Hydrogen production from electrochemical water splitting represents a highly promising technology for sustainable energy storage, but its widespread implementation heavily relies on the development of high-performance and cost-effective hydrogen evolution reaction (HER) electrocatalysts. Metal oxides, an important family of functional materials with diverse compositions and structures, were traditionally believed inactive towards HER. Encouragingly, the continuous breakthroughs and significant progress in recent years (mainly from 2015 onwards) make engineered metal oxides emerge as promising candidates for HER electrocatalysis. In this article, we present a comprehensive review of recent advances in metal oxide-based electrocatalysts for HER. We start with a brief description of some key fundamental concepts of HER, such as mechanisms, computational activity descriptors, and experimental parameters used to evaluate catalytic performance. This is followed by a overview of various types of metal oxide-based HER electrocatalysts reported so far, including single transition metal oxides, spinel oxides, perovskite oxides, metal (oxy)hydroxides, specially-structured metal oxides and oxide-containing hybrids, with special emphasis on designed strategies for promoting performance and property–activity correlation. Finally, some concluding remarks and perspectives about future opportunities of this exciting field are provided.

Metal oxide-based materials as an emerging family of hydrogen evolution electrocatalysts

Electrochemical water splitting technology for producing "green hydrogen" is important for the global mission of carbon neutrality. Electrocatalysts with decent performance at high current densities play a central role in the industrial implementation of this technology. This field has advanced immensely in recent years, as witnessed by many types of catalysts designed and synthesized toward industriallyrelevant current densities (>200 mA cm-2 ). By discussing recent advances in this field, several key aspects are summarized that affect the catalytic performance for high-current-density electrocatalysis, including dimensionality of catalysts, surface chemistry, electron transport path, morphology, and catalyst-electrolyte interplay. The multiscale design strategy that considers these aspects comprehensively for developing high-current-density electrocatalysts are highlighted. The perspectives on the future directions in this emerging field are also put forward. 

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Recent Advances in Design of Electrocatalysts for High‐Current‐Density Water Splitting

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis.

Heterogeneous Bimetallic Mo‐NiP x /NiS y as a Highly Efficient Electrocatalyst for Robust Overall Water Splitting

Metal-organic frameworks (MOFs) can act as precursors or templates to a myriad of nanostructured materials that are difficult to prepare In this study, Co-MOF nanorods (NRs) were prepared at room temperature followed by a calcination and hydrothermal sulfurization strategy to transform the MOF into CoS NRs on carbon cloth (CoS/CC) Intriguingly, the resultant 3D sulfide NRs can serve as scaffolds to electrodeposit layered double hydroxides (LDHs) on the surfaces Through combining the advantages of structure and composition, the as-fabricated CoS@CoNi-LDH/CC exhibits remarkable electrocatalytic activity for the hydrogen evolution reaction (HER) An overpotential of 124 mV is needed to reach a current density of 10 mA cm-2 with a Tafel slope of only 89 mV dec-1, which is superior to that of pure CoS/CC (141 mV along with 103 mV dec-1) and other reported cobalt-based catalysts Notably, after the chronopotentiometry test for 50 h, the overpotential of CoS@CoNi-LDH/CC increased by 17 mV only

MOF-Derived Sulfide-Based Electrocatalyst and Scaffold for Boosted Hydrogen Production.

The hydrogen evolution reaction (HER) is a critical process in the electrolysis of water. Recently, much effort has been dedicated to developing low-cost, highly efficient and stable electrocatalysts. Transition metal phosphides are investigated intensively due to their high electronic conductivity and optimized absorption energy of intermediates in acid electrolytes. However, the low stability of metal phosphide materials in air and during electrocatalytic processes causes a decay of performance and hinders the discovery of specific active sites. The HER in alkaline media is more intricate, which requires further delicate design due to the Volmer steps. In this work, we develop phosphorus modified monoclinic β-CoMoO4 as a low-cost, efficient and stable HER electrocatalyst for the electrolysis of water in alkaline media. The optimized catalyst shows a small overpotential of 94 mV to reach a current density of 10 mA cm-2 for the HER with high stability in KOH electrolyte, and an overpotential of 197 mV to reach a current density of 100 mA cm-2. Combined computational and in-situ spectroscopic techniques show P is present as a surface phosphate ion; that electron holes localise on the surface ions and both (P-O1-) and Co3+-OH- are prospective surface active sites for the HER.

The Role of Phosphate Group in Doped Cobalt Molybdate: Improved Electrocatalytic Hydrogen Evolution Performance

It remains a challenge to develop efficient electrocatalysts in neutral media for hydrogen evolution reaction (HER) due to the sluggish kinetics and switch of the rate determining step. Although metal phosphides are widely used HER catalysts, their structural stability is an issue due to oxidization, and the HER performance in neutral media requires improvement. Herein, a new material, i.e., grapevine-shaped N-doped iron phosphide on carbon nanotubes, as an efficient HER catalyst in neutral media is developed. The optimized catalyst shows an overpotential of 256 mV at a large current density of 65 mA cm−2, which is even 10 mV lower than that of the commercial 20% Pt/C catalyst. The excellent performance of the catalyst is further studied by combined computational and experimental techniques, which proves that the interaction between nitrogen and iron phosphides can provide more efficient active structures and stabilize the metal phosphide electrocatalysts for HER.

Enhancing Hydrogen Evolution Electrocatalytic Performance in Neutral Media via Nitrogen and Iron Phosphide Interactions

The electrolysis of water is one of the most promising strategies to produce renewable fuels and it is important to develop an energy-conserving, low-cost and easily prepared electrocatalyst for oxygen evolution reaction (OER). In this work, Ni foam supported Co4S3 (Co4S3/NF) was fabricated by a facile one-step approach at room temperature and exhibited excellent OER performance in alkaline media. Specifically, the Co4S3/NF electrocatalysts showed a small overpotential of only 340 mV to reach a current density of 100 mA cm−2 and a Tafel slope of 71.6 mV dec−1 in alkaline media. More importantly, excellent stability was achieved under a constant current density of 100 mA cm−2 for 100 h and the OER performance of the catalyst was improved after 1400 cycles of linear sweep voltammetry tests in alkaline media. Furthermore, the underpinning mechanism of action was studied by measuring the change of valence states for different elements to elucidate the structural evolution and active species during the electrocatalytic process.

Facile room-temperature synthesis of cobalt sulphide for efficient oxygen evolution reaction

Hydrogen Evolution: The Role of Phosphate Group in Doped Cobalt Molybdate: Improved Electrocatalytic Hydrogen Evolution Performance (Adv. Sci. 12/2020)

Manni Yang

Papers

The Role of Phosphate Group in Doped Cobalt Molybdate: Improved Electrocatalytic Hydrogen Evolution Performance

Enhancing Hydrogen Evolution Electrocatalytic Performance in Neutral Media via Nitrogen and Iron Phosphide Interactions

Facile room-temperature synthesis of cobalt sulphide for efficient oxygen evolution reaction

Hydrogen Evolution: The Role of Phosphate Group in Doped Cobalt Molybdate: Improved Electrocatalytic Hydrogen Evolution Performance (Adv. Sci. 12/2020)