Crystallinity Dependence of Ruthenium Nanocatalyst ...

Letter Cite This: ACS Catal. 2018, 8, 5714−5720

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Crystallinity Dependence of Ruthenium Nanocatalyst toward Hydrogen Evolution Reaction Yutong Li,†,∇ Lei A Zhang,‡,∇ Yong Qin,*,† Fuqiang Chu,† Yong Kong,† Yongxin Tao,† Yongxin Li,† Yunfei Bu,*,§ Dong Ding,‡ and Meilin Liu*,‡ †

Jiangsu Key Laboratory of Advanced Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China ‡ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-2045, United States § School of Environmental Science and Engineering, Nanjing University of Information & Technology, Nanjing, Jiangsu 210044, China S Supporting Information *

ABSTRACT: The development of highly active and durable inexpensive electrocatalysts for hydrogen evolution reaction (HER) is still a formidable challenge. Herein, an ordered hexagonal-closed-packed (hcp)-Ru nanocrystal coated with a thin layer of N-doped carbon (hcp-Ru@NC) was fabricated through the thermal annealing of polydopamine (PDA)-coated Ru nanoparticle (RuNP@PDA). As an alternative to Pt/C catalyst, the hcp-Ru@NC nanocatalyst exhibited the small overpotential of 27.5 mV at a current density of 10 mA cm−2, as well as long-term stability for HER in acid media. Interestingly, the HER performance of hcp-Ru is highly dependent on its crystallinity. The calculation from density functional theory (DFT) revealed that the difference in HER activity over various exposed surface causes the crystallinity-dependent property of hcp-Ru. The results provided clues to guide the design of Ru-based inexpensive HER electrocatalyst. KEYWORDS: Ruthenium nanocatalyst, crystallinity-dependence, hydrogen evolution reaction, N-doped graphene, electrocatalyst

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reaction (ORR),20 and oxygen evolution reaction (OER).21 Theoretically, Ru is also an ideal HER catalyst, because it possesses similar hydrogen bond strength (∼65 kcal mol−1) with Pt;22 unfortunately, it has failed to exhibit comparable HER activity with Pt for a long time.23,24 As is known, the practical activity of a heterogeneous catalyst is usually strongly affected by its crystal properties, such as crystalline phase, degree of crystallinity, and size. For instance, the crystallographic dependence of HER over silver indium sulfide catalyst,25 the size dependence of CO2 electroreduction over nanopalladium catalyst,26 and the crystallinity dependence of OER over Ni−B catalyst,27 have been extensively reported. Among those, a catalyst’s crystallinity is particularly concerning, because it usually has an unpredictable effect on its electrocatalytic activity. In some cases, the high crystallinity is favorable for electrocatalysis, such as HER on tungsten carbide catalyst,28 while in other cases, the poor crystallinity is more favorable, like OER on CoWO4 catalyst.29 Recently, Baek30 and Qiao31 group separately found that the composites of Ru nanocrystal with C2N or

INTRODUCTION The environmental deterioration and the forthcoming depletion of traditional fossil resources have triggered a burst of requirements for the development of clean and sustainable energies.1−3 As a renewable and abundant source of fuel on the Earth, hydrogen has been considered an ideal clean energy for the replacement of fossil energy in the future.4−6 The production of hydrogen from electrocatalytic water reduction via hydrogen evolution reaction (HER) represents one of the most advanced technologies for sustainable and economic hydrogen production,7,8 but a high voltage is usually required to drive the reaction smoothly; hence, it is imperative to develop a highly efficient electrocatalysts for HER that can work at low overpotentials. Currently, Pt is the most effective catalyst, but their prohibitive cost and limited availability are big hurdles for its large-scale application.9−11 Although transition metals and their derivatives have been largely proposed to replace Pt-based catalysts,12−17 their activity is still far from satisfactory. Ru is a 4d transition metal, which is also a member of the precious metals family, but its price is 10 times lower than that of Pt.18 Ruthenium-based material are the famous electrocatalyst for many classical reactions, such as hydrogen oxidation reaction (HOR),14 methanol oxidation reaction,19 oxygen reduction © XXXX American Chemical Society

Received: April 25, 2018 Revised: May 20, 2018 Published: May 22, 2018 5714

DOI: 10.1021/acscatal.8b01609 ACS Catal. 2018, 8, 5714−5720

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ACS Catalysis Scheme 1. Illustration of the Fabricating Process of hcp-Ru@NC on NG

a robust 3D structure, was hierarchically porous, and contained abundant N heteroatoms, which was proposed in our previous work,36 was selected as the support to load hcp-Ru@NC. The final products were denoted as hcp-Ru@NC-T (here, T represents the annealing temperature). The crystalline behavior of RuNP@PDA during annealing process was first investigated by powder XRD, as shown in Figure 1. All the materials show a wide peak centered at 26°,

C3N4 nanosheets showed unprecedented electrocatalytic performance toward HER, which demonstrated the great potentials of Ru-based HER electrocatalyst; however, how the crystallinity of Ru affects the HER activity still remains unknown. Ru nanoparticle (RuNP) fabricated under ambient conditions usually adopts the disordered amorphous structure. Thermal annealing at high temperature can enhance its crystallinity.31 To prevent the coalescence during annealing process, a protective coating is commonly suggested. As an emerging protective coating, polydopamine has drawn great concern, because of the controllable thickness, abundant N sources, versatile adhesion properties, and excellent carbon yield.32−34 Herein, an hexagonal-closed-packed (hcp)-Ru nanocrystal coated with a thin layer of NC (hcp-Ru@NC) was fabricated through the thermal annealing of polydopamine (PDA)-coated Ru nanoparticle (RuNP@ PDA). The in-situ-formed NC layer can effectively protect the agglomeration of RuNP during the annealing process, which helps to explore the effect of crystallinity on HER activity. It was found, for the first time, that the HER performance of hcp-Ru is greatly dependent on its crystallinity. The fully crystallized hcp-Ru exhibits the best HER performance involving high electrocatalytic activity and super long-term stability in acid media.

Figure 1. XRD pattern of RuNP@PDA and hcp-Ru@NC annealed at various temperatures ranged from 400 °C to 800 °C.

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RESULTS AND DISCUSSION The hcp-Ru@NC was fabricated through a surfactant-assisted and PDA-reduced process, as shown in Scheme 1. The Ru precursor, RuCl3·3H2O, was first mixed with dopamine (DA) and cetyltrimethylammonium bromide (CTAB) in a phosphate buffer solution (PBS, pH 9). The dopamine quickly selfpolymerized to polydopamine (PDA), which was then solubilized or dispersed by CTAB under ambient conditions. Because of the strong interaction between Ru3+ and the −NH2 group,35 Ru3+ was largely absorbed by PDA and subsequently reduced to RuNP after the mixture was heated to 80 °C, as evidenced by transmission electron microscopy (TEM) (Figures S1a and S1b in the Supporting Information), X-ray diffraction (XRD) (Figure S1c in the Supporting Information), and Fourier transform infrared (FT-IR) spectroscopy (Figure S1d in the Supporting Information) results. The as-obtained RuNP@ PDA was dispersed on a carbon support and annealed at various temperatures for crystallization. The PDA was converted to a layer of NC shell during the annealing process. Meanwhile, a three-dimensional (3D) N-doped graphene (NG) that had

which belongs to the C(002) diffraction peak of graphene. Apart from this peak, the RuNP@PDA gives another peak located at 43.8°, which is assigned to the diffraction peak of Ru(101).37 The fairly weak and broad property of Ru(101) reveals the poor crystallinity of the as-prepared RuNP. The RuNP annealed at 400 °C (hcp-Ru@NC-400) shows an XRD pattern that is basically identical to that of RuNP@PDA, indicating that RuNP does not crystallize before 400 °C. At 500 °C (hcp-Ru@NC-500), the Ru(101) peak becomes very significant, and an extra small Ru(100) peak appears, suggesting that RuNP starts to crystallize at this temperature. When the temperature was increased to 600 °C (hcp-Ru@NC-600), both Ru(100) and Ru(101) peaks became more and more sharper, and a new Ru(002) peak emerges, which is indicative of the further improved crystallinity. As the temperature was further increased to 700 °C (hcp-Ru@NC-700), these three peaks became more intense and sharper, and an additional set of Ru(102), Ru(110), Ru(103), Ru(112), and Ru(201) peaks appear. In comparison with the standard XRD card of Ru, it was found that the crystalline phase is assigned to the hcp 5715


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Figure 2. (a, b, c) TEM images of hcp-Ru@NC-700 and (d) the corresponding SAED. The inset in panel (b) is the size distribution curve.

on their surface. The carbonization of PDA is first observed at 400 °C by the clear lattice fringe of carbon (Figure S2c), however, none of the RuNP display clear lattice fringe at this temperature. At 500 °C, RuNP can be found clearly, and few particles begin to exhibit the clear lattice fringe (Figure S2d). The number of RuNP with clear lattice fringes shows a slight increase at 600 °C (Figure S2e), and reaches a maximum at 700 °C (Figure S2f), whereas has minor degradation at 800 °C (Figure S2g) and 900 °C (Figure S2h). Clearly, the crystalline behavior of RuNP observed by in situ TEM agrees well with the result from XRD. Apart from the crystalline process, the evolution of the RuNP size also can be readily observed. The average diameter of the hcp-Ru@NC increases slowly with the temperature rising from 300 °C to 700 °C. In comparison with the changes of RuNP without carbon shell before and after the annealing process (Figure S3 in the Supporting Information), it can be found that the outer carbon layer exactly plays a critical role in alleviating the coalescence of particles. Upon the temperature further increasing to >800 °C, the size of particles increases sharply, most likely because the collapse of the NC shell results in coalescence of the neighboring RuNPs.38 The electrocatalytic HER performance of hcp-Ru@NC annealed at various temperatures was evaluated on a glass carbon electrode (GCE) in acidic media (0.5 M H2SO4 solution) via a conventional three-electrode electrochemical cell. All the catalysts have the basically equal Ru loading of ∼17 wt %, as determined by inductively coupled plasma−atomic emission spectrometry (ICP-AES). For comparison, that of the commercial Pt/C (20 wt %) catalyst was also measured under the same conditions. Their polarization curves and the corresponding Tafel plots of the electrocatalysts were collected in Figures 3a and 3b, respectively. Interestingly, the HER activity of hcp-Ru@NC obviously varies with the annealing temperature. As the temperature increases from room temperature to 700 °C, the HER activity promotes appreciably. Electrochemical active surface

structure. In addition, the XRD pattern of hcp-Ru@NC-800 is basically identical to that of hcp-Ru@NC-700, evidencing that RuNP has been fully crystallized at 700 °C. Consequently, the degree of crystallinity of RuNP varies with the annealing temperature, which is improved with the increase of temperature below 700 °C. The morphology of hcp-Ru@NC was further examined by transmission electron microscopy (TEM). Figure 2 showed the TEM images of the fully crystallized hcp-Ru@NC-700 at various resolutions. It can be seen that the 3D interconnected graphene sheets are thin and transparent, which is favorable to watch RuNP clearly (Figure 2a). RuNP is uniformly anchored on graphene sheets, and the diameter is estimated to be ∼4 nm (Figure 2b). hcp-Ru@NC-700 shows highly crystallized features, as indicated by the clear crystal lattice fringes under the observation of high-resolution TEM (HRTEM). The spacing of adjacent fringes is mainly 0.21 and 0.23 nm (Figure 2b), which is indexed to the (100) and (101) crystalline plane of hcp-Ru@NC, respectively. The NC layer outside hcp-Ru can be clearly observed from the particles at the edge of the graphene sheet. As estimated, the thickness of NC carbon shell is ∼0.8 nm (Figure 2c). In addition, as found from the selectedarea electron diffraction (SAED) pattern, hcp-Ru@NC-700 gives clear diffraction spots of (100), (002), (101), (110), and (112) (Figure 2d), also verifying its good crystallinity. The crystalline process of RuNP was also observed via an in situ TEM technique, as shown in Figure S2 in the Supporting Information. It is noteworthy that a crowded area was selected intentionally to avoid the shift of single RuNP at high temperature (Figure S2a). Also, the range of annealing temperature is extended to 300−900 °C for the purpose of watching the evolution process as completely as possible. The crystallinity of Ru was estimated by the number of RuNP with clear lattice fringe. At 300 °C (Figure S2b), both the RuNP and the graphene sheet show unclear profiles, because of the PDA covering 5716


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Figure 3. (a) Polarization curves, (b) the corresponding Tafel plots, (c) TOF value at the overpotential of 25 mV of the hcp-Ru@NC catalysts, and (d) the cycle stability tests of the hcp-Ru@NC-700 in 0.5 M H2SO4.

Pt/C catalyst (22.5 mV) (Figure 3a and 3b). The Tafel slope of hcp-Ru@NC-700 is as small as 37 mV dec−1, approaching to that of Pt/C (33 mV dec−1) as well. The value of the Tafel slope is located between 30 mV dec−1 and 40 mV dec−1, suggesting that HER is performed through the very efficient Volmer− Tafel process.42 At an overpotential of 25 mV, the hcp-Ru@NC700 exhibits an extremely high TOF of 1.6 s−1, which is much larger than those of the recently reported Ru-based catalyst.30,31 Moreover, hcp-Ru@NC-700 displays even better HER activity than that of Pt/C (see Figure S5 in the Supporting Information) in alkaline media, which is, unfortunately, inferior to some outstanding non-noble-metal electrocatalysts reported previously.43 The electrochemical stability of the hcp-Ru@NC was further assessed by an accelerated durability test (ADT) in acidic media. As shown in Figure 3d, after 20 000 cyclic voltammetry (CV) cycles, the polarization curve of hcp-Ru@NC-700 exhibits an almost-negligible shift to achieve the current density of 100 mA cm−2. In striking contrast, that of Pt/C shows a significant potential shift of ∼23 mV (Figure S6a in the Supporting Information). By comparison with the polarization curve of RuNP and hcp-Ru without an NC shell, it can be found that the remarkable stability of hcp-Ru@NC is mostly attributed to the dual effect of the NC shell, as well as the good crystallinity (see Figures S6b and S6c in the Supporting Information). The continuous HER performance of hcp-Ru@NC-700 at a static overpotential of 27.5 mV (η10) was also performed, as shown in Figure S7 in the Supporting Information. Clearly, the current density does not display any decay after a long period of 20 000 s. Both long-term tests afford evidence for the super stability of

area (ECSA) tested via the copper underpotential deposition (Cu-upd) method31 also verified this trend (see Figure S4 in the Supporting Information). Turnover frequency (TOF), which is the best figure-of-merit to evaluate the HER activity of catalysts, was calculated based on the ECSA and the current density from the polarization curves. As expected, the TOF value of the catalysts increases as the annealing temperature increases from 400 °C to 700 °C following the LSV, Tafel, and ECSA behavior (Figure 3c); note that the size of the catalysts is slightly increased in this temperature region, as revealed by the in situ TEM previously. Generally, the smaller size of the nanocatalyst is more beneficial to enhance electrocatalytic activity. Nevertheless, hcp-Ru@NC exhibited the abnormally improved electrocatalytic activity with the increased size. Therefore, there exists some other factors to affect the HER activity of hcp-Ru beyond the size. By excluding the effect of the outer NC shell, since PDA has been carbonized at 400 °C,33 the enhanced HER activity is most likely attributed to the positive contribution of the crystallinity, because they really feature very strong interdependency. That is, the HER activity of hcp-Ru is mostly dependent on its crystallinity. The hcp-Ru@NC-700 holds the highest degree of crystallinity and, hence, exhibits the best HER activity. When the annealing temperature reaches 800 °C, the effect of the size begins to play a dominant role, which results in the HER degradation of hcp-Ru@NC-800. In addition, as found from Figure 3a, the overpotential required to achieve a 10 mA cm−2 cathodic current density (η10) by the hcp-Ru@ NC-700 is as low as 27.5 mV, which is not only much lower than that of the previously reported nonprecious metal electrocatalysts,39−41 but also very close to that of the state-of-the-art 5717


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Figure 4. (a) Most favorable adsorption sites of H on various planes of Ru by DFT calculation (silver gray denotes Ru; tiny pink denotes H); (b) the corresponding ΔG(H*) on the fcc (111), hcp (100), hcp (002), and hcp (101) surfaces, respectively.

considered upon designing a Ru-based HER electrocatalyst in the future. In summary, hcp-Ru@NC was fabricated through the thermal annealing of RuNP@PDA. The crystallinity of hcpRu is gradually enhanced during the annealing process as the temperature increases below 700 °C. It was found, for the first time, that the HER activity of hcp-Ru is highly dependent on its crystallinity. The hcp-Ru@NC prepared at 700 °C presented the best HER performance, because of its highest degree of crystallinity. As an inexpensive alternative to the Pt/C catalyst, the hcp-Ru@NC-700 exhibited the fairly small overpotential of 27.5 mV at the current density of 10 mA cm−2 and Tafel slope of 34 mV dec−1, an extremely high TOF of 1.6 s−1, and super long-term stability in acid media. In combination with the DFT calculation, the crystallinity-dependent property of hcp-Ru toward HER was mostly attributed to the gradually exposed more-efficient surface of (100) and 002 during the annealing process. The results provided new clues to understand the active site of Ru nanocrystal, which is inspired to develop novel highly efficient and inexpensive HER electrocatalysts.

hcp-Ru@NC-700. In this regard, the highly crystallized hcpRu@NC nanocatalyst is an extremely active and durable electrocatalyst for HER in acidic media. It presents comparable activity to the state-of-the-art Pt/C catalyst, but it is more stable, as well as being at least 10 times less expensive than Pt, which makes it hold great promise for future commercialization. In order to elucidate the intrinsic reason of the crystallinitydependent property of hcp-Ru toward HER activity, the HER energetics over various exposed surfaces of the hcp-Ru were carried out via DFT. Based on the Tafel slope obtained experimentally, HER catalyzed by the hcp-Ru follows the Volmer− Tafel mechanism; hence, the HER activity is evaluated by the three-state diagram consisting of H+ (initial state), H* (intermediate state), and 1/2 H2 (final state). The Gibbs free energy of adsorption under room temperature and an electric potential of 1.23 V was derived based on Norskov’s scheme.44 Theoretically, a good catalyst features a moderate free energy for H adsorption (ΔG(H*)) as well as desorption.13,45 Based on the lattice fringe from the HRTEM and in situ TEM images, Ru slabs with hcp (0001), (101̅0), and (101̅1) were modeled to represent the dominantly observed surfaces of hcp (002), (100), and (101), respectively. For comparison, the Ru slab with a face-centered cubic (fcc) (111) plane representing fcc-Ru was calculated as well. The various Ru slabs with H sitting on their high-symmetry sites were shown in Figure S8 in the Supporting Information, and the corresponding adsorption energy was plotted in Figure S9 in the Supporting Information. It can be seen that various adsorption sites give a strong hydrogen binding, with energy value spanning from approximately −0.5 eV to approximately −0.7 eV. From Figure S8, the optimal adsorption site on various surfaces can be readily found (Figure 4a). It is assumed that H is dominantly adsorbed on the surfaces with the most favorable mode to conduct HER. The (ΔG(H*) of the typical Ru surfaces was shown in Figure 4b. Clearly, the HER activity of Ru over various surface follows the order of hcp (100) > hcp (002) > hcp (101) > fcc (111), which means that hcp (100) and (002) are the more efficient surfaces than hcp (101) to catalyze HER. Based on this result, the crystallinitydependent property of hcp-Ru toward HER is readily understandable that the gradually exposed more-efficient surface of hcp (100) and (002) with the increase of crystallinity cause the promoted HER activity. Besides, an interesting result is that all of the surface of hcp-Ru exhibits better HER activity than fccRu(111), which offers a clue that hcp-Ru may be a better HER electrocatalyst than fcc-Ru. As a result, the choice of the crystalline phase, and the regulation of crystalline surface, should be

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01609. Experimental section, the characterization of Ru@PDA, the in situ TEM images, the HER performance of catalysts in alkaline media, and the detailed DFT calculation process (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong Qin: 0000-0003-4563-8828 Yong Kong: 0000-0003-1270-3232 Yunfei Bu: 0000-0002-1488-5925 Meilin Liu: 0000-0002-6188-2372 Author Contributions ∇

These authors contributed equally to the work.

Notes

The authors declare no competing financial interest. 5718


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ACKNOWLEDGMENTS The authors are thankful to Prof. Qiaobao Zhang from Xiamen University, China, for their contributions on the in situ TEM observation. The experimental work is financially supported by the Natural Science Foundation of Jiangsu Province, China (No. BK20161191), the National Natural Science Foundation of China (Nos. 21476031 and 21673024), the Advanced Catalysis and Green Manufacturing Collaborative Innovation Center of Jiangsu Province, and the Jiangsu Key Laboratory of Advanced Materials and Technology (No. BM2012110). The computational work used Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation (Grant No. TG-DMR140083), and National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy (under Contract No. DE-AC02-05CH11231).

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