A fuel cell is a device that converts a fuel, such as hydrogen, natural
gas, methanol or propane to electricity. There are no moving parts such as
in traditional setups for power generation (boiler, turbine generator),
resulting in high efficiencies up to approximately 80%.
Fuel cells and batteries have much in common. They both
contain two electrodes and function on the principle of
a chemical reduction/oxidation reaction (redox reaction). However, a
fuel cell typically runs on a liquid or gaseous fuel and oxidizer
that is fed from outside of the cell, whereas a battery stores its solid
fuel and oxidizer on plates inside the cell.
A schematic of a fuel cell is given below. The cell consists of two
electrodes, an anode and a cathode. The anode part is
fed with the fuel, for instance hydrogen. The anode itself consists of
a porous, conductive material such as carbon with a metal catalyst
(typically, finely dispersed platinum.) At the anode, the feed gas is
reacted to protons (H+) and
electrons (e-.) The protons migrate
through an electrolyte, which can either be a liquid (KOH / H2SO4), or a
conductive polymer membrane (the so called PEM, Polymer Electrolyte
Membrane.) The electrons leave the anode, perform work (e.g.
operate an electric motor), and enter the cathode. At the cathode,
oxygen is dissociated and reacted with the protons to form
water.
Anode Cathode
Feed e- -> Feed
H2 ________ O2
| |
_| |__| _____|__| |_
| |/|- - |/| |
| |/| - -|/| |
| |/|Elec|/| |
| |/| tro|/| |
| |/|lyte|/| |
| |/| - -|/| |
| |/|- - |/| |
| |/| - -|/| |
| |/|- - |/| |
|_ _|_|____|_|_ _|
| | | |
Anode Cathode
Vent Vent
H2 O2 + H2O
The partial (redox) reactions in the fuel cell are as follows:
Anode: H2 -> 2 H+ + 2 e-
Cathode: O2 -> 2 O
2 H+ + O + 2 e- -> H2O
A single H2/O2
fuel cell operates at approximately 0.7 volt. Manufacturers arrange
multiple cells in series to provide the desired electrical output. The
theoretical efficiency (Free Energy/Enthalpy) of this reaction is
approximately 82.9%. Thus, more than 80% of the chemical energy can be converted
into electricity. Practical efficiencies range from 40-80%, but this is
still much higher than the 25-40% obtained in steam turbines/generators.
The concept of fuel cells isn't new; it is almost as old as the field
of electrochemistry itself. However, the first practical fuel cells
were developed in the 1960s for the Gemini and Apollo Space programs. Fuel cell systems were advantageous for
spacecraft, since hydrogen was already present as rocket propellant, and
the fuel cell provided both electricity and drinking water to the
astronauts. Fuel cells are also in use in the Space Shuttle program.
Since the 1970s, fuel cells have also been used as power generators
for commercial buildings. This application has gained significant
interest over the last 10 years. The army is also investigating fuel
cell technology as means of portable power for combat needs. There is a
lot of development on (PEM) fuel cells for automotive applications,
because of the low emissions and high efficiency.
While fuel cell technology offers great promises for the future,
there are still several hurdles that need to be overcome. The first
problem is that of fuel storage. Especially for automotive applications,
storage of hydrogen in the form of a compressed gas is dangerous.
Cryogenic liquid hydrogen is expensive, and also dangerous when the
vehicle is involved in an accident. Several researchers are looking into
safer means of storing hydrogen: binding it as a metal
hydride, or adsorbing it on carbon nanofibers. The
storage capacity of these alternatives is still relatively low.
Currently, several companies are investigating using regular
gasoline as a hydrogen source for fuel cells. The fuel is processed
by steam reforming, before it enters the fuel cell:
CnHm + n H2O -> n CO + (m/2 + n) H2
CO + H2O -> CO2 + H2
This reaction can be conducted with a high conversion and yield,
and it has the advantage that a wide range of fuels can be used. Regular
gasoline is advantageous, since this would require little changes to the
current gasoline distribution network. Methanol
(CH3OH) is a good alternative fuel, because of
its high hydrogen content. Ethanol (ethyl alcohol) could be
synthesized by biochemical means. However, a major problem is the
presence of small amounts of unreacted carbon monoxide (CO) in the feed
stream of the fuel cell. The carbon monoxide acts as a poison to the
platinum catalysts at concentrations as low as 100 ppm. Some researchers
are investigating direct-methanol fuel cells that oxidize the liquid
feed directly, but these systems currently have a relatively low
efficiency.
A second issue is cost. Regular combustion engines are much cheaper
to build and do not require expensive platinum catalysts to operate.
There is a lot of development on fuel cells that use less of the
precious metal, or cheaper alternative catalysts. A car using 1980s
technology would require approximately $30 000 worth of platinum.
Currently, only $400 of the precious metal would be required.
Finally, reliability is also very important for automotive applications. Catalysts
can deactivate due to impurities in the fuel as was already mentioned,
but also due to sintering and agglomeration. The lifetime of the polymer
membrane is also an important consideration.
Fuel cells have a promising future for localized power generation and
automotive applications. However, a widespread acceptance of the
technology (the "Hydrogen Economy") requires a viable supply of
appropriate fuel, lower manufacturing costs, and higher performances.