JP2019175816A - Fuel cell activation method - Google Patents

Abstract

To activate a fuel cell by a simple process.SOLUTION: A potentiostat 44 is connected to a fuel cell 10 and a voltage is applied between an anode electrode 22 and a cathode electrode 24. After the voltage rises to a predetermined upper limit value, the voltage is maintained at a predetermined upper limit value, and then the voltage is lowered to a predetermined lower limit value. The voltage increase, maintenance, and decrease are repeated a predetermined number of times. If necessary, hydrogen pump reaction may be performed between the voltage increase and the voltage decrease.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for activating a fuel cell having an electrolyte membrane made of a solid polymer.

  As a kind of fuel cell, a so-called solid polymer fuel cell having an electrolyte membrane / electrode structure in which an electrolyte membrane made of a solid polymer is sandwiched between an anode electrode and a cathode electrode has become widely known.

  The anode electrode and the cathode electrode each have a catalyst layer containing a metal catalyst of a noble metal such as platinum. Immediately after the production of the fuel cell stack, the catalyst layer does not exhibit sufficient activity, and it is difficult to obtain good power generation characteristics. One reason for this is that the metal catalyst is covered with an impurity such as an oxide film or anion species.

  Therefore, an activation process is performed on the fuel cell stack in order to remove impurities and expose and hold the active metal surface. For example, Patent Document 1 discloses an activation method in which an oxygen-containing gas and an inert gas are alternately and repeatedly supplied to a cathode electrode while supplying hydrogen to the anode electrode.

  In addition, in the patent document 2, the present applicant moves the proton generated at the anode electrode to the cathode electrode through the electrolyte membrane, the first step of applying a variable voltage between the anode electrode and the cathode electrode, Have proposed an activation method in which a second step (so-called hydrogen pump reaction) in which electrons are combined with electrons at the cathode electrode and changed to hydrogen is repeated.

Japanese Unexamined Patent Publication No. 2017-079194 Japanese Patent No. 5526226

  Platinum or the like, which is a metal catalyst, is a noble metal, and if the amount used is large, the material cost of the fuel cell will increase. Therefore, attempts have been made to reduce the amount of metal catalyst used as much as possible. However, in this case, since sulfonate ions (anions) derived from the electrolyte membrane are mixed in the catalyst layer and reduce the activity of the metal catalyst, power generation is performed until the surface of the metal catalyst is sufficiently washed with the generated water generated by power generation. There is a problem that the characteristics are difficult to improve.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a fuel cell activation method capable of easily activating a fuel cell by a simple process.

To achieve the above object, the present invention provides an electrolyte membrane / electrode structure configured by sandwiching an electrolyte membrane made of a solid polymer between an anode electrode and a cathode electrode, and 1 sandwiching the electrolyte membrane / electrode structure 1 A fuel cell activation method for activating a fuel cell having a set of separators,
Supplying a voltage to the fuel cell while supplying a hydrogen-containing gas to the anode electrode and supplying an inert gas to the cathode electrode;
A voltage raising step for raising the voltage to a predetermined upper limit;
A voltage maintaining step of maintaining the voltage at the predetermined upper limit;
A voltage lowering step for lowering the voltage to a predetermined lower limit;
Have
The voltage increasing step, the voltage maintaining step, and the voltage decreasing step are repeated a plurality of times.

  By repeating such voltage application, in particular, the oxide covering the surface of the metal catalyst contained in the catalyst layer of the cathode electrode is removed. In addition, the surface area of the catalyst support material increases during the oxidation process exposed to a high potential, and the area of the reaction field where oxygen, protons and electrons coexist is increased. As a result, the active surface area of the metal catalyst located in the reaction field is increased.

  It is preferable that the hydrogen pump reaction be performed between the voltage lowering step and the next voltage increasing step while maintaining the voltage at a predetermined lower limit value. In particular, the catalyst layer of the cathode electrode is mixed with sulfonate ions derived from the electrolyte membrane, but when performing a hydrogen pump reaction, by supplying a wet gas to the anode electrode, the anode electrode side to the cathode electrode side. The proton-entrained water moves in the electrolyte membrane. This water is condensed on the catalyst layer of the cathode electrode, and is removed outside the cathode electrode through the gas diffusion layer while accompanying the sulfonate ions mixed in the catalyst layer. Thereby, the sulfonate ion in the cathode side catalyst layer is removed.

  In addition, water penetrates into the electrolyte membrane. For this reason, since the electrolyte membrane is sufficiently wet, proton conduction occurs actively. Therefore, the fuel cell has excellent power generation characteristics.

  When raising and lowering the voltage, it is preferable not to change the voltage greatly at once, but to change it over time. That is, it is preferable to change with time during the execution of each step. For example, the voltage may be changed linearly in proportion to the passage of time, or may be changed stepwise. Furthermore, it may be changed in a curved shape. In either case, the voltage can be reliably controlled within the upper limit value.

  If the predetermined upper limit is excessively increased, metal bonds in the catalyst layer tend to decrease while metal-oxygen bonds tend to increase. In order to avoid this, it is preferable to set the predetermined upper limit value to, for example, about 1V. The predetermined lower limit value may be set to approximately 0V, for example.

  The voltage maintaining step may be performed by confirming the effect of each step and setting the optimum time. For example, the voltage maintaining step may be performed for 30 to 300 seconds, more preferably 40 to 120 seconds. This is presumed to promote the active surface area growth of the metal catalyst in the catalyst layer. Oxides and sulfonate ions are mainly removed in the voltage drop process.

  According to the present invention, the voltage applied to the fuel cell is raised to the predetermined upper limit value and then maintained at the predetermined upper limit value. For this reason, the oxidation of the metal catalyst is promoted particularly in the catalyst layer of the cathode electrode, and the generated oxide and eluted sulfonate ions are removed in the voltage lowering process, so that the metal catalyst is purified and activated. The degree is further improved.

It is a principal part schematic sectional drawing which showed typically the structure for enforcing the activation method of the fuel cell which concerns on embodiment of this invention. It is a schematic flow of the activation method of the fuel cell which concerns on embodiment of this invention. It is a time chart which shows the change of a voltage.

  Hereinafter, preferred embodiments of a fuel cell activation method according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic cross-sectional view of a main part showing a fuel cell 10 as a unit cell. The actual fuel cell 10 is configured as a fuel cell stack in which a plurality of unit cells are stacked. However, in FIG. 1, the unit cell is shown as a unit cell for easy understanding. However, it can be activated by an activation method described later regardless of whether it is a unit cell or a fuel cell stack.

  The unit cell includes an electrolyte membrane / electrode structure 12 and a first separator 14 and a second separator 16 (a set of separators) that sandwich the electrolyte membrane / electrode structure 12.

  The electrolyte membrane / electrode structure 12 is configured by interposing an electrolyte membrane 20 between an anode electrode 22 and a cathode electrode 24. The electrolyte membrane 20 therein is formed of a solid polymer having proton conductivity. Examples of this type of polymer include perfluorosulfonic acid fluororesins. Alternatively, a hydrocarbon resin may be used.

  The anode electrode 22 is provided on one end surface of the electrolyte membrane 20. The anode electrode 22 includes an anode side gas diffusion layer 26 to which a fuel gas such as hydrogen is supplied, and an anode side catalyst layer 28 facing the electrolyte membrane 20.

  On the other hand, the cathode electrode 24 is provided on the other end surface of the electrolyte membrane 20. The cathode electrode 24 includes a cathode side gas diffusion layer 30 to which an oxidant gas such as air or oxygen is supplied during operation of the fuel cell 10, and a cathode side catalyst layer 32 facing the electrolyte membrane 20.

  The anode side gas diffusion layer 26 and the cathode side gas diffusion layer 30 are made of, for example, carbon paper, carbon cloth, a metal mesh, a metal sintered body, or the like. These are all porous bodies having conductivity, and the fuel gas or the oxidant gas diffuses easily.

  The anode-side catalyst layer 28 and the cathode-side catalyst layer 32 include, for example, porous carbon particles having metal catalyst particles such as platinum or an alloy thereof supported on the surface, and an ion-conductive polymer binder together with the anode-side gas diffusion layer 26. The cathode side gas diffusion layer 30 is uniformly coated on each surface (or both end surfaces of the electrolyte membrane 20). The anode side catalyst layer 28 and the cathode side catalyst layer 32 face each other with the electrolyte membrane 20 interposed therebetween. The metal catalyst particles may be applied alone to the anode side gas diffusion layer 26 and the cathode side gas diffusion layer 30 together with the ion conductive polymer binder without being supported on the porous carbon particles.

  The first separator 14 and the second separator 16 sandwiching the electrolyte membrane / electrode structure 12 are made of, for example, a metal plate such as a steel plate, a stainless steel plate, an aluminum plate, or a titanium plate. In some cases, the surface of these metal plates is subjected to anticorrosion surface treatment (plating treatment or the like) or a carbon plate.

  A fuel gas passage 34 for supplying fuel gas to the anode electrode 22 is provided on the end face of the first separator 14 facing the anode-side gas diffusion layer 26. Similarly, an oxidant gas flow path 36 for supplying an oxidant gas to the cathode electrode 24 during operation of the fuel cell 10 is formed on the end face of the second separator 16 facing the cathode side gas diffusion layer 30.

  The potentiostat 44 is electrically connected to the fuel cell 10 configured as described above through the current application lines 40a and 40b and the voltage application lines 42a and 42b. The voltage application lines 42a and 42b are provided with switches 43a and 43b. By switching the switches 43a and 43b, the polarity of the applied voltage of the fuel cell 10 can be reversed. The application lines 40a, 40b, 42a, and 42b are connected to current collector plates (not shown) that are individually adjacent to the first separator 14 and the second separator 16, but are electrically equivalent. In FIG. 1, application lines 40 a, 40 b, 42 a, 42 b are connected to the first separator 14 and the second separator 16.

   The potentiostat 44 makes it possible to change the voltage applied to the fuel cell 10 from a certain arbitrary value to another arbitrary value at a predetermined change rate, or to maintain it as necessary. . That is, the voltage applied to the fuel cell 10 is controlled. Instead of the potentiostat 44, a DC power source capable of applying a predetermined DC voltage may be used.

  The fuel cell 10 is further provided with an anode side gas supply line 46 for supplying fuel gas to the anode electrode 22 and an anode side gas discharge line 48 for discharging hydrogen gas from the anode electrode 22. Similarly, a cathode side gas supply line 50 for supplying an inert gas to the cathode electrode 24 during the activation process and a cathode side gas discharge line 52 for discharging nitrogen gas from the cathode electrode 24 are attached. The anode side gas supply line 46, the anode side gas discharge line 48, the cathode side gas supply line 50, and the cathode side gas discharge line 52 described above are all simplified.

  The anode side gas discharge line 48 is provided with a cutoff valve 54, and the cathode side gas supply line 50 is provided with a cutoff valve 56. The potentiostat 44 and the shut-off valves 54 and 56 are controlled by a control device 58.

  Next, a method for activating the fuel cell 10 according to the present embodiment will be described. Hereinafter, a case where wet hydrogen gas is used as the fuel gas and wet nitrogen gas is used as the inert gas will be exemplified.

  In the present embodiment, the activation process is performed on the fuel cell 10 (unit cell or fuel cell stack) immediately after manufacture. That is, first, the anode side gas supply line 46, the anode side gas discharge line 48, the cathode side gas supply line 50, and the cathode side gas discharge line 52 are assembled to the assembled fuel cell 10, and the current application lines 40a and 40b. The potentiostat 44 is electrically connected through the voltage application lines 42a and 42b. Furthermore, the potentiostat 44 and the shutoff valves 54 and 56 are electrically connected to the control circuit.

  Thereafter, wet hydrogen gas is supplied to the anode electrode 22 through the anode side gas supply line 46 with the shut-off valves 54 and 56 being opened, and wet nitrogen is supplied to the cathode electrode 24 through the cathode side gas supply line 50. Supply gas. The wet hydrogen gas flowing through the fuel gas flow path 34 of the first separator 14 diffuses through the anode side gas diffusion layer 26 and reaches the anode side catalyst layer 28, and the excess wet hydrogen gas enters the anode side gas discharge line 48. Discharged. Similarly, the wet nitrogen gas flowing through the oxidant gas flow path 36 of the second separator 16 diffuses through the cathode side gas diffusion layer 30 and reaches the cathode side catalyst layer 32, and the excess wet nitrogen gas is the cathode side gas. It is discharged to the discharge line 52. Excess wet hydrogen gas and wet nitrogen gas may be circulated and reused.

  By supplying and discharging the hydrogen gas and nitrogen gas, the air in the fuel gas channel 34 and the oxidant gas channel 36 is replaced with wet hydrogen gas and wet nitrogen gas, respectively. In this state, the voltage increasing process, the voltage maintaining process, and the voltage decreasing process shown in FIGS. 2 and 3 are performed. FIG. 3 also shows changes in current.

  That is, under the control action of the potentiostat 44, a voltage is applied to the fuel cell 10 in the same manner as the potential sweeping in the cyclic voltammetry. At this time, the anode electrode 22 is used as a reference, the anode electrode 22 is negative, the cathode electrode 24 is positive, and the potential difference is an applied voltage.

  In the voltage raising step, a variable voltage is applied. That is, the voltage is raised to a predetermined upper limit value. The predetermined upper limit value can be set to 1 V, for example. Note that the voltage is preferably increased gradually. For example, as shown in FIG. 3, it may be increased in proportion to the elapsed time. Alternatively, the voltage may be changed so as to increase stepwise or may be changed so as to increase in a curved manner. By changing the voltage in this way, the voltage can be reliably controlled within the range of the upper limit value.

  After the voltage reaches the predetermined upper limit value, the predetermined upper limit value is maintained. That is, the process proceeds to the voltage maintaining process. This voltage maintenance promotes the oxidation of the metal catalyst in the cathode side catalyst layer 32. The voltage may be maintained, for example, for 30 to 300 seconds, more preferably 40 to 120 seconds.

  After a predetermined time has elapsed from the start of the voltage maintaining step, the voltage starts to drop to a predetermined lower limit value. That is, the process proceeds to a voltage lowering process. It is considered that oxides and sulfonate ions are mainly removed in the voltage lowering step.

  The predetermined lower limit value can be set to 0 V, for example. As in the above, it is preferable to gradually decrease the voltage in order to prevent the current from fluctuating rapidly. For example, as shown in FIG. 3, it may be lowered in proportion to the elapsed time. Of course, it may be lowered stepwise or in a curved line as described above.

  It is preferable to perform the hydrogen pump reaction step after the voltage reaches a predetermined lower limit. Specifically, wet nitrogen gas is circulated through the cathode side gas supply line 50. However, the shutoff valve 56 provided in the cathode side gas supply line 50 may be closed to stop the supply of wet nitrogen gas to the oxidant gas flow path 36. On the other hand, the shut-off valve 54 provided in the anode-side gas discharge line 48 may be closed while supplying wet hydrogen gas from the anode-side gas supply line 46, or the fuel gas flow path 34 may be closed without closing the shut-off valve 54. The supply of wet hydrogen gas may be continued.

  Then, the polarity of the applied voltage is reversed, and a voltage is applied so that the anode electrode 22 is positive and the cathode electrode 24 is negative. As a result, a predetermined amount of current flows from the cathode electrode 24 to the anode electrode 22. For this reason, in the anode side catalyst layer 28, an oxidation reaction occurs in which hydrogen is ionized into protons and electrons. The proton moves to the cathode side catalyst layer 32 through the electrolyte membrane 20. On the other hand, electrons move from the anode electrode 22 to the cathode electrode 24 via the current application lines 40a and 40b (that is, current flows from the cathode electrode 24 toward the anode electrode 22).

  In the cathode-side catalyst layer 32, a reduction reaction occurs in which protons and electrons that have moved in this way are recombined. Hydrogen generated by this reduction reaction is discharged from the cathode side gas discharge line 52. At the same time as this reaction occurs, water permeates into the electrolyte membrane 20 derived from moisture contained in the wet hydrogen gas.

  After the hydrogen pump reaction is performed for a predetermined time, the polarity of the voltage applied to the fuel cell 10 is reversed by inverting the electrode of the potentiostat 44. At this time, the shutoff valves 54 and 56 may be opened, and the supply of wet nitrogen gas to the cathode electrode 24 may be resumed. Further, the applied voltage is increased, and the voltage increasing process in the second cycle is performed again. Thereafter, the above-described voltage maintaining step, voltage dropping step, and hydrogen pump reaction step are performed again. The activation process is completed by repeating a cycle including the voltage increase process, the voltage maintenance process, the voltage decrease process, and the hydrogen pump reaction process a predetermined number of times (for example, several tens to several hundreds).

  By performing the voltage raising step and the voltage lowering step, the oxide covering the surface of the metal catalyst particularly included in the cathode side catalyst layer 32 is removed. In addition, the surface area of the catalyst support material increases during the oxidation process exposed to a high potential, and the area of the reaction field where oxygen, protons and electrons coexist is increased. As a result, the active surface area of the metal catalyst located in the reaction field is increased.

  In particular, the cathode side catalyst layer 32 is mixed with sulfonate ions derived from the electrolyte membrane 20, but is accompanied by protons because wet hydrogen gas is supplied to the anode electrode 22 in the hydrogen pump reaction step. Moisture (proton-entrained water) moves through the electrolyte membrane 20 from the anode electrode 22 side toward the cathode electrode 24 side.

  This moisture is condensed in the cathode side catalyst layer 32. That is, condensed water is generated. The sulfonate ions mixed in the cathode side catalyst layer 32 are accompanied by the condensed water, and are discharged out of the cathode electrode 24 through the cathode side gas diffusion layer 30. The activity of the fuel cell 10 is also improved by removing the sulfonate ions mixed in the cathode side catalyst layer 32 in this manner.

  In the hydrogen pump reaction step, only hydrogen with a small overvoltage is involved in the reaction. For this reason, a large current can flow continuously. Therefore, the time required for the activation process can be shortened. Furthermore, since water permeates into the electrolyte membrane 20, the electrolyte membrane 20 is sufficiently wet.

  For the above reasons, the power generation characteristics of the fuel cell 10 are improved. In other words, the fuel cell 10 is activated effectively.

  In the above-described embodiment, the case where the activation process is performed on the fuel cell 10 immediately after manufacture has been described. However, the present invention is not particularly limited to this case. For example, the operation of the fuel cell 10 is performed. It can also be applied at the time of maintenance such as when (power generation) is temporarily stopped and then restarted, or when the activity of the catalyst is reduced after long-time operation.

DESCRIPTION OF SYMBOLS 10 ... Fuel cell 12 ... Electrolyte membrane electrode assembly 14, 16 ... Separator 20 ... Electrolyte membrane 22 ... Anode electrode 24 ... Cathode electrode 28 ... Anode side catalyst layer 32 ... Cathode side catalyst layer 34 ... Fuel gas flow path 36 ... Oxidation Agent gas flow path 44 ... Potentiostat 46 ... Anode side gas supply line 48 ... Anode side gas discharge line 50 ... Cathode side gas supply line 52 ... Cathode side gas discharge line 54, 56 ... Shut-off valve

Claims (5)

Fuel for activating a fuel cell having an electrolyte membrane / electrode structure configured by sandwiching an electrolyte membrane made of a solid polymer between an anode electrode and a cathode electrode, and a pair of separators sandwiching the electrolyte membrane / electrode structure A battery activation method comprising:
Supplying a voltage to the fuel cell while supplying a hydrogen-containing gas to the anode electrode and supplying an inert gas to the cathode electrode;
A voltage raising step for raising the voltage to a predetermined upper limit;
A voltage maintaining step of maintaining the voltage at the predetermined upper limit;
A voltage lowering step for lowering the voltage to a predetermined lower limit;
Have
A method for activating a fuel cell, comprising repeating the voltage increasing step, the voltage maintaining step, and the voltage decreasing step a plurality of times.   2. The activation method according to claim 1, further comprising a hydrogen pump reaction step that performs a hydrogen pump reaction while maintaining a voltage at the predetermined lower limit value between the voltage lowering step and the next voltage rising step. A method for activating a fuel cell.   3. The activation method according to claim 1, wherein the voltage increase in the voltage increase step and the voltage decrease in the voltage decrease step are performed as a change over time during the execution of each step. Activation method.   4. The activation method according to claim 1, wherein the predetermined upper limit value is set to 1V and the predetermined lower limit value is set to 0V.   5. The activation method according to claim 1, wherein the voltage maintaining step is performed for 30 to 300 seconds.

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