Note: Descriptions are shown in the official language in which they were submitted.
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COATING FOR METALLIC CELL ELEMENT MATERIALS OF AN
ELECTROLYSIS CELL
[0001] The
present invention relates to a cathode half shell, wherein the metallic
components have a specific electrically conductive coating as characterised in
the
preamble of claim 1.
[0002] In
electrochemical processes, chemical reactions are controlled by an
external electric current. Within the electrochemical cells, a conductive,
stable,
inexpensive conductor is needed to transport the electrons. Here, nickel has
proved to be
an ideal material for the electrodes. However, a disadvantage is the formation
of poorly or
non-conductive nickel surfaces when the electrodes are operated at potential
ranges in
which nickel oxide or nickel hydroxide species are formed. Due to these low-
level
potentials, oxide or hydroxide formation occurs in many processes.
[0003] Ohmic
losses at the surface of the nickel compromise the efficiency of the
whole system, such as, for example, zinc/air and nickel/metal hydride
batteries, oxygen
cathodes in chlor-alkali electrolysis or oxygen electrodes in alkaline fuel
cells.
[0004] These
poorly or non-conductive oxide or hydroxide layers are, for example, a
hindrance when pure nickel is used as an oxygen evolution electrode in
electrolysis. But
even in systems where, as a conductive mesh, expanded metal or sheet, nickel
comes
into contact with catalytically active material, such as carbon, platinised
carbon, etc., the
isolating layer has a negative effect. For example, the oxide or hydroxide
layers prevent
an optimum current flow even with oxygen-depolarised electrodes, and therefore
steps
are required to improve or maintain conductivity in industrial electrolysis.
[0005] In the
literature, numerous diagrams relating to electrochemical stability are
documented in the "Atlas of Electrochemical Equilibria in Aqueous Solutions"
by Marcel
Pourbaix (1974). Pourbaix's results show that under the pH conditions of 13-15
and
cathodic potentials above approximately 0.4-0.6 V, measured against NHE, which
exist
during chlor-alkali electrolysis with oxygen-depolarised cathodes under
electric load, the
familiar formation of nickel oxide occurs in the form of passivation.
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[0006] An additional problem results from the start-up and shutdown of
chlor-alkali
electrolysis cells, whereby soluble hydroxides may be formed when mid-range
potentials
of approximately 0.6 V, measured against NHE, are passed through. As the
Pourbaix
diagram does not allow statements regarding the kinetics to be made, it is not
possible to
predict the actual formation of these hydroxides due to decomposition
reactions, i.e.
corrosion. Practical electrolysis tests are therefore required in order to
ascertain the
behaviour of nickel under oxidising conditions, such as in case of chlor-
alkali electrolysis
with oxygen-depolarised cathodes.
[0007] Various patent specifications, for example EP 1 033 419 B1 or EP 1
092 789
A1, describe electrolysis cells for chlor-alkali electrolysis with oxygen-
depolarised
cathodes, in which nickel is used as the material for the metallic components
on the
cathode side. Nothing is said about the corrosion stability of the nickel
regarding the
formation of non-conductive oxide or hydroxide compounds.
[0008] EP 1 041 176 A1 describes a method for an electrolysis cell with a
gas-
diffusion electrode with the purpose of minimising ohmic losses during current
supply to
the oxygen-depolarised cathodes (here referred to as gas diffusion electrodes)
by means
of the metallic components for current distribution. It already includes a
description of a
coating with excellent conductivity that is metallic in nature. No further
details, especially
with regard to its corrosion stability, are given.
[0009] DE 10 2004 034 886 A1 describes a process for the production of
electrically
conductive nickel oxide surfaces. Here, the poor conductivity of the nickel
oxide surfaces
is significantly improved via subsequent chemical doping with alkali oxides at
low
temperature in the presence of hydrogen peroxide. This application is thus
particularly
suitable for the operating conditions in fuel cells, storage batteries and
chlor-alkali
electrolysis.
[0010] The process described in DE 10 2004 034 886 A1 was first used
successfully
for the operation of laboratory chlor-alkali electrolysis cells with oxygen
depolarisation.
Oxygen-depolarised cathodes whose manufacture is described, for example, in EP
1 402
587 B1 or DE 37 10 168 A1 were used for this. These electrodes consist of an
electrically
conductive grid, usually a nickel wire grid, onto which has been rolled a
catalyst strip
made up of a silver/PTFE or silver oxide/PTFE mixture. The grid of the gas
diffusion
electrode is in electrical contact with the nickel current distributor, the
conductivity of
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which has been improved as per the process described in DE 10 2004 034 886 A1.
During operation of these laboratory cells no voltage increase or signs of
corrosion in the
form of nickel decomposition were detected despite the cells being repeatedly
switched
off and it can therefore be assumed that DE 10 2004 034 886 A1describes an
effective
process for protecting the nickel from corrosion.
[0011] EP 1 601 817 A1 describes an electrolysis cell that is commercially
exploited
and used for conventional chlor-alkali electrolysis. US 7 670 472 B2 describes
an
electrolysis cell that represents a design configuration within the cathodic
compartment
which allows the electrolysis cell for chlor-alkali electrolysis to be
operated with oxygen-
depolarised cathodes.
[0012] The design of the electrolysis cell described in EP 1 601 817 A1 was
altered
on the basis of the technical features of US 7 670 472 B2 so as to allow the
resulting
electrolysis cell for chlor-alkali electrolysis to be operated with oxygen
depolarisation. For
this, an electrode consisting of a nickel grid onto which a catalyst strip
made of silver
oxide and PTFE has been rolled, as in principle described in DE 37 10 168 A1,
was used
as the oxygen-depolarised cathode. The current supply to the oxygen-
depolarised
cathode located in the cathode compartment was realised in such a manner that
a
lamella-type support structure positioned parallel to the back wall of the
cathode has been
inserted, said structure being electrically connected to the back wall via
vertically
arranged webs by means of welded joints. An elastic element is attached to
this support
structure so that when the cathode half shell and the anode half shell of the
cell are
screwed together, a press-fit is created with the wire grid of the oxygen-
depolarised
cathode, which ensures electrical contact and an even current distribution.
Such elastic
elements are already described in various patent specifications, for example
in EP 1 446
515 A2 and particularly EP 1 451 389 A2, and consist of various compressible
layers
made of metal wires, which, pressed together like a sandwich, ensure
elasticity.
[0013] The process for treating the nickel oxide surfaces described in DE
10 2004
034 886 A1 was used on the nickel components in order to ensure the
conductivity of the
nickel oxide surfaces resulting from passivation during operation.
[0014] In a test series 1 two such redesigned electrolysis cells having an
active
electrolysis surface of 2.7m2 with Flemion F8020 membranes were operated at a
current
density of 4I(A/m2, 88 C operating temperature, an NaCI anolyte concentration
of 210 g/I,
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an NaOH catholyte discharge concentration of 32%w/w and with saturated moist
oxygen
in a stoichiometric excess of 20%. Fla. 1 shows the voltage curve of two
electrolysis cells
over the first 65 days of operation. Different symbols are used for each of
the electrolysis
cells (shaded diamonds and unshaded triangles).
[0015] In the first 30 days of operation, the electrolysis cells showed a
stable cell
voltage. On the 30th day of operation, the current to the two electrolysis
cells was turned
off. After it had been switched back on and a current density of 4kA/m2 had
been
reached, both cells showed an increased ohmic resistance in the form of a
voltage
increase of up to 100mV. After another 4 days of operation, the electrolysis
cells were
switched off again. After they had been turned back on and a current density
of 4kA/m2
had been reached, the ohmic resistance had increased further, resulting in an
additional
voltage increase of approximately another 200nnV. After approximately another
30 days
of operation, the two electrolysis cells were switched off and the components
inspected.
This showed that the conductivity of the components made of nickel ¨ the
support
structure and elastic element ¨ had decreased considerably. The oxygen-
depolarised
cathodes used were checked in laboratory cells and compared with reference
specimens.
During laboratory operation, this component also exhibited an increased
voltage
compared to the reference specimens, which could at least in part be
attributed to
reduced conductivity of the nickel wire grid due to oxidations. The protective
effect of the
process described in DE 10 2004 034 886 A1 was thus not effective under
certain
potential and operating conditions, which obviously occur when the
electrolysis cells are
switched off.
[0016] Based on the thermodynamic equilibrium reactions for noble metals
such as
silver and gold, described in the 'Atlas of Electrochemical Equilibria in
Aqueous Solutions'
(1974), the electrochemical stability diagrams for chlor-alkali electrolysis
operating
conditions at 85 C were recalculated in order to obtain a detailed overview of
the
electrochemical conditions:
[0017] For nickel, the result for 10-6 mol/kg at 85 C against an NHE
(normal
hydrogen electrode) is, in simplified form, the stability diagram shown in
Fig. 2. Here,
environment A is characterised by passivation, environments B and C by
corrosion and
environment D by immunity. According to this, at 85 C the corrosive
environment for
hydroxide formation is always passed through in the start-up environment (load
and
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potential increase) and shutdown environment (load and potential decrease) and
these
therefore represent the critical operating conditions.
[0018] For gold, the result, for 10-6 mol/kg at 85 C against an NHE (normal
hydrogen
electrode) is, in simplified form, the stability diagram shown in Fiq. 3.
Here, environment A
is characterised by passivation, environment B by corrosion and environment D
by
immunity.
[0019] Similar to nickel in Fig. 2, the diagram shows a possible corrosive
environment in mid-range potentials, where hydroxide compounds may be formed.
However, tests using gold in strongly alkaline caustic soda solution show
hardly any signs
of decomposition. It can thus be concluded that there is a kinetic hindrance
and gold can
be regarded as a stable metal for chlor-alkali electrolysis under oxidising
conditions.
[0020] For silver, the result, for 10-6 mol/kg at 85 C against an NHE
(normal
hydrogen electrode) is, in simplified form, the stability diagram shown in
Fig. 4. Here,
environment A is characterised by passivation, environment B by corrosion and
environment D by immunity.
[0021] From Fig. 4 it is clear that silver also has a narrow corrosion
environment
albeit in the sour pH range. In the alkaline and in particular under oxidising
conditions
silver tends towards passivation through the formation of oxidising species.
Corrosion
stability would thus be given, the question of conductivity under chlor-alkali
electrolysis
conditions with oxygen-depolarised cathodes would need to be tested.
[0022] WO 01/57290 A1 "Electrolysis cell provided with gas diffusion
electrodes"
describes an electrolysis cell with gas diffusion electrodes, in which
attention is drawn to
the protective function of silver coatings under oxidising conditions. In
particular, it
describes a metal current conductor with openings, said conductor being made
of silver,
stainless steel or nickel, although nickel should preferably be coated with
silver.
[0023] As literature and the experience of various operators confirm the
stability of
silver on nickel, the nickel components of the electrolysis cells were electro-
silver-plated.
For this, a coating thickness of approximately lOpm was applied to the nickel.
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[0024] In a test series 2, two electrolysis cells were tested in continuous
operation in
a similar manner to test series 1. Both cells have an active electrolysis
surface of 2.7m2
and are equipped with Flemion F8020 membranes. The continuous current density
was
4kA/m2, the operating temperature 88 C, the NaCI anolyte concentration 210
g/I, the
NaOH catholyte discharge concentration 32%w/w and the stoichiometric excess of
saturated moist oxygen was again 20%. Fig. 5 shows the voltage curve over 80
days of
operation. Different symbols are used for each of the electrolysis cells
(shaded diamonds
and unshaded triangles).
[0025] The results of test series 2 according to Fig. 5 again show a
voltage increase.
This time it was continuous. The start-up and shutdown procedures which
occurred
repeatedly during the operating period had no perceptible effect on the cell
voltage, unlike
the observations made for test series 1 on the basis of Fig. 1.
[0026] The cell elements were inspected after 80 days of operation and the
condition
of the metallic support structure and the metallic elastic element analysed.
By way of
example, transverse micrographs of silver-plated nickel monofilaments of the
elastic
element are shown on a scale of 100:1 in Figure 6. The micorgraph clearly
shows the
silver spalling off in the filament sample from the bottom while the sample
from the top
exhibits a loosened silver coating and a reduction of approximately 50% in the
coating
thickness.
[0027] Material comparisons between samples from the upper and lower part
of the
cell element also show a transfer of decomposed silver, which dissolves at the
top due to
corrosion and is deposited again on the bottom part of the cell (data not
shown). It can
thus be seen that simply electroplating the nickel with a layer of silver
under oxidising
electrolysis conditions under no circumstances suffices to form an
electrochemically
stable connection.
[0028] These tests show that there is a further need to provide coatings
that result in
electrochemically stable connections in the form of stable conductivities of
the metallic
components of the cathode half shell without the above disadvantages
occurring.
[0029] The aim of the present invention is thus to achieve the following
objectives:
¨ Provision of an alternative corrosion protection coating for the metallic
cell element
components of the cathode half shell of an electrolysis cell
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¨ Guaranteeing an increased adhesive strength of the coating on the
surfaces of the cell
element components so that no non-conductive oxide layers can be formed
¨ Stable operation of an electrolysis cell in respect of the cell voltage
being as constant
as possible over a longer period of time at a given current load despite an
arbitrary
number of start-ups and shutdowns and thus a longer life cycle
¨ Minimisation of the ohmic and thus conductivity losses during current
conduction from
the metallic components to the metallic grid of the oxygen-depolarisation
cathode
[0030] The objective is achieved by a cathode half shell of an electrolysis
cell
comprising metallic cell element components comprising
- a metallic support structure welded to the back wall of the cathode half
shell and at
least one metallic elastic element positioned plane-parallel thereon,
- an oxygen-depolarised cathode which lies against at least one metallic
elastic element,
said oxygen-depolarised cathode comprising a perforated metallic grid and a
catalyst
strip made up of PTFE and silver oxide mechanically press-fitted thereto,
wherein the
silver oxide is reduced to silver during operation of the electrolysis plant
and in so
being generates a uniform connection/bond between the components of the oxygen-
depolarised cathode and the at least one elastic element, said connection/bond
being
characterised by high conductivity,
wherein at least one of the metallic components is provided with an
electrically conductive
coating comprising at least two layers, wherein
- a first layer, which is applied directly to the cell element materials,
is selected from a
group which contains Au, B-doped nickel, Ni sulphides and mixtures thereof,
this first
layer having a layer thickness of 0.005 to 0.2 pm, and
- a second layer, which is applied to the first layer, is made of silver, this
second layer
having a layer thickness of 0.1 to 30 pm.
[0031] The present invention also claims that all cell element components
which are
contained in the cathode half shell and conduct an electric current are
coated. Here,
preferably those cell element components of the cathode half shell of the
electrolysis cell
which are in contact with caustic soda solution have the inventive coating.
[0032] Also claimed is the use of the inventive cathode half shell of the
electrolysis
cell in chlor-alkali electrolysis.
[0033] The following figures are used to describe the invention in greater
detail.
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Fig. 1: Electrolysis cell voltage for test series 1: Shown is the voltage
curve over
the first 65 days of operation of an electrolysis cell using an electrode as
described in DE 10 2004 034 886 A1.
Fig. 2: Simplified stability diagram for Ni-H20 at 85 C against NHE
Fig. 3: Simplified stability diagram for Au-H20 at 85 C against NHE
Fig. 4: Simplified stability diagram for Ag-H20 at 85 C against NHE
Fig. 5: Electrolysis cell voltage for test series 2: shown is the voltage
curve over
80 days of operation of an electrolysis cell using metallic cell element
components that are provided with a 10 pm thick layer of silver.
Fig. 6: Transverse micrograph of a silver-plated nickel wire from test series
2 on a
scale of 100:1
Fig. 7: Electrolysis cell voltage for test series 3: shown is the voltage
curve over
240 days of operation of an electrolysis cell using metallic cell element
components that are coated with a 0.15 pm thin layer of gold and a 25 pm
thick layer of silver.
Fig. 8: Transverse micrograph of the silver-plated nickel wire with an
intermediate
layer of gold from test series 3 on a scale of 25:1
Fig. 9: Transverse micrograph of the silver-plated nickel wire with an
intermediate
layer of gold from test series 3 on a scale of 500:1
Fig. 10: SEM image of Ni-Ag binder layer with light layer of gold
Fig. 11: Basic configuration of the metallic cell element components in a
cathode
half shell which are provided with the inventive coating
[0034] From materials chemistry it is known that nickel and silver do not
bond. Even
above melting point, these metals cannot be mixed, they merely form a
monotectic
system. As this behaviour does not apply to mixtures of nickel/gold and
gold/silver,
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. .
. ,
coating tests of 3-layer systems were commenced. As a result the nickel
components
were first coated with a thin 0.15 pm layer of gold followed by a 25 pm-layer
of silver. The
nickel components which were thus prepared were installed in newly
manufactured chlor-
alkali electrolysis cells with oxygen-depolarised cathodes and subjected to a
continuous
load test in test series 3.
[0035] In test series 3, two electrolysis cells were tested in
continuous operation in a
similar manner to test series 2. Both cells have an active electrolysis
surface of 2.7m2 and
are equipped with Flemion F8020 membranes. The continuous current density was
4kA/m2, the operating temperature 88 C, the NaCI anolyte concentration 210 WI,
the
NaOH catholyte discharge concentration 32%w/w and the stoichiometric excess of
saturated moist oxygen was again 20%. Fig. 7 shows the voltage curve for test
series 3
over 240 days of operation. Different symbols are used for each of the
electrolysis cells
(shaded diamonds and unshaded triangles).
[0036] The results of test series 3 according to Fia. 7 show a
slight voltage increase
at the beginning which can be attributed to the characteristics of the oxygen-
depolarised
cathode used. This is then followed by a stable phase over more than 200 days
of
operation. A plurality of start-ups and shutdowns have no perceptible
influence on the cell
voltage.
[0037] After completing the test, the 3 metallic components ¨ the
support structure,
the elastic element and the oxygen-depolarised cathode including the wire grid
¨ were
inspected and the condition verified via micrographs. This is shown in Figures
8 and 9.
Significant loosening of the layers or spalling off was not observed. The
nickel support
structure is evenly electro-silver-plated, the surfaces are slightly
roughened.
[0038] Surprisingly, a physically uniform bond between the oxygen-
depolarised
cathode and the elastic element positioned plane-parallel below it was also
found during
the inspection. The silver oxide rolled into the catalyst strip of the oxygen-
depolarised
cathode is reduced to silver during the first start-up of the electrolysis
cell. In the process,
a physically extremely uniform bond results from the silver strip that forms,
the metallic
grid of the oxygen-depolarised cathode and the at least one elastic element
that has the
inventive coating, said bond being very difficult to separate during
disassembly as the
silver layers of the components have at least partially formed a chemical
bond. This type
of bond leads to low ohmic losses during current transport through the
electrolysis cell
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and a low and stable cell voltage that is not affected by start-ups and
shutdowns is thus
achieved during long-term operation.
[0039] Fig. 10 shows the condition of the intermediate gold layer which
forms the
binder layer between the nickel and silver. There is no noticeable corrosion
here either.
[0040] Finally, Fiq. 11 shows a basic configuration of the metallic cell
element
components provided with the inventive coating. The base is the cathode half
shell (1).
Metallic webs (2) that are welded to both the back wall as well as the current
distributor
(3) component, are arranged parallel to the narrow side wall. The elastic
element (4)
component is press-fitted between the current distributor (3) and the oxygen-
depolarised
cathode. The oxygen-depolarised cathode positioned plane-parallel thereto
consists of a
perforated metallic grid, or rather wire grid (5), onto which a catalyst strip
(6) is rolled,
which, during operation of the electrolysis cell for the intended purpose,
forms a
connection/bond with the metallic grid (5) and the elastic element (4), said
connection/bond being characterised by high conductivity and thus a low ohmic
resistance.
