Highly efficient and stable oxygen evolution from seawater enabled by a hierarchical NiMoS_x microcolumn@NiFe-layered double hydroxide nanosheet array

Abstract

Developing efficient and robust oxygen evolution reaction (OER) catalysts in seawater is important for green hydrogen generation but remains a significant challenge. Herein, we report a hierarchical core–shell OER electrocatalyst consisting of NiFe-layered double hydroxide nanosheets uniformly coated on a NiMoS_x microcolumn supported on Ni foam (NiMoS_x@NiFe-LDH/NF). Such NiMoS_x@NiFe-LDH/NF shows excellent OER activity with a low overpotential of 297 mV to drive an industrial-level current density of 500 mA cm⁻² in alkaline seawater and can operate continuously for 500 h without apparent activity degradation. In situ Raman spectroscopy studies indicate that the high-valent molybdate ions can promote the generation of disordered NiOOH active species and protect catalysts from Cl⁻ corrosion during seawater oxidation. Additionally, the integrated alkaline seawater electrolyzer (with NiMoS_x/NF as the cathode) is demonstrated to reach a current density of 100 mA cm⁻² with a low voltage of 1.61 V, outperforming the benchmark Pt/C/NF||RuO₂/NF.

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Water electrolysis using renewable energy as input is an attractive technology for the mass production of high-purity hydrogen without carbon emissions.^1–3 Most industrially available water electrolyzers, nonetheless, must employ high-purity freshwater feedstock, which necessitates the extra use of pre-treatment purification units, adding to the complexity of the technological process and power consumption.⁴ Direct electrolysis of seawater has become increasingly appealing due to the abundant seawater resource on our planet.^5–10 However, the abundant chloride ions (Cl⁻) in seawater can gradually corrode the electrode and directly lead to undesirable but kinetically quick chlorine evolution reaction (CER) at the anode, which significantly inhibits the development of seawater electrolysis technology.^11–13 Both chlorine gas and hypochlorite can be generated from the CER at high oxidation potentials, competing with the oxygen evolution reaction (OER) and causing severe anode corrosion.^14,15 Thus, it is crucial to explore highly active and robust OER catalysts, especially at industrial-level current densities (>500 mA cm⁻²), to facilitate research in large-scale seawater electrolysis.

Among the electrocatalysts explored for alkaline OER, NiFe-layered double hydroxide (NiFe-LDH) is considered as a classical and promising one due to its low cost and good intrinsic activity, but its poor stability at industrial-grade current densities restricts its further use.^16–19 Recently, various strategies have been proposed to overcome the corrosion problem of NiFe-LDH-based electrodes during seawater oxidation.^8,20–23 Sun's group recently reported an anti-corrosion strategy of PO₄³⁻ intercalation in NiFe-LDH, in which the highly negatively charged PO₄³⁻ can prevent the corrosion of Ni substrate by electrostatic repulsive force.²¹ Chen's group demonstrated that the adsorption of SO₄²⁻ on the anode surface can effectively alleviate the Cl⁻ corrosion, resulting in a significant increase in the operational stability of the anode in alkaline seawater.²² Although phosphate coating or polyatomic sulfate-rich passivating layers on the electrode surface to alleviate Cl⁻ corrosion has been exploited with modest success, it is plenty of room to boost the long-term stability of NiFe-LDH-based catalysts at industrially required current densities in alkaline seawater.

Herein, we employ NiMoS_x microcolumn array supported on Ni foam as a multifunctional conductive core to uniformly/densely grow OER-active NiFe-LDH nanosheets (NiMoS_x@NiFe-LDH/NF) for alkaline seawater oxidation. Noticeably, the 3D hierarchical NiMoS_x@NiFe-LDH/NF electrocatalyst exhibits efficient and stable OER catalytic performance, requiring a low overpotential of 297 mV to attain an industrial-level current density (j) of 500 mA cm⁻² and showing slight decay after 500 h of electrolysis in alkaline seawater. In situ Raman spectroscopy studies show that the high-valent molybdate ions adsorbed on the anode surface can promote the generation of disordered NiOOH active species and repel Cl⁻ during alkaline seawater oxidation. The critical role of adsorbed molybdate ions on the NiMoS_x/NF surface for hydrogen evolution reaction (HER) in alkaline seawater is also revealed by spectroscopic and electrochemical studies. When the NiMoS_x@NiFe-LDH/NF is paired with the NiMoS_x/NF in alkaline seawater, the integrated electrolyzer exhibits a low voltage of 1.61 V to deliver a j of 100 mA cm⁻² and excellent stability up to 150 h at a j of 500 mA cm⁻².

A schematic diagram for the synthesis of NiMoS_x@NiFe-LDH/NF is shown in Fig. 1a. In brief, NiMoO₄·xH₂O microcolumns were grown on nickel foam (NiMoO₄·xH₂O/NF) by hydrothermal method, followed by ion exchange reaction using Na₂S as sulfur source to obtain NiMoS_x microcolumns on nickel foam (NiMoS_x/NF), and finally NiFe-LDH nanosheets were electrodeposited on the fabricated NiMoS_x/NF. After being coated with NiMoO₄·xH₂O/NF, the NF turned from dark gray to yellow; the formation of NiMoS_x@NiFe-LDH/NF can be observed by the color change of yellow NiMoO₄·xH₂O/NF to dark black NiMoS_x/NF to blue-black after electrodeposition (Fig. S1†). The X-ray diffraction (XRD) pattern of NiMoO₄·xH₂O/NF in Fig. S2† shows the characteristic diffraction peaks of metal Ni at 44.4°, 52.4°, and 76.5°, and diffraction peaks of NiMoO₄·xH₂O were also detected, which has been reported.^24–26 As shown in Fig. 1b, the XRD pattern of NiMoS_x/NF indicates the successful formation of NiS (PDF No. 12-0041) and Ni₃S₂ (PDF No. 44-1418) components. The lack of XRD peaks related to molybdenum means the possible amorphous phase, which can be revealed later by Raman spectroscopy. No new XRD peaks appeared even after the electrodeposition of NiFe-LDH, which is due to the amorphous nature of the deposited NiFe-LDH.²⁷ Fig. S3† displays the Raman spectrum of NiMoS_x/NF, containing E_2g and A_1g vibrations of molybdenum sulfide species at 401.6 and 418.2 cm⁻¹, respectively.^28,29 The Raman peaks of NiMoS_x@NiFe-LDH/NF at 337.4 and 662.6 cm⁻¹ are assigned to the Fe–O vibrations in disordered FeOOH,^30,31 and the peak at 373.9 cm⁻¹ is associated with FeOOH bending modes.³¹ The band of NiMoS_x@NiFe-LDH/NF at 554.7 cm⁻¹ corresponds to the lattice vibrations of β-Ni_1−xFe_x(OH)₂.³² As shown in Fig. S4,† the scanning electron microscopy (SEM) image of NiMoO₄·xH₂O/NF shows numerous microcolumns with smooth surfaces. After the ion exchange reaction in the Na₂S, NiMoS_x/NF displays a well-preserved microcolumn morphology with rough surfaces (Fig. 1c). Fig. 1d indicates the highly wrinkled surface of NiMoS_x/NF after NiFe-LDH growth, where the NiMoS_x microcolumn core is uniformly covered by the shell composed of cross-linked NiFe-LDH nanosheets. Energy dispersive X-ray spectroscopy (EDX) elemental mapping images acquired from NiMoS_x@NiFe-LDH/NF (Fig. 1e) further identify the existence of Ni, Mo, S, Fe, and O elements with a uniform distribution. Transmission electron microscopy (TEM) image in Fig. 1f clearly shows the hierarchical structure of NiMoS_x@NiFe-LDH. Furthermore, the high-resolution TEM (HRTEM) images of NiMoS_x@NiFe-LDH (Fig. 1g and h) show that the spacing of 0.287 and 0.277 nm are well-indexed to the (110) and (300) lattice planes of Ni₃S₂ and NiS, respectively. The surface chemical state of the NiMoS_x@NiFe-LDH/NF can be revealed by X-ray photoelectron spectroscopy (XPS). Ni 2p spectrum of NiMoS_x@NiFe-LDH/NF (Fig. S5a†) displays two peaks at 856.9 and 874.9 eV for Ni 2p_3/2 and Ni 2p_1/2 of Ni³⁺, respectively, and peaks at 856.3 and 874.3 eV for Ni 2p_3/2 and Ni 2p_1/2 of Ni²⁺, respectively. The two additional peaks at 862.3 and 880.5 eV are the satellite peaks (Sat.).^33–35 The Fe 2p spectrum of NiMoS_x@NiFe-LDH/NF (Fig. S5b†) shows two dominant peaks at 712.7 eV for Fe 2p_3/2 and 726.7 eV for Fe 2p_1/2.^36,37 Two peaks in the O 1 s region (i.e., O1 at 532.4 eV and O2 at 531.2 eV) suggest that oxygen species of NiMoS_x@NiFe-LDH/NF were mainly composed of mental-OH and mental-O (Fig. S5c†), respectively.⁷ In the Mo 3d spectrum of NiMoS_x@NiFe-LDH/NF, two peaks at 232.5 eV and 233.2 eV can be ascribed to the Mo⁴⁺ species, while a peak at 230.1 eV is assigned to the Mo⁶⁺ species (Fig. S5d†).^38,39 In the high-resolution spectrum of S 2p (Fig. S5e†), the three peaks at 161.5 eV, 163.0 eV, and 168.8 eV are attributed to S 2p_3/2, S 2p_1/2, and Sat., respectively.^40,41

Fig. 1 Synthesis and structural analysis of materials. (a) Schematic fabrication procedure of NiMoS_x@NiFe-LDH/NF. (b) XRD patterns of NiMoS_x/NF and NiMoS_x@NiFe-LDH/NF. SEM images of (c) NiMoS_x/NF and (d) NiMoS_x@NiFe-LDH/NF. (e) SEM and corresponding EDX elemental mapping images of NiMoS_x@NiFe-LDH/NF. (f) TEM and (g and h) HRTEM images of NiMoS_x@NiFe-LDH.

The electrochemical OER activity of as-prepared catalysts was first evaluated in 1 M KOH. Fig. 2a presents the linear sweep voltammetry (LSV) curves of NiMoS_x@NiFe-LDH/NF, NiFe-LDH/NF, NiMoS_x/NF, NiMoO₄·xH₂O/NF, and the benchmark RuO₂/NF as well as bare NF. The overpotential at a j of 100 mA cm⁻² for NiMoS_x@NiFe-LDH/NF is 240 mV, which is much lower than those of NiFe-LDH/NF (273 mV), NiMoS_x/NF (315 mV), NiMoO₄·xH₂O/NF (367 mV), RuO₂/NF (361 mV), and bare NF (469 mV). Noticeably, the required overpotentials of NiMoS_x@NiFe-LDH/NF to attain j of 200 and 500 mA cm⁻² are only 260 and 296 mV, respectively (Fig. 2b), which are lower than those of NiFe-LDH/NF (298 and 353 mV) and NiMoS_x/NF (362 and 447 mV). Impressively, NiMoS_x@NiFe-LDH/NF shows the smallest Tafel slope of 34.77 mV dec⁻¹ compared to the NiFe-LDH/NF (48.01 mV dec⁻¹), NiMoS_x/NF (82.92 mV dec⁻¹) NiMoO₄·xH₂O/NF (91.84 mV dec⁻¹), RuO₂/NF (67.52 mV dec⁻¹), and bare NF (164.53 mV dec⁻¹), possessing the fastest OER kinetics (Fig. 2c). The intrinsic OER kinetics of electrodes are also investigated by electrochemical impedance spectroscopy (EIS) (Fig. 2d). NiMoS_x@NiFe-LDH/NF shows the smallest semicircle compared to NiMoS_x/NF and NiFe-LDH/NF, indicating the fastest charge transfer and OER kinetics. Cyclic voltammetry (CV) curves were used to evaluate the electrochemically active surface area, which is positively correlated with the double-layer capacitance (C_dl). As shown in Fig. S6,† the C_dl of NiMoS_x@NiFe-LDH/NF in 1 M KOH is 50.66 mF cm⁻², which is about 10 times larger than NiFe-LDH/NF (4.59 mF cm⁻²). After confirming the high OER activity baseline of NiMoS_x@NiFe-LDH/NF in 1 M KOH, NiMoS_x@NiFe-LDH/NF was used as an anode for water oxidation in the different electrolytes, i.e., alkaline simulated seawater and alkaline seawater. After switching pure KOH solution with alkaline simulated seawater, the OER activity of NiMoS_x@NiFe-LDH/NF does not show a great influence (blue curve in Fig. 2e), with overpotentials of 240, 257, and 282 mV to reach j of 100, 200, and 500 mA cm⁻², respectively. Even in alkaline seawater, the activity of NiMoS_x@NiFe-LDH/NF shows no significant degradation (Fig. S7†). Remarkably, the electrocatalytic OER activity (overpotential required to attain the j of 500 mA cm⁻²) of NiMoS_x@NiFe-LDH/NF exceeds those of many self-supported electrocatalysts reported to date in alkaline seawater (Fig. 2f and Table S1†). Furthermore, NiMoS_x@NiFe-LDH/NF also displays excellent stability in alkaline seawater. As indicated in Fig. 2g, NiMoS_x@NiFe-LDH/NF maintains a steady j output at a constant potential for 500 h with a high retention of 97.55%. The electrocatalytic OER stability of NiMoS_x@NiFe-LDH/NF (stable for more than 500 h at 500 mA cm⁻²) exceeds that of many self-supported electrocatalysts reported so far in alkaline seawater (Fig. 2h). It is noteworthy that the SEM image of the post-electrolysis NiMoS_x@NiFe-LDH/NF (Fig. S8†) confirms that the 3D structure is maintained after seawater oxidation reaction. However, the XRD peak intensity of the post-OER NiMoS_x@NiFe-LDH/NF looks weaker (Fig. S9†), indicating a more disordered structure that will be examined in more detail in the following section. High-resolution spectrum of post-electrolysis NiMoS_x@NiFe-LDH/NF in Ni 2p region shows a broader Ni 2p_3/2 peak and higher content of Ni³⁺ relative to Ni²⁺ in Fig. S10a.† Compared to the initial NiMoS_x@NiFe-LDH/NF, Mo in NiMoS_x@NiFe-LDH/NF evolved to more high-valent Mo species (Mo⁶⁺) after alkaline seawater oxidation process (Fig. S10d†). Indeed, HRTEM image for post-electrolysis NiMoS_x@NiFe-LDH (Fig. S11†) also confirms that the surface was covered with an amorphous NiOOH layer. Here, our observation of amorphous NiOOH in electrocatalysts serving as active sites for seawater oxidation has been reported in previous works.^8,42

Fig. 2 Oxygen evolution catalysis. (a) OER polarization curves, (b) overpotential comparison, and (c) Tafel plots of different catalysts in 1 M KOH. (d) Nyquist plots of NiMoS_x@NiFe-LDH/NF, NiFe-LDH/NF, and NiMoS_x/NF in 1 M KOH. (e) OER polarization curves of NiMoS_x@NiFe-LDH/NF in different electrolytes. (f) Comparison of overpotentials required to achieve a j of 500 mA cm⁻² between NiMoS_x@NiFe-LDH/NF and other self-supported catalysts in alkaline seawater. (g) Chronopotentiometry testing of NiMoS_x@NiFe-LDH/NF at a j of 500 mA cm⁻² in alkaline seawater. (h) Comparison of the stability of NiMoS_x@NiFe-LDH/NF in alkaline seawater with reported self-supported catalysts.

In situ Raman spectroscopy investigations were carried out to interrogate the evolution of catalyst structure during the seawater oxidation processes (Fig. S12†). Fig. 3a presents the Raman spectra of NiFe-LDH/NF at increasing oxidation potentials (1.2 V to 1.9 V) related to seawater oxidation reaction. The initial bands at 455 cm⁻¹ and 525 cm⁻¹, related to Ni–O(H) and Ni–O vibrations in NiFe-LDH phases,^32,43,44 can be observed. Raman peak at 455 cm⁻¹ gradually shifts to a higher wavenumber of 473 cm⁻¹, and the peak at 473 cm⁻¹ intensifies with increasing applied bias. Such red-shift and peak intensity change (Fig. 3a) are assignable to the escalated structure disorder, thus confirming the formation of the NiOOH during the oxidation reaction.^45,46 In comparison, the in situ Raman spectra of NiMoS_x@NiFe-LDH/NF are displayed in Fig. 3b. Some obvious signals at 244, 301, and 348 cm⁻¹ ascribed to Mo–O–Mo deformation mode, Mo=O stretching, and ν4 vibration modes of Mo–O are observed, respectively.^39,47,48 The intensity of these bands is almost constant when the potential is applied and gradually increased on NiMoS_x@NiFe-LDH/NF, indicating the high-valent molybdate ions (MoO₄²⁻ and Mo₂O₇²⁻) are always present at the electrode surface during the seawater oxidation process.³⁹ Similar structure changes in a potential-dependent manner also occur in NiMoS_x@NiFe-LDH/NF, where the appearance of the Raman band located at 473 cm⁻¹ is assigned to the E_g(Ni–O) bending vibrational mode at applied potentials above 1.6 V, indicating the formation of disordered NiOOH.⁴⁹ High-valent molybdate ions from NiMoS_x@NiFe-LDH/NF can influence the self-reconfiguration process of NiOOH. The intensity ratios of the bands at 473 and 557 cm⁻¹ (I₄₇₄/I₅₅₇) indicate the OER performance in Ni–Fe systems, with higher ratios corresponding to better OER catalytic activity.^43,49 When the applied potential was raised to 1.60 V, two bands appeared at 473 and 557 cm⁻¹, and the I₄₇₃/I₅₅₇ of NiMoS_x@NiFe-LDH/NF was higher than NiFe-LDH/NF (Fig. 3c). The higher I₄₇₃/I₅₅₇ is due to the structural disorder of NiOOH caused by the high-valent molybdate ions, which is more favorable to OER.⁵⁰ We present a schematic diagram of the surface reconfiguration of NiFe-LDH/NF and NiMoS_x@NiFe-LDH/NF during seawater oxidation to deeply understand the role of high-valent molybdate ions (Fig. 3d). In alkaline seawater oxidation, Ni sites can be converted to disordered NiOOH at the NiFe-LDH/NF surface. For NiMoS_x@NiFe-LDH/NF, similar to the case of NiFe-LDH/NF, the same disordered NiOOH conversion of low crystallinity NiFe-LDH occurs in the electrochemical activation process, and the presence of high-valent molybdate ions also leads to the rapid formation of disordered NiOOH on the electrode surface under electrochemical activation, ahead of the potential for surface amorphization of NiFe-LDH/NF. In addition, the adsorption of high-valent molybdate ions on the electrode surface can also push Cl⁻ away from the electrode surface by electrostatic repulsive forces like benzoate and sulfate anions.^8,22

Fig. 3 In situ Raman monitoring of the catalyst evolution in the seawater oxidation process. In situ Raman spectra of (a) NiFe-LDH/NF and (b) NiMoS_x@NiFe-LDH/NF collected in alkaline seawater. (c) Ratio of band intensity (I₄₇₃/I₅₂₅) obtained from in situ Raman spectra of NiFe-LDH/NF and NiMoS_x@NiFe-LDH/NF. (d) Schematic of the surface reconstruction process for NiFe-LDH/NF and NiMoS_x@NiFe-LDH/NF in alkaline seawater.

Sulfide-based catalysts usually possess excellent HER electrocatalytic performance. The LSV curves in Fig. 4a display that NiMoS_x/NF requires an overpotential of only 225 mV to attain a j of 500 mA cm⁻², which is far smaller than Pt/C/NF (341 mV), bare NF (504 mV), and NiMoO₄·xH₂O/NF (533 mV) (Fig. S13†), with a small Tafel slope of 70.36 mV dec⁻¹ (Fig. 4b). We also assessed the HER activity of the NiMoS_x/NF in alkaline simulated seawater and alkaline seawater electrolytes. As presented in Fig. 4c, the HER activity of NiMoS_x/NF is still prominent in alkaline simulated seawater, which requires overpotentials of 142, 172, and 240 mV to deliver j of 100, 200, and 500 mA cm⁻², respectively (Fig. 4d). The HER activity of NiMoS_x/NF, even in alkaline seawater, is very close to that in 1 M KOH, exceeding many of the self-supported electrocatalysts reported to date (Table S2†). Notably, the LSV curves of NiMoS_x/NF continually shift with successive scans in alkaline seawater (Fig. 4e). We hypothesize that the molybdates play a crucial role in NiMoS_x/NF for HER in seawater. The HER activity of Ni₂S/NF in alkaline seawater with the additive of molybdate was investigated. As shown in Fig. 4f, the HER activity of Ni₂S/NF is significantly promoted with the addition of 0.05 M MoO₄²⁻. The HER activity can be enhanced by further raising the MoO₄²⁻ concentration. Compared with the Ni₂S/NF in 1 M KOH + seawater, overpotential at the j of 100 mA cm⁻² in the presence of 0.1 M MoO₄²⁻ decreases from 283 to 259 mV (Fig. S14†). Owing to the more negative oxidation potential from Mo to MoO₄²⁻ than from H₂ to H⁺ (Fig. S15†),^39,52 Mo in NiMoS_x/NF is presumed to be oxidized in the seawater. The structural evaluation of NiMoS_x/NF in alkaline seawater is also studied by in situ Raman spectroscopy (Fig. 4g). A peak at 368 cm⁻¹ occurs at 0.2 V, which is assigned to symmetrical stretching vibrations of molybdenum in tetrahedron coordination of oxygen atoms in MoO₄ tetrahedron, indicating the formation of MoO₄²⁻ in the transformation of NiMoS_x/NF.⁵¹ Two peaks appear at 254 and 416 cm⁻¹ with the decrease of potential. The peak at 254 cm⁻¹ is a deformation mode of Mo–O–Mo,⁴⁶ while the peak at 416 cm⁻¹ is a symmetric stretching mode of Mo–O–Mo.⁵³ The appearance of these two new peaks indicated the dimerization of Mo₂O₇²⁻. Combining in situ Raman spectroscopy results and the thermodynamic stability of MoO₄²⁻ under alkaline conditions,⁵⁴ it can be concluded that the Mo in NiMoS_x/NF dissolves as MoO₄²⁻ and then adsorbs on the electrode surface to polymerize into Mo₂O₇²⁻. Under a constant j of 500 mA cm⁻², the potential displays a slight rise of 26 mV after 400 h electrolysis in alkaline seawater (Fig. 4h). In addition, SEM image of the post-HER NiMoS_x/NF catalyst further shows the microcolumn morphology is not significantly changed (Fig. S16†). The XRD pattern of NiMoS_x/NF catalyst after the durability test shows distinct NiS and Ni₃S₂ peaks (Fig. S17†). Noteworthily, the morphology and crystal structure of NiMoS_x/NF almost remain the same as before, revealing the excellent durability of NiMoS_x/NF in alkaline seawater. Fig. S18† displays the XPS spectra for NiMoS_x/NF before and after long-term durability test. No significant changes in the chemical state of the elements can be observed after chronopotentiometry testing, which indicates the chemical stability of NiMoS_x/NF.

Fig. 4 Hydrogen evolution catalysis. (a) LSV curves. (b) Tafel plots of the NiMoS_x/NF, NiMoO₄·xH₂O/NF, Pt/C/NF, and NF in 1 M KOH. (c) HER polarization curves, (d) the corresponding overpotential comparison of NiMoS_x/NF in different electrolytes. (e) The activation process of NiMoS_x/NF under HER condition in alkaline seawater. (f) LSV curves of Ni₂S/NF with the addition of different concentrations of MoO₄²⁻ in alkaline seawater. (g) In situ Raman spectra of NiMoS_x/NF collected in alkaline seawater under HER condition. (h) Chronopotentiometry testing of NiMoS_x/NF at a j of 500 mA cm⁻² in alkaline seawater.

Given the excellent OER activity of NiMoS_x@NiFe-LDH/NF and HER performance of NiMoS_x/NF in alkaline seawater, we further assembled a two-electrode electrolyzer with NiMoS_x@NiFe-LDH/NF as anode and NiMoS_x/NF as cathode for overall seawater splitting (Fig. 5a). Fig. 5b shows that NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF requires only a voltage of 1.61 V to drive a j of 100 mA cm⁻² in alkaline seawater, substantially lower than the benchmark RuO₂/NF||Pt/C/NF (1.93 V) and many of recently reported self-supported catalysts (Fig. 5c and Table S3†). Besides, the NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF can maintain its superior performance over 150 h of continuous operation at a j of 500 mA cm⁻² (Fig. 5d). The polarization curve of NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF after stability test is also close to the initial one (Fig. 5e).

Fig. 5 Overall seawater splitting performance. (a) Schematic illustration of the overall seawater splitting system. (b) Polarization curves of NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF and Pt/C/NF||RuO₂/NF in alkaline seawater. (c) Comparison of the voltages required to achieve a j of 100 mA cm⁻² in alkaline seawater between NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF and other seawater electrolyzers. (d) Chronopotentiometric curves of NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF at a j of 500 mA cm⁻² in alkaline seawater. (e) Polarization curves of NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF before and after stability test in alkaline seawater.

Conclusions

In summary, we reported a 3D core–shell NiMoS_x@NiFe-LDH/NF electrocatalyst for highly efficient and stable seawater oxidation. Such NiMoS_x@NiFe-LDH/NF exhibits a low overpotential of 297 mV to attain an industrial-level j of 500 mA cm⁻² and good stability up to 500 h at 500 mA cm⁻² in alkaline seawater. In situ Raman spectroscopy studies indicate that the high-valent molybdate ions can promote the generation of disordered NiOOH active species and protect catalysts from Cl⁻ corrosion during seawater oxidation. Meanwhile, the dissolution and polymerization of Mo in NiMoS_x/NF during the alkaline seawater HER process is also demonstrated by spectroscopic and electrochemical studies. The constructed NiMoS_x@NiFe-LDH/NF||NiMoS_x/NF requires only a voltage of 1.61 V to attain a j of 100 mA cm⁻² and operates stably at a j of 500 mA cm⁻² for more than 150 h, which has significant potential for industrial applications. Our work not only develops a highly active and robust catalyst for electrochemical seawater oxidation but also provides a new strategy for solving the anode corrosion problem in seawater electrolysis.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the Free Exploration Project of Frontier Technology for Laoshan Laboratory (No. 16-02) and the National Natural Science Foundation of China (No. 22072015).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and ESI figures. See DOI: https://doi.org/10.1039/d3qi00341h

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