Effect of the ammonium ion on proton conduction in porous ionic crystals based on Keggin-type silicododecatungstate

Abstract

Proton conduction in crystalline porous materials has received much attention from basic scientific research through to practical applications. Polyoxometalates (POMs) can efficiently transport protons because of their small superficial negative charge density. A simple method for enhancing proton conductivity is to introduce NH4+ into the crystal structure, because NH4+ can form hydrogen bonds and function as a proton carrier. According to these considerations, NH4+ was introduced into the porous structure of A2[Cr3O(OOCH)6(etpy)3]2[α‐SiW12O40]·nH2O (A = Li, Na, K and Cs; etpy = 4‐ethylpyridine) (I‐A+) via topotactic cation exchange. The resulting compound, diammonium tris(4‐ethylpyridine)hexaformatooxidotrichromium α‐silicododecatungstate hexahydrate, (NH4)2[Cr3(CHO2)6O(C7H9N)3]2[α‐SiW12O40]·6H2O, showed high proton conductivity and low activation energy under high relative humidity (RH), suggesting that protons migrate efficiently via rearrangement of the hydrogen‐bonding network formed by the NH4+ cations and the waters of crystallization (Grotthuss mechanism). The proton conductivity and activation energy greatly decreased and increased, respectively, with the decrease in RH, suggesting that protons migrate as NH4+ and/or H3O+ under low RH (vehicle mechanism).

Monoammonium salts of multiprotic acids as dopants for proton-conductive hydrogel membranes: the effects of anions

“… 11 The higher conductivities of the hydrogels doped with the monoammonium salts were attributed to the protonated ammonium cation, which can enhance proton conduction via both forming hydrogen bond and acting as proton carrier. 26 At 80 °C, ionic conductivities of HG-AHSO 4 and HG-AH 2 PO 4 climbed to 309 and 65 mS cm −1 , respectively.…”