9. Conclusion
Carbons characterized by highly extended surface area and pores which match with the size of electrolyte ions appear as the most suitable electrode materials for capacitive applications, where a high amount of charge should be efficiently accumulated in an EDL. The high interest in carbons is also on account of their high conductivity, versatility of morphologies, ability to tune the porous texture, controllability of surface functionalities, high natural abundance of precursors and low cost. Nevertheless, for each of the applications detailed in this review, it is important to look through the specific set of desires in order to select the most appropriate carbonaceous material. The parameters closely connected with the porous texture (specific surface area, pore volume, size and shape of pores, tortuosity) play an important role in the capacitive performance. One should be always aware that the EDL composition, and its formation mechanisms in the confined porosity of an electrode, is different from the models presented for an electrode flat surface. During charging, the EDL formed inside micropores is composed not only of single counter-ionic species, but of co-ions and solvent molecules. Moreover, the specific surface area of porous carbon electrodes estimated by use of a gas probe (the most often nitrogen) cannot be directly considered for predicting the capacitive performance in presence of electrolyte species of different size and affinity with pore walls. When considering the capacitive technologies, the importance of electrode architecture, thickness and other physical parameters cannot be ignored. For example, if the ion transport rate should be improved to ensure the charge storage, materials with a large external surface area (such as graphene, carbon onions or carbon nanotubes) should be preferred to provide a hierarchic transport pathway. When considering energy efficient water desalination with flowable carbon suspension electrodes, the particle shape and agglomeration, and viscosity of suspension become important factors to maximize the ratio of moles of salt removed from the water feedstock to the moles of electrons transferred during cell charging.
