Fuelling requirements

Cell Type

Fuelling requirements

Solution

AFC

Alkaline fuel cells, using KOH as the electrolyte, require pure H₂ and O₂ as their fuel. This is because even the small amounts of CO₂ in the air (about 300 ppm) are enough to prevent the cell from functioning properly. This is because CO₂ reacts with the KOH as follows:

    2KOH  +  CO₂ →  K₂CO₃  +  H₂O

The presence of potassium carbonate in the electrolyte may reduce cell activity by:

• Reducing the concentration of OH^–
• Increasing the viscosity of the solution, hindering the diffusion of ions through it.
• Reducing the solubility of oxygen in the solution
• Precipitation of the K₂CO₂, which would reduce the surface area of the electrodes.

Use of Pure Oxygen at the cathodes is necessary. This can be achieved by:

• Regenerative methods i.e. using some external power source (e.g. photovoltaic cells) to hydrolyse H₂O in situ and store the resulting gases.
• Remove the CO₂ from the air. One proposed method takes advantage of the heat exchanging stage necessary in using cryogenically stored hydrogen (the H₂ must be warmed and the cell must be cooled) to freeze the CO₂ out of the air.

Species:

H₂

CO

CH₄

CO₂ and H₂O

S (e.g. H₂S and COS)

Effect:

Fuel

Poison

Diluent

Poison

Unknown

PAFC

The phosphoric acid (H₃PO₄) systems are almost always fuelled by a hydrocarbon, which must undergo some sort of fuel processing (add link) to release the H₂ gas that reacts at the anodes. The PAFC systems can tolerate CO₂ and un-reacted hydrocarbons (e.g. methane). These act as diluents to the fuel and their concentrations should be minimised. CO gas however will poison the Pt catalysts at concentrations from 0.5%

• In Situ fuel possessing required (costly)
• CO removed by further processing and shift reaction.

Species:

H₂

CO

CH₄

CO₂/H₂O

S etc

Effect:

Fuel

Poison (>0.5%)

Diluent

Diluent

Poison (>50 ppm)

MCFC

CH4 can either be a diluent or it may be internally reformed (see SOFCs). Very low sulphur tolerance in the MCFC due to poisoning, especially in the catalysts that aid fuel reforming. The reactions of the MCFC requires CO₂ to be present in the fuel:
H₂ + ½O₂ + CO₂ (cathode) → H₂O + CO₂ (anode)

Recall that CO₃^2– ions are transported through the electrolyte from cathode to anode (see section on high temperature fuel cells). This requirement for CO₂ contrasts with the AFC where it must be excluded. The CO₂ is usually recycled externally by passing exhaust gases through a combustor to convert unused fuel into water and CO₂, which can be fed back to the cathode inlet. This also serves to preheat the reactant air.

• CO₂ recycled externally
• Exhaust gases combusted to yield more CO₂
• Sulphur removed from fuel stream by fuel processors.

Species:

H₂

CO

CH₄

CO₂/H₂O

S etc

Effect:

Fuel

Fuel (via shift reaction)

Can be internally reformed

Diluent

Poison (>0.5 ppm)

SOFC

SOFCs (and MCFCs) run at high enough temperatures to internally reform CO and hydrocarbons (e.g. petrol or methane) via reaction with H₂O, producing CO₂ and H₂ via “shift” (or oxygenolysis) reactions:

C_nH_m+nH₂O → nCO + (m/2 + n) H₂
CO + H₂O → CO₂ + H₂

• Sulphur removed from fuel stream by fuel processors.

Species:

H₂

CO

CH₄

CO₂/H₂O

S etc

Effect:

Fuel

Fuel (via shift reaction)

Can be internally reformed

Diluent

Poison (>1.0 ppm)

PEMFC

PEM Cells generally use pure Hydrogen as the fuel, especially in portable applications where complicated reforming apparatus would be impractical. CO poisons PEM systems very easily because they rely on platinum catalysts. CO has a high affinity for Pt and occupies catalytic sites, preventing hydrogen fuel from preaching them. The processing equipment needed to reduce CO partial pressures to less than 10 ppm adds considerably to the cost of the system.

Pure oxygen is used in air independent applications such as submarines and space shuttles. Whilst difficult to implement, use of pure O₂ improves the performance of PEM cells significantly:

• The open circuit voltage increases due to increased O₂ partial pressure, as described by the Nernst equation.
• Activation over-potential reduces because of better use of catalyst sites.
• Limiting current increases, reducing concentration over-potential losses, due to the absence of nitrogen gas.
  

• CO must be removed to ensure long life of cell.
• Pure Hydrogen can be used in either a Compressed form or a cryogenic or metal-hydride stored form.

Species:

H₂

CO

CH₄

CO₂/H₂O

S etc

Effect:

Fuel

Poison (>10 ppm)

Diluent

Diluent

Unknown