Iron porphyrin-based cathode catalysts for PEM fuel cells: Influence of pyrolysis gas on activity and stability

Fe-based catalysts for the O2 reduction in acidic medium were prepared by impregnating chloro-iron tetramethoxyphenylporphyrin (Cl-FeTMPP) on a non-microporous carbon black and heat-treating the resulting material at 950 °C either in NH3 (Mode 1) or in Ar (Mode 2). The most active catalyst using Mode 1 has a Fe bulk content of 0.4 wt% and an activity of 17 mA mg−1 at 0.8 V in fuel cell. This activity is controlled by the microporous surface area of the catalyst. The most active catalysts using Mode 2 has an Fe bulk content of 3.7 wt% and an activity of 1.9 mA mg−1 at 0.8 V in fuel cell. In Mode 2, the nitrogen and/or the iron surface concentrations control the activity. Concerning stability, Mode 1-catalysts are unstable while Mode 2-catalysts show stability for at least 15 h when at least 66 wt% Cl-FeTMPP is impregnated onto N330 and heat-treated at 950 °C in Ar. The catalyst made with 66 wt% Cl-FeTMPP has a bulk Fe content of 5.2 wt% and an activity of 1.3 mA mg−1 at 0.8 V in fuel cell. Thus, in the present study, pyrolysis in NH3 gives active but unstable catalysts while pyrolysis in argon gives less active but more stable catalysts at high Cl-FeTMPP loading. Graphitization of Cl-FeTMPP during pyrolysis in argon seems to impart stability. Mode 2-catalysts are stable in spite of a high peroxide yield of 26% while Mode 1-catalysts are unstable in spite of a low 5% peroxide yield.