The document summarizes a study that used synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (EXAFS) to characterize the local atomic structure of nickel-carbon catalyst materials with varying nickel concentrations (5-44% nickel). XRD showed different crystalline phases present before and after electrochemical treatment. EXAFS identified three structural regions - "Low" (5% Ni, large Ni-C bonds), "Medium" (11-24% Ni, mixture of Ni-Ni, Ni-C bonds), and "High" (35-44% Ni, crystalline Ni3C phase). The "High" Ni samples changed most after treatment, dissolving the Ni3C phase, while

1. A synchrotron EXAFS and XRD study of nickel carbon materials for fuel cell catalysis Bridget Ingham, IRL NZIP conference, 19 October 2011

2. Fuel cells Catalyst requirements: Electrocatalytically active Passive against corrosion in operating conditions +

3. Synthesis and electrochemical treatment Ni-C is sputtered from a single target of various Ni:C stoichiometry (5-44 at% Ni) Polarisation treatment used to condition the Ni-C electrodes: Open circuit potential +50 mV (NHE) +400 mV (NHE) -30 mV (NHE) XRD: Australian Synchrotron, Powder Diffraction beam line (crystalline phases) EXAFS: National Synchrotron Light Source (Brookhaven National Laboratory, NY), beam line X10C (short-range atomic order around Ni)

4. X-ray diffraction

5. X-ray diffraction: results As-prepared

6. X-ray diffraction: results After electrochemical treatment

7. Ni 3 C? S. Nagakura, J. Phys. Soc. Jap. 12 (1957) 482; 13 (1958) 1005 Electron diffraction is more sensitive to C relative to Ni than X-ray diffraction.

8. X-ray absorption spectroscopy Sample (working electrode) (O 2 ) (Thermometer) (Heating tape) Reference electrode Counter electrode

9. Data extraction EXAFS

10. EXAFS models Ideally, create a model from a known crystal structure -> the fitted parameters show the average local deviation from the crystal structure (bond distances, co-ordination #s)

11. EXAFS models What if no crystal structure exists? Single shell theory…

12. Results ‘ Low’ ‘ Medium’ ‘ High’

13. Summary

14. Summary

15. Summary ‘ High’ ‘ Medium’ ‘ Low’

16. Conclusions Three different structural regions have been identified in the Ni-C system: ‘ Low’, 5% Ni – large Ni-C nn with some Ni-Ni nn; no longer range order ‘ Medium’, 11-24% Ni – large Ni-Ni nn, some Ni-C nn, some Ni-Ni and Ni-C nnn, no longer range order ‘ High’, 35-44% Ni – crystalline Ni 3 C observed, EXAFS can be fitted with this plus Ni-Ni and Ni-C nn ‘ High’ Ni conc. samples show significant changes with treatment, Ni 3 C is dissolved (XRD) and significant loss of Ni (XANES) ‘ Low’ and ‘medium’ Ni conc. samples show very little change (EXAFS) after electrochemical passivation treatment -> suitable materials for electrocatalysts. Previous literature: 25% Ni optimal.

17. Acknowledgements Cambridge University, UK Kieran Fahy, Xiao Yao Chin, Eric Rees, Gareth Haslam, Zoe Barber, Tim Burstein Imperial College London, UK Mary Ryan Industrial Research Ltd, NZ Nicola Gaston, Christian Dotzler NSLS and BNL Hugh Isaacs, Kenneth Sutter, Larry Fareria, Mike Sansone Funding NZ FRST (NERF) C08X0409 EPSRC Portions of this research were undertaken on the Powder Diffraction beam line at the Australian Synchrotron, Victoria, Australia and beam line X10C at the National Synchrotron Light Source. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences.