Stable Phosphoric Acid Doped Sulfonated Poly(Ether Ether Ketone)Proton Exchange Membranes For Direct Methanol Fuel Cells

In recent years,fuel cells have been widely studied as an efficient and green electrochemical device which can directly generate electric energy through an electrochemical reaction of the fuel（hydrogen or methanol）and oxidant（air or oxygen）with low/zero emission and zero noise pollution.Among different kinds of fuel cells,direct methanol fuel cells（DMFCs）stand out benefitting from their advantages of high efficiency,fast power generation,convenient transportation and low cost.As one of the key components of DMFCs,the main functions of proton exchange membranes（PEMs）are to transport protons,block electrons,separate the fuel and oxidant,the performance of PEMs directly affect the performance and service life of DMFCs.Therefore,the design and development of PEMs with high proton conductivity,good mechanical strength,excellent thermal stability and low methanol penetration have become the research focus around the world.Sulfonated poly（ether ether ketone）（SPEEK）is currently considered to be one of the most potential candidates of commercial perfluorinated sulfonic acid proton exchange membranes（e.g.Nafion）.Although the SPEEK with high sulfonation degree exhibits high proton conductivity,it still has relatively high methanol permeability and low mechanical properties.In order to obtain PEMs with high proton conductivity,low methanol penetration,and satisfactory mechanical strength,a series of modificationed SPEEK membranes were designed and prepared for DMFC applications.This thesis provides new design ideas and solutions for the preparation of high-performance PEMs.（1）Flexible alkyl side chains containing Gemini quaternary ammonium（QA）ions were grafted onto SPEEK to obtain novel amphiphilic polymers（S-DMEAx-BPT）.Strong ionic crosslinking interactions were formed in the membrane through ion pairs between the QA and-SO₃H groups of SPEEK.In addition,the introduction of hydrophobic short chains can increase the aggregation of hydrophilic clusters（-SO₃H）in SPEEK.In order to further improve the proton conductivity of the polymer membrane,the S-DMEAx-BPT membranes were immersed in phosphoric acid（PA）and PA-doped S-DMEAx-BPT/PA membranes with low PA doping level（ADL）were obtained.PA molecules can self-dissociate to protonated H₂PO₄^-and H₄PO₄⁺.And a large number of acid-base pairs were formed between QA and H₂PO₄^-groups in the membranes,which can provide effective paths for proton transport,and also endow the membranes with excellent thermal stability and mechanical properties.S-DMEAx-BPT/PA membranes had excellent mechanical strength（>84.2 MPa）,the and proton conductivity of as high as 295.4 m S cm^-1（80 ^oC）that is 1.5 times that of commercial Nafion 212(191.8 m S cm^-1)membrane.In DMFC test,the cell equipped with S-DMEA₃₀-BPT/PA as the electrolyte membrane demonstrated the peak power density of 110.7 m W cm^-2,which is 2.1 times that of Nafion 212 under the same conditions.Moreover,the open-circuit voltage after 140 h remained as high as 82.4%of the initial value.（2）In order to further improve the ADL of the PEM,two nucleobase groups groups adenine（A）and cytosine（C）were introduced into SPEEK,and a series of PA-doped nucleobase modified PEMs SPEEK-A（C）x/PA were prepared.The introduction of nucleobase significantly improved the ADL values,and the ADL of SPEEK-A（C）x/PA membranes increased at least 4.1times compared with S-DMEAx-BPT/PA.Meanwhile,all the PA-doped membranes had excellent mechanical strength（>23.5 MPa）.SPEEK-A（C）x/PA exhibited extremely high proton conductivity(276.0 m S cm^-1)at 80 ^oC,which was 5.3 times of SPEEK under the same conditions.In DMFC test,the power density of the nucleobase modified membrane SPEEK-C₅₀/PA was as high as 141.7 m W cm^-2,which was 2.1 times of SPEEK(67.2 m W cm^-2).