An all-solid state asymmetric supercapacitor has been fabricated using carbon aerogel (CA) microspheres as the negative electrode and Co3O4 nanowires on nickel foam (Co3O4–NF) as the positive electrode separated by PVA-KOH membrane as the electrolyte. For the desirable porous structure, high specific capacitance and rate capability of CA and Co3O4–NF, broader potential window of the two electrodes, no binder and conductive agent added, the asymmetric supercapacitor can be cycled reversibly in a wide potential window of 0–1.5 V with an energy density of 17.9 Whkg−1 at a power density of 750 W kg−1. The energy density of the asymmetric supercapacitor is significantly improved in comparison with those of the symmetric supercapacitors based on CA (6.28 Whkg−1) and Co3O4–NF (2.42 Whkg−1). The asymmetric supercapacitor can also deliver a high energy density of 10.44 Whkg−1, even at a high power density of 7.5 kW kg−1. In addition, the asymmetric device shows good stability with approximately 85% of its initial capacitance after 1000 cycles.