Cathode for Chlor-Alkali Cells

The invention relates to an improved Raneyized hydrogen evolution cathode for chlor-alkali electrolytic cells.

The pertinent prior art has been discussed in great detail in parent application Ser. No. 025,153, (hereafter called simply "'153 application") and reference is made to that discussion as background for the present application.

One solution is the present invention which provides an improved low overvoltage electrode for use as a hydrogen evolution cathode in an electrolytic cell, the electrode being of the type that has a Raney metal surface layer in electrical contact with a conductive metal core, wherein said improvement comprises: said Raney metal surface is predominantly derived from an adherent Ni-Mo-Ti-Al quaternary crystalline precursory outer portion of said metal core.

Another solution provided by the invention is an improved low overvoltage electrode for use as a hydrogen evolution cathode in an electrolytic cell, the electrode being of the type that has a Raney metal surface layer in electrical contact with a conductive metal core, wherein the improvement comprises: said Raney metal surface layer is predominantly derived from adherent (Ni.sub.x Mo.sub.y TI.sub.z)Al.sub.3 crystalline precursory surface layer, where x is within the range from about 0.75 to about 0.94, y is within the range of from about 0.05 to about 0.20 and z is within the range of from about 0.01 to about 0.05 weight percent of the NiMoTi portion of the alloy.

A still further solution provided by the invention is an improved low overvoltage electrode for use in a hydrogen evolution cathode in an electrolytic cell, the electrode being of the type that has a Raney metal surface layer in electrical contact with a conductive metal core, wherein the improvement comprises: said Raney metal surface is derived from an adherent Ni-Mo-Ti-Al quaternary crystalline intermetallic layer stabilized by substitution of a stabilizing amount of molybdenum and titanium for some of the nickel in the crystalline structure of said crystalline layer.

Yet another solution provided by the invention is a method of producing a low overvoltage electrode for use as a hydrogen evolution cathode in an electrolytic cell which comprises the steps of:

(a) coating with aluminum the surface of a clean non-porous conductive base metal structure of an alloy of about 5-20 weight percent molybdenum, about 1-5 weight percent Ti and 75-94 percent nickel;

(b) heat treating said coated surface by maintaining said surface at a temperature of from 660.degree. to 750.degree. C. for a time sufficient to diffuse a portion of said aluminum into outer portions of said structure to produce an integral nickel-molybdenum-titanium-aluminum alloy layer in said outer portions consisting predominantly of NiAl.sub.3 type grains but insufficient to create a predominance of Ni.sub.2 Al.sub.3 type grains in said outer portions; and

(c) leaching out residual aluminum and intermetallics from the alloy layer until a Raney nickel-molybdenum-titanium layer is formed integral with said structure until the thickness of said Raney nickel-molybdenum-titanium layer is less than about 75 microns.

The invention will be better understood by reference to the attached drawing which is provided by way of illustration and in which the FIGURE is a graph of polarization potential versus time for a Raney NiMoTi cathode of the present invention as compared with a Raney NiMo cathode prepared according to the disclosure of the '153 application.

The FIGURE shows the overpotential curves versus current density for two catalytically coated cathodes, that of the present invention and that of the invention of the '153 application both prepared similarly from Beta phase precursor. Each has identical percent by weight of molybdenum (12%) and the same method (dipping) of depositing the aluminum prior to identical heat treatment for two hours at 725.degree. C. However, the cathode of the present invention has 2 percent by weight added titanium. The addition of two (2) percent Ti was found to produce, upon subsequent Raney treatment, a .beta.-Raney Ni-12Mo-2Ti cathode coating having about 50 millivolts less cathode overvoltage than that exhibited by a .beta.-Raney Ni-12Mo cathode coating. The test method was the same as in the '153 application. The reason for this startling difference is not known, although the result was confirmed. It is, however, clear that the difference in titanium content was responsible for the difference in potential since all other parameters were held identical.

It is also noted that, as with added molybdenum alone, an unexpected and surprising result is achieved when both molybdenum and titanium are added to a Beta phase (NiAl.sub.3) intermetallic. The Beta phase formation is stabilized by the addition of molybdenum and titanium in the amount of about 5-20 percent by weight and about 1-5 percent by weight, respectively, of the total weight of nickel, titanium and molybdenum. That is, the titanium does not harm this "Beta-stabilizing" effect of the molybdenum. Both molybdenum and titanium are apparently captured in the ordered orthorhombic Beta phase crystal structure such that the Beta phase can be represented by the forumla Ni.sub.x Mo.sub.y Ti.sub.z Al.sub.3 where x, y, and z are the weight percent nickel, molybdenum and titanium, respectively, in the total weight of nickel, titanium and molybdenum. By "stabilized" is meant that once the Beta phase forms it has less of a tendency to transform to a Gamma phase (Ni.sub.2 Al.sub.3) crystal structure thus the elevated heat treatment temperature can last longer without as much undesirable Gamma phase being formed. In fact, the heat treatment at the optimum 725.degree. C. can last for 2 hours, or 4 hours or even 6 hours with a .beta.-Raney Ni-Mo-Ti cathode still being produced. In fact, two hours was used on the samples in the FIGURE. Since it was shown in the '153 application that the Beta phase is the intermetallic of choice, this is an important advantage of the Ni-Mo-Ti-Al quaternary alloy over Ni-Al binary alloys.

One preferred electrode is a monolithic structure of a Ni-Mo-Ti alloy of 5-20 percent and most preferably from about 12-18 percent by weight molybdenum and about 75-94 percent and most preferably 77-87 percent by weight nickel with from about 1-5 percent by weight titanium which has been given a Raney treatment which preferably comprises coating a clean non-porous conductive base metal structure of the above defined alloy by dipping said structure into molten aluminum at a temperature within the range from about 650.degree. to about 675.degree. C. for a period of time ranging from about 1 to about 2 minutes. The alloy structure coating with molten aluminum is then heated. for about 1-360 minutes in an inert atmosphere at a temperature of from about 660.degree. C. to about 855.degree. C. produce a Beta phase crystal structure. A temperature of about 700.degree. C. to about 750.degree. C. and a time of about 1-30 or even 5-15 minutes at a temperature in the range from about 715.degree. to about 735.degree. C. are more preferred because this gives sufficient time for enough aluminum to interdiffuse into the nickel to provide maximum preponderance of NiAl.sub.3 or Beta phase over Gamma phase (Ni.sub.2 Al.sub.3) but does not allow enough time for the diffusion to result in the preponderance of undesirable Gamma phase (Ni.sub.2 Al.sub.3) as is specifically called for in U.S. Pat. No. 4,116,804.

Contrary to the disclosure of U.S. Pat. No. 4,116,804, it has been surprisingly found that the Beta phase NiAl.sub.3, with molybdenum and titantium added thereto, is not lost during leaching and in fact experiences no appreciable thinning during subsequent use in a chlor-alkali cell.

The inclusion of from about 1 to about 5 percent by weight titanium in the Ni-Mo alloy in order to produce a NiMoTi ternary alloy has given a further surprise in that a further reduction of 50 millivolts overvoltage (at 200 ma/cm.sup.2) in cathode overvoltage is achieved. Since the Raney NiMo alloy coating already exhibited such a low overvoltage it is most surprising that any additional lowering occured from added titanium. The thickness of the Raney nickel-molybdenum-titanium surface produced by the method of this invention is less than about 75 microns.

Advantageous use can be made of the electrodes of the invention, especially as hydrogen-evolution cathodes of cells intended for the electrolysis of brine, water or the like. The electrodes are particularly preferred for use in brine electrolysis cells, wherein the high electrochemical activity of the .beta.-Raney nickel-titanium-molybdenum surface remains constant for long periods of extended continuous use. When the electrode is intended for use in a brine-electrolysis diaphragm cell, the diaphragm can be applied directly to the porous nickel surface of the electrode as noted in the '153 application.

The various parameters associated with the present invention were measured by the techniques described in the '153 application.