Tethering Cations to Aromatic Polymers via Flexible Spacers to Enhance the Performance of Alkaline Fuel Cell Membranes

Alkaline membrane fuel cells (AMFCs) have recently received increasing attention in relation to proton-exchange membrane fuel cells because the alkaline operating conditions potentially bring significant advantages, including the use of non-precious metal catalysts and improved cell efficiencies to significantly reduce cost.¹ The polymeric anion-exchange membrane (AEM) is a critical component of AMFCs since its OH^- conductivity and alkaline stability to a great extent determine the performance and lifetime of the system. Hence, significant research efforts are currently committed to the molecular design, synthesis and characterization of robust and highly anion-conducting polymer materials.¹

Most studies of AEMs up until now have been performed on aromatic polymers functionalized with benzyltrimethylammonium groups positioned directly on the backbone.¹ These materials are readily accessed via, e.g., chloromethylation of suitable aromatic polymers, followed by quaternization with trimethylamine. However, several recent results on low-molecular weight model compounds indicate that polymers containing benzylic quaternary ammonium (QA) groups possess a significantly lower stability than corresponding polymers with alkyl-substituted QA groups in the presence of aqueous OH^-.² Moreover, the location of the benzylic QA groups directly on the aromatic polymer backbones restricts their mobility and hinders the efficient phase separation necessary to reach high anion conductivity. There is thus a current need for new synthetic approaches to attach cationic groups on aromatic polymers via alkyl spacers.

We have recently attached QA groups on poly(phenylene oxide) via flexible and non-hydrolysable alkyl spacers (PPO-7Q, Scheme 1) by using a straightforward synthetic route involving bromoalkylation and quaternization.³ AEMs based on these materials showed distinct small angle X-ray scattering maxima, so-called ionomer peaks, to indicate efficient phase separation (Fig. 1a). In contrast, corresponding AEMs based on polymers with QA groups placed in benzylic positions directly on the backbone (PPO-1Q, Scheme 1) showed very weak (if any) ionomer peaks. Moreover, the PPO-7Q showed a significantly enhanced OH^- conductivity (Fig. 1b) and far superior alkaline stability in relation to PPO-1Q. These results clearly demonstrate the significant advantages of the alkyl spacer concept in the molecular design of AEM materials for AMFCs.³ In the present paper we will discuss further recent findings regarding molecular design principles, synthetic procedures and important structure-property relationships of these new AEM materials.

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