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Thermodynamically Tuned Nanophase Materials for Reversible Hydrogen Storage: Structure and Kinetics of Nanoparticle and Model System
Materials
Bruce ClemensStanford University
April 15, 2005- Part of the DOE Metal Hydride Center of Excellence -
This presentation does not contain any proprietary or confidential information
Project ID: STP22
Overview
• Timeline– Project Start Date: FY05– Project End Date: FY09– Percent Complete: New Project
• Budget– Total project funding (expected)
• DOE share$778,828• Contractor share $199,093
– Funding for FY05:• DOE share $150,000• Contractor share 37,500
• Barriers Addressed– Hydrogen Capacity and Reversibility– Weight and Volume– Efficiency – Lack of Understanding of Hydrogen
Physisorption and Chemisorption• Targets
– 6% Gravimetric Capacity– .045 kg/L Volumetric Capacity– -30/80°C min/max Delivery Temp.
• Partners– DOE Metal Hydride Center of
Excellence Members– MHCoE sub-team on
thermodynamically tuned nanophase materials
• (Caltech, JPL, HRL, U. Hawaii)
Objectives
• Perform In-Situ Structural Studies of Hydrogen Storage Materials– Utilize high brightness x-ray source at Stanford Synchrotron Radiation
Laboratory– Construct Sieverts apparatus for in-situ control of hydrogen content– Demonstrate feasibility of in-situ synchrotron studies
• Investigate Light Metal Hydride Model Material Systems– Use engineered thin film model systems to investigate phase change and
catalytic processes associated with hydrogen cycling
• Develop Kinetic Model of Nanoparticle Phase Transformations– Build continuum models of nanoparticle kinetics to illuminate mechanisms of
hydride formation in nanoscale materials
Approach
• In-Situ Structural Studies– Real time structural analysis using high brightness synchrotron radiation – In-situ hydrogen charging of candidate materials– Correlate structural changes with hydrogen charging characteristics
• Model Material System Design and Synthesis– Design and grow model material systems using physical vapor deposition
techniques such as sputtering– Use input from MHCoE partners and kinetic modeling to select candidate
materials
• Kinetic Modeling of Nanoparticle Transformations– Model kinetic processes of phase transformations in nanoparticles to guide
future material selection and design– Apply existing thermodynamic data to new model to shed light on nanoscale
processes
Model Systems: Mg/Pd Nanostructured Films
• Pd- and Pd/Ti-capped Mg films• Pd cap:
– Catalyzes H2 dissociation– Rapid diffusion of H atoms– Source for atomic hydrogen
• Ti cap:– Suggested as another possible
catalyst candidate• Thin film vapor phase synthesis:
– Atomic scale control of composition– Engineer interface density and
catalyst geometry– One-dimensional diffusion geometry– Ideal for reaction kinetic studies
• Samples sent to HRL team for compositional analysis
– Determine impurity content (especially oxygen)
500 nm Mg Film
Si substrate
15 nmPd Cap
12 nm Pd + 9nm TiCap
200 nm Mg Film
Si substrate
Model Systems: Mg/Pd Nanostructured Films
• Films Analyzed Using X-Ray Diffraction (XRD) to Examine Structure/Composiition• Diffraction Results:
– Mg film with strong (002) texture• No presence of second orientation or phase (e.g. MgO)
– Rocking curve width 5.2°• Demonstrates Ability to Deposit Highly Textured Mg Thin Films
– Nanostructured model system: proof of concept
2
46
103
2
46
104
2
4
Inte
nsity
70605040302θ(deg)
Pd (111) Pd (200)
Mg(002)
Mg(004)Si (004)(detuned)
12x103
10
8
6
4
2
0
Inte
nsity
25201510
Rocking Angle (deg)
FWHM = 5.2°
Mg (002)
Model Systems: Mg/Pd Nanostructured Films
• SIMS Depth Profiling Data– Shows film composition as function of depth from surface– Deuterium (2H) signal shows hydriding behavior of Mg thin film
Model Systems: Mg/Pd Nanostructured Films
• Mg/Pd Multilayer Films:– Demonstrates atomic level control of composition and catalyst distribution through
controlled sputter deposition– High interface density ~ 300 m2/cm3
• Ideal structures for studies of:– Catalyst effects– Interface effects
Si substrate
Mg/Pd multilayer
Model Systems: Mg/Pd Nanostructured Films
• Small angle x-ray scattering– Peaks show strong composition modulation– Analysis allows calculation of bilayer periodicity
• Samples used to determine tooling factors in sputtering chamber
• Demonstrates Ability to Engineer Material Structures at the Nanoscale
10 2
10 3
10 4
10 5
10 6
Inte
nsity
6543212θ(deg)
Model Systems: Mg/Pd Nanostructured Films
• Tooling Factor Determination– Series of three multilayer samples with different bilayer periodicity
• MgPd_Multilayer1: (60Å Mg/ 90Å Pd) nominal bilayer periodicity of 150ÅMgPd_Multilayer2: (90Å Mg/ 60Å Pd) nominal bilayer periodicity of 150Å
• MgPd_Multilayer3: (60Å Mg/ 60Å Pd) nominal bilayer periodicity of 120Å– Measured bilayer periodicity (from low angle x-ray reflectivity scans) gives
tooling factors for Mg and Pd
60
40
20
0
Λm
eas(
�)
806040200
Nominal Thickness (�)
Mg Pd
fPd
fMg
hMg+δ0hPd+δ
0
δ - reflects change in volume due to interface reaction
Preliminary result:δ = -1.8 nm
(to be verified in future work)Significant reaction
Model Systems: Mg2Si Nanostructured Films
• Mg2Si film deposited by codeposition of Mg and Si• Determine whether sample can be charged with hydrogen
– Collaborative effort with HRL team• Nanostructured Model System Using Novel Material and Synthesis
Technique
500 nm Mg2Si Film
Si substrate
15 nmPd Cap
Model Systems: Mg2Si Nanostructured Films
• Diffraction Data Before Hydrogen Charging In RTA Furnace– Mg2Si (200) and Pd (111) peaks overlap nearly exactly
• Mg2Si (220): 40.156°• Pd (111): 40.149°
– Sharpness and intensity of peak indicates formation of the Mg2Si phase
102
103
10 4
105
106
Inte
nsity
2.52.01.51.00.5
2θ (deg)
High Angle Scan Low Angle Scan
10
2
4
68
100
2
4
68
Inte
nsity
90807060504030202θ(deg)
Pd (111)Mg2Si (220) Si (004)
(detuned)
Evidence for formation of Mg2Si phase Oscillations give thickness - used to monitor H-induced volume changes
Model Systems: Future Work
• Nanoscale Material Synthesis Methods For Future Work
– Physical vapor deposition of thin films
– Thin film growth on mesopouroussilica substrates
– Nanoparticle generation from condensing vapor
– Mechanical milling
• Controlled formation of nanostructures and nanometer-scale chemical features
– Investigate effect of nanometer scale chemistry and structure in hydrogen-storage systems (e.g. Mg-catalyst)
Mesoporous silica substrate formed from decomposition of block-copolymer/TEOS solution
Au nanoparticle array formed by deposition onto mesoporous substrate
In-Situ Structural Studies: Progress
• Received Sieverts apparatus– Controlled hydrogen charging– Determine hydrogen storage
capacity of model systems
• Plumbing and hydrogen safety safeguard installation ongoing
– County approved piping– Flammable gas cabinet for
compressed hydrogen storage
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
In-Situ Structural Studies: Capabilities
Synchrotron RadiationFacilities at SSRL • Characteristics
– High flux– Controlled environment
• Temperature: RT to 1500 C• Control of atmosphere (eg. H2
pressure)
• Parallel x-ray detection– Linear (existing) Area (planned)– Rapid collection of diffraction data – 1-10 second acquisition time
• Has Potential to Study– Reaction kinetics– Hydrogen-induced phase transitions– Study these as a function of
• Storage media size and morphology• Temperature and environment
In-Situ Structural Studies: Future Goals
• Future Work– Install hydrogen charging apparatus
on SSRL beamline– Perform real time structural studies
while material charges– Analyze data to correlate hydriding
behavior with:• Reaction kinetics• Structural changes
Kinetic Modeling: Nanoscale Phase Transitions
Mg
0.6
0.4
0.2
0.0
x H
3.02.52.01.51.00.50.0
z
MgH2
Smaller particles charge and discharge faster
Hydrogen
Reaction slows with time
Kinetic Modeling: Nanoparticle Thermodynamic Properties
• Due to interface and surface effects, thermodynamic properties (melting points, structure, phase formation) of nanoparticles are distinctly different from that of bulk materials
• Thermodynamics of nanoparticles are largely unknown.
Extended solubility
g
xB
α Phaseβ Phase
Two phase (bulk)
Two phase (nano)
~γ/r
Cost of interface drives up free energy of two-phase system for nanoparticles
Example: Extended solid solubility
fcc Au-Fe extended solid solution nanoparticle (Li, Sinclair, Dai)
Kinetic Modeling: Extended Solid Solubility of H in Mg Nanoparticles
Interface cost drives up the energy of two-phase configuration
Energy of two-phase system above that of supersaturated solution!
Interface energy γ
hcp Mg
MgH2
25
20
15
10
5
0Max
imum
Sol
ubili
ty (%
H)
1.00.80.60.40.20.0
γ(N/m)/r(nm)
g = γ(N/m)/r(nm)
-30
-25
-20
-15
-10
g(kJ
/mol
e)
1.00.80.60.40.20.0
xH
MgH2
hcp Mg
two phase(bulk)
two phase(nano)
g = 0
g = 1
r
Two phase nanoparticle
Size changes the phase diagram!Predicated supersaturation is 3 orders of magnitude above bulk
Thermodynamic parameters from: Zeng, Klassen, Oelerich, Bormann, Int. J. Hydrogen Eng. 24, 989 (1999)
Kinetic Modeling: Extended Solubility in Mg2Si Nanoparticles
-60
-40
-20
0
∆ g(k
J/m
ole)
1.00.80.60.40.20.0
xSi
Mg2Si
hcp
two phase(bulk)
two phase(nano)
-20
-15
-10
-5
0
5
∆ g(k
J/m
ole)
1.0x10 -10.80.60.40.20.0
xSi
hcp
two phase
g=0
g=1
3.0
2.5
2.0
1.5
1.0
0.5
0.0Max
imum
Sol
ubili
ty (%
Si)
1.00.80.60.40.20.0
g = γ(N/m)/r(nm)
Interface energy cost raises energy of two-phase nanoparticle
• Predicted solubility for nanoparticle is 1000 x that for bulk
• Dramatic consequences for reaction pathways involving phase changes in nanoparticles (egthermodynamically tuned systems such as MgSi)
Kinetic Modeling: Nanoparticle Nucleation
2
1
0
-1
²GT/
²G*
B
2.01.51.00.50.0
z/r
z
r = 5
r = 4
r = 3r = 2 r = 1 r = 1/2
For r < 3r* nucleation is easier than in bulk!
Easier nucleationLower driving forces needed for driving phase transitions
Transformation occurs as interface sweeps through particle
Energy as a function of interface positionNucleation of second phase has interface energy cost
Nanoparticles have:• Dramatically different thermodynamics
and phase stability• Faster reaction kinetics• Easier nucleation
Upcoming Work and Milestones
Figure 2. Projec t schedul e fo r tec hnica l e ffor t by S tanf ord Univers ity (G o/No -Go sho w n by soli d circles )
Yea r 1 Yea r 2 Yea r 3 Yea r 4 Yea r 5
TA SK AN D MILES TO N E 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
1. In -si tu St ructu ral St udies
In-si tu synch rotr on di ffrac tion studies Construct /acq uire Sieve rts apparatus for in-s itu synch rotron s tudies
In-si tu synch rotr on studies of k ine tics and pha se tran s itions du ring hydr ogen cycli ng
Nano s tructu red M g-bas ed a lloys Nano s tructu red Li -ba sed a lloys
2. Mod el Sys tem s
Desig n o f m odel sys tem arc hitec ture
Growth of thin film a nd m ultilaye r systems
Structur a l studies in M g/cat a lyst multilayers
Hydro gen reac tion kine tics in Mg/ca talyst mu ltilayers a nd isl anded films
Sincle crys tal films
Fe edback to m ate ria l des ign 3. Ki netic M ode ling o f
Nan opar tic le Pha se Tra nsfo rma tions
Ini tia l n ano -phase m od els Th eory group interact ion fo r mo del pa ra me ters
Exper im en tal guidanc e and m ateria l des ign
Deliv er able s Oral and written reports Cont r ibute to test sam ples to independent cha racte rizat ion lab
One -kg nanos tructu red sa mple to San dia fo r prototype tes ting