JP2020177786A - Method for recovering sulfur poisoning of anode catalyst in fuel battery cell - Google Patents

Abstract

To provide a method capable of recovering poisoning of an anode catalyst in an anode catalyst layer while preventing deterioration of an anode catalyst layer.SOLUTION: Disclosed is a method for recovering sulfur poisoning of an anode catalyst in a fuel cell, which includes: a step (a) of reducing the amount of fuel gas supplied to an anode while continuing the supply of oxidant gas to a cathode, thereby reducing a potential of the cathode to the anode to be equal to or less than - 0.2 V; and a step (b) of recovering the supply amount of the fuel gas after the step (a) so that a period in which the potential is continuously - 0.2 V or less becomes equal to or less than a predetermined period.SELECTED DRAWING: Figure 5

Description

The present disclosure relates to a method of recovering sulfur poisoning of an anode catalyst in a fuel cell.

Among the fuel cells, for example, the fuel cell (single cell), which is a constituent unit of a polymer electrolyte fuel cell (PEFC), is a membrane electrode assembly (MEA) sandwiched between a pair of separators (MEA: Membrane Electrode Assembly). have. The membrane electrode assembly is a power generator having a structure in which an electrolyte membrane is sandwiched between an anode and a cathode. The anode and the cathode each have a catalyst layer. The catalyst layer has, for example, a carbon carrier carrying platinum particles as a catalyst.

In a fuel cell, a fuel gas such as hydrogen (anode supply gas) and an oxidizing agent gas such as oxygen (cathode supply gas) are applied to an electrically connected anode (anode) and a cathode (air electrode), respectively. By supplying and electrochemically oxidizing the fuel, chemical energy is directly converted into electrical energy.

In the fuel cell, the reaction of the following equation (1) proceeds at the anode to which hydrogen is supplied.
_{^{H 2 → 2H + + 2e -}} ··· (1)

The electrons (e ⁻ ) generated by the above equation (1) reach the cathode after working with an external load via an external circuit. On the other hand, the proton (H ⁺ ) generated by the above equation (1) moves from the anode side to the cathode side in the electrolyte membrane by electroosmosis in a state of being hydrated with water.

On the other hand, at the cathode, the reaction of the following equation (2) proceeds.
2H ^＋ ＋ 1 / 2O ₂ ＋ 2e ⁻ → H ₂ O ・ ・ ・ (2)

Therefore, the chemical reaction shown in (3) below proceeds in the entire battery, electromotive force is generated, and electrical work is performed on the external load.
H ₂ + 1 / 2O ₂ → H ₂ O ・ ・ ・ (3)

The fuel gas supplied to the anode contains impurities such as carbon monoxide or sulfur compounds. Such impurities can interfere with the normal operation of the fuel cell. For example, hydrogen sulfide contained in hydrogen gas reacts with platinum in the catalyst layer, and sulfur accumulates on platinum.

The generally accepted dissociation and absorption mechanism of hydrogen sulfide in platinum is as follows.
H ₂ S + Pt → Pt-S + H ₂
H ₂ S + Pt-H → Pt-S + 3 / 2H ₂

Sulfur poisoning due to such accumulation of sulfur reduces the catalytic sites that catalyze the hydrogen oxidation reaction, resulting in a decrease in the power generation performance of the fuel cell.

In order to recover the poisoning of the catalyst by impurities, a technique for applying an external voltage to the anode has been developed.

Patent Document 1 discloses a method of operating a fuel cell. According to this method, a second circuit is provided outside the fuel cell. This second circuit comprises an electrical energy supply means for selectively supplying opposite DC potentials to the anode and cathode to bring the anode to a positive potential with respect to the cathode.

Patent Document 2 relates to a fuel cell system that restores battery performance by preventing carbon monoxide poisoning in a catalyst. The document states that the fuel cell system comprises a plurality of fuel cell stacks, that the fuel cell system comprises control means and voltage application means, and that the control means to any fuel cell stack. It is described that the supply of the fuel gas is temporarily stopped to stop the power generation, and the voltage is applied to the fuel cell stack whose power generation is stopped by the voltage applying means.

Patent Document 3 discloses an operation control method for a fuel cell, which includes a process of temporarily setting the electrode potential of the fuel electrode to a potential of +0.3 V or more noble with respect to the standard hydrogen electrode potential during power generation. The document describes that carbon monoxide (CO) adsorbed on the electrode catalyst was oxidatively removed.

Patent Document 4 discloses a method for decontaminating and regenerating a catalytic fuel cell electrode. The document states that this method reduces the fuel flow to reduce the value of the fuel stoichiometric coefficient (St _H2 ) to less than 1, cuts off the current, and increases the fuel flow to 1 again. It is described that the step of making the value of the above stoichiometric coefficient is included.

Japanese Unexamined Patent Publication No. 8-7905 Japanese Unexamined Patent Publication No. 2004-265692 Japanese Unexamined Patent Publication No. 11-219715 Special Table 2014-527260

The fuel gas supplied to the anode, for example, hydrogen gas, contains impurities such as hydrogen sulfide. According to the hydrogen quality standard ISO14687-2, which defines the permissible concentration of impurities in hydrogen gas for fuel cell vehicles, the permissible concentration of sulfur compounds such as hydrogen sulfide is 4 ppb. Such sulfur compounds present in hydrogen gas react with the platinum catalyst of the anode, and the sulfur atom bonded to platinum inhibits the hydrogen oxidation catalytic reaction.

It is experimentally known that an electrochemical potential of 1.25 V (vs. RHE) or higher is required for oxidative elimination of sulfur bound to platinum. On the other hand, during the operation of the fuel cell, the potential of the anode is always near 0 V (vs. SHE). Therefore, under normal fuel cell operating conditions, sulfur atoms bonded to platinum are not oxidatively desorbed, and as the poisoning and accumulation of platinum progresses, the battery performance deteriorates.

From the viewpoint of the widespread use of fuel cell vehicles, it is necessary to control the manufacturing cost of fuel cells, and in particular, it is required to reduce the amount of expensive platinum used. Therefore, it is important to recover the poisoning of the platinum catalyst by the sulfur compound in order to ensure sufficient catalytic performance with a limited amount of platinum.

As a method of recovering the poisoning of platinum, a method of temporarily raising the potential of the anode is known. However, an increase in the potential at the anode may cause an oxidation reaction of the carbon carrying the platinum catalyst in the catalyst layer, resulting in deterioration of the catalyst layer.

In addition, for example, in the method of using an oxidizing agent (ozone, etc.) or the method of forcibly applying a voltage to the anode from the outside of the fuel cell, it is necessary to additionally install a device for poison recovery, which is costly. May rise. Also, such additional devices can be difficult to install in real systems, especially in automobiles.

From this point of view, it is an object of the present disclosure to provide a method capable of recovering poisoning of the anode catalyst in the anode catalyst layer while preventing deterioration of the anode catalyst layer.

The present disclosure achieves the above object by the following means.

A method of recovering sulfur poisoning of an anode catalyst in a fuel cell, which comprises the following steps:
(A) While continuing to supply the oxidant gas to the cathode, the amount of fuel gas supplied to the anode is reduced so that the potential of the cathode with respect to the anode is lowered to -0.2 V or less. To
(B) After the step (a), the supply amount of the fuel gas is restored so that the period during which the potential is continuously −0.2 V or less is the predetermined period or less.

According to the method of the present disclosure, it is possible to recover the poisoning of the anode catalyst in the anode catalyst layer while preventing the deterioration of the anode catalyst layer.

FIG. 1 is a schematic flowchart of an embodiment of the method according to the present disclosure. FIG. 2 is a schematic configuration diagram of an evaluation device for a poison recovery experiment. FIG. 3 shows the measured value of the cathode potential with respect to the anode in the power generation performance evaluation. FIG. 4 shows the evaluation result of the electrochemical active specific surface area (ECSA). FIG. 5 shows the time course of the potential when the fuel cell is recovered from poisoning according to the method of the present disclosure. FIG. 6 is an enlargement of the portion of FIG. 5 from 3.0 hours to 3.5 hours.

Hereinafter, embodiments of the method according to the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist of the present disclosure. The forms shown in the drawings are exemplary of the present disclosure and are not intended to limit the present disclosure. In the following description, similar components are given the same reference numbers.

≪How to recover from sulfur poisoning≫
The present disclosure relates to a method of recovering sulfur poisoning of an anode catalyst in a fuel cell, which method comprises the following steps:
(A) While continuing to supply the oxidant gas to the cathode, the amount of fuel gas supplied to the anode is reduced so that the potential of the cathode with respect to the anode is lowered to -0.2 V or less. , Poison recovery process,
(B) After the step (a), the supply amount of the fuel gas is restored so that the period during which the potential of the cathode with respect to the anode is continuously −0.2 V or less is equal to or less than a predetermined period. Process.

Hereinafter, the method of the present disclosure will be described with reference to FIG.

FIG. 1 is a schematic flowchart of an embodiment of the method according to the present disclosure. In the embodiment shown in FIG. 1, first, it is determined whether or not the catalyst is in a poisoned state in the fuel cell that is operating normally. If the transition of the voltage drop of the fuel cell is within the range of the predicted transition, it is determined that the poisoning by hydrogen sulfide has occurred. The predicted transition can be obtained by examining in advance the relationship between the amount of hydrogen sulfide mixed in and the transition of the voltage drop of the fuel cell. The amount of hydrogen sulfide mixed is estimated from the concentration of the sulfide compound in the fuel and the amount of hydrogen consumed.

Then, when it is determined that the catalyst is in a poisoned state, the amount of fuel gas supplied to the anode is reduced while continuing to supply the oxidant gas to the cathode. Specifically, it is swept in the direction of lowering the stoichiometric coefficient (stoichiometry) of hydrogen. By this operation, the anode becomes hydrogen-deficient, the water electrolysis reaction is started, and the potential of the anode rises.

In the present disclosure, the potential is expressed by the potential of the cathode with respect to the anode, that is, by "cathode potential"-"anode potential". Therefore, an increase in potential at the anode is represented as a decrease in the potential of the cathode with respect to the anode.

In the method according to FIG. 1, the potential of the cathode with respect to the anode is lowered to −0.2 V or less by reducing the amount of fuel gas supplied to the anode. Since the cathode potential of the fuel cell during normal operation is 0.6V to 1.0V based on the standard hydrogen electrode potential, this operation causes the anode to have a potential of 0.8V or more based on the standard hydrogen electrode potential. Will be applied. As a result, it is considered that a reaction for oxidative elimination of the sulfur atom bonded to the platinum catalyst occurs at the anode.

Here, the increase in the potential at the anode can also cause the oxidation reaction of the carbon carrier at the anode (C + H ₂ O → CO + 2H ⁺ + 2e ⁻ and C + 2H ₂ O → CO ₂ + 4H ⁺ + 4e ⁻ ). Therefore, an increase in the potential at the anode may deteriorate the anode catalyst layer.

In the embodiment according to FIG. 1, when the potential of the anode reaches a predetermined value, the potential of the anode is restored to a normal value by restoring the hydrogen stoichiometric value to a reference value. As a result, the period during which the cathode potential with respect to the anode is continuously −0.2 V or less is set to be equal to or less than a predetermined period, and carbon corrosion due to an increase in potential is avoided.

As described above, according to the method of the present disclosure, it is possible to recover the catalyst poisoning in the anode catalyst while preventing the deterioration of the anode catalyst layer.

The method of the present disclosure will be described in detail below.

<Recovery of sulfur poisoning of anode catalyst in fuel cell>
The method according to the present disclosure is a method for recovering sulfur poisoning of an anode catalyst in a fuel cell.

(Fuel cell)
As the fuel cell, a known fuel cell may be used. The oxidant gas supplied to the cathode of the fuel cell may be, for example, air. The fuel gas supplied to the anode of the fuel cell may be, for example, hydrogen gas. The catalyst layer at the anode of the fuel cell has, for example, a platinum catalyst supported on a carbon carrier.

(Anode catalyst)
With respect to the present disclosure, the anode catalyst specifically means a catalyst contained in the catalyst layer of the anode, and particularly means a platinum catalyst supported on a carbon carrier in the catalyst layer of the anode. ..

(Sulfur poisoning)
With respect to the present disclosure, sulfur poisoning of an anode catalyst refers to the accumulation of sulfur in platinum present in the catalyst layer of the anode by a sulfur compound contained in the fuel gas supplied to the anode. Sulfur poisoning of the anode catalyst reduces the output of the fuel cell during normal operation.

(Recovery of sulfur poisoning)
With respect to the present disclosure, recovery of sulfur poisoning means removing sulfur from an anode catalyst in a sulfur poisoned state, particularly a platinum catalyst.

<Poison recovery process>
In the poisoning recovery step of the method according to the present disclosure, the amount of fuel gas supplied to the anode is reduced while continuing to supply the oxidant gas to the cathode, whereby the potential of the cathode with respect to the anode is -0.2 V. Try to reduce to:

(Gas supply)
The supply of gas to the cathode and the anode and the adjustment of the supply amount may be performed by a known method.

(Reduction in supply)
The decrease in the supply amount of the fuel gas for lowering the potential of the cathode with respect to the anode may be performed by supplying the fuel gas at a supply amount lower than the supply amount during normal operation. Specifically, for example, the supply amount of the fuel gas is adjusted so that the value of the stoichiometric coefficient (stoichiometry) of the fuel gas is less than 1.

In the method according to the present disclosure, the amount of fuel gas supplied to the anode is reduced while the supply of the oxidant gas to the cathode is continued. That is, the amount of fuel gas supplied is reduced during normal operation.

On the other hand, in the conventional method including cutting off the current while reducing the supply amount of the fuel gas, oxygen on the cathode side is not consumed and may permeate to the anode side. If oxygen is present on the anode side, an abnormal potential is generated at the anode when the fuel cell is restarted, causing carbon corrosion. As a result of repeated such operations, the catalyst layer may shrink, leading to deterioration in the performance of the fuel cell.

In the method according to the present disclosure, since the supply amount of fuel gas is reduced during normal operation, the above-mentioned adverse effects due to the non-consumption of oxidant gas do not occur.

Further, the method according to the present disclosure does not require an additional device, unlike the method of applying a voltage to the anode by a forced method from the outside. Therefore, for example, in automobile applications, it is possible to apply a voltage only in a vehicle system.

(Cathode potential with respect to anode)
In the poisoning recovery step of the method according to the present disclosure, the potential of the cathode with respect to the anode is lowered to −0.2 V or less.

The potential of the cathode with respect to the anode may be measured continuously or intermittently, for example, by a voltmeter or voltage sensor.

In the poisoning recovery step, the potential of the cathode with respect to the anode may be lowered to -0.2V or less, -0.4V or less, or -0.5V or less, and / or -1.0V or more, -0. It may be lowered to 8.8V or more, or -0.6V or more.

(Fuel cell temperature)
In another embodiment according to the present disclosure, the temperature of the fuel cell in the poisoning recovery step is 65 ° C. or higher. The temperature of the fuel cell in the poison recovery step may be more than 60 ° C, 62 ° C or higher, 64 ° C or higher, or 65 ° C or higher, and / or 75 ° C or lower, 70 ° C or lower, 68 ° C or lower, or It may be 67 ° C. or lower. The higher the temperature of the fuel cell, the more the amount of change in the potential of the cathode with respect to the anode in the poisoning recovery step is suppressed.

(Poison judgment)
In another embodiment of the method according to the present disclosure, it may be determined whether or not the catalyst is in a poisoned state in order to determine whether or not to perform the poisoning recovery step. In this determination, for example, the relationship between the amount of hydrogen sulfide mixed in the fuel gas and the voltage drop of the fuel cell is investigated in advance, and this information is compared with the information of the actual hydrogen sulfide concentration and the voltage drop of the fuel cell. It can be done by matching. Since the absolute value of hydrogen sulfide mixed in the fuel cell can be estimated from the information on the hydrogen sulfide concentration in the fuel gas, the transition of the voltage decrease of the fuel cell with respect to the hydrogen consumption can be predicted with high accuracy. If the actual transition of the voltage drop of the fuel cell is within the predicted range, it can be determined that poisoning due to hydrogen sulfide has occurred.

In one embodiment of the present disclosure, the supply of fuel gas to the anode is reduced when the catalyst is determined to be in a poisoned state. The operation of reducing the supply of fuel gas to the anode is preferably performed automatically.

<Return process>
In the restoration step according to the method of the present disclosure, the supply amount of fuel gas is restored after the poisoning recovery step. Here, the period during which the potential of the cathode with respect to the anode is continuously −0.2 V or less is set to be equal to or less than a predetermined period.

By recovering the supply amount of fuel gas, the potential of the anode is lowered and returned to the potential during normal operation.

(Recovery of supply)
The operation of recovering the supply amount of the fuel gas may be performed, for example, by adjusting the opening degree of the solenoid valve provided between the fuel gas tank and the fuel cell. The recovery of the fuel gas supply amount is performed, for example, by returning the fuel gas supply amount to the supply amount during normal operation.

(Negative voltage period)
In the method according to the present disclosure, the period during which the cathode potential with respect to the anode is continuously −0.2 V or less is a predetermined period or less.

The period during which the potential of the cathode with respect to the anode is continuously −0.2 V or less may be 0.1 seconds or more, 1 second or more, 5 seconds or more, 10 seconds or more, or 30 seconds or more, and / or. It may be 10 minutes or less, 5 minutes or less, 3 minutes or less, 2 minutes or less, 60 seconds or less, 45 seconds or less, or 30 seconds or less. Preferably, the period during which the cathode potential with respect to the anode is continuously −0.2 V or less is 60 seconds or less.

The length of the period during which the cathode potential with respect to the anode is continuously below -0.2 V sets an appropriate negative value _VA , and the cathode potential with respect to the anode becomes a negative value _VA , for example. In some cases, it can be controlled by restoring the fuel gas supply.

This negative value _VA can be determined by confirming the allowable negative voltage and its integration time by experiments in advance in consideration of the influence of carbon deterioration in the catalyst layer. The voltage and duration of the water electrolysis reaction that occur as the supply of fuel gas decreases, and the deterioration of the catalyst layer due to the application of negative voltage, the type of catalyst used, the amount of catalyst, temperature, humidity, etc. It can change according to the conditions of. Therefore, the relationship between the degree of recovery of the battery performance of the fuel cell and the effect on the catalyst layer structure caused by the application of the negative voltage to the anode was investigated in advance with respect to the application of various negative voltages, and the results were referred to. Above, it is preferable to determine the optimum negative value _VA .

According to one embodiment of the present disclosure, the negative value _VA is in the range of -0.2V to -1.0V. The negative value _VA may be -0.2V or less, -0.3V or less, -0.4V or less, or -0.5V or less, and / or -1.0V or more, -0.9V. It may be the above, or −0.8V or more.

In another embodiment of the present disclosure, poisoning recovery is performed by repeatedly applying a negative voltage such that the potential of the cathode with respect to the anode is continuously −0.2 V or less for a predetermined period or less. That is, after applying a negative voltage for a predetermined period or less to the anode, the operating state of the fuel cell is returned to normal, a negative voltage for a predetermined period or less is applied again, and this is repeated. Corrosion of the carbon carrier due to voltage application should be better avoided when negative charge is repeatedly applied for a short time as compared with the case where negative voltage is continuously applied and a relatively large negative voltage is applied. Is thought to be possible.

≪Sulfur poisoning recovery experiment and its evaluation≫
The conditions for recovery from sulfur poisoning, a sulfur poisoning recovery experiment, and its evaluation were conducted using a fuel cell cell evaluation device.

<Evaluation device>
FIG. 2 is a schematic configuration diagram of the evaluation device 100 for the poisoning recovery experiment. The oxidant gas tank 26 for supplying the oxidant gas is connected to the cathode side of the fuel cell 200 via a humidifier 266 by a connecting pipe. A fuel gas tank 22 for supplying fuel gas is connected to the anode side of the fuel cell 200 via a humidifier 226 by a connecting pipe. Each connecting pipe has a mass flow controller (MFC) 224, 264. The unreacted gas is exhausted to the outside of the system via the vents 230 and 232. The water generated in the fuel cell is sent to the drain water containers 228 and 268.

The evaluation device 100 further includes a line for introducing hydrogen sulfide into the fuel gas. Specifically, the hydrogen sulfide contamination gas tank 24 is connected to the connecting pipe for supplying fuel gas via the connecting pipe. The connecting pipe for hydrogen sulfide has a mass flow controller 244. The evaluation device 100 includes an anode CV line 20.

The evaluation device 100 includes a fuel cell 200. A JARI standard cell was used as the fuel cell. The fuel cell 200 had a 25 cm ² membrane electrode assembly (MEA) and a gas diffusion layer (GDL). As the anode catalyst, a catalyst in which Pt was supported on acetylene black was used. As the cathode catalyst, a PtCo catalyst supported on acetylene black was used. As the electrolyte membrane, a Gore Select (registered trademark) (W. L. Gore and Associates, Incorporated) membrane was used. Platinum basis weight measured by ICP analysis, 0.035 mg / cm ² in the anode was 0.40 mg / cm ² at the cathode.

Hydrogen gas was used as the fuel gas. A fine purer (manufactured by liquid gas) was used for ultra-high purity hydrogen (100 ppm or less), and the impurities were set to 1 ppb or less in a nominal value. The hydrogen sulfide / hydrogen balance gas was introduced by diluting 5.35 ppm of gas to 200 ppb (dilution rate: 26.75 times). Since the hydrogen sulfide / hydrogen balance gas is introduced in a dry state, dew point calibration was performed in the introduced state.

Prior to the experiment, first, a voltage of 0.9V to 0.1V was applied 100 times, and hydrogen gas / nitrogen gas purging was performed at the anode and cathode. Then, a constant current power generation of 2.0 A / cm ² was performed, and it was confirmed that the voltage of the fuel cell was stable.

≪Experiment 1: Applied voltage condition for poison recovery≫
The conditions were examined as to how much voltage should be applied to the anode to obtain the poisoning recovery effect. Specifically, after sulfur poisoning by the hydrogen sulfide contamination test, various values of voltages were applied to the anode, and it was investigated whether the battery performance was recovered.

<Hydrogen sulfide contamination test>
A hydrogen sulfide contamination test was performed to poison the anode with hydrogen sulfide. Specifically, the stoichiometric coefficient (stoichiometry) of hydrogen supplied to the anode is set to 2.2, and the current density is set to a constant value of 2.0 A / cm ² , while hydrogen sulfide is being introduced over 3 hours. , Generated electricity from the fuel cell. Details of the operating conditions of the fuel cell are shown in Table 1 below.

<Potentiometer galvanostat test>
Subsequently, a voltage was applied to the anode by potentiogalvanostat for 10 minutes according to the conditions in Table 2 below (Comparative Example 2, Examples 1 to 4).

The conditions for applying the voltage to the anode are shown in Table 3 below. Here, the applied voltage in Table 3 represents the potential with respect to the standard hydrogen electrode potential.

In Comparative Example 1, only sulfur poisoning by the hydrogen sulfide contamination test was performed, and no voltage was applied to the anode. In Reference Example 1, neither the hydrogen sulfide contamination test nor the application of the voltage to the anode was performed. Reference Example 1 corresponds to a fuel cell that is not poisoned with sulfur.

<Performance evaluation of fuel cell>
Following the above test, the potential of the cathode with respect to the anode at 2.0 A / cm ² was measured to evaluate the power generation performance. FIG. 3 shows the measured values of the cathode potential with respect to the anode in the power generation performance evaluation.

Based on the measurement results shown in FIG. 3, the fuel cell after recovery from poisoning was evaluated. Specifically, the case where the measured potential value of the cathode with respect to the anode at the time of power generation performance evaluation was relatively large as compared with Comparative Example 1 was set as “◯”. The case where the measured potential value of the cathode with respect to the anode at the time of power generation performance evaluation did not change as compared with Comparative Example 1 was designated as “x”. The results are shown in Table 3.

As can be seen in FIGS. 3 and 3, in Examples 1 to 4 in which a voltage of 0.8 V or higher was applied to the anode with reference to the standard hydrogen electrode potential after sulfur poisoning, the performance of the fuel cell was recovered. Was. On the other hand, in Comparative Example 2 in which a voltage of 0.6 V was applied to the anode with reference to the standard hydrogen electrode potential after sulfur poisoning, no recovery in the performance of the fuel cell was observed. Assuming that the standard hydrogen electrode potential at the cathode is 0.6V, the application of voltages of 0.6V, 0.8V, 1.0V, 1.2V and 1.3V to the anode is applied to the cathode by the anode. It corresponds to setting the potentials to 0V, -0.2V, -0.4V, -0.6V, and -0.7V, respectively.

<Evaluation of electrochemically active specific surface area>
In addition to the above performance evaluation, the electrode activity was evaluated. This was done by calculating the electrochemical active specific surface area (ECSA) from the anode CV waveform.

The results are shown in FIG.

FIG. 4 shows the evaluation result of the electrochemical active specific surface area (ECSA). As can be seen in FIG. 4, Examples 1 to 4 in which a voltage of 0.8 V or more was applied to the anode with reference to the standard hydrogen electrode after sulfur poisoning, that is, the potential of the cathode with respect to the anode was set to −0.2 V or less. In Examples 1 to 4, the ECSA value at the anode was relatively high as compared with Comparative Example 1 in which no voltage was applied after sulfur poisoning. On the other hand, in Comparative Example 2 in which a voltage of 0.6 V was applied to the anode with reference to the standard hydrogen electrode after sulfur poisoning, that is, in Comparative Example 2 in which the potential of the cathode with respect to the anode was set to 0 V, it was compared with Comparative Example 1. Therefore, no significant change in the ECSA value at the anode was observed.

From the above results, the recovery effect of sulfur poisoning can be obtained by applying a voltage of 0.8 V or more to the anode with reference to the standard hydrogen electrode, that is, by setting the potential of the cathode with respect to the anode to -0.2 V or less. It turned out to be.

≪Experiment 2: Poison recovery and recovery evaluation≫
The sulfur-poisoned fuel cell was recovered from poisoning according to the method of the present disclosure.

<Hydrogen sulfide contamination test>
First, the hydrogen sulfide contamination test described above was carried out to poison the anode with hydrogen sulfide.

<Recovery test>
Following the hydrogen sulfide contamination test described above, the fuel cell was recovered from poisoning according to the method of the present disclosure. Specifically, the hydrogen stoichiometric (St _H2 ) supplied to the anode is operated during normal operation while maintaining a load of 2.0 A / cm ² while continuing to supply the oxidant gas to the cathode. It was swept from 2.2 of the above at a rate of −0.01 St _H2 / sec. In this experiment, the fuel cell was controlled so that the hydrogen stoichiometric returned to 2.2 when the cathode potential with respect to the anode reached -1.0 V. In this experiment, the period during which the cathode potential with respect to the anode was continuously −0.2 V or less was 43 seconds.

During the hydrogen sulfide contamination test and recovery test described above, the potential of the cathode with respect to the anode was measured and recorded.

The results are shown in FIGS. 5 and 6.

FIG. 5 shows the time course of the potential when the fuel cell is recovered from poisoning according to the method of the present disclosure. The solid line shows the electric potential, and the dotted line shows the stoichiometric coefficient (stoichiometry) of hydrogen.

FIG. 6 is an enlargement of the portion of FIG. 5 from 3.0 hours to 3.5 hours. The time point of 3.0 hours corresponds to the time point when the reduction of hydrogen stoichiometricity at the anode was started.

As can be seen in FIG. 6, the potential of the cathode with respect to the anode changed around 0 V for 0.2 hours after the decrease of hydrogen stoichiometry was started. It is considered that this is because the water electrolysis reaction (2H ₂ O → 4H ⁺ + O ₂ + 4e ⁻ ) is in progress.

A sharp decrease in potential occurred around 0.2 hours (3.2 hours) after the start of the decrease in hydrogen stoichiometricity. After that, when the potential of the cathode with respect to the anode reached −1.0 V, the potential increased relatively slowly as the hydrogen stoichiometric returned to 2.2.

As can be seen in FIGS. 5 and 6, the voltage value after the recovery test was relatively high as compared with the voltage value immediately before reducing the hydrogen stoichiometry. Specifically, the average voltage value in the stable region after the recovery test was 258 mV with respect to the average voltage value of 168 mV immediately before reducing the hydrogen stoichiometric value.

Therefore, the power generation performance of the poisoned fuel cell by reducing the supply of fuel gas to the anode and thereby continuously applying a potential of −0.2 V or less to the anode for a period of less than a predetermined period. Was found to recover.

≪Experiment 3: Evaluation of anode catalyst after recovery from poisoning≫
It was confirmed whether or not the carbon carrier of the anode catalyst was deteriorated in the fuel cell in which the poisoning was recovered in the above experiment 2.

Specifically, the anode CV was measured before the hydrogen sulfide contamination test in Experiment 2 described above and after the recovery test according to the conditions shown in Table 4 below. Then, in order to estimate the double layer capacitance of carbon, the current density difference at 0.4 V of the CV waveform was compared.

As a result, no change was observed in the current density difference before the hydrogen sulfide contamination test and after applying the negative voltage. Therefore, in the above recovery test, it was found that the deterioration of the carbon carrier in the anode catalyst did not occur.

100 Evaluation device 200 Fuel cell cell 20 Anode CV line 22 Fuel gas tank 24 Gas tank for hydrogen sulfide contamination 26 Oxidizing agent gas tank 224,244,264 Mass flow controller 226,266 Humidifier 228,268 Drain water container 230,232 Vent

Claims (1)

A method of recovering sulfur poisoning of an anode catalyst in a fuel cell, which comprises the following steps:
(A) While continuing to supply the oxidant gas to the cathode, the amount of fuel gas supplied to the anode is reduced so that the potential of the cathode with respect to the anode is lowered to -0.2 V or less. To
(B) After the step (a), the supply amount of the fuel gas is restored so that the period during which the potential is continuously −0.2 V or less is the predetermined period or less.

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