Rational design of sustainable transition metal-based bifunctional electrocatalysts for oxygen reduction and evolution reactions

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two critical processes for metal-air batteries (MABs) that afford large energy capacity and high power density. However, both reactions suffer from sluggish kinetics. The availability of efficient electrocatalysts that could conquer activation energy barriers and accelerate reaction rates is crucial to tackling this issue. The noble metal-based electrocatalysts (e.g., Pt/C and Ru/IrO2) cannot meet the requirement for large-scale applications due to their scarce abundance, high cost, and insufficient stability. Transition metal-based materials can be a sustainable and promising candidate to replace the noble metal-based electrocatalysts for this purpose. This review intends to review the most recent, high performance, and sustainable strategies on tuning the catalytic activity of the transition metal-based materials toward ORR and OER. We also propose perspectives on remaining challenges and future research directions of transition metal-based bifunctional electrocatalysts toward practical applications. These fundamental design principles and strategies are crucial to guide and accelerate the design of the ideal catalysts and make possible the practical applications of MABs. (c) 2020 Published by Elsevier B.V.