There has been enormous research in ionic conducting materials that enable electrochemical devices. The vast majority of research has focused on single-phase materials capable of conducting a single ionic species . Some single-phase materials, such as rare-earth oxide doped perovskites, are capable of multi-species transport (e.g., [H ! ], [O "# ], and [e $ , h • ]) . However, it is not thermodynamically possible to tune the total conductivity of the singlephase with simultaneous transport of multiple charged species . Recently, the third class of ionic conductors has emerged where a composite consisting of multiple phases is able to accomplish uncoupled simultaneous transport of multiple ionic species. For example, Na-β′′alumina + yttria-stabilized zirconia (YSZ) , Na-β''-alumina + samaria-doped ceria (SDC) , sodium zirconogallate+YSZ and others are notable examples of composite materials having uncoupled cation and anion transport (e.g., Na ! and O "# ) occurring through two separate but contiguous phases. These composites could be utilized for other functions such as providing selective chemical species transport to inject or remove specific species from a gas stream or chemical reaction. Moreover, one could envision numerous composite systems with features such as an increased number of mobile species for enhanced electrochemical performance and ameliorated mechanical properties of brittle electrolytes, such as strengthening observed in Na -B'' -Alumina + YSZ . Sodium-ion storage devices have been considered the most promising candidate for largescale energy storage systems owing to sodium abundancy in nature and the low price of the resources compared to other candidates such as lithium . Hence, the development of highperformance sodium-ion devices for applications in energy storage devices, including sodium-ion batteries (SIBs) , sodium-sulfur batteries , sodium-air batteries , or sodium-ion hybrid capacitors is of great importance to improve synthesis and electrochemical properties for highly efficient utilization of renewable energies. High one-dimensional sodium ion conductivity has been observed when a tetravalent ion like Zr &! or Ti &! occupied some of Ga '! sites in sodium gallate, to form Na (.* Zr (.' Ga &.* O + (hereafter NZGO) or Na (.* Ti (.' Ga &.* O + (hereafter NTGO) . This fast sodium ion conductor contains one-dimensional tunnels (diameter ~2.5Å) serving as pathways for sodium-ion transport in a monoclinic crystal system . These channels are large enough to make the NZGO an excellent sodium ion conductor and additionally a good candidate for ion exchange for other monovalent cations such as K ! , Li ! , and Ag ! possibly using molten salts. The channels lie parallel to the b-axis with continuous hexagonal cross-section and low activation energy (E , ≈ 0.25eV) for sodium ions. Prior work on pure NZGO and NTGO noted limitations of these materials primarily due to the high energy cost of synthesis (>1600 °C for up to 48 hours) , formation of a liquid phase , the requirement for encapsulation of samples within platinum crucibles , and low mechanical strength of the pure single-phase NZGO and NZTO. Our prior work showed that the NZGO + YSZ composite could be successfully fabricated using Ga " O ' + YSZ mixture via a vapor phase process. The Electrochemical Impedance Spectroscopy (EIS) indicated the sodium-and oxygen-ion conductivity of the NZGO+YSZ. In this work, we attempt to study the effect of the microstructure of as-sintered Ga " O ' + YSZ samples on the vapor phase conversion kinetics, discussing the conversion mechanism and characterizing the converted NZGO + YSZ electrolyte by diffraction, microscopy, and spectroscopy techniques. The open-circuit voltage (OCV) of the cell fabricated using the NZGO + YSZ composite electrolyte corresponding to Goldman-Hodgkin-Katz potential was measured under oxygen and sodium chemical potential gradient and compared with the Nernst potential before and after reaching equilibrium time to show the effect of ion transient process in OCV measurements. Finally, the sample's degradation was studied after ~400 hours of operation at 1190 °C.