CA1246008A - Electrode with nickel substrate and coating of nickel and platinum group metal compounds - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A substrate is coated with a solution of metal oxide precursor compounds and an etchant for etching the substrate, the metal oxide precursor compounds are thermally concentrated by removing volatiles therefrom, and the so-concentrated metal oxides precursors are thermally oxidized in-situ on the substrate. The so-formed compositions are useful, e.g., as electrode material in electrochemical apparatuses and processes.

Description

~ 8 This invention pertains ~o a method for prepa~ing elec~rodes and to their use i~ electrolytic cells, for e~ample, brine electrolysis cells.

There are three general types of electrolytic cells used or the production of chlor-alkali: (1) the mercu ~ c~ll, (2~ ~he diaphragm-cell, and (3) the membr~n~ c~ll. Th~ operation of each of these cells is discu~sed i~ Volum~ I o~ the ThirdrEdition of the KIR~C-OTEIMER ENCYI::LOPEDIA OF C~qICAL T~ 3NOLOGY, page ~` ~0 799 ~t. s~q. Qther electroIytic cells which employ electro.des for electrolysis of agueous solutions are the so alled "chlorate cells" which do not use a divider or separator between the ca~hodes and anodes.
In the mercuxy cell, the alkali metal values produc~d by electrolyzing a~ alkali mekal salt form an amalgam ~Ith the mercury; the amalgam, when react~d wi~h water, produces N~O~ and ~ree~ the mercury which can be recovered and cycled back for fur~her use as a liguid cathode.

I~ many chlor-alkali electrolytic processes a b A ne solution (electrolyte) is ~lectrolyzed by passing 31,010-F

-2-electric current therethrough in a cell ha~ing a diaphraym or a membrane positioned between the cathode and the anode. Chlorine is produced at the anode while sodi.um hydroxide (NaOH~ and hydrogen (~2) are formed at ~he ca~hode. Brine is fed continuously to the cells, while C12, NaOH and H2 are continuously withdra~m from the cells.

~ he minimum voltage re~uired to electrolyze an electrol;yte into Cl2, NaO~ and H2 may be calculated using the thermodynamic data. However, in commercial practice, the theoretical amount of voltage is not achievable and higher voltages must be used to overcome the various resistances i~here~t in the various types of cells. To increase the efficiency of the operation of a ~iaphragm or a membrane cell one may attempt to reduce the over~oltages of the electrodes, to reduce ~he electrical resis-tance of the diaphragm or membrane, or re~uce the electrical resistanc~ o~ the brine b~ing electrol~ze~. The inventio~ herein described results in an el~c~rode.particularly useful as a cathode in the electrolysi~ o brine; cathode overvoltage is substantially r~duced, resulti~g in increased power efficiencies.

Because of the multi million-ton quantity of alkali metal haIides and water electrolyzed each year, even a reduction of as little as 0.05 volts in working voltage tr~nsla~es; ~o vexy meaningful energy savings.
Consequently, the industry has sought means to reduce the voltage requirement.

Throu~hout the development of chlor-alkali technoloyy, various methods have-be~n developed ~o reduce the~cell voltage. Some practitioners have 31.,010-F ~2-1~4~V(3B

concentrated on reducing cell voltage by modifying the physical design of the electrolytic cell, while others have concentrated their e~foxts on reduciny the over-voltage at the anode or the cathode. The present ~- 5 ~4~ ~ e pertains, in part, to a novel process to make an electrode that is characterized by a signifi-. e ~-cantly ~ overvoltage and to the use of these electrodes in electrolytic cells.

It has been disclosed that a~ electrode's overvoltage is a ~unc~ion of the curren~ density and i~s composi~ion (reference: PHYSICAL CHEMISTRY, 3rd ed., W. J. Moore, Prentice ~all (1962), pp. 406-408), where the current density refers ~o ~he amperage applied per unit of true surface area of an elec~rode and composition refers ~o the chemical and physical makeup o the electrode. Therefore, a process that will inc~ease a~ electrod~'s ~urface area should decrease its overvoltage at a give~ apparent current density.
I~ is also~ desirable ~o use a compositian of matter tha~ is a good electrocatalyst; this ~urther reduces the overvoltage.

It i~ well known in the art to use plasma or flame spraying ~o coat an electrode with an electro-conductive metal~ In U.S. Patent No. 1,263,959 it was 2S tau~ht that anodes may be coated by spraying fine nickel particles.onto an anode, wherein the particles ~re rendered molten and impacted on the iron substrate by means of a blast.

Cathodes, also, have been coated with electro-conductive metals.. In U.S. Patent No. 3,992,278,ca~hodes were coated by plasma spraying or ~lame spraying 31,010-F -3-an admixture o~ particulate cobalt and particulate zirconia. When these electrodes are used for the electrolysis of water or an aqueous alkali metal halide salt solution, they are said to give prolonged lowering of hydrogen overvoltage.

Various metals and combinations of metals have been used to coat electrodes by plasma or flame spraying: U.S. Patent No. 3,630,770 -teaches ~he use of lanthanum boride; U.S. Pate~t No. 3,649,355 teaches ~he use of tungsten ~r tungsten alloy; U.S. Patent No.
3t788.968 teaches the use o~ ti~anium car~ide or titanium nitride and at least one metal and/or metal oxide of the platinum group and a second oxide coating which is porous; U.S. Patent No. 3,945,907 teaches the use of rhenium; and U.S. Patent N~. 3,974,058 teaches the use of cobalt as ~ coating with an overcoat o~ ruthenium.

I~ i~, lik~ise, well known in the art to make porous ele~trode coa~in~s by seIective leaching.
Coating an electrode with particulat~ nickel, ~hen ~i~terin~ the nickel as taught in U.S. Patent Nos.
2,928,783 and 2,969,315; electrodepositing an alloy onto a substrate then leaching out one compon~nt of the alloy as~ taught in U.SO Pat. No. 3r272,788, pressing or cementing two or more components together or onto an electrode subs~rate and then selec~ively leaching out o~e or more of the coating components as illustrated by U.S;. Patent Nos. 3,316,159; 3,326,725; 3,427,204;

3,713,891 a~d 3,802,878.

It is also disclosed in the art to combine the steps of making electrodes by plasma- or ~lame-sprayin~ followed by leaching. It is alco disclosed to 3},010-F ~4~

61)0~

Combine the steps of electroplating followed by leaching.
Examples of known methods are illustrated in the following patents; U.S. Patent No. 3,219,730 (Institute of Gas Technology~ dated November 23, 1965 teaches coating a substrate wi~h a multiple oY~ide film coating then removing the substrate by leaching, thus forming an electrode;
U.S. Patent No. 3,4~3,057 ~Carrier Corporation) dated September 24, lg~8 teaches flame or plasma spraying a Raney alloy onto a substrate followed by leaching aluminum out of the alloy thus leaving a porous electrode; U.S.
Patent No. 3,492,720 (Badische Ani'in- & Soda-Fabrik Aktiengesellschaft~ dated February 3, 1970 teaches plasma spraying tungsten, titanium or alloys thereof along with aluminum, thorium and zirconium oxides onto a substrate.
The substrate was su~sequently removed, leaving a porous electrode.

U.S. Patent No. 3,497,425 (Imperial Metal Industries ~Kynoch) Ltd.~ dated February 24, 1970 teaches preparing porous electrodes ~y coating the substrate with a relatively insoluble metal followed by a coating of a more easily dissolvable metal. The teaching requires heat treating to cause inter-diffusion of the two coats, while optimum conditions re~uire separate heat treatments for each coat.
The dissolvable metal is subsequently leached out, leaving a porous electrode. U.S. Patent No. 3,618,136 (T. Fujita) dated November 2, 1971 teaches forming porous electrodes by coating a binary salt composition onto a substrate and leaching a soluble component from the system. The patent teaches that it is critical that the binary salt mixture is an eutectic composition and that optimum results are obtained when the same anions are used for both the active and the inactive salts, e.g. silver chloride -- sodium chloride.

31,010-F _5_ ~2~i0V~

Canadian Patent No. 1,07~,915 (~ooker Chemical & PlastiC Corp.~ teaches the preparation of porous cathodes by applying to a substrate a coating of at least one non-noble metal from the group of nickel, cobalt, chromium, manganese and iron, alloyed with a secondary, less noble, sacrificial metal. Specifically, the sacrificial metal is chosen from the group of zinc, aluminum, magnesium and tin. The sacrificial metal is removed by leaching with a lye solution or an acid solution.

U.S. Patent No. 4,279,709 discloses a method for making electrodes including electrodes having reduced ovexvoltage by applying an admixture of particulate metal and a particulate inorganic compound pore-former 1~ and then leaching out the pore-former to form pores.

Electrodes of film-forming metal substrates, especially titanium, coated with oxides of Group VIII
metals of the Periodic Table of The Elements have been taug~t, especially conjointly with other metal oxides, as being useful as anodes in electrolytic processes, such as in brine electrolysis. Ruthenium oxides, platinum oxides, and ~ther oxides of the "platinum 31,010-F -6-~` .

metal series", in association with various other metal oxides have received much acclaim as coatings for valve metal substrates (esp. Ti) ~or use as anodes. Patents relating to such anodes are, e.g. U.S. Patent Nos.
3,63~,498 and 3,711,3~5. These coatings may be applied in several ways, for example, U.S. Patent No. 3,869,312 teaches ~hat platinum group metal oxides, combined with film-forming metal oxides may be deposited on valve metal substrates by applying a mixture of the.rmally--decomposable compound~ of platinum group metals and a thermally-deco~posable organo compound o~ a film-forming metal in an organic liquid vehicle which may also optio~ally contain a r~ducing agent, to a support member, drying the coa~ing by evaporation of ~he organic vehicle, then heating the member in the range o~ 400-55GC
to ~orm metal oxides. Repeated coats are applied to increase the ~hickness of ~he coating. Also an o~ercoa~ing o a ilm-~orming metal oxide is applied. U.S. Paten~
3,632,498 teaches tha~ coati~gs o~ ~lnely divided oxid`es of platinu~ group metal& and film-forming metals may b~ produced by use of a plasma burner, by heating substxa~es which have been coated with thermally--decomposable compounds of platinum group metals and film-foDmi~g metals, b~ electrically depositing the metals in a galvanic bath followed by heating in air to form the oxid~, among others.

Some further patents relating to electrodes having metal oxide surfaces are, e.g., U.S. Patent Nos.
3,616,445; 4,003,817; 4,072,585; 3-,g77,958; 4,061,549;

4,073~,873; an~ 4,142,005.

The use of platinum group metal oxides, particularly ruthenium oxide, in active coatings for 31,010-~ -7-3~`~

the evolution o~ hydrogen is also known (ref. ~elendres, Carlos A., SPRI~G MEETING ELECTROCHEM. SOC., May 11~
1975). U~S. Paten~ No. 3,816,464 (Hodogaya Chemical Co., Ltd.~. dated November 17, 1~81refer to the use of a mixture of platinum group metal oxide(s~ with another metal oxide as active cathode coatings. U.S. Patent No. ~7 4,238,311 ~.Chlorine Engineers Corp., Ltd.) dated December 9, 1980 teaches that a cathode coating consisting of fine particles of platinum group metals and/or platinum group metal oxides in nickel is useful as a cathode coating.

In general, it is known by those skilled in the art that the use of oxides of platinum group metals as active catalysts for the evolution of hydrogen in modern electrolytic chlor-alkali cells employing permionic membranes is not useful because of extreme conditions of NaOH concentration and temperature now possible, wherein NaOH concentrations of 30 percent and temperatures exceeding 95C are not uncommon. Oxide coatings prepared according to the known art are found to decrepitate with use and.fail by loss of adherence to the substrate, accompanied presumably by substantial reduction, in some cases, to base metals.

It is also well known to those practiced in the art that catalytic coatings consisting of metals with intrinsically low hydrogen overvoltage properties are subject in actual practice to loss of catalytic activity due to overplating with metallic contaminants, such as iro,n or example, which are commonly present in brine and water employed in the process of electrolysis.
Consequently, active coatings found useful by those practiced in the art for evolution of hydrogen in modern electrolytic membrane chlor-alkali cells are 31,010-F -8-k g limited to the type characterized by high surface area, or porous coatings, with compo~itions resistant to some degree to chemical at~ack at these co~ditions, e.g.
nickel or various stainless steels.

In these cases, the ~ull effec~ of ~he catalytic nature of intrinsically low hydrogen overvoltage catalysts are not realized in practice, since, as is well known to those practiced in the art, the performance of these essentially high surface area coatings degrades in time to a level characterized by the eguivalent coating of the predominant metallic contaminant present in the brine or water Qmployed i~ the electrolytic process, usu211y Fe. Conse~uently, the Tafel slope charactexizing the electroly~ic activity of the applied coating changes to ess~ntially that of iron, with a resul~ing increase in hydrogen overvQltage, especially at higher urrent densities, 0.23 to 0.54 amp/cm~ ~1.5 to 3.5 amps/in2) and above, as ar~ common in modern membrane chlox-alkali c~llæ.. In con~rast, it i~ desirable to maintain the intrinsically low overvoltage properties of khose materials which are k~own- to be characterized by low Tafel slopes, i.e~ pla~i~um group metal oxides, particularly ru~henium oxide, during long-term operation i~ membxa~e chlor-aIkali cells. It has now been discovered, among other things, that active coatings of o~ide~ of platinum group metals and secondary electro-catalytic me~als when prepared according to the process of the i~vention/ exhibit unexpected properties of low hydro~en over~oltage, physical stability, and long~term efficacy as cathodes in the el~ctxolysis of brine at conditions of high NaO~ con~entrations, temperatures, and process pressures. It has also been discovered that the use of these electrodes in electrolytic 31,010-F -g-` ~16~

process wherein chlorine and cau.stic soda are produced at certain process conditions of temperature~ ~aOH
concentration, pressure, etc., results in reduced energy requirements not otherwise attainable in practice.
The invention particularly resides in a method o~ making a low hydrogen overvoltage cathode which comprises a nickel substrate having coated thereon an electrocatalytically-active heterogeneous mixture of nickel 02ide and a platinum group metal oxide, said method comprising the steps of (a) depositing on a nickel substrate, a coating solution containing a nickel compound which is effective as a precursor for nickel oxide, and at least one platinum group metal compound which is effective as a precursor for a platinum group metal oxide, said coating solution also containing an etchant capable of etching the surface of the substrate and/or any previously applied coating, (b) heating to remove volatiles from the~so-coated substrate to cause the metal values of the precursor compounds and those etched from the substrate or previous coating to be concentrated and recoated on the substrate or previously applied coating9 an~ (c) further heating, in the presence of oxygen, air or an oxidizing agent, to a temperature sufficient to oxidize the metal values, said steps (a), (b), and (c) being performed a plurality of timesO
3o The present invention also resides in a process for the electrolysis of aqueous solutions of sodium chloride in an electrolysis of aqueous solutions of sodium chloride in an elec~rolytic cell comprising an anolyte compartment and a catholyte compartment separated by a cation exchange membrane to produce an 31,010-F -10-h~

fi~3~8 -10a- 4693-3433 aqueous solution of sodium hydroxide in the catholyte compartment, and chlorine in the anolyte compartment, wherein the cathode of said proces~ is a low hydrogen overvoltage cathode made by applying to an electroconductive substrate a coating solution of nickel oxide and platinum group metal oxide precursor compound(s) and an etchant capable of etching the surface of the substrate and/or any previously applied coating, heating to remove volatiles from the so-coated substrate to cause the metal values of the precursor compounds and those etched from the substrate or previous coating to be concentrated and recoated on the substrate or previously applied coating, and further heating, in the presence of oxygen, air or an oxidizing agent~ to a temperature sufficient to oxidize the metal values, thereby obtaining on said substrate an electro-catalytically active heterogeneous coating o~ a mixture of nickel oxide and at least one platinum group metal oxide.
The present invention further resides in an electrode for use in an eletrochemical cell comprising a substrate having an electrocatalytically active coating deposited thereon, said coating comprising a heterogeneous mixture of metal oxides containing nickel oxide and at least one oxide of a metal selected from Ru, Rh, Pd, Os, Ir 7 and Pt.
The present invention further resides in a low hydrogen overvoltage cathode for use in a chlor-alkali electrolytic cell comprising a substrate having an electrocatalytically active coating deposited thereon, said coating comprising a heterogeneous mixture of metal oxides containing at least one oxide o~ a metal selected ~rom Ru, Rh, Pd, Os, Ir, and Pt, and nickel.

31,010-F -lOa-, .

.

3L24~ 8 -lOb-Figure 1 illustrates graphizally data from some of the tests described hereinafter.
Electrodes comprising an electrically conductive, or non-conductive substrate having a coating of heterogeneous oxide mixtures of platinum group metals and secondary electrocatalytic metals are prepared by applying soluble metal compounds and an etchant for the substrate, and, in cases of successive coats, etching the metal oxides previously applied to the substrate, thereby, it is believed, attacking and solubilizing the least chemically resistant portions of the coating, then, as the substrate is heated to oxidize the metal values, concentrating and redepositing the said metal 3o 31,010-F -lOb-.

values on the subs~rate, and oxidizing them to produce a substantially hard, stable mixture of heterogeneou~
oxides of the metal values.

The pre~errsd electrically-conductive sub-strate may be any metal structure which re~ains itsphysical integrity during the preparation o~ the electrode. Metal laminates may be usedt such as a ~errous metal coated with another metal, e.g., nickel or a film-forming metal (also known as.valve metal).
The subs.~ra~e may be a ~errous metal, such as iron, steel, stainless steel or other metal alloys wherein the major component is iron. The ubstrate may also be a non-ferrou~ metal, such as a film~forming metal or a non-film-forming metal, e.g., Ni. Film-forming metals are well known in these relevan~ ar~ as including, notably, titanium, tantalum, zirconium, niobium, tungsten and alloys o-~ these with each other and with minor amounts of othe~ metals~ Non-conductive sub-~trates ma~ ~ employed, especially if they are then 20 coated with a conductive layer o~to which the instant metal oxides are deposited.

Th~ shape or c:onfiguration of the substrate used in the present coating process may be a flat sheet, curved surface~ con~oluted surface, punched 25 plat~, woven wire, ~xpanded metal sheet, rod, tube, porous., no~-porous, sin~ered, filamentary, re~ular, or irr.e~lar~ The p~esent novel coating process-is not.
depe~dent on having ~ substrate of a particular shape, since the chemical and thermal steps in~olved are applicable ~o virtually any shape which could be useful as an electrode article. Many electrolytic cells contain foraminous (mesh) sheets or flat plate sheets;

31,010-F -11--these are sometimes ben-t to form "pocket" electrodes with substantially parallel sides in a spaced-apart relationship~

The preferred substrate configura~ion com-pris:es expanded mesh, punched plate, woven wire, sinteredmetal, plate, or sheet, with expanded mesh being one of the most preferred of the porous substxates.

~ he preferred composition of the substrate comprises nickel, iron, copper, steel, stainless steel, or ferrous metal laminated with nickel, with nickel bei~g especially preferred~. It will be understood that these substrates, onto which the me~al oxide coatings are to be- deposited, may themselves be supported or xeinforced by an undexlying substrate or me~ber, especial~ wh~r~ nickel, iron, or copper is carried by, or ~r an. underlying substrate or member. The substrate, o~o which th~ me~al oxide coating is ~o be deposi~ed, ma~ itseL~ be an outer la~er o~ ~ laminate or coated structure, and i~ may be, optio~ally, a non-conductive substrate.onto which the metal oxide coating is deposited.

Th~ pla~inum metal serie~ comprises Ru, Rh, Pd, Qs, Ir, and Pt.. Of these, the pre~erred metals are platLnum and ruthe~ium, w~h xuthenium b~ing most preferred. The soluble.platinum metal compound may be ~5. ~he halids, sulphate, nitrate or other soluble salt or soluble compou~d of th~ metal and is preferably the halide salt, such as RuCl3 hydrate, PtCl4 hydxate, and the like.

The seconda~y electrocatalytic metal oxide precursor o~ ~he present coating ma~ be at least one 31,010-F -12-derived from a soluble compound of Ni, Co, Fe, Cu, W, V, Mn, Mo, Nb, Ta, Ti, Zr, Cd, Cr, B, Sn, La, or si.
The preferred of these are Ni, Zr, and Ti, with Ni being khe most preferred.

The solution of the present invention contains at least one chemically active agen~ capable of etching ~he s~bstrate, and, in the case of second and later coatings, etching and solubilizing the most chemically--susceptible portions o~ the 02ides previously formed, while also, preferably as the ~emperature is elev~ted, vaporizing, in many cases, from the heated mixture, along with volatilized anions or negative-valence radicals from the platinum metal oxide precursor and th~ secondary electrocatalytic metal oxide precursor.
The preferred chemically active etchants comprise most common acids, such as hydrochloric acid, sulphuric acid, ~i~ric acid, p~osphoric acid; also hydrazine hydrosulphate, and the lik~, with hydrochloric acid and hydrazine~hydrosulphat~ being among the most preferred.

In general, ~he preferred method contemplated in the present in~ention comprises applying to the de~ired;substrate a solution containing at least one platinum me~al series compound, at leas~ one electro-catalytic me~al compound, and a chemical etchant, preferably containing a volatile organic vehicle, such as isopropanol, a~d allowing the volatile vehicle to evaporate, leaving the etcha~t and the dissolved metal values; the~ heatin~ ~he substrate to a temperature sufficient to concentrate the metal values, also sub-30 ~tantially driving out the volatilized etchant alongwith the anions or ne~ative-valence radicals released from th~-metal oxide precursors, and heating the 31,01Q-F -13-substrate in the presence of oxygen or air to a temper-ature sufficient to ~he~nally oxidize and convert the metals to metal oxides in-situ on the substrate. The steps may be repeated a plurality of times in order to attain the best full effect of the i~vention by increasi~g the thic~ness of the coating. Furthermore there is, at times, a benefit to be derived from laying do~m 2 or more layers of the metal oxide precursors between each thermal oxidation step.

In a particularly preferred embodiment an electrode material is prepared by applying a hetero-g.e~eou& metal oxide coating, said heterogeneous metal o~id~ coatin~ comprising nickel o~ide and a platinum group me~al oxide ~op~io~ally contai~ing a modifier metal oxide, e.g., ZrO2), onto a nickel metal layer (which.may be in ~he form-of a nickel layer on an elec~roconducti~e substrate~ by the process which compri~e~ (a) applyi~ ta said nickel metal layer a.
coating solutio~ compriæing a Dickel oxide precursor, a platinum group metal oxide precursor, an optional modifier metal o~ide precursor, and an etchant for dissolving the mos-t soluble portions of the nickel m~tal ~urface, (b) heating.to e~aporate volatile porti:ons o~ the coating solution, thereby concentrating and depo~iting ~he metal oxide precursors on the ~o-etched nickel metal surface, (c) heating i~ the presence of air or oxygen at a temperature hetween 3Q0C to 600C for ~ time suficient to oxidize the ~- metals of the m0tal oxide precursors, and (d) cooling ~he so-prepared electrode material. Additional coati~gs may be applied in similar manner so as to increase the thickness o~ ~he so-produced heterogeneous metal oxide coating on the nlckel metal surface, though the etchant 31,010-F -14-~2~

for ~he second and later coating applications may beneficially be the same as, or different from, the etchant used in the initial coating application. There is thus prepared an electrode material comprising a nickel metal layer ha~ing tigh~ly adhered thereto a heterogeneous metal oxide coating comprising nickel o~ide and a platinum group metal oxide, op~ionally also containing a modifier metal oxide. Preferably, the platinum group metal oxide is ru~henium oxide. The pre~erred optional modifier metal oxide is zirconium oxide.. A~ economical form of the nickel metal layer is that of a nickel layer on a less expensive electro-conductiv~ substrate, such as steel or iron alloys.
Such electrode material is particularly useful as cathodes in chlor-alkali cells.

Ordinarily-the temperatures at which thermal oxidatlon of the metals:is achieved is somewhat depen-den~ on the metaIs, but a temperature in the range o~
fr~m~ 3:00Q to 650C, more or less~, is generally e~fective.
It is; generally preferred ~hat the thermal oxidation be per~ormed at a temperature in the range of from 350 to 550C.

The effect of the invention is to produce a s~bstantially hard, adherent coating of heterogeneou6 oxides of th~ solubilized metals.

It is within ~he purvie~ of the present i~ventive co~cept that the ~olubilization, reconcen-tration, and in-situ deposition.of the solubilized - metals, usin~ chemical etchi~g of the previously deposited layers and/or substrate produces an intimate mixture of oxides which are mutually s~abili2ing and electrocatalytically compleme~tary.

31,010-F -15~

~16-The following examples illustrate particular embodiments, but the invention is not limited to the particular embodiments illustrated.

Exam~le 1 A solu~ion was prepared which con~isted of 1 part RuCl3 3H20, 1 part NiC12 6~20, 3.3 parts H2NNH2-H2SO4 (hydrazine hydrosulphate), 5 parts HzO, and 28 parts isopropanol. The solution was prepared by first mixing together all ingredients other than ~he isopropan~l by stirring overnight, then adding the isopropanol and continuing to s~ir for approximately 6 houxs.

A cathode was prepared which was constructed of a 40% expanded mesh of nickel. The cathode was first sa~dblasted, ~h~n etched in 1:1 ~Cl. I~ was subse~ue~tIy ri~sed, dipped in isopropanol and air dried~ T~ cathode was coated by dipping it into the coa~ing solution, alIowing it to air dry, ~hen baking it in a~ oYe~ at 375C for 20 minutes. In the same maDner, a total of 6 coats were applied. The ca~hode waæ immersed in a heated bath containing 35% NaOH at a ~empe*ature of 90C. A current was applied and potential measurements were take~ using ~-standard Calomel Reference Electrode (SCE) a~d a Luggi~ probe. The cathode potential was measured at -1145 millivol~s vs. SCE at a current density oi~ 2 amp5 per square inch (0.31 amps per cm2 ) .
The cathode was asæembled in a laboratory membrane chlorine cell and operated a~ 90C, producing C12 at the anod~ and E2 at the cathode, at 31-33% NaO~
concentration, operating at 0.31 ampjcm2 (2 amp/in current density. The pvten~ial of the cathode was monitore~ and averaged per week. The results are shown in Table I.

31,01Q-F -16-- ~Z~6~

.
Example 2 A solution was prepared which consisted of 1 part RuC13-3H20, 1 part Nicl2~6H2o~ and 3.3 parts concentrated HCl. It was allowed to mix overnight.
Subseguently, 33 parts isoprop~nol were added and mi~ing continued 2 hours. A cathode was prepared in accordance with the procedure of Example 1. The cathode was then coated in the same manner as Example 1 except baki~g was carried out at a temperature of 495-500C.
Ten coats were applied. The cathode potential was measured as in E~ample 1. The potential was -1135 millivolts vs. SCE. The cathode was assembled in a laboratory cell containing a commercially a~ailable NAFION* polymer ~*a tradeRame o~ E. I. duPont de Nemours) 15 membrane. The cell was operated a~ gOC, 31-33% NaOH, and 0.31 amp/Cm2 ~2 amp/in2) curren~ density. The pote~tial of the cathode was monitored and averaged per week. The resul~s ar.e shown in Table I.

E~ampl ~3 A so:lution wa~ prepared which consisted of 1 par~ N~20~C1, 5 parts concentrated RCl, 2 parts 10%
PtClS~6E~0, 1 par~ NiCl2 6H20, and 1 par~ RuCl3 3H~O.
The solution was~ allowed to mix ~or 12 hours. Then 75 parts isopropanol were added and mixing continued for 2 hours~ A cathode-was prepared according to Example l. The cathode wa-q then coated in ~he same manner as Example 1 except baking:was carried out at a tempera.ture of 470-480C. Five coats were applied.
sixth cQa~ was applied and the alectrode was baked for 30 minutes at a temperature o~ 470-480C~ The potential of the cathode was measured as in Example 1. The pote~tial was ~1108 millivolts vs. SCE. The cathode . was assembled in a laboratory membrane chlorine cell 31,010-F -17-containing a commercialy availcible membrane, as in Example 2. The cell was operated at 90C, 31-33% NaQ~, and 0.31 amp/cm2 (2 amp/in2) current density. The potential of the cathode was monitored and averaged per week. The results are shown in Table I.

A solution was prepared which consisted of 3 parts RuC13 3H2O, 3 parts NiClz 6H2O, 1 part ZrCl~, 5 parts concentrated ~Cl, and 42 parts isopropanol. The solution was allowed to mix 2 hours. The cathode was then coated in ~he same manner as Example 1 except baki~g was carried out at a temperature of 495-500C.
. Eight coats were applied. A ninth coa~ was appli~d and the electrode was baked for 30 minutes at a temperature o~ 470~480C. The po~ential of the cathode was measured as in Example 1. The pote~tial was. 1146 milli~olts vs. SCE. T~e~ca~h~d~ wa assembled in a laborato~y membrane~chloriné c~l-1 contai~in~ a.commercially available membrane, ~ in ~ample 2. The cell was operated at 90C, 31-33~ NaOX~ ~nd 0.31 amp/cm2 (2 amp/i~Z) current de~sity. The poten~ial of th~ cathode was monitored and a~araged per week. The results are shown in Table I.

Example 5 A cathode was prepared as in the previous e~amples, ~hen d~pped in a solution contai~iny 1 gxam of tetraisopropyl titanate in 100 ml of isopropan~l.
Th~ cathode waa then baked at a temperature of 475-500C for 10 minutes. Thre~ co~ts were applied.
A solutio~ was prepared as in Example 2. The cathode was dipped in the soluti~, air dried, and baked at a temperatur~ of 475-500C. Six coats were applied.

31,010-F- -18-3~4~

The potential of the cathode was measured as in khe previous examples. The potential was -1154 millivolts vs, SCE. The cathode was assembled in a laboratory membranP chlorine cell containing a commercially

5 available membrane, as in Example 2. The cell was operated a~ 90C, 31~33% NaOH, and 0.31 amp/cm2 ( 2 amp/in2 ) current density. The potential of the cathode was monitored and averaged per week. The results axe shown in Table I and also in Figure l. .

Exam~le_6 (Comparative Example) A 40% expanded mesh electrode of steel was prepared, but no~ coated, and assembled as the cathode in a laboratory cell as in ~xamples 2-5, using the same type membrane. The potential of the cathode was monitored and averaged per week. The results are shown in Tabl~ I.

Exame~e 7 (Comparativ~ E~ample) A 40% expanded mesh elec~rode of ~ickel was prepared, but not coate~, and assembled as the cathode in a laboratory cell as in E~amples 2-5, using the same type membrane. The poten~ial of the cathode was monitore& and a~eraged per week. The results are shown in Table I and also in Figure 1.

31,010~F -19-~24~

TABLE I
NegatiYe volkage* averaged ~ach week for No. oElectrodes No 1 thru 7 . .
Weeks Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 CEx. 6 CEx. 7 11.145 1.120 1.135 l.lZ0 1.140 1.~75 1.49~
21.1~0 1.1~0 1.150 1.130 1.130 1.460 1.475 31.150 1.125 1.160 1.150 1.110 1.455 1.470 41.155 1.130 1.150 1.155 1.080 1.455 1.470 51.155 1.130 1.150 1.150 1.070 1.465 1~475 61.150 1.130 1.~80 1.15~ 1.060 1.475 1.480 71.150 1.125 1.185 1.155 1.060 1.480 1.4g5 81.150 1.125 1~180 1.160 1.060 1.480 1.510 91.140 1.120 ~.160 1.155 1.070 1.480 1.510 lO1.130 1.110 1.185 1.160 1.080 1.475 1.510 111.115 1.11~ 1.190 1.170 1.080 1.480 1.515 121.1~0 1.1~0 1.190 1.165 1.080 1.490 1.520 13I.100 1.110 1.150 10165 1.080 1.485 1.520 ~ l00 1.115 l.l90~ 1.1701.080 - - 1.520 I5l.Og5 1.120 I.190 1~170 1.090 - - 1;525 l*~.090 1.120 1~190 1.170 1.090 - - 1.530 171 ~85~ 0 1.190 1.170 1.09~
18I.080 1.120 1~.190 1.1651.100 lg1.0~0 l.llO 1.190 1.160 1.100 - - - -1.080~ 1~110 1.190 -1.100 - - - -21l.Q80 l.110 1.190 221.090 - - 1.190 23l.Q90 - ~l.l9Q-24I.100 - l.lgO
251.100 26I.090 271.090 ~ - - - - - - - -CEx - Comparative Example * The voltages recorded in Table I were all measured in the sam~ manner, using a Luggin probe, thus are relevant 31,010-F 20-..

4~V~

to each other, though all are believed to be slighkly lower than what one should expect to find from a theo-retical calculation~ By thermodyn~nic calculations, the actual absolute reversible voltage should be about -1.093V for a cell at'90C, 31-33% NaOH, and at a current density of 0.31 amp/inZ t2 ~np/in2).

Example 8 The cells o~ Examples 2-7 were operated at 90C, 31-33% NaOHr and 0.31 amp/cm2 (2 amp/inZ) current de~sity while maintaini~g atmospheric pressures in the anolyte an~ catholyte compartments of the cell. Sodium chIoride brine and water were fed to the anolyte and catholyte compartments, respecti~ely, in order to main~ai~ anolyte concentrations i~ the range 180-200 grams per liter NaCl a~d 31-33% NaO~. Internal mi~ing of the cells was accomplished by natural gas lift due to evolution of hydrogen gas at the cathode and chlorine gas at the anode. Data including mass and energy balance& were~collected periodically over the period of 2Q operation o~ ~h~ celIs and energy requirements for the ~roductio~ o~ NaOH were caLcuIate~. The results are show~ in Tabl~ 2.

Electrode ~ Cathode KW~MT NaO~
2 coate~ 2208 3 coated 2221 4 coated 2229 coated 2259

6 steel 2497

7 nickel 2504 31,010-F -21-Example 9 In a large scale tes~, two series of pressure membrane chlorine cellq were con~tructed. The con-struction and de~ign of the cells were identical except that the series identified as Series 1 had a nickel-wall catholyte compartment and nickel electrode~ installed in the catholyte compartment of the cells, while the ~eries identified as Series 2 have a steel-wall cathode compartment and steel cathodes. The electrodes of Series 1 were coated according to the process of the invention, while those of Series 2 were uncoated. Both series were erected with a commercially available cation exchange membrane, as in Example 2. The two series were operated simultaneously at 90C, 0.31 amp/cm2 ~2 amp/in2) current denqity, and 31 to 33% sodium hydroxide in the catholyte chamber. The series were operated at pressures of 101,325 to 202,650 Pa (1 to 2 atmospheres) while recirculating the anolyte and the catholyte through the cells using centrifugal pumps.
The ratio of the catholyte flow to the anolyte flow was maintained at a value greater than 1. Energy and mass balance data were collected and average performance data were calculated over a period of 45 days. The results clearly show that the energy savings attained with the use of the electrodes of the present invention (Series 1) averaged greater than a 5% reduction in energy, compared with Series 2.
It is within the purview of the present invention to employ the present novel electrodes at temperatures encountered in cell~ operated at super-atmospheric pressures, as well as at atmospheric or subatmospheric pressures. The electrodes are especially suitable for opera~ion in the elevated temperature 31,010-F -22-range of from 85 to 105C. Pressures a~ around 101,325 Pa (1 a~m.), more or less, are ordinarily used iTl chlor-alkali cells, though pressures up to abouk 303,975 Pa (3 atm.) or more may be used.

The electrodes of the present invention are useful in cells wherein circulation within each electrolyte compar~ment is created by ~he gas~lift (displacement) actio~ of gaseous products produced therein, though in some cells, such as in electrolyte series flow from cell-to-cell, another pumping means may be provided to supplement, or substi~ute for, the gas-lift action. We fi~d it ad~isable, in some cases, to maintain the ratio o~ ~he volume o~ catholyte pumped to that of -the a~olyte volume pumped, at a ratio greater than unity.

The electrodes of this invention are useful i~ chlor-alkali electro~ytic cells in which the anoly~e -has, or i~. adju~ted:to haYe, a p~ in the range of from l-to.5~ su~h as when an acid~, ~.g. ~Cl, is added to the analyte.

31,010-F -23:-

Claims (36)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of making a low hydrogen over-voltage cathode which comprises a nickel substrate having coated thereon an electrocatalytically-active heterogeneous mixture of nickel oxide and a platinum group metal oxide, said method comprising the steps of (a) depositing on a nickel substrate a coating solution containing a nickel compound which is effective as a precursor for nickel oxide, and at least one platinum group metal compound which is effective as a precursor for a platinum group metal oxide, said coating solution also containing an etchant capable of etching the surface of the substrate and/or any previously applied coating, (b) heating to remove volatiles from the so-coated substrate to cause the metal values of the precursor compounds and those etched from the substrate or previous coating to be concentrated and recoated on the substrate or previously applied coating, and (c) further heating, in the presence of oxygen, air or an oxidizing agent, to a temperature sufficient to oxidize the metal values, said steps (a), (b), and (c) being performed a plurality of times.

2. The method of Claim 1 wherein the coating solution contains a metal compound which is effective as a precursor for a modifier metal oxide for modifying the heterogeneous mixture of nickel oxide and platinum group metal oxide.

3. The method of Claim 2 wherein the modifier metal oxide is zirconium oxide.

4. The method of Claim 1 wherein the platinum group metal is selected from platinum and ruthenium, and alloys thereof.

5. The method of Claim 1 wherein the nickel substrate is self-supporting.

6. The method of Claim 1 wherein the nickel substrate is supported by, carried by, or laminated to an underlying substrate selected from an electro-conductive and non-electroconductive material.

7. The method of Claim 1 wherein the metal oxide precursor compounds are selected from metal chlorides, nitrates, sulphates, and phosphates.

8. The method of Claim 1 wherein the etchant is selected from hydrochloric acid, sulphuric acid, nitric acid, phosphoric acid, and hydrazine hydrosulphate.

9. The method of Claim 1 wherein the temperature at which the oxidation of the metal values is carried out is in the range of from 300°C to 600°C.

10. The method of Claim 1 wherein the nickel metal substrate comprises a sheet of an expanded mesh.

11. A process for the electrolysis of aqueous solutions of sodium chloride in an electrolytic cell comprising an anolyte compartment and a catholyte compartment separated by a cation exchange membrane to produce an aqueous solution of sodium hydroxide in the catholyte compartment, and chlorine in the anolyte compartment, wherein the cathode of said process is a low hydrogen overvoltage cathode made by applying to an electroconductive substrate a coating solution of nickel and at least one platinum group precursor compound(s) and an etchant capable of etching the surface of the substrate and/or any previously applied coating, heating to remove volatiles from the so-coated substrate to cause the metal values of the precursor compounds and those etched from the substrate or previous coating to be concentrated and recoated on the substrate or previously applied coating, and further heating, in the presence of oxygen, air or an oxidizing agent, to a temperature sufficient to oxidize the metal values, thereby obtaining on said substrate an electro-catalytically active heterogeneous coating of a mixture of nickel oxide and at least one platinum group metal oxide.

12. The process of Claim 11 wherein the cathode comprises a layer of nickel having tightly adhered thereto said heterogeneous metal oxide coating.

13 The process of Claim 12 wherein said coating contains at least one oxide of said platinum group metal selected from Ru, Rh, Pd, Os, Ir, and Pt.

14. The process of Claim 13 wherein the platinum group metal oxide is selected from platinum oxide and ruthenium oxide.

15. The process of Claim 11 wherein the heterogeneous metal oxide coating includes a modifier metal oxide.

16. The process of Claim 15 wherein the modifier metal oxide is ZrO2..

17. The process of Claim 16 wherein the heterogeneous metal oxide coating comprises RuO2 and NiO.

18. The process of Claim 11, wherein the substrate is supported by an electroconductive base.

19. The process of Claim 11, wherein the substrate is supported by a substantially non-electro-conductive base.

20. The process of Claim 11, wherein the substrate has a layer of nickel between it and the heterogeneous metal oxide coating.

21. The process of Claim 11, wherein the substrate is supported by a substantially non-conductive base and wherein there is a layer of nickel between the substrate and the heterogeneous metal oxide coating.

22. The process of Claim 11, wherein there is a layer of nickel between the electroconductive substrate and the heterogeneous metal oxide coating.

23. The process of Claim 11, wherein the substrate has a layer of Ni between it and the heterogeneous metal oxide coating, and wherein said heterogeneous metal oxide coating comprises, predominantly, RuO2 and NiO, along with ZrO2 as a modifier metal oxide.

24. An electrode for use in an electro-chemical cell comprising a substrate having an electrocatalytically active coating deposited thereon, said coating comprising a heterogeneous mixture of metal oxides containing nickel oxide and at least one platinum group metal oxide.

25. The electrode of Claim 24, wherein said coating contains at least one oxide of said platinum group metal selected from Ru, Rh, Pd, Os, Ir, and Pt.

26. An electrode of Claim 24, wherein the mixture of metal oxides includes a minor amount of a modifier metal oxide.

27 The electrode of Claim 26, wherein the modifier metal oxide is ZrO2.

28. The electrode of Claim 24, wherein the substrate has a layer of nickel between it and the heterogeneous metal oxide mixture.

29. The electrode of Claim 24, wherein the substrate is supported by a substantially non-conductive base and wherein there is a layer of nickel between the substrate and the heterogeneous metal oxide coating.

30. The electrode of Claim 24, wherein the substrate has a layer of Ni between it and the heterogeneous metal oxide coating, and wherein said heterogeneous metal oxide coating comprises, predominantly, RuO2 and NiO, along with ZrO2 as a modifier metal oxide.

31. The electrode of Claim 24, wherein the substrate comprises an expanded mesh sheet.

32. A low hydrogen overvoltage cathode for use in a chlor-alkali electrolytic cell comprising a substrate having an electrocatalytically active coating deposited thereon, said coating comprising a heterogeneous mixture of metal oxides containing nickel oxide and at least one oxide of a metal selected from Ru, Rh, Pd, Os, Ir, and Pt.

33. The cathode of Claim 32, wherein the heterogeneous mixture of metal oxides comprises RuO2 and NiO.

34. The cathode of Claim 32 or 33 wherein the heterogeneous mixture of metal oxides includes a modifier metal oxide.

35. The cathode of Claim 34, wherein the modifier metal oxides is ZrO2.

36. The cathode of Claim 32, wherein the substrate has a layer of Ni between it and the heterogeneous mixture of metal oxides.