Patent Publication Number: US-2023141938-A1

Title: Solid oxide electrolyzer cell including electrolysis-tolerant air-side electrode

Description:
The present disclosure is directed generally to solid oxide electrolyzer cells, and more specifically, to electrolyzer cells including electrolysis-tolerant air-side electrodes. 
     BACKGROUND 
     Solid oxide reversible fuel cell (SORFC) systems may be operated in a fuel cell mode to generate electricity by oxidizing a fuel. SORFC systems may also be operated in an electrolysis mode to generate hydrogen by electrolyzing water. However, prior art SORFCs may suffer from air-side electrode degradation due to cell voltage increases that may occur during the electrolysis process. 
     SUMMARY 
     According to various embodiments a solid oxide electrolyzer cell (SOEC) comprises a solid oxide electrolyte, a fuel-side electrode disposed on a fuel side of the electrolyte, and an air-side electrode disposed on an air side of the electrolyte. The air-side electrode comprises a barrier layer disposed on the air side of the electrolyte and comprising a first doped ceria material, and a functional layer disposed on the barrier layer and comprising an electrically conductive material and a second doped ceria material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a perspective view of a SOEC stack, according to various embodiments of the present disclosure. 
         FIG.  1 B  is a cross-sectional view of a portion of the stack of  FIG.  1 A . 
         FIG.  2 A  is a plan view of an air side of an interconnect, according to various embodiments of the present disclosure. 
         FIG.  2 B  is a plan view of a fuel side of the interconnect of  FIG.  2 A . 
         FIG.  3 A  is a plan view of an air side of a SOEC cell, according to various embodiments of the present disclosure. 
         FIG.  3 B  is a plan view of a fuel side of the SOEC cell of  FIG.  3 A . 
         FIG.  4    is a photograph showing air electrode delamination. 
         FIG.  5    is a cross-sectional view of a SOEC stack including an electrolysis-tolerant SOEC cell, according to various embodiments of the present disclosure. 
         FIG.  6 A  is a chart showing the degradation rate of an air electrode of an SOEC cell according to various embodiments of the present disclosure. 
         FIG.  6 B  is a chart showing the degradation rate of a comparative SOEC cell. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims. 
     It will be understood that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or directly connected to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present. It will be understood that for the purposes of this disclosure, “at least one of X, Y, and Z” can be construed as X only, Y only, Z only, or any combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ, ZZ). 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. It will also be understood that the term “about” may refer to a minor measurement errors of, for example, 5 to 10%. In addition, weight percentages (wt. %) and atomic percentages (at. %) as used herein respectively refer to a percent of total weight or a percent of a total number of atoms of a corresponding composition. 
     Words such as “thereafter,” “then,” “next,” etc. are not necessarily intended to limit the order of the steps; these words may be used to guide the reader through the description of the methods. Further, any reference to claim elements in the singular, for example, using the articles “a,” “an” or “the” is not to be construed as limiting the element to the singular. 
     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. 
       FIG.  1 A  is a perspective view of an electrolyzer cell stack  100 , and  FIG.  1 B  is a sectional view of a portion of the stack  100 , according to various embodiments of the present disclosure. Referring to  FIGS.  1 A and  1 B , the stack  100  may be a solid oxide electrolyzer cell (SOEC) stack that includes solid oxide electrolyzer cells  1  separated by interconnects  10 . Referring to  FIG.  1 B , each electrolyzer cell  1  comprises an air-side electrode  3 , a solid oxide electrolyte  5 , and a fuel-side electrode  7 . 
     Electrolyzer cell stacks are frequently built from a multiplicity of electrolyzer cells  1  in the form of planar elements, tubes, or other geometries. Although the electrolyzer cell stack  100  in  FIG.  1 A  is vertically oriented, electrolyzer cell stacks may be oriented horizontally or in any other direction. For example, water may be provided through water conduits  22  (e.g., water riser openings) formed in each interconnect  10  and electrolyzer cell  1 , while oxygen may be provided from the side of the stack between air side ribs of the interconnects  10 . 
     Each interconnect  10  electrically connects adjacent electrolyzer cells  1  in the stack  100 . In particular, an interconnect  10  may electrically connect the fuel-side electrode  7  of one electrolyzer cell  1  to the air-side electrode  3  of an adjacent electrolyzer cell  1 .  FIG.  1 B  shows that the lower electrolyzer cell  1  is located between two interconnects  10 . A Ni mesh (not shown) may be used to electrically connect the interconnect  10  to the fuel-side electrode  7  of an adjacent electrolyzer cell  1 . 
     Each interconnect  10  includes fuel-side ribs  12 A that at least partially define fuel channels  8 A and air-side ribs  12 B that at least partially define oxidant (e.g., air) channels  8 B. The interconnect  10  may operate as a separator that separates water flowing to the fuel-side electrode of one cell  1  in the stack from oxygen flowing from the air-side electrode of an adjacent cell  1  in the stack. At either end of the stack  100 , there may be an air end plate or fuel end plate (not shown). 
     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 in the cells (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), and may electrically connect the fuel-side electrode  7  of one electrolyzer cell  1  to the air-side electrode  3  of an adjacent electrolyzer cell  1 . 
       FIG.  2 A  is a top view of the air side of the interconnect  10 , and  FIG.  2 B  is a top view of a fuel side of the interconnect  10 , according to various embodiments of the present disclosure. Referring to  FIGS.  1 B and  2 A , the air side includes the air channels  8 B that extend from opposing first and second edges of the interconnect  10 . Oxygen flows through the air channels  8 B from the air-side electrode  3  of an adjacent electrolyzer cell  1 . Ring seals  20  may surround fuel holes  22 A,  22 B of the interconnect  10 , to prevent water from contacting the air-side electrode  3 . Strip-shaped peripheral seals  24  are located on peripheral portions of the air side of the interconnect  10 . The seals  20 ,  24  may be formed of a glass or glass-ceramic material. The peripheral portions may be an elevated plateau which does not include ribs or channels. The surface of the peripheral regions may be coplanar with tops of the ribs  12 B. 
     Referring to  FIGS.  1 B and  2 B , the fuel side of the interconnect  10  may include the fuel channels  8 A and fuel manifolds  28 . Water flows from one of the fuel holes  22 A (e.g., inlet fuel hole that forms part of the fuel inlet riser), into the adjacent manifold  28 , through the fuel channels  8 A, and to the fuel-side electrode  7  of an adjacent electrolyzer cell  1 . Excess water may flow into the other fuel manifold  28  and then into the outlet fuel hole  22 B. A frame seal  26  is disposed on a peripheral region of the fuel side of the interconnect  10 . The peripheral region may be an elevated plateau which does not include ribs or channels. The surface of the peripheral region may be coplanar with tops of the ribs  12 A. 
       FIG.  3 A  is a plan view of the air side of the electrolyzer cell  1 , and  FIG.  3 B  is a plan view of the fuel side of the electrolyzer cell  1 , according to various embodiments of the present disclosure. Referring to  FIGS.  1 A,  2 A,  3 A, and  3 B , the electrolyzer cell  1  may include an inlet fuel hole  22 A, an outlet fuel hole  22 B, the electrolyte  5 , and the air-side electrode  3 . The air-side electrode  3  may be disposed on the air side of the electrolyte  5 . The fuel-side electrode  7  may be disposed on an opposing fuel (e.g., water) side of the electrolyte  5 . 
     The fuel holes  22 A,  22 B may extend through the electrolyte  5  and may be arranged to overlap with the fuel holes  22 A,  22 B of the interconnects  10 , when assembled in the electrolyzer cell stack  100 . The air-side electrode  3  may be printed on the electrolyte  5  so as not to overlap with the ring seals  20  and the peripheral seals  24  when assembled in the electrolyzer cell stack  100 . The fuel-side electrode  7  may have a similar shape as the air-side electrode  3 . The fuel-side electrode  7  may be disposed so as not to overlap with the frame seal  26 , when assembled in the stack  100 . In other words, the electrodes  3  and  7  may be recessed from the edges of the electrolyte  5 , such that corresponding edge regions of the electrolyte  5  may directly contact the corresponding seals  20 ,  24 ,  26 . 
     In one embodiment, the electrolyzer cell stack  100  may only be operated in the electrolysis mode. Thus the electrolyzer cell stack  100  is not operated in a fuel cell mode to generate power from a fuel and air provided to fuel-side and air-side electrodes, respectively. Alternatively, the electrolyzer cell stack  100  may comprise a solid oxide regenerative (i.e., reversible) fuel cell (SORFC) stack. SORFCs can be operated in a fuel cell (FC) mode (e.g., power generation mode), in order to generate electricity from fuel and air provided to fuel-side and air-side electrodes, respectively, and may be operated in an electrolyzer cell (EC) mode (e.g., electrolysis mode) in order to produce hydrogen and oxygen from water provided to the fuel-side electrode  7 . In the FC mode, oxygen ions are transported from the air-side (e.g., cathode) electrode  3  to the fuel-side (e.g., anode) electrode  7  of the SORFC to oxidize the fuel (e.g., hydrogen and/or hydrocarbon fuel, such as natural gas) and to generate electricity. In EC mode, a positive potential is applied to the air side of the cell, and the oxygen ions are transported from the water at the fuel-side electrode  7  through the electrolyte  5  to the air-side electrode  3 . Thus, water is electrolyzed into hydrogen at the fuel-side electrode  7  and oxygen at air-side electrode  3 . 
     The air-side electrode  3  and the fuel-side electrode  7  of a SORFC respectively operate as a cathode and an anode during FC mode, and respectively operate as an anode and a cathode during EC mode (i.e., a FC mode cathode is an EC mode anode, and a FC mode anode is an EC mode cathode). Accordingly, the SORFCs described herein may be referred to as having air-side electrodes and fuel-side electrodes. 
     During the EC mode, water in the fuel stream is reduced (H 2 O+2e→O 2   − +H 2 ) to form H 2  gas and O 2   −  ions, the O 2   −  ions are transported through the solid electrolyte, and then oxidized on the air-side electrode (O 2   −  oxidized to O 2 ) to produce molecular oxygen. Since the open circuit voltage for a SORFC operating with air and wet fuel (e.g., hydrogen and/or reformed natural gas) may be from about 0.9 to 1.0V (depending on water content), the positive voltage applied to the air-side electrode in EC mode increases the cell voltage to typical operating voltages of from about 1.1 to 1.3V. In constant current mode, the cell voltages may increase over time if there is degradation of the cell, which may result from both ohmic sources and electrode polarization. 
     One of the major hurdles encountered with state-of-the-art solid oxide electrolyzer cells and SORFCs is the delamination of the air electrode at high current densities. The degree of delamination increases with the current density and the flux of oxide ion transport. Without wishing to be bound by a particular theory, it is believed that the delamination may be caused by the precipitation of oxygen at the electrolyte/cathode interface, which can lead to high pressures resulting in air electrode delamination. 
       FIG.  4    is a photograph showing air electrode  3  delamination after operating a solid oxide electrolyzer cell in electrolysis mode for an extended time at a high current density. As shown in  FIG.  4   , the air-side electrode  3  may separate from the underlying electrolyte  5 , as indicated by the black area there between. 
       FIG.  5    is a cross-sectional view of an electrolyzer cell stack  500  including an electrolysis-tolerant solid oxide electrolyzer cell  502 , according to various embodiments of the present disclosure. The electrolyzer cell stack  500  is similar to the stack  100  of  FIGS.  1 A- 3 B . As such, only the differences there between will be discussed in detail. 
     Referring to  FIG.  5   , the electrolyzer cell stack  500  may include at least one electrolyzer cell  502  disposed between interconnects  10 . The electrolyzer cell  502  may operate only in the electrolysis mode (e.g., the cell may comprise a solid oxide electrolyzer cell (SOEC)), or may operate in both fuel cell and electrolysis modes (e.g., the electrolyzer cell  502  may comprise a SORFC). The electrolyzer cell  502  includes a solid oxide electrolyte  5 , an air-side electrode  3  disposed on an air side of the electrolyte  5 , and a fuel-side electrode  7  disposed on a fuel side of the electrolyte  5 . Air may be provided to the air-side electrode  3  by air channels  8 B in a fuel cell mode, and fuel may be provided to the fuel-side electrode  7  by fuel channels  8 A in the fuel cell mode, while water may be provided to the fuel-side electrode  7  by fuel channels  8 A in the electrolysis mode. 
     Various materials may be used for the solid oxide electrolyte  5 , the fuel-side electrode  7 , and air-side electrode  3 . In various embodiments, the electrolyte  5  may include an ionically conductive material or phase, such as a stabilized zirconia, for example 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. Alternatively, the electrolyte  5  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). In some embodiments, the electrolyte  5  may comprise a material 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 , wherein 0.09≤w≤0.11, 0&lt;x≤0.0125, a+b=z, and 0.0025≤z≤0.0125. In some embodiments, the electrolyte  5  may comprise (ZrO 2 ) 0.88 (Sc 2 O 3 ) 0.1 (CeO 2 ) 0.01 (Yb 2 O 3 ) 0.01  or (ZrO 2 ) 0.88 (Sc 2 O 3 ) 0.1 (CeO 2 ) 0.01 (Y 2 O 3 ) 0.01 . Alternatively, the electrolyte  5  may comprise (ZrO 2 ) 0.89 (Sc 2 O 3 ) 0.1 (CeO 2 ) 0.01 . 
     The fuel-side electrode  7  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-side electrode  7  may be annealed in a reducing atmosphere prior to operation of the electrolyzer cell  1 , to reduce the oxidized metal catalyst to a metallic state. 
     The metal-containing phase may consist entirely of nickel in a reduced state. This nickel-containing phase may form nickel oxide when it is in an oxidized state. Thus, the fuel-side electrode  7  is preferably annealed in a reducing atmosphere prior to operation to reduce the nickel oxide to nickel. 
     The ceramic phase of the fuel-side electrode  7  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. 
     Furthermore, if desired, an additional contact or current collector layer may be placed over the fuel-side electrodes  7 . For example, a Ni or nickel oxide anode contact layer may be formed on the fuel-side electrode  7 . 
     The air-side electrode  3  may include a barrier layer  30  disposed directly on an air side of the electrolyte  5 , a functional layer  32  disposed on the barrier layer  30 , and an optional current collector layer  34  disposed on the functional layer  32 . Thus, the functional layer  32  is located between the barrier layer  30  and the current collector layer  34 . 
     The barrier layer  30  may be sintered to the air side of the electrolyte  5 . The barrier layer  30  may comprise, consist essentially or, or consist of a doped ceria material. For example, the barrier layer may comprise from about 95 weight percent (wt. %) to about 100 wt. % of the doped ceria material, based on the total weight of the barrier layer  30 . The doped ceria material may include samarium-doped ceria (SDC) and/or gadolinium-doped ceria (GDC). 
     The SDC may be represented by the formula: Ce 1-x Sm x O 2-d , where x ranges from 0.1 to 0.3. For example, specific SDC materials may be represented by the formulas: Ce 0.8 Sm 0.2 O 2-d , Ce 0.9 Sm 0.1 O 2-d , and Ce 0.7 Sm 0.3 O 2-d , where d ranges from 0 to 0.2, such as from 0 to 0.1. 
     The GDC may be represented by the formula Ce 1-x Gd x O 2-d , where x ranges from 0.1 to 0.3 and d ranges from 0 to 0.2, such as from 0 to 0.1. For example, specific GDC materials may be represented by the formulas: Ce 0.9 Gd 0.1 O 2-d , Ce 0.8 Gd 0.2 O 2-d , and Ce 0.7 Gd 0.3 O 2-d , where d ranges from 0 to 0.2, such as from 0 to 0.1. 
     The functional layer  32  may include a mixture of an electrically conductive material and the doped ceria material. The electrically conductive material may comprise an electrically conductive perovskite material, such as lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium cobaltite (LSC), lanthanum strontium cobalt manganite (LSCM), lanthanum strontium ferrite (LSF), La 0.85 Sr 0.15 Cr 0.9 Ni 0.1 O 3  (LSCN), combinations thereof, or the like. In some embodiments, the electrically conductive material may preferably comprise LSM and/or LSCF. Alternatively, the electrically conductive material may comprise a metal, such as platinum. For example, the functional layer  32  may include from about 10 wt. % to about 90 wt. % of the electrically conductive material described above, and from about 10 wt. % to about 90 wt. % of the doped ceria material. 
     In various embodiments, the functional layer may include the LSM as the electrically conductive material. The LSM may be represented by the formula: (La 1-z Sr z ) q MnO 3-d , wherein z ranges from 0.1 to 0.4, q ranges from 0.94 to 1, such as 0.96 to 1, and d ranges from 0 to 0.2. For example, the LSM may comprise La 0.8 Sr 0.2 MnO 3-d  or A-site deficient LSM, such as (La 0.8 Sr 0.2 ) 0.98 MnO 3-d , wherein d ranges from 0 to 0.1. 
     In some embodiments, the functional layer may include the LSCF as the electrically conductive material. The LSCF may be represented by the formula: (La x Sr 1-x ) y Co z Fe 1-z O 3-δ , wherein x ranges from 0.4 to 0.8, y ranges from 0.94 to 1.0, z ranges from 0.01 to 0.99, and δ is the equilibrium oxygen deficiency which ranges from 0 to 0.1. For example, the LSCF may comprise La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ , (La 0.6 Sr 0.4 ) 0.98 Co 0.2 Fe 0.8 O 3-δ , or (La 0.6 Sr 0.4 ) 0.95 Co 0.2 Fe 0.8 O 3-δ , where δ is the equilibrium oxygen deficiency. 
     The barrier layer  30  and the functional layer  32  may include the same doped ceria material or different doped ceria materials. For example, the barrier layer  30  may include GDC and the functional layer  32  may include LSM and GDC, or LSM and SDC. In other embodiments, the barrier layer  30  may include SDC and the functional layer  32  may include LSM and SDC, or LSM and GDC. In other embodiments, the barrier layer may include SDC and the functional layer may include LSCF and SDC, or LSCF and GDC. 
     While not wishing to be bound by a particular theory, it is believed that the mixed oxide-ion and electronic conduction of the ceria phases of the barrier layer  30  reduces the overpotential at the interface between the barrier layer  30  and the functional layer  32 . The reduction in overpotential may suppress the air-side electrode  3  delamination from the electrolyte  5 . 
     The current collector layer  34  may include an electrically conductive material, such as an electrically conductive metal oxide, such as LSM. However, other conductive perovskites, such as LSC, LSCM, LSCF, LSF, LSCN, etc., or metals, such as Pt, may also be used. 
       FIG.  6 A  is a chart showing a voltage change of first embodiment SOEC cells and second embodiment SOEC cells, between beginning of life and 17 current cycles of operation in an embodiment SOEC stack.  FIG.  6 B  is a chart showing the voltage change of comparative SOEC cells between beginning of life and 17 current cycles of operation in a comparative SOEC stack, under similar conditions. The y-axis in both figures is the voltage change in volts, and the x-axis is a number of the SOEC cell in the respective stack. 
     Referring to  FIGS.  6 A and  6 B , the first and second embodiment SOEC cells have similar configurations, except that the first embodiment SOEC cells contain SDC barrier layers  30  and GDC/LSM cathode functional layers  32 , while the second embodiment SOEC cells contain SDC barrier layers  30  and SDC/LSM cathode functional layers  32 . The comparative SOEC cells include no barrier layers  30  and cathode functional layers that comprise YSZ/LSM. 
     Referring to  FIGS.  6 A and  6 B , higher cell voltage differences indicate higher cell over-potentials, and thus, higher cathode degradation. As can be seen in the charts, the comparative SOEC cells experience higher cell voltage differences (and thus an increase in cell over-potential), while the embodiment cells exhibit substantially lower cell voltage differences. Accordingly, the doped ceria-based barrier layer and cathode functional layer materials provide an unexpectedly improved protection against cell over-potential, which is expected to decrease delamination and/or general cathode degradation. 
     Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.