Graphene-like Layered Carbon Composite as an Efficient

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Ruthenium/graphene-like layered carbon composite as efficient hydrogen evolution reaction electrocatalyst Zhe Chen, Jinfeng Lu, Yue-Jie Ai, Yongfei Ji, Tadafumi Adschiri, and Li-Jun Wan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09331 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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ACS Applied Materials & Interfaces

Ruthenium /graphene-like layered carbon composite as efficient hydrogen evolution reaction electrocatalyst Zhe Chen,a, bJinfeng Lub, Yuejie Aia, Yongfei Jic, Tadafumi Adschirib* and Lijun Wand a.

School of Environmental and Chemical Engineering, North China Electric Power

University, Beijing, 102206, China b.

WPI, Advanced Institute for Materials Research, Tohoku University, Sendai,

980-8577, Japan c.

Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal

Institute of Technology, SE-10691 Stockholm, Sweden d.

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry Chinese

Academy of Sciences, Beijing100190, P. R. China

KEYWORDS: Graphene-like, Ruthenium, supercritical fluid, electrocatalysis, hydrogen evolution reaction (HER)

ABSTRACT: Efficient water splitting through electrocatalysis has been studied extensively in modern energy devices, while the development of catalysts with activity and stability comparable to Pt is still great challenge. In this work we successfully developed a facile route to synthesize graphene-like layered carbon 1 ACS Paragon Plus Environment


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(GLC) from a layered silicate template. The obtained GLC has similar layered structure with the template and can be used as support to load ultra-small Ru nanoparticles on it in supercritical water. The specific structure and surface properties of GLC enable Ru nanoparticles dispersed highly uniformly on it even at large loading amount (62wt %). When the novel Ru/GLC was used as catalyst on glass carbon electrode for hydrogen evolution reaction (HER) in 0.5 M H2SO4 solution, it exhibits an extremely low onset potential of only 3 mV and a small Tafel slope of 46 mV/decade. The outstanding performance proved that the Ru/GLC are highly active catalyst for HER, comparable with transition metal dichalcogenides or selenides. As the price of Ruthenium is much lower than Platinum, our study provides that the Ru/GLC might be a promising candidate as a HER catalyst in future energy applications.

1. Introduction Hydrogen evolution reaction (HER), which produces hydrogen by water electrolysis at a thermodynamic potential of ~1.23 V, is an important half reaction which is used to couple with oxygen evolution reaction in water splitting.1 To reduce the electrode overpotential of HER, additional electrocatalysts are required. The overpotential of HER reaction can be significantly decreased by Pt and alloys.2 However, the high cost of Pt restricts the development and practical implementation of electrochemical hydrogen production. Recently, lots of efforts have been devoted to developing the substitution of Pt catalysts, such as Mo, W, Ni and their molecular derivatives for HER.3-7 Despite many of them showed excellent catalytic activity on 2 ACS Paragon Plus Environment

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HER, generally, the synthesis process is quite complicated.8 What’s more, these catalysts are much inferior to the Pt-based catalyst, including higher overpotential and lower current density.3,9 Consequently the development of new catalysts with abilities comparable with Pt catalysts is still desperately needed to fulfill the practical hydrogen generation. Ruthenium and its oxide have been proved to be high active in OER (oxygen evolution reaction),10 although the reports about Ruthenium on HER catalysis are rare.11 In the previous reference, the relative large size and poor distribution of the carriers restrict its catalytic performance. As a result, how to synthesize uniform and well-dispersed Ru nanoparticle is the key point to be solved. To further enhance the HER activity, the carbon materials with good conductivity could be an ideal support to anchor Ru catalyst on it. Carbon based materials such as active carbon,12 carbon nanotubes,13 N-doping carbon14 and reduced graphene oxide (RGO)15have been developed to support noble metal or metal oxides nanoparticles to construct active HER catalyst. Due to its 2D structure, extraordinary electron transport properties and chemical stability, graphene-based material is more promising for the fabrication of the HER catalyst. However, due to its strong p-stacking and hydrophobic interactions, RGO tend to aggregate during the fabrication process. In our view, the strong p-stacking could be weakened if the space between two graphene layers elongated. In this work, a graphene-like layered carbon (GLC) was fabricated with layered silicate as hard template through a simple and scalable method. The GLC is composed 3 ACS Paragon Plus Environment


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by monolayer-patched graphene-like carbon layer with a space of ~1 nm. Then Ru nanoparticles with the size of 2-5nm were successfully loaded on GLC in supercritical water as a green solvent. The method is eco-friendly, low cost and requires short reaction times. In the Ru/GLC composite, Ru nanoparticles was dispersed highly uniformly on the GLC even in high loading amount. When the novel Ru/GLC was used as catalyst on glass carbon electrode for HER in 0.5 M H2SO4 solution, it exhibits highly active properties comparable to Pt with the onset potential of only 3 mV and a smaller Tafel slope of 46 mV/dec. 2. Experimental section Synthesis of Rub-15nanoplates:17.78 g TMAOH (Tetramethylammonium hydroxide, 25 wt% solution) was mixed with 12.22 g TEOS (Tetraethyl orthosilicate) at room temperature under stirring. The solution was stirred overnight. Then the solution was transferred to a 50 mL Teflon-lined autoclave and to be kept in the oven at the temperature of 140 ◦C for 14 days. The product was collected by filtration and washed with acetone till the pH is about 7. Synthesis of GLC from Rub-15 and glucose:

In a typical process, 500 mg Rub-15 and 750

mg glucose was dispersed in 5mL distilled water and stirred for overnight. Then 150 µL concentrated sulfuric acid was added into it. Afterwards the mixture was stirred for another 10 min, and heated at 110 °C for 10 h to produce GLC/Rub-15 composite. The GLC/Rub-15 composite was collected carefully, and calcined at 900 °C in Ar flow for 4 h (Rate 5 °C /min) to remove oxygen, organic species and improve the graphitization of GLC. Then the graphitized GLC/Rub-15 was stirred with 5 mol/L 4 ACS Paragon Plus Environment

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NaOH solution tenderly for 12 h. The GLC was harvested after centrifugation and washed with distilled water for 5 times until the pH is 7, and dried at 60 °C for 6 h. Production of Ru/GLC nanocomposite: In the batch-type synthesis of Ru/GLC in supercritical H2O, a pressure-resistant SUS316 vessel with a 5 cm3 volume was used. For the 10 wt% Ru/GLC composite, 30 mg GLC obtained from last step was first dispersed in the 2 ml H2O with ultrasonic for 20 min. Then 10 mg RuCl3was dissolved into 1 mL H2O and mixed with GLC solution. The 3 mL mixture solution was added into a batch-type vessel, in which the hydrothermal reaction was carried out at 400 °C for 10 min. Then the reactor was quenched into cold water to stop the reaction. The products Ru/GLC were collected and washed 5 times with water by centrifugation. The production route of 2 wt% and 62 wt% Ru/GLC was same except the added weight of RuCl3. The finally obtained Ru/GLC product was further employed as catalyst for HER reaction. Characterization: The phases of the produced nanoparticles were characterized using an X-ray diffraction meter (XRD-Rigaku) with Cu Kα radiation. The particle size and morphology of the products were examined using a Scanning electron microscopy (JEOL JSM-7800F) and transmission electron microscope (TEM-Hitachi). The average diameter was evaluated by measuring 100 particles on the TEM micrographs. Tapping mode AFM measurements were carried out on a Bruker Multimode AFM at ambient conditions. Nitrogen adsorption–desorption isotherms was obtained on Quantachrome Autosorb AS-1. The XPS spectra were measured

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using VG SCIENTA KKLaser, while Raman spectra were characterized by HORIBA, LabRAM HR-800. Electrochemical measurements: All electrochemical measurements were carried out via a standard three electrode cell system on a CHI model 760E electrochemical workstation (Shanghai Chenhua Instrument Factory, China) at room temperature. The embedded software of CHI 760E calculates the voltage drop derived from Ohmic resistance at the first cycle, and then automatic remediate the overpotential from the second cycle to the end (automatic iR Compensation command). Glass carbon rotating disk electrode (RDE) in a diameter of 3 mm coated with catalysts was used as work electrode. Standard Ag/AgCl (3 M KCl solution) reference electrode and Pt foil were used as reference electrode and counter electrode, respectively. Before each test, RDE was mechanically polished with 0.05 µm alumina slurry and then washed with ethanol to obtain a clean surface. For the preparation of working electrode, 2 mg catalyst was dispersed in 800 µL ethanol and sonicated for 15 minutes to form a homogeneous ink. 11 µL ink was loaded on clean glassy carbon electrode to achieve a catalyst loading of 400 µg/cm2. After that, 2 µL nafion (0.5wt.%) solution was dropped onto electrode surface. After dry in air, the electrode was ready for test. The electrocatalytic activities of the as-prepared electrodes were evaluated on the electrochemical workstation in a standard three electrode mode in a 0.5 M H2SO4. At room temperature (∼20 °C), the HER activities were evaluated by linear sweep


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voltammetry (LSV) at 5 mV/s. The electrolytes were purged with high pure Ar for 30 min prior to measurements. Results and discussion

Figure 1. Schematic illustration of the synthesis route of GLC layers and Ru/GLC composites.

As illustrated in Figure 1, a three step procedure was designed to produce the graphene-like

layered

carbon.

A

layered

silicate

RUB-15

([N(CH3)4]8[Si24O52(OH)4]·20H2O), was first prepared by a hydrothermal strategy reported previously.16 The SEM and TEM images of RUB-15 are shown in Figure S1. It can be figured out that RUB-15 is squared sheet-like layer, with a length of side of 5-10 µm and thickness of 100 nm approximately. The layered structure ofRUB-15 is similar with graphite, while the layer space is 1.40 nm proven by XRD (X-ray diffraction) patterns in Figure S1. This 1.40 nm silicate layer of RUB-15 allows carbon source such as glucose to diffuse in to form a RUB-15/carbon composite.17,18 Then the RUB-15/carbon precursor composite was calcined at 900°C in Ar to remove the oxygen functional groups and defects, assuring the graphitization of carbon. A layered carbon with very thin layers could be obtained after the RUB-15 removal by hot NaOH solution. Finally, small Ru nanoparticles was loaded on GLC in supercritical water without any additional reductant. 7 ACS Paragon Plus Environment


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Figure 2.a) The SEM image, b) The TEM image, c) The AFM image and d) The Raman spectrum of GLC.

Figure 2a and b are the SEM and TEM images of the product GLC. Both of the images indicate that the as-obtained carbon layer has almost the same morphology and size as the RUB-15 sheets. TEM image also show that the GLC is composed of many thinner graphene-like carbon layers, indicating that GLC was formed inside the layer space of the layered silicate RUB-15. The GLC was composed by monolayer-patched carbon layer, and quite a number was exfoliated as single or few-layers graphene. Meanwhile, the obvious layered structure as shown in Atomic Force Microscopy(AFM) image also confirmed that (Figure 2c). From the Raman spectra (Figure 2d), the D-band around 1350 cm-1 and a higher G-band around 1570cm-1 of graphitized carbon could be observed, suggesting that the GLC was largely graphitized.19


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XPS spectra of GLC also confirmed that the dominating bond in the product is the C=C (284.5 eV) and the C-C bond (285.8 eV), while rare C-O bond (287.8 eV) or O-C=O band (289.2 eV) can be detected (Figure S2). The result shows that the GLC prepared under this method is high graphitized as clear graphene-like surface with rarely impure C-O groups yielded, consistent with the conclusion derived from Raman spectroscopy. The N2 adsorption-desorption curve of GLC is revealed in Figure S3, from which Brunauer–Emmett–Teller (BET) surface area is calculated as 404.9 m2•g-1. This is a relative large value among that have been detected for graphene-like carbon, indicating that the obtained GLC have large surface area and abundant active sites to combine other reagents as an efficient support. The pore diameter with Density Functional Theory (DFT) method of the GLC is ~1 nm (Figure S3), which also supply support that layer space of the GLC was caused by the removal of layered silicate template. Ru nanoparticles can be loaded on supports by the metal organic chemical vapor deposition (MOCVD)20or by impregnation-reduction method and so on. The expensive organic Ruthenium salts were usually used and the final size of Ru was hard to control. Herein, a green supercritical technology was employed to load Ru nanoparticles with uniform size on the GLC from less expensive reagent. Typically, GLC and RuCl3 were dispersed in 3ml H2O, and then transferred to batch reactor and heated to 400°C for 10 min. Ru3+ was reduced by GLC at 400°C, yielding well-crystallized Ru nanoparticles.



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Figure 3. a-c) The TEM image of 10 wt% Ru/GLC, inset in c) is the HRTEM image of a single Ru nanoparticles; d) The TEM image of 2wt% Ru/GLC; e) The TEM image of 62wt% Ru/GLC; f) The XRD patterns of GLC and 10 wt% Ru/GLC.

Figure 3 a-c showed the TEM images of Ru/GLC composite with 10 wt% loading amount. The 2-5 nm Ru nanoparticles were highly dispersed on the GLC without any agglomeration. Even the Ru loading amount is raised as high as 62 wt%, the dispersion is still perfect (Figure 3e), proving super affinity between GLC and Ru nanoparticles. High-resolutions TEM (HRTEM) image of a selected section showed that lattice fringes of Ru (101) planes with a d spacing of 0.206 nm, indicating the crystallization of Ru nanoparticles (Figure 3c, the inset image). Figure 3f gives the XRD pattern of the Ru/GLC composite, GLC and standard hexagonal Ru. The XRD signal confirmed that the as-prepared composite consisted of Ru, corresponding to the HRTEM image in Figure 3c. Some works showed that hydrothermal treatment could 10 ACS Paragon Plus Environment

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improve the reduction from graphite oxide to graphene with lowering the oxygen and defect.21 And RuCl3 could be reduced by carbon nanotubes in supercritical H2O were also been reported.22 As a result, we speculate that the Ru nanoparticles have been obtained during the supercritical H2O treatment. We also tried the reaction of loading Ru on commercial graphene in supercritical H2O, yielding the seriously aggregated Ru nanoparticles on the graphene surface (Figure S4). As a result, we believed that the novel structure and surface affinity of GLC is crucial to yield the highly dispersed Ru nanoparticles.

Figure 4. Electrocatalysis performances of Ru/GLC, commercial Ru/C and commercial Pt/C. a) Polarization curve, b) Enlargement of onset part of polarization 11 ACS Paragon Plus Environment


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curve, c) Corresponding Tafel slopes and d) Polarization curves of Ru/GLC initially and after 1000 cycles scans.

To appraise the HER catalytic properties of the Ru/GLC composite, we evaluated it in 0.5 M H2SO4 solution using a three-electrode electrochemical system. Figure 4a-b shows the polarization curve of linear sweeps in a cathodic direction of Ru/GLC (10 wt%) together with commercial Ru/C (10 wt%) and Pt/C (10 wt%) as reference. The Ru/GLC exhibited large current and quite early onset of catalytic current. The Ru/GLC possesses onset overpotential of 3 mV,23 which is much lower than commercial Ru/C (22 mV) in the standard three electrode cell system (Figure 4b). Cathodic current density at a certain applied potential is also an important criterion to evaluate catalytic activities of HER electrocatalysts. At the current density of 10 mA·cm−2, which is an important metric according to solar fuel material fabrication,24 a small overpotential(η) can be achieved of 35 mV by Ru/GLC, while 69 mV by commercial Ru/C. For Ru/GLC, overpotential of 61 mV and 125 mV can lead to current densities of 20 and 50 mA·cm−2, respectively. Comparing with the reports of Pt/reduced graphene oxide,25 core/shell NiAu/Au nanoparticles,26 Au/MoS2 nanosheet,27 MoS2/nanoporous gold,28 the

Ru/GLC exhibit much lower onset

overpotential and higher current density at low applied potential, indicating higher catalytic activity. Although the current density on Pt/C grow more rapid than Ru/GLC with the increase of overpotential, these values of Ru/GLC is very impressive and endow it with potential application prospect.


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The super catalytic activity of Ru/GLC is also validated by Tafel plots (Figure 4c). The Tafel slope (46mV/decade) of the Ru/GLC is much lower than those of the commercial Ru/C (72 mV/decade), assuring the ability to gain large current at small overpotential.

The

HER

process

are

described

by

Volmer-Tafel

or

Volmer−Heyrovsky mechanism.29 Briefly, Volmer−Tafel mechanism suggests that the discharge reaction is fast and H2 is evolved by a rate-determining combination reaction, while Volmer−Heyrovsky mechanism occurs when the discharge reaction is relatively slow and H2 is evolved by a rate determining ion + atom reaction. The Tafel slope of the Ru/GLC is 46mV/decade, indicative of an excellent catalyst where the Heyrovsky mechanism dominates. Noted that recent references suggested that HER activity of Pt group metals (such as Pt, Ru, Pd, etc) measured by proton exchange membrane cell method30 or gas diffusion electrode 31 would surpass the mass transport limitation and get more accurate data than on RDE. However, current results on RDE still suggested that the HER activity of Ru/GLC was much better than Ru/C. To access the stability of the catalyst in acidic environment, we measured 1000 continuous cyclic voltammograms cycles at an accelerated scanning rate of 100 mV/s. As shown in Figure 4d, the polarization curve of Ru/GLC after 1000 cycles overlays almost with the initial one, which confirms the catalyst is highly stable. The excellent electrochemical performance could be explained by the structure feature of Ru/GLC. Electron migration could be accelerated on the conductive GLC substrate. The large specific surface area of GLC and the high loading amount of 2-5



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nm uniform Ru assured abundant catalytic site. Our synthesis route supplied a facial platform to fabricate 2D GLC composite for catalysis and other fields. Finally, Plane-wave-based DFT calculations on slab models are carried out by the exchange-correlation functional GGA-PBE, which supplies guiding theoretical interpretation for the superior HER performance.32 In our work, the adsorption energy Ead is defined to describe the HER catalytic activity of Ru or Pt. ∆Ead=Esurface-H-(Esurface+EH2/2)+0.24eV, where Esurface is the energy of super-cell for Ru (001) and Pt (111) surfaces; Esurface-H is the total energy of complex of hydrogen adsorption on surfaces; EH2 is the energy of hydrogen molecule, and 0.24 eV is added to the calculated binding energies (with respect to gaseous H2) to give adsorption free energies. 33 As shown in Figure S6 and Figure S7, we calculated the ∆Ead value of Ru (001) surface to be -0.37 eV while it is -0.27 for Pt (111) surface. The relatively small ∆Ead difference between Ru (001) and Pt (111) indicates that Ru could exhibit efficient HER performance as Pt-group metal. It’s worth note that we optimize Ru/GLC and Pt/C to ideal element Ru and Pt planar, ignoring the effect of different lattice plane, nanoparticle size, edge, step and the supporter GLC. Yan synthesized Ru nanocrystals with controlled high shape selectivity34, and demonstrated that the exposed facets, edges and corners have remarkable influence to their activity on the surface-enhanced Raman spectra and CO-selective methanation that consistent with first-principles calculation. Strasser reported that there is dramatic increase of activity and selectivity of Cu nanoparticle below 5 nm35, suppling DFT theoretical verification on the impact of size and different facets to catalytic performance. Thus, our DFT 14 ACS Paragon Plus Environment

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results just provides a guiding light, but still reflects the intrinsic H2 adsorption ability of element Ru. The superior electrocatalytic HER performance of Ru/GLC can be explained as follows: (1) the high surface area of GLC provide more active sites for reaction; (2) the 2-5nm uniformly crystallized Ru nanoparticles led to high catalytic ability and the exposure of more catalytic sites; (3) the high efficient, green synthesis method in supercritical H2O yielded clear GLC surface, assuring intimately contact between Ru and GLC without other impurity groups; (4) the flexible layered skeleton of GLC would promote electron transfer between the surface and layers, and improved the stability. 3. Conclusions In conclusion, we successfully developed a facile route to synthesize graphene-like carbon and load well-dispersed crystallized Ru nanoparticles on it in supercritical water. XRD pattern and TEM image of the Ru/GLC demonstrated that very small Ru nanoparticles with the diameter of about 2 nm was uniformly deposited on the graphene-like carbon. The Ru/GLC was employed as catalyst for HER in acidic environment and it exhibited a smaller Tafel slope of 46 mV/dec and a lower onset potential of 3 mV than commercial Ru/C, revealing that the Ru/GLC facilitates combination of two adsorbed H atoms and therefore the HER. ASSOCIATED CONTENT

Supporting Information. Computational models and methods and supporting Figures, “This material is available free of charge via the Internet at 15 ACS Paragon Plus Environment


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http://pubs.acs.org.” For instructions on what should be included in the Supporting Information, as well as how to prepare this material for publication, refer to the journal’s Instructions for Authors.

AUTHOR INFORMATION Corresponding Author *(T.Adschiri). E-mail: [email protected]

ACKNOWLEDGMENT We appreciate the financial support from Grant-in-Aid for Scientific Research (A) 25249108 (Japan), National Natural Science Foundation of China (NSFC 51502089, 51302008), the Fundamental Research Funds for the Central Universities (2016MS03). ABBREVIATIONS GLC, graphene-like carbon; HER, hydrogen evolution reaction

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Table of Contents Graphic


Graphene-like Layered Carbon Composite as an Efficient

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