PtRu alloy nanostructure electrodes for methanol electrooxidation

Article abstract of DOI:10.1016/j.jallcom.2008.06.012

In this work, PtRu nanostructure electrodes for methanol electrooxidation were electrodeposited by modulating an electrode potential. The PtRu nanostructures were characterized by scanning electron microscopy, transmission electron microscopy and X-ray di

Full text of DOI:10.1016/j.jallcom.2008.06.012

Journal of Alloys and Compounds 473 (2009) 516–520
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
PtRu alloy nanostructure electrodes for methanol electrooxidation
You-Jung Song, Sang-Beom Han, Jong-Min Lee, Kyung-Won Park∗
Department of Chemical and Environmental Engineering, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, PtRu nanostructure electrodes for methanol electrooxidation were electrodeposited by
modulating an electrode potential. The PtRu nanostructures were characterized by scanning electron
microscopy, transmission electron microscopy and X-ray diffraction method. To our surprise, it was found
that as the electrode potential was increased, the spherical shape of electrode could be transformed
into sharp tip formed by PtRu alloy nanoparticles. In particular, the PtRu nanostructure electrode elec-
trodeposited at −0.6 V showed an excellent catalytic activity for methanol electrooxidation due to both
Received 28 April 2008
Received in revised form 4 June 2008
Accepted 5 June 2008
Available online 18 July 2008
Keywords:
Fuel cells
Metals and alloys
Nanostructure materials
Nanofabrications
Catalysis
bifuctional mechanism and increased active surface area.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
electrochemical method [13–15]. Among them, especially, the elec-
trochemical deposition method has been extensively used because
Direct methanol fuel cells (DMFCs) using a proton exchange
membrane have been considered as suitable power sources for
portable applications [1,2]. Because they have a variety of mer-
its such as low operating temperatures, easy to handle a liquid
fuel, high energy density of methanol, and applications to micro-
sized fuel cells [3–9]. The excellent catalytic activity of platinum
for methanol oxidation, especially at low temperatures, makes this
metal electrocatalyst ideal for use as an anode in the DMFCs. How-
ever, since pure platinum is rapidly poisoned by intermediates
produced during methanol electrooxidation at low temperatures,
Pt-based alloy or nanocomposite catalysts with second or third
elements need to be designed and synthesized. Among them,
PtRu catalyst has been well known as the most effective anode
in the DMFCs due to the complete oxidation of methanol to
CO2 via a bifunctional mechanism. According to bifunctional
mechanism, the complete oxidation of CO-like intermediates to
CO2 can be promoted by OH group formed on neighbouring
Ru at relatively low potential [10–12]. Therefore, the PtRu alloy
structure is extremely essential for such an enhanced methanol
electrooxidation. In addition, many efforts have been reported
to modulate the composition and structure of PtRu nanoparti-
cles by means of physical deposition, chemical synthesis, and
of simple operation, low cost, high purity, and uniform deposi-
tion.
In this work, PtRu nanostructure electrodes were electrode-
posited with increased potentials. We found that the spherical
shape of the electrodes could be transformed into sharp tip with
PtRu nanophases with increased electrode potential. The effect of
morphology of the electrodes on their electrocatalytic properties
was discussed.
2. Experimental
PtRu nanostructure electrodes were electrodeposited on indium tin oxide (ITO)
glass in the solution of 0.2 M H3BO3, 5 mM H2PtCl6, and 0.5 mM RuCl3·3H2O at 25 ◦C.
The electrodeposition was carried out under an applied potential of −0.3, −0.6, −0.9,
or −1.2 V for 10 min in a typical electrochemical cell consisting of Pt wire, Ag/AgCl
and ITO glass as a counter, reference, and working electrode, respectively. In addition,
pure Pt and Ru electrodes were fabricated in the solution of 0.2 M H3BO3 + 5 mM
H2PtCl6, and 0.2 M H3BO3 + 0.5 mM RuCl3·3H2O, respectively.
The morphology and composition of the Pt, PtRu, and Ru electrodes were
observed by means of scanning electron microscopy (SEM, JEOL JSM-6360A) and
energy dispersive X-ray spectroscopy (EDX). The crystal structures of the Pt, PtRu
and Ru electrodes were confirmed by X-ray diffraction (Rigaku X-ray diffractome-
ter equipped with a Cu K␣ source at 40 kV and 100 mA) and transmission electron
microscopy (TEM-Phillips-F20 Electron Microscope at an accelerating voltage of
200 kV).
The conventional electrochemical cell consisting of Pt wire, Ag/AgCl and ITO
glass as a counter, reference, and working electrode, respectively, was used to eval-
uate electrochemical properties and catalytic activity for methanol electrooxidation.
Cyclic voltammograms (CVs) of the Pt and PtRu electrodes were obtained in 0.5 M
H2SO4 and 2 M CH3OH + 0.5 M H2SO4 at a scan rate of 50 mV/s. Chronoamperometry
(CA) of the PtRu electrodes were obtained at 0.4 V in 2 M CH3OH + 0.5 M H2SO4.
∗
Corresponding author. Tel.: +82 2 820 0613; fax: +82 2 812 5378.
E-mail address: kwpark@ssu.ac.kr (K.-W. Park).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.06.012

Y.-J. Song et al. / Journal of Alloys and Compounds 473 (2009) 516–520
517
Fig. 1. Scanning electron microscopy (SEM) images of Pt electrodes electrodeposited at (a) −0.3 V, (b) −0.6 V, (c) −0.9 V, (d) −1.2 V, PtRu electrodes electrodeposited at (e)
−0.3 V, (f) −0.6 V, (g) −0.9 V, (h) −1.2 V, and Ru electrodes electrodeposited at (i) −0.3 V, (j) −0.6 V, (k) −0.9 V and (l) −1.2 V, respectively.
Fig. 2. Low and high resolution transmission electron microscopy (TEM) images of the PtRu electrodes electrodeposited at −0.3 V (a and b) and −0.6 V (c and d).

518
Y.-J. Song et al. / Journal of Alloys and Compounds 473 (2009) 516–520
Fig. 3. (a) An energy dispersive X-ray (EDX) spectrum and (b) composition ratio of
Fig. 5. Cyclic voltammograms (CVs) of Pt and PtRu electrodes deposited at (a) −0.3 V
and (b) −0.6 V in 0.5 M H2SO4.
the PtRu electrodes.
3. Results and discussion
was carried out under applied potentials of −0.3, −0.6, −0.9, and
−1.2 V for 10 min. As shown in Fig. 1(a), (e), and (i), the spherical
shape of electrodes is formed at −0.3 V. However, as the poten-
tial is increased above −0.6 V, the spherical shape of electrodes is
transformed into sharp tip as shown in Fig. 1(b)–(d), (f)–(h) and
(j)–(l). In case of PtRu electrodes, the length of the tip is increased
with the increased applied potential, as shown in Fig. 1(f)–(h). Dur-
ing the electrodeposition process, platinum and ruthenium ions
in the electrolyte are reduced to metal lattice. Simultaneously, the
decomposition of water occurs as follows:
Fig. 1 shows SEM images of Pt ((a)–(d)), PtRu ((e)–(h)) and Ru
((i)–(l)) electrodes electrodeposited by means of applied poten-
tials. To observe the morphology of the electrodes, the deposition
Pt4+ + 4e− → Pt0
(1)
(2)
(3)
Ru3+ + 3e− → Ru0
2H2O + 2e− → H2 + 2OH−
At the higher electrode potentials, the faster water decompo-
sition consequently occurs with an abundant H2 evolution. The
hydrogen bubbles generated in the reaction adhere to the elec-
trode surface; metal nucleation then occurs at high-surface-energy
sites on the electrode–bubble interface affected by the local elec-
tric field. This enables deposition to extend laterally over the surface
covered by the entire bubble. Then, the sharp tip of the electrode
radically grows with respect to the configuration of the electric field
(Fig. 1(f)–(h)) [16]. It is considered that higher applied potential
could produce more tips in the PtRu electrodes due to H2 evo-
lution on the surface of the electrode. Fig. 2 shows TEM images
Fig. 4. X-ray diffraction patterns of PtRu electrodes electrodeposited at (a) −0.3 V,
(b) −0.6 V, (c) −0.9 V, (d) −1.2 V, and (e) pure Pt. The asterisk mark represents XRD
patterns of ITO glass.

Y.-J. Song et al. / Journal of Alloys and Compounds 473 (2009) 516–520
519
tials could result in the generation of PtRu nanoparticles caused by
fast nucleation and the lateral growth of the nanoparticles on the
surface covered by the H2 bubbles.
Fig. 3 shows EDX data of the PtRu electrodes in Fig. 1(e)–(h).
As shown in Fig. 3(b), all the ratios of Pt to Ru in the electrodes
is 86:14 at.% and are constant irrespective of applied potentials. In
order to confirm crystal structure of the PtRu and Pt electrodes, the
XRD patterns were obtained as shown in Fig. 4(a)–(d). The diffrac-
tion peaks at around 39.9◦, 46.4◦ and 67.7◦ correspond to Pt (1 1 1),
(2 0 0) and (2 2 0) planes, respectively, with these positions shifted
to higher angles compared to those of pure Pt (PCPDF #040802).
Assuming alloy formation between Pt and Ru based on substitu-
tional solid solution, such a shift as shown in Fig. 4 is due to the
difference of an atomic size between Pt and Ru atom. Also, from the
XRD patterns of the PtRu electrodes, it was found that the atomic
ratios of Pt to Ru of the electrodes were 80–20 at.% in good agree-
ment with that of EDX analysis. This indicates that all the Pt and Ru
elements in the electrodes are alloyed forming homogeneous solid
solution within the intended composition.
Fig. 5 shows typical CVs of Pt, PtRu-0.3 and PtRu-0.6. As shown
in Fig. 5(a), the Pt and PtRu-0.3 have electrochemical surface areas
of 17.71 and 18.14 cm2, respectively, measured by absorption sites
of hydrogen on platinum surface. On the other hand, as shown
in Fig. 5(b), the PtRu-0.6 has much larger electrochemical surface
area of 3.99 cm2 compared to that of 0.27 cm2 in the Pt electrode.
This may be attributed to the increased active surface area of the
electrode with the nanostructured tips consisting of PtRu alloy
nanoparticles. In the case of the PtRu alloy electrode, the existence
of Ru component in the electrode was confirmed by double layer
region at the potential range of 0.1–0.6 V thicker than one of pure
Pt due to hydrophilic properties of Ru.
Fig. 6 shows CVs of the Pt and PtRu alloy electrodes for methanol
electrooxidation in 2 M CH3OH + 0.5 M H2SO4 at 25 ◦C. As shown in
Fig. 6(a), the PtRu-0.3 shows superior catalytic activity to pure Pt,
that is, lower onset potential and higher oxidation current density,
due to the bifunctional mechanism. In addition, as shown in
Fig. 6(b), the PtRu-0.6 shows higher current density for methanol
electrooxidation than that of pure Pt. However, compared to
the PtRu-0.3, such an excellent catalytic activity the PtRu-0.6
is due to not only bifunctional mechanism but also increased
active surface area resulting from the nanostructured tips of the
electrode. However, although the higher potential increases active
surface area, the highest potential of −1.2 V does not exhibit
the most efficient performance in methanol electrooxidation.
It is considered that too much hydrogen evolution gives rise to
porous films with poor adhesion, thus, deteriorates the stability
of electrodeposited electrode. As shown in Fig. 7 using the PtRu
electrodes, plots of oxidation current vs. time (CA) were measured
in 2.0 M CH3OH + 0.5 M H2SO4 at 0.4 V, for 600 s [17,18]. The
current measured at PtRu-0.6 is higher than that obtained with
PtRu-0.3. By combing CV and CA, we can conclude that PtRu-0.6
represent the best alternative candidates for the DMFC anode
catalysts.
Fig. 6. Cyclic voltammograms (CVs) of Pt and PtRu electrodes deposited at (a) −0.3
and (b) −0.6 V for methanol electrooxidation in 2 M CH3OH + 0.5 M H2SO4.
of PtRu electrodes electrodeposited at −0.3 (PtRu-0.3) and −0.6 V
(PtRu-0.6). The spherical shape in the PtRu-0.3 is transformed into
sharp tip in the PtRu-0.6. As shown in Fig. 2(a) and (b), the PtRu-0.3
seems to be spherical and large polycrystalline structures which
are well consistent with SEM images. On the other hand, as shown
in Fig. 2(c) and (d), the PtRu-0.6 seems to be sharp tip consisting
of PtRu nanoparticles of 4–5 nm. This means that the higher poten-
4. Conclusion
We found that the growth of nanostructured tips in the PtRu
electrodes could be controlled by means of applied potential in
electrodeposition method due to H2 evolution on the surface of the
electrode. The PtRu electrode electrodeposited at −0.6 V showed
an excellent catalytic activity for methanol oxidation due to both
bifuctional mechanism and increased surface area resulting from
the nanostructured tips of the electrode. The PtRu nanostructure
electrode with the nanostructured tips could be electrodeposited
Fig. 7. Chronoamperometry (CA) of PtRu electrodes for methanol electrooxidation
at 0.4 V in 2 M CH3OH + 0.5 M H2SO4.

520
Y.-J. Song et al. / Journal of Alloys and Compounds 473 (2009) 516–520
at an optimum electrode potential and may be promising as an
anode in DMFC.
[7] H.-F. Oetjen, V.M. Schmidt, U. Stimming, F.J. Trila, J. Electrochem. Soc. 143 (1996)
3838.
[8] A. Hamnett, Catal. Today 38 (1997) 445.
[9] K.W. Park, J.H. Choi, B.K. Kwon, S.A. Lee, Y.E. Sung, H.Y. Ha, S.A. Hong, H. Kim, A.
Wieckowski, J. Phys. Chem. B 106 (2002) 1869.
[10] M. Watanebe, S. Motoo, J. Electroananl. Chem. 60 (1975) 267.
[11] J. Munk, P.A. Christensen, A. Hamnet, E. Skou, J. Electroanal. Chem. 401 (1996)
215.
Acknowledgment
This work was supported by Soongsil University Research Fund.
[12] H. Wang, C. Wingender, H. Baltruschat, M. Lopez, M.T. Reetz, J. Electroanal.
Chem. 509 (2001) 163.
[13] K.W. Park, J.H. Choi, K.S. Ahn, Y.E. Sung, J. Phys. Chem. B 108 (2004) 5989.
[14] M.S. Loffler, H. Natter, R. Hempelmann, K. Wippermann, Electrochim. Acta 48
(2003) 3047.
[15] J.S. Guo, G.Q. Sun, S. Sun, S. Yan, W. Yang, J. Qi, Y. Yan, Q. Xin, J. Power Sources
168 (2007) 299.
[16] W.L. Tsai, P.C. Hsu, Y. Hwu, C.H. Chen, L.W. Chang, J.H. Je, H.M. Lin, A. Groso, G.
Margaritondo, Nature 417 (2002) 139.
References
[1] S. Wasmus, A. Kuver, J. Electroanal. Chem. 461 (1999) 14.
[2] L. Carrette, J. Collins, A. Dickinson, U. Stimming, Bunsen Mag. 2 (2000) 27.
[3] E. Reddington, A. Sapienza, B. Gurau, R. Viswanathan, S. Sarangapani, E.S.
Smotkin, T.E. Mallouk, Science 280 (1998) 1735.
[4] P.N. Ross, in: J. Lipkowski, P.N. Ross (Eds.), Electrocatalysis, Wiley–VCH, New
York, 1998 (Chapter 2).
[5] A. Wieckowski, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel-
Dekker, New York, 1999 (Chapters 44–51).
[6] M.P. Hearth, G.A. Hards, Platinum Met. Rev. 40 (1996) 150.
[17] W. Chrzanowski, A. Wieckowski, Langmuir 14 (1998) 1967.
[18] F. Maillard, G.-Q. Lu, A. Wieckowski, U. Stimming, J. Phys. Chem. B 109 (2005)
16230.