JPH05121077A - Fuel cell with molten carbonate - Google Patents

Abstract

PURPOSE:To provide a molten carbonate type fuel cell, in which secular deterioration at normal service temp. is suppressed and which presents a high cell performance in the same degree as in the case where lithium nickel oxide is used as a cathode electrode. CONSTITUTION:A fuel cell with molten carbonate is equipped with a pair of gas diffusion electrodes, anode and cathode consisting of porous body and an electrolyte matrix which is pinched by these gas diffusion electrodes and holds the molten carbonate. Among them the cathode electrode is formed from a porous body consisting of a composite material in which at least one sort of metal particulates 12 of gold and platinum series metals are dispersed in an electroconductive oxide 11.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a molten carbonate fuel cell.

[0002]

2. Description of the Related Art A molten carbonate fuel cell is sandwiched by a pair of gas diffusion electrodes made of a porous material, that is, an anode electrode (fuel electrode) and a cathode electrode (oxidant electrode), and melted. A plurality of unit cells having an electrolyte matrix holding the alkali carbonates are electrically connected to each other, and are laminated via a separator serving as a passage forming portion of a reaction gas to each gas diffusion electrode. Is configured. The electromotive reaction is a reaction site in which the porous skeleton of the gas diffusion electrode and the molten carbonate and the reaction gas coexist by supplying a fuel cell gas containing hydrogen gas to the anode electrode and an oxidant gas to the cathode electrode. It occurs at (three-phase interface). The molten carbonate fuel cell takes out the electric energy converted and generated as a result from between a pair of electrodes.

By the way, the above electromotive reaction is a chemical reaction, and the higher the reaction temperature, the faster the reaction proceeds.
On the other hand, the deterioration and corrosion of the members constituting the porous gas diffusion electrode, the electrolyte matrix and the like are more likely to proceed as the reaction temperature is higher. Therefore, it is usually operated near 650 ℃. However, even at such temperatures, the problems of deterioration and corrosion have not been completely solved by conventional cell constituent materials.

For example, a porous sintered body of nickel oxide doped with lithium (hereinafter referred to as lithiation) has been generally used as the cathode electrode of the gas diffusion electrode. However, this lithiated nickel oxide has a problem that it gradually dissolves even at the above-mentioned operating temperature. When the nickel ions are dissolved, they react with hydrogen that has diffused from the anode side, nickel is deposited in the electrolyte, and the anode and cathode electrodes are short-circuited. Further, even if the short circuit between the electrodes does not occur, the dissolution of the nickel ions causes the wear of the cathode electrode and the decrease of the reaction sites, resulting in the deterioration of the cell performance.

Under these circumstances, it has been attempted to use a conductive oxide in place of lithiated nickel oxide as the cathode electrode. The conductive oxide used as an alternative is required to have a solubility that is equal to or lower than that of nickel oxide and that it is difficult to reduce hydrogen in the fuel gas near the anode. Therefore, lithium ferrite (LiFeO ₂ ) and lithium titanate (Li ₂ T
iO _3), lithium manganate (LiMnO _2), it has been proposed to use such as zinc oxide (ZnO). In order to improve the conductivity of these oxides, MgO, CaO, Z
It has also been proposed to add rO ₂ , ZnO, manganese oxide, etc. (National Fuel Cell Seminor, 1983p65,1985
p158, 1990 p227 etc.).

[0006]

However, in the molten carbonate fuel cell using the above-mentioned conductive oxide as the cathode electrode, it is possible to obtain sufficient cell performance although the stability against operating temperature is increased. There was a problem that I could not. Further, even when a cathode electrode made of a material added to improve conductivity is used, cell performance comparable to that of lithium nickel oxide is not obtained at present.

The present invention has been made in order to solve such a problem, and suppresses deterioration with time at normal operating temperature, and has the same high level as in the case of using lithium nickel oxide as a cathode electrode. It is an object of the present invention to provide a molten carbonate fuel cell capable of obtaining excellent cell performance.

[0008]

The molten carbonate fuel cell of the present invention comprises a pair of gas diffusion electrodes made of a porous body and an electrolyte matrix sandwiched between these gas diffusion electrodes and holding molten carbonate. In the molten carbonate fuel cell provided, the cathode of the gas diffusion electrode has at least 1 selected from a conductive oxide as gold and a platinum group metal.
It is characterized by being composed of a porous body of a composite material in which fine metal particles of a kind are dispersed.

[0009]

In the molten carbonate fuel cell of the present invention, as the skeleton of the cathode electrode, a conductive oxide having a low solubility and stable against hydrogen in the fuel gas is used.
Fine particles of catalytically active gold or platinum group metal are dispersed in this electrode skeleton. Since a conductive oxide having a small resistance loss is used for the skeleton of the cathode electrode itself, the resistance loss inside the electrode can be made sufficiently small. In addition, the presence of the catalytically active metal fine particles can increase the reaction area. Due to these effects, good cell performance can be obtained.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram schematically showing the structure of the main part of a molten carbonate fuel cell according to an embodiment of the present invention. In the figure, 1 is an electrolyte layer in which a molten electrolyte such as an alkali carbonate is held by a matrix such as LiAlO ₂ or BaO · 6Al ₂ O ₃ . On both main surfaces of this electrolyte layer 1,
For example, an anode electrode (fuel electrode) 2 and a cathode electrode (oxidizer electrode) 3 each made of a porous material of nickel are arranged.

The cathode 3 is lithium ferrite
(LiFeO ₂ ), lithium ferrite spinel, lithium titanate (Li ₂ TiO ₃ ), lithium manganate (LiMnO 2
₂ ), yttrium iron garnet, lithium cobaltite, etc. are used as a skeleton of a conductive oxide having a low solubility and stable to carbonates, and gold, Pt, Ru, Rh, Pd,
It is composed of a composite material in which at least one fine particle selected from platinum group metals such as Ir is dispersed. As a dispersion form of gold or platinum group metal particles, for example, as shown in FIG. 2, particles 1 of gold or platinum group metal are formed on the surface of the conductive oxide 11.
An example is one in which 2 is dispersed and adhered. It is also possible to deposit gold or a platinum group metal in layers. In this way, by dispersing catalytically active metal particles such as gold particles or platinum group metal particles in the conductive oxide, the reaction area is increased and the reaction at the cathode can be activated. At the same time, it becomes possible to improve conductivity as an electrode.

The amount of gold or platinum group metal dispersed and added to the conductive oxide should be small in terms of raw material cost, but if less than 0.05 wt% of the conductive oxide is added, the reaction area will be reduced. Since neither the effect of increasing the conductivity nor the effect of increasing conductivity is significant, it is preferable to add more than this value.
On the other hand, if it is added in an amount of more than about 40% by weight, the distance between the metal particles dispersed and adhered to the surface of the oxide is shortened and the particles agglomerate in a short time. Therefore, the amount of precious metal added is 0.05 to 40% by weight.
A weight% range is preferred.

The size of the gold or platinum group metal particles dispersed and attached to the surface of the conductive oxide is preferably about 5 nm to 70 nm. The size of these noble metal particles can be changed by adjusting the conditions for dispersing and attaching the noble metal particles to the conductive oxide.

The above-mentioned conductive oxide is not limited to the above-mentioned simple substance of oxide, but magnesium oxide (MgO) is added to the above-mentioned conductive oxide in order to improve conductivity.
It is also possible to use a composite oxide containing calcium oxide (CaO), zirconium oxide (ZrO ₂ ), zinc oxide (ZnO), tin oxide (SnO), manganese oxide and the like. As the addition amount of these oxides, the metal element in the oxide, for example, the amount of Mg in the case of magnesium oxide, the metal element serving as the main constituent element of the conductive oxide, for example lithium ferrite
It is preferable to adjust the amount of Fe in the range of 0.5 to 30 mol% with respect to Fe. This is because if it is less than 0.5 mol%, the effect of improving conductivity is not sufficiently obtained, and if it is 30 mol% or more, it does not form a solid solution in the conductive oxide that is the matrix and becomes an independent phase.
This is because the amount of gO and the like increases, and the conductivity decreases.

The cathode 3 as described above is manufactured, for example, as follows. First, gold or a platinum group metal, for example, is added as a salt to the dispersion liquid of the conductive oxide powder. Then, by adjusting the pH or the like, the hydroxide of gold or a platinum group metal is adsorbed on the surface of the conductive oxide powder. Then, by subjecting this powder to heat treatment, a composite material powder is obtained in which fine particles of gold or a platinum group metal are dispersed and adhered to the surface of the conductive oxide powder. The particle size of the gold or platinum group metal particles is adjusted to 5 nm by adjusting the pH of the mixed solution of oxide and gold, the stirring speed, the content of gold, etc., the heat treatment conditions, etc.
It can be about 70 nm.

In addition to these metal salts, in order to prevent aggregation (sintering) of fine particles of gold or the like dispersed or adhered to the surface of the conductive oxide and to accelerate the reduction reaction of oxygen,
A nitrate, chloride, hydroxide or the like of aluminum, zirconium, silicon or the like may be added so that the number of moles of these cation elements is 0.1 to 10% with respect to the number of moles of the added metal fine particles.

As the cathode in the present invention, a green sheet obtained by mixing the above-mentioned composite material powder with an organic or inorganic binder and molding the mixture into a predetermined shape by a doctor blade method or the like is used. A porous sintered body obtained by sintering the green sheet at 500 ° C to 1200 ° C in a reducing or vacuum atmosphere may be used as the cathode.
Further, the green sheet as the cathode electrode is not limited to the single layer form, and in order to quickly diffuse the reaction gas, a green sheet using coarse powder (dispersed powder of noble metal with a large particle size) is provided on the oxidant gas supply side and the electrolyte. On the matrix side, a green sheet using fine powder may be arranged in two layers to serve as a cathode.

In the cathode of the present invention, a porous sintered body is prepared in advance from a conductive oxide, and a hydroxide layer containing gold or a platinum group metal is electrochemically formed on the surface of the porous sintered body. It is also possible to apply a method of forming a metal layer by applying heat treatment. Further, it is also possible to electrochemically form a layer of gold or a platinum group metal directly on the surface of the porous sintered body of the conductive oxide.

In the molten carbonate fuel cell shown in FIG. 1, conductive separators 4 and 5 having fuel gas and oxidant gas supply paths 4a and 5a are provided outside the anode electrode 2 and the cathode electrode 3, respectively. Each is arranged to form a unit battery. Then, a plurality of unit batteries described above are stacked to form an electromotive section structure, and a gas supply manifold (not shown) is attached to the side surface of the electromotive section structure and tightened with a predetermined pressure from the stacking direction. A molten carbonate fuel cell is constructed.

Next, a concrete example of the molten carbonate fuel cell having the above-mentioned constitution and its evaluation result will be explained. Example 1 First, 5 mol% of magnesium was added to lithium ferrite (LiFeO ₂ ), which was a composite oxide (LiFe) having a specific surface area of 5 m ² / g.
_0.95 Mg _0.05 O ₂ ) powder is dispersed in water. Then, to this dispersion, a solution of chloroauric acid (HAuCl ₄ ) containing gold corresponding to 8% by weight with respect to the above composite oxide (preliminarily adjusted to pH 9 with sodium carbonate, potassium carbonate, etc.), After adding slowly, stir well. Further, sodium carbonate is added to adjust the pH to 10, and the mixture is sufficiently stirred and then left standing for 12 hours. By the above operation, gold hydroxide (Au (O
H) ₃ ) adsorbed complex oxide powder can be prepared.

The obtained powder is washed with ion-exchanged water until the filtrate has a pH of 7 to 7.5, and dried at 40 ° C. for 12 hours. In addition, heat-treat at 400 ℃ in air for 1 hour. By these treatments, Au (OH) ₃ becomes Au and the conductive oxide (LiFe _0.95 Mg
_0.05 O ₂ ) Gold fine particles are dispersed and adhered on the surface to obtain a composite material powder. The composite material powder thus obtained was subjected to the characteristic evaluation described later.

Example 2 Sodium carbonate was added to a mixture prepared by mixing iron nitrate, magnesium nitrate and chloroauric acid so that the ratio of the total moles of iron and magnesium to the moles of gold was 19: 1. hand
Pour into an aqueous solution adjusted to pH 8 and stir well. Sodium carbonate is further added to this aqueous solution to adjust the pH to 9.
The precipitate formed is filtered and washed thoroughly with ion-exchanged water.
Next, it heat-processes at 40 degreeC in air for 30 hours. Thus, a composite material powder in which fine gold particles are dispersed on the surface and inside of the oxide is obtained. This composite material powder was subjected to the characteristic evaluation described below.

Although the composite oxide of this example does not contain lithium, when the battery cell is constructed and the temperature is raised above the melting temperature of the electrolyte, lithium in the electrolyte diffuses and penetrates into the oxide. And becomes lithium oxide.

Next, a molten carbonate fuel cell was produced using each of the composite material powders obtained in Examples 1 and 2 above,
The results of evaluating the characteristics will be described. The powders obtained in Examples 1 and 2 above were analyzed by X-ray diffractometry (She
According to the method of Rerr), the particle size of the contained gold was measured to be about 9 nm each.

First, each powder obtained in Examples 1 and 2 above
To 80 g, 20 g of polyvinyl butyral (PVB) as an organic binder was added, and these mixtures were dispersed in butanol and mixed by a ball mill for 24 hours. These were poured onto a glass substrate and molded into a green sheet having a thickness of 0.2 mm by the doctor blade method. These sheets were cut into a predetermined size to obtain a porous body for a cathode electrode. Then, these cathode electrodes,
An anode electrode made of a porous material (porosity 55%) obtained by sintering nickel powder with 0.5% by weight of alumina dispersed therein, and an electrolyte matrix made of lithium aluminate (porosity 45%) with an average particle size of 0.2 μm. A unit cell was composed of a green sheet of a mixed carbonate of lithium carbonate and sodium carbonate and a battery holder, and its characteristics were measured. The results are shown in FIG.

As a comparison with the present invention, a unit cell (comparative example 1) using a nickel porous plate having a porosity of 70% and an average particle size of 10 μm for the cathode electrode and the gold component in Example 2 were not added. A unit cell (Comparative Example 2) using a cathode electrode formed by using powder as a raw material was assembled, and each unit cell characteristic was measured. The results are also shown in FIG.

As is apparent from FIG. 3, the characteristics of the unit cell according to the embodiment of the present invention are equal to or higher than those in the case of using the conventional cathode (Comparative Example 1), and the lithium ferrite to which gold is not added is used. It can be seen that it is significantly improved compared to the case (Comparative Example 2). That is, it was shown that the reduction of oxygen proceeds rapidly on the finely dispersed gold particles.

Incidentally, Mg, Ca, Zr, added to the composite oxide,
In order to prevent Zn, Mn, Sn, etc. from eluting from the electrode into the electrolyte matrix, it is possible to add about 2 to 15 mol% of carbonate or oxide of these elements in the matrix or electrolyte. Good. Further, the cathode electrode used in the present invention is not limited to that of a molten carbonate fuel cell, but can be used as a cathode electrode of an oxide solid electrolyte, for example.

[0030]

As described above, according to the molten carbonate fuel cell of the present invention, since the cathode electrode in which the catalytically active fine metal particles are dispersed on the surface or inside of the conductive oxide is used, Compared with a conventional cathode, in particular, a cathode containing lithium ferrite or the like as a main component, which is less likely to be eluted and consumed, the oxygen reduction reaction proceeds more rapidly, and therefore the cell performance can be significantly improved.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing a main part of a molten carbonate fuel cell according to an embodiment of the present invention.

FIG. 2 is a diagram conceptually showing a cathode electrode composite material powder used in one example of the present invention.

FIG. 3 is a diagram showing characteristics of a unit cell according to an embodiment of the present invention in comparison with a conventional example.

[Explanation of symbols]

 1 ... Electrolyte layer 2 ... Anode electrode 3 ... Cathode electrode 11 ... Conductive oxide powder 12 ... Fine particles of gold or platinum group metal

Claims (2)

[Claims] 1. A molten carbonate fuel cell comprising a pair of gas diffusion electrodes made of a porous body, and an electrolyte matrix sandwiched between these gas diffusion electrodes and holding molten carbonate, wherein Among them, the cathode electrode is a molten carbonate fuel cell characterized by comprising a porous body of a composite material in which at least one kind of metal fine particles selected from gold and platinum group metals is dispersed in a conductive oxide. 2. The molten carbonate fuel cell according to claim 1, wherein the conductive oxide is selected from lithium ferrite, lithium titanate, lithium manganate, lithium ferrite spinel, yttrium iron garnet and lithium cobaltite. At least one kind of oxide, or a composite oxide containing at least one kind selected from magnesium oxide, calcium oxide, zirconium oxide, zinc oxide, tin oxide and manganese oxide added to the oxide. Molten carbonate fuel cell.