catalyst

Abstract

The present invention provides a method for preparing a catalyst material, the catalyst material comprising electrocatalyst particles, a support material and graphite-phase carbon nitride, wherein the method comprises applying the graphite-phase carbon nitride to the catalyst material precursor. The present invention also provides a catalyst material comprising graphite-phase carbon nitride.

Description

Technical Field

[0001] The present invention provides a novel catalyst material and a method for preparing the material. The catalyst material comprises an electrocatalyst and graphite-phase carbon nitride. Background Art

[0002] A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel (e.g., hydrogen, an alcohol (such as methanol or ethanol) or formic acid) is supplied to the anode, and an oxidant (e.g., oxygen or air) is supplied to the cathode. An electrochemical reaction occurs at the electrodes, and the chemical energy of the fuel and oxidant is converted into electrical energy and heat. An electrocatalyst is used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

[0003] Fuel cells are generally classified according to the nature of the electrolyte used. The electrolyte is typically a solid polymer membrane that is electrically insulating but ionically conductive. In a proton exchange membrane fuel cell (PEMFC), the ion-conducting membrane is proton-conducting and transports protons generated at the anode across the membrane to the cathode, where they combine with oxygen to form water.

[0004] The main component of a PEMFC is the membrane electrode assembly (MEA), which essentially consists of five layers. The middle layer is a polymer ion-conducting membrane. On either side of the ion-conducting membrane are electrocatalyst layers containing electrocatalysts designed for a specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer are gas diffusion layers. The gas diffusion layers must allow reactants to reach the electrocatalyst layers and must conduct the current generated by the electrochemical reaction. Therefore, the gas diffusion layers must be porous and conductive.

[0005] The electrocatalyst layer generally also comprises a proton-conducting material, such as a proton-conducting polymer, to facilitate the transfer of protons from the anode electrocatalyst to the ion-conducting membrane and / or from the ion-conducting membrane to the cathode electrocatalyst.

[0006] Conventionally, membrane electrode assemblies can be constructed by a variety of methods. Typically, these methods involve applying one or two of the electrocatalyst layers to an ion-conducting membrane to form a catalyst-coated ion-conducting membrane. Subsequently, a gas diffusion layer is applied to the electrocatalyst layer. Alternatively, the electrocatalyst layer is applied to the gas diffusion layer to form a gas diffusion electrode, which is then combined with the ion-conducting membrane. The membrane electrode assembly can be prepared by a combination of these methods, for example, applying one electrocatalyst layer to an ion-conducting membrane to form a catalyst-coated ion-conducting membrane and applying another electrocatalyst layer as a gas diffusion electrode.

[0007] The electrocatalyst layer typically comprises an electrocatalyst material comprising a metal or metal alloy suitable for fuel oxidation or oxygen reduction reaction, depending on whether the layer is to be used for anode or cathode. Electrocatalysts for fuel oxidation and oxygen reduction are typically based on platinum or platinum alloyed with one or more other metals. Platinum or platinum alloy electrocatalysts can be in the form of unsupported nanoscale particles (e.g., metallic black), or can be deposited onto a support material (supported electrocatalyst) as discrete nanoparticles, resulting in a very high electrocatalyst surface area. Electrocatalysts can also be in the form of a coating or extended film deposited onto a support material. Over the past decade, a wide range of catalyst concepts have been studied, such as Pt binary alloys, Pt single-layer electrocatalysts, Pt surface electrocatalysts, and nanostructured thin film electrocatalysts. Another approach to the reported high-activity catalyst is the dealloyed Pt-M concept material obtained by synthesizing base metal-rich (M) particles, which are subjected to a process of selectively leaching less precious metals from the particles.

[0008] There is an ongoing search for electrocatalysts, particularly oxygen reduction electrocatalysts, that have improved activity and / or stability and thus more efficiently utilize expensive platinum electrocatalysts. This results in improved membrane electrode assembly performance, or reduced loading (and therefore cost) of the electrocatalyst used in the membrane electrode assembly, or a combination of these two beneficial effects.

[0009] One research approach involves using graphitic carbon nitride (g-C3N4) as a support material for platinum oxygen reduction reaction electrocatalysts. In this field, M. Kim et al. reported in J. Mater. Chem. 2007, 17, 1656-1659 a Pt-Ru electrocatalyst for a direct methanol fuel cell anode supported on g-C3N4. In addition, H. Huang et al. reported in Adv. Mater. 2014, 26, 5160-5165 that Pt-decorated graphene and g-C3N4 sheets can be used as methanol oxidation electrocatalysts. In addition, I. Lee et al. reported in Catal. B: Environ. 2018, 237, 318-326 an oxygen reduction reaction electrocatalyst comprising platinum dispersed on a support having a core-shell structure in which carbon black particles are coated with g-C3N4 sheets and comprise at least 17 wt% g-C3N4. Summary of the Invention

[0010] Thus, in a first aspect, the present invention provides a method of preparing a catalyst material comprising, preferably consisting essentially of, more preferably consisting of (or consisting only of) electrocatalyst particles, a support material and graphitic carbon nitride, the method comprising the steps of:

[0011] (i) providing a catalyst material precursor comprising, preferably consisting essentially of, more preferably consisting of (or consisting only of): electrocatalyst particles dispersed on a support material; and

[0012] (ii) applying graphite-phase carbon nitride to the catalyst material precursor provided in step (i).

[0013] In this context, the term "catalyst material precursor" refers to a material from which the catalyst material containing graphite-phase carbon nitride of the present invention can be prepared. Such a catalyst material precursor can itself be catalytically active and can be, for example, a conventional catalyst material comprising an electrocatalyst dispersed on a support material in the absence of graphite-phase carbon nitride. The catalyst material precursor does not contain graphite-phase carbon nitride.

[0014] In a second aspect, the present invention provides a catalyst material, which can be obtained by the method described in the first aspect of the present invention.

[0015] In a third aspect, the present invention provides a catalyst material comprising, preferably consisting essentially of, more preferably consisting of (or consisting only of): electrocatalyst particles, a support material, and graphitic carbon nitride, wherein the electrocatalyst particles are dispersed on the support material, and wherein the catalyst material comprises no more than 5 wt% graphitic carbon nitride, based on the total weight of the graphitic carbon nitride and the support material.

[0016] Surprisingly, the catalyst materials of the present invention (i.e., the catalyst materials of the second or third aspects) or the catalyst materials prepared according to the first aspect exhibit improved electrochemical performance relative to conventional catalyst materials comprising an electrocatalyst dispersed on a support material. In particular, surprisingly, by applying graphitic carbon nitride to the catalyst material, the electrochemical performance of such conventional catalyst materials can be improved.

[0017] The present invention also provides an electrocatalyst layer comprising the catalyst material of the present invention.

[0018] The present invention also provides a catalyst-coated ion-conducting membrane comprising the electrocatalyst layer of the present invention and an ion-conducting membrane.

[0019] The present invention also provides a gas diffusion electrode comprising the electrocatalyst layer of the present invention and a gas diffusion layer.

[0020] The present invention also provides a membrane electrode assembly comprising the catalyst layer of the present invention, the catalyst-coated ion-conducting membrane of the present invention, or the gas diffusion electrode of the present invention.

[0021] The present invention also provides a fuel cell comprising the electrocatalyst layer of the present invention, the catalyst-coated ion-conducting membrane of the present invention, the gas diffusion electrode of the present invention or the membrane electrode assembly of the present invention.

[0022] The present invention also provides a catalyzed transfer substrate comprising the electrocatalyst layer of the present invention and a transfer substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A to Figure 1 E shows an energy dispersive X-ray spectroscopy (EDS) elemental mapping image of the catalyst material of Example 1.

[0024] Figure 2 A and Figure 2 B shows the deconvoluted X-ray photoelectron spectroscopy (XPS) curves of C1s in the catalyst material of Comparative Example 1 (Image A) and the catalyst material of Example 1 (Image B).

[0025] Figure 3 A and Figure 3 B shows the deconvoluted XPS curves of Pt 4f in the catalyst material of Comparative Example 1 (Image A) and the catalyst material of Example 1 (Image B).

[0026] Figure 4 A shows a graph of cell potential versus current density for half-cell tests performed on a gas diffusion electrode containing the catalyst material of Comparative Example 1, and gas diffusion electrodes containing the catalyst materials of Examples 1 to 4.

[0027] Figure 4 B shows a graph of cell potential versus mass activity for half-cell tests performed on a gas diffusion electrode containing the catalyst material of Comparative Example 1, and gas diffusion electrodes containing the catalyst materials of Examples 1 to 4 with a pure oxygen feed.

[0028] Figure 5 A is a bar graph showing mass activities determined from half-cell tests of a gas diffusion electrode containing the catalyst material of Comparative Example 1 and gas diffusion electrodes containing the catalyst materials of Examples 1 to 4.

[0029] Figure 5 B is a bar graph showing specific activities determined from half-cell tests of a gas diffusion electrode containing the catalyst material of Comparative Example 1 and gas diffusion electrodes containing the catalyst materials of Examples 1 to 4.

[0030] Figure 6 A shows a graph of cell potential versus current density for single cell tests performed on a membrane electrode assembly containing the catalyst material of Comparative Example 1 and a membrane electrode assembly containing the catalyst material of Example 1, the membrane electrode assembly being corrected for internal resistance. The cathode was supplied with pure oxygen.

[0031] Figure 6 B shows a graph of potential versus current density for single cell tests performed on a membrane electrode assembly containing the catalyst material of Comparative Example 1 and a membrane electrode assembly containing the catalyst material of Example 1, the membrane electrode assembly being corrected for internal resistance. The cathode was supplied with air. DETAILED DESCRIPTION

[0032] The preferred and / or optional features of the present invention will now be described. Unless the context otherwise requires, any aspect of the present invention may be combined with any other aspect of the present invention. Unless the context otherwise requires, any of the preferred or optional features of any aspect may be combined with any aspect of the present invention individually or in combination. Reference to catalyst materials of the present invention includes catalyst materials prepared by the method of the first aspect of the present invention, as well as catalyst materials of the second and third aspects of the present invention.

[0033] The catalyst material of the present invention comprises electrocatalyst particles, preferably nanoparticles. The exact electrocatalyst used will depend on the reaction it is intended to catalyze, and its selection is within the ability of the skilled person. The electrocatalyst may preferably be a cathode or anode electrocatalyst of a fuel cell, more preferably a proton exchange membrane fuel cell. The electrocatalyst is suitably selected from:

[0034] (i) platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium);

[0035] (ii) gold or silver;

[0036] (iii) base metals;

[0037] or alloys or mixtures comprising one or more of these metals or their oxides. The base metal is tin or a transition metal that is not a noble metal. The noble metal is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium, or osmium) or gold. Preferred base metals are copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium, and tin.

[0038] Typically, the electrocatalyst comprises, preferably consists essentially of, more preferably consists of (or includes) a platinum group metal or an alloy of a platinum group metal, preferably with a base metal (preferred base metals as defined above). Specifically, the electrocatalyst comprises, preferably consists essentially of, more preferably consists of (or includes) platinum or an alloy of platinum with a base metal (preferred base metals as defined above), preferably nickel or cobalt, most preferably nickel. The atomic ratio of platinum to alloy metal is typically in the range of 3:1 to 1:3 and includes 3:1 to 1:3.

[0039] The electrocatalyst is dispersed on a support material. Thus, a skilled artisan will readily understand supported electrocatalysts and the term "supported." For example, it should be understood that the term "supported" includes binding or immobilizing the electrocatalyst to the support material via physical or chemical bonds. For example, the electrocatalyst may be bound or immobilized to the support material via ionic or covalent bonds, or non-specific interactions such as van der Waals forces.

[0040] Typically, a conductive carbon support material is used, preferably in powder form. The carbon support material can be, for example, commercially available carbon black (such as ( XC72R) or Akzo Nobel( Black series)) or graphitized versions of these carbon blacks or other commercially available carbon blacks such as acetylene black (available from Chevron Phillips (Shawinigan ) or those of Denka). The support material may also be a support material specifically designed for use in fuel cells, such as those described in WO 2013 / 045894.

[0041] Alternatively, the support material may be a metal oxide or mixed oxide (in particular a conductive mixed oxide, such as niobium-doped titanium dioxide, phosphorus-doped tin oxide and mixed platinum group metal oxides or mixed metal oxides (as disclosed in WO 2012 / 080726)), a carbide (for example, tungsten carbide, molybdenum carbide or titanium carbide, suitably tungsten carbide or titanium carbide), a nitride (in particular a conductive nitride (for example, titanium nitride or titanium aluminum nitride)).

[0042] The electrocatalyst loading in the catalyst material and catalyst material precursor may be expressed in terms of the weight percentage of active metal (e.g., platinum group metal) in the electrocatalyst relative to the total weight of the electrocatalyst and the support material (i.e., excluding any other materials present, including graphitic carbon nitride), which may be determined using inductively coupled plasma mass spectrometry (ICPMS). Thus, the loading may suitably be at least 10% by weight of active metal, e.g., platinum group metal. The loading of the electrocatalyst may suitably be no more than 90% by weight of active metal, e.g., platinum group metal, typically no more than 60% by weight of active metal, e.g., platinum group metal, for example no more than 40% by weight of active metal, e.g., platinum group metal, based on the total weight of the electrocatalyst and support material.

[0043] Graphitic carbon nitride, also denoted g-CN, is a well-known and well-studied graphite-like layered material composed of ordered tris-s-triazine or heptazine subunits connected by planar tertiary amino groups within the layers and weak van der Waals forces between the layers. Graphitic carbon nitride is prepared by condensing precursor compounds, such as melamine, dicyandiamide, cyanamide, urea, thiourea, and ammonium thiocyanate, for example, with the aid of sonication and / or heating. This condensation proceeds through polymeric CN structures known as melons, which further condense into graphitic carbon nitride with tris-s-triazine units as basic building blocks.

[0044] In the present invention, step (ii) can be carried out by contacting the catalyst material precursor with a graphite phase carbon nitride precursor compound (preferably a solution of the compound), and applying conditions sufficient to convert, preferably condense (i.e., convert by a condensation reaction) the graphite phase carbon nitride precursor into graphite phase carbon nitride. Suitably, the graphite phase carbon nitride is applied to the catalyst material precursor by mixing the catalyst material precursor with, for example, a solution of a desired graphite phase carbon nitride precursor in water, the desired graphite phase carbon nitride precursor being preferably melamine, dicyandiamide or cyanamide, more preferably cyanamide. Preferably, the mixture is ultrasonically treated to ensure that the catalyst material precursor is fully dispersed in the solution of the graphite phase carbon nitride. Most preferably, the catalyst material precursor is mixed with a solution of cyanamide in water. The concentration of the graphite phase carbon nitride precursor in the solution is not particularly limited and can be, for example, in the range of 25 wt % to 75 wt %, for example, 50 wt %. The skilled person knows the conditions sufficient to condense the graphite phase carbon nitride precursor to form graphite phase carbon nitride. Suitably, the mixture is dried and then calcined in an inert atmosphere, for example in an argon atmosphere, to a temperature in the range of 250°C to 750°C, preferably 400°C to 600°C and inclusive, and suitably at the desired temperature for a period of not more than 10 hours, preferably not more than 5 hours. The mixture is suitably calcined at the desired temperature for at least 1 hour. The drying step may, for example, be carried out by leaving the mixture in air for a period of at least 12 hours and not more than 48 hours, suitably not more than 36 hours or not more than 24 hours. However, the skilled person will be guided by the manner in which the drying step is carried out. The graphite phase carbon nitride in the catalyst material of the present invention is preferably bonded to the support material by covalent bonds, and therefore the method of the present invention preferably results in covalent bonding between the graphite phase carbon nitride and the support material. Such covalent bonds can, for example, be detected by x-ray photoelectron spectroscopy, as Figure 2 shown and discussed in the Examples section.

[0045] According to the total weight of the graphite phase carbon nitride and the support material (i.e., excluding any other materials present, including electrocatalysts), the loading of the graphite phase carbon nitride is suitably no more than 5% by weight, preferably no more than 3.5% by weight, and more preferably less than 2% by weight. According to the total weight of the graphite phase carbon nitride and the support material, the loading of the graphite phase carbon nitride is suitably at least 0.5% by weight. The skilled person knows methods that can be used to determine the elemental composition of a sample, and therefore knows the amount of graphite phase carbon nitride in the catalyst material. For example, the loading can be determined by conventional techniques, such as burning a sample and analyzing the product gas (i.e., combustion analysis), for example, using a CHNS / O analyzer (e.g., Perkin-Elmer 2400 series), which is based on a molar ratio of C to N = 3:4. In this way, the total N content can be determined, and then the carbon content according to the graphite phase carbon nitride can be calculated based on a molar ratio of C to N = 3:4, and then the weight of the graphite phase carbon nitride can be determined. The loading can be controlled by controlling the amount of the graphite phase carbon nitride precursor used in the method of the present invention.

[0046] Suitably, the graphite phase carbon nitride is dispersed, preferably uniformly dispersed, on the surface of the catalyst material. The graphite phase nitride is suitably dispersed on at least 80%, preferably at least 90%, more preferably at least 95% of the free surface of the support material (i.e., the surface of the support material that does not contain the electrocatalyst). The presence of nitrogen uniformly dispersed on the catalyst material can be confirmed by energy dispersive X-ray spectroscopy (EDS) elemental mapping of transmission electron microscopy (TEM) images, such as Figure 1 The formation of graphitic carbon nitride on the catalyst material can be detected, for example, by x-ray analysis, as shown. Figure 2 As shown in FIG. 1 and discussed in the Examples section.

[0047] The electrocatalyst layer of the present invention may preferably be a fuel cell, more preferably a cathode or anode of a proton exchange membrane fuel cell. The characteristics of the electrocatalyst layer, such as thickness, electrocatalyst loading, porosity, pore size distribution, average pore size and hydrophobicity, will depend on whether it is used for an anode or a cathode. Specifically, the electrocatalyst layer is used for a cathode. In the anode, the electrocatalyst layer thickness is suitably at least 1 μm, typically at least 5 μm. In the anode, the electrocatalyst layer thickness is suitably no more than 15 μm, typically no more than 10 μm. In the cathode, the electrocatalyst layer thickness is suitably at least 2 μm, typically at least 5 μm. In the cathode, the electrocatalyst layer thickness is suitably no more than 20 μm, typically no more than 15 μm.

[0048] The electrocatalyst loading in the electrocatalyst layer will also depend on the intended use. In this context, electrocatalyst loading means the amount of active metal (e.g., platinum group metal) in the electrocatalyst layer. Thus, when the electrocatalyst is an alloy of platinum, the electrocatalyst loading is the amount of platinum per unit area, expressed as mg / cm2 For example, in a fuel cell cathode comprising an electrocatalyst comprising platinum, the electrocatalyst loading is suitably at least 0.05 mg Pt / cm 2 , for example, no more than 0.5 mgPt / cm 2 , preferably not more than 0.3 mgPt / cm 2 In the fuel cell anode, the electrocatalyst loading is suitably at least 0.02 mg Pt / cm 2 , for example, no more than 0.2 mg / Ptcm 2 , preferably not more than 0.15 mgPt / cm 2 .

[0049] The electrocatalyst layer of the present invention preferably comprises an ion conducting polymer, such as a proton conducting polymer, to improve the ionic conductivity of the layer. Thus, the ion conducting material may comprise an ionomer, such as a perfluorosulfonic acid material (e.g. (Chemours Company), (Asahi Kasei)、 (Solvay SpecialtyPolymer), (Asahi Glass Co.), or ionomers based on partially fluorinated or non-fluorinated hydrocarbons as sulfonated or phosphonated polymers, such as those available from FuMA-Tech GmbH ( P, E or K series products), those of JSR Corporation, Toyobo Corporation, etc. Suitably, the ionomer is perfluorosulfonic acid, specifically fluorinated sulfonic acid available from Chemours Company. series (especially 1100EW), and purchased from Solvay series (especially 830EW).

[0050] The electrocatalyst layer of the present invention may contain additional components. Such components include, but are not limited to, oxygen evolution catalysts; hydrogen peroxide decomposition catalysts; hydrophobic additives (e.g., polymers (such as polytetrafluoroethylene (PTFE) or inorganic solids) with or without surface treatment or hydrophilic additives (e.g., polymers or inorganic solids such as oxides) to control the transport characteristics of reactants and water. The choice of additional components will depend on whether the electrocatalyst layer is for an anode or a cathode, and determining which additional components are appropriate is within the ability of the skilled person.

[0051] In order to prepare electrocatalyst layer, catalyst material of the present invention and any additional component are dispersed in aqueous solvent and / or organic solvent to prepare catalyst ink.If necessary, by methods known in the art, such as high shear mixing, grinding, ball milling, by microfluidic instrument etc. or their combination, carry out particle crushing, to realize suitable particle size distribution.After preparing catalyst ink, ink is deposited onto substrate (for example, gas diffusion layer, ion conducting membrane or carrier / transfer substrate) to form electrocatalyst layer. Ink can be deposited by any suitable technology known to those skilled in the art, and this technology includes but is not limited to gravure coating, slit type extrusion (slot, extrusion) coating, screen printing, rotary screen printing, inkjet printing, spraying, painting, rod coating, pad coating, gap coating technology such as carrying out knife or scraper coating and metering rod coating on roller.

[0052] The electrocatalyst layer can be deposited onto the gas diffusion layer to form the gas diffusion electrode of the present invention. The gas diffusion layer comprises a gas diffusion substrate and preferably a microporous layer. When the microporous layer is present, the electrocatalyst layer is deposited onto the microporous layer. Typical gas diffusion substrates include nonwoven paper or fiber webs comprising a network of carbon fibers and a thermosetting resin binder (e.g., TGP-H series carbon fiber papers available from Toray Industries Inc., Japan, or H2315 series available from Freudenberg FCCT KG, Germany, or TEFLON® available from SGL Technologies GmbH, Germany). series, or purchased from Ballard Power Systems Inc. series), or woven carbon cloth. Prior to manufacturing the electrodes and incorporating them into the membrane electrode assembly, the carbon paper, mesh or cloth may be provided with a pre-treatment to make it more wettable (hydrophilic) or more water-repellent (hydrophobic). The nature of any treatment will depend on the type of fuel cell and the operating conditions in which it will be used. The substrate may be made more wettable by incorporating a material such as amorphous carbon black via impregnation from a liquid suspension, or the substrate may be made more hydrophobic by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylene propylene (FEP), followed by drying and heating to above the melting point of the polymer. A typical microporous layer comprises a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE).

[0053] In the catalyst-coated ion-conducting membrane of the present invention, the electrocatalyst layer is deposited onto the ion-conducting membrane by applying the catalyst ink directly to the membrane, or the electrocatalyst layer is deposited onto the ion-conducting membrane indirectly by transferring from a transfer substrate to form the catalyst-coated ion-conducting membrane. The catalyst-coated ion-conducting membrane of the present invention may comprise a second electrocatalyst layer on its opposite side, which may be an electrocatalyst layer according to the present invention or another electrocatalyst layer. The ion-conducting membrane may suitably be any membrane suitable for use in a proton exchange membrane fuel cell, for example, the membrane may be based on a perfluorinated sulfonic acid material such as Nafion TM (Chemours Company), (Solvay Specialty Polymers)、 (Asahi Glass Group) and Aciplex TM (Asahi Kasei Chemicals Corp.). Alternatively, the membrane may be based on a sulfonated hydrocarbon membrane, such as P, E or K series products are those available from FuMA-Tech GmbH, from JSR Corporation, Toyobo Corporation, and the like.

[0054] The thickness of the ion-conducting membrane is not particularly limited and will depend on the intended application of the ion-conducting membrane. For example, a typical fuel cell ion-conducting membrane has a thickness of at least 5 μm, suitably at least 8 μm, preferably at least 10 μm. A typical fuel cell ion-conducting membrane has a thickness of no more than 50 μm, suitably no more than 30 μm, preferably no more than 20 μm. Thus, a typical fuel cell ion-conducting membrane has a thickness in the range of 5 μm to 50 μm, suitably 8 μm to 30 μm, preferably 10 μm to 20 μm and including these end values.

[0055] The ion-conducting membrane may include additional components, such as peroxide decomposition catalysts and / or free radical decomposition catalysts and / or recombination catalysts. The recombination catalyst catalyzes the reorganization of unreacted H2 and O2, which can diffuse into the ion-conducting membrane from the anode and cathode of the fuel cell, to produce water. The ion-conducting membrane may also include a reinforcing material embedded in the thickness of the ion-conducting membrane, such as a planar porous material (ePTFE (ePTFE) described in, for example, USRE37307), to provide improved mechanical strength for the ion-conducting membrane, such as enhanced tear resistance and reduced hydration and dehydration, and thus further increase the durability of the membrane electrode assembly and the lifespan of the fuel cell incorporating the catalytic ion-conducting membrane of the present invention. Other methods for forming an enhanced ion-conducting membrane include those disclosed in US 7,807,063 and US 7,867,669, wherein the reinforcing member is a rigid polymer film, such as polyimide, in which multiple holes are formed and then filled with PFSA ionomer. The graphene particles dispersed in the ion-conducting polymer layer may also be used as reinforcing material.

[0056] Any reinforcement present may extend over the entire thickness of the ion-conducting membrane, or may extend over only a portion of the thickness of the ion-conducting membrane. It will be appreciated that the thickness of the ion-conducting membrane extends perpendicular to the face of the ion-conducting membrane, for example, in the through-plane z-direction. It may also be advantageous to reinforce the periphery of the first and second surfaces of the ion-conducting membrane to a greater extent than the center plane of the first and second surfaces of the ion-conducting membrane. Conversely, it may be desirable to reinforce the center of the first or second surface of the ion-conducting membrane to a greater extent than the periphery of the first or second surface of the ion-conducting membrane.

[0057] When the electrocatalyst layer is deposited onto the transfer substrate by applying the catalyst ink onto the transfer substrate, it forms the catalyzed transfer substrate of the present invention. Additional layers may be deposited on the exposed surface of the electrocatalyst layer before removing the transfer substrate; for example, the ion-conducting ionomer layer may be applied from a dispersion of ionomer using any suitable deposition technique known as described above for the deposition of the electrocatalyst layer. Further additional layers may be added as needed, for example as described in PCT publication WO2015 / 145128. The transfer substrate is removed from the electrocatalyst layer at an appropriate time. The transfer substrate may be formed of any suitable material from which the electrocatalyst layer may be removed without damage. Examples of suitable materials include fluoropolymers such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymers (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene) and polyolefins such as biaxially oriented polypropylene (BOPP).

[0058] As will be appreciated by the skilled person, the membrane electrode assembly of the present invention may be constructed by a variety of methods, provided that it contains at least one electrocatalyst layer of the present invention. For example, the membrane electrode assembly may comprise an ion-conducting membrane of the present invention, comprising two electrocatalyst layers, at least one of which is an electrocatalyst layer of the present invention, wherein a gas diffusion layer is applied to each electrocatalyst layer. Alternatively, the membrane electrode assembly may comprise an ion-conducting membrane sandwiched between two gas diffusion electrodes, at least one of which is a gas diffusion electrode of the present invention. The membrane electrode assembly may also comprise a catalyst-coated ion-conducting membrane having one electrocatalyst layer, and a gas diffusion electrode on the opposite side of the ion-conducting membrane, wherein either or both of the electrocatalyst layer and the gas diffusion electrode are of the present invention.

[0059] The fuel cell of the present invention is preferably a proton exchange membrane fuel cell.

[0060] Example

[0061] Preparation of catalyst materials

[0062] Example 1: On a carbon black carrier ( XC72R) on platinum electrocatalyst (20 wt%), g-C3N4 (0.75 wt%).

[0063] Example 2: Carbon black carrier ( XC72R) on platinum electrocatalyst (20 wt%), g-C3N4 (1.4 wt%).

[0064] Example 3: Carbon black carrier ( XC72R) on platinum electrocatalyst (20 wt%), g-C3N4 (1.9 wt%).

[0065] Example 4: Carbon black carrier ( XC72R) on platinum electrocatalyst (20 wt%), g-C3N4 (3.1 wt%).

[0066] Comparative Example 1: Carbon black carrier ( XC72R) on a platinum electrocatalyst (20 wt%).

[0067] g-C3N4 = graphite phase carbon nitride.

[0068] For platinum electrocatalysts, the weight % quoted is relative to the total weight of the platinum electrocatalyst and the carbon black support.For graphitic carbon nitride, the weight % quoted is relative to the total weight of the graphitic carbon nitride and the carbon black support.

[0069] For Examples 1 to 4, g-C3N4@Pt (20 wt%) NP / VC (g-C3N4 = graphitic carbon nitride, NP = nanoparticles, XC72R carbon powder, obtained from Cabot Corporation) samples were synthesized using Pt (20 wt%) NP / VC as a catalyst material precursor and cyanamide (50 wt% H2O solution, Sigma-Aldrich) as a graphite phase carbon nitride precursor. For Example 1, 100 mg of Pt (20 wt%) NP / VC was mixed with 0.1 mL of cyanamide solution and 9.9 mL of H2O and ultrasonicated for 20 minutes. After being stored overnight under ambient conditions, the mixture was left to dry in air and then heated at 550°C in Ar for 3 hours to produce g-C3N4@Pt (20 wt%) NP / VC. For the synthesis of Examples 2, 3, and 4, 0.2 mL, 0.3 mL, and 0.5 mL of cyanamide solution were used, respectively.

[0070] Comparative Example 1 is Pt (20 wt %) NPs / VC, which is the same catalyst material precursor used to prepare Examples 1 to 4.

[0071] Pt (20 wt%) NP / VC is a granular carbon-supported nanoparticle material obtained from Cabot Corporation using a method similar to the general method for preparing carbon-supported catalysts described in WO2013 / 045894. Prepared by XC72R carbon powder.

[0072] Material characterization

[0073] Scanning electron microscopy (SEM) images were obtained using a FEI Helios NanoLab 650 FIB (focused ion beam)-SEM.

[0074] Energy dispersive X-ray spectroscopy (EDS) elemental mapping and transmission electron microscopy (TEM) imaging were performed using a FEI Tecnai G2 microscope operated at 200 kV.

[0075] The amounts of C and N were determined by combustion analysis using a CHNS / O analyzer (Perkin-Elmer 2400 series), and the content of g-C3N4 in the samples was calculated based on a molar ratio of C to N = 3:4.

[0076] X-ray photoelectron spectroscopy (XPS) analyses were performed in a Leybold MAX200 spectrometer using a Mg Ka source (1253.6 eV) operated at 15 kV and 20 mA. All binding energies for charged samples were corrected by reference to the adventitious root C1s peak at 285.0 eV.

[0077] Electrochemical measurements

[0078] Half-cell measurement

[0079] An electrochemical half-cell operated with a gaseous oxygen feed was used to evaluate the electrocatalytic activity of the catalyst materials. The cell achieved high current density (approximately 2 A cm -2 ) and thus allow users to simulate actual fuel cell operating environments.

[0080] The components and arrangement of the half-cell are described in B. Pinaud et al., Key considerations for high current fuel cell catalyst testing in an electrochemical half-cell, J. Electrochem. Soc., 2017, 164, F321-F327. Briefly, a flag-shaped counter electrode with a high surface area was fabricated using a large platinum wire mesh (52 mesh, 99.9%, Alfa Aesar) and a platinum wire (0.5 mm diameter, 99.95%, Alfa Aesar). An Hg / Hg2SO4 (filled with 0.5 MH2SO4 solution) electrode was used as a reference electrode. A catalyst-coated gas diffusion electrode (Sigracet 29BC) with a microporous layer was used as a working electrode having a 1.91 cm 2 The exposed active surface area of the working electrode was set to 0.1 mg Pt cm -2, and the actual electrocatalyst loading was determined by ICP-OES analysis. The electrolyte consisted of 1 M perchloric acid (HClO4). All half-cell measurements were performed at ambient temperature (approximately 20°C) to minimize the safety risks associated with hot HClO4 and to prevent loss of electrolyte due to evaporation. A glass frit (Ace dispersion tube, porosity D, Sigma-Aldrich) was used to purge the cell electrolyte and headspace with Ar or O2. All gases were delivered at a pressure of 1 atm (101 kPa absolute pressure) and had a purity of 99.99% or greater. A potentiostat (5A plate, Bio-Logic VMP3) was used for potential control and data acquisition. At each electrolyte concentration, the reference electrode was regularly calibrated to a reversible hydrogen electrode (RHE), and all potentials were reported relative to the RHE specifications unless otherwise stated.

[0081] Figure 5 The mass activity (mA / mg Pt ) data at various battery potentials directly from Figure 4 B was obtained (i.e., 0.80 V, 0.85 V, 0.90 V). The mass activity was determined by using Figure 4 The data shown in A are in the low current density region (0 mA / cm 2 Up to 100mA / cm 2 For the calculations, it was assumed that the catalyst-coated gas diffusion electrode used for the half-cell test had a 1.91 cm 2 In addition, the actual Pt loading in the catalyst-coated gas diffusion electrode was also determined by ICP-OES. Based on these parameters, Figure 4 The current density (mA / cm 2 ) can be easily converted into Figure 4 The mass activities shown in B (mA / mg Pt ). Figure 5 The specific activity data (mA / cm 2 )Data comes from Figure 5 The mass activity (mA / mg Pt ) data were divided by the electrochemical surface area (ECSA, cm) of each catalyst material. 2 / mg Pt Electrochemical surface area (ECSA) based on 0.21 mC cm on polycrystalline platinum -2 The monolayer hydrogen adsorption charge is determined by the integrated charge in the hydrogen adsorption region of the steady-state CV curve (after subtracting the capacitive contribution). Cyclic voltammogram (CV) measurements were performed at room temperature at 20 mV s -1The cyclic voltammograms (CVs) were 100% IR-corrected (95% in-situ compensation and 5% post-processing) by measuring the open circuit resistance at f = 10 kHz. The detailed information of the IR correction procedure for the constant current polarization curves is described in B. Pinaud et al. Key considerations for high current fuel cell catalyst testing in an electrochemical half-cell, J. Electrochem. Soc., 2017, 164, F321-F327.

[0082] Single cell measurement

[0083] A 50 cm2 membrane was prepared by applying a cathode gas diffusion electrode and an anode gas diffusion electrode to either side of a perfluorosulfonic acid (PFSA) reinforced membrane. 2 Total area and 14cm 2 The active area of the membrane electrode assembly is masked. The inactive area is covered. The cathode gas diffusion electrode contains a catalyst layer with a platinum loading of 0.16 mg / cm 2 The catalyst materials of the embodiments and comparative examples have a PFSA ionomer on a Sigracet 29BC gas diffusion layer. The anode gas diffusion electrode comprises a catalyst layer with a loading of 0.04 mg / cm 2 Anode electrocatalyst ( 3000 with a nominal platinum loading of 20 wt% on the carbon support) with a PFSA ionomer on a Sigracet 29BC gas diffusion layer.

[0084] For single cell measurements, use a 50 cm 2 Screener cells, which operate at 100% relative humidity on both the anode and cathode, are larger than 14 cm 2 The area is masked. The anode feed was H2 (stoichiometric ratio 1.5), and the cathode feed was O2 or air (stoichiometric ratio 2). The cell temperature was 80°C, the cell compression was 80 psi, and the cell gas pressure was 150 kPa for both the anode and cathode.

[0085] Results and Discussion

[0086] Figure 1EDS elemental mapping is used to show the location of platinum (Images D and E) and nitrogen (Images C and E) in the Example 1 catalyst material after deposition of graphitic carbon nitride. The data shows that nitrogen, and therefore graphitic carbon nitride, is uniformly dispersed across the catalyst material. In the raw data, nitrogen is represented by green and platinum by red. The raw data shows uniform dispersion of green and red across the carbon support material.

[0087] Figure 2 The deconvoluted XPS curve in shows that new C═O and CNC bonds have been generated after the application of the CCN. Figure 2 A is the data from the catalyst material of Comparative Example 1, and Figure 2 B is data from the catalyst material of Example 1. Thus, it is apparent that the process of the present invention results in the presence of graphitic carbon nitride and that the graphitic carbon nitride is covalently bonded to the support material.

[0088] Figure 3 The deconvoluted XPS curves in FIG. 1 show that for the catalyst material of Comparative Example 1 ( Figure 3 A) and Example 1 catalyst material ( Figure 3 B) For both samples, the main platinum species is Pt(0). Specifically, based on the area surrounded by the peaks of Pt(0), Pt(II), and Pt(IV), the percentage of each Pt species (i.e., Pt(0), Pt(II), and Pt(IV)) in the sample can be calculated. The main platinum species of both samples is Pt(0) (neither Pt(II) nor Pt(IV)), which means that the method of the present invention does not change the properties of the platinum electrocatalyst. This is advantageous because, for example, if the main platinum species is changed from Pt(0) to Pt(II) or Pt(IV), the electrocatalytic activity of the electrocatalyst can be significantly reduced.

[0089] Figure 4 A and Figure 4 The polarization curves and mass activity curves of the half-cell measurements shown in B respectively show that the presence of a maximum of graphite-phase carbon nitride results in better performance of the catalyst materials of Examples 1 to 4 than the catalyst material of Comparative Example 1. At high current density, a loading of less than 2 wt% (i.e., Examples 1 and 2) relative to the total weight of the graphite-phase carbon nitride and carbon black support provides better electrochemical performance than the catalyst material of Comparative Example 1. Figure 5 A and Figure 5 The mass activity and specific activity data in B show similar trends.

[0090] Figure 6 A and Figure 6The single cell test polarization curves of the catalyst materials of Comparative Example 1 and Example 1 shown in B show the surprising increase in the electrochemical performance of the catalyst materials of the present invention compared to conventional catalyst materials. Figure 6 The increase in performance seen under O2 shown in A (where the reaction is under kinetic control) shows an increase in the intrinsic catalytic activity of the catalyst material of the present invention. Figure 6 In B, under air, this increased catalytic activity is evident at low current densities, whereby kinetic control still dominates.

Claims

1. A method for preparing a catalyst material, wherein the catalyst material comprises electrocatalyst particles, a conductive carbon support material, and graphite-phase carbon nitride, the method comprising the following steps: (i) providing a catalyst material precursor comprising electrocatalyst particles dispersed on a conductive carbon support material; and then (ii) applying graphite-phase carbon nitride to the catalyst material precursor provided in step (i); The electrocatalyst particles comprise a platinum group metal or an alloy of a platinum group metal.

2. The method of claim 1 , wherein the catalyst material comprises no more than 5 wt% graphitic carbon nitride, based on the total weight of the graphitic carbon nitride and the support material.

3. A method according to claim 1 or claim 2, wherein step (ii) is carried out by contacting the catalyst material precursor with a graphite-phase carbon nitride precursor compound and applying conditions sufficient to convert the graphite-phase carbon nitride precursor into graphite-phase carbon nitride.

4. A catalyst material comprising electrocatalyst particles, a conductive carbon support material, and graphite-phase carbon nitride dispersed on a surface of the conductive carbon support material, wherein the electrocatalyst particles are dispersed on the conductive carbon support material, and wherein the catalyst material comprises no more than 2 wt% graphite-phase carbon nitride, based on the total weight of the graphite-phase carbon nitride and the conductive carbon support material, wherein the electrocatalyst particles comprise a platinum group metal or an alloy of a platinum group metal. 5 . An electrocatalyst layer comprising the catalyst material according to claim 4 .

6. A catalyst-coated ion-conducting membrane comprising the electrocatalyst layer according to claim 5 and an ion-conducting membrane. 7 . A gas diffusion electrode comprising the electrocatalyst layer according to claim 5 and a gas diffusion layer.

8. A membrane electrode assembly comprising the electrocatalyst layer according to claim 5, the catalyst-coated ion-conducting membrane according to claim 6, or the gas diffusion electrode according to claim 7.

9. A fuel cell comprising the electrocatalyst layer according to claim 5, the catalyst-coated ion-conducting membrane according to claim 6, the gas diffusion electrode according to claim 7, or the membrane electrode assembly according to claim 8.

10. The fuel cell according to claim 9, wherein the fuel cell is a proton exchange membrane fuel cell.

11. A catalyzed transfer substrate comprising the electrocatalyst layer according to claim 5 and a transfer substrate.

Images