PEFC Cell

Question 1

What are the PEFC stack components from left (anode side) to right?

Answer 1

• BP
  
  • End metal plate
  • H2 in and out
  • Current collector
  • Graphite plate
• Gasket (for gas tightness)
• Gas diffusion medium including Anodic Gas Diffusion Layer (GDL) and Micro porous layer (MPL)
• Gasket
• MEA (Membrane electrode Assembly), proton exchange membrane
• Gasket
• Gas diffusion medium with MPL then Cathodic GDL
• Gasket
• BP
  
  • Graphit
  • Current collector
  • Air/O2 in and Air/O2, water out
  • End metal plate

Question 2

What must be watched out for in PEFC?

Answer 2

Water management is important since the water requirement for the transport principle can lead to considerable swelling of the material. In addition, water is dragged from the anode side during the movement of protons, so water balance must be controlled to make sure that the anode doesn‘t become too dry and the cathode too wet.

Question 3

Compare isotopic and linear swelling in PEM Membrane

Answer 3

30% isotropic volume swelling ≈ 3.1% linear swelling
Swelling often does not occur isotropic, since the material is cast

Question 4

Compare swelling in Nafion 117 and 212

Answer 4

Nafion 212 swells more for same water content.

Question 5

What is the molecular weight of Nafion?

Answer 5

105-106 DA (1 DA = m_u = m(12C)/12 (Dalton; unit for mass often used for polymers))

Question 6

What is Equivalent weight (EW)?

Answer 6

The Equivalent Weight (EW) is the mass of a polymer electrolyte providing 1 mole of exchangeable protons

Question 7

What is the PEM for a typical PEM exhibit?

Answer 7

800 - 1200 g/mol

Question 8

What is the Ion Exchange Capacity (IEC) and what is it typically for PEMs?

Answer 8

IEX= 1000/EW, typicall 1,2-0,8 mol/g

Question 9

What are the material vales for Nafion 117?

Answer 9

EW 1100 and 7 mil in thickness (1 mil = 1 thou = 0.001 inch, so 0,007 inches)

Question 10

What is the structure of Nafion like and how does temperature affect it?

Answer 10

Polymers form crystalline and amorphous regions, so does the Nafion backbone. At the glass transition temperature T_g the crystalline structure disappears owing to a higher mobility of the chains at higher temperature and vice versa.
Dry Nafion shows two T_gs with the first one owing to the backbone at 125°C and a second one owing to the attractive forces of the -SO3- Groups at 195°C.
The softening of the backbone structure above 125°C is used during MEA assebly to gain a good interfacial stability between electrode and electrolyte by hot pressing. The proton conductivity suffers and the water uptake % decreases, though.

Question 11

Why is Nafion still hot pressed, even though it leads to proton conductivity losses?

Answer 11

The better interface formation at hot pressing slightly above the glass transition temperature Overcompensates the minimal conductivity losses of the electrolyte. Even higher hot-pressing temperatures result in strong performance losses owing to increased crystallization.

Question 12

What are the targets for optimisation of electrolyte properties?

Answer 12

• Mechanical stability
• Lower swelling
• Higher conductivity

Question 13

Describe Macroscopic heterogenous materials Gore Select.

Answer 13

• Inert polymer matrix
• Introduced ionomer
• Microstructure in the μm to mm range

Question 14

Describe Microscopic heterogenous materials Nafion.

Answer 14

• A hydrophobic macromolecule with hydrophilic groups
• Phase separation in the nm range

Question 15

Describe Homogenous materials Ballard (perfluorinated polystyrene polymer).

Answer 15

• A hydrophobic macromolecule with hydrophilic groups
• No phase separation

Question 16

What are some more Approaches to the Optimization of Electrolyte Properties.

Answer 16

• Blends of various fluorine-free base polymers (e.g., PEEK-PPS)
• Non-woven reinforcement
• Multi-layer systems