1

2007 DOE Hydrogen ProgramPEM Fuel Cell Freeze Durability and Cold

Start Project

Mike Perry, Tim Patterson, Jonathan O’Neill United Technologies Corporation

May 15, 2007Project ID # FCP5

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

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Overview

• Project start date: 5/1/2006• Project end date: 4/30/2007• Percent complete: 100%

• Start-up and shut-down time and energy

• Water transport within the stack

• Durability

• Total project funding– $990K DOE– $247K UTC Power

Budget

Timeline Barriers

• United Technologies Research Center (UTRC)

Partners

3

Objectives

• Improve cold-start, or Boot-Strap Start, (“BSS”) time by investigating the effects of cell properties and start procedures

• Subject a short stack to freeze/thaw cycling between - 40ºC and + 20ºC to investigate any possible damage mechanisms

• Subject single cells and/or short stacks to repeated cold starts (BSS), measure performance degradation, and investigate the mechanisms of any performance degradation

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Approach

• Task 1: Cold Start (BSS) Decay Studies– Investigate the effect of freeze and cold start (BSS) procedures

on performance decay– Alternative cell materials will be evaluated for their resistance to

performance loss with repeated cycles. • Task 2: Cold Survivability

– Conduct freeze/thaw cycling of short stack to -40 ºC– Conduct teardown analysis to characterize failure modes

• Task 3: Rapid Cold Start (BSS) Characterization – Investigate the effect of freeze and cold start procedures on BSS

capability– Investigate effect of alternate cell materials on BSS capability

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• Traditional, Solid-Plate Cell– Water movement is in the channels– External water management required

• Humidification and water recovery– Liquid water build-up is unavoidable

• In the channels and in the GDLs

Coolant channels

Coo

lant

cha

nnel

s

Coolant channels

Coo

lant

cha

nnel

s

• UTC’s Microporous-Plate Cell– Water movement is through the plate– Provides humidification and removal of excess

liquid water– Single-phase flow in the channels

• Low pressure drop• No local flooding/starvation

UTC’s PEM fuel-cell technology

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Baseline Performance:30-cell Stack (320-cm2 per cell)

0.4 A/cm2 BSS from -12 deg. C

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 30 60 90 120 150 180

Time (s)

Cel

l Vol

tage

(V)

AverageCathode EndAnode End

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 5 10 15 20 25 30

Cell Number

Res

ista

nce

(Ohm

*cm

2)

Room Temperature

Fast Freeze to -10

Slow Freeze to -10

Cathode End Anode End

• Poor BSS performance and high frozen resistance on anode end of stack.• Anode-end resistance is affected by rate of freeze.

Measured w/ high frequency (1 kHz) AC Impedance

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Water Movement During Freeze: Frost-Heave Mechanism

rPP

rP

gasliq

c

θγ

θγ

cos2

cos2

−=

=•As water freezes, effectively, the pore radius decreases, drops the liquid pressure, and induces liquid water movement towards the freezing point •In extreme cases, the excess of ice which is formed may push against the porous media, resulting in “frost heave”

r rfreezing

ice waterice

Pore cross-section

Proposed Performance-Loss Mechanism (1)

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Mem

brane

Anode C

atalyst Layer

Cathode C

atalyst Layer

Anode G

DL

Cathode G

DL

Anode W

TP

Cathode W

TP

Mem

brane

Anode C

atalyst Layer

Cathode C

atalyst Layer

Anode G

DL

Cathode G

DL

Anode W

TP

Cathode W

TP

Temperature

Cathode Side:Water transport towards cold cathode sideCathode catalyst layer water content decreases

Saturation% Saturation%

Anode Side:Water transport towards cold anode sideCathode catalyst layer water content increases

Initial Fill

Heat, water

Final Fill

Heat, water

Temperature

Proposed Performance-Loss Mechanism (2)

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Single-Cell Hardware Configuration:Glycol used to create freeze direction

-10°C glycol

Pressure Plate w/ glycol channels

Fuel Cell

Pressure Plate w/ glycol channels

CathodeAnode

Cathode-Side Freeze

-10°C glycolPressure Plate w

/ glycol channels

Fuel Cell

Pressure Plate w/ glycol channels

Anode Cathode

Anode-Side Freeze

10

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

0 300 600 900 1200Time (s)

Vol

tage

(V)

High Perm//High PermLow Perm//High PermHigh Perm//Low PermLow Perm//Low Perm

Anode GDL//Cathode GDL

Single-Cell Results:Effect of GDL Permeability

BSS after Anode Freeze

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

0 300 600 900 1200Time (s)

Vol

tage

(V)

High Perm//High PermLow Perm//High PermHigh Perm//Low PermLow Perm//Low Perm

Anode GDL//Cathode GDL

Air Stoichiometry:Initially 5,Decreased to 1.67

•Cell w/ High Permeability GDL performed better after cathode-side freeze•Cell w/ Low Permeability GDL performed better after anode-side freeze

Air Stoichiometry:Initially 5,Decreased to 1.67

BSS after Anode-Side FreezeBSS after Cathode-Side Freeze

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• Heating pads on each side mimic stack heat• Cell in freezer, no glycol• Heating one side during freeze

simulates freeze direction• Freezing rates more realistic to

those experienced in stacks• Variable heat on each side

during startup, pegged to current density

• Pyropel© insulation between cell and pressure plates and high-density Pyropel© manifolds

• Resistance is very high after anode freeze, due to H2O movement during freeze (see slides 7 & 8)

“Adiabatic” Single-Cell Hardware:Heat used to create freeze direction

0

1

2

3

4

5

6

7

8

-32 -16 0 16 32Heat Applied During Freeze (mW/cm2)

Res

ista

nce

(mill

iohm

s)

Cathode freeze

Anode freeze

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-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 15 30 45 60 75 90

Time (s)

Cel

l Vol

tage

(V)

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 15 30 45 60 75 90

Time (s)

Cel

l Vol

tage

(V)

Lo Perm GDL 1

Lo Perm GDL 2

Lo Perm GDL 3

• Cell w/ Low Perm GDL maintained positive voltage at 0.6A/cm2 after anode freeze to -30°C• There appears to be an optimum permeability. Results are dependent on rate of freeze.

BSS after Cathode-Side Freeze BSS after Anode-Side Freeze

“Adiabatic” Single-Cell Results:0.6 A/cm2 BSS from -30°C

Low Perm GDL

Lowest Perm GDL

Medium Perm GDL

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Effect of GDL on Short-Stack BSS

•When all cells had baseline GDL configuration, anode end performed poorly on 0.3 A/cm2 BSS.

•Inserting low permeability GDLs on the anode-end cells greatly improved their BSS performance.

Cathode End Anode-End Cell

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Avg. 1-27 Cell 28 Cell 29 Cell 30

Cel

l Vol

tage

(V)

RecoveredBaselineLo Perm GDLs on Cells 28-30

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-40°C Freeze/Thaw Survivability

Performance afterFreeze-Thaw Cycling-40 and +20 Celsius

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 50 100

Freeze-Thaw Cycle Number

Ave

rage

Cel

l Vol

tage

(V)

Open Circuit

0.2 A/cm2

0.4 A/cm2

0.6 A/cm2

0.8 A/cm2

UEA 2Cathode-end

UEA 10Middle

UEA 19Anode-end

UEA 2Cathode-end

UEA 10Middle

UEA 19Anode-end

After 100 cycles

Scanning Electron Micrographs (SEM) of Unitized Electrode Assemblies (UEA)

No changes observed in 20-cell stack after 100 freeze-thaw cycles to -40°C. SEM images same as as-received.

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-40°C Freeze/Thaw Survivability:Second 20-Cell Short Stack

• Excluding end-cells, negligible performance loss after 111 freeze/thaw cycles.• Mostly recoverable decay observed on end-cells.

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Current Density (A/cm2)

Ave

rage

Cel

l Vol

tage

(V)

Initial89 f/t cycles89, recoveredinitial (excluding end-cells)89 (excluding end-cells)89, recovered (excluding end-cells)111, recovered (excluding end-cells)

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Progress vs. 2010 DOE Targets

• DOE Target: BSS from -40°C– Achieved: BSS from -35°C with Short Stack

• With no air-side purge required on shutdown• Short fuel-side purge for system components (e.g., fuel regulator)

– Achieved: -40°C Freeze/Thaw Survivability• DOE Target: 50% rated power in 30s from -20°C

– Baseline Short Stack: 33% r.p.* in 30s from -12°C – Single Cell w/ Low Perm GDL:

• 65% r.p. in 30s from -10°C (anode freeze & cathode freeze)• 47% r.p. in 30s from -29°C (anode freeze)• 33% r.p. in 30s from -29°C (cathode freeze)

• DOE Target: 50% rated power in 5s from +20°C– Achieved: 94% r.p. in 5s from +23°C (single cell)

* UTC’s rated power is 0.65 W/cm2

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Future Work

• DOE-funded program completed on 4/30/2007• Freeze work continues at UTC Power & UTRC

– Both stack and system-level work ongoing– Additional optimization of cell design and materials

required to achieve faster cold-start times– Further system simplifications to improve robustness

• Results strongly depend on water management:– Amount of water present in cell on shutdown– Water movement during freeze– Water production, movement, and removal on start

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Summary

• Excellent BSS and freeze durability results have been achieved with UTC’s microporous-plate cells.

• Freeze-decay mechanism:– H2O moves down thermal gradient across GDL– Anode end of stack: More cathode flooding during freeze

• With low permeability GDL:– Less H2O movement during freeze than baseline cells– Excellent performance after both anode- and cathode-side freeze

• Notable short-stack results:– Cold starts (BSS) successfully conducted down to -35°C

• No purging of stack on shutdown required – 111 freeze/thaw cycles to -40°C with negligible performance loss