Hydrogenated graphene as support of Pd nanoparticles with improved electrocatalytic activity for ethanol oxidation reaction in alkaline media

Article abstract of DOI:10.1016/j.electacta.2018.12.058

A new class of ethanol electrooxidation catalyst was developed based on Pd nanoparticles supported by hydrogenated graphene (HG). The catalytic activities of Pd/HG with different hydrogenation degree (annealed at 60, 300, 800 °C respectively) were also carefully studied. Test results suggested that Pd/HG-60 exhibited super activity for electrocatalytic ethanol oxidation to Pd/HG-300 and Pd/HG-800. Moreover, Pd/HG-60 showed the highest electrochemically active surface area (EASA) among these obtained catalysts. The characterization data indicated that hydrogenated graphene with high hydrogenation degree was more favorable to promote the formation and dispersion of small Pd nanoparticles. In turn, highly dispersed Pd nanoparticles have strong electronic interaction with graphene support, which contributes to high EASA resulting in better electrocatalytic activity for ethanol oxidation reaction in alkaline media.

Full text of DOI:10.1016/j.electacta.2018.12.058

Electrochimica Acta 297 (2019) 856e863
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Hydrogenated graphene as support of Pd nanoparticles with improved
electrocatalytic activity for ethanol oxidation reaction in alkaline
media
He Xiao a , Jingjuan Zhang b, Man Zhao a b , Tianjun Hu a b, Jianfeng Jia a
,
b
a
,
,
,
*
,
, b, **
,
a, b
Haishun Wu
a
The School of Chemical and Material Science, Shanxi Normal University, No. 1, Gongyuan Street, Linfen, 041004, China
Key Laboratory of Magnetic Molecules & Magnetic Information Materials Ministry of Education, Shanxi Normal University, Linfen, 041004, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 September 2018
Received in revised form
A new class of ethanol electrooxidation catalyst was developed based on Pd nanoparticles supported by
hydrogenated graphene (HG). The catalytic activities of Pd/HG with different hydrogenation degree
ꢀ
(
annealed at 60, 300, 800 C respectively) were also carefully studied. Test results suggested that Pd/HG-
6
December 2018
60 exhibited super activity for electrocatalytic ethanol oxidation to Pd/HG-300 and Pd/HG-800. More-
Accepted 9 December 2018
Available online 10 December 2018
over, Pd/HG-60 showed the highest electrochemically active surface area (EASA) among these obtained
catalysts. The characterization data indicated that hydrogenated graphene with high hydrogenation
degree was more favorable to promote the formation and dispersion of small Pd nanoparticles. In turn,
highly dispersed Pd nanoparticles have strong electronic interaction with graphene support, which
contributes to high EASA resulting in better electrocatalytic activity for ethanol oxidation reaction in
alkaline media.
Keywords:
Hydrogenated graphene
Pd
Ethanol oxidation
©
2018 Elsevier Ltd. All rights reserved.
1. Introduction
Pd catalysts have recently been synthesized on different supporting
materials such as carbon materials [21e25]. Graphene, a sp -hy-
2
At present, direct ethanol fuel cells (DEFCs) are attracting in
electro-chemical power generation used in portable electronic
devices because of their superior properties such as high energy
density and energy conversion efficiency, low pollutant emission,
fuel availability and so on [1e7]. Previous reports [8e11] showed
that Pt and Pt-based alloys catalysts exhibited the best electro-
catalytic performance for alcohol oxidation, but their industrial
application is limited due to the high cost and easily poisoning
feature suffered from the oxidation intermediates and CO molecule.
In view of above, Pd and Pd-based catalysts were developed to
replace Pt because of their high abundance and good electro-
catalytic activity for alcohol oxidation in alkaline medium
bridized two-dimensional mono-layer sheet with unique electronic
and geometric properties [26], is widely used to support transition
metals as hybrid catalyst for electrocatalytic reactions. Ravindra
Nath Singh et al. [1] synthesized graphene nanosheets (GNS) as a
catalyst support of Pd nanoparticles for the electrooxidation of
ethanol and methanol. Their findings suggested that the enhanced
electrochemical activity of Pd/GNS for alcohol oxidation were
ascribed to the greatly enhanced electrochemically active surface
area due to improved dispersion of Pd nanoparticles on the GNS
support. Moreover, Jimin Xie et al. [12] modified the reduced gra-
phene oxide with dimethyldiallylammonium chloride (DMDAAC)
to support Pd nanoparticles, and the prepared Pd/DMDAAC-RGO
catalyst exhibited excellent activity and stability for ethanol
oxidation in alkaline media compared with Pd/RGO. Their results
indicated that DMDAAC played a crucial role in the dispersion and
stabilization of Pd nanoparticles on RGO sheets. It's obvious that the
catalytic activity of supported Pd catalyst for alcohol oxidation can
be attributed to the synergistic effect between Pd nanoparticles and
graphene support. When Pd nanoparticles serves as the main active
component, their size and dispersion are considered as the key
[
1,12e20]. In order to further improve the electrocatalytic activity,
*
Corresponding author. The School of Chemical and Material Science, Shanxi
Normal University, No. 1, Gongyuan Street, Linfen, 041004, China.
* Corresponding author. The School of Chemical and Material Science, Shanxi
*
Normal University, No. 1, Gongyuan Street, Linfen, 041004, China.
E-mail addresses: zm03030225@sxnu.edu.cn (M. Zhao), jiajf@dns.sxnu.edu.cn
(
J. Jia).
https://doi.org/10.1016/j.electacta.2018.12.058
013-4686/© 2018 Elsevier Ltd. All rights reserved.
0

H. Xiao et al. / Electrochimica Acta 297 (2019) 856e863
857
factors affecting the catalytic activity, which can be adjusted by
modifying graphene support [27].
subsequent stirring for 0.5 h. Then 100 mg hydrogenated graphene
support (HG-60 or HG-300 or HG-800) was added into H
2
PdCl
4
Previous studies [28,29] have suggested that graphene sheets
prepared under a more reduced condition could provide a more
effective metal-graphene interaction that would enhance the cat-
alytic performance of metal supported on them. In view of above,
here hydrogenated graphene was employed as substrate to support
Pd nanoparticles via a facile reduction method. The relatively
reducing atmosphere created by hydrogen desorption is probably
in favor of offering effective metal-graphene interaction, which is
benefit to improve the catalytic activity. The electrocatalytic per-
formances of prepared Pd/HG hybrid catalysts for ethanol oxidation
were tested in alkaline media using a conventional three-electrode
cell. To understand the origin of the enhanced catalytic perfor-
mance, characterization techniques such as XRD, FTIR, SEM, TEM,
XPS and Raman were used, and the structure-activity relationship
of these prepared catalysts was also investigated.
solution. After sonication and stirring, lysine solution and NaBH
4
solution were added dropwise to above solution respectively. Then,
3 mmol HCl (36%) was added to the solution and stirred for 24 h.
Finally, the obtained mixture was washed with deionized water
ꢀ
and ethanol, and then dried at 60 C for 8 h.
2.2. Characterization
X-ray diffraction (XRD) analysis was carried out on an X-ray
diffractometer (Ultima IV-185) with Cu Ka radiation. Raman spectra
were recorded on a Jobin Yvon Lab RAM HR800 micro-Raman
spectrometer using a laser excitation of 632 nm at room tempera-
ture. X-ray photoelectron spectroscopy (XPS) was performed on
Thermo Fisher SCIENTIFIC using Al Ka X-ray source. The morphol-
ogies of the samples were investigated by transmission electron
microscopy (TEM, FEI Tecnai F20) and scanning electron micro-
scopy (SEM, JSM-7001F). Fourier transform infrared (FTIR) spectra
2
. Experimental section
were recorded using
a
Brukerspectrometer. The thermal
2
.1. Chemicals and materials
gravimetry-mass spectrum (TG-MS) measurements were per-
ꢀ
formed from room temperature to 800 C under a flow of argon
ꢁ
1
Graphite powder (80 mesh) was purchased from Qingdao
(30 ml min ) using an Evolution and OMNI star equipment.
Graphite Company. Tetrabutylammoniumhexafluorophosphate
TBAPF , 99%, powder) was obtained from Sigma-Aldrich and used
(
6
2.3. Electrochemical measurements
as received. N, N-dimethylformamide (99.9%), and propylene car-
bonate (99.9%) was got from Aladdin, Anhydrous ethanol (99.5%)
purchased from Woke Company. NaBH (98%) and lysine (BR) were
4
bought from Woke Company. Commercial Pd/C (5%) was purchased
from Linshu Economic Development.
All electrochemical experiments were performed at room tem-
perature using a CHI660E electrochemical workstation. A conven-
tional three-electrode cell configuration was applied. Modified GC
electrode was used as working electrodes, and saturated Hg/HgO
and a platinum plate were used as reference and counter electrode,
respectively. Before measurement, GC electrode (3 mm in diam-
eter) surface was polished with 0.05 mm alumina powder and
washed subsequently with hydrous ethanol and deionized water.
The GC working electrode was coated with an ink. To form a ho-
2.1.1. Preparation of hydrogenated graphene
Graphite powder (50 mg) was put in a porous plastic tube with a
platinum plate inserted as negative electrode, another platinum
plate served as positive electrode. Both electrodes were immersed
in proprylene carbonate (PC) with tetra- butylammoniumhexa-
mogeneous ink, 5 mg catalysts and 20
were mixed in 1 ml of water/ethanol (v/v ¼ 3:1) by sonication for
30 min. Then, 5 L of the ink was loaded onto a glasscarbon disk
mL 5 wt% Nafion solution
6
fluorophosphate (TBA PF ; 0.1 M) and a voltage of 30 V was applied
for 12 h. After that, the sample was washed with DMF solution for
m
ꢀ
three times, and then dried in a vacuum oven for 8 h at 60 C, and
electrode by drop casting. The electrochemical impedance spec-
troscopy (EIS) was carried out on an IM6e electrochemical work-
the obtained sample was defined as HG-60. Besides, the samples
ꢀ
ꢀ
5
were annealed at 300 C and 800 C defined as HG-300 and HG-800
station over a frequency range from 10 to 1 Hz with a 5 mV
respectively.
amplitude.
2
.1.2. Preparation of Pd/HG
40 l 0.05 M H PdCl , 309.7 mg lysine and 265 mg NaBH
dispersed in 32 ml, 15 ml, 10 ml deionized water respectively with
3. Results and discussion
9
m
2
4
4
were
The morphology of prepared graphene supports and their Pd-
Fig. 1. SEM images of the HG-60(a), HG-300(b), HG-800(c), Pd/HG-60(d), Pd/HG-300(e) and Pd/HG-800(f).

858
H. Xiao et al. / Electrochimica Acta 297 (2019) 856e863
Fig. 2. TEM images of the Pd/HG-60(a), Pd/HG-300(b) and Pd/HG-800(c). Inset: particle-size distribution histogram of Pd nanoparticles. HRTEM images of the Pd/HG-60 (d), Pd/HG-
00(e) and Pd/HG-800(f).
3
loading catalysts were characterized using SEM. It can be seen from
the SEM images that HG-60 (Fig. 1a) presents thick and bulky
all show that translucent graphene sheets are covered with
homogenously distributed Pd nanoparticles with diameters of
1e14 nm. Particle-size distribution results (Insert in Fig. 2 aec)
indicates that the average diameter of the nanoparticles are
3.13 nm, 3.81 nm, 3.48 nm for Pd/HG-60, Pd/HG-300 and Pd/HG-
300, respectively. Apparently the Pd nanoparticles over Pd/HG-60
present the smallest average size among these Pd-loading cata-
lysts, indicating that HG-60 as the support is more benefit for
decreasing the size Pd particles than HG-300 and HG-800. Previous
reports [27] have showed that the dispersion and size of Pd
nanoparticles over support play a key role in electrochemical ac-
tivity for ethanol oxidation, thus here Pd/HG-60 with smaller
average size and high dispersion probably exhibits the better
ꢀ
ꢀ
morphology, after annealing at 300 C and 800 C, the obtained HG-
00 (Fig. 1b) and HG-800 (Fig. 1c) both exhibit aligned and
3
agglomerated stacks of graphene sheets occurring from the
expansion across the c-axis. With introduction of Pd to support, the
obtained Pd/HG-60 catalyst (Fig. 1d) presents similar bulky
morphology with HG-60 while Pd/HG-300 (Fig. 1e) and Pd/HG-800
(Fig. 1f) both show ultrathin graphene sheets. Moreover, the EDS
results (Fig. S1) of Pd/HG-60, Pd/HG-300 and Pd/HG-800 all show
that Pd has been loaded on the functional graphene support.
In order to observe obviously the size and distribution of Pd
nanoparticles on graphene support, TEM was applied. TEM images
of Pd/HG-60 (Fig. 2a), Pd/HG-300 (Fig. 2b) and Pd/HG-800 (Fig. 2c)
Fig. 3. XRD patterns of different samples.
Fig. 4. The FTIR spectra in the C-H stretching region of the as-prepared samples.

H. Xiao et al. / Electrochimica Acta 297 (2019) 856e863
859
to low Pd content, highly dispersed Pd and/or extra-crystalline Pd
nanoparticles on the surface. Combining with TEM results, it's
deduced that high dispersion contributed to one key role for that.
Furthermore, low Pd contents of Pd/HG-60, Pd/HG-300 and Pd/HG-
300 inferred from ICP-OES results (Table S1) turn into another
factor for explaining the no appearance of characteristic diffraction
peak of Pd.
The infrared spectra in the CeH stretching region for the cata-
lysts are shown in Fig. 4. As shown, HG-60 appears obvious peaks
ꢁ
1
between 2820 and 2970 cm assigned to asymmetric and sym-
3
metric sp CeH vibrations, indicating hydrogenated graphene can
be obtained by electrochemical cathodic method [31,32]. Compared
to HG-60, the intensities of these peaks over HG-300 and HG-800
are much lower, especially for HG-800, the CeH peaks almost
disappear because high-temperature treatment makes these CeH
groups break and the hydrogenation degree decrease. In other
words, the higher temperture treatment process applies, and the
lower hydrogenation degree the support will show, thus the HG-60
appears the highest hydrogenation degree. In addition, compared
with synthesized graphene support, almost no CeH peaks present
over corresponding Pd-loading catalysts, suggesting introduction of
Pd results in decrease of hydrogenation degree. In order to under-
stand the hydrogen composition change during the process of
thermal annealing, thermogravimetric analysis coupled with mass
spectrometry (TG-MS) has been applied. As depicted in Fig. S2a, the
Fig. 5. Raman spectra of different samples.
activity for ethanol oxidation than HG-300 and HG-800. Besides,
the interior lattice fringe spacing of Pd nanoparticles in the HRTEM
images for Pd/HG-60 (Fig. 2d) is measured to be about 0.22 nm
corresponding to (111) crystal face of palladium [15], meanwhile
Pd/HG-300 and Pd/HG-800 both show the similar results, further
verifying that it’s the Pd nanoparticles that highly disperse on the
graphene support.
Fig. 3 shows the XRD powder patterns of all samples. As seen
obviously, all the supports obtained through thermal treatment at
different temperatures present the same characteristic diffraction
peak (002) of graphite [30]. However, no XRD reflections ascribed
to Pd can be found on these Pd-loading catalysts. It is probably due
ꢀ
mass loss of the obtained support appeared from 160 C until about
ꢀ
700 C. The dehydrogenation of the sample can be seen inferred
from the mass spectrometric analysis (Fig. S2b) exhibiting two
ꢀ
ꢀ
peaks from about 250 C to 700 C for m/z ¼ 2, indicating the hy-
drogenated graphene can be obtained by this electrochemical
exfoliation method, and the hydrogen more easily desorbed from
hydrogenated graphene with the increase of thermal treatment.
Meanwhile, it provided a basis for choosing different thermal
ꢀ
ꢀ
ꢀ
temperature at 60 C, 300 C and 800 C after electrochemical
exfoliation process according to different dehydrogenation degree.
Fig. 6. Curve-fitting of C1s region in XPS spectra of HG-60(a), HG-300(b), HG-800(c), Pd/HG-60(d), Pd/HG-300(e) and Pd/HG-800(f).

860
H. Xiao et al. / Electrochimica Acta 297 (2019) 856e863
Raman spectra of graphene support and corresponding Pd-
loading samples were shown in Fig. 5. As for graphitic materials,
ꢁ
1
ꢁ1
three characteristic peaks at about 1350 cm , 1580 cm
2
and
ꢁ1
650 cm assigned to D, G and 2D bands respectively are often
investigated [31,32]. The D band is assigned to edges, other defects
and disordered carbon, while G band is attributed to the zone
2
center E2g mode corresponding to the ordered sp bonded carbon
atom [31,32]. Therefore, the I
and of G(I ) band is used to measure the disorder degree. The I
increases with the disorder degree for graphitic materials [1]. As for
these three supports, their I /I ratios follow the order: I /I (HG-
0) > I /I (HG-300) > I /I (HG-800), in accordance with their
/I
D G
ratio of the intensities of the D (I
D
)
G
/I
D G
D
G
D G
6
D
G
D G
hydrogenation degree inferred from FTIR results. It's concluded that
the hydrogenation in preparation process mainly contributed to the
defects on HG-60 agreeing with previous research [33]. That's to
say, like oxidation of graphene producing defects in the synthesis
process of graphene oxide, hydrogenation also leads to defects
D G
sites. Compared to HG-300 and HG-800, the I /I ratios in corre-
sponding Pd-loading catalysts (Pd/HG-300 and Pd/HG-800) in-
crease. Such an enhancement was also found in GO decorated with
metal nanoparticles, suggesting formation of strong chemical
interaction between the metal nanoparticles and graphene [34]. It
was further verified by the Pd 3d XPS results. Pd 3d BE value of Pd/
HG-60 (Fig. S3) is a little higher than that of Pd/HG-300 and Pd/HG-
8
00 reflecting stronger Pd-graphene support interactions. How-
ever, when compared Pd/HG-60 with HG-60, the I /I ratio
D
G
decreased, that's because the decrease of dehydrogenation degree
with introduction of Pd leads to drastic reduction of defects
deduced from FTIR results. In addition, the hydrogenation degree of
these three Pd-loading catalysts are very low, and their I
D
/I
G
ratios
(Pd/
follow the order: I /I (Pd/HG-60)> I /I (Pd/HG-300) > I
D
G
D
G
D
/I
G
2
HG-800), indicating smaller average size of the sp domains and
higher dispersion of Pd nanoparticles on Pd/HG-60 than that on Pd/
HG-300 and Pd/HG-800. In other words, Pd/HG-60 should show
the super catalytic activity for ethanol oxidation because of its
higher dispersion of Pd nanoparticles to Pd/HG-300 and Pd/HG-
8
00.
C1s spectra of graphene supports and corresponding Pd-loading
Fig. 7. Cyclic voltammograms of Pd/HG-60, Pd/HG-300, Pd/HG-800, commercial Pd/C
and Pd/GO in 1 M KOH (a) and 1 M KOHþ1 M ethanol (b) respectively at room
temperature.
samples were exhibited in Fig. 6. The spectrum of graphitic mate-
rials is usually fitted to five different peaks centered at 284.5, 285.5,
2
3
2
86.6, 287.2 and 288.7 eV, assigned to sp C (C]C), sp C(CeC/
CeH), CeOH, C]O and eCOOe groups, respectively [35]. But, as for
Table S1 for electrochemical activity comparison. Based on the
ethanol oxidation current peak values obtained from Fig. 7b and
Table S1, it's concluded that the rate of oxidation of ethanol is
3
HG-60 and HG-300, both their sp C peaks shift to about 285.1 eV
3
probably due to relatively high hydrogenation of graphene. The sp
2
3
ꢁ1
C/sp C ratios for three graphene samples follows the order: sp C/
higher on Pd/HG-60 (I z 3442 mA mg Pd) than that on Pd/HG-300
sp C (HG-60) > sp3 C/sp C (HG-300) > sp C/sp C (HG-800).
2
2
3
2
ꢁ1
ꢁ1
(I z 3197 mA mg Pd) or Pd/HG-800 (I z 2505 mA mg Pd) or
ꢁ
1
Compared to these graphene supports, as for the corresponding Pd-
commercial Pd/C (I z 532 mA mg Pd) under similar experimental
conditions, suggesting the Pd/HG-60 showed the best ethanol
oxidation activity among these samples. The test results verify the
conclusion inferred from TEM, Raman and FTIR results that small
size and high dispersion of Pd nanoparticles more easily formed on
hydrogenated graphene support contributes to high electro-
catalytic performance for ethanol oxidation. It's seen that EOR ac-
tivity for these catalysts increased with increase of the
hydrogenation. For verification, the EOR of Pd supported on non-
hydrogenated graphene (graphene oxide (GO)) was evaluated for
comparison. As seen from the Fig. 7b, the rate of oxidation of
ethanol is lowest on Pd/GO compared with other catalysts, sug-
gesting the crucial rule of hydrogenated graphene substrate for the
improvement of EOR activity.
3
2
loading catalysts, their sp C/sp C ratios all decrease. The high ratio
3
2
of sp C/sp C means the efficient hydrogenation of the graphene
support [36]. Thus, the above results suggest HG-60 show the
highest hydrogenation degree among these supports, and the hy-
drogenation degree decreases with the introduction of Pd, agreeing
with FTIR results.
Fig. 7a and b show CVs of Pd/HG-60, Pd/HG-300, Pd/HG-800,
commercial Pd/C and Pd/GO electrodes in the potential range
from 0.224 to 1.424 V in 1 M KOH and 1 M KOHþ1 M ethanol
respectively. As seen from the Fig. 7a, four CV curves determined in
1
M KOH are almost similar and don't show obvious characteristic
current peaks of ethanol oxidation. As for Pd-loading catalysts,
their CV curves (Fig. 7b) measured in 1 M KOHþ1 M ethanol also
appear to be similar, but each curve exhibits two characteristic
current peaks: one in the forward scan is under anodic condition
while the other in the reverse scan is under cathodic condition.
Moreover, values of the forward oxidation current peak and its
corresponding peak potential for ethanol oxidation are given in
Fig. 8 presents the potentiodynamic polarization plots obtained
ꢁ1
at a scan rate of 0.5 mV s for the Pd loading catalysts. The po-
larization currents of the ethanol oxidation on these catalysts are in
the order as follows: Pd/HG-60 > Pd/HG-300 > Pd/HG-800 > com-
mercial Pd/C at the anodic peak potential. The corresponding Tafel

H. Xiao et al. / Electrochimica Acta 297 (2019) 856e863
861
and other samples electrodes were shown in Fig. 9. The semi-
circular diameter of Pd/HG-60 was smaller than that of others
indicating a smaller charge transfer resistance and faster reaction
rate of Pd/HG-60 in accordance with Tafel results. In addition, the
electrochemically active surface area (EASA) of the electrodes is
obtained to examine the role of graphene support in the
improvement of electrocatalytic properties of Pd. The EASA of Pd
electrodes can be measured by determining the coulombic charge
for reduction of palladium oxide and was obtained using the rela-
tion: EASA ¼ Q/Sl [1,37,38]. Calculation results of EASA are shown in
2
Table S1. Results show that the EASA of Pd/HG-60 (703.2 cm
ꢁ1
2
ꢁ1
mg Pd) is higher than that of Pd/HG-300 (593.1 cm mg Pd), Pd/
HG-800 (392.6 cm mg Pd), Pd/GO (44.4 cm mg Pd) and com-
2
ꢁ1
2
ꢁ1
2
ꢁ1
mercial Pd/C (140.1 cm mg Pd). The high EASA of Pd/HG-60 is a
key role resulting in super ethanol oxidation activity. In combina-
tion with these TEM, Raman, FTIR, XPS results as well as electro-
catalytic test results, it can be concluded that the improved
electrocatalytic activity of the Pd/HG-60 compared to other sam-
ples is attributed to the improved EASA of Pd nanoparticles on the
HG-60 support.
Fig. 8. Potentiodynamic polarization plots with different working electrodes in solu-
ꢁ
1
tion of 1.0 M ethanol and 1.0 M KOH at a slow scan rate of 0.05 V s . The inset is the
Tafel plots.
In detail, the Pd nanoparticles highly dispersed on the surface of
graphene, and then an effective Pd-graphene interaction formed,
which improves the dispersion and reduces the size of Pd nano-
particles. Simultaneously, this effective synergistic effect between
Pd and graphene provided high EASA, finally resulting in high
electrocatalytic activity for ethanol oxidation reaction.
In view of the above discussion, we propose a probable sche-
matic diagram (Scheme 1). After the first electrochemical exfolia-
tion, the bulky graphite was exfoliated into larger thick corrugated
hydrogenated graphene sheets with much defects caused by hy-
ꢀ
drogenation. After thermal treatment at 60 C, the HG-60 support
was obtained. With introduction of Pd to HG-60 in the second
loading process, most hydrogen desorbed from CeH on HG-60
support leading to low hydrogenation degree and less defects
inferred from FTIR and XPS results. Meanwhile, under this reducing
condition created by dehydrogenation of HG-60 support well as
reducing agent, the Pd nanoparticles highly dispersed on the sur-
face of graphene support. The graphene support can generate an
extended confined space that could give rise to significant steric
hindrance. It can thus prohibit the formation of large species. Thus,
there is a strong and effective electronic interaction between gra-
phene and Pd nanoparticles because of the physical and chemical
adsorbance and charge transfer due to their different work func-
Fig. 9. Nyquist plots of EOR on Pd/HG-60, Pd/HG-300, Pd/HG-800 and commercial Pd/
C at a 0.78 V vs RHE at room temperature.
D G
tions, resulting in high I /I ratio inferred from Raman results. And
in turn, it supports more electrochemically active surface area
(EASA) for OER. In a word, HG-60 support with high hydrogenation
degree was favorable to promote the formation and dispersion of
small Pd nanoparticles, and the synergistic effect between highly
dispersed Pd nanoparticles and graphene support contributes to
high electrocatalytic activity for ethanol oxidation reaction in
alkaline media.
curves of these catalysts showed that the Tafel value
ꢁ1
(
(
161.6 mV$dec ) of Pd/HG-60 is smaller than that of Pd/HG-300
ꢁ
1
ꢁ1
174.4 mV$dec ), Pd/HG-800 (204.9 mV$dec ) and commercial
ꢁ
1
Pd/C (306.4 mV$dec ), suggesting the highest EOR rate over Pd/
HG-60. In addition, the electrochemical impedance spectroscopy
(
EIS) results for comparing the conductivity between the Pd/HG-60
Scheme 1. A probable schematic diagram for formation of Pd/HG-60.

8
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H. Xiao et al. / Electrochimica Acta 297 (2019) 856e863
110 (2006) 26055e26061, https://doi.org/10.1021/jp0651891.
4
. Conclusions
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A simple method for preparation of Pd/graphene catalyst for
electrocatalytic ethanol oxidation was established by one-step
electrochemical cathodic exfoliation and then loading process.
The prepared Pd/HG-60 catalyst showed remarkably superior
electrocatalytic activity for ethanol oxidation to Pd/HG-300 and Pd/
HG-800. Characterization results such as XRD, XPS, SEM, etc. sug-
gested the bulky graphite can be exfoliated into larger thick
corrugated hydrogenated graphene sheets with much defects
caused by hydrogenation. In addition, the hydrogenation degree of
hydrogenated graphene support decreased with the rise of thermal
temperture. As for HG-60, most hydrogen desorbed from CeH on
its surface leading to low hydrogenation degree and less defects
with introduction of Pd in the loading process. Moreover, HG-60
support with high hydrogenation degree was more favorable to
promote the formation and dispersion of small Pd nanoparticles
than HG-300 and HG-800. The highly dispersed Pd nanoparticles
with average size of 3.13 nm have strong electronic interaction with
HG-60 support, which produced high EASA for electrocatalytic
ethanol oxidation reaction in alkaline media. Moreover, this work
expanded the application of hydrogenated graphene as support.
[
[
[
[
[
14] Z. Wen, S. Yang, Y. Liang, W. He, H. Tong, L. Hao, X. Zhang, Q. Song, The
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Acknowledgements
This work is financially supported by the National Natural Sci-
ence Foundation of China (21373131, 21571110) and the Program for
New Century Excellent Talents in University (NCET-12-1035).
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.electacta.2018.12.058.
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