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Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical oxidation behavior of titanium nitride based electrocatalysts
under PEM fuel cell conditions
Bharat Avasarala ∗ , Pradeep Haldar
College of Nanoscale Science and Engineering, University at Albany, SUNY, 253 Fuller Road, Albany, NY 12203, USA
a r t i c l e
i n f o
Article history:
Received 26 May 2010
Received in revised form 7 July 2010
Accepted 9 August 2010
Available online xxx
Keywords:
Proton exchange membrane fuel cells
Catalyst supports
Electrochemical oxidation
Corrosion
Titanium nitride
Electrocatalyst
XPS
a b s t r a c t
Titanium nitride (TiN) is attracting attention as a promising material for low temperature proton exchange
membrane fuel cells. With its high electrical conductivity and resistance to oxidation, TiN has a potential
to act as a durable electrocatalyst material. Using electrochemical and spectroscopic techniques, the
electrochemical oxidation properties of TiN nanoparticles (NP) are studied under PEM fuel cell conditions
and compared with conventional carbon black supports. It is observed that TiN NP has a significantly lower
rate of electrochemical oxidation than carbon black due to its inert nature and the presence of a native
oxide/oxynitride layer on its surface. Depending on the temperature and the acidic media used in the
electrochemical conditions, the open circuit potential (OCP) curves shows the overlayer dissolved in the
acidic solution leading to the passivation of the exposed nitride surface. It is shown that TiN NP displays
passive behavior under the tested conditions. The XPS characterization further supports the dissolution
argument and shows that the surface becomes passivated with the O–H groups reducing the electrical
conductivity of TiN NP. The long-term stability of the Pt/TiN electrocatalysts is tested under PEM fuel
cell conditions and the trends of the measured electrochemical surface area at different temperatures is
shown to agree with the proposed passivation model.
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
In the past decade, fuel cell technology has made significant
strides towards commercialization but much work needs to be
done in many aspects of this promising alternative energy source
for it to compete against the conventional energy sources. Of the
various types of fuel cells, proton exchange membrane (PEM) fuel
cells have received broad attention due to their low operating temperature, low emissions and a quick startup time. But the cost and
the lifetime of a PEM fuel cell system are the major challenges that
are hindering its large-scale commercialization [1,2]. Lifetime of a
PEM fuel cell is mainly dependent on the durability of its material components [3]. As the PEM fuel cell operates under harsh
conditions, high-performance durable materials are required to
withstand the degradation caused due to these corrosive operating
conditions.
The current fuel cell durability demonstrated in vehicles is
1977 h (∼60,000 miles) while the requirement for large-scale commercialization is 5000 h (∼150,000 miles) [2], which the current
internal combustion (IC) engine based automotive vehicles can easily accomplish. Apart from the lifetime costs, durability can also
influence the reliability of the PEM fuel cell technology against its
∗ Corresponding author.
E-mail address: [email protected] (B. Avasarala).
counterparts.
Investigations have revealed that a considerable part of the
performance loss is due to degradation of the electrocatalyst [4]
during extended operation and repeated cycling [5], especially
for PEM fuel cells in automotive applications. Currently, the carbon black supported platinum nanoparticles (Pt/C) remains the
state of the art electrocatalyst for PEM fuel cells. Under the corrosive operating conditions, the Pt/C degrades via Pt dissolution, Pt
particle agglomeration and carbon support corrosion mechanisms
resulting, primarily, in the loss of electrochemical surface area. Furthermore, the catalytic metal, especially Pt, catalyzes the oxidation
of carbon [6,7] and the oxidation of carbon black accelerates Pt sintering [8]. Vulcan XC-72 carbon black is the most popular catalyst
support currently used in the Pt/C electrocatalysts but its durability,
under the oxidizing conditions of a PEM fuel cell, needs significant
improvement [9,10].
An ideal catalyst support material should have corrosion resistance properties under strongly oxidizing conditions of PEM fuel
cell: high water content, low pH (<1), high temperature (50–90 ◦ C),
high potentials (>0.9 V) and high oxygen concentration. But carbon is known to undergo electrochemical oxidation to form surface
oxides and CO/CO2 under these conditions [11–13]. Significant
oxidation of carbon support can be expected to decrease the
performance of a PEM fuel cell [13,14], due to the loss and/or
agglomeration of Pt particles caused by the carbon corrosion. It
has also been reported [14,15] that oxygen containing groups (e.g.,
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.08.035
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carboxyl, carbonyl, hydroxyl, phenol) can be formed on the carbon surface at high temperatures and/or high potentials (>1.0 V
vs. RHE at room temperature or >0.8 V vs. RHE at 65 ◦ C) which
decrease the conductivity of catalysts and weaken its interaction
with the support resulting in an accelerated Pt sintering [8,16,17];
thus drastically affecting the performance of PEM fuel cell. In our
report [9], we showed that carbon black supports have a higher rate
of corrosion under potential cycling compared to constant potential conditions and tend to form significant amount of surface oxide
groups. These studies indicate towards the vulnerability of carbon
support under prolonged fuel cell operating conditions.
In our recent publication [18], we introduced titanium nitride
(TiN) nanoparticles as catalyst supports for synthesizing Pt/TiN
electrocatalyst and showed that it can outperform the Pt/C electrocatalyst in electrochemical surface area and catalytic activity for the
same Pt particle size and loading [18]. With its unique properties
of higher electrical conductivity (relative to carbon) and an outstanding oxidation and acid corrosion resistance [19–21], TiN has a
high potential for developing durable electrocatalysts. Research is
also being pursued on the materials of transition metal carbide and
nitride family for their application in low temperature fuel cells,
as described in a review by Ham and Lee [22]. Recently Musthafa
and Sampath [23] reported the application of TiN films for direct
methanol based fuel cells and other groups [24,25] have reported
the application of Ti-based compounds for fuel cells [24,25]. But
the stability and the long-term durability aspect of the electrocatalyst material under above room temperature fuel cell operating
conditions is not discussed. As titanium nitride gains attention as
an electrocatalyst material, its behavior under PEM fuel cell conditions remains to be investigated. Considerable research has been
done in the past in electrochemically evaluating TiN thin films in
electrolytes of a wide-pH range. But behavior of TiN vis-à-vis –
high surface area porous material, above RT conditions, cyclic/high
potentials, highly acidic conditions (pH < 1) – which are the typical conditional variables of an electrocatalyst material in a fuel
cell, has not been reported in the literature. Through this paper,
we extend upon our research on the TiN based electrocatalysts and
report the electrochemical behavior of titanium nitride nanoparticles by evaluating them under the operating conditions of a PEM
fuel cell. Along with the bare supports, we also report the electrochemical behavior of synthesized Pt/TiN electrocatalyst and its
stability under prolonged exposure conditions.
2. Titanium nitride
Titanium nitride belongs to the transition metal nitride family
and displays similar formation and physicochemical properties of
other nitrides in the family. Many of its properties are caused by a
special feature of its electronic structure, i.e., transfer of part of sand d-valency electrons of the atoms of Ti to the 2p-states of the
N thereby depleting the electron population of the d-states of Ti,
which are responsible for its covalent bonding with foreign atoms
[26,27]. According to Ham and Lee [22], this deficiency in the dband occupation near the Fermi level should make the TiN surface
to have a reduced ability to donate d-electrons to adsorbates; thus
making it a better electron acceptor.
The triple bond between Ti and N and the mixed p and d characteristics of the interaction between Ti 3d and N 2p represents
an essential aspect of covalent bonding between titanium and
nitrogen atoms and is responsible for its inert nature, mechanical
hardness and high melting point [28]. The metallic character arises
with one unpaired electron going into a metal localized sp hybrid
orbital on Ti resulting in the non-zero electron density at Fermi
level (EF ) [28] contributing to its relatively high electrical conductivity. Oyama [19] reported the electrical conductivity of bulk TiN as
4000 S cm−1 and that of C as 1190 S cm−1 while other groups have
reported values ranging from 3125 S cm−1 [29] to 33,000 S cm−1
[30] for polycrystalline TiN thin films to ∼55,500 S cm−1 [28 and
Refs. within] for TiN single crystal, conductivities that are higher
than that of carbon.
TiN is well known for it oxidation resistance property, an aspect
contributed to the native oxide/oxynitride layer which partially
covers its surface and is formed due to atmospheric oxidation
[28,31–33]. The titanium–oxygen–nitrogen system [34] has been
well researched and the composition of the native layer has been
attributed to a non-uniformly distributed mixture of oxynitride
and oxide [28,31,32,35–38] components on its surface that prevent the surface from further oxidation. The oxide and oxynitride
components are also formed during the electrochemical oxidation
of titanium nitride under potential scanning conditions [28].
2.1. Oxidation
TiN undergoes a thermodynamically favorable oxidation reaction when exposed to air [28,34]:
2TiN + 2O2 → 2TiO2 + N2 ↑ G◦ = −611.8 kJ mol−1 (298 K)
(1)
The oxidation process takes place in three stages [35,39]: stage
1: oxygen diffuses into the TiN lattice resulting in replacement of N
by O, leading to the formation of titanium oxynitride. Stage 2: the
natural protective films of the phases of Tin O2n−1 types are formed
on the oxynitride layer during this stage, representing a diffusioncontrolled oxidation [40]. Stage 3: slow oxidation of the residual
material of TiN coated with TiO2 .
A major portion of the replaced nitrogen is released in the interstitial positions of surface oxide layers as molecular nitrogen, which
gets released from the surface upon further oxidation [35].
2.2. Electrochemical oxidation of TiN
Many research papers have evaluated the electrochemical
oxidation behavior of titanium nitride films in various media
[28,32,36–38], and showed the high stability of TiN against oxidation process in a wide-pH range. The electrochemical oxidation
resistive properties were attributed to the presence of the nitrogenenriched surface layer of titanium oxynitride with a large electron
density that screens the underlying titanium ions and inhibits the
oxidation reaction [20,28].
The electrochemical oxidation of TiN follows a similar path of
oxidation process, where: 0.5 V to 0.8–0.9 V: the formation and
growth of oxide/oxynitride films on the surface of TiN by reaction
(2) predominantly leads to the retardation (or passivation) of these
processes.
TiN + 2H2 O → TiO2 + 1/2N2 + 4H+ + 4e− [28, 41]
(2)
−
(3)
TiN = Ti
3+
+ 1/2N2 + 3e
∼1.0–1.5 V: oxidation resulting in formation of hydroxides [41],
TiN + 3H2 O → Ti(OH)3 + N2 + 3H+ + 3e−
+
TiN + 3H2 O → TiO2 ·H2 O + 1/2N2 + 4H + 4e
(4)
−
(5)
Milošev et al. [28] attributed the increase in current in this
potential region to the formation of TiO2 on the surface of TiN rather
than hydroxide (reaction (4)), but accompanied by a similar liberation of N2 . The reasoning being that the hydroxides are more
soluble and less protective than the oxides, and can cause the rise
of currents in this region. Above 2.0 V: at these potentials, oxygen
evolution takes place with a simultaneous oxidation of titanium
nitride to TiO2 .
Through the above discussion in oxidation and electrochemical
oxidation processes, we intend to better address the nature of TiN
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nanoparticles under PEM fuel cell conditions by fully considering
the reported electrochemical behavior of TiN in the literature.
3. Experimental
3.1. Catalyst support materials
The TiN nanoparticles or TiN NP that are used as catalyst supports in this study were commercially purchased (NanoAmor Inc.,
USA) and have an average particle size (APS) of 20 nm and a specific surface area (SSA) of 40–55 m2 g−1 [42]. The mean particle
size, derived from the transmission electron microscopic (TEM)
image (Fig. 1) and the X-ray diffraction pattern of TiN(1 1 1) peak
(Fig. 2) using Debye–Scherer equation, match closely to the vendor reported value. The purity ascribed to the TiN was >97% and
an energy-dispersive spectroscopy was performed to verify the
presence of impurities, as shown in Fig. A1 of Appendix. For a
comparative investigation with carbon, commercially purchased
Vulcan XC-72R carbon black (Cabot Inc., USA) of a known surface
area of ∼250 m2 g−1 is used.
Fig. 2. X-ray diffraction pattern of TiN nanoparticles.
3.2. Preparation of catalyst support coated electrodes and
electrochemical tests
For electrochemical comparison of TiN and C, the TiN NP is sonicated in ethanol and an aliquot is deposited on a polished glassy
carbon electrode (GCE, 5 mm dia., 0.1962 cm2 , Pine Instruments)
for a material loading of 0.15 mg cm−2 and the solvent is allowed
to evaporate. The TiN NP coated GCE is used as a working electrode
in a three-electrode cell set-up and a similar procedure is followed
for depositing carbon black (CB) on polished GCE.
For spectroscopic characterization of TiN, carbon cloth (E-Tek® )
is used as a substrate on which TiN NP is coated. The coating is
prepared by ultrasonically mixing TiN NP, isopropyl alcohol and
0.05 wt% Nafion® for a loading of 3 mg cm−2 , with 95 wt% of TiN
NP. Electrodes were dried at 80 ◦ C for 10 min for the evaporation of
any remaining solvent.
Electrochemical tests were performed in a standard threeelectrode cell set-up with a PARSTAT 2273 potentiostat (Princeton
Applied Research, USA). A reversible hydrogen electrode (RHE)
(Gaskatel GmBH, Germany) is used as a reference electrode with
a Pt mesh acting as a counter electrode. All potentials are reported
with respect to RHE. Using cyclic voltammetry (CV) techniques,
the GCE was cycled between 0 and 1.0 V potentials at 50 mV s−1
in Ar saturated 0.1 M HClO4 (pH < 0.5) electrolyte at 60 ◦ C for 24 h.
The maximum potential of 1.0 V is chosen as it is near to fuel cell
idle conditions. Similarly, the TiN NP coated carbon cloth electrode
is held vertically inside the electrolyte chamber under the same
conditions but cycled between 0 and 1.2 V potentials at 50 mV s−1
for 16 h. Potential cycling is chosen for the electrochemical evaluation as it is one of the extreme operating conditions in a PEM
fuel cell where the electrode is exposed to repeat variations in
load due to changing potentials that result in the application of
oxidation/reduction processes on the electrode surface.
3.3. Synthesis of Pt/TiN electrocatalyst
Fig. 1. (a) Transmission electron micrograph showing the TiN nanoparticles and (b)
a histogram showing the TiN nanoparticles distribution with average particle size
at 18.9 nm.
TiN NP is mixed in ethylene glycol (EG) and ultrasonically
treated before adding H2 PtCl6 ·6H2 O drop wise. Sodium hydroxide
(NaOH) is added to control the size of Pt nanoparticles by adjusting
the pH of the solution (∼12). The solution is stirred well before heating at 160 ◦ C for 3 h in a N2 atmosphere under refluxing conditions.
The solution is stirred overnight at room temperature, after which
it is washed and filtered. The filtrate removed powder is dried in
air at 80 ◦ C for 4 h.
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3.4. Preparation of Pt/TiN electrocatalyst coated electrodes and
half-cell performance
For determining the electrochemical surface area (ECSA) of
Pt/TiN, the CV technique is used. The electrocatalyst dispersions
are prepared by ultrasonically mixing the Pt/TiN in ethanol and
pipetting an aliquot of the dispersion onto the GCE for a catalyst loading of 20 mgPt cm−2 . After drying, 10 ␮l of aqueous Nafion
(5 wt%) solution is pipetted onto the electrode surface in order to
attach the catalyst particles onto the electrode surface and also to
create pathways for the H+ ions in the electrolyte to access the
catalytic sites. The Pt/TiN deposited GCE is immersed in the Ar saturated 0.1 M HClO4 which is at 60 ◦ C and scanned between 0 and
1.2 V at 50 mV s−1 . Half-cell performance is measured by the electrochemical surface area (ECSA) which is calculated from the area
under the hydrogen adsorption and desorption peaks of CV curves
after double-layer correction.
3.5. Surface characterization
X-ray photoelectron spectroscopy (XPS) analysis was performed using a ThermoFisher Theta Probe system which has a
2-dimensional detector that will allow simultaneous collection
of spectral data from all angles without tilting the sample. The
spectrometer was equipped with a hemispherical analyzer and
a monochromator and all XPS data presented here are acquired
using Al K␣ X-rays (1486.6 eV) operated at 100 W. Sputter cleaning was done with a differentially pumped Ar+ sputter gun. The
sample to analyzer takeoff angle was 45◦ . Survey spectra were collected at pass energy (PE) of 125 eV over the binding energy range
0–1300 eV. High binding energy resolution multiplex data for the
individual elements were collected at a PE of 29.55 eV. The XPS
spectra were background subtracted using the non-linear, Shirley
method. The binding energies of the Ti 2p, N 1s, O 1s and C 1s
of the TiN electrodes were calibrated with respect to C 1s peak
at 284.6 eV. XPS characterization was performed on the electrode
samples before and after the electrochemical treatments.
A two-theta X-ray scan is performed on the TiN NP using a X-ray
diffractometer XDS 2000 (Scintag Inc., USA), where the wavelength
of X-rays from Cu source is 1.54 Å.
4. Results and discussion
nitride and oxide peaks is attributed to the oxynitride which has an
oxidation state between TiN and TiO2 [28,35].
The native layer on the surface of TiN nanoparticles used in
our study is a combination of Stage 1 (TiOx Ny phase) and Stage
2 (Tin O2n−1 phase) of the oxidation process (Section 2.1), with the
latter being predominant while no complete phase separation to
TiO2 [28] is seen to occur from the XRD plot in Fig. 2.
Using the TiN NP with partially covered native oxide/oxynitride
layer, the Pt/TiN electrocatalyst was synthesized and its electrochemical evaluation has been reported in our previous paper [18].
The cyclic voltammogram (CV) or the oxygen reduction reaction
(ORR) curves for Pt/TiN showed no indication of an ohmic resistance
due to the oxide/oxynitride layer on the surface of TiN nanoparticles. In fact, 20 wt% Pt/TiN showed higher electrochemical surface
area and catalytic activity compared to the conventional 20 wt%
BASF® Pt/C [18] for the same Pt loading. If the oxide/oxynitride layer
were to act as a poor electrical conductor, then the ohmic losses
(due to the inhibition of electron transportation from the catalyst
particle to the support) should cause retardation in the kinetics of
the catalytic reactions; which was not observed in our tests suggesting an alternative possibility. In the later part of this report, an
explanation to this behavior and the role of the oxide/oxynitride
layer on TiN nanoparticles are discussed.
4.2. Electrochemical comparison of CB and TiN NP under PEM fuel
cell operating conditions
Fig. 3 shows the CV curves of TiN NP and CB electrodes, where the
current densities generated by TiN NP are significantly lower when
compared to those of CB under identical test conditions. The inert
nature of TiN and the presence of an oxide/oxynitride layer on its
surface attribute to its initial lower current densities (TiN (0.5 h) vs.
C (0.5 h) in Fig. 3). Since the native oxide/oxynitride layer on TiN NP
acts as a barrier, the low oxidation current densities observed are,
predominantly, a result of the electrochemical oxidation of nitride
component on the surface. Based on these two arguments, it can
be reasoned that the composition of oxynitride and oxide components will increase while that of nitride decreases after the potential
cycling (as the nitride essentially converts to oxide and oxynitride
components due to electrochemical oxidation).
In the case of CV curve for TiN NP electrode shown in Fig. 3,
there is a wide margin of current density drop after 0.5 h with the
later cycles 8 h, 16 h (not shown in Fig. 3) and 24 h having the same
4.1. Oxide/oxynitride on TiN nanoparticles
The presence of the oxide/oxynitride components on the TiN
nanoparticles is confirmed using X-ray diffraction (XRD) technique,
as shown in Fig. 2. The strong peaks of TiN seen in the diffraction
pattern can be indexed to a pure NaCl-type structure (S.G. Fm3m
(2 2 5)). The lattice constant is ‘a’ = 4.212 Å, which is calculated from
Bragg’s angle of the (1 1 1) diffraction plane of XRD spectra of TiN
NP (calculations in Appendix).
The oxynitride (TiOx Ny ) is an intermediate phase and can be considered as a solid solution of titanium nitride (TiN) and the cubic
titanium oxide (TiO) [43]. Hence, the cubic lattice constant ‘a’ of a
given TiOx Ny , according to Vegard’s law relationship [34,44], should
lie between the value of lattice constants of TiN (4.241 Å) and TiO
(4.185 Å). The lattice constant calculated for TiN nanoparticles is
‘a’ = 4.212 Å is found to lie between these two values thus implying
the existence of an oxynitride layer (TiOx Ny ) on the surface of TiN
nanoparticles [34,44,45]. A comparison is made between the TiN
NP and the standard stoichiometric TiN XRD peaks and the discussion is provided in Table A1 of Appendix. Further evidence of the
oxynitride layer on TiN NP is also shown in the X-ray photoelectron
spectra of Ti 2p3/2 in Fig. 6, where the deconvoluted peak between
Fig. 3. Cyclic voltammogram (CV) of TiN and CB after potential cycling between 0
and 1.0 V (50 mV s−1 ) in an inert-gas saturated 0.1 M HClO4 at 60 ◦ C using a threeelectrode cell set-up. Material loading on GCE is 0.15 mg cm−2 .
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Fig. 5. Relative atomic % concentrations of surface elements on TiN electrode:
untreated and treated, obtained using XPS (excluding C and F).
Fig. 4. Anodic curve of TiN and CB in an oxygen saturated 0.1 M HClO4 at 60 ◦ C using
a three-electrode cell set-up. Scan rate is 50 mV s−1 and values are recorded after
50 cycles. TiN or CB loading: 0.1 mg cm−2 (normalized to specific surface area).
magnitude of lower oxidation current densities. This shows that
TiN NP electrode attains a passivating surface much earlier unlike
carbon, which continues to corrode at the same rate.
An anodic corrosion curve, in Fig. A2 of Appendix, shows that
the starting oxidation potential of TiN NP is higher compared to CB,
implying that carbon tends to oxidize at much earlier potentials
than titanium nitride. While the starting potential for CB decreases
from 70 to 20 mV making it prone to corrosion at much lower potentials, the starting potential of TiN NP remains constant for the entire
duration indicating that titanium nitride has a higher resistance to
oxidation.
Considering that the catalyst support is frequently affected by
very high potentials on anode side during extreme conditions of
fuel starvation in a PEM fuel cell, a comparison is made between
TiN NP and CB at potentials of 1.3 and 1.5 V and the CV curves are
shown in Fig. 4. It can be seen that the oxidation of CB increases
steeply compared to TiN NP and so does the surface carbon oxidation at ∼0.55 V (hydoquinone/quinone peak). TiN shows highly
passivating behavior, due to its oxide/oxynitride components and
also, possibly due to the titanium dioxide and hydroxide groups
that can be formed at these potentials according to reactions (4)
and (5). To study the surface changes on the TiN NP due to electrochemical oxidation, XPS characterization is performed on TiN NP
coated electrodes.
4.3. Spectroscopic analysis of TiN NP electrodes
XPS is a surface analysis technique which can provide a better
understanding of the changes on the surface due to electrochemical oxidation, along with the atomic concentration of elements on
the TiN support surface. As the catalyst support corrosion in low
temperature fuel cells usually occurs at the surface, XPS can be a
useful technique in observing these changes. For XPS characterization and analysis, the TiN electrodes were exposed to two different
surface conditions, (A) and (B).
(A) Untreated: TiN nanoparticles (TiN NP), as is
(B) Treated: TiN NP coated carbon cloth, 16 h of potential cycling
between 0 and 1.2 V at 50 mV s−1 in 60 ◦ C argon saturated 0.1 M
HClO4 electrolyte.
The XPS survey spectra of untreated and treated TiN NP electrodes exhibited the characteristic Ti 2p, O 1s and N 1s peaks at
the corresponding binding energies of 528.2, 456.5 and 396.2 eV,
respectively, in accordance with values reported in literature
[28,31,33,35,37,38,46]. Based on the elemental peaks in the survey
spectra and the sensitivity factors of individual elements, the relative atomic concentration on the surface of untreated and treated
TiN electrodes are plotted in Fig. 5.
The initial oxygen composition on the ‘untreated’ TiN NP electrode (in Fig. 5) is attributed to the native oxide/oxynitride layer on
its surface; other surface groups such as TiN–O*, TiN–OH, adsorbed
H2 O may also be contributing to oxygen composition, although
minimally. The increase in the composition of Ti is a result of the
electrochemical oxidation of TiN into Ti ions (through reaction (3)).
Similarly, the decrease of N can be attributed to the decrease in
nitride composition due to its oxidation. From the electrochemical
reactions, the oxygen composition should increase as an outcome
of the oxidation of TiN due to potential cycling. But, as seen in Fig. 5,
a slight decrease of oxygen composition is observed for the ‘treated’
TiN NP electrode.
The atomic concentration ratios of the elements on ‘treated’ TiN
electrodes, drawn from their respective peak intensities are presented in Table 1. The O/Ti ratio of the TiN electrodes decreases
after the electrochemical treatment due to the relative increase of
Ti while the increase of O/N ratio indicates towards the relative
decrease of N over O. This suggests that the electrochemical oxidation does not necessarily replace one-to-one of N with O. Based
on these arguments, the decrease of O/TiN ratios from 0.39 to 0.35,
suggests a relative increase in TiN composition after the electrochemical treatment, the increase attributed to the increase in Ti
composition.
The significant increase in Ti/N ratio suggests that the N
decreases while Ti increases simultaneously, presumably from the
same compound. Since the oxidation of TiN inevitably results in
nitrogen as a by-product in the form of N2 , NO2 , NO, etc. [37] one
can see the decrease in nitrogen composition. The increase of Ti,
with a decrease of O and N, suggests that the surface is not predominated by the oxide layer after the electrochemical treatment.
Further investigation of the elemental peaks can provide information on the nature of surface groups formed on the surface of TiN
and their trends after the electrochemical treatment.
Table 1
The atomic concentration ratios of surface elements on TiN electrode: untreated and
treated.
Sample
O/Ti
O/N
O/TiN
Ti/N
Untreated
Treated
0.60
0.50
1.15
1.22
0.39
0.35
1.91
2.42
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Table 2
Assignments to the component peaks of the high-resolution XPS of the ‘untreated’
and ‘treated’ TiN NP electrodes.
Peak
Component
Binding energy (eV)
Untreated
Treated
Assignment
Ti
2p3/2
Ia
IIa
IIIa
A
455.6
456.9
459.1
–
455.6
457.2
459.0
460.7
TiN
TiOx Ny
TiO2
Ti(OH)2 2+
Ti
2p1/2
Ib
IIb
IIIb
B
461.7
463.4
465.0
–
–
462.1
464.4
466.2
TiN
TiOx Ny
TiO2
Ti(OH)2 2+
O
1s
IV
V
VI
VII
VIII
530.9
–
532.5
533.9
–
530.5
531.2
533.2
534.0
535.5
TiO2
O–H (Ti(OH)2 )2+ )
Adsorbed O2 /H2 O
TiOx Ny
NO
N
1s
IX
X
XI
396.2
397.2
399.1
396.5
397.3
399.4
O–N396 ev (TiOx Ny )
TiN
O–N399 ev (TiOx Ny )
The most probable assignments to the origin of the components
of high-resolution X-ray photoelectron spectra of Ti 2p, O 1s and N
1s are presented in Table 2 along with their peak assignments that
are based on the reported literature [28,31,33,35,37,38,46].
4.3.1. Ti 2p
The deconvoluted Ti 2p peaks of ‘untreated’ and ‘treated’ TiN
electrodes are shown in Fig. 6(a) and (b), respectively, where the
high-resolution Ti 2p spectra shows the spin orbit doublet Ti 2p3/2
and Ti 2p1/2 peaks. The peak positions and their assignments are
presented in Table 2. The presence of titanium nitride (Ia) and that
of oxynitride (IIa) and oxide (IIIa) components of Ti 2p3/2 formed
due to atmospheric oxidation can be seen in Fig. 6(a) whose binding energies, shown in Table 2, match well with the literature
[28,31,35,38,46]. Using XPS technique, which is strictly a surface
characterization technique, the oxide peak at 459.1 eV is assigned
to TiO2 (IIIa) but a distinct phase of the same is not observed in the
XRD plot in Fig. 2.
Comparing the spectra of Ti 2p in Fig. 6(a) and (b), one can
observe the changes in peak areas and heights after the electrochemical treatment. The binding energies of Ti 2p3/2 and 2p1/2 of
‘untreated’ electrode are assigned to TiN (Ia) and TiN (Ib), respectively, with 455.6 and 461.7 eV as their respective peak positions.
After the electrochemical treatment, both the binding energies of Ti
2p3/2 and 2p1/2 shifted to 460.7 eV (A) and 466.2 eV (B), respectively,
as shown in Fig. 6(b). These characteristic peaks of ‘A’ and ‘B’ are
related to the tetravalent titanium ion, Ti(OH)2 2+ , where the Ti ion
possibly oxidized to the states of higher valence during potential
cycling following reaction (6) [37]:
TiN + 3H2 O = Ti(OH)2 2+ + NO + 4H+ + 6e−
(6)
For Ti in acidic media, the Pourbaix diagram predicts that the
thermodynamically stable states of Ti ions under the condition of
anodic polarization have 3+ and 4+ valence, with the dissociated
titanium, most likely, as Ti3+ , Ti(OH)2+ and Ti(OH)2 2+ [36,37]. Considering the XPS peak positions of ‘A’ and ‘B’ of Ti 2p in Fig. 6(b) and
the tetravalence of Ti ion according to the Pourbaix diagram of Ti in
acidic media, it is reasoned that the Ti ion could be taking the form
of Ti(OH)2 2+ with a 4+ oxidation state, after the electrochemical
treatment.
The areas under the deconvoluted peaks of Ti 2p3/2 are presented in Table 3 where one can see a significant presence of oxide
and oxynitride components on the surface of the untreated TiN
electrode, suggesting that the process of atmospheric oxidation
has been initiated to certain extent, supporting the earlier statements. Due to their inert nature, the existing oxide/oxynitride layer
present on the surface of TiN NP does not oxidize any further. It
should be noted that the overlayer was never transformed to a
pure titanium oxide even after 2 years of air exposure, but rather
remained a mixture of oxynitride and oxide. This implies that the
oxidation mainly occurs on the nitride part of the surface on TiN
nanoparticles resulting in the decrease of its composition, with a
corresponding increase in the oxide and oxynitride components
[28] by the same magnitude.
In Table 3, it can be seen that the nitride component, TiN (Ia),
decreases by 17.2% after the electrochemical treatment, with a
10.4% increase in the relative composition of TiOx Ny . This increase
agrees well with the reported literature [28], where the electrochemical oxidation of TiN forms TiOx Ny compound on the
surface. The remaining percentage of decrease in nitride should be
attributed to the increase of oxide composition. But the increase in
the oxynitride composition is not accompanied by a corresponding increase in the oxide component. In fact, the oxide component,
as shown in Table 3, significantly decreases by a relative composition of 46.7%. The area ratio of nitride (Ia) and oxide (IIIa) peaks,
(ATiN /ATiO2 ), in Ti 2p3/2 (Fig. 6) increases from 0.51 to 0.95 suggesting the significant decrease of oxide after the electrochemical
Table 3
Approximate functional group distribution of deconvoluted Ti 2p3/2 for “untreated”
and “treated” TiN electrode (relative composition, %).
Functional groups
Fig. 6. Deconvoluted Ti 2p XPS spectra of: (a) untreated; (b) treated TiN NP electrode.
TiN
TiOx Ny
TiO2
Ti(OH)2 2+
Untreated (1)
31.5
6.8
61.5
–
Treated (2)
14.3
17.2
14.8
53.7
Change in relative
composition %,
(1)–(2)
17.2↓
10.4↑
46.7↓
–
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treatment. The oxide on TiN NP surface dissolves in the acidic
media exposing the nitride surface which further reacts with the
acidic media to form Ti–OH compound according to reaction (8).
The above proposed mechanism explains the significant increase
in Ti(OH)2 2+ , the magnitude (53.7%) of which can be equaled to
the dissolution of oxide component (↓46.7%) and the dissolution
of oxide (↓6.8%) which is newly formed due to the electrochemical
oxidation of nitride component. Although an elaborate discussion
on this topic is provided later in Section 4.4, it is emphasized that
electrochemical treatment under cycling potentials will lead to the
formation of oxide/oxynitride layer [28]. But, unlike in buffer solutions (pH ∼ 7), perchloric acid solution tends to dissolve the layer
exposing the nitride surface underneath.
4.3.2. O 1s
The deconvoluted O 1s XPS spectra for the ‘untreated’ and
‘treated’ TiN electrode are shown in Fig. 7(a) and (b), respectively. The dominant O 1s component (IV) for ‘untreated’ electrode
in Fig. 7(a) is located at 530.2 eV and is assigned to TiO2
[28,31,33,35,38,46,47] but the dominant O 1s peak for the ‘treated’
electrode has shifted to 531.2 eV, which is characteristic to O–H
group [28,35,37,46,48]. The significant increase of O–H peak (V)
of O 1s in Fig. 7, after the electrochemical treatment, supports the
earlier argument about the formation of Ti–OH groups indicated by
the peaks ‘A’ and ‘B’ of Ti 2p in Fig. 6(b) and the Pourbaix diagram
of Ti in acidic media. The remaining components of the O 1s peaks
are labeled in Table 3 and illustrated in Fig. 7. The dissolution of
titanium oxide in acidic media can also be seen from the significant
drop of the oxide peak (IV) at 530.9 eV. Similar to the oxynitride
component in Ti 2p and N 1s spectra (Fig. A3, Appendix), the TiOx Ny
(VII) peak in O 1s is seen to increase for the ‘treated’ TiN electrode.
NO (VIII) is a possible by-product formed during the electrochemical oxidation of TiN [36] along with NO2 and N2 and its peak can
be seen near 535.0 eV [49].
4.4. Dissolution of oxide/oxynitride layer
It is well known [50,51] that the oxides of titanium, due to their
strong basic character, dissolve in acidic media to form colorless
titanium (IV or 4+) oxysalts that decompose in water. In perchloric
acid media such as the one used in our case, Ti (IV) species can
be present as (Ti(OH)2 )2+ or TiO2 + that is derived from the salts of
TiOCl4 2− anions [51]. To confirm the oxide dissolution in perchloric
acid, a few ml of H2 O2 was added TiN NP dispersed in 0.1 M HClO4
and the solution turned to intense orange color. This is due to the
fact that colorless hydrated Ti(IV) ions in the solution get converted
in to orange hydrated (Ti(O2 )OH)+ ions according to reaction (7)
[50] thus confirming the presence of dissolved Ti(IV) species:
(Ti(OH)3 )+ + H2 O2 → (Ti(O2 )OH)+ + 2H2 O
(7)
The immediate change in color of the solution suggests the rapid
dissolution of titanium oxides in the acidic media. Further evidence
to the dissolution of oxide component is shown in the analysis of
XPS spectra of Ti 2p and O 1s in Figs. 6 and 7, where one can see the
decrease of oxide composition.
Unlike the oxides which were quick to dissolve in the acidic
media, it can be reasoned that the oxynitride takes a longer time to
dissolve due to its nitrogen composition. Fig. 8 shows the changes of
open circuit potential (OCP) of TiN NP with immersion time at various temperatures of perchloric acid. OCP or the corrosion potential
(Ecorr ) measures the equilibrium potential above which the oxidation is initiated on the surface. A stable OCP indicates the stable
nature of the electrode while the surface reactions cause the OCP
to fluctuate.
The OCP (at 60, 70, 80 ◦ C) of TiN NP in Fig. 8 shifted rapidly downwards during the initial stage of immersion. The downward shift of
Fig. 7. Deconvoluted O 1s XPS spectra of: (a) untreated; (b) treated TiN NP electrode.
OCP towards the active potentials is due to the self-activation of
TiN and is an indication of a dissolution of the oxynitride on the
surface of TiN [37,52], which increasingly exposes the nitride surface directly to acidic solution. The anodic chemical reaction that
can be related to TiN at these OCP conditions can be expressed as
[37]:
TiN + H2 O → Ti(OH)2+ + 1/2N2 + H+ + 3e−
(8)
The potential-pH (or Pourbaix) diagram of Ti-H2 O predicts that
the possible thermodynamically stable state of Ti ions under the
OCP conditions can be 3+ valence; and the reaction (8) indicates that
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leads to the formation of oxide and oxynitride components [26].
Thus the oxide/oxynitride components on the surface of TiN NP
undergo dissolution in the acidic media and also formation due to
oxidizing potentials during the potential cycling treatment. As a
result of the two parallel phenomena, one can see a net increase of
TiOx Ny component on TiN nanoparticles.
4.5. Electrical conductivity of oxide/oxynitride components on
TiN NP surface
Fig. 8. Shifts in open circuit potential (OCP) with the immersion time (4 h) for TiN
in 0.1 M HClO4 solution saturated with argon.
this dissociated titanium, Ti(III), is most likely Ti(OH)2+ [36,37]. As
the temperature of the perchloric acid solution in Fig. 8 increases
from 60 to 80 ◦ C, the hydrogen ion strength in the acid solution
increases thus causing a faster dissolution of oxynitride layer during the initial stage of immersion. The steeper slope and a lower
dissolution time with an increase in temperature in Fig. 8 are an
indication of faster rate of oxynitride dissolution due to increased
ionic strength of the acid solution.
The rapid decrease of OCP, for the curves in Fig. 8, in the initial
stage of immersion is related to the formation of the corrosion product of Ti(OH)2+ (according to reaction (8)) on the exposed nitride
surface, with its nature of positive charge [37]. The formation of
these corrosion products on the TiN surface leads to a subsequent
adsorption of other negative ions existing in the solution such as
ClO4 − . As a result, an adsorption configuration is likely to be setup where the surface area of TiN NP will be covered with such
anions, naturally prohibiting the dissolution of TiN and thus making the electrode passive and attain more noble potentials. The
OCP curves of 60, 70 and 80◦ C in Fig. 8 drop steeply in the initial
stages of immersion after which it gradually plateaus for the rest
of the immersion, in the passive region of the plot. This increase
towards nobler potentials is likely the result of subsequent adsorption of anions in the solution by the positively charged Ti(OH)2+
layer on the nanoparticles surface. This adsorption model, proposed
by Chyou et al. [37] for TiN thin films in varying concentrations of
sulfuric acid solutions, explains the electrochemical behavior of TiN
NP under 0.1 M perchloric acid at different temperatures.
The minimal decline of OCP curve at room temperature (RT,
23 ◦ C) in Fig. 8 indicates that the rate of dissolution of the oxynitride
layer on the surface of TiN NP is very low at the given conditions;
which imply that TiN NP will take a significantly longer time to
passivate under the RT conditions of 0.1 M HClO4 acid electrolyte.
It should be noted that the variation in the starting OCP potential
at various temperatures is most likely due to the varying thickness
of the oxynitride layer on the surface of TiN NP.
Although the OCP plot shows the decrease of oxynitride layer
due to its dissolution, Table 3 and the Ti 2p and O 1s spectra show
an increase of TiOx Ny composition for the ‘treated’ TiN NP electrode. Unlike potential cycling, no external potential is imposed on
the TiN NP electrode surface during the OCP conditions. Hence, the
dissolution of TiOx Ny in the initial stage and a passivation of nitride
surface in the later stage are observed. Potential cycling involves
imposing oxidizing potentials on the TiN NP electrode surface and
An aspect of concern for TiN nanoparticles is the change in
its electrical conductivity due to the presence of the oxide and
oxynitride components on the surface. Since the surface is not
predominated with TiO2 , the oxide component largely exists as
a sub-stoichiometric composition (Tin O2n−1 ) of Magneli phase,
which has electrically conductive properties [24]. With an increase
in surface oxygen composition due to electrochemical oxidation
at high potentials, the surface layers can become titanium dioxide
but get easily dissolved in the acidic conditions of fuel cell without
acting as an insulator.
The oxynitride component of the native layer, TiOx Ny , has a
relatively slower rate of dissolution compared to oxide and consists of lower percentage of oxygen composition than the oxide
component. TiOx Ny has an electrically conductive nature due to its
NaCl crystal structure which is similar to TiN and also a significant
presence of nitrogen in its lattice [34]. It has been reported [53]
that titanium oxynitride thin films with low oxygen contents (<40
at.%) are partly metallic suggesting that the oxynitride layer on the
surface of TiN nanoparticles may have relatively higher electrical
conductivity. To further support the statement, the elemental composition of oxygen, from oxide and oxynitride components, on the
‘untreated’ TiN nanoparticles surface was measured by XPS and was
found to be ∼35% atomic concentration. This means that the oxynitride component will have a much lower composition of oxygen
than the total value. It is reasonable to speculate that this conductive nature of oxynitride component on the surface of TiN NP
resulted in the minimal ohmic resistance (or poor retardation of
kinetics) from the catalyst supports during the initial cycles of CV
and ORR tests of Pt/TiN presented in our previous report [18].
4.6. Stability of Pt/TiN electrocatalyst
As the temperature affects the active or passive behavior of the
TiN NP, the durability of the Pt/TiN electrocatalyst (synthesized
using TiN NP) will also be invariably affected in a similar manner. The synthesized Pt/TiN electrocatalyst has an average platinum
particle size of 2.4 nm as measured by TEM (Fig. 9). To test the durability of the electrocatalyst, Pt/TiN was subjected to accelerated
durability test (ADT) where the electrocatalyst coated electrode
undergoes repeated potential cycling for a prolonged period of time
under simulated PEM fuel cell conditions. The durability performance in this case was monitored by the no. of active Pt sites on the
electrocatalyst i.e., the electrochemical surface area (ECSA), which
was measured at intervals of 100 cycles; the plot of ECSA of Pt/TiN
vs. no. of cycles at different temperatures is shown in Fig. 10.
The ECSA of Pt/TiN at 60 ◦ C in Fig. 10 has an initial high value
of 70 m2 g−1 Pt but falls to near zero within few hours of operation
and continues to plateau near zero for the remaining duration. This
trend can be explained by the OCP curve of TiN NP at 60 ◦ C (in Fig. 8)
where the dissolution of oxynitride layer takes place in the initial
stages and later passivation occurs due to the formation of Ti(OH)
layer on its surface. XPS characterization also revealed a similar
hydroxide layer on the surface of TiN NP after repeated potential
cycling, which is similar to ADT. This hydroxide layer prevents the
TiN NP supports (of Pt/TiN electrocatalyst) to conduct electrons
from Pt catalyst particles resulting in near zero ECSA as shown
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Fig. 9. (a and b) Transmission electron micrograph showing the Pt particles on TiN catalyst supports; (c) histogram showing the Pt nanoparticles distribution with mean
particle size at 2.4 nm.
Fig. 10. Decrease of ECSA of 20 wt% Pt/TiN after potential cycling between 0 and
1.3 V (50 mV s−1 ) in N2 saturated 0.1 M HClO4 for 1200 cycles or 16 h. Pt loading:
20 ␮g cm−2 .
Fig. 11. CV curves of 20 wt% Pt/TiN showing the 100 and 1200 cycles during the
potential cycling between 0 and 1.3 V (50 mV s−1 ) in N2 saturated 0.1 M HClO4 at
70 ◦ C. Pt loading: 20 ␮g cm−2 .
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Fig. 12. Scheme showing the surface condition of TiN nanoparticles at various stages during the exposure to PEM fuel cell conditions (acid electrolyte).
in Fig. 10. Passivation of TiN surface due to hydroxide layer and
the resulting poor conductivity of TiN particles has been reported
by Qiu and Gao [21] who measured that the electrical conductivity of TiN powder decreases by an order of magnitude due to the
hydrolyzed layer on the surface.
As mentioned earlier, the OCP curves in Fig. 8 show that an
increase in temperature leads to faster passivation of the TiN NP
surface. Hence, one can see a faster rate of decline of ECSA for test
at 70 ◦ C and significantly lower rate of decrease of ECSA at 23 ◦ C
(RT) in Fig. 10. This shows that the decreasing trends of ECSA of
Pt/TiN can be explained by the adsorption model of TiN described
using OCP curves.
It may be argued that the decline of ECSA for the Pt/TiN at
higher temperatures (60 and 70 ◦ C) of acidic media may also be
due to the complete loss of Pt catalyst and/or a sharp increase in
Pt particle size due to agglomeration. But an increase in Pt particle size does not amount to a corresponding rapid decline of ECSA
as seen in Fig. 10 and the presence of Pt (on the Pt/TiN coated
carbon cloth electrode) was still seen after the ADT using an energydispersive spectrometer. Also, an increased Pt particle shows lower
current densities but retains the typical shape of the platinum CV
curve unlike the ‘cyc 1100’ curve in Fig. 11 that shows negligible (hydrogen adsorption–desorption) HAD peaks with a complete
passivation suggesting the poor conduction of support.
It can be observed that the passivation of TiN surface (plateauing of OCP in Fig. 8 at 60 ◦ C) is attained in ∼1 h whereas
the passivation of TiN surface of Pt/TiN (plateauing of ECSA
in Fig. 10 at 60 ◦ C) is attained after ∼4 h (or 300 potential
cycles). The longer time taken by the TiN NP to passivate under
potential cycling conditions, compared to the OCP conditions,
agrees well with the earlier proposed mechanism of simultaneous dissolution and formation of oxide/oxynitride components
during potential cycling. The illustration of the process can be
seen in Fig. 12.
Though the above results of Pt/TiN electrocatalyst may seem to
suggest only the strong dependence of temperature on its stability,
a closer analysis indicates that factors such as concentration and
type of acid media can also influence the TiN surface. It is emphasized that the passivation behavior of TiN NP shown in this study
is specific to the temperature and concentration conditions of perchloric acid electrolyte. By changing the operating window to a
different set of parameters of—acidic media, temperature and concentration, crystallinity of TiN powder [54], an active phase or a
delayed passivation of TiN nanoparticles can be observed.
The passivation behavior of TiN NP under the prolonged electrochemical conditions may not be a limitation for its application
as a catalyst support in PEM fuel cells. Unlike carbon black, TiN
nanoparticles do not predominantly oxidize to gaseous oxides that
can lead to a significant mass loss but forms a passivating surface
that prevents further oxidation. If the chemical stability of the TiN
nanoparticles towards water in acidic media can be modified to
reduce surface hydrolysis, TiN NP can become a durable catalyst
support that can increase the lifetime of PEM fuel cells. Zhang and
Binner [55] proposed a coating of stearic acid on nitride powder
as a stabilizing agent against hydrolysis while Müller et al. [56]
described the protection of nanoscale non-oxidic particles from
oxygen uptake by coating with N2 -containing surfactants. Though
a carbon-based coating can provide a hydrophobic surface, there is
a possibility that it may undergo electrochemical oxidation similar
to carbon supports under PEM fuel cell conditions. Alternatively,
identifying an active phase of TiN, in a set of conditions that fall in
the operating window of a PEM fuel cell, can result in an increased
durability of the Pt/TiN electrocatalyst.
Though carbon black supports cost less than TiN NP, an increased
durability obtained from the novel TiN NP supports can reduce the
lifetime costs of a PEM fuel cell system by enabling the system to
function till its intended life span. Further research is being pursued
to effectively use the oxidation resistance properties of titanium
nitride nanoparticles for increasing the durability of electrocatalysts in PEM fuel cells. The research also opens different avenues
where TiN supports can be used in combination with other catalysts
or mixed with other conventional supports. But it is emphasized
that attention must be paid to the stability of the nitride support
over prolonged exposure under PEM fuel cell conditions as it provides the true understanding of the durability of the electrocatalyst.
5. Conclusion
The oxidation currents of TiN are significantly lower compared
to that of CB at operating potentials 1.0 and 1.2 V of a PEM fuel
cell thus indicating its corrosion resistance properties. XRD and XPS
characterization of TiN NP revealed an existing oxide and oxynitride
layer on the surface, which prevents further diffusion of oxygen into
the bulk TiN. It is reported that the nitride component decreases and
oxide/oxynitride components increase on the TiN surface during
the potential cycling but the oxide/oxynitride layer tends to dissolve in acidic solution depending on the media and temperature.
The exposed nitride surface reacts with the solution and using XPS
and OCP data, it is shown to form Ti(IV) hydroxide ions that passivates the surface by adsorbing oppositely charged anions in the
solution. The durability of Pt/TiN electrocatalyst also agrees well
with the adsorption model of TiN NP.
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Acknowledgements
The authors gratefully acknowledge Richard Moore for his assistance with XPS characterization of the TiN electrode samples and
Dr. Thomas Murray for transmission electron microscopy images
of TiN and Pt/TiN powders.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2010.08.035.
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Please cite this article in press as: B. Avasarala, P. Haldar, Electrochemical oxidation behavior of titanium nitride based electrocatalysts under
PEM fuel cell conditions, Electrochim. Acta (2010), doi:10.1016/j.electacta.2010.08.035