Polyoxometalates (POMs) are discrete early transition metal-oxide clusters. They have had a history of about two centuries since the •rst heteropoly compound was synthesized by Berzelius in 1826 [1]. Polyoxometalates not only have ideal geometrical morphology and structural diversity in symmetry and size, but also many potential applications in electrochemistry, catalysis, photochemistry, biomedicine, magnetism, and materials science, among others [2-7].