How does a phosphoric acid fuel cell differ from an alkaline fuel cell?
The proton exchange membrane is one of the most advanced fuel cell designs. Hydrogen gas under pressure is forced through a catalyst, typically made of platinum, on the anode (negative) side of the fuel cell. At this catalyst, electrons are stripped from the hydrogen atoms and carried by an external electric circuit to the cathode (positive) side. The positively charged hydrogen ions (protons) then pass through the proton exchange membrane to the catalyst on the cathode side, where they react with oxygen and the electrons from the electric circuit to form water vapour (H2O) and heat. The electric circuit is used to do work, such as power a motor.
Phosphoric acid fuel cells and alkaline fuel cells differ primarily in their electrolytes, operating temperatures, and construction materials.
- Electrolyte and Temperature: Phosphoric acid fuel cells use an orthophosphoric acid electrolyte and operate at temperatures up to approximately 200 °C (400 °F). In contrast, alkaline fuel cells utilize an aqueous solution of sodium or potassium hydroxide and typically operate at temperatures below 100 °C (212 °F).12
- Fuel Tolerance: Phosphoric acid cells can utilize hydrogen fuel that is contaminated with carbon dioxide. Alkaline fuel cells generally require pure hydrogen gas.12
- Construction: The framing for phosphoric acid cells is made of graphite, which contributes to higher costs. Alkaline fuel cells are constructed using metal and certain plastics, with electrodes made of carbon and metals such as nickel.12
- Applications: Due to their design, phosphoric acid cells have been tested for municipal power stations and remote generators. Alkaline fuel cells have been famously employed on U.S. space shuttle orbiters.12