Monolithic In‐Plane Integration of Gate‐Modulated Switchable Supercapacitors

Monolithic integration of iontronic devices is a key challenge for future miniaturization and system integration. The G‐Cap, a novel iontronic element, is a switchable supercapacitor with gating characteristics comparable to transistors in electronic circuits, but switching relies on ionic currents and ion electroadsorption. The first monolithic in‐plane G‐Cap integration through 3D‐inkjet printing of nanoporous carbon precursors is reported. The printed G‐Cap has a three‐electrode architecture integrating a symmetric “working” supercapacitor (W‐Cap) and a third “gate” electrode (G‐electrode) that reversibly depletes/injects electrolyte ions into the system, effectively controlling the “working” capacitance. The symmetric W‐Cap operates with a proton‐conducting hydrogel electrolyte PVA/H 2 SO 4 and shows a high capacitance (1.6 mF cm −2 ) that can be switched “on” and “off” by applying a DC bias potential (−1.0 V) at the G‐electrode. This effectively suppresses AC electroadsorption in the nanoporous carbon electrodes of the W‐Cap, resulting in a high capacitance drop from an “on” to an “off” state. The new monolithic structures achieve high rate performance, reversible on‐off switching with an off‐value reaching 0.5 %, which even surpasses recently reported values. Establishing technologies and device architectures for functional ionic electroadsorption devices is crucial for diverse fields ranging from microelectronics and iontronics to biointerfacing and neuromodulation.