Patent Application
20220190373
The present disclosure is directed generally to solid oxide electrolyzer cells, and more specifically, to electrolyzer cells including electrolysis-tolerant air-side electrodes.
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.
According to various embodiments, a solid oxide electrolyzer cell (SOEC) includes 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 includes a barrier layer disposed on the air side of the electrolyte and containing a stabilized zirconia material having a lower electrical conductivity than an electrical conductivity of the electrolyte, and a functional layer disposed on the barrier layer.
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.
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
Various materials may be used for the air-side electrode 3, solid oxide electrolyte 5, and fuel-side electrode 7. For example, 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.
The solid oxide electrolyte 5 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. Alternatively, the electrolyte 5 may comprise another ionically conductive material, such as a samaria-doped ceria (SDC), gadolinia-doped ceria (GDC), or yttria-doped ceria (YDC).
The air-side electrode 3 may comprise a layer of 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), La0.85Sr0.15Cr0.9Ni0.1O3 (LSCN), etc., or metals, such as Pt, may also be used.
In some embodiments, the air-side electrode 3 may comprise a mixture of the electrically conductive material and an ionically conductive material. For example, the air-side electrode 3 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: (ZrO2)1-w-x-z(Sc2O3)w(CeO2)x(Y2O3)a(Yb2O3)b, wherein 0.09≤w≤0.11, 0<x≤0.0125, a+b=z, and 0.0025≤z≤0.0125. In some embodiments, 0.009≤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.
Furthermore, if desired, additional contact or current collector layers may be placed over the air-side electrode 3 and the fuel-side electrodes 7. For example, a Ni or nickel oxide anode contact layer and an LSM or LSCo cathode contact layer may be formed on the fuel-side electrode 7 and the air-side electrode 3, respectively.
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.
Each interconnect 10 includes fuel-side ribs 12A that at least partially define fuel channels 8A and air-side ribs 12B that at least partially define oxidant (e.g., air) channels 8B. 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.
Referring to
The fuel holes 22A, 22B may extend through the electrolyte 5 and may be arranged to overlap with the fuel holes 22A, 22B 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 (H2O+2e→O2−+H2) to form H2 gas and O2− ions, the O2− ions are transported through the solid electrolyte, and then oxidized on the air-side electrode (O2− oxidized to O2) 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.
Referring to
In various embodiments, the electrolyte 5 may include an ionically conductive material or phase, such as a stabilized zirconia material as described above, such as SSZ, YSZ, SCSZ, SCYSZ, SCYbSZ, or the like. Alternatively, the electrolyte 5 may comprise another ionically conductive material, such as doped ceria, including scandia, gadolinia or yttria doped ceria (i.e., SDC, GDC or YDC). In some embodiments, the electrolyte 5 may comprise a material represented by the formula:
(ZrO2)1-w-x-z(Sc2O3)w(CeO2)x(Y2O3)a(Yb2O3)b,
wherein 0.09≤w≤0.11, 0<x≤0.0125, a+b=z, and 0.0025≤z≤0.0125. In some embodiments, the electrolyte 5 may comprise (ZrO2)0.88(Sc2O3)0.1(CeO2)0.01(Yb2O3)0.01 or (ZrO2)0.88(Sc2O3)0.1(CeO2)0.01(Y2O3)0.01. Alternatively, the electrolyte 5 may comprise (ZrO2)0.89(Sc2O3)0.1(CeO2)0.01.
The air-side electrode 3 may include a barrier layer 30 disposed 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. The functional layer 32 may include a mixture of an electrically conductive material and an ionically conductive material. For example, the functional layer 32 may include from about 10 weight percent (wt %) to about 90 wt % of the electrically conductive material described above, (e.g., LSM, LSC, LSCM, LSCF, LSF, LSCN, Pt, etc.) and from about 10 wt % to about 90 wt % of the ionically conductive material. Suitable ionically conductive materials include zirconia-based based materials. For example, the ionically conductive material may comprise yttria-stabilized zirconia (YSZ) or scandia-stabilized zirconia (SSZ) including at least one of yttria and/or ytterbia and optionally ceria. In some embodiments, the ionically conductive material may be represented by the formula: (ZrO2)1-w-x-z(Sc2O3)w(CeO2)x(Y2O3)a(Yb2O3)b, wherein 0≤w≤0.11, 0≤x≤0.0125, a+b=z, and 0.0025≤z≤0.11. In some embodiments, 0≤x≤0.011 and 0.009≤z≤0.0125, and optionally, one of a or b may be equal to zero, if the other one of a or b is not equal to zero.
In some embodiments, the functional layer 32 may include a mixture of LSM and at least one of SSZ, YSZ, scandia-ceria-ytterbia-stabilized zirconia (SCYbSZ), scandia-ceria-yttria-stabilized zirconia (SCYSZ), scandia-yttria-stabilized zirconia (SYSZ) or scandia-ytterbia-stabilized zirconia (SYbSZ). For example, YSZ may include 8 to 11 at % Y2O3 and 89 to 92 at % ZrO2, such as about 8 at % Y2O3 and about 92 at % ZrO2 SYSZ may include about 10 at % Sc2O3, about 1 at % Y2O3, and about 89 at % ZrO2 SCYbSZ may include about 10 at % Sc2O3, about 1 at % CeO2, about 1 at % Yb2O3, and about 88 at % ZrO2.
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.
The barrier layer 30 may be sintered to the air-side of the electrolyte 5 and may include at least about 95 at % of an ionically conductive material, such as from about 97 at % to about 100 at %, or from about 98 at % to about 100 at % of an ionically conductive material. The barrier layer 30 may have a relatively high ionic conductivity and a relatively low electrical conductivity. For example, the barrier layer 30 may be free of, or contain no more than a trace amount of an electrically conductive material. For example, the barrier layer 30 may comprise less than 1 at %, such as from 0 to 0.5 at %, or from 0 to 0.25 at % of an electrically material, such as a metal or electrically conductive oxide, such LSM, LSC, LSCM, LSCF, LSF, and LSCN, and less than 1 at %, such as from 0 to 0.5 at %, or from 0 to 0.25 at % ceria.
In some embodiments, the barrier layer 30 may have a lower electric conductivity than the electrolyte 5. While not wishing to be bound to any particular theory, the present inventors believe that such an electrical conductivity difference may operate to prevent and/or reduce an over-potential (e.g., increase in cell voltage) when the electrolyzer cell 500 is operated in EC mode. It is believed that preventing and/or reducing such a cell over-potential reduces and/or prevents delamination of the air-side electrode 3 during EC operation.
In some embodiments, the barrier layer 30 may include a stabilized or partially stabilized zirconia (ZrO2) material, such as a rare earth stabilized (e.g., doped) zirconia, such as scandia (Sc2O3) stabilized zirconia (SSZ), a yttria (Y2O3) stabilized zirconia (YSZ), and/or ytterbia (Yb2O3) stabilized zirconia (YbSZ). In various embodiments, the barrier layer 30 may include zirconia stabilized with any combination of yttria, ytterbia, and/or scandia. For example, the barrier layer 30 may include scandia-yttria-stabilized zirconia (SYSZ) or scandia-ytterbia-stabilized zirconia (SYbSZ). In other embodiments, the barrier layer 30 may include zirconia stabilized or doped with Mg, Ca, La, and/or oxides thereof.
In some embodiments, the barrier layer 30 may include a YSZ material represented by the formula: (ZrO2)1-x(Y2O3)x, wherein x ranges from 0.02 to 0.12, such as from 0.08 to 0.11. In some embodiments, the barrier layer 30 may include a partially stabilized YSZ material represented by the formula: (ZrO2)1-x(Y2O3)x, wherein x ranges from 0.02 to 0.07, such as from 0.03 to 0.05.
Alternatively, the barrier layer 30 may include a SYSZ material represented by the formula: (ZrO2)0.9+y−x(Sc2O3)0.1−y(Y2O3)x, wherein y ranges from 0 to 0.05, and x ranges from 0.01 to (0.05+y). In some embodiments, the barrier layer 30 may include a SYSZ material represented by the formula: (ZrO2)0.9−x(Sc2O3)0.1(Y2O3)x, wherein x ranges from 0.005 to 0.1, such as from 0.01 to 0.05. For example, the SYSZ material may comprise (ZrO2)0.89(Sc2O3)0.1(Y2O3)0.01. In general, the 10 mol % scandia doped zirconia with 1 to 5 mol % yttria doping may include 89 mol % ZrO2—10 mol % Sc2O3—1 mol % Y2O3, 88 mol % ZrO2—10 mol % Sc2O3—2 mol % Y2O3, 87 mol % ZrO2—10 mol % Sc2O3—3 mol % Y2O3, 86 mol % ZrO2—10 mol % Sc2O3—4 mol % Y2O3, or 85 mol % ZrO2—10 mol % Sc2O3—5 mol % Y2O3 compositions.
In some embodiments, the barrier layer 30 may include a SYSZ material represented by the formula: (ZrO2)0.91−x(Sc2O3)0.09(Y2O3)x, wherein x ranges from 0.005 to 0.1, such as from 0.01 to 0.06. For example, the SYSZ material may comprise (ZrO2)0.89(Sc2O3)0.09(Y2O3)0.02. In general, the 9 mol % scandia doped zirconia with 1 to 6 mol % yttria doping may include 90 mol % ZrO2—9 mol % Sc2O3—1 mol % Y2O3, 89 mol % ZrO2—9 mol % Sc2O3—2 mol % Y2O3, 88 mol % ZrO2—9 mol % Sc2O3—3 mol % Y2O3, 87 mol % ZrO2—9 mol % Sc2O3—4 mol % Y2O3, 86 mol % ZrO2—9 mol % Sc2O3—5 mol % Y2O3 or 85 mol % ZrO2—9 mol % Sc2O3—6 mol % Y2O3 compositions.
Alternatively, the barrier layer 30 may include a SYbSZ material represented by the formula: (ZrO2)0.9+y−x(Sc2O3)0.1−y(Yb2O3)x, wherein y ranges from 0 to 0.05, and x ranges from 0.01 to (0.05+y). In some embodiments, the barrier layer 30 may include a SYbSZ material represented by the formula: (ZrO2)0.9−x(Sc2O3)0.1(Yb2O3)x, wherein x ranges from 0.005 to 0.1, such as from 0.01 to 0.05. For example, the SYbSZ material may comprise (ZrO2)0.89(Sc2O3)0.1(Yb2O3)0.01. In general, the 10 mol % scandia doped zirconia with 1 to 5 mol % ytterbia doping may include 89 mol % ZrO2—10 mol % Sc2O3—1 mol % Yb2O3, 88 mol % ZrO2—10 mol % Sc2O3—2 mol % Yb2O3, 87 mol % ZrO2—10 mol % Sc2O3—3 mol % Yb2O3, 86 mol % ZrO2—10 mol % Sc2O3—4 mol % Yb2O3, or 85 mol % ZrO2—10 mol % Sc2O3—5 mol % Yb2O3 compositions.
In some embodiments, the barrier layer 30 may include a SYbSZ material represented by the formula: (ZrO2)0.91−x(Sc2O3)0.09(Yb2O3)x, wherein x ranges from 0.005 to 0.1, such as from 0.01 to 0.06. For example, the SYbSZ material may comprise (ZrO2)0.89(Sc2O3)0.09(Yb2O3)0.02. In general, the 9 mol % scandia doped zirconia with 1 to 6 mol % ytterbia doping may include 90 mol % ZrO2—9 mol % Sc2O3—1 mol % Yb2O3, 89 mol % ZrO2—9 mol % Sc2O3—2 mol % Yb2O3, 88 mol % ZrO2—9 mol % Sc2O3—3 mol % Yb2O3, 87 mol % ZrO2—9 mol % Sc2O3—4 mol % Yb2O3, 86 mol % ZrO2—9 mol % Sc2O3—5 mol % Yb2O3 or 85 mol % ZrO2—9 mol % Sc2O3—6 mol % Yb2O3 compositions.
In some embodiments, the barrier layer 30 may include 10 mol % scandia doped zirconia with 1-5 mol % Y2O3 or Yb2O3 doping, 9 mol % scandia doped zirconia with 1-6 mol % Y2O3 or Yb2O3 doping, 8 mol % scandia doped zirconia with 1-7 mol % Y2O3 or Yb2O3 doping, 7 mol % scandia doped zirconia with 1-8 mol % Y2O3 or Yb2O3 doping, 6 mol % scandia doped zirconia with 1-9 mol % Y2O3 or Yb2O3 doping, or 5 mol % scandia doped zirconia with 1-10 mol % Y2O3 or Yb2O3 doping.
Solid oxide electrolyzer cells of types A-G were fabricated and included air-side electrodes having functional layer and optionally barrier layer materials shown below in Table 1:
Each cell included an electrolyte comprising (ZrO2)0.88(Sc2O3)0.1(CeO2)0.1(Yb2O3)0.01. An electrolyzer cell stack was assembled including multiple cells of each of cell types A-G. The stack was tested in EC mode at a temperature of 850° C. at a current of 36 Amps
As shown in
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.
