J Mater Sci (2018) 53:11131–11150 REVIEW Review A review of the synthesis and characterization of anion exchange membranes 1 1 1, Kimberly F. L. Hagesteijn , Shanxue Jiang , and Bradley P. Ladewig * Barrer Centre, Department of Chemical Engineering, Imperial College London, London, UK Received: 21 March 2018 ABSTRACT Accepted: 5 May 2018 This review highlights advancements made in anion exchange membrane Published online: (AEM) head groups, polymer structures and membrane synthesis methods. 21 May 2018 Limitations of current analytical techniques for characterizing AEMs are also discussed. AEM research is primarily driven by the need to develop suit- The Author(s) 2018 able AEMs for the high-pH and high-temperature environments in anion exchange membrane fuel cells and anion exchange membrane water electrolysis applications. AEM head groups can be broadly classiﬁed as nitrogen based (e.g. quaternary ammonium), nitrogen free (e.g. phosphonium) and metal cations (e.g. ruthenium). Metal cation head groups show great promise for AEM due to their high stability and high valency. Through ‘‘rational polymer architecture’’, it is possible to synthesize AEMs with ion channels and improved chemical sta- bility. Heterogeneous membranes using porous supports or inorganic nanoparticles show great promise due to the ability to tune membrane charac- teristics based on the ratio of polymer to porou2s support or nanoparticles. Future research should investigate consolidating advancements in AEM head groups with an optimized polymer structure in heterogeneous membranes to bring together the valuable characteristics gained from using head groups with improved chemical stability, with the beneﬁts of a polymer structure with ion channels and improved membrane properties from using a porous support or nanoparticles. CEM Cation exchange membrane Abbreviations DSC Differential scanning calorimetry AAEM Alkaline anion exchange membrane IEC Ion exchange capacity (mmol/g) AEM Anion exchange membrane IEM Ion exchange membrane AEMFC Anion exchange membrane fuel cell PEMFC Proton exchange membrane fuel cell AEMWE Anion exchange membrane water electrolysis Address correspondence to E-mail: email@example.com https://doi.org/10.1007/s10853-018-2409-y J Mater Sci (2018) 53:11131–11150 and power generation applications [e.g. electrodial- PEMWE Proton exchange membrane water ysis (ED), electrodialysis reversal (EDR), electro- electrolysis deionization (EDI) and bipolar membrane electro- QA Quaternary ammonium dialysis (BMED)] [4–6]. Commercialized IEM appli- SR Swelling ratio (%) cations are also found in inorganic acid/base TGA Thermogravimetric analysis production (e.g. BMED) and acid/base recovery [e.g. WU Water uptake (%) diffusion dialysis (DD)] . Other water treatment processes under development include Donnan dial- List of symbols ysis to remove harmful pollutants and scaling species A Membrane cross-sectional area from water/wastewater streams [7–9] and ion (width 9 thickness) (cm ) exchange membrane bioreactor to combine the ben- C Acid concentration (mmol/ml) acid eﬁts of IEMs with biological treatment for ground- C AgNO concentration (mmol/ml) AgNO 3 water remediation and water/wastewater treatment C Base concentration (mmol/ml) base [10–12]. Driven by the need for sustainable energy c Water content (H O molecules/mobile generation and storage, innovative applications anion) under development include fuel cells, water elec- L Length between inner electrodes (cm) trolysis, reverse electrodialysis and redox ﬂow bat- l Membrane wet length (cm) teries [5, 13–15]. l Membrane dry length (cm) Speciﬁc to AEMs, research is focused on develop- m Membrane dry mass (g) ing AAEMs for high-pH and high-temperature m Membrane wet mass (g) applications such as anion exchange membrane fuel MW Molecular weight of water (g/mol) H O cells (AEMFC) and anion exchange membrane water R Membrane resistance (X) electrolysis (AEMWE) [13, 16, 17]. The principle r Conductivity (S/cm) behind fuel cells is to convert energy stored in V Acid volume added (ml) acid chemical bonds to generate electricity and produce V Base volume added (ml) base water as waste . On the other hand, water elec- V AgNO volume added (ml) AgNO 3 trolysis uses DC electricity to split water and generate hydrogen and oxygen gas . Together, these two Introduction technologies, in conjunction with other renewable energy sources (e.g. solar, wind), are viewed as a Ion exchange membranes (IEM) are semi-permeable potential solution to develop a ‘‘hydrogen economy’’ membranes composed of ionic head groups attached that utilizes renewable energy in place of fossil fuels to polymer matrices . They can be broadly classi- and does not produce CO [20, 21]. ﬁed as anion exchange membranes (AEM) and cation Researchers are motivated to advance AEMFC/ exchange membranes (CEM) depending on the type AEMWE technologies to be in line with comple- of ion that is permitted to cross the membrane layer mentary technologies that use CEMs: cation exchange [2, 3]. For example, AEMs contain positively charge membrane fuel cells (or proton exchange membrane head groups in the membrane which permit the fuel cells, PEMFC) and proton exchange membrane passage of anions while repelling cations . AEMs (or polymer electrolyte membrane) water electrolysis, can be further reﬁned based on the types of anions PEMWE) [13, 16, 22]. Figure 1 presents a schematic of they pass, with AEMs passing non-alkaline form - 2- 3- a typical AEMFC, PEMFC, AEMWE and PEMWE. In anions (e.g. Cl ,SO ,PO ) and alkaline anion 4 4 a fuel cell, fuels, usually hydrogen gas or low exchange membranes (AAEMs) passing alkaline - - 2- molecular weight alcohols (e.g. methanol, ethanol, form anions (e.g. OH , HCO ,CO ). 3 3 ethylene glycol), are fed into the fuel cell where it By exploiting the selective nature of IEMs, a variety contacts a catalytic layer that facilitates a chemical of applications exist for both AEMs and CEMs. reaction to generate electrons [18, 23]. Depending on Commercialized IEM applications are primarily ? - the type of IEM, either H or OH ions are trans- found in water/wastewater treatment applications ported across the membrane where a second catalytic such as desalination or high-purity water production layer facilitates a chemical reaction to produce water. in food & beverage, pharmaceutical, semiconductor In conjunction with these chemical reactions, J Mater Sci (2018) 53:11131–11150 11133 Figure 1 Schematic of anion exchange membrane fuel cell (a), proton exchange membrane fuel cell (b), anion exchange membrane water electrolysis (c), proton exchange membrane water electrolysis (d) and membrane electrode assembly (e). J Mater Sci (2018) 53:11131–11150 electrodes are connected on either side of the IEM to AEMFC/AEMWEs have several promising beneﬁts complete the electrical circuit and allow electrons to compared to PEMFC/PEMWEs, which is why travel from anode to cathode, which generates an research is actively addressing the issues impeding electric current [24, 25]. In water electrolysis, a DC AAEM commercializing for AEMFC/AEMWE. Key current is applied across two electrodes and splits beneﬁts of AEMFC/AEMWE over PEMFC/PEMWE water into near pure hydrogen gas (cathode) and include: oxygen gas (anode) streams . A membrane or • The ability to use cheaper non-platinum or non- diaphragm is used to prevent the hydrogen and precious metal-based catalysts. In PEMFC/ oxygen gas streams from mixing, which reduces the PEMWE, the acidic environment requires the use electrolyser efﬁciency . It is also permeable to H , of platinum catalysts and there is concern that OH and H O to keep charges in balance between widespread commercialization of these technolo- anode and cathode . gies will be hindered by insufﬁcient platinum Historically, PEMFC/PEMWE have seen greater supply [24, 33]. In AEMFC, the alkaline environ- advancements compared to AEMFC/AEMWE pri- ment permits more favourable oxygen reduction marily due to the ability to create CEMs with high H reaction kinetics, which allows for greater ﬂexi- conductivities. For example, Naﬁon 117 membrane bility in selecting non-platinum or non-precious by DuPont, one of the most commonly used CEMs, metal-based catalysts [16, 24, 25]. In AEMWE, the has a reported H conductivity of 78 mS/cm , alkaline environment permits a greater variety of whereas most anion conductivities in AAEMs have catalyst material selection, which could permit the been reported between 5 and 20 mS/cm [25, 28]. use of non-precious metals for the hydrogen Within the past 5 years, AAEM conductivities have evolution and oxygen evolution reactions [33, 34]. widely been reported from 50 to 100 mS/cm, with • The ability to use a variety of fuels in fuel cells. In some even as high as 200 mS/cm [29, 30]. This is in PEMFC, nitrogen-based fuels (e.g. hydrazine or part due to (a) a focus on optimizing AEM chemistry, ammonia) are not compatible with CEM and can (b) advancements in understanding the relationship severely deteriorate fuel cell performance even at between conductivity and water uptake and (c) im- 1 ppm ammonia . The alkaline environment in proved conductivity measurement techniques AEMFC has improved electro-oxidation kinetics [16, 29, 31, 32]. Given that higher AAEM conductiv- which permits the use of a greater variety of ities have been correlated with increased water liquid fuels including nitrogen-based fuels . update, up to a plateau around 100 mS/cm at which • The ability to use more concentrated fuels in fuel point the water update dilutes the ionic charge and cells. Unlike PEMFC, in AEMFC, ions and water reduces conductivity, by ensuring sufﬁcient hydra- move in opposite directions. As Fig. 1 shows, tion at the cathode of a fuel cell, improved conduc- water is both a reactant on the cathode side and tivities can be achieved . Furthermore, the product on the anode side . Water transport detrimental effect of carbonate and bicarbonate for- across the IEM is by two predominant mecha- mation on true OH conductivities has been shown nisms: electro-osmotic drag and back diffusion. In to be signiﬁcant with true OH conductivity values ? - electro-osmotic drag, when an H or OH ion measured via CO -free environment being double the passes through the IEM, it carries or ‘‘drags’’ a conductivity values measured via current procedures water molecule with it. In back diffusion, due to a in ambient air environments . Additionally, concentration gradient between anode and cath- CEMs have better chemical stability and higher sol- ode, water diffuses across the membrane to ubility in low boiling point solvents compared to establish equilibrium [35, 36]. AAEMs, which lead to easier and ‘‘greener’’ CEM Interestingly, alkaline fuel cells, which are like synthesis . As discussed in this review, the poor AEMFC in that they rely on the transport of OH chemical stability of AAEM in high-pH and high- temperature environments is a critical issue that has ions to generate electricity, were originally discov- prevented commercialization of AEMFC/AEMWE, ered in the 1930s by Francis T. Bacon; however, the since currently no AAEM exist which can stably main design shortcoming was the formation of car- operate in the high-pH and high-temperature envi- bonate precipitates (e.g. K CO ) in the electrolyte 2 3(s) ronments of AEMFC/AEMWE . Despite this, solution (e.g. KOH) . Unlike AEMFC, the original J Mater Sci (2018) 53:11131–11150 11135 alkaline fuel cell contained a liquid electrolyte solu- currents . In AEMFC, hydroxide is generated at tion and when air containing CO entered the fuel the cathode and transported across the AEM to the cell, it would react and form carbonates by the fol- anode. By operating at high currents, the hydroxide lowing reaction, which impeded fuel cell generation rate exceeds the carbonate formation rate, performance. resulting in the excess hydroxide in the AEM purging - 2- 2 the carbonate species (HCO ,CO ) from the þ þ 3 3 CO þ H O H CO HCO þ H CO þ 2H : 2 2 2 3 3 3 membrane to the anode, thus allowing the AEM to ð1Þ remain in the OH state and retain high conductivity Similarly, alkaline electrolysis, which utilizes an [39, 43]. Modelling work by Krewer et al.  sug- alkaline electrolyte (e.g. KOH) to improve ionic con- gests that operating AEMFC at current densities ductivity and partake in the electrochemical reac- greater than 1 A/cm can signiﬁcantly improve CO tions, is an established technology, but it faces a tolerance; however, this is awaiting experimental similar issue regarding electrode and membrane/di- validation. aphragm precipitates which reduce performance Additionally, recent work by Katayama et al.  2? [19, 37]. Electrolyte impurities such as calcium (Ca / has investigated feeding a gas blend (e.g. ammonia- 2? - Ca(OH) ) and magnesium (Mg /Mg(OH) ) have hydrogen) at the anode to facilitate a HCO con- 2 2 3 very low solubility products, which can lead to pre- sumption reaction and improving AEMFC CO tol- cipitation in the high-pH environments . erance. Katayama et al.  suggest that low CO By using an IEM, the cation head group is immo- tolerance in AEMFC is primarily due to carbonate bilized in the polymer matrix to minimize CO species adsorbing on the hydrogen oxidation reaction catalyst at the anode, so by facilitating HCO con- exposure and avoid the formation of carbonate pre- sumption at the anode, it removes the adsorbed cipitates in the AEMFC/AEMWE [2, 24, 38]. While no species and frees the catalyst to perform its function, carbonate precipitates may form, AEMFC/AEMWE thus retaining AEMFC performance. As Krewer et al. are sensitive to CO as CO ingress leads to a car- 2 2 bonation reaction between the ion-conducting group and Katayama et al. have shown, the area of (OH ) in the membrane and CO in the air/water AEMFC/AEMWE CO tolerance is rapidly evolving 2 2 - - 2- that converts the OH to HCO /CO via the fol- and shows great research potential to understand the 3 3 lowing reactions [39, 40]: carbonation mechanisms and mitigation strategies. AEM research is primarily driven by the need to CO þ OH HCO ð2Þ develop AAEMs for fuel cells and water electrolysis HCO þ OH CO þ H O: ð3Þ 3 3 applications [13, 16, 45]. Literature has suggested that the primary AEM research objective is targeting Reduced AEMFC/AEMWE performance is attrib- AAEMs with higher anion conductivity and uted to this carbonation reaction which increases improved chemical and mechanical stability. A sec- membrane resistance and also enables the adsorption ondary research objective is identifying alternative of carbonates on the anode catalyst layer [16, 39, 41]. non-platinum catalysts to reduce AEMFC/AEMWE CO exposure to the AEM results in the carbonation costs. The ﬁrst research objective is deemed most reaction converting the ion-conducting group, a critical as without a stable AAEM, there is no need to hydroxide ion, in the AEM to a larger carbonate ion develop non-platinum catalysts . To address the which is four to ﬁve times less conductive compared ﬁrst research objective, research has focused on two to hydroxide [39, 42]. Current laboratory-scale key areas, the anion exchange head group and strategies to minimize CO ingress include feeding polymer structure, and to a lesser extent, membrane pure oxygen or CO -free air into fuel cells and preparation techniques . degassing water supplies into water electrolysers; Using the Web of Science (SCI-EXPANDED) data- however, these are not practical solutions for large- base, an analysis of the number of journal article scale applications [39, 40]. By exploiting the self- publications from 2001 to 2018 was performed to purge mechanism in AEMFC, it is postulated that gauge research interest in AEMs. As Fig. 2 shows, the improved CO tolerance can be achieved by operat- number of AEM publications has been growing ing at higher currents and also reverse the detri- steadily since 2008, indicating a growing interest in mental effects of CO ingress from operating at low this research topic for the past 10 years. J Mater Sci (2018) 53:11131–11150 their relatively easy preparation and good stability . AEMs with QA can be formed by reacting the polymer containing a benzyl halide (e.g. chlorine) with an amine (e.g. triethylamine) to add the ammonia group, and then treating with an alkaline (e.g. potassium hydroxide) to convert the ammonia group to the salt form which can participate in anion exchange [5, 24]. In terms of stability, QA has been shown to have higher thermal and chemical stability compared to quaternary phosphonium and tertiary sulphonium . Additionally, the wide variety of tertiary amines permits the selection of diamines to act as both quaternization and cross-linking reagents when syn- thesizing AEMs. Notable tertiary diamine head groups include DABCO (1,4-diazabicyclo[2,2,2]oc- Figure 2 Annual journal article publications in anion exchange tane) and TMHDA (N,N,N,N-tetramethylhexane membranes (AEM), representing an average of 12% of annual ion diammonium) . The ability to self-cross-link is exchange membrane journal article publications. The total number important since it simpliﬁes the membrane synthesis of articles for 2018 (blue) is a projection based on the number of process and improves membrane mechanical stabil- articles published by April 2018 (red). ity. To compensate for the intrinsically lower elec- - ? trochemical mobility of OH compared to H , This review outlines advancements in anion research has focused on increasing the ion exchange exchange head groups, polymer structures and capacity (IEC) of AEM . However, the trade-off is membrane preparation methods. Analytical methods to characterize AEM performance is also discussed to that higher IEC increases the membrane swelling and comment on limitations with current testing reduces mechanical stability . By increasing cross- procedures. linking, it can mitigate, but not eliminate, these detrimental effects making these AEM good candi- dates for further development . Synthesis of anion exchange membranes The main drawback of QA AEMs is poor chemical stability due to the ammonium group’s susceptibility Advancements in AEM head groups to OH attack, leading to ammonium group degra- dation and reduced IEC . The OH attack occurs AEM head groups have traditionally been quaternary via one of the following reaction pathways: Hoffman ammonium (QA) ions; however, current research is elimination, nucleophile substitution (S 2) or ylide investigating other head groups such as tertiary intermediate formation [38, 46, 65]. Figure 3a–d diamines, phosphonium, sulphonium and metal highlights the respective degradation reaction path- cations [2, 5, 16]. Table 1 highlights common anion- ways. Given that all these reactions can be initiated conducting cations found in AEM head groups. by nucleophiles such as OH , the high-pH environ- AEMs were ﬁrst synthesized with QA because of ment in AEMFC/AEMWE makes it inevitable that Table 1 Common anion-conducting cations in AEM head group Nitrogen-containing groups Nitrogen-free groups Quaternary ammonium/tertiary diamines [2, 47–49] Phosphonium [58, 59] (Benz)Imidazolium [50–53] Sulphonium [60, 61] Guanidinium [54, 55] Metal cations [62, 63] (Ruthenium, Nickel, Cobalt) Pyridinium [56, 57] J Mater Sci (2018) 53:11131–11150 11137 Figure 3 Nucleophile (OH ) degradation mechanisms for quaternary ammonium (a–d) and imidazolium (e) based ion-conducting groups. J Mater Sci (2018) 53:11131–11150 the QA will be degraded over time [2, 13]. It has been functional groups improving imidazolium cation postulated that the cation chemical stability could be stability . Of these substitutions, N3 substitutions improved by adding large functional groups or are most promising as these imidazolium cations electron-donating groups. Large functional groups could be easily synthesized compared to C4- and C5- (e.g. phenyl groups) create a steric hindrance effect substituted imidazolium cations . From the imi- that blocks the OH from attacking the cation and dazolium-based cation head groups, benzimida- electron-donating groups (e.g. methoxy groups) help zolium cations (benzene group bound to an protect the cation group from OH attack [5, 51]. imidazolium group) have been shown to have Branching out from QA head groups, researchers improved stability, due to benzene ring resonance have investigated other nitrogen-containing cations structures, and improved anion conductivity, due to such as guanidinium, imidazolium and pyridinium ion cluster formation, compared to similar QA and [50–52, 54–57]. Of these, imidazolium-based head imidazolium-based AEM . As such, these head groups have shown the most promise due to their groups are promising and worthy of additional relatively easy synthesis method, adaptable structure research. which allows for the addition of various functional Through understanding the impacts of steric hin- groups and selective solubility in water-miscible drance and electron-donating groups, researchers solvents [5, 51, 66]. With respect to alkaline stability, have revisited phosphonium and sulphonium cations in addition to S 2 and deprotonation degradation with a focus on adding large electron-donating mechanisms, imidazolium-based head groups can groups surrounding the cation to improve chemical also be degraded via a ring opening mechanism stability . Phosphonium-based AEM can be syn- (Fig. 3e) [67, 68]. Multiple literature sources have thesized in similar methods to QA AEMs, except they reported that the electron-deﬁcient C2 position of use phosphine instead of amine for quaternization. imidazolium-based head groups is highly susceptible Research has shown that stable phosphonium- and to nucleophile attack, which could be mitigated sulphonium-based AEM can be synthesized through the addition of phenyl and methoxy groups to the through the addition of large functional groups to sterically hinder OH attack [67, 69, 70]. There is phosphorous and sulphur group to protect the some conﬂicting information as to the importance cations [58, 60]. While this work is relatively recent, it steric hindrance plays in protecting the C2 carbon. has demonstrated that nitrogen-free AEM mem- Price et al.  commented that imidazolium cation branes can be synthesized, and suggests that further stability can be increased primarily by competing research is needed to improve phosphonium- and reversible deprotonation reactions, followed by elec- sulphonium-based AEM performance to match and/ tronic stabilization of the C2 carbon through reso- or exceed nitrogen-based AEM performance. nance and ﬁnally by steric hindrance of the C2 A ﬁnal class of AEM head groups involves metal carbon. The proposed predominant stabilizing cations such as ruthenium, cobalt and nickel . The mechanism is from the presence of acidic protons ﬁrst metal cation-based AEM was synthesized using which the OH attacks to deprotonate in a reversible ruthenium, which was signiﬁcant as it is a divalent reaction, therefore protecting the imidazolium nitro- cation which can carry two anions per cation, as gen from being irreversibly degraded. Speciﬁcally, it opposed to all previous AEM cations which are was showed that imidazolium ions with a hydrogen monovalent . Given the lower electrochemical - ? at the C2 position was more stable than imidazolium mobility of OH compared to H , the ability to use ions with an isobutane group at the C2 position . multivalent cations can be a strategy to increase the This conﬂicts with the theory that large electron IEC of AEM. Most recently, it has been found that dense functional groups at the C2 carbon would nickel-based AEMs had the highest conductivity better stabilize the imidazolium cation, as shown by compared to ruthenium and cobalt-based AEMs, Wang et al. for imidazolium cations and by Thomas which suggests a new potential AEM head group and et al. for benzimidazolium cations [51, 70]. Addi- opportunity to explore other metals for AEM head tionally, Sun et al. summarized research done on groups . large functional group substitutions for the N3, C4 Overall, there is no consensus on the ‘‘best’’ AEM and C5 positions of imidazolium cations, which all head group as all head groups have inherent issues agreed with the trend of large electron dense with chemical stability and limited IEC; however, J Mater Sci (2018) 53:11131–11150 11139 there are promising head groups worthy of addi- benzyltrimethylammonium . As previously tional research. Imidazolium-based head groups, mentioned, there is a trade-off between increasing including benzimidazolium cations, are promising as IEC, through the number of ion exchange sites, and stability and performance can be improved using decreasing mechanical stability due to water update large electron-donating functional groups. There has and membrane swelling . Therefore, AEM poly- also been a focus on nitrogen-free AEM head groups, mer research focuses on increasing polymer cross- such as phosphonium, sulphonium, and metal linking and the formation of ion channels in poly- cations, to investigate other materials that could be mers with distinct hydrophilic and hydrophobic used in place of traditional QA cations in AEMs. regions . This is driven in part by the success of While research into metal cation-based head groups Naﬁon as a PEMFC membrane since it exhibits a is limited, this class of head groups shows great ‘‘comb like’’ structure with a PTFE backbone and promise due to their high stability and high valency regularly spaced perﬂuorovinyl ether side chain ter- which can address AEM shortcomings related to minated with a sulphonate group for ion exchange chemical stability and low IEC. . In an ideal situation, the ‘‘best’’ AEM head group Cross-linking is done to impart more favourable or membrane, is one that is both functional and thermal, mechanical and physiochemical properties practical. Functional in that it accomplishes the on a polymer. It can be done as a cross-linking step in purpose of the given AEM application. This may a polymerization reaction using high molecular include ensuring suitable ion exchange capacity and weight or directly cross-linkable oligomers (one-step hydroxide conductivity, stable long-term operation, synthesis) or as a post-cross-linking step after poly- chemical stability and adequate mechanical proper- merization (multi-step synthesis) [28, 46]. Most ties for routine operation (continuous and/or inter- (post)cross-linking steps involve covalent bonding mittent). Functionality relates to the material, and the use of heat, radiation and/or chemicals to whereas practicality refers to the synthesis proce- facilitate a cross-linking chemical reaction . Given dure for the head group/membrane. If this mem- the variety of monomers used to synthesize AEMs, brane is to be used in a commercial application, it there is no universal cross-linking mechanism, but will likely be manufactured at a large scale. Overly, rather a variety of cross-linking and post-cross-link- complex membrane chemistries using multi-step ing reactions. To achieve easier and ‘‘greener’’ AEM synthesis with harsh chemicals and operating con- synthesis, it is logical to expect that the one-step ditions requiring specialized equipment are not synthesis method is preferred compared to the post- practical. Therefore, in designing the ‘‘best’’ AEM cross-linking route. head group or membrane it is important to keep the Polymer backbones are commonly polysulfones or end goal of the application in mind to engineer a ﬂuorinated polymers [e.g. poly(vinylidene ﬂuoride)] cost-effective solution that will be functional and [28, 46]. Figure 4 highlights common polymer back- practical to use. bone degradation pathways for polysulfones and ﬂuorinated polymers. Polysulfones are susceptible to Advancements in AEM polymer structure ether hydrolysis and quaternary carbon hydrolysis due to hydroxide attack, while ﬂuorinated polymers In parallel with enhancing AEM head groups, are susceptible to dehydroﬂuorination [74–78]. research also focuses on polymer structure to Therefore, in addition to AEM head group alkaline improve IEC and chemical stability [28, 46]. Since degradation, AEM chemical stability is also affected AEM have traditionally used QA groups, most work by the polymer backbone design. Within these classes on AEM polymer structure involves polymers with of polymers, chemical modiﬁcations have allowed QA, with benzyltrimethylammonium being consid- more thermal and chemically stable polymer back- ered the benchmark for AEM head groups . bones to be designed and/or selected [79–81]. Recently, it has been suggested that benzyl-N- Inspired by Naﬁon , rather than just having the methylpyrrolidinium should be considered the new QA attached to the polymer backbone, polymers QA benchmark in AEM research as it exhibits were created with regularly spaced ﬂexible side improved alkali stability, conductivity and in situ chains containing one or multiple QA groups [5, 82]. fuel cell performance compared to Small improvements in stability were seen by J Mater Sci (2018) 53:11131–11150 Figure 4 Polymer degradation pathways for polysulfone (top) and poly(vinylidene ﬂuoride), PVDF (bottom). changing the polymer backbone to less polar poly- additional QA groups or chain length [86, 89]. This mers; however, the greatest stability improvements was likely due to the QA group proximity (limited were achieved by attaching the QA groups by a long ion-dissociating ability) and the over-assembly of ion aliphatic side chain . By grafting multiple QA clusters that resulted in separate ion-conducting groups on the side chains, regions of hydrophobicity regions. (polymer backbone) and hydrophilicity (polymer This demonstrates the need for ‘‘rational polymer side chains) developed which has been shown to architecture’’ to optimize the location, type and con- improve IEC and chemical stability [84, 85]. It has centration of anion-conducting groups and been reported that AEM with 99 mS/cm OH con- hydrophobic side chains to achieve optimal AEM ductivity at room temperature have been synthe- performance through effective hydrophobic/hy- sized, which is greater than conductivity values drophilic region interactions. In Fig. 5, the B scenario reported for Naﬁon . is what has been shown to be most effective as it Strategies to obtain AEM with ion channels include creates ion channels to facilitate higher anion con- locating ion-conducting groups at the ends of poly- ductivity while providing improved alkaline stability mer side chains, synthesizing polymer main chains since the polymer backbone is protected in the using multiblock co-polymers containing regions of hydrophobic region . ion-conducting groups [86, 87], monomers with Another factor to consider when synthesizing IEMs densely functionalized ion-conducting regions on the is Manning’s counterion condensation theory, which main chains  or separately attaching the suggests that counterions can condense on polyelec- hydrophobic side change and ion-conducting group trolytes if the linear charge density of the polyelec- to the polymer backbone . Work by Pan et al. and trolyte chain is greater than one [90, 91]. Due to Weiber et al. shows that increasing the number of QA counterion condensation, reduced effective charge is groups in the block copolymer or the hydrophobic seen compared to expected values from elemental side chain length improved the membrane’s IEC to a analysis since the counterion is effectively ‘‘screen- certain point, after which the IEC decreased with ing’’ the polyelectrolyte charge . While minimal J Mater Sci (2018) 53:11131–11150 11141 Figure 5 Development of ion channels in AEM. a Dispersed and overdeveloped ion channels with distinct hydrophilic/hydrophobic underdeveloped ion channels, b interconnected ion channels regions. Adapted from . conductive to the formation of ‘‘ionic highways’’, c segregated research has investigated this effect in AEMs, multi- techniques in that a polymer solution is either poured ple sources have demonstrated and modelled the over or immersed in a porous substrate allowing the signiﬁcance of this effect for sulphonated CEMs polymer matrix to ﬁll the porous substrate pores [92–94]. For example, Naﬁon 117 has been reported creating a membrane [96–99]. The porous substrate is to have approximately 80% of protons in the con- selected to be chemically inert and mechanically densed state . As research targets improvements stable (e.g. high-density polyethylene, polypropy- in ion exchange capacity, understanding and miti- lene, polystyrene, polyimide or similar porous poly- gating the effect of counterion condensation can oleﬁn) . This technique combines the beneﬁcial provide an opportunity for optimized IEM polymer characteristics of the polymer (high ion conductivity) and membrane structures. and porous support (mechanical strength and reduced membrane swelling) to produce membranes Advancements in AEM preparation with improved performance . While this method methods may involve repeated pouring and immersion steps, the literature sources have reported the ability to In terms of membrane synthesis, most AEMs are obtain both cation and anion exchange membranes homogenous membranes prepared by (a) direct with high IEC [97, 98, 101]. Work by Lee et al.  polymerization and cross-linking, (b) chemical mod- relating to anion exchange membranes is signiﬁcant iﬁcation of polymers by irradiation or grafting or as AEM with high IEC were achieved without sig- (c) chemical reactions to modify polymers . This niﬁcant membrane swelling, which was attributed to usually involves phase inversion methods where the use of the porous substrate. This produced solutions of membrane precursors are dissolved in membranes with improved mechanical strength polar solvents and casts on a plate after which the compared to similar AEMs synthesized without solvent is evaporated producing an IEM . Typi- porous supports. Additionally, by using a porous cally, there are multiple steps with harsh solvents support, AEMs could be synthesized with multiple (e.g. chloromethyl methyl ether which is carcinogenic narrow ion channels that allowed for the high OH for chloromethylation) or radiation sources (e.g. UV, conductivity . gamma or X-ray for grafting various head groups) Mixed matrix membranes are another promising [46, 95]. type of heterogeneous membranes due to the variety Alternatively, heterogeneous AEM can be pre- of inorganic nanoparticles and organic polymers that pared using (a) a pore ﬁlling or pore immersion can be blended to achieve desired membrane char- technique which synthesizes polymeric membranes acteristics . Examples of inorganic nanoparticles on a porous support or (b) mixed matrix membranes that have been used include metal ions, metal oxides, that ﬁx inorganic nanoparticles in organic polymers silica, functionalized nanoparticles (e.g. imidazolium- . Pore ﬁlling and pore immersion are similar functionalized silsesquioxane), graphene oxide and J Mater Sci (2018) 53:11131–11150 carbon nanotubes [48, 53, 102–104]. The inorganic including titration, spectroscopy to determine NO phase is selected to provide improved ion conduc- ion concentrations and ion selective electrodes (e.g. ? - tivity and thermal, chemical and mechanical stability, pH probe) to determine the presence of H /OH while the organic phase is selected to provide the ions in solution . Titration methods, either ﬂexibility to the membrane [5, 102]. Sol–gel tech- through acid/base titration or the Mohr method, are niques are typically used to prepare mixed matrix the most common methods to determine IEC. From a membranes, which stresses the importance of well- safety perspective, the acid/base titration method dispersed inorganic nanoparticles in the organic may be preferred since the Mohr method involves 2- phase to produce uniform membranes . hexavalent chromium (CrO ) which is a known Heterogeneous membranes synthesized using porous carcinogen [110, 111]; however, it has been postulated supports or inorganic nanoparticles are promising that there are inherent shortcomings with the acid/ methodologies to achieve AEM with high IEC with- base titration method related to CO poisoning of the - - out compromising mechanical strength. This OH groups. When the AEM in OH form is exposed methodology still uses volatile organic solvents, to CO -containing environments (e.g. ambient air or which suggests further research is needed to develop air-saturated water), these groups can convert to IEM synthesis pathways that can minimize harsh HCO form and alter the calculated IEC . This solvent usage. With the variety of porous supports inﬂuence may be minimal given the short exposure and inorganic nanoparticles available, ample research time to air, yet Karas et al.  demonstrated IEC opportunities are available to tune membrane prop- decreases of 3.5 and 2.0% per minute for homoge- erties for various applications. neous and heterogeneous AEMS, respectively, when exposed to air for 5 min. Therefore, efﬁcient proce- dures when performing acid/base titrations and Characterization of anion exchange rinsing AEMs with degassed DI water could help mitigate, but not eliminate, the risk of CO poisoning membranes when measuring IEC [109, 112]. AEM characterization methods primarily examine Furthermore, it has been suggested to measure the the chemical homogeneity, structure, stability and membrane IEC in the Cl form, which is the form the mechanical properties [106, 107]. Analytical methods membrane is typically synthesized in, to eliminate such as microscopy [scanning electron microscopy any deviations in IEC measurements due to pH (SEM)] and spectroscopy [energy-dispersive X-ray swings during acid/base titrations . To measure (ERD), nuclear magnetic resonance (NMR), Fourier- the AEM IEC in Cl form, the AEM is initially transformed infrared (FTIR), small-angle X-ray scat- equilibrated in a NaNO solution and then acidiﬁed tering (SAXS)] are used to characterize the molecular using HNO . The resulting solution containing the composition (e.g. uniform distribution of head displaced Cl ions is then titrated with AgNO using groups, formation of ion clusters) and structure of the Ag-titrodes to the endpoint, which is when all Cl membrane surfaces (e.g. pore structure, surface has been converted to AgCl. Using the following smoothness) [89, 101]. If asymmetrical membranes equation, where m is the membrane dry mass, are synthesized, comparisons can be made between which is the membrane mass after drying at 80 C for both membrane surfaces to understand the impact 48 h until there is no change in membrane mass, the surface differences have on membrane properties. IEC (Cl form) can be determined [47, 113]: AEM performance and chemical stability are typi- V C AgNO AgNO 3 3 IEC ¼ : ð4Þ cally assessed by measuring the IEC, swelling ratio, water uptake, water content, contact angle, conduc- For the acid/base titration method, various proce- tivity and alkaline stability [2, 28, 108]. dures are reported depending on acid/base strengths used and soaking times. The general premise is to Ion exchange capacity (IEC) soak the AEM in a strong base solution (e.g. 1 M The IEC is a measure of the number of exchangeable NaOH) to convert the AEM to the OH form and ions per membrane dry weight (meqiv/g or mmol/g) then soaking in a strong acid solution of known . It can be measured via different methods volume and concentration to convert the AEM to the J Mater Sci (2018) 53:11131–11150 11143 Cl form. Then, the AEM is removed and rinsed with procedures, it is possible to reduce the resulting DI water so the resulting diluted HCl solution is errors in IEC values obtained. More importantly, this titrated with standardized NaOH to the phenolph- demonstrates a need to develop a robust and uni- thalein endpoint. To calculate the number of versal IEC measurement procedure for AEMs to exchangeable ions (OH ) present, the moles of NaOH allow accurate comparisons between different AEMs. added are subtracted from the moles of HCl added. Swelling ratio (SR) This value is then divided by the membrane dry mass, and the resulting IEC calculated by the acid/ The swelling ratio is a measure of the linear expan- base titration method is [66, 70]: sion of the membranes when exposed to water . ðÞ V C ðÞ V C acid acid base base IEC ¼ : ð5Þ It is calculated as a per cent difference between wet and dry membrane lengths. The ‘‘dry’’ membrane With the Mohr method, an AEM is converted to the state is deﬁned the same as above for IEC. Cl form by soaking in a salt solution (e.g. 1 M NaCl). l l w d SR ¼ 100%: ð7Þ The AEM is then rinsed and equilibrated in a 0.5 M Na SO solution to facilitate the release of Cl . Using 2 4 a AgNO solution with K CrO indicator, the AEM/ 3 2 4 Water uptake (WU) Na SO solution is titrated until the K CrO endpoint, 2 4 2 4 which indicates when all the chlorides have been The water uptake is a measure of how the membrane precipitated and now Ag CrO forms. The resulting 2 4 mass changes when exposed to water . It is IEC calculated by the Mohr method is [110, 114]: calculated as a per cent difference between wet and V C AgNO AgNO 3 3 dry membrane masses. The ‘‘dry’’ membrane state is IEC ¼ : ð6Þ deﬁned the same as above for IEC. m m As with any titration, there are inherent human w d WU ¼ 100%: ð8Þ errors in determining the colour change at the end- point, which ultimately affects the calculated IEC. A further complication for titrations is the dilute nature Membrane water content (c) of the ion of interest targeted in the titrations. To ensure complete conversion of the AEM to the given The membrane water content is a measure of the form, strong bulk solutions (e.g. HCl, NaSO ) are number of water molecules per mobile anion and is needed for the titrations. For both the acid/base calculated by dividing the water uptake by the titration and Mohr method, the concentration of the molecular weight of water and IEC . Note that ion participating in the ion exchange is low relative to the WU is multiplied by 10 to account for the WU the bulk solution, resulting in challenges to accu- being reported in per cent and the IEC being reported rately determine the endpoint when titrating. To in mmol/g. improve accuracy and reduce variability when per- 10 WU forming titrations, it is possible to use an ISE (e.g. pH c ¼ : ð9Þ MW IEC H O probe) to determine the endpoint rather than relying on the visual colour change . Unlike determining IEC for CEM, IEC procedures Water contact angle (h) for AEM are less well deﬁned. While the most com- The water contact angle (h) is a measure of the wet- mon IEC procedures involve titrations, this may not tability of a membrane surface with large contact be the most accurate method. Karas et al.  angles indicating highly hydrophobic surfaces. This demonstrated that using UV–Vis spectroscopy to - - can be measured using the sessile-drop technique determine the NO ions that exchange with Cl ions . in an AEM produced IEC results in greatest agree- ment with theoretical IEC determined from elemental analysis of AEM composition. By understanding and mitigating the shortcomings with the different IEC J Mater Sci (2018) 53:11131–11150 Hydroxide conductivity (r) of the nucleophile (OH ); speciﬁcally, reducing hydration levels reduces alkaline stability . At Hydroxide conductivity can be calculated from elec- higher hydration levels, the water molecules ﬁll the trochemical impedance spectroscopy (EIS) and a two- solvation sphere surrounding the OH , in effect or four-electrode testing cell . After soaking an shielding it and reducing its nucleophilic character, AEM in DI water overnight, the membrane is secured resulting in improved alkaline stability. Ex situ in the testing cell and varying AC current is applied alkaline stability has been tested using KOH or to collect impedance data. Using nonlinear least NaOH solutions up to 10 M, which corresponds to a squares regression analysis, the membrane ionic water content of approximately 5 (c =5) resistance (R ) can be obtained and from that the m [66, 121, 122]. Higher KOH concentrations have lower conductivity (r) can be calculated from the following water contents; however, the higher viscosities may [70, 119]: adversely affect OH diffusivity and resulting mea- sured alkaline stability. In AEMFCs, as Fig. 1 shows, r ¼ : ð10Þ the cathode can become water-depleted, especially at R A higher current densities, thus exposing the AEM to Given that the aforementioned procedure is per- ultralow hydration levels (c = 0). Work by Dekel et al. formed in ambient atmosphere, the presence of CO demonstrated that QA groups had excellent stability presents challenges when measuring the true OH at c = 4; however, this stability was signiﬁcantly conductivity due to the rapid formation of carbonates reduced at c = 0, which was attributed to the change and bicarbonates (refer to Eq. 1)[31, 120]. This effect in S 2 reaction energies, which was the predominant was previously believed to be minimal; however, Ziv degradation mechanism for the QA group studied et al.  have shown that CO can signiﬁcantly . As the hydration level increased, the OH impact true OH conductivity measurements nucleophilicity decreased, resulting in higher activa- (* 50 mS/cm (conventional procedure) versus 103 tion energies and reaction energies. This demon- mS/cm (CO -free environment). Ziv et al. proposes strates that current ex situ alkalinity stability testing modifying conventional hydroxide conductivity using aqueous solutions may produce artiﬁcially testing procedure to ensure a CO -free environment high alkalinity stability values that would not be by subjecting the AEM to a nitrogen sweep gas ﬂow representative of in situ alkaline stability in AEMFC. in the testing cell. Then, a current is applied to gen- Therefore, Dekel et al.  proposed an alternative erate OH at the cathode and convert (bi)carbonates ex situ alkalinity stability testing procedure using to CO which are released at the anode. Once all the NMR and water-free hydroxide (crown ether/KOH) CO is released, the AEM would be in the pure OH 2 - solution where the water/OH ratio (c) could be form allowing for true OH conductivities to be controlled to assess alkaline stability at different measured and thus providing a standardized plat- hydration levels. form to compare hydroxide conductivity measure- To analyse AEM mechanical properties, properties ments between various AEMs . such as thermal stability and tensile strength are measured. Knowing the elevated operating temper- Alkaline stability atures of AEMFC (up to 200 C) and AEMWE (typi- cally 50–70 C), thermal stability of the membrane is The alkaline stability is a measure of how the AEM important and can be determined using thermo- performance changes over time when exposed to gravimetric analysis (TGA) and differential scanning high-pH environments . Testing conditions vary; calorimetry (DSC) [13, 101, 123]. TGA is used to however, the general premise is to soak the AEM in a assess thermal stability by monitoring the tempera- high-pH solution (e.g. 1–10 M KOH) at a given tem- ture at which membrane changes occurs due to water perature (room temperature or elevated temperature) loss, head group decomposition and/or polymer for extended periods of time and periodically testing decomposition [30, 123, 124]. DSC can be used to the membrane IEC to see how it changes with time evaluate the glass transition temperature, the effects . Inconsistencies in alkaline stability testing con- of thermal cycling and changes in polymer crys- ditions may be problematic, as it’s been shown that tallinity and cross-linking [112, 123–125]. By stretch- alkaline stability is inﬂuenced by the hydration level ing membrane samples in a universal testing J Mater Sci (2018) 53:11131–11150 11145 machine, various physical properties like tensile investigate consolidating advancements in AEM strength, stress–strain curves and elongation at break head groups with an optimized polymer structure in can be determined [103, 116, 125]. heterogeneous membranes. This could bring together the valuable characteristics gained from using a novel head group with improved chemical stability, with Conclusions the beneﬁts of a polymer structure with ion channels and improved membrane properties from using a AEM research is driven by the need to develop porous support or inorganic nanoparticles. AAEM for fuel cells and water electrolysis applica- tions since presently there are no suitable AAEMs which can stably operate in the high-pH and high- Acknowledgements temperature environments of AEMFC/AEMWE. B.P.L. gratefully acknowledges ﬁnancial support AEMFC/AEMWE are a promising source of clean from Imperial College London (ICL). S.J. gratefully energy and have several operational beneﬁts com- acknowledges ﬁnancial support from the Department pared to PEMFC/PEMWE, mainly in that catalysts of Chemical Engineering at ICL. K.F.L.H. gratefully can be platinum free. Given the limited focus on acknowledges ﬁnancial support from the Department AEM compared to CEM, it is a matter of time before of Chemical Engineering at ICL and Statoil. suitable AAEMs for AEMFC/AEMWE are developed. Compliance with ethical standards The principal AEM research objective is to improve AEM chemical and mechanical stability in high-pH Conﬂict of interest The authors declare that they and high-temperature environments. To achieve this, have no conﬂict of interest. research is focused on improving AEM head groups, polymer structure and membrane preparation meth- Open Access This article is distributed under the ods to produce AEM with high IEC and conductivity, terms of the Creative Commons Attribution 4.0 improved alkaline stability and improved mechanical International License (http://creativecommons.org/ stability to permit the commercialization of AEMFC/ licenses/by/4.0/), which permits unrestricted use, AEMWE. Given that no suitable AEM has been distribution, and reproduction in any medium, pro- synthesized to achieve these performance objectives vided you give appropriate credit to the original reliably, it demonstrates the need for further research author(s) and the source, provide a link to the Crea- in this ﬁeld. Progress has been made in using imi- tive Commons license, and indicate if changes were dazole and metal cation-based head groups to made. improve IEC and conductivity. 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Published: May 21, 2018
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