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Article
Atomic H-Induced Mo
2
C Hybrid as an Active
and Stable Bifunctional Electrocatalyst
Xiujun Fan, Yuanyue Liu, Zhiwei Peng, Zhenhua Zhang, Haiqing Zhou, Xianming Zhang,
Boris I. Yakobson, William A. Goddard, Xia Guo, Robert H. Hauge, and James M. Tour
ACS Nano
,
Just Accepted Manuscript
• DOI: 10.1021/acsnano.6b06089
• Publication Date (Web): 18 Dec 2016
Downloaded from http://pubs.acs.org on December 19, 2016
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1
Atomic H-Induced Mo
2
C
Hybrid as an Active and Stable
Bifunctional Electrocatalyst
Xiujun Fan,
†‡
⊥||
*
Yuanyue Liu,
¶ς
Zhiwei Peng,
⊥
Zhenhua Zhang,
#
Haiqing Zhou,
⊥||
Xianming Zhang,
†
Boris I. Yakobson,
⊥||
§
William A. Goddard III,
¶
Xia Guo,
‡*
Robert H.
Hauge
⊥||
1
and James M. Tour
⊥||
§*
†
Institute of Crystalline Materials, Shanxi Universi
ty, Taiyuan, Shanxi 030006, China
‡
College of Electronic Information and Control Engin
eering,
#
Institute of Microstructures and
Properties of Advanced Materials, Beijing Universit
y of Technology, Beijing 100124, China
⊥
Department of Chemistry,
||
NanoCarbon Center,
§
Department of Materials Science and
NanoEngineering, Rice University, Houston, Texas 77
005, United States
¶
Materials and Process Simulation Center,
ς
The Resnick Sustainability Institute, California
Institute of Technology, Pasadena, CA 91125, United
States
ECmail
:
tour@rice.edu, guo@bjut.edu.cn, fxiujun@gmail.com
Abstract
Mo
2
C nanocrystals (NCs) anchored on vertically aligned
graphene
nanoribbons (VA%GNR) as hybrid nanoelectrocatalysts
(Mo
2
C%GNR) are synthesized through the
direct carbonization of metallic Mo with atomic H t
reatment. The growth mechanism of Mo
2
C
NCs with atomic H treatment is discussed. The Mo
2
C%GNR hybrid exhibits highly active and
durable electrocatalytic performance for the hydrog
en evolution reaction (HER) and oxygen
reduction reaction (ORR). For HER, in an acidic sol
ution the Mo
2
C%GNR has an onset potential
1
Deceased March 17, 2016
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of 39 mV and a Tafel slope of 65 mV dec
%1
, in a basic solution Mo
2
C%GNR has an onset potential
of 53 mV, and Tafel slope of 54 mV dec
%1
. It is stable in both acidic and basic media. Mo
2
C%GNR
is a high activity ORR catalyst with a high peak cu
rrent density of 2.01 mA cm
%2
, an onset
potential of 0.93 V that is more positive
vs
reversible hydrogen electrode (RHE), a high electr
on
transfer number
n
(
∼
3.90) and long%term stability.
Keywords:
Mo
2
C, graphene nanoribbon, hydrogen evolution reaction
(HER), oxygen reduction
reaction (ORR), atomic H
The electrocatalytic hydrogen evolution reaction (H
ER) and oxygen reduction reaction
(ORR) hold tremendous promise as efficient and clea
n energy solutions. Although
platinum%group metals (PGMs) Pt,
1
Rh,
2
and Pd,
3
are highly active and stable electrocatalysts for
HER and ORR, their scarcity and high cost hinder th
eir large%scale commercial applications.
Hence, some earth%abundant transition metal carbide
s (TMCs), such as WC
4
and particularly
Mo
2
C
5
have emerged and they exhibit remarkable HER and O
RR performances due to their
electronic structures that are similar to those of
noble metals.
6
Mo
2
C%based materials with
various nanostructures have been designed to be HER
catalysts, including nanotubes,
7
nanowires,
8
and nanoparticles.
9
Recent studies have shown that composites consisti
ng of Mo
2
C
and carbon are active toward hydrogen evolution in
acidic electrolytes, such as Mo
2
C/carbon
xerogel (CXG),
10
Mo
x
C%Ni@N%doped carbon vesicle,
11
Mo
2
C–graphene,
12
Mo
2
C/reduced
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graphene oxide,
13
MoO
2
/Mo
2
C%N%doped carbon nanotubes (NCNTs),
14
Mo
2
C@nitrogen%rich
carbon,
15
and Mo
2
C/graphitic carbon sheets.
16
Nonetheless, the key issue for the development of
carbide catalysts is their synthesis, especially ca
talysts with nanocrystalline phases, because of
the expected aggregation and/or disproportionate gr
owth of Mo
2
C nanocrystals (NCs) at elevated
reaction temperatures, and the swift oxidation of M
o
2
C nanoparticles (NPs) surfaces to MoO
x
species on exposure to air.
17
The usual process to synthesize metal carbides invo
lves high temperature (
>
1000 °C)
carburization of metals with graphitic carbon, whic
h is not suitable to producing materials for
catalytic applications because the products have lo
w surface areas.
18
The
temperature%programmed reduction (TPR) method was d
eveloped in the 1980s and extensively
used to synthesize high surface area transition%met
al nitrides and carbides.
19
Nevertheless, the
TPR conditions are demanding. More recently, gas%ph
ase reactions of volatile metal
compounds,
5
pyrolysis of metal complexes
20
and solution reactions
21
were used to synthesize
metal carbides using costly or toxic reagents such
as MoF
6
22
or Mo(CO)
6
.
23
Unfortunately, chars
from the pyrolysis of the carbon usually contaminat
e the resultant carbide.
24
Coincidentally,
quite recently, functionalized 2D layered metal car
bides/carbonitrides (MXene) including Mo
2
C
have been synthesized by selective etching with con
centrated HF or a solution of LiF and HCl.
Such chemically derived Mo
2
C suffers from severe structural defects, is termin
ated by hydroxyl
and/or other oxygen%containing groups, and has fluo
rine present on the surface.
25
Furthermore,
hot%filament chemical vapor deposition (HF%CVD) is
one of the most promising techniques for
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growing high melt point carbide materials at relati
vely low substrate temperature, such as WC,
26
M
3
C (M:Fe,Co,Ni),
27
silicon carbide (SiC)
28
and so on. In the HF%CVD processing, the filament
was heated to over 2000
o
C for activation of the source gas. It is highly fl
exible and scalable,
easy to scale up to large%scale growth and, no less
important, utilizes metal rather than vaporized
metal%containing reagents as precursors, which intr
oduce no other impurity.
From the
applications point of view, although Mo
2
C%based materials have been extensively studied for
HER in acidic media, it is still challenging to dev
elop Mo
2
C%based HER catalysts that work
efficiently over a range of pH values. Moreover, Mo
2
C materials draw much less attention in
ORR.
29
Only recently have these carbides been shown to be
efficient noble metal%free catalysts
for ORR.
30
The dual use of the Mo
2
C%based materials for both HER and ORR applications
is a
challenging issue that has been scarcely reported.
Herein, we report a synthesis of a highly active an
d stable precious%metal%free hybrid
electrocatalyst consisting of Mo
2
C NCs anchored on vertically aligned graphene nanor
ibbon
(VA%GNR)
.
The VA%GNR supported%Mo
2
C (Mo
2
C%GNR) hybrid is prepared with atomic H
treatment through the direct reaction of metallic M
o with a carbon source. The Mo
2
C%GNR
hybrid demonstrates excellent HER performance both
in acidic and alkaline electrolytes, with the
overpotential for driving a current of 10 mA cm
%2
(
η
10
) of 152 mV, onset potential of 39 mV and
Tafel slop of 65 mV dec
%1
in acid solution, and 121 mV, 53 mV and 54 mV dec
%1
, respectively, in
alkaline media. The Mo
2
C%GNR is also a highly active catalyst for ORR with
a high peak current
density of 2.01 mA cm
%2
, a more positive onset potential of 0.93 V
vs
reversible hydrogen
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electrode (RHE) and a high electron transfer number
n
(
∼
3.90). The dual performance of
Mo
2
C%GNR in both HER and ORR suggests a material type
as nonprecious metal bifunctional
catalysts.
Results and Discussion
The experimental setup for the preparation of Mo
2
C%GNR is depicted in Figure 1a. First,
VA%GNR was made from vertically aligned carbon nano
tubes (VA%CNTs) through a hot filament
chemical vapor deposition (HF%CVD) process.
31
After a thin layer of Mo (~75 nm) was
deposited atop the VA%GNR, the prepared structure w
as subjected to atomic H treatment for
carburization of metallic Mo. Mo
2
C without VA%GNR support was prepared at the same t
ime for
comparison. Scanning electron microscopy (SEM) was
carried out to study the morphology of
VA%GNR before and after atomic H treatment (Figure
1b%e). As shown in Figure 1b
and
Figure
S1%S2, the unzipped VA%GNR have clumps of tips that
meet together, forming structures
resembling Native American “teepees”.
32
The teepees are distributed with enough open space
to
permit the Mo particles to pass through the VA%GNR
bundles and disperse on the surface of the
VA%GNR. Based on imaging, there is limited agglomer
ation of the Mo, which appears as a film
to the eye (Supporting Information, Figure S3). Wit
h atomic H treatment, Mo
2
C NCs form along
the surface of the VA%GNR, which maintains the vert
ical structural integrity and alignment
(Figure 1c and S4). The SEM top view image shows th
at the Mo
2
C grains are uniformly
dispersed without visible aggregation (Figure 1d),
and not stacked on the tips of VA%GNR, which
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is different from previous studies about iron group
metal carbide NCs prepared with same the
method.
27
This means that the properties of the elements them
selves such as the melting points
and vapor pressures may play a role in NCs growth w
ith HF%CVD process.
The cross%sectional
image reveals the porous structure of the VA%GNR fo
rest, while Mo
2
C grains are difficult to
distinguish due to their small size (Figure 1e). As
a control, Mo
2
C synthesized directly on Si
without the GNR support produced larger particles,
as shown in the SEM image (Supporting
Information, Figure S5). This control experiment de
monstrates the important role of VA%GNR in
the formation of Mo
2
C NCs with uniform dispersion and small particle si
ze, which are critical
for the high electrocatalytic activities.
Raman spectra (Figure 1f
and
S6a) and powder X%ray diffraction (XRD) patterns (F
igure 1g
and
S6b)
acquired from VA%GNR, Mo@VA%GNR and Mo
2
C%GNR are performed to studied the
physical and chemical structures evolution of these
carbon hybrid. From the Raman spectrum
and XRD pattern of Mo@VA%GNR in Figure S6, it is co
nfirmed that Mo layer deposited on
VA%GNR was partly converted to MoO
3
when the sample is exposed to air.
33
With atomic H
treated for 6 h, the Mo
2
C%GNR reveals peaks at 1592 cm
−1
(G%band) and 2676 cm
−1
(2D%band),
followed by a shoulder peak at 2927 cm
−1
(D+G) (Figure 1f). The area ratios of G%band to
D%band (
I
G
/
I
D
) are 1.34 and 1.27 for VA%GNR and Mo
2
C%GNR, respectively, indicating that litter
disorder is introduced by atomic H treatment.
16
There are no noticeable peaks in the low range
from 100 to 1200 cm
%1
for Mo
2
C%GNR (dashed circle in Figure 1f), indicating that
the sputtered
Mo and the relevant MoO
3
have been transformed into Mo
2
C with atomic H treatment. In
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Figure 1g, the diffraction peaks at 34.6°, 38.1°, 3
9.6° and 52.4° can be assigned to the (100),
(002), (101) and (102) crystal faces of Mo
2
C with a hexagonal closed%packed structure
(PDF#35%0787). The carbon peak of (101) is from the
GNR support, which corresponds to the
XRD result of VA%GNR. No any peaks from metallic Mo
or MoO
3
are detected, indicative of a
successful chemical conversion of Mo into Mo
2
C.
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Figure 1.
(a) The scheme depicting the key process for Mo
2
C NC growth, in which a layer of
Mo is deposited on VA%GNR using a sputter system. S
EM images of (b) VA%GNR, (c%e)
Mo
2
C%GNR hybrid. (f) Raman spectra and (g) XRD pattern
s of pristine VA%GNR and
Mo
2
C%GNR hybrid.
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Figure 2.
(a) XPS survey spectrum and (b) high resolution XP
S scan of the Mo 3d of
Mo@VA%GNR and Mo
2
C%GNR. (c) TGA curves of the VA%GNR and Mo
2
C%GNR hybrid
measured by heating samples in air flow to 900 °C a
t a ramp of 5 °C min
%1
. (d) TEM image of
Mo
2
C%GNR prepared at 850 °C for 6 h. Inset: Gaussian f
its of the size distribution of Mo
2
C NCs.
(e, f) HR%TEM images of individual Mo
2
C NCs with (100) plane and (002) plane. Thin layers
of
carbon species (indicated by yellow arrows) are obs
erved. The insets in (e, f) show the
corresponding fast Fourier transform (FFT) patterns
of the flat surface enclosed by the yellow
boxes on each image.
Further investigation of the composition and crysta
lline structure of the Mo
2
C%GNR hybrid
was performed by X%ray photoelectron spectroscopy (
XPS, Figure 2a,b), thermal gravimetric
analysis (TGA, Figure 2c) and transmission electron
microscope (TEM, Figure 2d%f). From
Figure 2a, elements of C, O and Mo can be clearly i
dentified. The main C1s peaks at 284.5 eV
are observed for both Mo@VA%GNR and Mo
2
C%GNR (Supporting Information, Figure S7a),
which is consistent with the sp
2
carbon of the GNR scaffold. For Mo@VA%GNR, the pea
k at
228.1 eV is attributed to metallic Mo, while the pe
aks at 232.1 and 235.3 eV are are indexed with
MoO
3
, which results from surface oxidation of the Mo wh
en the sample is exposed to air. For
Mo
2
C%GNR, the peaks located at binding energies of 228
.2 eV and 231.5 eV are assigned to
Mo
2+
(3d
5/2
and 3d
3/2
, respectively), which is consistent with the carbi
de phase (Figure 2b). The
O 1s peak at 530.4 eV is correspond to the lattice
O
2
%
in the MoO
3
(Supporting Information,
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Figure S7b). While, with atomic H treatment, the O
1s peak is obviously shifted from 530.4 eV
to 532.8 eV, which is attributed to absorbed oxygen
on the surface of Mo
2
C%GNR. This result is
also consistent with the Mo 3d XPS spectra of Mo
2
C%GNR in Figure 2b. The elemental
composition determined by the
inductively coupled plasma mass spectrometry (ICP%M
S) and the
surface atomic concentrations derived by energy%dis
persion X%ray spectroscopy (EDS, Figure S8)
are summarized in Table S1. Mo@VA%GNR and Mo
2
C%GNR have an almost constant Mo
content of ~ 31 wt%. Mo@VA%GNR has an O content of 5
0.6 at% derived from the EDS
measurements. In contrast, after atomic H treatment
the O content decreases significantly to 3.5
at%, which may be due to the reduction of MoO
3
and carbonization of Mo at VA%GNR in atomic
H at 850 °C.
34
Confirmed by TGA, no significant weight loss is det
ected below 500 °C in air for
Mo
2
C%GNR (Figure 2c). When the temperature rises above
500 °C, the decrease in weight starts
and continues until 645 °C, and displays a mass inc
rease, followed by a mass decrease, which
confirms the formation of MoO
3
solid residue.
34
Furthermore, all of the TEM observed Mo
2
C is
inlaid or anchored on the GNR; few of any free%stan
ding particles were found away from the
GNR support (Figure 2d). Some nanotube%like carbon
remains from unopened CNTs (yellow
arrow). A few layers of graphite shell surround the
se irregularly shaped granular particles, as
shown in Figure 2d. The shell structure is likely f
ormed during the cooling of the HF%CVD
system due to the active carbon dissolution%precipi
tation process for transition metals.
35
Covalent
bonding between the graphite shells and Mo
2
C could further downshift the d%band center of
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molybdenum, and thereby decrease its hydrogen bindi
ng energy.
24
A Gaussian fit gives a most
common diameter of 7.5 nm
(inset in Figure 2d). In contrast, the Mo
2
C on Si without GNR
support forms larger aggregates, as shown in the TE
M image (Supporting Information, Figure
S9). Therefore, it can be concluded that the VA%GNR
effectively prevented the aggregation and
growth of the reduced Mo
2
C NCs. The high%resolution TEM (HR%TEM) images allo
w us to
observe the crystalline nature of the Mo
2
C NCs produced. Figures 2e,f
exhibite that the lattice
fringes with a d%spacing of 0.26 and 0.24 nm, close
ly match the (100) and (002) planes of
hexagonal Mo
2
C, respectively, and are also consistent with the X
RD result (Figure 1f). The size
of the Mo
2
C NCs are vary with different atomic H treatment ti
me. With atomic H treatment for 3
h, the Mo
2
C NCs ranged from 3 to 5 nm and decorated the carbo
n layer without obvious
aggregation (Figure S10a,b). Upon increasing the tr
eatment time to 9 h, the Mo
2
C NCs exhibit
larger nanoparticles (Figure S10c,d). The larger si
ze could be due to the migration and ripening
of the NCs during the reduction. The continuous lat
tice fringes throughout the entire nanoparticle
shown in the HR%TEM image (Figure S10) together wit
h the sharp peaks in the XRD patterns
(Figure S11) imply the efficiency of the atomic H t
reatment to produce high crystallinity and
pure phase Mo
2
C NCs.
36
High%angle angular dark%field scanning transmissio
n electron
microscopy (HAADF%STEM) and the corresponding eleme
ntal mappings (EDS) reveal the
uniform distribution of C and O elements over the M
o
2
C%GNR prepared with atomic H treatment
for 9 h (Figure S12). Mo element is covered on the
interior of the C and O elements, also
suggesting that the Mo
2
C NCs are mainly attached over the carbon support t
o form a hybrid.
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In the TPR reaction, the mechanism for the MoO
3
to Mo
2
C conversion involves the
substitution of carbon for oxygen in the MoO
3
lattice, with little displacement of the Mo atoms.
22
However, in the HF%CVD process, the temperature of
the system is far above the temperature
regime where MoO
3
is unstable with respect to decomposition into MoO
2
followed by the MoO
2
itself decomposing to give Mo metal.
37
Therefore, The growth of Mo
2
C NCs is through the direct
carburization of Mo metal, without forming any reac
tion intermediate such as MoO
2
. That is,
MoO
3
is
reduced to metallic Mo mainly by atomic H, followed
by simultaneous C insertion at
high temperature. The addition of the hot filament
step in the synthetic route is critical to the
formation of Mo
2
C NCs. It not only assists to form active carbon th
at diffuses into the Mo
surface and reacts with Mo, but also creates excess
ive atomic H that activates the Mo surface
via
reduction of MoO
3
and catalyzes the crystallization of Mo
2
C. Eq 1%4 outline the mechanism of
the Mo
2
C formation with atomic H treatment. The activated
gas mixtures of atomic H (H*, eq 1)
and carbon%containing species (eq 2), are generated
under high temperature of the filament (
>
2000 °C) in the HF%CVD system. MoO
3
is reduced by H* to metallic Mo in eq 3.
38
The
generated C* is so active that it reacts directly w
ith metallic Mo to form Mo
2
C NCs at the high
temperature in eq 4.
→
∗
1
→
∗
+
∗
2
MoO
+
∗
→
Mo +
3
+
∗
∗
4
The specific surface area of the pristine VA%GNR an
d Mo
2
C%GNR measured by N
2
adsorption using Brunauer%Emmett%Teller (BET) measu
rements (see Supporting Information,
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Figure S13) are 936.4 and 641.1 m
2
g
%1
, respectively. The decrease of the specific surfac
e area of
Mo
2
C%GNR can be attributed to the decoration of Mo
2
C NCs on the surface of VA%GNR. This
high surface area can provide sufficient space for
electrocatalytic performance of HER and ORR.
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0.4
0.6
0.8
1.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
0.2
0.4
0.6
0.8
1.0
1.2
0.02
0.04
0.06
0.2
0.4
0.6
225 rpm
400 rpm
625 rpm
900 rpm
1225 rpm
1600 rpm
2025 rpm
Mo
2
C-GNR
j / mA cm
-2
E / V vs RHE
n = 3.90
0.4
0.6
0.8
1.0
-4
-2
0
Mo
2
C
VA-GNR
E / V vs RHE
f
e
d
b
a
j / mA cm
-2
c
Pt/C
Mo
2
C
-GNR
E
1/2
= 31 mV
E / V vs RHE
0
2
4
0.6
0.7
0.8
0.9
1.0
b = 115 mv dec
-1
Pt/C
Mo
2
C-GNR
Mo
2
C
VA-GNR
j / mA cm
-2
b = 119 mv dec
-1
b = 41 mv dec
-1
b = 43 mv dec
-1
0.4
0.6
0.8
1.0
-4
-2
0
Mo
2
C-GNR
Initial
After 1000 cycles
0.1 M KOH
15 mV
j / mA cm
-2
E / V vs RHE
0
5000
10000
15000
20000
0
20
40
60
80
100
0.1 M KOH
Pt/C
Mo
2
C-GNR
Normalized current (%)
Time / s
Potential / V vs RHE
VA-GNR
Mo
2
C
Mo
2
C-GNR
0.5 mA cm
-2
0.80 V
0.75 V
0.70 V
0.65 V
j
-1
/ mA
-1
cm
2
ω
-1/2
/ rpm
-1/2
Figure 3.
(a) CVs of Mo
2
C%GNR, Mo
2
C and VA%GNR in O
2
%saturated (solid lines) and
Ar%saturated (dashed lines) 0.1 M KOH electrolyte s
olutions at a scan rate of 5 mV s
−1
. (b) RDE
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voltammogram of Mo
2
C%GNR in O
2
%saturated 0.1 M KOH with a scan rate of 5 mV s
%1
at
different RDE rotation rates (rpm).
(c) RDE voltammogram of Mo
2
C, VA%GNR, Mo
2
C%GNR and
Pt/C in O
2
%saturated 0.1 M KOH with a sweep rate of 5 mV s
%1
at 1600 rpm, (d) the
corresponding Tafel slopes. (e) Endurance test of t
he Mo
2
C%GNR catalyst for 1000 cycles in
O
2
%saturated 0.1 M KOH. (f) Chronoamperometric respon
ses (percentage of current retained
vs
operation time) of Mo
2
C%GNR hybrid and Pt/C on carbon glass electrodes ke
pt at 0.70 V
vs
RHE
in O
2
%saturated 0.1 M KOH electrolytes.
To measure the electrocatalytic activity of Mo
2
C%GNR hybrid toward ORR, the as%prepared
electrodes are first loaded onto glassy carbon elec
trodes for cyclic voltammetry (CV) in
O
2
%saturated and Ar%saturated 0.1 M KOH (see
Supplementary Information
for experimental
details). The Mo
2
C%GNR catalyst exhibits a pronounced active ORR act
ivity, displaying a much
more positive onset potential (~ 0.93 V
vs
RHE) and peak potential (~ 0.83 V
vs
RHE), and a
higher peak current density (2.01 mA cm
%2
) than those of Mo
2
C (~ 0.84 V, 0.73 V and 0.085 mA
cm
%2
, respectively) (Figure 3a, Table S2). VA%GNR alone
without any Mo
2
C shows some ORR
catalytic activity, but the onset potential (0.82 V
) and peak potential (0.68 V) are more negative
than the hybrid materials. Note that the onset pote
ntial for Pt/C catalyst (20 wt% Pt on Vulcan
XC%72) is located at 0.96 V
vs
RHE, only ~ 30 mV more positive than that of the M
o
2
C%GNR
hybrid (Figure S14, Table S2). This suggests that M
o
2
C in the Mo
2
C%GNR hybrid enhances the
ORR catalytic activity to approach that of Pt/C in
0.1 M KOH. The rotating%disk electrode (RDE)
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measurements were employed to reveal the ORR kineti
cs of the Mo
2
C%GNR hybrid in 0.1 M
KOH (Figure 3b). The Koutecky%Levich plots are line
ar; the fitting lines are almost parallel,
which suggests first%order reaction kinetics toward
the concentration of dissolved oxygen and
similar electron transfer number (
n
) for ORR at different potentials (Figure 3b inset)
. The
n
calculated from the the slopes of the Koutecky%Levi
ch plots is 3.90 at 0.65%0.80 V, suggesting
that the Mo
2
C%GNR hybrid favors a 4 e
%
oxygen reduction process, similar to a high%qualit
y
commercial Pt/C ORR catalyst measured in the same 0
.1 M KOH electrolyte (
n
= 4.0 for Pt/C,
see Supporting Information, Figure S14). Consistent
results were obtained in rotating ring disk
electrode (RRDE) voltammetric measurements. Figure
S15a shows that the ring current (
I
r
) was
negligible compared to the disk current (
I
d
); the H
2
O
2
yield (Figure S15b) was below
∼
5% over
the potential range From 0.2 to 0.7 V and gave an
n
of
∼
3.93, suggesting a one%step,
four%electron oxygen reduction pathway. 20% Pt/C, a
s expected, shows an
n
value of
∼
4 from
low (onset,
n
= 3.98) to high overpotential (steady%state,
n
= 3.98) as it generally proceeds by an
efficient four%electron ORR mechanism (Figure S15c,
d). On the other hand, in acidic media (0.5
M H
2
SO
4
), the Mo
2
C%GNR possesses an onset potential (0.77 V
vs
RHE) and half%wave potential
(
E
1/2
) (0.60 V
vs
RHE) similar to that of Pt/C (0.82 V
vs
RHE and 0.66 V
vs
RHE, respectively,
Figure S16).
Figure 3c reveals the linear sweep voltammograms (L
SV) of the Mo
2
C, VA%GNR,
Mo
2
C%GNR and Pt/C in O
2
%saturated 0.1 M KOH solution, Mo
2
C and VA%GNR exhibit poor
electrocatalytic activity toward ORR in alkaline so
lution, while Mo
2
C%GNR represents high
electrocatalytic activity with a more positive onse
t potential.
The
E
1/2
of Mo
2
C%GNR at 1600 rpm
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is 0.80 V, similar to that of Pt/C at 0.83 V as see
n in Figure S14, and more positive than that of
Mo
2
C at 0.74 V shown in Figure 3c.
The excellent ORR activity of the Mo
2
C%GNR hybrid
catalyst is also indicated from the much smaller Ta
fel slope of 43 mV dec
%1
at low overpotential
(Figure 3d) than that measured with Mo
2
C (115 mV dec
%1
) in 0.1 M KOH, further confirming
that the Mo
2
C%GNR hybrid is an efficient electrochemical cataly
st for ORR.
In addition, the ORR performances of Mo
2
C based hybrids can be tailored
via
varying the
atomic H treated time. The Mo
2
C%GNR hybrid prepared with atomic H treatment for 3
h presents
a lower ORR activity, as evidenced in Figure S17a,
where the
E
1/2
,
onset potential and
n
are 0.73
V, 0.86 V and 3.27 at 1600 rpm, respectively. The
n
of Mo
2
C%GNR grown for 9 h and derived
from the Koutecky%Levich plot, drops to 2.64, indic
ating a more favorable 2e reduction of O
2
toward HO
2
%
(Figure S17b). The enhanced catalytic performance
with atomic H treatment
process for 6 h is thought to be due to the increas
ed crystalline structure and the improved
contact between Mo
2
C NCs and graphene, producing a strong synergistic
interaction. Due to the
high thermal and chemical stability of Mo
2
C%GNR hybrid obtained at high temperature, its ORR
durability is high. As shown in Figure 3e, further
extension of the stability test to 1000 cycles in
0.1 M KOH shows a small additional shift of the
E
1/2
by 15 mV for Mo
2
C%GNR. Moreover, the
Mo
2
C%GNR exhibits no attenuation after 20000 s in 0.1
M KOH solution, at which time 100.01 %
of the relative current persists; whereas Pt/C lose
s nearly 12 % of its initial activity (Figure 3f),
further confirming that the Mo
2
C%GNR hybrid has better stability than Pt/C.
In contrast to the traditional metal carbide and ot
her nanostructure electrocatalysts, which
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are active under either acidic or basic conditions,
39
the Mo
2
C%GNR catalyst prepared by
HF%CVD demonstrates favorable HER activity over a w
ide range of pH values. The
electrocatalytic activities of the Mo
2
C%GNR toward HER were determined in both acidic (0.
5 M
H
2
SO
4
) and alkaline (0.1 M KOH) aqueous solutions with V
A%GNR and commercial Pt for
comparison (Figure 4a%c
)
. The onset overpotentials of Pt are 5 and 6 mV wit
h Tafel slopes of 30
and 29 mV dec
%1
in acidic and alkaline solution, respectively. For
Mo
2
C%GNR, the onset
potentials are 39 and 53 mV, while the Tafel slopes
are
65 and 54 mV dec
%1
in acidic and alkaline
solution, respectively. The HER process occurs thro
ugh a Volmer%Heyrovsky mechanism, in
which a fast discharge of a proton is followed by r
ate%limiting electrochemical recombination
with an additional proton.
40
To achieve a 10 mA cm
%2
HER current density, the Mo
2
C%GNR
requires overpotentials (
η
10
) of ~152 and ~121 mV in acidic and alkaline solution
, respectively,
which are much smaller values than those of Mo
2
C, ~275 and ~266 mV, respectively (Table S3).
The HER efficiency shows a maxima when the Mo
2
C%GNR hybrid treated for 6 h, which is
confirmed by the cathodic polarization curves and c
orresponding Tafel plots
(Supporting
Information, Figure S18). The Mo
2
C%GNR hybrid electrode presents onset overpotential
which is
significant lower than those reported for nanoscale
Mo
2
C (Supporting Information, Table S3). At
an overpotential of 300 mV, the maximum kinetic cur
rent density normalized to the geometric
area can reach up to 106 mA cm
−2
, which is higher than that seen in Mo
2
C grown without GNR
support. The corresponding Tafel slopes of the Mo
2
C%GNR hybrids with various atomic H
treated time are in the range of 65 to 93 mV dec
−1
(Figure S18). Compared with CNT%GR
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supported Mo
2
C/CNT%GR composites,
5
Mo
2
C%NCNTs
41
and other Mo%based catalysts recently
reported (Supporting Information, Table S4), the Mo
2
C%GNR hybrids deliver higher kinetic
current densities and more positive onset overpoten
tials. The improved HER performances of
Mo
2
C%GNR are mainly ascribed to the high crystallinity
and purity of Mo
2
C NCs that facilitate
the exposure of more active edge sites and provide
more pathways for ion and mass transport.
42
Hence, Mo
2
C%GNR with a treatment time of 6 h displays the hig
hest HER activity with the most
positive onset overpotential and smallest Tafel slo
pe (Figure 4a%d) among the nanoscale class of
Mo%based materials.
VA%GNR without anchored Mo
2
C has negligible electrocatalytic activity, which i
ndicates the
the robust structure formed by Mo
2
C and GNR reduces the energy input needed to activa
te HER.
The exchange current densities (log (Current Densit
y) at 0 V
vs
RHE) of Mo
2
C%GNR and Mo
2
C
are 0.31 and 0.027 mA cm
%2
, respectively, which are expected to be proportion
al to the
catalytically active surface area. An alternative a
pproach to estimate the effective surface area is
to measure the capacitance of the double layer at t
he solid%liquid interface with CV. The
capacitances for Mo
2
C%GNR hybrid electrodes treated with atomic H under
various time for 3, 6
and 9 h are 12.05, 22.34 and 6.14 mF cm
−2
, respectively (Supporting Information, Figure S19)
.
As seen in Figure 4e and
S20, the capacitance of the Mo
2
C%GNR and Mo
2
C electrodes are 23.34
and 0.42 mF cm
%2
, respectively. Accordingly, the roughness factor o
f Mo
2
C%GNR and Mo
2
C are
1060 and 19, respectively (Figure 4f
)
. The electrochemical surface area serves as an app
roximate
guide for surface roughness within an order%of%magn
itude accuracy.
43
Therefore, the large
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exchange current density of the Mo
2
C%GNR electrode is associated with its high surface
area.
Moreover, NCs synthesis is essential for improving
the HER performance by increasing the
number of catalytically active sites.
44
It is known that Mo
2
C nanoparticles exhibit good
electrocatalytic activity toward HER.
45
Thus, the size effect of the GNR%supported Mo
2
C
contributed significantly to its electrochemical pe
rformance. Figure S21 shows the Nyquist plots
of Mo
2
C%GNR, Mo
2
C and VA%GNR. It is seen that although VA%GNR shows
a slightly larger
semicircle, the Faradaic impedance yielded by Mo
2
C%GNR is also very small, indicating that
Mo
2
C%GNR have good electron transfer ability. In addit
ion, integration of catalysts with carbon
will enhance the electroconductivity. These verify
that VA%GNR plays a significant role in
improving the activity of electrocatalysts. The Mo
2
C%GNR robust conjugation helps Mo
2
C NCs
coalesce strongly with the GNR (Figure 2e,f), provi
ding a lower resistance path suitable for fast
electron transfer and improve electrocatalytic acti
vity for HER.
To assess the durability of the catalyst in acidic
and alkaline environments, the practical
operation of the catalyst was examined by electroly
sis at fixed potentials over extended periods.
As illustrated in Figure 5a, at overpotentials of %
186 and %240 mV, the catalyst current densities
remain stable at
∼
20 and 50 mA cm
%2
for electrolysis over 30000 s. At a higher overpot
ential to
drive high current density of
∼
100 mA cm
%2
, a small loss in cathodic current density of
<
5% is
observed in the Mo
2
C%GNR electrodes during the first 1 h. The catalyti
c current then keeps
increasing with negligible degradation after 30000
s, revealing its excellent stability under HER
conditions. While in alkaline media, the catalytic
current first increases slightly, and then fully
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stabilizes for the rest of the potentiostatic elect
rolysis (Figure 5b). The as%measured
time%dependent curve shows a typical serrate shape,
which can be attributed to the alternate
processes of bubble accumulation and bubble release
(insets of Figure 5a,b). This exceptional
durability demonstrates promise for practical appli
cations of the catalysts over longer periods.
Furthermore, long%term potential cycling was perfor
med by taking continuous CVs at an
accelerated scanning rate of 50 mV s
%1
for 1000 cycles in both acidic and alkaline media.
The
Mo
2
C%GNR catalyst after 1000 cycles overlays almost ex
actly with the initial one with negligible
loss of cathodic current (Figure 5c,d). This confir
ms that the Mo
2
C%GNR catalyst is highly
resistant to accelerated degradation in both acidic
and alkaline media. The XPS spectra of
Mo
2
C%GNR with a 30000 s HER process show negligible ch
ange of the oxidation state, which
confirms the excellent stability of the NCs under t
he long%term electrochemical cycling process
(Supporting Information, Figure S22).
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-0.3
-0.2
-0.1
0.0
-100
-80
-60
-40
-20
0
a
0.5 M H
2
SO
4
VA-GNR
Pt
Mo
2
C
-GNR
Mo
2
C
j / mA cm
-2
Potential / V vs RHE
-0.5
0.0
0.5
1.0
1.5
2.0
0.0
0.1
0.2
0.3
0.4
0.5 M H
2
SO
4
b
Overpotential / V
Log j / mA cm
-2
65 mV dec
-1
31 mV dec
-1
129 mV dec
-1
Mo
2
C-GNR
Mo
2
C
Pt
-0.3
-0.2
-0.1
0.0
-30
-20
-10
0
c
0.1 M KOH
VA-GNR
Pt
Mo
2
C
-GNR
Mo
2
C
j / mA cm
-2
Potential / V vs RHE
0.0
0.5
1.0
1.5
2.0
0.0
0.1
0.2
0.3
0.1 M KOH
d
Overpotential / V
Log j / mA cm
-2
54 mV dec
-1
29 mV dec
-1
147 mV dec
-1
Pt
Mo
2
C-GNR
Mo
2
C
-0.04
-0.02
0.00
0.02
0.04
-2
0
2
e
j
G
(
mA cm
-
2
)
E / V vs RHE
40 mV/s
80 mV/s
120 mV/s
0
50
100
150
200
0
2
4
f
Mo
2
C-GNR
Mo
2
C
j
G
(
mA cm
-
2
)
Scan Rate / mV s
-1
C
dl
= 23.34 mF cm
-2
C
dl
= 0.42 mF cm
-2
Figure 4.
HER polarization curves and the corresponding Tafe
l plots of VA%GNR, Pt,
Mo
2
C%GNR and Mo
2
C in (a,b) 0.5 M H
2
SO
4
at pH 0 and (c,d) 0.1 M KOH at pH 13. (e) CV
curves of Mo
2
C%GNR and (f) corresponding differences of Mo
2
C and Mo
2
C%GNR in current
density plotted against scan rate. The current dens
ity is measured at 0.40 and 0 V
vs
RHE for
Mo
2
C and Mo
2
C%GNR, respectively.
Page 23 of 39
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