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

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 N, CO, and NO. 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 FeC sites, spanning across many typical size distributions and carbon environments. Removing FeC 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. DFT calculations prove that the FeC surface binds O and N intermediates too strongly to be catalytic. By settling the argument on the catalytic role of FeC in alkaline electrocatalysis, we hope to spur innovation in this critical field.