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Engineering LibreTexts

31.22: Questions

  • Page ID
    33007
  • Quick questions

    You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

    For each type of fuel cell below, what are the charge carrying species (electrolyte type)?

    (a) Polymeric exchange membrane

    (b) Immobilised alkaline solution

    (c) Phosphoric acid

    (d) Molten carbonate

    (e) Ceramic solid oxide

    Answer

    (a) H+

    (b) OH

    (c) H+

    (d) CO32–

    (e) O2–

    Which cells are able to internally reform?

    Answer

    High temperature, solid oxide or molten carbonate cells.

    The efficiency limit of H2 cells reduces as operating temperature is increased. Why are high efficiency systems often run at very high temperatures?

    Answer

    The heat produced can be more useful than the efficiency gained at lower temperatures. Running a system at a higher temperature allows the hot exhaust gasses to be utilised in a turbine-hybrid system, raising the overall efficiency of the system up as high as 80% or more.

    Which of the following statements are TRUE about the development of fuel cells?

    (a) Fuel cells have been around since the 19th century.

    (b) A fuel cell tractor was built in 1959.

    (c) PEM fuel cells are so expensive because of the platinum needed as a catalyst.

    (d) The first fuel cells were developed to run on hydrogen.

    (e) Fuel cell cars are at least a decade away.

    Answer

    (a) True. The phenomenon was discovered in 1800 and the first attempts to engineer a fuel cell were in 1889.

    (b) True.

    (c) False. The amount of platinum used in a fuel cell is actually small compared to the cost of the bipolar plates; typically about £10 for a 1 kW stack.

    (d) False. The first cells were developed to run on coal gas. It was never successful however. The first successful cell was run on pure hydrogen and oxygen by Bacon in 1932.

    (e) False. Honda are already marketing a fuel cell powered automobile.

    Give two reasons why we don’t all use fuel cells in our cars already?

    Answer

    (i) High temperature fuel cells are not easily portable and low temperature ones require hydrogen as the fuel in order to produce any useful power. Hydrogen is not yet a readily available fuel. It is also difficult to store.

    (ii) Fuel cells also suffer from low reaction rates and therefore low power output.

    Why is nickel used as the anode in SOFCs? How is it treated in order to aid bonding to YSZ?

    Answer

    It is cheap and performs well but flakes off unless mixed with zirconia.

    Deeper questions

    The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

    Give as many ways as you can think of to provide a mobile fuel cell with pure hydrogen.

    Answer

    (i) Pure H2 may be stored as used either under pressure, cryogenically, in a metal hydride, in a high surface area carbon nano-tube arrangement or even in glass nano-spheres.

    (ii) Many fuels may be carried and reformed before use e.g. alcohols, alkali metal hydrides, ammonia, sodium borohydride, methane and even hydrazine.

    (iii) Alternately the cell could be run directly on methanol.

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