JP4318887B2 - Molten carbonate fuel cell cathode and method for producing the same - Google Patents

Description

【００３５】
（実施例２）溶出特性評価
実施例１で得られたＭｇＦｅ₂Ｏ₄複合化Ｎｉ粉体を用いて、ドクターブレード法によりシート成形し、還元雰囲気下での焼成によりＭｇＦｅ₂Ｏ₄被覆Ｎｉカソードを作製した。このカソード（試験片サイズ3×10×0.７thick mm）について、Li/Na系溶融炭酸塩中でのNi溶出量を測定した。なお、当該カソードにおいては、溶融炭酸塩内においてその場酸化されて、ＮｉからＮｉＯが生成されている。
表２に測定条件を示す。圧力1.2MPa、温度650℃、10%CO₂/7%O₂/83%N₂カソード雰囲気下で（Li_0.52Na_0.48）₂CO₃溶融炭酸塩中に浸漬して約７５０時間の溶融試験を実施した。Ni溶出量は、溶融塩を定期的にサンプリングし、測定用試料調製後、ICP発光分光分析法により測定した。比較例として従来カソードNiOについて、同条件での溶出試験を実施しNi溶出量を測定した。
【表２】

【００３６】
なお、評価装置は、オートクレーブ、加熱ヒータ、試料サンプリング部、昇降式サンプリング管、温度及び圧力制御部から構成される。オートクレーブは、ステンレス製で内径１０４mm、容積約２５００ｃｍ³を有している。オートクレーブ内には、アルミナ坩堝が設置されており、この坩堝内に溶融炭酸塩を保持できるようになっているとともに、被験試料を完全に溶融炭酸塩に浸漬できるようになっている。また、坩堝内の溶融炭酸塩には、カソードガスを供給するガスバブリング管の先端が侵入された状態となっているとともに、熱電対が浸漬されて溶融炭酸塩の温度が測定可能となっている。
加熱方式は、オートクレーブ内部及び外部に備えたヒータにより加熱する方式であり、加圧方式はガスボンベから供給したガス圧により昇圧させ、圧力制御部により制御する方式である。溶融塩のサンプリングは、サンプリング室を加圧して、オートクレーブの内部圧力と同圧に調節した後、シャッターを開け、サンプリング（アルミナ）管をアルミナ坩堝中の溶融塩に浸漬して行う。また、サンプリング管の先端側部に開口するサンプリング口から連続的あるいは定期的にサンプリングできるようになっている。
【００３７】
図４にＭｇＦｅ₂Ｏ₄被覆ＮｉＯカソード及びＮｉＯのＮｉ溶出量の経時変化を示す。Ｎｉ溶出量はいずれのカソードも約５５０時間で平衡に達し、このときの値をNi溶解度とした。ＭｇＦｅ₂Ｏ₄被覆ＮｉカソードのＮｉ溶解度は約５×１０^-6（モル分率）となり、従来カソード（ＮｉＯ）の約１／４と大きく低減した。
また、図５に溶出試験後のＭｇＦｅ₂Ｏ₄被覆ＮｉＯカソードのＳＥＭ写真とＥＤＳ分析結果のグラフ図とを示す。溶出試験後においてもＮｉＯの表面には、Ｆｅ元素が確認されており、溶融炭酸塩中で固溶体層による被覆構造が保持されていることがわかった。これらのことから、当該被覆構造の保持によりＮｉの溶出が抑制されたことと考えられる。
以上の結果からＭｇＦｅ₂Ｏ₄被覆ＮｉＯカソードは１ＭＰａ以上の高圧下においても高い耐溶融塩耐食性を有することが確認され、本カソードの有効性が実証された。
【００３８】
（実施例３）電池性能評価
実施例２で作製したカソードを用いてＭＣＦＣを作製した。なお、アノードは、Ｎｉ−Ａｌ，Ｃｒとし、電解質としてはＬｉＡｌＯ₂、保持材にＬｉ／Ｎａ系炭酸塩を含浸したものを用いた。燃料電池の性能は、温度６５０℃、圧力０．１MPa、カソードガス：CO₂/air=30/70%（酸素利用率50%）、アノードガスH₂/CO₂/H₂O=64/16/20%（燃料利用率75%）の条件下で負荷電流を0〜150mA/cm₂通電したときの初期性能（電流−電圧特性）は、図６に示すとおりであった。図６に示すように、実施例のカソードについて、従来のカソード（ＮｉＯ）を用いた場合と同等の性能が得られた。
【００３９】
また、この電池を長期運転（10000時間相当）後における電解質中のNi量を分析した結果を図７に示す。
図７からは、実施例２で作製したカソードは、従来カソード（ＮｉＯ）に比べＮｉの溶出が約30％減少することを確認した。
【００４０】
【発明の効果】
本発明によれば、電極特性とＮｉＯの溶出抑制とを両立させうるカソード材料ならびにＭＣＦＣを提供することができる。
【図面の簡単な説明】
【図１】本発明の粉体を得るのに適した機械的処理の模式図である。
【図２】本発明の粉体を大量に得るのに適した機械的処理装置の概略を示す図である。
【図３】実施例で作製した粉体のＳＥＭ写真を示す図（ａ）と、この粉体のＥＤＳの元素分析結果（Ｆｅ）を示す図（ｂ）と、この粉体のＥＤＳ分析結果（Ｎｉ）を示す図（ｃ）とを組み合わせた図である。
【図４】ＭｇＦｅ₂Ｏ₄被覆ＮｉＯカソード及びＮｉＯのＮｉ溶出量の経時変化を示すグラフ図である。
【図５】溶出試験後のＭｇＦｅ₂Ｏ₄被覆ＮｉＯカソードのＳＥＭ写真とＥＤＳ分析結果のグラフ図とを組み合わせた図である。
【図６】実施例２で作製したカソードを用いて作製したＭＣＦＣの電池性能の評価結果を示す図である。
【図７】実施例３で作製したＭＣＦＣを長期運転（10000時間相当）した後における電解質中のNi量を分析した結果を示すグラフ図である。
【符号の説明】
２、１２ 回転槽
４、１４ キャビティ
６、１６ 回転翼
１０ 原料粉末
１８ 粉体収容部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a manufacturing technique of a molten carbonate fuel cell (hereinafter abbreviated as MCFC), and more particularly to a manufacturing technique of a cathode.
[0002]
[Prior art]
The MCFC is composed of two porous electrodes (fuel electrode and cathode), an electrolyte sandwiched between them, and an electrolyte plate holding the electrolyte. Conventionally, nickel oxide (NiO) has been used for the cathode (cathode) of MCFC. NiO used as a cathode material elutes in a small amount in an electrolyte molten carbonate during battery operation. Further, the eluted NiO is dissolved in nickel by the reaction of formula (1) in the molten carbonate. Diffuses as ions.
NiO + CO₂→ Ni²⁺+ CO_Three ^2-(1)
The diffused nickel ions are reduced by hydrogen dissolved in the molten carbonate and deposited as metallic nickel. The metallic nickel grows in the molten carbonate with the operation time, and eventually an internal short circuit of the cell occurs, and the operation as a battery becomes impossible. When considering long-term operation in MCFC, the suppression of NiO elution from the cathode is the most important issue.
[0003]
As techniques for suppressing the elution of NiO, there are JP-A-11-233121 and JP-A-6-349499. Japanese Patent Application Laid-Open No. 11-233121 describes that a cathode material is a powder composed of coated particles obtained by coating the surface of Ni particles with Co or Co oxide. According to this cathode material, LiCo is formed on the surface of NiO by oxidation in the battery.₂Mg-LiCo₂, Li (Co, Ni) O₂It is described that the molten salt resistance is exhibited by these, and the elution of NiO can be suppressed.
Japanese Patent Laid-Open No. 6-349499 discloses that Ni powder is mixed with iron oxide or cobalt oxide by adding an alkaline earth oxide and calcined to form a composite oxide. It is described that a cathode material is added to and mixed with powder. According to this cathode material, a Ni—Fe (or Co) oxide film having excellent dissolution resistance is formed on the NiO surface, and alkaline earth exists around the iron oxide film on the Ni surface. Thus, it is described that the carbonate is less likely to adhere to NiO and the elution of NiO is suppressed.
[0004]
[Problems to be solved by the invention]
However, according to the technique described in JP-A-11-233121, although the molten salt resistance is improved, elution suppression during battery operation has not been achieved. Further, according to the technique described in Japanese Patent Laid-Open No. 6-349499, the composite oxide on the NiO surface has impaired the conductivity of the cathode within a range in which the elution of NiO can be suppressed to some extent.
From the above, to date, no cathode material that can achieve both electrode characteristics and suppression of elution of NiO has been found, and a cathode material capable of realizing these has been desired.
[0005]
Accordingly, an object of the present invention is to provide a cathode material and an MCFC that can achieve both electrode characteristics and suppression of NiO elution.
[0006]
[Means for Solving the Problems]
The inventors of the present invention conducted various studies on a composite form of Ni and an oxide capable of imparting molten salt resistance to NiO in the cathode.₂O_FourThe inventors have found that by using Ni particle powder of a predetermined shape coated with particles as a cathode material, both NiO elution suppression and electrode performance can be achieved, and the above-described problems can be solved.
[0007]
That is, the inventors of the present invention considered that MgFe, which is considered to have a low possibility of being used as an impediment to electrode performance because it has high molten salt resistance but low electrical conductivity.₂O_FourThe composite form on the surface of the NiO particles was examined using less particles of MgFe than expected.₂O_FourNiO elution suppression effect can be realized with the amount used, and at the same time less MgFe₂O_FourThe present invention was completed by finding that sufficient electrode performance could be secured for the amount used.
[0008]
According to the studies by the present inventors, Ni powder containing Mg particles and MgFe are simply used, as in the conventional method, as the cathode powder disclosed in JP-A-6-349449.₂0_FourIn the powder obtained by mechanically mixing the particle-containing powder, during the oxidation in the electrode, MgFe₂0_FourIt cannot be said that the particles are present so as to cover the surface of the Ni particles, but are almost in a state of being in partial contact with the Ni surface.₂0_FourFe and Mg were diffused into the particles. For this reason, MgFe is selectively applied to the NiO surface layer.₂0_FourIt was found that no layer was formed.
The present invention is based on such studies, and the present inventors₂0_FourThis was achieved for the first time as a result of searching based on its elution characteristics (especially under high pressure) and electrode performance by preparing the composite-form Ni-particle-containing powder by using the particle-containing powder. .
That is, according to the present invention, the following means are provided.
[0009]
(1)Ni particles having an average particle diameter of 1 to 5 μm having surface ridges, and spherical MgFe having a particle diameter of 0.5 μm or less ₂ O _Four In a state where particles are made to flow, it is obtained by mixing while intermittently acting any one or more of frictional force, shearing force and compressive force,
  For Ni, MgFe ₂ O _Four Is added in an amount of 1 mass% to 10 mass%,
  Forming binders, dispersants, and pores in composite powders with Fe of 0.6 mol% to 6.5 mol% and Mg of 0.3 mol% to 3.3 mol% with respect to Ni A molten carbonate comprising: a step of adding and mixing an agent to prepare a slurry; a step of forming the slurry into a sheet; a step of reducing and baking the sheet; and a step of oxidizing the sheet after reduction and baking. Of manufacturing a cathode for a fuel cell.
(2)A cathode for a molten carbonate fuel cell obtained by the production method according to (1),
  A solid solution layer containing Fe-Mg-Ni-O is selectively coated only on the surface of a three-dimensional network-like porous skeleton in which a large number of chained bodies in which NiO particles are linked in a chain are branched,
  A cathode for a molten carbonate fuel cell, wherein the solid solution layer has a molar ratio of Fe to Mg of 0.5 to 2.
[0010]
According to these inventions, elution of NiO is remarkably suppressed as compared with the conventional cathode, and sufficiently high electrode performance can be maintained.
[0011]
According to the present inventors, the present cathode material is capable of effectively suppressing the elution of NiO in a small amount by forming the solid solution layer selectively on the NiO surface only during the oxidation in the electrode. In addition, since the solid solution layer is rich in Fe with respect to Mg, Fe generates a very stable Li—Fe oxide layer in the molten carbonate due to the reaction with Li in the molten carbonate. The surface microstructure of the electrode is stably maintained, and the effect is also small.₂O_FourIt is thought that it contributes to effective NiO suppression by. The inference does not bind the present invention.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
The MCFC cathode of the present invention is a cathode mainly composed of NiO and has a structure in which the NiO surface is coated with a solid solution layer containing Fe—Mg—Ni—O. Spherical MgFe on the surface of Ni particles having surface ridges₂O_FourIt can be obtained by a composite powder for a cathode of a molten carbonate fuel cell comprising particles. Further, the present powder includes Ni-particle-containing powder having a surface ridge, and MgFe₂0_FourManufacture of composite powder for cathode of molten carbonate fuel cell that is mixed with one or more of frictional force, shearing force and compressive force in a state of flowing particle-containing powder. Can be obtained by the method of
In addition, the electrode of the present invention can be produced by tape-molding, reducing and firing the powder, and oxidizing the battery.
[0013]
(Composite powder for cathode)
This powder consists of Ni particles, MgFe₂O_FourContaining particles. Further, at least some of these primary particles are formed on the surface of the Ni particles with MgFe.₂O_FourSecondary particles in which particles are present are formed. Preferably, such secondary particles are mainly used in the present powder.
[0014]
The Ni particles preferably have a surface ridge. The surface bulge is generally composed of, for example, one or a combination of wrinkles, creases and rod-like, needle-like or spike-like projections on the particle surface. As the Ni particles having surface bulges, spherical particles having a large number of spike-like projections on the surface are preferably used. More preferably, chain-like particles (hereinafter referred to as filament particles) in which a plurality (preferably about 5 to 30) of spherical particles having a large number of spike-like protrusions are connected in a chain form. Use. Filamentous particles are advantageous in that the filamentous particles can reliably maintain a contact state, can easily form a porous structure, and can easily ensure a large surface area. Filamentous Ni particles are commercially available, for example, as Ni powder Type 255 manufactured by INCO.
The particle size of the Ni particles is not particularly limited, but preferably the average particle size is 1 to 5 μm. This is because if the average particle size is less than 1 μm, it is difficult to form a covering structure on the electrode, and if the average particle size exceeds 5 μm, the specific surface area of the electrode is reduced and the electrode performance is reduced. More preferably, it is 2-3 micrometers. In particular, when the Ni particles are filamentous particles, the particle size of the single particles constituting the filament is preferably 1 μm or more and 5 μm or less, more preferably 2 μm or more and 3 μm or less. Further, the length of the filamentous particles is preferably mainly 30 μm or less, more preferably 20 μm or more and 25 μm or less. In the present specification, the particle size of Ni is indicated by “Fisher Subscribe Size”.
[0015]
MgFe₂O_FourAlthough the form of particle | grains is not specifically limited, Preferably it is a spherical particle. MgFe₂O_FourThe particles preferably have a particle size of 0.5 μm or less from the viewpoint that the surface of the Ni particles exists in a covering state. MgFe below the particle size₂O_FourIf it is a particle, it can be made to exist in a covering form in a small amount with respect to the surface bulge portion or the concave portion formed between the surface bulge portions of the Ni particles having the above average particle diameter range. More preferably, it is 0.1 μm or less.
[0016]
Ni particles and MgFe₂O_FourThe composite form with the particles is MgFe on the surface of the Ni particles.₂O_FourThis is a form in which particles are attached. Preferably, MgFe is formed in the convex portion of the surface formed by the surface raised portion, the portion where the surface raised portion is not formed, or the concave portion formed between the surface raised portions.₂O_FourParticles are attached. Preferably, MgFe is provided in the concave portion relative to the surface raised portion.₂O_FourParticles are attached embedded.
MgFe₂O_FourAs for the adhesion state of the particles, it is preferable that the particles are adhered to the entire surface of the Ni particles in a dispersed state. Such an adhesion state is referred to as a covering state in this specification.
[0017]
In the present powder, it is preferable that Fe is about 0.6 mol% or more and about 6.5 mol% or less and Mg is about 0.3 mol% or more and about 3.3 mol% or less with respect to Ni. This is because a solid solution layer exhibiting a sufficient Ni elution suppressing effect is formed when the content is in the mol% range. If Fe is less than 0.6 mol% and Mg is less than 0.3 mol%, it will be difficult to obtain a sufficient Ni elution suppression effect, Fe will exceed 6.5 mol%, and Mg will be 3. This is because if it exceeds 3 mol%, the conductivity is greatly lowered and the sinterability of the electrode is lowered. More preferably, Fe is 1.8 mol% or more and 3.1 mol% or less, and Mg is 0.9 mol% or more and 1.5 mol% or less.
At the same time, MgFe against Ni in this powder₂O_FourThe composite ratio of MgFe with respect to Ni₂O_FourIs preferably about 1 mass% or more and 10 mass% or less. If within this range, MgFe₂O_FourEven when is added, the electrode characteristics as a cathode can be maintained, and the molten salt resistance of the cathode can be sufficiently improved. If it is less than 1 mass%, it is difficult to sufficiently exhibit the molten salt resistance, and if it exceeds 10 mass%, the conductivity and sinterability of the resulting cathode will be too low. More preferably, it is about 3 mass% or more and about 5 mass% or less. This is because high molten salt resistance (Ni elution suppression effect) can be obtained within this range.
[0018]
Next, the manufacturing method of this powder is demonstrated.
The method for producing this powder is not particularly limited, and various conventionally known compositing means can be employed. However, MgFe is formed on the surface of Ni particles having surface bulges.₂O_FourIn order to combine the particles into a coating, a mixture of these primary particle-containing powders is combined by mechanical treatment.
Examples of mechanical processing methods include particle-combining devices such as hybridization systems (Nara Machinery Co., Ltd.), Cosmos (Kawasaki Heavy Industries), Mechano Fusion System (Hosokawa Micron), Mechano Mill (Okada Seiko), Theta Composer (Tokuju Seisakusho). A mixing method using can be employed. Regarding the mechanical particle composite technology, including the technology for creating coated composite particles, Makio Naito, present state and prospects of mechanical particle composite technology (powder and industry (VOL. 25, No. 5 (1994), No. 1) 31 to 42).
[0019]
In particular, in the present invention, it is preferable to mix these primary particle powders by mechanical treatment while maintaining the surface structure of the first particles. Here, the mechanical mixing treatment refers to a treatment in which any one or more of mechanical forces among a frictional force, a compressive force, and a shearing force are applied to the primary particle powder mixture.
In addition, such mechanical treatment includes Ni particle powder and MgFe₂O_FourIt is preferable to carry out in a state where the particle powder flows. Moreover, it is preferable to make the said mechanical force act intermittently and intermittently. Furthermore, it is preferable to carry out so that a shear force and a compressive force may be simultaneously given among the said mechanical force, More preferably, it is preferable to make these forces act intermittently.
[0020]
According to such a mechanical treatment, mixing and complexing of the primary particles can be achieved while maintaining the surface structure of the Ni particles. As a result, the surface structure of the Ni particles is maintained in the composite secondary particles.
Specifically, it is preferable to mix so as to repeatedly apply a shearing force and a compressive force instantaneously in a state where these primary particle powders are substantially flowing.
In particular, in the flow state of the powder, the shear force and the compressive force act instantaneously, so that the MgFe without breaking the surface structure of the Ni particles having surface ridges.₂O_FourThe compounding in which the particles are embedded in the surface recesses of the Ni particles is efficiently achieved.
[0021]
A schematic diagram of the mechanical treatment for carrying out such mixing is shown in FIG. In FIG. 1, a mixing device including a rotating tank 2 and rotating blades 6 provided in a cavity 4 of the rotating tank 2 is described. The rotary tank 2 and the rotary blade 6 are formed so as to be rotatable in the same direction or in the reverse direction, respectively, and the rotation speed can be controlled. Further, by rotating the rotating tub 2 and the rotating blade 6 so as not to rotate in the same direction and at the same speed (hereinafter referred to as relative rotation), the inner wall of the cavity 4 and the rotating blade 6 are rotated. It is formed so that the gap interval changes. In order to obtain such a mixed state, the sectional shape of the cavity 4 and the rotor blade 6 is made irregular, the rotor blade 6 is made uneven, or the rotational axis of the rotor blade 6 is displaced in the cavity 4. It can be achieved. In the form shown in FIG. 1, the shape inside the cavity 4 and the shape of the rotor blade 6 are elliptical shapes having different sizes.
[0022]
In the ceramic powder manufacturing method of the present invention, using such a mixing device, the raw material powder 10 containing the first particle powder and the second particle powder does not stick to the inner wall of the cavity 4 during the relative rotation. When the rotating tank 2 is rotated to the extent, the powder 10 in the rotating cavity 4 is free-falling while flowing against the inner wall, and as a result, a constantly flowing state is obtained. Due to such a fluid state, the raw material powder 10 is well mixed. (See Figure 1)
When the rotary blade 6 is rotated relative to the rotary tank 2, a state occurs in which the gap between the inner wall of the cavity 4 and the rotary blade 6 becomes narrow. At this timing, the raw material powder 10 that flows or falls along the inner wall is press-fitted into this narrow gap (see the lowermost drawing in FIG. 1). Thereby, a compressive force and a shear force act simultaneously on the raw material powder 10 in a fluid state. At this point, a mechanochemical reaction occurs between the particles.
Such a press-fitted state is maintained temporarily, that is, almost instantaneously, and the raw powder 10 is again returned to a free flowing state in which no pressure is applied by relative rotation. That is, the compressive force and shear force are alleviated. Then, after a certain time, the press-fitted state of the raw material powder 10 is formed again, and the compressive force and the shearing force are simultaneously applied, and thereafter such a mixed state is repeated.
According to this apparatus, sufficient mixing and a composite state preferable in the present invention can be easily obtained.
[0023]
Furthermore, according to the apparatus shown in FIG. 2, a large amount of primary powder can be easily processed and combined. In this apparatus, the rotary tank 12 includes a mixing cavity 14 including rotary blades 16 and a cavity capable of temporarily storing and discharging the mixed powder in the cavity 14 when the rotary tank 12 rotates. A powder container 18 is provided. The mixed powder falls into the powder container 18 as the rotating tank 12 rotates, or falls into the cavity 14 from the powder container 18, so that a large amount of powder can be obtained. The compounding process can be performed by the interaction between the inner wall of the rotary tank 12 and the rotary blade 16.
[0024]
MgFe against Ni particles₂O_FourThe amount and state of adhesion of the particles can be adjusted by mixing the raw material powder 10, the speed of the rotary tanks 2 and 12 and the rotary blades 6 and 16, the mixing time, and the like.
[0025]
By mixing the primary particle powder by such a mechanical treatment, the surface of the Ni particles is coated with MgFe.₂O_FourA composite particle powder to which particles are attached can be obtained. That is, in the powder obtained by this treatment, MgFe is formed on the surface protrusions and recesses formed by the surface raised portions of the Ni particles.₂O_FourParticles are attached. Preferably, MgFe is formed in the concave portion of the surface raised portion.₂O_FourParticles are attached embedded.
In particular, according to the present processing method, a relatively small amount of MgFe with respect to Ni particles.₂O_FourThe particles can be uniformly coated.
[0026]
(Production of cathode)
Next, production of an MCFC cathode using the composite powder will be described.
Various known methods can be employed to produce the MCFC cathode using this composite powder, but the doctor blade molding method is less likely to impair the shape of the Ni powder. It is preferable to use a technique.
For example, a binder, a dispersant, a pore forming agent, and the like are added to the present powder and mixed to prepare a slurry. This slurry is formed with a doctor blade forming apparatus and dried to form a sheet. The sheet can be fired in a reducing atmosphere and then oxidized (or in-situ or in-battery oxidation) to produce an electrode.
[0027]
When a cathode is produced using this powder, the composite particles contained in this powder each have electrode performance derived from Ni, and MgFe₂O_FourIt has molten salt resistance derived from Further, in the composite particles, MgFe is formed on the Ni particle surface.₂O_FourAlthough particles are present, many are present in the recesses between the surface ridges, and the surface ridges of the composite particles are partially exposed. For this reason, the composite particles are prevented from aggregating, and a network-like porous skeleton can be easily formed by sintering by the surface ridges partially exposed. At the same time, MgFe present mainly in the recesses between the surface ridges₂O_FourThe particles are fired so as to cover the surface of the obtained Ni network-like porous skeleton. Here, the network-like porous skeleton is a three-dimensional network skeleton in which the skeleton is a chain of Ni (NiO after oxidation treatment or at the time of electrode operation) and a large number of such chain bodies are branched. I mean.
That is, in this powder, MgFe is formed on the Ni particle surface.₂O_FourWhen the particles are partially present, the Ni network can be easily coated during firing. In particular, MgFe on the surface of filamentary Ni particles₂O_FourAccording to the powder in which the particles are combined, a more preferable network structure can be formed, and the NiO surface can be efficiently coated with the Mg—Fe—Ni—O solid solution layer. For this reason, even with a small amount of addition to Ni, MgFe in the cathode₂O_FourIt seems that the coating form by can be obtained.
[0028]
In the present cathode, Fe is preferably about 0.6 mol% or more and about 6.5 mol% or less, and Mg is about 0.3 mol% or more and about 3.3 mol% or less with respect to Ni. It is preferable. This is because a solid solution layer exhibiting a sufficient Ni elution suppressing effect is formed when the content is in the mol% range. If Fe is less than 0.6 mol% and Mg is less than 0.3 mol%, it will be difficult to obtain a sufficient Ni elution suppression effect, Fe will exceed 6.5 mol%, and Mg will be 3. This is because if it exceeds 3 mol%, the conductivity is greatly lowered and the sinterability of the electrode is lowered. More preferably, Fe is 1.8 mol% or more and 3.1 mol% or less, and Mg is 0.9 mol% or more and 1.5 mol% or less.
[0029]
Further, in this cathode, during the oxidation treatment, NiO particles and MgFe₂O_FourThe particles are in solid solution, and an Fe—Mg—Ni—O solid solution layer is formed on the surface of the NiO particles. In this solid solution layer, MgFe₂O_FourBecause it is derived from this layer, it is Fe rich. For this reason, a Li-Fe oxide can be produced | generated by reaction with Li in molten carbonate especially on the surface, and the stable corrosion-resistant layer can be formed. Simultaneously, MgO can suppress elution of NiO by eluting into molten carbonate. Such a solid solution layer is MgFe₂O_FourThus, the cathode can exhibit stable molten salt resistance. Further, since the surface NiO of the NiO skeleton is in solid solution with Fe—Mg—O, the elution of the surface NiO is suppressed.
In particular, according to the present cathode, it has been found that elution of NiO can be effectively suppressed during battery operation under a high pressure, for example, at a pressure of 1 MPa or 1.2 MPa or more. For example, 1.2 MPa, 650 ° C., CO₂/ O₂/ N₂= 10/7/83%, (Li_0.52Na_0.48CO_ThreeIn the results of the elution test, the present inventors confirmed that the elution of NiO was suppressed to about one-fourth as compared with the conventional electrode material (NiO).
[0030]
As described above, the present invention can take the following aspects.
(1) Filamentary Ni particles and spherical MgFe having a smaller particle size than the Ni particles₂O_FourA step of forming a composite particle-containing powder obtained by mixing while causing any one or more of frictional force, shearing force, and compressive force to act intermittently in a state of flowing particles;
A step of reducing and firing the sheet;
A step of oxidizing the sheet after reduction firing,
A method for producing a cathode for MCFC, comprising:
(2) A MCFC cathode obtained by the production method of (1).
(3) A porous body having a three-dimensional skeleton in which NiO particles are connected in a network, and MgFeO_FourMCFC cathode coated with a solid solution layer from the surface.
[0031]
As described above, by applying this technology, it is possible to dramatically extend the life of the molten carbonate fuel cell, and the corrosion resistance is maintained even during long-term battery operation, and a significant improvement in the battery life can be expected. .
[0032]
【Example】
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to a following example.
Example 1 MgFe against filamentary Ni particles₂O_FourParticle composite processing
MgFe using composite powder synthesizer₂O_FourA coated Ni powder was produced. FIG. 1 shows the principle of composite coating, and FIG. 2 shows an outline of the apparatus used. This apparatus is composed of a powder container (upper and lower) that rotates at a low speed and a rotor that rotates at a high speed in the opposite direction to the powder. In the powder container, coated particles (here, MgFe₂O_FourParticles) and core particles (here, Ni particles) are mixed and mixed, so that mechanical properties such as compaction and shearing force act on the powder at the clearance between the rotor rotating at high speed and the container wall surface. A mechanochemical reaction occurs between them. By making this act on the powder intermittently, it is possible to form a composite coating. The cavity in the rotor was 200 ml, and the volume of the upper and lower powder containers was 1000 ml.
[0033]
Table 1 shows the coating treatment conditions. Filamentary Ni (Type255, manufactured by INCO) having an average particle diameter of 2 to 3 μm as core particles, and MgFe having an average particle diameter of 0.5 μm as coated particles₂O_FourWas used. MgFe per 1000g NiFe₂O_Four3 mass% of the powder was added and placed in the powder container, and composite treatment was performed under the conditions of a rotor rotation speed of 2000 rpm and a powder container rotation speed of 5 rpm. Processing time was approximately 50 minutes.
MgFe obtained in FIG.₂O_FourThe SEM observation result of the composite Ni powder is shown, the elemental analysis result (Fe) by EDS of the powder is shown in FIG. 3 (b), and the elemental analysis result (Ni) by EDS is also shown in FIG. 3 (c). As shown in FIG.
As shown in FIG. 3, by intermittently applying a compression / shearing force to the mixed powder, MgFe is applied to the Ni particle surface.₂O_FourIt was clear that composite particles with uniform fine particles were obtained.
[0034]
[Table 1]

[0035]
(Example 2) Elution characteristic evaluation
MgFe obtained in Example 1₂O_FourThe composite Ni powder is used to form a sheet by the doctor blade method and fired in a reducing atmosphere to form MgFe₂O_FourA coated Ni cathode was prepared. For this cathode (test piece size 3 × 10 × 0.7 thick mm), the elution amount of Ni in Li / Na-based molten carbonate was measured. The cathode is oxidized in situ in molten carbonate to produce NiO from Ni.
Table 2 shows the measurement conditions. Pressure 1.2MPa, temperature 650 ℃, 10% CO₂/ 7% O₂/ 83% N₂Under cathode atmosphere (Li_0.52Na_0.48)₂CO_ThreeIt was immersed in molten carbonate and a melting test was conducted for about 750 hours. The amount of Ni elution was measured by ICP emission spectrometry after sampling the molten salt and preparing the measurement sample. As a comparative example, a conventional cathode NiO was subjected to an elution test under the same conditions, and the Ni elution amount was measured.
[Table 2]

[0036]
The evaluation apparatus includes an autoclave, a heater, a sample sampling unit, a liftable sampling tube, and a temperature and pressure control unit. The autoclave is made of stainless steel and has an inner diameter of 104 mm and a volume of about 2500 cm.^Threehave. An alumina crucible is installed in the autoclave. The molten carbonate can be held in the crucible, and the test sample can be completely immersed in the molten carbonate. In addition, the molten carbonate in the crucible is in a state in which the tip of a gas bubbling tube for supplying the cathode gas is intruded, and a thermocouple is immersed so that the temperature of the molten carbonate can be measured. .
The heating method is a method in which heating is performed by a heater provided inside and outside the autoclave, and the pressurization method is a method in which the pressure is increased by a gas pressure supplied from a gas cylinder and controlled by a pressure control unit. The sampling of the molten salt is performed by pressurizing the sampling chamber and adjusting it to the same pressure as the internal pressure of the autoclave, then opening the shutter and immersing the sampling (alumina) tube in the molten salt in the alumina crucible. In addition, sampling can be performed continuously or periodically from a sampling port opened at the tip side of the sampling tube.
[0037]
Figure 4 shows MgFe₂O_FourThe change over time of the Ni elution amount of the coated NiO cathode and NiO is shown. The amount of elution of Ni reached equilibrium in about 550 hours, and the value at this time was defined as Ni solubility. MgFe₂O_FourThe Ni solubility of the coated Ni cathode is about 5 × 10^-6(Molar fraction), which is about 1/4 of the conventional cathode (NiO).
FIG. 5 shows MgFe after the dissolution test.₂O_FourAn SEM photograph of a coated NiO cathode and a graph of EDS analysis results are shown. Even after the dissolution test, Fe element was confirmed on the surface of NiO, and it was found that the coating structure of the solid solution layer was retained in the molten carbonate. From these things, it is thought that elution of Ni was suppressed by holding | maintenance of the said coating structure.
From the above results, MgFe₂O_FourThe coated NiO cathode was confirmed to have high molten salt corrosion resistance even under a high pressure of 1 MPa or more, and the effectiveness of this cathode was demonstrated.
[0038]
(Example 3) Battery performance evaluation
MCFC was produced using the cathode produced in Example 2. The anode is Ni-Al, Cr, and the electrolyte is LiAlO.₂A holding material impregnated with Li / Na carbonate was used. The performance of the fuel cell is as follows: temperature 650 ° C., pressure 0.1 MPa, cathode gas: CO₂/ air = 30/70% (oxygen utilization rate 50%), anode gas H₂/ CO₂/ H₂The load current is 0 to 150mA / cm under the condition of O = 64/16/20% (75% fuel utilization)₂The initial performance (current-voltage characteristics) when energized was as shown in FIG. As shown in FIG. 6, for the cathode of the example, the same performance as that obtained when a conventional cathode (NiO) was used was obtained.
[0039]
FIG. 7 shows the result of analyzing the amount of Ni in the electrolyte after long-term operation (equivalent to 10,000 hours) of this battery.
From FIG. 7, it was confirmed that the elution of Ni was reduced by about 30% in the cathode produced in Example 2 compared to the conventional cathode (NiO).
[0040]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the cathode material and MCFC which can make an electrode characteristic and the elution suppression of NiO compatible can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic view of a mechanical treatment suitable for obtaining the powder of the present invention.
FIG. 2 is a schematic view of a mechanical processing apparatus suitable for obtaining a large amount of the powder of the present invention.
FIG. 3A is a diagram showing an SEM photograph of the powder produced in Example, FIG. 3B is a diagram showing an elemental analysis result (Fe) of EDS of this powder, and an EDS analysis result of this powder ( It is the figure which combined the figure (c) which shows Ni).
FIG. 4 MgFe₂O_FourIt is a graph which shows a time-dependent change of Ni elution amount of a covering NiO cathode and NiO.
FIG. 5: MgFe after dissolution test₂O_FourIt is the figure which combined the SEM photograph of a covering NiO cathode, and the graph figure of an EDS analysis result.
6 is a diagram showing the evaluation results of the battery performance of MCFC produced using the cathode produced in Example 2. FIG.
7 is a graph showing the results of analyzing the amount of Ni in the electrolyte after long-term operation (corresponding to 10,000 hours) of the MCFC produced in Example 3. FIG.
[Explanation of symbols]
2, 12 Rotating tank
4, 14 cavity
6,16 Rotor blade
10 Raw material powder
18 Powder container

Claims (2)

Ni particles having an average particle diameter of 1 to 5 μm having surface ridges, and spherical MgFe having a particle diameter of 0.5 μm or less ₂ O _Four In a state where particles are made to flow, it is obtained by mixing while intermittently acting any one or more of frictional force, shearing force and compressive force,
For Ni, MgFe ₂ O _Four Is added in an amount of 1 mass% to 10 mass%,
Forming binders, dispersants, and pores in composite powders with Fe of 0.6 mol% to 6.5 mol% and Mg of 0.3 mol% to 3.3 mol% with respect to Ni A molten carbonate comprising: a step of adding and mixing an agent to prepare a slurry; a step of forming the slurry into a sheet; a step of reducing and baking the sheet; and a step of oxidizing the sheet after reduction and baking. Of manufacturing a cathode for a fuel cell. A cathode for a molten carbonate fuel cell obtained by the production method according to claim 1,
A solid solution layer containing Fe-Mg-Ni-O is selectively coated only on the surface of a three-dimensional network-like porous skeleton in which a large number of chained bodies in which NiO particles are linked in a chain are branched,
A cathode for a molten carbonate fuel cell, wherein the solid solution layer has a molar ratio of Fe to Mg of 0.5 to 2.

Images