Influence of Carbon on Molybdenum Carbide Catalysts for the Hydrogen Evolution Reaction

Article abstract of DOI:10.1002/cctc.201600107

The influence of carbon on molybdenum carbide catalysts for the hydrogen evolution reaction (HER) is discussed. The carbon content is adjusted by varying the molar ratio of molybdenum and glucose sources and the holding time in the carbonization process. The carbon plays a crucial role in the determination of phase formation, surface area, and electrical resistance, which are associated with the final HER activity. There is a contradiction between the reduced active sites and improved electrical resistance that results from the reduced content of carbon, and a balance can be achieved with a holding time of 9 h to provide the best HER activity with a low Tafel slope of 55 mV dec^?1 and a high exchange current density of 0.047 mA cm^?2. Importantly, the transformation of the order of the free carbon improves the electrical conductivity remarkably to result in a great improvement in the final HER activity.

Full text of DOI:10.1002/cctc.201600107

DOI: 10.1002/cctc.201600107
Full Papers
Influence of Carbon on Molybdenum Carbide Catalysts for
the Hydrogen Evolution Reaction
Chaoyun Tang,[a] Zhuangzhi Wu,*[a, b] and Dezhi Wang*[a, b]
The influence of carbon on molybdenum carbide catalysts for
the hydrogen evolution reaction (HER) is discussed. The carbon
content is adjusted by varying the molar ratio of molybdenum
and glucose sources and the holding time in the carbonization
process. The carbon plays a crucial role in the determination of
phase formation, surface area, and electrical resistance, which
are associated with the final HER activity. There is a contradic-
tion between the reduced active sites and improved electrical
resistance that results from the reduced content of carbon,
and a balance can be achieved with a holding time of 9 h to
provide the best HER activity with a low Tafel slope of
55 mVdecÀ1 and
a high exchange current density of
0.047 mAcmÀ2. Importantly, the transformation of the order of
the free carbon improves the electrical conductivity remarkably
to result in a great improvement in the final HER activity.
Introduction
Hydrogen is considered to be an ideal fuel because it is clean,
energetic, and can be produced from renewable energy
sources.[1] Sustainable hydrogen production from water split-
ting is one of the most attractive methods.[2,3] The efficiency of
water electrolysis depends critically on cathode materials, es-
pecially in electrolyzers based on proton exchange membranes
(PEM).[4,5] Pt-based catalysts are considered to be the most ef-
fective electrocatalysts for the hydrogen evolution reaction
(HER), but the high cost and low abundance of Pt limits their
scalable industrial applications.[6–8] Various candidates have
been explored to replace Pt-based catalysts, which include
MoS2,[9–11] WS2,[6] MoB,[12] MoP,[13] and Mo2C.[14–18]
state reaction to synthesize graphitic carbon sheets decorated
with Mo2C nanoparticles, which exhibited a high HER activity
with a Tafel slope of 62.6 mVdecÀ1 [16]
Pan et al. prepared Mo2C
.
nanoparticles stabilized by a carbon layer on reduced gra-
phene oxide (RGO) sheets, which showed a Tafel slope of
57.3 mVdecÀ1 [17]
.
Clearly, recent efforts have mainly focused on increasing sur-
face areas and enhancing the electron transfer ability by
adopting highly conductive supports. However, there are few
reports on the effect of carbon, which plays a crucial role in
the determination of the final composites as well as the HER
activity over MoC catalysts.[20–24] On one hand, the carbon con-
tent determines the formation of various molybdenum car-
bides with different HER activities.[25,26] On the other hand,
a suitable content of carbon residue is helpful for the increase
of the surface area and the enhancement of conductivity.
Therefore, it is a critical task to clarify the influence of carbon
during the formation of molybdenum carbides.
Among them, molybdenum carbide (Mo2C) has been pro-
posed as a highly effective electrocatalyst for HER under both
acidic and basic conditions,[10,19] and great efforts have been
made to further improve the final HER activity. Xiao et al. pro-
posed molybdenum carbide nanorods with porous structures,
which exhibited excellent HER activity with a low Tafel slope of
58 mVdecÀ1 (IR-drop corrected).[14] Chen et al. presented bio-
mass-derived composites made from earth-abundant molybde-
num and humble soybeans, which exhibited a high HER activi-
ty with a Tafel slope of 66.4 mVdecÀ1[15] and adopted carbon
nanotubes as supports to reduce the Tafel slope to
Herein, we investigated the influence of carbon on the final
HER activity over Mo2C catalysts, by varying different synthesis
factors, which included the amount of carbon precursor added
and the carbonization duration. The optimized Mo2C nanopar-
ticles eventually exhibit excellent HER activity with a high cur-
rent density of 242 mAcmÀ2 at À400 mV and a high exchange
current density of 0.047 mAcmÀ2 with a low Tafel slope of
55 mVdecÀ1. Furthermore, this work proves the importance of
carbon for the phase formation and the resulting HER activity
over Mo2C catalysts, which provides an effective and feasible
path to further optimize the HER ability of transition metal car-
bides.
55.2 mVdecÀ1 [18]
Cui et al. used a biopolymer-derived solid-
.
[a] Dr. C. Tang, Prof. Z. Wu, Prof. D. Wang
School of Materials Science and Engineering
Central South University
Changsha 410083 (P.R. China)
[b] Prof. Z. Wu, Prof. D. Wang
Key Laboratory of Ministry of Education for Non-ferrous Materials Science
and Engineering
Central South University
Changsha 410083 (P.R. China)
E-mail: zwu2012@csu.edu.cn
Results and Discussion
The ratio of the molybdenum and carbon sources can deter-
mine the constitution of the final product. Therefore, molybde-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201600107.
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num carbide samples synthesized with different ammonium
heptamolybdate (AM)/glucose (GLC) molar ratios were investi-
gated at first. The XRD patterns of molybdenum carbide syn-
thesized at 8008C for 3 h with different AM and GLC molar
ratios are shown in Figure 1a. With an AM and GLC molar ratio
of 1:10 (Mo2C-1:10), only the mono-carbon phase of h-MoC
(JCPDS Card No. 65-3558) was detected with three typical dif-
fraction peaks of (002), (103), and (104), which indicates that
the carbon supply was insufficient during the carbonization
process. If the content of the carbon source is increased to
a molar ratio of 1:20 (Mo2C-1:20), b-Mo2C (JCPDS Card No.35-
0787) with three typical peaks of (100), (002), and (101) can
be obtained. However, with a further increase to 1:40 (Mo2C-
1:40), although b-Mo2C can also be obtained, a small peak at
2q=268 becomes more intense compared with that of Mo2C-
1:20 (Figure 1b), which indicates the increased content of
amorphous carbon residue. As a comparison, pure Mo2C with-
out amorphous carbon (Mo2C-TPRe) was also synthesized
through the traditional temperature-programmed reduction
route using a CH4/H2 mixture as the carbon source. This mate-
rial shows the same XRD pattern of b-Mo2C without the peak
of amorphous carbon at 2q=268. It can be concluded that the
amount of carbon residue is increased with the increased
amount of the carbon precursor, and a lower amount of
carbon precursor (1:10) will result in the formation of the
mono-carbon phase of h-MoC, which exhibits a worse HER ac-
tivity than the b-Mo2C phase.[25–27]
The holding time before the H2 flow is introduced is a crucial
procedure in which the carbon source transforms into amor-
phous carbon and molybdenum carbides. Therefore, molybde-
num carbides synthesized at 8008C with a molar ratio of 1:20
for different holding times were also investigated, and their
XRD patterns are shown in Figure 1c. All the samples show
typical XRD patterns of b-Mo2C, and the intensity of the peaks
at 2q=268 increases gradually as the holding time is pro-
longed from 0 to 9 h, which indicates a continuous increase of
amorphous carbon. Clearly, the carbon precursor requires a rel-
atively long holding time to be transformed into amorphous
carbon. However, if the holding time reaches 12 h, the XRD
pattern becomes very sharp but the peak at 2q=268 is not
visible at all, which suggests that the crystallinity of Mo2C is re-
markably improved and the content of amorphous carbon resi-
due is greatly reduced (Figure 1c and d).
The quantified carbon content of Mo2C was identified by
using a carbon-sulfur analyzer. The carbon content of Mo2C-
1:10 is only 4.65 wt%, less than the theoretical value of pure
Mo2C (5.89 wt%), which indicates a mono-carbon phase, co-
incident with the XRD analysis. The carbon content of Mo2C-
1:40 is ꢀ21.53 wt%, much higher than the theoretical value of
pure Mo2C, which indicates the abundant carbon residue.
However, Mo2C-1:20 shows a regular carbon content of
7.89 wt%, which suggests that there is ꢀ2 wt% free carbon.
Moreover, the free-carbon contents of Mo2C synthesized at
8008C with an AM/GLC molar ratio of 1:20 for different hold-
ing times are listed in Table 1. Mo2C-0 h shows a free-carbon
Figure 1. a) XRD patterns of molybdenum carbides synthesized by TPRe and synthesized with various AM/GLC molar ratios; c) XRD patterns of molybdenum
carbides synthesized with an AM/GLC molar ratio of 1:20 for various holding times; b, d) enlarged specific regions of a and c, respectively.
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Table 1. Summary of information for various catalysts.
Samples
Excessive carbon content
[wt%]
ID/IG
Specific surfaces area
Tafel slope
j0
Rct (h=À150 mV)
[W]
[mgmÀ2
]
[mAcmÀ2
]
Mo2C-0 h
Mo2C-3 h
Mo2C-6 h
Mo2C-9 h
Mo2C-12 h
3.23
2
1.97
1.93
1.49
1.14
0.97
0.92
0.89
0.88
81
120
113
101
8
57
55
55
55
61
0.011
0.033
0.039
0.047
0.009
132.7
47.2
21.4
16.1
1260
content of 3.23 wt%. There is a sharp decrease of the free-
carbon content after a holding time of 3 h, which may be
caused by the reaction with oxygen in the organic carbon
source. With an increase of the holding time, there is only
a slight decrease of the free-carbon content caused by the oxi-
dation reaction until 9 h. However, a further increase up to
12 h leads to a notable reduction (0.44 wt%), which could be
attributed to the aggregation of Mo2C. Generally, the existence
of amorphous carbon is helpful for the formation of mesopo-
rous structures with high surface areas.[28] If we consider the
XRD pattern and carbon content of Mo2C-12 h, it can be
speculated that the mesoporous structure of Mo2C is de-
stroyed to form a dense phase with better crystallinity, as re-
vealed by the XRD patterns (Figure 1c and d) and the sharply
reduced BET surface area (Table 1), and partially adsorbed free
carbon is released during this process, which leads to a notable
decrease of the free-carbon content.
Raman spectra of Mo2C samples synthesized at 8008C with
an AM/GLC molar ratio of 1:20 for different holding times are
shown in Figure 2. The characteristic peaks of Mo2C are found
at n˜ =820 and 995 cmÀ1.[14,29] The D and G bands of amor-
phous carbon at n˜ =1340 and 1604 cmÀ1 are observed clearly
for all the samples.[30] This demonstrates the existence of free-
carbon residue, which is amorphous and not visible in the XRD
patterns (Figure 1d). Generally, the relative ratio of the D and
G band intensities (ID/IG) is correlated to the degree of disorder
in the graphite, and the calculated ID/IG values are shown in
Table 1. The ID/IG values decrease gradually with the increase of
the holding time from 0 to 9 h, which means that the degree
of order in amorphous carbon increases continuously with the
increase of the holding time. The degree of order in carbon
materials can directly decide their electronic conductivity and
further influence their final HER activity. If the holding time is
12 h, the peaks of Mo2C are remarkably more intense than the
peaks of amorphous carbon, consistent with the XRD analysis,
which indicates a good crystallinity and low amount of free-
carbon residue.
Figure 2. Raman spectra of Mo2C synthesized at 8008C with an AM/GLC
molar ratio of 1:20 for different holding times.
served and measured to be 0.23 and 0.24 nm, which corre-
spond to the (101) and (002) crystal planes of b-Mo2C, respec-
tively. Meanwhile, the average particle size observed by high-
resolution transmission electron microscopy (HRTEM) is
ꢀ25 nm.
To obtain the BET surface areas, N2 adsorption–desorption
measurements were performed. The specific surface area
values of Mo2C synthesized at 8008C with an AM/GLC molar
ratio of 1:20 for different holding times are shown in Table 1.
The specific surface area of Mo2C-0 h is lower than that of
Mo2C-3 h, which implies that amorphous carbon requires more
time at high temperatures to form the porous structure in the
pyrolysis process. However, with an increase of the holding
time beyond 3 h, the specific surface area decreases gradually,
which can be attributed to the sustained damage of porous
structures because of the loss of amorphous carbon. Notably,
the specific surface area of Mo2C-12 h is decreased sharply,
which could be attributed to the collapse of the porous struc-
Typical SEM and TEM images of Mo2C-9 h and Mo2C-12 h are
shown in Figure 3. Mo2C-9 h has porous structures constructed
by aggregated nanoparticles with an average particle size of
ꢀ100 nm. However, the average particle size of Mo2C-12 h is
ꢀ300 nm, much bigger than that of Mo2C-9 h, which indicates
distinct crystal growth and agglomeration, consistent with the
XRD and Raman spectroscopy. To gain a deeper insight into
the microstructures, a high-magnification TEM image (Fig-
ure 3d) was also obtained. Clear interplanar distances were ob-
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characteristic doublet of the Mo2+ state of b-Mo2C. Characteris-
tic doublets of MoO3 and MoO2 can also be observed because
b-Mo2C is prone to oxidation on the surface. No metallic Mo is
detected. The C1s peaks at BE=283.8, 284.6, and 287.7 eV can
be assigned to the C atoms of Mo2C, amorphous carbon, and
carbon atoms bonded to oxygen atoms, respectively.[16,17]
Moreover, a comparison of the C1s peaks of molybdenum car-
bides obtained for various holding times is shown in Figure S1.
After a prolonged holding time, the ratio of the abundance
amorphous carbon to the carbon atoms of Mo2C decreased
gradually, coincident with the previous analysis.
The HER properties of various catalysts (Mo2C-1:10, Mo2C-
1:20, and Mo2C-1:40) were investigated in 0.5m H2SO4 solution.
Mo2C-TPRe was tested for comparison. The polarization curves
obtained at a scan rate of 2 mVsÀ1 are shown in Figure 5a.
Clearly, the Mo2C-1:20 catalyst shows the best HER activity
with the lowest onset potential (À80 mV) and the highest spe-
cific current density. Mo2C-1:10 shows a relatively higher onset
potential and a relatively lower current density at the same po-
tential, which can be ascribed to the insufficient carbide phase
of MoC. Without free amorphous carbon, Mo2C-TPRe shows
the worst HER activity with the highest onset potential and
the lowest specific current density. Therefore, we can conclude
Figure 3. SEM images of a) Mo2C-9 h and b) Mo2C-12 h; c, d) TEM images of
Mo2C-9 h.
tures associated with a high loss of free carbon, as revealed by
XRD and Raman spectroscopy.
X-ray photoelectron spectroscopy (XPS) spectra of the Mo2C-
9 h catalyst are shown in Figure 4.The selection of the position
of each peak for fitting is based on Ref. [31]. The Mo3d peaks
located at binding energies (BEs) of 228.1 and 231.3 eV exhibit
a spin energy separation of 3.2 eV, which demonstrates the
Figure 5. a) Polarization curves and b) Tafel plots of various catalysts with
Figure 4. a) Mo 3d and b) C1s XPS spectra of Mo2C-9 h.
different AM/GLC molar ratios.
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that the free amorphous carbon plays a crucial positive role in
the enhancement of the final HER activity. However, too much
free carbon will cover the exposed active sites and deteriorate
the HER activity, as revealed by the low specific current density
of Mo2C-1:40 (Figure 5a).
of Mo2C-6 h (À162 mV) and Mo2C-3 h (À165 mV), to demon-
strate a superior HER activity.
As a rule, the Tafel slope is an inherent property of the cata-
lyst determined by the rate-limiting step of the HER.[32] A lower
Tafel slope means a faster increase of the HER rate with in-
creased potential. To assess this, the Tafel slopes are shown in
Figures 5b and 6b and are fitted to the Tafel equation (h=
b log j+a, in which j is the current density, h is the overpoten-
tial, a is the intercept, and b is the Tafel slope). The Mo2C-TPRe
catalyst shows the worst activity with the highest Tafel slope
of 81 mVdecÀ1, associated with the lowest specific current
To further explore the influence of carbonization duration on
Mo2C catalysts, the HER properties of various catalysts ob-
tained after different holding times (Mo2C-0 h, Mo2C-3 h, Mo2C-
6 h, Mo2C-9 h, and Mo2C-12 h) were investigated in 0.5m
H2SO4, and the polarization curves are shown in Figure 6a. We
density in Figure 5a. Mo2C-1:20 shows
a Tafel slope of
55 mVdecÀ1, a little lower than that of Mo2C-1:10 (59 mVdecÀ1
)
and Mo2C-1:40 (56 mVdecÀ1). It seems that the content of free
carbon only has a slight effect on the Tafel slopes of the cata-
lysts. Meanwhile, from a comparison of Mo2C synthesized at
different holding times, we can see that all the samples show
Tafel slopes of 55–61 mVdecÀ1, and there are no significant
changes. If we consider the effects of the carbon precursor
ratio and holding time, we can conclude that the existence of
free amorphous carbon affects the specific current density re-
markably but only has a slight effect on the Tafel slopes, which
could be related to the BET surface areas and electrical resist-
ance and will be discussed later.
Generally, three possible reaction steps have been proposed
for the HER in acidic solutions:[33] the discharge reaction
(Volmer) [Eq. (1)]:
H3OþþeÀ ! HadþH2O
ð1Þ
the electrochemical desorption reaction (Heyrovsky) [Eq. (2)]:
HadþHþþeÀ ! H2
ð2Þ
and the combination reaction (Tafel) [Eq. (3)]:
HadþHad ! H2
ð3Þ
H2 can be produced if Reaction (1) is combined with Reac-
tion (2) or (3). If the first step is rate-limiting, it results in a Tafel
slope of 120 mVdecÀ1. If reaction (2) or (3) is the rate-limiting
step, it will lead to Tafel slopes of 40 or 30 mVdecÀ1, respec-
tively. Clearly, the Tafel slopes of 55–61 mVdecÀ1 obtained for
the catalysts in this work suggest that the HER takes place by
a rapid Volmer reaction followed by a rate-limiting Heyrovsky
step, which demonstrates the Volmer–Heyrovsky mechanism.
We used extrapolation methods from Tafel plots to determine
the exchange current density (j0; Table 1). Mo2C-9 h exhibits
the highest value of 0.047 mAcmÀ2, which is much higher than
that of molybdenum carbide nanorods.[14] At the same time, as
it has the lowest Tafel slope of 55 mVdecÀ1, the highest specif-
ic current density can be expected over Mo2C-9 h (Figure 6a).
Electrochemical impedance spectroscopy (EIS), which is well-
established technique to study the transport kinetics of elec-
trodes, was performed. Nyquist plots of b-Mo2C synthesized at
8008C with an AM/GLC molar ratio of 1:20 for different hold-
ing times at À150 mV are shown in Figure 7. All samples exhib-
Figure 6. a) Polarization curves and b) Tafel plots of various catalysts ob-
tained with different holding times.
can see that the current density is increased gradually with
a prolonged holding time from 0 to 9 h. At an overpotential of
À400 mV, Mo2C-9 h shows
a specific current density of
242 mAcmÀ2, which is higher than that of Mo2C-6 h
(223 mAcmÀ2) and Mo2C-3 h (198 mAcmÀ2). Mo2C-0 h shows
a much lower current density of 54 mAcmÀ2 because of the in-
sufficient carbonization. Mo2C-12 h shows the worst HER activi-
ty with the lowest current density of 32 mAcmÀ2, which can
be ascribed to the collapse and agglomeration of the porous
structures, as revealed by the sharply reduced BET surface area.
Meanwhile, to obtain a current density of 10 mAcmÀ2, Mo2C-
9 h only requires an overpotential of À157 mV, lower than that
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um heptamolybdate/glucose ratio of 1:20. Secondly, in the cal-
cination process, the pyrolysis of glucose produces amorphous
free carbon with a large surface area and accelerates the car-
bonization of the Mo source. With a prolonged holding time,
the content of amorphous free carbon decreases gradually,
which is associated with the reduced surface areas, relative
ratio of the D and G band intensities in the Raman spectra,
and electrical resistance, and presents less active sites and
a better electrical conductivity. Clearly, there is a contradiction
between these factors, and a balance can be achieved with
a holding time of 9 h, which provides the best HER activity
with a low Tafel slope of 55 mVdecÀ1 and a high exchange cur-
rent density of 0.047 mAcmÀ2.
Figure 7. Nyquist plots of various catalysts.
Experimental Section
it the classical two-time-constant behavior. The charge-transfer
resistance Rct, determined from the semicircle registered at low
frequencies, is shown in Table 1. The charge-transfer impe-
dance of Mo2C catalysts is reduced gradually with the pro-
longed holding time from 0 to 9 h, which suggests that the
Mo2C catalysts afford faster electron transport from the Mo2C
active sites to protons with a prolonged holding time. The
charge-transfer impedance of Mo2C-9 h is only 16.1 W, which
means a high rate of the electron transfer reaction over Mo2C-
9 h, which provides credible evidence for its superior HER ac-
tivity. If we take the discussions of various carbon source ratios
and holding times into account, it can be concluded that an
appropriate content of amorphous free carbon can not only
retain the porous structures with a large BET surface area to
expose more active sites but also reduce the electrical resist-
ance to accelerate the electron transfer, which ensures a great
improvement of the final HER activity. However, excessive free
carbon may block the active sites and deteriorate the HER ac-
tivity. The control of the holding time is an effective way to
adjust the content of free carbon, which is related closely to
the final HER activity of Mo2C catalysts derived from organic re-
sources.
Preparation
AM ((NH4)6Mo7O4·4H2O) as the molybdenum source and GLC as
the carbon source with molar ratios of 1:10, 1:20, and 1:40 were
dissolved in distilled water (50 mL) to form a homogeneous solu-
tion. After stirring for 10 min, the mixture solution was transferred
into a 100 mL Teflon-lined stainless autoclave, heated to 2008C,
and maintained for 10 h. After centrifugation and drying, a black
precursor was collected. This precursor was heated in a tube fur-
nace from RT to 8008C at a rate of 58C minÀ1 and kept for different
durations in an atmosphere of Ar. A successive H2 flow was provid-
ed for another 20 min. Finally, black molybdenum carbide was ob-
tained. Pure Mo2C without a carbon residue was synthesized by
traditional temperature-programmed reduction (TPRe) for compari-
son: the same amount of ammonium heptamolybdate instead of
the precursor was heated from RT in air to 5008C at a rate of
108CminÀ1 and kept for 1 h. Next, the sample was heated to
8008C at a rate of 58CminÀ1 in an atmosphere of Ar and then
switched to an atmosphere of 20 vol% CH4/H2 for 1 h to finish the
carbonization.[27]
Characterization
XRD patterns were recorded by using a D/max-2500 system with
a CuKa irradiation source (l=0.154 nm). Raman spectroscopy was
recorded by using a LabRAMHR-800 instrument (HORIBA) with an
excitation wavelength of 633 nm. The morphology and microstruc-
ture were observed by using SEM (FEI Sirion 200) and TEM (Tecnai
G220). XPS (ESCALAB 250Xi) was used to analyze the surface com-
positions. Specific surface areas (BET) were determined by N2 ad-
sorption at 77 K with the BET method by using a volumetric unit
(Quadrasorb SI-3MP). Moreover, the carbon content of samples
was tested by using a carbon-sulfur analyzer (American LECO CS-
444).
Stability is another critical factor in the evaluation of a HER
catalyst. To test the stability, cyclic voltammetry (CV) measure-
ments of Mo2C-9 h were performed continuously in 0.5m
H2SO4. After 1000 cycles, the catalyst exhibits similar polariza-
tion curves with a slight decay of cathodic currents (Figure S2),
which indicates an excellent durability in acidic electrolyte,
consistent with the Mo2C-based catalysts.[14–18]
Conclusions
Electrochemical measurements
The influence of carbon on Mo2C catalysts derived from an or-
ganic source for the hydrogen evolution reaction (HER) has
been discussed by varying the ratio of the carbon source and
the holding time. The carbon plays a crucial role in the deter-
mination of phase formation, surface area, and electrical resist-
ance associated with the final HER activity. Firstly, enough of
the carbon source can ensure full carbonization, but excessive
carbon will cover the active sites and deteriorate the HER activ-
ity. As a result, a suitable content is obtained with an ammoni-
Typically, catalyst (3 mg) and 5 wt% Nafion solution (80 mL) were
dispersed in a solution of deionized water and ethanol (4:1 v/v,
1 mL). After ultrasonication for 30 min, the catalyst slurry (5 mL)
was dropped onto smooth glassy carbon electrodes with a diame-
ter of 3 mm and dried in air. The mass loading was
ꢀ0.213 mgcmÀ2
.
All the electrochemical measurements were performed by using an
electrochemical workstation (CHI 660E). For the determination of
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activity toward the HER, a three-electrode electrochemical cell (i.e.,
a half-cell) was employed, and linear sweep voltammetry (LSV)
measurements were conducted from 0.1 to À0.4 V at RT in 0.5m
H2SO4 at a scan rate of 2 mVsÀ1 using a saturated calomel elec-
trode (SCE) as the reference electrode and glassy carbon as the
counter electrode. Before the electrochemical measurements, the
electrolyte solution was purged with N2 for 1 h to remove the
oxygen completely. The onset potential was determined as the po-
tential required to reach a specific current density of 1 mAcmÀ2
.
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Keywords: carbides · carbon · electrochemistry · hydrogen ·
molybdenum
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Received: January 27, 2016
Revised: March 8, 2016
Published online on May 11, 2016
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