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First Principles-based Atomistic Modeling in Hydrogen Fuel Cell Technologies: Promises and Challenges Gyeong S. Hwang Department of Chemical Engineering Center for Nano- and Molecular Science and Technology Texas Materials Institute of Theoretical Chemistry The University of Texas at Austin
GREEN POWER: Hydrogen Fuel Cells HYDROGEN + FUEL CELLS … combines hydrogen and oxygen to produce electricity, heat, and water. … address the two most important energy challenges: § reducing carbon dioxide emissions; § lowering dependence on non-renewable fossil fuels.
Rosy Outlook: Angstrom ü ü ü Electric and hybrid vehicles, Potable electronic devices, Base load power plants, Emergency power systems, …… Connecticut Toshiba Boeing Eco-Sailboat Fukuoka Hydrogen Town Hydrogen Fuel Cells
HYDROGEN FUEL INITIATIVE – yr 2003 "Tonight I am proposing $1. 2 billion (for 5 years) in research funding so that America can lead the world in developing clean, hydrogen-powered automobiles. " January 28, 2003 State of the Union Address The initiative’s goal is to develop the technologies by 2015 that will enable U. S. industry to make hydrogen-powered cars available to consumers by 2020. Korea: The government only started funding hydrogen-related R&D in 1998, but is emerging as a major player. A new program was launched in 2004 with a budget of $586 million through to 2011.
Technical Challenges: … that needs to be addressed through R&D to pave the way for commercialization of fuel cell and hydrogen infrastructure technologies Hydrogen Production and Delivery Hydrogen Storage Fuel Cell Cost and Durability
Recent Progress in R & D: Hydrogen program led by the Department of Energy (DOE) has made important progress in R & D: § reduce the cost of producing hydrogen from natural gas; § develop a sophisticated model to identify and optimize major elements of a projected hydrogen delivery infrastructure; § increase by 50% the storage capacity of hydrogen; § reduce the cost and improve the durability of fuel cells. MORE DIFFICULT CHALLEGES lie ahead: • find a technology that can store enough hydrogen on board a vehicle to achieve a 300 -mile driving range; • reduce the cost of delivering hydrogen to consumers; • further reduce the cost and improve the durability of fuel cells.
Fuel Cells: Fundamental Issues Reliable; Low-cost; High-performance Reliable Low-cost FC stack § High Temperature (> 100 o. C) Membranes § Water Management § Stack Cooling § High System Complexity § Uneven Reactant Distribution § …. ELECTRIC CIRCUIT (40%-60% Efficiency) Catalyst Research O 2 from Air Fuel H 2 2 H+ + 2 e– + ½O 2 H 2 O Heat (85 o. C) Used Fuel Recirculates H 2 O(g), Air Flow Field Plate Anode Catalyst Cathode Proton Exchange Membrane (PEM) Platinum (Pt) catalysts § Increasing catalytic activity, particularly for the oxygen reduction reaction (ORR) § Reducing CO poisoning § Developing cheaper, more abundant alternatives to Pt.
Catalyst Design: Magic or Science. . ? ? ! Alchemy 2007 Nobel Prize In Chemistry “Chemist Wins Nobel For Catalyst Studies” Gerhard Ertl Max Planck Society
Non-Platinum Catalysts for PEM Fuel Cells Develop low-Pt or Pt-free metal catalysts, with similar (or better) activity and performance durability to the currently used Pt-based catalysts … Pt monolayer supported on less expensive metals Pt-M alloys (M=Ni, Co, Fe, Ti, V, …) Core-Shell Nanoparticles Novel Pt-free Catalysts §Binary/ternary combinations of Pd, Au, Ag, Co … §… Our current research is focused on gold-based bimetallic alloys, such as gold-palladium group metals.
Why Gold-based Nanocatalysts ? Gold nanoparticles exhibit extraordinarily high activity for various catalytic oxidation processes at or below room temperature, while its bulk counter part has long been known to be chemically inert. Advantages of gold-based alloy nanocatalysts include: § much less sensitive to CO poisoning than Pt § less expensive than Pt § bifunctional catalytic activity ---- can be used for both anode and cathode reactions in fuel cells Kyle Hwang
Metal Alloy Nanoparticles for Catalysis Metal nanoparticles are ideal catalysts, exhibiting high activity and moreover their catalytic function can be controlled by tailoring their size and shape. Such tunability may allow design and synthesis of next generation catalysts with: 1. Higher activity Less catalytic usage 2. Higher selectivity Little or no byproducts and waste 3. Longer lifetime Reduced catalyst cost metal-metal interaction supported single metal nanoparticles (Pd, Pt, Au, . . ) bimetallic nanoparticles (Pd-Au, Pt-Au, . . ) 50 nm from G. G. Scherer e- diffusion layer e- low metal coordination metal-support interaction single crystal surfaces Pt. Co/C H 2 O H 2 H+ Anode O 2 Cathode electrolyte catalyst membrane
Nanocatalysts: Fundamental Issues q Catalytic properties are governed by – particle size and shape – particle surface composition and structure – particle-support interfacial interactions from Krumeich, ETH q Weak particle-support Interactions – shape change – sintering – loss of their unique properties q Therefore it is necessary to better understand: – synthesis, structure and thermal stability of supported metal nanoparticles – surface structure and chemistry of support materials – dependence on catalytic activity on particle structure, metal-metal interactions in metal particles, and metalsupport interfacial interactions from Goodman, TAMU “ Better understanding the fundamentals is key to developing more effective nanocatalysts”
Rational Design and Development of Novel Bimetallic Electrocatalysts for Fuel Cell Applications through First Principles-based Atomistic Modeling Develop a quantitative understanding of the nature and reactivity of Au-based bimetallic nanocatalysts, with particular focus on the effects of catalyst support materials and process conditions on: Nucleation, Growth and Structure Surface Composition and Configuration Catalytic Activity and Poisoning Rational Design and Synthesis Au. Pd/C
Gold-Palladium Nanocatalysts Bimetallic palladium-gold (Pd-Au) alloys have been found to significantly increase catalytic efficiency, compared to the monometallic Pd and Au counterparts, in various reactions including: § § Direct synthesis of H 2 O 2 from H 2 and O 2 Oxidation of carbon monoxide Production of vinyl acetate monomers …. . Pd-Au catalysts have also recently received much attention for hydrogen fuel cell applications, because they are much less sensitive to CO poisoning than pure Pt or even Pt-Ru. What alloying effects …. . ? ? ?
Direct H 2 O 2 Synthesis: Role of Pd Ensembles Pd monomer 0. 53 1. 55 Pd-Au Pt-Au Pd monomers surrounded by less active Au atoms that suppress O-O bond scission are primarily responsible for the significantly enhanced selectivity towards H 2 O 2 formation on Pd. Au alloys. Pure Pd 0. 24 0. 89 0. 51 Ham, Hwang et al. , J. Phys. Chem. C, in press (2009) in e. V
Gold Nanoparticles on Ti. O 2(110) … unusual catalytic activity for CO oxidation even below room temperature … Haruta, catalytic today (1997) O=O O = C= O O=O C=O single crystalline Au surfaces Ti. O 2 supported Au nanoparticles Tsurf < 300 K … shows strong size-dependent catalytic activity. Activity low metal coordination particle-support interaction. . . ? ? ? CO oxidation Goodman et al. Science (1998) Mean particle diameter, nm
Gold Nanoparticles on Ti. O 2: Surface Chemistry O 2 adsorption & diffusion Au particle growth & structure 1. 48 CO oxidation § § Gold particle – low coordination Au-Ti. O 2 interface
Supported Au Clusters ( Role of the cluster-support interface) O 2 dissociation … 0. 4 e. V 1. 4 e. V
Supported Au Clusters ( Role of the cluster-support interface) … no sizable barrier, in good agreement with CO oxidation experimental observations* *Haruta, Gold Bul. 37, 27 (2004) + CO(ad) 2. 0 e. V Haruta (2004)
Promise of Atomistic Modeling Science 227, 917 (1985) First principles-based atomistic modeling can complement experimental observations and also provide many valuable hints on how to control the structure and function of supported metal nanocatalysts, while current experimental techniques are often limited to providing complementary real space information. This further offers an invaluable guide to the rational design and synthesis of bimetallic nanoparticle-based materials for various catalytic applications
Supercapacitor energy storage Si/Ge nanowire: batteries, thermoelectrics Si. O 2 Si Au Synthesis Structure Nanocrystal memory …. ? ? ? Properties Computational Nanoengineering Lab Si UT – Austin (since Fall 2001) Au Si. O 2 Ti. O 2(110) Semiconductor processing Metal nanocatalysts Si
First Principles-based Atomistic Modeling q …. allows us to explore complex chemical and physical phenomena occurring at nanomaterials and nanosystems at the atomic scale. So it has emerged as an increasingly important area of research in nanoscale science and engineering. q Progress from the computation approach contributes greatly to realizing experimental control of materials properties in the nanoscale regime.
Exponential Growth in Computation In past 35+ years, computational power (driven by Moore’s Law) has increased by over 6+ orders of magnitude. ENIAC (1947 -1955) Supercomputer Computational modeling now ‘auto-catalyses’ its own progress exponentially!!!
Acknowledgements Current & Former Group Members: § Fourteen (14) Ph. D Students § Three (3) Post Doctoral Associates § Four (4) Visiting Professors Sponsors (Current & Past): § National Science Foundation (NIRT, CAREER, SGER, CBET) § Department of Energy (SISGR) § Robert A. Welch Foundation (2002 -present) § Semiconductor Research Corporation (CSR, BEP, FE) § International SEMATECH (AMRC) § Korea Institute of Science and Technology § Tokyo Electron, Inc. § Applied Materials, Inc. § SKC, KCC, Intel, University of Texas at Austin § Texas Advanced Computing Center