Institute of Fundamental Technological Research

This paper describes an ionotronic-based sensor that can detect changes in temperature using a solid polymer electrolyte (SPE)-based transparent electrochemical capacitor (EC). The EC developed using flexible ITO as an active electrode and polyvinyl alcohol (PVA)- lithium chloride (LiCl) gel composite-based SPE. The electrochemical performances of the sensors are investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charging and discharging (GCD) analysis. The EC was fabricated using a freestanding SPE, which exhibits a specific capacitance of 4.19 μF. cm-² at a scan rate of 5 mV. s-1. The direct coating of the SPE on the electrode enhances the specific capacitance and is found to be 18.70 μF.cm-², which is 12 times higher than the EC fabricated using freestanding SPE. When a temperature was applied to the top of the EC with directly coated SPE, we observed a variation in the device’s capacitance due to the change in the mobility of ions of the SPE, which is directly related to the temperature change. The EC exhibits a sensitivity of 0.30 µF/ °C (R2= 0.9694) for the temperature range of -10 to 50 °C. Due to its ionic reaction, the EC demonstrates a high capacitance value in the range of µF in the low frequency range, which shows its potential application in ionotronic-based sensing and as an energy storage for the next generation of wearable devices.

Ionotronic sensor, Temperature monitoring, Transparent electronics, Electrochemical capacitor

Advanced flexible ionotronic devices have found excellent applications in the next generation of electronic skin (e-skin) development for smart wearables, robotics, and prosthesis. In this work, we developed transparent ionotronic-based flexible electrochemical capacitors using gel electrolytes and indium tin oxide (ITO) based transparent flexible electrodes. Different gel electrolytes were prepared using various salts, including NaCl, KCl, and LiCl in a 1:1 ratio with a polyvinyl alcohol (PVA) solution and compared its electrochemical performances. The interaction between gel electrolytes and ITO electrodes was investigated through the development of transparent electrochemical capacitors (TEC). The stable and consistent supply of ions was provided by the gel, which is essential for the charge storage and discharge within the TEC. The total charge contribution of the developed TECs is found from the diffusion-controlled mechanism and is measured to be 4.59 mC cm−2 for a LiCl/PVA-based gel. The prepared TEC with LiCl/PVA gel electrolyte exhibited a specific capacitance of 6.61 mF cm−2 at 10 μA cm−2. The prepared electrolyte shows a transparency of 99% at 550 nm and the fabricated TEC using LiCl/PVA gel exhibited a direct bandgap of 5.34 eV. The primary benefits of such ionotronic-based TEC development point to its potential future applications in the manufacturing of transparent batteries, electrochromic energy storage devices, ionotronic-based sensors, and photoelectrochemical energy storage devices.