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.

Over the past few decades, the design and development of advanced electrocatalysts for efficient energy conversion technologies have been subjects of extensive study. With the discovery of graphene, two-dimensional (2D) nanomaterials have emerged as some of the most promising candidates for heterogeneous electrocatalysts due to their unique physical, chemical, and electronic properties. Here, we review 2D-nanomaterial-based electrocatalysts for selected electrocatalytic processes. We first discuss the unique advances in 2D electrocatalysts based on different compositions and functions followed by specific design principles. Following this overview, we discuss various 2D electrocatalysts for electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle from their fundamental conception to their functional application. We place a significant emphasis on different engineering strategies for 2D nanomaterials and the influence these strategies have on intrinsic material performance, ...

Emerging Two-Dimensional Nanomaterials for Electrocatalysis

Electrocatalytic oxygen evolution reaction for energy conversion and storage: A comprehensive review

Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

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A review on fundamentals for designing oxygen evolution electrocatalysts

As one of the most important chemicals and carbon-free energy carriers, ammonia (NH3) has a worldwide annual production of ∼150 million tons, and is mainly produced by the traditional high-temperature and high-pressure Haber–Bosch process which consumes massive amounts of energy. Very recently, electrocatalytic and photo(electro)catalytic reduction of N2 to NH3, which can be performed at ambient conditions using renewable energy, have received tremendous attention. The overall performance of these electrocatalytic and photo(electro)catalytic systems is largely dictated by their core components, catalysts. This perspective for the first time highlights the rational design of electrocatalysts and photo(electro)catalysts for N2 reduction to NH3 under ambient conditions. Fundamental theory of catalytic reaction pathways for the N2 reduction reaction and the corresponding material design principles are introduced first. Then, recently developed electrocatalysts and photo(electro)catalysts are summarized, with a special emphasis on the relationship between their physicochemical properties and NH3 production performance. Finally, the opportunities in this emerging research field, in particular, the strategy of combining experimental and theoretical techniques to design efficient and stable catalysts for NH3 production, are outlined.

Rational design of electrocatalysts and photo(electro)catalysts for nitrogen reduction to ammonia (NH3) under ambient conditions

Electrocatalysis is dominated by reaction at the solid-liquid-gas interface; surface properties of electrocatalysts determine the electrochemical behavior. The surface charge of active sites on catalysts modulate adsorption and desorption of intermediates. However, there is no direct evidence to bridge surface charge and catalytic activity of active sites. Defects (active sites) were created on a HOPG (highly oriented pyrolytic graphite) surface that broke the intrinsic sp2 -hybridization of graphite by plasma, inducing localization of surface charge onto defective active sites, as shown by scanning ion conductance microscopy (SICM) and Kelvin probe force microscopy (KPFM). An electrochemical test revealed enhanced intrinsic activity by the localized surface charge. DFT calculations confirmed the relationship between surface charge and catalytic activity. This work correlates surface charge and catalytic activity, providing insights into electrocatalytic behavior and guiding the design of advanced electrocatalysts.

Bridging the Surface Charge and Catalytic Activity of a Defective Carbon Electrocatalyst.

Activity Origins and Design Principles of Nickel-Based Catalysts for Nucleophile Electrooxidation

High entropy materials (HEMs) are attracting significant attention due to their unique structure and unprecedented application potential in many fields. Herein, a novel low temperature strategy for preparing high entropy (Co, Cu, Fe, Mn, Ni)3O4 oxides with a single-phase, the Fdm spinel structure, is developed via solvothermal synthesis followed by annealing. This is the first time that high entropy oxides were fabricated successfully at low temperature. The as-prepared high entropy oxides possess five highly dispersed metal species, and the average particle size is approximately 5 nm, which is the smallest nanometer size of the high entropy oxides that has been reported so far. By considering their unique physical and chemical properties, the resultant high entropy oxides exhibit an excellent performance for water oxidation. Hence, this work provides a new synthetic strategy for high entropy oxides to solve the bottlenecks encountered by traditional methods and broaden their application areas.

Low-temperature synthesis of small-sized high-entropy oxides for water oxidation

Water splitting has been considered as one of the most prospective technologies for enhancing sustainable output of hydrogen and reducing the release of carbon dioxide in the 21st century. Much att...

Defect Engineering of Cobalt-Based Materials for Electrocatalytic Water Splitting

Developing highly active electrocatalysts for the oxygen evolution reaction (OER) with a high surface area, high catalytic activity, low cost and high conductivity is a big challenge for various energy technologies. Herein, for the first time, we realized the simultaneous nitrogen doping and etching of Co3O4 nanosheets to produce N-doped nanoporous Co3O4 nanosheets with oxygen vacancies by N2 plasma. The increase in active sites in N-doped Co3O4 nanosheets and improved electronic conductivity with N doping and oxygen vacancies results in excellent electrocatalytic activity for the OER. Compared with pristine Co3O4 nanosheets, the N-doped Co3O4 nanosheets with oxygen vacancies have a much lower required potential of 1.54 V versus a reversible hydrogen electrode than the pristine Co3O4 nanosheets (1.79 V) to reach the current density of 10 mA cm-2. The N-doped and etched Co3O4 nanosheets have a much lower Tafel slope of 59 mV dec-1 than pristine Co3O4 nanosheets (234 mV dec-1). The enhanced electrocatalytic activity for the OER is caused by the increased surface area, N doping and the produced oxygen vacancies.

https://iopscience.iop.org/article/10.1088/1361-6528/aa6381/pdf

Zhaohui Xiao

Papers

Bridging the Surface Charge and Catalytic Activity of a Defective Carbon Electrocatalyst.

Activity Origins and Design Principles of Nickel-Based Catalysts for Nucleophile Electrooxidation

Low-temperature synthesis of small-sized high-entropy oxides for water oxidation

Defect Engineering of Cobalt-Based Materials for Electrocatalytic Water Splitting

N-doped nanoporous Co3O4 nanosheets with oxygen vacancies as oxygen evolving electrocatalysts