In situ synthesis of ultrafine beta-MnO2/polypyrrole nanorod composites for high-performance supercapacitors

We report a remarkable observation that is at odds with the established notion that beta-MnO2 was regarded as an undesirable candidate for supercapacitor applications. The specific capacitance of beta-MnO2 can reach as high as 294 F g(-1), which is comparable to the best crystallographic structure, like alpha-MnO2. The key is to substantially decrease the size of beta-MnO2 powders to ultra small regime. We demonstrate a facile, simple, and effective approach to synthesizing ultrafine (< 10 nm in diameter) beta-MnO2/polypyrrole nanorod composite powders for high-performance supercapacitor electrodes. Our observation may encourage a revisit of the other good or even bad candidate active materials if we can decrease their size to extremely small scales. In addition, the proposed synthetic mechanism and the developed synthetic strategy may provide design guidelines in synthesizing other energy storage materials toward ultrafine 1D nanostructures.