Fig. 2.

ORR electrochemical characterization. Measurements of Co_0.5, Fe_0.5, N-C and a Pt/C benchmark catalyst. Upper panels are RRDE measurements at various rotation rates and loadings performed in O₂-saturated 0.1 M H₂SO₄ aqueous solution. a ORR current density at the disk. The total catalyst loading for Co_0.5, N-C and Pt/C (5 wt% Pt/C) was 0.8 mg cm⁻². b % H₂O₂ measured during ORR at the ring. The total catalyst loading for Co_0.5 was 0.8 or 0.2 mg cm⁻². Lower panels are iR-corrected PEMFC polarization curves with Co_0.5, Fe_0.5 or Pt/C at the cathode, presented in linear scale c and semi-logarithmic scale d. The fuel cell temperature was 80 °C, pure H₂ and O₂ gases were fully humidified at the cell temperature, the gauge pressure was 1 bar. The cathode catalyst loading for Co_0.5 and Fe_0.5 was 4 mg cm⁻² (60 µg_metal per cm²). For Pt/C at the cathode, 5% Pt/C was diluted to 1% Pt with uncatalysed Vulcan carbon black, and 4 mg cm⁻² of 1% Pt/C (40 µg_Pt per cm²) deposited