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In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst equation.[1] A concentration cell produces a small voltage as it attempts to reach chemical equilibrium, which occurs when the concentration of reactant in both half-cells are equal. Because an order of magnitude concentration difference produces less than 60 millivolts at room temperature, concentration cells are not typically used for energy storage.
A concentration cell generates electricity from the reduction in the thermodynamic free energy of the electrochemical system as the difference in the chemical concentrations in the two half-cells is reduced. The same reaction occurs in the half-cells but in opposite directions, increasing the lower and decreasing the higher concentration. The energy is generated from thermal energy that the cell absorbs as heat, as the electricity flows. This generation of electricity from ambient thermal energy, without a temperature gradient, is possible because the convergence of the chemical concentrations in the two half-cells increases entropy, and this increase more than compensates for the entropy decrease when heat is converted into electrical energy.
Concentration cell methods of chemical analysis compare a solution of known concentration with an unknown, determining the concentration of the unknown via the Nernst Equation or comparison tables against a group of standards.
Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution. There are two general types of concentration cells.
Concentration cells can be electrode concentration cells or electrolyte concentration cells.
Electrolyte Concentration cell - In this particular electrochemical cell, the electrodes within both half-cells consist of identical substances, while the electrolyte comprises a solution of the same substance, albeit with varying concentrations.
Electrode Concentration cell - In this particular electrochemical cell, two electrodes composed of the same substance but with differing concentrations are immersed in a common solution.
- ^ Almost any textbook on physical chemistry, e.g. by I. N. Levine or P. W. Atkins, and also many general chemistry texts.