Magnetic enhancement of high-entropy oxide electrocatalysts for high areal-energy rechargeable zinc air batteries

Abstract

High-entropy spinel oxide (Cu_0.2Co_0.2Fe_0.2Mn_0.2Ni_0.2)₃O₄ nanoparticles were synthesized and confined in Vulcan carbon for use as a bifunctional OER/ORR catalyst in a rechargeable zinc–air battery (RZAB). A partially inverted spinel phase with a distorted O²⁻ lattice was found, with metals randomly distributed in M²⁺ and M³⁺ states. Copper was the exception, being found only as Cu²⁺. Strong metal oxide–support interactions were noted, as well as ferromagnetism. The composite exhibited moderate intrinsic catalytic activity, with overpotentials and current densities comparable to those of commercial platinum on carbon catalysts even at low loadings: an example being E_j=10 of 1.53 V. Magnetic enhancement was noted and associated with the final OER and initial ORR electron transfers. The performance of the test RZAB was greatly improved when an external magnetic field was applied, with peak power increasing from 101 to 169 mW cm⁻². We report the most significant magnetic enhancement in the RZAB power profile in the literature to date, as well as improved RZAB stability and areal energy, achieving 43.2 mWh cm⁻² for over 140 h during 36 h charge–discharge cycles. This work offers insights into the mechanism of magnetic enhancement in the case of high-entropy materials, and illustrates the use of combined strategies to achieve stable, cost-efficient, and effective bifunctional OER/ORR electrocatalysis.