Carbon-based bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions: Optimization strategies and mechanistic analysis

Electrocatalysts are one of the essential components for the devices of high-efficiency green energy storage and conversion, such as metal-air cells, fuel cells, and water electrolysis systems. While catalysts made from noble metals possess high catalytic performance in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), their scarcity and expensiveness significantly limit large-scale applications. In this regard, metal-free/non-noble metal carbon-based catalysts have become competitive alternatives to replace catalysts made of noble metals. Nevertheless, low catalytic ORR/OER performance is the challenge of carbon-based catalysts for the commercial applications of metal-air batteries. To solve the problem of poor catalytic performance, two strategies have been proposed: (1) controlling the microstructure of the catalysts to expose more active sites as the channels of rapid mass and electron transfer; and (2) reducing the reaction energy barrier by optimizing the electronic structures of the catalysts via surface engineering. Here, we review different types of bifunctional ORR/OER electrocatalysts with the activated surface sites. We focus on how the challenge can be overcome with different methods of material synthesis, structural and surface characterization, performance validation/optimization, to outline the principles of surface modifications behind catalyst designs. In particular, we provide critical analysis in the challenges that we are facing in structural design and surface engineering of bifunctional ORR/OER catalysts and indicate the possible solution for these problems, providing the society with clearer ideas on the practical prospects of noble-metal-free electrocatalysts for their future applications.