Synthetic approaches for anion exchange membranes with improved alkaline stability

Anion exchange membrane fuel cells (AEMFCs) are promising electrochemical devices to be employed e.g. in automotive applications that can help to reduce greenhouse gas emissions. A main issue that limits its large-scale implementation is the low chemical stability of the anion exchange membranes (AEMs) in strong alkaline conditions. In this thesis, we aimed to understand the degradation and to find innovative AEMs with improved durability and high performance. Initially, we focused on the study of ionomers based on poly(2,6-dimethyl-1,4- phenylene oxide) (PPO). We explored different approaches to mitigate the chemical degradation in alkaline conditions. 1) The influence of pendent methyl groups on the alkaline stability of PPO grafted with trimethylammonium groups was studied varying the synthesis procedure: i) by the bromination route, the ammonium groups are inserted on the structural methyl group of PPO (PPO-Br), ii) by the chloromethylation route, the ammonium groups are attached in the ortho-position to the methyl group of PPO (PPO-Cl). Membranes made by the chloromethylation route presented better mechanical properties before and after the alkaline test (2M NaOH, 80°C), but a faster loss in conductivity. Studies by 1H-NMR and FTIR spectroscopy and by TGA confirmed a chain scission during the aging treatment, which was related with the lower mechanical properties observed for membranes made by the bromination route. 2) The use of bulky and rigid amine, 1,4-diazabicyclo octane (DABCO), was explored to quaternize PPO-Cl. The obtained ionomer was the base of a composite membrane with 30% of Mg/Al lamellar double hydroxide particles (LDH) as inorganic filler. LDH, an intrinsic anion conductor, was added in high concentration to mitigate the conductivity loss and degradation. The ionic conductivity was measured in Clform for the pristine ionomer and its composite. The remaining conductivity of membranes based on PPO-DABCO and PPODABCO/LDH after alkaline treatment was higher than for ionomers prepared with TMA. 3) 1,5,7-triazabicyclo dec-5-ene (TBD) was explored as substituting amine, because it shows a very high basicity and could allow to synthesize an intrinsic ion conductor. To favour the SN2 reaction, TBD was activated with butyl lithium. The "intrinsic" anionic conductivity of PPO-TBD due to dissociation of grafted TBD was verified. The quaternized ionomer showed a larger ionic conductivity, indicating that the permanent charge favors the hydrophobic-hydrophilic phase separation and the formation of ionic domains. 4) The stability in alkaline conditions of ionomers based on PPO with a long side chain was higher than that of short side-chain ionomers with the same basic group. This improvement can be related to several hypothesis: i) the SN2 reaction is less effective, because the primary alkyl halide presents a much lower reactivity than the benzyl halide; ii) the ether linkage in the backbone is less destabilized due to the long alkyl chain that screens the positive charge; iii) the antiperiplanar conformation needed for the E2 reaction is less stable when a long chain is attached to ammonium. This approach was the most beneficial to enhance the alkaline stability of AEMs based on PPO. We also followed another approach with the synthesis of a new copolymer without ether linkage, which can be a weak point for chain scission. Poly (vinylbenzylchloride-co-hexene) was synthesized by Ziegler-Natta polymerization employing Zirconium (IV) chloride tetrahydrofuran complex (1:2) as catalyst. The ionomer properties were studied and compared with AEMs based on PPO. The results reported in this dissertation can help in understanding and selecting high performance AEMs for electrochemical energy technologies. The advantages and disadvantages of the different approaches to enhance the properties of anion exchange ionomers were also evaluated.