Isolating the effect of pore size distribution on electrochemical double-layer capacitance using activated fluid coke

•Porous carbons with distinct pore sizes were tested for electrochemical capacitors.•First comparison of carbons with similar surface areas, but different pore sizes.•Micropores boost normalized capacitance even in broad pore size distribution carbon.•A minimum voltage is needed to boost the capacitance in highly microporous carbons.•Results strength theory that the electrical double layer is thinner in micropores.

Electrochemical double-layer capacitors (EDLCs) use physical ion adsorption in the capacitive electrical double layer of high specific surface area (SSA) materials to store electrical energy. Previous work shows that the SSA-normalized capacitance increases when pore diameters are less than 1 nm. However, there still remains uncertainty about the charge storage mechanism since the enhanced SSA-normalized capacitance is not observed in all microporous materials. In previous studies, the total specific surface area and the chemical composition of the electrode materials were not controlled. The current work is the first reported study that systematically compares the performance of activated carbon prepared from the same raw material, with similar chemical composition and specific surface area, but different pore size distributions. Preparing samples with similar SSAs, but different pores sizes is not straightforward since increasing pore diameters results in decreasing the SSA. This study observes that the microporous activated carbon has a higher SSA-normalized capacitance, 14.1 μF cm−2, compared to the mesoporous material, 12.4 μF cm−2. However, this enhanced SSA-normalized capacitance is only observed above a threshold operating voltage. Therefore, it can be concluded that a minimum applied voltage is required to induce ion adsorption in these sub-nanometer micropores, which increases the capacitance.