₂, N₃O and N₄ as donor groups around the active sites, govern the OER/ORR catalytic performances. Co–SCs with N₂O₂ ligands exhibit the highest bifunctional catalytic activities. In particular, low limiting overpotentials of 0.22 V for OER and 0.33 V for ORR can be observed on Co sites, which are even superior to those of noble metal catalysts. Analyzing the linear relationships between the adsorption strength of intermediates and the overpotentials shows that the origin of excellent electrocatalytic performance is the smaller slope (0.86) for OOH1 vs OH1 on TM-SCs compared to metal surfaces, resulting in strengthened binding of the OOH1 intermediate. Besides, the adsorption energies of the intermediates bound on Co–N₂O₂ are close to the ideal values, while too strong on the Co–N₃O and Co–N₄ catalysts. By applying external strains, the adsorption strengths of reaction intermediates can be further modulated due to the tunable d-band centers, and the resulting ORR/OER activities are further boosted. Considering that the Co salen-based chain has been synthesized experimentally, this work highlights the excellent electrocatalytic performances of this new material and devises novel strategy by straining for catalyst optimizations.     

Molecular metal nanoclusters for ORR,HER and OER: Achievements,opportunities and challenges

With the rapid development of economy, the past decades have experienced more and more severe energy depletion and environmental pollution issues, hence it is urgent to develop more environmental-friendly energy devices, such as fuel cells, metal-air batteries, water electrolysis and so on. However, such devices have long been suffering from the sluggish reaction kinetics and high energy barriers, plus the conventional electrocatalysts used in these devices mostly are noble-metal-based materials, such as Pt/C, IrO₂, and RuO₂. These noble-metal-based electrocatalysts possess significant disadvantages such as high price, limited reserves, and undesirable stability, and these factors together hinder their large-scale industrial application and the inhomogeneity of the catalyst structure at the atomic level also impose great challenges to disclose the underlying catalytic mechanism. Noble metal nanoclusters, as a promising type of electrocatalyst with definitive composition and structure, have been attracting increasingly research efforts. This review aims to summarize the recent achievements of molecular metal nanoclusters employed in electrocatalytic processes, with particular elaboration on oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), as well as to unravel the catalytic mechanism and establish the relationship between its structure and functionality. Specifically, the size effect, the metal core configuration, charge effect, size effect, ligand effect, and metal-ligand binding motifs of the metal clusters that would impact the electrocatalytic performance are comprehensively discussed. In the end, the outlook and perspective including challenges and opportunities are proposed. We anticipate this review would be beneficial for gaining a deeper understanding of engineering nanoclusters for electrocatalysis and to expand its application in electrocatalysis.     

Combustion-derived BaNiO3 nanoparticles as a potential bifunctional electrocatalyst for overall water splitting

Electrochemical water electrolyser though an assuring solution for clean hydrogen production, the sluggish kinetics and high cost of existing precious metal electrocatalyst remains a barrier to its effective utilization. Herein, solution combustion route derived perovskite type barium nickelate (BaNiO₃) nanoparticles were developed and studied for their bifunctional electrocatalytic properties towards overall water splitting. The unannealed BaNiO₃ nanoparticles exhibited the highest OER and HER activity with overpotentials 253 mV and 427 mV respectively to attain 10 mAcm⁻² in 1.0 M KOH. Using unannealed BaNiO₃ as a bifunctional electrocatalyst in a two-electrode alkaline electrolyser, the cell was able to achieve the benchmark current density at a low cell voltage of 1.82 V. Impressively the setup's electrocatalytic performance improved 4.9% after continuous overall water splitting for 24 h at 30 mAcm⁻². Therefore, BaNiO₃ nanoparticles can be a low-cost and efficient alternative for noble metal electrocatalysts for clean H₂ production.     

Graphene-based electrocatalysts: Hydrogen evolution reactions and overall water splitting

Recently, carbon-based materials (e.g., graphene, carbon nanotubes, carbon quantum dots) have been used as electrocatalysts to catalyze the reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Among them, graphene has attracted attention as an electrocatalyst, and its electrocatalytic performances have been improved by doping with metals and non-metals, surface and defect engineering, and hybrid development. In this perspective, the present paper reviewed the recent advances (2018 onwards) on the progress of graphene-based electrocatalysts for HER and overall water splitting (OWS). It is emphasizing strategies for optimizing electrocatalytic properties followed by challenges and future outlook. This review will provide the essential ideas and strategies that can help design graphene-based electrocatalysts of high performance that can be implemented for sustainable energy application.     

Mixed-metal MOF-derived Co-doped Ni3C/Ni NPs embedded in carbon matrix as an efficient electrocatalyst for oxygen evolution reaction

The exploration of electrocatalysts with high oxygen evolution reaction (OER) activity is highly desirable and remains a significant challenge. Transition metal carbides (TMCs) have been investigated as remarkable hydrogen evolution reaction (HER) electrocatalysts but few used as oxygen evolution reaction (OER) electrocatalysts. Herein, a Co doped Ni₃C/Ni uniformly dispersed in a graphitic carbon matrix was prepared by pyrolysis of a metal organic framework (Co/Ni-MOF) under a flow of Ar/H₂ at 350 °C, and Ni₃C/Ni@C was also prepared for comparison. The various characterization techniques confirmed the successful preparation of the heteroatom doped TMCs-based catalysts by pyrolysis of MOFs. Co doped Ni₃C/Ni@C exhibited superior electrocatalytic properties for OER. For example, Co–Ni₃C/Ni@C depicts a lower overpotential and smaller Tafel slope than Ni₃C/Ni@C and IrO₂ during the OER in 1 M KOH solution, additionally, it shows a higher active surface area than Ni₃C/Ni@C. The outstanding electrocatalytic performance of Co-doped Ni₃C/Ni@C in the OER was mainly ascribed to the synergistic effect of the Co and Ni₃C/Ni active sites.     

Fe doped metal organic framework (Ni)/carbon black nanosheet as highly active electrocatalyst for oxygen evolution reaction

To alleviate the sluggish oxygen evolution reaction (OER) kinetics, it's urgent to develop electrocatalysts with high activity and low cost. In this work, Fe doped metal organic frameworks (Ni)/carbon black composites were synthesized via a facile hydrothermal method. Benefiting from the direct use of metal organic frameworks (MOFs) for OER, numerous and highly dispersed active sites are exposed to the electrolyte and reactants. By regulating Ni/Fe ratios, a high electrochemical active surface area (ECSA) and high relative surface content of active Ni³⁺ species are obtained, which mainly contribute to the high OER activity. Besides, the introduced carbon black (CB) was found to enhance the charge-transfer efficiency of the electrocatalysts, which is also favorable for OER. The optimal Ni9Fe1-BDC-0.15CB electrocatalyst shows excellent OER activity with the low overpotential of ~290 mV at 10 mA cm⁻² and the Tafel slope of ~76.1 mV dec⁻¹, which is comparable to RuO₂ and other MOFs-based OER electrocatalysts reported in recent years.     

Palladium nanoparticles grown on β-Mo2C nanotubes as dual functional electrocatalysts for both oxygen reduction reaction and hydrogen evolution reaction

Developing efficient dual functional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is critical for boosting the performance of fuel cells and metal air batteries, as well as production of clean and sustainable energy source. Herein, Pd nanoparticles grown on Mo₂C nanotubes were prepared as dual functional electrocatalysts for both ORR and HER. A series of samples with different Pd loadings were fabricated, while the morphology and the structural features were well examined by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Interestingly, both the ORR activity and HER activity first increased then decreased with the increasing of Pd loading, and the sample of Mo₂C-Pd-9% exhibited the best performance among the series, superior than commercial Pd/C in both ORR and HER tests. Furthermore it also exhibited markedly higher long-term stability than Pd/C for both electrocatalytic reactions. The results may shed light on rational design of novel bi-functional electrocatalysts in the renewable energy field.     

Engineering FeNi-based electrocatalysts conjoined with Mo2C grown on carbon spheres toward efficient water oxidation via structure and electronic modulation

The development of active non-noble electrocatalysts for oxygen evolution reaction (OER) is urgently desired to accomplish high-performance electrocatalytic water splitting. Here, we report a unique structure electrocatalyst composed of FeNi and Mo₂C heterojunction as spherical shell supported on carbon sphere as core for efficient OER. With the aid of Mo₂C incorporation, FeNi–Mo₂C@C shows increased specific surface area, more electrocatalytic active sites, improved surface water adsorption, and reduced energy barrier. Benefiting from the synergy of shell-core and heterojunction sturcture, the optimized FeNi–Mo₂C@C exhibits superior activity for OER with an overpotential of 283 mV at 10 mA cm⁻² as well as Tafel slope of 29.2 mV dec⁻¹, which is comparable to that of the benchmark ruthenium oxide. The feasible bond between structural design and electronic alteration enhances the charge transfer efficiency, conductivity, and catalytic kinetics, thus intrinsically boost the electrocatalytic performance. This study hence supports a viable strategy to develop highly-efficient non-precious OER electrocatalysts through structural and electronic modification.     

Nickel selenides as pre-catalysts for electrochemical oxygen evolution reaction: A review

Nickel selenide is an important class of nickel chalcogenide that has recently gained greater attention in electrochemical water splitting. Though other chalcogenides such as sulphides and tellurides have also been shown to possess appreciable electrocatalytic water splitting activity as both cathode and anode material, the electrocatalytic activity of nickel selenides in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is unmatched by the former one. In the case of HER, nickel selenides are good electrocatalysts in both acid and alkali with some metal dissolution in acid and surface modification to nickel hydroxide in alkali. In the case of OER, nickel selenides have been shown to behave in a unique way in which it acted better than simple nickel hydroxides/oxyhydroxides. This is quite intriguing to researchers and many studies have been recently carried out. However, there is still no exclusive review summarizing the recent development of this material for OER electrocatalysis with highlights on challenges and opportunities. Hence, we have dedicated to discuss the same in this review. In addition, the significance of OER electrocatalysis has been briefly introduced. Finally, all the studies that reported the OER activity of nickel selenides are benchmarked based on the Tafel slope values. With the collective information provided in this review, readers would be updated with the recent trend in utilizing nickel selenides as OER electrocatalysts.     

Interface regulation of Pt quantum dots doped nickel phosphide and cobalt hydroxide to promote electrocatalytic overall water splitting

The transition metal phosphates are earth-abundant minerals that have been shown to perform well in electrocatalytic water splitting, whereas these catalysts still tend to have excessively high overpotentials and slow kinetics in HER and OER processes. In the present work, hybrid catalysts consisting of Pt quantum dots doped NiP (NiP-Pt) nano-embroidery spheres and Co(OH)₂ nanosheets were successfully prepared by two-step electrodeposition method. The excellent catalytic performance of the catalyst relies principally on the synergistic interaction between NiP and Pt quantum dots. Additionally, the NiP-Pt exhibits strong electronic interactions at the interface with Co(OH)₂. Consequently, the catalyst has a strong catalytic performance in terms of HER and OER catalytic performance. In terms of HER, an overpotential of only 40 mV is required when the current density reaches 10 mA cm^?2, corresponding to a Tafel slope of 49.85 mV·dec^?1. At the same time, the catalyst also performs well at OER, with a current density of 10 mA cm^?2 at an overpotential of 186 mV and a Tafel slope of 53.049 mV·dec^?1 much less than most electrocatalysts. This study involving electrodeposition and doping of quantum dots provides a new idea for the efficient synthesis of fundamental HER and OER bifunctional catalysts.     

Spinel NiCo2O4 3-D nanoflowers supported on graphene nanosheets as efficient electrocatalyst for oxygen evolution reaction

Efficient oxygen evolution reaction (OER) electrocatalysts with non-noble metals are very critical for the large-scale exploitation of electrocatalytic hydrogen production systems. To improve the catalytic activity of OER electrocatalysts, several design strategies, such as construction of nanostructures, porous structures and composite materials have been proposed. Herein, spinel NiCo₂O₄ 3-D nanoflowers supported on graphene nanosheets (GNs) are prepared by a simple solvothermal synthesis method as non-noble metal electrocatalysts for OER. The present NiCo₂O₄/GNs composite integrates multiple advantages of nanostructures, porous structures and composite materials, including high surface area, abundant catalytic sites and high stability. Benefiting from the favorable features, the NiCo₂O₄/GNs composite exhibits a better OER performance than NiCo₂O₄ and RuO₂ in alkaline medium, which has a low onset potential (1.50 V), a small Tafel slope (137 mV dec⁻¹). The present work opens a new window for the construction of the carbon-supported 3-D nanostructure of transition metal catalysts with optimizable electrocatalytic performances for electrocatalytic hydrogen production.