EP0096837B1 - Process for the production of a tungsten carbide activated electrode - Google Patents

Abstract

A process is disclosed for the production of a tungsten carbide-activated electrode, particularly an electrode which is utilizable as a cathode, and which consists of an electrically-conductive substrate with an active surface layer of tungsten carbide. The tungsten carbide is adhesively bonded through chemical reaction to the surface of a substrate which is constituted of graphite or a graphite-like material. The graphite-like material is a graphite binder mixture, whose binder material components, other than the graphitic filler granules, are only carbonized, and may not be completely graphited.

Description

The invention relates to a method for producing a tungsten carbide-activated electrode, which can be used in particular as a cathode and which consists of an electrically conductive carrier material with a surface-active layer consisting exclusively of tungsten carbide.

Electrodes of the aforementioned type are required as cathodes in the electrolysis for hydrogen generation in an acidic environment in general and in particular in the electrolysis in sulfuric acid solution or in the cathodic generation of hydrogen with simultaneous anodic oxidation of sulfur dioxide in sulfuric acid electrolyte. Special examples are the electrochemical oxidation of S0 ₂ from exhaust gases in the production of H ₂ as well as the analog electrolysis in the sulfuric acid hybrid cycle.

A known method for producing tungsten carbide-activated electrodes is that tungsten carbide is first produced and this is applied to the carrier as a powder by means of a binder (polyimide or polysulphone). However, electrodes manufactured in this way do not show a sufficiently high electrochemical activity.

A further method is known by which cathodes with a sufficiently high electrochemical activity can be produced (P. Cavallotti in Hydrogen as an Energy Vector, edit. AA Strub and G. Imarisio, D. Reidel Publishing Company, Dordrecht / Boston / London, 1980, p. 408, EUR 6783). According to this known method, a tungsten carbide-Teflon binder coating is applied to a gold-coated carrier. However, this procedure is very expensive due to the use of the noble metal.

Another known method for producing a tungsten carbide-activated electrode consists in that active tungsten carbide with graphite powder is cold-pressed to the electrode (H. Böhm, Chem.-Ing.-Techn. 49 (1977, 328). However, the cathodes produced in this way only result in relatively low cathodic current densities, which suggests an unfavorable structure of the electrodes.

From DE-B-1 299 287 an electrode, in particular a cathode for the electrolytic decomposition of hydrochloric acid is also known, which has a coating containing tungsten carbide. However, this coating, which is applied to the carrier by flame spraying, contains metallic cobalt or nickel in addition to tungsten carbide.

It is an object of the invention to provide a method for producing a tungsten carbide-activated electrode of the type mentioned at the outset, in which the use of expensive precious metals is avoided and which nevertheless leads to electrodes with high electrochemical activity which are stable in the long term.

According to the invention, this object is achieved in that tungsten metal or a tungsten compound is applied to the surface of the carrier consisting of graphite or a graphite-like material and converted into tungsten carbide and thereby chemically adhesively bonded to the carrier. A graphite-like material is understood to mean a graphite binder mixture whose binder components - unlike the graphite filler grains - are only coked, but at least not completely graphitized.

The method according to the invention can be carried out in various ways. Thus, an adhesive bond between the support and the tungsten carbide layer, which is a bond between solids achieved by chemical reaction, is achieved by evaporating tungsten metal and then carburizing to form tungsten carbide, which as such is bonded to the support.

An advantageous procedure according to the invention is furthermore that tungsten oxide or a tungsten compound which is thermally decomposable to tungsten oxide is applied to the carrier, and that the tungsten oxide or the tungsten compound is converted to tungsten oxide by reducing and carburizing on the carrier into tungsten carbide and thereby thereby as such is connected to the carrier. The amount of tungsten oxide applied to the carrier is 50 to 350 mg per cm ² .

According to the method according to the invention, the tungsten compound (the tungsten oxide or the tungsten compound which can be decomposed to tungsten oxide) can be applied directly to the support - that is to say without using a binder - or using a coking binder. The procedure is advantageously such that a thin binder layer is first applied to the carrier and the tungsten compound is applied to this binder layer. The thickness of the layer is to be dimensioned such that it is just sufficient for the adhesion of the tungsten compound to be applied and for the formation of the adhesive compound, but this prevents a part of the tungsten oxide or the tungsten compound from being covered by binders. A particularly high electrochemical activity of the electrode is achieved by this procedure.

The binder is either completely consumed during the carburization phase or, insofar as part of the binder remains, for example as a layer between the tungsten carbide formed and the support, coked. Since - as has been shown - the coked binder is conductive, the formation of a layer consisting of coked binder between tungsten carbide and the carrier of the electrochemical activity is practically not detrimental. However, it may be advantageous to use a binder which has been admixed with graphite powder in order to increase the conductivity of the binder coked after carburizing. It may also be expedient for the binder to be a phenolic resin.

A particularly advantageous variant of the method according to the invention is that the carrier is a green body formed from a binder-containing graphite powder, the carrier having a binder content of 10 to 40% by weight. Because a binder is added to the filler grains of the green body, there is also a layer of binder on its surface after the carrier has been produced, so that in this case there is no need to apply an additional binder layer.

For applications in which a porous electrode is required, it may be appropriate for the support to be a porous diaphragm made of graphite. In such a case, the tungsten compound is expediently applied to the carrier using a binder.

A tungsten oxide powder which has been produced from para-ammonium tungstate via a precipitation reaction to tungstic acid and subsequent decomposition in air at approximately 550 ° C. or direct thermal decomposition of para-ammonium tungstate and whose specific surface area is greater than 10 is advantageously used for application to the carrier m ² / g. A further procedure according to the invention, however, is also that tungsten acid or para-ammonium tungstate is applied to the carrier, and this is thermally decomposed to tungsten oxide on the carrier before reducing and carburizing.

To carry out the step of reducing the tungsten oxide, the procedure is expediently carried out in such a way that the support with the tungsten oxide thereon is brought to reduce the tungsten oxide under a hydrogen atmosphere or a hydrogen inert gas atmosphere and is heated to a temperature of 370 to 580 ° C. The flowing gas is generally 1 to 3 l hydrogen / h per cm ² cross-sectional area of the vessel in which the carrier is located. The heating takes place continuously in 1 to 3 hours from 20 ° C. The tungsten oxide (W0 ₃ ) is reduced to a mixture of different oxide phases with the composition WO _X (0.33 ≦ x ≦ 3).

After reducing the tungsten oxide and before carburizing, the sample is expediently carried out in an inert gas atmosphere (e.g. about 21 / h flowing argon per cm ² cross-sectional area of the vessel) in about 0.7 to 2 hours at 1 to 2 ° K / min Heated up to 620 ° C. For carburization, the carrier with the tungsten oxide located thereon is then heated in the temperature range from 620 to 950 ° C. at a temperature rise of approximately 1 ° K / min under a flowing CO / CO ₂ atmosphere. The volume ratio of the CO to CO ₂ is expediently about 10: 1, the volume flow about 1.5 I / h per cm ² cross-sectional area of the vessel. After the final temperature has been reached, this is maintained for up to 4 hours, depending on the type of previous process steps, whether the tungsten compound has been applied, for example, with or without a binder, and the thickness of the binder layer. If possible, the carburization time should last at most until all tungsten oxide has been converted into tungsten carbide. In order to achieve the highest possible electrochemical activity of the electrode, however, it may be expedient not to convert the tungsten oxide applied completely into tungsten carbide, but to leave a portion, albeit very small, of tungsten oxide.

It may furthermore be expedient that after the carburization phase, the carrier with the tungsten carbide layer formed is kept at 750 ° C. under flowing hydrogen gas in order to reduce any carbon present on the carrier surface.

Advantageous uses of the electrode produced by the method according to the invention consist in its use as a cathode in an electrolysis cell for carrying out the sulfuric acid hybrid cycle process or as a cathode in an electrolysis cell for the anodic oxidation of SO ₂ exhaust gases with simultaneous cathodic H ₂ production.

Embodiment 1

A carrier was pre-pressed from a mixture of electrical and natural graphite powder with a binder content of 20% by weight (phenolic resin). About 170 mg / cm ^{2 of} tungsten oxide were applied uniformly to this support. The pre-pressed carrier with the applied tungsten oxide was then pressed under a pressure of 200 MPA for 15 seconds at room temperature. The compacts had a diameter of 16 mm (F = 2 cm ² ) and a thickness of about 2.5 mm.

The compacts were then continuously heated in a quartz tube (inner diameter = 35 mm, F = 10 cm 2) under flowing hydrogen (20 IH ₂ / h) from 20 ° C. to 370 ° C. in about 2 hours, so that the WO ₃ became WO _x (0.33 = x = 3) was reduced.

Argon (20 l / h) was then passed through the quartz tube and the compacts were further heated up to 500.degree. The heating rate was 2.5 ° K / min. For the carburization, the compacts were heated from 580 ° C with a temperature increase of 1.4 ° K / min to 760 ° C, whereby a gas mixture of CO and C0 ₂ (CO / C0 ₂ volume ratio 9: 1; volume flow up to 15 l / h) was passed through the pipe.

The finished electrodes with the tungsten carbide-activated surface were examined electrochemically at 80 ° C. in 50% by weight H ₂ SO ₄ . At -100 mV against the reversible hydrogen electrode (RHE) in the same solution, the cathodic current density of the hydrogen evolution was recorded as a measure of the activity of the electrode. A current density of 210 mA / cm ^{2 was} achieved under the test conditions mentioned. The electrode was operated under the aforementioned conditions for 1000 hours and remained practically without weight loss.

Embodiment 2

A carrier made of electrographite was coated with a 20% (w / w) solution of phenolic resin Spread binder in methanol. The resulting very thin binder layer was then coated uniformly with tungsten oxide powder (170 mg / cm ² ).

The reduction and carburization of the tungsten oxide layer was carried out as described in exemplary embodiment 1.

The subsequent cathodic measurement of the electrode under the test conditions described in Example 1 resulted in a current density of 150 mA / cm ² at -100 mV against RHE.

Embodiment 3

A vertically standing quartz tube with an inner diameter of 30 mm and a length of 900 mm was used as the device for carrying out the method, which could be heated in its lower part by means of a resistance furnace and in the upper part by means of an induction coil. A crucible with tungsten hexachloride (WC1 ₆ ) was placed in the middle of the resistance-heated area and a graphite disc with a diameter of 16 mm and a thickness of 2 mm hung with tungsten wire was placed in the area of the induction coil.

To avoid heating and cooling effects, the WC1 _{6 was} first heated to about 240 ° C in a stationary argon / hydrogen atmosphere and then the graphite disc was quickly inductively heated to about 650 ° C. The gas flow was then started from bottom to top and an argon / hydrogen gas mixture (4% hydrogen content) was passed through the quartz tube at a rate of 10 l / min. The exhaust gases were neutralized with NaOH in a wash bottle downstream of the quartz tube.

In this way, the WC1 ₆ vapor with the H _{2 of} the carrier gas on the surface of the graphite disc was reduced to tungsten metal and the surface was coated. Under the set conditions, a layer thickness of about 10 µm was generated in two minutes, regardless of the roughness of the surface of the graphite disc. With a coating time of about 3 minutes, a layer thickness of 15 _{1 1} m was achieved.

The graphite disc coated in this way was then held in the same quartz tube in a H ₂ / HCl gas mixture (mixing ratio 8: 2) with a proportion of about 1% propane at a temperature of 750 ° C. for 30 minutes, during which the metallic tungsten became tungsten carbide reacted and was thereby adhesively bound to the graphitic base.

The graphite disc coated with tungsten carbide was examined for its suitability as a cathode. At 80 ° C in 50 wt.% H ₂ S0 ₄ and at -100 mV voltage against the reversible hydrogen electrode (RHE), a cathodic current density of hydrogen evolution of 180 mA / cm ^{2 was} measured.

Claims (18)

1. Process for the production of a tungsten carbide activated electrode which is usable especially as a cathode and which consists of an electrically conductive support material with a surface- active layer consisting exclusively of tungsten carbide, characterised in that tungsten metal or a tungsten compound is applied on to the surface of the support consisting of graphite or of a graphite- like material, and converted into tungsten carbide, and in so doing is connected chemically adhesively to the support.

2. Process according to claim 1, characterised in that tungsten metal is applied by vapour coating on to the support, then the tungsten is converted to tungsten carbide by carburisation.

3. Process according to claim 1, characterised in that tungsten oxide is applied on to the support, and converted to tungsten carbide by reduction and carburisation on the support.

4. Process according to claim 1, characterised in that a tungsten compound thermally decomposable to tungsten oxide is applied to the support, thermally decomposed to give tungsten oxide, and converted to tungsten carbide by reduction and carburisation on the support.

5. Process according to claim 3, characterised in that the tungsten oxide powder is applied to the support with the use of a binder which is capable of being coked.

6. Process according to claim 5, characterised in that a binder is used into which graphite powder has been admixed for increasing the conductivity of the binder coked after the carburisation.

7. Process according to claim 5 or 6, characterised in that the binder is a phenol synthetic resin.

8. Process according to one of claims 3 to 7, characterised in that the support is a green body formed from a graphite powder containing binder.

9. Process according to claim 8, characterised in that the support has a binder content of 10 to 40% by weight.

10. Process according to one of claims 1 to 7, characterised in that the support is a porous graphite diaphragm.

11. Process according to one of claims 3 and also 5 to 10, characterised in that a tungsten oxide powder is used which has been produced from para-ammonium tungstate by way of a precipitation reaction to tungstic acid and subsequent decomposition in air at about 550 °C or by direct thermal decomposition of p-ammonium tungstate.

12. Process according to claim 11, characterised in that the specific surface of the tungsten oxide powder is greater than 10 m²/g.

13. Process according to one of claims 4 to 9, characterised in that tungstic acid or para-ammonium tungstate is applied on to the support, and thermally decomposed to give tungsten oxide before reduction and carburisation on the support.

14. Process according to one of claims 3 and 5 to 13, characterised in that the support with the tungsten oxide situated thereon is brought under a hydrogen atmosphere or hydrogen and inert gas atmosphere for reduction of the tungsten oxide, and heated to a temperature of 370 to 580 °C.

15. Process according to one of claims 3 to 14, characterised in that the support with the tungsten oxide situated thereon is heated, for the carburisation of said tungsten oxide, in the temperature range of 620 to 950 °C with a temperature rise of about 1 °K/min under a flowing CO/C0₂ atmosphere.

16. Process according to claim 15, characterised in that after the carburisation phase the support with the tungsten carbide layer formed thereon is held under flowing hydrogen gas at 750 °C for reduction of carbon possibly present at the support surface.

17. Use of a tungsten carbide activated electrode produced according to the preceding process claims as cathode in an electrolysis cell for carrying out the sulphuric acid hybrid cyclic process.

18. Use of a tungsten carbide activated electrode produced according to the preceding process claims as cathode in an electrolysis cell for anodic oxidation of S0₂ exhaust gases with simultaneous cathodic H₂ production.