High pressure pyrolyzed non-precious metal oxygen reduction catalysts for alkaline polymer electrolyte membrane fuel cells

Abstract

Non-precious metal catalysts, such as metal-coordinated to nitrogen doped-carbon, have shown reasonable oxygen reduction reaction (ORR) performances in alkaline fuel cells. In this report, we present the development of a highly active, stable and low-cost non-precious metal ORR catalyst by direct synthesis under autogenic-pressure conditions. Transmission electron microscopy studies show highly porous Fe–N–C and Co–N–C structures, which were further confirmed by Brunauer–Emmett–Teller surface area measurements. The surface areas of the Fe–N–C and Co–N–C catalysts were found to be 377.5 and 369.3 m² g⁻¹, respectively. XPS results show the possible existence of N–C and M–N_x structures, which are generally proposed to be the active sites in non-precious metal catalysts. The Fe–N–C electrocatalyst exhibits an ORR half-wave potential 20 mV higher than the reference Pt/C catalyst. The cycling durability test for Fe–N–C over 5000 cycles shows that the half-wave potential lost only 4 mV, whereas the half-wave potential of the Pt/C catalyst lost about 50 mV. The Fe–N–C catalyst exhibited an improved activity and stability compared to the reference Pt/C catalyst and it possesses a direct 4-electron transfer pathway for the ORR process. Further, the Fe–N–C catalyst produces extremely low HO₂⁻ content, as confirmed by the rotating ring-disk electrode measurements. In the alkaline fuel single cell tests, maximum power densities of 75 and 80 mW cm⁻² were observed for the Fe–N–C and Pt/C cathodes, respectively. Durability studies (100 h) showed that decay of the fuel cell current was more prominent for the Pt/C cathode catalyst compared to the Fe–N–C cathode catalyst. Therefore, the Fe–N–C catalyst appears to be a promising new class of non-precious metal catalysts prepared by an autogenic synthetic method.