Title of Invention

BIPOLAR PLATE FOR FILTER PRESS ELECTROLYZERS

Abstract Bipolar plate made of a composite material for use in a filter-press electrolyzer. Said plate comprises a central portion which is electrically conductive and is obtained by heat-pressing of a mixture of graphite or conductive carbon and a thermoplastic polymer powder resistant to corrosion and two terminal portions containing the distribution holes for the inlet of the fresh electrolytes and for the outlet of the exhausted electrolytes and electrolysis products. Said terminal portions are integral with the central portion and are obtained during said heat- pressing from a mixture of graphite or conductive carbon and said thermoplastic polymer powder with a ratio between said powders lower than that of the central portion. Said mixture of the terminal portions may further contain also a non-conductive compound powder, in which case the mixture may also be free from graphite or conductive carbon powder.
Full Text FIELD OF INVENTION
Membrane electrolysis processes of industrial interest such as chlorine
and caustic soda production from sodium chloride solutions and even
more for the production of chlorine from hydrochloric acid solutions or
directly from gaseous hydrochloric acid as described in US Patent
No. 5,411,641, J.A. Trainham III, C.G. Law Jr, J.S. Newman, K.B.
Keating, D.J. Eames, E. I. Du Pont de Nemours and Co. (USA), May 2,
1995, undergo extremely aggressive conditions.
In the process for the production of caustic soda and chlorine, the
anodic reaction produces chlorine gas which, as well known, is a
strongly corrosive agent. For this reason, in industrial practice usually
titanium is used for the anodic elements of the elementary cells forming
the electrolyzers. The use of titanium, in this case, is permitted by the
relatively modest acidity of the sodium chloride brine in contact with
said anodic parts. The acidity is kept at low levels for process reasons
and mainly not to damage the delicate ion-exchange membranes
separating with a high efficiency the produced caustic soda from the
acid brine Suppliers of this kind of membranes specify in fact that the
minimum pH for continuous operation must be kept around 2.
Titanium cannot be used for the construction of the cathodic parts of the
elementary cells forming the electrolyzer, as the hydrogen evolution,
which is the only cathodic reaction, would cause a dramatic

embrittlement. In most cases the cathodic parts of the elementary cells
are made of high-alloy stainless steels or even better nickel. As a
consequence, in bipolar electrolyzers, the bipolar elements which
coupled together in a filter-press arrangement form the elementary
cells, are made of two layers made of nickel and titanium connected
either mechanically (Indian Patent No. 166591, H. Schmitt, H. Schurig,
D. Bergner, K. Hannesen, Uhde GmbH, May 12, 1987) or by welding
(Indian- Patent No. 160 120 G.J.E. Morris, R.N. Beaver, S.
Grosshandler, H.D. Dang, J. R. Pimlott, The Dow Chemical Co.,
December 18, 1984), optionally with an internal layer directed to
ensure the electrical conductivity and necessary rigidity. These bipolar
elements obviously entail a complicated construction and therefore high
costs.
In the production of chlorine by electrolysis of hydrochloric acid, the
aggressivity is much greater due to the concurrent presence of chlorine
and high acidity. Under particular conditions (temperature below 60°C,
acid concentration below 20%, addition of passivating agents) a
titanium - 0.2% palladium alloy (ASTM B265, Grade 7) may be used
with the interstice areas suitably protected by a proper ceramic coating.
With temperatures and acid concentrations higher than the above
mentioned ones and in the absence of passivating agents, the only
suitable material for the construction of the anodic parts of the
electrolyzer is tantalum, an extremely expensive material which poses a

lot of problems for its working.
Anyway, tantalum, just as titanium, is not compatible with hydrogen and
therefore cannot be used for the cathodic parts. A possible solution is
given by the nickel alloys of Hastelloy B® type, but they are very
expensive and undergo corrosion during the shut-downs of the
electrolyzers. To avoid this severe inconvenience, it would be
necessary providing the electrolysis plants with polarization systems,
which would make scarcely practical the whole construction.
A possible alternative is offered by graphite, which is sufficiently
stable at the process conditions, both the anodic (chlorine evolution
with minor quantities of oxygen, in the presence of chlorides and
acidity), and the cathodic ones (hydrogen in the presence of caustic
soda - chlor-alkali electrolysis - or in the presence of acidity -
electrolysis of hydrochloric acid). Therefore graphite may be used in the
form of plates directly forming the elements which are then assembled
in a filter press-arrangement to form the elementary cells of
electrolyzers. In the case of bipolar electrolyzers the two faces of the
same graphite plate actually act as the cathodic wall of one cell and the
anodic wall of the adjacent cell. As graphite is intrinsically porous, the
mixing of chlorine and hydrogen, caused by diffusion through the pores,
may be avoided only making the graphite plates impermeable by
means of processes comprising filling under vacuum of the pores with a
liquid resin which is subsequently polymerized and makes the graphite

plate more stiff and enhances its chemical resistance characteristics.
Graphite plates of this type are currently used in the industrial process
known as "Uhde-Bayer" process for the electrolysis of hydrochloric
acid solutions. Impermeable graphite however is extremely fragile and
is not deemed acceptable for most chlorine producers, especially in
critical apparatuses such as electrolyzers for chlorine production.
An interesting alternative is disclosed by US Patent No. 4,214,969, R.J.
Lawrance, General Electric Company, July 29, 1980 directed to the
production of plates made of graphite powder and thermoplastic
fluorinated polymers. The product obtained by heating and pressing
the powders mixture is a composite having a minimum or no porosity,
exhibiting a suitable electrical conductivity. This last characteristic is
obviously necessary as the plates must provide for an efficient
electric current transmission to ensure a correct operation of the
electrolyzers. The advantage of the graphite-polymer composite over
impermeable graphite is its higher stiffness. In fact, the two requisites,
stiffness and electrical conductivity, are contradictory as a higher
stiffness involves a greater amount of polymer while a greater amount
of graphite would be necessary to enhance the electrical conductivity.
As; a consequence, an optimized product must be a compromise
between the two needs, a compromise which the above patent
indicates to be a function of the production parameters, in particular
pressure and temperature.

When the thermoplastic fluoropolymer is the polivinyldenfluoride, such
as Kynar® produced by da Pennwalt (USA), the best results in terms
of electrical conductivity and stiffness (measured as resistance to
bending) are obtained with contents of polymer in the range of 20-25%
by weight. Obviously, a composite plate obtained as above illustrated
and with the aforesaid material is intrinsically expensive.
A reduction of the total costs of an electrolyzer obtained by assembling
in a filter press-arrangement several plates may be achieved by
eliminating from each plate every external connection (threaded joints,
pipes, gaskets) for the circulation of the electrolytes and withdrawal of
the products. This simplified design certainly increases the operation
reliability of the electrolyzers, in particular when operating under
pressure. The elimination of the external connection requires that each
plate be provided with suitable internal holes provided with suitable
distribution systems, as described in details in U.S. Patent No
4,214,969. The multiplicity of plates of the filter-press electrolyzer
must have all the holes matching in order to form longitudinal channels
inside the electrolyzer structure. These channels (manifolds), which are
connected to suitable nozzles positioned on one or both sides of the
electrolyzer heads, provide for the internal distribution to the various
elementary cells of the fresh electrolytes and for the withdrawal of the
exhausted electrolytes and electrolysis products (for example chlorine
and oxygen). Said channels longitudinally crossing the electrolyzer are

therefore subjected to a remarkable electric potential gradient. Further,
if both the fresh and the exhausted electrolytes have a sufficient
electrical conductivity (hydrochloric acid, sodium chloride brine and
caustic soda are high conductive), then the channels are crossed by
consistent electric current, the so-called shunt current, which represent
an efficiency loss and cause electrolysis phenomena among the
surfaces of the plates facing the channels.
These electrolysis phenomena produce substantially two negative
effects, that is the reduced purity of the electrolysis products and the
corrosion of at least part of the composite plate surfaces. As a matter
of fact also the graphite particles forming the composite may undergo
corrosion and be progressively worn out and converted into carbon
monoxide and/or carbon hydroxide under the electrolysis conditions
typical of said channels. As a consequence, the composite looses its
major components and thus any mechanical solidity.
US Patent No. 4,371,433, E.N. Balko, L.C. Moulthrop, General Electric
Company, February 1, 1983, describes a method for reducing
parasitic shunt currents and eliminating corrosion phenomena. This
method foresees a particular profile of the manifolds in order to cause a
fractionating of the electrolyte flow in small droplets (increase of the
overall electrical resistance) housing particular gaskets inside the
manifolds. Substantially the surface of the composite plates facing the
manifold is internally lined with the gaskets and cannot get in contact

with the electrolytes. However, in view of the fact that these gaskets
have a complex geometry and are made of elastomeric fluorocarbon
materials which must ensure a high chemical resistance, such as
Viton® polyhexafluoropropylene rubber supplied by DuPont (USA), this
method is very expensive and therefore scarcely applicable in industrial
practice.
SUMMARY OF THE INVENTION
It is the aim of the present invention to overcome the problems of the
prior aht by providing for a method for protecting the composite graphite
(or conductive carbon) - thermoplastic (preferably, but not exclusively,
fluorinated) polymer in those areas where the surface of said plates
faces the longitudinal manifolds. The method of the invention has the
advantage of not increasing noticeably the production cost of a
common composite plate and may be realized in the production of said
plate.
The present invention solves the problem of localized corrosion in those
areas where the surface of said plates faces the longitudinal manifolds
by suitably decreasing, or even eliminating, the content of graphite
powder or conductive carbon powder in the terminal portions of said
bipolar plates. Said terminal portion contain the holes which, after
assembling in a filter-press arrangement of the bipolar plates, form the
longitudinal channels (manifolds).

DESCRIPTION OF THE PREFERRED EMBODIMENT
The present preferred embodiment of the invention will be now
described making reference to figure 1 which is a frontal view of the
bipolar plate.
With ref to Fig. 1, the bipolar plate 1 is provided with holes 2, 3, 4,
and 5 which, after assembling in a filter-press arrangement of adjacent
bipolar plate, form the longitudinal channels (manifolds) and with
longitudinal grooves 6 directed to favour the circulation and distribution
of electrolytes. Said grooves 6 may be also avoided and the bipolar
plate may alternatively have a flat surface.
The terminal portions 7 e 8 of the bipolar plate have a reduced content
of graphite powder or may even not contain graphite at all. The central
oortion 9 of the bipolar plate has a greater area with respect to terminal
portions 7 and 8 and is made of a composite with a high content of
graphite and thus highly conductive. Said central portion 9 is in fact
directed to transmit electric current to the electrodes (anodes and
cathodes) which are in contact with said central portion and
substantially have the same area.
E3y decreasing or even eliminating the content of graphite or conductive
carbon in the conductive areas 7 and 8, corrosion problems are
avoided. These corrosion problems are due to the fact that the surfaces
of the bipolar plate facing the longitudinal channels (manifolds)
(c rcumferential surfaces of the holes 2, 3, 4 e 5 in Fig. 1) may act as

electrodes and in particular as alternated anodes and cathodes due to
the effect of the electric potential gradient across the electrolyzer. On
the surfaces acting as cathodes hydrogen is evolved and no problem of
stability in the graphite or conductive carbon polymer is experienced.
On the surfaces acting as anodes the chloride ions discharge to form
chlorine. This reaction is characterized by high efficiency but not
100%, and involves a parasitic reaction of water discharge with oxygen
evolution. Under these conditions the graphite or conductive carbon
particles are slowly attacked and are converted into carbon monoxide
and/or carbon hydroxide. When the composite is conductive, the
graphite particles are so concentrated that it may be assumed that
statistically said particles get in contact with each other forming
conductive chains throughout all the plates thickness. Therefore when
corrosion causes the complete depletion of the plate the attack does
not stop but continues in the adjacent plate, giving rise to a porosity
crossing the composite bulk which consequently looses any
mechanical stiffness.
The most obvious solution would seem the complete elimination of the
graphite powder manufacturing the terminal portions 7 and 8 of the
bipolar plate 1 with the thermoplastic polymer powder only. As already
said, this is an extreme solution which may involve mechanical
problems. In fact in this case the composite plate would be made, as
aforementioned, by compression and heating of a mixture of graphite

and thermoplastic polymer powder (optionally in the form of pre-
formed pellets) spread on the central portion of the mold, and powder
or pellets of the polymer only spread in the areas of the mold
corresponding to the terminal portions 7 e 8 of the bipolar plate.
When a similar plate with portions having different content of graphite
powder cools down, severe distortions are frequently experienced,
caused by the different thermal expansion coefficients of the portions
having a different content of graphite. In particular, the terminal
portions made of thermoplastic polymer only are characterized by a
much greater thermal expansion coefficient. To avoid distortion
problems hindering the production of perfectly planar plates, the
graphite content must be reduced but not eliminated. To define the
exact content of graphite powder necessary to avoid the above
problems, the electrical resistivity values of various composites have
been measured and are listed in Table 1.

TABLE 1
Electrical resistivity of various composites comprising
polivinylidenfluoride and graphite powder (Stackpole A-905)

Similar results are obtained by substituting at least partially the
graphite powder with graphite fibers as disclosed by US Patent No.
4.339,322, E.N. Balko, R.J. Lawrance, General Electric Company, July
13, 1982. The production cycle comprises cold-compression at 145
bar, heating at 150°C, decreasing the pressure to 20 bar, increasing
tie temperature to 205°C, bringing back the pressure to 145 bar, with
s final phase of step-by-step reduction of pressure and temperature.
Table 1 clearly indicates that a substantial reduction of the graphite
powder content to 40% still leaves a minimum electrical conductivity
which means that the graphite particles (or their aggregates) at least
partially form electrical continuity bridges. Corrosion tests have been

carried out under current, that is using samples of composites
containing 40% by weight of graphite powder working as anodes in
sodium chloride brine and hydrochloric acid. It resulted that corrosions
affects only small areas, the ones where the infrequent conductivity
bridges exist, (chains of graphite particles in contact with each other).
As a consequence, the porosity of the composite is modest and the
mechanical characteristics are not affected.
It has been found that a complete immunity to the porosity caused by
corrosion may be obtained by further decreasing the content of
graphite powder, for example down to 20% by weight or even below.
However, in this case distortion phenomena are again present, typical
of bipolar plates with terminal portions 7 and 8 made of thermoplastic
polymer only, in particular when it is polyvinyldenfluoride characterized
by a particularly high thermal expansion coefficient. In fact, the thermal
expansion coefficient of the composite containing 20% by weight of
graphite is much higher than that of a composite having a high content
ot graphite (e.g. 80% by weight) used for central portion 9 of
bipolar plate 1.
It has been found that the above problem may be overcome if the
terminal portions 7 and 8 of the bipolar plate are produced with a
mixture comprising powders of graphite, in minor amounts (20% by
weight or less), of a thermoplastic polymer and of a non-conductive
corrosion resistant filling material.

The best results are obtained when the percentage of thermoplastic
polymer calculated on the total weight of the ternary mixture are the
same as those of the central portion 9 of the bipolar plate 1.
It has been further found that the filling material must be carefully
selected taking into consideration the chemical characteristics of the
thermoplastic polymer. In fact when the latter is a fluorinated polymer
(best preferred due to its high chemical inertness), a chemical reaction
between the polymer and the filling material may take place at the
temperatures reached during molding of the bipolar plate. For example
when the thermoplastic polymer is polyvinyldenfluoride, it may violently
react with silica powder or boro oxide and possibly form volatile
compounds such as silica tetrafluoride or boro trifluoride. Further, the
additional filling material must be stable in contact with the acidic
sodium chloride brines and the hydrochloric acid solutions containing
chlorine. It has been found that certain ceramic oxides, such as
niobium pentoxide, tantalum pentoxide, zirconium oxide, lanthanum
oxide, thorium oxide, rare earths ceramic oxides and some silicates
are suitable for use. Also suitable for use are certain insoluble salts,
such as for example barium sulphate.
Even if barium sulphate is quite satisfactory for the destination of the
bipolar plate of the invention, it has been found that the best
mechanical characteristics, particularly resistance to bending, are
obtained by using the various oxides or silicates as listed above. It may

The strips containing a small amount of graphite (20% by weight) and
an additional quantity of tantalum pentoxide or barium oxide were
immune from any attack. A similar result was obtained with samples
containing tantalum pentoxide, niobium pentoxide, barium oxide. The
relevant data are not included in Table 2.
TABLE 2
Behavior of various composites under anodic polarization in sodium
chloride solutions (220 grams per liter) and hydrochloric acid (5%).


WE CAIM
1. Bipolar plate for use in bipolar electrolyzer of the filter-press type, said
plate (1) comprising a central portion (9) made of a conductive composite
obtained from a mixture of graphite or conductive carbon powder or fibers
and powder of a corrosion resistant thermoplastic polymer, and two
nerrninal portions (7,8) made of a composite obtained from a mixture of
said graphite or conductive carbon powder or fibers and said powder of
the corrosion resistant thermoplastic polymer, said terminal portions
haying a higher electrical resistivity than the central portion and
containing holes (2, 3, 4f 5) for distribution of fresh electrolytes and the
withdrawal of exhausted electrolytes and electrolysis products,, said
centra! portion (9) and terminal portions (7, 8) forming an integral
element, wherein
said central portion (9) contains more than 60% by weight of said
graphite or conductive carbon powder or fibers/
said terminal portions (7, 8) have a low content of said graphite or
conductive carbon powder or fibers such that the electrical
resistivity of said terminal portions (7, 8) is at least ten times higher
than that of the central portion (9), and
said terminal portions (7, 3) further comprise an additional non-
conductive corrosion resistant material to reduce the difference in
the thermal expansion coefficient between said central portion (9)
and said terminal portions, (7, 8).
2 The bipolar plate of claim 1, wherein said additional non-conductive
material is selected from the group tantalum pentoxide, niobium
pentoxide, zirconium oxide,barium sulphate.
-18-

The bipolar plate of claim 1, wherein said composite of the terminal
portion is obtained from a mixture not containing graphite or conductive
caibon.
the bipolar plate of any of the previous claims, wherein said thermoplastic
polymer is a fluorinated polymer.
The bipolar plate of claim 4, wherein said thermoplastic polymer is
polivinyldenfluoride.

Bipolar plate made of a composite material for use in a filter-press
electrolyzer. Said plate comprises a central portion which is electrically
conductive and is obtained by heat-pressing of a mixture of graphite or
conductive carbon and a thermoplastic polymer powder resistant to
corrosion and two terminal portions containing the distribution holes for
the inlet of the fresh electrolytes and for the outlet of the exhausted
electrolytes and electrolysis products. Said terminal portions are
integral with the central portion and are obtained during said heat-
pressing from a mixture of graphite or conductive carbon and said
thermoplastic polymer powder with a ratio between said powders lower
than that of the central portion. Said mixture of the terminal portions
may further contain also a non-conductive compound powder, in which
case the mixture may also be free from graphite or conductive carbon
powder.

Documents:

697-cal-1997-granted-abstract.pdf

697-cal-1997-granted-claims.pdf

697-cal-1997-granted-correspondence.pdf

697-cal-1997-granted-description (complete).pdf

697-cal-1997-granted-drawings.pdf

697-cal-1997-granted-examination report.pdf

697-cal-1997-granted-form 1.pdf

697-cal-1997-granted-form 13.pdf

697-cal-1997-granted-form 2.pdf

697-cal-1997-granted-form 26.pdf

697-cal-1997-granted-form 3.pdf

697-cal-1997-granted-form 5.pdf

697-cal-1997-granted-pa.pdf

697-cal-1997-granted-priority document.pdf

697-cal-1997-granted-reply to examination report.pdf

697-cal-1997-granted-specification.pdf

697-cal-1997-granted-translated copy of priority document.pdf


Patent Number 246971
Indian Patent Application Number 697/CAL/1997
PG Journal Number 51/2008
Publication Date 19-Dec-2008
Grant Date 17-Dec-2008
Date of Filing 22-Apr-1997
Name of Patentee DE NORA ELETTRODI S. P. A.
Applicant Address VIA BISTOLFI 35, 20134 MILAN
Inventors:
# Inventor's Name Inventor's Address
1 FULVIO FEDERICO VIA SBOLLI, 4 29100 PIACENZA
PCT International Classification Number C25B 9/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 MI96A 000911 1996-05-07 Italy