Simulations of Cyclic Voltammetry for Electric Double Layers in Asymmetric Electrolytes: A Generalized Modified Poisson-Nernst-Planck Model

Journal Article · Fri Nov 15 00:00:00 EST 2013 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/jp402181e· OSTI ID:1096088

Wang, Hainan; Thiele, Alexander; Pilon, Laurent ^[1]

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1. UCLA

This paper presents a generalized modified Poisson–Nernst–Planck (MPNP) model derived from first principles based on excess chemical potential and Langmuir activity coefficient to simulate electric double-layer dynamics in asymmetric electrolytes. The model accounts simultaneously for (1) asymmetric electrolytes with (2) multiple ion species, (3) finite ion sizes, and (4) Stern and diffuse layers along with Ohmic potential drop in the electrode. It was used to simulate cyclic voltammetry (CV) measurements for binary asymmetric electrolytes. The results demonstrated that the current density increased significantly with decreasing ion diameter and/or increasing valency |z_i| of either ion species. By contrast, the ion diffusion coefficients affected the CV curves and capacitance only at large scan rates. Dimensional analysis was also performed, and 11 dimensionless numbers were identified to govern the CV measurements of the electric double layer in binary asymmetric electrolytes between two identical planar electrodes of finite thickness. A self-similar behavior was identified for the electric double-layer integral capacitance estimated from CV measurement simulations. Two regimes were identified by comparing the half cycle period τ_CV and the “RC time scale” τ_RC corresponding to the characteristic time of ions’ electrodiffusion. For τ_RC ← τ_CV, quasi-equilibrium conditions prevailed and the capacitance was diffusion-independent while for τ_RC → τ_CV, the capacitance was diffusion-limited. The effect of the electrode was captured by the dimensionless electrode electrical conductivity representing the ratio of characteristic times associated with charge transport in the electrolyte and that in the electrode. The model developed here will be useful for simulating and designing various practical electrochemical, colloidal, and biological systems for a wide range of applications.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: DOE - BASIC ENERGY SCIENCES
• OSTI ID: 1096088
• Journal Information: Journal of Physical Chemistry. C, Vol. 117, Issue (36) ; 09, 2013; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: ENGLISH