【作者】 张扬；

【导师】 吴雪梅； 贺高红；

【作者基本信息】 大连理工大学 ， 化学工程， 2021， 博士

【摘要】 阴离子交换膜燃料电池(AEMFCs)可实现氢能的高效利用,满足国家重大需求。碱性条件下,燃料电池具有更快的氧还原动力学,可使用非贵金属催化剂。然而,作为阴离子交换膜燃料电池中的关键组件,阴离子交换膜(AEMs)的OH-本征迁移率低,OH-进攻下阳离子基团和聚合物主链易降解,造成其离子电导率低、碱稳定性差。设计膜材料的分子结构促进微相分离,可以有效地构建OH-传导通道,进而提高膜的OH-电导率。采用碱稳定性高的阳离子基团,可以显著提高膜的耐碱性,从而延长膜的使用寿命。N-螺环季铵功能基团具有约束环构象,碱稳定性高于其他常用的咪唑、脂肪胺、环胺基阳离子。但N-螺环的双环结构高度刚性难以微相分离,导致存在成膜性差、电导率-溶胀平衡难、N-螺环基燃料电池功率密度低等关键问题。本文提出N-螺环基耐碱高传导聚砜阴离子交换膜的微相分离结构设计方法。设计具有醚氧间隔基、氢键二胺醚氧间隔基,以及氢键二胺醚氧N-螺环/烷基亲疏水链系列N-螺环侧链结构,调控膜内亲-疏水微相分离,氢键协同离子键促进OH-传导,显著提高N-螺环基阴离子交换膜性能。N-螺环阳离子碱稳定性高,但其闭环结构使其接枝困难、常规溴甲基接枝路线形成的苄基季铵N-螺环结构会导致功能基团和聚合物主链碱稳定性差。本章提出羟基取代N-螺环阳离子合成路线,通过Williamson反应接入聚砜主链,再利用N-螺环聚砜与氯甲基聚砜之间的原位热交联,制备出具有醚氧间隔基的N-螺环聚砜交联膜(Cr-ASD-PSF及Cr-ASU-PSF),显示出优异的热碱稳定性(1 M KOH,80℃浸泡720 h后,Cr-ASU-PSF 1.92膜的电导率仅下降4.4%),优于文献报道的苄基间隔N-螺环基阴离子交换膜。醚氧键旋转使N-螺环基聚砜形成微相分离结构(离子簇约为5.2nm),Cr-ASD-PSF 2.05膜在80℃下的电导率为85.7 mS cm-1、溶胀度为39.6%。醚氧键易旋转,但N-螺环的双环结构高度刚性,使未交联的醚氧间隔N-螺环基聚砜难以单独成膜,亟需提高N-螺环侧链移动性。论文设计含氢键的二胺间隔基,首先采用刚性远低于N-螺环的常用咪唑功能基团,研究氢键二胺间隔基对膜微相分离结构的调控作用和OH-传导机制。分子动力学模拟表明,胺基与水分子之间存在氢键网络,可以扩展OH-传导通道,协同离子键促进OH-传导。调控二胺间隔基长度可优化亲-疏水微相分离结构,随二胺间隔基长度增加,侧链移动性增强促进阳离子基团聚集,同时减弱侧链与聚合物主链之间不相容性,不利于阳离子基团聚集。以离子簇尺寸作为侧链长度优化的判据,当二胺间隔基长度为6时,氢键二胺咪唑基PSF-C6-DMIm膜具有较大离子簇(约为11.2nm)、较高电导率(80℃下为125.8 mS cm-1)和较低溶胀度(80℃下<20%)。进而,提出氢键二胺醚氧间隔N-螺环侧链,提高N-螺环侧链移动性,促进微相分离,提高电导率和机械性能。分子动力学模拟表明,虽然与氢键二胺咪唑侧链相比,氢键二胺N-螺环侧链的刚性增加、与主链不相容性降低导致微相分离程度降低,但相比于醚氧N-螺环侧链,氢键二胺N-螺环侧链具有更高的移动性和链缠结,显著提高了微相分离程度,聚合物成膜性良好,膜具有更高的电导率和机械性能。与氢键二胺咪唑侧链基膜类似,具有6个-CH2-间隔基的PSF-C6-ASD膜表现出最大的离子簇(约为8.5 nm)和最高的OH-电导率(在80℃时为107.1 mS cm-1)。柔性侧链间隔的链缠结作用显著提高了 N-螺环基聚合物的成膜性和膜的机械强度及韧性,PSF-C6-ASD膜的断裂伸长率提高至 Cr-ASD-PSF 2.05 膜的 3 倍。PSF-C6-ASD 膜在 1 M KOH,80℃ 浸泡 720 h 后,电导率保持率达98.3%,机械强度保持率达92.3%,最优测试条件下燃料电池峰值功率密度为 149.3 mW cm-2。氢键二胺醚氧间隔N-螺环侧链移动性高,但N-螺环的空间位阻效应使膜内产生较大的自由体积,电池测试中原料气速增加会造成电池开路电压大幅降低(气速为1000 mL min-1时,开路电压仅为0.71 V),表明膜中原料气渗透严重,限制了 N-螺环基燃料电池的功率密度。论文进一步设计了亲水氢键二胺醚氧间隔N-螺环侧链协同疏水烷基侧链的亲疏水链接枝N-螺环基膜结构,降低膜的溶胀度和气体渗透性。分子动力学模拟表明,疏水烷基侧链不仅可以促进刚性N-螺环基团聚集和微相分离,同时也填充了膜内自由体积有利于减少原料气渗透。优化疏水链长度后的正辛胺N-螺环基膜具有更好的电导率-溶胀平衡性(80℃下,OH-电导率为136.2 mS cm-1,溶胀度为15.4%,拉伸强度为28.5 MPa),使其燃料电池可以在更高气速下保持较高的开路电压(>1.0V)。优化测试条件后,电池功率密度最高可达850.1 mW cm-2,远高于文献报道的其他N-螺环基燃料电池,极大地提升了高碱稳定N-螺环基阴离子交换膜的应用潜质。更多还原

【Abstract】 Anion exchange membrane fuel cells(AEMFCs)can achieve efficient utilization of hydrogen energy to meet major national needs.Under alkaline conditions,fuel cells have faster oxygen reduction kinetics and can use non-noble metal catalysts.However,as the key component of AEMFCs,anion exchange membranes(AEMs)have low OH-intrinsic mobility.Cationic groups and polymer main chains are easily degraded under OH-attack,resulting in low ionic conductivity and poor alkali stability.Designing molecular structures of the membrane materials to promote microphase separation can effectively construct OHconduction channels,thereby increasing the OH-conductivity of the membranes.Applying the cationic groups with high alkali stability can significantly improve the alkali stability of the membranes,thereby prolonging the service life of the membranes.The alkali stability of N-spirocyclic quaternary ammonium functional groups is higher than the commonly used imidazole,aliphatic amine and cyclic amino cations due to their constrained ring conformation.However,the high rigidity of the bicyclic structures of the N-spirocyclic rings makes the membranes difficult to form microphase separation,which leads to key problems such as poor membrane-forming properties,difficulty in balancing the conductivity and swelling,low power density of the N-spirocyclic based fuel cells.This paper proposes the design methods of microphase separation structures for N-spirocyclic based alkaline stable and highly conductive polysulfone anion exchange membranes.N-spirocyclic side chain structures with ether oxygen spacers,hydrogen bond diamine ether oxygen spacers as well as hydrophilic hydrogen bond diamine ether oxygen N-spirocyclic/hydrophobic alkyl chains are designed,which regulate the hydrophilic-hydrophobic microphase separation in the membranes.The hydrogen bonds cooperate with the ionic bonds could promote OHconduction,which significantly improve the performance of N-spirocyclic based anion exchange membranes.N-spirocyclic cations have high stability,but the grafting of the N-spirocyclic cations is difficult due to their closed-loop structures.The conventional bromomethyl grafting routes will form N-spirocyclic side chains with benzyl structures,which could lead to the poor alkali stability of the functional groups and polymer main chains.The synthetic routes of hydroxyl-substituted N-spirocyclic cations are proposed to connect the N-spirocyclic cations to the polysulfone main chains through Williamson reaction in this chapter.Then the in-situ thermal crosslinking between N-spirocyclic based polysulfone and chloromethyl polysulfone is used to prepare N-spirocyclic based polysulfone cross-linked membranes(Cr-ASD-PSF and Cr-ASU-PSF)with ether oxygen spacers.The membranes show excellent thermal alkali stability(the conductivity of the Cr-ASU-PSF 1.92 membrane is only reduced by 4.4%after immersed in 1 M KOH at 80℃ for 720 h),which are better than the N-spirocyclic based anion exchange membranes with benzyl spacers reported in the literatures.The rotation of the ether oxygen bonds makes the N-spirocyclic based polysulfone membranes form microphase separation structures(the ion clusters are about 5.2 nm).The conductivity of the Cr-ASD-PSF 2.05 membrane is 85.7 mS cm-1 and the swelling ratio is 39.6%at 80℃.The ether oxygen bonds are easy to rotate,but the bicyclic structures of the N-spirocyclic cations are highly rigid,making it difficult for the uncrosslinked N-spirocyclic based polysulfone with ether oxygen spacers to form membranes separately.Therefore,it is urgent to improve the mobility of the N-spirocyclic side chains.In this thesis,diamine spacers with hydrogen bonds are designed.The commonly used imidazole functional group whose rigidity is much lower than that of N-spirocyclic rings is used to study the regulating effect of diamine spacers with hydrogen bonds on the microphase separation structures and the OH-conduction mechanism.Molecular dynamics simulations show that there are hydrogen bond networks between amine groups and water molecules,which can expand the OH-conduction channels and cooperate with ionic bonds to promote OH-conduction.Adjusting the length of the diamine spacers can optimize the hydrophilic-hydrophobic microphase separation structures.As the length of the diamine spacers increases,the mobility of the side chains increases to promote the aggregation of the cationic groups,while reducing the incompatibility between the side chains and the polymer main chains,which is not conducive to the aggregation of the cationic groups.Therefore,the size of the ion clusters can be used as the criterion for the optimization of the length of the side chains.When the length of the diamine spacers is 6,the imidazolium based PSF-C6-DMIm membrane containing hydrogen bond diamine spacers has large ion clusters(about 11.2 nm),high conductivity(125.8 mS cm-1 at 80℃)and low swelling ratio(<20%at 80℃).In addition,N-spirocyclic side chains with hydrogen bond diamine ether oxygen spacers are proposed to improve the mobility of the N-spirocyclic side chains,microphase separation,conductivity and mechanical properties.Molecular dynamics simulations show that,compared with the imidazolium side chains with hydrogen bond diamine spacers,the increase of the rigidity of the N-spirocyclic side chains with hydrogen bond diamine spacers and the decrease of their incompatibility with the main chains lead to the decrease of the microphase separation degree.But compared with the N-spirocyclic side chains with ether oxygen spacers,the N-spirocyclic side chains with hydrogen bond diamine spacers have higher chain mobility and entanglement,which significantly promote microphase separation.The polymers have good membrane-forming properties and the membranes have higher conductivity and mechanical properties.Similar to the imidazolium based membranes with hydrogen bond diamine spacers,the PSF-C6-ASD membrane with 6-CH2-spacers exhibits the largest ion clusters(about 8.5 nm)and the highest OH-conductivity(107.1 mS cm-1 at 80℃).The chain entanglement of the flexible spacers can significantly improve the membrane-forming properties of the N-spirocyclic polymers,mechanical strength and toughness of the membranes.The elongation at break of the PSF-C6-ASD membrane is 3 times of that of the Cr-ASD-PSF 2.05 membrane.After the PSF-C6-ASD membrane immersed in 1 M KOH at 80℃ for 720 hours,the retention rates of the conductivity and mechanical strength reach 98.3%and 92.3%,respectively.The peak power density of the fuel cell assembled with PSF-C6-ASD under the optimal test conditions is 149.3 mW cm-2.The N-spirocyclic side chains with hydrogen bond diamine ether oxygen spacers have high mobility.However,the steric hindrance effect of the N-spirocyclic cations leads to large free volume in the membranes.The increase of feed gas velocities could cause a significant decrease of the open circuit voltages(open circuit voltage is only 0.71 V,when gas velocity is 1000 mL min-1)during fuel cell tests,which indicates that the permeation of feed gas in the membranes is serious and limits the power density of N-spirocyclic based fuel cells.The N-spirocyclic based membrane structures with hydrophilic-hydrophobic grafts based on hydrophilic N-spirocyclic side chains with hydrogen bond diamine ether oxygen spacers and hydrophobic alkyl side chains are further designed in this thesis to reduce the swelling and gas permeability of the membranes.Molecular dynamics simulations show that the hydrophobic alkyl side chains can not only promote the aggregation of rigid N-spirocyclic groups and microphase separation,but also fill the free volume in the membranes,which is beneficial to reduce the permeation of feed gas.The N-spirocyclic based membrane with optimized n-octylamine hydrophobic chain length has better conductivity-swelling balance(the OH-conductivity and swelling ratio are 136.2 mS cm-1 and 15.4%at 80 ℃,respectively;the tensile strength is 28.5 MPa),which makes the fuel cells maintain high open circuit voltages(>1.0 V)at high gas velocities.The power density of the fuel cell can reach up to 850.1 mW cm-2 under the optimal test conditions,which is much higher than other N-spirocyclic based fuel cells reported in the literatures,greatly improving the application potential of the high alkaline stable N-spirocyclic based anion exchange membranes.更多还原