Probing the Effects of Acid Electrolyte Anions on Electrocatalyst Activity and Selectivity for the Oxygen Reduction Reaction

Abstract

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: HClO 4 > HNO 3 > H 2 SO 4 > H 3 PO 4 > HCl @ HBr, while Pd ORR activity trends as: HClO 4 > H 2 SO 4 > HNO 3 > H 3 PO 4 > HCl @ HBr. Moreover, rotating-ring-disk-electrode selectivity measurements demonstrate enhanced 4e À selectivity on both Ag and Pd, by up to 35 %H 2 O 2 and 10 %H 2 O 2 respectively, in HNO 3 compared to in HClO 4 . Relating physicsbased 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.

Acid Anion Electrolyte Effects on Platinum for Oxygen and Hydrogen Electrocatalysis

“…2a, and Supplementary Note 1). For continuous, dense, and planar surfaces, AFM is the most direct method to understand the real exposed catalyst surface area 22,[68][69][70][71] . Electrochemical surface area (ECSA) estimation by hydrogen under-potential deposition (H UPD ) methods is standard 37,67 , but electrolyte composition and surface structure can complicate such analysis (see the "Methods" section, Supplementary Fig.…”

“…One common acid that remains underexplored for hydrogen and oxygen electrocatalysis on Pt is nitric acid (HNO 3 ). Our previous work on Pd 22 , which is similar to Pt in that it binds oxygen adsorbates slightly more strongly than ideal 66 , indicates that HNO 3 enhances ORR activity over H 3 PO 4 , as characterized by an onset potential improvement of 35 mV at -0.1 mA cm À2 P d . In this work, we probe how anion identity (NO À 3 , HSO À 4 =SO 2À 4 , and ClO À 4 ) affects the performance of oxygen and hydrogen electrocatalysis on a well-defined Pt surface.…”

“…Microenvironment electrolyte/anion effects on electrocatalyst activity have been studied at a fundamental level for the ORR in several electrolytes on Pt and a few other catalysts [11][12][13][14][15][16][17][18][19][20][21][22] , while few studies have examined electrolyte effects for the OER 23 , HER [24][25][26][27] , and HOR [24][25][26][27] on Pt [28][29][30][31][32][33][34][35][36] . Based on the better-known electrolyte-activity relationships for the ORR on Pt, perchloric acid (HClO 4 ) has been noted as the standard 37 electrolyte for benchmarking performance 16,38 .…”

“…There are several near-surface phenomena 28,53,54 that could be expected in the electrolyte double layer region that can affect catalyst activity 22,55,56 . Electrolyte effects have been proposed to impact the catalyst surface in a number of different ways, including but not limited to the following six interrelated phenomena that occur within the double layer microenvironment:…”

“…In electrochemistry, this involves non-reaction species, such as anions in the electrolyte phase, adsorbing to the surface and blocking active sites, possibly also altering the local electronic structure of neighboring active sites 28,53,54,59 . The interaction of anions with the surface occurs through chemisorption or physisorption depending on the magnitude of the adsorption free energy 28,53,54 , a phenomenon that we have previously 22 explored with DFT calculations for other metals on the ORR. Potential drop redistribution 60 refers to the potential drop across the electric double layer region changing due to the presence of adsorbed species under an applied external potential.…”