Synthesis and Characterization of Aromatic Hydrocarbon based Polymer Electrolyte Membranes for Fuel Cells and Reverse Electrodialysis

Abstract

ABSTRACT

This dissertation focused on the various polymeric materials for polymer electrolyte membranes (PEMs) in order to apply for the energy generation. Specifically, sulfonated poly(sulfide sulfone imide), sulfonated polybenzothiazoles and sulfonated poly(benzothiazole-co-benzimidazole) copolymers were studied for proton exchange membrane fuel cell (PEMFC). Sulfonated poly(arylene ether sulfone) copolymers were studied for direct methanol fuel cell (DMFC) and reverse electrodialysis (RED). In addition, imidazolium functionalized poly(arylene ether sulfone) copolymer was studied for anion exchange membrane fuel cell (AEMFC). Detailed research data and optimized synthetic strategies for each applications are described for effective energy generation. This dissertation is organized as follows: Introduction, PEMs for proton exchange membrane fuel cell (PEMFC), PEMs for direct methanol fuel cell (DMFC), PEMs for anion exchange membrane fuel cell (AEMFC) and PEMs for reverse electrodialysis (RED) application.

Proton Exchange Membrane Fuel Cell (PEMFC) In Chapter 2: Sulfonated poly(sulfide sulfone imide) copolymers containing flexible sulfide bond and six-membered imide ring were synthesized by random polycondensation. Two types of membranes were prepared by using sulfide (S-PSI) and sulfide sulfone (S-PSFI) based non-sulfonated diamines to investigate the effects of the hydrophobic component. IECw values were controlled to 1.51-1.94 meq.g-1 depending on the degree of sulfonation (DS) which was in the range of 50 to 80%. The membrane series showed good thermal stability up to 310 oC and mechanical properties (tensile strength > 30 MPa). Dimensional stabilities were excellent with 23-35% increases, even at 100 oC. Proton conductivities of membranes composed of different hydrophobic diamines display a relatively good correlation with water content and morphology. In fuel cell tests, the S-PSI60 membrane shows relatively high current density of 250 mA cm-2 at 0.6 V and maximum power density of 175 mW cm-2 at 120 oC, 35% RH, 1.5 atm. In Chapter 3: Two series of sulfonated polybenzothiazoles containing naphthalene, derived from either 2,2-bis(4-carboxyphenyl) hexafluoropropane (6FA) or 2,6-naphthalene dicarboxylic acid (NA) were synthesized by polycondensation with non-sulfonated monomer 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) and sulfonated monomer 4,8-disulfonyl-2,6-naphthalene dicarboxylic acid (DSNA). The sPBT-6FA series polymers containing DSNA were soluble in polar aprotic solvents such as dimethyl sulfoxide or 1-methyl-2-pyrrolidinone, whereas sPBT-NA65 with NA was not soluble in common polar aprotic solvents. Therefore, the incorporation of both the naphthalene and flexible hexafluoroisopropylidene units enhanced the solubility of the sPBT polymer series. The polymers were evaluated as proton exchange membranes and exhibited excellent dimensional stability, high thermal and oxidative stabilities, good mechanical properties, and high proton conductivities. The proton conductivities of sPBT-6FA65 were 0.25 S cm-1 at 80 oC in water and 0.018 S cm-1 at 120 oC under 35% RH. In Chapter 4: Two series of random sulfonated poly(benzothiazole-co-benzimida zole) polymers (sPBT-BI) with 70 % and 60 % degree of sulfonation were evaluated as proton exchange membranes. Sulfonated poly(benzothiazole) (sPBT) was also prepared for a comparative study. The mechanical properties of sPBT-BI were greatly enhanced by incorporation of benzimidazole (BI); sPBT-BI70-10 showed the tensile strength of 125 Mpa and elongation at break of 38.9 %, an increase of 56.5 % and 145 %, respectively, compared with sPBT. The solubility, dimensional stability, thermal properties, and oxidative stability of sPBT-BI were also improved. The ionic clusters of sPBT-BI membranes in both AFM phase images and TEM images became narrower with increasing amounts of BI while containing the same molar amount of sulfonic acid groups. This resulted in lower dimensional swelling and higher mechanical strength, but the proton conductivity decreased. However, high proton conductivity was achieved by incorporating an appropriate content of BI. H2/air single cell performances and PEMFCs durabilities were improved by incorporation of 5 % of BI groups in sPBT. In Chapter 5: Sulfonated poly(arylene ether sulfone) copolymers having three different dihydroxynaphthalene isomers and biphenol (BP) moiety were synthesized according to the same weight-based ion exchange capacity (IECw). Membrane samples having two different IECw (1.82 and 2.2 meq g-1) were evaluated through thermal, mechanical and electrochemical analysis. In order to investigate the isomeric effects of naphthalene and BP units in polymer matrix for polymer electrolyte membrane fuel cell (PEMFC) applications. Interestingly, the prepared polymeric membranes comprising BP or two different naphthalene isomers exhibited considerably disparate ion transport properties in PEMFC applications. In addition, despite having the same IECw, a large difference in electrochemical performance was observed due to isomeric effects in the naphthalene based polymer backbone. Moreover, in PEMFC single cell tests at 80 oC under RH 100% and H2/Air fed condition, the membrane composed of 1,5-dihydroxynaphthlene (IECw = 2.2 meq g-1) having helical conformation achieved a promising current density (614 mA cm-2 at 0.6 V) among the tested membranes.

Direct Methanol Fuel Cell (DMFC) In Chapter 6: End-group crosslinkable sulfonated poly(arylene ether sulfone) copolymer (ESPAES) and imidazolium poly(arylene ether sulfone) copolymer (IPAES) are synthesized as a proton exchange membrane and ionic crosslinker, respectively. A novel dually cross-linked membrane (DCM) based on ESPAES is similar to an inter-penetrating network and is prepared via blending IPAES and thermal treatment for direct methanol fuel cell (DMFC) applications. The synergistic effects of end-group crosslinking and ionic crosslinking improves chemical and thermal stability and mechanical properties. In addition, the DMFC performance of the DCM outperforms that of the end-group cross-linked SPAES and Nafion® 212 due to its excellent fuel barrier property in spite of relatively low proton conductivity, which is derived from the content of the non-proton conducting IPAES copolymer. Consequently, the DCM has great potential as an electrolyte membrane for DMFC applications.

Anion Exchange Membrane Fuel Cell (AEMFC) In Chapter 7: End-group cross-linked anion exchange membranes (AEM) was demonstrated by physical and electrochemical analysis for the first time. A novel feature of the cross-linking reaction is that basic additives are not required to prevent gelation with the cationic functional groups. In this work, the sodium salt of 3-hydroxyphenylacetylene acted directly as the end-group cross-linker, and it was cross-linked by thermal treatment at 180 oC. The gel fraction and hydroxide conductivity of the cross-linked membranes (XE-Imds) depended on the cross-linking temperature and time. The prepared XE-Imd70 (70 refers to the degree of functionalization) membranes with an ion exchange capacity (IEC) of 2.2 meq g-1 achieved a high hydroxide conductivity (107 mS cm-1). This material also showed good single cell performance (XE-Imd70: 202 mA cm-2 at 0.6V and a maximum power density of 196.1 mW cm-2) at 80oC, 100% relative humidity (RH), and improved durability and alkaline stability. The excellent hydroxide conductivity and electrochemical performance of XE-Imd70 was due to the fact that the ion cluster size of XE-Imd membranes was larger (12.1 – 14 nm) than that of E-Imd (5.5 – 8.14 nm), indicating that XE-Imd membranes have a closely associated ion-clustered morphology, which was confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) measurements

REVERSE ELECTRODIALYSIS (RED) In Chapter 8: Three different dihydroxynaphthalene (DHN) isomers and biphenol (BP) monomer were used for sulfonated poly(arylene ether sulfone) (SPAES) materials. Three different IECw (1.42, 1.82 and 2.2 meq g-1) were controlled to evaluate ion transport properties such as permselectivity and resistance. In order to investigate the isomeric effects of DHNs and BP in polymer matrix for reverse electrodialysis (RED). Their Na+ ion transport properties were analyzed by testing electrochemical experimentals. In particular, three different DHN isomers exhibited considerably different ion transport. In RED application, 2,6-dihydroxynaphthalene containing SPAES showed high Na+ ion permselectivity and low membrane resistance due to dense and linear conformation.|국문 요지 고분자 전해질 분리막은 수소연료전지 (PEMFCs), 직접메탄올 연료전지 (DMFCs), 알카라인 연료전지(AEMFCs) 및 역전기분해 (RED) 시스템을 통한 에너지생산 프로세스에 있어 가장 중요한 핵심 요소 중 하나이다. 고분자 전해질 분리막은 시스템의 전기화학적 성능과 그 내구성에 영향을 주어 전체적인 시스템의 효율을 결정한다. 때문에 고분자 전해질 분리막의 소재 연구 및 그 화학적, 구조적 응용 연구는 연료전지 및 역전기분해 시스템의 상용화 및 성능 향상을 위해 중요하며, 최근까지 수많은 연구자들이기존의 물질들이 가진 한계점을 돌파할 수 있는 새로운 전해질 소재의 개발 및 응용에 힘써왔다. 본 학위 논문은 이러한 고분자 전해질 분리막을 위한 소재연구의 동향을 파악하고 전기화학적 성능을 극대화하기 위한 맞춤형 전략을 다룬다. 본 내용은 세부적으로 총 9장으로 구성되어 있으며, 1장 전반적 소개, 2장부터 8장까지는 고분자 전해질 분리막의 에너지 관련 응용을 위한 각각의 세부연구를 다루며, 마지막으로 9장 결론을 통해 행한 연구의 장점과 그 방향성, 한계를 점검하고 앞으로 연구가 이어질 방향을 제시한다. 본문 2장에서는 두 가지 소수성 단량체를 사용한 육각 이미드 고리와 유연한 설파이드 결합을 가진 술폰화 폴리 설파이드 술폰 이미드 공중합체 시리즈의 합성을 통해 중, 고온 수소연료전지 응용을 위한 술폰화 폴리이미드의 성능을 분석하였으며, 소수성 단량체의 차이에 따른 고분자 전해질 분리막의 특성 변화를 관찰하였다. 더불어 최정 단위전지성능시험을 통하여 단량체의 화학적 구조 변화를 통한 모폴로지의 조절로 수소이온전달에 유리한 이온채널 사이즈를 제어하는 법을 개발하였다. 3장에서는 나프탈렌 그룹과 유연한 프로필 불소그룹을 가지는 술폰화 폴리 벤즈싸이아졸을 합성하여 용매에 대한 용해도를 향상 시킨 고분자 전해질 분리막 소재를 개발하였으며, 폴리 벤즈 싸이아졸의 특성상, 훌륭한 치수안정성과 높은 열 안정성과 산화안정성을 나타내었다. 더불어 고온 저가습 조건에서의 연료전지 구동 또한 훌륭한 전기화학적 성능을 보였다. 4장은 술폰화 폴리 벤즈싸이아졸의 응용으로 폴리 벤즈이미다졸을 공중합체로 도입하여 합성을 하였다. 폴리 벤즈이미다졸의 부분적인 도입을 통해 더욱 나노사이즈로 상 분리된 이온채널을 얻을 수 있었으며, 또한 산-염기 상태의 이온결합효과를 통해 아주 훌륭한 기계적 강도 및 분리막의 내구성을 강화시킬 수 있었다. 5장에서는 고분자 전해질로 가장 널리 사용되고 있는 술폰화 폴리 아릴렌 에테르 술폰 소재에 나프탈렌 단량체 이성질체 시리즈를 도입하여, 각 이성질체에 따른 고분자 체인 매트릭스의 변화가 고분자 전해질 분리막의 다양한 특성에 미치는 영향을 연구하였다. 그 결과, 이성질체마다 고분자 사슬의 패킹 및 밀도 그리고 기계적 성질 등에 큰 변화를 주기 때문에 이온전달능력이 크게 달라지는 것을 확인하였다. 본 연구를 통하여 이성질체 영향이 큰 나프탈렌 그룹의 도입은 전기화학적 성능 최적화를 위하여 생각해볼 수 있는 유용한 대안임을 알 수 있었다. 6장에서는 직접메탄올 전지응용을 위한 맞춤형 가교전략이 연구되었다. 술폰화 폴리 아릴렌 에테르 고분자를 기반으로 이미다졸륨이 음이온 교환 작용기로 달린 폴리 아릴렌 에테르 고분자를 소량 블렌딩하여 산-염기 간 이온가교를 도입하였다. 또한 술폰화 고분자의 말단에 가교가 가능한 삼중결합을 추가하여 제막과정에서 열에 의한 고리화 반응을 통하여 고분자 말단가교를 동시에 실시함으로써 연료 투과도를 현저히 떨어트리고 이온 선택도의 향상을 이뤄내었다. 두 가지 가교법을 동시에 도입하여, 고분자 전해질 분리막의 전반적인 열적, 기계적 안정성의 향상을 가져왔으며 결과적으로 말단가교를 통한 모폴로지의 최적화로 고성능 직접메탄올 연료전지용 고분자 전해질 분리막을 개발하였다. 7장에서는 앞서 이용한 말단 가교법을 음이온교환 고분자 전해질 분리막에 처음으로 적용하였다. 양이온 교환수지와 달리 음이온 교환 수지의 합성과정의 특이점으로 맞춤형 전략으로 가교제를 만들어 만들어진 전해질막은 높은 이온교환능력을 가짐에도 불구하고 큰 수화안정성과 알칼라인 안정성을 나타내었고, 말단가교 과정에서 변화된 최적화된 이온채널의 형성을 통하여 최고수준의 이온전도도 및 단위전지 성능을 나타내었다. 8장에서는 역전기투석용 술폰화 폴리 아릴렌 에테르 술폰 양이온 교환막의 특성 분석 및 나프탈렌 이성질체 그룹이 고분자 사슬의 구조 및 분리막의 투과특성에 미치는 영향을 연구하였다.; sPBT-BI70-10 showed the tensile strength of 125 Mpa and elongation at break of 38.9 %, an increase of 56.5 % and 145 %, respectively, compared with sPBT. The solubility, dimensional stability, thermal properties, and oxidative stability of sPBT-BI were also improved. The ionic clusters of sPBT-BI membranes in both AFM phase images and TEM images became narrower with increasing amounts of BI while containing the same molar amount of sulfonic acid groups. This resulted in lower dimensional swelling and higher mechanical strength, but the proton conductivity decreased. However, high proton conductivity was achieved by incorporating an appropriate content of BI. H2/air single cell performances and PEMFCs durabilities were improved by incorporation of 5 % of BI groups in sPBT. In Chapter 5: Sulfonated poly(arylene ether sulfone) copolymers having three different dihydroxynaphthalene isomers and biphenol (BP) moiety were synthesized according to the same weight-based ion exchange capacity (IECw). Membrane samples having two different IECw (1.82 and 2.2 meq g-1) were evaluated through thermal, mechanical and electrochemical analysis. In order to investigate the isomeric effects of naphthalene and BP units in polymer matrix for polymer electrolyte membrane fuel cell (PEMFC) applications. Interestingly, the prepared polymeric membranes comprising BP or two different naphthalene isomers exhibited considerably disparate ion transport properties in PEMFC applications. In addition, despite having the same IECw, a large difference in electrochemical performance was observed due to isomeric effects in the naphthalene based polymer backbone. Moreover, in PEMFC single cell tests at 80 oC under RH 100% and H2/Air fed condition, the membrane composed of 1,5-dihydroxynaphthlene (IECw = 2.2 meq g-1) having helical conformation achieved a promising current density (614 mA cm-2 at 0.6 V) among the tested membranes.

Direct Methanol Fuel Cell (DMFC) In Chapter 6: End-group crosslinkable sulfonated poly(arylene ether sulfone) copolymer (ESPAES) and imidazolium poly(arylene ether sulfone) copolymer (IPAES) are synthesized as a proton exchange membrane and ionic crosslinker, respectively. A novel dually cross-linked membrane (DCM) based on ESPAES is similar to an inter-penetrating network and is prepared via blending IPAES and thermal treatment for direct methanol fuel cell (DMFC) applications. The synergistic effects of end-group crosslinking and ionic crosslinking improves chemical and thermal stability and mechanical properties. In addition, the DMFC performance of the DCM outperforms that of the end-group cross-linked SPAES and Nafion® 212 due to its excellent fuel barrier property in spite of relatively low proton conductivity, which is derived from the content of the non-proton conducting IPAES copolymer. Consequently, the DCM has great potential as an electrolyte membrane for DMFC applications.

Anion Exchange Membrane Fuel Cell (AEMFC) In Chapter 7: End-group cross-linked anion exchange membranes (AEM) was demonstrated by physical and electrochemical analysis for the first time. A novel feature of the cross-linking reaction is that basic additives are not required to prevent gelation with the cationic functional groups. In this work, the sodium salt of 3-hydroxyphenylacetylene acted directly as the end-group cross-linker, and it was cross-linked by thermal treatment at 180 oC. The gel fraction and hydroxide conductivity of the cross-linked membranes (XE-Imds) depended on the cross-linking temperature and time. The prepared XE-Imd70 (70 refers to the degree of functionalization) membranes with an ion exchange capacity (IEC) of 2.2 meq g-1 achieved a high hydroxide conductivity (107 mS cm-1). This material also showed good single cell performance (XE-Imd70: 202 mA cm-2 at 0.6V and a maximum power density of 196.1 mW cm-2) at 80oC, 100% relative humidity (RH), and improved durability and alkaline stability. The excellent hydroxide conductivity and electrochemical performance of XE-Imd70 was due to the fact that the ion cluster size of XE-Imd membranes was larger (12.1 – 14 nm) than that of E-Imd (5.5 – 8.14 nm), indicating that XE-Imd membranes have a closely associated ion-clustered morphology, which was confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) measurements

REVERSE ELECTRODIALYSIS (RED) In Chapter 8: Three different dihydroxynaphthalene (DHN) isomers and biphenol (BP) monomer were used for sulfonated poly(arylene ether sulfone) (SPAES) materials. Three different IECw (1.42, 1.82 and 2.2 meq g-1) were controlled to evaluate ion transport properties such as permselectivity and resistance. In order to investigate the isomeric effects of DHNs and BP in polymer matrix for reverse electrodialysis (RED). Their Na+ ion transport properties were analyzed by testing electrochemical experimentals. In particular, three different DHN isomers exhibited considerably different ion transport. In RED application, 2,6-dihydroxynaphthalene containing SPAES showed high Na+ ion permselectivity and low membrane resistance due to dense and linear conformation.