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

Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices Dengjie Chen,†,⊥,∇ Chi Chen,†,⊥ Zarah Medina Baiyee,† Zongping Shao,‡,§ and Francesco Ciucci*,†,∥ †Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, China Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices

NiFe-based (oxy)hydroxides are highly active catalysts for the oxygen evolution reaction in alkaline electrolyte solutions. These catalysts can be synthesized in different ways leading to nanomaterials and thin films with distinct morphologies, stoichiometries and long-range order. Notably, their structure evolves under oxygen evolution operating conditions with respect to the as-synthesized state. Therefore, many researchers have dedicated their efforts on the identification of the catalytic active sites employing in operando experimental methods and theoretical calculations. These investigations are pivotal to rationally design materials with outstanding performances that will constitute the anodes of practical commercial alkaline electrolyzers. The family of NiFe-based oxyhydroxide catalysts reported in recent years is addressed and the actual state of the research with special focus on the understanding of the oxygen-evolution-reaction active sites and phase is described. Finally, an overview on the proposed oxygen-evolution-reaction mechanisms occurring on NiFe-based oxyhydroxide electrocatalysts is provided.

NiFe‐Based (Oxy)hydroxide Catalysts for Oxygen Evolution Reaction in Non‐Acidic Electrolytes

Designing a highly active electrocatalyst with optimal stability at low cost is must and non-negotiable if large-scale implementations of fuel cells are to be fully realized. Zeolitic-imidazolate frameworks (ZIFs) offer rich platforms to design multifunctional materials due to their flexibility and ultrahigh surface area. Herein, an advanced Co–Nx/C nanorod array derived from 3D ZIF nanocrystals with superior electrocatalytic activity and stability toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) compared to commercial Pt/C and IrO2, respectively, is synthesized. Remarkably, as a bifunctional catalyst (Ej = 10 (OER) − E1/2 (ORR) ≈ 0.65 V), it further displays high performance of Zn–air batteries with high cycling stability even at a high current density. Such supercatalytic properties are largely attributed to the synergistic effect of the chemical composition, high surface area, and abundant active sites of the nanorods. The activity origin is clarified through post oxygen reduction X-ray photoelectron spectroscopy analysis and density functional theory studies. Undoubtedly, this approach opens a new avenue to strategically design highly active and performance-oriented electrocatalytic materials for wider electrochemical energy applications.

From 3D ZIF Nanocrystals to Co-N x /C Nanorod Array Electrocatalysts for ORR, OER, and Zn-Air Batteries

Development of cost-effective and efficient electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of prime importance to emerging renewable energy technologies. Here, we report a simple and effective strategy for enhancing ORR and OER electrocatalytic activity in alkaline solution by introducing A-site cation deficiency in LaFeO3 perovskite; the enhancement effect is more pronounced for the OER than the ORR. Among the A-site cation deficient perovskites studied, La0.95FeO3-δ (L0.95F) demonstrates the highest ORR and OER activity and, hence, the best bifunctionality. The dramatic enhancement is attributed to the creation of surface oxygen vacancies and a small amount of Fe4+ species. This work highlights the importance of tuning cation deficiency in perovskites as an effective strategy for enhancing ORR and OER activity for applications in various oxygen-based energy storage and conversion processes.

Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning Cation Deficiency for Oxygen Reduction and Evolution Reactions

Electrochemical water splitting is a promising method for storing light/electrical energy in the form of H2 fuel; however, it is limited by the sluggish anodic oxygen evolution reaction (OER). To improve the accessibility of H2 production, it is necessary to develop an efficient OER catalyst with large surface area, abundant active sites, and good stability, through a low-cost fabrication route. Herein, a facile solution reduction method using NaBH4 as a reductant is developed to prepare iron-cobalt oxide nanosheets (FexCoy-ONSs) with a large specific surface area (up to 261.1 m2 g−1), ultrathin thickness (1.2 nm), and, importantly, abundant oxygen vacancies. The mass activity of Fe1Co1-ONS measured at an overpotential of 350 mV reaches up to 54.9 A g−1, while its Tafel slope is 36.8 mV dec−1; both of which are superior to those of commercial RuO2, crystalline Fe1Co1-ONP, and most reported OER catalysts. The excellent OER catalytic activity of Fe1Co1-ONS can be attributed to its specific structure, e.g., ultrathin nanosheets that could facilitate mass diffusion/transport of OH− ions and provide more active sites for OER catalysis, and oxygen vacancies that could improve electronic conductivity and facilitate adsorption of H2O onto nearby Co3+ sites.

Ultrathin Iron-Cobalt Oxide Nanosheets with Abundant Oxygen Vacancies for the Oxygen Evolution Reaction.

Calcium-doped lanthanum nickelate layered perovskite and nickel oxide nano-hybrid for highly efficient water oxidation

A facile method to synthesize boron-doped Ni/Fe alloy nano-chains as electrocatalyst for water oxidation

The perovskite Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) is one of the best catalysts for the oxygen evolution reaction (OER), which is critical to many energy-storage applications. However, the catalytic activity of BSCF perovskites is constrained by a low specific surface area (0.5m(2)g(-1)). We report here, for the first time, a facile, in situ template method using tetraethoxysilane (TEOS) to synthesize porous BSCF with surface areas of up to 32.1m(2)g(-1), which to our knowledge is the highest reported surface area in a BSCF perovskite, and with excellent catalytic activity for the OER. For example, the BCSF prepared using a TEOS-to-BSCF ratio of 3.4 exhibited up to 35.2Ag(-1) at 1.63V versus a reversible hydrogen electrode (overpotential, =0.4V) and this current is 5.3 times that exhibited by nonporous BSCF (6.6Ag(-1)). The high activity of the porous BCSF is attributed to the additional catalytic surface sites available in the pores created by the in situ TEOS-template method. The general application of this technique to produce porous perovskites is demonstrated in the synthesis of a second example, porous LaMnO3.

In Situ Tetraethoxysilane‐Templated Porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ Perovskite for the Oxygen Evolution Reaction

We present a variety of amorphous transition-metal borides prepared at room temperature by a chemical reduction method as highly active catalysts for the oxygen evolution reaction (OER). The amorphous borides exhibit activities much higher than the corresponding crystalline (spinel, layered double hydroxide and perovskite) metal oxides containing the identical metal compositions, which have already been regarded as promising OER catalysts. The amorphous Ni/Fe borides showed the best mass normalized OER current density of 50 A g−1 at an overpotential of 0.35 V, transcending the performance of the state-of-the-art OER catalyst, RuO2. Amorphous transition-metal borides demonstrated extremely high active OER catalytic activity. The outstanding catalytic activity can be attributed to the amorphous structure, the large specific surface areas (above 110 m2 g−1) and the electron-enriched transition metal sites stemming from boron doping. The stoichiometry of the catalysts can be controlled precisely even for the synthesis of quaternary metal boride catalysts, which made it feasible to further optimize the catalytic activity. These results indicated that it is facile to prepare highly active OER catalysts by the one-step chemical reduction process at room temperature.

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Efficient water oxidation with amorphous transition metal boride catalysts synthesized by chemical reduction of metal nitrate salts at room temperature

Yisu Yang

Papers

Ultrathin Iron-Cobalt Oxide Nanosheets with Abundant Oxygen Vacancies for the Oxygen Evolution Reaction.

Calcium-doped lanthanum nickelate layered perovskite and nickel oxide nano-hybrid for highly efficient water oxidation

A facile method to synthesize boron-doped Ni/Fe alloy nano-chains as electrocatalyst for water oxidation

In Situ Tetraethoxysilane‐Templated Porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ Perovskite for the Oxygen Evolution Reaction

Efficient water oxidation with amorphous transition metal boride catalysts synthesized by chemical reduction of metal nitrate salts at room temperature