Fuel Cell Information

Fuel Cell Information Sheet (10/20/09)
(http://physics.indiana.edu/~brabson/p310/fuelcells.html) References:
1.) Fuel Cells: Energy Conversion for the Next Century, Sivan Kartha and Patrick Grimes, Physics Today, November 1994, pp 54-61.
2.) Devins, Energy, Its Physical Impact on the Environment, Krieger Publishing, 1982, pp. 134.
3.) Christenson, C.D., Fuel Cell System Technologies and Application Issues, Energy Engineering 94, No. 2, 1997, pp. 36-46..

Hydrogen/Oxygen Fuel Cell reactions:
The negative electrode reaction: H₂ ---> 2 H⁺ + 2 e^- (V = 0.00 volts)
The positive electrode reaction: O₂ + 4 H⁺ + 4 e^- ---> 2 H₂O (V = -1.23 volts)

Fuel Cell Characteristics:
    1.) Modularity: They can be made in essentially any size without loss of efficiency.
    2.) Efficiency: The overall fuel cell efficiency can be considerably higher than that of electric generation through high-temperature steam, subject to the Second Law of Thermodynamics. In a fuel cell the fuel and oxidant combine in a controlled electrochemical process which avoids the intermediate generation of heat and the corresponding inefficient conversion of heat energy to mechanical and electrical energy. Efficiencies as high as 60% have been achieved.
    3.) Maintenance: Fuel cells have no moving parts and correspondingly low maintenance costs.
    4.) Cost: Depending on the fuel used, fuel cells require both a catalyst (often expensive platinum) and a high-temperature environment, both needed to increase the rate of combination of hydrogen with oxygen.
    5.) Current densities: Current densities as high as 10 A/m² at a voltage of 0.7 volts have been reached.

Fuel Cells in Transportation:
    1.) Pollutants: Operation of a hydrogen fuel cell would emit only water and no local pollutants. Fuel cells can use other fuels such as methanol or hydrocarbons such as methane or even gasoline. Such a fuel cell requires a reformer that extracts gaseous hydrogen from these fuels. A vehicle powered by such a fuel cell would emit only about 1% of the local pollutants emitted by an ICE.
    2.) Reformation: Fuel cells used for transportation are able to use the thermochemical gasification of biomass resources such as agricultural residues. Methane can also be reformed into hydrogen using:
    CH₄ + 2 H₂O ---> CO₂ + 8 H⁺ + 8e^- at the anode and
    8e^- + 8H⁺ + 2O₂ ---> 4H₂O              at the cathode
Also, kerosene, hydrazine, and formaldehyde can be used as a primary fuel.

Fuel Cell Types: Fuel Cells are best distinguished by their electrolyte:
    1.) Proton Exchange Membrane Fuel Cells (PEM) use H⁺ ions to carry the charge. [60-120⁰C]
    2.) Phosphoric acid fuel cells (PAFC) also use H⁺ ions. [160 - 220 ⁰C]
    3.) Solid oxide ceramic fuel cells (SOFC) use O^--ions to carry charge. [900 - 1000 ⁰C]
    4.) Alkaline fuel cells use OH^- ions to carry charge. [60 - 120 ⁰C]
    5.) Molten-carbonate fuel cells (MCFC) use CO₃^-- ions to carry charge. [600 - 650 ⁰C]

Comparison of Various Types of Fuel Cells⁴

Advantages:

Phosphoric Acid FC	Molten Carbon FC	Solid Oxide FC	Proton Ex. Memb. FC
PAFC	MCFC	SOFC	PEMFC
H⁺ exchanged	CO3^-- exchanged	O^-- exchanged	H⁺ exchanged
250 mA/cm²	160 mA/cm²	?? mA/cm²	1000 mA/cm²
200^oC	600^oC	1000^oC	100^oC
Tolerant to CO₂	No precious metal required	No precious metal required	Tolerant to CO₂
Most advanced	CO is a usable fuel	CO is a usable fuel	Low working temp.
Applicable to small capacity plants and vehicular uses	Internal reforming in cells is feasible	Internal reforming cells in cells is feasible	Very high current density is achievable
	High grade heat is available	CO2 recycling is not required

⁴ Anahara, R., S. Yokokawa, and M. Sakurai, Proc. IEEE 81(3), 1993, pp. 399-408.

Disadvantages:

Catalyst needs precious metals ($$)	Material problem related to cell life and mechanical stability	High T presents severe constraints on cell materials	CO content in fuel is strictly prohibited and catalyst ($$)
Pt catalyst is deactivated by CO	CO2 source needed for cathode	Relatively high electrolyte resistivity	Water management in cell is difficult
Low conductivity electrolyte management	Phase change of electrolyte at high T		Only low grade heat is available

Ben's Fuel Cell Diagrams: http://physics.indiana.edu/~brabson/p310/FC1.tif,
http://physics.indiana.edu/~brabson/p310/FC2.tif

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