Simple cobalt modifying on flower-like NiO positive electrode active materials for electrochemical capacitors was investigated for improving their rate capability. Soaking an NiO-deposited Ni foam positive electrode in a cobalt acetate aqueous solution and the following heat treatment realized uniform modification of Co oxides on the NiO positive electrode. A hybrid capacitor (HC) cell was constructed with the activated carbon negative electrode and cobalt modified or non-modified NiO positive electrode in 10 M KOH aqueous solution. The HC cell with the cobalt modified NiO positive electrode showed excellent high-rate dischargeability. The cobalt modifying was effective for reducing cell resistance due to the formation of cobalt oxyhydroxide with higher electrical conductivity than NiO during the first charging. Consequently, the HC cell with the cobalt modified NiO electrode showed high specific power even at high discharge currents.
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