A mesoporous electrode strategy for probing ions at the electrode-electrolyte interface using in situ X-ray absorption spectroscopy

Electrolyte ions present at the electrode-electrolyte interface critically influence electrocatalytic reactions, yet direct, quantitative detection of interfacial ions under operating conditions remains challenging due to dominant bulk electrolyte signals. Here, we present an interface-sensitive soft X-ray absorption spectroscopy (XAS) approach that enables quantitative tracking of interfacial ion behavior at electrified platinum (Pt) surfaces. Mesoporous Pt films with high surface-area-to-volume ratios were fabricated via soft-templated electrodeposition onto SiN_x windows, strongly enhancing the relative contribution of the interfacial electrolyte within nanoscale pores while effectively attenuating signals from the bulk. Using Na⁺ as a model cation, Na K-edge XAS measurements reveal pronounced dependencies of interfacial ion concentration on applied potential, electrolyte pH, and bulk concentration. The observed trends highlight the role of surface chemistry in interfacial ion behavior. In addition to concentration changes, potential-dependent modifications of the Na⁺ hydration shell are identified, indicating re-organization of the solvation of the ions at the interface. These results highlight the capability of our methodology to quantitatively probe ion-specific behavior in the electrical double layer under operando conditions. We foresee that the method can be extended to a wide range of ionic and molecular species relevant to electrocatalysis and energy conversion.