Design And Study Of Novel Anion Exchange Membranes For Alkaline Fuel Cells

The demand for large-scale energy storage devices has increased as a result of the continuous growth of alternative energy sources and the need to store this energy.Fuel cells have emerged as an option for grid-scale energy storage owing to their high energy efficiency,reasonable costs,and low pollution levels.Among the different types of fuel cells,proton exchange membrane fuel cells?PEMFCs?have received particular attention worldwide owing to their high power density,high energy conversion efficiency,low starting temperature,and easy handling characteristics.Despite these advantages,PEMFCs suffer from a number of issues?e.g.,exclusive dependency on platinum catalysts,high cost of the perfluorinated membranes,and environmental incompatibility?which hinder their applications.Thus,there is growing interest in developing anion-exchange membranes?AEMs?for alkaline anion exchange membrane fuel cells?AEMFCs?,which can overcome many of the limitations of PEMFCs.Thus,these systems are more cost effective and technically feasible than PEMFCs since electrochemical reactions are usually favoured in alkaline media,and nonnoble metals can be used as catalysts.Furthermore,AEMFCs avoid fuel permeation because the conducting direction of the hydroxide ions is opposite to that of the fuel crossover.One of the obstacles for the development and commercial introduction of AEMFCs lies in the current lack of AEMs combining high hydroxide conductivity and sufficient mechanical and chemical stabilities under strong alkaline conditions.With the aim to meet these requirements,we desigined novel chemical structure of AEMs according to the relationship between the micro-structure and properties in the membranes.On the one hand,it's an effective way to improve ion conductivity by constructing connective ion channels in AEMs;on the other hand,the excellent alkaline stability needed to be improved by introducing the new groups and side chains in the chemical structure also.Therefore,we designed and prepared foure kinds of AEMs and studied their properties:?1?The fluorinated hydrophobic blocks were designed and prepared by using two fluorinated monomers namely,4-fluorostyrene?4FS?and 2,3,4,5,6-pentafluorostyrene?PFS?.The corresponding diblock copolymers were prepared by copolymerization of a poly?vinylbenzyl chloride??PVBC?macroinitiator with fluorinated monomers via the RAFT polymerization technique.The as-obtained hydrophilic chain is expected to be immiscible in the highly fluorinated blocks,thus driving membrane self-assembly to form nanoscale domains with enhanced local concentrations of QA groups,which facilitate effective hydroxide transport.The fluoro-substituted functional groups resulted in excellent mechanical properties,satisfactory dimensional stability and alkaline tolerance of AEMs as well.?2?The mid-quaternized tri-block copolymers having side-chain-type QA groups were prepared via RAFT polymerization with a difunctional polystyrene macroinitiator followed by N-alkylation.This synthetic method allows the incorporation of an alkyl spacer chain?n=3?in-between the aromatic ring and the QA cation with or without thermally cross-linkable styryl groups,which improved hydrophilic–hydrophobic separation and alkaline stability.This approach allowed an excellent and systematic control over the concentration of functional groups in PSs.Despite their low WU,these side-chain-type PS-based AEMs exhibited hydroxide conductivity values comparable to those of typical AEMs based on the benzyltrimethyl ammonium motif.The steric effects of the alkyl chains in the“side-chain type”architecture surrounding the QA center are likely responsible for the observed good alkaline stability.?3?A novel series of comb-shaped AEMs with different lengths of alkyl tail chains?from C1to C10?based on PSm-b-PDVPPAn diblock polymers were prepared using a facile and highly efficient Menshutkin reaction.The comb-shaped polymer has nanoscale-organized phase-separated morphology.However,the cation density in the ionic cluster decreased,and this resulted in slightly lower hydroxide conductivity.Considering the much lower WU?<30%?,the comparable conductivity suggests that the use of QA groups is highly efficient and that well-connected ion conducting channels form.Additionally,because of the presence of large volumetric alkyl tail chains,ammonium cations are protected from Hofmann elimination,and thus,good hydroxide stability?>84%of the initial conductivity?was also observed after the40-day test.?4?Polyolefin-based anion exchange membranes?AEMs?with well-defined ionic domains for anion transport were synthesized by introducing bulky poly?4-phenyl-1-butene??P4PB?moieties into quaternized polyolefinic copolymers via heterogeneous catalytic copolymerization of 4-phenyl-1-butene with 11-bromo-1-undecene over a Ziegler–Natta catalyst and subsequent quaternization.Well-defined hydrophobic-hydrophilic separation was observed for the as-obtained membranes.The membranes showed high hydroxide conductivity(10.4 m S cm^-1)despite their low ion exchange capacity(0.98 meq.g^-1).Moreover,excellent alkaline stabilities?>90%?were observed for the P4PB-based AEMs after 20 d in a 10 M Na OH solution at 80°C.The‘side chain-type'architecture surrounding the QA center is believed to be responsible for the excellent alkaline stability of these materials.