1. Developing High-performance Hydrogen Purification Membranes
For High Temperature Operation Using
Ternary Metal Films
David Sholl*, Chen Ling, Lymarie Semidey-Flecha
School of Chemical and Biomolecular Engineering
Georgia Tech, Atlanta, GA *
Sabina Gade, J. Douglas Way
Dept. of Chemical Engineering, Colorado School of Mines
Kent Coulter
Southwest Research Institute
Financial support: DOE National Energy Technology Laboratory
Chemical and
Biomolecular Engineering

2. Biomolecular Engineering
Ternary Alloys for Metal Membranes
Thin metal membranes are a useful technology for H2 purification at
elevated temperatures (e.g. for coal/biomass gasification)
Binary alloys have well known advantages relative to pure Pd
- avoid membrane embrittlement by making hydride phase less stable
- some binary alloys have higher permeability than pure Pd
- some binary alloys have better impurity resistance than pure Pd
Ternary alloys have potential for improving upon binary alloys, but choosing
appropriate alloys to test is a significant challenge
Collaborative work at Georgia Tech/SWRI/CSM
Selection of ternary alloy compositions using quantitative first-principles calculations of membrane permeability
Fabrication and testing of freestanding ultra-thin ternary alloy membranes
Chemical and
Biomolecular Engineering

3. Biomolecular Engineering
Theoretical Methods: First-Principles Calculations
Density Functional Theory (DFT) is well suited for quantitative calculations of interstitial H in metals without experimental input.
- Accurate prediction of interstitial binding energies1-3
- Accurate prediction of site to site diffusion barriers1-3
- Isotope effects1 and (at low T) tunneling effects4
- Free energies for solid phase hydride reactions5
DFT data must be combined with coarse-grained statistical mechanics descriptions to give information on macroscopic properties.1,4
P. Kamakoti, D.S. Sholl, J.Membr. Sci. 225, (2003)
P. Kamakoti, D.S. Sholl, Phys. Rev. B 71, (2005)
P. Kamakoti, et al., Science, 2005, 307, 569
B. Bhatia, D. S. Sholl, Phys. Rev. B 72, (2005)
S. Alapati et al., J. Phys. Chem. C (2007)
Chemical and
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4. Modeling Membrane Properties Using First-Principles Data
Understand and predict intrinsic membrane properties
Given: Alloy phase diagram
Hierarchial Approach
Use quantum chemical methods (Density Functional Theory) to calculate atomic level information: i.e. hydrogen binding energies, energy barriers, etc.
Input for coarse grained lattice gas models to calculate long-range transport properties
Predict macroscopic properties that can be directly compared with experimental data
P. Kamakoti, D.S. Sholl, J.Membr. Sci. 225, (2003)
P. Kamakoti, D.S. Sholl, Phys. Rev. B 71, (2005)
P. Kamakoti, et al., Science, 2005, 307, 569
Chemical and
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5. DFT Data for Ternary Alloys
An important challenge for screening ternary alloys is to generate DFT datasets
that span the range of physically realizable sites on a moderate time scale
We have developed heuristic algorithms to adapt known configurations from
detailed binary alloy calculations to ternary alloys using only
(relatively inexpensive) energy minimizations.
We have calculated the properties of hundreds of transition states with this approach
Chemical and
Biomolecular Engineering

6. Cluster Expansions for Energies of Intersititial H
Description of interstitial H must include O sites, T sites, and transition states (TS).
Previous work1,2 used simple linear correlations:
e.g., EO site = E1 + nNN E2 + nNNN E3
It is difficult to demonstrate the statistical validity of
this approach.
# of Pd atoms in surrounding shells
A more general method is to use a Cluster Expansion (CE)3:
4 body +…..
P. Kamakoti, D.S. Sholl, Phys. Rev. B 71, (2005)
P. Kamakoti, et al., Science, 2005, 307, 569
J. M. Sanchez et al., Physica A, 1984, 128, 734
Chemical and
Biomolecular Engineering

7. DFT + Cluster Expansion Methods
DFT calculations for
individual sites
Least squares fit for
all possible truncated
cluster expansions
Select best model
using Leave One Out
analysis
1 CE per alloy per site
Binary alloys Pd96M4
21 O sites
88 T sites
92 TS
Ternary alloys Pd70Cu26M4
54 O sites
108 T sites
432 TS
Binary alloys Pd96M4
16,396 O site expansions
32,752 T site expansions
268,435,427 TS expansions
Ternary alloys Pd70Cu26M4
1,048,555 O site expansions
8,388,584 T site expansions
≈ 67,108,837 TS expansions
Best model for Pd96Ag4 O-sites includes 4 terms (inc. two 2 body and one 3 body)
Mean-squared error relative to DFT data set: eV
Previous simple correlation gave error of eV
Chemical and
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8. Validation of Cluster Expansion Models
Define substitutionally random alloy
(66,325 atoms in volume)
66,325 O sites
Define each site with the CE model for the specific site and compare with DFT data.
Chemical and
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9. Experiments performed by Yoshihara et al.
Solubility in Binary Alloys
Semidey-Flecha and Sholl, J. Chem. Phys., in press
Sievert’s constant obtained from a statistical mechanics treatment of the O site using CE models for each alloy
Experimental work done by Boureau et al. on Pd90Ag10 showed these alloys to have greater solubility than pure Pd1
Experiments performed by Yoshihara et al.
H in Pd at 1000 K
Qexp = 0.009, our results
Qcomp =0.011
H in Pd96Cu4 at 663 K
Qexp = 0.02, our results
Qcomp =0.03
Results are normalized with respect to pure Pd
1. Boureau, G., O.J. Kleppa, and K.C. Hong, J. Chem. Phys, 1977, 67(8), 3437.
2. Yoshihara, M., McLellan, R.B, Acta. Metall, 1983, 31, 66.
Chemical and
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10. Diffusion and Permeability: Binary Alloys
Semidey-Flecha and Sholl, J. Chem. Phys., in press
Permeability:
1 Pd90Ag10 (Exp)
2 Pd86Cu16 (Exp)
Diffusion:
All results are normalized with respect to pure Pd
1. Gryaznov, V., Sep. Purif. Methods, 2000, 29(2), 171.
2. McKinley, D., US Patent
Chemical and
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11. Predicted H Solubility in Binary and Ternary Alloys
All results are normalized with respect to pure Pd
Enhanced solubility in PdCuAg relative to PdCu suggests Ag may be a useful “additive” to PdCu
Chemical and
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12. Predicted H Diffusivity in Binary and Ternary Alloys
All results are normalized with respect to pure Pd
Enhanced diffusivity in PdCuAg relative to PdCu suggests Ag may be a useful “additive” to PdCu
Chemical and
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13. Biomolecular Engineering
Collaborative work at Georgia Tech/SWRI/CSM
Selection of ternary alloy compositions using quantitative first-principles calculations of membrane permeability
Fabrication and testing of freestanding ultra-thin ternary alloy membranes
Chemical and
Biomolecular Engineering

14. Fabrication of Freestanding Thin Foils
Thin films of desired alloy composition
sputter deposited onto Si substrate
Removing film gives thin foil
Typical samples to date are
< 10 mm thick
Chemical and
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15. Testing Ternary Films – Preliminary Data
~8 micron thick foil with composition Pd80Cu17Ag4 (wt.%)
Chemical and
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16. Comparing Ternary and Binary Films – Preliminary Data
Two foils tested under identical conditions
Pd80Cu17Ag4 and Pd86Cu14 (wt.%)
T (oC) H2/N2 Selectivity Permeability Ratio
Binary Ternary
>
>
>
Adding a small amount of Ag appears to improve permeability of binary alloy, although
observed increase is less dramatic than predicted by theory for similar compositions
Theory suggests that permeability ratio will increase as T is lowered – this is not observed
in the current experiments
Chemical and
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17. Surface Resistances and Surface Segregation
Ling and Sholl, J. Membrane Sci., 2007
Previous DFT calculations for Pd75Cu25 (at.%) predicted surface resistances may
contribute to net flux when T < 300 oC for films thinner than 10 mm
Diffusion limited
Diffusion + desorption
Our next round of experiments will use somewhat thicker foils than our current
experiments to avoid possible surface resistances
Ag and Au expected to surface segregation in binary Pd alloys
This effect is likely to be stronger in PdCu alloys (based on lattice strain)
Surface segregation may be partially reversed in presence of H
Chemical and
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18. Summary and Ongoing Work
We have developed a rigorous strategy for predicting H permeability
through ternary alloys using first-principles calculations
Results shown today: Pd-Cu-Ag and Pd-Cu-Au
Calculations in progress: Pd-Cu-Ni and Pd-Cu-Pt
We have used sputter deposition to fabricate freestanding alloy films of
Pd, Pd-Cu, and Pd-Cu-Ag
Testing of pure Pd and Pd-Cu alloys give permeabilities in reasonable agreement with
prior literature
Preliminary tests indicate that adding ~4 wt.% Ag improves permeability of Pd-Cu by ~20-25%.
Further experimental testing with thicker foils (10-15 mm) will be used to
characterize ternary foils
Financial support: DOE National Energy Technology Laboratory
Chemical and
Biomolecular Engineering

19. Beyond Crystalline Films
Shiqiang Hao, Mike Widom and David Sholl, Phys. Rev. Lett., submitted
Amorphous films may have useful processing advantages over crystalline
films (see, e.g. Ockwig and Nenoff, Chem. Rev. 107 (2007) 4078)
We have developed first-principles methods to characterize H transport through
amorphous films
Results below compare amorphous and crystalline Fe3B
amorphous
crystalline
Chemical and
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20. Biomolecular Engineering
Acknowledgements
Georgia Tech: Lymarie Semidey-Flecha, Chen Ling, Shiqiang Hao
Southwest Research Institute: Kent Coulter
Colorado School of Mines: Abby Gade, Doug Way
Financial support: DOE National Energy Technology Laboratory
Chemical and
Biomolecular Engineering