In a previous study, we determined that the electrostatic surface potential had a profound influence on the gas reduction kinetics, where a relationship ∆χ ̸ = 0 is desirable. Few experiments have investigated the complex ∆χ -η act relationship, leaving this phenomenon poorly understood despite its kinetic merit. By analysing the shift in outer work function using in operando X-ray photoelectron spectroscopy (XPS) over an applied overpotential range, Feng et al. measured the ∆χ -η act relationship for three different electroreduction systems illustrated in Fig. . Mechanistic details given in Table and Table shows the overpotential derived from each reaction. Upon expanding the electrochemical potential terms we find the general solution neη act = ±T ∆S ± ne∆χ. Thus, each derivation finds a relationship consistent with which electrons and adsorbates interact. They also concluded that the space charge potential at the electrode surface was invariant under an applied overpotential (i.e. H 2 O and CO 2 on CeO 2 and O 2 on LaFeO 3 ). While this finding points towards the existence of an active mechanism controlling electrochemical performance of electrified interfaces, the theoretical understanding is still at its early stages. Here, we rationalise the mechanistic basis of these observations. Table : Steps of gas reduction at the MIEC surface for three systems investigated experimentally written in Kröger-Vink notation, where the reported rate limiting step for each process is given in bold. For H 2 O reduction, steam adsorption is an IT step which consumes an oxygen vacancy (V O ) on the surface. This is followed by the rate limiting ET step which consumes two polarons (Ce ′ Ce ) and forms H 2(g) . For CO 2 reduction, the first step forms an adsorbed neutral carbonate (CO x 3O ) which then undergoes the rate limiting CIET step forming CO (g) . For O 2 reduction, we assume fast dissociate adsorption and ET forming a negatively charged adsorbate (O ′ ads ) and a polaronic hole (Fe • Fe ). The second step is the rate determine incorporation step which includes a single electron transfer and consumes an oxygen vacancy. The activation overpotential equations are given for each reaction step derived in Table where ∆S conf is the change in configurational entropy under nonequilibrium.