Hainan Wang

Simulating Electric Double Layer Capacitance of Mesoporous Electrodes with Cylindrical Pores

This paper aims to numerically assess the effects of electrolyte properties and electrode morphology on the capacitance of electric double layer capacitors (EDLCs) made of mesoporous electrodes consisting of ordered cylindrical pores in non-aqueous electrolytes. Simulations solved a three-dimensional modified Poisson-Boltzmann model. They accounted for the finite size of ions and field-dependent electrolyte permittivity while the pores were perpendicular to the current collector. The effects of pore radius, porosity, effective ion diameter, and electrolyte field-dependent permittivity on the diffuse layer gravimetric capacitance were investigated systematically in order to determine key parameters affecting EDLCs’ performance. The simulations showed that reducing the ion effective diameter and the pore radius resulted in the strongest increase in diffuse layer gravimetric capacitance up to a critical radius below which the capacitance reaches a plateau. Increasing the electrode porosity also increased the diffuse layer gravimetric capacitance. Accounting for more realistic field-dependent permittivity was found to significantly reduce the predicted diffuse layer gravimetric capacitance. Finally, accounting for the contribution of the Stern layer to the total capacitance was essential in predicting experimental data for a wide range of porous activated carbon electrodes and non-aqueous electrolytes.

Scaling Analysis of Thermal Behavior of Electrical Double Layers

Charging of electric double layer capacitors (EDLCs) may cause significant heat generation. The resulting elevated temperatures lead to shortened cell life and increased self-discharge rates and cell pressure. Better understanding and accurate modeling of the fundamental physical phenomena involved are needed for developing thermal management strategies and for designing and optimizing the next generation of EDLCs. Existing thermal models of EDLCs rely on experimentally measured heat generation rates or cell electrical resistances. This makes them unsuitable for assessing new and untested designs. The present study aims to develop a physical model accounting for the dominant transport phenomena taking place in EDLCs. It accounts for the presence of the Stern layer, finite ion size, ion diffusion, and Joule heating. It solves the modified Poisson-Nernst-Planck model with a Stern layer and the heat diffusion equation. A dimensional analysis was performed and six dimensionless parameters governing electrodiffusion coupled with heat transfer were identified and physically interpreted. The scaling analysis was successfully validated numerically.Copyright