Functionalization of Carbon Surfaces Tunes the Redox Stability of Polyoxometalate@Carbon Electrodes

The use of redox-active polyoxometalates (POMs) offers unique opportunities for the development of advanced functional materials, but the use of such systems is often limited by their instability in operando. To enhance the functional stability of POMs, they are often immobilized onto solid supports, though complex POM hybridization strategies are usually required. Herein, we describe a simple and scalable method for the fabrication of POM-based redox materials. Our method involves functionalizing an inexpensive carbon black (Vulcan XC-72R) support with oxygenic groups, which act as stabilizing anchor points for the Wells–Dawson POM K6[P2W18O62]. X-ray photoelectron spectroscopy and electron microscopy confirm that oxidation of the carbon surfaces promotes interactions between the POMs and the carbon surfaces. Scanning electron microscopy shows that the surface area of carbon decreases upon oxidation, and this is confirmed by Brunauer–Emmett–Teller analysis, where the specific surface area of carbon decreases from 234 to 108 m2 g–1. Solid-state voltammetry shows that the electrochemical properties of the POMs are retained by the hybrid material. Four successive, reversible reductions of the attached POMs are observed when the composite is in contact with 1.0 mol dm–3 H2SO4, corresponding to the [P2W18O62]6–/[P2W18O62]7–, [P2W18O62]7–/[P2W18O62]8–, [P2W18O62]8–/[H2P2W18O62]8–, and [H2P2W18O62]8–/[H4P2W18O62]8– redox couples. In contrast, formation of a composite using unoxidized carbon only shows the [P2W18O62]6–/[P2W18O62]8–, [P2W18O62]8–/[H2P2W18O62]8–, and [H2P2W18O62]8–/[H4P2W18O62]8– couples. Finally, voltammetric analysis shows that the composite formed using oxidized carbon shows very little degradation in any of its 4 redox process over 500 charging/discharging cycles, whereas that formed using unoxidized carbon almost completely loses its redox activity after 250 cycles. These observations have significant implications for the sustainable synthesis of functional molecular redox materials for future energy storage technologies.