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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 with water. In 1800 Alessandro Volta invented the voltaic pile, and a few weeks later William Nicholson and Anthony Carlisle used it for the electrolysis of water. When Zénobe Gramme invented the Gramme machine 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 in 1888.
An electrical power source is connected to two electrodes, or two plates (typically made from some inert metal such as platinum, stainless steel or iridium) which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons enter the water), and oxygen will appear at the anode (the positively charged electrode). Assuming ideal faradaic efficiency, the amount of hydrogen generated is twice the amount of oxygen, and both are proportional 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 of pure water requires excess energy in the form of overpotential 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. Pure water has an electrical conductivity about one millionth that of seawater. Many electrolytic cells may also lack the requisite electrocatalysts. The efficiency of electrolysis is increased through the addition of an electrolyte (such as a salt, an acid or a base) and the use of electrocatalysts.
In pure water at the negatively charged cathode, a reduction reaction takes place, with electrons (e−) from the cathode being given to hydrogen cations to form hydrogen gas (the half reaction balanced with acid):
At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the anode to complete the circuit:
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.
Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen:
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.
Thus, the standard potential of the water electrolysis cell is -1.23 V at 25 °C at pH 0 (H+ = 1.0 M). At 25 °C with pH 7 (H+ = 1.0×10−7 M), the potential is unchanged based on the Nernst equation. However, electrolysis will not generally proceed at these voltages, as the electrical input must provide the full amount of enthalpy of the H2-O2 products (286 kJ per mol). This takes the theoretical and real observed threshold of electrolysis to (-)1.48 V. This is a standard value derived from basic energy conservation for H2 with a known molar enthalpy value of 286 kJ, (diatomic H2 having 2 Faraday units of charge per mol), therefore the ideal voltage becomes 286,000/(2*96485) = 1.48 V.
The negative voltage indicates the Gibbs free energy 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.
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 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, 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. An electrolyte anion with less standard electrode potential than hydroxide will be oxidized instead of the hydroxide, and no oxygen gas will be produced. A cation with a greater standard electrode potential 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+, Rb+, K+, Cs+, Ba2+, Sr2+, Ca2+, Na+, and Mg2+. Sodium and lithium are frequently used, as they form inexpensive, soluble salts.
If an acid 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 (SO2−
4), as it is very difficult to oxidize, with the standard potential for oxidation of this ion to the peroxydisulfate ion being −2.05 volts.
Strong acids such as sulfuric acid (H2SO4), and strong bases such as potassium hydroxide (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 and when applied with a special catalyst on each side of the membrane can efficiently split the water molecule with as little as 1.5 Volts. There are also a number of other solid electrolyte systems that have been trialled and developed with a number of electrolysis systems now available commercially that use solid electrolytes.
Two leads, 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 (table salt) in an electrolyte solution results in chlorine gas rather than oxygen due to a competing half-reaction. With the correct electrodes and correct electrolyte, such as baking soda, hydrogen and oxygen gases will stream from the oppositely charged electrodes. Oxygen will collect at the positively-charged electrode (anode) and hydrogen will collect at the negatively-charged electrode (cathode). Note that hydrogen is positively charged in the H2O molecule, so it ends up at the negative electrode. (And vice versa for oxygen.)
Note that an aqueous solution of water with chloride ions will, when electrolysed, either result in either OH− if the concentration of Cl− is low, OR in chlorine gas being preferentially discharged if the concentration of Cl− is greater than 25% by mass in the solution.
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 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. When current is run through the Hofmann voltameter, gaseous oxygen forms at the anode (positive) and gaseous hydrogen at the cathode (negative). Each gas displaces water and collects at the top of the two outer tubes, where it can be drawn off with a stopcock.
Many industrial electrolysis cells are very similar to Hofmann voltameters, 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 torches or when extremely high hydrogen purity or oxygen is desired. The vast majority of hydrogen is produced from hydrocarbons and as a result contains trace amounts of carbon monoxide among other impurities. The carbon monoxide impurity can be detrimental to various systems including many fuel cells.
High pressure electrolysis is the electrolysis of water with a compressed hydrogen output around 120-200 Bar (1740-2900 psi). By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor is eliminated, the average energy consumption for internal compression is around 3%.
High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for water electrolysis with a heat engine. High temperature electrolysis may be preferable to 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.
About five 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.
The electrolysis of brine (saltwater), a water sodium chloride mixture, is only half the electrolysis of water since the chloride ions are oxidized to chlorine 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.
The electrolysis of water in standard conditions requires a theoretical minimum of 237 kJ of electrical energy input to dissociate each mole of water, which is the standard Gibbs free energy of formation of water. Very often, although incorrectly, the standard enthalpy of formation of liquid water is used as a reference (286 kJ/mol), or possibly that of water vapour (242 kJ/mol). These are also often indicated as the higher heating value (HHV) and lower heating value (LHV) of hydrogen.
Since each mole of water requires two moles of electrons, and given that the Faraday constant F represents the charge of a mole of electrons (96485 C/mol), it follows that the minimum voltage necessary for electrolysis is about 1.23 V. If the HHV is used as a reference instead, the obtained voltage is 1.48 V, the thermoneutral voltage.
In the case of water electrolysis, Gibbs free energy represents the minimum work necessary for the reaction to proceed, and the reaction enthalpy is the amount of energy (both work and heat) that has to be provided so the reaction products are at the same temperature as the reactants (i.e. standard temperature for the values given above). This implies that an ideal, 100% efficient electrolyser would produce hydrogen and oxygen at a lower temperature than the provided water. An electrolyser operating at 1.48 V would be only 83% efficient, and would produce hydrogen and oxygen at the same temperature as provided water.
Real water electrolysers require higher voltages for the reaction to proceed. The part that exceeds 1.23 V is called overpotential or overvoltage, and represents any kind of loss and nonideality in the electrochemical process.
For a well designed cell the largest overpotential is the reaction overpotential for the four-electron oxidation of water to oxygen at the anode; electrocatalysts can facilitate this reaction, and platinum alloys are the state of the art for this oxidation. Developing a cheap, effective electrocatalyst for this reaction would be a great advance, and is a topic of current research, there are many approaches among them a 30 year old recipe for molybdenum sulfide, graphene quantum dots and perovskite The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no overpotential by platinum, or in theory a hydrogenase enzyme. If other, less effective, materials are used for the cathode (e.g. graphite), large overpotentials will appear.
Efficiency of modern hydrogen generators is measured by power consumed per standard volume of hydrogen (MJ/m3 or kWh/m3), assuming standard temperature and pressure of the H2. A 100%-efficient electrolyser would consume then 10.6 MJ/m3 (2.94 kWh/m3); the lower the actual power used, the higher efficiency.
Very often electrolyser vendors provide efficiencies based on enthalpy (LHV or HHV). These, being larger than the corresponding Gibbs free energy, make the calculated efficiency values appear significantly better than the actual values. To assess the claimed efficiency of an electrolyser it is therefore important to establish how it was defined by the vendor.
There are two main technologies available on the market, alkaline and proton exchange membrane (PEM) electrolysers. Alkaline electrolysers are cheaper in terms of investment, but less efficient; PEM electrolysers, conversely, are more expensive but also more efficient, and can be cheaper if the hydrogen production is large enough. Reported efficiencies are in the range 50–80%.
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