Fuel cells are the
next generation of alternative energy. While your standard
household or car batteries provide stable reliable power in a very wide
range of application, and your combustion engine will provide large
amounts of power as long as you continue to provide fuel, there are
obvious deficiencies to both. Batteries don't last forever, and engines
are undesireably inefficient. Fuel cells are the solution to both of
these problems, and they boast several other advantages too!
Fuel cells are
devices that convert chemical energy directly into electricity.
The easiest fuel cell system to think about is the
H2/O2
polymer electrolyte membrane fuel cell (PEMFC). In
this system there are two reactions to consider (just like any
battery),
the oxidation
reaction:
H2
→ 2H+
+ 2e-
&
the reduction
reaction: O2
+ 4H+
+ 4e-
→ 2H2O.
Thus,
using continous inputs of hydrogen gas and oxygen gas, you can
build a system that generates a very scalable source of power, like
batteries, without the inconvenience of running out of fuel. The
oxidation reaction occurs on the surface of a catalyst (usually Pt)
deposited on
the anode, while the reduction reaction occurs on a potentially
different catalyst (also usually Pt) deposited on the cathode. The
third and most important component in every electrochemical cell is the
electrolyte, which in this case must be condicting to protons,
electrically insulating
resistant to strong oxidants and reductants, thermally and mechanically
stable - it is a challenge difficult to accomplish. The anode, the
polymer electrolyte membrane and the cathode are sandwiched together to
make the fuel cell. The anode reaction produces protons and
electrons, which, as can be seen from this GIF file, are seperated: the
electrons (black) are routed through a wire to provide access to the
electrical energy and the protons (yellow) are allowed to pass through
the electrolyte membrane (here depicited by blue/brown
interface). The cathode reaction is the recombination of the
protons and electrons in the presence of oxygen (red) to make
water. Because this device does not rely on a thermal cycle the
theoretical efficiency is not limited by the carno cycle, and so fuel
cells can outpreform combustion engines by a signifigant amount.
One final remark, which will appeal to the environmentalists, is that
this energy device has the potential to replace petroleum powered
devices, and thus reduce the consumption of fossil fuels, and curb the
production of CO2. Unfortunately, all affordable sources of fuel
for fuel cells today are, in fact, derived from petroleum.
At typical operating temperatures of
around 80oC, problems of CO
poisoning (as a result of residual CO from the petroleum reformation)
of the plantinum anode catalyst, and thermal and water
management make the development of a simple and reliable fuel cell
system more difficult. Hydrocarbons, such as
natural gas, gasoline or alcohol are more economical fuel sources and
are more technically
viable in the near-term, but the problem of CO poisoning is exemplified
as the partial oxidation products of such fuels form high
concentrations of CO at that anode catalyst. One solution is
increasing the temperature of fuel
cell operation, as this reduces the adsorption of CO onto the platinum
electrocatalyst and improves performance (as long as you stay under the
phase transition temperatures of your membrane). In addition, the
heat loss through radiation of the fuel cell stack is
greatly enhanced at high temperatures, and minimizes the need for
costly heat exchange devices in large scale applications. Also,
at high temperatures, product water can be generated as a vapor at the
cathode, alleviating the problem of flooding the cell (which results in
inaccessable catalyst particles, and loss of voltage). However,
increasing the temperature above 100oC has a down-side for PEMFCs
because of the reliance of the
membrane on water for proton conduction. As the temperatures increases,
the evaporation of water from the membrane leads to a dramatic drop in
proton conductivity. In order to maintain adequate membrane hydration,
increases in temperature needs to be accompained by increases in
operating pressure to ensure that sufficient water vapor and
reactant gas is available in the reaction compartments. The practical
upper limit for the operating pressure is around 3-4 atm.
Current group interest in this project
include a focus on the advancement of a Direct Ethanol Fuel Cell, an
exploration into various non-fluoranated polymer membranes, and
an investigation into Solid Oxide
Fuel Cells using beta"-alumina as the proton conducting
electrolyte. Additionally, there are efforts to understand the
bulk mechanical properties of the electrolyte membrane materials in
order to ellucidate the microstructure. The first two of these
projects rely heavily on four fuel cell test stations equipped to
deliver humidified and non-humidified gases as well as liquids to
a single cell setup. Automated data collection using a programable
variable electronic load makes experiments far less tasking on the
students. The other projects depend on the test stations as well,
but also utilize electron microscopy and dynamic mechanical
analysis techniques.
Here are some of the important papers that have come out of this
project:
153.
"Ion
exchange resin/poly styrene sulfonate
composite
membranes for PEM fuel cells," S. L.
Chen, L.
Krishnan, S. Srinivasan, J. Benziger, A.
B. Bocarsly, Journal
of Membrane Science, 2004, 243, 327-33
152.
"A Comparison of
Physical Properties and Fuel
Cell
Performance of Nafion and Zirconium Phosphate/Nafion Composite
Membranes,"
C. Yang, S. Srinivasan, A. B. Bocarsly, S.
Tulyani and J. B. Benziger, J. Membrane
Science, 2004, 237,
145-161.
131.
"Silicon
Oxide/Nafion® Composite Membranes
for PEMFC Operation at 80 to 140o
C,"K. T. Adjemian,
S. J. Lee, S. Srinivasan, J. Benziger, and
A. B.
Bocarsly, J. Electrochem. Soc.,
2002, 149, A256.
Thus,
using continous inputs of hydrogen gas and oxygen gas, you can
build a system that generates a very scalable source of power, like
batteries, without the inconvenience of running out of fuel. The
oxidation reaction occurs on the surface of a catalyst (usually Pt)
deposited on
the anode, while the reduction reaction occurs on a potentially
different catalyst (also usually Pt) deposited on the cathode. The
third and most important component in every electrochemical cell is the
electrolyte, which in this case must be condicting to protons,
electrically insulating
resistant to strong oxidants and reductants, thermally and mechanically
stable - it is a challenge difficult to accomplish. The anode, the
polymer electrolyte membrane and the cathode are sandwiched together to
make the fuel cell. The anode reaction produces protons and
electrons, which, as can be seen from this GIF file, are seperated: the
electrons (black) are routed through a wire to provide access to the
electrical energy and the protons (yellow) are allowed to pass through
the electrolyte membrane (here depicited by blue/brown
interface). The cathode reaction is the recombination of the
protons and electrons in the presence of oxygen (red) to make
water. Because this device does not rely on a thermal cycle the
theoretical efficiency is not limited by the carno cycle, and so fuel
cells can outpreform combustion engines by a signifigant amount.
One final remark, which will appeal to the environmentalists, is that
this energy device has the potential to replace petroleum powered
devices, and thus reduce the consumption of fossil fuels, and curb the
production of CO2. Unfortunately, all affordable sources of fuel
for fuel cells today are, in fact, derived from petroleum.