Rechargeable zinc-air batteries (ZABs) require bifunctional electrocatalysts presenting high activity in oxygen reduction/evolution reactions (ORR/OER), but the single-site metal-N-C catalysts suffer from their low OER activity. Herein, we designed a series of single-site Fe-N-C catalysts, which present high surface area and good conductivity by incorporating into mesoporous carbon supported on carbon nanotubes, to study the doping effect of N and P on the bifunctional activity. The additional P-doping dramatically increased the content of active pyridine-N and introduced P-N/C/O sites, which not only act as extra active sites but also regulate the electron density of Fe centers to optimize the absorption of oxygenated intermediates, thereby ultimately improving the bifunctional activity of Fe-N-C sites. The optimized catalyst displayed a half-wave potential of 0.882 V for ORR and a low overpotential of 365 mV at 10 mA cm-2 for OER, which significantly outperforms the counterpart without P, as well as noble-metal-based catalysts. The ZABs with air cathodes containing the N,P-co-doped catalysts exhibited a high peak power density of 201 mW cm-2 and a long cycling stability beyond 600 h. Doping has shown to be an effective way to optimize the performance of single-site catalysts in bifunctional oxygen electrocatalysis, which can be extended to other catalyst systems.
Keywords: Zn−air batteries; bifunctional oxygen electrocatalyst; carbon nanotubes; codoping; phosphorus; single site catalysts.