Cleaning (de-poisining) PEMFC electrodes from strongly adsorbed species on the catalyst surface

Abstract

A method for cleaning the electrochemical catalyst of fuel cell electrodes that is performed by applying a power pulse, using a low-power supply, across the fuel cell electrodes. The power pulse removes chemisorbed chemical species from the electrochemical catalyst of the electrodes.

Description

RELATED APPLICATIONS

This application claims the benefit of provisional application No. 60/679,038 filed on May 6, 2005, titled “Cleaning (De-poisoning) PEMFC Electrodes from Strongly Adsorbed Species on the Catalyst Surface”.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No. W-7450-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to fuel cells, and, more particularly to a method for cleaning (de-poisoning) fuel cell electrodes from strongly adsorbed species on the catalyst surface.

BACKGROUND OF THE INVENTION

Proton Exchange Membranes Fuel Cells (PEMFC) are devices that generate electrical power from two complementary electrochemical reactions. Hydrogen is oxidized at the anode and oxygen is reduced at the cathode. Thus, efficient fuel cell operation relies on the availability of both the cleanest fuel and air possible. These reactions take place on the surface of highly dispersed Pt catalysts. The catalytic activity of the Pt surface is very sensitive to the presence of certain impurities. Therefore, PEMFC performance may be strongly affected by the presence of contaminants in the fuel and in the air stream. In the hydrogen fuel, the impurities can be present in the primary source of fuel or can be generated during the reforming process. For instance, reformation of hydrocarbon fuels such as methane or gasoline, besides H₂, may produce various impurities at levels that can be detrimental to fuel cell (FC) operation. Typical fuel impurities are carbon monoxide (CO), ammonia (NH₃) and hydrogen sulfide (H₂S).

Contaminant species, such as hydrogen sulfide, poisons Pt catalysts irreversibly. That is, a neat (impurity-free) hydrogen stream will not be able to clean a sulfur-poisoned Pt surface because of the high chemical affinity of H₂S with metals. FIG. 1, shows cyclic voltammograms (CV) of a fuel cell anode fully poisoned with H₂S. Two major features in this CV indicate the presence of sulfur species chemisorbed onto the Pt surface. Within the potential domain 0.1 to 0.4 V, in the first cycle the typical peaks of a clean Pt catalyst corresponding to H-desorption are totally absent because the active sites are blocked by sulfur species. The second feature is seen in the potential range 0.9 to 1.3 V, which appears as two major merging oxidation waves. These currents correspond to the electrochemical oxidation of chemisorbed sulfur to non-poisoning species. Subsequent cycles become similar to that obtained on a clean Pt surface. Consequently, FC performance after cleaning by cyclic voltammetry is the same as observed before contamination with H₂S.

Other impurities may be present as contaminants in the ambient air injected to the cathode during FC operation. For instance sulfur dioxide (SO₂) is a common air pollutant that comes from fossil fuel combustion and is particularly abundant in urban areas. Depending on the concentration, SO₂ presence in the FC cathode air stream may have fast and irreversible negative effects on FC performance.

The CV in FIG. 2 shows similar features to those described in FIG. 1 for H₂S, suggesting that the chemisorbed sulfur species are similar in both cases. Again, FC performance after cleaning by cyclic voltammetry is the same as that obtained before the cathode was contaminated with sulfur dioxide. The facts described above, advise that electrode contamination with either hydrogen sulfide or sulfur dioxide should be avoided by all means, and there is a clear need to address what to do if the catalyst (electrode) becomes inadvertently poisoned with one of these contaminants.

Cyclic voltammetry is an electroanalytical technique that not only provides information about the status of the surface of the Pt-catalyst, but also by applying it to a poisoned Pt-catalyst electrode results in full electrode cleaning. However, performing CV involves interrupting fuel cell operation for a considerable amount of time (at least 1 hour). In addition the electrode being probed has to be purged with an inert gas (N₂ or Ar), which is time consuming and requiring a potentiostat, which is a rather expensive instrument.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a method for cleaning the electrochemical catalyst of fuel cell electrodes that is performed by applying a power pulse, using a low-power supply, across the fuel cell electrodes. The power pulse removes chemisorbed chemical species from the electrochemical catalyst of the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiment(s) of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1 is a graph of cyclic voltammograms of an anode Pt catalyst after full poisoning with H₂S. A curve before poisoning is also shown for reference.

FIG. 2 is a graph of cyclic voltammograms (1) of a fuel cell cathode Pt catalyst after full poisoning with SO₂.

FIG. 3a is a schematic of the electrical connections from the power supply to the fuel cell electrodes, for electrochemical cleaning of the catalyst for cathode cleaning.

FIG. 3b is a schematic of the electrical connections from the power supply to the fuel cell electrodes, for electrochemical cleaning of the catalyst for anode cleaning.

FIG. 4 is a graph of the fuel cell current density transient recorded before, during, and after poisoning the FC cathode with 10 ppm of sulfur dioxide in the air stream. The plot also includes the result of the 5 s power pulse cleaning stage.

FIG. 5 is a graph of the current-voltage power pulse applied to a fuel cell poisoned with sulfur dioxide. The power supply was set at 2.0 A and 1.4 V and the result of this 5 s power pulse application on cell performance is also shown in FIG. 4.

FIG. 6 is a graph of the fuel cell current density transient recorded before, during, and after poisoning the FC anode with 2 ppm of hydrogen sulfide in the H₂ fuel stream. The hydrogen flow was interrupted prior to pulsing for 10 minutes. The plot also includes the result of the 20 s power pulse cleaning stage.

FIG. 7 is a graph of the fuel cell current density transient recorded before, during and after poisoning the FC anode with 2 ppm of hydrogen sulfide in the H₂ fuel stream. The hydrogen flow was uninterrupted. The plot also includes the result of the 20 s power pulse cleaning stage.

FIG. 8 is a graph of the current-voltage power pulse applied to a fuel cell poisoned with hydrogen sulfide. The power supply was set at 30 A and 1.4 V and the result of this 20 s power pulse application on cell performance is also shown in FIG. 7.

DETAILED DESCRIPTION

The present invention comprises a method for in-situ cleaning fuel cell electrodes whose electrochemical catalyst is poisoned with strongly chemisorbed chemical species. Example 1 below shows the technique applied to a SO₂-poisoned cathode Pt-catalyst. The procedure is also applicable to other chemical species that chemisorb on Pt-catalysts, other metals or alloys used as electrochemical catalysts, independent of their origin. These species include but are not limited to H₂S, HCN, olefins and aromatic compounds.

The method consists of applying a power pulse, for 1 to 20 seconds, using a low power supply (0.5 to 6.0 W/cm²), across the fuel cell electrodes. Because the adsorbed species on the catalyst surface usually is a monolayer of molecules, the total amount of electrical charge necessary for the electrochemical desorption of these species is small. Inherently, the power requirements are also small. A short voltage/current pulse is enough for cleaning the contaminated catalyst surface. Low and high limits on the applied voltage are imposed, the lower limit to ensure that the electrochemical desorption process occurs, and, the higher limit to avoid electrochemical reactions that may irreversibly damage the electrode materials. In a preferred embodiment the voltage limit is from 1.2 to 1.4 V. However, in another embodiment, larger currents (4.5 A/cm² or 6 W/cm²) may be used.

Cleaning an anode poisoned with H₂S, can be carried out using either of the following two options; a) with interruption of the H₂ flow prior to pulsing, which requires low currents (up to 0.4 A/cm²) and b) without interruption of H₂ flow, which requires high currents (up to 4.5 A/cm²). In this instance a large portion of the current is used in H₂ oxidation. In both cases, the voltage must be kept below 1.4 V for reasons mentioned above.

EXAMPLE 1

Cleaning a Fuel Cell Cathode Contaminated with SO₂

Poisoning with SO₂

FIG. 4 shows the cell current density as a function of time for a fuel cell experiment in which the cell operated at constant voltage (0.6 V). Initially the cell cathode ran on impurity-free air for 20 minutes and then operated with air contaminated with 10 ppm of SO₂ for 20 minutes. The negative effect of the impurity on performance was observed as soon as the SO₂ injection started and it is indicated by the sudden decrease in the current, which eventually dropped below 20% of the original value. Once the SO₂ injection was interrupted, the cell ran on neat air again for about 24 minutes. A slow and small recovery was observed. Numerous SO₂ poisoning tests indicate that the recovery does not improve even if the cell continued operating on clean air for several days.

Fuel Cell Cathode Cleaning

The fuel cell was momentarily turned off before the cleaning was started. Then, the positive terminal of the power supply was connected to the fuel cathode and the negative terminal to the fuel anode, as shown schematically in FIG. 3a. A power pulse was applied for 5 seconds. The power supply was fixed at 1.4 V and the current was recorded as a function of time as shown in FIG. 5. Immediately after the pulse, the cell was disconnected from the power supply and turned on. As shown in FIG. 4, the recovery of the fuel cell performance was quite fast and the cell current practically returned to the original value recorded prior to poisoning.

EXAMPLE 2

Cleaning a FC Anode Contaminated with H₂S

Anode Poisoning with H₂S and Cleaning Electrode with H₂ Flow Interruption

FIG. 6 shows a similar experiment to example 1, but this time for an anode whose hydrogen fuel supply was contaminated with 2 ppm of H₂S. Initially the cell ran on impurity-free hydrogen for 40 minutes, showing steady performance. Then it was exposed to H₂S-contaminated hydrogen for 30 minutes. The cell current dropped considerably to 56% of its initial value. As expected, after stopping the injection of H₂S the anode experienced insignificant recovery when the cell resumed operation on neat hydrogen again. Prior to applying the power pulse to the FC, the H₂ gas flow was completely interrupted while the air flow was significantly reduced for about 10 minutes. This allowed the existing hydrogen at the anode to be consumed, leaving only the chemisorbed electroactive species on the Pt catalyst to be electro-oxidized by the external power pulse. Thus, prior consumption of H₂ to pulsing reduces the power requirements from the power supply.

After most of the hydrogen at the anode was consumed, the cell was momentarily turned off and the positive terminal of the power supply was connected to the fuel cell anode and the negative terminal to the fuel cell cathode (see FIG. 3b). Then, the power supply was set at a fixed voltage of 1.4 V and a power pulse was applied to the cell for 20 s. Notice that in this example, the power supply terminals are connected opposite to the previous one. In this case the cell recovered 95% of the initial fuel cell current prior to poisoning.

Anode Poisoning with H₂S and Cleaning Electrode with H₂ Flow:

FIG. 7 shows a similar experiment to that of FIG. 6, except for a modification in the cleaning procedure. First, the cell ran on impurity-free H₂ for 50 minutes. Then, the cell continued running on H₂ contaminated with 2 ppm of H₂S for 40 minutes. As a result, the performance of the cell decreased to 40% of the initial value and did not recover even after again running on impurity-free H₂ for another 30 minutes.

Then the cell H₂ flow was decreased from 160 to 80 sccm (standard cubic centimeters per minute) and a 20 second electrical pulse was applied with the power supply settings at 1.4 V and 15 A. After the pulse application, the cell performance recovery was fast and complete, as indicated by comparison of the initial and final current values.

Full cell performance recovery is the main advantage of the procedure described. It appears that by keeping the fuel flowing during the applied pulse, the desorbed active sulfur-species washes away from the anode catalyst. Without fuel flowing, some sulfur-species may re-adsorb on the catalyst surface resulting in a partial catalyst cleaning and performance recovery. However, as explained above this option requires higher power because most of the supplied current is used in oxidizing the H₂ fuel. As shown in FIG. 8, only for a small fraction of the time the pulse reaches values above 0.9 V, which is the minimum voltage for initiating the catalyst cleaning.

These two examples demonstrate a simple cleaning method for reactivating fuel cell electrodes irreversibly poisoned with strongly chemisorbed species. These kinds of impurities can fully disable a fuel cell operation in short exposure times. The worse aspect of this ordeal is the irreversibility of the process. Once the catalyst is poisoned further operation with neat fuel in the anode or clean air in the cathode does not recover the original performance.

The injection of small amounts of air, a proven approach to increasing anode CO tolerance (Gottesfeld, U.S. Pat. No. 4,910,099), is not efficient in the case of H₂S poisoning due to the high potential required for electro-oxidation of the impurity.

The present technique provides: simplicity of application, low cost of the required equipment, short time length of the procedure, no requirement for inert gases, and minimal interruption of the fuel cell operation.

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

1. A method for cleaning the electrochemical catalyst of fuel cell electrodes, comprising: applying a power pulse using a low-power supply across said fuel cell electrodes for a period of time sufficient to remove chemisorbed chemical species from said electrochemical catalyst.

2. A method of removing a contaminant from a fuel cell cathode, comprising:

connecting a fuel cell cathode to a positive terminal,

connecting a fuel cell anode to a negative terminal, and

applying a fixed voltage low-power power supply across said fuel cell cathode for a predetermined period where said contaminant is removed from said fuel cell cathode.

3. The method of claim 2 where said fixed voltage ranges from about 1.2 to 1.4 volts.

4. The method of claim 2 where said predetermined period ranges from about 1 to 20 seconds.

5. The method of claim 2 where said low-power supply ranges from about 0.5 to 6.0 W/cm2.

6. The method of claim 2 where said fixed voltage is applied for a time sufficient to restore said fuel cell current to within 95 to 100% of initial current.

7. A method of removing a contaminant from a fuel cell anode, comprising:

connecting a fuel cell cathode to a negative terminal,

connecting a fuel cell anode to a positive terminal, and

applying a fixed voltage across said fuel cell anode for a predetermined period where said contaminant is removed from said fuel cell anode.

8. The method of claim 7 where said fixed voltage ranges from about 1.2 to 1.4 volts.

9. The method of claim 7 where said predetermined period ranges from about 1 to 20 seconds.

10. The method of claim 2 where said low-power supply ranges from about 0.5 to 6.0 W/cm2.

11. The method of claim 7 where said fixed voltage is applied for a time sufficient to restore said fuel cell current to within 95 to 100% of initial current.