JP2013206684A - Electrode activation method of solid oxide fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which allows for rated operation over a longer period, by minimizing voltage drop due to long-term use of a solid oxide fuel cell (SOFC), while shortening the time after start-up operation of the SOFC before stabilization of voltage.SOLUTION: In the electrode activation method of a solid oxide fuel cell including an electrically insulating electrolyte membrane exhibiting oxygen ion conductivity, a fuel electrode formed on one main surface of the electrolyte membrane, and an air electrode formed on the other main surface of the electrolyte membrane, the fuel electrode is placed under a first atmosphere and the air electrode is placed under a second atmosphere when start-up operation of the solid oxide fuel cell is started and when the output voltage drops. Subsequently, a pulse voltage is applied to at least one of the fuel electrode and the air electrode.

Description

  The present invention relates to an electrode activation method for a solid oxide fuel cell.

  A solid oxide fuel cell (SOFC) usually has a reducing agent (fuel gas; hydrogen or hydrogen) through an electrolyte membrane having ionic conductivity (oxygen ions or hydrogen ions) under an operating condition of about 600 to 900 ° C. This is a device that reacts a hydrocarbon (such as hydrocarbon) with an oxidant (such as oxygen) (fuel cell reaction) and extracts the energy as electricity.

  When such a SOFC is operated over a long period of time under a constant load current, the time-dependent characteristics of the voltage are examined. For example, the voltage gradually increases until about 400 hours immediately after the start of constant current operation. After the voltage stabilizes, the voltage is maintained for a relatively long time. After that, when about 700 to 2000 hours are passed, the general behavior is that the voltage gradually decreases.

  In a practical machine for a power plant, it is desired to shorten the time from the start of constant current operation until the voltage stabilizes. For this reason, the operation of gradually increasing the load current from the low load state to the high load state is repeated, and the active site (reaction site between oxygen ions and reducing agent (fuel gas)) at the electrode / electrolyte layer interface is determined. A method of shortening the time to rated load operation by increasing the number is adopted.

  Moreover, in the said practical machine, it is desired to perform the rated driving | running over a long period of time, suppressing the voltage fall in the latter stage of driving | operation. It is known that the voltage drop in the late stage of operation of the SOFC is mainly caused by a decrease in the catalytic activity of the fuel electrode and a decrease in the supply of fuel gas in the SOFC. The decrease in the catalytic activity and the supply of fuel gas are caused by the fact that the material constituting the SOFC anode is aggregated by heating during long-term operation, the surface area of the material is reduced, and the porosity in the material is reduced. Due to

  Increasing the porosity in the fuel electrode material can improve the supply of fuel gas. However, if the porosity exceeds a predetermined value, the surface area of the fuel electrode material decreases and the catalytic activity decreases. Will end up. On the other hand, if the surface area of the anode material is increased, the catalytic activity of the anode can be improved. However, if the surface area becomes larger than a predetermined value, the anode material is agglomerated by heating over the long term operation thereafter. As a result, the porosity in the material is reduced, and the supply of fuel gas is reduced. That is, the catalyst activity and fuel gas supply at the fuel electrode are in a trade-off relationship, and it is difficult to improve both simultaneously.

  Therefore, various means have been proposed to suppress aggregation of the material constituting the fuel electrode. For example, it has been proposed that the fuel electrode is made of nickel foam or nickel felt. In this case, the porosity of the fuel electrode is determined in advance by the porosity of nickel foam or nickel felt, but aggregation of nickel fibers such as nickel foam cannot be avoided by heating over a long period of operation.

  As a result, catalyst activity and fuel gas supply at the fuel electrode cannot be improved at the same time, voltage drop due to long-term use of SOFC cannot be suppressed, and rated operation should be performed for a longer period. I can't.

Special table 2007-531974

  The problem to be solved by the present invention is to suppress a voltage drop due to a long-term use of a solid oxide fuel cell (SOFC) and to perform a rated operation for a longer period of time. It is to provide a method for shortening the time from voltage stabilization to voltage stabilization.

  One embodiment of the present invention is an electrically insulating electrolyte membrane exhibiting oxygen ion conductivity, a fuel electrode formed on one main surface side of the electrolyte membrane, and the other main electrode of the electrolyte membrane. An electrode activation method for a solid oxide fuel cell comprising an air electrode formed on a surface side, wherein the fuel electrode is inserted into the first electrode at the start of operation of the solid oxide fuel cell and when the output voltage decreases. And disposing the air electrode in a second atmosphere, and applying a pulse voltage to at least one of the fuel electrode and the air electrode. To do.

  According to the present invention, voltage drop due to long-term use of a solid oxide fuel cell (SOFC) can be suppressed, rated operation can be performed for a longer period, and voltage stabilization from the start of SOFC operation. It is possible to provide a method for shortening the time until the time.

It is sectional drawing which shows schematic structure of the solid oxide fuel cell (SOFC) in 1st Embodiment. It is a schematic block diagram of the apparatus which performs the electrode activation method of SOFC shown in FIG. It is a figure which shows the waveform of the pulse voltage used for electrode activation. It is a figure which shows the waveform of the pulse voltage used for electrode activation. It is a graph which shows the time dependence of the voltage (electromotive force) of SOFC by activation of the fuel electrode in an Example. It is a graph which shows the time dependence of the voltage (electromotive force) of SOFC by activation of the fuel electrode in an Example.

  Hereinafter, it demonstrates based on embodiment, referring drawings.

(First embodiment)
FIG. 1 is a cross-sectional view showing a schematic configuration of a solid oxide fuel cell (SOFC) in the present embodiment, and FIG. 2 is a schematic configuration diagram of an apparatus for performing the SOFC electrode activation method shown in FIG. .

  An electrochemical cell 10 shown in FIG. 1 is electrically insulative and has an electrolyte membrane 11 exhibiting electronic insulation and oxygen ion conductivity, and a fuel formed on one main surface 11A side of the electrolyte membrane 11. The electrode 12, the air electrode 13 formed on the other main surface 11B side of the electrolyte membrane 11, and the fuel electrode 12 and the air electrode 13 respectively formed on the main surfaces 12A and 13A side opposite to the electrolyte membrane 11. A pair of current collectors 14 and 15 is included.

The electrolyte membrane 11 can be composed of, for example, stabilized zirconia. In this case, examples of the stabilizer include Y ₂ O ₃ , Sc ₂ O ₃ , Yb ₂ O ₃ , Gd ₂ O ₃ , Nd ₂ O ₃ , CaO, and MgO. These stabilizers are used as a solid solution in zirconia. Moreover, it can replace with stabilized zirconia and can also comprise perovskite type oxides, such as LaSrGaMg oxide, LaSrGaMgCo oxide, LaSrGaMgCoFe oxide, LaSrGaMgCoFe oxide. Furthermore, a ceria-based electrolyte solid solution in which Sm ₂ O ₃ , Gd ₂ O ₃ , Y ₂ O ₃ , La ₂ O _{3 and the} like are dissolved in CeO ₂ can also be used. However, the electrolyte membrane 11 is not limited to these materials, and may be composed of other materials.

  The thickness of the electrolyte membrane 11 can be arbitrarily set according to the purpose, but can be set in the range of 0.01 mm to 0.5 mm, for example.

As an example, 8 mol% Y ₂ O ₃ stabilized zirconia having a thickness of 0.01 mm can be used.

  The fuel electrode 12 can be composed of nickel particles or cermets of nickel oxide particles and at least one of ceria-based and zirconia-based ceramic particles. When the fuel electrode 12 is composed of cermet, before the reduction treatment, the mass mixing ratio of nickel oxide particles and ceramic particles is, for example, 60:40 to 30:70, and the average particle diameter of nickel oxide particles and the average of ceramic particles The ratio with the particle diameter is set to, for example, 100: 30 to 100: 1. Note that the nickel oxide particles are converted into nickel particles after the reduction treatment. The thickness of the fuel electrode 12 can be 5 μm to 50 μm.

Examples of the ceria-based ceramic particles include particles made of ceramics in which Sm ₂ O ₃ , Gd ₂ O ₃ , Y ₂ O ₃ , La ₂ O _{3 and the} like are dissolved in CeO ₂ . Moreover, as zirconia-based ceramic particles, stabilizers such as Y ₂ O ₃ , Sc ₂ O ₃ , Yb ₂ O ₃ , Gd ₂ O ₃ , Nd ₂ O ₃ , CaO, and MgO are dissolved in ZrO ₂ . There may be mentioned particles made of ceramics.

  As an example, nickel oxide particles having an average particle diameter of 1 μm and yttria-stabilized zirconia particles having an average particle diameter of 0.2 μm can be set to a mass mixing ratio of 50:50. In this case, the average particle diameter of the nickel oxide particles and the average particle diameter of the yttria-stabilized zirconia particles are 100: 20. The thickness can be 5 μm.

The air electrode 13, can be composed of any material used as an oxygen electrode of an electrochemical cell than before, preferably, the general formula _{Ln 1-x A x BO 3} -δ (Ln = rare earth elements; A = Sr, Ca, Ba; B = at least one of Cr, Mn, Fe, Co, and Ni). Such a composite oxide efficiently dissociates oxygen and has electronic conductivity. Therefore, an electrochemical reaction between oxygen ions and hydrogen gas via the catalyst in the fuel electrode 12 can be promoted, which contributes to high output of the solid oxide fuel cell 10.

  Although the thickness of the air electrode 13 can be set arbitrarily according to the purpose, it can be set in a range of 0.01 mm to 1 mm, for example.

As an example, a composite oxide of lanthanum, strontium, cobalt, and iron (La ₆ Sr ₄ Co ₂ Fe ₈ O ₃ ) having a thickness of 0.02 mm can be used.

  The current collectors 14 and 15 must be made of a material that does not oxidize under normal operating conditions. For example, in addition to noble metals such as gold, silver, and platinum, other base metals such as silver can be used. A coated one can be used. A ceramic material having conductivity can also be used. Further, a chromium-based alloy whose oxide film has conductivity can also be used.

Next, a method for activating the SOFC 10 electrode using the apparatus shown in FIG. 2 will be described.
As shown in FIG. 2, a potentiostat (or “power supply”) 21 is connected to the electrodes of the SOFC 10, that is, the fuel electrode 12 and the air electrode 13 via wirings 211 and 212, and this potentiostat (power supply) 21. Is connected to the waveform generator 22. Then, when the SOFC 10 is started up and when the output voltage is lowered, an inert gas is supplied to the fuel electrode 12 side, the fuel electrode 12 side is set to an inert gas atmosphere, and an oxygen-containing gas is supplied to the air electrode 13 side. And let the said air electrode 13 side be oxygen gas containing atmosphere.

  The inert gas is not particularly limited as long as the electrode activity as described below is performed, but nitrogen gas, rare gas, or the like can be used, and particularly inexpensive nitrogen gas is preferably used. . The oxygen-containing gas is not particularly limited as long as the electrode activity as described below is performed, and a gas in which oxygen gas is intentionally mixed in nitrogen gas, air, or the like can be used. In particular, air that is inexpensive and used when the SOFC 10 is driven is preferable. Further, by using air as the oxygen-containing gas, it is not necessary to provide a separate gas supply device in the SOFC 10 system system, and the configuration of the system system can be simplified.

In this embodiment, the fuel electrode 12 side is an inert gas atmosphere, and the air electrode 13 side is an oxygen gas-containing atmosphere. However, as long as the electrode activity described below is performed, the atmosphere on these electrode sides is particularly It is not limited. However, when the vacuum atmosphere or oxygen partial pressure ≦ 20%, the general formula Ln _1-x A _x BO _3−δ (Ln = rare earth element; A = Sr, Ca, Ba; In some cases, the oxygen in the composite oxide particles represented by at least one of Cr, Mn, Fe, Co, and Ni) is released, and the air electrode 13 may collapse. Therefore, it is preferable to avoid the air electrode 13 from being in a vacuum atmosphere or an oxygen partial pressure ≦ 20%.

  Next, a predetermined pulse voltage is applied from the power source 22 via the potentiostat 21 to the electrode on the side to be activated, that is, the fuel electrode 12 or the air electrode 13. As a result, the electrode on the side to which the pulse voltage is applied, that is, the fuel electrode 12 or the air electrode 13 is activated, and the time from when the SOFC 10 is started until it reaches a certain voltage, that is, until the rated operation is achieved. The time can be shortened and the voltage drop after long-term operation can be suppressed.

  The pulse voltage may be a triangular pulse voltage as shown in FIG. 3 or a sawtooth pulse voltage as shown in FIG. In this case, the pulse length is preferably 0.1 seconds to 30 minutes. If the pulse length is less than 0.1 seconds, the electrode activation effect may not be sufficiently obtained, and if the pulse length is 30 minutes or longer, it takes a long time to activate the electrode.

  The potential sweep rate of the pulse voltage is preferably 5 mV / second to 500 mV / second. If the potential sweep rate of the pulse voltage is less than 5 mV / second, it takes a long time for electrode activation, which is not preferable. In addition, if the potential sweep rate of the pulse voltage exceeds 500 mV / second, the electrode activation effect may not be sufficiently obtained. The potential sweep speed of the pulse voltage means the rising speed or the falling speed of the pulse voltage as shown in FIGS.

  The application time of the pulse voltage is preferably 10 minutes to 60 minutes. If the application time of the pulse voltage is less than 10 minutes, the electrode activation effect may not be sufficiently obtained. Even if the application time of the pulse voltage exceeds 60 minutes, it no longer contributes to the electrode activity and is simply treated. This is not preferable because the time is prolonged.

  The magnitude of the pulse voltage, that is, the magnitude of the applied voltage depends on the configuration of the SOFC 10 or the like, but is usually about several volts, preferably about 0.1 to 1V. Further, the polarity of the pulse voltage can be single, but the polarity can also be switched. For example, after applying a negative polarity pulse voltage so that the potential of the fuel electrode 12 becomes a negative potential, a positive polarity pulse voltage can be applied so that the potential of the fuel electrode 12 becomes a positive potential. Although the reason is not particularly clear, the voltage decrease after long-term operation of the SOFC 10 can be more effectively recovered by applying the pulse voltage with the polarity reversed as in the latter case.

  By shortening the time from when the electrode, that is, the fuel electrode 12 or the air electrode 13 is activated by the operation as described above until the SOFC 10 starts up to reach a certain voltage, that is, the time until the rated operation is performed. The reason why the voltage drop after long-term operation can be suppressed is not clear at present. However, application of the pulse voltage in the atmosphere described above causes movement and rearrangement of the materials constituting the fuel electrode 12 and the air electrode 13, such that the surface area and porosity of these electrodes are as originally designed. It is considered that the catalyst is close to the optimum state and the catalyst activity and the gas supply are close to the optimum state.

(Second Embodiment)
In the first embodiment, the fuel electrode 12 side is an inert gas atmosphere and the air electrode 13 side is an oxygen gas-containing atmosphere, but in this embodiment, the fuel electrode 12 side is a hydrogen gas-containing atmosphere and the air electrode 13 is. The side is an atmosphere containing oxygen gas. The hydrogen gas-containing atmosphere can be a mixed gas of hydrogen gas and water vapor or a mixed gas of hydrogen gas and hydrocarbon gas, that is, a fuel gas. Further, as described in the first embodiment, the oxygen gas-containing atmosphere can be air used when the SOFC 10 is driven.

  Therefore, in this embodiment, since the electrode, that is, the fuel electrode 12 or the air electrode 13 can be activated using the fuel gas and air when the SOFC 10 is driven, a separate gas supply device is provided in the system system of the SOFC 10. There is no need to provide it, and the configuration of the system system can be simplified.

  However, since the fuel gas is consumed in the fuel cell reaction (oxidation / reduction reaction) in the fuel electrode 12 during the activation process, the electrode activation effect is deteriorated as compared with the case of the first embodiment.

  The configuration of the SOFC 10 and the device configuration for electrode activation are the same as those in FIGS. 1 and 2 relating to the first embodiment. Further, the requirements regarding the pulse voltage to be applied are the same as those in the first embodiment.

Example 1
The fuel electrode 12 of the SOFC 10 as shown in FIG. 1 has a nitrogen atmosphere (nitrogen 60% by volume + hydrogen 40% by volume), and the air electrode 13 has an air atmosphere. Then, a potential sweep rate of 50 mV / second with respect to the fuel electrode 12 A triangular pulse voltage with a pulse width of 40 seconds was applied for 30 minutes. This operation was defined as one cycle, and a total of four cycles of operation were performed on the fuel electrode 12 to activate the fuel electrode 12. When the load current density between the SOFC 10, that is, the fuel electrode 12 and the air electrode 13 is set to 0.15 A / cm ² by the potentiostat 21, the potential of the fuel electrode 12 is changed to the potential of the air electrode 13 by applying a pulse voltage. As a reference, it varied between -1V and -0.3V.

The electrolyte membrane 11 is composed of 0.01 mol thick 8 mol% Y ₂ O ₃ stabilized zirconia, and the fuel electrode 12 is 5 μm thick, nickel oxide particles having an average particle size of 1 μm, and an average particle size of 0.2 μm. A yttria-stabilized zirconia particle and a porous layer formed by mixing at a mass mixing ratio of 50:50 were used. The air electrode 13 was a composite oxide of lanthanum, strontium, cobalt, and iron (La ₆ Sr ₄ Co ₂ Fe ₈ O ₃ ) having a thickness of 0.02 mm. In order to simplify the operation of the present embodiment, the current collectors 14 and 15 of the SOFC 10 are omitted.

  FIG. 5 is a graph showing the time dependence of the voltage (electromotive force) of the SOFC 10 due to the activation of the fuel electrode 12.

  As can be seen from FIG. 5, when the SOFC 10 is started, the fuel electrode 12 is activated by applying a pulse voltage to the fuel electrode 12, and the voltage of the SOFC 10, that is, the electromotive force increases for each cycle. It can be seen that the rated operation state is reached in about 250 hours (thick line in the figure). On the other hand, when no pulse voltage is applied to the fuel electrode 12, it can be seen that it takes about 500 hours from the start to the rated operation state (thin line in the figure). Therefore, it has been found that the activation of the fuel electrode 12 described above shortens the time from the activation of the SOFC 10 to the rated operation state by about half.

  As is clear from FIG. 5, when the activation of the fuel electrode 12 is not performed at the time of starting the SOFC 10, the voltage decreases after about 700 hours of operation, but the activation of the fuel electrode 12 is performed. Was found not to drop in voltage after about 700 hours of operation.

  Although not shown in particular, when a pulse is applied with the atmosphere on the fuel electrode 12 side being 100% nitrogen, compared to the above embodiment, the rated operation state is reached in about 200 hours from the start, and the voltage remains after about 700 operations. It turned out not to decline. That is, it was found that by setting the nitrogen to 100%, the rated operation state can be achieved in a shorter time and the rated operation state can be maintained for a longer period than in the case of containing hydrogen.

  Although not shown, when the air electrode 13 was activated under the same conditions as described above, almost the same result as that obtained when the fuel electrode 12 was activated was obtained.

(Example 2)
FIG. 6 is a graph showing a voltage drop of the SOFC 10 accompanying a long-term operation when the fuel electrode 12 is activated by applying a pulse voltage when the SOFC 10 is activated in the first embodiment.

  As shown in FIG. 6, even when the fuel electrode 12 is activated when the SOFC 10 is activated, a voltage decrease is observed after about 2500 hours. However, by adopting the same pulse voltage application condition as in Example 1, and applying the pulse voltage under this condition as one cycle and applying the pulse voltage for a total of three cycles, the above-mentioned voltage decrease can be recovered. I understand.

  Although not particularly shown, it has been found that applying a pulse voltage with the polarity reversed for 5 to 10 minutes after the pulse voltage of the above three cycles shortens the time required to recover the voltage decrease by about 100 hours.

  As described above, from Example 1 and Example 2, when the SOFC 10 is started up and the output voltage is reduced, the fuel electrode 12 is an inert atmosphere, the air electrode 13 is an oxygen gas-containing atmosphere, and the fuel electrode 12 or the air electrode 13 is connected. In contrast, by applying a pulse voltage under predetermined conditions, voltage drop due to long-term use of the SOFC 10 can be suppressed, rated operation can be performed for a longer period, and voltage stabilization from the start of operation of the SOFC 10 It can be seen that the time until conversion can be shortened.

  As mentioned above, although several embodiment of this invention was described, these embodiment was posted as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

10 Solid Oxide Fuel Cell 11 Electrolyte Membrane 12 Fuel Electrode 13 Air Electrodes 14 and 15 Current Collector 21 Potentiostat (Power Supply)
22 Waveform generator

Claims (8)

An electrolyte membrane that is electrically insulating and exhibits oxygen ion conductivity, a fuel electrode formed on one main surface side of the electrolyte membrane, and an air formed on the other main surface side of the electrolyte membrane An electrode activation method for a solid oxide fuel cell comprising an electrode,
Disposing the fuel electrode in a first atmosphere at the start of start-up operation of the solid oxide fuel cell and lowering the output voltage, and disposing the air electrode in a second atmosphere;
Applying a pulse voltage to at least one of the fuel electrode and the air electrode;
A method for activating an electrode of a solid oxide fuel cell, comprising:   2. The solid according to claim 1, wherein the first atmosphere is an inert gas atmosphere, the second atmosphere is an oxygen gas-containing atmosphere, and the pulse voltage is applied to the fuel electrode. An electrode activation method for an oxide fuel cell.   2. The solid according to claim 1, wherein the first atmosphere is an inert gas atmosphere, the second atmosphere is an oxygen gas-containing atmosphere, and the pulse voltage is applied to the air electrode. An electrode activation method for an oxide fuel cell.   2. The solid according to claim 1, wherein the first atmosphere is a hydrogen gas-containing atmosphere, the second atmosphere is an oxygen gas-containing atmosphere, and the pulse voltage is applied to the fuel electrode. An electrode activation method for an oxide fuel cell.   2. The solid according to claim 1, wherein the first atmosphere is a hydrogen gas-containing atmosphere, the second atmosphere is an oxygen gas-containing atmosphere, and the pulse voltage is applied to the air electrode. An electrode activation method for an oxide fuel cell.   6. The electrode activation method for a solid oxide fuel cell according to claim 1, wherein a pulse length of the pulse voltage is 0.1 second to 30 minutes.   The electrode activation method for a solid oxide fuel cell according to any one of claims 1 to 6, wherein a potential sweep rate of the pulse voltage is 5 mV / sec to 500 mV / sec.   8. The electrode activation method for a solid oxide fuel cell according to claim 1, wherein the pulse voltage is applied for 10 minutes to 60 minutes.

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