l-Alanine-Assisted Synthesis of a Hierarchically Porous Co@N–C/N-Ketjenblack Carbon Catalyst for Oxygen Reduction in Zn–Air Batteries

Abstract

The commercialization of zinc–air batteries (ZABs) and many types of fuel cells hinges on the discovery of non-precious metal catalysts with high activity and durability for the oxygen reduction reaction (ORR). Herein, we describe a simple and scalable l-alanine-assisted thermal pyrolysis strategy [utilizing l-alanine, urea, Ketjenblack carbon (KB), and CoCl2 as precursors] that yielded a Co@N–C/N–KB catalyst with outstanding ORR performance in alkaline media. The addition of l-alanine in the pyrolysis-step increased the proportion of pyridinic-N + graphitic-N in the Co@N–C/N–KB catalyst, with highly conductive KB-promoting electron transfer kinetics during ORR. These attributes, together with the hierarchical porosity of the catalyst [presence of micropores, mesopores (dominant), and macropores], gave Co@N–C/N–KB an onset potential of 0.91 V vs RHE, a half-wave potential of 0.84 V vs RHE, a limiting current density of −5.86 mA cm–2, a Tafel slope of 63.7 mV dec–1, and an excellent durability and methanol tolerance (superior to a commercial 20 wt % Pt/C catalyst in almost all these aspects). A ZAB constructed with Co@N–C/N–KB as the cathode catalyst delivered an impressive open-circuit voltage of 1.519 V, a high power density of 204.5 mW cm–2, an energy density up to 790 mA h gZn –1, and very stable operation with charge–discharge cycling, thus offering great promise for practical devices.