Nitrogen-doped graphitic carbons with encapsulated CoNi bimetallic nanoparticles as bifunctional electrocatalysts for rechargeable Zn-Air batteries

Highly efficient and low-cost bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are greatly desirable for rechargeable metal-air batteries. Herein, we propose a useful strategy of integrating a bimetallic nanoparticle into the N-doped porous carbon to synthesize highly active bifunctional oxygen catalyst (CoNi/NG). The bimetallic CoNi/NG catalyst exhibits a positive half-wave potential of 0.85 V (versus, RHE) for ORR, and a low operating potential of 1.6 V (versus, RHE) to achieve a 10 mA cm(-2) current density for OER. The excellent bifunctional properties of CoNi/NG are attributed to the various active sites (i.e., Co-N-C, Ni-N-C moieties and N-C sites) and its 2D graphene-like ultrathin structure, as well as synergistic effect of CoNi alloys. Importantly, the liquid Zn-air battery with CoNi/NG as oxygen electrode presents not only excellent charging and discharging curves and maximal power density of 130.5 mW cm(-2), but also robust durability with only 50 mV increase in the voltage gap (the voltage difference between the charging and discharging) after continuous 110 h cycle tests. This work provides a general design principle for development of bifunctional catalysts by integrating different bimetals responsible for different electrochemical reactions into the N-doped graphitic carbons. (C) 2018 Elsevier Ltd. All rights reserved.