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Humidity Influence on the CO 2 Response of Potentiometric Sensors Based on NASICON Pellets with New Compositions, Na 3 Zr 2−( x /4) Si 2− x P 1+ x O 12 ( x = 1.333)
Author(s) -
Aono Hiromichi,
Sadaoka Yoshihiko,
Montanaro Laura,
Bartolomeo Elisabetta,
Traversa Enrico
Publication year - 2002
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2002.tb00136.x
Subject(s) - fast ion conductor , electromotive force , analytical chemistry (journal) , pellets , potentiometric titration , conductivity , ceramic , relative humidity , desorption , electrolyte , materials science , gas composition , concentration cell , electrochemistry , chemistry , electrode , adsorption , chromatography , metallurgy , thermodynamics , physics , quantum mechanics , composite material
CO 2 ‐gas sensors were fabricated using NASICON‐type dense ceramics with new compositions in the Na 3 Zr 2−( x /4) Si 2− x P 1+ x O 12 system. The bulk ceramics were prepared from sol‐gel‐processed powders. Dense electrolytes were obtained for compositions with x = 0.667 and x = 1.333. An improved sinterability was observed with respect to NASICON with conventional composition, which was attributed to liquid‐phase sintering. The CO 2 ‐gas sensors using these dense samples showed a stable electromotive force (emf) response in dry atmosphere that was similar to the theoretical Nernstian value for a two‐electron electrochemical reaction. This was observed also for the NASICON sample with composition x = 1.333 that showed a conductivity far lower than that of the NASICON with conventional composition. The emf changed quickly with changes of the CO 2 ‐gas concentration, and steady‐state values were observed. The response time, in adsorption and desorption of CO 2 , was very fast, especially at high CO 2 concentrations. The influence of humidity on the CO 2 ‐sensing performance was investigated. A lower sensitivity and slower response were obtained in humid CO 2 gas, especially at low CO 2 concentrations. CO 2 ‐sensing measurements at various gas‐flow rates were performed to evaluate the reactions occurring at the measuring electrode. The occurrence of a side reaction on the measuring electrode was observed, i.e., the formation of sodium oxides. In dry gas, the reaction took place at low CO 2 concentrations and small flow rates, whereas the reaction was strongly enhanced in humid environments and occurred over the entire CO 2 concentration range. However, the sensor performance recovered after switching from humid gas to dry gas. This demonstrated that the humidity affects the emf because of the Na 2 O x formation at the electrode, and, thus, the solid electrolyte itself was not degraded by humidity.