CA2067719C - Nanocrystal powders of an electro-active alloy and preparation process thereof - Google Patents

Abstract

Powders are described comprising nanocrystals agglomerated with an electroactive alloy. The main element of said alloy can be nickel, cobalt, iron or mixtures of the latter, while the alloyed element consists of one or more transition metals, in particular Mo, W, V, the alloy comprising also oxygen. Preferably, the nanocrystals are made of an alloy of nickel, molybdenum and oxygen. An electrode is also disclosed which can be prepared by compacting the powders. In addition, there is disclosed a process for the preparation of powders from particles of nickel, cobalt and iron and / or the oxides thereof as well as particles of at least one transition metal (Mo, W, V) and / or the oxides thereof, mechanical high-energy grinding is carried out, in particular by ball grinding under conditions comprising oxygen and for a period of time allowing a nanocrystalline alloy to be obtained. The electrodes produced from these powders have an electro-catalytic activity for the production of hydrogen comparable or superior to the electrodes which are presently used in the electrochemical industry. In addition, these materials have excellent chemical, electrochemical and mechanical stability. When used as a cathode the powders are useful in water electrolysers, in chlorine-alkaline cells or others.

Description

208771 9 ~
This invention relates to powders which can be used for manufacturing electrodes for the production of hydrogen by electrolysis, especially in electrolysers water, chlorates, and can also be used in chlor-alkaline or other cells. More specifically, the invention relates to: the manufacture of nanocrystalline nickel alloy powders, molybdenum and oxygen by mechanical deformations intense, said powders having a high electro-catalytic activity for evolution of hydrogen when used under electrolysers water, chlorine-alkaline cells, chlorate or other.
We know that we can succeed: successfully air a alkaline water electrolysis using an electrode made of an alloy of an element chosen from nickel, cobalt, iron, an element chosen from Mo, W, V and oxygen. An electrode of this nature normally consists of a nickel alloy, molybdenum and oxygen, where nickel is used in predominant amount.
U.S. Patent No. 4,358,475 issued on November 9, 1982 to British PetrolE ~ um Company Limited discloses a method of producing electrodes metallic by coating a substrate with a homogeneous solution of iron, cobalt or nickel and molybdenum, tungsten or vanadium. The coated substrate is then decomposed thermally to obtain a coated substrate of oxides which is then treated at high temperature in a reducing atmosphere .. This method produces good electrodes but it is obviously complicated, expensive to execute and takes a lot of time. The same technology is also disclosed in the following publicat_Lons:

Int. J. Hydrogen Energy, Vol. 7, No. 5, pages 405-410, 1987, DE Brown. et al.
Electrochimica Acta, Vol. 29, No. 11, pages 1551-1556, 1984, DE Brown et al.
On the other hand, alloys of nickel and titanium, and nickel and niobium in the form of amorphous powders were produced by grinding mechanical in a laboratory bench mill, as disclosed in:
Appl. Phys. Lett. 49 (3), July 21, 1986, pages 146-148, Ricardo B. Schwarz and roll.
E. Hellstern et al., On the occasion of Symposium on "Multicomponent Ultrafine Microstructures "held in Boston, Mass on November 30 1988 discloses the nanocrystalline AlRu pie preparation in a ball mill. The process is essentially restricted to Ru and AlRu and we don't find no disclosure of product usefulness obtained by this process.
DE Brown et al., In "The Development of Low Overvoltage Cathodes, Electrode Coatings ", pages 233-245, disclose the possibility of using electrodes coated with a nickel alloy and molybdenum in chlor-alkaline cells.
Finally, AW Weeber et al. pass in review the production of alloys <~ morphs by grinding marbles in Physica B, Vol. 153, pages 93-135, 1988, AW Weeber and H. Bakker.
The prior art is therefore completely devoid of any disclosure of electrodes that may be manufactured by intense mechanical grinding.
An object of the present invention is to use powders that can be used with advantage for the production of electrodes which can be used in electrolytic production hydrogen.

-2-2os ~ 71 9 Another object of the present invention resides in powders with a morphology and a 'unique microstructure, which differ of those produced by other techniques and which can be advantageously used for the production of electrodes for manufacturing hydrogen.
Another object of the present invention lies in the manufacture of inexpensive cathodes which can be used for hydrogen production by a simple manufacturing technique without require chemical, thermal or electrochemical of the materials used.
Another object of the present invention resides in electrical powders:
electrode manufacturing without the need for a substrate during their manufacture.
Another object of the present invention resides in agglomerated nanocrystals of an alloy which can be used as cathodes for the chlorate production.
Another object of the present invention consists in making chlorates by performing electrolysis in an electrolytic cell whose cathode consists of the powders mentioned above.
Another object of the present invention resides in agglomerated nanocrystals of an alloy which can be used for manufacturing these electrodes which can be used in water electrolysers, chlorine-alkaline cells or others.
The present invention relates to powders comprising nanocrystals agglomerated from an alloy comprising at least a first element chosen from the

-3-~ a6771 9 group consisting of nickel, cobalt, iron and at least a second element chosen by Mo, W or V, said alloy also comprising o: xygene.
The present invention also relates to a process for making powders that can be use for the preparation of electrodes having electro-catalytic properties for hydrogen production. The process uses particles of at least a first element chosen in the group consisting of nickel, cobalt or iron and / or oxides thereof, and at least a second element chosen from Mo, W or 'V, and / or oxides of these, and involves subjecting the particles to high energy mechanical grinding in conditions such that oxygen is incorporated into the alloy during grinding if it is not already present, and for a time long enough to produce nanocrystals.
The term "nanocrystals.x" designates a crystal whose dimension is of the order of approximately 1 to 50 nanometers.
In practice, oxygen is introduced into powder by high energy mechanical grinding in presence of air or oxygen. It is also possible to obtain powders containing oxygen in mixing a certain amount of oxides and elements to be combined so as to give the quantity required oxygen.
The preferred combination of agglomerated nanocrystals contains nickel, molybdenum and oxygen.
Although the quantities of the various elements constituting the main alloy may vary from considerably, taking into account the high price of molybdenum compared to nickel, we preferred manufacture a main alloy comprising at least ~ Y
about 50 At. ~ of nickel, the rest comprising

-4-20ô7 ~ 9 molybdenum and oxygen. For example, an alloy main which has about 60 At. to about 85 At. $ Of nickel gave excellent results. A
typical alloy contains 60 At. ~ of nickel and 40 At. ~ Molybdenum excluding any quantity of oxygen which it can contain, ~ and another is that containing 85 At. ~ of nickel and 15 At. ~ of molybdenum, excluding any amount of oxygen that it can contain. These two concentrations of nickel, have been tested and found successful impressive as we will see later, indicating that this technique can be successfully applied over a fairly wide concentration range.
The powders obtained are pressed or consolidated cold or at moderate temperatures of so as to prevent any x: crystallization and segregation. We will therefore realize that the powders according to the invention may be sold as such for subsequently be transformed into electrodes.
Usually the electrode should be prepared in final form. In this case, it is simply necessary to obtain the powder: f, to press them on any medium in particular. a fence or a plate to constitute an electrode. For example, the filing or consolidation should be done at cold or moderate temperature to prevent recrystallization or segregation. This can be performed by a variety of techniques including electro-coding, laminating, painting or projection so as to constitute an electrode.
Finally, the surface of the pressed powder constituting an electrode can be post-treated, especially by oxidation-reduction, or treatment thermal at low temperature to give even more better results as it will appear to humans art.

- 5 -B

r.20677 ~ ~
As mentioned above, according to the invention, the process includes mechanical grinding at high energy to produce alloy powders especially nickel / molybdenum and oxygen, the - 5a -microstructure in this case is that of an agglomerate nanocrystals, that is to say crystals of E3 whose dimension is of the order of about 1 to 50 nanometers.
The expression high energy used in the present invention in connection with the term "grinding mechanical ", is meant to mean that grinding mechanical is intense enough to cause breaking of the alloy crystals as well as allow sufficient prohibition: broadcasting between elements.
'' In practice, the mechanical alloy according to the invention is made by ball milling although any other means, including grinding by others particle techniques where cold rolling thin elementary strips could also be used.
In practice, when using the ball grinding, the latter must be carried out in a crucible with balls that do not contaminate the final product. Grinding at b_Llles can be done under an oxygen-containing atmosphere if this element is not already present in the mixture of departure. We prefer to use quantities of oxygen greater than 2 ~ by weight. Dan: the present case, the ball milling takes place in a crucible in one carbide of a transition metal, with balls made of the same material. ~ A favorite material is tungsten carbide given its hardness and since this material is easily available and not detrimental to property electro-catalytic. We can also use molybdenum carbide.
Although the proportions of the particles of nickel and molybdenum can vary so considerable, they must be chosen for constitute an alloy of which the quantity of nickel and molybdenum is the one mentioned above,

-6-~~ 67719 in particular containing at least approximately 50 At. ~ of nickel, preferably between about 60 to 85 At. ~ of nickel and about 15 to 40 At .. $ of molybdenum to the exclusion of any amount of oxygen. Good results were obtained as indicated above with a main alloy comprising 60 At. ô of nickel and 40 At. ~ Of molybdenum with the exclusion of oxygen and a other alloy comprising 85 Fvt. ~ nickel and At. ~ Of molybdenum with the exclusion of oxygen, the 10 amount of oxygen being of the order of 1 to 15 ~ in weight.
During the grinding process, the speed beads is typically larger than about one meter per second. Good results have been obtained 15 when the transaction is carried out over a period of a few hours under these conditions.
When the production stage in the ball mill lasts longer (more than typically 25 hours), in addition. nanocrystals of nickel-molybdenum, we found small quantities of tungsten carbide, constituting an impurity from crucible. Presence of small quantities of this phase, however, does not appear to affect the electro-catalytic performance of the alloy as well as seen in Figure 1.
After obtaining powders from agglomerated nanocrystals of a: Nickel plating, molybdenum and oxygen, the powders can be moderate temperature presses to prevent recrystallization or phase segregation, under form of an electrode or on a support, in particular a grill or a plate intended to constitute a electrode. Other techniques, including painting, projection, electro-coding: ition and application can also be used.

, 2ofi7719 It is believed that the production of nanocrystals in the powders according to the invention, produces a large number of active sites, which are responsible for high electro-catalytic activity: of the electrode which results.
Molybdenum is responsible for the dilation of the network of Ni crystals. In others words, high-energy mechanical grinding, especially by ball milling, forces molybdenum to inside the network of nickel crystals where it remains despite the phase diagram. At all beginning of high energy mechanical grinding, the particles come into contact with each other and unite with each other. After a few hours of mechanical alloy, during which there are increased amount of deformation of nickel and molybdenum crystallites, we find diffusion of molybdenum atoms inside nickel crystals, the latter being fragmented into units that decrease continuously. After approximately twenty hours of deformation, the powder structure consists of an agglomerate of nickel crystals saturated with molybdenum, and also contains oxygen, the dimension of which is less than or equal to 50 nanometers. These nanocrystals can be mixed with a small amount of impurities from tungsten carbide balls and crucible.
The presence of oxygen in this invention ensures a gain of about 0.2 at 0.5 volts on the voltage actually used in each elementary electrochemical cell at 250 mA
cm 2. In a typical industrial electrolyser for the production of sodium chlorate we would make sure savings of up to half a million dollars for a tenth of a volt that we gain on voltage usually in use.
_G_ 206,771 9 The electrodes made from powders according to the invention have been presented during tests performed for the electrolysis of water at 70 ° C in KOH
30% by weight, electro-catalytic activity comparable or superior to ce7_le des electrodes currently used in industry electrochemical.
The overvoltage measured at 250 mA cm 2 in KOH 30% by weight is 60 mV and at 500 mA cm 2 it is around 90 mV.
These overvoltages are stable during first fifteen hours of operation. These performances were maintained after several interruptions or restarting of the cell.
The invention will now be illustrated by the appended drawings in which:
Figure 1 is a curve representing the overvoltage in a solution of KOH at 70oC by compared to the grinding time of alloys according to the invention respectively containing 15 At.% and 40 At.% Molybdenum excluding any quantity oxygen;
Figure 2 illustrates the dependence by relative to the time of the alloy overvoltage Ni ~ OMo400x according to the invention respectively at 500 and 250 mA cm 2;
Figure 3 is a curve representing the structure of an alloy containing 60 At.% nickel after two hours of ball milling;
Figure 4 is a curve similar to that of Figure 3 after twenty hours of grinding at beads;
Figure 5 is a curve similar to that of Figure 3 after thirty hours of grinding â
beads;

~ O ~~ ~~ has Figure 6 is a curve similar to that of Figure 3 after forty hours of grinding ball;
Figure 7 is a curve similar to that of Figure 3 for an alloy containing 85 At.% Nickel and 15 At.% Molybdenum at exclusion of oxygen.
Figure 8 is a curve similar to that of Figure 7 after eight hours of grinding;
Figure 9 is a curve similar to that of Figure 7 after twenty hours of grinding;
Figure 10 shows the morphology of a alloy according to the invention containing 85 At.% of nickel and 15 At.% molybdenum excluding oxygen after 20 hours of ball milling;
Figure 11 is a curve showing the amount of oxygen in the powder depending on the grinding time in air and e: ou an atmosphere argon; and Figure 12 shows the change in overvoltage as a function of tempo for a powder mixed under air and under argon.
Referring to Figure l, we will see that each of the alloys containing 15 At.% of molybdenum and 40 At.% molybdenum has an overvoltage acceptable already after about 10 hours of grinding. However, a surge actually satisfactory is obtained after eight o'clock and we note that the potential improves slightly at as long as the grinding time exceeds fifteen hours.
Referring to Figure 2, note that an alloy comprising 40 At.% of molybdenum shows a good overvoltage, i.e. less than l00 mV
even after a fifteen hour test at 500 mA cm 2.
Another indication of good behavior of an alloy according to the invention, is obtained by measuring the Tafel slope, which is a measure of 2p ~ 771 ~
the increase in potential that must be applied to the electrode to get an increase in current by a factor of 10. Table 1 shows that the alloys give slopes of Taf: el less than 70 mV after 20 and 40 hours of grinding time. The overvoltages calculated at 250 mA cm 2 (n250) confirm the high electro-catalytic activity of a ~ .liages.

20677 ~ 9 Tafell parameters for the reaction evolution of hydrogen in 30% by weight KOH, 70oC
on Ni-Mo alloys produced by ball milling intensive alloy Curve time T <~ fel I o _ N250 grinding (h) (mV) (mA cm 2) Ni60Mo40 0.25 166 14.8 204 Ni85Mo15 2.0 156 22 165 Ni85Mo15 10.0 73 15 89 Ni85Mo15 20.0 63 16 75 Ni60Mo40 20.0 50 17 58 Ni60Mo40 40, U 63 29 59 Ni60Mo40 melted arc 107 0.042 404 1 Obtained by a galvanodynamic method for a scan rate of 1 mA cm 2 s 1 from 250 to 10 mA
cm 2 after having kept the electrode at 250 mA cm 2 for 1800s.

~ "û6771 9 Referring to Figure 3, we shows the structure of the mixture after 2 hours of ball grinding. We will see that the molybdenum phase is clearly separated from the nickel phase.
Referring to Figure 4, we will see that Mo peaks decrease in intensity: ~ ity compared to corresponding peaks in Figure 3 indicating that the diffuse molybdenum in nickel, widening of peaks signifying that there is a reduction in the size of crystallites.
Referring to Figure 5, it will be seen that molybdenum peaks further decrease. it means that there is even more diffusion of molybdenum in nickel, which is also indicated because the peak (111) of nickel moves towards the left. We can also note the beginning of a appearance of a secondary phase, indicated by X, and identified as being made up of impurities from tungsten carbonate.
Referring to Figure 6, we see that there is an increase in the quantity of the phase secondary after 40 hours of grinding time.
Figures 7, 8 and 9 correspond to those given above for an alloy containing 60 At. ~ Of nickel, but this time we have a alloy containing 85 ~ nickel. We can see the same results.
The morphology illustrated in Figure 10 shows that the surface of the powder electrode consolidated according to the invention is fairly smooth at the microscopic scale. We could apply a treatment in order to increase the effective surface to make the electrode even more active.
Referring to Figure 11, we will see that The amount of oxygen present in the powders when the grinding takes place under argon does not vary 2os7? ~ 9 not. Presumably it represents all oxygen impurity already present in ~ nickel and molybdenum and / or argon before grinding.
On the other hand, we will realize that the overvoltage (mV) measured for powders which have undergone different grinding conditions (air vs argon) is not the same. It seems that the reduction in overvoltage is associated with the incorporation of oxygen present in powders. We will refer to Table 2, and in Figure 12.

Overvoltage samples (mV) Material Air time Under argon grinding Ni 0 332 Ni0 0 270 Ni75: Mo25 0 194 194 Ni75: Mo25 2 132 149 Ni75: Mo25 5 114 167 Ni75: Mo25 10 101 177 Ni75: Mo25 20 91 196 Ni75: Mo25 40 - 190 Ni75: Mo25 45 70 ---2 ~ ST7 ~ 9 'I'able 2 shows a marked improvement electro-catalytic properties of cathodes when oxygen is incorporated into the structure of powders.
We must therefore conclude that oxygen is mainly responsible for 7.activation of the alloy structure.

Boost Density (mV) current (mA / cm 2) Crushed Crushed 31 hours 41 hours (argon) (3 ~ _h argon + lOh air) Tafel slope 160 mV / drop 29 mV / drop Exchange current - 11.4 mA / cm 2 Amount of oxygen (% by weight) 0.25 5.0

Claims (29)

1. Powders comprising nanocrystals agglomerates of a main alloy containing at least a first element chosen from the group consisting of nickel, cobalt and iron, and at least one second element chosen from molybdenum, tungsten, vanadium, said powders also comprising oxygen, characterized in that the nanocrystals are obtained by grinding in an atmosphere containing oxygen and have a dimension of the order of 1 to 50 nm. 2. Powders according to claim 1, characterized in that the first element includes nickel, and the second element includes molybdenum, said powders also comprising oxygen. 3. Powders according to claim 2, characterized in that the main alloy comprises at least about 50 At.% nickel, the rest being consisting of molybdenum, and oxygen. 4. Powders according to claim 3, characterized in that the main alloy comprises between about 60 to 85 At.% nickel. 5. Powders according to claim 3, characterized in that the main alloy comprises about 60 At.% of nickel and about 40 At.% of molybdenum, excluding said oxygen. 6. Powders according to claim 3, characterized in that said main alloy contains approximately 85 At.% nickel and approximately 15 At.% Molybdenum, excluding said oxygen. 7. Powders according to claim 1, which are obtained by high mechanical grinding energy. 8. Powders according to claim 1, characterized in that they are pressed to moderate temperature so as to prevent any recrystallization and phase segregation. 9. Powders according to claim 8, characterized in that they are pressed in the form electrode. 10. Powders according to claim 8, characterized in that they are pressed on a support for constituting an electrode. 11. Electrode prepared by deposit, consolidation or pressing of powders such as defined in claim 1, on a support by electro-coding, rolling techniques, roller application, painting or projection. 12. An electrode according to claim 11, characterized in that said support comprises a fence. 13. An electrode according to claim 12, characterized in that said support comprises a plate. 14. An electrode according to claim 12, having a rough surface so as to increase the activity of said electrode. 15. An electrode according to claim 12, which is chemically, electrochemically and mechanically stable and whose overvoltage is less than 100 mV at 500 mA cm-2 and is stable for at minus the first 15 hours of operation in a 30% by weight KOH solution at 70 ° C. 16. Process for the preparation of powders intended to constitute electrodes whose electro-catalytic properties allow these to produce hydrogen by electrolysis, said process being characterized in that one prepares particles of at least a first chosen element in the group consisting of nickel, cobalt and iron, and at least one second element chosen from Mo, W and V, said particles are subjected to mechanical grinding at high energy under conditions and for a duration making sure to give alloys nanocrystalline of said elements and oxygen, the dimension of nanocrystalline alloys being less than 50 nm. 17. The method of claim 16, characterized in that mechanical grinding produces nanocrystals whose size varies between approximately 1 and 50 nanometers. 18. The method of claim 16, characterized in that said high mechanical grinding energy is carried out by ball milling of said particles in an atmosphere containing oxygen. 19. The method of claim 16, characterized in that said high mechanical alloy energy is carried out by grinding or cold rolling in an atmosphere containing oxygen. 20. The method of claim 16, characterized in that particles of nickel and molybdenum particles and / or oxides of these in a report capable of giving nanocrystals of a main alloy of nickel and molybdenum comprising at least about 50 At.% of nickel, the rest being molybdenum and oxygen. 21. The method of claim 20, characterized in that said main alloy comprises between around 60 and 85 At.% nickel and around 15 to 40 At.% Molybdenum, excluding said oxygen. 22. The method of claim 20, characterized in that said main alloy comprises about 60 At.% nickel and 40 At.% molybdenum, at exclusion of said oxygen. 23. The method as claimed in claim 20, characterized in that said alloy comprises approximately 85 At.% Nickel and 15 At.% Molybdenum, at exclusion of said oxygen. 24. The method of claim 18, including ball milling of the particles of nickel and particles of molybdenum or oxides of these, while adjusting the speed of said balls at a value greater than about one meter / second. 25. The method of claim 20, characterized in that said powders comprise agglomerated nanocrystals of a nickel alloy, molybdenum and oxygen and are consolidated to a temperature preventing any recrystallization and phase segregation of said alloy, allowing these last to constitute an electrode. 26. The method of claim 25, characterized in that said powders are pressed on a support comprising a mesh. 27. The method of claim 25, characterized in that said powders are pressed on a support comprising a plate. 28. The method of claim 25, characterized in that said powders are consolidated or deposited on a support by electro-coding, rolling, pressing, roller application, painting or projection. 29. Process for the production of hydrogen by electrolysis in an electrolytic cell comprising a cathode, characterized in that the cathode is as defined in claim 11.