Characterization of proton conducting Characterization of proton conducting polyphosphate compositepolyphosphate compositess

1894: Wilhelm Ostwald demonstrates that fuel cells are not limited by the Carnot efficiency.

D. Freude2, S. Haufe3, D. Prochnow2, H.Y. Tu1, U. Stimming1

1Technische Universität München, 2Universität Leipzig ,3Proton Motor Fuel Cell GmbH, Germany

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.

MAS Rotor 7 mm

CO2 Laser

Cryo Magnet

B0

2001: Composite electrolytes: preparation, characterization and investigation of the conductivity; PhD thesis by Stefan Haufe

Synthesis of polyphosphate compositeSynthesis of polyphosphate composite

nitrogenphosphorus

oxygen

Preparation of NH4PO3:

NH4H2PO4 + (NH2)2

NH4PO3 (modification I)

NH4PO3 (modification II)

200 °C

2 h, NH3

280 °C

24 h, NH3

Preparation of composite:

10 NH4PO3 + SiO2

6 NH4PO3 / (NH4)2SiP4O13

250°C

12h, NH3

XRD-structure of NH4PO3 XRD-structure of (NH4)2SiP4O13

silicon

Characterization by XRD, CA, REMCharacterization by XRD, CA, REM

XRD

5 10 15 20 25 30 35 40 45 50 55

rela

tive

inte

nsi

ty /

%

2q / °

0

10

20

30

40

50

60

70

80

90

100

- NH4PO

3 I (Shen et al.)

- NH4PO

3 II (Shen et al.)

- (NH4)

2SiP

4O

13

X-ray diffraction indicates the presence

of NH4PO3 in modifications I and II

and (NH4)2SiP4O13 as well.

Chemical analysis

Composition of the material is 3.7 wt% H,

11.5 wt% N, 29.6 wt% P and 2.9 wt% Si.

It yields [NH4PO3]6[(NH4)2SiP4O13]1.

REM

Particle size 5 – 15 m

C.Y. Shen, N.E. Stahlheber and D.R. Dyroff, J. Am. Chem. Soc. 91 (1969) 62-67

Termogravimetry was

performed with a heating rate

of 10 K/min and a helium flow

of 100 mL/min.

After an initial mass loss

(mostly NH3) of 7% the

material is thermally stable

upon cycling between 50 °C

and 300 °C.

Characterization by TGCharacterization by TG

50 100 150 200 250 300 90

92

94

96

98

100

first cycle second cycle

wt%

T / °C

1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20

-5

-4

-3

-2

-1

0

1

1st heating

1st cooling

2nd heating

2nd cooling

log( s

T / S

K c

m-1)

1000 K / T

650600 550 500 450 400 350

T / K

Arrhenius plot of conductivity measured by ac impedance spectroscopy in dry hydrogen

Increase in conductivity after heating from room temperature up to 300 °C parallelto the mass loss of NH3

observed by thermal gravimetric analysis.

The conductivity does not exhibit any significant changes with further heating-cooling cycles. The values reach from 1×10 S/cm at 50 °C to 2×10 S/cm at 300 °C.

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.

Conductivity measurementsConductivity measurements

1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20

-5

-4

-3

-2

-1

0

1

2

dry hydrogen dry oxygen dry argon humid hydrogen

log( s

T / S

K c

m-1)

1000 K / T

650600 550 500 450 400 350

T / K

Arrhenius plot of conductivity after activation of composite material measured in dry hydrogen,

dry oxygen, dry argon and humid hydrogen

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.

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.

Gas variationGas variation

/ppm -40 -30 -20 -10 0 10

Q0

Q1

Q2

/ppm 30 20 -10 0

Q0

Q1 Q2

31P 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.

NomenclatureQ0: isolated PO4-tetrahedrons, Q1: chain end groups, Q2: middle groups in chain anions

*

*

*

*

*

*

*

/ ppm 200 150 100 50 0 50 100 150

* * * *

/ppm 150 100 50 0 50 100

31P MAS NMRT = 297 K

One Q2-signal according to one non-crystallographic site in APP-II (cf. XRD)

Chain length about 150 Q-units

Four Q2-signals due to four non-crystallographic sites in ASiPP (cf. XRD)

Chain length about 500 Q-units in ASiPP

Q0-signal due to impurities

NMRNMR measurements measurements

31P MAS NMR spectrum of non-activated composite compared to the spectral addition of single components

31P MAS NMR

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.

Sum of the spectra of APP-II and ASiPP

/ppm 0 50

*

40

* * *

Composite (non-activated)

ASiPP

APP

1H MAS NMR spectrum of non-activated composite and its single components

Composite (non-activated)

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

1H MAS NMR

Composite activated

NMR spectra of the composite at NMR spectra of the composite at TT = 297 K = 297 K

100 0 /ppm

10 20 30 50 /ppm

1H MAS NMR between 297 K and 580 K1H MAS NMR between 297 K and 580 K

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.

T = 580K

T = 297K

T = 297K

0 20 40 60 80 /ppm

No further signals arise or vanish during cycling after activation. The 1H MAS NMR spectrum is reversible.

Second cycleActivation in the MAS rotor

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.

Chemical exchange and line mergingChemical exchange and line merging

/ppm

T = 351 K

T = 421 K

T = 441 K

T = 451 K

T = 491 K

3 6 9 12 15Theoretical dependence of the line shape on

the exchange rate k for a two-spin-system

Three cases:

for k « two lines are observed (slow exchange),

for k one very broad signal that often cannot be observed

for k » one narrow signal at the averaged line position is observed (fast exchange).

200 Hz

1 2

k=1

k=10

k=100

k=1000

The presence of cross peaks indicates the chemical exchange.

/ppm12.0 8.0 4.0

12.0

8.0

4.0

An Arrhenius-plot of k for temperatures above 370 K yields an activation energy of 0.8 eV

0 200 400 600 800 1000

12 ppm

7.5 ppm

mix/ms

Determination of exchange ratesDetermination of exchange rates

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.

2D-EXSY spectrum of an activated composite.

T = 297K, mix = 10 ms.

Peak intensities in deopendence on themixing time (T=320 K)

100

1000

2.2 2.4 2.6 2.8 3.0 3.2 3.4

exc

ha

ng

e r

ate

k/s

1000 T / K

Diffusion measurements with PFG and SFG NMRDiffusion measurements with PFG and SFG NMRsequence te Adiff

attenuation due to diffusionAr=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}

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.

t0 t0 + t0 + t0 + 20

G = 60 T/m

PFG

SFG

NMR

20

650 600 550 500 450 400

1.5 2.0 2.51E-13

1E-12

1E-11

1E-10

1E-9

PFG NMR SFG NMR

Ea=0,275 eV

Ea=0,308 eV

103 K / T

D /

m2 s1

T / K

ConclusionsConclusions 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 MAS NMR measurements demonstrate that (non-ammonium) bridging protons are created by the activation procedure of the composite.

31P 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

D. Prochnow, Thesis in preparation, University of Leipzig

S. Haufe, Thesis, Technical University of Munich, 2002