Ni17W3 Nanoparticles Decorated WO2 Nanohybrid Electrocatalyst for

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Ni17W3 Nanoparticles Decorated WO2 Nanohybrid Electrocatalyst for Highly Efficient Hydrogen Evolution Reaction Ce Han,†,‡ Dewen Wang,†,§ Qun Li,†,§ Zhicai Xing,*,† and Xiurong Yang*,†,§ †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡ School of Chemistry and Life Science, Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, Jilin, China § University of Science and Technology of China, Hefei 230026, China

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 5.101.219.242 on 03/21/19. For personal use only.

S Supporting Information *

ABSTRACT: Herein, Ni17W3 nanoparticles (NPs) decorated WO2 nanohybrid supported by Ni foam (Ni17W3/WO2/NF) is successfully prepared via annealing Ni4W6O21(OH)2·4H2O/NF under reductive atmosphere and applied as hydrogen evolution reaction (HER) electrocatalyst. The integration of WO2 and Ni17W3 derived from a single precursor simultaneously facilitates the water dissociation and H intermediates association during HER kinetics, while promotes the corresponding synergistic effect to form numerous exposed active sites scattered over the nanohybrid. Hence, the Ni17W3/WO2/NF behaves with superior HER performance delivering merely 35 mV overpotential at 10 mA cm−2 with a small Tafel slope of 41.6 mV dec−1 and good durability in alkaline medium, which is comparable to that for commercial Pt/C. This work may offer positive inspirations to stimulate the development of NiW-based highly efficient and stable electrocatalysts for various sustainable energy applications. KEYWORDS: nonprecious catalysts, electrocatalysis, Ni17W3, tungsten oxide, hydrogen evolution reaction

H

level and thus proper binding energy for metal−H bond, which favors desirable electrocatalytic HER performance.12,14,15 Meanwhile, the introduction of oxygen vacancies into transition metal oxides has been suggested to be an effective approach to facilitate the delocalization of electrons around metal atoms and the increase in carrier concentration, which can be conducive to the dissociation of water molecules, generate additional active sites, and further significantly improve the HER activity.16 As demonstrated by published literature, tungsten oxides possess many substoichiometric species with abundant oxygen vacancies, and the formation of oxygen vacancies-rich tungsten oxides can give rise to the appearance of a metallic phase in the materials, which enables the combination of NiW alloys and oxygen vacancies-rich tungsten oxides.17,18 Additionally, synthesizing nanohybrid from single precursor has been supposed to benefit the contact of active

ydrogen has been identified as an ideal energy source for replacing exhaustible fossil fuels owing to its inherent advantages of high energy density, recyclability, and carbonfree feature.1 Nowadays, the generation of hydrogen through electrochemical water splitting in alkaline media initiates a simple and sustainable route to efficiently produce hydrogen with high purity.2 Nevertheless, as the benchmark electrocatalyst for hydrogen evolution reaction (HER), platinum still has many drawbacks, such as inferior stability, low terrestrial abundance, and high cost, which severely impede widespread industrial implementation and thus necessarily urge the exploration of alternative high-performance nonprecious electrocatalysts based on earth-abundant elements.3 In past decades, variously nanostructured transition-metalbased materials have been reported as promising electrocatalysts with competitive HER activity.2,4−10 Among them, nickel-based alloys have recently stimulated massive research owing to the regulatable electrocatalytic performance arising from the synergistic effect of alloy components.5,11−13 In comparison to the mono-metallic samples, Ni-based binary alloys feature modified electron density of states at the Fermi © XXXX American Chemical Society

Received: January 24, 2019 Accepted: March 20, 2019 Published: March 20, 2019 A

DOI: 10.1021/acsaem.9b00170 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Figure 1. Scheme of the synthesis progress of the Ni17W3/WO2/NF electrode.

components and further enhance the synergistic effect between them to boost the electrocatalytic performance.19,20 Thus, it is anticipated that the fabrication for nanohybrid of oxygen vacancies-rich tungsten oxides and NiW alloys from nickel− tungsten hydroxide precursors should synchronously endow the material with prominent capacity to cleave a H−OH bond into H intermediates and adsorb H intermediates to generate H2, which demonstrates its promising potential for developing a novel catalyst with outstanding HER activity. In this letter, Ni17W3 nanoparticles (NPs) decorated WO2 nanohybrid supported by Ni foam (Ni17W3/WO2/NF) were designed and prepared by facilely pyrolyzing Ni4W6O21(OH)2· 4H2O/NF precursor under reductive atmosphere and utilized as highly efficient HER electrocatalyst in alkaline electrolyte. The presence of synergistic effect stemmed from Ni17W3, and oxygen vacancies-rich WO2 simultaneously promotes the acceleration of water dissociation and subsequent association of the H intermediates step, hence improving the entire HER kinetic process on Ni17W3/WO2/NF. Moreover, the fabrication of Ni17W3/WO2/NF from a single precursor can facilitate the sufficient contact of Ni17W3 NPs with WO2 at the nanoscale and prevent the aggregation of Ni17W3 NPs during the pyrolysis process, further favoring the enhancement of synergistic effect between dual active components. Consequently, the as-prepared Ni17W3/WO2/NF exhibits impressively high HER performance in 1 M KOH with low overpotential of 36.1 mV at 10 mA cm−2, small Tafel slope of 41.6 mV dec−1, and good stability, which is comparable to that for commercial Pt/C catalyst. It is believed that our work will offer positive inspirations for the development of highperformance and cost-effective HER electrocatalysts. As schematically depicted in Figure 1, the Ni4W6O21(OH)2· 4H2O/NF as a single precursor was first fabricated through a hydrothermal reaction of NF with Ni(NO3)2·6H2O and (NH4)6H2W12O40·xH2O; then Ni17W3/WO2/NF was obtained via annealing the hydrothermal resultant under reductive atmosphere (details are provided in the Supporting Information). X-ray diffraction (XRD) analysis was performed to characterize the crystallographic structure of as-synthesized materials peeled off from NF. As illustrated in Figure 2a, the hydrothermal product can be identified as the Ni4W6O21(OH)2·4H2O (JPCDS No. 47-0143). Meanwhile, the sample annealed at 600 °C displays detected characteristic peaks originating from the Ni17W3 (JPCDS No. 65-4828) and WO2 (JPCDS No. 05-0431) phases, indicating the successful fabrication of Ni17W3/WO2 nanohybrid on NF. In comparison, as the annealing temperature decreases to 500 °C, the new emerged peaks can be assigned to the NiWO4 (JPCDS No. 150755) species, elucidating the incomplete reduction of precursor (Figure S1a).21 In sharp contrast, the almost substitution of WO2 by metallic W (JPCDS No. 01-1204)

Figure 2. XRD patterns of the Ni17W3/WO2 and Ni4W6O21(OH)2· 4H 2 O powders peeled off from NF (a). SEM images of Ni4W6O21(OH)2·4H2O/NF (b) and Ni17W3/WO2/NF (c). TEM (d) and HRTEM images (e) of Ni17W3/WO2. Element distribution mappings of Ni, W, and O in Ni17W3/WO2 (f).

observed from Figure S1b signifies the excessive reduction of sample at the increased annealing temperature of 700 °C.22 In addition, the XRD pattern of the sample processed under Ar atmosphere (sample-Ar) can be ascribed to NiWO4 , reconfirming the formation of NiWO4 under insufficient reducing condition (Figure S1c). The scanning electron microscopy (SEM) image of hydrothermal product depicted in Figure 2b reveals the uniform Ni4W6O21(OH)2·4H2O layer densely packing over the NF without any specific morphology. As shown in Figure 2c, the Ni17W3/WO2/NF possesses a rough surface with numerous homogeneously distributed nanoparticles after the hydrogen reduction process. Similar morphology can also be detected in the SEM observation of the material synthesized at 500 °C (Figure S2a). Nevertheless, the apparently cracked surface of the loosely structured sample can be readily noticed from Figure S2b, probably due to the further reduction of precursor to form metallic W at 700 °C. Moreover, as presented by the transmission electron microscopy (TEM) image in Figure 2d, Ni17W3/WO2 displays in situ formed uniform nanoparticles evenly dispersed over the B

DOI: 10.1021/acsaem.9b00170 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials entire sample. Further high-resolution TEM investigation (Figure 2e) exhibits noticeable lattice fringes with interplanar distances of 0.35, 0.21, and 0.18 nm corresponding to the (110) plane of WO2 and the (111), (200) planes of Ni17W3 alloy, respectively, validating the construction of nanohybrid in the form of Ni17W3 NPs decorated WO2 supported by NF. In addition, the corresponding elemental mapping analyses given in Figure 2f verify uniform dispersion of W and O elements throughout the sample, while the well overlapping W and Ni signals can be detected from the nanoparticle areas, suggesting the closely interweaving of Ni17W3 and WO2 in the nanohybrid. It is worth mentioning that the homogeneous distribution of uniform Ni17W3 NPs anchored on WO2 can be advantageous for the synergistic effect derived from dual active components, which generate abundant exposed active sites and further accelerate the electrocatalytic HER process on the nanohybrid. The X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the detailed binding states of co nstituent el ements f or the Ni 1 7 W 3 / WO 2 and Ni4W6O21(OH)2·4H2O precursor. The Ni2+ peaks centered at 855.7 and 873.2 eV are presented in the high-resolution Ni 2p spectra of Ni17W3/WO2 and Ni4W6O21(OH)2·4H2O, accompanied by the satellite peaks at 861.9 and 880.3 eV (Figure 3a).23 Besides, another type component located at

Figure 4. LSV polarization curves (a) and corresponding Tafel plots (b) of NF, Pt/C/NF, Ni17W3/WO2/NF, and Ni4W6O21(OH)2· 4H2O/NF. Capacitive current densities at 0.25 V as a function of scan rate for NF, Ni17W3/WO2/NF, and Ni4W6O21(OH)2·4H2O/NF (c). Chronopotentiometric curve of Ni17W3/WO2/NF and Pt/C/NF at the constant current density of 20 mA cm−2 (d).

the obviously enhanced HER performance of Ni17W3/WO2/ NF arising from the presence of Ni17W3, WO2 and synergistic effect of Ni17W3 and WO2, which represents an overpotential of 35 mV at the current density of 10 mA cm −2 outperforming those of Ni4W6O21(OH)2·4H2O/NF (173 mV) and bare NF (228 mV). Meanwhile, the Pt/C/NF exhibits as-expected HER activity with an overpotential of 15 mV at 10 mA cm−2. Moreover, as depicted in Figure 4b, Ni17W3/WO2/NF manifests accelerated HER kinetics with a Tafel slope of 41.6 mV dec−1, which is similar to Pt/C/NF (35.5 mV dec−1) and far smaller than Ni4W6O21(OH)2·4H2O/ NF (94.7 mV dec−1) and NF (111.3 mV dec−1). The fitted results signify that the HER pathway proceeding on Ni17W3/ WO2/NF follows the Volmer−Heyrovsky mechanism with the H2 generation by electrochemical adsorption of H intermediates as the rate-determining step.28 Furthermore, the enhanced HER performance of Ni17W3/WO2/NF described above is comparable to that of Pt/C and various reported stateof-the-art alkaline HER electrocatalysts (Table S1). Additionally, electrochemical impedance spectroscopy (EIS) analyses recorded at overpotential of 100 mV certify the noticeably reduced charge-transfer resistance of Ni 17W3/WO2/NF exceeding those for Ni4W6O21(OH)2·4H2O/NF and NF, which implies the fast charge-transfer rate and hence improved HER kinetic process triggered by the formation of metallic Ni17W3 and oxygen vacancies-rich WO2 in the materials (Figure S3).12,17 Furthermore, as displayed in Figure S4, the HER performance collected from three batches of Ni17W3/ WO2/NF presents high similarity without obvious difference, illustrating the reproducibility of catalyst. The optimization of HER activity for Ni17W3/WO2/NF was implemented through the adjustment of pyrolysis temperature under identical reductive atmosphere. As observed from Figure S5, in comparison to the samples annealed at 500 and 700 °C, Ni17W3/WO2/NF synthesized at 600 °C possesses better HER activity with superior HER kinetics and charge-transfer resistance. Taking account of the aforementioned XRD results, it is inferred that the diminished HER performance should be mainly due to the absence of oxygen vacancies stemmed from

Figure 3. XPS results of Ni 2p (a) and W 4f (b) for Ni17W3/WO2 and Ni4W6O21(OH)2·4H2O.

852.4 and 869.5 eV can be related to the Ni0 derived from the Ni17W3 in Ni17W3/WO2.19 Meanwhile, only doublet W6+ peaks at 35.1 and 37.2 eV can be examined in the Ni4W6O21(OH)2· 4H2O (Figure 3b).24 For the postcalcinated sample, except for the W6+ species, the deconvolution of W 4f spectrum illustrates the emergence of W4+ (at 32.3 and 34.3 eV) and W0 (at 30.8 and 33.0 eV) signals, definitely verifying the existence of oxygen vacancies-rich WO2 and Ni17W3 in the material.25,26 As is known to all, the presence of oxygen vacancies has been declared to cause the reduction of the band gaps and formation of active sites arising from its electron-donating feature, thereby effectively improving the electrocatalytic activity and conductivity.26,27 Moreover, integrating the WO2 with Ni17W3 alloys should afford the nanohybrid fast HER kinetics stemmed from moderate metal−H bond strength14 and concurrently induce the synergistic effect of two components, which benefit the further improvement of HER performance. The electrocatalytic HER performance of as-fabricated materials was assessed in 1 M KOH at 25 °C with a scan rate of 5 mV s−1. Bare NF and 20 wt % Pt/C loaded NF (mass loading, 3 mg cm−2) were measured as contrast samples under identical test conditions. The iR compensated linear sweep voltammetry (LSV) curves displayed in Figure 4a demonstrate C

DOI: 10.1021/acsaem.9b00170 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

can favor the contact of two components and thereby reinforce the corresponding synergistic effect to provide massive catalytic sites for HER. In summary, we have successfully constructed Ni17W3 nanoparticles decorated WO2 nanohybrid supported by NF via a facile hydrothermal-hydrogen reduction process. The asprepared nanohybrid possesses significantly improved HER kinetic process derived from the accelerated water dissociation and H intermediates absorption−desorption step, which may be ascribed to the oxygen vacancies and moderate metal−H binding energy arising from the WO2 and Ni17W3 alloy. Moreover, the synergistic effect of two active components has been further strengthened by synthesizing the nanohybrid from single precursor, which generates massive exposed active sites uniformly distributed throughout the materials. Consequently, the resultant Ni17W3/WO2/NF has demonstrated excellent HER performance demanding a low overpotential of 35 mV to yield the current density of 10 mA cm−2 with small Tafel slope (41.6 mV dec−1) and good durability in alkaline electrolyte, which is comparable to the commercial Pt/C. Our work may provide a feasible approach to rationally design original NiWbased highly efficient and stable nonprecious electrocatalysts for diverse significant emerging energy applications.

the insufficient or excessive reduction of WO2 at inappropriate pyrolysis temperature, highlighting the significance of oxygen vacancies in the promotion of HER activity. Besides, considering the emergence of inferior HER active metallic W,29 the presence of Ni17W3 should be responsible for the reserved HER activity of the sample annealed at 700 °C, which facilitates the HER kinetics and thus enhance the electrocatalytic activity toward HER. Moreover, the effect of Ni and W precursors on the HER performance was also studied by adjusting the Ni precursor addition and keeping the W precursor addition during the synthetic process. As displayed in Figure S6, Ni17W3/WO2/NF demonstrates better HER performance compared to the other contrast samples (sample0.5Ni and sample-2Ni), indicating the appropriate Ni and W precursor addition of Ni17W3/WO2/NF. To further clarify the reason for improved electrocatalytic performance, the electrochemically active surface area (ECSA) of as-synthesized catalysts was estimated by the electrochemical double-layer capacitance (Cdl) calculated from cyclic voltammetry at various scan rates (Figures 4c and S7 and S8).19 Ni17W3/WO2/NF behaves with a pronouncedly larger Cdl (105.35 mF cm−2) than those of Ni4W6O21(OH)2·4H2O/ NF (6.24 mF cm−2) and NF (0.62 mF cm−2), revealing the plentiful accessible active sites resulting from the rough surface of Ni17W3/WO2/NF. Meanwhile, the specific surface of the electrode (Aelect.) for Ni17W3/WO2/NF and Pt/C/NF are calculated from the corresponding Cdl values.30 As depicted in Figure S9, the LSV curves with current densities normalized to Aelect. demonstrate an overpotential of 201 mV at the current density of 1.5 mA cm−2 for Ni17W3/WO2/NF, which is comparable to that of Pt/C/NF (182 mV). Moreover, the durability of Ni17W3/WO2/NF and Pt/C/NF was probed through the chronopotentiometry measured at constant current density of 20 mA cm−2 in 1 M KOH. After 12 h of continuous hydrogen production, Ni17W3/WO2/NF presents good durability with well-maintained potential; in contrast, the Pt/C/NF shows a distinct potential decline from −0.024 to −0.130 V (Figure 4d). Additionally, the SEM and XRD characterization following prolonged operation further elucidates the prominently robust structure of Ni17W3/WO2/NF for the HER in alkaline condition (Figure S10). To better investigate the origin of HER activity, sample-Ar was prepared by pyrolyzing Ni4W6O21(OH)2·4H2O/NF under Ar atmosphere as contrast sample. The SEM image of sampleAr (Figure S11a) manifests no distinct morphological variation after calcination. Meanwhile, owing to the lack of the active components Ni17W3 and WO2 as determined by the XRD analysis, the electrochemical measurements indicate an evident attenuation on the HER activity of sample-Ar (Figure S11b− d), further testifying that the Ni17W3 and WO2, as well as the synergistic effect between them, can effectively promote the electrocatalytic HER activity. According to the aforementioned results, the remarkable HER performance of Ni17W3/WO2/NF should be attributed to the following advantageous factors: (1) the homogeneous distribution of uniform Ni17W3 nanoparticles can promote the conductivity and afford suitable metal−H binding energy for the nanohybrid to improve the HER activity;14 (2) the abundant oxygen vacancies in WO2 can tune the electronic structure of nanohybrid and contribute to the water dissociation, which benefits the electron transfer and further facilitates the HER kinetic process; (3) the preparation of nanohybrid using single Ni4W6O21(OH)2·4H2O/NF precursor

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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/acsaem.9b00170. Experimental and characterization details, including synthesis of materials, characterization, electrochemical measurements, calculation of electrochemical surface area, specific surface of the electrode, and additional Figures S1−11and Table S1 mentioned in the main text (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(X.Y.) E-mail: [email protected]. *(Z.X.) E-mail: [email protected]. ORCID

Xiurong Yang: 0000-0003-0021-5135 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grant Nos. 21435005 and 21627808), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDY-SSW-SLH019), and the Development Project of Science and Technology of Jilin Province (Grant No. 20180520146JH) for financial support.

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DOI: 10.1021/acsaem.9b00170 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX