Performance Of Anion Exchange Membranes Research Articles

The impact of the catalyst layer structure on the performance of anion exchange membrane fuel cell

Intrinsic water imbalance in AEMFC complicates the mass transport phenomena in the electrodes and causes performance loss. Because an AEMFC electrode is a complex multi-component system composed of agglomerates of carbon-supported catalyst bound to ionomer, balancing these components is critical for better water management. Herein, we clarify the influence of the three electrode parameters (ionomer content, active material loading, and active material to carbon ratio) on AEMFC performance using commercial AEM and AEI (FAA-3–50 membrane and FAA-3 ionomer) with standard precious metal catalysts (PtRu/C and Pt/C). The influence of the electrode parameters of each electrode (anode and cathode) was investigated individually. Physical and electrochemical measurements revealed that the cell performance was highly dependent on each electrode parameter for both electrodes, and it was confirmed that the supply of water by back-diffusion rather than oxygen at the cathode, as well as water flooding at the anode, limited the performance of the AEMFC. In particular, we demonstrate commercial material-based AEMFC with specific power over 3 W mgPGM−1 with low PGM loading (0.2 mgPGM cm−2) by adjusting the anode and cathode electrode parameters.

Tailoring catalyst layer structures for anion exchange membrane fuel cells by controlling the size of ionomer aggreates in dispersion

The development of anion exchange membranes and ionomers plays a critical role in performance of anion exchange membrane fuel cells. However, the optimal design of catalyst layer structures still remains unexplored despite its significance. Herein, the rational design guide is presented for cathode and anode catalyst layers using m-TPN1 ionomer by controlling the size of ionomer aggregates in dispersion and comparing various catalyst layer structures. The size of m-TPN1 aggregates decreases, creating a more uniform ionomer distribution and a larger triple-phase-boundary. As ionomer distribution of the cathode catalyst layer becomes more uniform, the power density of membrane-electrode-assembly is enhanced. Also, the power density at fully humidified condition strongly correlates with the porosity of anode catalyst layer due to the limitation of hydrogen transport at anode by water flooding.

Interaction Regulation Between Ionomer Binder and Catalyst: Active Triple-Phase Boundary and High Performance Catalyst Layer for Anion Exchange Membrane Fuel Cells.

As one of the most crucial components, the catalyst layer (CL) plays a critical role in the performance of anion exchange membrane fuel cells (AEMFCs). However, the effect of the structural evolution of ionomer binder on the micromorphology and catalytic activity of CL is yet to be clarified. In this study, pyrrolidinum and quaternary ammonium cations are attached to the polyphenylene oxide (PPO) backbone through flexible spacer units (five, seven, or nine carbon atoms) with different terminal alkyl groups. The Van der Waals force and electrostatic repulsion between the ionomer binder and catalyst are regulated through the flexible spacer units and terminal alkyl groups to alleviate the agglomeration of catalyst particles and acquire a high catalytic activity. To evaluate the electrochemical stability of the cationic groups, the alkaline stability of the ionomer binder is tested under a constant voltage to simulate the true operational environment of the fuel cells. The results reveal that the degradation of the cation groups of ionomer binder is accelerated under a constant voltage condition. This phenomenon in neglect earlier, may serve as a useful reference for the synthesis and performance enhancement of ionomer binders.

Poly(vinyl alcohol)-Based Anion Exchange Membranes for Alkaline Direct Ethanol Fuel Cells

Crosslinked anion exchange membranes (AEMs) made from poly(vinyl alcohol) (PVA) as a backbone polymer and different approaches to functional group introduction were prepared by means of solution casting with thermal and chemical crosslinking. Membrane characterization was performed by SEM, FTIR, and thermogravimetric analyses. The performance of AEMs was evaluated by water uptake, swelling degree, ion exchange capacity, OH- conductivity, and single cell tests. A combination of quaternized ammonium poly(vinyl alcohol) (QPVA) and poly(diallyldimethylammonium chloride) (PDDMAC) showed the highest conductivity, water uptake, and swelling among other functional group sources. The AEM with a combined mass ratio of QPVA and PDDMAC of 1:0.5 (QPV/PDD0.5) has the highest hydroxide conductivity of 54.46 mS cm-1. The single fuel cell tests with QPV/PDD0.5 membrane yield the maximum power density and current density of 8.6 mW cm-2 and 47.6 mA cm-2 at 57 °C. This study demonstrates that PVA-based AEMs have the potential for alkaline direct ethanol fuel cells (ADEFCs) application.

Crown ether-based anion exchange membranes with highly efficient dual ion conducting pathways

Anion exchange membranes (AEMs) are a crucial constituent for alkaline fuel cells. As the core component of fuel cells, the low performance AEMs restrict the development and application of the fuel cells. Herein, the trade-off between the OH– conductivity and dimensional stability was solved by constructing AEMs with adequate OH– conductivity and satisfactory alkali resistance using Tröger’s base (TB) poly (crown ether)s (PCEs) as the main chain, the embedded quaternary ammonium (QA) and Na+-functionalized crown ether units as the cationic group. Crown ether is an electron donator, and can capture Na+ to form Na+-functionalized crown ether units to conveniently transfer OH– and significantly promote the alkaline stability of the AEMs. The influence of the Na+-functionalized crown ether units on the performance of AEMs was studied in detail. The PCEs based AEMs show an obvious hydrophobic-hydrophilic microphase separation. These features make them ideal platforms for the OH– conduction applications. As expected, the as-prepared PCEs-QA-100% (100% is the degree of cross-linking) AEM with an ionic exchange capacity (IEC) of 2.07 meq g−1 has a high OH– conductivity of 159 mS cm−1 at 80 °C. Furthermore, the membrane electrode assemblies fabricated using the PCEs-QA-100% AEM possess a maximum power density of 291 mW cm−2 under the current density of 500 mA cm−2.

Crosslinked naphthalene-based triblock polymer anion exchange membranes for fuel cells

To investigate the influence of main chain structure on the performance of anion exchange membranes (AEMs), crosslinked naphthalene-based block polymer AEMs are synthesized via nucleophilic polycondensation, Williamson reaction, atom transfer radical polymerization (ATRP) reaction, bromination, and quaternization. With a helical structure, the naphthalene-based triblock backbone can provide the ion channels inside AEMs, further enhance the ionic conductivity and dimensional stability. The crosslinked network of long alkyl chains ensures desirable mechanical properties of AEMs. The prepared NAPEK-PVP-X AEMs have a high alkaline stability, a low swelling ratio and a high ionic conductivity. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) are used to confirm the hydrophilic/hydrophobic microphase separated structure of the AEMs. More developed ion transport channels of naphthalene-based AEMs are shown by molecular dynamics (MD) simulations. A power density peak of 160.73 mW cm−2 is obtained by assembling the NAPEK-PVP-10-Q4 AEM into a single cell operating at 60 °C. The study provides a resource for future work on AEMs.

Radiation-Grafted Anion-Exchange Membrane for Fuel Cell and Electrolyzer Applications: A Mini Review.

This review discusses the roles of anion exchange membrane (AEM) as a solid-state electrolyte in fuel cell and electrolyzer applications. It highlights the advancement of existing fabrication methods and emphasizes the importance of radiation grafting methods in improving the properties of AEM. The development of AEM has been focused on the improvement of its physicochemical properties, including ionic conductivity, ion exchange capacity, water uptake, swelling ratio, etc., and its thermo-mechano-chemical stability in high-pH and high-temperature conditions. Generally, the AEM radiation grafting processes are considered green synthesis because they are usually performed at room temperature and practically eliminated the use of catalysts and toxic solvents, yet the final products are homogeneous and high quality. The radiation grafting technique is capable of modifying the hydrophilic and hydrophobic domains to control the ionic properties of membrane as well as its water uptake and swelling ratio without scarifying its mechanical properties. Researchers also showed that the chemical stability of AEMs can be improved by grafting spacers onto base polymers. The effects of irradiation dose and dose rate on the performance of AEM were discussed. The long-term stability of membrane in alkaline solutions remains the main challenge to commercial use.

Effect of Crosslinking on the Properties of QPVA/PDDA Anion Exchange Membranes for Fuel Cells Application

Anion exchange membranes (AEMs) consisting of quaternary ammonium poly(vinyl alcohol) (QPVA) and poly(diallyldimethylammonium chloride) (PDDA) were prepared by a solution casting method. The influence of the concentration of the chemical crosslinker on the properties and performance of AEMs was investigated. Morphology, chemical structures, thermal and mechanical properties of AEMs were characterized by SEM, FTIR, TGA, and UTM. The performance of AEMs was evaluated by water uptake, swelling degree, ion exchange capacity, and OH− conductivity measurement. The tensile strength, water uptake, and OH− conductivity of AEMs were enhanced with the increase of the crosslinker concentration. By introducing 12.5% glutaraldehyde (GA), the QPVA/PDDA AEMs achieved the highest tensile strength, water uptake, and OH− conductivity of 46.21 MPa, 90.6% and 53.09 ms cm−1 at ambient condition, respectively. The investigations show that crosslinked QPVA/PDDA AEMs are a potential candidate for anion exchange membrane fuel cells.

Facilitating ionic conduction for anion exchange membrane via employing star-shaped block copolymer

Anion exchange membrane (AEM) with high ionic conductivity and good stability is of vital importance for developing high-performance alkaline anion-exchange membrane fuel cell (AEMFC). In particular, the performance of AEM largely depends on the microstructure of the basic polymer material. Herein, we have developed a novel strategy to fabricate the AEMs, featuring a star topological structure, via living anionic polymerization and the subsequent functionalization. Thanks to the introduction of well-defined amphiphilic block structure, a distinct micro-phase separation was observed in the synthesized AEMs (AEM-SCP) which were based on star-shaped block copolymers. In comparison to the AEM-LCP membranes based on linear block copolymers, the AEM-SCP membranes exhibited relatively lower water uptake and swelling ratio, but much higher hydroxide conductivity (at 80 °C, 68.1 mS cm−1 for AEM-SCP-3 vs. 38.1 mS cm−1 for AEM-LCP). This is attributed to the efficient aggregation of ionic groups via employing star-shaped block copolymers, which can result in the formation of continuous ion-conducting channels and the facilitated ionic conduction in the AEMs. Furthermore, compared with the AEM-LCP membrane, the dimensional and alkaline stability of the resultant AEM-SCPs have been significantly enhanced.

Self-adjusting anode catalyst layer for smart water management in anion exchange membrane fuel cells

Summary Water management in the anode catalyst layer (ACL) during fuel cell operation is of importance to the performance of anion exchange membrane fuel cells (AEMFCs). Until now, only a few methods, such as controlling external conditions and adjusting other components like the gas diffusion layer (GDL), attempt to indirectly regulate water content within the ACL. Here, we report a self-regulating ACL having stratified, gradient pore sizes from 8 nm (membrane side) to 150 nm (GDL side). Self-regulating moisture behavior was observed for the gradient ACL, constituted individually of Pt/C layer and PtRu/C layer, which was distinctly different than that from catalyst layers (CLs) composed of homogeneous catalysts (Pt/C CL, PtRu/C CL, and mixed CL). The results of this study are anticipated to give an insight into the design of CLs with smart structures for improving AEMFC performance.

Modification of an anion exchange membrane based on rapid mussel-inspired deposition for improved antifouling performance

In this study, an anion exchange membrane (AEM) was modified by rapid deposition of polydopamine (PDA) and 2,5-diaminobenzenesulfonic acid (DSA), where DSA was further immobilized by mussel-inspired PDA on the membrane surface. The surface properties of the AEM were characterized by a series of analysis techniques such as ATR-FTIR, XPS, AFM and SEM analysis, contact angle and zeta potential measurements. The results indicated that a uniform membrane surface was obtained by the rapid deposition of PDA/DSA complexes, which endowed the membrane with negative charge and higher hydrophilicity without significant increase of surface roughness. The rapid deposition of PDA/DSA complexes on the membrane surface improved significantly the antifouling performance of AEM to an organic foulant, i.e., sodium dodecyl sulfate (SDS) in electrodialysis. According to the change of the area electrical resistance after the fouling experiment, the antifouling performance of the modified AEM could be improved by 50.6 % under optimal modification parameter compared with the pristine AEM. The surface negative charge and higher hydrophilicity reduced the adsorption of organic foulants on the membrane and restricted the formation of a fouling layer on the membrane surface, hence contributed to the improvement of antifouling performance of the modified AEM.

Dimensionally and oxidatively stable anion exchange membranes based on bication cross-linked poly(meta-terphenylene alkylene)s

Anion exchange membrane (AEM) is a key component for energy-conversion devices that enabling the use of non-noble electrocatalysts. After more than a decade of pursuit in alkaline-stable and high anion-conducting ionomers, aryl ether-free polyaromatics offer a promising candidate as an advanced AEM for use in fuel cell and electrolyzers. However, the traditional trade-off dilemma between conductivity and stability (e.g., chemical, dimensional, and mechanical stabilities etc.) of the membrane still remains. Here, we introduced a flexible bication cross-linker to doubly quaternary ammonium-tethered poly(meta-terphenylene alkylene) ionomers with high ion contents, aiming to improve the chemical and dimensional stabilities without compensate the ion-conducting merits. The designed flexible bication cross-linker is responsible for the microphase-separated morphology preservation in the cross-linked membranes. Although the water absorption decreased significantly as cross-linking content increasing, the ion conductivity of the cross-linked AEM was well retained, especially at elevated temperatures. Moreover, after cross-linking, a notable increase in chemical stability was observed, and the m-XPTPA40-2N with highest cross-linking density exhibited the best alkaline and oxidative stabilities. Owing to the mechanical robustness and high conductivity, a fuel cell fabricated with cross-linked membrane m-XPTPA40-2N showed a maximum power density of 302 mW cm−2 at a current density of 614 mA cm−2. Overall, the results of this study provide a simple but efficient strategy to improve the performance of AEM.

Preparation and characterization of high performance anion exchange membranes for acid recovery

Preparation and characterization of high performance anion exchange membranes for acid recovery

Development of high performance anion exchange membranes for diffusion dialysis process

Development of high performance anion exchange membranes for diffusion dialysis process

Ionomer dispersion solvent influence on the microstructure of Co–N–C catalyst layers for anion exchange membrane fuel cell

The controllable fabrication of non-precious metal cathode catalyst layer (CCL) to improve the water management is crucial to the performance of anion exchange membrane fuel cells (AEMFCs). Due to the higher porosity and larger particle size of M-N-C (M = Co, Fe) catalysts, compared with commercial Pt/C catalysts, the M-N-C layer is more complex. Here, we study the influence of solvent dispersion on the microstructure of Co–N–C CCLs. The solvent dielectric constants determine the aggregate size, while the relative volatilization rate dominates the final pore structure. The Co–N–C aggregate size in methanol is approximately two times larger than that in tetrahydrofuran or isopropanol. An interesting phenomenon is that ionomer tends to migrate and coalesce because of height differences in the CCL, demonstrating the importance of fast consolidation for achieving a homogenous ionomer distribution. By using ink containing tetrahydrofuran, the membrane electrode assembly from the Co–N–C CCL exhibits higher water adsorption ability in comparison with those using methanol, propanol, or isopropanol solvents, leading to a power density of 181.7 mW cm−2 at 50 °C, assembled with a commercial FAA-3-20 membrane. We anticipate our results can guide the design of Co–N–C CCLs with improved microstructure to achieve high performance AEMFCs.

(Invited) Partially Fluorinated Anion Exchange Membranes for Electrochemical Applications

There has been a considerable effort to develop high performance and durable anion exchange membranes (AEMs) for the electrochemical applications such as fuel cells, electrolyzers, and redox flow batteries. When these devices are operated under alkaline conditions, possible use of non-precious metal catalysts is advantageous. Although a number of AEMs have been proposed in the literature, chemical and mechanical stability in alkaline media still remains an issue for long-term, reliable operation of the devices. In the pursuit of more stable AEMs, we have recently reported some new polymer structures. Combination of absence of heteroatom linkages such as ether, sulfide, and sulfone in the main chain and employing pendant ammonium groups contributed to improvement of the alkaline and mechanical stability of AEMs. A typical example is QPAF-4 copolymers composed of perfluoroalkylene/phenylene main chain and hexylammonium head groups, which exhibited high hydroxide ion conductivity and alkaline stability.1 In the present paper, we report effect of other fluorinated components for our AEMs. We have developed a novel series of quaternized aromatic copolymers (BAF-QAFs) containing hexafluoroisopropylidene groups.2 Similar to QPAF-4 copolymer membranes, BAF-QAF copolymers had good solubility in polar organic solvents, membrane forming capability, and phase-separated morphology. Hexafluoroisopropylidene groups contributed to efficient anion transport as BAF-QAF membranes showed lower water uptake and higher hydroxide ion conductivity than those of QPAF-4 membranes. BAF-QAF membranes were thermally stable up to 95 °C and 90% RH with no obvious glass transition behavior in DMA analyses, while the elongation of BAF-QAF membranes were smaller than that of QPAF-4 membranes. BAF-QAF membranes were alkaline stable in 1 and 4 M KOH at 80 °C for 1000 h. A hydrogen/oxygen fuel cell with the BAF-QAF as a membrane and an electrode binder was successfully operated. The maximum power density reached 319 mW/cm2 at a current density of 702 mAcm2 with O2. The fuel cell with BAF-QAF copolymers were better in cathode performance than QPAF-4 cell most probably because of the higher hydroxide ion conductivity and better interfacial contact between the membrane and the catalyst layer. Acknowledgements This project was partly supported by NEDO through the Advanced Research Program for Energy and Environmental Technologies, JST through SICORP project, and JSPS/SNSF under the Joint Research Projects (JRPs) program. References Ono, T. Kimura, A. Takano, K. Asazawa, J. Miyake, J. Inukai, K. Miyatake, J. Mater. Chem. A, 5, 24804 (2017).Kimura, A. Matsumoto, J. Inukai, K. Miyatake, ACS Appl. Energy Mater., 3, 469-477 (2020). Figure 1

Engineering Catalysts for Anion Exchange Membrane Electrolyzer Anodes and Cathodes

The development of catalyst for both Oxygen Evolution and Hydrogen Evolution reactions (OER and HER, respectively) in Anion-Exchange Membrane (AEM) electrolyzers is proceeding along with the development of better solids electrolytes, as well as electrode and system designs.AEM-based electrolyzers are now achieving operating lives exceeding 1000s of hours and 5.0 > A/cm2 at 1.8V [1] with increasing evidence that catalyst/ionomer interactions greatly effect both catalyst performance and ionomer stability[2]. Further, the promise of PGM-free electrolysis continues even though PGM-based catalysts seem to be most stable.Pajarito Powder is developing catalysts explicitly designed for AEM electrolysis and has been working with AEM materials and systems developers to measure and understand key performance issues for electrolysis catalysts. Together with partners like LANL, Nel, GT, and WUSTL Pajarito catalysts are now showing that for stable HER available PGM-free catalysts lag PGM catalyst by about 100 mV, compared to a near 250 mV lag for OER.The best OER catalysts demonstrated however by Nel, GT, and PP are based on PbRuOx [3], and have outperformed commercialized IrOx by >140 mV for up to 1000 h has been shown and outperform PGM-free catalysts by 250+ mV. However, the combination of Pb and the rarity of Ru have motivated alternative formulations. Leading stable PGM-free catalysts in turn include multi-site electrocatalysts such as barium and lanthanum oxide systems doped with strontium, samarium, and niobium are combined with Cr, Mn, Fe, Co, and Ni in perovskite, mullite, spinel, and delaffosite crystal structures. The activity and electronic conductivity of these systems have been achieved simultaneously through balancing the structural strain of the La and Ba doped with Sr, Sm, and Nb while the doping and exchange of the transition metal component refines the electronic structure and therefore bonding of the catalysts. These documented materials have some limitation in terms of conductivity and stability, but there are indications that improvements are possible through refining these types of catalysts to including use of mesoporous and conductive supports that both bolster catalyst system conductivity and increase the active surface area through dispersion of the catalyst phase.For stable HER the performance of Ni-alloys are rapidly approaching Pt-alloys for the HER, where the in-cell performance only lags by about 50-100 mV and they are stable for >1000 h. At the same time, improvements in PGM-free OER catalysts continue to be made, though challenging.The current status of electrocatalysts for AEM, prospects for further catalysts improvements, and challenges will be presented and discussed.[1] Li, D., Park, E.J., Zhu, W. et al. Highly quaternized polystyrene ionomers for high performance anion exchange membrane water electrolysers. Nat Energy 5, 378–385 (2020). https://doi.org/10.1038/s41560-020-0577-x and[2] Maurya S., Lee, AS., Li, D., Park, EJ., Leonard, DP., Noh, S. On the origin of permanent performance loss of anion exchange membrane fuel cells: Electrochemical oxidation of phenyl group Journal of Power Sources, Volume 436, 1 October 2019, 226866[3] Horowitz, H. S., Longo, J.M., and Horowitz, H. H. Oxygen Electrocatalysis on Some Oxide Pyrochlores J. Electrochem. Soc. 1983 volume 130, issue 9, 1851-1859 and Pyrochlore electrocatalysts for efficient alkaline water electrolysis, Parrondo, J., George, M., Capuano, C., Ayers, C., Ramani, V. J. Mater. Chem. A, 2015,3, 10819-10828.

Flexible cationic side chains for enhancing the hydroxide ion conductivity of olefinic-type copolymer-based anion exchange membranes: An experimental and theoretical study

The performance of anion exchange membranes (AEMs) is closely related to their cationic groups, hydration degree, and nano-phase morphology. Here, mono- (and di-) quaternary ammonium, pyrrolidinium, and imidazolium cation-based AEMs were synthesized by one-step photo-polymerization. Through combination of experimental and molecular dynamics (MD) approaches, the effects of mono- (and di-) cationic functional groups on the properties of the AEMs, in terms of physicochemical, electrochemical properties, and alkaline stability were comprehensively investigated. It was concluded by the experimental data that AEMs with di-cations exhibit better water uptake, swelling ratio, alkaline stability, and higher ionic conductivity than those with mono-cations. Furthermore, mathematical models of the synthesized AEMs were established for MD simulations, and the calculated radial distribution function and mean square displacements between cationic functional groups, hydroxide ions, and water molecules obtained using the simulated molecular trajectories were examined for the solvation, transport properties, and alkaline stability of AEMs, which are in good qualitative agreement with the experimental results.

Poly(ether sulfone)s with pendent imidazolium for anion exchange membranes via click chemistry

A new monomer, propargyl-1-methylimidazolium bromide, is synthesized. A series of poly(ether sulfone)s with pendent imidazolium (PES-Im-x) are synthesized from poly(ether sulfone)s with azide via nucleophilic polycondensation, bromination, and azidation, which are then reacted with propargyl-1-methylimidazolium bromide via click chemistry. The membranes are prepared sequentially via solution casting and ion exchange, which exhibit good alkaline stability under basic and elevated temperature conditions. The micro-phase separation of the membranes is observed on transmission electron microscopy. The conductivities of the membranes achieve in the range of 3.53 × 10−2 to 5.76 × 10−2 S cm−1 at 80 °C, which are related to the introduction of dense imidazolium groups on the side chains of the polymers. The designed structure facilitates the aggregation and transport of the ions in the membranes. The membrane electrode assembly based on PES-Im-38 with a power density of 18.0 mW cm−2 at 60 °C is obtained. In addition, all these membranes show good dimensional stability and thermal stability. It is demonstrated that click reaction may provide a viable strategy for the further preparation of high performance anion exchange membranes.

A high-temperature anion-exchange membrane fuel cell

In the past few years, developments in anion exchange membranes (AEMs) have led to a significant increase in hydroxide conductivities, ultimately yielding striking improvements in the performance of anion exchange membrane fuel cells (AEMFCs) at low operating temperatures, usually at 40–80 °C. Aside from these remarkable achievements, the literature is void of any work on AEMFCs operated at temperatures above 100 °C, despite the consensus from various models remarking that working at higher cell temperatures may lead to many significant advantages. In this work, we present the first high-temperature AEMFC (HT-AEMFC) tested at 110 °C. The HT-AEMFC exhibits high performance, with a peak power density of 2.1 W cm −2 and a current density of as high as 574 mA cm −2 measured at 0.8 V. This initial work represents a significant landmark for the research and development of the fuel cell technology, opening a wide door for a new field of research we call hereafter, HT-AEMFCs . • For the first time, AEMFC performance was evaluated at high-temperature of 110 °C. • At 110 °C the AEM showed a record high hydroxide conductivity of ca. 300 mS cm −1 . • Excellent cell performance achieved – 2.1 W cm −2 peak and 574 mA cm −2 at 0.8 V. • The AEMFC showed good stability measured for 50 h at 110 °C. • This first test opens the doors for a new research field we call “HT-AEMFCs”.