Assembling Biodegradable Non-Metal Quinone-Based Supercapacitor with High Energy Density
Authors: Haba Nagihiro , Katsuyama Yuto , Morimoto Keisuke , Nakayasu Yuta 
Published: December 2024

Quinone-based materials have drawn a lot of attention particularly because of its large ion storage capacities from its low-molecular-weight moiety while having an eco-friendly element structure. Quinones, however, do not show any conductivity and are soluble to in organic solvent. Many reports have shown how to utilize such quinones as electrochemical capacitor’s active material, of which acetone impregnation is the simplest and the most practical method. However, battery cell configuration consisting of, current collector composed of metal, activated carbons (ACs) loaded with quinones synthesized from petroleum, and industrialized carbon black used as a conductive additive, are not eco-friendly. In this study, we conducted a thorough examination of the crucial parameters involved in the impregnation process of ACs to clarify the potential of biomass-based ACs in achieving enhanced electrochemical performance when employed as an organic battery. Subsequently, we applied the ACs to metal-free, organic supercapacitor, utilizing a carbon-based collector and woody biomass-based conductive additives.

It was prepared that 7 different AC samples, including a commercially available petroleum-derived AC sample called Maxsorb®, and impregnated chloranil (CHL) and 1,5 dichloroanthraquinone (DCAQ) to each of the samples. Subsequently, a half-cell test was conducted on the sample to determine the extent to what CHL and DCAQ functions as a redox-active material when impregnated into each sample. To characterize each sample, we performed a Brunauer-Emmett-Teller (BET) surface area measurement, Barrett-Joyner-Halenda (BJH) pore distribution measurement, electrical resistivity measurement, and X-ray photoelectron spectroscopy (XPS) analysis to discuss the crucial parameters of quinone impregnation. To create a high-voltage aqueous organic supercapacitor with a metal-free and wood-based structure, we conducted the half-cell on each sample and utilized the best AC sample, a current collector synthesized from natural graphite, a conductive additive derived from a bamboo charcoal to fabricate a practical-sized electrode.

From the BET surface area analysis and the half-cell test, one of the AC samples with a great peak in < 5 nm of diameter had a high utilization rate of 95.3% for the 30% CHL impregnated into the AC sample. Though the AC sample had only a surface area of 2271 m2/g compared to that of Maxsorb®which was 2989 m2/g, it achieved the utilization ratio comparable to that of Maxsorb® which was 94.6%. Samples with a peak diameter > 9 nm showed no redox capacity, while samples with a small broad peak in the range of 1–10nm, exhibited an approximately 60% utilization ratio. Hence, the pore size distribution was found to be a crucial aspect for quinone-impregnation, and the specific surface area plays a crucial role in determining the amount of CHL that can be effectively impregnated, as it provides more pore sites with diameters < 5 nm throughout the sample. As for DCAQ, it had a similar result compared to the CHL-impregnation, however in a slightly higher diameter range at about 5 nm for its optimized impregnation pore size. Therefore, identifying the pore size distribution of ACs is crucial in discovering new biomass-based material used for quinone-based supercapacitor.

From the half-cell test with CHL, it was observed that the overpotential significantly increased from gold to SUS316 to graphite sheet as current collectors. To reduce the overpotential, we introduced a bamboo charcoal as a conductive additive, which effectively reduced the overpotential from 0.1 V to 0 V for SUS316 and from > 1 V to 0 V for graphite sheet at 1 C, while maintaining > 95% utilization rate. Subsequently, we fabricated a practical sized electrode (9 cm x 9 cm, 1 g) with CHL impregnated cathode and DCAQ impregnated anode from the best biomass-based AC, graphite sheet, and biodegradable 3D-printed battery cell case. The resulting full-cell supercapacitors exhibited a high energy density, demonstrating the feasibility of eco-friendly, metal-free wood-based organic supercapacitors.