Abstract:
The present disclosure relates to a method for making a thin layer solid oxide fuel cell including at least an anode, an electrolyte and a cathode including at least the following steps of:
       magnetron sputtering deposition of an electrolyte on a first electrode, and of   magnetron sputtering deposition of a second electrode on the electrolyte,       
 
     at least one catalyst is incorporated into the first electrode and/or the second electrode during the deposition thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a National Phase Entry of International Application No. PCT/EP2008/067863, filed on Dec. 18, 2008, which claims priority to French Application 0760124, filed on Dec. 20, 2007, both of which are incorporated by reference herein. 
     
    
     BACKGROUND AND SUMMARY 
       [0002]    The present invention relates to the field of thin layer solid oxide fuel cells, so-called SOFCs (Solid Oxide Fuel Cells) and more particularly to the method for making them. 
         [0003]    It is well-known that fuel cells are used in many applications, and are notably considered as a possible alternative to the use of fossil fuels. Indeed, these cells allow direct conversion of a source of chemical energy for example hydrogen or ethanol into electric energy. 
         [0004]    A thin layer fuel cell of the SOFC type usually consists, with reference to  FIG. 1  which is a schematic illustration of a fuel cell, of an ion-conducting electrolyte  1 , in which are deposited on either side an anode  2  and a cathode  3 . The operating principle of such a cell is the following: the anode  2  is the center of the reaction 2H 2 +2O 2− →2H 2 O+4e − , the electrolyte  1  being responsible for transporting the O 2−  ion and the cathode  3  is the center of the following reaction: O 2 +4e − →2O 2−  when the cell is supplied with hydrogen (H 2 ) and oxygen (O 2 ). The anode  2  and the cathode  3  have to be obtained in a porous material in order to ensure accessibility of the gases and to provide discharge of the water produced by the cell. 
         [0005]    Moreover, the anode  2  and the cathode  3  have to be electrically conducting in order to ensure transport of the current. Further, the electrolyte  1  has to be obtained in a dense and ion-conducting material in order to provide the transport of the O 2−  ion. Thus, these fuel cells usually consist of an anode  2  in Cermet Ni—ZrO 2 -8% Y 2 O 3 , of an electrolyte  1  in ZrO 2 -8% Y 2 O 3 (YSZ) and a cathode  3  in LaSrMnO 3−δ  (LSM). The usual methods for making these SOFC fuel cells are the formation of successive layers forming the anode  2 , the electrolyte  1  and the cathode  3  by strip casting, by screen-printing, by spin coating, by thermal plasma projection or by flame spraying for example. 
         [0006]    However, the fuel cells obtained according to these methods have too high operating temperatures, comprised between 700 and 1,000° C., for applications in the fields of domestic power auxiliaries and transportation. One of the objects of the invention is therefore to find a remedy to these drawbacks by proposing a method for making fuel cells having a low operating temperature, i.e. below 400° C. 
         [0007]    According to the invention, a method for making a thin layer solid oxide fuel cell is proposed including at least an anode, an electrolyte and a cathode remarkable in that it includes at least the following steps of magnetron sputtering of an electrolyte on a first electrode, and then of magnetron sputtering of a second electrode on the electrolyte, and in that at least one catalyst is incorporated into the first electrode and/or the second electrode during their deposition. The first electrode and/or the second electrode therefore advantageously includes at least one catalyst distributed in said electrode. Said catalyst is preferably comprised in an element or a combination of at least two elements from the group comprising the platinum group, platinoid alloys such as platinum-ruthenium, platinum-molybdenum, platinum-tin, non-platinoid metals such as iron, nickel or cobalt. Said platinum group includes platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os) and iridium (Ir). 
         [0008]    The first electrode, the electrolyte and the second electrode are successively deposited in a chamber including at least  3  magnetron targets. The depositions are preferably carried out under an oxidizing atmosphere. According to a feature of the method according to the invention, the depositions are carried out with an ionized reactive magnetron plasma sputtering method. Said plasma is a plasma containing at least oxygen and preferably an argon-oxygen mixture. Moreover, the pressure in the chamber is variable. The first electrode is obtained by magnetron sputtering deposition on a supporting substrate. 
         [0009]    Said supporting substrate consists in a substrate capable of being dissolved in a liquid, said liquid not dissolving the electrodes and the electrolyte of the fuel cell. Said first electrode forming the anode of the fuel cell is obtained by magnetron sputtering of a Ni-YSZ or Sr 1−x Y x TiO 3  target under an oxidizing atmosphere. The bias of the target and/or the pressure of the plasmagen gas and/or the speed of rotation of the supporting substrate are continuously adjusted during sputtering in order to vary the porosity in the depth of the deposited layer forming the first electrode. 
         [0010]    According to an alternative embodiment, the first electrode forming the anode or the cathode forms a supporting substrate obtained in an electron conducting or ion/electron conducting and porous reducing oxide, on which are deposited the electrolyte and the cathode or respectively the anode. 
         [0011]    Moreover, the electrolyte of the fuel cell is obtained by magnetron sputtering of a yttriated zirconia target or CeO 2  doped with Sm 2 O 3  or Gd 2 O 3  under an oxidizing atmosphere. Sputtering is obtained by pulsed magnetron sputtering. The second electrode forming the cathode of the fuel cell is obtained by magnetron sputtering of a target of La x Sr 1−x MnO 3  (LSM) of LaNiO 4+δ  or Nd x NiO 4+δ  under an oxidizing atmosphere. The bias of said target and/or the pressure of the plasmagen gas and/or the speed of rotation of the supporting substrate is continuously adjusted during sputtering in order to vary porosity in the depth of the deposited layer forming the second electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Other advantages and characteristics will become better apparent from the description which follows, of several alternative embodiments, given as non-limiting examples, of the method for making a fuel cell of the SOFC type according to the invention, from appended drawings wherein: 
           [0013]      FIG. 1  is a schematic illustration of a fuel cell; and 
           [0014]      FIG. 2  is a schematic illustration of a vacuum chamber of a magnetron sputtering deposition device for applying the method according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    With reference to  FIG. 1 , the fuel cell consists of an ion-conducting electrolyte  1 , on which an electrode, more specifically an anode  2  and a cathode  3  are deposited on either side. The electrolyte  1  is preferably made in yttriated zirconia (YSZ), and more specifically in 8% yttriated zirconia (YSZ) having a high density in order to optimize conduction of O 2  ions in the fuel cell. Said density should be close to 6.10 g/cm 3 . Said electrolyte  1  may also be obtained in CeO 2  doped with Sm 2 O 3 or Gd 2 O 3  for example. 
         [0016]    The anode  2  is preferably made in yttriated zirconia, Cermet Ni-YSZ, the porosity of which is advantageously variable in the depth of the layer forming the anode  2 , the average porosity being of the order of 50%. Said anode  2  may advantageously include at least one catalyst distributed in said anode  2 . Said catalyst consists in an element or a combination of at least two elements from the group comprising the platinum group, platinoid alloys such as platinum-molybdenum, platinum-tin, and non-platinoid metals such as iron (Fe), nickel (Ni) or cobalt (Co). The group of platinum notably includes platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os) and iridium (Ir). 
         [0017]    Accessorily, the catalyst concentration increases from the outer face towards the inner face of the anode  2 , i.e. from the free face of the anode  2  towards the electrolyte  1 , in order to improve the efficiency of use of said catalyst. It will be observed that the anode  2  may also notably be obtained by deposition of Sr 1−x Y x TiO 3 . 
         [0018]    The cathode  3  is made in La x Sr 1−x MnO 3  (LSM) and advantageously includes a catalyst as described earlier for the anode  2 . Moreover, the average porosity of the cathode  3  is also of the order of 50%. Accessorily, and in the same way as for the anode  2 , the catalyst concentration increases from the outer face towards the inner face of the cathode  3 , i.e. from the free face of the cathode  3  towards the electrolyte  1 , in order to improve the efficiency of use of said catalyst. Further, said porosity may advantageously be variable in the depth of the layer forming the cathode  3 . It will be observed that the cathode  3  may also be obtained by deposition of LaNiO 4+δ  or Nd x NiO 4+δ . 
         [0019]    For making this fuel cell, with reference to  FIG. 2 , a possibly ionized reactive magnetron plasma sputtering device  10  is used. This magnetron sputtering device  10  consists of a vacuum chamber  11 , with a generally cylindrical shape for example, in which a support-holder  12  and at least three magnetron targets  13 ,  14  and  15  extend. The support-holder  12  is capable of being driven into rotation around the normal to the main face of the latter so as to allow uniform deposition of different materials. 
         [0020]    The magnetron targets  13 ,  14  and  15  are respectively biased with variable voltages V 13 , V 14  and V 15 . The first target  13  is for example a target of yttriated zirconia (YSZ), and more specifically in 8% yttriated zirconia (YSZ), for making the electrolyte  1 . The second target  14  is for example a target of yttriated zirconia, Cermet Ni-YSZ, for making the anode  2  and the third target  15  is a target of La x Sr 1−x MnO 3  (LSM) for making the cathode  3 . Accessorily, the device includes a fourth target, not illustrated in  FIG. 2 , for sputtering a catalyst simultaneously with the sputtering of the material of the anode  2  and/or of the cathode  3 . 
         [0021]    The device moreover includes a radiofrequency emission source  16 , such as a radiofrequency antenna, in order to generate additional plasma in the chamber  11 , preferably a plasma containing oxygen, such as an argon-oxygen plasma for example, and to control the oxidization rate of the layers. The oxygen flow may for example be comprised between 0 and 50% and the argon flow may be comprised between 1 and 50% for example. 
         [0022]    Moreover, the device includes one or more magnets  17 , permanent magnets and/or electromagnets, positioned under the support-holder  12  and capable of trapping the low pressure plasma in proximity to the support-holder  12 . Preferably this is a low pressure plasma of argon, or of any other gas having a mass close to the mass of the target. By low pressure plasma is meant a plasma for which the pressure is comprised between 0.1 and 100 mTorrs. Further, the device may advantageously include a computer  18  in which one or more time diagrams are recorded in memory and which is capable of controlling the variable voltages V 13 , V 14  and V 15  so as to obtain the desired profile. 
         [0023]    The making method consists of placing a supporting substrate on the support-holder  12  of the possibly ionized reactive magnetron plasma sputtering device  10 . Said supporting substrate may consist in a substrate capable of being dissolved in a liquid, said liquid not dissolving the electrodes  2 ,  3  and the electrolyte  1  of the fuel cell. The anode  2  is made by sputtering of the Ni-YSZ target  14  on the supporting substrate under an oxidizing atmosphere in order to be able to control the oxygen level, either with or without assistance from the radiofrequency emission source  16 . The thereby obtained anode  2  is in Cermet Ni-YSZ (yttriated zirconia), the porosity of which may be variable in the depth of the layer by continuously adjusting the parameters for biasing the magnetron sputtering target  14  and/or the pressure of the plasmagen gas and/or the speed of rotation of the support-holder, said biasing parameters being adjusted by means of the variable voltage V 14 . 
         [0024]    According to an alternative embodiment of the method according to the invention, the deposition of the layer forming the anode  2  is achieved by reactive magnetron sputtering of an Ni—Y—Zr alloy under a mixed argon-oxygen plasma, with or without the assistance of the radiofrequency emission source  16 . 
         [0025]    Moreover, it will be observed that with the variation of argon pressure and/or the variation of bias and/or the variation of the speed of rotation of the supporting substrate, the porosity of the anode  2  may be controlled, the average porosity being usually close to 50%. It will be noted that the person skilled in the art may easily adjust the porosity, by oxidation tests at this anode  2 , depending on the targeted application for the fuel cell. For low temperature operation, i.e. at a temperature below 400° C., it is easy to incorporate a catalyst into the Ni-YSZ layer forming the anode  2  during deposition by co-sputtering or by using an additional target not illustrated in the figures. 
         [0026]    Said catalyst consists in an element or in a combination of at least two elements from the group comprising the platinum group, platinoid alloys such as platinum-molybdenum, platinum-tin, and non-platinoid metals such as iron (Fe), nickel (Ni) or cobalt (Co). The platinum group notably includes platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os) and iridium (Ir). Accessorily, co-sputtering may be achieved in such a way that the platinum concentration decreases towards the inside of the anode  2  in order to reduce its amount and improve its efficiency of use. 
         [0027]    After the deposition of the anode  2  on the supporting substrate  2 , the target  13  of yttriated zirconia is sputtered under an oxygen atmosphere with or without the assistance of the radiofrequency emission source, preferably by pulsed magnetron sputtering, in order to deposit a layer forming the electrolyte  1  on the anode  2 . It will be observed that the thereby deposited electrolyte  1  should have high density, of about 6.10 g/cm 3 , in order to optimize conduction of O 2−  ions in the lattice of the electrolyte  1  of the fuel cell. Further, this pulsed magnetron sputtering technique is particularly suitable for sputtering insulating targets while retaining the performances of continuous sputtering depositions. 
         [0028]    Finally, the cathode  3  is deposited on the electrolyte  1  from the target La x Sr 1−x MnO 3  (LSM)  15 , said target  15  being sputtered under an oxidizing atmosphere in order to preserve oxygen stoichiometry. Said target  15  advantageously contains a catalyst for better operation at low temperature. In the same way as earlier, said catalyst consists in an element or a combination of at least two elements of the group comprising the platinum group, platinoid alloys such as platinum-molybdenum, platinum-tin, and non-platinoid metals such as iron (Fe), nickel (Ni) or cobalt (Co), the platinum group notably including platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os) and iridium (Ir). 
         [0029]    Moreover, it will be noted that by varying argon pressure and/or by varying bias and/or by varying the speed of rotation of the supporting substrate, it is possible to control the porosity of the cathode  3 , the average porosity being usually close to 50%. The anode  2 , electrolyte  1  and cathode  3  assembly is detached from the supporting substrate by any suitable means well-known to the person skilled in the art. 
         [0030]    According to an alternative embodiment of the method for making a fuel cell according to the invention, the electrolyte  1  and then the cathode  3  may be deposited on a supporting substrate consisting of a substrate forming the anode  2 , said substrate forming the anode  2  being obtained in an electron conducting or ion/electron conducting reducing oxide, such as yttriated zirconia Cermet Ni-YSZ, or Sr 1−x Y x TiO 3 , for example, without however departing from the scope of the invention. According to another alternative embodiment of the method for making a fuel cell according to the invention, the electrolyte  1  and then the anode  2  may be deposited on a supporting substrate consisting of a substrate forming the cathode  3 , said substrate forming the cathode  3  being obtained in an electron conducting or ion/electron conducting reducing oxide, such as in yttriated zirconia Cermet Ni-YSZ, or Sr 1−x Y x TiO 3  for example, without however departing from the scope of the invention. 
         [0031]    The electrolyte  1  may be obtained by magnetron sputtering deposition of CeO 2  doped with Sm 2 O 3  or Gd 2 O 3  for example, that the anode  2  may be obtained by magnetron sputtering deposition of Sr 1−x Y x TiO 3  and that the cathode  3  may be obtained by magnetron sputtering deposition of LaNiO 4+δ  or Nd x NiO 4+δ , the targets  13 ,  14  and  15  being adapted accordingly. Moreover, the electrolyte  1  may be obtained in any ion-conducting oxide and that the anode  2  and/or the cathode  3  may be obtained in any electron-conducting oxide and/or in any mixed electron/ion conducting oxide, without however departing from the scope of the invention. Finally, the examples which have just been given are only particular illustrations and by no means limiting as to the fields of application of the invention.