The remarkable electrochemical behavior of complete activated carbon/cobalt oxide cells is reported in the present work. Among the various weight ratios between the positive and negative electrodes evaluated, the best features are obtained with an overcapacitive cobalt oxide electrode. The energy densities obtained by this system (20 Wh kg−1 for a power density of 209 W kg−1) are twice higher than those measured for a activated carbon/activated carbon symmetric cell, in the same operating conditions. With discharge capacities around 62 F g−1, this system is among the best ones reported in the literature for this category. •Original conductive nano cobalt oxide is used in hybrid C/Co supercap.•The positive/negative ratio was optimized.•Best performances for a positive/negative weight ratio of 1.25.•This system is among the best ones of its category (61.6 F g−1).•The system exhibits excellent electrochemical ageing over 3000 cycles.