An Aqueous Organic Redox Supercapacitor Fabricated By Supercritical CO2 Impregnation of Quinones into Activated Carbon
Authors: Ooka Chie , Nakayasu Yuta , Sokabe Shu , Yamada Tomoya , Nagamura Naoka , Ozawa Kenichi , Watanabe Masaru 
Published: December 2024

The aqueous organic redox supercapacitor (OSC) is a metal-free, safe, and cost-effective energy storage device.[1] It utilizes activated carbons (AC) impregnated with two types of quinones as electrodes and an aqueous solution as the electrolyte. The redox reaction of quinones serves as the electron carrier. Conventional liquid impregnation methods for loading quinones onto AC have been used but such methods are limited by low quinone loading ratios and low specific capacities. In this study, a supercritical fluid (SCF) impregnation method is proposed to improve the quinone loading and the energy density of OSC. All OSC fabricated via this method were evaluated, and the impregnation effects compared with those in liquid impregnation method through thermogravimetric analysis (TG) and X-ray absorption fine structure (NEXAFS) measurements.
The cathodes were fabricated by placing AC and tetrachloro-1,4-benzoquinone (TCBQ) in a tube reactor, maintaining them at 105°C and 15 MPa for 24 hours, as per previous study.[2] The anodes were prepared by placing AC, 1, 5-dichloroanthraquinone and ethanol in a reactor and holding the mixture at 155°C and 25 MPa for 24 hours. For comparison, liquid impregnation method was used to fabricate respective electrodes.[1] The quinone loading in each sample was assessed through TG analysis while the electrochemical measurements were performed using a 0.5 mol/L sulfuric acid solution as the electrolyte. Specific capacities of half-cell (1A/g, -0.3-0.3V vs Ag/AgCl) and full cell (2A/g, 0-1.2V) configurations were determined via constant current measurements using a galvanostatic. Additionally, the quinone loading on AC surface of each sample was investigated using the NEXAFS method.
In comparison with the liquid impregnation method, cathodes and anodes produced through SCF impregnation exhibited a 37% and 83% increase in specific capacity, respectively. This enhancement was attributed to the amplified impregnation and loading of redox molecules. The superior solubility of redox molecules in supercritical CO2, along with its high diffusion and low viscosity, facilitated the impregnation and loading of redox molecules into the micropores of the porous carbon. In fact, TG analysis results showed that the supercritical quinone loading was higher in the supercritical impregnation method than in the liquid impregnation method, and the correlation between quinone loading and electrode properties was confirmed. Overpotential suppression was also confirmed, suggesting a decrease in interfacial resistance due to improved adsorption morphology.[3] Full cell measurements showed an energy density of 21 Wh/kg, 1.4 times higher than previous studies.[1] In a durability test of 1000 cycles, the energy density retention rate was 95%, indicating excellent electrode properties with high capacity and long life. Comparing the OSC properties in this study to other energy storage devices, the OSC performed higher than nickel-cadmium and lead-acid batteries for energy density and power density, and higher than cobalt-based lithium-ion batteries in terms of power density.[4] In the K-edge NEXAFS spectra of carbon, the transition peak to the 1s→π* orbital derived from TCBQ[5] was observed at a higher energy in the SCF impregnation compared to the in liquid impregnation. In this case, the peak appeared at a higher energy level due to the strong interactions between the TCBQ supported by the SCF impregnation and AC resulting in the increase in the ratio of π-bonded components and hence an increase in the effective oxidation number.

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