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Motivation for New Energy Technology

Motivation for New Energy Technology. Reduction of localized pollution in heavily populated areas Improved fuel efficiency Reduction in CO 2 emissions Conservation of energy resources PNGV - 3X improvement in fuel economy for automobiles between 1994 and 2004. Some Background on Fuel Cells.

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Motivation for New Energy Technology

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  1. Motivation for New Energy Technology • Reduction of localized pollution in heavily populated areas • Improved fuel efficiency • Reduction in CO2 emissions • Conservation of energy resources • PNGV - 3X improvement in fuel economy for automobiles between 1994 and 2004

  2. Some Background on Fuel Cells • First fuel cell - William Groves, 1839 • Convert chemical energy to electrical energy • Can be thought of as a battery to which the energetic reactants are continuously supplied • Have potential to revolutionize global energy production

  3. Established Applications of PEM Fuel Cells Power source for spacecraft applications Small scale terrestrial electric power generation Photo of Apollo 11, taken from Kennedy Space Center web site: http://images.jsc.nasa.gov/images/pao/AS11/10075233.jpg

  4. Potential Applications of PEM Fuel Cells • Fuel-cell-powered automobiles • High efficiency (45-65%) • Low tail-pipe emissions • Stationary power generation systems • Conserve fossil fuels • Reduce SOx emissions • Battery replacement devices • Fuel-cell-powered submarines Photo obtained from International Fuel Cells web site: http://www.internationalfuelcells.com/solution/trans/index.shtml

  5. Schematic of a PEM Fuel Cell Gottesfeld, S. Adv. Electrochem. Sci. Eng. (1997), 5, 198.

  6. PEM Fuel Cell Components • Bipolar gas flow plates • Gas distribution to electrodes • Current collection from MEA • Membrane Electrode Assembly (MEA)

  7. The Membrane Electrode Assembly (MEA) • MEA composition • Two electrodes • Carbon-fiber paper • Coated with catalyst • Polymer membrane electrolyte • conducts protons • electrical insulator • separates gases Gray et. al. Energy & Fuels (1998), 12, 1125.

  8. The Polymer Electrolyte Membrane • Perfluorocarbon sulfonic acid ionomer • Ex.: Nafion • + High protonic conductivity • + Excellent long-term chemical stability • - Temp < 100° C • - Cost

  9. Basic Electrochemistry of Hydrogen Fuel Cell • Anode: H2 2H+ + 2e- • Cathode: O2 +4H+ + 4e- 2H2O • Theoretical cell potential: 1.22 V

  10. Voltage Loss Processes • Actual voltage for H2 fuel cell is typically 0.6-0.7 V (compared to 1.22 V theoretical) • Cathode activation • Cell resistance • Anode activation • Mass transport resistance

  11. Hydrogen Fuel Cell Catalyst • Nature of catalyst • Historically- Pt catalyst used • Last 6 years - reduced Pt loading has substantially reduced the cost without lessening performance • Activity of catalyst • Anode: Dissociative adsorption of H2 • Cathode: O2 oxygen anions

  12. Fuel Storage • The figure depicts storage volume and weight in comparison to 55 L of gasoline. • Attractive candidates for autos • Methanol • Hydrocarbons Schmidt et. al. Journal of Power Sources (1994), 49, 302.

  13. Reforming of Methanol • Reaction: CH3OH + H2O CO2 + 3H2 • Side reactions: • CH3OH CO + 2H2 • CO + H2O CO2 + H2 • These three reactions are the origin of the CO and CO2 contaminants in hydrogen fuel cells

  14. Anode Catalyst Poisoning • CO • Produced in fuel reformation process • Strongly adsorbs to active sites on catalyst • CO2 • May generate CO via CO2 + H2 CO + H2O • Could form COH-type residues with adsorbed hydrogen

  15. Anode Poisoning by CO Chalk et. al. Journal of Power Sources (1998), 71, 31.

  16. Improving Fuel Cell Performance • Use a platinum alloy as the anode catalyst • Improves anode CO and CO2 tolerance • Reduces the overvoltage associated with the cell reaction • Platinum-ruthenium alloys have been found to be effective catalysts.

  17. Weight Percent Ruthenium 80 90 100 20 40 60 70 0 10 30 50 2400 2334 C 2200 L 2000 70 79 1800 1769 °C Temperature °C 1600 Pt + Ru Pt (fcc) Ru (hcp) 1400 1200 62 80 1000 100 50 60 80 90 20 10 30 40 0 70 Atomic Percent Ruthenium The Platinum-Ruthenium Phase Diagram

  18. Direct Methanol PEM Fuel Cells • Major advantage: hydrogen reformation is bypassed • In the past, DMFC’s shunned for low power density (orders of magnitude less than that of hydrogen fuel cell) • Recent improvements have been immense • Currently under intense study

  19. DMFC Redox Reactions • Reactions • Anode: CH3OH + H2O CO2 + 6H+ + 6e- • Cathode: 3O2 + 6H+ +6e- 3H2O • Catalysts • Anode: Pt-Ru nanoparticles • Cathode: Pt

  20. Fuel Cell Stack • Fuel cells must be combined to increase power capability • Combination: voltages additive

  21. PEM Fuel Cell Membrane Electrode Assembly (MEA) and Stack Gray et. al. Energy & Fuels (1998), 12, 1127.

  22. Economics of fuel cells • In parentheses is the max cost for which fuel cells would be cost effective in that application • Note reduction in Pt utilization with time • PEM fuel cells can not yet compete with gas-powered cars • PEMFC’s could be competitive in buses and stationary systems

  23. Advantages of PEM Fuel Cells • High power density (~6kW/m2) • High efficiency • Reliable • Materials are durable and benign • No emissions (from the fuel cell)

  24. Conclusions PEM fuel cells are very promising Tremendous advances have been made in the past 6 years Appear to be close to large-scale commercialization Much development still needed

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