U.S. patent application number 12/618007 was filed with the patent office on 2010-05-20 for method for coating metallic interconnect of solid oxide fuel cell.
This patent application is currently assigned to KOREA INSTITUTE OF ENERGY RESEARCH. Invention is credited to Jong-Eun Hong, Seung-Bok Lee, Tak-Hyung Lim, Seong-Soo Pyo, Dong-Ryul Shin, Rak-Hyun Song.
Application Number | 20100122911 12/618007 |
Document ID | / |
Family ID | 41355798 |
Filed Date | 2010-05-20 |
United States Patent
Application |
20100122911 |
Kind Code |
A1 |
Song; Rak-Hyun ; et
al. |
May 20, 2010 |
METHOD FOR COATING METALLIC INTERCONNECT OF SOLID OXIDE FUEL
CELL
Abstract
Disclosed is a method for coating a metallic interconnect for a
solid oxide fuel cell (SOFC), the method including the steps of:
carrying out pre-treatment for removing impurities adhered on a
surface of the metallic interconnect; and carrying out pulse
plating with cobalt as an anode, and the metallic interconnect as a
cathode, in which an average current density (I.sub.a) is set in a
room-temperature cobalt plating solution, and a maximum current
density (I.sub.p), a current-on time (T.sub.on) and a current-off
time (T.sub.off) are adjusted based on
I.sub.a=I.sub.p.times.T.sub.on/(T.sub.on+T.sub.off). Through the
disclosed method, it is possible to obtain a metallic interconnect
having a coating surface which has a high electrical conductivity
and a high chrome volatilization inhibiting property and can
minimize the occurrence of micro-cracks and micro-pores, thereby
improving the performance of the SOFC.
Inventors: |
Song; Rak-Hyun; (Daejeon,
KR) ; Shin; Dong-Ryul; (Daejeon, KR) ; Lim;
Tak-Hyung; (Daejeon, KR) ; Lee; Seung-Bok;
(Seoul, KR) ; Hong; Jong-Eun; (Daejeon, KR)
; Pyo; Seong-Soo; (Seoul, KR) |
Correspondence
Address: |
LAW OFFICE OF DELIO & PETERSON, LLC.
121 WHITNEY AVENUE, 3RD FLLOR
NEW HAVEN
CT
06510
US
|
Assignee: |
KOREA INSTITUTE OF ENERGY
RESEARCH
Daejeon
KR
|
Family ID: |
41355798 |
Appl. No.: |
12/618007 |
Filed: |
November 13, 2009 |
Current U.S.
Class: |
205/206 ;
205/205 |
Current CPC
Class: |
C25D 5/50 20130101; H01M
8/0228 20130101; Y02E 60/50 20130101; C25D 5/18 20130101; C25D 5/34
20130101; H01M 8/0206 20130101; C25D 3/12 20130101; H01M 2008/1293
20130101 |
Class at
Publication: |
205/206 ;
205/205 |
International
Class: |
C25D 5/34 20060101
C25D005/34 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2008 |
KR |
10-2008-0113157 |
Claims
1. A method for coating a metallic interconnect for a solid oxide
fuel cell (SOFC), the method comprising the steps of: carrying out
pre-treatment for removing impurities adhered on a surface of the
metallic interconnect; and carrying out pulse plating with cobalt
as an anode, and the metallic interconnect as a cathode, in which
an average current density (I.sub.a) is set in a room-temperature
cobalt plating solution, and a maximum current density (I.sub.p), a
current-on time (T.sub.on), and a current-off time (T.sub.off) are
adjusted based on a Mathematical Formula 1 described below.
I.sub.a=I.sub.p.times.T.sub.on/(T.sub.on+T.sub.off) [Mathematical
Formula 1]
2. The method as claimed in claim 1, wherein the pulse plating is
carried out under a condition of the current-on time (T.sub.on) of
0.002.about.0.005 seconds, the current-off time (T.sub.off) of
0.005.about.0.008 seconds, the maximum current density (I.sub.p) of
100.about.250 mA/cm.sup.2, and the average current density
(I.sub.a) of 30.about.50 mA/cm.sup.2.
3. The method as claimed in claim 1, wherein the step of carrying
out the pre-treatment comprises the steps of polishing the surface
of the metallic interconnect by silicon carbide abrasive paper,
washing off the impurities on the surface of the metallic
interconnect by 10% NaOH aqueous solution and acetone, removing a
fine scale on the surface of the metallic interconnect by 10% HCl
solution, and carrying out pickling for 30 to 60 seconds.
4. The method as claimed in claim 1, wherein a size of the anode is
1.about.1.5 times larger than a size of the cathode, and an
interval between the anode and the cathode is 1.about.2 times
larger than a width of the cathode.
5. The method as claimed in claim 1, wherein the plating solution
employs a Watts bath of cobalt sulfate (CoSO.sub.4.7H.sub.2O) and
cobalt chloride (CoCl.sub.2.6H.sub.2O) with pH of 2 to 4, the pH
being maintained by a cobalt hydroxide aqueous solution or a
diluted hydrochloric acid solution.
6. The method as claimed in claim 1, wherein after the pulse
plating, heat-treatment is carried out in a 800.degree. C. reducing
atmosphere (10% H.sub.2+90% N.sub.2) for 2.about.20 hours to
enhance a binding force between the metallic interconnect and a
plated coating layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a solid oxide fuel cell,
and more particularly to a coating method for forming a protective
coating on the surface of a metallic interconnect that
interconnects unit cells and collects current of a stack.
[0003] 2. Description of the Prior Art
[0004] As power demands show a tendency to gradually increase
according to a recent industrial development and economic growth,
environmental problems, including air pollution and earth shock,
have seriously arisen by the use of fossil fuels (such as
petroleum, or coal) required for power production. Especially,
since the exhaust of carbon dioxide by the use of fossil fuels is
pointed out as a main factor of global warming and various kinds of
environmental pollution, the development of solar light/heat
energy, bio energy, wind energy, and hydrogen energy, as clean
energy sources substituting for the fossil fuels, is being actively
conducted.
[0005] From among such clean energy sources, research on the field
of fuel cells using a hydrogen fuel is active. A fuel cell
technology is considered as a future electricity generation
technology because a fuel cell does not exhaust pollutants in
electricity generation, and has an advantage in that it does not
require a site for a power plant, a power transmission facility, or
a substation.
[0006] The fuel cell is divided into a phosphoric acid fuel cell
(PAFC), a molten carbonate fuel cell (MCFC), a solid acid oxide
fuel cell (SOFC), a solid polymer electrolyte fuel cell (a polymer
electrolyte fuel cell (PEFC) or a proton exchange membrane fuel
cell (PEMFC)), according to the type of electrolyte. Herein, the
phosphoric acid fuel cell has an operating temperature of about
200.degree. C., the molten carbonate fuel cell has about
650.degree. C., the solid oxide fuel cell has about 1000.degree.
C., and the solid polymer electrolyte fuel cell has an operating
temperature around 80.degree. C.
[0007] The SOFC, from among the cells, employs a solid oxide having
oxygen ion conductivity as an electrolyte. Thus, the SOFC has an
advantage in that it has the highest efficiency as a fuel cell, can
improve the efficiency by up to 85%, due to inclusion of the heat
generated by cogeneration with a gas turbine, and can use various
fuels. Also, since the electrolyte for the SOFC is in a solid
state, there is no loss in the electrolyte and thus no need to
supplement the electrolyte. Besides, there is no need to use a
noble metal catalyst, and it is easy to supply a fuel through
direct internal reforming.
[0008] The output performance of a unit cell of such an SOFC is
reduced by various factors, such as polarization loss. Also, when a
plurality of unit cells of the SOFC are layered between a metallic
interconnect, the output performance is influenced by the contact
resistance between the metallic interconnect and the cells.
[0009] In a fuel cell, the metallic interconnect mainly performs a
role of electrically interconnecting cells of a cell stack, and
preventing supplied gases within cells from mixing with each other,
and is referred to as a bipolar plate or a separator.
[0010] At present, as a material of a metallic interconnect for the
SOFC, a stainless steel, such as STS430, and STS444, is used. Also,
a newly developed Crofer 22 APU may be used. However, by these
materials, it is very difficult to achieve the durability of up to
40000 hours required for commercialization, and thus there is need
to develop a novel alloy and to research the application of
protective coating on the surface of a conventional material.
[0011] A material for protective coating on the metallic
interconnect for an SOFC may include various kinds of metal
materials, such as Perovskite-type ceramic materials based on
Lanthanum chromite (LaCrO.sub.3) having a high electrical
conductivity at a high temperature, spinel-type ceramic materials
based on (Mn,Co)304 which is known to have a high temperature
conductivity and a high chrome volatilization inhibiting property,
or transition elements forming a spinel structure (e.g.,
manganese.cndot.cobalt.cndot.nickel.cndot.copper.cndot.chrome).
Especially, examples of a method for forming a protective coating
of transition metals forming the spinel structure, from among the
above materials, include sputtering, slurry-spraying,
electrodeposition, chemical vapor deposition, or the like.
[0012] The electrodeposition, from among recently used methods, is
considered as a method appropriate for future mass production of
the metallic interconnect for an SOFC due to its simple equipment
and low cost. One of electrodeposition methods is a DC plating
method.
[0013] In coating by using the DC plating method, a high current
density is required to be applied to obtain a densified coating
layer with fine particles. However, at a higher current density
than a predetermined limitation, plating ions around a to-be-coated
substrate are depleted by mass transfer limiting conditions,
thereby causing concentration polarization. Accordingly, there is a
problem in that a plated surface is non-uniform and a densified
coating layer cannot be formed. Also, in cobalt protective coating
using a DC plating method, micro-cracks and micro-pores may be
formed within a coating layer due to coarse particles of the
coating layer. Such micro-cracks and micro-pores cause some
problems, including peeling of an oxide film on a metallic
interconnect surface, and pollution of a cathode, by volatilizing
chrome (Cr(IV)) gas from the metallic interconnect during the
operation of the SOFC, and thus operates as a factor inhibiting an
electrochemical reaction of the SOFC.
[0014] Therefore, research for establishing a technology of
protective coating of a metallic interconnect for an SOFC by using
electrodeposition (which has not been attempted) and establishing
plating conditions with improved protection features is
required.
SUMMARY OF THE INVENTION
[0015] Accordingly, the present invention has been made to solve
the above-mentioned problems occurring in the prior art, and the
present invention provides a method for coating a metallic
interconnect for a solid oxide fuel cell (SOFC), which can minimize
the occurrence of micro-cracks and micro-pores in a coating layer
by formation of densified protective coating.
[0016] In accordance with an aspect of the present invention, there
is provided a method for coating a metallic interconnect for a
solid oxide fuel cell (SOFC), the method including the steps of:
carrying out pre-treatment for removing impurities adhered on a
surface of the metallic interconnect; and carrying out pulse
plating with cobalt as an anode, and the metallic interconnect as a
cathode, in which an average current density (I.sub.a) is set in a
room-temperature cobalt plating solution, and a maximum current
density (I.sub.p), a current-on time (T.sub.on) and a current-off
time (T.sub.off) are adjusted based on a Mathematical Formula 1
described below.
I.sub.a=I.sub.p.times.T.sub.on/(T.sub.on+T.sub.off) Mathematical
Formula 1
[0017] Herein, the pulse plating may be carried out under a
condition of the current-on time (T.sub.on) of 0.002.about.0.005
seconds, the current-off time (T.sub.off) of 0.005.about.0.008
seconds, the maximum current density (I.sub.p) of 100.about.250
mA/cm.sup.2, and the average current density (I.sub.a) of
30.about.50 mA/cm.sup.2.
[0018] Also, the step of carrying out the pre-treatment may include
the steps of polishing the surface of the metallic interconnect by
silicon carbide abrasive paper, washing off the impurities on the
surface of the metallic interconnect by 10% NaOH aqueous solution
and acetone, removing a surface fine scale of the metallic
interconnect by 10% HCl solution, and carrying out pickling for 30
to 60 seconds.
[0019] The size of the anode may be 1.about.1.5 times larger than
that of the cathode, and an interval between the anode and the
cathode may be 1.about.2 times larger than a width of the
cathode.
[0020] Also, the plating solution may employ a Watts bath of cobalt
sulfate (CoSO.sub.4.7H.sub.2O) and cobalt chloride
(CoCl.sub.2.6H.sub.2O), in which pH is maintained from 2 to 4 by a
cobalt hydroxide aqueous solution or a diluted hydrochloric acid
solution.
[0021] Also, after the pulse plating, heat-treatment may be further
carried out in a 800.degree. C. reducing atmosphere (10%
H.sub.2+90% N.sub.2) for 2.about.20 hours to enhance a binding
force between the metallic interconnect and a plated coating
layer.
[0022] Through the method according to the present invention, it is
possible to obtain a metallic interconnect having a coating surface
which has a high electrical conductivity and a high chrome
volatilization inhibiting property and can minimize the occurrence
of micro-cracks and micro-pores, thereby improving the performance
of the SOFC.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The above and other objects, features and advantages of the
present invention will be more apparent from the following detailed
description taken in conjunction with the accompanying drawings, in
which:
[0024] FIG. 1 is a conceptual view illustrating a pulse plating
device for coating a metallic interconnect for a solid oxide fuel
cell (SOFC), according to the present invention;
[0025] FIG. 2 shows the shape of pulse current applied to pulse
plating according to the present invention, compared to the shape
of DC current;
[0026] FIG. 3 shows photographs of plated coating surfaces after
pulse plating according to the present invention;
[0027] FIG. 4 shows photographs of the surfaces of plated coating
layers, which illustrates the results of pulse plating according to
the change of a duty ratio of (T.sub.on/T.sub.off) in the present
invention;
[0028] FIG. 5 shows photographs of the surfaces of plated coating
layers, which illustrates the results of pulse plating according to
the change of a maximum current density (I.sub.p) and an average
current density (I.sub.a) in the present invention;
[0029] FIG. 6 shows photographs of the results of pulse plating
according to the present invention under the conditions of FIG. 5,
which were observed by AFM (Atomic Force Microscope);
[0030] FIG. 7 shows photographs of the surfaces of metallic
interconnect test samples, in which the test samples were coated by
conventional DC plating and pulse plating of the present invention
with the same quantity of electric charge;
[0031] FIG. 8 shows photographs of cross sections of metallic
interconnect test samples, in which the test samples were coated by
conventional DC plating and pulse plating of the present invention
with a same thickness;
[0032] FIG. 9 shows photographs of cross sections of metallic
interconnect test samples, in which the test samples coated with
cobalt by pulse plating according to the present invention were
subjected to oxidation evaluation in a 800.degree. C. oxidizing
atmosphere, and the extent of volatilization of chrome from the
test samples was observed;
[0033] FIG. 10 shows the result of X-ray diffraction analysis,
which was carried out to find out a phase change of the cobalt
protective coating by high temperature oxidation, after pulse
plating on a metallic interconnect according to the present
invention; and
[0034] FIG. 11 shows the measurement results of electrical
conductivity of materials for a metallic interconnect, in a state
where the materials were coated with cobalt by pulse plating
according to the present invention.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0035] Hereinafter, an exemplary embodiment of the present
invention will be described with reference to the accompanying
drawings. It is to be understood, however, that the following
embodiment is illustrative only, and the scope of the present
invention is not limited thereto. Also, those skilled in the art
will appreciate that various modifications, additions and
substitutions are possible.
[0036] In the operation of a solid oxide fuel cell (SOFC), a chrome
oxide scale formed on the surface of a metallic interconnect at a
high temperature causes some problems, such as reduction of a
sealing property by peeling of the scale, pollution of a cathode by
volatilization of chrome from the scale, or the like. These
problems reduce output performance and long-term durability of the
SOFC.
[0037] Accordingly, in the method according to the present
invention, on the surface of a metallic interconnect, cobalt, one
of transition metals for forming a spinel layer with a high
electrical conductivity and a high chrome volatilization inhibiting
property, is coated through pulse plating. This improves the
electrical conductivity in a high temperature oxidizing atmosphere,
and inhibits the pollution of a cathode by inhibiting growth and
volatilization of chrome oxide, so as to improve the output
performance and long-term durability of the SOFC.
[0038] Before pulse plating is carried out on a metallic
interconnect, pre-treatment for removing impurities adhered on the
surface of the metallic interconnect is performed. Hereinafter, the
pre-treatment will be described in detail. First, the surface of
the metallic interconnect is polished by using silicon carbide
(SiC) abrasive paper (preferably, abrasive paper with roughness of
#100.about.2000). Second, 10% NaOH aqueous solution and acetone are
used to wash off the surface impurities of the metallic
interconnect. Third, 10% HCl solution is used to remove a fine
scale on the surface of the metallic interconnect, and then
pickling is carried out for 30 to 60 seconds.
[0039] On the metallic interconnect which has been subjected to the
above described pre-treatment, pulse plating is carried out by
using a pulse plating device.
[0040] FIG. 1 is a conceptual view illustrating a pulse plating
device for coating a metallic interconnect for an SOFC, according
to the present invention. As shown, within a plating bath 1
containing a plating solution L, an anode 2 and a cathode 3 are
immersed, and the anode 2 and the cathode 3 are connected to a
pulse generating device 4.
[0041] The plating solution is a room-temperature cobalt plating
solution that employs a Watts bath of cobalt sulfate
(CoSO.sub.4.7H.sub.2O) and cobalt chloride (CoCl.sub.2.6H.sub.2O),
in which the pH is maintained from 2 to 4 by a cobalt hydroxide
aqueous solution or a diluted hydrochloric acid solution. The
cobalt chloride and the cobalt chloride improve current efficiency
and reduce pit density so as to obtain a uniform plated surface. To
the Watts bath, boric acid is preferably added so as to provide
buffering of pH, and to reduce stress. The acidity (pH) of a
plating solution is maintained between 2.about.4, so that a uniform
plated layer can be obtained.
[0042] As the anode 2, a mesh type cobalt plate is used, and as the
cathode 3, a stainless steel (STS430, STS444, Crofer 22 APU), that
is, a material of a metallic interconnect for an SOFC, is used. In
general, in an electrochemical reaction, it is known that a
potential difference and a local current density over an interface
between a plating solution and an electrode change along the
surface of the electrode. The current density is higher at a
protruded portion of an electrode, or is higher at edges when the
interval between an anode and a cathode is larger than the width of
an electrode. In this case, an `edge effect` which increases the
plating thickness of edge portions, compared to a center portion,
may occur, thereby having a bad influence on the uniformity of the
plating thickness. Accordingly, in order to obtain a uniform
plating thickness, it is preferable that electrodes have a similar
size, and the width of an electrode is same or similar to the
interval between electrodes. Thus, in the present invention, the
size of the anode 2 is 1.about.1.5 times larger than that of the
cathode 3, and the interval between the anode 2 and the cathode 3
is 1.about.2 times larger than the width of the cathode 3.
[0043] FIG. 2 shows the shape of pulse current applied to pulse
plating, compared to the shape of DC current. As shown, the shape
of pulse current is shown as a pulse waveform by a current-on time
(T.sub.on) and a current-off time (T.sub.off) based on the average
current density (I.sub.a).
[0044] Meanwhile, the pulse generating device 4 is used to set the
average current density (I.sub.a) in the room-temperature cobalt
plating solution, and a maximum current density (I.sub.p), a
current-on time (T.sub.on) and a current-off time (T.sub.off) are
adjusted based on the Mathematical Formula 1 described below to
carry out pulse plating.
I.sub.a=I.sub.p.times.T.sub.on/(T.sub.on+T.sub.off) Mathematical
Formula 1
[0045] Through the test, the inventor of the present invention
found that during pulse plating, it is possible to apply a higher
current density than an average current density of DC plating by
adjusting the current-on time (T.sub.on) and the current-off time
(T.sub.off), and to apply a maximum current density (I.sub.p)
according to an increase in the current-off time (T.sub.off), and
this makes it possible to obtain fine coating particles.
[0046] In other words, when the pulse plating was carried out under
the condition where (T.sub.on+T.sub.off) was changed from
(0.0005+0.0005).about.(5.0+5.0) seconds, the maximum current
density (I.sub.p) was fixed within a range of 100.about.250
mA/cm.sup.2, and the average current density (I.sub.a) was fixed
within a range of 30.about.50 mA/cm.sup.2, as can be seen from the
photograph of the coating surface shown in FIG. 3, the particle
size shows a tendency to increase according to an increase in
(T.sub.on+T.sub.off). Especially, under the condition where
(T.sub.on+T.sub.off) was (0.005+0.005) seconds, a fine-particle
coating film was formed.
[0047] As described above, after pulse plating according to the
present invention, when heat-treatment is carried out in a
800.degree. C. reducing atmosphere (10% H.sub.2+90% N.sub.2) for
2.about.20 hours, the binding force between the metallic
interconnect and the plated coating layer can be enhanced.
[0048] FIG. 4 shows photographs of the surfaces of plated coating
layers, which illustrates the results of pulse plating according to
the change of a duty ratio of (T.sub.on/T.sub.off) in the present
invention. In other words, under the condition where the average
current density (I.sub.a) had a fixed value of 50 mA/cm.sup.2, and
the maximum current density (I.sub.p) was adjusted between
62.5.about.250 mA/cm.sup.2 while the duty ratio of
(T.sub.on/T.sub.off) was changed to 20, 50, and 80%, the surface
particles of the plated coating layer show a tendency to become
fine according to a decrease in T.sub.on, and an increase in
T.sub.off. The reason for this is that plating ions can be
sufficiently re-diffused around the cathode during T.sub.off.
[0049] FIG. 5 shows photographs of the surfaces of plated coating
layers, which illustrates the results of pulse plating according to
the change of a maximum current density (I.sub.p) and an average
current density (I.sub.a) in the present invention. Under the
condition where a duty ratio of (T.sub.on/T.sub.off) was fixed at
20%, the maximum current density (I.sub.p) was changed to 250, 200,
150 mA/cm.sup.2, and the average current density (I.sub.a) was
changed to 50, 40, 30 mA/cm.sup.2, it is determined that a throwing
power of the surface becomes better as the maximum current density
increases. In general, since a much higher maximum current density
can be applied in pulse plating, compared to that in DC plating, it
is possible to obtain a high nucleation rate of plating particles,
thereby forming fine plating particles.
[0050] FIG. 6 shows photographs of the results of pulse plating
according to the present invention under the conditions of FIG. 5,
which were observed by AFM (Atomic Force Microscope), and it can be
seen that on the whole the surface roughness of the plated layer is
low.
[0051] In brief, through the test results of FIGS. 4 to 6, it can
be determined that the particles on a plated surface are fine and
uniform, and have a high throwing power under the conditions of a
current-on time (T.sub.on)=0.002.about.0.005 seconds, a current-off
time (T.sub.off)=0.005.about.0.008 seconds, a maximum current
density (I.sub.p)=100.about.250 mA/cm.sup.2, and an average current
density (I.sub.a)=30.about.50 mA/cm.sup.2.
[0052] Meanwhile, hereinafter, the case where pulse plating
according to the present invention is carried out to coat a
metallic interconnect for an SOFC, will be described, compared to
conventional DC plating, with reference to FIGS. 7 and 8.
[0053] FIG. 7 shows photographs of the surfaces of metallic
interconnect test samples, in which the test samples were coated by
conventional DC plating and pulse plating of the present invention
with the same quantity of electric charge (current.times.time), and
it can be seen that the test sample coated by the pulse plating has
a better throwing power, compared to the test sample coated by the
DC plating.
[0054] FIG. 8 shows photographs of cross sections of metallic
interconnect test samples, in which the test samples were coated by
DC plating and pulse plating with a same thickness. It can be seen
that pores exist in patches on the cross-section coated by DC
plating, while pores hardly exist on the cross-section coated by
pulse plating.
[0055] FIG. 9 shows photographs of cross sections of metallic
interconnect test samples, in which the test samples coated with
cobalt by pulse plating according to the present invention were
subjected to oxidation evaluation in a 800.degree. C. oxidizing
atmosphere and the extent of volatilization of chrome from the test
samples was observed. It can be seen that the volatilization of
chrome was effectively inhibited.
[0056] FIG. 10 shows the result of X-ray diffraction analysis,
which was carried out to find out a phase change of the cobalt
protective coating on a metallic interconnect test sample by high
temperature oxidation. FIG. 10a indicates the result just after
plating, and FIG. 10b indicates the result of X-ray diffraction
analysis after high temperature oxidation for 1000 hours. Through
the analysis, it was determined that a spinel phase having a high
electrical conductivity and a high chrome volatilization inhibiting
property was formed, which includes cobalt.
[0057] FIG. 11 shows the measurement results of electrical
conductivity of materials (STS430 and STS444) for a metallic
interconnect, in a state where the materials were coated with
cobalt by pulse plating according to the present invention. In
other words, when the area specific resistance was measured while
the materials were maintained in a 800.degree. C. oxidizing
atmosphere for 1000 hours, it was observed that a low value of
11.about.31 m.OMEGA.cm.sup.2 was maintained for about 1000 hours
during the evaluation of high temperature electrical
conductivity.
[0058] As described above, through the pulse plating according to
the present invention, it is possible to form a spinel phase
containing cobalt on the surface of a metallic interconnect for an
SOFC, thereby providing a coating layer having a high electrical
conductivity and a high chrome volatilization inhibiting property.
Also, it is possible to obtain densified protective coating in
which the occurrence of micro-cracks and micro-pores is
minimized.
[0059] Although an exemplary embodiment of the present invention
has been described for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *