Characterization of proton conducting
polyphosphate composites
D. Freude2, S. Haufe3, D. Prochnow 2, H.Y. Tu1, U. Stimming1
1Technische Universität München, 2Universität Leipzig ,
3Proton Motor Fuel Cell GmbH, Germany
1894: Wilhelm Ostwald demonstrates
that fuel cells are not limited by the
Carnot efficiency.
2001: Composite
electrolytes:
preparation,
characterization
and investigation of
the conductivity;
PhD thesis by
Stefan Haufe
B0
MAS Rotor
 7 mm
Cryo Magnet
CO2 Laser
2002: Solid-state MAS NMR
studies of composite material
were performed in the high
field up to 17 T (750 MHz)
and at temperatures of about
530 K (maximum: 850 K by
laser heating), PhD thesis by
Daniel Prochnow.
Synthesis of polyphosphate composite
silicon
nitrogen
phosphorus
oxygen
XRD-structure of NH4PO3
XRD-structure of (NH4)2SiP4O13
Preparation of NH4PO3:
Preparation of composite:
NH4H2PO4 + (NH2)2
10 NH4PO3 + SiO2
200 °C
2 h, NH3
280 °C
24 h, NH3
NH4PO3 (modification I)
NH4PO3 (modification II)
250°C
12h, NH3
6 NH4PO3 / (NH4)2SiP4O13
Characterization by XRD, CA, REM
XRD
Chemical analysis
100
90
Composition of the material is 3.7 wt% H,
relative intensity / %
80
- NH4PO3 I (Shen et al.)
- NH4PO3 II (Shen et al.)
- (NH4)2SiP4O13
70
60
50
11.5 wt% N, 29.6 wt% P and 2.9 wt% Si.
It yields [NH4PO3]6[(NH4)2SiP4O13]1.
40
30
20
10
0
REM
5
10
15
20
25
30
35
40
45
50
55
2q / °
X-ray diffraction indicates the presence
of NH4PO3 in modifications I and II
and (NH4)2SiP4O13 as well.
Particle size 5 – 15 mm
C.Y. Shen, N.E. Stahlheber and D.R. Dyroff, J. Am. Chem. Soc. 91 (1969) 62-67
Characterization by TG
Termogravimetry was
performed with a heating rate
of 10 K/min and a helium flow
of 100 mL/min.
100
98
first cycle
second cycle
96
wt%
After an initial mass loss
(mostly NH3) of 7% the
material is thermally stable
upon cycling between 50 °C
and 300 °C.
94
92
90
50
100
150
200
T / °C
250
300
Conductivity measurements
T/K
650 600 550
500
450
400
350
 Increase in conductivity after heating
from room temperature up to 300 °C
parallelto the mass loss of NH3
observed by thermal gravimetric
analysis.
1
0
 The conductivity does not exhibit any
significant changes with further
heating-cooling cycles. The values
reach from 1×10-7 S/cm at 50 °C to
2×10-2 S/cm at 300 °C.
-1
log( s T / S K cm )
-1
-2
-3
-4
st
1 heating
st
1 cooling
nd
2 heating
nd
2 cooling
-5
1.60
1.80
2.00
2.20
2.40
2.60
2.80
3.00
3.20
1000 K / T
Arrhenius plot of conductivity measured by
ac impedance spectroscopy in dry hydrogen
 The temperature dependent dc
conductivity measurements in a two
chamber hydrogen cell reveal that the
ionic conductivity is a proton
conductivity. The conductivities
measured by ac and dc techniques
coincide.
T/K
650 600 550
500
450
400
350
2
Gas variation
1
 Varying the gas environment
from dry to humid hydrogen
has a dramatic effect. Due to
water uptake of the sample,
the conductivity increases
reversibly by almost an order
of magnitude.
-1
log( s T / S K cm )
0
-1
-2
-3
dry hydrogen
dry oxygen
dry argon
humid hydrogen
-4
-5
1.60
1.80
2.00
2.20
2.40
2.60
2.80
3.00
3.20
1000 K / T
Arrhenius plot of conductivity after activation of
composite material measured in dry hydrogen,
dry oxygen, dry argon and humid hydrogen
 Activation energies vary from
0.5 eV to 1.0 eV in dry
atmosphere and 0.1 eV to
0.2 eV in humid atmosphere
at 300 °C and 50 °C,
respectively.
0
NMR measurements
Nomenclature
Q0: isolated PO4-tetrahedrons, Q1: chain end groups, Q2: middle groups in chain anions
Q
2
Q0
*
Q0
31P
MAS NMR
T = 297 K
*
1
Q2
Q1
Q
10
0
-10
-20

/ppm
-30
-40
30
*
-50
20
10
0
-10
 /ppm
-20
-30
-40
*
*
*
150
31P
100
50
0
-50
 / ppm
-100
-150
*
*
*
-200
100
50
*
-50
0
*
-100
-150
 /ppm
MAS NMR spectrum of APP-II at rot = 10 kHz.
Asterisks denote spinning side bands.
31P
MAS NMR spectrum of ASiPP at rot = 10 kHz.
Asterisks denote spinning side bands.
 One Q2-signal according to one noncrystallographic site in APP-II (cf. XRD)
 Four Q2-signals due to four noncrystallographic sites in ASiPP (cf. XRD)
 Chain length about 150 Q-units
 Chain length about 500 Q-units in ASiPP
 Q0-signal due to impurities
NMR spectra of the composite at T = 297 K
31P
MAS NMR
1H
Sum of the spectra
of APP-II and ASiPP
MAS NMR
Composite
(non-activated)
ASiPP
Composite activated
Composite
(non-activated)
*
0
 /ppm
-100
31P
MAS NMR spectrum of non-activated composite
compared to the spectral addition of single components
 Spectrum of (non-activated) composite
shows the same 31P resonance positions
with the same chemical shift anisotropies
as observed in the single components.
 Chain length dramatically decreased
upon composition (5 Q-units) and increases
again after activation up to 50 Q-units.
50
*
 /ppm
APP*
0
*
-40
1H
MAS NMR spectrum of non-activated
composite and its single components
 Proton resonance in spectra of APP is
assigned to NH4+ species ( = 7.0 ppm)
 Additional resonance at  = 9.0 ppm in
spectra of ASiPP is due to protons in
hydrogen bridges
 Only one signal at  = 7.3 ppm in the
spectrum of the non-activated composite
0
1H MAS NMR between 297 K and 580 K
Activation in the MAS rotor
Second cycle
T = 580K
T = 297K
50
 /ppm
100
30
20
10
T = 297K
First heating and subsequent cooling observed
by 1H MAS NMR. During the activation process
a second signal arises due to the ammoniac
loss. This new signal, which is assigned to
protons in “bridging positions”, seems to be
responsible for the high protonic conductivity.
80
60
40
 /ppm
20
0
No further signals arise or vanish during
cycling after activation. The 1H MAS NMR
spectrum is reversible.
Chemical exchange and line merging
T = 491 K
200 Hz
T = 451 K
k=1
k=10
k=100
k=1000
T = 441 K
T = 421 K
T = 351 K
1
2
Theoretical dependence of the line shape on
the exchange rate k for a two-spin-system
Three cases:
for k « D two lines are observed
(slow exchange),
for k  D one very broad signal that often
cannot be observed
for k » D one narrow signal at the averaged
line position is observed (fast exchange).
15

12
9
 /ppm
6
3
1H
MAS NMR spectrum of activated
composite shows two signals at 297 K.
 At higher temperatures the signals are
broadened and merge to one line.
 It can be concluded that a chemical
exchange takes place between the two
species.
4.0
Peak intensities in
deopendence on themixing
time (T=320 K)
8.0
7.5 ppm
12.0
12 ppm
12.0
8.0
4.0
 The presence of
cross peaks
indicates the
chemical exchange.
1000
100
 /ppm
0
2D-EXSY spectrum of
an activated composite.
T = 297K, mix = 10 ms.
exchange rate k/s-1
Determination of exchange rates
200 400 600 8001000
mix/ms
 Exchange rates k were
measured between 297 K
and 440 K using 1D
NOESY NMR.
 The analysis of the peak
intensities in dependence
on the mixing time gives
the exchange rates.
2.2 2.4 2.6 2.8 3.0 3.2 3.4
1000 T-1 / K-1
 An Arrhenius-plot of k for
temperatures above 370 K
yields an activation energy
of 0.8 eV
Diffusion measurements with PFG and SFG NMR
sequence
te
Adiff
attenuation due to diffusion
Ar=Ar1Ar2
attenuation due to relaxation
PFG
2t1+t2
exp{-g2G2Dd2(D-d/3)}
exp{-2t1/T2-t2/T1}
SFG
2t1+t2
exp{-g2G2D t12(t2 +2t1/3)}
exp{-2t1/T2-t2/T1}
650 600
t0
t0 + 
20
PFG
t0 + D t0 + D + 
SFG
G = 60 T/m
p/2
p/2
p/2
1+2
T/K
450
400
1E-9
Ea=0,275 eV
1E-10
1E-11
NMR
1
500
D / m 2s- 1
20
550
21+2
PFG NMR
SFG NMR
1E-12
Ea=0,308 eV
1E-13
1.5
2.0
3
2.5
10 K / T
 Proton diffusion measurements were performed by means of PFG (Pulsed Field Gradient) NMR
at L = 400 MHz up to 450 K and SFG (Stray Field Gradient) NMR at L = 118 MHz up to 600 K.
 The activation energy of the diffusion coefficient (about 0.3 eV) is to compare with the ac
conductivity activation energies varying from 0.5 eV to 1.0 eV in dry atmosphere.
Conclusions
 It is well-known that ammonium polyphosphate composites combine the high protonic conductivity and
mechanical stability and exhibit interesting properties as an electrolyte in the intermediate-temperature
fuel cells.
 The prepared ammonium polyphosphate composites contain the phases of (NH4)2SiP4O13 as well as of
NH4PO3, modification I and II. The composite shows thermo-chemical stability after the first heating
cycle.
 The composite also exhibits high conductivity in humid atmosphere. The change from humid to dry
atmosphere causes a reversible decrease in the electrical conductivity by some orders of magnitude.
 A comparison of ac and dc experiments reveals that the electrical conductivity relates to proton
conductivity.

1H

31P
MAS NMR measurements demonstrate that (non-ammonium) bridging protons are created by the
activation procedure of the composite.
MAS NMR measurements show that the phosphorous chain length of about 500 Q-units in APP
decreases upon composition to a value of 5 for ASiPP and increases again after activation up to 50.
 A chemical exchange between ammonium and bridging protons can be observed. Above 380 K the
activation energy of the exchange rate amounts to 0.8 eV.
 NMR diffusion coefficients yield an activation energy of about 0.3 eV. This is to compare with the ac
conductivity activation energies varying from 0.5 eV to 1.0 eV in dry atmosphere.
T. Kenjo and Y. Ogawa, Solid State Ionics 76 (1995) 29-34
S. Haufe, Thesis, Technical University of Munich, 2002
D. Prochnow, Thesis in preparation, University of Leipzig