where µ M,0 e is an intrinsic bulk material property while the activity of electrons (a e (r)) and the inner potential (ϕ(r)) vary spatially. For metallic electrodes, the activity term is usually omitted but it plays a crucial role in SCEs due to the low number of charge carriers which in turn leads to poor screening of the interfacial charge and the creation of a SCR. In particular, a e (r) and ϕ(r) are correlated because under equilibrium conditions μM e is constant throughout the system: this coupling leads to local variations in the electrostatic potential and charge distribution within the SCR as shown in Figure . It should also be stressed that the applied electrode potential is a bulk property and it is therefore measured from the "backside" or bulk of the SCE. This means that the electrode potential needs to be measured outside the SCR, i.e. when the bulk values of ϕ M and a e have been reached, see Figure .