# Direct Seawater Electrolysis with Stabilized High-Performance NiCr Anodes

Hiba Saada, Gwenaëlle Benoit, Joudi Dabboussi, Corinne Lagrost, Bruno Fabre, Gabriel Loget

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Hiba Saada, Gwenaëlle Benoit, Joudi Dabboussi, Corinne Lagrost, Bruno Fabre, et al. Direct Seawater Electrolysis with Stabilized High-Performance NiCr Anodes. Advanced Materials Interfaces, 2025, pp.2500305. 10.1002/admi.202500305. hal-05094786

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# Direct Seawater Electrolysis with Stabilized High-Performance NiCr Anodes

Hiba Saada, Gwenaëlle Benoit, Joudi Dabboussi, Corinne Lagrost, Bruno Fabre,* and Gabriel Loget*

Hydrogen ($H_2$) production through direct seawater electrolysis faces significant challenges due to the complex composition of seawater that induces undesirable reactions competing with the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Such competition ultimately has a considerable impact on both the efficiency and sustainability of this process. Indeed, $Cl^-$ and $Br^-$ anions in seawater contribute to corrosion, lower the catalytic efficiency, and compromise the stability of conventional electrocatalysts, thereby restricting the feasibility of large-scale direct seawater electrolysis at high pH through alkaline seawater electrolysis (ASE). In this work, it is shown that the use of an alloyed NiCr foam (NiCrF) anode, exhibiting enhanced durability compared to conventional Ni foam (NiF), is compatible with the ASE process if coated with an efficient and robust electrocatalyst. This is reported here by using a highly active NiFe-layered double hydroxide (NiFeLDH) deposited on the NiCr foam. It is demonstrated that this assembly shows high corrosion resistance during ASE, minimal Cr leaching in the electrolyte, and long-term efficiency. An ASE electrolyzer comprising the so-prepared anode and a cathode derivatized with $NiMoO_4$ is stable in operation at 50 °C for 270 h at a current density of $100 \\ \text{mA cm}^{-2}$ and an iR-corrected voltage of 1.6 V.

## 1. Introduction

The replacement of fossil fuels with sustainable and affordable energy sources is crucial to reducing greenhouse gases and improving human well-being. With a global energy demand expected to double by 2050 and triple by the end of the

century, $H_2$ is being considered a promising energy carrier for decarbonization.[1-4] At present, the use of freshwater for $H_2$ production faces a problem due to its increasing scarcity.[5] In contrast, seawater, which is abundant and economically viable, has been identified as an attractive alternative for green $H_2$ production.[6-8] Due to the development of numerous affordable non-noble metal catalysts for seawater electrolysis and the adoption of new electrolyzer configurations, ASE currently offers advantages over solid oxide electrolyzer cells (SOEC) or proton exchange membrane (PEM) electrolyzers, including the integration of a membrane into the cell to simplify seawater pretreatment.[9] However, ASE faces significant challenges due to the complex composition of the liquid phase, particularly the presence of dissolved ions that can impact the electrolysis process.[10-12] On the anode side, the chlorine evolution reaction (CLER) can compete with the oxygen evolution reaction (OER), as they

have relatively close thermodynamic equilibrium potentials.[7,10,13] The faster kinetics of CLER further complicates the process, leading to the formation of hypochlorite ($ClO^-$) with an onset potential of 480 mV versus the reversible hydrogen electrode (RHE) under alkaline conditions, which causes severe corrosion of the anode.[7,14] Yet, ASE offers a

H. Saada, J. Dabboussi, C. Lagrost, B. Fabre

Univ Rennes

CNRS

ISCR (Institut des Sciences Chimiques de Rennes) – UMR6226

Rennes F-35000, France

E-mail: bruno.fabre@univ-rennes.fr

H. Saada, G. Benoit

EKIUM

50 rue Antoine de Saint-Exupéry, Guipavas 29490, France

G. Loget

Univ. Bordeaux

CNRS

Bordeaux INP

ISM-UMR 5255

Pessac 33607, France

E-mail: gabriel.loget@cnrs.fr

C. Lagrost

Univ Rennes

CNRS

ScanMAT-UAR2025

Rennes F-35000, France

ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202500305

© 2025 The Author(s). Advanced Materials Interfaces published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/admi.202500305
promising pathway for H₂ production but still requires durable and chemically robust anodes to function efficiently and reliably in Cl⁻-rich environments.[15,16]

To generalize the use of ASE in industrial-scale H₂ production, it is crucial to enhance the stability of key components, such as the electrolyzer electrodes, with a special focus on the anode.[16-18] Ni, Co, Fe, and Mo are cost-effective and efficient materials for seawater electrolysis.[19,20] They have demonstrated high catalytic activity for both the HER and OER.[21] NiFeLDH (Figure S1, Supporting Information) has been recognized as a promising OER catalyst due to its high activity and low cost.[22] However, self-standing NiFeLDH is not suitable for industrial seawater electrolysis because of its low conductivity and poor stability at high current densities.[23] To enhance the OER performance and durability of NiFeLDH for seawater electrolysis in the presence of Cl⁻ and Br⁻ ions, various strategies have been employed including the introduction of supplementary elements into the catalyst or the modification of the surface structure to increase the number of active sites.[24] Often, NiFeLDH is covered with a protective layer to prevent corrosive ions from attacking the catalyst and/or the underlying electrode substrate.[25-30] Another strategy involves intercalating anions (e.g., phosphates PO₄³⁻ or benzoates -C₆H₅COO⁻) into the catalytic layer to repel halide ions.[31-33] Alternative methods included the use of N-doped carbon,[34] Ce,[35] Ag,[36] Te,[37] S,[38] Zr,[39] or Ba sites,[40] which are strongly coupled with NiFeLDH nanosheet networks to effectively facilitate the OER while avoiding the CLER. In another study, corrosion was prevented by replacing Fe in the NiFeLDH catalyst with a noble and expensive element like Ir.[41] On the other hand, Tang and coworkers enhanced the stability of NiFeLDH by integrating Cr sites, which resulted in low overpotentials and selective seawater oxidation in alkaline media.[15] They demonstrated that Cr(III) sites promote OH⁻ enrichment, while Cr(VI) sites repel Cl⁻ ions. Both types of sites are well-dispersed and positioned close to NiFe, enhancing charge transfer and overall electrode performance.[15] Cr-Doping modified the electronic structure, facilitating the surface reconstruction of the catalytic material to form oxides or hydroxides, which reduced activation energy and promoted O₂ generation. Cr also enhanced the synergy between Ni and Fe, boosting catalytic activity. Furthermore, Cr formed a protective rust layer in alkaline conditions, improving the corrosion resistance of Fe and the stability of composite catalysts.[42] However, introducing Cr into the anode always required a doping step.[15,42] Usually, these metal-doped OER electrocatalysts are deposited on high specific surface area metal foams, such as NiF, which are popular electrode supports showing unique properties, such as high porosity, high conductivity, thermal stability, and chemical compatibility with high pH electrolytes.[43] However, when operated in the anodic mode in alkalinized seawater at high current density (e.g. 500 mA cm⁻²), NiF was mostly destroyed after a short time of operation in Cl⁻-rich environments (<2 h).[44]

In this work, we demonstrate that NiFeLDH layers grown on another porous support, namely NiCrF, through a one-step hydrothermal method constitute highly active electrocatalytic and stable anodes for ASE. The NiCrF/NiFeLDH electrode exhibited a potential of 2.4 V versus RHE at an OER current density of 100 mA cm⁻² in alkaline seawater (i.e. 1.6 V with iR correction). When combined with a NiMoO₄-derivatized NiF cathode, a re-

markable activity was obtained for overall ASE (stability of 270 h at an operating current of 100 mA cm⁻²). These results open opportunities for direct seawater electrolysis and evidence of the synergistic interplay between the electrode support and the OER electrocatalyst for improving anode lifetime at high current densities in alkalinized seawater.

## 2. Results and Discussion

### 2.1. NiFeLDH-Coated NiCr Foam Anodes for Stable OER in Alkaline Seawater

**Figure 1a** summarizes the experimental conditions of the hydrothermal method used to synthesize the NiCrF/NiFeLDH anode. Here, a commercial NiCrF is used as the conductive support due to its high surface area, good electrical conductivity, and low cost. We first evaluated the OER activity of the NiCrF/NiFeLDH anode in a three-electrode configuration in alkalinized seawater at room temperature, using artificial seawater prepared according to the ASTM D 1141-98 standard.

The linear sweep voltammetry (LSV) curves shown in Figure 1b reveal, as expected, that the NiCrF/NiFeLDH anode electrocatalyzes OER more efficiently than the bare NiCrF electrode support, as supported by the lower potential measured at a current density of 100 mA cm⁻², 1.63 V against 1.69 V versus RHE. Furthermore, the catalytic response of this anode is not significantly different from that observed in 1 M KOH (Figure S2, Supporting Information). Scanning electron microscopy (SEM) images of the NiCrF/NiFeLDH surface before (Figure 1d) and after (Figure 1e) 100 h of ASE at 100 mA cm⁻² showed similar array structures, which confirmed the remarkable stability of the anode.

Additionally, the energy-dispersive spectroscopy (EDS) elemental spectra (Figure 1f) confirmed the presence of the expected elements (Ni, Fe, and Cr) before (black) and after (blue) ASE. Furthermore, particles of Ca, K, and Na were detected on the NiCrF/NiFeLDH after ASE (Figure 1f, blue spectrum). Importantly, the deposition of these particles did not affect the OER catalytic activity, as evidenced by potentials of 1.64 V and 1.66 V measured at 100 mA cm⁻² after 100 and 146 h of ASE, respectively (vide infra). The EDS analysis showed that the mass percentage ratio of Fe to Ni remained nearly unchanged after 100 h of ASE, varying from 0.23 to 0.24 (Tables S1 and S2, Supporting Information).

Complementary to EDS analyses, X-ray photoelectron spectroscopy (XPS) was employed to analyze the outermost surface composition, where the electrocatalytic reaction generally occurs. The survey spectra, recorded before and after 100 h ASE, show the presence of C, O, Ni, Fe, and Cr elemental species (Figure 1g). Fe was still observed after 100 h ASE, but at a lower concentration while no Fe was detected after ASE when the NiFeLDH was deposited onto NiF (Table S3, Supporting Information), indicating a superior stability of the NiCrF/NiFeLDH material. The high-resolution core level spectra in the Ni 2p region for NiCrF/NiFeLDH and NiF/NiFeLDH show similar patterns with the doublet structure for Ni 2p₃/2 and Ni 2p₁/₂ components at ≈856 and 873.3 eV, along with their satellites in the 860-866 eV and 873-879 eV regions. Interestingly, a significant shift (0.9 eV) of the main signal is observed for NiCrF/NiFeLDH toward the
<figure><img src="image_3.png" /><figcaption>**Figure 1.** NiCrF/NiFeLDH: Synthesis, characterization, and OER performance for alkaline seawater electrolysis. a) Scheme of the synthesis of NiCrF/NiFeLDH. b) iR-corrected OER polarization curves of NiCrF/NiFeLDH at 5 mV s⁻¹ before (black) and after 100 h (blue) or 146 h (orange) ASE at 100 mA cm⁻² in 1 M KOH + seawater. The polarization curve of bare NiCrF (grey) in 1 M KOH + seawater is shown for comparison. c) Stability test in 1 M KOH + seawater at a current density of 100 mA cm⁻². SEM images of NiCrF/NiFeLDH before (d) and after (e) the 100 h-long stability test. f) EDS spectra of NiCrF/NiFeLDH before (black) and after (blue) 100 h of ASE. g) XPS survey spectra of NiCrF/NiFeLDH before (black) and after (blue) ASE.</figcaption></figure>

lower binding energy as compared to NiF/NiFeLDH (Figure S3a, Supporting Information). The peak-fitting indicates the presence of metallic Ni at 852.5 eV and Ni²⁺ species at 856.2 eV for all samples (Figure S3b-d, Supporting Information).[45,46] A contribution due to CrNi₂ at 852.7 eV is observed for both NiCrF and NiCrF/NiFeLDH.[47] An additional contribution for Ni²⁺ species at 854.9 eV is added to account for the energy shift of the signal, which can be attributed to an interface NiO-NiFe/NiCr, suggesting an effective interaction between the NiFeLDH and NiCrF. After 100 h ASE, the components attributed to the metallic Ni and CrNi₂ were not identified (Figure S4, Supporting Information), but the contribution attributed to the interface NiO-NiFe/NiCr was still visible at 854.9 eV (Figure S5, Supporting Information), in line with the better stability of this anode material.

Comparison of the electrochemical response of NiCrF/NiFeLDH with that of the bare NiCrF anode provides essential information on the synergistic effect arising from the anode support/OER electrocatalyst combination. In the case of the bare NiCrF, a 100 h ASE led to a strong increase in the anodic peak attributed to the Ni³⁺/Ni²⁺ redox couple at ≈1.4 V versus RHE[48] (Figure 2a), which is not observed for NiCrF/NiFeLDH (Figure 1b). This is indicative of the dissolution of Cr and Ni, as supported by EDS (Figure 2b).

Indeed, the EDS analysis showed significant changes in the mass percentages of Ni and Cr before and after 100 h of ASE. Initially, the mass percentages of Ni and Cr were 53.5% and 27.3%, respectively, (Table S4, Supporting Information), decreasing to 40.1% and 6.3%, respectively, after 100 h of ASE (Table S5, Supporting Information). The mass ratio of Ni to Cr has therefore significantly increased from 2.0 to 6.3 after electrolysis, which is consistent with a stronger propensity of Cr to be dissolved

compared with Ni in our electrolytic conditions. The comparative SEM and optical images shown in Figure 2c,d evidence the metal corrosion and dissolution which are caused by Cl⁻ and Br⁻ ions present in seawater (the red arrows in Figure 2d highlight the visible corrosion of NiCrF).[44,49] In contrast, the NiCrF substrate covered with NiFeLDH was found to be protected against corrosion and showed no visible signs of deterioration. The LSV shape of NiCrF/NiFeLDH is not modified after 100 h of ASE (Figure 1b) and the post-electrolysis appearance and morphology of the electrode showed no signs of degradation (Figure 2e,f).

These results were confirmed by inductively coupled plasma mass spectrometry (ICP-MS). The ICP analysis of the alkalinized seawater electrolyte revealed the presence of a large amount of Cr (>70 mg L⁻¹) after ASE for bare NiCrF (Figure S6, Supporting Information). In contrast, the Cr content in the electrolyte was almost negligible (<1 mg L⁻¹) after 100 h of ASE for NiCrF/NiFeLDH (Figure S7, Supporting Information). This is further supported by the Brunauer–Emmett–Teller (BET) measurements in Table S6 (Supporting Information), which showed an increase in the surface area from 0.5 to 5.5 m² g⁻¹. In contrast, the BET measurements for NiCrF/NiFeLDH did not indicate any increase in the surface area after electrolysis.

XPS measurements of NiCrF samples were performed before and after 100 h ASE for comparison purposes. XPS spectra in the Cr 2p region clearly show that the full Cr layer for bare NiCrF has been removed through dissolution after ASE (Figure S8, Supporting Information) which is not the case for the NiCrF/NiFeLDH anode (Figure S9, Supporting Information). Moreover, the Ni 2p signal after 100 h ASE shows a contribution of Ni²⁺ species Ni(OH)₂ at 856.1 eV (Ni 2p₃/₂), in agreement with the disappearance of the Cr sublayer (Figure S10, Supporting Information).
<figure><img src="image_4.png" /><figcaption>**Figure 2.** NiCrF: Characterization and OER performance for alkaline seawater electrolysis. a) *iR*-corrected OER polarization curves of NiCrF at 5 mV s⁻¹ before (grey) and after (red) 100 h ASE at 100 mA cm⁻² in 1 M KOH + seawater. b) EDS spectra of NiCrF before (grey) and after (red) ASE. SEM images of NiCrF (c,d) and NiCrF/NiFeLDH (e,f) before (c,e) and after (d,f) the stability test. Insets are the photographs of the electrodes. The red arrows in the inset of Figure 2d highlight the visible corrosion of bare NiCrF.</figcaption></figure>

To sum up, these results led us to propose a reconstruction process of the NiCrF/NiFeLDH catalyst during the OER. It can be hypothesized that some cationic Cr species are intercalated into the basal planes of the NiFeLDH through electrostatic interactions, while others are uniformly dispersed across the nanosheet structure, forming an interface with the NiFeLDH.[15] This configuration enhances the electronic interactions between OH⁻ ions and the active sites, thereby boosting the OER activity.[50] Furthermore, these results reveal the beneficial role of the NiFeLDH coating in preventing the degradation of the underlying NiCrF support, enabling the use of the engineered electrode for achieving efficient and durable ASE.

## 2.2. NiMoO₄-H₂-Coated Ni Foam Cathodes for HER in Alkaline Seawater

The HER cathode (named thereafter NiF/NiMoO₄-H₂) was prepared by coating the commercially available NiF with a well-established HER-active material based on NiMoO₄ annealed under an Ar/H₂ reductive atmosphere.[51,52] As shown in **Figure 3b**, NiF/NiMoO₄-H₂ showed expectedly a HER activity higher than that of bare NiF, as supported by overpotential values at -100 mA cm⁻² of 270 against 430 mV versus RHE for the latter. Similar to NiCrF/NiFeLDH for OER, the NiF/NiMoO₄-H₂ exhibited remarkable stability for HER over 115 h at -100 mA cm⁻² in seawater (Figure 3c). Interestingly, the HER activity was even considerably improved after the stability test, as shown by the LSV measurements before and after ASE. Indeed, the overpotential of NiF/NiMoO₄-H₂ at -100 mA cm⁻² was decreased from

270 to 200 mV after ASE (Figure 3b). Such an improvement in activity may be attributed to some changes in the morphology of Ni/NiMoO₄-H₂, as supported by the SEM images before (Figure 3d) and after (Figure 3e) ASE. After ASE, the NiMoO₄ nanofibers adhered to each other and formed agglomerates.

Additionally, the EDS analysis revealed that the mass percentage ratio of Mo to Ni dramatically decreased from 0.53 to 0.046 after 100 h of ASE at -100 mA cm⁻² (Figure 3f; Tables S7 and S8, Supporting Information). This indicates a significant decrease in the proportion of Mo relative to Ni after electrolysis. These results suggest that Mo leached into the electrolyte, creating newly formed active sites, and the intrinsic activity of the catalyst increased significantly.[53] The presence of a few weight % of magnesium (Mg) and calcium (Ca) on the cathode surface, as observed by EDS, can be attributed to the adsorption of Mg(OH)₂ and Ca(OH)₂ precipitates which unavoidably form due to the employed electrolyte.

The chemical composition of the surface was further examined through XPS analysis (Figure 3g). The survey spectra display the photoelectron peaks of C 1s (≈285 eV), O 1s (≈530 eV), Ni 2p (≈856 eV), Mo 3p (≈395 eV), and Mo 3d (≈231 eV). The Mo/Ni atomic ratio decreases from 1.64 to 0.065 after 100 h ASE, confirming the leaching of Mo (Table S3, Supporting Information). The high-resolution core level spectra in the Mo 3d region were peak-fitted with doublet structures due to Mo 3d<sub>5/2</sub> and Mo 3d<sub>3/2</sub> components with an energy separation of 3.1 eV and an area ratio of 2/3 (Figure S11, Supporting Information). Before electrolysis, the Mo 3d signal shows a main component (52%) attributed to NiMoO₄ at 232.8 and 235.9 eV.[54] Two other contributions are assigned to MoO (12.5%, at 229.5 and 232.6 eV) and
<figure><img src="image_5.png" /><figcaption>**Figure 3.** NiF/NiMoO₄-H₂: Synthesis, characterization, and HER performance for alkaline seawater electrolysis. a) Scheme of the synthesis of NiF/NiMoO₄-H₂. b) *iR*-corrected HER polarization curves of NiF/NiMoO₄-H₂ at 5 mV s⁻¹ before (wine) and after (green) 115 h ASE at -100 mA cm⁻² in 1 M KOH + seawater. The polarization curve of bare NiF (pink) in 1 M KOH + seawater is shown for comparison. c) Stability test in 1 M KOH + seawater at the current density of -100 mA cm⁻². SEM images of NiF/NiMoO₄-H₂ before (d) and after (e) the stability test. f) EDS spectra of NiMoO₄-H₂ before (wine) and after (green) ASE. g) XPS survey spectra of NiF/NiMoO₄-H₂ before (wine) and after (green) ASE.</figcaption></figure>

MoO₂ (35.5%, at 230.7 and 233.8 eV). After ASE, the Mo 3d signal no longer evidence the contribution of NiMoO₄, and only MoO and MoO₂ signals are still visible. Such results fall in line with the EDS analysis. The atomic ratios Ni/C and Ni/O did not show any significant change before and after ASE (Table S3, Supporting Information). Overall, all characterization data suggest that the better HER electrocatalytic performance of the activated cathode is likely due to an increase in the porosity of the electrocatalytic coating after the dissolution of the NiMoO₄ layer.[52]

## 2.3. Overall Alkaline Seawater Electrolysis

Given the appealing catalytic performance of both NiCrF/NiFeLDH and NiF/NiMoO₄-H₂ electrodes for OER and HER, respectively, we further evaluated the overall seawater splitting performance by integrating these two electrodes into a commercial flow cell with NiCrF/NiFeLDH serving as the anode and NiF/NiMoO₄-H₂ as the cathode, separated by a diaphragm. Heated alkalinized seawater was pumped using two separate pumps to continuously supply the anodic and cathodic compartments with the electrolyte (Figure 4h). The electrolyzer remained stable in operation for up to 270 h with continuous flow and achieved current densities of 100 mA cm⁻² at *iR* corrected-cell voltages of only 1.6 and 1.4 V at 50 and 60 °C, respectively (Figure 4a). NiCrF/NiFeLDH, NiF/NiMoO₄,

and diaphragm exhibited no signs of degradation after 270 h of overall ASE (Figure 4b–f). In contrast, an anode composed of NiFeLDH grown on the conventional NiF degraded after less than 75 h of ASE under identical conditions (Figure 4g), whereas it remained stable in 1 M KOH/pure water (Figure S12, Supporting Information). The remarkable stability of the anode (NiCrF/NiFeLDH) and cathode (NiF/NiMoO₄) highlights their potential for long-term ASE at industrially relevant current densities.

## 3. Conclusion

In this work, we reported on the use of new catalytic electrodes for ASE under practical conditions. A key highlight of this study is that the herein-designed anode demonstrated excellent corrosion resistance, even in the presence of Cl⁻ and Br⁻ ions present in seawater. NiCrF/NiFeLDH, a robust catalytic anode for seawater oxidation, was produced by combining only two components: a porous electrode substrate (NiCrF) and a NiFeLDH layer, rather than the traditionally used three elements that are the electrode substrate, the catalytic layer, and the protective layer.[25,26,28,31] It is important to mention that the modification of NiCrF with the NiFeLDH electrocatalyst avoided the release of Cr ions in the electrolyte, which is crucial to envision practical applications of NiCr alloys in ASE. This simplification may lead to faster implementation of the anode and reduced production costs. The
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Overall alkaline seawater splitting cell performance at high temperature. a) iR-corrected chronopotentiometry curve for NiCrF/NiFeLDH||Zirfon||NiF/NiMoO₄·H₂ at 100 mA cm⁻² for 270 h in 1 M KOH + seawater. Photographs of NiCrF/NiFeLDH (b,c), NiF/NiMoO₄·H₂ (d,e) Zirfon diaphragm (f) and NiF/NiFeLDH (g) before (b,d) and after (c,e,f,g) the stability test. h) Photograph of the alkaline seawater electrolysis setup with heated seawater passing through pumps to the electrolysis cell (the exploded view of the cell is shown below the electrolyzer).</figcaption></figure>

beneficial effect of Cr against corrosion in seawater electrolysis is well known.[15] It can be hypothesized that the Cr present in the NiCrF substrate may have induced spontaneous doping in the NiFeLDH catalyst layer, eliminating the need for an additional step, as required in other published studies.[21,28–31,36,55–57]

**Table 1.** presents the lifespan of our OER catalyst in comparison with other state-of-the-art materials for alkaline seawater oxidation. In previous studies, to enhance electrical conductivity, charge and electron transport, chloride corrosion resistance, and overall anode durability, catalysts were typically engineered

<table><caption>**Table 1.** Durability, overpotential, and performance comparison of state-of-the-art OER electrocatalysts for direct seawater electrolysis at various current densities.</caption><thead><tr><th>Catalyst</th><th>Electrolyte</th><th>Overpotential</th><th>OER Stability</th><th>Refs.</th></tr></thead><tbody><tr><td>CrO<sub>4</sub><sup>2-</sup>-NiFe LDH/Cr<sub>2</sub>O<sub>3</sub>/NiF</td><td>Alkaline seawater</td><td>—</td><td>150 h at 2 A cm<sup>-2</sup></td><td>[15]</td></tr><tr><td>Ag-doped NiFeLDH</td><td>1 M KOH + seawater</td><td>246 mV at 100 mA cm<sup>-2</sup></td><td>1000 h at 1000 mA cm<sup>-2</sup></td><td>[36]</td></tr><tr><td>NiPO<sub>x</sub>/NiFeLDH/NiF</td><td>Alkaline seawater</td><td>352 mV at 1000 mA cm<sup>-2</sup></td><td>600 h at 1000 mA cm<sup>-2</sup></td><td>[55]</td></tr><tr><td>NiFe LDH/PTPA/NiF</td><td>1 M KOH + seawater</td><td>270 mV at 100 mA cm<sup>-2</sup></td><td>600 h at 1000 mA cm<sup>-2</sup></td><td>[13]</td></tr><tr><td>NiCoFe-Bi</td><td>Alkaline seawater (pH: 14)</td><td>314 mV at 100 mA cm<sup>-2</sup></td><td>1000 h at 500 mA cm<sup>-2</sup></td><td>[63]</td></tr><tr><td>Fe-modified Ni hydroxysulfide</td><td>1 M KOH + seawater</td><td>263 mV at 100 mA cm<sup>-2</sup></td><td>900 h at 500 mA cm<sup>-2</sup></td><td>[64]</td></tr><tr><td>NiMoS<sub>x</sub>/NiFe-LDH/NiF</td><td>Alkaline seawater</td><td>297 mV at 500 mA cm<sup>-2</sup></td><td>500 h at 500 mA cm<sup>-2</sup></td><td>[65]</td></tr><tr><td>NiCo/NiFeLDH</td><td>1 M KOH + seawater</td><td>222 mV at 100 mA cm<sup>-2</sup></td><td>100 h at 500 mA cm<sup>-2</sup></td><td>[66]</td></tr><tr><td>NiMoN/NiFeN core-shell</td><td>1 M KOH + natural seawater</td><td>369 mV at 500 mA cm<sup>-2</sup></td><td>100 h at 500 mA cm<sup>-2</sup></td><td>[24]</td></tr><tr><td>NiFeP/crystalline Ni<sub>3</sub>S<sub>2</sub></td><td>1 M KOH + seawater</td><td>354 mV at 800 mA cm<sup>-2</sup></td><td>600 h at 100 mA cm<sup>-2</sup></td><td>[67]</td></tr><tr><td>Mo-doped Ni<sub>3</sub>S<sub>2</sub></td><td>1 M KOH + natural seawater</td><td>291 mV at 100 mA cm<sup>-2</sup></td><td>500 h at 100 mA cm<sup>-2</sup></td><td>[58]</td></tr><tr><td>S-(Ni, Fe)OOH</td><td>1 M KOH + seawater</td><td>300 mV at 100 mA cm<sup>-2</sup></td><td>100 h at 100 mA cm<sup>-2</sup></td><td>[56]</td></tr><tr><td>B-Co<sub>2</sub>Fe-LDH</td><td>1 M KOH + seawater</td><td>310 mV at 100 mA cm<sup>-2</sup></td><td>100 h at 100 mA cm<sup>-2</sup></td><td>[60]</td></tr><tr><td>FeOOH/Ni(OH)<sub>2</sub></td><td>1 M KOH + seawater</td><td>325 mV at 100 mA cm<sup>-2</sup></td><td>100 h at 100 mA cm<sup>-2</sup></td><td>[62]</td></tr><tr><td>Cr-Co<sub>x</sub>P</td><td>1 M KOH + seawater</td><td>325 mV at 100 mA cm<sup>-2</sup></td><td>100 h at 100 mA cm<sup>-2</sup></td><td>[57]</td></tr><tr><td>CoP<sub>x</sub>/FeOOH</td><td>1 M KOH + seawater</td><td>283 mV at 100 mA cm<sup>-2</sup></td><td>80 h at 100 mA cm<sup>-2</sup></td><td>[59]</td></tr><tr><td>Commercial NiF</td><td>Acidic solution + alkali metal cations (Co<sup>2+</sup>, Na<sup>+</sup>)</td><td>—</td><td>93.8 h at -1&1 A cm<sup>-2</sup></td><td>[61]</td></tr><tr><td>NiCrF/NiFeLDH</td><td>1 M KOH + seawater</td><td>400 mV at 100 mA cm<sup>-2</sup></td><td>140 h at 100 mA cm<sup>-2</sup></td><td>This work</td></tr></tbody></table>
using multi-step strategies such as doping,[36,58,56,57] core-shell architectures,[24,59] heterostructure design,[56,60] or by introducing alkali metal cations into the electrolyte to modulate OER catalysts interfacial properties.[61] In contrast, our heterostructured electrode, synthesized through a one-step process, demonstrates comparable overpotential, stability, and durability to those reported in previous studies,[56,57,59,60,62] and holds strong potential for further optimization under industrial operating conditions.

The assembled two-electrode system comprising NiCrF/NiFeLDH and NiF/NiMoO₄·H₂, separated by a conventional diaphragm, achieves long-term stability for 270 h at 100 mA cm⁻² and a low voltage of 1.6 V at 50 °C. The development of new anodes to repel Cl⁻ and Br⁻ anions present in seawater, preventing their corrosion while enhancing catalytic activity and durability, is essential. It is critical to develop new 3D catalysts based on transition metals on the NiCrF substrate to improve overall performance. Additionally, strategies such as in situ growth on conductive substrates, surface wettability engineering, and the construction of 3D porous architectures can be employed to extend stability and to improve catalyst activity. These approaches can meet the demands of industrial electrolysis at high current densities and provide a long lifespan for seawater electrolysis.

## 4. Experimental Section

**Chemicals:** Iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O, ≥98%), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O ≥98.5%), ammonium fluoride (NH₄F), urea (CO(NH₂)₂), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), potassium hydroxide (KOH), sodium chloride (NaCl), sodium sulfate (Na₂SO₄), magnesium chloride (MgCl₂), calcium chloride (CaCl₂), potassium chloride (KCl), sodium bicarbonate (NaHCO₃), potassium bromide (KBr), sodium fluoride (NaF) and boric acid (H₃BO₃) were purchased from Sigma-Aldrich and used without further purification. NiF with an average pore size of 350 µm and NiCrF with an average pore size of 300 µm were purchased from Recemat B.V. Cell Material Engineering, The Netherlands. Ultrapure water (Veolia Water-STI) with a resistivity of 18.2 MΩ cm was used to prepare the aqueous solutions. Anhydrous ethanol (RSE electronic grade, Erbatron from Carlo Erba) was used without further purification.

**Synthesis of NiCrF/NiFeLDH:** NiCrF/NiFeLDH was synthesized via a simple one-step hydrothermal method adapted from D. Liu et al.[68] Prior to the self-hydrodeposition, NiCrF was cleaned using 3M HCl solution, ultrapure water, and absolute ethanol, with sonication for 10 min in each solution to remove the oxide layer. Following this pretreatment, pieces of NiCrF (4 × 5 cm²) were immersed in 17 mL of a solution containing 1.9 mM of Ni(NO₃)₂·6H₂O, 0.59 mM of Fe(NO₃)₃·9H₂O, 6.6 mM of NH₄F, and 42 mM of CH₄·N₂O, all dissolved in ultrapure water, within a PTFE-lined stainless-steel reactor. The reactor was then placed in an oven at 120 °C for 10 h.

**Synthesis of NiF/NiMoO₄·H₂:** NiF/NiMoO₄·H₂ was synthesized in two steps. Briefly, 0.09 M of Ni(NO₃)₂·6H₂O and 0.1 M of Na₂MoO₄·2H₂O were dissolved in 35 mL of deionized water, the solution was then transferred to a 50 mL PTFE-lined stainless-steel reactor containing the Ni foam. The reactor was then placed in an oven at 150 °C for 6 h. Afterward, the resulting NiMoO₄ was annealed under hydrogen at 400 °C for 2 h to obtain NiF/NiMoO₄·H₂.

**Electrolysis stack:** The cathode and anode placed in the electrolyzer both possessed a geometric area of 10 cm². The anode and cathode were positioned on their respective flow-field plates, separated by a Zirfon PERL UTP 500 diaphragm purchased from Electrocell, and then an equal compression torque was applied. A peristaltic pump was used to flow the filtered electrolyte (1 M KOH + seawater) through the stacks. The alkaline

seawater electrolyte was filtered to remove precipitates of Mg(OH)₂ and Ca(OH)₂, which could clog the components of the electrolyzer. The filtered electrolyte was transferred into a flask placed on a heating plate, and peristaltic pumps were used to circulate the filtered electrolyte (1 M KOH + seawater) at 50 °C through the stacks at a flow rate of 10 mL min⁻¹. The preparation of artificial seawater refers to the ASTMD 1141–98 standard (salinity 3.5 wt.%), and the specific components are shown in Table S9 (Supporting Information).[69]

**Physicochemical Characterizations:** JSM-7100F JEOL Scanning electron microscope (SEM) equipped with an X-Max 50 mm² EDS detector (Oxford Instruments) was used for structural characterization and elemental analysis.

XPS measurements were performed with a NEXSA G2 (ThermoFischer Scientific) spectrometer using the Al Kα X-ray source working at 1486.6 eV with a spot size of 200 µm². Survey spectra (0–1000 eV) were acquired with an analyzer pass energy of 200 eV (1 eV step⁻¹); high-resolution spectra used pass energy of 50 eV (0.1 eV step⁻¹). Binding energies were referenced to C 1s peak at 284.8 eV (adventitious carbon). The core level spectra were peak-fitted using the CasaXPS Software, Ltd. Version 2.3.25PR1.0. U2 Tougaard or Shirley background subtraction was used for the spectral analysis. The peak areas were normalized by the manufacturer-supplied sensitivity factor (S<sub>C1s</sub> = 1, S<sub>O1s</sub> = 2.88, S<sub>N12p</sub> = 20.76, S<sub>Fe3p</sub> = 1.49, S<sub>Mo3d</sub> = 11.008 and S<sub>Cr2p</sub> = 11.76). For each sample, XPS survey spectra were recorded at three different locations, and we did not observe any differences.

Time-varying leaching of Ni, Fe, and Cr from the samples in alkaline seawater was determined using inductively coupled plasma optical emission spectrometry (ICP-OES) (ICAP7400, Thermo Fisher).

**Electrochemical Measurements:** The electrochemical measurements were performed on a HTDS PARSTAT PMC-200 multichannel electrochemical workstation equipped with an alternating current (AC) frequency response analyzer (FRA). For the three-electrode configuration, a NiF and Hg/HgO (filled with 1.0 M KOH) were used as the counter and reference electrodes, respectively. All HER and OER polarization curves were recorded by linear sweep voltammetry (LSV) at 5 mV s⁻¹ with 95% iR compensation using the automated iR-correction function of the potentiostat. The current densities (j) were normalized by the geometric surface area of electrodes. All potentials measured versus Hg/HgO were converted versus the reversible hydrogen electrode (RHE) according to the equation given below:

$$E_{RHE} = E_{Hg/HgO} + 0.059pH + E^{0}_{Hg/HgO} \text{with } E^{0}_{Hg/HgO} = +0.102 \text{ V} \quad (1)$$

where E<sub>Hg/HgO</sub> is the applied potential versus the used reference electrode.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work has been supported by CNRS and EKUM in the frame of the France Relance Program (project "H2OMER", 2022–2024). The XPS measurements have been performed on the ASPHERYX platform (ScanMAT, UAR 2025 University of Rennes-CNRS; CPER-FEDER 2015–2020). Loic Joanny and Francis Gouttefangeas are fully acknowledged for SEM images and EDS analyses performed at CMEBA (ScanMAT, UAR 2025 University of Rennes-CNRS).

## Conflict of Interest

The authors declare no conflict of interest.
**Data Availability Statement**

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords**

hydrogen production, NiCr foam, NiFe layered double hydroxide, oxygen evolution reaction, seawater electrolysis

Received: April 7, 2025

Revised: May 9, 2025

Published online:

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