Preparation And Investigation Of Novel High Temperature Proton Exchange Membranes For Fuel Cell Applications

Fuel cell is a kind of energy conversion device, which can directly convert the chemical energy into electrical energy by electrochemical reactions. The fuel cell is regarded as "the ideal power generation device during 21st century". Among the various types of fuel cells, the proton exchange membrane fuel cell (PEMFC) is considered as the excellent power source candidate for using in electric vehicles, submarines and portable power sources due to its advantages such as fast start-up at room temperature, fast shift of the output power according to the load requirement. The PEMFC operating at 120-200? rather than 80-90? will bring several technical advantages, including high tolerance to CO and simple water and thermal management systems. Therefore, development of high temperature PEMFCs has become the most attractive research subject in recent years. As a key material of high temperature PEMFC, proton exchange membranes (PEMs) should possess high proton conductivity under elevated temperature and low humidification, and also occupy high thermal stability and mechanical strength. Developing high temperature PEMs with excellent physicochemical properties and high durability is an important issue for the progress of PEMFCs as well as its commercialization.The phosphoric acid (PA) doped polybenzimidazole (PBI) membranes as the electrolyte promoted PEMFC working at elevated temperatures, and thus are regarded as the most successful candidate of the high temperature PEM. Polybenzimidazoles are of the same category polymers, which contain aromatic heterocyclic unites with excellently thermal stability, film-forming and mechanical properties. The basic benzimidazole groups in the PBI structure have high affinity to acid molecules, which can be doped into the polymer backbones. The phosphoric acid doping level (ADL) of the PBI membrane determines its conductivity; the higher ADL, the higher conductivity of the membrane. However, the high ADLs may dramatically decrease the mechanical strength of PBI/PA membranes and thus remarkably affect the performance and life time of the fuel cell. It is therefore to say that the contrary effect of the ADL on the conductivity and mechanical strength of PA doped PBI membranes is a critical issue to be solved. The stability and durability of the natural PBI membrane, eapecially for long term use at elevated temperatures, also need to be improved.The aim of this research is to improve the physicochemical properties of PBI based membranes under high ADLs and elevated temperatures by chemically modifying the structure of PBIs for increased fuel cell durability. The chemical modifications include: modification of the monomer structure, chemical grafting, three-monomer copolymerization and crosslinking. In addition, high temperature PEMs based on ionic liquid modified polysulfone (PSU) and Nafion are also fabricated and investigated.A brief overview on the development of fuel cell and a comprehensive review of updated progress of high temperature PEMs including PBI membranes are given, respectively, in Chapter 1. All the experimental methods and techniques adopted in the present thesis are described in Chapter 2.The influence of the molecular weight of mPBI on its properties in terms of chemical stability, ADL and swelling, conductivity, mechanical strength and fuel cell performance is discussed in Chapter 3. The mPBI polymers with high molecular weights up to 94 kDa were synthesized through optimizing the polymerization reaction conditions. The durability of mPBI membranes with different molecular weights but with a similar ADL of around 10 was measured. The results indicated that mPBI membranes with high molecular weights showed superior mechanical strength, good fuel cell performance and improved durability. The mPBI membrane with a molecular weight of 78 kDa and an ADL of 10.8 exhibited tensile strength of 30.3 MPa at room temperature and 7.3 MPa at 130?, respectively. Fuel cell tests with H2 and air at 160? showed an open circuit voltage (OCV) of 0.942V, a peak power density of 295 mW cm-2, and a low degradation rate of 1.5 ?V h-1 at a constant load of 300 mA cm-2 within a period of 1580 h.In Chapter 4, novel PBI polymers with side-chain or main-chain benzimidazole groups and with main-chain hydroxyl pyridine groups synthesized via grafting reaction, modifying monomer and copolymerization reaction, respectively, are presented. A concept of molar acid conductivity was proposed to evaluate the effective conductivity contributed from the doping acids. The additional basic groups of benzimidzole or hydroxyl pyridine in the polymer structure resulted in higher density of basic sites, and therefore higher ADL, conductivity and better fuel cell performance. With a grafting degree of 5.3%, the mPBI-5.3%BeIm/13.1PA membrane exhibited a conductivity of 0.14 S cm-1 and a peak power density of 351 mW cm-2 feeding with H2-air at 160?. In addition, an improved tensile modulus at elevated temperatures than at room temperature was observed for acid doped hydroxyl pyridine containing PBI (OHPyPBI) membranes. The synthesis of PBI copolymer (AB-mPBI) was carried out with the assistance of microwave irradiation and the polymerization reaction could be performed within much shortter time.Studies on an aryl sufone containing PBI (SO2PBI) and its copolymers (Co-SO2PBIS) are presented in Chapter 5. A series of copolymers containing SO2PBI and para-phenylene (pPBI) repeat units were synthesized by means of three-monomer polycondensation. Compared with mPBI, the presence of the flexible aryl sulfone linkages resulted in an increased solubility in organic solvents and high stability of the copolymers; while the stiff para-phenylene in the structures ensured the membranes with excellent mechanical properties. Moreover, covalently cross-linked SO2PBI membranes (CL-SO2PBI) were fabricated using poly(vinylbenzyl chloride) as a crosslinker. Both Co-SO2PBI and CL-SO2PBI membranes could achieve high ADLs by performing the acid doping with high concentration PA solutions at elevated temperatures, hence to exhibit higher conductivity and better fuel cell performance. The test results indicated that the chemical stability and mechanical strength of the SO2PBI membranes could be improved by the crosslinking. Life time tests of the Membrane-Electrode-Assemble (MEA) based on Co-20%SO2PBI/11.5PA membranes under a constant current density of 300 mA cm-2 at 160? exhibited a degradation rate of 6.4?v h-1 during a period of 2400 h; and MEA based on CL-6.6%SO22PBI/13.1PA membrane showed an average degradation rate of 13.7?Vh-1 over 2200 h under the same testing condition.In Chapter 6, studies on a hexafluoropropylidene containing PBI (F6PBI) membranes crosslinked with a macromolecule crosslinker are introduced. The F6PBI polymer was easy to dissolve in organic solvents and possessed superior chemical stability. Nevertheless, a plastic deformation of the PA doped F6PBI membrane was observed at 130?. To solve this problem, crosslinking of the F6PBI membranes by using a polymeric crosslinker was proposed. The chloromethyl polysulfone was synthesized and employed as the polymeric crosslinker. The polymer crosslinked F6PBI had improved stability in high concentration PA solutions at elevated temperatures comparing with small molecule crosslinked F6PBI membranes with a similar crosslinking degree. Comprehensive characterizations on physiochemical properties of the crosslinked F6PBI membranes including fuel cell performance and life time were investigated. Durability tests at 160? with a constant current load of 300 mA cm-2 showed no noticeable performance degradation during a test period of 3500 h for MEA based on F6PBI crosslinked membranes. The proposed concept of the macromolecular crosslinker open-up a new rout for the improvement of the PEMs and fabrication of novel membrane material.High temperature PEMs based on ionic liquid modified polysulfone and Nafion are studied and given in Chapter 7. The composite membranes of imidazolium cations (Im or BMIm) modified polysulfone (ImPSU) and Nafion (Nafion/BMIm) were prepared by chemical grafting reaction and electrostatic assemble approach, respectively. Due to the presence of the imidazolium cations, the composite membranes had good ability of doping phosphoric acid. The influence of polymer structures on the swellings of the membrane was investigated by calculating the molar free volumes of different types of membranes. Compared with PA doped mPBI membrane, the two composite membranes showed much lower swellings at a similar ADL. The results of conductivity and mechanical strength demonstrated the composite membranes as high temperature PEMs. The MEA assembled by PA doped ImPUS membranes had an OCV of 0.96 V and peak power densities of 175-204 mW cm-2 at 150? under H2 and air without optimization of gas diffusion electrodes.