Effect of cross-linking agent and heteropolyacid (HPA) contents on physicochemical characteristics of covalently cross-linked sulfonated poly(ether ether ketone)/HPAs composite membranes for water electrolysis

In order to improve the mechanical properties while maintaining electrochemical characteristics, engineering plastic of poly(ether ether ketone) as polymer matrix was sulfonated and the organic–inorganic blend composite membranes was prepared by loading 20–50% of HPAs, including tungstophosphoric acid (TPA), molybdophosphoric acid (MoPA) and tungstosilicic acid (TSiA). And then, these composite membranes were covalently cross-linked with the variation of 0.005–0.015 mL of cross linking agent contents (CL-SPEEK/HPA). Also, the characteristics of membrane electrolyte assembly (MEA) were measured in polymer electrolyte membrane electrolysis (PEME) cells. Although cross-linking decreased the number of sulfonic acid groups available for proton transfer in the membrane, the addition of cross-linking agent and HPAs significantly improved the mechanical and electrochemical properties of the membrane.Consequently, the optimum conditions of CL-SPEEK/HPAs with 0.01 mL of cross-linking agent content were established and electrochemical characteristics, such as ion conductivity, was in the order of magnitude: CL-SPEEK/TPA30 (30 wt%) < CL-SPEEK/MoPA40 < CL-SPEEK/TSiA30 and mechanical characteristics such as tensile strength: Nafion117 < CL-SPEEK/TSiA30 < CL-SPEEK/MoPA40 < CL-SPEEK/TPA30. With increasing cross-linking agent content, tensile strength increased and ion conductivity decreased. CL-SPEEK/MoPA40 showed the best electrocatalytic activity of cell voltage 1.71 V at 80 °C among the prepared composite membranes. However, in consideration of high water content, low anti-oxidative property and weak mechanical properties of CL-SPEEK/MoPA40 and CL-SPEEK/TSiA30 membranes, in spite of its high proton conductivity and electrocatalytic activity, it is expected that the CL-SPEEK/TPA30 (1.75 V) was suitable as an alternative membrane in large scale polymer electrolyte membrane electrolysis system. The dual effect of higher proton conductivity and electrocatalytic activity with the addition of HPAs, causes a synergy effect.