1H–1H Double Quantum NMR Investigation of Proton Dynamics in

Article pubs.acs.org/JPCC

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H−1H Double Quantum NMR Investigation of Proton Dynamics in Solid Acids Nicole E. De Almeida,† Kristopher J. Harris,† Ago Samoson,‡ and Gillian R. Goward*,† †

Department of Chemistry & Chemical Biology, and Brockhouse Institute for Materials Research, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada ‡ NMR Instituut and Technomeedikum, University of Tallinn, 10120 Tallinn, Estonia ABSTRACT: Currently, the most popular proton exchange membrane (PEM) for fuel cell applications is Nafion. However, Nafion does not retain its high conductivity at high temperatures due to its dependence on water for proton transport. Because operational temperatures higher than the evaporation point of water are desirable, a family of solid acids was investigated. Cations known to transport protons were paired with anions to make acidic salts. Solid acids discussed here include imidazole paired with trifluoromethanesulfuric acid as well as imidazole, benzimidazole and adenine paired with methanesulfonic acid. Solid-state NMR was utilized to show the relative mobility of protons through double-quantum filter (DQF) experiments. The POST-C7 homonuclear dipolar-recoupling scheme was paired with DUMBO homonuclear decoupling to produce 1H double-quantum coherence buildup curves for the hydrogen-bonded protons of interest. Experimental buildup curves, which reflect both local structure as well as dynamics, are compared to theoretical curves of the static system. The SPINEVOLUTION-simulated curves utilized up to eight pairs of homonuclear dipolar couplings within a sphere of 7 Å diameter centered on the proton of interest. Steep buildup of the DQ curve and maxima at short recoupling times in the buildup curves indicate strong dipole−dipole coupling and are interpreted to indicate limited dynamics of the H-bonded protons. In contrast, shallower buildup curves and maxima at longer recoupling times imply that H-bonded protons (in an otherwise similar structure) are associated with local mobility, which reduces their local dipolar coupling and may facilitate proton transport. Bulk proton conductivities measured via electrochemical impedance spectroscopy were compared to DQF measurements to understand proton conduction within these materials.

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organic solid acid.6 These organic solid acids have demonstrated modest proton conductivity under anhydrous hightemperature conditions. Benzimidazolium and imidazolium phosphonates have been reported to retain proton conductivities on the order of 1 × 10−6 to 1 × 10−5 S/cm under anhydrous, high-temperature conditions;11 however, polymerization of the phosphate groups was found to limit long-term stability. Sulfonate groups are more resistant to polymerization and, as such, represent a promising alternative. To test this hypothesis, the sulfonated solid acids adenine-methanesulfonate (AMSA), benzimidazolium-methanesulfonate (BMSA), imidazolium-methanesulfonate (IMSA), and imidazolium-trifluoromethanesulfonate (IFMS) are studied herein. Proton conductivity, stability, and local proton dynamics in each material are reported. Here, the proton conductivity is investigated not only with the industry standard electrochemical impedance spectroscopy, EIS, but also by solid-state 1H NMR. While EIS is an excellent

INTRODUCTION Proton electrolyte fuel cells (PEFCs) are promising energy carriers for both automobiles and stationary applications. Current PEFCs use Nafion as an electrolyte due to its chemical resistance, durability, and high proton conductivity.1 Nafion conducts protons with a vehicle mechanism utilizing water as a proton carrier. Thus, PEFCs constructed with Nafion are optimal for operation only at temperatures 300 240 191 189

anhydrous conditions. Solid-state NMR is used here to probe locally hydrogen-bonded sites. The relative mobility of each Hbonding motif, probed by DQC buildup curves and their simulations, is correlated with the efficiency of that solid-acid structure in achieving bulk proton transport. Electrochemical Impedance Spectroscopy. Generally, solids with lower melting points are associated with more mobility and tend to display higher proton conductivities. The melting points of the solid acids can be found in Table 1, and given the melting points, it is predicted that IFMS would possess the highest proton conductivity. Table 1 lists the bulk proton conductivities of four organic solid acid salts measured at 25 °C under anhydrous conditions, where BMSA, IMSA, and IFMS were similar within error and AMSA was half the value of the other three solid acids. It is interesting to note that the conductivity BMSA possesses is similar to IMSA, in spite of the larger benzimidazolium cation. This indicates that the rotation of the cation is not rate limiting for proton transport in these two salts.24 It is important to note that when measuring conductivities of bulk materials, grain boundaries can alter the observed performance. It is therefore difficult to measure definitive results with EIS because different preparation procedures produce different grain boundaries. Here, we explore the use of solid-state NMR to probe local mobility in these salts that might otherwise be easily overlooked.

Figure 1. On the left is a 2D 1H DQC/SQC correlation spectrum performed with 12.5 kHz MAS using a supercycle with two loops of recoupling of glycine collected on an 11.7 T magnet at room temperature. On the right is a structure of glycine.

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Figure 3. On the left is a 2D 1H DQC/SQC correlation spectrum of BMSA performed with 12.5 kHz MAS, 64 scans per slice, using a supercycle with two loops of recoupling. On the right is an excerpt from the crystal structure of BMSA, highlighting the intermolecular couplings of interest.

Table 2. Slopes of DQC Buildup for BMSA, IMSA, and IFMS Spin Pairs (× 10−3) BMSA pair

exptl (1/μs)

sim (1/μs)

E−E B−D A−B

2.6 ± 0.8 3.4 ± 0.7 2.0 ± 0.2 IMSA

2.3 ± 0.1 3.7 ± 0.2 1.8 ± 0.1

pair

exptl (1/μs)

sim (1/μs)

A−B A−C B−D

2.8 ± 0.2 2.8 ± 0.2 2.7 ± 0.2 IFMS

2.8 ± 0.2 2.3 ± 0.3 2.5 ± 0.6

pair

exptl (1/μs)

sim (1/μs)

A−B A−C C−B

2.3 ± 0.2 2.3 ± 0.2 0.8 ± 0.1

3.8 ± 0.1 4.3 ± 0.4 2.7 ± 0.1

(peak A at 8.1 ppm) from the three equivalent NH3 group protons, and two peaks (peaks A and A′ at 4.2 and 3.1 ppm) from the two crystallographically inequivalent CH2 protons. Four DQCs resulting from the three sites in glycine are visible in the spectrum: those from A−A′, B−A, B−A′, and B−B spin pairs. For example, the B-A DQC appears at 12.3 ppm in the DQC dimension and is measured as a pair of peaks in the SQC dimension at 8.1 and 4.2 ppm. It is worth noting that the excellent resolution in both dimensions is only made possible by the DUMBO homonuclear decoupling sequence applied during these observation periods and is a stringent test of the decoupling method.17 The intensities of the peaks produced by each spin pair vary as a function of the DQC generation period. Three DQC buildup curves measured from a series of 2D DQC/SQC spectra of glycine are shown in Figure 2A. These represent DQCs from pairs of nuclei within each type of functional group (A−A′ and B−B DQCs), and from spin pairs between the two functional groups (B−A DQCs). As is typical, the DQC buildup curves have a steeper initial slope and peak at an earlier

Figure 4. POST-C7 DQC buildup curves of BMSA. (A) Simulated buildup curves and (B) experimental buildup curves collected at incremented recoupling times. Filled data points are used in the comparison of the initial rise. Open data points are included for completeness. 1 H NMR Methods. Before investigating a system that includes mobility, we first present data on glycine as a benchmark material that is not affected by motion (the dipeptide β-AspAla has also been used as a benchmark for structural studies13). The projection of the direct dimension of the DQC/SQC spectrum of glycine, Figure 1, shows one peak

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Figure 5. On the left is a 2D 1H DQC/SQC correlation spectrum of IMSA performed with 12.5 kHz MAS, 64 scans per slice, using a supercycle with two loops of recoupling. On the right is an excerpt of the crystal structure of IMSA.

point in time for the closer A−A′ and B−B spin pairs (H−H ≈ 1.8 Å) than for the pairs of spins with longer internuclear distances and lower overall homonuclear dipolar coupling (H− H ≈ 2.5 Å for the B−A sites). These characteristics of the DQC buildup curve provide easily interpreted information in simple cases. In cases when the network of strongly coupled protons is more complicated, comparison to theoretical curves taking into account all of the distances and information from the crystal structure are required. Theoretical DQC curves for these three sites, generated using numerical SPINEVOLUTION simulations including the 10 nearest protons in the known crystal structure, are shown in Figure 2A.25 The experimental glycine DQC buildup curves are shown in Figure 2B for intermolecular spin pairs (H−H ≈ 2.5 to 3.3 Å). While the full form of the curve is only partially reproduced, likely because of higherorder spin effects and relaxation dampening, the initial slopes and the positions of the maxima in the curves are well represented. This agreement is analogous to the results of a structural study of a dipeptide, characterized by DQC buildup curves.12 Normalization of the DQC buildup curves was applied by choosing the maximum DQC intensity of the points between τr = 0 and 400 μs, and normalizing to this data point (I = 1.0) for both the experimental and simulated data sets of a given solid acid. In each case we have plotted the normalized intensities of a selection of DQ correlations that include the H-bonded resonance (A) and the aromatic protons of the cationic rings. For each buildup curve, we compare the steepness of the initial buildup, as emphasized by the solid-data points. The decay portion of the curves are not expected to fit well to the simulated data sets, due to the impact of relaxation at these longer recoupling times. To de-emphasize these data points from the comparison, these data are plotted with open symbols. 1 H NMR of Solid Acids. The structure and DQC/SQC correlation spectrum of the solid acid BMSA are shown in

Figure 6. POST-C7 DQC buildup curves of IMSA. (A) Simulated buildup curves and (B) experimental buildup curves collected at incremented recoupling times. Filled data points are used in the comparison of the initial rise. Open data points are included for completeness.

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Figure 7. On the left is a 2D 1H DQC/SQC correlation spectrum performed with 12.5 kHz MAS, 16 scans per slice, using a supercycle with two loops of recoupling of IFMS. On the right is an excerpt of the crystal structure of IFMS.

Figure 4A. Experimental data sets for the same resonances are shown in Figure 4B. The correlations to peak C are suppressed, likely due to the lower population of this site. Somewhat surprisingly, the E−E correlation is predicted to be a weak correlation with a shallow buildup, but measured to be strongly coupled. This implies a limitation in the simulation perhaps due to a lack of correctly representing the effects of spin-diffusion, but since this resonance represents a self-correlation within the methyl group, it was not further investigated here. The buildup curves for both the A−B (H-bond to aromatic ring) and B−D (aromatic-ring to aromatic-ring) correlations have similar initial slopes for both the simulated and experimental data sets, and their maxima occur at the same point in recoupling time. Namely, the two green curves match and the two purple curves match. This indicates that neither of these dipolar couplings is impacted by local dynamics. More specifically, the H-bonded proton is not mobile and neither is the benzimidazole cation reorienting on a time scale comparable to the strength of the 1 H−1H dipolar couplings for these sites. Initial slope values are calculated for the initial buildup from data up to 400 μs recoupling time, and are tabulated in Table 2. Figure 5 shows the structure and correlation spectrum for the second solid acid of interest, IMSA. The SQ projection of the correlation spectrum shows four proton resonances including a hydrogen-bonded site at 13.0 ppm (A), aromatic sites at 7.6 ppm (B) and 9.1 ppm (C), and an aliphatic site at 1.7 ppm (D). Five DQCs are visible in the spectrum, from spin pairs in D−D, B−D, B−B, A−B, and A−C sites. Three representative buildup curves, A−C, A−B, both including the H-bonded proton A, and B−D, between protons on the aromatic ring, for the simulated and experimental data are shown in Figure 6. The corresponding slopes are listed in Table 2. Of the three, the simulated data sets build up most steeply for the B−D correlation, and this buildup is very similar for both simulated and experimental data sets. This indicates that ringreorientation is not averaging this homonuclear coupling. In contrast, the two correlations that include the H-bonded proton A both build up steeply in the simulated data set but are attenuated in the experimental data set, indicating local

Figure 8. POST-C7 DQC buildup curves of IFMS. (A) Simulated buildup curves and (B) experimental buildup curves collected at incremented recoupling times. Filled data points are used in the comparison of the initial rise. Open data points are included for completeness.

Figure 3. The spectrum is characterized by one hydrogenbonded site at 13 ppm (A), three aromatic sites at 8 ppm (B), 6 ppm (C), and 4 ppm (D), and one aliphatic site at 3 ppm (E). The three DQCs observed in the spectrum arise from spin pairs in A−B, B−D, and E−E sites. Simulated DQC buildup curves for three resonances (A−B, B−D, and E−E) are plotted in F

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Figure 9. (A) Two-dimensional 1H DQC/SQC correlation spectrum performed with 12.5 kHz MAS, 64 scans per slice, using a supercycle with two loops of recoupling of AMSA collected on an 11.7 T magnet at room temperature. (B) Two-dimensional 1H 100 kHz MAS back-to-back of AMSA collected on a 900 MHz spectrometer at room temperature using a 0.85 mm probe. (C) Excerpt of the crystal structure of AMSA. (D) AMSA structure with the spin pairs highlighted on the structure.

with short distances between cation and anion and six hydrogen-bonded protons. These constraints are proposed to prevent the cation rotation, which would be necessary for the structure to conduct protons. DQC/SQC correlations in AMSA were also studied, though different conditions had to be used for the spectroscopy. Initial attempts under the same conditions used above, i.e., DUMBO homonuclear decoupling at a moderate field and MAS rate, produced poorly resolved 2D spectra such as the one shown in Figure 9A. While the DUMBO method is extremely well developed and widely applicable, there are simply too many closely spaced peaks to resolve them under the conditions employed. Historically, the DUMBO method has been used because NMR probes capable of spinning the sample at rates much faster than the magnitude of the dipolar coupling were unavailable. This limitation can now be pushed with a specialty probe capable of MAS at the unusually rapid rate of 100 kHz that was built by the Samoson group. Tests of the 1H spectral resolution under 100 kHz MAS and a very high field (21.1 T) produced excellent results, vide infra. We note that the very rapid MAS rate is incompatible with POST-C7 generation of DQC states, so the BABA method was instead employed.20 The DQC/SQC correlation spectrum of AMSA collected under ultrafast MAS at an ultrahigh applied field, Figure 9B, shows extremely high resolution in both dimensions. Six proton sites are apparent: the anion’s methyl group at 1.7 ppm (D),

mobility that may contribute to proton transport is present in this case. The structure and correlation spectrum for the solid acid IFMS are shown in Figure 7. The 1H spectrum has three proton shifts: 11.4 ppm for the hydrogen-bonded site (A), 8.1 ppm for one aromatic proton (C), and 7.1 ppm for a second aromatic proton (B). Four DQCs are visible in the spectrum, composed of spin pairs in A−B, A−C, C−B, and B−B sites, and their buildup curves are shown in Figure 8B. In comparison to the simulated curves, Figure 8A, the experimental curves all have shallower slopes and maxima that occur at later times. Furthermore, the experimental curves for IFMS are shallow and peak at later times than those of all the other compounds studied here, despite the fact that the dipolar couplings are stronger in this tightly hydrogen-bonded material. The shape of the DQC buildup curves observed in IFMS therefore provide a clear indication of motion that averages the dipolar couplings. This observation suggests that IFMS could possess higher proton conductivity than BMSA and IMSA and demonstrates the potential of using DQC NMR to monitor local motion independently of grain boundary resistances. The next system studied by 1H NMR is AMSA, though we first report details of its structure. The crystal structure of AMSA was solved in space group P21/c with an R-factor of 3.19%, where cell lengths a = 7.2162(8) Å, b = 14.6646(15) Å, and c = 9.1094(10) Å were found. This structure is compact, G

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conductivity. Finally, fluorination of the anion’s methyl group, i.e., comparing IMSA to IFMS, again resulted in minimal changes in ion conductivity, but IFMS shown here possesses somewhat higher dynamics. The increase may be due to the change in electronegativity of the sulfonate group, producing a more equivalent sharing of the protons between the cations and anions, or a size effect, producing an intermolecular spacing that is better optimized for proton transport. In order to directly compare the local mobility within BMSA, IMSA, and IFMS, the experimental and simulated DQC buildup curves of the hydrogen bonded protons are shown in Figure 10A,B. It is clear that the experimental IFMS DQ response is the most attenuated relative to the simulation based on the rigid IFMS lattice. From this we conclude that the local dynamics within the H-bonded network of IFMS are fastest, IMSA shows intermediate dynamics, and BMSA shows the least local dynamics of these data sets. Since the bulk conductivities are similar for BMSA, IMSA, and IFMS, the higher dynamics observed for IFMS from NMR indicate that higher proton conductivities may be obtainable if sample preparation strategies can be optimized.

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CONCLUSIONS A series of solid acids that are potentially useful as proton conductors in fuel cells were studied using macroscale conductivity measurements combined with molecular-scale investigations using NMR spectroscopy. The distance and orientation dependence of 1H−1H dipolar couplings, which are very sensitive to motional averaging, were investigated through the generation of 1H DQC states that depend on such couplings. Comparisons of experimental and theoretical DQC excitation experiments show excellent agreement in the initial buildup of the DQC curves for stationary systems and attenuation relative to the calculated curves for other protons, which is indicative of local motional averaging. The solid acids BMSA and AMSA do not show significant signs of motion in the DQC excitation curves, while similar analysis of IMSA and IFMS shows clear evidence of motion. Proton conductivity measurements depicted limited changes between BMSA, IMSA, and IFMS; however, the attenuation observed in the DQC excitation curves indicate that IFMS could possess higher conductivity. The NMR method is independent of grainboundary resistances, external contacts, homogeneity, and other difficulties with macroscale techniques. 1H DQC excitation experiments appear to be an excellent method of investigating motion in the solid state, and the results here point to IFMS being a strong candidate for proton conduction applications.

Figure 10. Dipole−dipole coupled spin pairs for conductive protons in solid acid salts. IMSA and BMSA have similar dipolar buildup curves and also similar proton conductivity. IFMS possesses the highest proton conductivity and has an attenuated experimental DQC buildup compared to the simulated DQC buildup, indicating additional dynamics on this time scale.

aromatic C−H sites on the cation’s two rings at 7.9 and 8.3 ppm (C,C′), the primary amine site at 10.3 ppm (B), and the two secondary amines at 12.7 and 13.5 ppm (A,A′). A large number of DQCs are visible, and nuclei in all sites form observable DQCs with nuclei in other types of site. Of particular note are the DQCs involving C and B sites in neighboring cations that are strongly hydrogen bonded, N−H− N ≈ 2.5 Å, in the arrangement shown in Figure 9C. The fact that numerous intermolecular DQCs are observed, particularly those involving the amine sites that would be responsible for proton conduction, shows that the molecules are not undergoing the rapid motion that would be required for fast proton transport. This conclusion is consistent with the EIS data, Table 1, which shows AMSA to have very poor proton conductivity. Studying proton motion in this series of solid acids provides interesting relationships given their structural similarity. Modifying the benzene ring by removing the pendant amine and the off-center cationic site, i.e., comparing AMSA to BMSA, decreases the number of hydrogen-bonded pathways available for conduction, but results in an approximate doubling of the conductivity. The decreased number of hydrogen bonds may provide a beneficial lowering of activation energies; additionally, 180° jumps about the long axis of the BMSA cation result in an energy-equivalent structure, suggesting that such cation rotation may contribute to its comparatively higher conductivity. Removing the benzene ring from the cation, i.e., comparing BMSA to IMSA, results in very little change in the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-(905)-525-9140, ext. 24176. Fax: +1-(905)-522-2509. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Science and Engineering Research Council (NSERC) for funding this project. We are grateful to Prof. Brown (Warwick University) for hosting NDA for a short exchange, and useful discussions on DQC/ SQC data acquisition and processing. Also, we are thankful to Robin Stein (Bruker) and Prof. Alex Bain for helpful H

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(20) Schnell, I.; Spiess, H. W. High-Resolution 1H NMR Spectroscopy in the Solid State: Very Fast Sample Rotation and Multiple-Quantum Coherences. J. Magn. Reson. 2001, 149, 90−102. (21) Zaremba, S. K. Good Lattice Points, Discrepancy, and Numerical Integration. Annali di Matematica 1966, 73, 293−317. (22) Conroy, H. Molecular Schrödinger Equation. VIII. a New Method for the Evaluation of Multidimensional Integrals. J. Chem. Phys. 1967, 47, 5307−5318. (23) Cheng, V. B.; Suzukawa, H. H., Jr; Wolfsberg, M. Investigations of a Nonrandom Numerical Method for Multidimensional Integration. J. Chem. Phys. 1973, 59, 3992−3999. (24) Traer, J. W.; Britten, J. F.; Goward, G. R. A Solid-State NMR Study of Hydrogen-Bonding Networks and Ion Dynamics in Benzimidazole Salts. J. Phys. Chem. B 2007, 111, 5602−5609. (25) Albrecht, G.; Corey, R. B. The Crystal Structure of Glycine. J. Am. Chem. Soc. 1939, 61, 1087−1103.

discussions. Lastly, we are appreciative of the National Ultrahigh-Field NMR Facility for Solids, in Ottawa, Canada for aid in the collection of the 100 kHz data, especially of Vicktor Terskikh and Andreas Brinkmann.

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REFERENCES

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