Hydrogen fuel is considered as the cleanest renewable resource and the primary alternative to fossil fuels for future energy supply. Sustainable hydrogen generation is the major prerequisite to realize future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the vital step of water electrolysis to H2 production, has been the subject of extensive study over the past decades. In this comprehensive review, we first summarize the fundamentals of HER and review the recent state-of-the-art advances in the low-cost and high-performance catalysts based on noble and non-noble metals, as well as metal-free HER electrocatalysts. We systemically discuss the insights into the relationship among the catalytic activity, morphology, structure, composition, and synthetic method. Strategies for developing an effective catalyst, including increasing the intrinsic activity of active sites and/or increasing the number of active sites, are summarized and highlighted. Finally, the challenges, perspectives, and research directions of HER electrocatalysis are featured.

Recent Advances in Electrocatalytic Hydrogen Evolution Using Nanoparticles.

Seawater is one of the most abundant natural resources on our planet. Electrolysis of seawater is not only a promising approach to produce clean hydrogen energy, but also of great significance to seawater desalination. The implementation of seawater electrolysis requires robust and efficient electrocatalysts that can sustain seawater splitting without chloride corrosion, especially for the anode. Here we report a three-dimensional core-shell metal-nitride catalyst consisting of NiFeN nanoparticles uniformly decorated on NiMoN nanorods supported on Ni foam, which serves as an eminently active and durable oxygen evolution reaction catalyst for alkaline seawater electrolysis. Combined with an efficient hydrogen evolution reaction catalyst of NiMoN nanorods, we have achieved the industrially required current densities of 500 and 1000 mA cm−2 at record low voltages of 1.608 and 1.709 V, respectively, for overall alkaline seawater splitting at 60 °C. This discovery significantly advances the development of seawater electrolysis for large-scale hydrogen production. Seawater electrolysis is a promising approach to produce hydrogen fuel and is also of great significance to seawater desalination. Here, the authors prepare 3D core-shell metal-nitride catalysts from earth-abundant elements for high-performance alkaline seawater electrolysis.

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Non-noble metal-nitride based electrocatalysts for high-performance alkaline seawater electrolysis

Water-splitting electrolyzers that can convert electricity into storable hydrogen are a fascinating and scalable energy conversion technology for the utilization of renewable energies. To speed up the sluggish hydrogen and oxygen evolution reactions (HER and OER), electrocatalysts are essential for reducing their kinetic energy barriers and eventually improving the energy conversion efficiency. As efficient strategies for modulating the binding ability of water-splitting intermediates on electrocatalyst surface, the support effect and interface effect are drawing growing attention. Herein, some of the recent research progress on the support and interface effects in HER, OER, and overall water-splitting electrocatalysts is highlighted. Specifically, the correlation between the electronic interaction of the constituent components and the electrocatalytic water-splitting performance of electrocatalysts is profoundly discussed, with the aim of advancing the development of highly efficient water-splitting electrocatalysts, which may eventually replace the noble-metal-based electrocatalysts and bring the practically widespread utilization of water-splitting electrolyzers into a reality.

Support and Interface Effects in Water-Splitting Electrocatalysts

Overall water splitting driven by a low voltage is crucial for practical H2 evolution, but it is challenging. Herein, anion-modulation of 3D Ni-V-based transition metal interstitial compound (TMIC) heterojunctions supported on nickel foam (Ni3 N-VN/NF and Ni2 P-VP2 /NF) as coupled hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts for efficient overall water splitting is demonstrated. The heterointerface in Ni3 N-VN has a suitable H* absorption energy, being favorable for enhancing HER activity with onset overpotential (ηonset ) of zero and Tafel slope of 37 mV dec-1 in 1 m KOH (close to that of Pt/C/NF). For the OER, the synergy of Ni2 P-VP2 with oxide species can give enhanced activity with ηonset of 220 mV and Tafel slope of 49 mV dec-1 . The good activity is ascribed to heterointerface for activating the intermediates, good conductivity of TMICs for electron-transfer, and porous structure facilitation of mass-transport. Additionally, the minimal mutual influence of Ni3 N-VN/NF and Ni2 P-VP2 /NF allows easy coupling for efficient overall water splitting with a low driving voltage (≥1.43 V), a voltage of 1.51 V at 10 mA cm-2 , and remarkable durability for 100 h. It can be driven by a solar cell (1.5 V), indicating its potential to store intermittent energy.

Anion-Modulated HER and OER Activities of 3D Ni-V-Based Interstitial Compound Heterojunctions for High-Efficiency and Stable Overall Water Splitting.

With the increasing demands in energy consumption and increasing environmental concerns, it is of vital significance for developing renewable and clean energy sources to substitute traditional fossil fuels. As an outstanding candidate, hydrogen is recognized as a green energy carrier due to its high gravimetric energy density, zero carbon footprints, and earth-abundance. Currently, water splitting in alkaline electrolytes represents one of the most promising methods for sustainable hydrogen production, and the key challenge lies in the development of high-performance electrocatalysts for the hydrogen evolution reaction (HER). Given the rapid advances in the design and development of efficient catalysts towards the alkaline HER, especially capable transition metal (TM)-based materials, this review aims to summarise recent progress in the theoretical understanding of the alkaline HER and TM-based electrocatalysts. TM-based catalysts classified by their different anionic compositions (metals, alloys, oxides, hydroxides, sulfides, selenides, tellurides, nitrides, phosphides, carbides, and borides) are comprehensively showcased. Special attention is given to mainstream strategies that can improve the catalytic properties of each category, as well as the underlying structure–activity regimes. Additionally, the challenges for the future development of novel catalysts are also analyzed.

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Recent advances in transition metal-based electrocatalysts for alkaline hydrogen evolution

Electrocatalytic performance can be enhanced by engineering a purposely designed nanoheterojunction and fine-tuning the interface electronic structure. Herein a new approach of developing atomic epitaxial in-growth in Co-Ni3 N nanowires array is devised, where a nanoconfinement effect is reinforced at the interface. The Co-Ni3 N heterostructure array is formed by thermal annealing NiCo2 O4 precursor nanowires under an optimized condition, during which the nanowire morphology is retained. The epitaxial in-growth structure of Co-Ni3 N at nanometer scale facilitates the electron transfer between the two different domains at the epitaxial interface, leading to a significant enhancement in catalytic activities for both hydrogen and oxygen evolution reactions (10 and 16 times higher in the respective turn-over frequency compared to Ni3 N-alone nanorods). The interface transfer effect is verified by electronic binding energy shift and density functional theory (DFT) calculations. This nanoconfinement effect occurring during in situ atomic epitaxial in-growth of the two compatible materials shows an effective pathway toward high-performance electrocatalysis and energy storages.

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Jiani Kang

Papers

In Situ Grown Epitaxial Heterojunction Exhibits High-Performance Electrocatalytic Water Splitting