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 number of moles 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.
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.
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:
- 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
- Anode (oxidation): 2 H2O(l) → O2(g) + 4 H+(aq) + 4e− Eo
ox = -1.23 V (Eo
red = 1.23 ))
- Cathode (reduction): 2 H+(aq) + 2e− → H2(g) Eo
red = 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). 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 peroxodisulfate 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 is "pulled out" at the negative electrode. (And vice versa for oxygen.)
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
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 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 (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 requires a minimum of 286 kJ of electrical energy input to dissociate each mole. Since each mole of water requires two moles of electrons, the specific electrical energy required is 143 kJ/mole (8.9×1023 eV/mole). It follows then that a minimum electrical power input per ampere is implied, namely 1.48 W/ampere. In turn, the minimum electrolytic potential for electrolysis of water of 1.48 V (not 1.23 V). Thus, any current (I) at applied voltage (V) greater than 1.48 V is an overvoltage and results in waste heat which can be estimated as I×(V-1.48).
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 potentials. This excess potential accounts for various forms of overpotential by which the extra energy is eventually lost as heat. For a well designed cell the largest overpotential 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 and phosphate. 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. If other, less effective, materials are used for the cathode then another large overpotential must be paid.
Efficiency of modern hydrogen generators is measured by Power consumed per volume of [extracted hydrogen] mass (MJ/m3 or kWh/m3), assuming standard temperature and pressure of the H2. The lower this number the higher efficiency is. Efficiency percentage can be calculated by simplified equation:
where (in SI metric units):
|= efficiency in %|
|= Hydrogen density in normal conditions (25°C,1 atm) (0.0824 kg/m3)|
|E||= Hydrogen specific energy (144 MJ/kg (40.0 kWh/kg))|
|P||= Power consumed per volume of [extracted hydrogen] mass (MJ/m3)|
Here is a table with market standard efficiency parameters:
|P (MJ/m3)||Efficiency (%)|
The energy efficiency of water electrolysis varies widely. Some report 50–80%. These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. If one considers simply the electrical energy input to an electrolyser and the enthalpy of combustion of the H2 product (therefore the energy input and the energy output of the system), then voltage efficiency (HHV) of greater than 82% is achievable (using platinum catalysts and PEM technology, with H2 production occurring at 1.55V, with ideal Faraday efficiency being achieved). Confusingly, the historical 'lower heating value' of H2 (LHV) has occasionally been used to calculate the efficiency for electrolysers, and often for fuel cells. However, it is technically correct to use only HHV, as this value represents the total amount of enthalpy available from the H2 product (which is the enthalpy released during the combustion reaction of H2 with O2). This is generally observed correctly with electrolysis calculations, since LHV if used would give lower efficiency values for electrolysis. However, with fuel cells, LHV gives apparently higher efficiencies thus promoting the fuel cell and has crept into use; LHV is simply a benchmarking method for fuel cells, and not the true overall efficiency.
On a more general note to consider, the energy lost in generating the electricity for the electrolyser is not included in this figure. 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 are not usually allowed for in calculating the efficiency, so the former measure of 50–80% efficient is probably a more realistic efficiency.
Oscillators and electrolysis
There are many web sites that claim that an ocsillator running at 4.8 volts, oscillating at around 923Htz connected to an electrolysis unit can achive conversion rates of upwards of 1L/min. These claims are merely hoaxes as such production rates are not energy justified and would therefore violate the laws of physics, "Energy is neither created nor destroyed."
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|Wikimedia Commons has media related to: Water electrolysis|
- "Electrolysis of Water". Experiments on Electrochemistry. Retrieved November 20, 2005.
- "Electrolysis of Water". Do Chem 044. Retrieved November 20, 2005.
- EERE 2008 - 100 kgH2/day Trade Study
- NREL 2006 - Electrolysis technical report