Chapter 1 Introduction
1.3 Electrode-solution interface formation
We have discussed earlier each component of the interface separately (i.e. the electrolyte and the electrode) so, what happens when both of them come in contactv? In general, surfaces carry excess electric charge, when two dissimilar phases meet, an electrified interface is established and perturbation of the solution composition near the electrode occurs.23, 188, 189 At the interface, the forces experienced by ions and solvent
molecules are anisotropic.23, 190 Thus, a new solvent structure, which is different from
that of the bulk develops.191 There will be a net orientation of solvent dipoles and a net
excess charge in any volume of the solution adjacent to the electrode surface.7
As the interface is now electrified i.e. acquired a net/excess charge, an electric field is developed.192 For a metallic electrode, free electrons will move away or towards the
interface depending on the direction of the electric field.193 As a result, a net charge
will be induced on the electrode surface, which will be equal in magnitude and opposite in sign to that of the electrolyte (i.e. this is known as the electrical double layer (EDL), which can be described simply as a sheet of negative charges (electrode) next to a sheet of positive charges (solution) or vice versa).194 Consequently, charge separation occurs
across the boundary between the solid (electrode) and the liquid phase (solution or electrolyte).194 This gives rise to an interfacial potential difference (Galvani potential
difference, ∆φ).194, 195 Models have been developed to provide insights about the EDL,
these are (1) Helmhotz model; (2) Gouy–Chapman model (G-C); (3) Stern model, and (4) Esin and Markov, Grahame, and Bockris, Devanathan, Müller model, shown in Figure 1.13.10, 23, 88, 188, 196-198
In the Helmholtz model of the interface the solvated ions arrange themselves along the surface of the electrode but are held away from it by their hydration spheres (Figure 1.13(a)).23, 188, 195, 199, 200 The outer Helmholtz plane (OHP) is established and is known
layer bounded between the electrode surface and the OHP plane (Figure 1.13(a)).23, 188, 195, 199, 200 In an enhancement of the Helmholtz model, the inner Helmholtz plane (IHP)
was suggested to form when ions lose their solvating molecules and become attached to the electrode surface by chemical bonds.23, 188, 195, 199, 200 However, the Helmholtz
model ignores the disrupting effect of thermal motion, which tends to break up and scatter the rigid outer plane of charge. Figure 1.13(b) depicts the G-C model of the diffuse EDL, where the disordering effect of thermal motion is considered. The G-C model shows how the interfacial region concentrations of anions and cations differ in the G–C model from the bulk concentrations i.e. ions of opposite charge cluster close to the electrode and ions of the same charge are repelled from it.23, 32, 188, 195, 196, 199-203
In other terms, it is supposed that the disordering forces arising from the thermal energy of the ions should oppose the ordering tendency induced in the interphase region by electrostatic forces. Consequently instead of considering a simple compact layer (the Helmholtz layer), these thermal and electrostatic forces are assumed to result in an equilibrium, in which the excess surface charge density, σS, counter balancing the
surface charge density, φM, on the metal, is at a maximum close to the electrode surface.
This diminishes in an approximately exponential way with increasing distance from the electrode surface, thereby giving rise to a diffuse layer adjacent to the electrode. Detailed analysis indicates that the thickness of the latter region will depend on the potential applied to the electrode and on the concentration of ions in the electrolyte.23, 32, 188, 190, 196, 199-203
The Helmholtz model overemphasises the rigidity of the local (interface) solution, whereas the G–C model underemphasises the structure of the interface, 23, 32, 188, 190, 196, 198-203 see references 184-189. In a refinement of both models, the ions, which are close
to the electrode are constrained into a rigid Helmholtz plane, while the ions that are located outside that plane, are spread in a similar fashion to the G–C model, 23, 32, 188, 190, 196, 198-203 see references 32, 41, 174, 176, 182 as well. This model is known as the
Stern model and is shown in Figure 1.13(c).198 Stern considered ion sizes and avoided
treating ions as point charges only, where solvated ions can approach the electrode surface to a distance equal to its solvated radius. At the electrode surface, there is a region of high electric field and low dielectric constant (εr) with a row of firmly held
Chapter 1
Figure 1.13: Schematics of the different models for the EDL at electrode|electrolyte interface, for ion distribution and potential versus distance from the metal surface. (a) Helmhotz model; (b) Gouy–Chapman model; (c) Stern model, and (d) composite of models from Esin-Markov-Grahame, and Bockris-Devanathan-Müller model.194
Schematic potential change is superimposed.195
Beyond that there is the diffuse layer, where a balance between the ordering electrostatic force and disordering thermal motions exists. The dielectric constant increases rapidly with distance in this region. The line of demarcation between the
compact and diffuse regions is the OHP. Electrical potential drops linearly with distance within the inner compact region (IHP+OHP) after which it decreases in an approximate exponential manner with distance within the diffuse region, and approaching to zero in the bulk solution.23, 32, 188, 190, 196, 198-203
Further improvement to these models, led to the development of the Grahame model, which adds an IHP to the Stern model.32, 191, 199, 204 In the IHP, de-solvated ions that had
been adsorbed specifically to the surface are considered because they would be able to be within the immediate proximity of the surface. The potential difference between
points in the bulk metal and the bulk solution is the Galvani potential difference, Δφ.23, 32, 188, 190, 196, 198-203 Yet another level of sophistication is found in the triple layer model
displayed in Figures 1.13(d) and 1.14, 23, 31, 32, 36, 88, 91, 191, 199, 204-213 see references 32,
40, 41, 45, 86, 89, 177, 185 for further discussions. In this model, the IHP is introduced and regarded as the location of the electrical centres of specifically adsorbed ions. These ions are strongly adsorbed onto the electrode surface and are partially de- solvated. Thus, a reversal of electrostatic potential in the region between the IHP and the OHP establishes. Primary and secondary water layers are introduced with differing dielectric properties. The primary water layer is located immediately adjacent to the electrode surface. The secondary water layer is located as a hydration sphere around a solvated cation and anion. This model accounts for the solvent interactions with the electric field at the electrode, assigning orientations to dipoles of the solvent molecules which are dependent on the relative charge, for more information consult 32, 40, 41, 45, 86, 89, 129, 177, 185.23, 31, 32, 36, 88, 91, 129, 191, 199, 204-213
Efforts are undergoing to further develop better understanding of the electrified interface.203, 214-220 For example, in a recent work by Crumlin et al., ambient pressure
X-ray photoelectron spectroscopy (AXPS) and electrochemical modelling were used to probe the interface of a single crystal gold electrode. Using this method, information about the potential distribution in the EDL was extracted. The study showed that there is a potential drop over the compact layer and the structure within the double layer region is dependent on both the applied potential and the electrolyte solution.219
Chapter 1
Figure 1.14: A representation of the Bockris-Devanathan-Müller model for the electrical double layer at the electrode|electrolyte interface.23, 88, 96