Characterization of proton conducting polyphosphate composite s

1. Characterization of proton conducting polyphosphate composites D. Freude2, S. Haufe3, D. Prochnow2, H.Y. Tu1, U. Stimming11Technische 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 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.

2. 250°C 12h, NH3 200 °C 2 h, NH3 280 °C 24 h, NH3 Synthesis of polyphosphate composite silicon nitrogen phosphorus oxygen XRD-structure of NH4PO3 XRD-structure of (NH4)2SiP4O13 Preparation of NH4PO3:NH4H2PO4 + (NH2)2 NH4PO3 (modification I) NH4PO3 (modification II) Preparation of composite: 10 NH4PO3 + SiO2 6 NH4PO3 / (NH4)2SiP4O13

3. Characterization by XRD, CA, REM 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. XRD REM X-ray diffraction indicates the presence of NH4PO3 in modifications I and IIand (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

4. Characterization by TG 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.

5. Conductivity measurements • 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 1×10-7 S/cm at 50 °C to 2×10-2 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. Arrhenius plot of conductivity measured by ac impedance spectroscopy in dry hydrogen

6. Gas variation • 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. Arrhenius plot of conductivity after activation of composite material measured in dry hydrogen, dry oxygen, dry argon and humid hydrogen

7. NomenclatureQ0: isolated PO4-tetrahedrons, Q1: chain end groups, Q2: middle groups in chain anions NMR measurements 31P MAS NMRT = 297 K 31P MAS NMR spectrum of ASiPP at rot = 10 kHz. Asterisks denote spinning side bands. 31P MAS NMR spectrum of APP-II at rot = 10 kHz. Asterisks denote spinning side bands. • Four Q2-signals due to four non-crystallographic sites in ASiPP (cf. XRD) • Chain length about 500 Q-units in ASiPP • One Q2-signal according to one non-crystallographic site in APP-II (cf. XRD) • Chain length about 150 Q-units • Q0-signal due to impurities

8. 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) ASiPP Composite activated Composite (non-activated) APP 1H MAS NMR spectrum of non-activated composite and its single components 31P MAS NMR spectrum of non-activated composite compared to the spectral addition of single components • Proton resonance in spectra of APP is assigned to NH4+ species (d = 7.0 ppm) • Additional resonance at d = 9.0 ppm in spectra of ASiPP is due to protons in hydrogen bridges • Only one signal at d = 7.3 ppm in the spectrum of the non-activated composite • 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.

9. 1H MAS NMR between 297 K and 580 K Activation in the MAS rotor Second cycle T = 580K T = 297K 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. No further signals arise or vanish during cycling after activation. The 1H MAS NMR spectrum is reversible.

10. T = 491 K T = 451 K T = 441 K T = 421 K T = 351 K d /ppm 15 12 9 6 3 Chemical exchange and line merging Theoretical dependence of the line shape on the exchange rate k for a two-spin-system Three cases: for k« Dn two lines are observed (slow exchange), for kDn one very broad signal that often cannot be observed for k» Dn one narrow signal at the averaged line position is observed (fast exchange). • 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.

11. 4.0 1000 7.5 ppm 8.0 exchange rate k/s-1 12.0 100 12 ppm d /ppm 12.0 8.0 4.0 0 200 400 600 800 1000 2.2 2.4 2.6 2.8 3.0 3.2 3.4 mix/ms 1000 T-1 / K-1 Determination of exchange rates Peak intensities in deopendence on themixing time (T=320 K) 2D-EXSY spectrum of an activated composite. T = 297K, mix = 10 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. • An Arrhenius-plot of k for temperatures above 370 K yields an activation energy of 0.8 eV • The presence of cross peaks indicates the chemical exchange.

12. 650 600 550 500 450 400 20 PFG 20 1E-9 t0 + d t0 + D t0 t0 + D + d E =0,275 eV a 1E-10 SFG G = 60 T/m 1 - s 2 p/2 D / m p/2 p/2 1E-11 NMR PFG NMR 1E-12 SFG NMR E =0,308 eV t1+t2 2t1+t2 t1 a 1E-13 1.5 2.0 2.5 3 10 K / T Diffusion measurements with PFG and SFG NMR T / K • Proton diffusion measurements were performed by means of PFG (Pulsed Field Gradient) NMR at nL = 400 MHz up to 450 K and SFG (Stray Field Gradient) NMR at nL = 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.

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 (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 S. Haufe, Thesis, Technical University of Munich, 2002 D. Prochnow, Thesis in preparation, University of Leipzig