Durability of Sodium Selective Super Ionic Conductors in Sodium and Chloride Containing Aqueous Environments at pH 2-11

NASICON (Na3Zr2Si2PO12) ceramic specimens were synthesized by dry pressing and sintering Na2CO3, ZrP2O7, SiO2, and ZrO2 powders. The resulting NASICON membranes were exposed to 1M NaCl solutions titrated to pH values between 2 and 11. Accelerated electrochemical exposure testing was performed, which involved exposing thin discs of NASICON to both static and flowing 1M NaCl solutions with either HCl or NaOH solutions to pH 2, 7, or 11, under an applied effective current of 65 mA/cm2 to drive ion transport until failure was observed. Three and four electrode testing was performed to observe if any catastrophic failure would occur due to electrochemical exposure under the applied conditions. The three electrode set-up was a static solution set-up with 1M NaCl titrated to different pH for exposure conditions. The specimens were characterized by XRD, SEM, and XPS. At pH 7 and 11, no degradation of NASICON was observed, although current profiles suggest higher pH solutions tend to fail faster than the low pH. At pH 2, it was found that a buildup of Na on the surface and apparent depletion of Si/P from the surface lead to a degradation of the NASICON ceramic specimen