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Fuel Cells

Fuel Cells. The long awaited arrival. 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

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Fuel Cells

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  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 1 Identify Need 10 Commun -ication 2 Define Problem 9 Specification 3 Search 4 Constraints 8 Decision 5 Criteria 7 Analysis 6 Alternative Solutions • Many designs require a power source • Realize the benefits and the costs • Be aware of emerging technologies

  4. 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

  6. Basic principals of a H2-O2 fuel cell Schematic of a 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

  7. 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

  8. 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

  9. 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 Combustion=25% • Coal=30% • Nuclear=35% • Fuel cell=50-80%

  10. Fuel cell efficiency is not an completely defined term yet Vcell Vmax E = x100% voltage (µ)·[Vcell] Ћ x100% E = • 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 Vmax=1.23v @STP • The Change in Gibbs energy and Heat of formation of water form the basis for the most commonly used overall efficiency measure µ=0.95 Ћ=1.48 overall

  11. PEM fuel cells are already gaining everyday acceptance Consumer products Fuel cells power cameras Computers, even lawnmowers Ballard Inc, Ion Power Inc,.. Auto manufacturers PEMFC cars are being publicly released this year in Japan Toyota, Honda Power generation Powered flight Avista Laboratory's 7.5KW PEM residential fuel cell power plant. Carbon composite plane built by DynAero, fuel cells provide power to the motor

  12. Limited numbers of fuel cell-powered cars are publicly available in Japan and fleet vehicles are currently in use around the world Hy-Pump PEMFC Mass Transit Japanese H-Station

  13. 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 ……………………….

  14. Hydrogen presents its own special set of problems, but ….. hydrogen has the highest fuel content per mass of any substance in the universe <PERIOD>

  15. Today’s Lab Exercise • Safety • No ignition or heat sources • Don’t 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

  16. 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

  17. 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?

  18. PEMFC Efficiency Vcell 1.23 E = x100% voltage (0.95)·[Vcell] 1.48 100% E= • 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

  19. 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

  20. 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?

  21. Hope that you learned something and enjoyed the lab as well

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