Anion Exchange Membrane Materials Research Articles

Tethering Cations to Aromatic Polymers via Flexible Spacers to Enhance the Performance of Alkaline Fuel Cell Membranes

Alkaline membrane fuel cells (AMFCs) have recently received increasing attention in relation to proton-exchange membrane fuel cells because the alkaline operating conditions potentially bring significant advantages, including the use of non-precious metal catalysts and improved cell efficiencies to significantly reduce cost.1 The polymeric anion-exchange membrane (AEM) is a critical component of AMFCs since its OH- conductivity and alkaline stability to a great extent determine the performance and lifetime of the system. Hence, significant research efforts are currently committed to the molecular design, synthesis and characterization of robust and highly anion-conducting polymer materials.1 Most studies of AEMs up until now have been performed on aromatic polymers functionalized with benzyltrimethylammonium groups positioned directly on the backbone.1 These materials are readily accessed via, e.g., chloromethylation of suitable aromatic polymers, followed by quaternization with trimethylamine. However, several recent results on low-molecular weight model compounds indicate that polymers containing benzylic quaternary ammonium (QA) groups possess a significantly lower stability than corresponding polymers with alkyl-substituted QA groups in the presence of aqueous OH-.2 Moreover, the location of the benzylic QA groups directly on the aromatic polymer backbones restricts their mobility and hinders the efficient phase separation necessary to reach high anion conductivity. There is thus a current need for new synthetic approaches to attach cationic groups on aromatic polymers via alkyl spacers. We have recently attached QA groups on poly(phenylene oxide) via flexible and non-hydrolysable alkyl spacers (PPO-7Q, Scheme 1) by using a straightforward synthetic route involving bromoalkylation and quaternization.3 AEMs based on these materials showed distinct small angle X-ray scattering maxima, so-called ionomer peaks, to indicate efficient phase separation (Fig. 1a). In contrast, corresponding AEMs based on polymers with QA groups placed in benzylic positions directly on the backbone (PPO-1Q, Scheme 1) showed very weak (if any) ionomer peaks. Moreover, the PPO-7Q showed a significantly enhanced OH- conductivity (Fig. 1b) and far superior alkaline stability in relation to PPO-1Q. These results clearly demonstrate the significant advantages of the alkyl spacer concept in the molecular design of AEM materials for AMFCs.3 In the present paper we will discuss further recent findings regarding molecular design principles, synthetic procedures and important structure-property relationships of these new AEM materials. 1. a) N. W. Li and M. D. Guiver, Macromolecules, 2014, 47, 2175; b) J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. W. Xu and L. Zhuang, Energy Environ. Sci., 2014, 7, 3135. 2. a) M. G. Marino, J. P. Melchior, A. Wohlfarth and K. D. Kreuer, J. Membr. Sci., 2014, 464, 61; b) A. D. Mohanty and C. Bae, J. Mater. Chem. A, 2014, 2, 17314. 3. H. S. Dang, E. A. Weiber and P. Jannasch, J. Mater. Chem. A, 2015, 3, 5280. Figure 1

Molecular Engineering of Ion-Conducting Polymers for Fuel Cell Membrane Applications

Ion-conducting polymer membranes (cation exchange and anion exchange) are a key component in clean energy conversion devices (e.g., fuel cells and water electrolysis). For example, perfluorinated sulfonic acid polymers such as Nafion are state-of-the-art proton-conducting polymers, but they suffer from high cost, mediocre mechanical strength, and moderate operation temperature. Thus, it is highly desirable to develop readily tunable, high-performance ion-conducting membranes based on low-cost hydrocarbon polymers. Over the years we have developed a novel polymer functionalization method that allows incorporation of a variety of ionic groups to aromatic polymers. Using the method we synthesized highly proton-conductive fuel cell membranes containing sulfonate groups with different acidity strengths (fluoroalkylsulfonic, arylsulfonic, and alkylsulfonic acid groups) [1-2]. The effect of these different sulfonate groups of polymers on proton transport properties and fuel cell performance will be discussed. Recently, anion exchange membrane fuel cells (AEMFCs) have gained significant attention because they can efficiently and cleanly convert the chemical energy stored in fuels directly into electrical energy [3]. Since AEMFCs utilize a solid alkaline electrolyte [i.e. anion exchange membrane (AEM)] to transport hydroxide anions, they can avoid issues previously faced with the concentrated aqueous KOH electrolyte in alkaline fuel cells. However, significant shortcomings still remain in AEMFCs such as poor long-term alkaline stability and low hydroxide conductivity. In our continued effort to develop robust, high performance ion-conducting polymers, we prepared styrene-ethylene/butylene-styrene (SEBS)-based AEM materials bearing various ammonium-cation groups. Utilizing our groups’ recently developed metal-catalyzed synthetic strategy, involving C-H borylation followed by Suzuki coupling [4], we successfully introduced chemically stable ammonium-cation species to the aromatic units of SEBS. The synthesis, hydroxide transport properties, alkaline stability, and fuel cell performance of these SEBS-based AEMs will be presented.

Current State of the Art in Water Electrolysis Performance Based on Anion Exchange Membranes

Hydrogen represents a major chemical feedstock worldwide, and is currently manufactured primarily from fossil fuel-based sources. Obtaining hydrogen from non-fossil sources is thus an important part of the overall strategy to reduce greenhouse gas emissions, including the potential tie to transportation infrastructure. Large scale renewable hydrogen production is also a key part of the global energy storage strategy, to capture peak solar and wind and convert energy that would otherwise be stranded to a deployable source. Water electrolysis is the only technology currently mature enough to meet the scale requirements in the near term for renewable hydrogen. However, manufacturing and material cost need to be decreased to meet the economic requirements of these applications. Commercial options for water electrolysis are currently limited to liquid KOH electrolyte based systems, and proton exchange membrane (PEM)-based systems. Both have advantages and disadvantages. Anion exchange membrane (AEM) technology represents a potential hybrid approach which leverages the advantages of both legacy systems. However, the key obstacle in the AEM systems is the robustness of the polymers, both in the ionomer and membrane form. The ionomer is subject to chemical degradation leading to loss of integrity of the electrocatalyst layer, while the membrane can lose mechanical strength due to both thermal damage during processing and chemical attack of the polymer backbone during operation, leading to pinhole failures. The membrane can also lose efficiency through loss of active sites, either through carbonation of the hydroxide conducting sites, or chemical degradation. These membrane and ionomer materials are much less mature than the PEM analogs, and fundamental understanding is still being developed around polymer design and degradation methods. Proton OnSite has developed cell designs and manufacturing processes which improve the robustness of the existing AEM materials, and have also collaborated with several membrane partners to develop improved materials. Most recently, the Illinois Institute of Technology (IIT) performed mechanistic studies to determine potential membrane degradation pathways, while both IIT and Sandia provided materials for cell testing. In this talk, results of these studies will be discussed, as well as resulting performance in commercial stack hardware. Proton has currently achieved stable operation of the AEM cell for over 1700 hours, with over 1000 hours at 500 mA/cm2, and has scaled the technology to a 1 L/min prototype AEM generator. Comparison to state of the art in PEM technology and the ultimate outlook for each technology will also be discussed.

The Hybrid Bi-Cell Approach for Reducing Active Water Management in the Direct Methanol Fuel Cell

Direct methanol fuel cells (DMFCs) provide an avenue for developing high (specific) energy density, portable power sources. While the overall energy efficiencies that have been reported for DFMCs are rather modest (e.g., approximately 15-20%), interest in the technology stems from the specific energy methanol (6.1 kWhr/kg) which still enables very large system energy densities to be achieved (1, 2). Despite this key benefit, the size and cost of the DMFC technology remain key technical hurdles.To date, many if not most efforts to develop DMFC technologies have used membrane electrolyte assemblies (MEAs) which utilize proton exchange membrane (PEM) electrolyte materials. These acidic PEM materials tend to be quite robust ; however, their use has relegated the DMFC systems to use of relatively large balance of plant (BoP) components for active water/fuel management (3, 4) and expensive platinum based catalyst materials (5). In this talk, we review an approach which is intended to address the size of the BoP required for active water/fuel management as well as enable the removal of some of the platinum based catalysts. This approach uses a bi-cell, or pseudo-bipolar, configuration which has been previously shown to be well suited to portable power applications (6). The novelty of this approach is the combination of a first acidic MEA using PEM electrolyte materials and a second alkaline MEA using anion exchange membrane (AEM) electrolyte materials. This configuration naturally balances the water stoichiometry of the methanol oxidation and oxygen reduction reactions between the adjacent acid and alkaline MEAs, significantly improving the water balance within the system. The integration of alkaline AEM materials has the additional benefit of opening doors for alternatives to platinum based catalysts.A multi-phase, multi-component transport model is used as the basis of this work, which is intended to demonstrate the technical feasibility of the approach for miniaturizing portable DMFC system. A previous study by Kim et al. has shown the compatibility of the materials (7). In this talk, focus will be placed on comparing standard and hybrid configurations and the relative water/fuel management requirements. Additional attention will be placed on the treatment of direct oxidation of methanol in an alkaline MEA and the effect of MEA properties. 1. D. Chu and R. Jiang, Electrochim Acta 51(26), 5829 (2006).2. R. Jiang, C. Rong and D. Chu, J Power Sources 126(1-2), 119 (2004).3. R. Jiang and D. Chu, J Electrochem Soc 155(8), B798 (2008).4. R. Jiang and D. Chu, J Electrochem Soc 155(8), B804 (2008).5. S. Wasmus and A. Kuver, J Electroanal Chem 461(1-2), 14 (1999).6. R. Jiang and D. Chu, J Power Sources 93(1-2), 25 (2001).7. H. Kim, M. Unlu, J. F. Zhou, I. Anestis-Richard and P. A. Kohl, J Power Sources 195(21), 7289 (2010).

Anion Exchange Fuel Cell Membranes Prepared from C–H Borylation and Suzuki Coupling Reactions

Of the polymer materials investigated as anion exchange membranes, trimethylbenzylammonium-containing polysulfones are the most extensively studied. They are commonly prepared by chloromethylation of the aromatic backbone or radical bromination of benzylic carbons, followed by a Menshutkin reaction between a tertiary amine and the benzyl halide. To overcome synthetic limitations involved with these two methods, we report the preparation of an anion exchange membrane by means of iridium-catalyzed C–H borylation followed by palladium-catalyzed Suzuki coupling. Owing to the use of mild reaction conditions and high efficiency of these metal-catalyzed reactions, we were able to minimize side reactions and easily control the degree of functionalization for a series of trimethylbenzylammonium-containing polysulfones. The resulting membranes exhibited lower water uptake while maintaining similar hydroxide conductivity and functional group stability as compared to the corresponding chloromethylation-prepared anion exchange membrane materials.

Synthesis and characterization of anion exchange membranes based on poly(arylene ether sulfone)s containing various cations functioned tetraphenyl methane moieties

Anion exchange membranes (AEMs) based on novel poly(arylene ether sulfone)s containing imidazolium, pyridinium and trialkylamine quaternary ammonium hydroxide groups functioned tetraphenyl methane moieties were obtained by treating the poly(arylene ether sulfone) having chloromethylated tetraphenyl methane moieties with N-methylimidazole (MIm), pyridine (Py), trimethylamine (TMA) and triethylamine (TEA) and following by alkalization. The chemical structures of the ionized poly(arylene ether sulfone)s (IPAESs) were confirmed by 1H NMR. Dimension stability, hydroxide conductivity, mechanical properties, thermal stability and chemical stability of the IPAESs membranes were evaluated as AEMs materials. The IPAES-TMA and IPAES-MIm membranes exhibit better comprehensive properties than that of the IPAES-TEA and IPAES-Py membranes. Especially, the IPAES-MIm membrane with IEC value of 1.85 exhibits the highest hydroxide conductivity of 65.6 mS cm−1 at 80 °C, and the best thermal and chemical stability. The results demonstrate the IPAES-MIm membrane constitutes a good candidate for alkaline anion-exchange membrane fuel cell application.

Synthesis and structure–conductivity relationship of polystyrene‐block‐poly(vinyl benzyl trimethylammonium) for alkaline anion exchange membrane fuel cells

Block copolymers of polystyrene‐b‐poly(vinyl benzyl trimethylammonium tetrafluoroborate) (PS‐b‐[PVBTMA][BF4]) were synthesized by sequential monomer addition using atom transfer radical polymerization. Membranes of the block copolymers were prepared by drop casting from dimethylformamide. Initial evaluation of the microphase separation in these PS‐b‐[PVBTMA][BF4] materials via SAXS revealed the formation of spherical, cylindrical, and lamellar morphologies. Block copolymers of polystyrene‐b‐poly(vinyl benzyl trimethylammonium hydroxide) (PS‐b‐[PVBTMA][OH]) were prepared as polymeric alkaline anion exchange membranes materials by ion exchange from PS‐b‐[PVBTMA][BF4] with hydroxide in order to investigate the relationship between morphology and ionic conductivity. Studies of humidity [relative humidity (RH)]‐dependent conductivity at 80 °C showed that the conductivity increases with increasing humidity. Moreover, the investigation of the temperature‐dependent conductivity at RH = 50, 70, and 90% showed a significant effect of grain boundaries in the membranes against the formation of continuous conductive channels, which is an important requirement for achieving high ion conductivity. © 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1751–1760, 2013

Preparation of chloromethylated/quaternized poly(phthalazinone ether ketone) anion exchange membrane materials for vanadium redox flow battery applications

Chloromethylated poly(phthalazinone ether ketone) (CMPPEK) was prepared from poly(phthalazinone ether ketone), with chloromethyl methyl ether (CME) as the chlromethylating reagent and concentrated sulfuric acid as the solvent. The effects of CME quantity, reaction temperature, and reaction time on degree of chloromethylation (DCM) were investigated. CMPPEK with DCM ranging from 0.73 to 2.32 mmol g −1 were obtained. CMPPEK were characterized with 1H NMR and TGA. CMPPEK membranes were prepared from CMPPEK/ N-methyl-2-pyrrolidinone casting solutions. Quaternized poly(phthalazinone ether ketone) (QAPPEK) anion exchange membranes were prepared from CMPPEK membranes with different DCM. The ion exchange capacity (IEC) and water content (Wc) of QAPPEK membranes were studied. QAPPEK membranes exhibited IEC ranging from 0.70 to 2.04 mmol g −1 and Wc ranging from 12.9 to 52.3%. IEC and Wc of QAPPEK increased with an increase in DCM of CMPPEK. The vanadium ion permeability of QAPPEK membranes was much lower than that of Nafion117 membrane. The performance of vanadium redox flow battery (VRB) single cell with QAPPEK membranes was investigated. Compare to VRB cell with Nafion117 membrane, the VRB single cell with QAPPEK membranes exhibited higher columbic efficiency. The results show that QAPPEK membranes could be promising anion exchange membranes for VRB applications.