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Trends for fuel cell membrane development
Desalination 250 (2010) 1034–1037
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Trends for fuel cell membrane development☆ Lorenz Gubler ⁎, Günther G. Scherer Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
a r t i c l e
i n f o
Available online 15 October 2009 Keywords: Polymer electrolyte fuel cell (PEFC) Proton exchange membrane (PEM) Block coplymer Graft copolymer Polymer blend
a b s t r a c t Fuel cells are considered a promising energy conversion technology of the future owing to inherent advantages of electrochemical conversion over thermal combustion processes. In the polymer electrolyte fuel cell (PEFC) a proton-conducting polymer membrane is utilized as solid electrolyte, having to allow the transport of protons from anode to cathode yet block the passage of reactants (e.g. H2, O2) and electrons. Although PEFC technology has matured substantially over the past two decades, technological barriers, such as insufficient durability and high cost, still delay commercialization in many applications. In this contribution, we review current fuel cell membrane technology and outline approaches that are taken to improve the functionality as well as the chemical and mechanical stability of proton conducting polymers in fuel cells. © 2009 Elsevier B.V. All rights reserved.
1. Low temperature fuel cells Fuel cells are clean and efficient electrochemical energy conversion reactors. Among the various fuel cell types under consideration, with operating temperatures ranging up to 1000 °C for the solid oxide fuel cell (SOFC), the polymer electrolyte fuel cell (PEFC) is particularly attractive for applications with variable load profile and intermittent operation (portable electronics, remote power sources, vehicle propulsion). The PEFC typically operates at temperatures between 60 and 100 °C, in specific configurations around 180 °C. In a PEFC, the electrochemical reaction of a fuel – typically H2 – and O2, commonly taken from ambient air, takes place in two half-cell reactions separated by an electrolyte, which is a polymeric membrane with a thickness of 20 to 200 μm and proton conductivity on the order of 0.1 S cm− 1. Methanol can also be used as fuel in the direct methanol fuel cell (DMFC), having the advantage of easy storage, yet with a lower power density compared to the fuel cell operated on H2. The electrodes consist of porous carbon fiber layer structures with a thickness of a few hundred micrometers (Fig. 1). The hydrogen oxidation and oxygen reduction reactions take place at the anode–membrane and cathode–membrane interface, respectively, within the active layer (thickness ~ 10 μm) containing highly dispersed Pt nanoparticles, supported on high surface area carbon, as electrocatalyst [1]. In the PEFC, the solid polymer membrane has to fulfill several functions, with concomitant requirements for candidate ion-conducting materials:
☆ Presented at the 12th Aachener Membrane Kolloquium, Aachen, Germany, 29-30 October, 2008. ⁎ Corresponding author. Tel.: +41 56 310 2673; fax: +41 56 310 4416. E-mail address: [email protected] (L. Gubler). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.101
Requirements ❑ transport of protons ❑ electrical insulator ❑ oxygen and hydrogen gas barrier
Materials ❑ chemically stable (against HO / HOO radicals) ❑ thermally stable ❑ mechanically robust
Furthermore, the membrane may be part of the gasket system, requiring certain specific mechanical properties. This ensemble of specifications calls for a comprehensive approach in the membrane development for fuel cell application.
2. Membrane materials for fuel cells In general, PEFC electrolytes are classified into acidic or basic types, yet the acidic proton exchange membrane (PEM) is most widely used owing to its superior chemical stability. Generation of mobile protons is accomplished via the introduction of acid sites into the polymer, yielding an ‘ionomer’. As it is desired to have an acid with high dissociation constant, sulfonic acid is predominantly used for this purpose. Ion exchange membranes are widely used in separation technology (desalination, dialysis, filtration, etc.) and electrochemical processes (electrolysis, fuel cells) [2]. The PEM in the PEFC is subjected to particularly aggressive conditions, because the interaction of H2 and O2 on the surface of the Pt catalyst leads to the formation of aggressive HO• and HOO• radical species, which attack the polymer and lead to chain scission and thus membrane degradation [3]. For this reason, the majority of membranes used in fuel cells are perfluorosulfonic acid (PFSA) type membranes, known under the tradenames Nafion®, Flemion®, Hyflon® Ion, etc. Substantial efforts are devoted by the academia and industry to the development of novel fuel cell membrane materials, driven by the need for membranes with improved functionality (e.g., conductivity,
L. Gubler, G.G. Scherer / Desalination 250 (2010) 1034–1037
Fig. 1. Operating principle of a H2 / O2 fuel cell with acidic electrolyte membrane. Protons are transported from anode to cathode, where water is formed.
robustness) and more cost-efficient polymers [4]. Promising candidate materials are partially fluorinated or non-fluorinated ionomers containing aromatic units with attached –SO3H groups, either in the main polymer chain, or attached to an aliphatic main chain (Fig. 2). Examples of more ‘exotic’ polymers are polyphosphazenes and silicones. 3. Structure–property relationship The sulfonic acid based PEMs have a common characteristic: dissociation and formation of mobile protons require the presence of water, acting as ‘proton solvent’. The structure of water swollen PEMs,
1035
especially Nafion®, has been subject to numerous investigations. It has been established that Nafion®, as well as other well-working PEMs, have a nano-phase-separated structure, with a ‘polymer’ phase and an ‘aqueous’ phase [5,6]. The proton conduction takes place within water channels lined with sulfonate counter-ions (Fig. 3a). As a consequence, the mobility of the proton is strongly affected by the water content (Fig. 3b). For Nafion® a more or less linear increase of conductivity is observed as a function of membrane water content. The dissimilar structure–property relationship for sPEEK membranes leads to a different dependence of conductivity on water content [7]. PFSA type membranes, such as Nafion®, are copolymers of tetrafluoroethylene and a monomer with pendant –SO2F group, which is later hydrolyzed to yield –SO3H. Due to the different chemical nature of the PTFE-like backbone and the pendant acidbearing chains, the hydrated ionomer forms the phase-separated structure depicted in Fig. 3a. For alternative ionomer materials under investigation, it is important to impart the polymer with a property that will lead to the spontaneous formation of a multi-phase structure. This may be achieved by forming block copolymers with hydrophobic and hydrophilic blocks [9,10] (Fig. 4). Another approach to obtain phase-separated polymers is the formation of graft copolymers. Grafting involves the introduction of a polymer constituent into a pre-existing polymer film. Grafting can be accomplished by irradiating the base film to introduce radicals, which will serve as starting points for the growth of the grafted polymer [11]. Radiation grafting allows the formation of polymer combinations that are impossible to obtain with other methods, owing to the incompatibility of the constituents, e.g., a hydrophobic and a hydrophilic component. The process offers a broad range of possibilities to design the polymer architecture via careful choice of the base film, grafting monomers, irradiation and grafting conditions [12]. In yet another approach, polymers in solution with different target functionalities may be mixed and cast to yield a polymer blend [13]. One component,
Fig. 2. Polymeric materials used to form proton exchange membranes for fuel cells. The acid functionality is predominantly provided by sulfonic acid.
1036
L. Gubler, G.G. Scherer / Desalination 250 (2010) 1034–1037
Fig. 3. a) Nano-phase-separated structure of proton exchange membranes such as Nafion® (courtesy of K.D. Kreuer, MPI für Festkörperforschung, Stuttgart, Germany); b) proton conductivity of Nafion® membranes and a sulfonated PEEK membrane as a function of water content [8].
such as PVDF, may provide the mechanical integrity of the membrane, whereas the second component is an ionomer. Besides providing a proton transport medium, fuel cell membranes have to be mechanically robust. Fuel cells are mechanically compacted, typically in a filter-press configuration, to minimize contact resistance and prevent reactant leakage to the outside. Often, the membrane is part of the gasket arrangement for sealing purposes. Therefore, the polymer has to be ductile, have high strength and fracture toughness, as well as exhibit a low propensity to undergo creep deformation and be dimensionally stable. The mechanical properties are crucial, as mechanical membrane failure leads to catastrophic cell failure, yet largely underrated in the scientific literature. As an example, the tensile properties of a 25 μm thick Nafion® membrane are shown in Fig. 5a. The tensile strength decreases significantly with increasing temperature and level of hydration.
4. Challenges and future directions The relatively low mechanical strength of a hydrated PEM, and the fact that the membrane absorbs water, leading to residual stress in the membrane confined in the fuel cell hardware, is one of the major shortcomings. Therefore, reinforced membranes are being developed, consisting of a porous substrate of high strength, such as expanded PTFE (Fig. 6a) [14,15], or UHDPE [16,17], which is filled with an ionomer. The composite membrane is mechanically more resilient
and dimensionally stable, yielding substantially improved fuel cell lifetime. The requirement of absorbed water within the polymer structure to obtain mobile protons and the concomitant strong dependence of conductivity on relative humidity is another shortcoming of sulfonic acid based PEMs. Fuel cell applications such as remote power or car propulsion demand operating temperatures above 100 °C for system simplification reasons (e.g., smaller cooling units). However, active humidification of the fuel cell and membrane is prohibitive at these temperatures, requiring it to operate at lower relative humidity. Increasing the ionic content (decreasing the equivalent weight) is somewhat successful (Fig. 5b), yet at the expense of mechanical stability and durability. An approach to increase the water retention of the polymer at high temperature and low relative humidity is to include hydrophilic additives into the ionomer, such as phosphotungstic acid (Fig. 6b), Zr phosph(on)ates, SiO2 or TiO2 [18–20], which may lead to better cell performance if the composition and morphology of the additive and composite are well-designed. An entirely different approach is that of ‘water-free’ acid-doped polymers for operating at temperatures far beyond 100 °C. Water as the proton solvent is thereby replaced by the acid. In this realm, phosphoric acid (H3PO4) doped polybenzimidazole (PBI) membranes constitute the most advanced technology [21]. Membranes with extremely high H3PO4 content of 85% by weight are obtained through a sol–gel process [22]. The heterocycle of the PBI is involved in the proton transport process, as it provides free electron pairs for proton binding (Fig. 6c). The operating temperature of H3PO4 / PBI based PEFCs is between 160 and 200 °C. Although the material is essentially an intrinsic proton conductor, the presence of water greatly improves conductivity. One difficulty faced in the operation of such membranes is the transition through the liquid water regime below 100 °C during startup and shutdown, which leads to a leaching-out of the acid if water condensation is allowed to happen. 5. Conclusion
Fig. 4. Possible polymer configurations to facilitate the formation of a phase-separated structure.
Proton exchange membranes (PEMs) for fuel cells are required to transport protons from anode to cathode and block the passage of electrons and reactants. Due to the aggressive chemical environment, perfluorosulfonic acid membranes (e.g., Nafion®) are widely used in
L. Gubler, G.G. Scherer / Desalination 250 (2010) 1034–1037
1037
Fig. 5. a) Mechanical properties of a 25 μm Nafion membrane under different conditions of temperature and relative humidity (adapted from R. Solasi et al., J. Power Sources 167, 2007, 366, Copyright (2007), with permission from Elsevier); b) proton conductivity of perfluorinated membranes with different equivalent weight (EW) (source: C.S. Gittleman, General Motors, presentation at the DOE High Temperature Membrane Working Group, May 18, Washington, DC, 2009. Also available on the World Wide Web: http://www1.eere. energy.gov/hydrogenandfuelcells/pdfs/htmwg_may09_automotive_perspective.pdf).
Fig. 6. a) Structure of expanded PTFE (porosity 85%) used as substrate for composite membranes (Reprinted from Tang et al. [15], Copyright (2007), with permission from Elsevier); b) example of an inorganic material (phosphotungstic acid) that is incorporated into the ionomer for improved conductivity at high temperature (~120 °C)/low r.h.; c) proton conduction mechanism in phosphoric acid-doped polybenzimidazole (PBI) membranes for operating temperatures of ~180 °C.
fuel cells. There are ranges of novel and potentially cost-effective partially fluorinated or non-fluorinated ionomers under development with interesting properties. PEMs conduct protons when hydrated. To maximize conductivity at minimum water content, adequate control of polymer structure and morphology is required. Improved membrane designs involve reinforced polymers for higher mechanical robustness and inclusion of hydrophilic additives to improve water retention at low relative humidity. Water-free proton conduction is highly desirable and can be achieved in phosphoric acid-doped polymers operating at temperatures around 180 °C. Although substantial advances have been made in fuel cell membrane technology, a number of barriers hampering wide-spread commercialization remain to be overcome: conductivity of 0.1 S cm− 1 under hot and dry conditions, mechanical and chemical durability under fuel cell conditions, and last but not least: cost. References [1] G. Hoogers, Fuel Cell Technology Handbook, CRC Press, 2003. [2] S.P. Nunes, K.V. Peinemann, Membrane Technology in Chemical Industry, Wiley-VCH, 2001. [3] A.B. LaConti, H. Hamdan, R.C. McDonald, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), ‘Mechanisms of Membrane Degradation’, in Handbook of Fuel Cells—Fundamentals, Technology and Applications, Volume 3, John Wiley & Sons, 2003, pp. 647–662. [4] B. Smitha, S. Sridhar, A.A. Khan, J. Membr. Sci. 259 (2005) 10–26. [5] K.A. Mauritz, R.B. Moore, Chem. Rev. 104 (2004) 4535–4585. [6] K. Schmidt-Rohr, Q. Chen, Nature Materials 7 (2008) 75–83.
[7] K.D. Kreuer, J. Membr. Sci. 185 (2001) 29–39. [8] L. Gubler, T. Finsterwald, G.G. Scherer, PSI Electrochemistry Laboratory—Annual Report 2004, p. 111. Also available on the World Wide Web: http://ecl.web.psi.ch/ SciRep.html. [9] C.K. Shin, G. Maier, B. Andreaus, G.G. Scherer, J. Membr. Sci. 245 (2004) 147–161. [10] A. Roy, M.A. Hickner, X. Yu, Y. Li, T.E. Glass, J.E. McGrath, J. Polym. Sci. Part B: Polym. Phys. 44 (2006) 2226–2239. [11] L. Gubler, S. Alkan Gürsel, G.G. Scherer, Fuel Cells 5 (2005) 317–335. [12] S. Alkan Gürsel, L. Gubler, B. Gupta, G.G. Scherer, Adv. Polym. Sci. 215 (2008) 157–218. [13] L. Hedhli, L. Billon, Fluoropolymer resins containing ionic or ionizable groups and products containing the same, US Patent 6,780,935, Atofina Chemicals, 2004. [14] S.J.C. Cleghorn, D.K. Mayfield, D.A. Moore, J.C. Moore, G. Rusch, T.W. Sherman, N.T. Sisofo, U. Beuscher, J. Power Sources 158 (2006) 446–454. [15] H. Tang, M. Pan, S.P. Jiang, X. Wang, Y. Ruan, Electrochim. acta 52 (2007) 5304–5311. [16] D.E. Barnwell, P.J. Bouwman, Process for preparing a composite membrane, WO 2007/034233, Johnson Matthey plc, 2007. [17] T.R. Ralph, D.E. Barnwell, P.J. Bouwman, A.J. Hodgkinson, M.I. Petch, M. Pollington, J. Electrochem. Soc. 155 (2008) B411–B422. [18] J.-C. Lin, H.R. Kunz, J.M. Fenton, Membrane/electrode additives for low-humidification operation, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of Fuel Cells— Fundamentals, Technology and Applications, Volume 3, John Wiley & Sons, 2003, pp. 456–463. [19] D.J. Jones, J. Rozière, Inorganic/organic composite membranes, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of Fuel Cells—Fundamentals, Technology and Applications, Volume 3, John Wiley & Sons, 2003, pp. 447–455. [20] E. Chalkova, M.B. Pague, M.V. Fedkin, D.J. Wesolowski, S.N. Lvov, J. Electrochem. Soc. 152 (2005) A1035–A1040. [21] T.J. Schmidt, J. Baurmeister, Electrochem. Soc. Trans. 3 (1) (2006) 861–869. [22] L. Xiao, H. Zhang, E. Scanlon, L.S. Ramanathan, E.-W. Choe, D. Rogers, T. Apple, B.C. Benicewicz, Chem. Mater. 17 (2005) 5328–5333.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Trends for fuel cell membrane development☆ Lorenz Gubler ⁎, Günther G. Scherer Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
a r t i c l e
i n f o
Available online 15 October 2009 Keywords: Polymer electrolyte fuel cell (PEFC) Proton exchange membrane (PEM) Block coplymer Graft copolymer Polymer blend
a b s t r a c t Fuel cells are considered a promising energy conversion technology of the future owing to inherent advantages of electrochemical conversion over thermal combustion processes. In the polymer electrolyte fuel cell (PEFC) a proton-conducting polymer membrane is utilized as solid electrolyte, having to allow the transport of protons from anode to cathode yet block the passage of reactants (e.g. H2, O2) and electrons. Although PEFC technology has matured substantially over the past two decades, technological barriers, such as insufficient durability and high cost, still delay commercialization in many applications. In this contribution, we review current fuel cell membrane technology and outline approaches that are taken to improve the functionality as well as the chemical and mechanical stability of proton conducting polymers in fuel cells. © 2009 Elsevier B.V. All rights reserved.
1. Low temperature fuel cells Fuel cells are clean and efficient electrochemical energy conversion reactors. Among the various fuel cell types under consideration, with operating temperatures ranging up to 1000 °C for the solid oxide fuel cell (SOFC), the polymer electrolyte fuel cell (PEFC) is particularly attractive for applications with variable load profile and intermittent operation (portable electronics, remote power sources, vehicle propulsion). The PEFC typically operates at temperatures between 60 and 100 °C, in specific configurations around 180 °C. In a PEFC, the electrochemical reaction of a fuel – typically H2 – and O2, commonly taken from ambient air, takes place in two half-cell reactions separated by an electrolyte, which is a polymeric membrane with a thickness of 20 to 200 μm and proton conductivity on the order of 0.1 S cm− 1. Methanol can also be used as fuel in the direct methanol fuel cell (DMFC), having the advantage of easy storage, yet with a lower power density compared to the fuel cell operated on H2. The electrodes consist of porous carbon fiber layer structures with a thickness of a few hundred micrometers (Fig. 1). The hydrogen oxidation and oxygen reduction reactions take place at the anode–membrane and cathode–membrane interface, respectively, within the active layer (thickness ~ 10 μm) containing highly dispersed Pt nanoparticles, supported on high surface area carbon, as electrocatalyst [1]. In the PEFC, the solid polymer membrane has to fulfill several functions, with concomitant requirements for candidate ion-conducting materials:
☆ Presented at the 12th Aachener Membrane Kolloquium, Aachen, Germany, 29-30 October, 2008. ⁎ Corresponding author. Tel.: +41 56 310 2673; fax: +41 56 310 4416. E-mail address: [email protected] (L. Gubler). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.101
Requirements ❑ transport of protons ❑ electrical insulator ❑ oxygen and hydrogen gas barrier
Materials ❑ chemically stable (against HO / HOO radicals) ❑ thermally stable ❑ mechanically robust
Furthermore, the membrane may be part of the gasket system, requiring certain specific mechanical properties. This ensemble of specifications calls for a comprehensive approach in the membrane development for fuel cell application.
2. Membrane materials for fuel cells In general, PEFC electrolytes are classified into acidic or basic types, yet the acidic proton exchange membrane (PEM) is most widely used owing to its superior chemical stability. Generation of mobile protons is accomplished via the introduction of acid sites into the polymer, yielding an ‘ionomer’. As it is desired to have an acid with high dissociation constant, sulfonic acid is predominantly used for this purpose. Ion exchange membranes are widely used in separation technology (desalination, dialysis, filtration, etc.) and electrochemical processes (electrolysis, fuel cells) [2]. The PEM in the PEFC is subjected to particularly aggressive conditions, because the interaction of H2 and O2 on the surface of the Pt catalyst leads to the formation of aggressive HO• and HOO• radical species, which attack the polymer and lead to chain scission and thus membrane degradation [3]. For this reason, the majority of membranes used in fuel cells are perfluorosulfonic acid (PFSA) type membranes, known under the tradenames Nafion®, Flemion®, Hyflon® Ion, etc. Substantial efforts are devoted by the academia and industry to the development of novel fuel cell membrane materials, driven by the need for membranes with improved functionality (e.g., conductivity,
L. Gubler, G.G. Scherer / Desalination 250 (2010) 1034–1037
Fig. 1. Operating principle of a H2 / O2 fuel cell with acidic electrolyte membrane. Protons are transported from anode to cathode, where water is formed.
robustness) and more cost-efficient polymers [4]. Promising candidate materials are partially fluorinated or non-fluorinated ionomers containing aromatic units with attached –SO3H groups, either in the main polymer chain, or attached to an aliphatic main chain (Fig. 2). Examples of more ‘exotic’ polymers are polyphosphazenes and silicones. 3. Structure–property relationship The sulfonic acid based PEMs have a common characteristic: dissociation and formation of mobile protons require the presence of water, acting as ‘proton solvent’. The structure of water swollen PEMs,
1035
especially Nafion®, has been subject to numerous investigations. It has been established that Nafion®, as well as other well-working PEMs, have a nano-phase-separated structure, with a ‘polymer’ phase and an ‘aqueous’ phase [5,6]. The proton conduction takes place within water channels lined with sulfonate counter-ions (Fig. 3a). As a consequence, the mobility of the proton is strongly affected by the water content (Fig. 3b). For Nafion® a more or less linear increase of conductivity is observed as a function of membrane water content. The dissimilar structure–property relationship for sPEEK membranes leads to a different dependence of conductivity on water content [7]. PFSA type membranes, such as Nafion®, are copolymers of tetrafluoroethylene and a monomer with pendant –SO2F group, which is later hydrolyzed to yield –SO3H. Due to the different chemical nature of the PTFE-like backbone and the pendant acidbearing chains, the hydrated ionomer forms the phase-separated structure depicted in Fig. 3a. For alternative ionomer materials under investigation, it is important to impart the polymer with a property that will lead to the spontaneous formation of a multi-phase structure. This may be achieved by forming block copolymers with hydrophobic and hydrophilic blocks [9,10] (Fig. 4). Another approach to obtain phase-separated polymers is the formation of graft copolymers. Grafting involves the introduction of a polymer constituent into a pre-existing polymer film. Grafting can be accomplished by irradiating the base film to introduce radicals, which will serve as starting points for the growth of the grafted polymer [11]. Radiation grafting allows the formation of polymer combinations that are impossible to obtain with other methods, owing to the incompatibility of the constituents, e.g., a hydrophobic and a hydrophilic component. The process offers a broad range of possibilities to design the polymer architecture via careful choice of the base film, grafting monomers, irradiation and grafting conditions [12]. In yet another approach, polymers in solution with different target functionalities may be mixed and cast to yield a polymer blend [13]. One component,
Fig. 2. Polymeric materials used to form proton exchange membranes for fuel cells. The acid functionality is predominantly provided by sulfonic acid.
1036
L. Gubler, G.G. Scherer / Desalination 250 (2010) 1034–1037
Fig. 3. a) Nano-phase-separated structure of proton exchange membranes such as Nafion® (courtesy of K.D. Kreuer, MPI für Festkörperforschung, Stuttgart, Germany); b) proton conductivity of Nafion® membranes and a sulfonated PEEK membrane as a function of water content [8].
such as PVDF, may provide the mechanical integrity of the membrane, whereas the second component is an ionomer. Besides providing a proton transport medium, fuel cell membranes have to be mechanically robust. Fuel cells are mechanically compacted, typically in a filter-press configuration, to minimize contact resistance and prevent reactant leakage to the outside. Often, the membrane is part of the gasket arrangement for sealing purposes. Therefore, the polymer has to be ductile, have high strength and fracture toughness, as well as exhibit a low propensity to undergo creep deformation and be dimensionally stable. The mechanical properties are crucial, as mechanical membrane failure leads to catastrophic cell failure, yet largely underrated in the scientific literature. As an example, the tensile properties of a 25 μm thick Nafion® membrane are shown in Fig. 5a. The tensile strength decreases significantly with increasing temperature and level of hydration.
4. Challenges and future directions The relatively low mechanical strength of a hydrated PEM, and the fact that the membrane absorbs water, leading to residual stress in the membrane confined in the fuel cell hardware, is one of the major shortcomings. Therefore, reinforced membranes are being developed, consisting of a porous substrate of high strength, such as expanded PTFE (Fig. 6a) [14,15], or UHDPE [16,17], which is filled with an ionomer. The composite membrane is mechanically more resilient
and dimensionally stable, yielding substantially improved fuel cell lifetime. The requirement of absorbed water within the polymer structure to obtain mobile protons and the concomitant strong dependence of conductivity on relative humidity is another shortcoming of sulfonic acid based PEMs. Fuel cell applications such as remote power or car propulsion demand operating temperatures above 100 °C for system simplification reasons (e.g., smaller cooling units). However, active humidification of the fuel cell and membrane is prohibitive at these temperatures, requiring it to operate at lower relative humidity. Increasing the ionic content (decreasing the equivalent weight) is somewhat successful (Fig. 5b), yet at the expense of mechanical stability and durability. An approach to increase the water retention of the polymer at high temperature and low relative humidity is to include hydrophilic additives into the ionomer, such as phosphotungstic acid (Fig. 6b), Zr phosph(on)ates, SiO2 or TiO2 [18–20], which may lead to better cell performance if the composition and morphology of the additive and composite are well-designed. An entirely different approach is that of ‘water-free’ acid-doped polymers for operating at temperatures far beyond 100 °C. Water as the proton solvent is thereby replaced by the acid. In this realm, phosphoric acid (H3PO4) doped polybenzimidazole (PBI) membranes constitute the most advanced technology [21]. Membranes with extremely high H3PO4 content of 85% by weight are obtained through a sol–gel process [22]. The heterocycle of the PBI is involved in the proton transport process, as it provides free electron pairs for proton binding (Fig. 6c). The operating temperature of H3PO4 / PBI based PEFCs is between 160 and 200 °C. Although the material is essentially an intrinsic proton conductor, the presence of water greatly improves conductivity. One difficulty faced in the operation of such membranes is the transition through the liquid water regime below 100 °C during startup and shutdown, which leads to a leaching-out of the acid if water condensation is allowed to happen. 5. Conclusion
Fig. 4. Possible polymer configurations to facilitate the formation of a phase-separated structure.
Proton exchange membranes (PEMs) for fuel cells are required to transport protons from anode to cathode and block the passage of electrons and reactants. Due to the aggressive chemical environment, perfluorosulfonic acid membranes (e.g., Nafion®) are widely used in
L. Gubler, G.G. Scherer / Desalination 250 (2010) 1034–1037
1037
Fig. 5. a) Mechanical properties of a 25 μm Nafion membrane under different conditions of temperature and relative humidity (adapted from R. Solasi et al., J. Power Sources 167, 2007, 366, Copyright (2007), with permission from Elsevier); b) proton conductivity of perfluorinated membranes with different equivalent weight (EW) (source: C.S. Gittleman, General Motors, presentation at the DOE High Temperature Membrane Working Group, May 18, Washington, DC, 2009. Also available on the World Wide Web: http://www1.eere. energy.gov/hydrogenandfuelcells/pdfs/htmwg_may09_automotive_perspective.pdf).
Fig. 6. a) Structure of expanded PTFE (porosity 85%) used as substrate for composite membranes (Reprinted from Tang et al. [15], Copyright (2007), with permission from Elsevier); b) example of an inorganic material (phosphotungstic acid) that is incorporated into the ionomer for improved conductivity at high temperature (~120 °C)/low r.h.; c) proton conduction mechanism in phosphoric acid-doped polybenzimidazole (PBI) membranes for operating temperatures of ~180 °C.
fuel cells. There are ranges of novel and potentially cost-effective partially fluorinated or non-fluorinated ionomers under development with interesting properties. PEMs conduct protons when hydrated. To maximize conductivity at minimum water content, adequate control of polymer structure and morphology is required. Improved membrane designs involve reinforced polymers for higher mechanical robustness and inclusion of hydrophilic additives to improve water retention at low relative humidity. Water-free proton conduction is highly desirable and can be achieved in phosphoric acid-doped polymers operating at temperatures around 180 °C. Although substantial advances have been made in fuel cell membrane technology, a number of barriers hampering wide-spread commercialization remain to be overcome: conductivity of 0.1 S cm− 1 under hot and dry conditions, mechanical and chemical durability under fuel cell conditions, and last but not least: cost. References [1] G. Hoogers, Fuel Cell Technology Handbook, CRC Press, 2003. [2] S.P. Nunes, K.V. Peinemann, Membrane Technology in Chemical Industry, Wiley-VCH, 2001. [3] A.B. LaConti, H. Hamdan, R.C. McDonald, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), ‘Mechanisms of Membrane Degradation’, in Handbook of Fuel Cells—Fundamentals, Technology and Applications, Volume 3, John Wiley & Sons, 2003, pp. 647–662. [4] B. Smitha, S. Sridhar, A.A. Khan, J. Membr. Sci. 259 (2005) 10–26. [5] K.A. Mauritz, R.B. Moore, Chem. Rev. 104 (2004) 4535–4585. [6] K. Schmidt-Rohr, Q. Chen, Nature Materials 7 (2008) 75–83.
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