Unraveling the significance of FeN₄ and Fe₃S₄ active species for sustained direct ammonia fuel cells and zinc-air batteries

Transition metal electrocatalysts are deem to be alternatives for the replacement of commercial Pt/C as highly efficient oxygen reduction reaction (ORR) electrocatalysts in the applications of advanced energy conversion/storage technologies. Herein, we reprecipitate hemin on polypyrrole (PPy) modified carbon materials (R-Hm/PPy@C), which boosts up the ORR performances. A high half-wave potential (E_1/2) of 0.896 V (vs. RHE) and kinetic current density (J_K) of 40.9 mA·cm⁻² at 0.80 V (vs. RHE) of R-Hm/PPy@C outperforms these of commercial Pt/C in a 0.1 M KOH electrolyte. Moreover, the direct ammonia fuel cells (DAFC) and zinc-air batteries (ZAB) assembled by R-Hm/PPy@C deliver remarkable peak power densities of 288.4 mW·cm⁻² in an electrolyte containing 3 M NH₃ and 3 M KOH, and 180.2 mW·cm⁻² with a high capacity of 820 mAh·g_cat.⁻¹ in a 6 M KOH solution. Such superior electrocatalytic performances are not only due to the synergistic effect of Fe₃S₄ particles and FeN₄ single sites, but also are improved by enriched N-doped C species, further corroborated by theoretical calculations. Overall, this study delivers an efficacious approach to fabricate non-noble metal electrocatalysts (NNMEs) by engineering the synergistic effect of nanoparticles (NPs) and single-atom species with N-enriched dopants towards elevated electrocatalytic activities for energy conversion/storage devices.