Quantifying Local pH Changes in Carbonate Electrolyte during Copper-Catalysed CO2 Electroreduction Using In Operando 13C NMR

The electrochemical carbon dioxide reduction on copper attracted considerable attention within the last decade, since Cu is the only elemental transition metal that catalyses the formation of short-chain hydrocarbons and alcohols. Research in that field is mainly concentrated on understanding the reaction mechanism in terms of adsorbates and intermediates. Furthermore, dynamic changes in the microenvironment of the catalyst, i.e. local pH and CO2 concentration values, play an equivalently decisive role in the selectivity of product formation. In this study, we present an in operando 13C Nuclear Magnetic Resonance (NMR) technique that enables the simultaneous measurement of pH and CO2 concentration in electrode vicinity during electroreduction. The influence of applied potential and buffer capacity of the electrolyte on the formation of formate is demonstrated. Herewith, theoretical considerations are confirmed experimentally and the importance of the interplay between catalyst and electrolyte is highlighted.