U.S. patent application number 13/245395 was filed with the patent office on 2013-01-03 for fuel cell electrodes.
Invention is credited to Bostjan Genorio, Nenad Markovic, Vojislav Stamenkovic, Dusan Strmcnik.
Application Number | 20130004885 13/245395 |
Document ID | / |
Family ID | 47391015 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130004885 |
Kind Code |
A1 |
Strmcnik; Dusan ; et
al. |
January 3, 2013 |
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.
Inventors: |
Strmcnik; Dusan; (Woodridge,
IL) ; Genorio; Bostjan; (Ivancna Gorica, SI) ;
Stamenkovic; Vojislav; (Naperville, IL) ; Markovic;
Nenad; (Hinsdale, IL) |
Family ID: |
47391015 |
Appl. No.: |
13/245395 |
Filed: |
September 26, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61502412 |
Jun 29, 2011 |
|
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|
61512590 |
Jul 28, 2011 |
|
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Current U.S.
Class: |
429/524 ;
148/248; 148/249; 148/274; 148/280; 429/525; 429/526; 429/527;
429/530 |
Current CPC
Class: |
H01M 4/86 20130101; H01M
4/8621 20130101; C23C 22/02 20130101; Y02E 60/50 20130101; H01M
4/921 20130101; H01M 4/92 20130101 |
Class at
Publication: |
429/524 ;
429/530; 429/525; 429/526; 429/527; 148/280; 148/274; 148/248;
148/249 |
International
Class: |
H01M 4/92 20060101
H01M004/92; C23C 22/00 20060101 C23C022/00; C23C 22/78 20060101
C23C022/78; H01M 4/62 20060101 H01M004/62; C23C 8/04 20060101
C23C008/04 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with Government support under
Contract No. DE-AC02-06CH11357 awarded by the Department of Energy.
The Government has certain rights in this invention.
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 N.sub.2.
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 .mu.M to
about 500 .mu.M.
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.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Nos. 61/502,412, filed on Jun. 29, 2011; and
61/512,590, filed on Jul. 28, 2011, the entire disclosures of which
are incorporated herein by reference for any and all purposes.
FIELD
[0003] The present technology relates generally to modified
electrodes and methods of making and using the same.
BACKGROUND
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] DESCRIPTION OF THE DRAWINGS
[0016] 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.
[0017] FIGS. 2A and 2B, depict imaging of Pt(111) and Pt(111)-calix
electrodes by Scanning Tunneling Microscopy (STM), according to
various embodiments.
[0018] FIG. 3 is an illustrative model for adsorbed calix[4]arene
molecules on Pt(111), according to the examples.
[0019] FIGS. 4A and 4B illustrate relationships between surface
coverages by calix[4]arene molecules and H.sub.upd/OH.sub.ad on
Pt(111) in 0.1M HClO.sub.4, according to the examples. Potentials
of interest during startup and shutdown are shown in the shaded
region.
[0020] FIGS. 5A, 5B, and 5C illustrate the design of
O.sub.2-tolerant selective anode catalysts for the hydrogen
oxidation reaction (HOR) by controlling .THETA..sub.calix on
Pt(111), according to the examples. Potentials of interest during
startup and shutdown are shown in the shaded region.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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 .mu.g/cm.sup.2; 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.
[0025] FIG. 10 illustrates the stability of the Pt-calix system at
60.degree. C. in O.sub.2-saturated 0.1 M HClO.sub.4 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
[0026] 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.
[0027] The technology is described herein using several
definitions, as set forth throughout the specification.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] The term "hydroxyl" refers to --OH groups.
[0037] 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.
[0038] 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.
[0039] "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
[0040] 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.
[0041] 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
H.sub.2/O.sub.2 fuel cell (below the dashed line) driving an
electrolytic cell C/O.sub.2 (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 .mu.m 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 H.sub.2/air fuel
cell driving the C/O.sub.2 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/O.sub.2 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.
[0042] 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).
[0043] 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.
[0044] 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, Pt.sub.3Ni, Pt.sub.3Co, Pt.sub.3Fe,
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.
[0045] 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:
##STR00001##
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.times.200 nm.sup.2 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.times.100 nm.sup.2 image of
the calix[4]arene adlayer (surface coverage of .about.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.
[0046] According to some embodiments, each R is H or a
C.sub.1-C.sub.8 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, C.sub.1-C.sub.8 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.
[0047] 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.
[0048] 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 %.
[0049] 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 .mu.M to about
500 .mu.M.
[0050] In another aspect, a Pt-based electrode produced by such
methods is provided.
[0051] 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.
[0052] 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
O.sub.2-tolerant Pt-group metal sites that are very active for the
adsorption of H.sub.2 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.
[0053] 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).times.(100)}and
Pt(110){2(111).times.(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%.
[0054] 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
[0055] 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
[0056] 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.
##STR00002##
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).
[0057] Preparation of Pt(111), Pt(100) and Pt(Poly) and
self-assembly procedures. Pt electrodes were prepared by inductive
heating for 10 min at .about.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 .mu.M,
respectively, in addition to an unmodified Pt sample for each of
the Pt(111), Pt(100), and Pt(Poly).
[0058] For Pt(100) and Pt(Poly) electrodes, 600 .mu.M solutions
were used to demonstrate the validity of the CME approach even at
very high coverages. Calix[4]arene coverages were calculated by
H.sub.upd comparison between the clean Pt(111) surface and
chemically modified surfaces.
[0059] 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.
[0060] 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`.
[0061] 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.sub.ad can be obtained by monitoring how the
fractional coverages (.THETA.) of underpotentially deposited
hydrogen (H.sub.upd)and hydroxyl species (OH.sub.ad) are affected
by .THETA..sub.calix.
[0062] 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
HClO.sub.4. 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.
[0063] FIG. 4A illustrates the effect of .THETA..sub.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.sub.upd in region I,
double layer (region II) and OH.sub.ad adsorption in region III.
FIG. 5B illustrates corresponding charge density versus E curves
for H.sub.upd and OH.sub.ad, which are assessed on the basis of the
assumptions of one H.sub.upd per Pt on the Pt(111)-(1.times.1)
surface and that the H.sub.upd charge on Pt(111) in potential range
I is .about.160 .mu.C cm.sup.-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.sup.-1. FIG. 4A
shows that pseudo-capacitive features corresponding to H.sub.upd
formation, double-layer charging and OH.sub.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
.THETA..sub.calix leads to a marked decrease in availability of Pt
sites, that is, for H.sub.upd and OH.sub.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 .THETA..sub.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-US-00001 TABLE 1 Calix[4]arene coverages and turnover
frequencies (TOF) for the ORR and HOR for five samples. H.sub.upd
Calix Avail- Number of Min. Min. charge cover- able avail- TOF TOF
(.mu.C age surface able sites for ORR for HOR Sample cm.sup.-2)
(%)* (%)* (N cm.sup.-2).sup..dagger. @0.8 V.sup..dagger-dbl. @0.1
V.sup..dagger-dbl. Pt(111) 161 0 100 10.sup.15 9 8 Pt(111)- 26 84
16 1.6 .times. 10.sup.14 9 49 A Pt(111)- 9 94 6 5.5 .times.
10.sup.13 9 129 B Pt(111)- 7 96 4 4.2 .times. 10.sup.13 6 194 C
Pt(111)- 4 98 2 2.4 .times. 10.sup.13 6 388 D *Calix coverages and
available surface were calculated by comparing Hupd charge on the
bare Pt(111) and modified Pt(111).
.sup..dagger.Number of available sites was calculated assuming one
H per Pt adsorption and number of total sites on Pt(111) surface
1.5.times.10.sup.15..sup..dagger-dbl.Values are calculated by
taking the measured current density @0.8 and @0.1, using the
equation TOF=i.sub.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.
[0064] Having illustrated the effect of .THETA..sub.calix on the
formation of H.sub.upd and OH.sub.ad adlayers, the extent to which
the .THETA..sub.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.sub.E.sub.1=nFK.sub.1c.sub.reac(1-.THETA..sub.ad) (2)
where n is the number of electrons, K.sub.1 is a constant,
c.sub.reac. is the concentration of H.sub.2 or O.sub.2 in the
solution and
.THETA..sub.ad=.THETA.H.sub.upd+.THETA..sub.OHad+.THETA..sub.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--O.sub.2 and Pt--H.sub.2 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 H.sub.2 and O.sub.2; that is, the adsorption of H.sub.2 and
O.sub.2 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.sub.E1/nFN, and is summarized in Table 1.
[0065] FIG. 4A shows polarization curves for the HOR in 0.1M
HClO.sub.4 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
.THETA..sub.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.sup.-1 was used
in all experiments.
[0066] 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.sub.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.
[0067] The ORR is a more complex multi-electron reaction in which
O.sub.2 is being reduced in one of the following ways: to water
without peroxide formation (4e.sup.- reduction) or to water with
peroxide formation (a mixed 4e.sup.-+2e.sup.- reduction) or
completely to peroxide via a 2e.sup.- 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 .about.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.sup.- 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 O.sub.2 is reached at the negative
potential limit.
[0068] 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 .THETA..sub.calix (FIG.
5C and Table 1). The inhibition of O.sub.2 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.sub.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 H.sub.2O.sub.2 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.sub.ad. Furthermore, on this surface the
onset potential for peroxide formation is shifted .about.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.sup.-
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 O.sub.2 and the concomitant
formation of H.sub.2O.sub.2.
[0069] 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 O.sub.2 (that
is, the ORR) is much higher than that required for the adsorption
of H.sub.2 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 O.sub.2 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
HClO.sub.4. 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.sub.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
[0070] 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
CaH.sub.2. Reagent grade hexanes, 2-butanone (MEK),
CH.sub.2Cl.sub.2 (DCM), MeOH, and ethyl acetate (EtOAc) were used
without further distillation. Acetyl chloride (AcCI) was heated at
reflux with PCl.sub.5 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. .sup.1H NMR spectra were taken at 300 MHz.
.sup.13C NMR were recorded on the same instrument at 75.5 MHz.
Proton chemical shifts (.delta.) are reported in ppm downfield from
tetramethylsilane (TMS). Carbon was referenced to CDCl.sub.3 (77.23
ppm).
[0071] 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.degree. 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.degree. C., and acetyl chloride
was added and the mixture was allowed to warm to room temperature
and stir over night (.about.12 hours). The reaction mixture was
diluted with CH.sub.2Cl.sub.2 and transferred to a separatory
funnel and a saturated solution of NH.sub.4Cl was added. The
organic phase was separated and the aqueous layer was extracted
with CH.sub.2Cl.sub.2 (3.times.25 mL). The combined organic
extracts were dried over anhydrous Na.sub.2SO.sub.4, and the
solvents were removed in vacuo. The crude product was then purified
by column chromatography on silica gel.
[0072]
5,11,17,23,29,35-Hexabromo-37,38,39,40,41,42-hexabutoxycalix[6]aren-
e (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
.mu.L). The yellow solution was stirred at room temperature for 24
hours. The mixture was then stirred with 10% aqueous NaHSO.sub.3
(23 mL), and CH.sub.2Cl.sub.2 (50 mL) was added. The organic phase
was separated from aqueous phase which was extracted with
CH.sub.2Cl.sub.2 (3.times.20 mL). The combined organic extracts
were dried over anhydrous Na.sub.2SO.sub.4, and the solvents were
removed in vacuo. The crude product was then purified by
recrystallization from cold CH.sub.2Cl.sub.2 to yield colorless
crystals (810 mg, 91% yield). IR (KBr): 2959, 2932, 2870, 1574,
1452, 1381, 1195 cm.sup.-1. .sup.1H NMR (300 MHz, CDCl.sub.3):
.delta. 7.09 (br s, 12H), 3.83 in 3.49 (br s, 24H), 1.34 (br s,
24H), 0.84 (br s, 18H). .sup.13C NMR (75.5 MHz, CDCl.sub.3):
.delta. 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 C.sub.66H.sub.78Br.sub.6O.sub.6, C, 54.79%; H, 5.43%; Found: C,
55.17%; H, 5.62%.
[0073]
5,11,17,23,29,35-Hexakis(thioacetyl)-37,38,39,40,41,42-hexabutoxyca-
lix[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]ar-
ene 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.sup.-1. .sup.1H NMR (300 MHz, CDCl.sub.3):
.delta. 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). .sup.13C NMR (75.5 MHz,
CDCl.sub.3): .delta. 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 C.sub.78H.sub.96O.sub.12S.sub.6+Na.sup.+, 1439.5124;
found, 1439.5130.
[0074]
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.sup.-1. .sup.1H NMR (300 MHz, CDCl.sub.3): .delta.
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).
.sup.13C NMR (75.5 MHz, CDCl.sub.3): .delta. 194.9, 156.9, 135.4,
135.0, 122.9, 73.6, 32.4, 30.2, 19.4, 14.1. HRMS: calcd for
C.sub.104H.sub.128O.sub.16S.sub.8+H+, 1889.7046; found,
1889.7058.
[0075] 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 .mu.M to obtain samples with very high coverages of calix
on Pt surface. Coverages were estimated from the H.sub.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.
[0076] 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.sup.-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.degree. 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 .mu.M 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.
[0077] 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 HClO.sub.4.
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.
[0078] 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.sub.Et=nFK.sub.1c.sub.reac.(1-.THETA..sub.ad) (2)
where n is the number of electrons, K.sub.1 is a constant,
c.sub.reac is the concentration of H.sub.2 or O.sub.2 in the
solution and
.THETA..sub.ad=.THETA.H.sub.upd+.THETA..sub.OHad+.THETA..sub.calix
is the fraction of the surface masked by the site-blocking species,
that is, reaction intermediates are adsorbed at low coverages.
[0079] 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 .about.6-8% for nanocatalyst
surfaces (estimated from the H.sub.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.
[0080] 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 .THETA..sub.ad , surface
coverage of adsorbed species, is very large thereby driving the
kinetics for the ORR, driven by the (1-.THETA..sub.ad), to very
small values. This suggests that the ensemble of sites
necessary/required for adsorbing O.sub.2 and for performing ORR are
limited in number which results in low activities for O.sub.2
reduction.
[0081] NSTF vs. TKK H.sub.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.sub.updranged
from 78% to 90% going from TKK-calix system to NSTF-calix
systems.
[0082] 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.
[0083] 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.sub.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.
[0084] 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.
[0085] 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.degree. 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.
[0086] 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.
[0087] 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
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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.
* * * * *