Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
L~ 7
A LOW HYDROGEN OVERVOLTAGE CATHODE
AND NETHOD FOR PRODUCING THE SA~E
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention generally relates to a low hydrogen overvoltage
cathode for use in the electrolysis of water or alkali metal halides and a
method for producing the same. More particularly, it relates to a low
hydrogen overvolta~e cathode having low hydrogen-generating electric potential
10 and satisfactory durability and which is specifically suitable for
electrolysis of an aqueous alkali metal halide solution, and to a method of
production thereof.
2. Description of prior art
In the electrolysis of water or an aqueous alkali metal halide solution
using an asbestos diaphragm or an ion exchange membrane; punched mild steel
plates, mild steel mesh and the like have been used as the cathodes. These
materials are advantageous in respect of cost, alkali-resistance,
processability and the like, as compared with other materials. Horeo~er, mild
steel has a hydrogen overvoltage of from 0.3 to 0.4 Volt, which is relatively
20 low except for platinum group metals.
Nonetheless, recently a rapid increase in energy cost has increased the
need for greatly reducing hydrogen overvoltage of mild steel cathodes for use
in hydrogen generation in order to lower energy costs. A variety of cathodes
have been proposed. ~ost of those imprbved cathodes employ iron group metals
which are less expensive, easily processable and available as a cathode base,
on the surface of which a coating for reducing hydrogen overvoltage is formed.
For example, electrodes are known which are made by spray coating an iron
group cathode base with nickel or tungsten carbide powder (Japanese
non-examined Patent Publication No. 32832/77 of Argade et al published
30 12 March, 1977), electrodes which are spray coated with cobalt and zirconium
(Japanese non-examined Patent, Publication Uo. 36582/77 of Brannan et al
published 19 March, 1977), electrodes comprising nickel and cobalt which are
subjected to a leaching treatment after spray coating (Japanese non-examined
Patent Publication No. 36583/77 of srannan et al published 19 March, 1977),
electrodes which are obtained by spray coating an electrode base with
PAT 7489-1 - 1 -
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Raney-nickel and then leachin~ out with alkali a sacrieicial metal contained
in the coatin~ layer (Japanese non-examined Patent Publication ~o. 122887/80
of Kanetani et al published 20 September, 1980), electrodes which are obtained
by spray coating o$ an alkali-resistant metal on a cathode base and depositing
a platinum ~roup metal on the surface thereof (Japanese non-examined Patent
Publications Nos. 131189/80 of Nakagawa et al published 11 October, 1980,
158288/80 of Kanetani et al published 9 December, 1980), electrodes which are
obtained by fonming an activated layer by plating on a cathode base, e.S.,
electrodes obtained by dispersinK a platinum ~roup metal powder into nickel
(Japanese non-examined Patent Publication No. 110983/79 of Kazunori Kasuya
published 30 August, 1979) or dispersing Raney-nickel into nickel (Japanese
non-examined Patent Publication No. 112785/79 of Oda et al published
3 September, 1979) and so on.
These activated cathodes, however, involve disadvanta~es including
insufficient durability and hi8h cost. In particular, when exposed to hi~h
tewperatures and hi8h concentrations of caustic sods, these electrodes are far
from satisfactory as csthode6 for use in ion excban~e membrane electrolysis.
That i~, in, for example, a procegs for forming asney-nickel-containin6 nickel
or a nickel alloy coatin6 layer, when the content of electroconductive fine
particles is small, e.6., approximately less than 20~, the performance is
insufficient but a firm coating layer with stron~ adhesion is obtained. when
electroconductive fine particles are contained in large amounts, e.g., in
exco~s of 45~, a hi~hly activated coating layer is obtained but strength as
well as athesion is not satisfactory. Accordingly it is dlfflcult to provide
cathodes which are completely satisfactory in activity, adhesion and strength.
On the other hand, various methods for manufacturing low hydro~en
overvoltage cathodes have been proposed. As to the structure of a low
hytro~en overvoltage cathode, the cathode base is first considered. As a
material for the cathode base, carbon steel, stainless steel, nickel and the
like are known but carbon steel is normally used for economical
;~ ~ consiterations. On the cathode base an activated layer of low hydroKen
overvolta6e is deposited. In this case, when corrosion of the base is
po~siblè turin~ operation at low hydrogen overvolta~e, it is necessary to
provide a protective layer of alkali- ~ material between the base and
the activated layer. As the protective layer, nickel-plating bodies,
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copper-plating bodies and the like are normally employed and as the activated
co~rt~o.~
layer, it is ~ ne to employ alkali-resistant metals designed to have
large surface area. For example, included are a method for electroplating a
Raney alloy (Japanese Patent Examined Publication Nos. 4766/53 of Morii et al
published 24 September, 1953, 6611/56 of Sasaki et al published 7 August,
l9S6, Japanese Patent non-examined Publication No. 2527S/79 of Brannan et al
published 26 February, 1979), a method for codeposit plating of Raney-nickel
(Japanese Patent Non-examined Publication Nos. 68795/79 of Kawasaki et al
published 2 June, 197~, 112785/79), a method for depositing a Raney alloy by
spray coating, sintering and the like (Japanese Patent non-examined
Publication Nos. 36583/77, 81484/78 of Needes published 18 July, 1978,
79803/80 of Kawasaki published 16 June, 1980), a method for spray coating of
metals such as nickel (Japanese Patent non-examined Publication Nos. 32832/77,
131189/80), a method for depositing by electroplating a coating, a sacrificial
component of which is leached out during operation (Japanese Patent
non-examined Publication Nos. 115674/78 of Kajiyama et al published
6 February, 1981, 22161/78 of Takahashi published 1 ~arch, 1978, 102876/78 of
Oda et al published 7 September, 1978 and 100987/80 of Yanagihara et al
published 1 August, 1980) and the like.
Notwithstandine, those cathodes manufactured by the foregoing methods are
not always suited to industrial use as to performance, i.e., those with low
hydrogen overvoltage are inferior in durability for a prolonged period of
time, while those with satisfactory durability are high in hydrogen
overvoltage.
SUMMARY OF THE INVENTION
The present invention, however, provides a low hydrogen overvoltage
cathode with low hydrogen overvoltage as well as adequate durability and which
is especially suited to electrolysis of an aqueous alkali halide solution.
More ~pecifically, there is provided a low hydrogen overvoltage cathode having
a nickel or nickel alloy coating layer containing electroconductive fine
particles (e.g., Raney-nickel etc.) which contain a platinum group metal
and/or an oxide thereof.
The present invention also provides a method for producing the foregoing
low hydrogen overvoltage cathode which comprises the use of a codeposit
plating tank in which an anode and the object to be plated of a non-perforated
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flat structure are positioned parallel to each other, introducing a ~ispsrsant
slurry through one side of the tank and causins lt to flow in the space formed
between the anode and the object, then removing the slurry through the
opposite side, whereby codeposit platin~ is applied to only one surface of the
object.
These and other advantages of the present invention will become apparent
to those skilled in the art from the detailed disclosure of the present
invention as set forth hereinbelow.
BRIEF DESCRIPTION OF DRA.WINGS
~0 FIG. 1 and FIG. 2 are graphs showing the relationship between operating
days and hydrogen overvoltage in Examples 1 and 2, respectively.
FIG. 3 is a schematic representation illustrating an embodiment of a
method of production of the present invention.
FIG. 4(X) is a schematic representation of a codeposit plating tank used
in the present invention in which the surface to be plated is located
substantially horizontally and to facing upwardly, and FIG. 4(Y) is a
cross-sectional view taken on line A-A of FIG. 4(X).
FIG. S(~) is a schematic representation of a codeposit plating tank used
in the present invention in which two sheets of cathodes, the surfaces to be
plated bein6 located substantially vertically, are codeposit plated at the
s8 time ant FIG. S(Y) is a cross-sectional view taken on line B-B of
FIG. S(I).
' ; DeTAILeD DeSCRIPTION OF THE INVeNTIONi~ ~ The present invention encompasses allow hydrogen overvoltage cathode
hsvin6 a nickel or nickel alloy coating layer containing electroconductive
~; fine particles which contain a platinum group metal and/or an oxide thereof,
and methot for producing the same. The method comprises using a codeposit
~ plsting tank in which an anode and the object to be plated which is a
I non-perforsted flat structure are positioned parallel to each other,
introducing a dispersant slurry through one side of the tank to cause it to
flow in the space formed between the anode and the object, then removing the
slurry through the opposite side, whereby codeposit plating is applied to only
one surfsce of the object. The cathode of the present invention is coated
Y;~ d th electroconductive fine particles containing a platinum group metal and/or
sn oxlae thereof dispersea in nickel or a nickel alloy, and is capable of
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reducing hydrogen generation electric potential by 200 to 300 mV as compared
with conventional iron cathodes.
As the cathode base, iron, stainless steel, nickel and the liXe may be
suitably used and further, iron coated with nickel, a nickel alloy such as
Ni-Mo, Ni-W and the like may also be used.
As the coating layer, nickel or nicXel alloys such as ~ o, Ni-W and the
like may be suitably used and further, a mixture of nickel and oxides thereof
may also suitably be used.
As the active substance, a platinum group metal selected from the group
consisting of platinum, ruthenium, iridium, rhodium, palladium and osmium, and
an oxide thereof may be used singly or in combination of two or more.
Electroconductive fine particles should have electroconductivity and large
surface area and be superior in resistance to caustic alkali, which may be
exemplified by porous nickel particles synthesized from nickel formate or
nickel carbonyl, carbon particles such as activated carbon, Raney-nickel,
Raney-cobalt, Raney-silver and the like. When Raney-nickel alloy is employed,
it ls necessary to leach in known matter after formation of the coating
layer. Por example, adequate activity is obtained by immersing the coating
layer in a 10 to 30% aqueous caustic soda solution at 40 to 60C for more
than one hour.
As the process for preparing the electroconductive fine partlcles to hold
the active substance, known techniques may be employed including, for example,
(a) chemical plating in which the fine particles are dispersed in a platinum
group metal salt solution, then a reducing agent solution containing sodium
hypophosphite or the like is added to the resulting slurry to thus cause the
platinum group metal to deposit onto the surface of the fine particles~
tb) displacement plating in which the fine particles are dispersed in a
platinum group metal solution, and the platinum group metal is deposited onto
the surface of the fine particles through ionization tendency,
(c) electroplating in which a pacXed layer of the fine particles is sub~ected
to cathodic polarization and a platinum group metal solutlon is passed through
the packed layer to thus allow the platinum group metal to deposit onto the
surface of the fine particles,
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4~7
~d) vapor deposition in which ths fine particl~s are brou~ht into contact
with a platinum ~roup metal vapor to cause the plaSinum ~roup metal to deposit
onto the surface of the fine particles, and
(e) thermal melting in which the fine particles are mixed with platinum group
metal particles and the mixture is elevated to a high temperature whereby an
alloy of the metal of the flne partlcles and the platinum group metal is
formed.
The fore~oins fine particles should desirably be as fine as possible, and
uniformly dispersed in nickel or a nickel alloy to be used for coatin~. The
partlcle size of the fine partlcles should preferably be approximately 100
mesh or less, more preferably 200 mesh or less, though not limited in
particular. An amount of 0.01% or more of a platinum group metal and/or an
oxide thereof contained in the fine particles provides cathodes having
adequate activity. In excess of 50% leads to economical disadvantages. The
thickness of the coating layer is not specifically limited but should
preferably be 800 um or less, more preferably 400 um or less, takin8 into
consideration economy. With a v~ew to maintaining activity over a prolonged
period of time, the thickness should be at least 10 um or more, more
preferably 50 um or more.
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On the other hand, sn extensive series of studies has been made by the
p~esent inventors on a method for obtainSn~ low hydrosen overvoltage cathodes
satisfyin~ thc req~irements of low hydro~en overvolta~e and long-term
du~abi lity ~ and the follo~in~ conclusion has been resched.
Low durability of the cathodes is mainly due to low mechanical str~ngth
which is caused by many vacant spaces being present in the active ~ortion
or insufficiency of adhesive force among particles, whereas high hydrogen
overvoltage of the cathodes is caused by lack of active area in actual operationor activity per unit area.
Looking at the prior art from this viewpoint,
cathodes produced by electroplating and which contains a sacriflcial component
which dissolveg dur~n~ the electroly~ls, when the sacrlflcial component i8
present in lar~e smounts to lower hydro~en overvolta~e, deterloration in
mechanical stren8th occurs and thereby the electrode is inferlor in
durability. A process for codeposit plating of Raney-nickel is characterlzed
in that the mechanical strength of the active portion is hlgh but lt has been
found throu~h studies by the present lnventors that the electrode is still
deficlent in long-term durability. That is, for the purpose of minimizinB
hydroRen overvoltage, the content of Raney alloy in a codeposit plating
coating layer has to be increased, but the increased content of Raney-nickel
results in a decrease in the mechanical strength of the active portion.
A modification of the foregoing aaney alloy codeposit platlng method
producin~ low overvolta6e cathodes lsldisclosed by Japanese non-examined
Patent Publication No. 133387/83 of Oda et al published 9 August, 1983. In
this process, Raney alloy ant a platinum group metal are admixed as powders,
with which codeposit platin6 i5 carried out. However, the process is not
satisfactory thou6h providing cathodes which are not only stronger in
mechanical stren6th, and smallar in bydrogen overvoltage, as compared to
codeposlt platin6 usln6 Raney alloy alone. That may be because uniform
codeposit platln6 is difficult due to the differences in particle size,
specific 6ravity and tbe like between the Raney alloy and the platinum 6roup
metals. The platinum 6roup metals, different from Raney alloy, show no
activity when buried in a nickel matrix.
The present inventors have repeated studies of a new codeposit plating
;~ method which makes use of both the activitles of aaney alloy and of platlnum
~ roup metal~ nd have rriv-d t th- con¢-pt of adaln~ s thlrd compon-nt
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platinum group metals to th~ Kaney alloy. That is it has been discovered
that cathodes wi~h suf~icient strength, long-term durability and
satisfactorily low hydro~en overvoltage can be provided by employing a
three-component alloy as dispersed phase which is comprised of a first metal
selected from the ~roup consisting of nickel, cobalt and silver, a second
metal selected from the group consisting of aluminum, magnesium, zinc and tin,
and a third metal selected from the group consisting of platinum, palladium,
rhodium, ruthenium, iridium and osmium.
The method for producing low hydro~en overvoltage cathodes by codeposit
plating of the three-component alloy is characterized by using a codeposit
plating tank in which an anode and the object to be plated which is of a
non-perforated flat structure are positioned parallel to each other, supplyins
a dispersant slurry throuBh one side of the tank and ¢ause it to flow through the
space formed between the anode and the object, then removing the slurry
throù~h the opposite side, but more preferably, recirculating the removed
slurry back to a codeposit platin~ bath storaKe tank, whereby codeposit
plating is applied to only one surface of the object.
~ n apparatus and process for codeposit plating of Raney alloy is disclosed
by, for example, Japanese Patent non-examined Publication No. 104491~80 of Oda
et al published on August 9, 1980, and Uo. 31091/83 of Oda et al published on
February 23, 1983. According to these processes, codeposit plating is carried
out in a codeposit plating vessel in which a dispersant slurry is a~itated
vertically by the use of gas, a vibrating plate, a pump or the like. These
processes were carried out by the present inventors with a large
non-pereorated plate but a uniform and firm coatin~ was not obtained. First,
it i8 ~urmi~ed that when an object to be plated i8 non-perforated, the dis-
per~ant slurry is not dispersed uniformly durin~ the platin~, which results
in non-uniform contact of the fine particles with the ob~ect. ~or this
rea~on, the~e proces~es are not suited to a non-perforated plate which the present
~nv~nt~on ~ims at,-but only to a perforated plate.-Second, the reason
for failure to obtain the uniform coating is presumably attributable to
non-uniformity in chances of collision of dispersant particles with the
ob~ect. Generally, Raney alloy has a Rreater density than that of a PlatinB
bath so that it tends to precipitate toward the bottom.
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The present inventlon has been completed on the basis that if a dispersant
slurry flows horlzontally rather than verticslly, the chsnces of contsct snd
collision between dlsperssnt ~lurry partlcles and the ob~ect to be pl~ted
should be enhanced and thereby improve the deposltlon of the psrticles onto
the ob~ect
The cathode base ugable for actlvated cathodes of the present invent~on
may be in any form but a non-perforated flat plate is preferably e~ployed In
an electrolytic cell for use in the electrolysls of an aqueous alXali metal
halide solutlon which was converted from a conventLonal asbestos diaphragm
cell or msrcury cell to an ion exchan~e membrane cell, operatlon is often
carrLed out to save ener~y costs by reducin~ the anode-cathode distance to 3mm
or less, often 2 mm or less In those cases, non-perforated flat plate
cathodes are capable of providin~ unlform micro-distribution of current
density over the cation exchan6e membrane and hence are very desirable
~oreover, Ln the case of non-perforated flat plates, formatlon of a coatlng
layer may be sùltably achieved by conventional technlques incluting plasma
spray costin6, cbeml'cal plstins, electroplstin~ and the llke, if deposits can
be attained with 600t adhesion on the cathode base of electroconductive fine
particles containing platinum ~roup metals and/or oxides thereof together with
nickel or nickel alloys
. .
In pr-ctlcln~ th pr-s-nt lnv ntlo~, only th- n-c~ ry ~ra- of th
c-~hod- b--- n--d b w b~-ct-t to th- tr -tm nt, i - , only th- r--
approxim tely equ-l to'the cation exchan6e membrane nest be troatet
Further,-in th~ ca~e of perforst-d cathode, electrlc current, durin6
operatlon, i~ li-bl- to concentrste ln d8es in the vlclnlty of the
perforstion~ c-usln~ non-unlformlty ln cu~rent tensity over th- cathod- Por
this roason, ther re probl- w 'lnclutln~ psrtisl corroslon of nlck-l os
nlckel slloy ~ervln~ ~ a costin~ l-y-r ba-e Therefore, lt ls prefsrret to
spply the pros-nt technique to a c-thot- bs-e of a fl-t tructur- havln~
neither perfor-tion~ nor ed~-s
The csthot-- of th- pr-sent inventlon obtained in such manner as
aforo~-id re d-ptot for u-o - el-ctrot-- which ~en-r-t- hydro~en ta- ln,
for oxsmplo, th- ~lectrolysls of w-t-r', lksli metsl hslld-s nt tha lik-
- Th- m-thot for protucin6 low hydro~-n ovorvolts~- cathot-- of th- pr---nt
inventlon will bo oxplslnod by referrln~ to the trswints 111u-tr~t1n~
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embodiments.
FIG. 3 depicts a schematic representation showing an example in which
codeposit platin~ is effected accordin~ to the present invention. In a
codeposit plating bath storage tank (1), a dispersant is well stirred to give
a uniform slurry concentration. The dispersant slurry (2) is supplied by a
pump (3) to a codeposit plating tank (4~ through one side, then removed
through the other side. The removed dispersant slurry is returned back to the
codeposit plating bath storage tank (1) and recirculated between the plating
bath storage tank (1) and the plating tank (4). Although uniform codeposit
plating is possible without recirculating the dispersant slurry between the
plating bath stora~e tank (1) and the plating tank (4), it is preferred to
effect recirculation since a large quantity of dispersant slurry is needed.
The codeposit plating tank (4) is equipped with a cathode (6) to be plated
and an anode (5), both beinB positioned parallel to each other, to thus form a
closed codeposit plating chamber (7). The cathode (6) is, of course, located
so that the surface to be plated faces to the inside of the chamber. As the
anode tS), any known anode for use in electroplating may be used and the shape
is not specifically limited but includes a flat plate, a perforated plate, a
net, an aggregate of nickel tips and the like.
The amount of the dispersant contained in the codeposit plating coating
according to the present invention varies according to the direction in which
the object to be plated was placed, the concentration of the dispersant
slurry, the average flow rate of the dispersant slurry within the codeposit
plating chamber and the like. I
FIG. 4 and FIG. S are schematic representations showing the direction in
which the object to be plated is placed. In FIG. 4, the object is placed
horizontally and faces upward, as in the case of FIG. 3. In FIG. 5, the
object is placed vertically. A preferred embodiment is to place the object
substantially horizontally to face upward, as shown by FIG. 3 and FIG. 4. It
is also consideredlto place the object to face downwardly, but in this case
the same platsd object as in the case of FIG. 3 and FIG. 4 cannot be obtained
because of a decrease in deposition caused by gravity, unless the slurry
concentration is higher than in the case of FIG. 3 and FIG. 4.
An embodiment placing the objects as shown by FIG. S is useful when two
sheets of cathodes are produced at one time. That is, by placing two objects
PAT 7489-1 - 9 -
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so that the backs thereof are in contact with each other, two sheets of
hydro~en overvolta~e cathodes can be produced through one operstion of
codeposit platin~. In FIG. 5, the chances of collision between dispersant
particles and the object are somewhat reduced as compared with the cases of
FIG. 3 and FIG. 4, and hence codeposit platin~ should then desirably be made
with a higher slurry concentration.
The dispersant slurry should flow substantially horizontally in the
codeposit plsting tank. "Substantially horizontally~ means to the extent
within which an increase in deposition of the dispersant particles resulting
from gravity is achievable, i.e., the angle between the horizontal surface and
the slurry flowing line beinB within 45 degrees, more preferably 30 degrees,
regardless of upward or downward direction, most preferably 0 de8ree. The
dispersant slurry is normally supplied through one side of the tank and
removed throu~h the opposite side, but it is possible for the purpose of
uniformity to flow it in reverse direction during operation by changing the
inlet and the outlet. It is further desired to position dispersing plates at
the inlet and the outlet to improve uniformity.
The slurry concentration should preferably be not less than 0.01 g/l but
less than 3 8/1, more preferably not less than 0.05 g/l but less than 3 g~l.
When less than 0.01 8~1, only the plated object containi~ng dispertsafnt~ in~ low
amounts is obtained resulting in hi8h hydrogen overvoltage. Whenisreater than
3 g/l, the plated object contains dispersant in 8reater amounts, which shows
low initial hydrogen overvoltage but is poor in mechanical strength, thus
lacking in long-term durability. The a~era8e flow rate of the dispersant
slurry within the codeposit plating chamber should preferably be 0.05 m/sec or
more, more preferably less than 10 m/sec. When less than 0.05 m/sec, local
unbalance of the dispersant content becomes 8reat and thus the plated object
havin~ uniform composition is not obtained. When 10 m/sec or more the
dispersant content decreases and the plated object possesses hi8h hydrogen
overvoltage, further, equipment costs and energy costs increase due to the
increased amount of dispersant slurry recirculated.
i As the bath solution forming the dispersant slurry, the well-known nickel
plating bath may be suitably employed, including such as watts bath, all
nickel chloride bath, high nickel chloride bath and the like.
The particle size of the dispersant is not specifically limited, but
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should preferably be approximately 100-mesh or less, more preferably 200-mesh
or less.
When the object to be plated is of lar~e dimensions, it is separated into
several parts anq codeposit plated.
The dispers ~ ~ ~ ee'compr~sed of an optional combination of a first
metal selected from the group consistin~ of nickel, cobalt and silver, a
second metal selected from the group consisting of aluminium, masnesium, zinc
and tin, and a third metal selected from the group consisting of platinum,
palladium, rhodium, ruthenium, iridium and osmium. The second metal is
leached out by being immersed in an aqueous caustic alkali solution after
codeposit plating, whereby the coating layer is made porous and thus
activated. The content of the third metal is considered from both aspects of
cost and activity, but should desirably be not hi~her than 50 weight %. In
the case of less than 0.01 weight %, the effect of increasing activity is
negli~ible.
As far as chan~es in the dispersant slurry concentration are concerned, it
is possible to start with an initial 6iven concentration and end with a
concentration lower than the initial concentration, or to keep the
concentration constant from the be8inning to the end by supplylng the
dispersant to the codeposit plating bath tank continuously or periodically, or
to end with a hi8her concentration than the initial one.
Cathodes subjected to codeposit plating may be stored for a prolon~ed
period of time after bein8 washed and dried. To use the cathodes as low
hydrogen overvolta~e cathodes, the seco~d metals must be leached out by an
aqueous caustic alkali solution. This treatment may be made either before or
after installation of the cathodes in an electrolytic cell, but the latter is
preferred.
As cathodes used in the production of an aqueous alkali metal hydroxide
solution by an ion exchange membrane process or an asbestos diaphra~m process,
expanded metals, perforated plates or net structure cathodes have been
commonly employed. Notwithstanding, according to the studies made by the
present inventors, it has been made clear that cathodes of non-perforated flat
,~ plates, only one surface of which is codeposit plated provide the best
j results, when used as cathodes in a horizontal type ion exchange membrane
~ ~ electrolytic cell. Moreover, it has been also discovered that even when an
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asbestos diaphra~m electrolytic cell is retrofitted to an ion exchange
membrane electrolytic cell, cells equipped with cathodes of non-perforated
flat plates, only one surface of which is codeposit plated posses low cell
volta~e. The non-perforated flat plate structure has numerous merits
including highly uniform current distribution, reduction of electric
resistance, hi~h accuracy of dimensions on manufacturin~ and the like.
As stated above, the present invention is capable of production of
superior cells equipped with low hydrogen overvolta~e cathodes, contrary to
knowledge of persons skilled in the art that cells with non-perforated flat
plate cathodes show hi8h cell voltage (e.~., Japanese non-examined Patent
Publication No. 174477/82 of Nishizawa et al published 27 October, 1982, "SODA
AND CHLORINE", 32, 281, 1981), and are therefore exceedingly valuable in the
industry.
The present invention will be explained in more detail by way of Examples
and Comparative Examples that follow, to which the invention is no way limited.
EXA~PLE 1
Nickel particles having an average particle size of 8 um were
electroplatet with ruthenium to 4 wei~ht %.
The nicXel particles containinfi ruthenium, and nickel particles having an
average particle size of 54 um, were admixed in the proportions of 3:7, with
which sant blasted nickel plates of 4 cm x 4 cm were subjected to plasma spray
coating to 200 um in thickness.
The foregoing test piece was used as a cathode, a DSE (manufactured by
Permelec electrode Company) was used aslan anode, "NAFION* 901" (manufactured
by e.I. DuPont de Nemors 6 Co.) was served as a cation exchanee membrane in an
electrolysis cell and an aqueous sodium chloride solution was electrolysed
while controlling the catholyte concentration to 32%, current density to 25
A/t m2 and catholyte temperature to 90C. Hydrogen overvoltage was
measuret by the current interruptor method.
Hydrogen overvoltage of the cathode was 0.07 to 0.09 V and no degradation
,~ in performance could be observed even after continuous 1000-day operation or
more. In FIG. 1, the changes in hydrogen overvoltage are depicted.
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P~T 7489-1 - 12 -
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COHPARATIYE EXA~PLE 1
The same sand blasted nickel plates as in ~xample 1 were subjected to
plasma spray coatin~ with nickel particles of 8 um average particle size to
200 um in thickness.
The 10 sheets of test pieces were used for electrolysis under the same
conditions as in Example 1. The test pieces showed an initial hydrogen
overvoltage of 0.08 to 0.09 V but each of them showed an abrupt increase after
50 to 150 days. The section was observed by an electron microscope and cracks
could be seen on the inside of every test piece.
COHPARATIVE EXAMPLE 2
Nickel particles of a 54 um average particle size were applied by plasma
spray coating to the test pieces as used in comparative Example 1 and used for
electrolysis under the same conditions as in Example 1.
Hydrogen overvoltage was 0.15 V initially but increased to 0.23 V after
50-day operation.
eXAYPLe 2
A three-component-alloy Raney-nickel comprising 48.9 weight % of nickel,
S0 weight % of aluminium and 1.1 weight % of platinum was pulverized and
classified. The 200-mesh tbree-component Raney-nickel particles were
dispersed in a plating bath in a concentration of 1 g/l, the bath containing
300 g/l of ~iC12.6H20 and 40 g/l of H3B03 and aBed with stirring for
an hour.
Nickel plate test pieces 4 cm x 4 cm were immersed in the foregoing
platin~ bath and plated with stirring ab 50 C for 90 minutes. As the anode,
nickel was used and 0.48 A direct current was supplied. The content of
Raney-nickel was 25.6% and the thickness of the coating was 250 um.
The test pieces were then leached out at 80 C for 2 hours by being
immersed in a 20% aqueous caustic soda solution. The activated test pieces
were used as cathodes and the electrolysis was effected under the following
conditions. Hydrogen overvoltage was measured by the current interruptor
method.
PAT 7489-1 - 13 -
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Electrolysis conditions:
Anolyte Nacl
220 g~l
Catholyte NaOH
30~
Temperature
85 C
Anode insoluble anode
Ion exchange membrane "NAFION *901"
The cathodes produced accordin~ to the foregoing manner showed hydrogen
overvolta~es ran~in~ from 0.06 to 0.08 V and even after 800 days of operation,
no chsn~e in performance could be seen. The changes in hydrogen overvolta~e
are shown in FIG. 2.
XANPLE 3
Test pieces were prepared in a similar fashion to that of Example 2
excepting that a three-component-alloy Raney-nickel comprising 45.7 weight %
of nickel, 50 weight ~ of aluminium and 4.3 weight % of ruthenium was
employed, and the electrolysis was carried out under the same conditions.
Hydro~en overvolta~e was between 0.07 V and 0.08 V and even after 750 days
of operation no degradation in performance was observed. During that period,
operation was shut down five times for one week, in each case, but de~radation
in performance could not be seen before or after every shut-down.
EXAHPLE 4
Sand blasted test pieces of 4 cm x 4 cm were coated by plasma spraying
with 200-mesh three-component-alloy particles obtained in Examples 2 and 3 to
200 um in thickness.
Using the foregoing cathodes, the electrolysis was effected similarly to
example 2. The cathode using Al-Ni-Pt alloy showed 0.06 V hydrogen
overvoltage, while the cathode using A1-Ni-Ru alloy showed 0.07 V. No
degradation in performance was observed even after 650 days of operation in
each case. During that period, operation was shut down 15 times for one week,
in each case but changes in performance before or after each shut-down did not
occur.
CO~PARATIVE EXAHPLE 3
Codeposit plating was effected in a similar manner to that of Example 2,
PAT 7489-1 - 14 -
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exceptin~ that 200-mesh Raney-nickel comprising 50 weight % of nickel and 50
wei~ht % of aluminium was used, and the electrolysis was conducted similarly
to Example 2.
Hydrogen overvoltage w8s O.10 V and increased to 0.18 V after 100 days of
operation. During that period, operation was shut down five times for one
week, in each csse and degrsdation of 0.01 V on an average was observed every
shut-down.
EXAHPLE 5
Non-perforated flat plates of carbon steel, 660 mm x 2,000 mm, were
degreased, washed with an acid, and chemically plated with nickel to 30 um in
thickness.
Two sheets of the flat plates so treated were placed vertically so that
the backs were in contact with each other, as shown by FIG. 5. Anodes formed
by wrapping nickel tips for electroplating in a titanium mesh were arranged in
parallel and the non-perforated flat plates and the anodes were secured to
hard rubber-lined iron frames to form two codeposit plating chambers.
On the other hand, in a codeposit plating bath storage tank having an
inside capacity of 1.8 m3, a Raney alloy dispersant slurry (Al:Ni:Ru = 50
wt. %:45 wt. %:5 wt%, Particle size : 200-mesh) was dispersed in a nickel
plating bath (~iC12.6H20 300 8~1, H3B0338 ~ PH 2 2.5) to prepare
l.S m codeposit plating bath containing the slurry concentration of 2 g/l.
The codeposit plating bath was removed with stirring by a pump and
supplied into the codeposit platin6 chambers through one side to flow
~ horizontally. Codeposit plating was ca~ried out under the conditions;
;~ temperature 50 C, current den~ity 3 ~/d m , time 90 minutes and average
flow rate of the slurry within the chamber 1.0 m/sec. The plated coating so
obtained was hard and uniform in thickness.
Yrom the re~ultant flat plates, only one surface of which was codeposit
plated, in each case 26 sheets of cathodes were made under the same conditions
and installed in an asbestos diaphra8m electrolytic cell (H-4 type,
manufactured by Hooker Chem. Corp. Inc.). After assembling the cell, the
~ cathodes were subjected to leaching out treatment by being immersed in a 25%
't aqueous NaOH solution for three hours. As the cation exchange membrane
"NAFION* 901" was used. Cell voltage was 3.4 V under the conditions;
temperature 90C, current denfiity 23.5 ~/d m and NaOH concentration 32%.
PAT 7489-l - 15 -
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~ydrogen overvoltage was 0.07 V.
EXAHPLE 6
A cathode bottom plate (carbon steel), 1800 mm x 11,000 mm, used in a
mercury electrolytic cell was polished smooth and then chemically plated with
nickel to 30 um in thickness.
The obtained plate was separated into six parts lon~itudinally, by which
the codeposit plating tank is formed substantially horizontal to allow the
surface to be plated to face upward, as illustrated by FIG. 3 and FIG. 4.
Each part was codeposit plated in the same manner and the same conditions
as in Example 5. The plated coating was hard and was approximately uniform in
thickness in every part.
Usin~ this cathode, a cell was assembled and leaching out treatment was
performed under the same conditions as in Example 5. "~AFION* 901" was
positioned and the electrolysis was efeected. Cell voltage was 3.45 V under
the conditions; temperature 90 C, current density 50 A/d m and NaOH 32%,
and hydrogen overvoltage was 0.10 V.
PAT 7489-l - 16 -
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