In-situ X-ray Spectroscopy and Scattering Diagnostic Studies of PEFC Cathode Catalysts D. Myers, M. Smith, A.J. Kropf, M. Ferrandon, and J. Gilbert Argonne National Laboratory, Argonne, Illinois, United States G. Wu, J. Chlistunoff, C. Johnston, and P. Zelenay Los Alamos National Laboratory, Los Alamos, New Mexico, United States DIAGNOSTIC TOOLS FOR FUEL CELL TECHNOLOGIES Trondheim, Norway June 23-24, 2009
Why don’t we have “two fuel cell cars in every garage”? Major hurdles to overcome – Cost • 50% of cost of PEFC stack is due to Pt catalyst* – Durability • Pt and Pt alloy cathode electrocatalysts lose electrochemically-active surface area with time – Fuel storage, availability, and delivery e - e - e - e - How can we get there? H 2 H 2 O 2 O 2 N 2 N 2 – Materials and engineering advances O 2 O 2 H 2 H 2 N 2 N 2 H 2 H 2 N 2 N 2 • better utilization/performance H 2 H 2 O 2 O 2 O 2 O 2 N 2 N 2 H+ H+ • lower cost (e.g., PGM alternatives) H 2 H 2 O 2 O 2 N 2 N 2 – Fundamental studies of materials H 2 H 2 O 2 O 2 Electrolyte Electrolyte • how they work Cathode Cathode Anode Anode • what limits their performance *2007 Status, Directed Technologies Incorporated Study, Feb. 2008 2
How can we get the necessary information? What’s needed for rational design of catalysts: identity of active site; relationship between structure and degradation Must “see” inside the fuel cell while it’s running with 0.1-10 nm “vision” Probe must penetrate through flow field, gas diffusion layer, and ionomer to characterize catalyst on the atomic level X-rays can penetrate through low atomic number materials and have wavelengths on the order of atomic dimensions Synchrotron X-ray sources (high intensity, tunable wavelength), such as Argonne’s Advanced Photon Source, give us “X-ray vision” 3
X-ray Absorption Fine Structure (XAFS) Oxidation state of absorbing atom Distances between atoms h Number of neighboring atoms Identity of neighboring atoms Amount of absorbing material in beam 4
Small-Angle X-ray Scattering (SAXS) Gives information on particles 1 - 100 nm in size Shape Mean Size Size Distribution 5
Examples of systems studied with in-situ and ex-situ X-ray techniques Pt-based electrocatalyst degradation – Oxidation state and correlation of loss of Pt with voltage • X-ray absorption in an aqueous environment – Oxide formation and Pt particle growth as a function of potential cycling • Small angle X-ray scattering and anomalous small angle X-ray scattering • Aqueous environment and MEA Non-platinum group metal catalyst composition, structure, oxidation state, and amount of absorbing metal using X-ray absorption – During pyrolysis – Effect of post-pyrolysis acid treatment – As a function of potential in aqueous environment – In MEA during polarization 6
Cells for in situ X-ray studies of cathode catalysts 300 m thick window Potentiostat Potentiostat machined over three Reference Reference Working Working Counter Counter channels of single serpentine flow field* Fluorescence Fluorescence (modified Fuel Cell Detector Detector I f I f Technologies I t I t I t Hardware) I 0 I 0 X-ray X-ray In Situ In Situ APS APS Electrochemical Electrochemical Cell Cell *Based on published design: Principi, E.; Di Cicco, A.; Witkowski, A.; Marassi R. J. Synchrotron 7 Rad., 2007, 14, 276.
Aqueous in-situ XAFS shows potential dependence of Pt loss and Pt oxidation state Pt L Pt L 3 -edge XANES 3 -edge XANES 1.6 1.6 1.6 2 XAFS 1.4 V 1.4 V SCANS 1.4 V 10 mV/s 10 mV/s 1.1 V 1.1 V 1.1 V 1.1 V 1.5 1.5 1.5 Normalized Absorbance Normalized Absorbance Normalized Absorbance 0.8 V 0.8 V 0.8 V 0.8 V 1.4 1.4 1.4 0.5 V 0.5 V Open 1.3 1.3 1.3 Circuit 1.2 1.2 1.2 Potential cycling Š 1 st cycle Potential cycling Š 2 nd and subsequent cycles 1.1 1.1 1.1 0.5 V 1 1 1 Absorption edge loss over three cycles 0.9 0.9 0.9 0.8 0.8 0.8 11560 11560 11560 11560 11565 11565 11565 11565 11570 11570 11570 11570 11575 11575 11575 11575 11580 11580 11580 11580 11585 11585 11585 11585 11590 11590 11590 11590 Energy (eV) Energy (eV) Energy (eV) Height of “white line” extent of oxidation of Pt Height of Pt L 3 absorption edge amount of Pt in electrode 8
Platinum loss occurs during anodic and cathodic potential scans Greatest Pt loss observed in anodic step from 1.1 to 1.4 V 0.5 1.4 0 (% Loss) 2 0 1.2 % Loss Edge Step Height 4 6 -0.5 1 8 0.8 -1 10 Potential (V) 12 0.6 -1.5 14 -2 0.4 16 Potential Cycle 9
XAFS shows platinum loss and oxide formation are linked Pt loss is highest during oxide formation Approximately same extent of oxidation show different Pt loss rates – Evidence against major role of oxide dissolution – Evidence for dissolution of metal – “Time-resolved” experiments are underway Extent of Pt oxidation decreases with potential cycling - may be indicative of particle growth 10
SAXS studies shows Pt particle growth with cycling 100 20 wt% Pt/C 1 hr 90 2 hrs 80 3 hrs 70 5 hrs Frequency 60 7 hrs 50 10 hrs 40 12 hrs 30 14 hrs 20 20 wt% Pt/C 16 hrs 10 0 0 1 2 3 4 5 6 7 8 4.5 Particle Size (nm) 100 20 wt% Pt/C 4 Particle size (nm) 40 wt% Pt/C 80 Frequency 3.5 60 40 3 20 wt% Pt/C SAXS Analysis 20 2.5 TEM Analysis 0 2 0 1 2 3 4 5 6 7 8 9 10 = 40 cycles 0 2 4 6 8 10 12 14 16 Particle Size (nm) Cycle Time (hrs) M.C. Smith et al., J. Am. Chem. Soc., 2008. 11
Non-platinum group metal electrocatalysts Cobalt or iron either complexed with C-N polymer/molecule or pyrolyzed (J.P. Dodelet, Los Alamos NL, U. South Carolina, 3M, et al.) – Low cost • (Co ~US$ 3 /oz, abundance 20,000- 30,000 ppb in Earth’s crust vs 3-37 ppb for Pt) – Promising oxygen reduction activity, but lower than platinum group metals n – Good durability, but longer testing and H cycling tests are needed (>1000 hrs) N C Issues: Co R. Bashyam and P. Zelenay, Nature, 2006. – Identity of the active site is unknown • Metal center coordinated to pyridinic nitrogen Metal particle • Encapsulated metal catalyzes formation of active site – Metal leaches from catalyst during operation 12
XAFS analysis shows Co-polypyrrole (not pyrolyzed) catalyst changes with time/potential Slow break in: possible formation of ORR sites during operation or removal of site- blocking species Ex-situ XAFS data: as-prepared MEA contained a mixture of cobalt metal and a small oxide fraction In-situ XAFS data: cobalt metal fraction is removed and/or converted to higher oxidation state Three cobalt species observed in-situ : 1.5 1.92 Å 0.4 V, 0.3 V 2.05 Å 2.83 Å 1.0 Magnitude 3.10 Å 2.11 Å 2.06-2.08 Å 0.2 V, 0.1 V 0.5 0.1 V, low RH 0.2 V, low RH >0.3V, low RH >0.1 V, high RH 0.0 0 1 2 3 4 R (Å) H O/N Co 13
Los Alamos NL’s pyrolyzed polyaniline-Fe(Co)-C ORR catalysts Pt/C 14
Aqueous cell in-situ data for pyrolyzed polyaniline-Fe-C system 2.0 XAFS shows reversible reduction of Fe 3+ 1 catalyst component between 0.64 and 0.44 V 0.87 V 8 1.6 Fe is lost from the electrode with greatest loss 2 Absorbance observed during this reduction step 6 0.64 V 1.2 3 5 0.44 V 0.8 3.0 4 0.24 V 0.4 Fe 2 O 3 Fe 3 O 4 Wt% Fe 2.0 FeO 0.0 7050 7100 7150 7200 7250 FeS 2 1.0 FeSO 4 Energy (eV) Fe-phthalocyanine Fe metal 0.0 0.87 0.64 0.44 0.24 0.44 0.64 0.84 1.04 0.87 Potential (V vs. SHE) 15
Pyrolyzed polyaniline-Fe-C catalyst composition 1.4 1.4 MEA preparation: Acid-Treated Acid-Treated 1.2 1.2 Powder Powder – Removes metal coordination environ. coordination environ. Wt% Fe in indicated Wt% Fe in indicated MEA MEA 1.0 1.0 – Removes sulfides 0.8 0.8 – Oxidizes Fe 2+ to Fe 3+ 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 Fe metal Fe metal 1.5 1.5 FeS FeS FeS2 FeS2 Fresh MEA Fresh MEA Fe-pc Fe-pc Fe2O3 Fe2O3 Fe3O4 Fe3O4 FeO FeO FeSO4 FeSO4 MEA, 0.6 V for 200 h MEA, 0.6 V for 200 h 1.0 1.0 coordination Fe is lost from MEA during long- Wt% Fe in 0.5 0.5 indicated term polarization at 0.6 V environ. (approx. 50% loss) 0.0 0.0 Ratio of Fe 2 O 3 to Fe-pc coordination Fe2O3 Fe-pc Fe2O3 Fe-pc is approx. unchanged FeS2 FeS2 Fe3O4 Fe3O4 16
Summary In-situ X-ray absorption and scattering techniques are powerful for diagnosing the state of PEFC catalysts during operation New in-situ X-ray fuel cell block design allows XAFS studies in fluorescence mode – Enables study of very low loadings of low Z metals (e.g., Fe and Co) – Eliminates the need to modify flow field design – Allows the study of one electrode of a cell when the opposing electrode contains the same metal (e.g., can study Pt in a Pt cathode with a Pt anode) SAXS SAXS Future needs/experiments Detector Detector – Combination of scattering and absorption experiments with microsecond time resolution Sample Sample – Simultaneous spatio-temporal resolved (micrometer and microsecond) atomic, electronic, and particle size X-rays X-rays characterization for a wide range of metals (e.g., Pt XAFS XAFS Detector Detector and Co in Pt 3 Co catalyst) 17
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