Polyisatin derived ion-solvating blend membranes for alkaline water electrolysis

Abstract

It is a great challenge to develop membranes based on polyaromatic backbone chemistries that combine high alkaline resistance with high ionic conductivity and low gas crossover for alkaline water electrolysis. Hence, a new alkaline stable aromatic monomer containing side ion-solvating poly(ethylene oxide) (PEO) groups was synthesized and polymerized with isatin and biphenyl via super acid catalyzed hydroxyalkylation to yield ion-solvating copolymers. The prepared aryl-ether free backbone aromatic copolymers (P(IB-PEO)-y) have excellent film-forming properties, high thermal stability, but moderate KOH electrolyte uptake and relatively low conductivity. Therefore, to further enhance the electrolyte uptake and ionic conductivity, P(IB-PEO)-20 copolymers were blended with polybenzimidazole in ratios 80/20, 70/30, 60/40 and 50/50. The prepared blend membranes exhibit high electrolyte uptakes (up to 97 wt%) while the highest ionic conductivity of 110 mS cm−1 at 80 °C was observed for PBI80/P(IB-PEO) blend. The KOH doped PBI70/P(IB-PEO) membrane shows a tensile strength of 20 MPa and a significant increase in Young's modulus (131%) compared to that of PBI80/P(IB-PEO). The alkaline stability test demonstrated that PBI80/P(IB-PEO) membrane exhibits a substantially higher Young's modulus (144% increase) than non-aged analogue, after 1 month in 20 wt% KOH solution at 80 °C. Further, PBI80/P(IB-PEO) and PBI70/P(IB-PEO) membranes retained 96–98% of their original conductivity after aging, indicating their excellent alkaline resistance. Selected membranes were tested in a single cell electrolyzer to probe feasibility and crossover behaviour.