Transition from Nonpolarizable to Polarizable Regime in Ag/AgCl-Gated Electrochemical Devices

Organic mixed conductors (OMCs) are of great interest for a variety of electrochemical device applications because of their ability to support both ionic and electronic transport. Device operation relies on ion accumulation from the electrolyte within the bulk of the OMC, yielding a capacitance that scales with the film volume. However, this volume-dependent capacitance has been reported to hold only within limited ranges of voltage and film thickness, constraining device performance. Here, we investigate the origin of this limitation and its correlation with the Ag/AgCl-doping electrode properties. To this end, we fabricated capacitor-like devices based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with constant surface area and varying film thicknesses, employing commonly used nonpolarizable Ag/AgCl pellets as doping electrodes. Our results show that the breakdown of the linear capacitance–volume relationship arises from an increasing voltage drop at the Ag/AgCl–electrolyte interface. According to the Butler–Volmer equation, electrochemical reactions at a nonpolarizable electrode surface proceed at a finite rate. When the current demand surpasses this rate, the electrode can no longer behave ideally, resulting in increased charge-transfer resistance and accumulation of charge at the interface, resulting in a transition toward polarizable-like behavior. This transition is marked by the growing voltage drop at the Ag/AgCl–electrolyte interface, which reduces the effective channel voltage and, consequently, the capacitance of the OMC. These electrical findings were corroborated by depth-profile X-ray photoelectron spectroscopy (XPS), which revealed a significant decrease in the cation concentration with depth. Overall, this study underscores the critical role of doping electrode properties in the design and optimization of OMC-based electrochemical devices.