EP2507859A1

EP2507859A1 - Method for managing a fuel cell during sulfur compound pollution, and power supply device

Info

Publication number: EP2507859A1
Application number: EP10799069A
Authority: EP
Inventors: Olivier Lemaire; Benoît BARTHE; Alejandro Franco
Original assignee: Commissariat a lEnergie Atomique CEA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2009-12-03
Filing date: 2010-12-03
Publication date: 2012-10-10
Also published as: CA2782160A1; WO2011070242A1; JP2013513201A; CN102725899A; FR2953649A1; BR112012013115A2; US20120251907A1

Abstract

The fuel cell management method, comprising an active gas flowing in contact with an electrode, includes a step of comparing the concentration of a sulfur compound in the active gas to a threshold indicative of a sulfur compound pollution phase (P2) and moreover includes a step of temporarily adding (P3) an oxygenated non-sulfur polluting gas into the active gas if the concentration is greater than the threshold. The polluting gas can be added during or after the pollution phase (P2).

Description

METHOD FOR MANAGING A FUEL CELL DURING SULFUR COMPOUND POLLUTION AND POWER SUPPLY DEVICE Technical Field of the Invention

The invention relates to a method for managing a fuel cell comprising a circulating active gas in contact with an electrode. The invention also relates to a power supply device comprising a fuel cell.

State of the art

Fuel cells are electrochemical systems that convert chemical energy into electricity. For proton exchange membrane fuel cells (PEMFC), the chemical energy is, for example, in the form of hydrogen gas. The fuel cell is broken down into two compartments separated by a proton exchange membrane. One of the compartments is fed, for example, hydrogen or methanol, called fuel, and the other is supplied with oxygen or air, called oxidant. At the anode, the oxidation reaction of hydrogen produces protons and electrons. The protons cross the membrane while the electrons must pass into an external electrical circuit to reach the cathode. At the cathode occurs the reaction of reduction of oxygen, in the presence of protons and electrons. The core of the cell, also called membrane-electrode assembly (AME), consists of catalytic layers and the separating membrane. The catalytic layers are the seat of the oxidation and reduction reactions in the cell. Gaseous diffusion layers are arranged on either side of ΑΜΕ to ensure electrical conduction, the homogeneous arrival of the gases and the evacuation of the water produced by the reaction and the non-consumed reagents.

Oxidant and fuel pollution is one of the main factors responsible for the performance degradation of a PEMFC cell. The impurities contained in the hydrogen (combustible gas) are, for example, carbon oxides CO and C0 ₂ , sulfur compounds (H ₂ S in particular) and ammonia NH ₃ . These impurities come in particular from the process for manufacturing hydrogen. The pollutants of air or oxygen (oxidizing gas) are, for example, NO _x nitrogen oxides, SO _x sulfur oxides and CO _x carbon oxides. These pollutants generally come from motor vehicle exhausts, industrial and military sites.

These contaminants can access the chemical reaction zones of the cell and attach to the catalytic sites of the anode and cathode. The catalytic sites are then poisoned and no longer participate in the oxidation and reduction processes. In addition, the contaminants modify the structure and the properties of the core of the cell, for example by modifying its hydrophobic or hydrophilic character. Thus, the degradation of the performance of the cell is mainly due to the decrease in catalytic activity, the ohmic losses following the increase of the resistance of the components of the cell and to the losses of mass transport following the variations of the structure . Among the pollutants of the oxidizing gas stated above, the oxides of sulfur (SO _x ), sulfur dioxide S0 _{2 in} particular, are particularly harmful and greatly degrade the performance of the battery. Various electrochemical methods are used to regenerate the performance of a fuel cell after an episode of pollution with sulfur compounds. These methods involve applying an electric current or an electrical pulse to each of the contaminated electrodes to remove impurities on their surface. Another method is to impose a voltage that varies cyclically between -1.5 V and 1.5 V. These regeneration techniques can find a satisfactory level of performance. However, such techniques require a disabling of the battery. Although it may be brief, stopping the battery is detrimental to the device it feeds. In addition, the application of an electric current in the form of a pulse or a cycle can degrade the core components of the battery, including the catalyst. These techniques are therefore not appropriate.

Object of the invention

The invention relates to a fuel cell management method, simple and easy to implement, to find good performance after pollution sulfur compounds.

According to the invention, this objective is attained by the fact that the concentration of a sulfur compound in the active gas is compared with a single indicative of a phase of pollution with the sulfur compound and by the fact that an oxygen pollutant gas and non-sulfur is temporarily introduced into the active gas if the concentration of sulfur compound is greater than the threshold.

The invention also relates to a power supply device comprising means for comparing the concentration of a sulfur compound in the active gas to a threshold indicative of a pollution phase sulfur compound, a source of oxygen pollutant gas and not sulfur and means of introduction pollutant gas in the active gas if the concentration of the sulfur compound is above the threshold.

Brief description of the drawings

Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given by way of non-limiting example and represented in the accompanying drawings, in which:

- Figure 1 shows changes in the voltage across a battery in time, depending on the concentration of sulfur compound.

FIG. 2 schematically represents the variations in the performance of a battery with time, for a first embodiment of a management method according to the invention,

FIG. 3 represents, schematically, the variations of the performances of a stack with time, for a second embodiment,

FIG. 4 schematically represents the variations in the performance of a battery with time, for a variant embodiment of the embodiment of FIG. 3,

FIGS. 5 and 6 represent the variations of the voltage at the terminals of a battery with time, for a sulfur compound concentration of 1.5 ppm,

FIG. 7 represents the variations of the voltage at the terminals of a battery with time, for a sulfur compound concentration of 4 ppm, and

- Figure 8 shows a power supply device according to the invention. Description of particular embodiments

The article "The effect of ambient contamination on PEMFC performance" (Jing et al., Journal of Power Sources, 166, 172-176, 2007) describes the mechanism of pollution of the oxidizing gas by SO ₂ sulfur dioxide. Air containing sulfur dioxide is injected into the cell to determine the impact of such pollution on performance. Sulfur dioxide is adsorbed on the platinum catalyst layer, thereby reducing the active surface area and hence the catalytic activity. The performance of the battery decreases by about 35% after pollution for about 100 hours and the recovery rate is of the order of 84%.

The experiment is repeated with another pollutant of the oxidizing gas: nitrogen dioxide NO ₂ . In the same way, the nitrogen dioxide is fixed on the catalytic sites and reduces the performance of the stack by 10% after a pollution for about 100 hours and the recovery rate is of the order of 94%.

A third experiment is carried out with an oxidizing gas comprising NO ₂ nitrogen dioxide and SO ₂ sulfur dioxide. The decrease in performance is of the order of 23% and the recovery rate of 94%. NO ₂ adsorption and SO ₂ adsorption by the catalyst appear to be two competing mechanisms. Nitrogen dioxide is more easily adsorbed, which limits the adsorption of sulfur dioxide by the catalyst, thus explaining a lower performance reduction in the case of a mixture of the two pollutants than in the case of sulfur dioxide only. Nitrogen dioxide thus has an adsorption affinity to the catalyst greater than the affinity of sulfur dioxide. It is proposed here to develop the teachings of this article to implement them in an advantageous manner. In the case of pollution with a sulfur compound, of chemical formula SX _n , the sulfur can be adsorbed by platinum according to the following simplified formula:

SX _n + Pt = X _n + PtS

Sulfur binds to platinum and forms a compound of formula PtS.

FIG. 1 represents the evolution in time (t) of the voltage U at the terminals of a PEMFC stack in different cases of pollution. Phase P1 corresponds to a phase of operation without pollutants. The voltage U is maximum in this phase. Phase P2 corresponds to pollution by a sulfur compound, for example sulfur dioxide. The concentration of sulfur oxide S0 ₂ varies between 0.75 ppm (parts per million) and 4 ppm according to the different curves shown in FIG. 1. In this phase, the voltage U drops gradually.

It can be seen in FIG. 1 that the rate of decrease of the voltage increases with the pollutant concentration. For example, the voltage decreased by about 40 mV at the end of the P2 phase, for a SO ₂ concentration of 1 ppm, while for a concentration of 4 ppm, the decrease is about 150mV. At the end of the P2 pollution phase, the active gases become pure (phase P1 b) and the U voltage rises. Nevertheless, the voltage U does not go back completely to its initial level. Indeed, some of the active sites is irreversibly poisoned by the sulfur compound. A method of regenerating stack performance should be employed.

The inventors have discovered that certain oxygenated compounds, in particular nitrogen dioxide NO ₂ and carbon dioxide CO ₂ , can replace the sulfur occupying the catalytic sites and responsible for reducing the performance of a battery, for example voltage. This phenomenon is explained in that these oxygenated compounds have a higher adsorption affinity than the sulfur compounds, as previously described. The mechanism is described, in a simplified way, by the following equation, in the case of N0 ₂ :

PtS + N0 ₂ + Pt = "NOPt + PtO + S

The sulfur is replaced at the level of the contaminated catalytic site (PtS) by the radical NO coming from NO ₂ .

In the case of CO ₂₎ it is the radical CO which displaces the sulfur according to the reaction:

PtS + CO => COPt + S

There is provided a method of managing a fuel cell practicing this phenomenon. Such a method comprises the detection of pollution to a sulfur compound and the introduction of a healing gas during or after this pollution phase. This healing gas, oxygenated and not sulfurized, will allow the evacuation of particles including sulfur which poison the catalytic sites. The gas is called a healer because it allows a better regeneration of the performance of the battery during a return to pure active gases after pollution to the sulfur compound. Indeed, catalytic sites will be released in larger quantities and the performance will rise to a higher level. This is explained by the fact that oxygenated radicals desorb more easily than sulfur when returning to pure active gases.

The fuel cell traditionally comprises two active gases: an oxidizing gas, air for example, and a combustible gas, hydrogen, for example. Each of the oxidant and fuel active gases circulates in contact with an electrode, respectively the cathode and the anode. As soon as a sulfur compound is detected in one of the active gases, a pollution phase is identified. This detection can be performed by comparing the concentration of the sulfur compound in the active gas at a threshold indicative of a sulfur compound pollution phase. The sulfur compound is, for example, sulfur dioxide S0 ₂ , generally present in air, or hydrogen sulphide H ₂ S generally present in the fuel gas. The battery management method applies to the sulfur compounds that can be adsorbed by the catalyst, both on the anode side and on the cathode side.

The healing gas is then temporarily introduced into the polluted active gas if the concentration of sulfur compound is greater than the threshold. The threshold is preferably defined in relation to the performance degradation caused by the pollution. For example, a 10% decrease in performance due to pollution may provide the value of a first threshold. The healant gas may be selected from NO _x nitrogen oxides and CO _x carbon oxides, which are themselves common pollutants of PEMFC cells. Thus, the introduction of such a gas may, in the short term, worsen the drop in performance due to the sulfur compound, but during the return to pure active gases, the gas has helped to further regenerate the performance of the battery. The duration of introduction of the healing gas is preferably between 1 minute and 10 hours and may vary depending on the desired final level of performance. The duration of introduction may also be a function of the amount of healing gas. By way of example, it can vary from a few minutes, for a concentration of healing gas of the order of a few parts per million (ppm), to a few hours for a concentration of the order of a few parts per billion ( ppb). The amount of heal gas is preferably from 10 parts per billion to 10 parts per million based on the total amount of the gases, i.e., the active gas, the pollutant gas and the healer gas. FIG. 2 represents the evolution of the performances of a stack in time according to a first embodiment of the management method. A phase of involuntary pollution P2 succeeds a first phase without pollutants P1 a. The presence of a sulfur compound is detected between times t, and t ₂ . Between the instant t ₂ and a time t ₃ , the cell operates again with pure active gases (phase P1 b). The performances rise slightly and reach an intermediate value P _m , lower than the initial performances P |. The healing gas is introduced voluntarily at time t ₃ , corresponding to the healing phase P3. Performance drops again. Indeed, the gas introduced is also polluting. However, after stopping the introduction of the healer gas, the performance goes back, during a non-polluting phase P1 c, to a value P _f greater than the value P _m of the performance after the pollution phase P2. The introduction of the healing gas during a phase P3 thus allowed an improvement in the performance of P _f -P _m , represented by the arrow in FIG. 2. In the embodiment of FIG. 2, the healing gas is introduced. after the pollution phase with the sulfur compound (P2) and a phase of operation without pollutants (P1 b). In this case, the management method includes a step of detecting the end of the pollution phase and a waiting step of a time interval corresponding to the phase P1b.

In an alternative embodiment, the healing gas is introduced immediately after detecting the end of the pollution phase P2 sulfur compound. In this case also, the final performance is improved compared to the performance without use of healing gas.

FIG. 3 represents a second embodiment in which the healing gas is introduced during the pollution phase with the sulfur compound. The curve in solid line represents the case of a management method with introduction of a healing gas while the dashed curve represents the case of a process without introduction of the healing gas. The sulfur compound pollution occurs between times t, and t ₃ . At the instant t ₂ between ^ and t ₃ , the healing phase P3 starts. The performance then decreases further (curve in solid line). However, during the return under pure active gas, that is to say in phase P1 b, the performance goes back to a level P _f higher than that obtained without healing gas (dashed, level P _m ).

FIG. 4 represents a variant of the method of FIG. 3. The gas is introduced during the pollution phase P2 between the times t- and t ₂ during several disjointed time intervals. In the example of FIG. 4, three healing phases P3a to P3c are used. The duration of introduction of the healing gas of each phase P3 is variable, as is the duration between two successive phases P3. This division is advantageous because it allows an intermediate analysis of the performance reduction in order to adjust the amount of heal gas to be introduced to the next phase P3. This amount can be adjusted, for example, by changing the number of phases P3 and the duration of each of them.

FIGS. 5 to 7 illustrate examples of operation of a stack managed according to the various embodiments of the management method described. The operating conditions of this battery are as follows:

The loading of the electrodes with a catalyst, platinum for example, is of the order of 0.5 mg / cm ² .

The polymer membrane is, for example, a material marketed under the trademark Nafion by the company DuPont and a thickness of about 50 μτη.

The moisture content of the reactive gases at the anode and the cathode is approximately 60%.

The current density of the battery is of the order of 0.6 A / cm ² .

The pollutant gas is sulfur dioxide in the air.

The healing gas is nitrogen dioxide. FIG. 5 represents an example of operation with a healing phase P3 beginning at the end of a pollution phase P2. The curve in solid line represents the case of a management method with introduction of the healing gas while the dashed curve represents the case of a process without introduction of the healing gas (no phase P3 in this case). The pollution phase P2 sulfur dioxide, a concentration of 1, 5 ppm, lasts about 15 hours. It is followed by a healing phase P3 by nitrogen dioxide N0 _2, at a concentration of 1, 5 ppm. Nitrogen dioxide is introduced into the air for approximately 15 hours.

We take the voltage as a parameter representative of the performance of the battery. The final tension will be noted P _t . Similarly, the voltage obtained without having introduced a healing gas is noted P _m . The improvement P _f - P _m obtained in terms of voltage by the management method is of the order 17 mV.

FIG. 6 represents another example in which the introduction of the NO ₂ healer gas takes place after a pollution phase P2 at SO ₂ of 30 hours at 1.5 ppm and a phase of operation without pollutants P 1 b of approximately 30 hours. The healing phase P3 lasts approximately 30 hours. The performance improvement P _f - P _m corresponds to a voltage of 16 mV.

The management method with introduction of an oxygenated and non-sulfur healing gas applies regardless of the concentrations of pollutants. The concentration of the healing gas may also be adapted according to the desired performance improvement.

FIG. 7 represents an example of operation with an NO ₂ concentration of 4 ppm during the phase P 3 and a waiting phase P 1 b between the end of the pollution phase P2 and the introduction of the healing gas. The voltage P _f after the healing phase P3 has increased by one value up to 22 mV with respect to the voltage P _m obtained after the P2 pollution phase.

This management method with introduction of a healer gas will preferably apply as long as the performance is greater than 50% of the initial performance.

In order to implement this management method, a power supply device comprises means for comparing the concentration of a sulfur compound in the active gas with a threshold indicative of a phase of pollution with the sulfur compound and means for introducing an oxygenated and unsulfurized healer gas into the active gas if the concentration of the sulfur compound is greater than the threshold. The device can then automatically control the introduction of the healing gas according to a most suitable regeneration mode.

The feed device further comprises means for identifying the sulfur compound and means for calculating the amount of healer gas to be introduced. The computing means may also determine the rate of degradation of performance, the level of performance, the duration of introduction of the gas. The calculation means will thus determine the appropriate mode of introduction and control the means for introducing the healer gas, depending, for example, on the nature of the pollutant and / or the rate of degradation of performance.

Figure 8 shows an example of a feeding device. The device comprises a fuel cell 1 with proton exchange membrane (PEMFC). The cell 1 comprising a membrane-electrode assembly (AME) 2 constituting the core of the cell and gas diffusion layers 3a and 3b on either side of the assembly 2. Each gas diffusion layer (3a, 3b) includes an active gas inlet and an outlet for excess gas and the reaction products, respectively 4a and 5a for the fuel gas on the left in FIG. 8, and 4b and 5b for the oxidant gas on the right in FIG. 8. The device 1 further comprises a detector 7 for sulfur compounds SX _n . The detection may consist in comparing the concentration of the sulfur compound with a threshold indicative of a pollution. In FIG. 8, the supply device comprises two detectors 7a and 7b, respectively for the fuel gas and the combustion gas. An electronic control circuit 8, for example a microcontroller, makes it possible to determine the evolution of the performances of the cell, in particular from the measured voltage (V) and current (I) values. The control circuit 8 is connected to the output of the detectors 7a and 7b for controlling an introduction valve 9 of the healing gas, CO _x or NO _x, for example. The control circuit 8 is also connected to identification means 10a and 10b of the polluting gas. The identification means 10a and 10b may be incorporated in the detectors 7a and 7b of the polluting gas. The healer gas is introduced into the combustible active gas using a conduit 1 1 a and / or in the oxidizing active gas using a conduit 1 1 b. The ducts 1 1 a and 11 b are connected to the active gas inlet of the stack, respectively 4 a and 4 b

The fuel cell management process also applies if the two compartments, fuel and oxidant, are polluted simultaneously, by the same gas or by different gases. A healing gas is then injected into each compartment. The healing gases may be identical or different in nature from the fuel side and the combustion side. Finally, several healing gases can be used successively.

Claims

claims

A method for managing a fuel cell (1) comprising a circulating active gas in contact with an electrode, characterized in that it comprises the following steps:

comparing the concentration of a sulfur compound in the active gas with a threshold indicative of a pollution phase (P2) to said sulfur compound, and

- Temporarily introduce into the active gas an oxygen pollutant gas and not sulfur if the concentration of the sulfur compound is greater than the threshold.

2. Method according to claim 1, characterized in that the polluting gas is selected from nitrogen oxides and carbon oxides.

3. Method according to one of claims 1 and 2, characterized in that the polluting gas is introduced during the pollution phase (P2) to the sulfur compound.

4. Method according to claim 3, characterized in that the pollutant gas is introduced during several disjoint time intervals.

5. Method according to one of claims 1 and 2, characterized in that the polluting gas is introduced after the pollution phase (P2) to the sulfur compound.

6. Method according to claim 5, characterized in that it comprises the following steps before the step of introducing the polluting gas (P3):

detecting the end of the pollution phase (P2) with the sulfur compound, and

- wait for a time interval (P1 b).

7. Method according to any one of claims 1 to 6, characterized in that the duration of introduction of the pollutant gas is between 1 minute and 10 hours.

8. Process according to any one of claims 1 to 7, characterized in that the amount of pollutant gas is between 10 parts per billion and 10 parts per million relative to the total amount of gas.

9. Power supply device comprising a fuel cell (1), the cell comprising an active gas flowing in contact with an electrode, characterized in that it comprises:

means for comparing (7, 7a, 7b) the concentration of a sulfur compound in the active gas with a threshold indicative of a pollution phase (P2) to said sulfur compound,

- a source of pollutant gas oxygenated and not sulfur, and

means for introducing (9) the pollutant gas into the active gas if the concentration of the sulfur compound is greater than the threshold.

10. Device according to claim 9, characterized in that it comprises:

means for identifying (10a, 10b) the sulfur compound, and

- Calculation means (8) of the amount of pollutant gas to be introduced.