WO2018094266A1

WO2018094266A1 - Membrane electrode assembly with improved electrode

Info

Publication number: WO2018094266A1
Application number: PCT/US2017/062394
Authority: WO
Inventors: Alan Young; Rajesh Bashyam; Brooks FRIESS; Siyu Ye
Original assignee: Ballard Power Systems Inc.; Ballard Material Products Inc.
Priority date: 2016-11-18
Filing date: 2017-11-17
Publication date: 2018-05-24

Abstract

A membrane electrode assembly comprising a solid electrolyte interposed between an anode electrode and cathode electrode, the anode electrode comprising an anode catalyst layer adjacent at least a portion of a first major surface of the solid electrolyte, the cathode electrode comprising a cathode catalyst layer adjacent at least a portion of a second major surface of the solid electrolyte; wherein at least one of the anode and cathode catalyst layers comprises a component that has been treated with a polyethylene glycol-phosphonic acid compound.

Description

MEMBRANE ELECTRODE ASSEMBLY WITH IMPROVED ELECTRODE

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a membrane electrode assembly with an improved electrode for use in PEM fuel cells, and to catalyst-coated membranes and fuel cells comprising the improved electrode.

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. 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. Polymer electrolyte membrane fuel cells ("PEM fuel cell") employ a membrane electrode assembly ("MEA"), which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.

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. (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 effects sealing and provides 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.

In practice, fuel cells need to be robust to varying operating conditions, especially in applications that impose numerous on-off cycles and/or require dynamic, load-following power output, such as automotive applications. For many applications, high current density operation is of particular interest but it results in the mass transport losses under high relative humidity conditions due to high water production. Without proper water removal mechanisms, mass transport losses limit the maximum current output of fuel cell stacks. As a result, it is desirable to employ a hydrophobic treatment to the gas diffusion layers and/or catalyst layers to remove excess water produced at high current density. One common method is to use polytetrafluoroethylene (PTFE) in the gas diffusion layers and/or catalyst layers to render them hydrophobic.

However, for low relative humidity/high operating temperature applications, water retention in the electrode is desirable due to reduced water production, which may lead to insufficient membrane and/or ionomer hydration and, thus, reduce proton conductivity. Methods of improving water retention in the electrode include adding various hygroscopic additives, such as A1₂0₃, Ti0₂ and silica, or simply by increasing the ionomer content in the catalyst layer. However, such methods may also reduce fuel cell performance as such materials are typically electrically insulating and/or reduce electrochemical activity and/or may reduce MEA lifetime.

As a result, there exists a need for membrane electrode assemblies and fuel cells that are more robust to different operating conditions. The present invention addresses this need and provides associated benefits.

BRIEF SUMMARY OF THE INVENTION

In brief, a membrane electrode assembly comprises a solid electrolyte interposed between an anode electrode and cathode electrode, the anode electrode comprising an anode catalyst layer adjacent at least a portion of a first major surface of the solid electrolyte, the cathode electrode comprising a cathode catalyst layer adjacent at least a portion of a second major surface of the solid electrolyte; wherein at least one of the anode and cathode catalyst layers comprises a component that has been treated with a polyethylene glycol-phosphonic acid compound.

In one embodiment, the component is a metal catalyst. In specific embodiments, the metal catalyst is selected from the group consisting of platinum, gold, ruthenium, iridium, cobalt, nickel, molybdenum, palladium, iron, tin, titanium, manganese, cerium, chromium, copper, and tungsten, and alloys, solid solutions, and intermetallic compounds thereof.

In another embodiment, the component is an additive. In specific embodiments, the additive is a metal oxide. In further embodiments, the metal oxide is selected from the group consisting of an oxide of silicon, titanium, ruthenium, iridium, cerium, manganese, indium, tin, and composites and mixtures thereof.

In yet another embodiment, a method of making a treated component for a polymer electrolyte membrane electrode assembly comprises dissolving a polyethylene glycol-phosphonic acid compound in an aqueous solution to form a polyethylene glycol-phosphonic acid solution; dispersing a component in a second solution to form a component dispersion; providing the component dispersion to the polyethylene glycol-phosphonic acid solution to form a mixed acid component dispersion; and removing the first and second solutions from the mixed acid component dispersion to form a polyethylene glycol-phosphonic acid-treated component.

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

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a cross-section of an exemplary fuel cell according to one embodiment of the present description.

Figure 2 shows graphitized carbon dispersed in water, with and without a

PEG-phosphonic acid treatment.

Figure 3 shows the air polarization results of the MEAs with and without a PEG-phosphonic acid-treated metal catalyst at 100%RH.

Figure 4 shows the air polarization results of the MEAs with and without a PEG-phosphonic acid-treated metal catalyst at 60%RH. 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 invention. However, one skilled in the art will understand that the invention 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 invention.

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."

Furthermore, the term "acid" is meant to include substances that donate a proton in a chemical reaction to a base. The term "acid derivative" is meant to include materials that behave similarly to acids such as acid salts, and acid esters, particularly lower alkyl esters containing from 1 to 4 carbon atoms.

A "corrosion resistant support material" is at least as resistant to oxidative corrosion as Shawinigan acetylene black (Chevron Chemical Company, TX, USA).

A "metal catalyst" is a compound that promotes the hydrogen oxidation reaction and/or oxygen reduction reaction in polymer electrolyte membrane fuel cells. An "additive" is a compound that does not participate in the hydrogen oxidation reaction or oxygen reduction reaction in polymer electrolyte membrane fuel cells. In other words, an additive is non-catalytic for the hydrogen oxidation reaction and/or oxygen reduction reaction. The metal catalyst may be supported or unsupported.

A "polyethylene-glycol (PEG)-phosphonic acid compound" is a compound having the formula R^c(OCH₂CH₂)_xP(=0)(OR^a)(OR^b) or (OR^c)(OR^d)P(=0)CH₂CH₂(OCH₂CH₂)_xP(=0)(OR^b)(OR^c), wherein R^a, R^b, R^c and R^d are independently H or Ci-C₆ alkyl and x is an integer from 1 to 100. In some embodiments, x is selected such that the (PEG)-phosphonic acid compound has a molecular weight from about 250 to about 700. In some embodiments, R^a, R^b, R^c and R^d are independently H or ethyl. Salts (i.e., where one or more of R^a, R^b, R^c and R^d is a counter ion, such as Na⁺) of (PEG)-phosphonic acid compounds are included within the scope of (PEG)-phosphonic acid compounds.

"Alkyl" refers to a straight or branched hydrocarbon chain radical consisting solely of carbon and hydrogen atoms, which is saturated or unsaturated (i.e., contains one or more double and/or triple bonds), having from one to six carbon atoms (Ci-C₆ alkyl), and which is attached to the rest of the molecule by a single bond, e.g., methyl, ethyl, ^-propyl, and the like. Unless stated otherwise specifically in the specification, an alkyl, group is optionally substituted, for example by replacing a hydrogen atom with a bond to a non-hydrogen atom..

In one embodiment, an electrochemical fuel cell 2 includes a solid electrolyte 4 interposed between an anode electrode 6 and a cathode electrode 8, an anode catalyst layer 10 between solid electrolyte 4 and anode gas diffusion layer 12, and a cathode catalyst layer 14 between solid electrolyte 4 and cathode gas diffusion layer 16. A component in at least one of the anode and cathode catalyst layers has been treated with a polyethylene glycol (PEG)-phosphonic acid compound.

In one embodiment, the component is a metal catalyst, which may be supported or unsupported. Without being bound by theory, the PEG-phosphonic acid- treated metal catalyst renders the catalyst layer more hydrophilic than without the PEG- phosphonic acid treatment, and forms a self-assembled monolayer through covalent bonding that may help to alter the catalyst-ionomer interface such that the hydrophilic side chains orient towards the catalyst and provide a pathway for water to leave the surface of the metal catalyst instead of being trapped in the mesopores of the catalyst layer, thereby providing more active surface area on the metal catalyst for gas to access. In addition, such hydrophilic metal catalysts may be useful to improve the water retention properties of catalyst layers. For example, for low humidity and low pressure applications, there may be insufficient water production during operation. In this instance, a PEG-phosphonic acid-treated metal catalyst on the cathode may help with water retention to prevent the membrane electrode assembly from drying out. In addition, the PEG-phosphonic acid-treated metal catalyst may be useful for the anode to improve cell reversal tolerance. As described in U.S. Patent No. 6,936,370, fuel cells can also be made more tolerant to cell reversal by promoting water electrolysis over anode component oxidation at the anode. During reversal, water present in the anode catalyst layer can be electrolyzed and oxidation (corrosion) of anode components, including carbon catalyst supports, if present, can occur. It is preferred to have water electrolysis occur rather than component oxidation. Thus, by increasing water retention on the anode by using a PEG-phosphonic acid-treated metal catalyst so that water is more readily available for water electrolysis, the fuel cell can be made more tolerant to cell reversal, particularly for fuel cells operating at low relative humidity conditions. One skilled in the art will appreciate the various ways that a PEG-phosphonic acid- treated metal catalyst may be used on the anode and/or the cathode for various fuel cell applications.

The metal catalyst may comprise noble and non-noble metals, for example, but not limited to, platinum, gold, ruthenium, iridium, cobalt, nickel, molybdenum, palladium, iron, tin, titanium, manganese, chromium, copper, and tungsten, and alloys, solid solutions, and intermetallic compounds thereof.

In some embodiments, the metal catalyst may comprise a noble metal supported in dispersed form on a suitable electrically conducting particulate support. The support may be a carbonaceous support, such as activated carbon, carbon black, carbon that is at least partially graphitized, and graphite. For example, high surface area carbons, such as acetylene or furnace blacks, are commonly used as supports for such catalysts. Generally, the corrosion resistance of a carbon support material is related to its graphitic nature: the more graphitic the carbon support, the more corrosion resistant it is. Graphitized carbon BA (TKK, Tokyo, JP) has a similar BET surface area to Shawinigan acetylene carbon and is a suitable carbon support material in some embodiments. In other embodiments suitable carbon support materials may include nitrogen-, boron-, sulfur-, and/or phosphorous-doped carbons, carbon nanofibres, carbon nanotubes, carbon nanohorns, graphenes, and aerogels. The loading of the noble metal on the support material may range from about 20% to about 80% by weight, typically about 20% to about 50% by weight. Though a lower noble metal loading on the support is typically preferred in terms of electrochemical surface area per gram of platinum (ECA), a higher noble metal loading and coverage of the support appears preferable in terms of reducing corrosion of the support and in reducing noble metal loss during fuel cell operation.

Instead of carbon, carbides or electrically conductive metal oxides may be considered as a suitable high surface area support for the corrosion resistant support material. For instance, iridium, ruthenium, titanium, and niobium oxides may serve as a corrosion resistant support material in some embodiments. In this regard, other valve metal oxides might be considered as well if they have acceptable electronic conductivity when acting as supports.

In another embodiment, the component is an additive in the catalyst layer that is treated with PEG-phosphonic acid. Such catalyst layer additives do not participate in the catalytic reactions, namely, the hydrogen oxidation and oxygen reduction reactions, and do not act as a catalyst support material for these reactions as well.

For example, the additive may be a hydrophilic metal oxide that is added to increase the hydrophilicity of the catalyst layer, which is desirable for applications that operate under drier conditions, such as air-cooled stacks. Such hydrophilic catalyst layer additives include, but are not limited to, metal oxides such as silica (Si0₂), titania (Ti0₂) and indium tin oxide (InSn0₂). Without being bound by theory, functionalizing such hydrophilic metal oxide additives with a hydrophilic PEG phosphonic acid may further improve their hydrophilicity, thereby reducing the loading of the hydrophilic metal oxide, and reduce dissolution during fuel cell operation, thereby improving fuel cell lifetime.

In other examples, the additive may be a metal oxide, such as ruthenium oxide (Ru0₂), iridium oxide (Ir0₂) and ruthenium iridium oxide (IrRu0₂), which may be supported or unsupported. Such metal oxides are known to improve cell reversal tolerance by promoting the electrolysis of water, as discussed in the foregoing. Without being bound by theory, treating such metal oxides with a PEG phosphonic acid may increase the hydrophilicity of these metal oxides, thereby further improving water retention of the catalyst layer while still retaining the function of electrolyzing water for cell reversal tolerance purposes. One skilled in the art will appreciate that the loading of the PEG phosphonic acid on these metal oxides will need to be controlled so that their ability to electrolyze water is not significantly hindered.

In yet other examples, the catalyst layer additive may be cerium oxide, manganese oxide, and indium oxide. Such metal oxides are known to mitigate hydrogen peroxide formation in the fuel cell. Without being bound by theory, treating such metal oxides with a PEG phosphonic acid may increase the hydrophilicity of these metal oxides, thereby further improving water retention of the catalyst layer while still retaining the function of mitigating hydrogen peroxide formation. One skilled in the art will appreciate that the loading of the PEG phosphonic acid on these metal oxides will need to be controlled so that their ability to mitigate hydrogen peroxide formation is not significantly hindered.

In some embodiments, the average molecular weight of the PEG- phosphonic acid compound may range from about 250 to about 700. More specifically, the average molecular weight of the PEG-phosphonic acid compound may range from 272 to 645.

In some embodiments, the PEG-phosphonic acid compound may be a mono-PEG-phosphonic acid compound or a bis-PEG-phosphonic acid compound. Without being bound by theory, a bis-PEG-phosphonic acid compound may be preferable because multidentate adsorption on the surface of the metal catalyst or metal oxide can occur, which may increase the stability of metal catalyst or the metal oxide, and also possibly improve their hydrophilicity. In specific examples, the polyethylene glycol-phosphonic acid compound is selected from the group consisting of a polyethylene glycol-ethyl hydrogen phosphonate, a polyethylene glycol-diethyl phosphonate, a polyethylene glycol-bis(ethyl hydrogen phosphonate) and a polyethylene glycol-bis(diethyl phosphonate) The advantage of using a PEG- phosphonic acid compound to treat the catalyst material is that aqueous solvents may be used so the process is more environmentally friendly and safer as compared to compounds that require alcoholic solvents.

In one method to treat the component, the PEG-phosphonic acid compound is dissolved in water while stirring, the component is separately dispersed in water while stirring, and then the component dispersion is added drop-wise to PEG- phosphonic acid compound solution while stirring. The final mixed acid component dispersion is filtered and rinsed with deionized water until the filtrate reaches a neutral pH. The filtered product is then dried in vacuum at 50 degrees Celsius overnight. The PEG-phosphonic acid-treated component may contain about 2.5 wt% to about 20 wt% of the phosphonic acid compound.

In some embodiments, the amount of the PEG-phosphonic acid-treated component that is desirably incorporated into the anode and/or cathode catalyst layers will depend on such factors as the fuel cell stack construction and operating conditions (for example, current that may be expected in reversal), cost, desired lifetime, and so on. It is expected that some empirical trials will determine an optimum amount for a given application.

The anode and cathode catalyst layers typically further include a binder, such as an ionomer and/or hydrophobic agent.

To form an MEA, the anode and cathode catalyst layers may be applied to a gas diffusion layer (GDL) to form anode and cathode electrodes, or to a decal transfer sheet which is then decal transferred to a surface of the GDL or solid electrolyte, or applied directly to the surface of the solid electrolyte to form a catalyst- coated membrane (CCM). The electrodes or CCM can then be bonded with other components to form an MEA. Alternatively, the application of the catalyst layer on the desired substrate may occur at the same time the remaining MEA components are bonded together.

The present catalyst layers may be applied according to known methods. For example, the catalyst may be applied as a catalyst ink or slurry, or as a dry mixture. Catalyst inks 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 and decal transfer) to the surface of the solid electrolyte or GDL. Examples of dry deposition methods include electrostatic powder deposition techniques and decal transfer.

The selection of additional components for the catalyst ink/mixture and the choice of application method and GDL to which it is applied are not essential to the present invention, and will depend on the characteristics of the mixture and the substrate to which it will be applied, the application method and desired structure of the catalyst layer. Persons of ordinary skill in the art can readily select suitable catalyst mixtures and application methods for a given application.

EXAMPLES

Twelve grams of graphitic carbon powder (TKK, Japan) was slowly added to 125 ml water while stirring at 800 RPM. 308 mg of PEG-phosphonic acid was dissolved in 4 ml water and the PEG dispersion was slowly added drop-wise to the carbon dispersion while stirring. The mixture was stirred for 90 minutes at room temperature. The final mixture was filtered and rinsed with deionized water until the filtrate reached neutral pH. The filtered product was dried in vacuum at 50 degrees Celsius overnight.

Figure 2 shows the picture of treated graphitic carbon powder (right) in comparison to untreated graphitic carbon powder (left) dispersed in water. As shown, the treated graphitic carbon powder was hydrophilic as the powder dispersed well in water. In contrast, the untreated graphitic carbon powder was hydrophobic and water could not wet the surface and, hence, it floated on the water.

For MEA testing, 12 grams of 50% Pt supported on graphitized carbon black (TKK, Japan) was slowly added to 125 ml water while stirring at 800 RPM. 308 mg of PEG-phosphonic acid was dissolved in 4 ml water and the PEG dispersion was slowly added drop-wise to the carbon dispersion while stirring. The mixture was stirred for 90 minutes at room temperature. The final mixture was filtered and rinsed with deionized water until the filtrate reached neutral pH. The filtered product was dried in vacuum at 50 degrees Celsius overnight to form a 2.5 wt% PEG-phosphonic acid- treated Pt catalyst. The treated catalyst was then dispersed in a Nafion®/alcohol mixture with a platinum-based catalyst, then applied to a decal transfer film and subsequently decal transferred to a half CCM (with cathode catalyst) to form a CCM. Two MEAs were made with the following electrode structures as listed in Table 1, with Nafion® 211 as the solid electrolyte and carbon fibre paper with sublayers as the GDL. The active area of each of the MEAs was 45cm².

Table 1. Anode and cathode catalyst structures for MEAs

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/cm²:

Table 2. Conditioning parameters

Figures 3 and 4 show the polarization results at 100%RH and 60%RH, respectively. It is clear that the fuel cell with the PEG-phosphonic acid-treated Pt catalyst on the cathode showed much better performance than the fuel cell without the PEG-phosphonic acid-treated Pt catalyst on the cathode at lower relative humidity, thus confirming improved performance characteristics at drier conditions.

The MEAs were also evaluated for effective platinum surface area via CO stripping. The EPSA for the cathode of Comparative MEA was 186 while the EPSA for the cathode of MEA #1 was 188. Thus, the PEG-phosphonic acid compound did not adversely affect the electrochemically available surface area nor its kinetic performance.

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.

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, are incorporated herein by reference in their entirety. This application also claims the benefit of U.S. Provisional Patent Application No. 62/424,384, filed November 18, 2016, and is incorporated herein by reference in its entirety.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention 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 invention.

Claims

CLAIMS What is claimed is:

1. A membrane electrode assembly comprising a solid electrolyte interposed between an anode electrode and cathode electrode, the anode electrode comprising an anode catalyst layer adjacent at least a portion of a first major surface of the solid electrolyte, the cathode electrode comprising a cathode catalyst layer adjacent at least a portion of a second major surface of the solid electrolyte; wherein at least one of the anode and cathode catalyst layers comprises a component that has been treated with a polyethylene glycol-phosphonic acid compound.

2. The membrane electrode assembly of claim 1, wherein the polyethylene glycol-phosphonic acid compound has a molecular weight of about 250 to about 700.

3. The membrane electrode assembly of claim 2, wherein the polyethylene glycol-phosphonic acid compound has a molecular weight of 272 to 645.

4. The membrane electrode assembly of claim 1, wherein the polyethylene glycol-phosphonic acid compound is selected from the group consisting of a polyethylene glycol-ethyl hydrogen phosphonate, a polyethylene glycol-diethyl phosphonate, a polyethylene glycol-bis(ethyl hydrogen phosphonate) and a polyethylene glycol-bis(diethyl phosphonate).

5. The membrane electrode assembly of claim 1, wherein the component comprises a metal catalyst.

6. The membrane electrode assembly of claim 5, wherein the metal catalyst comprises a noble metal.

7. The membrane electrode assembly of claim 5, wherein the metal catalyst comprises a non-noble metal.

8. The membrane electrode assembly of claim 5, wherein the metal catalyst is selected from the group consisting of selected from the group consisting of platinum, gold, ruthenium, iridium, cobalt, nickel, molybdenum, palladium, iron, tin, titanium, manganese, chromium, copper, and tungsten, and alloys, solid solutions, and intermetallic compounds thereof.

9. The membrane electrode assembly of claim 5, wherein the treated metal catalyst comprises about 2.5 wt% to about 20 wt% of the phosphonic acid compound.

10. The membrane electrode assembly of claim 1, wherein the component comprises a catalyst layer additive.

11. The membrane electrode assembly of claim 10, wherein the catalyst layer additive is a metal oxide.

12. The membrane electrode assembly of claim 11, wherein the metal oxide is selected from the group consisting of an oxide of silicon, titanium, ruthenium, iridium, cerium, manganese, indium, tin, and composites and mixtures thereof.

13. A method of making a treated component for a polymer electrolyte membrane electrode assembly comprising: dissolving a polyethylene glycol-phosphonic acid compound in an aqueous solution to form a polyethylene glycol-phosphonic acid solution; dispersing a component in a second solution to form a component dispersion; providing the component dispersion to the polyethylene glycol- phosphonic acid solution to form a mixed acid component dispersion; removing the first and second solutions from the mixed acid component dispersion to form a polyethylene glycol-phosphonic acid-treated component.

14. The method of claim 13, wherein the component is a metal catalyst.

15. The method of claim 14, wherein the metal catalyst is a noble metal.

16. The method of claim 15, wherein the metal catalyst is selected from the group consisting of platinum, gold, ruthenium, iridium, cobalt, nickel, molybdenum, palladium, iron, tin, titanium, manganese, cerium, chromium, copper, and tungsten, and alloys, solid solutions, and intermetallic compounds thereof.

17. The method of claim 13, wherein the component is a catalyst layer additive.

18. The method of claim 17, wherein the catalyst layer additive is a metal oxide.

19. The method of claim 18, wherein the metal oxide is selected from the group consisting of an oxide of silicon, titanium, ruthenium, iridium, cerium, manganese, indium, tin, and composites and mixtures thereof.