Water Transport Exploratory Studies

2008 DOE Hydrogen Program ReviewJune 9-13, 2008Presented by: Rod Borup

Solicitation Partners:Los Alamos National Lab, National Institute of Standards and

Technology, Sandia National Lab, Oak Ridge National Lab, SGL Carbon, W.L. Gore, Case Western Reserve University

Additional Partners/Collaborations:University of Texas-Austin, 3M Company, Nuvera Fuel Cells

FC35 This presentation does not contain any proprietary, confidential, or otherwise restricted information

Project Overview

Barriers• New Project for FY07• 4 year Project Duration

Water management is critical for optimal operation of PEM Fuel Cells

• Energy efficiency• Power density• Specific power• Cost• Start up and shut down energy• Freeze Start Operation• Total project funding

– DOE Cost: $6,550,000(over 4 yrs)

– Cost Share: $290,811• Funding for FY08

LANL $1000kIndustrial Partners $300kOther National Labs $350k FY08 Total 1650

• Direct collaboration with Industry, Universities and other National Labs (see list)

• Interactions with other interested developers

• Project lead: Los Alamos National Lab

Timeline

Partners

Budget

Organizations / Partners• Los Alamos National Lab: Rod Borup, Rangachary Mukundan, John

Davey, Tom Springer, Yu Seung Kim, Jacob Spendelow, Tommy Rockward, Partha Mukherjee

• Sandia National Laboratory: Ken Chen & C.Y Wang (PSU)

• Oak Ridge National Lab: Karren More• Case Western Reserve University (sub-contract): Tom Zawodzinski,

Vladimir Gurau• SGL Carbon Group (sub-contract in progress): Peter Wilde• National Institute of Standards and Technology (no-cost): Daniel

Hussey, David Jacobson, Muhammad Arif• W. L. Gore and Associates, Inc. (PR basis): Will Johnson, Simon

Cleghorn• Univ. Texas-Austin (additional sub-contract): Jeremy Meyers• 3M: Mark Debe (Technical Assistance – providing NSTF materials)

• Nuvera: James Cross, Amedeo Conti, Olga Polevaya, Filippo Gambini(Technical Assistance – low temperature conductivity)

Objectives

• Develop understanding of water transport in PEM Fuel Cells (non-design-specific) – Evaluate structural and surface properties of materials affecting water

transport and performance– Develop (Enable) new components and operating methods – Accurately model water transport within the fuel cell– Develop a better understanding of the effects of freeze/thaw cycles

and operation– Develop models which accurately predict cell water content and

water distributions– Work with developers to better state-of-art– Present and publish results

Approach• Experimentally measure water in situ operating fuel cells

– Neutron Imaging of water– HFR, AC impedance measurements– Transient responses to water, water balance measurements– Freeze measurement / low temperature conductivity

• Understand the effects of freeze/thaw cycles and operation• Help guide mitigation strategies.

• Characterization of materials responsible for water transport– Evaluate structural and surface properties of materials affecting water transport

• Measure/model structural and surface properties of material components • Determine how material properties affect water transport (and performance)• Evaluate materials properties before/after operation

• Modeling of water transport within fuel cells– Water droplet detachment– Water profile in membranes, catalyst layers, GDLs– Water movement via electro-osmotic drag, diffusion, migration and removal

• Develop (enable) new components and operating methods– Evaluate materials effects on water transport

Neutron ImagingCross-Section Design for High Resolution Imaging

Design Considerations:• Maximum field of view is 2 cm X 2 cm for

the high resolution neutron detector. • Limits X dimension to 2 cm.

• Outermost edge to image = 3 cm from the detector for good focus.• Detector is 0.5 cm inset of the face

plate, 2.5 cm available• Active area 1.2 cm in width

• Entire cell is < 3 cm from detectorDesign:• 2.25 cm2 active area• No hydrocarbon materials• Metal hardware

• No plate porosity of hardware for water hold-up

• 1 cm linear water imaging length• Shallow single serpentine flowfield

• Attempt to simulate pressure drop of real flowfields

High resolution (~ 25 μm) cross-section cell

NeutronBeam

GDL Teflon Loading Effect on Water ContentMonitored by Neutron Imaging and AC Impedance

Co-Flow, 80 oC, 172 kPa (abs)Anode: 1.1 stoich. / 50 % RHCathode: 2.0 stoich / 100 % RH

• Charge transfer resistance• Decreases with increasing current• Greater for GDL with 23% PTFE in MPL

• Mass transfer resistance• Increases with increasing current• Greater for GDL with 23% PTFE in MPL

GDL Variation-0.7

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Real (z') - Ohm-cm2

GDL AGDL BGDL C

AC ImpedanceCross-section Neutron Imaging

GDL A = 5% Substrate23% MPL PTFE Loading

GDL B = 5% Substrate10% MPL PTFE Loading

GDL C = 20% Substrate10% MPL PTFE Loading

Cathode

Anode

OutletsInlets

Channel

Land

Cathode

Anode

OutletsInlets Incr

easi

ng w

ater

con

tent

5% PTFE Substrate, 23% PTFE MPL

5% PTFE Substrate, 10% PTFE MPL

Dry Image

Channel Land

• More PTFE in the MPL results in more water in GDLs and channels

• Mass transport limitations consistent with lower performance of fuel cells with high MPL Teflon loading at high current densities

Water Profiles Nafion 212Water content comparison for different operating conditions

• Variation of water content as a function of current density/anode stoichiometry• Anode stoich = 3 (simulating anode recycle), dry cathode has lower water content• Anode stoich = 1.2, dry cathode similar water content to fully humidified cell

• Measured Water content in Nafion lower than expected

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H2O

N212_80_80_0_0p5A_3stN212_80_80_0_1p0AN212_80_80_0_1p0A_3stN212_80_80_80_0p5AN212_80_80_80_1p0A

Membrane/CLGDL Edge

Flowfield Land

FlowfieldChannel Edge

• Membrane/Catalyst Layer is only ~ 5 pixels wide

• ~ 3 pixels for thinner MEAs• 1 pixel = 14.7 microns

GDL H2O condensation

• Low constant stoich (1.1/2.0)• Simulating anode recycle (3.0)• Flowfield co-flow

• Anode channel/GDL water:• With const. anode stoich ~ 1.1• Disappears with anode recycle • Anode GDL water may be water

condensation (heat pipe effect)

Anode Cathode

Water Profiles Delineated in Counter-Flow Orientation

• MEA shows highest water content in middle of cell (land)

• Right Land / Channel (anode out)

• Low water content in cell (compared with other materials)

• Water in channels at outlets

3M NSTF, Counter Flow, 40 oC, 0.59 A/cm2, Ta = 28, Tc = 28

Delineated Profiles of Channels/Lands

Anode InletCathode Inlet

Left Middle Right

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Left ChannelLeft LandRight LandRight ChannelMiddle ChannelMiddle Land

Profile direction

Water Content Comparison with Various Materials

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H2O

BPSH (Hydrocarbon)

Nafion 212

3M NSTF

GORE™ PRIMEA® MEASeries 57110

Membranes alligned at pixel = 51

Artifactpeak

• High resolution neutron images of different MEA materials under similar operating conditions.

• N212 high water content, low water content for 3M NSTF materials• Anode GDL water differs significantly• Significantly more water in MEA/GDLs at lower temperatures

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H2O

3M NSTF

GORE™ PRIMEA®MEA Series 57110

(40 oC)(80 oC)

GORE, and PRIMEA are trademarks of W. L. Gore & Associates, Inc.

Cell Length Water ProfilesCo-flow vs. Counter flow

100 / 0 % RH 1.2 / 2.0 St.

MEA GDL/MEA/GDL

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Water Den

sity / m

m H2O

Counter(An = 1.2St) MEA

Co‐Flow (An = 1.2St) MEA

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0 5 10 15 20Cell Length / mm

Water Den

sity / m

m H2O Counter(An = 1.2St) GDL/MEA/GDL

Co‐Flow (An = 1.2St) GDL/MEA/GDL

Counter Flow : I = 1.49 A/cm2; V = 0.27 VHFR = 0.064 Ohm.cm2

Co-flow : I = 1.41 A/cm2; V = 0.095 VHFR = 0.10 Ohm.cm2

• Higher membrane water with counter flow• Membrane water correlates to lower HFR and higher performance with counter flow

AnodeInlet

CathodeInlet

Profile direction

Counter Flow

CathodeProfile direction

Coflow Flow

Anode

GORE™ PRIMEA® MEA Series 57110GORE, and PRIMEA are trademarks of W. L. Gore & Associates, Inc.

Cell Length Water ProfilesAnode Stoich comparison

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Water Den

sity / m

m H2O Counter(An = 1.2St) MEA

Counter (An = 3.0St) MEA

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Cell Length / mm

Water Den

sity / m

m H2O Counter(An = 1.2St)

GDL/MEA/GDLCounter (An = 3.0St)GDL/MEA/GDL

Anode Inlet Cathode InletProfile direction

MEA GDL/MEA/GDL

Counter Flow : 1.2StI = 1.49; V = 0.27V; HFR = 0.064

Counter Flow : 3.0StI = 1.49; V = 0.27; HFR = 0.076

100 / 0 % RH, 1.2 vs. 3.0 st. simulating anode recycle

(1.2 st)

• MEA water content ~ same• Higher anode stoich: lower land water• Similar Performance

GORE™ PRIMEA® MEA Series 57110GORE, and PRIMEA are trademarks of W. L. Gore & Associates, Inc.

00.1

0.20.30.4

0.50.6

0.70.8

0 5 10 15 20Cell Length / mm

Water Den

sity / m

m H2O Counter(An = 1.2St)

GDL/MEA/GDLGravity Counterflow (Anode ontop = 1.2st) GDL/MEA/GDL

Cell Length Water ProfilesOrientation comparison

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Water Den

sity / mm H2O Counter(An = 1.2St) MEA

Gravity Counterflow (Anode on top = 1.2st)MEA

MEA GDL/MEA/GDL

Counter Flow Inverted: 1.2StI = 1.39; V = 0.385; HFR = 0.067

Counter Flow : 1.2StI = 1.49; V = 0.27V; HFR = 0.064

100 / 0 % RH 1.2/2.0 St. Orientation inverted

CathodeAnode

Profile direction

Counter Flow

CathodeProfile direction

Counter Flow InvertedAnode

• Membrane water content similar• Cathode on top shows flooding (gravity effect) and loss of performance• Cathode on bottom GDL water lower water content

GORE™ PRIMEA® MEA Series 57110GORE, and PRIMEA are trademarks of W. L. Gore & Associates, Inc.

Freeze Operation

012345678910

0 2 4 6 8 10Real Z'

Imaginary Z"

V = 0.78 (1)V =  0.77 (2)V = 0.77 (3)V = 0.76 (4)V = 0.75 (5)V = 0.75 (6)V = 0.73 (7)V = 0.70 (8)

T = ‐ 10 CAnode = H2 (500 sccm)Cathode = Air (500 sccm)

I = 1A (0.02 A/cm2)V = 0.87 to 0.89VAC Aplitude = 0.1A

‐0.005‐0.004‐0.003‐0.002‐0.0010.0000.0010.0020.0030.004

0.0 0.5 1.0 1.5

Voltage / V

Curren

t

Before 3rd start I(A/cm2)

After 3rd start I(A/cm2)

Before 4th start I(A/cm2)

After 4th start I(A/cm2)

00.10.20.30.40.50.60.70.80.91

0 10 20 30 40

0

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Cell Voltage

Cell Current Density

Time (min)

Voltage (V)

Current Density (A

/cm2)

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8

• Little change in HFR• Steady increase of Charge Transfer Resistance• Steep increase in Mass Transport Resistance when

cell voltage drops

Fuel Cell Start-up at -10 oC

• Performance decays quickly at -10 oC• Ice formation leads to mass-transport limitations• No change in ECSA at low temperatures • As operating time increases, AC Impedance resistance

shows mass-transport limitations• ECSA slightly increases after multiple runs at -10 / 80 oC

• Possible hydration of membrane or cell break-in

Impedance During Start-up at -10 oC

ECSA at -10 oC

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0 2 4 6 8 10

AC Impedance Scan #

Resist. / Ohm

‐cm2

HFRRctRmt

Neutron Imaging of Ice FormationDuring Operation at -10 oC

• Neutron imaging of ice formation in a 50 cm2 fuel cell operated at 0.5 V at -10 oC.

• Calculated/measured water/ice accumulation from current and neutron imaging in the fuel cells track

Water/Ice accumulation @ -10 oC

02468

101214

0 200 400 600 800 1000Time (sec)

Wat

er A

ccum

. (cm

3 cm

-2)

0.0E+00

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Cumulative Current Density

Measured water thickness

Calculated water volumeTota

l cha

rge

(C)

Incr

easi

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ater

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tent

0 - 100 sec 800 - 900 sec

MEA Freezing Conductivity

• At 25%RH @ 70 oC: • Hysteresis is seen; Cooling (Lower λ); Heating (higher λ)

• If cell is left at cold temperatures: membrane will rehydrate

• At 100% RH @ 70 oC: Membrane fully hydrated; No hysteresis in conductivity

• At 50% RH @ 70 oC: Membrane λ is lower, Conductivity is lower

• However, membrane hydrates at low temperatures (higher RH)

Nuvera Fuel Cells

MEA HFR Response to Transients

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Current Density(A/cm2)HFR(Ohms cm^2)

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Current Density (A/cm2)HFR(Ohms cm^2)

80 oC

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HFR

/ O

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cm2Current Density

(Amps/cm2)HFR(Ohms cm^2)

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Current Density(Amp/cm2)HFR(Ohms cm^2)

60 oC

GORE, and PRIMEA are trademarks of W. L. Gore & Associates, Inc.

020406080

100120140160

0 10 20 30

% PTFE Loading

Con

tact

Ang

le

AdvancingReceding

with MPL

without MPL

• Advancing more hydrophobic• Once wet; difficult to ‘de-wet’

0.1/0.2 GORE™ PRIMEA® MEA Series 57110100% RH Anode50% RH Cathode

Wilhelmy-PlateContact Angle

advancing vs. receding

• Wetting / dewetting show very different time constants in response to transient inputs

• MEA quickly hydrates / MEA slowly dehydrates• Contact angle characterization shows similar hysteresis

Cathode

Anode

Land

Channel

Symmetry

CFD Modeling of Water Removal from GDL

MEA

GDL

Channel

• Liquid water accumulates above the lands before exiting the GDL in the channel.

• Maximum saturation is above the lands.

• Liquid water streamlines converge towards the channel-land corners

• Sessile/pendant droplets form and leak down the channel walls.

• CFD results agree with Neutron Images

CFD simulations represent liquid water streamlines in diffusion media.

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50 / 050 / 5050 / 100100 / 100

Cathode channel water

Cathode GDL

Anode channel water

Anode GDL

Distance (μm)

Wat

er T

hick

ness

(mm

) Anode/Cathode • Liquid water saturation profiles

modeled in the cathode GDL and catalyst layer.

• Liquid water accumulates in diffusion media over time• When liquid pressure at GDL-channel interface reaches a threshold value (Young-Laplace) it exits GDL via channel.

• CFD modeling profiles agree with experimental results (magenta frame above) obtained by neutron imaging.

CFD Simulation Results

GDL Catalyst Layer

time

Liqu

id S

atur

atio

n

Water exit

Predicting Onset of Water-Droplet Detachment

Ken S. Chen (kschen@sandia.gov)

Motivation: droplet detachment from GDL/channel interface is a key mechanism for liquid-water removal in PEM fuel cells. Elucidating water-droplet detachment from GDL/channel interface and being able to predict the critical air-flow velocity required to detach droplets can provide useful design and operational guidelines.

Schematic of water-droplet growing and being deformed by flowing air drag at the GDL/flow-channel interface

Channel/droplet/pore dimensions:Channel height = 1 mm, Droplet diameter = 0.6 mm, Pore diameter = 100 μm

Sandia National Lab

Ken S. Chen (kschen@sandia.gov)

Simulated 3-D water-droplet deformation and detachment from GDL/channel interface

5 m/s (deformation visible)1 m/s (deformation not yet visible)

6.3 m/s (moments before detachment) 6.4 m/s (moments after detachment)

Sandia National Lab

Single-phase CFD model explaining neutron imaging patterns on water distribution*

11

16

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36

0.015 0.02 0.025 0.03 0.035 0.04 0.045

Relative position, m

Vel

ocity

mag

nitu

de, m

/s

SR = 4SR = 2

Zone 1 Zone 2 Zone 3

Flow Direction 4 stoic4 stoic2 stoic 2 stoic

Computed along-channel velocity component and neutron image through a corner of flow channel

Computed velocity vector plot and neutron image in a corner of the gas flow channel

Computed along-channel velocity component through a corner of the gas flow channel

• Regions where liquid water content is reduced corresponds high gas velocity.

• Computed velocity field indicates the presence of recirculation zones in the 90° bends.

• Low flow speed and circular nature of gas flow lead to reduction in water removal driving force and corresponding increase in water content.

*Reference: M. A. Hickner, K. S. Chen, N. P. Siegel, to appear inJournal of Fuel Cell Science and Technology (2008) Sandia National Lab

Future Work• NIST Neutron Imaging (June 12-18)

– NSTF Start-up, understand saturation water content of membrane, high resolution freeze, transients

• Transient operation– Simulate automotive operation, RH transients

• Segmented Cell operation– Measure water transport spatially in cell by HFR

• Freeze Measurement– in situ monitoring of ice formation

• Characterization– TEM characterization of aged GDL materials, surface spectroscopy of GDL

surfaces• Model development

– Develop multi-dimensional (quasi-3D) model of water transport and removal– Incorporate sub-models of liquid-water removal via droplet detachment and

evaporation

Milestones

Mon Yr Milestone

Dec 07 Quantify water content by HFR measurements in various cell components under steady-state operation

Dec 07 Accurate water balance measurements during steady-state operation

Mar 08 100 freeze/thaw cycles to -40oC on fully humidified cells using paper GDL (completed FY07)

New: Performance of fuel cells operated at –10oC

Jun 08 Report surface properties of GDL and the effect of aging

Sept 08 Direct observation of ice formation by neutron imaging (completed FY07)

In progress

Summary of Technical Accomplishments

• Experimentally measure water in situ operating fuel cells– Direct water imaging at NIST by neutrons

• High resolution (25 μm) imaging, Low resolution (150 μm) imaging– AC Impedance and HFR measurements– Freeze/Thaw

• Ice results in performance loss associated with increasing low freq. resistance – Ice formation limits gas access to the reaction sites

• Characterization–Hydrophobicity characterization, microscopic characterization, elemental

compositional– Varying GDL materials (MPL Teflon loading, GDL substrate Teflon loading)

• GDL wetting/dewetting properties help explain fuel cell performance hysteresis.

• Modeling of water transport within fuel cells– Delineation of mass transport loss from IR, kinetics, etc.– Modeling of water-droplet detachment from the GDL/channel interface.– CFD modeling simulates liquid water saturation profiles