High free-volume hyperbranched poly(aryl piperidinium) anion exchange membranes based on noncoplanar V-shaped Tröger Base-Carbazole derivative with synergistic enhancement of ionic conductivity and dimensional stability for water electrolysis

The practical application of poly(aryl piperidinium) AEMs in water electrolysis is constrained by two major technical bottlenecks: (1) conformational distortion of the piperidine group reducing alkaline stability; (2) tight packing of polymer main chain hindering OH − transport. In this study, the noncoplanar V-shaped Tröger Base-Carbazole derivative, 2,8-di( N -carbazolyl)-6H,12H-5,11-methano-dibenzo[b,f][1,5]diazocine (CTB), was designed and introduced into poly(aryl piperidinium) chain, thus obtaining a series of hyperbranched AEMs. Compared with linear polymer chain, the unique V-shaped three-dimensional structure in CTB inhibited tight packing of polymer and increased free volume within the membrane, thereby effectively enhancing the migration rate of OH − . At 80 °C, the QPCTB-PEG-TP-5 achieved a high ionic conductivity of 159.79 mS/cm with a low ion exchange capacity (1.108 mmol/g), while maintaining the swelling ratio below 8.5 %. Additionally, the electrostatic potential analysis and 1 H NMR spectroscopy revealed that the bridged methylene groups in CTB protected piperidine cationic groups from Hoffmann elimination. After treatment in 1 M KOH medium at 60 °C for 1000 h, the OH − conductivity retention rate and tensile strength reached 80.72 % and 21.71 MPa, respectively. The favorable OH − conductivity, mechanical strength, and alkaline stability strongly supported the application of AEM in water electrolysis. The AEMWE equipped with QPCTB-PEG-TP-5 type AEM achieved an instantaneous current density of 608 mA/cm 2 (60 °C, 1.51 V) and operated stably for 120 h without degradation at a current density of 250 mA/cm 2 . Therefore, the molecular structure design of hyperbranched poly(aryl piperidinium) AEMs based on noncoplanar V-shaped Tröger Base-Carbazole derivative provides an effective approach to solve the dual challenge of poor alkaline stability and low ionic conductivity.