Electrochemistry

Description

The electrochemical reactions are the phenomena which take place with the interface of the two conducting systems (electronics and ionic) during the transfer of load composed of one or more electrons. These transfers of charge are accompanied by modification of the states of oxidation of materials (oxidation or reduction) and thus of their physicochemical nature (metal deposit, gas evolution, formation of ridicalizing species, chemical reactions coupled…). The whole of the elementary reactions can thus reach an high level of complexity. Electrochemistry makes it possible to better apprehend the phenomena of Oxydo-réduction and Corrosion.

Note:: a junction PN between two Semi-conducteur S is not spring of electrochemistry.

History of electrochemistry

Great scopes of application

One generally classifies the industrial applications of electrochemistry in 5 main categories:

Electrosynthesis

The electrosynthesis is sometimes used in heavy chemical industry with the detriment of a synthesis per thermal way, the processes of electrosynthesis being generally more easily controllable and selective. The principal raw materials produced by electrosynthesis are: the Aluminum (approx. 24 Mtonnes/an), the di Chlore and the Soude (approx. 40 Mtonnes/an). One also produces in less quantity of the di Fluor, of the Lithium, the Sodium, the Magnésium and the Dihydrogène.

One will arrange in the same category the purification of certain metals by électroaffinage (in particular the Cuivre, the Zinc and the Aluminum).

Surface treatment and corrosion

The surface treatments and the corrosion . The surface treatments per electrochemical way are numerous because electrochemistry makes it possible to control well the nature and the quality of the deposit. This metal deposit (nickel, zinc, gold…) of some micrometers thickness (1 to 10 micrometers) an esthetic role or of protection against the Corrosion plays. The electrochemical methods can also be used with the restoration as old objects.

Storage and the transformation energy

The Piles and Electric fencer are Générateur S electrochemical. The accumulators are distinguished from the piles by the fact that they are electrically refillable. Their field is in full expansion.

In applications of the type “general public” like the batteries for cellphones.
In the professional applications, most current is the lead-acid batteries, they amongst other things ensure the auxiliary role of energy source of the vehicles Automobile S allowing, their startup.
Of other types of accumulators, more sophisticated, starts to play a great part in the hybrid vehicles ; they store the energy recovered via generators during brakings and, restore it with electric motors at the time of the phases of acceleration: example, certain models Toyota.
In addition, of many research is today carried out in the field of the combustible batteries in order to equip these same vehicles. This although the resource clean Hydrogène is still hypothetical.

The Supercondensateur S are condensers (capacities) ready to quickly accumulate a great quantity of electrical energy then to be used as generators. The electrodes are not reactive, one speaks about blocking electrode. The ions of the electrolyte accumulate with the electrodes with the load and restore the electroneutrality during the discharge.

Methods of analysis and of measurement

Because of their low costs, one uses more and more electrochemical Capteur S. Simplest of them is the electrode with pH. More used is the sensor with Dioxygène, in particular for the analysis of the Gaz of Combustion. The electrochemical sensors have also many applications in the field Biomédical or for the analysis of the Pollution.

The measuring device most useful for electrochemistry is called the Potentiostat or galvanostat.

The electrochemical cell most current is the cell with three electrodes: The electrode of work and against electrode between which the power is on. The electrode of reference (or impolarizable) which makes it possible to evaluate the potential difference between the electrode of work and the electrolyte.

Environment and biology

In this field in strong expansion, the electrochemical techniques allow separation (electrodialysis), the Récupération, the concentration or the Destruction of certain elements. An typical example of application is the desalination of the brackish water by electrodialysis.

Great concepts in electrochemistry

The complexity of the electrochemical phenomena combining at the stages of transfer of elementary charge (the departure or the arrival of a electron, according to whether one studies a Oxydation or a Réduction) of the chemical reactions of transfer of matter, can be studied in an analytical and detailed way or in a total way. In the first approach one will deal with each elementary stage, will study his kinetics and his interaction on the preceding or following stages. In the second approach one will study great balances of a total reaction, to carry out the energy assessments and thermics, it is the thermodynamic approach.

Thermodynamic electrochemical

Kinetic electrochemical

Mechanism of reaction. Square diagram

The most rigorous method to analyze all the reactional ways of an electrochemical reaction coupled to chemical reactions is to represent on a horizontal axis the elementary electrochemical reactions and on a vertical axis the coupled chemical reactions. (Method of Jacq).

Let us analyze for example the oxidation of the nickel hydroxide in alkaline aqueous medium:

It simply appears that is formed by monoelectronic oxidation and loss of a proton the form oxy-hydroxy NiOOH with valence III, then another oxidation with loss of the second proton led to oxide NiO₂ Nickel to valence IV. The existence of various reactional ways associated with various intermediate ionic species can be considered; their identification remains very complex in this case.

Notes and references of the article

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