Platinum Based Oxygen Reaction Reduction Catalyst

An oxygen reduction reaction catalyst and method for making the catalyst includes a graphitized carbon substrate with an amorphous metal oxide layer overlying the surface of the substrate. The amorphous metal oxide layer has a worm-like structure. A catalyst overlies the metal oxide layer.

TECHNICAL FIELD

The present disclosure relates to a platinum based oxygen reduction catalyst.

BACKGROUND

A durable, highly active oxygen reduction reaction (ORR) catalyst is an important candidate in developing proton exchange membrane fuel cell (PEMFC) vehicles. For many years, it has been known that platinum (Pt) based particles can be used as an oxygen reduction catalyst. Ways to improve the durability of the ORR and to enhance the reaction activity have been the focus of world-wide research for the past several decades.

SUMMARY

The present invention solves one or more problems of the prior art by providing in at least one embodiment an oxygen reduction reaction catalyst and method for making the catalyst. The catalyst comprises a graphitized carbon substrate with an amorphous metal oxide layer overlying the surface of the substrate. The amorphous metal oxide layer has a worm-like structure. A catalyst overlies the metal oxide layer.

In another embodiment, oxygen reduction reaction catalyst comprising platinum is provided. The catalyst comprises a substrate with an amorphous metal oxide layer overlying a surface of the substrate. The amorphous metal oxide layer has a worm-like structure. A platinum catalyst layer having a crystalline, 2-D connected film structure overlies the metal oxide layer.

In another embodiment, a method of forming an oxygen reduction reaction catalyst is provided. The method comprises depositing a metal oxide onto a substrate to form a metal oxide layer having a conductive, amorphous worm-like structure; and depositing a crystalline platinum film having a 2-D connected structure onto the metal oxide layer to form an oxygen reduction reaction catalyst.

DETAILED DESCRIPTION

There is still a need for improved platinum based ORR catalyst designs, and methods of making such catalysts. As a substrate for oxygen reduction reaction catalysts, graphene is the most stable carbon, but it is hard to obtain in spherical shape as nano-particles. Its application as the ORR catalyst primary support is impractical at present. Nano-particles are desired since they have a high surface area which results in an increase in reaction activity. Since the surface atoms of graphitized carbon are close to those of graphene in terms of surface carbon atomic arrangement and bonding, it can be used as a substitute for graphene. Platinum (Pt) based particles that are wet-chemically coated onto amorphous or graphitized carbon have been used as ORR catalysts. Graphitized carbon is a relatively stable support that imparts improvement of catalyst durability compared to Pt on Vulcan XC-72R, a carbon black manufactured by Cabot Corporation. A platinum based oxygen reduction catalyst on graphitized carbon, such as TKK EA carbon from Tanaka Kikinzoku Kogyo K.K. has improved durability however its ORR activity does not exhibit long term stability. The electrochemical surface area measured by hydrogen desorption (ECSA) and ORR activity at 0.9V decreases with increased potential cycling, indicating that the Pt is not interconnected and that agglomeration and dissolution still occur when graphitized carbon is used.

Fuel cell and energy storage devices lack efficient and stable catalysts. Embodiments of the present invention provide a platinum based oxygen reduction reaction catalyst that offers proven activity while maintaining exceptional durability. The use of different preparation methods is critical to achieve these attributes.

Referring now toFIG. 1, a schematic cross section of a platinum based oxygen reduction catalyst incorporating an amorphous metal oxide layer is provided. The catalyst can optionally be a component of a variety of electrochemical cells. Examples of anticipated applications include embodiments wherein the catalyst is incorporated into thin film batteries, supercapacitors, fuel cells and the like. Oxygen reduction catalyst10includes a substrate12, and a metal oxide layer14. Disposed over metal oxide layer14is a platinum catalyst16. The metal oxide layer14inhibits a reaction between the Pt catalyst16and the graphitized carbon substrate12that results in Pt agglomeration under repeated end use cycling. Further, the metal oxide layer14provides an open matrix or amorphous worm-like structure so that the overlying Pt catalyst has a large surface area for promoting the electrochemical reaction.

With reference toFIG. 2A, a scanning electron microscope20of the metal oxide layer on a substrate is provided. The substrate22is shown of graphitized carbon with an overlying metal oxide layer24having worm-like morphology28.FIG. 2Bshows a scanning electron microscope image of a 2-D connected platinum catalyst26overlying the metal oxide layer on the graphitized carbon substrate22. It can be seen that the Pt catalyst26is crystalline, and mainly forms around the junction between NbOx and graphitic carbon, and some has formed 2-D connected clusters, similar to the targeted 2-D connected Pt network morphology.

Many embodiments of the invention involve the substrate22comprising nanoparticles of graphitized carbon. The substrate22in this embodiment can promote the growth of the a worm-like structure in the overlying metal oxide layer24due to the nanoparticle arrangements to be coated.

The metal oxide layer24may be amorphous, worm-like or discontinuous, and may be referred to as a thin film layer. A thin film layer may be a continuous or discontinuous layer having a thickness from about 5 angstroms to about 1 um. The metal oxide layer24is of sufficient thickness to form a worm-like structure and is limited in thickness so as to not result in a continuous coverage of the substrate. Thicker metal oxide layers tend to form continuous coverage and can grow without the worm-like structure. The metal oxide layer24may have (e.g., is deposited at) a physical thickness of less than 1000 angstroms. In other embodiments, the layer24has a thickness of less than 500 angstroms, preferably less than 300 angstroms, and more preferably less than 100 Å.

Film24may consist essentially of, or consist of, a metal oxide. In other embodiments, film24may consist essentially of, or consist of, sub-stoichiometric metal oxide (MOx where x is less than 2). In a variation of the present embodiment, the metal oxide layer can comprise one or more materials, such as oxides of niobium, molybdenum, tungsten, tantalum, titanium, indium, zinc and tin or combinations thereof. Preferably a major percentage (e.g. by weight) of the film24is niobium. In a refinement, the metal oxide layer may contain a mixture of two or more oxides. In one embodiment, the metal oxide layer may be 100% niobium oxide. In another embodiment, the metal oxide layer is partially niobium oxide and the remaining composition is other oxides and dopants. The percent niobium oxide in the metal oxide layer can range from 0 to 100%, and in certain embodiments from 50% to 80% and in other embodiments more than 80%

In one embodiment, the metal oxide layer may be conductive. Conductivity can range from 102to 104/ohm centimeter. In a further refinement, the metal oxide layer may be doped to increase electrical conductivity. In yet another refinement, the metal oxide layer may be a cermet, containing both oxides and a metal for doping.

Solid materials and thin film layers may be characterized by their crystallographic atomic arrangement. Amorphous thin film layers lack long range order in contrast with the ordered atomic arrangement of crystalline materials. Selected Area X-ray Diffraction (SAED) is used to determine crystalline properties or the percent crystallinity of a material. Grazing angle X-ray diffraction is often used for thin films to increase the x-ray path length and accumulate enough signal to determine the presence or absence of a crystal structure.FIG. 3Ashows SAED results30for a niobium oxide layer on a graphene substrate. The diffraction pattern32is diffuse and does not show diffraction from ordered atomic arrangements indicative of crystallinity. The niobium oxide layer is therefore of amorphous structure.FIG. 3Bshows the SAED results34for the ORR Pt catalyst, here showing repeated diffractions36from atoms aligned as in the crystalline structure. The amorphous or crystalline growth of films is affected by the deposition temperature as described below and SAED serves as an important tool in determining device crystallinity and the resulting device function.

Structure zone models may be used to predict micro structure of thin films. Generally, the zone model predicts that thin films deposited at less than 30% of their melting temperature are an amorphous structure, and those deposited at temperatures greater than 30% of their melting temperature are crystalline. Deposition temperature plays a role in the resulting structure and in one embodiment, the metal oxide layer is niobium oxide because it is amorphous structurally and grows in a worm-like structure.

Referring again toFIG. 2B, the oxygen reduction reaction catalyst layer26can be Pt or can comprise Pt such as a Pt-based alloy. This Pt layer is deposited overlying the amorphous metal oxide layer24that has a worm-like structure. Unlike the worm-like structure of the metal oxide layer24, the Pt layer26has a continuous, 2D connected network structure. This is accomplished by depositing a very thin Pt layer overlying the wormlike metal oxide layer28, with thickness ranging from 5 angstroms to 100 angstroms, in one variation, ranging from 10 angstroms to 70 angstroms, and in another variation ranging from 20 angstroms to 50 angstroms.

Referring now toFIG. 4A, a schematic top view of the structure of the Pt ORR catalyst is shown. The carbon substrate42, the graphitized carbon44, the metal oxide worm-like layer46and the continuous Pt catalyst48according to one embodiment are illustrated. During the initial stages of growth, the Pt catalyst48tends to form at the junction of the interface50of the graphitized carbon substrate44and the amorphous metal oxide layer46, shown inFIG. 4C. The atomic deposition processes may occur under vacuum to enable the growing film, the Pt catalyst layer48, to form with a desired arrangement, which can follow the underlying structure, that of metal oxide layer46.FIG. 4Bis the side view of the schematic illustrated inFIG. 4A. Referring again toFIG. 4C, it is an expanded schematic top view ofFIG. 4Ashowing the graphitized carbon substrate44, the metal oxide worm-like layer46, the continuous Pt catalyst48and the interface50of the graphitized carbon substrate44and the amorphous metal oxide layer46shown.

Referring again toFIG. 1, the catalyst layer16is deposited onto the amorphous metal oxide layer14by any number of thin film vacuum deposition techniques known to those skilled in the art of thin film deposition. Examples of useful vacuum techniques include, but are not limited to, physical vapor deposition or sputtering, chemical vapor deposition, plasma assisted chemical vapor deposition, ion beam deposition and the like. In one embodiment, sputtering is found to be particularly useful because of its superior film uniformity enabling thin layers of the desired coverage. Furthermore, sputtering allows control of the process so that epitaxial growth can occur on the metal oxide layer46and at the interface50of the surface of the substrate44and the metal oxide layer46, as shown inFIG. 4C. This phenomenon is well known by those skilled in the art of sputtering.

The stability of the ECSA of the ORR catalyst is shown inFIG. 5. The ORR catalyst is cycled from 0.05 volts to 1.05 volt with a scan rate of 20 mV/sec. The stability of the catalyst with the Pt overlying a niobium oxide layer52is comparable to that of TKK-EA50 catalyst54which does not contain the metal oxide layer. The ORR activity loss is depicted in the plot ofFIG. 6. Again the cycling is from 0.05 volts to 1.05 volts with a scan rate of 20 mV/sec. The durability of the ORR activity of62is much superior to that of TKK EA5064in one or more embodiments.

Referring now toFIG. 7where the method of forming the platinum oxygen reduction reaction catalyst is shown schematically. The substrate72is coated74with a metal oxide layer having a conductive, amorphous worm-like structure. A platinum film having a 2-D connected structure is deposited76onto the metal oxide layer.

The following example illustrates the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims. For example, other coating methods can include mechanical barrel-type rotation to disperse the graphitized carbon. In certain embodiments, the graphitized carbon powders can be heated to temperatures ranging from 200° C. to 700° C. In other embodiments, the PVD sputtering can be DC magnetron sputtering of metallic targets in reactive gas mixtures such as oxygen and oxygen with argon.

An ORR catalyst with niobium oxide overlaying the substrate and a catalyst overlying the niobium oxide layer is coated as follows. Highly graphitized carbon powders of 30 nm particle size are loaded into a sample dispersion system inside a vacuum sputtering chamber. The vacuum chamber is pumped to 10−6Torr using turbo molecular pumps model Turbovac TMP 151 from Oerlikon Leybold Vacuum. Next the powders are heated to 500° C. and dispersed using ultrasonic vibration to yield a graphitized carbon substrate. The thin films are deposited onto the substrate by physical vapor deposition (PVD) using a cathode for DC magnetron sputtering. The source for the amorphous niobium oxide layer is a niobium oxide target 3 inches in diameter by 0.25 inches thick. A Pt-based target that is pure metal, and of the same dimensions, is used as the target for the platinum catalyst layer. One thousand standard cubic centimeters per minute (sccm) of argon gas is introduced into the vacuum chamber and pumped by a turbo molecular pump backed by a rotary piston mechanical pump to maintain a sputtering pressure of 5 mTorr. The sputtering is sequential, i.e., sputtering the amorphous niobium oxide first at 30 watts, followed by sputtering of the Pt catalyst at 30 watts.

The morphology of the ORR catalyst is shown inFIGS. 2A and 2B. InFIG. 2A, the NbOx24is amorphous, while the Pt26ofFIG. 2Bshows crystallinity and mainly forms around the junction between NbOx and graphitic carbon, while some has formed in 2-D connected clusters, similar to the targeted 2-D connected Pt network morphology.