energy of activation of an electrode reaction

This is defined by the equation:

$U ‡ = − R T ( ∂ ( ln I 0 ) ∂ T −1 ) p , c j , ...$

where $I 0$ is the exchange current. At any overpotential $η$ it is defined by the equation:

$U ‡ η = − R ( ∂ ( ln ( | I | ) ) ∂ T −1 ) p , η , c j , ...$

where $I$ is the current passing from the electrode into the electrolyte.

Source:

PAC, 1974, 37, 499 (Electrochemical nomenclature) on page 516