Galvani potential

Galvani potential (also called Galvani potential difference, or inner potential difference, Δφ, delta phi)                                                                   in electrochemistry, is the electric potential difference between two points in the bulk of two phases.                                                             These phases can be two different solids (e.g., two metals joint together), or a solid and a                                                                        liquid (e.g., a metal electrode submerged in an electrolyte).

Generally, the Galvani potential difference is measurable only when the two phases have                                                                         identical chemical composition.

The Galvani potential is named after Luigi Galvani.

Galvani potential between two metals

First, consider the Galvani potential between two metals. When two metals are                                                                                     electrically isolated from each other, an arbitrary voltage difference may exist                                                                                        between them. However, when two different metals are brought into electronic                                                                                            contact, electrons will flow from the metal with a lower voltage to the metal                                                                                                  with the higher voltage until the Fermi level of the electrons in the bulk of both                                                                                        phases are equal. The actual numbers of electrons that passes between the                                                                                              two phases is small (it depends on the capacitance between the objects),                                                                                                      and the occupancies of the electron bands are practically unaffected.                                                                                                       Rather, this small increase or decrease in charge results in a shift in all the energy                                                                                     levels in the metals. An electrical double layer is formed at the interface between the two phases.

The equality of the electrochemical potential between the two different phases in contact can be written as:

where:

•  is the electrochemical potential
• j denotes the species which are the carrier of electrical current in the system                                                                                      (which are electrons in metals)
• (1) and (2) denote phase 1 and phase 2, respectively.

Now, the electrochemical potential of a species is defined as a sum of its chemical                                                                                      potential and the local electrostatic potential:

where:

• μ is the chemical potential
• z is the electrical charge carried by a single charge carrier (unity for electrons)
• F is the Faraday constant
• Φ is the electrostatic potential

From the two equations above:

where the difference on the left-hand side is the Galvani potential difference between                                                                                          the phases (1) and (2). Thus, the Galvani potential difference is determined entirely                                                                                       by chemical identity of the two phases; specifically by the difference of the chemical                                                                                            potential of the charge carriers in the two phases.

The Galvani potential difference between an electrode and electrolyte (or between                                                                                        other two electrically conductive phases) forms in an analogous fashion, although                                                                                          the chemical potentials in the equation above may need to include all species                                                                                                involved in the electrochemical reaction at the interface.