Metal oxide Perovskite-Carbon composites as electrocatalysts for Zinc-air batteries. Optimization of ball-milling mixing parameters

Zn-air batteries (ZABs) are promising electrochemical devices to store energy. Metal oxide perovskites mixed with carbon materials are highlighted as interesting materials for this application because of their appropriate bifunctional performance in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The interaction between both components of the electrocatalyst is important in the bifunctional electrocatalytic activity, and the mixing method plays an important role in this interaction. Then, different mixing methods have been studied in this work (ball-milling, mortar and manual shaking). The use of different physicochemical techniques such as temperature programmed desorption (TPD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) in the materials characterization, allows us to conclude that the mixing method strongly influences the particle size and the interaction between both components, which determine the final electrocatalytic activity. The materials prepared by ball-milling displayed the best performance. Herein, the experimental conditions were optimized to obtain electrocatalysts with enhanced electrocatalytic activity for ORR and OER. Low rotating speed, air atmosphere and low ball-milling time generate electrocatalysts with a small nanoparticle size, more homogeneous and with a higher interaction between both components, which enhances electron transfer, and consequently, the overall oxygen-involved reactions. The best electrocatalyst obtained was studied as air-electrode in a Zn-air battery and it was compared to a commercial Pt/C electrocatalyst, obtaining higher cyclability (55.2 vs. 51.7%) for 30 h, and higher energy density at 5 mA/cm2 (764 mAh/g vs. 741 mAh/g). The authors would like to thank PID2019-105923RB-I00 project funded by MCIN/AEI/10.13039/501100011033. J.X. Flores-Lasluisa thanks the MICINN for the FPI contract (BES-2017-081598 for). M. García-Rodríguez thanks for the funding of the Research Initiation Scholarship from Vicerrectorado de Investigación y Transferencia de Conocimiento of the University of Alicante (AII-20-14) and Ministerio de Universidades for the FPU20-01746 grant.