Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TITLE: FUEL CELL CATALYST SUPPORT BASED ON DOPED TITANIUM
SUBOXIDES
FIELD
[0001] This disclosure relates generally to fuel cell electrocatalysts, and
more
specifically to fuel cell electrocatalysts having support structures based on
doped titanium
suboxides.
BACKGROUND
[0002] A fuel cell is an electrochemical cell that converts the chemical
energy from a
fuel into electricity through an electrochemical reaction. Fuel cells require
a continuous
source of fuel and oxygen (usually from air) to sustain the electrochemical
reaction and
can produce electricity continuously for as long as fuel and oxygen are
supplied.
[0003] There are several types of fuel cells currently under development,
each with its
own advantages, limitations, and potential applications. For instance, one
type of fuel cell
design currently under development uses a solid polymer electrolyte ("SPE")
membrane
or a proton exchange membrane ("PEM"), to provide ion transport between the
anode
and cathode. In PEM type fuel cells, hydrogen is supplied to the anode as fuel
and oxygen
is supplied to the cathode as the oxidant. The oxygen can either be in pure
form (02) or
air (a mixture of 02 and N2). PEM fuel cells typically have a membrane
electrode assembly
("MEA") in which a solid polymer membrane has an anode catalyst on one
surface, and
a cathode catalyst on the opposite surface. The anode and cathode layers of a
typical
PEM fuel cell are formed of porous conductive materials, such as woven
graphite,
graphitized sheets, or carbon paper to enable the fuel to disperse over the
surface of the
membrane facing the fuel supply electrode. Each electrode then has catalyst
particles
arranged thereon, supported on carbon particles, to promote ionization of
hydrogen at the
anode and reduction of oxygen at the cathode. Protons flow from the anode
through the
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ionically conductive polymer membrane to the cathode where they combine with
oxygen
to form water, which is discharged from the cell.
[0004] PEM fuel cells are generally clean, portable power sources that are
powered
by hydrogen or alcohols from secure and renewable sources and may be a
solution for
non-polluting and highly efficient vehicles and portable devices.
[0005] Current PEM fuel cells rely on platinum (Pt) electrocatalysts to
drive the anodic
and cathodic reactions. The support material onto which the Pt catalyst is
dispersed can
exert a significant influence on its electroactivity and durability. Normally,
Pt nanoparticles
(NPs) are dispersed onto a high surface area carbon support (Pt/C) to maximize
the active
surface area of the catalyst. Carbon black has been the de facto catalyst
support in fuel
cells over the last 30 years due to its high surface area and electrical
conductivity.
However, carbon can be a liability when it comes to durability since it is
prone to corrosion
under the highly acidic and oxidative operating conditions of a PEM fuel cell.
Carbon
corrosion can be detrimental to the long-term performance of a fuel cell.
Furthermore,
even when carbon corrosion does not occur, Pt aggregation can occur on carbon,
which
decreases the electrochemically active surface area (ECSA) of the catalyst,
and
subsequently the performance of the electrode. This is particularly evident
during
prolonged open-circuit potential (OCP) or under repeated start¨stop cycles.
[0006] Accordingly, there is a need to replace the traditional carbon
supports in PEM
fuel cells with a corrosion stable support.
[0007] Many metal oxides such as TiO2, NbOx, WON, and MoOx offer low
corrosion
and strong interaction with Pt NPs. In fact, many of these metal oxides also
enhance the
catalytic activity of Pt towards the oxygen reduction reaction (ORR) via an
electronic
interaction and can also promote homogeneous dispersion of Pt particles.
[0008] However, most metal oxides suffer from poor electronic conductivity,
which
limits their practical use in fuel cell devices. As such, many researchers
have investigated
composite supports, where metal oxides are combined with carbon black in order
to
harness the desirable properties of both materials.
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,
,
[0009] While these materials do show some promising activity for the
ORR,
researchers using this approach have faced challenges optimizing contact
between the
metal oxide and carbon in order to maintain sufficient electronic
conductivity.
SUMMARY
[0010] In accordance with a broad aspect, there is provided a fuel
cell electrocatalyst
including a support structure. The support structure includes at least one
titanium
suboxide, a first dopant and a second dopant. The first dopant is a metal and
the second
dopant is a Group IV element. The fuel cell electrocatalyst also includes a
metal catalyst
deposited on the support structure.
[0011] In at least one embodiment, the first dopant comprises one of
chromium,
molybdenum and tungsten.
[0012] In at least one embodiment, the first dopant is molybdenum.
[0013] In at least one embodiment, the second dopant comprises one of
silicon,
germanium, tin and lead.
[0014] In at least one embodiment, the second dopant is silicon.
[0015] In at least one embodiment, the support structure has the
formula Ti305-MoxSiy
and x and y are each less than 1.
[0016] In at least one embodiment, the support structure has the
formula Ti305-MoxSiy
and x and y are each less than 0.5.
[0017] In at least one embodiment, the support structure has the
formula Ti305-MoxSiy
and x is 0.2.
[0018] In at least one embodiment, the support structure has the
formula Ti305-MoxSiy
and y is 0.4.
[0019] In at least one embodiment, the support structure has the
formula Ti305-MoxSiy
and x is 0.2 and y is 0.4.
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[0020] In at least one embodiment, the support structure has a band gap
that is less
than 1 eV.
[0021] In at least one embodiment, the support structure has a band gap
that is about
0.31 eV.
[0022] In at least one embodiment, the catalyst layer is a metal catalyst.
[0023] In at least one embodiment, the metal catalyst is platinum or a
platinum alloy.
[0024] In accordance with a broad aspect, there is provided a fuel cell
electrocatalyst
support structure. The support structure includes at least one titanium
suboxide, a first
dopant and a second dopant. The first dopant is a metal and the second dopant
is Group
IV element.
[0025] These and other features and advantages of the present application
will
become apparent from the following detailed description taken together with
the
accompanying drawings. It should be understood, however, that the detailed
description
and the specific examples, while indicating preferred embodiments of the
application, are
given by way of illustration only, since various changes and modifications
within the spirit
and scope of the application will become apparent to those skilled in the art
from this
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] For a better understanding of the various embodiments described
herein, and
to show more clearly how these various embodiments may be carried into effect,
reference will be made, by way of example, to the accompanying drawings which
show
at least one example embodiment, and which are now described. The drawings are
not
intended to limit the scope of the teachings described herein.
[0027] FIG. 1A is an X-ray diffraction pattern obtained for a TOMS
electrocatalyst.
[0028] FIG. 1B is an X-ray diffraction pattern obtained for a Pt/TOMS
support
structure.
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[0029] FIG. 10 is a band gap energy determination of commercial TiO2 and
the TOMS
support structure.
[0030] FIG. 2A is an XPS spectrum of the Pt/TOMS catalyst showing the
composition
of Ti.
[0031] FIG. 2B is an XPS spectra of the Pt/TOMS catalyst showing the
composition of
Mo.
[0032] FIG. 20 is an XPS spectra of the Pt/TOMS catalyst showing the
composition
of Si.
[0033] FIG. 2D is an XPS spectra of the Pt/TOMS catalyst showing the
composition
of Pt.
[0034] FIG. 3A shows a secondary electron image of the Pt/TOMS
elecrocatalyst.
[0035] FIG. 3B shows a back scattered electron image of the Pt/TOMS
electrocatalyst.
[0036] FIG. 30 shows an EDX analysis of the Pt/TOMS electrocatalyst.
[0037] FIGS. 4A and 4B show TEM images of the TOMS support structure at
different
magnifications.
[0038] FIGS. 40 and 4D show TEM images of the Pt/TOMS electrocatalyst at
different
magnifications.
[0039] FIG. 5A is a chart showing variation in the CV response of the TOMS
support
structure.
[0040] FIG. 5B is a Nyquist plot of the TOMS support structure.
[0041] FIG. 50 is a capacitance plot of the TOMS support structure obtained
at a DC
bias potential of 0.425 V vs. RHE.
[0042] FIG. 6A shows a comparison of the CVs obtained for Pt/TOMS and Pt/C
recorded in N2-purged 0.5 m H2SO4 at 25 C and a scan rate of 10 my s-1.
[0043] FIG. 6B shows a comparison of the ORR activity of Pt/TOMS and Pt/C
recorded
in 02-saturated 0.5 m H2504 at 25 C and a scan rate of 5 my s-1 and 900 rpm.
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,
[0044] FIG. 7A shows variation in the CV response of Pt/TOMS during
the AST with
measurements being made in N2-purged 0.5 m H2SO4 (aq) at 25 C at a sweep rate
of
50 my s-1.
[0045] FIG. 7B shows variation in the CV response of commercial Pt/C
during the AST
with measurements being made in N2-purged 0.5 m H2SO4 (aq) at 25 C at a sweep
rate
of 50 my s-1.
[0046] FIG. 70 shows variation in ECSA with number of potential
cycles for each
catalyst.
[0047] FIG. 8A shows variation in the EIS response obtained by
Pt/TOMS during the
AST, shown as Nyquist plots.
[0048] FIG. 8B shows variation in the EIS response obtained by
Pt/TOMS during the
AST, shown as capacitance plots.
[0049] FIG. 80 shows variation in the EIS response obtained by Pt/C
during the AST,
shown as Nyquist plots.
[0050] FIG. 8D shows variation in the EIS response obtained by Pt/C
during the AST,
shown as capacitance plots.
[0051] FIG. 9A shows a comparison of the ORR activity of the Pt/TOMS
catalysts
before and after the 5000 cycle AST.
[0052] FIG. 9B shows a comparison of the ORR activity of the Pt/C
catalysts before
and after the 5000 cycle AST.
[0053] FIG. 10 shows a comparison of the fuel cell performance
obtained using the
Pt/TOMS and Pt/C electrodes with data being shown in (a) power density and (b)
polarization curves where measurements were made in a 5 cm2 single cell PEMFC
at 80
C, using a Pt loading of 0.2 mg cm-2, H2 flow rate of 100 nml min-1 100% rh 1
bar bp; 02
flow rate 200 nml min-1 100% rh 1 bar bp, and membrane = NRE212.
[0054] FIG. 11 shows fuel cell performance stability testing of the
Pt/TOMS MEA
where data being obtained at the beginning of life (BOL) and after 150 h and
250 h of
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,
,
testing and shown as (a) power density and (b) polarization curves with
measurements
being made in a 5 cm2 single cell PEMFC at 80 C, using a Pt loading of 0.2 mg
CM-2, H2
flow rate of 100 nml min-1 100% rh 1 bar bp; 02 flow rate 200 nml min-1 100%
rh 1 bar bp,
and membrane = NRE212.
[0055] Further aspects and features of the example embodiments
described herein
will appear from the following description taken together with the
accompanying drawings.
DESCRIPTION OF EXAMPLE EMBODIMENTS
[0056] Various apparatuses, methods and compositions are described
below to
provide an example of at least one embodiment of the claimed subject matter.
No
embodiment described below limits any claimed subject matter and any claimed
subject
matter may cover apparatuses and methods that differ from those described
below. The
claimed subject matter are not limited to apparatuses, methods and
compositions having
all of the features of any one apparatus, method or composition described
below or to
features common to multiple or all of the apparatuses, methods or compositions
described below. It is possible that an apparatus, method or composition
described below
is not an embodiment of any claimed subject matter. Any subject matter that is
disclosed
in an apparatus, method or composition described herein that is not claimed in
this
document may be the subject matter of another protective instrument, for
example, a
continuing patent application, and the applicant(s), inventor(s) and/or
owner(s) do not
intend to abandon, disclaim, or dedicate to the public any such invention by
its disclosure
in this document.
[0057] Furthermore, it will be appreciated that for simplicity and
clarity of illustration,
where considered appropriate, reference numerals may be repeated among the
figures
to indicate corresponding or analogous elements. In addition, numerous
specific details
are set forth in order to provide a thorough understanding of the example
embodiments
described herein. However, it will be understood by those of ordinary skill in
the art that
the example embodiments described herein may be practiced without these
specific
details. In other instances, well-known methods, procedures, and components
have not
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been described in detail so as not to obscure the example embodiments
described herein.
Also, the description is not to be considered as limiting the scope of the
example
embodiments described herein.
[0058] It should be noted that terms of degree such as "substantially",
"about" and
"approximately" as used herein mean a reasonable amount of deviation of the
modified
term such that the end result is not significantly changed. These terms of
degree should
be construed as including a deviation of the modified term, such as 1%, 2%,
5%, or 10%,
for example, if this deviation does not negate the meaning of the term it
modifies.
[0059] Furthermore, the recitation of any numerical ranges by endpoints
herein
includes all numbers and fractions subsumed within that range (e.g. 1 to 5
includes 1,
1.5, 2, 2.75, 3, 3.90, 4, and 5). It is also to be understood that all numbers
and fractions
thereof are presumed to be modified by the term "about" which means a
variation up to a
certain amount of the number to which reference is being made, such as 1%, 2%,
5%, or
10%, for example, if the end result is not significantly changed.
[0060] It should also be noted that, as used herein, the wording "and/or"
is intended to
represent an inclusive - or. That is, "X and/or Y" is intended to mean X or Y
or both, for
example. As a further example, "X, Y, and/or Z" is intended to mean X or Y or
Z or any
combination thereof.
[0061] The following description is not intended to limit or define any
claimed or as yet
unclaimed subject matter. Subject matter that may be claimed may reside in any
combination or sub-combination of the elements or process steps disclosed in
any part
of this document including its claims and figures. Accordingly, it will be
appreciated by a
person skilled in the art that an apparatus, system or method disclosed in
accordance
with the teachings herein may embody any one or more of the features contained
herein
and that the features may be used in any particular combination or sub-
combination that
is physically feasible and realizable for its intended purpose.
[0062] Recently, there has been a growing interest in creating metal oxides
with
sufficient electrical conductivity to be a practical catalyst support. Among
the available
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candidates, considerable attention has been paid to titanium suboxides (Tix02x-
1) since
they possess improved electronic conductivity compared to TiO2 and high
oxidative
stability. Furthermore, the addition of a dopant into the titanium suboxide
structure can
further enhance electronic conductivity. Recently, studies have reported a
process of
modifying TiO2 with Mo to form a Mo-doped titanium suboxide (-11305¨Mo). The
Pt/Ti305¨
Mo catalyst showed enhanced ORR activity and durability despite the support
still having
a sizable band gap (2.6 eV). However, in spite of the technologies that have
been
developed, there remains a need in the field for improvements in the
development of fuel
cell catalyst supports.
[0063] Herein, a metal oxide material with enhanced electronic conductivity
is
prepared and proposed for use as a catalyst support structure in
electrochemical devices.
The process of forming the catalyst support structure involves the use of
sequential
doping of TiO2 with two different elements to create oxygen vacancies within
the lattice
of the metal oxide. The resulting dual-doped suboxide has a substantially
lower band gap,
which is lower than doping with just a single element, rendering it suitable
for use in
electrochemical devices.
[0064] In one aspect, deposition of catalyst nanoparticles (e.g. Pt) onto
the surface of
the catalyst support structure creates a material that may be suitable for use
in PEM fuel
cells. Electrochemical testing revealed that the catalyst support structure
has high activity
and performance in a PEM fuel cell. Furthermore, accelerated stress testing
demonstrated that the dual-doped suboxide catalyst support structure is stable
and
durable under the harsh operating condition of PEM fuel cells.
[0065] According to one aspect of the teachings herein, a carbon-free
multifunctional
titanium suboxide with two dopants (i.e. doping elements) has been developed
as a fuel
cell support structure. The carbon-free multifunctional titanium suboxide
support structure
has high electronic conductivity for a metal oxide (as further described
below). As noted
above, Ti305¨Mo-based support structures, where Mo is a first dopant, are
generally
known in the art. However, in at least one embodiment in accordance with the
teachings
herein, to improve the conductivity of Ti305¨Mo-based support structures for
fuel cells, a
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second dopant has been introduced into the Ti305¨Mo-based support structure.
The
second dopant can be a Group IV element. For instance, the second dopant can
be silicon
(Si). More specifically, the support structure can be a Ti305-Moo2Sio.4 (TOMS)
support
structure and be used in various applications including, but not limited to,
as a fuel cell
catalyst support structure.
[0066] The introduction of Si as the second dopant generally has a
favorable influence
on the metal¨support interaction to further enhance the ORR and durability of
the Ti305¨
Mo-based support structure.
[0067] In at least one embodiment, the TOMS support structure can be used
with a
metal catalyst in a fuel cell. For instance, the metal catalyst can be
arranged on top of the
TOMS support structure. In at least one embodiment, the metal catalyst can be
platinum
or a platinum-based alloy. Hereinafter the terms a "Pt/TOMS support" or a
"Pt/TOMS
catalyst" will refer to a TOMS support structure with platinum or a platinum-
based alloy
arranged thereon.
[0068] In another aspect, a Pt/TOMS catalyst is also disclosed herein that
generally
provides improved activity towards the oxygen reduction reaction (ORR) when
compared
to a TOMS support structure and to a traditional PMS fuel cell catalyst. This
improved
activity may be attributed to a strong electronic interaction between the Pt
nanoparticles
and the TOMS support structure. Furthermore, this Pt/TOMS catalyst according
to at least
one aspect disclosed herein shows durability in accelerated stress tests,
losing only 10%
of its active surface area over the 5000 cycle accelerated stress test.
[0069] The TOMS support structure alone and the Pt/TOMS structure can be
characterized by X-ray diffraction (XRD), UV-Visible spectroscopy, X-ray
photoemission
spectroscopy (XPS), transmission electron microscopy (TEM), and scanning
electron
microscopy (SEM). The electrochemical properties of each can be compared to
those of
a typical commercial electrocatalyst (e.g. 20% Pt/carbon).
[0070] Detailed structural analysis has been performed on the TOMS support
structure
and the Pt/TOMS catalyst. Referring now to FIG. 1A, shown therein is an XRD
pattern
CA 3019718 2018-10-03
obtained from the TOMS support structure. The presence of Mo appears to favor
the
formation of the titanium suboxides in a reducing environment that creates Ti
cation
oxygen vacancies or stoichiometric reduction of Ti4+ to Ti3+. The
corresponding
diffractogram shows that the support structure consists of a mixture of Ti305
phases with
the main characteristic reflection at 26 = 25.51 , {110) and Ti60 at 26 =
39.65, with Ti305
being the prevailing phase. Mo is present either as metallic Mo (ICDD card no.
01-088-
2331) or M002 (ICDD card 00-021-0569). The M002 appears to have a rutile-type
crystal
structure, in which M06 octahedra share cores and edges. M002 has high
electronic
conductivity due to the short metal¨metal bond distance along the direction of
edge
sharing. Both metallic Si (ICDD card no. 01-078-2500) and 5i02 (ICDD card no.
00-046-
1242) can be identified.
[0071] Referring now to FIG. 1B, shown therein is a XRD pattern obtained
for the
Pt/TOMS electrocatalyst, with metallic platinum present in a face-centered
cubic (fcc)
structure (ICDD card no. 01-087-0640), with typical reflections at 21., =
40.61 {111},
46.92 {200}, 68.1 {220}, and 81.81 {311}, and 86.43 {222}. All
corresponding Pt peaks
were shifted toward higher angles, indicating a diminution of the lattice
spacing. This
phenomenon may be attributed to the strong interaction between Pt and the TOMS
support structure. Moreover, the peak at 20 = 40.61 appears broad and
intense, which
may signify that the Pt NPs are greatly oriented towards the Pt {111} plane,
which is the
most stable and highly active toward the ORR which contains hexagonally packed
Pt
atoms and does not undergo surface reconstruction, unlike Pt {100} and Pt
{110}
surfaces. The size of Pt crystallites over the TOMS and carbon supports was
calculated
from the width of the {220} and {222} peaks using the Scherrer¨Debye equation,
resulting
in a mean crystallite size of Pt equal to 4 and 2.5 nm for Pt/TOMS and Pt/C
electrocatalysts, respectively.
[0072] FIG. 1C shows the optical band gap of commercial TiO2 and the TOMS
support
structure measured through the absorption spectra of the diffuse reflectance
of both the
materials. The reflectance data has been converted into the absorption
coefficient values
followed by the creation of a Tauc plot. The band gap of the TOMS support
structure was
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measured to be 0.31 eV, which appears to be lower than those of commercial
TiO2 (3.35
eV) and Ti305¨Mo (2.6 eV). These measurements appear to indicate that the TOMS
support structure has conductivity approaching that of a metal conductor.
[0073] The electrical conductivity of these materials has been measured
using a 2-
point probe and the values are listed in Table 1, below. The TOMS support
structure
shows an electrical conductivity of 0.11 S cm-1, which is about two orders of
magnitude
higher than that of T1305¨Mo and only slightly lower than carbon black.
Table 1: Band gap and electronic conductivity of TiO2, Ti305¨Mo and TOMS
Band gap (eV) Electronic conductivity (S cm')
TiO2 3.35 1.4 x 10-6
Ti305-Mo 2.6 [ref. 17] 0.004
TOMS 0.31 0.11
Carbon black 0.83
[0074] The electronic interaction between Pt and the TOMS support structure
and the
chemical state of Ti, Mo, Si, and Pt were investigated by using XPS spectra
and the
results are shown in FIG. 2. The presence of Ti suboxides on the surface of
the support
is confirmed by the Ti3+ peak located at a low binding energy of 456.86 eV
(see FIG. 2A).
Also, the Ti double peaks of 2p1/2 and 2p3/2 levels were located at binding
energies of
464.67 eV and 458.97 eV, respectively. Pure TiO2 shows the Ti double peaks of
2p1/2
and 2p3/2 levels at 464.2 eV and 458.5 eV binding energies, while the reduced
form (e.g.
Magneli phase) has binding energies of 464.7 eV and 459.0 eV. The shift in the
Ti 2p
levels compared to defect-free TiO2 is caused by surface oxygen vacancy
defects in the
TOMS support structure. The incorporation of Mo and Si into the TiO2 lattice
appears to
create these oxygen vacancies resulting in the change of energy difference
between the
conduction and valence bands. Therefore, the shifting of Ti 2p levels towards
higher
binding energies may be attributed to the reduced Ti due to combined effects
between
molybdenum and Si with TiO2 which resulted in oxygen vacancies and Ti suboxide
formation.
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[0075] The Mo spectrum has peaks associated with Mo6+, Mo4+ and Mo (see
FIG.
2B): Mo is largely present in the form of MoOx on the surface of the TOMS
support
structure, with traces of metallic Mo. The Si spectrum has peaks associated
with Si4+ and
Si (see Fig. 2C): also, Si is largely present in the form of SiOx on the
surface of the TOMS
support structure (in agreement with XRD analysis), with traces of metallic Si
on the
surface. The Pt analysis appears to demonstrate spin¨orbit splitting doublet
peaks in the
4f region referring to 4f7/2and 4f512, where the deconvolution of the Pt
spectrum reveals
two pairs of doublet peaks in each region. The high intensity doublet peaks at
the binding
energies of 71.75 eV and 75.1 eV, respectively, appear to be attributed to
metallic Pt
(Pt ). The low-intensity doublet peaks at binding energies of 72.95 eV and
76.3 eV,
respectively, appear at a binding energy 1.2 eV higher than that of Pt ,
assigned to Pt2+
species due to surface oxide/hydroxide (see FIG. 2D). The binding energy of
71.75 eV
for Pt0 4f7/2 reveals 0.75 eV positive shifts towards higher binding energy
compared to
the 4f7/2 conventional value of Pt/C. This shift to higher binding energy
corresponds to
induced positive charge on the dispersed Pt NPs due to the interaction between
Pt NPs
and the TOMS support structure which positively influenced the d-band state of
Pt NPs.
This effect, which is also in line with the XRD results, shows an enhanced
interfacial
strong electronic interaction between the TOMS support structure and Pt NPs.
This
electron donation from the TOMS support structure to Pt NPs, due to the strong
metal¨
support interaction (SMSI), is expected to enhance the electroactivity of
Pt/TOMS.
[0076] FIG. 3 displays SEM images of Pt/TOMS, along with the corresponding
energy
dispersive X-ray spectra (EDX). The particles are spherical in nature, and
fairly
homogeneous in their shape and composition. The EDX spectrum confirms the
presence
of Si and Mo in the Pt/TOMS electrocatalyst.
[0077] FIG. 4 shows the TEM images obtained for TOMS and Pt/TOMS at
different
magnifications. The Pt NPs appear to be well dispersed over the TOMS support
structure
and are clearly identifiable from TEM images.
[0078] FIG. 5A presents the variation in the CV profile of the TOMS support
structure
at various points in the AST. The CV curves have rectangular shapes with small
reversible
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redox peaks at 0.6 V vs v RHE, typical for pseudo-capacitive materials. The
TOMS
support structure appears to remain stable, showing virtually no change over
the course
of 5000 cycles, giving no indication of Ti, Mo, and Si oxidation/corrosion.
[0079] FIG. 5B shows Nyquist plots obtained from the TOMS support structure
at
different stages during the durability test. Relatively short Warburg regions
were observed
which confirms that the TOMS layer has good electronic conductivity. The shape
of the
Nyquist plots was unchanged over the course of the AST, which is indicative of
no change
in either the electronic or ionic conductivity during the AST. Furthermore,
the capacitance
plots (see FIG. 50) showed no change over the course of the 5000 cycles,
indicating that
the surface area of the TOMS support structure was unchanged and remained
stable
during the AST.
[0080] FIG. 6A compares the CVs obtained for the Pt/TOMS and the commercial
Pt/C
catalysts. Both electrocatalysts exhibit the classical Pt CV shape, three
anodic peaks and
two cathodic peaks with good reversibility in the hydrogen region assigned to
the
uniformly dispersed polycrystalline Pt NPs over the surface of the TOMS
support
structure. The Pt/TOMS showed earlier reduction of adsorbed Pt oxides compared
to
Pt/C. The ECSA of each electrocatalyst was determined by integrating the
charge
associated with HUPD (210 pC cmpt -2), and the values are listed in Table 2.
Also listed
in Table 2 are the reported ECSA values for several other catalysts that
employ metal
oxide-based supports. The Pt/TOMS catalyst had a very high ECSA value of 87 m2
gpt-1,
which is one of the highest values reported in the literature for metal oxide
containing
supports. This confirms that catalyst particles are well dispersed onto the
TOMS support
structure. Furthermore, it also indicates that the catalyst layer created from
Pt/TOMS
creates a very high degree of accessibility to Pt active sites.
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Table 2: Electrochemical characterization of electrocatalysts
Pt ECSA I 0.9 *0.9 ECSA loss, cycle,
Catalyst [mg cm-2] Cm2g-il imA [mA cm-2] Media scan rate
[rn1/ s-1 Ref.
Pt/TOMS 0.025 87.33 66.5 1.57 0.5 M H2SO4 10.2%,
5000, 50 Current study
PVC 0.03 98.67 31.7 0.95 0.5 M H2SO4 79.4%,
5000, 50 Current study
Ptr11305-Mo 0.015 22.3 55.2 1.1 0.5 M I-I2SO4
11.2%, 5000, 50 17
Pt/Ti0 76405,302 0.221 81.07 3.17 0.65 0.5 M 1-12804 -
67
0.075 34.7 3.4 0.2 0.5 M H2SO4 58.8%,
5000, 20 68
Ptrri02-CN., 75 zt 10 0.5 M H2SO4 3%, 1000,
100 14
Ptirio.,Moo302 0.221 72.5 0.8 0.5 M 112804 -
21
Pt/Ta-TiO2 0.038 36.5 21 0.45 0.1 ttl HC104 26.3%,
10000, 20 69
Pt/Nb-Ti02. 0.087 36 2.4 0.15 0.1 M HC104 25%,
1000, 100 70
Pt/1'1107 0.0478 6 5.33 0.2 0.1 M FIC104 28%,
1000, 20 71
[0081] FIG. 6B compares the ORR activity of Pt/TOMS and Pt/C catalysts. The
Pt/TOMS electrocatalyst exhibits excellent ORR activity, with a high onset
potential for 02
reduction as well as a high half-wave potential of 0.9 V vs. RHE (compared to
a halfwave
potential of 0.86 V vs. RHE for the commercial Pt/C). The Pt/TOMS produced
1.57 mA
cm-2 at 0.9 V vs. RHE compared to only 0.95 mA cm-2 for Pt/C. Furthermore,
this appears
to represent an improvement over the ORR activity reported for Pt/Ti305-Mo
(1.1 mA cm
-
2 at 0.9 V). Such an enhancement in ORR activity may correlated with the
change in the
Pt-Pt interatomic distance. In fact, the distinctive electroactivity of
Pt/TOMS compared to
Pt/C is defined through changes in the Pt d-band length and smaller lattice
parameter
values induced by the metallic suboxide support and formation of hydrogen
molybdenum
bronze, which effectively promotes the direct 4-electron transfer ORR on the
Pt/TOMS.
The reduction of the Pt d-bond length is due to the SMSI between the TOMS
support
structure and Pt NPs that weakens the interaction between Pt and the adsorbed
oxygenated species that leads to higher electroactivity of Pt/TOMS compared
with that of
the commercial Pt/C catalyst. Since the TOMS support structure makes a stable
Pt NP
surface at high electrochemical potentials, the Pt/TOMS appears to possess a
lower
kinetic barrier for the ORR compared to Pt/C which is a key kinetic parameter
for ORR
activity. A summary of key electrochemical parameters and a comparison to the
ORR
activity of other catalysts that employ metal oxide based supports is shown in
Table 2.
CA 3019718 2018-10-03
[0082] Beside high electrocatalytic activity, the durability of the
electrocatalysts is an
important characteristic to identify the influence of electrochemical
variation over potential
cycling and its influence and contribution to the loss of ECSA and
consequently the
electrocatalyst activity. The AST was conducted by subjecting both the Pt/TOMS
and
commercial Pt/C electrocatalysts to 5000 potential cycles in the range of 0.05-
1.25 V vs.
RHE. Referring now to FIG. 7, shown therein is a comparison of the change in
the CV
response with potential cycling for each catalyst. As the test progressed, the
Pt/TOMS
remained quite stable, while the Pt/C decayed rapidly (see FIGS. 7A and 7B).
For the
Pt/TOMS electrocatalyst, the decay in ECSA after 5000 ASTs was 10.2% while
Pt/C
showed an ECSA decay of 79.4%. This durability for the Pt/TOMS catalyst
appears to be
a result of the ability of the support structure to mitigate the segregation
of Pt NPs, which
implies that Pt NPs anchor to the surface of the TOMS support structure
through the SMSI
that leads to improvement in both electroactivity and stability of the Pt
catalyst.
[0083] FIG. 8 shows the EIS response for Pt/TOMS and commercial Pt/C
electrocatalysts, shown as Nyquist and capacitance plots. The Nyquist and
capacitance
plots obtained for Pt/TOMS were virtually unchanged over the course of the
AST. This
appears to indicate that excellent ionic and electronic conductivities were
maintained
throughout the test, and that there was no decay or corrosion of the TOMS
support
structure. For the Pt/C catalyst, a small increase in the Warburg length was
observed
over the course of the AST, indicating a small increase of the catalyst layer
resistance
due to carbon corrosion. The capacitance plots for Pt/C showed an initial
increase in
limiting capacitance, which is most likely due to incomplete wetting of the
Pt/C
electrocatalyst surface at the initial stage of the measurements. Upon
cycling, the
capacitance plots for Pt/C showed decrease in limiting capacitance, which is
the
characteristic response when Pt dissolution and agglomeration are the dominant
degradation mechanisms.
[0084] To further examine the impact of the AST on the catalysts, ORR
activity was
reassessed repeatedly for both electrocatalysts after the completion of the
AST procedure
protocol (see FIG. 9). As expected, there was very little change in ORR
activity for the
16
CA 3019718 2018-10-03
Pt/TOMS catalyst, while a decline in ORR activity was observed for Pt/C due to
sintering
and agglomeration of Pt NPs. These results appear to show that the Pt/TOMS
catalyst
layer remained stable over the course of the AST and there was essentially no
change in
the elemental distribution of the Pt/TOMS catalyst layer components after the
AST.
[0085] FIG. 10 compares the fuel cell performance achieved using Pt/TOMS
electrodes with that obtained using Pt/C electrodes. The Pt/TOMS and Pt/C
electrodes
produced a maximum power density of 973 and 865 mW cm-2, respectively. This is
consistent with the obtained results from ORR activity and appears to
demonstrate that
the Pt/TOMS catalyst material is more durable than Pt/C and outperforms Pt/C
catalysts
at the beginning of life. The power density of Pt/TOMS appears to be
associated with the
charge transfer between Pt and the TOMS support structure which appears to
cause the
enhancement of the oxygen reduction kinetics. The obtained results confirmed
that the
support material influences the activity of electrocatalysts by promoting the
diffusion of
reactants and products, and this translates into higher performance in an
operating fuel
cell. Moreover, the stability of Pt/TOMS MEA was assessed over the course of
250 hours
of operation. FIG. 11 shows a comparison of the fuel cell performance at the
start of the
test to that obtained at the end of 250 hours. Minimal change in the
performance was
observed, with the polarization curves being virtually unchanged. This
demonstrates that
Pt/TOMS remained stable during the durability test and enhanced stability of
the TOMS
support structure translates into better long-term performance in an operating
fuel cell.
[0086] In some embodiments, the TOMS material may be suitable for use in
electrochemical devices, both for electrochemical energy conversion/storage
and
electrochemical analysis. For instance, as noted herein, the TOMS may be used
as a
catalyst support material for fuel cell electrodes and that the Pt/TOMS
catalyst
demonstrates improved electrocatalytic activity and stability of the oxygen
reduction
reaction, which is a key reaction in polymer electrolyte membrane fuel cells
(PEMFC).
[0087] In some embodiments, the TOMS material and/or the Pt/TOMS catalyst
may
be suitable for use as PEMFC electrocatalysts, both in acidic media and
alkaline media
(alkaline fuel cells) that employ hydrogen as a fuel. In these embodiments,
alloy catalysts
17
CA 3019718 2018-10-03
may be deposited on the TOMS material. For instance, alloy catalysts deposited
on
TOMS that are suitable for use as PEMFC electrocatalysts may include, but are
not
limited to, Pt-Ni/TOMS and/or Pt-Co/TOMS.
[0088] In some embodiments, the TOMS material and/or the Pt/TOMS catalyst
may
also be suitable for use in direct alcohol fuel cells, including direct
methanol and direct
ethanol fuel cells. In these embodiments, metal catalysts may be deposited on
the TOMS
materials. Metal catalysts deposited on TOMS suitable for use in direct
alcohol fuel cells,
including direct methanol and direct ethanol fuel cells, may include but are
not limited to,
Pt-Ru/TOMS and Pt-Sn/TOMS.
[0089] In some embodiments, the TOMS material and/or the Pt/TOMS catalyst may
be suitable for use in formic acid fuel cells. In these embodiments, the metal
catalysts
may be deposited on the TOMS materials. Metal catalysts deposited on TOMS
suitable
for use in formic acid fuel cells may include but are not limited to Pd/TOMS.
[0090] In some embodiments, the TOMS material and/or the Pt/TOMS catalyst
may
be suitable for use in water electrolyzers.
[0091] In some embodiments, the TOMS material and/or the Pt/TOMS catalyst
may
be suitable for use as supercapacitors/electrochemical capacitors.
[0092] In some embodiments, the TOMS material and/or the Pt/TOMS catalyst
may
be suitable for use as electrochemical sensors. For example, the TOMS material
and/or
the Pt/TOMS catalyst may be suitable for use as breath alcohol sensors (i.e.
breathalyzers) and glucose sensors.
EXAMPLES
[0093] Titanium(IV) oxide, anatase (TiO2) 99.8 wt%, chloroplatinic acid
hexahydrate
(H2PtC16.6H20) 37.50% Pt basis, sodium borohydride (NaBH4) 98 wt%, ethylene
glycol
98 wt% (EG), potassium hydroxide (KOH) 85 wt%, ammonium hydroxide (NH4OH)
28.0%
NH3 basis, polyvinylpyrrolidone (PVP40: (C6H9N0), average molar weight 40
000),
poly(ethylene glycol)-b/ock-poly(propylene glycol)-b/ock-poly(ethylene glycol)
(Pluronic
18
CA 3019718 2018-10-03
123, average molar weight 5800), sulfuric acid (H2SO4) 95-98 wt%, Naflon
perfluorinated resin solution 5 wt%, acetone (CH3-COCH3) 99.5 wt%, 2-propanol
(C3I-180)
99.5 wt%, ammonium molybdate (H24M07N6024.4H20), Silicon nano-powder (Si) 98%,
were purchased from Sigma-Aldrich. A commercial platinum catalyst 20 wt% on
carbon
black Johnson Matthey, HiSPEC 3000 was purchased from Alfa Aesar. A gas
diffusion
layer (GDL) Elat LT1400W single sided was purchased from NuVant Systems Inc. A
Naflon membrane NRE212 was purchased from Ion Power and nitrogen and oxygen
gases were supplied in cylinders by PRAXAIR with 99.999% purity. All aqueous
solutions
were prepared using ultrapure water obtained from a Millipore Milli-Q system
with
resistivity >18 mn cm-1.
Synthesis of the Ti305M0o.2Sio.4 (TOMS) support structure
[0094] The TOMS support structure was prepared by doping commercial
TiO2anatase
with Mo and Si. TiO2 was dispersed in a solution of (70:30 vol%) ultrapure
water and
ethanol, followed by the addition of 2 wt% Pluronic P123 surfactant. The
obtained solution
was stirred for 5 hours at ambient temperature. Then 20 wt% of MO
(H24Mo7N6024.4H20)
was added to the solution. The pH of the solution was held constant at pH = 9
by adding
NH4OH. The solution was continuously stirred at room temperature for another 5
h under
N2 purging, and dried at 80 C. The obtained powder was annealed at 850 C
(heating
rate of 10 C min-1) for 8 h under a reducing atmosphere (H2 : N2 10 : 90
vol%). The
obtained Ti305Mo0.2 powder dispersed in a solution of (50: 50 vol%) ultrapure
water and
ethanol followed by the addition of 2 wt% Pluronic P123 surfactant and 10 wt%
Si NPs.
The solution was stirred at room temperature for another 3 h under N2 purging,
and dried
at 80 C. The obtained powder was annealed at 550 C (heating rate of 10 C min-
1) for 5
h under a reducing atmosphere (H2: N2 10 : 90 vol%).
Synthesis of a Pt/Ti305Moo.2Sio.4 (Pt/TOMS) support
[0095] The synthesis of a Pt/TOMS electrocatalyst was performed through a
modified
polyol method. 200 mg of the TOMS support structure was added to a solution of
EG and
ethanol (80:20 ml). The mixture was left stirring for 1 h. Then 5 wt% PVP was
added to
the solution and left stirring for 3 h. A solution of H2PtC16-.6H20 (80 mg)
was dissolved in
19
CA 3019718 2018-10-03
EG (5 ml), added to the solution containing TOMS and left stirring for 3 h at
pH 11 by
adding 1 M KOH. The solution was heated at 90 C under a water-cooled reflux
condenser
for 5 h, and after that the solution was cooled to room temperature and then
stirred for 6
h. The pH of the solution was brought to 4 by adding 1 M HNO3, and left
stirring for 12 h.
The obtained solution was filtered, washed with ultrapure water, and
subsequently dried
at 80 C under N2 purging. The obtained sample of Pt/TOMS was annealed at 450
C
(heating rate of 5 C m1n-1) for 4 h under a reducing atmosphere (H2: N2 10 :
90 vol%).
Physical characterization of the electrocatalysts
[0096] The phases and lattice parameters of the TOMS and Pt/TOMS were
characterized by using X-ray diffraction (XRD) employing a Rigaku Ultima IV X-
ray
diffractometer system detector. This instrument employed Cu Ka radiation, (k =
0.15418
nm) operating at 40 kV and 44 mA. Diffuse reflectance UV-vis spectra of
TiO2(1V) oxide,
anatase and synthesized TOMS were recorded using a Perkin Elmer Lambda-750S
UV/VIS spectrometer. The optical absorption spectra were used to determine the
band
gap of each sample by applying the Tauc equation. The surface composition of
the
Pt/TOMS catalyst was studied by XPS, employing the Thermo Scientific K-Alpha
Angle-
Resolved system equipped with a monochromatic Al Ka (1486.7 eV) X-ray source
and a
180 double focusing hemispherical analyzer with a 128 channel detector with
effective
charge compensation. Transmission electron microscopy (TEM) images of the TOMS
support structure and the Pt/TOMS electrocatalyst were acquired using a Zeiss
Libra
200MC Transmission Electron Microscopy (TEM) system operating at 200 kV.
Scanning
Electron Microscopy (SEM) images were obtained using a Hitachi FlexSEM 1000
system
equipped with an energy dispersive X-ray analyzer. The electrical conductivity
of the
TOMS support structure was measured in the solid state phase via two-point
probe
measurements. The TOMS powder was pelletized under a manual press (15000
pounds),
resulting in the TOMS pellet with a diameter of 10 mm and a thickness of 1 mm.
The
TOMS pellet was placed between two copper probes with 9.3 mm cross section,
and then
the potential in the range of 0.1-1.2 V vs. RHE was applied in order to
measure the
produced current.
CA 3019718 2018-10-03
Electrochemical characterization of the electrocatalysts
[0097] The electrochemical evaluation of catalysts and catalyst supports
was
performed by immobilizing the sample onto the surface of a glass carbon
rotating disk
electrode (Pine Instruments). Inks were prepared by dispersing each catalyst
in a solution
containing ultrapure water and isopropanol alcohol (50-50 vol%), followed by
adding
Naflon at an ionomer-to catalyst ratio of 0.15. After mixing, 4 mL of ink was
deposited
onto the surface of the glassy carbon electrode (0.19625 cm2), and allowed to
dry for 20
minutes. This ink-coated electrode served as the working electrode and was
placed in a
solution of 0.5 M H2SO4 along with a Hg/HgSO4 reference electrode and either a
graphite
rod or a Pt wire counter electrode.
[0098] Electrochemical experiments were performed using either a Pine
WaveDriver
20 potentiostat or a Solartron 1470 multichannel potentiostat coupled to a
Solartron 1260
Frequency response analyzer. Cyclic voltammetry (CV) and electrochemical
impedance
spectroscopy (EIS) experiments were performed in N2-saturated solution.
Impedance
spectra were collected over a frequency range of 100 kHz to 0.1 Hz at a DC
bias potential
of 0.425 V vs. RHE. The ORR activity was assessed using linear sweep
voltammetry
using a rotating disk electrode in 02-saturated solution. Catalyst durability
was assessed
using an accelerated stress test (AST) that involved repeated cycling of the
working
electrode potential between 0.05 and 1.25 V vs. RHE at a scan rate of 50 mV s-
1, in an
N2-saturated 0.5 M H2504 solution. According to the United States Department
of Energy
testing protocols, this potential range assures the accelerated corrosion of
the support as
well as the sintering of Pt NPs. The electrode condition was monitored by
periodic CV
and EIS assessments throughout the AST. In addition, the ORR activity of each
electrocatalyst was assessed before and after the AST.
Membrane electrode assembly (MEA) preparation and testing
[0099] Both Pt/TOMS and Pt/C MEA were prepared through ink spray deposition
onto
the gas diffusion layer (GDL). The obtained electrodes were dried over a
vacuum plate
at 50 C for 2 hours, and then transferred to an oven at 80 C for 6 h. Both
electrodes had
Pt loadings of 0.20 mg cm2 and contained 30 wt% Naflon. MEAs were fabricated
by hot-
21
CA 3019718 2018-10-03
pressing (150 kg cm2 for 90 s at 110 C) the two identical electrodes across a
Naflon
NRE212 membrane. MEAs were tested in a 5 cm2 test fuel cell (Fuel Cell
Technologies)
on a commercial fuel cell test station (Fuel Cell Technologies) controlled
using Labview
software.
[0100]
While the applicant's teachings described herein are in conjunction with
various
embodiments for illustrative purposes, it is not intended that the applicant's
teachings be
limited to such embodiments as the embodiments described herein are intended
to be
examples. On the contrary, the applicant's teachings described and illustrated
herein
encompass various alternatives, modifications, and equivalents, without
departing from
the embodiments described herein, the general scope of which is defined in the
appended
claims.
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