Evidence of transient potentials in ion-selective electrodes based on thin-layer ion-exchange membranes

Abstract

Polymeric membranes with ion-exchange properties have found numerous applications in water treatment, dialysis, energy storage, chemical sensors, and bio-interfaces, among others. Notably, it is common to operate under non-equilibrium conditions while pursuing specific features (e.g., current generation) through an electron-to-ion mechanism. To maximize the final performance, it is crucial to understand the role of each interface within the system, which becomes complex when the device is tailored with several materials and films. This is the case for ion sensors based on thin membranes in backside contact with a redox active conducting polymer. Herein, we investigate such a system operating under a charge transfer mechanism, which features electroneutrality maintenance as the main driving force upon application of a linear sweep potential. This potential is modeled as being unequally distributed among the various system interfaces. Our results demonstrate and quantify the existence of a transient membrane potential at the membrane-electrolyte interface, owing to the implementation of a strategical measurement point on the buried membrane side and connected to a built-in electrometer for the exclusive acquisition of the potential difference at such an interface. The transient membrane potential was found to be <1 % of the total applied potential, meaning that the ion-transfer process at the electrolyte-membrane interface is less energetically costly that the electron transfer and doping processes occurring at the conducting polymer side. This small contribution can be potentiated by increasing the ion-exchange capacity of the membrane, which indirectly enlarges the system current and serves as a strategy for increasing the efficiency of the device.