Na4Co3P4O15 in situ transformation to Co(OH)2/CoO(OH) nanoforms for aqueous supercapacitor using redox additive electrolyte

•Cathode was fabricated by converting Na4Co3P4O15 in situ to Co(OH)2/CoOOH composite.•To prepare the anode, commercial carbon was activated with KOH at 850 °C.•Electrolyte KOH + KFC was used to increase the specific capacity of electrodes.•The supercapacitor has specific energy of 42 Wh⋅kg−1 at power density of 400 W⋅kg−1.•Supercapacitor capacity remains 78% after 5000 charge–discharge cycles.