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

The commercialization of zinc-air batteries (ZABs)and manytypes of fuel cells hinges on the discovery of non-precious metalcatalysts with high activity and durability for the oxygen reductionreaction (ORR). Herein, we describe a simple and scalable l-alanine-assisted thermal pyrolysis strategy [utilizing l-alanine, urea, Ketjenblack carbon (KB), and CoCl2 asprecursors] that yielded a Co@N-C/N-KB catalyst withoutstanding ORR performance in alkaline media. The addition of l-alanine in the pyrolysis-step increased the proportion ofpyridinic-N + graphitic-N in the Co@N-C/N-KB catalyst,with highly conductive KB-promoting electron transfer kinetics duringORR. These attributes, together with the hierarchical porosity ofthe 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 densityof -5.86 mA cm(-2), a Tafel slope of 63.7 mVdec(-1), and an excellent durability and methanoltolerance (superior to a commercial 20 wt % Pt/C catalyst in almostall these aspects). A ZAB constructed with Co@N-C/N-KBas the cathode catalyst delivered an impressive open-circuit voltageof 1.519 V, a high power density of 204.5 mW cm(-2), an energy density up to 790 mA h g(Zn) (-1), and very stable operation with charge-discharge cycling,thus offering great promise for practical devices.