Hydrogen Generation in Neutral Electrolyte of a Hybrid Bipolar Membrane Combining Ionic-Liquid Copolymer (PVAc- co -PVBIT) and Sulfonated Polybenzimidazole

Hybrid bipolar membranes (BPMs) combining sulfonated polybenzimidazole (SPBI-6F) and an imidazolium-containing ionic-liquid copolymer (PVAc-co-PVBIT) were fabricated and evaluated for hydrogen generation under neutral water electrolysis conditions. The SPBI-6F layer provides high chemical and thermal stability with controlled proton transport, while PVAc-co-PVBIT enables anion conduction through flexible imidazolium-rich domains. The SPBI-6F/PVAc-co-PVBIT BPM was assembled by heat pressing and integrated into a membrane–electrode assembly operated in 0.5 M Na2SO4 at 28 °C. Electrochemical impedance spectroscopy (EIS) fitted with R1 + (Q1∥R2) + (Q2∥R3) yielded R1 ≈ 4.4 Ω and two polarization-associated resistances (R2 ≈ 508 Ω; R3 ≈ 580 Ω), indicating that cell losses are dominated by interfacial processes rather than bulk ohmic resistance. Chronoamperometry (3.5 V) and chronopotentiometry (3–15 mA cm–2) showed stable current and voltage profiles with negligible drift, consistent with robust interlayer adhesion under sustained operation. Cross-sectional FE-SEM micrographs before and after electrolysis showed a continuous CEM/AEM interface without evidence of progressive interfacial separation, while both layers withstood 72 h of differential exposure to 1 M KOH and 1 M H2SO4 without visible dissolution or fragmentation. Linear sweep voltammetry showed well-defined activation and ohmic regimes, with an onset voltage estimated by the tangent-intersection method of Vonset,tan = 1.52 V and an apparent current rise around 1.8–2.0 V. At a current density of 8 mA cm–2, the BPM delivered a hydrogen production rate of 2.49 µmol min–1 cm–2 with 52% Faradaic efficiency. These results demonstrate that combining sulfonated and imidazolium-functionalized polymers enables stable hydrogen-generating BPM electrolysis under near-neutral electrolyte conditions without a catalyst at the bipolar junction.