a is a schematic representation of a second configuration for carrying out a method of removing at least one contaminant from fuel cell catalyst electrode of a fuel cell membrane electrode assembly (MEA);
b is a schematic representation showing the removal and replacement of the getter electrode shown in
a is a plot of long term performance of a MEA having a standard cathode;
b is a plot of long term performance of a MEA having a cathode that was cleaned using the present methods;
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as either comprising or consisting of at least one of a group of elements and combinations thereof, it is understood that the group may comprise or consist of any number of those elements recited, either individually or in combination with each other.
Referring to the drawings in general and to
In a second embodiment, shown in
The fuel cell may be a polymer electrolyte fuel cell such as, but not limited to, a direct methanol fuel cell, a neat hydrogen fuel cell, a reformed hydrogen fuel cell, and the like.
The composition of fuel cell catalyst electrode 110 depends on whether the fuel cell catalyst electrode 110 is an anode or a cathode. Where fuel cell catalyst electrode 110 is an anode, it may comprise platinum, or platinum alloys such as, but not limited to platinum/ruthenium alloys, platinum/nickel alloys, platinum/chromium alloys, and the like. The cathode may comprise at least one transition metal such as ruthenium, iron, chromium, cobalt, molybdenum, manganese, magnesium, nickel, osmium, rhodium, tungsten, rhenium, yttrium, titanium, and combinations thereof. Alternatively, cathode 350 may comprise platinum, gold, iridium, palladium, or a platinum alloy such as, but not limited to, platinum/chromium, platinum/cobalt and platinum/iron alloys.
In another embodiment, cathode 350 comprises a cathode catalyst that comprises at least one transition metal and at least one chalcogen (e.g., sulfur, selenium, or tellurium). The at least one transition metal is selected from the group consisting of ruthenium, osmium, rhenium, rhodium, platinum, palladium, cobalt, nickel, chromium, molybdenum, iron, manganese, vanadium, tungsten, and combinations thereof. The cathode catalyst may comprise a support that includes at least one transition metal in elemental form capable of reducing oxygen and at least one chalcogen in solid elemental form disposed on the support (e.g., elemental selenium disposed on a ruthenium support). Alternatively, the cathode catalyst may comprise a transition metal chalcogenide (e.g., RuSex).
In one embodiment, substrate 120, 220, 320 is an ionomeric membrane such as, but not limited to poly (perfluorosulphonic acid) membranes, which are commercially available as Nafion®, Aciplex®, or Flemion®. MEA 205, 305 may additionally include a gas diffusion layer (not shown) comprising either carbon cloth or carbon paper.
In one embodiment, the at least one contaminant 115, 215, 315 is a metal oxide. The metal oxide is typically selected from the group consisting of oxides of ruthenium, iron, chromium, cobalt, molybdenum, manganese, magnesium, nickel, osmium, rhodium, tungsten, rhenium, yttrium, titanium, platinum, gold, iridium, and palladium.
Getter electrode 130, 230, 310 is a metal electrode comprising at least one of ruthenium, iron, chromium, cobalt, molybdenum, manganese, magnesium, nickel, osmium, rhodium, tungsten, rhenium, yttrium, titanium, platinum, gold, iridium, palladium, and combinations thereof. In one embodiment, getter electrode 130, 230, 330 comprises at least one of platinum, gold, iridium, palladium, and combinations thereof.
The invention provides a method of removing at least one contaminant from a fuel cell catalyst. A flow chart outlining the steps of method 400 is shown in
A method of removing at least one contaminant from a membrane electrode assembly (MEA) of a fuel cell is also provided.
As previously described herein, the fuel cell catalyst electrode, in one embodiment, is a fuel cell anode. In this instance, the getter electrode is not a fuel cell catalyst electrode, but instead is one of a disposable electrode or an electrode that can be otherwise be regenerated. The getter electrode is replaced, if necessary or desired, at the end of the cleaning with an electrode free of accumulated contaminants. For example, the fuel cell catalyst electrode that is to be cleaned may be applied, by methods well known in the art, to a portion of the substrate to form a first half of a MEA. A second half of the MEA is formed by applying the getter electrode to a portion of a second substrate. The two halves are attached through the substrate surfaces that do not bear catalyst/electrode material to form a complete membrane electrode assembly. Following cleaning by either method 400 or method 500, the second half of the MEA, which includes the getter electrode (now containing the contaminant) of the MEA, may be removed and replaced by a half-MEA comprising an uncontaminated fuel cell electrode.
In those instances where the fuel cell catalyst electrode to be cleaned using the present methods is a fuel cell cathode and the getter electrode is the fuel cell anode, it is not necessary to replace the getter electrode.
By using methods 400, 500, the removal of mobile contaminants is carried out at a low potential (about 0.1 V) that is sufficient to permit selective migration of the contaminant to the “sacrificial” getter electrode. Removal of such contaminants improves electrode—and fuel cell—performance. Fuel cells in which contaminants have been removed from either the anode or cathode catalyst electrode by the methods described herein exhibit longer lifetimes, greater current densities, and greater output than similar catalyst electrodes that are not treated by these methods.
The following examples are intended to illustrate the features and advantages of the invention and in no way are intended to limit the invention thereto.
Membrane electrode assemblies were prepared by attaching two halves—i.e., membranes catalyzed on one side with either an anode catalyst or a cathode catalyst, with the other side left uncatalyzed—to form a complete MEA. All half-MEAs were prepared using Nafion® 1135 membranes, each having a thickness of about 90 μm, and standard catalyst inks. Unsupported Pt—Ru and Pt catalysts were applied each to Nafion® 1135 membrane using brush painting.
The standard anode ink composition was 86 wt % 1:1 Pt—Ru black (HiSPEC 6000, Johnson Matthey, Plc.) and 14 wt % Nafion® (1100 equivalent weight, Solution Technology, Inc.). The standard cathode ink composition was 90 wt % Pt black (HiSPEC 1000, Johnson Matthey, Plc.) and 10 wt % Nafion®. The catalyst inks were sonicated for about 90 seconds and then directly applied to pre-dried membrane by brush painting at 75° C. An ice bath was used to prevent overheating and minimize evaporation of solvents during painting. The anode and cathode catalyst loadings were approximately 10 mg/cm2 and 6 mg/cm2, respectively.
A reference MEA (i.e., not cleaned) was made by pressing together anode and cathode half-MEAs to form a complete MEA having the same thickness (about 180 μm) as a typical Nafion® 117 MEA. Single-side hydrophilic and double-sided hydrophobic carbon-cloth gas diffusion layers (E-TEK, Inc.), were used on the anode and cathode sides, respectively.
Using the methods described herein, oxide removal from the anode catalyst was carried out on another MEA at a potential of approximately 0.1 V against the fuel cell cathode (here serving as the getter electrode), which served as a quasi-reference/counter electrode, for five hours. High-purity nitrogen and hydrogen were flowed into the anode and cathode, respectively. After cleaning, the cathode half-MEA that served as the getter electrode was removed from the cell and replaced with a new cathode half-MEA.
The performance of the MEA having the cleaned anode catalyst was compared to that of the reference MEA.
a and 8b summarize DMFC performance of the reference MEA and the MEA having the cleaned anode, respectively, after 115-hour and 330-hour life tests. Compared to the reference MEA, the MEA having the cleaned anode shows less performance loss.
MEAs having standard (i.e., not cleaned) ruthenium cathodes and MEAs having Ru cathodes that were cleaned using the methods described herein were prepared using Nafion® 1135 membranes and standard catalyst inks. Unsupported Pt and Ru catalysts were applied to each side of the membranes using brush painting.
The standard anode ink composition was 90 wt % Pt black (HiSPEC 1000, Johnson Matthey, Plc.) and 10 wt % Nafion® (1100 equivalent, Solution Technology, Inc.). The standard cathode ink composition was 83 wt % Ru black (Johnson Matthey, Plc) and 17 wt % Nafion®. Catalyst inks were sonicated for about 90 seconds and then applied to one side of a pre-dried membrane by brush painting. When the first electrode was complete, the membrane was turned over and the other side was painted. An ice bath was used to prevent overheating and minimize evaporation of solvents during the painting. The anode and cathode catalyst loadings were each approximately 6 mg/cm2.
Single-sided hydrophilic and double-sided hydrophobic carbon-cloth GDLs (E-TEK, Inc.) were used on the anode and cathode sides of both MEAs, respectively. Cathode cleaning using the method described herein was carried out at a potential of approximately 0.10 V, measured against the fuel cell cathode (here serving as the getter electrode), which served as a quasi-reference/counter electrode, for five hours. Ultra-high purity hydrogen and nitrogen were flowed into the anode and cathode, respectively.
The performance of the MEA having the cleaned cathode catalyst was compared to the MEA having the standard Ru cathode.
The benefits of a steady-state cleaning of catalysts are evident from the comparison of the two H2-air polarization plots shown in
Long-term H2-air fuel cell performance for a MEA having a cleaned cathode is shown in
MEAs with Pt—Ru anode black (HiSPEC 6000, Johnson Matthey, Plc) and Ru—Se black cathode were prepared using Nafion® 1135 membranes. The anode ink composition was 86 wt % Pt—Ru black and 14 wt % Nafion (1100 equivalent, Solution Technology, Inc.). The cathode ink composition was 83 wt % Ru—Se black and 17 wt % Nafion. The MEA fabrication procedure described in Example 2 was used to prepare the standard (i.e., not cleaned) MEA.
Hydrogen/air fuel cell polarization curves obtained for a standard MEA (i.e., no cathode cleaning) and a MEA having a cathode cleaned using the present methods are compared in
While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention.
This invention was made with government support under Contract No. DE-AC 52-06 NA 25396, awarded by the U.S. Department of Energy. The government has certain rights in the invention.