Fuel cell Gibbs free energy

The theoretical efficiency of a fuel cell is given by the ratio between the Gibbs free energy (AG) which is the maximum electrical work that can be obtained, and the enthalpy (A//) of the fuel (Equation 6.3). 

Temperature and Pressure The effect of temperature and pressure on the ideal potential (E) of a fuel cell can be analyzed on the basis of changes in the Gibbs free energy with temperature and pressure. 

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 maximum electrical energy available from a fuel cell is determined by the Gibbs free energy difference across the electrolyte membrane, AG. This determines the equilibrium voltage of the cell, E, through the Nemst equation, which is nothing more than a restatement in electrical units of how AG (= 2FE) changes with pressure. 

In accordance with thermodynamic laws, only the Gibbs free energy, AG °, of the overall fuel cell reaction can be converted into the equivalent electric cell potential, AE° these two quantities are linked via... 

However, the efficiency of a fuel cell is determined by the thermodynamic equation of Gibbs free energy ... 

One of the emerging uses of solid-state electrocatalytic systems is in fuel cells, to convert a significant portion of the Gibbs free energy change of exothermic reactions into electricity rather than heat. The thermodynamic efficiency of such power generating schemes compares favorably with thermal power generation which is limited by Carnot-type constraints. 

Devise a fuel cell arrangement in which the reaction 2H2 + 02 - 2H20 may be carried out to a finite extent, such that the measured Gibbs free energy change is identical with AGd. 

Theoretically all the combustible materials can serve as a fuel to the SOFC via the direct electrochemical oxidation on the anode. Table 1 lists a number of fuel reactions with AG° (i.e., the change in the Gibbs free energy) and AH° (the change in the enthalpy), E° (theoretically reversible potential or voltage), and the fuel cell efficiency. Fuel cell efficiency is defined as ... 

If the change in the Gibbs free energy AG is greater than the changes in the enthalpy AH, the thermodynamic efficiency can exceed 1 (or 100%). Table 2 also lists the thermodynamic efficiencies for fuel cell reactions of interest under standard conditions. 

Fuel cells are galvanic cells, in which the free energy of a chemical reaction is converted into electrical energy via an electrical current. The Gibbs free energy change of a chemical reaction is related to the cell voltage via Equation 9-28 ... 

While the membrane represents the heart of the fuel cell, determining the type of cell and feasible operating conditions, the two catalyst layers are its pacemakers. They fix the rates of electrochemical conversion of reactants. The anode catalyst layer (ACL) separates hydrogen or hydrocarbon fuels into protons and electrons and directs them onto distinct pathways. The cathode catalyst layer (CCL) rejoins them with oxygen to form liquid water. This spatial separation of reduction and oxidation reactions enables the electrons to do work in external electrical appliances, making the Gibbs free energy of the net reaction, —AG, available to them. 

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. 

Yet, changing over to higher temperatures implies a certain diminution of thermodynamic indices of the fuel cells. The Gibbs free energy - AG of hydrogen oxidation by oxygen decreases with increasing temperature. It amounts to 1.23 eV at 25°C but... 

Equation (7.7) explicitly states the linear relationship between conversion efficiency and photocurrent density calculated under AM1.5G solar illumination. The use of the Gibbs free energy in these equations reflects chemical energy in the hydrogen that can be retrieved using an ideal fuel cell. This in effect, calculates the lower heating value (LHV), which is standard in practical comparisons between different fuels [63]. 

Gibbs Free Energy Change in Fuel Cell Reactions... 

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