Multiple roles of graphene in electrocatalysts for metal-air batteries

Metal-air batteries are the most promising next-generation energy conversion and storage devices due to their high theoretical energy densities. However, their practical energy densities and voltages are limited by the sluggish kinetics of the oxygen reduction and evolution reactions due to the multi-step electron transfer and the triple-phase mass transfer. To address this issue, a number of research efforts have been devoted to materials science and interface chemistry, in which graphene-based electrocatalysts have attracted much attention due to their tunable catalytic properties, high electrical conductivities, and large surface areas. In this review, we reveal the principles of oxygen electrocatalytic reactions and the requirements for efficient oxygen electrocatalysts with regards to the structural feature, intrinsic activity, and electrical conductivity. Afterwards, the morphological design of the electrode is clearly elucidated, followed by the ample discussion on the main research progresses and challenges of graphene-based materials as either electrocatalysts with intrinsic active sites or supports for other traditional metal or metal oxide catalysts. Furthermore, we outline the research directions for developing highly-efficient and low-cost graphene-based electrocatalysts.