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Effect of the ammonium ion on proton conduction in porous ionic crystals based on Keggin‐type silicododecatungstate
Author(s) -
Miyazawa Satoru,
Hosono Reina,
Osuga Ryota,
Kondo Junko Nomura,
Uchida Sayaka
Publication year - 2018
Publication title -
acta crystallographica section c
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.304
H-Index - 17
ISSN - 2053-2296
DOI - 10.1107/s2053229618008227
Subject(s) - proton , conductivity , activation energy , proton transport , ionic conductivity , chemistry , ionic bonding , ammonium , inorganic chemistry , crystal structure , hydrogen bond , crystallization , ion exchange , ion , crystallography , molecule , electrolyte , electrode , organic chemistry , physics , quantum mechanics
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 NH 4 + into the crystal structure, because NH 4 + can form hydrogen bonds and function as a proton carrier. According to these considerations, NH 4 + was introduced into the porous structure of A 2 [Cr 3 O(OOCH) 6 (etpy) 3 ] 2 [α‐SiW 12 O 40 ]· n H 2 O ( 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, (NH 4 ) 2 [Cr 3 (CHO 2 ) 6 O(C 7 H 9 N) 3 ] 2 [α‐SiW 12 O 40 ]·6H 2 O, 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 NH 4 + 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 NH 4 + and/or H 3 O + under low RH (vehicle mechanism).