Kinetics of Double-Layer Formation: Influence of Porous Structure and Pore Size Distribution

Porous carbon materials have been prepared from different precursors (anthracite and carbon fibers) by different activation methods (KOH and CO2), obtaining a series of samples with very different porous texture and porous structure. Electrochemical impedance spectroscopy has been used to analyze the kinetics of the charge-discharge process during the behavior of these materials as supercapacitors. Capacitance reduction percentages (CRPs) from the starting value (at the lowest frequency of the measurements) to the value at around 0.02 Hz frequency were calculated for all of the samples. The results pointed out that, from a kinetic point or view, porous materials with wider micropore size distribution (M PSI)) have a better performance, because porosity is accessible to the electrolyte even at high charge-discharge rates. In addition, quite different behavior between activated carbons (ACs) and activated carbon fibers (ACFs) has been observed when Na2SO4 is used its the electrolyte; a widening of porosity produces a lower CRP (better kinetic behavior) in ACF than in AC, which is most likely due to the existence of a smaller tortuosity of networks of porosity in ACF than in AC. This different behavior is much less important when H2SO4 is used as the electrolyte, which is due to the higher mobility of proton in solution compared to any other ion. Thus, kinetics of the double-layer formation is improved using (i) samples with wide MPSD, (ii) samples with a porous texture with low tortuosity, and (iii) acidic medium as the electrolyte.