Imidazolium-based anion exchange membranes for alkaline anion fuel cells: (2) elucidation of the ionic structure and its impact on conducting properties. Issue 45 (1st November 2017)

• Record Type: Journal Article
• Authors: Yoshimura, Kimio; Zhao, Yue; Hasegawa, Shin; Hiroki, Akihiro; Kishiyama, Yoshihiro; Shishitani, Hideyuki; Yamaguchi, Susumu; Tanaka, Hirohisa; Koizumi, Satoshi; Appavou, Marie-Sousai; Radulescu, Aurel; Richter, Dieter; Maekawa, Yasunari
• Is Part Of: Soft matter. Volume 13:Issue 45(2017)
• Page Start: 8463
• Page End: 8473
• Publication Date: 2017-11-01
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/c7sm01774j
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : We report a novel ionic structure of dispersed ions and its impact on conducting properties. Abstract : In our previous study ( Soft Matter, 2016, 12, 1567), the relationship between the morphology and properties of graft-type imidazolium-based anion exchange membranes (AEMs) was revealed, in that the semi-crystalline features of the polymer matrix maintain its mechanical properties and the formation of interconnected hydrophilic domains promotes the membrane conductivity. Here, we report a novel ionic structure of the same graft-type AEMs with different grafting degrees, analyzed using a small-angle X-ray scattering method under different relative humidity (RH) conditions. The characteristic "ionomer peak" with a corresponding correlation distance of approximately 1.0 nm was observed at RH < 80%. This distance is much smaller than the literature-reported mean distance between two ionic clusters, but close to the Bjerrum length of water. Since the representative number of water molecules per cation, n w, was small, we proposed that dissociated ion-pairs are distributed in the hydrophilic domains (ion-channels). At RH < 80%, ion-channels are disconnected, however in liquid water, they are well-connected as evidenced by the sharp increase in n w . The disconnected ion-channels even under relatively high RH conditions should be a substantial factor for the low power generation efficiency of AEM-type fuel cells.