Electrochemical cell

An electrochemical cell is a device used for creating an electron differential between two electrode ends to cause an electromotive force (voltage) and current from chemical reactions. The current is caused by the reactions releasing and accepting electrons at the different ends of a conductor. A common example of an electrochemical cell is a standard 1.5-volt battery.

Overview
Each half-cell consists of an electrode with atoms, and an electrolyte with ions that undergo either oxidation or reduction. In a full electrochemical cell, atoms from the electrode of one half-cell lose electrons (oxidation) and become aqueous ions, while ions from the electrolyte of other half-cell gain electrons (reduction) from their electrode and deposit on the electrode itself. The mass of the electrode is diminished in the first case and increased in the second. If the atoms/ions involved in the electrode reactions are metal, then the same metal can be used for each electrode. If the atoms/ions involved in the reaction at each half-cell are not metal, then no electrode can be constructed out of those ions in atomic form; nonreactive metals such as platinum then can be used as a substitute electrode (as in the standard hydrogen electrode). Finally, a salt bridge is often employed to provide electrical contact between two half-cells, allow electrons to flow between them, and prevent a charge imbalance from developing (this would prevent further flow of electrons). A salt bridge can simply be a strip of filter paper soaked in saturated potassium nitrate (V) solution or similar salt solution.

Each half-cell has a characteristic voltage. Different choices of substances for each half-cell give different potential differences. Each reaction is undergoing an equilibrium reaction between different oxidation states of the ions&mdash;when equilibrium is reached the cell cannot provide further voltage. In the half-cell which is undergoing oxidation, the closer the equilibrium lies to the ion/atom with the more positive oxidation state the more potential this reaction will provide. Similarly, in the reduction reaction, the further the equilibrium lies to the ion/atom with the more negative oxidation state the higher the potential.

This potential can be predicted quantitatively through the use of electrode potentials (the voltage measured when the substance is connected to hydrogen). The difference in voltage between electrode potentials gives a prediction for the potential measured. Spontaneity of a chemical reaction is determined by the overall cell potential Ecell. If Ecell>0, the reaction is spontaneous and if Ecell<0, the reaction will not be spontaneous.

The potential window is the electrochemical voltage range between which a substance does not get oxidized or reduced.

Cell types

 * Accumulator
 * Concentration cell
 * Electrolytic cell
 * Galvanic cell
 * Lasagna cell
 * Lemon battery