Multimodal In Situ Investigation of Anion-Dependent Surface Dynamics of Pd Electrocatalysts during the Oxygen Reduction Reaction in Acidic Media

The electrode–electrolyte interface plays a large role in steering electrocatalytic activity, selectivity, and stability. However, there is limited understanding of the general structure and dynamics of the reactive microenvironment and its effects on electrocatalyst performance. Herein, this study probes the effects of acid anion identity (pH 1, HClO4, HNO3, H2SO4, HCl, HBr) on Pd electrocatalyst stability, surface morphology, and crystal structure during the oxygen reduction reaction (ORR), using a multimodal in situ approach. The overall trend in Pd stability follows: HClO4 > HNO3 ∼H2SO4 > HCl ≫ HBr, paralleling observed ORR activity, highlighting the correlation between stability and electrocatalytic performance. To correlate the stability trends with ORR activity, dissolution rates were quantified using on-line inductively coupled plasma mass spectrometry (ICP-MS) under ORR-relevant conditions. Complementary in situ X-ray reflectivity (XRR) closely agrees with trends observed from on-line ICP-MS, while atomic force microscopy (AFM) revealed corresponding surface roughening behavior. In situ grazing incidence X-ray diffraction (GIXRD) further revealed facet-specific peak broadening for the (200), indicating early signs of anisotropic amorphization and potentially a route toward dissolution. Altogether, this work establishes a multimodal in situ framework for understanding anion-dependent dissolution and surface reconstruction of electrocatalysts under ORR operating conditions. These insights enable rational electrolyte design for enhanced stability in acidic media, bridging interfacial science with strategies to promote electrocatalyst durability and activity in concert.