Synthesis and Evaluation of Electrocatalysts for Fuel Cells

Synthesis and Evaluation of Electrocatalysts for Fuel Cells Jingguang Chen Center for Catalytic Science and Technology (CCST) Department of Chemical Engineering University of Delaware Newark, DE 19711 jgchen@udel.edu

PEM Fuel Cells: H 2 /Methanol/Glycerol Cathode: 6e - + 3/2 O 2 + 3H 2 O 6OH - 6OH - + 6H + 6H 2 O O2 and H 2 O feed stream Flow of protons Cathode Electrolyte (Ionomer Membrane) Anode H 2 O and CH 3 OH feed stream Electron flow via circuit Anode: CH 3 OH + H 2 O CO 2 + 6H + + 6e - CHEG-Related Challenges: Electroatalyst Issues: Cost of Pt or Pt/Ru; CO poisoning Membrane Issues: Low operating temperature (< 100 o C); cost of Nafion Transport Issues: Water management; three-phase region Fuel Issues: H 2 transportation and storage

Electrocatalyst Issues in PEM Fuel Cells Anode Reaction: CH 3 OH + H 2 O CO 2 + 6H + + 6e - Anode Electrocatalysts: Pt; Pt/Ru Issues: Cost of Pt/Ru; CO poisoning Cathode Reaction: 6e - + 3/2 O 2 + 3H 2 O 6OH - Cathode Electrocatalysts: Pt; Pt alloys Issues: Cost and low activity of Pt; stability of alloys

Reduction of Pt Contents in Electrocatalysts Bulk Metal Prices in the United States in 9/2005 Pt W Ti V Cr Mn Fe Co Ni Price ($/kg) 29,200 1.10 24.00 45.20 1.36 0.52 0.50 39.00 13.82 www.metalprices.com Potential advantages of bimetallic and carbide catalysts: Reduce cost Enhance activity

Bimetallic and Carbide Catalysts Ti V Cr Mn Fe Co Ni Cu Zr Nb Mo Tc Ru Rh Pd Ag Hf Ta W Re Os Ir Pt Au Metal carbides Pt-metal alloys Chemical/electronic properties are often tunable: Alloying with carbon or with another metal

Research Approach: Combining Model Surfaces, Theory and Electrocatalysis Single Crystal Model Surfaces - UHV studies - DFT modeling Bridging Materials Gap - Thin films - Supported catalyst Bridging Pressure Gap -Electrochemical evaluation - Objective: Do carbides possess the necessary activity AND stability for PEM fuel cells?

OUTLINE OF PRESENTATION: - UHV Studies and DFT Modeling on Carbide Films on Single Crystal Surfaces - Electrochemical Evaluation on Polycrystalline Tungsten Carbide Thin Films - Synthesis of Tungsten Carbide Films on Carbon Substrates (fiber, foam, cloth) Using PVD/CVD - Summary and Future Plans

Molecular Level Understanding Using Advanced Techniques Pt(111) Single Crystals Well-defined single crystal surfaces P chamber = 2 x 10-10 Torr Surface Spectroscopies: Elemental Composition (AES) Surface Order (LEED) Surface Structure (STM) Electronic Properties (NEXAFS; Valence Spectroscopies) Gas Phase Products (Mass Spectrometry) Surface Intermediates (Vib. Spectroscopy)

DFT of Surface d-band Center of Carbides WC W 2 C Controlling properties of bimetallic and carbide surfaces: Kitchin et al. Phys. Rev. Lett. 93 (2004) 156801 Kitchin et al. Catalysis Today 105 (2005) 66 Weigert et al. Topics in Catalysis 46 (2007) 349

Desirable/Prerequisite Properties for DMFC Electrocatalysts Decompose CH 3 OH to produce H (ads) Decompose H 2 O to produce H (ads) Desorb CO at room temperature Question: Does tungsten carbide have these properties?

Comparing C/W with Pt and Ru C/W (111) O/C/W (111) Pt/CW Pt (111) Ru (0001) CH 3 OH activity per metal atom 0.28 0.24 0.18 <0.03 <0.03 H 2 O activity per metal atom 0.18 0.10 0.06 Negligible CO desorption (K) 330 284 330 460 475 C/W, O/C/ and Pt/C/W are more active then Pt/Ru toward dissociation of methanol and water CO desorbs from tungsten carbides at lower temperatures than Pt/Ru more CO-tolerant J. Phys. Chem. B 105, 10037 (2001) [CH 3 OH on C/W(111)] J. Catal. 215, 254 (2003) [Pt/C/W(111)] J. Phys. Chem. B 107, 2029 (2003) [C/W(110)] J. Electrochem. Soc. 152 (2005) A1483 [Pt/C/W(110)]

Synthesis of WC and W 2 C Films CH 3 O C/W(111) CH 3 O W 2 C/WC films High S.A. W 2 C/WC powders Electrochemical testing Spectroscopic characterization

XRD Characterization of PVD Films 1400 D:\86-122a1.udf Intensity 1200 101 Single phase WC 1000 800 100 600 001 400 200 110 111 102 0 20 30 40 50 60 70 80 D:\86-132.udf Intensity 100000 10000 C Single phase W2C 1000 100 100 002 101 102 C 110 103 112 201 10 1 20 30 40 50 60 70 80 Zellner & Chen, Surface Science, 569 (2004) 89

Desorption of CO from Polycrystalline WC Polycrystalline Surface Pt WC ~0.8ML Pt/WC CO Desorption Temperature (K) Leading Edge 400 282 324 Peak 531 365 398 TPD studies of adsorbed CH 3 OH similar trend between single crystal surfaces and polycrystalline films.

Electrochemical Testing of Carbide Films

In-Situ XPS and Electrochemistry X-ray photoelectron spectroscopy (XPS) (1 x 10-8 Torr) transfer chamber (1 x 10-7 Torr) gate valve Manipulator with W-Foil working electrode gate valve Pt metal evaporation source sputtering gun three electrode electrochemical cell (purged with N 2 )

Surface Preparation on Polycrystalline W sputtering gas: C 2 H 4 Pt-wire on W Filament Pt deposition W Foil ~1200 K WC Surface ~600 K Pt / WC Surface Decomposition of ethylene over hot filament Annealing by resistive heating to ~ 1200 K to form WC Analysis of surface composition using XPS

In-Situ XPS and Electrochemistry X-ray photoelectron spectroscopy (XPS) (1 x 10-8 Torr) transfer chamber (1 Atm, N 2 ) gate valve Manipulator with W-Foil working electrode gate valve Pt metal evaporation source sputtering gun three electrode electrochemical cell (purged with N 2 )

X-ray photoelectron spectroscopy (XPS) (1 x 10-8 Torr) transfer chamber (1 Atm, N 2 ) gate valve Manipulator with W-Foil working electrode gate valve Pt metal evaporation source Deaerated room temperature electrolyte 0.05 M H 2 SO 4 (supporting) 0.2 M CH 3 OH (fuel molecule) sputtering gun three electrode electrochemical cell (purged with N 2 )

Cyclic Voltammetry (CV) of Pt, WC, Pt/WC Conditions: 0.2 M CH 3 OH in 0.05 M H 2 SO 4 Synergistic Effect: Pt-modified WC shows higher activity and stability for electro-oxidation of methanol Weigert et al. J. Phys. Chem. C, Letters, (2007)

Chronoamperometry (CA) of WC and Pt/WC Electrocatalysts for Fuel Cells Steady-state current of WC and 0.8 ML Pt/WC higher than Pt Needs in-situ study of active phases and stability of WC under fuel cell conditions Weigert & Chen, J. Phys. Chem. C, Letters, 111 (2007) 14617

Phase-Pure WC: PVD, CVD and TPR PVD and CVD synthesis of pure WC on various carbon substrates for fuel cell testing Supported WC particles produced by temperature programmed reaction (TPR) PVD and TPR: Weigert et al. J. Vac. Sci. Technol (2007) CVD: Beadle et al. Thin Solid Films (2007)

SEM of Tungsten Carbide Films Non-Reactive 450 C 100 nm Reactive 450 C Non-Reactive 1040 C Reactive 1040 C

Fuel Cell Test Conditions 2M methanol solution as fuel feed: 4 ml/min Air as oxidant: 500 sccm Serpentine channel flow-field Temperature range of (50 70 C) Temperature of cell, fuel, and oxidant streams Emphasis on anode performance Stability Open circuit potential (OCV) Effects of: Fuel concentration Fuel flow rate Operating temperature

Pt/WC as Anode in Fuel Cells Higher current density possible at 2M Improved current/power densities with Temp. Reaction-limited behavior Linear behavior of polarization curves Limitation of power densities by catalyst surface area E.C. Weigert et al. J. New Materials Electrochem. Systems (submitted)

Disadvantages of H 2 and Methanol - H 2 Fuel Cells: CO-free H 2 ; transportation; storage - Methanol Fuel Cells: toxicity; production of methanol Potential Advantages of Oxygenates CH 3 O H CH 2 O H CH 2 O H CH 3 CH 2 O H CH 2 O H CH 2 O H CH 2 O H methanol ethanol glycol glycerol Direct electrooxidation to e- and H + without reforming reaction

CH 3 O H CH 2 O H CH 3 CH 2 O H CH 2 O H CH 2 O H CH 2 O H CH 2 O H

Photoelectrochemical Cell (PEC): An electrochemical cell containing at least one photoelectrode through which it is able to convert light energy into electrical and/or chemical energy Load e - 3 Main Components 1. Photoelectrode 2. Counterelectrode 3. Electrolyte OX RED Electrolyte Redox Species Counterelectrode (Cathode) Photoelectrode (Anode) Carbides as potential counterelectrode in PEC

Conclusions and Future Efforts Case Study: Carbides as less expensive and more active electrocatalysts Single Crystal Model Surfaces - UHV studies - DFT modeling Bridging Materials Gap - Thin films - Supported catalyst Bridging Pressure Gap - Half-cell studies - Full-cell studies Other Research Efforts Related to Energy: - Carbides as Alternative Anode Electrocatalysts - Bimetallic Alloys as Alternative Cathode Electrocatalysts - Fuel Cells Using Biomass-Derived Molecules (glycerol, glycol) - Production of H 2 from Reforming of Oxygenates - Photoelectrochemical (PEC) for H 2 Production from Water

UD Energy Institute: Assessment of Fuel Cells and Electrocatalysts Basic Scientific Challenges UD Assets Energy Impact & Implications External Connections: R&D Deployment UD Needs Now Immediate Needs Identification of alternative electrocatalysts Production and storage of hydrogen Center for Catalytic Science and Technology UD Fuel Cell Center IEC More efficient and environmentfriendly utilization of fuels Fuel cell companies in Delaware and in other States New faculty with expertise in membrane Team of faculty interested fuel cells and PEC 5-10 Years Combination of solar devices with fuel cells for H 2 production from H 2 O Direct utilization of biomass-derived molecules as fuels Long Term Systems Integration Combination of solar devices with fuel cells for production of chemicals from H 2 O and CO 2