Pseudocapacitance

Depending on the electrode's structure or surface material, pseudocapacitance can originate when specifically adsorbed ions pervade the double-layer, proceeding in several one-electron stages. The electrons involved in the faradaic processes are transferred to or from the electrode's valence-electron states (orbitals) and flow through the external circuit to the opposite electrode where a second double-layer with an equal number of opposite-charged ions forms. The electrons remain in the strongly ionized and electrode surface's "electron hungry" transition-metal ions and are not transferred to the adsorbed ions. This kind of pseudocapacitance has a linear function within narrow limits and is determined by the potential-dependent degree of surface coverage of the adsorbed anions. The storage capacity of the pseudocapacitance is limited by the finite quantity of reagent or of available surface.

Systems that give rise to pseudocapacitance:^[5]

• Redox system: Ox + ze‾ ⇌ Red
• Intercalation system: Li⁺
  in "Ma
  ₂"
• Electrosorption, underpotential deposition of metal adatoms or H: M⁺
  + ze‾ + S ⇌ SM or H⁺
  + e‾ + S ⇌ SH (S = surface lattice sites)

All three types of electrochemical processes have appeared in supercapacitors.^[5][6]

When discharging pseudocapacitance, the charge transfer is reversed and the ions or atoms leave the double-layer and spread throughout the electrolyte.

Materials

Electrodes' ability to produce pseudocapacitance strongly depends on the electrode materials' chemical affinity to the ions adsorbed on the electrode surface as well as on the electrode pore structure and dimension. Materials exhibiting redox behavior for use as pseudocapacitor electrodes are

These materials provide high pseudocapacitance and were thoroughly studied by Conway.[1]^[7] Many oxides of transition metals like ruthenium (RuO
₂), iridium (IrO
₂), iron (Fe
₃O
₄), manganese (MnO
₂) or sulfides such as titanium sulfide (TiS
₂) or their combinations generate faradaic electron–transferring reactions with low conducting resistance.^{[citation needed]}

The electron transfer reaction takes place according to:

${\displaystyle \mathrm {RuO_{2}+xH^{+}+xe^{-}\leftrightarrow RuO_{2-x}(OH)_{x}} }$ where ${\displaystyle 0\leq x\leq 2}$^[8]

During charge and discharge, H⁺
(protons) are incorporated into or removed from the RuO
₂ crystal lattice, which generates storage of electrical energy without chemical transformation. The OH groups are deposited as a molecular layer on the electrode surface and remain in the region of the Helmholtz layer. Since the measurable voltage from the redox reaction is proportional to the charged state, the reaction behaves like a capacitor rather than a battery, whose voltage is largely independent of the state of charge.

Conducting polymers

Another type of material with a high amount of pseudocapacitance is electron-conducting polymers. Conductive polymer such as polyaniline, polythiophene, polypyrrole and polyacetylene have a lower reversibility of the redox processes involving faradaic charge transfer than transition metal oxides, and suffer from a limited stability during cycling.^{[citation needed]} Such electrodes employ electrochemical doping or dedoping of the polymers with anions and cations. Highest capacitance and power density are achieved with a n/p-type polymer configuration, with one negatively charged (n-doped) and one positively charged (p-doped) electrode.

Structure

Pseudocapacitance may originate from the electrode structure, especially from the material pore size. The use of

Pseudocapacitance properties can be expressed in a cyclic voltammogram. For an ideal double-layer capacitor, the current flow is reversed immediately upon reversing the potential yielding a rectangular-shaped voltammogram, with a current independent of the electrode potential. For double-layer capacitors with resistive losses, the shape changes to a parallelogram. In faradaic electrodes the electrical charge stored in the capacitor is strongly dependent on the potential, therefore, the voltammetry characteristics deviate from the parallelogram due to a delay while reversing the potential, ultimately coming from kinetic charging processes.^[12][13]

Applications

Pseudocapacitance is an important property in supercapacitors.

Literature

• Héctor D. Abruña; Yasuyuki Kiya; Jay C. Henderson (2008), "Batteries and electrochemical capacitors" (PDF), Physics Today, no. 12, pp. 43–47
• Béguin, Francois; Raymundo-Piñero, E.;
  ISBN 978-1-4200-5540-5
  .
• Müller, Klaus; Bockris, J. O'M.; Devanathan, M. A. V. (1965). "On the Structure of Charged Interfaces". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences. 274 (1356): 55–79.
  S2CID 94958336
  .
• B. E. Conway (1999), Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (in German), Berlin: Springer,
  ISBN 978-0306457364
• Leitner, K. W.; Winter, M.; Besenhard, J. O. (December 2003). "Composite supercapacitor electrodes". Journal of Solid State Electrochemistry. 8 (1): 15–16.
  S2CID 95416761
  .
• Yu M., Volfkovich; Serdyuk, T. M. (September 2002). "Electrochemical Capacitors". Russian Journal of Electrochemistry. 38 (9): 935–959.
  ISSN 1608-3342
  .
• Aiping Yu; Aaron Davies; Zhongwei Chen (2011). "8 - Electrochemical Supercapacitors". In Jiujun Zhang; Lei Zhang; Hansan Liu; Andy Sun; Ru-Shi Liu (eds.). Electrochemical Technologies for Energy Storage and Conversion, Band 1. Weinheim: Wiley-VCH. pp. 317–376.
  ISBN 978-3-527-32869-7
  .