A Cr-doped NiFeOOH catalyst with surface reconfiguration for chloride-resistant simulated seawater electrolysis in an anion-exchange membrane electrolyzer

Huajie Wu^abc, Xingran Wang^d, Tiejun Zhao^e, Xiaopeng Li*^d and Danlei Li*^abc
aDepartment of Chemistry and Materials Science, School of Science, Xi'an Jiaotong-Liverpool University, Suzhou 215123, P. R. China. E-mail: danlei.li@xjtlu.edu.cn
^bAdvanced Materials Research Center, Xi'an Jiaotong Liverpool University, 111 Ren'ai Road, Suzhou Industrial Park, Suzhou, P.R. China
^cDepartment of Chemistry, University of Liverpool, L69 7ZD, UK
^dState Key Laboratory of Advanced Fiber Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, P. R. China. E-mail: xiaopeng.li@dhu.edu.cn
^eJiangsu JITRI Institute of Advanced Catalytic Technologies, Suzhou 215123, P. R. China

First published on 8th August 2025

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1. Introduction

The transition to a carbon-neutral energy paradigm has driven worldwide interest in green hydrogen production via water electrolysis. According to the International Energy Agency (IEA), hydrogen demand could increase to 530 million tons annually by 2050 in net-zero scenarios;^1–3 however, over 95% of current production relies on fossil fuels with substantial CO₂ emissions.⁴ Recently, anion exchange membrane (AEM) electrolyzers have demonstrated excellent performance with non-precious metal catalysts and tolerance to lower water purity under alkaline conditions.⁵ However, they still suffer from insufficient catalyst durability under industrial operating conditions or high current densities.⁶

The oxygen evolution reaction (OER) is kinetically sluggish due to its four-electron transfer process (4OH⁻ → O₂ + 2H₂O + 4e⁻, E^θ = 1.23 V), with high overpotential requirements. High concentrations of chloride ions (∼0.5 M) in seawater often result in competitive chloride oxidation (ClER, 2Cl⁻ → Cl₂ + 2e⁻, E^θ = 1.36 V), which reduces OER efficiency, generates toxic byproducts and accelerates catalyst degradation via corrosion and poisoning.^7–11 Current research mainly focuses on the laboratory scale, neglecting industrial conditions like fluctuating local pH and hydrodynamic stress.¹² Bridging lab-scale material optimization and device-level validation under real-world scenarios is crucial for efficient seawater electrolysis, which requires the development of OER catalysts with high activity, selectivity, and corrosion resistance to Cl⁻.^13–19 To address these challenges, recent advancements in OER catalyst design for chloride-rich electrolytes have focused on three primary approaches: (1) applying protective surface layers to prevent chloride penetration;^20,21 (2) modifying the electronic structure through heteroatom doping;^22–25 and (3) creating selective active sites with built-in chloride-repelling abilities.²⁶ While these strategies improve certain aspects of chloride resistance, they often involve trade-offs between catalytic activity, stability and scalability. To address these challenges, we herein report a chromium-doped NiFe oxyhydroxide (Cr-NiFeOOH) catalyst, where Cr³⁺ induces electronic modulation to enhance OER activity. While heteroatom doping (e.g., Cr) often faces durability challenges due to dopant leaching, we demonstrate that moderate Cr incorporation can transiently optimize surface reactivity and chloride repulsion, even as long-term stability remains a critical consideration. Similarly, engineered active sites for chloride exclusion often lead to a loss in performance in dynamic seawater environments, where competing ion adsorption and pH changes dominate the interface behavior. A review of the literature shows that less than 10% of reported chloride-resistant OER catalysts have been tested in membrane electrode assemblies at industrially relevant current densities.^27,28 None of them can simultaneously achieve all of the following goals: low overpotential, over 95% inhibition of the chlorine oxidation reaction, and high stability in a flowing seawater electrolyzer.^29–31 This longstanding performance gap highlights the need for catalyst designs that combine electronic tuning, surface selectivity, and overall stability without sacrificing charge transfer efficiency—a challenge that current design methods haven't solved yet. While recent advances in transition metal-based catalysts (e.g., NiFe-layered double hydroxides, Co₃O₄ spinels) have improved OER activity in simulated seawater in a traditional three-electrode system, they remain limited in practical anion exchange membrane water electrolyzers (AEMWEs).^32–34 This performance gap arises mainly from the simplified assessment scheme used in most studies. The absence of mechanical stresses, such as bubble-induced shear in a running AEMWE, leads to an overestimation of catalyst stability.^35–37 A critical analysis of the literature shows that more than 80% of reported seawater-OER catalysts have been validated only in three-electrode cells in batch mode, while less than 5% have been tested in direct current (DC) membrane electrode assemblies at industrial current densities.^38,39 This discrepancy hinders the accurate evaluation of the catalytic performance of catalysts of interest under real conditions (e.g., Cl⁻ assisted metal leaching and decoupling of the ionophore from the catalyst).⁴⁰ To address these limitations, a paradigm shift toward device-integrated catalyst design, where the real-world operational conditions of the AEMWE system are used as a guide for material innovation, is imperative.^41,42

In this work, we successfully synthesized a Cr-doped Ni-FeOOH catalyst engineered to enhance both catalytic activity toward the OER and chloride tolerance in seawater. The introduction of Cr changed the electronic structure of Ni-FeOOH, optimizing the adsorption energetics of oxygen intermediates while promoting the formation of a hydroxyl-rich surface that effectively repels Cl⁻ ions. This synergistic effect suppressed the competing chloride evolution reaction (>95% selectivity) and enhanced its catalyst durability. Comprehensive electrochemical characterization studies demonstrate that the Cr-doped Ni-FeOOH catalyst achieves a low overpotential of 230 mV (85% iR-corrected) at 100 mA cm⁻² in simulated seawater (1 M KOH + 0.5 M NaCl) and a long duration (<3% decay over 100 h) in a traditional three-electrode system. Importantly, the catalyst was further integrated into an AEMWE and it achieved remarkable stability (>40 h at 200 mA cm⁻²) under seawater-relevant conditions (1 M KOH + 0.5 M NaCl). By integrating scalable synthesis (spray-assisted electrodeposition) with device-compatible electrode fabrication, this work establishes a rational catalyst design strategy that bridges the gap between fundamental material studies and practical electrolysis applications, offering a viable pathway toward durable, cost-effective simulated seawater electrolysis technologies.

2. Experimental section

2.1 Chemicals and reagents

Ferric chloride hexahydrate (FeCl₃·6H₂O; Sinopharm; >99%), urea (CO(NH₂)₂; Sinopharm; >99%), chromium nitrate (Cr(NO₃)₃·9H₂O; Sinopharm; >99%), hydrochloric acid (HCl; Sinopharm; 36%–38%), ethanol (C₂H₆O; Sinopharm; 78.9%–79.1%), potassium hydroxide (KOH; Sinopharm; >95%), and commercial nickel foam (NF) with a thickness of 1.7 mm obtained from Keshenghe Material (Suzhou, China) were used. All solutions were prepared using deionized water (Millipore) with a resistivity of 18.2 MΩ cm.

2.2 Nickel foam pre-treatment

A piece of commercial nickel foam (ca. 1 × 3 cm) was cut and sonicated in 3 M HCl for 10 min and then washed with ethanol and water until the solution was neutral. The cleaned nickel foam was then dried under vacuum at 60 °C for 12 h.

2.3 Synthesis of Cr_xNi-FeOOH/NF

1 mmol of FeCl₃ was dissolved in 40 mL of deionized water, and two pieces of nickel foam (1 × 3 cm) were immersed in the FeCl₃ solution and transferred to a hydrothermal reactor, where they were heated at 60 °C for 12 hours. After cooling to room temperature, 0.2 g of urea was added, along with 0.1, 0.5, and 0.7 mmol of Cr(NO₃)₃. The mixture was sonicated for 10 minutes to ensure uniform mixing and then heated in an oven at 120 °C for 12 hours. Finally, the obtained products were washed three times with deionized water and ethanol and vacuum-dried overnight at 60 °C. The resulting samples were named Cr_LNi-FeOOH/NF (low Cr, ∼5.3 atom%), Cr_MNi-FeOOH/NF (medium Cr, ∼11.2 atom%) and Cr_HNi-FeOOH/NF (high Cr, ∼21.4 atom%), respectively. Samples without Cr doping were prepared separately by the above method.

2.4 Material characterization

The crystal structure of the samples was analyzed using a Bruker X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.54 Å, 42 kV, 100 mA) over a 10° to 80° range and a scanning speed of 5° min⁻¹. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a PHI 5000 VERSAPROBE II spectrometer with an Al Kα X-ray source and a photon energy of 112 eV. The morphological characteristics of the samples were examined using a Phenom ProX scanning electron microscope (SEM). Structural and morphological analyses were further performed using a Tecnai G2 F20 S-Twin transmission electron microscope (TEM), and elemental quantitative analysis was conducted through X-ray energy dispersive spectroscopy (EDS) on an Aztec X-Max 80T inductively coupled plasma photoemission spectrometer. Raman spectra were recorded using a DXR Microscope laser Raman spectrometer with a 532 nm excitation wavelength.

2.5 Electrochemical measurements

Electrochemical measurements were conducted in a three-electrode system using an AMETEK electrochemical workstation (1470E Cell Test). The saturated mercury/mercury oxide (Hg/HgO) electrode and platinum electrode were used as the reference and counter electrodes, respectively. The working electrode consisted of catalyst-coated nickel foam (NF) with an area of 1.0 cm². Before each electrochemical measurement, the activation process was carried out using cyclic voltammetry (CV) in a potential range of 0 to 0.8 V versus Hg/HgO in 1.0 M KOH with 0.5 M NaCl solution. Electrochemical performance was subsequently evaluated using linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). LSV curves were recorded at a scan rate of 1 mV s⁻¹, while EIS was performed at 0.6 V versus Hg/HgO in a frequency range from 100 kHz to 10 mHz. Chronoamperometry (CA) was used to evaluate the stability of the samples at a current density of 200 mA cm⁻². The Faraday efficiency (FE) of Cr_MNi-FeOOH/NF at 2 V was estimated by comparing the volume of Cl₂ generated with the theoretical values. Since the chlorine evolution reaction (ClER) competes with the oxygen evolution reaction (OER), the Faraday efficiency of the OER catalyst was evaluated by comparing the catalytic performance under these competitive reaction conditions. The double-layer capacitance (C_dl) was evaluated by cyclic voltammetry curves performed in the non-faradaic region with an interval of 20 mV s⁻¹ over a scan rate range of 20 to 100 mV s⁻¹. To evaluate the selectivity towards the OER over ClER, the Faraday efficiency (FE) of ClER was measured. The gas generated during the reaction at 2 V in a 1 M KOH + 0.5 M NaCl solution for 1 h was bubbled through a trap containing 3 M NaOH solution to absorb Cl₂ gas. The concentration of Cl⁻ ions in the absorbed solution was then analyzed by ion chromatography (IC). The FE for ClER was calculated by comparing the measured amount of Cl⁻ (corresponding to the generated Cl₂) with the theoretical charge based on the total current passed during electrolysis, using the formula: where F is Faraday's constant (96485 C mol⁻¹). The ClER suppression efficiency is calculated as 100% − ClER FE. This method allows quantification of the suppression of the competitive chlorine evolution reaction by the catalyst.

The anion exchange membrane (AEM) used in the electrolysis test of the AEMWE is the EHYDRO® AEM-01 series product produced by Zhongke HydroEasy Company. The anode is composed of an integrated catalyst (Cr_MNi-FeOOH/NF) synthesized by the hydrothermal method, with a size of 1 × 1 cm². The cathode is 0.5 mm-thick 1 × 1 cm² nickel foam. The electrochemical test was conducted at a constant temperature of 25 °C in an AEMWE, with 1.0 M KOH + 0.5 M NaCl (simulating seawater) filled into the anode and cathode through a peristaltic pump, separated by the AEM. The polarization curve was recorded at a scanning rate of 5 mV s⁻¹; at the same time, the stability test of the constant current ammeter was carried out in real-time while monitoring the battery voltage, with a current density of 200 mA cm⁻².

3. Results and discussion

Self-supported Cr_xNi-FeOOH/NF electrodes were synthesized via a two-step hydrothermal method (Fig. 1a). In the first step, nickel foam (NF) served as both the substrate and the nickel source. During the hydrothermal process, the hydrolysis of Fe ions generated hydrogen ions, which corroded metallic Ni on the NF surface, releasing Ni ions into the solution. In the second step, Cr ions and urea were introduced as the dopant and precipitant, respectively. Under high temperature and pressure, urea decomposed to produce ammonium ions, which neutralized the acidic environment caused by Fe ion hydrolysis, resulting in an alkaline environment. This alkaline condition facilitated the co-precipitation of Fe and Ni ions, promoting their in situ deposition and growth on the NF surface. The Cr doping amount was optimized by evaluating the catalytic performance. Based on the TEM-EDS (Table S1 and Fig. S1e–g) and ICP (Table S2) results, the Cr content in Cr_L, Cr_M, and Cr_H samples was 5.3%, 11.2%, and 21.4% atomic ratio, respectively, and the samples were designated as Ni-FeOOH, Cr_LNi-FeOOH, Cr_MNi-FeOOH, and Cr_HNi-FeOOH, respectively.

To gain deeper insight into the morphological and structural evolution of Cr_xNi-FeOOH/NF, SEM and TEM were employed. The SEM images of Ni-FeOOH/NF reveal that the nickel foam (NF) scaffold was uniformly coated with vertically aligned nanosheets. These nanosheets exhibit smooth surfaces and an interconnected architecture, ensuring effective electron transport and ion diffusion, rather than forming randomly stacked aggregates that could hinder electrochemical performance (Fig. S1a). Upon Cr incorporation, the SEM images of Cr_xNi-FeOOH/NF demonstrate a distinct transformation in the nanosheet arrangement, forming a more densely packed and cross-linked nanoarray structure (Fig. S1b–d). Higher-magnification images further reveal the presence of smaller secondary nanosheets interspersed within the nanoarray, which suggests an increase in the density of active sites. Notably, variations in Cr doping concentration induce distinct morphological changes. In Cr_MNi-FeOOH, the formation of a greater number of ultrathin nanosheets is observed, which significantly enhances the specific surface area. This structural refinement is expected to facilitate mass transport of reactants and intermediates and improve the electrochemical performance. TEM images (Fig. 1b) showed well-defined lattice fringes with interplanar spacings of 0.296 nm and 0.252 nm, corresponding to the (220) and (311) planes of the FeOOH crystal structure, respectively. Furthermore, selected area electron diffraction (SAED) revealed a six-fold symmetric diffraction pattern, confirming the single-crystalline nature of the material (Fig. 1c). This crystallographic feature is expected to reduce grain boundary resistance and facilitate electron conduction, thereby improving electrocatalytic performance. EDS mapping further demonstrated the homogeneous distribution of Cr, Fe, and Ni throughout the nanosheet matrix, indicating uniform elemental incorporation (Fig. 1e–g).

The crystal structures of all the synthesized samples were systematically analyzed using XRD, as shown in Fig. 2a where XRD patterns confirm that the prepared samples predominantly exhibit the FeOOH phase. This is evidenced by the distinct diffraction peaks observed at 30.2°, 35.6°, and 43.2°, which correspond to the (220), (311), and (400) planes of FeOOH,^43,44 respectively, and is consistent with TEM results. Additionally, the presence of the NF substrate is confirmed by the characteristic diffraction peaks appearing at 25.7° and 43.3°. Notably, no additional peaks corresponding to other nickel-containing phases were detected, suggesting that Ni species exist in an amorphous or highly dispersed state within the FeOOH matrix.

The bonding configuration and surface chemistry of Cr-doped Ni-FeOOH/NF were investigated via Raman spectroscopy and XPS. Raman spectroscopic analysis revealed critical structural information through characteristic vibrational modes shown in Fig. 2b and c. Distinct Fe–O vibrations in FeOOH were identified at 300 cm⁻¹, 480 cm⁻¹, 550 cm⁻¹, and 680 cm⁻¹. Notably, while XRD characterization showed no detectable nickel-containing crystalline phases, Raman spectroscopy revealed three distinct Ni–O symmetric stretching vibrations at 370, 460, and 570 cm⁻¹.⁴⁵ This strongly suggests the successful incorporation of Ni species within amorphous/nanocrystalline domains of the FeOOH matrix. Simultaneously, Cr–O vibrational signatures at 540 cm⁻¹ and 690 cm⁻¹ confirmed chromium integration.⁴⁶ The coexistence of these three metal–oxygen bonding systems provides compelling evidence for the formation of a ternary Cr/Fe/Ni-based hydroxide structure, which critically modulates the material's electronic configuration and catalytic functionality.

Complementary XPS analysis was conducted to study the surface chemical states and interelement interactions in the catalyst system. High-resolution XPS spectra of fresh Cr_MNi-FeOOH/NF confirmed the presence of Cr³⁺ (Cr 2p_3/2 at 576.8 eV, Fig. 2d), with homogeneous integration into the Ni-FeOOH matrix. The high-resolution Ni 2p_3/2 spectra (Fig. 2f) exhibited peaks at 855.5 eV (Ni³⁺) and 856.3 eV (Ni³⁺ with Cr doping), while Fe 2p_3/2 spectra (Fig. 2e) showed peaks at 711.8 eV (Fe³⁺) and 712.4 eV (Fe³⁺ with Cr doping), respectively. A positive shift of 0.8 eV in Ni 2p_3/2 and a negative shift of 0.4 eV in Fe 2p_3/2 upon Cr incorporation indicated strong electronic coupling between Cr³⁺ and Ni/Fe centers, leading to electron-deficient Ni³⁺/Fe³⁺ active sites^45,47 that indicated strong electronic coupling between Cr³⁺ and Ni/Fe centers, leading to electron-deficient Ni³⁺/Fe³⁺ sites. This Cr-induced electronic modulation optimizes the adsorption energy of oxygen intermediates (e.g., *OOH), thereby accelerating OER kinetics.

O 1s XPS analysis (Fig. S9) revealed three characteristic peaks at 531.0 eV (M–OH) and 529.8 eV (M–O), with the M–OH/M–O ratio increasing after Cr doping. This enhancement in surface hydroxylation correlates with the formation of a Cl⁻-repelling hydroxyl layer, as evidenced by the >99.7% ClER suppression. The valence state distribution, validated by Raman spectroscopy's metal–oxygen bonding network, collectively confirms the successful synthesis of a Cr-modified Ni-FeOOH heterostructure on nickel foam.

This valence state distribution correlates with the metal–oxygen bonding network detected by Raman spectroscopy, collectively verifying the successful synthesis of a Cr-modified Ni-FeOOH heterostructure on nickel foam. Notably, the Ni 2p_3/2 binding energy in the Cr_MNi-FeOOH catalyst exhibits a positive shift relative to pristine Ni-FeOOH (Fig. 2f), indicating partial oxidation of Ni²⁺ to Ni³⁺ species.⁴⁸ Concurrently, the Fe 2p_3/2 peak displays an upshift upon Cr incorporation. These systematic binding energy changes demonstrate Cr-induced electron redistribution, creating electron-deficient Ni and Fe centers through strong intermetallic coupling. The synergistic interplay between these electronic effects enhances catalytic functionality through Cr-mediated electronic restructuring that optimizes charge transfer pathways, in combination with the increased exposure of activated metal centers in elevated oxidation states. This tailored electronic configuration accounts for the superior electrocatalytic performance of the ternary system compared to its binary Ni–Fe counterparts.

The electrocatalytic performance of the prepared samples was systematically evaluated in 1.0 M KOH with 0.5 M NaCl in a three-electrode cell using LSV. Fig. 3a shows a comparison of electrocatalytic activity of the synthesized materials toward the OER where the Cr_MNi-FeOOH/NF electrode demonstrated enhanced electrocatalytic activity compared to other electrodes with the lowest overpotential of 230 mV at a current density of 100 mA cm⁻² whilst the Cr_LFeOOH/NF, Cr_HFeOOH/NF, and Ni-FeOOH/NF electrodes required higher overpotentials of 250 mV, 270 mV, and 290 mV, respectively, under the same conditions. This comparison highlights the optimal balance of composition and electronic structure in Cr_MNi-FeOOH/NF, which facilitates more efficient OER kinetics. The improved catalytic activity of Cr_MNi-FeOOH/NF can be attributed to its synergistic composition, where moderate Cr doping effectively enhances electronic conductivity and optimizes the active sites for the OER. Excessive Cr doping, as observed in Cr_HFeOOH/NF, likely introduces structural distortions or electronic localization effects that hinder charge transfer, leading to a higher overpotential. On the other hand, Cr_LFeOOH/NF and Ni-FeOOH/NF, with lower or no Cr content, lack the beneficial electronic modulation provided by the optimal Cr incorporation.^49–51 These results collectively indicate that Cr_MNi-FeOOH/NF achieves a fine-tuned balance between conductivity, surface reactivity, and structural stability, making it a promising electrocatalyst for efficient and durable OER performance in alkaline-chloride electrolytes. Tafel analysis was conducted to investigate the kinetics of different catalysts toward the OER from the corresponding LSV curves. As illustrated in Fig. 3d, the Cr_MNi-FeOOH/NF catalyst exhibits a significantly lower Tafel slope of 57.1 mV dec⁻¹ compared to those of other synthesized materials (75.8 mV dec⁻¹ for Ni-FeOOH/NF, 75.6 mV dec⁻¹ for Cr_LNi-FeOOH/NF, and 99.1 mV dec⁻¹ for Cr_HNi-FeOOH/NF), further proving an increase in charge transfer efficiency and more favorable reaction kinetics. To validate the reproducibility of the catalytic performance, three repeated LSV tests were conducted for Cr_MNi-FeOOH/NF in 1.0 M KOH + 0.5 M NaCl (Fig. S10a). The overpotentials at 100 mA cm⁻² without iR-correction were 353 ± 3 mV, demonstrating excellent consistency with less than 2% variation across trials. Corresponding Tafel slope measurements further confirmed kinetic reproducibility, with values of 57.1 ± 0.8 mV dec⁻¹ (Fig. S10b).

This reproducibility highlights the reliability of the catalyst synthesis and electrochemical characterization, ensuring the validity of the reported activity and kinetics. To further contextualize the superior OER performance of Cr_MNi-FeOOH/NF in chloride-rich environments, a comparative analysis with state-of-the-art seawater OER catalysts is summarized in Table S4. The optimized Cr_MNi-FeOOH/NF exhibits a low overpotential of 200 mV at 10 mA cm⁻² and 230 mV at 100 mA cm⁻² in 1 M KOH + 0.5 M NaCl, outperforming most reported catalysts. For instance, it shows a 97 mV lower overpotential at 100 mA cm⁻² compared to Fe_0.05Mo-NiS_y (327 mV) (ref. 52) and a 43 mV advantage over CoCr_0.7Rh_1.3O₄ at 10 mA cm⁻² (273 mV).⁵³ Even compared to the high-performance CV-B-NiFe-LDH (236 mV at 100 mA cm⁻²),⁵⁴ our catalyst achieves a slightly lower overpotential, highlighting the efficacy of Cr doping in optimizing OER kinetics under chloride stress. Notably, a-NiFeOOH/N-CFP exhibits a lower overpotential at 10 mA cm⁻² (170 mV),⁵⁵ which may stem from its amorphous structure and high surface defect density—an area worthy of further optimization in our system. Additionally, Cr_MNi-FeOOH/NF maintains competitive performance in chloride-containing electrolytes, whereas many catalysts (e.g., CoS₂⁵⁶ and NiCo₂O₄/NiCoP⁵⁷) show significantly higher overpotentials or lack data at 100 mA cm⁻², underscoring its potential for practical seawater electrolysis.

In addition, the electrochemically active surface area (ECSA) of the prepared samples was calculated using CV in the non-faradaic region (Fig. S3). Ni-FeOOH exhibits the highest C_dl value, while Cr doping results in a decrease, as shown in Fig. 3b. Among the Cr-doped samples, Cr_MNi-FeOOH retains a relatively larger C_dl value, contributing to its enhanced catalytic activity. The incorporation of Cr³⁺ inhibits the growth of Ni-FeOOH along the short-axis crystal planes, leading to a denser surface morphology and a reduction in active regions available for charge storage within the double layer. Additionally, Cr³⁺ may form stable coordination structures with Cl⁻, preferentially occupying surface-active sites and reducing the adsorption of free Cl⁻ within the double layer. Furthermore, Cr³⁺ doping may promote Ni-FeOOH particle aggregation, further limiting the accessible surface area for Cl⁻ adsorption. As illustrated in Fig. 3c, the Cr_MNi-FeOOH/NF electrode exhibits the smallest semicircle radius, indicating the lowest charge transfer resistance. This finding suggests that Cr_MNi-FeOOH/NF possesses superior mass transfer properties during the OER process. As shown in Fig. 3e, Cr_MNi-FeOOH/NF maintains stable operation at 200 mA cm⁻² for 120 hours, with the cell potential increasing from 1.84 V to 1.97 V (7.07% degradation). Notably, Cr leaching contributes ≤30% to this potential rise, as confirmed by the ICP-MS analysis of electrolyte Cr ions (Fig. S6), indicating that the residual Ni-FeOOH matrix primarily sustains activity. This highlights the electrode's initial durability with <3% activity loss, despite dynamic surface reconstruction. This leaching behavior is deeply linked to the electronic structure modulation by Cr³⁺. XPS spectra (Fig. 2d–f) show that Cr doping induces a 0.8 eV positive shift in Ni 2p3/2 (855.5 → 856.3 eV) and a 0.4 eV negative shift in Fe 2p3/2 (712.4 → 712.0 eV), forming electron-deficient Ni³⁺/Fe³⁺ sites that strengthen Cr–O–Ni/Fe covalent bonds (Raman Cr–O vibration at 540 cm⁻¹, Fig. 2c). This electronic coupling suppresses initial Cr leaching (0.74 ppm h⁻¹ in 0–24 h, Fig. S6). However, a prolonged OER triggers a feedback loop: Cr loss weakens electronic modulation, as seen in post-OER Ni 2p3/2 shifting back to 855.8 eV (Fig. S5a), which accelerates leaching to 1.49 ppm h⁻¹ (48–72 h).

These results reveal that the initial electronic optimization by Cr³⁺ concurrently enhances activity and suppresses leaching, while long-term leaching disrupts the electronic structure, forming a dynamic balance between modulation and dissolution (Fig. S5a and S6). The NF scaffold, with its high mechanical strength, plays a critical role in this stability by effectively dispersing Cr_MNi-FeOOH nanosheets on its surface and providing robust structural support for long-term durability.

The structural and electronic changes before and after the OER duration test were investigated using XPS and Raman spectroscopy. While Cr leaching was confirmed by the disappearance of Cr signals in post-OER XPS and ICP-MS detection of Cr ions in the electrolyte, the catalyst maintained <3% activity loss over 100 hours. This suggests that Cr³⁺ initially induces favorable electronic restructuring (e.g., formation of oxygen vacancies and Ni³⁺/Fe³⁺ active sites), even as gradual Cr dissolution weakens its long-term efficacy. The remaining Ni–Fe oxyhydroxide matrix, however, retains sufficient structural integrity to sustain OER activity in the short term. Simultaneously, the Fe and Ni peaks exhibited a noticeable red shift, implying a redistribution of electronic density (Fig. S5b and d). As a dopant, Cr initially played a crucial role in modulating the electronic environment of Ni and Fe through electron transfer interactions. However, upon Cr leaching, the original electron transfer pathway was disrupted, resulting in an increase in the electronic density of Ni and Fe. This effect was evidenced by the observed decrease in XPS binding energy (Fig. S5a). The interruption of electron delocalization due to Cr loss may influence the oxidation states and catalytic performance of the remaining Ni–Fe active sites. Furthermore, in the Ni–Fe–Cr alloy system, the leaching of Cr could lead to a partial reduction in the oxidation states of Ni and Fe, contributing to the overall shift of XPS peaks toward lower binding energies. Notably, the deconvolution of the high-resolution O 1s spectrum reveals a significant presence of hydroxyl (M–OH) species (Fig. S5c), consistent with the hydroxyl-rich surface induced by Cr³⁺ doping in the fresh catalyst. These hydroxyl groups likely contribute to Cl⁻ repulsion and surface oxygen coordination unsaturation. The interplay between Cr leaching, electronic structure modulation, and surface oxygen vacancy formation provides valuable insights into the dynamic nature of the Cr_MNi-FeOOH catalyst under OER conditions. This Cr leaching was further proven via Raman spectra of the electrode used after the OER duration test (Fig. S4b) where only the characteristic peaks of Ni–O (475 cm⁻¹) and Fe–O (552 cm⁻¹) were observed (Fig. S4b). Notably, their peak shapes and positions remain unchanged compared to those of undoped Ni-FeOOH, indicating that the structural integrity of Ni-FeOOH is preserved (Fig. S4a). The absence of Cr-related signals strongly suggests that Cr leaches from the catalyst surface during the OER. To verify this phenomenon, inductively coupled plasma mass spectrometry (ICP-MS) was conducted to analyze the metal ion content in the electrolyte. The presence of Cr ions in solution provides direct evidence of Cr leaching (Fig. S6). Notably, Cr³⁺ leaching kinetics (Fig. S6) exhibits a biphasic trend, with concentrations increasing from 17.87 ppm in 24 h to 26.26 ppm in 48 h (leaching rate: 0.35 ppm h⁻¹) and surging to 62.21 ppm in 72 h (1.49 ppm h⁻¹) under 200 mA cm⁻². This correlates with the potential evolution from 1.87 V to 1.90 V, revealing a critical transition: the initial slow leaching phase (0–24 h) coincides with a potential rise rate of 1.25 × 10⁻³ V h⁻¹, likely reflecting Cr³⁺-mediated surface activation, whereas accelerated leaching post-48 h is accompanied by a decelerated potential rise (0.42 × 10⁻³ V h⁻¹). As shown in Fig. S4b, in situ reconstruction of γ-NiOOH layers (Fig. S4b) buffers Cr loss. This dynamic balance explains the <3% activity decay despite 62.21 ppm Cr dissolution at 72 h, highlighting the NiFeOOH matrix's self-repair capacity as the durability determinant. The remaining Ni-FeOOH matrix, with its robust nanosheet architecture and residual hydroxyl groups, maintains structural integrity and surface reactivity, thereby sustaining OER activity despite Cr leaching.

In summary, these findings confirm that Cr_MNi-FeOOH undergoes Cr leaching during the OER, leading to surface reconstruction marked by O 1s binding energy shifts and reduced hydroxyl density. This dynamic surface restructuring enhances catalytic activity, demonstrating the positive impact of Cr doping on OER performance. Building on the above analysis, the electrocatalytic OER performance of the Cr_MNi-FeOOH/NF electrode was evaluated in an alkaline saline solution containing 1.0 M KOH and 0.5 M NaCl to assess its feasibility for simulated seawater electrolysis. As shown in Fig. S7 and Table S3, the electrode exhibits selective OER activity in both alkaline and saline environments. A major challenge in simulated seawater electrolysis is the competing ClER at the anode. Notably, Cr doping enhances the selectivity toward the OER over CER, suppressing ClER activity by 99.7% under operational conditions. This remarkable inhibition of the competing chloride oxidation pathway demonstrates the material's exceptional potential for efficient simulated seawater electrolysis applications. This improved selectivity is primarily attributed to the presence of Cr(III) species, which effectively repel Cl⁻ ions and reduce their adsorption on the catalyst surface, thereby enhancing both stability and corrosion resistance. Specifically, while Cr³⁺ mitigates Cl⁻ adsorption through electrostatic repulsion. These synergistic effects contribute to the catalyst's improved selectivity and long-term durability in simulated seawater electrolysis, highlighting its potential for practical applications.

The AEMWE system used for evaluating the overall water-splitting performance of the Ni-FeOOH and Cr_MNi-FeOOH catalysts consisted of NF as the cathode material, Cr_MNi-FeOOH as the anode catalyst, and the EHYDRO® AEM-01 anion exchange membrane. The system was tested in 1.0 M KOH with a 0.5 M NaCl electrolyte using LSV. In Fig. 4b, the Cr_MNi-FeOOH catalyst at the cathode achieved a current density of 0.1 A cm⁻² at a remarkably low cell voltage of 1.92 V, outperforming the Ni-FeOOH catalyst. This enhanced performance is attributed to the effect of Cr doping, which modifies the electronic structure of Ni and Fe, improving charge transfer and catalytic efficiency. In addition to its superior electrocatalytic activity, the Cr_MNi-FeOOH catalyst demonstrated excellent stability. As shown in Fig. 4c, it maintained stable performance at 0.2 A cm⁻² in the AEMWE system for over 40 hours with an 18% increase in cell potential, indicating its durability and potential for long-term applications. This stability is likely due to the structural benefits provided by Cr doping, which helps preserve the active surface of the catalyst over extended periods of operation. These results collectively indicate that Cr_MNi-FeOOH is a highly efficient and durable catalyst for AEMWE applications, offering significant potential for large-scale hydrogen production and renewable energy technologies.

4. Conclusions

In this study, we demonstrated a Cr-doped Ni-FeOOH catalyst that addresses the dual challenges of activity–stability trade-offs and chloride-induced degradation in the seawater oxygen evolution reaction. By incorporating Cr³⁺ into the Ni-FeOOH lattice, the catalyst achieved a low overpotential of 230 mV (85% iR-corrected) at 100 mA cm⁻² in simulated seawater (1 M KOH + 0.5 M NaCl), supported by favorable reaction kinetics evidenced by a low Tafel slope of 57.1 mV dec⁻¹ along with exceptional durability (>100 h) in a three-electrode system. In summary, Cr doping transiently enhances OER activity and chloride resistance by optimizing the electronic structure and forming a Cl⁻-repelling hydroxyl layer. The observed Cr leaching (verified by ICP-MS detection of 0.12 ppm Cr ions in the electrolyte after 100 h, Fig. S6) indeed indicates that the catalyst undergoes dynamic surface reconstruction. While the activity loss remains <3%, this durability stems from the residual Ni–Fe oxyhydroxide matrix maintaining structural integrity, rather than persistent Cr retention. The term ‘transient enhancement’ here refers to Cr³⁺-induced electronic modulation that primes the catalyst surface for optimized OER kinetics, even as Cr dissolution gradually occurs. This work provides a foundation for designing dynamic, dopant-assisted catalysts for simulated seawater electrolysis, balancing initial performance with long-term stability challenges. This dual functionality suppresses the competitive ClER by 99.7%, effectively eliminating toxic Cl₂ byproducts. Crucially, when integrated into a flowing AEMWE, the catalyst exhibited excellent device-level stability, maintaining 0.2 A cm⁻² for over 40 hours under seawater-relevant conditions (1 M KOH + 0.5 M NaCl). For practical applications, the transient nature of Cr doping highlights the need for strategies to anchor Cr³⁺ in the lattice— e.g., via stronger M–O–M bonds (e.g., Cr–Ni–Fe triple metal sites) or protective hydroxide layers—to extend the duration of Cr-mediated activity. The current design demonstrates a proof-of-concept for dopant-induced surface engineering, but long-term stability remains a challenge that requires future optimization.

Author contributions

Huajie Wu: writing – original draft, methodology, investigation, and data analysis. Xingran Wang: investigation. Tiejun Zhao: supervision and resources. Xiaopeng Li: conceptualization, supervision, and writing – review & editing. Danlei Li: conceptualization, supervision, and writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the SI.

The information provided includes scanning electron microscopy (SEM) images; energy-dispersive X-ray spectroscopy (EDS) spectra; high-resolution X-ray photoelectron spectroscopy (XPS) spectra; cyclic voltammetry (CV) curves; linear sweep voltammetry (LSV) curves; Tafel slope curves; Raman spectra of catalysts before and after durability testing; variation of electrolyte ion content; chronoamperometric (I-t) curves; a photograph of the anion-exchange membrane water electrolyzer (AEMWE) device; tabulated data from EDS and inductively coupled plasma (ICP) analysis; Faraday efficiency data for chlorine gas generation; and a comparison table of oxygen evolution reaction (OER) activities with reported electrocatalysts.

The Supplementary Information file provides comprehensive supporting data for validating the findings in the main text, which focuses on Cr-doped NiFeOOH catalysts for chloride-resistant seawater electrolysis in anion-exchange membrane electrolyzers. It includes: scanning electron microscopy (SEM) images of various catalysts (Fig. S1) with accompanying energy-dispersive X-ray spectroscopy (EDS) spectra to confirm elemental distributions, high-resolution X-ray photoelectron spectroscopy (XPS) spectra (Fig. S2, S9) analyzing surface chemical states; cyclic voltammetry (CV) curves (Fig. S3) for evaluating electrochemical active surface areas, linear sweep voltammetry (LSV) curves without iR correction and Tafel slope curves from triplicate tests (Fig. S10) to verify oxygen evolution reaction (OER) activity reproducibility; Raman spectra (Fig. S4) and high-resolution XPS spectra (Fig. S5) assessing structural and surface reconfiguration stability during durability testing, time-dependent electrolyte ion content variation (Fig. S6) and chronoamperometric (I-t) curves (Fig. S7) demonstrating long-term stability and chloride tolerance; a photograph of the anion-exchange membrane water electrolyzer (AEMWE) device (Fig. S8), EDS and inductively coupled plasma (ICP) tables (Tables S1, S2) quantifying elemental concentrations, Faraday efficiency data for chlorine generation (Table S3), and a comparison table (Table S4) of OER activities with reported catalysts in seawater-like electrolytes. This information supports result reproducibility and offers insights into catalyst synthesis, structural evolution, and electrochemical behavior under chloride-containing alkaline conditions.See DOI: https://doi.org/10.1039/d5qi01304f.

Acknowledgements

D. Li acknowledges the support from the Research Development Fund (No. RDF-21-02-060) of Xi'an Jiaotong-Liverpool University. The authors acknowledge the support received from the Suzhou Industrial Park High Quality Innovation Platform of Functional Molecular Materials and Devices (YZCXPT2023105).

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