- Email: [email protected]
Membranes fit for a revolution
Membranes
fit for a revolution by George Marsh
Within the next few decades we will be driving cars powered by material ‘lumps’ that have no moving parts. Early fuel cell vehicles (FCVs), outriders of the hydrogen economy, are already with us, but only as costly concept prototypes. Affordable family FCVs remain out of reach, largely because a single crucial component at the heart of the ‘alternative engine’ has been exotic, easily damaged and, in common with other fuel cell materials, too expensive. With the emergence of a potential mass market, new answers are needed and it is falling to material scientists to supply them.
FCVs can potentially deliver 70% fuel-to-wheel conversion efficiencies – compared with 40% for hybrid electric (gasoline- or diesel-engine plus electric) vehicles, or the 30% maximum one can expect from a traditional internal combustion engine. They can be powered from totally renewable resources, their basic fuel – hydrogen – being available sustainably from water or from hydrocarbon precursor fuels. Emissions are minor and harmless. FCVs, therefore, hold out promise of a viable successor to fossil fuel-based transportation (Fig. 1). A challenge for materials scientists is to unlock these possibilities by developing fuel cell materials that are efficient, durable, and affordable. At the heart of fuel cells suitable for road vehicle ‘engines’ and a range of other applications is a thin static film-like element, the proton exchange membrane (PEM). PEM fuel cells operate on a cycle that is the inverse of the well known school experiment in which water is electrolyzed to form hydrogen and oxygen, 2H2 + O2 → 2H2O. The constituent reactions are as follows (Fig. 2), At the cathode: ½O2 + 2H+ + 2e- → H2O At the anode: H2 → 2H+ + 2eH2 can be derived from existing fuels such as natural gas, methanol, or gasoline, but the best long-term solution is to produce pure H2 from water by, for example, using heat from solar sources. The O2 can be atmospheric (not pure). The only waste product is environmentally harmless warm water.
38
March 2003
ISSN:1369 7021 © Elsevier Science Ltd 2003
APPLICATIONS FEATURE
The electrons, channeled through an external circuit, are the electric current that is converted to motive (or other) power. Electron flow has to be balanced by a corresponding proton flow through the electrolyte, from anode to cathode, to complete the electrochemical reaction. Central to the electrolyte is an ionic conductor, which acts as a barrier to everything except protons. Accordingly, while promoting maximum mobility for the protons, so that the overall reaction is not impeded, the PEM should bar the passage of fuel (H2), any contaminants within it such as carbon monoxide (CO) from precursor fuel, and water. The membrane and its qualities are therefore crucial to the fuel cell process.
Challenge to materials The challenge facing materials scientists is to engineer an affordable material that can fulfill these purposes effectively, while enduring prolonged exposure to the aggressive in-cell environment. The electrolyte membrane must resist oxidation, reduction, and hydrolysis. It must retain rigidity and other mechanical properties across a wide range of humidity and in temperatures approaching the boiling point of water. Indeed, some PEM fuel cells will have to be designed to operate at temperatures substantially higher than this. Since membranes are in close proximity to the electrodes, they have also to be able to resist attack from electrode materials and catalysts, while working productively with them. It must be possible to secure membranes, preferably with adhesive, to a support structure. Viable membrane materials will have to be compatible with volume-manufacturing processes. Although the first ever fuel cell (Fig. 3), devised by Sir William Grove in 1842, was very much a ‘reverse battery’ – complete with graphite
Fig. 1 The shape of future personal transportation might be this AUTOnomy concept car from General Motors. (Credit: General Motors.)
H2O Cathode 1
Anode
2 O2 + 2H +2e +
H2O
H2
2H+ +2e-
Fig. 2 Sub-reactions at fuel cell anodes add up to recombination of hydrogen and oxygen to form water – along with electricity and some heat. (Credit: Steven Holdcroft, Simon Fraser University.)
electrodes and sulfuric acid electrolyte – modern cells have as their active components layers of planar solid materials combined within a membrane-electrode assembly (MEA). These layered MEAs, each of which generates a particular potential drop, are placed in series in an MEA stack to build up the required voltage. For reasons of economical production, the ideal is to be able to produce these layered structures using continuous processes, preferably casting the material constituents as film. To meet the needs of transport and consumer markets, all these requirements must be met at costs very much less than those of today. Unfortunately, the membrane paragon to match this ideal does not exist. Some materials that come close are far too costly and may be short-lived outside particular closelycontrolled environments. NASA scientists seeking power sources for spacecraft back in the 1960s first experimented by modifying hydrocarbon backbones. Introducing sulfonate groups, by complexing with sulfonic acid, became a favored route since sulfonate sites turned out to be good ionic conductors. Crosslinked sulfonated polystyrenes showed promise but, to meet durability requirements in space, scientists turned to thermoplastics for their chemical, thermal, and physical resilience. A major focus was robust copolymers in which certain polymer blocks could be sulfonated, a process that leaves them weakened physically, while others were fluorinated to confer mechanical and thermal stability under fuel cell conditions. DuPont produced a sulfonated fluoropolymer for use on the American BIOS satellite. As a result, from the 1970s onwards, the company took a commanding lead with its Nafion™ material, which became – and remains to this day – the industry standard.
March 2003
39
APPLICATIONS FEATURE
Fig. 3 The William Grove fuel cell. (© The Royal Society.)
Subsequently, Asahi Chemicals and others have introduced similar products. Nafion copolymer has much to recommend it, being a good proton conductor with adequate mechanical and chemical properties, which, under favorable conditions, lasts well, resisting acids, bases, and oxidants. As a film-forming thermoplastic, it is easy to process and commercially available in several varieties. On the downside, it is sensitive to contaminants and permeable to methanol, the latter being used in some cells as a hydrogen precursor fuel. It loses performance if operated for long above 80°C. Over-hydration and excessive dehydration upset its morphology, causing it to swell disruptively in the first case, or shrink and form pinholes in the second. And, because its originators were primarily interested in power density and durability under orbital conditions, it is expensive. Figures around $600/m2 are often quoted, though this can vary according to purchasing ‘muscle’. Nevertheless, Nafion became the standard virtually by default, because it was available when little else was. It is still in demand for space and other highspecification applications but, despite its use in a few concept prototypes, cannot be seriously contemplated for everyday road vehicles or domestic applications. The effectiveness or otherwise of a membrane material is closely related to its microstructure. Acidic clusters (primarily sulfonate up to now) are key and much research has been
40
March 2003
directed at establishing how these clusters promote ion conduction. For example, Steven Holdcroft at Simon Fraser University in Canada has revealed the importance of the spacing between them1. He and his colleagues have shown how the clusters (Fig. 4), themselves only 10-100 nm in
Fig. 4 Transmission electron micrograph of a membrane showing acidic clusters. The spacing of these has a major influence on ionic conductivity. (Credit: Steven Holdcroft, Simon Fraser University.)
APPLICATIONS FEATURE
100 EW 306 EW 391
80
-Z'' (Ω)
EW 408 EW 470
60
Nafion 117
40 20 0 0
50
100
150
200
Z' (Ω) Fig. 5 Impedance plots for experimental membranes show that improvements in conductivity over Nafion are possible. (Credit: Steven Holdcroft, Simon Fraser University.)
diameter, can be spatially controlled to enhance conductivity. Holdcroft is also trying to understand the complex interactions that occur within the MEA at the interfaces between the membrane and electrodes. Using their results and knowledge of membrane morphology, the group has been able to improve on existing methods for selecting potential Nafion substitutes (impedance plots in Fig. 5 show that conductivity can be significantly improved) and have developed a rationale for membrane design. Other groups have shed light on the role played by moisture in ion conduction, and why Nafion-type membranes have a limiting range of hydration within which they can operate. A team at Uppsala University in Sweden, for example, have established that full proton transfer between sulfonate groups within the ionomer requires a water molecule to be associated with each group2. Significant drying, evidenced by the formation of crystals within the membrane, reduces proton transfer. Too much water, on the other hand, causes the membrane to swell and lose effectiveness. These findings parallel those of other workers, including noted polymer scientist James E. McGrath of Virginia Tech, who has likewise highlighted the combined role of sulfonate groups and water in promoting ionic conduction3. McGrath’s team has similarly observed how excessive hydration can upset material morphology, especially at high temperatures.
Mechanical properties A number of research groups have sought to engineer fluoropolymer materials that maintain their mechanical properties for longer or retain water better. One basic
approach is to reinforce the membrane with PTFE (Teflon®) fibers or fabric – as in products available from Asahi or US company WI Gore (GoreSelect). Such reinforced plastic solutions improve mechanical properties, and allow the preparation of thin (therefore, low-resistance) films that resist swelling and remain dimensionally stable. Fluoropolymer-coated glass fabrics are another option. A further possibility, examined by researchers particularly in Europe as well as the US, is to seek better polymer backbones. Robust thermoplastics such as polyetheretherketone (PEEK) and polyimides have proved amenable to sulfonation. Ballard, arguably the world’s leading fuel cell company, has membranes produced by sulfonating variants of PEEK thermoplastic, available commercially from Victrex. Use of such material, already produced in bulk, results in an affordable membrane having good mechanical, thermal, and chemical resistance properties, plus the potential ability to operate at higher temperatures. Other backbones have been investigated. A Russian group has developed sulfonated polyphenylquinoxalines4. Ethylenestyrene interpolymer, developed a few years ago by Dow Chemical (under the brand name Index), is the basis for an inexpensive class of membranes from Florida-based Dais Analytic, who partially sulfonate (30-60%) the cast film material using a proprietary process. Properties of this highly conductive material are manipulated to overcome deficiencies of thermal tolerance and longevity. Dais, with academic support from Virginia Commonwealth University, has also stepped back in time to revisit hydrocarbons5 and the class of materials space scientists abandoned four decades ago, polystyrenes. The company believes that alternatives to previously tried crosslinked polystyrenes could deliver attractive mechanical properties, at least for low temperature (up to 45°C) and pressure operation. Though these would always be less robust than perfluorinated polymers, Dais and its partners consider some loss of resilience may be acceptable as the price to pay for a system that avoids the complexities of active cooling, pressurization, and hydration mechanisms. Today Dais markets materials that exhibit high ionic conductivity along with mechanical properties adequate for modest operating temperatures and pressures. Basic membrane films can be produced at costs in the order of $20/m2. Other research teams have, on the other hand, persevered with the multiphase possibilities of copolymers. For example,
March 2003
41
APPLICATIONS FEATURE
Higher temperatures One option in PEM fuel cell design is to make an ally of higher temperatures, securing the benefits of faster electrochemical reactions and, therefore, higher currents. Once temperatures go above 100°C, though, systems are needed that can either operate under pressurized water conditions or support ionic conduction without water. This statement needs qualifying, since a team from Princeton University6 has found that introducing SiO2 into the nanostructures of perfluorinated membranes can help them retain water at higher operating temperatures. In fact, composite SiO2-Nafion membranes proved able to operate at 130°C or more, in contrast to unmodified Nafion 115 and recast Nafion membranes, both of which perform poorly and suffer irreversible heat damage above 100°C. Other inorganic additions may prove to have similar effects. As mentioned above, McGrath is principal investigator on a NASA Glenn-funded program to synthesize ion-containing, thermally stable polymers for use in PEM fuel cells intended
42
March 2003
=
O
O
Ar
O
S =
commercially available and affordable styrene/ethylenebutylene/styrene triblock polymers have been targeted. These contain a saturated carbon center block that resists sulfonation along with its mechanically weakening effect, while offering high ionic conductivity conferred by the sulfonated blocks. McGrath is keen to distinguish between generally used post-sulfonation techniques, which place acid radicals randomly on the polymer backbone, and his own team’s more targeted approach in which sulfonation of specific monomers at an earlier stage yields a conductivity-enhancing disulfonation benefit. Using such means to disperse sulfonate groups effectively throughout the copolymer it is possible, says McGrath, to optimize ion exchange capacity to a level just below that at which the membrane would start swelling because of excessive moisture absorption. McGrath’s novel research has attracted major backing, resulting in three major collaborative programs. One, funded by NASA Glenn Research Center (see below), still has a year to run (with a possible extension); a second, in collaboration with Los Alamos National Laboratory and funded by the Department of Energy, is nominally due to finish in October; and the third, for the Department of Defense, started in July and focuses on direct methanol and other fuel cell power for portable devices.
O
n
Fig. 6 Structure of poly(arylene ether sulfone) – one of a class of aromatic polymers that could substitute for sulfonated fluoropolymers. (Credit: James E. McGrath, Virginia Tech.)
for high temperature and humidity operation. Polymers shown to offer good conduction, durability, and film processability include certain wholly aromatic structures such as poly(arylene ether sulfone), as shown in Fig. 63. They have succeeded in introducing sulfonated ion-conducting sites both during direct polymerization and by post-sulfonation. Cast films of sulfonated materials have shown significant improvements over Nafion in terms of conductivity at ambient temperatures, while certain copolymers proved twice as conductive during prolonged operation at 120°C with pressurized water. Other researchers have expressed hope for the polyphosphazene platform. Using this, with sulfonimide side groups, a team from Penn State7 has delivered a combination of high proton conductivity and low small-molecule permeability. The phosphorus-nitrogen backbone is said to provide high thermal and chemical stability, while membrane properties can be optimized by subtle changes to the side group structure. Given bulk manufacture, the price should, it is claimed, be substantially less than Nafion. Despite a limited Tg, the material is seen as a candidate for PEM fuel cells, including direct methanol types, intended for operation at temperatures above 100°C. Another possibility – layered zirconium phosphonate – is also being investigated, notably by a team from Perugia University in Italy8. For operation at substantially higher temperatures, conduction mechanisms are needed that work without water. Tsuchida9 showed that this is possible, by demonstrating a non-hydrated polymer blend in conjunction with aromatic sulfonic acid, which yielded moderate conduction at 150°C. Complexing acids other than sulfonic provide yet another research avenue. Investigations of the use of heteropoly acids such as phosphotungstic acid (PTA), have demonstrated enhancement of both conductivity and mechanical modulus, yielding materials promising in the 150-170°C range. McGrath says that the resulting materials can be likened to film having an inorganic metal filler. With nanoparticles present, these materials can also be described as nanocomposites. Savinell and coworkers at Case Western
APPLICATIONS FEATURE
Reserve University10 achieved conductivity at high temperature using polybenzimidazoles (PBI) doped with phosphoric acid. PBI is, in any case, a promising backbone candidate because it is thermally tolerant, resistant to CO, and has interesting mechanical properties. PBI producer Celanese Ventures GmbH has developed membranes suitable for use at 150-190°C. Celanese Ventures is partnering Honda in the development of automotive fuel cells based on PBI membranes, and US company Plug Power for parallel development of cells for residential application. A Virginia Tech/Virginia Commonwealth University collaboration, benefiting from a strong background in phosphorus-based materials, has experimented with phosphine-containing polyimides, which complex easily with a number of potential ‘doping’ acids including sulfonic. Other phosphine derivatives, which complex readily with metals and probably with strong acids such as trifluoromethanesulfonic (Triflic), are among candidates for further investigation. The extent to which phosphorus and other complexing alternatives can deliver effective conducting sites is the subject of ongoing research.
Response to demand Meanwhile DuPont, conscious of the threat to its market dominance, is responding to the challenge with new developments to Nafion. It reacted to demand for thinner membranes, so that higher in-cell current densities could be achieved, by supplementing its standard 5 mil (127 µm) product with a 2 mil (51 µm) Nafion, and a 1 mil (26 µm) version is in the offing. The company is also working on
variants for use at higher temperatures. In the belief that multiple markets will represent a $10 billion industry within a decade, DuPont has formed a specific fuel cell business unit. Its stance over cost is that Nafion is not necessarily too expensive for the job it does and DuPont will aim to keep membrane cost levels down to the ~5% of total fuel cell cost that has been typical to date. But the quest for substitutes is accelerating. Membranes are a strong focus for the National Science Foundation’s ‘Advanced Materials for PEM-based Fuel Cell Systems’ partnership that includes Los Alamos National Laboratory, four universities (including Virginia Commonwealth and Virginia Tech) and several industrial partners – among them General Motors’ Global Alternative Propulsion Center, DuPont, BP, Motorola, Dais Analytic Corp., United Technologies Corp., Newport News Shipbuilding, Nanosonic Inc., Solvay Advanced Polymers, Acadia Elastomers Corp., and Air Products and Chemicals Inc. The Bush administration’s Freedom Cooperative Automotive Research (FreedomCAR) initiative also allies government and commercial interests to bring fuel cell powered vehicles to market. Parallel initiatives are under way in Europe and Japan. The attention gained by hydrogen-powered fuel cell concept cars, such as General Motors’ drive-by-wire AUTOnomy, demonstrates a public fascination with the technology that bodes well for its future. As McGrath aptly summarizes, “One of the big issues is making materials cheaper and less exotic than those NASA has been using for 30-40 years. Users would like such materials and as we develop them, we hope that some of our partners will commercialize them.” MT
REFERENCES
and their Use for Preparing either Filled Porous Membranes or Hybrid Ionomeric Membranes. ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’, Pacific Grove, California, February 23-27 2003*
1. Holdcroft, S., et al., J. New Mater. Electrochem. Sys. (2003), in press 2. Ludvigsson, M., (2000), Materials for future power sources, http://publications.uu.se/theses/91-554-4789-9/ 3. Wang, F., et al., J. Membrane Sci., (2002) 197, 231-242
9. Miyatake, K., et al., Macromolecules (1996) 29 (21), 6969
4. Rusanov, A., Preparation and Characterisation of Sulfonated Polyphenylquinoxalines. ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’, Pacific Grove, California, February 23-27 2003*
10. Savinell, R., Performance and Mechanistic Studies of the High Temperature PBI/phosphine PEM Membrane Electrolyte. ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’, Pacific Grove, California, February 23-27 2003*
5. Wnek, G., et al., Fuel Cells Bulletin (January 1999) 4, 6-8
*
6. Adjemian, K.T., et al., J. Electrochem. Soc. (2002) 149 (3) A256-A261 7. Hofmann, M. A., et al., Macromolecules (2002) 35, 6490-6493 8. Alberti, G., Layered Zirconium Phosphonates with High Protonic Conductivity
Details of the conference ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’ conference are available from James E. McGrath, Materials Research Institute and Department of Chemistry, Virginia Tech, 2108 Hahn Hall, Blacksberg VA 24061, USA. Email: [email protected] or www.visitasilomar.com
March 2003
43
fit for a revolution by George Marsh
Within the next few decades we will be driving cars powered by material ‘lumps’ that have no moving parts. Early fuel cell vehicles (FCVs), outriders of the hydrogen economy, are already with us, but only as costly concept prototypes. Affordable family FCVs remain out of reach, largely because a single crucial component at the heart of the ‘alternative engine’ has been exotic, easily damaged and, in common with other fuel cell materials, too expensive. With the emergence of a potential mass market, new answers are needed and it is falling to material scientists to supply them.
FCVs can potentially deliver 70% fuel-to-wheel conversion efficiencies – compared with 40% for hybrid electric (gasoline- or diesel-engine plus electric) vehicles, or the 30% maximum one can expect from a traditional internal combustion engine. They can be powered from totally renewable resources, their basic fuel – hydrogen – being available sustainably from water or from hydrocarbon precursor fuels. Emissions are minor and harmless. FCVs, therefore, hold out promise of a viable successor to fossil fuel-based transportation (Fig. 1). A challenge for materials scientists is to unlock these possibilities by developing fuel cell materials that are efficient, durable, and affordable. At the heart of fuel cells suitable for road vehicle ‘engines’ and a range of other applications is a thin static film-like element, the proton exchange membrane (PEM). PEM fuel cells operate on a cycle that is the inverse of the well known school experiment in which water is electrolyzed to form hydrogen and oxygen, 2H2 + O2 → 2H2O. The constituent reactions are as follows (Fig. 2), At the cathode: ½O2 + 2H+ + 2e- → H2O At the anode: H2 → 2H+ + 2eH2 can be derived from existing fuels such as natural gas, methanol, or gasoline, but the best long-term solution is to produce pure H2 from water by, for example, using heat from solar sources. The O2 can be atmospheric (not pure). The only waste product is environmentally harmless warm water.
38
March 2003
ISSN:1369 7021 © Elsevier Science Ltd 2003
APPLICATIONS FEATURE
The electrons, channeled through an external circuit, are the electric current that is converted to motive (or other) power. Electron flow has to be balanced by a corresponding proton flow through the electrolyte, from anode to cathode, to complete the electrochemical reaction. Central to the electrolyte is an ionic conductor, which acts as a barrier to everything except protons. Accordingly, while promoting maximum mobility for the protons, so that the overall reaction is not impeded, the PEM should bar the passage of fuel (H2), any contaminants within it such as carbon monoxide (CO) from precursor fuel, and water. The membrane and its qualities are therefore crucial to the fuel cell process.
Challenge to materials The challenge facing materials scientists is to engineer an affordable material that can fulfill these purposes effectively, while enduring prolonged exposure to the aggressive in-cell environment. The electrolyte membrane must resist oxidation, reduction, and hydrolysis. It must retain rigidity and other mechanical properties across a wide range of humidity and in temperatures approaching the boiling point of water. Indeed, some PEM fuel cells will have to be designed to operate at temperatures substantially higher than this. Since membranes are in close proximity to the electrodes, they have also to be able to resist attack from electrode materials and catalysts, while working productively with them. It must be possible to secure membranes, preferably with adhesive, to a support structure. Viable membrane materials will have to be compatible with volume-manufacturing processes. Although the first ever fuel cell (Fig. 3), devised by Sir William Grove in 1842, was very much a ‘reverse battery’ – complete with graphite
Fig. 1 The shape of future personal transportation might be this AUTOnomy concept car from General Motors. (Credit: General Motors.)
H2O Cathode 1
Anode
2 O2 + 2H +2e +
H2O
H2
2H+ +2e-
Fig. 2 Sub-reactions at fuel cell anodes add up to recombination of hydrogen and oxygen to form water – along with electricity and some heat. (Credit: Steven Holdcroft, Simon Fraser University.)
electrodes and sulfuric acid electrolyte – modern cells have as their active components layers of planar solid materials combined within a membrane-electrode assembly (MEA). These layered MEAs, each of which generates a particular potential drop, are placed in series in an MEA stack to build up the required voltage. For reasons of economical production, the ideal is to be able to produce these layered structures using continuous processes, preferably casting the material constituents as film. To meet the needs of transport and consumer markets, all these requirements must be met at costs very much less than those of today. Unfortunately, the membrane paragon to match this ideal does not exist. Some materials that come close are far too costly and may be short-lived outside particular closelycontrolled environments. NASA scientists seeking power sources for spacecraft back in the 1960s first experimented by modifying hydrocarbon backbones. Introducing sulfonate groups, by complexing with sulfonic acid, became a favored route since sulfonate sites turned out to be good ionic conductors. Crosslinked sulfonated polystyrenes showed promise but, to meet durability requirements in space, scientists turned to thermoplastics for their chemical, thermal, and physical resilience. A major focus was robust copolymers in which certain polymer blocks could be sulfonated, a process that leaves them weakened physically, while others were fluorinated to confer mechanical and thermal stability under fuel cell conditions. DuPont produced a sulfonated fluoropolymer for use on the American BIOS satellite. As a result, from the 1970s onwards, the company took a commanding lead with its Nafion™ material, which became – and remains to this day – the industry standard.
March 2003
39
APPLICATIONS FEATURE
Fig. 3 The William Grove fuel cell. (© The Royal Society.)
Subsequently, Asahi Chemicals and others have introduced similar products. Nafion copolymer has much to recommend it, being a good proton conductor with adequate mechanical and chemical properties, which, under favorable conditions, lasts well, resisting acids, bases, and oxidants. As a film-forming thermoplastic, it is easy to process and commercially available in several varieties. On the downside, it is sensitive to contaminants and permeable to methanol, the latter being used in some cells as a hydrogen precursor fuel. It loses performance if operated for long above 80°C. Over-hydration and excessive dehydration upset its morphology, causing it to swell disruptively in the first case, or shrink and form pinholes in the second. And, because its originators were primarily interested in power density and durability under orbital conditions, it is expensive. Figures around $600/m2 are often quoted, though this can vary according to purchasing ‘muscle’. Nevertheless, Nafion became the standard virtually by default, because it was available when little else was. It is still in demand for space and other highspecification applications but, despite its use in a few concept prototypes, cannot be seriously contemplated for everyday road vehicles or domestic applications. The effectiveness or otherwise of a membrane material is closely related to its microstructure. Acidic clusters (primarily sulfonate up to now) are key and much research has been
40
March 2003
directed at establishing how these clusters promote ion conduction. For example, Steven Holdcroft at Simon Fraser University in Canada has revealed the importance of the spacing between them1. He and his colleagues have shown how the clusters (Fig. 4), themselves only 10-100 nm in
Fig. 4 Transmission electron micrograph of a membrane showing acidic clusters. The spacing of these has a major influence on ionic conductivity. (Credit: Steven Holdcroft, Simon Fraser University.)
APPLICATIONS FEATURE
100 EW 306 EW 391
80
-Z'' (Ω)
EW 408 EW 470
60
Nafion 117
40 20 0 0
50
100
150
200
Z' (Ω) Fig. 5 Impedance plots for experimental membranes show that improvements in conductivity over Nafion are possible. (Credit: Steven Holdcroft, Simon Fraser University.)
diameter, can be spatially controlled to enhance conductivity. Holdcroft is also trying to understand the complex interactions that occur within the MEA at the interfaces between the membrane and electrodes. Using their results and knowledge of membrane morphology, the group has been able to improve on existing methods for selecting potential Nafion substitutes (impedance plots in Fig. 5 show that conductivity can be significantly improved) and have developed a rationale for membrane design. Other groups have shed light on the role played by moisture in ion conduction, and why Nafion-type membranes have a limiting range of hydration within which they can operate. A team at Uppsala University in Sweden, for example, have established that full proton transfer between sulfonate groups within the ionomer requires a water molecule to be associated with each group2. Significant drying, evidenced by the formation of crystals within the membrane, reduces proton transfer. Too much water, on the other hand, causes the membrane to swell and lose effectiveness. These findings parallel those of other workers, including noted polymer scientist James E. McGrath of Virginia Tech, who has likewise highlighted the combined role of sulfonate groups and water in promoting ionic conduction3. McGrath’s team has similarly observed how excessive hydration can upset material morphology, especially at high temperatures.
Mechanical properties A number of research groups have sought to engineer fluoropolymer materials that maintain their mechanical properties for longer or retain water better. One basic
approach is to reinforce the membrane with PTFE (Teflon®) fibers or fabric – as in products available from Asahi or US company WI Gore (GoreSelect). Such reinforced plastic solutions improve mechanical properties, and allow the preparation of thin (therefore, low-resistance) films that resist swelling and remain dimensionally stable. Fluoropolymer-coated glass fabrics are another option. A further possibility, examined by researchers particularly in Europe as well as the US, is to seek better polymer backbones. Robust thermoplastics such as polyetheretherketone (PEEK) and polyimides have proved amenable to sulfonation. Ballard, arguably the world’s leading fuel cell company, has membranes produced by sulfonating variants of PEEK thermoplastic, available commercially from Victrex. Use of such material, already produced in bulk, results in an affordable membrane having good mechanical, thermal, and chemical resistance properties, plus the potential ability to operate at higher temperatures. Other backbones have been investigated. A Russian group has developed sulfonated polyphenylquinoxalines4. Ethylenestyrene interpolymer, developed a few years ago by Dow Chemical (under the brand name Index), is the basis for an inexpensive class of membranes from Florida-based Dais Analytic, who partially sulfonate (30-60%) the cast film material using a proprietary process. Properties of this highly conductive material are manipulated to overcome deficiencies of thermal tolerance and longevity. Dais, with academic support from Virginia Commonwealth University, has also stepped back in time to revisit hydrocarbons5 and the class of materials space scientists abandoned four decades ago, polystyrenes. The company believes that alternatives to previously tried crosslinked polystyrenes could deliver attractive mechanical properties, at least for low temperature (up to 45°C) and pressure operation. Though these would always be less robust than perfluorinated polymers, Dais and its partners consider some loss of resilience may be acceptable as the price to pay for a system that avoids the complexities of active cooling, pressurization, and hydration mechanisms. Today Dais markets materials that exhibit high ionic conductivity along with mechanical properties adequate for modest operating temperatures and pressures. Basic membrane films can be produced at costs in the order of $20/m2. Other research teams have, on the other hand, persevered with the multiphase possibilities of copolymers. For example,
March 2003
41
APPLICATIONS FEATURE
Higher temperatures One option in PEM fuel cell design is to make an ally of higher temperatures, securing the benefits of faster electrochemical reactions and, therefore, higher currents. Once temperatures go above 100°C, though, systems are needed that can either operate under pressurized water conditions or support ionic conduction without water. This statement needs qualifying, since a team from Princeton University6 has found that introducing SiO2 into the nanostructures of perfluorinated membranes can help them retain water at higher operating temperatures. In fact, composite SiO2-Nafion membranes proved able to operate at 130°C or more, in contrast to unmodified Nafion 115 and recast Nafion membranes, both of which perform poorly and suffer irreversible heat damage above 100°C. Other inorganic additions may prove to have similar effects. As mentioned above, McGrath is principal investigator on a NASA Glenn-funded program to synthesize ion-containing, thermally stable polymers for use in PEM fuel cells intended
42
March 2003
=
O
O
Ar
O
S =
commercially available and affordable styrene/ethylenebutylene/styrene triblock polymers have been targeted. These contain a saturated carbon center block that resists sulfonation along with its mechanically weakening effect, while offering high ionic conductivity conferred by the sulfonated blocks. McGrath is keen to distinguish between generally used post-sulfonation techniques, which place acid radicals randomly on the polymer backbone, and his own team’s more targeted approach in which sulfonation of specific monomers at an earlier stage yields a conductivity-enhancing disulfonation benefit. Using such means to disperse sulfonate groups effectively throughout the copolymer it is possible, says McGrath, to optimize ion exchange capacity to a level just below that at which the membrane would start swelling because of excessive moisture absorption. McGrath’s novel research has attracted major backing, resulting in three major collaborative programs. One, funded by NASA Glenn Research Center (see below), still has a year to run (with a possible extension); a second, in collaboration with Los Alamos National Laboratory and funded by the Department of Energy, is nominally due to finish in October; and the third, for the Department of Defense, started in July and focuses on direct methanol and other fuel cell power for portable devices.
O
n
Fig. 6 Structure of poly(arylene ether sulfone) – one of a class of aromatic polymers that could substitute for sulfonated fluoropolymers. (Credit: James E. McGrath, Virginia Tech.)
for high temperature and humidity operation. Polymers shown to offer good conduction, durability, and film processability include certain wholly aromatic structures such as poly(arylene ether sulfone), as shown in Fig. 63. They have succeeded in introducing sulfonated ion-conducting sites both during direct polymerization and by post-sulfonation. Cast films of sulfonated materials have shown significant improvements over Nafion in terms of conductivity at ambient temperatures, while certain copolymers proved twice as conductive during prolonged operation at 120°C with pressurized water. Other researchers have expressed hope for the polyphosphazene platform. Using this, with sulfonimide side groups, a team from Penn State7 has delivered a combination of high proton conductivity and low small-molecule permeability. The phosphorus-nitrogen backbone is said to provide high thermal and chemical stability, while membrane properties can be optimized by subtle changes to the side group structure. Given bulk manufacture, the price should, it is claimed, be substantially less than Nafion. Despite a limited Tg, the material is seen as a candidate for PEM fuel cells, including direct methanol types, intended for operation at temperatures above 100°C. Another possibility – layered zirconium phosphonate – is also being investigated, notably by a team from Perugia University in Italy8. For operation at substantially higher temperatures, conduction mechanisms are needed that work without water. Tsuchida9 showed that this is possible, by demonstrating a non-hydrated polymer blend in conjunction with aromatic sulfonic acid, which yielded moderate conduction at 150°C. Complexing acids other than sulfonic provide yet another research avenue. Investigations of the use of heteropoly acids such as phosphotungstic acid (PTA), have demonstrated enhancement of both conductivity and mechanical modulus, yielding materials promising in the 150-170°C range. McGrath says that the resulting materials can be likened to film having an inorganic metal filler. With nanoparticles present, these materials can also be described as nanocomposites. Savinell and coworkers at Case Western
APPLICATIONS FEATURE
Reserve University10 achieved conductivity at high temperature using polybenzimidazoles (PBI) doped with phosphoric acid. PBI is, in any case, a promising backbone candidate because it is thermally tolerant, resistant to CO, and has interesting mechanical properties. PBI producer Celanese Ventures GmbH has developed membranes suitable for use at 150-190°C. Celanese Ventures is partnering Honda in the development of automotive fuel cells based on PBI membranes, and US company Plug Power for parallel development of cells for residential application. A Virginia Tech/Virginia Commonwealth University collaboration, benefiting from a strong background in phosphorus-based materials, has experimented with phosphine-containing polyimides, which complex easily with a number of potential ‘doping’ acids including sulfonic. Other phosphine derivatives, which complex readily with metals and probably with strong acids such as trifluoromethanesulfonic (Triflic), are among candidates for further investigation. The extent to which phosphorus and other complexing alternatives can deliver effective conducting sites is the subject of ongoing research.
Response to demand Meanwhile DuPont, conscious of the threat to its market dominance, is responding to the challenge with new developments to Nafion. It reacted to demand for thinner membranes, so that higher in-cell current densities could be achieved, by supplementing its standard 5 mil (127 µm) product with a 2 mil (51 µm) Nafion, and a 1 mil (26 µm) version is in the offing. The company is also working on
variants for use at higher temperatures. In the belief that multiple markets will represent a $10 billion industry within a decade, DuPont has formed a specific fuel cell business unit. Its stance over cost is that Nafion is not necessarily too expensive for the job it does and DuPont will aim to keep membrane cost levels down to the ~5% of total fuel cell cost that has been typical to date. But the quest for substitutes is accelerating. Membranes are a strong focus for the National Science Foundation’s ‘Advanced Materials for PEM-based Fuel Cell Systems’ partnership that includes Los Alamos National Laboratory, four universities (including Virginia Commonwealth and Virginia Tech) and several industrial partners – among them General Motors’ Global Alternative Propulsion Center, DuPont, BP, Motorola, Dais Analytic Corp., United Technologies Corp., Newport News Shipbuilding, Nanosonic Inc., Solvay Advanced Polymers, Acadia Elastomers Corp., and Air Products and Chemicals Inc. The Bush administration’s Freedom Cooperative Automotive Research (FreedomCAR) initiative also allies government and commercial interests to bring fuel cell powered vehicles to market. Parallel initiatives are under way in Europe and Japan. The attention gained by hydrogen-powered fuel cell concept cars, such as General Motors’ drive-by-wire AUTOnomy, demonstrates a public fascination with the technology that bodes well for its future. As McGrath aptly summarizes, “One of the big issues is making materials cheaper and less exotic than those NASA has been using for 30-40 years. Users would like such materials and as we develop them, we hope that some of our partners will commercialize them.” MT
REFERENCES
and their Use for Preparing either Filled Porous Membranes or Hybrid Ionomeric Membranes. ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’, Pacific Grove, California, February 23-27 2003*
1. Holdcroft, S., et al., J. New Mater. Electrochem. Sys. (2003), in press 2. Ludvigsson, M., (2000), Materials for future power sources, http://publications.uu.se/theses/91-554-4789-9/ 3. Wang, F., et al., J. Membrane Sci., (2002) 197, 231-242
9. Miyatake, K., et al., Macromolecules (1996) 29 (21), 6969
4. Rusanov, A., Preparation and Characterisation of Sulfonated Polyphenylquinoxalines. ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’, Pacific Grove, California, February 23-27 2003*
10. Savinell, R., Performance and Mechanistic Studies of the High Temperature PBI/phosphine PEM Membrane Electrolyte. ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’, Pacific Grove, California, February 23-27 2003*
5. Wnek, G., et al., Fuel Cells Bulletin (January 1999) 4, 6-8
*
6. Adjemian, K.T., et al., J. Electrochem. Soc. (2002) 149 (3) A256-A261 7. Hofmann, M. A., et al., Macromolecules (2002) 35, 6490-6493 8. Alberti, G., Layered Zirconium Phosphonates with High Protonic Conductivity
Details of the conference ‘Advances in Materials for Proton Exchange Membrane Fuel Cell Systems’ conference are available from James E. McGrath, Materials Research Institute and Department of Chemistry, Virginia Tech, 2108 Hahn Hall, Blacksberg VA 24061, USA. Email: [email protected] or www.visitasilomar.com
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