Electrical Double Layer Potential Distribution in Nanoporous Electrodes from Molecular Modeling and Classical Electrodynamics Analysis
When a solid surface is immersed in a solution, the surface can become charged in a variety of ways including crystal lattice defects, dissociation of atoms or molecules, ionization of surface groups, and adsorption of external ions or, in the case of an electrode, via the application of an external potential. The excess charge at the surface results in an accumulation of mobile ions in the solution near the surface. Relative to the charge on the surface, ions of opposite charge (counterions) build up near the surface to neutralize the surface charge, and ions of like charge (coions) are depleted in this region due to the effects of entropy and coulomb repulsion. The resulting distribution of charges in the solution creates an excess electric potential at the interface. This potential drops to the Galvani potential in the solution bulk as the distance from the surface increases. The cumulative result is a phenomenon ubiquitous in electrochemistry known as the electrical double layer (EDL). The EDL governs natural phenomena like colloidal stability and electrokinetic phenomena; it also affects the outcome of electrochemical reactions.
Classical Theory describes the EDL via the combination of the Poisson equation for electrical potential and the Boltzmann distribution of charge within an electric field next to a planar charged solid surface. This approach treats ions as non-interacting point charges in the vicinity of a single-charged surface in contact with a bulk solution. However, most current applications where EDL is relevant involve confinement within nanostructures, where the size of the ions is within one or two orders of magnitude of the size of the structure itself. In such cases, Classical Theory fails, and one must turn to either more complicated theories or molecular modeling techniques. This work focuses on Monte Carlo simulation of the EDL.
A method to describe the EDL in confinement that combines Grand Canonical Monte Carlo simulations (GCMC) with electrodynamics concepts to determine not only the EDL structure, but most importantly the distribution of electrical potential within charged slit-type pores is presented. A benefit of the method in this work is that it computes potential with respect to field-free virtual bulk electrolyte solution implicit in the Grand Canonical ensemble; i.e. it does not require a physical bulk to be included in the simulation model. The method was applied to systems for which Classical EDL Theory can be employed (two non-interacting planar surfaces) for comparison purposes, and then extended to increasingly confined spaces. In contrast to Classical EDL Theory, this method accounts for ion-size and ion-ion interactions, and provides insights into the EDL structure within nanostructures.
The GCMC simulations for slit-type pores considered in this work show that entropy effects lead to the exclusion of coions in a counterion layer which packs up against the charged walls of the pore. This, in conjunction with the fact that electrical effects are long range and extend out into space from the location of the source charge, lead to a semi-oscillatory behavior of the potential profiles not predicted by Classical Theory. The oscillatory behavior, comprised of local minima near the walls and a local maximum in the center of the pore, is diminished in smaller pores as the extent of the electrical effects of the coion exclusion regions begin to overlap even when concentration profiles do not overlap, and bulk conditions for concentration are achieved in the center of the pore. The magnitude of the oscillation is directly related to surface charge, and the local maximum in the pore center becomes a flat, zero-potential region as pore size increases, pointing towards a true bulk region in terms of distribution of ions and electrical potential.
Library of Congress Subject Headings
Electric double layer; Monte Carlo method; Surface chemistry
Mechanical Engineering (MS)
Department, Program, or Center
Mechanical Engineering (KGCOE)
Ney, Evan Marshall, "Electrical Double Layer Potential Distribution in Nanoporous Electrodes from Molecular Modeling and Classical Electrodynamics Analysis" (2016). Thesis. Rochester Institute of Technology. Accessed from
RIT – Main Campus
Physical copy available from RIT's Wallace Library at QC625 .N49 2016