Chemistry: electrical and wave energy – Processes and products
Patent
1986-09-29
1988-02-09
Tufariello, T. M.
Chemistry: electrical and wave energy
Processes and products
204 98, C25D 1500
Patent
active
047240520
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION OF THE INVENTION
The present invention relates to a method for preparing electrodes for use in electrochemical process, in particular for use in ion exchange membrane or permeable diaphragm cells for the electrolysis of alkali metal halides and more particularly as cathodes for hydrogen evolution in the presence of alkali metal hydroxide solutions.
Further, the present invention relates to the electrodes which are obtainable by the above method.
The main requisites for industrial cathodes are a low hydrogen overvoltage, which results in a reduction of energy consumption, as well as a suitable mechanical stability under the stresses which may occur during assembly or due to the turbulence of the liquids during operation.
Cathodes which fulfil the above requirements are constituted by a support of a suitable conductive material, such as iron, steel, stainless steel, nickel and alloys thereof, copper and alloys thereof, whereto an electrocatalytic conductive coating is applied.
Said electrocatalytic conductive coating may be applied, among various methods, by galvanic or electroless deposition of metal or metal alloys, which are electroconductive, but only partially electrocatalytic per se, such as nickel or alloys thereof, copper or alloys thereof, silver or alloys thereof, containing metals of the platinum group exhibiting low hydrogen overvoltages, these metals being present in the coating as a homogeneous phase, most probably as a solid solution.
As an alternative, the electrocatalytic coating may be obtained by galvanic or electroless deposition of an electrically conductive metal, only partially electrocatalytic per se, such as nickel, copper, silver and alloys thereof as aforementioned, which contains dispersed therein particles of an electrocatalytic material exhibiting a low overvoltage to hydrogen evolution. The electrocatalytic particles may consist of elements belonging to the group comprising: titanium, zirconium, niobium, hafnium, tantalum, metals of the platinum group, nickel, cobalt, tin, manganese, as metals or alloys thereof, oxides thereof, mixed oxides, borides, nitrides, carbides, sulphides, and are added and held in suspension in the plating baths utilized for the deposition.
Examples of electrodes having a coating containing dispersed electrocatalytic particles are illustrated in Belgian Pat. No. 848,458, corresponding to Italian patent application No. 29506 A/76, and in U.S. Pat. No. 4,465,580 which are incorporated herein by reference.
A particularly serious drawback connected to the use of the aforementioned electrodes, when used as cathodes in diaphragm or ion exchange membrane cells for alkali halides electrolysis, is constituted by the progressive poisoning of the catalytic surface caused by metal ions contained in the electrolyte, with the consequent gradual increase of the hydrogen overvoltage. The process efficiency results therefore negatively affected, which represents a particularly critical problem involving the necessity of periodical substitution of the cathodes.
Metal impurities which are normally responsible for the poisoning comprise Fe, Co, Ni, Pb, Hg, Sn, Sb or the like.
In the specific case of brine electrolysis in membrane cells, the metal impurities are more frequently represented by iron and mercury.
Iron impurities may have two origins: ferrocyanide, added as anti-caking agent. cathodic compartment and accessories thereof.
Mercury is found in the brine circuit after conversion of mercury cells to membrane cells.
As soon as these impurities, which are usually present in solution under a complex form, diffuse to the cathode surface, they are readily electroprecipitated to the metal state, so that a poorly electrocatalytic layer is built up in a relatively short time.
This catalytic aging, which depends on various factors such as the type of cathodic material (composition and structural), working conditions (temperature, catholyte concentration), and the nature of the impurity, results remarkable and irreversible soon after a short time of operation ev
REFERENCES:
patent: 4496442 (1985-01-01), Okazaki
Oronzio de Nora Impianti Elettrochimici S.p.A.
Tufariello T. M.
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