JP2013161662A - Evaluation method of fuel electrode - Google Patents

Abstract

The present invention makes it possible to accurately and accurately grasp the reduction state of nickel in a fuel electrode.
In step S101, a water vapor partial pressure in exhaust gas discharged from a fuel electrode including nickel of a solid oxide fuel cell is measured in time series. Next, in step S102, an integrated water vapor amount is calculated from the relationship between the amount of fuel gas supplied to the fuel electrode and the water vapor partial pressure in the exhaust gas, and the water vapor partial pressure measured in time series. Next, in step S103, the ratio of the integrated water vapor amount to the total amount of nickel present on the fuel electrode side is calculated as the reduction rate of the fuel electrode, and the fuel electrode is evaluated based on the calculated reduction rate.
[Selection] Figure 1

Description

  The present invention relates to a fuel electrode evaluation method for evaluating the state of a fuel electrode of a solid oxide fuel cell.

  In recent years, there has been an increasing interest in solid oxide fuel cells using oxygen ion conductors. In particular, from the viewpoint of effective use of energy, solid oxide fuel cells are not subject to the restrictions of Carnot efficiency, so they have essentially high energy conversion efficiency, and excellent environmental protection is expected. Has characteristics.

  Hereinafter, the solid oxide fuel cell will be briefly described with reference to FIG. FIG. 4 is a configuration diagram showing the configuration of the solid oxide fuel cell. The solid oxide fuel cell includes an electrolyte 401, a fuel electrode 402, and an air electrode 403. Hydrogen 404 is supplied as fuel to the fuel electrode 402, and air 406 containing oxygen as an oxidant is supplied to the air electrode 403, and a power generation operation is performed.

In the power generation operation, oxygen in the supplied air 406 is combined with electrons 408 at the air electrode 403 to become oxide ions (O ²⁻ ) 407. The oxide ions 407 pass through the electrolyte 401 and move to the fuel electrode 402. The oxide ions 407 that have moved to the fuel electrode 402 react with the hydrogen 404 supplied thereto to produce water (H ₂ O). At this time, electrons 408 are generated in the fuel electrode 402. The electrons 408 generated in this manner are taken out from the fuel electrode 402 to the outside. The electrons 408 taken out from the fuel electrode 402 pass through the electronic load 409 and are supplied to the air electrode 403.

  The movement of the oxide ions 407 and the electrons 408 described above becomes a current in the power generation of the solid oxide fuel cell. The water (water vapor) generated at the fuel electrode 402 during power generation in this way is discharged to the outside as the anode exhaust gas 405. Note that the anode exhaust gas 405 is a mixed gas of hydrogen and water vapor because it also contains unused hydrogen.

  Incidentally, the fuel electrode of a solid oxide fuel cell is generally composed of nickel and an electrolyte material. Nickel acts as a catalyst for the electrochemical oxidation reaction of hydrogen. Nickel also functions as a path (conduction path) through which electrons generated by the oxidation reaction of hydrogen are conducted.

  In addition, as shown in FIG. 5, the solid oxide fuel cell (SOFC) 501 is disposed inside the heat insulating container 505 in an actual operation, and operates using heat generated by the power generation operation. The required temperature is maintained. In addition, when the temperature cannot be maintained only by heat generated by power generation, such as at the start of operation, heat is applied by an electric heater or a burner.

  During operation, hydrogen gas 502 and air 504 are supplied from the outside of the heat insulating container 505 to generate power, and a mixed gas in which water vapor generated as a result of power generation and hydrogen that has not been used is mixed is used as exhaust gas 503. It is discharged outside the heat insulating container 505. Although not shown, the air used for power generation is also discharged outside the heat insulating container 505. In addition, the voltage of the SOFC 501 is measured by a voltage measurement line 506 and a voltmeter 507.

  Thus, in SOFC501 comprised as a power generation system, an air electrode, an electrolyte, a fuel electrode, etc. are integrated after baking each material which consists of metal oxides. In the fuel electrode, nickel as a constituent component is in an oxidized state in the initial stage of being incorporated in the power generation system as described above until it is reduced after being heated to start the power generation operation. As described above, in order to generate an electric power in the fuel cell by performing an electrode reaction for power generation, it is necessary to first reduce nickel oxide in the fuel electrode to nickel metal (refer to Patent Document 1 Acid). The initial nickel reduction process at the fuel electrode is a process that consumes a large amount of hydrogen and electric power without performing a power generation reaction or an exothermic reaction.

  This initial reduction process will be briefly described. First, when the supply of the hydrogen gas 502 and the air 504 to the SOFC 501 is started, a voltage is generated at the output end of the SOFC 501 due to the oxygen concentration difference between the fuel electrode side and the air electrode side. At the same time, nickel oxide in the fuel electrode side current collector constituting member such as the fuel electrode and nickel mesh is reduced. A mixed gas of hydrogen that has not been used for reduction and water vapor that is generated along with the reduction is discharged as exhaust gas 503. Although not shown in FIG. 5, the air after use is discharged to the outside of the heat insulating container 505. Further, the voltage of the SOFC 501 is measured using the voltage measurement line 506 and the voltmeter 507.

  In the initial reduction of nickel at the fuel electrode described above, the voltage of the SOFC 501 is detected, and the progress of the initial reduction of the fuel electrode is detected. Specifically, the voltage is observed using a voltmeter 507, and when the potential difference equal to the electromotive force E described by the following equation is obtained, the conductivity of nickel on the fuel electrode side is sufficiently increased, Judgment is complete. In addition, (DELTA) G in a type | formula is a standard Gibbs free energy change of the water production | generation in a certain temperature.

  For example, the initial reduction is performed by flowing a reducing atmosphere gas with a small flow rate of air and hydrogen, and a voltage of 1.05 V or more is obtained as a guideline for the end of the initial reduction process.

JP 2010-061829 A

Hiroaki Tagawa, “Solid Oxide Fuel Cell and Global Environment”, Agnes Jofusha Co., Ltd., 1st edition, 1st edition, pages 37-40, 1998.

  However, in the above-described technique, the measured voltage is, in addition to the gas equilibrium state on the fuel electrode side, the change in the fuel electrode side gas equilibrium state due to the hydrogen flow rate, the change in the gas equilibrium state on the air electrode side due to the air flow rate, etc. It depends on many factors (see Non-Patent Document 1), and there is a problem that it is difficult to distinguish from voltage changes due to these causes. In the above-described technique, the nickel reduction state in the fuel electrode in the initial reduction process cannot be detected with high sensitivity.

  Here, as a method of judging the reduction progress rate from other than the voltage, a method of judging from the material balance of the gas supply flow rate and the nickel content can be considered. However, not all hydrogen gas supplied to the fuel electrode is used for nickel reduction, and the time for completing the reduction differs depending on the fuel electrode side channel structure and the fine structure in the fuel electrode. It is difficult to determine the time required for reduction from the flow rate and the amount of nickel contained.

  As described above, conventionally, the reduction state of nickel at the fuel electrode cannot be accurately and accurately grasped, and the reduction process has been completed empirically with sufficient time, and the initial reduction of nickel at the fuel electrode. The process could not be performed for the minimum necessary time.

  The present invention has been made to solve the above problems, and an object thereof is to make it possible to accurately and accurately grasp the nickel reduction state in the fuel electrode.

  A fuel electrode evaluation method according to the present invention includes a first step of measuring a water vapor partial pressure in exhaust gas discharged from a fuel electrode including nickel of a solid oxide fuel cell in time series, and a fuel electrode. A second step of calculating the integrated water vapor amount from the relationship between the amount of fuel gas supplied to the water vapor and the water vapor partial pressure in the exhaust gas, and the water vapor partial pressure measured in time series, and nickel present on the fuel electrode side And a third step of evaluating the fuel electrode using the ratio of the accumulated water vapor amount to the total amount of the material as a reduction rate of the fuel electrode. It should be noted that the reduction of the fuel electrode may be completed when the reduction rate exceeds 90%.

  As described above, according to the present invention, it is possible to obtain an excellent effect that the reduction state of nickel in the fuel electrode can be accurately and accurately grasped.

FIG. 1 is a flowchart for explaining a fuel electrode evaluation method according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing a configuration of a power generation system using a solid oxide fuel cell. FIG. 3 shows the change over time (a) in the SOFC211 voltage after the temperature was raised to 800 ° C in the atmosphere and the gas supplied to the fuel electrode side was switched from nitrogen to hydrogen when the temperature of the SOFC211 was 800 ° C. It is a characteristic view which shows a time-dependent change (b) of a reduction rate. FIG. 4 is a configuration diagram showing the configuration of the solid oxide fuel cell. FIG. 5 is a configuration diagram showing an operating state using the solid oxide fuel cell 501.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a flowchart for explaining a fuel electrode evaluation method according to an embodiment of the present invention. First, in step S101, the water vapor partial pressure in the exhaust gas discharged from the fuel electrode configured to contain nickel of the solid oxide fuel cell is measured in time series. This is performed in the initial reduction process of nickel in the fuel electrode. For example, a solid oxide fuel cell immediately after manufacture is heated and heated in an air atmosphere, and after the temperature in the vicinity of the solid oxide fuel cell reaches 800 ° C., it is purged with nitrogen, and then hydrogen is introduced. From this, the time-series measurement may be started.

  Next, in step S102, an integrated water vapor amount is calculated from the relationship between the amount of fuel gas supplied to the fuel electrode and the water vapor partial pressure in the exhaust gas, and the water vapor partial pressure measured in time series.

  Next, in step S103, the ratio of the integrated water vapor amount to the total amount of nickel present on the fuel electrode side is calculated as the reduction rate of the fuel electrode, and the fuel electrode is evaluated based on the calculated reduction rate. The reduction rate is obtained on the assumption that the integrated water vapor amount is equal to the amount of substance obtained by reducing nickel oxide present on the fuel electrode side. For example, in this evaluation, it is determined that the calculated reduction rate exceeds 90%. Here, if it is determined that the reduction rate has exceeded 90% (Y in step S103), the process proceeds to step S104, and the reduction of the fuel electrode may be completed. Steps S101 to S103 may be repeated until the reduction rate exceeds 90%.

  A power generation system using a solid oxide fuel cell to which the above-described fuel electrode evaluation method is applied will be described with reference to FIG. FIG. 2 is a configuration diagram showing the configuration of the power generation system. This system includes a measurement unit 201 and a calculation / display unit 202.

  The measuring unit 201 measures the partial pressure of water vapor in the exhaust gas 213 discharged from the fuel electrode during the power generation operation of the SOFC 211. The measurement part 201 is comprised from the dew point meter which measures the dew point of the waste gas 213, for example.

  The calculation / display unit 202 calculates the integrated water vapor amount from the relationship between the amount of fuel gas supplied to the fuel electrode and the water vapor partial pressure in the exhaust gas 213 and the water vapor partial pressure measured in time series, and the fuel electrode. The ratio of the integrated water vapor amount to the total amount of nickel present on the side of the battery is calculated, and this is displayed as a reduction rate of the fuel electrode so as to be visible to the user. Further, when the calculated reduction rate reaches the set reference value (90%), the calculation / display unit 202 displays to the user that the reduction of the fuel electrode has been completed.

  The power generation system configured as described above is, for example, a computer device including a CPU, a main storage device, an external storage device, a network connection device, and the like, and the CPU is operated by a program developed in the main storage device. Each function mentioned above is implement | achieved.

  Note that the SOFC 211 to be measured and determined is disposed inside the heat insulating container 215, and maintains the temperature necessary for the operation by using the heat generated in the power generation operation during the power generation operation. However, when the temperature cannot be maintained only with the heat generated by power generation, such as at the start of operation or the initial reduction process, or when the heat generated by power generation cannot be used, heat is applied with an electric heater or burner. .

  During operation, hydrogen gas 212 and air 214 are supplied from the outside of the heat insulating container 215 to generate power, and a mixed gas in which water vapor generated as a result of power generation and hydrogen that has not been used is mixed is used as the exhaust gas 213. It is discharged outside the heat insulating container 215. Although not shown, the flow rate of each gas is controlled by, for example, a mass flow controller. Further, the air after being used for power generation is also discharged to the outside of the heat insulating container 215.

  In addition, current is extracted from the SOFC 211 during operation using the current extraction line 217. The extracted current is controlled by an electronic load 216 such as a variable resistor. Further, the voltage of the SOFC 211 is measured by a voltage measurement line 218 and a voltmeter 219.

Next, calculation of the reduction rate in the initial reduction process will be described in more detail. First, there is no current flowing through the electric circuit including the SOFC 211, and the electrolyte constituting the SOFC 211 has an ion transport number of 1.0. Under these conditions, the temperature of SOFC 211 is raised in the atmosphere, and once the temperature near SOFC 211 reaches 800 ° C., once purged with nitrogen, and then hydrogen is introduced, nickel oxide in the fuel electrode and in the fuel electrode side current collector Is reduced (initial reduction step). These reductions are represented by a reaction formula of “NiO + H ₂ → Ni + H ₂ O”.

Note that when hydrogen is introduced, some of the trace amount of hydrogen leaks between parts. Due to this gas leak, water vapor is generated in addition to the reduction of nickel oxide. In the system using the SOFC 211, the amount of water vapor [H ₂ O] leak generated in the fuel electrode exhaust gas per unit time due to the gas leak between the components described above is approximately 5 ml / min (approximately 2 × 10 ⁻⁴⁾ regardless of the hydrogen flow rate. mol / min) has been clarified by previous element tests. Therefore, the amount of water vapor generated by the gas leak is a value that can be ignored.

  Next, the amount of nickel oxide contained in the fuel electrode of the SOFC 211 can be determined in advance based on the amount of nickel oxide powder blended as a raw material when the SOFC 211 is manufactured. This is referred to as [NiO] anode. Further, since the temperature is raised to 800 ° C. in the air atmosphere, the fuel electrode side gas flow path component made of nickel metal such as nickel mesh is oxidized to nickel oxide in the temperature raising process.

  Therefore, it is considered that nickel oxide in the flow path component on the fuel electrode side is also reduced by the above-described reaction formula, and as a result, water vapor is generated. For this reason, the nickel oxide in the flow path component on the fuel electrode side is also considered. The amount of the nickel oxide material is [NiO] channel, 1073K. This amount is equal to the nickel substance amount ([Ni] channel, normal) in the fuel electrode side gas flow path component, and can be determined in advance by the composition and specific gravity of the component before assembly.

  In addition to nickel, heat-resistant stainless steel is used for the fuel electrode side gas flow path components, but these are heated to 800 ° C in an air atmosphere and form a passive film on the surface. It is not reduced by a normal pressure / pure hydrogen reduction atmosphere.

Based on the above, when the fuel electrode heated to 800 ° C. in the air atmosphere is once purged with nitrogen and then hydrogen is introduced as a reducing agent (initial reduction step), the fuel electrode exhaust gas per unit time Assuming that the amount of water vapor generated in [H ₂ O] anodegas is, a relationship “Σ ([H ₂ O] anodegas− [H ₂ O] leak) dt = [Ni] anodeside” holds at a certain time t. However, in this equation, [Ni] anodeside is the amount of nickel produced in the fuel electrode and the nickel metal fuel electrode side gas flow path component by time t.

Here, [H ₂ O] leak is a negligible amount as described above. Therefore, the relationship between the amount of water vapor and the amount of nickel can be expressed as “Σ ([H ₂ O] anodegas) dt = [Ni] anodeside”. Thus, the total amount of water vapor in the fuel electrode exhaust gas in the initial reduction step can be made equal to the amount of nickel substance on the fuel electrode side. Accordingly, when the reduction progress rate reaches 100%, the relationship “[Ni] anodeside = [NiO] anode + [NiO] channel, 1073K” is established.

  Therefore, for example, the water vapor partial pressure is measured every minute using the measuring unit 201, the measurement results up to time t are integrated, and the amount of fuel gas supplied to the fuel electrode and the water vapor in the exhaust gas are calculated from the integration result. If the amount of water vapor in the fuel electrode exhaust gas is calculated using the relationship with the partial pressure, the total amount of nickel oxide on the fuel electrode side is known, so check (evaluate) the progress of nickel reduction in the initial reduction process. Can do.

  Hereinafter, it will be described with reference to FIG. 3 that this reduction confirmation method has higher sensitivity than confirmation of reduction by voltage measurement. FIG. 3 shows the change over time (a) in the SOFC211 voltage after the temperature was raised to 800 ° C in the atmosphere and the gas supplied to the fuel electrode side was switched from nitrogen to hydrogen when the temperature of the SOFC211 was 800 ° C. It is a characteristic figure which shows the time-dependent change (b) of the reduction rate calculated | required by having mentioned above.

  As shown in FIG. 3, even after the output voltage of SOFC 211 is almost stabilized at 1.1 V, it can be seen that the reduction rate required by the above-mentioned increases and the reduction reaction of nickel oxide continues. . Thus, the completion of reduction cannot be confirmed only by the output voltage of the SOFC 211. This has been confirmed by experience, and it has been necessary to complete the reduction with a margin in the determination based on the measurement result of the output voltage.

  On the other hand, according to the present embodiment, it is possible to directly know the rate at which the reduction of nickel in the initial reduction step has progressed, and it is possible to confirm the state where necessary and sufficient reduction has been performed. As described above, according to the fuel electrode evaluation method in the present embodiment, it is possible to detect the completion of reduction more sensitively than the method based on voltage measurement.

  Next, it will be described how to determine that the reduction is completed at a reduction rate of 90%. The thickness of the fuel electrode side gas flow path component, for example, a plate-shaped nickel net provided with precision holes is 100 μm. The nickel mesh is gradually oxidized from the surface during the temperature rise in the atmosphere, but the central part of the mesh is difficult for oxygen to reach, so the nickel metal state is not oxidized during the general temperature rise process. Is maintained. Therefore, when such parts are provided, it is considered that the above-described reduction rate does not normally exceed 95%. Even if the reduction rate is less than 90%, the electrode functions sufficiently, but the effective electrode area becomes small, which is not preferable. From the above, it is desirable that the reduction rate for determining completion of reduction is 90% or more.

  Further, according to the method in the present embodiment, the reduction of nickel on the fuel electrode side can be selectively detected. The reason for this will be described below. As described when the power generation principle of the solid oxide fuel cell is described with reference to FIG. 4, the fuel electrode exhaust gas is a mixed gas of unused hydrogen and water vapor generated by a nickel oxide-hydrogen reaction. Therefore, in the method of this embodiment by measuring the water vapor partial pressure, the magnitude of the reaction between hydrogen and nickel at a certain time t is directly detected.

  As described above, in the present invention, the amount of water vapor obtained from the integrated value of the water vapor partial pressure in the exhaust gas discharged from the fuel electrode is generated by the reduction of nickel oxide on the fuel electrode side, and is oxidized. Based on the knowledge that nickel is equal to the amount of substance reduced, the reduction rate was calculated from the amount of water vapor to evaluate the fuel electrode. According to the present invention described above, the initial reduction progress rate of the fuel electrode can be selectively detected with high sensitivity, and an appropriate measure can be taken. Therefore, according to the present invention, when the solid oxide fuel cell power generation system is manufactured, the initial reduction operation of the solid oxide fuel cell portion can be more reliably and efficiently performed.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  DESCRIPTION OF SYMBOLS 201 ... Measuring part 202 ... Calculation / display part 202, 211 ... Solid oxide fuel cell (SOFC), 212 ... Hydrogen gas, 213 ... Exhaust gas, 214 ... Air, 215 ... Thermal insulation container, 216 ... Electronic load, 217 ... Current extraction line, 218 ... voltage measurement line, 219 ... voltmeter.

Claims (2)

A first step of measuring, in a time series, a water vapor partial pressure in exhaust gas discharged from a fuel electrode configured to contain nickel of a solid oxide fuel cell;
A second step of calculating an integrated water vapor amount from the relationship between the amount of fuel gas supplied to the fuel electrode and the water vapor partial pressure in the exhaust gas, and the water vapor partial pressure measured in time series;
And a third step of evaluating the fuel electrode using the ratio of the accumulated water vapor amount to the total amount of nickel present on the fuel electrode side as a reduction rate of the fuel electrode. . In the fuel electrode evaluation method according to claim 1,
The fuel electrode evaluation method, wherein the reduction of the fuel electrode is completed when the reduction rate exceeds 90%.

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