Charge transfer coefficient

Charge transfer coefficient, and symmetry factor (symbols α and β, respectively) are two related parameters used in description of the kinetics of electrochemical reactions. They appear in the Butler–Volmer equation and related expressions.

The symmetry factor and the charge transfer coefficient are dimensionless.^[1]

According to an IUPAC definition,^[2] for a reaction with a single rate-determining step, the charge transfer coefficient for a cathodic reaction (the cathodic transfer coefficient, α_c) is defined as:

${\displaystyle {\frac {\alpha _{\rm {c}}}{\nu }}=-{\frac {RT}{nF}}\left({\frac {\partial \ln |I_{\rm {red}}|}{\partial E}}\right)_{p,T,c_{i}^{\rm {interface}}}}$

The anodic transfer coefficient (α_a) is defined by analogy:^[3]

${\displaystyle {\frac {\alpha _{\rm {a}}}{\nu }}={\frac {RT}{nF}}\left({\frac {\partial \ln |I_{\rm {ox}}|}{\partial E}}\right)_{p,T,c_{i}^{\rm {interface}}}}$

where:

• ${\displaystyle \nu }$: stoichiometric number, i.e., the number of activated complexes formed and destroyed in the overall reaction (with n electrons)
• ${\displaystyle R}$: universal gas constant
• ${\displaystyle T}$: absolute temperature
• ${\displaystyle n}$: number of electrons involved in the electrode reaction
• ${\displaystyle F}$: Faraday constant
• ${\displaystyle E}$: electrode potential
• ${\displaystyle I}$: partial cathodic (anodic) current^[4][5]