Study of the mesopore size effect on the electrochemical capacitor behaviors of mesoporous carbon/quinone derivative hybrids

Abstract

A quinone derivative, 2,5-dichloro-1,4-benzoquinone (DCBQ), is hybridized with MgO-templated mesoporous carbons (MgO-Cs) with the average pore sizes of 5, 10, and 30 nm through gas-phase adsorption. The resulting hybrids are used for aqueous electrochemical capacitor electrodes due to the reversible redox reaction of DCBQ within the mesopores when using an aqueous H2SO4 electrolyte. This hybridization method precisely controls the weight ratio of porous carbons and DCBQ, and therefore, facilitates the discussion of the correlations between the amount of adsorbed DCBQ and the electrochemical capacitor behaviors of the MgO-C/DCBQ hybrids. Irrespective of the pore size of the MgO-Cs, the volumetric capacitance increases with increasing DCBQ content at current densities of 0.05−0.5 A g−1 because the hybridization of the redox-active DCBQ with porous carbons is not accompanied by the volumetric expansion of porous carbon particles. However, the DCBQ contents above 6 mmol per 1 g of MgO-C results in the decrease of the capacitance retention in the high current density region of ~10 A g−1. Not all the hybridized DCBQ molecules contribute to the capacitance enhancement, and the increase of the DCBQ content leads to a gradual decrease of the utilization ratio. In addition, the average pore sizes of 5 and 10 nm do not show the significant difference in the utilization ratio, but the MgO-C with the average pore size of 30 nm shows a decrease of the utilization ratio in comparison with those of the other MgO-Cs. This hybridization is applicable to any porous carbons, and the commercial activated carbon containing mesopores is appropriate for preparing high-performance electrochemical capacitors, due to its low-cost, large surface area for accommodating a large amount of DCBQ, and the absence of large mesopores as large as 30 nm.