| An ion exchange membrane fuel cells (AEMFCs) have been extensively investigated in recent years, due to their advantages such as low fuel permeation, facile oxygen reduction reaction in alkaline medium, convenient water management and the usability of non-noble metal catalyst. Anion exchange membrane, as the key component of AEMFCs, its low conductivity and poor chemical stability in high pH solution has limited further application of AEMFCs in many fields. As for these issues, many encouraging progresses have been achieved in recent years via experimental measures. However, there are not enough literatures on hydroxide ions transport mechanism in AEM and the ionic groups degradation mechanism. For that reason, researchers couldn’t primarily design the superior anion exchange groups and the polymer structure with high ionic conductivity. In the present study, the DFT calculation was applied to investigate the hydroxide ions transport mechanism in AEM based on quaternary ammonium functionalized polystyrene (QAPS-OH) which coupled with OH- and the degradation reaction mechanism of novel Guanidimidazolium ion in alkaline solution. Based on the above ideas, this thesis includes the following main contents:(1) Hydroxide ions transport mechanism in QAPS-AEM was studied by density functional theory (DFT). There were two processes about the minimum energy path of OH-transport in QAPS-AEM. The first process was the transport of OH" in water channel via the forming and breaking of continuous hydrogen bonds (H-bonds) line in H-bonds network. The second process was that OH" traversed across quaternary ammonium (QA) groups by following the rotation about C-CHt single bond, which was the rate determining step for OH-transport in QAPS-AEM. We also concluded that the water surroundings could reduce the rotation potential energy barriers, while the weak connectivity of H-bonds networks between two adjacent side chains would hinder OH- transport. All these conclusions were confirmed by experiments.(2) The degradation reaction mechanism of novel Guanidimidazolium ion in alkaline solution was explored by DFT. Meanwhile, the reaction mechanism for imidazolium and guanidinium attacked by OH" in water solution were also investigated. The whole transition states of every degradation reaction were searched and the crucial bonds length was analyzed for further understanding of the reaction nature. The rate determining step was confirmed by comparing the reaction barriers of each elementary reaction. Finally, we drew the conclusion that the chemical stability of ionic transfer group was dependent on the induced effect of electrons in the connected ionic groups. The chemical stability of the conjugated structure groups decreased when linking with an electron-drawing groups, while it increased when linking with an electron-donating groups.(3) Based on the results obtained from previous two parts, hexaminium-functional polysulfone membranes (HMPSf-OH) were synthesized using hexamine without β-H and containing multiple nitrogen atoms as quaternary ammonium agent. Ion exchange capacity (IEC) of HMPSf-OH membrane ranges from 1.40 to 2.25 mmol·g-1 and the degree of quaternarization is from 76.1 to 97.4%. Because of cross-linking structure, the swelling ratio change small along with the variation of water-uptake and IEC. The swelling ratio is only 10.75%, when the water uptake is up to 68.34%. The good performance of membrane in thermostability and chemical stability were confirmed by thermogravimetric analysis (TGA) and alkali-stability test.The crucial roles of H-bonds for hydroxide ions transport in AEM and the relation between the chemical stability of ionic groups and electron inductive dffects of linking groups were studied by computational chemistry. The conclusions provide novel ideal for experimenter to desigen high-performance membrane. |
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