Patent Publication Number: US-2023132773-A1

Title: Sealed porous structures for solid oxide fuel cells and methods of fabricating thereof

Description:
BACKGROUND 
     Solid oxide fuel cells (SOFCs) are used for various applications, e.g., auxiliary power units in electrical vehicles, stationary power generators, and the like. Similar to other fuel cells and unlike conventional heat engines, SOFCs are modular, scalable, efficient, and clean. Unlike internal combustion engines, SOFCs produce zero emissions. Furthermore, SOFCs can use a wide range of fuels and are generally more tolerant to various fuel contaminants than other types of fuel cells. SOFCs also produce high-temperature exhaust that can be used, for example, to power heaters and turbines. Finally, SOFCs can provide carbon capture opportunities due to the separation of fuel and oxidant streams in these cells. 
     Metal-supported SOFCs represent the latest development in the fuel cell field. A metal-supported cell is rugged, vibration tolerant, and thermal-shock tolerant that enables rapid startup capability. A metal-supported SOFC typically utilizes porous metal layers for transferring the fuel and oxidant to the electrolyte, positioned in between these porous metal layers. For example, the fuel and oxidant can be supplied through the supply ports extending through multiple SOFCs forming one assembly, which can be referred to internally manifolded cell/stack design. The fuel and oxidant independently flow through each porous metal layer to different portions of the electrolyte and away from their respective supply ports. However, porous cell structure can also result in direct intermixing of the fuel and oxidant if the distribution is not controlled. For example, each porous metal layer (i.e., on each side of the electrode) can extend to both fuel and oxidant supply ports. 
     What is needed are new SOFCs with controlled distribution of fuels and oxidants within porous metal layers of these cells. 
     SUMMARY 
     Described herein are solid oxide fuel cells (SOFCs), comprising anode-conductor seals and/or cathode-conductor seals used for sealing porous metal structures and controlling the distribution of fuel and oxidants within these porous structures. For example, a SOFC comprises an anode conductor, cathode conductor, and electrolyte, disposed between the anode and cathode conductors. The anode conductor comprises multiple porous portions (permeable to the fuel) and a non-porous portion. The SOFC also comprises an anode-conductor seal, forming a stack with the non-porous portion. This sealing stack extends between the electrolyte and current collector and separates two porous portions thereby preventing the fuel and oxidant migration between these portions. In some examples, the sealing stack forms an enclosed boundary around one porous portion of the anode conductor. In the same or other examples, another sealing stack is formed in the cathode conductor, e.g., surrounding a fuel port extending through the cathode conductor. 
     In some examples, a SOFC for electrochemically reacting fuel and oxidant and producing an electrical current is provided. The SOFC comprises an anode conductor, comprising anode-conductor porous portions and an anode-conductor non-porous portion. Each of the anode-conductor porous portions is permeable to the fuel. The SOFC also comprises an anode-conductor seal. The anode-conductor non-porous portion and the anode-conductor seal form a stack, impermeable to the fuel and forming an anode-conductor boundary around one of the anode-conductor porous portions. Furthermore, the SOFC comprises a cathode conductor, comprising a cathode-conductor porous portion permeable to the oxidant, and an electrolyte, disposed between the anode conductor and cathode conductor. The electrolyte is fluidically and electrically coupled to each of the anode conductor and the cathode conductor. Furthermore, the electrolyte is configured to electrochemically react the fuel and the oxidant to produce the electrical current between the anode conductor and cathode conductor. 
     In some examples, the anode-conductor non-porous portion is monolithic with the anode-conductor porous portions. In more specific examples, the anode-conductor non-porous portion and the anode-conductor porous portions are both formed stainless steel. The anode-conductor non-porous portion may have a porosity of less than 10%. In some examples, the anode-conductor non-porous portion is formed by compression or melting of the anode-conductor porous portions. 
     In some examples, the anode-conductor seal is formed from one or more materials selected glass or braze. In the same or other examples, the anode conductor comprises an anode-conductor first side and an anode-conductor second side, opposite of the anode-conductor first side and directly interfacing the electrolyte. A portion of the anode-conductor seal extends over a part of the anode-conductor first side. The height of the anode-conductor seal can be greater than the height of the anode-conductor non-porous portion in the stack. In some examples, the anode-conductor non-porous portion extends to and directly interfaces the electrolyte. 
     In some examples, the SOFC, or more generally, the SOFC stack comprises interconnecting plates. At least a part of the anode-conductor porous portions of the anode conductor directly interfaces and is electrically coupled to the first one of the interconnecting plates. At least a part of the cathode-conductor porous portions of the cathode conductor directly interfaces and is electrically coupled to the second one of the interconnecting plates. The anode-conductor seal extends to and is sealed against the first one of the interconnecting plates. In some examples, at least a part of the anode-conductor seal extends between the first one of the interconnecting plates and the anode-conductor porous portions. 
     In some examples, the SOFC further comprises a cathode-conductor seal. The cathode conductor further comprises a cathode-conductor non-porous portion. The cathode-conductor non-porous portion and the cathode-conductor seal form a cathode-conductor sealing stack impermeable to the fuel and forming a cathode-conductor boundary around at least a part of the cathode-conductor porous portion. In some examples, the cathode-conductor seal is laterally offset relative to the anode-conductor seal. The cathode-conductor boundary can be surrounded by the anode-conductor boundary. In some examples, the cathode-conductor boundary surrounds a fuel port, protruding through the SOFC. 
     Also provided is a method of forming a SOFC. In some examples, the method comprises providing a subassembly comprising an anode-conductor porous portion, a cathode-conductor porous portion, and an electrolyte disposed between the anode-conductor porous portion and the cathode-conductor porous portion. The method continues with forming an anode-conductor non-porous portion from a part of the anode-conductor porous portion, wherein forming the anode-conductor non-porous portion also forms an anode-conductor cavity. In some examples, the method also comprises forming an anode-conductor seal within the anode-conductor cavity. For example, forming the anode-conductor non-porous portion can comprise one or more techniques selected from the group (1) selective mechanical compression of the part of the anode-conductor porous portion, and (2) selective melting of the part of the anode-conductor porous portion. 
     In some examples, selective melting of the part of the anode-conductor porous portion is performed using a laser. 
     In some examples, forming the anode-conductor seal comprises filling the anode-conductor cavity with a seal-precursor material and melting the seal-precursor material within the anode-conductor cavity. 
     In some examples, the method further comprises forming a cathode-conductor non-porous portion from a part of the cathode-conductor porous portion. This operation also forms a cathode-conductor cavity. The method also comprises forming a cathode-conductor seal within the cathode-conductor cavity. 
     These and other embodiments are described further below with reference to the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a schematic exploded view of a SOFC assembly, illustrating various components of this assembly, in accordance with some examples. 
         FIG.  1 B  is a top schematic view of the SOFC in the SOFC assembly (in  FIG.  1 A ) further illustrating fuel and oxidant ports protruding through this SOFC, in accordance with some examples. 
         FIGS.  2 A and  2 B  are schematic cross-sectional views of two examples of a SOFC assembly, illustrating various internal components. 
         FIG.  2 C  is an expanded view of a portion of the SOFC assembly in  FIG.  2 B  illustrating an anode-conductor sealing stack, extending through the anode conductor, in accordance with some examples. 
         FIG.  2 D  is a top schematic view of the SOFC in the SOFC assembly (in  FIGS.  2 B and  2 C ) further illustrating the boundary formed by the anode-conductor sealing stack, in accordance with some examples. 
         FIG.  3 A  is a top schematic view expanded view of the cathode conductor of a SOFC, in accordance with some examples. 
         FIG.  3 B  is a side cross-sectional view of the SOFC active component comprising the anode conductor, illustrated in  FIG.  3 A , and the cathode conductor, illustrated in  FIG.  3 B , in accordance with some examples. 
         FIG.  4    is a process flowchart corresponding to a method of forming a SOFC, in accordance with some examples. 
         FIGS.  5 A- 5 E  are schematic cross-sectional views of different stages during the method of forming the SOFC in  FIG.  4   . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are outlined to provide a thorough understanding of the presented concepts. The presented concepts may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail to not unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific embodiments, it will be understood that these embodiments are not intended to be limiting. 
     Introduction 
     A metal-supported SOFC utilizes porous metal layers for fuel and oxidant delivery to the electrolyte. The electrolyte comprises an anode layer, a cathode layer, and an electrolyte layer, disposed between the anode and cathode layers. The anode and cathode layers comprise catalysts, enabling the electrochemical reaction to produce electrical current. The electrolyte layer transmits ions between the anode and cathode layers thereby balancing the electrical current. In addition to delivering the fuel and oxidant, the porous metal layers are also responsible for conducting this electrical current between the electrolyte and interconnecting plates. The porous metal layers can be referred to as an anode conductor and cathode conductor. Specifically, the anode conductor directly interfaces as well as fluidically and electronically coupled to the anode layer of the electrolyte. The anode conductor is responsible for fuel delivery to the anode layer. The cathode conductor directly interfaces as well as fluidically and electronically coupled to the cathode layer of the electrolyte. The cathode conductor is responsible for oxidant delivery to the cathode layer. 
     Uniform delivery of the fuel and oxidant as well as uniform distribution of the fuel and oxidant at the responsive layers is beneficial for the cell performance and allows for efficient utilization of the entire surface of the electrolyte. At the same time, direct mixing of the fuel and oxidant is not desirable. Considering the porous structure of the anode and anode conductors, controlled distribution can be challenging. Porous structures are specifically used to enable the fuel and oxidant flow through these structures. However, the same porous structure is exposed to both fuel and oxidant (e.g., bypassing both fuel and oxidant ports through this structure), fuel-oxidant mixing is possible if the distribution is not specifically controlled. 
     SOFCs described herein include anode-conductor seals and/or cathode-conductor seals to eliminate the fuel-oxidant mixing. Specifically, an anode-conductor seal forms a sealing stack together with a non-porous portion of the anode-conductor. This non-porous portion can be formed by selective mechanical compression and/or selective melting of the initial porous structure. The remaining parts of this initial porous structure, which are not compressed or melted, remain porous and can be referred to as anode-conductor porous portions. Therefore, the sealing stack extends between two adjacent porous portions and prevents fuel and/or oxidant transfer between these porous portions. Sealing of the cathode conductor can be performed similarly. 
       FIG.  1 A  is a schematic exploded view of one example of SOFC assembly  100 , illustrating various components of this assembly. Specifically, SOFC assembly  100  comprises one or more SOFCs  102 . Each SOFC  102  is positioned between two interconnecting plates  106 , collectively forming SOFC stack  104 . It should be noted that, when stacked, two adjacent SOFC stacks  104  share interconnecting plate  106 . In other words, interconnecting plate  106  is positioned between and interconnects two SOFCs  102 . In this example, all SOFCs  102  in SOFC assembly  100  are connected in series. 
     In some examples, SOFC assembly  100  comprises flow plates  107 , used for spacing other components and providing pathways for fuel and oxygen to SOFCs  102 . Flow plates  107  can be standalone components or be integrated with interconnecting plate  106 . It should be noted that away from the edge, interconnecting plate  106  and SOFCs  102  directly contact each other and are electrically interconnected. It should be noted that SOFCs  102  can protrude to and form a portion of the sides of SOFC assembly  100 . In other words, at least the edges of SOFCs  102  can be exposed to the environment. It should be also noted that each SOFC  102  comprises porous metal layers forming opposite sides of this SOFC  102  as further described below with reference to  FIGS.  2 A- 2 C . Finally, SOFCs  102  and interconnecting plates  106  can be stacked between first end-plate current-collector  101  and second end-plate current-collector  109 , defining the top-bottom boundaries of SOFC assembly  100 . First end-plate current-collector  101  and second end-plate current-collector  109  can be electrically coupled to an external electrical load, which SOFC assembly  100  powers. 
     Referring to  FIGS.  1 A and  1 B , in some examples, SOFC assembly  100  also comprises one or more fuel ports  190  and one or more oxidant ports  192  for supplying fuel and oxidant to each SOFC  102 . In this example, SOFC assembly  100  comprises two fuel ports  190  (positioned along the shorter sides) and four oxidant ports  192  (positioned in the corners). However, any number of ports and any positions of these ports are within the scope. Fuel ports  190  and oxidant ports  192  protrude through the entire stack, e.g., each SOFC  102  and interconnecting plate  106 .  FIG.  16    illustrates the position of fuel ports  190  and oxidant ports  192  in SOFC  102 . It should be noted that each side of each SOFC  102  (formed by porous metal layers) can be exposed to both fuel and oxidant (e.g., extend to one or more fuel ports  190  and one or more oxidant ports  192 ). Without controlled distribution of the fuel and oxidant within SOFC  102 , direct mixing of the fuel and oxidant can occur. The controlled distribution will now be described with reference to  FIGS.  2 A and  2 B , roughly representing the “A-A” cross-section identified in  FIG.  16   . 
     Specifically,  FIGS.  2 A and  2 B  illustrate two examples of SOFC assembly  100 . In these examples, SOFC assembly  100  comprises two SOFCs  102 . However, one having ordinary skill in the art would understand that any number of SOFCs  102  can form SOFC assembly  100 , e.g., one, two, three, four, or more. Each SOFC  102  comprises anode conductor  110 , cathode conductor  150 , and electrolyte  130  disposed between anode conductor  110  and cathode conductor  150 . Electrolyte  130  is fluidically and electrically coupled to each of the anode conductor  110  and cathode conductor  150 . During operation, the fuel is flown to the anode side of SOFC  102 , while oxidant (e.g., air) is flown to the cathode side. Anode conductor  110  is a porous metal structure, which allows the fuel to pass through anode conductor  110  and reach electrolyte  130 . Similarly, cathode conductor  150  is also a porous metal structure, which allows the oxidant to pass through cathode conductor  150  and to reach electrolyte  130 . Electrolyte  130  is configured such the fuel and oxidant electrochemically react upon contacting electrolyte  130  and produce an electrical current between anode conductor  110  and cathode conductor  150 . This electrical current is carried by interconnecting plate  106  to another SOFC  102  and/or external connectors. 
       FIGS.  2 A and  2 B  also illustrate oxidant ports  192  extending through SOFC assembly  100  and each SOFC  102 . More specifically, oxidant ports  192  extend anode conductor  110  and cathode conductor  150  of each SOFC  102 . Therefore, both anode conductor  110  and cathode conductor  150  of each SOFC  102  are exposed to oxidant port  192 . As noted above, anode conductor  110  is responsible for delivering the fuel to electrolyte  130 . As such, without the flow control, the oxidant from oxidant port  192  can mix with the fuel, that is believed delivered by and is provided in anode conductor  110 . While fuel port  190  is not shown in these cross-sectional views, one having ordinary skill in the art would understand that similar mixing can occur between the fuel (from fuel port  190 ) and the oxidant in cathode conductor  150 . Direct fuel-oxidant mixing can lower the cell performance/power, reduce the overall efficiency, and cause localized fuel auto-ignition, all of which are not desirable. 
     While this disclosure focuses on sealing porous structures in SOFC  102 , one having ordinary skill in the art would understand that this sealing approach can be used in any sealing structures, regardless of the application of these sealing structures. 
     Examples of Solid Oxide Fuel Cells with Porous Sealing 
     Referring to  FIGS.  2 B- 2 D , fuel-oxidant mixing can be mitigated by positioning anode-conductor seal  120  within anode conductor  110 . Furthermore, additional mitigation can be achieved by a cathode-conductor seal which is further described below with reference to  FIGS.  3 A and  3 B . These anode-conductor and cathode-conductor seals are designed to fluidically isolate different zones of anode conductor  110  and cathode conductor  150 . In some of these zones, the fuel is allowed while the oxidant is not allowed. In other zones, the oxidant is allowed while and the fuel is not allowed. Anode-conductor seal  120  will be now described with reference to  FIGS.  2 C and  2 D . 
     Referring to  FIG.  2 C , anode conductor  110  comprises anode-conductor porous portions  114  and anode-conductor non-porous portion  116 . In some examples, anode conductor  110  comprises multiple anode-conductor non-porous portions  116 . Each of anode-conductor porous portions  114  is a porous structure that is permeable to the fuel. It should be noted that anode-conductor porous portions  114  are also permeable to the oxidant. However, at least one of anode-conductor porous portions  114  is protected to the oxidant and is used to supply fuel from fuel port  190  to electrolyte assembly  130 . Another one of anode-conductor porous portions  114  can extend to oxidant port  192  and is fluidically isolated from the fuel-carrying portion. Anode-conductor non-porous portion  116  is positioned between these two types of anode-conductor porous portions  114 . 
     Anode-conductor porous portions  114  is a metal porous structure, formed from stainless steel, such 430 stainless steel alloy, 434 stainless steel alloy, Fr—Cr alloys (e.g., with various additives), and the like. In some examples, the porosity of anode-conductor porous portions  114  is between 20% and 60% or, more specifically, between 30 and 55%. The electronic conduction and the material transfer are both impacted by the current collector porosity. In some examples, anode-conductor porous portions  114  comprise a sintering-control agent, such as doped zirconia (X—ZrO 2 , wherein X can be yttrium (Y), scandium (Sc), cerium (Ce), and/or calcium (Ca)), alumina (Al 2 O 3 ), yttria (Y 2 O 3 ), calcium oxide (CaO), magnesium oxide (MgO). The amount of sintering-control agent can be between 0.1% by weight and 5% by weight or, more specifically, between 0.5% by weight and 2.5% by weight. The sintering-control agent helps to achieve finer metal grains, smaller pores, more uniform pore distribution, higher porosity. Overall, adding the sintering-control agent helps achieve higher flow rates of fuel, oxidant, and reaction products through the current collectors. 
     Anode-conductor non-porous portion  116  can be formed from a part of anode-conductor porous portion  114 . In other words, an initial metal porous structure is selectively processed (e.g., compressed, melted) to form anode-conductor non-porous portion  116 . The remaining unprocessed portions become anode-conductor porous portions  114 . As such, in some examples, anode-conductor non-porous portion  116  is monolithic with anode-conductor porous portions  114  or at least formed from the same material. However, the porosity of anode-conductor non-porous portion  116  is substantially lower than that of anode-conductor porous portions  114 . In some examples, anode-conductor non-porous portion  116  has a porosity of less than 10% or even less than 1%. This lower porosity ensures that anode-conductor non-porous portion  116  is impermeable to the fuel and oxidant. 
     Referring to  FIG.  2 C , anode-conductor non-porous portion  116  and anode-conductor seal  120  form sealing stack  122  that is impermeable to the fuel and the oxidant. In other words, each of anode-conductor non-porous portion  116  and anode-conductor seal  120  is impermeable to the fuel and the oxidant. Sealing stack  122  extends between two anode-conductor porous portions  114  and prevents the fuel and oxidant transfer between these portions. 
     Sealing stack  122  extends between electrolyte  130  and interconnecting plate  106 . Sealing stack  122  is sealed against electrolyte  130 , e.g., by the direct contact of anode-conductor non-porous portion  116  with electrolyte  130 . Furthermore, sealing stack  122  is sealed against interconnecting plate  106  by the direct contact of anode-conductor seal  120  with interconnecting plate  106 . It should be noted that neither electrolyte  130  nor interconnecting plate  106  are impermeable to the fuel and to the oxidant. Various examples of electrolyte  130  and interconnecting plate  106  will now be described. 
     As shown in  FIG.  2 C , electrolyte  130  comprises anode layer  132 , cathode layer  136 , and electrolyte layer  134  disposed between anode layer  132  and cathode layer  136 . In some examples, each of anode layer  132  and cathode layer  136  comprises a porous base with catalysts sites disposed within this porous base. The porous base can comprise yttria-stabilized zirconia (YSZ), which is a ceramic comprising zirconium dioxide (ZrO 2 ) and yttrium oxide (Y 2 O 3 ). Yttrium oxide helps to maintain zirconium dioxide in a cubic crystal structure over a wide temperature range. Other suitable additives in zirconia include, but are not limited to, scandium (Sc), ceria (CeO 2 ), and/or calcium. Other suitable materials for the porous base include, but are not limited to, ceria (CeO 2 ) dopes with gadolinium (Gd), samarium (Sm), lanthanum (La), calcium (La), and yttrium (Y). In some examples, the thickness of the porous base is anywhere from 5 micrometers to 40 micrometers or, more specifically, from 10 micrometers to 30 micrometers. The porosity of the porous base can be between 20% and 60% or, more specifically, between 30% and 50%. The pore size can be between 0.1 micrometers and 25 micrometers or, more specifically, between 0.5 micrometers and 20 micrometers. In some examples, the porous bases of anode layer  132  and cathode layer  136  have the same structure (e.g., the composition, thickness, and porosity). 
     In some examples, the cathode catalyst sites of cathode layer  136  comprise, but are not limited to, lanthanum strontium manganite (LSM with a general formula or La 1-x Sr x MnO 3 ), praseodymium oxide (e.g., Pr 2 O 3 , PrO 2 , Pr 6 O 11 ). A lanthanum strontium cobalt ferrite (LSCF with a general formula La x Sr 1-x Co y Fe 1-y O 3 ), and/or lanthanum strontium cobaltite (LSC, e.g., LaSrCoO 3 ). The material of the cathode catalyst sites is specifically selected to provide oxidant reduction. Furthermore, the cathode catalyst sites are at least partially responsible for the electronic conduction within cathode layer  136 . For example, LSM has a high electrical conductivity at higher temperatures (e.g., between about 100 S/cm and 500 S/cm at a temperature of 600° C. and 1000° C.). Furthermore, LSM does not react with YSZ, which helps with extending the operating lifetime of SOFC  102 . However, the ionic conductivity of LSM is low, which limits the activity of the cathode catalyst sites (e.g., to a triple-phase boundary). In some examples, a combination of LSM and YSZ is used as cathode catalyst sites to increase the size of this triple-phase boundary. In the same or other examples, the catalyst sites of anode layer  132  comprise nickel. The material of the anode catalyst sites is specifically selected to stimulate electrochemical fuel oxidation. Furthermore, the anode catalyst sites are at least partially responsible for the electronic conduction within anode layer  132 . 
     In some examples, electrolyte layer  134  is formed from ZrO and/CeO, e.g., doped with one or more of Sm, Y, Sc, Gd, Al, and/or La. The porosity of electrolyte layer  134  is less than 5% or even less than 1%. 
     In some examples, interconnecting plate  106  comprises are formed from stainless steel or other suitable materials. A portion of interconnecting plate  106  can have out-of-plane protrusions to form better direct mechanical and electrical contacts with adjacent SOFCs  102  or, more specifically, with anode conductor  110  of one SOFC  102  and with cathode conductor  150  of another SOFC  102 . For example, out-of-plane protrusions can have a wave profile as, e.g., is schematically shown in  FIGS.  2 A and  2 B . 
     Referring to  FIG.  2 C , anode conductor  110  comprises anode-conductor first side  111  and anode-conductor second side  112 . Anode-conductor second side  112  is opposite of anode-conductor first side  111  and directly interfaces electrolyte  130  or, more specifically, anode layer  132  of electrolyte  130 . Anode-conductor first side  111  is formed entirely by anode-conductor porous portions  114 . Anode-conductor second side  112  is formed by both anode-conductor porous portions  114  and anode-conductor non-porous portions  116 . 
     In some examples, a portion of anode-conductor seal  120  extends over anode-conductor first side  111 . This portion provides additional sealing around anode-conductor sealing stack  122  and helps with sealing against corresponding interconnecting plate  106 . It should be noted that this portion of anode-conductor seal  120  extends over a small portion of anode-conductor first side  111 , while the remaining portion anode-conductor first side  111  is free from anode-conductor seal  120  and forms a direct mechanical and electrical contact with interconnecting plate  106 . In some examples, the size of the portion of anode-conductor first side  111  covered with conductor seal  120  is between 0.5% and 10% or, more specifically, between 1% and 5% of the total surface of anode-conductor first side  111 . 
     Referring to  FIG.  2 C , in some examples, the height of anode-conductor seal  120  is greater than the height of anode-conductor non-porous portion  116  in anode-conductor sealing stack  122 . In general, anode-conductor seal  120  covers between 20% and 80% or, more specifically, between 30% and 70% of the total height of anode-conductor sealing stack  122 . These height proportions depend on the porosity of anode-conductor porous portion  114  and also on the porosity of anode-conductor non-porous portions  116 . For example, anode-conductor non-porous portions  116  can be formed by condensing (e.g., compressing, melting) a part of the continuous porous structure, the remaining parts of which become anode-conductor porous portions  114 . Various porosity examples are described above. 
     Referring to  FIG.  2 C , in some examples, anode-conductor non-porous portion  116  extends to and directly interfaces electrolyte  130 . Therefore, anode-conductor non-porous portion  116  forms a portion of the overall seal at the interface with electrolyte  130 . The position of anode-conductor non-porous portion  116  in anode-conductor sealing stack  122  is determined by the order of manufacturing operations. For example, anode-conductor non-porous portion  116  can be formed from a continuous porous structure after this structure has been stacked with electrolyte  130 . In other words, electrolyte  130  already interfaces anode-conductor second side  112 , prior to forming anode-conductor non-porous portion  116 . However, anode-conductor first side  111  is exposed and available for processing. As further described with reference to  FIG.  4   , a cavity is formed in anode-conductor first side  111  by selectively melting or compressing a portion of this continuous porous structure. Anode-conductor non-porous portion  116  is positioned under this cavity. Anode-conductor seal  120  is later formed in this cavity. 
     Referring to  FIGS.  2 B and  2 C , in some examples, at least a portion of anode-conductor porous portions  114  of anode conductor  110  directly interfaces and is electrically coupled to the first one of interconnecting plates  106 . Similarly, at least a portion of cathode-conductor porous portions  154  of cathode conductor  150  directly interfaces and is electrically coupled to the second one of interconnecting plates  106 . As described above, interconnecting plates  106  can have various out-of-plate features to enhance and maintain this electrical coupling. In some examples, anode-conductor seal  120  extends to and is sealed against the first one of interconnecting plates  106 . 
     Referring to  FIG.  2 D , in some examples, anode-conductor seal  120  or, more specifically, anode-conductor sealing stack  122  (formed by anode-conductor seal  120  and anode-conductor non-porous portion  116 ) forms a boundary around one of anode-conductor non-porous portions  116 . This boundary may be referred to as an anode-conductor boundary to differentiate from a cathode-conductor boundary. Specifically,  FIG.  2 D  is a planar schematic view of one side of SOFC  102  showing anode conductor  110 . Cathode conductor  150  is positioned below and is not visible in  FIG.  2 D . Anode conductor  110  comprises anode-conductor porous portions  114 , such as first anode-conductor porous portion  114   a  and second anode-conductor porous portion  114   b . First anode-conductor porous portion  114   a  is surrounded by anode-conductor sealing stack  122 , formed by anode-conductor seal  120  and anode-conductor non-porous portion  116 . First anode-conductor porous portion  114   a  is fluidically coupled to fuel ports  190 . The fuel is received (from fuel ports  190 ) and distributed by first anode-conductor porous portion  114   a . However, the fuel is restricted to first anode-conductor porous portion  114   a  and cannot pass through anode-conductor sealing stack  122 . As such, second anode-conductor porous portion  114   b , being fluidically isolated from first anode-conductor porous portion  114   a , can be exposed to the oxidant without risk of direct fuel-oxidant mixing. Furthermore, the fuel is not able to reach the edges of SOFC  102  and leak into the rest other parts of SOFC assembly  100  or the environment. As shown in  FIG.  2 D , second anode-conductor porous portion  114   b  surround oxidant ports  192 . This configuration of anode-conductor sealing stack  122  simplifies manufacturing of anode-conductor sealing stack  122  and provides new routing options for the fuel and oxidant. 
     Referring to  FIGS.  3 A and  3 B , in some examples, SOFC  102  further comprising cathode-conductor seal  160 . Specifically, cathode conductor  150  comprises cathode-conductor non-porous portion  156 . Cathode-conductor non-porous portion  156  and cathode-conductor seal  160  form cathode-conductor sealing stack  162  that is impermeable to the fuel and/or the air. Various aspects of anode-conductor seal  120 , described above, apply to cathode-conductor seal  160 . Specifically,  FIG.  3 A  is a planar schematic view of one side of SOFC  102  showing cathode conductor  150  is (opposite to the side shown in  FIG.  2 D ). In this view, anode conductor  110  is positioned below and is not visible. Specifically, cathode conductor  150  comprises cathode-conductor porous portions  154 , such as first cathode-conductor porous portion  154   a , second cathode-conductor porous portion  154   b , and third cathode-conductor porous portion  154   c . Each of second cathode-conductor porous portion  154   b  and third cathode-conductor porous portion  154   c  is surrounded by cathode-conductor sealing stack  162 , formed by cathode-conductor seal  160  and cathode-conductor non-porous portion  156 . As such, first cathode-conductor porous portion  154   a  is fluidically isolated from each of second cathode-conductor porous portion  154   b  and third cathode-conductor porous portion  154   c . First cathode-conductor porous portion  154   a  is fluidically coupled to oxidant ports  192 . The oxidant is received (from oxidant ports  192 ) and distributed by first cathode-conductor porous portion  154   a . However, the oxidant is restricted from going to second cathode-conductor porous portion  154   b  and third cathode-conductor porous portion  154   c . As such second cathode-conductor porous portion  154   b  and third cathode-conductor porous portion  154   c , being fluidically isolated from first cathode-conductor porous portion  154   a , can be exposed to the fuel without risk of direct fuel-oxidant mixing. As shown in  FIG.  3 A , each second cathode-conductor porous portion  154   b  and third cathode-conductor porous portion  154   c  surrounds one of fuel ports  190 . 
     Referring to  FIG.  3 B , in some examples, cathode-conductor seal  160  is laterally offset relative to anode-conductor seal  120 . This offset helps to preserve the overall integrity of SOFC  102  and provide support to electrolyte  130 . Specifically, electrolyte  130  is much thinner than each of anode conductor  110  and cathode conductor  150 . Anode conductor  110  and cathode conductor  150  provide mechanical primary mechanical support to the overall SOFC  102 . When cathode-conductor seal  160  protrudes through cathode conductor  150 , cathode conductor  150  is weakened at this point. Similarly, when anode-conductor seal  120  protrudes through anode conductor  110 , anode conductor  110  is weakened at this point. The offset between cathode-conductor seal  160  and anode-conductor seal  120  helps to space apart these weakened portions. 
     In some examples, the offset between cathode-conductor seal  160  and anode-conductor seal  120  is at least 1 millimeter or, more specifically, at least 2 millimeters. For comparison, the thickness of each anode conductor  110  and cathode conductor  150  is between 5 micrometers and 100 micrometers or, more specifically, between about 10 micrometers and 50 micrometers. The thickness of electrolyte  130  is between 5 micrometers and 100 micrometers or, more specifically, between about 10 micrometers and 50 micrometers. 
     Examples of Methods of Forming Solid Oxide Fuel Cell Stacks 
       FIG.  4    is a process flowchart corresponding to method  400  of forming SOFC  102 , in accordance with some examples. Various examples of SOFC  102  are described below. SOFC  102  comprises anode conductor  110 , cathode conductor  150 , and electrolyte assembly  130  positioned between anode conductor  110  and cathode conductor  150 . Furthermore, SOFC  102  comprises one or more anode-conductor seals  120  and/or one or more cathode-conductor seals  160 , used for controlling the flow of fuel and oxidant within anode conductor  110  and cathode conductor  150 . 
     Method  400  comprises (block  410 ) providing subassembly  500  comprising anode-conductor porous portion  114 , cathode-conductor porous portion  154 , and electrolyte  130 . Electrolyte  130  is disposed between anode-conductor porous portion  114  and cathode-conductor porous portion  154  as, e.g., schematically shown in  FIG.  5 A . At this stage, each of anode-conductor porous portion  114  and cathode-conductor porous portion  154  is a continuous porous structure. During later operations, one or more selected parts of anode-conductor porous portion  114  are converted into anode-conductor non-porous portions  116 . These anode-conductor non-porous portions  116  separate remaining anode-conductor porous portions  114 . Similarly, one or more selected parts of cathode-conductor porous portion  154  can be converted into cathode-conductor non-porous portions  156 . These cathode-conductor non-porous portions  156  separate remaining cathode-conductor porous portions  154 . 
     Method  400  proceeds with (block  420 ) forming one or more anode-conductor non-porous portions  116  from selected parts of anode-conductor porous portion  114 . Forming anode-conductor non-porous portions  116  also forms anode-conductor cavity  117  as, e.g., is schematically shown in  FIG.  5 B . For example, anode-conductor non-porous portions  116  can be formed using one or more techniques selected from the group: (1) selective mechanical compression of the part of anode-conductor porous portion  114  and (2) selective melting of the part of anode-conductor porous portion  114 . The location of anode-conductor non-porous portions  116  is described above with reference to  FIG.  1 B . 
     In some examples, method  400  also comprises (block  440 ) forming one or more cathode-conductor non-porous portions  156  from selected parts of cathode-conductor porous portion  154 . Forming cathode-conductor non-porous portions  156  also forms cathode-conductor cavity  157  as, e.g., is schematically shown in  FIG.  5 B . For example, cathode-conductor non-porous portions  156  can be formed using one or more techniques described above with reference to forming anode-conductor non-porous portions  116 . In some examples, anode-conductor non-porous portions  116  and cathode-conductor non-porous portions  156  are formed in the same operation or even simultaneously. When both anode-conductor non-porous portions  116  and cathode-conductor non-porous portions  156  are formed in the same subassembly  500 , two adjacent ones of adjacent anode-conductor non-porous portion  116  and cathode-conductor non-porous portion  156  are offset relative to each other as described above with reference to  FIG.  3 B . 
     Method  400  proceeds with (block  430 ) forming anode-conductor seal  120  within anode-conductor cavity  117 . For example, this anode-conductor seal forming operation comprises (block  432 ) filling anode-conductor cavity  117  with seal-precursor material  510  as, e.g., is schematically shown in  FIG.  5 C , and (block  434 ) melting seal-precursor material  510  within anode-conductor cavity  117  as, e.g., is schematically shown in  FIG.  5 D . Various examples of anode-conductor seal  120  are described above. In some examples, melting seal-precursor material  510  within anode-conductor cavity  117  can be performed during the overall processing (e.g., sintering) of SOFC assembly  100 . 
     In some examples, method  400  proceeds with (block  450 ) forming cathode-conductor seal  160 . In these examples, cathode-conductor seal  160  can be formed in a manner similar to forming anode-conductor seal  120 . In more specific examples, cathode-conductor seal  160  and anode-conductor seal  120  can be formed in the same operation. 
     In some examples, method  400  proceeds with (block  460 ) with positioning interconnecting plates  106  on the opposite sides of SOFC  102 . One interconnecting plate  106  can come in direct mechanical and electrical contact with anode-conductor porous portion  114  and also sealed against anode-conductor seal  120 . The other interconnecting plate  106  can come in direct mechanical and electrical contact with cathode-conductor porous portion  154  and also sealed against cathode-conductor seal  160 . In some examples, interconnecting plates  106  are positioned prior to melting the seal-precursor material (block  534 ) and are used to support the seal-precursor material within corresponding cavities. 
     CONCLUSION 
     Although the foregoing concepts have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatuses. Accordingly, the present embodiments are to be considered illustrative and not restrictive.