Ni-Co hydroxides have been extensively studied as electrode materials for hybrid supercapacitors. In this work, we report an effective method to significantly improve the rate performance of Ni-Co hydroxides through engineering the dual-channel structure and tuning the valence state of Cobalt, which is realized through a facile in suit chemical treatment of phosphites. The specific capacitance of the obtained flower-like Ni-Co hydroxide electrode is 1425Fg−1 at 20Ag−1, which is 83.9% of 1698Fg−1 at 1Ag−1. Simultaneously, a moderate cycling stability with an efficiency of 80.6% after cycling 4000 times at a current density of 10Ag−1 is obtained. Moreover, an energy density of 40.1 Wh kg−1 at a power density of 801.2Wkg−1 has been achieved in an assembled aqueous asymmetric supercapacitor, using Ni-Co hydroxide as positive electrode material and activated carbon as negative electrode material. Our study shows that the chemical treatment evokes morphology and phase transformation and induces partial Co2+ conversion to a more conductive Co3+ state. And the electrochemical performance has a significant relationship with the microstructure and valence state of Cobalt.