Pseudocapacitance

A pseudo capacitance stores electric energy with the aid of reversible redox reactions at the electrodes to a suitable electrochemical capacitor having a Helmholtz double layer. The redox reactions are connected to a Faraday charge exchange of the ions in the electrolyte to the metallic ions in the conductive electrode. In this case, only an electron from an ion adsorbed desolvatierten and is involved in each case. The adsorbed ion is not a chemical bond with the electrode. There is only one electron transfer.

A pseudo- capacity still only occurs in conjunction with a double-layer capacitance. You can add up in all electrochemical capacitors ( supercapacitors ) inextricably to a total size. They have, however, depending on the design of the electrodes, a highly variable proportion of the total capacity. The floating capacity of a suitable electrode can be larger by a factor of 100 as the double layer capacitance, for example, with the same surface of the electrode.

The amount of charge stored in a pseudo- capacitance energy is linearly proportional to the applied voltage. The unit of pseudo- capacitance is the farad.

History

For the history of the theoretical models for the pseudocapacitance see Electrochemical double layer.

For the history of the development of electrochemical capacitors see supercapacitor.

Operation of the electrochemical pseudocapacitance

Redox reactions with faradaic charge exchange are known from accumulators for decades. However, these chemical processes are associated with solid chemical bonds of the electrode material by the adsorbate from the electrolyte. Although the chemical processes are relatively reversible, leave the battery charge-discharge cycles in irreversible chemical compounds that limit the storage capacity and lifetime. In addition, the chemical reactions in batteries run quite slowly, so that a longer time is required for the Laden-/Entladen.

Pseudocapacitive redox reactions in electrochemical capacitors ( supercapacitors ) have been different. They are made with a physical adsorption ( physisorption ) of a charged molecule or atom ( ion) on the electrode surface, and is similar to a chemical equilibrium reaction. However, the adsorbed substance ( adsorbate ) with the surface is no chemical bond, but rather adheres by weaker forces similar to the adhesion. So only van der Waals forces occur in the rule. The adsorbing ions first have to overcome the divisive effect of the electrochemical double layer supercapacitor. They lose the surrounding solvation shell. In then following adsorption of ions from the electrolyte, a Faraday charge exchange at the surface of suitable electrode takes place. On the redox reactions only one electron is involved in each case. There is only one electron transfer ( one-electron exchange reaction ). These outer-sphere redox reactions no bonds are formed or broken. This process is reversible, ie, the capacitor discharges is the electron transfer in the opposite direction instead.

The ability of the capacitor electrodes to effect redox reactions for a pseudo capacity depends very much on the nature and structure of the electrode material. Electrode materials having pseudocapacitive properties, for example, metal oxides of transition metals, which are introduced in part by the doping in the electrode material, or inserted by means of an intercalation. And conductive polymers such as polyaniline, polythiophene or derivatives of which are applied to the structures of the carbon electrode, suitable for pseudo capacitors. However, carbon electrodes may have a pseudo capacity. The proportion of pseudocapacitive reactions at carbon electrodes can be significantly increased by tailor-made pore sizes.

There are three types of electrochemical energy storage with an electron transfer, which leads to a floating capacity, occur in supercapacitors:

• Redox ( oxidation-reduction reactions) of specifically adsorbed ions from the electrolyte on surfaces of the electrodes
• Intercalation, insertion of atoms in the lattice structure of the electrode
• Electrosorption under potential deposition of hydrogen atoms or metal atoms in ad surface lattice sites of the electrode grid structure

Description of the types of systems that contribute to pseudocapacitance:

• Redox system: Ox ze ~ ⇌ Red and O2 ~ H ˡ ⇌ in the lattice
• Interkalationssystem: Li ˡ in " Ma2 "
• Electrosorption, under potential deposition of metal adatoms: M ꞊ ˡ S ze ~ ⇌ SM or H ˡ e ~ S ⇌ SH (S = surface lattice sites )

The best studied and understood is the pseudo capacity at ruthenium oxide ( RuO2 ). Here there is a reversible redox reaction coupled with multiple oxidation states, their potential overlap. The electrons usually come from the valence orbitals of the electrode material and the electron transfer reaction is very fast, with high currents can flow according to the following reaction equation:

In this charge-transfer transition (charge - transfer transition ) can be stored during charging and discharging H protons in the ruthenium crystal lattice or away from him. There is a faradaic or electrochemical electrical energy storage without chemical transformation of the electrode material. The OH groups are deposited as molecular layer at the electrode surface. Since the measurable voltage of the redox reaction is proportional to the charge state, the behavior of the reaction of a capacitor and not for a battery, wherein the voltage is largely independent of the charging state.

These electron transfer reactions are very fast, much faster than the chemical processes in the battery. These reversible reactions, an electron is released to the surface atoms of the negative electrode, respectively. This electron flow through the external circuit to the positive electrode. At the same time an equal number of anions migrate through the electrolyte from the negative to the positive electrode. There, in electrodes consisting of transition metal oxides, but do not be enriched anions, the electron again, but the present there and strongly ionized in the charged state and therefore quite " electron hungry " transition metal ions. Since these reactions pseudocapacitive no solid chemical compounds are formed, they can theoretically be repeated indefinitely. That is the reason for the very high cycle strength of many supercapacitors with high pseudo- capacitance.

The pseudocapacitive property of a supercapacitor can be detected in cyclically varying voltage with a so-called " cyclic voltammogram ", the recording of the current course. The current waveform of a pseudo capacitor is quite different from that of an ideal, or of a lossy capacitor having pure static storage. The voltammogram of an ideal capacitor runs rectangular. For a lossy capacitor, the curve shifts to a parallelogram. For electrodes with Faraday exchange reactions is the electric charge stored in the capacitor, highly dependent on the potential of the electrode. Because the differing potential of the electrode relative to the potential in which the voltammetric measurement when reversing causes a delay, differs from the voltammogram of a pseudo capacitor of the shape of the parallelogram, see the diagram on the right.

As with double-layer electrodes results in the storage capacity of pseudo- capacitor electrodes from the potential-dependent coverage of the electrode surface with adsorbed ions. Since all the reactions pseudokapazitiv effective desolvated ions, i.e., not having the ball-shaped coating layer of solvent molecules, they are significantly smaller than the solvated ions that contribute to the double layer capacitance. Therefore, they require correspondingly less electrode surface thereby declares that the same electrode surface much more pseudocapacitance can arise as a double-layer capacitance. This potential-dependent holding capacity also results in the pseudo- capacitance, which, supercapacitors have a linear course of the capacitor voltage in response to the stored charge in contrast to the behavior of voltage batteries having a charge almost independent voltage curve.

In real-life electrochemical capacitors measurable at the condenser capacity is always a combination of double-layer and pseudocapacitance. Both types of memory are inextricably linked, and only by the curve shape of the cyclic voltammogram to detect. The amount of the pseudo capacitance of an electrode, if it consists of a pseudokapazitiv active material such as the transition metal oxides or conductive polymers can in the same electrode surface, and the same volume, a factor of 10 up to 100 value greater than that of the double layer capacitance.

Capacitors whose capacitance mainly comes from electrochemical reactions, are called pseudo- capacitors. Commercial super capacitors with very high pseudocapacitance combine a pseudo-capacitive electrode with a double-layer electrode and the family of hybrid capacitors are attributable.