Elucidation of the Morphology of Imidazolium-Based Anion Exchange Membranes for Alkaline Anion Fuel Cells

Abstract

Due to the growing concerns on the depletion of petroleum based energy resources and the climate change, polymer electrolyte membrane fuel cells (FCs) technologies have received much attention in recent years owing to their high efficiencies and low emissions. Among them, alkaline anion exchange membrane fuel cell (AEM-FC) is a good choice, because it avoids the consumption of acid-resistant precious metal catalysts and costs up. However, the biggest challenge in developing AEM-FCs is to fabricate AEM with high ion conductivity and mechanical stability without chemical deterioration at elevated pH and temperatures. So far, most strategies were focused on synthesizing new thermally and chemically durable fluorinated and aromatic polymers. For the first time, our group tried to develop the new type of 2Me-AEMs by radiation-induced grafting of 2-methyl-1-vinylimidazole and styrene into poly(ethylene-co-tetrafluoroethylene) (ETFE) films and a subsequent N-alkylation with methyliodide. The resultant AEMs exhibit high ion conductivity (> 100 mS/cm) and longer alkaline durability, owing to the fact that the methyl protecting group at 2-imidazole position prevented the ring-opening degradation. 2Me-AEM with an IEC(ion exchange capacity) of 1.82 mmol/g shows the best well-balanced properties required for fuel cell applications. All these findings on one hand, are the result of sample preparation procedure of the radiation grafting method and the introduction of alkylimidazolium cations as an anion conducting group, and on the other hand, are believed to be controlled by the microphase separated structures of the membranes in the hydrated state. In this work, we aim to elucidate the morphology of these 2Me-AEMs and understand the structure related unique properties such as the mechanical property and the anion conductivity. We investigated the morphology and swelling behavior of these new graft-type AEMs by using contrast variation small angle neutron scattering (SANS) technique, performed mainly on KWS-2 SANS diffractometer operated by Juelich Centre for Neutron Science at the neutron source Heinz Maier-Leibnitz (FRM II reactor) in Garching, Germany. Our results showed that the scattering intensity and the shape of the profiles vary significantly upon grafting, but change little upon alkylation; while the incorporation of water in the hydrophilic domains upon swelling, not only increases the intensity and domain size, but also leads to the excess scattering at high-q region with q being the scattering vector. From these results, we concluded 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 were 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. We further quantitatively studied the swelling behavior of the AEMs 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 to promote the conductivity.