Nanofibers versus Nanopores: A Comparison of the Electrochemical Performance of Hierarchically Ordered Porous Carbons

Two hierarchically organized carbon materials with an inverse mesopore structure were synthesized. Their nanostructure is either composed of hexagonally packed nanofibers (nanocast carbon) or cylindrical nanopores arranged on a hexagonal lattice (soft-templated carbon). Micropores were subsequently introduced by CO₂ activation with a concomitant increase in specific surface area. All materials were characterized regarding their structure using electron microscopy, gas adsorption, X-ray diffraction, and small-angle X-ray scattering. Electrochemical characterization was performed employing cyclic voltammetry and electrochemical impedance spectroscopy. It was shown that CO₂ activation not only influences the pore structure and specific capacitance but also the rate handling stability for high charging and discharging rates. For short activation times considerable differences in the rate handling capability of the two materials were observed, which are attributed to their entirely different nanostructures and connectivity. For optimum activation parameters, both materials outperform a purely microporous activated carbon reference material in terms of specific capacitance. In terms of rate handling capability, only the nanocast material shows a clearly better performance, while the soft-templated material behaves similarly to the reference. The superior performance of the nanocast material is attributed to an enhanced ion transport mediated by the optimized hierarchical pore structure.