| All vanadium redox flow battery?VRFB?is novel green energy storage system.Compared others,VRFB demonstrated many advantages,such as adjustable capacity and power,lossless deep discharge at high current,long life,simple operation,and so on.As one of the main components of VRFB,the proton conducting membrane provides the channel of proton transfer,and it also plays the role of separating positive and negative electrolytes and preventing the self discharge of battery.At present,perfluorosulfonic Nafion series membranes have been widely used in VRFBs due to their good chemical stabilities and high proton conductivities.However,since the proton/vanadium ion selectivities of Nafion membranes are not high,the crossing permeation of vanadium ions in positive and negative electrolytes during the charge-discharge process occurs unexpectedly.This leads to a series of problems,such as the declined coulomb efficiency and energy efficiency,the serious self discharge and the shortening of cycle life.etc.Besides,the price of Nafion membranes is extremely high,which greatly restricts their large-scale commercial application in VRFBs.Sulfonated polyimide?SPI?membranes have been widely studied and applied to VRFB due to their good vanadium resistance,thermal stability and proton conductivity as well as reasonable price.But similar to most non-fluorinated polymer membranes,SPI membrane is insufficient in the chemical stability for VRFB application.Therefore,it is necessary to modify SPI membrane.It is found that the branched polymer membranes have higher thermal stability and chemical stability than the linear polymer membranes.Inspired by this point,in this thesis,a series of branched SPI?bSPI?membranes with improved chemical stabilities were prepared for VRFB application by introducing non-sulfonated triamine monomer with branched structure and optimizing the non-sulfonated diamine monomer structure.Then,the inorganic filler was added into bSPI matrix to increase the vanadium ion resistance of the membrane,so that the novel membranes with high proton conductivities and chemical stabilities,low vanadium ion permeabilities and prices were obtained for VRFB usage.The main research work of this thesis is as follows:?1?A series of bSPI membranes with different non-sulfonated diamine monomers including 2-?4-aminophenyl?-5-amino benzimidazole?APABI?,4,4-diaminodiphenyl methane?MDA?,2,2-[4-?4-aminophenoxy?phenyl]propane?BAPP?were prepared respectively.1,4,5,8-naphthalenetetracarboxylicdianhydride?NTDA?and4,4'-diamino-biphenyl-2,2'-disulphonic acid?BDSA?were used as raw materials,and the triamine monomer 1,3,5-tris?4-aminophenoxy?benzene?TAPOB?was also introduced during the membrane preparation process.The structures of membranes were characterized using FTIR and 1H-NMR spectra.The morphologies of membranes were characterized using SEM images.The thermal stabilities of membranes were analyzed using TG curves,and the physico-chemical properties of membranes,such as water uptake,swelling ratio,ion exchange capacity and proton conductivity and so on,were also studied.The results show that all bSPI membranes have good thermal stabilities,their vanadium ion permeabilities are lower than that of Nafion 117 membrane,and their chemical stabilities are better than that of linear SPI?ODA?membrane.Among all bSPI membranes,the bSPI?BAPP?membrane has the highest proton selectivity(1.23×105 S min cm-3)and chemical stability.The VRFB assembled with bSPI?BAPP?membrane has a longer self-discharge time?135 h?than that assembled with Nafion 117 membrane?100h?.VRFBs with bSPI?BAPP?and Nafion 117 membranes were conducted 500-time cycling charge-discharge test at the current densities of 30–120 mA cm-2.The battery efficiencies?CE:97.199.7%;EE:61.680.7%?of VRFB with bSPI?BAPP?membrane are higher than those of VRFB with Nafion 117 membrane?CE:95.298.6%;EE:58.874.0%?.The former capacity retention rate is also higher than or comparable to the latter.?2?In order to improve the vanadium resistance and chemical stability of bSPI membrane,bSPI was synthesized from NTDA,BDSA,BAPP and TAPOB.In addition,MoS2 nanosheets were prepared by a liquid phase exfoliation method,and bSPI/MoS2nanosheets?bSPI/MoS2-ns?composite membrane was prepared by adding MoS2-ns to bSPI matrix.Also,both pure bSPI membrane and bSPI/MoS2 nanopowders?bSPI/MoS2-np?composite membrane were prepared for references.The morphology of MoS2-ns was characterized using SEM,TEM and SPM.The structures of inorganic and membranes were characterized using XRD and FTIR.The morphologies of membranes were characterized using SEM and AFM,and the physico-chemical properties of membranes were tested,including water uptake,swelling ratio,ion exchange capacity,proton conductivity,and so on.The results show that the proton conductivity(5.04×10-2S cm-1)of bSPI/MoS2-ns membrane is comparable to that of Nafion 117 membrane,and its proton selectivity(7.75×105 S min cm-3)is the highest among all the membranes.The VRFB assembled with bSPI/MoS2-ns membrane shows the longest self-discharge time?312 h?.VRFBs with bSPI/MoS2-ns and Nafion 117 membranes were conducted 500-time cycling charge-discharge test at the current densities of 20–60 mA cm-2.The VRFB assembled with bSPI/MoS2-ns membrane has higher battery efficiencies?CE:97.0–99.9%;EE:61.3–80.0%?than those assembled with Nafion 117 membrane?CE:96.0–99.0%;EE:55.0–72.9%?.And the discharge capacity retention rate of VRFB with bSPI/MoS2-ns membrane is also higher than or comparable to that of VRFB with Nafion117 membrane.?3?In order to investigate the effect of the content of inorganic filler on the properties and performance of bSPI membranes,a series of bSPI/SiC?PDMS?composite membranes were prepared by adding SiC?PDMS?to bSPI matrix.And SiC?PDMS?was obtained by using?,?-diaminopropyl polydimethylsiloxane?PDMS?to modify SiC.At the same time,pure bSPI membrane and bSPI/SiC composite membrane were also prepared for references.The morphology of SiC?PDMS?was characterized using SEM and TEM.The structure of SiC?PDMS?was characterized using XRD and FTIR.The morphologies of membranes were characterized using SEM images,and the physico-chemical properties of membranes were tested,such as water uptake,swelling ratio,ion exchange capacity,proton conductivity,and so on.The results show that the bSPI/SiC?PDMS?-1.5%membrane has the highest proton selectivity(2.99×105 S min cm-3)among all the bSPI/SiC?PDMS?membranes.The VRFB of bSPI/SiC?PDMS?-1.5%membrane has a longer self-discharge time?374 h?than that of Nafion 117membrane.And VRFBs of bSPI/SiC?PDMS?-1.5%and Nafion 117 membranes were conducted 500-time cycling charge-discharge test at the current densities of 20–60 mA cm-2.The VRFB using bSPI/SiC?PDMS?-1.5%membrane has higher or close CE?97.2–99.3%?and EE?55.0–84.4%?compared with that using Nafion 117 membrane?CE:96.0–99.0%;EE:55.0–72.9%?.In addition,the discharge capacity retention rate of VRFB with bSPI/SiC?PDMS?-1.5%membrane is also better than or close to the that of VRFB with Nafion 117 membrane. |
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