Regulating the spin state of triple-atom catalysts for improved oxygen electrocatalysis in zinc-air batteries at subzero temperature

<p>Triple-atom catalysts (TACs), building upon dual-atom catalysts, have demonstrated exceptional catalytic activity toward oxygen reduction reaction (ORR). Nevertheless, advancing the catalytic activity of TACs for application in low-temperature (Low-T) energy conversion devices remains a formidable challenge, primarily limited by the slow capture efficiency of oxygen reactants at catalytic sites. In this work, utilizing Fe-Fe double atomic pairs as a foundational research model, we plant the CoN<sub>4</sub> site adjacent to activate them to construct efficient ORR electrocatalysts (named as Fe<sub>2</sub>/Co-NHCS) featuring the Fe<sub>2</sub>N<sub>5</sub>+CoN<sub>4</sub> tri-atomic structure. The adjacent CoN<sub>4</sub> site can optimize the spin state of the Fe-Fe double atomic pairs from low to medium spin with <inline-formula><tex-math id="M1">${ {t}_{{2g}^{4}}} $</tex-math></inline-formula> <inline-formula><tex-math id="M2">${ {e}_{{g}^{1}} }$</tex-math></inline-formula>3d-electron configuration, which makes the catalysts bind with oxygen reactants more readily to complete ORR process. As anticipated, the Fe<sub>2</sub>/Co-NHCS achieved a high half-wave potential of 0.92 V. Moreover, the flexible Zn-air batteries powered by Fe<sub>2</sub>/Co-NHCS deliver a large peak power density of 57.3 mW cm<sup>-2</sup> and brilliant charge-discharge stability over 150 stable cycles at -40°C. This research advances ORR electrocatalysts by modulating the electron spin states at diatomic Fe sites, paving a viable route for expanding the application range of ZABs in low-temperature environment.</p>