Co/Co-Fe Nanoparticles/N-Doped Carbon Composite as Bifunctional Electrocatalyst for Rechargeable Metal-Air Batteries

Abstract

In secondary metal-air batteries, it is indispensable to develop novel bifunctional catalysts for the low overpotential oxygen reduction/oxygen evolution reaction to allow on the one hand very high energy density combined with high capacity and low price.1,2 Therefore, the widespread use of rechargeable metal-air batteries strongly depends on the substitution of precious metal-based bifunctional electrocatalysts towards the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) with catalysts from high-abundant materials.3 This seems to be possible since the electrolyte in typical Ni- or Zn-metal air batteries is highly alkaline. Here, we report a novel Co/Co-Fe nanoparticle/N-doped carbon composite (Co/CoxFey/NC) obtained by thermal treatment of a composite of Co-Fe layered double hydroxide (Co-Fe LDH) and polybenzoxaxine (pBO) under ammonia atmosphere. The latter undergoes thermal degradation to form a conductive nitrogen-doped carbon matrix encapsulating Co/Co-Fe nanoparticles thereby circumventing the use of any binder material to immobilize the catalyst particles on the electrode surface.4 Within this work, the influence of the temperature, duration of treatment and the nature of gas atmosphere employed during the thermal treatment, as well as the ratio Co-Fe LDH:pBO on the electrocatalytic activity performance were investigated, leading to identification of an optimal Co/CoxFey/NC catalyst exhibiting remarkable bifunctional activity towards ORR and OER in 0.1 M KOH with an overpotential difference between these two reaction as low as 0.76 V at -1 mA cm-2 for the ORR and at 10 mA cm-2 for the OER, respectively. Moreover, the stability of Co/CoxFey/NC catalyst was evaluated in an air-breathing set-up, alternating between the OER and the ORR at a current density of 10 mA cm-2 in 6 M KOH to simulate the charge discharge cycles in rechargeable metal-air batteries for a total of 120 charge-discharge cycles each lasting 10 minutes.