Tracking the Dynamics of a Ag-MnO _x Oxygen Reduction Catalyst Using In Situ and Operando X-ray Absorption Near-Edge Spectroscopy

Sustainable electricity generation via hydrogen fuelcells requiresthe development of efficient oxygen reduction reaction (ORR) catalysts. In situ/operando experiments are necessaryto uncover the extent of dynamic material changes during catalysis.Herein, we use in situ/operando X-rayabsorption near-edge spectroscopy to track Mn valence changes of apromising, ultrathin, porous MnO x layeron a Ag thin film. Mn-K-edge measurements as a function ofelectrochemical environment and ORR conditions reveal that, interestingly,when driving the ORR at 0.8 V-RHE, the Mn is distinctivelymore reduced and the MnO x redox is nonreversiblein contrast to measurements in N-2-saturated electrolyte. Ex situ inductively coupled plasma mass spectrometry, atomicforce microscopy, and X-ray photoelectron spectroscopy indicate thatthese phenomena do not correlate to metal dissolution but might beassociated with morphological surface reconstruction related to Agvalency. This study highlights how the microenvironment and catalysisplay a key role in the in situ/operando surface structure and chemical state.