JPH10302823A - Fuel cell characteristic diagnostic method and fuel cell operating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a characteristic diagnostic method of a fuel cell capable of easily avoiding damage of the fuel cell caused by disturbance in a field plant. SOLUTION: An air flow rate supplying to an air electrode is changed in plural levels of air flow rate containing the rated air flow rate, cell voltage and DC current in each level are measured. Each mean oxygen concentration is found from each oxygen concentration in the inlet and outlet of the air electrode obtained from each air flow rate and each DC current. An extrapolated value of the cell voltage in pure oxygen concentration is found by the primary regression of the mean oxygen concentration in each measured point and the cell voltage, and oxygen gain is found from the difference between the cell voltage and the rated cell voltage corresponding to the rated air flow rate. The gas diffusion performance of the air electrode is diagnosed from the oxygen gain.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for diagnosing characteristics of a fuel cell and a method for operating the fuel cell, in which fuel gas and air are supplied and electric power is taken out by an electrochemical reaction.

[0002]

2. Description of the Related Art In a fuel cell, particularly, in a phosphoric acid type fuel cell, an operating life of more than 20,000 hours has been proved in a demonstration machine, and a prospect of securing reliability in a practical machine is being obtained. The challenges for practical use of fuel cells at this stage include, apart from technological development at the design stage such as cost reduction and reliability improvement, optimization of the actual field operation method for the purpose of improving operation reliability. Technology development is also needed. In particular, excessive protection circuits and redundant designs cause cost hindrance, and should be concentrated on the minimum necessary protection functions according to the actual operation method in the actual field.

[0003] Incidentally, there are the following two factors that reduce the operational reliability of the fuel cell. (1) Deterioration of functions due to disturbance Malfunction or failure of sensors and control devices such as flow meters, and deterioration of external systems such as reformer and transformer performance (2) Deterioration of functions inside the fuel cell Deterioration of functions inside the cell, such as a decrease in gas diffusivity in the electrode, electrolyte shortage due to electrolyte dissipation, and the like. In many cases, the operability of the fuel cell is reduced. This is a case in which the function is deteriorated due to this. While preventing the occurrence of disturbance during driving, a method of early detection of occurrence of disturbance is also one of the important issues.

[0004] Conventional methods for early detection of disturbance occurrence include (1) a method of measuring a block voltage of a battery,
(2) A method based on gas composition analysis at a battery gas outlet, and (3) a method based on a change in pressure loss in a battery gas flow path are known. These are actually employed in actual machines as a method of detecting an abnormal situation such as "abrupt change", that is, "abnormal control is stopped from a certain moment" such as malfunction of a control system.

[0005]

SUMMARY OF THE INVENTION In a conventional fuel cell,
Although a protection function for detecting such “abrupt change” is necessary, a protection function for detecting “when the battery characteristics change over time” is also required. For example, the electrolyte may penetrate into the electrode due to the influence of disturbance, and the gas diffusion performance may be reduced. In such a case, if the operation is continued under the same operating conditions, the cell may be damaged. For this reason, a method of diagnosing a change in the fuel cell over time due to the influence of a disturbance is required.

As one of the methods, the increase in the penetration of the electrolyte into the fuel cell electrode can be determined by disassembling the fuel cell and analyzing the amount of the electrolyte absorbed by the electrode. That is, the situation diagnosis can be performed by the destructive inspection. However, in this destructive inspection, the operation as a plant must be abandoned. If a diagnosis can be made by a non-destructive inspection method, it is possible to take measures for recovering characteristics according to the diagnosis result.

One method of such non-destructive inspection is disclosed in
No. 122570. In this method, the output voltage during steady-state operation, and the output voltage in an operating state in which the oxygen concentration of the oxidizing gas is higher than that during steady-state operation are measured as needed, and each output voltage, and the steady-state operation and the high oxygen concentration operation This is a method for diagnosing a state change of a cell based on a temporal change pattern of an output voltage difference.

However, this method is not practical when applied to an actual plant. For example, 200
In a power plant having a kW capacity, the amount of air required for power generation (the amount supplied to the air electrode) is about 600 Nm ³ / h. In order to supply oxygen instead of air, it is necessary to secure a supply amount of about 130 Nm ³ / h, and when supplying this using an oxygen cylinder of 7 m ³ , 20 bottles per hour are required. There was a problem that the oxygen cylinder was consumed, which was not realistic, and that a complicated control change had to be performed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of diagnosing characteristics of a fuel cell which can easily avoid damage to the fuel cell due to disturbance in an actual plant, and to operate a fuel cell operating under appropriate operating conditions. The aim is to get the method.

[0010]

According to a first aspect of the present invention, there is provided a method for diagnosing characteristics of a fuel cell, wherein an air flow supplied to an air electrode is changed to a plurality of levels of air flow including a rated air flow. The voltage and DC current were measured, and then the average oxygen concentration was obtained from the oxygen concentration at the inlet and outlet of the cathode obtained from each air flow rate and each DC current, and then the average oxygen concentration at each measurement point and the battery The extrapolation value of the battery voltage at the pure oxygen concentration is obtained by linear regression with the voltage, the oxygen gain is obtained from the difference between this battery voltage and the rated battery voltage corresponding to the rated air flow, and the gas diffusion of the air electrode is obtained from the oxygen gain. Diagnosis of performance.

According to a second aspect of the present invention, there is provided a method for diagnosing characteristics of a fuel cell, wherein a series of steps from oxygen gain to diagnosing gas diffusion performance of an air electrode are sequence-controlled.

According to a third aspect of the present invention, there is provided a method for diagnosing characteristics of a fuel cell, wherein a fuel gas supplied to a fuel electrode is changed to a plurality of levels of a fuel gas flow including a rated fuel gas flow, and the respective cell voltage and DC current Then, each average hydrogen concentration is obtained from each hydrogen concentration at the inlet and outlet of the fuel electrode obtained from each fuel gas flow rate and each DC current, and then the average hydrogen concentration at each measurement point and the battery voltage are calculated. Calculate the extrapolated value of the battery voltage at the pure hydrogen concentration by linear regression, find the hydrogen gain from the difference between this battery voltage and the rated battery voltage corresponding to the rated hydrogen flow rate, and diagnose the gas diffusion performance of the fuel electrode from this hydrogen gain. Is what you do.

According to a fourth aspect of the invention, there is provided a method for diagnosing characteristics of a fuel cell, wherein a series of steps from a hydrogen gain to a diagnosis of gas diffusion performance of a fuel electrode are sequence-controlled.

According to a fifth aspect of the fuel cell characteristic diagnosis method,
The fuel cell is a phosphoric acid fuel cell.

According to a sixth aspect of the invention, there is provided a method for operating a fuel cell, wherein an air flow supplied to an air electrode is changed to a plurality of levels of air flow, and the respective cell voltages and DC currents are measured at that time.
Next, each average oxygen concentration is obtained from each oxygen concentration at the inlet and outlet of the air electrode obtained from each air flow rate and each DC current,
Then, a relational expression is obtained by linear regression between the average oxygen concentration at each measurement point and the battery voltage. Is to perform rated operation.

According to a seventh aspect of the present invention, there is provided a method for operating a fuel cell, in which an appropriate amount of air flow is derived, and a series of steps until a rated operation is performed with the derived air flow is sequence-controlled.

In the fuel cell operating method according to the present invention, the flow rate of the fuel gas supplied to the fuel electrode is changed to a plurality of levels, and the respective cell voltages and DC currents are measured at that time.
Next, each average hydrogen concentration is obtained from each hydrogen concentration at the inlet and the outlet of the fuel electrode obtained from each fuel gas flow rate and each DC current, and then the relationship between the average hydrogen concentration at each measurement point and the battery voltage by linear regression. An equation is obtained, and then an appropriate fuel gas flow rate is obtained from the relational equation, and the rated operation is performed at the fuel gas flow rate.

According to a ninth aspect of the present invention, there is provided a method of operating a fuel cell in which an appropriate amount of fuel gas flow rate is obtained, and a series of steps until a rated operation is performed at the fuel gas flow rate is sequence-controlled.

In the fuel cell operating method according to the tenth aspect, the fuel cell is a phosphoric acid type fuel cell.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. First, the principle of the fuel cell characteristic diagnosis method of the present invention will be described. FIG. 1 is a diagram showing the relationship between the actually measured flow rate of supply air to the phosphoric acid type fuel cell and the actually measured cell voltage in the initial, middle and final stages of operation of the actual plant. From FIG. 1, it can be seen that the cell voltage increases as the air flow rate increases. This is because a certain amount of oxygen in the air is consumed in the fuel cell by a certain load, and the oxygen partial pressure in the fuel cell increases as the supply air flow rate increases. This is the average value of the partial pressure and the oxygen partial pressure at the outlet of the fuel cell. The oxygen partial pressure at the outlet increases as the air flow increases.)
It is. A similar tendency is seen for the fuel gas, in which the cell voltage increases as the fuel supply gas flow rate increases.
It can also be seen that the rate of increase in cell voltage (increase in cell voltage with respect to increase in air flow rate) tends to increase at the end of operation, that is, as the operation time increases. This is presumed to be due to the fact that the gas diffusion performance in the electrode is reduced due to an increase in the permeation of the electrolyte into the electrode at the end of the operation and the like, and is easily affected by the air flow rate.

The cell characteristics (cell voltage) of a fuel cell depend on the oxygen partial pressure and the hydrogen partial pressure. This relationship is (Equation 1),
(Equation 2). VcellarufaLogPO ₂ using (Equation 1) VcellarufaLogPH ₂ (Equation 2) This relationship, the average partial pressure of certain were tested with varying each independently an air flow and fuel flow in the load, each of the electrodes Is plotted on the horizontal axis and the cell voltage on the vertical axis, a linear relationship is obtained. This is shown in FIG.

FIG. 2 shows a change in cell voltage when the air utilization rate (air flow rate) is changed. The average oxygen partial pressure (PO ₂ ) on the horizontal axis is obtained by calculating a logarithmic average of the oxygen partial pressure at the inlet of the fuel cell and the oxygen partial pressure at the outlet. "Rating" in Fig. 2
Point is the rated air utilization rate (rated air flow rate), and the point of "pure oxygen" is the oxygen partial pressure at the inlet when pure oxygen is supplied, taking into account the respective utilization rate and water vapor partial pressure. The logarithmic mean value with the oxygen partial pressure at the outlet is shown. Each plot point (◆) is an actually measured value, but since a linear relationship was obtained, extrapolation of this straight line was used to determine the oxygen gain (the rise in cell voltage when pure oxygen was supplied instead of air). Cell characteristics in pure oxygen can be estimated. That is, "pure oxygen"
The oxygen gain can be determined as the difference between the cell voltage at "1" and the cell voltage at "rated".

Next, an example of evaluating the validity of the above-described fuel cell characteristic diagnosis will be described. Verification tests were conducted on a short stack of phosphoric acid type fuel cells using actual size cells. In this verification test, assuming that a normal cell was subjected to disturbance, in order to simulate accelerated deterioration of the electrodes over time, the cells were configured with electrodes having variously changed water repellency. Evaluation of changes over time in cell characteristics for each water repellency, evaluation of changes over time in oxygen gain by periodic air utilization rate evaluation test (air flow rate change test), and absorption of phosphoric acid into the electrode by dismantling survey Was evaluated.

FIG. 3 shows the change over time of the oxygen gain for each electrode. A, B, and C in FIG. 3 indicate electrode specifications, the contents of which are as shown in FIG. The rate of increase of the oxygen gain differs for each electrode specification. The oxygen gain hardly changes with time in the electrode A which is a standard specification, but gradually increases with time in the electrodes B and C whose water repellency is weakened. These results are shown in FIG. Fig. 4 shows the results of long-term operation with the short stack, how much the rated characteristics (cell voltage) decreased, how much the oxygen gain decreased, and the amount of phosphoric acid absorbed by the electrodes for each electrode specification. How much increase (the initial
100%) is shown.

As shown in FIG. 4, an increase in the oxygen gain is observed in response to a decrease in the rated characteristics. It is presumed that the increase in the oxygen gain is solely due to the decrease in gas diffusion performance. It is considered that the gas diffusion performance decreased due to excessive absorption of phosphoric acid in the electrode. As a result of the decomposition investigation, it was confirmed that the amount of phosphoric acid absorbed in the electrode was increased from the initial stage. In addition, there is a tendency that the greater the increase in oxygen gain, the greater the amount of phosphoric acid absorbed. The following contents can be understood from these measurement results. (1) Evaluation of oxygen gain as a method of diagnosing the cause of a decrease in cell characteristics can be carried out even in an actual cell. (2) An increase in the oxygen gain in the actual cell has a correlation with an increase in the absorption of electrolyte (phosphoric acid) in the cell. This indicates that the characteristic diagnosis method according to the present invention is effective as a nondestructive inspection.

The hydrogen gain (the cell voltage rise value when pure hydrogen is supplied instead of the fuel gas) can be similarly derived by changing the fuel flow rate. The principle is shown in FIG. The difference between the cell voltage at the “rated” point and the “pure hydrogen” point in the figure is a value calculated as the hydrogen gain. From the value of the hydrogen gain, it is possible to quantitatively evaluate the decrease in gas diffusion performance at the fuel electrode.

As described above, according to the present invention, it is possible to quantitatively and correctly diagnose a situation in which the gas diffusion performance is reduced due to disturbance or the like in the actual cell.

FIG. 6 is a sequence flow of the automatic characteristic diagnosis of the fuel cell. Basically, the air flow rate is sequentially changed from the rated condition, and the cell voltage value and the DC current value at each point are measured. In this example, the width of change of each air flow rate is -3% to + 9%, but if allowed under other plant control conditions, the air flow rate to be changed is varied from -10% to about + 20%. Is more desirable. The measured direct current value at the air flow rate at each point is substituted into the following equation, and the average oxygen concentration can be calculated as follows.

[0029]

(Equation 1) Since the amount of steam discharge depends on the steam / carbon ratio and the flow rate of the reformed gas, it is necessary to separately calculate the amount of steam discharge under rated conditions.

The cell voltage is plotted for each logarithmic value of the calculated average oxygen concentration to obtain a linear regression line, an extrapolated value of the cell voltage at the average oxygen concentration in pure oxygen is obtained, and the oxygen gain is calculated. calculate. To evaluate the cell diffusion performance from the oxygen gain, the cell state such as gas diffusion performance is obtained from a database constructed with short stacks and laboratory-level small cells. Of course, without using such a database, it is possible to roughly evaluate the diffusion performance of the electrolyte only from the obtained oxygen gain. The fuel electrode side can be set in the same manner.

A series of diagnoses can be easily performed by registering them as a sequence program in the control device. In addition, a change over time can be performed by automatically setting a timer every 500 hours, for example. It is possible to obtain an accurate characteristic diagnosis result of the fuel cell by grasping it.

If the increase in the oxygen gain is extremely significant in addition to the increase in the air flow rate, a measure is taken to change the operation mode to discharge the electrolyte excessively absorbed in the electrode to the outside of the electrode. It can also be taken. In addition, if the oxygen voltage can be obtained for each block voltage by measuring the block voltage of several cells as a basic unit, as well as the voltage of the entire stack, the accuracy of diagnosis will increase, and measures for countermeasures will be taken. The effect can be expected to be higher.

Embodiment 2 FIG. Next, measures for recovering the fuel cell characteristics will be described. It is effective to take measures to increase the air flow rate as a means of restoring the lowered fuel cell characteristics. However, increasing the air flow rate unnecessarily causes a decrease in power generation efficiency due to an increase in power consumption of the air blower. Therefore, it is necessary to derive an appropriate amount of increase in the air flow rate according to the situation.

As shown in FIG. 1 described above, the cell voltage increment for the same air supply increase is Δ
ΔV _{2 at} the end of operation is larger than V ₁ .
The increase in auxiliary power with the same increase in air supply is the same in the medium term and the end, but the improvement in power generation efficiency due to the accompanying increase in voltage is greater at the end. This situation will be described with reference to the drawings.

FIG. 7 shows the relationship between the loss of power generation efficiency due to an increase in auxiliary power due to an increase in air supply and the improvement in power generation efficiency due to an improvement in cell characteristics with an increase in air supply at the electrode of specification B. An increase in power generation efficiency due to improved characteristics is seen against an increase in auxiliary power loss. The break-even line in FIG. 7 indicates that the loss and the improvement offset each other on the line. If the break-even line is above the break-even line, the overall efficiency is improved and the gain is obtained, but the break-even line is below the break-even line. Then, it means that the overall efficiency is reduced and a loss is caused.

In the initial stage of the operation, it is almost on the break even line, and even if the air supply amount is increased beyond the rated point, the increase in efficiency is small. However, the gain of the power generation efficiency is increasing as the operation is in the middle stage or the end stage. Therefore, it is possible to calculate the gain of the power generation efficiency, derive an appropriate increase in the air flow rate, and take an appropriate characteristic recovery measure.

With respect to hydrogen, an increase in the amount of supplied hydrogen is not an increase in the power of auxiliary equipment, but an increase in the amount of input raw materials. Therefore, the loss of power generation efficiency increases.
However, if the gas diffusion performance on the fuel electrode side is reduced due to the influence of disturbance or the like, to reduce the reliability of the fuel cell, the reduction in power generation efficiency is sacrificed according to the derived hydrogen gain value. Therefore, it is determined that it is unavoidable to increase the amount of supplied fuel gas.

FIG. 8 shows a sequence flow chart of a method of operating the fuel cell. In this example, a sequence diagram of a characteristic recovery process when it is determined that the cell characteristics are degraded based on the result of the automatic characteristic diagnosis shown in FIG. 6 is shown. Basically, when the oxygen gain is increasing, the degree of characteristic recovery with respect to an increase in air flow rate is increasing.
This is an automated procedure for deriving an increase in the air flow rate such that the maximum value of the power generation efficiency is obtained. That is, the procedure for deriving the air supply amount indicating the maximum power generation efficiency is automated based on the balance between the power loss of the auxiliary equipment due to the increase in the air flow rate and the power generation efficiency improvement due to the characteristic improvement.

It should be noted that the procedure for deriving the air supply amount that indicates the maximum power generation efficiency may be performed separately and independently without relating to the diagnosis of the characteristics of the fuel cell. By automating such a procedure, it is possible to always operate at the maximum power generation efficiency. Of course, there is a limitation due to the capacity of the plant equipment (such as the maximum air volume of the blower), and there is a case where the operation cannot be performed uniformly. In that case, by outputting the information to the operation monitoring staff, the countermeasures will be separately considered. In any case, by constantly monitoring such information and controlling the output, the reliability of the power plant is improved.

On the fuel electrode side, as described above, an increase in the fuel supply amount corresponding to the increase in the hydrogen gain causes a decrease in efficiency. In this case, even if the air amount is increased, the effect of reducing the hydrogen gain can be obtained. Therefore, it is necessary to first increase the air amount and then set up a sequence for suppressing an increase in the fuel supply amount even a little.

[0041]

As described above, according to the fuel cell characteristic diagnosis method of the present invention, the air flow supplied to the air electrode is changed to a plurality of levels of air flow including the rated air flow. Measure each battery voltage and DC current,
Next, each average oxygen concentration is obtained from each oxygen concentration at the inlet and outlet of the air electrode obtained from each air flow rate and each DC current,
After that, the extrapolation value of the battery voltage at the pure oxygen concentration was obtained by linear regression of the average oxygen concentration and the battery voltage at each measurement point, and the oxygen gain was obtained from the difference between this battery voltage and the rated battery voltage corresponding to the rated air flow rate. Since the gas diffusion performance of the air electrode is diagnosed based on the oxygen gain, it is possible to easily know that the gas diffusion performance has decreased due to the disturbance, and to avoid damage to the fuel cell due to the disturbance.

Further, when a series of steps from the oxygen gain to the diagnosis of the gas diffusion performance of the air electrode are sequence-controlled, accurate diagnosis can be easily performed, and the reliability of fuel cell characteristic diagnosis is improved. I do.

According to the fuel cell characteristic diagnosis method of the present invention, the fuel gas supplied to the fuel electrode is changed to a plurality of levels of fuel gas flow including the rated fuel gas flow, and the respective cell voltages at that time are changed. And the DC current, and then calculate the average hydrogen concentration from each hydrogen concentration at the inlet and outlet of the fuel electrode obtained from each fuel gas flow rate and each DC current, and then calculate the average hydrogen concentration at each measurement point and the battery The extrapolation value of the battery voltage at the pure hydrogen concentration is obtained by linear regression with the voltage, and the hydrogen gain is obtained from the difference between this battery voltage and the rated battery voltage corresponding to the rated hydrogen flow rate. Can be easily known, and damage to the fuel cell due to disturbance can be avoided.

Further, when a series of steps from the hydrogen gain to the diagnosis of the gas diffusion performance of the fuel electrode are sequence-controlled, accurate diagnosis can be easily performed, and the reliability of the fuel cell characteristic diagnosis is improved. I do.

According to the fuel cell operating method of the present invention, the air flow supplied to the air electrode is changed to a plurality of levels of air flow, and the respective cell voltages and DC currents at that time are measured. The average oxygen concentration is obtained from the oxygen concentration at the inlet and outlet of the air electrode obtained from each air flow rate and each DC current, and then the relational expression by linear regression between the average oxygen concentration at each measurement point and the battery voltage is obtained. After that, an appropriate amount of air flow was derived by calculating the gain of power generation efficiency with respect to the increase in air flow rate from this relational expression, and the rated operation was performed with the derived air flow rate. it can.

Further, when an appropriate amount of air flow is derived, and a series of steps until the rated operation is performed at the derived air flow is sequence-controlled, the fuel cell can be easily and accurately operated. .

Further, according to the fuel cell operating method of the present invention, the flow rate of the fuel gas supplied to the fuel electrode is changed to a plurality of levels, and the cell voltage and DC current at that time are measured. The average hydrogen concentration is obtained from each hydrogen concentration at the inlet and outlet of the fuel electrode obtained from each fuel gas flow rate and each DC current, and then the relational expression by linear regression between the average hydrogen concentration at each measurement point and the battery voltage Then, an appropriate amount of fuel gas flow rate is obtained from this relational expression, and the rated operation is performed at the fuel gas flow rate, so that an appropriate fuel cell operation can be performed.

Further, when an appropriate amount of fuel gas flow rate is obtained and a series of steps until the rated operation is performed at the fuel gas flow rate is sequence-controlled, the fuel cell can be easily and accurately operated.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a fuel cell supply air flow rate and a cell voltage.

FIG. 2 is a diagram showing a relationship between a logarithmic average value of a fuel cell air electrode side oxygen partial pressure and a cell voltage.

FIG. 3 is a diagram showing a change over time in oxygen gain for each type of electrode.

FIG. 4 is a diagram showing evaluation results obtained by characteristic diagnosis.

FIG. 5 is a diagram showing a relationship between a logarithmic average value of a fuel cell fuel electrode side hydrogen partial pressure and a cell voltage.

FIG. 6 is a sequence flowchart of a fuel cell automatic characteristic diagnosis.

FIG. 7 is a diagram illustrating a relationship between an auxiliary machine power loss and an increase in power generation efficiency due to an improvement in characteristics due to an increase in an air supply amount.

FIG. 8 is a sequence flow chart of rating condition automatic setting based on fuel cell automatic characteristic diagnosis.

Continuation of front page (72) Inventor Yoshiichi Matsumoto 3-3-22 Nakanoshima, Kita-ku, Osaka City, Osaka Kansai Electric Power Company

Claims (10)

[Claims] 1. A method for diagnosing characteristics of a fuel cell, wherein fuel gas and air are supplied and electric power is extracted to the outside by an electrochemical reaction. The flow rate was changed, the respective battery voltage and DC current at that time were measured, and then the respective average oxygen concentrations were calculated from the oxygen concentrations at the inlet and outlet of the cathode obtained from each air flow rate and each DC current. An extrapolated value of the battery voltage at the pure oxygen concentration is obtained by first-order regression of the average oxygen concentration at each measurement point and the battery voltage, and the oxygen gain is calculated from the difference between this battery voltage and the rated battery voltage corresponding to the rated air flow rate. And a fuel cell characteristic diagnosis method for diagnosing the gas diffusion performance of the air electrode from the oxygen gain. 2. The method for diagnosing characteristics of a fuel cell according to claim 1, wherein a series of steps from the oxygen gain to diagnosing the gas diffusion performance of the air electrode is sequence-controlled. 3. A method for diagnosing characteristics of a fuel cell in which fuel gas and air are supplied and electric power is extracted to the outside by an electrochemical reaction, wherein a fuel gas supplied to a fuel electrode has a plurality of levels including a rated fuel gas flow rate. The fuel cell flow rate was changed, and the respective battery voltage and DC current at that time were measured.Then, each average was obtained from each hydrogen concentration at the inlet and outlet of the fuel electrode obtained from each fuel gas flow rate and each DC current. Calculate the hydrogen concentration, then calculate the extrapolated value of the battery voltage at the pure hydrogen concentration by linear regression of the average hydrogen concentration at each measurement point and the battery voltage, and calculate the difference between this battery voltage and the rated battery voltage corresponding to the rated hydrogen flow rate. A fuel cell characteristic diagnosis method for obtaining a hydrogen gain from the fuel cell and diagnosing the gas diffusion performance of the fuel electrode from the hydrogen gain. 4. The fuel cell characteristic diagnosis method according to claim 3, wherein a series of steps from the hydrogen gain to the diagnosis of the gas diffusion performance of the fuel electrode are sequence-controlled. 5. The method according to claim 1, wherein the fuel cell is a phosphoric acid fuel cell. 6. A method of operating a fuel cell which supplies a fuel gas and air and takes out electric power by an electrochemical reaction, wherein a flow rate of air supplied to an air electrode is changed to a plurality of levels of air flow rate. At that time, each battery voltage and DC current were measured, and then each average oxygen concentration was obtained from each oxygen concentration at the inlet and outlet of the air electrode obtained from each air flow rate and each DC current. Of the average oxygen concentration of the battery and the battery voltage by linear regression.After that, the power generation efficiency gain with respect to the increase of the air flow rate is calculated from this relational expression to derive an appropriate air flow rate. How to operate the fuel cell. 7. The fuel cell operating method according to claim 6, wherein an appropriate amount of air flow is derived, and a series of steps until a rated operation is performed with the derived air flow is sequence-controlled. 8. A method of operating a fuel cell which supplies a fuel gas and air and takes out electric power by an electrochemical reaction, wherein a flow rate of a fuel gas supplied to a fuel electrode is changed to a flow rate of a plurality of levels. At that time, each battery voltage and DC current were measured, and then each average hydrogen concentration was obtained from each hydrogen concentration at the inlet and outlet of the fuel electrode obtained from each fuel gas flow rate and each DC current. A method of operating a fuel cell in which a relational expression based on linear regression between the average hydrogen concentration at a point and the cell voltage is obtained, and then an appropriate fuel gas flow rate is obtained from the relational expression, and the rated operation is performed at the fuel gas flow rate. 9. The fuel cell operating method according to claim 8, wherein an appropriate amount of fuel gas flow rate is obtained, and a series of steps until a rated operation is performed at the fuel gas flow rate is sequence-controlled. 10. The method for operating a fuel cell according to claim 6, wherein the fuel cell is a phosphoric acid type fuel cell.