In-situ immobilization strategy for single-atom iron sites to enhance oxygen reduction reaction in Zn–air batteries and microbial fuel cells

A straightforward and environmentally friendly approach was utilized to synthesize single-atomic Fe-N-C catalyst. In-situ immobilization of Fe3+ during the formation of polydopamine (PDA) and ZIF-8 in aqueous solution effectively prevent metal agglomeration. The mutual promotion of PDA and aqueous solution constructed the hybrid-level porous hollow structure in P-FeNC. This method extends the solution scope from organic solution system to water solution, thus providing fascinating opportunities for investigating metal-N-C catalyst by in-situ mixing various metal salt precursor and dopamine. The optimal P-FeNC catalyst demonstrates superlative oxygen reduction reaction (ORR) performance, with the half-wave potentials (E1/2) of 0.886 V, 0.790 V and 0.720 V in 0.1 M KOH, 0.5 M H2SO4 and 0.05 M PBS solutions, which exceeded commercial 20 wt% Pt/C. The combinatorial impacts of highly-dense FeN4 active sites and interconnected hierarchical mesopores/macropores are presumed to underpin the observed enhancement in ORR performance. Further evaluations show that the power densities of P-FeNC equipped zinc-air battery (ZAB) and microbial fuel cell (MFC) are 460 mW cm-2 and 3083 ± 10 mW m-2, respectively. This study confirms the feasibility to design the innovative single-atom catalyst in aqueous solution and provides constructive ideas to employ the high-performance P-FeNC electrocatalyst in ZABs and MFCs.