We investigated the morphology and swelling behavior of a new graft-type of anion exchange membrane (AEM) containing 2-methylimidazolium groups by using a contrast variation small angle neutron scattering (SANS) technique. These AEMs were prepared by radiation-induced grafting of 2-methyl-1-vinylimidazole and styrene into poly(ethylene-co-tetrafluoroethylene) (ETFE) films and subsequent N-alkylation with methyliodide, and possessed both high alkaline durability and high conductivity. Our results showed that the crystalline lamellar and crystallite structures originating from the pristine ETFE films were more or less conserved in these AEMs, but the lamellar d-spacing in both dry and wet membranes was enlarged, indicating an expansion of the amorphous lamellae due to the graft chains introduced in the grafting process and the water incorporated in the swelling process. For the first time, the swelling behavior of the AEMs was studied quantitatively in various water mixtures of water and deuterated water with different volume ratios (contrast variation method), and the morphology of these membranes was elucidated by three phases: phase (1) crystalline ETFE domains, which offer good mechanical properties; phase (2) hydrophobic amorphous domains, which are made up of amorphous ETFE chains and offer a matrix to create conducting regions; phase (3) interconnected hydrated domains, which are composed of the entire graft chains and water and play a key role in promoting the conductivity.

We found unprecedented reverse relationships in anion-exchange membranes (AEMs) for Pt-free alkaline fuel cell systems, i.e., the increase in hydrophobicity increased water uptake and susceptibility to hydrolysis. AEMs with graft copolymers that composed of anion-conducting 2-methyl-N-vinylimidazolium (Im) and hydrophobic styrene (St) units were employed. We characterized two new structures in these AEMs using a small-angle neutron scattering with a contrast variation method. (1) The distribution of graft polymers in conducting (ion channel) or non-conducting (hydrophobic amorphous poly(ethylene-co-tetrafluoroethylene) (ETFE)) phase was evaluated in a quantitative manner. High fraction in conducting layer for AEMs having high grafting degrees was found using the proposed structural model of "conducting/non-conducting two-phase system". (2) Assuming a hard-sphere fluid model, we found AEMs having high St contents and low alkaline durability possessed nanophase-separated water puddles with diameters of 3-4 nm. The AEM having a low St content and the best alkaline durability did not show evident nanophase separation. The above hierarchical structures elucidate the unexpected reverse relationships that the AEM having highly hydrophobic graft polymers was subjected to the morphological transition to give water puddles at nanoscale. The imidazolium groups that were located at the boundary between graft polymers and water puddles should be susceptible to hydrolysis.

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, 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. 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.

Graft-type anion-conducting electrolyte membranes (AEMs) with imidazolium cations on graft polymers were synthesized through radiation-induced graft polymerization of N-vinylimidazole (NVIm) on poly(ethylene-co-tetrafluoroethylene) (ETFE) films, followed by N-propylation and ion-exchange reactions. The N-propylation proceeded quantitatively, whereas the ion-exchange reactions in 1 M KOH at 60 • C were accompanied by partial β-elimination of the imidazolium cations to yield copolymer grafts consisting of poly-(1-vinyl-3-propylimidazolium hydroxide) and double bonds (polyethyne) (AEM2), which exhibited an ion-exchange capacity (IEC) of 0.85 mmol g −1 and ionic conductivity of 10 mS cm −1 . AEM2 showed alkaline stability at 60 • C but it gradually degraded at 80 • C for ca. 150 h. The copolymer-type AEM (AEM3) with an IEC of 1.20 mmol g −1 was prepared through the copolymerization of NVIm with styrene on ETFE films, followed by the same N-propylation and ion-exchange reactions. AEM3 was characterized to be a terpolymer, owing to partial β-elimination and showed higher alkaline durability in 1 M KOH at 80 • C. As a result, it exhibited higher conductivity (>10 mS cm −1 ) for 250 h. Therefore, alkylimidazolium cations in copolymer grafts are a promising anion conducting group for alkaline-durable AEMs. A maximum power density of 75 mW cm −2 is obtained for AEM3 in a direct hydrazine hydrate fuel cell.

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