Triptycene Branched Poly(Isatin-Biphenylene) Electrolytes for Bipolar Membrane Water Electrolyzers

Abstract Bipolar membrane water electrolysis (BPMWE) performance depends on the properties of two ion exchange layers and their interfacial compatibility. Herein, linear anion and cation exchange ionomers (PIB-QA and PIB-BS) are synthesized based on the same poly(biphenyl-isatin) backbone, and a linear BPM (PIB-L) with a nano-SnO2 catalytic interlayer is prepared via ultrasonic spray coating. A rigid triptycene moiety is introduced to prepare branched ionomers (PIB-QA-5% and PIB-BS-5%) and the corresponding BPM (PIB-Trip5%). The branching structure increases fractional free volume and ion-transport channel connectivity while providing high water uptake, low swelling, and dimensional stability, facilitating water dissociation, and interfacial stability. At 60 °C, PIB-Trip5% achieves 0.7 A cm−2 at 2.00 V and 2.29 V at 1.0 A cm−2 in pure-water electrolysis, and 1.5 A cm−2 at 2.00 V in an asymmetric acid-base electrolyzer. The electrolyzer operates at 0.5 A cm−2 for 200 h with a voltage decay rate of 0.87 mV h−1.