Development of a highly active Fe–N–C catalyst with the preferential formation of atomic iron sites for oxygen reduction in alkaline and acidic electrolytes

Nitrogen-doped porous carbons containing atomically dispersed iron are prime candidates for substituting platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. These carbon catalysts are classically synthesized via complicated routes involving multiple heat-treatment steps to form the desired Fe-N_x sites. We herein developed a highly active Fe–N–C catalyst comprising of exclusive Fe-N_x sites by a simplified solid-state synthesis protocol involving only a single heat-treatment. Imidazole is pyrolyzed in the presence of an inorganic salt-melt resulting in highly porous carbon sheets decorated with abundant Fe-N_x centers, which yielded a high density of electrochemically accessible active sites (1.36 × 10¹⁹ sites g⁻¹) as determined by the in situ nitrite stripping technique. The optimized catalyst delivered a remarkable ORR activity with a half-wave potential (E_1/2) of 0.905 V_RHE in alkaline electrolyte surpassing the benchmark Pt catalyst by 55 mV. In acidic electrolyte, an E_1/2 of 0.760 V_RHE is achieved at a low loading level (0.29 mg cm⁻²). In PEMFC tests, a current density of 2.3 mA cm⁻² is achieved at 0.90 V_iR-free under H₂–O₂ conditions, reflecting high kinetic activity of the optimized catalyst.