Supporting information

Mo2C/VC Heterojunction Embedded in Graphitic Carbon Network:

An Advanced Electrocatalyst for Hydrogen Evolution

Chao Huang,a, b‡ Xiaowei Miao,c‡ Chaoran Pi,b Biao Gao,a, b* Xuming Zhang,b Ping

Qin,b Kaifu Huo,d* Xiang Peng,e Paul K. Chua*

a Department of Physics and Department of Materials Science and Engineering City

University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

b The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced

Materials and Nanotechnology, Wuhan University of Science and Technology, Wuhan

430081, China

c Jiangsu Laboratory of Advanced Functional Materials, School of Chemistry and

Material Engineering, Changshu Institute of Technology, Changshu 215500, China

d Wuhan National Laboratory for Optoelectronics (WNLO) School of Optical and

Electronic Information, Huazhong University of Science and Technology, Wuhan

430074, China

e School of Materials Science and Engineering, Wuhan Institute of Technology,

Wuhan 430205, China

*Corresponding authors:

E-mail: gaobiao@wust.edu.cn (B. Gao); kfhuo@hust.edu.cn (K.F. Huo);

paul.chu@cityu.edu.hk (P.K. Chu)

Author contributions:

‡ Chao Huang and Xiaowei Miao contributed equally to this work.

Figure S1. (a) XRD pattern and (b) SEM image of V2MoO8.

Figure S2. SEM images of Mo2C/VC@C.

Figure S3. TEM image of Mo2C/VC@C.

Figure S4. SEM images of Mo2C@C.

Figure S5. SEM images of VC@C.

Figure S6. SEM images of the carbon network.

Figure S7. Raman scattering spectra of the products prepared with different precursors.

Figure S8. (a, b) XRD patterns of the products prepared with different precursors

(MoO3+Mg+NaHCO3), (V2O5+Mg+NaHCO3), (V2MoO8+Mg+NaHCO3), and

(Mg+NaHCO3); (c) Gibbs free energy changes as a function of the reaction

temperature in the MRT process

To investigate the formation mechanism in MRT, Mg/V2O5/NaHCO3,

Mg/MoO3/NaHCO3, Mg/NaHCO3, and Mg/V2MoO8/NaHCO3 react at 800 oC in

stainless steel sealed cans. The products of the four reactions are analyzed by XRD

which indicates that MgO and Na2CO3 are present in the four products after MRT.

NaHCO3 is thermally decomposed at 400–500 oC to form CO2, Na2CO3, and H2O as

shown in reaction S1[1] and CO2 is reduced into carbon by Mg to generate MgO

(reaction S2). As shown in Figure S8c, the Gibbs free energy change (ΔG) in

reaction 1 is more negative than those in reactions S3 and S4. In the

Mg/MoO3/NaHCO3 and Mg/V2O5/NaHCO3 systems, the in situ generated carbon

reacts with MoO3 and V2O5 to form Mo2C and VC as the temperature is raised, as

shown in Figures S8a and S8b. After removing the byproducts of MgO and Na2CO3,

Mo2C@C and VC@C are obtained. In the Mg/V2MoO8/NaHCO3 system, both

reactions S3 and S4 take place to generate the Mo2C/VC heterojunction which is

further wrapped by 3D carbon. The reactons in MTR are shown as follows:

2NaHCO3=Na2CO3+H2O+CO2(g) (S1)

2Mg+CO2(g)=2MgO+C (S2)

9C+2V2O5=4VC+5CO2(g) (S3)

4C+2MoO3=Mo2C+3CO2(g) (S4)

Figure S9. TEM images of Mo2C/VC@C.

Figure S10. (a) TEM image and (b) Electron energy loss spectroscopy (EELS)

spectra of Mo2C/VC@C.

Figure S11. Polarization curves of the Mo2C/VC@C electrocatalyst in 0.5 M H2SO4

with and without iR correction.

Figure S12. Polarization curves of the bare GCE, Mo2C/VC@C, and mixture of

Mo2C@C and VC@C.

Figure S13. LSV of bare GCE and carbon.

Figure S14. CV curves of (a) Mo2C@C, (b) VC@C, and (c) Mixture of Mo2C@C and

VC@C acquired at different scanning rates from 10 to 200 mV s-1.

Figure S15. Time-dependent current density of Mo2C/VC@C at 10 mA cm-2 for 20 h.

Figure S16. XPS spectra of (a) Mo 3d and (b) V 2p acquried from Mo2C/VC@C

before and after the 20-h test performed at a current density of 10 mA cm-2.

Figure S17. Theoretical models of (a) C-Mo2C, (b) Mo-Mo2C, (c) C-VC,

and (d) V-VC (Top view for H adsorption on the surface of the

samples in the upper images and the other pictures are the side

views for H adsorption on the surface of the samples).

Table S1. Comparison of the performance of representative precious metal-free HER

electrocatalysts.

Samples

Mass

loading

(mg cm-2)

Mediaη1

(mV)

η10

(mV)

Tafel slope

(mV dec-1)Ref.

Mo2C/VC@C 0.28 0.5 M H2SO4 75 122 43.8This

work

MoC-G 0.8 0.5 M H2SO4 -- 221 88 [2]

Mo2C-G 0.8 0.5 M H2SO4 -- 150 57 [2]

nanoMoC@GS(700) 0.76 0.5 M H2SO4 84 132 46 [3]

MoC/C 0.57 0.5 M H2SO4 -- 144 63.6 [4]

MoC@C 0.57 0.5 M H2SO4 -- 157 193.3 [4]

Mo2N-Mo2C/HGr-3 0.337 0.5 M H2SO4 11 157 55 [5]

b-Mo2C nanotubes 0.75 0.5 M H2SO4 -- 172 62 [6]

-Mo2C 0.102 0.5 M H2SO4 -- 198 56 [7]

-Mo2C 0.102 1 M KOH -- 176 58 [7]

MoCx octahedrons 0.8 0.5 M H2SO4 87 142 53 [8]

Mo2C/RGO 0.285 0.5 M H2SO4 -- 130 57.3 [9]

Mo2C/CNT-GR 0.65-0.67 0.5 M H2SO4 -- 130 58 [10]

Mo2C@GCSs 0.36 0.5 M H2SO4 120 200 62.6 [11]

N,P-Mo2C@C 0.9 0.5 M H2SO4 -- 141 56 [12]

nw-W4MoC 1.28 0.5 M H2SO4 -- ~135 52 [13]

Co-Mo2C 0.14 0.5 M H2SO4 -- 140 39 [14]

MoCN 0.4 0.5 M H2SO4 -- 140 46 [15]

Mo2C-NCNT 3.0 0.5 M H2SO4 -- 147 71 [16]

np-Mo2C NWs 0.21 0.5 M H2SO4 -- 200 53 [17]

MoSx@Mo2C-1:2 0.213 0.5 M H2SO4 -- 178 44 [18]

η1, defined as the overpotential at a current density of 1 mA cm-2

η10, defined as the overpotential at a current density of 10 mA cm-2

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