The local microenvironment at the electrode-electrolyte interface plays an important role in electrocatalytic performance. Herein, we investigate the effect of acid electrolyte anion identity on the oxygen reduction reaction (ORR) activity and selectivity of smooth Ag and Pd catalyst thin films. Cyclic voltammetry in perchloric, nitric, sulfuric, phosphoric, hydrochloric, and hydrobromic acid, at pH 1, reveals that Ag ORR activity trends as follows: HClO4>HNO3>H2SO4>H3PO4>HCl≫HBr, while Pd ORR activity trends as: HClO4>H2SO4>HNO3>H3PO4>HCl≫HBr. Moreover, rotating-ring-disk-electrode selectivity measurements demonstrate enhanced 4e− selectivity on both Ag and Pd, by up to 35 %H2O2 and 10 %H2O2 respectively, in HNO3 compared to in HClO4. Relating physics-based modeling and experimental results, we postulate that ORR performance depends greatly on anion-related phenomena in the double layer, for instance competitive adsorption and non-covalent interactions.
Bibliographical noteFunding Information:
This work was supported by the Toyota Research Institute. Some of the research efforts involving thin film synthesis were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program to the SUNCAT Center for Interface Science and Catalysis. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under Award ECCS‐2026822. The authors thank Victoria Chen for help in accessing the AFM, Jaime E. Avilés Acosta for performing AFM on a GC substrate, and Melissa E. Kreider for valuable discussions. J.A.Z.Z. gratefully acknowledges support of the Gates Millennium Graduate Fellowship/Scholarship and the Stanford's Office of VPGE EDGE Fellowship. G.A.K. gratefully acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. 1650114.