Preparation of PBI membranes with controlled thickness for use in PEMFCs and the impact on water and phosphoric acid uptake

Protonated poly (2,2′‐m‐phenylene‐5,5′‐bibenzimidazole) (PBI) membranes are good electrolytes for proton‐exchange membrane fuel cells (PEMFC) as they can be operated at high temperatures, thus favoring catalysis reaction kinetics, CO elimination, and water management. Electrolyte production from polymeric membranes with controlled thickness is important for the study of their properties and their use in fuel cells (FCs). When preparing PBI membranes by casting with controlled thickness, polymer solubility in different solvents was assessed as well as the relation between petri dish diameter and solution concentration. Membranes with different thicknesses were phosphonated and had their phosphoric acid (PA) and water uptake measured. A systematic procedure was established to prepare membranes with predetermined thickness using 12 mL of 5–8% PBI in N,N‐dimethylacetamide in petri dishes measuring 5–9 cm. Membranes with predicted thickness of 60 ± 20 μm and 200 ± 40 μm showed a maximum water uptake of 8.34 ± 0.01% and PA uptake of 32 ± 1% and 18 ± 2%, respectively. This study contributed significantly to systematize procedures for producing high‐quality membranes with predefined thickness and to understand the influence of their thickness on their degree of phosphonation.