Description
A Master of Science thesis in Chemical Engineering by Hanin Mohammed entitled, "Zirconium Phosphate/Ionic Liquid Proton Conductors for High Temperature Fuel Cell Applications," submitted in November 2017. Thesis advisor is Dr. Amani Al-Othman and thesis co-advisor is Dr. Paul Nancarrow. Soft and hard copy available.
Abstract
High temperature operation (> 120 °C) is preferred in proton exchange membrane (PEM) fuel cells. It enhances the electrode kinetics, improves the catalyst tolerance for impurities and allows the use of lower cost fuels such as hydrocarbons. However, high temperature operation is not possible using the conventional Nafion membranes. Their proton conductivity decreases dramatically beyond 90 °C. Therefore, this work aimed at developing Nafion-free proton conducting material, based on zirconium phosphates (ZrP) and ionic liquids (IL) to allow a high temperate operation. ZrP/IL proton conducting materials were prepared via the reaction of zirconium oxychloride ZrOCl₂ in an aqueous solution with phosphoric acid H₃PO₄ at room temperature. Seven different ionic liquids, were investigated in this work. The ionic liquid component was added to the ZrOCl₂ solution prior to the precipitation reaction with IL contents ranging from 0.4-5% by mass. The modified materials were investigated for their proton conductivity. The results of this work demonstrated that the addition of ionic liquids enhances the proton conductivity of the ZrP material by orders of magnitude. Among all the tested ionic liquids, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-butyl-3-methylimidazolium dicyanamide, and 1-butyl-3- methylimidazolium triflate produced the best results with conductivities of 2.26X 10⁻², 1.61X10⁻² and 1.36X 10⁻² S cm⁻¹, respectively. The proton conductivity of the unmodified ZrP prepared in this work was equal to 9.24X 10⁻⁴ S cm⁻¹. The modified samples were analyzed by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and scanning electron microscopy (SEM). Results showed the enhancement of water uptake properties by 40-60%, changes in morphology and changes in structure upon the introduction of the IL component. The modified samples were processed at high temperature (200 °C) under completely anhydrous conditions and showed a high anhydrous proton conductivity on the order of 10⁻⁴ S cm⁻¹. In conclusion, it appeared that the ionic liquids have formed hydrogen bonds with the ZrP molecules and hence, provided additional pathways for the proton transfer and effective proton hopping sites. The enhanced conductivity of the ZrP/IL materials make them good candidates as solid proton conductors for fuel cells applications.
