High Temperature Proton Exchange Membranes Based On Modified Perfluoronated Sufonic Acid Resins

Because of its high efficiency of energy conversion and zero pollution, proton-exchange membrane fuel cells (PEMFCs) show great potential in application for transportation and stationary power. PEMFCs operating at high temperature have attracted considerable attention in recent years because they could benefit from enhanced tolerance to impurity of the fuel gas, improved water and heat management, and increased reaction rates at both cathode and anode. The generally used PEMs for current PEMFC technology are perfluorosulfonic acid (PFSA) polymeric membranes, e.g. Nafion membrane. However, the drastic decrease in proton conductivity at low humidity and the low glass transition temperature of the PFSA limit the operating temperature of fuel cells to maximum of 100℃. In this thesis, we firstly analyzed the validity of the commonly used approach for proton conductivity test and further investigated the properties of membranes at high temperature based on modification of PFSA membranes, Nafion membranes incorporated with TiO2 nanoparticles and short-side-chain PFSA membrane. Some conclusions have been drawn as following:(1) A testing cell was designed with controllable temperature and humidity to test the proton conductivity of PEMs at different temperatures and relative humidity. During the AC impedance measurement, Q(RC) should be selected as the equivalent circuit, and the impedance spectra comprised a semicircle in the high frequency region, corresponding to the bulk membrane impedance, and an inclined spur in the low frequency region, corresponding to interfacial capacitance. Thus, the frequency should be set as high as possible to obtain the more region of semicircle (1MHz for the Autolab in our laboratory). Fitting of impedance data to the equivalent circuit is a more accurate method to analyze Nyquist plot compared to linear regression.(2) Crystallized anatase type titania nanoparticles with diameters of 4nm were in situ formed in Nafion solution through sol-gel process. Self-assembled Nafion molecules at surface of titania nanoparticles through electrostatic interactions prevented the further growth of initial formed particles. The Nafion-titania nanocomposite membranes were formed using a recasting process. It was found that the addition of titania nanoparticles had little effect on the crysatllinity and chemical structure of Nafion in the membrane significantly. The formed Nafion-titania nanocomposite membrane shows enhanced water retention ability and higher proton conductivity at high temperature compared to recast plain Nafion membrane. This work demonstrates the potential of Nafion-titania nanocomposite membranes for elevated temperature PEMFC applications.(3) By comparison the structure and performance of Aquivion short-side-chain membrane (EW790) with Nafion211 (EW1100), it has been found that both membranes have similar chemical and crystal structure, and the crysatllinity of Aquicion membrane is higher than the latter (19.8%to 15.5%). Furthermore, Aquicion membrane has better thermal stability and thermo-mechanical properties than Nafon211 membrane. The improved proton conductivity and single cell performance using Aquicion membrane have been observed compared to that using Nafion 211 membrane, indicating that the low EW Aquicion membrane has the potential to be applied for high temperature PEMFC electrolytes.