Solid oxide reversible fuel cell, reversible fuel cell system including the same, and method of operating the same

Abstract

To provide a solid oxide reversible fuel cell (r-SOC) in which electrode performance deterioration is less likely to occur even when switching between steam electrolysis (SOEC) mode and power generation (SOFC) mode resulting in a stable operation, and a reversible fuel cell system including the same.SOLUTION: The solid oxide reversible fuel cell has a fuel electrode, an air electrode, and a solid electrolyte placed between the fuel electrode and the air electrode. The fuel electrode is composed of an electron conductive oxide and an oxygen ion conductive oxide, and at least a portion of the oxygen ion-conducting oxide is a CeO2-based oxide. A reversible fuel cell system including the same is also provided.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to a reversible solid oxide fuel cell having an anode made substantially of only an oxide material, and to the related art. [Background technology]

[0002] The Reversible Solid Oxide Cell (hereinafter sometimes referred to as "r-SOC") system is an energy system that can reversibly produce hydrogen and generate electricity. The reversible solid oxide fuel cell system is a system that reversibly performs the power generation technology of solid oxide fuel cells (SOFC) and the hydrogen production technology of solid oxide steam electrolyzer (SOEC) in a single device, which are opposite reactions to each other, and is attracting attention as a next-generation energy system that can not only generate electricity using hydrogen as fuel, but also store energy using hydrogen produced by steam electrolysis.

[0003] As an example of a solid oxide reversible fuel cell, Patent Document 1 discloses a power generation system having a reversible fuel cell module that has an electrolysis function mode in which power is supplied from a renewable energy source to produce combustible gas, and a fuel cell function mode in which electricity is produced from the combustible gas, and discloses the use of a solid oxide reversible fuel cell that operates at 500 to 1000°C as the reversible fuel cell module.

[0004] Patent Document 2 discloses an energy storage device including a fuel cell main body having a fuel electrode, an air electrode, and an electrolyte portion containing a solid oxide and disposed between the fuel electrode and the air electrode, an auxiliary gas supply portion that supplies an auxiliary gas containing at least carbon dioxide and hydrogen to the fuel electrode, and a mode switching portion that switches between a charging mode in which power is received from the outside and water (water vapor) is electrolyzed by the fuel cell main body, and a discharging mode in which the fuel cell main body is discharged (generated electricity).

[0005] Furthermore, in Patent Document 3, the present inventors have developed a reversible solid oxide fuel cell equipped with a fuel electrode composed of an electrode skeleton composed of a Ti-containing perovskite-type oxide and an ion-conductive oxide, and a composite electrode catalyst composed of an electrode catalyst metal and an ion-conductive oxide supported on the surface of the electrode skeleton. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2018-517233 [Patent Document 2] Patent Publication No. 2021-34130 [Patent Document 3] Patent Application No. 2021-37193 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, the biggest bottleneck in the development of a solid oxide reversible fuel cell system is the durability of the electrode (fuel electrode) at high water vapor concentrations (partial pressures) to which hydrogen gas and water vapor are supplied as reactant gases (fuel or raw material). When a solid oxide reversible fuel cell system is used for water vapor decomposition (SOEC mode), it is used under conditions of greater potential fluctuations at higher potentials (approximately 1.1 V to 1.6 V) than for power generation (SOFC mode, approximately 0.6 V to 1.0 V), so the fuel electrode of a solid oxide reversible fuel cell system must have higher durability at higher potentials than general-purpose SOFC fuel electrodes.

[0008] Steam electrolysis can be considered the reverse operation of high-temperature solid oxide fuel cell (SOFC) power generation. The fuel electrode of existing SOFCs disclosed in Patent Documents 1 and 2 contains metallic Ni and stabilized zirconia (ZrO 2 ) porous composite "Ni-ZrO 2 Cermets have been widely used. However, metallic Ni begins to oxidize under conditions where the water vapor concentration exceeds 80% downstream of the fuel cell system (particularly when the current density becomes high). When oxidized to NiO, its volume expands by several tens of percent compared to metallic Ni. Conversely, when NiO is reduced back to metallic Ni, its volume shrinks significantly. Repeating this oxidation-reduction cycle (Redox cycle) causes the porous structure of the fuel electrode to break down, leading to the aggregation of Ni particles and accelerating performance degradation, which was an issue.

[0009] In addition, the fuel electrode of Patent Document 3 developed by the present inventors is less susceptible to volume changes because the electrode skeleton, which occupies most of the electrode volume, is made of a stable oxide. However, when Ni is used as the metal species of the composite electrode catalyst, problems such as agglomeration of Ni particles and thinning due to sublimation of Ni hydroxide cannot be completely avoided, and an expensive precious metal (Rh) must be used as the metal species, so further improvement was required.

[0010] In this way, the anode of a SOFC is often used as the anode of a reversible solid oxide fuel cell (r-SOC). However, the electrode atmosphere in the electrolysis of water vapor, which is the reverse reaction of power generation, is significantly different from the electrode atmosphere in a SOFC (electric potential, water vapor concentration, etc.). Therefore, when the anode of a current SOFC is used for an r-SOC, it is not suitable for water vapor electrolysis (SOEC).

[0011] Under these circumstances, an object of the present invention is to provide a reversible fuel cell system and an operating method thereof that are unlikely to deteriorate due to volume changes in the electrodes and can operate stably even when switching between SOEC mode (steam electrolysis) and SOFC mode (power generation). [Means for solving the problem]

[0012] As a result of intensive research aimed at solving the above problems, the inventors of the present invention have found that by forming the fuel electrode of a solid oxide reversible fuel cell from an electronically conductive oxide and an ionically conductive oxide, deterioration due to volume change of the electrode is unlikely to occur, and that the ionically conductive oxide is CeO 2The inventors have found that the use of such an oxide has excellent activity in power generation and steam electrolysis even without the inclusion of an electrode catalyst metal such as metallic Ni, and have thus completed the present invention.

[0013] That is, the present invention relates to the following inventions. <1> The fuel cell comprises an anode, an air electrode, and a solid electrolyte provided between the anode and the air electrode, the anode being composed of an electronically conductive oxide and an ionically conductive oxide, and at least a part of the ionically conductive oxide is CeO 2 A reversible solid oxide fuel cell that uses an oxide-based electrolyte. <2> The ion-conductive oxide in the anode is CeO 2 Consists of only oxides <1> The reversible solid oxide fuel cell according to claim 1. <3> The CeO 2 The oxide is Gd 2 O 3 Doped CeO 2 Or Sm 2 O 3 Doped CeO 2 is <1> or <2> The reversible solid oxide fuel cell according to claim 1. <4> The electronically conductive oxide in the fuel electrode has a composition formula of ABO 3 The perovskite oxide is represented by the formula: wherein the A site is at least one selected from the group consisting of Ca, Sr, Ba, and La, and the B site is Ti. <1> from <3> 2. A reversible solid oxide fuel cell according to claim 1 . <5> The fuel electrode is made of a sintered body of particulate electronic conductive oxide and particulate ion conductive oxide. <1> from <4> 2. A reversible solid oxide fuel cell according to claim 1 . <6> <1> from <5> a fuel supply unit that supplies a fuel gas containing at least hydrogen to the fuel electrode, an oxygen supply unit that supplies an oxygen-containing gas containing at least oxygen to the air electrode, and a water supply unit that supplies water to the fuel electrode or the air electrode. <7> <6> 2. A method for operating a reversible fuel cell system, comprising the steps of: repeatedly performing steam electrolysis and power generation in a temperature range of 300° C. or higher and 1000° C. or lower; Effect of the Invention

[0014] According to the present invention, a reversible fuel cell system is provided that can operate stably even when switching between SOEC (steam electrolysis) and SOFC (power generation). [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic diagram of a fuel electrode in a reversible solid oxide fuel cell (r-SOC) of the present invention. [Diagram 2] FIG. 1 is a diagram illustrating a reversible fuel cell (r-SOC) system according to an embodiment of the present invention. [Diagram 3] FIG. 2 is a configuration diagram of an r-SOC cell (single cell) according to an embodiment. [Figure 4] 1 is a schematic diagram of an r-SOC cell evaluation device according to an embodiment. [Diagram 5] 1 shows current-voltage (IV) characteristics (initial performance) of r-SOC cells according to examples (Example 1 (LST-GDC fuel electrode), Comparative Example 1 (LST fuel electrode), Comparative Example 2 (GDC fuel electrode), Reference Example 1 (Ni-ScSZ fuel electrode)). [Figure 6] 1 shows the impedance measurement results in SOEC mode (-0.2 A cm-2) and SOFC mode (0.2 A cm-2) of r-SOC cells according to the examples (Example 1 (LST-GDC fuel electrode), Comparative Example 1 (LST fuel electrode), Comparative Example 2 (GDC fuel electrode), Reference Example 1 (Ni-ScSZ fuel electrode)). [Figure 7] FIG. 1 is a diagram showing measurement conditions for a reversible cycle durability test. [Figure 8] FIG. 1 is a diagram showing the change over time in a reversible cycle durability test (1000 cycles) (Example 1 (LST-GDC fuel electrode) and Reference Example 1 (Ni-ScSZ fuel electrode)). [Figure 9]FIG. 1 is a diagram showing the change in impedance before and after a reversible cycle durability test (1000 cycles) (Example 1 (LST-GDC fuel electrode) and Reference Example 1 (Ni-ScSZ fuel electrode)). [Figure 10] 1 shows SEM cross-sectional photographs of the fuel electrode (Ni-ScSZ fuel electrode) of Reference Example 1 after a reversible cycle test, where (a) is before the test and (b) is after the test (after 1000 cycles). [Figure 11] 1 shows SEM cross-sectional photographs of the fuel electrode (LST-GDC fuel electrode) of Experimental Example 1 after a reversible cycle test, where (a) is before the test and (b) is after the test (after 1000 cycles). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples and can be modified as desired without departing from the gist of the present invention. In this specification, the word "to" is used as an expression including the numerical value or physical quantity before and after it.

[0017] In this specification, the term "power generation environment of a solid oxide fuel cell (SOFC)" refers to a temperature range of 300 to 1000°C and an oxygen partial pressure range of 10 -25 ~10 -10 The term "steam electrolysis environment of a solid oxide steam electrolysis device (SOEC)" refers to an atmosphere having a temperature range of 300 to 1000°C and a water vapor concentration of 1% to 100%.

[0018] <1. Solid oxide reversible fuel cell (r-SOC)> The reversible solid oxide fuel cell (r-SOC) of the present invention has an anode, an cathode, and a solid electrolyte provided between the anode and the cathode, and the anode is composed of an electronically conductive oxide and an ionically conductive oxide.

[0019] The r-SOC of the present invention may be a so-called electrolyte-supported r-SOC in which an anode and an cathode are baked onto a solid electrolyte substrate, or a so-called fuel-electrode-supported r-SOC in which an electrolyte membrane is formed on an anode and an air electrode is further formed on the electrolyte membrane.

[0020] The r-SOC of the present invention can be suitably used as a solid oxide reversible fuel cell for use in the reversible fuel cell system of the present invention described below. In addition, since the fuel electrode in the r-SOC of the present invention is stable in the power generation environment of the SOFC and in the steam electrolysis environment of the SOEC, it is possible to repeatedly perform steam electrolysis and power generation in the temperature range of 300°C to 1000°C using the reversible fuel cell system of the present invention.

[0021] Hereinafter, the components (fuel electrode, air electrode, solid electrolyte) constituting the r-SOC of the present invention will be described in detail with reference to examples, but the components can be modified as desired without departing from the gist of the present invention.

[0022] (1-1.Solid electrolyte) The solid electrolyte of the r-SOC of the present invention is selected from among known solid electrolytes for conventional SOFCs and SOECs, taking into consideration the reactivity with the constituent materials of the anode and cathode during manufacture and operation, and the long-term stability in the power generation environment of the SOFC and the steam electrolysis environment of the SOEC, etc.

[0023] As the solid electrolyte of the r-SOC of the present invention, for example, zirconia-based oxides doped with scandium or yttrium (ScSZ, YSZ, respectively), ceria-based oxides doped with gadolinium or samarium (GDC, SDC, respectively), lanthanum gallate-based oxides doped with strontium or magnesium, etc. can be used. Among these, ScSZ, which has high ion conductivity and high stability, and YSZ, which is inexpensive and highly stable, are preferred. In addition, the solid electrolyte may be the same type of oxygen ion conductive oxide as the oxygen ion conductive oxide in the electrode skeleton of the fuel electrode of the present invention described later.

[0024] The thickness of the solid electrolyte may be appropriately adjusted according to the required electrical conductivity and strength. For example, in the case of an electrolyte-supported cell, a thickness of 5 to 500 μm or less is preferably used for ScSZ.

[0025] (1-2. Air electrode) The air electrode of the r-SOC of the present invention is appropriately selected in consideration of the reactivity with the solid electrolyte during production and operation. For example, a metal oxide such as a perovskite-type oxide can be used for the air electrode. More specifically, (Sm,Sr)CoO 3 , (La,Sr)MnO 3 , (La,Sr)CoO 3 , (La,Sr)(Fe,Co)O 3 , (La,Sr)(Fe,Co,Ni)O 3 etc.

[0026] The thickness of the air electrode varies depending on the form of the r-SOC and the purpose of use, but is, for example, about 10 to 300 μm.

[0027] (1-3.Fuel electrode) The fuel electrode of the r-SOC of the present invention (hereinafter, sometimes referred to as the "fuel electrode of the present invention") is composed of an electronically conductive oxide and an ionically conductive oxide. Figure 1 shows a schematic diagram of the fuel electrode of the present invention.

[0028] In Fig. 1, the skeleton of the fuel electrode (hereinafter sometimes referred to as "electrode skeleton") is composed of electronically conductive oxide particles and ionically conductive oxide particles, and the respective particles are sintered and bonded to each other to form three-dimensionally continuous electronic conductive paths and ion conductive paths. In addition, the electrode skeleton has voids to the extent that gas can diffuse inside the fuel electrode. In FIG. 1, the electronically conductive oxide and the ionically conductive oxide are spherical particles, but they need not be spherical particles as long as they form three-dimensionally continuous electronically conductive paths and ionically conductive paths.

[0029] As mentioned above, in the anode containing metallic Ni used in conventional r-SOC, metallic Ni in the anode is oxidized to form Ni oxide and Ni hydroxide under conditions of high water vapor concentration, and is reduced to metallic Ni under conditions of low water vapor concentration. When this oxidation-reduction cycle (Redox cycle) is repeated, there are problems such as destruction of the porous structure of the anode and thinning due to sublimation of Ni hydroxide.

[0030] On the other hand, one of the features of the fuel electrode of the present invention is that it is a metal-free electrode that is composed only of oxide materials (electronically conductive oxide and ionically conductive oxide) and does not substantially contain electrode catalyst metals such as Ni. That is, the electrode skeleton of the fuel electrode of the present invention is composed of ionically conductive oxide and electronically conductive oxide that undergo almost no volume change in the power generation environment of SOFC and the steam electrolysis environment of SOEC, and since the electrode skeleton is composed only of oxide materials, there is almost no volume change due to metallic Ni, which is a problem with fuel electrodes of conventional SOFCs and SOECs, and destruction of the fuel electrode skeleton is avoided.

[0031] On the other hand, fuel electrodes that use electrode skeletons composed only of oxide materials do not have sufficient electrode activity to promote steam electrolysis reactions and electrochemical oxidation reactions of hydrogen, and it was thought that a separate electrode catalyst would be necessary. In the fuel electrode of the present invention, at least a part of the ion-conductive oxide constituting the fuel electrode is CeO 2 One of its features is that it is a type oxide, and despite not containing electrocatalyst metals such as Ni, it has excellent electrode activity that promotes steam electrolysis reactions and electrochemical oxidation reactions of hydrogen (especially steam electrolysis reactions).

[0032] Each of the components of the fuel electrode will now be described in detail.

[0033] The fuel electrode of the present invention may have porosity to an extent that hydrogen gas, water vapor, and generated gas can diffuse within the fuel electrode. The porosity of the fuel electrode of the present invention may be within a range that provides ionic conductivity and electronic conductivity and maintains the strength of the electrode, and is preferably 30% by volume or more and 70% by volume or less.

[0034] The thickness of the fuel electrode of the present invention varies depending on the form and purpose of use, but in the case of a solid electrolyte supported type, a suitable range is about 10 to 300 μm. In the case of a fuel electrode supported membrane type cell rather than an electrolyte supported type cell, the thickness of the fuel electrode is preferably 0.1 to 5 mm (particularly 0.5 to 2.5 mm).

[0035] The electrode skeleton in the fuel electrode of the present invention can be a sintered body obtained by sintering electronically conductive oxide particles and ionically conductive oxide particles. In the sintered body, electronically conductive oxides, or electronically conductive oxides and ionically conductive oxides, or ionically conductive oxides are partially sintered to an extent that allows the conduction paths of electrons and ions to be secured, so that the electronically conductive portions and the ionically conductive portions are three-dimensionally continuous.

[0036] The electrode skeleton of the fuel electrode of the present invention is composed of an electronically conductive oxide and an ionically conductive oxide, but may contain other components as long as the object of the present invention is not impaired. The type, particle size, and ratio of the electronically conductive oxide and the ionically conductive oxide constituting the electrode skeleton may be appropriately determined within the scope of achieving the object of the present invention, but the ratio of the electronically conductive oxide and the ionically conductive oxide constituting the electrode skeleton is usually 70:30 to 30:70 in volume ratio of electronically conductive oxide:ionically conductive oxide.

[0037] [Ion-conducting oxides that compose the electrode skeleton] The ion-conductive oxide constituting the fuel electrode of the present invention can be selected from oxides having oxygen ion conductivity that is highly chemically and thermally stable in the power generation environment of an SOFC and in the steam electrolysis environment of an SOEC.

[0038] When the electrode skeleton is a sintered body obtained by sintering electron conductive oxide particles and oxygen ion conductive oxide particles, the average particle size of the oxygen ion conductive oxide particles is about 0.5 to 10 μm. The "average particle size" of the oxygen ion conductive oxide particles in the electrode skeleton can be calculated by randomly selecting 50 particles and measuring the particle size (diameter) of each of them with a scanning electron microscope (SEM), and averaging the particle sizes of the 50 particles. When the particle shape is other than spherical, the perimeter of the particle in the microscope image is measured with analysis software, and the diameter when the perimeter is taken as the circumference is taken as the particle size.

[0039] The ion-conducting oxide constituting the electrode skeleton is at least ceria (CeO 2 )-based oxides. 2 )-based oxides include Gd 2 O 3 Doped CeO 2 (GDC) and Sm 2 O 3 Doped CeO 2 (SDC) is preferred.

[0040] Celia (CeO 2 The SiO2-based oxides are ion-conductive oxides having mixed conductivity. The term "mixed conductivity" used here means that the oxides have both oxygen ion conductivity and electronic conductivity.

[0041] The ceria (CeO 2 As an oxygen-ion conductive oxide other than the zirconia (ZrO 2 )-based oxides, lanthanum gallate (LaGaO 3 )-based oxides and the like can be used. Examples of zirconia-based oxides include stabilized zirconia such as scandia-stabilized zirconia (ScSZ), yttria-stabilized zirconia (YSZ), and calcia-stabilized zirconia (CSZ). Examples of lanthanum gallate-based oxides include lanthanum gallate doped with strontium or magnesium.

[0042] Among the ion-conductive oxides that make up the electrode skeleton, ceria (CeO 2 The ratio of the ceria (CeO 2 If the amount of ceria (CeO) is too small, the electrocatalytic activity will be insufficient. 2 When the total weight of the ceria (CeO 2 )-based oxides are 50% by weight or more, 70% by weight or more, or 90% by weight or more.

[0043] The ion-conductive oxide that constitutes the electrode skeleton is ceria (CeO 2 )-based oxides (i.e., ceria (CeO 2 )-based oxides 100% by weight.

[0044] [Other ion-conducting oxides] The fuel electrode of the present invention may contain an ion-conductive oxide other than the ion-conductive oxide constituting the electrode skeleton (hereinafter referred to as "another ion-conductive oxide"). The other ion conductive oxide may have any form as long as it does not impair the object of the present invention, and for example, the other ion conductive oxide may be supported on the surface of the ion conductive oxide particles or electronic conductive oxide particles that constitute the electrode skeleton, or other ion conductive oxide in the form of particles that do not constitute the electrode skeleton may be mixed and contained. Furthermore, the ratio of the electronic conductive oxide and ion conductive oxide (electrode skeleton material) that constitute the electrode skeleton to the other ion conductive oxide may be appropriately determined depending on the object and conditions of use.

[0045] [Electron-conductive oxides that compose the electrode skeleton] There are no particular limitations on the electronically conductive oxide, so long as it is an oxide that has high chemical and thermal stability and electronic conductivity in the power generation environment of the SOFC and the steam electrolysis environment of the SOEC, and it can be appropriately selected.

[0046] When the electrode skeleton is a sintered body obtained by sintering electronically conductive oxide particles and ionically conductive oxide particles, the electronically conductive oxide particles have an average particle size of about 0.5 to 10 μm. The method for determining the average particle size of the electronically conductive oxide particles is the same as that for the average particle size of the ionically conductive oxide particles described above.

[0047] As the electron-conductive oxide, particles of a perovskite-type oxide, a spinel-type oxide, or the like can be used, with the perovskite-type oxide being the preferred electron-conductive oxide. Here, perovskite oxides are oxides with the formula ABO 3 and has an element A located at the A site and an element B located at the B site. A is a rare earth element or an alkaline earth element, and B is a transition metal element.

[0048] In particular, one suitable electronic conductive oxide is a compound having the formula ABO 3 The perovskite oxide is represented by the formula: wherein the A site is at least one selected from the group consisting of Ca, Sr, Ba and La, and the B site is Ti.

[0049] The Ti-containing perovskite oxide may be a Ti-containing perovskite oxide in which the A site is Sr (which may be partially substituted with other atoms), or a Ti-containing perovskite oxide in which a portion of the B site is substituted with at least one element selected from the group consisting of Sb, Nb, Ta, W, Co, V, Cr, Mn, and Mo. An example of such an electronically conductive oxide is lanthanum-doped strontium titanate (LST).

[0050] That is, in the fuel electrode, an electrode skeleton in which the electronic conductive oxide is a Ti-containing perovskite-type oxide and the ion conductive oxide is a ceria-based oxide is one of the preferable electrode skeletons of the present invention. For example, a combination of LST, which is an electronic conductive oxide, and GDC, which is an ion conductive oxide, can be mentioned as one preferable combination.

[0051] The fuel electrode of the r-SOC of the present invention has excellent steam electrolysis activity, and therefore can be used as a fuel electrode for an SOEC. That is, a solid oxide electrochemical cell having the SOEC fuel electrode, an air electrode, and a solid electrolyte provided between the fuel electrode and the air electrode is also suitable as a solid oxide steam electrolysis device (SOEC).

[0052] <2. Reversible fuel cell system> The reversible fuel cell system of the present invention is characterized by comprising the above-mentioned solid oxide reversible fuel cell of the present invention, a fuel supply unit that supplies a fuel gas containing at least hydrogen to the fuel electrode, an oxygen supply unit that supplies an oxygen-containing gas containing at least oxygen to the air electrode, and a water supply unit that supplies water to the fuel electrode or the air electrode.

[0053] Hereinafter, an embodiment of the reversible fuel cell system of the present invention will be described in detail with reference to the drawings, however, the reversible fuel cell system of the present invention is not limited to this embodiment. In the drawings, elements having substantially the same functions and configurations are denoted by the same reference numerals, and elements not directly related to the present invention may be omitted. In the following description of the reversible fuel cell system, the solid oxide reversible fuel cell of the present invention will be referred to as the "r-SOC main body."

[0054] FIG. 2 is a diagram illustrating a reversible fuel cell system 100 according to an embodiment of the present invention. As shown in FIG. 2, the reversible fuel cell system 100 includes an r-SOC main body 110, a fuel supply unit 120, an oxygen supply unit 130, a water supply unit 140, a first exhaust unit 160, a second exhaust unit 170, a first heat exchanger 180, a second heat exchanger 182, a third heat exchanger 184, and a central control unit 190. In FIG. 2, dashed arrows indicate the flow of signals. In addition, in order to simplify the drawing, dashed lines indicating the flow of signals from the mode switching unit 192 to the blowers 124, 134, 144, 164, 174 and the on-off valves 126, 136, 146 are omitted in FIG. 2.

[0055] The r-SOC body 110 is the above-mentioned solid oxide reversible fuel cell of the present invention, and includes a fuel electrode 112, an air electrode 114, and an electrolyte part 116. The details of each component of the r-SOC body 110 (the fuel electrode 112, the air electrode 114, and the electrolyte part 116) are as described above, and therefore will not be described in detail again.

[0056] The fuel supply unit 120 supplies a fuel gas F to the fuel electrode 112. The fuel gas F contains at least hydrogen (H 2 ). The fuel supply unit 120 includes a fuel supply pipe 122, a blower 124, and an on-off valve 126. The fuel supply pipe 122 connects a supply source of the fuel gas F to a supply port (or a supply manifold) of the fuel electrode 112. The blower 124 is provided in the fuel supply pipe 122. The suction side of the blower 124 is connected to the supply source of the fuel gas F, and the discharge side is connected to the fuel electrode 112. The on-off valve 126 is provided in the fuel supply pipe 122 between the blower 124 and the fuel electrode 112. The on-off valve 126 opens and closes a flow path formed in the fuel supply pipe 122.

[0057] In this embodiment, the fuel gas F is hydrogen. In addition to hydrogen, the fuel gas F may be, for example, a hydrocarbon gas such as methane, an ammonia gas, or CO 2 Gas such as nitrous oxide, CO gas, a mixture of these, and a mixture of these with hydrogen can be used.

[0058] The oxygen supply unit 130 supplies an oxygen-containing gas S to the air electrode 114. The oxygen-containing gas S contains at least oxygen (O 2) in the present embodiment, the oxygen-containing gas S is air. The oxygen supply unit 130 includes an oxygen supply pipe 132, a blower 134, and an on-off valve 136. The oxygen supply pipe 132 connects a supply source of the oxygen-containing gas S to a supply port (or a supply manifold) of the air electrode 114. The blower 134 is provided in the oxygen supply pipe 132. The intake side of the blower 134 is connected to the supply source of the oxygen-containing gas S, and the discharge side is connected to the air electrode 114. The on-off valve 136 is provided in the oxygen supply pipe 132 between the blower 134 and the air electrode 114. The on-off valve 136 opens and closes a flow path formed in the oxygen supply pipe 132.

[0059] The water supply unit 140 supplies water vapor (water (H 2 O))W is supplied. The water supply unit 140 includes a water vapor supply pipe 142, a blower 144, and an on-off valve 146. The water vapor supply pipe 142 connects a supply source of water vapor W to a portion between the on-off valve 126 in the fuel supply pipe 122 and the fuel electrode 112. That is, the water vapor supply pipe 142 connects a supply source of water vapor W to a supply port of the fuel electrode 112. The blower 144 is provided in the water vapor supply pipe 142. The suction side of the blower 144 is connected to a supply source of water vapor W, and the discharge side is connected to the fuel electrode 112. The on-off valve 146 is provided in the water vapor supply pipe 142 between the blower 144 and the fuel electrode 112. The on-off valve 146 opens and closes a flow path formed in the water vapor supply pipe 142.

[0060] The first exhaust section 160 exhausts the anode exhaust gas EX1 from the anode 112. The first exhaust section 160 includes a first exhaust pipe 162, a blower 164, and an on-off valve 166. The first exhaust pipe 162 connects an exhaust port (or an exhaust manifold) of the anode 112 to a storage section 168 for the anode exhaust gas EX1. The blower 164 is provided in the first exhaust pipe 162. The suction side of the blower 164 is connected to the anode 112, and the discharge side is connected to the storage section 168. The on-off valve 166 is provided in the first exhaust pipe 162 between the blower 164 and the storage section 168. The on-off valve 166 opens and closes a flow path formed in the first exhaust pipe 162.

[0061] The second exhaust section 170 exhausts the air electrode exhaust gas EX2 from the air electrode 114. The second exhaust section 170 includes a second exhaust pipe 172, a blower 174, and an on-off valve 176. The second exhaust pipe 172 connects an exhaust port (or an exhaust manifold) of the air electrode 114 to a storage section 178 for the air electrode exhaust gas EX2. The blower 174 is provided in the second exhaust pipe 172. The suction side of the blower 174 is connected to the air electrode 114, and the discharge side is connected to the storage section 178. The on-off valve 176 is provided in the second exhaust pipe 172 between the blower 174 and the storage section 178. The on-off valve 176 opens and closes a flow path formed in the second exhaust pipe 172.

[0062] The first heat exchanger 180 exchanges heat between the gas (fuel gas F, water vapor W) supplied to the anode 112 and the anode exhaust gas EX1 exhausted from the anode 112. In this embodiment, the first heat exchanger 180 exchanges heat between the gas passing through the fuel supply pipe 122 and the anode exhaust gas EX1 passing through the first exhaust pipe 162.

[0063] The second heat exchanger 182 exchanges heat between the oxygen-containing gas S supplied to the air electrode 114 and the air electrode exhaust gas EX2 exhausted from the air electrode 114. In this embodiment, the second heat exchanger 182 exchanges heat between the oxygen-containing gas S passing through the oxygen supply pipe 132 and the air electrode exhaust gas EX2 passing through the second exhaust pipe 172.

[0064] The third heat exchanger 184 exchanges heat between the air electrode exhaust gas EX2 exhausted from the air electrode 114 and the water vapor W supplied by the water supply unit 140. In this embodiment, the reversible fuel cell system 100 includes a three-way valve 186 between the air electrode 114 and the second heat exchanger 182 in the second exhaust pipe 172. The reversible fuel cell system 100 also includes a bypass pipe 188 that connects the three-way valve 186 to a portion of the second exhaust pipe 172 between the second heat exchanger 182 and the blower 174. The third heat exchanger 184 is provided in the bypass pipe 188. The third heat exchanger 184 exchanges heat between the air electrode exhaust gas EX2 passing through the second exhaust pipe 172 and the water vapor W passing through the water vapor supply pipe 142.

[0065] The central control unit 190 is composed of a semiconductor integrated circuit including a CPU (Central Processing Unit). The central control unit 190 reads out programs and parameters for operating the CPU itself from the ROM. The central control unit 190 manages and controls the entire reversible fuel cell system 100 in cooperation with a RAM as a work area and other electronic circuits. In this embodiment, the central control unit 190 functions as a mode switching unit 192.

[0066] The mode switching unit 192 switches the operation mode of the r-SOC main body 110 between an SOEC mode (steam electrolysis) and an SOFC mode (power generation). The SOEC mode and the SOFC mode of this embodiment will be described in detail below.

[0067] [SOEC mode (steam electrolysis)] The SOEC mode is an operation mode in which power is received from an external source, steam electrolysis is performed by the r-SOC main body 110, and hydrogen is produced.

[0068] When the operation mode is set to the SOEC mode, the mode switching unit 192 drives the water supply unit 140, the first exhaust unit 160, and the second exhaust unit 170. Specifically, the mode switching unit 192 opens the on-off valves 146, 166, and 176, and drives the blowers 144, 164, and 174. The mode switching unit 192 also moves the three-way valve 186 to a position that connects the second exhaust pipe 172 and the bypass pipe 188 (bypasses the second heat exchanger 182). In other words, the mode switching unit 192 switches the three-way valve 186 so that the cathode exhaust gas EX2 passes through the third heat exchanger 184. The mode switching unit 192 also causes the power supply source 10 to supply power to the r-SOC main body 110. That is, the mode switching unit 192 energizes the r-SOC main body 110 and the power supply source 10. The power supply source 10 may be a power generation device that utilizes renewable energy, such as a solar power generation device, a hydroelectric power generation device, or a wind power generation device.

[0069] Then, water vapor W is supplied to the fuel electrode 112, and the reaction shown in the following formula (1) proceeds due to the received power. H 2 O + 2e - → H 2 + O 2- ...Formula (1)

[0070] And the oxide ion (O 2- ) is conducted (moves) through the electrolyte portion 116, whereby the reaction shown in the following formula (2) proceeds in the air electrode 114. O 2- → 1 / 2O 2 + 2e - ...Formula (2)

[0071] Thus, in the SOEC mode, water (water vapor) is electrolyzed at the fuel electrode 112 to generate hydrogen (the above formula (1)). Note that the fuel electrode exhaust gas EX1 also contains water vapor W that has not reacted at the fuel electrode 112.

[0072] The temperature in SOEC mode is determined taking into consideration the components and size of the r-SOC body, the required steam electrolysis reaction rate, etc., and is, for example, between 300°C and 1000°C, or between 500°C and 900°C.

[0073] As described above, oxygen is generated at the air electrode 114 in the SOEC mode (see formula (2) above). The oxygen generated at the air electrode 114 is sucked into the blower 174 as the air electrode exhaust gas EX2, passes through the third heat exchanger 184, and is then guided to the storage section 178. The oxygen guided to the storage section 178 is used as the oxygen-containing gas S in the SOFC mode. The third heat exchanger 184 can also provide the heat of the air electrode exhaust gas EX2 to either or both of the water vaporizer and the water vapor W. Therefore, the reversible fuel cell system 100 can reduce the energy required for either or both of generating the water vapor W (heat of vaporization) and heating the water vapor W (preheating).

[0074] [SOFC mode (power generation)] The SOFC mode is an operation mode in which the r-SOC main body 110 generates electricity. When the operation mode is set to the SOFC mode, the mode switching unit 192 drives the fuel supply unit 120, the oxygen supply unit 130, and the first exhaust unit 160. Specifically, the mode switching unit 192 opens the on-off valves 126, 136, and 166, and drives the blowers 124, 134, 164, and 174. The mode switching unit 192 also moves the three-way valve 186 to a position that connects the second exhaust pipe 172 and the second heat exchanger 182 (bypasses the third heat exchanger 184). In other words, the mode switching unit 192 switches the three-way valve 186 so that the cathode exhaust gas EX2 passes through the second heat exchanger 182. The mode switching unit 192 also connects the r-SOC main body 110 to the load 12.

[0075] Then, the fuel gas F is supplied to the fuel electrode 112, and the reaction shown in the following formula (3) proceeds: H 2 + O 2- → H 2 O + 2e - ...Formula (3)

[0076] Furthermore, an oxygen-containing gas S is supplied to the air electrode 114, and the reaction shown in the following formula (4) proceeds. 1 / 2O 2 + 2e - → O 2- ...Formula (4) And the oxide ion (O 2- ) is conducted (moved) through the electrolyte portion 116, whereby the r-SOC body 110 generates power. The generated power is supplied to the load 12 connected to the r-SOC body 110.

[0077] Furthermore, in the SOFC mode, water (water vapor) is generated at the fuel electrode 112 (see formula (3) above). The water vapor generated at the fuel electrode 112 is sucked in by the blower 164 as the fuel electrode exhaust gas EX1. The fuel electrode exhaust gas EX1 also contains fuel gas F that has not reacted at the fuel electrode 112. The sucked in fuel electrode exhaust gas EX1 is separated into fuel gas F and water by a cooler (separation section) and a gas-liquid separator (separation section), both of which are not shown. The separated water is then used as water vapor W in the SOEC mode.

[0078] In the SOFC mode, the air electrode exhaust gas EX2 exhausted from the air electrode 114 contains oxygen-containing gas S that has not reacted in the air electrode 114. The air electrode exhaust gas EX2 is sucked into the blower 174, passes through the second heat exchanger 182, and is then guided to the storage section 178.

[0079] The temperature in SOFC mode is determined taking into consideration the components and size of the r-SOC body, the required steam electrolysis reaction rate, etc., and is, for example, 300°C to 1000°C, or 500°C to 900°C.

[0080] The reversible fuel cell system and its operating method according to the embodiment of the present invention have been described above with reference to the drawings, but the matters disclosed herein are illustrative and not restrictive, except for the r-SOC main body. In particular, matters not explicitly disclosed in the matters disclosed herein, such as operating conditions, operating conditions, various parameters, dimensions, weights, volumes of components, etc., do not deviate from the scope of ordinary practice of a person skilled in the art, and values ​​that can be easily assumed by a person skilled in the art can be adopted. EXAMPLES

[0081] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0082] FIG. 3 shows a configuration diagram of an r-SOC cell (single cell) according to the embodiment.

[0083] 1. Preparation of r-SOC cell (single cell) (a) Solid electrolyte A flat (disk) type scandia-stabilized zirconia (ScSZ: 10mol%ScSZ) with a diameter of 20 mm and a thickness of 200 μm 2 O 3 -1mol%CeO 2 -89mol%ZrO 2 ) (manufactured by Daiichi Kigenso Kagaku Kogyo Co., Ltd.) was used. (b) Fuel electrode paste The electrode framework material is La 0.1 Sr 0.9 TiO 3 (LST, Kyoritsu Material Co., Ltd.) Powder, Gd 0.1 Ce 0.9 O 2 (GDC, Rhodia) powder was used. LST powder was used as the electronically conductive oxide, and GDC powder was used as the ionically conductive oxide. LST and GDC were mixed in a volume ratio of 50:50 (weight ratio of 5.26:7.21), and α-terpineol in which 6 wt% ethyl cellulose had been dissolved was weighed out in a weight ratio of approximately 70:30. The mixture was then mixed well to obtain a fuel electrode paste. (c) Air electrode paste LSCF((La 0.6 Sr 0.4 )(Co 0.2 Fe 0.8 )O 3 The air electrode paste was obtained by mixing and stirring the powder (manufactured by PRAXAIR) and the binder in a weight ratio of 6:4.

[0084] [Example 1] (LST-GDC fuel electrode) The r-SOC cell (single cell) of Example 1 was fabricated by applying an electrode paste, sintering, and heat treating it using a screen printing method in the following steps (1) to (12). The same paste was applied to both the second and third layers of the fuel electrode. (1) Apply the first layer of fuel electrode paste (GDC, 5 μm) to the electrolyte plate. (2) The fuel electrode is baked (1400°C, 2 hours). (3) The second layer of fuel electrode paste (LST-GDC, 30 μm) is applied on the first layer. (4) Dry in a dryer (approximately 100°C) for approximately 20 minutes. (5) Apply the third layer of fuel electrode paste (LST-GDC, 30 μm) on top of the second layer. (6) After embedding the mesh part of the current collector in the fuel electrode part, it is pressed lightly from above to adhere to the electrolyte plate. (7) The fuel electrode is baked (1300°C, 3 hours). (8) Apply the first layer of cathode paste (GDC, 10 μm) to the electrolyte plate. (9) The air electrode is baked (1300°C, 2 hours). (10) Apply a second layer of air electrode paste (LSCF, 30 μm) on top of the first layer. (11) After embedding the mesh part of the current collector in the air electrode part, it is pressed lightly from above to adhere to the electrolyte plate. (12) The air electrode is baked (1100°C, 2 hours). The reference electrode was prepared by coating the r-SOC cell with platinum paste at a position 2 mm away from the air electrode.

[0085] [Comparative example 1] (LST fuel electrode) An r-SOC cell (single cell) of Comparative Example 1 was obtained in the same manner as in Example 1, except that in the manufacturing method of the r-SOC cell of Experimental Example 1 above, a fuel electrode paste of only LST powder was used instead of the fuel electrode paste of LST-GDC powder.

[0086] [Comparative example 2] (GDC fuel electrode) An r-SOC cell (single cell) of Comparative Example 2 was obtained in the same manner as in Example 1, except that in the manufacturing method of the r-SOC cell of Experimental Example 1 above, a fuel electrode paste of only GDC powder was used instead of the fuel electrode paste of LST-GDC powder.

[0087] [Reference example 1] (Ni-ScSZ fuel electrode) A fuel electrode paste was prepared in the same manner as in Example 1, except that nickel oxide powder (NiO, manufactured by Kanto Chemical, particle size 1 to 5 μm) and ScSZ powder (manufactured by Daiichi Kigenso Kagaku Kogyo, product name "10Sc1CeSZ") (56 wt %: 44 wt %) were used as the raw material powders for the fuel electrode instead of the LST powder and GDC powder. Next, the fuel electrode paste was baked on one side of the solid electrolyte at 1300°C for 3 hours. Next, an air electrode was prepared on the opposite surface of the fuel electrode in the same manner as in Example 1, to obtain an r-SOC cell (single cell) of Comparative Example 1.

[0088] 2. Evaluation 2-1. Electrochemical property evaluation (initial performance) The electrochemical characteristics (initial performance evaluation) were evaluated for the r-SOC cells of Example 1, Comparative Examples 1 and 2, and Reference Example 1. A schematic diagram of the evaluation device is shown in FIG. 50% H as fuel gas 2 -50%H 2 Humidified hydrogen of 1000 m2 was supplied to the fuel electrode, and dry air was supplied to the air electrode, and current-voltage (IV) characteristics and impedance measurements were performed at 800 °C. The amount of water vapor was adjusted by passing the supplied fuel through a humidifier and adjusting the temperature of the humidifier. The IR loss and non-ohmic overpotential on the fuel electrode side were separated and evaluated by measuring the impedance using an impedance analyzer (EIS) (Solatron, 1255WB) with potentiostat / galvanostat functions.

[0089] For the r-SOC cells of Example 1, Comparative Examples 1 and 2, and Reference Example 1, the current density was changed to switch between the SOFC mode (positive current density) and the SOEC mode (negative current density). Figure 5 shows the current-voltage (IV) characteristics of each r-SOC cell. Figure 6 shows the current-voltage (IV) characteristics of the r-SOC cell in the SOEC mode (-0.2 A cm -2 ) and SOFC mode (0.2A cm -2 ) shows the impedance measurement results. As shown in Fig. 5, Example 1 (LST-GDC fuel electrode) had a lower electrode potential than Reference Example 1 (Ni-ScSZ fuel electrode) in SOEC mode, and showed an electrode potential that was almost equivalent to that of Reference Example 1 (Ni-ScSZ fuel electrode) in SOFC mode. In addition, as shown in Fig. 6, it was confirmed that Example 1 (LST-GDC fuel electrode) had a smaller overvoltage in SOEC mode than Reference Example 1 (Ni-ScSZ fuel electrode). These results show that Example 1 (LST-GDC fuel electrode) has higher performance in steam electrolysis (SOEC mode) than Reference Example 1 (Ni-ScSZ fuel electrode), which is widely used in SOFCs, and has equivalent performance in power generation.

[0090] Furthermore, in Comparative Example 1 (LST fuel electrode) and Comparative Example 2 (GDC fuel electrode), the electrode potential was higher than that of Example 1 in the SOEC mode and lower than that of Example 1 in the SOFC mode (see FIG. 5). The overvoltage was higher in both the SOEC and SOFC operation modes (see FIG. 6). This indicates that Example 1 (LST-GDC fuel electrode) has improved steam electrolysis and power generation characteristics due to the combination of LST and GDC.

[0091] 2-2.SOFC / SOEC reversible cycle durability test A reversible cycle durability test (1000 cycles) was carried out on the r-SOC cells of Example 1 and Reference Example 1. Fig. 7 shows the measurement conditions for the reversible cycle durability test (temperature: 800°C), and Fig. 8 shows the change over time in the reversible cycle durability test.

[0092] As shown in Fig. 8, in the r-SOC cell (Ni-ScSZ fuel electrode) of Reference Example 1, as the number of cycles increases, the electrode potential increases in SOEC mode and decreases in SOFC mode, whereas in the r-SOC cell (LST-GDC fuel electrode) of Example 1, the potential change is small even with an increase in the number of cycles. This result confirms that the r-SOC cell having the fuel electrode of Example 1 can operate stably even when switching between SOEC (steam electrolysis) and SOFC (power generation) compared to the r-SOC cell (Reference Example) equipped with a conventional fuel electrode.

[0093] 2-3. Change in impedance before and after reversible cycle durability test Also shown is the change in impedance under OCV before and after a reversible cycle durability test (1000 cycles). From the change in impedance before and after the reversible cycle durability test in Figure 9, the IR loss in Reference Example 1 (Ni-ScSZ fuel electrode) was approximately 0.12 Ω cm 2 to about 1.2Ωcm 2 In contrast, in Example 1 (LST-GDC anode), the resistance increased significantly (by a factor of about 10) to about 0.30 Ωcm 2 to about 0.32 Ω cm 2 There was almost no change. On the other hand, the non-ohmic overvoltage before and after the reversible cycle durability test was about 0.72 Ω cm for Reference Example 1 (Ni-ScSZ fuel electrode). 2 to about 0.81 Ω cm 2 In contrast, in Example 1 (LST-GDC anode), the increase was about 0.14 Ω cm 2 to about 0.37 Ω cm 2 This was an increase of 10%.

[0094] 2-4. Microstructural observation after reversible cycle testing The anodes of Reference Example 1 and Experimental Example 1 before and after the reversible cycle durability test (1000 cycles) were sampled using a focused ion beam scanning electron microscope (FIB-SEM) and their microstructures were evaluated using a scanning transmission electron microscope (STEM). The results are shown in Figures 10 and 11, respectively. In Reference Example 1 (Ni-ScSZ fuel electrode), little change was observed in the ScSZ particles before (Fig. 10(a)) and after (Fig. 10(b)) the cycle test, but a noticeable change appeared in the Ni particles, and their thickness reduction was confirmed. On the other hand, in Example 1 (LST-GDC fuel electrode), no noticeable change was observed in the LST particles and GDC particles before (Fig. 11(a)) and after (Fig. 11(b)) the cycle test.

[0095] The above results show that Example 1 (LST-GDC fuel electrode) has superior durability against repetition of SOFC and SOEC modes compared to the conventional Reference Example 1 (Ni-ScSZ fuel electrode). [Industrial Applicability]

[0096] The reversible solid oxide fuel cell (r-SOC) of the present invention can operate stably for long periods of time even when switching between SOEC and SOFC, so a reversible fuel cell system equipped with this is industrially promising as an energy system that can generate electricity and store energy using hydrogen produced by steam electrolysis. [Explanation of symbols]

[0097] 100 Reversible Fuel Cell System 110 r-SOC body 120 Fuel supply section 130 Oxygen supply unit 140 Water supply section 160 First exhaust section 170 Second exhaust section 180 1st heat exchanger 182 Second heat exchanger 184 Third heat exchanger 190 Central Control Unit 192 Mode switching section

Claims

1. The fuel cell comprises an anode, an air electrode, and a solid electrolyte provided between the anode and the air electrode, the anode being composed of an electronically conductive oxide and an ionically conductive oxide, and at least a part of the ionically conductive oxide is CeO 2 a reversible solid oxide fuel cell using a lithium-ion battery; A reversible fuel cell system comprising: a fuel supply unit that supplies a fuel gas containing at least hydrogen to the fuel electrode; an oxygen supply unit that supplies an oxygen-containing gas containing at least oxygen to the air electrode; and a water supply unit that supplies water to the fuel electrode or the air electrode.

2. The ion-conductive oxide in the fuel electrode is CeO 2 2. The reversible fuel cell system according to claim 1, which is composed solely of base oxides.

3. The CeO 2 The Gd-based oxide 2 O 3 Doped CeO 2 Or Sm 2 O 3 Doped CeO 2 3. The reversible fuel cell system according to claim 1 or 2,

4. The electronic conductive oxide in the fuel electrode has a composition formula of ABO 3 4. The reversible fuel cell system according to claim 1, which is a Ti-containing perovskite oxide represented by the formula (1) in which the A site is at least one element selected from the group consisting of Ca, Sr, Ba and La, and the B site is Ti.

5. 5. The reversible fuel cell system according to claim 1, wherein the fuel electrode is made of a sintered body of particulate electronically conductive oxide and particulate ionically conductive oxide.

6. 6. A method for operating a reversible fuel cell system according to claim 1, which comprises repeatedly performing steam electrolysis and power generation in a temperature range of 300° C. or higher and 1000° C. or lower.

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