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Ruthenium-nickel sandwiched nanoplates for efficient water splitting electrocatalysis
Author’s Accepted Manuscript Ruthenium-Nickel Sandwiched Nanoplates for Efficient Water Splitting Electrocatalysis Jiabao Ding, Qi Shao, Yonggang Feng, Xiaoqing Huang www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(18)30084-3 https://doi.org/10.1016/j.nanoen.2018.02.017 NANOEN2504
To appear in: Nano Energy Received date: 17 November 2017 Revised date: 6 February 2018 Accepted date: 8 February 2018 Cite this article as: Jiabao Ding, Qi Shao, Yonggang Feng and Xiaoqing Huang, Ruthenium-Nickel Sandwiched Nanoplates for Efficient Water Splitting Electrocatalysis, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.02.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ruthenium-Nickel Sandwiched Nanoplates for Efficient Water Splitting Electrocatalysis Jiabao Ding1†, Qi Shao1†, Yonggang Feng, Xiaoqing Huang*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China. *
To whom correspondence should be addressed. E-mail: [email protected]
Abstract Since the interface in catalyst directly influences the catalytic performance, it is imperative to create hierarchical nanostructures with modulated atomic arrangements at the interfaces for catalysis optimization. Herein, we report an unprecedented ruthenium-nickel (Ru-Ni) heterostructure
with
phase-segregated
sandwich-like
morphology
for
boosting
electrocatalysis. The Ru selectively grows at the two ends of the Ni pillar, in which intimate interfaces is formed between Ru and Ni domains. We found such Ru-Ni sandwiched nanoplates (SNs) are highly efficient for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolytes with very low overpotentials and Tafel slopes. Significantly, the optimized Ru-Ni SNs deliver a low onset potential of only 1.45 V and enhanced durability in the overall water splitting device, indicating a promising electrocatalyst towards the practical alkaline electrolysis.
Graphical abstract
1
Jiabao Ding and Qi Shao contributed equally
1
Keywords: Phase segregation, Interface, Sandwiched nanoplates, Hydrogen evolution, Oxygen evolution, water splitting
1. Introduction Water electrolysis is one of the most promising approaches to produce hydrogen and oxygen in high purity.[1] It is composed of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), both of which require efficient catalysts to reduce the reaction overpotentials and accelerate the process.[2] Pt is still the most active metal for HER due to its zero onset overpotential and low Tafel slope,[3-5] whereas the stateof-the-art OER electrocatalysts are still Ir or Ru based materials.[6-8] However, the practical applications of noble metals for water splitting are severely hindered, owing to their scarcities and high cost.[9] Therefore, achieving highly efficient Pt (or Ru or Ir) based nanocatalysts for water splitting electrolysis is highly desirable but still a great challenge. Recently, the creation of interface in nanocatalysts is attracting more attention due to its critical role in tuning the catalytic activity and stability.[10-12] The integration of noble metals with non-noble metals can not only decrease the consumption of noble metals, but also enhance the performances by tailoring the atomic arrangements at the interfaces.[13-17] In this regard, maximizing the atomic (or nano) interface contact/or interaction between two kinds of materials becomes very important for maximizing the electrocatalytic performance, however, achieving such target is still a great challenge. There have been only a few papers on engineering Ru-based bimetallic nanocatalyst with the application only in OER.[18]
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Herein, we report a facile wet chemical strategy to create unique sandwich-like Ru-Ni heterostructures with two hexagonal Ru rings caped on the two ends of a Ni pillar to achieve more efficient water splitting electrocatalyst in alkaline electrolytes. The Ru2Ni2 sandwiched nanoplates (SNs) with optimized interface show the lowest HER overpotential of 40 mV at 10 mA cm-2 and Tafel slope of 23 mV dec-1 in 1 M KOH, much lower than those of the commercial Pt/C (20% Pt). The Ru2Ni2 SNs are also more active for OER than the commercial Pt/C. More significantly, when applied in the alkaline overall water splitting electrolysis, the Ru2Ni2 SNs exhibit low onset potential of only 1.45 V, much better than that of the Pt/C, which makes them among the best water splitting electrocatalysts. They are also stable for water splitting with limited overpotential changes. The present works highlights the importance of interfacial modulating in boosting multicomponent water splitting electrocatalysis. 2. Experimental section Chemicals: Ruthenium(III) acetylacetonate (Ru(acac)3, 97%) was purchased from SigmaAldrich (USA). Nickel(II) acetylacetonate (Ni(acac)2, 96%) and polyvinyl pyrrolidone (PVP, AR) were obtained from J&K Scientific Ltd. (Shanghai, China). Potassium hydroxide (KOH, 85%) and benzyl alcohol (C6H5CH2OH, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Pt/C (20 wt% Pt) was purchased from Johnson Matthey Company. Ir/C (20% wt Ir) was purchased from Premetek Co. Nafion (5 wt%) were from Sigma-Aldrich. All the chemicals were used as received without further purification. The water (18 MΩ cm-1) used in all experiments was prepared by an ultra-pure purification system. Synthesis of Ru-Ni sandwiched nanoplates (Ru-Ni SNs): In a typical preparation of Ru-Ni SNs, 10 mg of Ru(acac)3, 6.6 mg of Ni(acac)2, 160 mg of PVP and 10 mL benzyl alcohol were added in a vial (volume: 30 mL). The vial was capped and ultrasonicated for around 30
3
min. The resulting red solution was then heated at 170 ºC for 5 h in an oil bath before it cooled down naturally. The black product was collected by centrifugation and washed with the ethanol/acetone mixtures for several times. Characterization: Low-magnification transmission electron microscopy (TEM) was conducted on a HITACHI HT7700TEM at an acceleration voltage of 120 kV. Highmagnification TEM and scanning TEM (STEM) were conducted on an FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. Energy-dispersive Xray spectroscopy (EDS) patterns were taken from a HITACHI S-4700 cold field emission scanning electron microscope operated at 15 kV. X-ray diffraction (XRD) pattern was collected on X’Pert-Pro MPD diffractometer (Netherlands PANalytical) with a Cu Kα X-ray source (λ = 1.540598 Å). X-ray photoelectron spectroscopy (XPS) spectra were conducted on a Thermo Scientific ESCALAB 250 XI X-ray photoelectron spectrometer. The concentration of Ru and Ni in the after stability test was determined by the inductively coupled plasma optical emission spectrometer (Varian 710-ES). Electrochemical measurements: For all the electrochemical tests, a three-electrode system was used to conduct the electrochemical measurements at an electrochemical workstation (CHI660E). A saturated calomel electrode, a glassy carbon electrode (diameter: 5 mm, area: 0.196 cm2) coated with catalyst and a graphite rod were used as the reference electrode, the working electrode and the counter electrode, respectively. To prepare the working electrode, the Ru-Ni SNs were loaded on 10 mg carbon black (VXC-72, carbot) and denoted as asprepared RuxNiy SNs/C. The as-prepared RuxNiy SNs/C were then treated in an oven at 250 ºC for 1 h and termed as RuxNiy SNs/C. The RuxNiy SNs/C (2 mg) and Nafion solution (5 wt %, 10 μL) were dispersed in isopropanol to form a homogeneous ink with the assistance of sonication. 10 μL of the catalyst ink were then loaded onto a glassy carbon electrode (catalyst loading is 0.10 mg cm-2). Linear sweep voltammetries of both hydrogen evolution reaction
4
(HER) and oxygen evolution reaction (OER) were performed in 0.1 M and 1 M KOH solution at scan rate of 5 mV s-1 and all the polarization curves were iR-corrected. The overall water splitting was conducted with a two-electrode system at scan rate of 5 mV s-1 in 50 mL of 1 M KOH. For the long-term stability test, 8 mg of the catalyst was dispersed in the solution of 920 μL isopropanol and 80 μL of the Nafion. 375 μL of the ink was dropped onto the carbon fiber paper (area: 1 × 0.5 cm-2) with catalyst loading of 3 mg cm-2. The catalyst loadings of the Ir/C and the Pt/C are 1 mg cm-2. All the potentials were calibrated and reported respect to the reversible hydrogen electrode (RHE). 3. Results and Discussion The synthesis of Ru-Ni SNs was realized by a colloid chemistry method using ruthenium(III) acetylacetonate (Ru(acac)3) and nickel(II) acetylacetonate (Ni(acac)2) as the metal precursors, polyvinyl pyrrolidone (PVP) as the surfactant and benzyl alcohol as the solvent, respectively (see Supporting Information for details). The composition and structure were characterized by energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The atomic ratio of the as-prepared Ru2Ni2 SNs is 53.5/46.5 (Figure S1a). The TEM images reveal that the Ru2Ni2 SNs are composed of sandwich-like morphology with hexagonal outline at the first glance (Figures 1a&S2). The sharp contrast between the ring and the hexagonal center indicates the formation of different components between them. The other projection further displays that a pillar is sandwiched by two nanoplates at each end (Figure 1a). The diameter based on the length of the diagonal across the two terminal hexagonal nanoplates of these nanostructures is averaged at 16 nm (Figure S3). The morphology of the Ru2Ni2 SNs is also vividly displayed in the high-angle annular dark-field scanning TEM (HAADF-STEM) images, demonstrating the sharp contrast of the terminal plates (Figures 1b&S2f&S4a). The XRD pattern of the Ru2Ni2 SNs shows that a small peak locates at 39° indexed to the Ru (100) facet, while a faint small peak at 51.8°
5
can be attributed to Ni (200) facet. The broadened peak at around 43.5°corresponds to the Ru (002) and (101) facets (JCPDS 06-0663) (Figure S1b).[19] The measured interplanar distance is 0.23 nm, corresponding to the Ru (100) facet (Figure 1c). The projection along the <001> direction reveals that the interplanar distance is 0.21 nm, attributed to the Ru (002) facet (Figure 1d). To reveal the elemental distributions of Ru and Ni, the line-scanning profiles and elemental mappings were carefully collected (Figures 1e-f). The compositional line-scanning profiles across the three lines further reveal the inhomogeneous elemental distributions of Ru and Ni, where Ru largely distributes at the two plates and Ni mainly concentrates in the middle (Figure 1e). The elemental mappings turn out that Ru and Ni are not uniformly distributed through the Ru2Ni2 SN with Ni located at the center and Ru mainly existed in the ring region. Therefore, this unique structure is considered as a phase segregated heterostructure with distributing at the two ends of the Ni pillar.
6
Figure 1. (a) TEM image (inset is an ideal model), (b) HAADF-STEM image, and (c, d) HRTEM images with (c) along and (d) perpendicular to the <001> direction of Ru2Ni2 SNs. The insets in (c) and (d) are the ideal models projected (c) along and (d) perpendicular to the <001> direction, respectively. (e) Line-scanning profiles across Ru2Ni2 SNs. (f-h) EDS elemental mappings of (f) Ru2Ni2 SNs, (g) Ru2Ni1 SNs and (h) Ru2Ni3 SNs).
7
The unique structure of Ru-Ni SNs with Ru growing along the edges of the pillar is the most striking feature of our synthesis. To comprehend the growth process, the intermediates collected at different time were also characterized. Before 27 min, no product could be collected by centrifugation. Hereafter, the solution color changed into black quickly, producing small quasi-hexagonal pillars with the average size of 6.9 nm. The atomic ratio of Ni to Ru was as high as about 90%, indicating much higher reduction rate of Ni than that of Ru at the initial stage (Figures S5a&S5f&S6a). The fringe spacing of the product is 0.206 nm, close to the interplanar distance of the (111) facet in fcc Ni (0.203 nm) (Figure S7ab).[20] The elemental mappings reveal the phase-segregated structure of this intermediate (Figure S7c). With 1 more min reaction, the Ni content in the product decreased to 75%, while its average size increased to 7.7 nm (Figures S5b&S6b). As the reaction time reached 0.5 h, the morphology of the product changed into the sandwiched-like feature with nanoplates growing at the ends and the Ni content down to 62% (Figures S5c&S6c). With increasing the reaction time to 1 h, a rapid increase in the average size of Ru2Ni2 SNs was observed, along with the atomic ratio of Ni promptly dropped to 49 % (Figure S6d). Further prolonging the reaction time to 5 h, no obvious changes were observed in the size and the Ni content, indicating the reaction completed after first 1 h (Figures S6e-g). The XRD patterns of the intermediates display the enhancements of the peak intensities at 39.0°, 70.2° and 85.6° as the reaction proceeded, indicating the increase of Ru (Figure S6h). In the present synthesis, no product could be obtained without the addition of Ru precursor while other parameters unchanged, showing that Ru can play a significant role in inducing the reduction of Ni in the reaction process.[21] As previously demonstrated, Ru preferred to selectively grow on the corners and edges of the seeds.[22-25] Since Ni and Pd locate at the same column of the periodic table, similar properties are expected. Therefore, Ru prefers to grow on the Ni substrate, thus controlling the morphology of Ni.[26] Notably, hexagonal Ni
8
prism formed in a very short time may play a role of template with its vertex and edges as the active sites for the growth of Ru. It should be noted that the bulk Ru and most of the Ru nanocrystals keep the hcp structure while the Ni remains the fcc structure in the bulk.[19, 2729] Hence, the Ru can take the hcp phase, and Ni can possess the fcc phase, as observed in the XRD patterns (Figure S1b). Since the Ru (001) facet has lower surface energy than the Ru (100) facet, the Ru SNs will expose the Ru (001) facet to minimize the surface energy.[19] By combining the time-dependent investigations, an evolution process is proposed, which contains two stages: the formation of a hexagonal prism of Ni by the axial growth under the control of Ru and the emerging of the Ru ring by the radial growth at the corners and edges. Through adjusting the molar ratio of Ru(acac)3 to Ni(acac)2 at 2/1 and 2/3, the Ru2Ni1 SNs and Ru2Ni3 SNs could also be made, as determined by the SEM-EDS with the molar ratios of Ru to Ni being 68.6/31.4 and 44.2/55.8 for Ru2Ni1 SNs and Ru2Ni3 SNs, which matches well with the molar ratio of Ru precursor to Ni precursor (Figure S8). These Ru-Ni SNs with different compositions have similar morphologies, with the average size of 15 nm and 16 nm, respectively (Figures S2a-c&g-i&S3a,3c). The XRD patterns of Ru2Ni1 SNs and Ru2Ni3 SNs are similar to that of Ru2Ni2 SNs (Figure S9). The peak at 38.7° can be indexed to the (100) facet of Ru (JCPDS Card. 06-0663), in which the intensity decreases with the rise of the Ni content. The peak at 50.3° assigned to Ni (200) facet becomes more obvious. The TEM and HRTEM images demonstrate the structures of both Ru2Ni1 SNs and Ru2Ni3 SNs with interplanar distance of 0.23 nm along the <001> direction and 0.21 nm perpendicular to the <001> direction, respectively (Figure S10). The EDS elemental mappings (Figures 1g-h) and line-scanning profiles (Figures S11) of Ru2Ni1 SNs and Ru2Ni3 SNs determine the phase segregated structure with Ru mainly dispersed in the ring region of the terminal plates and Ni concentrated at the center. Comparing the EDS elemental mappings of Ru2Ni3 SNs with those of Ru2Ni2 SNs and Ru2Ni1 SNs, we observe that the Ni in the center region increases with
9
increasing the Ni content in the SNs, vividly revealed by the ideal models in the insets of Figures 1f-h. Considering the structural and compositional anisotropy of the Ru-Ni SNs, we studied the HER and OER activities of the Ru-Ni SNs in alkaline solution. The commercial Pt/C and Ir/C were selected as the reference. Before the electrocatalytic measurements, the Ru-Ni SNs were loaded onto the carbon black (Carbon, VXC-72) (denoted as as-prepared Ru-Ni SNs/C, Figures S12a-b). The amounts of the Ru-Ni SNs load on carbon black are 5.9%, 5.5% and 6.7% (wt% Ru) for the Ru2Ni1 SNs, Ru2Ni2 SNs and Ru2Ni3 SNs, respectively (Table S1). The Ru-Ni SNs/C was then annealed in air to clean the surface of the Ru-Ni SNs (renamed as Ru-Ni SNs/C, Figures S12c-d). The HER and OER activities of the treated Ru-Ni SNs/C are better than the as-prepared Ru-Ni SNs/C (Figure S13). Figure 2a reveals the iR-corrected polarization curves of Ru-Ni SNs/C, the Pt/C, and the Ir/C for HER in 0.1 M KOH. All the Ru-Ni SNs/C exhibit near zero onset potentials, lower than those of the Pt/C and Ir/C. The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples were summarized in Figure 2b. It intuitively demonstrates that the Ru2Ni1 SNs/C, Ru2Ni2 SNs/C and Ru2Ni3 SNs/C possess average overpotentials of 42.7 mV, 39.3 mV and 44.3 mV and average Tafel slopes of 27.9 mV dec-1, 25.0 mV dec-1 and 29.5mV dec-1 respectively, which are much better than those of Pt/C (59.0 mV and 33.9 mV dec-1) and Ir/C (66.3 mV and 37.4 mV dec-1), indicating the best catalytic performance of Ru2Ni2 SNs/C (Figure 2b). The polarization curves in higher alkaline environment (1 M KOH) were also collected. The onset potential of Ru2Ni2 SNs/C is as low as 11 mV (Figure 2c). The corresponding average overpotentials at 10 mA cm-2 and the average Tafel slopes for different catalysts are presented in Figure 2d. We can see that the average overpotentials at 10 mA cm-2 of Ru2Ni1 SNs/C, Ru2Ni2 SNs/C, Ru2Ni3 SNs/C, the Pt/C and the Ir/C are 47.7 mV, 40.0 mV, 47.7 mV, 56.7 mV and 65.3 mV,
10
respectively. The Ru-Ni SNs possess the NiOx/Ru interface, facilitating the water dissociation by the synergistic effect.[30] The lowest average overpotential and average Tafel slope of 23.4 mV dec-1 in 1 M KOH of Ru2Ni2 SNs/C suggests the Volmer-Heyrovsky mechanism took place in the Ru-Ni SNs (Figure 2d).[31]
Figure 2. (a) Polarization curves of various Ru-Ni SNs/C, the Pt/C and the Ir/C for HER in 0.1 M KOH. (b) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples. (c) Polarization curves of various Ru-Ni SNs/C, the Pt/C and the Ir/C for HER in 1 M KOH. (d) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples. All the polarization curves are iR-corrected. The error bars represent the standard deviations based on triplicate measurements.
The Ru-Ni SNs are also active for OER. As presented in Figure 3a, the Ru2Ni2 SNs/C
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exhibit the lowest onset potential for OER in 0.1 M KOH (1.45 V). The average overpotential of Ru2Ni2 SNs/C at 10 mA cm-2 for OER is 357 mV (Figure 3b), which is the lowest in all the investigated catalysts in this work. The polarization curves in 1 M KOH illustrate that the Ru-Ni SNs/C have lower onset potentials than those of the Pt/C and the Ir/C (Figure 3c). The average overpotentials at 10 mA cm-2 for the three Ru-Ni SNs/C are similar with that of Ru2Ni2 SNs/C being the lowest, much lower than that for the commercial Pt/C and Ir/C (Figure 3d).
Figure 3. (a) Polarization curves of various Ru-Ni SNs/C, the Pt/C and the Ir/C for OER in 0.1 M KOH. (b) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of calculated Tafel slopes obtained from at least three electrodes samples. (c) Polarization curves of Ru-Ni SNs/C, the Pt/C and the Ir/C for OER in 1 M KOH. (d) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples. All the
12
polarization curves are iR-corrected. The error bars represent the standard deviations based on at least triplicate measurements.
Based on their highly efficient bifunctional properties, the Ru-Ni SNs/C was further used as catalysts for overall water splitting in alkaline condition. Figure 4a displays the iRcorrected polarization curves of various Ru-Ni SNs/C and the Pt/C for HER and OER in 0.1 M KOH. The voltage difference (△V) between OER and HER of Ru2Ni2 SNs/C is 1.63V. In 1 M KOH, Ru2Ni2 SNs/C exhibits the △V of 1.58 V at 10 mA cm-2, much lower than that of the commercial Pt/C (1.805 V), and outperforming most of the previous overall water splitting catalysts (Figure 4b) (Table S2). As shown in Figure 4c, the iR-corrected polarization curve of Ru2Ni2 SNs is basically the same as the voltage difference (ΔV) between HER and OER with the onset potential of 1.45 V, which is close to the calculated voltage difference (ΔV) between theoretical onset potentials of OER and HER.[32] The longterm stability of Ru2Ni2 SNs/C was then evaluated in two-electrode configuration in 1 M KOH, where slight decrease in potential (12 mV) was observed after 40 h, showing its excellent long-term stability (Figure 4d). The Ru2Ni2 SNs/C shows better stability with the potential changing from 1.68 V to 1.97 V after 105 h, while Ir/C and Pt/C almost loss the electrochemical activity after 90 h measurement (Figure S14). The morphology of the Ru2Ni2 SNs/C at the cathode after the stability test was also checked by TEM, revealing that while the shape was largely maintained, the molar ratio of Ru to Ni was changed into 3 (Figures S12e-f&Figure S15), indicating the partial dissolution of Ni during the reaction. The ICP measurement of the solution after durability testing was also conducted, which turned out that the Ru leached out was only 13.8 % of the Ru loaded on the electrode, indicating the enhanced durability of Ru.
13
Figure 4. (a) IR-corrected polarization curves of various Ru-Ni SNs/C and the Pt/C for HER and OER in (a) 0.1 M KOH and (b) 1 M KOH. (c) The IR-corrected polarization curves for overall water splitting of Ru2Ni2 SNs/C tested in a two-electrode configuration. The inset is the iR-corrected polarization curves of Ru2Ni2 SNs/C for HER and OER. (d) The chronopotentiometric curve of Ru2Ni2 SNs/C and Ir/C and Pt/C in the water electrolysis with two-electrode configuration at 10 mA cm-2.
The X-ray photoelectron spectroscopy (XPS) was applied to investigate the structure of the Ru-Ni SNs/C (Figures 5a-b&S16). The surface Ru/Ni ratios are 35.8/64.2 for Ru2Ni1 SNs/C, 28.3/71.7 for Ru2Ni2 SNs/C and 10.9/89.1 for Ru2Ni3 SNs/C, respectively (Table S3). The deconvoluted spectra of Ru 3d and Ni 2p reveal that the coexistence of the Ru4+ and Ru0, and Nix+ and Ni0. The binding energies of Ru0 3d5/2 and Ru0 3d3/2 are 280.43 eV and 284.62 eV, respectively, corresponding to the metallic Ru.[25] The peak at the binding energy of 281.14 can be ascribed to Ru4+ 3d5/2.[33] The binding energies of Ni0 2p3/2 and Nix+ 2p3/2 for Ru-Ni SNs are 852.9 eV and 855.94 eV.[9] As can be seen from Figure 5c-d&Table S4, the Ru2Ni2
14
SNs/C has the largest ratios of Ru4+/Ru0 and Nix+/Ni0. The volcano-like molar ratio change of Ru4+/Ru0 and Nix+/Ni0 is well consistent with the water splitting activity change trend, suggesting that the higher ratios of Ru4+/Ru0 and Nix+/Ni0 in Ru2Ni2 SNs/C is probably the key in boosting the water splitting activity.
Figure 5. (a) Ru 3d and (b) Ni 2p XPS spectra of Ru2Ni2 SNs/C. Area ratios of (c) Ru4+/Ru0 and (d) Nix+/Ni0 for Ru-Ni SNs obtained from their corresponding XPS spectra.
4. Conclusion To summarize, we demonstrate a facile wet-chemical strategy for the preparation of unique Ru-Ni SNs with a Ni pillar inserting into two hexagonal Ru rings. We found that the formation of the Ru-Ni SNs underwent the initial growth of quasi-hexagonal Ni pillars by the axial growth under the control of Ru and the subsequent formation of the Ru ring at the corners and edges by the radial growth. When applied as electrocatalyst towards alkaline
15
HER, the Ru2Ni2 SNs/C with optimized interface exhibit low overpotential as well as Tafel slope in KOH. For OER, the Ru2Ni2 SNs/C also show much lower overpotential and lower Tafel slope than the Pt/C. When applied as alkaline overall water splitting electrocatalyst, the Ru2Ni2 SNs/C exhibit very low onset potential of 1.45 V and also excellent durability after long-term chronopotentiometry, showing a promising electrocatalyst for practical alkaline electrolysis and beyond.
ASSOCIATED CONTENT Supporting Information. Figure S1-16&Table S1-4. This material is available free of charge via the Internet at https://www.journals.elsevier.com/nano-energy/. Corresponding Author [email protected] ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, the start-up supports from Soochow University, Natural Science Foundation of Jiangsu Higher Education Institutions (17KJB150032) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). J. Ding and Q. Shao contribute equally to this work.
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Recent personal portrait photo and biosketch of all the authors.
Jiabao Ding received his Master’s degree in Xiamen University, China in 2014. He is currently a Ph.D. candidate under the supervision of Prof. Huang in College of Chemistry, Chemical Engineering and Materials Science in Soochow University. His major is inorganic
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chemistry and his research interests are the design of nanomaterials for energy storage and conversion.
Dr. Qi Shao received her Ph.D. degree in applied physics from City University of Hong Kong in 2016. Now she is an assistant professor in Professor Huang’s group at College of Chemistry, Chemical Engineering and Materials Science, Soochow University. Her current research interests are focusing on non-noble metal based catalysts for electrochemical applications.
Yonggang Feng is currently a M.S. candidate under the supervision of Prof. Huang in College of Chemistry, Chemical Engineering and Materials Science in Soochow University. He received his B.S degree in Jiaxing University, China in 2015. His major is inorganic chemistry and he is now focused on the design of noble metal nanomaterials for heterogenous catalysis and electrocatalysis.
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Dr. Xiaoqing Huang is currently a Professor at College of Chemistry, Chemical Engineering and Materials Science, Soochow University. He obtained his B.Sc. in chemistry education from Southwest Normal University (2005) and Ph.D. in organic chemistry from Xiamen University (2011) under the supervision of Profs. Nanfeng Zheng and Lansun Zheng. Then he joined Profs. Yu Huang and Xiangfeng Duan’s group as a postdoctoral research associate from September 2011 to June 2014 at University of California, Los Angeles. His current research interests are in the design of nanoscale materials for heterogenous catalysis, electrocatalysis, energy conversion and beyond.
Highlights
For the first time, a spool-like Ru-Ni nanocrystal (NC) with Ru distributed in the hexagonal ring and Ni concentrated in the pillar was successfully created. This unique Ru-Ni NC exhibits outstanding activity and enhanced stability toward both hydrogen evolution reaction and oxygen evolution reaction, much higher than those of the commercial Pt/C. Significantly, the optimized Ru-Ni SNs deliver a low onset potential of only 1.45 V and enhanced durability in the overall water splitting device, indicating a promising electrocatalyst towards the practical alkaline electrolysis.
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PII: DOI: Reference:
S2211-2855(18)30084-3 https://doi.org/10.1016/j.nanoen.2018.02.017 NANOEN2504
To appear in: Nano Energy Received date: 17 November 2017 Revised date: 6 February 2018 Accepted date: 8 February 2018 Cite this article as: Jiabao Ding, Qi Shao, Yonggang Feng and Xiaoqing Huang, Ruthenium-Nickel Sandwiched Nanoplates for Efficient Water Splitting Electrocatalysis, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.02.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ruthenium-Nickel Sandwiched Nanoplates for Efficient Water Splitting Electrocatalysis Jiabao Ding1†, Qi Shao1†, Yonggang Feng, Xiaoqing Huang*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China. *
To whom correspondence should be addressed. E-mail: [email protected]
Abstract Since the interface in catalyst directly influences the catalytic performance, it is imperative to create hierarchical nanostructures with modulated atomic arrangements at the interfaces for catalysis optimization. Herein, we report an unprecedented ruthenium-nickel (Ru-Ni) heterostructure
with
phase-segregated
sandwich-like
morphology
for
boosting
electrocatalysis. The Ru selectively grows at the two ends of the Ni pillar, in which intimate interfaces is formed between Ru and Ni domains. We found such Ru-Ni sandwiched nanoplates (SNs) are highly efficient for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolytes with very low overpotentials and Tafel slopes. Significantly, the optimized Ru-Ni SNs deliver a low onset potential of only 1.45 V and enhanced durability in the overall water splitting device, indicating a promising electrocatalyst towards the practical alkaline electrolysis.
Graphical abstract
1
Jiabao Ding and Qi Shao contributed equally
1
Keywords: Phase segregation, Interface, Sandwiched nanoplates, Hydrogen evolution, Oxygen evolution, water splitting
1. Introduction Water electrolysis is one of the most promising approaches to produce hydrogen and oxygen in high purity.[1] It is composed of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), both of which require efficient catalysts to reduce the reaction overpotentials and accelerate the process.[2] Pt is still the most active metal for HER due to its zero onset overpotential and low Tafel slope,[3-5] whereas the stateof-the-art OER electrocatalysts are still Ir or Ru based materials.[6-8] However, the practical applications of noble metals for water splitting are severely hindered, owing to their scarcities and high cost.[9] Therefore, achieving highly efficient Pt (or Ru or Ir) based nanocatalysts for water splitting electrolysis is highly desirable but still a great challenge. Recently, the creation of interface in nanocatalysts is attracting more attention due to its critical role in tuning the catalytic activity and stability.[10-12] The integration of noble metals with non-noble metals can not only decrease the consumption of noble metals, but also enhance the performances by tailoring the atomic arrangements at the interfaces.[13-17] In this regard, maximizing the atomic (or nano) interface contact/or interaction between two kinds of materials becomes very important for maximizing the electrocatalytic performance, however, achieving such target is still a great challenge. There have been only a few papers on engineering Ru-based bimetallic nanocatalyst with the application only in OER.[18]
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Herein, we report a facile wet chemical strategy to create unique sandwich-like Ru-Ni heterostructures with two hexagonal Ru rings caped on the two ends of a Ni pillar to achieve more efficient water splitting electrocatalyst in alkaline electrolytes. The Ru2Ni2 sandwiched nanoplates (SNs) with optimized interface show the lowest HER overpotential of 40 mV at 10 mA cm-2 and Tafel slope of 23 mV dec-1 in 1 M KOH, much lower than those of the commercial Pt/C (20% Pt). The Ru2Ni2 SNs are also more active for OER than the commercial Pt/C. More significantly, when applied in the alkaline overall water splitting electrolysis, the Ru2Ni2 SNs exhibit low onset potential of only 1.45 V, much better than that of the Pt/C, which makes them among the best water splitting electrocatalysts. They are also stable for water splitting with limited overpotential changes. The present works highlights the importance of interfacial modulating in boosting multicomponent water splitting electrocatalysis. 2. Experimental section Chemicals: Ruthenium(III) acetylacetonate (Ru(acac)3, 97%) was purchased from SigmaAldrich (USA). Nickel(II) acetylacetonate (Ni(acac)2, 96%) and polyvinyl pyrrolidone (PVP, AR) were obtained from J&K Scientific Ltd. (Shanghai, China). Potassium hydroxide (KOH, 85%) and benzyl alcohol (C6H5CH2OH, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Pt/C (20 wt% Pt) was purchased from Johnson Matthey Company. Ir/C (20% wt Ir) was purchased from Premetek Co. Nafion (5 wt%) were from Sigma-Aldrich. All the chemicals were used as received without further purification. The water (18 MΩ cm-1) used in all experiments was prepared by an ultra-pure purification system. Synthesis of Ru-Ni sandwiched nanoplates (Ru-Ni SNs): In a typical preparation of Ru-Ni SNs, 10 mg of Ru(acac)3, 6.6 mg of Ni(acac)2, 160 mg of PVP and 10 mL benzyl alcohol were added in a vial (volume: 30 mL). The vial was capped and ultrasonicated for around 30
3
min. The resulting red solution was then heated at 170 ºC for 5 h in an oil bath before it cooled down naturally. The black product was collected by centrifugation and washed with the ethanol/acetone mixtures for several times. Characterization: Low-magnification transmission electron microscopy (TEM) was conducted on a HITACHI HT7700TEM at an acceleration voltage of 120 kV. Highmagnification TEM and scanning TEM (STEM) were conducted on an FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. Energy-dispersive Xray spectroscopy (EDS) patterns were taken from a HITACHI S-4700 cold field emission scanning electron microscope operated at 15 kV. X-ray diffraction (XRD) pattern was collected on X’Pert-Pro MPD diffractometer (Netherlands PANalytical) with a Cu Kα X-ray source (λ = 1.540598 Å). X-ray photoelectron spectroscopy (XPS) spectra were conducted on a Thermo Scientific ESCALAB 250 XI X-ray photoelectron spectrometer. The concentration of Ru and Ni in the after stability test was determined by the inductively coupled plasma optical emission spectrometer (Varian 710-ES). Electrochemical measurements: For all the electrochemical tests, a three-electrode system was used to conduct the electrochemical measurements at an electrochemical workstation (CHI660E). A saturated calomel electrode, a glassy carbon electrode (diameter: 5 mm, area: 0.196 cm2) coated with catalyst and a graphite rod were used as the reference electrode, the working electrode and the counter electrode, respectively. To prepare the working electrode, the Ru-Ni SNs were loaded on 10 mg carbon black (VXC-72, carbot) and denoted as asprepared RuxNiy SNs/C. The as-prepared RuxNiy SNs/C were then treated in an oven at 250 ºC for 1 h and termed as RuxNiy SNs/C. The RuxNiy SNs/C (2 mg) and Nafion solution (5 wt %, 10 μL) were dispersed in isopropanol to form a homogeneous ink with the assistance of sonication. 10 μL of the catalyst ink were then loaded onto a glassy carbon electrode (catalyst loading is 0.10 mg cm-2). Linear sweep voltammetries of both hydrogen evolution reaction
4
(HER) and oxygen evolution reaction (OER) were performed in 0.1 M and 1 M KOH solution at scan rate of 5 mV s-1 and all the polarization curves were iR-corrected. The overall water splitting was conducted with a two-electrode system at scan rate of 5 mV s-1 in 50 mL of 1 M KOH. For the long-term stability test, 8 mg of the catalyst was dispersed in the solution of 920 μL isopropanol and 80 μL of the Nafion. 375 μL of the ink was dropped onto the carbon fiber paper (area: 1 × 0.5 cm-2) with catalyst loading of 3 mg cm-2. The catalyst loadings of the Ir/C and the Pt/C are 1 mg cm-2. All the potentials were calibrated and reported respect to the reversible hydrogen electrode (RHE). 3. Results and Discussion The synthesis of Ru-Ni SNs was realized by a colloid chemistry method using ruthenium(III) acetylacetonate (Ru(acac)3) and nickel(II) acetylacetonate (Ni(acac)2) as the metal precursors, polyvinyl pyrrolidone (PVP) as the surfactant and benzyl alcohol as the solvent, respectively (see Supporting Information for details). The composition and structure were characterized by energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The atomic ratio of the as-prepared Ru2Ni2 SNs is 53.5/46.5 (Figure S1a). The TEM images reveal that the Ru2Ni2 SNs are composed of sandwich-like morphology with hexagonal outline at the first glance (Figures 1a&S2). The sharp contrast between the ring and the hexagonal center indicates the formation of different components between them. The other projection further displays that a pillar is sandwiched by two nanoplates at each end (Figure 1a). The diameter based on the length of the diagonal across the two terminal hexagonal nanoplates of these nanostructures is averaged at 16 nm (Figure S3). The morphology of the Ru2Ni2 SNs is also vividly displayed in the high-angle annular dark-field scanning TEM (HAADF-STEM) images, demonstrating the sharp contrast of the terminal plates (Figures 1b&S2f&S4a). The XRD pattern of the Ru2Ni2 SNs shows that a small peak locates at 39° indexed to the Ru (100) facet, while a faint small peak at 51.8°
5
can be attributed to Ni (200) facet. The broadened peak at around 43.5°corresponds to the Ru (002) and (101) facets (JCPDS 06-0663) (Figure S1b).[19] The measured interplanar distance is 0.23 nm, corresponding to the Ru (100) facet (Figure 1c). The projection along the <001> direction reveals that the interplanar distance is 0.21 nm, attributed to the Ru (002) facet (Figure 1d). To reveal the elemental distributions of Ru and Ni, the line-scanning profiles and elemental mappings were carefully collected (Figures 1e-f). The compositional line-scanning profiles across the three lines further reveal the inhomogeneous elemental distributions of Ru and Ni, where Ru largely distributes at the two plates and Ni mainly concentrates in the middle (Figure 1e). The elemental mappings turn out that Ru and Ni are not uniformly distributed through the Ru2Ni2 SN with Ni located at the center and Ru mainly existed in the ring region. Therefore, this unique structure is considered as a phase segregated heterostructure with distributing at the two ends of the Ni pillar.
6
Figure 1. (a) TEM image (inset is an ideal model), (b) HAADF-STEM image, and (c, d) HRTEM images with (c) along and (d) perpendicular to the <001> direction of Ru2Ni2 SNs. The insets in (c) and (d) are the ideal models projected (c) along and (d) perpendicular to the <001> direction, respectively. (e) Line-scanning profiles across Ru2Ni2 SNs. (f-h) EDS elemental mappings of (f) Ru2Ni2 SNs, (g) Ru2Ni1 SNs and (h) Ru2Ni3 SNs).
7
The unique structure of Ru-Ni SNs with Ru growing along the edges of the pillar is the most striking feature of our synthesis. To comprehend the growth process, the intermediates collected at different time were also characterized. Before 27 min, no product could be collected by centrifugation. Hereafter, the solution color changed into black quickly, producing small quasi-hexagonal pillars with the average size of 6.9 nm. The atomic ratio of Ni to Ru was as high as about 90%, indicating much higher reduction rate of Ni than that of Ru at the initial stage (Figures S5a&S5f&S6a). The fringe spacing of the product is 0.206 nm, close to the interplanar distance of the (111) facet in fcc Ni (0.203 nm) (Figure S7ab).[20] The elemental mappings reveal the phase-segregated structure of this intermediate (Figure S7c). With 1 more min reaction, the Ni content in the product decreased to 75%, while its average size increased to 7.7 nm (Figures S5b&S6b). As the reaction time reached 0.5 h, the morphology of the product changed into the sandwiched-like feature with nanoplates growing at the ends and the Ni content down to 62% (Figures S5c&S6c). With increasing the reaction time to 1 h, a rapid increase in the average size of Ru2Ni2 SNs was observed, along with the atomic ratio of Ni promptly dropped to 49 % (Figure S6d). Further prolonging the reaction time to 5 h, no obvious changes were observed in the size and the Ni content, indicating the reaction completed after first 1 h (Figures S6e-g). The XRD patterns of the intermediates display the enhancements of the peak intensities at 39.0°, 70.2° and 85.6° as the reaction proceeded, indicating the increase of Ru (Figure S6h). In the present synthesis, no product could be obtained without the addition of Ru precursor while other parameters unchanged, showing that Ru can play a significant role in inducing the reduction of Ni in the reaction process.[21] As previously demonstrated, Ru preferred to selectively grow on the corners and edges of the seeds.[22-25] Since Ni and Pd locate at the same column of the periodic table, similar properties are expected. Therefore, Ru prefers to grow on the Ni substrate, thus controlling the morphology of Ni.[26] Notably, hexagonal Ni
8
prism formed in a very short time may play a role of template with its vertex and edges as the active sites for the growth of Ru. It should be noted that the bulk Ru and most of the Ru nanocrystals keep the hcp structure while the Ni remains the fcc structure in the bulk.[19, 2729] Hence, the Ru can take the hcp phase, and Ni can possess the fcc phase, as observed in the XRD patterns (Figure S1b). Since the Ru (001) facet has lower surface energy than the Ru (100) facet, the Ru SNs will expose the Ru (001) facet to minimize the surface energy.[19] By combining the time-dependent investigations, an evolution process is proposed, which contains two stages: the formation of a hexagonal prism of Ni by the axial growth under the control of Ru and the emerging of the Ru ring by the radial growth at the corners and edges. Through adjusting the molar ratio of Ru(acac)3 to Ni(acac)2 at 2/1 and 2/3, the Ru2Ni1 SNs and Ru2Ni3 SNs could also be made, as determined by the SEM-EDS with the molar ratios of Ru to Ni being 68.6/31.4 and 44.2/55.8 for Ru2Ni1 SNs and Ru2Ni3 SNs, which matches well with the molar ratio of Ru precursor to Ni precursor (Figure S8). These Ru-Ni SNs with different compositions have similar morphologies, with the average size of 15 nm and 16 nm, respectively (Figures S2a-c&g-i&S3a,3c). The XRD patterns of Ru2Ni1 SNs and Ru2Ni3 SNs are similar to that of Ru2Ni2 SNs (Figure S9). The peak at 38.7° can be indexed to the (100) facet of Ru (JCPDS Card. 06-0663), in which the intensity decreases with the rise of the Ni content. The peak at 50.3° assigned to Ni (200) facet becomes more obvious. The TEM and HRTEM images demonstrate the structures of both Ru2Ni1 SNs and Ru2Ni3 SNs with interplanar distance of 0.23 nm along the <001> direction and 0.21 nm perpendicular to the <001> direction, respectively (Figure S10). The EDS elemental mappings (Figures 1g-h) and line-scanning profiles (Figures S11) of Ru2Ni1 SNs and Ru2Ni3 SNs determine the phase segregated structure with Ru mainly dispersed in the ring region of the terminal plates and Ni concentrated at the center. Comparing the EDS elemental mappings of Ru2Ni3 SNs with those of Ru2Ni2 SNs and Ru2Ni1 SNs, we observe that the Ni in the center region increases with
9
increasing the Ni content in the SNs, vividly revealed by the ideal models in the insets of Figures 1f-h. Considering the structural and compositional anisotropy of the Ru-Ni SNs, we studied the HER and OER activities of the Ru-Ni SNs in alkaline solution. The commercial Pt/C and Ir/C were selected as the reference. Before the electrocatalytic measurements, the Ru-Ni SNs were loaded onto the carbon black (Carbon, VXC-72) (denoted as as-prepared Ru-Ni SNs/C, Figures S12a-b). The amounts of the Ru-Ni SNs load on carbon black are 5.9%, 5.5% and 6.7% (wt% Ru) for the Ru2Ni1 SNs, Ru2Ni2 SNs and Ru2Ni3 SNs, respectively (Table S1). The Ru-Ni SNs/C was then annealed in air to clean the surface of the Ru-Ni SNs (renamed as Ru-Ni SNs/C, Figures S12c-d). The HER and OER activities of the treated Ru-Ni SNs/C are better than the as-prepared Ru-Ni SNs/C (Figure S13). Figure 2a reveals the iR-corrected polarization curves of Ru-Ni SNs/C, the Pt/C, and the Ir/C for HER in 0.1 M KOH. All the Ru-Ni SNs/C exhibit near zero onset potentials, lower than those of the Pt/C and Ir/C. The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples were summarized in Figure 2b. It intuitively demonstrates that the Ru2Ni1 SNs/C, Ru2Ni2 SNs/C and Ru2Ni3 SNs/C possess average overpotentials of 42.7 mV, 39.3 mV and 44.3 mV and average Tafel slopes of 27.9 mV dec-1, 25.0 mV dec-1 and 29.5mV dec-1 respectively, which are much better than those of Pt/C (59.0 mV and 33.9 mV dec-1) and Ir/C (66.3 mV and 37.4 mV dec-1), indicating the best catalytic performance of Ru2Ni2 SNs/C (Figure 2b). The polarization curves in higher alkaline environment (1 M KOH) were also collected. The onset potential of Ru2Ni2 SNs/C is as low as 11 mV (Figure 2c). The corresponding average overpotentials at 10 mA cm-2 and the average Tafel slopes for different catalysts are presented in Figure 2d. We can see that the average overpotentials at 10 mA cm-2 of Ru2Ni1 SNs/C, Ru2Ni2 SNs/C, Ru2Ni3 SNs/C, the Pt/C and the Ir/C are 47.7 mV, 40.0 mV, 47.7 mV, 56.7 mV and 65.3 mV,
10
respectively. The Ru-Ni SNs possess the NiOx/Ru interface, facilitating the water dissociation by the synergistic effect.[30] The lowest average overpotential and average Tafel slope of 23.4 mV dec-1 in 1 M KOH of Ru2Ni2 SNs/C suggests the Volmer-Heyrovsky mechanism took place in the Ru-Ni SNs (Figure 2d).[31]
Figure 2. (a) Polarization curves of various Ru-Ni SNs/C, the Pt/C and the Ir/C for HER in 0.1 M KOH. (b) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples. (c) Polarization curves of various Ru-Ni SNs/C, the Pt/C and the Ir/C for HER in 1 M KOH. (d) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples. All the polarization curves are iR-corrected. The error bars represent the standard deviations based on triplicate measurements.
The Ru-Ni SNs are also active for OER. As presented in Figure 3a, the Ru2Ni2 SNs/C
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exhibit the lowest onset potential for OER in 0.1 M KOH (1.45 V). The average overpotential of Ru2Ni2 SNs/C at 10 mA cm-2 for OER is 357 mV (Figure 3b), which is the lowest in all the investigated catalysts in this work. The polarization curves in 1 M KOH illustrate that the Ru-Ni SNs/C have lower onset potentials than those of the Pt/C and the Ir/C (Figure 3c). The average overpotentials at 10 mA cm-2 for the three Ru-Ni SNs/C are similar with that of Ru2Ni2 SNs/C being the lowest, much lower than that for the commercial Pt/C and Ir/C (Figure 3d).
Figure 3. (a) Polarization curves of various Ru-Ni SNs/C, the Pt/C and the Ir/C for OER in 0.1 M KOH. (b) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of calculated Tafel slopes obtained from at least three electrodes samples. (c) Polarization curves of Ru-Ni SNs/C, the Pt/C and the Ir/C for OER in 1 M KOH. (d) The corresponding average of the overpotentials at 10 mA cm-2 and the corresponding average of the calculated Tafel slopes obtained from at least three electrodes samples. All the
12
polarization curves are iR-corrected. The error bars represent the standard deviations based on at least triplicate measurements.
Based on their highly efficient bifunctional properties, the Ru-Ni SNs/C was further used as catalysts for overall water splitting in alkaline condition. Figure 4a displays the iRcorrected polarization curves of various Ru-Ni SNs/C and the Pt/C for HER and OER in 0.1 M KOH. The voltage difference (△V) between OER and HER of Ru2Ni2 SNs/C is 1.63V. In 1 M KOH, Ru2Ni2 SNs/C exhibits the △V of 1.58 V at 10 mA cm-2, much lower than that of the commercial Pt/C (1.805 V), and outperforming most of the previous overall water splitting catalysts (Figure 4b) (Table S2). As shown in Figure 4c, the iR-corrected polarization curve of Ru2Ni2 SNs is basically the same as the voltage difference (ΔV) between HER and OER with the onset potential of 1.45 V, which is close to the calculated voltage difference (ΔV) between theoretical onset potentials of OER and HER.[32] The longterm stability of Ru2Ni2 SNs/C was then evaluated in two-electrode configuration in 1 M KOH, where slight decrease in potential (12 mV) was observed after 40 h, showing its excellent long-term stability (Figure 4d). The Ru2Ni2 SNs/C shows better stability with the potential changing from 1.68 V to 1.97 V after 105 h, while Ir/C and Pt/C almost loss the electrochemical activity after 90 h measurement (Figure S14). The morphology of the Ru2Ni2 SNs/C at the cathode after the stability test was also checked by TEM, revealing that while the shape was largely maintained, the molar ratio of Ru to Ni was changed into 3 (Figures S12e-f&Figure S15), indicating the partial dissolution of Ni during the reaction. The ICP measurement of the solution after durability testing was also conducted, which turned out that the Ru leached out was only 13.8 % of the Ru loaded on the electrode, indicating the enhanced durability of Ru.
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Figure 4. (a) IR-corrected polarization curves of various Ru-Ni SNs/C and the Pt/C for HER and OER in (a) 0.1 M KOH and (b) 1 M KOH. (c) The IR-corrected polarization curves for overall water splitting of Ru2Ni2 SNs/C tested in a two-electrode configuration. The inset is the iR-corrected polarization curves of Ru2Ni2 SNs/C for HER and OER. (d) The chronopotentiometric curve of Ru2Ni2 SNs/C and Ir/C and Pt/C in the water electrolysis with two-electrode configuration at 10 mA cm-2.
The X-ray photoelectron spectroscopy (XPS) was applied to investigate the structure of the Ru-Ni SNs/C (Figures 5a-b&S16). The surface Ru/Ni ratios are 35.8/64.2 for Ru2Ni1 SNs/C, 28.3/71.7 for Ru2Ni2 SNs/C and 10.9/89.1 for Ru2Ni3 SNs/C, respectively (Table S3). The deconvoluted spectra of Ru 3d and Ni 2p reveal that the coexistence of the Ru4+ and Ru0, and Nix+ and Ni0. The binding energies of Ru0 3d5/2 and Ru0 3d3/2 are 280.43 eV and 284.62 eV, respectively, corresponding to the metallic Ru.[25] The peak at the binding energy of 281.14 can be ascribed to Ru4+ 3d5/2.[33] The binding energies of Ni0 2p3/2 and Nix+ 2p3/2 for Ru-Ni SNs are 852.9 eV and 855.94 eV.[9] As can be seen from Figure 5c-d&Table S4, the Ru2Ni2
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SNs/C has the largest ratios of Ru4+/Ru0 and Nix+/Ni0. The volcano-like molar ratio change of Ru4+/Ru0 and Nix+/Ni0 is well consistent with the water splitting activity change trend, suggesting that the higher ratios of Ru4+/Ru0 and Nix+/Ni0 in Ru2Ni2 SNs/C is probably the key in boosting the water splitting activity.
Figure 5. (a) Ru 3d and (b) Ni 2p XPS spectra of Ru2Ni2 SNs/C. Area ratios of (c) Ru4+/Ru0 and (d) Nix+/Ni0 for Ru-Ni SNs obtained from their corresponding XPS spectra.
4. Conclusion To summarize, we demonstrate a facile wet-chemical strategy for the preparation of unique Ru-Ni SNs with a Ni pillar inserting into two hexagonal Ru rings. We found that the formation of the Ru-Ni SNs underwent the initial growth of quasi-hexagonal Ni pillars by the axial growth under the control of Ru and the subsequent formation of the Ru ring at the corners and edges by the radial growth. When applied as electrocatalyst towards alkaline
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HER, the Ru2Ni2 SNs/C with optimized interface exhibit low overpotential as well as Tafel slope in KOH. For OER, the Ru2Ni2 SNs/C also show much lower overpotential and lower Tafel slope than the Pt/C. When applied as alkaline overall water splitting electrocatalyst, the Ru2Ni2 SNs/C exhibit very low onset potential of 1.45 V and also excellent durability after long-term chronopotentiometry, showing a promising electrocatalyst for practical alkaline electrolysis and beyond.
ASSOCIATED CONTENT Supporting Information. Figure S1-16&Table S1-4. This material is available free of charge via the Internet at https://www.journals.elsevier.com/nano-energy/. Corresponding Author [email protected] ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, the start-up supports from Soochow University, Natural Science Foundation of Jiangsu Higher Education Institutions (17KJB150032) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). J. Ding and Q. Shao contribute equally to this work.
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Recent personal portrait photo and biosketch of all the authors.
Jiabao Ding received his Master’s degree in Xiamen University, China in 2014. He is currently a Ph.D. candidate under the supervision of Prof. Huang in College of Chemistry, Chemical Engineering and Materials Science in Soochow University. His major is inorganic
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chemistry and his research interests are the design of nanomaterials for energy storage and conversion.
Dr. Qi Shao received her Ph.D. degree in applied physics from City University of Hong Kong in 2016. Now she is an assistant professor in Professor Huang’s group at College of Chemistry, Chemical Engineering and Materials Science, Soochow University. Her current research interests are focusing on non-noble metal based catalysts for electrochemical applications.
Yonggang Feng is currently a M.S. candidate under the supervision of Prof. Huang in College of Chemistry, Chemical Engineering and Materials Science in Soochow University. He received his B.S degree in Jiaxing University, China in 2015. His major is inorganic chemistry and he is now focused on the design of noble metal nanomaterials for heterogenous catalysis and electrocatalysis.
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Dr. Xiaoqing Huang is currently a Professor at College of Chemistry, Chemical Engineering and Materials Science, Soochow University. He obtained his B.Sc. in chemistry education from Southwest Normal University (2005) and Ph.D. in organic chemistry from Xiamen University (2011) under the supervision of Profs. Nanfeng Zheng and Lansun Zheng. Then he joined Profs. Yu Huang and Xiangfeng Duan’s group as a postdoctoral research associate from September 2011 to June 2014 at University of California, Los Angeles. His current research interests are in the design of nanoscale materials for heterogenous catalysis, electrocatalysis, energy conversion and beyond.
Highlights
For the first time, a spool-like Ru-Ni nanocrystal (NC) with Ru distributed in the hexagonal ring and Ni concentrated in the pillar was successfully created. This unique Ru-Ni NC exhibits outstanding activity and enhanced stability toward both hydrogen evolution reaction and oxygen evolution reaction, much higher than those of the commercial Pt/C. Significantly, the optimized Ru-Ni SNs deliver a low onset potential of only 1.45 V and enhanced durability in the overall water splitting device, indicating a promising electrocatalyst towards the practical alkaline electrolysis.
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