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 57 Fe cryogenic Mössbauer spectroscopy and X-ray absorption spectroscopy. Site density values measured by in situ nitrite stripping and ex situ CO chemisorption methods are 4.7 × 10 19 and 7.8 × 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 H 2 SO 4 electrolyte. The catalyst delivers an excellent proton exchange membrane fuel cell performance with current densities of 41.3 mA cm −2 at 0.90 V iR -free using H 2 –O 2 and 145 mA cm −2 at 0.80 V (199 mA cm −2 at 0.80 V iR -free ) using H 2 –air.