Electrochemical maximum electric energy

Equation 1.3 indicates that, for an electrochemical reaction, part of the reaction energy (AH) is used to generate electrical energy (AG), and the other part is used to produce heat (TAS). In a fuel cell system, the most useful energy is electrical energy, while the heat produced is sometimes not desired. Therefore, electrical efficiency (or the reversible or thermod5mamic efficiency), can be defined as the ratio of the maximum electrical energy from the cell reaction to the reaction enthalpy. This represents the theoretical upper limit for fuel cell electrical efficiency. 

The Gibbs free energy - AG of the reaction amounts to 237.1 kJ/mol. This value corresponds to the maximum electrical energy that could theoretically be gained from the reaction when following the electrochemical mechanism. This means that the maximum attainable thermodynamic efficiency Tjaierm of energy conversion in this reaction is 83%. 

The maximum electric energy that can be delivered by the chemicals that are stored within or supplied to the electrodes in the cell depends on the change in free energy AG of the electrochemical couple, as shown in Eq. (2.5) and discussed in Sec. 2.2. 

Maximum Intrinsic Efficiency in Electrochemical Conversion of the Energy of a Chemical Reaction to Electric Energy... 

It is in this sense it is said that in an electrochemical energy converter, the ideal maximum efficiency is 100% for, as in the above idealized situation, if one could carry out reactions in such a way that the electrode potentials were infinitely near the equilibrium values, the electrical energy one could draw2 from the reaction would be nFVe and this is all of the free-energy change AG, which is the maximum amount of useful work one can obtain from a chemical reaction. 

The maximum electrical work (M ei) of a fuel cell is given by the change in the free energy of the overall (combined anodic and cathodic) electrochemical reaction... 

Tf n gram-equivalents of substances are consumed during an electrochemical reaction the quantity of electricity nF coulombs will flow through the cell, and will he transfered to a potential level which is higher by E volts. The product of both quantities indicates the maximum electrical work IVmM, which may be gained during this process from the chemical energy, thus ... 

In an ideal electrochemical cell, the Gibbs energy of reaction is converted to electrical energy, then the intrinsic maximum energy conversion efficiency of an electrochemical cell is... 

AG is the electric energy that can be converted from the system and AH is the enthalpy change of the electrochemical reaction, indicating that if the fuel was burned, this amount of heat could be produced in the reaction. However, not all the enthalpy change from the reaction can be converted to electric energy, as some is lost due to the change in entropy. Thus, the maximum efficiency ( cre ) can be calculated by... 

As = surface area of a semiconductor contact [A ] = concentration of the reduced form of a redox couple in solution [A] = concentration of the oxidized form of a redox couple in solution A" = effective Richardson constant (A/A ) = electrochemical potential of a solution cb = energy of the conduction band edge Ep = Fermi level EF,m = Fermi level of a metal f,sc = Fermi level of a semiconductor SjA/A") = redox potential of a solution ° (A/A ) = formal redox potential of a solution = electric field max = maximum electric field at a semiconductor interface e = number of electrons transferred per molecule oxidized or reduced F = Faraday constant / = current /o = exchange current k = Boltzmann constant = intrinsic rate constant for electron transfer at a semiconductor/liquid interface k = forward electron transfer rate constant = reverse electron transfer rate constant = concentration of donor atoms in an n-type semiconductor NHE = normal hydrogen electrode n = electron concentration b = electron concentration in the bulk of a semiconductor ... 

For a fuel cell the maximum thermodynamic efficiency can be calculated from the Gibbs energy (AG) and the enthalpy change (AH) of the electrochemical reaction. Ideally, the free energy of the reaction can be completely converted into electrical energy and the efficiency e is given by ... 

The pure compound capacity accounts for the amount of charge that can be released by the fuel, it is independent of E° and proportional to the ratio njM. Therefore exhibits the same trend as E p. The theoretical energy conversion efficiency is the ratio between the reversible (maximum) electric work that can be obtained by electrochemical oxidation of the fuel and the heat released by direct combustion with oxygen, that is ... 

How is the relationship between maximum electrical work and cell potential used to determine the free energy change for electrochemical cells ... 

For a given electrochemical reaction, the theoretical open-circuit voltage, the Nernst potential, is an important, if not the most important, parameter that affects and measures a fuel cell s performance. The Nernst potential is affected by the operating conditions of the SOFC, such as the temperature, pressure, and fuel composition, and is calculated from the maximum electrical work obtainable, the Gibbs free energy of the reaction (AG) ... 

Towards the end of the nineteenth century Walther Nernst had used the powerful system of energy relations known as thermodynamics to show how one could calculate the maximum amount of electrical energy that could be obtained from a chemical reaction occurring in an electrochemical cell. These predictions can often be achieved in practice but only if the current flow is very small, that is the process is carried out very slowly. This observation shows that electrochemical reactions like all chemical reactions occur at a finite rate. The study of the rates of reaction is called kinetics and the understanding of the kinetics of electrochemical reactions has been the central preoccupation of electrochemists this century. 

The maximum electric work done could achieve about 1100 kJ.moH, when the reaction would be reversible at room temperature. That is to say, the power generation leaching would create a probability that the chemical energy of an electrochemical reaction transfers to the useful electric work. 

The theoretical maximum energy available as electric energy in any electrochemical cell is equal to AjC for the reaction. The maximum energy release when a fuel is burned is Aj.H°. One of the measures used to evaluate a fuel cell is the efficiency value, e = A,.G°/Aj.H°. For the hydrogen-oxygen fuel cell, e = -474.4 kj morV-571.6 kj mol" = 0.83. 

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