Unraveling the significance of FeN <sub>4</sub> and Fe <sub>3</sub>S <sub>4</sub> active species for sustained direct ammonia fuel cells and zinc-air batteries
Zhiwen Li, Yan Xie, Yangkai Han, Hezhen Wang, Jianxin Gao, Su Chen, Ling Xu, Linguo Lu, Yun Zhao, Erdong Wang, Wenbin Li
Abstract
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 (<em>E</em><sub>1/2</sub>) of 0.896 V (vs. RHE) and kinetic current density (<em>J</em><sub>K</sub>) of 40.9 mA·cm<sup>−2</sup> 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<sup>−2</sup> in an electrolyte containing 3 M NH<sub>3</sub> and 3 M KOH, and 180.2 mW·cm<sup>−2</sup> with a high capacity of 820 mAh·g<sub>cat.</sub><sup>−1</sup> in a 6 M KOH solution. Such superior electrocatalytic performances are not only due to the synergistic effect of Fe<sub>3</sub>S<sub>4</sub> particles and FeN<sub>4</sub> 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.