Performance of microporous carbon electrodes for supercapacitors: Comparing graphene with disordered materials

Over the past decades, the specific surface area and the pore size distribution have been identified as the main structural features that govern the performance of carbon-based supercapacitors. As a consequence, graphene nanostructures have been identified as strong candidates for maximizing their capacitance. However, this hypothesis could not be thoroughly tested so far due to the difficulty of synthesizing perfect materials with high pore accessibility and a sufficiently large density. Here we perform molecular simulations of a series of perforated graphene electrodes with single pore sizes ranging from 7 to 10 angstrom in contact with an adsorbed ionic liquid, and compare the capacitances (using various metrics) to the one obtained with a typical disordered nanoporous carbon. The latter displays better performances, an observation that we explain by analyzing the structure of the liquid inside the pores. It appears that although the smaller pores are responsible for the largest surface charges, larger ones are also necessary to store the co-ions and avoid the formation of detrimental opposite charges on the carbon. These results rationalize the need for disordered or activated carbon materials to design efficient supercapacitors.