Fuel Cell Electrode Having Increased Oxygen Concentration and Methods of Preparing Electrode

ABSTRACT

A membrane electrode assembly includes a membrane, a gas diffusion layer and a catalyst layer between the membrane and the gas diffusion layer. The catalyst layer comprises catalyst comprising active catalyst particles supported on support particles, a proton conducting ionomer and a phospholipid containing soluble oxygen. One method of preparation includes preparing a catalyst solution comprising a solvent and catalyst, adding proton conducting ionomer to the catalyst solution to form a catalyst ink, saturating a solution of solvent and a phospholipid with oxygen and mixing the saturated phospholipid with the catalyst ink.
TECHNICAL FIELD

This disclosure relates to fuel cell electrodes having improved oxygentransport and methods of making the fuel cell electrodes. In particular,the electrodes include a phospholipid having soluble oxygen to improveoxygen transport to active catalyst material.
BACKGROUND

Fuel cells efficiently and electrochemically convert fuel into electriccurrent, which may then be used to power electric circuits, such asdrive systems for vehicles. A fuel cell containing a proton exchangemembrane is an electrochemical device that converts chemical energy toelectrical energy using, for example, hydrogen or methane as fuel andoxygen/air as oxidant.
Fuel cells have membrane electrode assemblies comprising a membrane withan anode on one side and a cathode on the other side. On the anode side,the catalyst enables hydrogen molecules to be split into protons andelectrons. On the cathode side, the catalyst enables oxygen reduction byreacting with the protons generated by the anode, producing water. Dueto pressure to reduce cost of fuel cells, amounts of active catalystmaterial such as platinum have been reduced while balancing fuel cellefficiency. Poor oxygen transport to the active catalyst particlesimpacts the efficiency of the fuel cell.
SUMMARY

Disclosed herein are implementations of fuel cell electrodes havingincreased oxygen concentration in the catalyst layer.
 One embodiment of a membrane electrode assembly for a fuel cell includesa membrane, a gas diffusion layer and a catalyst layer between themembrane and the gas diffusion layer. The catalyst layer comprisescatalyst particles comprising active catalyst particles supported onsupport particles, a proton conducting ionomer and a phospholipidcontaining soluble oxygen.
Also disclosed are methods of preparing the membrane electrodeassemblies for a fuel cell. One method of preparation includes preparinga catalyst solution comprising a solvent and catalyst, adding protonconducting ionomer to the catalyst solution to form a catalyst ink,saturating a solution of solvent and a phospholipid with oxygen andmixing the saturated phospholipid with the catalyst ink.
Another method of preparing a membrane electrode assembly for a fuelcell includes preparing a catalyst solution comprising a solvent andcatalyst, saturating a solution of solvent and a phospholipid withoxygen, mixing proton conducting ionomer to the saturated phospholipid,depositing a catalyst solution comprising a solvent and catalyst on oneof a membrane and a gas diffusion layer and depositing the mixture ofproton conducting ionomer and the saturated phospholipid on thedeposited catalyst solution.
BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Itis emphasized that, according to common practice, the various featuresof the drawings are not to-scale. On the contrary, the dimensions of thevarious features are arbitrarily expanded or reduced for clarity.
FIG. 1 is a schematic of a cross-section of a fuel cell stack.
FIG. 2 is schematic of a membrane electrode assembly.
FIG. 3 is a schematic of a membrane electrode assembly as disclosedherein.
FIG. 4 is a schematic of another membrane electrode assembly asdisclosed herein.
FIG. 5 is a flow diagram of a method of making a membrane electrodeassembly as disclosed herein.
 FIG. 6 is a flow diagram of another method of making a membraneelectrode assembly as disclosed herein.
DETAILED DESCRIPTION

FIG. 1 shows a schematic cross-sectional illustration of a portion of afuel cell stack 10. The illustration is provided as an example and isnot meant to be limiting. The fuel cell stack 10 is comprised ofmultiple membrane electrode assemblies 20. Fuel 30 such as hydrogen isfed to the anode side of a membrane electrode assembly 20, while anoxidant 40 such as oxygen or air is fed to the cathode side of themembrane electrode assembly 20. Coolant 50 is supplied between the fuel30 and oxidant 40, the coolant 50 separated from the fuel 30 and oxidant40 by separators 60.
FIG. 2 is an illustration of one of the plurality of fuel cells 70 inthe fuel cell stack 10. The fuel cell 70 is comprised of a singlemembrane electrode assembly 20. The membrane 80 has a catalyst layer 84formed on opposing surfaces of the membrane 80, such that whenassembled, the catalyst layers 84 are each between the membrane 80 and agas diffusion layer 82. Alternatively, a gas diffusion electrode is madeby forming a catalyst layer 84 on a surface of each of two gas diffusionlayers 82 and sandwiching the membrane 80 between the gas diffusionlayers 82 such that the catalyst layers 84 contact the membrane 80.
 Fuel cell catalyst typically comprises a support material such as carbonor another conductive material, with an active catalyst materialsupported on the support material. The active catalyst material can beplatinum or other noble metals, transition metals, metal oxides, oralloys or a combination thereof. Non-limiting examples of activecatalyst material include Pt, Pt—Co, Pt—Ni, Pt—Cu and Pt—Fe. Thecatalyst is typically mixed with an ionomer and sprayed or otherwiselayered on one of the membrane 80 and the gas diffusion layer 82 to formthe catalyst layer 84. Due in part to the cost of the active catalystmaterial, as little as possible is used to achieve the requisite fuelcell performance. Oxygen in the oxidant 40 is required at the site ofthe active catalyst material for the reduction reaction.
 To increase oxygen concentration in the catalyst layer, the membraneelectrode assemblies disclosed herein incorporate material having highoxygen solubility into the catalyst layer. These materials lower theoxygen transport resistance and improve fuel cell performance,particularly at high current densities. The material can be saturatedwith oxygen prior to addition into the catalyst layer. However,pre-saturation is not necessary as oxygen coming into the catalyst layerin the oxidant during use will be readily solubilized in the material.As used herein, “high oxygen solubility” refers to oxygen solubilitygreater than conventional ionomers, such as perfluorosulfonic acid(PFSA), that are currently used in catalyst ink for fuel cells. Oxygensolubility is dependent on at least pressure, temperature and humidity,so the oxygen solubility of the materials having high oxygen solubilityat one pressure and temperature is greater than the oxygen solubility ofPFSA at that same pressure and temperature. As a non-limiting example,Nafion™ ionomer has an oxygen solubility of about 10-11 mmol/liter at80° C. and 1 atm. The oxygen solubility of the material having highoxygen solubility can be greater than 12 mmol/liter and up to about 50mmol/liter at 80° C. and 1 atm.
One embodiment of a membrane electrode assembly 100 for a fuel cell isillustrated in FIG. 3 and includes a membrane 102, a gas diffusion layer104 and a catalyst layer 106 between the membrane 102 and the gasdiffusion layer 104. The catalyst layer 106 comprises catalyst 108comprising active catalyst particles supported on support particles, aproton conducting ionomer and a phospholipid 110 as the material havinghigh oxygen solubility.
 The phospholipid in the catalyst layer traps and stores oxygen, allowingenhanced localized oxygen concentration at the active catalystparticles. Oxygen is required at the cathode for the reduction reaction.High oxygen concentrations assist fuel cells with low loading of activecatalyst particles at higher current densities. The phospholipid alsoacts as a surfactant to assist dispersion of the catalyst throughout thecatalyst layer. Some phospholipids will also conduct protons, but theconductivity of the phospholipids is lower than the ionomer used in thecatalyst layer. Therefore, ionomer is used in the catalyst layer alongwith the phospholipid to provide the requisite proton conduction.
The phospholipid can be a phosphocholine. Phophocholine has thefollowing chemical structure:

The phosphocholine can be one of1,2-dilauroyl-sn-glycero-3-phosphocholine,1,2-dimyristoyl-sn-glycero-3-phosphocholine,1,2-Dipalmitoyl-sn-glycero-3-phosphocholine,1,2-Distearoyl-sn-glycero-3-phosphocholine, and1,2-Dioleoyl-sn-glycero-3-phosphocholine and mixtures thereof. Thephosphocholine can also be one or more of the following:

-   1,2-Dipalmitoyl-rac-glycero-3-phosphocholine-   1,2-Dierucoyl-sn-glycero-3-phosphocholine-   1,2-Dimyristoyl-rac-glycero-3-phosphocholine-   1,2-Distearoyl-sn-glycero-3-phosphocholine-   1-Palmitoyl-sn-glycero-3-phosphocholine-   1-Oleoyl-sn-glycero-3-phosphocholine-   1-Stearoyl-sn-glycero-3-phosphocholine-   1,2-Diarachidoyl-sn-glycero-3-phosphocholine-   1-O-Palmityl-sn-glycero-3-phosphocholine-   1,2-Didodecanoyl-sn-glycero-3-phosphocholine-   1,2-Dilinoleoyl-sn-glycero-3-phosphocholine-   1,2-Didecanoyl-sn-glycero-3-phosphocholine-   1,2-Didodecanoyl-rac-glycero-3-phosphocholine-   1,2-Dihexanoyl-sn-glycero-3-phosphocholine-   1-Dodecanoyl-sn-glycero-3-phosphocholine-   2,3-Dipalmitoyl-sn-glycero-1-phosphocholine-   1,2-Dieicosapentaenoyl-sn-glycero-3-phosphocholine-   1,2-Dihexadecyl-rac-glycero-3-phosphocholine-   1,2-Dipentadecanoyl-sn-glycero-3-phosphocholine-   1,2-Didocosanoyl-sn-glycero-3-phosphocholine-   1,2-Dielaidoyl-sn-glycero-3-phosphocholine-   1-O-Palmityl-rac-glycero-3-phosphocholine-   1,2-Dimyristoyl-rac-glycero-3-phosphocholine-d₇₂ hydrate-   1,2-Dinonanoyl-sn-glycero-3-phosphocholine

The phospholipid can be saturated with oxygen prior to adding to thecatalyst layer to immediately provide additional oxygen to the activecatalyst particles in addition to that provided by the oxidant.Alternatively, the phospholipid can be added without saturation, thephospholipid trapping and storing oxygen from the oxidant.
The amount of phospholipid used in the catalyst layer will depend on theamount of active catalyst used. The ratio of the weight percent ofactive catalyst particles to the weight percent of phospholipid isgreater than or equal to 0.2 and less than or equal to 0.4. The ratio ofweight percent phospholipid to weight percent ionomer I the catalystlayer is greater than or equal to 0.2 and less than or equal to 0.6.
 The amount of ionomer used in the catalyst layer is typically dependenton the amount of catalyst used. However, because the weight of thesupport particles is significantly greater than the weight of the activecatalyst particles, the amount of ionomer can be dependent on the amountof support particles in the catalyst layer. For the embodiments herein,the ratio of the proton conducting ionomer plus the phospholipid, byweight, to the support particles by weight is greater than or equal to0.6 and less than or equal to 1.2.
As a non-limiting example of a catalyst ink recipe, in terms of weightpercent, 20% catalyst (Pt or Pt alloys) is dispersed on carbon support.One gram of catalyst dispersed on carbon contains 20% catalyst and 80%carbon. From Equations 1-3, one can calculate the amount (and percentageof) phospholipids and ionomer ranges:

${{Equation}\mspace{14mu} 1} = {0.2 \leq \frac{Catalyst}{Phospholipids} \leq 0.4}$${{Equation}\mspace{14mu} 2} = {0.2 \leq \frac{Phospholipids}{Ionomer} \leq 0.6}$${{Equation}\mspace{14mu} 3} = {0.6 \leq \frac{{Ionomer} + {Phospholipids}}{Support} \leq 1.2}$

As noted, the phospholipid may provide some proton conduction, buttypically has a lower conductivity than the ionomer used in the catalystlayer. Proton conduction happens at the phospholipid-water interface dueto interfacial water and its associated hydrogen bond network. However,proton conduction by the phospholipid can be improved by surfacemodification with a functional group. When a functionalized phospholipidis used in the catalyst layer, less ionomer is required to meet therequisite proton conduction. The ratio of a functionalized hydrogenconducting phospholipid by weight to ionomer by weight is adjusted togreater than or equal to 0.5 and less than or equal to 1.0.
 The phospholipid can be modified, for example, with polyethylene glycol(PEG). The PEG lipids are immobilized onto the surface of thephospholipid vesicles by adding an aqueous dispersion of the PEG lipidsto the vesicle dispersion. To prevent dissociation of the PEG lipids,PEGS with at least two and four acyl chains are used with a molecularweight of about 5000 and 12,500, respectively.
The phospholipid can also be modified with the addition of proteinaromatics. For example, one or more fluorinated aromatic amino acids canbe incorporated. Fluorinated aromatic amino acids destabilize thecation-π interactions by altering electrostatics of the aromatic ringwhile their increased hydrophobicity enhances membrane insertion.Non-limiting examples of the fluorinated aromatic amino acids includepentafluorophenylalanine and difluorotyrosine.
The phospholipid can also be modified with a fatty acid bound on a polarhead of the phospholipid with an ether linkage. The fatty acid boundwith an either rather than an ester linkage provide intercalated watermolecules hydrogen bonded to the carboxyl group. Non-limiting examplesof the fatty acids are lauric acid, myristic acid, palmitic acid, oleicacid and phytanic acid.
 To further increase the oxygen concentration at the active catalystparticles, the catalyst layer 206 can have an active catalyst particlegradient such that a concentration of catalyst 208 is greatest towardthe gas diffusion layer 104 and lowest toward the membrane 100, asillustrated in the membrane electrode assembly 200 in FIG. 4. Theoxidant enters through the gas diffusion layer, so having an increasedconcentration of active catalyst particles near the gas diffusion layerimproves cathode performance. Accordingly, the catalyst layer can alsohave a phospholipid gradient such that a concentration of thephospholipid 210 is greatest toward the gas diffusion layer 104 andlowest toward the membrane 102. Alternative to a true gradient, two ormore layers can be used, with the layer closest to the gas diffusionlayer having the highest concentration of both catalyst andphospholipid, and the layer closest to the membrane having the highestconcentration of ionomer. As a non-limiting example, two layers can beused, with the layer closest to the gas diffusion layer being about 4microns in thickness and the layer closest to the membrane being about10 microns in thickness.
 Methods of preparing the membrane electrode assemblies are alsodisclosed. One method of preparation, shown in FIG. 5, includespreparing a catalyst solution comprising a solvent and catalyst (300),and adding the proton conducting ionomer to the catalyst solution toform a catalyst ink (302). A solution of solvent and a phospholipid issaturated with oxygen (304) and is then mixed with the catalyst ink(306). Saturating the phospholipid can be done by bubbling gascontaining oxygen through the solution for a period of time, such as twohours or more, as a non-limiting example. The mixed catalyst ink andsaturated phospholipid are layered onto one of a membrane and a gasdiffusion layer (308). The layering can occur using using traditionaldeposition via a spray system such as an automated robotic system or anair brush. Phospholipids typically are high molecular weight molecules,so electrospinning can be used for the deposition of the catalyst layer.Electrospinning can result in phospholipids with a fiber-like structure,which enhances oxygen transport. Electrospraying can also be used,providing a more uniform layer than traditional deposition.
As shown in FIG. 6, another method of preparing a membrane electrodeassembly for a fuel cell includes preparing a catalyst solutioncomprising a solvent and catalyst (400), saturating a solution ofsolvent and a phospholipid with oxygen (402), mixing proton conductingionomer with the saturated phospholipid (404), depositing the catalystsolution comprising the solvent and catalyst on one of a membrane and agas diffusion layer (406) and depositing the mixture of protonconducting ionomer and the saturated phospholipid on the depositedcatalyst solution (408).
The depositing can be performed using the same methods disclosed or adifferent method can be used for each layer. The solvent can be water orany other solvent known by those skilled in the art. If a modifiedphospholipid is used, the modification or functionalization will occurprior to saturating the phospholipid.
 Alternative to the phospholipid or in addition to phospholipids, anionic liquid can be used. Examples of ionic liquids having high oxygensolubility that can be used in the membrane electrode assembliesdisclosed herein include, but are not limited to, fluoroalkyl sulfonicacids with different chain lengths. Non-limiting examples of fluoroalkylsulfonic acids include:

The ionic liquids can be used in the catalyst layers disclosed herein inplace of the phospholipids or in addition to the phospholipids. Inaddition, the catalyst layer can have an ionic liquid gradient such thata concentration of ionic liquid is greatest toward the gas diffusionlayer and lowest toward the membrane to further increase the oxygenconcentration near the active catalyst particles.
For simplicity of explanation, although the figures and descriptionsherein may include sequences or series of steps or stages, elements ofthe methods disclosed herein may occur in various orders orconcurrently. Additionally, elements of the methods disclosed herein mayoccur with other elements not explicitly presented and described herein.Furthermore, not all elements of the methods described herein may berequired to implement a method in accordance with this disclosure.Although aspects, features, and elements are described herein inparticular combinations, each aspect, feature, or element may be usedindependently or in various combinations with or without other aspects,features, and elements.
As used herein, the terminology “example,” “embodiment,”“implementation,” “aspect,” “feature,” or “element” indicate serving asan example, instance, or illustration. Unless expressly indicated, anyexample, embodiment, implementation, aspect, feature, or element isindependent of each other example, embodiment, implementation, aspect,feature, or element and may be used in combination with any otherexample, embodiment, implementation, aspect, feature, or element.
 As used herein, the terminology “or” is intended to mean an inclusive“or” rather than an exclusive “or.” That is, unless specified otherwise,or clear from context, “X includes A or B” is intended to indicate anyof the natural inclusive permutations. If X includes A; X includes B; orX includes both A and B, then “X includes A or B” is satisfied under anyof the foregoing instances. In addition, the articles “a” and “an” asused in this application and the appended claims should generally beconstrued to mean “one or more” unless specified otherwise or clear fromcontext to be directed to a singular form.
While the disclosure has been described in connection with certainembodiments, it is to be understood that the disclosure is not to belimited to the disclosed embodiments but, on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the scope of the appended claims, which scope is to be accordedthe broadest interpretation so as to encompass all such modificationsand equivalent structures as is permitted under the law.
What is claimed is:
 1. A membrane electrode assembly, comprising: amembrane; a gas diffusion layer; and a catalyst layer between themembrane and the gas diffusion layer, the catalyst layer comprising:catalyst comprising active catalyst particles supported on supportparticles; a proton conducting ionomer; and a phospholipid containingsoluble oxygen.
2. The membrane electrode assembly of claim 1, whereinthe phospholipid is a phosphocholine.
3. The membrane electrode assemblyof claim 2, wherein the phosphocholine is one of1,2-dilauroyl-sn-glycero-3-phosphocholine,1,2-dimyristoyl-sn-glycero-3-phosphocholine,1,2-Dipalmitoyl-sn-glycero-3-phosphocholine,1,2-Distearoyl-sn-glycero-3-phosphocholine, and1,2-Dioleoyl-sn-glycero-3-phosphocholine and mixtures thereof.
4. Themembrane electrode assembly of claim 1, wherein the active catalystparticles are platinum, and a weight ratio of active catalyst particlesto the phospholipid is greater than or equal to 0.2 and less than orequal to 0.4.
 5. The membrane electrode assembly of claim 1, wherein aweight ratio of the proton conducting ionomer with the phospholipid tothe support particles is greater than or equal to 0.6 and less than orequal to 1.2.
6. The membrane electrode assembly of claim 1, wherein aweight ratio of phospholipid to ionomer is greater than or equal to 0.2and less than or equal to 0.6.
7. The membrane electrode assembly ofclaim 1, wherein the phospholipid is also a hydrogen conductingphospholipid, a weight ratio of hydrogen conducting phospholipid toionomer being greater than or equal to 0.5 and less than or equal to1.0.
8. The membrane electrode assembly of claim 7, wherein the hydrogenconducting phospholipid is a polyethylene glycol modified phospholipid.9. The membrane electrode assembly of claim 7, wherein the hydrogenconducting phospholipid is a phospholipid into which a fluorinatedaromatic amino acid is incorporated.
10. The membrane electrode assemblyof claim 9, wherein the fluorinated aromatic amino acid is one ofpentafluorophenylalanine and difluorotyrosine.
11. The membraneelectrode assembly of claim 7, wherein the hydrogen conductingphospholipid is a phospholipid with a fatty acid bound on a polar headof the phospholipid with an ether linkage.
12. The membrane electrodeassembly of claim 11, wherein the fatty acid is one or more of lauricacid, myristic acid, palmitic acid, oleic acid and phytanic acid. 13.The membrane electrode assembly of claim 1, wherein the catalyst layerhas an active catalyst particle gradient such that a concentration ofactive catalyst particles is greatest toward the gas diffusion layer andlowest toward the membrane.
14. The membrane electrode assembly of claim13, wherein the catalyst layer has a phospholipid gradient such that aconcentration of the phospholipid is greatest toward the gas diffusionlayer and lowest toward the membrane.
 15. A method of preparing amembrane electrode assembly, comprising: preparing a catalyst solutioncomprising a solvent and catalyst; adding proton conducting ionomer tothe catalyst solution to form a catalyst ink; saturating a solution ofsolvent and a phospholipid with oxygen; mixing the saturatedphospholipid with the catalyst ink.
16. The method of claim 15, whereinthe saturating comprises bubbling gas containing oxygen through thesolution for at least two hours.
17. The method of claim 15, furthercomprising layering the mixed catalyst ink and saturated phospholipidonto one of a membrane and a gas diffusion layer using eitherelectrospinning or electrospraying.
18. The method of claim 15, whereinthe phospholipid is one of 1,2-dilauroyl-sn-glycero-3-phosphocholine,1,2-dimyristoyl-sn-glycero-3-phosphocholine,1,2-Dipalmitoyl-sn-glycero-3-phosphocholine,1,2-Distearoyl-sn-glycero-3-phosphocholine, and1,2-Dioleoyl-sn-glycero-3-phosphocholine and mixtures thereof.
19. Themethod of claim 15, wherein the active catalyst particles are platinum,and a weight ratio of active catalyst particles to the phospholipid isgreater than or equal to 0.2 and less than or equal to 0.4.
20. A methodof preparing a membrane electrode assembly, comprising: preparing acatalyst solution comprising a solvent and catalyst; saturating asolution of solvent and a phospholipid with oxygen; mixing protonconducting ionomer to the saturated phospholipid; depositing a catalystsolution comprising the solvent and the catalyst on one of a membraneand a gas diffusion layer; and depositing the mixture of protonconducting ionomer and the saturated phospholipid on the depositedcatalyst solution..
