Highly Active Non-Precious-Metal Hydrogen-Evolution Electrocatalyst:
Ultrafine Molybdenum Carbide Nanoparticles Embedding into 3D
Nitrogen Implanted Carbon Matrix
Preparation of N doped carbon: The 500 mg of activated ZIF-8 powder was heated to 700 oC at
a heating rate of 5 oC/min and carbonized at 700 oC for 2 h under flowing Ar atmosphere, and
cooled to room temperature naturally to obtain porous carbon materials. In order to remove
unevaporated zinc component, the samples were treated in 0.5 M H2SO4 aqueous solution at 60 oC
followed by a repeatedly filtered and washed process in deionized water. The synthesis of ZIF-8
nanocrystals was based on a previous procedure with some modifications.[S1] Typically,
Zn(NO3)2·6H2O (1.68 g) was dissolved in 80 mL of methanol. A mixture of 2-methylimidazole
(3.70 g) with 80 mL methanol was added to the above solution with vigorous stirring for 24 h. The
product was separated by centrifugation and washed thoroughly with methanol for twice, and
finally dried overnight at 50 oC.
Electrochemically active surface area (EASA) evaluation: The electrochemically active surface
area was estimated from the electrochemical double-layer capacitance of the hybrids. The
electrochemical double-layer capacitance was determined from the CV curves measured in the
potential range of 0−0.1 V (vs RHE) without redox processes according to the following equation:
CdI = Ic/ν (1)
where Cdl, Ic, and ν are the double-layer capacitance (F cm−2 ) of the electroactive materials,
charging current (mA cm−2), and scan rate (mV s−1 ). In the present work, the capacitive currents
(taken at the potential of 0.05 V vs RHE) as a function of scan rate for MoCN-2D and MoCN-3D
have been investigated.
TOF Calculation.
The mass activity and turnover frequency (TOF) of the catalysts are calculated as following:
𝐦𝐚𝐬𝐬 𝐚𝐜𝐭𝐢𝐯𝐢𝐭𝐲 =
𝒋
𝒎
(𝟏)
𝐓𝐎𝐅 =
𝒋∙𝑺
𝟐𝑭 ∙ 𝒏
(𝟐)
Here, j is the measured current density (mA cm-2), m is the catalyst loading (mg cm-2), S is the surface
area of the GC electrode, the number 2 in the TOF calculation means 2 electrons required for one H 2
molecule evolution, F is Faraday’s constant (96485.3 C mol-1), and n is the moles of metal atom on the
electrode.
The calculation of free energies: For all the studied system, the free energies were evaluated by the
formula ΔG(H*) =ΔE(H*)+ΔZPE − TΔS, where ΔE(H*), ΔZPE and ΔS are the binding energy, zero point
energy change and entropy change of H* adsorption, respectively. In this work, the TΔS and ΔZPE are
obtained by the following the scheme proposed by Norskov et al.[S2] Specifically, ΔS can be got by the
equation ΔS = S(H*) − 1/2 S(H2) ≈ −1/2 S(H2), in view of the negligible vibrational entropy of the H*.
Thus we can easily conclude that the corresponding TΔS is -0.205 eV, since TS(H2) is known to be 0.41
eV for H2 at 300 K and 1 atm. Additionally, the equation ΔZPE = ZPE(H*) – 1/2 ZPE(H2) was employed
to estimate ΔZPE for H*. The ZPE(H2) was set to 0.27 eV according to previously report [S2].
Theoretical models
We have constructed the correlative theoretical models to simulate the hybrid Mo2C@NC
nanomaterial (Figure 4a), as well as the composited Mo2C@C, Mo2C, C and NC systems for comparative
purpose. Specifically, the (001) facet with Mo-termination is adopted to act as the active surface for the βMo2C nanoparticle, which is modeled by the slab with four layers of Mo-C atoms. It is worth mention that
this facet is flat and the most densely packed surface with unified active centers for β-Mo2C, which has
been extensively employed in theoretical studies on the adsorption and reaction of small species on the
surfaces. In addition, the ultrathin pure carbon layer is simulated by the single-layer graphene (C). We have
sampled the N-doped carbon layer with the N/C atomic ratio of 1:8.6 obtained in experimental result, where
the diversified possible distributions of N atoms were considered to obtain the more favorable configuration
in energy. Further, the composited Mo2C@NC and Mo2C@C models were constructed by covering the
respective NC and C layers on the (001) facet of the Mo2C slab. For a reasonable repeated period, 4 × 4
supercell for the subunit Mo2C is used in Mo2C@C and Mo2C@CN model.
Reference:
[S1] Tang, J., Salunkhe, R. R., Liu, J., Torad, N. L., Imura, M., Furukawa, S., Yamauchi, Y. Thermal
Conversion of Core–Shell Metal–Organic Frameworks: A New Method for Selectively Functionalized
Nanoporous Hybrid Carbon, J. Am. Chem. Soc. 2015 137, 1572–1580.
[S2] Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. J.
Electrochem. Soc. 2005, 152, J23−J26.
B
Figure S1. Structure comparison between ZIF-8 and HZIF, Zn(im)4 was partially substituted by MoO4 unit
in HZIF. Single crystal X-ray diffraction analysis reveals that the hybrid zeolite imidazole
framework (HZIF) crystallizes in the cubic Im3m space group.[S3] The HZIF is constructed from
two kinds of tetrahedral building blocks (MoO42-unit and Zn2+ node) and contain two kinds of
connectivity, and combine structural features of both zeolites and ZIFs. A prominent structural
feature of this sdt-type framework is to interconnect the truncated octahedral cages of [Zn24(2mim)36] by the inorganic MoO4 units (Figure S1d). The large [Zn24(2-mim)36] cage with effective
diameter of 12.5 Å and pore aperture of 3.3 Å in HZIF is the same as the subunit in ZIF-8, a well
known framework with zeolitic sodalite (SOD) topology, and thus the whole structure of HZIF
can also be considered as partly substitution of Zn(im)42- units in ZIF-8 by MoO42- nodes.
Reference:
[S3] Wang, F., Liu, Z-S., Yang, H., Tan, Y-X., Zhang, J. Angew. Chem. Int. Ed. 2011, 50, 450–
453.
Figure S2, Wide-angle XRD patterns of precursors HZIF.
Figure S3, N2 adsorption-desorption isotherms for HZIF.
Interpretation for Figure S2 and S3
The XRD patterns of HZIF was recorded at room temperature. The peak positions of simulated and
experimental patterns are in good agreement with each other, indicating the phase purity of the as-
synthesized samples (Figure. S2). The porosity for HZIF has also been investigated by N2 adsorption. As
shown in Figure S3, the samples displayed type I isotherms with steep N2 uptakes at low relative pressure,
which is typically associated with microporosity. The Brunauer−Emmett−Teller (BET) surface areas for
HZIF was confirmed as 197.4 m2/g, these results confirmed the successful construction of HZIF frame.
Figure S4, Energy dispersive X-ray (EDX) spectra of C, N, O, Mo and Zn of MoCN-3D
Interpretation for Figure S4:
In order to remove unevaporated zinc component, the MoCN-3D samples were treated in 0.5
M H2SO4 aqueous solution at 60 oC followed by a repeatedly filtered and washed process in
deionized water. As zinc based compounds were not stable in acid environment, the zinc
component can be removed almost completely (<0.06% (atom %)). TEM mapping
characterization confirmed this point. Besides, EDX spectrum characterization further verified this
fact.
Figure S5. SEM images of the newly constructed MoCN-3D, the SEM image clearly shows the porous
framework of MoCN-3D.
Figure S6. (a) TEM images of the newly constructed MoCN-3D, and (b) corresponding particle sizes
distribution of the molybdenum carbide nanoparticles.
Figure S7 Wide-angle XRD patterns of MoCN-3D.
Figure S8. (a) SEM image for the precursor of MoCN-2D. (b-d) TEM and HRTEM images of MoCN-2D
sample.
Figure S9 Wide-angle XRD patterns of MoCN-2D.
Figure S10. The pore-size distribution curve for MoCN-3D.
Figure S11. TGA (Thermal gravimetric analysis) curves of MoCN-3D and MoCN-2D catalysts measured
under air atmosphere
Interpretation for Figure S11.
Catalyst MoCN-3D and MoCN-2D undergoes the process of gradual oxidation of Mo2C to MoO3,
and the combustion of carbon. Assuming that the sample is composed of stoichiometric Mo2C and carbon,
and converts to only MoO3 after heating to 700 oC, the carbon content is estimated according to the
following equation:
Carbon (Mass) = catalyst (Mass) - Mo2C (Mass)
= 100% -
(Residual mass)∗M (𝛽−molybdenum carbide)
2 M (molybdenum trioxide)
For MoCN-3D, m (Mo2C) = 39.1*204/(2*144) = 27.7%. Thus, m (carbon) = 72.3%. The Mo content
in atomic ratio is calculated to be 4.2 at%. For MoCN-2D, m (Mo2C) = 43.4%*204/(2*144) = 30.7%. Thus,
m (carbon) = 69.3%. The Mo content in atomic ratio is calculated to be 4.83 at%.
The contents of Mo elements for MoCN-3D and MoCN-2D calculated from the TGA investigation are
consistent with the results derived from ICP measurement (Table S1).
Figure S12. The polarization curves of MoCN-3D after 2000 CV cycles in the stability test over in 0.5 M
H2SO4 media.
Figure S13 (a) Time dependence of cathodic current density over MoCN-3D during electrolysis at −0.2 V
in 1M KOH media, (b) The polarization curves of MoCN-3D after 2000 CV cycles in the stability test over
in 1M KOH media.
Figure S14. (a) N2 adsorption-desorption isotherms for MoCN-3D-800. (b) N2 adsorption-desorption
isotherms for MoCN-3D-900. (c) Polarization curves of MoCN-3D calcined at different temperature in 0.5
M H2SO4.
Interpretation for Figure S14
To optimize the catalytic performance of MoCN-3D catalyst, catalytic performance for sample calcined
at different temperature have been evaluated. The hybrid catalysts MoCN-3D-700, MoCN-3D-800, and
MoCN-3D-900 all exhibited apparent electrocatalytic activities for HER, much higher than com-Mo2C,
which clearly indicated that coupling for carbon layers and Mo2C can effectively modified the work
function of Mo2C and eventually influence its catalytic performance. As the pyrolysis temperature is raised
from 700 to 900 oC, the catalytic activity of the materials decrease. As seen from the result that MoCN3D-700 produce the best catalytic performance and exhibit an onset potential of about 45 mV (RHE). This
onset potential is similar in magnitude to that of Pt, and notably more positive than that of MoCN-3D-800
(~ -92 mV) and MoCN-3D-900 (~ -114 mV). Combined with Figure S14a and Figure S14b, which clearly
indicated that higher temperature will lead to serious decrease of surface area. Based on these results, we
may safely draw the conclusion that the surface area is an important parameter to influence the catalytic
activity of the final hybrids.
Figure S15. N1s core level (a) and Mo 3d (b) XPS spectrum of MoCN-3D before and after
electrocatalysis.
Figure S16. HRTEM images of surface regions of MoCN-3D before (a) and after electrocatalysis (b).
Figure S17 Three-dimensional charge density difference for the Mo2C-C composite with an isovalue of
0.0006 e/bohr3. Yellow and blue isosurfaces represent charge accumulation and depletion in the space
with respect to isolated Mo2C and C layer.
Table S1. Chemical compositions of MoCN-2D, and MoCN-3D hybrids calculated from
elemental Vairo EL III analyzer and ICP measurements.
Sample
C(atom %)a
N(atom %) a
O (atom %) b
H (atom %)a
Zn (atom %)c
Mo (atom %)c
MoCN-2D
83.97
6.49
4.27
0.13
0.11
5.03
MoCN-3D
83.17
6.86
4.89
0.09
0.1
4.89
a
C, N and H, contents were detected by elemental Vairo EL III analyzer; bO content was confirmed by
theoretical calculation: content of oxygen was confirmed as O (atom %) = 100 - C(atom %) N(atom %) - Zn(atom %) - Mo(atom %);
c
Zn and Mo contents were detected by ICP.
Table S2. Presents all the computed ZPE(H*) for H* adsorbed on the surface of different models
and the corresponding ΔZPE, as well as ΔE(H*) values.
Sample
Delta-E
ZPE
Delta-ZPE
Delta-G
Mo2C
-0.9874
0.18213
0.04713
-0.7353
C
1.42018
0.29503
0.16003
1.78521
Mo2C-C
-0.0884
0.307
0.172
0.28858
N-doped C
0.17937
0.31021
0.17521
0.55958
Mo2C- N-doped C
-0.3877
0.3113
0.1763
-0.0064
Table S3. Summary of representative HER catalysts in acidic electrolyte
Author
Catalyst
η
(mV)
Tafel slope
(mV/dec)
This work
MoCN-3D
~46
51.4
mv/dec
Bao et al. Energy &
Environ. Sci 2014, 7,
FeCo@NC
1919–1923.
NTs-NH
Sasaki et al.
Angew. Chem. Int. Ed.
2012, 51, 6131–6135
Zhang et al. J. Am.
Chem. Soc., 2015, 137,
15070–15073
NiMoNx/C
Co−C−N
Asefa. et al. Angew.
Chem. Int. Ed. 2014, 53,
Co-
4372–4376
NRCNTs
Qiao. et al. ACS NANO,
2014, 5, 5290-5296.
N, P
graphene
Chen. et al.
Adv. Funct. Mater. 2015,
25, 872–882
PCP//NRG
O
Besenbacher. et.al
Nature Chem. 2014, 6,
[Mo3S13]2/Graphene
Overpotential
at the 10
mA cm-2
89 mv
Electrolyte
0.5 M H2SO4
70
74
mv/dec
0.5 M H2SO4
78
35.9
mv/dec
~101
55
mv/dec
50
69
mv/dec
~276
91
mv/dec
58
127
mv/dec
100
40
mv/dec
180 mV
80
60
mv/dec
230 mV
0.5 M H2SO4
110
51.5
mv/dec
240 mV
0.5 M H2SO4
～275 mV
0.1 M HClO4
～283 mV
0.5 M H2SO4
138 mV
0.5 M H2SO4
140 mV
0.5 M H2SO4
420 mV
0.5 M H2SO4
229 mV
0.5 M H2SO4
248
Chhowalla. et al.
Nature Mater. 2013, 12,
850
Qiao. et al.
Nature Commun. 2014, 5,
3783.
WS2
nanosheets
C3N4@NG
Table S4. Comparison of the electrocatalytic activity of the MoCN-3D reported here with some
representative Mo based catalysts.
Author
This work
Sasaki et al.
Energy Environ. Sci.,
2013, 6, 943–951
Girault et al
Energy Environ. Sci.,
2014, 7, 387–392
Sun et al
Adv. Mater. 2014, 26,
5702–5707
Zou et al
Angew. Chem. Int. Ed.
2015 54, 10752–10757
Lee et al
Acs Nano 2014 8
5164–5173
Chhowalla et al
Nano Lett. 2013, 13,
6222−6227
Sun et al
Applied Catalysis B:
2015, 164, 144–150
Leonard et al
Angew. Chem. Int. Ed.
2014, 53, 6407 –6410
Lou et al Angew.
Chem. Int. Ed. 2015,
54, 15395 –15399
Yeo et.al
J. Mater. Chem. A,
2015, 3, 8361–8368
η
(mV)
Tafel slope
(mV/dec)
Overpotential
at the 10
mA cm-2
Electrolyte
~46
51.4
mv/dec
89 mv
0.5 M H2SO4
Mo2C/CNT
~74
87.6
mv/dec
150 mV
0.1 M HClO4
Mo2C
nanowire
~70
~78
mv/dec
～131 mV
0.5 M H2SO4
MoP
Net work
~40
54
mv/dec
～125 mV
0.5 M H2SO4
Mo2C@NC
~60
60
mv/dec
～124 mV
0.5 M H2SO4
Mo2C/CNTgraphene
~62
58
mv/dec
1T-MoS2
~119
～179 mV
0.5 M H2SO4
MoP
nanosheets
~100
40
mV/decade
56.4
mv/dec
～ 189 mV
0.5 M H2SO4
β-Mo2C
~183
120
mV/decade
～ 189 mV
0.1m HClO4
β-Mo2C
nanotubes
~82
62
mV/decade
～ 157 mV
0.5 M H2SO4
γ-Mo2N
~279
～ 381 mV
0.5 M H2SO4
Catalyst
MoCN-3D
108
mV/decade
～136 mV
0.5 M H2SO4