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
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:
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
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
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
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.
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.
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+
1313C CP {C CP {11H} MAS NMR spectroscopyH} MAS NMR spectroscopy
Imidazole-MCM-41
SiHO3S
SO3H-MCM-41
NN
Si
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.
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.
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.
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.
Diffusion Fundamentals IV
Basic Principles of Theory, Experiment and Application
August 21rd - 24th, 2011Troy, NY, USA
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