Fabrication of a hierarchical NiTe@NiFe-LDH core-shell array for high-efficiency alkaline seawater oxidation

Summary Herein, a hierarchical NiTe@NiFe-LDH core-shell array on Ni foam (NiTe@NiFe-LDH/NF) demonstrates its effectiveness for oxygen evolution reaction (OER) in alkaline seawater electrolyte. This NiTe@NiFe-LDH/NF array showcases remarkably low overpotentials of 277 mV and 359 mV for achieving current densities of 100 and 500 mA cm−2, respectively. Also, it shows a low Tafel slope of 68.66 mV dec−1. Notably, the electrocatalyst maintains robust stability over continuous electrolysis for at least 50 h at 100 mA cm−2. The remarkable performance and hierarchical structure advantages of NiTe@NiFe-LDH/NF offer innovative insights for designing efficient seawater oxidation electrocatalysts.


INTRODUCTION
Hydrogen, a high energy density green energy source, with water as its sole combustion product, offers a compelling alternative to fossil fuels. 1,24][5][6][7] Its progress, nevertheless, is constrained by the limited availability of freshwater resources on Earth.][10][11][12][13] Yet, the efficiency of seawater electrolysis encounters a challenge.Abundant chlorine ions (Cl À ) in seawater tend to outcompete the oxygen evolution reaction (OER) at the anode for the kinetically swift two-electron chlorine evolution reaction (CER).5][16][17] Significantly, in alkaline electrolytes (e.g., 1 M KOH + seawater), the thermodynamic potential difference between the OER and CER can exceed 480 mV. 18,19To mitigate Cl À interference, it becomes plausible to achieve high current densities within a thermodynamic potential range of %480 mV.
7][28] The poor electrical conductivity of NiFe-LDH remains a limitation, which leads to inefficient electron-ion transport at its interface. 28Various strategies such as heteroatom doping, 29,30 anion exchange, 31,32 hybridizations with carbon, 33,34 etc., have been proposed to enhance the catalytic performance of NiFe-LDH.6][37][38][39] Wang et al. reported the fabrication of NiFe-LDH on NiMoO 4 nanorod, increasing the active surface area, allowing for faster kinetics. 35Our research group further introduced NiFe-LDH on NiMoS x microcolumn, which exhibited superior OER activity. 39Tellurium, as a chalcogenide, is more metallic and thus more conductive than its fellow oxygen and sulfur. 40,411][42][43] The assembly of a hierarchical structure comprising NiFe-LDH shell and NiTe core is anticipated to be an ideal seawater oxidation electrocatalyst, a proposition that remains unprecedented.
In this work, we present the fabrication of well-dispersed NiFe-LDH nanosheet on NiTe nanorod array supported on Ni Foam (NiTe@NiFe-LDH/NF) as a highly active seawater oxidation electrocatalyst.To drive a current density (j) of 100 mA cm À2 , such NiTe@NiFe-LDH/NF requires an overpotential of 277 mV in alkaline seawater.Furthermore, the catalyst also demonstrates remarkable electrochemical stability, maintaining its performance for a continuous duration of 50 h without any observable deterioration.

RESULTS AND DISCUSSION
Figure 1A shows a two-step synthesis of NiTe@NiFe-LDH/NF.Firstly, NiTe nanorods were grown on NF (NiTe/NF) by a hydrothermal method.NiFe-LDH nanosheets were then electrodeposited on NiTe/NF to get NiTe@NiFe-LDH/NF.In Figure 1B, the X-ray diffraction (XRD) pattern of NiTe/NF indicates the formation of NiTe nanorods on NF (JCPDS No. 38-1393), 41 three distinct peaks at 44.4 , 52.4 , and 76.5 correspond to characteristic diffraction peaks of metallic Ni within the NF substrate (JCPDS No. 40-0850).No discernible alterations are observed after the electrodeposition process, which is consistent with the inherent amorphous nature of the deposited NiFe-LDH. 44The morphology of NiTe nanorods is displayed T-shape in Figure 1C, while in Figure 1D, NiFe-LDH nanosheets are uniformly deposited on the surface of NiTe nanorods.For comparison, the morphology of NiFe-LDH/NF, displayed in Figure S1, exhibits that nanosheets are evenly distributed on NF.Transmission electron microscopy (TEM) image reveals a distinct core-shell structure of NiTe@NiFe-LDH, with clearly discernible boundaries between NiTe and NiFe-LDH, as indicated by green dashed lines in Figure 1E.In Figures 1F and 1A high-resolution TEM (HRTEM) image displays a crystal lattice distance of 0.269 nm, corresponding to the (002) plane of NiTe, and a clear demarcation between crystalline NiTe and amorphous NiFe-LDH.High-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure 1G) further confirms the core-shell configuration of NiTe@NiFe-LDH.Energy dispersive spectrometer (EDS) elemental mapping images in Figure 1H exhibit the even distribution of Ni, Fe, Te, and O elements for NiTe@NiFe-LDH.This elemental distribution provides conclusive evidence that NiTe nanorod is uniformly enveloped by NiFe-LDH.
The surface chemical state of the NiTe@NiFe-LDH/NF can be elucidated through X-ray photoelectron spectroscopy (XPS).The Ni 2p spectrum of NiTe@NiFe-LDH/NF (Figure 2A) reveals two discernible peaks at 855.7 and 873.3 eV, corresponding to Ni 2p 3/2 and Ni 2p 1/2 of Ni 2+ .The dual peaks located at 861.3 and 879.2 eV are attributed to satellite peaks (Sat.).Besides, additional peaks at 852.4 and 869.5 eV align with Ni 0 of NF. [45][46][47] The Fe 2p spectrum (Figure 2B) illustrates two prominent peaks at 711.5 eV for Fe 3+ 2p 3/2 and 725.0 eV for Fe 3+ 2p 1/2 .The additional two peaks at 718.1 and 733.0 eV correspond to Sat. [47][48][49] Regarding the Te 3d spectrum shown in Figure 2C, two peaks at 572.7 and 583.1 eV can be attributed to Te 2À 3d 5/2 and Te 2À 3d 3/2 of Te 2À in NiTe, respectively.The presence of two additional Te 3d peaks with binding energies of 576.1 and 586.5 eV suggests a potential association with the surface oxidation of Te. 40 The O 1s region indicates two distinguishable peaks, labeled as O1 and O2, situated at 530.4 and 532.5 eV, respectively.These peaks correspond to metal-OH (M-OH) and surface-adsorbed oxygen species (Figure 2D). 50,51he electrochemical performance of NiTe@NiFe-LDH/NF for OER was initially assessed in a 1 M KOH electrolyte.Based on the Linear Sweep Voltammetry (LSV) curves obtained for different electrodeposition times (as illustrated in Figure S2), it was determined that a 60-s electrodeposition time provided the most favorable conditions for OER.LSV curves for NiTe@NiFe-LDH/NF, NiFe-LDH/NF, NiTe/NF, NF, and commercially available RuO 2 loaded on NF (RuO 2 /NF) were presented in Figure 3A.Remarkably, NiTe@NiFe-LDH/NF exhibited significantly enhanced electrocatalytic activity compared to the other four electrode configurations.At a j of 100 mA cm À2 , NiTe@NiFe-LDH/NF demonstrated a particularly low overpotential of 257 mV.This overpotential was substantially lower than those for NiFe-LDH/NF (280 mV), NiTe/NF (451 mV), benchmark RuO 2 /NF (370 mV), and NF (517 mV).In Figure 3B, the Tafel slope value of NiTe@NiFe-LDH/NF was found to be the lowest at 53.41 mV dec À1 compared to NiFe-LDH/NF (64.14 mV dec À1 ), NiTe/NF (115.08 mV dec À1 ), RuO 2 /NF (83.40 mV dec À1 ), and NF (143.57mV dec À1 ).This suggests that NiTe@NiFe-LDH/NF exhibits faster kinetics during the OER process.Electrochemical impedance spectroscopy then revealed a smaller charge transfer resistance for NiTe@NiFe-LDH/NF (1.37U) compared to that of NiFe-LDH/NF (7.74U) (Figure S3), indicating improved charge transfer and reaction kinetics for NiTe@NiFe-LDH/NF.As depicted in Figures 3C and S4, NiTe@NiFe-LDH/NF possessed a double-layer capacitance (C dl ) of 6.8 mF cm À2 in 1 M KOH, nearly double that of NiFe-LDH/NF (3.3 mF cm À2 ).Consequently, the increased electrochemical active surface area and active sites of NiTe@NiFe-LDH/NF contribute to its enhanced OER performance.The integration of a conductive NiTe core with NiFe-LDH not only improves conductivity and reaction kinetics but also introduces more active sites, thus enhancing the OER activity of the anode catalyst.Moreover, a multi-step chronopotentiometric curve (from 40 mA cm À2 to 240 mA cm À2 ) was presented in Figure 3D, suggesting efficient mass transportation, excellent mechanical durability, and conductivity of the catalyst.
Following the evaluation of NiTe@NiFe-LDH/NF's OER catalytic activity in 1 M KOH, its performance was tested as an anode for OER in alkaline simulated seawater (1 M KOH +0.5 M NaCl) and alkaline seawater (1 M KOH + seawater).Illustrated in Figure 4A, the LSV curves from the two mentioned electrolytes closely resembled the curve in 1 M KOH.Notably, the overpotential in alkaline seawater exhibited a mere 20 mV increase compared to that in 1 M KOH.The OER catalytic kinetics of NiTe@NiFe-LDH/NF were further assessed by Tafel slopes in alkaline simulated seawater (59.79 mV dec À1 ) and alkaline seawater (68.66 mV dec À1 ), slightly surpassing that in 1 M KOH (Figure 4B).This trend suggests the swift OER catalytic kinetics of NiTe@NiFe-LDH/NF.Remarkably, the overpotentials required to achieve the j of 100, 200, and 500 mA cm À2 for NiTe@NiFe-LDH/NF were 277, 309, and 359 mV, respectively (Figure S5).These values surpass those of many reported self-supported electrocatalysts for OER in alkaline seawater (Figure 4C; Table S1).To assess the stability of NiTe@NiFe-LDH/NF in alkaline seawater, an LSV curve was obtained after 1000 CV cycles (Figure S6).No discernible current loss was observed compared to the initial curve before cycling.Furthermore, in Figure 4D, the stability of NiTe@NiFe-LDH/NF was assessed by chronopotentiometry at a j of 100 mA cm À2 over a span of 50 h in alkaline seawater.Especially, there was no evident decay in overpotential, indicating the excellent stability of NiTe@NiFe-LDH/NF.After 50-h stability test, the morphology of NiTe@NiFe-LDH/NF in Figure S7 demonstrated the preservation of the core-shell structure.Additionally, the XRD pattern of NiTe@NiFe-LDH/NF (Figure S8) displayed that no additional peaks were observed compared with the initial catalyst.The XPS spectrum (Figure S9) in the Ni 2P region revealed two additional peaks at 857.7 and 874.5 eV, corresponding to Ni 3+ in NiOOH.3][54] The XPS spectra of the O 1s region unveiled an additional peak at 530.0 eV for metal-O (referred to as O3). 55,56

Conclusions
In summary, we have successfully fabricated a core-shell NiTe@NiFe-LDH electrocatalyst customized for seawater oxidation.To drive a j of 100 mA cm À2 in alkaline seawater, NiTe@NiFe-LDH/NF only necessitates a low overpotential of 277 mV.Compared to NiFe-LDH directly grown on NF, the incorporated NiTe nanorod core provides an elevated electrochemical surface area, thereby facilitating an increased number of active sites.Furthermore, the real active species NiOOH for alkaline seawater oxidation are generated through reconstruction.Importantly, it exhibited robust stability during an extended 50-h assessment.This work not only advances the field by presenting a proficient OER electrocatalyst but also broadens the potential for future exploration into hierarchical nanostructures tailored for seawater oxidation applications.

Limitations of the study
Our work has demonstrated an excellent electrocatalyst for seawater oxidation with low overpotential and robust stability.Based on the combination of EDS and XPS, increased active surface area and improved conductivity have been interpreted as the key factors that facilitate OER performance.However, an in-depth understanding of the catalytic process and an analysis of the oxidation states of the catalyst remain significant.Hence, we will make a further exploration by in situ characterizations and theoretical calculations.iScience Article Characterizations X-ray diffraction (XRD) data was acquired from a LabX XRD-6100 X-ray diffractometer with a Cu Ka radiation (40 kV, 30 mA) of wavelength of 0.154 nm (SHIMADZU, Japan).Scanning electron microscope (SEM) images were collected on a GeminiSEM 300 scanning electron microscope (ZEISS, Germany) at an accelerating voltage of 5 kV.Transmission electron microscopy (TEM), high-angle annular dark-field scanning TEM (HAADF-STEM), and Energy dispersive spectrometer (EDS) images were acquired on a JEM-2800 electron microscope (JEOL, Japan) operated at 200 kV.X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALABMKII X-ray photoelectron spectrometer using Mg as the exciting source.

Electrochemical measurements
Electrochemical OER experiments were performed with the CHI 760E electrochemical workstation, using the prepared samples (1 3 0.5 cm 2 ), carbon rod, and Hg/HgO as the working electrode, counter electrode, and reference electrode, respectively.All the potentials in this experiment are presented as reversible hydrogen electrodes (RHE): E (vs.RHE) = E (vs.Hg/HgO) + 0.0591 3 pH + 0.098.Electrochemical impedance spectroscopy (EIS) measurements were performed at 1.52 V vs. RHE from 10 5 to 0.01 Hz with an amplitude of 5 mV.The iR-compensated potential was obtained after the correction of solution resistance measured following the equation: E corr = E À iR, where E is the original potential, R the solution resistance, i the corresponding current, and E corr the iR-compensated potential.

QUANTIFICATION AND STATISTICAL ANALYSIS
We did not perform any statistical analysis to filter out relevant data.

Figure 2 .
Figure 2. XPS spectra of NiTe@NiFe-LDH/NF (A) XPS spectra in the Ni 2p region.(B) XPS spectra in the Fe 2p region.(C) XPS spectra in the Te 3d region.(D) XPS spectra in the O 1s region.

Figure 4 .
Figure 4. Electrochemical tests in alkaline seawater (A) LSV curves and (B) corresponding Tafel plots of NiTe@NiFe-LDH/NF in 1 M KOH, alkaline stimulated seawater, and alkaline seawater.(C) Comparison of overpotentials required to obtain the j of 100 mA cm À2 between NiTe@NiFe-LDH/NF and previously reported self-supported catalysts in alkaline seawater.(D) Chronopotentiometry test of NiTe@NiFe-LDH/NF at the j of 100 mA cm À2 in alkaline seawater (without iR correction).

TABLE
d RESOURCE AVAILABILITY B Lead contact B Materials availability B Data and code availability d EXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS d METHOD DETAILS B Synthesis of NiTe/NF B Synthesis of NiTe@NiFe-LDH/NF B Characterizations B Electrochemical measurements d QUANTIFICATION AND STATISTICAL ANALYSIS