Sharply expanding single-atomically dispersed Fe–N active sites through bidirectional coordination for oxygen reduction

Abstract

For Fe–NC systems, high-density Fe–N sites are the basis for high-efficiency oxygen reduction reaction (ORR), and P doping can further lower the reaction energy barrier, especially in the form of metal–P bonding. However, limited to the irregular agglomeration of metal atoms at high temperatures, Fe–P bonds and high-density Fe–N cannot be guaranteed simultaneously. Here, to escape the random and violent agglomeration of Fe species during high-temperature carbonization, triphenylphosphine and 2-methylimidazole with a strong metal coordination capability are introduced together to confine Fe growth. With the aid of such bidirectional coordination, the high-density Fe–N site with Fe–P bonds is realized by in situ phosphorylation of Fe in an Fe–NC system (Fe–P–NC) at high temperatures. Impressively, the content of single-atomically dispersed Fe sites for Fe–P–NC dramatically increases from 2.8% to 65.3% compared with that of pure Fe–NC, greatly improving the ORR activity in acidic and alkaline electrolytes. The theoretical calculation results show that the generated Fe₂P can simultaneously facilitate the adsorption of intermediates to Fe–N₄ sites and the electron transfer, thereby reducing the reaction energy barrier and obtaining superior ORR activity.