Probing of Near-Surface Cations during the Oxygen Evolution Reaction (OER) Using Operando XAS

Abstract

Electrified solid-liquid interfaces are ubiquitous in technologies ranging from colloidal science to electrochemistry. Hence, an atomic-level characterization of the electrode-electrolyte interface is crucial for total control and optimization of the processes involved. While the electrode side of the interface has been quite extensively studied, the electrolyte side is comparatively poorly understood1. Specifically probing the near-surface electrolyte species during an electrochemical reaction is a major technical challenge in the field of catalysis and surface chemistry.We have developed an interface-sensitive X-ray absorption spectroscopy (XAS) approach which allows us to directly probe the near-surface cations and anions of the electrolyte under applied potentials. In this approach, we employ a mesoporous electrocatalyst film coated on a 𝑆𝑖𝑁𝑥 X-ray window. These mesoporous films exhibit an extremely high electrode-electrolyte interface area, enabling us to specifically probe the behaviour of interfacial ions via their K-edge spectra. Using this approach, we have recently investigated the interaction of Na+ cations with IrOx during the oxygen evolution reaction (OER), which is a bottleneck in major electrochemical processes like green hydrogen production and CO2 reduction. The cations and anions of the electrolyte reportedly influence the OER activity of electrodes during OER2,3. We used operando Na K-edge XAS to directly probe the concentration and coordination environment of the near-surface Na+ cations. Simultaneously, operando O K-edge XAS monitored the interfacial water structure and the evolution of catalyst’s surface structure. Contrary to expectations, we discovered that the positively charged Na+ ions are drawn to the IrOx surface by more positive potentials only at alkaline pH. This finding cannot be explained by any of the electrolyte theories4 put up thus far, emphasizing the necessity of a detailed investigation of interfacial electrolyte structures.