Electrolysis

A process in which an electric current forces a redox reaction is called electrolysis. It is for example used for the extraction of metals or for the manufacture of materials whose extraction would be more expensive by purely chemical processes or hardly possible. Examples of important electrolysis are the recovery of hydrogen, aluminum, chlorine and caustic soda.

An electrolysis requires a DC voltage source, which supplies the electrical energy, and chemical reactions drives. A part of the electrical energy is converted into chemical energy. Exactly the opposite purpose, the conversion of chemical energy into electrical, serve batteries, accumulators or fuel cells: they are used as a voltage source. When loading an accumulator, runs from electrolysis that makes the chemical processes during discharge back. Electrolysis can therefore serve energy storage, for example, in the electrolysis of water, which yields hydrogen and oxygen, which have been proposed as an energy source of a hydrogen economy. By reversing the electrolysis of water in a fuel cell can be about 40% of the energy originally used to be recovered.

The deposition of metals from a solution containing the corresponding metal ion, with a externally impressed current is also an electrolysis. This can be used to produce layers of metal, for example chromium plating; this type of electrolysis are the subject of electroplating. The electrolytic dissolution and re- deposition of metals is used for cleaning, for example of copper, and electrolytic refining is mentioned.

The chemical reactions which occur during the electrolysis, electrons are transferred. There are therefore always redox reactions, the oxidation at the anode (positive pole), the reduction at the cathode (negative pole) run; Oxidation- reduction processes, and are therefore at least partially spatially separated from one another.

• 2.1 Electrolysis of water
• 2.2 Electrolysis of zinc iodide

• 5.1 extractive
• 5.2 electroplating
• 5.3 Electrolytic refining
• 5.4 Water decomposition
• 5.5 Kolbe electrolysis

Principle

By means of two electrodes, an electric direct current in a conductive liquid (see electrolyte) passed. At the electrodes caused by the electrolysis reaction products of the substances contained in the electrolyte.

The voltage source causes an electron deficiency in the connected with the positive electrode ( anode ) and an excess of electrons in the other, associated with the negative electrode ( cathode). The solution between the cathode and the anode contains positively and negatively charged ions as electrolytes. Positively charged ions (cations ), or electrically neutral substances take on electrons at the cathode and be reduced. At the anode, the opposite process takes place, the supply of electrons to the electrode, wherein substances, such as anions which are oxidized. The amount of transmitted electrons at the anode is equal to the transmitted to the cathode.

The transport of substances to the electrodes is accomplished by convective mass transfer (diffusion within the liquid with superimposed flow of the liquid ) and, as far as ions, in addition through migration ( migration by the action of the electric field between the electrodes).

The voltage that must be applied to the electrolysis at least is called the decomposition voltage ( Uz or Ez ). This or a higher voltage must be applied so that the electrolysis takes place at all. If this minimum voltage is not reached, the electrolyte or its interface operates to the electrodes, which are also referred to as electrochemical double layer insulating.

For each substance, for any conversion of ions to two or polyatomic molecules, the decomposition voltage, the deposition potential on the basis of the redox potential can be determined. From the redox potential can also get some more information on how to electrolytic decomposition of metal electrodes in acid or for the reduction of decomposition voltage by changing the pH. So can be calculated by the redox potential, the anodic oxygen evolution in water electrolysis of water in basic solution (decomposition voltage: 0,401 V) at lower voltage will be reached in acid (decomposition voltage: 1.23 V) or neutral (decomposition voltage: 0.815 V) solution at the cathode, however, they produce hydrogen more easily under acidic conditions than under neutral or basic conditions.

In an electrolyte solution are more reducible cations present, first, the cations are reduced, which in the redox series (power series ) have a more positive ( less negative ) potential. In the electrolysis of an aqueous saline solution usually forms at the cathode and hydrogen is not sodium. Even in the presence of a plurality of anionic species that may be oxidized first to come into play those that are as close as possible in the redox voltage zero point, thus having a weaker positive redox potential.

After the decomposition voltage is exceeded grows proportionately, the amperage with voltage gain. Faraday by the amount by weight of a substance is proportional to the electrolytically formed flowed electric charge ( current multiplied by the time, see Faraday's law ). For the formation of 1 g of hydrogen ( about 11.2 liters, in the formation of a hydrogen molecule two electrons are required) from an aqueous solution, an electric charge of 96 485 C (1 1 A = C · s ) is required. At a current of 1 A, the formation of 11.2 liters of hydrogen thus takes 26 hours and 48 minutes.

In addition to the redox potential is still the overvoltage ( the overpotential ) is important. On the basis of kinetic inhibition at one electrode often requires a much higher voltage than can be calculated from the calculation of the redox potential. Will also change the redox series, so that other ions are oxidized or reduced than would have been expected from the redox potential - The overvoltage effects - depending on the material properties of the electrodes.

Soon after switching off an electrolysis can detect a current with an ammeter deflection in the other direction. In this brief phase of the reverse process of electrolysis, the formation of a galvanic cell is committed. In this case, current is not consumed for the reaction, but it is briefly generated current; This principle is used in fuel cells.

Sometimes it is advisable to separate from each other to prevent undesirable chemical reactions cathode chamber and the anode chamber and the charge exchange between the anode and cathode compartment only by a porous diaphragm - to take place - often an ion exchange resin. In the industrial electrolysis for the production of caustic soda, this is quite important. In pursuit of metabolism, rates of migration of ions also the knowledge of molar conductivities boundary may be important.

When forces by electrolysis a separation of individual molecules or bonds, the same effect a galvanic element that counteracts the voltage of the electrolysis. This voltage is referred to as bias voltage.

Electrodes

There are only a few anode materials, which remain inert during the electrolysis, thus not go into solution, such as platinum and carbon. Some metals dissolve despite strong negative redox potential is not on, this property is referred to as " passivity ". In acidic solution, the majority of the metals would dissolve with cations and hydrogen formation according to the Nernst equation. Except for copper, silver, gold, platinum, palladium, almost all metal / metal cation pairs have a negative redox potential and would be for electrolysis in an acidic medium unsuitable because the equilibrium (metal atom and protons) forming cations and hydrogen shifts. In the sulfuric acid environment, lead is an inexpensive and popular cathode material as the anode can both lead and lead oxide are used ( also used in car batteries ). Lead sulfate is poorly soluble, so that the lead electrodes hardly dissolve.

Iron, nickel can sometimes be used because of the passivity as anodes in acidic medium, but also these anode materials are preferably used in a basic medium. An iron anode, which was treated with concentrated nitric acid, will not loosen up, go through the passivation no iron ( II) - and iron (III ) ions in solution. It has a very thin and stable oxide layer formed (similar to aluminum), which prevents further dissolution of the electrode. However, chloride ions or higher temperatures may cancel the passivity.

Iron anodes have, in comparison to other anode materials, only a small overvoltage for oxygen evolution, so they are preferably used in the production of oxygen.

Inhibition phenomena at the anode, which result in the formation of oxygen to surge, observed in coal and platinum anodes. The over-voltage can be used to produce chlorine in the electrolysis of aqueous sodium chloride solution instead of oxygen.

Of zinc, lead and mercury cathodes show particularly protons significant over-voltage and the formation of hydrogen takes place only at a much higher voltage. Significant surge of hydrogen is bound to the mercury cathode, the sodium amalgam and as such the equilibrium is to be withdrawn is utilized for the commercial production of caustic soda. Due to the significant over- voltage at this electrode in the hydrogen form, the redox changes, rather than protons are hiking sodium cations to the mercury cathode.

Suitable electrode materials:

( ) Well suited ( ) suitable, (-) not suitable

Overload

Both the cathode and at the anode overvoltage may occur, thus increasing the required voltage with respect to the calculations according to the Nernst equation. The surges are for gas formations ( eg, hydrogen and oxygen formation) sometimes considerably. The applied surge energy is lost as heat, so does not contribute to metabolism. Depending on the type of metal and surface properties of the electrodes vary the overvoltage. Amperage and temperature also affect the overvoltage. An increasing flow rate is slightly increased, the overvoltage, whereas an increase in temperature lowers the overvoltage.

The following tables provide a brief overview regarding the surge in the anodic oxygen evolution and cathodic hydrogen evolution ( the experiments were however carried out at different pH values ​​for the calculation of pH - changes, see Nernst equation)

Overvoltage oxygen formation

Conditions: 1 N aq. KOH, 20 ° C, measured after 20 min.

Overvoltage hydrogen formation

Conditions: 1 N aq. HCl, 16 ° C.

In other electrolytic reductions (no gas formation), the diffusion overvoltage can be important. If after some minutes, falls, the concentration of electrolytic substance to be reacted against the electrode, more power must be applied to achieve the same current. By continuous stirring or with rotating discs, cylinder electrode, the diffusion overvoltage can be reduced.

The hydrogen and the oxygen overvoltage does not remain constant in many metals. They sometimes even increase after 60 minutes slightly.

Cell resistance

The electrical resistance of an electrolytic cell impedes the flow of current ( Ohm's law ) and should therefore be minimized, otherwise is energy in the form of heat is lost. The resistance of an electrolytic cell depends on the electrode spacing, the size of the electrode surface and on the conductivity.

Generally apply to the calculation of the resistance of an electrolytic cell:

The resistance becomes very high - - In distilled water, the conductivity is very low and a bad electrolysis possible.

Conductivity of several solutions:

The conductivities of solutions of low concentrations can be calculated via the specific electrolytic conductivity or the equivalent conductivities of the ions. The conductivity of solutions of very high concentration must be determined experimentally. Although strong acids, the conductivity is higher than in the basic solutions of the same concentration, many electrolysis - mainly performed in a basic medium - due to the anodic dissolution processes or delayed formation of oxygen or halogen oxidation in the acidic range.

Current density

To increase the efficiency of the electrolytic process, the process should be carried out at the highest possible current densities. This is achieved by increasing the conductivity by the addition of salt or by increasing the temperature (depending on degree of temperature increase in the conductivity increases by about 1-2 %). Frequently, the current density is limited by the diffusion limiting current. From knowledge of the diffusion limiting current is dimensionless parameters can be determined in order to calculate the turnover for larger plants can. There is one for each electrolysis Costing optimum current density, it is not for the most part, the maximum current density.

In order to preserve clean, compact metal deposits, should be carried out at low current density. This is especially important for gold, silver and copper covers. Metal deposits at high current densities form of so-called skewers, sticks, trees, and this can lead to short circuits.

Often - especially in organic chemistry - are superior to the electrolytic process, thermal process due to the higher material revenue per unit time.

Migration rates of ions in electrolysis

During the electrolysis, cations can be reduced at the cathode and oxidized at the anode anions. Since dense charge changes occur prior to the electrode by reduction or oxidation, the charge difference must be compensated by migration processes in the electrode chamber. Cations and anions must be present in the electrode chamber at an identical concentration, there should be no excess of positive or negative ions. The balance of ions in an electrolysis cell is caused by the migration of ions. The migration rate depends on the applied cell voltage and the type of ions. The loss of cations front of the cathode can be compensated by the migration of excess cations from the anode compartment, or vice versa of excess anions from the cathode compartment. In general, a compromise between these two directions is a hike. The migration rates can be calculated from the limiting conductivities of the ionic species. With the transference number of the change in ionic composition can be determined directly.

There are ions such as H or OH -, which migrate very quickly in an electrolyte solution. Due to the different migration speeds ionic species can accumulate in the half-cell of the electrolytic cell during the electrolysis.

At a temperature increase of 1 ° C, the conductivity increases by about 1-2.5%. The increase in the migration rate could be justified with a lower viscosity of the solvation shell around the ion or even with a decrease in the solvation shell around the ion.

Electrolysis of water

The electrolysis of the water separated in this the elements of oxygen and hydrogen. Like all electrolysis it consists of two partial reactions that take place at the two electrodes ( cathode and anode compartments ). The overall reaction scheme of this redox reaction is:

The electrodes are immersed in water, which is made ​​by the addition of acid or alkali, better conductive. Are the partial reactions

Cathode compartment: 2 H3O 2 e- → H2 2 H2O ( for acidic solutions) or: 2 H2O 2 e- → H2 2 OH - ( for basic solutions )

Anode compartment: 6 H2O → O2 4 H3O 4 e - ( for acidic solutions) or 4 - OH → O2 2 H2O 4 e - ( for basic solutions )

As a demonstration experiment, this reaction can be carried out in the Hofmann water electrolysis unit.

Electrolysis of zinc iodide

The electrolysis of zinc iodide this decomposed into the elements zinc and iodine. Like all electrolysis is also this part of two reactions taking place at the two electrodes ( cathode and anode chamber ). The overall reaction scheme of this redox reaction is:

The reactions of the individual electrode areas are:

Cathode compartment: Zn Zn2 2 e- →

Anode: 2 I- → I2 2 e-

Due to the energy supply, the individual ions move towards the electrodes. Zinc cations migrate to the cathode, it will be taken up by the zinc cation two electrons ( reduction), and is formed of elemental zinc. The iodine anions migrate to the anode and are oxidized to elemental iodine.

The History

The electrolysis was discovered in 1800, which invented by Alessandro Volta first usable battery was used, the Voltaic pile. The electrolysis newfound allowed Humphry Davy, in 1807 and 1808 several base metals first elementary manufacture over the years, for example, sodium and calcium. Michael Faraday investigated the electrolysis accurate and discovered its fundamental laws, namely the dependence of the unconverted masses of the charge amount and the molecular weight. At his suggestion, the terms electrolysis, electrode, electrolyte, anode, cathode, anion and cation were created. After the invention of powerful electrical generators led electrolysis end of the 19th century a rapid development in science and technology, such as in the electrolytic production of aluminum, chlorine and alkalis, and in explaining the behavior of electrolytes, including acids and bases are.

Experimental setup for electrolysis in an aqueous medium

Applications

Material recovery

The metals aluminum and magnesium are produced electrolytically using the fused-salt electrolysis. Electrochemical copper, silver and gold are also obtained, as well as to a large extent, zinc and nickel. Other alkali metals and alkaline earth metals, most are also obtained by electrolysis.

The halogens fluorine, bromine and chlorine are free of both case as well as in electrolysis in aqueous media, depending on the starting material, which are used in large scale for further synthesis.

In the chlor-alkali electrolysis chlorine, hydrogen and caustic soda is made from rock salt.

Electroplating

Electrolytic metal deposits are among the most important applications, either for the production of metallic coatings with galvanizing ( zinc galvanizing, chrome plating, etc.) or for the production and amplification of traces in the PCB production.

Electrolytic refining

The purification and separation of metals is achieved in the electrorefining by the fact that triggers an ( impure ) anode by an electric current selectively deposited only the purified metal at the cathode. This process is used in particular for the production of electrolytic copper, nickel and lead. Copper, because of its great ability in the liquid state to solve other substances are difficult to clean otherwise. Electrolytic copper having a purity of > 99.5% and is primarily used for electrical conductors. This purity is necessary, as in the lower copper dissolved impurities (oxygen, other metals), the conductivity greatly. Already in 1847 the possibility of electrolytic copper recovery of Maximilian Duke of Leuchtenberg was described. Only with the development of the dynamo M. Kiliani (1885 ) was able to demonstrate the technical feasibility.

In copper refining the cell voltage is about 280 mV ( mainly caused by power surges and the cell resistance ), the current density of about 0.21 A/cm2.

The electrorefining of copper provides the residues (sludge) on the bottom of Elektrolysiergefäßes other valuable materials, especially the noble metals gold and silver, as well as selenium and antimony. As alloying constituents they are not ionized by during the copper refining process on the anode voltage and low for them to fall during the dissolution of the anode to the ground. This anode mud is worked up in further separation steps on its valuable components out.

In refining crude lead is used ( introduction of the method in 1903 ) for the separation of arsenic, antimony and bismuth. Normally, the purified lead contains about 1% antimony, 0.5% bismuth and 0.01 % arsenic. The current density is approximately 0.16 to 0.2 cm 2, in this refining process.

In the nickel refining either crude nickel or nickel sulphide ( Ni2S3 ) is used as an anode.

Water decomposition

The elements hydrogen and oxygen are obtainable by water electrolysis in the electrolyzer principle. In general, it is cheaper to gain the basic chemical hydrogen from petroleum or natural gas and oxygen from the air, but may electrowinning at locations to be profitable where cheap electricity is available, eg in the vicinity of large hydropower plants. If natural gas and oil are scarce, it is expected that in the future the electrolysis of water increases by means of electrical energy to produce hydrogen as a chemical raw material and also as an energy carrier in importance. The energy efficiency of the electrolysis of water is over 70 %. For further information see the article electrolysis of water.

Kolbe electrolysis

The Kolbe electrolysis is the oldest example of an organic electrochemical reaction. In this electrolysis two carboxylic acid molecules are coupled with CO2 capture.

Other types

• Qualitative analysis, reaction kinetics: voltammetry and polarography.

Here we used the measurement of the electrolysis current depending on the voltage in order to obtain information on the chemical composition of the electrolyte.

• Quantitative analysis: Electrogravimetry and coulometry

The decomposition of the electrolyte caused by electric current is applied in the Electrogravimetry and coulometry to obtain information on the metal content of a sample.

• Wastewater treatment

In addition to the hydroxide precipitation and purification of waste water with ion exchangers electroplating, dye, pharmaceutical industry for the purification of polluted wastewater from the metal processing industry, applied electrochemical cleaning methods. At the anode, cyanide salts, organic compounds are rendered harmless by oxidation. At the cathode, for example, lead, arsenic and copper are removed by reduction, chromate is reduced to Cr3 . However, the problem with this method is the low conductivity and low concentration of metal ions. The so-called ECO - cell and cell - Chemelec have proven to wastewater treatment. The power requirement is, however, at the ECO cell per cubic meter of water about 4.5 kWh, including the power generation for the rotation. In this case, silver or copper concentrations of about 100 ppm can be reduced to 2 ppm.

• Electrochemical machining (ECM )

Electrochemical machining is also called electrochemical metal processing. The workpiece is connected as anode, and the metal then dissolves due to close proximity to the cathode. By the shape of the cathode, the separation can be affected at the anode. Suitable metals are aluminum, cobalt, molybdenum, nickel, titanium, tungsten, steel and iron alloys are. The electrolyte is sodium nitrate or sodium hydroxide. The current densities are in the process to 160 A/cm2.

• Isotope separation

In natural water some deuterium is included. Since deuterium reacts much more slowly than hydrogen at the cathode for mixed gas molecule deuterium hydrogen, deuterium can be electrolytically accumulate.

Poorly documented, the electrodynamic electrolysis, in which the ions are accelerated in a pulsating electric and magnetic field.

To optimize the efficiency of the electrolysis and increasing the temperature and photon irradiation can find use ( Solarhydrolyse ). Also, vacuum can be applied efficiency increasing in the extraction of gaseous electrolysis products.

Economy

According to the Federal Statistical Office, the following amounts of metals or chemicals in Germany were made in 2007.

In the U.S., the electrolysis products produced are higher by a factor of 2-3. There, about 5 % of total electricity production for the electrolysis are needed.

View

Electrolysis, with their very high levels of efficiency in material transformations could be next to renewable energy to a synergetic key method for mankind to future use of electricity from renewable sources for important chemical processes. Unlike the burning of finite fossil reserves of raw materials ( oil and gas) can be customized with solar power and inorganic and organic transformations perform in which the carbon dioxide emissions does not increase.

Possible the conversion of solar or wind energy into electricity. The current can be used for electrolytically producing hydrogen and oxygen. The reduction of carbon dioxide is also conceivable.