by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

1. rotor with sample in the rf coil zr B0 = 9  21 T rot  10 kHz θ NMR Diffusometry and MAS NMR Spectroscopy of Functionalized Mesoporous Proton Conductors Magic-Angle Spinning Pulsed Field Gradient Nuclear Magnetic Resonance as a New Tool for Diffusometry of Interface Materials • by Dieter Freude1, Monir Sharifi2, and Michael Wark2 • 1Universität Leipzig, Inst. für Experimentelle Physik, Linnéstraße 5, 04103 Leipzig, Germany • 2Leibniz Universität Hannover, Inst. für Phys. Chem. und Elektrochemie, Callinstraße 3a, 30167 Hannover, Germany gradient coils forpulsed field gradients, maximum 1 T / m

2. B z B B B z z z 0 0 0 0 y y y y 5 1 4 2 M M x x 3 3 1 2 5 4 x x Introduction to pulsed field gradient (PFG) NMR Spin recovery by Hahn echo without diffusion of nuclei: p p /2 r.f. pulse t gradient pulse t gmax = 25 T / m d free induction Hahn echo y magnetization t D D

3. p/2 p p/2 p/2 p/2 p p/2 rf pulses free induction decay, FID, amplitude S t t t t g gradient pulses d d tecd  PFG NMR, signal decay by diffusion of the nuclei PFG NMR diffusion measurements baseon radio frequency (rf) pulse sequences. They generate a spin echo, like the Hahn echo (two pulses) orthe stimulated spin echo (three pulses). At right, a sequence for alternatingsine shaped gradient pulses andlongitudinal eddy current delay (LED) consisting of 7 rf pulses, 4 magnetic field gradient pulses of duration , intensity g, observation time , and 2 eddy current quench pulses is presented. The self-diffusion coefficient D of molecules is obtained from the decay of the amplitude S of the FID in dependence on the field gradient intensity g by the equation

4. High-resolution solid-state MAS NMR Fast rotation (1-60 kHz) of the sample about an axis oriented at the angle54.7° (magic-angle) with respect to the static magnetic field removes all broadening effects with an angular dependency of zr B0 rot θ Chemical shift anisotropy,internuclear dipolar interactions,first-order quadrupole interactions, and inhomogeneities of the magnetic susceptibility are averaged out. It results an enhancement in spectral resolution by line narrowing for solids and for soft matter. The transverse relaxation time is prolonged.

5. CH (iso) 3 CH (n - but) 3 = 0.4 ppm δ Δ but) CH (n - 2 FAU Na-X , n-butane + isobutane CH (iso) ωr = 0 kHz gradient strength 4 2 0 -2 d / ppm δ = 0.02 ppm ωr = 10 kHz 1.0 2.0 d / ppm 2.0 1.5 1.0 0.5 d / ppm Δδ MAS PFG NMR  diffusometry with spectral resolution Spectral resolution is necessary for studies of mixture diffusion and functionalized mesoporous proton conductors as well. From left: 1H MAS NMR spectra of imidazol composite b, hydrated composite c, and sulfonic acid functionalized composite

6. Functionalized mesoporous proton conductors R. Marschall, M. Sharifi, M. Wark: Proton conductivity of imidazole functionalized ordered mesoporous silica, Microporous Mesoporous Mater. 123 (2009) 21–29: The proton conductivity of highly ordered high surface mesoporous silica material Si-MCM-41 functionalized with imidazole groups was studied by impedance spectroscopy in the temperature range of 60–140 C. Samples were characterized by X-ray diffraction, nitrogen adsorption and FT-infrared spectroscopy in addition. The degree of functionalization, spacer chain length between silica host and functional imidazole group, and the relative humidity was varied. R. Marschall, I. Bannat, A. Feldhoff, L. Wang, G. Q. Lu, M. Wark: SO3H-functionalized Si-MCM-41 with superior proton conductivity, small 5 (2009) 854–859: Mesoporous silica particles of around 100 nm diameter functionalized with sulfonic acid groups are prepared using a simple and fast in situ co-condensation procedure. Structural data are determined via electron microscopy, nitrogen adsorption, and X-ray diffraction. Proton conductivity values of the functionalized samples are measured via impedance spectroscopy.

7. Magic-angle spinning NMR spectroscopy on 1H, 13C, and 29Si nuclei in the functionalized mesoporous proton conducting materials was performed in the fields of 9.4 and 17.6 Tesla mainly at room temperature. Solid-state NMR spectroscopy

8. N Si N OH 1H MAS NMR spectroscopy HO3S Si OH H2O + H+ H3O+ H3O+  10 H2O Imidazole-MCM-41 SO3H-MCM-41

9. N Si N HO3S Si 13C CP {1H} MAS NMR spectroscopy SO3H-MCM-41 Imidazole-MCM-41

10. 29Si and 29Si CP {1H} MAS NMR spectroscopy Imidazole-MCM-41 29Si CP {1H} MAS NMR Si (OSi-)3 (OH)1 Si (OSi-)2 (OH)2 Si (OSi-)4 -CH2Si (OSi-)2 (OH)1 29Si MAS NMR (one-pulse) -CH2Si (OSi-)3 29Si MAS NMR Bloch decay spectra yield quantitative information about linking of functional groups. 100% 5% 5% relative concentration

11. 1H MAS PFG NMR diffusometry 2D-presentation of the signal decay of sample SO3H-MCM-41 (grafting) measured at 353 K. The self-diffusion coefficient is obtained from the decay of the 7-ppm-signal. Methylen signals in the range 1-4 ppm are relatively increased, since their relaxation times are longer. The diffusion time was 20 ms and 1-ms-alternating-gradient-pulses were used. The figure left demonstrates the advantage of MAS PFG NMR diffusometry with respect to the well-established PFG NMR diffusometry. The latter would consider the sum of all unresolved signals for the determination of the self-diffusion coefficient. Fitting of the values S for the 7-ppm-signal yields a self-diffusion coefficient of D = 7.9  10-9 m2s-1.

12. Nernst-Einstein equationand conductivity models The Nernst-Einstein equation gives the direct-current conductivity sdc as a function of the concentration C of the proton vehicles, the charge e of a single vehicle, the self-diffusion coefficient D and the temperature T, with kB as Boltzmann constant:1 The concentration can be obtained from solid-state NMR data and weight and volume of the sample in the NMR rotor. Then we obtain from the equation above sdc = 0.036 S cm-1. A comparison with the value obtained directly by impedance spectroscopy [R. Marschall, J. Rathousky, M. Wark, Ordered functionalized silica materials with high proton conductivity, Chem. Mater. 19 (2007) 6401-6407] shows that the calculated values are higher by one order of magnitude. Models of the conductivity in solid ionic conductors describe a macroscopic behavior. Diffusion can be studied by several techniques giving a macroscopic or microscopic picture. NMR diffusometry monitors diffusion path lengths in the order of magnitude of micrometer during observation times 1-1000 ms. The comparison of conductivities, which were directly measured, with those obtained by the Nernst-Einstein equationfrom NMR diffusivity data, can be used for the verification of conductivity models. 1 P. Colomban, A. Novak, Proton Conductors: classification and conductivity, in: Proton coductors. Solids, membranes and gels – materials and devices, (P. Colomban, Eds.), Cambridge University Press, 1992, p. 38-60

13. The development of functionalized mesoporous materials for proton exchange membrane fuel cells (PEM cells) at higher temperatures (140 °C) is a key area in the research for new environmentally friendly ways of energy generation. • A conductivity of s = 10-3 S cm-1 can be obtained at 140 °C for the sulfonic acid functionalized mesoporous material Si-MCM-41. • 1H MAS NMR spectroscopy yield information about the spacer and the nature of the proton vehicle for the conductivity • 13C CP MAS NMR shows the structure of the spacer and functional group • 29Si MAS NMR gives quantitative results about the anchorage of the spacer to the mesoporous host material. • 1H MAS PFG diffusometry determines selectively the diffusivity of the proton vehicles in the cell material. • A comparison between conductivities, which were directly measured by impedance spectroscopy, with values obtained by the Nernst-Einstein equation from the self-diffusion coefficient, which was obtained by 1H MAS PFG NMR, is helpful for the evaluation of conductivity models. Conclusions

14. Diffusion Fundamentals IV Basic Principles of Theory, Experiment and ApplicationAugust 21rd - 24th, 2011Troy, NY, USA