Understanding how local reaction environments control product selectivity is essential for advancing bicarbonate-fed CO₂ electrolyzers, which directly integrate CO₂ capture and conversion. In a bicarbonate electrolyzer, protonation of HCO₃^- enables the in-situ release of CO₂ (i-CO₂) for electroreduction. Enabling C-C products is challenging in these systems due to the lower availability of reactive CO₂ compared to the gas fed systems.

In a bicarbonate electrolyzer, the overall performance of the system is determined by the interplay of three key factors: the proton flux, the chemical potential of HCO₃^- in the electrolyte and the chemical potential of surface bound hydroxide (OH^-). The proton flux is proportional to the current density and determines the CO₂ generated at the interface of membrane and electrolyte. In addition to being the effective CO₂ feed, HCO₃^- is a proton buffer that limits the local pH increase at the surface hindering CO₂ reduction reaction efficiency. Since the bulk chemical potential of HCO₃^- and proton flux are externally controlled input variables, we focus on the role of surface bound OH^- and its impact on C₂ product selectivity.

Here, we employ operando Raman spectroscopy to directly monitor the evolution of surface-bound OH- as a function of current density and bicarbonate concentrations, enabling the first real-time visualization of so OH^- behaviour under industrially relevant operating currents (>200 mA·cm^-2). These measurements reveal the conditions under which OH^- accumulation is either suppressed by bicarbonate buffering or amplified to create highly alkaline microenvironments conducive to C-C coupling. Analysis of OH^- band position and intensity further uncover how the local interfacial environment interacts with bulk electrolyte chemistry, defining the operating boundaries for maximising C₂ selectivity in bicarbonate electrolysis.

By functionally modifying the cathode catalyst, we observe distinct OH^- dynamics that directly correlate with improved ethylene faradaic efficiencies, demonstrating that bypassing the bicarbonate buffer can sustain the high local pH required for C₂ pathways.

Complementary galvanostatic electrochemical impedance spectroscopy (EIS) provides an additional handle on interfacial processes, with equivalent-circuit fits supporting the Raman-derived picture of how OH^- behaviour governs product selectivity.

Together, these results provide the first operando spectroscopic evidence linking OH^- interfacial dynamics to C₂ formation in bicarbonate electrolyzers, offering mechanistic insight and design principles for next-generation reactive-capture CO₂ conversion systems.