Recent research on electrochemical capacitors using transition metal hydroxides as electrode materials is reviewed.

We synthesized porous Co 3 O 4 with different hierarchical morphologies and studied the effects on their capacitor properties when they were used as electrode materials. By employing ammonia as the source of the hydroxide anion (OH 2 ), an effective and robust precipitation route, free of surfactants, was developed to synthesize cobalt hydroxides with different hierarchical structures. When ammonia was used, the insufficient OH 2 supply could tune the formation of cobalt hydroxide to have different hierarchical structures. The (0001) cobalt hydroxide nanosheet is found to be the first product in the nucleation stage, and the basic building block for the hierarchical structures. Three types of anions were tested: the chloride (Cl 2 ) anion prompted the formation of large plates with a similar aspect ratio to the basic building block; the nitrate (NO 32 ) anion formed large plates at the beginning, but this soon led to the formation of nanocolumn structures with a high aspect ratio; and the acetate (C 2 H 3 OO 2 ) anion led to the formation of a flower-shaped hierarchical morphology with stacking of curved (0001) cobalt hydroxide nanosheets. Subsequent calcination transformed the cobalt hydroxides into porous cobalt oxide while the hierarchical morphology remained the same. Cobalt oxides with such complex hierarchical structures have a better capacitor performance, with higher specific capacitance, than Co 3 O 4 nanoparticles. Among them, the Co 3 O 4 hierarchical structure made with the nitrate anion shows the highest capacitance. The mechanism for forming the different hierarchical structures is discussed based on an electron microscopy investigation.

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