Description of a Fuel Cell

The conversion of chemical energy of fuels into electric energy requires three steps in the process that is utilized in many power plants today  

C) Conversion of mechanical energy into electric energy. 

For combustion engines, steps A and B are combined in the well known way. The efficiency of step B is limited since the efficiency of a closed-cycle heat engine cannot surpass a certain value at given temperatures for the input and output of heat as derived by Carnot on thermodynamic grounds. Total efficiencies of up to 41% have been achieved for the conversion of chemical energy into electric energy in modern units. 

A different approach was taken in the attempt to convert chemical energy into electric energy by electrochemical reactions with continuous supply of the reactants in a galvanic cell. In contrast to batteries, an electrical recharging of the cell after a certain period of use is not required. A device for this type of energy conversion is called Fuel Cell . Some of the features that make fuel cells attractive are cited below  

Schematically, a fuel cell may be represented as a system of two electrodes separated by electrolyte (see Fig. 1). Liquid or solid electrolytes are used in different types of cells. Fuel is supplied to the anode and oxygen or air to the cathode. The two electrodes are connected by a resistive load. The electrochemical oxidation of the fuel at the anode produces electrons. The electrons flow through the external circuit to the cathode on which oxygen is reduced. The ionic and neutral species that participate in the electrochemical reactions are different for the 

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Swiss chemist Christian Schonbein (1799-1868) publishes the first description of a fuel cell. 

The most basic description of a fuel cell is that of an electrochemical cell that has reactants supplied from an external source. An electrochemical cell consists of two electrodes, an anode and a cathode, to which reactants (oxidant and reductant, normally referred to as the fuel) are supplied and then react... 

A brief description of various electrolyte cells of interest follows. A detailed description of these fuel cells may be found in References (1) and (2). 

A logical first step in understanding the operation of a fuel cell is to define its ideal performance. Once the ideal performance is determined, losses can be calculated and then deducted from the ideal performance to describe the actual operation. Section 2.1.1 is a description of the thermodynamics that characterize ideal performance. Actual performance is addressed in Section 2.1.2. Section 2.1.3 provides a lead-in to the development of equations in Section 3 through Section 8 that quantify the actual cell performance as a function of operating conditions for PEM, PAFC, AFC, ITSOFC, MCFC, and SOFC, respectively. 

The electrochemical eflBdency gives more information about fuel cells than the thermodynamic efHciency as it is directly related to the performance of the cell. For a complete description of the fuel cell efHciency further factors associated with practical operation have to be considered. These include the faradaic efHciency ep which is deHned as the ratio of the actual current fact and the maximum possible current Imax- This faradaic efficiency considers the possibility of parallel reactions which can lead to a lower current yield than expected theoretically. Furthermore, the total efficiency of the cell requires the consideration of practical aspects concerned with the specific fuel used. In most cases fuel cells are not operated with 100 % fuel utilization in order to avoid fuel depletion in some areas of the electrodes. Therefore the fuel utilization should be included in the total efficiency, given as... 

Description. House Bill 2845, passed by the 77th Legislature, directed the Texas State Energy Conservation Office (SECO) "to develop a plan for the acceleration of the commercialization of fuel cells in Texas. The bill also called on SECO to appoint an advisory committee to help with this task. The duty of the Fuel Cell Initiative Advisory Committee (FCIAC) was to help develop the plan that offers policy options to the legislature to ensure the viability of a fuel cell industry in Texas now and in the future." The FCIAC has a number of their published reports online at www.seco.cpa.state.tx.us /fciachome.htm. 

The chemistry of electrocatalytic reactions at either electrode and the operating temperature of a fuel cell eCMR are determined mainly by the nature of the chargecarrying ion and the EM. To ftiUy appreciate the broad potential of the fuel cell eCMR, it is useful to start with a generic description. 

Equations describing global properties of fuel cells are denoted as zerodimensional models. An example is the description of the current-voltage characteristics of a fuel cell where the different overvoltages are considered, for example, in the form (Ticianelli et al., 1988)... 

Under conditions of constant current, namely, under steady-state operation, reactants must be supplied continuously and at a constant rate to precisely balance the rate of reactant consumption in electrode reactions. The coupled fluxes of electrons, protons, and gaseous reactants are subdued to two fundamental conservation laws, which complete the description of the fuel cell principle conservation of charge and mass. These laws allow balance or continuity equations to be written among all involved... 

The anode and cathode reaction characteristics, however, vary for different types of fuel cells. While a detailed discussion of these fuel cells is given in Chapter 8, brief descriptions of these fuel cells along with their associated reactions are given here. 

There are a number of fuel cells, which can convert the energy of a fuel (usually hydrogen) and an oxidant (usually oxygen) to electricity. We will not cover all fuel cells but consider just a few of them that have some particular historical or technological significance. A detailed description of other fuel cells can be found in [4],... 

The first and the second law of thermodynamics allow the description of a reversible fuel cell, whereas in particular the second law of thermodynamics governs the reversibility of the transport processes. The fuel and the air are separated within the fuel cell as non-mixed gases consisting of the different components. The assumption of a reversible operating fuel cell presupposes that the chemical potentials of the fluids at the anode and the cathode are converted into electrical potentials at each specific gas composition. This implies that no diffusion occurs in the gaseous phases. The reactants deliver the total enthalpy J2 ni Hi to the fuel cell and the total enthalpy J2 ni Hj leaves the cell (Figure 2.1). 

This chapter presents the design and application of a two-stage combinatorial and high-throughput screening electrochemical workflow for the development of new fuel cell electrocatalysts. First, a brief description of combinatorial methodologies in electrocatalysis is presented. Then, the primary and secondary electrochemical workflows are described in detail. Finally, a case study on ternary methanol oxidation catalysts for DMFC anodes illustrates the application of the workflow to fuel cell research. 

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