A High Performance Ru@C4N Electrocatalyst for Hydrogen Evolution

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Energy, Environmental, and Catalysis Applications

A High Performance Ru@C4N Electrocatalyst for Hydrogen Evolution Reaction in Both Acidic and Alkaline Solutions Shu-Wen Sun, Gao-Feng Wang, Yue Zhou, Feng-Bin Wang, and Xing-Hua Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04255 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

A High Performance Ru@C4N Electrocatalyst for Hydrogen Evolution Reaction in Both Acidic and Alkaline Solutions Shu-Wen Suna,b, Gao-Feng Wangb, Yue Zhoua, Feng-Bin Wanga, Xing-Hua Xiaa* a

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry

and Chemical Engineering, Nanjing University, Nanjing 210023, China. b

Department of Applied Chemistry, Yuncheng University, Yuncheng 044000, China.

ABSTRACT: We report a high performance Ru@C4N electrocatalyst for hydrogen evolution reaction in both acidic and alkaline solutions. This catalyst is synthesized by annealing a complex of covalent organic framework (COF) compound coordinated with ruthenium synthesized by a “one-pot” solvothermal method. This Ru@C4N catalyst shows excellent electrocatalytic activity toward the hydrogen evolution reaction (HER) in both acidic and alkaline solutions with very low overpotentials at 10 mA cm-2 (6 mV in 0.5 M H2SO4 solution; 7 mV in 1.0 M KOH solution), which outperform commercial catalyst Pt/C. The Ru@C4N electrocatalyst also exhibits high HER turnover frequencies of 0.93 H2 s-1 in 0.5 M H2SO4 and 0.65 H2 s-1 in 1.0 M KOH solutions at 25 mV as well as superior performance stability. KEYWORDS:

Hydrogen

evolution

reaction

(HER);

Ru@C4N;

ruthenium

nanoparticles; electrocatalyst; high performance. INTRODUCTION Hydrogen energy has been considered as one of the most potential clean energies. Electrolysis of water is an important and efficient approach to produce high purity hydrogen. A major premise for abundant hydrogen is to obtain low cost and high active catalysts as the electrode materials of hydrogen evolution reaction (HER). Platinum (Pt) and Pt-based noble metal catalysts1-3 are the most ideal electrode materials which show active HER efficiency. However, their rocketing cost and limited supply on the earth 1

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limit their widely industrialization in a large scale. Thus, it is highly desirable to design alternative efficient non-Pt metal-based catalysts.4-5 Up to date, various non-Pt metal-based catalysts have been reported in the energy conversion field, mainly including transition metal oxide,6-7 carbide,8-10 nitride,11-12 sulfide,13-15 selenide,16-17phosphide,18-20 hydroxide.21-22 These catalysts yet suffer from insufficient stability and easier deactivation, which limit their practical application for industry. Ruthenium (Ru) and carbon complex as middle-priced catalysts have the advantages of high hydrogen turnover efficiency and long-term performance stability. It has been reported that nitrogen doping increases the electron-donor properties of the carbon-based materials, improving the interactions between the carbon and metallic catalysts. For instance, two-dimensional material C2N can efficiently disperse Ru nanoparticles (Ru NPs) inside the molecular holes.23 However, the synthesis of this catalyst requires harsh process involving anhydrous anaerobic synthesis method and strong reductive agent of NaBH4. Solvothermal method performs under relatively mild conditions, showing more advantages like easy and faster as compared to the anhydrous anaerobic synthesis method. In addition, the mild conditions could improve the stability of the carbon-based metal complex and intensify the host-guest interactions, however, the metal particles size cannot be easily controlled. In this study, we propose a “one pot” solvothermal method (Figure S1) to synthesize covalent organic framework (COF) as C4N precursor via the polycondensation reaction of 3, 3', 4, 4'-biphenyltetramine (BPTA) with hexaketocyclohexane (HKH) in the presence of MeOH and N-methyl-2-pyrrolidone (NMP). In this synthesis approach, ruthenium chloride (RuCl3) as the Ru precursor can be coordinated into the C4N layers in situ, forming an initial product (Raw Ru@C4N, Figure 1). After washing, the Raw Ru@C4N was annealed in Ar atmosphere at high temperature. The product was labeled as Ru@C4N. The resulting Ru@C4N catalyst shows an extremely high activity toward HER in both acidic and alkaline solutions.

2


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Figure 1. Schematic illustration of the synthesis and the structure of Raw M@C4N (M = Ru, Co, Ni). EXPERIMENTAL Reagents and Apparatus. Ruthenium (III) chloride hydrate (RuCl3 3H2O, 99.99% metal basis), 3, 3', 4, 4'-biphenyltetramine (BPTA), hexaketocyclohexane (HKH) and Nafion (5 wt%) were purchased from Sigma-Aldrich. Cobalt chloride hexahydrate (CoCl2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), concentrated sulfuric acid, isopropanol, methanol and ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were of analytical grade and used without further purification. Solutions were freshly prepared with Millipore water with a resistivity of 18.2 MΩ (Purelab Classic Corp., USA). The infrared absorption spectra in the range of 4000-400 cm-1 were recorded on a TERSOR 27 Fourier transform infrared spectrometer (Germany). Thermal stability test was carried out in air atmosphere on a thermogravimetric analysis (TGA) (NETZSCH STA449F3, Germany). The morphologies of samples were collected using a fieldemission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) with an accelerating voltage of 10 kV. Thickness was measured on an atomic force microscope (AFM, Bruker, Germany) with tapping mode. Transmission electron microscopic and HRTEM images were collected on Cu-grids with a transmission electron microscopy (TEM, JEM-200CX, Japan) and a Hitachi-2100 TEM facility using a 200 kV accelerating voltage, respectively. X-ray patterns were recorded on an X-ray powder diffractometer (XRD, Shimadzu, X-6000, Cu KR radiation). Raman scattering 3



characterization was performed on a Renishaw InVia micro Raman system (excitation wavelength of 514 nm) at room temperature. X-ray photoelectric spectroscopic (XPS) characterization was carried out on a PHI 5000 Versa Probe (Japan). The high resolution spectra were fitted using a ~20% of Lorentzian-Gaussian peak shape. Brunauer Emmett Teller (BET) analysis was performed on a Micromeritics ASAP2020 (USA) to calculate the specific surface area and pore size distribution of the samples. All NMR characterizations were carried out on a Varian Infinity-plus 400 Spectrometer (USA) with an operating magnetic field strength of 9.4 T. The resonance frequencies of 13C and 1H at this field strength were 100.6 and 400.1 MHz, respectively. Carbon and Nitrogen weight percentages were measured on a Heraeus CHN-O-Rapid elemental analyzer (Germany). Inductively coupled plasma-atomic emission spectroscopic (ICPAES) was measured on an Optima 5300DV plasma spectral mass spectrometer (PE, USA). Synthesis of C4N. BPTA (160 mg, 0.75 mmol) and HKH (156 mg, 0.5 mmol) were added to 25 ml mixed solution (MeOH: NMP, 1: 4) and dispersed by sonication for 10 min, then transferred to a reaction kettle and kept at 175 oC for 24 h. After that, the solution was centrifuged. The residue was washed by water and methanol for 3 times, respectively. The dried black solid sample was named as Raw C4N. The Raw C4N was annealed at 900 oC in argon (Ar) atmosphere at a heating rate of 10 oC/min. After slowly cooling to room temperature, C4N-900 was obtained. Synthesis of M@C4N (M=Ru, Co, Ni). Raw Ru@C4N was synthesized by using the same method for Raw C4N besides that RuCl3·3H2O (54 mg) was added to the mixed solution during the initial solvothermal reaction. After washing and drying, the products were annealed at different temperatures of 800, 900 and 1000 oC, and the final products were labeled as Ru@C4N-800, Ru@C4N-900 and Ru@C4N-1000, respectively. Co@C4N-900 and Ni@C4N-900 were prepared like Ru@C4N with annealing temperature of 900 oC except CoCl2·6H2O (62 mg) and NiCl2·6H2O (62 mg) were respectively added instead of RuCl3·3H2O during the initial solvothermal reaction. Electrochemical characterization. Electrochemical experiments were performed with 4


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a three-electrode configuration on a CHI 660C electrochemical workstation (CHI Instruments, China). To prepare the working electrode, 4 mg electrocatalyst and 7.5 µL Nafion were first ultrasonically dispersed in 250 µL water/isopropanol (v:v=3:1). The suspension (12 µL, corresponding to a loading amount of 2.7 mg/cm2) was casted on a cleaned glassy carbon electrode (GCE, 3 mm diameter). Linear sweep voltammetry (LSV) at 2 mV/s was carried out in N2 saturated 0.5 M H2SO4 or 1.0 M KOH aqueous solution. An Ag/AgCl (3M KCl) electrode and graphite rod (Alfa Aesar, 99.9995%) were respectively used as the reference and counter electrodes. The modified GCE was used as the working electrode. The potential of the reference electrode was calibrated to the reversible hydrogen electrode (RHE) to be 0.238 V in 0.5 M H2SO4 and 1.024 V in 1.0 M KOH aqueous solutions, respectively. All the potentials were converted to values verse RHE. All polarization curves were collected after 30 min N2 purge with iR correction. Electrochemical impedance spectroscopy (EIS) measurement was carried out within a frequency range 0.01-100,000 Hz. Electrochemical active site density and turnover frequency (TOF) were determined according to the literatures.24-25 The electrochemical active site density of Ru samples was determined from the method of copper underpotential deposition (UPD). The Ru@C4N-900 modified GCE (Ru@C4N900/GCE) was polarized at 0.190~0.225V in a solution of 0.5 M H2SO4 + 20 mM CuSO4 + 60 mM NaCl for 100 s to form the UPD layers. After that, LSVs were carried out for stripping the Cu UPD layers. Plotting the charge of the stripping peaks as a function of the polarization potential occurs a plateau. At the deposition potentials corresponding to the plateau, a full monolayer of Cu UPD can be formed. The charge at the plateau was used to calculate the electrochemical active site density. RESULTS AND DISCUSSION Characterization of C4N and Ru@C4N. To confirm the synthesis of covalent organic framework (COF) compounds C4N and Ru@C4N via a one pot polycondensation reaction, characterizations with infrared spectroscopy and

13

C solid-state NMR were

performed. The IR spectra of the starting material HKH (Figure S2a) and BPTA (Figure 5



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S2b) were collected for comparison, which were obviously different from the one for Raw C4N product. As is shown in Figure S2c, both the Raw C4N and C4N-900 samples show similar IR spectra. The appearance of an absorption peak at 1637 cm-1 corresponding to the stretching vibration of C=N indicates the successful formation of C4N structure that remains after annealing process. When ruthenium is complexed with C4N, the absorption peak for C=N stretching mode shifts from 1637 in C4N to 1641 cm1

in Raw Ru@C4N, possible owing to the electronic interactions between C=N bond

and ruthenium. After annealing, with the reduction of Ru3+ ions to Ru0, the absorption peak slightly shifts to low wavenumber (1635 cm-1, Figure S2d), indicating the electronic interactions between C=N bond and ruthenium is weakened. The broad peaks in the range of 3000-4000 cm-1 in the IR spectra (Figure 2Sc-d) are attributed to H2O. The formation of Raw C4N structure has been further confirmed by comparison of the 13

C solid-state NMR spectra of Raw C4N with its starting material (BPTA). As shown

in Figure S3, a broad signal between 135 and 150 ppm is distinguishable in the

13

C

NMR spectrum of Raw C4N. This new chemical shift centered at approximately δ=142.16 ppm is far from those of BPTA molecule, indicating the presence of C=N double bonds. Element analysis (Table S1) verifies the molar ratio (4:1) of C to N atom in Raw C4N, which is well agreement with the structure displayed in Figure S1. The SEM images of C4N-900 and Ru@C4N-900 are shown in Figure 2a and b. Both the samples display layer-stacked structure with a thickness of about 150 nm as determined by atomic force microscopy (Figure S4), demonstrating that encapsulation of Ru will not change the layer structure of C4N-900 besides that the layer size of Ru@C4N-900 decreases. The element energy-dispersive X-ray spectroscopic (SEMEDS) spectrum in Figure 2c shows the presence of C, N, O and Ru in Ru@C4N-900 with atom percentages (At %) of 83.96, 7.6, 5.24 and 3.21, respectively. The mass percentages (wt%) are: 66.22, 6.99, 5.51 and 21.28, respectively. Selected SEM-EDS mapping of the Ru@C4N-900 sample in Figure S5 verifies the homogeneous distribution of C, N and Ru in the sample. TEM image of C4N-900 (Figure 2d) shows sheet-like structure, indicating a strong space continuity of the conjugate structure. The 6


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TEM image of Ru@C4N-900 in Figure 2e shows Ru nanoparticles (Ru NPs) with size ranging from 4-33 nm uniformly encapsulated between C4N layers. The magnified high resolution TEM (HRTEM) in Figure 2f displays the crystal structures of Ru NPs and carbon sheet with lattice fringe spacing of 0.206 nm (green) and 0.370 nm (red), respectively.

Figure 2. SEM images of C4N-900 (a) and (b) Ru@C4N-900. (c) SEM-EDS spectrum of Ru@C4N-900. TEM images of C4N-900 (d) and Ru@C4N-900 (e), and amplified HRTEM image of Ru@C4N-900 (f). The crystalline phase of Raw C4N, Raw Ru@C4N and their annealing products were analyzed by XRD. As shown by the XRD patterns in Figure 3a, the diffraction angle (2θ) of carbon in Raw C4N decreases from 26° to 24° (C4N-900), corresponding to the change of C4N crystal face. For Raw Ru@C4N, the XRD pattern (Figure 3b, 7



black curve) does not show a clear diffraction peak, which might be owing to the introduction of Ru in the structure. The diffraction peak at 24° for Ru@C4N-900 becomes obvious after annealing (Figure 3b, red curve). From this peak, an interlayer distance of 0.372 nm was estimated using the Bragg's equation. In addition, the diffraction peak at 43.6o for Ru was clearly observed (Figure 3b). The lattice spacing of 0.206 nm was estimated, which corresponded to the Ru (101) plane as compared with the PDF card #06-0663. The existence of graphite-like structure in Raw C4N and C4N-900 was further evidenced by the appearance of the characteristic D and G bands in Raman spectra (Figure 3c). The ratio of D band (~1384 cm-1) to G band (~1588 cm1

) increases from 0.66 for Raw C4N to 0.96 for C4N-900, demonstrating the decrease in

the average size of the sp2 domains and increase in defects upon annealing.26 The annealing products with Ru encapsulated at different temperatures also show characteristic features of graphite carbon with similar D/G ratios (Figure 3d).

Figure 3. XRD patterns of (a) Raw C4N and C4N-900, (b) Raw Ru@C4N and Ru@C4N-900. (c) Raman spectra of Raw C4N and Ru@C4N-900, (d) Ru@C4N 8


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samples annealed at different temperatures. The nitrogen adsorption/desorption isotherms of Ru@C4N-900 exhibit the type I features (Figure S6a, b). The corresponding Brunauer-Emmett-Teller (BET) analysis shows that the Ru@C4N-900 catalyst has large surface area of 225.6 m2/g and porous structure with average pore size of 2.520 nm. Appearance of a sharp increase at P/P0 < 0.1 and P/P0=0.7~1 in the isotherms further indicates the existence of lots of micropores, which might be attributed to the sheets stacking structure of Raw C4N. In addition, appearance of a hysteresis loop indicates the existence of mesoporous structure that would facilitate mass transport for reaction. While the specific surface area of C4N-900 is only 15.4 m2/g with average pore size of 2.524 nm. The results demonstrate that introduction of Ru nanoparticles increases the specific surface area of the C4N-900. The composition and chemical configuration of the samples were analyzed by XPS characterizations. The binding energies of C, N, O and Ru elements are listed in Table S2. It is found that the annealing process does not change the binding energy of O which mainly comes from physical adsorbed oxygen. However, the binding energy of nitrogen in the samples increases, demonstrating possible conformation change from pyrazine-like nitrogen to graphite nitrogen upon annealing. After annealing, the binding energy of Ru3p increases from 484.7 (Raw Ru@C4N) to 487 eV (Ru@C4N-900), while the binding energy of Ru3d decreases from 282.4 eV to 280.7 eV. The shifts in binding energies indicate that the coordination bond of Ru3+-N is weakened and more Ru ions are reduced to Ruo upon annealing. The XPS spectrum of Ru@C4N-900 with full range scan (Figure S7) shows the presence of C1s, N1s, O1s and Ru3p/Ru3d with binding energies at 284.6, 400.7, 532.0, and 462.3/487.0 eV, respectively. All the high resolution scans are subtracted by the Shirley background and fitted by Lorentzian-Gaussian function. The C1s peak can be deconvoluted into three sub-peaks at 284.56, 285.4 and 280.7 eV (Figure 4a), which are assigned to sp2 hybridization C=C, C=N and the overlapped Ru3d5/2 (Ru0), respectively. Similarly, the high resolution scan for N1s peak can be fitted into three components (Figure 4b). The three bands located at 398.15, 399.15 and 400.65 eV are 9



ascribed to the “C-NH2”, “pyrazine-like C-N” and “graphitic” nitrogen configurations, respectively. Integral of the three nitrogen configurations demonstrates the “graphitic” N atoms exists as the dominant configuration. The high resolution band of O1s at 532 eV is fitted into two bands (Figure 4c). The One located at 531.1 eV is related to the surface adsorbed O2, while the other at 532.3 eV is attributed to the residual C=O group. Two peaks at ~462.6 and ~487.0 eV for Ru3p (Figure 4d) are attributed to Ru3p3/2 and Ru3p1/2 (Ru0), respectively. The XPS characterizations of Raw Ru@C4N, Raw C4N, and C4N-900 refers to Figures S8-S10.

Figure 4. XPS spectra of Ru@C4N-900. (a) High resolution scans and corresponding fittings of C1s+Ru3d, N1s (b), O1s (c), and Ru3p (d). The TGA measurement of Ru@C4N-900 in air (Figure S11) shows that this sample is stable below 300 oC and then rapidly decomposes to reach an agravic plateau with the increase of temperature. The percentage of residue (RuO2) accounts for 24.3%, namely the Ru content in the product is calculated to be about 18.6 wt%, which is close to the estimated value from SEM-EDS result (21.28%). This value is also in good 10


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agreement with the result from the inductively coupled plasma-atomic emission spectroscopic (ICP-AES) measurement (20.85 wt%). Electrocatalytic activity of Ru@C4N toward HER. The catalytic performance of Ru@C4N for HER in 0.5 M H2SO4 solution shows that the prepared catalyst has excellent HER electrocatalytic activity (Figure 5a). The HER electrocatalytic activity increases with the annealing temperature to 900 oC and shows decrease with higher annealing temperature owing to evaporation of nitrogen which changes the coordination environment of the Ru catalyst. Thus, the HER electrocatalytic activity of Ru@C4N-900, C4N-900, Co@C4N-900 (mass loading of 2.7 mg cm-2), Ni@ C4N-900 (mass loading of 2.7 mg cm-2) and the commercially available benchmark Pt/C catalyst (20% Pt, mass loading of 2 mg cm-2) are compared under the same experimental conditions. It is evident from the polarization curves in Figure 5b, the synthesized Ru@C4N-900 catalyst shows the best HER performance among the C4N catalysts, and is comparable to the commercially available benchmark Pt/C catalyst. In addition, Ru@C4N-900 exhibits the smallest overpotential (6 mV) at the current density of 10 mA cm-2, which is lower than Pt/C (12 mV), Co@ C4N-900 (276 mV), Ni@ C4N-900 (451 mV) and that of the reported Ru@C2N (28.7%, 12 mV) and Ru/C(10%, 127 mV).23 The Ru@C4N also shows long-term HER performance stability in acidic solution as evidenced by the minor difference between the initial linear scan voltammetric (LSV) curve and the one after 10000 times scans (Figure 5c). In acidic solution, HER can be divided into two main steps.26 The first step is the Volmer reaction (H3O+ + e- → Hads + H2O) with a Tafel slope of 120 mV/dec. The second one is the Heyrovsky reaction (Hads + H3O+ + e- → H2 + H2O) with a Tafel slope of 40 mV/dec or Tafel reaction (Hads + Hads→ H2) with a Tafel slope of 30 mV/dec. The Tafel slop of Co@ C4N-900, Ni@ C4N-900, Ru@C4N-900 and Pt/C in 0.5 M H2SO4 are 122, 107, 26 and 27 mV/dec, respectively (Figure 5d). The observed smaller Tafel slop for Ru@C4N comparable to Pt/C indicates that the rate controlling step of HER is the Tafel reaction.

11



Figure 5. Polarization curves of the Ru@C4N catalysts annealed at different temperatures (800 oC, 900 oC and 1000 oC) (a) and the C4N-900, Co@C4N-900, Ni@C4N-900, Ru@C4N-900 and Pt/C catalysts in 0.5 M H2SO4 (b). (c) Initial and the 10000th polarization curves of Ru@C4N-900 recorded in 0.5 M H2SO4 in a potential window from 0.2 to -0.1 V (vs. RHE). (d) Tafel plots obtained from the data in (a). The HER performance of various catalysts was further studied in alkaline solution (1.0 M KOH). As shown in Figure 6a, the catalysts of Co@ C4N-900, Ni@ C4N-900, Ru@C4N-900 and Pt/C show different HER activity with the overpotentials of 264, 300, 7 and 92 mV at the current density of 10 mA/cm2, respectively. In alkaline condition, the overpotential of Ru@C4N is 85 mV lower than that of Pt/C at 10 mA/cm2, demonstrating that Ru@C4N-900 exhibits more excellent HER electrocatalytic activity than Pt/C in alkaline media. In addition, this catalyst also shows good long-term HER stability as evidenced by the small change between the initial and 10000th LSVs in 1.0 M KOH solution (Figure 6b). The Tafel slop of Co@ C4N-900, Ni@ C4N-900, Ru@C4N-900 and Pt/C in alkaline solution are determined to be 119, 129, 18 and 113 12


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mV/dec, respectively (Figure 6c). The lowest Tafel slop indicates the Tafel reaction as the rate determining step for HER on Ru@C4N-900. In addition, the Tafel slope of Ru@C4N-900 is even smaller than that of Pt/C, demonstrating the great potential application of Ru@C4N-900 instead of Pt/C in alkaline media.

Figure 6. Polarization curves of C4N-900, Co@C4N-900, Ni@C4N-900, Ru@C4N-900 and Pt/C catalysts in 1 M KOH. (b) Initial and the 10000th polarization curves of Ru@C4N-900 recorded in 1.0 M KOH in a potential window from 0.2 to -0.1 V (vs. RHE). (c) Tafel plots obtained from the data in (a). The surface Ru atoms of catalysts are believed to be the active centers for hydrogen evolution reaction. It is assumed that each surface Ru atom can located one copper adatom due to their comparable atomic radius. Thus, to demonstrate the excellent HER performance of Ru@C4N-900, the electrochemical active site density of the catalyst was estimated to be 1.44×1017 sites/cm2 by using the copper underpotential deposition method (Figure S12).27-28 The turnover frequency (TOF) of Ru@C4N-900 for HER at an overpotential of 25 mV can then be calculated as 0.93 s-1 and 0.65 s-1 in 0.5 M H2SO4 and 1.0 M KOH, respectively. The TOF of our catalyst in acidic solution shows higher than the reported one (0.67 s-1) of Ru@C2N at an overpotential of 25 mV.23 A detailed comparison of the Tafel slope and TOF on Ru@C4N-900 and the catalysts in literature23, 25, 29-36 is provided in Table S3. It is clear that the synthesized Ru@C4N-900 shows superior HER performance than the reported hybrid catalysts. The excellent HER performance of Ru@C4N-900 could be due to the improved electronic structure of surface Ru atoms coordinated by nitrogen atoms. To

analysis

the

HER

kinetics

at

the

electrode/electrolyte

interface,

Electrochemical Impedance Spectroscopy (EIS) was conducted and the results are 13



displayed in Figure S13.24 The charge transfer resistance (Rct) of the Ru@C4N-900 catalyst is 1.435 Ω in 0.5 M H2SO4 and 0.9478 Ω in 1.0 M KOH. The Rct of the Co@C4N-900 catalyst is fitted as 1.012 Ω in 0.5 M H2SO4 and 1.046 Ω in 1.0 M KOH (Figure S13b). The Rct of the Ni@C4N-900 catalyst is fitted as 1.163 Ω in 0.5 M H2SO4 and 1.194 Ω in 1.0 M KOH (Figure S13c). All the Rct values are fitted with ZSimpWin software and the same equivalent circuit in Figure S13d. The Rct values for all the samples are very small and do not show significant difference in both acid and alkaline solutions, demonstrating the excellent C4N carrier for catalysts. The observed superior HER performance of the synthesized Ru@C4N-900 catalyst could be due to the synergistic effects of optimal electronic structure of Ru coordinated by N in C4N structure and the highly dispersed Ru NPs offered by C4N as the excellent dispersing agent. CONCLUSION In summary, we have synthesized a new carbon nitride material Ru@C4N by solvothermal reaction method followed by annealing. The synthesized Ru@C4N electrocatalyst shows excellent HER performance in both acidic and alkaline solutions as evidenced by the lower overpotentials at 10 mA/cm2 and higher TOF values as compared to the ones for the reported ruthenium-based catalysts, such as Ru/C and Ru@C2N. This might be attributed to the optimal electronic structure of Ru coordinated by N in C4N structure. In addition, the Ru@C4N electrocatalyst shows good long-term HER stability, demonstrating its potential application for hydrogen production. This study suggests that nitrogen contained covalent organic frameworks might be a new kind of substrates for designing high performance catalysts. ASSOCIATED CONTENT Supporting Information IR spectra, NMR, AFM image and thickness, table of elemental analysis, SEM images and element mapping, BET, table of binding energies, XPS spectra, thermogravimetric analysis curves, active site density testing, Nyquist plots and supplementary references. AUTHOR INFORMATION 14


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Corresponding Author * E-mail: xhxia@ nju.edu.cn (X.H. Xia) Notes The authors declare no competing financial interest. Author Contributions Shu-Wen Sun and Gao-Feng Wang contributed equally. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was financially supported by the grants from the National Key Research and Development Program of China (2017YFA0206500), the National Natural Science Foundation of China (21635004, 21675079). REFERENCES (1) Lin, L.; Sun, Z.; Yuan, M.; He, J.; Long, R.; Li, H.; Nan, C.; Sun, G.; Ma, S., Significant Enhancement of the Performance of Hydrogen Evolution Reaction through Shape-Controlled Synthesis of Hierarchical Dendrite-Like Platinum. J. Mater. Chem. A 2018, 6, 8068-8077. (2) Ganci, F.; Lombardo, S.; Sunseri, C.; Inguanta, R., Nanostructured Electrodes for Hydrogen Production in Alkaline Electrolyzer. Renew. Energy. 2018, 123, 117-124. (3) Prasad, A. K.; Sahoo, P. K.; Dhara, S.; Dash, S.; Tyagi, A. K., Differences in Hydrogen Absorption over Pd and Pt Functionalized CVD-Grown GaN Nanowires. Mater. Chem. Phys. 2018, 211, 355-360. (4) Yu, P.; Wang, L.; Xie, Y.; Tian, C.; Sun, F.; Ma, J.; Tong, M.; Zhou, W.; Li, J.; Fu, H., High-Efficient, Stable Electrocatalytic Hydrogen Evolution in Acid Media by Amorphous FexP Coating Fe2N Supported on Reduced Graphene Oxide. Small 2018, 14, 1801717. (5) Wang, X.; Zheng, B.; Yu, B.; Wang, B.; Hou, W.; Zhang, W.; Chen, Y., In Situ Synthesis of Hierarchical MoSe2-CoSe2 Nanotubes as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction in Both Acidic and Alkaline Media. J. Mater. Chem. 15



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A High Performance Ru@C4N Electrocatalyst for Hydrogen Evolution

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