PAFC FCs

Question 1

How is the PAFC cell produced?

Answer 1

Two equal half cellsare produced, filled with H3PO4 in between and laminated together.

Question 2

How is the PAFC cell structured?

Answer 2

• Substrate/ gas diffusion layer
• Electrode
• Half of a matrix
  This is the structure of the half cell, which is then equally produced for the other side and joined together.

Question 3

Describe the matrix in the PAFC cell.

Answer 3

SiC with 2-5% PTFE filled with H3PO4.
Half of the matrix is printed on either Electrode; both sides are laminated together.

Question 4

Describe the electrodes in the PAFC cell.

Answer 4

Pt/C – PTFE
Anode 0.25 mg/cm2 Pt; Cathode 0.5 mg/cm2 Pt-alloy

Electrode is cast on substrate, dried Rolled, sintered at 209°C and above
PTFE crystalline melting point at 337°C

Question 5

Describe the Substrate/ gas diffusion layer in the PAFC cell.

Answer 5

Toray carbon paper, porosity 80% filled with carbon black to increase electrolyte storage capacity (no catalyst)

Question 6

Why does corrosion take place in PAFC?

Answer 6

• Corrosion owing to vaporized electrolyte acid in the BoP
• Pressurization increases the cell efficiency, yet promotes corrosion via an increase of H3PO4
partial pressure

Question 7

Where does corrosion take place in PAFC?

Answer 7

• Corrosion takes place in areas other than the active cell area, e.g. manifold and system
• Carbon and platinum corrosion takes place above 0.8 V/cell
• Hence, partial load at very low current densities / idle operation at OCV are to be prevented

Question 8

Why does freezing damage in PAFC occur?

Answer 8

• At 205 °C, the electrolyte is close to 100% H3PO4
• At 42.5 °C, the acid crystallizes

Question 9

What are the freezing damages caused or the consequences of freezing in PAFC?

Answer 9

• The acid frequently super cools / crystallizes rapidly when solidifying / damages electrode structure
• Loss in voltage after restart is chiefly due to:
- increase in concentration polarization (the electrolyte clogs pores that served as gas passages)
- crystals disrupt the interface between the catalyst layer and substrate

Question 10

What is the solution to Freezing in PAFC?

Answer 10

• maintained at temperature during shutdown
• conditioned to 80% H3PO4 by wetting though the air supply
• Wetting can prevent the stack from freezing even under 0°C (75% H3PO4 -20°C)
• Individual cells incorporate electrolyte reservoir plates (ERPs) to accommodate changes in electrolyte volume from:
  
  • load changes
  • reactant humidification

Question 11

Describe the corrosion at the cathode in PAFC

Answer 11

• The cathode potential is generally higher than 0.7 V w/r to a hydrogen reference electrode
• Thus, carbon will oxidize at the cathode
• Corrosion induces higher cathodic overpotentials
• Operation at part load to idle operation above 0.8V is to be prevented

Question 12

Describe the three phase boundary in PAFCs.

Answer 12

• A longer pathway for diffusion is created to create high resistance to diffusion
  The variables related to the boundary are:
• Contact angle
• O2 solubility in the electrolyte
• Diffusion coefficient

Question 13

What must be watched out for when starting up / shutting down PAFCs.

Answer 13

• Electrodes shall never exceed a potential of 0.8 V
• Electrodes may never be exposed to oxygen (air) when hot and at OCV
• Irregularities will result in platinum dissolution and carbon corrosion
• During shutdown, the stack and piping are to be flushed with nitrogen.
• To start up, hydrogen is first flushed through the cold stack. Once the fuel supply is established, the stack temperature is raised by circulating the heated coolant.
• The nitrogen in the anode and cathode is replaced by hydrogen and air, respectively.
• Simultaneously, an auxiliary load is connected across the output terminals to keep the Chair for Fuel Cells voltages of the cells at 0.8 V until the stack reached its operating temperature.

Question 14

How are reactants utilised in PAFC and why?

Answer 14

Fuel utilization: 85%
Air utilization: 70%
Limited utilization to prevent electrode starving and subsequent irreversible damage

Question 15

Describe Air starving at cathode.

Answer 15

Protons will keep moving through the electrolyte
Hydrogen evolution replaces oxygen reduction at the cathode (electrolysis process) From small performance loss, which equals loss of the FC contribution plus energy
consumption for overcoming the ohmic resistance up to permanent damage of the fuel cell
Safety hazard may occur when the hydrogen concentration in the air exit stream becomes too high

Question 16

Describe fuel starving at anode.

Answer 16

Current keeps passing through the cell
Oxidation needs to take place, otherwise the current cannot be furnished. Hence, potential of the anode is raised to a level at which cell components get oxidized (separator plate, catalyst support, etc.)
Catastrophic failure occurs.

Question 17

Describe impurities at cathode in PAFC.

Answer 17

• Removes particulates
• Use oil-free compressor

Question 18

Describe impurities at anode in PAFC.

Answer 18

• Chlorides, sulfides and ammonia are to be avoided
• H2S above 50 ppm severely and irreversibly poisons the catalyst
• Chloride is introduced through the manufacturing steps of the cell, e.g., chloride containing process water and chloroplatinic acid to catalyze the electrodes
• Chlorides are no permanent poisons
• Cl- is reduced to HCl that exits the stack in the anode exhaust
• Ammonia may occur in the air supply or may be produced in the reformer
• Ammonia combines with the phosphoric acid [(NH3)3PO4] and clogs the electrode
inducing increased concentration polarization at the cathode
• CO2, N2 and H2O are merely diluents
• CO up to 3% can be sustained