Typical design problems associated with electrochemical power sources are characterized not only by the usual intricacies of the interfacial problem, but also by the complex composite character of the materials used as electrodes and electrolytes. A battery may consist of one or more cells. For example, a 12V car battery is composed of six 2V cells in series; a flash light usually has a battery composed of two separate 1.5V D-size cells, but the individual cells are often loosely referred to as batteries.
Instead of mixing two reactants and heating them until they burn, they can be separated by an electrolyte, such as in the aluminum-air cell shown here. The cell has three main components:
An anode, or negative electrode: This is the fuel electrode which gives up electrons to the external circuit during the reaction and becomes oxidized. It is usually a metal, such as aluminum, zinc, lead, or lithium.
A cathode, or positive electrode, which accepts electrons from the external circuit and is reduced during the reaction. It is usually a metal , e.g. manganese dioxide, silver oxide or, as in the aluminum-air cell, oxygen from the ambient air.
Some electrolyte, typically a liquid with dissolved salts, acids or alkalis, e.g. sulfuric acid or potassium hydroxide, whose molecules break up into positive and negative ions to carry electric charge between the electrodes within the cell as the reaction proceeds.
Reactions with the electrolyte take place at the anode and the cathode only so long as an external electrical circuit is completed between the electrodes, which removes or supplies the required electrons. Several forms of construction are commonly adopted:
Bobbin: common for general purpose cylindrical cells (lower current than spirally wound).
Flat: commonly used in lead acid automobile batteries.
Button: common for hearing aids, watches, calculators, night sights.
Spirally wound: common for lithium cells and sealed rechargeable cells.
The electrodes must be kept apart to avoid short circuiting but the separator must be permeable to the electrolyte to allow ion movement. Cells are usually sealed to prevent the electrolyte from leaking and the cell drying-out. There is also a safety valve to vent any gases that may be given off when a problem occurs, so that dangerous internal pressures do not build up.
Several research topics fall under the electrochemical umbrella, such as heterogeneous electrocatalysis, electron and ion transfer, as well as side reactions like electrode dissolution, nucleation, deposition, and electrolyte reduction and oxidation. Substantial advances in surface-science techniques are having a great impact on the development of fields such as electrochemical conversion and storage of energy, photoelectrochemistry, bioelectrochemistry, and materials stability.
In parallel, design using computational techniques based on first-principles theory is becoming an increasingly important tool in all areas of science and engineering. Recent work has set initial steps in the direction of a theory-guided design for the development of new electrode and electrolyte materials, the understanding of catalytic and electrocatalytic mechanisms, and the design and fabrication of catalysts and electrocatalysts.
Many pages of the Corrosion Doctors Web site discuss specific issues related to the electrochemical power source design issues. The following are references to some of these pages: