PPT Slide

Phosphoric Acid Fuel Cell

Figure 8. How a Phosphoric Acid Fuel Cell Works. Smithsonian Institute, 2001.

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Phosphoric Acid Fuel Cell

HOW IT WORKS

• The PAFC uses liquid phosphoric acid as the electrolyte.

• The phosphoric acid is contained in a Teflon bonded silicone carbide matrix. The small pore structure of this matrix preferentially keeps the acid in place through capillary action.

• Some acid may be entrained in the fuel or oxidant streams and addition of acid may be required after many hours of operation.

• Platinum catalyzed, porous carbon electrodes are used on both the fuel (anode) and oxidant (cathode) sides of the electrolyte.

• Fuel and oxidant gases are supplied to the backs of the porous electrodes by parallel grooves formed into carbon or carbon-composite plates. These plates are electrically conductive and conduct electrons from an anode to the cathode of the adjacent cell. In most designs, the plates are "bi-polar" in that they have grooves on both sides - one side supplies fuel to the anode of one cell, while the other side supplies air or oxygen to the cathode of the adjacent cell.

• The byproduct water is removed as steam on the cathode (air or oxygen) side of each cell by flowing excess oxidant past the backs of the electrodes. This water removal procedure requires that the system be operated at temperatures around 375°F (190°C). At lower temperatures, the product water will dissolve in the electrolyte and not be removed as steam. At approximately 410°F (210°C), the phosphoric acid begins to decompose.

• Excess heat is removed from the fuel cell stack by providing carbon plates containing cooling channels every few cells. Either air or a liquid coolant, such as water, can be passed through these channels to remove excess heat.

• At the anode, hydrogen is split into two hydrogen ions (H+), which pass through the electrolyte to the cathode, and two electrons which pass through the external circuit (electric load) to the cathode. At the cathode, the hydrogen, electrons and oxygen combine to form water. This is shown below.