Fuel Cells

1
Fuel Cells

• The long awaited arrival

2
Fuel cells have been just 10 more years away from
everyday use over 50 years

• Several problems have contributed to this
• Materials
• Hydrogen storage safety
• Economics
• Recent advances in engineered materials have made
  fuel cells a viable design option
• Proton exchange membranes (PEM)
• Composite and nanotechnology
• Fuel cell technology is being used today
• Busses, motorcycles, remotely operated aircraft
• Lawnmowers, laptops, cameras
• Home power generation units (up to 10 KW)

3
Fuel cells are a power source that is becoming
increasingly desirable

• Many designs require a power source
• Realize the benefits
• and the costs
• Be aware of emerging technologies

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So what exactly is a fuel cell?

• Demonstrated in 1839 by Sir William Grove
• The principals involved have not changed since,
  but
• the materials and cell configurations have
• Fuel cell An electrochemical device, which
  converts chemical energy to electrical energy
  without combustion and has its fuel /or oxidant
  supplied externally.
• This encompasses a wide array of devices
• Five commonly accepted categories
• Classified by the electrolyte used

5
Fuel cells are typically classified by the type
of electrolyte they use
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Basic principals of a H2-O2 fuel cell

• Most familiar is the hydrogen oxygen fuel cell
• Cathode
• O2 4 e- 2 H2O 4 OH-
• Anode
• 2 H2 4 OH- 4 H2O 4 e-
• Overall
• 2 H2 O2 2 H2O
• Current must be routed
• externally to be used
• PEMFC has the greatest
• immediate potential

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So, how does a PEMFC work?

• The electrolyte is replaced by a highly
  engineered
• polymer
• Allows the passage of H ions form anode to
  cathode
• Does not allow electrons to flow in the opposite
  direction
• Hydrogen must be supplied to the anode surface
• Pt catalyst are used to activate H2
• Reformer units crack ethanol or methane to
  obtain their hydrogen
• Oxygen must be supplied to the cathode surface
• Pt catalyzes the formation of water also
• Oxygen or air feed
• Due to small Vcell, MEA units are stacked in
  series

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The PEM is an amazing feat of materials
engineering

• Nafion is the patented DuPont polymer exchange
  membrane
• 300/m2 ( up)
• Costs vary by application
• Surface Pt facilitates the reactions at both
  electrodes
• Carbon nanotubes conduct protons from one
  electrode interface to the other
• PEM Functions as an electrolyte solution
• Allows H migration
• Prevents electron migration

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Chemical and thermodynamic irreversibility lower
fuel cell efficiency

• Some energy is lost to heat
• Reaction conditions effect efficiency
• Ample gas must be provided to the electrodes
• Low PEM protonic (H) resistance
• Resistance of cell circuitry lowers efficiency
• Typical fuel-based efficiencies
• Internal Combustion25
• Coal30
• Nuclear35
• Fuel cell50-80

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Fuel cell efficiency is not an completely defined
term yet

• Voltage efficiency tends to over estimate the
  amount of work based on caloric fuel content
• Max voltage the reversible reaction potential
  at the operating conditions
• Emax 100, not realistic because of heat losses
• 
  Vmax1.23v _at_STP
• The Change in Gibbs energy and Heat of formation
  of water form the basis for the most commonly
  used overall efficiency measure

• µ0.95
• ?1.48

overall
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PEM fuel cells are already gaining everyday
acceptance
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Limited numbers of fuel cell-powered cars are
publicly available in Japan and fleet vehicles
are currently in use around the world
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But there is no such thing as a free lunch

• Hydrogen must be produced, stored, and
  distributed
• This requires electricity for electrolysis of
  water
• Solar power is being perused for this application
• Construction of a PEMFC requires
• Platinum catalyst
• Exotic membrane materials
• Carbon electrodes
• Polymers
• Copper for wiring
• Low voltages necessitate fuel cell stacking
• And, lets not forget the biggest potential
  difficulty with using hydrogen as a fuel
• .

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Hydrogen presents its own special set of
problems, but ..
hydrogen has the highest fuel content per mass of
any substance in the universe ltPERIODgt
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Todays Lab Exercise

• Safety
• No ignition or heat sources
• Dont inhale the hydrogen or blow into the
  balloons
• Wear safety glasses at all times
• Keep the hydrogen balloons above head level as
  much as practical

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Set Up

• Disconnect yardstick from the stand
• Fill the balloon to about 6 dia. with H2
• Turn needle valves off/on as needed
• The valve will keep the H2 from leaking out too
  rapidly
• Keep the balloons high, and re-connect to the
  stands
• Plug in the tube to the upper H2 connection on
  the PEMFC

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Connection to the Motor

• Connect the leads to the motor and open the
  needle valve 1/4 turn (or less)
• Observe the motor for a couple of minutes
• Adjust the flow back. Very little flow is needed
• Now turn off the valve and wait 1 min
• What happens?
• Why or why not?
• What would happen if the vents on the air side of
  the PEMFC were blocked?

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PEMFC Efficiency

• Connect the multimeter to the fuel cell
• Set to read 2 volts
• Record the volts generated by your fuel cell, Vc
• Calculate the efficiency of your PEMFC unit using
  each of the following equations
• maximum efficiency of PEMFC under these
  conditions is 80

overall
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Fuel Cell Stacks and Hydrogen Consumption

• Connect your PEMFC in series with the other one
  at your table
• What do you expect the combined voltage to be?
• Measure the combined voltage
• Calculate overall efficiency of the stack

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Short answer questions

• Inspect the dissembled unit before leaving today
• Look at the additional PEM at your table
• Please try not to touch the black area of the PEM
  
• At 5 ml/min, how long will the H2 balloon run the
  motor?
• Calculate the mass of Pt in the PEM
• (assume 0.3 mg Pt/cm2)
• What factors must be considered when comparing
  the costs of H2 and gasoline as fuel sources?

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Hope that you learned something and enjoyed the
lab as well