Preparation And Properties Of Novel Anhydrous Acid-base Proton Exchange Membranes Based On Polysilsesquioxane

Proton exchange membrane fuel cells (PEMFCs) are the fifth generation of the fuel cells. There are a lot of advantages of the PEMFCs, such as efficient energy transformation, fast initiation, high power density and high specific energy. It is accepted that PEMFCs are one of the most promising electrochemical energy conversion devices. PEMFCs are suitable for portable and distributed power applications. PEM is one of the key items of the PEMFCs. It directly influences the cost and the efficiency of the PEMFCs. The research on the PEM will be helpful to the wide application of the PEMFCs.Hybrid materials is one of the hot materials in recent years, it combines the advantages of the organic and inorganic materials. So, hybrid materials have the excellent performance comparing with common materials. The Polysilsesquioxane is an important class in the hybrid materials. It is usually synthesized by sol-gel method. Polysilsesquioxane based materials typically have good heat resistance, corrosion resistance, easy functionalization, and so on. Thus, application of polysilsesquioxane to areas of high temperature proton exchange membrane is a meaningful and challenging work. In this paper, a series of acid-base type of high temperature proton exchange membranes based on basic polysilsesquioxane system were studied. Particularly the mechanism of the proton conduction in these membranes, as follows:(1) Inorganic-organic hybrid proton exchange membranes were prepared via sol-gel reaction of N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AAS) in a sulfuric or phosphoric acid aqueous solution. The chemical structures of these membranes are characterized by means of Fourier transform infrared (FTIR) and 29Si cross polarization nuclear magnetic resonance (29Si CP NMR). Those acid-doped membranes were stable at temperatures up to about 300℃and showed varied conductivities at different temperature ranges. Optical Microscopy and Scan Electron Microscopy (SEM) analysis revealed that the introduction of H2SO4 led to the generation of nanoparticles in situ. Atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS) results indicated that these nanoparticles were wrapped by the soft organic side chains. Root-mean-square (RMS) roughness measured by AFM demonstrated that lower water addition during synthesis led to rougher surface and higher conductivity of H2SO4-doped membrane, while the surface of H3PO4-doped membrane remained smooth and clean, and the conductivity did not show significant change by varying water additions. All those results demonstrated that the higher conductivity of the H2SO4-doped membrane achieved was contributed to not only the dissociation of the counter anion but also the morphology of the membrane. Finally, we proposed a potential mechanism for the proton conduction in such acid-doped membranes. This mechanism could possibly provide a method to construct effective proton channels in the cross-linked anhydrous proton exchange membranes.(2) In the present work, H3PO4-doped 1,2,4-triazole-polysiloxane proton conducting membrane is successfully prepared by sol-gel reaction. The molecular structure of the PGA-xH3PO4 is confirmed via FTIR. Thermogravimetry (TG) analysis shows that the samples were thermally stable up to approximately 250℃. The fracture surface morphology of the materials is characterized by SEM. The temperature dependence of proton conductivity of all the membranes exhibits an Arrhenius behavior. The proton conductivities of these membranes increase with dopant concentration and the temperature. In an anhydrous state, the proton conductivity of PGA-1H3PO4,PGA-2H3PO4 and PGA-3H3PO4 is 1.48×10-3 1.07×10-2,1.43×10-2 S/cm at 200℃, respectively. According to FTIR results, the added H3PO4 is presumed to break up the hydrogen-bonding network of pure PGA, facilitating ring-reorientation and thus Grotthus mechanism proton transport. In PGA-2H3PO4, the extra H3PO4 may act as a bridge providing effective proton conduction. Therefore the proton conductivity of PGA-2H3PO4 is greatly improved compared with PGA-1H3PO4.(3) The development of anhydrous proton exchange membrane is critical for the polymer electrolyte membrane fuel cell PEMFC operating at intermediate temperature (100～200℃). In the present work, inorganic-organic hybrid proton exchange membranes were prepared via sol-gel reaction of N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AAS) and PVA in a phosphoric acid aqueous solution. The chemical structure of the hybrid membrane was confirmed by FTIR. The film-forming capacity and acid content of these membranes were enhanced by the introduction of the PVA. Those PVA-doped acid-base membranes were stable at temperatures up to about 300℃.The proton conductivities of the membranes were measured under anhydrous conditions. The dependence of proton conductivity to temperature of all membranes showed an Arrehnius behavior. There was a visible drop of conductivity from 10% to 15% PVA content. The surface and fracture surface morphology of the membrane was observed by AFM and SEM, respectively. A large scale phase separation was appeared at 15% or 20% PVA content which led to the decreased tensile strength and proton conductivity of the membranes. All the results suggest that appropriate PVA doping level (≤10%) was helpful to the membrane formation and improved the mechanical property of the membrane but with little effect on the proton conductivity.(4) Amphibious proton exchange membranes which could be used under both wet and dry conditions were prepared by sol-gel method in this work. Those novel hybrid PEMs were constructed by three parts:1) Polysiloxane with two basic sites (-NH-on the pendant and=N-in the triazole); 2) H3PO4 as the proton source; 3) Poly(vinyl alcohol) (PVA) which had good film-forming capacity and ability to anchor the H3PO4 under wet conditions. The resulting hybrid membranes were thermally stabilized up to 200℃. A matrix-change between polysiloxane and PVA could be observed at a high PVA doping level. The proton conducting property of these membranes was investigated under both hydrous and anhydrous conditions. From 25 to 120℃, the non-soaked and soaked (soaked in the water) membranes showed the proton conductivity of 0.019～0.068 and 0.009～0.031 S/cm at 100% relative humidity (RH), respectively. Under completely dry conditions, the proton conductivity of these membranes showed large dependence on the temperature and the proton conductivity of 0.0047～0.021 S/cm was achieved at 150℃for these membranes. The excellent performance of these hybrid membranes under both wet and dry conditions demonstrated that they have potential use as electrolytes in PEMFCs operating either in a watery or in a water-free environment and so called "amphibious" proton conducting membranes.