| As an energy conversion device,proton exchange membrane fuel cell(PEMFC)has many advantages,such as high energy conversion efficiency and environment friendly zero emission,and has great potential in solving the unbalanced distribution of global green energy.Many studies have shown that the kinetics of the relevant reactions involved in proton exchange membrane fuel cells are directly related to the operating temperature of the fuel cell,and increasing the operating temperature of the fuel cell can effectively improve the dynamics of the relevant reactions.Moreover,the tolerance of the electrocatalysts to poisonous gases,including CO and H2 S,can be strongly improved.Most importantly,the double-phase fluid issue,which is caused by the simultaneous existence of liquid water and gas reactants on the cathode side,of low-temperature PEMFCs can be facilely solved by increasing the working temperature to over 100 oC.Moreover,the practical high-temperature application of PEMFC is therefore hindered,and significant efforts have been made to overcome this shortcoming.In order to realize the stable operation of proton exchange membrane materials in high-temperature proton exchange membrane fuel cells,researchers have also developed some new types of proton exchange membrane materials,such as polybenzimidazolephosphoric acid(PBI)materials.The leakage of phosphoric acid and its corrosion to fuel cell components hinder its further development.In general,high-temperature modification of the existing mature Nafion membrane to achieve stable operation above100 oC is the most direct and effective way to solve the high-temperature operation of proton exchange membrane fuel cells.Based on the above considerations,carry out the following tasks:1.For the modification of Nafion membrane at high temperature,the most commonly used method is to blend modified materials with Nafion to prepare composite membrane,such as silicon dioxide,titanium dioxide and zirconia.Although the high temperature proton conductivity of the composite membrane prepared by the above strategy can be significantly improved,the stability of the composite membrane,especially the mechanical stability,is often greatly affected.In order to avoid the above problems and maintain the integrity of the nanophase separation structure in the membrane materials,silica/Nafion composite membranes were prepared nondestructively using the "swelling-filling" modification strategy.Due to the continuous and ordered phase separation structure in the composite membrane and the excellent water retention ability of silicon dioxide,the high temperature proton conductivity of the composite membrane was significantly enhanced.At 110 oC and 60%RH,the proton conductivity of the composite membrane reached 33 m S/cm,which was about 30%higher than that of the pure Nafion membrane.Under the condition of low relative humidity,the maximum output power of the fuel cell was increased by 37%.2.As an efficient modified material,the silica has been widely used for the high temperature modification of Nafion.Various silica-Nafion composite membrane has been prepared by different modification strategies.Nevertheless,poor mechanical stability of composite membranes caused by particle agglomeration or excessive swelling and bed compatibility with matrices greatly affect fuel cell performance.In order to avoid above issue,the silica nanofibers-Nafion composite membrane were prepared with the silica nanofibers as the modified material in this work.On the one hand,silica nanofibers can enhance the high temperature proton conductivity of composite membrane due to excellent water retention capacity;on the other hand,nanofiber is beneficial to improvement of mechanical stability.Finally,the proton conductivity of the silica nanofiber–Nafion composite membrane at 110 °C is therefore almost doubled compared with that of a pristine Nafion membrane,while the mechanical stability of the composite Nafion membrane is enhanced by 44%.3.The random distribution and bed compatibility with Nafion matrix of silica would lead to the fuel crossover and weakened mechanical stability.Avoiding the random distribution and bed compatibility issue,the modified in-situ “sol-gel” method was used to prepared the silica/Nafion composite.The TEOS solution,acting as the silica precursor,was swelled into the Nafion framework together with the solvent and filled into the –SO3H ionic cluster.Herein,the –SO3H groups instead of extra acid were used as catalysts for the TEOS hydrolysis.Therefore,the silica network was grown inside the –SO3H ionic cluster.The proton conductivity of Nafion was therefore doubled at elevated temperatures,and the high-temperature fuel cell performance was significantly improved by 45%.4.In addition to inorganic oxide modified materials such as silicon dioxide,heteropolyacids,such as phosphotungstic acid,are also commonly used in hightemperature modification of Nafion membrane.Phosphotungstic acid has good thermal stability and excellent proton conductivity at high temperature.However,the biggest obstacle to its further development in Nafion composite membrane is its stability in the composite membrane caused by its water solubility.Based on the previous work,twocomponent high-temperature modification of Nafion membrane by silicon dioxide and phosphotungstic acid was realized in this part of work.Under the synergistic effect of the two-component modifiers,the high-temperature performance of the composite film was obviously improved.Meanwhile,the stability of phosphotungstic acid in the composite membrane was improved due to the in-situ coating of silicon dioxide.The proton conductivity of the highly stable composite membrane reached 58 m S/cm at 110 oC and60%RH,2.4 times that of the pure Nafion membrane.In summary,"swelling-filling" method can avoid the damage to the original structure of Nafion membrane to the greatest extent,so that the composite membrane can still maintain good mechanical stability.On the other hand,in-situ targeted filling can really achieve molecular level filling,greatly improving the efficiency of modification. |
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