Method for activating fuel cell

A method for activating a polymer electrolyte type fuel cell comprising at least one unit cell which is configured by including a proton conductive polymer electrolyte, an electrode layer having a catalytic activity arranged on the both faces of said polymer electrolyte membrane and a gas-supplying path is disclosed. This method comprises at least one of the step (a) of enhancing the catalytic activity of said electrode and the step (b) of giving a wetting condition to said polymer electrolyte. According to the present invention, it is possible to readily activate the fuel cell and to cause the same to demonstrate a high cell performance.

BACKGROUND OF THE INVENTION
 The present invention relates to fuel cells and, in particular, to a method
 for activating solid polymer electrolyte type fuel cells.
 In recent years, with the social trend for the growing concern about the
 environmental protection problems, the development on the solid polymer
 electrolyte type fuel cell (hereinafter, to be briefly referred to as
 "PEFC") is under remarkable progress in the field of the fuel cells.
 Although the fuel cells are in their step of just being to put to
 practical uses, they are still not satisfactory due to many reasons.
 The conventional PEFC is configured by including a proton conductive
 polymer electrolyte membrane, a pair of positive and negative electrodes,
 bipolar plates made of carbon or metal, and cooling plates. Each electrode
 is configured by including a mixture of carbon powder with catalyst powder
 such as Pt. And, if required, a water-repelling agent such as a
 fluorocarbon compound is added to the mixture. The electrode is configured
 by joining on a gas-diffusing layer and the electrode is combined with the
 proton conductive polymer film. In a case of using pure hydrogen as the
 fuel gas, it is possible to use the same material for configuring both the
 positive electrode and the negative electrode.
 (1) Problems due to the fuel.
 Usually, fuel gas or a fuel gas obtained by reforming of methanol or
 methane gas is used for the fuel cells. However, in the particular case of
 PEFC which usually employs a platinum catalyst in the electrode, there is
 a problem of poisoning the platinum catalyst by carbon monoxide in fuel
 gas, thereby decreasing the catalytic activity and inviting a serious
 deterioration in the cell performance.
 In order to avoid this problem, there have been proposed various methods.
 As one of them, there is a hydrogen separating method, whereby the carbon
 monoxide in the fuel gas is removed by the use of a Pd thin membrane in
 advance of the introduction of the fuel gas into the PEFC. In this method,
 hydrogen is selectively caused to pass through a Pd membrane selectively
 by applying a certain pressure at one side of the thin membrane. This
 method is already used in a plant for manufacturing semiconductor devices
 or the like, and is under development also for the PEFC.
 As another method for decreasing the CO concentration of the fuel gas, a
 so-called CO-denaturing method is proposed. In this method, after
 reforming methanol or methane gas, the CO was removed from the reformed
 gas by the use of a CO-denaturing catalyst (CO+H.sub.2 O.fwdarw.CO.sub.2
 +H.sub.2). By this method, it is usually possible to decrease the CO
 concentration of the reformed gas down to 0.4 to 1.4%. If the CO
 concentration can be decreased to this extent, the reformed gas can be
 used for a phosphoric acid type-fuel cell which also employs the same Pt
 electrode catalyst. However, in order to prevent the possible poisoning of
 the platinum catalyst in PEFC, the CO concentration should be decreased
 down to a level of at least several ppm, and thus the above-mentioned
 CO-denaturing method is still not satisfactory.
 Under the stated circumstances, another method is proposed for further
 removing the CO in the CO-denatured gas by further introducing oxygen
 (air) into the CO-denatured gas, thereby oxidizing the CO by the use of an
 oxidizing catalyst at 200 to 300.degree. C. For the oxidizing catalyst
 used in this method, an alumina catalyst which carries a noble metal is
 proposed, for example. It is however very difficult to selectively and
 completely oxidize the CO in the hydrogen.
 Further, although various investigations of an alloy catalyst are conducted
 to have higher resistance against the poisoning by CO, the performance of
 such an electrode catalyst is not satisfactory and, thus, it is difficult
 to develop an electrode catalyst which does not completely adsorb CO.
 Moreover even if, the CO-oxidizing method and the method of mixing air with
 the fuel gas are employed, it is difficult to sufficiently decrease the CO
 concentration down to the extent for the PEFC. There is a hazard of
 including a large amount of CO at a start-up stage of the fuel cell. Thus,
 there is a need for a long time period in order to be stabilized before
 introducing the fuel gas, or a need for separately providing a hydrogen
 reservoir (bomb) solely for the start-up. In addition, the CO is gradually
 accumulated in the fuel electrode even under normal operating conditions
 and the cell performance is gradually deteriorated. Once the cell
 performance has been deteriorated, it cannot be recovered automatically,
 and there is a need for removing the accumulated CO by oxidizing the CO by
 temporarily suspending the operation of the fuel cell and introducing a
 large amount of air, or replacing the whole electrode assembly with fresh
 one.
 (2) Problems due to the water-repelling property of the polymer electrolyte
 membrane
 Incidentally, a compound having a main chain (repetitive unit) of
 --CF.sub.3 -- and a side chain containing sulfonic groups (--SO.sub.3 H)
 at the end functional group is generally used as the proton conductive
 polymer electrolyte. This type of electrolyte has a proton conductivity
 with water, which must be supplied from the outside. For that reason, the
 electrolyte must be constantly attached (in contact) with water under the
 operating condition of the cell. But the electrolyte has a strong acidity
 with containing water. Material of any parts and components of the cell,
 which are in direct contact with the electrolyte, should therefore have
 acid resistance.
 Since the polymer electrolyte needs water, in a case of operating the PEFC,
 it is required to humidify the fuel and the air to a dew point before they
 are supplied to the cell. In particular, the higher the operating
 temperature of the cell is, the more important becomes the humidity
 control on the supplying gases.
 In a case of loading a PEFC to operate just after the assembly, or in
 another case of loading the PEFC again which had been standing still in
 non-operated state for a long period, it is generally difficult to
 immediately obtain sufficient performance. The cause for this phenomenon
 is due to the fact that a long time is required for hydrating an electrode
 diffusing layer completely, because the electrode diffusing layer of the
 PEFC has been treated for water-repelling.
 In addition, a long time is also required for sufficiently wetting the
 material, which is the same case of polymer electrolyte, contained in the
 electrode catalyst. Further, even if the cell is kept to a moderate
 temperature and the gases, which are adjusted to a moderate temperature
 and humidity, the electrode diffusing layer is hard to hydrate when the
 cell is left in the no-loaded state. Moreover, the material contained in
 the electrode catalyst is hard to humidify and, thus, it becomes hardly
 possible to derive the sufficient cell output unless the cell is
 continuously subjected to generate the electricity generation at a high
 current density for several days.
 For the reasons mentioned above, in order to derive the cell with the high
 performance at an early stage, an activating treatment was conventionally
 practiced by, for instance, generating electricity at a higher current
 density with a pure oxygen, or by maintaining the cell voltage at about 0
 V by regulating the potential while supplying a large amount of the fuel
 gases. Even with these methods, there is still a problem to be solved that
 a time period of more than several hours is required to derive the cell
 with the high performance.
 It is therefore the primary object of the present invention to solve the
 above-mentioned problems (1) and (2). More specific objects of the present
 invention are to provide an easy and highly effective method for
 activating the fuel cell by preventing the deterioration in the cell
 performance due to CO poisoning or restoring the deteriorated cell
 performance, and a method for activating the fuel cell by reducing the
 delay in demonstrating the cell performance due to the water-repelling
 property of the polymer electrolyte membrane.
 SUMMARY OF THE INVENTION
 In order to solve the above-mentioned problems, the present invention
 provides a method for activating a polymer electrolyte type fuel cell
 comprising at least one unit cell which is configured by including a
 proton conductive polymer electrolyte, an electrode layer having a
 catalytic activity arranged on the both faces of said polymer electrolyte
 membrane and a gas-supplying path; comprising at least one of the step (a)
 of enhancing the catalytic activity of said electrode and the step (b) of
 giving a wetting condition to said polymer electrolyte.
 In the present invention, the catalytic activity of said electrode is
 preferably enhanced by compulsively decreasing the output voltage of said
 polymer electrolyte type fuel cell in said step (a).
 In concrete, the output voltage is preferably decreased down to 0 to 0.3 V
 per unit cell.
 In this case, the output voltage of the unit cell may be intermittently
 decreased in succession. In other words, the output voltage may be
 decreased stepwise with some intervals.
 Further, a wetting condition to said polymer electrolyte is preferably
 given by immersing and boiling said polymer electorolyte type fuel cell in
 a deionized water or a weakly acidic aqueous solution and boiling in said
 step (b).
 A wetting condition to said polymer electrolyte may be given by introducing
 a deionized water or a weakly acidic aqueous solution having a temperature
 higher than an operating temperature of said polymer electrolyte type fuel
 cell into the gas-supplying path in said step (b).
 It is preferable that an alcohol be introduced into a gas-supplying path
 before a deionized water or a weakly acidic aqueous solution in said step
 (b).
 And, it is preferable that said weakly acidic aqueous solution is hydrogen
 peroxide water.
 Further, it is preferable that an ion-exchange group in said polymer
 electrolyte is --SO.sub.3 H and said weakly acidic aqueous solution is a
 diluted sulfuric acid aqueous solution.
 It is noted that, in the present invention, the structure of the
 above-mentioned fuel cell is not specifically limited to those disclosed.
 In this process, it is preferable to set the pressure of the deionized
 water or the weakly acidic aqueous solution to be introduced into the
 gas-supplying path to not less than 0.1 kgf/cm.sup.2.
 In another preferred embodiment, the above-mentioned step (a) is a step of
 performing electricity generation at an oxygen utilization rate of not
 less than 50% and, then, applying a voltage to the fuel cell so as to make
 the average cell voltage not more than 0.3 V per unit cell.

DETAILED DESCRIPTION OF THE INVENTION
 The method for activating the PEFC in accordance with the present invention
 is characterized by including the step (a) of enhancing the catalytic
 activity of the electrode which constitutes the PEFC, and the step (b) of
 giving a wetting condition to the polymer electrolyte.
 The above-mentioned step (a) is provided for solving the above-mentioned
 problems (1), and the above-mentioned step (b) is provided for solving the
 above-mentioned problems (2).
 In the operation of the PEFC for generating DC power, by introducing an
 oxidant gas and a fuel gas obtained by reforming a raw material gas of a
 hydrocarbon into the cell, the above-mentioned step (a) restores the
 catalytic activity of the electrode and maintains the output
 characteristic of the cell by compulsively decreasing the output voltage
 of the cell at an appropriate time to remove carbon monoxide adsorbed in
 the catalytically reactive portion of the electrode. If this step is
 adopted, it is possible to omit a hydrogen treatment which is required in
 the start-up stage of the cell. In addition, even if the performance of
 the fuel cell has once been deteriorated by a possible poisoning with CO,
 it is possible to easily remove the CO adsorbed on the catalyst and to
 restore the initial cell performance.
 Further, the above-mentioned step (b) makes it possible to simply derive
 the sufficient cell output for the high performance, which is inherently
 held by the cell in a short period of time, by boiling the PEFC in a
 deionized water or in a weakly acidic aqueous solution. In this step, by
 boiling in the weakly acidic aqueous solution, impurity ions contained in
 the electrolyte membrane or in the material equivalent to the polymer
 electrolyte in the electrode catalytic layer are exchanged with protons
 and, thus, it is possible to derive a higher performance of the PEFC.
 However, it seems rather difficult to boil the PEFC having a large area and
 a high lamination from the viewpoints of the capacity of the container to
 be employed and of the handling in the process. For that reason, by
 introducing the deionized water or the weakly acidic aqueous solution
 having a higher temperature than a previously determined cell operating
 temperature into the gas-supplying path of the PEFC, instead of using the
 container for the boiling process, it is possible to simply derive the
 cell output for the high performance inherently held by the cell in a
 short time period. Further preferably, by setting the pressure of the
 water or solution to be introduced to not less than 0.1 kgf/cm.sup.2, it
 is possible to derive the cell output sufficient for the high performance
 in a shorter time period.
 Alternatively, by introducing an alcohol, which has some affinity to the
 carbon materials at the diffusing layer of the electrode, into the
 gas-supplying path of the PEFC, it is also possible to instantly replace
 alcohol by water at the diffusing layer. Thereafter, by washing the PEFC
 with the deionized water or the weakly acid aqueous solution, it is
 possible to simply hydrate the electrode diffusing layer in a short time
 period, thereby deriving the cell output sufficient for the high
 performance which is inherently held by the cell.
 During this procedure, when a residual alcohol is present at the side of
 the fuel electrode, the alcohol is oxidized by the electrode catalyst to
 produce a substance, which may poison the electrode. In order to derive
 the cell output sufficient for the high performance which is inherently
 held by the cell, it is important to thoroughly hydrate the electrode
 diffusing layer at the positive electrode side rather than at the fuel
 electrode side. For that reason, a satisfactory effect can be obtained by
 supplying the alcohol only to the positive electrode side, to which the
 air is supplied. In addition, it is further desirable to supply an oxidant
 gas to the fuel electrode side after the activating treatment for some
 period of time so as to further oxidize and remove the electrode poisoning
 substance and, thereafter, to supply the fuel gas.
 Further, in a case of using a compound having a main chain (repetitive
 unit) of --CF.sub.3 -- and side chain containing a sulfonic group
 (--SO.sub.3 H) as the end functional group for the polymer electrolyte, it
 is desirable to employ an aqueous solution of diluted sulfuric acid as the
 weakly acidic aqueous solution for activating the compound. The reason for
 this is due to the fact that any sulfuric acid ion may not remain even if
 the diluted sulfuric acid is introduced into the polymer electrolyte,
 because the ion exchanging group of the polymer electrolyte is --SO.sub.3
 H. As such a compound, the compound represented by the formula:
 ##STR1##
 my be exemplified preferably.
 In addition, there is a need for removing any metal ion from the deionized
 water or the weakly acidic aqueous solution to be introduced into the PEFC
 at the activating step. The reason for this is due to the fact that if a
 metal ion exists in the water or solution, the ion exchanging group in the
 polymer electrolyte, for instance, --SO.sub.3 H group combines with the
 metal ion to produce --SO.sub.3 Me (Me is a metal element), thereby to
 losing the ion exchanging ability. In order to prevent this adverse
 phenomenon, it is particularly useful to use hydrogen peroxide water which
 consists of pure water and hydrogen as the weakly acidic aqueous solution.
 Separate from this, by causing the PEFC to generate electricity at an
 oxygen utilization rate of not less than 50%, in other words by making the
 positive electrode side of the cell a half-suffocated state, and by
 keeping the average cell voltage to not higher than 0.3 V thereby evolving
 the required water vapor from the cell, it is possible to simply derive
 the cell output sufficient for the high performance which is inherently
 held by the cell in a short time period.
 In the following paragraphs, a description will be made of the method for
 activating the fuel cell in accordance with the present invention by way
 of example, but the present invention should not be construed to be
 limited to these examples.
 EXAMPLES
 Examples 1 Through 3
 First, a PEFC to be used in this example was configured in the following
 manner. By causing a carbon powder of acetylene black to carry platinum
 particles having an average particle diameter of about 30 .ANG. at 25 wt
 %, the catalyst for the reacting electrode was prepared. By mixing a
 dispersion prepared by dispersing the catalyst powder in isopropanol and a
 dispersion prepared by dispersing a powder of perfluorocarbon sulfonic
 acid in ethanol, a paste like product was obtained. On one face of a
 non-woven fabric, which was made of carbon and had a thickness of 250
 .mu.m, a catalyst-reacting layer was formed by painting the paste by means
 of screen printing process. During this process, the amount of the
 platinum contained in the formed reacting electrode was adjusted to 0.5
 mg/cm.sup.2, and the amount of the perfluorocarbon sulfonic acid was
 adjusted to 1.2 mg/cm.sup.2.
 The same configuration was adopted to both the positive electrode and the
 negative electrode, and the electrode plate was formed to have a one-size
 larger area than that of the electrodes. Next, as a proton-conductive
 polymer electrolyte, a thin film of perfluorocarbon sulfonic acid having a
 thickness of 25 .mu.m was used. And, the electrode plates were joined with
 the electrolyte film by means of hot-pressing process so as to make the
 printed catalyst layers in close contact with the electrolyte film at
 their central portions, thereby producing an
 electrode/electrolyte/electrode (MEA, Membrane Electrode Assembly).
 Next, a gasket-like sealing, which has ports for the electrode and the
 gas-manifold, was prepared by molding into a plate-shape. A portion of the
 electrolyte film around the peripheral of the MEA was sandwiched between
 the two gasket-like sealings so that the reacting electrode portion of the
 MEA was fitted to the port for the reacting electrode, which was
 positioned in the center of the gasket-like sealing. Further, the MEA and
 the gasket-like sealing were sandwiched between two bipolar plates in an
 arrangement wherein the gas-flowing path in one bipolar plate confronted
 with that of the other bipolar plate, thereby configuring a unit cell of
 polymer electrolyte type fuel cell(PEFC). The gasket-like sealing in the
 form of a molded sheet was made by punching a sheet of butyl rubber with a
 thickness of 250 .mu.m to have the required ports.
 By providing a heater in the form of plate having the ports for the
 gas-manifold, a current collector, an insulating plate, and end plates on
 both the outer sides of this MEA and, then, by fastening the cell while
 applying a pressure of 20 kg/cm.sup.2 for the unit area of the electrode
 between the outermost end plates with bolts, springs and nuts, a unit cell
 of the PEFC was configured. By laminating 50 pieces of this unit cells, a
 PEFC module was produced.
 While maintaining the module thus produced at 75.degree. C., hydrogen gas
 heated and humidified so as to have a dew point of 73.degree. C. was
 supplied to one of the electrode sides and air heated and humidified so as
 to have a dew point of 68.degree. C. was supplied to the other of the
 electrode sides. As the result, an open circuit voltage of 0.98 V was
 obtained.
 FIG. 1 schematically shows a structure of a fuel cell system used in this
 example. In FIG. 1, a methane gas was introduced into the reforming unit 1
 at a S/C (steam/carbon) ratio of 3 to reform with steam and, then, the CO
 in the reformed gas was denatured. The reformed gas exhausted from the
 reforming unit 1 was then introduced into a CO-oxidizing and removing unit
 2 so as to make the O.sub.2 /CO ratio in the gas 1, and finally introduced
 into the cell module 3.
 During an operation in the steady state, the CO concentration of the fuel
 gas to be introduced into the cell module was not more than 100 ppm. At
 the start-up stage, the CO concentration of the fuel gas, however, was not
 less than 1%. The operating temperature of the PEFC was set to 80.degree.
 C. And, when the gas was humidified, the temperature of the fuel gas was
 set to 75.degree. C. and the temperature of the oxidant gas (air) was set
 to 65.degree. C.
 First, an evaluation was conducted on the characteristic of the fuel cell
 in a case of introducing the fuel gas into the PEFC immediately after the
 starting-up without subjecting the fuel gas to any treatment. FIG. 2 shows
 the current-voltage characteristic obtained in the case of the
 above-evaluation in comparison with that obtained in another case of
 introducing pure hydrogen. Based on this evaluation, it was found that the
 cell characteristic is remarkably deteriorated when a fuel gas in the
 untreated state was introduced into the PEFC immediately after the
 starting-up.
 Subsequently, an evaluation was conducted on the activating method in
 accordance with the present invention (Example 1). That is, the cell
 performance was examined in the state where the closed-circuit voltage of
 the PEFC at the start-up stage was decreased to 0.2 V for 2 seconds by
 connecting an appropriate resistance between the output terminals of the
 cell. FIG. 3 shows the current-voltage characteristic obtained in the case
 of the above examination in comparison with that obtained in the case of
 no treatment. Based on the evaluation, it was found that the cell
 characteristic was improved by temporarily decreasing (short-cutting) the
 cell voltage.
 This phenomenon is caused by the fact that the electrode potential in the
 MEA is decreased down to an oxidation potential of CO by compulsively
 decreasing the output voltage of the PEFC, thereby oxidizing the CO
 adsorbed on the Pt catalyst.
 Next, an investigation was conducted on the cell performance by varying the
 duration of the time period to decreases the cell voltage of the PEFC
 (Example 2). Table 1 below shows the relationship between the duration and
 the cell voltage. The duration was the period of time that the output
 voltage of the fuel cell was maintained to 10 V, i.e., 0.2 V per unit cell
 after the starting-up of the fuel cell. And, the cell voltage was the
 closed-circuit voltage of the fuel cell at the time when the output
 current was set to 500 mA/cm.sup.2 after the compulsive decreasing. From
 Table 1, it is appreciated that when the duration for compulsively
 decreasing the cell voltage is not more than 10 seconds, the cell
 performance is improved. Whereas, if the duration is made longer than
 this, the cell performance is deteriorated in contrast.
 TABLE 1
 VOLTAGE DECREASING CELL VOLTAGE
 DURATION (sec) (V)
 PURE HYDROGEN 33
 0.01 29
 0.5 30
 1 31
 5 32
 10 33
 15 31
 20 29
 Next, an investigation was conducted on the cell performance obtained by
 varying the duration of time when the output voltage of the PEFC was
 compulsively decreased (Example 3). Table 2. below shows the relation
 between the decreased voltages of the fuel cell obtained by fixing the
 duration of compulsively decreasing the cell voltage to 5 seconds, and the
 closed-circuit voltage of the fuel cell when the output current is set to
 200 mA/cm.sup.2, obtained after subjecting the cell to this step. When a
 range of the compulsively decreased voltage was 0 to 0.3 V per unit cell,
 the cell demonstrated almost equivalent performance to the case of using
 pure hydrogen. But a restoring rate of the cell voltage was small outside
 the above range and the voltage become not less than 5 V lower than that
 in the case of using pure hydrogen.
 TABLE 2
 VOLTAGE WHEN DECREASED CELL VOLTAGE
 COMPULSIVELY (V) (V)
 PURE HYDROGEN 36
 40 2.5
 30 10
 20 30
 15 34
 10 34
 5 35
 2.5 35
 0 35
 Based on the above evaluations, it is found that there are effective ranges
 in the duration and the voltage for compulsively decreasing the output
 voltage of the PEFC, which may be selected or determined by considering
 the configuration of the cell. It is believed that if the duration for
 compulsively decreasing the cell voltage is unnecessarily elongated, the
 closed-circuit voltages of some cells in the serially-connected 50 unit
 cells might become less than 0 V and minus volt, i.e., a polarity-inverted
 state. As a means for avoiding the occurrence of such a state, it is
 effective to adopt a method of compulsively decreasing the output voltage
 of the fuel cell module as a whole while monitoring the voltages of the
 respective unit cells within a range where no polarity-inverted cell is
 produced.
 Example 4
 In this example, the PEFC produced in the same manner as in Example 1 was
 used. At the start-up stage, the cell stood still for not less than 1 hour
 before introducing the fuel gas into the fuel cell. And, the gases were
 introduced after the CO concentration of the fuel gas became less than 100
 ppm. Then, the PEFC was discharged continuously at 500 mA/cm.sup.2 for
 1,000 hours. FIG. 4 shows the relation between the voltage and the
 duration of the continuous discharging.
 From FIG. 4, it is appreciated that the voltage was decreased by 10% as
 compared with that at the initial period. Therefore, an attempt was made
 at this time point for decreasing the cell voltage temporarily. After
 decreasing the output voltage of the PEFC from 47 V to 5 V for 5 seconds,
 the discharging at 500 mA/cm.sup.2 was performed again. As the result, it
 was appreciated that the cell voltage was greatly improved as compared
 with that immediately before this treatment.
 Next, an investigation was conducted on the characteristic of the PEFC
 after the treatment by changing the number of the treatment. Table 3 shows
 the cell voltage after each number of the treatment. From this result, it
 is appreciated that the larger the number of the temporal decreasing of
 the cell voltage is, the higher the effect thereof becomes.
 TABLE 3
 TREATMENT NUMBER CELL VOLTAGE
 (times) (V)
 1 32
 2 32
 5 33
 7 33
 10 33
 15 34
 20 34
 Based on the results, it is found that by temporarily and compulsively
 decreasing the cell output voltage of the PEFC, it is possible to solve
 the problem of poisoning the electrode catalyst by CO at the start-up
 stage of the PEFC which had conventionally been left to be solved, and to
 use the fuel gas as it is.
 In addition, it is also found that in a case where the performance of the
 PEFC in the steady state operation is deteriorated by CO poisoning, it is
 possible to restore the cell performance up to the performance almost
 equal to that at the initial stage by temporarily and compulsively
 decreasing the cell output voltage.
 Furthermore, it was also found that in a case wherein the duration for
 temporarily decreasing the cell output voltage was not more than 10
 seconds or the number is not less than 2, or the voltage for the temporal
 decrease was in a range of 0 to 0.3 V for a unit cell, the effect of the
 treatment was obtained. In the foregoing examples, a fuel gas obtained by
 reforming a methane gas was used, but the present invention is not
 specifically limited thereto. In addition, any catalyst other than those
 disclosed such as an alloy catalyst or the like can also be used as the
 electrode catalyst for the PEFC of the present invention.
 Example 5 and Comparative Example 1
 A unit cell of a PEFC was configured as in the same manner as in Example 1.
 The obtained unit cell was boiled in a ion-exchanged and distilled water
 for one hour.
 Thereafter, while keeping the unit cell (PEFC) at 75.degree. C., hydrogen
 gas, which was humidified and heated so as to have a dew point of
 73.degree. C., was supplied to one of the electrode sides and air, which
 was humidified and heated so as to have a dew point of 68.degree. C. was
 supplied to the other of the electrode sides. A non-loaded cell voltage of
 0.98 V was obtained with the unit cell in this state. In addition, when a
 continuous electricity generating test was conducted on this unit cell
 under the conditions of a fuel utilization rate of 80%, an oxygen
 utilization rate of 40% and a current density of 0.3 A/cm.sup.2, a cell
 voltage of not less than 0.7 V was obtained immediately after the start of
 the test. Moreover, the cell was able to generate electricity for further
 5,000 hours or longer while maintaining the cell voltage of not less than
 0.7 V without any deterioration in the cell voltage.
 For comparison, another PEFC of the same configuration was produced without
 subjecting the same to the activation treatment, i.e., without being
 boiled in the deionized and distilled water, and an electricity generating
 test was conducted on the cell under the same conditions. As the result,
 it was found that a non-loaded cell voltage of only 0.93 V was obtained.
 In addition, this unit cell did not operate in the initial stage under the
 conditions of a fuel utilization rate of 80%, an oxygen utilization rate
 of 40% and a current density of 0.3 A/cm.sup.2, and when the unit cell was
 compulsively loaded, the output cell voltage of this cell was decreased
 down to 0 V or lower. Therefore, an electricity generating test was
 conducted on this unit cell under the conditions of a fuel utilization
 rate of 70%, an oxygen utilization rate of 20% and a current density of
 0.1 A/cm.sup.2, and after recognizing a gradual improvement in the cell
 performance, the load was increased up to 0.7 A/cm.sup.2 stepwise. The
 above-mentioned procedure was repeated three times and the gas utilization
 rate and the like was turned to the initial conditions. Thereafter, it
 took about three days to obtain the cell voltage of above 0.7 V at a load
 of 0.3 A/cm.sup.2.
 Although the case of boiling the cell in a deionized and distilled water
 was disclosed in this example, a similar technical advantage was obtained
 with a cell, which was stored in a hydrogen peroxide water having a pH of
 5 for 2 hours.
 Example 6
 By closely stacking and serially connecting 100 unit cells of the PEFC, a
 laminated cell was produced. Each unit cell was produced in the same
 manner as in Example 5. Then, current collectors and insulating plates,
 each provided with ports required for the gas manifolds and the cooling
 water manifolds, respectively, as well as end plates were provided on the
 both outside ends of the obtained laminated cell for combining the
 laminated unit cells together. And, then, the combined body was fastened
 between both the outermost end plates by the uses of bolts, springs and
 nuts, at a pressure of 20 kg/cm.sup.2 for the unit area of the electrode
 to obtain a PEFC module.
 An aqueous solution of sulfuric acid of 0.01 N and 95.degree. C. was
 introduced into the module through both the gas inlet openings at the
 positive electrode side and the negative electrode side for 30 minutes. At
 that time, by narrowing an exhausting outlet, the pressure of the
 introducing aqueous solution was adjused to 0.1 kgf/cm.sup.2.
 Thereafter, while maintaining the temperature of the PEFC module at
 75.degree. C. by circulating cooling water, hydrogen gas, which was
 humidified and heated to have a dew point of 73.degree. C., was supplied
 to one of the electrode sides and an air, which was humidified and heated
 to have a dew point of 68.degree. C., was supplied to the other of the
 electrode sides. Thereby, the open-circuit voltage of 0.98 V was obtained.
 In addition, as the result of conducting a continuous electricity
 generating test on this cell module under the conditions of a fuel
 utilization rate of 80%, an oxygen utilization rate of 40% and a current
 density of 0.3 A/cm.sup.2, a cell voltage of above 0.7 V was obtained
 immediately after the start of the test. Moreover, the cell was able to
 generate electricity for further 5,000 hours or longer while maintaining
 the cell voltage of above 0.7 V without any deterioration in the cell
 voltage.
 Although the case of activating the cell by introducing the sulfuric acid
 aqueous solution of 0.01 N and 95.degree. C. into both the gas inlet
 openings at the positive electrode side and the negative electrode side of
 the unit cell for 30 minutes is disclosed in this example, a similar
 technical advantage was obtained with a cell, which was activated by
 introducing a hydrogen peroxide water having a pH of 5 and a temperature
 of 90.degree. C. for 1 hour. In addition, a similar technical advantage
 was also obtained with a cell, which was activated by introducing a
 deionized water of 95.degree. C. for 3 hours.
 Example 7
 After supplying about 100 cc of methanol to the unit cell of the PEFC,
 which was produced in the same manner as in Example 5 through the gas
 supplying inlet, the unit cell was washed by supplying ion-exchanged and
 distilled water. Thereafter, the PEFC was kept at a temperature of
 75.degree. C., and an air, which was humidified and heated to have a dew
 point of 70.degree. C., was supplied to both the electrode sides for 1
 hour, and the gas at the fuel electrode side was replaced with nitrogen.
 Subsequently, a hydrogen gas, which was humidified and heated to have a
 dew point of 73.degree. C., was supplied to the fuel electrode side and an
 air, which was humidified and heated to have a dew point of 68.degree. C.,
 was supplied to the air electrode side. Thereby, the cell voltage of 0.98
 V was obtained in the non-loaded state. In addition, as the result of
 conducting a continuous electricity generating test on this cell under the
 conditions of a fuel utilization rate of 80%, an oxygen utilization rate
 of 40% and a current density of 0.3 A/cm.sup.2, a cell voltage of above
 0.7 V was obtained immediately after the start of the test. Moreover, the
 cell was able to generate electricity for further 5,000 hours or longer
 while maintaining the cell voltage of not less than 0.7 V without any
 deterioration in the cell voltage.
 In this example, although the cell was activated by supplying the
 ion-exchanged and distilled water, after the supply of methanol in this
 example, a similar technical advantage was obtained with a cell, which was
 activated by introducing a hydrogen peroxide water having a pH of 5 for 1
 hour. In addition, a similar technical advantage was also obtained by
 using an aqueous solution of sulfuric acid having a pH of 5.
 Example 8
 The unit cell of the PEFC, which was produced in the same manner as in
 Example 5, was heated up to 75.degree. C. without subjecting the same to
 any activating treatment. And, hydrogen, which was humidified and heated
 to have a dew point of 73.degree. C., was supplied to the fuel electrode
 side and an air, which was humidified and heated to have a dew point of
 68.degree. C., was supplied to the air electrode side. Thereby, the
 open-circuit voltage of 0.93 V was obtained. Next, this cell was
 maintained to generate electricity at low potential of the cell voltage of
 0.1 V for one hour, while adjusting the gas flow rate so as to have the
 fuel utilization 90% and the oxygen utilization rate 60%.
 Thereafter, as the result of conducting a continuous electricity generating
 test on this cell under the conditions of a fuel utilization rate of 90%,
 an oxygen utilization rate of 60% and a constant current density of 0.3
 A/cm.sup.2, a cell voltage of not less than 0.7 V was obtained immediately
 after the start of the test. Moreover, the cell was able to generate
 electricity for further 5,000 hours or longer while maintaining the cell
 voltage of above 0.7 V without any deterioration in the cell voltage.
 In this example, although the voltage applied to the cell for activation
 was set to 0.1 V for a unit cell, the technical advantage was remarkably
 deteriorated at a voltage higher than 0.3 V. In addition, if the cell
 voltage was lowered to below 0 V and the lowered voltage was applied for a
 long time period, the output characteristic of the cell was deteriorated.
 This is believed to be due to the fact that if the applied voltage is
 lowered to less than 0 V, a so-called polarity-inverted phenomenon of the
 cell occurs to break a part of the cell reaction component.
 INDUSTRIAL APPLICABILITY
 As clearly disclosed in the foregoing examples, according to the present
 invention, it is possible to introduce the fuel gas into the cell at the
 start-up stage as it is. In addition, even if the performance of the fuel
 cell was once deteriorated by CO, it is possible to remove the CO adsorbed
 on the fuel electrode with ease and to restore the cell performance, by
 temporarily decreasing the cell voltage.
 Further, according to the present invention, it is possible to simply
 derive the cell output at the high performance which has inherently been
 held by the PEFC in a short time, by boiling the PEFC in a deionized water
 or in a weakly acidic aqueous solution. In addition, it is also possible
 to simply derive the cell output at its high performance which has
 inherently been held by the PEFC in a short time, by introducing a
 deionized water or a weakly acidic aqueous solution at a temperature
 higher than the previously determined cell operating temperature into the
 gas supply paths of the PEFC. Further preferably, by pressurizing the
 water head at that time up to not less than 0.1 kgf/cm.sup.2, it is
 possible to derive the cell output at its high performance in a shorter
 time period.
 Moreover, after introducing an alcohol into the gas supply paths of the
 PEFC, by washing the gas supply paths with a deionized water or a weakly
 acidic aqueous solution, it is also possible to simply derive the cell
 output at its high performance which has inherently been held by the cell
 in a short time.
 In addition, by operating the PEFC for generating electricity at an oxygen
 utilization rate of above 50% and keeping it to a state of an average cell
 voltage of below 0.3 V per unit cell for more than 10 seconds, it is also
 possible to simply derive the cell output at its high performance which
 has inherently been held by the cell in a short time.
 It is understood that various other modifications will be apparent to and
 can be readily made by those skilled in the art without departing from the
 scope and spirit of this invention. Accordingly, it is not intended that
 the scope of the claims appended hereto be limited to the description as
 set forth herein, but rather that the claims be construed as encompassing
 all the features of patentable novelty that reside in the present
 invention, including all features that would be treated as equivalents
 thereof by those skilled in the art to which this invention pertains.