Construction And Properties Of Fast Proton Transport Channels For Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell（PEMFC）has drawn extensive interest for its advantages of friendly environment,high efficiency and low pollutant emission.It is considered as a promising new type of clean power-conversion device,which is expected to alleviate energy shortage and environmental pollution.As one of the key components of a PEMFC,proton exchange membrane（PEM）determines the output power and service life of fuel cells directly.Generally,the most direct way to improve the proton conductivity of PEMs is to increase its sulfonation degree.However,PEMs with high sulfonation degree will leads to poor physical and chemical stability,which severely restricts the commercial applications.Incorporating hydrophilic nanofillers into membrane has been proved to be a simple and effective strategy to enhance the overall performance of PEMs.In this dissertation,sulfonated poly（arylene ether sulfone）（SPAES）,which has a wide range of sources,low cost and good stability,is selected as the matrix.New high-efficiency proton transport channels are constructed with single-filler and double-filler doping strategy.By synergistically optimizing the physical and chemical microenvironment of the doping materials,and integrating the Grotthuss and Vehicle mechanisms of PEMs,a series of novel composite membranes with high proton conductivity,low fuel permeability and high stability are developed.Towards the breaking of the“trade-off”between stability and proton conductivity of SPAES-based PEMs,a strategy of compositing with the superacids of sulfated Sn O₂（ST）and Sn O₂-x Ce O₂（CSTx）is designed and conducted.By the solution casting method,the SPAES/ST and SPAES/CSTx membranes are obtained in a homogenous state and show excellent thermal,mechanical strength,dimensional-chemical stability.ST or CSTx in the composite membranes provides excess active sites and forms additional proton-conducting channels by hydrogen-bonding networks.Despite with low IEC values,the composite membranes show enhanced water absorption and proton conductivity.Especially,the SPAES-2CST3 membrane achieves a proton conductivity of 147 m S cm^-1,which is 45%higher than the control membrane.In a H₂/O₂ fuel cell test,the maximum power density of the SPAES-2CST3 membrane reaches 674m W cm^-2 at 80^oC/100%relative humidity（RH）,which is much higher than the control membrane(482 m W cm^-2).After the accelerated stress test（168 h,80℃/100%RH）,the cell voltage（CV）decay rate of the SPAES-2CST3 membrane is only 0.27 m V h^-1,while the value is much lower for the control SPAES membrane(0.49 m V h^-1)and Nafion^?112(0.43 m V h^-1);the aged SPAES-2CST3 membrane exhibits better cell performance and lower hydrogen crossover than the aged control SPAES membrane.The smaller the size of the doped particles,the higher even dispersity tendency into the membrane matrix.To further improve the performance of the membrane,carbon nano-onions（CNOs,～5 nm）matching the size of the proton transport channels are prepared,and then the highly sulfonated carbon nano-onions（SP-CNOs）are prepared by introducing high content of sulfonated phenyls on the surface of the CNOs.The obtained mesoporous SP-CNOs possess high specific surface and mesoporous structure,which endow the nanoparticles with good hydrophilicity and dispersivity.The composite membranes all show excellent mechanical toughness and greatly enhanced water-retention capacity,dimensional,thermal and oxidative stability owing to the good interfacial compatibility and the formation of hydrogen-bonding interaction between SPAES and SP-CNOs.Induced by hydrophilic nanoparticles,the ion clusters aggregate and become larger,which effectively promotes the microphase separation and forms a continuous proton transport channels.The SPAES/SP-CNOs-1.5 membrane achieves the highest proton conductivity of 181.2 m S cm^-1 at 90^oC;the maximum power density of the H₂/O₂ fuel cell reaches 735 m W cm^-2 and 355 m W cm^-2at 80^oC and 100%/60%RHs,which are 33.4%and 84.0%higher than those of the SPAES membrane.After the accelerated stress test,the SPAES/SP-CNOs-1.5 membrane exhibits lower CV/OCV decay rate and hydrogen crossover than the control one.To further improve the proton conductivity and fuel cell performance of the membrane at low humidity,a double-filler of phosphorylated carbon nano-onions（P-CNOs）and adenosine triphosphate（ATP）is incorporated into SPAES by facile solution mixing to design a bionic proton-conduction channels for high-performance PEMs.Both the P-CNOs and ATP fillers can provide abundant proton conducting sites via acid-base pairs and hydrogen-bonding networks,thus improving the membrane proton conductivity and stability.Benefiting from the formation of ionic interaction between P-CNOs/ATP and their interaction with SPAES substrates,the composite membranes demonstrate better mechanical-dimensional-chemical stability,higher proton conductivity and power output than the control and single-filler SPAES membranes.Among them,the SPAES/ATP/P-CNOs-2 membrane exhibits high proton conductivity of 196m S cm^-1 at 90^oC in water and 33 m S cm^-1 at 80^oC/50%RH.The SPAES/ATP/P-CNOs-2membrane displays an excellent cell performance of 752 m W cm^-2 and 302 m W cm^-2 at 80^oC and 100%/50%RHs,which are 45%and 110%higher than those of the SPAES membrane,and comparable to Nafion^?112.It also holds stable voltages,experiences the minimal cell performance loss and lowest hydrogen crossover after the durability test for 144 h and 80^oC/60%RH.Two-dimensional nanosheets with high aspect ratio and large specific surface area can provide more sites and long-range continuous channels for proton transport,which can effectively reduce proton conductive resistance.To further improve the comprehensive properties of PEMs,phosphotungstic acid（HPW）is chemically grafted to mesoporous carbon nitride nanosheets（PCN）and then introduced into the membrane matrix to prepare a series of SPAES/PCN membranes.The acid-base interaction and hydrogen-bonding network between PCN and SPAES make the composite membranes possess good physicochemical stability,high water absorption and excellent proton conductivity.At 90 ^oC,the water absorption and proton conductivity of the SPAES/PCN-7.5 membrane reach 83.8%and 250 m S cm^-1,while the in-plane swelling ratio is only 10.6%.The good interfacial compatibility between PCN and polymer facilitates the formation of well-defined microphase-separation morphology and long-range continuous proton transport channels in the membrane.In addition,due to the strong acidity and water-retention capacity of HPW,the proton conductivity of the composite membrane under low humidity is significantly improved.The SPAES/PCN-7.5 membrane achieves the highest proton conductivity of 46 m S cm^-1 at 80^oC/50%RH,which is 84%higher than the control SPAES membrane.The SPAES/PCN-7.5 membrane displays an excellent H₂/O₂ fuel cell performance of 824 m W cm^-2 and 404 m W cm^-2 at 80^oC and 100%/50%RHs,much higher than the control SPAES membrane(546 m W cm^-2 and 207 m W cm^-2)and slightly higher than Nafion（?）112(723 m W cm^-2 and 359 m W cm^-2).It still remains stable voltages and cell performance,as well as lower hydrogen crossover after the durability test.