(670f) Understanding the Effects of Microenvironment and Mass Transport on Electrochemical CO2 Reduction Using Operando ATR-Seiras

Electrochemical CO₂ reduction (CO₂R) on Cu enables the sustainable production of fuels and chemicals, though often with non-ideal selectivity. Mass transport plays a key role in the product selectivity and activity by controlling the local microenvironment and availability of reactants. Attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) enables the selective study of the microenvironment within 10 nm of a nanostructured metal thin film. For CO₂R on nanostructured Cu thin films, ATR-SEIRAS provides measurements of the local CO₂ concentration, relative coverage of *CO, and local pH via the measured concentrations of HCO₃^- and CO₃^2-. We have developed a flow reactor (Figure A) capable of operando ATR-SEIRAS, where reaction products can be quantified concurrently with spectroscopic measurements. The flow reactor utilizes impinging electrolyte jets for well-defined and tunable mass transport, with flow rates of 10-100 mL/min providing a 5x difference in the effective boundary layer thickness.

Using ATR-SEIRAS, we detect the CO₂ concentration and pH at the catalyst-electrolyte interface, finding significant variation from their bulk values (Figure B). The CO₂ concentration can decrease by a factor of 3 or more and the pH can increase by 3 or more units. The differences in local pH and CO₂ concentration from the bulk are highly dependent on the flow rate, as the flow controls the rate at which CO₂ and HCO₃^- are replenished and basic species are carried away. Our work also elucidates the effect of flow rate on reaction selectivity, such as the observed increased production of CO relative to C₂H₄ on Cu foil at higher flow rates. These relationships between flow rate and selectivity are rationalized based on the observed microenvironment changes. The application of surface-sensitive spectroscopy to a CO₂R reactor with well-defined and tunable flow illuminates the significant impact of mass transport and catalyst microenvironment on CO₂R device performance.