Polymer Electrolyte Fuel Cells (PEFC) Flashcards
PEFCs – Cell Structure and Components
Which ones are focused on performance optimization, and which ones in weight/volume? Which ones in both?
Presentation 1 Slide 120
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Define Ionomer
Polymer composed of macromolecules in which a small but significant proportion of the constitutional units has ionic or ionizable groups, or both.
These ionic groups can cause micro-phase separation of ionic domains from the continuous polymer phase.
Targets, advantages and disadvantages of an Ionomer
- Targets: High ionic conductivity, electrically insulation and no gas cross-over.
- Advantages: High corrosion stability, require low T, robust and bendable (it can bear more load)
Which are the relevant parameters to describe a membrane?
- EW (Equivalent Weight) = Mass of dry polymer/ion exchange site.
- Ion conductivity
- Water uptake
- Thickness
Challenges of PFSA membranes
Challenges:
1. Humidification
2. Only work below 100ºC
What is the main goal of the catalyst layers?
- They need to speed up hydrogen oxidation and oxygen reduction reaction (HOR and ORR)
- Provide the reaction site
- Provide the membrane with everything necessary from the GDL
Which is the most common material for the catalysts (current ones)?
Which are the research directions?
- Platinium
Research directions: make better use of Pt, and replace Pt (it is expensive).
Main requirements of the Catalyst Support Materials
- High surface area
- Porous design
- High conductivity
Gas Diffusion Layer (GDL). Function, Materials and Challenges
- Function:
1. Electronic connection between bipolar plate and Catalyst Layer
2. Heat water removal
3. Mechanical support - Materials:
1. Carbon fiber - Challenges:
1. Complex manufacturing of cabron fibers
2. Limited electrical/thermal conductivity
3. Reduced diffusivity
What is the Microporpus Layer (MPL)?
Additional diffusion media sandwiched inbetween CL (Catalyst Layer) and GDL (Gas Diffusion Layer).
Functions of the Bipolar Plates
- Mechanical support
- Heat Management
- Electrical connection
Targets of Bipolar Plates
- High mechanical strength
- High corrosion resistance
- High electrical conductivity
- High thermal conductivity
- Low gas permeability.
–> Basically everything that needs to make good its jobs.
Bipolar plates material and characteristics of each material
Materials BipolarPlatesTable
Main challenges of PEFCs in Aviation
- Smaller power density compared to turbines
- Difficult heat integration
- Size
- Electrification (high voltage levels, power management…)
PEFCs - Aging and Failure Modes
PEFCs - Aging and Failure Modes table
PEFC Degradation by Type
- Mechanical degradation: non-uniform mechanical stress caused by: structural differences, different thermal coefficients, pressure differences in cell…
- Chemical degradation: degradation of materials due to internal or external factors (temperature, humidity…), non-uniform current distribution.
What does ECSA mean?
In which scenario can it be lost?
ECSA = Electrochemical Surface Area
This is basically, the ‘usable’ surface you can use for your reaction
This can be lost when the degradation of the catalyst. This leads to performance loss.
What is the Carbon Corrosion?
Which are the its main causes?
Layers affected?
Consequences?
Carbon corrosion: When the particles lose their detachment. Basically the cell is losing the reaction.
Main causes: Flooding, rapid load change.
Layers affected: Catalyst Layer, GDL, MPL.
Consequences: Loss of catalyst surface area, delamination, loss of mass transport.
it is an irreversible damage causing voltage and power loss, and reducing device lifetime.
Main cause of the membrane degradation?
Mechanical stress
Main challenges of the Bipolar Plate Degradation
- Formation of resistive surface layer; leading to high ohmic resistance.
- Mechanical stress; leading to fracture
What is the Clamping Force?
Mechanical force applied to compress the fuel cell components to ensure proper contact, leading to a higher conductivity.
It helps to avoid mechanical stress.
