공융 갈륨-인듐 액체금속 전극 기반 전기이중층 커패시터

An Electric Double-Layer Capacitor Based on Eutectic Gallium-Indium Liquid Metal Electrodes

Abstract

Gallium-based liquid metal, e.g., eutectic gallium-indium (EGaIn), is highly attractive as an electrode material for flexible and stretchable devices. On the liquid metal, oxide layer is spontaneously formed, which has a wide band-gap, and therefore is electrically insulating. In this paper, we fabricate a capacitor based on eutectic gallium-indium (EGaIn) liquid metal and investigate its cyclic voltammetry (CV) behavior. The EGaIn capacitor is composed of two EGaIn electrodes and electrolyte. CV curves reveal that the EGaIn capacitor shows the behavior of electric double-layer capacitors (EDLC), where the oxide layers on the EGaIn electrodes serves as the dielectric layer of EDLC. The oxide thicker than the spontaneously-formed native oxide decreases the capacitance of the EGaIn capacitor, due to increased voltage loss across the oxide layer. The EGaIn capacitor without oxide layer exhibits unstable CV curves during the repeated cycles, where self-repair characteristic of the oxide was observed. Finally, the electrolyte concentration is optimized by comparing the CV curves at various electrolyte concentrations.

Fig. 4. Stability of the EGaIn capacitors depending on the thickness of oxide layer. (a) CV curves of the oxide-free EGaIn capacitor during 200 cycles. (b) Comparison of CV curve of the oxide-free EGaIn capacitor at 200th cycle, to that of the EGaIn capacitor with the native oxide at the first cycle. (c) CV curve of the EGaIn capacitor with thicker oxide by 1 V

Fig. 5. Capacitance for 200 cycles at difference oxide thickness. The scan rate is 500 mV/s

Fig. 6. Effect of electrolyte concentrations on CV curves of EGaIn based capacitors; (a) 0.01 M, (b) 0.1 M, (c) 1 M. For the stable CV measurement, oxide skin was further oxidized under 1 V for 30 seconds. Aqueous Na₂SO₄ solutions were used as the electrolyte

Fig. 1. (a) Experimental setup of a EGaIn capacitor. (b) Electrochemical process for removal or formation of the oxide layer on EGaIn electrodes by bias application

Fig. 2. (a) A top-view image of the EGaIn liquid metal electrodes of the EGaIn capacitor. Scale bar=0.5 mm. (b) Chargingdischarging mechanism of the EGaIn capacitor. (c) CV curves of the EGaIn capacitor with native oxide at different sweep rates. 1 M of Na₂SO₄ aqueous solution was used as electrolyte

Fig. 3. (a) Top-view images of EGaIn liquid metal electrodes after further oxidation with different oxidative voltages. Scale bar=0.5 mm. Each oxidative voltage was applied for 30 s. (b)CV curves of the capacitors with the EGaIn electrodes in (a). The scan rate is 200 mV/s