Enhanced Capacitive Energy Storage in Polyoxometalate-Doped Polypyrrole

High-performance batteries and supercapacitors require the molecular-level linkage of charge transport components and charge storage components. This study shows how redox-tunable Lindqvist-type molecular metal oxide anions [VnM6-nO19]((2+n))-(M = W(VI) or Mo(VI); n = 0, 1, 2) can be incorporated in cationic polypyrrole (PPy) conductive polymer films by means of electrochemical polymerization. Electron microscopy and (spectro-) electrochemistry show that the electroactivity and morphology of the composites can be tuned by Lindqvist anion incorporation. Reductive electrochemical " activation" of the Lindqvist-PPy composites leads to significantly increased electrical capacitance (range: approximate to 25-38 F g(-1), increase up to approximate to 25x), highlighting that this general synthetic route gives access to promising capacitive materials with suitable long-term stability. Electrochemical, electron microscopic, and Raman spectroscopic analyses together with density functional theory (DFT) calculations provide molecular-level insight into the effects of Lindqvist anion incorporation in PPy films and their role during reductive activation. The study therefore provides fundamental understanding of the principles governing the bottom-up integration of molecular components into nano-structured composites for electrochemical energy storage.