Beyond Capacitance: Rethinking the Stability of Ion-Selective Electrodes With Carbon-Based Solid Contacts

Large-surface-area carbon materials have been studied widely as solid-contact (SC) materials for ion-selective electrodes (ISEs), as their high nonfaradaic capacitance has been associated with high potential stability. However, recent work highlighted that the very slow potential discharge of single-walled carbon nanotube (SWCNT) solid-contact interfaces as a result of slow, unexpected redox processes of the SWCNTs causes potential drift, which, albeit of very low magnitude, limits the long-term stability of SC-ISEs. Successive use of chronopotentiometry (CP), chronoamperometry (CA), and long-term open-circuit potential measurements (Pot) in a CP-CA-CP-Pot-CP sequence provides the temporal resolution to distinguish between such redox reactions of the high-surface-area carbon and the charge redistribution artifacts that may bias capacitance measurements by chronopotentiometry. We discuss here differences in the charge redistribution and redox processes of solid contacts made of nanographite, mesoporous carbon nanospheres (MCN), and SWCNTs, as studied with the CP-CA-CP-Pot-CP technique. After brief applications of small voltages to mimic effects of the finite input impedance of real-life potentiometers, MCN and nanographite interfaces exhibited no changes in capacitances, as they were previously observed for SWCNT interfaces. However, contact angle measurements after application of a small voltage over 1 day suggest that SWCNTs, MCN, and nanographite all undergo surface oxidation to some extent, nanographite being most sensitive to oxygen. These results demonstrate that discharge mechanisms vary across different carbon materials and that a large capacitance cannot guarantee greater electrode stability unless redox reactions are effectively suppressed. The development of high-surface-area solid contacts with minimal redox reactivity will be critical for the further improvement of SC-ISEs with high long-term stability and calibration-free measurements.