Phosphoric acid fuel cell

The phosphoric acid fuel cell (English Phosphoric Acid Fuel Cell, PAFC ) is a medium-temperature fuel cell.

Principle

The PAFC is different from other fuel cells in that it works with phosphoric acid as the electrolyte. The highly concentrated phosphoric acid (90 -100%) is fixed in a PTFE fiber structure. As all fuel cell produces electricity through the oxidation of a fuel gas. The fuel gas is hydrogen. As the oxidant may be air or pure oxygen may be used. It must be used no pure gases. Traces of carbon dioxide in the gases submitted to the PAFC in contrast to the AFC not a problem The phosphoric acid fuel cell operates in a temperature range of 135 to 200 ° C.

Due to the temperature and the use of phosphoric acid high demands on the quality and resilience of the components apply. Usually carbon and graphite parts to be used for the electrodes. The advantage of this type is that it is relatively insensitive to contamination of the fuel gas.

As an electrolyte, phosphoric acid H3PO4 will be used. A particular advantage of the phosphoric acid is the low vapor pressure, but which can not be ignored, ie liquid phosphoric acid evaporates still slow. Therefore, the exhaust gas usually in a cooling zone to 160 -. To 180 ° C and therefore cooled to about 30 ° C. Operating Thus, the electrolyte is condensed before it exits the cell and can be passed back. The phosphoric acid is injected in a matrix of silicon carbide, which is disposed between the anode and cathode and through which migrate the protons.

As a catalyst, platinum or a Platinlegeriung is usually used because they are stable in hot phosphoric acid H3PO4, have catalytic properties and in sufficient quantity on the earth are available to use it industrially for the Elektrodenbau. In more recent developments, for example, a platinum-ruthenium mixture was used as a catalyst, whereby the carbon monoxide tolerance of the electrode is increased. Today it takes about 0.25 mg of platinum per cm ² for the anode, and 0.5 mg of platinum per cm ² of the cathode.

Various cooling methods include liquid cooling, air cooling and water cooling, the water cooling is the most widely used. The water cooling differs in principle back in two ways. In both species, the water is introduced in liquid form into the cooling system. When the water comes back out of a liquid, but what large amounts of water needed because only a small proportion can be added to heat. The other water occurs in two phases (liquid / gas) of the cooling system, which, although a higher expense in the construction of the cooling system is formed for less water is needed for cooling.

Reaction equations

The fuel of a PAFC is hydrogen and hydrogen -rich gas. Hydrogen is fed to the anode where the electrochemical reaction takes place. Hydrogen is oxidized into hydrogen ions (H ), the electrons ( e-) are captured by a collector electrode, and directed through a load to the cathode. The protons migrate through the electrolyte from the anode to the cathode. At the cathode, oxygen is supplied, it results in an electrochemical reaction between oxygen, hydrogen ions and electrons; there is water and heat.

Advantages and disadvantages compared to other fuel cells

Advantages of a PAFC are:

  • Robust
  • Increased tolerance to contamination of the fuel gas
  • On the cathode side (no clean gas required), air can be used
  • Good dynamic performance
  • CO2 - tolerant

Disadvantages are:

  • Low power density
  • Low life (due to the extremely aggressive electrolytes )
  • CO is a catalyst poison, the tolerance to carbon monoxide is between (1-3 %) decreases with increasing operating temperature.
  • Plant efficiency is rather low

SAFC

In the solid - acid fuel cells ( engl: Solid acid fuel cell, SAFC ) solid phosphate salts form the electrolyte. Above 220 ° C, the proton conductivity of 0.05 above ( ohm-cm ) -1 and almost reaches the value of PEFC ( to 0.1 ( ohm-cm ) -1). Salt formation of phosphoric acid is introduced by the addition of alkali metal phosphates:

Currently (2011) is examined, SAFC run on hydrogen, which is obtained by reformation of gasoline.

Catalyst poisoning
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