JP5916013B2 - Multi-stage electrochemical reactor - Google Patents

Description

  The present invention relates to a multistage electrochemical reactor.

2. Description of the Related Art Conventionally, in an electrochemical reactor represented by a fuel cell, a steam electrolysis cell, an electrochemical NOx decomposition cell, etc., a structure in which single cells are connected by an interconnect is known.
(Patent Documents 1 and 2, Non-Patent Document 1).

  In addition, the electrochemical reaction cell structure in which single cells are connected in series in the form of horizontal stripes on the surface of the hollow support has the advantage that an alloy-based separator is not required, and is currently under investigation.

  Such a structure has a structure in which single cells are subdivided and the subdivided single cells are connected in series at a short distance compared to an electrode-supported electrochemical reaction cell structure using an electrode material as a support. Therefore, the current density can be suppressed, and the resistance loss due to the connection resistance between the interconnect and the cell can be reduced.

  In a horizontal striped multistage electrochemical reactor, various kinds of electrochemical reactions such as power generation and material synthesis are caused by interposing an electrolyte membrane and allowing different gases to flow through each space.

  However, in the prior art as described above, for example, among the electrochemical reactors, since the general operating temperature of a solid oxide fuel cell (hereinafter referred to as SOFC) is as high as 700 to 1000 ° C. or higher, a single cell The interconnects connecting them had to be formed only from conductive ceramic materials with high melting points and softening points.

  In this case, for example, as shown in FIG. 8, the interconnect (FIG. 1) for connecting the single cells is formed of a dense and conductive material, but the gas sealing property at the interface between the single cells and the interconnect is insufficient. Therefore, there is a problem that an open circuit voltage that is 10 to 20% lower than the theoretical value, that is, 10 to 20% of energy is lost.

  In recent years, attempts have been made to perform electrochemical material synthesis or material conversion using unused energy typified by sunlight, etc. In order to increase reaction efficiency, the airtightness of the electrolyte and interconnect is essential. The cross leak at the connection interface has a problem that the reaction efficiency is lowered.

JP-A-6-44983 JP-A-8-78041

“Effect of Cathode Sheet Resistance on Segmented-in-series SOFC Power Density,” Journal of Power Sources, 164 (2007) 742-745

  So far, a structure has been proposed in which electrochemical reaction cells are arranged in a horizontal stripe pattern on a porous substrate in a similar structure, and the cells are connected in series. In order to advance the existing electrochemical reaction such as SOFC efficiently, high temperature operation around 800 ° C is required. Therefore, the material constituting the connection part is not softened or melted even in the operating region, and is conductive and dense. Thus, conductive ceramic materials having excellent chemical stability even in oxidizing and reducing atmospheres have been used. However, many of these materials are expensive, and the process for densification is limited, which is costly.

  In addition, the electrode of the electrochemical reaction cell is porous, and even if the connection layer is formed of a conductive ceramic material, the interface between the connection layer and the porous electrode can be completely bonded (bonded). Since it is difficult, the gas sealing property at the bonding interface becomes insufficient.

  As a result, the air electrode gas and the fuel electrode gas that should be completely separated from each other by the electrolyte and the connection layer cross-leak at the connection interface, and the open circuit voltage is 10 to 20% lower than the theoretical value, that is, 10 to 20 % Energy is lost, and the stability of the electrochemical cell is impaired, for example, by promoting the deterioration mode of the cell interface. Moreover, in the electrochemical reactor using an electrochemical reaction, the purity of the reaction product is reduced due to cross leak, and as a result, there is a problem that the reaction efficiency is lowered.

In order to solve the problems as described above, the present invention provides a plurality of electrochemical reaction cells having a structure in which an electrolyte layer is sandwiched between a fuel electrode layer and an air electrode layer on the surface of a porous body having a tubular structure, independently formed, are electrically connected in series with a conductive layer of the air electrode layer and the porous electrochemical reaction cells where the fuel electrode layer independent respective electrochemical reaction cells are adjacent, electrically conductive layer of the porous A multi-stage electrochemical reactor is provided in which a gas seal layer is laminated on.

  Moreover, the reaction method operated at 300-700 degreeC using these multistage electrochemical reactors, Preferably it is 500 to 650 degreeC is provided.

  The connection structure of the electrochemical reactor cell of the present invention is theoretically because an airtight layer can be formed with a sealing material at a corresponding place after connecting and wiring the electrochemical cells arranged in a horizontal stripe shape in series with a conductive material. There is an advantage that stable operation can be performed while minimizing the drop from the open circuit voltage.

  In addition, since it is no longer necessary to determine the density of the conductive material, it is possible to select a conductive material with good conductivity, so that the performance loss due to resistance loss can be suppressed, and the choice of conductive material is widened, so that low-cost materials can be applied. Therefore, an electrochemical reactor that is industrially superior and excellent in power generation performance and electrolysis performance is provided.

  According to the reaction method of the present invention, an electrochemical reaction can be performed efficiently at a relatively low temperature.

Connection structure in the present invention Structure of the substrate 2 in the production process in the present invention In the present invention, the structure after the conductive material layer is formed Evaluation method for multistage electrochemical reactor Non-patent literature and power generation performance per cell in the present invention Power generation and electrolytic cyclic voltammetry performance in the present invention Power generation and electrolytic cycle performance in the present invention Connection structure in the prior art

<Configuration of multi-stage electrochemical reactor>
The main feature of the present invention is that a laminated structure of a conductive material and a sealing material is applied as a method for connecting a plurality of electrochemical reaction cells in a multistage electrochemical reactor.

  The configuration of the multistage electrochemical reactor according to the present invention will be described. FIG. 4 is a schematic view illustrating a part of a multistage electrochemical reactor according to the present invention. FIG. 1 is a partial cross-sectional view in the longitudinal direction of the multistage electrochemical reactor illustrated in FIG. The fuel electrode layers 1 and 2, the electrolyte layers 1 and 2, and the air electrode layers 1 and 2 are laminated on a base material 1 that is a tubular porous body.

  The fuel electrode layer 1 and the air electrode layer 2 are connected by a conductive material layer, and the conductive material layer is covered with a gas seal layer. In this way, the electrochemical reactor (cell) 1 and the electrochemical reactor (cell) 2 are electrically connected, the conductive material layer is prevented from coming into contact with air, and the oxidative deterioration of the conductive material layer is suppressed. . In FIG. 2, two electrochemical reactors (cells) are connected, but three or more can be combined depending on the purpose.

  For example, it is necessary to use a material having a high insulating property (especially an electronic insulating property is essential) as the material of the porous body used as the substrate 1, and is preferably an oxide typified by zirconia or magnesia.

Among them, stabilized zirconia stabilized with a stabilizer such as yttria (Y ₂ O ₃ ), calcia (CaO), magnesia (MgO), ytterbia (Yb ₂ O ₃ ), erbia (Er ₂ O ₃ ), etc. A suitable example is magnesia. Stabilized zirconia can be stabilized by one or more other stabilizers, and can also be a composite with alumina (Al ₂ O ₃ ).

For example, it is necessary to use a material having high ionic conductivity as the electrolyte material for forming the electrolyte layers 1 and 2, and a ceria-based oxide: Ce _1-x Ln _x O _{2−x / 2} (however, Ln Includes at least one of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and x is 0.05 to 0.50), yttria stable zirconia oxide: 5 to 10 mol% yttria, scandia-stabilized zirconia _{oxide: a mol% Sc 2 O 3} -b mol% CeO 2 -c mol% ZrO 2 ( where, a is 8 to 15, b is 0 to 2, and a + b + c = 100) , lanthanum gallate _{oxide: La 1-m Sr m Ga} 1-n Mg n O 3 ( provided that, m is 0.05 to 0.3, n is 0 or more One .3 or less), or a two or more complex it is desirable.

  Specifically, stabilized zirconia that does not cause defects such as decomposition even with respect to steam electrolysis is particularly preferable.

  The cermet electrode material can be considered as the fuel electrode layers 1 and 2, for example. For these materials, the above-described electrolyte material and a composite containing Ni, Cu, Co, Mn, Fe, Ag, Pt, Pd, Rh, Ir, La, Sr, Ti and one or more of these elements or Cermet is a suitable representative example.

  Specifically, (Ni, Fe, Cu) -stabilized zirconia having a wide stable region with respect to the redox cycle is particularly preferable.

  For example, the air electrode materials constituting the air electrode layers 1 and 2 include Ag, La, Sr, Mn, Co, Fe, Sm, Ca, Ba, Ni, Mg, and one or more of these elements. A typical example is a suitable oxide.

  Specifically, a (LaSr) (CoFe) O 3 oxide that effectively acts on both the oxidation reaction and the reduction reaction is particularly preferable.

  Moreover, in order to improve the electrode reaction activity of the said air electrode or a fuel electrode, it is also possible to combine an auxiliary material. As a combination method, an appropriate method may be selected according to the purpose, such as mixing the active auxiliary material directly into the fuel electrode material or inserting an intermediate layer containing the active auxiliary material between the fuel electrode and the electrolyte. it can.

  In addition, as a reaction activity auxiliary material directly mixed with the fuel electrode material, Pt, Pd, Ag, Ba, Sr, Ca, Mg, K, Na, Mn, Fe, Co, Ni, Cu, Zn, Ti, Al, A metal containing at least one element of Ga, Nb, Ta, V, and La, or an oxide containing one or more of these elements is desirable. The active layer inserted between the air electrode and the electrolyte is preferably a composite containing one or more of the above-described electrolyte material or the above-described reaction activity auxiliary material with respect to the air electrode material.

  Specifically, Pt, Pd, Ag, Mn, Cu, and Fe are particularly preferable.

  As the conductive material layer, one or more of Ag, Pt, Pd, La, Sr, Ti, Nb and a composite containing one or more of these elements, or a heat-resistant alloy such as ferritic stainless steel, Inconel, etc. It is preferable that it is a material containing.

  As the gas seal layer, any material having a low gas permeability such as air may be used. However, silicate glass can seal pores of the conductive material layer and the electrode layer by utilizing its softening behavior. This is a suitable representative example. Furthermore, a silicate glass that does not contain an element that promotes electrode deterioration such as B, Na, K, and P is more preferable. In addition, according to the operating temperature of the said multistage electrochemical reactor, a suitable structural component can be selected and softening temperature can be controlled suitably.

Among them, by using a silicate glass having a thermal expansion coefficient in the range of 9 × 10 ^{−6 to} 12 × 10 ⁻⁶ (° C. ⁻¹ ) and a softening temperature of 600 ° C. or higher, residual stress destruction is prevented, and the fuel electrode The cross gas leak between the air electrode and the air electrode can be reduced to 10% or less.
<Production method of multi-stage electrochemical reactor>
In general, the multistage electrochemical reactor of the present invention having the above-described configuration can be produced, for example, by the following method.

  To the porous material, a pore-forming agent such as a cellulose-based binder and carbon powder is added, and the mixture is kneaded with water. The obtained plastic mixture is extruded into a predetermined tube diameter and tube length. The tube is formed into a tubular molded body having a thickness. Usually, the tube outer diameter is 2 to 5 mm, and the tube inner diameter is 0.5 to 4.5 mm.

  Next, a cermet fuel electrode material is formed in a horizontal stripe pattern on the tubular molded body and laminated. Next, an electrolyte material is formed and laminated on the fuel electrode except for a part of the cermet fuel electrode (see FIG. 2). Typical examples of the film forming method include a dip coating method, a thermal spraying method, an ink jet method, a transfer method, and a printing method.

  For example, in dip coat film formation, the film thickness of each layer can be controlled by masking a region where no film is formed and adjusting the pulling speed within a range of 0.1 mm / sec to 3 mm / sec. The particle dispersion slurry of the fuel electrode material and the electrolyte material used for the coating may be prepared by ball mill mixing the material powder constituting each layer together with an ethanol main solvent and further mixing a commercially available PVB binder and a surfactant. Also in other film forming methods, a suitable method may be selected as appropriate.

  The tubular molded body in which the cermet fuel electrode layer and the electrolyte layer are laminated is fired at 1000 ° C. to 1600 ° C., preferably 1200 ° C. to 1400 ° C. to obtain a tubular porous body (see FIG. 2).

Here, the porosity of the porous body will be described. The open porosity of the tubular porous body is preferably 10% to 50%, particularly preferably 30 to 40%. If it is less than 10%, the gas permeability coefficient becomes insufficient, and if it exceeds 50%, the mechanical strength as a support becomes insufficient. In order for all the formed electrochemical reaction cells to function, the raw material gas (or fuel gas in the case of the SOFC fuel electrode) passes through the base material 1 in FIG. 2, reacts in the fuel electrode layer, and the reaction product ( In the case of a fuel electrode material for SOFC, water vapor, CO, and CO ₂ gas) and the raw material gas must smoothly and continuously interdiffuse.

  The porous support wall is required to have gas permeability, and preferably has a porosity of 30% or more. At the same time, since the porous support forms the basic skeleton of the multistage electrochemical reactor, the maximum porosity should be 50% or less, particularly 20 to 40% in order to achieve the strength retention. Is preferred. If it exceeds 50%, the support tube wall strength will be insufficient.

  In consideration of the in-plane conductive resistance, the width of each cell formed in a horizontal stripe shape is preferably within a range of 0.01 to 5 cm. From the viewpoint of reducing the connection resistance, the distance between the cells is 0. It is preferable to be within the range of 01 to 0.5 mm.

  The distance between the electrochemical reaction cells is preferably in the range of 0.01 to 5 mm. However, if the distance is less than 0.01 mm, high positional accuracy is required for coating of the fuel electrode layer, the electrolyte layer, and the conductive material layer, leading to high manufacturing costs. If it exceeds 5 mm, it leads to an increase in electrical resistance of the connecting portion. In particular, 0.5-2 mm is preferable.

  Next, a conductive material slurryed with an organic solvent system is formed between the fuel electrode layer 1 and the planned air electrode layer 2 to form a conductive layer. The fuel electrode layer 1 and the air electrode layer 2 are electrically connected in series. (See FIG. 3). As a film forming method, as described above, a dip coating method, a thermal spraying method, an ink jet method, a transfer method, a printing method, and the like can be given as representative examples. The thickness of the conductive layer 1 is 1 micron or more, preferably 10 microns or more. If it is less than 10 microns, the electrical resistance increases due to insufficient film thickness.

  Then, an air electrode layer is formed at a predetermined position and fired at 800 ° C. to 1300 ° C., preferably 1000 ° C. to 1200 ° C. (see FIG. 3). As a film forming method, as described above, a dip coating method, a thermal spraying method, an ink jet method, a transfer method, a printing method, and the like can be given as representative examples. A gas seal layer material slurried in a solvent system is formed on the conductive layer and baked at 600 ° C. or higher, preferably 800 ° C. to 900 ° C., which is higher than the expected operating temperature.

  In this way, the fuel electrode layer 1 and the air electrode layer 2 are electrically connected by the conductive material layer, and the multistage electric circuit having a connection structure formed of a laminated structure in which the gas seal layer is formed and laminated so as to cover the conductive material layer. A chemical reactor can be made (see FIGS. 1 and 4). As described above, the cermet fuel electrode layer, the electrolyte layer, the air electrode layer, the conductive material layer, and the gas seal layer may be formed by dip coating, printing, transfer, spraying, spray coating, etc. It is not limited to that.

  Thus, the target performance is obtained by combining a plurality of electrochemical reaction cells in series. For example, under fuel cell operating conditions, a voltage of single cell voltage × number of series connections is obtained. Similarly, regarding the electrolysis reaction, the electrolysis reaction of the target substance can be performed by applying a voltage of single cell voltage × the number of series connections.

  Conventionally, the interconnect layer had to be dense, but in the present invention, the conductive material layer is made porous so that the conductive material becomes a reducing atmosphere, so even when a metal material such as an alloy is used. There is an effect of suppressing oxidation, that is, as a protective layer of a conductive material. Further, the sealing material layer penetrates not only the conductive material layer but also the adjacent air electrode layer, and a part of the air electrode can function as a gas barrier layer.

  Details of the examples will be described below, but the present invention is not limited to the following examples.

  In the present invention, first, a substrate 1 (FIG. 2) made of a porous molded body was produced according to the following procedure. Yttria-stabilized zirconia (hereinafter referred to as 3YSZ) to which 3 mol% of yttria is added as a stabilizer, which is a kind of electronic insulating ceramic material, is kneaded with water and commercially available ethyl cellulose, and pressure-extruded to obtain a tube outer diameter. A tubular molded body having a diameter of 5.8 mm and a tube inner diameter of 5.0 mm was obtained. The tubular molded body was cut into a length of about 6 cm to obtain a tubular porous molded body.

  NiO and 8 mol% yttria-stabilized zirconia (hereinafter referred to as 8YSZ) mixture is selected as the fuel electrode material, 8YSZ is selected as the electrolyte material, the material powder is ball milled with ethanol main solvent, a commercially available PVB binder, Each particle-dispersed slurry was prepared by further mixing the surfactant.

  Next, the non-film-formed region of the tubular molded body was masked, immersed in a particle-dispersed slurry of cermet fuel electrode material, and then the fuel electrode layer was formed and laminated in a horizontal stripe pattern by dip coating (pickup speed: 1 mm / sec). Next, the tubular molded body masked with a part of the cermet fuel electrode layer was immersed in a particle-dispersed slurry of electrolyte material, and then formed and laminated by dip coating (pickup speed: 2 mm / sec) (see FIG. 2).

  A tubular molded body in which the cermet fuel electrode layer and the electrolyte layer are laminated is fired at 1300 ° C., and the base material 2 (porosity of the porous substrate is 36%) in which the cermet fuel electrode layer and the electrolyte layer are laminated on the porous body. Obtained (see FIG. 2).

  Subsequently, a metal paste (metal is Pt) was printed and applied so as to be electrically connected in series between the fuel electrode layer 1 and the planned air electrode layer 2 to form a conductive material layer. Next, the air electrode layer on the conductive material layer was printed and applied, and the conductive material layer and the air electrode layer were baked at 1000 ° C. Next, a gas seal layer was printed and applied so as to cover the conductive material layer including a part of the electrode layer, and baked at 850 ° C. to obtain a tubular two-stage electrochemical reactor (see FIG. 1).

The thicknesses of the cermet fuel electrode layer, the electrolyte layer, and the air electrode layer were 100 microns, 10 microns, and 50 microns, respectively. The thermal expansion coefficient of the conductive layer was about 11 × 10 ⁻⁶ (° C. ⁻¹ ) and the sheet resistance was 300 (mΩ / cm ² ).

The thermal expansion coefficient of the sealing layer was 10.5 × 10 ⁻⁶ (° C. ⁻¹ ), and the softening temperature was about 700 ° C.
<Comparative Example 1>

  FIG. 3 shows a state of a sample obtained by forming and firing a cermet fuel electrode layer, an electrolyte layer, a conductive material layer, and an air electrode layer on a porous substrate tube in the same manner as in Example 1. Compared to FIG. 1 obtained in Example 1, the gas seal layer is not formed in Comparative Example 1.

  Next, the electrochemical evaluation results of the multistage electrochemical reactor produced by Example 1 and Comparative Example 1 will be described.

  One of the methods for determining the degree of gas leakage at the connection is to evaluate in the SOFC mode and grasp the amount of decrease from the theoretical open circuit voltage. This time, as shown in FIG. 4, room temperature humidified hydrogen is supplied into the tube at a flow rate of 20 cc / min, and air is supplied to the outside of the tube at 100 cc / min, and a potential line is connected to the terminal fuel electrode layer and the air electrode layer. The electrochemical evaluation was performed at 550 ° C and 600 ° C.

  In addition, in order to make it easy to evaluate the sealing performance of the connection portion, a 5-series electrochemical cell is used, and four connection portions are included in both the examples and the comparative examples.

  Table 1 summarizes changes in the open circuit voltage during the reduction of the fuel electrode, measured in the process of raising the temperature from room temperature to 550 ° C. by passing a predetermined gas through the fuel electrode and the air electrode. In Comparative Example 1 using the existing technology, the maximum voltage was 2.4 V, and voltage disturbance occurred from about 100 minutes.

  In contrast, in the case of Example 1, the open circuit voltage steadily increased as the temperature increased, and reached a maximum open circuit voltage of 5.55 V (1.11 V per cell) at around 500 ° C., as shown in Table 1. Thus, at 550 ° C., the open circuit voltage was stable at 5.45 V (1.09 V per cell).

Since the theoretical open circuit voltage is 1.12 V at 500 ° C. and 1.11 V at 550 ° C., it can be seen that the gas leak is greatly improved by applying the gas seal layer. Further, even in the open circuit voltage of the preceding comparative example, as shown in FIG. 5, the open circuit voltage per cell is 1 V or less, and the gas seal is incomplete, so that the cell reliability of the present invention is improved. The effect is obvious.

FIG. 5 shows a current-voltage curve measured at 600 ° C. for the five-stage series-connected cell of Example 1, and the power generation output density increases with increasing current density from an open circuit voltage of about 1.1 V per cell. There rises, to obtain a power density 90 mW / cm ² with a maximum at 0.125 a / cm ² (in the preceding comparative example, the maximum 70mW / ^cm 2).

  The steam electrolysis characteristics of the multistage electrochemical reactor produced in Example 1 will be described.

  The same evaluation setting as in Example 2 was performed, room temperature humidified hydrogen was supplied into the tube at a flow rate of 50 cc / min, and air was supplied to the outside of the tube at 100 cc / min, and a potential line was connected to the terminal fuel electrode layer and the air electrode layer. Then, power generation and electrolytic cyclic voltammetry evaluation were performed at 600 ° C. In addition, in order to make it easy to evaluate the sealing performance of the connection part as in Example 2, a 5-series electrochemical cell was used.

  As shown in FIG. 6, with the open circuit voltage of 5.4 V as a boundary, power generation is ideally operated at a potential of 5.4 V or lower, and the steam decomposition reaction is ideally operated at a potential of 5.4 V or higher. While the theoretical open circuit voltage is 5.5 V at 600 ° C., the fabricated five-stage series-connected cell obtains 5.4 V, and the effect of improving the cell reliability of the present invention appears.

  Furthermore, the time change of the electric current and voltage which repeated the power generation measured at 600 degreeC and the electrolysis cyclic voltammetry 100 cycles is shown in FIG. After about 10 cycles from the beginning, the current density became stable, and both the power generation mode and the electrolysis mode succeeded in obtaining stable performance at 5.4V as a boundary.

  As described above in detail, the present invention relates to a multistage electrochemical reactor in which electrochemical cells formed in a horizontal stripe pattern on a porous substrate are connected in series with a laminated structure of a conductive material layer and a seal layer. Thus, the electrochemical reactor can be made highly efficient by the production method described in the present invention.

  The width of each cell to be formed in a horizontal stripe is in the range of 0.01 to 5 cm, and from the viewpoint of reducing connection resistance, the distance between each cell is in the range of 0.01 to 0.5 mm, so that it operates at a lower temperature than the prior art. At the same time, by optimizing the material system and cell structure, it becomes possible to realize a further high-performance electrochemical reactor. INDUSTRIAL APPLICABILITY The present invention is useful for providing new technologies and new products related to electrochemical reaction systems such as solid oxide fuel cells.

Claims (8)

A plurality of electrochemical reaction cells having a structure in which an electrolyte layer is sandwiched between a fuel electrode layer and an air electrode layer are independently formed on the surface of a porous body having a tubular structure. A fuel electrode layer is electrically connected in series by an air electrode layer and a porous conductive layer of an adjacent electrochemical reaction cell, and a gas seal layer is laminated on the porous conductive layer. Type electrochemical reactor.   2. The multistage electrochemical reactor according to claim 1, wherein the fuel electrode layer of the electrochemical reaction cell is in contact with the porous body. The multistage electrochemical reactor according to claim 1 or 2, wherein the porous body has a porosity of 30% or more and 50% or less and is electronically insulating.   The multistage electrochemical reactor according to any one of claims 1 to 3, wherein the electrochemical reaction cell has a length in a series direction of 0.01 to 5 cm.   The multistage electrochemical reactor according to any one of claims 1 to 4, wherein a distance between the electrochemical reaction cells is in a range of 0.01 to 5 mm.   The multistage electrochemical reactor according to any one of claims 1 to 5, wherein the electrochemical reaction cell has an outer diameter of 0.2 to 5 cm and an inner diameter of 0.1 to 4.9 cm. . The multistage electrochemical reactor according to any one of claims 1 to 6, wherein the gas seal layer is made of silicate glass. It operates at 300-700 degreeC using the multistage electrochemical reactor as described in any one of Claim 1 to 7, The electrochemical reaction method characterized by the above-mentioned.