Double layer capacitance
A double layer capacitance is formed by the application of an electrical voltage by charge separation in a Helmholtz - double layer at the phase boundary between the electrode surface and electrolyte . The electrical energy in the double layer capacitance is statically stored in an electrical field .
Inseparably together with a static double-layer capacitance, an electrochemical storage of the electrical energy occurs in these electrochemical capacitors with the so-called pseudocapacitance .
The double-layer capacitance is implemented and used in supercapacitors , also called "double-layer capacitors".
history
For the history of the theoretical models of the double layer phenomenon, see Electrochemical double layer .
For the history of the development of electrochemical capacitors, see Supercapacitor .
functionality
With the description of the electrical phenomena at the interfaces between a metallic or metallically conductive electrode and a liquid electrolyte , Helmholtz first described the “ double-layer effect ”. With this electrical phenomenon at the phase boundary between the surface of the electrode and the electrolyte, two ionized layers are created when a voltage is applied , one in the surface area of the metallically conductive electrode and a second layer of opposite polarity in the adjacent area of the liquid electrolyte consisting of dissociated and solvated ions. These two layers of ionized electrical charges are separated by a molecular layer made up of polar molecules of the electrolyte solvent , ie in the case of aqueous electrolytes made up of water molecules that adhere firmly to the surface of the electrode through physical adsorption and have an electrically insulating effect. They form the "Inner Helmholtz Layer (IHP)", which, like the dielectric in a conventional capacitor, has a charge-separating effect.
Adsorption is a physical process in which a substance, in this case the molecules of the solvent, stick to the surface of another substance, in this case the electrode, and accumulate on its surface. The forces that cause attachment are not chemical bonds, but physical forces similar to adhesion . Chemical bonds within the adsorbed molecules remain, but they are polarized .
The amount of charge, ie the number of ions that can come from the electrolyte and accumulate in a double layer, depends on the concentration of the ions in the electrolyte and the surface of the electrode. It is linearly dependent on the applied voltage up to a limit value, the so-called decomposition voltage of the electrolyte. The number of charge carriers in the electrode is balanced by a corresponding number of ions of opposite polarity, the counter ions , in the electrolyte. A static electric field forms between the charge carriers, which polarizes the solvent molecules between them. This charge separation in the double layer causes the storage of electrical energy.
The "thickness" of a charged electrochemical double layer ; H. the mean dimension perpendicular to the surface is about 0.1 nm in the metallic electrode. It mainly depends on the electron density , since the atomic cores in fixed electrodes are immobile. In the electrolyte, it depends on the size of the molecules in the solvent and on the mobility and concentration of the ions in the solvent. In the electrolyte it is about 0.1 to 10 nm and is described by the Debye length . Both “thicknesses” together result in the total thickness of a double layer.
The Helmholtz double layer in a double layer capacitor is effective like a plate capacitor with the spacing of the thickness of a solvent molecule. The capacity of a double layer is calculated according to the formula of the plate capacitor .
This means that the capacitance " C " of a capacitor is greater, the larger the electrode area " A " and the permittivity " ε " and the thinner the dielectric " d ". The highly roughened, very large surface of the electrodes of these capacitors and the extremely thin inner Helmholtz layer on the order of a few nanometers together produce the very large double-layer capacitance.
The extremely small thickness of the Helmholtz double layer creates a very strong electric field E in it . With a potential difference of, for example, U = 2 V and a molecular distance of d = 0.4 nm, the electric field strength is
In order to be able to classify this value, a comparison with an aluminum electrolytic capacitor is given here. The dielectric strength of the aluminum oxide layer is about 1.4 nm / V. With a 6.3 V capacitor, the thickness of the dielectric is then around 8.8 nm. From this, the field strength in the aluminum oxide is calculated as 6.3 V / 8.8 nm = around 0.7 V / nm.
A field strength of 5 V / nm, which occurs in the molecules in the inner Helmholtz layer, cannot be achieved in a capacitor with a conventional dielectric. No dielectric could prevent the charge carriers from breaking through. In a double layer, the chemical stability of the molecular bond of the separating solvent molecule prevents breakdown. However, the extremely strong field strength has a major influence on the permittivity of the material from which the separating inner Helmholtz layer is formed. For example, the permittivity of water is normally around 80. Under the influence of the extremely high field strength, it decreases to a value of around 6.
Each double-layer capacitor now has two electrodes, which are protected against mechanical contact by a separator . The electrolyte, permeated with its positive and negative ions, which make the electrolyte conductive, connects the two electrodes with one another. After applying a voltage, a double layer is formed on each of the two electrodes. The applied voltage causes the dissolved ions statistically distributed in the electrolyte to migrate to the electrode with the opposite polarity. There they form a double layer with the ions in the electrode, separated by the position of the solvent molecules. The charge distribution on one electrode is mirrored on the second electrode of the capacitor. Both double layers act like two capacitors connected in series. In the case of symmetrically constructed capacitors in which both electrodes have approximately the same capacitance, the total capacitance of the capacitor is therefore equal to half the value of an electrode.
After switching off the voltage, the ions are distributed again statistically in the electrolyte.
The electrically separating effect of a Helmholtz double layer is only effective for a relatively small voltage range of around 1.2 to 3 V, depending on the electrolyte system. If the voltage rises above the decomposition voltage of the electrolyte (see also electrolysis ), the separating effect of the Helmholtz double layer collapses and a short circuit occurs.
Individual evidence
- ↑ a b Zbigniew Stojek: The Electrical Double Layer and Its Structure . In: Fritz Scholz (Ed.): Electroanalytical Methods: Guide to Experiments and Applications . Springer, Berlin / Heidelberg 2010, ISBN 978-3-642-02914-1 , p. 3-10 ( online ).
- ↑ a b Marin S. Halper, James C. Ellenbogen: Supercapacitors: A Brief Overview. In: MITER Nanosystems Group. March 2006, accessed May 14, 2013 . (last accessed on July 27, 2013)
- ^ E. Frackowiak, F. Beguin: Carbon Materials For The Electrochemical Storage Of Energy In Capacitors. In: CARBON. 39, 2001, pp. 937-950 ( PDF ) and E. Frackowiak, K. Jurewicz, S. Delpeux, F. Béguin: Nanotubular Materials For Supercapacitors. In: Journal of Power Sources . Volumes 97-98, July 2001, pp. 822-825, doi : 10.1016 / S0378-7753 (01) 00736-4 .
- ↑ Adam Marcus Namisnyk and JG Zhu: A Survey of Electrochemical super-capacitor technology . 2003 ( PDF [accessed December 7, 2015] Bachelor thesis; University of Technology, Sydney; 2003).
- ^ Daniel Gräser, Christoph Schmid: Supercap, Basics - Properties - Applications. Bern University of Applied Sciences, term paper in technology and German PDF
- ↑ S. Srinivasan, Fuel Cells, From Fundamentals to Applications, Springer eBooks, 2006, ISBN 978-0-387-35402-6 , Download CHAPTER 2 , ELECTRODE / ELECTROLYTE INTERFACES: STRUCTURE AND KINETICS OF CHARGE TRANSFER (pdf, 769 kB) (last accessed on July 30, 2013)
- ^ Stefan Woelki, Theory of the Electric Double Layer, ISBN 3-89675-568-4 , page 36, Fig. 3-3 Google Books