Patent Publication Number: US-2010122911-A1

Title: Method for coating metallic interconnect of solid oxide fuel cell

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Description of the Prior Art 
     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. 
     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. 
     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° C., the molten carbonate fuel cell has about 650° C., the solid oxide fuel cell has about 1000° C., and the solid polymer electrolyte fuel cell has an operating temperature around 80° C. 
     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. 
     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. 
     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. 
     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. 
     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 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•cobalt•nickel•copper•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. 
     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. 
     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. 
     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 
     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. 
     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 a ) is set in a room-temperature cobalt plating solution, and a maximum current density (I p ), a current-on time (T on ) and a current-off time (T off ) are adjusted based on a Mathematical Formula 1 described below. 
         I   a   =I   p   ×T   on /( T   on   +T   off )  Mathematical Formula 1 
     Herein, the pulse plating may be carried out under a condition of the current-on time (T on ) of 0.002˜0.005 seconds, the current-off time (T off ) of 0.005˜0.008 seconds, the maximum current density (I p ) of 100˜250 mA/cm 2 , and the average current density (I a ) of 30˜50 mA/cm 2 . 
     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. 
     The size of the anode may be 1˜1.5 times larger than that of the cathode, and an interval between the anode and the cathode may be 1˜2 times larger than a width of the cathode. 
     Also, the plating solution may employ a Watts bath of cobalt sulfate (CoSO 4 .7H 2 O) and cobalt chloride (CoCl 2 .6H 2 O), in which pH is maintained from 2 to 4 by a cobalt hydroxide aqueous solution or a diluted hydrochloric acid solution. 
     Also, after the pulse plating, heat-treatment may be further carried out in a 800° C. reducing atmosphere (10% H 2 +90% N 2 ) for 2˜20 hours to enhance a binding force between the metallic interconnect and a plated coating layer. 
     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 
       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: 
         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; 
         FIG. 2  shows the shape of pulse current applied to pulse plating according to the present invention, compared to the shape of DC current; 
         FIG. 3  shows photographs of plated coating surfaces after pulse plating according to the present invention; 
         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 on /T off ) in the present invention; 
         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 p ) and an average current density (I a ) in the present invention; 
         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); 
         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; 
         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; 
         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° C. oxidizing atmosphere, and the extent of volatilization of chrome from the test samples was observed; 
         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 
         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 
     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. 
     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. 
     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. 
     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˜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. 
     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. 
       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 . 
     The plating solution is a room-temperature cobalt plating solution that employs a Watts bath of cobalt sulfate (CoSO 4 .7H 2 O) and cobalt chloride (CoCl 2 .6H 2 O), 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˜4, so that a uniform plated layer can be obtained. 
     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˜1.5 times larger than that of the cathode  3 , and the interval between the anode  2  and the cathode  3  is 1˜2 times larger than the width of the cathode  3 . 
       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 on ) and a current-off time (T off ) based on the average current density (I a ). 
     Meanwhile, the pulse generating device  4  is used to set the average current density (I a ) in the room-temperature cobalt plating solution, and a maximum current density (I p ), a current-on time (T on ) and a current-off time (T off ) are adjusted based on the Mathematical Formula 1 described below to carry out pulse plating. 
         I   a   =I   p   ×T   on /( T   on   +T   off )  Mathematical Formula 1 
     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 on ) and the current-off time (T off ), and to apply a maximum current density (I p ) according to an increase in the current-off time (T off ), and this makes it possible to obtain fine coating particles. 
     In other words, when the pulse plating was carried out under the condition where (T on +T off ) was changed from (0.0005+0.0005)˜(5.0+5.0) seconds, the maximum current density (I p ) was fixed within a range of 100˜250 mA/cm 2 , and the average current density (I a ) was fixed within a range of 30˜50 mA/cm 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 on +T off ). Especially, under the condition where (T on +T off ) was (0.005+0.005) seconds, a fine-particle coating film was formed. 
     As described above, after pulse plating according to the present invention, when heat-treatment is carried out in a 800° C. reducing atmosphere (10% H 2 +90% N 2 ) for 2˜20 hours, the binding force between the metallic interconnect and the plated coating layer can be enhanced. 
       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 on /T off ) in the present invention. In other words, under the condition where the average current density (I a ) had a fixed value of 50 mA/cm 2 , and the maximum current density (I p ) was adjusted between 62.5˜250 mA/cm 2  while the duty ratio of (T on /T 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 on , and an increase in T off . The reason for this is that plating ions can be sufficiently re-diffused around the cathode during T off . 
       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 p ) and an average current density (I a ) in the present invention. Under the condition where a duty ratio of (T on /T off ) was fixed at 20%, the maximum current density (I p ) was changed to 250, 200, 150 mA/cm 2 , and the average current density (I a ) was changed to 50, 40, 30 mA/cm 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. 
       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. 
     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 on )=0.002˜0.005 seconds, a current-off time (T off )=0.005˜0.008 seconds, a maximum current density (I p )=100˜250 mA/cm 2 , and an average current density (I a )=30˜50 mA/cm 2 . 
     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 . 
       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×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. 
       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. 
       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° 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. 
       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. 10   a  indicates the result just after plating, and  FIG. 10   b  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. 
       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° C. oxidizing atmosphere for 1000 hours, it was observed that a low value of 11˜31 mΩcm 2  was maintained for about 1000 hours during the evaluation of high temperature electrical conductivity. 
     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. 
     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.