JP6155682B2 - Operation method of fuel cell - Google Patents

Description

  The present invention relates to a method for operating a fuel cell, and more particularly to a technique for reducing the inside of a fuel cell during initial operation of the fuel cell.

  A solid oxide fuel cell (SOFC; hereinafter referred to as “fuel cell”) is composed of a fuel electrode and an air electrode in which two kinds of gases, a fuel (reducing agent) and an oxidizing agent, are separated by an oxygen ion conductive solid electrolyte. , And an electrochemical reaction is caused to proceed at each electrode to extract electric power to the outside. A power generator in which battery constituent materials are stacked and combined is referred to as a cell.

  Since such a fuel cell uses a metal oxide as a fuel electrode, it is necessary to remove (reduce) oxygen from the oxide when starting operation. Therefore, conventionally, an operation method has been adopted in which oxygen is removed by reacting with oxygen by supplying a reducing agent to the fuel electrode side in the fuel cell at the initial operation (at the start of the initial operation).

  As a conventional example of a fuel cell that performs such reduction, for example, one described in Patent Document 1 is known. In Patent Document 1, it is considered that the reduction treatment is preferably performed at the operating temperature of the fuel cell from the viewpoint of simplifying the work process.

  On the other hand, the cell provided in the fuel cell is formed as a joined body (unit) joined to a casing constituting a gas flow path and a supporting member mainly made of metal for each layer. As a material used for this joining, glass that crystallizes and solidifies at a high temperature is generally used. Since the glass must be solidified in a high temperature / oxidized state (air atmosphere, etc.), as a process sequence when assembling the fuel cell, the glass cells are joined and then the units are stacked to form a so-called stack (laminate). When the operation is started, a reducing agent is supplied to the fuel electrode side to increase the temperature, and the inside of the fuel cell is reduced.

Japanese Patent Laying-Open No. 2005-166481

  As described above, in the conventional example disclosed in Patent Document 1, the fuel cell is reduced at the operating temperature of the fuel cell or lower. The glass transition point of the glass used for joining the fuel cells is generally around 650 ° C. For example, in a fuel cell operating at 650 ° C., reduction is performed at this temperature. During the reduction, the cell is contracted by removing oxygen from the metal oxide constituting the cell. At this time, if the shrinkage rate of the cell is large, the glass is rigid in the vicinity of the glass transition point, so that stress is generated inside the glass. For this reason, glass may break.

  The present invention has been made to solve such a conventional problem, and an object of the present invention is to provide a method of operating a fuel cell capable of reducing stress applied to glass during reduction of the fuel cell. There is to do.

In order to achieve the above object, the present invention relates to the temperature of the fuel cell when reducing the inside of the fuel cell during the initial operation of the fuel cell, and the transition point of the glass that joins the cell and the casing provided in the fuel cell. Above the temperature at which the pour point of the glass is reached .

  In the fuel cell operating method according to the present invention, when the inside of the fuel cell is reduced in the initial stage of the fuel cell, the glass can be prevented from being broken due to the stress applied to the glass due to the shrinkage of the cell.

It is explanatory drawing which shows typically the cross section of the unit structure of the fuel cell which concerns on one Embodiment of this invention. It is sectional drawing which shows the stack structure of the fuel cell which concerns on one Embodiment of this invention. It is explanatory drawing which expanded the junction part vicinity of the cell and housing | casing of the fuel cell which concerns on one Embodiment of this invention. It is a characteristic view which shows the relationship between the temperature of glass and the viscosity in 1st Example of the fuel cell which concerns on one Embodiment of this invention. It is a characteristic view which shows the relationship between the temperature and viscosity of the glass in 2nd Example of the fuel cell which concerns on one Embodiment of this invention. It is a figure which shows the experimental result when not employ | adopting the case where the operating method of the fuel cell by this invention is employ | adopted.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Description of Embodiment of the Present Invention]
FIG. 1 is an explanatory view schematically showing a cross section of a fuel cell unit according to an embodiment of the present invention. As shown in FIG. 1, the fuel cell unit 10 has a disk shape in which a cavity serving as a gas flow path is formed at the center. As shown in FIG. 2, a plurality of fuel cell units 10 are stacked to form a stack 30.

  As shown in FIG. 1, the fuel cell unit 10 includes a housing 13. The housing 13 plays a role of supporting the cells 11, forming a gas flow path on the surface of the cells 11, conducting between the cells 11 (between the stacked upper and lower cells), etc. It is formed by a combination of parts. The cell 11 is bonded to the casing 13 by glass 12. Further, a current collector 14 is formed integrally with the housing 13 or separately from the housing 13 in order to assist the conduction between the cells 11. The current collector 14 has a gap for gas flow.

  FIG. 2 is a cross-sectional view showing the structure of a fuel cell stack according to an embodiment of the present invention. A stack 30 is formed by stacking a plurality of fuel cell units 10 shown in FIG. In this manner, by stacking the units 10 in the direction perpendicular to the cell plane, independent fuel flow paths and air flow paths are formed on the fuel electrode side and the air electrode side on both surfaces of the cell.

  FIG. 3 is an explanatory diagram showing, in an enlarged manner, the vicinity of the glass joint portion 20 between the cell 11 and the housing 13 of the unit 10 shown in FIG. FIG. 3A schematically shows a state before the stack is reduced (before the reducing agent is supplied), and FIG. 3B schematically shows a state after the reduction.

  As shown in FIG. 3A, in the state before supplying the reducing agent, the distance from the center of the end of the cell 11 is initially set to R0. Further, the distance from the center of the end portion of the glass 12 is also R0. And if a reducing agent is supplied in a stack and oxygen in the cell 11 is removed, the cell 11 will shrink | contract inside and the distance from a center will become short. That is, as shown in FIG. 3B, the distance from the center of the end of the cell 11 changes from R0 to R1 (R1 <R0). At this time, since the housing 13 does not contract, the position of the housing 13 is maintained in the state before reduction. Therefore, the glass 12 that joins the cell 11 and the housing 13 is deformed by receiving a shearing force due to the shrinkage of the cell 11. Depending on the conditions for receiving the shearing force, there arises a problem that the glass 12 is broken.

  Hereinafter, the conditions under which the glass 12 does not break when the stack is reduced will be described.

  FIG. 4 is a characteristic diagram showing the relationship between the temperature and the viscosity of the glass 12, where the horizontal axis represents temperature and the vertical axis represents logarithmic viscosity [log (Pa · s)]. The viscosity of the glass 12 is indicated by a curve S1, and the critical viscosity is indicated by a curve S2. In FIG. 4, Tf represents the pour point of the glass 12, Ts represents the softening point of the glass 12, and Tg represents the glass transition point.

  Here, the critical viscosity ηs can be expressed by the following equation (1).

ηs = τs / γ (1)
Where τs is the allowable shear stress of the glass 12, and γ is the cell deformation shear rate. Here, the allowable shear stress τs is a numerical value that varies depending on the temperature of the glass 12, and is determined by the breaking strength of the glass at that temperature. That is, the allowable shear stress τs is a function of temperature. The cell deformation shear rate γ is a numerical value that varies depending on the temperature during reduction, the concentration of the reducing agent (reducing gas), and the flow rate. Accordingly, the critical viscosity ηs of the glass 12 is a numerical value determined by the allowable shear stress τs and the cell deformation shear rate γ, and is a numerical value that changes with a temperature change during reduction if the concentration and flow rate of the reducing agent are constant. It becomes.

  A curve S2 shown in FIG. 4 shows the characteristic of the critical viscosity ηs with respect to temperature change of the glass 12 when the concentration and flow rate of the reducing agent are set to predetermined values. When the glass 12 is broken, the viscosity η of the glass 12 exceeds the critical viscosity ηs (= τs / γ) that gives the allowable shear stress τs at the temperature at that time.

  In other words, in the temperature range where the viscosity η of the glass 12 is lower than the critical viscosity ηs, the bonding of the glass 12 is not broken and the bonding state between the cell 11 and the housing 13 is maintained. On the other hand, in the temperature range where the viscosity η of the glass 12 exceeds the critical viscosity ηs, the possibility that the glass 12 is broken by the shearing force generated in the glass 12 increases.

  Therefore, in the present embodiment, the characteristic indicating the relationship between the critical viscosity ηs and the temperature is obtained by the above-described equation (1), and the stack temperature at the time of reduction is adjusted so that the temperature range becomes “η <ηs”. . Specifically, the temperature of the reducing agent is adjusted to be within this temperature range. Here, auxiliary means such as flowing hot air or heating with a heater may be used in combination.

  In addition, when the stack temperature is equal to or lower than the temperature at which the glass transition point (Tg), which is the temperature at which the glass transition occurs (the temperature at which the supercooled state changes to the glass state), is given, the rigidity is high and the glass 12 has fluidity. Since it does not have, it is necessary to set stack temperature so that it may become more than the temperature which gives a glass transition point (Tg). Furthermore, when the stack temperature exceeds the flow start point (Tf; temperature at which the glass fluidizes) of the glass 12, the fluidization of the glass 12 becomes large and becomes too soft, so that it is less than the temperature that gives this flow start point (Tf). It is necessary to.

  Therefore, in this embodiment, the stack temperature (fuel cell temperature) at the time of reduction is equal to or higher than the temperature that gives the transition point (Tg) of the glass 12, and is in a temperature range that satisfies “η <ηs”, and The temperature is lower than the temperature at which the flow start point (Tf) of the glass 12 is given. By doing so, it is possible to prevent the glass 12 from being broken at the time of reduction, and the bonding state between the cell 11 and the housing 13 shown in FIG. 1 can be reliably maintained.

  In the example shown in FIG. 4, the curve S1 indicating the viscosity of the glass 12 and the curve S2 indicating the critical viscosity ηs of the glass 12 intersect at around 700 ° C., and “η <ηs” in the temperature range exceeding 700 ° C. The relationship is satisfied. Moreover, the temperature which gives a glass transition point (Tg) is 650 degreeC, and the pour point (Tf) of the glass 12 is 850 degreeC. For this reason, since “η <ηs” does not hold in the temperature range r1 of less than 700 ° C., the glass 12 is highly likely to break during the reduction, and in the temperature range r3 above 850 ° C., the glass 12 during the reduction. Fluidizes and cannot maintain the bonding. Therefore, if the reduction treatment is performed with the stack temperature in the temperature range r2 of 700 ° C. to 850 ° C., the glass 12 does not break and the glass 12 can be prevented from fluidizing.

  Thus, in the fuel cell operating method according to the present embodiment, the fuel cell temperature (stack temperature) at the time of reduction is changed to the transition point of the glass 12 that joins the cell 11 and the housing 13 provided in the fuel cell ( Tg) or higher. Furthermore, the temperature range in which the viscosity η of the glass 12 is equal to or lower than the critical viscosity ηs (= τs / γ) (τs is the allowable shear stress of the glass, and γ is the cell deformation shear rate). Specifically, since the temperature of the reducing gas is set in a range of 700 ° C. to 850 ° C. during the reduction, even when the cell 11 contracts due to the removal of oxygen in the cell 11 (FIG. 3B). )), And the glass 12 can be prevented from breaking.

  Further, if the viscosity of the glass 12 is reduced at a temperature lower than the temperature indicating the softening point (Ts) within the temperature range r2 (temperature giving a viscosity higher than the softening point), an external force such as vibration is applied to the cell. Even when a force larger than the stress caused by the reduction acts between the housing 11 and the casing 13, the viscous deformation is suppressed by the high viscosity, and the further effect that the positional deviation before and after the reduction hardly occurs is also obtained. Be expected.

[Description of modification]
Next, an embodiment in which glass 12 having different viscosity characteristics is used will be described. FIG. 5 is a characteristic diagram showing the relationship between the viscosity η and the critical viscosity ηs when a glass 12 having a viscosity characteristic different from that of the above-described embodiment is used. A curve S11 shown in FIG. 5 shows the characteristic of the viscosity η with respect to the temperature change, and a curve S12 shows the characteristic of the critical viscosity ηs with respect to the temperature change. In this example, the transition point (Tg) of the glass 12 is 600 ° C., the softening point (Ts) is 800 ° C., the pour point (Tf) is 850 ° C., and the temperature range where “η <ηs” is 650 ° C. or more. ing. For this reason, in the temperature range r11 where the temperature is less than 650 ° C., the glass 12 is likely to break during the reduction, and in the temperature range r13 exceeding 850 ° C., the glass 12 is fluidized during the reduction and the bonding cannot be maintained. Therefore, if reduction is performed in the temperature range r12 of 650 ° C. to 850 ° C., the glass 12 is not broken and can be prevented from fluidizing.

  Next, examples and comparative examples of the fuel cell operation method according to the present invention will be described. The inventors performed the stack reduction process under a plurality of conditions, respectively, when the temperature range is set by the operation method of the present embodiment and when the temperature range is other than this. When the state of gas leak was inspected, the following results were obtained.

[Example 1]
Under the condition that the glass transition point is 650 ° C., the softening point is 750 ° C., and the temperature at which η = ηs is 700 ° C. (condition shown in FIG. 4), the reduction temperature is 720 ° C. (within the temperature range r2 in FIG. 4). did. When three test pieces were inspected, the number of gas leaks after reduction was 0 out of 3, and it was determined that no gas leak occurred. Moreover, when the joint part by the glass 12 was confirmed visually, generation | occurrence | production of a fracture | rupture was not confirmed.

[Example 2]
The reduction temperature is set to 800 ° C. (within the temperature range r 2 in FIG. 4) under the condition that the glass transition point is 650 ° C., the softening point is 750 ° C., and the temperature at which η = ηs is 700 ° C. (the condition shown in FIG. 4). did. When three test pieces were inspected, the number of gas leaks after reduction was 0 out of 3, and it was determined that no gas leak occurred. Moreover, when the joint part by the glass 12 was confirmed visually, generation | occurrence | production of a fracture | rupture was not confirmed.

[Example 3]
The glass transition point is 600 ° C., the softening point is 800 ° C., and the temperature at which η = ηs is 650 ° C. (the conditions shown in FIG. 5). The reduction temperature is 670 ° C. (the temperature within the temperature range r12 in FIG. 5). ). When three test pieces were inspected, the number of gas leaks after reduction was 0 out of 3, and it was determined that no gas leak occurred. Moreover, when the joint part by the glass 12 was confirmed visually, generation | occurrence | production of a fracture | rupture was not confirmed.

[Comparative Example 1]
The reduction temperature is 650 ° C. (outside the temperature range r 2 in FIG. 4) under the condition that the glass transition point is 650 ° C., the softening point is 750 ° C., and the temperature at which η = ηs is 700 ° C. (the condition shown in FIG. 4). did. When three test pieces were inspected, the number of gas leaks after the reduction was three out of three, and it was determined that a gas leak occurred. Moreover, when the joint part by the glass 12 was confirmed visually, generation | occurrence | production of a fracture | rupture was confirmed in many places.

[Comparative Example 2]
The glass transition point is 600 ° C., the softening point is 800 ° C., and the temperature at which η = ηs is 650 ° C. (the conditions shown in FIG. 5), and the reduction temperature is 620 ° C. (temperature outside the temperature range r12 in FIG. 5). ). When three test pieces were inspected, the number of gas leaks after reduction was two out of three, and it was determined that a gas leak occurred. Moreover, when the joint part by the glass 12 was confirmed visually, generation | occurrence | production of a fracture | rupture was confirmed in several places.

  And as understood by comparing Examples 1 to 3 and Comparative Examples 1 and 2 described above, the glass 12 can be prevented from being broken by performing the reduction treatment in the temperature range shown in the present invention. confirmed.

  Thus, in the method for operating the fuel cell according to the present embodiment, when the cell 11 is subjected to the reduction treatment in the initial stage of the stack operation, the temperature at the time of reduction is set to the glass that joins the cell 11 and the housing 13. The temperature is equal to or higher than 12 transition points (Tg). Furthermore, since the viscosity η of the glass 12 is in a temperature range lower than the critical viscosity ηs (= τs / γ), even when the cell 11 contracts during reduction, the glass 12 can be prevented from being broken, and the cell 11 and the housing It becomes possible to maintain the joining of the body 13.

  Moreover, since the temperature at the time of reduction is less than the pour point (Tf), the glass 12 is not fluidized and the joined state between the cell 11 and the housing 13 can be maintained.

  Here, as described above, the “cell deformation shear rate γ” used in the equation (1) is a numerical value that varies depending on the concentration and flow rate of the combustion gas. Therefore, depending on how the combustion gas concentration and flow rate are selected, there may be no temperature range in which the viscosity η is lower than the critical viscosity ηs, as shown in FIG. Specifically, as the value of γ increases, the critical viscosity ηs decreases, so the temperature range r2 shown in FIG. 4 may not exist.

Therefore, in the present invention, by appropriately setting the concentration and flow rate of the combustion gas, the cell deformation shear rate γ is set to an appropriate value, and the temperature range in which the viscosity η is lower than the critical viscosity ηs (see FIG. 4). The temperature range r2) shown is present. By doing so, it is possible to more reliably prevent the glass 12 from being broken during the stack reduction. The fuel cell operating method of the present invention has been described based on the illustrated embodiment. However, the configuration of each unit can be replaced with any configuration having the same function.

  The present invention can be used to prevent breakage of glass during a reduction process performed at the initial stage of operating a fuel cell.

DESCRIPTION OF SYMBOLS 10 Fuel cell unit 11 Cell 12 Glass 13 Case 14 Current collector 20 Glass joint part 30 Stack

Claims (3)

In the initial operation of the fuel cell, in the operation method when reducing by supplying a reducing agent into the fuel cell,
The fuel cell temperature during reduction
A method for operating a fuel cell, wherein the temperature is equal to or higher than a temperature at which a glass transition point (Tg) is bonded between a cell and a casing provided in the fuel cell, and is equal to or lower than a temperature at which the glass flows . 2. The fuel cell operating method according to claim 1, wherein the glass has a viscosity η within a temperature range equal to or lower than a critical viscosity ηs determined by the following formula (1).
ηs = τs / γ (1)
Where τs is the allowable shear stress of glass and γ is the cell deformation shear rate.   The flow rate and concentration of the reducing agent supplied into the fuel cell are set so that there is a temperature range where the viscosity η of the glass is equal to or lower than the critical viscosity ηs in a temperature range where the temperature becomes the pour point or lower. The method for operating a fuel cell according to claim 2, wherein:

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