Novel Imidazolium Ionic Liquid Based Membranes For Energy Conversion

Ionic liquids (ILs) have attracted the extensive attention of the researchers from various fields all over the world, because they have favorable properties. As a kind of new electrolyte, they are being used in many areas, such as electrodeposition, lithium batteries, fuel cells, thermochromic and so on. ILs have many advantages, such as negligible vapour pressure, non-inflammability, wide electrochemical window, good thermal and electrochemical stability, so it could be used as the proton carrier in intermediate-temperature proton exchange membrane fuel cell (PEMFC). However, there are many challenges in ILs composite membranes, such as low proton transfer number and insufficient mechanical strength. In our work, the nanoparticles and hydroxyl functional ionic liquid were introduced into composite membrane, by doing this anhydrous hydrogen-bond networks forming in composite membrane. Besides, a kind of low-temperature reversible thermochromism film based on ILs was also prepared. The results are as follows:Firstly, we prepared a ternary intermediate-temperature proton exchange membrane (Nafion/BmimBF4(50)/SiO2(10)) with Nafion, BmimBF4 and SiO2. It showed that SiO2 not only could effectively improve the conductivity and proton transfer number of the membrane, but also elevate the thermal stability and mechanical strength. The anhydrous conductivity at 160 ? was 38.34 mS cm-1 in the Nafion/BmimBF4(50)/SiO2(10) film. We designed an electrochemical hydrogen pump to determine the real proton conductivities or proton transfer number (th), the value calculated at 160 ? was 0.79, which was higher than 0.64 of the Nafion/BmimBF4 composite film, but it was less than 1.0 of the traditional water swollen Nafion. The tensile strength reached about 12 Mpa, higher than that of the recast Nafion. When the single cell was assembled with this membrane, the output power density could reach 250 mW cm-2 at 160 ? in anhydrous condition. These performance are having a certain degree of increasing based ionic liquids for intermediate-temperature proton exchange membranes reported at present.Secondly, we investigated the proton diffusion in BmimBF4 and hydroxyl functional ionic liquid (C3OHmimBF4). The diffusion coefficient of H+and Cl` in the C3OHmimBF4 and BmimBF4 had been studied by chronoamperometry, using CF3SO3H (denoted as HTfO hereinafter) as the proton source and C3OHmimCl, BmimCl as the chloridion source. The results showed that proton could transmit via hydrogen-bond networks formed in the C3OHmimBF4. The diffusion coefficient of H+and C1- were 6.33 x 10-7,3.39 x 10-7 cm2 s"1 respectively in BmimBF4+0.22 M HTfO+0.22 M BmimCl system, and the value of H+, although was slightly larger than Cl', it was still in the same order of magnitudes with Cl". It inferred that the conduct modes of proton should be the same with Cl', and the proton transfer faster may be due to its smaller radius. However, the diffusion coefficient of H+and Cl- in the system (C3OHmimBF4+0.22 M HTfO+0.22 M C3OHmimCl) were 1.68 x 10-6, 1.87 x 10-7 cm2 s'1 respectively, and the diffusion coefficient of H+was about 10 times higher than that of Cl", it indicated that proton could transfer via hydrogen-bond networks, and 1H NMR had been provided to support the formation of highly interconnected hydrogen-bond networks in C3OHmimBF4. The proton transfer in the C3OHmimBF4 system was about 5 times higher than that in the BmimBF4 system if normalized to the respective data of Cl". These results were very important for developing proton exchange membrane based this kind of ILs.Thirdly, we builded a high performance anhydrous proton exchange membranes "humidified" by hydroxyl-functionalized ionic liquid in a nano-SiO2 doped Nafion. In traditional water swelled Nafion membrane, its high ionic conductivity was contributed by the proton hopping mechanism in hydrogen-bond networks. In hydroxyl functionalized ionic liquid C3OHmimBF4 (denoted as IL-OH hereinafter), its hydroxyl can act as both proton acceptor and donor, so here we designed a PEM with IL-OH as a key component to form an anhydrous hydrogen-bond networks. FTIR showed that IL-OH had strong hydrogen bonding interaction with both Nafion and nano-SiO2, resulting in an effective interconnected hydrogen-bond networks in the Nafion/IL-OH(50)/SiO2(10) membrane. The anhydrous conductivity of this membrane reached about 39.5 mS cm-1 at 160 ?, and the tH value was about 0.9. Beside that, in our work, we had also found that IL-OH and SiO2 had a synergetic enhanced effect in mechanical strength of the Nafion membrane, it endowed the PEM with high mechanical properties, the tensile strength (-20 Mpa) of the Nafion/IL-OH(50)/SiO2(10) membrane reached, almost two times than that of the pure recast Nafion. The performance of fuel cell was significantly better, at 160 ? and 180 ?, a maximum power density 340 and 420 mW cm-2 was reached, the cell remained fairly stable over the 100 h observation with a power output of~220 mW cm-2 when it operated at 300 mA cm-2. In summary, the anhydrous hydrogen-bond fusing strategy may have initiated a new generation of hybrid sulfonated polymer PEMs for intermediate-temperature electrochemical devices.Fourthly, a kind of low-temperature reversible thermochromism film was prepared, choosing PVDF as framework, [Bmim]2NiCl4 as thermochromic materials, BmimBF4 as the additive reagent. Absorption spectra showed the film undergoed reversible thermochromism via release or coordinate of the metal-bound water molecules. Moreover, the results indicated that the addition of BmimBF4 could not only reduce the thermochromism temperature and time, but also increase the sensitivity, and the best quality ratio of PVDF, [Bmim]2NiCl4, BmimBF4 was 70:10:20. The film after continuous high temperature, low temperature frozen, repeated cycle after 100 weeks, illumination, the light transmittance value kept well. We presented a simulation test using it, the temperature difference between inside and outside cabinet could reach 9 ?, the effect of prevent rising temperature was obvious. It will be helpful in the area of energy-efficient buildings.