Electrochemical characterization: Electrochemical characterization was carried out under two different scenarios. Open-Circuit Voltage (OCV) measurements were made under applied oxygen chemical potential gradient and sodium chemical potential gradient. In the first series of experiments, an oxygen chemical potential gradient was achieved by exposing one side of the sample to a reducing atmosphere consisting of oxygen, hydrogen and water vapor (where, p / ! 0 , p / ! ≫ p 0 ! ) while the other side was exposed to air (p 0 ! ≈ 0.21 atm). In the second series, a sodium chemical potential gradient was achieved by fabricating two different electrodes with a variation of sodium chemical potential in two sets. The first set of experiments was conducted using Ga " O ' + Na-β''-gallia and 𝛽+𝛽''-gallia where synthesis methods are described elsewhere and the second set using Na-β/β''-alumina and Al " O ' + Na-β''-alumina. A fine Platinum gauze (Alfa Aesar) was placed over the screen-printed Pt paste (Heraeus) as current collectors on both sides of the fully converted sample (NZGO + YSZ) and the device was mechanically pressed together with screws. For all experiments, pristine NZGO + YSZ pellets were used. Diffusional mass transport in these devices needs to reach equilibrium. Therefore, the experiments were conducted before and after the necessary relaxing time to reach equilibrium. The time to reach the equilibrium, τ, is a function of sample thickness, l, and chemical diffusion coefficient of the mobile species, D b , which according to Thangadurai and Weppner , in the absence of the electrical field, can be estimated by