Stability Of Anion Exchange Membranes Research Articles

Biomass‐based crosslinked composite anion exchange membranes using bacterial cellulose template towards enhanced ionic conductivity and alkaline stability

AbstractA balanced relationship among ionic conductivity, mechanical properties and alkaline stability of anion exchange membranes (AEMs) is essential for their practical application in fuel cells. Herein, a porous substrate (BC@LDH) with hierarchical structure was constructed using natural bacterial cellulose (BC) as a template and then in situ grew inorganic hydroxide ion conductor (layered double hydroxide, LDH). The membrane was fabricated via a filling together with sol–gel crosslinking strategy after impregnating an ionomer containing both quaternary ammonium and hydroxyl groups into the porous substrate and then crosslinked by organosiloxane. Due to the synergistic enhancement effect of BC, LDH and cross‐linking, the obtained membrane exhibited a tensile strength of 47.48 MPa that was 88.3% higher than that of the pure ionomer membrane, and showed an ionic conductivity of 70.7 mS cm−1 (80°C) which benefited from the introduction of ionic conductor LDH. More importantly, its alkaline stability was significantly improved and the residual conductivity ratio was still as high as 81.5% even after hot alkali treatment for 580 h. The direct methanol fuel cell assembled with this membrane exhibited a peak power density of 13.6 mW cm−2 with an open‐circuit voltage of 0.72 V, indicating its potential application in biomass‐based AEMs for fuel cells.Highlights Hierarchical porous substrate comprising LDHs anchored BC fiber was prepared. LDH@BC served as a reinforcing substrate and an ion transport medium. The crosslinked membrane showed significantly improved alkaline stability. The membrane had high ionic conductivity and mechanical strength. Providing a new way to design biomass‐based AEMs with high performance.

Triphenylamine branched poly(aryl piperidinium) anion exchange membrane for alkaline water electrolyzers

Coordinating the “trade-off” effect between ion conductivity and dimensional stability of anion exchange membranes (AEMs) is a key issue for the industrialization of AEM water electrolyzers (AEMWEs). To resolve this dilemma, a series of branched quaternized poly(p-terphenyl-triphenylamine-piperidinium)-x (QPTAP-x) AEMs are fabricated by inserting various molar percentage of triphenylamine into ether-free poly(p-terphenyl piperidinium). This branched polymer structure not only promotes the fabrication of the microphase separation structure, reducing ion transport resistance, but also contributes to the generation of the branched network structure, improving the dimensional stability and mechanical properties of the membrane. QPTAP-5.0%, a membrane with a triphenylamine content of 5.0%, exhibits the highest conductivity (174.63 mS cm−1 at 80 °C), low water swelling ratio (17.59% at 80 °C), and excellent mechanical properties (TS of 48.30 MPa and EB of 40.32%). Furthermore, it exhibits excellent alkaline stability and maintains 74.8% of conductivity after treated in 3 M KOH at 80 °C for 1440 h. The AEMWE based on QPTAP-5.0% achieves a current density of 1.6 A cm−2 at 2.2 V. The results suggest the significant potential of the triphenylamine-branched QPTAP-x for future AEMWE applications.

Chemical stability of anion exchange membranes for alkaline fuel cells and water electrolysers: Experiment, multi-dimensional modeling and hybrid simulations

The unstable nature of anion exchange membranes (AEMs) has hindered the progress of alkaline fuel cells and water electrolyzers. Density functional theory (DFT) can explore the correlation between the structure and properties of model compounds. However, theoretical explorations of AEMs remain immensely insufficient owing to the complexity of modeling the realistic solution system which caused by multi-dimensional parameters such as polymer chain entanglement, steric hindrance, and migration of ions. In this study, molecular dynamic simulation was utilized to obtain the multi-dimensional parameters of polyelectrolytes, which were considered when calculating the degradation energy barrier by DFT, thus leading to more accurate results and optimizing the conventional DFT calculations. Combined with the computational works, pyridinium functionalized polyolefin and polyether sulfone were synthesized to verify the feasibility of the simulation scheme. This optimized scheme fills the gap in the DFT calculations scale and is easy to extend to other types of AEMs, which help design the chemically stable AEMs and promote the development of alkaline fuel cells and water electrolysers.

Fluorine-Containing Poly(Fluorene)-Based Anion Exchange Membrane with High Hydroxide Conductivity and Physicochemical Stability for Water Electrolysis.

Improving the hydroxide conductivity and dimensional stability of anion exchange membranes (AEMs) while retaining their high alkaline stability is necessary to realize the commercialization of AEM water electrolysis (AEMWE). A strategy for improving the hydroxide conductivity and dimensional stability of AEMs by inserting fluorine atoms in the core structure of the backbone is reported, which not only reduces the glass transition temperature of the polymer due to steric strain, but also induces distinct phase separation by inducing polarity discrimination to facilitate the formation of ion transport channels. The resulting PFPFTP-QA AEM with fluorine into the core structure shows high hydroxide conductivity (>159 mS cm-1 at 80°C), favorable dimensional stability (>25% at 80°C), and excellent alkaline stability for 1000h in 2m KOH solution at 80°C. Moreover, the PFPFTP-QA is used to construct an AEMWE cell with a platinum group metal (PGM)-free NiFe anode, which exhibits the current density of 6.86 A cm-2 at 1.9V at 80°C, the highest performance in Pt/C cathode and PGM-free anode reports so far and operates stably for over 100h at a constant current of 0.5 A cm-2.

Novel Fluorinated Anion Exchange Membranes Based on Poly(Pentafluorophenyl-Carbazole) with High Ionic Conductivity and Alkaline Stability for Fuel Cell Applications.

Constructing good microphase separation structures by designing different polymer backbones and ion-conducting groups is an effective strategy for improving the ionic conductivity and chemical stability of anion exchange membranes (AEMs). In this study, a series of AEMs based on the poly(pentafluorophenylcarbazole) backbone grafted with different cationic groups are designed and prepared to construct well-defined microphase separation morphology and improve the trade-off between the properties of AEMs. Highly hydrophobic fluorinated backbone and alkyl spaces enhance phase separation and construct interconnected hydrophilic channels for anion transport. The ionic conductivity of the PC-PF-QA membrane is 123 mS cm-1 at 80°C, and the ionic conductivity of the PC-PF-QA membrane decreased by only 6% after 960h of immersion at 60°C in 1M NaOH aqueous solution. The maximum peak power density of the single cell based on PC-PF-QA is 214mW cm-2 at 60°C.

Current Challenges on the Alkaline Stability of Anion Exchange Membranes for Fuel Cells

AbstractAnion exchange membranes (AEMs), which consist of positively charged groups, not only allow the directional transport of OH− but can also separate anode and cathode reactions. Over the past decades, significant progress has been made in the preparation of high‐performance polymer electrolyte membranes with high conductivity, good mechanical properties, and satisfactory dimensional stability. However, the wide application of AEMs remains limited by various factors, especially alkaline stability, thereby hindering the development of fuel cells. This minireview provides an overview of recent strategies for improving the alkaline stability of AEMs. The influence of various polymer molecular structures on alkaline stability is discussed in detail. These insights into key factors affecting alkaline stability can act as guidelines for the design of novel AEMs with enhanced performance.

Effect of polyelectrolyte architecture on chemical stability in alkaline medium of anion exchange membranes

The chemical stability of anion exchange membranes (AEM) is crucial for the durability of the electrochemical devicesusing them. In this context, poly(epichlorohydrin)-based AEM were subjected to chemical degradation in the presence of a corrosive alkaline medium. Linear and cross-linked polyelectrolytes grafted with ammonium cations based on Quinuclidine and/or DABCO were first synthesized. They were characterized by an IEC comprising between 1.3 and 3.1 mmol/g, an ionic conductivity above 2 mS/cm and transport numbers between 0.71 and 0.89. Their chemical stability was then evaluated after immersion in 1 M to 8 M KOH solutions for 672 h at 50 °C. The evolutions of these characteristics as well as the molecular structure of the materials during this period inform on the chemical resistance of the polymer backbone as well as that of the cationic sites. The main degradation reactions have been identified as chains scission and Hofmann elimination, leading to the loss of the cationic groups. Nevertheless, these reactions can be limited by a high crosslinking density of the membrane, enhancing then its chemical stability despite a high ammonium group content.

Correlation between the Humidity-Dependent Surface Microstructure and Macroconductivity of Anion Exchange Membranes.

Anion exchange membrane (AEM) fuel cells have gained significant interest in recent years due to their promising applications in cost-effective and environmentally friendly energy conversion. Among various factors that affect their performance, water content plays an important role in the conductivity and stability of AEMs. However, the effect of the hydration level on the microstructure of AEMs and the correlation between the microstructure and macroconductivity have not been systematically investigated. In this work, four AEMs, quaternary ammonia polysulfone, quaternary ammonia poly(N-methyl-piperidine-co-p-terphenyl) (QAPPT), and bromoalkyl-tethered poly(biphenyl alkylene)s PBPA and PBPA-co-BPP, have been studied by atomic force microscopy and electrochemical impedance spectroscopy to elucidate the correlation between the humidity-dependent surface microstructure and macroconductivity of the AEMs. We obtained phase images by atomic force microscopy and identified hydrophilic and hydrophobic domains by fitting the distribution curve of phase images, which reasonably distinguishes hydrophilic domains from hydrophobic domains of the membrane surface, and thus, the surface hydrophilic area ratio and average size could be quantitatively analyzed. The conductivities of the membranes were then measured by electrochemical impedance spectroscopy at various humidities. The joint results from atomic force microscopy and electrochemical measurements help clarify the effect of the hydration level on the microphase separation and ionic conduction of the membranes.

Challenges and Strategies of Anion Exchange Membranes in Hydrogen-electricity Energy Conversion Devices.

Alkaline hydrogen-electricity energy conversion technologies, involving anion exchange membrane fuel cells (AEMFCs) and anion exchange membrane water electrolyzers (AEMWEs) are more appealing than the acidic counterparts due to the elimination of precious metal catalysts. However, the physicochemical properties of anion exchange membrane (AEMs), i.e., ionic conductivity, mechanical strength, stability, etc., are inferior to that of proton exchange membranes (PEMs), thus hindering these alkaline technologies from practical employment. To promote their development, we summarize the main challenges and the corresponding strategies of AEMs for the application of AEMFCs and AEMWEs in this review. The hydroxide transportation mechanism, ion exchange capacity, hydration and microscopic morphology that are relevant to the ionic conductivity are discussed firstly. Following the ionic conductivity, another obstacle, stability of AEMs is comprehensively described in terms of alkaline stability, mechanical stability and electrochemical stability. Upon integrating into the devices, water management, carbonation effect and membrane-electrode interface that are critical to the cell performance are highlighted as well. This review is anticipated to provide insights into the AEM design for hydrogen-electric energy conversion devices, thus accelerating the widespread commercialization of these promising technologies.

Preparation and Properties of Quaternary Ammonium Anion Exchange Membranes with Flexible Side Chains for the Vanadium Redox Flow Battery

In order to promote the conduction and stability of anion exchange membranes (AEMs) for the vanadium redox flow battery (VRFB), a kind of quaternary ammonium AEMs with flexible side chains (PAES-4N-xx, xx represents the proportion of monomers containing an amino structure) are prepared via aromatic nucleophilic polycondensation and grafting reactions. The ion exchange capacity (IEC) values of PAES-4N-xx are in the range of 1.58–2.10 mmol g–1, and their water uptake, swelling ratio, SO42– conductivity, and VO2+ permeability at 30 °C are in the range of 15–52%, 5–17%, 13.5–22.4 mS cm–1, and 8.6–43.3 cm2 min–1, respectively. In addition, PAES-4N-xx display good stability, and their retention ratio of IEC and conductivity reach more than 95% after soaking in a solution of 1.5 mol L–1 VO2+/3 mol L–1 H2SO4 at RT for 30 days. The typical battery assembled with PAES-4N-40 shows high efficiency; its Coulomb efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) are in the range of 93.5%–97.8%, 88.6%–70.5%, and 69.0%–84.9% at the fixed current density which is between 40 mA cm–2 and 200 mA cm–2, respectively. The battery also has good stability for 100 cycles of charging and discharging at a current density of 40 mA cm–2, and its retention of EE reaches 96.5%. The introduction of flexible side chains and dense cationic structures can effectively improve the performance of the VRFB.

“Thiol-ene” crosslinked polybenzimidazoles anion exchange membrane with enhanced performance and durability

To address the “trade-off” between conductivity and stability of anion exchange membranes (AEMs), we developed a series of crosslinked AEMs by using polybenzimidazole with norbornene (cPBI-Nb) as backbone and the crosslinked structure was fabricated by adopting click chemical between thiol and vinyl-group. Meanwhile, the hydrophilic properties of the dithiol cross-linker were regulated to explore the effect for micro-phase separation morphology and hydroxide ion conductivity. As result, the AEMs with hydrophilic crosslinked structure (PcPBI-Nb-C2) not only had apparent micro-phase separation morphology and high OH− conductivity of 105.54 mS/cm at 80 °C, but also exhibited improved mechanical properties, dimensional stability (swelling ratio < 15%) and chemical stability (90.22 % mass maintaining in Fenton’s reagent at 80 °C for 24 h, 78.30 % conductivity keeping in 2 M NaOH at 80 °C for 2016 h). In addition, the anion exchange membranes water electrolysis (AEMWEs) using PcPBI-Nb-C2 as AEMs achieved the current density of 368 mA/cm2 at 2.1 V and the durability over 500 min operated at 150 mA/cm2 under 60 °C. Therefore, this work paves the way for constructing AEMs by introduction of norbornene into polybenzimidazole and formation of hydrophilic crosslinked structure based on “thiol-ene”.

Micro-block poly(arylene ether sulfone)s with densely quaternized units for anion exchange membranes: Effects of benzyl N-methylpiperidinium and benzyl trimethyl ammonium cations

The piperidinium cation exhibited a substantial impact on the improvement of alkaline stability of anion exchange membranes (AEMs), yet the research regarding the micro-block polymer AEMs modified with it has not been conducted. Herein, we report two series of micro-block poly(arylene ether sulfone)s containing densely benzyl-N-methylpiperidinium (TTP-n-PIPPES) and benzyl-trimethyl-ammonium (TTP-n-QAPES) functionalized units for AMEs, respectively, where the n represents the molar ratio of monomer containing tri-tetraphenyl to the employed bisphenol monomers. The content and arrangement of the two types of cations in both polymer backbones were the same. The hydrophilic phases of TTP-n-PIPPES membranes exhibited isolated island states, wrapped with coherent hydrophobic phases. While the TTP-n-QAPES membranes formed continuous hydrophilic channels but fragmented hydrophobic phases. The discrepancy in the microscopic morphology of membranes and the intrinsic nature of cations thus rendered two types of AEMs with different advantages in macroscopic properties. The TTP-n-PIPPES membranes performed better dimensional stability and alkaline stability (low temperature). In contrast, the preferable capabilities of TTP-n-QAPES membranes were water affinity, hydroxide conductivity, and alkaline stability (high temperature). For instance, the TTP-18.2-PIPPES membrane was 11.7%, 11.4%, and 27.4% lower than the TTP-18.2-QAPES membrane at 80 °C in terms of dimensional change, water uptake, and hydroxide conductivity, respectively.

New Insights into High-Temperature Anion-Exchange Membrane Fuel Cells

Until a few years ago, the operation of anion-exchange membrane (AEM) fuel cells (AEMFCs) at temperatures above 70 oC was a real challenge, mainly due to the temperature limitation of the AEMs. Recently, with the remarkable progress made in the development of stable AEMs, more and more AEMFC tests could be performed at higher cell temperatures, mainly at 70-85 oC. Very recently, very few studies even reported the operation of AEMFCs at 90-95 oC. In the past year, we presented the first results of what we call high-temperature AEMFCs (HT-AEMFCs) tested at cell temperatures above 100 oC. At these temperatures, we could obtain hydroxide conductivities close to 300 mS cm-1, the highest hydroxide conductivity ever measured for an AEM. The first HT-AEMFC results are very encouraging and represent a significant landmark for the research and development of fuel cell technology, opening a wide door for a new field of research - the HT-AEMFCs. At this initial stage, however, there are many open questions and unknowns, mainly regarding the stability of the polymeric materials under these high temperatures. In this talk, this topic will be discussed, and new exciting and surprising insights will be presented.References “A high-temperature anion-exchange membrane fuel cell”; John C. Douglin, John R. Varcoe, and Dario R. Dekel; J. Power Sources Advances 5, 100023, 2020.“Quantifying the critical effect of water diffusivity in anion exchange membranes for fuel cell applications”; Karam Yassin, Igal G. Rasin, Simon Brandon, and Dario R. Dekel; J. Membrane Sci. 608, 118206, 2020.“A High-Temperature Anion-Exchange Membrane Fuel Cell with a Critical Raw Material-free Nitrogen-doped Carbon Cathode”; John C. Douglin, Ramesh K. Singh, Saja Haj, Songlin Li, Jasper Biemolt, Ning Yan, John R. Varcoe, Gadi Rothenberg, and Dario R. Dekel; Chemical Engineering J. Adv. 8, 100153, 2021.“A surprising relation between operating temperature and stability of anion exchange membrane fuel cells”; Karam Yassin, Igal G. Rasin, Sapir Willdorf-Cohen, Charles E. Diesendruck, Simon Brandon, and Dario R. Dekel; J. Power Sources Adv. 11, 100066, 2021.“Magnetic-field-oriented mixed-valence-stabilized ferrocenium anion-exchange membrane”; Xin Liu, Na Xie, Jiandang Xue, Mengyuan Li, Chenyang Zheng, Junfeng Zhang, Yanzhou Qin, Yan Yin, Dario R. Dekel, Michael D. Guiver; Nature Energy, just accepted (https://doi.org/10.1038/s41560-022-00978-y) 2022.“Non-Monotonic Temperature Dependence of Hydroxide Ion Diffusion in Anion Exchange Membranes”; Tamar Zelovich, Leslie Vogt-Maranto, Cataldo Simari, Isabella Nicotera, Michael Hickner, Stephen J. Paddison, Chulsung Bae, Dario R. Dekel, and Mark E. Tuckerman; Chemistry Mater., Chem. Mater. 34, 5, 2133-2145, 2022.

Which Properties Should Anion-Exchange Membranes Have to Achieve a Longer Fuel Cell Lifetime?

The substantial advancements and availability of new cost-effective materials have drawn significant attention to the technology of anion-exchange membrane fuel cells (AEMFCs). The anion exchange membrane (AEM) is a core component in AEMFCs, essential for conducting hydroxide ions and, more importantly, controlling water transport between the fuel cell electrodes. In this contribution, we apply the computational methodology outlined in Refs. [1,2] to explore the effects of various membrane properties on AEMFC performance and its stability. We specifically consider the following parameters: (A) water diffusivity, (B) membrane thickness, (C) ion exchange capacity (IEC), (D) maximum hydration level (λmax) corresponding to ionomer/water contact (maximum number of water molecules per functional group), (E) hydroxide conductivity, and (F) degradation rate constant of the AEM functional group.First, we describe our modeling approach, which includes a one-dimensional isothermal and time-dependent model of AEMFC operations. The model considers the chemical degradation process of the ionomeric materials in the cell, transport phenomena through the anode and cathode gas diffusion layers, anode and cathode catalyst layers, and the membrane. Finally, the electrochemical reactions, hydrogen oxidation in the anode and oxygen reduction in the cathode, are described by Butler-Volmer kinetics. We present our main findings, demonstrating that membrane water diffusivity and λmax have the most significant impact on AEMFC lifetime, followed by membrane thickness and IEC, attributed to enhanced water distribution across the cell. We also demonstrate the significance of improving functional group stability to performance stability, while AEM hydroxide conductivity shows a negligible effect on AEMFC lifetime. Finally, we perform dimensional analysis and provide an analytical, useful correlation to estimate the AEMFC lifetime of selected membranes from the literature and compare it to the measured operation time. In conclusion, this work highlights important AEM parameters aimed at improving AEM designs in order to enhance the performance stability of AEMFCs.[1] D.R. Dekel, I.G. Rasin, S. Brandon, Predicting performance stability of anion exchange membrane fuel cells, J. Power Sources. 420 (2019) 118–123. https://doi.org/10.1016/j.jpowsour.2019.02.069.[2] K. Yassin, I.G. Rasin, S. Brandon, D.R. Dekel, Elucidating the role of anion-exchange ionomer conductivity within the cathode catalytic layer of anion-exchange membrane fuel cells, J. Power Sources. 524 (2022) 231083. https://doi.org/10.1016/j.jpowsour.2022.231083.

Construction of macromolecule cross-linked anion exchange membranes containing free radical inhibitor groups for superior chemical stability

In order to achieve high chemical stability of anion exchange membranes (AEMs), polystyrene (PVE) containing free radical inhibitor groups is first used to cross-link fluorinated poly(aryl ether ketone) (PAEK). The crosslinked PAEK is afterwards quaternized with 6-bromohexyltrimethylammonium bromide (QA) to fabricate AEMs. The prepared AEMs show obvious microphase separation with anionic cluster spacing within 8.8–9.3 nm according to the SAXS analysis results. The hydroxide conductivity of the proposed AEMs is in the range of 76.7–97.6 mS cm−1 at 80 °C. It is found that the free radical inhibitor groups as well as the dense cross-linked structure endow the AEMs with excellent resistance against the attack of OH‾ and free radicals. After immersed in 1 mol L−1 KOH at 60 °C for 1000 h, the QPAEK-30PVE membrane exhibits about 75% and 82% of the conductivity and IEC retention rate. Using this membrane as the electrolyte, a peak power density of 329 mW cm−2 is achieved under a cell voltage of 0.66 V at 60 °C by fueling the cell with humidified H2 and O2 without back pressure.

(Invited) Towards Entirely Platinum Group Metal-Free Water Electrolyzers: Innovative Electrocatalysts for Oxygen Evolution and Hydrogen Evolution Reactions

Making the production of “green” hydrogen (H2) cost-effective requires the development of high-performance and affordable low-temperature water electrolyzers (LTWE).1 Currently, the most mature technology for H2 production using renewable electricity is the liquid alkaline electrolysis (AE). This technology suffers several major drawbacks such as gas crossover, relatively low current density, and the use of highly corrosive concentrated alkaline solutions (20-40% KOH). Proton exchange membrane (PEM) electrolysis, a valid alternative to AE technology, already commercialized on a large scale, enables operation on pure water thus eliminating corrosion, reducing gas crossover and allowing higher current density. However, the main drawback of PEM electrolyzers is the need of very expensive and rare platinum group metals (PGMs) such as Ir and Pt as catalysts for oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode, respectively.2 Recent advancements in performance and stability of anion exchange membranes (AEMs) have enabled a new type of alkaline membrane-based LTWE operating on pure water and with PGM-free catalysts.3,4 If successful, this new AEM-LTWE technology will allow to overcome the drawbacks of AEs and PEM-LTWEs while benefiting from their respective advantages in a major breakthrough in the production of “green” H2 at a low cost. In this scenario crucial is the development of high-performance PGM-free electrocatalysts for both OER and HER.Due to the operation at high potentials, carbon-based catalysts and supports cannot be used at the anode. Therefore, the most common PGM-free anode catalysts are based on transition metal oxides, which suffer, however, from low surface area and electronic conductivity, limiting the electrocatalytic performance.5,6 The catalysts with the most promising OER activity in alkaline environment are Ni-based alloys, oxides, and (oxy)hydroxides.7 The combination of Ni with other first-row transition metals such as Fe and Co was found to increase the OER catalytic activity.8,9 In this work, we present a new method for synthesizing NiFe OER catalysts. The catalyst was synthesized via a sol-gel method, followed by a thermal treatment. The impact on the OER activity in alkaline liquid electrolyte of different synthesis parameters such as the Ni-to-Fe atomic ratio, the addition of a third transition metal (e.g., Co, Mn), the thermal treatment temperature and atmosphere were investigated. Then, the most promising electrocatalysts were tested in an AEM-LTWE operating with pure water and supporting electrolyte solution.Bimetallic HER PGM-free catalysts were also developed by combining one a first-row transition metal, e.g. Ni, with a second-row transition metal, e.g. Mo. These HER catalysts were synthesized by either (i) using the sol-gel approach described above or (ii) via a metal organic framework (MOF) method similar to the one used in the synthesis of “atomically dispersed” M-N-C catalysts for oxygen reduction reaction.10 References A. M. Oliveira, R. R. Beswick, and Y. Yan, Curr. Opin. Chem. Eng., 33, 100701 (2021).H. A. Miller et al., Sustain. Energy Fuels, 4, 2114–2133 (2020).J. Xiao et al., ACS Catal., 11, 264–270 (2021).D. Li et al., Nat. Energy, 5, 378–385 (2020).Q. Gao et al., Chem. Eng. J., 331, 185–193 (2018).D. Xu et al., ACS Catal., 9, 7–15 (2019).S. Fu et al., Nano Energy, 44, 319–326 (2018).G. Zhang et al., Appl. Catal. B Environ., 286, 119902 (2021).P. Chen and X. Hu, Adv. Energy Mater., 10, 1–6 (2020).Y. He et al., Energy Environ. Sci., 12, 250–260 (2019).

High alkaline stability and long-term durability of imidazole functionalized poly(ether ether ketone) by incorporating graphene oxide/metal-organic framework complex

The poly(ether ether ketone) (PEEK) was prepared as organic matrix. ZIF-8 and GO/ZIF-8 were used as fillers. A series of novel new anion exchange membranes (AEMs) were fabricated with imidazole functionalized PEEK and GO/ZIF-8. The structure of ZIF-8, GO/ZIF-8 and polymers are verified by 1H NMR, FT-IR and SEM. This series of hybrid membranes showed good thermal stability, mechanical properties and alkaline stability. The ionic conductivities of hybrid membranes are in the range of 39.38 mS cm−1–43.64 mS cm−1 at 30 °C, 100% RH and 59.21 mS cm−1–86.87 mS cm−1 at 80 °C, 100% RH, respectively. Im-PEEK/GO/ZIF-8-1% which means the mass percent of GO/ZIF-8 compound in Im-PEEK polymers is 1%, showed the higher ionic conductivity of 86.87 mS cm−1 at 80 °C and tensile strength (38.21 MPa) than that of pure membrane (59.21 mS cm−1 at 80 °C and 19.47 MPa). After alkaline treatment (in 2 M NaOH solution at 60 °C for 400 h), the ionic conductivity of Im-PEEK/GO/ZIF-8-1% could also maintain 92.01% of the original ionic conductivity. The results show that hybrid membranes possess the ability to coordinate trade-off effect between ionic conductivity and alkaline stability of anion exchange membranes. The excellent performances make this series of hybrid membranes become good candidate for application as AEMs in fuel cells.

Improving the conductivity and dimensional stability of anion exchange membranes by grafting of quaternized dendrons

AbstractHigh conductivity is critical for the practical applications of anion exchange membranes (AEMs) in fuel cells. In this study, a new strategy for enhanced conductivity and dimensional stability of AEMs by incorporating quaternized dendrons is proposed. Thanks to the introduced quaternized dendrons, distinct nanoscale phase separation and well‐connected ion conductive channels are formed in the as‐prepared membranes (PPO‐QG‐x). As a result, PPO‐QG‐x AEMs achieve high hydroxide conductivities up to 65.5 mS cm−1 at 20 °C and 121.5 mS cm−1 at 80 °C (IEC = 1.95 mmol g−1), while possessing good dimensional stability. Meanwhile, PPO‐QG‐x AEMs show good alkaline stability with the maximum loss in conductivity of 15.1% after treated in 2 M NaOH at 80 °C for 960 h. In addition, the single‐cell assembled with PPO‐QG‐12 membrane exhibit a peak power density of 249.4 mW cm−2 at 60 °C. Overall, this work provides a new insight to achieve high conductivity of AEMs.

Highly alkaline stable fully-interpenetrating network poly(styrene-co-4-vinyl pyridine)/polyquaternium-10 anion exchange membrane without aryl ether linkages

To avoid the detrimental effect of aryl ethers on the alkali stability of anion exchange membrane (AEM), elaborately designed and synthesized poly(styrene-co-4-vinyl pyridine) (PS4VP) copolymer without aryl ether linkages is used to prepare AEMs. By introducing commercialized polyquaternium-10 (PQ-10) as the main ion-conducting molecule, the fully-interpenetrating polymer network quaternized PS4VP/PQ-10 (F-IPN QPS4VP/PQ-10) AEMs are prepared by cross-linking PS4VP and PQ-10, respectively. The as-prepared F-IPN QPS4VP/PQ-10 AEMs have obvious nanoscale microphase-separated morphologies, which ensure the membranes have good mechanical properties and dimensional stability. With optimized component ratios, F-IPN QPS4VP/PQ-10 AEM exhibits high ionic conductivity (74.29 mS/cm at 80 °C) and power density (111.83 mW/cm2 at 60 °C), as well as excellent chemical stabilities (94.36% retaining of initial mass after immersion in Fenton reagent at 30 °C for 10 days, and 92.28% retaining of original ionic conductivity after immersion in 1 M NaOH solution at 60 °C for 30 days), which are greater than those of semi-interpenetrating polymer network QPS4VP/PQ-10 AEM. In summary, a combination of fully-interpenetrating polymer network and stable polymer chains and ion-conducting moieties is found to effectively overcome the trade-off between high ionic conductivity and good dimensional/chemical stabilities.

Assessing the Oxidative Stability of Anion Exchange Membranes in Oxygen Saturated Aqueous Alkaline Solutions

While anion exchange membrane water electrolyzers show promise as a source of green hydrogen using low-temperatures and non-platinum group metal catalysts, many concerns must be addressed. A primary challenge for the development of high-performance anion exchange membrane water electrolyzers is the fabrication of a stable membrane that will be able to survive long-term stability test while maintaining high anion conductivity, which is a necessity for a durable water electrolyzer. This method will present a standardized protocol that can be used by researchers to assess the quality of their AEM materials and be able to provide insight into how materials may be degrading and how to improve the quality of AEMs. Using Mohr or Volhard titration to measure ion-exchange capacity, EIS to determine the ionic conductivity, NMR and FTIR to understand the extent of membrane degradation over time, and mechanical analyzers to determine the strength of their AEMs, this standardized protocol will guide researchers to determining and improving upon the long-term durability and performance of their AEM materials.