Electrolysis of water
Under water electrolysis means the decomposition of water into hydrogen and oxygen by means of an electric current. The most important application of this electrolysis is the production of hydrogen, which is however is used technically only if favorable electrical energy is available, otherwise other production methods are more favorable, for example, starting from crude oil or coal. When these raw materials and energy are scarce, the electrolysis of water in the hydrogen economy, which uses hydrogen as an energy carrier, could be significant.
Due to the strong expansion of renewable energy water electrolysis is gaining increasing importance as a method of production of so-called EE- gas. With hydrogen as an energy storage device is the improved continuity of electricity from renewable energy sources, especially wind power and photovoltaics, promoted. By using production peaks from these renewable energy is used and supplied either directly in the form of hydrogen or after subsequent methanation than the methane gas network by electrolysis, the excess energy.
The electrolysis of water is also important as a demonstration test; it is often used the Hofmann water electrolysis unit. Another application of the electrolysis of water is the enrichment of deuterium. Furthermore, the water electrolysis is the most important side reaction of many technical electrolysis, such as the chlor-alkali electrolysis.
Reactions and their equations
The electrolysis of water consists of two reactions that occur at the two electrodes ( cathode and anode spaces ). The overall reaction scheme of this redox reaction is:
The electrodes are immersed in water, which is made more conductive by the addition of a little acid, preferably sulfuric acid, or alkali. The use of sodium chloride as an electrolyte is also possible, however, depending on the electrode and current density in addition to or instead of oxygen also produced chlorine. Large-scale, commercial applications are still missing. For existing facilities, a highly concentrated aqueous KOH solution is used in the electrolysis of water.
The positively charged oxonium ions ( H3O ) migrate in the electric field to the negatively charged electrode (cathode ) where they each take one electron. This produces hydrogen atoms combine with one another, caused by reduction of H- atom to form a hydrogen molecule. Remains is water molecules.
Cathode compartment: 2 H3O 2 e- → H2 2 H2O or: 2 H2O 2 e- → H2 2 OH -
The separated gaseous hydrogen rises to the cathode, wherein the cathode chamber is basic. The negatively charged hydroxide anions migrate toward the positive anode - if this is not prevented by the division of anode and cathode chamber or the use of a conductive salt or ion exchangers - which neutralize negative hydroxide ions with protons to form water or at the anode under electron donation to oxygen convert.
Anode: 4 OH → O2 2 H2O 4 e - or 6 H2O → O2 4 H3O 4 e -
Again, the separated oxygen increases as a gas at the anode, at the same time, the anode chamber is acidic. The resulting protons migrate towards the cathode - analogous to the processes in the cathode compartment.
The total reaction equation is the electrolysis of water:
4 H3O 4 OH - → 2 H2 O2 6 H2O
The subject on the left side of hydronium ( new: " oxonium " ) and hydroxide ions come from the autoionization of water:
8 H2O → 4 H3O 4 OH -
It is the electrolysis equation therefore also be written as follows:
8 H2O → 2 H2 O2 6 H2O
Or by shortening of the water:
2 H2O → 2 H2 O2
Technical water electrolysis
The energy efficiency of the electrolysis of water is over 70 %. Several equipment manufacturers (eg electrolyser Corp., Brown Boveri, Lurgi, De Nora, Epoch Energy Technology Corp.. . ) Offer great electrolyzers with an even higher efficiency - over 80 % - of. As the electrolyte concentration and the temperature of an electrolyte solution great influence on the cell resistance and thus on the cost of energy, is used a 25-30 % strength potassium hydroxide solution at modern plants, the temperature is about 70-90 ° C. The current density is about 0.15 to 0.5 A/cm2, the voltage at about 1.90 V. For the production of 1 m3 of hydrogen (at atmospheric pressure ) is in modern equipment, an electric power of 4.3 to 4, 9 kWh needed. A large pressure electrolyzer of Lurgi has a capacity of 760 m3 / h of hydrogen at about 3.5 MW (stack power) and about 4.5 MW input power ( AC). By electro- catalysts (for example, Ni-Co - Zn at the cathode, Ni-Mo, wherein the anode: nickel - lanthanum - perovskite, nickel -cobalt spinel), the surge can be reduced by about 80 mV.
There is also the possibility to split distilled water by electrolysis. The SPE - hydrogen electrolysis, a proton- loaded Nafion membrane used. The thin perforated electrodes are located on the surface layer (english zero gap, gap -free cell geometry ') of the membrane. As an electrode material, for example Rutheniumoxidhydrate (anode) and platinum (cathode) can be used. The SPE electrolysis seems to prevail as a market for Kleinelektrolyseure.
Currently research is being conducted ( at 800 to 1000 ° C) solid electrolyte and on the high-temperature steam electrolysis. As a solid electrolyte may be a calcium - yttrium - zirconium oxide, or a perovskite may be used ( for example, LaCrO3 ). With such systems, the required voltage to 1.30 V was lower, the current density was 0.4 A/cm2. The efficiency is especially important for the use of hydrogen as an energy store when a stable power supply is to be established by means of fuel cells, for example, seasonal, regional or daytime due fluctuating renewable energy sources. Another technical application with a similar background is the generation of wind gas (solar gas ) and the use of hydrogen production vessel Hydrogen Challenger.
Side reactions