(589b) Transport in Electrochemical Capacitors: Effects of Porous Geometry, Electrolyte Asymmetry, and Redox Reactions

Electrochemical capacitors, such as supercapacitors and pseudocapacitors, are energy storage devices characterized by their high power densities. They consist of porous electrodes, typically made up of dispersions of activated carbon spheres, immersed in aqueous, organic, or ionic electrolytes. These devices store energy primarily through the electrostatic attraction of ions in electrical double layers. However, the electrode material may also consist of metallic oxides to store additional energy through an oxidation/reduction mechanism. Over the past two decades, there have been significant advancements in the materials of electrodes and electrolytes for electrochemical capacitors. The field has tended to focus on finding different combinations of materials to optimize the energy and power densities. However, an understanding of various transport processes that govern the dynamics of charging/discharging remains unclear.

In this talk, we describe the theoretical frameworks we have developed to understand the charging dynamics of electrochemical capacitors. First, we have analyzed the charging of electrical double layers inside a cylindrical pore with an asymmetric electrolyte and arbitrary pore size [1, 2]. By utilizing perturbation analysis and direct numerical simulations of the Poisson-Nernst-Planck equations, we are able to predict the evolution of electrical potential and ion concentrations in both the radial and axial directions. Our analysis reveals that asymmetric ionic diffusivities and valences can yield salt migration, even at low potentials, and control the charging timescale of the pores. Second, we will describe an approach to capture the impact of Faradaic reactions on the charging of electrical double layers. This method, also based in perturbation analysis, predicts electrolyte dynamics for an arbitrary number of ion types and asymmetries in valence and diffusivity in the thin double layer limit [3]. We find that electrochemical reactions directly impact the dynamics, and show that the effective double layer thickness can be controlled using reaction kinetics.

[1] Henrique, F., Zuk, P. J., & Gupta, A. (2022). Charging Dynamics of Electrical Double Layers Inside a Cylindrical Pore: Predicting the Effects of Arbitrary Pore Size. Soft Matter, 18(1), 198-213

[2] Henrique, F., Zuk, P. J., & Gupta, A. (2022), Effects of Asymmetry in Valence and Diffusivities on Transport of a Binary Electrolyte in a Cylindrical Pore, submitted

[3] Jarvey, N., Henrique, F., & Gupta, A. (2022). Charging of an Electrochemical Cell: Theoretical Framework to Couple Dynamics of Double Layers and Redox Reactions for an Arbitrary Number of Ions, submitted