Selective synthesis of dense high-spin D1 active sites via engineered less-graphitized carbon environments†

Fe–N–C catalysts are the most promising alternative to Pt for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, the mixture of two distinct active sites—highly active but unstable D1 and less active but more stable D2—has complicated the study of site-specific catalytic behaviors. Here we report a synthetic procedure to maximize the D1 site by introducing ascorbic acid (AA) as a perturbing molecule. The AA not only increases the single-atom loading, but also creates a less-graphitized carbon environment, featuring increased carbon defects and mesoporosity, which favors D1 site formation. This resulting catalyst exhibits over 80% D1 site and a substantially high D1 concentration of 2.13 wt%. The denser D1 sites, as well as the increased mesoporosity, enables a current density of 151 mA cm⁻² at 0.8 V and a peak power density of 803 mW cm⁻² at 1.5 bar air. Meanwhile, the catalyst loses 93% of its initial power within 50 hours. Both the activity and stability behaviors meet the characteristics of the D1 site. The study paves the way for the precise exploration of the D1 active site, not only for the ORR but also potentially for other catalytic processes.