Preparation,Characterization And Application Of Nafion-CDs Hybrid Membranes For Energy Devices

Ion Exchange Membrane（IEM）is a polymer membrane containing ionic groups with the ability to permeate ions selectivelly.It is widely used in chemical separation,water electrolysis,chlor-alkali,seawater desalination,fuel cell and flow battery.In these devices,ion exchange membranes generally play the role of separating the anode and cathode,preventing short circuit and conducting ions selectively.After years of development,Du Pont company developed a perfluorinated sulfonic acid membrane（trade name Nafion）using PTFE as the main chain,sulfonic acid group as the guide ion side chain group in the 1970s.Nafion membrane has high mechanical property,chemical stability and excellent proton conduction performance,but it also has the disadvantages of poor high-temperature stability and high fuel permeability,which limit the application field of Nafion membrane and the improvement of device performance.Using silica,metal oxides and carbon nanoparticles as fillers can improve the mechanical properties and thermal stability of Nafion membranes.However,the problem of molecular transport across membranes has not been effectively solved due to the presence of a large number of 4 nm pores in the microstructure of Nafion membranes.In this paper,the Nafion-CDs hybrid membranes were prepared by blending method based on the characteristics of small size（2-5 nm）and rich polar groups on the surface of Carbon Dots（CDs）.The pore size distribution and ion transport performance of the hybrid membranes were regulated by the assembly behavior of Carbon Dots.The mechanical properties,thermal stability,ionic conductivity,methanol and gas permeability of the hybrid membranes were studied in detail,and the membranes were applied to the construction of water electrolysis device and methanol fuel cell.The specific content of this paper is as follows:（1）Using cheap citric acid and urea as raw materials,carbon spots were prepared by microwave synthesis,high speed centrifugation,dialysis and freeze drying.The dissolution conditions of commercial Nafion 115 membrane were studied.A series of Nafion-CDs hybrid membranes with different doping amounts were prepared.The morphology,homogeneity and functional groups of the carbon dots and the hybrid membranes were characterized.The thermal stability and mechanical properties of the membranes were studied.（2）In the water electrolysis device constructed by using the prepared Nafion-CDs hybrid membranes as membrane,the purity of hydrogen and oxygen produced by water electrolysis at high current density of 1000 m A cm^-2 is 99.98%and 99.93%,respectively,which is significantly higher than the hydrogen（99.95%）and oxygen（99.90%）purity produced by water electrolysis using commercial Nafion membranes.The purity of hydrogen produced by Nafion membrane has met the requirements of the national standard for pure hydrogen of fuel cells（99.97%）.The mechanism of producing high purity hydrogen was deduced by comparing the pore size distribution and other parameters.（3）The methanol permeability of Nafion-CDs hybrid membrane was measured by electrochemical method at 80℃.The methanol permeability of the hybrid membrane was 3.229×10^-6 cm² s^-1,lower than that of the undoped Nafion membrane(3.467×10^-6cm² s^-1),indicating that the hybrid membrane can improve the methanol permeation behavior across the membrane.The methanol fuel cell was constructed by Nafion-CDs hybrid membrane and research the battery performance systematacially,the results showed that open circuit voltage of membrane electrode prepared by the hybrid membrane is improved significantly,the maximum output efficiency of fuel cell at 80℃is 100 m W cm^-2,is higher than the performance of membrane electrode prepared by commercial nafion membrane by about 20%.In this study,we proposed a method to regulate the pore size and ion transport capacity of proton exchange membrane by doping carbon dots,and demonstrated excellent performance in energy devices such as water electrolysis and fuel cell,providing a new idea for the construction of a new generation of energy devices.