An efficient and ph-universal ruthenium-based catalyst for the hydrogen evolution reaction

Transcription

1 In the format provided by the authors and unedited. DOI: /NNANO An efficient and ph-universal ruthenium-based catalyst for the hydrogen evolution reaction Javeed Mahmood, Feng Li, Sun-Min Jung, Mahmut Sait Okyay, Ishfaq Ahmad, Seok-Jin Kim, Noejung Park*, Hu Young Jeong*, Jong-Beom Baek* NATURE NANOTECHNOLOGY 1

2 Table of the contents 1. Materials and instrumentations 2. Supplementary figures 3. Supplementary tables 4. Supplementary References NATURE NANOTECHNOLOGY 2

3 1. Materials and Instrumentations. All solvents, chemicals and reagents were purchased from Aldrich Chemical Inc., unless otherwise stated. Solvents were degassed with nitrogen purging prior to use. All reactions were performed under nitrogen atmosphere using oven dried glassware. 1,2,3,4,5,6-hexaaminobenzene (HAB) was synthesized according to a procedure described in the literature 1. Thermogravimetric analysis (TGA) was conducted in air and in nitrogen atmosphere at a ramping rate of 10 C/min using a Thermogravimetric Analyzer Q200 TA Instrument, USA. The surface area was calculated by nitrogen adsorption-desorption isotherms using the Brunauer- Emmett-Teller (BET) method on BELSORP-max (BEL Japan, Inc., Japan). Scanning electron microscope (SEM) images were taken with a Field Emission Scanning Electron Microscope Nanonova 230 FEI, USA. X-ray photoelectron spectroscopy (XPS) was performed on an X-ray Photoelectron Spectrometer Thermo Fisher K-alpha (UK). X-ray diffraction (XRD) studies were taken on a High-Power X-Ray Diffractometer D/MAZX 2500V/PC (Cu Kα radiation, 35 kv, 20 ma, λ = Å) Rigaku, Japan. High-resolution transmission electron microscopy (HR-TEM) was performed by using a JEM-2100F microscope (JEOL, Japan) under an operating voltage of 200 kev. Atomic-resolution transmission electron microscopy (AR-TEM) images were taken using FEI Titan G equipped with an imaging-side spherical aberration (Cs) corrector operating at an accelerating voltage of 80 kv. Electron energy loss spectroscopy (EELS) spectra were obtained using a Gatan Imaging Filter (Quantum 965). Elemental analysis was conducted with a Thermo Scientific Flash 2000 Analyzer. NATURE NANOTECHNOLOGY 3

4 Supplementary Fig. 1 Photograph of as-prepared Ru@C2N catalyst on filter paper. NATURE NANOTECHNOLOGY 4

5 Supplementary Fig. 2 SEM images of Ru@C2N at different magnification. a, b, NATURE NANOTECHNOLOGY 5

6 Supplementary Fig. 3 XPS spectra of as-prepared Ru@C 2N: a, survey spectra, b, C 1s and Ru 3d, c, N 1s, d, O 1s and e, Ru 3p. Note that it is difficult to analyze the energy range around 285 ev accurately, which can be attributed to overlapping of the C 1s and Ru 3d3/2 peaks. The Ru(0) was further confirmed by the peaks at about and ev, belonging to Ru 3p3/2 and Ru 3p1/2 of Ru(0). NATURE NANOTECHNOLOGY 6

7 Supplementary Fig. 4 XPS spectra of C 2N: a, full survey spectra, b, C 1s, c, N 1s and d, O 1s. NATURE NANOTECHNOLOGY 7

8 Supplementary Fig. 5 a, b, HAADF and BF STEM images of Ru@C2N. c, d, High resolution HAADF and BF STEM images of Ru@C2N. NATURE NANOTECHNOLOGY 8

9 Supplementary Fig. 6 TGA curves of as-prepared Ru@C2N under air and nitrogen atmosphere with a ramping rate of 10 C min 1. NATURE NANOTECHNOLOGY 9

10 Supplementary Fig. 7 TGA thermograms of as-prepared samples under air atmosphere with a ramping rate of 10 C min 1. a, Ni@C2N, b, Co@C2N, c, Pd@C2N and d, Pt@C2N NATURE NANOTECHNOLOGY 10

11 Supplementary Fig. 8 N2 adsorption/desorption isotherms of C2N. The inset is the corresponding pore size distribution. NATURE NANOTECHNOLOGY 11

12 Supplementary Fig. 9 Nyquist plots of Ru@C 2N annealed at 900 C: a, in 0.5 M aq. H2SO4, b, in 1.0 M aq. KOH. The solution resistances (Rs) are approximately 9 Ω in both 0.5 M aq. H2SO4 and 1.0 M aq. KOH solutions. NATURE NANOTECHNOLOGY 12

13 Supplementary Fig. 10 a, Polarization curves of C2N, Ru/C and electrocatalysts in 0.5 M H2SO4 solution. b, Metal mass normalized polarization curves of Ru/C and Ru@C2N electrocatalysts in 0.5 M H2SO4 solution. Electrode rotation speed: 1600 r.p.m.; scan rate: 5 mv s 1. The HER performances of C2N and Ru/C (~ 10 wt%) were also investigated. As shown, C2N showed little activity even at an overpotential of 0.5 V, while Ru/C exhibited relatively worse performance (Supplementary Fig. 10a). Polarization curves of Ru/C and Ru@C2N were further normalized by the metal mass, indicating the much better mass activity of Ru@C2N (Supplementary Fig. 10b). NATURE NANOTECHNOLOGY 13

14 Supplementary Fig. 11 Polarization curves of electrocatalysts with respect to annealed temperature at 800, 900 and 1000 C. NATURE NANOTECHNOLOGY 14

15 Supplementary Fig. 12 a, Electrochemical impedance spectroscopy (EIS) curves of various electrocatalysts. b, Calculated charge transfer resistances of various electrocatalysts. NATURE NANOTECHNOLOGY 15

16 Supplementary Fig. 13 Durability test of annealed at 900 C in 0.5 M aq. H2SO4 and 1.0 M aq. KOH solutions respectively. The polarization curves were recorded before and after potential cycles from 0.2 to 0.1 V (vs. RHE). NATURE NANOTECHNOLOGY 16

17 Supplementary Fig. 14 Current-time curve of before and after the addition of SCN ions in 0.5 M aq. H2SO4 solution. Before the addition of SCN ions, the active sites of the Ru nanoparticles are available for proton adsorption and the release of H2. After the addition of SCN ions, the SCN ions bind to the Ru active sites of Ru@C2N and block the active sites for HER catalysis, which further suppresses the HER activity of Ru@C2N. NATURE NANOTECHNOLOGY 17

18 Active sites measurements: The underpotential deposition (UPD) of copper on Pt and Ru has proven to be an ideal method for qualifying their corresponding active sites 2-4. In this approach, the number of active sites (n) can be qualified based on the UPD copper stripping charge (QCu, Cuupd Cu e ) with the following equation: n=qcu/2f where F is the Faraday constant (C mol 1 ). Calculation of the turnover frequency (TOF) The TOF (s 1 ) can be calculated with the following equation: TOF=I/(2Fn) where I is the current (A) during linear sweep measurement, F is the Faraday constant (C mol 1 ), n is the number of active sites (mol). The factor 1 /2 is based on the consideration that two electrons are required to form one hydrogen molecule. NATURE NANOTECHNOLOGY 18

19 Supplementary Fig. 15 a, Copper UPD in 0.5 M H2SO4 in the (I) absence and (II-VI) presence of 5 mm CuSO4 on Ru@C2N. For II-VI, the electrode was polarized at 0.225, 0.220, 0.215, and V for 100 s to form the UPD layers, respectively. b, Copper UPD in 0.5 M H2SO4 in the (I, II) absence and (III) presence of 5 mm CuSO4 on Ru@C2N. For II and III, the electrode was polarized at V for 100 s to form the UPD layer. c, Copper UPD in 0.5 M H2SO4 in the (I, II) absence and (III) presence of 5 mm CuSO4 on Pt/C. For II and III, the electrode was polarized at 0.3 V for 100 s to form the UPD layer. d, Cycling voltammetry of Pt/C in 0.5 M H2SO4. Scan rate: 10 mv s 1. NATURE NANOTECHNOLOGY 19

20 As shown in Supplementary Fig. 15a, was first polarized at 0.225, 0.220, 0.215, and V for 100 s, respectively. Under the polarization potentials of 0.225, and V, there is only one oxidation peak around 0.36 V, which belongs to the underpotentially deposited mono- or submonolayer copper. When the potential was decreased to and V, another oxidation peak appeared, which can be attributed to the oxidation of bulk copper. To obtain a monolayer copper, V was selected in the following test of Ru@C2N (Supplementary Fig. 15b). Pt/C was also investigated using the same method (Supplementary Fig. 15c). As a comparison, Pt/C was further characterized by the underpotential deposition of hydrogen (Supplementary Fig. 15d). NATURE NANOTECHNOLOGY 20

21 Supplementary Fig. 16 The number of active sites in and Pt/C are calculated from the Cu-UPD and H-UPD in Supplementary Fig. 15. Based on the above information, the number of active sites in and Pt/C were calculated to be mol g 1 (Cu-UPD) and 0.474/ mol g 1 (Cu-UPD/H-UPD), respectively. The difference between the number of active sites calculated using Cu-UPD and H- UPD is less than 2%, indicating that the number of active sites obtained from Cu-UPD is reliable. NATURE NANOTECHNOLOGY 21

22 Supplementary Fig. 17 a, CO stripping voltammetry of Pt/C in 0.5 M aq. H2SO4. Stripping of a monolayer of CO in the first cycle (orange curve). Following cycle after the stripping of CO (black curve). b, CO stripping voltammetry of Ru@C2N in 0.5 M aq. H2SO4. Stripping of a monolayer of CO in the first cycle (purple curve). Following cycle after the stripping of CO (black curve). Scan rate: 10 mv s 1. CO stripping experiment was carried out with the method reported by Gasteiger et al. 5-7 CO adsorption was conducted in 0.5 M H2SO4 with CO bubbling for 20 min. After equilibrated for 10 min, the electrolyte was saturated with nitrogen by bubbling nitrogen for 30 min. During all the above processes, the potential was held at 75 mv. As shown in Supplementary Figure 17a, the oxidation peak at 0.79 V can be attributed to the CO stripping peak. The number of active sites in Pt/C was further calculated to be mol g 1, while that in Ru@C2N is mol g 1. The numbers of active sites obtained from CO stripping showed similar values with those obtained from Cu-UPD ( mol g 1 for Pt/C, mol g 1 for Ru@C2N). NATURE NANOTECHNOLOGY 22

23 Supplementary Fig. 18 Hydrogen adsorption configuration on the surfaces: a, Pt(111) facet, b, Ru(001) facet, c, Ru4, d, Ru13, e, Ru55 nanoparticles and f, as well as corresponding H binding energies in acidic solution. Purple, orange, light grey, sky blue and dark grey ivory balls represent platinum, ruthenium, hydrogen, nitrogen and carbon atoms, respectively. Ru nanoparticles contain 4, 13 and 55 Ru atoms are denoted as Ru4, Ru13 and Ru55, respectively. NATURE NANOTECHNOLOGY 23

24 Supplementary Fig. 19 Hydrogen adsorption configuration on the holes of C2N with and without the Ru13, as well as corresponding H binding energies. Dark grey, orange, sky blue and light grey ivory balls represent carbon, ruthenium, nitrogen, and hydrogen atoms, respectively. NATURE NANOTECHNOLOGY 24

25 Supplementary Fig. 20 H 2O adsorption configuration on the surfaces: a, Ru(001) facet, b, Ru55 nanoparticles c, Ru55@C2N (top), d, Ru55@C2N (near surface), e, Pt55 nanoparticles and f, Pt(111) facet, as well as corresponding binding energies in alkaline solution. Purple, orange, light grey, sky blue and dark grey ivory balls represent platinum, ruthenium, hydrogen, nitrogen and carbon atoms, respectively. NATURE NANOTECHNOLOGY 25

26 Supplementary Fig. 21 Adsorbed H 2O dissociation configuration on the surfaces: a, Ru(001) facet, b, Ru55 nanoparticles c, Ru55@C2N (top), d, Pt55 nanoparticles and e, Pt(111) facet, as well as corresponding binding energies in alkaline solution. Purple, orange, light grey, sky blue and dark grey ivory balls represent platinum, ruthenium, hydrogen, nitrogen and carbon atoms, respectively. NATURE NANOTECHNOLOGY 26

27 Supplementary Fig. 22 Hydrogen adsorption configuration on the surfaces: a, Ru(001) facet, b, Ru55 nanoparticles c, (top), d, (near surface), e, Pt55 nanoparticles and f, Pt(111) facet, as well as corresponding hydrogen binding energies in alkaline solution. Purple, orange, light grey, sky blue and dark grey ivory balls represent platinum, ruthenium, hydrogen, nitrogen and carbon atoms, respectively. NATURE NANOTECHNOLOGY 27

28 Supplementary Fig. 23 OH adsorption configuration on the surfaces: a, Ru(001) facet, b, Ru55 nanoparticles c, (top), d, (near surface), e, Pt55 nanoparticles and f, Pt(111) facet, as well as corresponding binding energies in alkaline solution. Purple, orange, light grey, sky blue and dark grey ivory balls represent platinum, ruthenium, hydrogen, nitrogen and carbon atoms, respectively. We additionally investigated the HER reactivity of Pt and Ru metal surfaces and nanoparticle surfaces in alkaline solution using density functional theory (DFT) energetic calculations. The binding energies of the H, OH and H2O, as well as H2O dissociation were calculated on the above mentioned surfaces. As model systems, three layers of 3 3 Pt(111) and Ru(0001) were used for NATURE NANOTECHNOLOGY 28

29 the metal surfaces, Pt55 and Ru55 were used for the nanoparticles, and Ru55 placed in one hole of a 2 2 C2N sheet was used for Ru(NP)@C2N. The nanoparticle size was set at 55, which is identical to nm diameter, since the experimental results show a nm average nanoparticle diameter. HER (2H + + e H2) is a multi-step electrochemical process taking place on the surface of an electrode that generates gaseous hydrogen. The generally accepted reaction mechanisms in acidic and alkaline solutions are: Electrochemical hydrogen adsorption, the Volmer reaction: H + + M + e M-H* H2O + M + e M-H* + OH (acidic solution) (alkaline solution) Followed by chemical desorption, the Tafel reaction: 2M-H* 2M + H2 (both acidic and alkaline solution) The reaction pathway in alkaline solution involves the adsorption of H2O molecule, electrochemical reduction of adsorbed H2O into adsorbed OH and H atom, desorption of OH to refresh the surface and formation of H adsorbed intermediate for H2 generation The bonding energies of H2O, H and OH, as well as H2O dissociation on Pt and Ru metal and nanoparticle surfaces were examined to find the HER activity in alkaline solution based on electrochemical hydrogen adsorption (the Volmer reaction). The goal of simulation is to produce M-H* from an M substrate in alkaline solution. That's why the Volmer reaction should proceed from left to right. To examine the three bonds strengths and H2O dissociation as well as their roles in determining the direction of the reaction. First, if the M-H2O binding energy is low, the reaction would prefer the left hand side, which is not the desired case. A higher M-H2O binding energy is preferable, since the attraction between the metal surface and H2O increases the proton source supply, which NATURE NANOTECHNOLOGY 29

30 will further increase the probability of shifting the Volmer reaction from the left to the right side. Second, the dissociation of H2O into H and OH on the surface of catalyst should be fast enough, leading to fast proton supply for the HER. Third, the M-H bond needs to be strong enough to expedite M-H* creation. Finally, the M-OH binding energy should be low, making the refresh of the surface much easier. Pt metal and nanoparticle surfaces have moderate H and H2O binding energies and quite low OH binding energy as summarized in Supplementary Table 3. From all bonding point of views, Pt is a good candidate for HER in both acidic and alkaline solutions. Ru metal and nanoparticle surfaces also have moderate H and H2O biding energies. However, Ru shows strong attraction to OH compared with Pt. Even though it is less than the Ru-H strength, the attraction decreases HER efficiency. When Ru nanoparticles are placed in holes of the C2N sheet in a basic solution, the C2N helps stabilize Ru by increasing its attraction to H2O. The bond strengths of H and OH are not significantly different in the presence or absence of C2N. However, H2O binding energy is dramatically increased in Ru@C2N. This strong attraction stabilizes nanoparticles on the C2N surface and increases the H2O capture rate of the Ru nanoparticle. Furthermore, the dissociation of H2O on the surface of Ru is much easier than that on the surface of Pt, as shown in Supplementary Figure 22, leading to the much faster proton supply for the HER. These help to overcome the efficiency loss in the Volmer reaction caused by strong OH binding. These results are in consistent with the experimental findings. NATURE NANOTECHNOLOGY 30

31 Supplementary Fig. 24 Current potential curves of Pt black in highly pure H2-saturated 0.5 M aq. H2SO4 and 1.0 M aq. KOH solutions, used for calibration of the Ag/AgCl electrode with respect to RHE. Scan rate: 1 mv s 1. NATURE NANOTECHNOLOGY 31

32 Supplementary Table 1 Summary of some recently reported representative HER electrocatalysts in acidic electrolytes Catalyst Ru@C2N [a] [Mo3S13] 2 clusters [a] CoPS nanoplate [a] Catalyst loading amount mg cm 2 Current density (ma cm 2 ) Overpotential at corresponding j (mv) Tafel slope (mv decade 1 ) 0.1 mg cm SV-MoS2 [a] Exfoliated WS2 nanosheets [a] 6.5 μg cm 2 10 ~ CoMoSx [b] 50 μg cm 2 5 ~ Mesoporous MoS2 [a] A-Ni C [a] 60 μg cm 2 10 ~ mg cm CoS P/CNT [a] 1.6 mg cm References 30 This work 41 Chem. 6, (2014) Mater. 14, (2015) Mater. 15, (2016) Mater. 12, (2013) Mater. 15, (2016) Mater. 11, (2012) Comm. 7, (2016) Comm. 7, (2016) NATURE NANOTECHNOLOGY 32

33 (Continue) M-MoS2 [a] 43 μg cm WO2.9 [a] MoS2/CoSe2 [a ] mg cm mg cm CoNx/C [a] 2 mg cm Edgeterminated MoS2 [a] 0.28 mg cm C3N4@NG [a] 0.1 mg cm Co-NG [a] Pt NWs/SL- Ni(OH)2 [a] MoCx nanooctahedrons [a] mg cm mg cm Pt-MoS2 [a] 75 μg cm Pt MLAg NF/Ni foam [a] Ni-Mo-S nanosheet [a] ~ mg cm 2 [a] The acidic electrolyte is 0.5 M aq. H2SO Comm. 7, (2016) Comm. 6, 8064 (2015) Comm. 6, 5982 (2015) Comm. 6, 7992 (2015) Comm. 6, 7493 (2015) Comm. 5, 3783 (2014) Comm. 6, 8668 (2015) Comm. 6, 6430 (2015) Comm. 6, 6512 (2015) Comm. 4, 1444 (2013) Sci. Adv. DOI: /sciadv Sci. Adv. DOI: /sciadv [b] The acidic electrolyte is 0.1 M aq. HClO4. NATURE NANOTECHNOLOGY 33

34 Supplementary Table 2 Summary of some recently reported representative HER electrocatalysts in alkaline electrolytes Catalyst Ru@C2N [a] CoMoSx [b] Co(OH)2/Pt(111) [b] Catalyst loading amount mg cm 2 50 μg cm 2 Current density (ma cm 2 ) Overpotential at corresponding j (mv) Tafel slope (mv decade 1 ) 5 ~ ~ np-cuti [b] ~ CoNx/C [b] MoCx nanooctahedrons [a] NiO/Ni-CNT [a] NiFeOx/CFP [a] 2 mg cm mg cm mg cm mg cm 2 [a] The basic electrolyte is 1.0 M aq. KOH. [b] The basic electrolyte is 0.1 M aq. KOH ~ References 38 This work Mater. 15, (2016) Mater. 11, (2012) Comm. 6, 6567 (2015) Comm. 6, 7992 (2015) Comm. 6, 6512 (2015) Comm. 5, 4695 (2014) Comm. 6, 7261 (2015) NATURE NANOTECHNOLOGY 34

35 Supplementary Table 3 Binding energies of H2O, H and OH on Pt and Ru substrates in alkaline solution Pt(111) (ev) Pt55 (ev) Ru(001) (ev) Ru55 (ev) Ru55@C2N (top) (ev) Ru55@C2N (near surface) (ev) H2O H OH Binding energies are the negative of the reaction energy ( E) which is calculated by E = Eproduct Ereactant. 1 /2H2 is taken as the reference reactant for H bonding, and OH is for the OH bonding. The binding energy varies depending on the location of the adsorption site on the nanoparticle, between the top of the nanoparticle on C2N, and close to the C2N. NATURE NANOTECHNOLOGY 35

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