Fe–N–C electrocatalysts in the oxygen and nitrogen cycles in alkaline media: the role of iron carbide

Abstract

Fe–N–C electrocatalysts hold a great promise for Pt-free energy conversion, driving the electrocatalysis of oxygen reduction and evolution, oxidation of nitrogen fuels, and reduction of N₂, CO₂, and NO_x. Nevertheless, the catalytic role of iron carbide, a component of nearly every pyrolytic Fe–N–C material, is at the focus of a heated controversy. We now resolve the debate by examining a broad range of Fe₃C sites, spanning across many typical size distributions and carbon environments. Removing Fe₃C selectively by a non-oxidizing acid reveals its inactivity towards two representative reactions in alkaline media, oxygen reduction and hydrazine oxidation. The activity is assigned to other pre-existing sites, most probably Fe–N_x. DFT calculations prove that the Fe₃C surface binds O and N intermediates too strongly to be catalytic. By settling the argument on the catalytic role of Fe₃C in alkaline electrocatalysis, we hope to spur innovation in this critical field.