AMaSiS 2018 Workshop: Abstracts

Thermodynamic modeling of electrolytes and their boundary conditions to electrodes

Manuel Landstorfer

Weierstrass Institute for Applied Analysis and Stochastics, Berlin

In this talk I will give an overview on the modeling procedure of electrolytes and their boundary conditions to various electrodes. Several characteristic quantities of electrolytes are discussed on the basis of non-equilibrium thermodynamics.

Starting from a general free energy for an electrolyte mixture, I will address the importance of solvation effects. It turns out that this has an enormous impact to the dissociation degree of electrolytes [7] and questions the concept of complete dissociation, even at moderate bulk concentrations.

Next we consider an electrolyte in contact with a metal electrode, which leads to the double layer capacity, a quantity which is widely modeled with Poisson–Boltzmann type equations. It was shown that a consistent incorporation of incompressibility [1], solvation effects [2] and adsorption [3] leads to a broad qualitative and quantitative accordance to experimental data [3]. Based on this validated equilibrium model, thermodynamically consistent boundary conditions for the electroneutral electrolyte can be derived, taking into account charge transfer reactions and capacitive effects [4, 5, 6].

Subsequent transport models of electrolytes are discussed, which are essentially generalized Nernst-Planck fluxes. I will show a remarkable relationship between the double layer capacity and the thermodynamic factor of an electrolyte and discuss the impact of incomplete dissociation. Further I will show that the concentration dependence of the molar conductivity and the transference number can be well understood in terms of cross-coefficient terms of general flux relations [8].

Finally, the electrolyte model with corresponding boundary conditions is summarized and selected numerical simulations are shown to show the range of applicability.

Acknowledgments: The author acknowledges funding for the project MALLi${}^2$ from the BMBF within the initiative Mathematik für Innovationen als Beitrag zur Energiewende.