Tri- and tetra-metallic oxides anchored to nitrogen-doped carbon nanotubes as bifunctional electrocatalysts for rechargeable zinc-air batteries

Electrochemical catalysts for the air electrode of Zn-air rechargeable batteries were fabricated from nitrogen-doped carbon nanotubes (N-CNTs), combined with either tri-metallic (Ni–Mn–Co) or tetra-metallic (Zn–Ni–Mn–Co) oxides, through a simple impregnation method into carbon-based, gas diffusion layers (GDL). Metal oxide compositions were selected based on previous results, preliminary electrochemical testing, and statistical design of experiments. Microstructural characterization was done using electron microscopy and X-ray photoelectron spectroscopy. Samples were electrochemically tested and the best candidates were subjected to full cell testing and bifunctional cycling for 200 charge/discharge cycles at 10 mA cm−2. The overall bifunctional efficiency, after cycling, of the best NiMnCoOx/N-CNT and ZnNiMnCoOx/N-CNT catalysts was 53.3% and 56.4%, respectively; both outperformed Pt–Ru/C in both overall bifunctional efficiency (38%) and cycling stability. The maximum power density of one of the tetra-metallic oxides exceeded that of Pt–Ru/C (110 mW cm−2) at 134 mW cm−2. The addition of Zn with Ni–Mn–Co oxide particles showed improved cycling stability and overall bifunctional efficiency.