Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Molecular Engineering of Hydroxide Conducting Polymers for Anion Exchange Membranes in Electrochemical Energy Conversion Technology Sangtaik Noh,† Jong Yeob Jeon,† Santosh Adhikari,† Yu Seung Kim,§ and Chulsung Bae*,†,‡
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†
Department of Chemistry & Chemical Biology and ‡Department of Chemical & Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States § MPA-11: Materials Synthesis & Integrated Devices, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
CONSPECTUS: Anion exchange membranes (AEMs) based on hydroxide-conducting polymers (HCPs) are a key component for anion-based electrochemical energy technology such as fuel cells, electrolyzers, and advanced batteries. Although these alkaline electrochemical applications offer a promising alternative to acidic proton exchange membrane electrochemical devices, access to alkaline-stable and high-performing polymer electrolyte materials has remained elusive until now. Despite vigorous research of AEM polymer design, literature examples of high-performance polymers with good alkaline stability at an elevated temperature are uncommon. Traditional aromatic polymers used in AEM applications contain a heteroatomic backbone linkage, such as an aryl ether bond, which is prone to degradation via nucleophilic attack by hydroxide ion. In this Account, we highlight some of the progress our group has made in the development of advanced HCPs for applications in AEMs and electrode ionomers. We propose that a synthetic polymer design with an all C−C bond backbone and a flexible chain-tethered quaternary ammonium group provides an effective solution to the problem of alkaline stability. Because of the critical demand for such a polymer system, we have established new synthetic strategies for polymer functionalization and polycondensation using an acid catalyst. The first approach is to graft a cationic tethered alkyl group to pre-existing, commercially available styrene-based block copolymers. The second approach is to synthesize high-molecular-weight aromatic backbone polymers using acid-catalyzed polycondensation of arene monomers and a functionalized trifluoromethyl ketone substrate. Both strategies involve a simple two-step reaction process and avoid the use of expensive metal-based catalysts and toxic chemicals, thereby making the synthetic processes easily scalable to large industrial quantities. Both polymer systems were found to have excellent alkaline stability, confirmed by the preservation of ion exchange capacity and ion conductivity of the membrane after an alkaline test under conditions of 1 M NaOH at 80−95 °C. In addition, the advantage of good solvent processability and convenient scalability of the reaction process generates considerable interest in these polymers as commercial standard AEM candidates. AEM fuel cell and electrolyzer tests of some of the developed polymer membranes showed excellent performance, suggesting that this new class of HCPs opens a new avenue to electrochemical devices with real-world applications.
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INTRODUCTION
Among sustainable energy solutions, hydrogen production from renewable sources and its use as a fuel have been touted © XXXX American Chemical Society
Received: June 30, 2019
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DOI: 10.1021/acs.accounts.9b00355 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Schematic diagram of AEM fuel cell and electrolyzer.
efficiently transporting hydroxide ion across the electrodes (i.e., solid hydroxide ion conductor) and preventing the passage of fuels and other ions (i.e., separator). Although the adoption of AEMs in electrochemical energy device applications can bring unprecedented benefits, this potential has not yet been realized, primarily owing to the lack of HCPs that are chemically and mechanically stable under high-pH conditions. Although Nafion has been used as a standard polymer in PEMs for decades, there is no advanced HCP that could serve as a standard AEM until now. Despite the relatively short history of AEMs, however, the research community has recently witnessed explosive evolution of HCPs in terms of durability, synthetic versatility, and feasibility in device applications. For comprehensive information about AEM evolution, we refer readers to a number of excellent reviews.16−22 In this Account, we discuss our molecular engineering approach to the identification of optimized cation head groups, synthetic method development for advanced HCPs, and the applications of these HCPs in AEM fuel cells and electrolyzers.
as an attractive, environmentally friendly, and easily scalable technology. For example in hydrogen fuel cells, the chemical energy stored in fuels (H2 and O2 from air) is converted directly to electrical energy via electrochemical reactions without producing greenhouse gases. Among currently available hydrogen production technologies, polymer electrolyte membrane based low-temperature (130 wt % at 25 °C) due to its high IEC. By switching biphenyl to more hydrophobic terphenyl structures in the backbone,55 the swelling ratio in water was significantly reduced: 16% and 21% for p-TPN1 and m-TPN1, respectively. Although both metaand para-terphenyl polymers have almost identical IECs of approximately 2.1 mequiv/g, the meta-substituted one showed better solubility and higher hydroxide ion conductivity (112 mS/cm at 80 °C), which is close to that of BPN1 with an IEC of 2.70 mequiv/g. Although further study is needed, the higher ion conductivity of the meta-substituted polymer compared with its para-substituted counterpart can be attributed to more efficient creation of ion-conducting channels through proper arrangement of the polymer backbone in m-TPN1. Once the acidic hydrogens at the 9-position of fluorene are substituted with either an alkyl or a bromoalkyl group, the disubstituted fluorenes can be used as arene monomers in acidcatalyzed polycondensation (FLN-m in Figure 7, where m indicates the mol % of the hexyltrimethylammonium functionalized disubstituted fluorene unit).56 When these anionic fluorene polymers were evaluated as an ionomer for electrode binder material, they showed remarkably enhanced maximum power density in an AEM fuel cell (1.5 W/cm2 with H2/O2 at 80 °C) compared with other aromatic polymer ionomer binder materials. Experimental and DFT calculation data suggest that the nonrotatable fluorene backbone structure greatly lowers the phenyl group adsorption parallel to the electrocatalyst surface and decreases interaction adsorption energy between the catalyst surface and the ionomer.57 Similar to the difference in membrane properties of biphenyl and terphenyl polymers, this significant boost in fuel cell performance highlights the importance of judicious optimization of conformational structures of aromatic polymer backbones at the molecular level.
Sandia National Laboratories.37,42 The random configurations of phenyl groups along the polymer backbone improves solubility in common organic solvents, greatly increasing the molecular weights of the polymer. Traditionally, condensation polymers are prepared through a nucleophilic aromatic substitution reaction that employs a basic reaction medium. Pioneered by Zolotukhin and coworkers since the early 2000s,53 the acid-catalyzed polycondensation reactions of highly electrophilic ketones with activated arenes have been reported to produce highmolecular-weight aromatic polymers that are difficult to synthesize otherwise. Because alkyl bromide functionality is quite stable under acidic conditions, we postulated that acidcatalyzed polycondensation of 5-bromopentyl trifluoromethyl ketone with electron-rich arenes would generate a polymer backbone made of a rigid aromatic ring and a tetrahedral carbon containing a terminal bromide at the side chain (e.g., BPBr in Figure 7).54 Subsequent substitution reaction of the bromide with trimethylamine would generate alkyltrimethylammonium-tethered HCPs. Our first trial using biphenyl as an arene monomer produced BPN1 in quantitative yield. Although the repeating unit of BPN1 is composed of a rigid aromatic backbone structure, it was obtained in high molecular weight (Mw = 120−160 kg/mol for precursor polymer BPBr) while remaining soluble in a variety of organic solvents including DMF, DMSO, methanol, ethanol, and an aqueous isopropanol solution. Satisfied with the initial success of our strategy, we decided to pursue more diverse structural modifications using pterphenyl, m-terphenyl, and 9,9-disubstituted fluorenes as arene monomers (p-TPN1, m-TPN1, and FLN-m, respectively, in Figure 7). Similar to BPN1, these polymers have high molecular weights (Mw > 100 kg/mol) and good solubility in alcohol solvents. They also have a long alkyl spacer between the charged quaternary ammonium group and the polymer backbone, which is beneficial not only for enhancing the alkaline stability of the cation but also for promoting phase separation between the hydrophobic backbone and the hydrophilic cationic group. All of these aromatic backbone polymers showed excellent alkaline stability with no noticeable changes in IEC or hydroxide ion conductivity after testing with 1 M NaOH solution at 80 °C for more than 500 h (see Table 1). A major drawback of BPN1 is its high swelling ratio (40%
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PRACTICAL APPLICATIONS IN AEM FUEL CELL AND ELECTROLYZER In addition to the materials properties of ideal HCPs mentioned earlier, there are other factors that are crucial for application in device level but not frequently addressed in literature. Representative examples include solubility and synthetic scalability. G
DOI: 10.1021/acs.accounts.9b00355 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 8. (a) Polarization curve (blue) and power density versus current density plot (black) measured for AEM fuel cell performance [conditions: H2/CO2-free air, 80 °C cell temperature, 1.5 bar cathode pressure, reinforced composite membrane (∼25 μm thickness), proprietary ionomer binder, and low loading platinum group metal catalyst at anode], (b) polarization curves measured for AEM electrolyzer performance [conditions: pure water, 0.1 M NaOH or 1 M NaOH flowed in the anode, 60 °C cell temperature, ambient pressure, SEBS AEM (∼30 μm thickness). Anode, IrO2 (1 mgIr/cm2); cathode, PtRu/C (0.5 mgPt/cm2); ionomer binder, FLN-55; I/C ratio, 1:8 (wt)].
polyethylene, 8% polybutylene) is shown in Figure 8b. Although higher performance of AEM electrolyzers is typically obtained with the addition of a small amount of alkaline electrolyte, we have obtained ∼1 A/cm2 at 2 V under an elevated temperature (60 °C) with pure deionized water. More engineering optimization at the molecular level and a degradation study at the device level are necessary to address further performance improvement and device durability challenges in the future.
The solubility of our quaternary ammonium functionalized aromatic polymers (BPN1, p-TPN1, m-TPN1, FLN-m) in alcohol solvents is quite unique given that most reported HCPs are soluble only in more hazardous organic solvents (e.g., DMF), which imposes difficulty in manufacturing the membrane products on a large scale. Moreover, this solubility in alcohol solvents is important in the application of ionomer binders at the catalyst layer of electrodes. Good compatibility at the membrane−catalyst layer is necessary to minimize ohmic resistance of the electrochemical cell device system. Although Nafion in alcohol solution has been widely used as an effective ionomer binder in the PEM field for decades, an ionomer solution that could be easily processable in AEM fuel cells and electrolyzers has been rare until now. If the synthetic process of HCPs and manufacturing process of AEMs are difficult to scale up, testing in real-world devices and engineering optimizations to enhance device performance and durability cannot be pursued. Notably, the easy access to Nafion (in membrane and ionomer solution forms) from commercial sources has been a major reason that PEM fuel cells and electrolyzers have been extensively studied in research laboratories and industry for decades. Among the aromatic HCPs we have developed to date, m-TPN1 has a good balance of (i) materials properties (high ion conductivity, good dimensional stability, and low swelling in water), (ii) device processability (good solubility), and (iii) synthetic scalability (two-step synthetic process, no use of toxic chemicals, and commercially available raw materials). The various product forms of m-TPN1 are now commercially available under the trade name OrionTM1. In collaboration with industrial and national laboratory partners, some of the HCPs developed by us have been evaluated as a membrane material by integrating into the membrane electrode assembly (MEA) of an AEM fuel cell and electrolyzer. Figure 8a illustrates fuel cell performance demonstrated using a reinforced AEM with a H2/CO2-free air feed at 80 °C. The maximum power density of 1.2 W/cm2 is almost comparable to the performance of the state-of-the-art PEM fuel cells obtained with H2/air feeds under similar conditions.19,58 In spite of a short development time, the performance of AEM electrolyzers has significantly improved over the past few years. AEM electrolysis performance based on a SEBS AEM (25 mol % polystyrene, 67 mol %
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CONCLUSIONS AND PERSPECTIVES Development of affordable alkaline-stable HCPs is indispensable to the success of AEM-based electrochemical energy conversion and storage technology. While the alkaline stability of cationic head groups in HCPs has been frequently discussed, initial efforts to develop alkaline-stable polymer backbones have yielded limited synthetic options. Because the presence of aryl ether bonds in the backbone potentially causes vulnerable bond cleavage via hydroxide attack, an all C−C bond backbone could be a viable solution. We explored two synthetic strategies for the preparation of HCPs composed of such backbone structures. The first approach uses acid-catalyzed grafting of a bromoalkyl side chain to pre-existing aromatic rings of SEBS. In the second approach, acid-catalyzed polycondensation of arene with trifluoromethyl ketone monomers produces polymer backbones with alternating aromatic rings and tetrahedral carbons. As anticipated, these aromatic polymers with all C−C bond backbones and a tethered alkyltrimethylammonium structure show excellent alkaline stability and good performance in fuel cell and electrolyzer when used as AEMs. Good solubility and scalability of the polymers suggest great potential for other electrochemical applications. Despite tremendous progress in the field of AEM research since the early 2010s, systematic studies of stability and degradation in operating electrochemical cell environments are still rare. This is primarily due to the lack of standard AEMs that satisfy multiple interrelated requirements simultaneously (e.g., high ion conductivity, good alkaline stability, robust mechanical stability, good solvent processability, and convenient synthetic scalability). Although m-TPN1 is not a perfect AEM material, it fulfills most of the important criteria, offering great potential for pursuit as a first-generation standard H
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applications for their invaluable intellectual and experimental contributions over the years. This research has been financially supported by the U.S. Department of Energy, Office of Efficiency and Renewable Energy (EERE), Fuel Cell Technology Office (FCTO; with Dr. Nancy Garland and Dr. David Peterson as program managers), and the HydroGEN Advanced Water Splitting Materials Consortium, established as part of the Energy Materials Network (DE-AC5206NA25396), ARPA-E (IONICS DE-AR0000769 and REFUEL DE-AR0000805), and NSF (CHE 1534289 and DMR 1506245). The authors also thank Xergy for providing reinforced fuel cell membranes based on ionic polymers developed from this research, POCell Tech for providing fuel cell MEA performance data, and Proton Onsite for providing insightful feedback of the manuscript and electrolyzer tests. Los Alamos National Laboratory (LANL) is operated by Triad National Security, LLC. under U.S. Department of Energy Contract Number 89233218CNA000001.
AEM. Convenient access to advanced HCP and AEM materials will open new avenues for device-level systems and operation optimization of alkaline electrochemical devices, which will in turn help to accelerate the enhancements of device performance and durability.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yu Seung Kim: 0000-0002-5446-3890 Chulsung Bae: 0000-0002-9026-3319 Notes
The authors declare no competing financial interest. Biographies Sangtaik Noh received B.S. (2008) and M.S. (2010) in chemistry at Seoul National University, Korea, and Ph.D. (2016) at the University of Southern California. He worked at Rensselaer Polytechnic Institute as a postdoctoral research associate until 2018 and is currently an information scientist at CAS, a division of American Chemical Society.
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REFERENCES
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Jong Yeob Jeon obtained his B.S. (2011) and Ph.D. (2016) degrees from Ajou University, South Korea. He is currently a postdoctoral research associate at Rensselaer Polytechnic Institute. His research interests include development of olefin polymerizations and polymer modifications for energy conversion technology. Santosh Adhikari received his B.Sc. and M.Sc. from Tribhuvan University, Nepal, and his Ph.D. (2018) in Chemistry from Oklahoma State University. He is currently working as a postdoctoral research associate at Rensselaer Polytechnic Institute. His major research interests include design, synthesis, and characterization of advanced organic materials for energy conversion application. Yu Seung Kim is a staff scientist at Materials Synthesis and Integrated Devices (MPA-11), Los Alamos National Laboratory. He received his Ph.D. from Korea Advanced Institute of Science and Technology in 1999. After three years of postdoctoral training at Virginia Tech, he joined LANL fuel cell team in 2003. He presently leads the alkaline membrane fuel cell and high temperature membrane fuel cell projects funded by US Department of Energy. His research focuses on the fundamental and applied science of fuel cells, including the development of ion exchange polymer electrolytes and understanding of catalyst−ionomer interface for polymer electrolyte membrane fuel cells. Chulsung Bae obtained his Ph.D. at the University of Southern California in 2002 under the guidance of G. K. Surya Prakash and Nobel laureate George A. Olah. After postdoctoral research with John F. Hartwig at Yale University, he started an independent academic career as an assistant professor at University of Nevada Las Vegas in 2004. He moved to Rensselaer Polytechnic Institute in 2012, where he is currently a professor in the Department of Chemistry & Chemical Biology and the Department of Chemical & Biological Engineering (joint appointment). He is primarily interested in the development of novel functional polymers that can play crucial roles in next-generation energy conversion and storage technology and the discovery of energy-efficient separation processes.
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ACKNOWLEDGMENTS We sincerely thank all co-workers from the Bae laboratory who have worked on the development of HCPs and their AEM I
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DOI: 10.1021/acs.accounts.9b00355 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.accounts.9b00355 Acc. Chem. Res. XXXX, XXX, XXX−XXX
