<i>In operando</i> NMR investigations of electrolyte chemistry during CO₂ electrolysis

The electrolytic reduction of CO2 in aqueous media is a powerful method for the large scale utilization of carbon dioxide. There have been large advances in catalyst and cell design, but only little progress has been made regarding in operando and in situ methods for the research of chemical and physical processes taking during electrolysis. NMR spectroscopy is a powerful method for the investigation of chemical systems, and has been applied regularly for the study of catalytically reactions. In general, NMR is thought to be incompatible with electrochemical reactors, i.e. electrolysis cells, due to the distortion of the main magnetic and radio frequency field by conductive cell components. In this study, we overcame these challenges and developed an in operando electrolysis setup which enables NMR investigations of the electrolytic CO2 reduction, which is compatible with standard hardware and pulse sequences. The setup was applied to the examination of physical and chemical properties of solvated CO2 and a commonly used bicarbonate salt electrolyte during three electrochemical stages with increasingly negative potential. In addition to acquiring 13C NMR spectra over course of the electrolysis, T1 and T2 relaxation as well as 1D exchange spectroscopy (EXSY) experiments were utilized in order to learn about the dynamics and equilibrium reactions of the involved species. It was found that in a concentrated electrolyte solution, the electrolyte anions and cations exist both as free ions and ion pairs, where the exchange rate between both forms decreases with increasingly negative potential. The effect of the potential on the exchange rate was confirmed by repeating the experiments at different magnetic field strengths. Furthermore, it was shown via analysis of the relaxation data that solvated CO2 is preferentially in a dynamic equilibrium with bicarbonate anions in form of an ion pair, and that the CO2/bicarbonate equilibrium reaction rate increases with increasingly stable ion pairs in solution. Finally, a catalytic mechanism was proposed on this basis, where the electrolyte cation stabilizes the transition of bicarbonate to CO2 comparable to the enzyme carbonic anhydrase in biological systems