First Principles-based Atomistic Modeling in Hydrogen Fuel Cell Technologies: Promises and Challenges Gyeong S. Hwang Department of Chemical Engineering.

First Principles-based Atomistic Modeling First Principles-based Atomistic Modeling

in Hydrogen Fuel Cell Technologies:in Hydrogen Fuel Cell Technologies: Promises and ChallengesPromises and Challenges

Gyeong S. HwangGyeong S. Hwang

Department of Chemical EngineeringDepartment of Chemical EngineeringCenter for Nano- and Molecular Science and Center for Nano- and Molecular Science and

TechnologyTechnologyTexas Materials InstituteTexas Materials Institute

Institute of Theoretical ChemistryInstitute of Theoretical Chemistry

The University of Texas at AustinThe 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.

HydrogenFuel Cells

Fukuoka Hydrogen Town

ConnecticutAngstrom

Eco-Sailboat

Boeing

Toshiba

Rosy Outlook: Electric and hybrid vehicles, Potable electronic devices, Base load power plants, Emergency power systems, ……

"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."

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.

January 28, 2003State of the Union Address

HYDROGEN FUEL INITIATIVE – yr 2003

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

Fuel Cell Cost and Durability

Hydrogen Production and Delivery

Hydrogen Storage

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

ReliableReliable; Low-costLow-cost; High-performanceHigh-performance

Catalyst Research

Increasing catalytic activity, particularly for the oxygen reduction reaction (ORR) Reducing CO poisoning Developing cheaper, more abundant alternatives to Pt.

H2 2H+ + 2e–

2H+ + 2e– + ½O2 H2O

Platinum (Pt) catalysts

High Temperature (> 100oC) Membranes Water Management Stack Cooling High System Complexity Uneven Reactant Distribution ….

O2 from Air

Heat (85 oC)

H2O(g), Air

Fuel H2

Used Fuel Recirculates

Flow Field Plate

Anode Cathode

Proton Exchange Membrane (PEM)

Catalyst

ELECTRIC CIRCUIT(40%-60% Efficiency)

FC stack

Catalyst Design: Magic or Science ..??!

“Chemist Wins Nobel For Catalyst Studies”

Alchemy

Gerhard ErtlMax Planck Society

2007 Nobel PrizeIn Chemistry

Our current research is focused on gold-based bimetallic alloys, such as gold-palladium group metals.

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 CatalystsBinary/ternary combinations of Pd, Au, Ag, Co … …

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.

Kyle Hwang

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

Metal Alloy Nanoparticles for Catalysis

50 nmfrom G.G. Scherer

PtCo/C

Anode Cathode

e-e-

H2O

O2

H2

H+

electrolyte membrane

diffusion layer

catalyst

metal-metal interaction

bimetallic nanoparticles(Pd-Au, Pt-Au, ..)

low metal coordination metal-support interaction

supported single metal nanoparticles(Pd, Pt, Au, ..)

single crystal surfaces

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 usage2. Higher selectivity Little or no byproducts and waste3. Longer lifetime Reduced catalyst cost

Nanocatalysts: Fundamental Issues

Catalytic properties are governed by

– particle size and shape

– particle surface composition and structure

– particle-support interfacial interactions

Weak particle-support Interactions

– shape change

– sintering

– loss of their unique properties

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 metal-support interfacial interactions

from Krumeich, ETH

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:

Rational Design and Synthesis

AuPd/C

Nucleation, Growth and Structure

Surface Composition and Configuration

Catalytic Activity and Poisoning

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:

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.

H2 O2

OH+OH

H2O2

O+OH

H2 O2

OH+OH

H2O2

O+OH

Direct synthesis of H2O2 from H2 and O2

Oxidation of carbon monoxide

Production of vinyl acetate monomers

…..

What alloying effects ….. ???

Direct H2O2 Synthesis: Role of Pd Ensembles

Pd-Au Pt-Au

0.24

in eV

Pd monomer

Pure Pd

0.53

1.55

0.89

0.51

Pd monomers surrounded by less active Au atoms that suppress O-O bond scission are primarily responsible for the significantly enhanced selectivity towards H2O2 formation on PdAu alloys.

Ham, Hwang et al., J. Phys. Chem. C, in press (2009)

Gold Nanoparticles on TiO2(110)

… shows strong size-dependent catalytic activity.

TiO2 supported Au nanoparticles

single crystalline Au surfaces

Tsurf < 300 K

C=O

O=O

O=O

C=O

O=C=O

… unusual catalytic activity for CO oxidation even below room temperature … Haruta, catalytic today (1997)

low metal coordination particle-support interaction ... ???

Mean particle diameter, nm

Act

ivit

y

CO oxidation

Goodman et al.Science (1998)

Gold Nanoparticles on TiO2: Surface Chemistry

Gold particle – low coordination

Au-TiO2 interface

o

A1.48

O2 adsorption & diffusion Au particle growth & structure

CO oxidation

0.4 eV

1.4 eV

Supported Au Clusters( Role of the cluster-support interface)

O2 dissociation …

CO oxidation

Supported Au Clusters ( Role of the cluster-support interface)

2.0 eV

… no sizable barrier, in good agreement with

experimental observations*

*Haruta, Gold Bul. 37, 27 (2004)

+ CO(ad)

Haruta (2004)

Promise of Atomistic Modeling

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

Science 227, 917 (1985)

Computational Nanoengineering LabComputational Nanoengineering LabUT – Austin (since Fall 2001)

Synthesis Structure PropertiesSynthesis Structure Properties ……. ???. ???

TiO2(110)

Au

Metal nanocatalystsMetal nanocatalysts Semiconductor processingSemiconductor processing

Nanocrystal memoryNanocrystal memory

SiO2

Si

Supercapacitor energy storageSupercapacitor energy storage Si/Ge nanowire: batteries, thermoelectricsSi/Ge nanowire: batteries, thermoelectrics

SiAu

Si

SiO2

…. 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.

Progress from the computation approach contributes greatly to realizing experimental control of materials properties in the nanoscale regime.

First Principles-based Atomistic Modeling

In past 35+ years, computational power (driven by Moore’s Law) has increased

by over 6+ orders of magnitude.

Computational modeling now ‘auto-catalyses’ its own progress exponentially!!!

Supercomputer

ENIAC (1947-1955)

Exponential Growth in Computation

AcknowledgementsCurrent & Former Group Members: Current & Former Group Members:

Fourteen (14) PhD Students Fourteen (14) PhD Students Three (3) Post Doctoral AssociatesThree (3) Post Doctoral Associates Four (4) Visiting Professors Four (4) Visiting Professors

National Science Foundation (NIRT, CAREER, SGER, CBET)National Science Foundation (NIRT, CAREER, SGER, CBET) Department of Energy (SISGR)Department of Energy (SISGR) Robert A. Welch Foundation (2002-present)Robert A. Welch Foundation (2002-present) Semiconductor Research Corporation (CSR, BEP, FE) Semiconductor Research Corporation (CSR, BEP, FE) International SEMATECH (AMRC)International SEMATECH (AMRC) Korea Institute of Science and TechnologyKorea Institute of Science and Technology Tokyo Electron, Inc. Tokyo Electron, Inc. Applied Materials, Inc. Applied Materials, Inc. SKC, KCC, Intel, University of Texas at AustinSKC, KCC, Intel, University of Texas at Austin Texas Advanced Computing CenterTexas Advanced Computing Center

Sponsors (Current & Past):Sponsors (Current & Past):