Tuning properties of PEO-functionalized ion-solvating blend membranes via PEO side chain length: Impact on alkaline water electrolysis performance

Abstract

Ion-solvating membranes (ISMs) based on chemical stable polyoxindole copolymers bearing long side PEO groups (MW750) were synthesized via a PEO-functionalization monomer strategy followed by super acid polyhydroxyalkylation. The synthesized copolymer membranes displayed very high electrolyte uptakes even at low KOH concentrations (10 wt%) due to deprotonation of oxindole groups enabling high KOH absorption. However, the increased plasticization resulted in deterioration of mechanical strength and the copolymers were therefore blended with m-PBI to yield mechanical robust, nanophase separated ISMs. Their physicochemical properties were tuned by adjusting the blend composition. The prepared blend membranes showed high KOH and water absorption in 30 wt% KOH concentration even higher than that of pure m-PBI. The presence of long hydrophilic side PEO chains facilitates both KOH and water uptake due to increased free volume and induced phase separation. This in turn resulted in high ionic conductivity exceeding 100 mS cm⁻¹ at 80 °C. Long term stability in 30 wt% KOH at 80 °C for blend PBI80/P(IB-PEO₇₅₀) was excellent: the conductivity remained unchanged (at room temperature), the thermal stability was improved, while the membrane retained its flexibility and the tensile strength and Young's modulus remained high after the 2 months (1440 h) test. The excellent alkaline stability was attributed to the stabilization of blend membranes via strong attractive electrostatic interactions between m-PBI's imidazolide and isatin or PEO groups with K⁺. The blend PBI80/P(IB-PEO₇₅₀) was evaluated under alkaline electrolysis conditions using a 30 wt% KOH feed solution at 80 °C. It exhibited a high current density of 1.06 A cm⁻² at 1.8 V. In comparison, the corresponding blend with short PEO groups PBI80/P(IB-PEO₃₅₀) showed higher current density of 1.36 A cm⁻² at the same voltage, which is comparable to the excellent performance of m-PBI. Long-term durability tests revealed that the cell with PBI80/P(IB-PEO₇₅₀) membrane successfully run for 250 h at 80 °C under a constant current density of 0.5 A cm⁻², in contrast to the cell with PBI80/P(IB-PEO₃₅₀) membrane, which failed after 160 h, showing its applicability in harsh alkaline AWE conditions. In addition, the H₂ in O₂ content for both cells with different blend membranes was low, in the range of 1.4–1.65 vol%, indicating low gas impurities for both cells. This work provides a simple blending strategy for designing chemically stable, with promising performance membranes for alkaline water electrolysis.