Double layer (interfacial)

Electrochemical double layer, electrolytic double layer or double layer short are common names for boundary layers, in which face each other electrically charged layers separated.

Construction

In general is understood to mean the electrochemical double layer, the interface between an electron conductor ( the electrode) and an ion conductor ( electrolyte ). And at the liquid-liquid phase boundary of the electrolyte occurs not miscible "double layer " on. Typically, stand at the phase boundary in the charged state against two charge layers, which - as in each capacitor - opposite sign wear (double layer capacitor). The discharged double layer on the electrodes carrying the so-called zero charge potential, wherein said metal side uncharged and the solution side carries no net charge. The " thickness" of the charged layers, i.e., the mean extent perpendicular to the surface, is about 0.1 nm in metals, in the solution from 0.1 to 10 nm; it is described by the Debye length. In the solution is dependent on the mobility of the ions and the concentration of the solution in the metal, especially of the electron density because the ions do not move in the fixed electrodes.

Historical development

Helmholtz model

Is a metal or an electrically conductive solid material (electrode) with a conductive liquid ( electrolyte) is brought into contact, is formed at the phase boundary, an electrochemical double layer comprising electrically - charge -separating properties. 1853 Hermann von Helmholtz discovered this particular electrical behavior at the limit of metallic electrodes and an electrolyte. Only after exceeding a limit value of an electric voltage, an electric current between the electrodes begins to flow. If the applied voltage is below this limit, then this arrangement behaves like a capacitor, which positively and negatively charged ions deposited from the electrolyte mirror image and voltage dependent on the respective opposite electrode. Thereby forming between the ions in the electrolyte and the electrode in which an electrical field. The concentration of the sorbed ions, that is, the capacitance of this capacitor, up to the threshold is linearly dependent on the applied voltage.

1879 Advanced Helmholtz understanding the electroosmotic flow to the description of the electrokinetic transport of colloidal suspensions of the boundary surfaces of electrodes. It was assumed that, when applying a voltage to the interface of a metal electrode, and a liquid ( electrolyte), a surface charge in the electrode, and a layer of counter-ions in the electrolyte can be formed. The charge of the counter ions in the electrolyte compensated, according to his idea, just the surface charge in the metallic electrode. The resulting electric field between the charges is limited to the thickness of a few molecular layers in the electrolyte. This phenomenon of opposite polarity charge between the layer in the metallic electrode and the liquid he called " double layer " effect.

Gouy -Chapman model

Early Helmholtz model described only a constant capacitance, regardless of the differential charge density depends only on the dielectric constant and thickness of the double layer. But this model is a good basis for the description of the charge separation. They do not take into account important factors as the diffusion of ions in the solvent and mixing the possibility of adsorption of ions on the surface of the electrode and the interaction of dipole moments in the solvent and in the electrode.

Therefore, it was further developed the theory of Helmholtz in 1910 by Louis Georges Gouy and 1913 by David Leonard Chapman. They were based on a thermal motion of the counter-ions in the electrolyte, leading to the formation of an extended over several molecular layers diffuse layer, the so-called double Gouy -Chapman layer is voltage dependent and also dependent on the concentration of the ions. In this model, the charge distribution of ions in the electrolyte is to be understood as a function of distance from the metal surface and can be described by the Maxwell -Boltzmann distribution. This means that the electric potential decreases exponentially from the surface of the liquid.

Star bilayer

However, the Gouy -Chapman model fails for highly charged bilayer. 1924 Otto Stern united the ideas of Helmholtz with that of Gouy and Chapman, when he realized that the double layer is composed of both a rigid and of a diffuse layer, the so -called star bilayer. The star bilayer into account the fact that ions have a finite size. Consequently, the closest approximation of the ions to the electrode in the order of the ionic radius.

Grahame

But the model of star has some restrictions, for example, the ions are only modeled as a point charge, the only significant interaction in the diffuse layer is an electric charge, as well as the permittivity is set via the double layer to be constant advance as well as the viscosity of the electrolyte.

Therefore, added David C. Grahame, 1947, the star model by about an outer Helmholtz layer. Its model is characterized by the existence of three layers. The first layer (english inner Helmholtz plane, IHP ) "inner Helmholtz plane " passes through the midpoints of the solvated molecules of the electrolyte solvent goes. The second layer is " outer Helmholtz plane " (english outer Helmholtz plane, OHP) called and passes through the midpoints of the solvated ions in their distance of closest approach to the electrode. The third layer is the area which is outside of the OHP and is called diffuse layer. In addition, Graham first described the effect of ions that have stripped their encasing solvation and the surface of the electrodes touched, although actually the electrode surface should be covered with solvated molecules of the electrolyte solvent completely. In which the metallic surface of an electrode he called " nonspecific adsorption " means the effect of these ions.

Bockris - Devanathan - Müller model

1963, formulated the later Nobel Prize winner John O'Mara Bockris along with Klaus Müller and Michael Angelo Vincent Devanathan the still generally accepted model of the different storage principles in electrical double layers. This model also included yet the influence of the solvent on the overall effect of the double layer. This follows the order of authors' names in the publication called " BMD " model was specifically adsorbed anions with the description of the redox reaction, the basis of the pseudo- capacity, described in more detail.

In the picture the BMD model is represented graphically. To the loaded electrode, the adsorbed on the electrode surface solvent molecules form the inner Helmholtz plane. The solvated cations in the outer Helmholtz layer deposited directly on the inner Helmholtz layer are the counter-ions to the ions in the electrode and form the double layer capacitance. In between a specifically adsorbed cation has penetrated the inner Helmholtz layer, delivered by a redox reaction its charge to the electrode ( pseudocapacitance ) and has thus become a anion.

Trasatti - Buzzanca

The further research on double layers with electrodes of ruthenium dioxide led in 1971 by Sergio Trasatti and Giovanni Buzzanca to the conclusion that the electrochemical behavior of charge specifically adsorbed ions at low voltages the balances of capacitors. The specifically adsorbed ions provided a charge transfer between the ion and the electrode and later provided a so-called " pseudo- capacitance ". It was the first step towards pseudo- capacitors.

Conway

Between 1975 and 1980 operational Brian Evans Conway basic research on redox processes on doped with ruthenium oxide electrodes. He described the 1991 transition of the behavior of a capacitor to a ( rechargeable ) battery (From Supercapacitor to Battery ) in the electrochemical energy storage and in 1999 he coined the term " super-capacitor " (English Supercapacitor ) to identify those capacitors by using the Faraday charge storage redox reactions on the electrode surfaces with respect to the static double layer capacitance have a significantly higher pseudocapacitance.

The condenser type, for the Conway coined the term super capacitor, the electric charge stored predominantly in the form of pseudo- capacity, a term used to Conway in 1962. Electrodes which were provided with metal oxides or conductive polymers, provided particularly high values ​​of pseudo capacity. The pseudo- capacity, Conway was able to determine, however, was based not only on " specifically adsorbed ions". Other research results provide three main sources for the pseudo- capacity, redox reactions, intercalation and Elektrosorbtion, which is one among potentials deposition of ad - atoms.

Marcus theory

The physical and mathematical foundations of the electron charge transfer without chemical bonds, which is the basis of the pseudo- capacity, were described by Rudolph A. Marcus. The eponymous Marcus theory describes redox reactions ( Einelektronenaustauschreaktionen ), in which the solvent is decisive during the reaction and allows the calculation of the Gibbs free energy of activation of the polarization properties of the solvent, the size and spacing of the reactants in the electron transfer and the free enthalpy the redox reaction. Marcus received for this theory in 1992 the Nobel Prize for Chemistry.

Applications

The existence of an electrochemical double layer, the biological macromolecules (proteins, nucleic acids) surrounds in low molecular weight electrolytes ( buffers ), is essential for all the methods of electrophoresis, was used for the biochemistry of the macromolecules essential (see SDS- gel electrophoresis, DNA sequencing ). The same applies to the in practice less important, often disturbing phenomenon of electroosmosis. In the electrical double layer of the phenomenon takes place in super capacitors, also known as " double-layer capacitors ," the sum of the double-layer capacitance and pseudo- capacitance a key role.