Performance Requirements of Bipolar Plates for

Automotive Fuel Cells

R. K. Ahluwalia, D. D. Papadias

Argonne National Laboratory

J. K. Thompson, H. M. Meyer III, M. P. Brady

Oak Ridge National Laboratory

H. Wang, J. A. Turner,

National Renewable Energy Laboratory

R. Mukundan, R. Borup

Los Alamos National Laboratory

IEA Advanced Fuel Cells

Annex 34: Fuel Cells for Transportation

Vienna, Austria

November 11, 2015

2

Bipolar Plates for Automotive Fuel Cells: Technical Targets

Characteristic Units Status 2020 Target

Plate costa $/kW 4

b,c 3

Plate weight kg/kW <0.4c,d 0.4

Plate H2 permeation

coefficiente

Std cm3/(sec cm

2 Pa)

@ 80⁰C,

3 atm 100% RH

<2 × 10-6 f

1.3 × 10-14

Corrosion anodeg

μA/cm2

no active peakh 1 and no active peak

Corrosion cathodei

μA/cm2 <0.1 1

Electrical conductivity S/cm >100j 100

Areal specific resistancek

Ohm cm2

0.006h 0.01

Flexural strengthl MPa >34 (carbon plate) 25

Forming elongationm See note m 20-40

n See note m

Table 6. Technical Targets: Bipolar Plates

a Guideline based on 2010 dollars and costs projected to high-volume production (500,000 fuel cell stacks per year), assuming MEA meets

performance target of 1,000 mW/cm2.

b Based on 50% utilization of active area on the whole plate surface, stainless steel foil cost at historical average of $2/lb, 1 W/cm

2 power density, and

projected 500,000 fuel cell stacks/year production.

c C.H. Wang, Treadstone, “Low-cost PEM Fuel Cell Metal Bipolar Plates,” DOE Hydrogen and Fuel Cells Program 2012 Annual Progress Report,

http://www.hydrogen.energy.gov/pdfs/progress12/ v_h_1_wang_2012.pdf.

d Based on the 0.1 mm thick stainless steel foil.

g Guideline, not to be used as a pass/fail criterion: pH 3, 0.1 ppm HF, 80°C, potentiodynamic test at 0.1 mV/s, -0.4 V to +0.6 V [Ag/AgCl], de-aerated

with argon purge.

i Guideline, not to be used as a pass/fail criterion: pH 3, 0.1 ppm HF, 80°C, potentiostatic test at +0.6 V [Ag/AgCl] for >24 hours, aerated solution.

Status reference: C.H. Wang, Treadstone, “Low-cost PEM Fuel Cell Metal Bipolar Plates,” DOE Hydrogen and Fuel Cells Program 2012 Annual

Progress Report, http://www.hydrogen.energy.gov/pdfs/progress12/v_h_1_wang_2012.pdf.

k Measured across the bipolar plate; includes interfacial contact resistance (on as received and after potentiostatic test), measured both sides at 200

pounds per square inch (138 N/cm2), H. Wang, M. Sweikart, and J. Turner, “Stainless steel as bipolar plate material for polymer electrolyte membrane

fuel cells,” Journal of Power Sources 115 (2003): 243-251.

http://www1.eere.energy.gov/vehiclesandfuels/pdfs/program/fctt_roadmap_june2013.pdf

3

Austenitic Stainless Steel (316L): Potentiostatic Polarization Curves

10-2

10-1

100

101

102

10-2

10-1

100

101

102

0 10 20 30 4010

-2

10-1

100

101

102

0 10 20 30 4010

-2

10-1

100

101

102

0 10 20 30 4010

-2

10-1

100

101

102

0 10 20 30 40

0 10 20 30 40

eHOOH 222/1 22

-

-

-

Electrochemical cell at ORNL

Specimen: 3-mm thick,

1 cm2 exposed surface area

Reference electrode: saturated

calomel electrode (offset

+0.224 V vs. SHE)

Electrolyte: 0.1 ppm HF

solution, adjusted to pH=3

by adding H2SO4

Passive film forms in ~6 h, after

which the current density

approaches a constant value of

~0.4 mA/cm2 over 0.2–1.0 V

Secondary passivation at

1.5 V

Current density may be

considerably higher over 1.0-

1.5 V due to transpassive

dissolution

D.D. Papadias, R.K. Ahluwalia, et. al., J. Power Sources 273 (2015) 1237-1249.

4

Barrier layer (MOx/2)Metal

(1b)

(2a)

(3a)

Solutionx=L x=0

)

(2b)

(3b)

(4)

: metal cation in solution cation vacancy on the barrier layer

: oxygen anion on the barrier layer

: interstitial cation : oxygen vacancy

: barrier layer thickness

Lattice non-conservative reactions

lattice conservative reactions

Migration flux

Point Defect Model (PDM)1 for Potentiostatic Current Transients and

Growth of Anodic Passive Film in 316L

Oxygen and metallic species are transported in the film through a vacancy mechanism.

Reactions (1) and (2) correspond to metal dissolution in the presence of the barrier layer.

Reactions (3a) and (3b) produce and consume oxygen vacancy (𝑉𝑂2−) and lead to the

growth of the oxide layer.

Reaction (4) results in the dissolution of the film

1) Macdonald (2011), Electrochimica Acta, 56, 1761-1772

5

Cumulative Ion Release (ICP-MS Data)

Region I: Passive H2 environment

Region II: Passive air environment

Region III: Trans-passive air

environment

SS 316L Composition (wt%): 0.02 C, 16.45 Cr, 0.14 Cu, 10.32 Ni, 2.02 Mo, 1.37 Mn, 0.51 Si, 68.93 Fe

6

Steady-State Corrosion Rate

0.21 0.20 0.21 0.21 0.210.25

0.110.17 0.17 0.16 0.15

0.10

0.010.02 0.02 0.03 0.04

0.02

0.050.05

0.03 0.03 0.02

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.0 V 0.2 V 0.5 V 0.8 V 1.0 V 1.5 V

Mn Cr Ni Fe

-

-

-

Point defect model (PDM) for potentiostatic and potentiodynamic release of

cations from 316L

Cr-rich film (XPS data) at potentials below 1 V. Fe-enriched film above 1.5 V.

Cr film dissolves at all potentials (0 – 1.0 V), rate function of pH

Potentiostatic release rates: Fe > Ni >Mn > Cr

7

Oxide Film Thickness

10-2

100

102

0 2 4 6 8 10 120

0.5

1

1.5

2

-

In the passive region, the thickness of Cr2O3 film varies linearly with potential

(2.5 nm/V).

Chemical dissolution of Cr2O3 is independent of voltage.

At steady state, film dissolution and growth rate are equal.

Step change in

potential from -0.02 V

(OCP) to 0.8 V

8

Interfacial Contact Resistance (2XICR)

Direct correlation between 2XICR and exposure potential

In air sparged environment, resistance is lowest at 0.2 V and highest at 1.5 V.

Resistance of the sample exposed in Ar-H2 at 0 V is close to the resistance of the

aerated sample at 0.2 V.

Measured resistance is specific to the specimen geometry and test conditions

9

Modeled Interfacial Contact Resistance (2XICR)

Electrical analog of the sample mounted in the fixture for measuring 2XICR

Re: area specific resistance (ASR) of the 1-cm diameter face (side 1) exposed to

the electrolyte

Ru: ASR of the annular face (side 1) not in contact with the electrolyte and side 2

not submerged in the electrolyte

Regression analysis to determine Ru, assuming that it depends only on the

applied pressure and is independent of the cell voltage

0 0.5 1 1.5 2 2.5 3400

600

800

1000

1200

1400

1600

1800

2000

2200

Applied pressure, P (MPa)

2X

ICR

(m

Oh

mc

m2)

Line = model fit

Symbol = experimental data

1.5 V

0.8 V

0.5 V

0.2 V

2X ICR

RE/Ae RU/AU

RU/AN

10

Interfacial Contact Resistance (1XICR)

ICR correlation assuming that ICR is a combination of three resistances in series

Rmc: Resistance due to small (micro) asperities in contact with surface, inversely

proportional to pressure

RMC: Resistance due to macro contacts, exponentially decreases with pressure

Rf: Resistance due to bulk resistivity and thickness of the oxide film, asymptotic

value of measured resistance extrapolated to high pressure

11

Dissolution Rates: Nitrided G35 vs. 316L

Fe, Mn and Cr cation release rates from nitrided G35 are lower at potentials below

0.5 V but are much higher at potentials above 1 V.

Ni cation release rates from nitrided G35 are same or higher at potentials below

0.5 V, but are much higher at potentials above 1 V.

0 20 40 60 80 1000

5

10

15

20

0V (Fe)

G35

316LAm

ou

nt

dis

solv

ed

, μg

cm-2

Time (h)

0 20 40 60 80 1000

2

4

6

8

10

12

0V (Ni)

G35

316L

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

0.5

1

1.5

2

2.5

3

G35

316L

0V Mn

Cr

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

5

10

15

20

0.5V (Fe) G35

316L

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

2

4

6

8

10

12

0.5V (Ni)

G35

316L

Am

ou

nt

dis

solv

ed

, μg

cm-2

Time (h)

0 20 40 60 80 1000

0.5

1

1.5

2

2.5

3

Mn

Cr0.5V

G35

316L

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

20

40

60

80

100

1.0V (Fe)

G35

316L

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

100

200

300

400

500

600

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

100

200

300

400

500

600

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

10

20

30

40

50

1.5V (Fe)

G35

316L

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

2000

4000

6000

8000

10000

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

0 20 40 60 80 1000

1000

2000

3000

4000

5000

6000

1.5VG35Mn

Cr

Mo

Am

ou

nt

dis

solv

ed, μ

g cm

-2

Time (h)

1.0 V (Ni) 1.5 V (Ni)

1.0 V

12

Surface Chemistry Reaction Model1 for Corrosion of G35 Plates

1Keddam et al. (1985). Transpassive dissolution of Ni in acidic sulfate media. A kinetic model ,

J. Electrochemical Society, 132, 2561-2566

Skeletal reaction model without explicit reaction chemistry

K1 step denotes the formation of a passive film (e.g. hydroxide layer)

K2 step denotes the chemical dissolution of the film

K3 step denotes the transformation of the passive film into a more soluble

species leading to transpassive dissolution (K4 step)

K5 step accounts for the secondary passivation (if any) with eventual O2

evolution at high enough potentials

M 𝑀𝑎𝑑𝑥+

𝑀𝑠𝑜𝑙𝛿+

Transpassive

dissolution

K3

K-3

𝑀𝑎𝑑𝑥+ ∗

K2 K4

PassivitySecondary

Passivation

K5

K-5

(𝑀𝑎𝑑𝑘+)

K1

K6

2H+ + 0.5O2

H2O

(Metal)

10-9

10-8

10-7

10-6

10-5

10-4

10-3

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

G35 Potentiostatic and Potentiodynamic Current Densities

13

0 10 20 30 40 50 60 7010

-8

10-7

10-6

10-5

10-4

10-3

1.5 V Potentiodynamic data (O2/N2 purge, T=80 °C, sweep rate = 0.1mV s-1)

Cu

rren

t d

ensi

ty (

A c

m-2

)

Calculated anodic(dissolution) current

Measured current

Potentiostatic data (O2/N2 purge, T=80 °C)

Current density (A cm-2)

Pote

nti

al (

V)

Time (h)

1.0 V0.5 V

o Solid line represents mixed current

Im = Ianodic+Icathodic 𝐼𝑐𝑎𝑡ℎ𝑜𝑑𝑖𝑐(𝐴𝑐𝑚−2) =

− 9.3𝑒−11 × 10−8×(𝐸−1.09)

o Calculated dissolution current under-predicts

measured current at E > 1 V

o The increase in measured current density at

~1 V may be due to a transpassive film

formation reaction

o Current calculated as a function of mean

surface fraction and metallic fraction

o Valence of ejected cation evaluated from

standard Porbaix diagrams

o Under-prediction of current at 1.5 V

possibly due to oxygen evolution reaction

not included in the model

Calculated anodic (dissolution) current

Nitrided G35 Interfacial Contact Resistance (2XICR)

14

Complex dependence of 2XICR on potential

Nitrided G35 shows an increase in resistance from 1 to 1.5 V, which may be

due to a precipitated oxide layer.

The lowest resistance measured at 1 V for G35 may be due to the onset of

transpassive dissolution.

What is the cause of resistance being higher at 0 V than at 1.5 V?

0 0.5 1 1.5 2 2.5 30

100

200

300

400

500

600

700

2X

ICR

(mΩ

cm2)

Compaction pressure (MPa)

E= Surface exposed to electrolyte

U = Unexposed surface

Assumptions:

ASR of unexposed surfaces is equal

on both sides of the sample.

RU equal to control sample of G35

(not polarized)

RE/AE RU/AU

RU/AN

0 V (H2/Ar)

1.5 V (Air)

0.5 V (Air)

1.0 V (Air)

Line = model fit

Symbol = experimental data

2X ICR

0 0.5 1 1.5 2 2.5 310

1

102

103

316L vs. Nitrided G35 Bipolar Plate Contact Resistance

15

316L

0 0.5 1 1.5 2 2.5 310

2

103

1.5 V

1.0 V

0.8 V

0.5 V

0.2 V

(300)

(1030)

Nitrided G35

ICR derived from 2XICR data for specimens exposed at different potentials, and

correlated with compaction force and potential

316L ICR as function of potential (0.2-1V) correlates well with modeled changes

in oxide layer thickness (passive film).

Nitrided G35 ICR shows a more complex behavior than 316L. Surface

properties (roughness, phases) affect resistance. Reduction in ICR between

0.5-1 V may be correlated with surface coverage (preliminary analysis)

The models developed need further improvement to correlate ICR above 1 V.

Compaction pressure (MPa)

ICR

(m

Ωcm

-2)

ICR

(m

Ωcm

-2)

316L

Compaction pressure (MPa)

1.0 V

0.5 V

1.5 V

(20)

(67)

16

Next Steps

Effect of Potential and Temperature on Electrochemical Corrosion of

Metallic Bipolar Plates for HT-PEFCs, Vitali Weißbecker, FZ Jülich

Advanced MEAs for Automotive Applications, Madeleine Odgaard, IRD

Fuel Cells

Interconnectors, Christian Bienert, Plansee

Recent Developments in Water Management – Pressure Drop, Water

Removal, Droplet Dynamics and Transient Performance, Satish Kandlikar,

Rochester Institute of Technology (tbc)

Bipolar Plates for PEM Electrolysis: Challenges vs. Fuel Cells, Kathy

Ayers, Proton OnSite

Metal Bipolar Plate Coating for PEM Fuel Cells, Conghua Wang,

TreadStone Technologies

R&D for Automotive Fuel Cell Systems – Bipolar Plates, Shinichi Hirano,

Ford Motor Company

Sandvik Surface Technology - Commercializing bipolar plate production,

Hanna Bramfeldt, Sandvik

Ceramic MaxPhase™ - a highly conductive, low cost, and corrosion

resistant coating on metal bipolar plates for PEM fuel cell, Henrik

Ijungcrantz, Impact Coatings

0 20 40 60 80 10010

-2

10-1

100

101

102

103

104

Nitrided G35 Bipolar Plate Corrosion Model

2-step lumped surface reaction model satisfactorily describes dissolution rates at

all potentials investigated (0-1.5 V)

Rate controlling step assumed to be at the surface/electrolyte interface

Chemical dissolution (mainly Fe and Ni) of passive surface (E < 0.9 V) as

function of surface coverage.

Transpassive dissolution rates (E > 0.9 V) as function of surface coverage

and applied potential. Ni and Cr preferentially dissolved species

Dissolution of individual species (Cr, Ni, Fe, Mn, Mo) correlated from ICP

measurements and polarization data

17

0 20 40 60 80 10010

-2

10-1

100

101

102

103

104

Ni

20 40 60 80 100

Fe

Am

ou

nt d

isso

lve

d, μ

g c

m-2

Time (h)

1.5 V

1.0V

0-0.5 V

1.5 V

1.0V

0-0.5 V

1.5 V

1.0V

0-0.5 V

Time (h) Time (h)

Cr