How Fuel Cells Work

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How Fuel Cells Work

• Fuel Cells (????)
• Making power more efficiently and with less
  pollution.

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Fuel Cell- an electrochemical energy conversion
device

• To convert the chemicals hydrogen and oxygen into
  water, and in the process it produces
  electricity.
• Battery (??) the other electrochemical device
  that we are all familiar.
• A battery has all of its chemicals stored inside,
  and it converts those chemicals into electricity
  too.
• This means that a battery eventually "goes dead"
  and you either throw it away or recharge it.

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For a fuel cell

• Chemicals constantly flow into the cell so it
  never goes dead.
• As long as there is a flow of chemicals into the
  cell,
• the electricity flows out of the cell.
• Most fuel cells in use today use hydrogen and
  oxygen as the chemicals.

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Fuel Cell Descriptions

• Fuel Cells generate electricity through an
  electrochemical process
• In which the energy stored in a fuel is converted
  directly into DC electricity.
• Because electrical energy is generated without
  combusting fuel,
• Fuel cells are extremely attractive from an
  environmental stand point.

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Attractive characteristics of Fuel Cell

• High energy conversion efficiency
• Modular design
• Very low chemical and acoustical pollution
• Fuel flexibility
• Cogeneration capability
• Rapid load response

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A functioning cell in a Solid Oxide Fuel Cell
stack
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• It consists of three components - a cathode, an
  anode, and an electrolyte sandwiched between the
  two.
• Oxygen from the air flows through the cathode
• A fuel gas containing hydrogen, such as methane,
  flows past the anode.
• Negatively charged oxygen ions migrate through
  the electrolyte membrane react with the hydrogen
  to form water,
• The reacts with
• the methane fuel
• to form hydrogen (H2)
• carbon dioxide (CO2).

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• This electrochemical reaction generates
  electrons, which flow from the anode to an
  external load and back to the cathode,
• a final step that both completes the circuit and
  supplies electric power.
• To increase voltage output, several fuel cells
  are stacked together to form the heart of a clean
  power generator.

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Cool Fuel Cells

• Fuel cells promise to be the environmentally-frien
  dly power source of the future,
• but some types run too hot to be practical.
  NASA-funded research may have a solution.

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All fuel cells have the same basic operating
principle.

• An input fuel is catalytically reacted (electrons
  removed from the fuel elements) in the fuel cell
  to create an electric current.

• Fuel cells consist of an electrolyte material
  which is sandwiched in between two thin
  electrodes (porous anode and cathode).
• The input fuel passes over the anode (and oxygen
  over the cathode) where it catalytically splits
  into ions and electrons.
• The electrons go through an external circuit to
  serve an electric load while the ions move
  through the electrolyte toward the oppositely
  charged electrode.
• At the electrode, ions combine to create
  by-products, primarily water and CO2. Depending
  on the input fuel and electrolyte, different
  chemical reactions will occur.

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Basic Configuration
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PEMFC
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Animation of PEMFC
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SUSTAINABLE Transport
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SOLID OXIDE FUEL CELL STACK PROVIDER

• HTceramix's SOFConnexTM based stack

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Applications of Fuel cells
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Woking Park Fuel Cell CHP schematic
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Four primary types of fuel cells

• They are based on the electrolyte employed
• Phosphoric Acid Fuel Cell
• Molten Carbonate Fuel Cell
• Solid Oxide Fuel Cell
• Proton Exchange Membrane Fuel Cell

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Phosphoric Acid Fuel Cells -PAFCs

• The most mature fuel cell technology in terms of
  system development and commercialization
  activities.
• Has been under development for more than 20 years
• Has received a total worldwide investment in the
  development and demonstration of the technology
  in excess of 500 million.
• The PAFC was selected for substantial development
  a number of years ago because of the belief that,
  among the low temperature fuel cells,
• It was the only technology which showed relative
  tolerance for reformed hydrocarbon fuels and thus
  could have widespread applicability in the near
  term.

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PAFC Design and Operation

• The PAFC uses liquid phosphoric acid as the
  electrolyte.
• The phosphoric acid is contained in a Teflon
  bonded silicone carbide matrix.
• The small pore structure of this matrix
  preferentially keeps the acid in place through
  capillary action.
• Some acid may be entrained in the fuel or oxidant
  streams and addition of acid may be required
  after many hours of operation.
• Platinum catalyzed, porous carbon electrodes are
  used on both the fuel (anode) and oxidant
  (cathode) sides of the electrolyte.

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• Fuel and oxidant gases are supplied to the backs
  of the porous electrodes by parallel grooves
  formed into carbon or carbon-composite plates.
• These plates are electrically conductive and
  conduct electrons from an anode to the cathode of
  the adjacent cell.
• In most designs, the plates are "bi-polar" in
  that they have grooves on both sides - one side
  supplies fuel to the anode of one cell, while the
  other side supplies air or oxygen to the cathode
  of the adjacent cell.
• The byproduct water is removed as steam on the
  cathode (air or oxygen) side of each cell by
  flowing excess oxidant past the backs of the
  electrodes.
• This water removal procedure requires that the
  system be operated at temperatures around 375oF
  (190oC).
• At lower temperatures, the product water will
  dissolve in the electrolyte and not be removed as
  steam. At approximately 410oF (210oC), the
  phosphoric acid begins to decompose.

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• The byproduct water is removed as steam on the
  cathode (air or oxygen) side of each cell by
  flowing excess oxidant past the backs of the
  electrodes.
• This water removal procedure requires that the
  system be operated at temperatures around 375oF
  (190oC).
• At lower temperatures, the product water will
  dissolve in the electrolyte and not be removed as
  steam. At approximately 410oF (210oC), the
  phosphoric acid begins to decompose.
• Excess heat is removed from the fuel cell stack
  by providing carbon plates containing cooling
  channels every few cells.
• Either air or a liquid coolant, such as water,
  can be passed through these channels to remove
  excess heat.

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Electrochemical reactions in PAFC

• At the anode
• Hydrogen is split into two hydrogen ions (H),
  which pass through the electrolyte to the
  cathode, and
• two electrons which pass through the external
  circuit (electric load) to the cathode.
• At the cathode
• the hydrogen, electrons and oxygen combine to
  form water.

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Electrochemical reactions in PAFC
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PAFC Performance Characteristics

• PAFC power plant designs show electrical
  efficiencies in the range from 36 (HHV) to 42
  (HHV).
• The higher efficiency designs operate with
  pressurized reactants.
• The higher efficiency pressurized design requires
  more components and likely higher cost.
• PAFC power plants supply usable thermal energy at
  an efficiency of 37 (HHV) to 41 (HHV).
• A portion of the thermal energy can be supplied
  at temperatures of 250oF to 300oF.
• However, the majority of the thermal energy is
  supplied at 150oF.
• The PAFC has a power density of 160-175 watts/ft2
  of active cell area

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Molten Carbonate Fuel Cells - MCFC

• A molten carbonate salt mixture is used as its
  electrolyte.
• They evolved from work in the 1960's aimed at
  producing a fuel cell which would operated
  directly on coal.
• While direct operation on coal seems less likely
  today,
• The operation on coal-derived fuel gases or
  natural gas is viable.

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Molten Carbonate Salt used as Electrolyte in MCFC

• A molten carbonate salt mixture is used as its
  electrolyte.
• The composition of the electrolyte (molten
  carbonate salt mixture) varies, but usually
  consists of lithium carbonate and potassium
  carbonate.
• At the operating temperature of about 650oC
  (1200oF), the salt mixture is liquid and a good
  ionic conductor.
• The electrolyte is suspended in a porous,
  insulating and chemically inert ceramic (LiAlO3)
  matrix.

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Reactions in MCFC

• The anode process involves a reaction between
  hydrogen and carbonate ions (CO3) from the
  electrolyte.
• The reaction produces water and carbon dioxide
  (CO2) while releasing electrons to the anode.

• The cathode process combines oxygen and CO2 from
  the oxidant stream with electrons from the
  cathode to produce carbonate ions which enter the
  electrolyte.
• The need for CO2 in the oxidant stream requires a
  system for collecting CO2 from the anode exhaust
  and mixing it with the cathode feed stream.

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Reactions in MCFC
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Description of reactions in MCFCs

• The anode process involves a reaction between
  hydrogen and carbonate ions (CO3) from the
  electrolyte.
• The reaction produces water and carbon dioxide
  (CO2) while releasing electrons to the anode.
• The cathode process combines oxygen and CO2 from
  the oxidant stream with electrons from the
  cathode to produce carbonate ions which enter the
  electrolyte.
• The need for CO2 in the oxidant stream requires a
  system for collecting CO2 from the anode exhaust
  and mixing it with the cathode feed stream.

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• As the operating temperature increases,
• the theoretical operating voltage for a fuel cell
  decreases and with it the maximum theoretical
  fuel efficiency.
• On the other hand, increasing the operating
  temperature increases the rate of the
  electrochemical reaction and
• Thus increases the current which can be obtained
  at a given voltage.
• The net effect for the MCFC is that the real
  operating voltage is higher than the operating
  voltage for the PAFC at the same current density.
  
• The higher operating voltage of the MCFC means
  that more power is available at a higher fuel
  efficiency from a MCFC than from a PAFC of the
  same electrode area.
• As size and cost scale roughly with electrode
  area, this suggests that a MCFC should be smaller
  and less expensive than a "comparable" PAFC.

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• As size and cost scale roughly with electrode
  area, this suggests that a MCFC should be smaller
  and less expensive than a "comparable" PAFC.
• The MCFC also produces excess heat at a
  temperature which is high enough to yield high
  pressure steam which may be fed to a turbine to
  generate additional electricity.
• In combined cycle operation, electrical
  efficiencies in excess of 60 (HHV) have been
  suggested for mature MCFC systems.
• The MCFC operates at between 1110F (600C) and
  1200F (650C) which is necessary to achieve
  sufficient conductivity of the electrolyte.
• To maintain this operating temperature, a higher
  volume of air is passed through the cathode for
  cooling purposes.

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• As mentioned above, the high operating
  temperature of the MCFC offers the possibility
  that it could operate directly on gaseous
  hydrocarbon fuels such as natural gas.
• The natural gas would be reformed to produce
  hydrogen within the fuel cell itself.
• The need for CO2 in the oxidant stream requires
  that CO2 from the spent anode gas be collected
  and mixed with the incoming air stream.
• Before this can be done, any residual hydrogen in
  the spent fuel stream must be burned.
• Future systems may incorporate membrane
  separators to remove the hydrogen for
  recirculation back to the fuel stream.

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• At cell operating temperatures of 650oC (1200oF)
  noble metal catalysts are not required.
• The anode is a highly porous sintered nickel
  powder, alloyed with chromium to prevent
  agglomeration and creep at operating
  temperatures.
• The cathode is a porous nickel oxide material
  doped with lithium.
• Significant technology has been developed to
  provide electrode structures which position the
  electrolyte with respect to the electrodes and
  maintain that position while allowing for some
  electrolyte boil-off during operation.
• The electrolyte boil-off has an insignificant
  impact on cell stack life.

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• A more significant factor of life expectancy has
  to do with corrosion of the cathode.
• The MCFC operating temperature is about 650oC
  (1200oF).
• At this temperature the salt mixture is liquid
  and is a good conductor.
• The cell performance is sensitive to operating
  temperature.
• A change in cell temperature from 650oC (1200oF)
  to 600oC (1110oF) results in a drop in cell
  voltage of almost 15.
• The reduction in cell voltage is due to increased
  ionic and electrical resistance and a reduction
  in electrode kinetics.

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Solid Oxide Fuel Cells

• The Solid Oxide Fuel Cell (SOFC) uses a ceramic,
  solid-phase electrolyte which reduces corrosion
  considerations and eliminates the electrolyte
  management problems associated with the liquid
  electrolyte fuel cells.
• To achieve adequate ionic conductivity in such a
  ceramic, however, the system must operate at
  about 1000oC (1830oF).
• At that temperature, internal reforming of
  carbonaceous fuels should be possible, and the
  waste heat from such a device would be easily
  utilized by conventional thermal electricity
  generating plants to yield excellent fuel
  efficiency.

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• The fuel cell will compete with many other types
  of energy conversion devices, including
• the gas turbine in city's power plant,
• the gasoline engine in your car and
• the battery in your laptop.
• Combustion engines like the turbine and the
  gasoline engine burn fuels and
• use the pressure created by the expansion of the
  gases to do mechanical work.
• Batteries converted chemical energy back into
  electrical energy when needed.
• Fuel cells should do both tasks more efficiently.
  
• A fuel cell provides a DC (direct current)
  voltage that can be used to power motors, lights
  or any number of electrical appliances.

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Classification of Fuel Cells

• 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.
• Some types of fuel cells work well for use in
  stationary power generation plants.
• Others may be useful for small portable
  applications or for powering cars.
• The proton exchange membrane fuel cell (PEMFC) is
  one of the most promising technologies.
• This is the type of fuel cell that will end up
  powering cars, buses and maybe even your house.
  Let's take a look at how they work...

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Tiny Fuel Cell to Power Sensors

• A fuel cell prototype that is the size of a
  pencil eraser and can deliver small amounts of
  electricity was developed at Case Western Reserve
  University (CWRU).

• The fuel cells are 5 mm3 in volume and generate
  10 mW of power with short pulses of up to 100 mW.
• The cell power is so limited
• There is no practical consumer use yet.
• A cell phone, e.g., needs 500 mW.
• The first use will be in sensors for the
  military.

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Microfuel cell

• The prototype microfuel cell uses an
  electrochemical process to directly convert
  energy from hydrogen into electricity.
• The fuel cell works like a battery, using an
  anode and cathode, positive and negative
  electrodes (solid electrical conductors), with an
  electrolyte.
• The electrolyte can be made of various materials
  or solutions. The hydrogen flows into the anode
  and the molecules are split into protons and
  electrons.
• The protons flow through the electrolyte, while
  the electrons take a different path, creating an
  electrical current.
• At the other end of the fuel cell, oxygen is
  pulled in from the air and flows into the
  cathode.
• The hydrogen protons and electrons reunite in the
  cathode and chemically bond with the oxygen atoms
  to form water molecules.
• Theoretically, the only waste product produced by
  a fuel cell is water.
• Fuel cells that extract hydrogen from natural gas
  or another hydrocarbon will emit some carbon
  dioxide as a byproduct, but in much smaller
  amounts than those produced by traditional energy
  sources.

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PEMFC Proton Exchange Membrane Fuel Cell
Animation fuel-cell-animation.swf

• The cell uses one of the simplest reactions of
  any fuel cell.

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Four Basic Elements in a PEMFC

• Anode the negative post of the fuel cell, has
  several jobs.
• It conducts the electrons that are freed from the
  hydrogen molecules
• so that they can be used in an external circuit.
• It has channels etched into it that disperse the
  hydrogen gas equally over the surface of the
  catalyst.
• Cathode the positive post of the fuel cell,
• has channels etched into it that distribute 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.

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Four Basic Elements in a PEMFC

• 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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Chemistry of a Fuel Cell

• Anode side 2H2 ? 4H 4e-
• Cathode side O2 4H 4e- ? 2H2O
• Net reaction 2H2 O2 ? 2H2O

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Animation of a fuel cell workingfuel-cell-animati
on.swf

• The pressurized hydrogen gas (H2) entering the
  fuel cell on the anode side.
• This gas is forced through the catalyst by the
  pressure. When an H2 molecule comes in contact
  with the platinum on the catalyst, it splits into
  two H ions and two electrons (e-).
• The electrons are conducted through the anode,
  where they make their way through the external
  circuit (doing useful work such as turning a
  motor) and return to the cathode side of the fuel
  cell.

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• Meanwhile, on the cathode side of the fuel cell,
  oxygen gas (O2) is being forced through the
  catalyst, where it forms two oxygen atoms.
• Each of these atoms has a strong negative charge.
  
• This negative charge attracts the two H ions
  through the membrane, where they combine with an
  oxygen atom and two of the electrons from the
  external circuit to form a water molecule (H2O).
• This reaction in a single fuel cell produces only
  about 0.7 volts.
• To get this voltage up to a reasonable level,
  many separate fuel cells must be combined to form
  a fuel-cell stack (???).

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• PEMFCs operate at a fairly low temperature (about
  176oF80oC),
• It 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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Problems with Fuel Cells

• The fuel cell uses oxygen and hydrogen to produce
  electricity.
• The oxygen required for a fuel cell comes from
  the air.
• In fact, in the PEM fuel cell, ordinary air is
  pumped into the cathode.
• The hydrogen is not so readily available,
  however.
• Hydrogen has some limitations that make it
  impractical for use in most applications.
• For instance, you don't have a hydrogen pipeline
  coming to your house, and you can't pull up to a
  hydrogen pump at your local gas station.
• Hydrogen is difficult to store and distribute, so
  it would be much more convenient if fuel cells
  could use fuels that are more readily available.
• This problem is addressed by a device called a
  reformer.
• A reformer turns hydrocarbon or alcohol fuels
  into hydrogen, which is then fed to the fuel
  cell.