U.S. patent application number 11/328898 was filed with the patent office on 2006-11-09 for cleaning (de-poisining) pemfc electrodes from strongly adsorbed species on the catalyst surface.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Tommy Q.T. Rockward, Francisco A. Uribe.
Application Number | 20060249399 11/328898 |
Document ID | / |
Family ID | 37393120 |
Filed Date | 2006-11-09 |
United States Patent
Application |
20060249399 |
Kind Code |
A1 |
Uribe; Francisco A. ; et
al. |
November 9, 2006 |
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.
Inventors: |
Uribe; Francisco A.; (Los
Alamos, NM) ; Rockward; Tommy Q.T.; (Rio Rancho,
NM) |
Correspondence
Address: |
LOS ALAMOS NATIONAL SECURITY, LLC
LOS ALAMOS NATIONAL LABORATORY
PPO. BOX 1663, LC/IP, MS A187
LOS ALAMOS
NM
87545
US
|
Assignee: |
The Regents of the University of
California
|
Family ID: |
37393120 |
Appl. No.: |
11/328898 |
Filed: |
January 9, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60679038 |
May 6, 2005 |
|
|
|
Current U.S.
Class: |
205/705 |
Current CPC
Class: |
H01M 8/043 20160201;
H01M 2008/1095 20130101; Y02E 60/50 20130101; H01M 8/04223
20130101 |
Class at
Publication: |
205/705 |
International
Class: |
C25F 1/00 20060101
C25F001/00 |
Goverment Interests
STATEMENT REGARDING FEDERAL RIGHTS
[0002] 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.
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/cm.sup.2.
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/cm.sup.2.
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.
Description
RELATED APPLICATIONS
[0001] 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".
FIELD OF THE INVENTION
[0003] 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
[0004] 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 H.sub.2, may produce various impurities at levels that can
be detrimental to fuel cell (FC) operation. Typical fuel impurities
are carbon monoxide (CO), ammonia (NH.sub.3) and hydrogen sulfide
(H.sub.2S).
[0005] 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 H.sub.2S with metals. FIG.
1, shows cyclic voltammograms (CV) of a fuel cell anode fully
poisoned with H.sub.2S. 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 H.sub.2S.
[0006] Other impurities may be present as contaminants in the
ambient air injected to the cathode during FC operation. For
instance sulfur dioxide (SO.sub.2) is a common air pollutant that
comes from fossil fuel combustion and is particularly abundant in
urban areas. Depending on the concentration, SO.sub.2 presence in
the FC cathode air stream may have fast and irreversible negative
effects on FC performance.
[0007] The CV in FIG. 2 shows similar features to those described
in FIG. 1 for H.sub.2S, 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.
[0008] 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 (N.sub.2 or Ar), which is
time consuming and requiring a potentiostat, which is a rather
expensive instrument.
[0009] 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
[0010] 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
[0011] 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:
[0012] FIG. 1 is a graph of cyclic voltammograms of an anode Pt
catalyst after full poisoning with H.sub.2S. A curve before
poisoning is also shown for reference.
[0013] FIG. 2 is a graph of cyclic voltammograms (1) of a fuel cell
cathode Pt catalyst after full poisoning with SO.sub.2.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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 H.sub.2 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.
[0019] 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 H.sub.2 fuel stream. The hydrogen flow
was uninterrupted. The plot also includes the result of the 20 s
power pulse cleaning stage.
[0020] 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
[0021] 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 SO.sub.2-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 H.sub.2S, HCN, olefins and
aromatic compounds.
[0022] The method consists of applying a power pulse, for 1 to 20
seconds, using a low power supply (0.5 to 6.0 W/cm.sup.2), 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/cm.sup.2 or 6
W/cm.sup.2) may be used.
[0023] Cleaning an anode poisoned with H.sub.2S, can be carried out
using either of the following two options; a) with interruption of
the H.sub.2 flow prior to pulsing, which requires low currents (up
to 0.4 A/cm.sup.2) and b) without interruption of H.sub.2 flow,
which requires high currents (up to 4.5 A/cm.sup.2). In this
instance a large portion of the current is used in H.sub.2
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 SO.sub.2
Poisoning with SO.sub.2
[0024] 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 SO.sub.2 for 20 minutes. The negative effect of the impurity
on performance was observed as soon as the SO.sub.2 injection
started and it is indicated by the sudden decrease in the current,
which eventually dropped below 20% of the original value. Once the
SO.sub.2 injection was interrupted, the cell ran on neat air again
for about 24 minutes. A slow and small recovery was observed.
Numerous SO.sub.2 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
[0025] 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 H.sub.2S
Anode Poisoning with H.sub.2S and Cleaning Electrode with H.sub.2
Flow Interruption
[0026] 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 H.sub.2S. Initially the cell ran on impurity-free hydrogen
for 40 minutes, showing steady performance. Then it was exposed to
H.sub.2S-contaminated hydrogen for 30 minutes. The cell current
dropped considerably to 56% of its initial value. As expected,
after stopping the injection of H.sub.2S the anode experienced
insignificant recovery when the cell resumed operation on neat
hydrogen again. Prior to applying the power pulse to the FC, the
H.sub.2 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 H.sub.2 to pulsing reduces the power requirements
from the power supply.
[0027] 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 H.sub.2S and Cleaning Electrode with H.sub.2
Flow:
[0028] 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 H.sub.2 for 50 minutes. Then, the cell continued
running on H.sub.2 contaminated with 2 ppm of H.sub.2S 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 H.sub.2 for another 30 minutes.
[0029] Then the cell H.sub.2 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.
[0030] 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 H.sub.2 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.
[0031] 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.
[0032] 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 H.sub.2S poisoning due
to the high potential required for electro-oxidation of the
impurity.
[0033] 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.
[0034] 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.
[0035] 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.
* * * * *