Probing Interfaces in Membrane Electrode Assemblies via <i>Operando</i> Infrared Spectroscopy at Model Gas–Liquid–Solid Triple-Phase Boundaries

Interrogation of the gas–liquid–solid triple-phase boundary (TPB) in membrane electrode assemblies (MEAs) operating at industrially relevant rates of CO 2 electroreduction is highly desirable but remains challenging. Here, we develop an operando MEA-type surface-enhanced infrared absorption spectroscopy (SEIRAS) platform that enables selective detection of the buried catalyst–membrane interface at current densities up to 137 mA/cm 2 on a Cu deposited-Au film substrate and up to 247 mA/cm 2 using a sputtered Cu film. CO adsorption persists at high current densities and reaches near-saturation coverage under ambient pressure at the TPB. Deconvolution of the O–H stretching region reveals preferential depletion of cation-coordinated water, while CO 2 reduction induces carbonate formation that reorganizes the hydrogen-bonding network toward a less reactive ice-like structure. These findings provide molecular-level evidence of adsorbate and hydration dynamics at the TPB and establish operando MEA-SEIRAS as a general approach for probing buried electrochemical interfaces.