Porous carbon–carbon composite electrodes for vanadium redox flow batteries synthesized by twin polymerization

Highly porous carbon-carbon composite electrodes have been synthesized by surface twin polymerization on a macroporous polyacrylonitrile (PAN)-based substrate. For this purpose the compound 2,2'-spirobi[benzo-4H-1,3,2-dioxasiline] (Spiro), being a molecular precursor for phenolic resin and silica, was polymerized onto PAN-based felts with subsequent thermal transformation of the hybrid material-coated felt into silica-containing carbon. The following etching step led to high surface carbon-carbon composite materials, where each carbon component served a different function in the battery electrode: the carbon fiber substrate possesses a high electron conductivity, while the amorphous carbon coating provides the catalytic function. For characterization of the composite materials with respect to structure, porosity and pore size distribution scanning electron microscopy (SEM) as well as nitrogen sorption measurements (BET) were performed. The electrochemical performance of the carbon felts (CF) for application in all-vanadium redox flow batteries was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared to the pristine PAN-based felt the composite electrodes show significantly enhanced surface areas (up to 35 times higher), which increases the amount of vanadium ions that could be adsorbed onto the surface and thus contributes to an increased performance.