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

• Ageing of pyrolysed Mn/PPy nanoparticles under oxygen reduction reaction is studied.
• Chemical state of single electrocatalyst nanoparticle followed by quasi-in situ XPS
• Scanning photoelectron microscopy captures nanoparticle morphochemical evolution.
• Raman and XPS disclose molecular aspects of progressive nano-electrocatalyst ageing.
• New nanocatalyst ageing model based on electrochemical and spectroscopic results

This study deals with the morphological 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 the Mn-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 the MnOx particles that are initially ~ 30 nm in diameter, then shrink to ~ 10 nm and subsequently grow to ~ 45 nm after 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.

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