Boosting the catalysis of AuCuMo for oxygen reduction: Important roles of an optimized electronic structure and surface electrochemical stability

Article abstract of DOI:10.1016/j.jallcom.2020.155552

The slow kinetics of the oxygen reduction reaction (ORR) remains a great challenge in many energy storage and conversion devices, such as alkaline fuel cells and metal-air batteries. Herein, a self-supported Au-based alloy (AuCuMo) was successfully synthesized by a one-pot hydrothermal method. By combining Cu and Mo, the electronic structure of Au was finely tuned. Furthermore, the electrochemical stability of surface Cu was enhanced by the incorporation of Mo. Benefitting from these advantages, the reaction of oxygen and oxygenated intermediates on AuCuMo was optimized, and the intrinsic activity was improved. AuCuMo thereby exhibited superior ORR activity and stability compared to commercial Pt/C catalysts.

Full text of DOI:10.1016/j.jallcom.2020.155552

Journal Pre-proof
Boosting the catalysis of AuCuMo for oxygen reduction: Important roles of an
optimized electronic structure and surface electrochemical stability
Hongyu Gong, Ruizhi Yang, Bo Yang, Fan Li, Ling Li
PII:
S0925-8388(20)31916-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.155552
JALCOM 155552
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 29 February 2020
Revised Date: 7 May 2020
Accepted Date: 7 May 2020
Please cite this article as: H. Gong, R. Yang, B. Yang, F. Li, L. Li, Boosting the catalysis of AuCuMo
for oxygen reduction: Important roles of an optimized electronic structure and surface electrochemical
stability, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.155552.
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© 2020 Published by Elsevier B.V.

Hongyu Gong: Conceptualization, Methodology, Data curation, Writing-Original
draft preparation. Ruizhi Yang: Conceptualization, Reviewing and Editing, Funding
acquisition. Bo Yang: Methodology. Fan Li: Investigation,Visualization. Ling Li:
Methodology ,Validation.

Boosting the catalysis of AuCuMo for oxygen reduction: Important
roles of an optimized electronic structure and surface electrochemical
stability
Hongyu Gonga,b, Ruizhi Yangb,*, Bo Yanga, Fan Lic, Ling Lid
aAdvanced Materials Institute, Qilu University of Technology (Shandong Academy of Sciences),
Jinan, 250014, China
bCollege of Energy, Soochow Institute for Energy and Materials InnovationS, Soochow University,
Suzhou, Jiangsu 215006, China
cBeijing Key Laboratory for Green Catalysis and Separation, School of Environmental and Energy
Engineering, Beijing University of Technology, Beijing, 100124, China
dSchool of Materials Science and Engineering, Shandong Jianzhu University, Jinan, 250101,
China
Corresponding Author * E-mail: yangrz@suda.edu.cn

Abstract: The slow kinetics of the oxygen reduction reaction (ORR) remains a great
challenge in many energy storage and conversion devices, such as alkaline fuel cells
and metal-air batteries. Herein, a self-supported Au-based alloy (AuCuMo) was
successfully synthesized by a one-pot hydrothermal method. By combining Cu and
Mo, the electronic structure of Au was finely tuned. Furthermore, the electrochemical
stability of surface Cu was enhanced by the incorporation of Mo. Benefitting from
these advantages, the reaction of oxygen and oxygenated intermediates on AuCuMo
was optimized, and the intrinsic activity was improved. AuCuMo thereby exhibited
superior ORR activity and stability compared to commercial Pt/C catalysts.
Keywords: oxygen reduction reaction; gold-based alloy; surface electrochemical
stability; intrinsic activity.

Introduction
As an important reaction in metal-air batteries and alkaline fuel cells, the oxygen
reduction reaction (ORR) in alkaline media has received much attention recently.[1-3]
However, the sluggish kinetics of the ORR have hindered the development of these
energy storage and conversion devices. Extensive efforts have been made to address
this challenge. The Pt/C catalyst is well known to exhibit excellent activity. However,
the limited natural resources and low stability of Pt restrict its wide applications.[4, 5]
Therefore, various electrocatalysts, including transition metal oxides[6] and
heteroatom-doped carbon[7, 8], have been exploited. However, the low electronic
conductivity of transition metal oxides and the poor stability of heteroatom-doped
carbon during operation (for example, start-up and shut-down during fuel cell
operation) remain great challenges.[9] As potential candidates for Pt-based
electrocatalysts, Au-based catalysts have the advantages of high conductivity, high
stability and relatively high storage but suffer from low activity due to their weak
oxygen-binding ability. Hence, continuous work has focused on the activity
optimization of Au-based electrocatalysts. In earlier research, alloying precious metals
with inexpensive metals has been proposed to reduce the cost and improve the
catalytic activity of precious metal catalysts. [10-14] As a feasible and effective
strategy, it also works for Au-based catalysts. Hence, Au-based bi-/trimetallic
nanomaterials were developed, and the addition of a second/third metal could tailor
the electronic structure of Au and generate a synergistic effect. [15-18] For example,
several studies have focused on Au-Cu-based catalysts, where the addition of Cu,

which has a strong oxygen-binding ability, could enhance the absorption of oxygen
and thereby the activity of the catalyst. However, the stability of the Au-Cu-based
catalyst needs to be improved.[15, 16] Herein, a trimetallic AuCuMo nanobranched
alloy was developed. The addition of Mo as a third element enhanced the
electrochemical stability of Au-Cu. Furthermore, the addition of Cu and Mo
effectively optimized the electronic structure of Au; hence, the activity and stability of
AuCuMo towards ORR electrocatalysis was significantly improved.
Scheme 1. Schematic of AuCuMo synthesis.
Experimental
Chemicals
Nafion (5% w/w) and commercial Pt/C (20 wt. %) were obtained from Alfa
Aesar. Octadecylamine (95 wt. %), HAuCl4·4H2O (99 wt. %), CuCl2·2H2O (99
wt. %), (NH4)6Mo7O24·4H2O (99 wt. %), DMSO (99 wt. %), n-hexane (95 wt. %),
ethanol (99 wt. %) and KOH (95 wt. %) were obtained from Shanghai Aladdin
Biochemical Technology Co., Ltd. All reagents were used without further purification.

Sample synthetic methods
Scheme 1 shows the synthetic route of the AuCuMo catalyst. The precursor
solution containing the three metal salts and octadecylamine was thoroughly and
homogeneously mixed by magnetic stirring. The hydrothermal process was performed
in a drying oven. Octadecylamine was used as the surfactant and reducing agent and
removed by repeated centrifugation.
In a typical procedure, octadecylamine (7 mmol), CuCl2·2H2O (0.2 mmol), and
(NH4)6Mo7O24·4H2O (0.05 mmol) were dispersed in a mixed solution (DMSO: H2O =
20 mL: 42.48 mL). Next, a HAuCl4·4H2O solution (0.2 mmol of HAuCl4·4H2O in
8.32 mL aqueous solution) was added, followed by magnetic stirring for 24 hr. Then,
the solution was transferred to an autoclave, and a hydrothermal process at 120 °C
was carried out for 48 hr. Afterwards, the sample was filtered and washed with
ethanol and n-hexane three times.
The synthesis of AuCu was the same as the synthesis of AuCuMo but without the
addition of (NH4)6Mo7O24·4H2O.
Material characterization
To obtain structure and morphology information, X-ray diffraction (XRD, D8
Advance), scanning electron microscopy (SEM, Hitachi SU8010) and transmission
electron microscopy (TEM; FEI Talos F200s and Tecnai G2 F20) were applied. To
obtain electronic structure environment information, X-ray photoelectron
spectroscopy (XPS, VG ESCALAB MKII) was applied. ICP (Vista MPX) was

applied to obtain bulk component information.
Electrochemical measurements
A CHI760E electrochemical workstation and RRDE-3A apparatus were
employed to perform the electrochemical measurements at 25 °C. For the electrode
preparation, 1 mg catalyst (AuCuMo, AuCu and commercial Pt/C) was dispersed in a
solution of 1.6 mL ethanol and 0.4 mL Nafion (0.2 % w/w in a mixture of ethanol),
and the solution was ultrasonically mixed for 30 minutes. Next, for RDE (3 mm in
diameter), 13 µL solution was dip-coated on the surface; for RRDE (4 mm in
diameter), 25 µL solution was dip-coated on the surface. After drying under ambient
conditions, RDE or RRDE was used as the working electrode in the three-electrode
system, and Ag/AgCl (saturated KCl solution) and Pt foil were used as reference and
counter electrodes, respectively. Here, in 0.1 M KOH, all potentials were referenced
to the standard reversible hydrogen electrode (RHE) according to E (RHE) = E
(Ag/AgCl, saturated KCl solution) + 0.965 V. For the electrochemical test, the details
were as follows: First, cyclic voltammetry (CV) was conducted from 0.05 to 1.1 V at
50 mV s-1 in argon without rotation for catalyst activation. Then, for ORR background
correction, CV was performed from 0.2 to 1.1 V at 10 mV s-1 in argon with a rotating
speed of 1600 rpm. Next, the CV was measured between 0.05 and 1.1 V at 50 mV s-1
in O2 without rotation for the ORR. Finally, linear sweep voltammetry (LSV) was
performed from 1.1 to 0.2 V at 1600 rpm to evaluate the ORR performance. For the
RRDE test, the ring potential was set at 1.2 V. For the chronoamperometric (CA) test,
the i-t curve was obtained under a constant potential of 0.765 V for 15000 s. To

determine the electron transfer number (n), the rotating speeds ranged from 900 to
2025 rpm, and n was calculated according to Koutecky–Levich (K-L) equations (1)
and (2):
1/j = 1/jk + 1/jd = 1/jk + 1/ (Bω1/2)
B = 0.62nFDO 2/3υ −1/ 6CO
(1)
(2)
2
2
where j is the measured current density, jk is the kinetic-limiting current density, jd is
D O
2
the diffusion-limiting current density, and ω is the electrode rotation rate.
is
DO
−5
2
the diffusion coefficient of O2 (
= 1.86 ×10 cm2 s−1), F is the Faraday constant
CO
CO
(96485 C mol-1),
is the concentration of O2 dissolved in the electrolyte (
=
2
2
1.21 × 10−6 mol cm−3), and υ is the kinetic viscosity of the solution (υ = 0.01 cm2 s−1).
The RRDE test was further applied to measure the hydrogen peroxide yield (%
-
HO2 ) and the electron transfer number (n) during the ORR.
abs( jD )
n = 4×
abs( jD ) + jR / N
(3)
(4)
2 jR / N
abs( jD ) + jR / N
−
%HO2 =100×
where jD and jR could be determined from polarization curves directly and the
collection efficiency (N) was 0.424.

Results and discussion
Figure 1. (A) SEM image, (B) TEM image of AuCuMo. (C, D) Enlarged HRTEM
images of AuCuMo. (E, F, G, H, I, J) HAADF-STEM image and elemental mappings
of AuCuMo.
The hydrothermal method was used to synthesize AuCuMo branched
nanostructures, where a soft template was adopted. [19] In our previous work, it was
found that the coexistence of gold and copper precursors has great significance in the
synthesis of nanobranches. [18, 20, 21] In this work, a molybdenum precursor was
added to form AuCuMo nanostructures. As shown in the SEM image (Figure 1A) and
TEM image (Figure 1B, C), the as-synthesized AuCuMo shows a branched
nanostructure, similar to that of AuCu (Figure S1). The typical interplanar distance of
0.217 nm (Figure 1D) corresponds well to the d-spacing of the (111) crystal plane of
Au-based alloys as a result of the addition of smaller Cu and Mo atoms to the (111)
planes of Au (0.235 nm). Moreover, different growth directions and dislocations are

observed in the distorted part of the AuCuMo branch in Figure 1D, and twin crystals
are observed for AuCu in Figure S1. These defects can act as active sites in
electrocatalysis.[22] Elemental mapping shows a homogeneous distribution of Au, Cu
and Mo in the nanobranch (Figure 1G, H, I, J). Similar morphology and features are
observed for AuCu (Figure S1). The XRD patterns of AuCuMo and AuCu are shown
in Figure S2. AuCuMo and AuCu show similar XRD diffraction patterns, and
diffraction peaks of gold, copper and molybdenum monocomponents are not observed;
hence, the synthesis of a single-phase alloy is further verified.
Figure 2. (A) 1-50 cycle CV curves of AuCuMo and AuCu. (B) CV curves of
AuCuMo after the activation process. (C) CV curves of AuCu after the activation
process.
In a surface-sensitive reaction such as the ORR, the surface condition is important
for electrocatalysis.[23] Here, the CV patterns in Ar-saturated 0.1 M KOH are useful
for analysing the surface conditions of AuCuMo and AuCu. Initially, a 50-cycle CV
(sweep rate: 50 mV s-1) served as an activation process, and the potential window
ranged from 0.05 to 1.665 V (Figure 2A). After activation, as shown in Figure 2B, a
stable curve was obtained at a scan rate of 10 mV s-1 on negative scanning, and the

characteristic reduction of Au and Cu oxide was observed. The peaks located at 0.85
V and 1.08 V correspond to Au oxide, and the peaks located at 0.31 V and 0.62 V
correspond to Cu oxide, respectively.
For AuCu, the same procedure was performed, and the surface conditions are
shown in Figure 2C. It is worth noting that the surface electrochemical stability is
important for ORR electrocatalysis. [24] Here, the electrochemical stability of surface
Cu in AuCuMo and AuCu is obviously different. Taking the peak corresponding to
Cu(II)-to-Cu(I) reduction as an example, during the activation process shown in
Figure 2A, the Cu(II)-to-Cu(I) reduction peak area undergoes a decrease of 59.7 % for
AuCuMo, while for AuCu, the peak undergoes a decrease of 97.4 %. This sharp
contrast illustrates that the electrochemical stability of surface Cu was distinctly
enhanced as a result of the addition of Mo into the Au-Cu system. In addition, as
shown in Figure 2C, the ratio of the Cu reduction peak to the Au reduction peak for
AuCu is much lower than that for AuCuMo in Figure 2B, and the electroactive and
electrostable Au and Cu in AuCuMo could be favourable for catalytic activity.
The ORR on AuCuMo, AuCu and commercial Pt/C was studied via linear sweep
voltammetry (LSV) in 0.1 M KOH saturated with O2. The onset potential (Eonset) and
the half-wave potential (E1/2) are important descriptors for the assessment of ORR
performance.[25] Here, for AuCuMo, the Eonset is 1.03 V, which is 30 mV and 50 mV
more positive than those of Pt/C (1.00 V) and AuCu (0.98 V), respectively (Figure
3A). Furthermore, the E1/2 of AuCuMo is 0.875 V, which is positively shifted by 16
mV and 50 mV compared to Pt/C (0.859 V) and AuCu (0.825 V), respectively (Figure

3A). To further understand the reaction kinetics for the ORR, mass transport-corrected
Tafel plots were obtained. Figure 3B shows that AuCuMo, AuCu and Pt/C possess
Tafel slopes of 47.7, 44.7 and 68 mV dec-1, respectively. The smaller Tafel slopes of
AuCuMo and AuCu indicate faster kinetic behaviour. [26] To obtain the electron
transfer number (n), LSV measurements were recorded at different rotation rates
(Figure 3C). The corresponding Koutecky-Levich (K-L) plots are shown in Figure 3D.
Based on the K-L equation, the electron transfer number n is calculated to be
approximately 4, which is further confirmed by rotating ring disk electrode (RRDE)
tests (Figure 3E, F). The results indicate a quasi-four-electron ORR pathway and a
low amount of H2O2 (< 3 %) for AuCuMo.
Figure 3. (A) RDE polarization curves of AuCuMo, AuCu and commercial Pt/C. (B)
Tafel plots of AuCuMo, AuCu and commercial Pt/C. (C) RDE polarization curves of
AuCuMo at different rotation rates. (D) K-L plots of the ORR on AuCuMo. (E) Disk
and ring currents for AuCuMo. (F) Electron transfer number (n; top) and H2O2 yield

(bottom) for AuCuMo.
The durability of AuCuMo, AuCu and commercial Pt/C was assessed by a CA
method. Strikingly, there is no obvious decay (< 0.5 %) of AuCuMo after the whole
testing time (15000 s) (Figure 4A), demonstrating that the AuCuMo catalyst exhibits
better durability than AuCu and Pt/C. To account for electrocatalytic stability, the CV
was performed before and after the CA test and compared. As shown in Figure 4B, no
obvious change in the CV curve was observed, especially for the reduction peaks of
Au and Cu; hence, the surface electrochemical stability of Au and Cu was achieved on
the AuCuMo surface, and the durability was distinctly enhanced.
Figure 4. (A) Comparison of ORR stability on AuCuMo, AuCu and commercial Pt/C.
(B) CV curves of AuCuMo before and after the stability test.
The enhanced ORR performance of the AuCuMo catalyst can be rationalized as
follows. Initially, Mo is considered to be important for the surface stability of
crystals.[27, 28] By adding Mo, Au and Cu become electrochemically stable on the
surface of AuCuMo after CV activation, while for AuCu, the Cu content at the surface
is very low after CV activation. For ORR electrocatalysis, Cu exhibits stronger
oxygen adsorption than Au, and the existence of stable Cu at the surface could thereby

enhance the adsorption of oxygen.[15]
Next, to investigate the variations in the desorption of oxygenated intermediates
from the catalyst surface, XPS was performed (Figure 5A, S3). As shown in Figure
5A, the Au4f high-resolution XPS spectrum of AuCuMo displays two peaks
corresponding to Au0(4f7/2) and Au0(4f5/2) at 84.22 and 87.92 eV, respectively.
Compared to those of pure Au (83.42 and 87.12 eV) and AuCu (83.62 and 87.32 eV),
the binding energies of Au4f for AuCuMo shift to higher values as a result of the
addition of Cu/Mo (Figure 5B), leading to the electronic structure optimization of Au
and rendering the desorption of oxygenated intermediates from the surface easier.[29]
Furthermore, the metal valence-band structure, usually described quantitatively in
terms of the d-band centre energy, can be significantly affected by the addition of the
second/third metal, and to determine the d-band centre position of Au, the surface
electronic structure of our bi-/trimetallic catalysts was studied using valence band
XPS evaluations (Figure 5C, Figure S4)[30, 31]. After subtraction of the Shirley
backgrounds, the intensity signals of the valence-band spectra are proportional to the
density of states. Upon addition of Cu/Mo, the valence d-band centre changed , from
-3.88 eV for Au, to -4.09 eV for AuCu, and -4.68 eV for AuCuMo. Such a change in
the d-band centres is crucial in the precise regulation/reduction of binding strength
between the catalysts and adsorbates. This will remove strongly adsorbed species
from the Au-based catalyst surface and boost the intrinsic activity.

Figure 5. (A) XPS spectra of Au4f in AuCuMo, AuCu and Au. (B) Binding energies
of Au0(4f7/2) in AuCuMo, AuCu and Au. (C) d-band centre energies of AuCuMo,
AuCu and Au. (D) ICP quantitative analysis and XPS semi-quantitative analysis of the
element content in AuCuMo. (E) ICP quantitative analysis and XPS semi-quantitative
analysis of the element content in AuCu.
In addition, as shown in Figure 5D and 5E, for AuCuMo, from the ICP
quantitative analysis, the Au: Cu: Mo ratio was 4.173: 10.088: 1 for the bulk catalyst;
from the XPS semiquantitative analysis, the Au: Cu: Mo ratio was 8.296: 11.028: 1
for the catalyst surface, the Au content was 27.34 % for the bulk and 40.82 % for the
surface, and more Au existed at the surface. For AuCu, the Au: Cu ratio was 1: 1 for
the bulk catalyst and 0.454: 1 for the catalyst surface. The Au content was 50 % for
the bulk catalyst and 31.21 % for the surface, and more Au existed inside the catalyst.
The obvious difference in the number of surface active sites explains the increasing

ORR activity of AuCuMo. It is worth noting that, as shown in Table S1, compared to
our previous research on Au-Cu-M (M= Co, Ni) trimetallic electrocatalysts[18, 29],
AuCuMo exhibits the best ORR activity, which confirms the importance of the
addition of Mo, and the continuous optimization of Au-Cu-M-based electrocatalysts
will be carried out in future work.
Conclusion
In this work, a promising AuCuMo catalyst for the ORR in alkaline media was
synthesized via a facile method. The enhanced properties originate from the improved
intrinsic activity of Au boosted by the optimized electronic structure of Au by adding
Cu and Mo and the enhanced electrochemically stable surface promoted by Mo. This
work opens a new avenue to the preparation of active and stable Au-based
electrocatalysts in the field of alkaline fuel cells and metal-air batteries.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Nos.
51972220
and
51472009)
and
the
Natural Science Foundation
of
Shandong Province (No. ZR2019BEM036).
Appendix A. Supplementary materials
Supplementary
data
to
this
article
can
be
found
online
at
https://doi.org/10.1016/j.jallcom.2020.xxxxxx.

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Highlights
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Self-supported AuCuMo alloy exhibits high activity and stability for ORR.
By incorporating Cu and Mo, the electronic structure of Au is finely tuned.
Electro-chemical stability of surface Cu is enhanced by incorporation of Mo
Electronic structure and surface electrochemical stability are crucial for ORR.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: