Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate

A New Solid-State Proton Conductor: The Salt Hydrate Based on Imidazolium and 12-Tungstophosphate

“…Upon the following cooling scan (i.e., from +145 to −50 °C), the conductivity shows a linear dependence on inverse temperature and is associated with a reduced activation energy of only 0.26 eV. This value is comparable to that measured for other proton conducting Keggin-ion based salts and acids, such as MgHWP•12H 2 O (0.20 eV for temperatures between 32 and 50 °C; 0.28 eV for temperatures below 32 °C), 8 BaHWP• 7H 2 O (0.27 eV for temperatures between 10 and 50 °C), 9 and H 3 WPA•29H 2 O (0.23 eV around 40 °C). 5 (Note: For the H 3 WPA•nH 2 O hydrate, the activation energy of proton transport increases upon dehydration; from 0.23 eV for 29 water molecules up to 1.07 eV for 6 water molecules.…”

“…It is notable that despite the lower degree of hydration (i.e., n≈4), Imid 3 WP shows proton conductivities higher than any other salt prepared from H 3 WPA (for instance by substituting one or more protons with positively charged ions like Ba 2+ or Mg 2+ ), , as shown by comparing to a selection of already reported data, reproduced in Figure for ease of comparison. This observation is even more important considering that the proton conductivity in the referred materials was measured under a relative humidity of 30% or higher, while our data were obtained under strictly anhydrous conditions (i.e., under flow of dry N 2 gas).…”

“…Furthermore, the reduced activation energy upon dehydration, that is, the change from 0.50 eV for Imid 3 WP•4H 2 O to 0.26 eV for Imid 3 WP•2H 2 O, is consistent with an increased contribution from proton hopping events and with the expected shift of the proton from the imidazolium cation toward the WP anion upon loss of water. 56 It is notable that despite the lower degree of hydration (i.e., n≈4), Imid 3 WP shows proton conductivities higher than any other salt prepared from H 3 WPA (for instance by substituting one or more protons with positively charged ions like Ba 2+ or Mg 2+ ), 8,9 as shown by comparing to a selection of already reported data, reproduced in Figure 4 for ease of comparison. This observation is even more important considering that the proton conductivity in the referred materials was measured under a relative humidity of 30% or higher, while our data were obtained under strictly anhydrous conditions (i.e., under flow of dry N 2 gas).…”

“…As an example, proton conductivity decays of four or five decades have been regularly reported when the number of coordinating water molecules decreases from 21 to 6 . To circumvent these limitations, novel structures based on the (partial) substitution of the acidic protons with metal cations, − or on the incorporation of the heteropolyacid in mesoporous membranes, have been investigated as alternative, more stable solid-state proton conductors. Moreover, the degree of hydration in these solid acids influences the mechanism of proton transport, which varies from mainly vehicular for a high number of water molecules to Grotthuss for the pseudoanhydrous or totally anhydrous state .…”