Preparation And Study Of Mesoporous Superacidic Sulfated Zirconia And Hybrid Membranes

With the demand for clean and renewable energy,fuel cell has been widely studied and applied in the field of transportation and power generation.As one of the most important components of proton exchange membrane fuel cell(PEMFC),proton exchange membrane greatly limits the commercialization of PEMFC owing to its performance and cost.In order to reduce the cost,a large number of studies have been carried out on the formation of composite films by using the micro-porous support materials with high specific surface area.In this paper,by using ammonium sulfate and zirconium oxychloride as the precursor blended with BP2000,adjustable mesoporous superacidic sulfated zirconia(S-ZrO2)with high surface area could be obtained.When prepared by introducing S-ZrO2 as filler to commercial Nafion,hybrid proton exchange membrane could get a very high proton conduction,which is expected to be the low-cost and high-performance proton exchange membrane used in commercial fuel cell.Zirconia is a common solid super acid,and its sulfonic acid group has the potential for proton conduction.However,the specific surface area of the traditional zirconia materials is relatively low,and the degree of surface functionalization is not high for more application.In this paper,a new synthesis method is proposed,which is based on the use of ammonium sulfate and zirconium oxychloride as precursor and prepared with BP2000 by high temperature calcination.S-ZrO2 is formed with different crystal types at different calcination temperatures,X-ray diffraction shows that only when the temperature is between 600 ? and 650 ?,the S-ZrO2 has a tetragonal phase for better proton conduction.Through Fourier transform infrared spectroscopy analysis,a large number of sulfonic acid groups are confirmed.The morphology of S-ZrO2 is observed and S-ZrO2 powder is found with the average particle size of about 8 nm by aggregation of nanocrystal.The nanocrystal boundaries and several nano-scale holesprove that mesoporous materials have been successfully prepared.In this paper,we uses BET method to measure surface area and porosity of S-ZrO2 powder and the titration method to determinate the ion exchange capacity(IEC).By adding the regulation of BP2000,we can obtain S-ZrO2 powder with high specific surface areas and the highest specific surface area is 181 m' g-1,which is much higher than most of the traditional zirconia materials and the corresponding IEC value is as high as 0.48?equiv m-2.At the same time,it can be found that the specific surface area and IEC of the sample can be adjusted by changing the ratio of BP2000 and precursor,and the ratio of them is fixed.The results show that the change of the specific surface area of S-ZrO2 does not affect the chemical properties of the surface and the surface density of sulfonic acid groups is consistent.In order to improve the performance of commercial Nafion membrane,mesoporous S-Zr02 is used as the filler and introduced into the Nafion matrix to prepare S-ZrO2/Nafion composite membrane.By observing the S-ZrO2/Nafion composite membrane under scanning electron microscopy(SEM),we find that the average thickness of them is 20.5 ?m.By observing the S-ZrO2/Nafion composite membrane under transmission electron microscopy(TEM),we find that the high density of the ion clusters aggregate in the interface between S-ZrO2 and Nafion,whichshows that the surface of S-ZrO2 is functionalized.When a large number of S-ZrO2 powders are introduced into the composite films,the ion clusters in the Nafion aggregated onto the surface of S-ZrO2,forming an ion channel with an average diameter of about lOnm.A large number of clusters gathered between S-ZrO2 and Nafion interface to form an ion pathway,which ensured the high efficiency of transmission through the film.So,combining mesoporous S-ZrO2 and Nafion can effectively improve the proton conductivity.The best proton conductivity of the prepared S-ZrO2/Nafion composite film is 0.83 S cm-1,which is about 7 times as much as commercial Nafion 211(0.12 S cm-1).Benefit from its high water retention rate,the proton conductive of S-ZrO2/Nafion composite membrane is significantly higher than that of Nafion 211 in all 20?100%relative humidity at 80 ?.When the film is applied to the fuel cell,the current density and power density of the optimal S-ZrO2/Nafion film are 1310 mA cm-2 and 786 mW cm-2 at 0.6 V,much higher than commercial Nafion 211.Its maximum power density is 988 mW cm-2,which is also much higher than that of commercial Nafion 211(726 mW cm-2).Due to its water retention properties,the power density of S-ZrO2/Nafion composite membrane is much higher than the commercial Nafion 211 under low humidity conditions.It can be seen that the S-ZrO2/Nafion composite film has excellent performance and can adapt to different humidity operating conditions,which is much better than the current commercial Nafion 211.