micro fuel cell vent membranes...polymer 1 w.j.koros, c.m. zimmerman. transprot and barrier...
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Micro Fuel Cell Vent Membranes
A Study in CO2/Methanol Selectivity
GA Tech. Step-Up Summer 2006Jeff Burmester
Peachtree Ridge High SchoolGwinnett County Georgia
Purpose
• Why fuel cells?– More devices are going mobile– The energy needs of mobile devices are increasing– No one wants to spend time recharging
• What are fuel cells?– Electrochemical devices that convert chemical energy
to electrical energy• Batteries vs. Fuel Cells?
– No time lag in recharging (they refuel)– Higher energy density– Battery technology may be reaching maturity
http://pd.pennnet.com/Articles/Article_Display.cfm?ARTICLE_ID=247264&p=21&cat=CONS
How does a DMFC work?
Note: CO2 is generated at the anodeNote: CO2 is generated at the anode
OHCOOOHCH 2223 223
+→+
In a micro fuel cell the venting of CO2 is critical• 1:1 molar ratio of MeOH and CO2
Example:– Fuel cell with 20µA output current– Generation of CO2: 3 x 10-6 moles/day– Head Space: 1 cm3
– Pressure built: ~1 PSI /day
• 30 days: Pressure ~ 28-30 PSI• Need to design a system which would preferentially allow
release of CO2 without releasing methanolShruti Prakash Dissertation Proposal 2006 unpublished
Requirement: A vent that lets CO2out but keeps methanol in… it must by highly selective
B
A
PPySelectivit =
AreaA
gradientpressurep
cmthicknesst
timemolesN
AptNP
⇒
⇒∆
⇒
⇒
•∆•
=
)(
/
Permeability (P) is defined as:
Current Status:• Sylgard ( Dow Corning): Poly dimethyl siloxane (PDMS)
– Widely studied for gas separation.– Has high CO2 permeability:– No knowledge of MeOH permeation
3230540-600Silicone rubber (PDMS)
13124Natural Rubber
9.52.2Poly ethylene
Permeability at STP (Barrers)1
O2 CO2
Polymer
1 W.J.Koros, C.M. Zimmerman. Transprot and Barrier Properties. Comprehensive Desk Reference of Polymer Characterization and Analysis (2003), 680-699.
Shruti Prakash Dissertation Proposal 2006 unpublished
Scientific Issues:• What helps Selectivity?
– Intersegmental attraction dictates permeation
– Nature of the membrane:• PDMS is Hydrophobic
– Nature of the permeate:• CO2: Non-Polar molecule• MeOH: Polar molecule
• How can we improve this?– Novel membrane: additives to make PDMS more hydrophobic
Shruti Prakash Dissertation Proposal 2006 unpublished
• Hydrophobic Additives– Methyl: CH3
– Fluoride: F
• Vinyl terminated ends
F FF
F F F
FFF
F FF
F
F
F
1,9-decadiene
1,6-divinyl perfluro hexane
1,6-divinylpermethylhexane
3,3-dimethyl butene
3,3,3-trifluoro propene
3,3-dimethyl pent-1,4-diene
Shruti Prakash Dissertation Proposal 2006 unpublished
Experimental Process
PDMS Base
PDMS Base
PDMS HardnerPDMS
Hardner
mixmix
Addative1. 1,9 Decadiene2. 1,6 Divinylperfluorohexane3. Trifluoropropyl
Methyldichloralsilane
Addative1. 1,9 Decadiene2. 1,6 Divinylperfluorohexane3. Trifluoropropyl
Methyldichloralsilane
Degas under
vacuum
Degas under
vacuum
Cast membrane
Cast membrane
Select Substrate
• Cu clad• glass• teflon
Select Substrate
• Cu clad• glass• teflon
Clean substrateClean
substrate
Cure membrane at 100C under
vacuum
Cure membrane at 100C under
vacuum
Separate membrane
from substrate
Separate membrane
from substrate
Cut membrane into samples
Cut membrane into samples
Measure thickness of each sample
Measure thickness of each sample
Makeup MeOHtest bottle and load sample
Makeup MeOHtest bottle and load sample
Fill bottle with MeOH and seal
with epoxy
Fill bottle with MeOH and seal
with epoxy
Measure the mass of each sample bottle
each day
Measure the mass of each sample bottle
each day
Calculate the permeability of membrane to
MeOH
Calculate the permeability of membrane to
MeOH
Load sample membrane into
CO2 test cell
Load sample membrane into
CO2 test cell
Connect to CO2 and open
valves
Connect to CO2 and open
valves
Measure the downstream
pressure once each minute
Measure the downstream
pressure once each minute
Calculate the permeability of membrane to
CO2
Calculate the permeability of membrane to
CO2
Calculate the selectivity of
membrane for CO2 over
MeOH
Calculate the selectivity of
membrane for CO2 over
MeOH
Experimental Setup
Perme
CO2
Pressu
PPressu
P
MeOH
Membrane
Permeation cell for MeOH
Permeation cell
CO2
Pressure GuageP
Pressure Transducer
P
Permeation cell for CO2
Shruti Prakash Dissertation Proposal 2006 unpublished
Laboratory measurement of downstream CO2 pressure over time
CO2 Diffusion through Sample 6
-3
-2
-1
0
1
2
3
4
5
6
0 5 10 15 20 25
Time (min)
Gua
ge P
ress
ure
(psi
)
Permeability was then calculated
CO2 Permeability over Time for Sample 60% addative
0.0E+00
2.0E-10
4.0E-10
6.0E-10
8.0E-10
1.0E-09
1.2E-09
1.4E-09
1.6E-09
1.8E-09
2.0E-09
0 5 10 15 20 25
Time (min)
Prem
eabi
lity
(mol
-cm
/day
-cm
2-pa
)
CO2 and MeOH permeability were combined to find selectivity
Concentration Permeability Permeability
Sample # % by wt mol-cm/day-cm2-pa
mol-cm/day-cm2-pa
Selectivity (CO2/MeOH)
Pure PDMS1 0 5.0E-10 4.5 3.7E-10 17.3 1.36 forward 0 9.0E-10 5.6 4.8E-10 16.3 1.96 backward 0 9.0E-10 7.9 4.8E-10 16.3 1.96b forward 0 1.0E-09 6.0 4.8E-10 16.3 2.1
1,6 divinylperfluro hexane 2 8.5 8.1E-10 5.3 7.3E-10 11.0 1.13 12.5 1.0E-09 5.6 5.5E-10 11.2 1.84 16.7 1.3E-09 9.4 5.3E-10 11.7 2.55 23 1.5E-09 6.1 4.5E-10 11.1 3.48 forward 27.8 8.1E-10 6.6 5.8E-10 10.1 1.48 backward 27.8 1.3E-09 9.1 5.8E-10 10.1 2.38 rerun forward 27.8 6.1E-10 3.1 5.8E-10 10.1 1.18 rerun backward 27.8 1.9E-09 8.1 5.8E-10 10.1 3.27 forward 29.6 1.6E-09 9.0 6.2E-10 14.4 2.57 backward 29.6 2.5E-09 6.4 6.2E-10 14.4 4.07b forward 29.6 1.5E-09 1.9 6.2E-10 14.4 2.47b backward 29.6 1.6E-09 10.3 6.2E-10 14.4 2.510 forward 37 2.7E-09 8.0 5.2E-10 10.1 5.110 backward 37 1.3E-09 7.8 5.2E-10 10.1 2.610b forward 37 1.5E-09 7.5 5.2E-10 10.1 2.9
1,9 Decadiene12b forward 16 1.2E-09 3.4 5.2E-10 10.7 2.312 forward 16 1.6E-09 4.9 5.2E-10 10.7 3.014 forward 22 1.4E-09 10.2 5.1E-10 13.1 2.814 backward 22 8.1E-10 5.7 5.1E-10 13.1 1.614 rerun forward 22 1.9E-09 4.6 5.1E-10 13.1 3.715 forward 30 1.7E-09 5.6 8.5E-10 12.5 2.015 backward 30 5.0E-09 3.7 8.5E-10 12.5 5.8
CO2
Perm/St. Dev.
MeOH
Perm/St. Dev.
1,6 Divinylperfluorohexane seems to improve the selectivity of PDMS
Average Selectivity (CO2/MeOH) vs Concentration of Flourinated PDMS
0.E+00
5.E-01
1.E+00
2.E+00
2.E+00
3.E+00
3.E+00
4.E+00
4.E+00
5.E+00
0 5 10 15 20 25 30 35 40
Fl compound concentration by weight (%)
Sele
ctiv
ity
1,9 Decadiene seems to improve the selectivity of PDMS
Average Se lectiv ity (CO2/M eOH) vs Concentration of PDM S with Decadiene
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 5 10 15 20 25 30 35
Decadiene compound concentration by weight (%)
Sele
ctiv
ity
Next Step: A more rigorous study of membrane selectivity• Multiple upstream phases – in fuel cells the vent will be exposed to
methanol in both liquid and gas phases• Multiple upstream materials – it is widely documented that permeability of a
particular gas through a membrane depends on the other gases present –the selectivity of the membrane should be measured with carbon dioxide and methanol together
• Ability to precisely control the concentration/pressure of upstream materials at the membrane face – as different material permeate through the membrane at different rates the upstream concentrations can change
• Analytics to measure quantity and makeup of materials that have crossed over the membrane – typically done with a GC or mass spec.
• Ability to support the membrane so that it does not deform under pressure –often achieve by supporting the membrane with sintered glass or metal
• Ability to evacuate both sides of the cell to eliminate atmospheric gases• Ability to precondition the membrane prior to the experiment – the
permeability of the membrane depends on what materials it already contains
• Improved consistency of membrane thickness and precision of thickness measurement
Acknowledgements• I would like to thank Dr. Paul Kohl for offering me the
opportunity to work in his lab, identifying a value-adding project and treating me like a member of his team.
• I would like to thank Shruti Prakash for all the time she spent teaching, helping and answering my questions at a time when she had lots of other pressing things to do.
• I would like to thank Dr. A.F. Burmester for helping me with both the theory and the mechanics of membrane permeability experimentation.
• I would like to thank Dr. Leyla Conrad and Dr. Edward Conrad and everyone else that made the STEP-UP program possible, it has been hugely valuable to both me and my future students.
Spin Coater – Used to spread polymer into a uniform filmCee 100CB Coat-Bake System• Features the Cee 100 spin
coater and Cee 1110 hotplate in one compact and microprocessor controlled bench-top unit
• Up to 200mm round or 6" square substrates;
• 0-6000 rpm spin range; • 1-30,000 rpm/sec acceleration,
unloaded • Repeatability is ±5 rpm with a
resolution of 1 rpm for the spincoater
CO2 Permeability Cell – Used to measure the rate at which CO2 diffuses through the membraneCustom designed and built at
Ga. Tech.• Made from stainless steel• Membrane is inserted between
two halves of the cell • An o-ring on each side of the
membrane creates the seal• Six bolts around the perimeter
hold the two halves together• Cavity on down-stream side of
the membrane is of known volume
• Pressure transducer is connected to cavity
Permeability Test Cell• Custom cell designed by Shruti
Prakash and custom fabricated of stainless steel in the Georgia Tech Chemical Engineering machine shop
• A) CO2 inlet • B) test membrane• C) rubber O-ring• D) pressure transducer
A
CB
D
CO2 Upstream Pressure Regulator – Used to maintain a constant CO2 pressure on the membraneMatheson© 3510 regulator• Single-stage• CGA Inlet size 660• Capacity 4-100 psig delivery
pressure • 316 stainless steel
construction• Nickel-plated brass bonnet• PFA seats, metal to metal
seals• 3,000 psig max. inlet pressure• Temp. range: -40-165 °F
Pressure Gauge – Used to measure the downstream CO2pressureFisherbrand© Traceable©
Pressure/Vacuum Gauge• Measures in millimeters of mercury,
pounds per square inch, bar, meters of water, and atmospheres
• Transducer has a 1/4 NPT male-threaded stainless-steel end
• Accuracy is 1% full-scale +1 digit
• Range from -736 to +1500mmHg
Balance – used to measure polymer and additivesMettler AE200 Analytical Balance• Range: 0 to 205 Gram by 0.0001
Grams• The Mettler AE Series of analytical
balances is considered by many to be the best analytical balances ever produced
• Extremely easy to use, all balance functions are controlled by the front panel control bar- automatic taring, integration time selection, calibration, and to turn the balance display on or off
• Capacity/Weighing Range: 0 to 205g• Taring Range: 0 to 205g • Readability: 0.1mg• Reproducibility: 0.1mg• Linearity: +/- 0.3mg• Stabilization Time: ~5 sec,
Vacuum Oven – Used to degas and harden membranesFisher Isotemp© Model 281A
Vacuum Oven• Dial-in temperatures up to
280°C • Maintains 30 in.Hg vacuum
with less than 1/2 in.Hg loss per day
• 1000-watt wraparound element • Full-view window permits
continuous monitoring• Separate inlet and outlet
connectors/controls allow replacement of air with inert gas
Caliper – Used to measure the thickness of the membraneMitutoyo Dial Caliper• 0-6 inch range• 0.05mm accuracy• Capable of inside and outside
measurements
Applications
• Remote sensing • Back-up power • Remote security applications• Remote communications networks• Recreational, outdoor products• Remote electronics
Shruti Prakash Dissertation Proposal 2006 unpublished
Crosslinking Mechanism of PDMS1
1Tummala, R.R, Fundamentals of Microsystem Packaging. McGraw- Hill 2001
Shruti Prakash Dissertation Proposal 2006 unpublished
Key approach
• Fuel cells are power limited, while batteries are energy limited
• Current results, high performance reliability of batteries
• Target areas: where batteries cannot“do the job”
– REMOTE APPLICATIONS IS THE KEY– Extended operation period– Go HYBRID!
Shruti Prakash Dissertation Proposal 2006 unpublished
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