The splitting of a compound into simpler forms by the input of electrical energy. When water is electrolysed between inert electrodes, the following two half-reactions take place: 
In chemistry and manufacturing, electrolysis is a method of separating bonded elements and compounds by passing an electric current through them.
Overview
An ionic compound is dissolved with an appropriate solvent, or otherwise melted by heat, so that its ions are available in the liquid. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply.
In electrolysis, the anode is the positive electrode, meaning it has a deficit of electrons;
The amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. In most cases the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input. (It is worth noting that the maximum theoretic efficiency of a fuel cell is the inverse of that of electrolysis. See water fuel cell for an example of such an attempt.)
A higher current flow (amperage) through the cell means it will be passing more electrons through it at any given time.
A higher potential difference (voltage) applied to the cell means the cathode will have more energy to bring about reduction, and the anode will have more energy to bring about oxidation.
The following technologies are related to electrolysis:
Electrochemical cells, including the hydrogen fuel cell, use the reverse of this process. Gel electrophoresis is an electrolysis where the solvent is a gel: it is used to separate substances, such as DNA strands, based on their electrical charge.Electrolysis of water
One important use of electrolysis is to produce hydrogen. The reaction that occurs is
2H2O(aq) → 2H2(g) + O2(g)This has been suggested as a way of shifting society toward using hydrogen as an energy carrier for powering electric motors and internal combustion engines. (See hydrogen economy.) Electrolysis of water can be achieved in a simple hands-on project, where electricity from a battery or low-voltage DC power supply (e.g. Using platinum electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode. For example using iron electrodes in a sodium chloride solution electrolyte, iron oxide will be produced at the anode, which will react to form iron hydroxide. When producing large quantites of hydrogen, this can significantly contaminate the electrolytic cell - which is why iron is not used for commercial electrolysis.
The energy efficiency of water electrolysis varies widely. The efficiency is a measure of what fraction of electrical energy used is actually contained within the hydrogen. Some reports quote efficiencies between 50–70% This efficiency is based on the Lower Heating Value of Hydrogen. The Lower Heating Value of Hydrogen is thermal energy released when Hydrogen is combusted. This does not represent the total amount of energy within the Hydrogen, hence the efficiency is lower than a more strict definition. Other reports quote the theoretical maximum efficiency of electrolysis. The theoretical maximum considers the total amount of energy absorbed by both the hydrogen and oxygen. These values only refer to the efficiency of converting electrical energy into hydrogen's chemical energy. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more like 25–40%.
About four percent of hydrogen gas produced worldwide is created by electrolysis, and normally used onsite. There is some speculation about future development of hydrogen as an energy carrier.
Experimenters
Scientific pioneers of electrolysis included:
Humphry Davy Michael Faraday Paul Héroult Svante Arrhenius Adolph Wilhelm Hermann KolbeMore recently, electrolysis of heavy water was performed by Fleischmann and Pons in their famous experiment, resulting in anomalous heat generation and the controversial claim of cold fusion.
First law of electrolysis
In 1832, Michael Faraday reported that the quantity of elements separated by passing an electrical current through a molten or dissolved salt was proportional to the quantity of electric charge passed through the circuit. This became the basis of the first law of electrolysis.
Second law of electrolysis
Faraday also discovered that the mass of the resulting separated elements was directly proportional to the atomic masses of the elements when an appropriate integral divisor was applied. Manufacture of hydrogen for hydrogen cars and fuel cells. High-temperature electrolysis is also being used for this. Coulometric techniques can be used to determine the amount of matter transformed during electrolysis by measuring the amount of electricity required to perform the electrolysis.
Military uses
As well as producing hydrogen, electrolysis also produces oxygen.
Space stations can also use electrolysis to produce amounts of extra oxygen from waste water or surplus water produced from the Space Shuttle fuel cells.
Examples
Electrolysis of an aqueous solution of table salt (NaCl, or sodium chloride) produces aqueous sodium hydroxide and chlorine, although usually only in minute amounts.
Electrolysis can be conducted relatively safely in the house, with use of any low voltage battery and a solution of salt water.
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