DE102020006961A1 - Method for operating at least one fuel cell - Google Patents

Abstract

The invention relates to a method for operating at least one fuel cell (6) with a solid electrolyte membrane (9), a cathode (8) and an anode (7), the anode (7) having at least one OER catalyst, the fuel cell (6 ) is monitored for possible polarity reversal. The method according to the invention is characterized in that after a polarity reversal is detected, a process for regenerating the OER catalytic converter is carried out. A fuel cell system with a controller (16) for carrying out the method is also described. A vehicle (1) with such a fuel cell system is also described.

Description

The invention relates to a method for operating at least one fuel cell according to the type defined in more detail in the preamble of claim 1. The invention also relates to a fuel cell system with such a fuel cell and a vehicle with such a fuel cell system.

Fuel cell systems, which have fuel cells, typically in the form of a stack of individual cells, are known from the prior art. Such fuel cell systems can be used for a variety of power supply purposes. In particular, they can be used to supply power to a vehicle, ie as a hydrogen-electric drive system for a vehicle. In such applications in particular, so-called PEM fuel cells are typically used, which have a solid electrolyte membrane between a cathode and an anode, with the cathode being supplied with air and the anode with hydrogen, for example from a compressed gas store, a cryogenic store or the like.

From the U.S. 2017/0309924 A1 it is known to use so-called OER catalysts (oxygen evolution reaction) in fuel cells. These OER catalysts mostly consist of iridium, ruthenium and/or their oxides. In the ideal case, the OER catalyst has no negative effect on the regular operation of the fuel cell and the catalytic activity of the primary catalysts required for the oxidation of the hydrogen, such as platinum in particular.

The primary mechanism of damage to the fuel cell, which can be cushioned via the OER catalyst, is damage to the individual cells as a result of polarity reversal. This type of damage, also known as cell reversal, always occurs when an individual cell is undersupplied with hydrogen. In the practical use of the fuel cell in a large fuel cell stack with, for example, 300 or 400 individual cells, this cannot always be avoided. Such a polarity reversal of one or more individual cells can occur, for example, when the hydrogen supply is correspondingly impaired. This means that individual cells are not or not sufficiently supplied with hydrogen, for example as a result of product water produced during operation or as a result of ice formation, in particular when the fuel cell system is started. They then change polarity due to the neighboring cells, resulting in oxidation of the carbonaceous support of the platinum catalyst as the primary catalyst. By using the OER catalyst used, this can be cushioned to a certain extent and the resulting damage processes can be contained. As soon as the problem underlying the lack of fuel has been rectified, i.e. that ice has thawed or the water has been blown out accordingly, the cell can then work normally again. Otherwise, suitable countermeasures must be taken, up to and including shutting down the entire fuel cell stack.

It is also known to those skilled in the art of fuel cell systems that when such a fuel cell system is started, particularly after a lengthy downtime, when the residual hydrogen remaining in the anode has diffused out, a start with air or oxygen takes place both in the anode and in the cathode. By starting the hydrogen metering, this air in the anode is displaced by a hydrogen front during the start-up of the fuel cell and, as it were, pushed out of the anode. This ensures that a hydrogen/oxygen front runs across the anode. This leads to extreme potential differences in the area of this hydrogen/oxygen front, since the fuel cell is not yet electrochemically active in front of the front and the full operating voltage is already present immediately after the front. This can pose a significant problem for the service life of the fuel cell, since the primary catalysts required in the anode, such as platinum, are usually provided on carbon-containing carriers, with these carriers oxidizing through the hydrogen/oxygen front, so that the catalyst is also discharged in addition to the carrier material, and the performance of the fuel cell, and in particular its anode, decreases accordingly over the service life and the increasing number of air/air starts, i.e. starts with air or oxygen both in the anode and in the cathode.

Numerous methods are therefore known for counteracting the disadvantageous effect of air/air starts by complying with special procedures when the fuel cell system is at a standstill in order to achieve the longest possible hydrogen protection time (hydrogen protection time). One strategy can be cyclical replenishment of hydrogen during standstill. If, after a certain period of time, further dosing of hydrogen is not advisable or not desired, then an air/air start must be consciously accepted. In this context, describes the DE 10 2019 001 388 A1 a method to actively cancel the hydrogen protection time. The harmful effects of the air/air start provoked in this way are minimized by prior cooling of the fuel cell.

The object of the present invention consists in specifying an improved method for operating at least one fuel cell with an OER catalyst.

According to the invention, this object is achieved by a method having the features in claim 1, and here in particular in the characterizing part of claim 1. Advantageous refinements and developments of the method according to the invention result from the dependent subclaims. In addition, a fuel cell system with the features in claim 8 solves the problem. Here, too, a particularly favorable configuration of the fuel cell system results from the dependent claim. Ultimately, a vehicle with such a fuel cell system can also solve the task.

The inventors have now extensively examined the behavior of PEM fuel cells in the event of cell polarity reversal, ie cell reversal. In the case of fuel cells with an OER catalyst as a secondary catalyst in the anode, it has been noticed that short-term polarity reversal events are relatively uncritical and can be largely cushioned by the OER catalyst without damaging the fuel cell. However, long-term polarity reversals have led to the fact that the problem cannot be compensated for by the OER catalytic converter or can no longer be compensated so well over time. This appears to be due to increasing "poisoning" of the OER catalyst during reverse polarity operation. Presumably, carbon or carbon compounds accumulate here on the OER catalyst, which result from the parallel oxidation of the carbon-containing support of the primary catalyst, which can never be completely avoided. With increasing "poisoning" of the OER catalyst, the mode of operation becomes worse and worse, so that a correspondingly high volume fraction of the secondary catalyst is required for reliable functionality, which makes the structure relatively complex and expensive, and possibly also adverse effects on the electrochemical Processes in the fuel cell can have during regular operation, since the comparatively large amount of OER catalyst required can impair the functionality and accessibility of the primary catalyst such as platinum.

However, the inventors were able to recognize that this poisoning can be counteracted by regenerating the OER catalyst. The method according to the invention for operating at least one fuel cell with a solid electrolyte membrane, a cathode and an anode, the anode having at least one OER catalyst, accordingly provides that the fuel cell is monitored for any polarity reversal, with a polarity reversal being detected Procedure for regenerating the OER catalyst is carried out. If, during operation of the at least one fuel cell, typically a stack of fuel cells in practice, a reversal of polarity of one or more of the individual cells has been detected, then, for example after the fuel cell stack has been switched off or in particular during a regular restart of the fuel cell stack, a sequence will occur performed to regenerate the OER catalyst. The poisoning can be reversed in whole or in large part via such a system, so that the OER catalyst is again available with its desired functionality during further operation.

According to a very advantageous development of the method according to the invention, the time of the polarity reversals that occurred can be taken into account. If these polarity reversals have occurred for a longer period of time, regeneration is necessary in any case and should be carried out completely. If the polarity reversal only occurred for short periods of time, then the regeneration can be delayed again, for example by setting a counter and, after a preset value has been reached, during the subsequent restart or standstill, the regeneration is carried out accordingly.

According to an extraordinarily favorable development of the method according to the invention, it can be provided that the procedure for regenerating the OER catalyst includes bringing about a start of the fuel cell with oxygen in the area of the anode and the cathode. A deliberate air/air start of the fuel cell is initiated here in order to oxidize the impurities in the area of the OER catalyst and to clean it again. This can be designed in such a way that, for example, a switch-off procedure or a procedure that runs while the fuel cell stack is at a standstill, which is actually intended to increase the hydrogen protection time, is suspended after a detected polarity reversal of individual cells during the last operating phase, so that an air/ Air start is deliberately accepted here in order to clean the OER catalyst accordingly, with the disadvantageous effects of the air/air start also being correspondingly minimized by this cleaning effect, since now primarily the impurities deposited on the OER catalyst are caused by carbon or carbon compounds are oxidized and just not or not to such a large extent as otherwise the carbon of the support of the primary catalyst.

According to an extraordinarily favorable development of the method, provision can also be made for air to be actively introduced into the anode before the fuel cell is started. In this variant, not only is the air/air start consciously accepted, but it is consciously and actively brought about, for example in the event that only very short downtimes of the fuel cell system occur, which typically do not result in an air/air start would lead. In this case, air can be actively directed into the area of the anode, for example by providing a suitable connection via a line with a valve between the anode side and the cathode side and this valve is always opened when a polarity reversal of individual cells is detected during previous operation has been.

In addition to this effect of an air/air start, which cleans the OER catalytic converter, the inventors have also recognized that there are also other possibilities for regenerating the OER catalytic converter. Thus, according to an extraordinarily favorable variant of the method according to the invention, it can be provided that the procedure for regenerating the OER catalytic converter includes impressing a voltage corresponding to the normal polarity at least on the polarity-reversed fuel cell. Such an imposition of a voltage pulse on at least the polarized individual cell or, in practice, often also on the entire stack of individual cells in the magnitude and polarity of the normal voltage, typically in the magnitude of more than 0.8 V per individual cell compared to the reversible hydrogen electrode (RHE ), a positive effect can also be achieved with regard to the regeneration of the OER catalyst. The required voltage can come from an external voltage source. In a vehicle drive system, the fuel cell is typically supported by a hybrid battery or the fuel cell is designed as a so-called range extender and recharges this battery while the drive is purely electrically powered from this battery. Such a battery typically offers sufficient power to enable the voltage required to impress a voltage pulse on the fuel cell, which is then at a standstill. If necessary, the voltage can also be increased to the required level using power electronic components such as a step-up converter or the like.

A fuel cell system according to the invention now comprises a stack of fuel cells which have an OER catalyst in their anode and also provides a device for detecting cell polarity reversals that occur. Such a fuel cell system is then provided with a controller which is set up to carry out the method described above in one variant or the other. In particular, according to a very advantageous development, the fuel cell system can also have a hybrid battery, which can be used as an external voltage source, in particular to impress a voltage pulse on at least one of the fuel cells, but in particular on the entire fuel cell stack, to regenerate the OER catalyst.

Furthermore, a vehicle can be provided with such a fuel cell system. The corresponding method can be ideally used in applications in vehicles in which the fuel cell itself or the fuel cell system often has to be restarted and stopped, since the possibilities for intervention to regenerate the OER catalyst are correspondingly frequent and typically in the statistical coincidence very soon after the occurrence of a polarity reversal.

Furthermore, it is the case that fuel cell systems in vehicles are often operated under adverse conditions, so that, for example, due to cool ambient temperatures, the condensation and/or freezing of product water is a problem that should not be underestimated, which can contribute to an increased occurrence of cell polarity reversals, so that particularly here on the one hand, the use of the OER catalyst and the method according to the invention for its regeneration is particularly advantageous and sensible.

Further advantageous configurations of a suitable fuel cell system and of the method according to the invention also result from the exemplary embodiment, which is described in more detail below with reference to the figures.

• 1 eine schematische Ansicht eines möglichen Fahrzeugs gemäß der Erfindung;
• 2 ein Diagramm der Zellspannung über der Zeit bei verschiedenen Fällen einer Umpolung; und
• 3 ein Diagramm der Zellspannung über der auf einen Fall normierten Zeit der Umpolung einer Einzelzelle.

show:

• 1 a schematic view of a possible vehicle according to the invention;
• 2 a diagram of the cell voltage over time in different cases of polarity reversal; and
• 3 a diagram of the cell voltage versus the polarity reversal time of a single cell normalized to one case.

In the representation of 1 a vehicle 1 is indicated schematically. The vehicle 1 is to be driven electrically by a traction motor 2 . For this purpose, the traction motor 2 drives two of the wheels indicated schematically. These drive wheels are denoted by 3. The control of electric traction motor 2 via power electronics 4, which is connected to a hybrid battery 5 on the one hand and a fuel cell stack 6 of a fuel cell system in the vehicle 1 on the other hand. The fuel cell stack 6 is designed as a stack of PEM fuel cells. Purely by way of example, an anode 7, a cathode 8 and a solid electrolyte membrane 9 lying between them are indicated schematically. The anode 7 of the fuel cell stack 6 shown here should have a so-called OER catalyst as a secondary catalyst in addition to the primary catalyst required for the electrochemical reaction of the fuel cell, typically platinum. This OER catalyst enables a tolerance of the individual cells of the fuel cell stack 6 against a polarity reversal of the voltage, a so-called cell reversal. It typically consists of iridium, ruthenium or their oxides and is known from the prior art.

In the exemplary embodiment shown here, the anode 7 is now supplied with hydrogen from a compressed gas storage device 10 via a pressure control and metering valve 11 . The cathode 8 is supplied with air from an indicated air conveying device 12 as an oxygen supplier. Unused exhaust air passes through an exhaust air line 13 into the surroundings of the vehicle 1. An exhaust line designated 14 from the anode 7 also leads into this exhaust line, shown here purely by way of example . Of course, further components such as humidifiers, an anode circuit, a system bypass and the like could be provided. This is of secondary importance for the present invention and is therefore not presented or discussed further.

In the individual fuel cells of the fuel cell stack 6 there is always the fundamental risk of a so-called cell reversal, ie an electrical polarity reversal of one or more of the individual cells. In general, the mechanism that can cause this problematic polarity reversal of the individual cells is a reduced supply of fuel, for example due to flow channels in the fuel cell stack 6 or the individual cell being blocked by water or ice. In principle, however, other mechanisms are also conceivable. A device for detecting such polarity reversals or cell reversals is shown in FIG 1 indicated and provided with the reference numeral 15. On the one hand, it is connected to the fuel cell stack 6, for example a so-called individual cell monitor within the fuel cell stack 6, and on the other hand to a controller 16, via which the corresponding processes of the fuel cell system and the electric drive system can be controlled. The controller 16 is connected on the one hand to the fuel cell stack 6 and to the hybrid battery 5 and the power electronics 4 . A connection to the device 15 is also provided, here in particular in such a way that the device 15 provides input for the controller 16 .

In the representation of 2 a diagram is shown in which the cell voltage of a single cell examined here with a secondary OER catalyst in the anode 7 is shown on the Y-axis, and the passage of time on the X-axis. The solid line shows operation with short-term polarity reversals of the individual cell examined here, and the dashed line shows a longer-term provoked polarity reversal of the individual cell.

3 picks this up again. Here, the solid line again shows the short-term, multiple recurring polarity reversal of the individual cell. With a dashed line, the longer-term polarity reversal of the single cell as a counterpart to the representation in the diagram 2 , where in the representation of the 3 the x-axis now shows the time normalized to the time measured in the process, ie a predetermined limit voltage of -1.5 V is reached here at normalized point in time 1. A possibility for intervening in the operation of the fuel cell, which will be explained later, is shown with a dot-dash line.

From the diagrams in the 2 and 3 it can be seen that a polarity reversal of the individual cell that occurs cyclically in short succession has largely no negative effects. However, longer-term polarity reversals lead to a massive drop in the recorded voltage after a certain period of time. From this it can be seen that in this situation the activity of the OER catalyst obviously decreases. This is presumably caused by carbon oxidation of the support of the primary catalyst, which always occurs parallel to the electrolysis of water in the area of the anode, with the resulting carbon species accumulating on the secondary catalyst and quasi "poisoning" it. This does not happen with cyclic polarity reversal. Obviously, the oxygen produced in the area of the anode during polarity reversal is the decisive factor, since this in turn oxidizes the impurities and thus virtually cleans or regenerates the OER catalyst.

According to the method, this can be done in the fuel cell system, as is the case, for example, in 1 exemplified vehicle 1 is used, now implement in such a way that if polarity reversals have been registered on the device 15, with a subsequent restart of the fuel cell stack 6, for example after this was switched off because the vehicle 1 was driven purely electrically from the hybrid battery 5, or because it was not driven at all, a regeneration of the OER catalytic converter is initiated. In this case, a so-called air/air start, which is not desirable per se with regard to the service life of the fuel cell stack 6, can then be carried out. With such a start with air both in the area of the anode 7 and the cathode 8, an air or oxygen/hydrogen front runs over the anode 7 when the metering of the hydrogen is started. In their area there is a high potential difference, which in this case now ensures that the poisoning of the OER catalyst is oxidized and this catalyst is thus regenerated. There is therefore a possibility of active regeneration of the catalytic converter if required, or passive regeneration of the catalytic converter by allowing such an undesirable air/air start whenever polarity reversals of individual cells in the fuel cell stack 6 have previously been detected have been.

In the representation of 1 a connecting line 17 between the side of the air supply and the side of the hydrogen supply can now also be seen. If necessary, air and thus oxygen can be introduced into the anode 7 via a valve 18 in this connecting line 17 in order to actively trigger such an air/air start for the regeneration of the OER catalytic converter.

As an alternative to this, it is also possible, as the inventors have recognized, to carry out the regeneration by impressing a voltage pulse via an external voltage source. This mechanism is shown in the diagram of 3 shown. A course of the voltage is indicated by a dot-dash line when a voltage pulse of, for example, +0.1 to 1.0 V against RHE is impressed at the normalized point in time 1 . In the further course, it can be seen very clearly that the massive drop in voltage to the value of -1.5 V selected here as the limit value occurs at a much later point in time than if this voltage pulse, which is shown in the illustration in the 3 is provided with the reference numeral 19, would have been dispensed with. As can be seen from the diagram, the voltage drop only occurs at the normalized time value of approx. 2.5, which means it can be more than doubled compared to operation without such a voltage pulse.

Depending on the needs and situation, you can choose between the various options for regenerating the OER catalytic converter. For example, a voltage pulse can be impressed, which can typically be impressed on the entire fuel cell stack 6 via the hybrid battery 5 as an external voltage source. If the nominal voltage of the battery is lower than the voltage of at least +0.8 V per individual cell required for the corresponding number of fuel cells, a step-up converter or the like can be provided in the area of the power electronics 4 in order to provide the required voltage. The controller 16 then triggers this pulse accordingly and connects the battery and the fuel cell to one another in the desired way. The alternative would be allowing or actively inducing an air/air launch. In principle, both options could also be combined.

Since the inventors have now shown that very short-term polarity reversals of the individual cells are less critical than long-term polarity reversals, the time period during which the polarity reversal occurred can also be recorded via the device 15 . In this case, it would be possible to react more quickly if the polarity reversals occurred over a longer period of time and to wait for short-term polarity reversals, for example by delaying an air/air start or a voltage pulse for one or two standstill phases, in particular until a total default time of polarity reversals has been reached has been reached.

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 2017/0309924 A1 [0003]
• DE 102019001388 A1 [0006]

Claims (9)

Method for operating at least one fuel cell (6) with a solid electrolyte membrane (9), a cathode (8) and an anode (7), wherein the anode (7) has at least one OER catalyst, the fuel cell (6) on a possible Polarity reversal is monitored, characterized in that after a detected polarity reversal, a process for regenerating the OER catalytic converter is carried out. procedure after claim 1 characterized in that the procedure for regenerating the OER catalyst includes causing the fuel cell (6) to start with oxygen in the region of the anode (7) and the cathode (8). procedure after claim 2 , characterized in that before the start of the fuel cell (6) air is actively introduced into the anode (7). procedure after claim 1 , 2 or 3 characterized in that the procedure for regenerating the OER catalytic converter includes impressing a voltage corresponding to the normal polarity at least on the previously reversed individual cell. procedure after claim 4 , characterized in that the applied voltage per individual cell is at least 0.6 V, preferably at least 0.8 V, or more compared to the reversible hydrogen electrode. procedure after claim 4 or 5 , characterized in that the voltage from an external voltage source (5) is applied. Fuel cell system with a stack (6) of individual cells which have an OER catalyst in their anode (7), with a device (15) for detecting occurring polarity reversals of individual cells and with a controller (16) which is set up for this purpose after one of Claims 1 until 6 to perform. fuel cell system claim 7 , characterized in that a hybrid battery (5) is also provided, which can be used as an external voltage source. Vehicle (1) with a fuel cell system according to one of Claims 7 or 8th , for providing at least part of the electrical drive power for the vehicle.

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