Effect of molecular grafting on the pore size distribution and the double layer capacitance of activated carbon for electrochemical double layer capacitors

High specific surface carbon (Black Pearls 2000) has been modified by electroactive anthraquinone (AQ) groups by spontaneous reduction of the corresponding in situ generated diazonium derivative. A set of modified carbons with different AQ loadings have been synthesized to study the effect of the grafted groups on the pore texture and on the double layer capacitance of the modified carbons. Grafting of a small amount of AQ molecules caused a significant loss of the BET surface area and affected mainly the ultramicroporosity of the carbon powder. A correlation of the pore texture with the electrochemical data allows to conclude that pores smaller than 0.65 nm do not significantly contribute to the double layer capacitance in H2SO4 electrolyte and therefore are not efficiently accessible to the ions of this electrolyte. Above an AQ loading of 7 wt.%, both microporosity and mesoporosity are strongly affected but the decrease of double layer capacitance seems to be directly related to the coverage of the carbon surface by AQ molecules.