MAS NMR Spectroscopy and NMR Diffusometry of Functionalized Mesoporous Proton Conductors

1. MAS NMR Spectroscopy and NMR Diffusometryof Functionalized Mesoporous Proton Conductors rotor with sample in the rf coil zr B0 = 9  21 T rot  10 kHz θ Investigation of the mechanism of the proton conductivity in composite materials by magic-angle spinning nuclear magnetic resonance spectroscopy and diffusometry in addition to impedance spectroscopy Dieter Freude Universität Leipzig InstitutfürExperimentellePhysik Linnéstraße 5, 04103 Leipzig, Germany gradient coils forpulsed field gradients, maximum 1 T / m

2. Introduction to pulsed field gradient (PFG) NMR 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 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. 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 angle 54.7° (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. MAS NMR spectroscopy and MAS PFG NMR diffusometry . Magic-angle spinning NMR spectroscopy on 1H, 13C, and 29Si nuclei and MAS PFG NMR diffusometry was performed in the field 17.6 Tesla.

6. MAS PFG NMR  diffusometry with spectral resolution Spectral resolution is necessary for studies of materials consisting of different proton species. The spectra show the methylene groups of the functional spacer in the range 0-4 ppm and the signal of the hydroxy group at about 6 ppm. PFG NMR without MAS cannot resolve the signals of mobile and fixed species.

7. Functionalized mesoporous proton conductors The following part of the lecture is based on a recent publication by M. Sharifi, M. Wark, D. Freude, J. Haase: Highly proton conducting sulfonic acid functionalized mesoporous materials studied by impedance spectroscopy, MAS NMR spectroscopy and MAS PFG NMR diffusometry, Microporous Mesoporous Materials.Distribution of the preprints DOI: 0.1016/j.micromeso.2012.02.019 to the audience. Successful incorporation of mercaptopropyltrimethoxysilane (MPMS) into the mesoporous framework of Si-MCM-41 was proven by 29Si MAS NMR measurements. 13C CP MAS NMR spectroscopy confirms that that the majority of the organic functional groups remained intact after the oxidation in 30% H2O2. 1H MAS NMR spectroscopy characterizes the hydroxo species. Conductivity differences of some orders of magnitude between 20% and 40% functionalization result in corresponding differences of the diffusion coefficients of the charge carrier by application of the Nernst-Einstein equation. But only small differences of the self-diffusion coefficient were measured by magic-angle spinning pulsed field gradient diffusometry. This gives a large Haven factor. The proton conductivity in functionalized MCM-41 is explained by structure diffusion. The drastic increase of conductivity (from 9.51 to 260 × 10-5 S cm-1 at 353 K ) from 20% to 40% functionalization is caused by the reduction of the activation energy of the charge relocation in a denser lattice of proton donator sites.

8. Conductivity of the samples under study Proton conductivities measured at 100% relative humidity (RH) of the materials Si-MCM-41 (--), Si-MCM-41-SO3H-20% (-■-), Si-MCM-41-SO3H-30% (--), Si-MCM-41-SO3H-40% (-▲-).

9. 1H MAS NMR spectroscopy HO3S H2O + H+ H3O+ 1H MAS NMR spectra of the sample F 20 loaded with water (100% RH at 353 K, dotted line) and of sample F 20 loaded with a smaller amount of water (33% RH at 353 K, solid line). Spectra were measured at 353 K.  4 H2O H3O+ 11 ppm 1H MAS NMR spectroscopy characterizes functional groups and hydroxo species.

10. 13C CP {1H} MAS NMR spectroscopy HO3S Si Carbon atoms in disulfide species(-R-S-S-R-) carbon atom adjacent to the SH in thiol-MCM-41 defect-free SO3H-MCM-41 Defects of the functional groups can be monitored by 13C CP {1H} MAS NMR spectroscopy.

11. 29Si CP {1H}and 29Si MAS NMR spectroscopy 29Si MAS NMR Bloch decay spectra yield quantitative information about the linking of functional groups. 29Si CP {1H} MAS NMR of sample F 30 29Si MAS NMR (one-pulse)of sample F 40 Si (OSi-)4 Si (OSi-)2 (OH)2 Si (OSi-)3 (OH)1 -CH2Si (OSi-)2 (OH)1 -CH2Si (OSi-)3

12. 1H MAS PFG NMR diffusometry 2D-presentation of the signal decay with linearly increasing strength of the gradient pulses for sample F 30 at 303 K, at left, and F 40 at 353 K, at right. The water loading corresponds to 100% RH at 353 K. Fitting of the values S for the 7-ppm-signal decay in dependence of the field gradient strength yields self-diffusion coefficients of D = 1.0 and 2.9  10-9 m2s-1 for F 30 at 303 K (left) and F 40 at 353 K (right). 1H MAS PFG NMR diffusometryyields the self-diffusion coefficients of the hydroxo species which can be compared with the diffusion coefficients of the charge carriers.

13. Nernst-Einstein equation 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 diffusion coefficient Ds of the charge carrier and the temperature T, with kB as Boltzmann constant:1 But also the diffusion coefficient Ds of the charge carrier could be determined by the transposed Nernst-Einstein equation using the experimentally obtained values of the direct-current conductivities sdc. The concentration C of the charged proton vehicles were determined from the ion exchange capacities and the densities of the samples. It result the following diffusion coefficients Ds of the charge carrier at 353 K: 8.5 × 10-13 m2s-1, 3.6 × 10-12m2s-1, 8.8 × 10-12 m2s-1, for F 20, F30, F40, respectively. But 1H MAS PFG self-diffusion coefficients of the hydroxo species are at 353 K : 4.8 × 10-9 m2s-1, 3.8 × 10-9m2s-1, 2.9 × 10-9 m2s-1, for F 20, F30, F40, respectively. Thus, we obtain Haven-factors HRin the range from 329 (F 40) to 5 660 (F 20) 1 P. Colomban, A. Novak, Proton Conductors: classificationandconductivity, in: Proton coductors. Solids, membranesandgels – materialsanddevices, (P. Colomban, Eds.), Cambridge University Press, 1992, p. 38-60

14. Conductivity model The lower part of the figure shows the dissociated functional group without the acid proton (–O–)3Si–CH2– CH2– CH2–SO3, which is linked to the MCM-41 framework via three ≡Si-O-Si≡ bonds. The upper part shows on the left a Zündel ion, H5O2+, ten hydrogen bonded water molecules and a single water molecule coming along. In the middle we see an Eigen ion, H9O4+, and nine hydrogen bonded water molecules. On the right we see the Zündel ion on a different location, again ten hydrogen bonded water molecules and a single water molecule going away. Comparing the left and right Zündel ion in the figure above, we see the relocation of the charge into a distance of about 0.1 nm. This illustrates the elementary step of the charge transport. The upper part of the figure is taken from Fig. 1 of K.D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster, Chem. Rev. 104 (2004) 4637-4678.

15. Conductivity model The proton conduction can be considered as structural diffusion, since the elementary step of the charge transport can be considered as formation and decomposition of an Eigen ion from and to Zündel ions. The elementary step includes a prolongation of the distance between the fixed negative charge of the SO3- group and the mobile charged hydroxo species. The additional Coulomb energy explains why the mobility of uncharged water molecules is higher than that of the charge carriers. The upper part of the figure is taken from Fig. 1 of K.D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster, Chem. Rev. 104 (2004) 4637-4678.

16. Conclusions • 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 = 0.2 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 structure and defects 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 hydroxo species in the cell material. • The drastic increase of conductivity from 20% to 40% functionalization is caused by the reduction of the contribution of the Coulomb energy to the activation energy of the charge relocation in a denser lattice of proton donator sites.