Metal-free carbon based air electrodes for Zn-air batteries: Recent advances and perspective

Zn-air batteries (ZABs) have been recognized as one of the most efficient, cost effective and environmentally benign energy storage devices that may play an important role in future sustainable energy system. Air electrode is a key part of ZABs, where oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) appear during discharge and charge processes which largely determine the performance, such as durability, rate performance and round-trip efficiency. However, the sluggish kinetics of ORR/OER may cause high cell overpotential, thus requiring certain electrocatalyst to speed up both reactions. Based on several considerations, such as cost, activity, stability, and conductivity, metal-free carbon materials have received considerable attentions as electrocatalysts of air electrode in ZABs to boost both OER and ORR. In this review article, the recent progress in applying metal-free carbon materials as air electrode in ZABs is summarized. The two main ways to tune the properties of carbon materials and eventually the catalytic performance for ORR and OER, i.e., heteroatom doping (N, P, B, S, F, etc.) and defect engineering, are focused. The performance indicators of ZABs based on the carbon materials were also tabulated. Finally, the latest challenges and future opportunities associated with metal-free carbon materials as air electrode in ZABs are discussed.