High-performance Anion Exchange Membranes Research Articles

Understanding mass and charge transports to create anion-ionomer-free high-performance alkaline direct formate fuel cells

The disadvantage of anion ionomer that possesses low hydroxide conductivity, and thermal and chemical instability hinders the development of the high-performance anion-exchange membrane direct liquid fuel cells. Instead of adding additional base and synthesizing high-conductivity ionomer material, by gaining insight into species transports, herein, we propose an anion-ionomer-free anion-exchange membrane direct formate fuel cell (AEM DFFC). Experimental result reveals that this conceptual anion-ionomer-free AEM DFFC can operate stably within a 6-h constant-current discharge at 10 mA cm−2, mainly because formate hydrolysis renders a high OH− conductivity. It was also found that the anion-ionomer-free AEM DFFC yields a peak power density as high as 41 mW cm−2 at 40 °C, 40% higher than that of the conventional quaternary ammonia polysulfone anion-ionomer AEM DFFC. This can be attributed to the fact that the OH−-containing formate solution facilitates the mass and charge transports, thereby enlarging the triple-phase boundary for both anodic formate oxidation reaction and cathodic oxygen reduction reaction.

Selectivity enhancement of quaternized poly(arylene ether ketone) membranes by ion segregation for vanadium redox flow batteries

Quaternary ammonium densely functionalized octa-benzylmethyl-containing poly(arylene ether ketone)s (QA-OMPAEKs) with ion exchange capacities (IECs) ranging from 1.23 to 2.21 mmol g−1 were synthesized from: (1) Ullmann coupling extension of tetra-benzylmethyl-containing bisphenol A; (2) condensation polymerization with activated dihalide in the presence of K2CO3; (3) selective bromination using N -bromosuccinimide; and (4) quantitative quaternization using trimethylamine. Both small-angle X-ray scattering (SAXS) and transmission electron microscope (TEM) characterizations revealed distinct nano-phase separation in QA-OMPAEKs as a result of the dense quaternization. The QA-OMPAEK-20 with an IEC of 1.98 mmol g−1 exhibited a high SO42− conductivity of 11.4 mS cm−1 and a low VO2+ permeability of 0.06× 10−12 m2 s−1 at room temperature, leading to a dramatically higher ion selectivity than Nafion N212. Consequently, the vanadium redox flow battery (VRFB) assembled with QA-OMPAEK-20 achieved a Coulombic efficiency of 96.9% and an energy efficiency of 84.8% at a current density of 50 mA cm−2, which were much higher than those of the batteries assembled with Nafion N212 and a home-made control membrane without distinct nano-phase separation. Therefore, ion segregation is demonstrated to be a strategical route for the design of high performance anion exchange membranes (AEMs) for VRFBs.

Development of a high-performance anion exchange membrane using poly(isatin biphenylene) with flexible heterocyclic quaternary ammonium cations for alkaline fuel cells

A series of quaternary ammonium-tethered poly(isatin biphenylene)s (PIBs) were prepared devoid of alkaline labile aryl ether bondsviasuperacid-catalyzed polymerization and explored as highly alkaline stable backbones of anion exchange membranes (AEMs).

Understanding how high-performance anion exchange membrane fuel cells were achieved: Component, interfacial, and cell-level factors

In recent years, it has become clear that there are 4 pillars to achieve high performing practical Anion exchange membrane fuel cells (AEMFCs): (1) alkaline stable, high conductivity, high water transport membranes; (2) controlling the content and balance of water in operating cells; (3) electrode development and optimization; and (4) minimizing the effects of CO2. The primary focus of this perspective article is (2) and (3), where the component-level advances as well as the cell-level and interfacial factors that contribute to overall cell performance will be discussed – with a perspective on understanding how state-of-the-art AEMFC performance of ∼2.5 W cm−2 (with H2/O2 reacting gases) has been achieved. The hope is that others in the field can use this paper to not only achieve the same performance, but to exceed it. Also, several recommendations to guide the future work of the field are made.

High-performance anion-exchange membrane water electrolysis

In this study, we developed high-performance anion-exchange membrane water electrolysis (AEMWE) by investigating various parameters. To enhance AEMWE performance, the effects of the membrane-electrode assembly (MEA) fabrication method, operating conditions, and MEA parameters were examined. As a result of the optimization and the use of FAA-3-50 and FAA-3-Br as the AEM and anion-exchange ionomer, respectively, AEMWE showed the highest performance reported thus far. Single-cell tests and electrochemical impedance spectroscopy results indicate that optimized AEMWE minimized the ohmic, charge-transfer, and mass transport resistances. Overall, by examining the AEMWE parameters, we were able to improve the electrochemical reactions, namely, the oxygen evolution reaction and hydrogen evolution reaction, and achieved outstanding performance.

Advances in alkaline stable guanidinium based anion exchange membranes

Anion exchange membranes (AEMs) are solid polymer electrolytes that contain cationic groups covalently bound to or embedded in a polymer backbone. They act as key role of hydroxide conduction and separator between two electrodes in anion exchange membrane fuel cells (AEMFCs). To achieve high performance in AEMFCs, ideal AEMs should own high ion conductivity and good alkaline stability. Considering to the alkaline stability of AEMs, ionic groups play very important role in determining the overall stability for the AEMs apart from polymer backbones. To date, various ion conductive moieties have been investigated as new tethered ionic groups to replace conventional quaternary ammoniums (QAs) in AEMs because of the multiple degradation pathways under alkaline conditions including: (1) nucleophilic substitution (SN2), (2) Hoffmann elimination (E2), or (3) nitrogen ylide formation. Guanidiniums are found to be a kind of novel ionic groups for the application of AEMs. In the structure of guanidinium, the positive charge is uniformly distributed over the central carbon atom and three nitrogen atoms, leading to stabilized charge delocalization and good thermal and chemical stability. There are six substitutions distributed over three N atoms which can be easily tuned by a mature and feasible route. Additionally, the kinetics of hydrogen evolution/oxidation reaction (HER/HOR) and ORR catalyzed by Pt and carbon-based non precious metal could be greatly enhanced through using guanidiniums. Guanidinium cations can be introduced into the polymer backbone easily by the synthetic methods as follows: (1) halomethylation of a phenyl group and subsequent amination with pentalkylguanidine; (2) nucleophilic substitution of a Vilsmeier salt and a secondary amine; (3) activated fluoroamine reaction; (4) nucleophilic substitution of alkyl halid and guanidine to prepare guanidinium functional monomer; (5) polycondensation between guanidine hydrochloride and diamine. Recently, several types of guanidinium-functionalized AEMs have been explored including benzylic guanidinium AEMs, alkylic guanidinium AEMs and phenylic guanidinium AEMs. Some of the AEMs have been proven to possess excellent ion conductivity, and some have been used in AEMFCs and show good electrochemical performance. Considering to degradation mechanism of guanidinium, it can be concluded as a nucleophilic addition-elimination reaction: firstly the center C atom is attacked by OH- and an intermediate is obtained (addition reaction); then one of N atoms combines with the H atom and eliminates from the intermediate (elimination reaction); finally the degradation products urea and amine are formed. It is worth noting that the degradation process of guanidinium is evidently different from quaternary ammoniums. Their degradation reaction sites only occur at the central carbon atom rather than the benzyl carbon which indicate the promising stability for guanidinium. Though the stabilities of guanidiniums are under debate by now, the usability of guanidiniums as ionic group is still a topic worthy of study. On the one hand, almost all of the studies are confined to guanidiniums with methyl groups substituted at the N1 and N3 positions, hence comprehensive and systematic analysis of the structure-stability relationship of guanidinium cations in alkaline media is still rarely developed. On the other hand, the degradation process of guanidiniums is only speculated by a few studies in theoretical way, more specific experimental studies of this process are badly needed to prove it. Therefore, revealing the structure-stability relationship and the detailed degradation mechanism are vital to design alkaline stable guanidinums which can be applied to AEMs preparation in the future. In addition, the polymer backbone is also very important to improve the stability of guanidiniums based AEMs. More guanidiniums based AEMs with various structures should be designed and prepared. The ion conductivity, alkaline stability and cell performace should be assessed comprehensively to figure out the relationship between the structures and properties of AEMs, and finally the preparation of high performance anion exchange membranes can be achieved.

Mid-block quaternized polystyrene-b-polybutadiene-b-polystyrene triblock copolymers as anion exchange membranes

A novel mid-block, side-chain-type quaternized triblock copolymer was prepared from commercial polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymers by Markovnikov addition reaction with HBr, azidation, and Cu(I)-catalyzed click reaction for high-performance anion exchange membranes (AEM). After the optimization of reaction conditions, the bromination, azidation and subsequently Cu(I)-catalyzed click reactions have been confirmed to be quantitative according to the NMR results. The anion-conductive, mid-block triblock copolymer (SBS-c-QA) membrane was obtained from casting its DMF solution. As a control experiment, the mid-block quaternized SBS (SBS-QA) membrane was also synthesized by direct quaternization of brominated SBS with trimethylamine. The SBS-c-QA membrane with pendent quaternary ammounium groups showed much lower water uptake than that of SBS-QA membrane. However, comparable bicarbonate conductivities of 5.6 mS/cm at 20 °C and 20.5 mS/cm at 80 °C were obtained for SBS-c-QA membrane in spite of its low ion exchange capacity (IEC) (1.10 meq./g) and low water uptake (18 wt%). Thus, the SBS-c-QA membrane displayed high IEC-normalized conductivity which can be attributed to the side-chain architecture of the triblock copolymer. The alkaline stability testing indicated that SBS-c-QA membrane showed a higher alkaline stability than SBS-QA membrane. A high retention of bicarbonate conductivity (70%) was observed after degradation testing in 1 M NaOH at 80 °C for 216 h. These results suggested that incorporating the side-chain-type QA cations into the mid-block of the triblock copolymer was an effective method to fabricate AEMs with both high ionic conductivity and excellent alkaline stability.

Densely quaternized anion exchange membranes synthesized from Ullmann coupling extension of ionic segments for vanadium redox flow batteries

Membranes with high ion conductivity and selectivity are important for vanadium redox flow batteries. Herein, densely quaternized anion exchange membranes based on quaternary ammonium functionalized octa-benzylmethyl-containing poly(fluorenyl ether ketone)s (QA-OMPFEKs) were prepared from the (i) condensation polymerization of a newly developed octa-benzylmethyl-containing bisphenol monomer via Ullmann coupling, (ii) bromination at the benzylmethyl sites using N-bromosuccinimide, and (iii) quaternization of the bromomethyl groups using trimethylamine. The QA-OMPFEK-20 with an ion exchange capacity (IEC) of 1.66 mmol g−1 exhibited a higher SO42− conductivity (9.62 mS cm−1) than that of the QA-TMPFEK-40 (4.82 mS cm−1) at room temperature, which had a slightly higher IEC of 1.73 mmol g−1 but much lower QA density. The enhanced SO42− conductivity of QA-OMPFEK-20 was attributed to the ion-segregated structure arising from the densely anchored QA groups, which was validated by SAXS observation. Furthermore, the QA-OMPFEK-20 showed much lower VO2+ permeability (1.24×10−14 m2 s−1) than QA-TMPFEK-40 (5.40×10−13 m2 s−1) and Nafion N212 (5.36×10−12 m2 s−1), leading to improved Coulombic and energy efficiencies in Vanadium redox flow batteries (VRFBs). Therefore, the Ullmann coupling extension is a valuable approach for the development of high performance anion exchange membranes for VRFBs.

Anion conductive poly(2,6-dimethyl phenylene oxide)s with clicked bulky quaternary phosphonium groups

A series of anion exchange membranes (AEMs) having pendent bulky quaternary phosphonium groups (based on the tris(2,4,6-trimethoxyphenyl)phosphane) (TPP-x and LTPP-x) have been designed and prepared by Cu(I)-catalyzed click chemistry for H2/O2 alkaline fuel cell application. In spite of numerous attempts, the high degree of functionalization (DF) copolymer displayed very poor film forming ability. Thus, the tough and transparent membranes were obtained only at IEC level as low as ~ 1.0 meq./g. The as-obtained TPP and LTPP AEMs having bulky phosphonium groups showed lower water uptake than that of the clicked CQA and LCQA membranes based on quaternary ammonium groups in spite of their similar IEC values. Lower ion conductivities were observed for all of the AEMs due to the lower water uptake. However, the bulky phosphonium groups could protect efficiently the core atom in organic cations against hydroxide attack and thus induced excellent alkaline stability of AEMs even at high NaOH concentration of 10 M at 80 °C for 200 h, although possibly SN2 substitution degradation was occurred according to the 1H NMR results. Interestingly, these AEMs with bulky quaternary phosphonium groups had not initial performance, e.g. no open circuit voltage (OCV) when it was employed as membrane in fuel cell probably because no efficient phase boundary forming between the catalyst layer and membrane resulting from the poor compatibility between poly(2,6-dimethyl phenylene oxide)s (PPOs) backbone and bulk phosphonium groups, although they were served as an ionomer successfully for the fuel cell in catalyst layer. This new finding that contrasts material alkaline stability and device operability is extremely important and gives us directions for new polymer designs for high performance anion exchange membrane for alkaline fuel cells.

High-Performing Hydroxide Exchange Membranes with Flexible Tetra-Piperidinium Side Chains Linked by Alkyl Spacers

The objective of the present work is to, in a single material, combine a number of molecular design strategies that have proven successful in the preparation of high-performance anion-exchange membranes (AEMs) for alkaline fuel cells. Hence, we here report on highly conductive and alkali-stable poly(phenylene oxide)s carrying flexible side chains attached via alkyl spacer units, where each side chain contains four quaternary piperidinium (QPip) cations interconnected with alkyl chain segments. These materials are completely soluble in, e.g., methanol and form mechanically tough transparent AEMs with efficiently segregated ions, as indicated by X-ray scattering. At 80 °C, the hydroxide ion conductivity reaches up to 170 and 221 mS cm–1 at ion-exchange capacities (IECs) of 2.1 and 2.6 mequiv g–1, respectively. Taking into account the IEC value and water uptake, the tetra-QPip side chain AEMs are found to be significantly more efficient hydroxide ion conductors than corresponding AEMs with mono-QPip side cha...

High Performance Anion-Exchange Membranes and Ionomers for Use in Alkaline Membrane Fuel Cells

This presentation will describe the latest research at the University of Surrey on the development of high-performance anion-exchange membranes (AEM) and ionomers (AEI) for use in alkaline membrane fuel cells (AEMFC). These anion-exchange polymer electrolyte matreials are produced using the peroxidation pre-irradiation grafting method (RG). Recent achievements that will be discussed in this presentation include: Synthesis of RG-AEMs with new chemistries that are more stable to alkali than those containing the benzyltrimethylammonium benchmark;The ability to produce RG-AEMs based on low density polyethylene (LDPE) as well as poly(ethylene-co-tetrafluoroethylene) (ETFE);Synthesis of thinner AEMs (< 30 μm thickness);Synthesis of an interesting class of RG-based solid-state AEI powders;AEMs with high hydroxide conductivities and high performances in hydrogen-fuelled AEMFC;Advanced use of Raman micro-spectroscopy for studying radiation-grafted anion-exchange polymer electrolytes. We will demonstrate that changes in head-group chemistry can switch off several alkali degradation mechanisms for RG-AEMs. We will also show that 2 W cm-2 peak power densities can be achieved in H2/O2 AEMFCs containing Pt cathodes and > 1 W cm-2 can be achieved with non-platinum-group-metal cathodes. The figure below shows the performance of a thin ETFE-based RG-AEM in H2/O2 and H2/air(CO2-free) AEMFCs when Ag/C cathode is used as a cathode electrocatalyst. Figure 1

Densely quaternized poly(arylene ether)s with distinct phase separation for highly anion-conductive membranes

To develop high performance anion exchange membranes (AEMs), a novel bisphenol monomer bearing eight benzylmethyl groups at the outer edge of the molecule was synthesized, which after condensation polymerization with various amounts of 4,4′-dihydroxydiphenylsulfone and 4,4′-difluorobenzophenone yielded novel poly(arylene ether)s with densely located benzylmethyl groups. These benzylmethyl groups were then converted to quaternary ammonium groups by radical-initiated bromination and quaternization in tandem, leading to the emergence of densely quaternized poly(arylene ether sulfone)s (QA-PAEs) with controlled ion exchange capacities (IECs) ranging from 1.61 to 2.32 mmol g−1. Both small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) studies revealed distinct phase separation in the QA-PAEs. The QA-PAE-40 with an IEC of 2.32 mmol g−1 exhibited a Br− conductivity of 9.2 mS cm−1 and a SO42− conductivity of 14.0 mS cm−1 at room temperature, much higher than those of a control membrane with a similar IEC but without obvious phase separation. Therefore, phase separation of AEMs was validated to be advantageous for the efficient conducting of anions. The experimental results also showed that the QA-PAEs were promising AEM materials, especially for non-alkaline applications.

Highly conductive anion exchange membranes with long flexible multication spacer

Poly(ether ketone)s bearing multication side chains are synthesized via nucleophilic polycondensation, demethylation and the Williamson reaction to prepare high performance anion exchange membranes (AEMs). The introduction of multication side chains is found to promote the formation of hydrophilic/hydrophobic phase segregated morphology, as confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The TQAPEK-0.5 membrane with ionic exchange capacity (IEC) of 1.94 meq g−1 shows the highest hydroxide conductivity of 132.8 mS cm−1 at 80 °C. The first cationic group on the side chain is linked to the backbone via hexyleneoxy groups to render the AEM a good alkaline stability. As a result, TQAPEK-0.5 has only a 22.4% decrease in the conductivity after immersed in a 2 M aqueous KOH solution at 60 °C for 600 h. Besides, the peak power density of a single cell using the TQAPEK-0.5 membrane is observed to be 206 mW cm−2 at a current density of 400 mA cm−2. Furthermore, the as-prepared AEMs also exhibit good mechanical property and thermal stability, and hold promise for application in fuel cells.

Crosslinked high-performance anion exchange membranes based on poly(styrene-b-(ethylene-co-butylene)-b-styrene)

Development of anion exchange membranes with good dimension, chemical stability, high conductivity and long life-time simultaneously is still a challenge for practical application of anion exchange membrane fuel cells. Herein, a mechanically and chemically stable poly(styrene-b-(ethylene-co-butylene)-b-styrene) triblock copolymer based crosslinked membranes are designed, prepared and characterized. N,N,N′,N′-tetramethyl-1,6-hexanediamine is introduced into the membranes as the crosslinker and functional reagent, which exhibits the significant improvement in water uptake (less than 7%), swelling ratio (8–5%, greatly smaller than uncrosslinked membranes) and tensile strength (higher than 18MPa). The hydroxide conductivity (14.9mScm−1 at 30°C) of the obtained membrane is obtained due to hydrophilic/hydrophobic microphase separation formed between the structure of crosslinking and quaternary ammonium groups. It demonstrates that incorporating stable crosslinking structure to polymer backbone greatly improves the chemical stability under Fenton and alkaline long-term tests. Furthermore, the peak power density of an H2/O2 single fuel cells reaches high peak power density of 416mWcm−2 at 0.514V. Based on these outstanding properties, the developed crosslinked membranes will be a promising candidate material for anion exchange membrane fuel cells.

High performance polyvinyl alcohol/calcium titanate nanocomposite anion-exchange membranes as separators in redox flow batteries

Low ionic conductivity and poor chemical stability are the two key parameters that limit the use of many anion-exchange membranes in electrochemical applications like rechargeable batteries and fuel cells. Herein we report a method for the synthesis of a high performance anion-exchange membrane fabricated by incorporating calcium titanate nanoparticles (CaTiO3) into polyvinyl alcohol (PVA) matrix. The CaTiO3 was synthesized by a new co-precipitation method from a solution of two simple precursors, viz potassium titanyl oxalate and calcium chloride. The XRD data of the synthesized nanoparticles indicate a phase pure orthorhombic perovskite structure. Morphological features investigated with SEM and TEM studies, reveal that the CaTiO3 is having spherical shape with a diameter of approximately 200 nm. The PVA/CaTiO3 nanocomposite membranes were fabricated by solution casting method from a well dispersed suspension of CaTiO3 in PVA and characterized by FT-IR spectroscopy, TGA, SEM, AC impedance analysis and tensile strength measurements. The membranes with 30 wt% CaTiO3 content possess ionic conductivity of 66 mS cm−1 at room temperature. The electrochemical performance of an all-iron redox flow cell was studied using galvanostatic charge–discharge tests using the above nanocomposite membrane as separator and the system exhibited a coulombic efficiency of 75% during the charge–discharge cycles.

Development of a novel durable aromatic anion exchange membrane using a thermally convertible precursor

We describe a new approach for obtaining high-performance anion exchange membranes by using a thermally convertible precursor. A new insoluble all-aromatic polymer containing anthracene units and benzyl trimethyl ammonium was successfully prepared from a highly soluble precursor polymer. The resulting polymer shows excellent chemical durability and conductivity.

High performance anion exchange membrane with proton transport pathways for diffusion dialysis

Abstract High performance anion exchange membranes (AEM) with proton transport pathways were fabricated through cationic functionalization of bromo-methylated poly (phenylene oxide) (BPPO) by a nucleophile dimethylamino pyridine (DMAP). The reaction was selectively occurred at dimethylamino functionalities of DMAP to form quaternary amine groups. The pyridine groups on DMAP having a dibasic nature promoted the formation of proton transport pathways and contributed to hydrophilicity of the membranes. The obtained membranes had the water uptake (WU) in the range of 12–57%, the ion exchange capacity (IEC) from 0.84 to 2.07 mmol g−1, acid dialysis coefficient (UH) from 0.37 to 20.3 (10−3 m h−1) at 25 °C. Interestingly, these membranes exhibited superior separation factor (S) values of 73–351 compared to a commercial DF-120 membrane and most reported membranes in literature. Moreover, the prepared membranes showed excellent thermal, chemical and mechanical stability.

N3-adamantyl imidazolium cations: Alkaline stability assessment and the corresponding comb-shaped anion exchange membranes

In the pursuit of alkaline stable cationic ions as promising candidates for high performance anion exchange membranes, two novel imidazoliums with N3-adamantyl substituent along with four imidazoliums with N3-methyl and butyl-substituents were synthesized and systematically assessed the impact of N3- and C2-substitutions on their chemical stabilities. Substituent at the N3-position prevented the degradation of imidazolium, and bulky and rigid adamantyl substituent was the most effective. The 1-adamantyl-2-methyl-3-ethylimidazolium cation (AdMEIm) exhibited the highest alkaline stability among the six cationic ions, which is stable under 5M NaOH aqueous solution at 80°C for 168h. Combined with the highly chemical stability of N3-adamantyl substituted imidazolium and excellent chemical and thermal stability of poly(2,6-dimethyl phenylene oxide) (PPO), the PPO-based AEMs functionalized with N3-adamantyl substituted imidazolium were further synthesized and characterized. However, unlike the small-molecule imidazolium salt with exceptional alkaline stability, a considerable ring-opening degradation of imidazolium was taken place for AEM functionalized with the stable N3-adamantyl substituent when exposed to 1M NaOH aqueous solution at 60°C for over 48h, demonstrating that the alkaline stabilities of modular cation and the corresponding AEM seems less correlated. A single H2-O2 fuel cell using PPO-AdIm-34 membrane showed a peak power density of 46.0mWcm−2 and an open circuit voltage (OCV) of 0.90V at 45°C without any optimization of the electrodes, demonstrating that the as-prepared membranes have potential application in AEMFCs.

Highly conductive alkaline anion exchange membrane containing imidazolium-functionalized octaphenyl polyhedral oligomeric silsesquioxane filler

Imidazolium-functionalized octaphenyl polyhedral oligomeric silsesquioxane nanoparticles were incorporated into imidazolium-functionalized poly(ether ether ketone) (ImPEEK) to fabricate high-performance anion exchange membranes with tuneable microphase separation structures and efficient ion transport channels. High-resolution transmission electron microscopy observations revealed the formation of ion channels in the membranes due to the aggregation of imidazolium groups. The transport of hydroxide ions in the composite membranes was facilitated by the increase in the effective ion mobility from 2.27×10−4 to 3.09×10−4cm2s−1V−1, while their hydroxide conductivity at 30°C (36.22–44.11mScm−1) was significantly higher than that of an ImPEEK control membrane (30.66mScm−1).

Click mediated high-performance anion exchange membranes with improved water uptake

HLTEI and BPPO based novel high-performance comb-shaped AEMs were fabricated. The prepared membrane showed improved water uptake.