Fuel Cells: Efficiency and Materials

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Fuel CellsEfficiency and Materials

• By Tony Phan
• Ari Erman
• Michael Hair
• Mike Countis
• Vince Biccoca

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Topics

• What is a Fuel Cell?
• Types of Fuel Cells
• Proton Exchange Membrane
• Direct Methanol
• Zinc Air
• Regenerative
• Alkaline
• Biological
• Sugar
• The Problem of Platinum
• Conclusion

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FUEL CELL

• DEFINITION. An electrochemical device that
  continuously changes the chemical energy of a
  fuel (hydrogen) and oxidant (oxygen) directly to
  electrical energy and heat, without combustion.
• PRINCIPLE. The electrical process causes hydrogen
  atoms to give up their electrons. It is similar
  to a battery in that it has electrodes, an
  electrolyte, and positive and negative terminals.
• USAGE. A fuel cell provides a DC (direct current)
  voltage that can be used to power motors, lights
  or any number of electrical appliances. There are
  several different types of fuel cells, each using
  a different chemistry. Fuel cells are usually
  classified by the type of electrolyte they use.
• ADVANTAGES. No combustion therefore few
  emissions. No moving parts therefore quiet.

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Proton Exchange Membrane

• Proton exchange membrane fuel cell uses one of
  the simplest reactions of any fuel cell.
• Anode side 2H2 gt 4H 4e-
• Cathode side O2 4H 4e- gt 2H2O
• Net reaction 2H2 O2 gt 2H2O

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Proton Membrane

• The anode conducts the electrons that are freed
  from the hydrogen molecules. It disperse the
  hydrogen gas equally over the surface of the
  catalyst.
• The cathode, the positive post of the fuel cell,
  distributes the oxygen to the surface of the
  catalyst. It also conducts the electrons back
  from the external circuit to the catalyst, where
  they can recombine with the hydrogen ions and
  oxygen to form water.
• The electrolyte is the proton exchange membrane.
  This specially treated material, which looks
  something like ordinary kitchen plastic wrap,
  only conducts positively charged ions. The
  membrane blocks electrons.
• The catalyst is a special material that
  facilitates the reaction of oxygen and hydrogen.
  It is usually made of platinum powder very thinly
  coated onto carbon paper or cloth. The catalyst
  is rough and porous so that the maximum surface
  area of the platinum can be exposed to the
  hydrogen or oxygen. The platinum-coated side of
  the catalyst faces the PEM.

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Advantages of PEMFC

• PEMFCs operate at a fairly low temperature (about
  176 degrees Fahrenheit, 80 degrees Celsius),
  which means they warm up quickly and don't
  require expensive containment structures.
  Constant improvements in the engineering and
  materials used in these cells have increased the
  power density to a level where a device about the
  size of a small piece of luggage can power a car.

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Direct Methanol Fuel Cells
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The scenario

• Coal is used as a fuel and primarily for
  electrical power. It is also far more abundant,
  energy wise, in the United States than either
  petroleum or natural gas. Even though coal is not
  widely used as a transportation fuel feedstock,
  commercial processes have been developed to
  produce both methanol and hydrogen from coal.
  This is clearly a scenario of interest since
  these processes also carry the possibility of
  transportation energy independence.
• Natural gas and coal are the only energy sources
  currently available in quantities comparable to
  petroleum for transportation. However these
  resources have limited quantities

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The solution

• Using the hydrogen and methanol from burning coal
  and combining it with water we can turn it into a
  new form of energy.
• Methanol is the most desirable liquid hydrocarbon
  fuel for fuel cells and can be effectively
  utilized in internal combustion engines using
  existing technologies.
• While all alternative fuels are expected to be
  more expensive to the consumer than present-day
  gasoline, methanol produced from coal is likely
  to be the least expensive of the fuels
  considered, if natural gas prices increase as
  projected.

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DMFC

• Uses a membrane as electrolyte and produce
  electricity directly from liquid methanol.
• Efficient of fuel utilization
• Simpler less costly fuel cell systems

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Principles of DMFC

• When provided with current, methanol is
  electrochemically oxidized at the anode electro
  catalyst to produce electrons which travel
  through the external circuit to the cathode and
  consumed with oxygen to form a reaction.
• CH3OH 3/2O2 CO2 2H2O

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• Bob Hockaday holds a Motorola cell phone battery
  for scale in this photo taken at MHTX H.Q. in Los
  Alamos, December 1998. To his right is his "test
  rack" micro-DMFC apparatus which provides the
  electricity to operate the small black Nokia cell
  phone in the foreground. The hypo syringes on top
  of the rack are used to squirt windshield washer
  fluid (blue juice) fuel into the direct methanol
  fuel cell.

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DMFCs for Potential Transportation Applications

• A 5-cell stack operated at 100 Celsius with 2.8
  atm air generated 1 kW per liter of active stack
  volume.
• With 0.50 V per cell, the 5-cell achieved a fuel
  utilization of 90 about 37 efficiency.
• Catalyst loading was lowered to 5 gPt per kw,
  compared to about 2gPt per kW (current)

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A Methanol Environment
Indy Race Car
Methanol Power plant
Methanol Fueled Vehicle
Methanol Pump
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Zinc Air
Fuel cell modules contain 47 individual
air-breathing zinc-air cells, connected in a
series they discharge 17.4kWh before being
refueled. Electric fueled transit buses carry
three trays of 6 modules. This yields 312kWh of
on-board energy. Transportation Programs in the
U.S. and Germany are currently in place testing
Zinc-Air Electric transit bussing. A Zinc-Air
fuel cell yields a practical specific energy of
around 200Wh/kg and a specific peak power of
90W/kg. Lead-acid batteries typically achieve 30
Wh/kg. Nickel-metal hydride typically achieves
70Wh/kg.
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Regenerative Fuel Cell
Unitized Regenerative Fuel Cells (URFC) operate
as both a generator and electrolyzer. When
coupled with lightweight fuel storage, URFCs
operate with an energy density of about
450Wh/kg. Transportation Regenerative Fuel Cells
act as energy storage systems for alternative
fuel systems. Example Photovoltaic systems
generate more power than what is need to fly an
airplane when the sun high. The energy storage
systems absorbs the excess power and accumulates
it during the day. The stored energy can then be
released at night. Possible Applications
automobiles, solar-powered aircraft, satellites,
and microspace-craft propulsion.
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Alkaline

• This is one of the oldest designs. It has been
  used in the U.S. space program since the 1960s.
  The AFC is very susceptible to contamination, so
  it requires pure hydrogen and oxygen.
• High-temperature AFCs operate at temperatures
  between 100ºC and 250ºC (212ºF and 482ºF).
  However, more-recent AFC designs operate at lower
  temperatures of roughly 23ºC to 70ºC (74ºF to
  158ºF).

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Alkaline Pro/Con

• AFCs are high-performance fuel cells due to the
  rate at which chemical reactions take place in
  the cell. They are also very efficient, reaching
  efficiencies of 60 percent in space applications.
  
• The disadvantage of this fuel cell type is that
  it is easily poisoned by carbon dioxide (CO2). In
  fact, even the small amount of CO2 in the air can
  affect the cell's operation, making it necessary
  to purify both the hydrogen and oxygen used in
  the cell. This purification process is costly.
  Susceptibility to poisoning also affects the
  cell's lifetime.
• AFC stacks have been shown to maintain
  sufficiently stable operation for more than 8,000
  operating hours.

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Biological Fuel Cells

• Microorganisms or enzymes replace catalysts

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Sugar in Fuel Cells

• Anodes are covered in a conducting polymer
• Prototypes produce 1.5 thousandths of an amp
• The layer blocks large molecules but lets
  hydrogen through
• Keeps the anode cleaner
• Only useful for small applications

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Platinum Anodes

• The DoE confirms a sufficient supply of platinum
• The DoE also confirms the expansion of the
  platinum industry

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Platinum Replacements

• Expensive, rare
• Research is being done with silver anodes
• John McMahon
• Silver anodes require a rough pitted surface
• Light is used to accelerate the reaction
• These cells must be designed for use with light

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Platinum Anode Poisoning

• With traditional fuel cell designs platinum
  anodes get poisoned
• This poisoning can be reduced with the use of
  platinum alloy anodes (Ru, Mo, W, Os1-3)
• Platinum and ruthenium alloys prove most promising

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Conclusion

• Researching the materials used in fuel cells is
  crucial to the development of a hydrogen fueled
  economy
• The PEMFC is the most widely used now
• With more research, liquid methanol could produce
  enough energy to power a car
• Since methanol is extracted from coal and coal
  reserves are five times larger than natural gas,
  methanol would be a very effective alternative
  power source
• Regenerative and zinc based fuel cells arent
  very useful for transportation
• Alkaline is strictly used in space applications
  because CO2 contaminates the oxygen and hydrogen
  in the cell
• Biological and sugar cells are promising but put
  out low power not yet suitable for autos
• Anode poisoning can be slowed with the use of
  platinum alloys in place of pure platinum