Electrochemical Kinetics

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The electrochemical kinetics describes how to run fast electrochemical processes and is therefore a branch of electrochemistry and kinetics . Since the processes at the electrodes are linked to an electric current and a voltage between the electrodes, typical questions of electrochemical kinetics are: “How large are the currents through an electrochemical cell for a given voltage?” Or vice versa: “What voltage is used for requires a certain amperage? ” B. relevant in electroplating or generally in electrolysis . For galvanic cells , the electrochemical kinetics deals with the questions about the current strength or the voltage during the operation of a battery (discharging) or an accumulator (charging or discharging). The rest voltage of a cell, on the other hand, is not dealt with in electrochemical kinetics, but rather determined by electrochemical thermodynamics within the framework of the Nernst equation .

Central terms and equations

Important terms in electrochemical kinetics are overvoltage , i.e. H. the deviation from equilibrium potential, and the current density j , d. H. the measured current I divided by the electrode area A : j = I / A . The charge passage is understood as the flow of electrical charge across the electrode / electrolyte phase boundary . In the process, electrons are transferred - for example from the electrode into the electrolyte (where a reduction reaction then takes place ) or, conversely, from the electrolyte (in which at least one component is oxidized ) into the electrode. Metal ions can also be reduced and bound to the electrode as a metal atom, or conversely, a metal atom can go into solution as an ion during the oxidation.

The speed of the reaction and thus the current density depend both on the charge passage and on the transport of the charges in the solution.

Current density j according to the Butler-Volmer equation and the two contributions that add up to j

A central result of the electrochemical kinetics is the Butler-Volmer equation , which describes the dependence of the current density on the overvoltage. According to her, the currents at an electrode for a given electrode process depend exponentially on the potential if the passage of charge determines the current strength. For currents that are not too small, the approximately valid Tafel equation can be used instead of the Butler-Volmer equation . In addition to the overvoltage caused by the passage of charge, there are also overvoltages in a cell due to the transport processes, which can lead to a concentration overvoltage. Sometimes chemical reactions, crystallization or adsorption processes also determine the rate.

As in chemical kinetics, many reactions in electrochemical kinetics can also be influenced by catalysts . One then speaks of electrocatalysis .

Historical

Julius Tafel published his study of the evolution of hydrogen in 1905. The Butler-Volmer equation in the form we know today was first published in 1930 by Tibor Erdey-Grúz and Max Volmer . John Alfred Valentine Butler published it in 1932. In that year, Alexander Frumkin began his extensive studies on electrochemical kinetics. In 1952 he published "Kinetics of Electrode Processes" (Кинетика электродных процессов, Kinetika ėlektrodnych processov) the first textbook in the field. Klaus J. Vetter published his book "Elektrochemische Kinetik" in 1961; it appeared in 1967 in translated and expanded English and Russian editions. Wolfgang Forker published another book with the same title in 1966 (2nd revised edition 1989). With these monographs, the expansion of this subject reached a climax and a preliminary conclusion.

Rudolph Marcus published his theory of electron transfer in solution from 1956 and later expanded it to include electrode reactions . In 1965 he published a unified theory of both cases. In 1992 he received the Nobel Prize in Chemistry for this .

literature

Web links

Individual evidence

  1. ↑ Julius Tafel : About the polarization during cathodic hydrogen evolution . In: Wilhelm Ostwald, JH van't Hoff (ed.): Journal for physical chemistry, stoichiometry and kinship theory . 50U, no. 6 . Wilhelm Engelmann, February 7, 1905, ISSN  2196-7156 , p. 641–712 , doi : 10.1515 / zpch-1905-5043 ( online in the Internet Archive ).
  2. ^ Academician AN Frumkin and Modern Advances in Electrochemistry. Retrieved September 11, 2018 .
  3. ^ Klaus Jürgen Vetter: Electrochemical Kinetics: Theoretical Aspects. Academic Press, Elsevier Science, Burlington 1967, ISBN 978-1-4832-6639-8 ( elsevier.com [accessed September 12, 2018]).
  4. ↑ "The Historical Development of Electrochemistry". Retrieved September 11, 2018 .
  5. ↑ Rudolph Arthur Marcus: On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. I . In: American Institute of Physics AIP Publishing (Ed.): The Journal of Chemical Physics . tape 24 , no. 5 , May 1956, ISSN  0021-9606 , pp. 966-978 , doi : 10.1063 / 1.1742723 ( scitation.org ).
  6. ↑ Rudolph Arthur Marcus: On the Theory of Electron Transfer Reactions. VI. Unified Treatment for Homogeneous and Electrode Reactions . In: American Institute of Physics AIP Publishing (Ed.): The Journal of Chemical Physics . tape 43 , no. 2 , July 15, 1965, ISSN  0021-9606 , p. 679-701 , doi : 10.1063 / 1.1696792 ( core.ac.uk [PDF]).
  7. ↑ Rudolph A. Marcus: Rudolph A. Marcus - Facts. In: nobelprize.org. Retrieved September 16, 2018 .