Supercapacitor-based ionic computing devices: A vision for future fluidic iontronics

The development of diodes, transistors, and integrated circuits represents three major milestones in the advancement of electronic technology, driving the rapid evolution of modern electronic devices.¹ In the early 20th century, British physicist John Fleming invented the “vacuum diode” [Figs. 1(b) and 1(c)], enabling unidirectional current flow.¹ In 1947, the invention of the transistor by Shockley, Bardeen, and Brattain at Bell Labs provided amplification and switching functions [Fig. 1(d)], replacing the bulky vacuum tubes.² The transistor laid the foundation for the development of computers, telecommunications, and other fields. In 1959, Robert Noyce of Fairchild Semiconductor invented the integrated circuit (IC), integrating multiple transistors onto a silicon chip [Figs. 1(e) and 1(f)].³ This innovation significantly increased integration, reduced device size and cost, and provided the driving force for the development of modern electronic products.³ 

Traditional electronics rely on electron flow for signal transmission, while biological systems use ions, highlighting their fundamental differences. Iontronics bridge artificial and biological systems by combining ionic conduction with electronic properties. Advances in nanofluidics enable the development of ionic diodes and rectifiers. Kim et al. synthesized ionoelastomers with opposing charges, forming an ionic double layer (IDL), resulting in stretchable diodes with strong ionic rectification and a wide electrochemical window [Fig. 1(g)].⁴ In addition, Toner and co-workers developed a microfluidic transistor that mimics the flow-pressure properties of electronic transistors,⁵ enabling amplification and integration into basic circuits [Fig. 1(h)]. Recent studies have shown that fluidic memristors simulate the memory function of the human brain via ion transport in nanoscale 2D slits.⁶ Robin et al. used MoS₂ and graphene devices to demonstrate ion behavior, revealing an 8-shaped I–V curve [Fig. 1(i)].⁶ This behavior is driven by the formation of Bjerrum ion pairs, enrichment–depletion effects, and the adsorption–desorption equilibrium, all of which contribute to the long-term memory phenomenon. Xiong et al. used polymer-brush nanotubes, observing frequency-dependent I–V characteristics that mimic synaptic signaling.⁷ These advances open new possibilities for ionic computing devices.

Similarly to the ion-computing devices, supercapacitors store energy through ion transport. Electric double-layer capacitors store energy via a double layer of ions at the electrode surface, while pseudocapacitors rely on oxidation–reduction or intercalation reactions. Both rely on charge transfer between the electrode and electrolyte. Inspired by ionic diodes, transistors, and memristors, our team proposed “confined ion transport in supercapacitors”.⁸ This concept optimizes pore size and surface charge distribution and uses field-effect techniques at the electrode/electrolyte interface to selectively transport ions between electrodes and electrolyte. The strategy restricts ion movement to achieve unidirectional, selective ion transfer, extending supercapacitors to the iontronic field.

In 2019, Zhang and co-workers used microporous carbon to sieve large cations, constructing asymmetric supercapacitors that integrated diode-like unidirectional conductivity [Fig. 1(j)].⁹ This led to the introduction of the concept of supercapacitor diodes (CAPodes). Kaskel’s team further constructed switchable supercapacitors with three- and four-terminal structures, giving these devices transistor-like functions [Fig. 1(l)].^10,11 Our team used nonlinear ion transport in a metal–organic framework to construct a novel supercapacitor memristor.¹² In aqueous pseudocapacitors, OH⁻ ion accumulation and release at different voltages induced a hysteresis effect, enabling memristor behavior. These innovations have led to a new class of ion-based devices: supercapacitor-based ionic devices. Supercapacitors, relying on ion transmission for charge storage, can now exhibit selective unidirectional ion flow and serve as integrated ionic diodes or transistors in single devices. These advances hold promise for ionic logic circuits, wireless charging, neuromorphic computing, and brain–machine interfaces.

Supercapacitors, with “confined ion transport” properties, can integrate traditional charge storage functions along with the capabilities of ionic computing devices, such as rectification, switching, and logic operations. This makes them an important bridge between emerging energy storage and information technology fields. The integration of logic computation functions in supercapacitors paves the way for the development of computer architectures, logic structures, and switchable devices based on the transition from electronic to ionic, and further to capacitive ionic devices.