Practical implementation of bis-six-membered N-cyclic quaternary ammonium cations in advanced anion exchange membranes for fuel cells: Synthesis and durability

Practical implementation of anion exchange membranes (AEMs) having N-cyclic quaternary ammonium (QA) in alkaline fuel cells is demonstrated. Two kinds of QAs are incorporated along PPO backbone, including six-membered (DMP) and bis-six-membered N-cyclic QA (ASU). PPO-ASU membranes with bulky ring size display the highest hydroxide conductivity of 76.5 mS cm(-1) at 80 degrees C, due to more developed microphase separation. Unfortunately, AEMs with N-cyclic QA cations undergo chemical degradation with>45% ionic conductivity loss for PPO-ASU membrane after storage in 10M NaOH at 80 degrees C for 250 h. Post-analysis of these AEMs reveals that Hoffmann elimination and ring-opening by SN2 substitution are the dominant degradation mechanisms, which is in agreement with the results for model compounds. ASU-based AEMs exhibit a maximum power density of 124.7mWcm(-2) in a single H-2/O-2 fuel cell at 60 degrees C. However, durability testing of fuel cell with the AEMs shows a 90% performance loss after operating at 0.3 V for 40 h. Further NMR analysis suggests that SN2 substitution triggered ring-opening is the dominant pathway for the degradation of AEMs, leading to the unsatisfactory fuel cell durability. These findings that compare the in situ and ex situ stability of AEMs give some insights on future directions for developing advanced AEMs.