Patent Application
20230271840
Proton exchange membrane fuel cells (PEMFCs) have attracted attention because of their high energy efficiency and environmental friendliness. Progress has been achieved during recent years, but improvements in several components, especially catalysts for cathode, are still needed to meet the cost and durability targets for large-scale fuel cell commercialization. State of the art catalysts are based on Pt and Pt alloy nanoparticles dispersed on high-surface area carbon supports, which are able to stabilize the nanoparticles to increase catalyst utilization and provide effective mass transport and electronic conductivity. However, catalyst durability with low Pt loadings remains insufficient for practical applications due to the existences of several degradation mechanisms. Poor support stability, which leads to the detachment of Pt nanoparticles, particle agglomeration, and the compaction and then the loss of electrode porosity, is a critical limitation on durability of current catalysts. A variety of conductive materials have been studied as the support for Pt, including nanostructured carbons, conductive diamonds, conductive oxides and carbides. Among them, nanostructured carbon materials have been the most successful due to their high surface area, high electrical conductivity, good interaction with Pt, and reasonable stability in acidic media. Currently, XC-72 and Ketjen black are among the most promising supports, and are commercially used for Pt/C catalysts. However, they are not electrochemically stable under the corrosive conditions in PEMFCs, which include high oxygen concentration, high water content, low pH, elevated temperature up to 100° C., and high electrode potential. Thus, severe carbon corrosion has been observed in fuel cell catalysts, leading to unacceptable catalyst durability.
To address this challenge, many kinds of carbon materials have been investigated as catalyst supports, and it has been observed that structural properties of carbon can have a significant effect on the catalytic activity and durability. Among them, carbon nanotubes and carbon nanofibers (CNTs and CNFs) attracted attention in recent years, and improved activity and stability have been claimed. However, due to the limitation of specific surface area and porous structure, the electrochemical surface area of Pt could be restricted in the case of CNTs and CNFs, which could limit the catalytic performance in a fuel cell. Although enhanced stability was observed in aqueous acidic electrolytes, poor dispersion of Nafion ionomer in CNT supported Pt cathodes usually leads to poor fuel cell performance, especially at high current density. Another candidate support is graphitized carbon, which has been suggested to be more favorable in terms of the decrease of electrical resistance and enhancement of carbon corrosion resistance. Unfortunately, most graphitization approaches compromise the porosity and specific surface area of carbon, while also weakening interactions between carbon and platinum, making it difficult to uniformly disperse Pt nanoparticles on the support. Therefore, significant challenges still remain to develop support materials for PEM fuel cells.
The oxygen reduction reaction (ORR) is critical but sluggish in proton-exchange membrane fuel cells (PEMFCs) and it requires enough catalysts to promote at the cathode. Platinum (Pt) is the only metal catalyst showing promising performance along with feasibility in real application scenario. Unfortunately, the high cost and scarcity of Pt dramatically limit the popularization of PEMFCs and have driven intensive efforts to reduce Pt usage regrading to catalysts development.
Typically, there are two approaches in response to Pt usage reduction. The first strategy explored is to alloy Pt with another first-row transition metal (M), such as cobalt (Co), nickel (Ni); lead (Pb), and iron (Fe). With smaller atomic radius, the incorporation of M atoms in the Pt-based alloy brings beneficial strain and alloy effects that are significant to improving the ORR performance of PtM catalyst. With improved intrinsic activity, intensive research has been conducted in terms of optimization of PtM alloy nanoparticles (NPs) structures. Compared to the common solid solution Al-structure, a PtM alloy with the specific Pt/M composition can adopt an ordered intermetallic structures, which can be cubic L12 (Pt3M) or tetragonal L10 (PtM). The ordered intermetallic structure is formed when there is a strong 3d-5d orbital interaction between M and Pt, which is capable to stabilize M much better by Pt in the more close-packed structure, resulting in less M etching and reasonable stability under acidic fuel cell conditions. Unlike the disordered Al-structure, the cubic L12 and the tetragonal L10 structures are normally obtained by thermal annealing of the Al-counterparts at high temperature (>700° C.). However, under the exact composition of Pt and M during synthesis, undesirable agglomeration of NPs at high temperature can result in a smaller number of shaped but large crystallites, which are not enough to spread over the electrode surface to encounter all the O2 and produce high current density. This results in significant drop of fuel cell current, especially under low fuel cell polarization voltage.
Another approach is to use a platinum group metal (PGM)-free catalyst, which could eliminate the Pt usage altogether, thus attracts intensive researchers to tackle this high-risk but high-reward task. Generally, such catalysts are prepared from earth-abundant elements such as Fe and Co embedded in nitrogen-carbon composites (M-N—C). The state-of-the-art PGM-free catalysts are located at the Fe-based ones produced from zeolitic imidazolate framework-8 (ZIF-8), have demonstrated promising ORR activity approaching that of Pt. The claimed active sites FeNx disperse densely and uniformly throughout the electrode, easily accessible by O2 fluxes. However, their poor stability under PEMFC operations becomes the fatal drawback, placing the development of PGM-free catalysts into an awkward scenario. Due to the ongoing debate about the nature of the active sites, the mechanism of PGM-free catalyst degradation is still poorly understood.
The present disclosure provides compositions, graphitic carbon materials and methods of making graphitic carbon materials; compositions comprising graphitic carbon materials and nanoparticles and methods of making the compositions; platinum cobalt nanoparticles and methods of making platinum cobalt nanoparticles. Also provided are catalyst materials and uses of the graphitic carbon materials, platinum cobalt nanoparticles, catalyst materials, and devices.
In an aspect, the present disclosure provides a graphitic carbon material. The graphitic carbon materials have a desirable amount of graphitization and porosity.
In an aspect, the present disclosure provides compositions. A composition may comprise a graphitic carbon material of the present disclosure and a plurality of nanoparticles. The graphitic carbon material may be a graphitic carbon material as described herein.
In an aspect, the present disclosure provides a method of making a graphitic carbon material of the present disclosure. The graphitic carbon material may be used to make a composition of the present disclosure.
In an aspect, the present disclosure provides methods of a making a composition. A composition may comprise a graphitic carbon material of the present disclosure and a plurality of nanoparticles.
In an aspect, the present disclosure provides devices. The device may comprise a graphitic carbon material of the present disclosure or a composition of the present disclosure. For example the device may be an electrode. The electrode may comprise an electrolyte membrane, a gas diffusion membrane, or a combination thereof. In other examples, the device is a fuel cell, an electrolysis device, or a battery. A battery may be a primary battery or a secondary battery. Non-limiting examples of batteries include ion-conducting batteries, such as, for example, lithium-ion batteries, and the like.
For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying figures.
Although claimed subject matter is described in terms of certain examples, other examples, including examples that do not necessarily provide all of the benefits and features set forth herein, are also within the scope of this disclosure. Various structural, logical, and process step changes may be made without departing from the scope of the disclosure.
Ranges of values are disclosed herein. The ranges set out a lower limit value and an upper limit value. Unless otherwise stated, the ranges include all values to the magnitude of the smallest value (either lower limit value or upper limit value) and ranges between the values of the stated range.
As used herein, unless otherwise stated, the term “group” refers to a chemical entity that is monovalent (i.e., has one terminus that can be covalently bonded to other chemical species), divalent, or polyvalent (i.e., has two or more termini that can be covalently bonded to other chemical species). The term “group” also includes radicals (e.g., monovalent and multivalent, such as, for example, divalent radicals, trivalent radicals, and the like). Illustrative examples of groups include:
Throughout this application, the use of the singular form encompasses the plural form and vice versa. For example, “a”, or “an” also includes a plurality of the referenced items, unless otherwise indicated.
The present disclosure provides compositions, graphitic carbon materials and methods of making graphitic carbon materials; compositions comprising graphitic carbon materials and nanoparticles and methods of making the compositions; platinum cobalt nanoparticles and methods of making platinum cobalt nanoparticles. Also provided are catalyst materials and uses of the graphitic carbon materials, platinum cobalt nanoparticles, catalyst materials, and devices.
In an aspect, the present disclosure provides a graphitic carbon material. The graphitic carbon materials have a desirable amount of graphitization and porosity.
A graphitic carbon material of the present disclosure may comprise various domains of graphitic carbon and amorphous carbon. In various examples, at least 85% of the carbon is graphitic carbon (e.g., 85-95%, inclusive (including all 0.1% values and ranges therebetween)) and the remainder may be amorphous carbon (e.g., 5-15%, inclusive (including all 0.1% values and ranges therebetween)). The graphitic carbon material may be defined by its graphitic content. The graphitic content may be represented by the ratio of the intensity of the D band (e.g., Raman peak maximum about 1350 cm−1 (e.g., 1340-1360 cm−1)) and G band (e.g., Raman peak maximum about 1590 cm−1 (e.g., 1580-1600 cm−1)) as determined by Raman spectroscopy (ID/IG). The D band is associated with structural defects in graphene and graphene-like materials and the G band is associated with C—C bond stretching of graphitic carbon. Graphitic carbon material of the present disclosure may have an ID/IG of 1-10, including all 0.01 ratio values and ranges therebetween (e.g., 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0). The total carbon content of the graphitic carbon material is at least 90 at % carbon (e.g., at least 90, at least 91, at least 92, at least 93, at least 94, or at least 95 at %).
The graphitic carbon material of the present disclosure may have a hierarchical porosity and has a desirable surface area. The term hierarchical porosity, refers to the graphitic carbon materials may have two or more pore scale ranges. For example, the graphitic carbon material may have any combination of mesopores, macropores, and micropores (e.g., i) mesopores, macropores, and micropores; ii) mesopores and macropores; iii) mesopores and micropores; or iv) micropores and macropores). The graphitic carbon material may be defined by its porosity (or a combination of its porosity and graphitic content). For example, the graphitic carbon may have a specific surface area of 350-550 m2/g, inclusive (including all 0.1 m2/g values and ranges therebetween) (e.g., 450±50 m2/g) and/or a cumulative pore volume of 0.70±0.1 cm3/g. In various other examples, the surface area is up to 700 m2/g. The graphitic carbon material may have mesopores (2-50 nm in size), macropores (greater than 50 nm in size), micropores (less than 2 nm in size), or a combination thereof. The terms “mesopores,” “macropores,” and “micropores” are used as defined by International Union of Pure and Applied Chemistry (IUPAC). Each pore of the plurality of pores may have a longest linear dimension or diameter of 1-75 nm, inclusive (including all 0.1 nm values and ranges therebetween). The pores may be formed by the removal of a metal graphitization catalyst (e.g., Mn). Various metal graphitization catalysts may be used.
The graphitic carbon material may be a plurality of carbon particles. The carbon particles may have a longest linear dimension of 100-300 nm, inclusive (including all 0.1 nm values and ranges therebetween). In other examples, the carbon particles may have a longest linear dimension of 20 to 1000 nm, inclusive (including all 0.1 nm values and ranges therebetween). The carbon material may be a three-dimensional (3D) carbon material. In various examples, the carbon material is a monolith, a film, or the like. The carbon material may have curly multilayer structures, flower (rose)-like structures, or the like, or a combination thereof. The graphitic carbon material may exhibit irregularly folded carbon layers, flower-like graphitic carbon structures, curly multilayer graphitic carbon structures, or the like, or a combination thereof.
The graphitic carbon material may be doped. For example, the graphitic carbon material may be N-doped and/or comprise a metal (e.g., metal binding sites (e.g., M-Nx groups, where M is a metal, such as, for example, Fe or Co)). In various examples, the graphitic material comprises a metal (e.g., metal-doped, such as, for example Fe-doped) and is also N-doped. In various other examples, the graphitic material is only N-doped. When the graphitic carbon material is N-doped, it may be doped by one or more N-dopants. The one or more N-dopants may be chosen from graphitic N-dopants, pyridinic N-dopants, NOx species, and combinations thereof. Examples of these groups include pyridinic-N at edges of carbon planes, graphitic-N doped in the interior of the graphitic planes, and oxidized pyridinic-N associated with oxygen. Such N-doped graphitic carbon materials may be formed by a method of the present disclosure. Such a method may include formation from polymerization of aniline (e.g., polyaniline (PANT)) and pyrrole (e.g., polypyrrole (PPy)) with a graphitization metal catalyst (e.g., Mn) and subsequent heat treatment and/or acid leaching. The N-dopant may be present at 0.2-0.5 at %, inclusive (including all 0.01% values and ranges therebetween). In various other examples, the graphitic carbon material comprises a metal (e.g., metal binding sites or active site). The metals may part of M-Nx groups, where M is a metal, such as, for example, Fe or Co and x is 1 to 10 (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, (e.g., 1-4)). An example of an M-Nx group is FeN4. These groups may be formed from a derivatized ZIF-8 support material. For example, ZIF supports (e.g., ZIF-8) may be doped with iron (e.g., Fe3+) and pyrolyzed to form an Fe-doped ZIF support having FeN4 active sites. Examples of graphitic carbon materials that are metal- and N-doped include ZIF derivatized materials as described herein.
In an aspect, the present disclosure provides compositions. A composition may comprise a graphitic carbon material of the present disclosure and a plurality of nanoparticles. The graphitic carbon material may be a graphitic carbon material as described herein.
The composition may comprise various nanoparticles. The metal nanoparticles may be an alloy of two or more metals. Non-limiting examples of metal nanoparticles include platinum nanoparticles, platinum/transition metal (TM) nanoparticles, and the like, and combinations thereof. A Pt/TM nanoparticle may comprise platinum atoms and one or more first row transition metal atoms(s), where the platinum atoms and first row transition metal atoms form an intermetallic structure. The intermetallic structure may be an ordered intermetallic structure. In various examples, the platinum atoms and first row transition metal atoms are not randomly oriented and/or disordered. Non-limiting examples of first row transition metal atoms include cobalt, iron, nickel, and the like, and combinations thereof. In various examples, the Pt/TM nanoparticle is a Pt/Co nanoparticle. The Pt/TM nanoparticle may be disposed on a carbon support material (e.g., a graphitic carbon material of the present disclosure). The nanoparticle may be spherical or the like. Non-limiting examples of platinum/TM nanoparticles include intermetallic L10 PtCo nanoparticles, L12 Pt3Co nanoparticles, and the like, and combinations thereof. The nanoparticles may be present at 5 to 80% by weight (e.g., 10 to 70%, 10 to 80%, 15 to 55%, or 15 to 655% by weight) (based on the total weight of the carbon material and metal nanoparticles), including all 0.1% by weigh values and ranges therebetween. A nanoparticle has at least one dimension (which may be a longest dimension) of 1 to 10 nm, including all 0.1 nm values and ranges therebetween (e.g., 3 to 10 nm). The nanoparticle may have a cubic structure, tetragonal structure, or the like. The nanoparticles may have uniform size distribution. For example, the nanoparticles have a size of uniform distribution with an average size of 2-5 nm, inclusive (including all 0.1 nm values and ranges therebetween) (e.g., 2.4 nm or 4.2 nm).
The structure of intermetallic nanoparticles (e.g., Pt3Co nanoparticles) may be controlled by its synthesis method. To control the intermetallic structures, a second step annealing process is utilized (e.g., annealing at 650° C. in an inert atmosphere (e.g., Ar atmosphere)). Excess cobalt of Pt3Co nanoparticles may be removed using acid washes as described herein.
The composition may be referred to as a catalyst material. The catalyst materials may be used in devices such as, for example, fuel cells, electrolysis devices, batteries (which may be primary batteries or secondary batteries, such as, for example, an ion-conducting batteries (e.g., lithium-ion batteries), and the like. The compositions may be used in oxygen reduction reaction (ORR) applications. The catalyst materials may be ORR catalyst materials. The composition may have a conductivity of 1.5 to 2.5×105 σ (S/m), inclusive (including all 0.01×105 σ values and ranges therebetween).
In an aspect, the present disclosure provides a method of making a graphitic carbon material of the present disclosure. The graphitic carbon material may be used to make a composition of the present disclosure.
A method of making a graphitic carbon material of the present disclosure may comprise providing a mixture (e.g., a hydrogel comprising water) and thermally treating the mixture (e.g., heating the mixture to 1050-1110° C., inclusive (including all 0.1° C. values and ranges therebetween) (e.g., 1090-1110° C.). The reaction mixture may comprise one or more polyanilines; one or more polypyrroles; and a metal graphitization catalyst (e.g., Fe, Co, Ni, Mn, or the like). In various examples, the metal graphitization catalyst is Mn. The mixture may comprise crosslinked (e.g., highly crosslinked) polyaniline(s) and/or polypyrrole(s). The crosslinking may be intrachain crosslinking, interchain crosslinking, or a combination thereof. In various examples, the mixture is a network of crosslinked polyaniline(s) and polypyrrole(s). The mixture may comprise folded polymer nanostructures. Without intending to be bound by any particular theory, it is considered that the metal graphitization catalyst(s) provide(s) desirable graphitization during the thermal treatment of the mixture, which may be a hydrogel. In various examples, the mixture is a polyaniline-polypyrrole hydrogel. In various examples, the hydrogel comprises 60 to 80 wt % water (based on the total weight of the reaction mixture), including all 0.1 wt. % values and ranges therebetween. In various examples, the polyaniline(s) have a molecular weight (e.g., Mw and/or Mn) of 180,000 g/mol. The polyanilines and/or polypyrroles may be prepared in situ. The mixture may be prepared by providing a reaction mixture comprising aniline, pyrrole, a metal graphitization catalyst (e.g., Fe, Co, Ni, Mn, or the like) and, optionally, one or more polymerization catalysts, and, optionally, one or more solvents and the reaction mixture may be held at a temperature of 18-24° C., inclusive (including all 0.1° C. values and ranges therebetween), where the polyanilines and polypyrroles are formed. A polymerization catalyst may catalyze a radical polymerization, thermal polymerization, ionic polymerization, or the like. In various examples, a polymerization catalyst is a radical polymerization catalyst, a thermal polymerization catalyst, a ionic polymerization catalyst, or the like. Suitable examples of catalysts are known in the art. Non-limiting examples of polymerization catalysts include radical polymerization catalysts, such as, for example, persulfates (such as, for example, ammonium persulfate, and the like), hydrogen peroxide, metal ions (such as, for example, ferric ions (Fe+) and the like), and the like, and combinations thereof. In various examples, the polyaniline:polypyrrole ratio is from 4 to 2, including all 0.1 ratio values and ranges therebetween. Non-limiting examples of solvents include HCl solutions, H2SO4 solutions, and the like, and combinations thereof. A solution may be a dilute acid solution. A polymerization reaction may be carried out at room temperature (e.g., 18-24° C.) and/or for about 24 hours. The method further comprises removing a portion of water from the mixture (e.g., removing water from the hydrogel). The removed portion of water may be substantially all or all of the water of the hydrogel. By “substantially all” it is meant that at least 99%, at least 99.5%, or at least 99.9% of the water is removed from the hydrogel. The method may further comprise acid washing the graphitic carbon material. Acid washing may remove the metal graphitization catalyst. Following acid washing, the graphitic carbon material may be further thermally treated (e.g., heated at a temperature of 900-1110° C., inclusive (including all 0.1° C. values and ranges therebetween)).
The balance of porosity and graphitization may be tuned by adjusting the method parameters. For example, varying the ratio of PANI and PPy. Additionally, increasing the temperature during the method may increase the graphitic content while lowering the porosity, whereas decreasing the temperature may increase the porosity while decreasing the graphitic content.
In an aspect, the present disclosure provides methods of a making a composition. A composition may comprise a graphitic carbon material of the present disclosure and a plurality of nanoparticles.
A method of making a composition may comprise forming a reaction mixture, dehydrating the reaction mixture to form a powder, thermally treating the powder, and annealing the powder. The reaction mixture may comprise an aqueous solution of the graphitic carbon material and one or more nanoparticle sources (e.g., metal sources, such as, for example, a platinum source and/or a cobalt source). The thermally treating may be performed in a reducing atmosphere. In various examples, the Pt/TM nanoparticle/nanoparticles are formed in situ in the presence of the carbon material. Non-limiting examples of platinum sources include acids, such as, for example, hexachloroplatinic acid, and the like, and combinations thereof, platinum salts, and the like, and combinations thereof. The platinum source(s) may be water soluble. Non-limiting examples of cobalt sources include cobalt salts, such as, for example, cobalt(II) chloride, cobalt (II) nitrate, and the like, and combinations thereof. A cobalt salt may be a hydrate. The cobalt source(s) may be water soluble. Non-limiting examples of carbon materials include ZIF-8_Fe derived support materials, other carbon materials described herein, and the like, and combinations thereof. In various examples the platinum source:cobalt source molar ratio is from 0.33 to 0.5, inclusive (including all 0.01 ratio values and ranges therebetween). It may be desirable to have a molar excess of platinum source(s) relative to the amount of cobalt source(s). The dehydration may be carried out by freeze-drying, or the like. The Pt/TM nanoparticle/nanoparticles may be annealed in a gas atmosphere. Normal acid leaching may be conducted (e.g., to remove excess transition metal species), followed by post treatment under an inert atmosphere (e.g., under argon at 400° C. for 1 hour). The structure of intermetallic nanoparticles (e.g., Pt3Co nanoparticles) may be controlled by its synthesis method. To control the intermetallic structures, a second step annealing process is utilized (e.g., annealing at 650° C. in an inert atmosphere (e.g., Ar atmosphere)). Excess cobalt of Pt3Co nanoparticles may be removed using acid washes as described herein.
The thermal treatment of the powder is carried out in a reducing atmosphere. Non-limiting examples of reducing atmospheres include a hydrogen gas atmosphere, forming gas (a mixture of hydrogen and argon), and the like, and combinations thereof. In various examples, the thermal treatment is carried out at a temperature of 200 to 350° C., including all 0.1° C. values and ranges therebetween, and/or for 3-6 hours, including all 0.1 hour values and ranges therebetween.
Thermal treatment of the thermally-treated powder (e.g., annealing) may be carried out in an inert gas atmosphere (such as, for example, argon, or the like, or a combination thereof), a reducing gas atmosphere, or the like. In various examples, the thermal treatment of the thermally-treated powder is carried out at a temperature of 600° C. or less (e.g., 550° C. to 700° C.) and for 3-6 hours. Without intending to be bound by any particular theory, it is considered that selection of the gas of the gas atmosphere can provide desired nanoparticle structure. Normal acid leaching (e.g., using diluted HClO4) may be conducted to remove excess transition metal species, followed by post treatment (e.g., under argon at 400° C. for 1 hour).
In various examples, nanoparticles may be applied to the graphitic carbon materials through deposition methods known in the art. For example, impregnation is used.
In an aspect, the present disclosure provides devices. The device may comprise a graphitic carbon material of the present disclosure or a composition of the present disclosure. For example the device may be an electrode. The electrode may comprise an electrolyte membrane, a gas diffusion membrane, or a combination thereof. In other examples, the device is a fuel cell, an electrolysis device, or a battery. A battery may be a primary battery or a secondary battery. Non-limiting examples of batteries include ion-conducting batteries, such as, for example, lithium-ion batteries, and the like.
The steps of the method described in the various examples disclosed herein are sufficient to carry out the methods of the present disclosure. Thus, in an example, a method consists essentially of a combination of steps of the methods disclosed herein. In another example, a method consists of such steps.
The following Examples provide various embodiments of the present disclosure:
The following examples are presented to illustrate the present disclosure. They are not intended to be limiting in any matter.
This example provides a description of carbon materials and catalyst materials of the present disclosure and methods of making same. Additionally, characterization and use of the carbon materials and catalyst materials are described.
In this example, a highly durable and active Pt catalyst supported on a three-dimensional (3D) porous graphitic carbon (PGC) derived from polymer hydrogel is described. Hydrogels, which have a 3D network of crosslinked polymer chains containing large amounts of water, has been extensively studied as a carbon precursor. The hydrogel precursors yield porous support architectures, which provide improvements in active site density, mass/charge transfer, and structural integrity. The polymers selected to prepare the hydrogel were crosslinked polyaniline (PANI) and polypyrrole (PPy), which have been proved to be effective nitrogen/carbon precursors for catalysts. In particular, PANI is rich in aromatic structures similar to graphitized carbon, and abundant carbon and nitrogen sources help direct conversion to graphitized carbon. By adding pyrrole, highly folded and contorted graphitic structures with high uniformity and porosity can be produced from PANI-PPy composite. The PANI-PPy hydrogel composite was used to facilitate increased nitrogen doping in the resulting PGC, which is believed to provide significant improvements of activity and stability for Pt/C catalysts. In addition, metal precursors including Fe, Co, Ni, or Mn were introduced into the polymerization process to take advantage of metal-catalyzed graphitization. Among these metals, Mn is the most suitable for fuel cell applications since it does not cause degradation processes associated with the Fenton reactions. At the same time, Mn was demonstrated to be an effective catalyst for graphitization of polymer-derived carbon. Whereas conventional high temperature treatment typically requires temperatures up to 3000° C. to produce highly graphitized carbon, the Mn-assisted hydrogel method is able to achieve a high degree of graphitization at only 1100° C., making this method attractive from a manufacturing standpoint. Deposition of Pt nanoparticles onto the hydrogel-derived PGC resulted in a catalyst with dramatically enhanced electrochemical stability compared to commercial Pt/C catalysts, including TEC10V20E (Vulcan support), TEC10EA20E (graphitized carbon support) and TEC10E20E (high-surface-area carbon black). The present disclosure demonstrates highly graphitized and porous carbon may have exceptional stability and performance under real fuel cell operating conditions. Without intending to be bound by any particular theory, it is considered that these PGCs address carbon corrosion issues that have limited widespread deployment of PEMFCs and other electrochemical technologies.)
Results and Discussion.
Synthesis, structure, and morphology of PGC supports and Pt catalysts. The polymer hydrogel approach to preparing PGC supports is illustrated in
To study the carbon structure and the degree of graphitization, Raman spectra for relevant carbon samples are compared in
Rose-like graphitic carbon nanostructures were dominant, which is in agreement with the morphology of PGC in STEM images (
To dynamically study the evolution of Mn-PANI-PPy hydrogel from precursor to carbon material in real time, in-situ HRTEM and STEM-EDS analysis was performed and simulated the heat treatment conditions to monitor the carbonization process.
Promotional role of N doping in strengthening metal-support interactions. The XPS N 1s and Pt 4f spectra are shown in
Further characterization of local structure through XAS was used to provide increased understanding of the improved activity and stability of Pt/Mn-PANI-PPy-PGC in comparison to conventional Pt/C. Examination of normalized Pt-L3 edge XANES spectra for Pt/Mn-PANI-PPy-PGC and Pt/C (
A series of EXAFS fittings were performed to characterize the local Pt structure of Pt/Mn-PANI-PPy-PGC and Pt/C using combinations of Pt—Pt, Pt—N, Pt—C and Pt—O scattering paths (
Density functional theory (DFT) calculations were performed to provide atomistic/electronic insights into the effects of N doping on the interaction between Pt and carbon supports. A model system consisting of a single Pt atom and a graphene layer with or structures of various possible adsorption configurations and the corresponding binding energies of a Pt atom on the graphene are shown in
Point defects in graphene, such as graphitic N, are known to modify the local electronic structure. The charge density difference of an N-doped graphene layer is plotted in
Factors to stability enhancement of PGCs. To fully elucidate the key factors to the encouraging stability enhancement observed with the PGC support, the synthetic chemistry of PGCs was studied and assessed correlations among synthesis, structure, and properties. The parameters examined during the synthesis include carbonization temperature, type of metals (Fe, Co, Ni, or Mn) as catalysts, heat treatment temperature (800-1100° C.), duration (1 to 3 hours), and the post annealing treatment.
As the first heat treatment temperature can influence graphitization and morphology of carbon, the stability of PANI-Mn-derived PGC-supported Pt catalysts was compared as a function of heating temperature from 900° C. to 1100° C. (
Post annealing treatment at 800° C. was applied to further stabilize the PGC-supported Pt catalysts. After post annealing treatment, the activity of Pt/Mn-PANI-PPy-PGC was improved slightly (
Catalyst activity and stability for the ORR. Electrochemical performance of the PGC-supported Pt and various commercial Pt/C catalysts, including TEC10V20E (Vulcan support), TEC10EA20E (graphitized carbon support), and TEC10E20E (high-surface-area carbon black), were measured in 0.1 M HClO4 solution using a rotating disk electrode (RDE) for the ORR (
Accelerated stress tests (ASTs) were conducted on RDEs with a Pt loading of 20 μgPt/cm2 by cycling the potentials in both low (0.6-1.0 V, 50 mV/s) and high (1.0-1.5 V, 500 mV/s) potential ranges to evaluate degradation of the Pt nanoparticles and the carbon support, respectively. To simulate the harsh environment in a fuel cell, electrolyte temperature was increased to 60° C. to study the carbon corrosion in high potential ranges.
TEM and STEM images of the Pt/Mn-PANI-PPy-PGC catalyst after various ASTs, including both high and low potential cycling, are shown in
Fuel cell stability evaluation and carbon corrosion analysis. Given the significant difference of conditions between traditional RDE and fuel cells, the PGC-supported Pt catalysts were carefully evaluated in MEAS under a real fuel cell environment and compared with commercially available Pt/C catalysts. Polarization performance was measured in H2/air, and ASTs were applied under H2/N2 in both high (1.0-1.5 V) and low (0.6-0.95 V) potential ranges at 80° C. and 100% RH. In addition to slightly enhanced initial fuel cell performance, the PGC-supported Pt catalyst, especially the Pt/Mn-PANI-PPy-PGC, exhibited significantly enhanced stability. In particular, after 5000 cycles from 1.0-1.5 V, the commercial Pt/C cathode (TEC10V20E) suffers from a serious degradation at a current density of 1.5 A/cm2 (
To address the grand stability challenges of Pt/C catalysts for fuel cell applications, a highly stable and favorable carbon support for Pt nanoparticles based on a polymer composite hydrogel precursor comprising PANI and PPy as carbon/nitrogen sources in combination with Mn as a graphitization catalyst was developed. The stability enhancement was carefully and comprehensively evaluated in both aqueous acidic electrolyte-based RDE and real fuel cell conditions by using a variety of accelerated stress test protocols recommended by U.S. DOE. The Mn-PANI-PPy hydrogel-derived carbon provides high graphitization degree and good morphology (e.g., sufficient surface area, and porosity), enabling exceptional catalytic stability. Among many findings, the importance of binary PANI and PPy polymer hydrogel was discovered and the unique role of Mn during the carbonization, which yield dramatically increased degree of graphitization and favorable hierarchical pore morphology for increased Pt utilization and strengthened metal-support interactions. Compared to other possible metals (e.g., Fe, Co, or Ni) as catalysts during the graphitization process, Mn was found to play an indispensable role in forming the highest degree of graphitization at optimized temperature and duration of the carbonization process. In addition to their excellent electrochemical properties as catalyst supports, these PGC materials are more suitable for low-cost manufacturing, since temperatures of only 1100° C. are needed, compared with heat treatments up to 3000° C. required for conventional graphitized carbons. In-situ HR-TEM and STEM further reveal that Mn is able to be uniformly dispersed into the hydrogel precursors at high temperature and effectively catalyze the graphitization process. Unlike other transition metals, Mn itself does not alloy with Pt and can be removed during the subsequent acidic leaching treatment. Post annealing treatment was found to strengthen Pt-support interactions, further enhancing stability. Importantly, the promotional role of nitrogen doping in facilitating the activity and stability enhancement was validated through high-resolution microscopy and X-ray absorption spectra in combination with theoretical DFT calculations. A high likelihood of Pt—N interaction is due to the possible electron transfer from Pt nanoparticles to N dopant in carbon support, leading to strengthened interactions of metal and supports by binding Pt atoms strongly to the graphitic N, while electron transfer from C to adjacent N atoms results in stronger interaction between Pt and C.
The high surface area, abundant porosity, and N doping present in PGCs create a favorable environment to disperse Pt nanoparticles and prevent agglomeration. Meanwhile, the remarkably improved degree of graphitization enhances carbon corrosion resistance in fuel cell cathodes. The well balanced porosity and graphitization of PGCs provide unique structural and morphological advantages to produce highly active and stable carbon supported Pt catalysts for PEMFCs. This new type of PGC-supported Pt catalyst significantly surpasses the state-of-the-art Pt/C and provides exceptionally enhanced stability with minimized carbon loss at high potentials. The advanced PGC represents a new class of carbon for fuel cells with extraordinarily enhanced durability.
Experimental Details.
Synthesis of Mn-hydrogel derived porous graphitic carbons. To prepare Mn-PANI hydrogel-derived PGC, 1.32 g (14.16 mmol) aniline and 1.62 g (7.08 mmol) ammonium persulfate (APS) was dissolved in separate 2.0 M hydrochloric acid (HCl) solutions (6 mL each). These two solutions were denoted as solution A (aniline solution) and solution B (APS solution). Subsequently, 2.8 g (14.16 mmol) manganese chloride tetrahydrate was dissolved into solution A. Then, solution B was gradually added into solution A, and shaken gently in a vial for 30 seconds. The resulting gel-like mixture was aged at room temperature for 24 h. Freeze-drying was used to remove solvent while retaining the porous structure of the PANI hydrogel composite. The resulting solid powder was processed by thorough grinding followed by a heat treatment at 900, 1000, or 1100° C. for 1 h under nitrogen (N2) flow with a ramp rate of 3° C./min. The pyrolyzed solid powder was leached with 0.5 M H2SO4 at 80° C. for 5 h and then dried at 60° C. in a vacuum oven for 12 h. A second heat treatment was then carried out at 900° C. for 3 h under N2 flow with a ramp rate of 3° C./min. The obtained sample heated at 1100° C. (the first heat treatment) is denoted as Mn-PANI-PGC. For Mn-PANI-PPy-PGC, 0.47 g (7.08 mmol) pyrrole was added together with aniline in solution A, and other steps and procedures remained the same.
Method to deposit Pt nanoparticles. Pt nanoparticle deposition onto the Mn-hydrogel-derived PGC supports was performed through an ethylene glycol (EG) reduction method with a controlled Pt mass loading of 20 wt %. The carbon support powder was dispersed in EG by sonication for 1 hour to form a homogeneous complex suspension. Then, a given amount of hexachloroplatinic acid solution (10 mg/mL) was added into EG solution under stirring for 20 minutes with N2 bubbling. The suspension was refluxed for 4 hours at 130° C. under continuous stirring. The catalysts were washed with Millipore water until no Cl− could be detected by AgNO3 solution and dried at 60° C. in a vacuum oven for 12 hours. The as-prepared samples were subsequently heat-treated in N2 at 800° C. for 30 mins. The final catalysts were identified as Pt/Mn-PANI-PGC or Pt/Mn-PANI-PPy-PGC when Mn-PANI and Mn-PANI-PPy hydrogel were used for carbon preparation, respectively.
Physical characterization. Raman spectra were collected on a Renishaw Raman system at 514 nm laser source to analyze carbon structures. Excitation power was held constant at ˜150 μW for all samples, which were prepared as powders on a glass surface. The excitation laser was focused through a 100× microscope objective for a total interrogation spot size of ˜1 micron diameter. Scattered light was collected in backscatter configuration into an optical fiber and then dispersed through the Renishaw spectrometer and projected onto a CCD camera. Brunauer-Emmett-Teller (BET) surface area and porosity were measured by using N2 adsorption/desorption at 77 K on a Micromeritics Tri Star II. Scanning electron microscopy (SEM) images were obtained on a Hitachi SU 70 microscope at a working voltage of 5 kV. Bright field and high-resolution transmission electron microscopy (HRTEM) images, and scanning TEM-energy dispersive spectroscopy (STEM-EDS) elemental maps were obtained with a Talos F200X (Thermo Fisher Scientific) at an accelerating voltage of 200 kV.
For in-situ analysis, samples were firstly dispersed in methanol and the suspension was deposited directly onto a thermal chip (DENS Solutions). The temperature was controlled with a MEMS heating stage from DENS Solutions. The in-situ electron microscopy was performed on an aberration-corrected transmission electron microscopy (FEI Titan 80/300), operating at 300 kV. The beam was blanked during the in-situ heating processes and the samples were only exposed to the beam during date setup and acquisition processes. The element mapping was conducted on a high-resolution analytical scanning/transmission electron microscope (S/TEM, FEI Talos F200X) operating at 200 keV. The elemental mappings were acquired with a four-quadrant 0.9-sr energy dispersive X-ray spectrometer (Super EDS).
X-ray diffraction (XRD) was conducted by using a Rigaku Ultima IV diffractometer with Cu K-α X-rays. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos AXIS Ultra DLD XPS equipped with a hemispherical energy analyzer and a monochromatic Al Kα source operated at 15 keV and 150 W and pass energy was fixed at 40 eV for the high-resolution scans. Samples were prepared as pressed powder supported on a metal bar for the measurements. The FWHM of the major XPS peaks ranged from 0.3 eV to 1.7 eV for the relevant elements. All the instrument parameters were constant including FWHMs, peak shapes, instrument design factors, chemical shifts, experimental settings and sample factors. The binding energy of Au was used as the reference. Pt particle size distributions were measured by TEM images of more than 200 particles for different catalysts. Pt L3-edge X-ray absorption spectroscopy (XAS) including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) experiments were carried out at beamline 20-BM at the Advanced Photon Source, Argonne National Laboratory. The EXAFS data were collected in transmission mode and the energy scale was using a Pt foil. Data analysis was performed using the Athena and Artemis software packages.
Electrochemical measurements. All electrochemical measurements were performed on a CHI Electrochemical Station (Model 760b) equipped with high-speed rotators from Pine Instruments. A rotating ring disk electrode (RRDE) from Pine Research Instrumentation (model: AFE7R9GCPT, USA) was used as the working electrode, containing glassy carbon disk and platinum ring: disk OD=5.61 mm; ring OD=7.92 mm; ID=6.25 mm. An Hg/HgSO4 reference electrode and a graphite rod counter electrode with a diameter of 0.250 inches and a length of 12 inches were used to complete the cell. To prepare the working electrode, 10 mg catalyst was dispersed ultrasonically in a 1.0 mL mixture of isopropanol and Nafion (5 wt. %) solution to form an ink. Then the ink was drop-casted on the disk electrode with a designed loading of 20 μgPt/cm2 or 60 μgPt/cm2 and dried at room temperature to yield a thin-film electrode. All the cyclic voltammetry (CV) and ORR polarization curves were recorded in 0.1 M HClO4 and the ORR activity was measured in 0.1 M HClO4 saturated with O2 at 900 rpm or 1600 rpm using steady-state polarization plots by holding each potential for 30 s (s=second(s)) with potential step of 30 mV. The accelerated stress tests (ASTs) were applied to evaluate catalyst stability by cycling the potentials in both low (0.6-1.0 V, 50 mV/s, 25° C.) and high (1.0-1.5 fd V, 500 mV/s, 60° C.) potential ranges in 0.1 M HClO4 saturated with N2 by using RDE. All reference potentials have been converted to reversible hydrogen electrode (RHE). As comparison, three kinds of Pt/C catalyst from TKK were studied regarding to activity and durability, including TEC10V20E, TEC10EA20E and TEC10E20E.
Fuel Cell Fabrication and Testing. Catalysts were incorporated into MEAs by spraying of a water/n-propanol based ink onto a 5 cm2 area of a Nafion 211 membrane. Each electrode was prepared with Pt loading of 0.1 mgPt/cm2, and 29BC gas diffusion layers (SGL Carbon) were used on both anode and cathode. H2-air fuel cell testing was carried out in a single cell using a commercial fuel cell test system (Fuel Cell Technologies Inc.). The MEA was sandwiched between two graphite plates with straight parallel flow channels machined in them. The cell was operated at 80° C., with 150 kPaabs H2/air or H2/O2, and a gas flow rate of 500/2000 sccm for anode/cathode, respectively. Catalyst mass activity was measured via the DOE/FCTT protocol (potential step from 0.6 V to 0.9 V and 15 min hold, current averaged during last 1 min) in 150 kPaabs H2/O2 (80° C., 100% RH, 500/2000 sccm) with correction for measured H2 crossover. The ECSA was obtained by calculating H adsorption charge in CV curves between 0.1-0.4 V (0.45-0.55 V background subtracted) at 30-35° C. with 500 sccm H2 on the anode and stagnant N2 on the cathode, assuming a value 210 μC/cm2 for the adsorption of a H monolayer on Pt. The low-potential catalyst AST was conducted by using trapezoidal wave cycling from 0.6 V to 0.95 V with 0.5 s rise time and 2.5 s hold time, while the high-potential support AST was conducted using triangle wave cycling from 1.0 to 1.5 V (150 kPaabs H2/N2, 80° C., 100% RH, 200/200 sccm H2/N2). Carbon corrosion rates were determined through measurement of CO2 concentration in the cathode effluent gas by non-dispersive infrared spectroscopy.
Computational methods. The spin-polarized density functional theory (DFT) calculations were performed using plane wave basis and Projector Augmented Wave (PAW) formalism, as implemented in the Vienna Ab-initio Simulation Package (VASP). The generalized-gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) functionals were employed to evaluate the exchange-correlation energy. The kinetic energy cutoff of 500 eV was used for plane wave expansion and the total energy was converged to 10−6 eV. The structures were optimized until the force acting on each atom was below 0.01 eV/A. The carbon support was modeled using a hexagonal 7×7 supercell of graphene layer containing 98 carbon atoms, with the in-plane lattice constant equal to the optimized value of 2.468 Å. The Brillouin zone was sampled using a Gamma centered k-point mesh of 2×2×1. A vacuum layer of 20 Å was added above the graphene layer to avoid the interaction between periodic images. One N atom was doped into the modeled graphene layer, giving a nominal doping concentration of about 1 at %. Single Pt atom and Pt13 cluster were allowed to adsorb on the undoped and N-doped graphene (N—C) layer. The binding energy Eb is defined as
E
b(Pt/C)=EPt/C−EPt−EC
where EPt/C is the total energy of the Pt/graphene system, EPt is the total energy of Pt atom or Pt cluster, and EC is the total energy of the graphene layer. The metric adopted to evaluate the relative stability of Pt adsorption is the binding energy difference between the defective graphene Eb(Pt/N−C) and pristine graphene Eb(Pt/C), which was calculated as follows
Additional physical characterization and electrochemical measurements are described in
This example provides a description of Pt/Co nanoparticles and catalyst materials of the present disclosure and methods of making same. Also, characterization and use of the nanoparticles and catalyst materials is described.
The reduction of platinum use and improvement of its corresponding catalytic performance have become of the most important steps to accelerate the development of proton-exchange membrane fuel cells (PEMFCs). In this example, a novel but facile method to boost the Pt-cased catalysts performance by integrating with iron based active sites FeNx. Synergistic catalysis between Pt(PtCo) nanoparticles over a platinum-group metal (PGM)-free catalytic substrate derived from iron doped zeolitic imidazolate framework-8 (ZIF-8) led to excellent oxygen reduction reaction performance under both rotating disk electrode (RDE) and fuel cell testing is described. Besides, easy phase transfer during synthesis between ordered intermetallic structures L10 PtCo and L12 Pt3Co was achieved and comprehensive comparison between them regrading to catalytic performance was established.
As for a typical Fe-doped ZIF-8-derived PGM-free catalyst (ZIF-8_Fe) (
Design and synthesis of synergistic ORR catalysts, containing Pt or PtCo NPs deposited on highly active ZIF-8_Fe carbon support, denoted as Pt(PtCo)/Z8_Fe, is described. The pyrolysis process during the synthesis of ZIF-8_Fe was optimized, achieving higher degree of graphitization and promising ORR catalytic performance with half-wave potential (E1/2) of 0.87 V vs RHE under RDE test. (
First, the ORR performance of the catalysts in the liquid half-cell were evaluated by the RDE method. As a result, Pt/Z8_Fe showed enhanced performance with E1/2 of 0.9 V vs RHE and mass activity (MA) of 0.57 A/mgPT at 0.9 V vs RHE, exceeding commercial Pt/C catalysts, indicating the FeNx active sites existing in ZIF-8_Fe contribute the intrinsic activity enhancement of Pt/Z8_Fe through synergistic interaction. Both PtCo(L12)/Z8_Fe and PtCo(L10)/Z8_Fe showed superior activity with E1/2 of around 0.95 V vs RHE (
In order to evaluate the catalysts performance in operating fuel cell environments of different mass and charge transport limitations, the performance of Pt/Z8_Fe was tested in membrane electrode assembly (MEA) with H2-Air. Without intending to be bound by any particular theory, it is considered particle size of M doped ZIF-8 could have impact on the catalyst performance, particularly under MEA tests. So, Pt was deposited onto different sizes ZIF-8_Fe, including 50 nm, 100 nm and 200 nm, and evaluated their performance under MEA tests. It clearly shows in
In summary, a series of synergistic ORR catalysts were designed and synthesized by depositing Pt or PtCo particles on highly active PGM-free ZIF-8_Fe carbon support. Much higher performance of Pt/Z8_Fe catalysts was demonstrated compared with commercial Pt/C under both RDE and MEA tests, indicating the advantages of ZIF-8_Fe support material. In the cases of PtCo catalysts, a method was developed to enables a facile control of intermetallic structures of PtCo and an apple to apple comparison between L10 (PtCo) and L12 (Pt3Co) catalysts. With similar initial performance, PtCo(L10)/Z8_Fe exhibited higher stability than PtCo(L12)/Z8_Fe under ADT tests by RDE.
Experimental Details
Catalysts Synthesis. Synthesis of ZIF-8_Fe derived carbon support. Synthesis of active ZIF-8_Fe carbon material is based on a known synthesis. Typical synthesis procedure of 100 nm ZIF-8_Fe carbon material is described below with a few modifications. Zinc nitrate hexahydrate (3.39 g) and iron nitrate nonahydrate (100 mg) were dissolved in 300 mL methanol in a round-bottom flask as solution 1; 2-Methylimidazole (3.94 g) was dissolved in another 300 mL methanol as solution 2. Then, two solutions were mixed gradually into the bottom-flask and it was sealed with a rubber stopper along with a cable tie. The mixture was then put into an oven and heated from 25° C. to 60° C. in 20 mins. The oven was kept on constant temperature at 60° C. for 24 h. After cooling, the resulting suspension was separated by centrifuging at 9000 rpm (10-15 mins each time) to collect all precipitant and washing with ethanol three times. All precipitant was collected and dried at 60° C. in a vacuum oven for 12 h. The dried light-yellow powder was then finely ground and heated at 1100° C. in a tube furnace under N2 flow for 3 h. After heat treatment, the furnace was cooled down to 25° C. The obtained black powder was finally ground to be the as-synthesized ZIF-8_Fe carbon support.
Synthesis of Pt (PtCo)/Z8_Fe catalysts. PtCo nanoparticle deposition onto the ZIF-8_Fe carbon support was performed through a forming gas (hydrogen (10%)+argon) reduction method with a controlled Pt mass loading of 20 wt %. The carbon support powder was dispersed in Milli-Q water by ultrasonic treatment for 1 hour to form a homogeneous complex suspension. Then, a given amount of hexachloroplatinic acid solution and hexahydrate cobalt (II) chloride (both 10 mg/mL) were added into the previous suspension solution under stirring for 20 minutes with N2 bubbling. The new suspension solution was further homogenized by ultrasonic treatment for 1 hour and quick-freeze by using liquid nitrogen, followed by freeze drying for overnight. The dried powder was then heated at 200° C. in a tube furnace under forming gas flow for 6 h (h=hour(s)). After cooling down to 25° C., the furnace was reheated to 650° C. for another 6 hours, under argon or forming gas for ordering L12 Pt3Co or L10 PtCo intermetallic structures, respectively. 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.
Electrochemical measurements. All electrochemical measurements were performed on a CHI Electrochemical Station (Model 760b) equipped with high-speed rotators from Pine Instruments. A rotating ring disk electrode (RRDE) from Pine Research Instrumentation (model: AFE7R9GCPT, USA) was used as the working electrode, containing glassy carbon disk and platinum ring: disk OD=5.61 mm; ring OD=7.92 mm; ID=6.25 mm. An Hg/HgSO4 reference electrode and a graphite rod counter electrode with a diameter of 0.250 inches and a length of 12 inches were used to complete the cell. To prepare the working electrode, 10 mg catalyst was dispersed ultrasonically in a 1.0 mL mixture of isopropanol and Nafion (5 wt. %) solution to form an ink. Then the ink was drop-casted on the disk electrode with a designed loading of 20 μgPt/cm2 and dried at room temperature to yield a thin-film electrode. All the cyclic voltammetry (CV) and ORR polarization curves were recorded in 0.1 M HClO4 and the ORR activity was measured in 0.1 M HClO4 saturated with O2 at 1600 rpm using linear sweep voltammetry (LSV) polarization plots at a scan rate of 10 mV·s−1. The accelerated stress tests (ASTs) were applied to evaluate catalyst stability by cycling the potentials ranging from 0.6 to 1.0 V, (scan rate: 50 mV/s) at 60° C. in 0.1 M HClO4 saturated with N2 by using RDE. All reference potentials have been converted to reversible hydrogen electrode (RHE).
Fuel Cell Fabrication and Testing. Catalysts were incorporated into MEAs by spraying of a water/n-propanol based ink onto a 5 cm2 area of a Nafion 211 membrane. Each electrode was prepared with Pt loading of 0.1 mgPt/cm2, and 29BC gas diffusion layers (SGL Carbon) were used on both anode and cathode. H2-air fuel cell testing was carried out in a single cell using a commercial fuel cell test system (Fuel Cell Technologies Inc.). The MEA was sandwiched between two graphite plates with straight parallel flow channels machined in them. The cell was operated at 80° C., with 150 kPaabs H2/air or H2/O2, and a gas flow rate of 500/2000 sccm for anode/cathode, respectively. Catalyst mass activity was measured via the DOE/FCTT protocol (potential step from 0.6 V to 0.9 V and 15 min (minute(s)) hold, current averaged during last 1 min) in 150 kPaabs H2/O2 (80° C., 100% RH, 500/2000 sccm) with correction for measured H2 crossover. The ECSA was obtained by calculating H adsorption charge in CV curves between 0.1-0.4 V (0.45-0.55 V background subtracted) at 30-35° C. with 500 sccm H2 on the anode and stagnant N2 on the cathode, assuming a value 210 μC/cm2 for the adsorption of a H monolayer on Pt.
This example provides a description of Pt/Co nanoparticles and catalyst materials of the present disclosure and methods of making same. Also, characterization and use of the nanoparticles and catalyst materials is described.
The fuel cell performance for L10-CoPt/NPGC, L10-CoPt/HSC, and Pt/HSC catalysts was studied. At the beginning-of-life (BOL) cycle, the L10-CoPt/NPGC exhibited higher current densities in the entire range from 0.4-1.0 V under Hz-Air condition (
Moreover, L10-CoPt/NPGC performed desirable stability in MEA testing. Accelerated stress test (AST) was carried by repeatedly sweeping from 0.6 to 1.0 V based on DOE catalyst stability evaluation protocols. The end-of-life (EOL) polarization curves are shown in
The fuel cell performances for the L10-CoPt/NPGC, L10-CoPt/Vulcan, and Pt/Vulcan catalysts were compared (
This example provides a description of Pt/Co nanoparticles and catalyst materials of the present disclosure and methods of making same. Also, characterization and use of the nanoparticles and catalyst materials is described.
Provided is a concept to design hybrid ORR catalysts by integrating PGM NPs, and FeN4 site-rich Fe—N—C carbon denoted as Pt/FeN4—C or PtCo/FeN4—C. As a comparison, the Fe-free nitrogen-doped carbon (NC) derived from carbonized ZIF-8, and the CoN4—C from cobalt-doped ZIF-8 were also studied to justify the effectiveness of FeN4 in carbon to enhance Pt and PtCo catalyst performance dramatically. Furthermore, during the synthesis of PtCo/FeN4—C catalysts, an effective approach to preparing L12 Pt3Co intermetallic structures: through controlled high-temperature annealing treatments was developed. The favorable porous carbon structure of FeN4—C and its abundant nitrogen doping enables a uniform Pt and Pt3Co NP distribution presenting average particle sizes of 3 to 4 nm. Unlike large particle sizes of ordered PtCo intermetallic reported before, the use of the FeN4 site-rich Fe—N—C carbon can significantly reduce the particle size while achieving an ordered structure. The newly developed Pt/FeN4—C and Pt—Co/FeN4—C catalysts show excellent performance and durability in rotating disk electrode (RDE) and membrane electrode assembly (MEA) studies. Compared to traditional carbon black-supported Pt (Pt/C), the Pt/FeN4—C achieved significantly improved ORR mass activity (MA) of 0.451 A/mgPT and retained 80% of the initial value after 30,000 accelerated stress test (AST) voltage cycles in an MEA with low cathode loading of 0.1 mgPT/cm2, exceeding the DOE 2020 targets even without using an alloy. Furthermore, the Pt3Co/FeN4—C achieved much higher ORR mass activities of 0.72 A/mgPT. The Pt3Co/FeN4 catalyst reached a power density of 824 mW/cm2 at 0.67 V and only lost 23 mV at 1.0 A/cm2 after 30,000 voltage cycles in an MEA. The DFT calculation further predicted the possible synergistic mechanism of Pt sites and FeN4 sites to enhance the intrinsic activity of Pt concerning O2 adsorption energy and activation energy to break O—O bonds during the ORR. The promotional role of the FeN4 site in boosting the activity and stability of Pt catalysts demonstrated an effective strategy to reduce Pt loading for high-performance low-PGM electrodes in PEMFCs.
Results and Discussion
Catalysts Synthesis and Structures
Fe—N—C catalysts containing highly active FeN4 sites through multiple effective methods are described. A chemical doping of Fe3+ ions into ZIF-8 nanocrystals and partially replaced Zn to form Fe—N4 coordination followed by subsequent pyrolysis in an Ar atmosphere to convert the Fe-doped ZIF-8 to FeN4 active sites uniformly dispersed into partially graphitized carbon was used. The carbon phase, derived from the hydrocarbon in ZIF-8, is partially graphitized and has a surface area up to 700 m2/g, containing atomically dispersed FeN4 sites with a significant micropore volume connected to hierarchical porous structures. More importantly, the carbon particle size can be easily tuned during the synthesis, ranging from 20 to 1000 nm, which provides an excellent opportunity to design electrode structures in MEAs. Therefore, the FeN4—C was applied as support to synthesize Pt and PtCo catalysts. The degree of the graphitization of carbon support is critical to Pt catalyst stability. Thus, unlike traditional Fe—N—C catalysts with significant amorphous carbon, the pyrolysis duration was prolonged from one to three hours at 1100° C. to graphitize the FeN4—C, aiming to increase catalyst stability. The graphitized layer structure of the FeN4—C is apparent in the STEM images (
During the subsequent Pt deposition, an impregnation method was applied with freeze-drying to disperse Pt nanoparticles on the FeN4—C support. A forming gas (5% H2 in Ar) was applied as a reductant to prepare the Pt/FeN4—C catalyst. This method minimizes the possible damage of FeN4 sites by avoiding a complicated wet chemistry synthesis. This catalyst synthesis scheme is illustrated in
PtCo intermetallic NP catalysts represent one of the most active ORR catalysts. Intermetallic ordering can improve the performance and durability. Pt3Co intermetallic NPs were integrated with the active FeN4—C support using an impregnation method followed by a reduction under forming gas at 200° C. (
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used to study structures of Pt—Co NPs. Significant atomic number difference between Co (Z=27) and Pt (Z=78) enables clear visualization of atomic ordering in bimetallic Pt—Co NPs through the atomic number (Z) contrast imaging.
Also investigated were the FeN4—C supported Pt and Pt3Co catalysts' electronic structures using X-ray photoelectron spectroscopy (XPS). Possible electron transfer from Pt to the FeN4—C support causes a positive shift in the Pt 4f binding energy in the Pt/FeN4—C catalyst (
The small-angle X-ray scattering (SAXS) regions of the X-ray scattering curves are shown in
Electrocatalysis Study for ORR in Aqueous Acidic Electrolytes.
The ORR activity and stability of FeN4—C supported Pt and Pt3Co catalysts in 0.1 M HClO4 electrolyte was evaluated using the RDE method (
The stability for all studied catalysts was evaluated in an acidic electrolyte using RDE. The Pt/FeN4—C catalyst exhibited superior stability during accelerated stress tests (ASTs) under 30000 potential cycles from 0.6 to 1.0 V at 60° C. (
MEA Tests in Fuel Cells.
The catalyst performance was evaluated in fuel cell environments, which involved incorporating the catalyst with solid-state ionomer for proton-conduction and the formation of porous electrode structures. Different FeN4—C-supported Pt and Pt3Co catalysts were tested as the cathode in membrane electrode assemblies (MEAs) under H2-air (FIG. 60). According to previous MEA studies on PGM-free catalysts, the particle size of FeN4—C carbon may affect the catalyst's MEA performance. Therefore, Pt NPs were deposited onto FeN4—C carbon supports with various sizes of 50, 100, and 200 nm, and evaluated their MEA performances. Results shown in
Based on the optimal 100 nm FeN4—C support, the MEA performance of the Pt/FeN4—C and the Pt3Co/FeN4—C catalysts was compared with commercial Pt/C catalysts, as shown in
In good agreement with RDE tests in acidic electrolytes, the Pt3Co/FeN4—C catalyst performed much better in MEAs than the Pt/FeN4—C catalyst. The Pt3Co/FeN4—C cathode exhibited excellent MA at 0.9 V of 0.72 A/mgPT, significantly exceeding the DOE target at 0.440 mA/mgPT (
These Pt and PtCo catalysts were also subjected to AST for 30,000 voltage cycles from 0.6 to 0.95 V with a 0.5 s rise time and 2.5 s dwell time at each potential under H2/N2 atmosphere. As shown in
Pt/FeN
4
—C
0.1
150
451
361
1400
245
679
kPa
Pt
3
Co/
0.1
150
720
441
1605
356
22
822
FeN
4
—C
kPa
Catalyst Degradation Mechanisms and XAS Study.
The morphology, structure, and chemical composition of aged FeN4—C supported PGM catalysts were further studied using advanced electron microscopy. As shown in
Similarly, the coexistence of FeN4 sites and Pt3Co NPs remained in the AST-aged catalysts (
XAFS spectra were processed and fit with the Demeter software package (
Pt—Co
Pt—Pt
Pt—Co
Pt—Pt
Pt—Co
Pt—Pt
Co—Co
Co—Pt
Co—Co
Co—Pt
Co—Co
Co—Pt
The aged catalysts show increased NPt-M and lower NPt—O coordination numbers than the fresh Pt3Co/FeN4—C catalyst, indicating particle growth. They also show a lower Pt—Co fraction, which suggests significant Co dissociation from Pt. In other words, there is a higher average Pt—Pt coordination in the aged catalysts. Increases in average Pt—Pt bond length up to +0.039 Å correspond to larger particles with less cobalt, consistent with the changes in coordination number. It isn't possible to determine whether the changes are due to the growth of separate Pt-rich particles, loss of Co from the growing PtxCo particles, or a rearrangement of the atoms to form a core-shell structure based on just the Pt EXAFS. The cobalt K edge EXAFS results are shown in Table 10. Uncertainties for the measured E0 are +/−0.4 eV. S02 and ΔE0 were found to be 0.775 and +7.12 eV, respectively, from fits to a cobalt metal foil. For the catalysts, it is clear from inspection that multiple paths overlap between 1.5 and 3.0 Å. However, the shorter k data range compared to Pt made it challenging to resolve them. It was noted that the Pt—Co bond length, R, and σ2 from the Pt EXAFS must equal the R and σ2 for Co—Pt from the Co EXAFS. These constraints may be applied to stabilize the fits. Therefore, except for TEC 36V32, the Co—Pt distance was fixed, while σ2 was allowed to vary. For TEC 36V32 powder, the Co results confirm R and σ2 are the same. Therefore, the alternative strategy of simultaneous fits was not pursued. Significant Co—Co coordination is observed at −2.62 Å, much longer than for monometallic Co (R=2.507 Å) and shorter than Pt—Pt (˜2.70 Å), offering further evidence of alloying over both intermetallic and a separate class of Co-rich nanoparticles. A fraction of the particles may consist of intermetallic Pt3Co, but within the sensitivity of the EXAFS measurements, more of the Co is alloyed.
In addition, the Pt3Co/FeN4—C powder exhibited some Co oxidation. The Co-M and Co—O coordination numbers are the same as for Pt, which suggests the Pt and Co are initially similarly distributed between the surface and interior. All other samples had a low-R structure but fitting it to Co—O resulted in unstable fits with physically unreasonable parameters. Therefore, only Co-metal paths were fit in those samples. In the MEAs, NCo-M does not increase as much as NPt-M, which suggests that the processes responsible for the change in Co coordination are not in concert with those responsible for the Pt coordination change. One explanation is that some Pt-rich particles have grown quite large, consistent with the increased average Pt—Pt bond length and the larger Pt—Pt fraction.
DFT Study of FeN4—C Supported Pt Catalysts.
The first-principles density functional theory (DFT) calculations were performed to understand the synergy that MN4 (M: Fe or Co) and N sites in carbon modify catalytic properties of Pt sites. A computational model was constructed consisting of a thirteen-atom cuboctahedral Pt13 cluster and a graphene layer with a FeN4 (Pt/FeN4—C), a CoN4 (Pt/CoN4—C), an N4 moiety (Pt/NC), or no dopants (Pt/C). The optimized atomistic structures of Pt/FeN4—C, Pt/CoN4—C, Pt/NC, and Pt/C are shown in
To further distinguish the most favorable binding sites to a Pt NP, the DFT calculations were performed to predict the binding energy of a four-atom tetrahedral Pt4 cluster adsorbed on various graphene locations containing a FeN4 moiety (
The DFT calculations were conducted to investigate how the FeN4 site affects the intrinsic ORR activity of Pt sites. Previous studies once indicated that the Pt NP could assist the CoN4 site to break O—O bond, resulting in the enhancement of the CoN4 site's ORR activity. Others have suggested Pt(100) could enhance the ORR activity of FeN4 active site through tailoring local charge distribution near the FeN4 site. These studies found that Pt will enhance the ORR activity at MN4 sites. However, considering the ORR activity measured herein on the Pt/FeN4—C, Pt/CoN4—C, and Pt/NC catalysts are much higher than PGM-free MN4 sites (
Conclusions.
Demonstrated herein is a design of high-performance low-PGM fuel cell catalysts by integrating the highly stable Pt3Co intermetallic nanoparticle and the most promising PGM-free FeN4 site-rich carbon catalyst. The high surface area, porous morphology, controlled graphitization degree, and adjustable carbon particle size dramatically improve the Pt and PtCo nanoparticle dispersion with uniform and narrow size distribution, promoting high catalytic activity and Pt utilization. In addition, the dense FeN4 sites likely significantly strengthen the interaction between Pt and carbon, thus preventing nanoparticle agglomeration, which enhances catalyst stability. Significantly, the FeN4 sites around the Pt sites can weaken the adsorption of O2 and intermediates during the ORR, intrinsically improving the catalytic activity of Pt for the ORR.
Atomically dispersed FeN4 carbon-supported Pt and the ordered cubic L12 (Pt3Co) intermetallic catalysts were synthesized. Compared to the common solid solution Al-structure, PtCo intermetallics with strong Pt-M interaction are particularly promising as new fuel cell catalysts due to their superior M-stabilization in the corrosive ORR conditions. Comprehensive RDE and MEA studies verified that the FeN4-rich carbon is superior to traditional nitrogen-doped carbon and carbon black concerning ORR activity and stability. In particular, the Pt/FeN4—C catalyst has achieved compelling activity and stability with 30 mV positive shift in half-wave potential relative to a Pt/C (i.e., Vulcan XC-72) catalyst and only 10 mV loss after 30 k potential cycles. MEA performance further demonstrated outstanding mass activity at 0.9 V (0.45 A/mgPT) and durability (20% loss in MA at 0.9 V and 8 mV loss at 0.8 A/cm2 MEA studies), achieved the challenging DOE targets by using Pt even without alloying.
The Pt3Co intermetallic catalyst on the FeN4-carbon achieved a high ORR activity with half-wave potentials above 0.95 V, representing one of the most active PGM catalysts. The Pt3Co/FeN4 MEA reached a power density of 824 mW/cm2 at 0.67 V and only lost 23 mV at 1.0 A/cm2 after 30,000 voltage cycles in an MEA. Further engineering electrode structures by optimizing ionomer/carbon ratios can balance ORR mass activity, power density, and durability. Thus, the effective approach to leveraging the most promising PGM-free FeN4 sites in the design of ordered PtCo intermetallics for high-performance low-PGM catalysts may be a new avenue to advance fuel cell catalyst technologies for the high impact transportation application.
Experimental Details:
Catalysts Synthesis.
Synthesis of the ZIF-8_Fe derived carbon support. The synthesis of active ZIF-8_Fe carbon material is based on our previous publication. The typical synthesis procedure of 100 nm ZIF-8_Fe carbon material is described below with a few modifications. Zinc nitrate hexahydrate (3.39 g) and iron nitrate nonahydrate (100 mg) were dissolved in 300 mL methanol in a round-bottom flask as solution 1; 2-Methylimidazole (3.94 g) was dissolved in another 300 mL methanol as solution 2. Two solutions were then mixed gradually into the bottom-flask, and it was sealed with a rubber stopper along with a cable tie. The mixture was then put into an oven and heated from 25° C. to 60° C. in 20 mins. The oven was kept at a constant temperature at 60° C. for 24 h. After cooling, the resulting suspension was separated by centrifuging at 9000 rpm (10-15 mins each time) to collect all precipitant and washing with ethanol three times. All precipitant was collected and dried at 60° C. in a vacuum oven for 12 h. The dried light-yellow powder was then finely ground and heated at 1100° C. in a tube furnace under N2 flow for three hours. After heat treatment, the furnace was cooled down to 25° C. The obtained black powder was finely ground to be the as-synthesized FeN4—C carbon support.
Synthesis of the Pt (Pt—Co)/FeN4—C catalysts. Pt—Co nanoparticle deposition onto the FeN4—C carbon support was performed through a forming gas (hydrogen (10%)+argon) reduction method with a controlled Pt mass loading of 20 wt %. The carbon support powder was dispersed in Milli-Q water by ultrasonic treatment for 1 hour to form a homogeneous complex suspension.
Pt: Then, a given amount of hexachloroplatinic acid solution (10 mg/mL) was added into the previous suspension solution under stirring for 20 minutes with N2 bubbling. The new suspension solution was further homogenized by ultrasonic treatment for 1 hour and quick-freeze using liquid nitrogen, followed by freeze-drying overnight. The dried powder was then heated at 200° C. in a tube furnace under forming gas flow for six hours. After cooling down to 25° C., the furnace was reheated to 600° C. for another 1 hours, under argon to obtain the final catalyst.
Pt—Co: Then, a given amount of hexachloroplatinic acid solution and hexahydrate cobalt (II) chloride (both 10 mg/mL) were added into the previous suspension solution under stirring for 20 minutes with N2 bubbling. The new suspension solution was further homogenized by ultrasonic treatment for 1 hour and quick-freeze by using liquid nitrogen, followed by freeze-drying overnight. The dried powder was then heated at 200° C. in a tube furnace under forming gas flow for six h. After cooling down to 25° C., the furnace was reheated to 650° C. for another 6 hours, under argon or forming gas for ordering L12 Pt3Co. The resulting powder was leached by 0.1 M HClO4 at 60° C. for 6 hours and post-treated at 400° C. under argon to obtain the final catalyst.
Physical Characterizations.
Raman spectra were collected on a Renishaw Raman system at 514 nm laser source to analyze carbon structures. Excitation power was held constant at ˜150 μW for all samples prepared as powders on a glass surface. The excitation laser was focused through a 100× microscope objective for a total interrogation spot size of ˜1 micron diameter. The scattered light was collected in backscatter configuration into an optical fiber and then dispersed through the Renishaw spectrometer and projected onto a CCD camera. Scanning electron microscopy (SEM) images were obtained on a Hitachi SU 70 microscope at a working voltage of 5 kV. Bright-field and high-resolution transmission electron microscopy (HRTEM) images and scanning TEM-energy dispersive spectroscopy (STEM-EDS) elemental maps were obtained with a Talos F200X (Thermo Fisher Scientific) at an accelerating voltage of 200 kV.
X-ray diffraction (XRD) was conducted by using a Rigaku Ultima IV diffractometer with Cu K-α X-rays. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos AXIS Ultra DLD XPS equipped with a hemispherical energy analyzer, and a monochromatic Al Kα source operated at 15 keV and 150 W and pass energy was fixed at 40 eV for the high-resolution scans. Samples were prepared as pressed powder supported on a metal bar for the measurements. The FWHM of the major XPS peaks ranged from 0.3 eV to 1.7 eV for the relevant elements. All the instrument parameters were constant, including FWHMs, peak shapes, instrument design factors, chemical shifts, experimental settings, and sample factors. The binding energy of Au was used as the reference. Pt particle size distributions were measured by TEM images of more than 120 particles for different catalysts.
X-ray scattering at beam line 9-ID-C at the Advanced Photon Source (APS), Argonne National Laboratory, was utilized to determine the catalyst particle size and lattice spacings of the metal particles in the as-prepared catalyst powders and in the cathodes of the AST-cycled MEAs. Monochromatic X-rays with an energy of 21 keV were used for the X-ray scattering measurements. The scattered X-ray intensity was obtained over a range of scattering angles/scatterer dimensions: ultra-small-angle X-ray scattering (USAXS), pinhole small-angle X-ray scattering (pinSAXS), and wide-angle X-ray scattering (WAXS). A Bonse-Hart camera was used for the USAXS region data collection and a Pilatus 100 K detector for pinhole SAXS and WAXS. The complete scattered intensity, I(q), was then obtained by combining the USAXS (10−4 to 6×10−2 Å−1) and the pinhole SAXS (3×10−2 to 1 Å−1). The scattering data were corrected and reduced with the NIKA software package, and data analysis was conducted using the IRENA software package. Both packages were run on IGOR Pro 7.0 (Wavemetrics).
Metal particle size distributions were obtained from the measured scattering data using the maximum entropy (MaxEnt) method, which involves a constrained optimization of parameters to solve the scattering equation:
I(q)=|ΔQ|2∫|F(q,r)|2(V(r))2Np(r)drint
Where I(q) is the scattered intensity, Q is the scattering length density of the particle, F (q, r) is the scattering function at scattering vector q of a particle of characteristic dimension r. V is the volume of the particle, and Np is the number density of particles in the scattering volume.
The WAXS data covered a d-spacing range from approximately 6 Å to 0.8 Å. The background scattering taken for the mounting tape was subtracted from the scattering data for each sample. The WAXS data analysis utilized powder diffraction multi peak fitting 2.0, an Irena macro. The positions of the (111), (200), (220), and (311) scattering peaks were utilized to determine the lattice spacing and this spacing was then utilized to calculate the Pt to Co ratio in the crystalline portions of the catalyst particles using Vegard's law and the nearest neighbor (NN) distances of 2.775 Å and 2.492 Å for Pt and Co, respectively. The Pt—Pt nearest neighbor distance was determined by fitting the WAXS region of data acquired for the as-prepared Pt/Fe—N—C catalyst. The Co—Co nearest neighbor distance was determined from the EXAFS fits for Co foil.
XAFS measurements were made at Materials Research Collaborative Access Team (MRCAT) beam lines 10ID and 10BM at The Advanced Photon Source, Argonne National Laboratory. Co and Fe K edge XAFS were measured at 10ID using a gas ionization chamber with Soller slits and the appropriate filter. Harmonic rejection was accomplished using the uncoated mirror. Pt L3 edge XAFS were measured at 10BM using a Vortex ME4 silicon drift detector. The monochromator 2nd crystal was detuned to 50% of the maximum intensity for harmonic rejection. Double crystal Si (111) monochromators were used at both beam lines. The vertical beam slit on 10BM was set to limit energy resolution degradation to less than +10%. The energies were calibrated within ±0.05 eV to known in the art.
Electrochemical Measurements. All electrochemical measurements were performed on a CHI Electrochemical Station (Model 760b) equipped with high-speed rotators from Pine Instruments. A rotating ring disk electrode (RRDE) from Pine Research Instrumentation (model: AFE7R9GCPT, USA) was used as the working electrode, containing glassy carbon disk and platinum ring: disk OD=5.61 mm; ring OD=7.92 mm; ID=6.25 mm. An Hg/HgSO4 reference electrode and a graphite rod counter electrode with a diameter of 0.250 inches and a length of 12 inches were used to complete the cell. To prepare the working electrode, 10 mg catalyst was dispersed ultrasonically in a 1.0 mL mixture of isopropanol and Nafion (5 wt. %) solution to form an ink. The ink was then drop-casted on the disk electrode with a designed loading of 20 μgPt/cm2 and dried at room temperature to yield a thin-film electrode. All the cyclic voltammetry (CV) and ORR polarization curves were recorded in 0.1 M HClO4, and the ORR activity was measured in 0.1 M HClO4 saturated with O2 at 1600 rpm using linear sweep voltammetry (LSV) polarization plots at a scan rate of 10 mV·s−1. The accelerated stress tests (ASTs) were applied to evaluate catalyst stability by cycling the potentials ranging from 0.6 to 1.0 V, (scan rate: 50 mV/s) at 60° C. in 0.1 M HClO4 saturated with N2 by using RDE. All reference potentials have been converted to a reversible hydrogen electrode (RHE).
Fuel Cell Fabrication and Testing. As-synthesized catalysts were incorporated into the membrane electrode assembly (MEA) by directly spraying a water/n-propanol based ink onto a Nafion 211 membrane. The MEA was prepared with a Pt loading of ˜0.1 mgPt cm−2 on the cathode side. H2-air fuel cell testing was carried out in a single cell using a commercial fuel cell test system (Fuel Cell Technologies Inc.). The MEA was sandwiched between two graphite plates with straight parallel flow channels machined in them. The cell was operated at 80° C., with 150 kPaabs H2/air or H2/O2, and a gas flow rate of 500/1000 sccm for anode/cathode, respectively. Catalyst mass activity was measured via the current DOE/FCTT protocol (potential step from 0.6 V to 0.9 V and 15 min hold, current averaged during last 1 min) and by measuring the current at 0.9 V (IR-free) in 150 kPaabs H2/O2 (80° C., 100% relative humidity (RH), 500/1000 sccm) with correction for measured H2 crossover. The electrochemical active surface area was obtained by calculating underpotentially-deposited hydrogen (HUPD) charge in CV curves 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 Pt (CV curves were obtained under 150 kPaabs H2/N2, 30° C., >100% RH, 500/1000 sccm). The potential cycling accelerated stability test (AST) 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 kPaabs H2/N2, 80° C., 100% RH, 200/200 sccm).
Computational method. The first-principles density functional theory (DFT) calculations with a plane-wave basis set were performed using the Vienna ab initio simulation package (VASP) software. In all calculations, the plane-wave basis set's cutoff energy was set as 500 eV for plane wave expansion. The generalized gradient approximation (GGA) in the form of the Perdew, Burke, and Ernzernhof (PBE) functionals was used to describe the electronic exchange and correlation energy. The projector augmented wave (PAW) pseudopotential was used to describe the core electrons. Metal, nitrogen co-doped carbon was modeled with a hexagonal 7×7 graphene layer containing the metal and nitrogen dopants. Platinum clusters were modeled using a thirteen-atom cuboctahedral particle and a four-atom tetrahedral particle. The platinum catalyst was modeled with a p(4×4) Pt(111) surface slab. The Brillouin zone was sampled using Monkhorst scheme with 2×2×1 k-point grid for model Pt/FeN4—C, Pt/CoN4—C, and Pt/N4—C, 3×3×1 k-point grid for model FeN4@Pt(111), CoN4@Pt(111), and N4@Pt(111). A vacuum layer of 12 Å perpendicular to the surface was added to avoid periodic images' interaction.
Although the present disclosure has been described with respect to one or more particular embodiments and/or examples, it will be understood that other embodiments and/or examples of the present disclosure may be made without departing from the scope of the present disclosure.
This application claims priority to U.S. Provisional Application No. 63/051,703, filed on Jul. 14, 2021, the disclosure of which is incorporated herein by reference.
This invention was made with government support under contract no. DE-AC52-06NA25396 awarded by the Department of Energy. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/041689 | 7/14/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63051703 | Jul 2020 | US |
