There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

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

Co3O4, which is of mixed valences Co2+ and Co3+, has been extensively investigated as an efficient electrocatalyst for the oxygen evolution reaction (OER). The proper control of Co2+/Co3+ ratio in Co3O4 could lead to modifications on its electronic and thus catalytic properties. Herein, we designed an efficient Co3O4-based OER electrocatalyst by a plasma-engraving strategy, which not only produced higher surface area, but also generated oxygen vacancies on Co3O4 surface with more Co2+ formed. The increased surface area ensures the Co3O4 has more sites for OER, and generated oxygen vacancies on Co3O4 surface improve the electronic conductivity and create more active defects for OER. Compared to pristine Co3O4, the engraved Co3O4 exhibits a much higher current density and a lower onset potential. The specific activity of the plasma-engraved Co3O4 nanosheets (0.055 mA cm−2BET at 1.6 V) is 10 times higher than that of pristine Co3O4, which is contributed by the surface oxygen vacancies.

Plasma-Engraved Co3 O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction.

Black phosphorus (BP) as a new 2D material has attracted extensive attention because of its unique electronic, optical, and structural properties. However, the difficulties associated with BP synthesis severely hinder the further development of BP for any potential applications. On the other hand, searching for other potential applications of BP is also a big challenge. A facile strategy was developed for preparation of BP supported on Ti foil (BP-Ti) in a thin-film form. Surprisingly, the as-prepared BP shows advanced electrocatalytic activity for the oxygen evolution reaction (OER). To improve the OER activity of the electrocatalyst, BP was grown on a carbon nanotube network (BP-CNT), showing even better activity. The results demonstrate that BP can be prepared by a facile method and may be applied as an electrocatalyst.

Facile Synthesis of Black Phosphorus: an Efficient Electrocatalyst for the Oxygen Evolving Reaction

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

Lei Xu

Papers

Plasma-Engraved Co3 O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction.

Facile Synthesis of Black Phosphorus: an Efficient Electrocatalyst for the Oxygen Evolving Reaction

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

Transforming Co3O4 nanosheets into porous N-doped CoxOy nanosheets with oxygen vacancies for the oxygen evolution reaction

Edge-selectively phosphorus-doped few-layer graphene as an efficient metal-free electrocatalyst for the oxygen evolution reaction