Fuel Cell MEA's Dayton

1st Symposium on MEA Manufacturing, Dayton 8/05

Overview of Fuel Cell MEAsat Los Alamos National Laboratory

ByJohn Davey, M. Wilson, J. Valerio, and G. Bender


Two Basic Types of MEAsAre Made in Limited Numbers at LANL

• At Los Alamos National Laboratory (LANL), membrane electrode assemblies (MEAs) are made in small quantities for single cell and small stack basic research investigations.

• Two basic types of MEAs are used at LANL– carbon supported catalyst thin film electrodes on polymer electrolyte membranes (PEMs) for H2/Air fuel cells and unsupported catalyst thin film electrodes on PEMs for direct methanol fuel cells (DMFCs).


Hydrogen Fuel Cell MEA

Ionomer Membrane

Anode GDL/MPL

Cathode GDL/MPL

Anode Flow Channel

Cathode Flow Channel

H2O LoadH+ e-

Cathode Electrocatalyst Layer

Anode Electrocatalyst Layer

Air (O2)

H2

H H e2 2 2→ ++ −

4 4 22 2H e O H O+ −+ + →


Hydrogen Fuel Cell MEA Fabrication

Clean and CationExchange PEM to Na+ Form.

Dry on Heated Vacuum TableFor decal Hot Pressing

Paint and Oven Dry Decals:Repeat Until Desired Catalyst

Loading Achieved

Make Carbon SupportedCatalyst, TBA Ionomer Ink

Hot Press DecalsOnto the PEM

Cation Exchange PEMw/Electrodes to H+ Form

Insert PEM w/Electrodes and GasDiffusion Layers (GDLs) [the MEA]into Fuel Cell Hardware for Testing


Making Carbon Supported Catalyst, TBA Ink

* Measure out desired amount of 5% Nafion® 1100EWsolution (Solution Technologies) into a vial & recordthe weight (e.g. 1.0 g)

* Add 20 wt% Pt carbon supported catalyst, 2.5 X dryweight Nafion® e.g. 2.5 X 1.0/20= 0.125 g, and stirwith stir bar for at least 1 hour. (E-Tek C2-20 20 wt%Pt XC-72R)

* Add glycerol, half the amount of 5% Nafion®

solution , e.g. 0.5 g, and stir for at least 15 minutes.

* Cation exchange the solubilized Nafion® to TBA+ form.Add Tetra Butyl Ammonium hydroxide (TBA-OH)in 1 M methanol, 50 mg per gram of 5% Nafion®

solution, using a micropipette or syringe (e.g. 50 mg X1.0 = 50 mg). Stir for at least 1 hour.

* Add glycerol again, half the amount of 5% Nafion®

solution , and stir for at least 24 hours.


Paint and Oven Dry Decals

* Clean fiberglass reinforced Teflon (Furon) decalblanks (5, 25, 50, or 100 cm2 sizes) with iso-propanol and dry them.

* Lightly spray them with a Teflon release agentand, when they are dry, label them and recordtheir weights.

* With an appropriate sized camel hair brush, painta coat of catalyst ink on one side of each decaland put them in a 140°C drying oven for atleast 1 hour.

* Remove the decals from oven, check theirweights and repeat the painting (paint 90° toprevious direction) and drying until the desiredloading is achieved (typically 0.2 mg Pt/cm2, 14.3% of the dried ink is Pt). The final weighingshould be after a minimum of 24 hours drying.


Clean and Cation Exchange PEM to Na+ Form.Dry on Heated Vacuum Table for Hot Pressing

* Cut Nafion® 112 PEM to the desired size.

* Clean the membranes by boiling them in a 3%H2O2 solution for 1 hr., rinsing with DI H2O, andboiling in DI H2O for another hour.

* Exchange membranes to Na+ form by boiling ina 1% NaOH solution for 1 hr., rinsing with DIH2O, and boiling in DI H2O for another hour.

* Store the membranes in DI H2O, until neededfor hot pressing.

* Dry membranes for hot pressing by placing wetmembranes on a heated vacuum table that is atroom temp., heat to 130°C for 5 – 10 minutes,cool to room temp. with vacuum still on, remove,and store in plastic bags for hot pressing.


Hot Press Decals Onto the PEM

* The painted side of two decals are placed facingeach other on either side of a dried Na+ formmembrane. The decal/membrane assembly isenveloped between two Furon sheets with a thin,high durometer silicone rubber sheet outside oneof the Furon sheets to assure even pressuredistribution. This whole assembly is situated withina hinged stainless steel holder.

* The stainless steel holder is placed in a 210°Cpreheated hot press and compressed to 110 lbs/cm2

of decal area for 5 min., removed from the press,and cooled with a weight on the stainless steelholder.

* The decals are then peeled away from themembrane leaving the catalyst electrodes fusedto the membrane.


Cation Exchange PEM w/Electrodes to H+ Form

* Boil PEM w/Electrodes in 0.5 M H2SO4 for 1 hr.

* Rinse PEM w/Electrodes with DI H2O and thenboil in DI H2O for 1 hr.

* Dry the protonated PEM w/Electrodes on a heatedvacuum table at 60°C for 15 minutes, cool to roomtemperature under vacuum, remove, and store inplastic bag for fuel cell assembly.


Insert PEM w/Electrodes and GDLs, the MEA,Into Fuel Cell Hardware For Testing

* GDLs are cut to fit the particular size electrodes.(Anodes of all sizes and 5 cm2 cathodes: E-TEK’s2-sided GDL LT 2500W; all other cathodes:E-TEK’s 1-sided ELAT V2.1 Type 20)

* Gaskets are cut for the particular size MEAand fuel cell hardware. We typically use a 0.010”thick fiberglass reinforced silicone rubber anodegasket and a 0.010” thick Furon cathode gasket.We also put a 0.001” thick windowed Teflonhydrogen crossover barrier on each side of theMEA.

* The components are assembled in the appropriateorder in the fuel cell hardware.

* The fuel cell is ready for testing.


Typical H2/Air Fuel Cell Performance

Polarization Curve For Typical 0.2 mg Pt/cm2

H2/Air Fuel Cell(Cell 80C; H2: 160 sccm, 105C, 30 psig; Air: 500 sccm, 80C, 30 psig)

Current Density (amps cm-2)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Volta

ge (v

olts

)

0.0

0.2

0.4

0.6

0.8

1.0


Features of H2/Air Fuel Cell Inks and MEAs

• The ink is a smooth homogeneous mixture in which the carbon/catalyst particles remain in suspension due to the low density of the 20 wt% Pt carbon supported catalyst and the high viscosity of the glycerol which also effectively “wets” the carbon/catalyst particles.

• The low concentration of Pt on the carbon support and low evaporation rate of the glycerol allow for oven drying of the transfer decals, without the complication of spontaneous combustion.

• The 20 wt%Pt on XC-72 carbon : Nafion® ionomer ratios in the electrodes are 2.5 : 1 by weight and 1.8 : 1 by volume.

• With the electrode decal transfer technique, the exact catalyst loading canbe determined.

• The TBA+ exchanged ionomer (Nafion® ) in the electrodes becomesthermoplastic during hot pressing and “flows”, making effective contactwith the catalyst particles and a good bond to the membrane. It alsoimparts a robust pseudo crystalline structure to the ionomer in the catalyst layer.


Direct Methanol Fuel Cell

CH3OH(l) + 3/2 O2 → 2 H2O(l) + CO2↑

V = 1.21 V (at 25°C) ∆G° = - 702.5 kJ mol-1 = 6.1 kWh kg-1

CH OH O (Air), H O

-

H+

e -

Anode Cathode

3 (aq) 2 2

CH3OH + H 2O

CO2 + 6 H+ + 6 e-

H2O

CH3OH

Electroosmoticdrag

6 H+ + 3/2 O2 + 6 e

H2O

CH OH O (Air), H O

-

H+

e -

Anode Cathode

3 (aq) 2 2

CH3OH + H 2O

CO2 + 6 H+ + 6 e-

H2O

CH3OH

Electroosmoticdrag

6 H+ + 3/2 O2 + 6 e

H2O

CH OH O (Air), H O

-

H+

e -

Anode Cathode

3 (aq) 2 2

CH3OH + H 2O

CO2 + 6 H+ + 6 e-

H2O

CH3OH

Electroosmoticdrag

6 H+ + 3/2 O2 + 6 e

H2OH+

e -

Anode Cathode

3 (aq) 2 2

CH3OH + H 2O

CO2 + 6 H+ + 6 e-

H2O

CH3OH

Electroosmoticdrag

6 H+ + 3/2 O2 + 6 e

H2O

Anode: Pt-Ru black, ~80 m2/g Cathode: Pt black, ~28 m2/g Membranes: Nafion® 117, 115, 1135


DMFC MEA Fabrication

Clean and CationExchange PEM to H+ Form

Make Unsupported Pt-black Cathode Ink

Paint Cathode Ink Directly on PEMHeld on a Heated Vacuum Table,

then Cure and Cool

Make Unsupported Pt/Ru-blackAnode Ink

Insert the PEM w/Electrodes and theGas Diffusion Layers(GDLs) [the MEA]into the Fuel Cell Hardware for Testing

Flip-Over the PEM w/Cathode andPaint On the Anode Ink. Then Cure,

Cool, and Remove the PEM w/Electrodes


Make Unsupported Pt-black Cathode Ink

* Measure into a vial/jar the amount of Pt-blackcatalyst (Johnson Matthey HiSpec 1000) needed forthe desired loading (typically 6 mg Pt/cm2) and thesize and the number of cathodes to be painted. Awaste factor also is included.

* Add DI H2O, 10 X catalyst wt., and sonicate thismixture in a ice bath for 10 min.

* Add 5% Nafion® 1100EW solution, such that dryNafion® will be 10 wt% of the dry electrode, andagain sonicate in an ice bath for 10 min.

* Weigh out the appropriate amount of ink into avial/jar for the desired loading (waste factorincluded) of each cathode to be painted, cap thevial/jar, and store it in an ice bath for painting.


Clean and Cation Exchange PEM to H+ Form

* Cut the Nafion® 117 PEM to the desired size.

* Clean the membranes by boiling them in a 3%H2O2 solution for 1 hr., rinsing with DI H2O, andboiling in DI H2O for another hour.

* Exchange the membranes to H+ form by boiling ina 0.5 M H2SO4 solution for 1 hr., rinsing with DIH2O, and boiling in DI H2O for another hour.

* Store the membranes in DI H2O, until neededfor direct painting on the membranes.


Paint Cathode Ink Directly on PEM Held on aHeated Vacuum Table, then Cure and Cool

* Place wet, pre-cut Nafion® 117 membrane(s) on a roomtemperature vacuum table with a porous Teflon sheetbeneath it. Place a white silicone mask over themembrane(s) covering all but the electrode areas to bepainted. Tape the edges of area to be painted.

* Turn on the vacuum and heat the vacuum table to 70°Cfor painting. Turn on the infared lamp directed onvacuum table.

* With the appropriate size camel hair brush, paint all thecathode ink in the vial/jar onto membrane. Applynumerous thin coats (about 20), successively at 90° toeach other. Each coat should be dry before thesubsequent coat is applied.

* Set the vacuum table to 80°C for 20-40 min. of curingturn off the heat and allow the table to cool to roomtemperature with the aide of a fan.


Make Unsupported Pt/Ru-black Anode Ink

* Measure into a vial/jar Pt/Ru-black catalyst (JohnsonMatthey HiSpec 6000) needed for the desiredloading (typically 10 mg Pt-Ru/cm2) and the sizeand the number of anodes to be painted. A wastefactor is included.

* Add DI H2O, 10 X catalyst wt., and sonicate thismixture in a ice bath for 10 min.

* Add 5% Nafion® 1100EW solution, such that dryNafion® will be 15 wt% of the dry electrode, andagain sonicate in an ice bath for 10 min.

* Weigh out the appropriate quantity of ink into avial/jar for the desired loading (waste factorincluded) of each anode to be painted. Cap thevial/jar and store it in an ice bath for painting.


Flip-Over the PEM w/Cathode and Paint On the Anode Ink. Then Cure, Cool, and Remove the PEM w/Electrodes

* Flip-over the PEM w/cathode, mask with the siliconesheet, turn on the vacuum, and heat the vacuumtable to 70°C for painting.

* Paint all of the anode ink, in the vial/jar onto themasked membrane, using an appropriate size camelhair brush, . Apply numerous thin coats (>30),successively at 90° to each other. Each coat shouldbe dry before the subsequent coat is applied.

* Set the vacuum table to 80°C for 20-40 minutes ofcuring; turn off the heat and allow the vacuum tableto cool to room temperature with the aide of a fan;turn off the vacuum, remove the PEM w/electrodes,and store in a plastic bag until needed.


Insert PEM w/Electrodes and GDLs, the MEA, intoFuel Cell Hardware for Testing

* Cut GDLs to fit the particular size MEA.(Anodes: E-TEK’s ELAT 1-sided V2.02 || Cathodes:E-TEK’s 2-sided GDL LT 2500W)

* Cut gaskets for the particular size MEA and fuel cellhardware. This is typically a 0.010” thick fiberglassreinforced silicone rubber anode gasket and a0.010” thick Furon cathode gasket. Also, put a0.001” thick Teflon MeOH crossover barrier, withan electrode window, on each side of the MEA.

* Assemble the components, in the appropriate order,in the fuel cell hardware.

* The fuel cell is ready for testing.


Typical DMFC Fuel Cell Performance

Cell 80C, 100 cm2 ; MeOH:14 ml min-1 ; Air: 900 sccm, 0 psig

Current Density (amps cm-2)

0.0 0.1 0.2 0.3 0.4

Volta

ge (v

olts

)

0.0

0.2

0.4

0.6

0.8

1.0


Features of DMFC Fuel Cell Inks and MEAs

• To achieve optimum performance in DMFCs, high catalyst loadings are required: 5 mg Pt/cm2 on the cathode and 10 mg Pt-Ru/cm2 on the anode.

• Supported catalyst are not used because, at the high catalyst loadings necessary for good performance, the electrodes are so thick that mass transport limitations dominate.

• The primary solvent in the inks is H2O because spontaneous combustion can occur in the presence of concentrated, finely dispersed catalyst and organic solvents, particularly during the electrode drying process.

• These high density unsupported catalyst, H2O-based inks do not form the smooth homogenous consistency of the H2/Air fuel cell ink. The DMFC ink must be painted directly after being made because the dense catalyst particles are progressively settling out.

• TBA+ exchange of the ink ionomer is problematic. The TBA-OH in 1 M MeOH solution causes agglomeration of the ink catalyst in these H2O-based inks. We are working on a solution for this.


Features of DMFC Fuel Cell Inks and MEAs (contin.)

• Pt/Ru catalyst is used on the anode because the Ru catalyzes thedisassociation of H2O, at a lower overpotential, providing OH- for the oxidation of the CO intermediate to CO2. With Pt alone, this is the rate limiting step and the fuel cell operates at lower cell voltages.

• Methanol crossover from the anode to the cathode wastes methanol and competes with protons at the cathode resulting in a mixed potential effect that can lower the cell potential by as much as 100 mV.

• Cathode Pt-black electrode : Nafion® weight ratio is 9 : 1 and volume ratio is 0.84 : 1.00. The anode electrode Pt/Ru-black : Nafion® weight ratio is 5.67 : 1.00 and volume ratio is 0.67 : 1.00


Brush Painting Application Method

Advantages-* It is simple* Complex apparatus is not needed* Quantity of waste is small* Efficient for making small numbers of MEAs

Disadvantages-* Uniformity of catalyst over the electrode is

not always good with same painter andbetween painters may be worse

* Prohibitive requirement of man hours atscaled-up production levels

* Waste might become significant at scaled-up production


Decal Transfer Application Method

Advantages-* It is easy to achieve more precise catalyst

loadings* Numerous decals could be painted (sprayed or

roller applied) side by side increasingproduction and reducing over-painting waste

* Lends itself to hot pressing, which produces arobust electrode that is strongly bonded to thePEM

Disadvantages-* It is a slow, many step process– painting

decals, drying decals, hot pressing decals,protonating PEM w/electrodes, and drying PEMw/electrodes

* It may be difficult to automate for scaled upproduction


Doctor Blading Application Method

Advantages-* Good catalyst uniformity using a

reproducible process* Reduced number of coats required

Disadvantages-* Automation may require expensive

apparatus* Nature of the ink must be such that it can

be applied and dried in small number ofthicker coats without any sacrifice ofperformance

* May not be the best method forautomated production

XRF Image of Pt Distribution on Anode of Segmented MEA

(ten equivalent 7.7 cm2 segments)


Doctor Blading Application Method (2)

0.0 0.2 0.4 0.6 0.8 1.0 1.20.4

0.5

0.6

0.7

0.8

0.9

1.0

Seg01, Seg06Seg02, Seg07Seg03, Seg08Seg04, Seg09Seg05, Seg10

Current density / A cm-2

Cel

l Vol

tage

/ V

Anode: 810 sccm H2 , TA=105°C, p=30psig

Cathode: 4000 sccm Air, TC=80°C, p=30psig

Cell temp. = 80°C, 0.2 mg Pt/cm2 at each electrode

Equivalent VIR Curves for the Different Segmentsof the Segmented Fuel Cell


Ultrasonic Spraying Application Method

Advantages-* Good catalyst distribution uniformity* Good reproducibility* Good application for numerous thin coats of

ink* Lends itself to automation (computer

controlled positioning of spray nozzle)

Disadvantages-* Complex and expensive* Not good for ink mixtures where the

components may easily separate out (e.g. not good for our DMFC inks)

* Catalyst waste may be significant if sprayingmasked PEMs rather than adjacent transferdecals


Publications

[1] M. Wilson and S. Gottesfeld, High performance catalyzed membranes of ultra-low Pt loadings for polymer electrolyte fuel cells, J. Electrochem. Soc., 139, L28 (1992).

[2] M. Wilson, J. Valerio and S. Gottesfeld, Low platinum loading electrodes for polymer electrolyte fuel cells fabricated using thermoplastic ionomers, Electrochimica Acta, 40, 355 (1995).

[3] P. Zelenay et al. US Patent 6,696,382 B1, 2/24/2004[4] G. Bender, Characterizing Spatial Conditions Within a PEFC Using the Segmented Cell

Approach, Technical University of Munich, 2005


Acknowledgements

• Thanks to the DOE EERE Hydrogen Program, managed by Nancy Garland, for funding of fuel cell research at LANL over the years

• Thanks to the LANL fuel cell team for all their contributions in developing the MEA fabrication process over the years

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Fuel Cell MEA's Dayton

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