基于芳醚型聚苯并咪唑的高温质子交换膜的制备及性能研究

发布时间：2018-08-16 14:56

【摘要】：在过去的几十年里,质子交换膜燃料电池(PEMFC)作为一种高效、环保的电化学能量转化装置得到了科学家们的广泛关注。近些年,随着研究的不断深入,开发能够在高温和低湿度环境下工作的PEMFC成为研究的热点。与传统的低温质子交换膜燃料电池(工作温度100℃)相比,在100-200℃下操作的高温质子交换膜燃料电池(HT-PEMFC)具有很多优势,例如,提高了催化剂对CO的耐受性,提高了催化剂效率以及简化了水/热管理等。聚苯并咪唑(PBI)由于具有优异的机械性能和热稳定性,以及具有可以同时作为质子供体和受体的特殊碱性杂环结构,使其成为高温质子交换膜(HT-PEM)的候选材料。PBI本身的质子传导率很低,只有10-9 m S·cm-1,不能作为独立的固体电解质。磷酸(PA)是一种具有高热稳定性的良好电解质,它在200℃下的质子传导率高达800m S·cm-1。在20世纪90年代,Wainright等人首次将磷酸掺杂到PBI中制备了磷酸掺杂聚苯并咪唑(PA-PBI)型质子交换膜,其在高温下运行表现出许多优异的性能。这个里程碑式的研究,以及随后的很多相关研究,使PA-PBI膜逐渐演变为最具潜力的HT-PEM候选材料。在过去的二十年里,最为广泛研究的PBI基体是已经商业化的聚[2,2'-间-(亚苯基)-5,5'-二苯并咪唑](m-PBI)。对于m-PBI来说,由于咪唑基团间强烈的氢键作用导致其具有较差的溶解性,为了保证薄膜的均匀性,避免不溶性残留物的存在,一般采用线性分子量相对较低(23-40 k Da,对应于0.5-1.0 d L·g-1的特性粘度)的PBI来制备PA-PBI膜。但较低的分子量导致聚合物薄膜的机械性能相对较差,仅允许PBI薄膜获得较低的磷酸掺杂量(通常每摩尔PBI重复单元掺杂6-10摩尔的磷酸分子),从而导致较低的质子传导率(100 m S·cm-1)。因此,研究者们采用了多种方法来提高膜的性能,包括接枝、交联、无机掺杂、溶胶-凝胶法制膜以及新型PBI材料的合成等。虽然前期工作在提高PBI膜性能上作出了很多贡献,但是PA-PBI型高温质子交换膜若想实现商业化仍然面临着几个关键性的挑战,如更高的质子传导率、更优异的机械性能和更稳定的磷酸保持能力等。PA-PBI膜的质子传导率与膜的磷酸掺杂水平(ADL)息息相关,我们可以直接通过提高PBI薄膜的磷酸掺杂水平来制备高传导性的PA-PBI膜,但是,高的ADL会导致薄膜产生较大的尺寸溶胀,相应的降低其机械强度,从而降低薄膜的耐久性。此外,在高的ADL下,磷酸容易从膜中流失,这也会影响燃料电池的性能和使用寿命。所以,研制能够在高ADL即高质子传导率下保持良好尺寸稳定性和机械稳定性以及具有高酸保持能力的PBI膜材料,是目前PA-PBI型高温质子交换膜研究中所需要解决的关键问题。针对以上问题,本论文的研究主要包括以下四个部分:首先,我们设计并合成了两种高分子量的含有柔性醚键和不对称大体积侧基(苯基和甲基苯基)的芳醚型PBI(Ph-PBI和Me-PBI),并且制备了相应的磷酸掺杂膜。通过芳醚型PBI薄膜与已有的不含侧基的聚[2,2'-(对-氧联二苯基)-5,5'-联苯并咪唑](OPBI)薄膜之间的性能比较,我们发现,Ph-PBI和Me-PBI薄膜表现出与OPBI薄膜不同的磷酸掺杂行为,具有更高的磷酸掺杂水平、更好的尺寸和机械稳定性以及更高的质子传导率和更加优异的燃料电池性能。其中Ph-PBI在160℃下经72 h的磷酸掺杂后,所得到的PA-PBI(Ph-72)膜在200℃、干态下的质子传导率为138 m S·cm-1,体积溶胀仅为188%,机械强度为9.7 MPa。Ph-72膜的H2/O2燃料电池在160℃、没有加湿情况下的最大功率密度为279m W·cm-2。基于上一部分工作中合成的具有优异尺寸和机械稳定性的Ph-PBI,为了进一步提升PBI膜的传导性能和电池性能,在研究的第二部分里,我们将2-氯甲基苯并咪唑接枝到Ph-PBI上,制备了一系列具有不同接枝度的聚苯并咪唑薄膜。额外的咪唑基团的引入使Ph-PBI分子链上的碱性基团增加,提高了Ph-PBI膜与磷酸分子之间的相互作用,进而使膜获得了更高的磷酸掺杂水平、更高的质子传导率以及更加优异的电池性能。当接枝度为20%时,磷酸掺杂量为341%,PBI膜在200℃、干态下的传导率达到212 m S·cm-1,拉伸强度为6.5 MPa。该膜的H2/O2燃料电池在160℃、没有加湿情况下的最大功率密度为443 m W·cm-2。继续提高接枝度,磷酸掺杂膜的强度变弱。在第三部分里,为了同时提高PA-PBI膜的质子传导率和机械强度,我们以Ph-PBI为基体,设计并合成了一种新型的含有咪唑基团的交联剂2,2'-双(氯甲基)-5,5'-联苯并咪唑,制备了一系列具有不同交联度的交联型PBI薄膜。交联薄膜在高的磷酸掺杂水平下表现出比原始Ph-PBI膜和第二部分中的接枝薄膜更好的尺寸和机械稳定性,从而获得了更高的质子传导率和更优异的燃料电池性能。所有磷酸掺杂交联膜的机械强度均在10 MPa以上。当交联度为30%时,交联膜的ADL为354%,200℃、无水条件下的传导率达到253 m S·cm-1,其H2/O2燃料电池在160℃、没有加湿情况下的最大功率密度高达533 m W·cm-2。在研究的最后一部分,为了增强PA-PBI膜对磷酸的保持能力,我们将具有保酸性的多羟基纳米Si O2粒子引入到上一部分所制备的具有优异性能的交联PBI膜中,制备了无机-有机复合型质子交换膜(Si O2/PBI)。通过扫描电子显微镜研究了复合膜的断面形貌。结果表明Si O2粒子均匀的分散在交联的PBI基质中。具有2 wt%Si O2含量的Si O2/PBI复合膜的磷酸掺杂水平为350%,其在200℃、无水条件下的质子传导率为244 m S·cm-1。该膜的H2/O2燃料电池在160℃、没有加湿情况下的最大功率密度为497 m W·cm-2。随着纳米Si O2含量的增加,PBI膜的酸保持能力和传导率稳定性均有提高。综上所述,我们从PBI基体结构设计的角度出发,制备了具有优异尺寸和机械稳定性的芳醚型PBI薄膜,通过引入更多的咪唑基团和交联膜的制备来进一步提升PBI膜的传导性能和机械强度,并通过无机-有机复合膜的制备来提升薄膜的保酸能力,最终获得了一系列具有优异综合性能的PA-PBI型高温质子交换膜。
[Abstract]:In the past few decades, proton exchange membrane fuel cell (PEMFC) as a highly efficient and environmentally friendly electrochemical energy conversion device has attracted wide attention of scientists. In recent years, with the deepening of research, the development of PEMFC which can work in high temperature and low humidity environment has become a research hotspot. High temperature proton exchange membrane fuel cell (HT-PEMFC) operated at 100-200 C has many advantages over membrane fuel cell (100 C). For example, it improves the tolerance of catalyst to CO, improves the efficiency of catalyst and simplifies the water/heat management. Polybenzimidazole (PBI) has excellent mechanical properties and thermal stability. The proton conductivity of PBI itself is very low, only 10-9 m S. cm-1, and it can not be used as an independent solid electrolyte. Phosphoric acid (PA) is a good electrolyte with high thermal stability, it is at 200 C. In the 1990s, Wainright et al. first doped phosphoric acid into PBI to prepare phosphoric acid-doped polybenzimidazole (PA-PBI) proton exchange membranes, which exhibited many excellent performances at high temperatures. This landmark study, as well as many subsequent related studies, led to PA-PBI films one by one. In the past two decades, the most widely studied PBI substrates have been commercialized poly [2,2'-m-(phenylene)-5,5'-dibenzimidazole] (m-PBI). For m-PBI, the strong hydrogen bonding between imidazole groups leads to poor solubility in order to ensure uniformity of the films. PA-PBI films are prepared by PBI with relatively low linear molecular weight (23-40 K Da, corresponding to the intrinsic viscosity of 0.5-1.0 D L.g-1). However, the mechanical properties of the polymer films are relatively poor due to the lower molecular weight, which only allows the PBI films to obtain a lower phosphoric acid doping amount (usually repeated per mole of PBI). The proton conductivity (100 m S cm 1) is low due to the doping of 6-10 moles of phosphoric acid with the unit. Therefore, researchers have adopted a variety of methods to improve the performance of the membrane, including grafting, crosslinking, inorganic doping, sol-gel method and synthesis of new PBI materials. The PA-PBI high-temperature proton exchange membranes still face several key challenges if they are to be commercialized, such as higher proton conductivity, better mechanical properties and more stable phosphoric acid retention capacity. High conductivity PA-PBI films are prepared by phosphoric acid doping of the films. However, high ADL leads to large size swelling, which correspondingly reduces the mechanical strength and durability of the films. PBI membrane materials with good dimensional stability and mechanical stability and high acid retention ability under high ADL or high proton conductivity are the key problems to be solved in the study of PA-PBI high temperature proton exchange membrane. Two kinds of high molecular weight aryl ether PBI (Ph-PBI and ME-PBI) containing flexible ether bonds and asymmetric large-volume side groups (phenyl and methyl phenyl) were synthesized and the corresponding phosphoric acid doped films were prepared. Compared with OPBI films, Ph-PBI and Me-PBI films exhibit different phosphoric acid doping behaviors, higher phosphoric acid doping levels, better dimensional and mechanical stability, higher proton conductivity and better fuel cell performance. Among them, the PA-PBI (Ph-PBI) obtained by phosphoric acid doping of Ph-PBI at 160 C for 72 h has higher proton conductivity and better fuel cell performance. - 72) The proton conductivity of the membrane is 138 m S. cm - 1, the volume swelling is only 188%, and the mechanical strength of the membrane is 9.7 MPa. Ph - 72. The maximum power density of the H2 / O2 fuel cell is 279 m W. cm - 2 without humidification. Based on the Ph - PBI with excellent size and mechanical stability synthesized in the previous part of the work, in order to make progress In the second part of the study, we grafted 2-chloromethylbenzimidazole onto Ph-PBI and prepared a series of polybenzimidazole films with different grafting degrees. The introduction of additional imidazole groups increased the basic groups on the chain of Ph-PBI and increased the ratio of Ph-PBI film to phosphoric acid. When the grafting degree is 20%, the phosphoric acid doping content is 341%, the conductivity of PBI film is 212 m S. cm - 1, and the tensile strength is 6.5 MPa in dry state. In the third part, in order to improve both proton conductivity and mechanical strength of PA-PBI film, we designed and synthesized a novel crosslinking agent 2,2'-bis (chloromethyl) -5,5'-biphenyl containing imidazole group on the basis of Ph-PBI. A series of cross-linked PBI thin films with different crosslinking degrees were prepared. The cross-linked thin films exhibited better size and mechanical stability than the original Ph-PBI films and the grafted thin films in the second part at high phosphoric acid doping levels, resulting in higher proton conductivity and better fuel cell performance. The mechanical strength of the cross-linked membranes is above 10 MPa. When the degree of cross-linking is 30%, the ADL of the cross-linked membranes is 354%, and the conductivity of the cross-linked membranes is 253 m S. cm-1 under anhydrous condition. The maximum power density of the H2/O2 fuel cell is 533 m W. cm-2 under 160 ~C and no humidification condition. In the last part of the study, in order to enhance the protection of PA-PBI membrane to phosphoric acid. The acidic polyhydroxy nano-Si O2 particles were introduced into the crosslinked PBI films prepared in the previous part to prepare inorganic-organic composite proton exchange membranes (Si O2/PBI). The cross-section morphology of the composite films was studied by scanning electron microscopy (SEM). The results showed that the Si O2 particles were uniformly dispersed in the crosslinked PBI films. The phosphoric acid doping level of the SiO 2/PBI composite membrane with 2 wt% SiO 2 content is 350%, and its proton conductivity is 244 m S cm 1 at 200 C and anhydrous condition. The maximum power density of the H2/O 2 fuel cell with this membrane is 497 m W cm 2 at 160 C and without humidification. With the increase of the nano Si 2 content, the acid of the PBI membrane remains. In summary, we have prepared aryl ether PBI films with excellent size and mechanical stability from the point of view of the structure design of PBI matrix. By introducing more imidazole groups and cross-linking membranes, the conductivity and mechanical strength of PBI films have been further improved, and by Inorganic-Organic methods. A series of PA-PBI high temperature proton exchange membranes with excellent comprehensive properties were obtained.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ425.236

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