Nanometer Scale Proton Conductivity and Dynamics of CsHSO₄ and H₃PW₁₂O₄₀ Composites under Non-Humidified Conditions

Abstract

We prepared composites of CsHSO4 and H3PW12O40·6H2O (WPA-6H2O), xCsHSO4·(100 − x)WPA-6H2O (mol %) by mechanical milling (MM). The nanometer scale proton dynamics and the non-humidified proton conductivity were investigated using 1H nuclear magnetic resonance (NMR) and alternating current impedance spectroscopies. The non-humidified proton conductivities at 100 °C for pure CsHSO4 and WPA-6H2O were 4.0 × 10−7 and 1.0 × 10−7 S/cm, respectively. On the other hand, the proton conductivity increased significantly to approximately 3 × 10−3 S/cm for the x = 80 and x = 90 composites. Proton conductivity, 1H spin−lattice relaxation time (T 1) and theoretical calculation results suggested the formation of a reacted region with high proton conductivity around the WPA-6H2O cluster after the MM. T 1 measurements for the 90CsHSO4·10WPA-6H2O (mol %) composite revealed that there were two relaxation components, and that the long T 1 and short T 1 components could be correlated to proton motion inside the WPA-6H2O cluster and inside the reacted region, respectively. From an analysis of the short T 1, we found that the proton jumping rate at 100 °C of CsHSO4 increased approximately 1,600 times after mixing CsHSO4 and WPA-6H2O. The proton conductivity of the reacted region was estimated to be 10 × 10−3 S/cm from a theoretical calculation using the effective medium theory.

Phase transitions in CsHSO4 up to 2.5GPa: Impedance spectroscopy under pressure

“…From other side, the 1 H MAS NMR data [11] demonstrated about the same activation energy for the proton mean residence time ~0.36 eV in superprotonic phase at room pressure and twice less activation energy 0.26 eV in ferroelastic phase CsHSO 4 (II). In [19] these results have been reproduced, the values 0.35 eV and 0.3 eV for superprotonic and phase II were reported, respectively. This means that in superprotonic phase I the density of mobile protons is fixed and does not depend on pressure and temperature, N= 8.48·10 27 m -3 which was calculated from the crystal structure [19].…”

“…In [19] these results have been reproduced, the values 0.35 eV and 0.3 eV for superprotonic and phase II were reported, respectively. This means that in superprotonic phase I the density of mobile protons is fixed and does not depend on pressure and temperature, N= 8.48·10 27 m -3 which was calculated from the crystal structure [19]. Thus, the super ionic phase I is a typical protonic conductor, which appears above T c .…”

“…The dielectric relaxation time on pressure at constant temperature increases within 1 order of magnitude from 0.1 MPa to 0.75 GPa. The slope of the temperature dependence of dielectric relaxation time on heating at 0.1 MPa, 0.5 GPa and 0.75 GPa is about the same as for the temperature dependence of the mean relaxation time of protons (τ H ) estimated from NMR [19]. The absolute values of dielectric relaxation time τ during heating are less than τ H for about 1 order of magnitude at 0.1 MPa, and 0.5 orders of magnitude at 0.5 and 0.75 GPa.…”

“…Here we use the conventional nomenclature of phases taken from [7,10,11], which is traditionally used for CsHSO 4 polymorphs. The range of dielectric relaxation times in phase III and II corresponds within one order of magnitude to the range of the mean residence time from 1 H MAS NMR [19]. At temperature above T c =T II-I transition the dielectric relaxation time cannot be derived from the electrical impedance measurements.…”

Diffusion in Solid Proton Conductors: Theoretical Aspects and Nuclear Magnetic Resonance Analysis

“…Matsuda et al studied proton conductivities under nonhumidified conditions of CsHSO 4 and dodecatungstophosphoric acid (WPA-6H 2 O) composites [80,81]. The proton dynamics was explored by determination of the spin-lattice relaxation time, T 1 , to explain the significant enhancement of the proton conductivity under dry conditions obtained by mixing CsHSO 4 and WPA-6H 2 O.…”