challenges for material and component development for … · challenges for material and component...

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Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering

Challenges for material and component development for PEM fuel cells

W. Lehnert1,2, H. Janßen1, J. Supra1, V. Weißbecker1

1 Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Germany 2 RWTH, Aachen University, Faculty of Mechanical Engineering, Germany

Institute of Electrochemical Process Engineering

Outline

• Interaction between bipolar plate

and GDL, some challenges

• Graphitic vers. metallic bipolar plates, thermal mangement

• Corrosion

• Gaskets

Material - Metallic - graphitic

GDL / flowfield - Geometry - Pressure drop

Gasket - Elastic - incompressible

Thermal management - Air - liquid


Polarization curves / different BiP design

Polarization curve for different channel/rib geometries

Ch. Hartnig, L. Jörissen, J. Kerres, W. Lehnert, J. Scholta, Polymer electrolyte membrane fuel cells (PEMFC), in: Materials for Fuel Cells, Ed. M. Gasik, Woodhead Publishing Limited, 2008, pp 101-184


• different gas diffusion layers • same flowfield

Polarization curves / different GDLs

Membrane: Gore 5510

p anode ambient p cathode ambient

Humidification anode dry cathode recirculation Stacktemperature 48 °C

Stoichiometry: λ anode 1,1 λ cathode 4,0

number of cells: 1 active area [cm²]: 126

C. Hartnig, L. Jörissen, J. Scholta, W. Lehnert, Gas diffusion media, flowfields and system aspects in low temperature fuel cells. In: C. Hartnig, C. Roth, Polymer electrolyte membrane and direct methanol fuel cell technology, Volume 1: Fundamentals and performance of low temperature fuel cells, Woodhead Publishing Limited, 2012, pp 81-116


Bipolar Plates

Requirements for Bipolar Plates • High electronic conductivity. • Low interfacial contact resistance to the GDL. • Providing a flow path for gas transport with low pressure drop but uniform distribution over the cell

area. • Providing mechanical strength and rigidity. • Providing thermal conduction in order to regulate the thermal management. • Has to be corrosion resistant.

some material properties graphitic BiP metallic BiP • thermal conductivity / W m-1 K-1 : ~ 20 – 50 ~ 15 – 60 e.g. 1.4301 (FeCr18Ni10): 15 • density / g cm-3 : ~ 1.9 ~ 7.9 (1.4301)

• spec. electr. resistivity / Ω mm2 m-1: ~ 10 ~ 0.73 (1.4301) • electric conductivity / S m-1: ~ 1∙105 ~ 1.4 ∙106

for comparison: Cu: 400 W m-1 K-1 Al: 235 W m-1 K-1 Cu: 0.017 Ω mm2 m-1 Al: 0.027 Ω mm2 m-1 [Ω mm2 m-1 = 10-6 Ωm]


Gas Diffusion Layers

Requirements for Gas Diffusion Layers • High electronic conductivity. • Heat must be transported through the material. Therefore a high heat conductivity is desirable. • Mechanically supporting the MEA. • The GDL should provide gas access from the flow-field channels to the catalyst layer and

allow removal of gaseous products. • The GDL should provide a passage for water removal from the electrode to the flow field.

some material properties of GDLs w/o MPL (material unisotropic, compression dependent) • effective thermal conductivity, xy (z): ~ 1 – (15) W m-1 K-1 • density: ~ 0.4 – 0.45 g cm-3

• electrical resistivity (through plane): < 12 mΩ cm²

• porosity ~ 78 - 85% • Air permeability ~ 20 – 200 cm³ cm-2 s-1

[1] Data sheet Freudenberg FCCT SE & Co. KG [2] Data sheet SGL TECHNOLOGIES GmbH [3] Data sheet Toray Industries

Teertstra et al. , Electrochimica Acta 56 (2011) 1670–1675 Sadegh et al. , J.Power Sources 196 (2011) 246–254, JPS 196 (2011) 3565–3571


Interaction between BiP / GDL


ε = 0,7 ε = 0,5 ε = 0,3 ε = 0,2

Influence of GDL thickness on fuel cell behaviour (idealized case)

0.6 V; porosity 0.7; channel width: 1mm; rib width: 1 mm; (only valid for specified flowfield)

curr

ent d

ensi

ty /

mA

cm-2

GDL thickness / µm


uncompressed compressed too much compression (e.g. stiff paper)

Membrane Electrode Gas diffusion layer

channel

land

Cross section of a cell

GDL compression (the truth)

Figure left: Ch. Hartnig, L. Jörissen, J. Kerres, W. Lehnert, J. Scholta, Polymer electrolyte membrane fuel cells (PEMFC), in: Materials for Fuel Cells, Ed. M. Gasik, Woodhead Publishing Limited, 2008, pp 101-184

compression under the lands uncompressed under the channel


GDL morphology underneath channel and land area

compression [%]

aver

age

poro

sity

Compression rate : 30%



compression [%]

thic

knes

s µm

Ch. Tötzke, G. Gaiselmann, I. Manke, A. Hilger, T. Arlt, H. Markötter, F. Wieder, M. Osenberg, J. Bohner, W. Lehnert, V. Schmidt, A. Kupsch, B.R. Müller, J. Banhart; Synchrotron tomographic study on the inhomogeneous compression of gas diffusion layers in fuel cells, ModVal 10 19.-20.03.2013 Bad Boll/Stuttgart


Influence of GDL on pressure drop

F. Liu, M. Kvesić, K. Wippermann, U. Reimer, W. Lehnert Effect of Gas Distribution on Performance and Durability of HT-PEFCs Journal of The Electrochemical Society, 160 (8) F892-F897 (2013)

Serpentine and spiral flow field plates. Both flow fields have an identical land width, and channel width and depth: 1.0 mm.

Pressure drop in the flow fields with and without GDL


Thermal management


Thermal management

Influence of the bipolarplate material on thermal management

Graphitic BIP Metallic BIP

Influence • of geometry → hydraulic diameter • of different contact area between BiP and GDL • of thermal conductivity • of thickness of the material

• channel depth: depending on the width • thermal conductivity of steel: λ=15-60 W m-1 K-1

• thickness of the material: < 0,2 mm

• flexible geometry • thermal conductivity of graphite/phenolic resin-

composit: λ=20-50 W m-1 K-1 • thickness of the material: ≥ 1 mm

http://www.graebener-maschinentechnik.de

www.reinz.com


3D-CFD simulation: thermal management

Graphitic BIP Metallic BIP

0.2 mm

3 mm

cooling channel anode gas channel

cathode gas channel

5 m

m

6 mm cathode gas channel

anode gas channel

cooling channel

GDL

2 mm 1.5 mm

CFD model: • 2 cooling channels L=100mm • homogeneous heat source on GDL: �̇�=0,75 W cm-2 • hydrogen / air operation: λH2/Air=2/2 ; Tin,H2/Air=160°C • cooling media: air (Tin =27°C, �̇�𝑎𝑎𝑎 = 3 ∙ 10−6 kg s−1cm−21) or

heat transfer fluid (Tin =160°C, �̇�𝑙𝑎𝑙𝑙𝑎𝑙 = 3.3 ∙ 10−5 kg s−1cm−2 1) 1 per cm2 GDL area

not to scale possible geometries different hydraulic diameter


3D-CFD simulation: Temperature distribution on cathode GDL

Simulated temperature distribution for a HT-PEFC, air-liquid cooled


Corrosion


Challenges of metallic BiP

Metallic BiP

Poisoning of membrane and catalyst

Blocking of functional groups in PEM and/or the catalyst due to metal ion release. Decrease of performance

Corrosion

wet / acidic environment (NT-TEM, HT-PEM and impact of T) Potential

Passivation

Formation of non-conductive passivation layers on metal surface (metale oxides/phosphates) Rise of interfacial contact resistance


Corrosion rates with polarization

1E-2

1E-1

1E+0

1E+1

1E+2

1E+3

1E+4

0.0 0.2 0.4 0.6 0.8 1.0

j / µA∙cm

-2

η / V

1.43011.43721.44041.4571

1E-2

1E-1

1E+0

1E+1

1E+2

1E+3

1E+4

0.0 0.2 0.4 0.6 0.8 1.0

j / µA∙cm

-2

η / V

1.45391.48762.48562.4869

Stainless Steels: - jcor = 45 µA∙cm-2 (current at Ecor) - Passive region (0.2 – 1.0 V): j = 31 µA∙cm-2

Alloys: - 2.4869 (Cr20Ni80): best performance - jcor = 0,7 µA∙cm-2 - Passive region: 16 µA∙cm-2 (at 0.6 V)

85 wt. % H3PO4 at 130 °C

Fe ~ 65 %

85 wt. % H3PO4 at 130 °C

Fe < 50 %

DOE: Corrosion rate < 1 µA∙cm-2


Possibility of Corrosion Protection

Metallic BiP

Stainless steels/alloys

- High Cr and Ni content with addings of Mo, Nb, W, Ta, La, Zr,Cu

Passivation layer

Cr/Ni

Metallic materials - Noble metals with - Metals with conduct. and stable oxide layer (Ti, Nb, Ta) Costs

Coating Surface treatment Bare substrates

- Enrichment of Cr on metal surface using pack cementation process (Cr oxide layers on substrate) - Thermal nitriding (CrN on surface)

Interfacial contact resistance

- Thin gold layers - Ceramic coatings - Cladding with Nb - Graphitic, diamond-like coatings - Polymeres (Polypyrrole,

Polyaniline) Costs, Defects and stability of

coating

metallic BiP should be coated


Coatings for Metallic BiP

Requirements for coatings • High electronic and thermal conductivity • Corrosion resistant, also under polarization • Impermeable to electrolyte • Heat-resistant (180 °C, HT-PEM) • Adequate adhesion on substrate • Free of defects (cracks, pinholes,…) • Should be flexible according to thermal expansion of metallic substrate at elevated temp.

Constrains of coatings • Corrosion values vary strongly in literature (as a function of pH, T, material composition,

surface finishing, accomplishment of experiments,…) • Missing of long-term results • Most promising results for nitriding and CrN, TiN (for NT-PEM) • Main challenges are defect-free coatings and costs • More results and publications for NT-PEM than for HT-PEM


Gaskets


Gaskets

Requirements for Gaskets • Sealing of stack components to avoid crossover and leakage of fluids • Compensation of tolerances (manufacturing inaccuracy, stack deflection, thermal

expansion) • Functionality must be fulfilled under operation and at standstill for the announced

stack lifetime

Material properties of gaskets • Electric conductivity < 5 µS cm-1

• H2 permeability < 2 • 10-6 cm³ s-1 cm-2 • Temperature stability > 100 °C (PEFC), > 200 °C (HT-PEFC) • Chemical resistance Reformate (H2), air (O2), H3PO4, cooling media


Gaskets - Requirements


Gasket materials

temp. range chem. resistance Elastomer Silicone -60 °C…(200) °C oil, water, ozone Fluorocarbon (FKM) -15 °C…200 °C (silicone) oil, ozone Fluorosilicone -60 °C…220 °C (silicone) oil, water, petrol Perfluoroelastomer (FFKM) -15 °C…310 °C solvents, steam, water Thermoplastic Perfluoroalkoxyalkan (PFA) -200 °C…260 °C hydrocarbons, petrol Polyetheretherketone (PEEK) -70 °C…260 °C hydrocarbons, aromatics, water, steam, coolant Polyimide (PI) -240 °C…280 °C oil, org. solvents

Compression: „Incompressible“ gasket Hardstop functionality by the gasket material „elastic“ gasket Hardstop functionality by the BIP construction


The selected examples show: that the interaction of the cell components have to be taken into account. • mechanical properties

• chemical properties

• transport properties have to be adjusted carefully in order to design proper cells and stacks.


Thank you for your attention

Mathematik und Industrie www.mathematik-21.de

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