Post on 11-May-2015
description
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.