Butler–Volmer equation

The Butler–Volmer equation (also known as Erdey-Grúz-Volmer equation) is one of the most fundamental relationships in electrochemical kinetics. It describes how the electrical current on an electrode depends on the electrode potential, considering that both a cathodic and an anodic reaction occur on the same electrode:

 j =  j_0 \cdot \left\{ \exp \left[ \frac { \alpha_a zF } {RT} (E - E_{eq}) \right] - \exp \left[ - { \frac { \alpha_c zF } {RT}} (E - E_{eq}) \right] \right\}

or in a more compact form:

j= j_0 \cdot \left\{ \exp \left[ \frac { \alpha_a zF \eta} {RT} \right] - \exp \left[ - { \frac { \alpha_c zF \eta} {RT}} \right] \right\}
The upper graph shows the current density as function of the overpotential η . The anodic and cathodic current densities are shown as ja and jc, respectively for α=αac=0.5 and j0 =1mAcm-2 (close to values for platinum and palladium). The lower graph shows the logarithmic plot for different values of α


where:

The right hand figure shows plots valid for αa=1-αc.

The equation is named after chemists John Alfred Valentine Butler[1] and Max Volmer.

Mass-transfer control

The previous form of Butler–Volmer equation is valid when the electrode reaction is controlled by electrical charge transfer at the electrode (and not by the mass transfer to or from the electrode surface from or to the bulk electrolyte). Nevertheless, the utility of the Butler–Volmer equation in electrochemistry is wide, and it is often considered to be "central in the phenomenological electrode kinetics".[2]

In the region of the limiting current, when the electrode process is mass-transfer controlled, the value of the current density is:

i_{limiting} = \frac {zFD} {\delta} C^*

where:

The above form simplifies to the conventional one (shown at the top of the article) when the concentration of the electroactive species at the surface is equal to that in the bulk.

The limiting cases

There are two limiting cases of the Butler–Volmer equation:

i=i_0\frac {zF} {RT} (E-E_{eq});
E-E_{eq} = a - b \log(i) for a cathodic reaction, when E << Eeq, or
E-E_{eq} = a + b \log(i) for an anodic reaction, when E >> Eeq

where a and b are constants (for a given reaction and temperature) and are called the Tafel equation constants. The theoretical values of a and b are different for the cathodic and anodic processes.

See also

References

  1. Mayneord, W. V. (1979). "John Alfred Valentine Butler. 14 February 1899-16 July 1977". Biographical Memoirs of Fellows of the Royal Society 25: 144–126. doi:10.1098/rsbm.1979.0004.
  2. J. O'M. Bockris, A.K.N.Reddy, and M. Gamboa-Aldeco, "Modern Electrochemistry 2A. Fundamentals of Electrodics.", Second Edition, Kluwer Academic/Plenum Publishers, p.1083, 2000.
This article is issued from Wikipedia - version of the Friday, February 19, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.