Influence of Hydrophilicity on the Performance of Bifunctional Cobalt-Based Catalysts in Zn–Air Batteries

Zinc–air batteries are promising for portable applications: they use earth-abundant elements, work at low cost, are lightweight, and are also safe in application. For the air-breathing side (cathode), bifunctional catalysts for the oxygen reduction reaction (ORR, for discharging) and the oxygen evolution reaction (OER, for charging) are required and face their own challenges. Metal–nitrogen–carbon (MNC) catalysts and specific cobalt-based CoNCs show very good performance for both the ORR and the OER. However, their performance might be improved by optimizing the surface properties for a balanced interaction with water, as it is a reactant in one reaction but a product in the counterreaction. This work focused on the use of a CoNC catalyst in Zn–air batteries that is prepared by microwave pyrolysis. We systematically varied the surface properties of the catalyst with hydrogen peroxide treatments. Contact angle measurements and Raman spectroscopy were used to follow the impact on hydrophobicity and changes in the carbon morphology. By rotating disc electrode experiments, the optimal H2O2 treatment condition was identified. In a Zn–air battery cell, the performance changes in the initial polarization curve and the round-trip efficiency were evaluated for a period of in total 54 h. Electrode characterization of the catalysts prepared by mild and strong peroxide treatments indicates that the changes induced by cycling were very different but always resulted in the reconstruction of the catalyst. The results show that a treatment with hydrogen peroxide was effective to improve the bifunctional properties of the Co-based catalyst.