Interplay of Local pH and Cation Hydrolysis during Electrochemical CO₂ Reduction Visualized by In Operando Chemical Shift-Resolved Magnetic Resonance Imaging

The Cu-catalyzed electrochemical CO2 reduction enables the conversion of greenhouse gas emissions to fuels or platform chemicals with prospects of storing intermittent energy from renewable sources. While current research in tuning catalyst activity and product selectivity is often mired in finding electrode engineering solutions, the importance of electrolyte engineering is mostly overlooked. This study presents a method for measuring local pH profiles in electrode proximity and correlating them to cation-induced buffering effects. Magnetic resonance imaging (MRI) techniques were applied to evaluate the local pH values using spatially resolved C-13 resonances of the CO2/HCO3-/CO32- equilibrium. The buffering effect of cation hydrolysis is substantiated by local shifts of the Na-23 resonance of Na+ in the NaHCO3 electrolytes. Steeper local pH gradients, compared to experiments with KHCO3, account for increased selectivity for acetate formation from the solution-based reaction. Proven itself capable of elucidating the effect of cations on local pH values, our presented method supports tailoring the electrode-electrolyte interface to selectively generate value-added products.