Designing Co–N/C Cathode Catalysts with Dense Atomic Cobalt Sites for Enhanced PEMFC Performance

Metal-nitrogen/carbon (M-N/C) catalysts, particularly those incorporating Fe, Co, or Mn, are among the most promising non-platinum group catalysts for the acidic oxygen reduction reaction (ORR) in fuel cells. This study reports a Co-N/C catalyst featuring high (3 wt%) cobalt content exclusively present as atomic sites. Extended X-ray absorption fine structure analysis confirms a tetrapyridinic Co-N₄ coordination environment in the optimized (3.0)Co-N/C^Δ catalyst. The high cobalt loading leads to a significant density of electrochemically accessible active sites, 3.58 × 10¹⁹ sites g^-1, quantified via the nitrite stripping method. The catalyst demonstrates excellent ORR activity in a rotating ring-disk electrode setup, achieving a half-wave potential (E_1/2) of 0.76 V at a low loading of 0.2 mg cm^-2 and a mass activity of 3.5 A g^-1 at 0.80 V_RHE. Single-cell hydrogen-oxygen PEMFC tests achieve a peak power density exceeding 1.3 W cm^-2 (iR-corrected). Under hydrogen-air condition, the catalyst delivers 0.54 A cm^-2 at 0.60 V (0.39 W cm^-2). Despite the intrinsically higher turnover frequency of Fe-based sites, the optimized (3.0)Co-N/C^Δ catalyst achieves similar fuel cell performance to that of Fe-N/C, highlighting the critical role of site density in overall activity.