Direct Conversion of MnO₂ into Atomic Mn Sites for Oxygen Reduction

Atomic metal sites, such as Fe, Co, and Mn, coordinated with N and embedded in carbon, are the most promising platinum-group-metal (PGM)-free catalysts for acidic oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Among others, Mn sites are preferable to minimize potential Fenton reactions within the electrode. Herein, we demonstrate a facile and scalable synthetic method to prepare atomic Mn-N-C catalysts by directly converting manganese oxides into active, stable MnN₄ sites via solid-state reactions. Post-treatment with ammonium chloride can enhance the intrinsic ORR activity of MnN_x moieties by introducing additional nitrogen groups and defects. Subsequent post-treatment with organic molecules, such as benzimidazole, promotes the formation of a robust carbon structure and significantly improves catalyst stability. The resulting Mn-N-C catalyst exhibits promising ORR activity, achieving a half-wave potential of 0.83 V vs RHE in aqueous 0.5 M H₂SO₄ electrolyte, outperforming most PGM-free ORR catalysts. A corresponding membrane electrode assembly (MEA) generated a current density of ∼400 mA cm⁻² at 0.67 V and a peak power density of 0.46 W cm⁻². Significantly, post-treatment with benzimidazole improved catalyst stability, retaining 85% of the MEA performance, although the initial performance was compromised.