BACKGROUND

Technical Field

The present disclosure relates to a method of making an electrode, specifically, an electrode via atomic layer deposition.

Description of the Related Art

Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of delivering power economically and with environmental and other benefits. Various types of fuel cells include, but are not limited to, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. To be commercially viable, however, fuel cell systems should exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside their preferred operating ranges.

Fuel cells convert reactants, namely, fuel and oxidant, to generate electric power and reaction products. In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (For most fuel cell systems, end plate assemblies are placed at each end of the stack to hold the stack together and to compress the stack components together. Compressive force provides sealing and adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.

Each fuel cell employs a membrane electrode assembly (“MEA”), which comprises an electrolyte disposed between the two electrodes, namely a cathode and an anode. The electrolyte will vary based on the type of fuel cell. For example, a polymer electrolyte fuel cell will employ a polymer electrolyte membrane; a phosphoric acid fuel cell will employ a liquid phosphoric acid; a molten carbonate fuel cell will employ a molten carbonate electrolyte; and a solid oxide fuel cell will typically employ an yttria-stabilized zirconia electrolyte. Fuel cells may further include separator or flow field plates for directing the reactants across one surface of each electrode substrate, which are disposed on each side of the electrode assembly to form a fuel cell.

Polymer electrolyte membrane (“PEM”) fuel cells and their MEAs typically employ noble metal catalysts for their electrodes to induce the desired electrochemical reactions. The noble metal catalyst may be deposited directly on the membrane to form a catalyst-coated membrane (“CCM”) or directly on a gas diffusion layer (“GDL”) to form a gas diffusion electrode (“GDE”). Conventional large scale production methods of depositing the catalyst include spraying, screen printing, knife coating, slot die, and microgravure coating.

However, such methods are not suitable for producing ultra-low platinum loading electrodes as the fuel cell industry moves towards lower platinum loadings to reduce cost. As a result, there has been much development of new methods to produce electrodes with low platinum loadings while maintaining high catalyst activity and durability. More recently, alternative methods of depositing ultra-low platinum loadings onto either the membrane or GDL have been explored, including atomic layer deposition (“ALD”), chemical vapor deposition (“CVD”) and physical vapor deposition (“PVD”) methods.

ALD has been applied to deposit Pt on various supports and shows the capability of controlling the catalyst size from single atoms to subnanometer clusters to nanoparticles, such as that described in Cheng et al. in Nano Energy. Vol. 29. 2016. p.220-242. One advantage of ALD is the deposition of metals or metal oxides on 3D materials. Such direct deposition of noble metals or metal oxides during MEA fabrication will improve the Pt utilization and decrease the cost of MEA. In particular, Shu et al. in Electrochemica Acta Vol 2015. p. 168- 173. describes a high- performance membrane electrode assembly with low platinum loading that was successfully prepared using an ALD technique in which platinum was directly deposited onto a gas diffusion layer, which was pretreated with citric acid, to form a catalyst layer. The results showed that the active component, Pt, was highly dispersed in the ALD anode, and the MEA with the ALD anode showed excellent activity and stability. The mass activity reached 4.80 kW g Pt ^-1 , which was 2.53 times higher than that of the MEA with the anode prepared using the commercial catalyst and a conventional screen printing method.

U.S. Patent Application Publication number 2009/0218311 discloses a method of fabricating layer-structured catalysts at the electrode/electrolyte interface of a fuel cell. The method includes providing a substrate, depositing an electrolyte layer on the substrate, depositing a catalyst bonding layer to the electrolyte layer, depositing a catalyst layer to the catalyst bonding layer, and depositing a microstructure stabilizing layer to the catalyst layer. The catalyst layer is deposited via atomic layer deposition. However, because the electrolyte is used as a support for the catalyst that is applied via atomic layer deposition, the electrolyte must necessarily be able to tolerate the relatively high processing temperatures of about 200 to 350 degrees Celsius for ALD, which is above the decomposition temperature of the electrolyte for most PEM fuel cell electrolytes, including the widely-used Nafion® electrolyte membrane by Dupont. Furthermore, the use of a catalyst bonding layer on the surface of the electrolyte layer before deposition of the catalyst onto the catalyst bonding layer may inhibit catalyst activity as the catalyst may not be in contact with the electrolyte.

While advances have been made in recent years with respect to methods of making electrodes for fuel cells, there still exists a need to explore new methods of making electrodes that will continue to reduce noble metal loading while maintaining high catalyst activity and durability. Embodiments of the present disclosure address this need and provide associated benefits.

BRIEF SUMMARY

In brief, one embodiment provides a method of making an electrode assembly comprising the steps of: providing a non-porous substrate; providing a carbonaceous material onto the non-porous substrate to form a microporous carbonaceous layer; and providing a noble metal catalyst onto the microporous carbonaceous layer to form a catalyst layer; wherein the noble metal catalyst is provided via atomic layer deposition onto the microporous carbonaceous layer.

In specific embodiments, the microporous carbonaceous layer is a non- platinum-containing microporous carbonaceous layer, that is, essentially free of platinum, before atomic layer deposition.

In some embodiments, the microporous carbonaceous layer may include at least one additive. In one embodiment, the additive is a capable of oxidizing hydrogen and/or reducing oxygen. In another embodiment, the additive is capable of catalyzing reactions other than those for electricity generation during fuel cell operation, that is, other than hydrogen oxidation on the anode or oxygen reduction on the cathode.

These and other aspects of the disclosure are evident upon reference in the attached drawings and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a membrane electrode assembly according to one embodiment.

Figure 2 shows a membrane electrode assembly according to another embodiment.

Figure 3 shows membrane electrode assembly according to another embodiment.

Figure 4 shows the performance loss of a comparative fuel cell and an example fuel cell of the present description after anode potential cycling.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of the various embodiments of the disclosure. However, one skilled in the art will understand that embodiments of the disclosure may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, batteries and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the disclosure.

Unless the context requires otherwise, throughout the specification and claims which follow, the word“comprise” and variations thereof, such as,“comprises” and“comprising” are to be construed in an open, inclusive sense, that is as“including, but not limited to.”

A“non-platinum-containing layer” means that the layer is essentially free of platinum.

In the present context, a“graphitic” material means that it is partially graphitized or graphite. Furthermore, carbonaceous or graphitic materials that are “partially graphitized” means that the surface of these materials comprise at least some graphitic carbon.

As shown in Figure 1, a membrane electrode assembly 2 including an anode electrode 4 having an anode gas diffusion layer 6 with anode microporous layer 8 and an anode catalyst layer 10; a cathode electrode 12 having a cathode gas diffusion layer 14 with cathode microporous layer 16 and a cathode catalyst layer 18; and an electrolyte membrane 20 interposed between anode catalyst layer 8 and cathode catalyst layer 18.

In one method of making an electrode assembly, as shown in Figure 2, a carbonaceous material is provided onto a non-porous substrate 22 to form a

microporous carbonaceous layer 24. Thereafter, a noble metal catalyst is provided onto microporous carbonaceous layer 24 via atomic layer deposition to form a catalyst layer 26. To form a catalyzed membrane, catalyst layer 26 is decal transferred to one surface of electrolyte membrane 20, and then non-porous substrate 22 is removed to form a catalyzed membrane with a microporous carbonaceous layer.

In one embodiment, the concentration of the noble metal through the thickness of the catalyst layer when deposited via ALD is non-uniform. For example, with reference to Figure 3, the concentration of the noble metal in catalyst layer 26 is higher on exposed surface 28 opposite from non-porous substrate 22 than non-exposed surface 30, which is adjacent microporous carbonaceous layer 24.

Without being bound by theory, the inventors posit that when platinum is deposited via ALD, the platinum distribution in the resulting noble metal-containing catalyst layer is dependent on the physical properties of the microporous carbonaceous layer, such as porosity, conductivity, surface area, roughness, tortuosity, and thickness, which, in turn, influence the fluid diffusivity and permeability characteristics of the microporous carbonaceous layer. For example, if the diffusivity and/or permeability of the gas diffusion layer is too high ( e.g ., when carbon fiber paper is used to support the microporous carbonaceous layer rather than a non-porous substrate), platinum was deposited on the other side of the microporous carbonaceous layer, rendering them catalytically inactive for hydrogen oxidation and oxygen reduction as platinum is not in contact with an ionomer.

However, the inventors surprisingly discovered that by using different carbonaceous materials in the microporous carbonaceous layer and applying the carbonaceous material on a non-porous substrate prior to ALD, the platinum

distribution through the thickness of the resulting catalyst layer can be controlled so that the platinum concentration is higher on the exposed surface rather than having a platinum throughout the thickness of the catalyst layer. This non-uniform distribution of platinum through the thickness is desirable as the exposed surface with the higher platinum concentration can be decal transferred to the polymer electrolyte membrane to form a catalyzed membrane. Without being bound by theory, the increased proximity of platinum to the electrolyte membrane improves the three-phase boundary of catalyst, electrolyte and reactant, which improves the performance of the fuel cell. Therefore, it is believed that tailoring the physical properties of the microporous carbonaceous layer to control of the diffusivity and/or permeability of the microporous layer, as well as using a non-porous substrate to support the microporous carbonaceous layer to additionally limit the penetration of platinum via ALD, will help achieve the desired distribution of platinum (or other noble metal) in the resulting catalyst layer when deposited via ALD. The non-porous substrate may be any suitable substrate so long as it is suitable for decal-transferring of the electrode assembly to the electrolyte membrane. The non-porous substrate should also be inert and resistant to the high processing temperatures of ALD, as well as the high sintering temperatures of the binder in the microporous carbonaceous layer. For example, the non-porous substrate may be a release sheet such as, but not limited to, polytetrafluoroethylene films (such as Teflon® (DuPont)), polyethylene naphthalate films (such as Teonex® (DuPont)), polyethylene terephthalate films (such as Mylar® (DuPont)), and polyimide films (such as Kapton® (DuPont)). One skilled in the art can readily select a non-porous film suitable for the present method.

The microporous carbonaceous layer includes at least one type of carbonaceous material that is electrically conductive. Suitable carbonaceous materials include, but are not limited to, particles, fibers, hollow spheres, whiskers, nanofibers, nanotubes, and nanowhiskers, that may be carbon, partially graphitized carbon, or graphite, and combinations thereof. In some examples, the carbonaceous material is a carbon black, such as Vulcan XC72 and Denka, an activated carbon, a graphitized carbon, graphite, graphene, and combinations thereof. For example, a high surface area carbon can be mixed with graphitized carbon particles to produce a microporous layer that is corrosion resistant as well as provides a higher surface area surface for atomic layer deposition of platinum. In some embodiments, non-carbonaceous materials, such as conductive oxides, may also be incorporated in the microporous carbonaceous layer.

To enhance the hydrophobic and/or hydrophilic properties of the microporous carbonaceous layer, a hydrophobic and/or hydrophilic binder material may also be used in the ink of the microporous carbonaceous layer, such as, but not limited to, polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),

polyvinylidene fluoride (PVDF), perfluoroalkoxy alkanes (PFA), an ionomer, and combinations thereof. The binder materials used in the microporous carbonaceous layer are preferably selected such that they are to withstand the relatively high ALD processing temperatures. Typical amounts of binder in the microporous carbonaceous layer may range from about 5 wt% to about 50 wt %, preferably about 10wt% to 40wt%, depending on the type of ionomer and carbonaceous material, as well as the operating conditions. Without being bound by theory, the combination of the binder materials in the microporous carbonaceous layer can be optimized to control the physical properties thereof, as well as the surface properties of the pores, to result in the desired gas and liquid transport through the microporous carbonaceous layer.

In some embodiments, the microporous carbonaceous layer is a non- platinum-containing microporous carbonaceous layer, that is, essentially free of platinum, before the step of atomic layer deposition of the noble metal catalyst.

In some embodiments, the microporous carbonaceous layer may include at least one additive. Without being bound by theory, it may be desirable to incorporate the additive separately from the noble metal catalyst because the additive catalyst may not be suitable for atomic layer deposition or may interfere with the deposition of the noble metal catalyst.

In one embodiment, the additive is a capable of oxidizing hydrogen and/or reducing oxygen. In one example, the additive may be a non-precious metal catalyst, such as, but not limited to, cobalt, iron, molybdenum, nickel, tantalum, tin, tungsten, palladium, titanium, zirconium, and osmium; and compounds, alloys, solid solutions, and mixtures thereof.

In another embodiment, the additive is capable of catalyzing reactions other than those for electricity generation during fuel cell operation, that is, other than hydrogen oxidation on the anode or oxygen reduction on the cathode.

For example, the additive may be capable of oxidizing carbon monoxide that is typically in the fuel stream when directly reformed from an on-board fuel reformer. Carbon monoxide poisoning of the platinum catalyst reduces the anode catalyst activity by blocking active catalyst sites normally available for hydrogen oxidation. Suitable second catalysts for oxidizing carbon monoxide may include, but are not limited to, oxides and/or organometallic complexes ruthenium, iridium, nickel, tungsten, and chromium. In another example, the additive may be capable of decomposing or scavenging peroxide radicals, which may form during fuel cell operation and negatively affect fuel cell durability, such as that described in U.S. Patent No. 7,537,857. Suitable additives for this function include, but are not limited to, salts, oxides and/or organometallic complexes of Ce, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn and W. In specific embodiments, the additive is a cerium-based metal oxide or a manganese-based metal oxide.

In some embodiments, the additive is supported on an additive support. Suitable additive supports include carbon and graphitic materials, such as high surface area carbons, partially graphitized carbons, and graphite. Other suitable carbon supports may include nitrogen-, boron-, sulfur-, and/or phosphorous-doped carbons, carbon nanofibres, carbon nanotubes, carbon nanohoms, graphenes, and aerogels. Instead of carbon, carbides or electrically conductive metal oxides may also be considered as a suitable catalyst support, such as titanium and niobium oxides.

The microporous carbonaceous layer may be formed on the non-porous substrate by any suitable method known in the art. For example, the carbonaceous material and, optionally, the binders, pore formers and/or additive, may first be dispersed in a suitable dispersant to form an ink or slurry, or as a dry mixture. The ink may be applied using a variety of suitable techniques ( e.g ., hand and machine methods, including hand brushing, notch bar coating, fluid bearing die coating, wire-wound rod coating, fluid bearing coating, slot-fed knife coating, three-roll coating, screen-printing, ultrasonic spray, inkjet, and decal transfer) to the non-porous substrate. Examples of dry deposition methods include spraying, vacuum deposition, and electrostatic powder deposition techniques. The microporous carbonaceous layer may optionally be heat- treated at an elevated temperature during or after application onto the non-porous substrate, for example, to sinter the binders and/or to remove the pore formers. Suitable pore formers include methyl cellulose and sublimating pore-forming agents such as durene, camphene, camphor and naphthalene.

To improve chemical bonding of the gaseous precursor to the microporous carbonaceous layer during the ALD process, the microporous carbonaceous layer may be functionalized after application onto the non-porous substrate. Such functionalization treatments include applying a plasma treatment, a hydrogen treatment, an ozone treatment, an acid treatment, or a peroxide treatment to the microporous carbonaceous layer. For example, the microporous carbonaceous layer may be chemically oxidized by citric acid to obtain surface oxides, such as carboxyl and hydroxyl groups, such as the method as described in Poh et al. in Journal of Power Sources , Vol. 176, 2008, p.70-75.

The noble metal-containing catalyst layer contains a noble metal, such as, platinum, gold, ruthenium, iridium, osmium, palladium, and silver, or alloys or mixtures thereof, including, but not limited to, platinum alloys with one or more of cobalt, nickel, manganese, and iron. The noble metal-containing catalyst layer is formed via atomic layer deposition of a precursor of the noble metal onto and/or into the non-platinum-containing microporous layer. Typical atomic layer deposition methods involve four steps: (1) adsorption of the gaseous precursor of the noble metal onto the surface of the deposition substrate; (2) purging the excess precursor and its byproducts from the sample; (3) introduction of a gaseous reactant to remove ligands and regenerate sites; and (4) purging out the excess reactants and their byproducts.

The gaseous precursor of the noble metal may be any suitable precursor. For example, the platinum precursor is methylcyclopentadienyl trimethylplatinum (MeCpPtMes) and the oxidative precursor is oxygen. Other suitable precursors for platinum include, but are not limited to, platinum (II) acetylacetonate [Pt(acac)2] and platinum(II) hexafluoroactylacetonate [Pt(hfac)2]. The platinum precursor may be introduced by pulse, which makes them adsorb to the surface of the microporous carbonaceous layer. Oxygen is introduced, which reacts with the platinum precursor to form platinum atoms that are deposited microporous carbonaceous layer with very high platinum dispersion.

The carrier and purging gas may be, for example, nitrogen. The deposition may be done under vacuum and at elevated temperatures, for example, between about 100 and about 300 degrees Celsius. The deposition may be done over a controlled number of cycles to control the platinum loading, particle size and penetration depth. The resulting platinum loading from ALD may range from, for example, about 0.01 mg/cm ² to about 0.1 mg/cm ² .

Optionally, an ionomer layer may be applied onto the noble metal- containing catalyst layer prior to bonding to the electrolyte membrane. The ionomer layer may be applied by any suitable method known in the art, such as, but not limited to, spraying, knife coating, and microgravure coating.

Thereafter, the electrode assembly comprising the microporous layer and the noble metal-containing catalyst layer, with or without the ionomer layer, may be decal transferred to an electrolyte membrane such that the catalyst layer is interposed between the microporous layer and the electrolyte membrane. If an ionomer layer is used, the ionomer layer will be interposed between the catalyst layer and the electrolyte membrane. The non-porous substrate may then be removed from the microporous layer of the electrode assembly to form a catalyzed electrolyte membrane. Any suitable electrolyte membrane may be used, such as those that are sold under the Nafion® (DuPont), Aciplex® (Asahi Kasei Corporation), and Aquivion® (Solvay Plastics) tradenames. The electrolyte membrane may be perfluorinated, partially fluorinated or hydrocarbon-based. A second electrode assembly comprising a second catalyst layer formed by the same method or different, may also be formed on the opposing surface of the catalyzed electrolyte membrane, simultaneously, before, or after the first electrode assembly is decal-transferred, to form a dual-sided catalyzed electrolyte membrane.

Gas diffusion layers may be placed on the outer surfaces of the CCM to form a membrane electrode assembly.

EXAMPLES

CCMs were made with the following electrode structures as listed in

Table 1.

Table 1. Anode and cathode catalyst structures for CCMs

For Comparative CCM, the anode catalyst ink was applied onto a non- porous PTFE substrate by microgravure, and then dried to form an anode catalyst- coated film (CCF). The cathode catalyst was directly applied onto one surface of a Nafion® membrane to form a half-CCM. The anode CCF was then decal-transferred to the opposing membrane surface of the half-CCM to form a full CCM.

For Example CCM, graphitized carbon was mixed with Nafion® ionomer, then coated onto a non-porous PTFE substrate by microgravure at a loading of 0.2mg/cm ² and dried. Platinum was then deposited via ALD onto the dry carbon layer until a loading of about 0.01 mg/cm ² to form an anode catalyst layer (about 20 cycles for a total of about 10 minutes). A Nafion® ionomer was then sprayed onto the anode catalyst layer at a loading of about 0.1 mg/cm ² to form an anode CCF. The cathode catalyst was directly applied onto one surface of a Nafion® membrane to form a half- CCM. The anode CCF was then decal -transferred to the opposing membrane surface of the half-CCM to form a full CCM.

Each of the CCMs were sandwiched between two AvCarb® GDLs (AvCarb Materials Solutions, Lowell, Massachusetts) to form MEAs. The active area of each of the MEAs was 2.85 cm ² .

The MEAs were then tested in a Ballard Standard Test Cell (STC) test fixture with graphite plates. The fuel cells were first conditioned overnight under the following conditions at 1.3 A/cm ² :

Table 2. Conditioning parameters

Anode potential cycling

The fuel cells were conditioned overnight at 1.3 A/cm ² at the conditions listed in Table 2. A beginning of life (BOL) polarization was obtained for each MEA.

The fuel cell was then potential cycled between 0.1V and 1.0V on the anode side under nitrogen and hydrogen on the anode and cathode, respectively, for 60 seconds at 0.1V and 30 seconds at 1.0 V. After 2000 cycles, an end of life (EOL) polarization was obtained for each of the MEAs. The EOL polarization data was then subtracted from the BOL polarization data to determine the performance loss, as shown in Figure 3. It is clear that the fuel cell with the Comparative CCM showed a lot higher performance loss than the fuel cell with the Example CCM.

While the present electrodes have been described for use in PEM fuel cells, it is anticipated that they may be useful in other fuel cells having an operating temperature below about 250 °C. They are particularly suited for acid electrolyte fuel cells, including phosphoric acid, PEM and liquid feed fuel cells. It is also contemplated that this treatment may also be useful for other metal oxides comprising ruthenium.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, including, but not limited to U.S. Provisional Patent Application No. 62/815,872 filed on March 8, 2019, are incorporated herein by reference in their entirety.

While particular elements, embodiments and applications of the present disclosure have been shown and described, it will be understood, of course, that the disclosure is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications that incorporate those features coming within the scope of the present disclosure.