Effect of pore morphology on the electrochemical properties of electric double layer carbon cryogel supercapacitors

In this study, a group of carbon cryogels have been synthesized using resorcinol formaldehyde as precursors, and altered via catalysis and activation, to obtain varied nanostructures and pore size distributions. To understand the relation between structure and electrochemical properties, an alternate approach to de Levi's cylindrical pore, transmission line method was utilized. Using electrochemical impedance spectroscopy, the capacitor can be studied as a dielectric system composed of a porous electrode and the electrolyte (tetraethylammonium tetrafluoroborate in propylene carbonate). The complex capacitance and power are used to study the behavior of the system below the relaxation frequency f(0) (phi=-45 degrees). Therefore, the relaxation of the capacitor system at the low frequency range, f < f(0), may be used as a measure of pore/electrolyte interaction. The approach proposed here also allows for a direct experimental characterization of the capacitance and power at low frequencies, where small pores are likely to affect the diffusion kinetics and dynamics of the electrolyte molecules. The results suggest a correlation between the occurrence of small micropores and that of high power losses that are related to the resistive element at the low frequency range. Moreover, the impact of the micropore structure on the supercapacitor's performance is apparent in its capacitance and energy as well. In addition to the complex power and capacitance, other measurements including nitrogen physisorption, cyclic voltammetry, galvanic cycling, and x-ray Raman scattering were used to characterize the samples and support these results. (C) 2008 American Institute of Physics.