Polybenzimidazole-based Ion Solvating Membranes for Electrochemical Energy Conversion and Storage

국문 초록 (Abstract)

Polybenzimidazole 기반 이온 용매화막을 이용한 전기화학적 에너지 변환 및 저장 작성자: Muhammad Mara Ikhsan 에너지공학, KIST-School 과학기술연합대학원대학교 지도교수: Prof. Dr. Dirk Henkensmeier 급격한 기후 변화로 인해 화석 연료 기반 에너지에서 재생 가능한 에너지로의 전환이 가속화 되고있다. 재생에너지를 이용해 생성된 전기에너지를 저장하고 다시 변환하기 위해 배터리, 수전해, 연료 전지와 같은 전기화학적 에너지 장치가 유망한 후보로 평가받고있다. Polybenzimidazole (PBI)은 양쪽성 특성, 우수한 화학적 안정성, 불소가 없는 화학 구조로 인해 전기화학적 에너지 장치의 막으로 특히 주목받고 있다. 바나듐 레독스 흐름 전지 (VRFB)에서 양성자가 도핑된 PBI는 Donnan 효과와 크기 배제 효과를 통해 바나듐 이온 교차를 감소시키는 이점을 제공하지만, 이온 전도도가 2-5 mS cm⁻¹로 낮다는 한계가 있다. 본 연구에서는 메타-PBI(mPBI) 막의 전도성을 향상시키기 위해 황산(H2SO4)을 이용한 고농도 전처리를 수행했다. 25 °C에서 10 M 황산을 이용한 전처리를 통해 최적의 성능과 높은 선택성을 얻을 수 있었으며, 80 mA cm⁻²에서 500 사이클 동안 99.5%의 쿨롬 효율(CE)과 89.6%의 에너지 효율(EE)을 기록했다. 또 다른 이온 전도도 향상 방법으로는 얇은 PBI를 바나듐 차단층으로 사용해 저항을 줄이면서 초기 좁은 형태를 유지하는 것이 있다. 그러나 전도성이 없는 다공성 지지체 위에 박막을 제작할 시, Shadowing 효과가 발생해 박막을 얇고 균일하게 유지하기 어렵다는 문제가 있다. 이를 해결하기 위해 42% 술폰화된 폴리스타이렌(S)과 1-2 μm 두께의 PBI 층(P)을 결합한 이중층 막을 개발하였다. 이 막은 높은 화학적 안정성, 감소된 바나듐 투과성, 그리고 나피온보다 높은 선택성을 보여주었다. 최적화된 막인 PSSP(1-25-25-1)은 100 mA cm⁻²에서 88.5%의 EE와 3500 사이클 이상의 안정성을 유지했다. 추가적으로, VRFB의 막 의존성을 제거하기 위해 PBI 바나듐 차단층을 탄소 전극에 직접 적용했다. 황산화(sulfonated)된 SEBS(sSEBS)를 접착제로 사용하여 PBI 차단 탄소 전극을 제작하였다. 위 전극은 젤 형태의 비선택적 저저항 PBI 막과 함께 VRFB 단일 셀 테스트에서 우수한 성능을 보였으며, 75 mA cm⁻²에서 CE, VE, EE는 각각 97.6%, 89.7%, 87.6%를 기록했다. (What is VE?) 알칼리 전해질에서 부분 황산화된 OPBI(50SOPBI)는 우수한 치수 안정성과 전도성을 보여주었으며, 1 M KOH에서 80 °C에서 358 mS cm⁻¹의 전도성을 기록했다. 장기 안정성 테스트에서도 열화 없이 전도도가 증가했으며, AEMWE는 2 V, 3 M KOH, 80 °C, Pt/NiFe 조건에서 4.82 A cm⁻²를 달성했다. 산화전극에서 발생하는 산소에 포함 된 수소를 측정하였을 때 수소는 산소의 2% 미만으로 유지되었으며, 비귀금속 전극 및 PPS 보강 막을 사용한 셀은 1000시간 이상의 내구성을 보였다. 비용 분석 결과, PBI 기반 막인 50SOPBI는 m²당 비용이 $59이므로 상업용 Piperion의 m²당 비용이 $1400인 것을 고려하면 막 제작비를 96%가량 인하할 수 있다. 이러한 연구 결과는 VRFB 및 AEMWE 응용 분야에서 PBI 기반 막이 뛰어난 성능, 내구성 및 경제적 이점을 제공하며 비용 효율적인 친환경 수소 생산의 길을 열어줄 유망한 후보로 자리 잡고 있음을 보여준다. 주요단어(Keywords) : 이온 용매화막, polybenzimidazole, 바나듐 레독 스 흐름 전지, 물 전기분해.

다국어 초록 (Multilingual Abstract)

Polybenzimidazole-based Ion Solvating Membranes for Electrochemical Energy Conversion and Storage By Muhammad Mara Ikhsan Energy Engineering, KIST-School University of Science and Technology Advised by Prof. Dr. Dirk Henkensmeier The rapid climate change has been driving an energy transition from fossil-based to a clean and renewable energy. For storing and converting back the excess generated energy, electrochemical energy devices such as flow batteries, electrolysers, and fuel cells are viewed as strong candidates. Polybenzimidazole (PBI) has gained significant attention as a versatile material, especially as membranes for electrochemical energy devices due to its amphoteric nature, excellent chemical stability, and fluorine-free chemistry. In vanadium redox flow batteries (VRFBs), protonated PBI offers reduced vanadium ion crossover through Donnan and size exclusion effects but faces challenges with low ionic conductivity in the range of 2-5 mS cm-1. In this study we enhanced the conductivity of meta-PBI (mPBI) membranes through pre-treatment with a higher concentration of H₂SO₄, achieving optimal performance with a 10 M H₂SO₄ treatment at 25 °C, leading to high selectivity and improved performance metrics, including Coulombic efficiency (CE) of 99.5% and energy efficiency (EE) of 89.6% at 80 mA cm-2 over 500 cycles. Another approach to reduce the resistance of PBI but still keeping the initial narrow morphology is by using very thin PBI as vanadium blocking layer. This is challenged by the high fragility of free-standing thin layers. To eliminate shadow effects from non-conductive porous supports, bilayer membranes combining sulfonated polystyrene with 42 degree of sulfonation (S) and 1-2 μm thin PBI layers (P) were developed. They demonstrated remarkable chemical stability, reduced vanadium permeability, and higher selectivity than Nafion. The optimized PSSP (1-25-25-1) configuration achieved an EE of 88.5% at 100 mA cm-2 and stability over 3500 cycles at significantly reduced material costs. Additionally, to further eliminate the dependency of VRFB on membrane properties, PBI vanadium blocking layers were directly applied on carbon electrodes. PBI-laminated carbon electrodes, fabricated using a sulfonated SEBS (sSEBS) as adhesive, exhibited excellent performance in VRFB single-cell tests with gel PBI as non- selective low-resistance membrane. CE, VE, and EE at 75 mA cm- 2 reached 97.6%, 89.7%, and 87.6%, respectively. In alkaline environments, partially sulfonated OPBI (50SOPBI) showed enhanced dimensional stability and conductivity, reaching 358 mS cm⁻¹ at 80 °C in 1 M KOH. Long-term stability tests demonstrated no degradation, with conductivity increasing over time. An AEMWE reached 4.82 A cm-2 at 2 V, 3 M KOH, 80 °C, Pt/NiFe. H2 in O2 remained <2%, and with non-PGM electrodes and PPS-reinforced membranes, a cell operated for >1000 hours without failure. A cost analysis revealed that PBI-based membranes significantly reduce material expenses, with 50SOPBI costing $59 m⁻² compared to commercial Piperion at $1400 m⁻². These findings position PBI-based membranes as promising candidates for VRFB and AEMWE applications, offering exceptional performance, durability, and economic advantages, paving the way for cost-effective green hydrogen production. Keywords: Ion-solvating membranes, polybenzimidazole, vanadium redox flow batteries, water electrolysis * A thesis submitted to committee of the University of Science and Technology in a partial fulfillment of the requirement for the degree of Doctor of Philosophy conferred in February 2025

목차 (Table of Contents)

• ABSTRACT i
• 1. Introduction 1
• 1.1. Transition to Clean Renewable Energy Sources 1
• 1.2. Electrochemical Energy Conversion and Storage 2
• 1.3. Vanadium Redox Flow Batteries 3
• 1.4. Water Electrolysis 7
• 1.5. Polybenzimidazole 9
• 1.6. Research Objectives 10
• References 12
• 2. Fundamentals 15
• 2.1. The Electrochemical Cell 15
• 2.2. Role of Membrane on Performance of Electrochemical Devices . 16
• 2.3. Membranes in Vanadium Redox Flow Batteries 17
• 2.3.1.Proton Exchange Membranes. 18
• 2.3.2.Anion Exchange Membranes 20
• 2.3.3.Amphoteric Membranes 20
• 2.3.4.Polybenzimidazole 21
• 2.4. Membranes for Water Electrolysis in Alkaline Conditions 22
• 2.4.1.Porous diaphragms: Alkaline Water Electrolysis. 22
• 2.4.2.Anion Exchange Membranes 23
• 2.4.2.1.Developments on cationic functional groups and alkaline stability
• . 25
• 2.4.2.2.Conductivity 25
• vi
• 2.4.3.PBI: Ion Solvating Membranes for Water Electrolysis 26
• 2.4.4.Beyond meta PBI: Broad operational range of PBI-based ISM 27
• References 27
• 3. Polybenzimidazole membranes for vanadium redox flow batteries:
• Effect of sulfuric acid doping conditions 32
• 3.1. Abstract 33
• 3.2. Introduction 34
• 3.3. Experimental 36
• 3.3.1.Materials 36
• 3.3.2.Preparation of PBI Membranes 37
• 3.3.3.Membrane acid pre-treatment and doping 37
• 3.3.4.Membrane characterization 37
• 3.3.4.1.Uptake measuremen t37
• 3.3.4.2.Dimensional stability 38
• 3.3.4.3.Through-plane ionic conductivity 38
• 3.3.4.4.Vanadium permeability and proton selectivity 39
• 3.3.4.5.Mechanical Properties 40
• 3.3.4.6.WAXS measurements 40
• 3.3.4.7.GPC measurements 40
• 3.3.5.VRFB Single Test 40
• 3.4. Results 41
• 3.4.1.Membrane Preparation 41
• 3.4.2.Acid Uptake and Swelling 42
• 3.4.3.Conductivity and Permeability 48
• 3.4.4.Membrane Morphology. 52
• 3.4.5.Mechanical Properties 58
• vii
• 3.4.6.Performance in VRFB Single Test 60
• 3.5. Conclusion 65
• 3.6. Acknowledgements 66
• References 66
• 3.7. Supporting Information 70
• 4. Sulfonated Polystyrene/Polybenzimidazole Bilayer Membranes for
• Vanadium Redox Flow Batteries 74
• 4.1. Abstract 75
• 4.2. Introduction 76
• 4.3. Experimental 79
• 4.3.1.Materials 79
• 4.3.2.Synthesis of sulfonated polymer s80
• 4.3.3.Preparation of Bilayer Membranes 80
• 4.3.4.Characterization 81
• 4.3.4.1.FTIR 81
• 4.3.4.2.NMR81
• 4.3.4.3.Ion Exchange Capacity (IEC) 81
• 4.3.4.4.Ionic Conductivity 82
• 4.3.4.5.Vanadium Ion Permeation 83
• 4.3.4.6.Chemical Stability 83
• 4.3.5.VRFB Single Cell Tests 84
• 4.4. Results 85
• 4.4.1.Sulfonated Polystyrene 85
• 4.4.2.Ion Exchange Capacity and Conductivity 85
• 4.4.3.Dimensional Change and Chemical Stability. 89
• 4.4.4.Layered Membrane Fabrication and Characterization 93
• viii
• 4.4.4.1.Conductivity and Area-Specific Resistance 95
• 4.4.4.2.Permeability of Layered Membranes 96
• 4.4.5.VRFB Tests 98
• 4.4.6.Long-Term Stability 104
• 4.5. Conclusion 105
• 4.6. Acknowledgements 106
• References 106
• 4.7. Supporting Information 111
• 5. Electrode laminated with ion-selective blocking layer for use in
• vanadium redox flow batteries 124
• 5.1. Abstract 125
• 5.2. Introduction 126
• 5.3. Experimental 128
• 5.3.1.Materials 128
• 5.3.2.Synthesis of Sulfonated SEBS Gluing Solution 128
• 5.3.3.Electrodes with an ion-selective blocking layer 129
• 5.3.4.Synthesis of Gel PBI Membrane 129
• 5.3.5.Characterization techniques 130
• 5.3.6.VRFB Single Test 132
• 5.5. Results 134
• 5.5.1.Fabrication of carbon paper electrodes with ion-selective blocking
• layer 134
• 5.5.1.1.Enhancing adhesion by using PBI/DMAc solution as glue 137
• 5.5.1.2.Enhancing adhesion by using sulfonated SEBS as adhesive .. 138
• 5.5.1.3.Enhancing adhesion by pre-swelling PBI with phosphoric acid
• 141
• 5.5.2.Area specific resistance and vanadium permeation 142
• ix
• 5.6. VRFB performance 146
• 5.7. Long-term stability test 151
• 5.8. Conclusion 154
• 5.9. Acknowledgements 155
• References 156
• 5.10.Supporting Information160
• 6. Low-Alkalinity Ion Solvating Membrane Water Electrolysis (LA
• ISMWE) 175
• 6.1. Abstract 176
• 6.2. Introduction 177
• 6.3. Experimental 179
• 6.3.1.Materials 179
• 6.3.2.Synthesis of sodium 6,6'-oxybis(3-carboxybenzenesulfonate)
• (SODBA) 179
• 6.3.3.Synthesis of SOPBI copolymers 180
• 6.3.4.Membrane Fabrication 180
• 6.3.5.Gel Permeation Chromatography 181
• 6.3.6.Conductivity and Alkaline Stability 181
• 6.3.7.Swelling Ratio 182
• 6.3.8.Water and KOH Uptake 182
• 6.3.9.Activation of 50SOPBI Membranes 182
• 6.3.10.Electrolyzer Test 183
• 6.4. Results 185
• 6.4.1.Polymer Synthesis and Membrane Preparation 185
• 6.4.2.Dimensional Stability and KOH Uptake 186
• 6.4.3.Ionic Conductivity 188
• x
• 6.4.4.Membrane Morphology 191
• 6.4.5.Chemical Stability 191
• 6.4.6.Water Electrolysis Performance and Durability. 193
• 6.4.6.1.Performance in a PGM-free anode and PGM cathode cell (type 1)
• 193
• 6.4.6.2.Performance in a PGM-free cell (type 2) 195
• 6.4.6.3.H2 Permeation Test (cell type 3) 198
• 6.5. Conclusions. 198
• References 200
• 6.6. Supporting Information 204
• 7. Conclusions and Future Works 222
• 7.1. Conclusions. 222
• 7.2. Future Works 226
• Acknowledgments 228
• xi
• Declaration of Self-citation
• This Ph.D. thesis is written as a cumulative thesis which combines three peer-
• review articles and one submitted article in a peer-reviewed journal. These
• articles were reproduced in Chapter 3-6. Additionally, Chapter 1,2, and 7
• were newly written specifically for this work. The details are as follows:
• Chapter Publications
• 3 Muhammad Mara Ikhsan, Saleem Abbas, Xuan Huy Do, Seung
• Young Choi, Kobra Azizi, Hans Aage Hjuler, Jong Hyun Jang,
• Heung Yong Ha3*, Dirk Henkensmeier
• Polybenzimidazole membranes for vanadium redox flow
• batteries: Effect of sulfuric acid doping conditions
• Chemical Engineering Journal 2022
• DOI: 10.1016/j.cej.2022.134902
• 4 Muhammad Mara Ikhsan, Saleem Abbas, Xuan Huy Do, Heung
• Yong Ha, Kobra Azizi, Dirk Henkensmeier
• Sulfonated Polystyrene/Polybenzimidazole Bilayer
• Membranes for Vanadium Redox Flow Batteries
• Advanced Energy Materials 2024
• DOI: 10.1002/aenm.202400139
• 5 Muhammad Mara Ikhsan, Saleem Abbas, Seung-Young Choi,
• Xuan Huy Do, Heung Yong Ha, Anders Bentien, Kobra Azizi,
• Hans Aage Hjuler, Dirk Henkensmeier
• Electrode laminated with ion-selective blocking layer for use
• in vanadium redox flow batteries
• Materials Today Chemistry 2024
• DOI: 10.1016/j.mtchem.2023.101830
• 6 Muhammad Mara Ikhsan, Chaeyon Yang, Kamal Ghotia, Franz
• Egert, Syed-Asif Ansar, Mikkel Rykær Kraglund, David Aili,
• Hyun S. Park, Fatemeh Razmjooei, Dirk Henkensmeier
• Low-Alkalinity Ion Solvating Membrane Water Electrolysis
• (LA ISMWE)
• Submitted