Morphochemical evolution during ageing of pyrolysed Mn/polypyrrole nanocomposite oxygen reduction electrocatalysts: A study based on quasi-in situ photoelectron spectromicroscopy

This study dealswith themorphological and chemical-state changes caused by the degradation of nanocomposite electrocatalysts – fabricated by pulsed potentiostatic co-electrodeposition and subsequently pyrolysed – under oxygen reduction reaction (ORR) conditions in aqueous alkaline solution. Variations in shape, dimensions and chemical state of theMn-centres were followed by quasi-in situ synchrotron-based scanning photoelectron microscopy with submicron lateral resolution, combined with ex situ Raman measurements, in correspondence of different cyclovoltammetric ageing stages. The decline of the electrocatalytic performance is accompanied by size variations of theMnOx particles that are initially ~30nmin diameter, then shrink to ~10nmand subsequently grow to ~45 nmafter prolonged ORR. Concerning chemical state, the pristine Mn0,II nanoparticles are converted to MnIII,IV oxy-hydroxides as a result of a dissolution/redeposition process favoured by the oxygen environment.