Preparation And Properities Of Cross-linked Polymer Menbranes For Proton Exchange Membrane Fuel Cells

Fuel Cell is an electrochemical device which converts chemical energy of fuel and oxidizer derectly into electrochemical energy with high efficiency and low emission of pollutants. It is a new type of green energy with theoretical conversion up to 60%-80%. There has been a growing interest in the development of proton exchange membrane fuel cells (PEMFCs) over the past decade because they have high energy efficiency, low emission, compact cell design and other benefits, as well as they have great potential in portable devices and vehicles.As a key component of PEMFCs, proton exchange membranes (PEMs) transport protons from the anode to the cathode, as well as separate the fuel and the oxidant, and much of the research focus on developing polymer electrolyte membranes. For its effect on the material used as a PEM, put forward the following requirements:a high proton conductivity, low water and gas permeability, strong electrochemical and mechanical stability, good processability and reasonable price. Currently, the membrane materials commercially utilized in PEMFCs are perfluorosulfonic acid membranes, such as Nafion. These membranes show good chemical and physical stability and high proton conductivity. However, their poor thermal properties, expensive cost, high methanol crossover, and limited operation temperature (T<80℃) hinder its widespread commercial use. Considerable efforts to develop alternative PEM materials have been proposed.Recent developments in high-temperature PEMFCs (HT-PEMFCs) suggest that they may be more commercially viable among all the different types of PEMFCs. Operating a PEMFC at temperatures above 100℃without humidification has many advantages, such as a simple water management and cooling system, higher catalytic activity, enhanced electrochemical kinetics of the electrode reactions, recovery of waste heat, and dramatically increased CO tolerance. Therefore, the development of PEMs with high stability and proton conductivity at elevated temperatures is a major challenge. Polybenzimidazole (PBI) is a kind of high-performance polymer that exhibits good mechanical and thermal properties. On the other hand, benzimidazole rings possess both donor and acceptor hydrogen bonding sites due to its amphoteric nature. At temperatures from 100℃to 200℃, PA-PBI membranes exhibited high proton conductivities, low gas permeabilities, and strong mechanical properties. In the PA-PBI membranes, higher PA doping levels lead to higher proton conductivities. However, at a high acid doping level, although the proton conductivities are high, both the chemical and mechanical stabilities of the PA-PBI membranes deteriorate.In the chapter 1, the two different polymerization methods of melt and solution polymerization were used to the synthesis of PBI. The reaction conditions were optimized. The structure of PBI was characterized by using'H-NMR and FT-IR, and their thermal properties and solubility were tested and compared. Finally, it summarized how to select an appropriate polymerization method according to the different purposes.To overcome issues, cross-linked PBI-TMBP membranes were prepared by a facile and general heating method using 4,4'-diglycidyl(3,3',5,5'-tetramethylbiphenyl) epoxy resin (TMBP) as a cross-linker. Several of the properties of the cross-linked membranes that are important during operation in HT-PEMFCs, such as their solubility, swelling in the PA solution, oxidative stability, mechanical properties, and proton conductivity, were studied in detail. In particular, the proton conductivity of the cross-linked PBI-TMBP 20% membrane after Fenton's test (30% H2O2,20 ppm Fe2+,85℃) was investigated for the first time.Recently, a lot of work has been focused on the fabrication of novel alternative PEMs based on sulfonated aromatic polymers as possible substitutes for Nafion, due to their well-known high thermal and chemical stabilities and excellent mechanical properties for many applications as well as low cost, easy availability and easily functionalized. However, in order to achieve sufficient proton conductivity, the sulfonated aromatic polymer membranes should possess a high sulfonation level. The increasing sulfonation level of the membranes leads to overfull swelling in water, as well as high methanol crossover. Cross-linking is an efficient way to limit excess water uptake and methanol crossover. In addition, benzimidazole rings possess both donor and acceptor hydrogen bonding sites due to its amphoteric nature, a lot of work has focus on introducing benzimidazole groups into sulfonated aromatic polymers for the above purpose by different means, such as the preparation of composite membranes based on benzimidazole, imidazole or polybenzimidazole (PBI) and sulfonated poly(ether ether ketone) (SPEEK), as well as the synthesis of sulfonated aromatic polymers bearing benzimidazole pendant groups.In this work, the strategy to introduce benzimidazole groups into the SPEEK by crosslinking was firstly proposed. The aim of this study is to investigate how the different way of cross-linking (semi-IPN and covalent cross-linking) and different structure of cross-linking agent could influence the performance of cross-linked membrane. The specific contents are as follows:Firstly, a diamine-terminated polybenzimidazole oligomer (o-PBI) was synthesized via the melt polymerization, and sulfonated poly(ether ether ketone) (SPEEK) with sulfonation degree (SD) of 1.4 and the 4,4'-diglycidyl(3,3',5,5'-tetramethylbiphenyl) epoxy resin (TMBP) with the epoxy equivalent of 177 were synthesized. The SPEEK/o-PBI/TMBP composite membranes (also known as semi-IPN cross-linked membrane) with in situ polymerization between o-PBI and TMBP in SPEEK membrane were prepared for the purpose of improving the performance of SPEEK membranes with high ion-exchange capacities (IEC) for the usage in the PEMFC. The three-dimensional network structure was made through a cross-linking reaction between PBI oligomer and TMBP and the acid-base interaction between sulfonic acid groups and benzimidazole groups. Resulting membranes show a significantly increasing of all of the properties, such as high proton conductivity (0.14 S cm-1 at 80℃), low methanol permeability (2.38×10-8 cm2 s-1), low water uptake (25.66% at 80℃) and swelling ratio (4.11% at 80℃), thermal and oxidative stability, and mechanical properties. Higher selectivity was been found for the composite membranes in comparison with SPEEK.In chapter 5, a new strategy to introduce benzimidazole groups into SPEEK by covalent crosslink-king was proposed. A carboxyl-terminated benzimidazole trimer (rigid-BI) was synthesized as a crosslinker by controlling different ratio of 3,3'-Diaminobenzidine and isophthalic acid. Composite membranes were obtained by mixing the benzimidazole trimer and sulfonated poly(ether ether ketone) (SPEEK) together. Cross-linked membranes were obtained by heating the composite membranes at 160℃. We prepared composite membranes and cross-linked membranes with a majority partner, SPEEK with a high degree of sulfonation (Ds=0.76), and a minority partner, benzimidazole trimer. The majority partner guaranteed good proton conductivity of the membranes and the minority partner improved the mechanical properties and dimensional change, meanwhile, the appropriate ratio of benzimidazole groups as proton donors and acceptors should help to enhance proton conductivity. The introduction of benzimidazole groups into SPEEK by crosslinking avoided basic groups leaching out from the membranes in liquid water, as well as incompatibility behavior among the different components of the blended membranes. The composite and cross-linked membranes were investigated as PEMs used in PEMFCs. All of the properties of the cross-linked membranes showed significantly increment compared with SPEEK, such as thermal, mechanical and dimensional stabilities, methanol resistance, and proton conductivity, due to more compact structure compare with pure SPEEK and composite membranes. Transmission electron microscopy (TEM) analysis revealed different microphase separated structure between composite and cross-linked membranes. This also resulting in their differences of performance. And the results showed that this method of crosslinking was suitable for further research of PEMs.In chapter 6, we synthesized a novel benzimidazole trimer (alkyl-BI) with an alkyl compound as crosslinker based on the monomer,3,3'-dia-minobenzidine, and succinic acid, which replace the isophthalic acid in the chapter 5. The SPEEK with the same degree of sulfonation (Ds=0.76) was also used. The cross-linked membranes (c-SPEEK-Xs) based on the alkyl-BI as crosslinker were also prepared by a heating method. We systematically studied the water uptake and swelling ratio, thermal and chemical stability, mechanical properties, proton conductivity, and methanol permeability of cross-linked membranes as a function of diffenrent loading ratios of the alkyl-BI trimer. All of the properties of the cross-linked membranes were significantly improved over membranes consisting of unmodified SPEEK. In order to investigate how crosslinkers with different structures affectde the morphology and properties of the membranes, the performance of the c-SPEEK-Xs membranes was compared with that of a cross-linked membrane, c-SPEEK-BI, which was prepared by using rigid-BI as the cross-linker. The transmission electron microscopy (TEM) analysis showed that c-SPEEK-BI and the c-SPEEK-X possess different hydrophilic/hydrophobic two-phase separation morphologies, which had dramatic effects on the proton conductivity and methanol permeability. c-SPEEK-X showed lower proton conductivity but higher methanol resistance compare with c-SPEEK-BI, which resulting in the enhanced selectivity of c-SPEEK-X.