Degradation Studies of Functionalized Polyphenylene Oxide for Anion Exchange Membrane

Anion exchange membranes (AEM) have garnered increased attention in recent years due to their potential use in energy conversion and storage devices. Fuel cells (FC) utilizing AEMs as the exchange media are expected to be less costly than the standard proton exchange membrane fuel cells due to the use of a less noble catalyst¹. AEMs are also attractive for use in metal air batteries, which have much higher theoretical energy densities than the current generation of lithium ion batteries². The use of an anion exchange membrane in a metal air battery would eliminate the need for liquid electrolyte, reducing the size and weight of the cell.

Anion exchange membranes reported in the literature to date suffer from chemical instability in alkaline environments, which are found in both AEMFCs and metal air batteries. The chemical instability is primarily a result of the degradation of the cationic moieties responsible for the transfer of the hydroxide ions.

For this work AEMs based on functionalized polyphenylene oxide (PPO) were synthesized. PPO was selected because it is commercially available, inexpensive, and demonstrates good stability in alkaline environments^3,4. AEMs were made by functionalizing PPO with quaternary ammonium groups; dimethylhexylamine, dimethyldecylamine, or trimethylamine. For each of these groups a membrane was synthesized with a high and low degree of functionalization. The resulting AEMs were studied for physical and electrochemical properties and compared against a commercially available AEM.

The conductivity of the membranes equilibrated in solutions between 1 and 10M potassium hydroxide and at different humidity levels was determined through electrochemical impedance spectroscopy. The equilibrium concentration of water and base within the membranes was studied. The mechanical properties of the membranes were determined with Dynamic Mechanical Analysis.

Degradation of the polymers was monitored by immersing the membranes in solutions between 1 and 10M potassium hydroxide at elevated temperature. The conductivity of the membranes was measured over time to determine loss in electrical properties. Membranes were also tested in situ in anion exchange membrane fuel cells and zinc air batteries.

We found that membranes based on the trimethylamine demonstrated the highest conductivity while those based on dimethyldecylamine demonstrated the greatest stability.

The membranes in lower molarity solutions were found to become more brittle over time. Thin layer chromatography was conducted on the solutions in which the membranes were held during degradation studies. The results indicated that polyols were forming, indicating that the cationic groups cause the polymer to undergo a SN2 reaction.

Figure 1: AEMs studied in this report: a) PPO-TMA, b) PPO-DMHA, b) PPO-DMDA

Acknowledgement

We gratefully acknowledge the support of ZAF Energy Systems for their support of this work.