Molecule-Assisted Synthesis of Highly Dispersed Ultrasmall RuO2

Aug 21, 2018 - 96, JinZhai Road Baohe District, Hefei , Anhui 230026 , P. R. China. ‡ Department of Breast and Thyroid Surgery, The First Affiliated...
0 downloads 0 Views 3MB Size
Download PDF
Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Molecule-Assisted Synthesis of Highly Dispersed Ultrasmall RuO2 Nanoparticles on Nitrogen-Doped Carbon Matrix as Ultraefficient Bifunctional Electrocatalysts for Overall Water Splitting Cheng-Zong Yuan,† Yi-Fan Jiang,† Zhi-Wei Zhao,† Sheng-Jie Zhao,† Xiao Zhou,† Tuck-Yun Cheang,*,‡ and An-Wu Xu*,†

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/29/18. For personal use only.

†

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, No. 96, JinZhai Road Baohe District, Hefei, Anhui 230026, P. R. China ‡ Department of Breast and Thyroid Surgery, The First Affiliated Hospital of Sun Yat-Sen University, No. 135, Xingang Xi Road, Guangzhou, 510275, P. R. China S Supporting Information *

ABSTRACT: Facile synthesis of ultrasmall metal-based materials as highly efficient bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is of vital importance to energy storage and conversion technologies. Here, a novel moleculeassisted strategy for synthesis of ultrasmall ruthenium oxide on nitrogen-doped carbon matrix catalyst (RuO2/N−C) is presented. 1,10-Phenanthroline molecular (Phen), which contains an abundant nitrogen element and can strongly coordinate with metal ions, is selected as the assistant to prepare RuO2/N−C. The space confinement effect in this process gives RuO2 nanoparticles (NPs) with size ca. 1.7 nm, making them ultraefficient bifunctional electrocatalysts for both HER and OER. The resultant RuO2/N−C catalyst exhibits striking catalytic performances in strong alkaline solution, affording a current density of 10 mA cm−2 at low overpotentials of 40 mV for HER and 280 mV for OER, respectively. Experimental results and density functional theory (DFT) calculations reveal that the excellent performances are due to the ultrasmall size of RuO2 NPs and the synergistic effect between RuO2 and N−C. More importantly, these RuO2/N−C composites can be utilized as bifunctional electrocatalysts as both anode and cathode and display superior water splitting activity to that of the commercial Pt/C and RuO2 catalysts couple, suggesting the potential applications of our catalyst for large-scale H2 and O2 production in the future. KEYWORDS: Molecule-assisted strategy, RuO2/N−C, Highly efficient, Bifunctional electrocatalysts, Overall water splitting

■

INTRODUCTION

To address these challenges, numerous efforts have been made to develop efficient OER and HER catalysts based on earth-abundant elements, such as transition metal sulfides,12,13 oxides,14,15 phosphates16,17 for OER, and chalcogenides,18 phosphides,19,20 and carbides21 for HER. Even though many of them displayed good catalytic activities, their synthesis processes are relatively complicated and their performances are much inferior to that of noble-metal-based catalysts. What is worse, these H2-evolving and O2-evolving electrocatalysts often function well in different media. When pairing different catalysts in the same electrolyte solutions, such an incompatible integration may lead to poor efficiency of overall water electrolysis. Thus, it is highly demanded but remains challenging to develop ultraefficient and durable bifunctional electrocatalysts to catalyze overall water splitting.

Hydrogen (H2), as a clean, sustainable, and promising chemical energy carrier alternative to fossil fuels, has attracted much global interest in its production through different routes.1−3 Electrocatalytic water splitting, consisting of two half reactions: oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode, is one of the most promising methods for large scale hydrogen and oxygen production in the future.4,5 Unluckily, both OER and HER in the overall water splitting are intrinsically efficiencylimited, because both of them have high energy barrier and involve a complex electron transfer process.6−9 To lower the overpotential and reach a considerable efficiency of water splitting, noble-metal-based materials (e.g., Pt for HER, IrO2 and RuO2 for OER) are the state-of-the-art catalysts.10,11 However, the high cost and low earth abundance of these noble metals seriously limit their widespread practical applications. © XXXX American Chemical Society

Received: April 15, 2018 Revised: July 22, 2018

A

DOI: 10.1021/acssuschemeng.8b01709 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Characterization. The morphology of as-obtained products was confirmed by transmission electron microscopy (TEM, JEOL ARM200F) operated at a 200 kV accelerating voltage. The phase of the samples was characterized via powder X-ray diffraction (XRD) using a Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å). Raman spectra were taken on a PerkinElmer 400F Raman spectrometer using a 514.5 nm laser beam. X-ray photoelectron spectroscopy (XPS) measurement was determined at the photoemission endstation in the National Synchrotron Radiation Laboratory (NSRL, Hefei, China). XPS spectra were fitted with the XPSPEAK41 software. Linear-type background and asymmetric Gaussian−Lorentzian profile functions were used to fit the peaks. The inductively coupled plasma atomic emission spectroscopy (ICPAES) method was performed on a PerkinElmer Optima 8000 ICPAES/ICP-OES spectrometer. Electrochemical Measurements. The electrochemical performances of all samples were analyzed using a traditional standard threeelectrode electrochemical cell, which was controlled by a CHI760E electrochemical workstation (Shanghai Chenhua, China). The Pt wire and a saturated Ag/AgCl electrode were used as the counter electrode and reference electrode, with 1 M KOH aqueous solution (pH = 14) as the electrolyte. The rotating disk electrode made of glassy carbon (PINE, 5 mm diameter, 0.196 cm2) loaded with catalysts was used as the working electrode. The working electrode was prepared as follows: catalyst (3 mg) was dispersed in 1 mL of ethanol and 10 μL of Nafion (5%), and the solution was sonicated until well dispersed ink was achieved. Then 20 μL of the ink was dropped onto the glassy carbon electrode, which was thoroughly cleaned. The activities toward HER and OER of catalysts were tested by linear sweep voltammetry (LSV) with a scan rate of 5 mV s−1 at room temperature. The stabilities of the samples were investigated by LSV polarization curves obtained before and after 1000 potential cycles and an amperometric i−t curve. Electrochemical impedance spectra (EIS) were obtained by applying an AC voltage with 5 mV amplitude in a frequency range from 0.01 Hz to 100 kHz. All potentials reported in this work were calibrated to a reversible hydrogen electrode (RHE) scale according to the Nernst equation (ERHE = EAg/AgCl + 0.059 × pH + 0.197) without any iR-correction. DFT Calculations. DFT simulation calculations were performed by the Vienna Ab Initio Simulation Package (VASP),28,29 and the generalized gradient approximation (GGA) of Perdew−Becke− Ernzerhof (PBE) was used for the exchange-correlation term.30 A nitrogen-doped graphitic carbon model containing 55 carbon atoms was constructed as the N−C to support the RuO2 cluster. The 400 eV kinetic energy cutoff was used for plane-wave basis set and the total energy and residual forces of 2 × 10−5 eV and −0.02 eV Å−1 were chosen for geometry optimization. All of the models were optimized in a periodically repeated slab with the vacuum layer of 15 Å to avoid the layer−layer interactions and Monhorst−Pack k-point sampling was employed in the calculation process. For the adsorption of H2O, O2, and H2 molecule on the N−C, RuO2 cluster, and RuO2/N−C surface, the adsorption energy is defined as Eads = Etotal − Eslab − Emole, where the Etotal and Eslab are the total energy with and without the molecule and Emole is the energy of the molecule including H2O, O2, and H2.

Owning to its electronic structure and high intrinsic activity, ruthenium (Ru) has attracted intense attention as the promising catalyst, recently.22−24 Ru and its oxides have been generally considered to be highly active toward OER,25 but the reports about Ru as HER catalysis are relatively rare. To the best of our knowledge, the relative large size and poor conductivity of the metal oxides materials are two main factors that seriously restrict their catalytic performances. Moreover, in order to synthesize uniform and well-dispersed catalyst/ support composites, some complicated or special methods were adopted. For instance, the Amir’s group reported a solution combustion synthesis method to prepare nanostructured and highly dispersible IrO2-based materials.26 To further enhance their conductivity and dispersity, some carbon materials (such as, active carbon, carbon nanotubes, N-doped carbon, and reduced graphene oxide) were always selected as ideal supports to anchor the metal oxides catalysts. Just as Adschiri and his co-workers used graphene-like layered carbon as the support to synthesize a novel Ru/GLC catalyst for HER, which showed superior performance, but the synthetic process is relatively complicated and the loading amount of Ru is very high (62%).27 Consequently, developing a facile method to synthesize ultrasmall and well-dispersed nanocatalysts on high conductive support is the key to efficiently catalyzing overall water splitting. In this work, we designed a molecule-assistant pyrolysis and subsequent oxidation strategy for in situ synthesis of ultrasmall RuO2 NPs on nitrogen-doped carbon matrix electrocatalysts. Thanks to the ultrasmall size, sufficient active sites, high conductivity, and the synergistic effect of RuO2 NPs and N−C, the resultant RuO2/N−C composite could serve as an ultraefficient bifunctional electrocatalyst for both HER and OER. As a result, the RuO2/N−C exhibits striking catalytic performances in 1 M KOH solution, reaching a current density of 10 mA cm−2 at low overpotentials of 40 mV for HER and 280 mV for OER, respectively. What is more, these RuO2/N− C composites can be utilized as bifunctional catalysts as both anode and cathode for overall water splitting and display excellent activity, which suggests their potential applications for H2 and O2 production.

■

EXPERIMENTAL SECTION

Materials. 1,10-Phenanthroline (Phen), ruthenium trichloride (RuCl3·xH2O), acetone, ethanol, and potassium hydroxide (KOH) were all purchased from Sinopharm Chemical Reagent Co. Ltd. Water used in experiments was purified using ion exchange (Milli-Q, Millipore). All reagents were used directly without further purification. Preparation of RuO2/N−C Catalysts. In a typical procedure, 20 mg of RuCl3·xH2O and 500 mg of Phen were dissolved in 10 and 90 mL of acetone at room temperature, respectively. Then the former salt solution was dropped into the latter ligand solution using a syringe under vigorous stirring. The solution rapidly became reddish brown, indicating the formation of Ru−Phen complexes. By centrifugation, the precursor was obtained and dried in vacuum at 60 °C for 12 h. After that, as-obtained precursors were placed in the crucible and calcined at 700 °C for 2 h with a ramp rate of 2 °C min in N2 atmosphere. During this process, Phen was thermally transformed into N-doped carbon species, and RuCl3 was reduced into Ru nanoparticles anchored on the N−C matrix. Finally, the produced black powders were kept at 250 °C for 3 h in air to get the RuO2/N−C sample. RuO2 particles were prepared as follows: 50 mg of RuCl3·xH2O powders were put in a furnace and directly oxidized at 350 °C for 3 h in an air atmosphere.

■

RESULTS AND DISCUSSION The RuO2/N−C sample was fabricated by a molecule-assisted pyrolysis and subsequent oxidation strategy as shown in Scheme 1, and the corresponding optical photographs were exhibited in Figure S1. 1,10-Phenanthroline molecular (Phen) containing abundant elemental nitrogen first coordinated with Ru3+ in acetone solution, which was verified by the color change of the solution from transparent color to reddish brown (Figure S1). During the subsequent pyrolysis process, Ru3+ was reduced into Ru, which can be proven by the XRD pattern shown in Figure S2. At the same time, Phen was converted into nitrogen-doped carbon (N−C) and restricted the free growth B

DOI: 10.1021/acssuschemeng.8b01709 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

strategy to anchor RuO2 nanoparticles onto the conductive N−C matrix enhances the conductivity of RuO2-based material, which may cause the obtained RuO2/N−C catalyst to possess excellent catalytic performance. Energy dispersive spectroscopy (EDS) analysis of this RuO2/N−C was also performed (Figure S4), which verifies the existence of Ru, O, N, and C elements on the surface of RuO2/N−C sample. The relative mass ratio of Ru, O, N, and C is about 1:2.3:0.8:18, which is the average value of two measurements taken in different regions of the sample. From the HRTEM image, RuO2 NPs can be clearly resolved, as displayed in Figure 1d. The lattice spacing was measured to be 0.32 nm, corresponding to the d-spacing of the (110) crystal plane in RuO2, indicating that the RuO2/N−C catalyst was successfully prepared by this route. To confirm the crystallographic phase information on the asobtained RuO2 and RuO2/N−C catalysts, X-ray diffraction (XRD) analysis was carried out. As shown in Figure S5 and Figure 2a, the XRD patterns of both samples are in good

Scheme 1. Schematic Illustration of the Synthetic Process for the RuO2/N−C Catalyst

of Ru particles, leading to ultrasmall Ru nanoparticles anchored on N−C. To confirm the space confinement effect, RuO2 particles were also prepared by direct oxidation of RuCl3 powder in air without adding Phen as the assistant. The morphologies of the RuO2 particles and RuO2/N−C samples were characterized by transmission electron microscopy (TEM). As shown in Figure 1a, the obtained RuO2 sample

Figure 2. (a) X-ray diffraction patterns of RuO2 particles and RuO2/ N−C; (b) Raman spectra of the obtained RuO2 particles (blue line); and RuO2/N−C samples (red line).

agreement with the standard patterns of RuO2 (JCPDS no. 401290) and the peaks observed at 28.0°, 35.1°, 40.0°, 54.2°, and 57.9° are from the diffraction planes of (110), (101), (200), (211), and (220) of the rutile RuO2. No other peaks were detected in the XRD patterns, confirming the formation of pure RuO2. It could be seen that the width of reflection peaks of RuO2/N−C is wider than that of pure RuO2, confirming the size of RuO2 in RuO2/N−C is smaller judged through the Debye−Scherrer formula (1.79 and 27 nm, respectively), which is consistent with the TEM observation. The compositions and structures of the RuO2/N−C catalyst were further studied by Raman spectroscopy. The Raman spectrum (Figure 2b) of RuO2/N−C exhibits the same pattern as that of RuO2 in range of 300−900 cm−1, where two characteristic peaks attributed to the first order Eg and A1g phonon bands of the RuO2 structure appeared at around 510 and 625 cm−1, respectively.31,32 Moreover, the RuO2/N−C sample shows the clear D-band and G-band peaks, which are attributed to the carbon matrix. A broad D-band appearing at 1341 cm−1 is ascribed to the disorder induced mode or defective graphitic structures,33 and a G-band appearing at 1598 cm−1 is attributed to the E2g-mode from the sp2 carbon domain. The IG/ID in the RuO2/N−C is about 1.03, indicating the formation of a large amount of defect in the carbon matrix, suggesting the obtained carbon was doped with N atoms after the pyrolysis process of the Ru−Phen complex. The surface elemental compositions and corresponding elemental valence states of the RuO2/N−C sample were explored by X-ray photoelectron spectroscopy (XPS). Figure

Figure 1. (a) TEM image of RuO2 particles, (b and c) TEM images of RuO2/N−C, and (d) HRTEM image of the RuO2/N−C sample.

displays irregular particles morphology with a size range between 20 and 100 nm. However, the TEM images (Figure 1b,c) of the RuO2/N−C catalyst clearly indicate that the samples consisted of RuO2 nanoparticles with an average size of 1.7 nm highly dispersed on the N−C matrix. The result demonstrates Phen can give a space confinement effect, which results in highly dispersed RuO2 NPs with ultrasmall sizes. The particle size distribution of the RuO2 NPs on the N−C matrix determined from the TEM images is presented in Figure S3, in which about 120 NPs were measured to establish the particle size distribution. Most particles fall within the size range of 1− 3 nm, with an average size of 1.7 nm. This small size morphology could prompt RuO2 to expose more active sites and the conductive N−C matrix can significantly enhance the electron transport in the catalyst. Moreover, this artful synthetic method generates a strong interaction between ultrasmall RuO2 NPs and highly conductive N−C support. As we all know, the conductivity of semiconductor RuO2 is much poor than that of N-doped graphene; therefore, our in situ C

DOI: 10.1021/acssuschemeng.8b01709 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering 3a displays the overall XPS survey spectrum of RuO2/N−C, also clearly revealing the existence of Ru, O, N, and C elements

Figure 3. (a) XPS survey spectrum of the RuO2/N−C, (b) Ru 3d, (c) O 1s, and (d) N 1s XPS spectra of the RuO2/N−C sample. Figure 4. Electrochemical performances of the samples in 1 M KOH solution with the scan rate of 5 mV s−1. (a) Polarization curves toward OER and (b) corresponding Tafel slope plots of the RuO2/N−C and RuO2 particles. (c) Linear sweep voltammograms of the RuO2/N−C for OER obtained before and after 1000 potential cycles. (d) Polarization curves toward HER and (e) corresponding Tafel slope plots of the RuO2/N−C and commercial Pt/C catalysts. (f) Linear sweep voltammograms of the RuO2/N−C for HER obtained before and after 1000 potential cycles.

in this sample. In addition, the atomic mass ratio of Ru, O, N, and C concluded from XPS analysis is nearly 1:2.2:0.85:17.1, which remains consistent with the result of the EDS and ICPAES analysis (Ru 4.5 wt %). The Ru 3d spectrum (Figure 3b) exhibits doublet peaks at binding energies of about 280.7 and 284.9 eV, which are contributed to the spin−orbit splitting of Ru (3d) in the RuO2 phase with two satellites peaks.34 From the O 1s spectrum (Figure 3c), two peaks at the binding energies of 529.9 and 531.4 eV are clearly detected, which are assigned to the O 1s core levels in rutile RuO2.35−37 Furthermore, the high-resolution N 1s spectrum of RuO2/ N−C (Figure 3d) can be fitted to three peaks at 398.7, 399.6, and 401 eV, confirming the presence of pyridinic-type, pyrrolic-type, and graphitic-type nitrogen atoms doped in carbon.38 This result indicates the successful formation of RuO2/N−C catalyst using the artful method. Synthesis of potential catalysts with excellent OER performances is still a big challenge, because oxygen evolution is a complex four-electron transfer process involving O − H bond breaking and O = O bond formation.39−41 The OER activity of the as-prepared RuO2/N−C catalyst was evaluated in 1 M KOH solution (pH = 14), and for comparison, RuO2 particles was also examined under the same conditions. Figure 4a shows the linear sweep voltammetry (LSV) curves of RuO2/N−C and RuO2, and as expected, the synthesized RuO2/N−C displays higher activity than that of RuO2 particles. According to the LSV, the OER onset potential of RuO2/N−C is near 1.45 V, which is same as that of RuO2 particles. However, this RuO2/N−C catalyst requires a small overpotential of only 280 mV to reach a kinetic current density of 10 mA cm−2, which is obviously better than that of RuO2 particles (290 mV). To further explore the inherent kinetics property, the Tafel slope derived from polarization curves was implemented, as shown in Figure 4b. The RuO2/N−C catalyst possesses a Tafel slope of 56 mV dec−1, which is also lower than that of RuO2 (59 mV dec−1). The small Tafel slope value implys a more rapid OER rate on the RuO2/N−C, which is in agreement with related charge-transfer resistance. As shown in Figure S6, the RuO2/

N−C displays a much smaller charge transfer impedance (12.3 Ω) in the electrochemical impedance spectra (EIS) than RuO2 (13.9 Ω). Furthermore, the stability of the catalyst is another crucial factor for practical application in water splitting. A negligible difference is detected in the polarization curves before and after continuous CV scanning of 1000 cycles from 1.0 to 1.7 V (scan rate of 100 mV s−1) shown in Figure 4c, demonstrating its excellent catalytic stability. These good performances could be attributed to the ultrasmall size of RuO2 and the synergistic effect of RuO2 NPs and conductive N−C support. These excellent catalytic performances and stability suggest that the RuO2/N−C is a remarkable electrocatalyst for OER and superior to almost all state-ofthe-art OER catalysts (Table S1). Besides acting well as an OER electrocatalyst, the asprepared RuO2/N−C material also shows outstanding performance toward HER in alkaline media. Figure 4d shows the polarization curves, where the commercial Pt/C catalyst shows excellent activity with a small overpotential of 49 mV to afford a current density of 10 mA cm−2. Strikingly, thanks to the ultrasmall size of active RuO2 NPs and the strong interaction between RuO2 NPs and highly conductive N−C matrix, the RuO2/N−C exhibits higher performance than commercial Pt/C achieving a current density 10 mA cm−2 with a smaller overpotential of 40 mV. More importantly, the corresponding Tafel slope of this RuO2/N−C catalyst (Figure 4e) is only 44 mV dec−1, which is also smaller than that of commercial Pt/C (47 mV dec−1), suggesting the RuO2/N−C catalyst is more active for HER. What is more, the HER D

DOI: 10.1021/acssuschemeng.8b01709 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering polarization curve of RuO2/N−C obtained after 1000 cycles of continuous CV from 0.1 to −0.3 V (scan rate of 100 mV s−1) shows negligible attenuation compared with the initial one (Figure 4f), confirming the good stability of the RuO2/N−C material. The low overpotential, small Tafel slope, and satisfactory catalytic stability demonstrate that the obtained RuO2/N−C material is a remarkable catalyst to replace commercial Pt/C or many other metal-based catalysts for practical hydrogen evolution (Table S2). These high catalytic performances of our RuO2/N−C samples toward both OER and HER inspire us to evaluate the possibility of using them for practical water splitting. In previous reports, carbon paper was often used to support catalysts, in view of its low cost and high conductivity.42−44 Therefore, we loaded the RuO2/N−C catalysts on carbon papers (1 cm × 2 cm) with a loading mass of 0.5 mg cm−2 and used them as both cathode and anode in a two-electrode configuration to check their performances for water splitting. For comparison, the integration of RuO2 particles and commercial Pt/C couple was also prepared and examined, which are the state-of-the-art catalysts for OER and HER. According to the polarization curves (Figure 5a), it is clear that

Figure 6. (a) Schematic illustration of water splitting on RuO2/N−C catalyst. (b) The adsorption energies of H2, O2, and H2O molecules on N−C, RuO2 cluster, and RuO2/N−C surface.

(Figure S7), the adsorption energy for RuO2/N−C is −1.55 eV, which is much lower than that of the RuO2 cluster and N− C (−0.62 and −0.07 eV) as shown in Figure 6b. Therefore, we can deduce that the synergistic effect between conductive N− C support and active RuO2 promotes the adsorption of H2O molecules onto the surface of the RuO2/N−C, which contributes to the water splitting performance. Moreover, the relative adsorption energies of H2 and O2 on the surfaces of catalysts were also calculated and displayed in Figure 6b. The synergistic effect has a small influence on the adsorption energy of H2, but it can reduce the adsorption energy of O2, suggesting that it is easier for generated O2 desorption from the surface of RuO2/N−C. The calculation results and the excellent activities of overall water splitting reveal that our prepared RuO2/N−C materials could serve as bifunctional catalysts for highly efficient large-scale hydrogen and oxygen production in the future.

■

Figure 5. (a) Polarization curves for water electrolysis of RuO2/N− C||RuO2/N−C and commercial Pt/C||RuO2 particles catalysts in 1 M KOH solution at a scan rate of 5 mV s−1. (b) Time-dependent current density curve of RuO2/N−C || RuO2/N−C couple under constant potential of 1.55 V vs RHE. The inset in panel b is the corresponding photograph of H2 and O2 bubbles.

CONCLUSIONS We have developed a novel molecule-assistant pyrolysis and subsequent oxidation strategy to fabricate ultrasmall RuO2 NPs on N-doped carbon matrix as an excellent bifunctional electrocatalyst for overall water splitting. Benefiting from the ultrasmall size of RuO2 NPs, a large amount of active sites and the synergistic effect between RuO2 and highly conductive N− C, the obtained catalyst exhibits excellent catalytic performances and outstanding stability toward both HER and OER in strong alkaline solution. Significantly, the RuO2/N−C can be loaded onto carbon paper and used as bifunctional electrodes to efficiently catalyze water splitting in a homemade electrolyzer. The RuO2/N−C||RuO2/N−C system displays superior catalytic performance and durability to that of the integrated Pt/C and RuO2 couple, suggesting that this RuO2/N−C sample is a promising catalyst for overall water splitting. Additionally, the large amount of active sites and the synergistic effect of the RuO2/N−C electrocatalyst have been explored by experimental results and DFT calculations. This work provides a promising strategy for in situ synthesis of ultrasmall metal-based or oxides NPs on a nitrogen-doped carbon matrix as bifunctional electrocatalysts for energy storage and conversion applications.

the two systems display almost the same onset potential, but the RuO2/N−C||RuO2/N−C system needs only an overpotential of 304 mV to achieve a current density of 10 mA cm−2, which is smaller than that of the Pt/C||RuO2 system (381 mV). More importantly, the RuO2/N−C||RuO2/N−C couple could give a higher current density than Pt/C||RuO2 system when a large potential (>1.8 V) was applied, which is critical for practical application. Furthermore, the durability test of the system was also carried out to assess the long-term stability. Figure 5b shows that RuO2/N−C||RuO2/N−C system remains a very high activity at the potential of 1.55 V even after 12 h, which indicates that the RuO2/N−C is a pretty stable for long time water splitting even in such extremely alkaline solution. To explore the contribution of the synergistic effect between conductive N−C support and active RuO2 nanoparticles toward overall water splitting, density functional theory (DFT) simulation calculations were performed. Figure 6a shows the schema of water dissociation in alkaline solutions, where H2O molecules are first adsorbed onto the surface of the RuO2/N−C. It is reasonable to consider that the adsorption of H2O molecules on the active sites plays a crucial role in water splitting activity of the catalysts.45,46 Given that three models including N−C, RuO 2 cluster, and RuO 2 /N−C were constructed to calculate the adsorption energies of H2O

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b01709. E

DOI: 10.1021/acssuschemeng.8b01709 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

■

Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Commun. 2016, 52, 1486−1489. (14) Zhou, X.; Xia, Z.; Tian, Z.; Ma, Y.; Qu, Y. Ultrathin Porous Co3O4 Nanoplates as Highly Efficient Oxygen Evolution Catalysts. J. Mater. Chem. A 2015, 3, 8107−8114. (15) Zhao, Z.; Wu, H.; He, H.; Xua, X.; Jin, Y. Self-standing Nonnoble Metal (Ni−Fe) Oxide Nanotube Array Anode Catalysts with Synergistic Reactivity for High-performance Water Oxidation. J. Mater. Chem. A 2015, 3, 7179−7186. (16) Cobo, S.; Heidkamp, J.; Jacques, P.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; Fontecave, M.; Artero, V. A Janus Cobalt-based Catalytic Material for Electrosplitting of Water. Nat. Mater. 2012, 11, 802−807. (17) Yuan, C.; Jiang, Y.; Wang, Z.; Xie, X.; Yang, Z.; Yousaf, A.; Xu, A. Cobalt Phosphate Nanoparticles Decorated with Nitrogen-doped Carbon Layers as Highly Active and Stable Electrocatalysts for the Oxygen Evolution Reaction. J. Mater. Chem. A 2016, 4, 8155−8160. (18) Xie, J.; Zhang, F.; Li, H.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X.; Xie, Y. Defect-rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2013, 25, 5807−5813. (19) Zhang, R.; Wang, X.; Yu, S.; Wen, T.; Zhu, X.; Yang, F.; Sun, X.; Wang, X.; Hu, W. Ternary NiCo2Px Nanowires as pH-Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction. Adv. Mater. 2017, 29, 1605502. (20) Yuan, C.; Zhong, S.; Jiang, Y.; Yang, Z.; Zhao, Z.; Zhao, S.; Jiang, N.; Xu, A. Direct Growth of Cobalt-rich Cobalt Phosphide Catalysts on Cobalt Foil: An Efficient and Self-supported Bifunctional Electrode for Overall Water Splitting in Alkaline Media. J. Mater. Chem. A 2017, 5, 10561−10566. (21) Liu, Y.; Huang, B.; Xie, Z. Hydrothermal Synthesis of Coreshell MoO2/α-Mo2C Heterojunctio as High Performance Electrocatalyst for Hydrogen Evolution Eeaction. Appl. Surf. Sci. 2018, 427, 693−701. (22) Ohyama, J.; Sato, T.; Yamamoto, Y.; Arai, S.; Satsuma, A. Size Specifically High Activity of Ru Nanoparticles for Hydrogen Oxidation Reaction in Alkaline Electrolyte. J. Am. Chem. Soc. 2013, 135, 8016−8021. (23) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L.; Han, Y.; Chen, Y.; Jaroniec, M.; Qiao, S. High Electrocatalytic Hydrogen Evolution Activity of An Anomalous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 16174− 16181. (24) Mahmood, J.; Li, F.; Jung, S.; Okyay, M.; Ahmad, I.; Kim, S.; Park, N.; Jeong, H.; Baek, J. An Efficient and pH-universal Ruthenium-based Catalyst for the Hydrogen Evolution Reaction. Nat. Nanotechnol. 2017, 12, 441−446. (25) Danilovic, N.; Subbaraman, R.; Chang, K.; Chang, S.; Kang, Y.; Snyder, J.; Paulikas, A.; Strmcnik, D.; Kim, Y.; Myers, D.; Stamenkovic, V.; Markovic, N. Using Surface Segregation to Design Stable Ru-Ir Oxides for the Oxygen Evolution Reaction in Acidic Environments. Angew. Chem., Int. Ed. 2014, 53, 14016−14021. (26) Chourashiya, M.; Urakawa, A. Solution Combustion Synthesis of Highly Dispersible and Dispersed Iridium Oxide as An Anode Catalyst in PEM Water Electrolysis. J. Mater. Chem. A 2017, 5, 4774− 4778. (27) Chen, Z.; Lu, J.; Ai, Y.; Ji, Y.; Adschiri, T.; Wan, L. Solution Combustion Synthesis of Highly Dispersible and Dispersed Iridium Oxide as an Anode Catalyst in PEM Water Electrolysis. ACS Appl. Mater. Interfaces 2016, 8, 35132−35137. (28) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (29) Hoshino, K.; Shimojo, F. Ab Initio Molecular Dynamics for Expanded and Compressed Liquid Alkali Metals. J. Phys.: Condens. Matter 1996, 8, 9315−9319. (30) Perdew, J.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (31) Lee, Y.; Ye, B.; Yu, H.; Lee, J.; Kim, M.; Baik, J. Facile Synthesis of Single Crystalline Metallic RuO2 Nanowires and Electromigration-

XRD patterns, TEM images, N2 sorption isotherm curve, corresponding calculation models and catalytic performances of the catalysts (PDF)

AUTHOR INFORMATION

Corresponding Authors

*T.-Y.C.: Email: [email protected]. *A.-W.X.: Email: [email protected]. ORCID

An-Wu Xu: 0000-0002-4950-0490 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the special funding support from the National Natural Science Foundation of China (51572253, 21771171), Scientific Research Grant of Hefei National Synchrotron Radiation Laboratory (UN2017LHJJ), and the Fundamental Research Funds for the Central Universities. The cooperation between NSFC and Netherlands Organization for Scientific Research (51561135011) is acknowledged.

■

REFERENCES

(1) Sun, J.; Zhong, D.; Gamelin, D. Composite Photoanodes for Photoelectrochemical Solar Water Splitting. Energy Environ. Sci. 2010, 3, 1252−1261. (2) Huang, B.; Liu, Y.; Xie, Z. Biomass Derived 2D Carbons via a Hydrothermal Carbonization Method as Efficient Bifunctional ORR/ HER Electrocatalysts. J. Mater. Chem. A 2017, 5, 23481−23488. (3) Ma, T.; Dai, S.; Qiao, S. Self-supported Electrocatalysts for Advanced Energy Conversion Processes. Mater. Today 2016, 19, 265−273. (4) Zhao, G.; Sun, Y.; Zhou, W.; Wang, X.; Chang, K.; Liu, G.; Liu, H.; Kako, T.; Ye, J. Superior Photocatalytic H2 Production with Cocatalytic Co/Ni Species Anchored on Sulfide Semiconductor. Adv. Mater. 2017, 29, 1703258. (5) Zhao, G.; Zhou, W.; Sun, Y.; Wang, X.; Liu, H.; Meng, X.; Chang, K.; Ye, J. Efficient Photocatalytic CO2 Reduction over Co(II) species Modified CdS in Aqueous Solution. Appl. Catal., B 2018, 226, 252−257. (6) Gray, H. Powering the Planet with Solar Fuel. Nat. Chem. 2009, 1, 7. (7) Antolini, E. Palladium in Fuel Cell Catalysis. Energy Environ. Sci. 2009, 2, 915−931. (8) McCrory, C.; Jung, S.; Peters, J.; Jaramillo, T. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (9) Ma, T.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. PhosphorusDoped Graphitic Carbon Nitrides Grown In Situ on Carbon-Fiber Paper: Flexible and Reversible Oxygen Electrodes. Angew. Chem., Int. Ed. 2015, 54, 4646−4650. (10) Xu, Y.; Kraft, M.; Xu, R. Metal-Free Carbonaceous Electrocatalysts and Photocatalysts for Water Splitting. Chem. Soc. Rev. 2016, 45, 3039−3052. (11) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Design of Electrocatalysts for Oxygen-and Hydrogen-involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060−2086. (12) Yuan, C.; Sun, Z.; Jiang, Y.; Yang, Z.; Jiang, N.; Zhao, Z.; Qazi, U.; Zhang, W.; Xu, A. One-Step In Situ Growth of Iron-Nickel Sulfide Nanosheets on FeNi Alloy Foils: High-Performance and SelfSupported Electrodes for Water Oxidation. Small 2017, 13, 1604161. (13) Zhu, W.; Yue, X.; Zhang, W.; Yu, S.; Zhang, Y.; Wang, J.; Wang, J. Nickel Sulfide Microsphere Film on Ni Foam as An Efficient F

DOI: 10.1021/acssuschemeng.8b01709 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Induced Transport Properties. J. Phys. Chem. C 2011, 115, 4611− 4615. (32) Kim, M.; Baik, J.; Lee, S.; Shin, H.; Lee, J.; Yoon, S.; Stucky, G.; Moskovits, M.; Wodtke, A. Growth Direction Determination of A Single Nanowire by Polarized Raman Spectroscopy. Appl. Phys. Lett. 2010, 96, 213108. (33) Liu, Y.; Huang, B.; Lin, X.; Xie, Z. Biomass-derived Hierarchical Porous Carbons: Boosting the Energy Density of Supercapacitors via an Ionothermal Approach. J. Mater. Chem. A 2017, 5, 13009−13018. (34) Näslund, L.; Ingason, Á .; Holmin, S.; Rosen, J. Formation of RuO(OH)2 on RuO2-Based Electrodes for Hydrogen Production. J. Phys. Chem. C 2014, 118, 15315−15323. (35) Pietron, J.; Pomfret, M.; Chervin, C.; Long, J.; Rolison, D. Direct Methanol Oxidation at Low Overpotentials Using Pt Nanoparticles Electrodeposited at Ultrathin Conductive RuO2 Nanoskins. J. Mater. Chem. 2012, 22, 5197−5024. (36) Audichon, T.; Guenot, B.; Baranton, S.; Cretin, M.; Lamy, C.; Coutanceau, C. Preparation and Characterization of Supported RuxIr(1‑x)O2 Nano-oxides Using a Modified Polyol Synthesis Assisted by Microwave Activation for Energy Storage Applications. Appl. Catal., B 2017, 200, 493−502. (37) McCafferty, E.; Wightman, J. Determination of the Concentration of Surface Hydroxyl Groups on Metal Oxide Films by a Quantitative XPS Method. Surf. Interface Anal. 1998, 26, 549− 564. (38) Zhang, J.; Xia, Z.; Dai, L. Carbon-based Electrocatalysts for Advanced Energy Conversion and Storage. Sci. Adv. 2015, 1, e1500564. (39) Subbaraman, R.; Tripkovic, D.; Chang, K.; Strmcnik, D.; Paulikas, A.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. Trends in Activity for the Water Electrolyser Reactions on 3d M (Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550−557. (40) Walter, M.; Warren, E.; McKone, J.; Boettcher, S.; Mi, Q.; Santori, E.; Lewis, N. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (41) Zou, X.; Su, J.; Silva, R.; Goswami, A.; Sathe, B.; Asefa, T. Efficient Oxygen Evolution Reaction Catalyzed by Low-density Nidoped Co3O4 Nanomaterials Derived from Metal-embedded Graphitic C3N4. Chem. Commun. 2013, 49, 7522−7524. (42) Zhang, S.; Yu, X.; Yan, F.; Li, C.; Zhang, X.; Chen, Y. N-Doped Graphene-supported Co@CoO Core−shell Nanoparticles as Highperformance Bifunctional Electrocatalysts for Overall Water Splitting. J. Mater. Chem. A 2016, 4, 12046−12053. (43) Fu, G.; Cui, Z.; Chen, Y.; Li, Y.; Tang, Y.; Goodenough, J. Ni3Fe-Doped Carbon Sheets as a Bifunctional Electrocatalyst for Air Cathodes. Adv. Energy Mater. 2017, 7, 1601172. (44) Zhang, Z.; Yi, Z.; Wang, J.; Tian, X.; Xu, P.; Shi, G.; Wang, S. Nitrogen-enriched Polydopamine Analogue-derived Defect-rich Porous Carbon as a Bifunctional Metal-free Electrocatalyst for Highly Efficient Overall Water Splitting. J. Mater. Chem. A 2017, 5, 17064− 17072. (45) Bao, J.; Zhang, X.; Fan, B.; Zhang, J.; Zhou, M.; Yang, W.; Hu, X.; Wang, H.; Pan, B.; Xie, Y. Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew. Chem. 2015, 127, 7507−7512. (46) Al-Mamun, M.; Su, X.; Zhang, H.; Yin, H.; Liu, P.; Yang, H.; Wang, D.; Tang, Z.; Wang, Y.; Zhao, H. Strongly Coupled CoCr2O4/ carbon Nanosheets as High Performance Electrocatalysts for Oxygen Evolution Reaction. Small 2016, 12, 2866−2871.

G

DOI: 10.1021/acssuschemeng.8b01709 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX