Proton Conduction Mechanism in H3PO4 Imbibed PEMs: The Effect of Chemical Structure and Steam

The proton conduction mechanism was studied in H3PO4 acid imbibed electrolytes by means of AC impedance spectroscopy for conductivity and polarization resistance measurements and thermogravimetric analysis to determine the hydration level of the membrane. It was found that the enthalpy of hydration varies between -73 and -119kJ/mol for 0mol H2O/mol H3PO4 hydration level, higher than that of H2O and H3PO4 alone, which demotes strong interaction of the water molecules with the polymer matrix. Proton conductivity is enhanced even by an order of magnitude by exposing the polymer electrolyte to water partial pressure up to 12kPa, while the activation energy of the process varies between 11 and 22kJ/mol. The significant increase in activation energy can be attributed to the strengthening of the H bonds of the H2O/H3PO4/membrane network. This increases the barrier of hydroniun/water molecules reorientation for the accomplishment of the structural diffusion/transport of proton.