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

1
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
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
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
?
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.
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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
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Functionalized mesoporous proton conductors
R. Marschall, M. Sharifi, M. Wark Proton
conductivity of imidazole functionalized ordered
mesoporous silica, Microporous Mesoporous Mater.
123 (2009) 2129 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 60140 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)
854859 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.
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Solid-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.
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1H MAS NMR spectroscopy
HO3S
Si
OH
H2O H ? H3O
H3O
? 10
H2O
Imidazole-MCM-41
SO3H-MCM-41
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13C CP 1H MAS NMR spectroscopy
SO3H-MCM-41
Imidazole-MCM-41
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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
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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-gra
dient-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.
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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 constant1
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
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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 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.

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