An electrode is a conductor that passes an electrical current from one medium to another, usually from a power source to a device or material. It can take a number of different forms, including a wire, a plate, or a rod, and is most commonly made of metal, such as copper, silver, lead, or zinc, but can also be made of a non-metallic substance that conducts electricity, such as graphite.
An electrode in an electrochemical cell is referred to as either an anode or a cathode. The anode is now defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode as the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode depending on the direction of current through the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell.
Anodes and Cathodes
In the case of a direct (DC) current, electrodes come in pairs, and are known as anodes and cathodes. For a DC source, the cathode is defined as the electrode from which the current leaves, and the anode as the point where it returns. For reasons that are historical rather than scientific, electricity in a circuit is, by convention, depicted as traveling from positive to negative, so that it is seen as a flow of positive charge out from the cathode, and into the anode. An electrical current, however, consists of a flow of tiny negatively charged particles called electrons, so this flow is actually in the opposite direction. In this context, it is probably better to think simply in terms of positive and negative terminals.
To obtain a given element, an ionic compound of that element can be electrolyzed. An example is the production of sodium metal from molten salt, or sodium chloride. When the current flows, positively charged sodium ions are attracted to the negative electrode, or cathode, where they gain electrons, forming sodium metal. Negatively charged chloride ions are attracted to the anode, where they lose electrons, forming chlorine gas, which is also collected as a by-product.
In this process, a metal object is coated with another metal to improve its corrosion resistance or appearance. The object to be coated forms the cathode in an electrolysis process by being immersed in a solution of a soluble compound of the metal that will form the coating, with the anode also made of this metal. When the current flows, positive metal ions from the solution are attracted to the cathode, and form a deposit on it. As the ions in solution are used up, they are replaced by ions that form from the anode. Sometimes, the anode is made from a different material that is not used up; in this method, the metal ions have to be replaced by topping up the solution.
Electrodes for medical purposes, such as EEG (for recording brain activity), ECG (recording heart beats), ECT (electrical brain stimulation), defibrillator (recording and delivering cardiac stimulation);
Electrodes for electrophysiology techniques in biomedical research;