Aggregation Microstructure Control The Property Of Proton/Anion Exchange Membrane Based On Ionic Liquid And Polyvinyl Alcohol

Fuel cell is one of the most attractive energy conversion systems, which can transform chemical energy directly into electricity. In fuel cells, the membrane serves as the electrode separator in addition to its primary role as a continuous medium for conducting ionic. As commercially available proton exchange membrane (PEM), Nafion has been widely used due to high proton conductivity, excellent chemical and mechanical properties. However, there are still some drawbacks for the use of Nafion in fuel cell. So, many studies have been carried out to develop new kinds of proton/anion conducting membranes in recent years. Most of these studies emphasized on the influence of macroscopic factors on the proton conductivity and swelling property of proton/anion exchange membrane, such as the influence of proton donor doping level. But the studies about the influence of microscopic factors, such as the influence of the ionic clusters on the property of membrane are limited. So, it should be paid more attention on the deep-seated influence factor-microstructure.In order to investigate the effects of different microstructure on the property of proton/anion exchange membrane, different molecule architecture was designed based on ionic liquid and polyvinyl alcohol (PVA). These different molecule architectures are employed to manipulate the ion-aggregating structures of proton/anion exchange membranes, which is crucial to improve the property of membrane. The outline and contents of this dissertation are as follows:1. The high temperature PEM is needed in the fuel cell. A new approach to increase the operating temperature of the membrane is the introduction of ionic liquids (ILs) into the membranes. ILs, possessing unique properties including high thermal stability, high ionic conductivity and wide electrochemical window, are such perfect proton-donating materials to improve the property of membrane at high temperatures. Following investigations were applied:(1) PVA PEM were prepared using succinic acid (SA) as a cross-linking agent and Br(?)nsted acidic ionic liquid (BAIL) as a proton source. The incorporated BAILs resulted in a relatively high proton conductivity compared with PVA/SA membrane without BAILs. The proton conductivities of PVA/SA/BAILs composite membranes increased versus the BAIL content. In addition, the optimal resultant proton conductivity of PVA/SA/BAILs composite membrane under dry condition could reach 0.4 mS·cm-1 at 140℃. It was notable that the PVA/SA/BAILs composite membranes could reach high thermal stability up to 150℃, which was higher than that of traditional PVA membranes (below 80℃).(2) A series of novel anhydrous PEM (poly (vinyl alcohol)-citric acid-ionic liquid (PVA-CA-IL)) were prepared using the low cost ionic liquids of ethylammonium nitrate (EAN), diethylammonium nitrate (DEAN) and triethylammonium nitrate (TEAN) as conductive fillers in PVA support membrane. The properties of the PVA-CA-IL membranes can be controlled by changing the molar ratio of the PVA, ILs and CA. The effects of temperature, ILs and crossliker dosage on proton conductivity were also systematically investigated. The results showed that the PVA-CA-IL membranes had excellent performance, which can be used at 160℃. The proton conductivity of PVA-CA-EAN (mole ratio= 1:0.05:0.4) could reach up to 7.8 mS·cm-1 at 140℃. The introduction of ionic liquid into PVA membrane constituted a new and efficient kind of anhydrous proton exchange membrane.2. To produce proton ordered conducting and lower methanol permeable ultrathin membrane for fuel cell, the imidazolium salt as both cross-linking agent and proton donor has been designed to build a proton ordered conducting channel in a PVA-based membrane. Meanwhile, these proton ordered conducting membranes resulted in higher proton conductivity, water uptake and better oxidative stability compared with proton unordered conducting membrane. The methanol permeability of proton ordered conducting membrane is very low, which is about twice smaller than that of Nafion117. The IEC value of proton ordered conducting membrane was slightly larger than that of proton unordered conducting membrane, but the proton conductivity of proton ordered conducting membrane presents almost double than that of proton unordered conducting membrane, which indicated the importance of the proton ordered conducting.3. The relationship between the grafting ratio of hydrophobic side chain and the microstructure of PEMs which can affect the property of the membrane has been investigated carefully. When the hydrophobic side chains were grafted onto PVA, the SAXS results showed that the PIL/PVA/CA-x (x=0.05,0.1,0.15) can generate some ionic clusters which is benefit for the proton conduction. Meanwhile, the membrane of PIL/PVA/CA-0.05 showed the higher proton conductivity at the same temperature compared with other membranes; this is attributed to the aggregation of bigger ionic clusters which can facilitate the formation of interconnected broad ionic channels. The highest proton conductivity of PIL/PVA/CA-0.05 was 158 mS·cm-1 which was higher than that of Nafion117. Moreover, the higher grafting density would reduce the tolerance of the PEM to the attack of OH’, but lower water uptake is benefit for enhancing the oxidative stability, which is induced by the entanglement of side chains under high grafting density.4. Three implementation strategies are designed to investigate the effects of different ion-aggregating structures on the property of PEM. Hydrophobic alkyl chains are grafted onto polyvinyl alcohol locating at different places between protons and polymer backbones, which are employed to develop PEMs with different ion-aggregating structures. The key PEM parameters, such as ion exchange capacity, proton conductivity, and methanol permeability, are significantly different among these membranes. Notably, the PEM with pendent-type side chain (M2) is in favor of driving the ionic clusters to aggregate and form broad ionic channels, which is confirmed by SAXS; it displays good proton conductivity (67.8 mS·cm-1 at 80℃). Meanwhile, the PEM with pendulum-type side chain (M3) is beneficial to display excellent tensile strength (22.4 MPa) and alcohol resistance performance (8.98×10-7 cm2·s-1).5. A novel poly(ionic liquid) (PIL) based on bis-imidazolium is designed to improve the property of alkaline anion exchange membrane (AAEM). As a result, the bis-imidazolium-based PILs AAEMs (PVA/DCnPIL) exhibit lower swelling ratio (higher dimensional stability) and higher chemical stability than that of mono-imidazolium-based PIL AAEM (PVA/C4PIL) under the same level of IEC value. Meanwhile, the length of the side chain of PILs was changed between the two imidazolium salts, which induces different microstructures that can affect the ionic conductivity and stability obviously. The SAXS results show that the PVA/DC8PIL can generate larger ionic clusters for facilitating the formation of interconnected broad ionic channels, which leads to higher conductivity in comparison with the PVA/DCnPIL (n=4,12) membranes. These observations unambiguously indicate that designed bis-imidazolium-based PIL with appropriate length of hydrophobic chain is an effective approach to increase the ion conductivity, dimensional and chemical stability of AAEMs.