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Polymers in Energy Applications
10.32
Polymers in Energy Applications
MA Hickner, The Pennsylvania State University, University Park, PA, USA JE McGrath, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA © 2012 Elsevier B.V. All rights reserved.
10.32.1 10.32.2
Introduction Chapter Summaries
597 598
10.32.1 Introduction Many next-generation energy conversion and storage devices will rely on solid polymer electrolytes to provide increased operational flexibility and long lifetimes for a wide range of applications. Fuel cells, batteries, capacitors, and electrolyzers all were initially conceived with liquid electrolytes serving to transfer charge between the anode and the cathode. These types of liquid electrolytes consisting of aqueous or high dielectric constant solutions with added salt, acid, or base set the internal pH of the device and facilitate rapid ion transport between electrodes to decrease resistive losses. Liquid electrolytes are inexpensive, widely available, and often used to characterize the fundamental response of the electrodes and electrocata lysts. However, liquid electrolytes are prone to leakage, can evaporate, and often accelerate corrosion of cell hardware and balance-of-plant systems. For these reasons, many advanced device concepts invoke solid electrolytes and often functiona lized polymer membranes as an ideal configuration for a solid-state electrochemical device. New solid polymer electrolytes for fuel cells has been an important topic in the development of fuel cells from laboratory curiosities, to space mission equipment, to candi dates for automobile power plants and mobile phone power sources. The case for fuel cells is simple: they can convert energy-dense fuels directly into electrical energy at low tem peratures. Even the most pessimistic estimates of energy density for fuel cell systems place them well ahead of bat teries in terms of how much energy can be stored in an automobile or in a portable electronic device. Liquid fuels such as methanol and ethanol contain 4–6 kWh L−1, and even 4 wt.% hydrogen storage in a pressurized tank has a volumetric energy density that is competitive for energy sto rage applications. Clearly, chemical fuels are needed for long run-time devices. The performance of fuel cells has increased at least 10-fold in the last 15 years spurred on by investment from the United States Department of Energy, the Japanese New Energy Development Organization, and the European Commission. New companies have entered the fuel cell market, and the US military is an early adopter of portable methanol fuel cells to replace batteries. However, the largest barrier to fuel cells capturing a significant share of the energy conversion device market for portable and mobile power is cost. The standard fuel cell materials are perfluorinated ion exchange membranes and platinum-based catalysts, both of which are expensive. Current research efforts in nonfluori nated membranes and nonprecious metal catalysts are directly addressing the cost issue, but these new materials still require advances in performance before they challenge the current benchmark materials. Polymer Science: A Comprehensive Reference, Volume 10
Alkaline fuel cell technology holds the promise of making fuel cells cost competitive with common alkaline batteries. New membranes based on commodity polymers and high-activity nonprecious metal catalysts may finally unlock fuel cell technology in the consumer world. Older generations of alkaline fuel cells have employed concentrated sodium or potassium hydroxide as the electrolyte. The performance of these types of cells is as good as or better than platinum-based proton exchange membrane fuel cells because the electrocata lytic reactions are facilitated at high pH. However, system corrosion from the highly basic electrolyte and potassium or sodium carbonate precipitation upon exposure to atmospheric carbon dioxide hampered the commercial prospects of these systems. The recent key innovation in alkaline fuel cells is new cationic membranes that do not suffer from carbonate precipi tation and highly dispersed, nonprecious metal catalysts that facilitate fabrication of a membrane electrodes assembly. As these types of fuel cells in their current embodiment are quite new and evolving rapidly, a chapter on their characteristics has not been included here; however, the reader is directed to Varcoe and Slade, Fuel Cells 2005, for a primer on this impor tant new area. Batteries are another area where solid electrolytes may offer distinct advantages over current practice. In Li-ion batteries, a liquid electrolyte, most often consisting of a mixture of ethy lene carbonate and dimethyl carbonate with added salt, is employed. The liquid electrolyte facilitates high charge mobi lity and inexpensive fabrication. However, the electrolyte is flammable and, combined with high energy density electrodes, can lead to fires during shorting or other types of catastrophic failures. Battery scientists across the globe are working on safer materials that prevent thermal runaway and battery fires, espe cially in the context of batteries for plug-in hybrid electric vehicles. Solid-state electrolytes may help to improve the safety of these devices. New directions are beginning to emerge in solid-state Li-ion battery solid electrolytes through the use of single-ion conductors. The standard solid polymer electrolyte explored in the context of Li-ion batteries is poly(ethylene oxide) (PEO) that contains added salt. The low Tg of PEO imparts high mobility to dissociated ions from the salt. Since PEO has only a moderate dielectric constant and does not have the high molecular mobility of a liquid, the conductivity of PEO-based electrolytes is at least an order of magnitude or two too low for application in Li-ion batteries. However, these types of polymeric electrolytes have been employed effectively in thin film batteries. Due to the recent proliferation of interest in new battery technology, new strategies for polymer-based electrolytes in batteries are beginning to emerge. However, there is still much work needed in this area and readers can learn more from Kerr et al., J. Power Sources 2002.
doi:10.1016/B978-0-444-53349-4.00282-X
597
598
Polymers in Energy Applications | Polymers in Energy Applications
Energy devices such as capacitors and fuel cells already widely employ polymer thin films or membranes in high-performance devices. The chapters included here review the state of the art in these areas and include directions for future research. Importantly, common threads between the chapters should be recognized which will provide practitioners in this area inspiration for new ideas and allow other research ers to recognize that their expertise can be applied to problems in these areas.
10.32.2 Chapter Summaries Perfluorinated membranes as described in Chapter 10.33 by Yandrasits and Hamrock are the standard ion exchange mem branes used in fuel cells and other electrochemical applications such as electrolyzers. NAFION®, a product of DuPont, has been on the market since the 1960s, but recent innovations in perfluorinated polymers from Solvay-Solexis and 3M and increased research investment in perfluorinated membranes have advanced the field in the last 10 years. As operating con ditions for fuel cells have tended toward less humidification, the superacid character of perfluorosulfonic acid makes perfluorinated membranes desirable platforms for continued improvements of these types of materials. Membranes based on aromatic architectures are leading candidates as perfluorinated membrane replacements. Much work has been performed to supplant NAFION®, and aromatic membranes have been demonstrated in operating devices for thousands of hours. Both step growth and radical polymeriza tion methods have been used to create new proton exchange membranes as reviewed in Chapter 10.34 by Bae, Miyatake, and Watanabe and in Chapter 10.35 by Tsang and Holdcroft. The high degree of microstructural control afforded by con trolled free radical and other living polymerization techniques has fostered some key insights in the field. Block structures are now being sought in step growth polymers to increase the hydrophilic–hydrophobic phase separation within the material at the nanoscale to affect high proton conductivities and robust mechanical properties. In addition to high-performance membranes, the proton-conducting medium in the electrodes must be consid ered in order to construct high-performance devices as detailed by Kim et al. in Chapter 10.36. Aromatic polymers need to be optimized to perform well in a porous electrode, which requires new polymers that may have different chemical com positions in relation to the membrane. Moreover, the processing of the catalyst layers can have a large influence on their performance. In general, aromatic polymers tend to have low oxygen permeability and therefore impart additional mass transfer resistance at the reaction interface. By choosing the correct casting solvents, electrode deposition techniques, and treatment steps, aromatic polymers in the electrode can have similar performance to more established perfluorinated mem branes. More work is needed to understand the basic principles that govern proton-conducting polymer performance in the electrodes. Key to the function of many proton-conducting membranes is the membrane nanophase structure. Much debate has occurred on the exact geometry of the hydrophilic and hydro phobic phase structure of NAFION®, but it remains clear that a
phase-separated structure on the nanoscale, with concentrated hydrophilic groups forming ionic domains for water and pro ton transport, is one of the important ingredients of a high-performance membrane as described by Osborn and Moore in Chapter 10.37. New block copolymer proton exchange membranes have attempted to emulate the phase-separated nanostructure of NAFION®, but the combi nation of ductility, high proton conductivity, and high water transport has proven difficult to replicate in non-PFSA materi als. As multiphase materials such as composites continue to be pursued for improvements over commercially available membranes, NAFION®’s phase-separated structure continues to be a model for how to design high-performing materials. As fuel cell systems are being pushed to their absolute limits of performance and durability, the degradation of the mem brane becomes an important factor in determining the overall lifetime of the device. Significant gains in lifetime of poly (perfluorosulfonic acid) membranes have been realized through stabilization of the backbone endgroups. Now that devices are operating over 10 000 h, other degradation points along the backbone and side chain have become the ‘weak links’ in membrane stability during fuel cell operation as described by Schiraldi and Savant in Chapter 10.38. While the stability of poly(perfluorosulfonic acid) materials is unpre cedented, the mechanistic studies conducted so far have set a template for how to attack the problem of degradation in poly (aromatic) membranes, which is an important topic for the future, as replacements for perfluorinated membranes are sought. Insight into the proton transport process in polymers has been developed by multiscale simulations as reviewed by Habenicht and Paddison in Chapter 10.39. Electronic structure calculations of polymer fragments and the hydration of the acidic group and proton have informed the community of the basic mechanisms of proton, water, and polymer structure and motion at the atomic level. Larger-scale simulations have been used to probe how the nanoscale morphology is devel oped in these materials and how the connectivity of the hydrophilic domains evolves with water content. The difficulty in describing explicit water molecules and the shuttling of protons within a water network has been addressed in small systems, but more work is required to expand these smaller simulations to length scales and time scales that are relevant to proton conductivity across micron-scale membranes. Capacitors will be needed for any highly efficient hybrid electrical storage or conversion system that incorporates a bat tery, fuel cell, or internal combustion engine. Polymer dielectric capacitors provide an important route to high energy density systems and have distinct advantages in processing and break down strength compared to traditional ceramic capacitors as detailed by Han and Wang in Chapter 10.40. Polymer films are also unique in that they have self-healing capabilities and are light and flexible which may provide interesting opportunities for new form factor devices. PVDF and nanocomposite-based dielectric polymer capacitors have greatly accelerated innova tions in this area in the last 10 years, but there are still innovations needed in methods to decrease loss and improve the energy density of these devices. We have also included a chapter on water treatment mem branes by Mickols in Chapter 10.41 as energy needs and clean water are inextricably linked. It takes energy to purify water and
Polymers in Energy Applications | Polymers in Energy Applications
clean water is needed in power plants for cooling and for biofuel production. While energy issues are front-and-center in 2011, the next great challenge will be clean water. Water treatment is a mature industry and new innovations in polymer membranes for reverse osmosis and nanofiltration have signif icantly reduced the cost of membrane-based water treatment and made reverse osmosis as the preferred technology for producing drinkable water from seawater. Innovations in the polymer chemistry of thin film composites, improvements in their chlorine resistance, and further optimization of materials processing parameters and systems design will continue to push reverse osmosis and membrane-mediated water treat ment technology to the fore of the water industry. Finally, electrolyzer technology as described by Mittelsteadt and Staser in Chapter 10.42 has not benefited from the large gains in performance that have been realized in fuel cells. Electrolyzer technology is the simplest method for producing hydrogen and is the most efficient method for generating hydrogen from non-fossil sources. Water electrolysis is
599
expensive due to the high cost of electricity and the high cost of the materials used in the devices. Commercialization of electrolyzers for hydrogen cooling of power plants, hydrogen gas generation for flame ionization detectors for gas chromato graphy, and process gases for metal processing will help to push the cost of this technology down for energy applications. While NAFION® is still the standard membrane in electrolyzer applications, new types of electrolyzers, such as the sulfur dioxide depolarized anode electrolyzer for the hybrid sulfur thermochemical cycle, require membranes with high thermal stability and low crossover. New electrolyzer membranes will provide a route for less expensive devices with higher perfor mance across a range of devices and operating conditions. In all, these chapters provide a detailed picture of where polymer science has contributed to the core understanding of electrolytes and dielectrics in electrochemical devices. These chapters build on the fundamental knowledge conveyed in the previous volumes and add to the message of sustainable polymers in the first part of Volume 10.
600
Polymers in Energy Applications | Polymers in Energy Applications
Biographical Sketches Michael A. Hickner received a BS in chemical engineering from Michigan Tech in 1999 and a MEng in 2002 and PhD in chemical engineering from Virginia Tech in 2003. In graduate school, he worked under the direction of James E. McGrath and also spent time in the fuel cell group at Los Alamos National Laboratory, developing novel aromatic proton exchange membranes for both hydrogen and direct methanol fuel cells. Before joining the Department of Materials Science and Engineering at Penn State in July 2007, he was a postdoctoral researcher and subsequently became a staff member at Sandia National Laboratories in Albuquerque, NM, where he conducted experimental investigations and modeling studies of liquid water transport in fuel cells and porous media, properties of ion-containing membranes, electrochemical reactors, and nanoporous membranes for water treatment applications. His research group at Penn State is focused on the synthesis and properties of ion-containing polymers, measurement of water–polymer interactions using spectroscopic techniques, and the study of self- and directed assembly of polymeric nanostructures for fast transport. He has ongoing projects in new polymer synthesis, fuel cells, batteries, water treatment membranes, and organic photovoltaic materials. He is currently an assistant professor and the Virginia S. and Philip L. Walker Jr. Faculty Fellow in the Materials Science Department at Penn State. Hickner’s work has been recognized by a Powe Junior Faculty Enhancement Award (2008), Young Investigator Awards from ONR and ARO (2008), a 3M Non-tenured Faculty Grant (2009), and a Presidential Early Career Award for Scientists and Engineers from President Obama in 2009. He has five US and international patents and over 60 peer-reviewed publications since 2001 that have been cited more than 2900 times as of 2011.
James E. McGrath received his BS in chemistry from Siena College in New York (1956) and his MS (1964) and PhD (1967) in polymer science from the University of Akron, where he worked on emulsion and anionic polymerization of synthetic rubbers, ozone cracking, and triblock copolymer thermoplastic elastomers. After 19 years in industry (Rayonier (cellulose), Goodyear (synthetic rubbers), and Union Carbide (engineering thermoplastics, polyolefins)], he joined the Chemistry Department at Virginia Tech in 1975. He is now Ethyl Chair and a University Distinguished Professor. He was director of the first group of NSF Science and Technology Centers from 1989 to 2000 on Structural Adhesives and Composites and focused on high-temperature polymers including polyimides, polysulfones, and toughened epoxy polymeric matrix resins for carbon fiber composites. He has many contributions to the anionic and ring-opening polymerization of dienes, epoxides, and organosiloxanes. His current focus is on polymeric materials for carbon fibers and membranes, including fuel cells, reverse osmosis water purification, and gas separation systems. He has 50 patents and over 500 publications and has received numerous awards, including election to the National Academy of Engineers (1994), The International SPE award, the Plastics Hall of Fame, and the ACS awards in Applied Polymer Science (2002) and Polymer Chemistry (2008). He has graduated more than 100 PhD chemists and engineers and remains one of the leaders in polymer science and engineering, with a current group (2011) of 13 students and postdoctoral fellows.
Polymers in Energy Applications
MA Hickner, The Pennsylvania State University, University Park, PA, USA JE McGrath, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA © 2012 Elsevier B.V. All rights reserved.
10.32.1 10.32.2
Introduction Chapter Summaries
597 598
10.32.1 Introduction Many next-generation energy conversion and storage devices will rely on solid polymer electrolytes to provide increased operational flexibility and long lifetimes for a wide range of applications. Fuel cells, batteries, capacitors, and electrolyzers all were initially conceived with liquid electrolytes serving to transfer charge between the anode and the cathode. These types of liquid electrolytes consisting of aqueous or high dielectric constant solutions with added salt, acid, or base set the internal pH of the device and facilitate rapid ion transport between electrodes to decrease resistive losses. Liquid electrolytes are inexpensive, widely available, and often used to characterize the fundamental response of the electrodes and electrocata lysts. However, liquid electrolytes are prone to leakage, can evaporate, and often accelerate corrosion of cell hardware and balance-of-plant systems. For these reasons, many advanced device concepts invoke solid electrolytes and often functiona lized polymer membranes as an ideal configuration for a solid-state electrochemical device. New solid polymer electrolytes for fuel cells has been an important topic in the development of fuel cells from laboratory curiosities, to space mission equipment, to candi dates for automobile power plants and mobile phone power sources. The case for fuel cells is simple: they can convert energy-dense fuels directly into electrical energy at low tem peratures. Even the most pessimistic estimates of energy density for fuel cell systems place them well ahead of bat teries in terms of how much energy can be stored in an automobile or in a portable electronic device. Liquid fuels such as methanol and ethanol contain 4–6 kWh L−1, and even 4 wt.% hydrogen storage in a pressurized tank has a volumetric energy density that is competitive for energy sto rage applications. Clearly, chemical fuels are needed for long run-time devices. The performance of fuel cells has increased at least 10-fold in the last 15 years spurred on by investment from the United States Department of Energy, the Japanese New Energy Development Organization, and the European Commission. New companies have entered the fuel cell market, and the US military is an early adopter of portable methanol fuel cells to replace batteries. However, the largest barrier to fuel cells capturing a significant share of the energy conversion device market for portable and mobile power is cost. The standard fuel cell materials are perfluorinated ion exchange membranes and platinum-based catalysts, both of which are expensive. Current research efforts in nonfluori nated membranes and nonprecious metal catalysts are directly addressing the cost issue, but these new materials still require advances in performance before they challenge the current benchmark materials. Polymer Science: A Comprehensive Reference, Volume 10
Alkaline fuel cell technology holds the promise of making fuel cells cost competitive with common alkaline batteries. New membranes based on commodity polymers and high-activity nonprecious metal catalysts may finally unlock fuel cell technology in the consumer world. Older generations of alkaline fuel cells have employed concentrated sodium or potassium hydroxide as the electrolyte. The performance of these types of cells is as good as or better than platinum-based proton exchange membrane fuel cells because the electrocata lytic reactions are facilitated at high pH. However, system corrosion from the highly basic electrolyte and potassium or sodium carbonate precipitation upon exposure to atmospheric carbon dioxide hampered the commercial prospects of these systems. The recent key innovation in alkaline fuel cells is new cationic membranes that do not suffer from carbonate precipi tation and highly dispersed, nonprecious metal catalysts that facilitate fabrication of a membrane electrodes assembly. As these types of fuel cells in their current embodiment are quite new and evolving rapidly, a chapter on their characteristics has not been included here; however, the reader is directed to Varcoe and Slade, Fuel Cells 2005, for a primer on this impor tant new area. Batteries are another area where solid electrolytes may offer distinct advantages over current practice. In Li-ion batteries, a liquid electrolyte, most often consisting of a mixture of ethy lene carbonate and dimethyl carbonate with added salt, is employed. The liquid electrolyte facilitates high charge mobi lity and inexpensive fabrication. However, the electrolyte is flammable and, combined with high energy density electrodes, can lead to fires during shorting or other types of catastrophic failures. Battery scientists across the globe are working on safer materials that prevent thermal runaway and battery fires, espe cially in the context of batteries for plug-in hybrid electric vehicles. Solid-state electrolytes may help to improve the safety of these devices. New directions are beginning to emerge in solid-state Li-ion battery solid electrolytes through the use of single-ion conductors. The standard solid polymer electrolyte explored in the context of Li-ion batteries is poly(ethylene oxide) (PEO) that contains added salt. The low Tg of PEO imparts high mobility to dissociated ions from the salt. Since PEO has only a moderate dielectric constant and does not have the high molecular mobility of a liquid, the conductivity of PEO-based electrolytes is at least an order of magnitude or two too low for application in Li-ion batteries. However, these types of polymeric electrolytes have been employed effectively in thin film batteries. Due to the recent proliferation of interest in new battery technology, new strategies for polymer-based electrolytes in batteries are beginning to emerge. However, there is still much work needed in this area and readers can learn more from Kerr et al., J. Power Sources 2002.
doi:10.1016/B978-0-444-53349-4.00282-X
597
598
Polymers in Energy Applications | Polymers in Energy Applications
Energy devices such as capacitors and fuel cells already widely employ polymer thin films or membranes in high-performance devices. The chapters included here review the state of the art in these areas and include directions for future research. Importantly, common threads between the chapters should be recognized which will provide practitioners in this area inspiration for new ideas and allow other research ers to recognize that their expertise can be applied to problems in these areas.
10.32.2 Chapter Summaries Perfluorinated membranes as described in Chapter 10.33 by Yandrasits and Hamrock are the standard ion exchange mem branes used in fuel cells and other electrochemical applications such as electrolyzers. NAFION®, a product of DuPont, has been on the market since the 1960s, but recent innovations in perfluorinated polymers from Solvay-Solexis and 3M and increased research investment in perfluorinated membranes have advanced the field in the last 10 years. As operating con ditions for fuel cells have tended toward less humidification, the superacid character of perfluorosulfonic acid makes perfluorinated membranes desirable platforms for continued improvements of these types of materials. Membranes based on aromatic architectures are leading candidates as perfluorinated membrane replacements. Much work has been performed to supplant NAFION®, and aromatic membranes have been demonstrated in operating devices for thousands of hours. Both step growth and radical polymeriza tion methods have been used to create new proton exchange membranes as reviewed in Chapter 10.34 by Bae, Miyatake, and Watanabe and in Chapter 10.35 by Tsang and Holdcroft. The high degree of microstructural control afforded by con trolled free radical and other living polymerization techniques has fostered some key insights in the field. Block structures are now being sought in step growth polymers to increase the hydrophilic–hydrophobic phase separation within the material at the nanoscale to affect high proton conductivities and robust mechanical properties. In addition to high-performance membranes, the proton-conducting medium in the electrodes must be consid ered in order to construct high-performance devices as detailed by Kim et al. in Chapter 10.36. Aromatic polymers need to be optimized to perform well in a porous electrode, which requires new polymers that may have different chemical com positions in relation to the membrane. Moreover, the processing of the catalyst layers can have a large influence on their performance. In general, aromatic polymers tend to have low oxygen permeability and therefore impart additional mass transfer resistance at the reaction interface. By choosing the correct casting solvents, electrode deposition techniques, and treatment steps, aromatic polymers in the electrode can have similar performance to more established perfluorinated mem branes. More work is needed to understand the basic principles that govern proton-conducting polymer performance in the electrodes. Key to the function of many proton-conducting membranes is the membrane nanophase structure. Much debate has occurred on the exact geometry of the hydrophilic and hydro phobic phase structure of NAFION®, but it remains clear that a
phase-separated structure on the nanoscale, with concentrated hydrophilic groups forming ionic domains for water and pro ton transport, is one of the important ingredients of a high-performance membrane as described by Osborn and Moore in Chapter 10.37. New block copolymer proton exchange membranes have attempted to emulate the phase-separated nanostructure of NAFION®, but the combi nation of ductility, high proton conductivity, and high water transport has proven difficult to replicate in non-PFSA materi als. As multiphase materials such as composites continue to be pursued for improvements over commercially available membranes, NAFION®’s phase-separated structure continues to be a model for how to design high-performing materials. As fuel cell systems are being pushed to their absolute limits of performance and durability, the degradation of the mem brane becomes an important factor in determining the overall lifetime of the device. Significant gains in lifetime of poly (perfluorosulfonic acid) membranes have been realized through stabilization of the backbone endgroups. Now that devices are operating over 10 000 h, other degradation points along the backbone and side chain have become the ‘weak links’ in membrane stability during fuel cell operation as described by Schiraldi and Savant in Chapter 10.38. While the stability of poly(perfluorosulfonic acid) materials is unpre cedented, the mechanistic studies conducted so far have set a template for how to attack the problem of degradation in poly (aromatic) membranes, which is an important topic for the future, as replacements for perfluorinated membranes are sought. Insight into the proton transport process in polymers has been developed by multiscale simulations as reviewed by Habenicht and Paddison in Chapter 10.39. Electronic structure calculations of polymer fragments and the hydration of the acidic group and proton have informed the community of the basic mechanisms of proton, water, and polymer structure and motion at the atomic level. Larger-scale simulations have been used to probe how the nanoscale morphology is devel oped in these materials and how the connectivity of the hydrophilic domains evolves with water content. The difficulty in describing explicit water molecules and the shuttling of protons within a water network has been addressed in small systems, but more work is required to expand these smaller simulations to length scales and time scales that are relevant to proton conductivity across micron-scale membranes. Capacitors will be needed for any highly efficient hybrid electrical storage or conversion system that incorporates a bat tery, fuel cell, or internal combustion engine. Polymer dielectric capacitors provide an important route to high energy density systems and have distinct advantages in processing and break down strength compared to traditional ceramic capacitors as detailed by Han and Wang in Chapter 10.40. Polymer films are also unique in that they have self-healing capabilities and are light and flexible which may provide interesting opportunities for new form factor devices. PVDF and nanocomposite-based dielectric polymer capacitors have greatly accelerated innova tions in this area in the last 10 years, but there are still innovations needed in methods to decrease loss and improve the energy density of these devices. We have also included a chapter on water treatment mem branes by Mickols in Chapter 10.41 as energy needs and clean water are inextricably linked. It takes energy to purify water and
Polymers in Energy Applications | Polymers in Energy Applications
clean water is needed in power plants for cooling and for biofuel production. While energy issues are front-and-center in 2011, the next great challenge will be clean water. Water treatment is a mature industry and new innovations in polymer membranes for reverse osmosis and nanofiltration have signif icantly reduced the cost of membrane-based water treatment and made reverse osmosis as the preferred technology for producing drinkable water from seawater. Innovations in the polymer chemistry of thin film composites, improvements in their chlorine resistance, and further optimization of materials processing parameters and systems design will continue to push reverse osmosis and membrane-mediated water treat ment technology to the fore of the water industry. Finally, electrolyzer technology as described by Mittelsteadt and Staser in Chapter 10.42 has not benefited from the large gains in performance that have been realized in fuel cells. Electrolyzer technology is the simplest method for producing hydrogen and is the most efficient method for generating hydrogen from non-fossil sources. Water electrolysis is
599
expensive due to the high cost of electricity and the high cost of the materials used in the devices. Commercialization of electrolyzers for hydrogen cooling of power plants, hydrogen gas generation for flame ionization detectors for gas chromato graphy, and process gases for metal processing will help to push the cost of this technology down for energy applications. While NAFION® is still the standard membrane in electrolyzer applications, new types of electrolyzers, such as the sulfur dioxide depolarized anode electrolyzer for the hybrid sulfur thermochemical cycle, require membranes with high thermal stability and low crossover. New electrolyzer membranes will provide a route for less expensive devices with higher perfor mance across a range of devices and operating conditions. In all, these chapters provide a detailed picture of where polymer science has contributed to the core understanding of electrolytes and dielectrics in electrochemical devices. These chapters build on the fundamental knowledge conveyed in the previous volumes and add to the message of sustainable polymers in the first part of Volume 10.
600
Polymers in Energy Applications | Polymers in Energy Applications
Biographical Sketches Michael A. Hickner received a BS in chemical engineering from Michigan Tech in 1999 and a MEng in 2002 and PhD in chemical engineering from Virginia Tech in 2003. In graduate school, he worked under the direction of James E. McGrath and also spent time in the fuel cell group at Los Alamos National Laboratory, developing novel aromatic proton exchange membranes for both hydrogen and direct methanol fuel cells. Before joining the Department of Materials Science and Engineering at Penn State in July 2007, he was a postdoctoral researcher and subsequently became a staff member at Sandia National Laboratories in Albuquerque, NM, where he conducted experimental investigations and modeling studies of liquid water transport in fuel cells and porous media, properties of ion-containing membranes, electrochemical reactors, and nanoporous membranes for water treatment applications. His research group at Penn State is focused on the synthesis and properties of ion-containing polymers, measurement of water–polymer interactions using spectroscopic techniques, and the study of self- and directed assembly of polymeric nanostructures for fast transport. He has ongoing projects in new polymer synthesis, fuel cells, batteries, water treatment membranes, and organic photovoltaic materials. He is currently an assistant professor and the Virginia S. and Philip L. Walker Jr. Faculty Fellow in the Materials Science Department at Penn State. Hickner’s work has been recognized by a Powe Junior Faculty Enhancement Award (2008), Young Investigator Awards from ONR and ARO (2008), a 3M Non-tenured Faculty Grant (2009), and a Presidential Early Career Award for Scientists and Engineers from President Obama in 2009. He has five US and international patents and over 60 peer-reviewed publications since 2001 that have been cited more than 2900 times as of 2011.
James E. McGrath received his BS in chemistry from Siena College in New York (1956) and his MS (1964) and PhD (1967) in polymer science from the University of Akron, where he worked on emulsion and anionic polymerization of synthetic rubbers, ozone cracking, and triblock copolymer thermoplastic elastomers. After 19 years in industry (Rayonier (cellulose), Goodyear (synthetic rubbers), and Union Carbide (engineering thermoplastics, polyolefins)], he joined the Chemistry Department at Virginia Tech in 1975. He is now Ethyl Chair and a University Distinguished Professor. He was director of the first group of NSF Science and Technology Centers from 1989 to 2000 on Structural Adhesives and Composites and focused on high-temperature polymers including polyimides, polysulfones, and toughened epoxy polymeric matrix resins for carbon fiber composites. He has many contributions to the anionic and ring-opening polymerization of dienes, epoxides, and organosiloxanes. His current focus is on polymeric materials for carbon fibers and membranes, including fuel cells, reverse osmosis water purification, and gas separation systems. He has 50 patents and over 500 publications and has received numerous awards, including election to the National Academy of Engineers (1994), The International SPE award, the Plastics Hall of Fame, and the ACS awards in Applied Polymer Science (2002) and Polymer Chemistry (2008). He has graduated more than 100 PhD chemists and engineers and remains one of the leaders in polymer science and engineering, with a current group (2011) of 13 students and postdoctoral fellows.