U.S. patent application number 15/380098 was filed with the patent office on 2017-06-15 for dopant alloying of titanium to suppress oxygen reduction catalysis.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Rachel Anderson, Carlos Hangarter, Derek Horton, Steve Policastro, James A. Wollmershauser.
Application Number | 20170167034 15/380098 |
Document ID | / |
Family ID | 59019572 |
Filed Date | 2017-06-15 |
United States Patent
Application |
20170167034 |
Kind Code |
A1 |
Policastro; Steve ; et
al. |
June 15, 2017 |
DOPANT ALLOYING OF TITANIUM TO SUPPRESS OXYGEN REDUCTION
CATALYSIS
Abstract
An alloy having the formula Ti.sub.1-xM.sub.x. M is Co, Sn, Cr,
or a combination. The value x is from 0.001 to 0.02. A method of
combining titanium metal and a dopant metal to form the alloy.
Inventors: |
Policastro; Steve; (Waldorf,
MD) ; Horton; Derek; (Alexandria, VA) ;
Hangarter; Carlos; (Alexandria, VA) ; Wollmershauser;
James A.; (Alexandria, VA) ; Anderson; Rachel;
(Alexandria, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Arlington
VA
|
Family ID: |
59019572 |
Appl. No.: |
15/380098 |
Filed: |
December 15, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62267457 |
Dec 15, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 14/00 20130101 |
International
Class: |
C23F 11/18 20060101
C23F011/18; C22C 14/00 20060101 C22C014/00 |
Claims
1. A composition comprising: an alloy having the formula
Ti.sub.1-xM.sub.x; wherein M is a dopant selected from Co, Sn, Cr,
and combinations thereof; wherein x is from 0.001 to 0.02.
2. The composition of claim 1; wherein M is Co, Sn, or Cr; and
wherein x is from 0.005 to 0.015.
3. The composition of claim 1, wherein the composition comprises at
least 90 wt % of the alloy.
4. The composition of claim 1, wherein the composition comprises at
least 99 wt % of the alloy.
5. An article comprising: the composition of claim 1; and an oxide
of the alloy comprising the dopant on a surface of the article.
6. The article of claim 5, wherein the article is a fastener.
7. A method comprising: positioning the article of claim 5 in
electrical contact with a metal component; and positioning the
article in contact with an electrolyte that is in contact with the
metal component; wherein the metal component has a lower electrode
potential in the electrolyte than titanium.
8. A method comprising: combining titanium metal and a dopant metal
to form an alloy having the formula Ti.sub.1-xM.sub.x; wherein M is
selected from Co, Sn, Cr, and combinations thereof; wherein x is
from 0.001 to 0.02.
9. The method of claim 8; wherein M is Co, Sn, or Cr; and wherein x
is from 0.005 to 0.015.
10. The method of claim 8, further comprising: forming an article
comprising the alloy.
11. The method of claim 8, wherein the article comprises at least
90 wt % of the alloy.
12. The method of claim 8, wherein the article comprises at least
99 wt % of the alloy.
13. The method of claim 10, further comprising: forming an oxide
comprising the dopant on the surface of the article.
14. The method of claim 13, wherein the article is a fastener.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/267,457, filed on Dec. 15, 2015. The provisional
application and all other publications and patent documents
referred to throughout this nonprovisional application are
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present disclosure is generally related to titanium
alloys.
DESCRIPTION OF RELATED ART
[0003] Galvanic corrosion generally refers to the corrosion damage
that occurs when two dissimilar metals are electrically connected
in the presence of a corrosive electrolyte (Jones, Principles and
Prevention of Corrosion, Upper Saddle River, New Jersey: Prentice
Hall, 1996, pp. 229-230; Mansfeld et al., "Galvanic corrosion of Al
alloys--I. Effect of dissimilar metal" Corrosion, vol. 30, pp.
343-353, 1974; Mansfeld et al., "Galvanic corrosion of Al
alloys--II. Effect of solution composition" Corrosion Science, vol.
15, pp. 183-198, 1975). It has been observed that titanium-alloy
fasteners contribute to increased corrosion in structural aluminum
alloys, on aircraft exposed to the environment, due to galvanically
driven corrosion (Matzdorf et al., "Galvanic test panels for
accelerated corrosion testing of coated al alloys: part I--concept"
Corrosion, vol. 69, pp. 1240-1246, 2013; Feng et al., "Galvanic
test panels for accelerated corrosion testing of coated al alloys:
part II--measurement of galvanic interaction" Corrosion, vol. 70,
pp. 95-106, 2014). The corrosive electrolytes in atmospheric
environments are aqueous in nature; that is, water acts as the
solvent for various ionic and gaseous constituents that then impact
anodic and cathodic reaction rates. The ionic components of the
atmospheric electrolytes are commonly the result of various kinds
of salt aerosols while the gaseous constituents (CO.sub.2, etc.)
diffuse in at the electrolyte-atmosphere boundary (Nishikata et
al., "Influence of electrolyte layer thickness and pH on the
initial stage of the atmospheric corrosion of iron" Journal of the
Electrochemical Society, vol. 144, pp. 1244-1252, 1997; Vera Cruz
et al., "Pitting corrosion mechanism of stainless steels under
wet-dry exposure in chloride-containing environments" Corrosion
Science, vol. 40, pp. 125-139, 1998; Wang et al., "Atmospheric
corrosion of aluminium alloy 2024-T3 exposed to salt lake
environment in Western China" Corrosion Science, vol. 59, pp.
63-70, 2012; Young et al., "Stages of damage evolution for al
2024-T3 around fasteners in marine atmosphere" Corrosion, vol. 71,
pp. 1278-1293, 2015).
[0004] Aqueous corrosion, both fully immersed and atmospheric,
requires a coupled oxidation-reduction reaction, which may occur on
neighboring regions on the same surface, an electronic conduction
path, and the aforementioned water-based electrolyte for ionic
transport between the reaction sites. In the case of galvanic
corrosion, the dominant sites for metal oxidation and oxygen
reduction are located on the different materials of the galvanic
couple; thereby requiring the establishment of an electron
conduction path that extends:
[0005] From the oxidation site
[0006] Through the bulk of the active metal or alloy
[0007] Across the galvanic junction
[0008] Through the bulk of the noble metal or alloy
[0009] Across the noble metal oxide
[0010] To the reduction reaction site
[0011] In the case of electrolytes that arise from atmospheric
processes, the electrolyte is usually aqueous, thin, and may be
localized on the surface. Chemistries and pH in the electrolyte can
vary greatly and the oxygen content of the electrolytes can be
high. The discrete nature of the atmospheric electrolyte makes
cathodic protection difficult and galvanic corrosion damage under
these conditions can be difficult to detect. FIG. 1 illustrates the
components and processes of atmospheric galvanic corrosion. The
junction of the two plates of metal 1 with a noble metal fastener
establishes the galvanic junction of dissimilar metals that can
cause corrosion once a droplet of water forms on the surface. The
high surface area-to-volume ratio of the droplet allows a high
dissolved oxygen concentration to obtain even once the reduction
reaction begins consuming oxygen. As a result, the electrolyte near
the cathode becomes alkaline while the electrolyte near the anode
becomes more acidic.
[0012] The corrosion rate of the more active material in a galvanic
couple is strongly influenced by such conditions as the presence of
aggressive anions, the amount of exposed surface area of the
cathode and by the ability of the cathode to support a suitable
reduction reaction, including hydrogen evolution and oxygen
reduction as shown in Table 1 (Hamann et al., Electrochemistry,
Weinheim: Wiley-VCH, 1998; Curioni et al., "The Mechanism of
Hydrogen Evolution during Anodic Polarization of Aluminium"
Electrochimica Acta, vol. 180, pp. 712-721, 2015). In the case of
noble fasteners in structural materials, the overall corrosion
damage is determined by, among other influences, the ability of the
noble materials to support cathodic reactions (Mansfeld et al.,
"Galvanic corrosion of bare and coated Al alloys coupled to
stainless steel 304 or Ti-6Al-4V" Corrosion Science, vol. 13, pp.
605-621, 1973; Zhang et al., "Transitions between pitting and
intergranular corrosion in AA2024" Electrochimica Acta, vol. 48,
pp. 1193-2010, 2003; Zhou et al., "Study of localized corrosion in
AA2024 aluminium alloy using electron tomography" Corrosion
Science, vol. 58, pp. 299-306, 2012). The two reduction reaction
rates are themselves influenced by the concentrations of the
reacting species in the electrolyte and by the catalytic properties
of the material surface. In the case of the oxide on titanium, the
electronic structure plays an important role in determining the
kinetics of the reduction reactions.
TABLE-US-00001 TABLE 1 Reactions, pH ranges, and potential ranges
for reduction reactions pH Potential Reaction range range
(V.sub.SHE) 1/2O.sub.2 + H.sub.2O + 2e.sup.- 20H.sup.- 7-14
+0.820-+0.401 2H.sub.2O + 2e.sup.- H.sub.2 + 20H.sup.- 7-14
-0.413--0.828
BRIEF SUMMARY
[0013] Disclosed herein is a composition comprising: an alloy
having the formula Ti.sub.1-xM.sub.x. M is a dopant selected from
Co, Sn, Cr, and combinations thereof. The value x is from 0.001 to
0.02.
[0014] Also disclosed herein is a method comprising: combining
titanium metal and the dopant metal to form the above alloy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A more complete appreciation will be readily obtained by
reference to the following Description of the Example Embodiments
and the accompanying drawings.
[0016] FIG. 1 shows an illustration of galvanic corrosion in an
atmospheric environment.
[0017] FIG. 2 shows a representation of the metal oxide-electrolyte
interface as an n-type semi-conductor energy-level diagram overlaid
on an array of redox potentials.
[0018] FIG. 3 shows a schematic illustration of the simplified
solution-oxide-metal system.
[0019] FIG. 4 shows ORR reaction energies for four different
surface sites (labeled 1-4) plotted at an applied potential of 0
and 1.23 VSHE. The intermediates correspond to the reactions in
Eqs. (1)-(4). The four curves from top to bottom are 2, Site 3,
Site 4, and Site 1.
[0020] FIG. 5 shows a Sabatier volcano plot of computationally
predicted dopant overpotentials. Dopants that were predicted and
tested are V(III), Mn(II), Cr(III), Co(II), Ag(I), Undoped, Sn(IV),
V(V), and Al(III), and dopants not yet experimentally verified are
Ni(III), Nb(V), Cu(II), Ge(IV), Zn(II), Ga(III), and Si(IV).
[0021] FIG. 6 shows a plot of annealed alloy unit cell volume as a
function of dopant concentration for Ti--V, Ti--Sn, Ti--Co, and
Ti--Cr alloys as determined by XRD.
[0022] FIG. 7 shows plots of the cathodic polarization curves for
Ti and the three binary alloys: Ti.sub.99Co.sub.1,
Ti.sub.99Sn.sub.1, and Ti.sub.99Cr.sub.1, in 0.6 M NaCl+0.01 M KOH
solution, highlighting regions of the current response to the
applied potential where different reaction mechanisms were
dominant. The four curves from top to bottom on the left are
Ti.sub.99Co.sub.1, pure Ti, Ti.sub.99Cr.sub.1, and
Ti.sub.99Sn.sub.1.
[0023] FIG. 8 shows a plot of |Z.sub.imag| vs. frequency for pure
titanium and the three titanium alloys in 0.1 M KOH. The single
time constant response to the voltage perturbation suggests that a
constant-phase element equivalent circuit is applicable for
capturing the oxide behavior.
[0024] FIG. 9 shows a Mott-Schottky plot of C.sup.-2 vs. applied
potential for Ti and the three binary alloys: Ti.sub.99Co.sub.1,
Ti.sub.99Sn.sub.1, and Ti.sub.99Cr.sub.1 in 0.1 M KOH solution.
[0025] FIG. 10 shows a plot of I.sub.mod as a function of frequency
for the pure titanium sample for light wavelengths from 310 nm to
1450 nm and modulation frequencies from 5000 Hz to 0.1 Hz.
[0026] FIG. 11 shows overlays of the photoelectrochemical responses
of pure Ti, Ti.sub.99Co.sub.1, Ti.sub.99Sn.sub.1, and
Ti.sub.99Cr.sub.1 to 310 nm wavelength UV light with modulation
frequencies from 5000 Hz to 0.1 Hz.
[0027] FIG. 12 shows a plot of current density vs. potential for
pure Ti, Ti.sub.99Co.sub.1, Ti.sub.99Sn.sub.1, and
Ti.sub.99Cr.sub.1 in 0.6 M NaCl with 0.01 M KOH at 1600 RPM with a
scan rate of 5 mV/s.
[0028] FIG. 13 shows an equivalent circuit diagram for a
constant-phase-element model of a metal oxide-solution
interface.
[0029] FIG. 14 shows an example Mott-Schottky plot of 1/C.sup.2 vs.
potential for Ti.sub.99Cr.sub.1. Expressions for the slope and
intercept are given that are related to the flatband potential and
donor density.
[0030] FIG. 15 shows normalized I.sub.mod as a function of photon
energy for the oxide on Ti.sub.99Sn.sub.1.
[0031] FIG. 16 shows a plot of (I.sub.mod.times.h.nu.).sup.0.5 vs.
photon energy for the different oxides. The linear best-fit line
for each oxide was determined by fitting through the
(I.sub.mod.times.h.nu.).sup.0.5 values from 3.5 eV to 2.7 eV.
[0032] FIG. 17 shows the overlay of band energies for the various
oxides on redox potentials to illustrate reduction reactions that
can be catalyzed on the oxides by electrons supplied from corrosion
oxidation reactions.
[0033] FIG. 18 shows cathodic polarizations scans of the undoped
titanium and all of the 1 at % doped titanium samples. The curves
from top to bottom on the left are undoped, Sn, Cr, V, Ag, Co, Mn,
and Al.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0034] In the following description, for purposes of explanation
and not limitation, specific details are set forth in order to
provide a thorough understanding of the present disclosure.
However, it will be apparent to one skilled in the art that the
present subject matter may be practiced in other embodiments that
depart from these specific details. In other instances, detailed
descriptions of well-known methods and devices are omitted so as to
not obscure the present disclosure with unnecessary detail.
[0035] Disclosed is a method to alter the electronic structure of
the native oxides formed on titanium by doping (alloying in low
concentrations) with specific elements, such as chromium and tin,
in order to disrupt the oxygen reduction reaction (ORR) and inhibit
corrosion in galvanic couples with these doped alloys.
[0036] Many structural aircraft alloys, such as AA2024 and AA7075,
have stable, corrosion resistant oxides. However, corrosion damage
is frequently seen in areas that are near galvanic contacts between
the aluminum alloys and fasteners made of titanium or steel alloys.
The fastener material, which is more noble than the aluminum,
catalyzes the reduction reaction from species present in the
electrolyte and drives oxidation in the structural alloy.
[0037] The corrosion rate of the more active material is strongly
influenced by the exposed surface area of the cathode and by the
ability of the noble material to support a suitable reduction
reaction such as hydrogen evolution or oxygen reduction. These two
reduction reaction rates are themselves influenced by the
concentrations of the reacting species in the electrolyte and by
the catalytic properties of the material surface. Of particular
interest are metal oxides in which the oxide microstructure plays a
role in affecting the kinetics of the reduction reactions.
[0038] The protective oxide that forms on titanium is of interest
because it is thermodynamically stable up to +1.5 V.sub.SHE (+1.25
V.sub.SCE) at pH 12 (Pourbaix, Atlas of Electrochemical Equilibria
in Aqueous Solutions, Houston: NACE, 1974). For the alkaline
conditions expected when acting as the noble material in a galvanic
couple, the electrochemical properties of the oxide dominate the
metal's cathodic behavior. In an alkaline environment, both oxygen
reduction and the hydrogen evolution reactions are expected to be
catalyzed on the oxide.
[0039] FIG. 2 illustrates a general framework for considering a
metal oxide-electrolyte system (Rajeshwar, "Charge transfer in
photoelectrochemical devices via interface states: unified model
and comparison with experimental data" Journal of the
Electrochemical Society, vol. 129, pp. 1003-1008, 1982). The
natively-formed metal oxide is represented by the energy-level
diagram of an n-type semi-conductor. The electrolyte is represented
by a line of relative redox potentials and adsorbed surface states
at the oxide-electrolyte interface. Electrons released in the
oxidation reaction are presumed to travel across the galvanic
junction and eventually scatter into the noble metal oxide
conduction band and then into an adsorbed surface state in order to
participate in the requisite reduction reaction.
[0040] This framework raises the question to what extent the
electrons scattered into the conduction band can be trapped or
disrupted from crossing the oxide-electrolyte interface and thereby
prevented from participating in reduction reactions. As an example,
some microstructural defects in titanium oxide are known to act as
traps for photoexcited electrons, thereby inhibiting catalysis
(Muhich et al., "Increasing the photcatalytic acitivity of anatase
TiO.sub.2 through B,C, and N doping" Journal of Physcial Chemisty
C, vol. 118, pp. 27415-27427, 2014). The approach taken here for
creating microstructural defects in the titanium oxide band
structure was to introduce dopant atoms of different valence states
or radii into the titanium oxide in order to disrupt reduction
reaction rates and thereby reduce the active metal corrosion rate
in associated galvanic couples.
[0041] Pure titanium was selected because, as a valve metal, it
formed a very stable oxide that was well characterized and the
titanium alloy, Ti 6Al-4V, has numerous aircraft applications. The
oxide that forms on pure titanium is thermodynamically stable up to
a range of .about.+1.6-+1.4 V.sub.SHE (+1.36-+1.16 V.sub.SCE) for
pH 10-13 and, thus, for the alkaline conditions expected to obtain
when acting as the noble material in a galvanic couple, the
electrochemical properties of the oxide dominate its cathodic
behavior. A schematic illustration of the electrochemical system is
shown in FIG. 3.
[0042] Computer simulations of oxygen reduction reactions on
various transition metal oxides (Man et al., "Universality in
oxygen evolution electrocatalysis on oxide surfaces" ChemCatChem,
vol. 3, pp. 1159-1165, 2011) suggested that cobalt, tin, and
chromium would be useful elements to consider as alloying elements
in titanium. The electrochemical properties of the oxides of the
various alloys were investigated using potentiodynamic
polarization, rotating-disk-electrode (RDE) cyclic voltammetry,
electrochemical impedance spectroscopy (EIS), Mott-Schottky (MS)
tests, and intensity modulated photocurrent spectroscopy (IMPS)
experiments.
[0043] Computational chemistry was used to obtain guiding design
principles. Computational catalysis studies often report calculable
thermodynamic descriptors and Sabatier volcano curves to identify
optimal catalysts near the top of the activity volcano (see e.g.,
Norskov et al., J. Phys. Chem. B 2004, 108, 17886-17892; Fabbri et
al., J. Catal. Sci. Tech. 2014, 4, 3800-3821; Greeley et al.,
Energy Environ. Sci. 2012, 5, 9246-9256; Morales-Guio et al., Chem.
Soc. Rev. 2014, 43, 6555-6569.). While this level of modeling works
well in predicting dopants that maximize the catalytic activity of
a material, it remains an open question if these in silico models
are also robust for predicting dopants that minimize catalytic
activity, i.e. dopants that lie near the bottom of the Sabatier
volcano plots. This work confirms that computational catalysis
modeling is also robust in this regime. Disclosed is an integrated
computational and experimental study that demonstrates that simple
Sabatier volcano descriptors can be used to identify metal dopants
that decrease oxygen reduction currents by as much as 77% when
impregnated in amorphous TiO.sub.2 at doping concentrations of
1%.
[0044] Calculating reaction overpotentials with the computational
hydrogen electrode model (Norskov et al., J. Phys. Chem. B 2004,
108, 17886-17892) is routinely the first step toward understanding
electrocatalytic activity. Although this model is not normally used
for calculating reaction barrier heights and rate constants, it can
yield robust insight into trends in electrocatalytic reaction
rates. This methodology was used to calculate reaction
overpotentials for the dissociative ORR mechanism shown. (The
symbol "*" denotes an empty surface site on the material.) Because
the hydrogen evolution reaction (1/2 H.sub.2H.sup.++e.sup.-) is in
equilibrium at 0 V vs. the standard hydrogen electrode (VSHE), the
energies of protons and electrons in electrochemical reduction
steps were modeled as half the energy of an H.sub.2 molecule plus a
linear energy correction to account for an applied potential. Using
these energy corrections, the theoretical reaction overpotential
was calculated by finding the applied potential at which all four
reaction steps are downhill in energy. Mathematically, this was
determined by the most uphill reaction step at the equilibrium
potential for the ORR (1.23 VSHE). For example, sites 1-4 in FIG. 4
had predicted overpotentials of 0.50, 0.95 1.01, and 1.24 V.
*+O.sub.2+H.sup.++e.sup.-*OOH (1)
*OOH+H.sup.++e.sup.-*O+H.sub.2O (2)
*O+H.sup.++e.sup.-*OH (3)
*OH+H.sup.++e.sup.+*+H.sub.2O (4)
[0045] Calculated energies from Kohn-Sham density functional theory
(DFT) were obtained using the Vienna ab initio simulation package
(VASP) (Kresse et al., J. Phys. Rev. B 1996, 54, 11169-11186;
Kresse et al., J. Comput. Mater. Sci. 1996, 6, 15-50; Kresse et
al., J. Phys. Rev. B 1994, 49, 14251-14269; Kresse et al., J. Phys.
Rev. B 1993, 47, 558-561) utilizing the Perdew-Burke-Ernzerhof
(PBE) (Perdew et al. Phys. Rev. Lett. 1996, 77, 3865-3868; Perdew
et al., Phys. Rev. Lett. 1997, 78, 1396-1396) GGA exchange
correlation functional and the projector augmented wave (PAW)
method (Blochl, Phys. Rev. B 1994, 50, 17953-17979; Kresse et al.,
Phys. Rev. B 1999, 59, 1758-1775) with spin polarization. Planewave
energy cutoffs of 450 eV and a 2.times.2.times.1 k-point grid gave
well-converged intermediate energies. The zero point energy,
entropic, and solvation free energy contributions were approximated
by using the values predicted by Valdes et al., J. Phys. Chem. C
2008, 112, 9872-9879 for the ORR intermediates adsorbed to
TiO.sub.2.
[0046] An atomistic reactive forcefield (ReaxFF) (Kim et al.,
Langmuir 2013, 29, 7838-7846; van Duin et al., J. Phys. Chem. A
2001, 105, 9396-9409) was used to create an amorphous oxide surface
model as had been done by others (Ewing et al., Langmuir 2014, 30,
5133-5141). Crystalline TiO.sub.2 surfaces were annealed using
ReaxFF (Aktulga et al., Parallel Comput. 2012, 38, 245-259) as
implemented in LAMMPS, (Plimpton, Comput. Phys. 1995, 117, 1-19)
and the resulting annealed structure was then geometrically relaxed
using Kohn-Sham density functional theory (DFT) calculations as
described below. Unit cells of these systems containing about 160
atoms were found to reasonably match experimental x-ray diffraction
patterns for TiO.sub.2 nanoparticles showing this to be a
reasonable model for an amorphous TiO.sub.2 surface (Petkov et al.,
Non-Cryst. Solids 1998, 231, 17-30). On this surface, four possible
sites were found accessible for ORR catalysis (FIG. 4). Modeling
the ORR reaction energies on these four sites yielded
overpotentials that vary by nearly 0.8 V, but the most active site
(Site 1) had a predicted overpotential in good agreement with the
experimental overpotential for TiO.sub.2
(.eta..sub.predicted.sup.ORR=0.5 V and .eta..sub.exp..sup.ORR 0.45
V) (Arashi et al., Catal. Today 2014, 233, 181-186). This validated
that the amorphous TiO.sub.2 model could be used to study ORR
mechanisms.
[0047] Different metal dopants were considered that could be
introduced into the oxide to increase ORR overpotentials. Each
dopant atom was embedded into the surface at its preferred
oxidation state given by experimental Pourbaix diagrams (Takeno,
Geological survey of Japan open file report 2005, 419, 1-102) at
the corrosion experiment operating conditions, -0.8 V vs the
saturated calomel electrode (VSCE) at pH 12. The stability of each
dopant was compared at all four different active sites shown in
FIG. 4 to identify the most stable substitution site. It was
assumed that the most thermodynamically stable site would reflect
the atomic configuration that would be least likely to reconstruct
and therefore participate in electrochemical ORR. Following work by
Carter and co-workers (Liao et al., J. Am. Chem. Soc. 2012, 134,
13296-13309), the maximum impact of each dopant on the ORR activity
was predicted by modeling the ORR intermediates adsorbed directly
to the dopant atom embedded in the amorphous surface at its most
stable site.
[0048] The predicted overpotential for each metal dopant is
displayed in a Sabatier volcano diagram (FIG. 5). Unlike work in
fuel cell catalysis where the ideal catalyst is found at the top of
the activity volcano, dopants at the bottom of the volcano that
would optimally reduce ORR rates are or more interest. Even though
the ORR overpotential directly on the doped site was calculated,
the reaction activity will be controlled by the most active sites
on the surface. It is known that dopants in oxides can affect
adsorbate binding energies multiple sites away (Liao et al., J. Am.
Chem. Soc. 2012, 134, 13296-13309). However, the influence of the
dopant on the overpotential is expected to decrease when the dopant
is further away from the binding site. Thus, even though the
overpotential is site dependent and not all of the sites will have
as high an overpotential as shown in FIG. 5, the overall trend for
how dopants affect ORR activity will be reflected by the activity
volcano.
[0049] The alloys include titanium and a dopant metal that may be
any of cobalt, tin, chromium, aluminum, manganese, vanadium,
silver, any other metal disclosed herein, any other metal that
reduces galvanic corrosion when used as described herein, or any
combination of these metals. The alloy may be free of any other
metals, or it may contain a trace amount of contaminating metals.
The contaminants may be present in an amount less than the total
molar amount of the dopant, less than 1 mol %, or less than 0.1 mol
%. The alloy has the formula Ti.sub.1-xM.sub.x, where M is the
dopant or dopants and x is from 0.001 to 0.02, or from 0.005 to
0.015. The alloy may have the formula Ti.sub.99M.sub.1. The alloy
may be included in a composition that is at least 90 or 99 wt % of
the alloy. The alloys may be made by the methods disclosed herein
or by any other method for producing alloys.
[0050] As titanium tends to oxidize on exposure to air, an article
made from the alloy or a composition containing the alloy will have
an oxide on its surface. The oxide includes atoms of both titanium
and the dopant. Such an article may be a fastener such as a bolt or
a screw.
[0051] When the fastener is used for fastening metal components, it
will typically be positioned in electrical contact with one of the
metal components. Electrolytes, such as aqueous electrolytes, may
come in electrical contact with both the fastener and the metal
component. For example, rainwater or condensation may collect to
form a droplet electrical connecting the metal component and the
fastener. When the metal component has a lower electrode potential
in the electrolyte than titanium, galvanic corrosion may occur in
the metal. However, the amount of corrosion is reduced due to the
presence of the dopant.
[0052] An advantage of this process is that the titanium oxide
spontaneously forms on contact with air because of the high
reactivity of pure titanium and the oxide is very tough so that it
is resistant to damage and can re-form on its own. In addition, by
bulk alloying the desired dopant atoms, the dopant atoms are
incorporated into the oxide when it forms, rather than needing to
be reapplied from the outside.
[0053] The following examples are given to illustrate specific
applications. These specific examples are not intended to limit the
scope of the disclosure in this application.
[0054] Titanium (99.995% purity) and Ti-based minor solute alloy
ingots were produced at solute concentration of 1 at % using the
arc-melting technique using high purity metals (Co 99.995%, Sn
99.999%, Cr 99.996%). Ingots were subsequently suction cast into a
custom copper mold consisting of two cylindrical regions of 1 cm
and 0.6 cm diameter the latter for RDE test specimens, which were
then machined and ground to 4 mm discs of 5.05 mm diameter. After
casting and machining, for Ti, Ti.sub.99Sn.sub.1, and
Ti.sub.99Cr.sub.1, a four hour solution anneal at 827.degree. C.,
within the single phase HCP region, was performed followed by a
water quench. In the case of Ti.sub.99Co.sub.1, a four hour
solution anneal at 685.degree. C. was used due to the shift in the
HCP single phase field to a lower temperature and lower solubility
(Murray, "The Co--Ti(Cobalt-Titanium) system" Bulletin of Alloy
Phase Diagrams, vol. 3, pp. 74-85, 1982). Similar alloys of Ag, Al,
Mn, and V were also made by the same or similar methods. X-ray
diffraction, Cu k-alpha, was used to verify single phase structure
and determine the lattice coefficients using whole pattern fits.
Prior to electrochemical testing, samples were polished in
successively finer grits to 1200 grit using SiC paper. For baseline
electrochemical testing, samples were mounted in insulating
epoxy.
[0055] Potentiodynamic polarization characterization of the various
titanium alloys was performed in 0.6 M NaCl+0.01M KOH (pH 12)
electrolyte with a platinum wire counter electrode and a saturated
calomel reference cell. The titanium alloy oxides were equilibrated
at room temperature and ambient aeration for 1 hour. After an
18-hour open circuit (OC) hold, the potentiodynamic polarizations
were performed over a range of potentials starting from +0.02 V
above the equilibrium potential to -1.5 V.sub.SCE or -2.0 V.sub.SCE
using a graphite counter electrode. The potentials were stepped at
a rate of -0.167 mV/sec.
[0056] Electrochemical impedance spectroscopy (EIS)
characterization of the various titanium alloys was performed in
0.1 M KOH (pH 13) with a platinum wire counter electrode and a
silver-oxide pseudo-reference cell (Austin et al., "Review of
fundamental investigations of silver oxide electrodes" United
States Army Materiel Commande, Washington, D.C., 1965). The
titanium alloy oxides were equilibrated at room temperature and
ambient aeration for 1 hour. The EIS scans were performed at the
oxides' equilibrium potentials with a 5 m V.sub.RMS perturbation at
frequencies from 100 kHz to 0.1 Hz.
[0057] Mott-Schottky analysis tests of the various oxides were
performed in 0.1 M KOH (pH 13) with a platinum wire counter
electrode and a silver-oxide pseudo-reference cell. The titanium
alloy oxides were equilibrated at room temperature and ambient
aeration for 1 hour. The applied DC potentials were stepped in 100
mV increments from +0.5 V.sub.SCE to -1.5 V.sub.SCE. The
oscillating potential was applied at 100 Hz at .+-.5 m
V.sub.RMS.
[0058] Intensity modulated photocurrent spectroscopy (IMPS)
characterization (Peter, "Dynamic aspectgs of semiconductor
photoelectrochemistry" Chemical Reviews, vol. 90, pp. 753-769,
1990) of the various oxides was also performed in 0.1 M KOH (pH 13)
with a platinum wire counter electrode and a silver-oxide
pseudo-reference cell. The titanium alloy oxides were equilibrated
at room temperature and ambient aeration for 1 hour. Two
potentiostats were used for these characterizations. The primary
potentiostat applied the baseline DC current and AC modulation to
the LED while the secondary potentiostat received the clock signal
from the primary and measured the response of the sample. The AC
current supplied to the LED varied in frequency from 5000 Hz to 0.1
Hz while the magnitude of the oscillation was varied to account for
the maximum current permitted through the LED. A dozen LEDs,
identified by the emitted wavelength with the highest intensity,
were used that spanned the spectrum from infrared to ultraviolet
light: 1450 nm, 1200 nm, 850 nm, 660 nm, 617 nm, 590 nm, 530 nm,
505 nm, 470 nm, 420 nm, 405 nm, and 310 nm.
[0059] Lastly, cyclic voltammetry (CV), using rotating disc
electrode (RDE) measurements, of the catalytic activity of the
various oxides were performed in 0.6 M NaCl+0.01 M KOH (pH 12)
under conditions in which the electrolyte was aerated with
pressurized air and de-oxygenated with nitrogen. The discs were
rotated at 100, 400, 900, 1600, and 2500 rpm. The potential was
scanned at a rate of 5 mV/sec from +0.25 V.sub.SCE to -1.75
V.sub.SCE.
[0060] Alloy Characterization--
[0061] Diffraction indicated that annealing Ti, Ti.sub.99Sn.sub.1,
Ti.sub.99Cr.sub.1, and Ti.sub.99Co.sub.1 produced a single HCP
phase. Unit cell determinations based on the diffraction results is
shown in FIG. 6. For alloying additions of Cr and Sn, a clear
decrease unit cell volume is seen. In the case of
Ti.sub.99Co.sub.1, the HCP lattice expands slightly upon alloying
likely due to Co having an atomic radius >20% smaller than Ti
and adopting an interstitial alloying site. In the case of both Cr
and Sn the atomic radii are only slight smaller than Ti
(.about.13%) and substitutional alloying would likely reduce the
unit cell volume. In either case, the trend in unit cell volumes
follows Hume-Rothery rules.
[0062] Potentiodynamic Polarization Scans--
[0063] Examples of the results from the cathodic polarizations for
each of the oxides are shown in FIG. 7. Various regions of the
current response of each oxide to the applied potential are
indicated on the plot, including the most likely dominant reduction
reaction and the most likely controlling mechanism. Each alloy
shows clear mixed activation and diffusion control regimes for
oxygen reduction at potentials above -1 V.sub.SCE and a clearly
defined mass transport limited current density at potentials
between -1 V and -1.4 V.sub.SCE before hydrogen reduction dominates
the cathodic reaction at lower potentials. Cathodic current
densities of each alloyed titanium show a reduction in the mass
transport limiting current density.
[0064] EIS and Mott-Schottky Scans--
[0065] A modified Bode plot of the oxide responses to the EIS scans
for pure titanium and the alloys are shown in FIG. 8. |Z.sub.imag|
is plotted as a function of frequency in order to detect
capacitance or inductance effects in the oxide response to the
oscillating applied potential. For each alloy, a constant phase
element equivalent circuit is applicable for use in analyzing the
oxide response.
[0066] Examples of the results from the Mott-Schottky tests for
each of the oxides are shown in FIG. 9. The increases in 1/C.sup.2
as potential increased from .about.-0.3 V.sub.SCE to .about.1.1
V.sub.SCE indicate a thickening of the oxide. The maxima in
1/C.sup.2 for all of the response curves around 1.2 V.sub.SCE is
most likely due to oxygen formation on the oxide, while the minima
near -0.4 V.sub.SCE indicate changes in the electronic structure of
the oxide.
[0067] IMPS Scans--
[0068] The photoelectrochemical response, as a Bode plot
representation, of the titanium oxide to a range of photon energies
and light modulation frequencies is shown in FIG. 10. The frequency
corresponding to the maximum photo-currents indicate time-constants
ranging from 2-8.times.10.sup.-3 s for electron transport to the
interfacial reaction sites under the assumption that the
electron-hole recombination rate is slowest at that modulation
frequency. The long tails, especially for the ultraviolet
wavelengths at the low modulation frequencies indicate that the
light intensity was too high.
[0069] The photoelectrochemical response, using a Nyquist plot
representation, of the oxides of pure Ti, Ti.sub.99Co.sub.1,
Ti.sub.99Sn.sub.1, and Ti.sub.99Cr.sub.1 to 310 nm wavelength UV
light with modulation frequencies from 5000 Hz to 0.1 Hz is shown
in FIG. 11. The Nyquist plots are in keeping with the
Ponomareve-Peter model for charge transport in the oxide band
structure (Ponomarev et al., "A generalized theory of intensity
modulated photocurrent spectroscopy (IMPS)" Journal of
Electroanalytical Chemistry, vol. 396, pp. 219-226, 1995.
[0070] CV Measurements--
[0071] In deoxygenated and aerated environments, portions of the CV
scans on pure titanium, Ti.sub.99Co.sub.1, Ti.sub.99Sn.sub.1, and
Ti.sub.99Cr.sub.1 are shown in FIG. 12. The cathodic-to-anodic
portion of the second potential sweep is shown and the y-axis has a
log scale instead of the more usual linear scale for current so
that the differences in the currents at the lower potentials can be
more easily seen.
[0072] Because only single time constants are present in the EIS
responses in FIG. 8, the constant-phase-element (CPE) equivalent
circuit model, shown in FIG. 13, can be used to capture the
behavior of the oxide. From the equivalent circuit models, values
for the solution resistance, charge-transfer resistance, and the
constant-phase-element parameters can be obtained, as shown in
Table 2. In addition, using Eq. (5), values for oxide capacitance
can be calculated and are shown in the final column of Table 2. C
represents the oxide film capacitance, R.sub.P the charge transfer
resistance, Y.sub.0 is the admittance of an ideal capacitor, and a
represents an empirical constant to capture the deviation of the
CPE from an ideal capacitor.
C = ( Y 0 R p ) 1 .alpha. R p ( 5 ) ##EQU00001##
TABLE-US-00002 TABLE 2 Values determined from a CPE equivalent
circuit model fit to EIS data obtained from pure titanium oxide and
oxides of titanium alloys along with the calculated oxide film
capacitance Calculated Fit Parameter Capacitance Metal Oxide
.alpha. Y.sub.0 (S*s.sup..alpha.) R.sub.P (k.OMEGA.) (.mu.F) Pure
Ti 0.931 22.21 .times. 10.sup.-6 85.4 8.4 Ti.sub.99Co.sub.1 0.890
12.61 .times. 10.sup.-6 267.5 2.7 Ti.sub.99Sn.sub.1 0.909 7.59
.times. 10.sup.-6 1110 2.4 Ti.sub.99Cr.sub.1 0.909 12.97 .times.
10.sup.-6 1035 4.2
[0073] In order to address how the electronic structure of the
oxide affects reduction reaction kinetics, the Mott-Schottky
relationship (Mantia et al., "A critical assessment of the
Mott-Schottky analysis for the characterization of passive
film-electrolyte junctions" Russian Journal of Electrochemistry,
vol. 11, pp. 1306-1322, 2010; Azumi et al., "Mott-Schottky plot of
the passive film formed on iron in neutral borate and phosphate
solution" Journal of the Electrochemical Society, vol. 134, pp.
1352-1357, 1987), Eq. (6), was employed to determine the flatband
potential of the oxides of the pure titanium and binary alloy
oxides.
1 C 2 = 2 * ( V app - E FB - k B T e ) .epsilon..epsilon. 0 eN D A
2 ( 6 ) ##EQU00002##
where E.sub.FB is the flatband potential, C is the interfacial
capacitance, e is the charge on the electron, A is the interfacial
contact area, .di-elect cons. is the dielectric constant of the
oxide, .di-elect cons..sub.0 is vacuum permittivity, and V.sub.app
is the externally applied potential. The Mott-Schottky equation
proposes an inverse relationship between capacitance and applied
potential with the slope related to the donor concentration,
N.sub.D. An example of the analysis for Ti.sub.99Cr.sub.1 is
presented in FIG. 14.
[0074] The dielectric constants, .di-elect cons., for the oxides
can be obtained from Eq. (7).
C = .epsilon..epsilon. 0 A D ( 7 ) ##EQU00003##
where C is the capacitance, .di-elect cons. is the dielectric
constant of the oxide, .di-elect cons..sub.0 is vacuum
permittivity, and A is the exposed area, and D is the oxide
thickness. The exposed areas were roughly 0.20 cm.sup.2 and the
oxide thickness was estimated to be 2 nm. From the equivalent CPE
circuit fits in FIG. 13 the dielectric constants in Table 3 were
obtained.
TABLE-US-00003 TABLE 3 Calculated dielectric constants for the
metal oxides. Metal Oxide Capacitance (.mu.F) Dielectric Constant
Pure Ti 8.4 97 Ti.sub.99Co.sub.1 2.7 31 Ti.sub.99Sn.sub.1 2.4 28
Ti.sub.99Cr.sub.1 4.2 48
[0075] The donor concentration can be obtained from the following
equation:
Slope = 2 .epsilon..epsilon. 0 eA 2 N D ( 8 ) ##EQU00004##
where the slope is from the linear best-fit line to the 1/C.sup.2
vs. potential plot for each oxide from the Mott-Schottky
measurements, as shown in FIG. 14. The flatband potential can be
obtained from the following:
V app = E FB + k B T e ( 9 ) ##EQU00005##
[0076] The donor concentration values suggest the oxidation states
shown in Table 4 for the oxide components. The oxidation states
imply that the three alloying elements to the bulk Ti: Co, Sn, and
Cr, are all p-type dopants in the oxides.
TABLE-US-00004 TABLE 4 Estimated oxidation states for the elements
comprising the various oxides. Element Oxidation State O -2 Ti +4
Co +3 Sn +2 Cr +3
[0077] The flatband potentials (Table 5) fix the lower bound of the
conduction band in the oxides but the band gaps are also needed to
determine the top edge of the valence bands. The response of the
oxide films to the IMPS experiments, as shown in FIG. 10, can be
used to determine the band gaps. FIG. 15 shows a plot of normalized
I.sub.mod for Ti.sub.99Sn.sub.1. That is, the maximum I.sub.mod
value was normalized to 1.0 and that ratio was applied to the other
I.sub.mod values. The I.sub.mod values were obtained from the oxide
film response to incident on-off light at 0.1 Hz.
TABLE-US-00005 TABLE 5 Values for E.sub.FB, the flatband potential,
and the dopant concentration, N.sub.D, determined from the analysis
of the Mott-Schottky tests performed on the pure titanium oxide and
oxides of titanium alloys. Metal Oxide E.sub.FB (V.sub.SCE) N.sub.D
(cm.sup.-3) Pure Ti -0.31 7.0 .times. 10.sup.21 Ti.sub.99Co.sub.1
-0.19 5.6 .times. 10.sup.21 Ti.sub.99Sn.sub.1 -0.18 13 .times.
10.sup.21 Ti.sub.99Cr.sub.1 -0.24 6.3 .times. 10.sup.21
[0078] The plot in FIG. 15 suggests a smaller band gap than the
band gap of 3.0 eV (Birch et al., "Oxides formed on titanium by
polishing, etching, anodizing, or thermal oxidizing" Corrosion,
vol. 56, pp. 1233-1241, 2000) for rutile TiO.sub.2. Following the
approach in Goosens ("Intensity-modulated photocurrent spectroscopy
of thin anodic films on titanium" Surface Science, vol. 365, pp.
662-671, 1996), the band gaps were determined from the x-intercepts
of the linear best-fit lines to plots of
(I.sub.mod.times.E.sub.h.nu.).sup.0.5 vs. photon energy, as shown
in FIG. 16.
[0079] The top of the valence band energies can then be obtained by
subtracting the band gap energy from the bottom of the conduction
band energies, with the results shown in Table 6.
TABLE-US-00006 TABLE 6 Values for the band gap energies and valence
band energies for titanium oxide and the oxides of the titanium
alloys. Metal Oxide E.sub.g (eV) E.sub.VB (V.sub.SHE) Pure Ti 2.90
2.59 Ti.sub.99Co.sub.1 2.82 2.63 Ti.sub.99Sn.sub.1 2.88 2.70
Ti.sub.99Cr.sub.1 2.78 2.54
[0080] The above analysis can be summarized in the chart shown in
FIG. 17 in which the band energies of the various oxides are
overlaid on redox reaction potentials of reduction reactions that
can be catalyzed on the oxides. In contrast to what was observed
elsewhere (Goosens, "Intensity-modulated photocurrent spectroscopy
of thin anodic films on titanium" Surface Science, vol. 365, pp.
662-671, 1996), Table and FIG. 17 indicate that the band gap energy
for the pure TiO.sub.2 is close to the accepted value of 3.0 eV for
rutile TiO.sub.2. However, the band gap energies for the oxides on
the low-alloyed titanium are slightly smaller than 3.0 eV. This
reduction may occur because the dopant atoms can act as strain
relief centers for the TiO.sub.2 while providing electron acceptor
states within the band gap. In addition, the conduction and valence
band edges are shifted as a result of the alloying indicating that
the H.sub.2/H.sub.2O reaction from Table 1 will not occur on these
oxides at this pH at the equilibrium potential (Ma et al.,
"Titanium dioxide-based nanomaterials for photocatalytic fuel
generations" Chemical Reviews, vol. 114, pp. 9987-10043, 2014).
[0081] The chart in FIG. 17 indicates that even low concentrations
of alloying elements in metallic titanium can alter the electronic
structure of the oxide that forms on the metal surface and suggests
that, from a thermodynamic standpoint, there are other surface
states available on the oxides that can compete with the oxygen
reduction reaction (ORR) for collection of electrons supplied by
oxidation reactions.
[0082] Measurements of the oxygen reduction reaction kinetics on
each of the oxide surfaces, as shown in FIG. 12, indicate that the
ORR is suppressed by the addition of these alloying elements. For
example, at -0.9 V.sub.SCE, a potential at which the oxygen
reduction reaction would occur on these oxides, the currents
obtained from the RDE experiments, shown in Table 7, suggest that
the cathodic current can be suppressed up to 99% in a high pH
environment.
TABLE-US-00007 TABLE 7 Reduction currents measured during RDE
experiments on each oxide at -0.9 V.sub.SCE, at 1600 rpm rotation
speed in 0.6M NaCl + 0.01M KOH and compared to the baseline of the
pure TiO.sub.2. Potential Reduction current % Oxide (V.sub.SCE)
(.mu.A/cm.sup.2) change Pure Ti -0.9 -414 0 Ti.sub.99Co.sub.1 -0.9
-173 -58 Ti.sub.99Sn.sub.1 -0.9 -14 -97 Ti.sub.99Cr.sub.1 -0.9 -6
-99
[0083] These data suggest that the presence of these alloying
elements in the oxide disrupt its catalytic capability. A similar
trend is seen in the Nyquist plot of the IMPS results in FIG. 11,
with the lowest response also occurring on the oxide of the
Ti.sub.99Cr.sub.1 alloy.
[0084] Computational modeling predicted that Mn and Al would bring
the highest ORR overpotentials of the cases considered and thus
would be the best dopants for suppressing ORR activity and
corrosion. Co, Sn, and Cr in turn should be moderate inhibitors,
and Nb and Ag should increase ORR activity relative to pure
amorphous TiO.sub.2. This is consistent with previous work by
Arashi and coworkers that showed Nb doped amorphous TiO.sub.2 has a
slightly lower overpotential than the undoped material (Arashi et
al., Catal. Today 2014, 233, 181-186). Vanadium is a more
challenging dopant to characterize because it has two stable
oxidations states V.sup.3+ and V.sup.5+ that lie near the
experimental conditions. Thus, V dopants are likely present as a
mixture of V.sup.3+ and V.sup.5+. At more negative applied
potentials, the ratio of V.sup.3+/V.sup.5+ should increase to favor
V.sup.3+ and the ORR activity of the oxide should decrease. This
suggests that different dopants could have different capacities to
suppress corrosion at different applied potentials in the
experiments. The Pourbaix diagrams for all other considered dopants
have only one stable oxidation state near the experimental
conditions. Trends for seven dopants were experimentally verified,
but Ga, Zn, and Si were also computationally predicted to
destabilize ORR intermediates on the surface and potentially result
in even better ORR inhibition than Al and Mn.
[0085] FIG. 18 shows the cathodic polarization scans for all of the
alloys. The current density values at -0.8 VSCE, a value in keeping
with galvanic corrosion potentials between Ti and Al alloys, were
taken for at least three replicates and then averaged. The percent
change for each alloy with respect to the undoped Ti is shown in
Table 8. Previous X-ray photoelectron spectroscopy studies have
indicated that the dopant metal is present in the oxide (Policastro
et al., J. Electrochem. Soc. 2016, 163, C269-C274).
TABLE-US-00008 TABLE 8 Percent change in current at -0.8 VSCE of
alloy samples versus the undoped Ti Current density Alloy
(.mu.A/cm.sup.2) % change Ti 8.8 -- Ti.sub.99V.sub.1 2.0 -77 .+-. 3
Ti.sub.99Mn.sub.1 3.1 -65 .+-. 4 Ti.sub.99Al.sub.1 3.4 -61 .+-. 11
Ti.sub.99Co.sub.1 4.7 -47 .+-. 5 Ti.sub.99Cr.sub.1 6.2 -30 .+-. 8
Ti.sub.99Sn.sub.1 6.9 -21 .+-. 11 Ti.sub.99Ag.sub.1 17.3 +95 .+-.
13
[0086] The quantum chemistry predictions almost exactly mirror the
experimental results. The trend in dopant performance predicted by
computational modeling was:
Ag>undoped>Sn>Co>Cr>Al>Mn>V
while experimental linear sweep voltammetry measurements found
almost exactly the same ranking:
Ag>undoped>Sn>Cr>Co>Al>Mn>V
[0087] The model appears to overestimate the effect of Cr relative
to Co, but these fall quite close on the volcano plot within 0.2
eV. This signifies that not only are Sabatier analyses useful for
discerning materials with high catalytic activity (as is done for
fuel cell catalysis), but such modeling is also robust enough to
identify dopants for materials having low catalytic activities.
[0088] The ability of the dopant atoms to bind the ORR
intermediates was hypothesized to correlate with the total charge
of each intermediate after it is bound to the surface. Bader charge
analysis shows that *OOH bound to Al.sup.3+, Ag.sup.+, Mn.sup.2+,
or V.sup.3+ has a charge of -0.70, -0.86, -1.02, or -1.05,
respectively. Note that dopants with larger degrees of charge
transfer would bind the intermediates more tightly, and thus more
effectively poison the surface for chemical reactions. Dopants
falling on the left side of the activity volcano and their
overpotentials are due to the energy required to remove *OH from
the surface (Eq. (4)). This is similar to the findings of Markovi
et al. (Nat. Chem. 2009, 1, 466-472) who showed that electrolyte
ions can (de)stabilize *OH on the surface and alter ORR rates by an
order of magnitude. On the other hand, dopants that transfer too
little charge will form weaker bonds that are not strong enough to
form reaction intermediates. The overpotential of these dopants is
determined by the energy required to form *OOH (Eq. (1)), and they
are located on the right side of the activity volcano.
[0089] For dopants with intermediate oxidation states (i.e.
V.sup.3+, Mn.sup.2+, Cr.sup.3+, Co.sup.2+, and Ag.sup.+), the
ability to donate electron density appears to correlate with their
approximate redox potentials from Pourbaix diagrams in the
literature (V.sub.2O.sub.3V.sub.3O.sub.5 at E.sup.0=-0.5 VSHE,
Mn.sup.2+Mn.sub.2O.sub.3 at E.sup.0=0.3 VSHE,
Cr.sub.2O.sub.3CrO.sub.4.sup.2- at E.sup.0=0.2 VSHE,
CoOCo.sub.3O.sub.4 at E.sup.0=1.0 VSHE, and Ag.sup.+Ag.sub.2O.sub.3
at E.sup.0=1.2 VSHE (Takeno, Geological survey of Japan open file
report 2005, 419, 1-102)). For dopants at their highest oxidation
state, (i.e. Nb.sup.5+, Ti.sup.4+, Sn.sup.4+, and Al.sup.3+) the
ability to bind to ORR intermediates correlates surprisingly well
with the calculated atomic radii of the dopants (Nb=1.98 .ANG.,
Ti=1.76 .ANG., Sn=1.45 .ANG., and Al=1.18 .ANG.). Bonding orbitals
in smaller dopants (such as Sn and Al) have less orbital overlap
which makes it more difficult to transfer electron density to the
adsorbed intermediates than larger dopants (such as Nb). This
results in weaker bonds and higher overpotentials. Although this
trend might be coincidental, Ge, Zn, Ga, and Si all have atomic
radii similar to Al (Ge=1.25 .ANG., Zn=1.42 .ANG., Ga=1.36 .ANG.,
and Si=1.11 .ANG.) which is consistent with the predictions that
these dopants may indeed suppress ORR activity more than Al and
Mn.
[0090] Obviously, many modifications and variations are possible in
light of the above teachings. It is therefore to be understood that
the claimed subject matter may be practiced otherwise than as
specifically described. Any reference to claim elements in the
singular, e.g., using the articles "a", "an", "the", or "said" is
not construed as limiting the element to the singular.
* * * * *