micro fuel cell vent membranes...polymer 1 w.j.koros, c.m. zimmerman. transprot and barrier...

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release of CO2 without releasing methanolShruti Prakash Dissertation Proposal 2006 unpublished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Shruti Prakash Dissertation Proposal 2006 unpublished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


• 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


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


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


CO2


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


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


Crosslinking Mechanism of PDMS1

1Tummala, R.R, Fundamentals of Microsystem Packaging. McGraw- Hill 2001


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!


micro fuel cell vent membranes...polymer 1 w.j.koros, c.m. zimmerman. transprot and barrier...

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