Monolithic Carbon Spherogels as Freestanding Electrodes for Supercapacitors

This study presents a direct application for freestanding and monolithic carbon spherogels, which are highly porous carbon aerogels solely composed of hollow nanospheres, as binder-free electrode materials for supercapacitors. By applying polystyrene nanospheres as templating agents in the size range of 277–907 nm and concentrations from 1.5 to 12 wt % in the sol–gel polymerization process of resorcinol–formaldehyde, we can distinctly tailor the hollow spheres' pore arrangement and thickness of the microporous carbon walls. Furthermore, the required synthesis conditions for mechanically processable (sliceable) carbon spherogels are explored. Following physical activation with carbon dioxide, only 277 nm templated samples, featuring one thin-walled (<20 nm) and one thick-walled (>60 nm) variant, retain a freestanding bulk constitution and hence are suitable for the application as binder-free electrodes. The electrochemical evaluation demonstrates the advantage of hollow carbon sphere aerogels in comparison to conventional carbon aerogels with respect to capacitance, particularly at high scan rates. The analysis of the electrolyte diffusion kinetics and the network morphology after 10,000 charge/discharge cycles reveals the necessity of wall thickness optimization. Thick-walled variants are favored due to a much higher rate capability of 30 kW kg–1 compared to 10 kW kg–1). Additionally, a post-mortem TEM analysis reveals the loss of a central cavity and sphere degradation in thin-walled samples.