Milk powder-derived bifunctional oxygen electrocatalysts for rechargeable Zn-air battery

Sustainable oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts are desirable for many energy storage and conversion systems. Australian dairy industry generates a large amount of expired milk powders every year, which are often dumped as food wastes. Here, we report a highly efficient bifunctional O₂ electrocatalyst by incorporating mixed metal oxide (NiFeO_x) nanoparticles into N (8.5 at%) and P (1.9 at%) dually doped carbon materials derived from milk powders. This catalyst demonstrates an onset potential of 0.90 V vs. reversible hydrogen electrode (RHE), a kinetic limiting current density of 12.9 mA cm⁻² at 0.4 V vs. RHE, and an electron transfer number of 3.86 for ORR, it also shows an onset potential of 1.48 V vs. RHE, an overpotential of 0.32 V at 10 mA cm⁻², a Tafel slope of 59.03 mV/dec in 1.0 M KOH for OER. Its catalytic performance is comparable to that of the best metal-free carbon-based ORR electrocatalysts and OER electrocatalysts based on hybrids of metal oxides and carbon materials. Rechargeable Zn-air batteries were also fabricated using air electrodes made of this catalyst, demonstrating an open-circuit potential of 1.39 V, a specific capacity of 688 mAh g⁻¹ (corresponding to an energy density of 853 Wh kg_Zn⁻¹), and excellent rechargeability over 150 cycles with a small performance loss. The milk powder-derived carbon materials can be used to produce eco-friendly and high value-added electrocatalysts for energy conversion and storage applications while reducing food wastage.