CA1168185A - Raney alloy coated cathode for chlor-alkali cells - Google Patents

Abstract

IMPROVED RANEY ALLOY COATED CATHODE FOR CHLOR-ALKALI CELLS Abstract of the Disclosure An improved cathode with a conductive metal core and a Raney-type catalytic surface predominantly derived from an adherent Beta Nickel (NiAl3) crystalline precursory outer portion of the metal core is disclosed. The precursory outer portion preferably has molybdenum added to give a precursor alloy having the formula NixMol-xAl3 where x is within the range of from about S to about 15 weight percent. Also dis-closed is a method of producing a low overvoltage cathode. The method includes the steps of taking a Ni-Mo core or sub-strate having about 5-20 weight percentage of Mo and coating it with aluminum then heat treating to form a Ni-Mo-Al alloy with mostly a Beta Nickel structure and then leaching out the Al to produce a Raney surface.

Description

IMPROVED R~NEY ALLOY COATED CATHODE
FOR CHLOR-ALKALI CELLS

Field of Invent The invention relates to an im~roved Raneyized hydrogen evolution cathode for chlo.r-alkali el~ctrolytic cells.

Prior Art Statement .

In view o the phenomenal jump in energy costs and the increased scarcity of indusl:rial fuel supplies, there has been and continu~s to be a flurry of research activity in the electrolysis field to find ways to reduce the amount of power used in electrolysis processes.
For many ~ears it ha~ been customary to use steel cathodes in chlor-alkali diaphragm cells, even though a substantial amount o~ power is used in overcoming what is called "hydrogen overvoltage" at the ~athode.
Hydrogen overvoltage is largely an inherent characteristic of the metallic suxface in contact with the electrolyte so th~re is a continual need and desire to cGme up with better cathode surfaces to reduce this overvoltage and thereby decrease the power consumption of the cell.
It is known ~hat active, porous nickel can be produced by selectively dissolving a soluble componellt, such as aluminum or zinc, out of an alloy of nickel and ~he soluhle component. A porous nickel of this ` ~

type and the alloy from which it is produced are generally called "Raney nickel" or "Raney alloy" afte.r their inventor. Ses U.S. Patent NosO 1,563,787 (1925), 1,628,191 (1927) and 1,915,473 (1933)~ There are various methods for producing this Raney nickel, and various applications for this metal are known.
It is also known to use such Raney nickel surfaces on cathodes for chlor-alkali cells. For example, U.S.
Patent No. 4,116,804 filed Nove~ber 17, 1976 and issued September 26, 1978 to C~ Needes and assigned to DuPont de Nemours describes an electrode, hereafter "Needes electrode", for use as a hydrogen evolution cathode in electrolytic cells in which a cohesive surface layer of Raney nickel is in elec~rical contact with a conduc-tive metal core having an ou~er layer of at least 15 percent nickel (see Table 4 thereof), characteri~ed in that the surace layer of Raney nickel is thicker than 75 ~Im and has a mean porosity of at least 11 percent. The ca~alytic surface layer consists predominantly of Ni2A13 grains from which at least 60 percent of aluminum has been leached out with an aqueous bas~. An overvoltage of about 60 millivolts is alleged. ~o phrase the s~me thing relative to con~entional cathodes, reductions of 315 to 345 millivolts in hydrogen overvol~age as compared with mild steel cathodes is alleged. However, subsequent testing indicates much higher overvoltages and actual reductions of only 100-150 millivolts. Furthermore, spalling or delamination of the coating has been observed upon additional testing. The patent teaches that any Raney nickel which forms from the NiA13 phase is mechanically weak and does not adhere well and is generally lost during leaching~ The patent also teaches that Ni2A13 (Gamma phase) is the pre~exred intermetallic precursor and governs the activity of the coating and that the heat treatment should be such that the proportion of Ni2A13 is maximized.

-~L6~ 5 This mechanical weakn~ss of Raney nickel from Ni~13 is unfoxtunate because it was previously known that Raney Ni rom NiA13 (Beta phase) is more active for hydrogen desorption than is Raney Ni from Ni2A13 (Gamma phase). See for example A~ A. Zavoxin et al, Kinetika i Kataliz, Vol. 18, No. 4, pp. 988_994, (USSR, July-August, 1977) which explains hydrogen is more weakly "bonded" in Raney Ni from NiA13 than fxom Ni2A13, that there are more hydrogen adsorption cPnters in Raney Ni from NiA13 than Ni2A13 and that the heat o~ desorptio~ i5 low~r for Raney Ni from NiA13 than Ni2A13.
Golin, Karaseva and Serykh in Elektrokhimiya, Vol. 13, No. 7, pp. 1052-1056 (~SSR, July, 1977) disclose a 10 percent Mo, 45 percent Ni, 45 percent Al alloy which, upon leaching, yields a Raney catalytic surface with ~xtremely low acti~ation energy for hydrogen oxidation such as would occur in a hydrogen-oxygen fuel cell. No men~ion of hydrogen evolution (i.e. hydrogen reduction) catalysis is yiven o~r suggested.
Austria~ Patent 206,867 i~sued December 28, 1959 to Ruhrchemie A. G. and Steinkohlen Electrizitat A. G.
gives a detailed discussion of preparation of ~hin foil electrodes with a "double-skeletal catalyst"
~5 coating o 20~80 p~rcent Raney metal with 80-20 pe.rcent skeletal material (e.g. Ni powder). Page 3, column 2 lists a number of sintered powder metal alloys suitable for catalytic coatings on the foil. German Auslegeschrift 1,094,723 by W. ~ielstich, E. Justi and A. Winsel-Ruhrchemie A. G. published December 15, 1960 suggests (page 3, lines 24-70) use of such a "double skeletal catalyst" coated foil improved by adding (page 3, lines 54-63) 1-20 percent of a Group VIII metal as the cathode of an amalgam decomposer of a mercury type chlor-alkali cell system. However, such sintered coatings have been found to delaminate after relatively short use as diaphragm or membrane cell cathodes.

~P~ 35 ~4--Baird and Steffgen in Ind. En~. Chem., Prod. Res.
Dev., Vol. 16, No. 2 (1977) in an article entitled "Methanation Studies on Nickel Flame-Sprayed Catalysts", describe the temperature ranges for the various intermetallics and say NiA13 i5 the major phase produced during heat treatments for 1, 10 or 30 minutes at about 725C and that no more than 10 minutes is required at 725C for alloying. When heat treated at 725C, the alloy was found to have the greate3t activity for carhon monoxide conversion catalysis ~see FIGURE 2 ~hereof~. NiA13 is described as believed to be the most active intermetallic phase "as shown by Petrov et al (1969)" and pho omicro-graphs are provided to show the structure.
U.S. Patent No. 4,033,837 ~ Kuo et al issued July 5, 1977 teaches use of a Ni-Mo-V catalytic coated copper cathode which achieves a relatively low over-voltage. While this cathode has a significantly lower overvoltaqe than a steel electrode, copper-fouling or iron-fouling can be a problem unless ~le catholyte solution is kept free of iron. No mention of Raney treatment is madeO
U.S. Patent No. 3,291,714 issued December 12, 1966 to Hall disclo~es a number of coatings for steel or titanium cathodes, among such coatings a Ni-Mo coating and a Fe-Ni-Mo coating were found most desirable. Heat treatment of the elec~rodeposited coating was re~uired to avoid delamination of the coatings. Moderately low overvoltages were alleged. No mention of Raney treatment is given.
West ~erman Offenlengungsschrift 2,704,213 published August 11, 1977 claiming priority of U.S.
Serial No. 655,429 filed ~ebruary 2, 1976 by Macmullin discloses a Raney-nickel cathode in the form of a plate or a porous Raney-Ni coated perforated nickel plate. The cathode is designed for chlor-alkali membrane cells, but was, as ~tated in the example ~6~
--5~

therein, apparently only tested in "a small laboratory cell". The cathode is prepared by creating a nickel-aluminum alloy, pouring a plate of the alloy and then leaching out the aluminum. Molybdenum is not mentioned.
W. Vielstich in , Vol. 33, pp. 75 79, (1961) describes a "dual-framel' electrode made of Raney nickel~ which is prepared by mixing a powdered ~aney alloy ~e.g~ of nickel and an alloying component, such as aluminum) with a frame metal con~isting of pure metal powder (e.g. carbonyl-nickel), pressing, si~texing, and then dissol~ing out the alloying compo~ent from which the Raney alloy is prepared.. The surface layer of such an electrode consist~ of a dispersion of active Raney nickel lS particles, which is embedded in a frame made of inac~i~e solid nickel particles. This electrode is used, among other thi~gsl as a hydrogen evolution cathode in a chlorine-alkali electrolysis diaphragm cell. Double-frame electrodes produced by the methods of powder metallurgy, however, have insufficient mechanical strength to be suitable for producing large mesh electrodes such as those which are desired for industrial scale electrolysis of sodium chloride solutions.
One process for producing flat material from Raney nickel consists of the fact that fused particles of a Raney alloy precursor (e.g., an alloy of nickel and aluminum) are spraye~ onto a metallic carrier, and the aluminum is then selec~ively dissolved out; see U.S. Patent No.
3,637,437. This material i5 suggested as a material for catalytic cathodes of fuel cells. Cathodes pro-duced according to this method, however, generally have sur~aces of low porosity and have a tendency to break apart.

~6~

U.S. Patent No. 3,272,728 and German Offenlegung~
sschri~t No. 2,527,386 (based on U.S. Patent Application Serial No. 489,284) describe electrodes with Raney nickel surfaces which are produced by simultaneously electrodepositing nickel and zinc from an inorganic electrolyte bath on a metal carrier (such as stPel) and then selectively dissolving zinc out of the Ni-Zn alloy thus produced. This elec~rode treatment is supposed to reduce hydrogen overvoltage of steel cathodes by ùp to I50 millivolts. U.S. Patent No.
4,104,133 i~sued ~ugust 1, 1978 discloses one method alleged to be useful to put this Ni-Zn Raney coating technology into commercial practice by use of metallic plating anodes for deliberately electroplating a Ni-Zn coating onto the cathode in-s:it~ in a chlor-alkali cell and subsequently leaching the zinc out to give a Raney nickel surface and :Lower the hydro~en over~oltage of the chlor-alkali cel]L. However, only layers of a very crude temporary ~aney alloy form.
Permanent coatings o~ greater overvoltage reductiors are desired.
British Patent No. 1.,239,751 describes a process for pxoducing porous nickel electrodes for electrochemi-cal cells or fuel cells by electrodeposition of aluminum from an electrolyte containing an organoaluminum complex on a support made of nickel or a nickel alloy, wherein some of the aluminum deposited diffuses into the nickel, forming an alloy, from which aluminum is then leached.
The diffusion is carried out ovex a period of 1 or 2 hours in an inert atmosphere at a temp~rature of less than 659C, preferably between 350 and 650C. Very thin electrodeposited layers, 5-20 ~m thick are described.

--7~

J. Yasamura and T. Yoshino in a report on "Laminated Raney Nickel Catalysts" in Ind. Chem. Prod.
Res. Dev., Vol. ll, No. 3, pp. 290~293, 1972, describe the production of Raney nickel plates, though not in connection with electrodes, by sprayiny molten aluminum onto a nickel plate, heating for l hour in a nitrogen atmospher2 at 700C to form a 0.2 mm-thick layer of NiAl3 and dissolving aluminum out o~ the layer. The product thus obtained is supposed to be usable as a hydrogenation (i.e_ hydrogen oxidation) catalyst.
Another method of prepariny molded articles from Raney nickel or use as hydrogenation catalysts is described in U.S. Patent No.. 3,846,344. According to this patent, a nickel-plated metal pipe is coated with an aluminum layer at least 0.02 mm thick, then the aluminum is permitted to diffuse into the nickel by heat treating for at least 30 mi.nutes at a tempera-ture of at least about 480C, and t:hen the aluminum is selectively dissolved out of the diffusion layer.
Example 5 of the patent describes how a 25 mm-diameter pipe with a 1 mm-thick electrodeposited nickel layer, on which a 0.5 mm thick aluminum layer has been deposited by flame spraying, is subjected to 6 hours of diffusion heat txeatment at 650C, in order to - produce a diffl~sion layer at least 0,05 mm thick. The pipe is than activated by imm~rsing for 8 hours in 25 percent a~ueous sodium hydroxide solution. The patent states that the surface displays a high degxee of efficacy for the catalytic hydrogenation of cyclohexane.
U.S. Patent No. 3,407,231 describes a process for producing a negative electrode with an active porous nickel surface for use in alkaline batteries.
~ccording to the patent, the electrode is produced by bxinging aluminum into contact with the surface of a r.-icke~containing core ak an elevated temperature, so that nickel and aluminum interdiffuse to form a layer of Gamma phase nickel aluminide (Ni2A13), after which the aluminum which has diffused in is dissolved out with alkali hydroxide and a layer of acti~e nickel is obtained, whi~h is metallurgically bonded to the core.
The patent mentions diffusion ~emp~ra~ures o~ 625 to 900C, diffusion times of 8 to 16 hours, dissolution temperatures of 20 to 100C, dissolution times of 1 to 32 hours, and coating thicknesses of 200 to 300~Im. In particular, the process is supposed to be carried out by placing a nickel sheet in a packet made o~ a mixture of about 58 percent A12O3, 40 percent al~minum powder, and 2 percent NH4Cl and heating the packet for 8 hours in a reducing atmosphere at 800C, so that a 200~Im-thick layer of Ni2A13 forms on each side of the nickel sheet, after which the coated nickel core is immersed in 6 N sodium hydroxide for about 16 hours at 80C, in order to dissolve out at least 85 pexcent of the aluminum. However, it has been found that Xaney nickel surfaces of electrodes produced ~0 according to this special method have low porosity.
The patent sugges~s that the nickel sheet be rolled hetween two aluminum sheets in order to produce a metallic bond, and the sandwich be heated in a reducing atmosphere at 543C. Although temperatures below 649C are pxeferred in this particular embodiment, the patent also suggests temperatures of as high as 872C.
I~ has been found, however, that in the case of bonding by rolling the desired metallic bond does not form.
It is an object of this invention to provide a solution to the problem of producing a cathode for a chlor-alkali membrane or diaphragm cell which has a lower cathode polarization potential ("hydrogen over-voltage") for a longer period than the prior art electrodes noted above.

g SUMMAR~ OF THE INVENTION

One 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 NixMol xA13 crystalline precursory surface layer, where x is less than 0.95.
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 a molybdenum alloy of a weight percent molybdenum within the range of from about 5 to about 20 and a weight percent nickel within the range of from about ~0 to about 95;
b) heat treating said coated surface by main~
taining said surface at a temperature within the range of from about 660 to about 750~C
for a time sufficient to diffuse a portion of said aluminum into ou-ter portions of said structure to produce an integral nickel-moly-bdenum-aluminum alloy layer in said outer portions consisting predominantly of Beta structured grains but insufficient to create a predominance of Gamma structured grains in said outer portions; and 6~ 35 c) leaching out residual aluminum and inter~
metallics from the alloy layer until a Raney nickel-molybdenum layer is formed integral with said structure.

~L~6~ S

Brief Description of the Drawin~

The in~ention will be better understood by reference to the attached drawing which is provided by way of illustration and in which:
S FIGURE 1 is a graph of polarization potential (ref. standard hydrogen electrode~ veræus time for a number of cathodes.
FIGURE 2 is a graph of polarization potentional (ref. standard hydrogen electrode~ versus current density for two cathodes of the invention.
FIGURE 3 is a graph of polarization potential versus time for three cathodes of the invention.
FIGURE 4 is ~ compariso~ graph of IR Free polarization potentials for the cathode of the invention and prior art cathodes.
FIGURE 5 is a photomicrograph of the coating of a cathode of the invention showing a predominance of Raney Ni-15 Mo formed from N 85Mo 1!~ A13 ~eta phase) precursor, as it appears just after the Raney treatment.
FIGURE 6 is a photomicrograph of the coating of the cathode of FIGURE 5 as it appeared after the electrolytic test of FIGURE 1 showing the Beta phase ~tructure still largely intact and wi~h essentially no iron overplating and no thinning of the coating.
FIGURE ~ is a microprobe photograph and readout showing ~he alumi~um, Beta (Ni~13), Gamma (Ni~A13) and nickel phase precursors prior to leaching.
FIGURE 8 is a vertical cross section through an exemplary laboratory electrolysis cell with which the invention may be used.

~6~

Detailed Descriptlon _f Preferred Embodiments FIGURE 1 graphically shows the cathode polarization potentials using three different Raney-trea~ed cathodes in a typical chlor-alkali cell environment. The Raney nickel coatings of the presen~ invention which were produced from Beta phase ~NiA13) precursors had 150 250 less potential than the Raney nickel from a Gamma phase (Ni2A13) precursor. FIGURE 1 also shows ~ha~ ~he Raney nickel cathode of the invention with 15 percent by weight molybdenum from a molybdenum enriched Beta phase (NixMol xA13) precursor (hereafter ~-Raney Ni-15Mo) exhibited about 80 to 120 millivolts less cathode polar-ization potential and hence 80O120 mV less overvoltage.
Also, the ~-Raney Ni-15Mo had a constant overvoltage of approxima~ely 60 millivolts OVeI' the entire seven week period shown. This is in cont:ras to all the other coatings tested in FIGURE 1 which exhibited significant potential increases. As noted before, the ~-Raney Ni-15Mo did not exhibit: any iron-fouling and did not have any appreciable thinning. The constant low overvoltage level is believed to be a result of this surprisingly unexpected constant nature of the coating during actual performance. It is seen that the mild steel sample, which started at about 540 milLivolts overvoltage (i.e. /(-0.94)-~-1.500)~volts), actually decreased in overpotential and then started rising. The explanation is the overplating of iron which has been recently found by others to cause increased roughness and hence lower actual curxent density and therefore lower overvoltage. It is well known that ovexpotential generally decreases when cuxrent density decreases. (See FIGURES 2 and 4~.

~68~

FXGURE 1 further shows that a major problem exists with prior art Raney nickel prepared from a purely Gamma phase inte~metallic structure (hereafter G-Raney Ni). The prior art G-~aney Ni cathode exhibited both significant spalling and iron pick-up.
FIGURES 2 and 3 show the overpotential curves versus current densi~y and time, respectively, for two catalytically coated cathodes of the invention, all prepared fxom Beta phase precursor. Each has a different percent by weight of molybdenum (10% for Ni-lOMo, 15 for Ni~15Mo) and a different method (plasma spraying and dipping) of depositing the aluminum prior to identical heat txeatment. Dipping a Ni-15Mo ~ubstrate in molten aluminum was found to produce, upon subsequent Raney txeatment, a ~-Raney ~i-lSMo cathode having ahout 20-40 mullivolts less cathode overvoltage than ~hat exhibited by a ~-Raney Ni-lOMo cathode wi~h a Ni-lOMo substrate on which the aluminum had been plasma sprayed prior to Raney treatment. The reason for this difference is not known, although the result was confirmed. It is believed that the difference in molybdenum content was primarily responsible.
FIGURE 4 is a polarization versus current density graph showing the relative overpotentials of ~Raney Ni cathod~s of the invention and prior art G-Raney Ni cathodes ~howing that ~-Raney Ni is initially about 60 milli~olts lower in overpotential than G-Raney Ni.
Re~erring to FIGIJRE l, it is seen that this difference increases with time.
FIGURE 5 i5 a 700X magnification cross-sectional view of a ~-Raney Ni 15Mo coating of the invention taken with a scanning electron microscope (SEM) showing at the bottom the core or substrate of nickel alloy with 15%
by weisht molybdenum (Ni-15Mo); a 40 micron layer of the Gamma phase (from Ni2A13 precursor) Raney Ni-15Mo or "G-~aney Ni-15Mo", immediately above the core and a 120 micron layer of the Beta phase (from NiA13 precursor~

~6~

Raney Ni-15Mo or "~-Raney Ni-15Mo" atop the G-Raney Ni-lSMo layer~ A portion of a conventiona:L medium in which the metal specimen was mounted appears at the upper right hand corner. It is s~en that the ~-Raney Ni 15Mo layer is three times as thick as the G-Raney Ni-15Mo layer and the ~-Raney Ni-15Mo layer is the outer layex and thus will be the layer in contact with any electrolyte in which the coated core is placed. Thus the ~-Raney Ni-15Mo controls the activity of the coating. Further, the ~-Raney Ni-15Mo does not fall off in th~ leaching step. Since the ~ Raney Ni-15Mo pr~dominates,this whole coating of FIGURE 5 is callectively called a ~-Raney Ni-15Mo coating.
FIGURE 6 is a 700X magnification SEM photo-micrograph substantially identical to FIGURE 5 except that it is taken after the coated core of FIGURE 5 was operated for o~er six weeks in a laboratory scale membrane cell under conditions sim~llating a typical commercial chlor-alkali diaphragm cell. The ~-Raney Ni-15Mo coating did not experience any appreciable thinning ater six weeks in a diaphragm cell catholyte, thus demonstrating that the ~-Raney Ni-15Mo does not fall off.
FIGURE 7 shows how the int~rdiffusion of nickel and aluminum proceeds at 610C. A given weight of Ni~Al3 has about 50 percent less~aluminum than the same weight of NiAl3. When there is an unlimited reservoir of aluminum and the alloying temperature is within the 660C to B60C range of the invention, an NiAl3 layer forms adjacent ~he aluminum reservoir and an Ni2A13 underneath. In FIGU~E 7 the aluminum is at the fax left side of the microphotograph, while nickel is at the ~ar ri~ht. This is seen to occur even at temperatures as low as 610C if the trea~ment is long enough. However, in FIGURE 7 the NiAl3 (Beta) layer ls only 5-lO microns thick while the Ni2A13 (Gamma) layer is about 35 microns thick as is proven by the microprobe readout. The solid white horizontal line on the photograph i5 the "scan" line along which the microprobe scanned and the white dots are the relative atomic percent nickel found at the corre-sponding location on the scan line, The corresponding location on the scan line is that point on the scan line which is directly above the corresponding dot. This preponderance of G~mma phase is similarly pronounced at higher ~emperatures and similar heat treatment times~
However, in FIGURES S and 6 where a Ni-15Mo alloy was used it was found that the Beta phase predominates. It is thus believed the molybdenum stabilizes the NiA13 phase so as to yield a constant surprisingly low over-voltage upon subsequent leaching.
Tha overvoltage reductions are based on operation of the electrode as ~he cathode in a brine electrolysis cell at a current density of 200 milliamps per square centimeter (i.e. 200 ma/cm2 or 2 RA/m2), which is typical of current densities found in conventional diaphragm chlor-alkali cells.
All voltage values quoted herein are based on ~he 200 ma/cm2 current density, although the electrodes are equally suitable for operation ovex a broad range of o~her current densities.
The porous nickel surface lay~r of the electrode of the present invention is formed on a metallic nickel core with which it is in electrical contact. The core material may be a~y conductive metal or alloy, but is preferably nickel or a nickel-molybdenum alloy so that the substrate itsel forms the coating after Raney treatment.
The electrode can be in the form of any conveniently shaped plate or screen. For commercial brine electrolysis cells, e~panded metal screens are preferred.
The electrode of the present invention may also bear a~ optional, very thin coating o~ nickel atop the porou~ nickel surface. The very thin coating, which is preferably S to 10 microns thick, improves the mechanical strength and surface stability of the porous nickel layer, without diminishing its electrochemical activity.

S

Electrodes of the invention are prepared by a~
improved process wherein an interdiffused nickel-aluminum alloy layer is formed, from which aluminum is subsequently selectively leached. The process includes the step~ of ~a) preparing a metallic core with a nickel-bearing outer layer, tb) aluminiziny the surface of the core, (c) interdi~fusing the aluminum and nickel, ~d~ selectively leaching alumin~m from the inter-diffu~ed material, (e) optionally chemically treating to prevent poten~ial pyrophoricity and (f) optionally coating with ni~kel t~ improve the mechanical properties of the final surface.
The m~tallic core which comprises the starting mater-ial for ~he electrode is prepared to have a nickel-bearing outer layer in which th~ nickel concen~ration is at least 15 percent, and pre~rably at ~east 18 percent by weight. When the core is of substantially pure ~ickel or an ~ppropriate nickel-bearing alloy such as Inconel 600, ~a~telloy C or 310 Stainles~ Steel, the core inheren$1y has the desired nic~el-bearing outer layer.
It i~ m~st preferred to have outer portions of the "core" (~core" is usea interchangeably herein wi~h n substrate") itself serve as the nickel-bearing outer layer, since thi~ helps eliminate or reduce spalli~g of the coating by eliminating or reducing the possibility of corrosion at the inter~ace between the outer layer and core by making the interface much less abrupt.
For cores of oth2rmetals or alloys, a nickel-coating can be deposited on the core by known techniques, such as metal dipping, electroplating, electroless plating and the like. The nickel-bearing outer layer of the core, whether provided by the core metal itself or as a deposited coatiny, is conveniently at least 100 microns thick, and preferably at least 150 microns thick. The maximum thickness of the nickel-bearing outer layer is a matter of convenience and economic choice. Although *Trade Mark ~68~5 cores in the form of screens or plates, especially screens/ are preferred, cores mad~ from foils, wires, tubes or expanded metal are also suitable. The nickel-bearing surface of the core, prior to further processing is thoroughly cleaned by conventional means, such as chemical cleaning and/or gritblasting, to improve the bond between the nickel-bearing surface of ~he core and subsequently applied layers.
The cleaned surface of the core is subjected to an aluminizing treatment~ By "aluminizing", as used herein, is meant tha~ aluminum is brought in~o initmate contact with the nickel-bearing material at the surface of the core so that when heated during the interdiffusion step the desired nickel-aluminum alloy iayer is formed. The aluminizing can be accomplished by any of several known methods, such as flame spraying aluminum onto the surface of the core, dipping t~e core into an aluminum melt or by use of fused salt electlolysis. Dipping is prefeIred since it has been found to yield the lowest overvsItage caating upon subsequent: Raney treatment.
When using these methods of aluminizing, an aluminum layer of at least 100 micron thickne~s is deposi~ed on the nickel-bearing surface of the core. Much thicker aluminum layers, of, for example,-greater than 500 micron thickness, perform satisfactorily in the process, but for reasons of economy, aluminum layer thicknesses of between about 150- and 300-microns are preferred.
The interdiffusion step, which is usually the next step in the process, is carried out at a temperature of at least 660C, i.e~, above the normal melting point of aluminum. Higher temperatures, under 750C are suitable, with temperatures within the range of from about 700C to about 750C and particularly from about 715 to about 735C
being most preferred. Usually the interdi~fusion is carried out in an atmosphere of hydrogen, nitrogen or an inert gas. This interdiffusion treatment is continued for a time sufficient for the aluminum and nickel to interdiffuse and form a nickel-aluminum alloy layer of at least 40 microns and preferably at least 80 ~61~ 5 microns in thickness. When the outer layer of the core is of substantially pure nickel, an interdiffused alloy layer of NiA13 forms in 1-10 minutes. Interdiffuse~
nickel-aluminum alloy layers of 100-400 microns in thickness are preferred, with best results being obtained when the thicknesses are between 150- and 300-microns. Unless molybdenum is added to the nickel-containing layer, ~he treatment is stopped by about 30 minutes 50 that only a minimum amount of Ni2A13 (Gamma phase) will form.
Since NiA13 has a higher proportion of aluminum than Ni2A13, it is believed that temperature should be high enough to allow relatively fast interdiffusion yet not so high that the supply of aluminum is used I5 up completely, because once the supply of "reservoir"
aluminum in ~he alum1num layer is used up, further diffusion merely encourages the diffused aluminum to diffuse or spread out more thinly ~md thus encourages formation of Ni2~13 or other less clesirable inter-metallics ha~ing a lower aluminum c:ontent than NiA13.
As noted above, a temperature within the range of from about 6~0C to 860C satisfies ~his need. Similarly, the interdif fusion time should be long enough to build up an interdiffused nickel alloy layer of suitable thickness but not so long as to deplete the aluminum reservoir. An interdif~usion time within the ran~e of from abou~ 1 minute to about 30 minutes satisfies this need.
FIGURE 5 presents a photomicrograph of a cross section of the ~-Raney Ni-15Mo cathode formed from an interdiffused nickel-aluminum Beta phase alloy layer that wa~ formed by dipping a Ni 15MQ substrate into molten aluminum and interdiffusing the ni~kel and aluminum at about 725C for about 10 minutes. The photomicrography shows the Ni-lSMo core, upon which is a relatively thin layer of Raneyed Ni2A13, atop of which is comparatively thick layer of Raneyed NiA13. In FIGURE 5, the ~rRaney Ni 15-Mo cathode that is formed by leaching is derived almost entirely from the Ni 85MO 15Al3 phase. Nic~el formed from the Ni 85M l5Ai3 phase is not lost from the active surface during the subsequent leaching step.
It is found that the Raney surface layer derived from Ni ~5Mo 15Al3 is stabilized by the 15 percent by wei~ht molybdenum. From about 5-20 percent by weight Mo is sufficient to stabilize the 8eta phase intermetallic.
The size of the Ni2A13 grains and the rat~ at which the thickness of the Ni2A13-containing layer grows are highly dependent on whether the aluminum layer is depleted the length of heat treatment as well as on the temperature a~ which the aluminum and nickel are interdiffused. Larger grain size and much faster buildup of the Ni2A13-containing layer accompany the use of lS temperatures of 750C or more.
Referriny now to the prior art~ FIGURE 6 of U.S.
Patent No. 4,116,804 shows the dependency of the average size OL the Ni2A13 grains on the temperature of inter-diffusion. Note the rapid increase in grain size that occurs at interdiffusion temperatures of above 660C, especially abo~e 700C. At interdiffusion temperatures below 660C, the size of the Ni2A13 grains are smaller than those found desirable for the later formation of the active porous nickel layer. Also, FIGURE 7 of U.S. Patent No. 4,116,804 shows, as a function of temperature, the time required for a 125 micron thick layer of Ni2A13 to form on a nickel core that had been flame-sprayed with aluminum. Note the rapidly increasing times that are required to obtain a given thickness of the Ni2A13 layer as the temperatures of interdiffusio~ are decreased. For the conditions shown in said FIGURE 7, formation of the Gamma phase layer thickness requires over 74 hours at 560C, over 29 hours at 600C, over 4 hours at 725C and over 1 hour at 860C. Thus the 1-30 minute time of heat treating in the present invention is much less and therefore less wasteful of fuel supplie~ and yet as noted elsewhere gives a coating that also uses less power in operation.

During the trea~ment at temperatures above 660C, excessively long interdiffusion times~ e.g.
one hour or more, are avoided for technical, as well as economic reasons. Temperatures above about 860C are generally avoided because the equilibrium phase diagram for Al-Ni, shows that Beta phase transforms into liquid and Gamma pha~e (Ni2A13) above about 85~DC. Also, fox coatings on a substrate differing in ¢omposition from the coating, extended heat ~reatment such as axe needed to produce Gamma phase coating~ might damage the substrate or form undesirable brittle intermetallics of the coating-substrate interface. For example~ luminum is diffused into a nickel-coated-steel core, excessive interdif~usion time or temperature can result in the alumi~um "breaking through" to the steel base o~ the oore, i~e~, the all~m;num di~fl~ses all th~ way through the nickel ~nto ~he steel core. Break-through is accompanied by the formation of a very bri~tle FeA13 ~5 intermetallic phase, which can significantly undermine the strength of the bond between the core ~nd the inter-diffused layer. Also, if interdiffusion is conti~ued too long, all of the available aluminum can be diffused in*o the nick~l such that ~here is still a large excess of nickel in the interdiffused material. Under these latter circumstances, and also frequently ~hen inter-diffusion temperatures of above about 1000C are used, an inte~metallic pha~e form~ r which does not permit satis~actory subsequent leaching of the aluminum from the intermetallic, and consequently, a highly acti~e porous nickel does not form. By providins sufficient ` quantities of nickel and al~minumt while avoiding excessively long treatments or e~cessively high temperatures durin~ interdiffusion, break-through and formation of the undesired intermetallics are a~7Oided.
As described above, the aluminizing and inter-diffusion steps are carried out ~equentially. Mowever, the steps can also be performed ~imultaneously by a pack-diffusion technique. For example, a mixture of aluminum and alumina powders and an activator can be packed around a nickel core and then heated in a hydro-gen atmosphere at a temperature of 750C for about 8 hours to form the desired nickel~alu~.inum alloy layQr.
The formation of the desired nickel-aluminum alloy layer is followed by a selective leaching step, wherein sufficient aluminum is removed from the surface and the nickel-aluminum alloy layer to form an active nickel surface layer. The average size of the active nickel agglomerates is generally less than 35 microns. Such an active layer is shown in cross section in the scanning-electron micrographs of FI~URES 5 and 6.
Generally, a strong aqueous base, such as NaOH, KOH
or other strongly basic solution capable of dissolving aluminum, is used in the selective leaching step.
Preferably, the salective leaching is carried out in aqueous caustic solutions containing about l to about 30 weight percent NaOH. For example, a selective leaching treatment of 20 hours in 10 percent NaOH at ambient conditions (i.e., temperature is not controlled) or a treatment of 14 hours in 10 percent NaOH at ambient temperatures followed by 6 hours in 30 percent NaOH at 100C has been found satisfactory for producing porous nickel surfaces of the invention. A preferred selective leaching procedure is carried out first for 2 hours in l percent NaOH, then for 20 hours in 10 percent ~aOH, both of these substeps under conditions in which temperature is not controlled, and finally for 4 hours in 30 percent NaOH at 100C. The leaching procedure ~68P85 removes at least about 60 percent, and prefer~bly between about 75 and about 95 percent, of the aluminum from the interdiffused alloy layer and provides a po~ous nickel surface o~ unusually high electrochemical activity. It is recognized that the leaching conditisns can be varied from those mentioned above to achieve effecti~e selective dissolution of the aluminum.
After the selective leaching/ the active nickel coatings may exhibit a ~endency ~o heat when exposed to air. This self-heating tendency could possibly lead to problems of pyrophoricity. However, an optional step of chemically treating the porous nickel layer can be used to eliminate this potential problem. Convenient methods for ~his chemical treatment include immersing the porous nickel for at least l hour and usually less than 4 hours in a dilute aqueous solution containing, for example, by weight (a) 3 percent NaN03 or (b) 3 percent K2Cr2O7 or (c) 3 percent NaClO3 and 10 percen~
NaOH. These treatments eliminatP the self-heating tendency of the porous nickel or n:ickel-molybdenum surface without diminishing its electrochemical activity or mechanical properties.
Although the ac~ive porous nickel sur~ace layers, as prepared by the preceding steps, have satisfactory mechani-cal properties and low tendency to spall, compared with many of the Raney nickel surfaces of the prior art, the mechanical properties o~ khe layer can,be improved by op-tionally coating a very thin layer of nickel onto the porous surface. This nickel layer, which is preferably 5 to 10 microns thick and can be applied from conventional electroless nickel or nickel electroplating baths, enhances the mechanical strength of the porous nickel layer without diminishing its electrochemical activity.

8~35 Some Advanta~es of the Invention Contamînation o~ low overvoltage coatings by over-platings of higher overvoltage metals has now been found to be one of the major obstacles to a long-life low overvolta~e cathode, and was initially felt to be an insurmountable obstacle. However, overplating has now, in the cathodes of the invention, surprisingly and unexpectedly been virtually eliminated. Thi~ is a major technical breakthrough in enabling long life, low overvoltaye coati~gs. It has now been found that there is a threshold potential for such o~erplating and that when the cathode polarization potential is redu~ed below about -1.100 volts (as measured against a standard mercury mercury oxide hydrogen electrode), i.e. below about 140 millivolts over~oltage, that fouling with higher overvoltage metals, such as for example iron and copper, are substantially eliminated.
While it is practical to remove ~any metal contaminants other than iron and copper ~rom the catholyte, there is somewhat more of a probl~m with iron and copper removal since the plumbing pipes carrying water to the ca~hode chamher are often coppe~, iron or steel and the cell housing itself is often made in whole or part of iron or steel for str~ngth and electrical connections of the cell are often mad~ of copper because of its relatively high conductiYity. In conventional chlor-alkali cells, ~his contamination comes m~stly from iron.
It has surprisingly and unexpectedly been found that this elimination o iron-fouling occurs in the most active cathodes, that is the cathodes having the lowest overvoltage. Thus the present invention gives the best o~ both worlds, an amazingly low overvoltage and an amazingly long life. Heretofore the two were felt to be incompatible objectives. Namely, it was believed that low overpotential cathode coatings suffered from short life. This is txue even though stable metal anodes of low overpotential are known, because the problem with anodes was corrosion, not overplating. By its very nature, an anode tands to corrode while a cathode tends to receive deposits.
Solving cathode corrosion alone does not give low overvoltage or even stable over~oltage. The explana~
tion for this elimination of iron fouling is not definitely known.
One possible explanation is the anodic shift of~handedly reported in a recondite Ru~sian fuel cell research article, Golin et al, "Connection Between Chemical and Electrochemical Activity of Raney Nickel Catalysts", Electrokhimiy~, Vol. 13~ No. 7, pp. 1052-1056 (USS~ Julyl 1977). If this shift is such tha~
the catalyst at rest has about the same potential as steel and then shi~ts 300 mV or so anodically in solution, there may be little or no residual electrical potential between steel and the coating during cell shut-downs to cause iron-fouling a.nd yet when the cath~de is in operation the anodic shift may lower overvoltage to near theoretical (as compared to steel cathodes which may not experience such a shift). Whatever the cause may actually be, the surpri~ing result i5 a "non-fouling" cathode, i.e. one that does not e~perience anyfsubstantial-iron ouling.
It has also now been found that yet another unexpect~d and surprising result is achieved when molybdenum is added to a Beta phase (NiA13) intermetallic.
The Beta phase formation is stabilized by the addition of molybdenum in the amount of about 5-20 percent by weight of the total weight of nickel and molybdenum.
This molybdenum is apparently captured in the ordered orthorhombic Beta phase crystal structure such that the Beta phase can be represented by the fonmula ~6~5 Ni~Mol xAl3 where x is th weight percent nic~sel in the total weight of nickel and molybdenum. By "stabili~ed" is meant that once the Beta phase for~ns th~rP is less of a tenden~ to transfo3im t~ Gamma phase S (Ni2A13) arld t:hus the elevated heat treat~nent tempera-t~re can last longer wi~hout as much ~:amma phase being f ormed . ~n f act, the heat treatment at the optimum 725C car~ la~;t fox 2 hour~, or 4 hours or e~en 6 hour~
with a ~-Raney ~i-Mo c:athode ~till beiIlg produced.
Since it i~ llDW sh~awn that ths Beta phase is the ir~termetallic of choice, . this is an important ad-vanta~e of the Ni-Mo-Al ternary alloy o~er the ~i-Al binary alloy.
It has further been found lthat the use oiE Mo in the coating reduces the heat of desorption of hyd:rogen (dete~nined by gas phase desorption) and khat this reduction correlates directly with the reduced over-potentlal of the Raney nickel with 5-2D percent by weight mol~bdenum as compared with p~re Raney nicke~, both having been prepared fro~ Beta phase inter~etallic prec~rsors~ It has further been fou~d that the Beta phase nickel indeed has a lower heat of desorption than the Gamma phase, as previously reported in the Za~rin et al Russian article noted above.
The preferred elec~rode is a monolithic structure of a Ni-Mo alloy of 5-20 percent and most preferably from about I2-18 percent by weight molybdenum and about 80-95 percent and most preferably 8~-88 percent by weight nickel which has been give~ a Raney treatment by dipping in molten aluminum and heating for about 1-30 munutes in an inert a~n~sphere at a temperature of froSn about 660C ~o about 855C. A ~emperature o~

about 700C to about 750C and a time of about 5-15 minutes are more preferred because this gives sufficient time for enough alumlnum to interdiffuse into the nickel to provide maximum preponderance of NiA13 or Beta phase over Gamma phase (Ni2A13) but does no~ allow enough time for the difXu ion to result in the preponderance of Gamma phase (Ni2A13) as was specifically called for in U.5. Patent No. 4,116,804, noted above.
Contra~y to the di.sclosure of U.S. Patent No.
4,116~804, it has been surprisingly found that the ~
Beta phase Ni~13 when molybdenum is added thereto, is not lost during leaching and in fact experiences no appreciable thinning during subsequent use in a chlor-alkali. cell. (see FIGU~ES 5 and 6~.
It was initially hypothesized that the non-fouling nature of the NiA13 surface la~rer was due to a gradual slow erosion and falling off of the individual OUteDSt NiA13 grains along with any iron which had been deposited thereon. In fact, it was ven thought that such slow continual erosiQn would be desirable to prevent iron buildup on the cathode surface, even though such erosion would make for a shorter cathode life than if ~here were no such erosion. However, it is most unexpected ~nd startling to find that there was no substantial erosion of the ~oating and that even though there was no substantial exosion the cathode did not pick up any substantial amount of iron. The addi-tional molybdenum had apparently lowered the overvoltage below some threshold le~el where iron fouling ceases to occur.

The surprising non-thinning of the NiA13 type coatings indicates there i8 probably some othex as yet unknown cause for the unexpected superior resis-tance of the NiA13 coating to iron fouling. In view of the teaching of U.S. Patent No. 4,116,804 that NiA13 is mechanically weak, it was not expected that molybd~num addition would re~ult in a stabilized layer.
In fa~t, U.S~ Patent No. 3,947,331, issued March 30, 1976 tv ANVA~ with Kinh and Montvelle as inven~ors, teaches that codeposits of nickel and molybdenum con-ventionally give layers of little mechanical strength, porous, fissured and incompa~ible with any practical industrial usen Such fissuring mi~ht conceivably be useful in some crystalline coatings which are not given a Raney treatment since such coatings might benefit rom the increased surf ace area generated by such fissures. Howevex, with a Raney surface fur~her fissuring would seem to be harmful., rather than helpful, because such fissuring would seeminyly tend to make the already ragged Raney mlcrostructure break apart and fall off. In Raney coatings, the ordinary artisan seaks strength, not weakness. Thus it was not only ~urprising bu-t rather startling that the Ran~y nickel coa~ing with added molybdenum could survive in a typical chlor-alkali cell enviro~ment without any appreciabl~ thinning of the coating.

~L6~35

-2~

Advantageous use can be made of the electrodes of the in~ention, especially as hydrogen-e~olution cathodes of cells intended for the electrolysis of brine, watex or the like. The electrodes are parti~u-larly preferred for use in brine electrolysis cells, wharein the high electrochen~cal activity of the ~-Raney nickel or nickel-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. For example, a tubulax screen electrode of the in~ention, with suction established through the inside of the tube, can be immersed in an aqueous dispersion of polytetrafluoro-ethylene fibers and asbestos fibers. The fibers are sucked onto the outer surface of the screen until a diaphragm of the desired thickness is formed. After removal of the suction, water is removed from the assembly, as for example, by heating at 95C for 5 hours. The assembly is then heated at 350C for about one-half hour in an inert atmosphere, to complete the diaphra~n fabrication. As is knowm in the art, the satisfactory operating lifetime of such diaphragms i5 not nearly-as long as that of th~ cathodes of the brine elec~roIysis cells. Economlcs dictates tha~ the diaphragms must be changed sever 1 times during the operating life of the cathode. With electrodes of the present invention, the diaphragms can be readily stripped from the porous nickel surface and replaced many times with insignificant detrimen~ to the electro-chemical activity or mechanical properties of the electrode. Similarly satisfactory results are also obtained with other diaphragm materials and with membrane materials (such as cationic exchange membranes of hydrophilic phosphonated, sulfonated or carboxylated fluorocarbontelomers blended with inert fibers such as asbestos, glass, tetrafluoroethylene and polytetrafluoro-ethylene).

~6~ 1~5 Test Methods The various parametexs associated with the present invention are measured by the techniques described below.

Thi ~

Scanning electron micrograph cross sections are prepared perpendicular to the surface of the electrode.
Micrographs are taken of typical areas of the cross section. A convenient magnification, usually between 150 and 700X permits inclusion of the entire thickness of the porous nickel layer in the photomicrograph.
The thickness o the porous nickel layer is determined by measuring the layer thickness depicted in the photomicrograph and dividing ~y the magnification.
At least five such measurements axe made on at least three micrographs and then aver~ged to obtain the thicknes~ o~ tha porous nickel layer of the electrode.
For electrodes of the invention, t:his provides thickness mea~urements having a coefficient of variation of generally le9s than 5 percent. Photomicrographs of the type that can be used to make these thickness measuremen$s are given in FIGURES 5 and 60

3~6~ 5 Scanning electron micrographs are pxepaxed o~
randomly selected areas of the surface of the porous nic~el layer of the electrode. The magnification is conveniently set between about 100 and 500X. The micrograph is printed on photographic paper of uniform weight. As can be seen from the scanning el~ctron micrograph of FIGURE 5 or 6, the individual agglomexates of the porous nickel of the electrodes of the invention (labelled "A"~ are readily identifiablei the dark areas between and within the agglomerates (labelled "B") depict the porous regions. Generally, a magnification is selected so tha~ at least five full agglomerates are displayed in the photomicrograph. The surface porosity and the average agglomerate size can be measured from ~he micrographs as follows:
1. From a prepared micrograph depicting a typical area of the surface of the electrode, cut away the agglomerates that are only partially shown at the edges of the micrograph. In deleting the partial agglomerates, cut along the centerline of the porous region between the partial agglomerate and the closes~ whole agglomer-ate. Measure the area of the remaining portion of the micrograph. The area of the micrograph divid~d by t~e 2S square of the magnification equals the actual area, S, of the surface being analyzed~ Determ~ne the weight, W, of the cut-out area. 2. Count the number of agglomer-ates, N, within the cut-out area of the micrograph that lie in the plane of the surface of the specimen. Those that clearly lie in a plane below the surface of the specimen are not counted.

~8:~5 3. Subdivide the cut-out portion of the micrograph into subcutting~ of areas depicting the agglomerates in the plane of the specimen surface and areas depicting porous regions in the plane of the specimen sur~ace. In making these subdivisions care is taken to include in the poxous area of the specimen surface, the are~s of the out-out micrograph that are shown as (a~ black regions, (b) agglomerates that are clearly benea~h the plane of the specimen sur~ace and (c) sides of agglomera~es, lying below and at an angle to the plane of the specimen surface (usually appearing as a somewhat liyhter shad~
than the black porous regions~ .

4. Weigh.the cuttings representing the agglomerates, wa, and the cuttings representing the porous regions, wp, and assure that no cuttings were lost by checking that wa + wp = W.

5. Calculate the surface porosity, P, expressed ~s a percentage, from P = (wp/W) 100.

6. Calculate the agglomerate size, D, from 2Q D - [S/N(l-P/100)~
, 7~ Make rPplicate measurements to determine average values for the samples studied. For.electrode of the invention, five replicate m~asurements are usually sufficient to result in average values of P and D ha~ing coefficients o~ variations of less than 10 percent.
An alternative method for measuring the average agglomerate size, D, is to (1) cut out at least five typical agglomerates from each of five micrographs taken at the same magnification, X, of the surface of the electrode; (2) determine the total weight, w, and number, n, of the cut-out agglomerates; (3) measure K, the wei~ht per unit area of the micrograph paper; and (4) calculate the average agglomerate size from D = [(w/n)/K~[l/X~

Separate micrographs of the intermetallic precursors are not needed since the structure can be asrertained readily from the leached sample. The leaching is seldom f if ever, continued long enough to leach clear through to the core or substrate, as that would tend to dissolve any intermetallics at the core coating interface and thus tend to loosen the coating from the coreu Thus a layer of unleached alloy is generally available for viewing in the micrographs of the Raneyed coating.
~owever, separate micrographs of the unleached layer could be made as follows:
Metallographic cross sec~ions are prepared perpen-dicular to the surface of the precursor of the electrode r that is, after the interdiffusion treatment but prior to the selec~ive leaching step. Plane polarized light is used~ Photomicrographs are taken of typical areas of ~he cross section to include the layer cont~ining the nickel-alu~inum alloy. Convenient magnifications are between lS0 and 700X. The th:ickness of the nickel- `~
alumlnum binary or nickel~molybdenum-aluminum ternary alloy layer is then determined in the same manner as described above for tne thickness of the porous nickel layer~
When the nickel-aluminum alloy layer is of Ni2A13 or NiA13, measurement of the size of the grains i5 facilitated by superimposing a grid on the photomicro-graph of the layex. Ten squares of the grid are randomly selected from the middle 80 percent of the NiA13 or Ni2A13 containing layer. The total n~mber of grains, Z, within the boundary of each square is counted. The area of the grid on the photomicrograph divided by the square of the magnification is the actual area, A~ of the layer under examination. The size, d, o the NiA13 or Ni2A13 grains in the layer, for each grid are examined, ~L~L68~5 is then calculated ~rom d = (A/Z)~. This formula.holds for the layers that consist essentially of NiA13 or Ni2A13 grains. The average NiAl~ or Ni2A13 grain size for a gi~en sampl~ is then simply the average of the size of the grain~ for each o ~he 10 grids. To characterize the NiA13 or:Ni2A13 grain siæe in the precursor of the electrode of the invention, at least three photomicrographs of the cross section axe sub-jected to the aboYe analysis and result in a measurement that has a coefficient of variation of less than 5 percent.

~6~

Metallogra ~ ens The cross sections to be subjected to the micro-yraphic examinations described above are prepared as follows. A sample is cut and sectioned by use o~ a diamond saw operating at low speed. The specimen is then.mounted in an epoxy resin. Convenient dimensions for the cross section of the specimen are about 6 by 13 millimeters. Primaxy polishing of the specimen is carried out on a polish ng wheel equipped with silicon caxbide papers of grades 240A, 400A and 600A. Fine polishin~
is then accomplished by use of (a) 1.0 micron levigated a-alumina on a felt-covered wheel and then (b) 0.05 micron levigated ~-alumina on a mirro-~loth covered wheel.

Elec~rocb_ ical Cell FIGURE 8 shows the stxuc~ure of a test cell used for measuring the cathode potentials of the various plate electrodes o~ the samples given below.

Elec~xochemical ~easurements A schematic diagram of an elec~rochemical test cell, used for measuring the cathode.potentials of the various plate electrodes of the examples below, is given in FIGURE 8.

Test cell 1, made of tetrafluoroethylene ("TFE"), is divided by diaphragm 2 into two ch~mbers, cathode ch~mber 10 and anode chamber 20. The diaphragm, which is placed between two TFE separators sealed in place by caustic-resistant gaskets, is mad~ of Nafion~ 227, which is a homogeneous fi~m 7 mils thick of 1200 equivalent-weight perfluorosulfonic acid resin which has been chemically modi~ied by ethylene diamine converting a depth of 1.5 mils to the perfluorosulfonamide laminated with a "T-12" tetra-fluoroethylene filament fabric marketed by duPon~.
A circular titanium anode 21 of two square centimeters area coated with a titanium oxide-ruthenium oxide mixed crystal is ins~alled at the end of the cathode in the anode chamber. Test electrode 11/ which becomes the cathode of the test cell, is installed at the end of the cathode chamher by means of flanges and gasket~
(not shown). Perforated tetrafluoroethylene separators 3 and 4 are placed between membranle 2 and anode 21 and cathode 11, respectivelyO
A circular area of one square centimeter of the porous nickel surface of the test electrode is exposed to the interior of the cathode compartment. The cathode and anode are connected electrically ~o control-lable voltage source by cathode current collector 12 and anode current collector 22. An ammeter is connected in the line between the two electrodes. The entire cell 10 is then immersed-in a thermostated liquid bath so as to give a constant operating tempe~ature (e.g., 85C).
Catholyte, consisting of an aqueous solution, containing 11 weighk percent sodium hydroxide and 15 weight percent sodium chloride, is pumped through inlet 13 into the cathode compartment at a rate which establishes an overflow through outlet 14. The catholyte is main-tained at 85C. Similarly, anolyte, consisting of an aqueous solution of 1.5 pH containing 24-26 weight percent sodium chloride is pumped through inlet 23 into the anode compartment and overflowed through outlet 24.

~61~ 8~

~36-The salt concentra~ions of the catholyte and anolyte are typical of that encountered in commercial brine electrolysis cells. The use of separate catholyte and anolyte feeds, rather than a single brine feed, assures better control of the desired catholyte composition.
The catholyte and anolyte flows are controlled so that there is a small ~low of solution from the anode to the cathode compartment, which flow is su~ficient to assure ionic conductivity across the cell, but insufficient ~o to significantly affect the catholyt~ composition.
Luggin tetra~luoroethylene capillary 25, installed in the.cathode chamber 10 and Luggin capillary 15 installed in the cathode chamber 10 and positioned 1/2 mm from the surface are each connected to a respec-tive mercury-mercury oxide reference electrode or "S.~.E."
(not shown), which in turn is connected through volt-meter 6, to the other electrode of cell 10. A Luggin capillary i~ a probe which, in making ionic or electrolytic contact between the anode or cathode and the re~erence electrod~, mini~izes the voltage drop due to solu~ion resistance and permits direct measureme~ of the anode or cathode potential with respect to ~he reference electrode.
~o determine the cathode potential of a test electrode, a voltage is impressed between the anode and test electrode (i.e~, cathode), such that a current density of 200 ma/cm2 is es~ablished at the cathode.
The current density is the current measured by the ammeter in milliamps divided by the area (i.e., 1 cm23 of the porous nickel surface of the test electrode exposed to catholyte. Thus 200 ma would be applied to cathode 11 to achieve a current density of 200 ma/cm .
Hydrogen gas, generated at the cathode is removed from the cathode compartmen~ through catholyte outlet 14.
Chlorine gas, generated at the platinum anode, i~
similarly removed through anolyte outlet 24. The cell is operated in this manner for at least 2 hours prior to reading the cathode potential directly ~rom the voltmeter.

~6&~ 5 In each of the examples, electrodes are prepared and tested as cathodes in brine electrolysis test cells.
All characterizations are carried out in accordance with the test procedures described above. Unless stated otherwise, all compositions are given as weight percentages.

EX~XPL~ 1 .

Five groups of test electrodes are prepared as 1~ ~ollows.:
1. Mild.Steel.
A thoroughly cleaned mild steel coupon.
2. G-Raney Ni on nickel core (prior art).
A 1.6-mm-thick nickel 200 sheet, assaying at least 99 percent nic~el, is ~ut.into a coupon measuring about one cm . The coupon which is to become the core of the electrode is thoroughly cleaned by degreasing with acetone, lightly etching with 10 percent ~ICl, rinsing with water and after drying, grit blasting with No. 24 grit Al2O3 at a pressure of 3.4 kg/cm2 (50 psi).
The cleaned nickel coupon is alumi~ized by flame-spraying a 305-micron-thick coating of aluminum on the s~rface of the nickel coupon~ A conventional plasma-arc spray gun operating at 13 to 16 ki:lowatts at a distance about lO cm from the coupon is used with aluminum powder of -200 to ~325 mesh~
The aluminized nickel coupon is heat treated at 760C for 8 hours in a nitrogen a~mosphere to interdiffuse the nickel and aluminum and form a layer 30~ which is pxedominantly Gamma phase (Ni2A13) nickel aluminide. After heat treating, the coupon is allowed to cool in a current of nitrogen for about 2 hours.
This produces a predominantly Ni2A13 interdiffused layer.

8~

The remaining coupon is then subjected to a leaching treatmen~ wherein the aluminum i5 selectively ~emoved from the interdiffused layer to leave an active porous nickel sur~ace on the coupon~ The leaching treatment consist~ of immer~ing the interdiffused coupon in 10 percent NaOH for 20 hours, without temperature control, followed by 4 hour~ in 30 percent MaOH at 100C. The coupon i~ then rinsed with water for 30 minutes.
3. ~-~aney ~i on nickel core (plasma sprayed)O
A 1.6-mm-thick nickel 200 sheet, assaying at least 99 percent nickel, i5 cut inko a coupon measurin~ about one cm2~ The coupon which is to become the core of the electrode is thoroughly cleaned by degreasing with acetone, lightly etching with 10 percent HCl 9 rinsing with water and after drying, grit blasting with No. 24.grit Al2O3 at a pressure of 3.4 kg/cm2 (50 psi).
The cleaned nickel coupon is aluminized by ~lame-spraying a 305-micron~thick coati~g of aluminum on the surface of the nickel coupon. A conventional plasma-arc spray gun operati~g at 13 to 1.6 kilowatts at a distance about 10 cm from the coupon is used with aluminum powder of -200 to ~325 mesh.
The aluminized nickel coupon is heat treated ~t 725C for 10 minutes in a nitrogen a~mosphere to interdiffuse the nickel and alumin~ and orm a layer which is predomin~ntly Beta phase (NlAl3) nickel alumlnide. After heat treating, the coupon is allowed to cool in a current of nitrogen for about 2 hours.
This produces a predominantly NiAl3 interdiffused layer.
The remaining coupon is then subjected to a leaching treatment wherein the aluminum i5 se~ectively removed from the interdiffused layer tv lea~e an active porous nickel surface on ~he coupon. The leaching treatment consists of immersing the interdiffused coupon in 10 percent NaOH for 20 hours, without temperature control, followed by 4 hours in 30 percent - 3a -~L~68:~35 NaOH at 100C. The coupon is then rinsed with water for 30 minutes.
4. ~Raney ~i on nickel core (dipped~. A 1O6-mm-thick nickel 200 sheet, assaying at least 99 percent ~ickel, is cut into a coupo~ measuring about one cm2.
The coupon which is to become the core of the electrode is thoxoughly cleaned by degreasing with acetone, lightly etching with 10 percent HCl, rinsing with water and after drying, grit blasting with No. 24 grit A1203 lQ at a pressure of 3.4 kg/Gm ~50 psi).
The clea~ed nickel coupon is aluminized by applying a commercial flux and then dipping in a pot of molten aluminum for a sufficient time to entirely coat the . coupon with aluminum.
1~ The alumlnized nickel coupon is heat treated at 725C or 10 minutes i~ a nitrogen atmosphere to interdiffuse the nickel and aluminum and form a layer which is predominantly Beta phase (NiA13) nickel aluminide. After heat tr2atins, the coupon is allowed to cool in a current of nitrogen for about 2 hours.
This produces a predominantly NiA13 i~terdiffused layer.
The remaining coupon i~ then subjected to a leaching treatment wherein the aluminum is selectively remo~ed from the interdiffused layer to leave an active ~orous nickel surface on the coupon. The leaching treatment consists of immersing the interdif~used coupon in 10 percent NaOH for 20 hours, without temperature control, followed by 4 hours in 30 percent NaCH at .
100C. The coupon is then rinsed with water for 30 minutes.
5. ~-Raney Ni-15Mo on Ni-lSMo core (dipped).
A 1.6-mm-thick sheet of an alloy assaying at least 84 percent nickel and 15.0 ~ 0.1 percent Mo (Ni-15Mo) is cut into a circular coupon measuring about one cm2.
The coupon which is to become the core of the electrode is thoroughly cleaned by degreasing with acetone, lightly etching with 10 percent HCl, rinsing with water and after drying~ grit bla~ti~g with No. 24 grit A1203 at a pressure of 3.4 kg/cm~ (50 psi~.

s The cleaned nickel-molybdenum coupon is aluminized by applying a commercial ~lux and then dippiny in a pot of molte~ aluminum for a sufficient time to entir~ly coat the coupon with aluminum.
The aluminized nickel-molybdenum coupon i5 heat treated at 725C for 10 minutes in a nitrogen atmosphere to interdiffuse ~he nickel and aluminum and form,a la~er which iq predcmlnantl~ Eeta phase nickel mol~bdenum alu~i~
nide ((Ni-15~)A13). A~ter heat treating, the coupon is allowed to cool in a current of nitrogen for about 2 hoursO This produces a predominahtlv Ni-15~oA13 lnterdiffused la~er.
The remaining coupon is then subjected to a leaching treatment wherein the aluminum is selectively removed from the interdiffused layer to leave an active porous nickel-molybdenum surface on the coupon~ The leaching treatmen~ consists of immlersing the inter-diffused coupon in 10 percent NaOEl for 20 hours, without temperature control~ *ollowed by 4 houxs in 30 pe~cent NaOH at 100~C. The coupon is then rinsed with w~ter for 30 minutPs.

The cathoae potentials are monitored fox 45 days to determine if the potential experiences a steady increase or i~stead levels out at some valueO
2~ The xesults are plotted in FIGURE 1~ It is seen that ~hQ Raney Ni-15Mo from Beta phase precursor is consta~t from start to finish at a lower level than the other four samples and that the Gamma phase sample, which in.itially had a potential of about 120 mV more cathodïc than the R~ney Ni~-lSMo~ after 45 days has about 320 m~ more cathodic potential. Also, the Raney Ni (without molybdenum) from Beta phase initially has 50 mV and 90 mV more cathodic potential than the Raney Ni-15Mo from Beta precursor (dipped) depending on whether dipped or plasma sprayed. However, .

40 ~

~6~ 5 the Raney Ni from Beta precursor without added molybdenum experiences an increase in cathodic pote~tial of about 50 millivo.lts ov~r the 45 day testO It is also seen that clean mild steel initially had a potential drop of about 150 mV and then slowly increased back to its original starting value of about -l.SOO volts. The constant low overpotential of 60 millivolts for the Raney Ni-lSMo from Beta phase precursor is unexpected.

EXAMPL~ 2 A ~-Raney Ni-lSMo coupon of the invention is prepared by the same procedure as for coupon 5 of Example 1. A second coupon of the invention is pre-pared b~ the same procedure as for coupon 2 of Example 1 except that instead of a 99 percent+ nickel sheet a Ni go~Mo lO sheet is used instead, so as ~o produce a ~-Raney Ni-lOMo (plasma sprayed).
- The results are plotted in FIGU~E 2 as cathode polarization potential (IR Free) versus current density.
~-Raney Ni~15Mo has 20~40 mlllivolts less polarization, i.e., less overvoltage. At a typical current density for diaphragm of 200 ma/cm2, the cathodic potential is a~out 0.97 volts for ~-Raney Ni-lOMo (plasma sprayed) and about -0.93 volts for ~-Raney Ni-15Mo (dipped).
At 200 ma/cm~ a typical IR Free cathodic potential ~or the mild steel electrode of Example 1 was -1~28 volts (see FIGURE 4).

-~2-EX~MPLE 3 The oupons of Example 2 are tested for 45 days at 200 ma/cm current density in the standard catholyte (15 percent NaCl, 11 percent NaOH, 0.1 percent NaC103, 73.9 perc~nt H20 at 85C) and measured against a mercury, mercury oxide ("Standard Hydrogen Electrode" or "S.H.E.") by the electro~hemical measurement technique noted above.
Two coupons of ~-~aney Ni-15Mo (dipped) and one coupon of ~ Raney Ni-lOMo (plasma sprayed) are used~ The ~-Raney Ni~lSMo (dipped) coupons each have a constant cathodic potential of ~1.03 volts (90 millivolts overpotential) while the ~-Raney Ni-lOMo (plasma sprayed~ has a slowly fluctuating cathodic potential of -1.04 to -1.140 volts versus the S.H.E. The potential ~-Raney Ni-lOMo (plasma sprayed) levels out after about 4 weeks and remains steady at -1.08 volts ~140 millivolts) overpotential.

EXAMP~E 4 A first coupon is prepared according to the same procedure as prescribed for coupon 2 of Example 1 to yield ~ prior art G-Raney Ni coated Ni cathode. A second coupon is prepared according to the same procedure as prescribed for coupon 3 of Example 1 to yield a ~-Raney Ni coated Ni cathode of the invention~ A third coupon i~
prepared according to the method prescribed for coupon 1 of Example 1 to yield a mild steel cathode. The IR Fxee polariæation curves versus current density are determined by electrochemical measurements for the three coupons in a standard catholyte as described above. The ~-Raney Ni cathode of the invention has about 60 millivolts less polarization potential at 200 ma/cm2 than the prior art G-Raney Ni cathodP. Also plotted for reference is the g-Raney Ni-15Mo (dipped) coated Ni-15Mo cathode o~ FIGURE
2 and Example 2. The ~Raney Ni-15Mo (dipped) cathode of the invention has about 110 millivolts less overpotential at 200 ma/cm2.

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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 pre-dominantly derived from an integral adherent NixMol-xAl3 cry-stalline precursory surface layer structure, where x is no more than 0.95.

2. The electrode of claim 1 wherein x is within the range of from about 0.80 to about 0.95.

3. The electrode of claim 2 wherein the thickness of said Raney metal surface is less than about 7.5 x 10-5 meters (i.e., 75 microns or 3 mils).

4. 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 a molybdenum alloy of a weight percent molybdenum within the range of from about 5 to about 20 and a weight percent nickel within the range of from about 80 to about 95;
b) heat treating said coated surface by main-taining said surface at a temperature within the range of from about 660° to about 750°C
for a time sufficient to diffuse a portion of said aluminum into outer portions of said structure to produce an integral nickel-molybdenum-aluminum alloy layer in said outer portions consisting predominantly of Beta structured grains but insufficient to create a predominance of Gamma structured grains in said outer portions; and c) leaching from about 75 to about 95% of the aluminum from said surface with a strong aqueous base so that a Raney nickel-molybdenum layer is formed integral with said structure.

5. The method of claim 4 wherein said time is no more than two hours.

6. The method of claim 4 wherein said temperature main-tained during heat treating is within the range of from about 700°C to about 750°C.

7. The method of claim 6 wherein said temperature is within the range of from about 715°C to about 735°C.

8. The method of claim 4 wherein said coating step is applied by dipping said structure into molten aluminum at a temperature within the range of from about 650°C to about 675°C
for 1-2 minutes.

9. 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 being predom-inantly derived from an adherent integral crystalline precursory outer portion stabilized by inclusion of a stabilizing amount of a stabilizing metal together with nickel in the crystalline structure of said outer portion.

10. The electrode of claim 9 wherein said stabilizing metal is molybdenum and said stabilizing amount is within the range of from about 5 to about 20 weight percent of the total amount of nickel and molybdenum in said crystalline structure.

11. The electrode of claim 10 wherein said stabilizing amount is within the range of from about 12 to about 18 weight percent of the total amount of nickel and molybdenum in said crystalline structure.