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Electrodes conduct electric current between mediums and come in various shapes and materials. In direct current, anodes and cathodes are used, with reduction occurring at the cathode and oxidation at the anode. In electrolysis, a direct current flows through a conducting liquid medium from a cathode to an anode, enabling reduction and oxidation processes. Electrodes are also used in galvanotechnics, welding, and medicine.
An electrode is a conductor that passes an electric current from one medium to another, usually from a power source to a device or material. It can take a number of different shapes, including a wire, plate or 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. Electrodes are used in welding, electroplating, batteries, medicine and industry for processes involving electrolysis.
Anodes and cathodes
In the case of a direct current (DC), the electrodes come in pairs and are known as anodes and cathodes. For a battery, or other DC source, the cathode is defined as the electrode from which the current leaves and the anode as the point where it returns. For historical rather than scientific reasons, 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 of the cathode and into the anode. An electric 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’s probably best to think simply in terms of positive and negative terminals.
Inside a battery, or electrochemical cell, the electrodes are made of different materials, one of which releases electrons more easily than the other. They are held in contact with a conductive chemical that can split into positively and negatively charged ions. When a circuit is completed, in other words, when the battery is connected to an electrical device, such as a light bulb, a redox reaction takes place inside the cell. This means that the conducting chemical gains electrons on one electrode – a process known as reduction – and loses them on the other – a process called oxidation – with the result that the electrons flow as a current around the circuit. Reduction always occurs at the cathode and oxidation at the anode.
In a rechargeable battery, this process is reversed while the battery is charging. An electric current from another source is used to power a redox reaction in the opposite direction, meaning the anode becomes the cathode and vice versa. It is still the case that reduction occurs at the cathode and oxidation at the anode, but the direction of current is reversed, so which electrode is negative and which is positive depends on whether the battery is supplying current or is recharging. Sometimes cells are connected to each other by an electrode, which acts as an anode for one cell and a cathode for the other. This is known as a bipolar electrode.
In the case of an alternating current (AC), there is no distinction between anode and cathode. This is because the current is constantly reversing direction, many times per second. An electrode using this type of current would therefore constantly switch between negative and positive.
Electrolysis
In this process, a direct current flows through a conducting liquid medium from a cathode to an anode, enabling reduction and oxidation processes. This is a very useful way to produce certain chemicals and, in particular, to isolate chemical elements from their compounds. In the case of some very responsive elements, it’s the only practical way to do it.
To obtain a given element, an ionic compound of that element can be electrolysed. An example is the production of metallic sodium from molten salt or sodium chloride. As current flows, positively charged sodium ions are attracted to the negative electrode, or cathode, where they acquire electrons, forming metallic sodium. The negatively charged chloride ions are attracted to the anode, where they lose electrons, forming chlorine gas, which is also collected as a by-product.
galvanotechnics
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 current flows, positive metal ions in the solution are attracted to the cathode and form a deposit on it. As the ions in solution are depleted, they are replaced by ions that form from the anode. Sometimes, the anode is made from a different material that isn’t used; in this method, the metal ions must be replaced by topping up the solution.
other uses
Electrodes are used in arc welding, a technique for joining two pieces of metal together using a large electric current. A consumable electrode melts and supplies the material that joins metals. The non-consumable type is made from a material with a very high melting point, such as tungsten, and simply supplies the heat to melt another material that forms the joint. In medicine, electrodes can be used in an emergency to apply an electrical current to the heart in a technique known as defibrillation. They are also used to record electrical activity in the brain during an electroencephalogram (EEG).
