Electronic Supplementary Material (ESI) for Energy & Environmental Science. This journal is The Royal Society of Chemistry 2018 Supporting Information Highly uniform Ru nanoparticles over N-doped carbon: ph and temperature-universal hydrogen release from water reduction Jing Wang, Zhongzhe Wei, Shanjun Mao, Haoran Li and Yong Wang * Advanced Materials and Catalysis Group, ZJU-NHU United R&D Center, Institute of Catalysis, Zhejiang University, Hangzhou, 310028, P. R. China. 1
Experimental Section Characterization SEM images were obtained on a SU-70 microscope. High-resolution TEM (HRTEM) was performed on Tecnai G2 F30 S-Twin at an acceleration voltage of 300 KV. Powder X-ray diffraction (XRD) patterns were measured on a D/tex-Ultima TV wide angle X-ray diffractometer equipped with Cu Kα radiation (1.54 Å). The X-ray photoelectron spectra (XPS) were obtained with an ESCALAB MARK II spherical analyzer using an aluminum anode (Al 1486.6 ev) X-ray source. The Raman spectra were collected on a Raman spectrometer (JY, HR 800) using 514-nm laser. N 2 adsorption analysis was performed at 77 K using a Micromeritics ASAP 2020 to access the surface areas and pore distributions. All the samples were outgassed at 150 o C for 8 h. The specific surface area was calculated by the conventional Brunauere-Emmette-Teller (BET) method. The pore size distribution (PSD) plot was recorded by the BJH method. The Ru content was determined by ICP-AES (Perkin Elmer Optima OES 8000) and aqua regia was used to dissolve the sample. The Ru content of Ru@CN-0.16 was 3.18 wt%. Thermogravimetric analysis (TGA) experiments were performed with METTLER TOLEDO TGA/DSC 1100SF. The sample was heated to 800 o C with a temperature ramp of 10 o C min -1 in a 20 ml min -1 N 2 or O 2 flow. Materials D-glucosamine hydrochloride (GAH), melamine and RuCl 3 were purchased from Aladdin. All the chemicals were used as delivered without further treatment. Methods Synthesis of Ru@CN: Ru@CN was synthesized by annealing the mixture of GAH-melamine-Ru, and the graphene-like morphology was formed during the pyrolysis process. Briefly, mixture solid of GAH, melamine and RuCl 3 was grinded into powder, then directly calcined by flowing N 2 of 400 ml min -1. After that, the samples were cooled to room temperature under N 2 ambient. Finally, the hybrid materials were collected from the crucible. Taking the synthesis of Ru@CN-0.16 as an example: Firstly, melamine (0.16 mole), GAH (1 g) and 1.5 ml RuCl 3 solution (Ru 10 mg ml -1 ) were mixed together adequately. The mixture was then transferred into a 30 ml-crucible. Then, the solid mixture was calcined to 800 o C (calcination process is as shown in Scheme S1) followed by an isothermal hold period of 6 h in a Muffle furnace in N 2 flow (400 ml min -1 ). After it cooled down to room temperature, loose black solid (Ru@CN) was gained. 2
The synthetic process of the contrastive samples was similar to that of Ru@CN-0.16. The contrastive samples can be prepared by tailoring the mass ratio between GAH, melamine and RuCl 3 or the calcination process. The corresponding parameters of preparation process of the contrastive samples were shown in Table S1. Scheme S1. Calcination process for samples. Computational setup The calculations reported here are performed by using periodic, spin-polarized DFT as implemented in the Vienna ab initio program package (VASP). The PBEsol functional with the van der Waals interaction correction involved and a (2 2 1) k-point grid are used to obtain more precise results. A plane wave basis set with an energy cutoff of 400 ev is used. A p (5 5) surpercell containing a four-layer slab with 100 atoms was modeled. The periodic condition is employed along the x and y directions. The vacuum space along the z direction was set to be 13 Å. The relaxation is stopped when the force residue on the atom is smaller than 0.02 ev Å 1. Table S1. The preparation parameters of different samples. Entry Samples Melamine GAH RuCl 3 Temperature Holding Time [mol] [g] [ml] [ o C] [h] 1 Ru@CN-0 0 1.0 1.5 800 6 2 Ru@CN-0.08 0.08 1.0 1.5 800 6 3 Ru@CN-0.16 0.16 1.0 1.5 800 6 4 Ru@CN-0.32 0.32 1.0 1.5 800 6 5 Ru@CN-600 0.16 1.0 1.5 600 1 3
Electrochemical measurements. The electrocatalytic properties of Ru@CN for the hydrogen evolution reaction (HER) were evaluated with a three electrode system using a CHI instrument (model 750E). Ru@CN served as working electrode and the counter- and the reference electrodes were a graphite rod electrode and a saturated calomel electrode (SCE), respectively. The potential, measured against an SCE electrode, was converted to the potential versus the reversible hydrogen electrode (RHE) according to E vs RHE = E vs SCE + E o SCE + 0.059 ph. Fabrication of the working electrode was done by pasting catalyst ink on a glassy carbon electrode (5 mm). The carbon ink was formed by dispersing 3 mg sample into the mixed solution of 10 μl ~5 wt% Nafion and 300 μl alcohol under ultra-sonication treatment for 20 min. 5 μl uniform suspension was dropped on the surface of the glassy carbon electrode, and the electrode was allowed to dry at room temperature before measurement. Linear sweep voltammetry with scan rate of 5 mv s -1 was conducted in 1 M KOH, 0.5 M H 2 SO 4 and 1 M PBS solutions. EIS impedance measurements were carried out in the configuration from 10 5-0.01 Hz in 1 M KOH. All the polarization curves are the steady-state ones after several cycles. All polarization curves were IR corrected. H 2 O + e - H* + OH - (1) Volmer (120 mv dec -1 ) H 2 O+ e - + H* H 2 + OH - (2) Heyrovsky (40 mv dec -1 ) H* + H* H 2 (3) Tafel (30 mv dec -1 ) Scheme S2. Hydrogen evolution reaction mechanism in alkaline medium. Table S2. Textural properties of the catalysts. catalyst BET surface area (m 2 /g) pore volume (cm 3 /g) pore size (nm) Ru@CN-0 120 0.01 51.1 Ru@CN-0.08 493 0.36 7.1 Ru@CN-0.16 632 0.48 6.7 Ru@CN-0.32 534 0.41 10.3 4
Table S3. Elemental analysis of the catalysts. catalyst N % C % H % Ru@CN-0 7.47 74.98 1.81 Ru@CN-0.08 9.91 64.59 2.04 Ru@CN-0.16 8.68 63.97 2.00 Ru@CN-0.32 6.28 55.50 2.32 Table S4. Ru 0 / Ru n+ content of the catalysts. catalyst a Ru 0 / % a Ru n+ / % b Ru 0 / % b Ru n+ / % Ru@CN-0.08 50.6 49.4 42.3 57.7 Ru@CN-0.16 70 30 55 45 Ru@CN-0.32 58.2 41.8 48 52 [a] The Ru content was calculated from Ru 3p spectra. [b] The Ru content was calculated from Ru 3d spectra. Table S5. Electrochemical parameters of the catalysts. catalyst Overpotential (mv) Tafel slope (mv dec -1 ) C dl (mf cm -2 ) Ru@CN-0 124 102 -- Ru@CN-0.08 104 92 24.5 Ru@CN-0.16 32 53 40.4 Ru@CN-0.32 59 64 30.6 [a] Overpotential was calculated at 10 ma cm -2. [b] C dl represented electrochemical double layer capacitances. 5
Table S6. Summary of the recently reported noble metal-based HER catalysts. Catalyst ƞ-j a Electrolyte solution Reference Ru@CN-0.16 32-10 100-10 126-10 1.0 M KOH 1.0 M PBS 0.5 M H 2 SO 4 Ru-MoO 2 29-10 1.0 M KOH Ru/C 3 N 4 /C 79-10 0.1 M KOH This work J. Mater. Chem. A. 2017, 5, 5475 J. Am. Chem. Soc. 2016, 138, 16174 Cu 2-x S@Ru 82-10 1.0 M KOH Small 2017, 13, 1700052 Pt@2D-Ni(OH) 2 123-4.2 0.1 M KOH Nano Energy 2017, 31, 456 Pd-CNx 180-5 0.5 M KOH ACS Catal. 2016, 6, 1929 Ru@C 2 N 17-10 1.0 M KOH PtNi NWs/C ~47-10 1.0 M KOH Nat. Nanotech. 2017, 12, 441 Angew. Chem. Int. Ed. 2016, 55, 12859 PNCH 22-5 0.1 M KOH Nanoscale, 2016, 8, 16379 Pt/Ni@NGNTs 193-50 1.0 M KOH Cu c -Pt s NW 650-1.15 0.2 M PBS Pt-Ru-Mo 196-10 Seawater Au-aerogel-CN x 185-10 0.5 M H 2 SO 4 J. Mater. Chem. A 2017, 5, 16249 Energy Environ. Sci., 2014, 7, 1461 J. Mater. Chem. A. 2016, 4, 6513 J. Mater. Chem. A. 2015, 3, 23120 Au@NC 130-10 0.5 M H 2 SO 4 Angew. Chem. Int. Ed. 2016, 55, 8416 Ru/SiNWs 200-10 0.5 M H 2 SO 4 Electrochem. Commun. 2015, 52, 29 NiAu 175-10 0.5 M H 2 SO 4 J. Am. Chem. Soc. 2015, 137, 5859 Ru/GLC 35-10 0.5 M H 2 SO 4 ACS Appl. Mater. Interfaces 2016, 8, 35132 Ru-CNT 63-4.874 0.5 M H 2 SO 4 Int. J. Hydrogen Energy 2016, 41, 23007 PdCu@Pd 68-10 0.5 M H 2 SO 4 ACS Appl. Mater. Interfaces 2017, 9, 8151 GCE-S-CNs-1000-C B-Ru ~80-10 0.5 M H 2 SO 4 Carbon 2015, 93, 762 [a] ƞ represents the overpotential calculated at the current density of j (j/ma cm -2 ). 6
Fig. S1. HRTEM images of Ru@CN-0.16. Fig. S2. XRD patterns of Ru@CN-0.16. 7
Fig. S3. a) C 1s spectra of Ru@CN. b) O 1s spectra of Ru@CN. c) Ru 3d spectra of Ru@CN. d) The enlarged Ru 3d 5/2 spectra of Ru@CN. The O 1s spectra in Fig. S3b showed that the oxygen species in Ru@CN belonged to the di-ketonic edge groups in N-doped carbon, physically adsorbed and/or trapped oxygen and moisture. 1 Besides, Fig. S3c showed the characteristic binding energy for Ru 3d 3/2 at 284.5 ev in the Ru@CN, which was partly overlapped with C 1s signal. 2 The composition of Ru was only studied via the Ru 3d 5/2 XPS signal. From Fig. S3d, the Ru@CN contains Ru n+ and Ru 0 with 3d 5/2 binding energies of ~281.3 ev and ~280.3 ev, respectively. 3 By calculating from the deconvoluted Ru 3d 5/2 spectra (Table S4), the Ru 0 content was gradually increased in an order of Ru@CN-0.08, Ru@CN-0.32, Ru@CN-0.16, which was in line with the catalytic performance. Combined with the data of Ru 3p and Ru 3d spectra, we can conclude that the inherent high Ru 0 ratio plays a vital role in the outstanding HER catalytic performance. 8
Fig. S4. N 2 adsorption/desorption isotherms: a) Ru@CN-0.08; b) Ru@CN-0.16; c) Ru@CN-0.32. Fig. S5. XRD pattern of Ru@CN-600. 9
Fig. S6. FTIR spectrum of Ru@CN-600. Fig. S7. TEM images of a) Ru@CN-0, b) Ru@CN-0.08, d) Ru@CN-0.32. c, e) Metal particle size distribution histogram of Ru@CN-0.08, Ru@CN-0.32. 10
Fig. S8. a, c, e) Cyclic voltammetry curves of Ru@CN-0.08, Ru@CN-0.16, Ru@CN-0.32, respectively. The purple arrow indicates the scan rate from 10 mv to 50 mv. b, d, f) The differences in current density variation ( J=Ja-Jc) at an overpotential of 0.152 V plotted against scan rate fitted to a linear regression enables the estimation of C dl, where the slope is twice C dl. 11
Fig. S9. Tafel curves of Ru@CN-0 in 1 M KOH. Fig. S10. LSV curves of Ru@CN-0.16 under different initial voltages. 12
Fig. S11. Polarization curves for commercial Pt/C under different temperature. Fig. S12. Polarization curves for commercial Pt/C and Ru@CN-0.16 under 60 o C. 13
Fig. S13. i-t test for Ru@CN-0.16 under room temperature, the blue arrow represents the electrode was washed and the solution was replaced with fresh electrolytes. Fig. S14. a) Chronoamperometric response of Ru@CN-0.16. b) Chronoamperometric response of Pt/C. c) Relative current-time (i-t) chronoamperometric response of Ru@CN-0.16 and Pt/C. The test was operated in 1 M KOH, 60 o C. 14
Fig. S15. a) TEM image of Ru@CN-0.16 after continuous two times long-term test under room temperature, in 1 M KOH. b) The corresponding Ru particle size distribution histogram from figure a. Fig. S16. a, b) TEM images of Ru@CN-0.16 after long-term test under 60 o C, in 1 M KOH. c) HRTEM image of Ru@CN-0.16 after long-term test under 60 o C, in 1 M KOH. d) Ru particle size distribution histogram of Ru@CN-0.16 after long-term test under 60 o C, in 1 M KOH. 15
Fig. S17. a, b) TGA curves of Ru@CN-0.16 under O 2 and N 2 atmosphere, respectively. In order to access the metal loading, the Ru@CN-0.16 was heated to 800 o C in oxygen flow to burn off the carbon residue. As shown in Fig. S17a, in the initial stage of heating, the mass loss was attributed to the adsorption of H 2 O in the Ru@CN-0.16. Further rising the temperature to ~400 o C, there is a sharp weight loss owing to the carbon burn off. When the temperature went up to around 475 o C, the weight of residue (namely RuO 2 ) remained stable. On the basis of TGA data, the calculated Ru loading in the material was 3.0 wt%, which was close to the result of ICP (3.18 wt%) data. Additionally, the stability of Ru@CN-0.16 was evaluated by TGA in N 2. From Fig. S17b, the mass loss in the initial stage of heating was attributed to the adsorption of H 2 O. Further increasing the temperature to 800 o C, there is about ~15% weitht loss, suggesting the good stability of Ru@CN-0.16. Reference 1. J. Mahmood, F. Li, S.-M. Jung, M. S. Okyay, I. Ahmad, S.-J. Kim, N. Park, H. Y. Jeong and J.-B. Baek, Nat. Nanotech., 2017, 12, 441-446. 2. W. Luo, M. Sankar, A. M. Beale, Q. He, C. J. Kiely, P. C. A. Bruijnincx and B. M. Weckhuysen, Nat. Commun., 2015, 6, 6540-6549. 3. J. Ftouni, A. Munoz-Murillo, A. Goryachev, J. P. Hofmann, E. J. M. Hensen, L. Lu, C. J. Kiely, P. C. A. Bruijnincx and B. M. Weckhuysen, ACS Catal., 2016, 6, 5462-5472. 16