Polymeric materials as anion-exchange membranes for alkaline fuel cells

After summarizing the different fuel cells systems, including advantages and drawbacks, this review focuses on the preparation of copolymers and polymeric materials as starting materials for solid alkaline fuel cells membranes. The requirements for such membranes are also summarized. Then, different strategies are given to synthesize anion-exchange polymeric materials containing cationic (especially ammonium) groups. The first pathway focuses on heterogeneous membranes that consist in: (i) polymer blends and composites based on poly(alkene oxide)s and hydroxide salts or polybenzimidazole doped with potassium hydroxide, (ii) organic-inorganic hybrid membranes especially those synthesized via a sol-gel process, and (iii) (semi)interpenetrated networks based on poly(epichlorhydrine), poly(acrylonitrile) and polyvinyl alcohol for example, that have led to new polymeric materials for anion-exchange membranes. The second and main part concerns the homogeneous membranes divided into three categories. The first one consists in materials synthesized from (co)polymers obtained via direct (co)polymerization, for example membranes based on poly(diallyldimethylammonium chloride). The second pathway concerns the modification of polymeric materials via radiografting or chemical reactions. These polymeric materials can be hydrogenated or halogenated. The radiografting of membranes means the irradiation via various sources - electron beam, X and γ rays, 60Co and 137Cs that lead to trapped radicals or macromolecular peroxides or hydroperoxides, followed by the radical graft polymerization of specific monomers such as chloromethyl styrene. The third route deals with the chemical modifications of commercially available hydrogenated aliphatic and aromatic (co)polymers, and the syntheses of fluorinated (co)polymers such as carboxylic and sulfonic perfluoropolymers. In addition, several approaches for the crosslinking of above-mentioned polymeric materials are also reported as this process enhances the properties of the resulting membranes. Moreover, electrochemical and thermal properties of various above ionomers are given and discussed.