The process of gas reduction at a MIEC surface can be generalised as: Ox (g) + V n+ ode + ne - ode ⇀ ↽ Red ode (3) where Ox (g) , V n+ ode , e - ode and Red ode represent the oxidised gas species (i.e. O 2(g) or H 2 O (g) ), vacant surface site, mobile electrons and the reduced species, respectively. Superscript n represents the charge of the vacant site and the number of electrons consumed by ambipolar charge transfer reaction. In the theory outlined by Fraggedakis et al., Faradaic reactions at an electrode-electrolyte interface can proceed through a coupled ion-electron transfer (CIET) mechanism. The excess chemical potential landscape in Fig. illustrates three scenarios of ion-electron transfer. When the barrier for ion transfer (IT) is significantly lower than that for the possible electron transfer (ET) step (Fig. ), the charge transfer process is decoupled. As such, the adsorbate will hold the charge of the vacancy which it filled and will develope an electrostatic surface potential. In the second case (Fig. ), both IT and ET can occur simultaneously through a concerted mechanism. In such a situation, there is no stable intermediate state and no charged adsorbate exists on the electrode surface. Therefore, charge separation does not occur and no electrostatic surface potential is observed. Last, when the energy barrier for IT is significantly larger compared to ET, the gas species will be reduced before the rate limiting IT step (Fig. ). Similarly to the ET-limited case, the charge transfer process is decoupled and the adsorbate will hold a charge equal to the number of electrons transferred in the ET step, resulting in an electrostatic surface potential. The white plots represent the 1D chemical potential landscape explored by the reduction reaction. R ads is the adsorption step, R ET is the electron transfer (ET) step, R IT is the IT step and R CIET is the CIET step. The right hand column details a schematic illustration of the shift in electric potential of an electrode experiencing a negative overpotential, where the black and red lines represent the potential at the point of zero charge (PZC) and under a negative applied electric field, respectively. (d) Positive adsorbate induces an electric field vector pointing into the electrode surface where ∆χ -η act has a positive correlation, (e) neutral adsorbate induces no electric field with no correlation, and (f) negative adsorbate induces an electric field vector pointing away from the electrode surface yielding a negative ∆χ -η act correlation.