Chemically Driven Ion Exchanging Synthesis of Na₅YSi₄O₁₂-Based Glass-Ceramic Proton Conductors

We have developed 12-membered silica-tetrahedra-ringed Na₅YSi₄O₁₂-type sodium ion conducting glass-ceramics on the basis of the composition Na_3+3x-yR_1-xP_ySi_3-yO₉ (R: rare earth elements; denoted as Narpsio); especially, the material of Na₄Y_0.6P_0.2Si_2.8O₉ with the combined parameters of (x, y) = (0.4, 0.2) gives rise to the maximum conductivity of 1 × 10^-1 Scm^-1 at 300 °C. Because glass-ceramics generally have the advantage of structural rigidity and chemical durability over sintered polycrystalline ceramics, the present study employed glass-ceramic Narpsio to perform chemically driven ion exchange of Na⁺ with protonated water molecules with an aim to produce a proton conductor. The ion exchange was carried out in a hydrochloric acid solution by changing immersion time, temperature, and acid concentration. The ion exchanged Na₄Y_0.6P_0.2Si_2.8O₉-based glass-ceramics were analyzed by the complex impedance method, and the proton conductivity was found to exhibit 3 × 10^-4 Scm^-1 at 300 °C with the activation energy of 59 kJ/mol. The dependence of humidity-sensitive conductivity of the ion exchanged bulk glass-ceramics was also examined; the conductivity increased almost linearly from 0.6 × 10^-4 Scm^-1 in dry air to 1.5 × 10^-4 Scm^-1 in 75% humid ambience at 300 °C. Thus, the ion exchanged glass-ceramics can be considered to be high temperature proton conductors as well as humidity sensors.