Preparation And Characteristics Of Poly(Ether Ether Ketone)-based Hydroxide Exchange Membranes

As a new type of fuel cell technology, hydroxide exchange membrane fuel cells (HEMFCs) have the ability to offer high electrode kinetics, usability of non-precious catalysts, and good catalyst stability. Hydroxide exchange membrane (HEM) is one of the key components in HEMFCs and it serves as fuel/oxidant separator and hydroxide conductor. So far, commercial HEMs cannot match the requirements of HEMFCs, due to their low hydroxide conductivity and poor chemical stability. In this thesis, poly(ether ether ketone)(PEEK) was chosen as the matrix to prepare highly conductive HEMs. PEEK has a unique two strongly electron-donating ether bonds-constructed chemical structure which endows the benzene ring of PEEK with fairly high electron-density, thus increasing the basicity of the linked hydroxide conducting functional group.The chloromethylation of PEEK was for the first time carried out by using sulfuric acid as solvent and catalyst simultaneously. The controllability of the reaction was studied. It was found that to avoid the gelation of reaction solution, sulfuric acid/polymer ratio has to be no more than60mL g-1, leading to a fast reaction. As a result, a stringently sub-ambient reaction temperature, e.g.-10℃, is required for the good controllability of DCDC of CMPEEK. Then, a new methanesulfonic acid-dissolving route was proposed to conduct the chloromethylation of PEEK, where methanesulfonic acid was used as the solvent, decreasing the sulfuric acid/polymer ratio, further reducing the reaction rate. For the sulfuric acid/polymer ratio of20mL g-1, the reaction rate constant is1.29h-1at25℃which is even lower than that at-10℃(1.88h-1) for sulfuric acid-dissolving route. The DC (0.30-1.29) of CMPEEK could be easily controlled at room temperature.PEEK-QAOH membranes with ion exchange capacity (IEC) of0.43-1.35mmol g-1were prepared by preparing CMPEEK membrane by a solution casting method and then immersion in trimethylamine aqueous solution and ion-exchange in1M KOH solution. The conductivity of PEEK-QAOH membranes increases with IEC, and the highest conductivity the PEEK-QAOH membrane reaches is17mS cm-1. Compared with all other trimethylamine aqueous solution quaternized aromatic polymer based HEMs with～1.0mmol g-1of IEC, hydroxide conductivities (12mS cm-1at～30℃) of PEEK-QAOH is comparable to the highest value (13mS cm-1) reported. PEEK-QAOH is stable in1M KOH at20℃, but unstable at30-60℃. Soluble quaternary-phosphonium functionalized PEEKs (PEEK-QPCls) were synthesized by the reaction of CMPEEKs and tri(2,4,6-trimethoxyphenyl)phosphine, and the corresponding PEEK-QPOH membranes were prepared by a solution casting of PEEK-QPCl and subsequent ion-exchange in1M KOH. IEC of PEEK-QPOH membranes arranges from0.89to1.19mmol g-1. The conductivity of PEEK-QPOH membrane with IEC of1.19mmol g-1is up to61mS cm-1at20℃, much higher than that of PEEK-QAOH membrane (17mS cm-1). PEEK-QPOH shows good solubility in low-boiling-point solvents:soluble in methanol, ethanol, n-propanol, and their aqueous solution. In addition, PEEK-QPOH has good chemical stability at20-60℃.Imidazolium-functionalized PEEKs (PEEK-ImCl) were synthesized by the Menshtkin reaction of CMPEEKs and1-methylimidazole, and then the PEEK-ImOH membranes with IEC of1.56-2.24mmol g-1were prepared by the solution casting method. The conductivity of PEEK-ImOH with IEC of2.03mmol g-1is up to52mS cm-1at20℃. Hydrated PEEK-ImOH membrane has a high tensile strength (18MPa) and good flexibility (elongation-to-break of167%). After drying, the tensile strength increases to78MPa and the flexibility was maintained (elongation-to-break of168%). PEEK-ImOHs have desired selective solubility:insoluble in alcohols but soluble in50vol.%aqueous solutions of acetone or tetrahydrofuran. PEEK-ImOH membranes exhibit low methanol permeability of1.3-6.9×10-7cm2s-1which are much lower than that of Nafion212membrane (6.5×10-6cm2s-1). The H2/O2single cell employing PEEK-ImOH as both the membrane and the electrode ionomer shows a high open circuit voltage of0.84V and a high power density of31mW cm-2. PEEK-ImOH membrane is stable in1M KOH at20℃, but unstable at30-60℃.To enhance the chemical stability of imidazolium-functionalized membranes, PEEK-AelmBrs were synthesized by the reaction of CMPEEKs and an imidazolium with an amine group, and the corresponding PEEK-AelmOH membranes were prepared by a solution cast method. The hydroxide conductivity of PEEK-AelmOH membrane with a theoretical IEC of2.07mmol g-1reaches48mS cm-1at20℃that increases to71mS cm-1at60℃. PEEK-AeImOH membrane has a good chemical stability at20-60℃.