In view of the rapid growth of energy demands and graduallyexhausted nonrenewable energy sources, clean, efficient, andrenewable energy-storage systems are very important forsatisfying the low-carbon and sustainable economy of thefuture development.1−6 Among various energy-storage devices,supercapacitors, known as electrochemical capacitors (ECs),act as unique and outstanding candidates for energy storagenow and in the near future.7−10The electrode material is a key factor for the performance ofsupercapacitors. Polyoxometalates (POMs),11−14 a uniqueclass of inorganic metal oxide clusters, are very goodcandidates as electrode materials for the following reasons:(i) POMs have 6 to 368 metal ions in a single cluster;therefore, their monodisperse size can be tuned from severalangstroms up to 10 nm.15−17 (ii) Besides their structural role,POMs are considered to be an “electron reservoir”18−20 or an“electron sponge”,18,21 which can participate in rapid reversiblemultielectron transfer reactions. The structural diversity, good