Patent Publication Number: US-2022231317-A1

Title: Method of manufacturing solid oxide electrolyzer cells using a continuous furnace

Description:
PRIORITY 
     This application is a non-provisional application that claims the benefit of U.S. Provisional Application No. 63/137,941, filed on Jan. 15, 2021, the contents of which are herein incorporated by reference in their entirety. 
    
    
     FIELD 
     The present embodiments of the present disclosure are directed to methods of manufacturing an electrolyzer cell using a continuous furnace. 
     BACKGROUND 
     Solid oxide electrolyzer cells (SOEC) may be used to produce hydrogen. However, SOECs are relatively expensive to manufacture. 
     SUMMARY 
     In various embodiments a method of manufacturing a SOEC comprises removing a binder from the SOEC using microwave radiation while the SOEC is disposed in a first zone of a furnace, and sintering the SOEC while the SOEC is disposed in a second zone of the furnace. 
    
    
     
       FIGURES 
         FIG. 1  is a diagram showing the operation of a solid oxide electrolyzer cell, according to various embodiments of the present disclosure. 
         FIG. 2A  is a perspective view of a solid oxide electrolyzer cell stack, and  FIG. 2B  is a side cross-sectional view of a portion of the stack of  FIG. 2A . 
         FIG. 3  is a diagram illustrating a method of forming a SOEC, according to various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Solid oxide fuel cells (SOFC) can be operated as SOECs in order to produce hydrogen and oxygen from water using electrolysis. In SOFC mode, oxide ions are transported from the cathode side (air) to the anode side (fuel) and the driving force is the chemical gradient of partial pressure of oxygen across the electrolyte. In SOEC mode, a positive potential (e.g., 1 to 1.5 V) is applied to the air side of the cell and the oxide ions are now transported from the “fuel” (e.g., water) side to the air side. Since the cathode and anode are reversed between SOFC and SOEC (i.e. a SOFC cathode is a SOEC anode, and a SOFC anode is a SOEC cathode), going forward, the SOFC cathode (SOEC anode) will be referred to as the air electrode, and the SOFC anode (SOEC cathode) will be referred to as the fuel electrode. 
       FIG. 1  is a diagram illustrating the operation of a SOEC  20 , according to various embodiments of the present disclosure. Referring to  FIG. 1 , an air stream is provided to the air electrode  23  while a fuel stream containing water (e.g., a water stream, a mixed water and hydrogen stream, etc.) is provided to the fuel electrode  27 . The water is reduced to form H 2  gas and O 2-  ions at the fuel electrode  27  according to the formula: H 2 O+2e→O 2- +H 2 . The O 2-  ions are transported through the solid oxide electrolyte  25 , and then oxidized at the air electrode  23  (O 2-  to O 2 ) to produce molecular oxygen. 
       FIG. 2A  is a perspective view of a solid oxide cell electrolyzer stack  100 , and  FIG. 2B  is a side cross-sectional view of a portion of the stack  100  of  FIG. 2A . The term “electrolyzer cell stack,” as used herein, means a plurality of stacked electrolyzer cells that can optionally share a common water inlet and exhaust passages or risers. The “electrolyzer cell stack,” as used herein, includes a distinct electrical entity which contains two end plates which are connected directly to power conditioning equipment and the power (i.e., electricity) input of the stack or comprises a portion of an electrolyzer cell column that contains terminal plates which provide electrical input. 
     For example, the stack  100  may include multiple SOECs  20  that are separated by interconnects  10 , which may also be referred to as gas flow separator plates or bipolar plates. Each SOEC  20  includes an air electrode  23 , a solid oxide electrolyte  25 , and a fuel electrode  27 . The stack  100  may optionally also include internal fuel (e.g., water) riser channels  22 . 
     The air electrode  23  may comprise an electrically conductive material, such as an electrically conductive perovskite material, such as lanthanum strontium manganite (LSM). Other conductive perovskites, such as lanthanum strontium cobaltite (LSC), lanthanum strontium cobalt manganite (LSCM), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium ferrite (LSF), La 0.85 Sr 0.15 Cr 0.9 Ni 0.1 O 3  (LSCN), etc., or metals, such as Pt, may also be used. In some embodiments, the air electrode  23  may comprise a mixture of the electrically conductive material and an ionically conductive ceramic material. For example, the air electrode  23  may include from about 10 wt % to about 90 wt % of the electrically conductive material described above, (e.g., LSM, etc.) and from about 10 wt % to about 90 wt % of the ionically conductive material. Suitable ionically conductive materials include zirconia-based and/or ceria based materials. For example, the ionically conductive material may comprise scandia-stabilized zirconia (SSZ), ceria, and at least one of yttria and ytterbia. In some embodiments, the ionically conductive material may be represented by the formula: (ZrO 2 ) 1-w-x-z (Sc 2 O 3 ) w (CeO 2 ) x (Y 2 O 3 ) a (Yb 2 O 3 ) b , where 0.09≤w≤0.11, 0&lt;x≤0.0125, a+b=z, and 0.0025≤z≤0.0125. In some embodiments, 0.009&lt;x≤0.011 and 0.009≤z≤0.011, and optionally either a or b may equal to zero if the other one of a or b does not equal to zero. 
     The electrolyte  25  may comprise a stabilized zirconia, such as scandia stabilized zirconia (SSZ), yttria stabilized zirconia (YSZ) scandia-ceria-stabilized zirconia (SCSZ), scandia-ceria-yttria-stabilized zirconia (SCYSZ), scandia-ceria-ytterbia-stabilized zirconia (SCYbSZ), or the like. For example, the electrolyte  25  may comprise a scandia and ceria stabilized zirconia, comprising 5 to 12 mol % scandia, 1 to 7 mol % ceria, and 80 to 94 mol % zirconia, and optionally 0.5 to 3 mol % ytterbia. Alternatively, the electrolyte  25  may comprise a yttria and ceria stabilized zirconia comprising 3 to 10 mol % yttria, 1 to 6 mol % ceria, and 84 to 96 mol % zirconia. Alternatively, the electrolyte  25  may comprise another ionically conductive material, such as a doped ceria, for example samaria-doped ceria (SDC), gadolinia-doped ceria (GDC), or yttria-doped ceria (YDC). 
     The fuel electrode  27  may comprise a cermet layer comprising a metal-containing phase and a ceramic phase. The metal-containing phase may include a metal catalyst, such as nickel (Ni), cobalt (Co), copper (Cu), alloys thereof, or the like, which operates as an electron conductor. The metal catalyst may be in a metallic state or may be in an oxide state. For example, the metal catalyst forms a metal oxide when it is in an oxidized state. Thus, the fuel electrode  27  may be annealed in a reducing atmosphere prior to operation of the SOEC, to reduce the oxidized metal catalyst to a metallic state. The ceramic phase of the fuel electrode  37  may include, but is not limited to gadolinia-doped ceria (GDC), samaria-doped ceria (SDC), ytterbia-doped ceria (YDC), scandia-stabilized zirconia (SSZ), ytterbia-ceria-scandia-stabilized zirconia (YbCSSZ), or the like. In the YbCSSZ, scandia may be present in an amount equal to 9 to 11 mol %, such as 10 mol %, ceria may present in amount greater than 0 (e.g., at least 0.5 mol %) and equal to or less than 2.5 mol %, such as 1 mol %, and at least one of yttria and ytterbia may be present in an amount greater than 0 and equal to or less than 2.5 mol %, such as 1 mol %, as disclosed in U.S. Pat. No. 8,580,456, which is incorporated herein, by reference. 
     Each interconnect  10  electrically connects adjacent SOECs  20  in the stack  100 . In particular, an interconnect  10  may electrically connect the air electrode  23  of one SOEC  20  to the fuel electrode  27  of an adjacent SOEC  20 .  FIG. 1B  shows that the lower SOEC  20  is located between two interconnects  10 . Each interconnect  10  includes ribs  12  that at least partially define fuel channels  18 A and air channels  18 B. The interconnect  10  may operate as a reactant separator that separates a first reactant, such as a water containing fuel stream, from a second reactant such as air, which are provided to adjacent SOECs  20 . At either end of the stack  100 , there may be an air end plate or fuel end plate (not shown) for providing air or fuel, respectively, to the end electrode. Each interconnect  10  may be made of or may contain electrically conductive material, such as a metal alloy (e.g., chromium-iron alloy) which has a similar coefficient of thermal expansion to that of the solid oxide electrolyte  5  (e.g., a difference of 0-10%). For example, the interconnects  10  may comprise a metal (e.g., a chromium-iron alloy, such as 4-6 weight percent iron (e.g., 5 wt % iron), optionally 1 or less weight percent yttrium and balance chromium alloy). 
       FIG. 3  is a process diagram showing an exemplary method of making SOECs by screen printing fuel and air electrodes on the electrolyte, drying, and firing. Referring to  FIG. 3 , bare, plate shaped solid oxide electrolytes  25 , such as stabilized zirconia electrolytes are unpackaged and placed into a slotted cassette  101 . Each cassette  101  is installed into an elevator  103  which positions the individual electrolytes  25  onto a walking beam conveyor  105 . The walking beam conveyor  105  transports the electrolytes  25  to a printing tool plate  107 , while exposing the electrolytes  25  to minimal abrasion during the process. A pickup head  109  may be used to place the electrolytes  25  from the end of the walking beam conveyor  105  onto the tool plate  107 . The pickup head  109  may be configured with a Bernoulli pad or vacuum pogo pin array to pick up the electrolyte  25 . 
     A screen print cycle begins once the pickup head  109  lowers the electrolyte  25  onto the tool plate  107 . First, multiple snugger alignment pins  111  collapse inward toward the electrolyte  25 . The pins  111  may be fixed and/or pressure loaded. The combined inward movements position the electrolyte  25  to a predetermined alignment position. A small amount of tool plate  107  vacuum is utilized to keep the electrolyte  25  from oscillating between the alignment pins  111 . Once the electrolyte  25  is positioned, a hold down vacuum is applied and the alignment pins  111  retract out and away from the work area. Afterward, the tool plate  107  carriage  113  shuttles below a mesh screen  115  (e.g., a calendared mesh with a high wire density) and the print cycle initiates. The print cycle includes the screen printing process using screen printing suitable ink, such as an ink with a relatively high solids loading of 80 to 93 weight percent, and screen tooling which defines the deposited ink image and ink deposition characteristic. The ink contains an organic binder and electrode active material powder particles in a solvent. 
     After the electrode screen printing is completed, the printed electrolyte  25  is lifted off the tool plate  107  either manually or by any suitable machine or device, such as a pick up head  109 . The tool plate  107  returns to its home position to receive the next electrolyte  25 . The previously printed electrolyte  25  is transported down a conveyor  117  to a predetermined pick up location. Another pick up head  119 , such as a robotic pick up head, lowers and surrounds the electrolyte  25  with two or more cleats. The cleats do not apply pressure to the substrate in order to minimize chipping or damage to the ink printed electrolyte  25 . The pickup head  119  raises and secures the electrolyte  25  with the force of gravity. The pickup head  119  then transports the electrolyte  25  to a dryer belt  121  and releases the electrolyte  25  onto the dryer belt  121 . 
     Any suitable dryer may be used. For example, the dryer  120  may include the dryer conveyor belt  121 , such as a woven stainless steel belt or other suitable conveyor belt travelling through an infrared heating zone  123  heated by one or more infrared heating lamps  125 . The electrolyte  25  is transported by the belt  121  to the heating zone  123  and heated in the heating zone by the heating lamp(s)  125 . During the heating process, a percentage of the ink organics are released from the electrode(s), which prepares the substrate for further processing. 
     The belt  121  may remain continuously moving while carrying the electrolytes  25  through the heating zone. Alternatively, the belt  121  may transport the electrolyte  25  to the heating zone, then stop while the substrate is being heated, followed by moving the electrolyte  25  out of the heating zone after completion of the heating. 
     If desired, the dryer may comprise two or more belts and/or two or more heating zones. In case of two or more belts  121 , the pickup head  119  may be pre-programmed or controlled by an operator or control system to sequentially place the substrates on different belts to dry the substrates in parallel rather than in series. The drying steps may be conducted at a temperature of less than 150° C., such as 50 to 100° C., for example 70 to 80° C. 
     The dried printed electrolyte  25  is then removed from the dryer  120  ether manually or by machine. Any suitable machine may be used. For example, a robotic Bernoulli pad or vacuum pickup head  127  with configured pogo pins may be positioned near the dryer exit. The pad or head removes the electrolyte  25  from the dryer belt and places it on a walking beam conveyor  129 . The walking beam conveyor  129  transports the substrate to an exit elevator  131 , which then loads the electrolyte  25  into a cassette for subsequent processing. 
     In particular, the above processes may be repeated to apply additional electrode ink layers to the same sides of the electrolytes  25 , and/or to print one or more electrode ink layers on opposing sides of the electrolytes  25 , in order to form the SOEC  20 . In various embodiments, different electrode inks may be applied to each side of the electrolytes  25  and/or to the same sides of the electrolytes  25 , in order to form the SOEC. 
     Conventionally, electrolytes printed with one or more electrode inks are thermally processed in a batch furnace to remove (e.g., burnout) ink binders and sinter the ink printed on the electrolytes into an electrode. However, conventional binder burnout processes may require long thermal processing times, in order to avoid electrode delamination, and may require mechanical electrolyte containment, in order to prevent excessive electrolyte warping. The equipment needed to constrain electrolytes may also increase the thermal mass of the process, which also increases processing time and reduces process throughput. 
     After sintering is complete, conventional batch furnaces are cooled before a new batch of electrolytes can be processed, which also extends processing time and increases energy costs. Further, conventional electrical batch furnaces are also vulnerable to heating element failure, which may results in the scraping of an entire furnace load of electrolytes and significant cost increases. 
     In view of the above and/or other drawbacks of conventional methods, various embodiments provide an improved method of binder removal and electrode sintering using microwave heating. The electrolyte  25  may be subjected to microwave heating with one dried electrode ink (e.g., fuel or air electrode ink) layer printed on one side of the electrolyte  25 , or with two dried electrode ink (e.g., both fuel and air electrode inks) printed on respective, opposing sides of the electrolyte  25 . 
     In particular, after electrode ink printing on one or both sides of the electrolyte  25  is complete, the SOECs  20  may be loaded into a ceramic frame or support  140 . In particular, the support  140  may include one or more rails or rings that support the periphery of each SOEC  20 , such that the dried electrode inks are exposed (e.g., do not contact adjacent SOECs  20 ). 
     The support  140  may be loaded into a continuous furnace  150 , such as a continuous pusher or roller hearth kiln or the like. The furnace  150  may include ceramic rollers  152  that are driven to move the supports  140  through the furnace  150 , at a continuous or a variable rate. The furnace  150  may include at least one microwave generator  162  and at least one additional heating element  172 . The furnace  150  may be divided into a first zone  160  and a second zone  170 . The furnace  150  may be configured to maintain a selected atmosphere and/or temperature within the first zone  160  and/or the second zone  170 . For example, the furnace  150  may be configured to maintain an internal atmosphere having a selected oxygen partial pressure. 
     In operation, supports  140  loaded with the SOECs  20  (e.g., the printed electrolytes  25 ) may be provided to the first zone  160  for binder removal (e.g., burnout). In particular, the microwave generator  162  may include one or more microwave sources configured to radiate microwaves to the SOECs  20 , as the support  140  moves through the first zone  160 . The microwave generator  162  may be configured to heat the SOECs  20  to a sufficient temperature in order to drive out (e.g., volatize) binders from the electrode inks printed thereon. For example, the SOECs  20  may be debindered at a temperature greater than 300° C., such as a temperature ranging from about 400° C. to about 800° C. In some embodiments, the first zone  160  may be heated by heat generated in the second zone  170 , in order to increase the rate of binder burnout. As such, the binder removal process may be referred to as a microwave-assisted binder burnout process. 
     After binder removal, the support  140  may be moved into the second zone  170 . The second zone  170  may be heated by the heating element  172 . In some embodiments, the heating element  172  may include one or more gas heating elements (e.g., gas-fired burners). However, in other embodiments, the heating element  172  may optionally include one or more resistive heating elements. The gas heating elements may be disposed in a separate chamber of the furnace, such that combustion gasses do not affect the internal atmosphere of the furnace  150 . For example, hot combustion gasses may be routed around the second zone  170 , in order to heat sidewalls of the furnace  150 , and thereby indirectly heat the electrolytes  25 . 
     The second zone  170  may be maintained at a temperature sufficient to sinter the SOECs  20 , as the SOECs  20  pass through the second zone  170 . In other words, the ceramic or cermet electrodes ( 23  and/or  27 ) are sintered to the electrolyte  25 . For example, the SOECs  20  may be sintered at a temperature greater than 1000° C., such as a temperature ranging from about 1100° C. to about 1400° C. 
     After sintering, the supports  140  may be removed from the furnace  150  and allowed to cool. After cooling, sintered SOECs  20  may be removed from the supports  140 . 
     According to various embodiments, the disclosed continuous binder burnout and sintering processes may result in unexpected cost reductions, as compared to conventional batch processing. For example, manufacturing output of SOECs may be increased by a factor of 4, or more, at the same or similar operating costs as conventional batch processing. For example, the continuous furnaces used in the present embodiments may have a smaller cross-section, as compared to conventional batch furnaces, which reduces thermal mass and the associated energy consumption costs. In addition, the continuous furnaces of the present embodiments may provide significantly lower failure rates, since resistive heating elements are not required to endure thermal cycling. Finally, utilizing a continuous furnace may also provide a lower overall maintenance costs. 
     The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.