Cr-Doped NiCo-Layered Double Hydroxide/N-Doped Graphene for a Bifunctional Electrocatalyst for Rechargeable Zinc Air Batteries

Rechargeable zinc air batteries (RZABs), which repeatedly store and release charges through the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) using oxygen electrocatalysts, are considered a promising energy storage system owing to their low cost and safety. Controlling the chemical structure and morphology of multifunctional electrocatalysts is crucial for improving their electrocatalytic activities and stabilities for high performance RZABs. Herein, we report hierarchically structured Cr-doped NiCo-layered double hydroxides (LDHs) nanoplates chemically grown on the surface of nitrogen-doped reduced graphene oxide nanosheets (NiCoCr LDH/N-rGO) for an application into bifunctional electrocatalysts of RZABs. As verified by the spectroscopic and electrochemical characterizations, Cr doping modulates the electronic configuration and surface structure of both LDHs and N-rGO nanosheets, thereby improving bifunctional catalytic activity and reaction kinetics. In particular, the electrocatalytic activity and kinetics of NiCoCr LDH/N-rGO for both ORR and OER are greatly improved owing to the increased active sites by Cr doping and hierarchical porous structure as demonstrated by low overpotential and Tafel's slope. Thus, the electrochemical performance of RZAB with NiCoCr LDH/N-rGO catalyst is superior to that with Pt/C+RuO2, as confirmed by higher power density of 97 mW cm-2 and better cycling stability of 18 h for the former than 59 mW cm-2 and 6 h for the latter.