Influence of Au contents of AuPt anode catalyst on the performance of direct formic acid fuel cell

Article abstract of DOI:10.1016/j.electacta.2007.12.031

We reported that various compositions of AuPt nanoparticles synthesized as an anode material for formic acid fuel cell were investigated. Its surface characteristics were systematically analyzed using XRD and TEM and anodic electrocatalytic activity was studied using a linear sweep voltammetry technique in 0.5 M H₂SO₄ + 1 M HCOOH. In addition, the voltage-current curve and power density of home-made AuPt-based membrane-electrode-assembly (MEA) and commercial Pt_0.5Ru_0.5-based MEA was measured at 60 °C in 9 M formic acid. The maximum power density of Au_0.6Pt_0.4-based MEA was 30% higher than that of PtRu-based MEA which were 200 mW cm^-2 and 155 mW cm^-2, respectively.

Full text of DOI:10.1016/j.electacta.2007.12.031

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 3474–3478
Influence of Au contents of AuPt anode catalyst on the performance
of direct formic acid fuel cell
a,b
b,∗
c
c
Jae Kwang Lee , Jaeyoung Lee , Jonghee Han , Tae-Hoon Lim ,
d
a,∗
Yung-Eun Sung , Yongsug Tak
Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea
a
b
Electrochemical Reaction and Technology Laboratory (ERTL), Department of Environmental Science and Engineering,
Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea
c
Center for Fuel Cell Research, Korea Institute of Science and Technology,Seoul 136-791, Republic of Korea
d
School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea
Received 21 August 2007; received in revised form 2 December 2007; accepted 3 December 2007
Available online 15 December 2007
Abstract
We reported that various compositions of AuPt nanoparticles synthesized as an anode material for formic acid fuel cell were investigated. Its
surface characteristics were systematically analyzed using XRD and TEM and anodic electrocatalytic activity was studied using a linear sweep
voltammetry technique in 0.5 M H
2
SO
4
+ 1 M HCOOH. In addition, the voltage–current curve and power density of home-made AuPt-based
◦
membrane–electrode-assembly (MEA) and commercial Pt0.5Ru0.5-based MEA was measured at 60 C in 9 M formic acid. The maximum power
−
2
−2
density of Au0.6Pt0.4-based MEA was 30% higher than that of PtRu-based MEA which were 200 mW cm and 155 mW cm , respectively.
2007 Elsevier Ltd. All rights reserved.
©
Keywords: AuPt nanoparticle; Impregnation reduction method; Formic acid oxidation; Fuel cell performance; Optimal combination
1
. Introduction
enhancement. In particular, the modification by palladium as
a Pt/Pd alloy [23], Pd “decorated” polycrystalline Pt surfaces
[10], and Pt single crystals covered with palladium [24,25], have
produced an increased reactivity. The single crystal Au(h k l)/Pd
[25], andAu/Pdalloy[26]havealsobeeninvestigated. Enhanced
reactivity induced by adatoms has been accounted for by a ‘third
body effect’ [26,27], the third body preventing surface poison
(chemisorbed CO) to form on the surface. The chemistry of the
Pt/Pd electrolyte interface has also been investigated in relation
to anion effects [28], and CO chemisorption [29]. Of late, Sung
and co-workers investigated synthesized PtAu alloy as an anodic
catalyst in the direct liquid (methanol and formic acid) fuel cell
[30], presenting higher activity than that of commercial PtRu,
but only one composition of PtAu alloy was investigated.
In this work, we tried to understand the influence of Au on
AuPt alloys and did more systematic approach of the effect of Au
contents in the electro-oxidation of formic acid. We measured
electrocatalytic oxidation activity of formic acid on a various
composited AuPt catalysts, which were compared with commer-
cial Pt and PtRu. Their fuel cell performances were also tested
applying 9 M formic acid and humidified oxygen. As mentioned
The electro-oxidation of small organic molecules is very sen-
sitive to the properties of the electrode surface. Thus, these
reactions are influenced by the properties of the electrode mate-
rial, usually a pure metal or an alloy [1,2]. In addition, a surface
modified by adatoms often reveals enhanced oxidation proper-
ties compared to the pure metal [3].
Amongsmallorganicmolecules, manyresearchershavestud-
ied the fundamental features of formic acid oxidation. Formic
acid oxidation was studied, for example, on polycrystalline
[
4–10] and single crystal Pt [11–13], rhodium, palladium, and
gold [1]. In addition, many efforts have been made to enhance
oxidation rates of formic acid on platinum by adding a vari-
ety of surface additives (modifiers). Many modifiers such as
Ru [14,15], Sn [16–18], Pb [17], As [17], Sb [17,19,20], or Bi
[
17,19,21,22] have been tested as sources of possible catalytic
∗
Corresponding author. Tel.: +82 62 970 2440; fax: +82 62 970 2434.
E-mail addresses: jaeyoung@gist.ac.kr (J. Lee), ystak@inha.ac.kr (Y. Tak).
0
013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.12.031

J.K. Lee et al. / Electrochimica Acta 53 (2008) 3474–3478
3475
the geometrically active area of the electrode was 10 cm2.
Membrane–electrode assemblies (MEAs) were prepared by hot
above, platinum has been attracting considerable attention due
to its electrocatalytic characteristics, but its limited resource on
the earth is also a considerable issue. On the other hand, Au is
more abundant than Pt, and has been known as a catalyst able to
enhance oxidation kinetics of formic acid via active intermediate
species.
◦
pressing with Nafion 115 at a temperature of 140 C and a
−
2
pressure of 30 kgf cm for 300 s. Current–voltage curves were
measured galvanostatically by using an electronic load (EL-
200P, Daegil Electronics).
2
. Experimental
3. Results and discussion
2
.1. Preparation and analysis of Au and AuPt nanoparticles
The characteristics of synthesized AuPt catalysts prepared by
the impregnation reduction method were investigated. Fig. 1(a)
and (b) shows the XRD patterns of pure Pt, Au and AuPt
alloys and those containing a dosage of Au in home-made AuPt
nanoparticles, respectively. The peaks in Fig. 1(a) represent
Unsupported Au, AuPt nanoparticles were prepared by an
impregnation reduction method. Metal salts (H2PtCl ·xH2O and
6
HAuCl4·3H2O) were dissolved in deionized water. After stirring
for 1 h, reducing agent (NaBH4) was injected into the metal salt
solution and the solution stirred for 1 h. After completion of the
reduction, the precipitated particles were washed three times
using deionized water and the resulting materials were freeze-
dried using a liquid N2 atmosphere without any heat-treatment.
X-ray diffraction measurements were made using a Rigaku
◦
−1
D/max-IIIA X-ray diffractometer with a scan speed of 0.08 s
,
◦
in the scan range of 20–80 using Cu K␣ as an X-ray source. The
size of AuPt nanoparticles was investigated using a transmission
electron microscope (Philips (CM200)) operating at a 200-kV
accelerating potential.
2
.2. Half cell measurements
To investigate the electrocatalytic activity of Pt, PtRu, and
AuPt catalysts, half cell tests were carried out. Catalysts sprayed
on gas diffusion layer (GDL)/carbon paper were used as the
working electrodes (WEs). The geometric area of the WE was
2
0
.2 cm . The Pt plate (99.99%) was used as counter electrode
(
CE) and silver–silver chloride (Ag/AgCl) saturated KCl was
used as reference electrode (RE). Cyclic voltammetric curve
between −0.2 V and 1.2 V was recorded in 0.5 M H2SO4 and the
activity test for formic acid oxidation was performed in 0.5 M
H2SO4 in the presence of 1 M HCOOH solution. A potentio-
stat/galvanostat (EG&G 273A) was used for all linear sweep
voltammetry (LSV) experiments and the data were transferred
to an IBM compatible PC controlled by a GPIB interface.
2
.3. Single cell measurements
To compare the different catalysts, such as AuPt, Pt black
(
Johnson Matthey, Hi-SPEC 1000) and PtRu black (Johnson
Matthey, Hi-SPEC 6000) were used as the anodes and Pt black
Johnson Matthey, Hi-SPEC 1000) as the cathode. All elec-
(
trodes were prepared by dispersing appropriate amounts of
unsupported catalysts powder in deionized water, 5% Nafion
solution (Aldrich), 2-propanol, and 1-propanol, sonicating the
solution for 90 s and catalyst inks were coated on a GDL/carbon
◦
paper by spraying at 80 C. Then, additional ionomer solu-
tion was sprayed onto the catalyst layer of each electrode in
order to decrease the contact resistance with polymer elec-
trolyte membrane (Nafion 115, DuPont). The catalyst loading
Fig. 1. (a) XRD pattern and (b) portion of Au of synthesized AuPt catalysts,
and (c) potential–current plot for Pt, PtRu, and AuPt in 0.5 M H2SO4 (scan rate:
10 mV s ).
−
2
−1
for the anode and cathode was approximately 3 mg cm and

3
476
J.K. Lee et al. / Electrochimica Acta 53 (2008) 3474–3478
Table 1
atom is larger than a Ru atom and such type of peak shift indicate
good alloy formation between Pt and Ru. PtRu (1:1) is known
to alloy well. Therefore, we suggest that the structure of PtAu
nanoparticles is the crystal structure having fcc metallic state
Particle size of AuPt catalysts calculated from XRD data using the
Debye–Scherrer equation
Ratio of Pt precursor and
Au precursor
Portion of Au of
the alloy
Particle size
(nm)
[32].
0
1
1
1
1
2
.75:1
:1
.25:1
.5:1
.75:1
:1
0.71
0.63
0.60
0.51
0.48
0.45
2.43
1.83
2.43
1.30
1.30
2.44
In order to investigate the real particle size of synthesized
AuPt nanoparticles as a function of Pt and Au precursor ratio,
tunnelling electron microscopy (TEM) measurements were car-
ried out. They were presented in Fig. 2. As shown in Fig. 2, the
particle size of synthesized AuPt is below 10 nm, in good agree-
ment with the XRD data, but most particles are agglomerated
into clumps. Interestingly, the higher the amount of Au in AuPt
nanoparticles increases, the more AuPt nanoparticles become
agglomerated; this might be due to the self-assembling property
of Au [33]. Analysis of XRD and TEM data shows that AuPt
particles could be controlled on the nano-scale.
(
1 1 1), (2 0 0), (2 2 0) reflection from left to right. The (1 1 1)
peaks for Pt and Au nanoparticles synthesized by the impregna-
tion reduction method were located at the original peak positions
corresponding to their fcc metallic states, respectively. The 2θ
of the (1 1 1) for AuPt nanoparticles was located between the
Fig. 3 illustrates the linear sweep voltammetry of Pt, PtRu
and AuPt alloys for each anode catalyst in 0.5 M H SO + 1 M
(
1 1 1) peaks of Pt and Au nanoparticles. As the proportion of
2
4
Au was lowered, the diffraction peaks in the AuPt alloy were
shifted to higher 2θ values with respect to the same reflections
of Pt. Assuming alloy formation between Pt and Au based on a
substitutional solid solution, such a shift can be attributed to the
difference in atomic size [31]. Therefore, the higher the ratio of
Au in an alloy becomes, the more peaks the shift in proportion
to the amount of Au added. The amount of Au alloyed can be
determined from this since the lattice parameter is linear with
the composition of Au. Fig. 1(b) presents the portion of Au in the
alloy with the ratio of Pt and Au precursors; it was calculated
from the (1 1 1) peaks. As shown in Fig. 1(b), the Au portion
in AuPt catalysts changes linearly with the ratio of Pt and Au
precursors.
HCOOH at room temperature. Current densities in the voltam-
mogram were normalized to the active surface area (see
Fig. 1(c)). The oxidation of formic acid on AuPt occurs at the
lower overpotential applied and the oxidation current density of
AuPt is larger than PtRu and Pt below 0.2 V, which is the most
interesting region for catalytic activity in the measurement of
formic acid fuel cell performance. The mechanistic origin of
the electrocatalytic oxidation of formic acid on AuPt alloys has
been explained by the third-body effect [34]. Poisoning does
not occur on the Au surface and the Au has a little ability for
electro-oxidation of formic acid [35].
Fig. 4 shows the voltage–current curve and power density
of a commercial PtRu, home-made AuPt anode and Pt cathode
◦
Table 1 presents the particle size of synthesized AuPt
nanoparticles. The particle size of synthesized nanoparticles can
be calculated using Debye–Scherrer equation (1) by fitting the
applying humidified oxygen at 60 C using 9 M formic acid. We
observe that the open-circuit voltages of PtRu and Pt0.41Au0.59
catalysts are 0.57 V and 0.72 V. In addition, the maximum power
density of Pt0.41Au0.59 catalyst was higher than that of PtRu
(
2 2 0) peak:
−
2
−2
whichare200 mW cm and155 mW cm , respectively. These
findings support the evidence for the higher catalytic activity of
AuPt for formic acid oxidation compared to PtRu. As per the
results presented above, the AuPt catalyst shows higher formic
acid oxidation than Pt and PtRu when the Au portion in the alloy
is above 0.59. This indicates that formic acid is relatively rapidly
oxidized directly to CO2 on AuPt catalyst.
0
.94λKα1
L =
(1)
B(2θ) cos θB
where L is the average particle size, λKα is the X-ray wave-
1
length, θB is the angle of the (2 2 0) peak and B(2θ) is the peak
broadening (FWHM). The average size of synthesized AuPt
nanoparticles is smaller than 10 nm. The active surface area was
calculated assuming the charge for the oxidation of a complete
The general mechanisms for the electro-oxidation of
HCOOH have been proposed by both Capon and Wieckowski
[6,7]. The first reaction of HCOOH with Pt involves the one-
electron transfer:
−
2
hydrogen monolayer as being 210 ␮C cm as derived before
for polycrystalline Pt. The active surface area of each electrode
2
was calculated as 270 ± 30 cm . However, for Pt0.37Au
a
.63
0
lower active surface area was obtained than for other electrodes.
We assume that AuPt nanoparticles are agglomerated as the
amount of Au in the AuPt nanoparticles increases. As mentioned
above, the (1 1 1) peaks for Pt and Au nanoparticles synthesized
by impregnation reduction method were located at 39.76 and
8.16 , respectively, having the original peak positions corre-
+
−
HCOOH → (COOH)ads + H + e
(2)
The next step is the decomposition of the unstable interme-
diate COOH to CO2 or CO, dependent upon the potential. At
potentials within the broad double layer region on Pt, it is most
likely that the weakly bound COOH is involved in the main
oxidation pathway:
◦
◦
3
sponding to their fcc metallic states. The 2θ of Pt0.37Au0.63 is
◦
3
8.72 . The (1 1 1) peak of Pt0.37Au0 was shifted between
.63
Pt and Au (1 1 1) peaks. Assuming alloy formation between Pt
and Au based on a substitutional solid solution, such a shift can
be attributed to the difference in atomic size. For example, a Pt
+
−
(
COOH)ads → CO2 + H + e
(3)

J.K. Lee et al. / Electrochimica Acta 53 (2008) 3474–3478
3477
Fig. 3. Potential–current plot for Pt, PtRu, and AuPt in 0.5 M H2SO4 + 1 M
−
1
HCOOH (scan rate: 10 mV s ).
Bulk HCOOH may interact with COOH to produce the poi-
son.
HCOOH + (COOH)ads → (COH)ads + H2O + CO2
(4)
Competition between reactions (3) and (4) causes the rate of
intermediate formation to be an inverse function of the rate of
the oxidation reaction.
Pt alone is insufficient for use in a formic acid because
CO/(COH)ads poison hinders its catalytic activity. Accordingly,
newmaterial is neededto improve activity offormic acid electro-
oxidation and hinder CO/(COH)ads poison on Pt surface. Au is a
poor catalyst; it cannot easily adsorb neither reactant nor inter-
mediates. The addition of Au to Pt can improve CO/(COH)ads
tolerance of the Pt surface due to the decrease in the number of
surface sites available to CO/(COH)ads adsorption [36]. Also,
the electro-oxidation of HCOOH on Au takes place only at high
overpotentials and in the presence of oxide [1]. We believe that
the activity and performance of AuPt catalyst could be improved
because the presence of Au in catalyst prevents CO/(COH)ads
poisoning and causes it to be relatively rapidly oxidized directly
to CO2 via a weakly bound reactive intermediate on the AuPt
catalyst surface. This occurs although the real surface area of
Fig. 2. TEM images of AuPt catalysts: (a) Pt0.37Au0.63, (b) Pt0.40Au0.60, and (c)
Pt0.53Au0.47.
Fig. 4. Single cell performance of Pt-based Pt, PtRu-based and AuPt-based
◦
MEA using 9 M formic acid and oxygen at 60 C.

3
478
J.K. Lee et al. / Electrochimica Acta 53 (2008) 3474–3478
AuPt catalyst is decreased with increase of the Au portion in the
alloy.
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[
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4
. Conclusions
[
1
AuPt catalysts were synthesized by an impregnation reduc-
tion method. The synthesized AuPt nanoparticles are generally
10 nm in diameter but many of them agglomerate into clumps.
(
[
17] M. Watanabe, M. Horiuchi, S. Motoo, J. Electroanal. Chem. Interf. Elec-
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<
The AuPt catalyst provided a higher activity than that of Pt and
PtRu, showing a lower onset potential and large current density
and as the proportion of Au in the AuPt particle was increased,
the current–potential polarization curve indicated its possible
potential as an anodic catalyst in direct formic acid fuel cells.
[
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5
Acknowledgements
(
This work was supported by the Korea Science and
Engineering Foundation(KOSEF) grant funded by the Korea
government(MOST) (R01-2007-000-20290-0).
2
(
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