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).
Keywords: ammonium ion; crystal structure; polyoxometalate; porous crystal; proton conduction; silicododecatungstate.