The ordered side chain imidazole functionalized cross-linked anion exchange membrane

Abstract

In recent years, the development of alkaline anion exchange membrane fuel cells (AEMFCs) has received widespread attention. On the one hand, non-noble metals can be used as catalysts in alkaline anion exchange membrane fuel cells (AEMFCs), such as silver and nickel which reduced the cost of fuel cell devices. On the other hand, the hydroxide ions move in the opposite direction with the fuel, which reduce fuel permeability and the cost on other aspects. Therefore, the development prospect of alkaline fuel cells is good, while anion exchange membrane, as the core component of alkaline fuel cells, has problems such as the insufficient ion conductivity and alkaline stability, which hinder the successful realization of AEMFCs technology. Until now, many cationic groups have been widely investigated, such as quaternary ammonium, imidazolium, guanidinium and phosphonium. Quaternary ammonium group and imidazolium group are the most commonly used cationic group. The experimental results suggest that imidazole-type anion exchange membrane have better performance. Due to the existence of the conjugated π bond in the unique N -heterocyclic structure, imidazolium grafted membranes exhibit not only high ionic conductivity, but also excellent chemical and thermal stability. The formation of cross-linked network structure is advantageous in suppressing the swelling of the membrane, thereby improving the dimensional stability of the membrane. Meantime, we introduce 1-vinylimidazole to improve the performance of the 1-methylimidazole grafted membrane. This monomer contains not only imidazolium cation groups, but also double bonds can be cross-linked. The imidazole group can transfer OH- ions. The dense network structure formed by double bonds cross-linking makes membranes have good mechanical properties and dimensional stability, and conducive to defense strong nucleophile OH- attack. First, a benzylmethyl-containing poly ether (ether ether ketone) (PEEK) copolymer was synthesized through condensation polymerization of 4,4′-difluorobenzopheno and methylhydroquinone. Then, dissolved the PEEK by 1,1,2,2-tetra- chloroethane, the yellow precipitate bromination of polyether ether ketone (BPEEK) was synthesized by adding NBS and BPO into the solution and reacted at 80°C for 5 h under the nitrogen atmosphere. The BPEEK was chosen to react directly with the functional reagent, 1-methyl imidazole and 1-vinylimidazole and obtained poly (ether ether ketone) crosslinked anion exchange membranes. The structure was characterized by 1H NMR spectroscopy. The morphology and microstructure of the resulting samples were characterized using transmission electron microscopy (TEM). The ion conductivity of the Im-PEEK- x membrane was measured by four-probe alternating current (AC) impedance technique. The thermal stability was tested by thermogravimetric analysis (TGA). The alkaline solution stability of the Im-PEEK- x membrane was evaluated by measuring the changes in IEC values after immersed in 1 mol/L NaOH for a certain time. Water uptake, swelling ratio and mechanical properties of the membranes were also measured. The results indicated that the Im-PEEK- x membrane had been synthesized. After grafting of 1-vinylimidazole, for the membrane of Im-PEEK-0.7, the maximum stress is 60.03 MPa, and the swelling rate is 18.2% at 80°C. After soaking in 1 mol L–1 NaOH solution for 200 h at 80°C, the IEC value was about 81.5% of the initial, which exhibited excellent alkaline stability.