In situ XAFS studies of carbon supported Pt and PtNi(1:1) catalysts for the oxygen reduction reaction in PEMFCs

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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

Nov. , ૧૩ ૨૦૦૯

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

Nov. 13, 2009

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!