Preparation And Properties Of Proton Exchange Membrane Reinforced By Metal-organic Framework/Ionic Liquid

As one of the most common types of fuel cells being developed today,proton exchange membrane fuel cells（PEMFC）have become an important candidate for transportation applications due to their high efficiency,higher power density,quiet operation,and environmental friendliness.Proton exchange membranes（PEMs）have excellent proton conductivity,high mechanical strength,and low methanol permeability,which are crucial for their application in PEMFC.Today,perfluorinated sulfonic acid commercial membranes such as Nafion^?membranes are most commonly used in PEMFC.Although they have significant proton conductivity,they have drawbacks such as high production costs,high fuel permeability,relying on water to achieve good performance,and can only operate at temperatures below 80℃.Polyether ether ketone（PEEK）has become a promising material for aromatic PEMs due to its high thermal and mechanical stability,as well as flexibility.In particular,sulfonated PEEK（SPEEK）is a proton conducting polymer with certain proton conductivity and thermodynamic properties.However,it still has the disadvantage of low proton conductivity and needs to be modified to improve its performance.In this paper,sulfonyl-1-methylimidazole ionic liquids（[IM1][HSO₄]）,sulfonyl-4-phenylimidazole ionic liquids（[IM2][HSO₄]）,phosphoryl-1-methylimidazole ionic liquids（[IM1][H₂PO₄]）,and phosphoryl-4-phenylimidazole ionic liquids([IM2][H₂PO_4])were prepared by acid-base neutralization method,and characterized by Fourier transform infrared spectroscopy（FT-IR）and nuclear magnetic resonance hydrogen spectroscopy（¹H NMR）to verify the successful preparation of ionic liquid（IL）.Binary composite membranes of[IM1][HSO₄]/SPEEK,[IM2][HSO₄]/SPEEK,[IM1][H₂PO₄]/SPEEK,and[IM2][H₂PO₄]/SPEEK were prepared by solution casting method.Scanning electron microscope（SEM）images show that IL had been completely doped into SPEEK.FT-IR and X-ray photoelectron spectroscopy（XPS）showed that SPEEK and IL formed intermolecular hydrogen bonds.The thermogravimetric analysis（TG）results showed that the IL/SPEEK binary composite membrane remained stable even at 300℃.Research on proton conductivity showed that IL doping helps to improve the proton conductivity of SPEEK membranes.At 120℃,the conductivity of the[IM2][H₂PO₄]/SPEEK binary composite membrane with 30 wt.%IL doping is the highest,at 24.89 m S·cm^-1.To further enhance the proton conductivity of SPEEK membrane and reduce the loss rate of IL in the IL/SPEEK binary composite membrane,a single metal Co-MOF-74/[IM2][H₂PO₄]/SPEEK ternary composite membrane was prepared by solution casting method.It can be observed a small amount of agglomeration on the surface of MOF/IL/SPEEK ternary composite membrane with an increase in Co-MOF-74 content,and the cross-section presents a gear like structure from SEM.Through characterization and performance testing of the single metal MOF/IL/SPEEK ternary composite membrane,it was found that the proton conductivity of the ternary composite membrane increased.At 120℃and the doping amount of Co-MOF-74 was 2.5 wt.%,the proton conductivity of the single metal MOF/IL/SPEEK ternary composite membrane reached25.96 m S·cm^-1,the IL loss rate in the single metal MOF/IL/SPEEK ternary composite membrane ranged from 30%to 35%.The IL/SPEEK binary composite membrane could be applied at 300℃,while the single metal MOF/IL/SPEEK ternary composite membrane can be applied within 320℃.And when the temperature reaches 800℃,the final weight of the IL/SPEEK binary composite membrane is 50%,while the final weight of the single metal MOF/IL/SPEEK ternary composite membrane reaches 55%.This indicates that encapsulating IL in MOF not only increases the proton conductivity of the SPEEK membrane,but also improves the thermal stability of the SPEEK membrane.In order to further reduce the loss rate of IL in IL/SPEEK membranes and improve the proton conductivity of SPEEK composite membranes,bimetallic materials Cd@Co-MOF-74/[IM2][H₂PO₄]/SPEEK ternary composite membrane were prepared by solution casting method.Through characterization and performance testing of bimetallic MOF/IL/SPEEK ternary composite membranes,it was found that at 120℃,the doping amount of Cd@Co-MOF-74 is 1.5 wt.%,the proton conductivity of the bimetallic MOF/IL/SPEEK ternary composite membrane is the highest,reaching 26.93 m S·cm^-1,which is higher than that of the single metal MOF/IL/SPEEK ternary composite membrane.The IL loss rate of the bimetallic MOF/IL/SPEEK ternary composite membrane mainly occurs within 1 hour,and after 1 hour,almost no loss occurs.The IL loss rate of the bimetallic MOF/IL/SPEEK ternary composite membrane with different contents is generally between 20%and 25%.Compared with the single metal MOF/IL/SPEEK ternary composite membrane,the swelling rate of the bimetallic MOF/IL/SPEEK ternary composite membrane is reduced due to the formation of hydrogen bonds between the imidazole ring in the ionic liquid and the-OH or carboxylic acid groups in the MOF,and the imidazole molecules are anchored on the pore wall of the MOF.The above results indicate that doping bimetallic MOF within the IL/SPEEK membrane not only ensures a high proton conductivity of the SPEEK membrane,but also reduces the swelling rate of the IL/SPEEK composite membrane,thereby increasing the service life of the IL/SPEEK composite membrane.