High utilization efficiency of tetramethylbenzoquinone hybridized in the pores of activated carbon for high-performance electrochemical capacitor electrodes

Abstract

High utilization efficiencies of hybridized redox-active materials are of great importance for increasing the capacitance of electrochemical capacitor electrodes. Herein, a redox-active alkylbenzoquinone, tetramethyl-1,4-benzoquinone (TMBQ), is hybridized in the gas phase in activated carbon (AC) pores with sizes of ∼4 nm, and the electrochemical capacitor performance of the obtained hybrids is evaluated using aqueous 1 M H₂SO₄ as electrolyte. The amount of hybridized TMBQ investigated herein is 4–10 mmol per gram of AC, and the utilization efficiencies of TMBQ in the resulting AC/TMBQ hybrids are ≥85%. The hybridization is not associated with the volume expansion of the AC particles, and therefore, the volumetric capacitance of the hybrids increases with increasing amount of TMBQ. Consequently, 1 mmol of TMBQ per gram of AC afforded the volumetric capacitance of ∼50 F cm⁻³ and the volumetric capacitance of the hybrid containing 10 mmol of TMBQ per gram of AC was 581 F cm⁻³ at the current density of 0.2 A g⁻¹, which is significantly higher than the volumetric capacitance of pristine AC (i.e., 99 F cm⁻³). Moreover, despite the substantial contribution of the reversible redox reaction of TMBQ to the increase in the capacitance, capacitance retentions of the hybrids up to a TMBQ amount of 8 mmol per gram of AC were higher at 10 A g⁻¹ than that of AC. The gas-phase adsorption does not require any organic solvents, concomitant solvent removal, or the drying process, and enables complete adsorption of TMBQ for the TMBQ amounts below the saturation capacity of AC. Therefore, this hybridization method is promising not only for enhancing the electrochemical capacitor performance but also for waste minimization.