3283
2008, 112, 3283-3286 Published on Web 01/17/2008
Nanoscale Building Blocks for the Development of Novel Proton Exchange Membrane Fuel Cells Eunja Kim,*,† Philippe F. Weck,‡ Naduvalath Balakrishnan,‡ and Chulsung Bae‡ Department of Physics and Astronomy, UniVersity of NeVada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NeVada 89154, and Department of Chemistry, UniVersity of NeVada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NeVada 89154 ReceiVed: December 7, 2007; In Final Form: January 4, 2008
We propose a new type of sulfonated aromatic polyarylenes as candidate building blocks for proton exchange membranes. Density functional theory calculations and ab initio molecular dynamics simulations suggest that desulfonation is limited at high temperatures, owing to the strong aryl-SO3H bond induced by the electrondeficient aromatic ring, and that the proposed polymers exhibit good thermomechanical stability due to the robust aromatic main-chain repeating unit. Simulations also emphasize the importance of the Grotthuss-type mechanism, with interconversion between Eigen (H9O4+) and Zundel cations (H5O2+) as limiting structures, for the hydrated proton transport in the vicinity of the sulfonic acid groups.
Developing efficient, reliable, environmentally friendly energy sources is one of the most challenging tasks of the 21st century. Among available alternative energy options, fuel cells are recognized as an ideal energy solution for many applications, including transportation and portable electronics.1,2 Among the types of fuel cells under active development, proton exchange membrane fuel cells (PEMFCs) are generally considered the most attractive power source for automotive use.2 The proton exchange membrane (PEM), which separates the fuel from the oxidant but allows for proton transport from the anode to the cathode, is the most important component of PEMFCs.3 For proton-conducting materials (typically a polymer electrolyte or ionomer) to be used successfully as PEMs in PEMFCs, they must have good chemical and electrochemical stability under fuel cell operating conditions, good mechanical stability in both dry and hydrated states, high proton conductivity, zero electric conductivity, and low production cost.4,5 Although sulfonated perfluoropolymers such as Nafion (DuPont) (cf. Figure 1a) have been dominantly used as a PEM in PEMFCs for the past three decades, their shortcomings seriously limit their wide application as stationary or mobile power sources. These drawbacks include low proton conductivity and poor mechanical stability at high temperature (>100 °C), high cost, and high methanol permeability in direct methanol fuel cells.4-6 Thus, the development of alternative, low-cost, hightemperature, polymer-based electrolytes that have good chemical resistance, good mechanical stability, and sufficient proton conductivity is currently an active research area.7-10 Among currently known alternative membranes, BAM3G from Ballard Advanced Materials11 (cf. Figure 1b) has been semi-commercialized and is considered the best available membrane in terms of performance and chemical stability.5 BAM3G is a * Corresponding author. E-mail:
[email protected]. † Department of Physics and Astronomy. ‡ Department of Chemistry.
10.1021/jp711568f CCC: $40.75
Figure 1. Chemical structures of proton exchange membranes: (a) Nafion (DuPont); (b) BAM3G (Ballard Advanced Materials); (c) proposed sulfonated polyarylenes, SPA-1 and SPA-2.
partially fluorinated polystyrene-like electrolyte membrane in which C-F bonds are substituted at the benzylic position. The presence of an electron-withdrawing group (-CF-) at the benzylic position of the aromatic ring renders the sulfonic acid group a stronger acid than typical aryl sulfonic acid. Despite this advantage, it is unsuitable for high-temperature use,12 owing to the flexible main-chain structure of this polystyrene-like polymer. Because aromatic main-chain polymers (e.g., engineering polymers) are known to have good thermal stability, their sulfonated hydrocarbon polymers have recently been investigated as alternative PEMs. They include sulfonated poly(arylene ether sulfone),13 sulfonated poly(ether ether ketone),14 and sulfonated poly(phenylene).15 In this article, we propose a new type of sulfonated aromatic polymers as PEMs, sulfonated polyarylenes (SPA-1 and SPA-2 in Figure 1c), and investigate their electronic and structural properties by density functional calculations. The inclusion of short electron-withdrawing groups (-CF2- and -CF2CF2- for SPA-1 and SPA-2, respectively) between aromatic rings is expected to significantly increase the acidity of the sulfonic acid group and hence the proton conductivity as in the case of BAM3G. The electron-deficient aromatic ring will also induce a stronger aryl-SO3H bond, which makes desulfonation less probable at high temperatures. © 2008 American Chemical Society
3284 J. Phys. Chem. B, Vol. 112, No. 11, 2008 In addition, SPA-1 and SPA-2 are expected to offer good thermomechanical stability because of the robust aromatic mainchain repeating unit. First-principles all-electron calculations of the total energies and optimized geometries were performed using the spinpolarized density functional theory (DFT) as implemented in the DMol3 software.16 The exchange correlation energy was calculated using the generalized gradient approximation (GGA) employing the Becke-Lee-Yang-Parr (BLYP) density functional.17,18 Double numerical basis sets including polarization functions on all atoms (DNP) were used in the calculations. The DNP basis set corresponds to a double-ζ quality basis set with a p-type polarization function added to hydrogen and d-type polarization functions added to heavier atoms. The DNP basis set is comparable to 6-31G** Gaussian basis sets19 with a better accuracy for a similar basis set size.16 In the generation of the numerical basis sets, a global orbital cutoff of 4.0 Å was used. The energy tolerance in the self-consistent field calculations was set to 10-6 hartree. Optimized geometries were obtained using the direct inversion in a subspace method (DIIS) without symmetry constraints with an energy convergence tolerance of 10-5 hartree and a gradient convergence of 2 × 10-3 hartree/Å. Molecular dynamics simulations were carried out using the ab initio planewave-pseudopotential Car-Parrinello method.20 The ionic and electronic forces were derived separately from an effective Lagrangian based on the BLYP GGA exchange correlation functional. This theoretical approach was used successfully to describe water dimers,21 protonated water dimers,22 and the proton mobility in water,23 relevant to the study of proton conductivity in hydrated PEMs. The simulations were performed in a periodic cubic box of 30 Å lattice constant, with a Γ-point planewave expansion of the valence orbitals up to 50 Ry. Norm-conserving pseudopotentials were used to treat the interaction between valence and core electrons.24 The time step of the ionic motion and the fictitious mass of the electronic degrees of freedom were 3 au (0.07 fs) and 500 au, respectively. The average temperature was set by initializing the kinetic energy and then rescaling the velocities of the atoms whenever the instantaneous temperature deviates from the target temperature by more than 20%. Snapshots used for the analysis of our MD simulations were taken every 100 simulation steps (7 fs). For the present investigation, we selected model fragments of SPA-1 and SPA-2 polymers consisting of sequences of seven aromatic rings for the backbone with hydrogen termination of the polymeric chains. These model systems were found to retain the electronic and structural properties of longer polymeric fragments. Fully optimized geometries of dry fragments of a reference SPA-1/SPA-2-analog polymer without difluoromethylene units along the backbone and the SPA-1 and SPA-2 polymers are displayed in Figure 2. The introduction of difluoromethylene units into the sulfonated poly(phenylene) backbone of the reference polymer shown in Figure 2a results in markedly different backbone conformations for the SPA-1 and SPA-2 polymers. The SPA-1 polymer is characterized by a partially folded backbone, due to the presence of ∼114° wide C-CF2-C angles (compared to 109° for C sp3) and C-CC-C dihedral angles ranging from ∼36° between successive phenylene units to ∼63° between adjacent aryl-SO3H and phenylene units. The SPA-2 polymer conformation appears rather elongated and linear, with the -CF2CF2- units forming “steps” along the backbone, while no significant change appears in C-C-C-C dihedral angles compared to the SPA-1 back-
Letters
Figure 2. Fully optimized geometries of dry fragments of (a) a reference SPA-1/SPA-2-analog polymer without difluoromethylene units along the backbone, (b) SPA-1 polymer, and (c) SPA-2 polymer. Color legend for atoms: carbon (gray); oxygen (red); hydrogen (white); fluorine (light blue).
bone. Complementary GGA DFT calculations performed for these polymer fragments using the parametrization of Perdew and Wang25 (PW91) lead only to minute deformations in the minimum energy structures displayed in Figure 2. The kink linkages caused by the presence of -CF2- units in the polymer backbone are expected to lower the rigidity of the polymers and possibly diminish the aggregation of the polymers. A standard indicator of kinetic stability and chemical hardness of molecular systems is given by the energy separation between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO); a large energy gap implies high kinetic stability and low chemical reactivity.26,27 The calculated HOMO-LUMO energy gap, ∆E, increases as more difluoromethylene units are introduced into the backbone of the polymeric fragments: ∆E ) 2.82 eV for the reference fragment in Figure 2a, ∆E ) 3.24 eV for SPA-1, and ∆E ) 3.40 eV for SPA-2. Thus, owing to their robust main-chain repeating units, SPA-1 and SPA-2 are expected to offer better thermomechanical and chemical stability than sulfonated poly(phenylene).15 These large values of ∆E also indicate that the proposed polymers have zero electric conductivity. In addition to the possible thermal decomposition of the polymer backbone, desulfonation of the polymer can be highly detrimental to the performance of the PEM at high temperatures. Figure 3 depicts the projected electronic charge densities of SPA-1 and SPA-2 polymeric fragments onto a plane containing a sulfonated ring. The calculated charge density is continuous and uniformly distributed along the aryl-SO3H bond and the backbone. The electron-deficient aromatic ring induces a strong aryl-SO3H bond, which lowers the probability of desulfonation at high temperatures. For both SPA-1 and SPA-2 fragments, the calculated C-S bond length is 1.82 Å, with charges of -0.02e on the C atom and +0.49e on the S atom calculated using the Hirshfeld partitioning of the electron density. For the sake of comparison, calculations performed at the same level of theory on a model fragment of Nafion 117 resulted in a longer/weaker C-S bond length of 1.94 Å, with charges of
Letters
J. Phys. Chem. B, Vol. 112, No. 11, 2008 3285
Figure 3. Projected electronic charge densities of (a) SPA-1 and (b) SPA-2 polymeric fragments. Charge densities are plotted in e/Å3 units.
+0.16e on the C atom and +0.47e on the S atom. From these results, we can infer that the desulfonation of SPA-1 and SPA-2 at high temperatures should be reduced compared to Nafion 117. Molecular dynamics simulations of solvated SPA-1 and SPA-2 polymeric fragments have been performed at 300 K for up to 3 ps. In our MD simulations, concentrations of 12-15 water molecules per sulfonated group were chosen as starting configurations of the solvation models. This choice is based on the experimental hydration number λ ) 15 H2O/SO3H for the H form of Nafion 117 at 300 K (∼20% water content), which is the concentration expected when operating the PEMFC in an atmosphere with ∼60% humidity.28 This hydration level was found to lead to a percolated nanophase structure of the hydrophilic domain in hydrated Nafion 117, with the initial assumption of complete dissociation of the protons from the sulfonate groups, in recent classical MD simulations.29 No desulfonation or backbone alterations of the solvated SPA-1 and SPA-2 fragments occurred during our simulations at 300 K, and complementary simulations at 400 K, thus confirming the good thermomechanical and chemical stability of the proposed polymers in a hydrated state. In addition, MD simulations of solvated SPA-1 and SPA-2 polyarylenes emphasize the importance of the Grotthuss-type mechanism23,30 for the hydrated proton transport in the vicinity of the sulfonic acid groups. Figure 4 shows snapshots of the proton transport mechanisms in the vicinity of hydrated sulfonic acid protogenic groups of the SPA-1 fragment at 300 K. Similar results were obtained with simulations on the SPA-2 polymeric fragment under identical conditions. Using our solvation models, the dissociation and transfer of the proton from the acidic site to the aqueous medium occur almost instantaneously, typically within 0.1-0.2 ps. The dissociated -SO3- group subject the confined water to a strong electrostatic field that increases the spatial and orientational order of the neighboring water molecules,31 with the hydrated proton forming a fluxional defect in the hydrogen-bonded network surrounding the protogenic
Figure 4. Snapshots of molecular dynamics simulations showing the proton transport mechanisms in the vicinity of hydrated sulfonic acid protogenic groups of the SPA-1 fragment at 300 K. The snapshots correspond to (a) 0.1 ps, (b) 0.7 ps, and (c) 1.1 ps. The dashed lines depict the hydrogen-bonded network of water molecules.
group. After dissociation, a tricoordinate SO3-‚H3O+‚(H2O)2 Eigen-type complex32 is formed (Figure 4a), which is further transformed 0.5 ps later via proton transfer into a shared proton complex or Zundel cation,33 i.e., [H2O‚‚‚H‚‚‚OH2]+ (Figure 4b). The following step for the transport of the topological defect in the form of an excess proton takes place within 0.4 ps with the formation of a solvated hydronium complex or Eigen cation,32 i.e., H3O+‚(H2O)3 (Figure 4c). Similar to the Grotthuss mechanism in bulk water,23 the rate of structural charge diffusion appears to be driven by thermally induced hydrogen-bond breaking in the second solvation shell of H3O+. In particular, the rate-determining step in the fluxional defect conduction is the rotation of water molecules to assume the particular H-bonded mutual orientation necessary for the actual proton flip. Our MD simulations, using λ ) 12-15 H2O/SO3H, suggest that the dominant transport mechanism consists of sequences of Eigen-Zundel-Eigen transformations, in agreement with the scenario for proton mobility in liquid water proposed recently by Markovitch and Agmon based on multistate empirical valence-bond calculations and NMR experimental data.34 However, for water contents of less than λ ) 3 H2O/SO3H, the Zundel cation and the Zundel-Zundel transformation are expected to feature importantly in the transfer of protons in PEMs with such minimal hydration.31 The importance of the Zundel cation for minimally hydrated PEMs was illustrated in recent ab initio MD simulations of proton transport in a trifluoromethane sulfonic acid monohydrate solid35 and in a model short-side-chain perfluorosulfonic acid membrane.36 While our ab initio MD simulations provide important insight into the mechanism of proton transport in the vicinity of the sulfonic acid groups, further computational effort involving
3286 J. Phys. Chem. B, Vol. 112, No. 11, 2008 larger scale systems and longer simulation time is needed in order to get complete mechanistic information of the long range proton transport. Experimental efforts to synthesize and characterize these novel sulfonated aromatic polyarylenes are also underway. Acknowledgment. This work was supported by DOE grant DE-FG36-05GO85028. References and Notes (1) Steele, B. C. H.; Heinzel, A. Materials for fuel-cell technologies. Nature 2001, 414 (6861), 345-352. (2) Winter, M.; Brodd, R. J. What are batteries, fuel cells, and supercapacitors? Chem. ReV. 2004, 104 (10), 4245-4269. (3) Rikukawa, M.; Sanui, K. Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Prog. Polym. Sci. 2000, 25 (10), 1463-1502. (4) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Alternative polymer systems for proton exchange membranes (PEMs). Chem. ReV. 2004, 104 (10), 4587-4611. (5) Savadogo, O. Emerging membranes for electrochemical systems: (I) solid polymer electrolyte membranes for fuel cell systems. J. New Mater. Electrochem. Syst. 1998, 1 (1), 47-66. (6) Kerres, J. A. Development of ionomer membranes for fuel cells. J. Membr. Sci. 2001, 185 (1), 3-27. (7) Li, Q. F.; He, R. H.; Jensen, J. O.; Bjerrum, N. J. Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 degrees C. Chem. Mater. 2003, 15 (26), 4896-4915. (8) Prakash, G. K. S.; Smart, M. C.; Wang, Q. J.; Atti, A.; Pleynet, V.; Yang, B.; McGrath, K.; Olah, G. A.; Narayanan, S. R.; Chun, W.; Valdez, T.; Surampudi, S. High efficiency direct methanol fuel cell based on poly(styrenesulfonic) acid (PSSA)-poly(vinylidene fluoride) (PVDF) composite membranes. J. Fluorine Chem. 2004, 125 (8), 1217-1230. (9) Jannasch, P. Recent developments in high-temperature proton conducting polymer electrolyte membranes. Curr. Opin. Colloid Interface Sci. 2003, 8 (1), 96-102. (10) Yang, C.; Costamagna, P.; Srinivasan, S.; Benziger, J.; Bocarsly, A. B. Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells. J. Power Sources 2001, 103 (1), 1-9. (11) Steck, A. E.; Stone, C. Proceedings of the 2nd International Symposium on New Materials for Fuel Cell and Modern Battery Systems; Ecole Polytechnique de Montreal: Montreal, 1997; p 792. (12) Sanchez, J.-Y.; Alloin, F.; Iojoiu, C. Fluorinated organic chemicals: Prospects in new electrochemical energy technologies. J. Fluorine Chem. 2006, 127 (11), 1471-1478. (13) Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E. Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes. J. Membr. Sci. 2002, 197 (1-2), 231-242. (14) Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Wang, K.; Kaliaguine, S. Synthesis and characterization of sulfonated poly(ether ether ketone) for proton exchange membranes. J. Membr. Sci. 2004, 229 (1-2), 95-106. (15) Fujimoto, C. H.; Hickner, M. A.; Cornelius, C. J.; Loy, D. A. Ionomeric poly(phenylene) prepared by diels-alder polymerization: Syn-
Letters thesis and physical properties of a novel polyelectrolyte. Macromolecules 2005, 38 (12), 5010-5016. (16) Delley, B. From molecules to solids with the DMol3 approach. J. Chem. Phys. 2000, 113 (18), 7756-7764. (17) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic-behavior. Phys. ReV. A 1988, 38 (6), 3098-3100. (18) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron-density. Phys. ReV. B 1988, 37 (2), 785-789. (19) Hehre, W. J.; Radom, L.; Schleyer, P. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (20) Car, R.; Parrinello, M. Unified approach for molecular-dynamics and density-functional theory. Phys. ReV. Lett. 1985, 55 (22), 2471-2474. (21) Sprik, M.; Hutter, J.; Parrinello, M. Ab initio molecular dynamics simulation of liquid water: Comparison of three gradient-corrected density functionals. J. Chem. Phys. 1996, 105 (3), 1142-1152. (22) Tuckerman, M. E.; Marx, D.; Klein, M. L.; Parrinello, M. On the quantum nature of the shared proton in hydrogen bonds. Science 1997, 275 (5301), 817-820. (23) Marx, D.; Tuckerman, M. E.; Hutter, J.; Parrinello, M. The nature of the hydrated excess proton in water. Nature 1999, 397 (6720), 601604. (24) Goedecker, S.; Hutter, J.; Teter, M. Separable dual-space Gaussian pseudopotentials. Phys. ReV. B 1996, 54 (3), 1703-1710. (25) Wang, Y.; Perdew, J. P. Accurate and simple analytic representation of the electron-gas correlation-energy. Phys. ReV. B 1992, 45 (23), 1324413249. (26) Manolopoulos, D. E.; May, J. C.; Down, S. E. Theoretical studies of the fullerenes C34 to C70. Chem. Phys. Lett. 1991, 181 (2-3), 105111. (27) Pearson, R. G. Hard and Soft Acids and Bases; Dowden, Hutchinson and Ross: Stroudsburg, PA, 1973. (28) Morris, D. R.; Sun, X. Water-sorption and transport-properties of Nafion-117-H. J. Appl. Polym. Sci. 1993, 50 (8), 1445-1452. (29) Jang, S. S.; Molinero, V.; Cagin, T.; Goddard, W. A. Nanophasesegregation and transport in Nafion 117 from molecular dynamics simulations: Effect of monomeric sequence. J. Phys. Chem. B 2004, 108 (10), 3149-3157. (30) de Grotthuss, C. J. T. Sur la decompostion de l’eau et des corps qu’elle tient en dissolution a l’aide de l’electricite galvanique. Ann. Chim. B 1806, LVIII, 54-74. (31) Paddison, S. J.; Elliott, J. A. On the consequences of side chain flexibility and backbone conformation on hydration and proton dissociation in perfluorosulfonic acid membranes. Phys. Chem. Chem. Phys. 2006, 8 (18), 2193-2203. (32) Eigen, M. Proton transfer, acid-base catalysis and enzymatic hydrolysis. Angew. Chem., Int. Ed. Engl. 1964, 3 (1), 1-19. (33) Zundel, G.; Metzger, H. Energieba¨nder der tunnelnden U ¨ berschussProtenon in flu¨ssigen Sa¨uren. Z. Phys. Chem. 1968, 58 (5-6), 225-245. (34) Markovitch, O.; Agmon, N. Structure and energetics of the hydronium hydration shells. J. Phys. Chem. A 2007, 111 (12), 2253-2256. (35) Eikerling, M.; Paddison, S. J.; Pratt, L. R.; Zawodzinski, T. A., Jr. Defect structure for proton transport in a triflic acid monohydrate solid. Chem. Phys. Lett. 2003, 368, 108-114. (36) Paddison, S. J.; Elliott, J. A. Molecular modeling of the short-sidechain perfluorosulfonic acid membrane. J. Phys. Chem. A 2005, 109, 75837593.
