CA1195654A - Low overvoltage hydrogen cathodes - Google Patents

Abstract

ABSTRACT

An activated cathode for use in electro-lytic processes has at least part of its surface portion of said cathode composed of a codeposit of a first metal selected from the group consisting of iron, cobalt, nickel, and mixtures thereof, and a second metal selected from the group consisting of cadmium, mercury, lead, silver, thallium, bismuth, copper, and mixtures thereof; the outer surface of the cathode is characterized as having a high rough-ness factor; the cathode has a lower hydrogen over-voltage and results in more efficient operation of the electrolyte cell.

Description

The present invention xelates to improved cathodes for use in elec-trolytic cells.
This application is a division of Canadian Patent Application Serial Number 365,389, filed November 25, 1980.
The cathodes of this invention have improved surface coatings on their active sides which substantially lowers the hydrogen overvoltage and results in a more efficient operation of the electrolytic cell. The cathodes of the present invention are particularly useful in the electrolysis of aqueous solution~ of alkali metal halides to produce alkali metal hydroxides and halogens, or :in the electrolysis of aqueous solutions of alkali metal halides to produce alkali metal halates, or in water electrolysis to produce hydrogen.
In an electrochemical cell, large quantities of electricity axe consumed to produce alkali me~al hydroxides, halogens, hydrogen, and alkali me-tal halates in electrochemical processes familiar to those ski.lled in the art. With increased cost of energy and fuel, -the savings of electricity, even in relat.ively min~r amounts, is of great economic advantage to the commercial operator of the ceLl.
Therefore, the ability to efEect savings in elec tricity t:hrough cell oper~tion, cell design, or im~)rovement in components, such as anodes and cathodes, is of increasing significance~

In such electrolytic pro~esses3 hydrcgen is evolYed at the cathode. and the overall reaction may be the~retically represented ~s:
~ 2~ ~ 2 e~ ~ H~ ~ 2 ~
However~ the cathode reac~ion actually produces monoatomic hydrogen on -the cathode surface, and consecutive stages of re~ction (13 can be represented as follows:
H O ~ e ~ H ~ OH

(2) 2 ff ~ 2 or as:
~2~ ~ e~ ~ H ~ OH

(3) ~ H2~ + e ~ + OH
The ~onatomic hydrogen generated as shown in reactions ~2) or (3) is adsorbed on the surface of the cathode and desorbed as hydrogen gas.
The Yoltage or potential that is rPquired in the operation of an electrolytic cell includes the total of the decoMposition voltage of ~he compound being electrolyzed, the voltage required to overcome the resistance of the electrolyte9 and the ~oltage required to overconle the resistance of the electrical connections w~th~n the cell. In ~ddition, a potential, known as ~'overvoltage", is ~lso required. ~he cathode overvoltage is the di~ference be-tween the thermodynamic potent;ial 4f the hydrogen electrode ~a~
equilibrium) and the potential oF an electrode on which hydrogen is evolved due to an impressed electr7c current. The cathode overvoltage is related to such factors as the ~echanism of hy-~5 drogen evolution and desorption, the current density, the tempe rature and composition of the electrolyte. the cathode ~terial and the surface area of the cathodeO

In recent years, ;ncreas;ng attention has been directed to-ward ;mprov;ng the hydrogen overYol~age characteristlcs of electro-ly~ic cell cathodes~ In ~ddition to having a reduced hydr~gen overvoltage, a oathode should also be constructed From materials that are inexpensiYe, easy to fabricate~ mechanically stronga and capable o~ withstanding the environmental conditions oF the electro-lytic cell. Iron or steel fulfills many of these requlrelnents, and has been the traditionzl material used comrnercially for cathode fabrication 1n the chlor-alkali indus~ry. When a chlor-alkali cell is by-passed. or in an open c;rcuit condition, ~he iron or steel cathodes become prone to electrolyte att~ck and their useful life is thereby significantly decreased.
Steel cathodes generally exhibit a cathode overvoltaye in the range of from about 300 ~o about 500 millivolts under typical cell operating conditions9 for example, at a temperature o-F about 100C.
and a current density of betw~en about 100 and about 200 ~illi-amperes per square centimeter. Efforts to decrease the hydrogen overvoltage of such cathodes have generally Focused on improving the catalytic effect of the surface material or providing a larger effectiYe surface area. In practice~ these efforts have frequently been frustrated by cathodes or cathode coatings which have been found to be either too expensive or which have only a l`;mited use-ful life ln commercial operation.
Various co~tings have been sug~ested to improve the hydrogen overYolta(Je characteristics of electroltyic cell cathodes in an economically viable manner. A si~nificant number of the prior art coatings have included nickel9 nr mixtures, allQys or intermetallic compounds of nickel with ~arious other metalsO Frequently9 when n~ckel is employed in admixture with another metal or compound~
the second metal or compound can be leached or extracted in a solution of~ for example, sodium hydroxide~ to provide a high surface area coatings9 such as Raney nickel coatings.

RepresentatiVe coatings of the prior art are dis losed in U.S. Patent 3,2919714, issued December 13~ 19669 and U.S. Patent 3,350,2g4, issued October 31, 1967. These patents disclose _ter alia cathode coatings comprising alloys of nickel-molybdenum or __ nickel-molybdenum-tungsten electroplated on iron or steel sub-strates. The electro-deposition of nickel-moly~denu~ alloys utilizing a pyrophosphate bath is also discussed by Havey, Krohn, and Hanneken in " The Electrodeposition of Nickel~Molybdenum Alloys", Journal ~ Soc;et~, Yol. 110~ page 362, (1963).
Other attempts ha~e been made in the prior art to produce coat;ngs of this general variety which offer an acceptable com-promise ~etween coating life and low overvoltage characteristics.
U.S. Patent 4,105,532, issued August 8D 1978, and U.S. Patent l~ ~,152,240, issued May 1~ 1979, are representative of these attempts d~sclosing, respectively, alloys of nickel-Molybdenum-vanadium and nickel-molybdenum using specially selected substrate and intermedia~e coatings of copper and/or dendritic copper.
Simllar coatings are also disclosed in U.S. Patents 4,033,837 and 3,291D714.
The surface trea~nent of a Raney nickel electrode with a cadmi~m n;trate solution for the purpose of reducing nydrogen overvoltage has been investigated by Korovin, Kozlowa and Savel'eva ~n "Effect of the Trea~ent of Surface Raney Nickel with Cadmium 2~ Nitrate on the Cathodic Evolution of Hydrogen", 50viet Electro-hemis~, Yol~ 14, page 1366 (1978). Although the initial results of such a coating exhibit a good reduction in h~drogen overvoltage, it has ~een found thak the overvoltage increases rapidly to the original 1evel after a short period of opera~ion~
30 i.e. about ~ hours~ -Even though many of the coatings described above have been successful in reducing hydrogen overvoltage, they have not proven entirely satisfac-tory due to rapid deteriora-tion of the coating in caustic environments, ultimately~leading to -the separation of the coating from the substrate material.
It is thus a primary object of the present invention ko provide cathodes suitable for use in electrolytic c011s that are economical to prepare, have reduced hyclrogen overvoltage charac-teristics, and exhibit minimal deterioration after prolonged operat:ion in electrolytic environments.
I~ere are d.isclosed cathodes for use in electrolytic processes, and a method for producing such cathodes. Such a cathode has at least part of its surface portion formed ~rom a aodeposit of a first metal selected from the group consisting of .iron, cobalt, nickel, and mixtures thereof, a second metal selected from the group consisting of cadmium, mercury, lead, silver, thallium, bismuth, copper, and mi~ture~ thereof, and a leachable third metal or metal ox.ide, preferably se].ected from the group consist.ing of molyhdenu~rl, manganese, titanium, tunysten, vanadium,indillm, chromium, zinc . -their 2S ox.ides, and combinations thereof. Prefe~rably, thi.s composition is applied as a coating to at least a portion of a substrate material sui.tably selectPd from cathode substrates known in the art such as, for example, nickel, tltanium, or a ferrous metal, such as iron or steel. The coatings are produced by codepositing, preferably using an electroplating bath or solution, a mixture of the three metals ~5 or metal oxides on the substrate surfaceO If the substrate is other than nickel, the substrate may be coated with a thin intermediate layer of nickel or alloys thereof, prior to depositing the active cathode surface. The third metal or metal oxide i5 subsequently removed, suitahly by leaching using an alkaline solution, such as an aqueous solution of an alkali metal hydroxide to produce a cathode of the invention. The leaching operation can be performed prior to placing the cathod~in operation in an elec-trolytic cell, or during actual operation in the cell by virtue of the presence of an alkali metal hydroxide in the electrolyte. Optionally, the cathodes of the present invention can he heat treated either before or a-fter at least partial leaching to improve the performance even further. The preferred coating of the present .invention comprises a codeposit of nickel and cadmium.
The cathode of the invention comprises at least an active surface portion formed fxorn a codeposit of a first metal selected from the group consisting of iron, cobalt, nickel, and mixtures thereof, and a second metal selected from the group con.sisting of cadmium, m~rcury, lead, silver, thalli.um, bismuth, copper, and mixtures thereof. ~he first and second meta:Ls are characteriæed as being substantia~ly nonleachable, i.e. they are removed very slowl~, if at al.l., by Leaching or extraction in an alkaline solution. Cathodes can also be obtained with a 3Q third component namely a metal or metal oxide.preferably selected from the group consisting of molybdenum, manganese, titanium, tungsten, vanadium, indium, ~ 3~

chromium, zinc, their oxides, and combinations thereof~
The third component is a leachable comp~nent, i.e~ at least a substa~ial p~rtion of this component is removable by leaching in an al~aline solution.
Hence, the proportions of the metals in the composition can be initially expected to change during operation in the cell, primarily due to the e~traction or leaching of the third component. The leaching action may be so extensive that virtually all of the third component is removed from the codeposita Under such circumstances, the absence of measurable amounts of the third component does not have an adverse effect on the performance of the cat'hode. In fact, leaching actually improves the perforrnance of the cathode by increasing the roughness and surface area of the cathode surface.
The present invention i5 concerned with cathodes having measurable quantities of only the first and second metal components in the codeposit after such leac'hing.
~ hus in accordance with the invention, suitable cathodes can be formed frorll a codeposit initially containing only the fir.st and second metal components, provided that the surface of the cathode has a rou~lness factor (defined as the ratio of the measurable surEace area to the geometrical surface area) 5uff'iciently hic~h enough to provide the d~sired clecrease in hydrogen overvoltage~ An 3(~ acceptable surface roughness factor in the context of this invention would be at least about 100, and 5~

preferably at least about l,000c Such cathodes can be prepared, for example, using chemica~ vapor deposition techniques, or by more conventional techniques, such as thermal fusion of the metals and subsequently etching the surface with a strong mineral acid. In this particular embodiment, the composition of the codeposit preferably contains from about 0.5 to about 25 atomic percent, and most preferably from about 1 to about 10 atomic percent, of the second metal component.
In addition to the two metal components, the composition may also include additional elements or compounds due to the particular method utilized for preparing the cathode. Such additional materials may be present in amounts of up to about 50% based on the total weight of the composition, and are perfectly acceptable provided they do not adversely afect the performance of the cathode.
The preferred metals of the present invention are nickel,and cadmium, present in the range of from about 0.5 to about 25 atomic percent, and preferably 1 to about 10 atomic percent, of cadmium, b~ased on the combined weight of nickel and caclmium, the nickel comprising the halance of the codepo~it.
Such a cathode has been found to produce surprisingly good results when utilized to electrolyze sodium chloride. For example, hydrogen overvoltages in the range of about 120 millivQlts of 150 ma/cm2 without heat treatment, and 80 millivolts at 150 ma~cm2 after heat treatment, are easily achievable using the cathode surface of this invention w~en applied to a standaxd ferrous substrate. These results can be even further improved by the appropriate selection of substrate material and cathode configuration, such as a woven wire mesh, a foraminous sheet, or a perforated and/or expanded me~al sheet. Furthermore, simulated life testing of this cathode for a pe~iod of 90 days in a 150 gr./liter caustic solution produces a relatively constant cell voltage, indicating suitability for long term operation in a cell.
Although the cathodes of the present invention rnay be formed entirely from the compositions described hereinabove, it is desirable, both from the standpoint of mechanical durability and reduced costs, to apply the codeposit in the form of a coating to a suitable substrate material. The sub-strate may be selected from any suitable materialhaving the requi.red electrical and mechanical pro-per.ties, and the chemical resistance to the parti-cular electrolytic solution in which it is to be used.
Generally conductive metals or alloys are useful, ~uch as ~errous metals (iron or steel), nickel, copper, or vaLve metals such as tungsten, titanium, -tantalum, niobium, vanadium, or alloys of these metals, such as a titanium/palladium alloy containin~
0~2% pal.ladium. Because of their mechanical pro-p~rties, eas0 of fabri.cation, and cost, ferrousme~tals, ~uch as iron or steel, are comrnonly used in chloî-alkali cells. However, in chlorate cells wh~e corrosion of the substrate material is a significant problem, titanium or titanium alloys are preferred.
It may also be desirable to apply an intermediate layer to the substrate material to protect the sub-strate from corrosion in the electrolytic cell environment. Suitable intermediate layers for this purpose include nickel, nickel codeposited with cadmium, and nickel codeposited with cadmium and f bismuth.
The preferred method for applying the sur-face coating to the substrate material is by electro-deposition in a suitable electroplating solution or bath. Although electrodeposition is a preferred method of preparation primarily due to the favorable economics of this particular proced~re, other methods o~ applica-tion, such as vapor deposition, thermal deposi.tion, plasma spraying or flame spraying are also within the scope of this invention~
Prior to coating the substrate in the plating bath, the substrate is preferably cleaned to insure good a~lesion of th~ coating. Techniques for such preparatory cleaning are conventional and well known in the art. For example, vapor degreasing or sand or grit blastinc; may be utilized, or the substrate may be etched in an acidic solution or cathodically cleaned in a caustic solution. If a substrate material other than nickel is utilized in the present invention, a plating of,nickë:L:~ suitably electrodeposited, may be initially applied to the portion of the substrate that is to be coated with the cathode surfac~.
After cleaning, the substrate can then be directly immersed in a plating bath to simultaneously cod~posit the components~ The basic electroplating t~chnique which can be utilized in this invention is 3~ known in the prior art. For example, U.S. Patent

4,105,532, issued August 8, 1978, and Ha~ey, Krohn, and Hannekin in "The Electrodeposition of ~ickel-Molybdenum Alloys", Journal of_the Electroche_ical , ~ol. 110, page 362 (1963), describe, respectively~ typical sulfate and pyrophosphate plating solutions. By way of illustration, a suitable plating bath for codepositing a coating of nickel, molybdenum and cadmium is described below:
Na2MoO~ 0002 M
~iC12 0.0~ ~
Cd(~03)~ 3.0 x 10 M
Na4P207 0~13 M
NaHC03 0.89 M
~2H4.H2S04 0~025 M
pH 7.5-9.0 Temperature 20~C.
Current Density 0~5 ASI
Plating Time 30 minutes In general, the pH level of the plating solution is significaht in the terms of the efficiency o the platincJ operation. p~I levels in the range of from about 7.5 to about 9.5 are preferred since a pH
of less than about 7.5 will tend to produce a coating having a higher hydrogen overvoltaye, wh:ile a pH of greater than about 9.5 will tencl to prec.ipitate nickel hy~roxide which, being nonconducti.ve, will 5 also increas~ the hyclroyen overvoltage.
Generally, other sources of nickel, molyb-clenum, ~nd cadmium may be employed in -the plating bath other than those speci-fically described above~ Other soluble salts of the corresponding metals are acceptable.
Other complexing agents, such as citrates, other

5~

buffering agents and supporting electrolytes, and other reducing agents may also be suitably utilized in substitution for the corresponding ingredients prescribed aboveO
The actual thickness of the coating will depend, at least in part, on the duration of the electroplating procedure. Coating thicknesses of from about 2 to about 200 micr~ns are acceptable, although thicknesses oE ~rom about 10 to about 50 microns are perhaps more useful. Coatings of less than about 10 microns in thickness usually do not have acceptable durability, and coatings of more than 50 microns usually do not produce any additional operating advantages.
Although the concentrations and relative proportions oE the various ingredients of the plating bath are not critical, particularly good coating~
are produced when ~he concentration of the cadmium ions in the bath is within the range of from about 1.5 x 10 4 M to ahout 6.0 x 10 4 M, and when the relative proportion of molybdenum ions to nickel ions in the bath is maintained at about 1~2. Such coatings contain less than about 40 atomic percent o molybdenum prior to leaching. It has also heen 2$ ~ound that small quantities of a soluble lead salt when aclded to the plating bath advantageously improve the efficiency oE the plating operation.
~le codeposit of the components may be in the ~orm of a mixture, an alloy, or an inter-metallic compound, depending on the particular conditions utilized in preparing the codepositr The term "codeposît", as used in the present speci-fication and claims, includes any of the various alloys, compourlds and inter~metallic phases of the components and does not imply any particular method or process of formulationO
After the coating has been deposited on the substrate material, the third me~al component of the coating, e.g. molybdenum, can then be removed.
q~is may be accomplished by immersing the coated cathode in an alkaline solution to leach the molyb-denum component. Typi~ally, a 2 to 20% by weight aqueous solution of sodium or potassium hydroxide for a period of about 2 - 100 hours, suitably at about ambient temperature, can be utilized. If stronger alkaline solutions are employed, or if the alkaline solution is heated, for instance from 50C.
to 70C. shorter leaching periods are pos~ible.
Alternatively, the electroplated cathode can be placed directly into service in an electrolytic cell,with - the leaching or extraction being carried out in situ in the cell by the electrolyte during cell operation~
Pa-~ticularly good coatings have been obtained by heat treating the coating either before, during or after removal of a portion of the molybdenum com~)ollent. Generally, the heat treatment can be carried out at temperatures of from about 100C, to about 350C. or a period of from about 1/2 hour to about 10 hours. The heat treatment is preferably carried out in an atmosphere in which the coating is inert, for example, argon, nitrogen, helium or neon are applicable, although o~ygen-containing atmospheres, ~ s~

can be used for convenience~
It is particularly advantageous and con-venient to heat treat the coated cathode con~urrently with a polymer-reinforced diaphragm which has been deposited on the cathode, In fact, it is perfectly acceptable to perform the entire plating operation in a diaphragm cell container using con~entional dimensionally stable anodes. Under these conditions, the heat treatment can be accomplished in about one hour at about 275C.
I~e cathodes of the present invention have applications in many types of electrolytic cells and can function effectively in various electrolytes.
Cathodes haviny an assortment of configurations and designs can be easily coated using the electroplatin~
technique of this invention, as will be unde~stood by those skilled in the art.
~ he following examples further illustrate and describe the various aspects ~f the invention, but are not intended to limit it~ Various modifi-cations can be made in the invention without departing from the spirit and scope thexeof, a~s will be read:ily appreciated hy those skilled in the art. Such modi-fications and variations are considered to be within th~ purview and scope of the appended claims.
Unless otherwise specified, temperatures in the followin~ examples are in dec~rees centigrade, and all parts and pexcentages are ~y weight. Hydrogen overvoltages were measured using a reversible hydrogen reference electrode.

~ 5 Two nickel plates were cleaned and i~mersed respectively in two 267 millil;ter Hull cellsO The first Hull cell contained an aqueous bath of 0.02 M Na2MoO4; 0.04 M NiCl~; 0.13 M Na4P~07;
0.89 M NaHC03; and 0.02~ M N2H4.H2S04. The second Hull cell contained the same bath but also included 3.0 x 10 4 M Cd~N0332.
Both Hull cells were connected in series, and the pla~ing was carried out at 20C. at a total curren~ of 4A for 30 minutes.
Two 2 x 2 cm. plated electrodes were cut out of each of the nickel plates9 and were leached in 20X NaOH for 15 hours at 70C. The electrodes were tested as hydrogen cathodes in a solution of 150 9.~1 NaOH ancl 170 g./l. NaCl at 95C. and a current densi~y of 300 ma/cm2. A hydrogen overvoltage of 184 mv. was recorded for the control electrode plated in the first Hull cell withDut cad-mium, and a hydrogen overvoltage,of 144 mv. was recorded for the electrode plated in the Hull cell containing cadmium.

The procedure of Example 1 was repeated to codeposit nickel, molybdenu~ ~nd cadmium on a nickel expanded mesh screen (50~ open~
at an average impressed current of 0.65 ~ or 30 minutes~ The electrode was subsequently leached in 20% NaOH at 70C. for 15 hours, and heat treated at 275C. for 1 hour. The electrode~ was tested as a hydrogen cathode ~ollowin9 the proceclure of Example 1, and a hydrogen overvoltage of 87 mv. was recorclecl.
EXAM~PIE 3 ?~r' The procedure of Example 2 was repeated except that the cadmium content of the bath was reduced to 1.5 x 10 4 M. The electrode was again tested as a hydrogen cathode fo'llowing the procedure of Example 2, and a hydrogen overvol-tage of 108 mY. ~las recorded .

A comparison of the results illustrated in Examples 1-3 demonstrates ~he improvement in hydrogen overvoltage obtained by the cathodes o~ the present invention as compared to the contrDl cathode of the prior art. In particular, Example 1 demonstrates that a 40 millivolt reduction in hydrogen overvo1tage is achieved by the cathodes of the present invention. Further improvements obtained by heat treating the cathodes ~re demonstra~ed in Examples 2 and 3 for varying concentrations o~ cadmium in the plating ba~h.

Claims (11)

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

1. An activated cathode for use in electrolytic processes, at least part of the surface portion of said cathode comprising a codeposit of a first metal selected from the group consisting of iron, cobalt, nickel, and mixtures thereof, and a second metal selected from the group consisting of cadmium, mercury, lead, silver, thallium, bismuth, copper and mixtures thereof, the outer surface of said cathode characterized as having a roughness factor of at least about 100.

2. The cathode of claim 1, wherein the codeposit was applied as a coating to at least a portion of a substrate material.

3. The cathode of claim 1, wherein the first metal is nickel and the second metal is cadmium.

4. The cathode of claim 3, wherein the cadmium is present in the range of from about 1 to about 10 atomic percent.

5. The cathode of claim 1, 2 or 3, wherein the roughness factor is at least about 1000.

6. The cathode of claim 4, wherein the roughness factor is at least about 1000.

7. An activated cathode for use in electro-lytic processes, at least part of the surface portion of said cathode comprising a codeposit of a first metal selected from the group consisting of iron, cobalt, nickel, and mixtures thereof, and a sub-stantially nonleachable second metal selected from the group consisting of cadmium, mercury, lead, silver, thallium, bismuth, copper, and mixtures thereof, said second metal being present in the range of from about 1 to about 10 atomic percent, the outer sur-face of said cathode characterized as having a rough-ness factor of at least about 100.

8. The cathode of claim 7 wherein the first metal is nickel and the second metal is cadmium.

9. The cathode of claim 7 wherein the rough-ness factor is at least about 1000.

10. An activated cathode for use in electro-lytic processes, at least part of the surface of said cathode comprising a codeposit of nickel and cadmium, the cadmium being substantially nonleachable and being present in the range of from about 1 to about 10 atomic percent, the outer surface of said cathode characterized as having a roughness factor of at least about 100.

11. A process for preparing an activated cathode comprising the steps of:
a) forming a codeposit of a first metal selected from the group consisting of iron, cobalt, nickel, and mixtures there-of, a second metal selected from the group consisting of cadmium, mercury, lead, silver, thallium, bismuth, copper, and mixtures thereof, and a third metal or metal oxide selected from the group consisting of molybdenum, manganese, titanium, tungsten, vanadium, indium, chromium, zinc, their oxides, and combinations thereof, and b) removing the third metal from the codeposit.