High loading of single atomic iron sites in Fe-NC oxygen reduction catalysts for proton exchange membrane fuel cells

Non-precious iron-based catalysts (Fe-NCs) require high active site density to meet the performance targets as cathode catalysts in proton exchange membrane fuel cells. Site density is generally limited to that achieved at a 1-3 wt%(Fe) loading due to the undesired formation of iron-containing nanoparticles at higher loadings. Here we show that by preforming a carbon-nitrogen matrix using a sacrificial metal (Zn) in the initial synthesis step and then exchanging iron into this preformed matrix we achieve 7 wt% iron coordinated solely as single-atom Fe-N-4 sites, as identified by Fe-57 cryogenic Mossbauer spectroscopy and X-ray absorption spectroscopy. Site density values measured by in situ nitrite stripping and ex situ CO chemisorption methods are 4.7 x 10(19) and 7.8 x 10(19) sites g(-1), with a turnover frequency of 5.4 electrons sites(-1) s(-1) at 0.80 V in a 0.5 M H2SO4 electrolyte. The catalyst delivers an excellent proton exchange membrane fuel cell performance with current densities of 41.3 mA cm(-2) at 0.90 ViR-free using H-2-O-2 and 145 mA cm(-2) at 0.80 V (199 mA cm(-2) at 0.80 ViR-free) using H-2-air.