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. Additionally, using ‘‘rational polymer architecture’’ to design polymer backbones, several AEMs have been synthesized with References ion channels that have demonstrated high IEC and -  Peterson DS (2014) Ion exchange membranes. In: Li D (ed) conductivity and improved OH stability due to the Encyclopedia of microﬂuidics and nanoﬂuidics. Springer, formation of hydrophobic and hydrophilic regions in Boston, MA the membrane. Finally, heterogeneous membrane  Varcoe JR, Atanassov P, Dekel DR et al (2014) Anion- preparation techniques (e.g. pore-ﬁlled/immersed exchange membranes in electrochemical energy systems. membranes, mixed matrix membranes) are promis- Energy Environ Sci 7:3135–3191. https://doi.org/10.1039/ ing methodologies to tune membrane characteristics C4EE01303D by optimizing the ratio of polymer to porous support  Xu T (2005) Ion exchange membranes: state of their or nanoparticles. development and perspective. J Membr Sci 263:1–29. While there have been developments of nitrogen- https://doi.org/10.1016/j.memsci.2005.05.002 free and metal cation-based AEM head groups,  Strathmann H (2010) Electrodialysis, a mature technology research on polymer structure and membrane with a multitude of new applications. Desalination preparation methods continue to focus on AEM with 264:268–288. https://doi.org/10.1016/j.desal.2010.04.069 QA head groups. In conjunction with the principle AEM research objective, future research should J Mater Sci (2018) 53:11131–11150  Ran J, Wu L, He Y et al (2017) Ion exchange membranes: challenges. J Power Sources 375:170–184. https://doi.org/ new developments and applications. J Membr Sci 10.1016/j.jpowsour.2017.08.010 522:267–291. https://doi.org/10.1016/j.memsci.2016.09.  Marini S, Salvi P, Nelli P et al (2012) Advanced alkaline 033 water electrolysis. Electrochim Acta 82:384–391. https://  Strathmann H, Grabowski A, Eigenberger G (2013) Ion- doi.org/10.1016/j.electacta.2012.05.011 exchange membranes in the chemical process industry. Ind  Varcoe JR, Slade RCT (2005) Prospects for alkaline anion- Eng Chem Res 52:10364–10379. https://doi.org/10.1021/ exchange membranes in low temperature fuel cells. Fuel ie4002102 Cells 5:187–200. https://doi.org/10.1002/fuce.200400045  Ben Hamouda S, Touati K, Ben Amor M (2012) Donnan  Ursua A, Gandia LM, Sanchis P (2012) Hydrogen pro- dialysis as membrane process for nitrate removal from duction from water electrolysis: current status and future drinking water: membrane structure effect. Arab J Chem. trends. Proc IEEE 100:410–426. https://doi.org/10.1109/ https://doi.org/10.1016/j.arabjc.2012.07.035 JPROC.2011.2156750  Rozanska A, Wisniewski J, Winnicki T (2006) Donnan  Santos DMF, Sequeira CAC, Figueiredo JL (2013) dialysis with anion-exchange membranes in a water Hydrogen production by alkaline water electrolysis. Quim desalination system. Desalination 198:236–246. https://doi. Nova 36:1176–1193. https://doi.org/10.1590/S0100- org/10.1016/j.desal.2006.02.006 40422013000800017  Akretche DE, Kerdjoudj H (2000) Donnan dialysis of  Zeng K, Zhang D (2010) Recent progress in alkaline water copper, gold and silver cyanides with various anion electrolysis for hydrogen production and applications. Prog exchange membranes. Talanta 51:281–289. https://doi.org/ Energy Combust Sci 36:307–326. https://doi.org/10.1016/j. 10.1016/S0039-9140(99)00261-1 pecs.2009.11.002  Fonseca AD, Crespo JG, Almeida JS, Reis MA (2000)  Leng Y, Chen G, Mendoza AJ et al (2012) Solid-state water Drinking water denitriﬁcation using a novel ion-exchange electrolysis with an alkaline membrane. J Am Chem Soc membrane bioreactor. Environ Sci Technol 34:1557–1562. 134:9054–9057. https://doi.org/10.1021/ja302439z https://doi.org/10.1021/es9910762  Antolini E, Gonzalez ER (2010) Alkaline direct alcohol  Fox S, Oren Y, Ronen Z, Gilron J (2014) Ion exchange fuel cells. J Power Sources 195:3431–3450. https://doi.org/ membrane bioreactor for treating groundwater contami- 10.1016/j.jpowsour.2009.11.145 nated with high perchlorate concentrations. J Hazard Mater  Arges CG, Ramani V, Pintauro PN (2010) Anion exchange 264:552–559. https://doi.org/10.1016/j.jhazmat.2013.10. membrane fuel cells. Elctrochem Soc Interface 19:31–35. 050 https://doi.org/10.3384/ecp110571227  Matos CT, Sequeira AM, Velizarov S, Reis MA (2009)  Dekel DR (2018) Review of cell performance in anion Nitrate removal in a closed marine system through the ion exchange membrane fuel cells. J Power Sources exchange membrane bioreactor. J Hazard Mater 375:158–169. https://doi.org/10.1016/j.jpowsour.2017.07. 166:428–434. https://doi.org/10.1016/j.jhazmat.2008.11. 117 038  Carmo M, Fritz DL, Mergel J, Stolten D (2013) A com-  Vincent I, Bessarabov D (2018) Low cost hydrogen pro- prehensive review on PEM water electrolysis. Int J Hydrog duction by anion exchange membrane electrolysis: a Energy 38:4901–4934. https://doi.org/10.1016/j.ijhydene. review. Renew Sustain Energy Rev 81:1690–1704. https:// 2013.01.151 doi.org/10.1016/j.rser.2017.05.258  Sone Y (1996) Proton conductivity of Naﬁon 117 as  Zlotorowicz A, Strand RV, Burheim OS et al (2017) The measured by a four-electrode AC impedance method. permselectivity and water transference number of ion J Electrochem Soc 143:1254–1259. https://doi.org/10.1149/ exchange membranes in reverse electrodialysis. J Membr 1.1836625 Sci 523:402–408. https://doi.org/10.1016/j.memsci.2016.  Merle G, Wessling M, Nijmeijer K (2011) Anion exchange 10.003 membranes for alkaline fuel cells: a review. J Membr Sci  Leong JX, Daud WRW, Ghasemi M et al (2013) Ion 377:1–35. https://doi.org/10.1016/j.memsci.2011.04.043 exchange membranes as separators in microbial fuel cells  Zheng Y, Ash U, Pandey RP et al (2018) Water uptake for bioenergy conversion: a comprehensive review. Renew study of anion exchange membranes. Macromolecules. Sustain Energy Rev 28:575–587. https://doi.org/10.1016/j. https://doi.org/10.1021/acs.macromol.8b00034 rser.2013.08.052  Li Z, He X, Jiang Z et al (2017) Enhancing hydroxide  Gottesfeld S, Dekel DR, Page M et al (2017) Anion conductivity and stability of anion exchange membrane by exchange membrane fuel cells: current status and remaining blending quaternary ammonium functionalized polymers. J Mater Sci (2018) 53:11131–11150 11147 Electrochim Acta 240:486–494. https://doi.org/10.1016/j. 88:552–558. https://doi.org/10.1016/j.electacta.2012.10. electacta.2017.04.109 105  Ziv N, Dekel DR (2018) A practical method for measuring  Krewer U, Weinzierl C, Ziv N, Dekel DR (2018) Impact of the true hydroxide conductivity of anion exchange mem- carbonation processes in anion exchange membrane fuel branes. Electrochem Commun 88:109–113. https://doi.org/ cells. Electrochim Acta 263:433–446. https://doi.org/10. 10.1016/j.elecom.2018.01.021 1016/j.electacta.2017.12.093  Dekel DR, Rasin IG, Page M, Brandon S (2018) Steady  Hickner MA, Herring AM, Coughlin EB (2013) Anion state and transient simulation of anion exchange membrane exchange membranes: current status and moving forward. fuel cells. J Power Sources 375:191–204. https://doi.org/10. J Polym Sci Part B: Polym Phys 51:1727–1735. https://doi. 1016/j.jpowsour.2017.07.012 org/10.1002/polb.23395  Seh ZW, Kibsgaard J, Dickens CF et al (2017) Combining  Couture G, Alaaeddine A, Boschet F, Ameduri B (2011) theory and experiment in electrocatalysis: insights into Polymeric materials as anion-exchange membranes for materials design. Science 355:eaad4998. https://doi.org/10. alkaline fuel cells. Prog Polym Sci 36:1521–1557. https:// 1126/science.aad4998 doi.org/10.1016/j.progpolymsci.2011.04.004  Seitz LC, Hersbach TJP, Nordlund D, Jaramillo TF (2015)  Ponce-Gonza´lez J, Whelligan DK, Wang L et al (2016) Enhancement effect of noble metals on manganese oxide High performance aliphatic-heterocyclic benzyl-quaternary for the oxygen evolution reaction. J Phys Chem Lett ammonium radiation-grafted anion-exchange membranes. 6:4178–4183. https://doi.org/10.1021/acs.jpclett.5b01928 Energy Environ Sci 9:3724–3735. https://doi.org/10.1039/  Huo S, Deng H, Chang Y, Jiao K (2012) Water manage- C6EE01958G ment in alkaline anion exchange membrane fuel cell anode.  Dai P, Mo Z-H, Xu R-W et al (2016) Development of a Int J Hydrog Energy 37:18389–18402. https://doi.org/10. cross-linked quaternized poly(styrene-b-isobutylene-b- 1016/j.ijhydene.2012.09.074 styrene)/graphene oxide composite anion exchange mem-  Spiegel C (2007) Designing and building fuel cells. brane for direct alkaline methanol fuel cell application. McGraw Hill Professional, New York RSC Adv 6:52122–52130. https://doi.org/10.1039/  Ayers KE, Anderson EB, Capuano C et al (2010) Research C6RA08037E Advances towards low cost, high efﬁciency PEM electrol-  Wang J, He G, Wu X et al (2014) Crosslinked poly (ether ysis. ECS Trans 33:3–15 ether ketone) hydroxide exchange membranes with  Slade RCT, Kizewski JP, Poynton SD et al (2012) Ency- improved conductivity. J Membr Sci 459:86–95. https://doi. clopedia of sustainability science and technology. https:// org/10.1016/j.memsci.2014.01.068 doi.org/10.1007/978-1-4419-0851-3  Lin X, Liang X, Poynton SD et al (2013) Novel alkaline  Ziv N, Mustain WE, Dekel DR (2018) Review of ambient anion exchange membranes containing pendant benzimi- CO effect on anion exchange membranes fuel cells. dazolium groups for alkaline fuel cells. J Membr Sci Chemsuschem. https://doi.org/10.1002/cssc.201702330 443:193–200. https://doi.org/10.1016/j.memsci.2013.04.  Parrondo J, Arges CG, Niedzwiecki M et al (2014) 059 Degradation of anion exchange membranes used for  Wang J, Gu S, Kaspar RB et al (2013) Stabilizing the hydrogen production by ultrapure water electrolysis. RSC imidazolium cation in hydroxide-exchange membranes for Adv 4:9875–9879. https://doi.org/10.1039/c3ra46630b fuel cells. Chemsuschem 6:2079–2082. https://doi.org/10.  Katayama Y, Yamauchi K, Hayashi K et al (2017) Anion- 1002/cssc.201300285 exchange membrane fuel cells with improved CO toler-  Guo D, Lai AN, Lin CX et al (2016) Imidazolium-func- ance: impact of chemically induced bicarbonate ion con- tionalized poly(arylene ether sulfone) anion-exchange sumption. ACS Appl Mater Interfaces 9:28650–28658. membranes densely grafted with ﬂexible side chains for https://doi.org/10.1021/acsami.7b09877 fuel cells. ACS Appl Mater Interfaces 8:25279–25288.  Marino MG, Melchior JP, Wohlfarth A, Kreuer KD (2014) https://doi.org/10.1021/acsami.6b07711 Hydroxide, halide and water transport in a model anion  Li Z, Zhang Y, Cao T et al (2017) Highly conductive exchange membrane. J Membr Sci 464:61–71. https://doi. alkaline anion exchange membrane containing imida- org/10.1016/j.memsci.2014.04.003 zolium-functionalized octaphenyl polyhedral oligomeric  Suzuki S, Muroyama H, Matsui T, Eguchi K (2013) silsesquioxane ﬁller. J Membr Sci 541:474–482. https://doi. Inﬂuence of CO dissolution into anion exchange mem- org/10.1016/j.memsci.2017.07.037 brane on fuel cell performance. Electrochim Acta  Zhang Q, Li S, Zhang S (2010) A novel guanidinium grafted poly(aryl ether sulfone) for high-performance J Mater Sci (2018) 53:11131–11150 hydroxide exchange membranes. Chem Commun  Lin B, Qiu L, Lu J, Yan F (2010) Cross-linked alkaline 46:7495–7497. https://doi.org/10.1039/c0cc01834a ionic liquid-based polymer electrolytes for alkaline fuel cell  Liu L, Li Q, Dai J et al (2014) A facile strategy for the applications. Chem Mater 22:6718–6725. https://doi.org/ synthesis of guanidinium-functionalized polymer as alka- 10.1021/cm102957g line anion exchange membrane with improved alkaline  Hugar KM, Kostalik HA, Coates W (2015) Imidazolium stability. J Membr Sci 453:52–60. https://doi.org/10.1016/j. cations with exceptional alkaline stability: a systematic memsci.2013.10.054 study of structure–stability relationships. J Am Chem Soc  Li Y, Xu T, Gong M (2006) Fundamental studies of a new 137:8730–8737. https://doi.org/10.1021/jacs.5b02879 series of anion exchange membranes: membranes prepared  Dong H, Gu F, Li M et al (2014) Improving the alkaline from bromomethylated poly(2,6-dimethyl-1,4-phenylene stability of imidazolium cations by substitution. Chem- oxide) (BPPO) and pyridine. J Membr Sci 279:200–208. PhysChem 15:3006–3014. https://doi.org/10.1002/cphc. https://doi.org/10.1016/j.memsci.2005.12.006 201402262  Sata T, Yamane Y, Matsusaki K (1998) Preparation and  Price SC, Williams KS, Beyer FL (2014) Relationships properties of anion exchange membranes having pyri- between structure and alkaline stability of imidazolium dinium or pyridinium derivatives as anion exchange cations for fuel cell membrane applications. ACS Macro groups. J Polym Sci Part A Polym Chem 36:49–58. https:// Lett 3:160–165. https://doi.org/10.1021/mz4005452 doi.org/10.1002/(SICI)1099-0518(19980115)36:1%3C49::  Thomas OD, Soo KJWY, Peckham TJ et al (2012) A AID-POLA8%3E3.0.CO;2-X stable hydroxide-conducting polymer. J Am Chem Soc  Gu S, Cai R, Yan Y (2011) Self-crosslinking for dimen- 134:10753–10756. https://doi.org/10.1021/ja303067t sionally stable and solvent-resistant quaternary phospho-  Sun Z, Lin B, Yan F (2017) Anion-exchange membranes nium based hydroxide exchange membranes. Chem for alkaline fuel-cell applications: the effects of cations. Commun 47:2856–2858. https://doi.org/10.1039/ Chemsuschem. https://doi.org/10.1002/cssc.201701600 c0cc04335d  Heitner-Wirguin C (1996) Recent advances in perﬂuori-  Noonan KJT, Hugar KM, Kostalik HA et al (2012) Phos- nated ionomer membranes: structure, properties and appli- phonium-functionalized polyethylene: a new class of base- cations. J Membr Sci 120:1–33 stable alkaline anion exchange membranes. J Am Chem  Tillet G, Boutevin B, Ameduri B (2011) Chemical reactions Soc 134:18161–18164. https://doi.org/10.1021/ja307466s of polymer crosslinking and post-crosslinking at room and  Zhang B, Gu S, Wang J et al (2012) Tertiary sulfonium as a medium temperature. Prog Polym Sci 36:191–217. https:// cationic functional group for hydroxide exchange mem- doi.org/10.1016/j.progpolymsci.2010.08.003 branes. RSC Adv 2:12683–12685. https://doi.org/10.1039/  Arges CG, Ramani V (2013) Two-dimensional NMR c2ra21402d spectroscopy reveals cation-triggered backbone degradation  Hossain MA, Jang H, Sutradhar SC et al (2016) Novel in polysulfone-based anion exchange membranes. Proc hydroxide conducting sulfonium-based anion exchange Natl Acad Sci 110:2490–2495. https://doi.org/10.1073/ membrane for alkaline fuel cell applications. Int J Hydrog pnas.1217215110 Energy 41:10458–10465. https://doi.org/10.1016/j.ijhy  Nun˜ez SA, Hickner MA (2013) Quantitative H NMR dene.2016.01.051 analysis of chemical stabilities in anion-exchange mem-  Zha Y, Disabb-Miller ML, Johnson ZD et al (2012) Metal- branes. ACS Macro Lett 2:49–52. https://doi.org/10.1021/ cation-based anion exchange membranes. J Am Chem Soc mz300486h 134:4493–4496. https://doi.org/10.1021/ja211365r  Sata T, Tsujimoto M, Yamaguchi T, Matsusaki K (1996)  Kwasny MT, Tew GN (2017) Expanding metal cation Change of anion exchange membranes in an aqueous options in polymeric anion exchange membranes. J Mater sodium hydroxide solution at high temperature. J Membr Chem A 5:1400–1405. https://doi.org/10.1039/ Sci 112:161–170. https://doi.org/10.1016/0376- C6TA07990C 7388(95)00292-8  Pan J, Chen C, Zhuang L, Lu J (2012) Designing advanced  Ameduri B (2009) From vinylidene ﬂuoride (VDF) to the alkaline polymer electrolytes for fuel cell applications. Acc applications of VDF-containing copolymers: recent devel- Chem Res 45:473–481. https://doi.org/10.1021/ar200201x opments and future trends. Chem Rev 109:6632–6686.  Chempath S, Einsla BR, Pratt LR et al (2008) Mechanism https://doi.org/10.1021/cr800187m> of tetraalkylammonium headgroup degradation in alkaline  Miyanishi S, Yamaguchi T (2016) Ether cleavage-triggered fuel cell membranes. J Phys Chem C 112:3179–3182. degradation of benzyl alkylammonium cations for https://doi.org/10.1021/jp7115577 polyethersulfone anion exchange membranes. Phys Chem J Mater Sci (2018) 53:11131–11150 11149 Chem Phys 18:12009–12023. https://doi.org/10.1039/  Kamcev J, Paul DR, Freeman BD (2015) Ion activity C6CP00579A coefﬁcients in ion exchange polymers: applicability of  Han J, Peng H, Pan J et al (2013) Highly stable alkaline Manning’s counterion condensation theory. Macro- polymer electrolyte based on a poly(ether ether ketone) molecules 48:8011–8024. https://doi.org/10.1021/acs.mac backbone. ACS Appl Mater Interfaces 5:13405–13411. romol.5b01654 https://doi.org/10.1021/am4043257  Muthukumar M (2004) Theory of counter-ion condensation  Fujimoto C, Kim DS, Hibbs M et al (2012) Backbone on ﬂexible polyelectrolytes: adsorption mechanism. J Chem stability of quaternized polyaromatics for alkaline mem- Phys 120:9343–9350. https://doi.org/10.1063/1.1701839 2?- brane fuel cells. J Membr Sci 423–424:438–449. https://  Rivas BL, Moreno-Villoslada I (1998) Binding of Cd doi.org/10.1016/j.memsci.2012.08.045 and Na ions by poly(sodium 4-styrenesulfonate) ana-  Ponce-Gonza´lez J, Ouachan I, Varcoe JR, Whelligan DK lyzed by ultraﬁltration and its relation with the counterion (2018) Radiation-induced grafting of a butyl-spacer styre- condensation theory. J Phys Chem B 102:6994–6999. nic monomer onto ETFE: the synthesis of the most alkali https://doi.org/10.1021/jp980941m stable radiation-grafted anion-exchange membrane to date.  Beers KM, Hallinan DT, Wang X et al (2011) Counterion J Mater Chem A. https://doi.org/10.1039/C7TA10222D condensation in Naﬁon. Macromolecules 44:8866–8870.  Zhu L, Pan J, Wang Y et al (2016) Multication side chain https://doi.org/10.1021/ma2015084 anion exchange membranes. Macromolecules 49:815–824.  Qaisrani NA, Ma Y, Ma L et al (2018) Facile and green https://doi.org/10.1021/acs.macromol.5b02671 fabrication of polybenzoxazine-based composite anion-ex-  Han J, Liu Q, Li X et al (2015) An effective approach for change membranes with a self-cross-linked structure. Ionics alleviating cation-induced backbone degradation in aro- (Kiel). https://doi.org/10.1007/s11581-017-2433-y matic ether-based alkaline polymer electrolytes. ACS Appl  Pandey AK, Childs RF, West M et al (2001) Formation of Mater Interfaces 7:2809–2816. https://doi.org/10.1021/ pore-ﬁlled ion-exchange membranes within situ crosslink- am508009z ing: poly(vinylbenzyl ammonium salt)-ﬁlled membranes.  Ran J, Wu L, Wei B et al (2014) Simultaneous enhance- J Polym Sci Part A Polym Chem 39:807–820. https://doi. ments of conductivity and stability for anion exchange org/10.1002/1099-0518(20010315)39:6%3C807::AID- membranes (AEMs) through precise structure design. Sci POLA1054%3E3.0.CO;2-2. Rep 4:1–5. https://doi.org/10.1038/srep06486  Kuzume A, Miki Y, Ito M (2013) Characterisation of  He Y, Pan J, Wu L et al (2015) A novel methodology to PAMPS–PSS pore-ﬁlling membrane for direct methanol synthesize highly conductive anion exchange membranes. fuel cell. J Membr Sci 446:92–98. https://doi.org/10.1016/j. Sci Rep 5:1–7. https://doi.org/10.1038/srep13417 memsci.2013.06.032  Weiber EA, Meis D, Jannasch P (2015) Anion conducting  Lee M-S, Kim T, Park S-H et al (2012) A highly durable multiblock poly(arylene ether sulfone)s containing hydro- cross-linked hydroxide ion conducting pore-ﬁlling mem- philic segments densely functionalized with quaternary brane. J Mater Chem 22:13928–13931. https://doi.org/10. ammonium groups. Polym Chem 6:1986–1996. https://doi. 1039/c2jm32628k org/10.1039/C4PY01588F  Maurya S, Shin S, Kim M et al (2013) Stability of com-  Ren X, Price SC, Jackson AC et al (2014) Highly con- posite anion exchange membranes with various functional ductive anion exchange membrane for high power density groups and their performance for energy conversion. fuel-cell performance. ACS Appl Mater Interfaces J Membr Sci 443:28–35. https://doi.org/10.1016/j.memsci. 6:13330–13333. https://doi.org/10.1021/am503870g 2013.04.035  Chen D, Hickner MA (2013) Ion clustering in quaternary  Yamaguchi T, Miyata F, Nakao SI (2003) Pore-ﬁlling type ammonium functionalized benzylmethyl containing poly(- polymer electrolyte membranes for a direct methanol fuel arylene ether ketone)s. Macromolecules 46:9270–9278. cell. J Membr Sci 214:283–292. https://doi.org/10.1016/ https://doi.org/10.1021/ma401620m S0376-7388(02)00579-3  Pan J, Chen C, Li Y et al (2014) Constructing ionic high-  Jiang S, Ladewig BP (2017) High ion-exchange capacity way in alkaline polymer electrolytes. Energy Environ Sci semihomogeneous cation exchange membranes prepared 7:354–360. https://doi.org/10.1039/C3EE43275K via a novel polymerization and sulfonation approach in  Manning GS (1969) Limiting laws and counterion con- porous polypropylene. ACS Appl Mater Interfaces densation in polyelectrolyte solutions. III. An analysis 9:38612–38620. https://doi.org/10.1021/acsami.7b13076 based on the mayer ionic solution theory. J Chem Phys  Kickelbick G (2003) Concepts for the incorporation of 51:3249–3252. https://doi.org/10.1063/1.1672502 inorganic building blocks into organic polymers on a J Mater Sci (2018) 53:11131–11150 nanoscale. Prog Polym Sci 28:83–114. https://doi.org/10. desalination. Mater (Basel) 9:1–14. https://doi.org/10.3390/ 1016/S0079-6700(02)00019-9 ma9050365  Wu Y, Wu C, Xu T, Fu Y (2009) Novel anion-exchange  Tanaka Y (2007) Ion exchange membranes: fundamentals organic-inorganic hybrid membranes prepared through sol– and applications. Elsevier, Amsterdam gel reaction of multi-alkoxy precursors. J Membr Sci  Strathmann H (2004) Ion-exchange membrane separation 329:236–245. https://doi.org/10.1016/j.memsci.2008.12. processes. Elsevier, Amsterdam 056  Luo X, Wright A, Weissbach T, Holdcroft S (2018) Water  Bakangura E, Wu L, Ge L et al (2016) Progress in polymer permeation through anion exchange membranes. J Power science mixed matrix proton exchange membranes for fuel Sources 375:442–451. https://doi.org/10.1016/j.jpowsour. cells: state of the art and perspectives. Prog Polym Sci 2017.05.030 57:103–152. https://doi.org/10.1016/j.progpolymsci.2015.  Park J-S, Choi J-H, Woo J-J, Moon S-H (2006) An elec- 11.004 trical impedance spectroscopic (EIS) study on transport  Laberty-Robert C, Valle´ K, Pereira F, Sanchez C (2011) characteristics of ion-exchange membrane systems. J Col- Design and properties of functional hybrid organic–inor- loid Interface Sci 300:655–662. https://doi.org/10.1016/j. ganic membranes for fuel cells. Chem Soc Rev jcis.2006.04.040 40:961–1005. https://doi.org/10.1039/c0cs00144a  Mu¨ller F, Ferreira CA, Azambuja DS et al (2014) Mea-  Giorno L, Drioli E, Strathmann H (2016) Ion-exchange suring the proton conductivity of ion-exchange membranes membrane characterization. In: Drioli E, Giorno L (eds) using electrochemical impedance spectroscopy and Encyclopedia of Membranes. Springer, Berlin, Heidelberg through-plane cell. J Phys Chem B 118:1102–1112. https://  Berezina NP, Kononenko NA, Dyomina OA, Gnusin NP doi.org/10.1021/jp409675z (2008) Characterization of ion-exchange membrane mate-  Wright AG, Fan J, Britton B et al (2016) Hexamethyl-p- rials: properties vs structure. Adv Colloid Interface Sci terphenyl poly(benzimidazolium): a universal hydroxide- 139:3–28. https://doi.org/10.1016/j.cis.2008.01.002 conducting polymer for energy conversion devices. Energy  Sata T (2007) Ion exchange membranes: preparation, Environ Sci 9:2130–2142. https://doi.org/10.1039/ characterization, modiﬁcation and application. Royal C6EE00656F Society of Chemistry, Cambridge  Dekel DR, Amar M, Willdorf S et al (2017) Effect of water  Karas F, Hna´t J, Paidar M et al (2014) Determination of the on the stability of quaternary ammonium groups for anion ion-exchange capacity of anion-selective membranes. Int J exchange membrane fuel cell applications. Chem Mater Hydrog Energy 39:5054–5062. https://doi.org/10.1016/j. 29:4425–4431. https://doi.org/10.1021/acs.chemmater. ijhydene.2014.01.074 7b00958  Hong T-K, Kim M-H, Czae M-Z (2010) Determination of  Fang J, Yang Y, Lu X et al (2012) Cross-linked, ETFE- chlorinity of water without the use of chromate indicator. derived and radiation grafted membranes for anion Int J Anal Chem 2010:1–7. https://doi.org/10.1155/2010/ exchange membrane fuel cell applications. Int J Hydrog 602939 Energy 37:594–602. https://doi.org/10.1016/j.ijhydene.  Leng G (2002) Chromium and its compounds [MAK Value 2011.09.112 Documentation, 1992]. In: The MAK-Collection for  Stokes KK, Orlicki JA, Beyer FL (2010) RAFT polymer- Occupational Health and Safety. Wiley-VCH Verlag GmbH ization and thermal behavior of trimethylphosphonium & Co. KGaA polystyrenes for anion exchange membranes. Polym Chem  Faraj M, Boccia M, Miller H et al (2012) New LDPE based 2:80–82. https://doi.org/10.1039/C0PY00293C anion-exchange membranes for alkaline solid polymeric  Mabrouk W, Ogier L, Vidal S et al (2014) Ion exchange electrolyte water electrolysis. Int J Hydrog Energy membranes based upon crosslinked sulfonated polyether- 37:14992–15002. https://doi.org/10.1016/j.ijhydene.2012. sulfone for electrochemical applications. J Membr Sci 08.012 452:263–270. https://doi.org/10.1016/j.memsci.2013.10.  Deavin OI, Murphy S, Ong AL et al (2012) Anion-ex- 006 change membranes for alkaline polymer electrolyte fuel  Narducci R, Chailan J-F, Fahs A et al (2016) Mechanical cells: comparison of pendent benzyltrimethylammonium- properties of anion exchange membranes by combination of and benzylmethylimidazolium-head-groups. Energy Envi- tensile stress–strain tests and dynamic mechanical analysis. ron Sci 5:8584–8597. https://doi.org/10.1039/c2ee22466f J Polym Sci Part B Polym Phys 54:1180–1187. https://doi.  Khan MI, Luque R, Akhtar S et al (2016) Design of anion org/10.1002/polb.24025 exchange membranes and electrodialysis studies for water
Journal of Materials Science – Springer Journals
Published: May 21, 2018
It’s your single place to instantly
discover and read the research
that matters to you.
Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.
All for just $49/month
Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly
Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.
Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.
Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.
All the latest content is available, no embargo periods.
“Hi guys, I cannot tell you how much I love this resource. Incredible. I really believe you've hit the nail on the head with this site in regards to solving the research-purchase issue.”Daniel C.
“Whoa! It’s like Spotify but for academic articles.”@Phil_Robichaud
“I must say, @deepdyve is a fabulous solution to the independent researcher's problem of #access to #information.”@deepthiw
“My last article couldn't be possible without the platform @deepdyve that makes journal papers cheaper.”@JoseServera