Bimetallic Zero-Valent Alloy with Measured High-Valent Surface States to Reinforce the Bifunctional Activity in Rechargeable Zinc-Air Batteries

Abstract

Among the earth-abundant catalysts for the air-electrode of metal-air batteries, success with one-component catalyst systems driving the bifunctional oxygen evolution and reduction reactions (OER and ORR) remains elusive. Herein, a cobalt–copper couple addresses this gap-area where the best performing catalyst consists of an electrically conducting backbone of 57 wt % Cu0.9Co0.1 alloy and 10 wt % Co with higher valence surface states of CoO, Cu2O, CuO, and Co3O4 with 19.7, 10, 2.9, and 0.3 wt %, respectively. Computational studies show the Co-center as the active site for OER and ORR, while charge transfer from the Co to Cu center accumulates the electronic charge at the junction between the surface CoO and the alloy core facilitating the adsorption of oxygenated species. The 24 h air-oxidized electrodeposited-catalyst shows OER overpotentials of 225, 290, and 312 mV at 10, 50, and 100 mA cm–2, respectively, with at least 150 h operational stability at 1.55 V versus reversible hydrogen electrode (RHE). By electrodepositing the catalyst on high-priced Ni-foam, record-low overpotentials of 190, 281, and 298 mV are observed at 10, 50, and 100 mA cm–2. Since the ORR overpotential is only 550 mV at −1 mA cm–2 on CFP, the bifunctionality index (BI) subsides to 0.775 V. The rechargeable zinc-air battery (ZAB) delivers a stellar 948 mA·h·gZn–1 specific capacity, 68.8 mW cm–2 power density, and uninterrupted one-week operational stability. Even in the presence of Cl– ions in alkaline artificial seawater, the OER overpotential is retained at 240 mV at 10 mA cm–2 and the ZAB specific capacity is slightly reduced to 759.5 mA·h·gZn–1. A synergistic bimetallic composition, measured higher valence surface states with singly occupied eg orbitals, and branched morphology, facilitates the optimum adsorption and desorption of the intermediates.

Topics & Concepts