On the oxygen reduction reaction catalyzed by Ti

On the oxygen reduction reaction catalyzed by Ti

On the oxygen reduction reaction catalyzed by Ti -Cu binary films in
0.5 M sulfuric acid solution
Jing-Chie Lin*, Chien-Ming Lai, Hang-Chung Chu, Cheng-Lin Chuang, Yu-Sheng
Chen
Department of Mechanical Engineering/Institute of Materials Science and
Engineering, National Central University,
No.300 , Jhongda Rd, Jhongli City, Taoyuan Country 320, Taiwan.
*Corresponding author：[email protected]
Abstract
Ti-Cu binary films co-sputtered in vacuum are catalytic active for the oxygen
reduction in 0.5M H2SO4. The activity of the oxygen reduction reaction (ORR)
increased with increasing the Cu-content in the Ti-Cu films and it reached to a
maximum with the copper composition up to 90 at. %. Through investigation of Tafel
polarization, the Ti-Cu films revealed a constant Tafel slope (i.e., 190 mV/decade)
similar to that of ORR on the pure Cu film. This infers that the electrochemical
reduction of oxygen is predominated on the Cu-sites in the film. In the cyclic
voltammograms, the strong broad peak should be arisen from the oxidation of Cu to
Cu+ and Cu2+ ions. This oxidation indicated that the Ti-Cu films are unstable and the
Cu-component is susceptible to dissolution in 0.5M H2SO4. This dissolution caused a
loss of catalytic activity in the films. Preparing the Ti-Cu films enriched in Ti will
stabilize these films to prevent the Cu-dissolution.
Keywords：Ti-Cu; binary film; oxygen reduction reaction; catalytic activity;
1
1. Introduction
One of the challenging problems in the fuel cell is oxygen reduction reaction at
the cathode [1]. Platinum is still considered the best electrocatalyst for the reduction
of oxygen in aqueous electrolytes due to the lowest overpotentials and the best
stability. The high cost and limited world’s supply of platinum are the major issues for
a widespread commercialization of the fuel cell technology. These difficulties have
created enormous interest in the search for less expensive, more efficient
electrocatalysts as well as in lowering the catalyst loading [2-6]. With respect to
identifying alternative electrocatalysts, non-platinum based metal combinations [7-13],
metal oxides [14-18], metal carbides [19], and metal oxynitrides [20] have been
investigated over the years for the ORR. These materials have some catalytic activity
for the ORR and are stable in acid media, although it is still not sufficient when
compared to platinum. Therefore, it is necessary to continuously investigate the
non-platinum catalyst for the commercialization of the PEFC.
Titanium and its alloys are technically superior and cost-effective construction
materials for a wide variety of aerospace, industrial, marine, medical and commercial
applications [21]. The kinetics of titanium metal dissolution is quite slow in the
oxidant-containing aqueous media. Such a behavior is attributed to the surface of the
2
oxide film (amorphous, mainly TiO2) present in air as well as in aqueous solutions
[22-25]. As a search of new ORR electrocatalysts, TiO2 catalysts were studied for
their catalytic activity during the ORR and stability in acid media. It was reported that
TiO2 supported the oxygen reaction in acid media [14]. The catalytic activity for the
ORR of the TiO2 catalysts was low and the ORR occurred below about 0.0V (versus
SHE) in a solution with pH at 1.05 [14]. However, titanium and its oxide are worth
being studied as a non-platinum electrocatalyst for the PEFC cathode, because it is an
abundant natural resource, and supports the electrochemical reaction such as the
oxygen evolution reaction. Bard and his co-workers [11] proposed a guideline for
developing the design to improve the bimetallic electrocatalysts for the ORR in acidic
media. This guideline is based on the thermodynamics correlated with oxygen
adsorption and its subsequent reduction on a bimetallic film. The oxygen-oxygen
bond would be weakened as the molecular oxygen adsorbed on one component of
film. This facilitates oxygen dissociation into atoms that could readily be reduced by
the other component in the film.
According to Bard’s criteria of the bimetallic catalyst, copper could be selected
to improve the activity of the titanium. In this work, Ti-Cu binary films varying in
Cu-composition
were
prepared
by
magnetic
sputtering
in
vacuum.
The
electrochemical behavior of those Ti-Cu films for the ORR in acid solution has been
3
investigated. The electrochemical characteristics of Ti-Cu films were explored using
by slow scan voltammetry (SSV), Tafel polarization and cyclic voltammetry (CV).
2. Experimental
2.1 Preparation of Ti-Cu catalysts
The Ti-Cu films were sputtered on the end of a glassy carbon (GC, Alfa Aesare,
USA.) rod substrate (6.15mm diameter × 10mm length) by DC magnetic sputtering
system (ARC-12M, Plasma Science Inc., USA.). The Ti and Cu targets (99.9% purity,
Gredmann, Well-Being Enterprise Co., Ltd, Taiwan) were fixed on a cathode. The
sputtering chamber was pre-evacuated down to 8×10-6 torr, then the sputtering was
performed at an argon pressure of 6×10-3 torr by supplying the power of 60 W and
20~60 W for the Ti and Cu targets, respectively. The binary films were deposited to
vary their copper composition at 50, 70 and 90 at.% through controlling the sputtering
power on the Cu target and the sputtering time. The thickness of the film was
controlled and monitored about 200 nm by a quartz crystal microbalance (QCM) film
thickness meter. The composition of the films was confirmed by energy dispersive
4
X-ray analysis (EDX, Kevex Level 3, Hitachi Co., Japan). The same sputtering
procedure was employed to deposit the monotonic catalyst of pure films, such as Ti
and Cu on the GC.
2.2 Electrochemical instruments and measurements
The electrochemical experiments were conducted in a three electrode glass cell
connected with a potentiostat (model 263, EG&G, USA.) to perform the following
measurements: (i) slow scan voltammetry (SSV, in the potential ranging from 1.0 to
0.0 V with a scan rate of 5 mV s−1); (ii) Tafel polarization (Tafel, within potentials of
the open circuit potential± 250 mV in a scan rate of 1 mV s−1); (iii) cyclic
voltammetry (CV, in the range from 0 to 1.2 V with a scan rate at 50 mV s−1). The
potential in this work was measured against a reference electrode of saturated calomel
electrode (SCE) but it was reported versus the standard hydrogen electrode (SHE) for
convenient comparison. Prior to the test, the electrolyte solution (0.5 M H2SO4) was
saturated with oxygen or nitrogen at room temperature. The current densities were in
terms of the current on the geometric surface area of the electrodes.
3. Results and discussion
5
Figure 1 show the SSV curves of (a) pure Ti and (b) Ti-Cu thin-film catalyst in
0.5 M H2SO4 saturated with N2 and O2 atmospheres. The potential at which the
reduction current with Ti under O2 atmospheres slightly increased than that under N2
was about 0.1 V vs SHE, as shown as Fig. 1(a). This indicated that Ti had a very
lower catalytic activity for the ORR. However, as shown in Fig 1 (b), an apparent
increase of the reduction current was observed under O2 atmosphere compared to the
N2 atmosphere. It indicated that the catalytic activity of the ORR was improved when
Cu was added in Ti to form a binary Ti-Cu catalyst.
Figure 2 displays the SSV for various thin-films in 0.5M H2SO4 saturated with
O2 atmosphere. In Fig. 2, the current density increases rapidly with the potential from
0.4 to 0.0 V. This SSV diagram provides an estimation of catalytic activity for ORR
on different films. A film with higher onset potential would catalyze ORR more
readily; the film exhibiting higher reduction current is more active. According to Fig.
2, the onset potential shifted slightly to higher potentials with increasing the
Cu-composition in the binary films. The oxygen reduction current (µA) at 0.2V vs
SHE decreases in the order of Cu (219.0) > Ti-90Cu (54.5) > Ti-70Cu (10.5) >
Ti-50Cu (8.0) > Ti (1.8). Obviously, the catalytic activity for the ORR is higher on the
Ti-Cu binary films than on the pure Ti film alone. The activity of the ORR increases
6
with increasing the Cu-composition from 50 to 90 at % in Ti-Cu films system.
Figure 3 (a) shows the cathodic Tafel plot of different films in 0.5M H2SO4
saturated with O2 atmosphere. The Tafel slope and exchange current density could be
estimated from Fig. 3 (a) and the data was collected and re-plotted in Fig. 3(b). Figure
3 (b) depicted the Tafel slope and exchange current density for the ORR on different
films. The Tafel slopes on Ti-Cu films are roughly at constant (i.e., 190 mV/decade)
similar to that on the pure Cu film (i.e., 198 mV/decade). Based on constant Tafel
slope, we infer that the reduction of oxygen on the Cu-sites will predominate the ORR.
The exchange current density of ORR (i0,ORR) was estimated by extending the Tafel
slope to the current density at the potential of the oxygen reduction (i.e., 1.23 V). The
magnitude of i0,ORR increases with increasing the Cu-concentration in the films. As a
result, the involvement of Cu in the Ti-Cu films moves the equilibrium of ORR to
higher current so that the rate of both forward and reverse reaction is accelerated.
Figure 4 shows the cyclic voltammograms (CV) for the films in 0.5 M H2SO4
saturated with N2 atmosphere. In Fig.4, the enormous anodic peak in a wide range of
0.2–1.0 V may arise from the dissolution of Cu to form Cu+ or/and Cu2+ ion,
especially in the films with higher Cu-composition. Evidently, the dissolution of
copper results in instability of the binary films in the acid solution, especially for the
films with higher Cu-composition. The intensity of the anodic peak decreases with
7
decreasing the Cu-composition in the films. The peak responsible for anodic
dissolution of copper diminishes with decreasing the copper content, and it eventually
disappears as the Cu-content in the Ti-Cu film is lower than 50 at.%. This implies that
the dissolution of Cu will be inhibited in the Ti-Cu films enriched with Ti. The ionic
concentration of copper in acid solution could be determined by ICP to confirm this
inhibition. When a specimen of binary film enriched in Ti (i.e., Ti50Cu50) was
immersed in 0.5M H2SO4 saturated with N2 at 0.5 V for 10 min to compare with a the
pure Cu, the concentration of copper ion was measured at 0.0318 and 0.2380 ppm,
respectively. This fact confirms that copper dissolution from binary films is inhibited
in the presence of Ti. Therefore, Ti co-existed in the binary films stabilizes the
system.
4. Conclusions
The catalytic activity of the oxygen reduction reaction (ORR) on a variety of
Ti-Cu films in the 0.5M H2SO4 has been investigated by using electrochemical
technologies such as slow scan voltammetry (SSV), Tafel-Plot (Tafel) and cyclic
voltammetry (CV). The results of SSV indicated that a combination of binary Cu-Ti
films reveals higher catalytic active for ORR. The catalytic activity increases with
8
increasing the concentration of Cu (from 50 to 90 at%) in the films. Ti10Cu90
revealed higher reduction current density (i.e., 54.5 µA/cm2) than any other binary
film at 0.2V. Based on the data of Tafel slope, ORR reaction on the Ti-Cu binary
films is determined predominantly on the Cu-sites. In CV diagram, an enormous
anodic peak in the range of 0.2–1.0 V is resulted from the oxidation of Cu to Cu+ and
Cu2+ ions. An unstable film was susceptible to Cu-dissolution in 0.5M H2SO4. The
anodic peak diminishes with decreasing the Cu-composition in the binary film.
Enrichment of Ti in the binary film will stabilize the films.
Acknowledgements
The financial support of this work by the National Science Council of Republic
of China under contract number NSC 95-2221-E-008-023 is gratefully acknowledged.
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11
Figure captions:
Figure 1. SSV curves of (a) Ti and (b) Ti-Cu film in 0.5 M H2SO4 saturated with N2
and O2 atmospheres.
Figure 2. SSV curves for different thin-film catalysts in 0.5M H2SO4 saturated with
O2 atmosphere.
Figure 3. (a) The potential (mV) as a function of logarithmic current density (mA
cm-2), in 0.5M H2SO4 saturated with O2 atmosphere, for ORR on the pure Ti,
Cu single films and Ti-xCu (x=50, 70, 90 at.%) films. (b) Tafel slope (bc)
and exchange current density (i0) as a function of the Cu-composition.
Figure 4. CV curves for the pure Ti, Cu film and Ti-xCu (x=50, 70, 90 at.%) films in
in 0.5M H2SO4 saturated with N2 atmosphere.
12
100
(a) Ti
I(µA)
50
0
N2
O2
-50
-100
0.0
0.2
0.4
0.6
0.8
1.0
0.8
1.0
E(V) vs SHE
50
(b) Ti-Cu
I (µA)
0
N2
-50
O2
-100
-150
0.0
0.2
0.4
0.6
E(V) vs SHE
Figure 1
13
I (µA)
0
-200
Ti
Cu
Ti50 Cu50
Ti30 Cu70
Ti10 Cu90
-400
-600
0.0
0.2
0.4
0.6
E (V) vs SHE
Figure 2
14
0.8
1.0
0.28
E (V) vs SHE
0.26
0.24
0.22
Cu
0.20
Ti
0.18
-6.5
-6.0
Ti50
Cu50
-5.5
Ti30
Cu70
-5.0
Ti10
Cu90
-4.5
-4.0
-3.5
log I (A)
102
160
101
i0 (A/cm2) x10
-10
bc (mA/decade)
200
120
100
0
20
40
60
Cu content (at.%)
Figure 3
15
80
100
1.00
Ti
Ti 50 Cu50
Ti 30 Cu70
i (mΑ/cm2)
0.75
Cu70
0.50
0.25
Cu50
0.00
Ti
-0.25
-0.50
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E(V) v.s. SHE
200
Cu
Ti10 Cu90
Cu
i (mΑ/cm2)
150
100
50
Cu90
0
-50
0.0
0.2
0.4
0.6
0.8
E(V) v.s. SHE
Figure 4
16
1.0
1.2