Transition metal polyoxometalates with reduced graphene oxide for high-performance air-electrode of metal-air batteries

Abstract

The potential of polyoxometalates (POMs) as alternatives to noble metal-based bifunctional electrocatalysts for air electrode, i.e., for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), was analyzed herein. Five POMs containing transition metal (Mn, Fe, Co, Ni, or Cu) were prepared and combined with reduced graphene oxide (rGO). Kenning structure of POMs with nanolamellae-like morphology and multi-layered graphene sheets of rGO were confirmed by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, transmission electron microscopy and Raman spectroscopy analysis. Different POM:rGO ratios were tested to optimize the composition, and a ratio of 1:5 Ni-POM:rGO was found to be the most electrocatalytically active for OER (Tafel slope as low as 42 mV dec−1, overpotential at 10 mA cm−2 of 354 mV and current density at 400 mV overpotential as high as 83.4 mA cm−2), notably better than IrO2. As for ORR, Co-POM/rGO carried the lowest Tafel slope of 165 mV dec−1, half-wave potential of 0.711 V and a mixed 2- and 4-electron ORR mechanism. Moreover, these non-precious metal electrocatalysts showed stability in alkaline media with no change in their structure.