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

         B0
                     2002: Solid-state MAS NMR
                     studies of composite material
         MAS Rotor
                     were performed in the high
           7 mm     field up to 17 T (750 MHz)
                     and at temperatures of about
    Cryo Magnet
                     530 K (maximum: 850 K by
                     laser heating), PhD thesis by
       CO2 Laser     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                                      250°C
             NH4PO3 (modification I)                    6 NH4PO3 / (NH4)2SiP4O13
 2 h, NH3                                    12h, NH3
 280 °C
             NH4PO3 (modification II)
 24 h, NH3
                                         Characterization by XRD, CA, REM
                               XRD                                                                 Chemical analysis
                         100
                         90
                         80
                                                                                                   Composition of the material is 3.7 wt% H,
                         70                                  - NH4PO3 I (Shen et al.)
                                                                                                   11.5 wt% N, 29.6 wt% P and 2.9 wt% Si.
relative intensity / %




                         60
                                                             - NH4PO3 II (Shen et al.)
                                                             - (NH4)2SiP4O13
                         50
                         40                                                                        It yields [NH4PO3]6[(NH4)2SiP4O13]1.
                         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


100                                                Termogravimetry was
                                                   performed with a heating rate
  98
                                                   of 10 K/min and a helium flow
            first cycle
            second cycle                           of 100 mL/min.
  96
                                                   After an initial mass loss
wt%




  94                                               (mostly NH3) of 7% the
                                                   material is thermally stable
  92                                               upon cycling between 50 °C
                                                   and 300 °C.
  90
       50   100        150       200   250   300

                             T / °C
                                                    Conductivity measurements
                                                          T/K
                           650 600 550      500    450       400         350
                                                                                              Increase in conductivity after heating
                      1                                                                        from room temperature up to 300 °C
                                                                                               parallelto the mass loss of NH3
                      0
                                                                                               observed by thermal gravimetric
                                                                                               analysis.
                      -1
log( s T / S K cm )




                                                                                              The conductivity does not exhibit any
-1




                      -2
                                                                                               significant changes with further
                                                                                               heating-cooling cycles. The values
                      -3
                                                                                               reach from 1×10-7 S/cm at 50 °C to
                                                                                               2×10-2 S/cm at 300 °C.
                      -4                   st
                                          1 heating
                                           st
                                          1 cooling
                                           nd
                                          2 heating
                                           nd
                                                                                              The temperature dependent dc
                      -5                  2 cooling
                                                                                               conductivity measurements in a two
                            1.60   1.80     2.00   2.20   2.40   2.60   2.80   3.00   3.20     chamber hydrogen cell reveal that the
                                                    1000 K / T
                                                                                               ionic conductivity is a proton
                       Arrhenius plot of conductivity measured by
                                                                                               conductivity. The conductivities
                       ac impedance spectroscopy in dry hydrogen
                                                                                               measured by ac and dc techniques
                                                                                               coincide.
                                                        T/K
                           650 600 550    500    450       400         350
                      2
                                                                                               Gas variation
                      1


                                                                                            Varying the gas environment
                      0
                                                                                             from dry to humid hydrogen
                                                                                             has a dramatic effect. Due to
                      -1                                                                     water uptake of the sample,
log( s T / S K cm )
-1




                                                                                             the conductivity increases
                      -2                                                                     reversibly by almost an order
                                                                                             of magnitude.
                      -3
                                                                                            Activation energies vary from
                                     dry hydrogen
                                     dry oxygen
                                                                                             0.5 eV to 1.0 eV in dry
                      -4             dry argon
                                     humid hydrogen
                                                                                             atmosphere and 0.1 eV to
                                                                                             0.2 eV in humid atmosphere
                      -5                                                                     at 300 °C and 50 °C,
                            1.60   1.80   2.00   2.20   2.40   2.60   2.80   3.00   3.20
                                                                                             respectively.
                                                  1000 K / T

                             Arrhenius plot of conductivity after activation of
                             composite material measured in dry hydrogen,
                              dry oxygen, dry argon and humid hydrogen
                                                                             NMR measurements
          Nomenclature
          Q0: isolated PO4-tetrahedrons, Q1: chain end groups, Q2: middle groups in chain anions
                                 Q2                                                                                                                             0
                                                                                                                                                                Q
                                                  *


               Q0
                                                                                                  31P     MAS NMR
                                                                                                         T = 297 K                                                                    Q2
                                                                        *                                                                                                  1
                       Q1                                                                                                                                            Q
                                                                                                                                        30      20    10    0        -10       -20     -30   -40
0   10         0       -10
                         
                              -20     -30   -40   -50                              *                                                                             /ppm
                         /ppm

                                    *

                                                                                              *
                   *
                                                                                                                           *        *                       *                  *
                                                                                                         *
         150               100              50          0         -50       -100       -150       -200              100        50        0            -50           -100             -150
                                                             / ppm                                                                           /ppm

           31P          MAS NMR spectrum of APP-II at rot = 10 kHz.                                          31P   MAS NMR spectrum of ASiPP at rot = 10 kHz.
                         Asterisks denote spinning side bands.                                                       Asterisks denote spinning side bands.


      One Q2-signal according to one non-                                                                     Four Q2-signals due to four non-
       crystallographic site in APP-II (cf. XRD)                                                                crystallographic 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   MAS NMR
                           Sum of the spectra
                           of APP-II and ASiPP                                  Composite
                                                                                (non-activated)



                              Composite activated                                ASiPP
                               Composite
                               (non-activated)
                                                               *      *          APP*        *

                            -100                             50                 0           -40
            0
                   /ppm                                             /ppm
31P                                                           1HMAS NMR spectrum of non-activated
   MAS NMR spectrum of non-activated composite
compared to the spectral addition of single components        composite and its single components

 Spectrum of (non-activated) composite                   Proton resonance in spectra of APP is
  shows the same 31P resonance positions                   assigned to NH4+ species ( = 7.0 ppm)
  with the same chemical shift anisotropies               Additional resonance at  = 9.0 ppm in
  as observed in the single components.                    spectra of ASiPP is due to protons in
 Chain length dramatically decreased                      hydrogen bridges
  upon composition (5 Q-units) and increases              Only one signal at  = 7.3 ppm in the
  again after activation up to 50 Q-units.                 spectrum of the non-activated composite
               1H MAS NMR between 297 K and 580 K
        Activation in the MAS rotor                                    Second cycle


                                                  T = 580K




                                                  T = 297K




                                                      100    80        60        40         20         0
                                                                             /ppm
0       50      /ppm    30      20       10      T = 297K
    First heating and subsequent cooling observed                 No further signals arise or vanish during
    by 1H MAS NMR. During the activation process                  cycling after activation. The 1H MAS NMR
    a second signal arises due to the ammoniac                    spectrum is reversible.
    loss. This new signal, which is assigned to
    protons in “bridging positions”, seems to be
    responsible for the high protonic conductivity.
               Chemical exchange and line merging

                                                                           T = 491 K
                 200 Hz
                                                                           T = 451 K

                                       k=1
                                                                           T = 441 K
                                       k=10
                                       k=100
                                                                           T = 421 K
                                       k=1000
                                                                           T = 351 K

          1                2
                                                      15     12     9        6         3
Theoretical dependence of the line shape on                        /ppm
 the exchange rate k for a two-spin-system
                                                   1HMAS NMR spectrum of activated
                                                    composite shows two signals at 297 K.
Three cases:
                                                 At higher temperatures the signals are
for k « D two lines are observed
                                                  broadened and merge to one line.
(slow exchange),
for k  D one very broad signal that often      It can be concluded that a chemical
cannot be observed                                exchange takes place between the two
                                                  species.
for k » D one narrow signal at the averaged
line position is observed (fast exchange).
                Determination of exchange rates

                            4.0           Peak intensities in
                                          deopendence on themixing
                                                                                           1000




                                                                     exchange rate k/s-1
                                          time (T=320 K)

                            8.0
                                                       7.5 ppm

                            12.0                                                           100
                                                        12 ppm

     12.0     8.0     4.0  /ppm
                                     0   200 400 600 8001000
   2D-EXSY spectrum of                        mix/ms                                             2.2 2.4 2.6 2.8 3.0 3.2 3.4
  an activated composite.                                                                                  1000 T-1 / K-1
  T = 297K, mix = 10 ms.           Exchange rates k were
                                     measured between 297 K                             An Arrhenius-plot of k for
 The presence of                    and 440 K using 1D
  cross peaks                                                                            temperatures above 370 K
                                     NOESY NMR.                                          yields an activation energy
  indicates the
  chemical exchange.                The analysis of the peak                            of 0.8 eV
                                     intensities in dependence
                                     on the mixing time gives
                                     the exchange rates.
    Diffusion measurements with PFG and SFG NMR
    sequence       te        Adiff                           Ar=Ar1Ar2
                             attenuation due to diffusion    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}

                                                                                                               T/K
                                                                                         650 600   550   500       450      400


        20                                                                       1E-9
                                          20       PFG
   t0        t0 +                  t0 + D t0 + D +                                                 Ea=0,275 eV
                                                                              1E-10
                                                            SFG




                                                                       D / m 2s- 1
                        G = 60 T/m
 p/2         p/2                      p/2                                     1E-11


                                                                                                     PFG NMR
                                                            NMR               1E-12                  SFG NMR
                                                                                                                     Ea=0,308 eV
             1                     1+2       21+2
                                                                              1E-13
                                                                                        1.5              2.0                2.5
                                                                                                               3
                                                                                                          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 MAS NMR measurements demonstrate that (non-ammonium) bridging protons are created by the
    activation procedure of the composite.
   31PMAS 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