Fuel cell irreversibilities

Fuel Cell Irreversibilities - Causes of Voltage Drop... 

Ultradeep desulfurization of fuel oils is used for producing not only clean fuels but also sulfur-free hydrogen used in fuel-cell systems, in which the hydrogen can be produced potentially through the reforming of fuel oils. Fuel-cell systems must be run with little-to-no sulfur content, because sulfur can irreversibly poison the precious metal catalysts and electrodes used [12]. 

Comprehensive discussions of fuel cells and Camot engines Nemst law analytical fuel cell modeling reversible losses and Nemst loss and irreversible losses, multistage oxidation, and equipartition of driving forces. Includes new developments and applications of fuel cells in trigeneration systems coal/biomass fuel cell systems indirect carbon fuel cells and direct carbon fuel cells. 

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

In the ideal case of an electrochemical converter, such as a fuel cell, the change in Gibbs free energy, AG, (Section 2.2.3) of the reaction is available as useful electric energy at the temperature of the conversion. The ideal efficiency of a fuel cell, operating irreversibly, is then... 

The heat resulting from these irreversibilities must then be removed in order to maintain the fuel cells at a desired operating temperature. Irreversibilities and the resulting quantity of heat produced can be reduced, in general, by increasing the active area of the fuel cells, heat exchangers, and fuel reformer but increased equipment costs result. 

This reaction is thermally neutral. The heat absorbed in the CH4 reforming reaction is released by the subsequent reaction of the H2 product at the anode of the fuel cell. If, therefore, the reforming process can be carried out in close proximity to and in thermal contact with the anode process, the thermal neutrality of the overall CH4 oxidation process can be approximated. And the heat removal and recovery process for the fuel cell system can deal merely with the heat produced by its operational irreversibilities. 

Among the three heat-generation terms, the irreversible and reversible heat sources of ORR are dominant. For a straight-channel cell shown in Figure 12, the total amount of heat release is 2.57 W, of which the irreversible heat is 55.3%, the reversible heat 35.4%, and the Joule heat only 9.3% The total heat released from the fuel cell can also be estimated from the overall energy balance, i.e. 

The quotient of the total power and reversible power at operating temperature gives the relation between the converted power at irreversible conditions and the maximum convertible reversible power of the fuel cell. 

Then, useful electric energy is attained from a fuel cell when current is extracted however, during this process, the real cell potential is reduced from its equilibrium potential because of the irreversible losses previously explained. 

The thermal efficiency of an actual fuel cell, operating irreversibly at temperature T. reduces to ... 

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