Self-Discharge Mechanism of Carbon-Based Ion-Selective Electrode Solid Contacts

Carbon materials have been of great interest as ion-to-electron transducers in the development of solid-contact ion-selective electrodes (SC-ISEs), offering an alternative to the conductive polymer species that have been used for this purpose previously. The enhanced stability of these SC-ISEs arises from the carbon material’s large capacitance, which results from the formation of an electrochemical double layer between the solid-contact and ion-selective membrane. Though a higher capacitance has empirically been found to relate to more stable electrode potential measurements, studies of charged SC-ISEs with high-capacitance carbon solid contacts show an exponential potential discharge over multiple days after the application of a DC bias potential. This phenomenon does not manifest itself when traditional chronopotentiometry is used to characterize SC-ISEs due to the comparatively short time scale of this technique. We herein investigate the discharge mechanism of ISE solid contacts when charged to ±100 mV from their open circuit potential. A combination of potential discharge from charge redistribution as well as possible redox reactions are studied. as similarly observed for electrostatic capacitors charged to high voltages. The extent of self-discharging differs among carbon materials, that is, nanographite, CIM carbon, and SWCNT, and high surface area gold. The effect of atmosphere, temperature, and membrane composition are evaluated to better understand the stability and capacitive properties of these charged electrodes.