English

• 1. INTRODUCTION

  The growing demand for energy and the increasing environmental pollution issues caused by fossil fuels have attracted the attention of research, which urgently requires the design and development of sustainable energy storage and conversion equipment^[1-6]. Hydrogen (H₂), as one of the important green energy sources, has been believed to be an alternative for fossil fuels^[7-10]. Now H₂ production is mainly produced by fossil fuel industry that is a low purity and high cost way. Another alternative renewable approach of producing H₂ is electrochemical water splitting, only using water as a protons source will improve the purity of H₂ and reduce manufacturing cost^[11-14]. Oxygen evolution reaction (OER) is undoubtedly a crucial half-reaction of water splitting, which has attracted tremendous efforts and intensive study^[15-22]. However, the kinetics of OER is sluggish due to a complicated multistep electron transfer pathways^{[23, 24]}. In order to accelerate the electrochemical reaction and therefore promote the energy conversion efficiency, it is needed to develop a highly active OER electrocatalysts^[25]. To date, there has been increasing interest in noble-metals (e.g, Ru and Ir-based materials) due to their low overpotential and Tafel slop towards OER^{[26, 27]}. But their practical applications are deeply hampered because of its low abundance and high price, and thus it cannot be widely used in the field of water electrolysis to produce hydrogen power^[28]. Therefore, it is necessary to design the economical OER electrocatalysts with low overpotential and excellent activity to enhance the performance of OER.

  Layered double hydroxides (LDH) are known as hydrotalcite-like lamellar materials that consist of layered metal cation surrounded by hydroxide anions and intercalated anions (e.g. CO₃^2–, NO₃^–, Cl^–, Br^–) in the layer region^[29-33]. Increasing the number of LDH materials containing 3d transition metals has been reported to possess superior catalytic activity for oxygen evolution because it is highly accessible to turn metal elements and ratios^[34-37]. However, low electrical conductivity of LDH is a common issue^[38] restricting the promotion of OER activity. To overcome this issue, many strategies have been designed to improve the activity of OER catalyst. One straightforward way is to load conduction carbon composites. It turns out that NiFe-LDH/conduction carbon composites exhibit superior electrochemical activity and stability, which can be comparable to noble-metals (e.g., IrO₂ and RuO₂)^{[39, 40]}. Another effective method is to make an ultrathin two- dimensional structure to improve conductivity^[41-44]. Furthermore, enormous researches have been made to combine heterogeneous 3d transition metals into catalysts for enhancing OER energy conversion efficiency due to the enhanced conductivity and structure disorder^[45-49]. At present, binary and ternary LDH materials are mainly studied, while quaternary LDH, which may provide better electrocatalytic performance, is rarely studied^[50-53].

  Inspired by the above facts, it is considered that the complex of multielement LDH ultrathin nanosheets and highly conductive carbon nanotube can be an excellent OER electrocatalyst. Herein, the carbon nanotube (CNT) supported FeCoNiW-LDH nanosheets (FeCoNiW-LDH/CNT) are prepared by one-pot hydrothermal methods. FeCoNiWLDH/CNT is a superior OER electrocatalyst with a low Tafel slope (41 mV·decade^–1) in alkaline electrolyte, which can outperform noble metal RuO₂. We addresses that the loaded CNT and W⁶⁺ embedded in FeCoNiW-LDH/CNT can enhance the conductivity of electrocatalysts and therefore facilities electron-transport in the OER electrochemical process.

  2. EXPERIMENTAL

  2.1 Synthesis of the sample

  FeCoNiW-LDH/CNT was prepared by one-pot hydrothermal method. 0.12 mmol FeCl₃·2H₂O, 0.1 mmol CoCl₂·2H₂O, 0.18 mmol NiCl₂·2H₂O, 0.17 mmol WCl₆ and 0.4 mmol carbon nanotube were dissolved in a mixture of 32 mL ethylene glycol and 8 mL water. The pH value of the solution was adjusted to about 8 by using 14 mol·L^–1 NH₄OH under vigorous stirring. Then, the solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and followed by hydrothermal reaction at 130 ℃ for 24 hours. After that, the product was collected after several times of water washing and dried by vacuum freeze dryer. FeCoNi-LDH/CNT was prepared by a similar procedure of FeCoNiW-LDH/CNT except without the addition of WCl₆. FeCoNi-LDH was also synthesized as a reference by repeating the procedures of FeCoNiW-LDH/CNT except without adding WCl₆ and carbon nanotube.

  2.2 Characterization

  X-ray powder diffraction patterns were collected on an XRD instrument (Miniflex6000, Rigaku) using a CuKα (λ = 1.54178 Å) radiation source, which was operated at a scan rate of 3 °·min^–1 in the 2θ range from 5° to 65°. TEM and HRTEM characterizations were operated on a FEI F20 microscope with an acceleration voltage of 200 kV. Atomic force microscopy (AFM) measurement was carried out using Bruker's Dimension icon Scanning Probe Microscope (SPM) systems. XPS analysis was conducted by ESCALAB 250Xi XPS (spectrometer with AlKα source), and XPS data were obtained by correcting the binding energies of C1s to 284.8 eV. The Raman spectra were obtained by using LabRAM HR with 532 nm laser.

  2.3 Electrochemical measurements

  The electrochemical measurements of catalyst were studied by using a CHI 760D electrochemistry workstation with a three-electrode system. OER electrochemical tests were carried out in 1 M KOH electrolyte. Hg/HgO electrodes and graphite rod were used as reference and counter electrodes. Typically, 4 mg catalyst and 40 μL Nafion solution (10 wt %) were dissolved in 2 mL mixture of ethanol and water, which was supersonic for one hour to form a uniform catalyst ink. Then, working electrodes were prepared by coating the glassy carbon electrode with 3.5 uL catalyst ink. The cyclic voltammetry measurements (CV) and Linear sweep voltammetry (LSV) of OER were conducted at scan rates of 50 and 5 mV·s^–1, respectively. The IR-corrected LSV with resistance compensation of 85% was obtained. Electrochemical impedance spectroscopy (EIS) measurement was performed at the potential of 1.45 V vs. RHE. All potentials were converted to a reversible hydrogen electrode.

  3. RESULTS AND DISCUSSION

  We herein report the fabrication of W atoms doped FeCoNi-LDH supported on carbon nanotube (FeCoNiWLDH/CNT) by the one-pot hydrothermal method. As described in Fig. 1, the desired FeCoNiW-LDH/CNT material was prepared by using the mixture solution of metal chloride, NH₄OH and carbon nanotube as precursors, and the subsequent hydrothermal reaction of mixture solution results in bottom-up self-assembly of the crystallization of ultrathin LDH nanosheets on the surface of reduced carbon nanotubes. Moreover, for the preparation of a series of contrasted samples, the products of FeCoNi-LDH/CNT were formed in a similar method but under the absence of WCl₆, and FeCoNi-LDH was obtained without adding WCl₆ and CNT.

  Figure 1

  

  Figure 1.  Schematic illustration of the preparation of FeCoNiW-LDH/CNT

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  X-ray diffraction (XRD) (Fig. 2) is carried out to identify the crystal structural features of FeCoNiW-LDH/CNT, FeCoNi-LDH/CNT and FeCoNi-LDH. All samples only present the sharp (003) and (006) characteristic peaks, which are consistent with the standard XRD pattern of α-Ni(OH)₂ (PDF# 38-0715) that is a typical LDH structure compound. This result indicates that only LDH compounds with ternary or quaternary element were prepared, and the doping of W and the loading of carbon nanotubes did not change the LDH phase.

  Figure 2

  

  Figure 2.  X-ray diffraction patterns of FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT

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  The graphite structure of FeCoNi-LDH/CNT was confirmed by Raman measurements, and the reduced CNT was also performed as the reference in Fig. 3a. Raman spectra of FeCoNi-LDH/CNT and CNT present the D (1344 cm^–1) and G (1574 cm^–1) bands. The intensity ratio I_D/I_G for FeCoNi-LDH/CNT was calculated to be 1.04, which is similar to that of the reduced CNT (I_D/I_G = 0.98), suggesting that reduced CNT with a high degree of graphitization was successfully loaded. The composition and valence state information were confirmed by X-ray photoelectron spectroscopy (XPS). The elementary composition of iron (Fe), cobalt (Co), and nickel(Ni) was firstly identified by the typical wide-scan XPS survey spectrum for FeCoNiWLDH/CNT, FeCoNi-LDH/CNT and FeCoNi-LDH samples in Fig. 3b^{[54, 55]}. The presence of W 4d peak in the FeCoNiW-LDH/CNT XPS survey reveals the successful preparation of W doping FeCoNi-LDH, and therefore the signals of Fe, Co and Ni in XPS survey spectrum for FeCoNiW-LDH/CNT were affected. The surface elemental ratio of Fe: Co: Ni in FeCoNiW-LDH/CNT is determined to be about 1.2:1:1.8 by XPS survey (Table 1), which is close to that of FeCoNi-LDH/CNT, FeCoNi-LDH. Likewise, the content of W in FeCoNiW-LDH/CNT is about 30.7%, similar to the feeding content. As shown in the highresolution W 4f core level lines of FeCoNiW-LDH/CNT, two characteristic peaks were clearly observed. The deconvoluted peaks of W4f_7/2 and W4f_5/2 are located at 35.43 and 37.53 eV, respectively, suggesting that the W species in FeCoNiW-LDH/CNT is W⁶⁺ (Fig. 3c)^[56]. The 2p scans of Fe, Co and Ni for FeCoNiW-LDH/CNT, FeCoNi-LDH/CNT and FeCoNi-LDH are present in Fig. 3d-3f, respectively. The coincidence curves of FeCoNiW-LDH/CNT, FeCoNi-LDH/CNT and FeCoNi-LDH were observed. It proves that there is no chemical state change in samples. In detail, the Fe 2p spectrum presents the Fe²⁺ (710.4 eV) and Fe³⁺ (712.8 eV) peaks^[57]. Similarly, Ni²⁺ (855.0 eV) and Ni³⁺ (856.1 eV) peaks were identified in the Ni 2p spectrum^[57]. As can be seen from the Co 2p spectra, two peaks at Co 2p_3/2 (780.0 eV) and Co 2p_1/2 (795.5 eV) are associated to signals of Co³⁺, and the deconvoluted peaks at Co 2p_3/2 (781.5 eV) and Co 2p_1/2 (796.8 eV) agree well with Co^2+[58]. Therefore, the above XPS results demonstrated that Fe, Co, and Ni with high and low oxidation states coexist in the LDH-based materials.

  Figure 3

  

  Figure 3.  (a) Raman spectra of FeCoNi-LDH/CNT and the reduced CNT (RCNT) and (b) XPS surveys of FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT. XPS high-resolution spectra of (c) W 4f in FeCoNiW-LDH/CNT. XPS highresolution spectra of (d) Fe 2p, (e) Ni 2p and (f) Co 2p in the FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT

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  Table 1

  

  Table 1.  Ratio of Fe, Co and Ni of FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT Determined by XPS

  DownLoad: CSV

  Sample	Atomic% (XPS)
  	Fe	Co	Ni	W
  FeCoNi-LDH	30.0	23.2	46.8	0
  FeCoNi-LDH/CNT	29.1	23.5	47.4	0
  FeCoNiW-LDH/CNT	20.9	17.4	31.0	30.7

  The size and morphology of the as-synthesized materials were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). For FeCoNiLDH sample, ultrathin nanosheets with a hundred-nanometer in diameter were observed by TEM in Fig. 4a. AFM is widely used as an important technique to determine the thickness of layered materials. AFM image and the corresponding cross-sectional profile of FeCoNi-LDH nanosheets are shown in Fig. 4b and the inset. As expected, the thickness of ultrathin FeCoNi-LDH nanosheets was measured to be about 0.82 nm. The TEM images of FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT (Fig. 4c-f) show that ultrathin LDH nanosheets were successfully grown over CNT support to form a scaly-like hybrid structure. Compared with FeCoNi-LDH nanosheets, the morphology of 2-D nanosheets retained well for CNT modified FeCoNi-LDH and FeCoNiW-LDH samples. As shown in the high resolution TEM (HRTEM) image of FeCoNiW-LDH/CNT (Fig. 4g), FeCoNiW-LDH nanosheet with 1 nm thickness is supported on a multi-wall CNT. The interlayer spacing of CNT is determined to be about 0.33 nm (yellow arrow), while the lattice fringe of FeCoNiW-LDH nanosheets (red arrow) is about 0.15 nm corresponding to the (110) face of LDH phase. As shown in Fig. 4h~n, the STEM-HAADF and the corresponding elemental mapping verify a coincided distribution of C, O, Fe, Co, Ni and W across the carbon nanotubes. The presence of W signals demonstrated the W⁶⁺ was uniformly doped into LDH nanosheets, which agreed well with the XPS results. According to the above results, after modification with multi-wall CNT, the thickness of multi-element-LDH nanosheet remains at about 1 nm. Meanwhile, the LDH nanosheet is tightly attached to CNT, which will significantly shorten the electron transfer path and improve the conductivity.

  Figure 4

  

  Figure 4.  (a) TEM image FeCoNi-LDH nanosheet (b) AFM image of FeCoNi-LDH nanosheet, inset showing the corresponding cross-sectional profile. (c, d) TEM images of FeCoNi-LDH/CNT at different magnification, (e, f) TEM images ofFeCoNiW-LDH/CNT at different magnification, (g) High-resolution TEM image, the yellow and red arrows represent the crystal lattices of carbon nanotubes and FeCoNiW-LDH nanosheets, respectively. (h) HAADF-STEM image and (i-n) the corresponding STEM-EDX elemental mapping images of the As-prepared FeCoNiW-LDH/CNT sample

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  The performance of FeCoNiW-LDH/CNT serving as OER electrocatalysts was systematically investigated by using a three-electrode system in 1 M KOH (see details in the experimental section) to state the intrinsic catalytic-activity. The working electrode was fabricated by coating the as-prepared sample on glassy carbon electrode. FeCoNi-LDH and FeCoNi-LDH/CNT samples were also measured for comparison under the same condition. The iR-corrected polarization curves of samples are shown in Fig. 5a. For the linear sweep voltammetry curves of FeCoNi-LDH and FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT, the oxidation peak prior to the sharp rise in the polarization curve was a result of the transition metal Ni²⁺/Ni^{3+ or 4+} redox reaction in the LDH nanosheets^{[59, 60]}. In addition, onset potential, which is one of the important parameters for evaluating the catalytic performance, gives the direct comparison of catalytic activity^[61]. The onset potential @ 1.5 mA·cm^–2 of FeCoNiW-LDH/CNT, FeCoNi-LDH/CNT, FeCoNi-LDH and RuO₂ are 1.450, 1.455, 1.458 and 1.474 V, respectively. Apparently, all of the LDH-based catalysts show approximate onset potential values but lower than that of commercial RuO₂ catalysts. In needs to be noted that the current density of FeCoNiW-LDH/CNT is about 41.8 mA·cm^–2 at a small overpotential of 300 mV, which is 1.8 times higher than that of FeCoNi-LDH/CNT and 2.5 times higher than that of pristine FeCoNi-LDH. In addition, the overpotential at a current density of 10 mA·cm^–2, approximately corresponding to the current density for a 10% efficient water-splitting conversion equipment, is an important fundamental index of OER electrocatalysts^{[62, 63]}. The overpotential at 10 mA·cm^–2 for samples is shown in Fig. 5b. It is uncovered that FeCoNi-LDH/CNT, FeCoNi-LDH and RuO₂ show high overpotential of 270, 280 and 310 mV, respectively. Notably, when W⁶⁺ has been doped into the FeCoNi-LDH/CNT sample, the electrochemical OER activity of catalyst was dramatically improved. The overpotential of FeCoNiW-LDH/CNT is lowered to 258 mV at a current density of 10 mA·cm^–2, comparable to the metal oxides and hydroxide electrocatalyst reported previously^[64-69]. The Tafel slope represents the effect of the applied potential on the change in current or current density. Therefore, in order to evaluate catalytic kinetics towards OER, the Tafel data of samples were calculated from LSV curves and shown in Fig. 5c. It is clear that FeCoNiLDH/CNT with Tafel slope of 56 mV·dec^-1 exhibits faster OER kinetics than FeCoNi-LDH and RuO₂ reference (63 and 64 mV·dec^–1, respectively). It is worth noting that Tafel slope of FeCoNiW-LDH/CNT is reduced to 41 mV·dec^–1, indicating that W⁶⁺ plays a key role in reducing the Tafel slope of OER. The Tafel data agree well with the results of polarization curves (Fig. 5a), that is, when the same potential was applied, FeCoNi-LDH/CNT presents superior current density to FeCoNi-LDH. The above results emphasize that the loaded CNT and the doped W⁶⁺ in FeCoNiW-LDH/CNT can significantly boost the electron transfer kinetics towards OER, and therefore improve the OER electrochemical properties.

  Figure 5

  

  Figure 5.  (a) OER polarization curves (b) Overpotential for OER obtained at 10 mA·cm^–2 and (c) Tafel plots of RuO₂, FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT in 1 M KOH electrolyte

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  To further confirm the effect of carbon nanotubes and LDH nanosheets on improving the capacity of electron transfer in the OER electrochemical process, electrochemical impedance spectroscopy (EIS) measurements and Tafel slope tests at different scan rates were performed. In detail, electrochemical impedance spectroscopy (EIS) measurements of FeCoNi-LDH/CNT and FeCoNi-LDH were performed at the potential of 1.45 V vs. RHE (Fig. 6a). It revealed the FeCoNi-LDH/CNT possesses much smaller electron transport resistance than FeCoNi-LDH, suggesting that the combination highly conductive carbon nanotubes and FeCoNi-LDH can be benefit to the electron transfer. Moreover, according to the literature reports, the lower Tafel slope of catalysts is usually caused by electron and mass transfer^[70]. To eliminate the possibility of mass transfer influence on OER activity, Tafel slope tests at different scan rates were studied (Fig. 6b). The Tafel slope of FeCoNiW-LDH/CNT is almost unchanged as the scan rate increases from 0.05 to 0.2 mV·s^–1, indicating that the mass transfer is fast enough, and the smaller Tafel slope of FeCoNiW-LDH/CNT is caused by electron transfer in the OER electrochemical process^[71]. As the above results revealed, the further reduction in Tafel slope of FeCoNiW-LDH/CNT is due to the W⁶⁺ doping in the LDH nanosheet, which can adjust the electronic structure and enhance the electron transfer capacity.

  Figure 6

  

  Figure 6.  (a) Electrochemical impedance spectroscopy of FeCoNi-LDH and FeCoNi-LDH/CNT at 1.45 V vs. RHE in 1 M KOH electrolyte. (b) Tafel slope curves of the NiFeCoW-LDH/CNT tested under various scan rates

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  4. CONCLUSION

  In summary, quaternary FeCoNiW-LDH/CNT was successfully fabricated by one-pot hydrothermal methods. The asprepared FeCoNiW-LDH/CNT achieved remarkable electrocatalytic OER activity with a small overpotential of 258 mV at 10 mA·cm^–2, and low Tafel slope of 41 mV·decade^–1. The enhanced electrochemical OER performance of FeCoNiW-LDH/CNT can be attributed to the synergistic effect of modified CNT and doped W⁶⁺ atoms. Firstly, the carbon nanotubes with removing oxygen-containing functional groups can closely bind to the LDH nanosheet and greatly increase the conductivity of catalysts. Secondly, the doped W⁶⁺ with high valence state in LDH nanosheet can regulate the electronic structure of 3d transition metal elements and enhance the electron transfer rate of active sites. This work opens a new venue for designing OER electrocatalyst, which can also be used for the application of sustainable energy storage and conversion technologies.

  
  
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• 

  Figure 1  Schematic illustration of the preparation of FeCoNiW-LDH/CNT

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  Figure 2  X-ray diffraction patterns of FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT

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  Figure 3  (a) Raman spectra of FeCoNi-LDH/CNT and the reduced CNT (RCNT) and (b) XPS surveys of FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT. XPS high-resolution spectra of (c) W 4f in FeCoNiW-LDH/CNT. XPS highresolution spectra of (d) Fe 2p, (e) Ni 2p and (f) Co 2p in the FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT

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  Figure 4  (a) TEM image FeCoNi-LDH nanosheet (b) AFM image of FeCoNi-LDH nanosheet, inset showing the corresponding cross-sectional profile. (c, d) TEM images of FeCoNi-LDH/CNT at different magnification, (e, f) TEM images ofFeCoNiW-LDH/CNT at different magnification, (g) High-resolution TEM image, the yellow and red arrows represent the crystal lattices of carbon nanotubes and FeCoNiW-LDH nanosheets, respectively. (h) HAADF-STEM image and (i-n) the corresponding STEM-EDX elemental mapping images of the As-prepared FeCoNiW-LDH/CNT sample

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  Figure 5  (a) OER polarization curves (b) Overpotential for OER obtained at 10 mA·cm^–2 and (c) Tafel plots of RuO₂, FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT in 1 M KOH electrolyte

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  Figure 6  (a) Electrochemical impedance spectroscopy of FeCoNi-LDH and FeCoNi-LDH/CNT at 1.45 V vs. RHE in 1 M KOH electrolyte. (b) Tafel slope curves of the NiFeCoW-LDH/CNT tested under various scan rates

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  Table 1.  Ratio of Fe, Co and Ni of FeCoNi-LDH, FeCoNi-LDH/CNT and FeCoNiW-LDH/CNT Determined by XPS

  Sample	Atomic% (XPS)
  	Fe	Co	Ni	W
  FeCoNi-LDH	30.0	23.2	46.8	0
  FeCoNi-LDH/CNT	29.1	23.5	47.4	0
  FeCoNiW-LDH/CNT	20.9	17.4	31.0	30.7

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