Redox properties have been imparted to activated carbon with a high degree of functionalization by chemical grafting of 2-amino-4,5-dimethoxybenzoic add in situ diazotized. The diazotization reaction was accomplished in the presence or in the absence of HCl for estimating the positive or negative effect of the carboxylic acid substituent on the grafting yield. Thermal gravimetric analysis, X-ray photoelectron spectroscopy and cyclic voltammetry experiments show that when the carboxylic acid group participates to the diazotization reaction. the grafting yield is improved and becomes even better than when the carboxylic group is not present, increasing the capacitance of pristine carbon electrode from 120 to 200 F/g.