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Unformatted text preview: Last class - Thermodynamics of fuel cells This class - Various types of fuel cells 1 Review last class! 1. Internal energy is originated from kinetic energy (molecular movement & vibration) and chemical energy (Bonding between atoms) 2. dU = dQ – dW Means that the change in the internal energy of a closed system (dU) is equal to the heat transferred to the system (dQ) minus the work done by the system (dW) 3. Helmholtz free energy F = U – TS Enthalpy H = U + PV Gibbs free energy G = U + PV -TS
2 4. There are polarizations in a practical fuel cell, which are originated from three sources: (1) activation polarization, (2) ohmic polarization, (3) concentration polarization. 5. The work done by a Carnot engine increases with increasing temperature. (True of False) 6. The efficiency of fuel cells increases with increasing temperature. (True of False) 3 Review last class! 7. The electrochemical potential of fuel cells is dependent on the concentrations (pressures) of H2 and O2 gases. (True of False) 8. What is the name of equation related to #4? 9. The electrochemical potential of the H2-O2 fuel cell decreases as the operating temperature increases. (True of False) 10. The electrochemical potential of the hydrogen fuel cell is same as that of the methanol fuel cell. (True of False) 4 5 □ Fuel cell configuration - Bipolar plate : The plate may be made of metal or a conductive polymer (which may be a carbon-filled composite). The plate usually incorporates flow channels for the fluid feeds and may also contain conduits for heat transfer. - Gasket: prevents the leakage of reactive gases. - Gas diffusion layer: help reactive gases flow through MEA efficiently. - MEA: membrane electrode assembly
6 □ MEA configuration 7 □ How the fuel cell works □ Various types of fuel cells – Electrolyte determines the type of fuel cells and operation temperature. : Operation temperature significantly affects the use of other components such as catalyst.
9 □ Various types of fuel cells portable home EV Stationary power generator 10 □ Polymer Electrolyte Membrane Fuel Cell (PEMFC) Hydrogen fuel cell Methanol fuel cell
11 • The use of organic cation exchange membrane polymers in fuel cells was originally conceived by William T. Grubbs in 1959. The desired function of the ion membrane was to provide an ion conductive gas barrier. • The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water; thus, corrosion problems are minimal. • A critical requirement of these cells is maintaining a high water content in the electrolyte to ensure high ionic conductivity. The ionic conductivity of the electrolyte is higher when the membrane is fully saturated, and this offers a low resistance to current flow and increases overall efficiency. Water management in the membrane is critical for efficient performance. the fuel cell must operate under conditions where the byproduct water does not evaporate faster than it is produced because the membrane must be hydrated.
12 • The cell is able to sustain operation at very high current densities. These attributes lead to a fast start capability and the ability to make a compact and lightweight cell. • Other beneficial attributes of the cell include no corrosive fluid hazard and lower sensitivity to orientation. As a result, the PEMFC is particularly suited for vehicular power application. • Because of the intrinsic nature of the materials used, low-temperature operation of approximately 80oC is possible. • The lower operating temperature of a PEMFC results in both advantages and disadvantages. Low temperature operation is advantageous because the cell can start from ambient conditions quickly, especially when pure hydrogen fuel is available. It is a disadvantage in that platinum catalysts are required to promote the electrochemical reaction.
13 • Carbon monoxide (CO) binds strongly to platinum sites at temperatures below 150oC, which reduces the sites available for hydrogen chemisorption and electro-oxidation. Due to CO affecting the anode, only a few ppm of CO can be tolerated with the platinum catalysis at 80oC. Because reformed and shifted hydrocarbons contain about one percent of CO, a mechanism to reduce the level of CO in the fuel gas is needed. Because of the limitation on the operating temperature imposed by the polymer, usually less than 120oC, and because of problems with water balance, a H2-rich gas with minimal or no CO (a poison at low temperature) is used. 14 • The standard electrolyte material presently used in PEMFCs is a fully fluorinated Teflon-based material produced by E.I. DuPont de Nemours for space application in the mid-1960s. The DuPont electrolytes have the generic brand name Nafion. The Nafion membranes, which are fully fluorinated polymers, exhibit exceptionally high chemical and thermal stability and are stable against chemical attack in strong bases, strong oxidizing and reducing acids, H2O2, Cl2, H2, and O2 at temperatures up to 125oC. • Operating temperature has a significant influence on PEMFC performance. An increase in temperature lowers the internal resistance of the cell, mainly by decreasing the ohmic resistance of the electrolyte. In addition, mass transport limitations are reduced at higher temperatures. The overall result is an improvement in cell performance. • Operating at higher temperatures also reduces the chemisorption of CO because this reaction is exothermic. However, improving the cell performance through an increase in temperature is limited by the high vapor pressure of water in the ion exchange membrane. This is due to the membrane’s susceptibility to dehydration and the subsequent loss 15 of ionic conductivity. □ Major application of PEMFC 16 □ Direct Methanol Fuel Cell (DMFC) 17 □ Direct Methanol Fuel Cell (DMFC) 18 □ Direct Methanol Fuel Cell (DMFC) • The large potential market for fuel cell vehicle applications has generated a strong interest in a fuel cell that can run directly on methanol. • Operation on liquid fuel would assist in rapid introduction of fuel cell technology into commercial markets, because it would greatly simplify the on-board system as well as reduce the infrastructure needed to supply fuel to passenger cars and commercial fleets. 19 □ Problems with Nafion DMFC 1. Methanol crossover from anode to cathode - Reduces fuel utilization Dilution (5-15% in water) - Polarizes the cathode (poisons catalytic sites for O2) 2. Poor methanol oxidation kinetics - Anode polarization dominates cell performance - Induce high activation overpotential Need for good anode catalyst for higher power applications (stationary, transportation) 3. High cost - Reduce or eliminate precious metal catalysts 4. Pressurization of gases - Use of 3 atm air to minimize mass transport losses 20 □ Phosphoric Acid Fuel Cell (PAFC) 21 □ Phosphoric Acid Fuel Cell (PAFC) • Phosphoric acid concentrated to 100% is used for the electrolyte in this fuel cell, which operates at 150 to 220oC. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the Pt electrocatalyst in the anode becomes severe. • The relative stability of concentrated phosphoric acid is high compared to other common acids; consequently the PAFC is capable of operating at the high end of the acid temperature range (100 to 220oC). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure so water management in the cell is not difficult. • The matrix universally used to retain the acid is silicon carbide, and the electrocatalyst in both the anode and cathode is Pt. 22 □ Alkaline Fuel Cell (AFC) The Alkaline Fuel Cell (AFC) was one of the first modern fuel cells to be developed, beginning in 1960. The application at that time was to provide on-board electric power for the Apollo space vehicle.
23 □ Alkaline Fuel Cell (AFC) • The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel cells operated at high temperature (~250oC), or less concentrated (3550 wt%) KOH for lower temperature (<120oC) operation. • The electrolyte is retained in a matrix (usually asbestos, silicate minerals), and a wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with nonprecious, relatively inexpensive metals. • The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air must be considered with the alkaline cell. 24 □ Molten Carbonate Fuel Cell (MCFC) Besides the reaction involving H2 and O2 to produce H2O, the equation shows a transfer of CO2 from the cathode gas stream to the anode gas stream, with 1 mole CO2 transferred along with two Faradays of charge or 2 gram moles of electrons. 25 • The electrolyte in this fuel cell is usually a combination of alkali carbonates, which is retained in a ceramic matrix of LiAlO2. The fuel cell operates at 600 to 700oC where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. • The high operating temperature is needed to achieve sufficient conductivity of its carbonate electrolyte yet allow the use low cost metal cell components. At the high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) are adequate to promote reaction. Noble metals are not required. • Gasified coal is expected to be the major source of fuel gas for MCFCs. Because coal contains many contaminants in a wide range of concentrations, fuel derived from this source also contains a considerable number of contaminants. A critical concern with these contaminants is the concentration levels that can be tolerated by MCFCs without suffering significant degradation in performance or reduction in cell life. sulfur compounds in low ppm (parts per million) concentrations in fuel gas are detrimental to MCFCs 26 □ Application of Molten Carbonate Fuel Cells (MCFC) • Molten carbonate fuel cells are being developed for natural gas and coal-based power plants for industrial, electrical utility, and military applications
27 □ Solid Oxide Fuel Cell (SOFC) 28 □ Solid Oxide Fuel Cell (SOFC) • Solid oxide fuel cells (SOFCs) have grown in recognition as a viable high temperature fuel cell technology. There is no liquid electrolyte with its attendant material corrosion and electrolyte management problems. The operating temperature of >800oC allows internal reforming, promotes rapid kinetics with nonprecious materials, and produces high quality byproduct heat for cogeneration or for use in a bottoming cycle, similar to the MCFC. • However, the high temperature of the SOFC places stringent requirements on its materials. The development of suitable low cost materials and the low cost fabrication of ceramic structures are presently the key technical challenges facing SOFCs. • The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y2O3-stabilized ZrO2. The cell operates at 1000oC where ionic conduction by oxygen ions takes place. Typically, the anode is Co-ZrO2 or Ni-ZrO2 cermet, and the cathode is Sr-doped LaMnO3.
29 □ Application of Solid Oxide Fuel Cells (SOFC) Major efforts are concentrated on the improvement of SOFCs for stationary dispersed power plants and on-site cogeneration power plants. 30 □ Solid Oxide Fuel Cell (SOFC): Potential applications 31 This class - Introduced various types of fuel cells Next class - Recent research trend in fuel cells 32 ...
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- Spring '10