About Batteries
Gary L. Bertrand
Professor of Chemistry
University of Missouri-Rolla
Simulation     Back to Start

A battery consists of one or more electrochemical cells. Each cell contains two metal electrodes and at least one electrolyte solution (a solution containing ions that can conduct electricity). The battery operates through electrochemical reactions called oxidation and reduction. These reactions involve the exchange of electrons between chemical species. If a chemical species loses one or more electrons, this is called oxidation. The opposite process, the gain of electrons, is called reduction.

Oxidation occurs at the Anode.

Reduction occurs at the Cathode.

If the reactive components of an electrochemical cell are placed in contact with each other, they will react by direct transfer of electrons (an oxidation - reduction reaction) and there is no way to harness this energy to do electrical work.  Most of the energy of the reaction is released as heat.  The heat released is closely related to the standard enthalpy change (delta-) of the reaction.


In most batteries, there are different materials at the two electrodes, such that they want to react with one material being oxidized and the other being reduced.  In the cell below, Zinc is used for the electrode on the left (the Anode) in contact with a solution of Zinc (II) ions, possibly a solution of Zinc NitrateCopper is used for the electrode on the right (the Cathode) in contact with a solution containing Copper (II) ions, perhaps Cupric Nitrate.  By keeping the materials separated, the electrons being produced by the oxidation at the Anode could be used to do electrical work as they are transferred to the Cathode where they will be consumed by the reduction process.  The amount of electrical work that a battery may produce is closely related to the standard free energy change (delta-) of the reaction.

However, the oxidation process either produces positive ions or removes negative ions from the solution at the anode (or it may change one ion to a more positive one), and the reduction process either removes positive ions or produces negative ions in the solution at the cathode.  This produces electrically charged solutions, and very quickly stops the process before a measureable number of electrons are transferred.

There must be a path for the ions to move between the two solutions in order for electrons to flow continuously through the wire.  This produces an "ion current" within the battery with cations (positively - charged ions) moving from anode to cathode, and anions (negatively - charged ions) moving from the cathode toward the anode.

This path may be provided by having the two solutions in contact with each other, but this allows diffusion of all of the ions and "runs down" the battery pretty quickly.  This diffusion can be slowed down by separating the solutions with a membrane or a porous plug.  All of these can lead to a "liquid junction potential" due to differing rates of movement by the cations and anions.  A "salt bridge" can be used to separate the two solutions with a third concentrated solution of well - matched cations and anions, completely eliminating the "liquid junction potential".  In a few cases, it is possible to design a battery so that both electrodes can be placed in the same container with only one solution.


The voltage of a cell may depend on many factors: the electrode materials, the components and concentrations of the solutions, the type of liquid junction, the temperature, and the pressure.  The voltage also depends on the electrical current being drawn from the cell.  The voltage (E) and the current (I) are related to the resistance (R) through Ohm's Law:
E = IR
The current is directly related to the rate at which electrons are pumped through the wire and any resistances in the circuit.  As the resistance is lowered to zero (a short-circuit), the current increases and the voltage of the cell decreases to zero. As the resistance is increased, the current decreases, and the voltage increases toward a limiting value.  In Chemistry, we are primarily interested in this limiting value, the maximum voltage that the electrochemical cell can deliver.  This maximum voltage or electrochemical potential is a measure of the maximum electrical work that can be obtained from the chemical reaction occurring within the cell, and this can be related to the Gibbs' Free Energy Change associated with the chemical reaction.

Before we leave this discussion to discuss the thermodynamics of batteries, we need to address the effects of concentration on the voltage of a cell.  This can get somewhat complicated and confusing.  We are going to avoid these problems by focusing on cells with a very specific type of chemical reaction.


In the cell above, the electrons are produced by the lead metal being oxidized to lead (II) ions, and by copper (II) ions being reduced to copper metal.  Even with the ions moving across the boundary between the solutions, there is an increase in the concentration of lead ions on the left and a decrease in copper ions on the right.  This causes the voltage of the battery to decrease, and eventually the voltage will decrease to zero.  Some batteries are designed to be re-chargeable by forcing electrons to flow backwards through the cell, reversing the chemical reaction.

The Nernst Equation describes the effects of concentrations on the maximum voltage that the chemical reaction can produce by relating the voltage to the Standard Electrochemical Potential (E°).  This Standard Electrochemical Potential represents the maximum voltage the reaction can produce with all of the components in their standard states or at unit activity.


The remainder of  this discussion will be concerned with  electrochemical cells that do not involve changes in the concentrations of ions or gases.  In these cells, the Standard Electrochemical Potential can be measured directly.

One way to do this is by using Metal/Metal Salt electrodes which are prepared by coating the metal with one of its insoluble salts (or an oxide), as in Silver/Silver Chloride, Lead/Lead Sulfate, or Mercury/Mercurous Chloride (Calomel) electrodes.  These are usually a solid metal and a solid salt, though in the case of Mercury, the metal is a pure liquid.  Electrical contact is usually made through a platinum wire in contact with the mercury.

This cell is constructed with a Lead/Lead Sulfate anode and a Silver/Silver Sulfate cathode, both in a solution of Sodium Sulfate.  The two solutions are separated by an anion exchange membrane, which allows negatively - charged ions to go through it, but positively - charged ions cannot.  The voltage of this cell still depends on the current being drawn from it, and on the temperature.  At any fixed temperature, however, the maximum voltage (at very low current) is independent of the concentration of the electrolyte, and is equal to the Standard Electrochemical Potential for this reaction.