Achieving pH-universal oxygen electrolysis via synergistic density and coordination tuning over biomass-derived Fe single-atom catalyst

Renewable biomass serves as a cost-effective source of carbon matrix to carry single-atom catalysts (SACs). However, the natural abundant oxygen in these materials hinders the sufficient dispersion of element with high oxygen affinity such iron (Fe). The lowered-density and oxidized SACs greatly limits their catalytic applications. Here we develop a facile continuous activation (CA) approach for synthesizing robust biomass-derived Fe-SACs. Comparing to the traditional pyrolysis method, the CA approach significantly increases the Fe loading density from 1.13 atoms nm^-2 to 4.70 atoms nm^-2. Simultaneously, the CA approach induces a distinct coordination tuning from dominated Fe-O to Fe-N moieties. We observe a pH-universal oxygen reduction reaction (ORR) performance over the CA-derived Fe-SACs with a half-wave potential of 0.93 V and 0.78 V vs. RHE in alkaline and acidic electrolyte, respectively. Density functional theory calculations further reveal that the increased Fe-N coordination effectively reduces the energy barriers for the ORR, thus enhancing the catalytic activity. The Fe-SACs-based zinc-air batteries show a specific capacity of 792 mA·h·g_Zn^-1 and ultra-long life span of over 650 h at 5 mA cm^-2.