Abstract:
To provide effective seals between the separator plates of fuel cells, particularly planar solid oxide fuel cells (SOFC&#39;s), a method of applying a glass-ceramic coating to such a separator plate comprises providing a laminar body incorporating a glass powder, e.g., a tape-cast sheet, forming a bond between the laminar body and the separator plate to form. an assembly comprising the separator plate and the laminar body, and heat-treating the assembly to convert the glass powder to a glass-ceramic.

Description:
BACKGROUND OF THE INVENTION 
     This invention is concerned with a method of forming a glass-ceramic coating on a substrate and to coatings produced thereby, such coatings having particular but not exclusive utility in the provision of sealing arrangements between non-porous separator plates of fuel cells, particularly planar solid oxide fuel cells (SOFC&#39;s), and includes methods of producing such sealing arrangements. 
     The present invention has particular advantages when used to produce seals between separator plates which are metal or metallic: in the context of this specification the terms ‘metal’ and ‘metallic’ are to be interpreted as meaning not just plates made of metals and exclusively metal alloys, but also of oxide dispersion strengthened metal alloys which include a relatively small percentage of an oxide or oxides incorporated therein. 
     A planar SOFC comprises a stack of vertically spaced impermeable separator plates. These separator plates separate the reactant gases and also provide electrical connection between adjacent cells. In the space between each adjacent pair of plates is held one or more cells each comprising a solid electrolyte having an anode and a cathode. Clearly, in view of their separator function, the separator plates must not be porous as they comprise part of a gas-tight assembly. The reactant gases comprise a fuel gas (e.g. hydrogen or carbon monoxide) and an oxidant (e.g. oxygen or air) and are respectively supplied to the anode and the cathode by suitable ducts which may, for example, be provided by channels in the upper and lower surfaces of the adjacent separator plates. As is known, the reactions at the electrode cause a voltage. Connection between the electrodes and adjacent separator plates can be either by direct contact or via an electrically conducting interlayer. For example a current collector (e.g. a nickel grid) may be provided adjacent the anode and a conductive porous sheet may be provided adjacent the cathode or the cathode may contact a conductive coating on the separator plate. 
     SOFC&#39;s usually operates at temperatures in the range 750° C.-1000° C., though it is envisaged that they could operate at lower temperatures, possibly as low as 650° C. In a planar SOFC stack, high-performance seals between adjacent plates are required to ensure separation and containment of the reactant gases. It is known to use glass-ceramic materials to produce such seals, since glass ceramics can be formulated to be (a) stable in the oxidizing and reducing atmospheres of the stack at high temperatures and (b) un-reactive towards adjoining components during operation of the stack. However, difficulties have been experienced in creating seal arrangements which are additionally capable of bonding with high integrity to the separator plates without raising high stresses due to differing thermal expansion characteristics of the seals and the adjoining materials. A requirement has also emerged to facilitate the creation of glass-ceramic seals of sufficient thickness to accommodate the thickness of the cells and current contacts. 
     In a stack where a plurality of laterally adjacent cells (e.g., an array of four cells) are sandwiched between adjacent separator plates in the stack, the seal should also provide high temperature electrical insulation between adjacent bipolar plates. However, such electrical insulation is not required in stack designs in which only one cell is sandwiched between adjacent plates, because in such stacks the electrolyte separates the entire area of the bipolar plates, thereby providing the required electronic insulation. 
     One of the problems in the manufacture of planar SOFC&#39;s using glass ceramics as a means of sealing between adjacent separator plates in the stack is the need to ensure that the cell components remain in electrical contact in all parts of the stack throughout the process of assembling and sealing the stack. This can be difficult, because a glass-ceramic, once it has crystallized, does not deform appreciably, whereas the rest of the manufacturing process can involve volume changes at elevated temperatures in the layers of the stack. This is because the oxide mixtures used to form the anodes and the anode contacts are partially reduced by passing a reforming gas such as hydrogen through the stack. 
     SUMMARY OF THE INVENTION 
     An aim of the invention is to provide a glass-ceramic coating with improved bonding to a substrate, particularly a metallic substrate of the type used for separator plates in planar SOFC&#39;s. 
     Another aim of the invention is to provide an effective high-performance non-porous glass-ceramic seal between adjacent non-porous separator plates of planar SOFC&#39;s. 
     A further aim is to produce such a seal capable of electrically insulating adjacent biploar plates from each other and preventing electronic leakage therebetween. 
     Another aim is to provide such a seal which accommodates change in dimension of the stack during its manufacture. 
     A further aim of the invention is to provide a method of applying a glass-ceramic coating to a separator plate for a solid oxide fuel cell, so providing a base layer for at least one further layer required to complete a seal between confronting surfaces of adjacent separator plates. 
     It is to be understood that a glass-ceramic is an inorganic, polycrystalline material formed by the controlled crystallization of a glass; a glass on the other hand is an inorganic material formed by fusion but wherein the material has cooled to a rigid condition without crystallizing. 
     According to a first aspect of the invention, a method of providing a glass-ceramic coating having improved bonding to a substrate comprises the steps of: depositing a first bonding layer of glass powder mixed with a binder directly onto the substrate (preferably using a screen printing or spraying process); adhering a laminar body comprising glass powder mixed with a binder to the first layer to form a second layer which is substantially thicker than the first layer, the glass powder in both layers being of a composition such as to form a glass-ceramic on heat treatment; and heat treating the resultant green coating on the substrate to drive off the binder and convert the glass powder layers to glassceramic layers. 
     According to a second aspect of the invention, in a fuel cell, a high-performance seal between confronting faces of adjacent non-porous separator plates comprises at least one glass-ceramic layer on at least one of the confronting faces and at least one glass seal layer interposed between the at least one glass-ceramic layer and the other separator plate. 
     Preferably, the at least one glass-ceramic layer is a duplex layer, comprising a first glass-ceramic layer for bonding the seal to the separator plate and a second glass-ceramic layer superimposed on the first glass-ceramic layer, the glass seal layer being interposed between the second glass-ceramic layer and the adjacent separator plate, the second glass-ceramic layer being substantially thicker than the first glass-ceramic layer. 
     Glass-ceramic layers are may be provided on both confronting faces of the separator plates, the glass seal layer being interposed therebetween. 
     The at least one glass-ceramic layer may for example comprise compositions in the SiO 2 —CaO—MgO—Al 2 O 3  system, the composition being adjusted to optimize its ability to bond with the separator plate surface and/or to optimize its thermal expansion coefficient with respect to the thermal expansion coefficient of the separator plate to which it is attached. Where a duplex glass-ceramic layer is utilized, the first and second layers are preferably of different compositions to optimize bonding of the seal to the separator plate surface in the first layer while also optimzing the thermal expansion coefficient of the second layer. 
     The at least one glass seal layer may for example comprise compositions in the SiO 2  —BaO—CaO-Al 2 O 3  system. 
     The invention further includes a method of forming a glass-ceramic coating on a substrate comprising a separator plate of a solid oxide fuel cell, which method comprises providing a laminar body incorporating a glass powder, bringing the laminar body into contact with the substrate, forming a bond between the laminar body and the substrate to form an assembly comprising the substrate and the laminar body and heat treating the assembly to convert the glass-powder to a dense glass-ceramic layer. 
     The laminar body preferably incorporates a binder and prior to bringing the laminar body and the substrate into contact a solvent is applied to the substrate and/or to the laminar body whereby when the laminar body and the substrate are brought into contact an adhesive bond is formed between the laminar body and the substrate; during heat treating of the assembly the binder is burned out before formation of the glass-ceramic. 
     Alternatively, prior to bringing the laminar body and the substrate into contact, a thin bonding layer incorporating a glass powder of composition such as to form a glass-ceramic upon heat treatment is applied to the substrate to provide an bonding layer to which the laminar body is then bonded. In this embodiment the bonding layer may be applied by spraying or screen-printing, the laminar body then being applied while both layers are in the green condition. During heat treatment, the glass-powder in the bonding layer becomes a glass-ceramic layer and forms a bond between the substrate and the layer produced by the laminar body. The glass-powder of the bonding layer may have a different composition to that of the glass powder incorporated in the laminar body, the glass-powder compositions of the laminar body being optimized to reduce thermal stress between the substrate and the seal on formation of the glass-ceramic and the glass-powder compositions of the bonding layer being optimized so as to flow upon melting and wet the substrate before crystallization occurs. 
     It is particularly envisaged that the above method is utilized to form a seal between two 
     The or each separator plate may be formed of a metal or a metal alloy, e.g., a ferritic stainless steel or a high chrome alloy; such a chrome alloy may have a composition including Cr, Fe and Y 2 O 3 , for example 95%Cr, 5%Fe and 1% Y 2 O 3 . Further, the or each plate may be coated with an alloy or an oxide e.g. with an oxide of formula La x Sr 1−x CrO 3    
     The laminar body mentioned above comprises a mixture of glass powder and a binder in the form of a tape or sheet of material (produced, e.g., by tape-casting or calendering). The tape or sheet can be stamped out, prior to application, so that the coating covers a defined area. Applying coatings via such a stamped out unfired (or green) tape enables complex, defined areas to be coated onto the (planar) substrate. By fixing the tape to the substrate in the green state, there is no or at least negligible in-plane shrinkage. Further, the coating thickness can be closely controlled, say in the range 100 μm-3 mm, by appropriate selection of green tape thickness. To ensure adhesion of the coating during heat-treatment and conversion to a dense glass-ceramic, a bond is required to fix the unfired (green) tape to the substrate. Two methods have been found especially effective to achieve this: 
     (i) In a first method the tape incorporates an organic binder of a composition such that application of a suitable solvent to the surface of the substrate and/or tape prior to contact therebetween will make the binder sufficiently tacky to cause a green-state bond between the tape and substrate. The intimate contact this causes between the glass powder and substrate surface is maintained during bum-out of the binder and fusion of the glass to form the glass-ceramic. 
     (ii) In a second method a duplex glass-ceramic layer is produced because a glass powder bonding layer is first applied to the substrate (e.g. by spraying or screen-printing) and while this bonding layer is still “wet ”, a further green tape layer is adhered to the bonding layer, thus securing both layers to the substrate. The powders in both layers fuse during subsequent heat-treatment to bond with the substrate. The glass powder in the bonding layer can be the same or of a different composition to that in the overlying tape. By using a glass powder of different composition to that in the tape, a graded interface is obtained which offers the following advantages: 
     improved adhesion; by utilized a glass of any appropriate composition which, in the glass state prior to crystallization; flows and wets the substrate more effectively than the glass in the overlying layer; 
     improved oxidation resistance at the substrate/glass-ceramic interface since the amount of porosity at this interface which could allow oxidation of the substrate, specifically a metal substrate, is reduced due to the more effective wetting of the substrate; 
     reduced stresses by grading differences in thermal expansion between the substrate and the overlying glass-ceramic layer. 
     In providing a seal for an SOFC, a glass-ceramic coating is formed as indicated above (single or duplex layers), and a sealing glass layer is then provided so that the seal is effectively a double layer comprising the glass-ceramic coating and a sealing glass layer. The combination of glass and glassceramic layers provide a gas-tight seal to separate and contain the reactant gases and electrically isolate adjacent bipolar plates. The main function of the glass-ceramic layer is to provide high temperature electrical insulation between the bipolar plates although it also must be gas-tight to contain the reactant gases. Conversely, the glass seal layer provides some measure of electrical insulation although not having as high electrical resistance as the glass-ceramic at the operating temperature. By using a glass rather than a glass-ceramic for the actual sealing stage of stack assembly the seal can continue to deform after sealing (under weight of the stack) to ensure the cell components remain in electrical contact through the stack. A glass is able to deform by viscous flow whereas a glass-ceramic, once crystallized, does not deform appreciably. The glass seal can be applied as a sheet or as a powder glass/binder mixture. 
     By using a glass powder composition that gives a glass seal layer of high viscosity during the heat-treatment, flow of the glass is minimized ensuring that coating thickness can be closely controlled and that the coating will closely conform to the stamped out pattern of the laminar body. 
     Compositions and heat-treatments of the glass-ceramic layers are selected so that their A. thermal expansions are closely matched to that of the separator plates to minimize stresses during thermal cycling. This may be achieved, for example, by producing two different glass powders in the CaO—MgO—Al 2 O 3 —SiO 2  system which have widely different coefficients of thermal expansion. The two glass powders may then be mixed together in the appropriate ratio to give a glass-ceramic coating composition of the required thermal expansion. The coefficients of thermal expansion can for example be varied in the range 8.5-11.5×10 −6 K −1 , 25-1000° C. 
     The glass-ceramic layer provides high temperature electrical insulation between adjacent bipolar plates is stable in the SOFC operating environment (750-1000° C. and oxidizing/reducing atmospheres) and is gas-tight. 
     The bipolar plate could be made of a Cr alloy, e.g. having a composition 94%Cr, 5%Fe, 1%Y 2 O 3 . The plate may be coated with an oxide layer, e.g. La x Sr 1−x CrO 3 , or another corrosion resistant alloy or metal such as ferrite steel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
     FIG. 1 shows a view of a planar array solid oxide fuel cell with one cell unit of the stack being illustrated in exploded view 
     FIGS.  2 ( a ) to ( c ) illustrate a first method of applying a glass-ceramic coating; 
     FIGS.  3 ( a ) to ( e ) illustrate a second method of applying a glass-ceramic coating; 
     FIGS.  4 ( a ) to ( d ) show partial cross-sections of four solid oxide fuel cells, each with a different seal arrangement between their adjacent separator plates. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIGS.  1  and  4 ( a ), the cell assembly  1  is of rectangular section and comprises a stack of cell units each of which comprises a current generating and collecting assembly between adjacent separator plates  11 . Plates  11  are also referred to as bipolar plates because each of them contacts (directly or indirectly) solid oxide cathode elements  17  on their lower or cathode-contacting faces  12  and anode elements  18  on their upper or anode-contacting faces  14 . 
     Reference numeral  13  in FIG.  4 ( a ) indicates a conductive layer applied to face  12  to improve electrical contact between the separator plate and the cathode  17 ; the layer  13  may take the form of a porous sheet. 
     Reference numeral  15  diagrammatically illustrates a layered assembly comprising an array of solid electrolytes  16  each with an oxide cathode layer  17  on one (the upper) surface and an anode layer  18  on the other (lower) surface. As shown in FIG. 1 the layered assembly takes the form of a two-by-two electrode array but other arrangements are possible, e.g. the layered assembly may be in the form of a single electrode or it could be in 3×3 or 2×4 arrays, for example. The porous layer(s) or sheet(s)  13  will be dimensioned to correspond with the array. As shown in FIG.  4 ( a ), a current collector  19 , e.g. in the form of a nickel grid, is affixed below the anode  18 , on top of anode-contacting surface  14  of plate  11 . 
     As seen in FIGS.  1  and  4 ( a ), each bipolar or separator plate  11  is formed with a gas flow channel arrangement  20 ,  21  formed respectively on its upper surface and its lower surface, through which channel arrangements flow the fuel gas and the oxidant gas respectively. The channel arrangements  20 ,  21  take the form of parallel channels  22  in the upper surface and parallel channels  23  in the lower surface, the channels in the respective surfaces being oriented transversely relatively to each other. 
     The gas flow channels  22  in the upper surface distribute a fuel gas (e.g. hydrogen, carbon monoxide, methane, or natural gas) entirely and evenly over the adjacent anode  18  and the gas flow channels  23  in the lower surface distribute the oxidant gas (e.g. oxygen, air) entirely and evenly on the adjacent cathode  17 . 
     The separator plates are formed with apertures  24 ,  25 ,  26  and  27  therethrough, so that when the stack of cells is assembled they form, respectively, passages for fuel gas to reach channels  22 , passages for oxidant gas to reach channels  23 , passages for the exhaust of spent and unused fuel gas and passages for the exhaust of spent and unused oxidant gas. 
     Reference  29  indicates a sealing arrangement between adjacent separator plates and comprises a layer  30  of glass-ceramic insulation and a sealing layer  40  of glass or of glass and glass ceramic. 
     The glass-ceramic layer  30  is deposited onto the cathode-contacting face  12  of the separator plate  11  prior to assembly of the SOFC stack and the glass (or glass and glass-ceramic) layer  40  bonds together adjacent separator plates and seals the electrolyte assembly to the separator plates during manufacture of the stack at elevated temperatures. Both layers  30  and  40  are of course shaped as required to accommodate the chosen geometry of the SOFC&#39;s components. As previously explained, the glass-ceramic layer  30  is formed utilizing a laminar body (e.g. in the form of sheet or tape) which incorporates a suitable glass powder and an organic binder. The laminar body is pre-shaped to the required geometry of layer  30  (e.g. by stamping) as necessary. 
     In FIG.  2 ( a ) the substrate  50  (e.g. fuel cell separator plate) and/or the tape (or sheet)  60  are shown as having a suitable solvent  70 ,  71  (e.g. ethanol or methanol) applied to one or both surfaces  51 ,  61  thereof, which solvent renders the binder in the tape tacky to hold the tape in place when brought into contact with the substrate, see FIG.  2 ( b ). FIG.  2 ( c ) shows the assembly after heat treatment to form a glass-ceramic coating  62 . Alternatively, in FIG.  3 ( a ) a thin bonding layer  80  comprising glass powder in a binder is first applied as a bond layer to the substrate surface  51 , e.g. by spraying or screen-printing, and the green tape  60  is then applied thereto, see FIG.  3 ( b ). The composition of the glass powder in layer  80 , like that of layer  60 , is such as to produce a glass-ceramic layer after heat treatment. However, it is nevertheless advantageous if the composition of layer  80  is different from that of layer  60  so as to encourage ready wetting of the surface  51  by the molten glass in layer  80  during heating and the subsequent formation of a graded glass-ceramic coating. FIG.  3 ( c ) illustrates the finished coating after heat treatment, comprising a thick outer glass-ceramic layer  62 , say between about 100 μm and 3 μm in thickness, and a thinner inner glass-ceramic bond layer  82 , say less than 50 μm in thickness. These layers are shown as distinct, but in reality during heat treatment would grade into each other. 
     FIGS.  4 ( a ) to ( d ) illustrate details of various forms of seals in fuel cell units. FIGS.  4 ( b ), ( c ), and ( d ) are identical to FIG.  4 ( a ), except with regard to the inter-plate sealing arrangement  29 . Hence, reference numerals are only provided in FIGS.  4 ( b ) to ( d ) where necessary to identify differences. 
     In the embodiment of FIG.  4 ( a ) is shown the fuel cell unit of FIG. 1 with a seal formed by a glass-ceramic layer  30  formed as described with reference to FIG. 2 or FIG. 3 and a glass layer  40 , with the glass layer being of sufficient area to bind and seal to the solid electrolyte  16 , as shown at interface  42 . 
     In FIG.  4 ( b ) is shown an arrangement which in addition to the layers  30 ,  40  of FIG. 4 a  comprises a screen-printed glass-ceramic bond layer  45  formed on the cathode face  12  of the separator plate. 
     In FIG.  4 ( c ) screen-printed glass-ceramic bond layers  45 ,  46  are provided respectively on both cathode- and anode-contacting faces of the separator plates, in addition to the layers  30 ,  40 . However, such a layer  46  may be of more value as a protective layer than as a bonding layer, by forming a barrier between the seal layer  40  and the separator plate, to obviate the possibility of unwanted reactions between the seal layer  40  and the separator plate. 
     FIG.  4 ( d ) shows a modification of the embodiment of FIG.  4 ( c ) but in this embodiment an additional screen-printed glass-ceramic layer  41  is provided between sealing glass layer  40  and glass-ceramic bond layer  46 . 
     It will be seen from the above that by suitable selection of the number and composition of the glass layers in the inter-plate sealing arrangement 291 d , it is possible to tailor their properties to simultaneously achieve, during manufacture and service of the SOFC stack, good bonding to the plates  11 ; good matching of thermal expansion coefficients; good sealing and insulation between plates  11  and between adjacent cells in each planar array of cells; and good electrical contacts within the cells. 
     The following Table gives exemplary compositions of glasses and glass-ceramics useful for putting the present invention into effect. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Weight percent of oxide 
                 SiO 2   
                 Al 2 O 3   
                 CaO 
                 MgO 
                 BaO 
                 TiO 2   
               
               
                   
               
             
             
               
                 Sealing glass layer 
                 43.9 
                 6.6 
                 13.2 
                   
                 36.3 
                   
               
               
                 Glass-ceramic bond layer 
                 52.8 
                 6.9 
                 17.2 
                 19.1 
                   
                 4.0 
               
               
                 Glass-ceramic 1 
                 59.0 
                 7.0 
                 20.8 
                 13.2 
               
               
                 Glass-ceramic 2 
                 55.0 
                 5.0 
                 10.0 
                 30.0 
               
               
                   
               
             
          
         
       
     
     Glass-ceramics  1  and  2  are examples of glass-ceramics having thermal expansion coefficients within the range 8.5-11.5×10 −6 K −1 , 25-1000° C., as required to enable close matching of the expansion characteristics of a multi-layer sealing arrangement such as 29 1  to the separator plates  11 . Powders of such differing compositions and thermal expansion coefficients can also be mixed with each other to produce glass-ceramic layers with thermal expansion characteristics intermediate the two extremes. 
     It should be understood that although the sealing glass layers such as layer  40  in FIGS.  4 ( a ) to ( d ) are formulated to remain in the glassy state during manufacture of the SOFC stack, so as to accommodate any dimensional changes during manufacturing processes involving high temperatures, it is nevertheless likely, and in fact preferred, that during subsequent service or heat-treatment, the glass sealing layers will progressively crytallize into the glass-ceramic state, so giving a stronger and less reactive sealing arrangement between the separator plates. The essential requirement for the sealing layer during manufacture of the SOFC stack is that the sealing layer must remain in the glassy viscous molten state until the anode reduction process has been completed and the consequent volume changes have ceased.