GB2129829A

GB2129829A - Catalytic activation of electrodes by "in-situ" formation of electrocatalysts

Info

Publication number: GB2129829A
Application number: GB08329793A
Authority: GB
Inventors: Hartmut Wendt; Jurgen Kuelps; Helmut Schneider; Hans Hofmann
Original assignee: European Atomic Energy Community Euratom
Current assignee: European Atomic Energy Community Euratom
Priority date: 1982-11-12
Filing date: 1983-11-08
Publication date: 1984-05-23
Also published as: GB8329793D0; FR2536091A1; DE3339835A1; IT1170575B; LU84466A1

Abstract

A process for the catalytic activation of electrodes, is characterised in that a layer of the electrocatalyst adhering to the surface of the electrode is applied directly in the electrolyzer and under operating conditions (pressure, temperature, electrolyte composition, current density and/or electrode potential) which are substantially equivalent to the normal operating conditions of the desired electrolysis.

Description

SPECIFICATION Process for the catalytic activation of anodes and cathodes by "in-situ" formation of electrocatalysts under indentical or approximate process conditions The invention relates to a process for the catalytic activation of electrodes.

Electrode activation is carried out in a variety of processes. One example of cathode activation is the coating of electrodes by cathodic deposition of an electro-catalyticallyactive metal or a metal mixture or alloy, one component of which, for the purpose of increasing the specific surface, can also be dissolved out subsequently either chemically or electrochemically, as well as coating by plasma spraying, vacuum depositin or iron implantation.

The so-called spray-sinter process is mentioned by way of example for anode activation. In this, solutions of the salts of metals, whose ions are required for the formation of the-typically-oxidic catalyst, are applied on to the electrode surface by spraying, painting or dipping, then dried and converted by heating or sintering under controlled conditions of temperature and atmosphere into oxide layers, for example, which constitute the catalyst and have a more or less accurately defined composition.

All these processes are characterised in that the activation of the electrodes is carried out at a separate time and place from the electro lysis process.

Only in exceptional cases is it possible, in the formation particularly of oxidic catalysts, to produce a catalyst which is thermodynamically and mechanically stable during subsequent use in the electrolysis process under the specified process conditions (electrode potential, electrolyte composition, etc.).

Frequently, the result of this is the unsatisfactory or not fully satisfactory long-term stability of the electrocatalyst, which necessitates frequent reactivation of the catalyst.

The process according to the-invention is characterised in that the layer of the electrocatalyst adhering to the surface of the electrode is applied directly in the electrolyzer and under operating conditions (pressure, temperature, electrolyte composition, current density and/or electrode potential) which are substantially equivalent to the normal operating conditions of the desired electrolysis.

In the process according to the invention the electrolysis is performed or continued under only slightly modified conditions in which, for example, metal ions are added to the electrolyte, for example ions of transition metals or other cations or anion or molecules, which are necessary for chemical or electrochemical formation of the desired electrocata lyst, in the form of salts or complexes or in the form of more or less soluble compounds in an as low as possible concentration and in the minimum quantity sufficient to form an effective catalyst layer. For this purpose, in many cases it is possible to perform the activation within relatively short periods, without substantially altering the composition of the electrolyte. As a result of such anodic (e.g.

for oxidic catalysts) or cathodic (e.g. for metallic catalysts) in-situ formation of electrocatalysts, the components of the catalyst arrive at the electrodes along a path via the electrolyte (i.e. via the chain: homogeneous solution, material transport to the electrode, formation by cathodic or anodic creation and deposition of the electrocatalyst), which renders superfluous any additional mechanical operation, for example spraying.

As a result of the in-situ coating according to the invention, it is achieved that in accordance with the formation conditions, which substantially correspond to the working conditions of electrolysis, the electrocatalyst is deposited on the electrode in a form (chemical composition, modification, atomic, microscopic and macroscopic structure) in which it exhibits the greatest possible chemical and mechanical stability under the desired operating conditions.

Therefore, besides the formation of the thermodynamically most stable phase, the deposition of a mechanically very stable structure takes place, since the deposition of the electrocatalyst occurs under mechanical conditions which also substantially correspond to the practical electrolysis conditions (e.g. bubbling at gas-evolving electrodes or high relative movement of the electrolyte in relation to the electrode as a result of recirculation or gassiphoning action of evolved gases).

With the two prerequisites of a) thermodynamic stability of the electrocatalyst, and b) mechanical stability of the electrocatalyst, under the desired electrolysis conditions, which are possible as a result of the in-situ formation of electrocatalysts according to the invention, there is given the greatest possible assurance of the long-term stability of the electrocatalyst against chemically and electrochemically-induced modification, deactivation and also mechanical erosion and abrasion.

Moreover, the process of the invention enables reactivation of the electrode to be effected at any time, after possible deactivation has occurred, without interrupting the ongoing electrolysis operation.

Example 1 Simultaneous "in-situ" activation of anodes and cathodes for alkaline water electrolysis.

In a 4 cm2 microcell with a sandwich arrangement of anode, diaphragm and cathode, water was electrolysed in 50% by weight of KOH at 90"C. The nominal current density amounted to 1 A/cm2.

After 200 working hours the cathode had an over-voltage of - 350 millivolt and the anode had an overvoltage of + 350 mV.

Cobalt nitrate solution was added to the electrolyte (1 1, 50% by weight of KOH) in a quantity which corresponded to 65 mg of cobalt. The cobalt was immediately dissolved in the electrolyte in the form of a deep-blue coloured cobalt-ll-hydroxy complex, the colour of which disappeared within a few minutes.

After 40 minutes, the overvoltage had improved by 20 millivolt, whereas the improvement of the anode potential amounted to 50 millivolt. During the next 300 working hours no further increase of the anodic and cathodic overvoltage was observed.

Example 2 Activation of the anode for alkaline water electrolysis by "in-situ" deposition of a cobaltmanganese mixed oxide on the anode.

In an experiment, as described in Example 1, after 350 working hours cobalt nitrate and manganese sulphate were added to 1 1 alkaline electrolyte in an amount which corresponded to 20 mg of cobalt and 20 mg of manganese. The salts were dissolved in 2 1 water and during the following 30 hours were added to the electrolyte in proportion to the loss of water, the concentrations of both cobalt and manganese in the alkaline electrolyte never exceeding 10-5 mol/1. During the following 40 hours the anodic overvoltage decreased by 90 mV and remained unchanged for the next 200 hours.

Example 3 Activation of the cathode for alkaline water electrolysis by "in-situ" deposition of finely divided iron whiskers on the cathode.

In an experiment, as described in Example 1, 1 gm of Fe203, which has a solubility of about 10-3 mol/l at 90"C in 50% by weight of KOH, was added to 1 1 of the electrolyte.

After 30 hours, fine iron needles had been deposited on the cathode and the cathodic overvoltage had dropped by 100 mV (to-250 mV).

Example 4 Activation of the cathode for alkaline water electrolysis by "in-situ" deposition of zinc on a cathode already coated with finely divided Raney nickel.

In a experiment, as described in Example 1, a perforateds nickel sheet was used as cathode, on to which was deposited 11 5 mg per cm2 of finely divided Raney nickel (an application according to [2]). After 100 working hours, the cathodic overvoltage amounted to - 225 millivolt (90"C, 50% by weight of KOH, 1 A/cm2).

After adding ZnO (which immediately dissolved as a zinc hydroxo-complex) in an amount which corresponds to 0.01 mol/l of zinc in alkaline electrolyte, the cathodic overvoltage dropped, after a temporary rise, by a further 60 mV within 20 hours and remained unchanged for the next 50 hours.

Claims

1. A process for the catalytic activation of electrodes, characterised in that a layer of the electrocatalyst adhering to the surface of the electrode is applied directly in the electrolyzer and under operating conditions (pressure, temperature, electrolyte composition, current density and/or electrode potential) which are substantially equivalent to the normal operating conditions of the desired electrolysis.

2. A process as claimed in claim 1 wherein the electrocatalyst comprises a transition metal.

3. A process as claimed in claim 1 substantially as hereinbefore described.

4. A process as claimed in claim 1 substantially as hereinbefore described in any one of the specific Examples.