Phosphoric acid fuel cell

The PAFC uses an electrolyte composed of pure phosphoric acid, H3PO4. This acid is a relatively poor conductor of hydrogen ions but is stable up to 200°C and can form the basis of a fuel cell which operates using hydrogen and oxygen supplied as air.

Table 7.1 Fuel cell characteristics

Operating Catalyst Efficiency (%)

temperature (°C)

Table 7.1 Fuel cell characteristics

Operating Catalyst Efficiency (%)

temperature (°C)

Alkaline cell

150-

-200

Platinum

70

PAFC

150-

-200

Platinum based

35-

42

PEM

80

Platinum based

42-

60

MCFC

650

None needed

50-

60

SOFC

750-

-1000

None needed

50-

60

Direct methanol

90-

-95

Platinum-ruthenium

The electrolyte is contained in a silicon carbide matrix where it is held within the pores of the material by capillary action. Since the electrolyte is a liquid, care must be taken to control evaporation or migration as this will impair the operation of the cell.

Electrodes made from porous carbon are bonded to the electrolyte-containing matrix using teflon mounts and the carbon is covered with a fine coating of platinum. There are groves in the back of the electrodes which carry the hydrogen or oxygen to each cell. Carbon is a good electrical conductor, so it can be used to transport the current from the cell. Each cell produces a voltage of around 0.65 V. Cells can be connected back-to-back, in series, to build what is known as a stack. Stacks are then connected in parallel to provide the required current and voltage output.

The PAFC requires hydrogen and oxygen, the latter from air. The cell must be heated to around 200°C before it can be started but once it starts operating the cell reaction produces sufficient heat to maintain its temperature. Water generated by the cell reaction is swept away from the cell in the air stream feeding oxygen to the cathode.

At the cell operating temperature the hydrogen-oxygen reaction will proceed sufficiently swiftly with a platinum catalyst to sustain cell operation. However the concentration of carbon monoxide in the hydrogen must be kept below 1.5% to prevent catalyst poisoning.

Hydrogen for cell operation is normally provided by reforming natural gas, though other sources such as biogas have been employed. Waste heat generated by the cell during operation can be used to carry out this reaction. The remainder of the energy which emerges as heat can be used to heat water or for space heating. The cell has a theoretical efficiency of around 42% but practical cells have not achieved this. Configured as a co-generation unit, a PAFC unit can achieve up to 87% efficiency.

PAFCs were the first type of fuel cell to achieve commercial status. A number of prototype units were field tested during the 1980s and the beginning of the 1990s. The most successful programme was carried out by International Fuel Cells3 which produced a commercial 200-kW unit, launched in 1992. Since then over 200 have been installed across the world.

The 200-kW unit has an electrical efficiency of 36%. In addition to generating power it can supply 200 kW of hot water. As a result of its low emissions and low noise, the unit is suitable for use in urban areas and can compete with engine-based co-generation system for small commercial applications. It has the advantage of producing a greater proportion of electricity than the typical engine-based package co-generation unit.

Although these units are commercially available, they have proved costly, with a unit price in 2003 of around $900,000.4 As a consequence, manufacture of them is likely to be discontinued and the company expects to switch to a PEM-based system for future stationary applications. Some development of the PAFC continues in Japan. However the technology may eventually be abandoned for one of the technologies below.

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