JP5152948B2 - Switching operation of reversible cell stack - Google Patents

Description

  The present invention relates to an operation switching method for a reversible cell stack having a plurality of reversible cells in which a solid polymer water electrolyzer (WE) and a fuel cell (FC) are integrated.

  A solid polymer reversible cell is a device in which the functions of a water electrolysis device and a fuel cell of the same shape are integrated. As disclosed in Patent Document 1, research on optimization of components has been conducted. However, it has not yet been put to practical use. This is because, on a practical scale, there are technical difficulties in establishing and controlling the opposite physical phenomena of water electrolysis device and fuel cell with the same member.

  That is, there are two cases in which the operation mode is switched: fuel cell operation → water electrolyzer operation and vice versa. Among these, when switching from the former fuel cell operation to the water electrolysis device operation, the switching judgment is not particularly necessary. In other words, in this case, it is necessary to change the internal base material of the reversible cell that is water-repellent during fuel cell operation, such as an ion exchange membrane and a power supply / current collector, to the hydrophilicity necessary for water electrolyzer operation. This is because these internal base materials can be easily made hydrophilic simply by supplying electrolyzed water to the inside of the cell, and electrolyzed water can be supplied to the electrode surface so that the water electrolyzer can be operated.

However, when switching from the latter water electrolyzer operation to the fuel cell operation, an appropriate switching judgment is required. This is because, when switching from water electrolyzer operation to fuel cell operation, even if the reaction gas is supplied to the cell internal substrate that has become hydrophilic in the water electrolyzer operation and loses gas permeability, This is because sufficient gas is not supplied to the surface and fuel cell operation is impossible. Even if the gas permeability is partially recovered, the other parts do not effectively act (react) as electrodes, and the apparent electrode area is reduced. As a result, the reaction occurs intensively only in the gas permeable part, resulting in performance degradation. In such a case, depending on the area of the electrode that acts effectively, a sudden drop in output voltage occurs immediately after the start of load operation, the ion exchange membrane dries due to a temperature rise accompanying the reaction, etc., and the hydrogen gas is in the H ⁺ (proton) state. Rather, it penetrates to the cathode side in the state of molecules (cross leak), and when the permeated hydrogen gas and the oxidant gas supplied to the cathode react on the electrode, an explosion or other combustion occurs, so an ion exchange membrane There is also a possibility of causing damage.

  As described above, when the operation mode is switched to the fuel cell operation, if the inside of the cell does not reach a dry state having a predetermined water repellency, the above-described problem occurs. In addition, for example, when electrolyzed water remains in the channel inside the cell, the electrolyzed water may not completely evaporate. Even if a drying process is performed to dry the inside of the cell, the remaining electrolyzed water evaporates. It may not be possible to do it. Moreover, in a reversible cell stack with multiple reversible cells, if each cell is not in the same dry state, the intended ability cannot be achieved or some cells may be damaged. There is a fear.

JP 2004-134134 A

  The present invention has been made in view of the above, and in a reversible cell stack including a plurality of reversible cells in which a solid polymer water electrolyzer (WE) and a fuel cell (FC) are integrated, When switching the operation mode from the electrolyzer operation to the fuel cell operation, the operation mode is switched safely, and the variation in the dry state between cells is suppressed, and the function of the entire stack is exhibited safely and efficiently. The purpose is that.

The present invention relates to a reversible cell stack having a plurality of polymer electrolyte reversible cells capable of switching between water electrolyzer operation and fuel cell operation, and changes the operation mode from water electrolyzer operation to fuel cell operation. In the switching method,
After the operation of the water electrolyzer and before the operation of the fuel cell, an inert gas is supplied into the channel inside each reversible cell, and the electrolyzed water remaining inside the cell due to the pressure difference between the channel inlet and outlet is supplied to the cell. A discharge step of discharging from the inside, and a drying step of drying the inside of each reversible cell after the discharge step,
When there is a reversible cell that has not yet reached the predetermined dry state after the drying process is completed, the humidified reaction gas is supplied to all the reversible cells, and the fuel cell operation with a lower load than the rated operation is performed. It is characterized in that the fuel cell operation is performed after performing a predetermined time and then performing the drying process again.

Fuel cell operation lower low load than the rated operation, for example when the current density is carried out low-load operation such that 0.1 A / cm ² or less. Here, the current density is the current density with respect to the effective electrode area. Thereafter, the drying process is performed again. By applying the load in this way, an amount of gas necessary for the reaction part of each cell to flow current according to the load is forcibly sucked. As a result, the reaction gas is reliably supplied to the channel portion, and as a result, the blockage of the flow path can be eliminated. As a result, the initial state of each cell is made uniform, so that by drying the cell again, there is no variation in the degree of drying between the cells, and safe and reliable operation switching is possible. In such a case, even if the inside of the cell is not completely dry, the gas supply rate to the reaction section in low-load operation is sufficiently slow so that there is no problem in maintaining the reaction, and even if a local reaction occurs, heat is generated. Since the amount is small, the heat generated at that time acts in the direction of drying the inside of the cell, and the possibility of damaging the exchange membrane is extremely low. At this time, it is also effective to perform a low-load operation while providing a dry effect inside the cell by supplying the reaction gas with low humidification.

  According to the present invention, operation modes can be switched safely and reliably in a reversible cell stack having a plurality of reversible cells in which a solid polymer water electrolyzer and a fuel cell are integrated. In addition, variations in the dry state between cells can be suppressed, and the function as a stack can be exhibited safely and efficiently.

  Hereinafter, preferred embodiments of the present invention will be described. FIG. 1 shows an outline of the configuration of the entire reversible cell stack 1 that implements the switching method according to the embodiment. This reversible cell stack 1 has end plates 2 and 3 arranged at both ends, and has, for example, 10 reversible cells C1 to C10 between power supply and current collecting plates 4 and 5 arranged inside thereof. ing. FIG. 2 shows a horizontal section of the reversible cell C1 located at the end.

  Since each reversible cell C1-C10 is the same structure, the structure is demonstrated taking the reversible cell C1 as an example, for example. As shown in FIGS. 1 and 2, the reversible cell C <b> 1 has two separators 11 and 12 and a composite plate 13 sandwiched between the separators 11 and 12. The composite plate 13 has a power supply / current collector 14, 15 and an MEA 16 sandwiched between the power supply / current collector 14, 15. The MEA 16 is a combined power generation unit in which an ion exchange membrane 16c made of a solid electrolyte material is disposed between electrode portions 16a and 16b made of two electrode catalyst layers. The supply / current collectors 14 and 15 are made of, for example, a porous material. When the water electrolysis apparatus is operated, the electrode portion 16a becomes a cathode and the electrode portion 16b becomes an anode.

  Each of the separators 11 and 12 has a corrugated cross section. The separators 11 and 12 and the separators 11 and 12 or the separators 11 and 12 are connected to the flow channels 21 and 23 isolated from the MEA 16. Forming. The flow paths 21 and 23 are cooling water flow paths. In addition, a plurality of channels (flow paths) 22 constituted by the separator 11 and the supply / current collector 14 and a plurality of channels (flow paths) 24 constituted by the separator 12 and the supply / current collector 15 are provided for the flow of the reaction gas. It becomes a road.

  FIG. 3 schematically shows, for example, the upper and lower flow configurations of each channel 22, and an upper header 31 communicating with each channel 22 is provided above each channel 22, and below each channel 22, A lower header 32 communicating with each channel 22 is provided. The upper header 31 communicates with the inlet manifold 33 and the lower header 32 communicates with the outlet manifold 34. Similarly, each channel 24 in the reversible cell C includes an upper header, a lower header, an inlet manifold, and an outlet manifold. However, the channels 22 and 24 are independent and supply and circulate gas of different systems. That is, during operation of the fuel cell, the humidified fuel gas (hydrogen gas) flows through the channel 22, and the humidified oxidant gas (oxygen) flows through the channel 24.

  The gas supply / discharge system and the cooling water supply / discharge system will be described with reference to FIG. 1 again. The inlet manifold 33 leading to the upper header 31 of the channel 22 of each reversible cell C is humidified from the outside of the end plate 3. To the first gas supply path 41 for supplying the fuel gas (hydrogen gas). The first gas supply passage 41 communicates with the inlet manifold of each reversible cell C in order from the reversible cell C10 side located at the end on the end plate 3 side, and communicates with the inlet manifold 33 of the reversible cell C1 located on the end plate 2 side. doing. As a result, the gas supplied from the first gas supply path 41 flows from the upper header 31 of each reversible cell C through each channel 22 to the lower header 32. The gas in the lower header 32 is collected at the outlet manifold 34, and finally passes through the first gas discharge passage 42 set at the lower part of each reversible cell C, and finally the reversible cell C10 located at the end on the end plate 3 side. It is communicated with the outlet manifold and discharged from the outside of the end plate 3. In this way, each channel 22 of each reversible cell C1 to C10 has a flow path structure capable of uniform flow. The first gas supply path 41 is connected to an inert gas supply source, for example, a nitrogen gas supply source 43.

  On the other hand, the channel 24 side of each reversible cell C is exactly the same, and the inlet manifold 33 that communicates with the header 31 of the channel 24 supplies a humidified oxidant gas (oxygen) from the outside of the end plate 3. Two gas supply paths 44 are connected. The second gas supply path 44 communicates with each reversible cell C in order from the reversible cell C10 located at the end on the end plate 3 side, and an inlet manifold 33 communicated with the channel 24 of the reversible cell C1 located on the end plate 2 side. Communicate. As a result, the gas supplied from the second gas supply path 44 flows from the upper header 31 of each reversible cell C through the channels 24 to the lower header 32. The gas in the lower header 32 is gathered at the outlet manifold 34, and finally passes through the second gas discharge passage 45 set at the lower part of each reversible cell C, and finally the reversible cell C10 located at the end on the end plate 3 side. It is communicated with the outlet manifold and discharged from the outside of the end plate 3. In this way, the channels 24 of the reversible cells C1 to C10 have a flow path structure that enables uniform distribution. An inert gas supply source, for example, a nitrogen gas supply source 46 is connected to the second gas supply path 41.

  As shown in FIG. 1, the cooling water is also supplied from the outside of the end plate 3 to the flow paths 21 and 23 from the upper side of the reversible cells C10 to C1 by the cooling water supply path 47. The cooling water that has flowed through 21 and 23 is discharged to the outside of the end plate 3 through a cooling water discharge passage 48 disposed on the lower side of each of the reversible cells C10 to C1.

The conductor resistance between the separators 11 and 12 of each reversible cell C1 to C10 (the increased value of the conductor resistance = the increased value of the membrane resistance is measured for each cell by the AC resistance measuring device 51 by the four-terminal measurement method. ing.

  The reversible cell stack 1 has the above main configuration. Next, a method of switching from the water electrolysis apparatus operation of the reversible cell stack 1 having such a configuration to the fuel cell operation will be described.

  When the water electrolysis apparatus is operated, the electrolyzed water is supplied into the channels 22 and 24, and the electrolyzed water is electrolyzed by supplying electric power to the supply / collector plates 4 and 5 from the outside. Oxygen is generated.

  When switching from the water electrolysis operation to the fuel cell operation, the inside of the reversible cells C1 to C10 of the reversible cell stack 1, particularly the inside of the channels 22 and 24, is completely wetted. Since the battery cannot be operated, the inside of the reversible cell 1 needs to be dried.

Therefore, it is necessary to first restore the water repellency by drying the inside of each channel 22, 24. In this case, for example, an inert gas may be supplied into the channels 22 and 24 and dried, and the dry state can be known from the resistance value measured by the AC resistance measuring device 51 for each reversible cell C. I can do it. That is, based on the resistance value when the inside of the cell is completely wet, the dry state inside the cell can be determined from the resistance increase value (= resistance increase value of the ion exchange membrane) generated by supplying the inert gas. . According to the knowledge of the inventors, if the resistance increase value of each reversible cell C is 0.1 [Ω · cm ² ] or more, it is possible to safely and safely carry out the operation without switching to the fuel cell operation. it can. However, this value varies depending on the thickness of the solid polymer film and the switching temperature. When the film thickness is t [cm] and the film conductivity is σ [1 / Ω · cm ² ], the resistance value referred to here is expressed by t / σ.

  By the way, even if the flow path structure is capable of uniform distribution to the reversible cells C1 to C10 in the dry state, the reversible cells C1 to C10 are switched at the time of switching from the water electrolysis device operation to the fuel cell operation. If there is a variation in drainage between the cells, there will be a difference in flow resistance between cells. Then, in the step of supplying the inert gas and drying the channels 22 and 24, the flow rate of the inert gas for drying supplied between the cells is different, so that the dry state between the cells (= film) The resistance rise value: ΔR) varies. Furthermore, the fact that the drainage state varies among cells means that the amount of water remaining in each cell is different, so the amount of water to be dried in each cell also differs. As a result, the variation in the degree of drying is further accelerated, and in an extreme case, a cell to which almost no drying gas is supplied appears.

As a result of the inventors actually examining the dry state of each reversible cell C1 to C10 of the reversible cell stack 1 of FIG. 1 from the resistance increase value of the AC resistance, the result is as shown in FIG. The drying conditions at this time were an inert gas temperature (T _PG ) of 68 ° C., a supply pressure (P _PG ) of 0.1 Mpa, and a supply flow rate (V _PG ) of 3 → 20 Nl / min. As can be seen from FIG. 4, when the lower limit dry value (switchable lower limit value) L from the water electrolyzer operation to the fuel cell operation is 0.1 [Ω · cm ² ], the drying starts (inert gas About 20 minutes after the start of supply), all other reversible cells except the reversible cell C7 clear the switchable lower limit L. However, with respect to the reversible cell C7, it is assumed that almost no dry gas is supplied.

If the fuel cell operation is performed in this state as it is, as shown in FIG. 6, the performance between the cells varies extremely from the low current density region, so operation at a higher current density or continued operation is also possible. Is impossible. The operating conditions at this time are an operating temperature (TFC) of 68 ° C., an operating pressure (PFC) of 0.1 MPa, and a hydrogen gas / oxygen flow rate (MH ₂ / O ₂ ) of 20/20 Nl / min.

  The reason why the AC resistance value hardly increases (does not dry) even when an inert gas for drying is supplied as in the reversible cell C7 is that the electrolytic water is contained in the channel 22 (or channel 24) as shown in FIG. It is considered that D remains and blocks the channel 22 (or the channel 24).

  The electrolyzed water D remaining in the channel 22 (or the channel 24) cannot be artificially discharged unless a certain level of force is applied due to the surface tension acting between the walls. Therefore, in order to discharge the electrolyzed water remaining in the channel uniformly and reliably, the upstream and downstream sides of the channel, that is, between the upper header 31 and the lower header 32 (upstream and downstream of the header) across the blocking portion. In addition, it is considered that the remaining electrolyzed water D can be blown away by applying a force (differential pressure) that exceeds this surface tension.

The discharge conditions (conditions for blowing off) at this time can be calculated based on the following balance of forces. That is, as shown in FIGS.
Residual water weight: F ₁ = mg
Surface tension acting on the channel part: F ₂ = σL
Force based on dynamic pressure of dry gas: F ₃ = 1 / 2ρV ² A
Force based on static pressure of channel upstream header: F ₄ = P ₁ A
Force based on static pressure in channel downstream header: F ₅ = P ₂ A
When
F ₁ + F ₃ + F ₄ > F ₂ + F ₅ (1)
If the above condition is satisfied, the remaining electrolyzed water D can be blown off.
here,
m: Mass of residual electrolyzed water [kg]
g: Gravity acceleration [m / s ² ]
σ: surface tension [N / m]
L: Interface length between water and wall surface (= perimeter of channel cross section × 2) [m]
ρ: Dry gas density [kg / m ³ ]
V: Average speed of dry gas (= supply flow rate / cross-sectional area) [m / s]
A: Channel cross-sectional area [m ² ]
P ₁ : Static pressure in the channel upstream header [Pa]
P ₂ : Static pressure in channel downstream header [Pa]
It is. However, in reality, it is extremely difficult to determine each specification so as to satisfy Equation (1).

At this time, it can be easily imagined that the flow rate of the inert gas for drying is different for each channel, considering that the clogging situation is different for each channel. Here, when the difference in static pressure between the upstream and downstream of the header is ΔP [Pa], if ΔP · A is F ₂ or more, it is possible to reliably eliminate the blockage of the channel regardless of the blockage of the channel. It becomes. That is,
1 / 2ρV ² · A> F ₂ (2)
As long as the conditions are satisfied, even if there is a variation in flow rate between the respective channels, all the channels are connected by the header 31, and the headers 31 of all the cells are connected by the first gas supply path 41. because you are, pushing pressure P ₁ of all the channels, all the cells become the same, but the lowest (when fully uniform flow), since the 1 / 2ρV ² or more, it is possible to reliably eliminate the passage closing.

  Therefore, after performing such a discharge process, if the drying process is performed by supplying a dry inert gas into the channel, the reversible cells C1 to C10 can be dried without variation, and the water electrolysis apparatus It is possible to perform fuel cell operation safely and reliably from operation, and it is possible to realize efficient operation.

  It should be noted that the flow rate of the inert gas for drying after the residual water inside the cell is completely discharged is not necessarily the same as the conditions for the above-described discharging process, taking into consideration the time for which drying is to be completed. And may be changed arbitrarily. That is, after the discharge process is completed, the flow rate may be arbitrarily changed, and the drying process may be performed as it is.

  By the way, even if the above-described discharging process is performed, it is fully conceivable that in the reversible cell stack 1, a variation in drying between the reversible cells C1 to C10 occurs due to some accidental phenomenon. Therefore, a method for switching from the water electrolysis device operation to the fuel cell operation safely and surely even if such a situation occurs will be described below.

  If drying variation occurs between the reversible cells C1 to C10, even if all the cells have reached the specified dry state by performing the drying for a long time, the cells may be overdried as a price. (For example, reversible cell C5 in FIG. 4). Then, as soon as the humidified reaction gas is supplied thereafter, the cell may be damaged due to a cross leak of hydrogen gas or oxygen gas. In such a case, after all the cells have reached the prescribed dry state, the humidified inert gas is supplied to the reversible cells C1 to C10 before supplying the humidified reaction gas and operating the fuel cell. By supplying the channels 22 and 24, the dry state between the cells can be made uniform safely and reliably.

  At this time, if the humidified reaction gas, that is, humidified oxygen gas or air, is supplied only to the cathode side, that is, according to FIG.

  In this way, when some variation in drying occurs, the above countermeasures are sufficient. However, if there is a cell that does not dry at all (not reach the specified dry state) (for example, the reversible cell C7 in FIG. 4), a new countermeasure is required.

In such a case, the supply of the inert gas for drying is temporarily stopped, and the humidified reaction gas (hydrogen gas and oxygen) is caused to flow through the corresponding channels 22 and 24, respectively. Low load operation, for example, low load operation in which the current density is 0.1 A / cm ² or less is performed for a certain time. Then, by supplying again the inert gas for drying to the channels 22 and 24, the dry state inside the cell can be made uniform. For example, the certain time may be determined by the integrated amount of the supplied gas.

For the reversible cell stack 1 having the drying results shown in FIG. 4, the inventors have operated the fuel cell at a low load so that the current density is 0.1 A / cm ² for a predetermined time, for example, 10 to 10. After carrying out for 20 minutes, the drying process with the inert gas was performed again under the conditions of the inert gas temperature (T _PG ) of 68 ° C., the supply pressure (P _PG ) of 0.1 Mpa, and the supply flow rate (V _PG ) of 10 Nl / min. The results are shown in FIG.

  According to this, even in the case of the reversible cell C7 that could not clear the operable drying lower limit (switchable lower limit) L in FIG. Possible lower limit (L) is cleared. Then, the result of carrying out the fuel cell operation is as shown in FIG. 8, and the operation without variation is realized from the stage of low current density.

  In this case, by applying a load, the reaction part of each cell forcibly sucks in an amount of gas necessary for flowing a current in accordance with the load. Then, the reaction gas is surely supplied to the channel, and as a result, the blockage of the flow path can be solved. This is because the initial state of each of the reversible cells C1 to C10 is made uniform, so that drying variation between the cells is eliminated by drying the cells again, thereby enabling safe and reliable operation switching.

Here, even if the inside of the cell is not completely dry, the gas supply rate to the reaction part (channel) in the low load operation is, for example, that the entire electrode area can be used. ^-7 [mol / s · cm ² ], and the oxygen gas side is 2.6 × 10 ⁻⁷ [mol / s · cm ² ], which is sufficiently slow so that it does not pose a problem for the duration of the reaction, and local reactions Since the amount of heat generated is small even if a spill occurs, the heat acts in the direction of drying the inside of the cell, so the possibility of damaging the membrane is extremely low. At this time, it is also effective to perform a low load operation while providing a drying effect inside the cell by supplying the reaction gas with low humidification.

  As described above, according to the present invention, even in a reversible cell stack, the reversible cell stack can be reliably switched without providing any special equipment for switching the operation. Moreover, in order to completely discharge the electrolyzed water remaining in the channel, which is the most important for switching the stack operation, it is possible to calculate the necessary conditions according to the shape of any shape of the cell and realize it. It is.

  The present invention is useful when switching from water electrolyzer operation to fuel cell operation in a reversible cell stack having a plurality of reversible cells.

It is explanatory drawing which shows the outline of a structure of the reversible cell stack used in embodiment. It is a partial expanded horizontal sectional view of the reversible cell stack used in the embodiment. It is explanatory drawing which showed typically the channel system of the channel inside the reversible cell used in embodiment. It is a graph which shows the membrane resistance rise value of each reversible cell with drying time. It is explanatory drawing explaining the balance of the force of the residual electrolyzed water which has obstruct | occluded the channel. 5 is a graph showing the relationship between the average current density and the output voltage when the reversible cell stack having the dry state of FIG. 4 is operated as a fuel cell. It is a graph which shows the membrane resistance rise value of each reversible cell accompanying the drying time after implementing low load operation once. It is a graph which shows the relationship between the average current density at the time of carrying out a fuel cell operation of the reversible cell stack which has the dry state of FIG.

Explanation of symbols

1 Reversible cell stack 4,5 Supply / collection plate 11,12 Separator 13 Composite plate 14,15 Supply / collector 16 MEA
16a, 16b Electrocatalyst layer 16c Ion exchange membrane 22.24 Channel 31 Upper header 32 Stock header 41 First gas supply path 42 First gas discharge path 43, 46 Inert gas supply source 44 Second gas supply path 45 Second gas Discharge path 47 Cooling water supply path 48 Cooling water discharge path 51 AC resistance measuring device C1 to C10 Reversible cell

Claims (2)

In a method of switching an operation mode from a water electrolysis device operation to a fuel cell operation, a reversible cell stack having a plurality of polymer electrolyte reversible cells capable of switching between an operation mode of a water electrolysis device and a fuel cell operation,
After the operation of the water electrolyzer, before operating the fuel cell,
A discharge step of supplying an inert gas into the channel inside each reversible cell and discharging the electrolyzed water remaining inside the cell due to a pressure difference between the channel inlet and the outlet from the inside of the cell;
A drying step of drying the inside of each reversible cell after the discharging step;
Have
When there is a reversible cell that has not yet reached the predetermined dry state after the drying process is completed, the humidified reaction gas is supplied to all the reversible cells, and the fuel cell operation with a lower load than the rated operation is performed. A method of switching operation of a reversible cell stack, characterized in that the fuel cell operation is performed after a predetermined time, and then a drying process is performed again . The reversible cell stack operation switching method according to claim 1, wherein the low load operation has a current density of 0.1 A / cm ² or less .

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