Adsorption process of phosphoric acid on polybenzimidazole membranes: a crucial step inside operational high temperature PEM fuel cells

High temperature polymer electrolyte fuel cells (HT-PEFC) typically work at 120-200°C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like poly¬benzimidazole (PBI). Although PA doped PBI membranes were investigated in several ex¬perimental studies, the kinetic of the adsorption process, the mo¬lecular interactions bet¬ween the PA molecules and the polymer chains, poly¬con¬densation equilibria of the PA molecules as well as the implications on the proton conductivity is not finally illuminated. In this work, we have investigated the adsorption process of PA on m-PBI and a commercial AB-PBI derivative (Fumapem AM-55). A number of membranes (cross and uncross-linked) has been prepared at different doping level and analysed to elucidate the adsorption process of the PA as function of temperature, acid concentration and chemical equilibria between PA derivatives. Impedance measurements, Karl Fischer titration method and RAMAN spectra have been used to characterise the membranes. The thermodynamic aspects related to the adsorption process have been analysed with different kinetic models. Considering own and literature data on non-crosslinked m-PBI, a BET-like adsorption kinetics explains the behaviour of this polymer type satisfactorily. Using the RAMAN data, different regions in the BET-like isotherm can be correlated to the protonation of the polymer chain, formation of H-bonds directly to the chain and indirectly to still adsorbed PA molecules.