Alkali doped electrospun polybenzimidazole membranes for anion exchange membrane water electrolysis

Alkali doped electrospun polybenzimidazole membranes for anion exchange membrane water electrolysis Introduction Hydrogen production through water electrolysis coupled with renewable energies is raising in scientific and geo-political interest year by year. In this scenario, anion exchange membranes (AEMs) and other technologies for anion exchange water electrolysis (AEMWE) are quickly developing [1]. Many challenges associated with membranes stability and ion conductivity have already been partially solved optimizing polymers’ architecture and processing. In this context, polybenzimidazole (PBI) based membranes, both cationic and anionic, gained attention as promising materials for ion-exchange applications due to their good mechanical and chemical properties. As far as AEMs are concerned, poly(2,2’-(m-phenylene)-5,5’bibenzimidazole) (m-PBI) can become conductive by two different strategies. The first one is the methylation reaction to form a quaternary ammonium group on the imidazolium moiety, while the second one involves the doping of the polymer with KOH solutions with a concentration of at least 15 wt% in order to deprotonate the imidazolium moiety. While this second strategy is faster and easier, its application has been limited to alkaline water electrolysis (AWE) due to the high pH needed to maintain membrane’s conductivity [2]. In this study, we employ a mixed strategy to manufacture KOH doped PBI membranes which are conductive even in the milder conditions of AEMWE (0.5-1 M KOH, 50 °C) and stable over time. These membranes are mats made of random nanometric fibers produced by the electrospinning technique which makes them extremely porous and with an enhanced surface area available for the alkali doping procedure [3]. Methodology A commercial PBI solution (S26, PBI Product©, Charlotte, NC, USA, Mw = 28 kDa), with a viscosity of 2100 ± 200 Poise, containing 26 wt%. of PBI in dimethylacetamide (DMAc) and LiCl (4 %wt.), was diluted to a concentration of 16 wt%. The electrospinning operating conditions were 40 °C and 15% RH in the chamber, with a flow rate of 2 mL/h, a 40 kV applied potential between the ejector and the collector, which were positioned at 135 mm of distance. The obtained membranes were carefully washed with boiling water for 6h to remove any trace of residual solvent or salt and then doped with KOH solutions with concentrations of 10, 15 and 20 wt%. The doping procedure was performed by soaking the membranes in the KOH solutions for 30 min at RT. Alkali doping level (ADL), meaning the number of KOH molecules per PBI unit, was evaluated both gravimetrically and by titration. * Corresponding author: [email protected] 2 After alkali impregnation, each membrane was covered with a thin layer of ionomer by dropping off 500 l per side of a 15 wt% Fumion in EtOH solution (Fig. 1) to make them impermeable to water and KOH and to prevent any leakage of the KOH doping solution, which would decrease their conductivity over time [4]. Figure 1: Electrospun PBI membrane impregnated with KOH 20wt% before (a) and after Fumion deposition (b). The evolution of the membrane throughout the whole process was monitored and characterised through ATR-FTIR, SEM and optical microscopy analysis, stress-strain and air permeability tests. After a conditioning step by immersion overnight in NaCl 0.5 M at RT, membranes were cut into circular specimens with an active area of 7.07 cm2 and electrochemically characterised in a H-cell kit (Scribner, USA), measuring specific resistance and conductivity in NaCl 0.5 M at RT. Finally, a 5 cm2 membrane-electrode assembly (MEA) made with the best performing membrane and commercial electrodes was tested in a AEMWE test station at different temperatures, recording electrochemical impedance spectroscopy (EIS) and polarization curves. The overall performances of the homemade MEA were compared with solvent cast based PBI MEA and with a commercial benchmark. Discussion The main goal of this study was to manufacture a conductive and stable PBI based membrane for AEMWE without resorting to the methylation reaction. As showed by the optical micrograph, electrospun membranes made of fibers with diameters up to 200 nm have been successfully produced (Fig 2). Figure 2: 5000x optical enlargement of a pristine electrospun membrane. Reference bar of 10 m. 3 The achieved high porosity allows to reach ADL values up to 6 KOH molecules per PBI repeating unit, which is two to three times higher than the solvent cast samples, underlining the great KOH uptake of these membranes. The deposition of the ionomer layer turned the membranes, which were easily permeable to air, water and KOH, into impermeable, as the Gurley air permeability tests confirmed. Conductivity values as high as 4.78 ± 0.44 mS cm-1 have been reached with the 20 wt% KOH doped membranes, being comparable with Fumasep commercial benchmark. Conclusions In this study we successfully manufactured a novel PBI based anionic membrane avoiding complex chemical reactions to perform or harsh working conditions thanks to the employment of the electrospinning technique and the Fumion ionomer. Moreover, we achieved ion conductivity values comparable to a commercial AEM while employing a backbone polymer well-known for its stability under the mildly alkali conditions of AEMWE. Acknowledgments This research was supported by the “POR H2 - W.P. 3.1 - Ricerca e sviluppo di tecnologie di stack, componenti e processi, per migliorarne le prestazioni e ridurne i costi. LA 3.1.1 Sviluppo di processi per la produzione di membrane a conduzione protonica ad alta temperature”. References [1] N. Du, C. Roy, R. Peach, M. Turnbull, S. Thiele and C. Bock, “Anion-Exchange Membrane Water Electrolyzers,” Chem. Rev., vol. 122, pp. 11830-11895, 2022. [2] D. Henkensmeier, W.-C. Cho, P. Jannasch, J. Stojadinovic, Q. Li, D. Aili and J. O. Jensen, “Separators and Membranes for Advanced Alkaline Water Electrolysis,” Chem. Rev., vol. 124, no. 10, pp. 6393-6443, 2024. [3] E. D. Gregorio, G. Roviello, V. Naticchioni, V. Cigolotti, A. Pozio, L. A. Hein, C. D. Luca, C. Ferone, A. Rinaldi and O. Tarallo, “Electrospun Polybenzimidazole Membranes: Fabrication and Fine-Tuning Through Physical and Statistical Approaches,” Polymers, vol. 17, no. 12, 2025. [4] S. Rakhshani, L. A. H. Rodolfo Araneo, A. Rinaldi and A. Pozio, “Fabrication and Characterization of Anionic Composite Membranes Produced by Electrospinning Method,” Polymers, vol. 17, no. 12, 2025.