A synchrotron EXAFS and XRD study of nickel carbon materials for fuel cell catalysis

Bridget Ingham, IRL NZIP conference, 19 October 2011

Fuel cells

Catalyst requirements:• Electrocatalytically active• Passive against corrosion

in operating conditions

+

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)

X-ray diffraction

X-ray diffraction: resultsAs-prepared

X-ray diffraction: resultsAfter electrochemical treatment

Ni3C?

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.

X-ray absorption spectroscopy

Sample(working

electrode)

(O2)

(Thermometer)

(Heating tape)

Reference electrode

Counter electrode

Data extraction

EXAFS

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)

EXAFS models

What if no crystal structure exists?

Single shell theory…

Results

‘Low’

‘Medium’

‘High’

Summary

Summary

Summary

‘High’

‘Medium’

‘Low’

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 Ni3C observed, EXAFS can be fitted with this plus Ni-Ni and Ni-C nn

• ‘High’ Ni conc. samples show significant changes with treatment, Ni3C 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.

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.