Abstract: 

Charged polymers deposited on electrocatalysts are known to alter electrochemical redox reaction selectivity. Interpretation of this multistep process, however, requires knowledge of fundamental thermodynamic and species transport properties, which many times are difficult to measure. To bridge this capability gap, herein we report a suite of electroanalytical methods that together allow us to quantify the Donnan electric potential difference at polymer|liquid interfaces, the chemical potential of protonic species at electrode surfaces – as pH or pOH, ionic transport properties within polymer layers, and electrical double layer capacitances at electrocatalyst|polymer interfaces. Our work reveals that coupled aqueous and polymer properties control ion transport flux and the extent of specific adsorption of ions to electrocatalyst surfaces. Moreover, our methods are broadly applicable and straightforward to implement. We expect them to aid electrochemists in understanding and controlling ionic properties that influence electrochemical reaction selectivity.