A phenomenological treatment of surface kinetics is meant to provide a simple, yet effective, ansatz for the resulting current density J surf at the surface. Common formulations here include a potential-dependent Butler-Volmer (BV) expression, or just the simple assumption that this J surf is a constant. In the latter case, J surf is simply approximated based on a specific faradaic efficiency and directly fed as input into the transport model with no effect from the applied potential U or interfacial species' concentration C. On the other hand, the BV formalism (which simplifies into a Tafel expression at large overpotentials) represents the more sophisticated ansatz, but comes at the price of relying upon a set of empirical parameters that must be a priori defined. These parameters primarily include the exchange current density J 0 and cathodic/anodic transfer (or symmetry) coefficient α within the standard BV formalism. Most often the basic BV formulation is extended by an additional concentration-dependent term that allows for a more advanced, transport-aware model :