Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors

Electrochemical supercapacitors can deliver high levels of electrical power and offer long operating lifetimes(1-8), but their energy storage density is too low for many important applications(2,3). Pseudocapacitive transition-metal oxides such as MnO2 could be used to make electrodes in such supercapacitors, because they are predicted to have a high capacitance for storing electrical charge while also being inexpensive and not harmful to the environment(9,10). However, the poor conductivity of MnO2 (10(-5)-10(-6) S cm(-1)) limits the charge/discharge rate for high-power applications(10,11). Here, we show that hybrid structures made of nanoporous gold and nanocrystalline MnO2 have enhanced conductivity, resulting in a specific capacitance of the constituent MnO2 (similar to 1,145 F g(-1)) that is close to the theoretical value(9). The nanoporous gold allows electron transport through the MnO2, and facilitates fast ion diffusion between the MnO2 and the electrolytes while also acting as a double-layer capacitor. The high specific capacitances and charge/discharge rates offered by such hybrid structures make them promising candidates as electrodes in supercapacitors, combining high-energy storage densities with high levels of power delivery.