WO1979000233A1

WO1979000233A1 - Method of catalysis,hydrogen produced by the method,and a porous electrode

Info

Publication number: WO1979000233A1
Application number: PCT/GB1978/000027
Authority: WO
Inventors: A Tseung; M Man
Original assignee: Nat Res Dev; A Tseung; M Man
Priority date: 1977-10-25
Filing date: 1978-10-20
Publication date: 1979-05-03
Also published as: IT7869445A0; DE2861417D1; GB1556452A; EP0006933B1; CA1137921A; IT1108757B; US4279713A; EP0006933A1; BE871328A

Abstract

In chlor-alkali cells, the cathode may consist of a nickel screen carrying 15.6 mg Co2NiS4 bonded by 4.4 mg polytetrafluoroethylene per cm2.

Description

METHOD OF CATALYSIS, HYDROGEN PRODUCED BY THE METHOD, AND A

POROUS ELECTRODE This invention relates to a method of catalysis, hydrogen produced by the method and to a porous electrode (intended to be suitable for evolving gas). The invention may be used in industrial catalysis, for example in producing hydrogen from brine or chlor-alkali solutions.

Many electrolysers use planar or mesh electrodes and as such can only give low current densities. A porous electrode which could ensure that most of the electrode surfaces continue to function during gas evolution reaction would give significantly higher current densities. At present, in for example the field of alkali (including chlor-alkali) electrolysis, anodes can be such that the performance of a cell is limited by the cathode, at which hydrogen gas forms. Electrodes including a mixed cobalt/nickel oxide compound have been briefly described in UK Patent Specification No. 1461764, but it would be desirable to have electrodes with a higher activity. This invention arises from modifying that compound.

The invention is a method of ccatalysis using, as a catalyst, particles whose surfaces (to a depth of at least 2θX) are compounds between sulphur optionally including oxygen and at least two of cobalt, nickel, iron and manganese. For example, evolution of gaseous hydrogen (e.g. formed by electrolysing water) may be thus catalysed. Another aspect of the invention is operating an aqueous alkali electrolysis cell using the catalyst, preferably bonded together in porous fashion by a chemically inert polymeric binder, as a cathode, optionally permitting air to contact the cathode from time to time, in which cell hydrogen is evolved at the cathode.

The invention in another aspect is an electrode made from particles whose surfaces (to a depth of at least 20Å) are compounds between sulphur optionally including oxygen and at least two of cobalt, nickel, iron and manganese bonded together in porous fashion by a chemically inert polymeric binder. The compounds are preferably A_xB_4-2xS_{3.6 4}O_0.4-0 where x is from

0.05 to 1.95 and where A and B are any different two of cobalt, nickel, iron and manganese, for example cobalt and nickel. The binder may be polytetrafluoroethylene, and may represent from 1 to 10 parts (by weight) per 10 parts of the total compounds, preferably 2 to 6 parts.

Thus, a most preferred electrode has 3 parts of polytetrafluoroethylene binding 10 parts of Co₂NiS₄. The compounds may be made by treating the corresponding oxides with a sulphur-bearing compound, e.g. H S. The oxides may themselves have been made by a method ensuring small particle size, for example freeze-drying, and are described in UK Patent Specification No. 1461764. The invention will now be described by way of example.

In the accompanying drawings, Figure 1 is a graph illustrating performance obtained according to Example 1, and Figure 2 is a graph illustrating performance obtained according to Example 2. EXAMPLE 1

A 100 ml solution containing 39.49g of Co (NO₃)₂.6H₂O and

19.79g of Ni(NO₃)₂.6H₂O was sprayed onto liquid nitrogen. The frozen metallic salt solution was rapidly transferred to roundbottomed flasks containing liquid nitrogen and subjected to freeze-drying. After drying, the mixed nitrate powder was subjected to vacuum decomposition for three hours at 250ºC followed by thermal treatment in hydrogen sulphide at 350ºC for

8 hours, giving a compound approximating to Co₂NiS₄ , in practice about Co₂NiS₃ O_0.4.

Ten parts of the Co₂NiS₄ , which has a particle size in the region of 0.1μm, were mixed with 3 parts of polytetrafluoroethylene, in the form of a dispersion (60% PTFE content) sold by Imperial Chemical Industries of Britain under the trade mark ICI Fluon GPl, and with just enough de-ionised water to make into a paste-like slurry. The slurry was dispersed ultrasonically and then painted onto a 100 B.S. mesh nickel screen, allowed to dry in air for one hour at 100ºC and then cured in air at 300ºC for an hour. The cured assembly represents the desired electrode, and offered a Co₂NiS₄, loading of 15.6 mg (and 4.4mg polytetrafluoroethylene) per square centimetre.

The electrode was held potentiostatically at -173mV with reference to a dynamic hydrogen electrode in 5M KOH at 70ºC, with iR correction, an excessively large nickel screen being provided as anode. As may be seen from Figure 1, on the first day, the electrode passed about 750mA/cm². After being exposed overnight to air at 250, however, the electrode passed 1300mA/cm². This recovery even after exposure to air, shown in both

Examples, is an important advantage.

EXAMPLE 2

150 ml of an aqueous solution contained 24.4g CoCl₂.6H₂O and 12.13g of NiCl₂.6H₂O. This solution was added with constant stirring to 100 ml of 5M KOH, and the pH was adjusted until chloride ion could not be detected in the filtrate and finally the clean precipitate was heated in an over (containing air) at 400ºC for 21 hours, giving Co₂NiO₄.

The Co₂NiO₄ was heated to 500ºC and exposed for 5 hours to excess hydrogen sulphide, thus giving Co₂NiS₄ as was confirmed by analysis. In any event, it is the superficial composition (i.e. the top 20Å layer) which influences the electrode behaviour and whose composition must therefore be as defined.

Alternatively, and equally successfully, the freeze-drying method of Example 1 could have been used. The Co₂NiS₄ was made into a slurry, painted onto a nickel screen and cured, in similar fashion to Example 1.

The cured assembly represents the desired electrode, and in this case offered a Co₂Nis₄ loading of 22mg (and 9.3 mg polytetrafluoroethylene) per square centimetre.

The electrode was held potentiostatically at -300mV with reference to a dynamic hydrogen electrode in 5-M KOH at 70ºC, with iR correction, an excessively large nickel screen being provided as the counter electrode (anode). As may be seen from Figure 2, the electrode was able to pass a current of 1150mA/cm² even after

10 hours use. Initially, the current was somewhat lower, at about

1050mA/cm²; if the electrode was used and then left in air for 24 hours, the performance on resuming use was 850mA/cm², rising to 1050mA/cm after about 6 hours. EXAMPLE 3

Example 1 was repeated with the difference that in making the paste-like slurry, methanol was used in place of the de-ionised water. The Co NiS, loading was also much higher, at about 40 to 60 mg/cm² on the electrode. The electrode was held potentiostatically at -75 eaV with reference to a reversible hydrogen electrode, at 70ºC NaOH (but otherwise as in Example 1), and gave 250mA/cm², (iR corrected) a significant improvement on mild steel cathodes.

In another experiment, the electrode was held at 95ºC in a typical chlor-alkali solution 15% NaOH+17%NaCl) and set to allow a steady 250mA/cm to pass. This current density was sustained for over 400 hours, with a reasonably steady half cell voltage (i.e. - 80mV with reference to a reversible hydrogen electrode). These results suggest that the invention could be exploited in industry by, for example, providing an alternative to mild steel cathodes in chlor-alkali electrolysis.

Claims

1. A method of catalysis, using as catalyst, particles whose surfaces (to depth of at least 20A) are compounds between sulphur optionally including oxygen and at least two of cobalt, nickel, iron and manganese.

2. A method according to Claim 1, wherein what is catalysed is evolution of gaseous hydrogen.

3- A method according to Claim 2, wherein the hydrogen is formed by electrolysing water.

4. A method according to Claim 3, when performed by operating an aqueous alkali electrolysis cell, in which the hydrogen is evolved at the cathode, which comprises the catalyst.

5. A method according to Claim 4, wherein the cathode comprises said particles bonded together in porous fashion by a chemically inert polymeric binder.

6. A method according to Claim 4 or 5, wherein air is permitted to contact the cathode from time to time.

7. Hydrogen obtained by a method according to any of Claims 2 to 6.

8. An electrode made from particles whose surfaces (to a depth of at least 20A) are compounds between sulphur optionally including oxygen and at least two of cobalt, nickel, iron and manganese bonded together in porous fashion by a chemically inert polymeric binder.

9. An electrode as claimed in Claim 8, wherein the compounds are A_xB_4-2xS_{3.6 - 4}O_{0.4 - 0} where x is from 0.05 to 1.95 and where A and B are any different two of cobalt, nickel, iron and manganese.

10. An electrode as claimed in Claim 9, wherein A is nickel, and B is cobalt.

11. An electrode as claimed in Claim 8, 9 or 10, wherein the binder is polytetrafluoroethylene.

12. An electrode as claimed in any of Claims 8 to 11, wherein the binder represents from 1 to 10 parts by weight per 10 parts of the total compounds.

13. An electrode as claimed in Claim 12, wherein the binder represents from 2 to 6 parts per 10 of the total compounds.

14. An electrode as claimed in Claim 13, wherein substantially 3 parts of polytetrafluoroethylene bind 10 parts of Co₂NiS₄.

15- An electrode as claimed in any of Claims 8 to 14, made by treating the corresponding oxide compounds with a sulphur-bearing compound such as H₂S. l6. A method according to any of Claims 1 to 6 when performed using an electrode as claimed in any of Claims 8 to 15.