1. Characterization of proton conducting polyphosphate composites 1894: Wilhelm Ostwald demonstrates that fuel cells are not limited by the Carnot efficiency. D. Freude 2, S. Haufe 3, D. Prochnow 2, H.Y. Tu 1, U. Stimming 1 1 Technische Universität München, 2 Universität Leipzig, 3 Proton 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. 2001: Composite electrolytes: preparation, characterization and investigation of the conductivity; PhD thesis by Stefan Haufe

2. Synthesis of polyphosphate composite nitrogen phosphorus oxygen Preparation of NH 4 PO 3 : NH 4 H 2 PO 4 + (NH 2 ) 2 NH 4 PO 3 (modification I) NH 4 PO 3 (modification II) 200 °C 2 h, NH 3 280 °C 24 h, NH 3 Preparation of composite: 10 NH 4 PO 3 + SiO 2 6 NH 4 PO 3 / (NH 4 ) 2 SiP 4 O 13 250°C 12h, NH 3 XRD-structure of NH 4 PO 3 XRD-structure of (NH 4 ) 2 SiP 4 O 13 silicon

3. Characterization by XRD, CA, REM XRD X-ray diffraction indicates the presence of NH 4 PO 3 in modifications I and II and (NH 4 ) 2 SiP 4 O 13 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 [NH 4 PO 3 ] 6 [(NH 4 ) 2 SiP 4 O 13 ] 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

4. 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 NH 3 ) of 7% the material is thermally stable upon cycling between 50 °C and 300 °C. Characterization by TG

5. 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 NH 3 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 measurements

6. 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 variation

7. 31 P MAS NMR spectrum of APP-II at rot = 10 kHz. Asterisks denote spinning side bands. 31 P MAS NMR spectrum of ASiPP at rot = 10 kHz. Asterisks denote spinning side bands. Nomenclature Q 0 : isolated PO 4 -tetrahedrons, Q 1 : chain end groups, Q 2 : middle groups in chain anions 31 P MAS NMR T = 297 K  One Q 2 -signal according to one non- crystallographic site in APP-II (cf. XRD)  Chain length about 150 Q-units  Four Q 2 -signals due to four non- crystallographic sites in ASiPP (cf. XRD)  Chain length about 500 Q-units in ASiPP  Q 0 -signal due to impurities NMR measurements

8. 31 P MAS NMR spectrum of non-activated composite compared to the spectral addition of single components 31 P MAS NMR  Spectrum of (non-activated) composite shows the same 31 P 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 Composite (non-activated) ASiPP APP 1 H MAS NMR spectrum of non-activated composite and its single components Composite (non-activated)  Proton resonance in spectra of APP is assigned to NH 4 + 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 1 H MAS NMR Composite activated NMR spectra of the composite at T = 297 K

9. 1H MAS NMR between 297 K and 580 K First heating and subsequent cooling observed by 1 H 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 No further signals arise or vanish during cycling after activation. The 1 H MAS NMR spectrum is reversible. Second cycleActivation in the MAS rotor

10.  1 H 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 merging  / ppm T = 351 K T = 421 K T = 441 K T = 451 K T = 491 K 3 6 9 12 15 Theoretical 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).

11.  The presence of cross peaks indicates the chemical exchange.  /ppm 12.08.04.0 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 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 exchange rate k/s  1000 T  / K 

12. Diffusion measurements with PFG and SFG NMR sequence tete A diff attenuation due to diffusion A r =A r1 A r2 attenuation due to relaxation PFG 2t1+t22t1+t2 exp{-g 2 G 2 Dd 2 (D-d/3)}exp{-2t 1 /T 2 -t 2 /T 1 } SFG 2t1+t22t1+t2 exp{-g 2 G 2 D t 1 2 (t 2 +2t 1 /3)}exp{-2t 1 /T 2 -t 2 /T 1 }  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. t0t0 t 0 +  t 0 +  t 0 +  20 G = 60 T/m  PFG SFG NMR 20    T / K

13. 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 (NH 4 ) 2 SiP 4 O 13 as well as of NH 4 PO 3, 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.  1 H MAS NMR measurements demonstrate that (non-ammonium) bridging protons are created by the activation procedure of the composite.  31 P 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