The microenvironment is crucial for controlling processes across electrochemical interfaces. At the electrocatalyst surface, the microenvironment consists of factors such as the electric double layer potential, electrolyte, and catalyst composition. Benchtop electrochemical measurements often use weak-adsorbing electrolytes to minimize competitive adsorption in water reduction reactions. However, in PEMFCs, ionomer introduction can induce complex microenvironments where electrocatalytic performance varies. Recognizing differences between lab-scale and industrial applications, it’s crucial to investigate the electrolyte’s impact on the electrocatalyst and microenvironment at a fundamental level.

This study probes the electrochemical behavior of noble metal electrodes across different electrolytes, focusing on ionic effects in oxygen reduction reactions (ORR) in acidic media. For Pt surfaces, ORR activity decreases in oxyacid and halide anions in the order: HClO₄ > HNO₃ > H₂SO₄ > HCl > HBr. Introducing tetraalkylammonium cations as a hydrophobic interface, results in a decrease in ORR activity, resembling those in H₂SO₄. Au surfaces show similar trends, though slight variations in onset potentials and current densities suggest changes in ORR activity and selectivity. Ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) are used to probe surface morphology and chemical states. AFM shows stable roughness values of 1-2 nm for Pt and Au across a potential range of -0.2 V_RHE to 1.0 V_RHE, with no significant changes. XPS confirms minimal shifts in oxidation states, suggesting chemical stability across electrolyte environments.

Synchrotron-based X-ray techniques, including X-ray Reflectivity (XRR), are employed to probe the in situ electrocatalyst surface during ORR. In oxyacidic media, the electrodes do not roughen significantly, but halide electrolytes cause transient roughening, observed via Kiessig fringe changes and supported by AFM measurements. XRR data also reveal changes in scattering length density profiles near the electrode-electrolyte interface, highlighting potential-dependent ion distribution at the metal surfaces. These insights into ion distribution across electrocatalyst surfaces may lead to further mechanistic understanding of interfacial ion dynamics. By studying these fundamental processes, we aim to contribute to the development of more efficient electrochemical technologies, supporting the transition to clean energy.

Colin Crago

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