Building Atomic Scale and Dense Fe─N 4 Edge Sites of Highly Efficient Fe─N─C Oxygen Reduction Catalysts Using a Sacrificial Bimetallic Pyrolysis Strategy.

Replacing high-cost and scarce platinum (Pt) with transition metal and nitrogen co-doped carbon (M/N/C, M = Fe, Co, Mn, and so on) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells has largely been impeded by the unsatisfactory ORR activity of M/N/C due to the low site utilization and inferior intrinsic activity of the M─N 4 active center. Here, these limits are overcome by using a sacrificial bimetallic pyrolysis strategy to synthesize Fe─N─C catalyst by implanting the Cd ions in the backbone of ZIF-8, leading to exposure of inaccessible FeN 4 edge sites (that is, increasing active site density (SD)) and high fast mass transport at the catalyst layer of cathode. As a result, the final obtained Fe(Cd)─N─C catalyst has an active site density of 33.01 µmol g -1 (with 33.01% site utilization) over 5.8 times higher than that of Fe─N─C catalyst. Specially, the optimal catalyst delivers a high ORR performance with a half-wave potential of 0.837 (vs RHE) in a 0.1 m HClO 4 electrolyte, which surpasses most of Fe-based catalysts.