Charge transfer coefficient, and symmetry factor (symbols ''α'' and ''β'', respectively) are two related parameters used in description of the

kinetics Kinetics ( grc, κίνησις, , kinesis, ''movement'' or ''to move'') may refer to: Science and medicine * Kinetics (physics), the study of motion and its causes ** Rigid body kinetics, the study of the motion of rigid bodies * Chemical ki ...

electrochemical reaction Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

s. They appear in the Butler–Volmer equation and related expressions. The symmetry factor and the charge transfer coefficient are

dimensionless A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no physical dimension is assigned, with a corresponding SI unit of measurement of one (or 1) ...

. According to an

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

definition, 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: :

\frac   = - \frac    \left( \frac  \right)_

The anodic transfer coefficient (''α_a'') is defined by analogy: :

\frac   =  \frac    \left( \frac  \right)_

where: *

\nu

stoichiometric number Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals ...

, i.e., the number of activated complexes formed and destroyed in the overall reaction (with ''n'' electrons) *

R

universal gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...

T

absolute temperature Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics. Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic wor ...

n

: number of electrons involved in the electrode reaction *

F

Faraday constant In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of S ...

E

electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode (S ...

I

: partial cathodic (anodic) currentIUPAC. ''ELECTRODE REACTION ORDERS, TRANSFER COEFFICIENTS AND RATE CONSTANTS. APPLICATION OF DEFINITIONS AND RECOMMENDATIONS FOR PUBLICATION OF PARAMETERS.'' Pure & Appi. Chern., Vol. 52, pp.233—240, 1979
(pdf)
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Significance

The charge transfer coefficient signifies the fraction of the interfacial potential at an

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

interface that helps in lowering the free

energy barrier In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...

for the electrochemical reaction. The electroactive

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

present in the interfacial region experiences the interfacial potential and electrostatic work is done on the ion by ''a part'' of the interfacial electric field. It is charge transfer coefficient that signifies ''this part'' that is utilized in activating the ion to the top of the free energy barrier.

Batteries and fuel cells

In operating batteries and

fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...

s, charge transfer coefficient is the parameter that signifies the fraction of

overpotential In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly relat ...

that affects the

current density In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional ar ...

. This parameter has had a mysterious significance in

electrochemical kinetics Electrochemical kinetics is the field of electrochemistry that studies the rate of electrochemical processes. This includes the study of how process conditions, such as concentration and electric potential, influence the rate of oxidation and red ...

for over three quarters of the previous century. It can also be said that charge transfer coefficient is the heart of electrode kinetics.

Symmetry factor

The symmetry factor (or barrier symmetry factor) is a coefficient similar to the transfer coefficient, but applicable only to single-step reactions. The sum of anodic symmetry factor and cathodic symmetry factor is equal to one: :

\beta_ + \beta_ = 1

References

{{Reflist Dimensionless numbers of chemistry Electrochemistry