Hydrogen Fuel CellA hydrogen fuel cell is an electrochemical cell that uses a spontaneous redox reaction to produce current that can do work. The net reaction is exothermic. Combining the 2 half cell potentials for the electrochemical reaction gives a positive cell potential.
Methanol Fuel CellMethanol is a liquid fuel that can be produced from renewable resources through fermentation. Direct methanol fuel cells use the electrochemical reactions below:
The electrochemical reaction produces carbon dioxide, a greenhouse gas. The anodic reaction requires a catalyst that contains expensive, precious metals. A mixture of ruthenium and palladium is typical. Another drawback is the toxicity of methanol.
Methanol can also be used with hydrogen fuel cells. Steam reforming of methanol at 250 deg C produces CO2 and H2 along with a small amount of CO.
Efficiency of Fuel CellsBased on G/H for the formation of water from hydrogen and oxygen, the efficiency of a hydrogen fuel cell should be nearly 80 % (corresponding to a cell potential of 1.23 V). In practice, the maximum cell potential of a hydrogen fuel cell is 0.95 - 1.0 V initially because of polarization losses as the electrode surface. Under a load, losses of energy due to resistance are significant and the true cell potential is between 0.6 and 0.8 V. The actual efficiency is approximately 0.7 V/1.48 V, less than 50 %. The indirect methanol fuel cells are much less efficient because of the heat loss in the reforming reaction.
What if the hydrogen fuel cells are used to power an automobile? We have to consider the loss in energy in the electrolysis of water at a central station, energy required to compress the hydrogen gas that is formed, loss of hydrogen and its energy in transfer from the point of production and losses at the fill-up station, efficiency of the fuel cell, energy use (parasitic loss) to power systems critical to the fuel cell, and electric losses in the drive train. The numbers below come from "Efficiency of Hydrogen Fuel Cell, Diesel-SOFC-Hybrid and Battery Electric Vehicles" by Ulf Bossel. (See more.)
10% losses for compression of hydrogen: factor 0.90
10% losses for distribution of gaseous hydrogen: factor 0.90
3% losses for hydrogen transfer: factor 0.97
50% for conversion to electricity in fuel cells: factor 0.50
10% parasitic losses for the hydrogen fuel cell system: factor 0.90
10% electric losses in the drive-train between battery and wheels: factor 0.90
The "power-plant-to-wheel" efficiency of a fuel cell vehicle operated on compressed gaseous hydrogen will be in the vicinity of 22%.