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

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rotor with sample in the rf coil. z r. B 0 = 9  21 T.  rot  10 kHz. θ. - PowerPoint PPT Presentation
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by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2 1 Universität Leipzig, Inst. für Experimentelle Physik, Linnéstraße 5, 04103 Leipzig, Germany 2 Leibniz Universität Hannover, Inst. für Phys. Chem. und Elektrochemie, Callinstraße 3a, 30167 Hannover, Germany NMR Diffusometry and MAS NMR NMR Diffusometry and MAS NMR Spectroscopy Spectroscopy of Functionalized Mesoporous Proton of Functionalized Mesoporous Proton Conductors Conductors Magic-Angle Spinning Pulsed Field Gradient Nuclear Magnetic Magic-Angle Spinning Pulsed Field Gradient Nuclear Magnetic Resonance Resonance as a New Tool for Diffusometry of Interface Materials as a New Tool for Diffusometry of Interface Materials gradient coils for pulsed field gradients, rotor with sample in the rf coil z r rot 10 kHz θ B 0 = 9 21 T
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Page 1: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

by Dieter Freude1, Monir Sharifi2, and Michael Wark2

1Universität Leipzig, Inst. für Experimentelle Physik, Linnéstraße 5, 04103 Leipzig, Germany2Leibniz Universität Hannover, Inst. für Phys. Chem. und Elektrochemie, Callinstraße 3a, 30167 Hannover, Germany

NMR Diffusometry and MAS NMR Spectroscopy NMR Diffusometry and MAS NMR Spectroscopy of Functionalized Mesoporous Proton Conductors of Functionalized Mesoporous Proton Conductors

Magic-Angle Spinning Pulsed Field Gradient Nuclear Magnetic Resonance Magic-Angle Spinning Pulsed Field Gradient Nuclear Magnetic Resonance as a New Tool for Diffusometry of Interface Materialsas a New Tool for Diffusometry of Interface Materials

gradient coils forpulsed field gradients,

maximum 1 T / m

rotor with samplein the rf coil zr

rot 10 kHz

θ

B0 = 9 21 T

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

Introduction to pulsed field gradient (PFG) NMRIntroduction to pulsed field gradient (PFG) NMR

r.f. pulse t

/2

gradient pulse tgmax = 25 T / m

magnetization

t

free induction Hahn echo

B0

M x

y

z B0

x

y

z

5 4

1 2

3

B0

x

y

z

1 2

5 4

3

B0

M x

y

z

Spin recovery by Hahn echo without diffusion of nuclei:

Page 3: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

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.

PFG NMR, signal decay by diffusion of the nucleiPFG NMR, signal decay by diffusion of the nuclei

free induction decay, FID, amplitude S

rf pulses

gradient pulses

g

ecd

kDSpg

DSS

exp2

4exp 0

2

0

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

Page 4: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

Fast rotation (160 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

o7.543

1cosarc

Chemical shift anisotropy,internuclear dipolar interactions,first-order quadrupole interactions, and inhomogeneities of the magnetic susceptibilityare averaged out.

It results an enhancement in spectral resolution by line narrowing for solids and for soft matter.The transverse relaxation time is prolonged.

High-resolution solid-state MAS NMRHigh-resolution solid-state MAS NMR

.2

1cos3 2 rot

zr

θ

B0

Page 5: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

MAS PFG NMR MAS PFG NMR diffusometry with spectral resolution diffusometry with spectral resolution

Spectral resolution is necessary for studies of mixture diffusion

and functionalized mesoporous proton conductorsfunctionalized mesoporous proton conductors as well.

ωr = 0 kHz

ωr = 10 kHz0.51.01.52.0

δ = 0.02 ppm

ppm

-2024ppm

FAU Na-X , n-butane + isobutane

Δδ 1.0 2.0 / ppm

CH3 (n-but)

CH3 (iso)

CH2 (n-but) CH (iso)

Δδ = 0.4 ppm

gradient strength

From left: 1H MAS NMR spectra of imidazol composite b, hydrated composite c, and sulfonic acid functionalized composite

Page 6: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

Functionalized mesoporous proton conductorsFunctionalized 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.

Page 7: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

Solid-state NMR spectroscopySolid-state NMR spectroscopy

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.

Page 8: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

11H MAS NMR spectroscopyH MAS NMR spectroscopy

Imidazole-MCM-41

Si

N

OH

N Si

OH

HO3S

SO3H-MCM-41

10H2O

H3O+

H2O + H+ H3O+

Page 9: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

1313C CP {C CP {11H} MAS NMR spectroscopyH} MAS NMR spectroscopy

Imidazole-MCM-41

SiHO3S

SO3H-MCM-41

NN

Si

Page 10: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

2929Si and Si and 2929Si CP {1H} MAS NMR spectroscopySi CP {1H} MAS NMR spectroscopy

Imidazole-MCM-41

29Si CP {1H} MAS NMR

29Si MAS NMR (one-pulse)

Si (OSi)3 (OH)1

Si (OSiSi (OSi))44

Si (OSi)2 (OH)2

CH2Si (OSi)2 (OH)1

CH2Si (OSi)3

100%5% 5%

relative concentration

29Si MAS NMR Bloch decay spectra yield quantitative information about linking of functional groups.

Page 11: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

11H MAS PFG NMR diffusometryH 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 14 ppm are relatively increased, since their relaxation times are longer. The diffusion time was 20 ms and 1-ms-alternating-gradient-pulses were used.

Fitting of the values S for the 7-ppm-signal yields a self-diffusion coefficient of D = 7.9 10-9 m2s-1.

kDSpg

DSS

exp2

4exp 0

2

0

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.

Page 12: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

Nernst-Einstein equationNernst-Einstein equationand conductivity modelsand 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

The Nernst-Einstein equation gives the direct-current conductivity dc 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 dc = 0.036 S cm1. 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 11000 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.

Page 13: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

ConclusionsConclusionsThe 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 = 10 S cm 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.

Page 14: by Dieter Freude 1 , Monir Sharifi 2 , and Michael Wark 2

Diffusion Fundamentals IV

Basic Principles of Theory, Experiment and Application

August 21rd - 24th, 2011Troy, NY, USA


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