We demonstrate here a novel type of electrode material for electrochemical capacitors (ECs) with improved capacitance properties that is prepared in a form of carbon nanotubes (CNTs) modified with ultra-thin films of a Keggin type polyoxometallate (POM), namely phosphododecamolybdic acid, H₃PMo₁₂O₄₀ (PMo₁₂). Such features of POMs as their well-defined structures, the ability to undergo fast and reversible redox reactions, specific photoelectrochemical properties made them model systems not only for nanometric metal (e.g. Mo or W) oxide particles but also for inorganic functional materials of potential utility in electrocatalysis, molecular electronics, sensing and in energy storage applications. Here we refer to such attractive properties of PMo₁₂ as its ability to adsorb irreversibly in forms of monolayers on solid surfaces as platinum, gold and carbon. It is notworthy that monolayers of such Keggin type POM’s are formed not only on solid electrodes but also on metal (e.g. Pt) nanoparticles, as well as on carbon particles (carbon black) and CNTs. In this communication, we explore the possibility of modification of CNT surfaces through adsorption of the anionic PMo₁₂ monolayers. It is apparent from three distinct test experiments (based on cyclic voltammetry, galavanostatic charging-discharging and AC impedance) that capacitors utilizing H₃PMo₁₂O₄₀-modified carbon nanotubes are characterized by higher specific capacitances and energy densities than systems built from bare (pristine) carbon nanotubes. It is reasonable to expect that multi-walled CNTs modified through the interfacial adsorption of Keggin-type polyoxometallate (PMo₁₂) monolayers seem to exhibit both double-layer type capacitive effects as well as pseudo-capacitance properties originating from the fast and reversible multi-electron redox reactions of PMo₁₂ that significantly contribute to the observed overall capacitance. When it comes to the determination of specific capacitance, a good correlation between such three electrochemical methods as cyclic voltammetry, galvanostatic charging-discharging and AC impedance has been found.

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