Patent Description:
A solid oxide electrochemical cell can have sufficient reaction speed without the use of an expensive precious metal catalyst due to its high operation temperature (<NUM> to <NUM>). For this reason, the solid oxide electrochemical cell, when operating as a solid oxide fuel cell (SOFC), has the highest power generation efficiency and generates less CO<NUM> as compared with other types of fuel cells. Therefore, the solid oxide electrochemical cell is expected as a clean power generation system of the next generation.

The solid oxide electrochemical cell, when operating as a solid oxide electrolysis cell (SOEC), can produce hydrogen theoretically at a low electrolytic voltage due to its high operation temperature. Therefore, the solid oxide electrochemical cell is expected as a high-efficiency hydrogen production device. Additionally, the solid oxide electrochemical cell is examined for use as a power storage system with SOFC/SOEC.

For an oxygen electrode of the solid oxide electrochemical cell, a perovskite oxide having high conductivity is generally used. For example, a lanthania-manganese based oxide (LaMnO<NUM> based) is often used for an oxygen electrode of high temperature operation type and a lanthania-cobalt based oxide (LaCoO<NUM> based) is often used for an oxygen electrode of middle-and-low temperature operation type. The LaCoO<NUM> based oxide has higher electric conductivity and higher electrode catalytic activity compared with those of the LaMnO<NUM> based oxide. On the other hand, the LaCoO<NUM> based oxide has higher reactivity with a zirconia based oxide (ZrO<NUM> based) generally used as an electrolyte of the solid oxide electrochemical cell. For this reason, a solid-phase reaction may occur during firing in the cell manufacture. In this case, a high resistance phase composed of La<NUM>Zr<NUM>O<NUM> or the like may be formed such that the cell performance may deteriorate.

A known method to prevent the formation of the high resistance phase is to form a dense and thin barrier-layer made of a CeO<NUM>-based oxide between the electrolyte and the oxygen electrode.

When the electrochemical cell having the above-mentioned barrier-layer formed therein operates at a high temperature, a component of the electrolyte and a component of the oxygen electrode diffuse and the form of the CeO<NUM>-based oxide is changed such that the cell performance may deteriorate. Relevant background : <CIT> refers to a solid oxide fuel cell having a porous region and a thickness between <NUM> and <NUM>. <CIT> refers to a solid oxide fuel cell having improved performance with an intermediate layer with a thickness between <NUM> and <NUM>. <CIT> refers to an electrolyte-electrode assembly having an intermediate layer with a thickness between <NUM> and <NUM>.

Hereinafter, electrochemical cells according to the present invention are described. However, the present invention should not be construed as being limited to the following embodiment and examples. The schematic views referred to in the following description are intended to illustrate the positional relationships between components, and the size of particles, the thickness ratio of layers, and the like do not necessarily correspond to actual ones.

An electrochemical cell according to an embodiment includes a hydrogen electrode, an electrolyte laminated on the hydrogen electrode, a barrier-layer laminated on the electrolyte, and an oxygen electrode laminated on the barrier-layer. The barrier-layer has a porous structure having a thickness of <NUM> or greater and <NUM> or less and a porosity of greater than <NUM>% determined by scanning electron microscopy.

Additionally, an electrochemical cell stack according to an embodiment includes an electrochemical cell according to claims <NUM> to <NUM>. <FIG> is a simplified cross sectional view of a structure of a part of an electrochemical cell according to an embodiment. The electrochemical cell <NUM> according to the present embodiment is a solid oxide electrochemical cell of hydrogen electrode support type. In the electrochemical cell <NUM>, a hydrogen electrode <NUM>, an electrolyte <NUM>, a barrier-layer <NUM>, and an oxygen electrode <NUM> are laminated in this order.

The hydrogen electrode <NUM> is composed of a substrate <NUM> and an active layer <NUM> laminated on the substrate <NUM>. The substrate <NUM> may be a porous layer or may have the same structure as that of the active layer <NUM>. For the substrate <NUM> and the active layer <NUM>, a sintered body containing a metal particulate and a metal oxide may be used. The metal particulate contained in the sintered body or contained in the oxide in the form of a solid solution includes, for example, one or more metals selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe), and copper (Cu), or alloys containing these metals.

The metal oxide includes, for example, a stabilized zirconia containing one or more stabilizers selected from the group consisting of yttrium oxide (Y<NUM>O<NUM>), scandium oxide (Sc<NUM>O<NUM>), ytterbium oxide (Yb<NUM>O<NUM>), gadolinium oxide (Gd<NUM>O<NUM>), calcium oxide (CaO), magnesium oxide (MgO), cerium oxide (CeO<NUM>), and the like in the form of a solid solution, as well as a doped ceria in which one or more oxides selected from the group consisting of samarium oxide (Sm<NUM>O<NUM>), Gd<NUM>O<NUM>, Y<NUM>O<NUM>, and the like and CeO<NUM> form a solid solution.

The electrolyte <NUM> is composed of a stabilized zirconia containing one or more stabilizers selected from the group consisting of Y<NUM>O<NUM>, Sc<NUM>O<NUM>, Yb<NUM>O<NUM>, Gd<NUM>O<NUM>, CaO, MgO, CeO<NUM>, and the like in the form of a solid solution, or a doped ceria in which one or more oxides selected from the group consisting of Sm<NUM>O<NUM>, Gd<NUM>O<NUM>, Y<NUM>O<NUM>, and the like and CeO<NUM> form a solid solution.

The barrier-layer <NUM> is composed of a doped ceria in which one or more oxides selected from the group consisting of Sm<NUM>O<NUM>, Gd<NUM>O<NUM>, Y<NUM>O<NUM>, and the like and CeO<NUM> form a solid solution.

The oxygen electrode <NUM> is composed of a sintered body containing a perovskite oxide. The perovskite oxide is mainly represented by Ln<NUM>-xAxB<NUM>-yCyO<NUM>-δ. "Ln" includes rare earth elements such as lanthania (La), for example. "A" includes strontium (Sr), calcium (Ca), and barium (Ba), for example. "B" and "C" include chromium (Cr), manganese (Mn), Co, Fe, and Ni, for example. For the perovskite oxide, x, y and δ satisfy <NUM> ≤ x ≤ <NUM>, <NUM> ≤ y ≤ <NUM>, and <NUM> ≤ δ ≤ <NUM>. In addition to the perovskite oxide, the oxygen electrode <NUM> may further contain ceria in which one or more oxides selected from the group consisting of Sm<NUM>O<NUM>, Gd<NUM>O<NUM>, Y<NUM>O<NUM>, and the like are doped in CeO<NUM>.

A laminated body, in which a plurality of electrochemical cells <NUM> configured as described above are laminated, is an electrochemical cell stack. In the electrochemical cell stack, all layers do not need to be the electrochemical cells <NUM>, and at least one layer may be the electrochemical cell <NUM>.

Hereinafter, a method of manufacturing the electrochemical cell <NUM> will be specifically described with reference to <FIG> according to the following examples.

First, a substrate precursor <NUM> is made as illustrated in <FIG>. In this example, a powder is prepared, which is obtained by mixing, at a weight ratio of <NUM>: <NUM>, nickel oxide (NiO) and Gd<NUM>O<NUM>-doped ceria (GDC) in which Gd<NUM>O<NUM> is doped in ceria to give a composition of (Gd<NUM>O<NUM>)<NUM>(CeO<NUM>)<NUM>. Subsequently, a paste made from the powder is formed into a sheet, and thereby the substrate precursor <NUM> is completed.

Next, as shown in <FIG>, an active layer precursor <NUM> is formed on the substrate precursor <NUM>, and the electrolyte <NUM> is then formed on the active layer precursor <NUM>. In this example, the active layer precursor <NUM> is made of a mixture of NiO and GDC. The electrolyte <NUM> is made using yttria-stabilized zirconia.

Next, as illustrated in <FIG>, the barrier-layer <NUM> is formed on the electrolyte <NUM>. In this example, GDC is fired, and the fired product is then formed into a porous structure having a thickness of <NUM> and a porosity of about <NUM>%. The porosity represents a ratio of voids per unit volume. The porosity can be adjusted, for example, by adding a pore-forming material or by setting a porous pattern with a ceramic 3D printer and the like.

Next, the substrate precursor <NUM>, the active layer precursor <NUM>, the electrolyte <NUM>, and the barrier-layer <NUM> are fired under a temperature condition of <NUM> or higher and <NUM> or lower. This firing step is performed until sufficient strength is obtained in the layers and between the layers.

Next, as illustrated in <FIG>, the oxygen electrode <NUM> is formed on the barrier-layer <NUM>. In this example, a layer composed of La(Sr)Co(Fe)O<NUM>-δ is formed on the barrier-layer <NUM>, and then fired within a range of <NUM> or higher and <NUM> or lower. The oxygen electrode <NUM> can be thereby adhered firmly to the barrier-layer <NUM>.

Next, the laminated body consisted of the substrate precursor <NUM>, the active layer precursor <NUM>, the electrolyte <NUM>, the barrier-layer <NUM>, and the oxygen electrode <NUM> is set in a hydrogen electrode output characteristic evaluation device. When dry hydrogen is circulated on the substrate precursor <NUM> side and an N<NUM>/O<NUM>-mixture gas obtained by mixing N<NUM> and O<NUM> at a volume ratio of <NUM>:<NUM> is circulated on the oxygen electrode <NUM> side, in the hydrogen electrode output characteristic evaluation device at <NUM> or higher, the substrate precursor <NUM> and the active layer precursor <NUM> are reduced, and the substrate <NUM> and the active layer <NUM> are formed. The electrochemical cell <NUM> illustrated in <FIG> is thereby completed.

The hydrogen electrode output characteristic evaluation device controls the concentration of vapor on the hydrogen electrode side and operates the electrochemical cell <NUM> in the SOFC mode or the SOEC mode so that the I-V characteristics indicating a relationship between a current and a voltage at that time can be evaluated. After the reduction reaction, the electrochemical cell <NUM> is operated as SOEC at a measurement temperature, and the initial I-V characteristic evaluation is performed.

After the I-V characteristic evaluation, the oxygen electrode <NUM> is separated and the pressure loss of the barrier-layer <NUM> is measured. In addition, a section of the electrochemical cell <NUM> is prepared to observe the structure of the barrier-layer <NUM> with a scanning electron microscope (SEM). From the obtained SEM image, the porosity of the barrier-layer <NUM> is calculated.

In Example <NUM>, on the substrate precursor <NUM>, the active layer precursor <NUM>, the electrolyte <NUM>, the barrier-layer <NUM>, and the oxygen electrode <NUM> are sequentially laminated by a manufacturing method similar to that in Example <NUM> described above. However, in this example, the thickness of the barrier-layer <NUM> is designed to be <NUM>, which is thinner than that in Example <NUM>.

Next, the substrate precursor <NUM> and the active layer precursor <NUM> are reduced to form the substrate <NUM> and the active layer <NUM> by a manufacturing method also similar to that in Example <NUM>. An electrochemical cell according to Example <NUM> is thereby completed.

Subsequently, the I-V characteristic evaluation of the electrochemical cell <NUM> in the initial state is performed as in Example <NUM>. In addition, the pressure loss and the porosity of the barrier-layer <NUM> are also measured.

Also in Example <NUM>, on the substrate precursor <NUM>, the active layer precursor <NUM>, the electrolyte <NUM>, the barrier-layer <NUM>, and the oxygen electrode <NUM> are sequentially laminated by a manufacturing method similar to that in Example <NUM> described above. However, in this example, the thickness of the barrier-layer <NUM> is designed to be <NUM>, which is thicker than that in Example <NUM>.

Also in Example <NUM>, on the substrate precursor <NUM>, the active layer precursor <NUM>, the electrolyte <NUM>, the barrier-layer <NUM>, and the oxygen electrode <NUM> are sequentially laminated by a manufacturing method similar to that in Example <NUM> described above. However, in this example, the thickness of the barrier-layer <NUM> is designed to be <NUM>, which is thinner than that in Example <NUM>. In addition, the porosity of the barrier-layer <NUM> is designed to be about <NUM>%, which porosity is smaller than that in Example <NUM>, by adjusting the porous pattern with a ceramic 3D printer.

In Comparative Example <NUM>, a forming method of the barrier-layer <NUM> is different from that in Example <NUM> described above. In this comparative example, a slurry containing the GDC particle is coated on the electrolyte <NUM> using a screen printing method or a tape casting method. At that time, the thickness of the barrier-layer <NUM> is designed to be <NUM>, which is significantly thinner than that in Example <NUM>.

After forming the barrier-layer <NUM> as described above, the oxygen electrode <NUM> is laminated on the barrier-layer <NUM> as in Example <NUM>. The substrate precursor <NUM> and the active layer precursor <NUM> are then reduced to form the substrate <NUM> and the active layer <NUM>. An electrochemical cell according to Comparative Example <NUM> is thereby completed.

Subsequently, the I-V characteristic evaluation of the electrochemical cell in the initial state is performed as in Example <NUM>. In addition, the pressure loss and the porosity of the barrier-layer <NUM> are also measured.

Also in Comparative Example <NUM>, a forming method of the barrier-layer <NUM> is different from that in Example <NUM> described above. In this comparative example, the barrier-layer <NUM> is formed by coating a slurry containing the GDC particle on the electrolyte <NUM> as in Comparative Example <NUM> described above. However, in this comparative example, the thickness of the barrier-layer <NUM> is designed to be <NUM>, which is thicker than that in Comparative Example <NUM> and thinner than that in Example <NUM>.

<FIG> is an enlarged cross sectional view of the barrier-layer <NUM> formed in Examples <NUM> to <NUM>. <FIG> is an enlarged cross sectional view of the barrier-layer <NUM> formed in Comparative Examples <NUM> and <NUM>. <FIG> is a table showing evaluation measurement results of Examples and Comparative Examples.

The table in <FIG> shows the thickness, the porosity, and the pressure loss of the barrier-layer <NUM> and the current density of the electrochemical cell for each of Examples and Comparative Examples. The current density, which is one of the I-V characteristics, is a current density when the electrochemical cell is operated as SOEC at a voltage of <NUM> V. Each thickness of the barrier-layer <NUM> shown in the table is a measured value and is in agreement with each designed value.

The barrier-layer <NUM> formed in Examples <NUM> to <NUM> is thick as illustrated in <FIG>, and has high porosity. Therefore, a component of the electrolyte <NUM> and a component of the oxygen electrode <NUM> are hard to diffuse. In particular, when the thickness is <NUM> to <NUM>, the porosity is <NUM>% or greater, and the pressure loss is within a range of <NUM> to <NUM> MPa/m, the current density is at least <NUM> A/cm<NUM> or greater. As a result, the cell performance is improved.

On the other hand, the barrier-layer <NUM> formed in Comparative Examples <NUM> and <NUM> is thin as illustrated in <FIG>, and has low porosity. In this case, the diffusion of a component of the electrolyte <NUM> and a component of the oxygen electrode <NUM> cannot be sufficiently suppressed, and thus the current density is <NUM> A/cm<NUM> or less. As a result, the cell performance is insufficient.

Claim 1:
An electrochemical cell (<NUM>) comprising:
a hydrogen electrode (<NUM>);
an electrolyte (<NUM>) laminated on the hydrogen electrode (<NUM>);
a barrier-layer (<NUM>) laminated on the electrolyte (<NUM>); and
an oxygen electrode (<NUM>) laminated on the barrier-layer (<NUM>),
wherein the barrier-layer (<NUM>) has a porous structure having a thickness of <NUM> or greater and <NUM> or less and a porosity of greater than <NUM>%, determined by scanning electron microscopy.