Phase-controlled synthesis of MnO2 nanocrystals by anodic electrodeposition:: Implications for high-rate capability electrochemical supercapacitors

The crystal structure of anodically electrodeposited MnO2 nanocrystals can be manipulated by introducing complexing agents in the electrodeposition solutions. MnO2 nanocrystals with three types of crystal structures were observed: hexagonal epsilon-MnO2 (complex-free), defective rock salt MnO2 (ethylenediaminetetraacetic acid), and defective antifluorite MnO2 (citrate). The capacitive performance of the MnO2 nanocrystals depends strongly on their crystal structures. MnO2 with defective rock salt and antifluorite structures exhibit better capacitive properties than epsilon-MnO2. The electrochemical capacitance differences can be explained in terms of the crystal chemistry. In both the defective rock salt and antifluorite MnO2, an anomalous trend was observed. The specific capacitance does not decrease with increasing scanning rate. A possible reason is that certain physicochemical changes, such as phase transformations or morphology changes, occur preferentially at high cycling rates.