Electrolysis of water
Overview
 
Electrolysis of water is the decomposition of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 (H2O) into oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 (O2) and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas (H2) due to an electric current
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 being passed through the water.

An electrical power source is connected to two electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s, or two plates (typically made from some inert metal such as platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 or stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

) which are placed in the water. Hydrogen will appear at the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 (the negatively charged electrode, where electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s enter the water), and oxygen will appear at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 (the positively charged electrode).
Encyclopedia
Electrolysis of water is the decomposition of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 (H2O) into oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 (O2) and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas (H2) due to an electric current
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 being passed through the water.

Principle

An electrical power source is connected to two electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s, or two plates (typically made from some inert metal such as platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 or stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

) which are placed in the water. Hydrogen will appear at the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 (the negatively charged electrode, where electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s enter the water), and oxygen will appear at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 (the positively charged electrode). Assuming ideal faradaic efficiency, the amount of hydrogen generated is twice the number of moles
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

 of oxygen, and both are proportional
Proportionality (mathematics)
In mathematics, two variable quantities are proportional if one of them is always the product of the other and a constant quantity, called the coefficient of proportionality or proportionality constant. In other words, are proportional if the ratio \tfrac yx is constant. We also say that one...

 to the total electrical charge conducted by the solution. However, in many cells competing side reactions dominate, resulting in different products and less than ideal faradaic efficiency.

Electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 of pure water requires excess energy in the form of overpotential
Overpotential
Overpotential is an electrochemical term which refers to the potential difference between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency...

 to overcome various activation barriers. Without the excess energy the electrolysis of pure water occurs very slowly or not at all. This is in part due to the limited self-ionization of water
Self-ionization of water
The self-ionization of water is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−...

. Pure water has an electrical conductivity about one millionth that of seawater. Many electrolytic cell
Electrolytic cell
An electrolytic cell decomposes chemical compounds by means of electrical energy, in a process called electrolysis; the Greek word lysis means to break up. The result is that the chemical energy is increased...

s may also lack the requisite electrocatalyst
Electrocatalyst
An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the...

s. The efficiency of electrolysis is increased through the addition of an electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

 (such as a salt, an acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 or a base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

) and the use of electrocatalyst
Electrocatalyst
An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the...

s.

Currently the electrolytic process
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 is rarely used in industrial applications since hydrogen can currently be produced more affordably from fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

s.

History

Jan Rudolph Deiman and Adriaan Paets van Troostwijk used in 1789 an electrostatic machine to produce electricity which was discharged on gold electrodes in a Leyden jar
Leyden jar
A Leyden jar, or Leiden jar, is a device that "stores" static electricity between two electrodes on the inside and outside of a jar. It was invented independently by German cleric Ewald Georg von Kleist on 11 October 1745 and by Dutch scientist Pieter van Musschenbroek of Leiden in 1745–1746. The...

 with water. In 1800 Alessandro Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...

 invented the voltaic pile
Voltaic pile
A voltaic pile is a set of individual Galvanic cells placed in series. The voltaic pile, invented by Alessandro Volta in 1800, was the first electric battery...

, a few weeks later William Nicholson
William Nicholson (chemist)
William Nicholson was a renowned English chemist and writer on "natural philosophy" and chemistry, as well as a translator, journalist, publisher, scientist, and inventor.-Early life:...

 and Anthony Carlisle
Anthony Carlisle
Sir Anthony Carlisle FRCS, FRS was an English surgeon.He was born in Stillington, County Durham, the third son of Thomas Carlisle and his first wife, and the half-brother of Nicholas Carlisle, FRS. He was apprenticed to medical practitioners in York and Durham, including his uncle Anthony Hubback...

 used it for the electrolysis of water. When Zénobe Gramme
Zénobe Gramme
Zénobe Théophile Gramme was a Belgian electrical engineer. He invented the Gramme machine, a type of direct current dynamo capable of generating smoother and much higher voltages than the dynamos known to that point.In 1873 he and Hippolyte Fontaine accidentally discovered that the device was...

 invented the Gramme machine
Gramme machine
A Gramme machine, Gramme ring, Gramme magneto, or Gramme dynamo is an electrical generator which produces direct current, named for its Belgian inventor, Zénobe Gramme, and was built as either a dynamo or a magneto. It was the first generator to produce power on a commercial scale for industry...

 in 1869 electrolysis of water became a cheap method for the production of hydrogen. A method of industrial synthesis of hydrogen and oxygen through electrolysis was developed by Dmitry Lachinov
Dmitry Lachinov
Dmitry Aleksandrovich Lachinov was a Russian physicist, electrical engineer, inventor, meteorologist and climatologist.Dmitry Lachinov studied in the St. Petersburg University, where he was a pupil of Heinrich Lenz, Pafnuty Chebyshev and Feodor Petrushevsky...

 in 1888.

Equations

In the pure water at the negatively charged cathode, a reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 reaction takes place, with electrons (e) from the cathode being given to hydrogen cations to form hydrogen gas (the half reaction balanced with acid):
Reduction at cathode: 2 H+(aq) + 2e → H2(g)


At the positively charged anode, an oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 reaction occurs, generating oxygen gas and giving electrons to the anode to complete the circuit:
Anode (oxidation): 2 H2O(l) → O2(g) + 4 H+(aq) + 4e


The same half reactions can also be balanced with base as listed below. Not all half reactions must be balanced with acid or base. Many do like the oxidation or reduction of water listed here. To add half reactions they must both be balanced with either acid or base.
Cathode (reduction): 2 H2O(l) + 2e → H2(g) + 2 OH-(aq)

Anode (oxidation): 4 OH- (aq) → O2(g) + 2 H2O(l) + 4 e


Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen:
Overall reaction: 2 H2O(l) → 2 H2(g) + O2(g)


The number of hydrogen molecules produced is thus twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas. The number of electrons pushed through the water is twice the number of generated hydrogen molecules and four times the number of generated oxygen molecules.

Thermodynamics of the process

Decomposition of pure water into hydrogen and oxygen at standard temperature and pressure
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

 is not favorable in thermodynamical
Thermochemistry
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...

 terms.
Anode (oxidation): 2 H2O(l) → O2(g) + 4 H+(aq) + 4e    E = -1.23 V (E = 1.23 ))
Cathode (reduction): 2 H+(aq) + 2e → H2(g)    E = 0.00 V


Thus, the standard potential of the water electrolysis cell is -1.23 V at 25 °C at pH 0 (H+ = 1.0 M). It is also -1.23 V at 25 °C at pH 7 (H+ = 1.0 M) based on the Nernst Equation
Nernst equation
In electrochemistry, the Nernst equation is an equation that can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...

.

The negative voltage indicates the Gibbs free energy
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...

 for electrolysis of water is greater than zero for these reactions. This can be found using the G = -nFE equation from chemical kinetics, where n is the moles of electrons and F is the Faraday constant. The reaction cannot occur without adding necessary energy, usually supplied by an external electrical power source.

Electrolyte selection

If the above described processes occur in pure water, H+ cations will accumulate at the anode and OH anions will accumulate at the cathode. This can be verified by adding a pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...

 to the water: the water near the anode is acidic while the water near the cathode is basic. The negative hydroxyl ions that approach the anode mostly combine with the positive hydronium ions (H3O+) to form water. The positive hydronium ions that approach the negative cathode mostly combine with negative hydroxyl ions to form water. Relatively few hydronium (hydroxyl) ions reach the cathode (anode). This can cause a concentration overpotential at both electrodes.

Pure water is a fairly good insulator since it has a low autoionization
Self-ionization of water
The self-ionization of water is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−...

, Kw = 1.0 x 10−14 at room temperature and thus pure water conducts current poorly, 0.055 µS·cm−1. Unless a very large potential is applied to cause an increase in the autoionization of water the electrolysis of pure water proceeds very slowly limited by the overall conductivity.

If a water-soluble electrolyte is added, the conductivity of the water rises considerably. The electrolyte disassociates into cations and anions; the anions rush towards the anode and neutralize the buildup of positively charged H+ there; similarly, the cations rush towards the cathode and neutralize the buildup of negatively charged OH there. This allows the continued flow of electricity.

Care must be taken in choosing an electrolyte, since an anion from the electrolyte is in competition with the hydroxide ions to give up an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

. An electrolyte anion with less standard electrode potential
Standard electrode potential
In electrochemistry, the standard electrode potential, abbreviated E° or E , is the measure of individual potential of a reversible electrode at standard state, which is with solutes at an effective concentration of 1 mol dm−3, and gases at a pressure of 1 atm...

 than hydroxide will be oxidized instead of the hydroxide, and no oxygen gas will be produced. A cation with a greater standard electrode potential
Standard electrode potential
In electrochemistry, the standard electrode potential, abbreviated E° or E , is the measure of individual potential of a reversible electrode at standard state, which is with solutes at an effective concentration of 1 mol dm−3, and gases at a pressure of 1 atm...

 than a hydrogen ion will be reduced in its stead, and no hydrogen gas will be produced.

The following cations have lower electrode potential than H+ and are therefore suitable for use as electrolyte cations: Li
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

+, Rb
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...

+, K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

+, Cs
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

+, Ba
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...

2+, Sr
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...

2+, Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

2+, Na
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

+, and Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

2+. Sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 and lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

 are frequently used, as they form inexpensive, soluble salts.

If an acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 is used as the electrolyte, the cation is H+, and there is no competitor for the H+ created by disassociating water. The most commonly used anion is sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

 , as it is very difficult to oxidize, with the standard potential for oxidation of this ion to the peroxodisulfate
Peroxodisulfate
The peroxodisulfate ion, S2O82−, is a sulfur oxoanion. It is commonly referred to as the persulfate ion, but this term also refers to the peroxomonosulfate ion, SO52−.-Compounds containing peroxodisulfate:* Na2S2O8* K2S2O8* 2S2O8...

 ion being −2.05 volts.

Strong acids such as sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 (H2SO4), and strong bases such as potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

 (KOH), and sodium hydroxide (NaOH) are frequently used as electrolytes due to their strong conducting abilities.

A solid polymer electrolyte can also be used such as Nafion
Nafion
Nafion is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Walther Grot of DuPont. It is the first of a class of synthetic polymers with ionic properties which are called ionomers...

 and when applied with a special catalyst on each side of the membrane can efficiently split the water molecule with as little as 1.8 Volts.

Fundamental demonstration

Two leads
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

, running from the terminals of a battery, are placed in a cup of water with a quantity of electrolyte to establish conductivity in the solution. Using NaCl as an electrolyte results in chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 gas rather than oxygen due to a competing half-reaction. With the correct electrodes and correct electrolyte hydrogen and oxygen gases will stream from the oppositely charged electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s. Oxygen will collect at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 and hydrogen will collect at the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

.

Hofmann voltameter

The Hofmann voltameter is often used as a small-scale electrolytic cell. It consists of three joined upright cylinders. The inner cylinder is open at the top to allow the addition of water and the electrolyte. A platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 electrode is placed at the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

. When current is run through the Hofmann voltameter, gaseous oxygen forms at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

(positive) and gaseous hydrogen at the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

(negative). Each gas displaces water and collects at the top of the two outer tubes, where it can be drawn off with a stopcock.

Industrial electrolysis

Many industrial electrolysis cells are very similar to Hofmann voltameter
Hofmann voltameter
A Hofmann voltameter is an apparatus for electrolyzing water, invented by August Wilhelm von Hofmann in 1866. It consists of three joined upright cylinders, usually glass. The inner cylinder is open at the top to allow addition of water and an ionic compound to improve conductivity, such as a...

s, with complex platinum plates or honeycombs as electrodes. Generally the only time hydrogen is intentionally produced from electrolysis is for specific point of use application such as is the case with oxyhydrogen
Oxyhydrogen
Oxyhydrogen is a mixture of hydrogen and oxygen gases, typically in a 2:1 molar ratio, the same proportion as water. This gaseous mixture is used for torches for the processing of refractory materials and was the first gaseous mixture used for welding...

 torches or when extremely high hydrogen purity
Hydrogen purity
Hydrogen purity or hydrogen quality is a term to describe the lack of impurities in hydrogen as a fuel gas. The purity requirement varies with the application, for example a H2 ICE can tolerate low hydrogen purity where a hydrogen fuel cell requires high hydrogen purity to prevent catalyst...

 or oxygen is desired. The vast majority of hydrogen is produced from hydrocarbons and as a result contains trace amounts of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 among other impurities. The carbon monoxide impurity can be detrimental to various systems including many fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

s.

High pressure electrolysis

High pressure electrolysis is the electrolysis of water with a compressed hydrogen
Compressed hydrogen
Compressed hydrogen is the gaseous state of the element hydrogen kept under pressure. Compressed hydrogen in hydrogen tanks at 350 bar and 700 bar is used for mobile hydrogen storage in hydrogen vehicles...

 output around 120-200 Bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 (1740-2900 psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...

). By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor
Hydrogen compressor
A hydrogen compressor is a device that increases the pressure of hydrogen by reducing its volume. Compression of hydrogen gas naturally increases its temperature, due to Charles' Law....

 is eliminated, the average energy consumption for internal compression is around 3%.

High-temperature electrolysis

High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for water electrolysis with a heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

. High temperature electrolysis is more efficient than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures.

Applications

About four percent of hydrogen gas produced worldwide is created by electrolysis. The majority of this hydrogen produced through electrolysis is a side product in the production of chlorine. This is a prime example of a competing side reaction.
2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH


The electrolysis of brine
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...

 (saltwater), a water sodium chloride mixture, is only half the electrolysis of water since the chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

 ions are oxidized to chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 rather than water being oxidized to oxygen. The hydrogen produced from this process is either burned (converting it back to water), used for the production of specialty chemicals, or various other small scale applications.

Efficiency

Water electrolysis does not convert 100% of the electrical energy into the chemical energy of hydrogen. The process requires more extreme potentials than what would be expected based on the cell's total reversible reduction potential
Reduction potential
Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts , or millivolts...

s. This excess potential accounts for various forms of overpotential
Overpotential
Overpotential is an electrochemical term which refers to the potential difference between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency...

 by which the extra energy is eventually lost as heat. For a well designed cell the largest overpotential
Overpotential
Overpotential is an electrochemical term which refers to the potential difference between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency...

 is the reaction overpotential for the four electron oxidation of water to oxygen at the anode. An effective electrocatalyst to facilitate this reaction has not been developed. Platinum alloys are the default state of the art for this oxidation. Developing a cheap effective electrocatalyst for this reaction would be a great advance (see also). In 2008, a group led by Daniel Nocera announced the development of an electrocatalyst composed of the abundant metal cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

 and phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

. Other researchers are pursuing carbon-based catalysts.

The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no reaction overpotential by platinum or in theory a hydrogenase enzyme
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....

. If other, less effective, materials are used for the cathode then another large overpotential must be paid.

The energy efficiency
Energy conversion efficiency
Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...

 of water electrolysis varies widely with the numbers cited below on the optimistic side. Some report 50–80%.
These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency may be closer to 30–45%, although the inefficiencies of powerplants in turning heat into electrical energy is not usually included in efficiency, so the former measure of 50-80% efficient is probably a more realistic efficiency.

See also

  • Electrochemistry
    Electrochemistry
    Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...

  • Electrolysis
    Electrolysis
    In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

  • Hydrogen production
    Hydrogen production
    Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...

  • Noryl
    Noryl
    Noryl is a blend of polyphenylene oxide and polystyrene that was developed by General Electric Plastics in 1966 . It is a rare example of a homogeneous mixture of two polymers. Most polymers are incompatible with one another, so tend to produce separate phases when mixed...

  • Gas cracker
    Gas cracker
    A gas cracker is any device that splits the molecules in a gas or liquid, usually by electrolysis, into atoms. The end product is usually a gas. A hydrocracker is an example of a gas cracker. In nature, molecules are split often, such as in food digestion and microbial digestion activity. A gas...

  • Photocatalytic water splitting
    Photocatalytic water splitting
    Photocatalytic water splitting is the term for the production of hydrogen and oxygen from water by directly utilizing the energy from light. Hydrogen fuel production has gained increasing attention as oil and other nonrenewable fuels become increasingly depleted and expensive...

  • Water purification
    Water purification
    Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose...

  • Timeline of hydrogen technologies
    Timeline of hydrogen technologies
    Timeline of hydrogen technologies — A timeline of the history of hydrogen technology.-1600s:* 1625 - First description of hydrogen by Johann Baptista van Helmont...



External links

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