Design And Preparation Of Anion Exchange Membranes For Alkaline Fuel Cells

Polymer electrolyte membrane fuel cells attract increasing attention due to their advantages of low operation temperature, quick start and high power density. Generally, polymer electrolyte membrane fuel cells can be divided into two categories:proton exchange membrane fuel cells (PEMFCs) and alkaline anion exchange membrane fuel cells (AAEMFCs). Compared with widely developed PEMFCs, AAEMFCs possess inherent superiorities, including the improved kinetics of the oxygen reduction reaction, more possibilities for cathode catalysts based on nonprecious metals and low fuel crossover. Anion exchange membranes (AEMs) act as one of the most important components of AAEMFCs, which can provide supports for catalysts, create environments for the electrode reaction, isolate the fuels of two chambers, and offer necessary channels for hydroxide ions conduction. AEMs will determine the fuel cells’ properties, lifetime and efficiency directly. To date, there are no readily available AEMs that serve as the standard-bearer, while commercially available Nafion (Dupont) membranes as state-of-the-art proton exchange membranes (PEMs) have driven the progresses of PEMFCs. Low hydroxide conductivity, insufficient dimension and alkaline stability are major challenges existing in AEMs, which seriously limit the development of AAEMFCs. Hence, the preparation of high hydroxide conductivity and alkaline stability is an urgent study for the development of AAEMFCs.By investigating the relationships between the micro-structures and properties of Nafion membranes, constructing connective ion channels is the principal reason for improving the proton conductivity. Consequently, the design of graft copolymers bearing cation groups, which can form well-developed ionic channels by self-assembly. High hydroxide conductivity can be expected from these copolymers. On the other hand, the design of novel alkaline functional groups is an efficient route to address the issues of alkaline stability in AEMs. In this context, the main contents are summarized as follows:(1) By atom transfer radical polymerization (ATRP)’grafting from’ method, graft type AEMs based on poly(vinylidene fluoride)(PVDF) can be obtained. Micro-phase separation could occur between hydrophobic main chains and hydrophilic graft chains, and thus the resulting AEMs displayed relatively high hydroxide conductivity. In addition, the membranes also presented elegant mechanical and thermal stability due to crystaline nature of PVDF.(2) By activators regenerated by electro transfer ATRP ’grafting from’ approach, comb-shaped AEMs based on bro mo methylated poly(phenylene oxide)(BPPO) can be obtained. Comb-shaped AEMs possessed high graft density and low graft length, which could induce the formation of desirable phase separation morphologies. Therefore, the hydroxide conductive properties of comb-shaped AEMs were more promising than other reported AEMs, and the comb-shaped AEMs could attain high hydroxide conductivity of100mS/cm at80℃. However, other fuel cell related properties should be optimized furthermore.(3) Following above research, this study continued adjusting the structure parameter of graft copolymers. Low graft density and high graft length of graft type AEMs have been prepared, using low brominated of BPPO. In this process, the amounts of catalysts should be adjusted carefully. These membranes were termed as Rod-Coil AEMs. Similar to comb-shaped AEMs, Rod-coil AEMs could also achieve clear phase separation micro-morphologies, and thus the resulting AEMs show high hydroxide conductivity (198mS/cm at90℃). In addition, Rod-Coil AEMs also presented improving alkaline stability, attributed to the low graft density. Low graft density can decrease the adverse effects of graft chains on the main chains. Such an accurate molecular design simultaneously improved the hydroxide conductivity and alkaline stability.(4) It is widely accepted that cross-linking can limit the AEMs’ excessive swelling, and improve the AEMs’ alkaline as well as thermal stability. However, cross-linking will hamper the development of ion channels, and decrease the AEMs’ hydroxide conductivity. In this study, the graft type AEMs can be achieved by adding macro molecular cross-linkers, which had the same component with the main chains of graft type AEMs. The design of cross-linking not only constructed firm cross-linking network in the matrix, but also didn’t destroy the micro-phase separation morphology. The resulting cross-linking AEMs displayed higher stability compared with uncross-linking ones, while the hydroxide conductivity were almost the same as the uncross-linking ones. This study provide a novel thinking about the design of rational cross-linking for AEMs.(5) Imidazolium (Im) type AEMs can be obtained by functionalizing the BPPO using1-methylimdazole (MIm). The degradation of imidazolium cations in alkaline environments was minimized due to the presence of steric hindrances and the π-conjugated structure of five-member heterocyclic ring. In addition, Im type AEMs also displayed higher thermal stability due to the special structure of Im rings. Compared with traditional quaternary ammonium groups, Im type AEMs show enhanced alkaline and thermal stability. The peak power density of a single cell using membrane Im type AEMs could reach30mW/cm2.