http://pubs.acs.org/journal/aelccp
1D/1D Hierarchical Nickel Sulfide/Phosphide Nanostructures for Electrocatalytic Water Oxidation Huai Qin Fu,† Le Zhang,† Chong Wu Wang,† Li Rong Zheng,‡ Peng Fei Liu,*,† and Hua Gui Yang*,†
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†
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: The sluggish kinetics of the oxygen evolution reaction (OER) limits the efficiencies of solar-powered electrical-conversion applications, such as water splitting and carbon dioxide reduction. Herein, we rationally designed a metallic nanostructured nickel sulfide/phosphide hybrid (NiSxPy) as an efficient precatalyst for OER, with one-dimensional (1D) nanowires grown on 1D nanorods. The resulting metallic hybrid NiSxPy catalyst can accelerate the electron transfer process and expose abundant in situ-generated NiOOH species during OER (NiSxPy−O). Therefore, NiSxPy−O exhibits a low overpotential of 192 mV (with 100% iR compensation; this value should be 200 mV without compensation) to achieve an O2 partial current density (jO2) of 10 mA cm−2 and a robust stability over 135 h without obvious degradation. Moreover, a jO2 of 10 mA cm−2 at an overpotential of 315 mV (with 100% iR compensation; this value should be 365 mV without compensation) is attained in near-neutral conditions. These results may pave a new way to design metallic precatalysts with 1D/1D hierarchical nanostructures to boost the OER. Zou et al. have fabricated amorphous Ni−Fe−OH 2D films supported on Ni3S2 2D nanosheets, exhibiting high catalytic activity toward the OER.18 Yang et al. have reported that the electrocatalyst with MoS2 2D nanosheets anchored in Ni3S2 one-dimensional (1D) nanorods can achieve a j of 10 mA cm−2 at a quite low cell voltage of 1.50 V for overall water splitting.19 Likewise, hierarchical nanostructures of 1D/2D Co(OH)F, 2D/1D CuO nanowire@Co3O4 nanosheet, and 0D/2D graphene dots/Co0.8Ni0.2P have also been prepared to boost the OER process.20−22 Noteworthy, on the basis of these discovered hierarchical nanostructures, we anticipate that 1D/ 1D subunit integrated structures could expose abundant active sites, simultaneously facilitating charge transport along the axial direction.19,23,24 However, such 1D/1D electrocatalysts have never been reported in this field, to the best of our knowledge. As highlighted in recent reports, phosphidation treatment has been proposed as an effective strategy to promote the growth of 1D nanostructures,25−27 which makes it possible to fabricate hierarchical nanostructures via phosphating 1D
W
ith the increasing depletion of fossil fuels and crisis of climate change, renewable energy systems, such as carbon dioxide reduction and water splitting electrolysis technologies, play key roles in energy conversion and storage.1−4 Unfortunately, the anodic oxygen evolution reaction (OER) is strikingly blocked to extend these applications because of a complicated four-electron transfer process and sluggish oxygen−oxygen bond formation,5−7 and even the most effective precious metal catalysts (i.e., IrO2 or RuO2) still need a large overpotential to reach the required current density (j) of 10 mA cm−2.8 Therefore, researchers have explored a series of earth-abundant and cost-effective electrocatalysts as alternatives, including 3d-metal-based oxyhydroxides, sulfides, and phosphides.9−12 Their electrocatalytic performances are correlated with the number of active sites and intrinsic activity, strongly depending on the specific morphology and electronic structures (e.g., valence state and conductivity), respectively.13 Following this, further efforts should be devoted to exposing more active sites for materials that possess highly intrinsic catalytic activity for OER. Architecting hierarchical nanostructures has been regarded as the most promising way to increase the number of active sites.14−16 For instance, Xu et al. have designed NixFe1−xSe2 two-dimensional (2D) nanoplates decorated with numerous zero-dimensional (0D) nanoparticles to motivate the OER.17 © XXXX American Chemical Society
Received: June 12, 2018 Accepted: July 10, 2018
2021
DOI: 10.1021/acsenergylett.8b00982 ACS Energy Lett. 2018, 3, 2021−2029
Letter
Cite This: ACS Energy Lett. 2018, 3, 2021−2029
Letter
ACS Energy Letters
Scheme 1. Schematic Illustration Describing the Formation Procedure of 1D/1D Hierarchical Nanostructures of Metallic Hybrid NiSxPya
a
The Ni3S2 nanorod arrays were prepared via a hydrothermal reaction. The 1D/1D hybrid NiSxPy was then fabricated after phosphidizing the Ni3S2 nanorod at 300 °C for 4 h.
Figure 1. (a−c) FESEM images of hybrid NiSxPy at different magnifications. (d) TEM image of hybrid NiSxPy sonicated off from the NF. The inset is the electron diffraction pattern of the nanowire. (e) HRTEM image of hybrid NiSxPy and (f) corresponding enlarged parts of the square in (e). (g) Corresponding SAED pattern of the nanorod in the square of (d). (h) STEM image of hybrid NiSxPy and corresponding element mapping images of (i) Ni, (j) P, and (k) S elements.
O2 Faradaic efficiency (η10 = 192 mV; and this value should be 200 mV without compensation), with a Tafel slope of 95.0 mV dec−1 in alkaline electrolyte (1.0 M KOH, pH = 13.6). Meanwhile, NiSxPy−O needs an η10 of about 315 mV (with 100% iR compensation; this value should be 365 mV without compensation) in near-neutral electrolyte (0.5 M KHCO3, pH = 8.54). We anticipate that hierarchical nanostructures engineering with metallic precatalysts could propose new insights to design efficient and durable OER catalysts for largescale alkaline and neutral electrolyzers. To fabricate the 1D/1D hierarchical nanostructures, comprehensive sulfidization−phosphidation treatments were conducted (Scheme 1). The 1D Ni3S2 nanorod arrays grown on a Ni foam substrate (NF) were prepared via a hydrothermal reaction (see more details in the Experimental Section in the
nanostructures. On this basis, unique 1D/1D nanostructures of a metallic nickel sulfide/phosphide hybrid (NiSxPy, consisting of highly dispersed Ni3S2 and Ni12P5) have been rationally designed by phosphating typical 1D Ni3S2 nanorods, with uniform distribution of 1D nanowires grown on 1D nanorods. The 1D/1D nanostructures integrated with metallic features of hybrid NiSxPy supply an electron highway, and the in situgenerated oxyhydroxides on the surface serve as OER active sites. Moreover, hierarchical nanostructures could expose more active sites, combining intrinsically improved OER activity, further synergistically boosting the OER process. Consequently, the oxidized NiSxPy catalyst (NiSxPy−O) exhibits extraordinary OER performance, requiring a low overpotential of 192 mV (with 100% iR compensation) to reach an O2 partial current density (jO2) of 10 mA cm−2, corrected by the 2022
DOI: 10.1021/acsenergylett.8b00982 ACS Energy Lett. 2018, 3, 2021−2029
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ACS Energy Letters
Figure 2. (a) XRD patterns of hybrid NiSxPy samples before and after a stability test for 20 h. (b) Ni 2p, (c) P 2p, and (d) S 2p highresolution XPS spectra of hybrid NiSxPy before and after 20, 50, as well as 130 h potentiostatic measurements at an overpotential of 200 mV.
temperature is 300 °C, at which the nanorod structure would be retained to finely control the growth of nanowires. Furthermore, the influence of phosphidation time was studied at the same temperature of 300 °C. Nanowires gradually grew on nanorods within 4 h and vanished for 6 h. In general, the well-arranged 1D/1D hierarchical structures would be obtained at a phosphidation temperature of 300 °C for 4 h (Figure 1a−c). In addition, the chemical composition of the hybrid NiSxPy nanostructure was probed by SEM energydispersive X-ray spectroscopy (EDX) elemental analysis. The elemental mapping images display that the elements including Ni, S, and P are uniformly distributed throughout the electrode (Figure S3). Transition electron microscopy (TEM) was carried out to confirm the hierarchical nanostructures. the TEM image shows that the extended nanowires features lengths ranging from 350 to 400 nm and diameters of around 15−20 nm (Figures 1d and S4). The high-resolution TEM (HRTEM) images present lattice fringes with spacings of 2.87 and 1.93 Å, which coincide with the (110) facet of Ni3S2 and (420) facet of Ni12P5 (Figure 1e,f), respectively.28−30 The corresponding selected area electron diffraction (SAED) pattern of the nanowire (inset of Figure 1d) shows two bright rings made up of discrete spots,
Supporting Information). Consequently, the as-grown Ni3S2 catalyst was phosphidized under the strongly reductive PH3 atmosphere induced by heating sodium hypophosphite, with phosphorus replacing sulfur to form a uniformly distributed Ni3S2/Ni12P5 hybrid, simultaneously promoting the growth of nanowires on nanorods. Therefore, a metallic electrocatalyst with well-defined hierarchical nanostructures where 1D nanowires were anchored on 1D nanorods could be finally obtained. Field-emission scanning electron microscopy (FESEM) was performed to identify the morphologies of hybrid NiSxPy and controlled samples. As shown in Figure 1a−c, FESEM images indicate that dense nanowires are vertically grown on nanorods. Only nanorods were obtained in the sample of Ni3S2 without further phosphidation treatment (Figure S1). In comparison, a series of controlled experiments were performed with different phosphidation temperatures and treatment times. As a result, temperature-dependent experiments for the same treatment time showed that no well-defined nanorods were observed at a temperature of 250 °C (Figure S2). At the same time, the higher phosphidation temperature of 400 °C would destroy the hierarchical structures due to the stronger reduction property of PH3 (Figure S2). The optimal treatment 2023
DOI: 10.1021/acsenergylett.8b00982 ACS Energy Lett. 2018, 3, 2021−2029
Letter
ACS Energy Letters
Figure 3. (a) Comparison of Ni K-edge XANES spectra collected on different samples. (b) Comparison of Ni EXAFS data in R-space collected on different samples. (c) Locally enlarged image of (a). (d) Locally enlarged image of (b). (e) Schematic illustration of fast electron transport resulting from the 1D/1D hierarchical nanostructures, simultaneously schematically illustrating the working mechanism of hybrid NiSxPy for the OER. The in situ-generated NiOOH on the surface of metallic hybrid NiSxPy serves as the active site.
the XRD patterns of the above-obtained samples could be ascribed to pure Ni3S2 or Ni2P (Figure S6), respectively. Different phosphidation treatment time has negligible effects on crystalline components (Figure S7). Furthermore, as exhibited in Figure 2a, the XRD pattern after a 20 h chronometric test at a constant overpotential of 200 mV also matches well with the mixed phases of Ni3S2 and Ni12P5, which demonstrates that the metallic bulk structure is stable during a long-term test. Raman spectroscopic measurements were performed to further detect the chemical composition and structure. As displayed in Figure S8, the characteristic broad Raman band at 540 cm−1 is associated with Ni−O vibrations in NiOOH, confirming the formation of oxyhydroxide on the surface during the OER process,31−33 which is usually regarded as the active centers. The chemical composition and surface valence states were further characterized by X-ray photoelectron spectroscopy (XPS). The survey scan spectrum reveals the co-occurrence of Ni, S, P, C, and O elements (Figure S9). In the high-resolution XPS spectra of Ni 2p region (Figure 2b), the pristine sample exhibits four prominent peaks. The peaks at 855.7 and 873.7 eV, accompanied by their shakeup satellites, indicate the
further matching well with the (110) plane of Ni3S2 and the (420) plane of Ni12P5. A fast Fourier transform (FFT) image was obtained from the selected area marked by white dashed in Figure 1d to further verify the crystal structure of the nanorod. As shown in Figure 1g, the FFT image displays two-set regularly arranged diffraction spots that are assigned to the reflections of planes for Ni3S2 and Ni12P5, indicating that the Ni3S2 and Ni12P5 are highly distributed on both nanowires and nanorods. To investigate the spatial distribution of Ni, S, and P components, element mapping analysis was carried out on the junction of nanowires and nanorods under scanning transmission electron microscope (STEM) mode. The STEM-EDX mapping images illuminate that Ni, P, and S elements are uniformly dispersed in the electrode, which suggests that nickel phosphide and nickel sulfide coexist in the whole region of hierarchical structures (Figure 1i−k). The crystalline components of hybrid NiS xP y were evidenced by X-ray diffraction (XRD) analysis. The XRD pattern of the as-prepared sample exhibits characteristic peaks indexed to Ni3S2 and Ni12P5 phases (Figures 2a and S5). For comparison, when the NF was solely vulcanized in the hydrothermal reaction or phosphidized in the PH3 atmosphere, 2024
DOI: 10.1021/acsenergylett.8b00982 ACS Energy Lett. 2018, 3, 2021−2029
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ACS Energy Letters
Figure 4. (a) Pristine LSV curves. The polarization curves were obtained with backward scan at a rate of 1 mV s−1 in 1.0 M KOH. (b) Fitted J−V curve showing the O2 partial current density of NiSxPy−O as a function of the potential. (c) Tafel plots of different catalysts. (d) Nyquist plots of hybrid NiSxPy and controlled Ni2P and Ni3S2 samples at an overpotential of 300 mV. (e) Half-charge current density differences (Δj = ja − jc)/2 plotted against scan rates for different samples. (f) Comparison of OER performances, evaluating the overpotential needed to reach the current density of 10 mA cm−2 and Tafel slopes. (g) Chronoamperometric curve obtained at a constant overpotential of 200 mV for over 135 h.
be attributed to the emergence of surface electron reorganization during the OER test.45 With regard to S 2p core level spectra (Figure 2d), the peaks at 161.8 and 162.8 eV are ascribed to S2− bonded to Ni atoms.46 The peaks at 162.9 and 163.8 eV are assigned to oxysulfide,47 and the doublet peak centered at 168.3 eV is associated with SO42− species.48 After the stability test, the signals of sulfide become weaker, while the percentage of oxidized sulfur species increases. The XPS ratios of elements before and after 20, 50, as well as 130 h potentiostatic measurements at an overpotential of 200 mV are compared in Table S1. The ratios of S and P decrease gradually, with the S content decreasing from 5.51 to 2.39% and the P content decreasing from 12.94 to 1.36%, but the O content increases from 43.63 to 52.18%. These variations for S, P, and O elements imply the in situ formation of oxide or oxyhydroxide. Additionally, the surface Ni valences of hybrid NiSxPy and Ni3S2 were further compared. The deconvolutions of the Ni 2p spectra show that the surface valence of Ni for hybrid NiSxPy is higher than that of Ni3S2 (Figure S10),
presence of nickel−oxygen species on the surface, most probably Ni(OH)2 and Ni−POx.34,35 The peaks located at 852.3 and 869.9 eV can be assigned to Ni−S or Ni−P bonds.19,36 From the XPS results of Ni spectra after the OER test, a newly generated peak at around 853.7 eV corresponds to NiO,37,38 along with disappearance of the peak at around 852.3 eV, indicating that nickel sulfide and phosphide are gradually oxidized. The peak of NiO becomes smaller and the peaks at 855.7 and 873.7 eV shift to higher binding energy with prolonged time, demonstrating that Ni is oxidized to a higher valence state after the OER test. The fitting analysis of the pristine P 2p spectrum shows notable features of metallic phosphide, with characteristic peaks at 129.5 and 130.5 eV (Figure 2c),39 accompanying the peak at around 134.5 eV ascribed to PO3−.40,41 Prolonging the reaction time, the intensity of the peak for phosphide gradually decreases, but all samples exhibit signals of PO43− at around 133.1 eV (Figures 2c and S9).42−44 The peak position of P 2p shifted negatively, and the Ni 2p peak shifted positively. This phenomenon may 2025
DOI: 10.1021/acsenergylett.8b00982 ACS Energy Lett. 2018, 3, 2021−2029
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ACS Energy Letters
a benchmark j of 10 mA cm−2 (without correction by Faradaic efficiency), while 370 mV is required for Ni3S2−O and 340 mV for Ni2P−O. Moreover, considering the possible existence of non-Faradaic current, the O2 Faradaic efficiencies were measured using gas chromatography under various operating overpotentials. As shown in Figure S13, the Faradaic efficiencies of NiSxPy−O for OER are all over 98% in the overpotential region from 240 to 570 mV. However, relatively low Faradaic efficiencies (
