632 Fundamental Limitations

The ideal performance of a fuel cell depends upon the electrochemical reactions that occur within the fuel cell. Table 6.2 presents a summary of the electrochemical reactions that occur within the various fuel cell types. The lower-temperature fuel cells (AFC, PAFC, and PEMFC) all require noble metal (e.g., platinum) electro-catalysts to achieve practical reaction rates at the anode and cathode, and they typically require hydrogen fuel. The higher-temperature fuel cells (MCFC and SOFC) typically use nickel-based materials to accomplish the electrochemistry described in Table 6.2. In addition, as indicated in Table 6.2, higher-temperature fuel cells can electrochemically react with hydrogen as well as other fuels (e.g., CO and CH4). Note that carbon monoxide poisons the noble metal catalysts of lower-temperature fuel cells, but serves as a source of fuel (H2) for the higher-temperature fuel cells. Also note that the reactions of CO and CH4 in Table 6.2 are presented as anodic electrochemical reactions. In reality, these reactions may not occur on the anode surface but, rather, through water-gas shift, and steam reformation chemical reactions likely produce hydrogen in the gas phase (Hirschenhofer et al., 1998).

TABLE 6.2

Fuel Cell Anode and Cathode Reactions

Fuel Cell Type

Anode Reactions

Cathode Reactions

Alkaline (AFC) Molten Carbonate (MCFC)

Phosphoric Acid (PAFC) Proton Exchange Membrane (PEMFC) Solid Oxide (SOFC)

1/2O2 + CO2 + 2e- ^ COf" 1/2O2 + 2H+ + 2e- ^ H2O

The ideal performance of a fuel cell is determined by the potential voltage level that it can theoretically produce. This potential voltage is called the Nernst potential and is defined by the Nernst equation. For the general reaction aA + bB ^ cC + dD

the Nernst equation can be expressed as

where Eo is the reversible standard potential for a cell reaction, E is the ideal equilibrium potential, T is temperature, T is Faraday's constant, and P is pressure. Therefore, for each of the fuel cell types, there is a theoretical voltage level that can be achieved which is determined by the Nernst equation for each of the electrochemical reactions that occur within the cell. Note that according to the Nernst equation, the ideal cell voltage can be increased by operation at higher pressures for a given temperature.

Table 6.3 presents fuel cell electrochemical reactions and their corresponding Nernst equations. The reaction of hydrogen and oxygen produces water, but when a carbon-containing fuel is used at the anode, carbon dioxide is also produced. For MCFCs, CO2 is required at the cathode to maintain a constant carbonate concentration in the electrolyte. Because CO2 is produced at the anode and consumed at the cathode in MCFCs, the partial pressure of CO2 is included in both the anode and cathode Nernst equations of Table 6.3.

TABLE 6.3

Fuel Cell Reactions and Corresponding Nernst Equations

Fuel Cell Reaction Nernst Equation

H2 + 1/2O2 ^ H2O E = E0 + (RT/2F) ln [Ph2/Ph2o] + (RT/2F) ln [Po21/2]

H2 + 1/2O2 + CO2(c) ^ H2O + E = Eo + (RT/2F) ln [PHH2/I,H2o(pCo2)a] + (RT/2F) CO2(a) ln [Po21/2(Pco2)]c

CO + 1/2O2 ^ CO2 E = Eo + (RT/2F) ln [Pco/Pco2] + (RT/2F) ln [Po21/2] CH4 + 2O2 ^ 2H2O + CO2_E = Eo + (RT/8F) ln [Pch4/P2h20Pco2] + (RT/8F) ln [Po21/2]

The ideal standard potential of a hydrogen oxygen fuel cell is 1.229 volts. Figure 6.4 presents the ideal potential for each of the cell reactions versus temperature. Note that the ideal potential for some of the primary fuel cell reactions increases with decreasing temperature. This is very different from all of the typical generation technologies based upon heat engine designs, which exhibit decreased performance with reductions in temperature.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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