Adsorption of Phosphoric Acid on Electrospun PBI Membranes: A Thermodynamic and Kinetic Insight

This study presents, for the first time, a comprehensive investigation of phosphoric acid impregnation in electrospun poly[2,2’-(m-phenylene)-5,5′-bibenzimidazole] (m-PBI) membranes and a comparison with the dense counterpart. The kinetics and thermodynamics of acid adsorption were systematically explored by varying concentration, temperature, and contact time. To this aim, a tailored experimental protocol was developed to dynamically quantify the amount of adsorbed acid and to prevent acid leaching phenomena. Electrospun membranes were found to reach Acid Doping Levels (ADL) as high as 12.9 mol H3PO4/mol PBI within only 10 min, far exceeding the typical maximum of 2.3 mol acid/mol polymer observed in conventional cast membranes after 24 h of immersion. A postdoping washing step with methanol used to distinguish between “free” (loosely held) phosphoric acid and “bound” (chemically or strongly hydrogen-bonded) acid showed that electrospun membranes retained a significant amount of bound acid compared to cast membranes, highlighting their superior stability. Adsorption isotherms were effectively described using Liu’s model, which allowed the extraction of equilibrium constants and thermodynamic parameters, indicating an endothermic adsorption mechanism. Kinetic analysis based on a reactive adsorption model yielded a reaction order of 1.91 and an activation energy of 26.6 kJ/mol. In addition, preliminary in-plane proton conductivity measurements under anhydrous conditions showed promising electrochemical performances for the acid-doped electrospun membranes, with conductivities up to 132 mS/cm at 150 °C, significantly higher than those of cast membranes (91 mS/cm). These results demonstrate a direct correlation between fibrous morphology, adsorption mechanisms, and functional properties of the membranes for next-generation fuel cell applications.