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Overpotential

The additional potential required to drive an electrochemical reaction beyond its equilibrium potential, impacting battery efficiency.
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The statement of the theorem

Let EeqE_{eq} be the equilibrium potential and EactualE_{actual} be the measured potential under non-equilibrium conditions. The overpotential η\eta is defined as the deviation from equilibrium:\nη=Eactual−Eeqor, in terms of current density j and Tafel slope b (for high overpotentials):η=±blog⁡(∣j∣j0) \eta = E_{actual} - E_{eq} \quad \text{or, in terms of current density } j \text{ and Tafel slope } b \text{ (for high overpotentials):} \quad \eta = \pm b \log \left( \frac{|j|}{j_0} \right) \nwhere j0j_0 is the exchange current density.