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Engineering LibreTexts

31.4: Types of fuel cells

  • Page ID
    32965
  • Fuel cells are categorised according to their type of electrolyte, since it is the property-determining component. The six main types of fuel cells are outlined below.

    Fuel cell type

    DMFC

    PEMFC

    AFC

    PAFC

    MCFC

    SOFC

    Electrolyte type

    Polymeric ion exchange membrane

    Polymeric ion exchange membrane

    Immobilised alkaline salt solution

    Immobilised liquid phosphoric acid

    Immobilised liquid molten carbonate

    Ceramic

    Operating temperature (°C)

    20 – 90

    30 – 100

    50 – 200

    ~220

    ~650

    500 – 1000

    Charge carrier

    H+

    H+

    OH

    H+

    H32–

    O2–

    Power range (W)

    1 – 100

    1 – 100k

    500 – 10k

    10k – 1M

    100k – 10M+

    1k – 10M+

    Applications and main advantages:

    Portable electronics

    Higher energy density than batteries and faster recharge.

    Cars boats and spaceships

    Zero emissions and higher efficiency.

    Domestic CHP

    Efficiency and reliability

    Distributed power generation, CHP, busses

    Efficiency, emissions and less noise

    Able to internally reform CH4
    (see Fuelling section)

    ×

    ×

    ×

    ×

    The phosphoric acid fuel cell (PAFC) is not covered in detail in this package. It was however the first type of fuel cell to be commercially produced and enjoys widespread terrestrial use. Many 200 kW systems are in place in USA and Europe.

    This package also doesn’t cover the alkaline fuel cell (AFC) in any detail. These particular cells use potassium hydroxide solution as the electrolyte. This means that any CO2 at the cathode, even the levels present in the air, will react with the OH in the solution to produce carbonates and prevent the cell functioning. This isn’t a huge problem in spacecraft, where pure oxygen can be supplied to the cathode reliably, but this characteristic flaw makes the AFC unsuitable for practical terrestrial use.

    The diagram below shows the mechanisms by which the different fuel cell types operate:

    What’s the catch?

    As fossil fuel resources become more and more pressed upon to deliver the worlds energy needs, as CO2 and global warming loom ever nearer and as cities become ever increasingly crowded with polluting automobiles the fuel cell seems to offer a golden solution to the world's energy problems. It’s efficient, it’s clean, hydrogen can be produced by renewable energy and the technology wouldn’t require any huge change in our way of life.

    So why don’t we all drive fuel cell cars already? The technology has two fundamental flaws:

    • Slow reaction rate, leading to low currents and power.
    • Hydrogen is not a readily available, or easily stored fuel.

    We’ll discuss ways of getting around these problems in the package. Each type of fuel cell has a different solution, but also brings its own set of difficulties.

    Temperature differences

    High temperature cells (solid oxide and molten carbonate electrolytes) operate by very different mechanisms to low temperature cells, and have different applications accordingly. The requirements of the “balance of plant” (i.e. the additional fuel processing equipment necessary to fuel a fuel cell) are also different. We therefore split the TLP in two and consider high temperature cells separately from low temperature cells.

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