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1 Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is The Royal Society of Chemistry 2019 Supplementary Information for Highly Efficient Nitrogen and Carbon Coordinated N-Co-C Electrocatalyst on Reduced Graphene Oxide Derived from Vitamin-B12 for Hydrogen Evolution Reaction Palani Sabhapathy a, b, d, e, Chen-Cheng Liao b, g, Wei-Fu Chen b, *, Tsu-Chin Chou b, Indrajit Shown a, Amr Sabbah a, d, e, Yan-Gu Lin c, Jyh-Fu Lee c, Ming-Kang Tsai g, Kuei-Hsien Chen a, b, * and Li-Chyong Chen b, f, * a. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei-10617, Taiwan. * chenkh@pub.iams.sinica.edu.tw b. Center for Condensed Matter Sciences, National Taiwan University, Taipei-10617, Taiwan. * wfchen@ntu.edu.tw, chenlc@ntu.edu.tw c. National Synchrotron Radiation Research Center, Hsinchu-30076, Taiwan. d. Department of Chemistry, National Tsing Hua University, Hsinchu-30013, Taiwan. e. Molecular Science and Technology, Taiwan International Graduate Program, Academia Sinica, Taipei-11529, Taiwan. f. Center of Atomic Initiative for New Materials, National Taiwan University, Taipei-10617, Taiwan. g. Department of Chemistry, National Taiwan Normal University, Taipei-11677, Taiwan.

2 EXPERIMENTAL SECTION Chemicals and Materials: Vitamin B12 (99 %, cyanocobalamin) and potassium hydroxide (KOH, AR) were purchased from Sigma-Aldrich. Graphene was purchased from ACS materials. Sulfuric acid (H2SO4, GR) was purchased from Scharlau chemicals. Pt/C (20 wt. %) was purchased from Alfa Aesar. Nafion solution (5 wt. %) was purchased from DuPont. All chemicals were used without further purification. Preparation of B12/GX and B12/G800A: 30 mg of vitamin B12 (99%, Aldrich), was dissolved in 10 ml of de-ionized water with constant stirring for 30 minutes at room temperature. Then, 20 mg of graphene (ACS materials) was added to the vitamin B12 solution, again with constant stirring for 30 minutes at room temperature. The solution was heated using steam to 80 C to eliminate the water. Subsequently, the suspension was dried at 60 C under vacuum for 12 h to obtain the slurry. Asprepared slurry was then pyrolyzed under flowing N2 (50 sccm) at a heating rate of 20 C min 1 to desired temperatures ( C) and the temperature was held for 2 h (herein, pyrolyzed samples are designated as B12/GX, where X=Temperature). Finally, the pyrolyzed product was soaked in 0.5 M H2SO4 for 24 h and post annealing ( C, 2 h) has been done to remove unstable cobalt species (B12/GXA, where X=Temperature). Preparation of pyrolyzed B12/C and B12: Pyrolyzed B12/C and pyrolyzed B12 were prepared to compare their electrochemical activity with B12/G. B12/C was prepared as follows: 60 mg of vitamin B12 (99%, Aldrich), was dissolved in 10 ml of de-ionized water with constant stirring for 30 minutes at room temperature. Then, 40 mg of Vulcan XC-72 was added to the vitamin B12 solution, again with constant stirring for 30 minutes at room temperature. The solution was heated using steam to 80 C to eliminate the water. Subsequently, the suspension was dried at 60 C under vacuum for 12 h to obtain the slurry. As-prepared slurry was then pyrolyzed under flowing N2 (50 sccm) at a heating rate of 20 C min 1 to 800 C and the temperature was held for 2 h.

3 The py-b12 was prepared as follows: 100 mg of vitamin B12 (99%, Aldrich), loaded on a fused aluminium oxide boat, was introduced into a quartz tube, which was placed in a tubular furnace. Then pyrolyzed under flowing N2 (50 sccm) at a heating rate of 20 C min 1 to 800 C and the temperature was held for 2 h. Characterization: The X-ray diffraction (XRD) measurements were carried out using a Bruker D2 Phaser X-ray diffractometer with Cu K radiation. Both scanning and transmission electron microscopies (SEM, JEOL 6700F and TEM, JEOL JEM-2100) were used to characterize the morphology and microstructure of materials. The X-ray photoelectron spectroscopy (XPS) was recorded at beam line 08A1 of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were performed on a TJA (Thermo Jarrell Ash) Atomscan Advantage instrument. The X-ray absorption near edge structure (XANES) at the Co K-edge was recorded at beam line 17C1 of the NSRRC, which is based on a multi-pole wiggler source with a critical energy of 2.7 kev. The electron storage ring was operated at energy of 1.5 GeV with a beam current of ma. The beam line employs a double Si (1 1 1)-crystal monochromator for energy selection with a resolution (ΔE/E) of better than in the energy range 5 15 kev. All spectra were obtained at room temperature in a transmission mode, and the intensities of incident and transmitted X-ray beams were measured using gas-filled ionization chambers. The catalyst powder was pressed into a slot of the stainless-steel holder and then placed in a cell for treatment under the desired conditions. The standard material, cobalt foil was measured simultaneously in the third ionization chamber to enable energy calibration scan by scan. The XANES data were processed, involving background subtraction, normalization with respect to the edge jump, Fourier transformation, and curve fitting, using computer programs that were implemented in the IFEFFIT software package. Additionally, the theoretical phase shifts and backscattering amplitudes for particular atom pairs were calculated using the FEFF7 code.

4 Supplementary Note 1: Electrochemical Measurements Electrochemical measurements were made in a three-compartment cell using a potentiostat/ galvanostat instrument (Biologic Bi-stat) at room temperature. The working electrode was a glassy carbon (GC). The counter electrode and reference electrode were Pt foil and a silver chloride electrode (0.197 V vs. NHE), respectively. The polarization curves were obtained in 0.5 M H2SO4 (ph=0) and 1 M KOH (ph=14) with a scan rate of 5 mv s -1 at room temperature unless otherwise noted. The working electrode bearing the catalyst was rotated at 1,600 r.p.m. to remove any H2 produced on the disk radially. All potentials were ir-compensated and converted to a reversible hydrogen electrode (RHE) scale via calibration. And the presented current density was normalized to the geometric surface area of the GC (0.196 cm -2 ). All the polarization curves are the ones reaching steady states after several cycles. The electrolyte solution was saturated with N2 for 20 min before each test. Catalyst ink was prepared by mixing catalyst (10 mg) with deionized water (1 ml). Then the desired amount of catalyst ink and Nafion solution (5 μ L, 0.1 wt. %) was dropped onto the GC disk, which was then left to dry in air at room temperature. Before the catalyst ink was deposited onto the GC disk, the GC disk had been polished to a mirror-like finish using 0.3 and then 0.05 μm alumina slurry. The electrochemical stability of the catalyst was evaluated by cycling the electrode 1000 times; each cycle started at V and ended at 0.25 V vs. RHE with a scan rate of 10 mv s -1 while rotating the working electrode at 1600 rpm. Chronopotentiometric measurements of the catalysts on GCE electrodes kept at a constant current density of 10, 30 and 50 ma cm -2 in N2- saturated 0.5 M H2SO4 and 10 ma cm -2 in 1 M KOH. Graphite rod was used as a counter electrode to avoid platinum dissolution during stability test. Electrochemical impedance spectroscopy was performed when the working electrode was biased at a constant 0 to V vs. RHE while sweeping the frequency from 5 MHz to 10 mhz with a 5 mv AC dither in 0.5 M

5 H2SO4. The impedance data were fit to a simplified Randles circuit to extract the series and charge transfer resistances. Supplementary Note 2: Double layer capacitances (Cdl) The electrochemical double layer capacitances (Cdl) of catalysts were measured by using a simple CV method 1. It is known that Cdl value is expected to be linearly proportional to the electrochemically active surface area of the electrode. A potential range of V vs. RHE was selected for the capacitance measurements because no obvious electrochemical features corresponding to the Faradic current were observed in this region for each catalyst. Then, the capacitive currents of Ja-Jc ΔJ V/2 were plotted as a function of the CV scan rate of 10, 30, 50, 70, 90, and 110 mv s -1. The working electrode was held at each potential vertex for 10 to 15 s before beginning the next sweep. The double-layer charging current is equal to the product of the scan rate, v, and the electrochemical double-layer capacitance, Cdl, as given by eq. 1. i c = vc dl (1) Supplementary Note 3: Turnover frequency (TOF) calculations The per-site TOF value was calculated according to the following equation: TOF (H 2 s) = #total hydrogen turnover per geometric area #activesites per geometric area (2) The number of total hydrogen turnovers was calculated from the current density extracted from the LSV polarization curve according to: 2,3

6 #total hydrogen turnovers = [ j ma cm 2] [ 1C s 1 mol e 1 mol ] [ ] [ 1000mA C 2 mol e ] [ molecules H 2 ] 1 mol H 2 = x H 2 s ma per cm2 cm 2 (3) The number of active sites in B12/G800A catalyst was calculated from the mass loading on the glassy carbon electrode, the Co contents and the Co atomic weight, assuming each Co center accounts for one active site: catalyst loading per geometric area (x g cm2 # active sites = [ Co M W (g mol) ) X Co wt% ] [ X 1023 Co atoms 1 mol Co ] (4) = [ X 10 3 g cm 2 X wt% ] [ X 1023 Co atoms ] g mol 1 mol Co = X Co sites per cm 2 Finally, the current density from the LSV polarization curve can be converted into TOF values according to: TOF = 3.12 X X X j = X j (5)

7 Supplementary Note 4: Active sites density measurements The electrochemical double-layer capacitance (Cdl) was determined from the scan-rate dependence of CVs in a potential range where there is no Faradic current. For N-Co-C, Cdl = 61.6 mf cm -2. The ECSA can be calculated from the Cdl according to: ECSA = C dl C s (6) where Cs is the specific capacitance of a flat standard electrode with 1 cm 2 of real surface area, which is generally in the range of 20 to 60 μf cm If we use the averaged value of 40 μf cm -2 for the flat electrode, we obtain: ECSA = C dl C s = 61.6 mf cm 2 40 µf cm 2 = 1540 cm 2 ECSA (7) If we divide the as-obtained ECSA by the loading density of Co centers on the electrode (Co sites per cm 2), we can get the averaged area to find one Co center (cm 2 per site): 2 2 ECSA A ECSA per site = # active sites = 1540 cm ECSA per cm real 23.4 X = 6.58 X cm Co sites per cm ECSA per Co (or)6.58 nm ECSA per Co (8) real The active sites density can be obtained by the inverse of the AECSA per site:

8 1 Active sites density (sites cm 2 ) = = 1.52 X sites cm 2 (9) A ECSA per site Supplementary Note 5: Computational methods for the d-band model calculation All of the DFT calculations have considered spin-polarization and were performed with Vienna Ab-initio Simulation Package (VASP) version Generalized gradient approximation (GGA) with revised Perdew-Burke-Ernzerh (rpbe) exchange-correlation functional 9 and projectoraugmented wave (PAW) method 10,11 were used. The kinetic cut off energy was 450 ev and the Monkhorst-Pack mesh k-points that we chose was 3x3x1 for optimization and 11x11x1 for density of state calculations. For structural optimization, structures were fully relaxed until the force s difference was less than 0.05 ev/å. The calculated Co-N and N-Co-C models are shown in Figure S18. Based on M-N4 (vitamin B12) macrocyclic compounds, to represent experimental Co-N structure, cobalt atom was embedded in 4x4 graphene supercell with 4 nitrogen atoms substitution. Three nitrogen atoms in Co-N model were replaced by carbon atoms to represent N-Co-C structure (Figure S19). For slab models, 15 Å vacuum was added to the model from the top layer.

9 Table S1. Elemental composition measured by XPS and ICP-AES of the B12/G800 and B12/G800A catalysts. Co-ICP (at%) N-XPS (at%) O-XPS (at%) C-XPS (at%) B12/G B12/G800A Table S2. Quantitative nitrogen moiety distributions and relative percentages of the total nitrogen contents of the B12/G800 and B12/G800A catalysts. Pyridinic-N (%) Pyrrolic-N (%) Graphitic-N (%) N-Oxide (%) B12/G B12/G800A

10 Table S3. Comparison of cobalt based catalysts for HER in acidic electrolyte Catalyst HER Tafel Slope Reference 10 ma cm -2 CoNi@NC Angew. Chem. Int. Ed., 2015, 54, Co-NRCNTs Angew. Chem. Int. Ed., 2014, 53, Co-C-N J. Am. Chem. Soc., 2015, 137, Co-NG Nat. Commun., 2015, 6, 8668 FeCo@NCNTs-NH Energy Environ. Sci., 2014, 7, CoS2 NW J. Am. Chem. Soc., 2014, 136, CoSe2 /CP J. Am. Chem. Soc., 2014, 136, CoS2 /RGO-CNT Angew. Chem. Int. Ed., 2014, 53, CoP/CNT Angew. Chem. Int. Ed., 2014, 53, Co0.6Mo1.4N J. Am. Chem. Soc., 2013, 135, Mo2C@NC Angew. Chem. Int. Ed., 2015, 54, D-TiO2/Co@NCT Nano Res., 2017, 10(8), MoS2/RGO J. Am. Chem. Soc., 2011, 133, Co:WS Energy Environ. Sci., 2018, 11, B12/G This work B12/G800A

11 Table S4. Comparison of cobalt based catalysts for HER in alkaline electrolyte Catalyst HER Tafel Slope Reference 10 ma cm -2 Co-NRCNTs Angew. Chem. Int. Ed., 2014, 53, Co-C-N J. Am. Chem. Soc., 2015, 137, Co-NG Nat. Commun., 2015, 6, 8668 Co-P film Angew. Chem. Int. Ed. 2015, 54, CoOx@CN J. Am. Chem. Soc. 2015, 137, CoP/CC J. Am. Chem. Soc., 2014, 136, Fe0.5Co0.5@NC/NCNS J. Mater. Chem. A, 2017, 5, Co@CNF Nano Energy, 2016, 22, CP/CTs/Co-S ACS Nano, 2016, 10, CoNiP@NF J. Mater. Chem. A, 2016, 4, Co-Ni-P Chem. Commun., 2016, 52, B12/G B12/G800A This work

12 Figure S1 (A) XRD patterns of B12/G samples prepared at different pyrolysis temperatures (600 o C, 700 o C, 800 o C and 1000 o C) and RGO. Comparative XRD patterns: (B) between B12/G800 and B12/C, i.e., using reduced graphene oxide and active carbon, respectively, as support for B12; (C) before and after the acid treatment of B12/G800 sample.

13 Figure S2 TEM images of (A) B12/G600, (B) B12/G1000, (C) Pristine reduced graphene oxide. The scale bar is 100 nm. SEM images of (D) B12/G600, (E) B12/G1000, (F) Pristine reduced graphene oxide. Figure S3 HR-TEM images of (A) B12/G600, (B) B12/G800 and (C) B12/G1000.

14 Figure S4 (A) SEM, (B) TEM and (C) HR-TEM image of B12/G800A. Figure S5 Raman spectra of (A) B12/C, (B) B12/G800 and (C) B12/G800A samples.

15 Figure S6 (A) XPS survey spectra of the B12/G800 and B12/G800A. High-resolution XPS C 1s spectra of the B12/G800 and B12/G800A, i.e., (B) before and (C) after acid treatment, respectively.

16 Figure S7 SEM image of B12/G800A and corresponding EDS elemental mapping images for cobalt, nitrogen, and oxygen (scale bar, 3µm).

17 Figure S8 (A) XANES spectra and (B) k 2 -weighted EXAFS Fourier transform magnitudes at the Co K edge from the B12/G1000, B12/G800, B12/G600, raw B12 and a Co foil (k=the photoelectron wavenumber). Figure S9 Fourier transformed magnitudes of the k 2 -weighted Co K-edge EXAFS data and firstshell fit for B12/G800A (k=the photoelectron wavenumber).

18 Figure S10 (A) The polarization curves and (B) overpotentials at 10 ma cm -2 ( 10) vs. pyrolysis temperature of B12/G at 600 o C, 700 o C, 800 o C and 1000 o C in 0.5 M H2SO4 (ph = 0, scan rate: 5 mv s -1 ). (C) The polarization curves of B12/G800 catalyst with different loading amount of vitamin B12 on glassy carbon and (D) the dependence of 10 to the loading amount.

19 Figure S11 The HER polarization curves of B12/G600A, B12/G700A, B12/G800A and B12/G1000A in 0.5 M H2SO4 (ph = 0, scan rate: 5 mv s -1 ). Figure S12 The HER polarization curves of B12/G800A in 0.5 M H2SO4 by using Pt wire (black curve) and graphite (red curve) as the counter electrode, respectively. Figure S13 Tafel plots of B12/G pyrolyzed at 600 o C, 700 o C, 800 o C and 1000 o C in 0.5 M H2SO4

20 Figure S14 Cyclic voltammograms in the region of V vs. RHE at various scan rates and the corresponding linear fitting of the capacitive currents vs. scan rates to estimate the Cdl, for various samples: (A) and (B) for B12/C; (C) and (D) for B12/G600; (E) and (F) for B12/G800; (G) and (H) for B12/G1000.

21 Figure S15 Cyclic voltammograms in the region of V vs. RHE at various scan rates and the corresponding linear fitting of the capacitive currents vs. scan rates to estimate the Cdl, for B12/G800 samples with various loadings: (A) and (B) for mg cm -2 ; (C) and (D) for 1.02 mg cm -2 ; (E) and (F) for mg cm -2 ; (G) and (H) for 1.53 mg cm -2 ; and the calculated Cdl values are shown in the insets.

22 Figure S16 Electrochemical Impedance Spectroscopy of (A) B12/G800, (B) B12/C and (C) B12/G800A on GCE in 0.5 M H2SO4. Figure S17 Equivalent circuit models with two time constants for fitting the EIS response of hydrogen evolution reaction on B12/G800A electrodes, where Rs is the series resistance, Rct is the charge transfer resistance, Rp related to the porosity of the electrode surface, and the double layer capacitance is represented by the elements Cd1 and Cd2. Here Rs contains components arising from the resistance in the wiring (Rwiring), the resistance of the GCE (Rcp), the resistance of B12/G800A (RB12/G800A), the resistance between the interface of B12/G800A and GCE (Rint), and the solution resistance (Rsoln).

23 Figure S18. The calculated density of states (DOS) curves of (A) Co-N and (B) N-Co-C. Figure S19. Top view of (A) Co-N (B) N-Co-C models.

24 Figure S20 Pictures showing the water contact angles (CA) of the surface of (a) G800, (b) G800A, (c) B12/G800 and (d) B12/G800A It can be seen that both B12/G800 and RGO exhibit hydrophilic property after carbonization. However, after acid soaking, both become completely different. G800A and B12/G800A exhibit more hydrophilic nature, which demonstrates that acid treatment changes the wettability of catalyst surface. It is believed that wettability of carbons is determined by surface roughness and chemical composition. The hydrophilic property will greatly facilitate the reactants transport, further leading to enhanced activity of the B12/G800A catalyst.

25 Figure S21 The time-dependent potential curve under a constant current density of 30 and 50 ma cm -2 for 20 h.

26 Figure S22 Electrochemical performances of catalysts toward HER in the alkaline electrolytes of 1 M KOH (ph = 14). (A) Linear sweep voltammetry (LSV) curves of Pt/C, B12/G800A, B12/G800 and B12/C catalyzed HER (5 mv s -1 ), (B) Tafel plots of Pt/C, B12/G800A, B12/G and B12/C catalyzed HER, (C) LSV curves of B12/G800A on HER initial and after 1000 continuous cycles (10 mv s -1 ), and (D) Chronopotentiometry test of B12/G800A at the current density of 10 ma/cm 2.

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