Activity and stability of non-precious metal catalysts for oxygen reduction in acid and alkaline electrolytes

The activity and stability of non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) in both acid and alkaline electrolytes were studied by the rotating disk electrode technique. The NPMCs were prepared through the pyrolysis of cobalt–iron–nitrogen chelate followed by combination of pyrolysis, acid leaching, and re-pyrolysis. In both environments, the catalysts heat-treated at 800–900 °C exhibited relatively high activity. Particularly, an onset potential of 0.92 V and a well-defined limiting current plateau for the ORR was observed in alkaline medium. The potential cycling stability test revealed the poor stability of NPMCs in acid solution with an exponential increase in the performance degradation as a function of the number of potential cycling. In contrast, the NPMCs demonstrated exceptional stability in alkaline solution. The numbers of electron transferred during the ORR on the NPMCs in acid and alkaline electrolytes were 3.65 and 3.92, respectively, and these numbers did not change before and after the stability test. XPS analysis indicated that the N-containing sites of catalysts are stable before and after the stability test when in alkaline solution but not in acid solution.