ELECTROSPUN POLYBENZIMIDAZOLE: OPTIMIZATION OF A NOVEL PROTON EXCHANGE MEMBRANE FOR HIGH TEMPERATURE FUEL CELLS

Polybenzimidazole based membranes are promising materials currently investigated for their application in high temperature proton exchange membrane fuel cells, due to their great thermal and chemical stability. In this study we investigated the possibility to enhance its conductivity by manufacturing an electrospun mat of polybenzimidazole nanofibers, starting from a synthetic material instead of a more conventional commercial one. Optimized membranes have then been produced through electrospinning technique and characterized in their morphological, mechanical and electrochemical properties. The obtained mats are made of fibers with diameters around 200 nm and show an increased conductivity compared to solvent casted ones. Introduction: Recently, polybenzimidazole (PBI) based membranes have gained attention as promising materials for ion-exchange applications, both cationic and anionic, due to their good mechanical and conductive properties. In particular, phosphoric acid (PA) doped PBI has already been employed in high temperature proton exchange fuel cells (HT-PEMFC), since it retains its excellent properties even in the harsh conditions of 140-160 °C and 0% relative humidity needed in such devices [1]. Objectives: Starting from the polymer synthesis, the aim of this work is to enhance even further the material’s great properties of mechanical strength and conductivity by manufacturing an electrospun mat of PBI nanofibers. In this way, the greater surface to volume ratio combined with a good pore interconnectivity should make the following PA doping and the consequent proton conduction mechanism even more efficient. Material and methods: PBI has been synthetised by the polycondensation of isophthalic acid and 3,3’-diaminobenzidine with polyphosphoric acid (PPA) as catalyst [2]. Since molecular weight and its distribution are fundamental parameters for both processability and membrane performances, the synthetised polymer has been then characterized through gel permeation cromathography. To evaluate physical-chemical properties the synthetised material was also characterized by thermal-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Starting from a synthetic PBI, several spinning conditions, pre- treatments of the sample and polymer/dimethylacetamide concentrations have been investigated, together with characterization of the microscopic morphology and the fibers diameters of the electrospun materials through optical and scanning electron microscopy. Finally, the optimized membranes have been mechanically and electrochemically characterized through stress strain, specific resistance and conductivity measurements. Results: An optimized synthesis procedure, with good reproducibility, was established at 200 °C, from which a molecular weight of around 30 kDa was obtained. The establishment of a consistent design of experiments led to the achievement of reproducible PBI electrospun mats made of nanofibers with a diameter of around 200 nm, obtained from a 16 wt % polymer solution and 1 wt % of LiCl in dimethylacetamide. The optimized membranes have shown values of conductivity at 140-160 °C and 0 % RH higher than 100 mS/cm. Conclusions: Lowering synthesis temperature down to 200 °C has limited the polycondensation reaction and reduced polymer’s molecular weight, allowing to process it with the electrospinning technique. In this way, the already good mechanical and electrochemical properties of PBI have been improved even further. In particular, the high surface to volume ratio given by the nanometric fibers has enhanced the PA doping, leading EFCH2 2025 - European Fuel Cell and Hydrogen Piero Lunghi Conference 2025 - Capri/Italy – September 17th-19th to interesting values of conductivity in the HT-PEMFC working conditions. According to the presented results, electrospun PBI surely is a promising candidate for proton exchange membranes in high temperature fuel cells, even though new solutions to strengthen its mechanical properties could be implemented, for example by introducing inorganic fillers. Acknowledgment 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] Jahangiri S, Aravi İ, Işıkel Şanlı L, Menceloğlu YZ, Özden-Yenigün E. Fabrication and optimization of proton conductive polybenzimidazole electrospun nanofiber membranes. Polym Adv Technol. 2018; 29: 594–602. https://doi.org/10.1002/pat.4169 [2] Yoda, N. and Kurihara, M. (1971), New polymers of aromatic heterocycles by polyphosphoric acid solution methods. J. Polym. Sci. Macromol. Rev., 5: 109- 193. https://doi.org/10.1002/pol.1971.230050102