Cross-linking of poly (ether ether ketone) and its sulfonated form: A spectroscopic study

Sulfonated poly(ether ether ketone) (SPEEK) stands as both a high-performance and low-cost polymer electrolyte membrane (PEM) that is the focus of many recent researches as a promising candidate to replace Nafion® membranes in potential applications such as fuel cells and redox flow batteries. In this work, the effects of various membrane treatments including washing, reduction of the carbonyl groups, and drying were investigated. The possibility of cross-linking PEEK and SPEEK without affecting the final degree of sulfonation (DS) was achieved by the use of carbonyl-selective reducing reagents, i.e. sodium borohydride. The modified PEEK samples did not dissolve in concentrated sulfuric acid and sulfonation occurred in the solid phase, which might indicate the co-occurrence of cross-linking reactions through the newly formed hydroxyl groups. Moreover, FTIR spectroscopy indicated the formation of ether links between the SPEEK polymer chains. In addition, the effects of thermal drying and washing of the SPEEK membranes were investigated and the conditions of cross-links formations were optimized. It is recommended to start with high DS PEMs when SPEEK membranes are applied in harsh environments such as fuel cells. This is due to the loss of active protons via both thermal and oxidative mechanisms, which will eventually lead to deterioration in membrane properties including conductivity and mechanical integrity.