FUEL CELL ELECTRODES

Abstract

A process includes patterning a surface of a platinum group metal-based electrode by contacting the electrode with an adsorbate to form a patterned platinum group metal-based electrode including platinum group metal sites blocked with adsorbate molecules and platinum group metal sites which are not blocked.

Description

FIELD

The present technology relates generally to modified electrodes and methods of making and using the same.

BACKGROUND

The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present technology.

The development of new materials that can solve the challenging problems of clean energy production, conversion and storage is of paramount importance in the quest for alternatives to fossil fuel use. One promising candidate is a fuel cell, a device that converts chemical energy directly into electrical energy. In a polymer electrolyte membrane fuel cell (PEMFC), an alkaline fuel cell (AFC), or a phosphoric acid fuel cell (PAFC), the main fuel is hydrogen, which, when reacted with oxygen, produces water as the only reaction product. However, to make hydrogen-based energy systems viable on a large scale, many problems still need to be resolved. These are mainly connected with new catalyst materials focusing primarily on three characteristics: activity, stability and selectivity. Improvement of these features presents the major roadblock to a wide commercialization of fuel cells.

Presently the state of the art approach for changing these properties undoubtedly entails changing the electronic properties of the catalyst in some way, shape or form. This approach rests on the premises that changing the catalyst's electronic structure will (i) change the adsorption free energy of reactants and products thus increasing the activity for a desired reaction, (ii) change the stability of catalyst by making the metal (or other active material) less soluble in relatively aggressive electrolytes and (iii) only effect activity for one reaction at the catalyst's surface. The possible beneficial effect of this approach has been extensively supported and advertised by theoretical work.

In the recent past, it has been shown many times that for platinum group and platinum based catalyst, the activity is determined by the solution side rather than the metal side of the catalyst. The term spectator species has been introduced for molecules, which come from the supporting electrolyte and essentially block the surface sites so that they are unavailable for the electrochemical reaction. These species do not alter the electronic properties of the surface nor do they participate in the reaction, hence they are spectators. In general these species greatly influence all three characteristics of the catalyst. By introducing the concept of chemically modified electrodes (CME) it is possible to enhance catalyst's activity, stability and selectivity without changing its electronic properties.

SUMMARY

In one aspect, a process is provided including patterning a surface of a platinum group metal-based electrode by contacting the electrode with an adsorbate to form a patterned platinum group metal-based electrode having platinum group metal sites blocked with adsorbate molecules and platinum group metal sites which are not blocked. As used herein a platinum group metal is one or more of the metals in the platinum group as are known in the art. In any of the processes of this paragraph, the adsorbate includes a calix[n]arene, cyclodextrine, a cucurbit[n′]uril, a porphyrin, a crown ether, a calix[n]pyrrole, a pyrogallol[n]arene, a calix[4]resorcinarene, and derivatives thereof, where n is 4, 6, or 8 and n′ is the number of glycoluril units and is 5, 6, 7, or 8.

In any of the processes of the preceding paragraph, the patterning includes heating the platinum group metal-based electrode to an annealing temperature from about 500 K to about 1500 K under a reducing atmosphere, cooling the electrode, and immersing the electrode in a solution that includes the adsorbate. The solution may include an organic solvent and a calix[n]arene, wherein n is 4, 6, or 8. In such embodiments, the organic solvent includes tetrahydrofuran, diethyl ether, methyl butyl ether, 1,2-dichlorobenzene, 1,4-dichlorobenzene, dimethylsulfoxide, dimethylformamide, ethanol, methanol or a mixture of any two or more such solvents. In such embodiments, from about 90% to about 99% of the platinum group metal sites are blocked by the calix[n]arene molecules and from about 10% to about 1% of the platinum group metal sites are free of the calix[n]arene molecules. The platinum group metal-based electrode may include, but is not limited to, surfaces of Pt(100), Pt(11l), Pt(1099), or polycrystalline Pt.

In another aspect, an electrode is provided including a platinum group metal-based substrate including adsorbate molecules wherein a portion of the platinum group metal sites are blocked by adsorbate molecules and a portion of the platinum group metal sites are not blocked. In one embodiment, the adsorbate molecules include calix[n]arene molecules, wherein n is 4, 6, or 8.

In another aspect, a fuel cell is provided including any of the above electrodes or any of the above electrodes produced by the described processes.

In one aspect, calix[4]arene-modified electrodes, fuel cells including calix[4]arene-modified electrodes, and methods of hydrogen oxidation and oxygen reduction are provided. In some embodiments the electrode is a metal selected from the platinum group metals, such as platinum, rhodium, palladium, ruthenium, osmium, or palladium. In some embodiments, the electrode is a anode. In some embodiments, the electrode includes a self-assembled monolayer of calix[4]arene. In some embodiments the electrode is Pt(111). In other embodiments, the electrode is Pt(100) or polycrystalline platinum. In other embodiments, the electrode is Pt(1099) or polycrystalline platinum. In some embodiments, the hydrogen oxidation at the calix[4]arene-modified electrode is tolerant of oxygen.

According to another aspect, calix[n]arene-modified nanocatalysts, fuel cells including calix[n]arene-modified nanocatalysts, and methods of hydrogen oxidation and oxygen reduction are provided. In some embodiments n is 4, 6, or 8. In some embodiments, the calix[4]arene includes thiol groups or are otherwise thiolated. In some embodiments the nanocatalyst is a metal selected from the platinum group metals, such as platinum, rhodium, iridium, palladium, ruthenium, osmium, or palladium. In some such embodiments, the nanocatalyst is a platinum nanocatalyst, such as a 3M nanostructured thin film (NSTF) or Tanaka 5 nm Pt/C (TKK) catalyst. In some embodiments, the nanocatalyst includes platinum nanoparticles supported on carbon. In some such embodiments, the platinum nanoparticles have an average diameter of 2-10 nm. In some embodiments, the carbon is amorphous carbon black. In some embodiments, the hydrogen oxidation at the calix[4]arene-modified electrode is tolerant of oxygen.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments and features described above, further aspects, embodiments and features will become apparent by reference to the following drawings and the detailed description.

DESCRIPTION OF THE DRAWINGS

FIG. 1A shows TEM images of Pt supported on carbon, as well as a STM image of Pt(111), according to various embodiments while FIGS. 1B, and 1C are illustrations showing the situation during startup/shutdown of a typical proton-exchange membrane fuel cell, according to various embodiments.

FIGS. 2A and 2B, depict imaging of Pt(111) and Pt(111)-calix electrodes by Scanning Tunneling Microscopy (STM), according to various embodiments.

FIG. 3 is an illustrative model for adsorbed calix[4]arene molecules on Pt(111), according to the examples.

FIGS. 4A and 4B illustrate relationships between surface coverages by calix[4]arene molecules and H_upd/OH_ad on Pt(111) in 0.1M HClO₄, according to the examples. Potentials of interest during startup and shutdown are shown in the shaded region.

FIGS. 5A, 5B, and 5C illustrate the design of O₂-tolerant selective anode catalysts for the hydrogen oxidation reaction (HOR) by controlling Θ_calix on Pt(111), according to the examples. Potentials of interest during startup and shutdown are shown in the shaded region.

FIGS. 6A, 6B, 6C, 6D, 6E, and 6F show that high selectivity of the ORR and HOR is also observed on the calix[4]arene-covered Pt(100) and polycrystalline Pt (“Pt(Poly)”) electrodes, according to the examples. Potentials of interest during startup and shutdown are shown in the shaded region.

FIGS. 7A and 7B show the electrochemical characteristics of calix-modified stepped surfaces: Pt(1099) (shown in FIG. 7A) and Pt(110) (shown in FIG. 7B). Potentials of interest during startup and shutdown are shown in the shaded region.

FIG. 8A shows an SEM image of an unmodified Pt nanowhisker, according to the examples. FIG. 8B shows Transmission Electron Microscopy (TEM) morphology of typical TKK nanocatalysts, according to the examples. Calix molecules are not visible by electron microscopy. FIG. 8C is an illustrative model morphology of a calix[4]arene-modified Pt NSTF nanowhisker. FIG. 8D is an illustrative model for a TKK nanocatalyst chemically modified with calix[4]arene.

FIGS. 9A and 9B show the electrochemical characteristics of calix-modified Pt nanocatalysts NSTF (FIG. 9A) and TKK (50% Pt loading, FIG. 9B), according to the examples. Catalyst loadings were approximately 14-16 μg/cm²; lower loadings were also used to mimic anode catalyst performance, yielding similar qualitative results. All current densities are given with respect to the disk geometric area.

FIG. 10 illustrates the stability of the Pt-calix system at 60° C. in O₂-saturated 0.1 M HClO₄ at 0.8 V. The inset of FIG. 10 shows ORR curves for unmodified surface and Pt-calix surface both before and after the stability test. Note: the HOR remains unchanged for the duration of the experiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

The technology is described herein using several definitions, as set forth throughout the specification.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing the elements (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.

As used herein, “about” will be understood by persons of ordinary skill in the art and will vary to some extent depending upon the context in which it is used. If there are uses of the term which are not clear to persons of ordinary skill in the art, given the context in which it is used, “about” will mean up to plus or minus 10% of the particular term.

Alkyl groups include straight chain and branched alkyl groups having from 1 to about 20 carbon atoms, and typically from 1 to 12 carbons or, in some embodiments, from 1 to 8 carbon atoms. As employed herein, “alkyl groups” include cycloalkyl groups as defined below. Examples of straight chain alkyl groups include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, and n-octyl groups. Examples of branched alkyl groups include, but are not limited to, isopropyl, sec-butyl, t-butyl, neopentyl, and isopentyl groups. Representative substituted alkyl groups may be substituted one or more times with, for example, amino, thio, hydroxy, cyano, alkoxy, and/or halo groups such as F, CI, Br, and I groups. As used herein the term haloalkyl is an alkyl group having one or more halo groups. In some embodiments, haloalkyl refers to a per-haloalkyl group.

Cycloalkyl groups are cyclic alkyl groups such as, but not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl groups. In some embodiments, the cycloalkyl group has 3 to 8 ring members, whereas in other embodiments the number of ring carbon atoms range from 3 to 5, 6, or 7. Cycloalkyl groups further include polycyclic cycloalkyl groups such as, but not limited to, norbornyl, adamantyl, bornyl, camphenyl, isocamphenyl, and carenyl groups, and fused rings such as, but not limited to, decalinyl, and the like. Cycloalkyl groups also include rings that are substituted with straight or branched chain alkyl groups as defined above. Representative substituted cycloalkyl groups may be mono-substituted or substituted more than once, such as, but not limited to: 2,2-; 2,3-; 2,4-; 2,5-; or 2,6-disubstituted cyclohexyl groups or mono-, di-, or tri-substituted norbornyl or cycloheptyl groups, which may be substituted with, for example, alkyl, alkoxy, amino, thio, hydroxy, cyano, and/or halo groups.

Aryl groups are cyclic aromatic hydrocarbons of 6 to 14 carbons that do not contain heteroatoms. Aryl groups herein include monocyclic, bicyclic and tricyclic ring systems. Thus, aryl groups include, but are not limited to, phenyl, azulenyl, heptalenyl, biphenyl, fluorenyl, phenanthrenyl, anthracenyl, indenyl, indanyl, pentalenyl, and naphthyl groups. In some embodiments, aryl groups contain from 6 to 12 or even 6 to 10 carbon atoms in the ring portions of the groups. In some embodiments, the aryl groups are phenyl or naphthyl. Although the phrase “aryl groups” includes groups containing fused rings, such as fused aromatic-aliphatic ring systems (e.g., indanyl, tetrahydronaphthyl, and the like), it does not include aryl groups that have other groups, such as alkyl or halogen groups, bonded to one of the ring members. Rather, groups such as tolyl are referred to as substituted aryl groups. Representative substituted aryl groups may be mono-substituted or substituted more than once. For example, monosubstituted aryl groups include, but are not limited to, 2-, 3-, 4-, 5-, or 6-substituted phenyl or naphthyl groups, which may be substituted with substituents such as those listed above.

Alkoxy groups are hydroxyl groups (—OH) in which the bond to the hydrogen atom is replaced by a bond to a carbon atom of an alkyl group as defined above. Examples of linear alkoxy groups include but are not limited to methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy, and the like. Examples of branched alkoxy groups include but are not limited to isopropoxy, sec-butoxy, tert-butoxy, isopentoxy, isohexoxy, and the like. Representative substituted alkoxy groups may be substituted one or more times with substituents such as those listed above.

The term “ester” as used herein refers to —COOR groups, where R is a substituted or unsubstituted alkyl, cycloalkyl, alkenyl, alkynyl, aryl, or aralkyl group as defined herein.

The term “thiol” refers to —SR groups, where R is H, a substituted or unsubstituted alkyl, cycloalkyl, alkenyl, alkynyl, aryl or aralkyl group as defined herein.

The term “hydroxyl” refers to —OH groups.

Heterocyclyl groups includes non-aromatic ring compounds containing 3 or more ring members, of which one or more is a heteroatom such as, but not limited to, N, O, and S. In some embodiments, heterocyclyl groups include 3 to 20 ring members, whereas other such groups have 3 to 6, 3 to 10, 3 to 12, or 3 to 15 ring members. Heterocyclyl groups encompass unsaturated, partially saturated and saturated ring systems, such as, for example, imidazolyl, imidazolinyl and imidazolidinyl groups. Heterocyclyl groups may be substituted or unsubstituted. Heterocyclyl groups include, but are not limited to, aziridinyl, azetidinyl, pyrrolidinyl, imidazolidinyl, pyrazolidinyl, thiazolidinyl, tetrahydrothiophenyl, tetrahydrofuranyl, dioxolyl, furanyl, thiophenyl, pyrrolyl, pyrrolinyl, imidazolyl, imidazolinyl, pyrazolyl, pyrazolinyl, triazolyl, tetrazolyl, oxazolyl, isoxazolyl, thiazolyl, thiazolinyl, isothiazolyl, thiadiazolyl, oxadiazolyl, piperidyl, piperazinyl, morpholinyl, thiomorpholinyl, tetrahydropyranyl, tetrahydrothiopyranyl, oxathiane, dioxyl, dithianyl, pyranyl, pyridyl, pyrimidinyl, pyridazinyl, pyrazinyl, triazinyl, dihydropyridyl, dihydrodithiinyl, dihydrodithionyl, homopiperazinyl, quinuclidyl, indolyl, indolinyl, isoindolyl,azaindolyl (pyrrolopyridyl), indazolyl, indolizinyl, benzotriazolyl, benzimidazolyl, benzofuranyl, benzothiophenyl, benzthiazolyl, benzoxadiazolyl, benzoxazinyl, benzodithiinyl, benzoxathiinyl, benzothiazinyl, benzoxazolyl, benzothiazolyl, benzothiadiazolyl, benzo[1,3]dioxolyl, pyrazolopyridyl, imidazopyridyl (azabenzimidazolyl), triazolopyridyl, isoxazolopyridyl, purinyl, xanthinyl, adeninyl, guaninyl, quinolinyl, isoquinolinyl, quinolizinyl, quinoxalinyl, quinazolinyl, cinnolinyl, phthalazinyl, naphthyridinyl, pteridinyl, thianaphthalenyl, dihydrobenzothiazinyl, dihydrobenzofuranyl, dihydroindolyl, dihydrobenzodioxinyl, tetrahydroindolyl, tetrahydroindazolyl, tetrahydrobenzimidazolyl, tetrahydrobenzotriazolyl, tetrahydropyrrolopyridyl, tetrahydropyrazolopyridyl, tetrahydroimidazopyridyl, tetrahydrotriazolopyridyl, and tetrahydroquinolinyl groups. Representative substituted heterocyclyl groups may be mono-substituted or substituted more than once, such as, but not limited to, pyridyl or morpholinyl groups, which are 2-, 3-, 4-, 5-, or 6-substituted, or disubstituted with various substituents such as those listed above.

Heteroaryl groups are aromatic ring compounds containing 5 or more ring members, of which, one or more is a heteroatom such as, but not limited to, N, O, and S. Heteroaryl groups may be substituted or unsubstituted. Heteroaryl groups include, but are not limited to, groups such as pyrrolyl, pyrazolyl, triazolyl, tetrazolyl, oxazolyl, isoxazolyl, thiazolyl, pyridyl, pyridazinyl, pyrimidinyl, pyrazinyl, thiophenyl, benzothiophenyl, furanyl, benzofuranyl, indolyl, azaindolyl (pyrrolopyridyl), indazolyl, benzimidazolyl, imidazopyridyl (azabenzimidazolyl), pyrazolopyridyl, triazolopyridyl, benzotriazolyl, benzoxazolyl, benzothiazolyl, benzothiadiazolyl, imidazopyridyl, isoxazolopyridyl, thianaphthalenyl, purinyl, xanthinyl, adeninyl, guaninyl, quinolinyl, isoquinolinyl, tetrahydroquinolinyl, quinoxalinyl, and quinazolinyl groups.

“Substituted” refers to a chemical group as described herein that further includes one or more substituents, such as lower alkyl (including substituted lower alkyl such as haloalkyl, hydroxyalkyl, aminoalkyl), aryl (including substituted aryl), acyl, halogen, hydroxy, amino, alkoxy, alkylamino, acylamino, thioamido, acyloxy, aryloxy, aryloxyalkyl, carboxy, thiol, sulfide, sulfonyl, oxo, both saturated and unsaturated cyclic hydrocarbons (e.g., cycloalkyl, cycloalkenyl), cycloheteroalkyls and the like. These groups may be attached to any carbon or substituent of the alkyl, alkenyl, alkynyl, aryl, cycloheteroalkyl, alkylene, alkenylene, alkynylene, arylene, hetero moieties. Additionally, the substituents may be pendent from, or integral to, the carbon chain itself.

I. Selectivity of the Anode Catalyst for HOR vs. the ORR Based on Patterning of Metal Surfaces with Adsorbates

Automotive durability requirements for PEM fuel demand about 5,000 hours of operation time and about 30,000 startup/shutdown cycles. In each of these cycles hydrogen is purged from the anode with air (shutdown) or air is purged from the anode with hydrogen (startup), creating conditions, where both air and hydrogen are present at the anode at the same time, which are beneficial for cathode degradation.

In short, the situation during startup/shutdown is summarized in FIG. 1. FIG. 1B is an illustration showing the situation during startup/shutdown of a typical proton-exchange membrane fuel cell. When the air is passing through the anode or out of the anode compartment, an air-hydrogen front is temporarily created—shown as a dashed line. This temporarily creates a H₂/O₂ fuel cell (below the dashed line) driving an electrolytic cell C/O₂ (above the dashed line), leading to the degradation of carbon support on the cathode side which diminishes the effective amount of the catalyst. The temporary conditions are, in part, enabled by the fact that the ionic conductivity of the protons is much lower in direction perpendicular to the electrodes (only 100 μm of polymer) then in the lateral direction (mm or cm of polymer). The conditions are further presented in FIG. 1C, where the polarization curves represent the behavior of H₂/air fuel cell driving the C/O₂ electrolytic cell. Essentially, the current running through both cells completing the electrical circuit is about the same and equals the current represented in FIG. 1C as the start/stop current. This is the degradation current of the carbon support, and reducing it means reducing the degradation of the cathode. Clearly, pushing the C/O₂ cell's polarization curve to more positive values means reducing the start/stop current. This can be achieved in two ways, pushing the oxidation of carbon support to more positive values, or as in the present case pushing the reduction of oxygen on the anode catalyst more negative, i.e. deactivating the catalyst for the ORR. The biggest problem in deactivating the ORR on Pt catalyst is to keep the HOR activity intact.

In one aspect, a chemically modified electrode (CME) is provided which is a Pt-based (platinum-based) or a platinum group metal electrode patterned with an adsorbate that includes calix[n]arenes, cyclodextrines, cucurbit[n′]uril, porphyrins, crown ethers, calix[n]pyrroles, pyrogallol[n]arenes, and calix[4]resorcinarenes, and derivatives thereof, where n is 4, 6, or 8 and n′ is the number of glycoluril units and is 5, 6, 7, or 8. Such electrodes provide for an improved, selective anode catalyst that will overcome the shutdown and startup limitations associated with polymer electrolyte membrane fuel cells (PEMFC).

Thus, in one embodiment, an electrode includes a Pt-based (which includes platinum group metal based) substrate to which calix[n]arene adsorbate molecules are attached, wherein the n is 4, 6, or 8. In some embodiments, the calix[n]arene adsorbate is a calix[4]arene. The calix[n]arene adsorbate molecules block a portion of the Pt metal sites and thereby suppressing the ORR, while allowing hydrogen to migrate to non-blocked Pt metal sites for the HOR. The amount of blocked and non-blocked Pt sites may vary. However, in one embodiment, from about 90% to about 99% of the Pt metal sites are blocked by the adsorbate molecules and from about 1% to about 10% of the Pt metal sites are adsorbate free.

The Pt-based substrate may be a platinum group metal or alloy thereof, a pure Pt substrate or it may be a Pt alloy with one or more of Co, Ni, Fe, Ti, Cr, V, or Mn. Illustrative Pt alloys include but are not limited to, Pt₃Ni, Pt₃Co, Pt₃Fe, PtNi, PtCo, and PtFe. As noted, the Pt-based substrate may be a pure platinum metal. Such metals include those having a Pt(100), Pt(1099), or Pt(111) surface, or those that are a polycrystalline Pt (i.e. Pt(Poly)). As used herein a platinum group metal is any of Pt, Pd, Ir, or Rh, or a alloy thereof. For example, the platinum group metal may include Pt, Pd, Ir, or Rh or an alloy of any of these with one or more of Co, Ni, Fe, Ti, Cr, V, or Mn. In some embodiments, the platinum group metal includes Pt, or Pd, or an alloy Pt or Pd with one or more of Co, Ni, Fe, Ti, Cr, V, or Mn. In some embodiments, the platinum group metal-based electrode includes a surface of Pt(100), Pt(111), Pt(1099), or polycrystalline Pt.

Suitable adsorbates for the Pt-based substrate include, but are not limited to, calix[n]arene molecules, where n is the number of repeat arene units in the compound. In some embodiments, n is 4, 6, or 8. As an illustrative example of a calix[4]arene the following Formula I may is representative:

According to this illustrative example, the calix[n]arenes may be thiol-alkoxy or thiol-hydroxyl arenes. That is to say, at one annular face of the calix[n]arene, thiol groups, —SR′, are present, and at the other annular face of the calix[n]arene, —OR groups are present. In the illustrative example, each R is independently a H, alkyl, aryl, heteroaryl, or heterocyclyl. In the illustrative example, each R′ is independently a H, alkyl, aryl, heteroaryl, or heterocyclyl. Due to the methylene linkers between each of the phenyl groups of the calix[n]arene being proximal to the —OR groups, the face defined by the —OR groups has a smaller radius than the face of the calix[n]arene with the thiol groups. The thiol groups, however, readily coordinate to metal surfaces, such as the Pt-based substrate in a self-assemble monolayer type structure. Accordingly, the larger diameter face of the calix[n]arene is the face that binds to the surface of the Pt-based substrate (e.g. see FIG. 2C for a schematic illustration). FIG. 2A shows a 200×200 nm² image of an as-prepared Pt(111) showing large terraces, divided by mono-atomic steps and covered with a small number of Pt adislands (the average size of the clusters is 2 nm). FIG. 2B shows a 100×100 nm² image of the calix[4]arene adlayer (surface coverage of ˜0.98 ML) showing long-range order of the self-assembled molecules. FIG. 3 shows a schematic representation of calix[4]arene molecules attached to the surface via —SH groups located on the molecule's wide rim.

According to some embodiments, each R is H or a C₁-C₈ alkyl. In other embodiments, each R is H, methyl, ethyl, n-propyl, iso-propyl, or n-butyl. According to some embodiments, each R′ is H, C₁-C₈ alkyl, or —C(O)alkyl. In other embodiments, each R′ is H, methyl, ethyl, n-propyl, iso-propyl, or n-butyl. According to some embodiments, each R′ is H or —C(O)alkyl. In one embodiment, the calix[n]arene is a calix[4]arene of Formula I, where each R is butyl, and each R′ is acetyl.

In another aspect, a process is provided for preparing a Pt-based electrode. Such processes include patterning a surface of a Pt-based electrode by contacting the electrode with a calix[n]arene adsorbate to form a patterned Pt-based electrode. The electrode includes both Pt metal sites blocked with adsorbate molecules and Pt metal sites which are not blocked. The Pt-based electrode and calix[n]arene adsorbate may be as described above for the Pt-based (platinum-based) electrode patterned with a calix[n]arene molecules.

The process of patterning may include heating the Pt-based electrode to an annealing temperature from about 500 K to about 1500 K under a reducing atmosphere to anneal the Pt surface and activate it toward reaction with the adsorbate. In some embodiments, the annealing temperature is from about 1000 K to about 1200 K. The reducing atmosphere may include hydrogen gas, or hydrogen gas mixed with an inert gas. Illustrative inert gases include, but are not limited to, nitrogen, neon, helium, and argon. Where the hydrogen is mixed with the inert gas, the ratio of hydrogen to inert gas may be from about 0.5 vol % to 50 vol.%. In some embodiments the ratio is from about 1 vol % to about 10 vol %. In yet other embodiments, the ratio is from about 1 vol % to about 5 vol %.

After heating, the annealed Pt electrode is then cooled to ambient temperature. The cooled, annealed Pt-based electrode may then be covered by a droplet of water to protect the annealed surface before contacting the electrode with the calix[n]arene, or the cooled, annealed Pt-based electrode may then be contacted directly with the calix[n]arene. The calix[n]arene may be the neat compound or it may be in solution. For example, in one embodiment, the calix[n]arene is present in a solution of an organic solvent. Suitable organic solvents include, but are not limited to tetrahydrofuran, diethyl ether, methyl butyl ether, 1,2-dichlorobenzene, 1,4-dichlorobenzene, dimethylsulfoxide, dimethylformamide, ethanol, methanol or any mixture of two or more such solvents. According to various embodiments, the concentration of the calix[n]arene in the solvent is from about 10 μM to about 500 μM.

In another aspect, a Pt-based electrode produced by such methods is provided.

As noted above, the electrodes described above, both as described and as produced by the described methods, may be used in fuel cells. Accordingly, in one aspect, a PEM fuel cell is provided including the Pt-based electrode comprising the calix[n]arene adsorbate. For example, in one embodiment, a fuel cell includes a cathode, an anode and a proton exchange membrane serving as an electrolyte. The anode catalyst is platinum or a platinum group metal or an alloy thereof modified with a calixarene. The cathode catalyst is platinum or a platinum group metal or an alloy thereof.

It has been demonstrated that a chemically modified Pt electrode with a self-assembled monolayer (SAM) of calix[4]arene molecules can selectively block the ORR, but in such a way that the HOR proceeds with Pt-like activity. The optimum selectivity has been achieved by fine tuning the surface coverage of calix[4]arene molecules, leading to the formation of a critical ensemble of O₂-tolerant Pt-group metal sites that are very active for the adsorption of H₂ and consequent H—H bond breaking. The chemically-modified electrode (CME) approach outlined herein is not restricted to the Pt-calix systems, and may have many applications in analytical, synthetic and materials chemistry as well as in chemical energy conversion, selective fuel production and energy storage.

In one aspect, a platinum substrate modified with a calix[4]arene (calix) is provided. High selectivity of the HOR on calix-modified Pt(1099){10(111)×(100)}and Pt(110){2(111)×(10 0)} step surfaces is demonstrated. A methodology for the preparation of highly selective and stable SAMs of calix molecules on nanocatalysts is also provided. It has been found that if the synthesis is precisely controlled, the selectivity of nanoparticles for the ORR in the presence of hydrogen under conditions relevant to PEMFC operations is nearly 100%.

The present technology, thus generally described, will be understood more readily by reference to the following examples, which are provided by way of illustration and are not intended to be limiting.

EXAMPLES

As a general note, all gases used in the following examples were of 5N5 quality and were purchased from Airgas. The sweep rate for all RRDE measurements was 50 mV For the ORR measurements, the electrode was rotated at 1,600 rpm. Electrode potentials are given versus the RHE.

Example 1

Selective Catalysts for the Hydrogen Oxidation Reaction (HOR) and the ORR by Patterning of Platinum with Calyx[4]Arene Molecules

Synthesis of calix[4]arenes. The quadropod (i.e. 4 groups capable of binding to a substrate) anchoring compounds were synthesized using a three-step reaction from the corresponding calix[4]arene, according to Scheme 1.

As shown, an alkyl-protected calix[4]arene 1 was brominated to yield bromo derivative 2. Lithiation with t-butyllithium, followed by introduction of sulfur and protection of the thiol group in the form of thiolacetate, gave a reasonable yield of the final compound 3. For the detailed procedure on the synthesis of the compounds 1-3 in Scheme 1, see Genorio, B. et al. Langmuir 24, 11523-11532 (2008).

Preparation of Pt(111), Pt(100) and Pt(Poly) and self-assembly procedures. Pt electrodes were prepared by inductive heating for 10 min at ˜1,100 K in an argon hydrogen flow (3% hydrogen). The annealed specimen was cooled slowly to room temperature in the flow stream and immediately covered by a droplet of water. The electrode was then immersed in a tetrahydrofuran solution of a calix[4]arene for 24 hours, allowing for the formation of a calix[4]arene self-assembled monolayer (SAM). Four samples, A, B, C, and D were prepared from calix[4]arene concentrations in tetrahydrofuran of 90, 130, 250 and 400 μM, respectively, in addition to an unmodified Pt sample for each of the Pt(111), Pt(100), and Pt(Poly).

For Pt(100) and Pt(Poly) electrodes, 600 μM solutions were used to demonstrate the validity of the CME approach even at very high coverages. Calix[4]arene coverages were calculated by H_upd comparison between the clean Pt(111) surface and chemically modified surfaces.

STM Images. For the as-prepared surfaces, the STM images were acquired with a Digital Instruments Multi-Mode Dimension STM controlled by a Nanoscope III control station. During the measurement, the microscope with the sample was enclosed in a pressurized cylinder with a CO atmosphere. For modified surfaces, STM measurements were carried out on a home-built low-temperature STM equipped with an RHK SPM1000 controller. The samples were prepared according to the method described above and transferred to a helium glove box. Any water drops remaining on the sample were removed by blowing the surface with helium gas. The sample was then mounted on the STM stage and the STM head was sealed and transferred to the cryostat. The STM was cooled to 4.2 K and the surface was scanned at a bias voltage of 500 mV and tunneling current of 20 pA. Measurements at 4.2 K provided minimum drift during scanning (less than a few angstroms per hour). A high tunneling resistance was necessary to ensure that the tip did not touch calix[4]arene molecules on the surface.

The following analysis refers to the scanning tunneling microscopy (STM) images of bare Pt(111) and Pt(111) modified with a calix[4]arene adlayer in FIGS. 2A and 2B. FIG. 2A shows that a Pt(111) surface includes small adislands of Pt atoms that are separated by well-resolved monoatomic terrace-edge-step sites running roughly parallel to the (111) substrate direction. The STM image for a surface modified by the highest coverage of calix[4]arene (FIG. 2B) is characterized by a close-packed, long-range ordered monolayer of parallel arrays of molecules that almost completely cover the (111) terrace sites. FIG. 3 is a schematic representation of the Pt(111)/calix interface, in which the wide rim of the cone (representing the anchoring groups) serves as an electrode surface protector, the narrow rim of the cone as a molecular sieve and the lateral surface of the ‘truncated cone’ as a ‘blocking wall’.

The driving force for ordering such large molecules is presumably governed by a synergy between the strong chemical interaction of Pt with calix[4]arene molecules and the surface homogeneity of Pt(111) that results in collective interaction of adsorbed molecules on terrace sites. It should be recognized, however, that on the basis of STM data alone it was difficult to deduce either the number of calix[4]arene-free Pt atoms and the nature (coordination) of the remaining bare Pt atoms. Without being bound by theory, it is reasonable to suggest that as a result of steric effects most of the step-edges observed in FIG. 2A are not decorated with these large molecules. In contrast, it appears that smaller Pt adislands formed on the terraces of annealed Pt(111) may be buried under the large calix[4]arene molecules. The determination of the number of available Pt sites is, however, less challenging given that a reasonable assessment of bare Pt atoms on Pt(111)-calix_ad can be obtained by monitoring how the fractional coverages (Θ) of underpotentially deposited hydrogen (H_upd)and hydroxyl species (OH_ad) are affected by Θ_calix.

RRDE Method, Electrolytes And Electrochemical Set-Up. The Pt(111), Pt(100), and Pt(Poly) electrodes were embedded into a rotating ring disc electrode (RRDE), which was then place in a standard three-compartment electrochemical cell containing 0.1M HClO₄. In each experiment, the electrode was immersed at 0.07 V in a solution saturated with argon. After obtaining a stable cycle between 0.07 V and 0.7 V the polarization curve for the ORR was recorded on the disc, whereas the peroxide oxidation signal was measured at the ring, which was held at 1.1 V versus the reversible hydrogen electrode (RHE). Peroxide currents presented are already corrected for the collection efficiency of 0.24. Subsequently, oxygen was purged from the solution, replaced with hydrogen and HOR polarization curves were measured. Finally, the voltammetric response was again recorded in an argon-purged solution to confirm that the calix coverage had not changed significantly.

FIG. 4A illustrates the effect of Θ_calix (curve A with 84% coverage, curve B with 95% coverage, curve C with 96% coverage and curve D with 98% coverage) on cyclic voltammetry of Pt(111) (black dashed line), including the H_upd in region I, double layer (region II) and OH_ad adsorption in region III. FIG. 5B illustrates corresponding charge density versus E curves for H_upd and OH_ad, which are assessed on the basis of the assumptions of one H_upd per Pt on the Pt(111)-(1×1) surface and that the H_upd charge on Pt(111) in potential range I is ˜160 μC cm⁻². A gray potential region in this and all other figures represents the potential window of importance to the anode selectivity. Sweep rates were 50 mV s⁻¹. FIG. 4A shows that pseudo-capacitive features corresponding to H_upd formation, double-layer charging and OH_ad formation on Pt(111) are almost completely suppressed on the surface covered by organic molecules, confirming a high coverage of calix[4]arene molecules. FIG. 4B also reveals that a systematic increase in Θ_calix leads to a marked decrease in availability of Pt sites, that is, for H_upd and OH_ad from 16% on the least covered surface to 2% on a surface with the most closely packed calix[4]arene adlayer. When the free Pt sites on the Pt(111)-calix surfaces are expressed as density of active sites (N), then a noticeable effect of Θ_calix on the availability of Pt becomes even more apparent, as evidenced in Table 1. This suggests that the ions/water from the supporting electrolyte are unable to penetrate the narrow rim of calix[4]arene molecules. On the basis of these observations, along with the structural insight derived from the STM images, it is proposed that the only sites available for adsorption of electrolyte components are the relatively small number of Pt unmodified step-edges and/or the small ensembles of terrace sites between the anchoring groups.

TABLE 1
Calix[4]arene coverages and turnover frequencies
(TOF) for the ORR and HOR for five samples.
	Hupd	Calix	Avail-	Number of	Min.	Min.
	charge	cover-	able	avail-	TOF	TOF
	(μC	age	surface	able sites	for ORR	for HOR
Sample	cm−2)	(%)*	(%)*	(N cm−2)†	@0.8 V‡	@0.1 V‡
Pt(111)	161	0	100	1015	9	8
Pt(111)-	26	84	16	1.6 × 1014	9	49
A
Pt(111)-	9	94	6	5.5 × 1013	9	129
B
Pt(111)-	7	96	4	4.2 × 1013	6	194
C
Pt(111)-	4	98	2	2.4 × 1013	6	388
D
*Calix coverages and available surface were calculated by comparing Hupd charge on the bare Pt(111) and modified Pt(111).

^†Number of available sites was calculated assuming one H per Pt adsorption and number of total sites on Pt(111) surface 1.5×10¹⁵.^‡Values are calculated by taking the measured current density @0.8 and @0.1, using the equation TOF=i_E1/nFN. As current is under diffusion control for the HOR and in some cases for the ORR, the values are presented as minimum TOFs for the reactions.

Having illustrated the effect of Θ_calix on the formation of H_upd and OH_ad adlayers, the extent to which the Θ_Hupd/OHad versus E curves (FIG. 4B) can be used for analyzing the polarization curves for the HOR and the ORR in FIG. 5 was investigated. In this analysis the general rate expression is used in which activity (current density of the ORR and the HOR is dependent on the number of available Pt sites as shown in Equation 2:

I _{E 1} =nFK ₁ c _reac(1−Θ_ad)   (2)

where n is the number of electrons, K₁ is a constant, c_reac. is the concentration of H₂ or O₂ in the solution and Θ_ad=ΘH_upd+Θ_OHad+Θ_calix is the fraction of the surface masked by the site-blocking species, that is, reaction intermediates are adsorbed at low coverages. Equation (2) was developed on the basis of a simple assumption that the Pt—O₂ and Pt—H₂ energetics as well as their reaction intermediates on bare Pt atoms is not affected by the surrounding calix[4]arene molecules. It is also assumed that if ions/water from the supporting electrolytes are unable to penetrate through the narrow end (see FIG. 3), then the same should be valid for H₂ and O₂; that is, the adsorption of H₂ and O₂ occurs on a small number of calix[4]arene-free Pt sites. To demonstrate that the number of active sites required for the maximum rates of the HOR and ORR is extremely low, the reactivity of Pt(111)-calix surface was converted into a turnover frequency (TOF), TOF=i_E1/nFN, and is summarized in Table 1.

FIG. 4A shows polarization curves for the HOR in 0.1M HClO₄ on Pt(111) are the same as for all Pt(111)-calix modified surfaces. The curves for Pt(111)-B (gray) and -C (blue) are the same but they are omitted for clarity. Small variations may be observed in the hydrogen evolution region. These arise from variability in the iR correction (voltage drop correction due to solution resistance) for the electrode. For electrodes with high Θ_calix values, the variability in iR (current-resistance) values for the HOR can alter the overall hydrogen evolution reaction currents because of the large values of these currents. FIG. 5B shows peroxide oxidation currents recorded at 1.1 V versus RHE on the ring electrode during the ORR on the Pt(111)-calix disc electrodes. FIG. 5C shows corresponding polarization curves for the ORR. The rotation rate for all measurements was 1,600 rpm; a sweep rate of 50 mV s⁻¹ was used in all experiments.

The analysis of polarization curves for the HOR on Pt(111) and Pt(111) modified with calix[4]arene begins with FIG. 5A. The HOR on Pt(111) (unmodified) is an extremely fast process that, below 0.1 V, is determined predominantly by the surface coverage of spectator species H_upd. Above this potential, the reaction rate is always under pure diffusion control. An important observation from FIGS. 5A-5C is that for various surface coverages of calix[4]arene molecules the HOR is essentially the same. This suggests that the required number of Pt sites for the maximum rates of the HOR is rather small, that is, calculated on the basis of 2% surface site availability, the minimum TOF for HOR on Pt sites could be as high as 388 molecules per site per second. Notably, this result has fulfilled the first requirement for designing highly active anode catalysts under operating PEMFC conditions—a Pt-like activity for the HOR.

The ORR is a more complex multi-electron reaction in which O₂ is being reduced in one of the following ways: to water without peroxide formation (4e⁻ reduction) or to water with peroxide formation (a mixed 4e⁻+2e⁻ reduction) or completely to peroxide via a 2e⁻ reduction process. To analyze possible reaction pathways (neglecting any rigorous kinetic analyses) of the ORR on P4111) and Pt(111)-calix[n]arene surfaces, the RRDE method was used. This method provides information on both the total currents for the ORR on the disc electrode (FIG. 5C) as well as the concomitant production of peroxide on the ring electrode (FIG. 5B). For Pt(111), starting at ˜0.95 V and sweeping the Pt(111) disc potential in the negative direction to 0.45 V, the ring currents were essentially zero, implying that in this potential region the ORR proceeds entirely through the direct 4e⁻ pathway. FIG. 5B shows that the appearance of peroxide oxidation currents on the ring electrode begins at potentials below 0.45V and the limiting current corresponding to an exactly two-electron reduction of O₂ is reached at the negative potential limit.

There are two general observations concerning the ORR on Pt(111)-calix systems. First, the disc currents show that the ORR is inhibited on the calix[4]arene-modified surfaces and the deactivation increases with an increase of Θ_calix (FIG. 5C and Table 1). The inhibition of O₂ adsorption is so strong that the theoretical diffusion-limited currents corresponding to the disc geometric area are never reached. Without being bound by theory, it is proposed that the currents observed on Pt(111)-calix_ad surfaces are related to the ORR on uncovered or partially covered active Pt patches. Given that the relative number of these patches is too small to allow a full overlap between the adjacent diffusion zones (spherical diffusion regions), it is reasonable to suggest that the currents below 0.6 V are diffusion-limited currents; but for conditions of highly blocked surfaces. Second, peroxide formation is hardly observed in the potential region of significance for startup/shutdown conditions (E>0.6 V). This is an important result as H₂O₂ may affect degradation of the Nafion membrane. As revealed in FIG. 5B, below 0.6 V a monotonic increase in the peroxide production is observed on Pt(111)-calix_ad. Furthermore, on this surface the onset potential for peroxide formation is shifted ˜300 mV positive relative to Pt(111), indicating a change in the reaction pathway on these two surfaces. In short, below 0.6 V the 2e⁻ reduction process is predominant on calix[4]arene-modified Pt(111) surfaces. This is consistent with the proposition that larger ensembles of Pt sites are required for efficient cleavage of the O—O bond than for the adsorption of O₂ and the concomitant formation of H₂O₂.

FIG. 5 also shows that on the 98% covered electrode, the ORR is completely inhibited between 0.6 and 0.85 V; however, on the same surface and within the same potential window the HOR is under pure diffusion control. This unique selectivity of such CMEs may be attributed to very strong ensemble effects in which the critical number of bare Pt atoms required for adsorption of O₂ (that is, the ORR) is much higher than that required for the adsorption of H₂ molecules and a subsequent HOR. Furthermore, the established selectivity was possible only because the required number of active sites for maximal rates of the HOR is, in fact, extremely small and just 2% of the available active surface sites are sufficient to reach the diffusion limiting currents. Finally, it is important to emphasize that the observed O₂ selectivity is not unique to the Pt(111)-calix system and, as summarized in FIGS. 6A-6F, an exceptional anode selectivity for the ORR and HOR is also observed on the Pt(100) and polycrystalline Pt (Pt(Poly))electrodes. This suggests that Pt-calix systems are of broad fundamental and technological importance. FIGS. 6A and 6B show polarization curves for the HOR on bare and covered Pt(100) (FIG. 6A) and Pt(Poly) (FIG. 6B). FIGS. 6C and 6D show ORR on bare and calix[4]arene-covered Pt(100) (FIG. 6C) and Pt(Poly) (FIG. 6D) electrodes. FIGS. 6E and 6F show cyclic voltammograms for bare and covered Pt(100) (FIG. 6E) and Pt(Poly) (FIG. 6F) in 0.1M HClO₄. Slight kinetic inhibition in HOR between 0.0 and 0.15 V observed for Pt(100) and Pt(Poly) surfaces is most likely the result of the high coverage of surface species (H_upd) as well as calix for these electrodes. Both Pt(100) and Pt(Poly) electrodes exhibit the same diffusion-limiting currents for HOR as the bare surfaces while showing significant deactivation for the oxygen reduction reaction at potentials >0.6 V.

Example 2

Tailoring the Selectivity and Stability of Chemically Modified Platinum Nanocatalysts to Design Highly Durable Cathodes for PEM Fuel Cells

Synthesis of thiolated derivatives of calix[4]arenes. Reactions were performed in dried glassware under nitrogen atmosphere. Precursor calix[n]arenes were prepared according to published procedures: calix[4]arene (Gutsche, C. D. et al. Organic Syntheses 68, 234 (1990); and Gutsche, C. D et. al. Tetrahedron 42, 1633 (1986)); calix[6]arene (Gutsche, C. D et. al. Tetrahedron 42, 1633 (1986); and Gutsche, C. D. et al. Organic Syntheses 68, 238 (1990)), and calix[8]arene (Gutsche, C. D et. al. Tetrahedron 42, 1633 (1986); and Munch, J. H. et al. Organic Syntheses CV8, 80) were synthesized according to published procedures. Calixarenes 1a, 1b, 1c, 2a, 2c, and 3a were prepared according to literature procedures (see 1a: Kenis, P. J. A. et al. Chemistry—a European Journal 4, 1225 (1998); 1b: Markowitz, M. A. et al. J. Am. Chem. Soc. 111, 8192 (1989); 1c, 2c: Perret, F. et al. New J. Chem. 31, 893 (2007); 2a: Gutsche, C. D. et al. J. Org. Chem. 50, 5795 (1985); and 3a: Genorio, B. et al. Langmuir 24, 11523 (2008)). Reagent grade tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl. N,N-dimethylformamide (DMF) was distilled over CaH₂. Reagent grade hexanes, 2-butanone (MEK), CH₂Cl₂ (DCM), MeOH, and ethyl acetate (EtOAc) were used without further distillation. Acetyl chloride (AcCI) was heated at reflux with PCl₅ and then distilled prior to use. tert-Butyllithium was obtained from Aldrich (1.7 M solution in pentane). All other commercially available reagents were used as received. Flash column chromatography was performed using Zeoprep 60 Eco 40-63 silica gel. ¹H NMR spectra were taken at 300 MHz. ¹³C NMR were recorded on the same instrument at 75.5 MHz. Proton chemical shifts (δ) are reported in ppm downfield from tetramethylsilane (TMS). Carbon was referenced to CDCl₃ (77.23 ppm).

General procedure for introducing protected thiol groups to the wide-rim of calixarene macrocycle (3a-c). A solution of n-bromocalix[n]arene 2a-c in THF was cooled to −78° C. and tert-BuLi (1.7 M solution in pentane) was added in 15 min. The reaction was stirred for 2 hours and sulfur was added. The reaction was allowed to warm to room temperature and stirred for 30 minutes. The mixture was then cooled to −20° C., and acetyl chloride was added and the mixture was allowed to warm to room temperature and stir over night (˜12 hours). The reaction mixture was diluted with CH₂Cl₂ and transferred to a separatory funnel and a saturated solution of NH₄Cl was added. The organic phase was separated and the aqueous layer was extracted with CH₂Cl₂ (3×25 mL). The combined organic extracts were dried over anhydrous Na₂SO₄, and the solvents were removed in vacuo. The crude product was then purified by column chromatography on silica gel.

5,11,17,23,29,35-Hexabromo-37,38,39,40,41,42-hexabutoxycalix[6]arene (2b). To a solution of,37,38,39,40,41,42-hexabutoxycalix[6]arene 1b (600 mg, 0.62 mmol) in 2-butanone (28 mL), NBS (1.1 g, 6.16 mmol) and catalytic amount of 48% wt aqueous HBr were added (30 μL). The yellow solution was stirred at room temperature for 24 hours. The mixture was then stirred with 10% aqueous NaHSO₃ (23 mL), and CH₂Cl₂ (50 mL) was added. The organic phase was separated from aqueous phase which was extracted with CH₂Cl₂ (3×20 mL). The combined organic extracts were dried over anhydrous Na₂SO₄, and the solvents were removed in vacuo. The crude product was then purified by recrystallization from cold CH₂Cl₂ to yield colorless crystals (810 mg, 91% yield). IR (KBr): 2959, 2932, 2870, 1574, 1452, 1381, 1195 cm⁻¹. ¹H NMR (300 MHz, CDCl₃): δ 7.09 (br s, 12H), 3.83 in 3.49 (br s, 24H), 1.34 (br s, 24H), 0.84 (br s, 18H). ¹³C NMR (75.5 MHz, CDCl₃): δ 154.6, 135.8, 132.3, 122.6, 119.3, 116.9, 109.3, 73.6, 32.4, 32.3, 30.8, 19.6, 14.2. HRMS: No signal for M+. Anal. Calcd for C₆₆H₇₈Br₆O₆, C, 54.79%; H, 5.43%; Found: C, 55.17%; H, 5.62%.

5,11,17,23,29,35-Hexakis(thioacetyl)-37,38,39,40,41,42-hexabutoxycalix[6]arene (3b). Following the procedure for introducing protected thiol groups, 5,11,17,23,29,35-hexabromo-37,38,39,40,41,42-hexabutoxycalix[6]arene 2b (400 mg, 0.28 mmol), THF (50 mL), tert-BuLi (2.93 mL of a 1.7 M solution in pentane, 4.98 mmol), sulfur (160 mg, 4.98 mmol) and acetyl chloride (0.59 mL, 8.29 mmol) were used. Column chromatography (EtOAc:hexanes=2:7) afforded the title compound as a white powder (155 mg, 40% yield). IR (KBr): 2955, 2931, 2870, 1706, 1451, 1201, 1115 cm⁻¹. ¹H NMR (300 MHz, CDCl₃): δ 7.04 (br s, 12H), 3.88-3.55 (br m, 24H), 2.30 (s, 18H), 1.31 (br, 24H), 0.82 (br s, 18H). ¹³C NMR (75.5 MHz, CDCl₃): δ 195.2, 156.5, 135.5, 134.8, 122.4, 122.2, 119.0, 115.7, 109.0, 73.1, 32.1, 30.6, 29.9, 19.3, 14.0. HRMS: calcd for C₇₈H₉₆O₁₂S₆+Na⁺, 1439.5124; found, 1439.5130.

5,11,17,23,29,35,41,47-Octakis(thioacetyl)-40,50,51,52,53,54,55,56-octabutoxycalix[8]arene (3c). Following the procedure for introducing protected thiol groups, 5,11,17,23,29,35,41,47-octabromo-40,50,51,52,53,54,55,56-octabutoxycalix[8]arene 2c (600 mg, 0.31 mmol), THF (35 mL), tert-BuLi (4.39 mL of a 1.7 M solution in pentane, 7.47 mmol), sulfur (239 mg, 7.47 mmol) and acetyl chloride (0.89 mL, 12.44 mmol) were used. Column chromatography (EtOAc:hexanes=2:5) afforded the title compound as a white powder (80 mg, 14% yield). IR (KBr): 2958, 2932, 2871, 1708, 1453, 1115 cm⁻¹. ¹H NMR (300 MHz, CDCl₃): δ 6.93 (s, 16H), 4.00 (s, 16H), 3.66 (t, J=6.6 Hz, 16H), 2.25 (s, 24H), 1.61 (m, 16H), 1.32 (m, 16H), 0.80 (t, J=7.3 Hz, 24H). ¹³C NMR (75.5 MHz, CDCl₃): δ 194.9, 156.9, 135.4, 135.0, 122.9, 73.6, 32.4, 30.2, 19.4, 14.1. HRMS: calcd for C₁₀₄H₁₂₈O₁₆S₈+H+, 1889.7046; found, 1889.7058.

Preparation Of Pt(1099), Pt(110), And Pt(Polycrystalline) Surfaces And Self-Assembly. Pt electrodes were prepared by inductive heating for 10 min at approximately 1100 K in an argon-hydrogen flow (3% hydrogen). The annealed specimen was cooled slowly to room temperature in this flow stream and immediately covered by a droplet of water. The electrode was then immersed in a THF solution of calix[4]arene for 24 hours, allowing the formation of a calix[4]arene SAM. The concentration of calix[4]arene in THF was 600 μM to obtain samples with very high coverages of calix on Pt surface. Coverages were estimated from the H_upd measurements. The coverages can be modified by either varying the concentrations of the calix/THF solution or the exposure time to the high-concentration solution. After SAM preparation, the crystals were washed thoroughly with deionized water before assembly and immersion in the electrochemical cell.

Preparation Of NSTF And TKK Catalyst Electrodes And Their Self-Assembly. NSTF and TKK are commercially available catalysts. NSTF is a nanostructured thin film available from 3M and TKK is a 5 nm Pt/C catalyst available from Tanaka. The catalysts were mixed with water at a concentration of 1 mg mL⁻¹. This dispersion was then ultrasonically mixed for one hour, after which a stable suspension was obtained. A glassy carbon disk (6 mm diameter) was then mechanically polished. Known volumes of the suspensions were added using a micropipette onto the glassy carbon disk electrode. The electrode was dried at 60° C. in an inert atmosphere. The suspension was applied so that it uniformly coated the surface of the electrode. Once dry, the electrodes were washed with water to verify the adhesion of particles to the glassy carbon substrate. Subsequently, the electrodes were immersed in 1000 μM solution of the calixarene in THF. A high concentration of calixarene was chosen owing to the larger surface area of Pt compared to the disk electrodes. The systems were equilibrated for 24 hours. Another method involved assembly of the disk electrode in a hanging meniscus arrangement with subsequent immersion of the electrode in the calixarene solution with rotation (600 rpm) for 4 hours. Both of these methods yield similar coverages. After equilibration, the samples were washed thoroughly with water before being immersed in the electrochemical cell.

RDE Method, Electrolytes, And Electrochemical Setup. After extensive rinsing, the electrode was embedded into a rotating-disk electrode (RDE), which was then placed in a standard three-compartment electrochemical cell containing 0.1M HClO₄. In each experiment, the electrode was immersed at 0.07 V in solution saturated with argon. After obtaining a stable voltammogram between 0.07 and 0.7 V the polarization curve for the ORR was recorded on the disk on the disk electrode. Subsequently, oxygen was purged out of the solution and replaced with hydrogen, and HOR polarization curves were measured. Finally, the voltammetric response was recorded in argon-purged solution to confirm that the calixarene coverage had not changed significantly.

Discussion of HOR and ORR kinetics. Generally, to describe the activity (current) of HOR and ORR, the simple rate law of Equation (2) can be used, where the surface coverage of Pt is treated as the primary variable controlling the reactions.

I _Et =nFK ₁ c _reac.(1−Θ_ad)   (2)

where n is the number of electrons, K₁ is a constant, c_reac is the concentration of H₂ or O₂ in the solution and Θ_ad=ΘH_upd+Θ_OHad+Θ_calix is the fraction of the surface masked by the site-blocking species, that is, reaction intermediates are adsorbed at low coverages.

The HOR activities are not affected by the presence of calix[4]arene molecules. The fraction of free sites is on the order of 5% for stepped surfaces and ˜6-8% for nanocatalyst surfaces (estimated from the H_upd for the calix-covered surfaces). This suggests that in order to achieve true diffusion limited currents for HOR, as can be seen in the HOR curves in FIGS. 7 and 9, the turn-over frequency that needs to be achieved is >50. Such large turn-over frequencies are typical for HOR reaction as has been reported previously. This helps in maintaining the reactivity of the Pt-catalysts even in the presence of calix-molecules.

The ORR activities on the other hand are significantly suppressed by the presence of calix[4]arene molecules. Both the diffusion controlled currents and the kinetic currents are significantly decreased. The decrease in diffusion limited current suggests that the patches of free platinum sites (present between the adsorbed calix molecule), which are viable for the reaction, are few and far apart which leads to a decrease in the diffusion controlled currents due to the limited overlap between such regions. Kinetics of ORR on the other hand are driven primarily by the surface coverage due to the spectator species. This can be explained based on Equation (2). The Θ_ad , surface coverage of adsorbed species, is very large thereby driving the kinetics for the ORR, driven by the (1−Θ_ad), to very small values. This suggests that the ensemble of sites necessary/required for adsorbing O₂ and for performing ORR are limited in number which results in low activities for O₂ reduction.

NSTF vs. TKK H_upd Suppression By Calix[4]arene. The relative coverages, for similar methods of preparation are slightly different. This brings up an important point regarding the surface morphology of the catalyst and the size/characteristic of the adsorbing calix molecule. Supported nanoparticles, as in the case of TKK, are known to exist in either cuboctahedrons or distorted cuboctahedron geometries. Other shapes are also possible depending on the synthesis methodology used. Such structures often exhibit very short range order, surface features and hence the number of molecules that be adsorbed co-planar is limited by the relative size of the molecule. Adsorption is also possible across the edges and vertices of such particles, however with lower probabilities. Hence, when using a CME approach, it is necessary to tailor the organic molecules to fit the surface morphology of interest. The coverage obtained is a variable parameter dependent on the size of the molecules as well as the tolerable ORR activities for the HDA catalysts. NSTF catalysts are typically larger domains and exhibit a fiber like geometry. These particles exhibit a slightly larger domains of ordered surfaces and hence can conceivably be a bridge between extended surfaces and supported nanocatalysts. The relatively higher surface coverage of calix molecules observed with NSTF catalysts is due to the “match” between the calix molecule used and the particle sizes of these catalysts. For the cases described herein, the relative suppression of the H_updranged from 78% to 90% going from TKK-calix system to NSTF-calix systems.

To encompass a wide range of electrocatalyst designs and properties, the two most commonly used commercial electrocatalysts were analyzed. The TKK catalyst and the 3M NSTF catalyst were both studied (FIG. 8). The TKK catalyst represents supported nanocatalysts, where platinum nanoparticles 2-10 nm in diameter are supported on amorphous carbon black. NSTF catalysts, comprised of a unique catalyst structure which is free of carbon support, are usually applied directly to the membrane to provide a compact membrane electrode assembly structure (FIG. 7). Aqueous electrochemical experiments conducted using the RDE/RRDE (RDE=rotating disk electrode, RRDE=rotating ring disk electrode) methods for these nanocatalysts correlate well with operating fuel-cell systems.

Presented herein are results obtained from the RDE study that are relevant for operating fuel-cell systems. Various modifications of the calix molecules were studied, including the thiolated derivatives of calix[6]arenes and calix[8]arenes. As can be seen in FIG. 9, the calix[4]arene molecules are found to suppress the H_upd region (0.05-0.4 V) for both NSTF and TKK catalysts. The relative coverages for similar methods of preparation are slightly different, but the net results appear to be the same: an exceptional selectivity for the HOR versus ORR. As for stepped surfaces discussed above, the diffusion-limiting currents for the HOR are observed at potentials above 0.1 V and the activities. below 0.1 V are, within the experimental limits, almost identical.

Furthermore, the ORR polarization curves show limited or insignificant currents in the potential region of interest for the anode-side catalyst. As was shown in the earlier study with Pt(111), the peroxide yield on all extended and nanoparticle Pt-calix systems is negligible above 0.6 V, and the overall ORR behavior of these surfaces mimic ORR on uncovered or partially covered patches. All of these observations suggest that SAMs of calix molecule can be used to tailor the selectivity of the nanocatalyst toward ORR while preserving the HOR activity, the goal for an ideal anode catalyst. It is also important that the established selectivity was possible only because the required number of active sites for maximal rates of the HOR is extremely small but is sufficient to provide enough sites for the diffusion-limiting currents.

In addition to selectivity of CME, both thermal and electrochemical stability of these electrodes are important properties that need to be addressed to evaluate the anode catalyst's applicability to PEMFC. In order to study the stability of calix-modified electrodes, a Pt-calix system was tested in an oxygen-rich environment at 0.8 V for approximately 14 hours in solution at 60° C. These conditions are expected to be harsher than those experienced by the electrode in a real fuel-cell system. The exposure of the anode catalyst to high potentials (E<0.8 V for anode) in an air (oxygen)-rich atmosphere during startup and shutdown is expected to last between tens of seconds and a few minutes a day. The temperatures are expected to be similar to those used in present test conditions.

FIG. 10 shows the current-time relationship for the CME held at 0.8 V in an oxygen-rich atmosphere. The ORR current actually shows a small decay, thus suggesting that there is no loss of the calix molecules from the surface owing to oxidation. (Removal of the molecules by oxidation or desorption would increase the reduction current.) A similar experiment was also performed for the nanocatalysts (TKK) modified with calix[4]arene molecules, which show qualitatively similar results. This finding suggests that the calix[4]-arene-modified electrodes are stable under these operating conditions. Moreover, during the long-term experiments, the HOR (results not shown) is not affected at all.

CMEs prepared by modifying Pt with calix[4]arene molecules are highly stable and can effectively tune the selectivity of anode catalysts for ORR without altering the maximum activity of the HOR. This behavior is highly transformational, extending from long-range-ordered stepped single-crystal surfaces to nanocatalysts. The CME approach is not restricted to a Pt-calix system, and it is envisioned that this approach will provide many applications in analytical, synthetic, and materials chemistry as well as in chemical energy conversion and storage.

Equivalents

The embodiments, illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising,” “including,” “containing,” etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claimed technology. Additionally, the phrase “consisting essentially of” will be understood to include those elements specifically recited and those additional elements that do not materially affect the basic and novel characteristics of the claimed technology. The phrase “consisting of” excludes any element not specified.

The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent compositions, apparatuses, and methods within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, reagents, compounds compositions or biological systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.

As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least'equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like, include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.

While certain embodiments have been illustrated and described, it should be understood that changes and modifications can be made therein in accordance with ordinary skill in the art without departing from the technology in its broader aspects as defined in the following claims.

All publications, patent applications, issued patents, and other documents referred to in this specification are herein incorporated by reference as if each individual publication, patent application, issued patent, or other document was specifically and individually indicated to be incorporated by reference in its entirety. Definitions that are contained in text incorporated by reference are excluded to the extent that they contradict definitions in this disclosure.

Claims

1. A process comprising: patterning a surface of a platinum group metal-based electrode by contacting the electrode with an adsorbate to form a patterned platinum group metal-based electrode;wherein: the adsorbate comprises a calix[n]arene, cyclodextrine, a cucurbit[n′]uril, a porphyrin, a crown ether, a calix[n]pyrrole, a pyrogallol[n]arene, and a calix[4]resorcinarene, where n is 4, 6, or 8 and n′ is the number of glycoluril units and is 5, 6, 7, or 8;and the patterned platinum group metal-based electrode comprises platinum group metal sites blocked with the adsorbate and platinum group metal sites which are not blocked.

2. The process of claim 1, wherein the platinum group metal-based electrode comprises Pt, Pd, Ir, or Rh, or a Pt alloy with one or more of Co, Ni, Fe, Ti, Cr, V, or Mn.

3. The process of claim 1, wherein the platinum group metal-based electrode comprises Pt(100), Pt(111), Pt(1099), or polycrystalline Pt.

4. The process of claim 1, wherein the adsorbate comprises calix[4]arene.

5. The process of claim 1, wherein the adsorbate comprises a calix[n]arene which is a thiol-modified calix[n]arene.

6. The process of claim 1, wherein the patterning comprises heating the platinum group metal-based electrode to an annealing temperature from about 500 K to about 1500 K under a reducing atmosphere, cooling the electrode, and immersing the electrode in a solution comprising the adsorbate.

7. The process of claim 6, wherein the annealing temperature is from about 1000 K to about 1200 K.

8. The process of claim 6, wherein the reducing atmosphere comprises hydrogen gas.

9. The process of claim 6, wherein the reducing atmosphere comprises hydrogen gas and an inert gas.

10. The process of claim 9, wherein the inert gas comprises He, Ne, Ar, or N2.

11. The process of claim 6, wherein the electrode is cooled to about ambient temperature.

12. The process of claim 6, wherein the solution comprises a solvent and the adsorbate.

13. The process of claim 6, wherein the solution further comprises an organic solvent.

14. The process of claim 13, wherein the organic solvent comprises tetrahydrofuran, diethyl ether, methyl butyl ether, 1,2-dichlorobenzene, 1,4-dichlorobenzene , dimethylsulfoxide, dimethylformamide, ethanol, methanol or a mixture of any two or more thereof.

15. The process of claim 13, wherein a concentration of the calix[n]arene in the organic solvent is from about 10 μM to about 500 μM.

16. The process of claim 1, wherein from about 90% to about 99% of the Pt metal sites are blocked by the calix[n]arene molecules and from about 10% to about 1% of the Pt metal sites are not blocked by the calix[n]arene molecules.

17. A platinum group metal-based electrode produced by the process of claim 1.

18. A platinum group metal-based electrode of claim 17 which comprises Pt(100), Pt(111), Pt(1099), or polycrystalline Pt.

19. An electrode comprising a platinum group metal-based substrate comprising adsorbate molecules wherein a portion of the platinum group metal sites are blocked by adsorbate molecules and a portion of the platinum group metal sites are unblocked.

20. The electrode of claim 19, wherein from about 90% to about 99% of the Pt metal sites are blocked by the adsorbate and from about 10% to about 1% of the Pt metal sites are free of adsorbate.

21. The electrode of claim 19, platinum group metal-based substrate comprises Pt, Pd, Ir, or Rh, or a Pt alloy with one or more of Co, Ni, Fe, Ti, Cr, V, or Mn.

22. The electrode of claim 19, wherein the platinum group metal-based substrate comprises Pt(100), Pt(111), Pt(1099), or polycrystalline Pt.

23. A fuel cell comprising the electrode of claim 19.