High-performance Anion Exchange Membranes Research Articles

High-Performance Anion Exchange Membrane Water Electrolyzers Enabled by Highly Gas Permeable and Dimensionally Stable Anion Exchange Ionomers.

Designing suitable anion exchange ionomers is critical to improving the performance and in situ durability of anion exchange membrane water electrolyzers (AEMWEs) as one of the promising devices for producing green hydrogen. Herein, highly gas-permeable and dimensionally stable anion exchange ionomers (QC6xBA and QC6xPA) are developed, in which bulky cyclohexyl (C6) groups are introduced into the polymer backbones. QC650BA-2.1 containing 50mol% C6 composition shows 16.6 times higher H2 permeability and 22.3 times higher O2 permeability than that of QC60BA-2.1 without C6 groups. Through-plane swelling of QC650BA-2.1 decreases to 12.5% from 31.1% (QC60BA-2.1)while OH- conductivity slightly decreases (64.9 and 56.2mScm-1 for QC60BA-2.1 and QC650BA-2.1, respectively, at 30 °C). The water electrolysis cell using the highly gas permeable QC650BA-2.1 ionomer and Ni0.8Co0.2O in the anode catalyst layer achieves two times higher performance (2.0Acm-2 at 1.69V, IR-included) than those of the previous cell using in-house ionomer (QPAF-4-2.0) (1.0Acm-2 at 1.69V, IR-included). During 1000h operation at 1.0Acm-2, the QC650BA-2.1 cell exhibits nearly constant cell voltage with a decay rate of 1.1µVh-1 after the initial increase of the cell voltage, proving the effectiveness of the highly gas permeable and dimensionally stable ionomer in AEMWEs.

Quaternized chitosan/polyvinyl alcohol anion exchange membrane enhanced by functionalized attapulgite clay with an ionic “chain-ball” surface structure

Biomass chitosan has garnered considerable interest for alkaline anion exchange membranes (AEMs) due to its eco-friendly and sustainable characteristics, low reactant permeability and easily modifiable nature, but it still faces the trade-off between high hydroxide conductivity and sufficient mechanical properties. Herein, a novel functionalized attapulgite clay (f-ATP) with a unique ionic “chain-ball” surface structure was prepared and incorporated with quaternized chitosan (QCS)/polyvinyl alcohol (PVA) matrix to fabricate high-performance composite AEMs. Due to the strengthened interfacial bonding between f-ATP nanofillers and the QCS/PVA matrix, composite membranes are synergistically reinforced and toughened, achieving peak tensile strength and elongation at break of 24.62 MPa and 33.8 %. Meanwhile, abundant ion pairs on f-ATP surface facilitate ion transport in the composite AEMs, with the maximum OH− conductivity of 46 mS cm−1 at 80 °C and the highest residual IEC of 83 % after alkaline treatment for 120 h. Moreover, the assembled alkaline direct methanol fuel cell exhibits a remarkable power density of 49.3 mW cm−2 at 80 °C. This work provides a new strategy for fabricating high-performance anion exchange membranes.

Anisotropic polyphenylene-based anion-exchange membranes with flexible side-chains via click reaction for high-performance water electrolysis

Anion-exchange membrane water electrolysis (AEMWE) represents a promising and cost-effective electrochemical energy conversion system for green hydrogen production owing to its utilization of earth-abundant metals for various cell components (catalysts, porous transport layers, bipolar plates) and the zero-gap design. However, the lack of high-performance anion-exchange membranes (AEMs) has hampered the advancement of AEMWE technology. Here, by using colon coupling and click reactions, the ether-free polyphenylene-based AEMs (QPPBTF-TMA-w) with flexible side-chains containing triazole linkers. The QPPBTF-TMA-w membranes stand out for their lower activation energy for hydroxide conduction which is attributed to the well-structured ion-conducting channels via distinct phase separation facilitated by the triazole linker, which optimizes hydrophilicity and ensures efficient ion transport within the membrane. In addition, the QPPBTF-TMA-w membranes exhibit unique anisotropic swelling and ion-conduction behaviors, such as lower in-plane swelling and higher through-plane conductivity, which are advantageous for practical cell applications. Therefore, despite having a modest ion exchange capacity (IEC) value of 2.0, the QPPBTF-TMA-w membrane delivers high-performance in anion-exchange membrane water electrolysis (AEMWE), achieving 6.4 A/cm2 at 2.0 V at 70 °C using a 1.0 M KOH solution.

Non-precious high entropy alloys and highly alkali-resistant composite membranes based high performance anion exchange membrane water electrolyzers

Cost-effective highly efficient and stable catalysts and alkali-resistant anion exchange membranes (AEMs) are critical for practical applications of anion exchange membrane water electrolyzers (AEMWEs). Here, bifunctional non-precious FeCoNiCuMo high entropy alloys (HEAs), electrochemically deposited on nickel mesh as an anode (FeCoNiCuMo/NM) and on carbon paper as a cathode (FeCoNiCuMo/CP), together with a NiFe layered double hydroxide (LDH) loaded AEM (AEM/LDH), were developed for high performance AEMWEs. The AEMWE thus fabricated delivered an ultrahigh current density of 1777 mA cm−2 at 2 V and exhibited remarkable stability with a less than 0.1% cell voltage increase after a 100-hour operation at 100 mA cm−2 at 50 °C, largely outperforming those of the precious metal based AEMWE, composed of IrO2/NM as the anode and Pt/C/CP as the cathode, 1231 mA cm−2@2 V and 4.57% increase in cell voltages. Composition of the alkali-resistant, hydrophilic, and water electrolysis active NiFe LDH into the AEM improved not only the electrolyte uptake and operational stability, but also the efficiency of the AEMWE, with a 10% boost in current density achieved at 2 V and a reduction in cell voltage increases from 2.94% to 0.0951%. The present development demonstrates the practicality of AEMWEs toward green hydrogen production.

Bacillus Subtilis-Derived Carbon Nanorods as Efficient ORR Electrocatalysts

Developing high-performance anion-exchange membrane fuel cells requires stable and highly active oxygen reduction (ORR) catalysts. To fabricate cheap and efficient porous carbon materials for ORR, heteroatom-doped carbon nanorods were synthesized by pyrolyzing Bacillus subtilis. The obtained carbon material maintained the uniformly distributed nanorods as original Bacillus subtilis and had inherent doped heteroatom. The obtained BS-2.0 had the highest specific surface area (209.04 m2 g−1) and the maximum ID/IG (1.0372). Its starting potential (0.93 V vs RHE) and semi-wave potential (0.81 V vs RHE) were close to 20% commercial Pt/C, and its ultimate current density was 3.98 mA cm−2@1600 rpm. Meanwhile, the stability of cycling potentiates polarization, and methanol tolerance of BS-2.0 were all greater than 20% commercial Pt/C. It was the inherent heteroatom, well-distributed nanorods, abundant pore distribution, and large surface area that contributed to its excellent electrochemical performance.

High-Performance Asymmetric Porous Anion Exchange Membrane with Controllable Morphology via Solvent Evaporation and Water Etching

High-Performance Asymmetric Porous Anion Exchange Membrane with Controllable Morphology via Solvent Evaporation and Water Etching

Polycyclic norbornene based anion-exchange membranes with high ionic conductivity and chemical stability

Polycyclic norbornene derivatives, dicyclopentadiene (DCPD) and tricyclopentadiene (TCPD), have rigid structures with ease of accessibility and high chemical reactivity. After vinylic-addition polymerization, the resultant polynorbornenes (PNBs) possess high thermal stability, excellent mechanical integrity, and chemical inertness due to the retention of the rigid polycyclic norbornene motifs along the polymer backbones. Herein, we first report polycyclic PNBs composing DCPD or TCPD and an alkyl-bromide-substituted norbornene as comonomers to prepare high performance anion-exchange membranes (AEMs). The two double bonds in DCPD/TCPD can be used for vinylic-addition polymerization and thiol-ene click cross-linking reactions, respectively. By comparing with previously reported cross-linked PNB AEMs without polycyclic structures, it is unambiguously confirmed that the involvement of DCPD/TCPD significantly improved the mechanical strength (the stress-at-break of 34 MPa vs 28 MPa), the hydroxide conductivity (212 mS cm−1vs 199 mS cm−1 at 90 °C), and the alkaline stability (93 % vs 81 % conductivity remaining after treating in 1 M KOH at 80 °C for 1200 h). The successful application of the AEMs with excellent durability for over 500 h prove that the involvement of polycyclic structures is a convenient strategy to enhance the performance of aromatic-free AEMs.

Pt single atoms coupled with Ru nanoclusters enable robust hydrogen oxidation for high-performance anion exchange membrane fuel cells

Pt single atoms coupled with Ru nanoclusters enable robust hydrogen oxidation for high-performance anion exchange membrane fuel cells

Intrinsic microporous poly(phenyl-alkane)s with pendent piperazinium for high performance anion exchange membrane fuel cells

Exploring anion exchange membranes (AEMs) with high conductivity and satisfactory stability is very meaningful for the advancement of alkaline fuel cells. Herein, a novel class of intrinsic microporous AEMs with pendent piperazinium are systematically fabricated for alkaline fuel cell performance testing. Contorted and rigid spirobisindane backbone structure expands the free volume of AEMs, inducing broad and interconnected ion transport channels. This unique feature endows membranes a leading advantage for the efficient transmission of hydroxide ions. Consequently, the prepared membrane displays excellent normalized hydroxide conductivity of 78.99 mS g cm−1 mmol−1 at 80 °C. Meanwhile, the hydroxide conductivity degrades by only 8.3 % after treatment in 1 M NaOH for 1000 h at 80 °C. Moreover, the peak power density of H2/O2 single cell employing this membrane exhibits 1.06 W cm−2 at 80 °C and the cell can preserve 93.0 % of its initial voltage after operating for 18 h.

An improvement on the electrocatalytic performance of ZIF-67 by in situ self-growing CNTs on surface

Efficient and robust oxygen reduction reaction (ORR) catalysts are essential for the development of high-performance anion-exchange membrane fuel cells (AEMFC). To enhance the electrochemical performance of metal–organic frameworks of cobalt-based zeolite imidazolium skeleton (ZIF-67), this study reported a novel ZIF-67-4@CNT by in situ growing carbon nanotubes (CNTs) on the surface of ZIF-67 via a mild two-step pyrolysis/oxidation treatment. The electrochemical results showed that the as-prepared ZIF-67-4@CNT after CTAB modification exhibited excellent catalytic activity with good stability, with Eonset, E1/2, and Ilimit, respectively were 0.98 V (versus RHE), 0.87 V (versus RHE) and 6.04 mA cm−2@1600 rpm, and a current retention rate of about 94.21% after polarized at 0.80 V for 10 000 s, which were all superior to that of the commercial 20 wt% Pt/C. The excellent ORR catalytic performance was mainly attributed to the large amount of the in situ growing CNTs on the surface, encapsulated with a wide range of valence states of metallic cobalt.

Solvent Effects on the Catalyst Ink and Layer Microstructure for Anion Exchange Membrane Fuel Cells.

Understanding the complex solvent effects on the microstructures of ink and catalyst layer (CL) is crucial for the development of high-performance anion exchange membrane fuel cells (AEMFCs). Herein, we study the solvent effects within the binary solvent ink system composed of water, isopropyl alcohol (IPA), commercial anion exchange ionomer, and Pt/C catalyst. The results show that the Pt/C particles and ionomer tend to form large aggregates wrapped with a thick ionomer layer in IPA-rich ink and promote the formation of large mesopores within the CL. With the increase of the water content in the ink, Pt/C particles are more likely to bridge to each other through wrapped FAA to form a well-connected three-dimensional network. The CL fabricated using water-rich ink shows smaller pores, higher porosity, and a more homogeneous ionomer network without the formation of large aggregates. Based on these results, we propose that the properties of the solvent mixture, including dielectric constant (ε) and solubility parameter (δ), affect the coulomb interaction of charged particles and surface tension at interfaces, which in turn affects the microstructure of ink and CL. By leveraging the solvent effects, we optimize the CL microstructures and improve the performance of AEMFC. These results may guide the rational design and fabrication of AEMFCs.

High-performance and durable anion-exchange membrane water electrolysers with high-molecular-weight polycarbazole-based anion-conducting polymer

Development of high-performance and durable anion-exchange membrane water electrolysis enabled by chain-extender derived high-molecular-weight polycarbazole-based anion-conductive polymer.

Poly(fluorenyl alkylene)-based anion exchange membranes for high-performance water electrolysis

Durable and high-performance anion exchange membranes (AEMs) are a pivotal issue for the industrial application of AEM water electrolysis (AEMWE). In this study, a series of poly(fluorenyl alkylene) copolymers with different contents of biphenyls and p-terphenyls are synthesized for the high-performance AEMWE. The resulting AEM with an appropriate content of p-terphenyl well facilitates the microphase separation to get a well-defined microstructure that effectively improves the ionic conductivity and reduces the swelling ratio. The AEM with 20% p-terphenyl possesses an OH– conductivity of 155.7 mS cm−1, a water uptake of 128.7%, and a swelling ratio of 33.8% at 80 °C. Moreover, it also shows durable alkaline stability and retains 83.5% of cationic groups after immersing in 5 M NaOH at 80 °C for 1440 h. The assembled AEMWE cell achieves an excellent current density of 4.9 A cm−2 at 2 V in 1 M KOH (80 °C) and 2200 h of long-term constant current (1 A cm−2) operation with a low voltage rising rate of 77 μV h−1. After this long-term operation, only 13.7% of cationic groups are degraded in the AEM, showing excellent in-situ durability. Therefore, the developed AEMs should have great potential for applications in the AEMWE.

(Poster Award - Honorable Mention) Durable Polybenzimidazole Anion Exchange Membranes for Alkaline Water Electrolyzers

Water electrolysis under alkaline conditions allows the use of inexpensive non-platinum-group catalysts such as nickel.1, 2 Conventional alkaline electrolyzers usually employ a highly concentrated aq. KOH solution as electrolyte (5-7 M) with a porous diaphragm as separator. Alternatively, protolysable polymers such as polybenzimidazole (PBI) may be swollen with electrolyte and used as ion-solvating membranes.2, 3 However, the chemical stability of the polymers can become a serious issue under these harsh conditions. Operation under more dilute conditions, e.g., up to 2 M KOH, may bring the benefits of the alkaline conditions combined with a longer cell lifetime. Under these conditions, aliphatic cyclic quaternary ammonium (QA) cations, such as piperidinium, have shown excellent stability.In the present work, performed as part of the EU project “NEXTAEC”, we have designed and synthesized m-PBI grafted with mono- and bis-piperidinium functionalized side chains, respectively (Figure 1a-b). AEMs were prepared from polymers with varying ion exchange capacities and studied at 80 °C containing KOH solutions in the concentration range 0.5-2 M KOH. The membranes were characterized with respect to, e.g., electrolyte solution uptake, alkaline stability, and electrolyser performance (Figure 1c). The uptake showed a strong inverse correlation with KOH concentration, which also affected the electrochemical performance. The stability as evaluated by 1HNMR spectroscopy was excellent showing, in the worst case, less than 8 % ionic loss after 6 months storage in 2 M KOH at 80 °C. Li, D.; Park, E. J.; Zhu, W.; Shi, Q.; Zhou, Y.; Tian, H.; Lin, Y.; Serov, A.; Zulevi, B.; Baca, E. D.; Fujimoto, C.; Chung, H. T.; Kim, Y. S., Highly quaternized polystyrene ionomers for high performance anion exchange membrane water electrolysers. Nature Energy 2020, 5 (5), 378-385.Aili, D.; Kraglund, M. R.; Rajappan, S. C.; Serhiichuk, D.; Xia, Y.; Deimede, V.; Kallitsis, J.; Bae, C.; Jannasch, P.; Henkensmeier, D.; Jensen, J. O., Electrode Separators for the Next-Generation Alkaline Water Electrolyzers. ACS Energy Letters 2023, 8 (4),1900-1910.Aili, D.; Yang, J.; Jankova, K.; Henkensmeier, D.; Li, Q., From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides. Journal of Materials Chemistry A 2020, 8 (26), 12854-12886.Marino, M. G.; Kreuer, K. D., Alkaline stability of quaternary ammonium cations for alkaline fuel cell membranes and ionic liquids. ChemSusChem 2015, 8 (3), 513-23. Figure 1

Hydrogenated TiO2 Carbon Support for PtRu Anode Catalyst in High-Performance Anion-Exchange Membrane Fuel Cells.

The availability of durable, high-performance electrocatalysts for the hydrogen oxidation reaction (HOR) is currently a constraint for anion-exchange membrane fuel cells (AEMFCs). Herein, a rapid microwave-assisted synthesis method is used to develop a core-shell catalyst support based on a hydrogenated TiO2/carbon for PtRu nanoparticles (NPs). The hydrogenated TiO2 provides a strong metal-support interactionwith the PtRu NPs, which improves the catalyst's oxophilicity and HOR activity compared to commercial PtRu/C and enables greater size control of the catalyst NPs. The as-synthesized PtRu/TiO2/C-400 electrocatalyst exhibits respectable performance in an AEMFC operated at 80 °C, yielding the highest current density (up to 3× higher) within the catalytic region (compared at 0.80-0.90V) and voltage efficiency (68%@ 0.5 A cm-2) values in the compared literature. In addition, the cell demonstrates promising short-term voltage stability with a minor voltage decay of 1.5mV h-1. This "first-of-its-kind in alkaline" work may open further research avenues to develop rapid synthesis methods to prepare advanced core-shell metal-oxide/carbon supports for electrocatalysts for use in the next-generation of AEMFCs with potential applicability to the broader electrochemical systems research community.

Sulfidation of CoCuOx Supported on Nickel Foam to Form a Heterostructure and Oxygen Vacancies for a High-Performance Anion-Exchange Membrane Water Electrolyzer.

Anion-exchange membrane water electrolyzer (AEMWE) is attracting attention for hydrogen production owing to its ability to employ nonprecious metal catalysts and high energy conversion efficiency. Spinel-structured transition metal oxides exhibit excellent potential in oxygen evolution reaction (OERs). Nevertheless, the research on highly active and durable spinel-structured electrodes for the anodic OER of AEMWE is deficient. Herein, a self-supported S-CoCu oxide/nickel foam (S-CoCuOx/NF) anode was synthesized through a two-step method (electrodeposition and sulfidation). The formation of abundant oxygen vacancies and heterostructure collaboratively enhances the electron and mass transfer, resulting in an overpotential of 313 mV at 100 mA cm-2 for OER. For the lab-scale AEMWE system with the S-CoCuOx/NF anode, a current density of 1 A cm-2 was obtained at 1.87 V (cell voltage) with high durability for 110 h (1 A cm-2) at 60 °C. The results will provide insights into developing the spinel structure-derived anode for high-performance AEMWE.

High-performance anion exchange membranes based on poly(aryl piperidinium): With interpenetrating ion transport channels induced by fluorinated crosslinking and side chains

Currently, the trade-off between mechanical stability and electrical conductivity has become one of the challenges in developing high-performance anion exchange membranes. Here, we propose a fluorinated crosslinking strategy to improve the mechanical properties of the membrane while achieving high conductivity. The introduction of hydrophobic fluorination units provides a further driving force for the aggregation of hydrophilic ion clusters within the membrane, forming continuous ion transfer channels within the membrane. The results show that the crosslinked membranes maintain a low swelling rate as well as a high OH– conductivity at 80 °C. Among them, the FPTBP-TMA-7 membrane showed the most comprehensive performance, with a OH– conductivity of 116.9 mS cm−1 and a swelling rate of only 14.1% at 80 °C. The chemical structure of the membranes maintained high stability after immersion in highly concentrated alkaline solutions (2 M NaOH at 80 °C for 720 h), with a conductivity retention rate of over 91.8%. In addition, the FPTBP-TMA-7 based fuel cell achieves a peak power density of 264.2 mA cm−2 at 80 °C, showing an attractive application prospect in alkaline fuel cells.

Fundamental Insight into Phenyl-Free Polynorbornene Ionomers Enables High Performance Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) are seen as a possible successor to proton exchange membrane fuel cell technologies. A major motivation behind AEMFC development is the potential to use less costly materials, such as non-platinum group metal catalysts, thus reducing the stack cost. Anion exchange ionomers (AEIs) are polymers that facilitate ion transport in the catalyst layer play and a critical role in the performance of fuel cells. In fact, cell performance is profoundly affected by fundamental interactions between the catalyst surface and the AEI. Two such interactions are of particular importance: phenyl adsorption on hydrogen oxidation catalysts and electrochemical oxidation of phenyl moieties on oxygen evolution catalysts. Both are detrimental to an alkaline device’s performance and durability.We compared the adsorption energy of phenyl-containing ionomers for implementing phenyl-free ionomers. Density functional theory calculations indicated that the norbornane fragment has minimal adsorption energy on Pt(111) due to the absence of aromatic electrons. A soluble quaternized polynorbornene ionomer was prepared by vinyl addition polymerization. This ionomer enables high performance in fuel cells, (peak power density > 2 W cm-2), proving the advantage of the phenyl-free structure. This study establishes the phenyl adsorption energy-electrode performance relationship, highlighting the importance of material interactions between the catalysts and ionomers.

PGM-Based Ammonia Oxidation Catalysts for Anion-Exchange Membrane Direct Ammonia Fuel Cells

Low-temperature direct ammonia fuel cells (DAFCs) use carbon-neutral ammonia as a fuel, which has attracted increasing attention recently due to ammonia's low source-to-tank energy cost, easy transport and storage, and wide availability. However, current DAFC technologies are greatly limited by the kinetically sluggish ammonia oxidation reaction (AOR) at the anode. Herein, we report an AOR catalyst, in which ternary PtIrZn nanoparticles with an average size of 2.3 ± 0.2 nm were highly dispersed on a binary composite support comprising cerium oxide (CeO2) and zeolitic imidazolate framework-8 (ZIF-8)-derived carbon (PtIrZn/CeO2-ZIF-8) through a sonochemical-assisted synthesis method. The PtIrZn alloy, with the aid of abundant OHad provided by CeO2 and uniform particle dispersibility contributed by porous ZIF-8 carbon (surface area: ∼600 m2 g−1), has shown highly efficient catalytic activity for the AOR in alkaline media, superior to that of commercial PtIr/C. The rotating disk electrode (RDE) results indicate a lower onset potential (0.35 vs. 0.43 V), relative to the reversible hydrogen electrode at room temperature, and a decreased activation energy (∼36.7 vs. 50.8 kJ mol−1) relative to the PtIr/C catalyst. Notably, the PtIrZn/CeO2-ZIF-8 catalyst was assembled with a high-performance hydroxide anion-exchange membrane to fabricate an alkaline DAFC, reaching a peak power density of 91 mW cm−2. Unlike in aqueous electrolytes, supports play a critical role in improving uniform ionomer distribution and mass transport in the anode. PtIrZn nanoparticles on silicon dioxide (SiO2) integrated with carboxyl-functionalized carbon nanotubes (CNT–COOH) were further studied as the anode in a DAFC. A significantly enhanced peak power density of 314 mW cm−2 was achieved. Density functional theory calculations elucidated that Zn atoms in the PtIr alloy can reduce the theoretical limiting potential of *NH2 dehydrogenation to *NH by ∼0.1 V, which can be attributed to a Zn-modulated upshift of the Pt–Ir d-band that facilitates the N–H bond breakage.

Ethylene–Styrene Multiblock Copolymers with Rigid Side Chains Densely Terminated by Piperidinium for High-Performance Anion Exchange Membranes

Ethylene–Styrene Multiblock Copolymers with Rigid Side Chains Densely Terminated by Piperidinium for High-Performance Anion Exchange Membranes