Coal remains
one of our most important sources of energy. Worldwide coal reserves are
considerably larger than petroleum and natural gas deposits. If it were
possible to "tease out" the energy stored in coal in a more efficient and
environmentally friendly way than in conventional power plants, we could
tackle one of the most pressing problems of our civilization. George M.
Whitesides and his Harvard team have addressed this important problem.
They have developed a model of a low-temperature coal-driven fuel cell,
showing that there might be an alternative to high-temperature cells, which
are under development in other laboratories.
Normally, fuels such as coal, petroleum,
or natural gas are burned with oxygen from the air. This releases energy
in the form of heat, which is used to generate steam. This steam expands
within a turbine, producing pressure, which drives a generator (mechanical
energy). The generator produces current, electrons that speed through a
power line. In an electrochemical cell, the electrons can be "drawn" directly
out of the chemical reaction, without burning the fuel-a much cleaner process.
Until now, most fuel cell systems
have worked with hydrogen, which has to be extracted from fossil fuels,
methanol, or methane. Can coal feed a fuel cell? Attempts to extract power
from coal without combustion go back a long way, but have not been very
successful-the electrochemical oxidation of coal is just too slow at easily
attainable temperatures. High-temperature cells may be a solution, yet
have to struggle with severe technical problems.
Whitesides and his team have now
found a way out: some metals are able to oxidize coal very easily. The
researchers made a slurry of coal powder in sulfuric acid and added iron
ions with three positive charges. The iron ions react with coal to produce
carbon dioxide and a reduced form of iron ions. The iron ions, which now
have only two positive charges, contribute their extra electron to the
electrical circuit at an electrode, the anode. The ions are then available
to react again. This completes the anodic half-cell of a prototypical coal
fuel cell. A soluble system based on vanadium ions served as the necessary
cathodic half-cell. At 100°C this model cell delivered electricity
for one thousand hours without loss of power.
Says Whitesides, "Our model cell
will not be practical, but it provides evidence that coal can, in principle,
be directly converted to electricity at temperatures as low as boiling
water-one small step towards a practical low-temperature fuel cell."
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