Patent Application
20220416260
The present invention relates generally to catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells and relates more particularly to a novel such catalyst.
Fuel cells, particularly proton exchange membrane (PEM) fuel cells, represent a promising sustainable and clean energy conversion technology for a number of different applications including, but not limited to, the field of transportation. In a PEM fuel cell, the chemical energy of a fuel, typically hydrogen, and of an oxidizing agent, typically oxygen, is converted into electricity through a pair of redox reactions. Where oxygen is used as the oxidizing agent, the redox reaction involving oxygen is often referred to as the oxygen reduction reaction and typically results in the reduction of oxygen to water. As can be appreciated, the oxygen reduction reaction represents a critical process in the operation of a PEM fuel cell and requires an effective and durable catalyst to attain efficient energy conversion. Typically, platinum-group metals (i.e., platinum and five other noble, precious metal elements clustered with platinum in the periodic table) have been used as such a catalyst, and such metals have shown promising performance and durability in real applications. Unfortunately, however, the high cost and scarcity of platinum-group metals have limited their large-scale deployment in PEM fuel cells and have driven efforts to reduce the usage of platinum-group metals in fuel cell catalysts.
One such approach to reducing the usage of platinum-group metals has been to alloy platinum with a first-row transition metal (M), such as cobalt, nickel, or iron. Because first-row transition metal atoms have a smaller atomic radius than does platinum, incorporating such metal atoms into a platinum-based alloy brings beneficial strain and alloy effects that are significant to optimize O2/intermediates adsorption and to improve the intrinsic oxygen reduction reaction activity. Compared to the common solid solution Al-structure, which is disordered, certain platinum-metal (PtM) alloys with specific Pt/M compositions can form ordered intermetallic structures, including a cubic L12 (Pt3M) and a tetragonal L10 (PtM). These types of ordered intermetallic structures are attributed to a negative enthalpy change often derived from a strong 3d-5d orbital interaction between M and Pt, which enables stabilization of M. Compared to traditional fcc Pt alloys, an ordered intermetallic structure results in less M leaching and improved stability under acidic fuel cell conditions. Unlike the disordered A1-structure, the cubic L12 and the tetragonal L10 structures usually are obtained by thermal annealing at high temperatures (>700° C.). However, an agglomeration of nanoparticles at such high temperatures can result in large particle sizes, which do not provide adequate electrochemically active surface areas, thus limiting performance at high current density.
Another approach for reducing platinum usage in catalysts has been to try to develop a platinum group metal (PGM)-free catalyst. Generally, such catalysts are prepared from earth-abundant elements, such as atomically dispersed metal (M: Fe and Co) single sites coordinated with nitrogen and embedded within a carbon matrix, thereby creating M—N—C catalysts. More specifically, the production of such M—N—C catalysts typically includes two stages, namely, the synthesis of a catalyst precursor and, then, the high temperature treatment or carbonization of the catalyst precursor to form active sites to be occupied by MN4 moieties. See, for example, Zhang et al., “Engineering nanostructures of PGM-free oxygen-reduction catalysts using metal-organic frameworks,” Nano Energy, 31:331-350 (2017), which is incorporated herein by reference. Current M—N—C catalysts are typically derived from zinc-based zeolitic imidazolate frameworks (ZIFs), a subfamily of metal-organic frameworks (MOFs). An example of a ZIF is 2-methylimidazole zinc salt (ZIF-8), which is typically in crystal form. ZIF-8-derived carbon materials synthesized via carbonization at high temperature (e.g., 1100° C.) possess an abundance of micropores and defects.
Some of the more promising M—N—C catalysts are derived from Fe and ZIF-8 precursors and demonstrate encouraging oxygen reduction reaction activity that approaches that of platinum catalysts. In such catalysts, the FeN4 moieties, which are believed to be the oxygen reduction reaction active sites, are dispersed uniformly throughout the carbon matrix. Unfortunately, however, the insufficient stability of these iron-containing catalysts during proton exchange membrane fuel cell (PEMFC) operation has been a significant drawback, severely limiting the viability of an approach involving PGM-free catalysts.
Documents that may be of interest may include the following, all of which are incorporated herein by reference: U.S. Patent Application Publication No. US 2022/0069315 A1, inventors Wu et al., published Mar. 3, 2022; U.S. Patent Application Publication No. US 2022/0190356 A1, inventors Wu et al., published Jun. 16, 2022; PCT International Publication No. WO 2022/015888 A2, published Jan. 20, 2022; Chen et al., “High-Platinum-Content Catalysts on Atomically Dispersed and Nitrogen Coordinated Single Manganese Site Carbons for Heavy-Duty Fuel Cells,” Journal of The Electrochemical Society, 169(3):034510 (March 2022); and Qiao et al., “Atomically dispersed single iron sites for promoting Pt and Pt3Co fuel cell catalysts: performance and durability improvements,” Energy Environ. Sci., 14:4948-4960 (2021).
It is an object of the present invention to provide a novel catalyst suitable for use in the oxygen reduction reaction in a proton exchange membrane fuel cell.
It is another object of the present invention to provide a catalyst as described above that overcomes at least some of the shortcomings associated with at least some of the existing catalysts for use in the oxygen reduction reaction in a proton exchange membrane fuel cell.
Therefore, according to one aspect of the invention, there is provided a hybrid catalyst suitable for use in an oxygen reduction reaction in a proton exchange membrane fuel cell, the hybrid catalyst comprising (a) a support, the support comprising an Mn—N—C support; and (b) platinum-containing nanoparticles dispersed on the Mn—N—C support.
In a more detailed feature of the invention, the Mn—N—C support may comprise atomically dispersed and nitrogen coordinated MnN4 moieties.
In a more detailed feature of the invention, the platinum-containing nanoparticles may have a particle size ranging from about 2 to 8 nm.
In a more detailed feature of the invention, the Mn—N—C support may have a particle size ranging from about 30 to 200 nm.
In a more detailed feature of the invention, the platinum-containing nanoparticles may be present with a loading ranging from about 10 to 60 wt. % against the Mn—N—C support.
In a more detailed feature of the invention, the platinum-containing nanoparticles may be present with a loading ranging from about 20 to 40 wt. % against the Mn—N—C support.
In a more detailed feature of the invention, the platinum-containing nanoparticles may be present with a loading of about 20 wt. % against the Mn—N—C support.
In a more detailed feature of the invention, the platinum-containing nanoparticles may be present with a loading of about 40 wt. % against the Mn—N—C support.
In a more detailed feature of the invention, the platinum-containing nanoparticles may comprise nanoparticles of a platinum alloy.
In a more detailed feature of the invention, the platinum alloy may be a platinum-cobalt alloy.
In a more detailed feature of the invention, the platinum-cobalt alloy may be a platinum-cobalt intermetallic alloy.
In a more detailed feature of the invention, the platinum-cobalt intermetallic alloy may be a cubic L12 Pt3Co alloy.
In a more detailed feature of the invention, the platinum-cobalt intermetallic alloy may be a tetragonal L10 PtCo alloy.
In a more detailed feature of the invention, the platinum-containing nanoparticles may be platinum nanoparticles.
In a more detailed feature of the invention, the Mn—N—C support may further comprise a sulfur dopant.
In a more detailed feature of the invention, the Mn—N—C support may be devoid of a dopant other than the platinum-containing nanoparticles.
According to another aspect of the invention, there is provided a membrane electrode assembly suitable for use in a proton exchange membrane fuel cell, the membrane electrode assembly comprising (a) a proton exchange membrane, the proton exchange membrane having first and second faces on opposite sides; (b) a cathode operatively coupled to the first face of the proton exchange membrane, the cathode comprising the above-described hybrid catalyst; and (c) an anode operatively coupled to the second face of the proton exchange membrane.
According to yet another aspect of the invention, there is provided a method of preparing a hybrid catalyst comprising platinum nanoparticles dispersed on an Mn—N—C support, the method comprising the steps of (a) combining a quantity of a hexachloroplatinic acid solution with a quantity of an Mn—N—C support to form a mixture; (b) sonicating the mixture in an ice bath; (c) then, freeze-drying the product of step (b); (d) then, calcinating the product of step (c) under a forming gas; and (e) then, heating the product of step (d).
According to still yet another aspect of the invention, there is provided a method of preparing a hybrid catalyst comprising nanoparticles of a cubic L12 Pt3Co alloy dispersed on an Mn—N—C support, the method comprising the steps of (a) combining quantities of a hexachloroplatinic acid solution, CoCl2.6H2O, and an Mn—N—C support to form a mixture; (b) sonicating the mixture in an ice bath; (c) then, freeze-drying the product of step (b); (d) then, calcinating the product of step (c) under a forming gas; (e) then, heating the product of step (d); (f) then, leaching the product of step (e) in perchloric acid; (g) then, vacuum-drying the product of step (f); and (h) then, post-treating the product of step (g) at an elevated temperature under argon.
According to a further aspect of the invention, there is provided a method of preparing a hybrid catalyst comprising nanoparticles of a tetragonal L10 PtCo alloy dispersed on an Mn—N—C support, the method comprising the steps of (a) combining quantities of a hexachloroplatinic acid solution, CoCl2.6H2O, and an Mn—N—C support to form a mixture; (b) sonicating the mixture in an ice bath to form a homogeneous complex suspension; (c) then, quickly freezing the product of step (b), followed by freeze-drying overnight; (d) then, heating the product of step (c) under forming gas flow; (e) then, allowing the product of step (d) to cool to room temperature; (f) then, heating the product of step (e) under forming gas; (g) then, leaching the product of step (f) in perchloric acid; and (h) then, post-treating the product of step (g) at an elevated temperature under argon.
Additional objects, as well as aspects, features and advantages, of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration various embodiments for practicing the invention. The embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.
The accompanying drawings, which are hereby incorporated into and constitute a part of this specification, illustrate various embodiments of the invention and, together with the description, serve to explain the principles of the invention. These drawings are not necessarily drawn to scale, and certain components may have undersized and/or oversized dimensions for purposes of explication or may omit certain features for purposes of clarity. In the drawings wherein like reference numerals represent like parts:
The present invention is based, at least in part, on the discovery of a novel catalyst that is suitable for use in an electrochemical cell, such as, but not limited to, a proton exchange membrane fuel cell. More specifically, the present invention is based, at least in part, on the discovery that such a catalyst may be a hybrid catalyst and that said hybrid catalyst may comprise a carbon-based Mn—N—C catalyst (sometimes alternatively referred to herein as an Mn—NC catalyst) that may be used as a support for dispersed platinum or platinum-alloy nanoparticles. The aforementioned Mn—N—C support may be desirable in that it may provide platinum group metal (PGM)-free active sites (e.g., MnN4 sites) that may serve to reduce platinum loading in an oxygen reduction reaction (ORR) cathode. In addition, the nitrogen dopants of the Mn—N—C support may stabilize dispersed platinum nanoparticles with strengthened metal-support interactions. Moreover, the possible synergy between platinum and PGM-free MnN4 sites at the atomic level could lead to performance enhancement.
Therefore, according to one aspect of the present invention, a hybrid catalyst suitable for use as an oxygen reduction reaction catalyst in a proton exchange membrane fuel cell may be made by integrating platinum group metal-containing nanoparticles (PGM NPs) and MnN4 site-rich Mn—N—C carbon with high surface area, the combination of which may be denoted herein as Pt@HS Mn—N—C, where the platinum group metal-containing nanoparticles are platinum nanoparticles, or, alternatively, as PtxCoy@HS Mn—N—C, where the platinum group metal-containing nanoparticles are platinum-cobalt alloy nanoparticles.
As will be discussed further below, MnN4 in carbon is highly effective in dramatically enhancing platinum and platinum-cobalt (Pt—Co) catalyst performance. More specifically, the favorable porous carbon structure of MnN4—C and its abundant nitrogen doping enable a uniform Pt—Co nanoparticle distribution, presenting average particle sizes of 3.0 nm for L12 intermetallic nanoparticles. Unlike the large particle sizes of ordered PtCo intermetallics, the use of the MnN4 site-rich Mn—N—C carbon can significantly reduce particle size while achieving a defined ordered structure. The Pt@HS Mn—N—C and Pt3Co@HS Mn—N—C hybrid catalysts of the present invention show excellent performance and durability in rotating disk electrode (RDE) and membrane electrode assembly (MEA) studies. Compared to traditional carbon black-supported Pt catalysts (Pt/C), a Pt@HS Mn—N—C hybrid catalyst of the present invention may achieve a significantly improved oxygen reduction reaction mass activity (MA) of 0.43 A/mgPt and may retain 83.7% of the initial value after 30,000 accelerated stress test (AST) voltage cycles in an MEA with low cathode loading of 0.1 mgPt cm−2, thereby approaching U.S. Department of Energy (DOE) 2020 targets without using an alloy. Furthermore, a Pt3Co@HS Mn—N—C hybrid catalyst of the present invention may achieve an even higher oxygen reduction reaction mass activity of 0.58 A mgPt−1. In fact, a Pt3Co@HS Mn—N—C catalyst of the present invention may reach a power density of 1132 mW cm−2 at 0.67 V, exceeding the DOE target transportation application of 1.0 W cm−2.
In view of the above, the hybrid catalysts of the present invention may comprise two components, namely, (i) an Mn—N—C support; and (ii) platinum-containing nanoparticles dispersed on the Mn—N—C support. Additional information concerning these two components is provided below.
The Mn—N—C support may be similar or identical to the Mn—N—C support disclosed in U.S. Patent Application Publication No. US 2022/0069315 A1, inventors Wu et al., published Mar. 3, 2022 and/or in Chen et al., “Atomically Dispersed MnN4 Catalysts via Environmentally Benign Aqueous Synthesis for Oxygen Reduction: Mechanistic Understanding of Activity and Stability Improvements,” ACS Catalysis, 10(18):10523-10534 (2020), both of which are incorporated herein by reference.
More specifically, according to one embodiment of the invention, the Mn—N—C support may be prepared by a method that first comprises preparing an Mn-doped ZIF-8 precursor. The Mn-doped ZIF-8 precursor preferably is prepared using one or more aqueous solvents, such as an acid solution, which may be, for example, a hydrochloric acid/water solution or a nitric acid/water solution, and preferably is devoid of any organic solvents. For example, the Mn-doped ZIF-8 precursor may be prepared, for example, by reacting (a) a first solution comprising zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and manganese chloride (MnCl2) dissolved in a hydrochloric acid/water solution with (b) a second solution comprising 2-methylimidazole dissolved in water. The molar ratio of Mn2+ to Zn2+ used for the synthesis can range from greater than 0 up to about 10%. Preferably, the molar ratio among MnCl2, Zn(NO3)2.6H2O, and 2-methylimidazole is about 0.015:1:47, respectively. The particle size of the Mn/ZIF precursor may be adjusted by modifying the concentration of chemicals in the synthesis reaction. For example, the concentration of Zn2+ in the aqueous solution may be adjusted at 66.9 mmol l−1, 96.1 mmol l−1, and 122.8 mmol l−1 to prepare Mn catalysts with 30 nm, 85 nm, and 200 nm particle sizes, respectively. In other words, as the molar ratio of water to Zn2+ decreases, the catalyst particle size increases.
After collecting the precipitate by centrifugation, followed by washing with alcohol, the precipitate may be dried, for example, by heating at 60° C. in an oven overnight.
Next, the Mn-doped ZIF-8 precursor may be thermally activated (i.e., carbonized). The thermal activation may take place in a single pyrolysis step, such as by heating at an elevated temperature, such as 1100° C., or may take place in a multi-step pyrolysis process, such as by heating at a first temperature (e.g., 600° C.-1000° C.) for a first interval and then heating at a second temperature, which may be a higher temperature (e.g., 900° C.-1100° C.) for a second interval. Preferably, the foregoing heating takes place in an argon atmosphere.
After the aforementioned carbonization has taken place, the catalyst material may optionally be subjected to further processing. In one embodiment, such further processing may comprise an absorption process, followed by a second thermal activation step. The absorption process may comprise, for example, dispersing the catalyst material in a solution, wherein the solution may comprise manganese chloride and urea in a solvent comprising hydrochloric acid and isopropanol, preferably in equal amounts. Next, the solution may be ultrasonicated for a period of time, such as 30 minutes, and then may be subjected to magnetic stirring for a period of time, such as two hours. Next, the mixture may be subjected to centrifugation and then dried, for example, at 60° C. in a vacuum oven for 12 hours. The second thermal activation step may be conducted, for example, at a temperature of 1100° C. in an argon atmosphere for one hour.
The Mn—N—C support of the present invention may be in particulate form and may have a particle size ranging from about 30 nm to about 200 nm.
A specific example of a protocol for making such a support may be as follows: First, a mass of 33.9 g of 2-methylimidazole may be dissolved in 80 mL of deionized water to produce a Solution A. Then, masses of 2.631 g of Zn(NO)2.6H2O and 16.0 mg of MnCl2 may be dissolved in a mixture of 0.5 mL of 2 M HCl solution and 10.5 mL of H2O to produce a Solution B. Then, Solution B may be added into Solution A under stirring, and stirring may be continued for 6 hours. The reaction may then be kept at room temperature. Next, the white product may be centrifuged out at 9000 rpm, and the Mn-ZIF-8 may be washed with methanol 5 times. All of the precipitant may be collected and dried at 60° C. in a vacuum oven for 12 hours. The dried white powder may then be finely ground and heated at 800° C. in a tube furnace under N2 flow for 2 hours. Then, the tube furnace may be heated to 1100° C. within 30 minutes. Pyrolysis at 1100° C. for 1 hour may occur; then, the product may be allowed to cool down to room temperature naturally. A mass of 40.0 mg of the thus prepared Mn—N—C may be combined with a mass of 200.0 mg of urea and a mass of 40.0 mg of MnCl2 in a glass vial. Next, 2.0 mL of 0.25M HCl and 2.0 mL of IPA may be added to the vial. Next, the mixture may be ultrasonicated for 1 hour and then stirred for another 2 hours. Next, the Mn—N—C adsorbed with urea and MnCl2 may be centrifuged out, and the product may be vacuum-dried overnight at 60° C. A mass of 40 mg of Mn—N—C adsorbed with urea and MnCl2 may be added into a crucible boat and then pyrolyzed in Ar at 1100° C. for 1 hour. The as-prepared Mn—N—C may be denoted “HS Mn—N—C-1100,” in which the prefix “HS” refers to high surface area. The suffix “1100,” as used in Mn—N—C-1100, as well as similar suffixes used throughout the present application, refers to a thermal activation or pyrolysis temperature used in the fabrication of the support. HS Mn—N—C-1100 is especially referred to the Mn—N—C prepared by the method mentioned above, as its surface area is higher than 1200 m2 gcat−1.
For purposes of the present invention, Mn—N—C supports may include, in addition to the platinum-containing nanoparticles, one or more dopants, an example of which may include sulfur. An example of such a support may be Mn—S—N—C-1100, in which thiourea may be used as a precursor to dope the Mn—N—C matrix with sulfur. Other types of Mn—N—C supports may include Mn—N—C x3AC-1100 and 125-0.12Mn—N—C x4AC-1100, both of which are prepared by a solid synthesis strategy. More specifically, Mn—N—C x3AC-1100 may use MnO2 as a manganese source and also may use an ammonium chloride post-treatment. By contrast, 125-0.12Mn—N—C x4AC-1100 may use Mn-doped ZnO2 nanoparticles as a manganese source and also may use an ammonium chloride post-treatment.
The platinum-containing nanoparticles of the present invention may have a particle size ranging from about 2 nm to about 8 nm. In addition, the platinum-containing nanoparticles of the present invention may be present in the hybrid catalyst with loadings ranging from about 10 to 60 wt. % (e.g., 40 wt. %) against the Mn—N—C-support.
In one embodiment, the platinum-containing nanoparticles of the present invention may consist of or may comprise nanoparticles of platinum. As an example, according to one embodiment, a catalyst comprising such nanoparticles of platinum may be made, for example, by a forming gas reduction method (10 vol. % H2 balanced with argon) with a controlled platinum loading (e.g., 20 wt. %). More specifically, 2.43 mL hexachloroplatinic acid solution (10.0 g L−1) may be dropped into a glass vial containing 36.0 mg of an Mn—N—C support. The mixture may then be sonicated in an ice bath for 1 h and then freeze-dried for 12 hours. The fine powder may be transferred into a crucible boat, and the precursor calcinated at 200° C. for 6 hours (10 min reach to 200° C.) under forming gas. Then, the temperature may be ramped to 600° C. and held for 1 h (30 min reach to 600° C.). Such catalysts may be denoted herein as Pt@Mn—N—C-600. For comparison, the catalysts may alternatively be annealed at 700° C., 800° C. or 900° C., with such catalysts being denoted according to the thermal annealing temperatures.
In another embodiment, the platinum-containing nanoparticles may consist of or may comprise nanoparticles of a platinum alloy. The platinum alloy may be a platinum-cobalt alloy and, more particularly, may be a platinum-cobalt intermetallic alloy. For example, and without limitation, the platinum-cobalt intermetallic alloy may be a cubic L12 Pt3Co alloy or may be a tetragonal L10 PtCo alloy.
Where the platinum-cobalt intermetallic alloy is a cubic L12 Pt3Co alloy, a catalyst comprising such an alloy may be made, for example, by a forming gas reduction method (10 vol. % H2 balanced with argon) with a controlled platinum loading (e.g., 20 wt. %). More specifically, 2.43 mL hexachloroplatinic acid solution (10.0 g L−) and 60 μL CoCl2.2H2O (150.0 g L−1) may be dropped into a glass vial containing 38.5 mg of an Mn—N—C support. The mixture may be sonicated in an ice bath for 1 h and then freeze-dried for 12 hours. The fine powder may then be transferred into a crucible boat, and the precursor calcinated at 350° C. for 2 hours (30 min reach to 350° C.) under forming gas. Then, the temperature may be ramped to 800° C. and held for 2 hours (25 min reach to 800° C.). The product may then be leached in 0.1M HClO4 at 60° C. for 12 hours and then vacuum-dried at 60° C. for 12 hours. The leached product may then be post-treated at 400° C. under argon for 1 hour to obtain the final catalyst. Such a catalyst may be denoted herein as L12 Pt3Co@Mn—N—C-800.
Where the platinum-cobalt intermetallic alloy is a tetragonal L10 PtCo alloy, a catalyst comprising such an alloy may be made, for example, by a forming gas reduction method (10 vol. % H2 balanced with argon) with a controlled platinum loading (e.g., 20 wt. %). More specifically, 2.505 mL hexachloroplatinic acid solution (10 mg/mL) may be dropped into a vial containing 37.0 mg HS Mn—N—C-1100 and 27.5 mg CoCl2.6H2O. The mixture may be sonicated in an ice bath for 1 hour to form a homogeneous complex suspension. The suspension may then be quickly frozen with liquid nitrogen, followed by freeze-drying overnight. The dried powder may then be heated at 350° C. in a tube furnace under forming gas flow for 2 hours. After cooling down to 25° C., the furnace may be reheated to 750° C. for another 3 hours under forming gas for ordering L10 PtCo intermetallic structures. The resulting powder may be leached by 0.1M HClO4 at 60° C. for 6 hours and post-treated at 400° C. under argon to obtain the final catalyst.
As will be discussed further below, the hybrid catalyst of the present invention may be used to make a membrane electrode assembly (MEA), such as, but not limited to an MEA of the type comprising a proton exchange membrane. For example, in one embodiment, an MEA of the type that comprises a proton exchange membrane and that is suitable for use in a fuel cell may be fabricated as follows: An ink may be prepared, the ink comprising the hybrid catalyst of the present invention and one or more suitable ionomers. The ink may then be directly applied, for example, by spray-coating, painting, or any other suitable technique, to one surface of the proton exchange membrane. The ink may then be fused to the proton exchange membrane by hot-pressing or any other suitable technique, thereby forming a catalyst coating directly on the proton exchange membrane. Said coating may serve, for example, as the cathode for the oxygen reduction reaction of a fuel cell. A suitable catalyst coating that may be used as the anode for the hydrogen oxidization reaction of a fuel cell may be directly applied and fused in an analogous fashion to the opposite surface of the proton exchange membrane. The foregoing membrane electrode assembly may be regarded as being of the membrane electrode assembly type commonly referred to in the art as a catalyst coated membrane.
Referring now to
Membrane electrode assembly (MEA) 11, which may be suitable for use in, for example, a fuel cell or other electrochemical cell, may be regarded as a catalyst coated membrane. MEA 11 may comprise a proton exchange membrane (also sometimes referred to as a solid polymer electrolyte membrane) (PEM) 13. PEM 13 is preferably a non-porous, ionically-conductive, electrically-non-conductive, liquid permeable and substantially gas-impermeable membrane. PEM 13 may consist of or comprise a homogeneous perfluorosulfonic acid (PFSA) polymer. Said PFSA polymer may be formed by the copolymerization of tetrafluoroethylene and perfluorovinylether sulfonic acid. See e.g., U.S. Pat. No. 3,282,875, inventors Connolly et al., issued Nov. 1, 1966; U.S. Pat. No. 4,470,889, inventors Ezzell et. al., issued Sep. 11, 1984; U.S. Pat. No. 4,478,695, inventors Ezzell et. al., issued Oct. 23, 1984; U.S. Pat. No. 6,492,431, inventor Cisar, issued Dec. 10, 2002; and U.S. Pat. No. 9,595,727 B2, inventors Mittelsteadt et al., issued Mar. 14, 2017, all of which are incorporated herein by reference in their entireties. A commercial embodiment of a PFSA polymer electrolyte membrane is manufactured by The Chemours Company FC, LLC (Fayetteville, N.C.) as NAFION™ extrusion cast PFSA polymer membrane.
MEA 11 may further comprise a cathode 15 and an anode 17. Cathode 15 and anode 17 may be positioned along two opposing major faces of PEM 13. In the present embodiment, cathode 15 is shown positioned along the top face of PEM 13, and anode 17 is shown positioned along the bottom face of PEM 13; however, it is to be understood that the positions of cathode 15 and anode 17 relative to PEM 13 could be reversed.
Cathode 15 may consist of or may comprise a catalyst layer comprising the hybrid catalyst of the present invention and may be formed in the manner described above by being applied directly to PEM 13.
Anode 17, which may be a catalyst layer of the type conventionally used in a PEM-based fuel cell for the hydrogen oxidation reaction, may comprise electrocatalyst particles in the form of a finely divided electrically-conductive and, optionally, ionically-conductive material (e.g., a metal powder) which can sustain a high rate of electrochemical reaction. The electrocatalyst particles may be distributed within anode 17 along with a binder, which is preferably ionically-conductive, to provide mechanical fixation. Anode 17 may be formed in the conventional manner by being applied directly to PEM 13.
Although the catalyst coatings of the above-described MEA are applied directly to the proton exchange membrane, it is to be understood that the MEA of the present invention is not limited to such a construction. For example, in another embodiment, the respective catalyst coatings may be applied to suitable substrates, such as gas diffusion media (e.g., carbon paper), and two such coated substrates may then be positioned relative to the proton exchange membrane so that their catalyst coatings directly contact opposing surfaces of the proton exchange membrane. Then, the coated substrates may be fused to the proton exchange membrane by hot-pressing or another suitable technique. Such a membrane electrode assembly may be regarded as including a plurality of catalyst coated substrates.
For example, referring now to
MEA 31 may comprise a PEM 33, which may be similar or identical to PEM 13 of MEA 11. MEA 31 may further comprise a cathode 35 and an anode 37. Cathode 35 and anode 37 may be positioned along two opposing major faces of PEM 33. In the present embodiment, cathode 35 is shown positioned along the top face of PEM 33, and anode 37 is shown positioned along the bottom face of PEM 33; however, it is to be understood that the positions of cathode 35 and anode 37 relative to PEM 33 could be reversed.
Cathode 35, in turn, may comprise a cathode electrocatalyst layer 39 and a cathode support 41. Cathode electrocatalyst layer 39, which may be similar or identical to cathode 15, may be positioned in direct contact with PEM 33, and, in the present embodiment, is shown as being positioned directly above and in contact with the top side of PEM 33.
Cathode support 41, which may be a cathode support of the type conventionally used in a PEM-based fuel cell, preferably comprises a material that is sufficiently porous to allow fluid (gas and/or liquid) transfer between cathode electrocatalyst layer 39 and some fluid conveying tube, cavity, or structure. In addition, cathode support 41 is preferably electrically-conductive to provide electrical connectivity between cathode electrocatalyst layer 39 and a cathode current collector or similar structure. Moreover, cathode support 41 is also preferably ionically-non-conductive. Cathode support 41 may be positioned in direct contact with cathode electrocatalyst layer 39 and, in the present embodiment, is shown as being positioned directly above cathode electrocatalyst layer 39 such that cathode electrocatalyst layer 39 may be sandwiched between and in contact with PEM 33 and cathode support 41. Cathode support 41 may be dimensioned to entirely cover a surface (e.g., the top surface) of cathode electrocatalyst layer 39, and, in fact, cathode 35 may be fabricated by depositing cathode electrocatalyst layer 39 on cathode support 41. Cathode 35 may then be coupled to PEM 33 by hot-pressing or another suitable technique.
Anode 37 may comprise an anode electrocatalyst layer 43 and an anode support 45. Anode electrocatalyst layer 43, which may be similar or identical to anode 17, may be positioned in direct contact with PEM 33, and, in the present embodiment, is shown as being positioned directly below and in contact with the bottom of PEM 33.
Anode support 45, which may be an anode support of the type conventionally used in a PEM-based fuel cell and may be, for example, a film or sheet of porous carbon, preferably comprises a material that is sufficiently porous to allow fluid (gas and/or liquid) transfer between anode electrocatalyst layer 43 and some fluid conveying tube, cavity, or structure. In addition, anode support 45 is electrically-conductive to provide electrical connectivity between anode electrocatalyst layer 43 and an anode current collector. Moreover, anode support 45 is also preferably ionically-non-conductive. Anode support 45 may be positioned in direct contact with anode electrocatalyst layer 43 and, in the present embodiment, is shown as being positioned directly below anode electrocatalyst layer 43 such that anode electrocatalyst layer 43 may be sandwiched between and in contact with PEM 33 and anode support 45. Anode support 45 may be dimensioned to entirely cover a surface (e.g., the top surface) of anode electrocatalyst layer 43, and, in fact, anode 37 may be fabricated by depositing anode electrocatalyst layer 43 on anode support 35. Anode 37 may then be coupled to PEM 33 by hot-pressing or another suitable technique.
The following examples are given for illustrative purposes only and are not meant to be a limitation on the invention described herein or on the claims appended hereto.
The hybrid catalyst of the present invention may be made by various methods. For example, according to one embodiment, the Mn—N—C support may be made using an environmentally benign aqueous synthesis approach in which Mn-doped zeolitic imidazolate frameworks (ZIFs) are synthesized in water. This technique is disclosed, for example, in U.S. Patent Application Publication No. US 2022/0069315 A1 and in Chen et al., “Atomically Dispersed MnN4 Catalysts via Environmentally Benign Aqueous Synthesis for Oxygen Reduction: Mechanistic Understanding of Activity and Stability Improvements,” ACS Catalysts, 10(18):10523-10534 (2020), both of which are incorporated herein by reference. In the subsequent platinum nanoparticle deposition, an impregnation method with freeze-drying may be used to disperse the platinum nanoparticles on the Mn—N—C support. A forming gas (10 vol. % H2 in argon) may be applied as a reductant to prepare the Pt/MnN4—C catalyst. The foregoing method may be advantageous in that it may minimize the possible damage of MnN4 sites by avoiding a complicated wet chemistry synthesis.
The morphology and composition of Pt@Mn—N—C-600, which was made as described above, were investigated by high-resolution transmission electron microscopy (HR-TEM). More specifically, referring to
The motif of synergistic electrocatalysis between platinum and manganese single atom catalysts (SACs) was further verified by anchoring platinum on different kinds of Mn—N—C catalysts. For example, one alternative Mn—N—C catalyst, namely, Mn—SNC-1100, was prepared by an approach reported by Guo et al., “Promoting Atomically Dispersed MnN4 Sites via Sulfur Doping for Oxygen Reduction: Unveiling Intrinsic Activity and Degradation in Fuel Cells,” ACS Nano, 15(4):6886-6899 (2021), which is incorporated herein by reference. In this catalyst, thiourea may be used as a precursor to dope an Mn—N—C matrix with sulfur. More specifically, in one embodiment, ZIF-8 may be subjected to a thermal activation at 900° C. under argon flow for 1 h in a tube furnace, with the heating rate being 30° C. min−1. The product may be collected when the temperature cools down to room temperature, this product being denoted as NC. Thiourea (130 mg), and MnCl2 (20 mg) may be dissolved in 2 mL solution (1 mL isopropanol, 1 mL 0.25 M HCl), followed by sonicating for 3 min, this solution being labeled as Solution A. Solution A may be added to a vial containing 20 mg of NC powder. Then, 0.2 mL of 2 M HCl solution may be added to the vial to avoid hydrolysis of MnCl2. The mixture may be sonicated for 30 min below 18° C. and stirred for at least 1 h. Then, the precipitants may be collected by centrifugation without washing and then dried under a vacuum oven at 50° C. for 12 h. The dried precipitants may then be treated at 1100° C. for 1 h under argon flow, with the heating rate being 33° C. min−1, the resulting product being Mn—SNC.
Another alternative Mn—N—C catalyst, namely, Mn—NC x3AC-1100, may be prepared by a solid synthesis strategy, in which MnO2 may be employed as a manganese source, and an ammonium chloride (AC) post-treatment may be employed to further increase the density of manganese single atoms in the catalyst. More specifically, in one embodiment, 6.78 g Zn(NO)2.6H2O and 50 mg MnO2 may be dispersed in 200 mL methanol in a round-bottom flask, followed by 20 min ultra-sonication. 2-methylimidazole (7.88 g) may be dissolved in another 200 mL methanol to form a Solution A. Solution A may be poured gradually into the round-bottom flask. The flask may be sealed with a rubber stopper along with a cable tie. The mixture may be transferred into a 60° C. oven. The oven may be kept at a constant temperature of 60° C. for 24 h. After cooling, the resulting suspension may be separated into four centrifuge tubes by centrifuging at 9000 rpm (10-15 mins each time) to collect all precipitant and washing with ethanol three times. 30 mL of ethanol for each tube may be used to wash the precipitant each time. The precipitant may be collected and dried at 60° C. in a vacuum oven for 12 h. The dried powder may then be finely ground to produce MnO2-ZIF-8. 300 mg of the MnO2-ZIF8 may be heated at 800° C. for 1 h. After heat treatment, the furnace may be cooled down to 25° C. in 30 minutes in program. Then, 100 mg of obtained carbon may be mixed with 300 mg NH4Cl (x3AC) and heat-treated at 1100° C. for 1 h in a tube furnace under argon flow with a ramping rate of 35.8° C. min−1. Then, the product may be allowed to cool down to room temperature naturally. Then, the obtained black powder may be ground and denoted as Mn—NC x3AC.
Yet another alternative Mn—N—C catalyst, namely, 125-0.12Mn—NC x4AC-1100, may be prepared using Mn-doped ZnO2 nanoparticles, in which an ammonium chloride post-treatment may also be employed. For example, in one embodiment, 4.23 g Zn(NO)2.6H2O, 0.15 g MnCl2, and 12.0 g sodium citrate may be dissolved in 180 mL deionized water under 60° C. Then, 4 g sodium hydroxide may be added and maintained for 1.5 h. After carefully washing and centrifuging, the as-prepared white powders may be vacuum-dried at 60° C. overnight and denoted as 0.15Mn—ZnO. 6.78 g Zn(NO)2.6H2O and 125 mg 0.15Mn—ZnO nanoparticles may be dispersed in 200 mL methanol in a round-bottom flask followed by 20 min ultra-sonication. 2-methylimidazole (7.88 g) may be dissolved in another 200 mL methanol and denoted as Solution A. Solution A may be gradually poured into the round-bottom flask. The flask may then be sealed with a rubber stopper along with a cable tie. The mixture may then be transferred into a 60° C. oven, the oven being kept at a constant temperature of 60° C. for 24 h. After cooling, the resulting suspension may be separated into four centrifuge tubes by centrifuging at 9000 rpm (10-15 mins each time) to collect all precipitant and washing with ethanol for three times. 30 mL of ethanol for each tube may be used to wash the precipitant each time. The precipitant may be collected and dried at 60° C. in a vacuum oven for 12 h. The dried powder may then be finely ground to produce 0.15MnZnO-ZIF-8. 300 mg of 0.15MnZnO-ZIF-8 may be heated at 800° C. for 1 h. After heat treatment, the furnace may be cooled down to 25° C. in 30 minutes in program. Then, 100 mg of obtained carbon may be mixed with 400 mg NH4Cl (x4AC) and heat-treated at 1100° C. for 1 h in a tube furnace under argon flow with a ramping rate of 35.8° C. min−1. The product may then be allowed to cool down to room temperature naturally. The obtained black powder may then be ground and denoted as 125-0.15Mn—NC x4AC.
The porosity and microstructure of the aforementioned Mn—N—C catalysts were characterized by BET (Brunauer-Emmett-Teller), XRD (X-ray diffraction), and Raman spectra. In particular, referring to
Platinum-cobalt (Pt—Co) intermetallic nanoparticle catalysts represent one of the most active oxygen reduction reaction catalysts known. In the experiments below, Pt—Co intermetallic nanoparticles were integrated with an active Mn—N—C support by using an impregnation method, followed by a reduction under forming gas at 350° C. Examples of Pt—Co intermetallic nanoparticles that were used include cubic L12 Pt3Co intermetallic nanoparticles and tetragonal L10 PtCo intermetallic nanoparticles.
To prepare the L12 Pt3Co intermetallic structures, a method for controlling their intermetallic structures was employed, said method involving annealing in a forming gas (10% H2 in argon). It is believed that, by using the foregoing technique, the forming gas may enlarge the lattice spacing of platinum by changing the coordination environment of platinum, which allows the incorporation of more cobalt into the platinum lattice. Excess cobalt may behave as an obstacle to the formation of nanoparticle agglomerates during the annealing and can be easily removed using subsequent acid treatment. The foregoing method enables effective control of L12 Pt3Co intermetallic structures on the Mn—N—C support, which allows a direct comparison of their catalytic activity, stability, and MEA (membrane electrode assembly) performance.
The morphology and structure of L12 Pt3Co@HS Mn—NC-800 have been studied by high-resolution transmission electron microscopy (HR-TEM) imaging and diffraction techniques. In
To prepare tetragonal L10 PtCo intermetallic structures, PtCo nanoparticles may be deposited onto HS (high surface area) Mn—N—C-1100 through a forming gas (hydrogen (10%)+argon) reduction method, for example, with a controlled platinum mass loading of 20 wt. %. More specifically, in one example, 2.505 mL hexachloroplatinic acid solution (10 mg/mL) was dropped into a vial containing 37.0 mg HS Mn—N—C-1100 and 27.5 mg CoCl2.6H2O. The mixture was sonicated in an ice bath for 1 hour to form a homogeneous complex suspension. The suspension was quickly frozen with liquid nitrogen, followed by freeze-drying overnight. The dried powder was then heated at 350° C. in a tube furnace under forming gas flow for 2 h. After cooling down to 25° C., the furnace was reheated to 750° C. for another 3 hours under forming gas to prepare ordered L10 PtCo intermetallic structures. The resulting powder was leached by 0.1M HClO4 at 60° C. for 6 hours and post-treated at 400° C. under argon to obtain the final catalyst.
Alternatively, the cubic crystal structure of L12 Pt3Co can be transformed into the tetragonal crystal structure of L10 PtCo by further adding cobalt in the formation of intermetallics.
In order to identify the promise of an Mn—N—C catalyst as a supporting material for a Pt-based catalyst, identical synthesis methods were used to produce corresponding Co-based and Ni-based supports, and the oxygen reduction reaction (ORR) catalytic performances for the various catalysts were studied and compared. In
In
Next, platinum catalysts were prepared using Mn—N—Cs as supports. In
The impact of thermal annealing on the oxygen reduction reaction performance was investigated by using high surface area (HS) Mn—NC-1100 as a support. In
To increase the activity and stability of a hybrid platinum/Mn—N—C catalyst of the present invention, cobalt was introduced into the synthesis of the catalyst nanoparticles. For example, in one case, L12 Pt3Co intermetallics were synthesized. A high annealing temperature was employed to promote the formation of Pt3Co intermetallics, instead of a random Pt3Co alloy. As can be seen in
In addition to activity, durability is another important criterion for platinum-based catalysts to be used in proton-exchange membrane fuel cells (PEMFCs). The activity and durability of L12 Pt3Co@HS Mn—NC-800 and Pt@HS Mn—NC-600 were further investigated in a single cell test. As can be seen in
In practical applications, the current density above 0.67 V is more meaningful. For Pt@HS Mn—NC-600, the current density was 1.16 A cm−2 whereas for L12 Pt3Co@HS Mn—NC-800, the current density increased to 1.69 A cm−2. The voltage losses at 0.8 A cm−2 were 14 and 22 mV for Pt@HS Mn—NC-600 and L12 Pt3Co@HS Mn—NC-800, respectively. The high activity and durability of both Pt and L12 P3Co supported on HS MN—NC are very desirable.
The aged L12 Pt3Co@HS Mn—NC-800 was investigated with high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive x-ray spectra (EDS). It was found that the particle size of L12 Pt3Co@HS Mn—NC-800 increased after intensive accelerated durability test (ADT) cycling. The ultrafine Pt3Co nanoparticles agglomerated, leading to a sparse distribution of platinum-based nanoparticles on the Mn—N—C support. In one case, two Pt3Co nanoparticles merged into one, with a partial retention of ordered structure. Electron energy loss spectra (EELS) indicated that Mn atoms were still atomically dispersed in the N-doped carbon matrix. In addition, there is a heterogeneous distribution of PtCo particles on the Mn—N—C support. Energy-dispersive X-ray spectra (EDS) mapping indicated that cobalt leached away during accelerated stress test (AST) cycles, with the atomic ratio of platinum to cobalt increasing to 92.55:7.45.
As noted above, the cubic crystal structure of L12 Pt3Co can be transformed into tetragonal crystals by further adding cobalt during the formation of intermetallics. As shown in
Recently, driven by the distinct scalability of fuel cells concerning energy and power, more significant attention regarding fuel cell applications has been transferred from light-duty vehicles (LDVs) to heavy-duty vehicles (HDVs). (Heavy-duty vehicles may be regarded as vehicles having a weight above a certain threshold, such as 26,001 pounds.) In heavy-duty vehicles, the dimension of the fuel cell stack or hydrogen tank can be increased with a small additional weight/volume penalty. Moreover, fewer hydrogen stations are needed because of predesigned routes for heavy-duty vehicles, resulting in less infrastructure investment. Additionally, given the high loading of precious metal used in current diesel trucks, the shift to heavy-duty vehicles allows for platinum loadings as high as around 0.3 mg cm−2 without a significant increase in cost. However, durability issues associated with heavy-duty vehicles are more challenging because they require an extended lifespan (more than 25,000 hours), as compared to light-duty vehicles (around 5,000 hours). Moreover, higher operating voltages and temperatures for high efficiency and much longer lifespan tend to accelerate cell degradation. Therefore, new materials (membranes, ionomers, and catalysts) are needed to meet the challenging efficiency and durability requirements posed by heavy-duty vehicles.
The loss of catalyst performance in membrane electrode assemblies (MEAs) significantly contributes to the durability issue. For example, power density loss mainly results from the loss of catalytic electrochemical surface area (ECSA), which is a consequence of platinum nanoparticle agglomeration and dissolution. The carbon support plays a vital role in achieving fine platinum nanoparticle (NP) dispersion, compatibility with an ionomer, sufficient ECSA, and strengthened Pt-carbon interaction, directly governing performance and durability. In addition, oxygen transport, the reaction kinetics of oxygen reduction reaction (ORR), and ECSA can be well-controlled by tuning the pore structure of carbon supports. Generally, the carbon supports can be divided into three main categories: (i) high surface area and relatively amorphous mesoporous carbon; (ii) highly graphitized but less porous carbon; (iii) carbon supports that combine the advantages of the above two kinds of carbons, together with the trade-off of the graphitic structure and porosity. Typically, a high-surface-area carbon support improves the mass activity and accessibility to reactants of platinum nanoparticles. However, it generally contains dominant defects and amorphous structures, which are prone to oxidization. In contrast, a high degree of graphitization can reduce carbon oxidation and improve durability, but is less favorable for improving platinum nanoparticle dispersion and strengthening platinum-carbon interactions.
Besides controlling the size and dispersion of platinum nanoparticles, the intrinsic activity of a platinum catalyst can be improved by preparing the PtM (wherein M is a transition metal) alloy nanostructure. However, transition metals are typically leached out during fuel cell operation, resulting in a loss of benefit of introducing the transition metals into platinum. Even worse, the leached transition metals can contaminate the membrane and ionomer, which mitigates proton and oxygen transports.
Compared to traditional 20 wt. % Pt/C catalysts, a high platinum content on carbon support (e.g., 40 wt. %) is more desirable for heavy-duty membrane electrode assemblies (MEAs) because the higher platinum loading in a heavy-duty MEA cathode (0.2 mgPt/cm2) causes thick cathode layers and significantly increases the mass transfer resistance. Thus, high platinum content on carbon support is more suitable for heavy-duty MEAs, which can achieve high platinum loadings without a mass transport penalty. Therefore, in this example, two platinum catalysts were fabricated with a high platinum loading content (40 wt. %). One such catalyst is a highly graphitized porous graphitic carbon (PGC) with a relatively lower porosity than a commercially available high-surface-area TKK carbon (Tanaka Holdings Co., Ltd., Tokyo, Japan). The other is a platinum group metal (PGM)-free carbon catalyst derived from Mn-doped ZIFs, containing atomically dispersed and nitrogen coordinated single Mn sites embedded in a high-surface-area and partially graphitic carbon structure (Mn—N—C). Both supports were doped with N dopants, resulting in a basic property and enhanced π-bonding, due to the nature of nitrogen as an electron donor, which plays an essential role in anchoring platinum nanoparticles firmly and mitigating platinum nanoparticle migration. Importantly, the atomically dispersed single metal site (e.g., FeN4 or CoN4) could generate a synergy to boost the oxygen reduction reaction (ORR) activity of platinum catalysts. As will be discussed below, a comparison of the foregoing platinum catalysts to a commercial TKK 40 wt. % Pt/C (TEC10E40E) catalyst (Tanaka Holdings Co., Ltd., Tokyo, Japan) reveals that the Mn—N—C carbon is more advantageous than the aforementioned TKK Pt/C catalyst for at least the reason that it is more capable of significantly promoting catalytic activity and stability of platinum nanoparticles in both aqueous acids and MEAs. Extensive characterization further elucidates that the Mn—N—C carbon support provides much-strengthened interaction between platinum nanoparticles and the carbon support, therefore mitigating possible platinum nanoparticle agglomeration or de-attachment under heavy-duty fuel cell operating conditions.
Catalyst synthesis: The synthesis procedure for the Mn—N—C support is based on the procedure discussed above, as well as that disclosed in Chen et al., “Atomically Dispersed MnN4 Catalysts via Environmentally Benign Aqueous Synthesis for Oxygen Reduction: Mechanistic Understanding of Activity and Stability Improvements,” ACS Catalysis, 10:10523-10534 (2020), which is incorporated herein by reference. The synthesis for the PGC support is based on the procedure disclosed in Qiao et al., “3D porous graphitic nanocarbon for enhancing the performance and durability of Pt catalysts: a balance between graphitization and hierarchical porosity, Energy & Environmental Science, 12:2830-2841 (2019), which is incorporated herein by reference.
The platinum nanoparticle deposition may be carried out using a modified ethylene glycol (EG) reduction method with a controllable platinum mass content of 40 wt. %. More specifically, in one embodiment, the carbon support (e.g., 40 mg) may be dispersed in an EG/H2O (1:1) solution (e.g., 160 ml) first, followed by sonicating for one hour to form a homogeneous suspension. Then, a certain amount of chloroplatinic acid solution may be added to the suspension under sonicating for 30 minutes purged with N2. The carbon suspension containing platinum sources may be refluxed at 130° C. for four hours under continuous purging with N2 bubbling. The resulting catalysts may be filtered with Millipore water and dried at 60° C. in a vacuum oven for 24 hours. Finally, the resultant catalyst may be post-treated at 800° C. under argon for one hour.
Physical characterization: X-ray diffraction (XRD) pattern was performed on a Rigaku Ultima IV diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα X-rays to study crystal phases in each sample. Raman spectroscopy was performed using a Renishaw Raman system (Renishaw, Inc., West Dundee, Ill.) at 514 nm excitation. Samples were prepared as ink on a standard microscope glass slide, with the excitation laser focused through a 50× microscope objective for a total interrogation spot size of 1.0-micron diameter. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos AXIS Ultra DLD XPS system (Kratos Analytical Limited, Manchester, UK) equipped with a hemispherical energy analyzer and a monochromatic Al Kα source. The monochromatic Al Kα source was operated at 15 keV, and 150 W. Pass energy was fixed at 40 eV for the high-resolution scans. All samples were prepared as pressed powders supported on a metal bar for the XPS measurements. Transmission electron microscope (TEM) images were obtained on a Thermo-Fisher Talos F200X transmission electron microscope (Thermo Fischer Scientific, Waltham, Mass.). Secondary electron and medium-angle annular dark-field-scanning TEM (MAADF-STEM) images were acquired with a Hitachi HD2700C dedicated STEM (Hitachi High-Tech America, Inc., Schaumburg, Ill.) with a probe Cs corrector. Both microscopes were operated at an accelerating voltage of 200 kV.
Electrochemical measurements: All electrochemical measurements were conducted using a CHI electrochemical workstation (CHI760b) coupled with a rotating-ring disk electrode (RRDE) in a three-electrode system. A graphite rod and a Hg/Hg2SO4 (K2SO4-sat.) electrode were used as the counter and reference electrodes, respectively. A glassy carbon disk covered by a thin film of the catalyst was used as the working electrode. Each catalyst powder (5 mg) was ultrasonically dispersed in a 1.0 mL mixture of isopropanol and NAFION® sulfonated tetrafluoroethylene based fluoropolymer-copolymer (5 wt. %) solution to produce a catalyst ink. The ink was then drop-casted on the rotating ring disk electrode with a mass loading of 20 μgPt/cm2. The catalyst-coated disk working electrode was subjected to cyclic voltammetry (CV) in N2-saturated 0.1 M HClO4 to activate the platinum catalysts being studied. The electrochemically active surface area (ECSA) calculation was based on underpotentially-deposited hydrogen (HUPD) charge in cyclic voltammetry (CV) curves (20 mV s−1) in an N2-saturated electrolyte between 0.1-0.4 V (0.4-0.45 V background subtracted), assuming a value of 210 μC/cm2 for the adsorption of a hydrogen monolayer on platinum. The electrocatalytic activity for the oxygen reduction reaction was tested by a linear sweep voltammetry (LSV) technique at room temperature, a rotation rate of 1600 rpm, and a scan rate of 5 mV/s. The stability of the catalyst was studied by potential cycling from 0.6 V to 0.95 V in 0.1 M HClO4 electrolyte at a scan rate of 50 mV/s at room temperature.
Fuel cell MEA studies: All electrodes were fabricated using the catalyst-coated membrane (CCM) method. First, the anode electrode was fabricated using a Pt/Vulcan catalyst. The catalyst ink was prepared by mixing platinum catalysts with ionomer using deionized water (DI-water) and 1-propanol (nPA) in a bath sonicator for 30 min. Then, a Sono-Tek spray machine (Sono-Tek Corporation, Milton, N.Y.) was used to coat the anode layer with a loading of 0.1 mgPt/cm2 on a proton exchange membrane (Gore MX20.15, W. L. Gore & Associates, Inc., Newark, Del.). Various cathode electrodes were prepared using 40 wt. % Pt/C catalysts, including homemade Pt/Mn—N—C and Pt/PGC catalysts, as well as a TKK Pt/C catalyst. The same spray coating was applied to fabricate cathode layers with a loading of 0.2 mgPt/cm2 on the opposite side of the membrane coated with the anode electrode. Finally, the resulting MEAs were tested using a 5 cm2 differential cell at 80° C. and under constant flow rates: 500 standard cubic centimeters per minute (sccm) H2 for the anode and 2000 sccm air for the cathode. The applied backpressure was 250 kPa, following the protocol for HDV MEAs. During the fuel cell MEA tests, US DOE fuel cell testing protocols were followed by holding constant voltages from 0.35 V to open-circuit voltage (OCV) at 50 mV/step and 60 seconds hold per step. Before the polarization test, 16 hr break-in was required. The break-in protocol was to scan voltage between 0.7 V and 0.35 V, 50 mV/ step and 5 min hold per step. Fuel cell MEA stability was evaluated based on the voltage cycling accelerated stability test (AST) suggested by the U.S. DOE, which was conducted using the trapezoidal wave method from 0.6 V to 0.95 V with 0.5 s rise time and 2.5 s hold time (150 kPa, H2/N2, 80° C., 100% RH, 200/200 sccm).
Catalyst morphology and nanostructure: Oxygen reduction reaction (ORR)-active, platinum group metal (PGM)-free, and iron-free carbon catalysts were explored as a support for dispersing platinum nanoparticles, due to their beneficial nitrogen dopants, the richness of micropores and carbon defects, and the possible synergy between single metal sites and platinum for promoting the oxygen reduction reaction. One such material explored was the partially graphitic carbon support (Mn—N—C) derived from zeolitic imidazolate frameworks (ZIFs), which material has demonstrated a specific surface area up to 1500 m2/g with atomically dispersed nitrogen coordinated Mn sites (i.e., MnN4). Another material explored was porous graphitic carbon (PGC) derived from polyaniline hydrogel, which material achieves an optimal balance between porosity (˜450 m2/g with dominant mesopores) and graphitization. In this example, platinum nanoparticles (NPs) were dispersed onto these two carbon-based supports using the ethylene glycol (EG) reduction method, followed by a post-heat treatment to improve stability. This method simplifies the synthesis process and guarantees that the high content of platinum (40 wt. %) can be achieved.
The morphology and nanostructure of the Mn—N—C supported platinum catalysts with a high content, denoted herein as Pt (40 wt. %)/Mn—N—C, were comprehensively studied using different electron microscopy techniques. According to transmission electron microscopy (TEM) images and platinum particle size distribution, it was determined that platinum nanoparticles with an average size of 3.7 nm were uniformly distributed on the polyhedron Mn—N—C carbon support particles (80-100 nm) (
The morphology and nanostructures of the Pt (40 wt. %)/porous graphitic carbon (PGC) catalysts were also comprehensively studied using multiple electron microscopy. Unlike the well-defined polyhedral particles observed with the Mn—N—C support, the PGC presents a porous sheet-like morphology (
The crystal structure of the Pt (40 wt. %)/Mn—N—C catalyst, the Pt (40 wt. %)/PGC catalyst, and a commercial TKK catalyst (40 wt. % Pt/C) were compared using X-ray diffraction (XRD) patterns (
ORR activity and stability in aqueous electrolyte: Oxygen reduction reaction (ORR) activity and stability of the above-mentioned three platinum catalysts with 40 wt. % high content were studied using a rotating ring disk electrode (RRDE) in a 0.1 M HClO4 electrolyte at room temperature (see
The catalyst durability was studied by potential cycling from 0.6-0.95 V vs. RHE (see
Catalyst degradation mechanisms: To understand the degradation mechanism of various Pt/C catalysts with a high content (40 wt. %), an analysis was conducted of platinum nanoparticles and carbon supports at the beginning of life (BOL) and at the end of life (EOL), said analysis using advanced electron microscopy techniques, including secondary electron (SE) and medium-angle annular dark-field-scanning transmission electron microscopy (MAADF-STEM) images. The SE images provide surface morphology and structures of these catalysts, while the MAADF-STEM images show the 2-dimensional projection of 3-dimensional objects. By comparing images from two techniques, one can identify whether platinum nanoparticles are still at the surface or not after the stability accelerated stress test (AST). By comparing
To the contrary, for the TKK 40 wt. % Pt/C sample, secondary electron images after the accelerated stress test indicate that the platinum nanoparticle density and coverage at the carbon surface are much lower than for the initial catalyst (
By contrast, the density of platinum nanoparticles at the surface and the original particle sizes can be well-reserved for the Mn—N—C support, which agrees with their minor activity losses. In fact, based on analysis, the elemental distribution of the Pt (40 wt. %)/Mn—N—C catalyst after the stability accelerated stress test shows that, in addition to the well-reserved fine platinum nanoparticles, carbon structures, N dopants, and atomically dispersed single Mn sites remain intact, verifying the excellent stability of the catalyst. The robust carbon structures of the Mn—N—C support could mitigate possible carbon corrosion and enhance the stability of platinum nanoparticles. Therefore, compared to the TKK high-surface-area carbon used for 40 wt % Pt/C catalyst, the Mn—N—C support is advanced to stabilize platinum nanoparticles during the stability accelerated stress test, likely due to strengthened interaction originating from the possible single metal site and N dopants in carbon.
Fuel cell MEA performance: These above-discussed catalysts were studied in a solid-state polymer electrolyte-based membrane electrode assembly (MEA) as the cathode to evaluate their fuel cell performance. Unlike traditional ultra-low platinum group metal (PGM) MEAs, a high PGM loading (0.2 mgPt/cm2) for the cathode under high back pressure (250 kPa) was applied for the heavy-duty vehicle applications. It was clear that the Pt (40 wt. %)/Mn—N—C cathode exhibited the best fuel cell performance in terms of both kinetic region and mass transfer region (
As heavy-duty vehicles are desirable to be operated at low relative humidity (RH), the effect of relative humidity on the MEA performance for the Pt (40 wt. %)/Mn—N—C cathode was also studied (
Referring now to
Additional advantages, features, and observations regarding the present invention are set forth below.
The embodiments of the present invention described above are intended to be merely exemplary and those skilled in the art shall be able to make numerous variations and modifications to it without departing from the spirit of the present invention. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.
The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 63/214,629, inventors Gang Wu et al., filed Jun. 24, 2021, the disclosure of which is incorporated herein by reference in its entirety.
This invention was made with government support under DOE DE-EE0008075 Cooperative Agreement entitled “Durable Mn-Based PGM-Free Catalysts for Polymer Electrolyte Membrane Fuel Cells” awarded by the US Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63214629 | Jun 2021 | US |
