Atomic H-Induced Mo 2 C Hybrid as an Active and Stable Bifunctional Electrocatalyst Supporting Information 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 University, Taiyuan, Shanxi 030006, China College of Electronic Information and Control Engineering, # Institute of Microstructures and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, China Department of Chemistry, NanoCarbon Center, Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States Materials and Process Simulation Center, ς The Resnick Sustainability Institute, California Institute of Technology, Pasadena, CA 91125, USA E-mail: tour@rice.edu, guo@bjut.edu.cn, fxiujun@gmail.com 1 Deceased March 17, 2016 S1
Figure S1. SEM images of VA-GNR. S2
Figure S2. TEM images of VA-GNR showing that with atomic hydrogen treatment, VA-CNTs were unzipped and transformed into VA-GNR. S3
Figure S3. Photograph of Mo@Si (left) and Mo@VA-GNR (right). S4
Figure S4. SEM images of Mo2C-GNR hybrid grown with various growth time (a,b) for 3 h and (c,d) for 9 h. S5
Figure S5. SEM image of Mo2C on Si. S6
a Mo@VA-GNR Intensity (a.u.) 223 294 342 779 931 D 1340 G 1577 2660 2D D+G 2943 b 500 1000 1500 2000 2500 3000 Raman shift (cm -1 ) (020) Mo@VA-GNR Intensity (a.u.) (002) (111) (060) : Carbon : MoO 3 : Mo (141) (101) (220) (210) (002) (112) (042) (171) 10 20 30 40 50 60 2 / ( o ) Figure S6. (a) Raman spectra of VA-GNR with molybdenum deposited on the top layers. Mo@VA-GNR has characteristic peaks at 223, 294, 342, 779, and 931 cm 1 that can be assigned to MoO3. (b) XRD patterns of Mo@VA-GNR. S7
(a) 284.5 ev C1s peak Intensity (a.u.) Mo@VA-GNR Mo 2 C-GNR 294 292 290 288 286 284 282 280 Binding energy (ev) (b) Mo 2 C-GNR O1s Intensity (a.u.) Mo@VA-GNR 538 536 534 532 530 528 Binding energy (ev) Figure S7. XPS spectrum of Mo@VA-GNR and Mo 2 C-GNR hybrid for (a) C 1s and (b) O 1s. The O 1s spectrum (b) for the Mo@VA-GNR contained a characteristic signal at 530.4 ev that is - assigned to O 2 in MoO 3. 1 For Mo 2 C-GNR, the O 1s peak becomes prominent and shifts to 532.8 ev, which can be attributed to physisorbed O. S8
C Mo Intensity (au) O Mo Mo@VA-GNR Mo 2 C-GNR O C 0 2 4 6 8 10 Energy (KeV) Figure S8. The results of qualitative EDS of (black curve) Mo@VA-GNR and (red curve) Mo2C-GNR. S9
Table S1. Mo content determined by ICP-MS and quantitative surface analysis by EDS. Samples Mo loading (wt%) Surface atomic concentration (at%) C O Mo Mo@VA-GNR 31.5 21.0 50.6 28.4 Mo2C-GNR 31.3 66.3 3.5 30.2 S10
Figure S9. TEM images of Mo2C. S11
Figure S10. The TEM images of Mo2C-GNR grown with various growth time, (a-b) for 3 h and (c-d) for 9 h. S12
Intensity (a.u.) : Carbon : Mo 2 C : MoO 3 (002) (100) (002) (101) (101) (004) (102) 9h 6h 10 20 30 40 50 60 2 / ( o ) Figure S11. XRD patterns of Mo2C-GNR grown with various growth time, from 3 to 9 h. From this data, it is confirmed that Mo2C was successfully synthesized on GNRs in the Mo2C-GNR hybrid. 3h S13
Figure S12. (a) TEM, (b) HAADF image of Mo2C-GNR prepared with atomic H treatment for 9 h and element maps of (c) C, (d) Mo, and (e) O. S14
Amount absorbed (cm 3 g -1 STP) 2000 VA-GNR, S BET =936.4 m 2 g -1 1500 Mo 2 C-GNR, S BET =641.1 m 2 g -1 1000 500 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P 0 ) Figure S13. Nitrogen sorption isotherms of VA-GNR and Mo2C-GNR hybrid. S15
(a) j / ma cm -2 0.4 0.2 0.0-0.2-0.4 Ar O 2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential / V vs. RHE (b) j (ma cm -2 ) 3.0 1.5 0.0-1.5-3.0-4.5 J -1 (ma -1 cm 2 ) 0.6 0.5 0.4 0.3 0.030 0.045 0.060-1/2 (rpm -1/2 ) 0.70 V 0.75 V 0.80 V 0.85 V n = 4.0 Pt/C 225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 0.4 0.6 0.8 1.0 1.2 E (V vs. RHE) Figure S14. (a) CV of Pt/C in Ar- (red) and O2- (black) saturated electrolyte. (b) LSVs of Pt/C in O2-saturated 0.1 M KOH at a scan rate of 5 mv s -1 at different RDE rotation rates (in rpm). Inset reveals corresponding Koutecky-Levich plots (J -1 vs rpm -1-2 ) at different potentials. S16
Percentage of peroxide (%) Percentage of peroxide (%) a j / ma cm -2 0 Ring -1 Mo -2 2 C-GNR -3-4 -5 Disk 0.2 0.4 0.6 0.8 E / V vs RHE 1.0 b Electron Transfer Number (n) 4.0 3.8 3.6 3.4 3.2 3.0 0.2 0.4 0.6 0.8 E / V vs RHE 40 30 20 10 0 c j / ma cm -2 0 Ring -1-2 Pt/C -3-4 Disk -5 0.2 0.4 0.6 0.8 E / V vs RHE d Electron Transfer Number (n) 4.0 3.8 3.6 0.2 0.4 0.6 E / V vs RHE 0.8 10 8 6 4 2 0-2 -4 Figure S15. (a,c) Rotating ring disk electrode (RRDE) voltammograms of ORR on Mo2C-GNR and Pt/C electrode, respectively, with a sweep rate of 5 mv s -1 at 1600 rpm. (b,d) Percentage of peroxide and the electron transfer number (n) of Mo2C-GNR and Pt/C at different potentials derived from the RRDE data. S17
a 3 1.2 0.5 M H 2 SO 4 1.0 2 Mo 2 C-GNR j / ma cm -2 1 0-1 -2-3 j -1 / ma -1 cm 2 0.8 0.6 0.4 0.02 0.03 0.04 0.05 0.06 0.07-1/2 / rpm -1/2 0.60 V 0.55 V 0.50 V 0.45 V n = 3.78 225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm bj / ma cm-2 3 2 1 0-1 -2-3 j -1 / ma -1 cm 2 1.2 1.0 0.8 0.6 0.4 0.02 0.03 0.04 0.05 0.06 0.07-1/2 / rpm -1/2 0.65 V 0.60 V 0.55 V 0.50 V 0.5 M H 2 SO 4 Pt/C n = 3.96 225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 E / V vs RHE E / V vs RHE Figure S16. LSVs at 5 mv s -1 in the presence of oxygen with rotation speed from 225 to 2025 rpm in 0.5 M H2SO4 for (a) Mo2C-GNR grown with 6 h and (b) Pt/C. The insets in each panel are the corresponding Koutecky-Levich plots (J -1 vs rpm -1/2 ) at different potentials. S18
a j / ma cm -2 3 2 1 0-1 -2-3 -4 j -1 (ma -1 cm 2 ) 1.0 0.8 0.6 0.4 0.75 V 0.70 V 0.65 V 0.60 V 0.02 0.03 0.04 0.05 0.06 0.07-1/2 (rpm -1/2 ) 0.4 0.6 0.8 1.0 E / V vs RHE 3 h n = 3.27 225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm b j / ma cm -2 3 2 1 0-1 -2-3 j -1 (ma -1 cm 2 ) 1.2 1.0 0.8 0.6 0.75 V 0.70 V 0.65 V 0.4 0.60 V 0.02 0.03 0.04 0.05 0.06 0.07-1/2 (rpm -1/2 ) 9 h n = 2.64-4 0.4 0.6 0.8 1.0 E / V vs RHE 225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm Figure S17. Rotating-disk voltammograms of Mo2C-GNR grown with various time in O2-saturated 0.1 M KOH at a scan rate of 5 mv s -1 at different rotating speeds, (a) 3 h and (b) 9 h. Insets are the corresponding Koutecky-Levich plots at different potentials. S19
ORR Activity Calculations The working electrode was scanned cathodically at a rate of 5 mv s 1 with varying rotating speed from 225 to 2025 rpm. Koutecky Levich plots (J 1 vs ω 1/2 ) were analyzed at various electrode potentials. The slopes of their best linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky Levich eq 1-3: 1 J = 1 + 1 = 1 + 1 J K J L J K Bω 1/2 (1) B = 0.62nFC 0 D 0 2/3 γ 1/6 (2) J K = nfkc 0 (3) where J is the measured current density, Jk and JL are the kinetic- and diffusion-limiting current densities, ω is the angular velocity, n is transferred electron number, F is the Faraday constant (96 485 C mol 1 ), C0 is the bulk concentration of O2 (1.2 10 6 mol cm 3 ), and ν is the kinetic viscosity of the electrolyte (0.01 cm 2 s 1 for both 0.5 M H2SO4 solution and 0.1 M KOH solution), D0 is the O2 diffusion coefficient (1.9 10 5 cm 2 s 1 ), and k is the electron-transfer rate constant. The number of electrons transferred (n) and Jk can be obtained from the slope and intercept of the Koutecky Levich plots, respectively. For the RRDE measurements, catalyst inks and electrodes were prepared by the same method as those of RDE. The disk electrode was scanned at a rate of 5 mv s -1 and the ring potential was kept constant at 0.5 V vs. Ag/AgCl. The H2O2 eq 4 and 5 H 2 O 2 (%) = 100 2I r N I d + I r N (4) S20
n = 4 I d I d + I r N (5) Here, Id is disk current, Ir is ring current and N = 0.36 is collection efficiency (N). S21
a j / ma cm -2 0-20 -40-60 -80 3 h 6 h 9 h 0.5 M H 2 SO 4-100 -0.3-0.2-0.1 0.0 0.1 Potential / V vs RHE b Overpotential / V 0.4 0.3 0.2 0.1 0.0 3 h 6 h 9 h 93 mv dec -1 84 mv dec -1 65 mv dec -1 0.5 M H 2 SO 4-1.0-0.5 0.0 0.5 1.0 1.5 2.0 Log j / ma cm -2 Figure S18. (a) HER polarization curves and (b) the corresponding Tafel plots of Mo2C-GNR grown with various growth time. S22
a 3 Increasing scan rate b 3 j G (ma cm -2 ) 2 1 0-1 -2 j G (ma cm -2 ) 2 1 Slope = 0.01249 C dl = 12.05 mf cm -2-3 -0.02 0.00 0.02 0.04 0.06 0 0 50 100 150 200 E vs RHE (V) Scan Rate (mv s -1 ) c j G (ma cm -2 ) 1.5 1.0 0.5 0.0-0.5-1.0 Increasing scan rate d j G (ma cm -2 ) 1.5 1.0 0.5 Slope = 0.00612 C dl = 6.14 mf cm -2-1.5 0.36 0.38 0.40 0.42 0.44 0.0 0 50 100 150 200 E vs RHE (V) Scan Rate (mv s -1 ) Figure S19. Cyclic voltammograms and capacitive currents plotted as a function of scan rate in 0.5 M H2SO4 at scan rates of 10, 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 mv s -1 for Mo2C-GNR grown with various time, (a,b) 3 h and (c,d) 9 h. The effective surface areas of Mo2C-GNR grown at various times were compared by estimating their electrochemical double layer capacitances (Cdl) with cyclic voltammograms (CVs). CVs were performed at a potential range of (-0.03) to 0.07 V vs RHE and 0.35 to 0.45 V vs RHE, respectively, where no obvious electrochemical features corresponding to the Faradic current were observed (Figure S19 a and c). The capacitive currents, j(ja-jc)@0.02 V and S23
j(ja-jc)@0.40 V, were plotted against the scan rate (Figure S19 b and d). The linear relationships were observed with the slopes twice the Cdl value. Accordingly, the Cdl values for Mo2C-GNR grown with 3 h and 9 h were calculated to be 12.05 and 6.14 mf cm -2, respectively. S24
0.05 Mo 2 C 0.00 j G (ma cm -2 ) -0.05-0.10-0.15 120 mv/s 80 mv/s 40 mv/s -0.20 0.34 0.36 0.38 0.40 0.42 0.44 E vs. RHE (V) Figure S20. CV curves of Mo2C in acidic medium. HER Activity Calculations The electrochemically active surface area (EASA) was estimated from the electrochemical double-layer capacitance of the nanoporous layers. The double layer capacitance (Cdl) was determined with a simple cyclic volatammetry (CV) method. The EASA is then calculated from the double-layer capacitance according to eq 6: EASA = C dl C s (6) Where Cs is the capacitance of an atomically smooth planar surface of the material per unit area under identical electrolyte conditions. An average value of Cs = 22 μf cm -2 is used in this work. The roughness factor (RF) is then calculated by dividing the estimated EASA by the geometric area of the electrode. S25
a -Z'' / 50 40 30 20 0.5 M H 2 SO 4 VA-GNR Mo 2 C Mo 2 C-GNR b -Z'' / 180 150 120 90 60 VA-GNR Mo 2 C Mo 2 C-GNR 0.1 M KOH 10 30 0 0 5 10 15 20 25 30 35 Z' / 0 0 20 40 60 80 100 120 140 Z' / Figure S21. Nyquist plots with an equivalent circuit in (a) acidic and (b) alkaline medium. S26
Mo 3d 5/2 228.2 Mo 3d Intensity (a.u.) Mo 3d 3/2 231.4 240 238 236 234 232 230 228 226 Binding energy (ev) Figure S22. Mo 3d XPS spectrum of Mo2C-GNR after a 30000 s durability test. S27
Table S2. ORR electrochemical analysis of Mo2C, Mo2C-GNR, VA-GNR and Pt/C catalyst. Catalysts Surface area a m 2 g -1 Sheet resistance b Ω -1 Epeak V jpeak ma cm -2 Eonset V Half-wave potential c V j d ma cm -2 Mo2C 28 456 0.73 0.09 0.84 0.71 2.76 Mo2C-GNR 641.1 76.8 0.83 2.01 0.93 0.81 4.55 VA-GNR 936.4 1065.2 0.68 0.41 0.82 0.73 1.43 Pt/C - - 0.82 0.41 0.96 0.84 4.40 a From BET method. b From 4-point probe method. c The half-wave potential represents the potential at which the current is half of the limiting current in the LSV curve. d Measured at 0.5 V vs RHE, 1600 rpm. S28
Table S3. HER electrochemical properties of Mo2C, Mo2C-GNR, and Pt catalysts. Catalyst @ 10 ma cm -2 mv Onset potential mv J @ 300 mv ma cm -2 Tafel slope mv dec -1 Rct a Ω Mo2C 0.5 M H2SO4 275 106 14.1 129 12.9 0.1 KOH 266 124 14.6 147 68.9 Mo2C-GNR 0.5 M H2SO4 152 39 106.2 69 10.5 0.1 KOH 121 53 31.2 59 47.8 Pt 0.5 M H2SO4 26.5 5 6030 30-0.1 KOH 15 6 1510 29 - a Extracted from fitting electrochemical impedance spectra measured at η = 5 mv to an equivalent circuit. S29
Table S4. Comparison of HER activity of some Mo-based catalysts. Catalysts Electrolyte Tafel slope mv dec 1 Onset overpotential mv Metal precursor ref Mo2C-GNR 0.5 M H2SO4 65 39 Mo This work Mo2C-GNR 0.1 M KOH 59 53 Mo This work Mo2C/GCSc Mo2C/CNTs Mo2C/XC Mo2C/CNTs-GR Mo2C-RGO Mo2C-NWs 0.5 M H2SO4 0.1 M HClO4 0.1 M HClO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 62.6 120 (NH4)6Mo7O24 4H2O 2 55.2 63 (NH4)6Mo7O24 4H2O 3 59.4 105 (NH4)6Mo7O24 4H2O 3 58 62 MoCl5 4 54 ~70 (NH4)6Mo7O24 4H2O 5 55.8 ~160 (NH4)6Mo7O24 4H2O 6 Mo2C-NSs 0.5 M 64.5 ~160 (NH4)6Mo7O24 4H2O 6 S30
Catalysts Electrolyte Tafel slope mv dec 1 Onset overpotential mv Metal precursor ref H2SO4 Np-Mo2C NWs Mo2C-NCNTs MoS2/Mo2C-NCNTs 0.5 M H2SO4 0. 5 M H2SO4 0. 5 M H2SO4 54 ~70 (NH4)6Mo7O24 4H2O 7 71 72 MoO3 8 69 145 MoO3 9 S31
Figure S23. Structure model for Mo2C-graphene hybrid. The blue lines show the boundaries of the periodic cell. S32
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