Alkaline and Phosphoric Acid Fuel Cells

1. Alkaline and Phosphoric Acid Fuel Cells By

2. Alkaline Fuel Cell • The alkaline fuel cell (AFC) is also known as the Bacon fuel cell after its British inventor. • AFCs consume hydrogen and pure oxygen producing potable water, heat, and electricity. • They are among the most efficient fuel cells, having the potential to reach 70%.

3. Chemistry of Alkaline Fuel Cell • Anode Reaction: 2 H2 + 4 OH- => 4 H2O + 4 e- • Cathode Reaction: O2 + 2 H2O + 4 e- => 4 OH- • Overall Net Reaction: 2 H2 + O2 => 2 H2O

4. Design • Two main variants of AFCs exist: static electrolyte and flowing electrolyte. • Static, or immobilized, electrolyte cells of the type used in the Apollo space craft and the Space Shuttle typically use an asbestos separator saturated in potassium hydroxide. • Water production is managed by evaporation out the anode which produces pure water that may be reclaimed for other uses.

5. Interesting Information! • AFCs are the cheapest of fuel cells to manufacture. The catalyst required for the electrodes can be any of a number of different chemicals that are inexpensive compared to those required for other types of fuel cells. • The commercial prospects for AFCs lie largely with the recently developed bi-polar plate version of this technology, considerably superior in performance to earlier mono-plate versions.

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8. Applications • NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. • The world's first Fuel Cell Ship HYDRA used an AFC system with 6.5 kW net output.

9. Phosphoric Acid Fuel Cells • Phosphoric acid fuel cells (PAFC) are a type of fuel cell that uses liquid phosphoric acid as an electrolyte. • The electrodes are made of carbon paper coated with a finely dispersed platinum catalyst, which make them expensive to manufacture. • They are not affected by carbon monoxide impurities in the hydrogen stream. • Phosphoric acid solidifies at a temperature of 40 °C, making startup difficult and restraining PAFCs to continuous operation.

10. Chemistry of PAFC • Anode Reaction: 2 H2 => 4 H+ + 4 e- • Cathode Reaction: O2(g) + 4 H+ + 4 e- => 2 H2O • Overall Cell Reaction: 2 H2 + O2 => 2 H2O

11. Interesting Information • The PAFC uses an electrolyte that is phosphoric acid (H3PO4) that can approach 100% concentration. • The ionic conductivity of phosphoric acid is low at low temperatures, so PAFCs are operated at the upper end of the range 150ºC–220ºC. • The PAFC operates at greater than 40% efficiency in generating electricity. • When operating in cogeneration applications, the overall efficiency is approximately 85%.

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14. Applications • Typical installations include buildings, hotels, hospitals, and electric utilities in Japan, Europe and the United States. • One particular program sponsored by CERL was to demonstrate 30 PAFCs generating 200 kW apiece at military installations. • The objective of this program was to gain experience with PAFC operation for maintenance, reliability and performance.

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16. Resources • http://en.wikipedia.org/wiki/Alkaline_fuel_cell • http://www.fctec.com/fctec_types_afc.asp • http://en.wikipedia.org/wiki/Phosphoric_acid_fuel_cell • http://www.fctec.com/fctec_types_pafc.asp • http://www.facebook.com/?ref=home