ARTICLE pubs.acs.org/JPCB
Characterization of Water in Proton-Conducting Membranes by Deuterium NMR T1 Relaxation David K. Lee,† Tomonori Saito,‡ Alan J. Benesi,† Michael A. Hickner,‡ and Harry R. Allcock*,† †
Department of Chemistry and‡Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
ABSTRACT: 2H T1 NMR relaxation was used to characterize the molecular motion of deuterated water (2 H 2 O) in Aquivion E87-05, Nafion 117, and sulfonated-Radel proton-exchange membranes. The presence of bound water with solid character was confirmed by the dependence of the 2 H T 1 relaxation on the magnetic field of the spectrometer. By comparing the 2 H T 1 relaxation times of the different membranes that were equilibrated in varying humidities, the factors that influence the state of water in the membranes were identified. At low levels of hydration, the molecular motion of 2 H 2 O is affected by the acidity and mobility of the sulfonic acid groups to which the water molecules are coordinated. At higher levels of hydration, the molecular motion of 2 H 2 O is affected by the phase separation of the hydrophilic/hydrophobic domains and the size of the hydrophilic domains.
’ INTRODUCTION The proton-conducting membrane is an integral component of a polymer electrolyte fuel cell. The function of the membrane is to transport protons generated at the anode to the cathode, where oxygen reduction occurs. Moreover, the membrane must form a robust barrier between electrodes to separate the fuel from the oxidant as well as to prevent internal short-circuiting of the cell. The high proton conductivity and excellent chemical stability of Nafion make this material the quintessential protonconducting membrane.1 Nafion's high proton conductivity is due to the presence of hydrophilic, sulfonate-rich domains within the membrane. Water molecules that reside in these 4-10 nm wide hydrophilic channels conduct protons by facilitating hopping transport (Grotthuss mechanism) when the rotational mobility of water is high.2 However, the proton conductivity of Nafion decreases dramatically at low relative humidity because of the loss of water. Furthermore, the flexible structure of Nafion renders it unsuitable for high-temperature operation, where macroscopic deformation of the membrane and disruption of the ionic domains can occur.3-5 Therefore, an incentive exists to develop alternative membranes that can perform better than Nafion across a wide range of temperatures and relative humidities. Membranes that are able to conduct protons at low humidity will simplify on-board automotive humidification schemes and enable fuel cells to operate at higher temperatures (>100 °C), which would increase the activity of the platinum catalyst and reduce poisoning of the catalyst by carbon monoxide. Moreover, higher operating temperatures may allow expensive platinum r 2011 American Chemical Society
catalysts to be replaced by less expensive metal oxide catalysts.6,7 Some polymer designs that are being investigated for low humidity proton conduction incorporate heterocycles, such as imidazoles8,9 or triazoles,10, because of their amphotericity - the ability to donate and accept protons. However, the proton conductivity of these systems is low because the rate of proton conduction is dependent on the dynamics of the larger donor-acceptor molecules. Another approach to low-humidity proton conductivity is to synthesize ion-containing block copolymers that form distinct hydrophobic and hydrophilic phases due to a highly sulfonated hydrophilic block and a hydrophobic block.11-15 The function of the hydrophobic phase is to provide mechanical stability to the hydrophilic, sulfonated phase, which would have poor mechanically stability when hydrated. The proton conductivites of these block copolymers are comparable to those of Nafion when hydrated, but improvement over Nafion at low humidity has proven to be difficult to attain.11 Many studies on the nanophase structure of proton-conducting membranes have been carried out to correlate the size and connectivity of the hydrophilic domains with proton conductivity.1,16-18 However, the characterization of water that is absorbed in the membrane is equally important because water is the protonconducting medium. The nature of water in proton-conducting membranes has been probed using different techniques such as dielectric spectroscopy,19,20 differential scanning calorimetry,21,22 Received: July 20, 2010 Revised: December 7, 2010 Published: January 10, 2011 776
dx.doi.org/10.1021/jp106757b | J. Phys. Chem. B 2011, 115, 776–783
The Journal of Physical Chemistry B
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infrared spectroscopy,23 and nuclear magnetic resonance.24,25 Computational methods have also been used to model the molecular motions of water in proton-conducting membranes.26,27 It has been proposed that at least two types of water are present in fuel cell membranes, which are termed “free” water and “bound” water. Free water is water within the hydrophilic pores of the membrane that is uncoordinated to the polymer and displays characteristics that are similar to those of bulk water. Bound water is strongly coordinated to the sulfonic acid groups of the polymer. Because of strong local forces, for example, hydrogen bonding and solvation of the ions, the water has limited mobility and has characteristics that resemble solid water at room temperature. This form of water has been reported in various systems including inorganic solids28-30 and polymer systems.31,32 Studies have shown that the bound water molecules are difficult to remove by heating because of their strong coordination to the sulfonic acid groups.33 Despite its tight binding, it is conceivable that bound water can be used in a highly sulfonated polymer system for proton conduction at high temperatures. Proton conduction can occur through mechanisms that have been observed in doped ice, where proton exchange between “ionic defects” (e.g., H3Oþ) and H2O, combined with Bjerrum defects that promote tetrahedral jumps of water molecules, allow for proton transport.34 Therefore, understanding the water dynamics in proton exchange membranes in relation to the chemical structure of the polymer and the morphology of the membrane will provide valuable information for planning synthetic strategies for future protonconducting polymers. The intent of this work was to focus on the nature and role of water (2H2O) in proton-conducting membrane materials. We have developed Mathematica analysis routines35 that can be used to quantitatively calculate deuterium quadrupolar dynamic powder lineshapes, T1, and T2 values for specific jump geometries, jump rates, or rotational diffusion coefficients. Using these tools, it has been possible to devise reasonable models of O-2H bond motion that match the experimental deuterium NMR data over a wide range of temperatures and magnetic fields for 2H2O in porous silicates and aluminosilicates.28-30 The match between experimental and theoretical spectra and relaxation times was used to show that the O-2H bond motion consists of a mixture of C2 and tetrahedral symmetry jumps in 2H2O-synthesized kanemite and in 2 H2O-hydrated Zeolite A and that the 2H2O in these materials is solid, even well above ambient temperatures.29 The specific objective in the work presented here was to use deuterium spectra and deuterium NMR T1 relaxation times as a means of elucidating the dynamics of water in several different 2H2O hydrated protonconducting membrane materials with varying chemical properties that would be expected to influence the molecular motion of water. 2 H NMR offers distinct advantages over 1H NMR for the characterization of angular dynamics because it is dominated by the single nucleus 2H quadrupolar interaction. The quadrupolar interaction derives from the coupling of the electric field gradient tensor (arising from the electron density distribution at the nucleus) with the nuclear electric quadrupole moment, Q. The energy of the interaction is expressed via the quadrupole coupling constant (qcc e2qQ/h, where eq is the principal component of the electric field gradient tensor, h is Planck's constant, e is the elementary charge, and qcc is expressed in frequency units).29 The electric field gradient tensor in 2H2O has nearly axial symmetry (η = 0.1) about the O-2H covalent bond axis and yields a qcc of 215 kHz.34 The spectrum and relaxation times for 2 H2O can therefore be used to monitor the time dependence of
the angle made by the molecular O-2H covalent bond with the magnetic field.30 The situation for 1H NMR, where the intramolecular 1H-1H dipolar interactions, intermolecular 1H-1H dipolar interactions, and heteronuclear (X-1H) dipolar interactions must be considered, is much more complicated and for that reason was not used in these studies.36 A repertoire of deuterium NMR techniques are sensitive to O-2H motions with frequencies (νmotion) of 1 10-2 < νmotion < 1 1015 s-1.37,38 These encompass most of the range of frequencies of reorientational and translational motions of water and other molecules in condensed phases. Almost all experimentally derived knowledge about the detailed angular dynamics of water has been derived from deuterium NMR experiments. For bulk liquid and supercooled liquid water, the 2H T1 and T2 relaxation times show that the O-2H bonds experience isotropic rotational diffusion. At 292 K and atmospheric pressure, the deuterium T1 is 400 ms, independent of the magnetic field strength, with a rotational rate constant for liquid 2H2O of 2.8 1011 s-1 (= 6Drot, where Drot is the rotational diffusion coefficient),39 whereas for supercooled liquid water at pressures >2000 atm and temperatures between 188 and 210 K, the T1 values are field-dependent and in the 2-20 ms range, with rotational rate constants on the order of 107 to 109 s-1.38,40 Although isotropic rotational diffusion occurs in some solids with spherically symmetric molecules,41 this type of motion is typical of liquids composed of small molecules. In contrast, for pure solid ice Ih at atmospheric pressure, 2H NMR line shape analysis and stimulated echo experiments show that the O-2H bonds experience tetrahedral jumps (jumprate ≈ 104 s-1 at several degrees Celsius below freezing) due to the diffusion of Bjerrum defects through the ice lattice.42 On a slower time scale, there is also a seven-site reorientiation mediated by proton transfer and interstitial translational diffusion.34 For most crystalline hydrates (CaSO4 3 2H2O, for example), variable temperature 2H NMR line shape analysis shows that the O-2H bonds experience rapid C2 symmetry jumps around the bisector of the HOH bond angle with jump rates g106 s-1 at ambient temperatures.43,44 In a few cases, spectral analysis for crystalline hydrates shows that the water molecules are rigid on the “NMR time scale” (νjumps , qcc) except for librations that reduce the effective 2H qcc. 2 H NMR stimulated echo experiments on tetrahydrofuran clathrate hydrate show that the O-2H bonds of the water molecules experience tetrahedral jumps (ascribed to Bjerrum defects) on a distorted tetrahedral lattice as well as a slower randomization process ascribed to a combination of Bjerrum and ionic defects.45 Dynamics characterized by rigidity or jumps rather than rotational diffusion provide atomic level evidence of the solid state. Recent 2H NMR studies in our laboratories have found solidstate water that exhibits both tetrahedral jumps (like ice Ih) and C2 symmetry jumps (like crystalline hydrates) at room temperature and higher in the layered silicate kanemite (NaHSi2O5 3 3H2O) synthesized with 2H2O and in the nearly spherical porous cavities of 2H2O-hydrated Naþ-Zeolite A (Na12Al12Si12O48 3 27H2O).28-30 In these samples, where the interlayer spacing and maximum pore diameter are 10.2 and 14 Å, respectively, all of the internal water was shown to be in the solid state, hydrogenbonded to and extending several layers away from the negatively charged silicate or aluminosilicate solid surfaces or coordinated to Naþ cations adjacent to the negative surface. In both of these hydrated solids, at ambient and higher temperatures, there is fast exchange of deuterons between two populations of water molecules: those that experience tetrahedral jumps of their O-2H 777
dx.doi.org/10.1021/jp106757b |J. Phys. Chem. B 2011, 115, 776–783
The Journal of Physical Chemistry B
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covalent bonds and those coordinated to Naþ that experience C2 symmetry jumps of their O-2H bonds about the 2H-O-2H bond angle bisector. C2 symmetry jump rates were
