Unraveling the interfacial degradation mechanism of metal oxide electrocatalyst/gas diffusion layer in Zn-Air batteries through FIB-SEM analysis

Abstract

Focused Ion Beam -Scanning Electron Microscopy (FIB-SEM) was employed to investigate the Gas Diffusion Layer (GDL), electrocatalyst, and electrolyte interface in rechargeable Zn-Air Batteries (ZABs) using Fresh and Used samples (before and after long-term electrocatalytic activity, respectively). The electrocatalyst was based on (hydro-)oxides of manganese, cobalt and lanthanum and carbon material. The results reveal a loss of material compaction in the Used sample, accompanied by the formation of pores and irregular gaps, as well as enhanced permeation of the electrocatalyst through the GDL, as evidenced in the tomogram. Interestingly, the analysis of the distribution of elements in the Used sample shows important differences, strongly dependent on their chemical properties in the electrolyte used. Mn and Co, the more electroactive elements for the oxygen-involved reactions, remain in proximity to the electrolyte interface, while La forms a preferential region more distant and parallel to the electrolyte. XPS results indicate that the formation of this region is associated with the generation of lanthanum acetate species, which are responsible for the decrease in conductivity of the Used sample, as demonstrated by electrochemical impedance spectroscopy (EIS) experiments, in addition to impairing O 2 diffusion along the GDL. Furthermore, DFT calculations support that the formation of lanthanum acetate species from the metal (hydro)oxides is energetically favorable in presence of zinc acetate electrolyte in ZABs.