Polymer Electrolyte Fuel Cells (PEFC) Flashcards
1
Q
Quick Facts
A
- State-of-the-art for mobile applications
- basen on polymeric electrolyte
- also called PEM FC (Proton Exchange Membrane FC)
- developed in 60s, widely adopted, still in use
2
Q
Cell Structure & Components
A
- Bipolar plates
- mechanical support
- electrical connection
- has distribution & product removal
- heat management
- of high relevance for weight/volume performance optimisation
- Gas Diffusion Layers
- gas & water transfer
- electrical connection
- heat transfer
- mechanical support
- of high relevance for weight/volume, echem performance optimisation
- Catalyst Layers
- provide reaction sites
- transfer electrons, protons & reactant gases to/from reaction site
- of high relevance for echem performance optimisation
- Membrane
- proton transfer
- electrical insulation
- prevent gas crossover
- water & heat distribution
- sustain & distribute mechanical force
- of high relevance for echem performance optimisation
- Gaskets
- mechanical support
- prevent gas mixing
- structure: BP, GDL, CL, Membrane, CL, GDL, BP
3
Q
Targets for use of Polymeric Electrolytes for PEFCs
A
- high ionic conductivity
- electrically insulating
- no gas cross-over
- chemically stable
- thermally stable
- mechanical robustness
- easy manufacturability
- abundant, inexpensive, sage
4
Q
Disadvantages/Advantages for use of Polymeric Electrolytes for PEFCs
A
- robust bendable
- easy cell construction, less mechanical degradation
- easy manufacturability, opportunities for favourable membrane-electrode interface engineering
- high corrosion stability
- enable fast startup
- require comparatively low T -> slow kinetics
- gas crossover not fully prevented
5
Q
PFSA electrolytes (e.g. Nafion)
A
- most commonly used membrane for PEFCs
- proton transport through Gotthuss mechanism (Proton hopping)
- backbone: tetrafluoroethylene (Teflon)
- very resistant to chemical attack
- side chains: specific type of per fluorinated sulfonic acid
- swells due to easy hydration
- operation sensitive to water content
- requires water management
- swells due to easy hydration
- used in a variety of thicknesses (20 to 200𝜇m)
- additional mechanical reinforcement esp. for diff. pressure operation
6
Q
Membrane Descriptors
A
- equivalent weight EW = mass of dry polymer per ion exchange site (g/mol)
- ion conductivity (as fct of water uptake or humidity λ (mol water/mol acid))
- water uptake (as a fct of relative humidity Pw/Pwvap)
- thickness
- commercial Nation membranes often named according to EQW & thickness
7
Q
Challenges of PFSA membranes
A
- only conduct when humidified
- only work below 100°C
- certain PFSA “chemical building blocks” under discussion for health & environmental concerns
- fabrication is expensive
- -> search for alternatives
8
Q
Research direction for membrane
A
- change the material
- change the manufacturing process (for macroscopic properties)
- both
9
Q
Alternative Membraner Materials
A
- nano composites
- Sulfonated hydrocarbon polymers
- (partially) non-fluorinated polymers
10
Q
Alternative Membrane Materials - nano composites
A
-> adding hygroscopic inorganic materials to PFSA
- retain water
- allow higher T
11
Q
Alternative Membrane Materials - Sulfonated hydrocarbon polymers
A
- polysulfones (PSF) or sulfonated polyetheretherketones (SPEEK)
- low cost
- lower proton conductivity
- good stabilities
- thermal
- mechanical
- chemical
12
Q
Alternative Membrane Materials - (partially) non-fluorinated polymers
A
- high heat resistance
- acid provides proton conductivity -> no humidification needed
- often used in HT-PEFCs
- e.g. phosphoric acid doped polybenzimidazole (PA PBI)
13
Q
Catalyst Layer
A
- made up of Catalyst & Support if needed
- needed to speed up HOR & ORR
- often immobilised on a support material -> adequate conductivity & mass transport, keeps material cost low
14
Q
Catalyst Layer - Targets
A
- high overall activity
- enable effective electron, proton & reactant transport to/from reaction site
- high electronic conductivity
- proper porous design
- adequate ionomer-catalyst interface
- limited cost
- easy manufacturability
15
Q
Descriptors of Catalysts
A
- mass activity at a certain voltage [A/gcatalyst]
- turn over frequency (catalytic cycles per catalytic site per time)
- selectivity
- poison tolerance (esp. CO, sulfur)
- stability (upon electrochemical cycling)
- durability (in operating environment)
- kinetic descriptors (Tafel slope, exchange current density i0)
16
Q
Catalyst Materials
A
- most commonly employed: Platin (Pt) based
- in mobile applications currently around 0.2 mgPT/cm^2, most of it on cathode -> scarce & expensive -> targets around 0.1 mgPT/cm^2
- typically in the form of nanoparticles deposited on a high surface area, porous carbon support
17
Q
Research directions for Catalyst materials
A
- make better use of Pt
- alloying
- ultra high surface are approaches
- synthesis of specific nanoparticles shape to expose more active surfaces
- defect engineering
- replace Pt
- use of abundant materials like transition metal complexes (Fe/Co-NC)
- most research focuses on more sluggish OR Reaction
18
Q
Support Layer - main requirements
A
- high surface area
- porous design
- high conductivity
- cheap
19
Q
Support Layer - Materials
A
- various carbon based materials
- carbon black (20-30nm particles)
- carbon nanofibers
- carbon nanotubes
- graphene
-> limited corrosion resistance, but cheap, high conductivity, tuneable pore structure
- ceramics (titanium oxide, cerium oxide)
- > limited conductivity, very high corrosion resistance - mixed
20
Q
Catalyst Layer Incorporation
A
- catalyst layer is incorporated in different ways:
- depositing on the membrane (catalyst coated membrane CCM), various deposition techniques
- deposition on the GDL (gas diffusion electrode GDE)
- self standing (very uncommon)
- optimal thickness depends on kinetics & transport properties within layer
- Pt-based: often < 10𝜇m
- non-noble metal based: often > 10𝜇m
21
Q
Challenges of Water Transport in PEFC
A
- flooding -> excess of water in membrane-electrode assembly (MEA)
- hinders reactant delivery (esp. O2)
- reduces utilisable electrochemically active surface area
- causes performance decrease
- may cause material degradation
- water & reactant transport via open porosity & ionomer network
- local GDL/MPL/CL structural properties greatly influence distribution
- currently limited understanding as experimental detection methods reach resolution limits
- material heterogeneity/mixed-wetability complexes modelling
-> fundamental understanding of two-phase flow (gas/water) is essential to material developments
22
Q
Gas Diffusion Layer - Function
A
- electronic connection between bipolar plate & CL
- mass transport - connects channels of flow field to CL
- heat/water removal
- corrosion & erosion protection for CL
23
Q
Gas Diffusion Layer - Materials
A
- carbon fibre papers/fleeces/cloths
- fibre thickness ≈ 10𝜇m, pore sizes ≈ 20𝜇m
- made hydropphobic by coating e.g. with fluoropolymers
24
Q
Gas Diffusion Layer - Challenges
A
- complex manufacturing of carbon
- limited electrical/thermal conductivity (esp. after polymeric coating)
- reduced diffusivity after polymeric coating
25
Gas Diffusion Layer - Research Direction
- improved application methods for hydrophobic coatings
- patterning to facilitate water management/gas transport
- material alternatives e.g. metallic GDLs
- highly conductive
- easily machinable
- not corrosion resistant, coatings needed
26
Microporous Layer
- between CL & GDL
- additional diffusion media -> improves flow characteristic
- fine pore structure
- specifically helps water management
- pore sizes < 1𝜇m
27
Bipolar Plates - Function
- cells need to be connected in series -> anode 1 to cathode
- provide electrical connection over entire electrode area
- host gas channels to provide the reactants to each cell
- host coolant channels
28
Bipolar Plates - Targets
- high
- mechanical strength
- corrosion resistance
- electrical conductivity
- thermal conductivity
- low
- gas permeability
- cost
- weight
- effective flow channel design
- easy manufacturability for ultra thin designs
29
Materials for Bipolar Plates
- graphite
- high
- corrosion resistance
- conductivity
- gas permeability (bad)
- difficult to machine bc of brittleness
-> challenging mass production
- carbon composites
- finding balance between enough conductivity & adequate stability is challenging
- aluminium, stainless, titanium
- have many advantages but aren't corrosion resistant
- Currently significant research efforts on thin but performant protective coatings
- have higher density -> need to be very thin due to weight improvement
30
Cell -> Stack
- fuel cell system consists of large number of individual cells
- connected in series to give one or several stacks
- required to reach practicable output voltages
- reasonable stack size/voltage/power depends on application
- Additional stack components (major weight contributors)
- current collectors
- endplates (incl. sensing, gas inlets, gas outlets, coolant port...)
- compression/clamping hardware
31
Stack performance f
- Governing properties
- Electrochemical reactions
- Mass transfer
- Ionic transport
- Electronic transport
- Water management
- thermal management
- targets published by
- US department of Energy
- EU Clean Hydrogen JU
32
Applications of PEFC
- stationary
- for hydrogen re-electrification
- limited durability & high cost compared to others
- mobility
- forklifts
- light duty passenger vehicles
- heavy duty vehicles
- trains
- Aviation (considered in wing, pods, fuselage)
- load following +
- high power density to other fuel cells but small compared to turbines
- high specific power comp. to fuel cells +
- gap of current & required performance -
- very difficult heat integration -
- system size -
33
Key Issues for Heavy duty Transport
- needs to reliably deliver high durability in their respective operating environment
- continuous use for maximum vehicle earnings
- reliable operation
- avoidance of (unplanned) maintenance times
- certification
- lifetime cost
34
CL failure modes
- loss of usable reaction sites due to aggregation, corrosion...
- loss/corrosion of catalyst support
- decrease in mass transport
- Degradation
- loss of ECSA -> less catalytic sites -> performance loss
- done by flooding, ionomer degradation, Pt-particle growth, CO/Sulfur adsorption, Carbon Corrosion
35
GDL failure modes
- conductivity loss
- hydrophobicity loss
- losses in mass transport efficiency
-> performance loss
36
BP failure modes
- conductivity loss
- fracture/deformation
-> performance loss
-> device failure
- Degradation
- main challenges
- formation of resistive surface layer -> high ohmic resistance
- mechanical stress -> fracture/deformation
- dissolution metal cations from bipolar plates -> impurities potentially harming other cell components
37
Membrane failure modes
- insufficient humidification
-> performance loss
- thinning/rupture through
- mechanical/chemical/thermal degradation
- short circuiting/device failure
- Degradation
- main challenges
- membrane thinning
- delamination
- pinhole formation
- caused by
- mechanical stress
- peroxide/radical attack (Fenton reaction)
38
Mechanical Degradation
- due to non-uniform mechanical stress caused by:
- local material/structural differences (fabrication)
- different thermal coefficients
- differing extension/shrinkage with humidity
- (local) pressure differences in cell
- cell-to-cell variations (thermal/pressure/humidity) causing strain over stack
39
Chemical degradation
- ageing/degradation of materials due to internal or external factors
- natural material ageing in operating environment -> elevated temperature, high relative humidity, harsh potentials & low pH lead to corrosion of materials
- contaminants
- enhanced ageing due to non-uniform current distribution
40
Chemical Degradation Mechanisms
- Radial attack (high T/fuel starvation)
- Flooding (high humidity/start/stop cycling)
- Ionomer degradation (high T)
- Membrane dehydration (low Humidity)
- Pt-particle growth (potential cycling/high humidity/high T)
- CO adsorption (low T/CO/CO2 Fuel Impurities)
- Sulfur adsorption (Sulfur fuel impurities)
- Carbon corrosion(high T/Potential Cycling/Fuel Starvation)
41
Carbon Corrosion
- affects CL mainly, but also GDL & MPL
- main causes
- flooding
- rapid load change
- start-stop
-> causes starvation conditions
- electrochemical carbon Corrosion due to starvation = H2 lack at anode (overall/local) -> Carbon is oxidised instead of H2
- chemical carbon corrosion
-> hydrogen peroxide may form, reacts with the carbon
- consequences
- detachment of Pt-particles
- loss of catalytic surface area
- alteration of hydrophobic surface of GDL & MPL
- delamination/wrinkling
- collapse of porous structure -> loss of mass transport
- degradation of graphitic bipolar plates
-> causes voltage & power loss, reducing device lifetime, irreversible
42
High Clamping Force - GDL
- Components
- Positive effects
- Improve contact resistance between GDL & other components
- Improve bulk conductivity
- negative effects
- Decrease in porosity
- Decrease in permeability
- Operational
- positive effects
- lower electrical & thermal resistance in GDL
- lower the interfacial resistance
- negative effects
- Higher mass over potential
- Lower gas permeability
43
Higher Clamping Force - CL
- Decreases the mass transfer rate
44
Higher Clamping Force - Membrane
- Components
- Developing mechanical strategic -> contributes to hydration/dehydration strain; results in
- Buckling or delamination of the membrane
- pinhole/cracks
- Operational
- negative effects
- Lowering the operating voltage
- Reactants leak
45
Higher Clamping Force - End plates
- Lower the contact resistance (positive)
46
Freezing Water
- presence is required for function proton transport
- freezing water expands -> mechanical stress, mass transport hindered
- "Cold Start" considered an abusive condition for PEFC but required for automotive application
47
Accelerated Stress Test AST
- testing protocols used to simulate material degradation in shorter time frame (more cost effective)
- depending on testing protocol degradation rates differ quite substantially -> ASTs should be application specific
- sota tests have been developed for automotive fuels cells -> aviation specific ASTs lacking
- component specific failure rates often unkown
48
Cost Breakdown
- Light Duty Vehicles
- main Catalyst Layer 41%
- least GDLs 6%
- Heavy duty
- main Catalyst Layer 60% (viel hilft viel)
- least GDL & Balance of Stack 3%
49
Take away Messages D)
- PEFC is complex electrochemical device made up of several individual components
- each component is associated with unique challenges
- for PEFC optimisation application is key
- PEFCs are well-suited for mobile applications bc
- fast startup
- excellent transient performance
- comparatively high power density
- comparatively high specific power
- research directions on component level are typically targeting higher performance &/ reduced cost
- durability & reliability are of high relevance, esp. in aviation
- mechanical & chemical degradation can affect all components & lead to fast performance decrease
