顧 梁, 孫 哲, 徐 丹, 嚴 鋒(蘇州大學(xué)材料與化學(xué)化工學(xué)部,江蘇蘇州215123)
特約綜述
堿性陰離子交換聚合物膜研究進展
顧 梁, 孫 哲, 徐 丹, 嚴 鋒
(蘇州大學(xué)材料與化學(xué)化工學(xué)部,江蘇蘇州215123)
堿性燃料電池(AFCs)是一種直接將化學(xué)能轉(zhuǎn)化為電能的發(fā)電裝置,因其高效、環(huán)保等優(yōu)點,得到了科學(xué)界與工業(yè)界的廣泛關(guān)注。陰離子交換聚合物膜作為堿性陰離子交換膜燃料電池的核心組成部分,要求其具備優(yōu)異的電導(dǎo)率、良好的化學(xué)穩(wěn)定性及力學(xué)強度。本文主要從聚合物主鏈及陽離子官能團的結(jié)構(gòu)與性能之間的關(guān)系及調(diào)控方式方面,綜述了堿性陰離子交換膜的研究進展。
堿性陰離子交換膜燃料電池;陽離子;耐堿穩(wěn)定性;理論計算
聚電解質(zhì)膜燃料電池是一種能夠直接將燃料中的化學(xué)能轉(zhuǎn)化為電能的發(fā)電裝置,具有高能量轉(zhuǎn)化效率、高比功率、高比能量、負荷響應(yīng)快等特點,被認為是最有應(yīng)用前景的清潔能源之一[1-3]。因此,研制無污染、高能效的燃料電池,對解決化石能源所造成的環(huán)境污染有著十分重要的意義?;陔娊赓|(zhì)的分類,聚電解質(zhì)膜燃料電池主要分為質(zhì)子交換膜燃料電池(PEMFCs)及堿性陰離子交換膜燃料電池(AEMFCs)等[4-6]。其中,PEMFCs具有功率密度高、啟動速率快等優(yōu)點,并已在車用動力能源、通訊設(shè)備用電源、移動電源等領(lǐng)域得到廣泛應(yīng)用[7]。然而,PFMFCs需要使用昂貴的鉑等貴金屬作為催化劑,限制了該燃料電池的大面積推廣應(yīng)用[8]。鑒于此,近年來,使用非貴金屬(如Ag、Ni等)催化劑,且對CO2具有較強耐受性的AEMFCs開始受到人們的重視[9]。
AEMFCs主要由電極、催化劑、電解液及陰離子交換膜組成,其結(jié)構(gòu)示意圖如圖1所示。在電池中,高濃度的強堿性溶液既可當作電解液,又可作冷卻劑。堿性陰離子交換膜在燃料電池中起著分隔電極和傳導(dǎo)氫氧根離子的作用,其性能的優(yōu)劣直接影響著燃料電池的工作性能[11-12]。為滿足堿性燃料電池的應(yīng)用需求,陰離子交換膜必須具備較高的離子電導(dǎo)率、良好的熱穩(wěn)定性,優(yōu)良的力學(xué)性能及堿性溶液中良好的耐堿穩(wěn)定性能[13-14]。因此,研發(fā)性能優(yōu)異的陰離子交換膜對堿性燃料電池的推廣應(yīng)用具有至關(guān)重要的意義[15-20]。
圖1 堿性陰離子交換膜燃料電池結(jié)構(gòu)示意圖Fig.1 Structures of alkaline anion exchange membrane fuel cells
從化學(xué)結(jié)構(gòu)上看,堿性陰離子交換膜主要由聚合物主鏈和陽離子基團組成。通常認為聚合物主鏈結(jié)構(gòu)決定膜的力學(xué)性能、熱穩(wěn)定性能等;而陽離子基團則影響膜的電導(dǎo)率及耐堿穩(wěn)定性。近年來,科研人員對陰離子交換膜進行了一系列的深入研究,分別從聚合物主鏈和陽離子官能團的設(shè)計入手,合成制備了高性能的新型陰離子交換膜[21-26]。本文主要論述堿性陰離子交換膜聚合物主鏈與陽離子官能團化學(xué)結(jié)構(gòu)與性能之間的關(guān)系及其調(diào)控方法。
近年來,基于聚烯烴[27-30]、聚亞芳基[31-33]、聚苯醚[34-35]、聚亞苯基[36]和聚酰亞胺[37]以及聚(苯乙烯-丙烯腈)等共聚物的陰離子交換膜[38],由于具有優(yōu)異的熱穩(wěn)定性、良好的力學(xué)性能以及較低的水溶脹性,已得到廣泛研究,其結(jié)構(gòu)示意圖如圖2所示。
圖2 不同主鏈結(jié)構(gòu)的陰離子交換膜結(jié)構(gòu)Fig.2 Anion exchange membranes with various backbone structures
較之陽離子官能團,關(guān)于聚合物主鏈在堿性溶液中穩(wěn)定性的研究較少。一般認為,聚合物主鏈對于堿性溶液的穩(wěn)定性要優(yōu)于陽離子官能團,但也有研究表明氫氧根離子的進攻會導(dǎo)致聚合物主鏈發(fā)生降解反應(yīng)[9]。此外,聚合物主鏈結(jié)構(gòu)也決定著陰離子交換膜的力學(xué)強度、水溶脹性能及離子電導(dǎo)率[39]。
最早應(yīng)用于制備陰離子交換聚合物膜的是烯烴類聚合物。Varcoe課題組[40]首先報道了基于氟化聚鏈烯烴陰離子交換膜。該類聚合物膜在60~100℃具有較好的熱穩(wěn)定性。Coates[41]通過開環(huán)易位聚合制備了聚烯烴的交聯(lián)聚合物膜材料。化學(xué)交聯(lián)所形成的聚合物鏈網(wǎng)絡(luò)結(jié)構(gòu)有利于氫氧根離子的傳導(dǎo)。聚亞芳醚聚合物包括聚亞芳基醚砜和聚亞芳基醚酮。這類聚合物具有較高的玻璃化轉(zhuǎn)變溫度(Tg)和優(yōu)異的熱穩(wěn)定性,并且在有機溶劑中具有良好的溶解性能,因此在陰離子交換膜研究領(lǐng)域得到了廣泛關(guān)注[42-43]。Yan等[44]使用N-溴代丁二酰亞胺對聚砜主鏈進行溴化,制備了主鏈結(jié)構(gòu)為聚砜的陰離子交換膜。該結(jié)構(gòu)可以通過控制聚砜主鏈的溴化程度調(diào)節(jié)膜的吸水能力和離子電導(dǎo)率(圖3)。Watanabe等[45]制備了一系列含有季銨化芴基取代基的聚亞芳基醚陰離子交換膜,該類聚合物膜在堿性燃料電池的應(yīng)用中展現(xiàn)出了優(yōu)異的電導(dǎo)率和力學(xué)性能。
圖3 基于聚砜主鏈陰離子交換膜Fig.3 Anion exchange membranes based on polysulfone backbone
作為優(yōu)異的工程塑料,聚(2,6-二甲基-1,4-苯醚)同樣因具有高玻璃化轉(zhuǎn)變溫度(Tg)和優(yōu)異的化學(xué)穩(wěn)定性而備受關(guān)注。Xu等[46]制備了基于聚苯醚的陰離子交換膜,進一步將該陰離子交換膜與含氯代乙?;鶊F的聚苯醚交聯(lián)反應(yīng),從而提高其力學(xué)性能。Hickner[47]合成了聚亞苯基主鏈的陰離子交換膜,研究表明亞苯環(huán)結(jié)構(gòu)的主鏈因其非共面的特性形成了剛性結(jié)構(gòu),使聚合物膜在500℃下仍保持優(yōu)良的熱穩(wěn)定性和力學(xué)性能。Hibbs[48]通過Diels-Alder反應(yīng)制備了聚亞苯基為主鏈的陰離子交換膜,其在水溶液中的離子電導(dǎo)率達到了50 mS/cm,并且在高溫堿性溶液中具有良好的堿穩(wěn)定性(圖4)。聚酰亞胺具有優(yōu)異的熱穩(wěn)定性、化學(xué)穩(wěn)定性和力學(xué)性能,故被廣泛應(yīng)用于膜材料的制備。Chen等[49]制備了聚(醚酰亞胺)陰離子交換膜,該聚合物膜在室溫條件下的8 mol/L堿溶液中仍具備相當高的化學(xué)穩(wěn)定性。
圖4 Diels-Alder聚合反應(yīng)制備的聚亞苯基主鏈陰離子交換膜Fig.4 Poly(phenylene)type anion exchange membranes prepared by Diels-Alder polymerization
近年來,利用幾種單體共聚所制備的陰離子交換膜得到了廣泛的研究。嚴鋒課題組[50]將苯乙烯、丙烯腈與咪唑鹽單體共聚制備了交聯(lián)型的陰離子交換膜。該陰離子交換聚合物膜具備較高的力學(xué)性能和電導(dǎo)率(在室溫條件下達到了10 mS/cm以上),并且顯示出良好的耐堿穩(wěn)定性。該課題組還通過光交聯(lián)1-乙烯基-3-甲基咪唑、苯乙烯和丙烯腈制備了新型交聯(lián)陰離子交換膜。經(jīng)400 h強堿溶液中穩(wěn)定性測試后,電導(dǎo)率沒有明顯降低,展現(xiàn)出了優(yōu)異的化學(xué)穩(wěn)定性[38]。
陰離子交換膜在堿性燃料電池中起著傳導(dǎo)氫氧根離子的作用,傳統(tǒng)的提高離子交換容量(IEC,ionic exchange capacity)的方法,雖然能夠增加膜的電導(dǎo)率,但同時也會使膜在水溶液中吸收過量的水,從而導(dǎo)致明顯的水溶脹,所以更為有效的方式是提高氫氧根離子的有效遷移率[39]。由于氫氧根離子的傳導(dǎo)主要依賴于聚合物主鏈中親水基團附近存在的水分子,因此聚合物主鏈的結(jié)構(gòu)框架會對氫氧根離子的遷移率產(chǎn)生明顯影響。Nafion膜優(yōu)異的離子傳導(dǎo)率正是因為其形態(tài)結(jié)構(gòu)上形成的相分離:聚合物主鏈結(jié)構(gòu)上同時存在的疏水碳鏈骨架和包含離子基團的親水鏈促進了親水-疏水相分離結(jié)構(gòu)的形成,并由親水區(qū)域的相互重疊形成了內(nèi)部的離子通道[51]。因此,在聚合物主鏈結(jié)構(gòu)上構(gòu)建相分離可以有效地提高陰離子交換膜的離子電導(dǎo)率。
圖5 1-乙烯基-3-甲基咪唑、苯乙烯和丙烯腈通過原位交聯(lián)制備的陰離子交換膜Fig.5 Anion exchange membranes prepared via in situ cross-linking of 1-vinyl-3-methyl-imidazolium iodide([VMIm][I])with styrene and acrylonitrile
常見的構(gòu)建相分離結(jié)構(gòu)的方式有增加聚合物主鏈與陽離子基團之間的碳鏈長度,例如將季銨陽離子通過長支鏈連接到聚合物主鏈上,但是該方法通常需要使用高度致癌的試劑如氯甲基乙醚。Chen等[52]將季銨鹽類的親水性離子基團按一定順序和規(guī)律分布在疏水性的聚合物主鏈上,離子基團與聚合物主鏈兩者相互作用形成了微小的離子通道,形成了明顯的微相分離。圖6(a)與圖6(d)分別是干燥狀態(tài)下的季銨聚砜(QAPSF)和支鏈陽離子季銨聚砜(p QAPSF)的TEM照片。圖6(a)中陽離子分布在聚砜主鏈上,親水基團均勻地分散在干燥的膜表面,表明膜中無微相分離存在。通過將支鏈陽離子引入季銨聚砜主鏈(圖6(d)),親水基團出現(xiàn)了明顯的聚集并且不均勻地分布在疏水主鏈上,形成了明顯的微相分離。通過粗?;肿觿恿W(xué)(coarse-grainedmoleculardynamics)進一步對QAPSF和p QAPSF模擬后表明,QAPSF無論在干燥還是濕潤狀態(tài)(圖6(c))下都無明顯的微相分離結(jié)構(gòu)存在。相比較而言,p QAPSF的干燥(圖6(e))和濕潤狀態(tài)(圖6(f))則存在明顯的微相分離結(jié)構(gòu)。該微相分離結(jié)構(gòu)的離子交換膜可以在聚合物主鏈附近形成親水區(qū)域,并增加此區(qū)域的氫氧根濃度,使聚合物膜在堿性溶液中的離子電導(dǎo)率提高,同時也使離子交換膜兼具良好的耐水溶脹性能和優(yōu)良的力學(xué)性能。
圖6 QAPSF和p QAPSF膜的透射電子顯微鏡照片(a,d);QAPSF和p QAPSF在干燥狀態(tài)(b,e)和在濕潤狀態(tài)(c,f)下的CGMD模擬照片F(xiàn)ig.6 TEM images for QAPSF and p QAPSF membrane in dry states(a,d);CGMD simulation images of QAPSF and p QAPSF in dry states(b,e)and in wet states(c,f)
Chao等[53]制備并合成了苯乙烯和異戊二烯嵌段的陰離子交換膜。該主鏈結(jié)構(gòu)中的微相分離使其與傳統(tǒng)的陰離子交換膜相比,呈現(xiàn)出了優(yōu)異的離子電導(dǎo)率。另外,對于聚合物膜還可以采取化學(xué)交聯(lián)的方法提高其力學(xué)性能和耐水溶脹性。例如,嚴鋒課題組[54]以二乙烯基苯(DVB)作為交聯(lián)劑,所制備的聚合物陰離子交換膜具有優(yōu)良的力學(xué)性能,并且在堿性溶液中也具備優(yōu)良的耐堿性。
陽離子官能團作為陰離子交換膜的一個重要組成部分,對于其結(jié)構(gòu)性能的研究必不可少,它的耐堿穩(wěn)定性決定著離子交換膜的耐堿穩(wěn)定性。在已有的研究報道中,基于N原子的季銨鹽,由于具有合成簡單,易于分子設(shè)計等特點而成為研究最廣泛的一類陽離子[55]。然而,由于季銨鹽陽離子化合物(QAs)在堿性溶液中易于被氫氧根進攻而發(fā)生霍夫曼降解反應(yīng)以及SN2親核取代反應(yīng) (圖7),因此,近年來研究人員致力于探索合成基于新型陽離子的陰離子交換聚合物膜。
圖7 季銨鹽陽離子化合物在堿性溶液中的降解機理Fig.7 Degradation mechanism of QAs in basic solution
近年來,科研人員對包括基于季銨鹽[56-58]、季磷鹽[59]、胍鹽[60]、咪唑鹽[61]、金屬陽離子[62]以及苯并咪唑[63]等陽離子的陰離子交換膜進行了一系列的研究(圖8),通過選擇設(shè)計不同的陽離子結(jié)構(gòu),提高陰離子交換膜的耐堿穩(wěn)定性。
圖8 常見的應(yīng)用于陰離子交換膜的陽離子官能團Fig.8 Cation groups used in the anion exchange membrane
Ramani教授課題組[64]用三甲基膦代替三甲胺修飾氯甲基化聚砜,實驗研究中觀察到,用三甲基膦修飾的聚合物耐堿性反而降低。Yan課題組[65]合成了新型三甲氧基苯基磷型季磷陽離子堿性陰離子交換膜,結(jié)果表明苯環(huán)具有明顯的位阻效應(yīng),可以阻礙氫氧根的親核進攻,從而提高季磷鹽陽離子的耐堿穩(wěn)定性。長春應(yīng)化所張所波教授課題組[66]合成了基于胍鹽的堿性陰離子交換聚合物膜,胍鹽的共振結(jié)構(gòu)使得陽離子電子云分布均勻,減弱了氫氧根的親核進攻,從而提高了陽離子的耐堿性。
較之于季銨鹽類陽離子,咪唑鹽陽離子因其環(huán)狀共軛結(jié)構(gòu),降低了陽離子電子云密度,減弱了對氫氧根離子的吸引,從而增強了陽離子的耐堿穩(wěn)定性。嚴鋒課題組系統(tǒng)地研究了取代基對咪唑鹽陽離子耐堿穩(wěn)定性的影響[38,54,67]。研究結(jié)果表明,取代基的吸-供電子能力、離域效應(yīng)和位阻效應(yīng)等都會影響OH-對陽離子的進攻。進一步的研究表明,咪唑環(huán)上取代基的位置對其穩(wěn)定性的影響,即取代基在咪唑環(huán)的C-2位上比在N-3位上能更有效地提高其耐堿穩(wěn)定性[68]。Zhang等[69]制備了聚砜主鏈上接枝了甲基咪唑陽離子的陰離子交換聚合物膜,其電導(dǎo)率、IEC、熱穩(wěn)定性都優(yōu)于傳統(tǒng)的季銨鹽陰離子交換膜。最近,嚴鋒課題組[70]還考察了基于雜環(huán)吡咯烷陽離子的陰離子交換聚合物膜。研究表明,1-乙基-1-甲基吡咯陽離子具有很好的堿穩(wěn)定性。核磁共振結(jié)果表明,其在80℃下、4 mol/L的NaOH溶液中浸泡96 h后,降解不足8%,最終制得的陰離子交換膜,其電導(dǎo)率在80℃下,于1 mol/L NaOH中浸泡了18 d后并無明顯下降。
僅僅通過實驗表征手段較難預(yù)測陽離子官能團的化學(xué)穩(wěn)定性。鑒于此,很多研究采用理論計算與實驗表征相結(jié)合的方法研究陽離子官能團的耐堿穩(wěn)定性。應(yīng)用得比較多的計算方法是密度泛函理論(DFT,density functional theory)。鑒于陽離子在強堿溶液中發(fā)生降解反應(yīng)的復(fù)雜性和多樣性,運用密度泛函理論(DFT)對陽離子進行理論計算,可以有效預(yù)測陽離子的降解程度和耐堿性。嚴鋒課題組[54,67]對咪唑鹽陽離子在堿性環(huán)境下的穩(wěn)定性進行計算。研究結(jié)果表明,可以通過陽離子LUMO的大小判斷咪唑陽離子耐堿穩(wěn)定性。陽離子的LUMO越高,咪唑鹽陽離子越難在堿性條件下降解。計算結(jié)果與實驗測試結(jié)果很好地吻合(表1)。
表1 80℃下,咪唑陽離子的LUMO與其在堿溶液中的降解率[67]Table 1 LUMO value and degradation of imidazolium salts in basic solutions at 80℃[67]
陽離子官能團的耐堿性與氫氧根離子反應(yīng)的能量壁壘也有關(guān),因此可以通過計算陽離子在氫氧根進攻下過渡態(tài)(transition state)與初始狀態(tài)(initial state)的能量差(Ebarrier)來預(yù)測(圖9)。Ebarrier越大,則氫氧根對于陽離子的進攻難度越大,即陽離子的耐堿穩(wěn)定性越好。其他的理論計算還有Mulliken電荷布居分析等,這些理論計算都大大豐富了陽離子耐堿穩(wěn)定性的檢測手段。
圖9 咪唑陽離子反應(yīng)的能量壁壘(1 kcal=4.187 kJ)Fig.9 Reaction energy barriers for imidazolium cations(1 kcal=4.187 kJ)
目前,陰離子交換膜因堿性燃料電池在能源領(lǐng)域頗受關(guān)注而得到了快速地發(fā)展。理想的陰離子交換膜需要滿足較高的電導(dǎo)率、力學(xué)性能、耐水溶脹性、耐堿穩(wěn)定性等性能。其中,耐堿穩(wěn)定性成為聚合物陰離子交換膜能否實用化的一個主要參數(shù)之一。在關(guān)注聚合物主鏈結(jié)構(gòu)影響的同時,也深入研究陽離子官能團的分子結(jié)構(gòu)對其化學(xué)穩(wěn)定性的影響。將實驗檢測手段與理論計算相結(jié)合是研究陰離子交換膜化學(xué)穩(wěn)定性的有效方法。相信隨著高電導(dǎo)率、高穩(wěn)定性陰離子交換膜的研制成功,將推動堿性陰離子交換膜燃料電池的實際應(yīng)用。
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Progress of Alkaline Anion Exchange Membranes
GU Liang, SUN Zhe, XU Dan, YAN Feng
(College of Chemistry,Chemical Engineering and Materials Science,Soochow University,Suzhou 215123,Jiangsu,China)
Alkaline fuel cells(AFCs),a kind of devices which can convert chemical energy into electric energy directly,have attracted considerable attention in the fields of science and industry with the advantages of high energy-conversion efficiencies and environmental protection.As the key component of AFCs,the anion exchange membranes require excellent ionic conductivity,high chemical stability,mechanical strength and low water swelling.This review reports the research progress of the anion exchange membranes,especially on the structure-alkaline stability relationships of polymer backbones and cations.
alkaline anion exchange membrane fuel cells;cations;alkaline stability;theoretical calculations
O63
A
1008-9357(2016)02-0153-010DOI: 10.14133/j.cnki.1008-9357.2016.02.003
2016-04-19
國家杰出青年基金(21425417);國家自然科學(xué)基金(21274101);江蘇高校優(yōu)勢學(xué)科建設(shè)工程資助項目
顧 梁(1991-),男,江蘇蘇州人,碩士生,主要研究方向為堿性燃料電池陰離子交換膜。E-mail:18351086532@163.com
嚴 鋒,E-mail:fyan@suda.edu.cn