POTENTIALS AND THERMODYNAMICS OF CELLS

Since thermodynamics can strictly encompass only systems at equilibrium, the concept of reversibility is important in treating real processes thermodynamically. After all, the concept of equilibrium involves the idea that a process can move in either of two opposite directions from the equilibrium position. Thus, the adjective reversible is an essential one. Unfortunately, it takes on several different, but related, meanings in the electrochemical literature, and we need to distinguish three of them now. 

Experimentally, one finds that the difference in potential between the silver wire and the platinum wire is 0.222 V when all substances are in their standard states. Furthermore, the platinum wire is the negative electrode, and when the two electrodes are shorted together, the following reaction takes place  

If one overcomes the cell voltage by opposing it with the output of a battery or other direct current (dc) source, the current flow through the cell will reverse, and the new cell reaction is 

Reversing the cell current merely reverses the cell reaction. No new reactions appear, thus the cell is termed chemically reversible. 

By applying an opposing voltage larger than the cell voltage, the current flow reverses, but the reactions observed are 

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To recall the basic concepts of the thermodynamics of cell operation, such as the electrode potential E, the standard electrode potential and the electromotive force (emf). 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

Measurements of the potentials of galvanic cells at open circuit give information about the thermodynamics of cells and cell reactions. For example, the potential of the cell in Figure 1, when the solution concentrations are 1 molar (1 M) at 25°C, is 1.10 V. This is called the standard potential of the cell and is represented by E°. The available energy (the Gibb s free energy AG°) of the cell reaction given in equation (3) is related to E° by... 

Electromotive force measurements of the cell Pt, H2 HBr(m), X% alcohol, Y% water AgBr-Ag were made at 25°, 35°, and 45°C in the following solvent systems (1) water, (2) water-ethanol (30%, 60%, 90%, 99% ethanol), (3) anhydrous ethanol, (4) water-tert-butanol (30%, 60%, 91% and 99% tert-butanol), and (5) anhydrous tert-butanol. Calculations of standard cell potential were made using the Debye-Huckel theory as extended by Gronwall, LaMer, and Sandved. Gibbs free energy, enthalpy, entropy changes, and mean ionic activity coefficients were calculated for each solvent mixture and temperature. Relationships of the stand-ard potentials and thermodynamic functons with respect to solvent compositions in the two mixed-solvent systems and the pure solvents were discussed. 

The electrode reactions taking place at the electrodes of DMFCs, the overall current-producing reactions and the corresponding thermodynamic values of equilibrium electrode potentials and EMF of the fuel cell are as follows ... 

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