In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal–air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.

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Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis

Methanol oxidation and direct methanol fuel cells: a selective review

Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ 18O isotope labelling mass spectrometry, we provide direct experimental evidence that the O2 generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton-electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal-oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton-electron transfers during OER.

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Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution

With the development of renewable energy and electrified transportation, electrochemical energy storage will be more important in the future than it has ever been in the past. Although lithium-ion batteries (LIBs) are traditionally considered to be the most likeliest candidate thanks to their relatively long cycle life and high energy efficiency, their limited energy density as well as cost are still causing a bottleneck for their long-term application. Alternatively, metal–air batteries have been proposed as a very promising large-scale electricity storage technology with the replacement of the intercalation reaction mechanism by the catalytic redox reaction of a light weight metal–oxygen couple. Generally, based on the electrolyte, these metal–air batteries can be divided into aqueous and nonaqueous systems, corresponding to two typical batteries of Zn–air and Li–air, respectively. The prominent feature of both batteries are their extremely high theoretical energy density, especially for nonaqueous Li–air batteries, which far exceeds the best that can be achieved with LIBs. In this review, we focus on the major obstacle of sluggish kinetics of the cathode in both batteries, and summarize the fundamentals and recent advances related to the oxygen catalyst materials. According to the electrolyte, the aqueous and nonaqueous electrocatalytic mechanisms of the oxygen reduction and evolution reactions are discussed. Subsequently, seven groups of oxygen catalysts, which have played catalytic roles in both systems, are selectively reviewed, including transition metal oxides (single-metal oxides and mixed-metal oxides), functional carbon materials (nanostructured carbons and doped carbons), metal oxide–nanocarbon hybrid materials, metal–nitrogen complexes (non-pyrolyzed and pyrolyzed), transition metal nitrides, conductive polymers, and precious metals (alloys). Nonaqueous systems have the advantages of energy density and rechargeability over aqueous systems and have gradually become the research focus of metal–air batteries. However, there are considerable challenges beyond catalysts from aqueous to nonaqueous electrolytes, which are also discussed in this review. Finally, several future research directions are proposed based on the results achieved in this field, with emphasis on nonaqueous Li–air batteries.

Oxygen electrocatalysts in metal–air batteries: from aqueous to nonaqueous electrolytes

Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle Size, Shape, and Composition Manipulation and Their Impact to Activity Yan-Jie Wang,†,‡ Nana Zhao,‡ Baizeng Fang,† Hui Li,* Xiaotao T. Bi,*,† and Haijiang Wang* †Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC Canada V6T 1Z3 ‡Vancouver International Clean-Tech Research Institute Inc., 4475 Wayburne Drive, Burnaby, Canada V5G 4X4 Electrochemical Materials, Energy, Mining and Environment, National Research Council Canada, 4250 Wesbrook Mall, Vancouver, BC, Canada V6T 1W5

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Carbon-Supported Pt-Based Alloy Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle Size, Shape, and Composition Manipulation and Their Impact to Activity

Measurements of the pH dependence of the surface-charge density on oxide particles are correlated with the pH dependence of the cyclic voltammograms and of the chemical activity of the oxide for electrolysis/fuel cell reactions. Studies of the pyrochlores Pb{sub 2}M{sub 2{minus}x}Pb{sub x}O{sub 7{minus}y} and the rutiles in MO{sub 2} (M = Ru or Ir) and of the perovskite Sr{sub 1{minus}x}NbO{sub 3{minus}{delta}} show that (1) the oxygen-reduction reaction, found on the pyrochlores in alkaline solution, occurs by exchange of a surface OH{sup {minus}} species at an O{prime} site with an adsorbed O{sub 2}{sup {minus}} solution species, (2) the oxygen-evolution and chlorine-evolution reactions occur at a surface O{sup {minus}} species made accessible by surface oxidation of a redox couple lying close to the top of the O{sup 2{minus}}:2p{sup 6} valence band, and (3) the hydrogen-evolution reaction occurs at the surface OH{sub 2} species bonded to cations with a surface redox couple lying close to the H{sup +}/H{sub 2} level in solution.

Ramasamy Manoharan

Papers

Surface protonation and electrochemical activity of oxides in aqueous solution

Methanol oxidation on unsupported and carbon supported Pt + Ru anodes

Intraalloy electron transfer and catalyst performance: a spectroscopic and electrochemical study

Porous carbon anodes for the direct methanol fuel cell—I. The role of the reduction method for carbon supported platinum electrodes

Pt nanoparticles supported on NiTiO3/C as electrocatalyst towards high performance Methanol Oxidation Reaction