Comparative study of random and block SPEEK copolymers for high-temperature proton exchange membrane electrolysis

Traditionally, High-temperature proton Exchange Membrane Electrolysis (HT-PEME) has relied on costly and exceptionally hydrogen-permeable PFSA ionomers like Nafion®, Flemion®, and Aciplex®. Sulfonated Poly (ether ether ketone) (SPEEK) membranes have been developed as potential substitutes for the highly hydrogen permeable Nafion® membranes. The ion exchange capacity (IEC) of SPEEK membranes has been varied, ranging from 0.5 to 2.2 meq/g, to enhance their performance. Furthermore, the incorporation of ion-conducting units into these polymers is explored using both random and block copolymerization approaches. At 75 °C, SPEEK membranes exhibit significantly lower hydrogen diffusivity (35–50 barriers) compared to Nafion® and Flemion® membranes (∼202 barriers). Among SPEEK yttria-stabilized zirconia membranes (YSZM), those produced through random copolymerization exhibit marginally greater proton-to-hydrogen selectivity than YSZM derived from block SPEEK copolymers with a similar ion exchange capacity (IEC). This could be ascribed to the improved hydrogen-blocking capability resulting from an underdeveloped hydrophilic component. Among the SPEEK membranes tested, the random SPEEK membrane with an IEC of approximately 1.9 meq/g demonstrates superior performance (6.8 A/cm2) compared to a similarly thick (∼45 μm) Flemion® membrane (4.4 A/cm2) at 2.2 V. However, during an accelerated stress test involving 410 successive fluctuating flow densities of 2.8 and 0.04 A/cm2 imitating the alternating period's cycles of a water electrolyzer, the random SPEEK membrane exhibits a higher degradation rate (860 μV/h) compared to Flemion® and Nafion® (505 μV/h).