Optimized structure of nanoporous carbon-based double-layer capacitors

Electrochemical double-layer capacitors with micro-to-nanoporous electrodes based on activated carbon utilize the gross interface between carbon and the electrolyte, which fills the pores of two main classes, (i) micrometer voids between agglomerates of carbon grains and (ii) micro-to-nanometer pores inside agglomerates and grains. The bigger pores provide good transport of ions throughout the layer whereas the smaller pores generate a large interfacial area. This essentially bifunctional architecture prompted us to combine two complementary concepts in the pertinent theory of double-layer capacitors, viz. linear transmission line models and self-affine fractal models. The merit of this structure-based approach, involving also the effect of hindrance of ionic conductivity in nanopores, is that it relates basic structural characteristics to the dynamic performance. Practically, with no fitting parameters, it reproduces the main features of recently reported complex impedance data. This refers to the effect of electrode thickness, constant phase angle (CPA) and crossover between CPA and non-CPA behavior. Routes toward the optimization of the capacitor architecture for the largest possible static capacitances and rapid charging-discharging are explored. The comparison of calculated optimum capacitance values with recent experimental data reveals remarkable reserves for advanced structural design. (C) 2004 The Electrochemical Society.