Fuel cell electromotive force

The portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

As shown in Figure 18, the potential is almost proportional to the logarithm of H2 concentration diluted in air. When H2 is diluted in N2, the observed potential corresponds to the electromotive force of a H2-02 fuel cell, and in fact the EMF was as large as about 1.0 V with a theoretical slope of 30 mV/decade, as shown in the same figure. It has been shown that in the case of H2 diluted in air, the following electrode reaction, i.e., electrochemical oxidation of hydrogen (2) and electrochemical reduction of oxygen (3), are important. 

Moore compared the technological branch of solar energy conversion, essentially photovoltaics, with the biological branch. He explained how a standard fuel cell that operates on oxygen and hydrogen produces water and electromotive force. A typical human-engineered fuel cell operates at 50-60 percent power conversion efficiency and uses platinum or other noble metals as catalysts. 

Electrochemical gas sensors detect gases based on the electromotive force(EMF) or the current of an electrochemical cell due to the electrochemical reaction of a particular gas. Solid electrolyte which a specific ion can selectively permeate is used as a diaphragm. Potentiometric type gas sensors have been most widely adopted. Among them potentiometric oxygen sensors composed of partial stabilized zirconia have already had practical application and heen extensively used for the feedback control of the air-fuel ratio of automobile engines. The oxygen sensor elements are composed of the following electrochemical cell. 

Under conditions of electrochemical equilibrium, the chemical composition determines the electromotive force (EMF) of the complete cell. The EMF is given by the Nernst-equation, i.e. for an H2IO2 fuel cell as the simplest system. 

The different fuel-cell systems differ in the nature of the components selected, and thus in the nature of the current-producing chemical reaction. Each reaction is associated with a particular value of enthalpy and Gibbs free energy -AG) of the reaction and thus also with a particular value of the heat of reaction and of the thermodynamic electromotive force (EMF) e. 

The thermodynamic electromotive force of a polymer electrolyte membrane fuel cell at a temperature of 25°C is given by e = 1.229 V. The open-circuit voltage (OCV) of a hydrogen-oxygen polymer electrolyte membrane fuel cell has values between 0.95 and 1.02 V, depending on the temperature and gas pressures. 

The electrode reactions taking place at the electrodes of direct methanol fuel cells, the overall current-producing reactions, and the corresponding thermodynamic values of equilibrium electrode potentials EP and electromotive force (EMF) of the direct methanol fuel cell are given as follows ... 

Ethanol is considered as the ideal fuel for the so-called direct alcohol fuel cells (DAFCs). This is because ethanol has a number of advantages over methanol it can be produced in a sustainable manner, easily stored and transported, and is less toxic or corrosive than methanol. The theoretical mass energy of ethanol is 8.0 kWh kg compared to 6.1 kWh kg" for methanol. The complete oxidation of ethanol releases 12 electrons per molecule its standard electromotive force E° q =1145V, is similar to that of methanol. 

The decrease in free energy of the system in a spontaneous redox reaction is equal to the electrical work done by the system on the surroundings, or AG = nFE. The equilibrium constant for a redox reaction can be found from the standard electromotive force of a cell. 10. The Nernst equation gives the relationship between the cell emf and the concentrations of the reactants and products under non-standard-state conditions. Batteries, which consist of one or more galvanic cells, are used widely as self-contained power sources. Some of the better-known batteries are the dry cell, such as the Leclanche cell, the mercury battery, and the lead storage battery used in automobiles. Fuel cells produce electrical energy from a continuous supply of reactants. 

Electric circuit is needed for a constant Vs output signal to control Ip as a series of operations. Actual Vs output is controlled at a constant voltage of 450 mV. As a result. Vs output is equivalent to the electromotive force generated by oxygen concentration cell at the stoichiometric point as is mentioned in the previous section. In other words, gas detection chamber is always maintained at the stoichiometric air/fuel ratio even though exhaust gas is under any atmosphere. Ip current for the retention corresponds to the equation below. 

All in all, the fuel cell principle explains how an electrostatic potential gradient or electromotive force is created and maintained by controlling the unequal composition of feed components. The current flowing through the load is uniquely determined by the coupled and balanced rates of reactant supply through diffusion media, rates of anode and cathode reactions at electrodes, and electron and proton fluxes through their respective conduction media. 

In a fuel cell, the difference in reactant gas compositions at the two electrodes leads to the formation of a difference in Galvani potential between anode and cathode, as discussed in the section Electromotive Force. Thereby, the Gibbs energy AG of the net fuel cell reaction is transformed directly into electrical work. Under ideal operation, with no parasitic heat loss of kinetic and transport processes involved, the reaction Gibbs energy can be converted completely into electrical energy, leading to the theoretical thermodynamic efficiency of the cell. 

This fundamental equation gives the electromotive force (EMF) or reversible open circuit voltage of the hydrogen fuel cell. 

Among such aqueous fuels, formic acid and methanol with energy densities of 2.08 and 4.69 kWh 1 attracted more attention for the use in membraneless LFFCs due to the ease of access and well-studied electrocatalysis. A formic acid/ O2 fuel cell has a high theoretical electromotive force of 1.45 V, while the corresponding value of methanol is 1.2 V. 

The conceptual theme of hierarchically stractured materials that are specifically designed to maximize anode or cathode performance can be separately considered in the following components (1) the bulk electrode materials, (2) pores in the bulk material that accommodate fluid flow, (3) mesoporous or structural material to transition from the bulk material to the nanometric dimensions, (4) the nanostmctured transducer, (5) the bio-nano interface, and (6) the biocatalyst (Figure 10.1) [23]. All of the elements must be coupled to achieve electron flow from the electron donor through the architecture to the final electron acceptor. In the anodic half-cell, fuel is oxidized and the liberated electrons are transferred to the electrode and through an external load. The cathode provides the BFC with electromotive force by catalyzing the reduction of oxygen at relatively positive potentials. 

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