Ditungsten Carbide Nanoparticles Encapsulated by Ultrathin. Graphitic Layers with Excellent Hydrogen-Evolution. Electrocatalytic Properties

Transcription

1 Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is The Royal Society of Chemistry 216 Supporting Information Ditungsten Carbide Nanoparticles Encapsulated by Ultrathin Graphitic Layers with Excellent Hydrogen-Evolution Electrocatalytic Properties Yao Zhou, 1 Ruguang Ma, 1 Pengxi Li, 1 Yongfang Chen, 1 Qian Liu, *,1 Guozhong Cao, *,2,3 and Jiacheng Wang *,1 1 The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 25, P. R. China 2 Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA 3 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 183, P. R. China * Author for correspondence. gzcao@u.washington.edu; qianliu@sunm.shcnc.ac.cn; jiacheng.wang@mail.sic.ac.cn, Tel: , Fax:

2 (a) H 2 /Ar Dicyandiamide+ phosphato-tungstic acid () Temperature/ C (b) 2 C/min 5 o C/min 12 min W-11 W-9 W-7 24 min Natural Cooling Time/ min Figure S1. (a) Schematic diagram of the apparatus used for the synthesis of W-7, W- 9 and W-11; (b) temperature program applied for the synthesis of nanoelectrocatalyst. Intensity (a.u.) W-11 W-9 W-7 W-O-W W=O Raman shift (cm -1 ) Figure S2. Raman spectra of samples W-7, W-9 and W-11 at low Raman shift area. The spectrum of W-7 and W-9 samples displays characteristic Raman peaks at 26, 77 and 88 cm -1, that can be assigned to W-O-W deformation mode, the W=O bending mode and the W=O stretching mode in WO 3, repectively. 1 With pyrolysis treatment at 9 C for 4 h, the peaks assigned to WO 3 get weaker. At 11 C, the WO 3 peaks are completely vanished, which verifies the successful chemical conversion from 2

3 WO 3 to W 2 C via pyrolysis process. Intensity (a.u.) W-7 P2p Binding Energy (ev) Figure S3. The high resolution XPS spectra of P 2p for sample W-7. Phosphorus, which should have been introduced from precursor (phosphotungstic acid) is not detected in W-7 with pyrolysis temperature at 7 C. It is believed that phosphorus is unstable and ready to be evaporated with heat-treatment. 2 The complete phosphorus removal of the W-7 suggests no existence of P in the samples with pyrolysis temperatures above 7 C. 3

4 Peak Analysis Peak Analysis Data Set:[Book7]AverageResult2!Averaged Y Data Set:[Book7]AverageResult3!Averaged Y Date:215/12/14 (a) Adj. R-Square= E-1 Degree of Freedom= x1 ID/IG=.82 5.x12 Fitting Results Peak Type Lorentz Lorentz Gaussian Gaussian Area Intg Chi^2= E+2 Adj. R-Square= E-1 SS=5.9456E+5 Degree of Freedom=991. (b) (a)w-7 G I. 1 Peak Index # of Data Points=1. D D" 2 8.x1 ID/IG=.8 # of Data Points=1. (b) D I 2 4.x1 Peak Analysis Data Set:[Book1]AverageResult1!Averaged Y. W-9 G D" Date:215/12/14 Baseline:ExpDec # of Data Chi^2=7.8512E+2-1 Adj. R-Square= E-1 Points=1. Raman shift (cm ) Degree offitting SS=7.7836E+5 Freedom=991. Raman shift (cm-1) Results FWHM (c) 1.x1 3 Max Height Intensity (a.u.) Intensity (a.u.) SS=1.851E+6 Intensity (a.u.) Chi^2=1.187E+3 Date:215/12/14 Baseline:ExpDec1 Baseline:Exponential Center Grvty Peak Index Area IntgP ID/IG=.78 Peak Type Lorentz Lorentz Gaussian Gaussian D 5.x12 I D" Area Intg FWHM W-11 Max Height Center Grvty Area IntgP G. 1 Fitting Results Peak Index Peak Type Lorentz Lorentz Gaussian Gaussian Area Intg Raman shift (cm ) FWHM Max Height Center Grvty Area IntgP Figure S4. Deconvoluted Raman spectra of (a) W-7, (b) W-9, and (c) W-11 showing D, G, I and D bands with values of ID/IG. All samples had the similar main features of graphitic materials. The G band stems from in-plane vibrations and has E2g symmetry. This is observed in all sp2 carbon systems. The D band stems from a double resonance process involving a phonon and a defect. After fitting process, additional bands are labeled as I and D. The I band is linked with disorder in the graphitic lattice, sp2-sp3 bonds or the presence of polyenes at ~ cm-1.3 The D band (~15 cm-1) is known to occur in the presence of amorphous carbon.4 4

5 Table S1. Elemental composition of samples W-x. Sample C (at%) W (at%) O (at%) W W W Current Density (ma/cm 2 ) Before IR corrected (a) After IR corrected Current Density (ma/cm 2 ) Before IR corrected (b) After IR corrected Current Density (ma/cm 2 ) Potential (V vs. RHE) Before IR corrected (c) After IR corrected Current Density (ma/cm 2 ) Potential (V vs. RHE) Potential (V vs. RHE) 5 Before IR corrected (d) After IR corrected Potential (V vs. RHE) Current Density (ma/cm 2 ) Before IR corrected After IR corrected (e) Potential (V vs. RHE) Figure S5. Polarization curves of (a) W-7, (b) W-9, (c) W-11 (d) pristine graphene and (e) 2% Pt/C before and after 9% ir compensation. 5

6 Figure S6. TEM image of as-prepared pristine graphene. Table S2. Nitrogen sorption data from pristine graphene. Sample ID Prstine graphene S BET (m 2 /g) S Meso S Micro V Meso (cm 3 /g) V Micro It shows that the specific surface area of pristine graphene (17. m 2 /g) is larger than those of W-x samples. It is believed that the higher surface area of catalyst facilitate to increased effective active surface, leading to enhanced catalytic performance. Without the promoting effect of W 2 C phase, only considering the contribution of surface area, pristine graphene should have exhibited the best catalytic performance. However, W-x samples all perform better than pristine graphene, indicating the apparent enhanced catalytic activity induced by W-containing nanoparticles. 6

7 Adsorbed Volume (cm 3 /g) W-7 W-9 W Relative Pressure (P/P) Figure S7. N 2 adsorption-desorption isotherms of samples W-x. Table S3. Nitrogen sorption data from samples W-7, W-9 and W-11. Sample ID S BET (m 2 /g) S Meso S Micro V Meso (cm 3 /g) W W W V Micro Nitrogen sorption was used to determine the surface area and pore volume of WO 3 /W 2 C@GL samples. The samples exhibit type IV isotherms with shape H4 type hysteresis loop. It is believed that the H4 is associated with the existence of narrow slitlike pores. 29 In addition, with increased pyrolysis temperature, micropores tends to be melted due to the vanishing of diverge of isotherms at relative pressures (P/P) below.2. 7

8 Overpotential (V vs. RHE) mv/dec Log j (ma/cm 2 ) Figure S8. Tafel plot of W-9 in.5 M H 2 SO 4 acidic solution. 8

9 Table S4. Comparison of the electrocatalytic activity of the W-11 reported here under Sample ID acidic conditions with other non-noble transition metal HER electrocatalysts. j (ma cm -2 ) η 1 (mv) Oneset potential (mv) Loading of catalyst (mg cm -2 ) Tafel slope (mv dec -1 ) W This work W 2 C microspheres W 2 C nanoparticles Ref. WC/aligned carbon tubes β-mo 2 C γ-mo 2 C Commercial Mo 2 C β-mo 2 C/reduced graphene oxide WC nanocrystals Mo 2 C/CNT-graphene WS 2 nanosheets Chemically exfoliated WS 2 nanosheets Metallic MoS 2 nanosheets Co@NCNTs FeCo@NCNTs Ni 2 P nanoparticles Co 2 P nanorods

10 MoP P-WN/rGO Metallic WO 2 /carbon MoCN NPs Commercial MoB > MoP Amorphous MoP nanoparticles CoP nanoparticles Ni-Mo nitrides

11 Table S5. HER catalytic properties of W-x samples. Sample ID η 1 Oneset potential Tafel slope (mv) (mv) (mv dec -1 ) W W W Figure S9. Electrical equivalent circuit model for fitting the EIS response of hydrogen evolution reaction on W-x samples with different pyrolysis temperatures and amount of phosphotungstic acid. The whole model follows the two-time constant model, where R s, is the series resistance, R ct denotes the charge transfer resistance, R p related to is the porosity the electrode surface. The model consists of a series resistance, Rs, in series with two parallel branches; one is related to the charge-transfer process (CPE 1 -R ct ); the other is on the surface porosity. W-11 exhibits a much lower charge transfer resistance (16.9 Ω) than W-7 (42.3 Ω) and W-9 (32.4 Ω). 11

12 (a) (b) (2.36 Å) W 2 C (2) (c) Intensity (a.u.) (e) Overpotential (V vs. RHE) nm Raw Data Fitted Curve Background W4f 5/2 (W 3 C) W4f 7/2 (W 3 C) Binding Energy (ev) 14 mv dec Log j (ma/cm 2 ) (d) Current Density (ma/cm 2 ) Z'' ( ) nm 1 =.2 V (f) Potential (V vs. RHE) Z' ( ) R ct =4.7 Figure S1. (a), (b) TEM images of W 2 C NP without GL wrapping; (c) high resolution XPS spectrum of W 4f for the f-w 2 C-11; (d) LSV polarization curve and (e) corresponding Tafel plot of W 2 C NP without GL wrapping pyrolyzed at 11 C in.5 M H 2 SO 4 at scan rates of 2 mv s

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