Sulfur atom modulated Fe-Nx species embedded in hollow porous carbon spheres for efficient oxygen reduction and high-performance zinc-air batteries

A nitrogen (N) and sulfur (S) co-doped FeS/FeN-NC-X4 (X represents the molar ratio of Fe and S added) catalyst with an abundant S-doped Fe-Nx species is designed rationally for enhanced oxygen reduction reaction (ORR). And these valuable activation sites are embedded into a hollow porous carbon shell for increasing the limiting current density and increasing the onset potential. The precise regulation of the Fe/S ratio also highlights the influence of appropriate S doping on the catalytic performance of atomically dispersed Fe-N-C-based catalysts for high-performance Zinc-air batteries. All of these factors contribute to the remarkable catalytic performance of FeS/FeN-NC-X4 for the ORR with a half-wave potential of 0.863 V, which is superior to most non-precious metal catalysts previously reported. Simultaneously, the synthesized material is used as a cathode catalyst to assemble zinc-air batteries to illustrate its practical application. The results show that it has a high open-circuit voltage (1.44 V), extremely high maximum power density (156 mW cm-2) and excellent stability (stable operation for 400 h at a current density of 5 mA cm-2). The assembled portable and flexible batteries also have a high open circuit voltage of 1.40 V and a maximum power density of 68.1 mW cm-2. This work therefore presents a new method for the synthesis of atomically dispersed iron catalysts with high efficiency and metal content that are appropriate for practical energy storage applications.