Current yield (electrochemistry)

In electrochemistry and thus also in electrolysis , including electroplating , the current yield is the proportion of the electrical charge that leads to the desired process.

Basics

The current efficiency η is defined as

${\displaystyle \eta \ ={\frac {Q_{\mathrm {nutz} }}{Q_{\mathrm {gesamt} }}}}$

with the theoretical charge Q util required for the _{use process} for the total charge Q _total . Accordingly, the current efficiency is a dimensionless number between 0 and 1.

In the case of electroplating with a metal , the current yield describes that portion of the charge that is used to deposit the desired metal. Undesired side reactions, often e.g. B. the evolution of hydrogen, consume part of the electricity and thus reduce the power yield. In the case of accumulators , the current yield is usually referred to as Coulomb efficiency.

Both the actually obtained, experimentally determined or determinable mass of the desired product m _exp , e.g. B. the mass of the metal layer deposited in the electroplating, as well as the theoretically expected mass m _th based on the total charge are proportional to the respective charges according to Faraday's laws :

${\displaystyle m_{\mathrm {nutz} }={\ddot {A}}_{e}Q_{\mathrm {nutz} }}$

and

${\displaystyle m_{\mathrm {th} }={\ddot {A}}_{e}Q_{\mathrm {ges} }}$

with the electrochemical equivalent Ä _e . Therefore the following applies to the current yield:

${\displaystyle \eta \ ={\frac {m_{\mathrm {exp} }}{m_{\mathrm {th} }}}={\frac {m_{\mathrm {exp} }}{{\ddot {A}}_{e}Q_{\mathrm {ges} }}}={\frac {zFm_{\mathrm {exp} }}{MQ_{\mathrm {ges} }}}}$.

Current yield in galvanic coating processes

The deposition of silver ( E ° = +0.8  V for Ag ⁺ / Ag) and copper ( E ° = +0.35 V for Cu ²⁺ / Cu) takes place at low current densities with practically no hydrogen evolution ( E ° = 0 V) and can therefore achieve current yields of one ( η = 1 within the scope of the measurement accuracy) or almost one ( η ≈ 1). This was previously used in silver coulometers and copper coulometers to measure charge. The electrochemical deposition of chromium ( E ° = −0.76 V for Cr ³⁺ / Cr), which is important for chrome plating, on the other hand, must take place at potentials at which hydrogen evolution is unavoidable. Therefore, the current efficiency in chrome plating is small. For an as usual on the electrode surface, i. H. the surface of the material to be coated, related current density of 50 A / dm², it is around 18%.

Current yields depend heavily on the current densities used. Due to the associated high overvoltages, high current densities favor the development of hydrogen, since the potential of the working electrode can drop below 0 V even during copper or silver deposition. In addition, current yields depend on the electrolyte composition and the temperature .

Individual evidence

1. ↑ Klaus J. Vetter : Electrochemical Kinetics . 1st edition. Springer, Berlin, Heidelberg 1961, ISBN 978-3-642-86547-3 , 5th mixed potentials and electrolytic corrosion, § 177 current yield, p.  590–591 ( limited preview in Google Book search).
2. ↑ ^{a b c} Klaus-Michael Mangold: Glossary: ​​Basic terms of electrochemistry. DECHEMA, Society for Chemical Technology and Biotechnology eV, 2008, accessed on April 13, 2020 .  
3. ^ Basics of electrochemistry. In: Formel-Sammlung.de> Chemistry> Electrochemistry. Adeos Media GmbH, accessed on April 15, 2020 .  
4. ↑ current efficiency. In: Lexicon of Chemistry. Spektrum Akademischer Verlag, Heidelberg, 1998, accessed on April 13, 2020 .