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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 Technology2 Chemistry Department, Northeastern University
3 Nuvant Systems, Inc.
Sponsors: Department of Energy & Army Research Office
Nov. , ૧૩ ૨૦૦૯
Outline
• Motivation• Experimental Details• XANES Results• EXAFS Interpretation• Conclusions• Further Work
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Anode : 2H2 → 4H+ + 4e- (0V SHE)
Cathode :O2 + 4H+ + 4e- → 2H
2O (1.229V SHE)
(a) O2 + Pt → Pt―O
2
(b) Pt-O2 + H+ + e- → Pt―O
2H
(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)
Polymer Exchange Membrane Fuel Cell
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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
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Operando Fuel Cell Experiment
• Version 2 operando cell• Air-breathing cathode• Pd on anode• 1.2 mg/cm2 loading• 50°C operating temperature• Pt L
3 and Ni K edges
• Continuous scan (1-2 min) mode @
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Fuel Cell Performance
• PtNi/Pd shows higher open circuit voltage• Pt/Pd and PtNi/Pd have similar performance
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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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Data Quality: Ni edge in PtNi/C
• Merge of 10-20 scans• Data good to k=11
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Pt/C and PtNi/C Comparison
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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 catalystsPt-O path not stable in PtNi/CMetal-Oxygen σ2 large
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Structural Model: part 2
• 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 #
Attempt to get global information about oxygen
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Example Fits
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Fit Results: metal core
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Fit Results: oxygen coordination
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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)
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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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Thank You!