Alkali Doped Electrospun Polybenzimidazole Membranes for Anion Exchange Membrane Water Electrolysis

Polybenzimidazoles (PBIs) are a class of engineering plastics widely employed in hydrogen research field for the manufacturing of both proton and anion exchange membranes due to its amphoteric nature and its excellent chemical and thermal resistance. As far as anion exchange membranes are concerned, pristine PBI is non-conductive and needs to be methylated or doped with high concentrations of KOH (≥15 wt%) [1]. In this study, an integrated strategy to manufacture KOH doped PBI membranes suitable for anion exchange membrane water electrolysis (AEMWE) is presented. Starting from a commercial low molecular weight PBI solution, electrospinning technique was employed to manufacture porous membranes made of nanometric fibers (𝑑≈150 𝑛𝑚) with enhanced open porosity, available for the doping procedure [2]. They were then washed with boiling water, oven-dried and doped with 20 wt% KOH for 30 minutes at RT. Their alkali doping level (ADL) was determined both gravimetrically and by titration with 0.1 M HCl. After alkali impregnation, membranes’ surface was coated with a layer of ionomer by dropping off 0.5 ml of a 15 wt% Fumion® FAA-3 in EtOH solution per side [3], becoming impermeable to both water and KOH and preventing any leakage of the taken-up KOH, which would decrease their conductivity over time. Membranes’ impermeability was confirmed through air, water and KOH (1.0 M) permeability measurements in H-cell. The obtained membranes (τ≈150 μm) were all characterised through ATR-FTIR, Optical Microscopy, SEM, stress-strain tests, while also monitoring ex-situ 𝐶𝑙− conductivity and chemical stability in 1 M KOH at 60 °C. Conductivity values (0.5 M NaCl, RT) as high as 6.20 ± 0.44 mS cm-1 have been reached. Electrochemical performance was evaluated manufacturing a membrane-electrode assembly (MEA) made of a membrane sandwiched between stainless steel anode and cathode. MEAs were tested in a lab-scale 2 cm2 electrolyser working at RT and fed at the anode with 0.5 M KOH through EIS spectroscopy, polarization curves and durability tests. Other commercial AEMs were characterised and tested with the same experimental procedures and results show that this integrated approach of electrospinning, KOH doping and surface coating produces promising membranes for green hydrogen production, superior to commercial benchmarks like Fumasep-PK-130.