Micro Fuel Cell Vent Membranes A Study in CO2/Methanol Selectivity GA Tech. Step-Up Summer 2006 Jeff Burmester Peachtree Ridge High School Gwinnett 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
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How does a DMFC work?
3 CH 3OH + O2 → CO2 + 2 H 2O 2 Note: Note: CO2 CO2is isgenerated generatedat atthe theanode anode
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which would preferentially allow release of CO2 without releasing methanol Shruti Prakash Dissertation Proposal 2006 unpublished
Requirement: A vent that lets CO2 out but keeps methanol in… it must by highly selective
PA Selectivity = PB
Permeability (P) is defined as: N •t P= ∆p • A
N ⇒ moles / time t ⇒ thickness(cm) ∆p ⇒ pressure gradient A ⇒ Area
Current Status: • Sylgard ( Dow Corning): Poly dimethyl siloxane (PDMS) – Widely studied for gas separation. – Has high CO2 permeability: – No knowledge of MeOH permeation Polymer Poly ethylene
Permeability at STP (Barrers)1 O2 CO2 2.2 9.5
Natural Rubber
24
131
Silicone rubber (PDMS)
540-600
3230
W.J.Koros, C.M. Zimmerman. Transprot and Barrier Properties. Comprehensive Desk Reference of Polymer Characterization and Analysis (2003), 680-699. 1
Shruti Prakash Dissertation Proposal 2006 unpublished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Shruti Prakash Dissertation Proposal 2006 unpublished
1,9-decadiene F
• Hydrophobic Additives
F
F
F
F
F
1,6-divinyl perfluro hexane F
F
F
– Methyl: CH3 – Fluoride: F
• Vinyl terminated ends
F
F
F
1,6-divinylpermethylhexane
3,3-dimethyl butene
F 3,3,3-trifluoro propene F F
3,3-dimethyl pent-1,4-diene
Shruti Prakash Dissertation Proposal 2006 unpublished
Experimental Process Addative Addative
1. 1,9 Decadiene 1,9Divinylperfluorohexane Decadiene 2.1.1,6 1,6 Divinylperfluorohexane 3.2.Trifluoropropyl 3.Methyldichloralsilane Trifluoropropyl Methyldichloralsilane
PDMS PDMS Base Base
mix mix
PDMS PDMS Hardner Hardner Select Select Substrate Substrate • Cu clad • Cu clad • glass • glass • teflon • teflon
Calculate Calculatethe the selectivity selectivityofof membrane membranefor for CO CO2 2over over MeOH MeOH
Degas Degas under under vacuum vacuum
Clean Clean substrate substrate
Cast Cast membrane membrane
Cure Cure membrane membraneatat 100C 100Cunder under vacuum vacuum
Separate Separate membrane membrane from from substrate substrate
Cut Cutmembrane membrane into intosamples samples
Calculate Calculatethe the permeability permeabilityofof membrane membranetoto MeOH MeOH
Measure Measurethe the mass massofofeach each sample samplebottle bottle each eachday day
Fill Fillbottle bottlewith with MeOH MeOHand andseal seal with withepoxy epoxy
Makeup MakeupMeOH MeOH test testbottle bottleand and load loadsample sample
Calculate Calculatethe the permeability permeabilityofof membrane membranetoto CO CO2
Measure Measurethe the downstream downstream pressure pressureonce once each eachminute minute
Connect Connecttoto CO CO2 2and andopen open valves valves
Load Loadsample sample membrane membraneinto into CO CO2 test testcell cell
2
2
Measure Measure thickness thicknessofof each eachsample sample
Experimental Setup P Pressure Guage Permeation cell
CO2
Membrane
P Pressure Transducer
MeOH
P C rO P e 2 s r s m u e
Permeation cell for CO2
Permeation cell for MeOH Shruti Prakash Dissertation Proposal 2006 unpublished
Laboratory measurement of downstream CO2 pressure over time CO2 Diffusion through Sample 6 6 5
Guage Pressure (psi)
4 3 2 1 0 -1
0
5
10
15
-2 -3 Time (min)
20
25
Permeability was then calculated CO2 Permeability over Time for Sample 6 0% addative 2.0E-09 1.8E-09 Premeability (mol-cm/day-cm2-pa)
1.6E-09 1.4E-09 1.2E-09 1.0E-09 8.0E-10 6.0E-10 4.0E-10 2.0E-10 0.0E+00 0
5
10
15
Time (min)
20
25
CO2 and MeOH permeability were combined to find selectivity CO2 Concentration Permeability Sample #
% by wt
Pure PDMS 1 6 forward 6 backward 6b forward 1,6 divinylperfluro hexane 2 3 4 5 8 forward 8 backward 8 rerun forward 8 rerun backward 7 forward 7 backward 7b forward 7b backward 10 forward 10 backward 10b forward 1,9 Decadiene 12b forward 12 forward 14 forward 14 backward 14 rerun forward 15 forward 15 backward
mol-cm/daycm2-pa
Perm/St. Dev.
MeOH Permeability Perm/St. mol-cm/dayDev. cm2-pa
Selectivity (CO2/MeOH)
0 0 0 0
5.0E-10 9.0E-10 9.0E-10 1.0E-09
4.5 5.6 7.9 6.0
3.7E-10 4.8E-10 4.8E-10 4.8E-10
17.3 16.3 16.3 16.3
1.3 1.9 1.9 2.1
8.5 12.5 16.7 23 27.8 27.8 27.8 27.8 29.6 29.6 29.6 29.6 37 37 37
8.1E-10 1.0E-09 1.3E-09 1.5E-09 8.1E-10 1.3E-09 6.1E-10 1.9E-09 1.6E-09 2.5E-09 1.5E-09 1.6E-09 2.7E-09 1.3E-09 1.5E-09
5.3 5.6 9.4 6.1 6.6 9.1 3.1 8.1 9.0 6.4 1.9 10.3 8.0 7.8 7.5
7.3E-10 5.5E-10 5.3E-10 4.5E-10 5.8E-10 5.8E-10 5.8E-10 5.8E-10 6.2E-10 6.2E-10 6.2E-10 6.2E-10 5.2E-10 5.2E-10 5.2E-10
11.0 11.2 11.7 11.1 10.1 10.1 10.1 10.1 14.4 14.4 14.4 14.4 10.1 10.1 10.1
1.1 1.8 2.5 3.4 1.4 2.3 1.1 3.2 2.5 4.0 2.4 2.5 5.1 2.6 2.9
16 16 22 22 22 30 30
1.2E-09 1.6E-09 1.4E-09 8.1E-10 1.9E-09 1.7E-09 5.0E-09
3.4 4.9 10.2 5.7 4.6 5.6 3.7
5.2E-10 5.2E-10 5.1E-10 5.1E-10 5.1E-10 8.5E-10 8.5E-10
10.7 10.7 13.1 13.1 13.1 12.5 12.5
2.3 3.0 2.8 1.6 3.7 2.0 5.8
1,6 Divinylperfluorohexane seems to improve the selectivity of PDMS Average Selectivity (CO2/MeOH) vs Concentration of Flourinated PDMS 5.E+00 4.E+00 4.E+00
Selectivity
3.E+00 3.E+00 2.E+00 2.E+00 1.E+00 5.E-01 0.E+00 0
5
10
15
20
25
30
Fl compound concentration by w eight (%)
35
40
1,9 Decadiene seems to improve the selectivity of PDMS Av e rage Se le ctiv ity (CO2/M e OH) v s Conce ntration of PDM S with De cadie ne 3.5 3.0
Selectivity
2.5 2.0 1.5 1.0 0.5 0.0 0
5
10
15
20
25
Decadiene compound concentration by w eight (%)
30
35
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 film Cee 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 membrane Custom 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
B
C
D
CO2 Upstream Pressure Regulator – Used to maintain a constant CO2 pressure on the membrane Matheson© 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 CO2 pressure Fisherbrand 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 malethreaded stainless-steel end Accuracy is 1% full-scale +1 digit
•
Range from -736 to +1500mmHg
•
Balance – used to measure polymer and additives Mettler 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 membranes Fisher 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 membrane Mitutoyo 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
1
Tummala, 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
