A New Picture of the Electrochemical Double Layer Based on Modern Continuum Thermodynamics

We will revise the widely used concept of inner- and outer Helmholtz planes in electrochemical double layers based on modern continuum thermodynamics. At the metal/electrolyte interface various layers of solvated ions form, namely the inner Helmholtz plane for specifically adsorbed and partially solvated ions, and the outer Helmholtz plane for completely solvated ions closest to the metal surface[1]. It is assumed that all the ionic charge is stored on the respective layer, and that they have a constant width. The potential drop would thus be linear between the inner and the outer Helmholtz layer, which motivates the model concept of a simple plate capacitor.

Based on our new continuum mechanical mixture theory for solvated ions[2] we provide new insights to this model conception. It turns out that neither the inner, nor the outer Helmholtz plane can be modeled as rigid planes which are supposed to behave like simple plate capacitors. The picture of the inner Helmholtz plane (IHP) we draw is based on a surface mixture theory, which incorporates partial solvation, and shows that there is actually no potential drop across the metal/IHP layer. The outer Helmholtz plane results from the saturation of solvated ions closest to the metal surface and grows with an increasing applied potential. Hence, in both cases the model concept of simple plate capacitors is doubtable.

Evidence of this picture comes from a remarkable agreement to single crystal experimental data regarding the computed differential capacity C(U)[3-4].

In consequence, the different types of layers in the space charge region are naturally incorporated in our new mixture theories and an explicit modeling or descriptionof the space charge layer is not necessary.

[1] D. M. Kolb, Angewandte Chemie, 2001, 113, 1198–1220.

[2] W. Dreyer, C. Guhlke, M. Landstorfer, Electrochemistry Communications, 2014, DOI: 10.1016/j.elecom.2014.03.015

[3] G. Valette, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1981, 122, 285 – 297.

[4] G. Valette, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1982, 138, 37 – 54.