| Polymer electrolyte fuel cells are expected to become a prominent technology in a variety of power generation applications due to their high efficiencies for energy conversion along with low pollution levels, noise and maintenance costs. Currently, the most widely investigated fuel cells are proton exchange membrane fuel cells (PEMFCs), in which Nafion-type (perfluorosulfonic acid) membranes act as polymeric proton conductors. However, the high cost of the expensive Nafion membrane and platinum catalysts is still an economic barrier to the widespread application of PEMFCs. However, alkaline polymer electrolyte fuel cells (APEFCs) operate at high pHs and offer advantages such as higher reaction kinetics at the cathodes and permitting the use of a lower quantities of a noble metal catalysts or non-precious metal catalysts (to replace Pt). Hence, worldwide efforts on developing APEFCs have recently intensified with a significant focus on the core anion exchange membrane (AEM) component. However, the current reality is that AEM performances have not been able to match those of PEMs [used in PEMFCs].To date, the majority of AEMs have been based on polymer electrolytes containing quaternary ammonium (QA) cationic functional groups (polymer-N+R3). They are commonly prepared via post-amination of pre-formed polymer films due to the poor solubility of QA salts in organic solvents. However, the low alkaline stabilities and ionic conductivities of the cationic polymer electrolytes continue to be major challenges for APEFCs. The former is due to the well-know QA degradation pathways deriving from the attack of the strong nucleophile OH-, while the latter is often due to the inability to form ionic aggregates [from phase separation] during the heterogeneous amination process. Due to these issues, alternative anion exchange [cationic] head-group chemistries should be proposed with the aim of enhanced alkaline stabilities and solubilities in organic solvents.Moreover, for the common methods of AEM preparaiton, a large number of organic solvents are used. This excess use of toxic organic solvents should not only harm human health but also raise costs of the AEMs. Additonly, the resultant large quantities of waste present serious hazards and the risk of pollution to the environment. Therefore, there is a strong desire to develop a method of AEM preparation that does not involve any organic solvents.On the basis of above considerations, three types of AEMs with different anion exchange groups (guanidinium, benzimidazolium, dimethylimidazolium-based) but quaternary amonium groups have been preapred based on poly phenyleneoxide)(PPO) main chains. The ion exchange capacity (IEC) of the resultant AEMs will be controlled and optimized by the amount of anion exchange goups in the membrane matrix. Also, the main fuel cells-related properties of these AEMs have been investigaed in detail and found to be comparalbe to some reported AEMs. Moreover, Novel solvent-free route for anion exchange membranes (AEMs) preparation has been established by employing an in situ bulk polymerization strategy. Apart from endowing the resultant AEMs with gratifying performances, the method employed herein is environment-friendly particularly due to the avoidance of organic solvents.The main conclusions based on this research are as follows:(1) Novel alkaline anion-exchange membranes (AAEMs) containing pendant quaternary guanidinium groups were synthesized from poly (2,6-dimethyl-1,4-phenylene oxide)(PPO) through benzyl bromination followed by reaction with1,1,2,3,3-pentamethylguanidine (PMG). The AAEMs exhibit high ionic conductivities (up to71mS cm-1at room temperature), their thermal and alkaline stabilities are also good due to the presence of the π electron conjugated systems.(2) Novel benzimidazole (BIm) functionalized anion exchange membranes (AEMs) based on PPO are synthesized and characterized for alkaline fuel cells (AFCs). Such BIm-PPO AEMs show good mechanical and thermal stabilities as well as the favorable fuel cell-related indicators, including high ion exchange capacity, proper water uptake and high ionic conductivity. In addition, a single H2/O2fuel cell test by employing the optimal BIm-PPO-0.54AEM yields a peak power density of13mW cm-2at35℃, indicating the potential application in AFCs.(3) Novel anion exchange membranes (AEMs), based on poly(phenylene oxide)(PPO) chains linked to pendant1,2-dimethylimidazolium (DIm) functional groups, have been prepared for evaluation in alkaline polymer electrolyte membrane fuel cells (APEFCs). The ionic conductivities of the resulting DIm-PPO AEMs at30℃are in the range of10-40mS cm-1and18-75mS cm-1at60℃. Moreover, the maximum power densities produced in simple beginning-of-life single cell H2/O2fuel cell tests by employing the optimal DIm-PPO-0.43and DIm-PPO-0.54are30mW cm-2and56mW cm-2, respectively.(4) Novel solvent-free route for anion exchange membranes (AEMs) preparation has been established by employing an in situ bulk polymerization strategy. The preparation conditions of the AEMs have been fully investigated and under the optimized conditions, the obtained AEMs exhibited high IEC (1.43～1.76mmol.g-1), favorable water uptake (57.9～82.5%) and hydroxide conductivities (0.019～0.022S.cm-1), excellent mechanical strength (Ts>50MPa) and low methanol permeability (2.82×10-7cm), which are comparable to or better than those of some reported AEMs and implicate great potential for the application in alkaline anion exchange membrane fuel cells (AAEMFCs).(5) Novel cross-linked anion exchange membranes (AEMs) have been prepared using a solvent free strategy using tetraethylenepentamine (TEPA) as cross-linker. Apart from improving the compatibility of the mains pahse in membrane matrix, TEPA also effectively enhances the charge density, ionic conductivity and alkaline stabilities of the resulting AEMs, whilst simultaneously inhibiting the swelling ratio. A power density of6mW cm-2is achieved in a H2/O2fuel cell test at50℃... |
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