JP5504726B2 - Fuel cell system and fuel cell characteristic recovery method - Google Patents

Description

  The present invention relates to a direct fuel cell system capable of generating electricity by directly supplying an organic substance or an organic solution to a fuel electrode and a method for recovering the characteristics of the fuel cell.

  A polymer electrolyte fuel cell (hereinafter referred to as “fuel cell stack”) has a structure in which cells called “unit cells” are stacked. This unit cell is composed of a cell in which a polymer electrolyte is sandwiched between a positive electrode (cathode) and a negative electrode (anode) provided with a catalyst layer, and a polymer electrolyte membrane and a catalyst layer are joined together. Electricity is generated by supplying air as an agent and fuel (for example, an aqueous methanol solution) as a reducing agent.

  When a fuel cell system equipped with such a fuel cell stack is operated for a long time, its output characteristics gradually deteriorate. This reason is considered to be because the catalyst metal on the oxidant electrode side is oxidized. Therefore, a method for recovering the output characteristics of the lowered fuel cell stack afterwards has been proposed. For example, Patent Document 1 describes that power degradation can be “recovered” by generating power with a cathode potential (that is, potential on the oxidant electrode side) lower than that during normal operation ( 5th paragraph). Similarly, Patent Document 2 describes that in a fuel cell vehicle using a fuel cell system as an in-vehicle power source, “the catalyst activation treatment can be performed by forcibly reducing the output voltage of the fuel cell stack”. (4th paragraph, etc.)

  That is, these documents disclose knowledge about activating the fuel cell catalyst by reducing the cathode potential to near the reduction potential at which the platinum catalyst to be oxidized is reduced by generating electricity.

Special table 2003-536232 gazette JP 2008-192468 A

However, the following problems may occur when the process of lowering the cathode potential by generating electricity is performed.
(1) The cathode potential may not be lowered to a level necessary for recovery. This problem is caused by fuel diffusion rate limiting at the anode. When diffusion rate control occurs, power generation at a current density higher than the limit current density is impossible. Therefore, if the cathode potential is noble at the current density above the above level, recovery of the oxidizer electrode is impossible within the scope of the prior art. is there. This problem remarkably occurs in the case of a liquid fuel type direct fuel cell using an alcohol aqueous solution as a fuel, as compared with a fuel cell employing a gaseous fuel system such as hydrogen gas. This is because liquid fuel has an extremely low limit current density as compared with gas fuel.
(2) When applied to a fuel cell stack manufactured by laminating a plurality of fuel cells in series, inversion of some fuel cells occurs. Inversion can occur when diffusion limited occurs at the anode of the fuel cell.
(3) When the flow rate of the oxidizing agent is lowered, hydrogen is generated from the anode. In addition, hydrogen can be generated even in a gaseous fuel system using hydrogen gas or the like as a fuel, but since hydrogen gas is used as a fuel, it is not a problem because it is designed to be highly airtight.

Here, the above (1) will be described with reference to FIG. This figure shows the relationship between the current density and the potential with reference to the reversible hydrogen electrode. Each point in the graph is plotted by measuring the cathode and anode potentials for each current density when power is generated using air as the oxidant and aqueous methanol as the fuel. The solid line is the cathode potential and the dotted line is the anode potential. The symbol ○ indicates that the concentration of the aqueous methanol solution is 5.0%, and the symbol Δ indicates 2.0%. In the case of 2.0%, it can be seen that when the current density is increased, the fuel diffusion rate is controlled at the anode. Although not measured for electrode protection, it is expected that the potential increases rapidly when the current density is further increased. For example, when a battery plotted with Δ is generated at 400 mAcm ⁻² , the anode potential becomes 800 mV, and thereafter, the elution reaction of the catalytic metal occurs, so it is expected that the potential will remain unchanged for a while. This is because the catalytic metal is oxidized instead of the missing fuel. For example, 800 mV corresponds to a potential at which Ru is oxidized. That is, the potential is shifted to the potential at which the catalytic metal elutes.

  In other words, the conventional fuel cell recovery method obtains the target cell voltage (cathode potential) at a relatively high current density comparable to that during normal operation. Even when the target cell voltage is not possible, the cathode potential may not reach the level necessary for recovery. When a plurality of unit cells are stacked, in the case of a so-called fuel cell stack, there is a risk that some of the unit cells are reversed and the catalyst metal is eluted. These problems can trigger the output of the fuel cell stack.

  The present invention has been made in view of the above, and an object of the present invention is to propose a fuel cell system that reliably brings the cathode potential to a level necessary for recovery. Further, it is an additional technical problem to propose a fuel cell system capable of recovering output characteristics without reversing the unit cell.

The fuel cell system according to the present invention includes a fuel cell including a fuel electrode carrying a catalyst and an oxidant electrode, means for supplying and discharging fuel to the fuel electrode, and means for supplying and discharging oxidant to the oxidant electrode. In a fuel cell system comprising:
The fuel cell is connected to an external load, and includes a control means for switching between a normal operation mode for supplying electric power for driving the load and a recovery operation mode for generating power below a preset cell voltage value,
The control means includes means for bringing a reducing agent into contact with the catalyst supported on the oxidant electrode in the recovery operation mode.

Recovery operation mode is a mode in which power generation of the fuel cell at a voltage value less than the single cell voltage set beforehand. The fuel cell after operating in this mode has recovered its output performance. Recovery refers to power generation voltage of the fuel cell after undergoing this mode, except voltage is to be higher than the power generation voltage before undergoing this mode when power generation by the same power generation conditions. Therefore, the “preset voltage value” means a voltage value set in advance so that this recovery can be achieved. This “preset voltage value” is a value that changes depending on conditions such as the type of fuel and the operating temperature of the fuel cell. This recovery is presumed to be due to the negative shift of the cathode potential in conjunction with the lower single cell voltage. When the cathode potential shifts to a base level until the potential at which the oxide film formed on the catalyst surface is removed, the removal of the oxide film on the catalyst surface proceeds and the activation overvoltage of the cathode reaction is reduced.

Normal operation mode is a mode in which power generation of the fuel cell in conditions other than the recovery operation mode. For example, a mode in which power generation of the fuel cell at a higher than "preset voltage value" single cell voltage.

  The inversion is considered to be caused by the non-uniform amounts of aqueous methanol solution and air supplied to each unit cell. Therefore, as described above, by supplying the reducing agent to the catalyst on the oxidant electrode side, the cathode potential is forcibly lowered, and the recovery operation is performed by setting the target cell voltage at a lower current density. . According to such a configuration, inversion is less likely to occur than in the case where recovery operation is performed by setting the target cell voltage at a high current density.

  As an operation during the recovery operation, the cell voltage may be lowered to a predetermined value (for example, 0.2 V or less) while continuing power generation in a state where it is connected to an external load. The cell voltage may be lowered to a predetermined voltage in a state where the internal load is further provided, and the external load is disconnected and connected only to the internal load.

  In the present invention, the “means for bringing a small amount of reducing agent into contact with the catalyst supported on the oxidant electrode” means that the aqueous alcohol solution and the oxidant are directly supplied to the fuel electrode and the oxidant electrode, respectively. Alternatively, the flow path of the aqueous alcohol solution may be controlled so as to be supplied also to the oxidizer electrode. Alternatively, the fuel concentration of the aqueous alcohol solution may be made to reach the oxidant electrode side indirectly from the fuel electrode side by efficiently crossing over by making it higher than that during normal operation.

  By any means, the alcohol aqueous solution is supplied to the oxidizer electrode side, thereby bringing about an effect of forcibly lowering the cathode potential while maintaining a low current density. Therefore, according to the above method, inversion can be suppressed, the catalyst metal on the oxidant electrode side can be reduced, and the output characteristics can be recovered.

  According to the present invention, by reducing the potential of the oxidant electrode by forcing the reducing agent in contact with the catalyst supported on the oxidant electrode, the output per unit cell at a current density lower than the limit current density of the anode. The voltage can be made lower than a predetermined voltage. Therefore, the potential of the cathode can be surely brought to a level necessary for recovery. In the fuel cell stack, the performance of the fuel cell stack can be recovered without reversing the unit cell.

1 is a schematic configuration diagram showing an example of a fuel cell system according to the present invention. Schematic of the structure of the unit cell which comprises the fuel cell stack of FIG. Diagram showing an example of operation procedure of fuel cell stack Operation procedure of fuel cell system of embodiment 1 Operation procedure of fuel cell system of Example 2 The figure which shows the result of the process of Experimental example 1 thru | or 4 and Comparative example 1 thru | or 5. The figure which shows the relationship between an electric current density and the electric potential on the basis of a reversible hydrogen electrode.

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

(First Embodiment)-Fuel Cell System-
FIG. 1 is a schematic configuration diagram showing an example of a fuel cell system according to the present invention.
The fuel cell stack 1 in this system is composed of a direct methanol fuel cell (DMFC) that generates electricity by directly contacting an air as an oxidant and a fuel (in particular, a liquid fuel such as an aqueous alcohol solution) as a reducing agent. The In the fuel cell stack, a heat exchanger 2, a water tank 3, a fuel tank 4, a circulation tank 5, and the like are connected to auxiliary equipment such as various pumps (water pump 6, fuel pump 7, circulation pump 8), and alcohol is contained in the system. The aqueous solution is circulating. Electricity is generated by supplying oxygen from the air pump 9 to the fuel cell stack 1.

  The heat exchanger 2 cools and collects water in the gas discharged from the oxidant electrode of the fuel cell stack 1. The water tank 3 is a tank for storing water collected by the heat exchanger, and the fuel tank 4 is a tank for storing a methanol aqueous solution having a higher concentration than that during normal operation. The circulation tank 5 has a concentration sensor 10 for measuring the concentration of the aqueous methanol solution, and controls the operation of machinery such as the water pump 6, the fuel pump 7 and the valve V1 in order to arbitrarily adjust the concentration of the aqueous methanol solution. The control unit 11 is connected.

Furthermore, a discharge unit 12 that functions as a discharge circuit that controls the extraction of electric power and an external load 13 that outputs electric power obtained from the fuel cell stack 1 are connected to the output terminal of the fuel cell stack 1. Discharge unit 12, which functions as a discharge circuit for power generation of the fuel cell stack below a predetermined voltage value, the external load 13, for holding of the fuel cell system, for convenience "internal load" Call it. Further, a control circuit 14 is provided for electronically controlling the connection between the external load and the internal load and the operation of each electrical element in the entire system. Moreover, the flow path 15 which can supply a part of alcohol aqueous solution also to the oxidizing agent electrode side may be provided. In this case, by using the valves V1 and V2 and the buffer 16, the aqueous alcohol solution can be supplied also to the oxidant electrode side via the flow path 15 during the recovery operation. Here, the buffer 16 is used in order to prevent the oxidizing agent from flowing into the flow path 15. The supply method is as follows. First, the valve V1 is opened to put the aqueous alcohol solution into the buffer 16, then the valve V1 is closed and then V2 is opened (hereinafter referred to as valves V1 and V2 are opened). Thereby, the alcohol aqueous solution is mixed with the oxidant and supplied to the oxidant electrode.

  In addition to the above, the fuel cell system includes a fan and a valve for cooling the heat exchanger 2, but these are omitted in FIG.

  FIG. 2 is a diagram showing an outline of the structure of a unit cell constituting the fuel cell stack of FIG. The unit cell 100 is sandwiched between an electrolyte membrane 101, a fuel electrode 102a and an oxidant electrode 102b provided on both sides thereof, and a membrane electrode assembly comprising gas diffusion layers 103a and 103b provided on the outside thereof. Further, on the surface facing the membrane electrode assembly, there are provided a separator 104a provided with a flow path for flowing a methanol aqueous solution supplied to the fuel electrode 102a and a flow path for flowing air supplied to the oxidant electrode 102b. Separator 104b. The fuel stack 1 in FIG. 1 is configured by stacking, for example, 40 such unit cells 100.

  The catalyst layers of the fuel electrode 102a and the oxidant electrode 102b are composed of a catalytic metal material, an ion exchange resin, an ion exchange membrane, a carbon material, and the like. The catalyst layer of the fuel electrode 102a may contain a powdery methanol clathrate compound. Further, a water-repellent material such as PTFE (polytetrafluoroethylene) or FEP (fluoroethylenepropylene) can be used as necessary for both the catalyst layer of the fuel electrode 102a and the catalyst layer of the oxidant electrode 102b.

  The catalyst metal material used in the present invention preferably has a catalytic action that promotes the oxidation reaction of methanol and the reduction reaction of oxygen. For example, platinum group metals such as platinum, ruthenium, iridium, rhodium, palladium, osmium, or alloys thereof are applicable.

  Further, as the ion exchange resin and the ion exchange membrane, a resin exhibiting proton conductivity can be used. For example, perfluorocarbon sulfonic acid resins, styrene-divinylbenzene sulfonic acid resins, or ion exchange resins having carboxylic acid groups, phosphonic acid groups, and phosphoric acid groups as ion exchange groups are preferred. In addition, a resin in which a sulfonic acid group is formed on polyolefin or polyimide can also be used. In addition, as a method of dispersing the ion exchange resin in the catalyst layer, the constituent material of the catalyst layer is mixed and molded, or the constituent materials other than the ion exchange resin are mixed and molded in advance and then impregnated with the ion exchange resin. And how to do it. As the carbon material, a material having high electron conductivity is preferable. In addition to carbon black such as furnace black, channel black, and acetylene black, vapor grown carbon, activated carbon, graphite, or the like may be used.

(Second Embodiment) -Recovery processing of fuel cell stack-
Next, the fuel cell stack recovery process will be described with reference to the drawings. In this embodiment, as a method of recovery treatment by reduction of the platinum catalyst, the cathode potential is forcibly lowered in order to achieve a target cell voltage at a current density lower than that during normal operation.

  FIG. 3 shows an example of the operation procedure of the fuel cell stack. The control circuit 14 constantly measures the stack voltage of the fuel cell stack during operation of the fuel cell system (step S11).

Next, it is determined whether or not the lower limit voltage has been reached (step S12). Here, the “lower limit voltage” is defined as a predetermined voltage obtained in advance from the number of stacks. For example, if power generation voltage in the normal operation mode is about 0.4V / cell, the lower limit voltage may be set 0.3V (single cell voltage). In this case, the operation is performed with a flow of switching to the recovery operation mode when the single cell voltage reaches 0.3V. The lower limit voltage value is a value appropriately set by the user or the system designer. When the lower limit voltage is reached, the normal operation mode is shifted to the recovery operation mode in order to recover the characteristics of the fuel cell stack 1 (step S13).

  However, the normal operation mode and the recovery operation mode are distinguished depending on whether or not the cell voltage is decreased to a preset voltage value, for example, about 0.2V. That is, it does not matter whether it is connected to an external load or an internal load. When operating with the single cell voltage lowered to 0.2 V by a method described later while generating power while connected to an external load, the vehicle is operating in the “recovery operation mode”. This 0.2 V is changed depending on conditions such as the type of fuel and the operating temperature of the fuel cell.

  However, when the single cell voltage varies and temporarily falls below 0.2 V due to fluctuations in the state of the load during normal operation, it is formally operated in the recovery operation mode. For example, if the operation is performed so that the “recovery operation mode” is set in a state where the single cell voltage is lowered to 0.2 V, the catalyst metal is reduced by the power generation, and thus the “recovery operation mode” is set. I can say that.

When operating in the recovery operation mode, in order to operate the single cell voltage at a “constant voltage” of 0.2 V or less, the external load is disconnected and the internal load, that is, the fuel cell stack such as the discharge unit 12 is set to a predetermined voltage. it is preferably connected to a discharge circuit for power generation at a current density of a value or less. The discharge unit 12 is specifically configured by such as a constant voltage circuit, the output voltage of the fuel cell stack at a predetermined voltage can be power generation.

  As described above, the operation mode of the “recovery operation mode” proposed by the present invention is a state in which the cathode potential is forcibly lowered in order to obtain a target cell voltage at a lower current density than during normal operation. It is assumed that the power is generated at a low current density, and a means for generating power while preventing inversion at a low current density, specifically, a small amount is applied to the oxidizer electrode by valve operation (V1, V2 in FIG. 1). The aqueous alcohol solution (reducing agent) is directly supplied (Example 1), or the alcohol aqueous solution having a higher concentration than that in normal operation is supplied to the fuel electrode by the concentration adjusting means of the aqueous alcohol solution, thereby crossing the fuel electrode through the fuel electrode. Thus, it is important to take measures (which are realized by indirectly supplying an aqueous alcohol solution (reducing agent) to the catalyst metal of the oxidant electrode (Example 2)).

  In any case (Embodiments 1 and 2), by supplying an alcohol aqueous solution to the oxidant electrode side, the cathode potential is forcibly lowered while maintaining a low current density. Therefore, it is possible to reduce the catalytic metal on the oxidizer electrode side while suppressing inversion, and to recover the output characteristics. Two examples will be described below.

Example 1
In the fuel cell system of FIG. 1, as a transition operation to the recovery operation mode, the control unit 11 opens the valves V1 and V2 so that a small amount of an alcohol aqueous solution is brought into contact with the oxidant electrode. In this way, the cathode potential of the fuel cell stack is forcibly lowered, and the cell voltage per unit cell is forcibly reduced to about 0.2 V, thereby preventing inversion at a low current density. Perform recovery processing.

  FIG. 4 shows an example of the recovery operation procedure of the fuel cell system according to the first embodiment. In order to start the recovery operation mode, the valves V1 and V2 are set to open by the control unit 11, and the flow rate is controlled as necessary (S21). In this state, power is generated at a current density lower than the current density at which the cell voltage per unit cell is set to a constant voltage of 0.2 V (or lower) and the same cell voltage is obtained in the normal operation mode (S22). By continuing the operation for a predetermined time (S23), the recovery operation mode is ended (S23).

  When the valve V1, V2 is opened as it is without being connected to the internal load and connected to the external load (in normal operation), the alcohol aqueous solution is supplied not only to the fuel electrode but also to the oxidant electrode. The recovery process may be performed by lowering the cathode potential and forcibly reducing the voltage per unit cell to about 0.2 V, but disconnecting the external load and It is desirable to perform control so as to obtain a predetermined current and voltage necessary for recovery of the catalyst metal by operating by connecting to an internal load such as the control unit 12.

  In order to shift to the normal operation mode again, it is necessary to close the valves V1 and V2 and operate for a while so that the aqueous alcohol solution directly supplied to the oxidizer electrode is eliminated.

(Example 2)
In the fuel cell system of FIG. 1, as an operation at the time of the recovery operation, the control unit 11 oxidizes from the fuel electrode side indirectly by efficiently crossing over by making the fuel concentration of the alcohol aqueous solution higher than that at the normal operation. You may employ | adopt the structure which makes it reach | attain the agent electrode side.

  That is, during normal operation, an aqueous alcohol solution adjusted to an appropriate concentration is supplied to the fuel electrode side via the circulation pump 8 with the valve V1 closed, and is supplied to the fuel electrode side of the fuel cell stack. Is generated with air supplied through an air pump and connected to an external load with the discharge unit 12 disconnected.

  FIG. 5 shows an example of the recovery operation of the fuel cell system according to the second embodiment. In order to start the recovery operation mode, the water pump 6 and the fuel pump 7 are adjusted by the control unit 11, and the concentration of the methanol aqueous solution is set to be higher than the concentration during normal operation (S31). The concentration sensor 10 confirms that the target concentration has been reached (S32). In this state, the cell voltage per unit cell is set to a constant voltage of 0.2V (or lower), and the same cell voltage is set in the normal operation mode. Operation is performed at a current density lower than the obtained current density (S33). By operating for a predetermined time (S34), the recovery operation mode is terminated (S35).

  As in Example 1, the concentration of the aqueous alcohol solution is adjusted as it is without being connected to the internal load and connected to the external load (in normal operation), and the cathode potential is forcibly set to about 0.2 V per unit cell. The recovery process may be performed by operating in a state reduced to a low level, but preferably the external metal load is disconnected and connected to an internal load such as the electric control unit 12 to recover the catalyst metal. Control is performed to obtain the required predetermined current and voltage. In this way, the recovery process is completed more effectively.

  In order to shift to the normal operation mode again, it is necessary to set the concentration to the original concentration by the control unit 11.

-Experimental example-
In order to evaluate the characteristics of the fuel cell stack by the recovery operation of the fuel cell system according to the present invention, a fuel cell stack was first manufactured by the following procedure.

1. Production of fuel electrode (1) Cation exchange of perfluorocarbon polymer having 1.0 g of platinum-ruthenium catalyst-supported carbon (Tanaka Kikinzoku Kogyo Co., Ltd., 29.2 mass% platinum, 22.6 mass% ruthenium) and sulfonic acid group 9.6 g of resin (manufactured by Dupont, 5 mass% solution), 0.5 g of water repellent (manufactured by Mitsui / DuPont Fluorochemicals, PTFE, 60 mass% dispersion) and 8.0 g of purified water and stirring rod And further kneading using a planetary ball mill to prepare a fuel electrode slurry.
(2) Carbon vapor (made by Toray Industries Inc., thickness 200 μm) cut into 5.0 cm on one side and 5.0 cm on the other side is used as a water repellent (PTFE, 60 mass% dispersion) manufactured by Mitsui DuPont Fluorochemical Co., Ltd. After impregnating and drying, carbon paper with water repellency was prepared by firing at 360 ° C.
(3) The fuel electrode was formed by applying and drying the slurry for fuel electrode (1) on one side of the carbon paper of (2) with a spray coating machine. The amount of catalyst supported on this fuel electrode was 3.0 mg (Pt—Ru) / cm ² .

2. Production of oxidizer electrode (1) Platinum catalyst supported carbon (Tanaka Kikinzoku Kogyo Co., Ltd., platinum 46.5 mass%) 1.0 g, cation exchange resin (Dupont 5 mass% solution) 7.1 g and water repellent ( Mitsui-DuPont Fluoro Chemical Co., Ltd. PTFE, 60 mass% dispersion) 0.5 g and purified water 8.0 g were weighed and mixed with a stir bar, and further kneaded using a planetary ball mill. An electrode slurry was prepared.
(2) Carbon vapor (made by Toray Industries Inc., thickness 200 μm) cut into 5.0 cm on one side and 5.0 cm on the other side is used as a water repellent (PTFE, 60 mass% dispersion) manufactured by Mitsui DuPont Fluorochemical Co., Ltd. After impregnating and drying, carbon paper with water repellency was formed by firing at 360 ° C.
(3) The oxidant electrode was formed by applying and drying the oxidant electrode slurry of (1) on one side of the carbon vapor of (2) with a spray coating machine. The amount of catalyst supported on the oxidant electrode was 3.0 mg (Pt) / cm ² .

3. Production of membrane electrode assembly 1. On one side of a cation exchange membrane (manufactured by Dupont, thickness 175 μm) made of a perfluorocarbon polymer having a sulfonic acid group. 1 on the other side. After each of the oxidizer electrodes was disposed, hot pressing was performed under conditions of 10 MPa and 150 ° C. to form a membrane electrode assembly.

4). 2. Production of fuel stack (1) The unit cell was formed by sandwiching the membrane electrode assembly between a carbon separator having a flow path for flowing fuel and a carbon separator having a flow path for flowing an oxidizing agent.
(2) The unit battery of (1) is stacked as 40 cells, both ends in the stacking direction are sandwiched between stainless end plates, bolts are passed between the end plates, and the bolts are tightened with nuts. A fuel cell stack was formed.

5. Experiment Next, using the fuel cell stack manufactured according to the above procedure, a temperature of 65 ° C., a 1 mol / l methanol aqueous solution was supplied to the fuel electrode at a flow rate of 200 ml / min, and air was supplied to the air electrode at a flow rate of 20000 ml / min. The fuel cell system was operated for 100 cycles of 10 hours of continuous operation at a current density of 130 mA / cm ² , that is, for a total of 1000 hours. Using this fuel cell stack, the following experimental examples 1 to 4 and comparative examples 1 to 5 were processed.

<Experimental example 1>
Supplying 1.5 mol / l aqueous methanol solution to the fuel electrode stack at a flow rate of 200 ml / min and air to the air electrode at a flow rate of 20000 ml / min to the fuel cell stack after 1000 hours of operation under the above conditions, the output voltage per unit cell Power was generated at a constant voltage for 2 hours so that the voltage became 0.15V.

<Experimental example 2>
Supplying 1.5 mol / l aqueous methanol solution to the fuel electrode stack at a flow rate of 200 ml / min and air to the air electrode at a flow rate of 20000 ml / min to the fuel cell stack after 1000 hours of operation under the above conditions, the output voltage per unit cell Was generated at a constant voltage for 2 hours so that the voltage became 0.2V.

<Experimental example 3>
Under the above conditions, a 2.0 mol / l aqueous methanol solution is supplied to the fuel electrode stack at a flow rate of 200 ml / min and air is supplied to the air electrode at a flow rate of 20000 ml / min. Power was generated at a constant voltage for 2 hours so that the voltage became 0.15V.

<Experimental example 4>
Under the above conditions, a 2.0 mol / l aqueous methanol solution is supplied to the fuel electrode stack at a flow rate of 200 ml / min and air is supplied to the air electrode at a flow rate of 20000 ml / min. Was generated at a constant voltage for 2 hours so that the voltage became 0.2V.

<Comparative Example 1>
Supplying 1.5 mol / l aqueous methanol solution to the fuel electrode stack at a flow rate of 200 ml / min and air to the air electrode at a flow rate of 20000 ml / min to the fuel cell stack after 1000 hours of operation under the above conditions, the output voltage per unit cell Power was generated at a constant voltage for 2 hours so that the voltage became 0.25V.

<Comparative example 2>
Under the above conditions, a 2.0 mol / l aqueous methanol solution is supplied to the fuel electrode stack at a flow rate of 200 ml / min and air is supplied to the air electrode at a flow rate of 20000 ml / min. Power was generated at a constant voltage for 2 hours so that the voltage became 0.25V.

<Comparative Example 3>
Supplying 1.0 mol / l aqueous methanol solution to the fuel electrode at a flow rate of 200 ml / min and air to the air electrode at a flow rate of 20000 ml / min to the fuel cell stack after 1000 hours of operation under the above conditions, the output voltage per unit cell Power was generated at a constant voltage for 2 hours so that the voltage became 0.15V.

<Comparative Example 4>
Supplying 1.0 mol / l aqueous methanol solution to the fuel electrode at a flow rate of 200 ml / min and air to the air electrode at a flow rate of 20000 ml / min to the fuel cell stack after 1000 hours of operation under the above conditions, the output voltage per unit cell Was generated at a constant voltage for 2 hours so that the voltage became 0.2V.

<Comparative Example 5>
Supplying 1.0 mol / l aqueous methanol solution to the fuel electrode at a flow rate of 200 ml / min and air to the air electrode at a flow rate of 20000 ml / min to the fuel cell stack after 1000 hours of operation under the above conditions, the output voltage per unit cell Power was generated at a constant voltage for 2 hours so that the voltage became 0.25V.

6). Evaluation Results Using the fuel cell stacks processed in the above Experimental Examples 1 to 4 and Comparative Examples 1 to 5, a temperature of 65 ° C., a 1 mol / l methanol aqueous solution at a flow rate of 200 ml / min to the fuel electrode, and air to 20000 ml / min. The fuel cell system was operated for one cycle at a current density of 130 mA / cm ² for 10 hours, that is, for a total of 10 hours.

  In order to evaluate Experimental Examples 1 to 4 and Comparative Examples 1 to 5, the average voltage of the fuel cell stack in the 10th continuous operation of the first cycle and the 100th cycle when the 100 cycles were operated continuously, and the experimental example The average voltage of the fuel cell stack when continuously operating for 10 hours after the processing of 1 to 4 and Comparative Examples 1 to 5, and the average value of the current density during the processing of Experimental Examples 1 to 4 and Comparative Examples 1 to 5 And got.

  FIG. 6 is a diagram illustrating the results of the processes of Experimental Examples 1 to 4 and Comparative Examples 1 to 5. There is a variation in average voltage between the first cycle and the 100th cycle before the treatment of Experimental Examples 1 to 4 and Comparative Examples 1 to 5, but the characteristics of the fuel cell stack deteriorated by performing 100 cycles of 10 hours of continuous operation. You can see that In addition, it can be seen that the treatments in Experimental Examples 1 to 4 recover the characteristics of the fuel cell stack more than the treatments in Comparative Examples 1 to 5.

  The decrease in the characteristics of the fuel cell stack is presumed to be caused by the fact that the platinum catalyst on the oxidizer electrode side was oxidized, and the reactivity with respect to the oxygen reduction reaction decreased. In Experimental Examples 1 to 4, since a methanol aqueous solution having a higher concentration than that during normal operation is supplied and power is generated at a current that causes the output voltage per unit cell to be 0.2 V or less, the potential on the oxidant electrode side (cathode) It is presumed that the lowered characteristics of the fuel cell stack were recovered because the potential) was reduced and the oxidized platinum on the oxidant electrode side was reduced. On the other hand, in the fuel cell stack after the processing of Comparative Examples 1 and 2, a methanol aqueous solution having a higher concentration than that in the normal operation is supplied, and power is generated at a current at which the output voltage per unit cell is 0.25 V. Since the potential on the oxidizer electrode side is high to reduce oxidized platinum, it is assumed that the oxidized platinum on the oxidizer electrode side is not reduced and the degraded characteristics of the fuel cell stack are not recovered. The

  From FIG. 6, it can be seen that the characteristics of the fuel cell stacks after the treatment of Comparative Examples 3 and 4 are lower than those before the treatment. In other words, if a methanol aqueous solution having a concentration during normal operation is supplied to the fuel electrode of the fuel cell stack and power is generated at a current that causes the output voltage per unit cell to be 0.2 V or less, the characteristics of the fuel cell stack may be further deteriorated. Recognize.

  In the treatments of Comparative Examples 3 and 4, the average value of the current density during the treatment is higher than that of Experimental Examples 1 to 4, so that the fuel or air is distributed in the fuel cell stack during the treatments of Comparative Examples 3 and 4. It is inferred that the deteriorated unit cell was reversed and the characteristics of the fuel cell stack treated in Comparative Examples 3 and 4 were deteriorated. On the other hand, in Experimental Examples 1 to 4 of the present invention, since the concentration of the aqueous methanol solution is higher than that during normal operation, the permeation amount of the aqueous methanol solution from the fuel electrode to the oxidant electrode increases, so that the current density is low. The output voltage per unit battery can be 0.2V or less. Therefore, in the fuel cell stack that has been processed in Experimental Examples 1 to 4, it is presumed that the degraded characteristics of the fuel cell stack can be recovered without reversing the unit cells in the fuel cell stack.

  From FIG. 6, it can be seen that the characteristics of the fuel cell stack after the treatment of Comparative Example 5 are lower than those before the treatment. In other words, it is understood that the characteristics of the fuel cell stack are further deteriorated when a methanol aqueous solution having a concentration during normal operation is supplied to the fuel electrode of the fuel cell stack and electric power is generated at a current at which the output voltage per unit cell is 0.25 V . Also in the process of Comparative Example 5, it is presumed that the characteristics of the fuel cell stack deteriorated for the same reason as in Comparative Examples 3 and 4.

  The fuel cell system according to the present invention has extremely great applicability in the electrode industry as it can efficiently reverse the output characteristics by preventing the reversal of the unit cell of the fuel cell stack.

DESCRIPTION OF SYMBOLS 1 Fuel cell stack 100 Unit cell 101 Electrolyte membrane 102a Fuel electrode 102b Oxidant electrode 103a Fuel electrode gas diffusion layer 103b Oxidant electrode gas diffusion layer 104a Fuel electrode separator 104b Oxidant electrode separator 2 Heat exchanger 3 Water tank 4 Fuel tank 5 Circulation tank 6 Water pump 7 Fuel pump 8 Circulation pump 9 Air pump 10 Concentration sensor 11 Control unit 12 Discharge unit 13 External load 14 Control circuit 15 Flow path 16 Buffer

Claims (6)

A fuel cell including a fuel electrode carrying a catalyst and an oxidant electrode;
Means for supplying and discharging an aqueous alcohol solution to the fuel electrode;
Means for supplying and discharging an oxidant to the oxidant electrode;
In a fuel cell system comprising:
Means for directly supplying the aqueous alcohol solution to the oxidant electrode;
And control means for switching between recovery operation mode in which the fuel cell generates electricity in the following normal cell voltage value set in advance as the operation mode for supplying the power for driving the connected the load to the external load,
The control means, in the recovery operation mode,
A fuel cell system , wherein the aqueous alcohol solution is directly supplied to the oxidant electrode while the supply of the aqueous alcohol solution and the oxidant to the fuel electrode and the oxidant electrode is continued . Means for directly supplying the aqueous alcohol solution to the oxidizer electrode is:
A buffer for temporarily holding the aqueous alcohol solution;
At least one valve respectively upstream and downstream of the buffer;
The fuel cell system according to claim 1, comprising: Further comprising an internal load consisting of a discharge circuit for causing the electricity generation of the fuel cell at a current density equal to or less than a predetermined voltage value,
The control means, in the recovery operation mode,
The fuel cell system according to claim 1 or 2 , wherein the recovery operation mode is executed in a state where the external load is disconnected and connected to the internal load. The control means, in the recovery operation mode,
The fuel cell system according to any one of claims 1 to 3, wherein the fuel cell is configured to generate power at a lower current density than in the normal operation mode. A fuel cell stack in which unit cells including a fuel electrode supporting a catalyst and an oxidant electrode are stacked;
Means for supplying and discharging an aqueous alcohol solution to the fuel electrode;
Means for supplying and discharging an oxidant to the oxidant electrode;
Means for directly supplying an aqueous alcohol solution to the oxidant electrode;
Discharge circuit (A) and in order to generate electricity at a current density comprised the fuel cell stack below a predetermined voltage value,
In order to switch between a normal operation mode in which the fuel cell stack is connected to an external load and a recovery operation mode of the catalyst that is operated in a state connected to the discharge circuit,
In the normal operation mode, while supplying the aqueous alcohol solution to the fuel electrode, supplying an oxidant to the oxidant electrode,
In the recovery operation mode, control means for controlling the flow path of the aqueous alcohol solution so as to supply the oxidizing agent electrode and the aqueous alcohol solution while the supply of the aqueous alcohol solution to the fuel electrode is continued. B) and a fuel cell system comprising:
Means for directly supplying the aqueous alcohol solution to the oxidizer electrode,
A buffer for temporarily holding the aqueous alcohol solution;
At least one valve respectively upstream and downstream of the buffer;
With
The control means, in the recovery operation mode,
With the downstream valve closed, the upstream valve is opened to supply the aqueous alcohol solution to the buffer;
Thereafter, the downstream valve is opened with the upstream valve closed, and the aqueous alcohol solution temporarily held in the buffer is discharged toward the oxidizer electrode.
By repeating the above, the alcohol aqueous solution is supplied to the oxidizer electrode,
The control means is
A power generation ending step of cutting off the external load and ending power generation from a normal operation mode in which the fuel cell stack is connected to an external load; and
Supplying the aqueous alcohol solution to both the fuel electrode and the oxidant electrode;
The fuel cell stack connected to the discharge circuit, performance recovery of the fuel cell stack, characterized by comprising a power generation initiating operation. The control means, in the recovery operation mode,
6. The method for recovering characteristics of a fuel cell stack according to claim 5, wherein the fuel cell stack is caused to generate power at a lower current density than in the normal operation mode.

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