Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors

Abstract

Electrochemical supercapacitors can deliver high levels of electrical power and offer long operating lifetimes^{1,2,3,4,5,6,7,8}, 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 ( ∼ 1,145 F g^–1) that is close to the theoretical value⁹. 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.