Well-Defined Ru Nanoclusters Anchored on Carbon: Facile Synthesis

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Well-Defined Ru Nanoclusters Anchored on Carbon: Facile Synthesis and High Electrochemical Activity toward Alkaline Water Splitting Xia Cheng, Hua Wang, Mei Ming, Wenxiu Luo, Yi Wang, Yingchun Yang, Yun Zhang, Daojiang Gao, Jian Bi, and Guangyin Fan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01581 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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ACS Sustainable Chemistry & Engineering

Well-Defined Ru Nanoclusters Anchored on Carbon: Facile Synthesis and High Electrochemical Activity toward Alkaline Water Splitting Xia Cheng†, Hua Wang†, Mei Ming†, Wenxiu Luo†, Yi Wang†, Yingchun Yang‡, Yun Zhang†,* Daojiang Gao†, Jian Bi†, and Guangyin Fan†, *

†

College of Chemistry and Materials Science, Sichuan Normal University, 5 Jingan Road,

Chengdu 610068, China ‡

College of resources and environment, Chengdu University of Information Technology, 10

Xingfu Road, Chengdu 610225, China

* Corresponding author

Email: [email protected](Y. Zhang)

Email: [email protected](G.Y. Fan)

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ACS Paragon Plus Environment

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ABSTRACT

Developing efficient and affordable alternatives for Pt-based electrocatalysts to promote the procedure of hydrogen evolution reaction (HER) is of significant importance. Herein, a facile and convenient strategy using tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate as precursor is developed for synthesizing ultrafine and highly dispersed ruthenium nanoclusters on carbon support. It is discovered that the pre-adsorption of Ru precursor on carbon substrate and followed by pyrolysis are very crucial for the synthesis of such Ru nanoclusters. Among as-prepared samples, Ru/CN-800 shows the best HER electrocatalytic performance in alkaline solution, which outperforms the benchmark Pt/C and recently reported catalysts. The excellent performance of Ru/CN-800 was presumably attributed to the dominant metallic Ru as well as the ultra-small Ru nanoclusters with a satisfied dispersion, which provides the massive highly active sites for HER. This facile and environmental-friendly pathway for synthesis of ultrasmall Ru nanoclusters with high HER activity shows a promise in practical applications.

KEYWORDS: Ultrafine ruthenium nanoclusters; Electrocatalyst; Hydrogen evolution reaction; Water splitting

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INTRODUCTION

Hydrogen is a favorable alternative for traditional energies due to its high density, green and renewable properties. Hydrogen evolution from electrochemical water splitting has been reported as the sustainable and advanced technique owing to its high efficiency as well as high-purity product.1 Yet, high overpotential of hydrogen evolution reaction (HER) greatly restricts this strategy extensive application. Hitherto, Platinum-based materials are the best electrocatalysts for HER. Unfortunately, the expensive price and limited availability of Pt prohibit them practical applications in large-scale. Therefore, design and synthesis of efficient and affordable alternatives for Pt-based electrocatalysts to drive HER are urgently required for the practical utilizations of hydrogen energy.2

Compared with Pt, ruthenium (Ru) is a promising candidate for HER because of its economic advantage and the similar bond strength with hydrogen.3-4 Therefore, Ru-based HER electrocatalysts has attracted increasing attention in recent years.5-13 Despite these advances, developing Ru-based materials with low Ru loading and more accessible active sites for HER remains a great challenge for practical applications. Of special note, ultrafine and highly dispersed metal nanoparticles (NPs) have captured intensively attention in heterogeneous catalysis since they can display some special natures, including unique geometrical structure, distinctive electronic feature, more surface atoms. However, severe aggregation of metal NPs with a high surface energy generally leads to the significant loss of catalytic activity and reusability.14-15 Capped metal NPs by organic stabilizing agents can effectively avoid the aggregations, whereas most of catalytic active sites are covered by these 3



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capping agents, decreasing the catalytic activity. Recently, nitrogen(N)-doped carbon matrixes are identified as effective supports to synthesize non-capped metal NPs for various catalytic utilizations. N-containing groups of support can act as essential coordination sites to promote the formation of ultrafine NPs and then solidly anchor these NPs on the matrixes, deducing a high catalytic performance.16 However, most of strategies for preparing such NPs often need pre-N-functional step for the supports. This step can make the synthesis procedure time-consuming and tedious. To this end, one-pot facile preparation of uniformly dispersed small Ru NPs with a low loading firmly stabilized on N-doped carbon is largely expected for both fundamental researches and industrial applications.

Herein, highly dispersed and small-sized Ru nanoclusters on N-doped carbon were directly synthesized

through

a

well-designed

strategy

by

direct

pyrolysis

of

tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate (TBA), which has been pre-absorbed uniformly on carbon substrate. Among the obtained samples, the Ru/CN prepared at 800 oC was demonstrated as a prominent HER electrocatalyst with a low overpotential of 14 mV at 10 mA cm-2 in alkaline medium.

EXPERIMENTAL SECTION

Synthesis of Samples

First, 0.037 g TBA (Aladdin) was dissolved in 20 mL methanol (Aladdin) and then 0.1 g carbon black (Vulcan XC-72R, Sigma-Aldrich) was added. After being stirred for 2 h, the suspension transferred into an oven and dried under vacuum for 24 h. The obtained black 4


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powder was put into a porcelain boat and pyrolyzed at different temperatures for 2 h under Ar atmosphere to get Ru/CN. The samples pyrolyzed at temperatures of 700, 800, and 900 oC were denoted as Ru/CN-700, -800, and -900, respectively. For comparison, Ru-800 were prepared in parallel by the same route as that for Ru/CN-800 except for no addition of carbon black; Ru/C-800 were prepared in parallel by the same route as that for Ru/CN-800 except for using the same molar amount of RuCl3 (Aladdin) instead of TBA.

Material Characterizations

Morphologies of samples were first determined on a transmission electron microscope (TEM, JEM-2100F, JEOL), and then on a JEOL ARM200F instrument. Crystal structures of samples were measured by X-ray diffraction (XRD) operated on a Regaku D/Max-2500 diffractometer. Surface compositions and element chemical states of samples were obtained by X-ray photoelectron spectroscopy (XPS) on a Thermo ESCALAB 250 Axis Ultra spectrometer. Nitrogen adsorption-desorption isotherms were carried out on an Automated Gas Sorption Analyser (Quanta Autosorb-IQ). The metal loading determined by thermal gravimetric analysis (TGA) was performed on a Netzsch DSC214 equipment. The generated gas was monitored by an Agilent 7890B gas chromatography.

Electrochemical Measurements

The electrochemical impedance spectroscopy measurements were conducted on an Autolab workstation (PGSTAT 302N). Other electrochemical experiments were done on the standard three-electrode system collected with CHI 760E setup. Hg/HgO electrode is the reference 5



electrode and graphite rod is the counter electrode. Synthesis of working electrode (WE): first, 0.002 g catalyst was added into the solvent containing 0.8 mL of alcohol and 0.008 mL of 5% Nafion (Sigma-Aldrich), and resulted mixture was treated by ultrasonication for 20 min. After that, 25 µL ink was then loading on the glassy carbon electrode (GCE, 4 mm) to achieve the loading of 0.498 mg cm-2 for WE. Unless stated, all Linear sweep voltammetry (LSV) curves were iR corrected and all potentials were calibrated to RHE. To evaluate the catalytic performance, the Faradaic efficiency of the Ru/CN-800 for HER was determined according to our previously method.17

RESULTS AND DISCUSSION

Scheme 1 illustrate the synthetic route of Ru/CN-800. Typically, TBA was firstly dissolved into methanol under stirring and then a desired amount of carbon black was added with continued stirring. After evaporating solvent, the obtained black powder was pyrolyzed at the desirable temperature (800 oC).

Scheme 1 The route for the preparation of Ru/CN-800. The intrinsic structure of Ru/CN-800 was firstly characterized by TEM. TEM images shows that a lots of metal nanoclusters are uniformly dispersed on the support with an anverage diameter of 1.12 nm (Figures 1a-c), which should ascribe to the excellent anchoring 6


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ability of electronegative N-group on the carbon support.3, 18 The lattice spacing of one metal nanocluster is calculated to be about 0.24 nm, well indexed to the (101) plane of Ru crsystal (JCPDS 06-0663)(Figure 1b inset). These results verify the successful preparation of highly dispersed ultrafine Ru nanoclusters in Ru/CN-800. This could be further confirmed by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) images, where the Ru nanoclusters are clearly distinguished whereas few of single atoms are observed (Figure S1). Moreover, the STEM and energy-dispersive X-ray spectrometry (EDS) mapping images shown in Figures 1d-g further indicate that these Ru nanoclusters in a samll size are highly distributed on the surface of carbon support.

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Figure 1. (a, b) TEM images (inset in b: HRTEM image), (c) particle size distribution of Ru/CN-800, and (d) STEM and EDX mapping images of (e) N, (f) C and (g) Ru for Ru/CN-800. Figure 2a is the XPS full scan spectrum of Ru/CN-800, where Ru, C, and N are distinctively observed. This again confirms the successful synthesis of Ru/CN-800 by our developed strategy. Moreover, the different oxygen-containing groups have been well identified by peak fitting the XPS spectra of C1s and O1s (Figure 2b and S2). The different N types (graphitic quaternary-N and pyrrolic-N) are confirmed by the high-resolution XPS of N1s (Figure 2c). It is believed that both oxygen and N-containing groups of CN are beneficial for Ru nanoclusters in a high dispersion and ultrafine size as well excellent stabilization. The high-resolution XPS spectrum of Ru3p shows a doublet situated at 462.5 and 484.6 eV, assigning to Ru0 3p3/2 and Ru0 3p1/2, respectively (Figure 2d and Table S1). This result further demonstrates these nanoclusters on CN in Ru/CN-800 are Ru0 species, which are highly expected for showing excellent electrocatalytic activity for HER. The contents of Ru, C, N, and O in Ru/CN-800 recorded by XPS are 3.59, 91.05, 0.65, and 4.71 wt%, respectively. As Ru nanoclusters are ultrafine and highly dispersed on the surface of carbon support, the Ru content measured by XPS is similar with that measured by EDX (3.15 wt%, Figure 1d-g) and TGA (3.84 wt%, Figure S3). Furthermore, the texture structure of Ru/CN-800 was analyzed by N2 adsorption-desorption isotherm (Figure S4). After analyzed, Ru/CN-800 displays a high Brunauer-Emmett-Teller (BET) surface area of 173.1 m2/g as well as massive nanopores, which guarantees that abundant active sites can be exposed at

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three phase-interfaces, therefore, promoting Ru/CN-800 with an excellent catalytic activity for HER.

Figure 2. (a) XPS full scan and high-resolution XPS spectra of (b) C 1s, (c) N 1s, and (d) Ru 3p for Ru/CN-800.

Figure 3a shows the HER polarization curves of Ru/CN-800 and control samples in 1.0 M KOH and corresponding performance data are summarized in Figure 3b. As expected, the bare GCE is electrocatalytically inactive, whereas Ru/CN-800 shows an excellent activity for HER, which achieves only overpotential of 14 mV at 10 mA cm-2. This outstanding 9



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performance of Ru/CN-800 is more than the state-of-the-art Pt/C (61 mV for 5 wt%, 26 mV for 10 wt% and 17 mV for 20 wt%). Specifically, the mass activity of Ru@CN-800 is 5.8, 4.7, and 6.0 higher than that of commercial 5, 10, and 20 wt% Pt/C, respectively. This must result from the small size and high dispersion of Ru nanoclusters as well as its excellent intrinsic activity.2, 4, 19-20 Furthermore, the activity toward HER over Ru/CN-800 is also much higher than most of recently reported Ru-based electrocatalysts. As shown in the Figure 3c and Table S2, the overpotential of Ru/CN-800 at 10 mA cm-2 is more inferior than Cu2-x@RuNPS (82 mV),

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Ru/CP(78 mV),8 RuP2@NPC (52 mV),7 Ru/GLC (35 mV),22

RuCo/NC(28 mV),23 and Ru@C2N(17 mV),24 but a litter bit higher than the Ru/NG (8 mV)25 and Ru/MoS2 (13 mV).8 The superior catalytic activity of Ru/CN-800 is consistent with the excellent nature of metal Ru toward water dissociation and hydrogen adsorption/desorption, both of which are the key of HER steps in alkaline solution.3,

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Metallic Ru has been

calculated to possess a moderate Ru-H* bonding energy and shows great potential for HER;24,

27-28

the higher metallic Ru ratio, the faster adsorption/desorption of H*, which

promotes the rapid formation of Ru-H* intermediate and H2. Moreover, the more metallic Ru species would result in a fast water capture and low energy barrier for water dissociation, and then a much easier water dissociation. Additionally, the highly catalytic activity of Ru/CN-800 is also in line with its high density of active site, which are dominated by the metal dispersion of catalyst. The larger metal dispersion means more exposed active sites. Considering the large Ru dispersion from the ultrasmall Ru nanoclusters basis on TEM observation, the highly electrocatalytic activity of Ru/CN-800 can be achieved for HER (Figures 1a-c and Table S1). 10


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Figure 3. (a) HER polarization curves of Ru-800, Ru/C-800, Ru/CN-800, commercial Pt/C (5, 10, and 20 wt%), and GCE. (b) corresponding Tafel slopes and overpotentials at 10 mA cm-2. (c) Comparison of the overpotential (at 10 mA cm-2) of Ru/CN-800 and recently reported Ru-based catalysts. (d) The Faradaic efficiency vs. time for Ru/CN-800. (e) HER polarization curves of Ru/CN-800 before and after 1000 CV cycles (inset: V-t curve at 10 mA cm-2 for Ru/CN-800). (f) EIS Nyquist plots of Ru-800, Ru/C-800, and Ru/CN-800 at overpotential of 30 mV.

Tafel plot was further constructed to identify the HER kinetic favourable and uncover the reaction mechanism over Ru/CN-800 (Figure S5). The Tafel slopes were achieved by fitting the Tafel plots on basis of the Tafel equation (η = b log j + a) (Figure 3b). Obtained Tafel slope of Ru/CN-800 is 30.02 mV·dec-1, higher than Pt/C. This result suggests the HER favourable kinetic over Ru/CN-800. As proposed previously, hydrogen evolution in basic 11



medium involves three typically fundamental pathway. The Volmer reaction with a Tafel slope of 120 mV dec-1 is the first step, in which the H* species are formed via water dissociation. Then, the H* species may undergo two parallel reactions to produce hydrogen including Heyrovsky reaction (Tafel slope: 40 mV dec-1) and Tafel reaction (Tafel slope: 30 mV dec-1).3 According to Tafel slope range, the HER catalysed by Ru/CN-800 in this system is probably performed by a Volmer-Tafel mechanism. Moreover, the Faradaic efficiency of Ru/CN-800 for HER is measured to be nearly 100%, again demonstrating the excellent activity of Ru/CN-800 for HER (Figure 3d). Considering the superior performance for electrochemical HER, the stability of Ru/CN-800 were further investigated. Figure 3e shows the LSV curves of Ru/CN-800 before and after 1000 CV cycles as well as the V-t curve at 10 mA cm-2. After stability tests, no obvious changes are observed in both polarization and V-t curves, indicating Ru/CN-800 with excellent HER durability in basic electrolyte.

To investigate the importance of carbon matrix for Ru/CN-800 in a high dispersion and small size, a control sample of Ru/C-800 was prepared by directly pyrolysis of TBA using the same procedure as that for Ru/CN-800 except for no addition of carbon matrix. As shown in Figures 3a,b, the Ru-800 delivers a bad activity in terms of an overpotential of 120 mV at 10 mA cm-2, which can be attributed to the less active centers from large-sized Ru NPs as observed by TEM image (Figure S6). This result indicates that introduction of carbon matrix during catalyst preparation can uniformly distribute and solidly anchor TBA precursor, leading to the formation of small Ru nanoclusters and protecting them overgrowth, therefore, supplying enough active sites for Ru/CN-800 with excellent HER. Besides, the effects of

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N-doping on NP size and carbon matrix were further studied to illustrate the internal factors that aroused the activity differences. On the one hand, N-modified carbon can modify the size and dispersion of NP. For demonstrating this, a control sample of Ru supported on carbon without nitrogen-doped was prepared in parallel by the same method as that for Ru/CN-800 except for using the same mole amount of RuCl3 instead of TBA. As shown in TEM images of Ru/C-800 (Figure S7), it can be clearly seen that the Ru NPs with a diameter of 5.08 nm in a wide range of size are randomly distributed on the carbon. This should be attributed to the rapid formation of Ru NPs on carbon when they lose N protection. As presence of massive large Ru NPs, Ru/C-800 has fewer active sites for HER, thus, showing a lower catalytic activity than Ru/CN-800 (Figures 3a,b). Additionally, electrical impedance spectroscopy experiments discover that all of three samples (Ru/CN-800, Ru/C-800, and Ru-800) have a similar solution resistance (Rs) whereas they show a different charge transfer resistance (Rct), Ru/CN-800 with the smallest Rct of 13.7 Ω and Ru/C-800 with the middle Rct of 14.7 Ω, in agreement with the development of HER catalytic activity (Figure 3f). Furthermore, N dopant of carbon support can activate the adjacent C atoms to enhance the density of active sites;29 graphitic quaternary N is beneficial for the electron transportation in carbon skeleton, enhancing the electronic conductivity of Ru/CN-800.30

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Figure 4. (a) XRD patterns, (b) HER polarization curves, (c) corresponding Tafel plots, and (d) the comparison of Tafel slopes, Cdl values, and overpotentials at 10 mA cm-2 for Ru/CN-700, -800, and -900. The XRD pattern of carbon was also presented in Figure 4a as the reference.

To explore the influence of pyrolysis temperature on the catalysts for HER electrocatalytic performance, three samples were achieved at different temperatures. The samples pyrolyzed at temperatures of 700, 800, and 900 oC were denoted as Ru/CN-700, -800, and -900, 14


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respectively. The HER performances of the three samples are first researched and obtained data are presented in Figure 4. The Ru/CN-700 shows a moderate electrocatalytic activity for HER with an overpotential of 24 mV at 10 mA cm-2, lower than Ru/CN-800 (14 mV) but better than Ru/CN-900 (51 mV). As shown in Figures 2a-c and S8, the average diameter of Ru nanoclusters in the samples slightly increases with the elevation of pyrolysis temperature (Table S1). As can be seen from Figure 4a, no visible peaks corresponding to Ru and Ru oxides are observed in the XRD patterns of the three samples. Combining with TEM observations, it can be concluded that these Ru NPs are relatively small and highly-dispersed on carbon support in three samples. The relative large Ru NPs in Ru/C-900 catalyst are assigned to the aggregation of Ru nanoclusters at high pyrolysis temperature, which results in the decrease of number of active sites, leading to a worse HER electrocatalytic activity for Ru/C-900. Although Ru/CN-700 has a smaller size of Ru nanoclusters than that Ru/CN-800, it still possesses a lower electrocatalytic activity toward HER because it is lack of highly active component (Ru0 species) (Table S1). These different HER activity in three samples can be identified by determined the electrochemical surface areas (ECSA) via testing the capacitances of double layer (Cdl) in non-faradic region since it is in line with ECSA (Figure S9).31-32 Among three samples, Ru/CN-800 displays the highest Cdl of 12.6 mF cm-2 (9.5 mF cm-2 for Ru/CN-700 and 6.1 mF cm-2 for Ru/CN-900) corresponding to the largest ECSA, indicating its largest active surface area. Moreover, kinetic studies discover that the Ru/CN-800 possesses the lowest Tafel slope of 30.02 mV dec-1 compared with Ru/C-700 and Ru/C-900 (Figures 4c and d). This result shows that the HER catalysis over Ru/CN-800 is the most kinetic favorable in alkaline medium. Overall, the pyrolysis temperature in this system 15



can not only tune the NP size but influence the chemical state of active component, thus modulating the HER electrocatalytic activity of Ru/CN.

CONCLUSIONS

In summary, uniformly dispersed ultrafine Ru nanoclusters supported on carbon matrix was efficiently synthesized by a convenient and simple strategy through pyrolysis of the TBA/carbon mixture. It was found that the carbon matrix and N-doping were the key to prepare such Ru nanoclusters. Among the as-prepared catalysts, Ru/CN-800 showed the outstanding electrocatalytic activity toward HER in basic solution with a low overpotential of 14 mV at 10 mA cm-2. Specifically, Ru/CN-800 also displayed the excellent mass activity outperformed commercial Pt/C. The high HER performance was presumably ascribed to the dominant metallic Ru, large specific surface area, and high dispersion of small Ru nanoclusters. The present work may open a facile and environmental-friendly pathway to synthesize noble metal catalysts with highly electrochemical HER efficiency.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website:

HAADF-STEM images, additional XPS spectrum, TGA curve, N2 sorption isotherm curve, Tafel plots, additional TEM images, and Table S1 and S2.

AUTHOR INFORMATION 16


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Corresponding Author

*Tel: +28 8476-0802 email: [email protected] (Y Zhang).

*Tel: +28 8476-0802 email: [email protected] (G Fan)

Conflicts of interest

There are no conflicts of interest to declare.

ACKNOWLEDGEMENTS

This work was supported by the Sichuan Youth Science and Technology Foundation (2016JQ0052) and the National Natural Science Foundation of China (21777109). REFERENCES (1) Zhang, Y.; Ma, Y.; Chen, Y.-Y.; Zhao, L.; Huang, L.-B.; Luo, H.; Jiang, W.-J.; Zhang, X.; Niu, S.; Gao, D.; Bi, J.; Fan, G.; Hu, J.-S. Encased copper boosts the electrocatalytic activity of N-doped carbon nanotubes for hydrogen evolution. ACS Appl. Mater. Interfaces 2017, 9 (42), DOI 10.1021/acsami.7b11748. (2) Huang, L. B.; Zhao, L.; Zhang, Y.; Chen, Y. Y.; Zhang, Q. H.; Luo, H.; Zhang, X.; Tang, T.; Gu, L.; Hu, J. S. Self‐limited on‐site conversion of MoO3 nanodots into vertically aligned ultrasmall monolayer MoS2 for efficient hydrogen evolution. Adv. Energy Mater. 2018, 8 (21) DOI 10.1002/aenm.201800734. (3) Wang, J.; Wei, Z.; Mao, S.; Li, H.; Wang, Y. Highly uniform Ru nanoparticles over N-doped carbon: pH and temperature-universal hydrogen release from water reduction.

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Ultrafine and highly dispersed Ru naoclusters on N-doped carbon synthesized by facile and convenient strategy shows outstanding performance for HER.

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Well-Defined Ru Nanoclusters Anchored on Carbon: Facile Synthesis

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