Review—Recent Progress in Highly Efficient Oxygen Reduction Electrocatalysts: From Structural Engineering to Performance Optimization

Oxygen reduction reaction (ORR) is one of the most important reactions in practical electrochemical energy devices such as fuel cells and metal-air batteries. In this paper, the recent advancements in platinum-group-metals-based alloys including Pt and/or Pd alloys with the late transitional metals for ORR electrocatalysis are reviewed in terms of catalyst synthesis, characterization, functional mechanism and the validation of performance (activities and stabilities) in both acidic and alkaline electrolytes. The electronic tuning and structural design/engineering for inducing lattice strain, favorable coordination environment, defects, vacancies, etc. for catalytic ORR active sites are emphasized. Morphologically, zero- to three-dimensional ORR catalysts with remarkable performances are introduced. For facilitating further research and development, several challenges are analyzed and the corresponding research directions for overcoming the challenges are also proposed.