Effect of Micromorphology on Alkaline Polymer Electrolyte Stability

Recent studies demonstrated that the chemical stability of alkaline polymer electrolytes (APEs) could be improved by reducing the inductive effect between cations and backbones. Therefore, pendent cations were recommended. However, microphase-separated morphologies would be generated by elongating the spacer between cations and backbones, which have a significant influence on the chemical stability of APEs too. In order to analyze how the patterns of micromorphology affect the chemical stability of the materials, in the present work, four APEs (a1-QAPS, a3-QAPS, a5-QAPS, and a7-QAPS) with different lengths of side chain between polysulfone and quaternary ammonium are synthesized. The longer the side chain is, the more obvious the microphase separation for the ax-QAPS membranes is observed. After immersing in a hot alkaline solution (80 °C, 1 M KOH) for 30 days, a5-QAPS exhibits the highest chemical stability. The ion exchange capacity and ionic conductivity of a5-QAPS film are reduced by 10.0 and 10.5%, respectively. The weight loss of a5-QAPS membrane is 8.0%, which is similar to the value of the pristine backbone. The increased chemical stability can be ascribed to the suitable micromorphology constructed in a5-QAPS sample. Besides, a5-QAPS membrane shows a high conductivity of 75.5 mS cm–1, whereas the swelling ratio is limited to 15.0% in liquid water at 80 °C. In addition, a peak power density of 339.1 mW cm–2 is obtained by applying a5-QAPS as the APE to the H2–O2 fuel cell at 60 °C.