HK1219074B - Non-pgm catalysts for orr based on charge transfer organic complexes - Google Patents

Description

Non-PGM catalysts for redox reactions based on charge-transfer organic complexes

CROSS-REFERENCE TO RELATED APPLICATIONS

The following application claims the benefit of U.S. Provisional Application No. 61/753,123, filed January 16, 2013, the entire contents of which are incorporated herein by reference.

Background of the Invention

Fuel cells are gaining increasing attention as a viable energy alternative. Generally, fuel cells convert electrochemical energy into electricity in an environmentally clean and efficient manner. They are seen as a potential energy source for everything from small electronics to cars and homes. To meet diverse energy needs, a variety of different fuel cell types exist, each with distinct chemistries, requirements, and uses.

As an example, a direct methanol fuel cell (DMFC) relies on the oxidation of methanol to form carbon dioxide on an electrocatalyst layer. Water is consumed at the anode and produced at the cathode. Positive ions (H ⁺ ) are transported across a proton exchange membrane to the cathode, where they react with oxygen to produce water. Electrons can then be transported from the anode to the cathode via an external circuit to provide power to an external source.

As another example, polymer electrolyte membrane (PEM) fuel cells (also known as proton exchange membrane fuel cells) use pure hydrogen (typically supplied from a hydrogen tank) as fuel. A hydrogen stream is delivered to the anode side of the membrane-electrode assembly (MEA), where it is catalytically broken down into protons and electrons. As with DMFCs, positive ions are transported across the proton exchange membrane to the cathode, where they react with oxygen to produce water.

Currently, one of the limiting factors for the large-scale commercialization of PEM and DMFC fuel cells is the cost associated with precious metals. Both DMFC and PEM fuel cells typically use platinum as an electrocatalyst. Precious metals such as platinum are required at the cathode to catalyze the slow redox reaction (ORR). One of the main ways to overcome this limitation is to increase the utilization of platinum in precious metal-based electrocatalysts. Another possible approach is to use cheaper but still sufficiently active catalysts in larger quantities. Several classes of non-platinum electrocatalysts have been identified that have sufficient redox activity to be considered as possible electrocatalysts in commercial fuel cell applications.

Typically, known non-platinum electrocatalysts are supported on high-surface-area carbon black. This improves the dispersion, active surface area, and conductivity of the catalytic layer. The synthesis procedure typically involves depositing precursor molecules onto a supporting substrate and pyrolyzing the supported precursor.

Metal-nitrogen-carbon (MNC) catalysts have been shown to be very promising for electrochemical oxygen reduction applications in fuel cell membrane electrode assemblies (MEAs), stacks, and fuel cell systems. Key aspects of these materials include the presence of metal particles, a conjugated carbon-nitrogen-oxide-metal network, and nitrogen-bonded carbon. The metal phase includes metal, oxide, carbide, nitride, and mixtures of these states. The chemical state and bonding of the N/C/M and N/C networks influence performance; for example, increased overall nitrogen content improves ORR performance. However, these systems still suffer from several significant drawbacks, including low stability in acidic environments, low durability in acidic and alkaline environments, the high cost of nitrogen precursors, and low ORR activity compared to platinum. The low stability in acid is associated with metal leaching from the carbon-nitrogen network. The low durability in acidic and alkaline solutions can be explained by the release of significant _amounts of _H₂O₂ in these environments, which is corrosive to both the metal and the carbon-nitrogen network. The low activity may be due to the low metal loading and hence the low concentration of active sites in such catalysts due to the use of external carbon sources (high surface carbon such as Vulcan, Ketjen Black, etc.).

SUMMARY OF THE INVENTION

Generally speaking, the present disclosure provides novel materials and methods of making the same.

According to one embodiment, the present disclosure provides a method for preparing novel non-platinum group metal (PGM) catalytic materials using a sacrificial support-based approach and using inexpensive and readily available precursors, including precursors of transition metals and nitrogen-rich charge transfer salts that can be used in different applications including fuel cells.

According to another embodiment, the present disclosure provides a method for preparing novel non-platinum group metal materials using a mechanosynthesis-based approach.

According to yet another embodiment, the present disclosure provides a method for preparing novel non-platinum group metal materials using a combination of mechanosynthesis and sacrificial support-based approaches.

According to yet another embodiment, the present disclosure provides a novel non-platinum group metal catalytic material formed by the above method.

Summary of the Figures

FIG1 is a SEM image of a Fe-NCB catalyst prepared using the method described herein.

FIG2 is a TEM image of the Fe-NCB catalyst of FIG1 .

FIG3 is a high-resolution TEM image of the Fe-NCB catalyst of FIG1 and FIG2.

Figure 4 shows the RRDE data (ring current - top and disk current - bottom) for catalysts prepared using the methods described herein with various thermal treatment schemes.

Figure 5 shows the RDE measurements of the durability of the catalysts prepared using the methods described herein, measured using the protocol proposed by the DOE Durability Working Group (DWG).

FIG6 shows RDE measurements of the durability of catalysts prepared using the methods described herein, measured using a loading cycling protocol.

Figure 7 shows the MEA performance data of Fe-NCB catalysts prepared using the method described in this article with different Nafion contents under the recommended DOE conditions of _H2 / _O2 operation, 100% RH and 1 bar _O2 partial pressure (total pressure 1.5 bar or back pressure 0.5 bar).

Figure 8 shows the kinetic current density of Fe-NCB catalysts prepared using the method described in this paper with different Nafion contents under the recommended DOE conditions of H ₂ /O ₂ operation, 100% RH and 1 bar O ₂ partial pressure (total pressure 1.5 bar or back pressure 0.5 bar).

Figure 9 shows the reproducibility of the dynamic current density for three different MEAs containing Fe-NCB catalysts prepared using the methods described herein in 55% Nafion. Conditions: _Tcell = 80°C, 100% RH, 0.5 bar backpressure.

Figure 10 shows the durability data of the Fe-NCB non-PGM catalyst prepared using the method described herein with 45% Nafion under a loading cycling regime. Conditions: _Tcell = 80°C, 100% RH, 0.5 bar back pressure.

Detailed Description of the Invention

In general, the present disclosure provides novel materials and methods for making the same. According to one embodiment, the present disclosure provides novel catalysts and catalytic materials and methods for making the same. Unlike many previously described methods for making M-N-C-based catalytic materials, which involve dispersing precursor materials on a solid support, the present disclosure provides a sacrificial support-based method, a mechanical synthesis-based method, and a combined sacrificial support/mechanical synthesis-based method that can make supported or unsupported catalytic materials and/or synthesize catalytic materials from soluble and insoluble materials. In addition, because the methods disclosed herein can be used to make catalytic materials with specific morphologies, especially specific porous morphologies, the catalytic materials described herein can be customized to meet the specific needs of the application in terms of size, shape, and activity.

For clarity, in this application, the term "catalyst" is used to refer to a catalytically active end product suitable for use in, for example, a fuel cell. The catalyst can include a variety of materials, some of which may not be catalytically active themselves (e.g., a supporting material). The term "catalytic material" refers to any material that is catalytically active, either alone or as part of a catalyst.

According to a more specific example, the catalytic materials of the present disclosure can be synthesized using a sacrificial support-based method. For the purposes of this disclosure, the term "sacrificial support" is intended to refer to a material used to provide temporary structural support during the synthesis process, but the material is largely or completely removed during the synthesis step. According to one embodiment of this particular method, the sacrificial support is an impregnated M-N-C precursor, wherein the metal is provided by one or more transition metal precursors and nitrogen and carbon are provided by one or more charge transfer salt precursors. According to some specific embodiments, the transition metal can be iron. Suitable iron precursors include, but are not limited to, iron nitrate, iron sulfate, iron acetate, iron chloride, and the like. In addition, it is understood that other transition metals such as Ce, Cr, Cu, Mo, Ni, Ru, Ta, Ti, V, W, and Zr can be substituted for iron by simply using precursors of those alternative metals. Exemplary transition metal precursors include, but are not limited to, cerium nitrate, chromium nitrate, copper nitrate, ammonium molybdate, nickel nitrate, ruthenium chloride, tantalum isopropoxide, titanium ethoxide, vanadium sulfate, ammonium tungstate, and zirconium nitrate. In addition, according to some embodiments, the presently described method can employ precursors of two or more metals to produce a multi-metal catalyst. Generally, a charge transfer salt is defined as an association of two or more molecules or atoms, or different parts of a macromolecule, wherein a portion of the electronic charge is transferred between the molecular or atomic entities. According to some specific embodiments, the charge transfer salt can be a nitrogen-rich charge transfer salt such as nicarbazine. Other suitable charge transfer salts include, but are not limited to, tetracyanoquinodimethane, tetrathiafulvalene, and multiferroic materials.

For the purposes of this disclosure, the term "precursor" is used to refer to a compound that participates in a chemical reaction by donating one or more atoms to a compound formed as a product of the chemical reaction or otherwise contributing to the formation of the product. For example, participating in the formation of a gaseous product that creates pores or voids in the final product or contributing to the formation of a chemical structure of the final product, such as in the case of nickel nanoparticles that lead to the growth of carbon fibers.

It is understood that the sacrificial support can be synthesized and impregnated in a single synthesis step, or the sacrificial support can be first synthesized (or otherwise obtained) and then impregnated with one or more charge transfer salt precursors and one or more transition metal precursors as appropriate/desired. The impregnated sacrificial support is then subjected to a thermal treatment (e.g., pyrolysis) in an inert ( _N2 , Ar, He, etc.) or reactive ( _NH3 , acetonitrile, etc.) atmosphere.

Of course, it will be appreciated that, given the high temperatures applied to the sacrificial support during the synthesis method, it is important to select a sacrificial support that is non-reactive to the catalytic material under the specific synthesis conditions used. Therefore, it will be appreciated that silica is a preferred material for the sacrificial support, but other suitable materials may be used. Other suitable sacrificial supports include, but are not limited to, zeolites, alumina, and other metal oxides, sulfides, nitrides, or mixtures. The support may take the form of spheres, particles, or other two-dimensional or three-dimensional regular, irregular, or amorphous shapes. The spheres, particles, or other shapes may be monodisperse or of irregular size. The spheres, particles, or other shapes may or may not have pores, and such pores may have the same or different sizes and shapes.

It should be understood that the size and shape of the silica particles can be selected based on the desired shape and size of the voids in the electrocatalyst material. Therefore, by selecting a specific size and shape of silica particles, an electrocatalyst with voids of predictable size and shape can be manufactured. For example, if the silica particles are pellets, the electrocatalyst will contain a plurality of spherical voids. Those skilled in the art are familiar with the electrocatalyst Pt-Ru black, which is composed of a plurality of platinum-ruthenium alloy pellets. The electrocatalyst formed using silica pellets using the above method looks similar to a negative image of the Pt-Ru black; the spaces that exist in the Pt-Ru black as voids are filled with metal electrocatalyst, and the spaces that exist in the Pt-Ru black as metal electrocatalyst are voids.

As described above, according to some embodiments, silica spheres of any diameter can be used. In some preferred embodiments, silica particles having a characteristic length of 1 nm to 100 nm can be used, in more preferred embodiments, silica particles having a characteristic length of 100 nm to 1000 nm can be used, and in other preferred embodiments, silica particles having a characteristic length of 1 mm to 10 mm can be used. In addition, mesoporous silica can also be used in template synthesis methods. In this case, templating involves embedding the mesopores of the material and obtaining a self-supported electrocatalyst with a porosity in the range of 2-20 nanometers. In one specific embodiment, the silica template is Cab-O-Sil amorphous fumed silica (325 m2/g). As described above, because the spheres serve as a template for forming the electrocatalyst, in embodiments in which silica particles with an average diameter of 20 nanometers are used, the spherical voids in the electrocatalyst will typically have a diameter of approximately 20 nanometers. Those skilled in the art will be familiar with a variety of commercially available silica particles and can use such particles. Alternatively, known methods of forming silica particles may be employed to obtain particles having the desired shape and/or size.

As described above, after depositing and/or impregnating the charge transfer salt and metal precursor onto the sacrificial support, the material is heat-treated in an inert atmosphere, such as _N₂ , Ar, or He, or in a reactive atmosphere, such as _NH₃ or acetonitrile. An inert atmosphere is typically used when the impregnated material is nitrogen-rich, as it allows for the generation of a large number of active sites containing Fe (or other metal) _N₄ centers. However, if the impregnated material is carbon-rich and nitrogen-poor, a nitrogen-rich atmosphere may be desirable, as it allows for the generation of Fe (or other metal)N₄ (including _N₄ ) centers. As described in more detail in the Experimental Section below, according to some preferred embodiments, the materials of the present invention are heat-treated in a reactive atmosphere.

According to some embodiments, particularly those in which a single-step synthesis method is used, the optimal temperature for heat treatment is generally between 500° C. and 1100° C. According to some embodiments, heat treatment is preferably between 800° C. and 1000° C., or more preferably between 875° C. and 925° C. In some embodiments, heat treatment at approximately 900° C. is preferred, as our experimental data indicates that heat treatment of material at this temperature for 1 hour produces a catalyst with a high catalytic activity for certain specific materials (see the Experimental section below).

After heat treatment, the sacrificial support is removed to obtain a porous, non-supported catalytic material. In some cases, the porous, non-supported catalytic material is composed only of materials derived from the starting precursor material. Any suitable means can be used to remove the sacrificial support. For example, the sacrificial support can be removed via chemical or thermal etching. Examples of suitable etchants include NaOH, KOH, and HF. According to some embodiments, KOH is preferably used because it retains all metals and metal oxides in the catalyst, and if the species is catalytically active, the use of KOH can in fact improve catalytic activity. Alternatively, in some embodiments, HF may be preferred because it is extremely corrosive and can be used to remove certain toxic species from the catalyst surface. Therefore, those skilled in the art will be able to select the required etchant according to the specific requirements of the specific catalytic material formed.

As described above, the presently described catalytic materials can also be synthesized using a dual thermal treatment method. In this method, the charge transfer salt and metal precursor are impregnated into the sacrificial support, which is then subjected to a first thermal treatment step, such as pyrolysis, to produce an intermediate material rich in unreacted iron. According to some embodiments, the sacrificial support can be removed after the first thermal treatment using chemical etching as described above or other suitable means. The intermediate material is then subjected to a second thermal treatment step, which can be, for example, a second pyrolysis treatment, to obtain newly formed active sites. This second thermal treatment step can also be used to remove any volatile species (such as HF) that may have been introduced during the chemical etching process (if performed) that may have introduced desirable surface defects and expanded the open pore structure initially generated by the sacrificial support. If the sacrificial support is not removed after the first thermal treatment step, it can be removed again using the above-described method after the second thermal treatment step.

In embodiments utilizing a dual heat treatment schedule, it may be desirable for the different heat treatment steps to be performed under different conditions, such as at different temperatures and/or for different durations. For example, a first heat treatment step may be performed at a higher temperature, such as 800° C., for 1 hour, and a second heat treatment step may be performed at a temperature of 800 to 1000° C. for a period of 10 minutes to 1 hour.

It is understood that in some applications, a single metal catalyst may not be sufficiently stable or active to replace conventional platinum or platinum alloy based catalysts. Therefore, as described above, according to some embodiments, the presently described methods may combine the use of multiple metal precursors to achieve the desired stability and/or activity.

According to some embodiments, it is desirable to manufacture large quantities of the catalysts described herein, for example, in a batch process. Therefore, the present disclosure further provides methods for large-scale preparation of the catalysts described herein. According to one embodiment, the present disclosure provides a method that combines a sacrificial support-based method with spray pyrolysis to produce a self-supported catalyst. According to this method, the spray pyrolysis method is a continuous method, while the sacrificial support-based method is carried out in batches. According to an exemplary method, the charge transfer salts and metal precursor materials described herein are mixed with a silica support, atomized and dried in a tube furnace. The powder obtained by the procedure is then collected on a filter. The collected powder is then heat-treated. Finally, the sacrificial support is removed, for example, by leaching it out with HF or KOH.

It is to be understood that the large-scale manufacturing methods described above are applicable to a variety of precursors and materials and, thus, need not be limited to the catalysts disclosed herein.

According to another embodiment, the present disclosure provides a method for forming a non-PGM catalytic material using a mechanosynthesis-based method. The mechanosynthesis-based method described herein is, for example, capable of preparing a variety of materials, including but not limited to catalytic materials formed from insoluble materials. The method employs ball milling and may or may not use a support, which may or may not be a sacrificial support. It will be understood that while the method does not require the addition of a solvent, a solvent may be used if desired.

Ball milling has been previously described as a method for filling the pores of a carbon support with a pore filler with reference to the synthesis of M-N-C catalyst materials. See, for example, Jaouen et al. [44]. However, in the methods described in the present disclosure, ball milling is employed to enable mechanical synthesis, reducing the need for solvent-based preparation methods. For the purposes of this disclosure, the term "ball mill" is used to refer to any type of grinder or pulverizer that uses grinding media such as silica abrasives or edged parts such as burrs to grind the material into a fine powder and/or introduce sufficient energy into the system to initiate the solid-state chemical reaction that leads to the formation of the catalyst. Generally, for the purposes of this disclosure, the ball mill used should be capable of generating sufficient energy to initiate the desired chemical reaction or achieve the desired level of mixing.

Generally, the presently described methods utilize the energy generated by ball milling various precursor materials to drive chemical reactions between the precursors. According to a more specific example, the catalytic materials of the present disclosure can be synthesized by ball milling the charge transfer salt and transition metal precursor under conditions sufficient to initiate polymerization of the various precursors, thereby forming (or initiating the formation of) a MNC polymer. This MNC polymer is then subjected to a thermal treatment (e.g., pyrolysis) in an inert ( _N2 , Ar, He, etc.) or reactive ( _NH3 , acetonitrile, etc.) atmosphere at a sufficient temperature to produce the catalytic material. According to some embodiments, the entire process is performed dry, meaning that no solvent is added. According to one embodiment of the solvent-free method, all reactants (i.e., precursors) are combined in powder form in a ball mill, and the entire process is performed without the addition of any liquid. According to some embodiments, a support material, which may or may not be sacrificial, may also be included. For the purposes of this disclosure, a powder is a dry, loose solid composed of a large number of very fine particles, which flows freely when shaken or tilted. Because this method can be performed in the absence of any solvent, it is capable of synthesizing catalysts formed from insoluble materials. Examples of insoluble materials that may be used to form catalysts according to the present disclosure include, but are not limited to, polyacrylonitrile, melamine, polyurethane, and the like.

Exemplary properties that may be considered in selecting nitrogen, carbon, or nitrogen-carbon precursors for use in making catalytic materials as described herein include, but are not limited to: (1) carbon content; (2) nitrogen content; and (3) thermal stability, i.e., the volatility of the molecule and its resistance to decomposition due to heating. The degree of carbon content is related to the porosity of the final product, with carbon content being inversely related to a more open final structure. For example, according to some embodiments, if the carbon precursor contains an average of at least 5 carbon atoms per molecule, a porous, open-framework matrix will be formed. The nitrogen richness of the precursor may need to be considered depending on whether the synthesis is to be performed in an inert environment or in a nitrogen-rich environment. For example, if the synthesis is to be performed in an inert atmosphere, the precursor must have a significant amount of nitrogen because all active MN _x centers must be formed from nitrogen contained in the precursor itself. Finally, a precursor should be selected that will remain stable under the thermal conditions to be used. For example, if the process to be used requires pyrolysis at temperatures above 700°C (the minimum temperature generally required to form active sites), it is important that the precursor remain stable at temperatures above 700°C.

According to some embodiments, the MNC precursors described herein are ball-milled in the presence of a support material to allow for infusion of the MNC precursor onto, around, and through the support material (if the support material is porous). Examples of suitable support materials include, but are not limited to, carbon black, carbon nanotubes, conductive oxides or nitrides such as indium tin oxide or molybdenum nitride, and the like, or materials that are initially non-conductive but can become conductive after processing, such as _TiO2, which can become conductive after chemical or thermal reduction, oxygen addition, or post-synthesis doping. Including a support material in the ball-milling process provides a supported catalytic material. The support material can be active or inert and may or may not contribute to the catalytic activity of the catalytic material.

According to yet another embodiment, a method combining the above-described ball milling with a sacrificial support-based technique can be used to form a non-PGM catalytic material. According to these embodiments, the M-N-C precursor described herein is ball milled in the presence of a sacrificial support, which is subsequently removed after pyrolysis as described above, to obtain a porous, unsupported catalytic material. In some cases, the porous, unsupported catalytic material is composed solely of materials derived from the initial precursor material.

The specific methods and compositions described herein represent preferred embodiments and are exemplary and not intended to limit the scope of the invention. For example, although much of the above description is directed to catalytic materials for fuel cells, it should be understood that the materials and methods disclosed herein can be used for other catalytic or non-catalytic materials and for other applications, which may or may not involve catalysis. As non-limiting examples, the materials disclosed herein can be used for liquid storage or as absorbents. Other objects, aspects, and embodiments will occur to those skilled in the art when considering this specification, and such other objects, aspects, and embodiments are encompassed within the spirit of the invention as defined by the scope of the claims. It will be apparent to those skilled in the art that various substitutions and modifications can be made to the invention disclosed herein without departing from the scope and spirit of the invention. The inventions suitably described exemplarily herein can be implemented in the absence of any one or more elements or one or more limitations that are not specifically disclosed herein as essential. The methods and processes suitably described exemplarily herein can be implemented in different step orders, and they are not necessarily limited to the order of steps described herein or in the claims. As used herein and in the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly indicates otherwise. Thus, for example, reference to "a catalyst" includes a plurality of such catalysts, and so forth.

The terms and expressions that have been used are used as terms of description rather than limitation, and it is not intended that such terms and expressions be used to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention as claimed. Thus, it is understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modifications and changes may be made to the concepts disclosed herein by those skilled in the art, and such modifications and changes are considered to be within the scope of the invention as defined by the appended claims.

All patents and publications cited below and/or mentioned herein are indicative of the levels of skill of those skilled in the art to which the invention pertains, and each such cited patent or publication is hereby incorporated by reference to the same extent as if individually incorporated by reference or set forth in its entirety herein. Applicants reserve the right to incorporate into this specification any and all materials and information from any such cited patent or publication as the case may be.

Additional information can be gleaned from the Examples section below. The reaction experiments shown and described in the accompanying figures and in the following Examples clearly demonstrate that the catalysts prepared using the described methods exhibit high redox activity in acidic media. Furthermore, the mechanism of oxygen reduction demonstrates that oxygen is directly reduced to water via a four-electron pathway, preventing the formation of corrosive peroxides and thereby improving the stability and durability of the catalyst. Thus, catalysts having this composition and using the preparation methods described herein, including but not limited to the materials described herein, are effective catalysts for oxygen reduction.

Example I: Synthesis of catalytic materials from iron and nicarbazine precursors using a sacrificial support-based approach

First, a calculated amount of silica (Cab-O-Sil® M5P, surface area 125 m²/g) was dispersed in water using a high-energy ultrasonic probe. Subsequently, a suspension of nicarbazine (Sigma-Aldrich) in acetone was added to the silica and sonicated in an ultrasonic bath for 20 minutes. Finally, a solution of ferric nitrate (Fe( _NO₃ ) _₃ · _9H₂O , Sigma-Aldrich) was added to the SiO₂ _- NCB solution and sonicated for 8 hours (the total metal loading on the silica was calculated to be ~20 wt%). After sonication, the viscous gel of silica and Fe-NCB was dried overnight at 85°C. The resulting solid was ground into a fine powder in an agate mortar and pestle and then subjected to heat treatment (HT). Typical HT conditions were UHP nitrogen gas (flow rate 100 cm³/min) and a heating rate of 20°C/min. The experimental variables of the heat-time trajectory were the temperature and duration of the HT (900°C, 1 hour; 950°C, 30 minutes, and 950°C, 1 hour). After the heat treatment, the silica was leached overnight using 25 wt% HF. Finally, the Fe-NCB catalyst was washed with DI water until a neutral pH was achieved and then dried at T = 85°C. A second heat treatment was performed at T = 950°C in a reactive ( _NH3 ) atmosphere.

The SEM image in Figure 1 shows that the Fe-NCB catalyst has several levels of porosity, resulting from the removal of _SiO2 nanoparticles and morphological defects formed during the decomposition of nicarbazine. TEM (Figure 2) reveals a very transparent, open structure with repeating morphological units. High-resolution TEM (Figure 3) reveals graphitic planes with amorphous carbon. EDS analysis confirms the presence of Fe, while the absence of observable metal particles in the TEM image suggests the presence of very small, uniformly distributed iron particles throughout the nitrogen-rich carbon network. High-resolution XPS spectroscopy reveals amounts of nitrogen (4.7 atom %) and iron (0.39 atom %) similar to those of other MNC electrocatalysts. This sample has a significant amount of pyridinic nitrogen (398.8 eV) and Fe- _Nx centers (399.6 eV), which have previously been associated with higher ORR electrocatalytic activity. Figure 4 shows RDE data for various thermal treatments. As shown, Fe-NCB heat treated at T = 900°C for 1 hour has a value of E1 _/2 = 0.8 volts, which is significantly higher than many other non-PGM catalysts tested under the same conditions.

A batch of Fe-NCB materials was synthesized using the above method using a first heat treatment step at T = 900 °C for 1 hour and tested to verify the high performance and durability of this promising catalyst under automotive performance and durability cycles simulating actual stack conditions.

RDE measurements of the catalyst sample (Figures 5 and 6) using the protocols proposed by the DOE Durability Working Group (DWG) (Figure 5) and the loading cycle protocol (Figure 6) revealed a high kinetic current density of i _k = 4.6 mA/cm² at 0.8 V, with a Tafel slope of 52 mV/decade. The Fe-NCB sample also exhibited an active reduction peak at approximately 0.75 V, likely related to the catalyst's active sites. Under durability testing, the catalyst exhibited an E _1/2 decrease of only 3-4% of the initial value, demonstrating excellent durability.

RDE evaluation is a powerful tool for measuring catalyst activity, but MEA testing in an operating fuel cell provides a more realistic assessment of overall performance. Figures 7 and 8 show the MEA performance of the Fe-NCB catalyst under the recommended DOE conditions of _H2 / _O2 operation, 100% RH, and 1 bar O2 _partial pressure (1.5 bar total pressure or 0.5 bar back pressure). Three MEAs with the same catalyst loading of 4 mg/cm2 but varying Nafion content were studied. The open circuit voltage (OCV) was 0.92 volts and did not change with increasing Nafion content. Figure 7 shows that increasing the ionomer content from 35% to 55% significantly altered the IV performance. The poor IV performance of the 35 wt% Nafion MEA can be attributed to incomplete coverage of the non-PGM active sites by Nafion. Better ionomer coverage was achieved when the Nafion content was increased to 45% and 55%, as indicated by the significant improvement in the IV curves. Increasing the ionomer content from 45% to 55% resulted in a further increase in power current. As shown in Figure 8, an MEA containing this Fe-NCB catalyst with 55% Nafion delivered a power current of 100 mA/cm² at 0.8 ViR-free. This is the first report of fuel cell performance that meets current DOE design targets for non-PGM cathode PEMFC catalysts for potential future automotive applications. As shown in Figure 9, this result was reproduced using three MEAs from different catalyst batches. The reproducibility of the high current densities achieved with this catalyst was confirmed by overlaid Tafel plots. To our knowledge, this is the first report of a non-PGM catalyst achieving such high current density values at 0.8 ViR-free using a Nafion NRE211 membrane (a significantly thinner membrane than Nafion 115 or Nafion 117, which are commonly used by other research groups working on non-PGM catalysts).

We also evaluated the durability of this Fe-NCB catalyst using an automotive accelerated stress test (AST) that simulates actual stack conditions in FECV operation. The catalyst demonstrated excellent durability, with minimal change in polarization performance after 10,000 potential cycles (shown in Figure 10 for the 45% Nafion sample). All MEAs tested under the loading cycling regime exhibited similar durability, regardless of Nafion content. The differences in the beginning of life (BoL) between the iV curves in Figure 10 and the corresponding curve for the 45% Nafion MEA in Figure 9 are attributed to MEA-to-MEA variations.

Claims (15)

1. A method for forming a metal-nitrogen-carbon (M-N-C) catalytic material, comprising: Under suitable conditions, a transition metal precursor is combined with a nitrogen-rich charge-transfer salt precursor to synthesize the catalytic material comprising a nitrogen-rich charge-transfer salt and a transition metal; and then subjected to heat treatment. 2. The method of claim 1, wherein the metal precursor is an iron precursor. 3. The method of claim 1, wherein the heat treatment comprises pyrolysis. 4. The method of claim 1, wherein the transition metal precursor and the nitrogen-rich charge-transfer salt precursor are combined in the presence of a support material. 5. The method of claim 4, wherein the carrier material is a sacrificial carrier. 6. The method of claim 5, further comprising removing the sacrificial support to produce an unsupported catalytic material. 7. The method of claim 1, wherein the nitrogen-rich charge-transfer salt is insoluble. 8. The method of claim 4, wherein the carrier material is insoluble. 9. The method of claim 1, wherein the nitrogen-rich charge-transfer salt is nicarbazin. 10. A metal-nitrogen-carbon (M-N-C) catalytic material comprising a metal and a substantial portion of nitrogen-bonded carbon derived from a nitrogen-rich charge-transfer salt. 11. The catalytic material of claim 10, wherein the catalytic material is unsupported. 12. The catalytic material of claim 10, wherein the nitrogen-rich charge-transfer salt is nicarbazin. 13. The catalytic material of claim 10, having a high current density of 0.8 ViR-free when using a Nafion NRE211 membrane. 14. The catalytic material of claim 10, wherein at least a portion of the catalytic material is insoluble. 15. A metal-nitrogen-carbon (M-N-C) catalytic material, which is formed by the following steps: Provide sacrificial template particles; The metal precursor is reacted with a nitrogen-rich charge-transfer salt onto the sacrificial template particles to produce a dispersed precursor; Heat treatment of the dispersed precursor; and The sacrificial template particles are removed to produce a self-supported high surface area catalytic material.