Processes at Electrodes

The electrode interacts with ions in the electrolyte solution at its interface. The rate of oxidation and reduction depends on the speed at which electrons can transfer through this interface. As ions attach to or leave the electrode surface, the electrode acquires a charge, and an electrical potential forms across the interface, making the process more difficult to reach equilibrium. The charge on the electrode affects the local ion concentrations in the solution, though thermal motion disrupts this. Using excess supporting electrolytes minimizes these local changes. Early models treated the interface as a simple electrical double layer of positive and negative sheets of charge.

More advanced models introduce a gradual change in structure, like the Helmholtz layer of solvated ions at the interface or the diffuse double layer of the Gouy–Chapman model. The Stern model combines the rigid Helmholtz layer near the electrode surface with the dispersed ions of Gouy–Chapman beyond it. The potential difference driving current is called the overpotential, denoted by η.

The current density, j, relates the rate of electron transfer occurring at an electrode to the overpotential and can be either cathodic j_c or anodic j_a. The net current density is defined as the difference between the cathodic and anodic current densities. When j_a > j_c, the net current is anodic, resulting in net oxidation of the species in solution. Conversely, when j_c > j_a, the net current is cathodic, and the overall process is the reduction. The current density at an electrode is expressed by the Butler–Volmer equation. Within this equation, α represents the transfer coefficient whose value lies in the range of 0 to 1, and 𝑓 is defined as F/RT, where F is the faraday constant, R is the universal gas constant, and T is the absolute temperature in kelvin (K). A value of 0 for α indicates that the activated complex resembles the reactants, while a value of 1 indicates that it closely resembles the products. Empirical observation often finds α to be approximately 0.5.