Covalently Linked Polyoxometalate-Polypyrrole Hybrids: Electropolymer Materials with Dual-Mode Enhanced Capacitive Energy Storage

Abstract

Lindqvist-type polyoxometalates (POMs) derivatized with pyrrole (Py) via an aryl-imido linkage [Mo6O18NPhPy]2- (1) and [Mo6O18NPhCCPhPy]2- (2) undergo coelectropolymerization with pyrrole, producing the first electropolymer films with covalently attached POM "molecular metal oxides". X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) elemental analyses indicate that the loadings of POM achieved are far higher than in conventional, noncovalent inclusion films, and covalent attachment prevents loss of POM on initial reduction cycles. Cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge measurements together indicate that the POMs enhance the specific capacitance (up to 5×) and decrease the charge-transfer resistance of the films by both modifying the behavior of the polypyrrole (PPy) film and introducing a substantial additional faradaic contribution through the POM redox processes. Increasing the length of the POM-PPy linker improves both capacitance and stability, with PPy-2 retaining 95% of its initial capacitance over 1200 cycles.