Solid-state bi-electrolyte carbon dioxide sensors with an yttira stabilized zirconia (YSZ) and sodium beta alumina (NBA) heterojunction were fabricated using the tape casting method to measure CO2 concentrations in the air. The enhanced interface between NBA, expressed by (1 + x) Na2O 11 Al2O3 and YSZ was made by the liquid phase sintering method while varying the ratio of Na2O/Al2O3 from x = 0 to x = 0.6. YSZ/Pt electrode was used as an air reference, maintaining a constant oxygen concentration of 21%, and Na2CO3 was used as a CO2 sensing electrode. The responses of the sensors to various concentrations of CO2 were evaluated at temperatures ranging from 550 degrees C to 650 degrees C. The electric potential difference of the sensors exhibited Nernstian behavior with sensitivity greater than 99% of the theoretical value at 650 degrees C. The activity of Na2O at the bi-electrolyte junction remained constant, showing invariance with the CO2 concentrations tested. The time for the initial stabilization was as short as 66 s. (C) 2017 Elsevier B.V. All rights reserved.