it\'s a presentation for APS Meeting In Iowa. It mainly introduces our work of rationalizing the superior reactivity of certain commercial alloy nanocatalysts by probing their physical and chemical properties through x-ray experiments and theoretical model simulation.

1. In situ XAFS studies of carbon supported Pt and PtNi(1:1)
catalysts for the oxygen reduction reaction in PEMFCs
Qingying Jia1, Emily Lewis2, Corey Grice3, Eugene Smotkin2, Carlo Segre1
1
CSRRI & BCPS Dept., Illinois Institute of Technology
2
Chemistry Department, Northeastern University
3
Nuvant Systems, Inc.
Sponsors: Department of Energy & Army Research Office
Nov. ૧૩, ૨૦૦૯

2. Outline
• Motivation
• Experimental Details
• XANES Results
• EXAFS Interpretation
• Conclusions
• Further Work
Nov. ૧૩, ૨૦૦૯

3. Polymer Exchange Membrane Fuel Cell
Anode : 2H2 → 4H+ + 4e- (0V SHE)
Cathode :O2 + 4H+ + 4e- → 2H2O (1.229V SHE)
(a) O2 + Pt → Pt―O2
(b) Pt-O2 + H+ + e- → Pt―O2H
(c) Pt―O2H + Pt → Pt―OH + Pt―O
(d) Pt―OH + Pt―O + 3H+ + 3e- → 2Pt + 2H2O
U.S. Department of Defense (DoD) Fuel Cell Test and Evaluation Center (FCTec)
Takako Toda et al. J. Electrochem Soc. 146 (10) 3750-3756 (1999)
Nov. ૧૩, ૨૦૦૯

4. Why PtNi?
• Uses less platinum (less expensive)
• Oxygen reduction reaction is improved
– Pt electronic structure modified
– Pt catalyst geometric structure is modified
– Static oxygen adsorbates inhibited
– Pt segregation onto Surface
– Overpotential reduced
XAFS can address these
Nov. 13, 2009

5. Operando Fuel Cell Experiment
• Version 2 operando cell
• Air-breathing cathode
• Pd on anode
• 1.2 mg/cm2 loading
• 50°C operating temperature
• Pt L3 and Ni K edges
• Continuous scan (1-2 min)
mode @
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6. Fuel Cell Performance
• PtNi/Pd shows higher open circuit voltage
• Pt/Pd and PtNi/Pd have similar performance
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7. Data Quality: Pt edge in Pt/C
• Merge of 10-20 scans
• Data good to k=13
• Oxygen peak just
below 2 Å
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8. Data Quality: Ni edge in PtNi/C
• Merge of 10-20 scans
• Data good to k=11
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9. Pt/C and PtNi/C Comparison
Nov. 13, 2009

10. Structural Model: part 1
• “First” shell model
– Pt/C: Pt-Pt; Pt-O
– PtNi/C: Pt-Pt; Pt-Ni; Ni-Ni; Ni-O
• 2.76 Å- 1 < k < 10.83 Å- 1
• 1.3 Å < R < 3.3 Å
• Fit each edge & potential individually
Metal cluster core is constant for both catalysts
Pt-O path not stable in PtNi/C
Metal-Oxygen σ2 large
Nov. 13, 2009

11. Structural Model: part 2
Attempt to get global information about oxygen
• Fit all potentials with same metal core parameters for
each catalyst
• Simultaneous fit of Pt and Ni edges in PtNi/C with
constraint on Pt-Ni path
• Fit in k, k2, and k3 weighting simultaneously
• M-O path constraints
– length common across potentials
– σ2 fixed to 0.01
– Pt-O in PtNi/C are refined with a single occupation #
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12. Example Fits
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13. Fit Results: metal core
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14. Fit Results: oxygen coordination
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15. Conclusions: what does Ni do?
• Resides predominantly in metal core of nanoparticle
• Eliminates static Pt-O bonds at all potentials
• Number of O near neighbors “increases” with potential
• Lengthens Pt-O and shortens Pt-Pt bond
• Reduces Pt white line in most reduced state (0 mV)
• Open circuit voltage is increased (reduction in overpotential)
Nov. 13, 2009

16. Further Work
– Understand strain and ligand effects by Feff calculations
– Analyzable data in just 1 scan (can look at time evolution)
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17. Thank You!
Nov. 13, 2009