Plasma-assisted nitrogen-doped NiHf nanoalloy for efficient seawater electrolysis

Abstract

Hydrogen production through seawater electrolysis has gained increasing attention, yet it faces significant challenges, including cathode poisoning by impurities and severe chloride (Cl⁻) corrosion. Herein, a plasma-assisted strategy for nitrogen-doping and alloying was developed to enable the in situ growth of nitrogen-doped NiHf nanoalloy catalysts on processed nickel foam substrates (N-NiHf@NF) for the hydrogen evolution reaction (HER). This approach combines abundant resource availability, high electrical conductivity, and exceptional corrosion resistance, making it a highly promising candidate for seawater electrolysis. Nitrogen doping effectively modulates the electronic environment of the metal nanoalloys, enabling highly efficient and corrosion-resistant hydrogen evolution. Compared to undoped NiHf nanoalloy catalysts, the N-NiHf@NF electrocatalysts exhibit enhanced intrinsic HER performance, achieving a low overpotential of 68 mV at 10 mA cm⁻² in alkaline seawater electrolyte. This enhanced performance primarily stems from the synergistic effect between nitrogen dopants and the nanoalloy matrix. Furthermore, N-NiHf@NF demonstrates exceptional electrochemical stability and corrosion resistance. Structural and elemental analyses confirm that the catalyst maintains its original architecture even after prolonged HER testing. The corrosion resistance mechanism is systematically investigated through electrochemical methods. Potentiodynamic polarization and stability tests reveal outstanding long-term durability and chloride corrosion resistance. This work highlights the great potential of nitrogen-doped NiHf nanoalloy catalysts for seawater electrolysis and provides new insights into surface modification strategies for nanoalloy-based electrocatalysts.

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Hao Tian

Dr Hao Tian is a research associate at the Centre for Clean Energy Technology, University of Technology Sydney. He received his B.S. and M.S. from Lanzhou University in 2009 and 2012, his M.S. from the University of New South Wales in 2014, and his PhD from Curtin University in 2018. His current research mainly focuses on the design and synthesis of nanostructured materials in the fields of lithium-ion batteries, lithium–sulfur batteries, zinc–air batteries, electrocatalysis, thermal-catalysis and photocatalysis. He also serves as an Assistant Editor for Nano-Micro Letters.

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1. Introduction

Seawater electrolysis, a sustainable hydrogen production strategy, offers a promising approach to generating green and clean energy while helping alleviate freshwater scarcity.^1–5 However, the presence of high concentrations of chloride (Cl⁻), microorganisms and other impurities in seawater causes severe electrode corrosion, significantly reduces electrolysis efficiency and impairs long-term stability.^6,7 Addressing these challenges requires careful optimization of the catalysts in alkaline seawater electrolysis systems. Such optimization aims to inhibit chloride-induced corrosion and improve the selectivity of the hydrogen evolution reaction (HER) by tailoring the catalysts' electronic structure, active site distribution and corrosion resistance.⁸ Although Pt/C catalysts remain the benchmark for HER, their prohibitive cost and vulnerability to chloride-induced deactivation hinder their widespread deployment.⁹ Recent studies have shown that non-precious metal catalysts are viable alternatives to platinum-based catalysts, owing to their tunable electronic structures and inherent tolerance to chloride. Among them, nickel-based alloy catalysts have attracted considerable attention for seawater electrolysis due to their inherent high activity, good electrical conductivity and chemical stability, which translate into excellent hydrogen production performance and durability.^10–14

Recently, nickel-based alloy catalysts (e.g., NiCo,¹⁵ NiCoCu,¹⁶ and FeCoNi¹⁷) have demonstrated remarkable electrocatalytic performance toward the HER. However, these conventional alloys face significant challenges in direct seawater electrolysis. Chloride-induced corrosion and the formation of insoluble precipitates from impurity cations (Ca²⁺ and Mg²⁺) on the electrode surface lead to active site poisoning and performance degradation.¹⁸ One strategy to overcome these problems is to optimize and regulate the alloy composition. As indicated by our research, hafnium (Hf) possesses the following advantages: (i) Hf exhibits multiple stable oxidation states (0 to +4), favoring enhanced electronic synergistic interactions among the active metals; (ii) the stable Hf⁴⁺ enhances charge transfer kinetics during the HER process; (iii) owing to its excellent corrosion resistance, Hf contributes significantly to the structural stability of the electrocatalyst in alkaline electrolytes.¹⁹ Therefore, incorporating Hf into Ni-based alloys might mitigate catalyst poisoning and improve durability. Furthermore, non-metal composition optimization, particularly nitrogen doping, enables simultaneous tuning of both the electronic structure and energy band structure, which is difficult to achieve through metal composition adjustments alone.^20,21 For example, Gao et al. demonstrated that nitrogen incorporation in NiFeCo alloys significantly shifts the metal d-band center, optimizing hydrogen adsorption free energy (ΔG_H*) and thereby enhancing HER activity.²² Nitrogen doping can be achieved through various methods such as chemical vapor deposition (CVD),²³ hydrothermal synthesis,²⁴ and template-based techniques.²⁵ Among these methods, plasma-assisted strategies stand out for their robust surface modification effects. The intense energy and flux of reactive species in plasma interact with the material surfaces to form conductive nitride phases that preserve the original nanostructure while simultaneously enhancing alloy electron mobility and catalytic durability.^26,27 Nonetheless, plasma-assisted co-doping of Ni-based alloys with Hf and N has seldom been reported.

In this work, we report the in situ grown nitrogen-doped NiHf nanoalloy catalyst (N-NiHf@NF) on a pre-treated nickel foam substrate via a plasma-assisted synthesis strategy for efficient hydrogen production from seawater. The catalysts offer the following potential advantages over conventional alloy materials: (i) alloying highly HER-active Ni with corrosion-resistant Hf, combined with N-induced electronic modification, creates synergistic active sites. This configuration significantly enhances both the adsorption and the dissociation of water molecules; (ii) the introduction of N and Hf forms a conductive nitride phase. These N–Hf synergistic sites improve the alloy's electron mobility, accelerating the kinetics of HER;²⁸ (iii) the surface nitride layer inhibits oxidation, passivates grain boundaries, and reduces the penetration of corrosive species, endowing the material with excellent corrosion resistance and durability in alkaline seawater. Electrochemical measurements show that N-NiHf@NF catalysts require an overpotential of only 68 mV to achieve a current density of 10 mA cm⁻² in alkaline seawater electrolyte, and it maintains stable operation for over 120 h. Additionally, the catalysts exhibit high stability in chloride-containing electrolytes, indicating excellent corrosion resistance under harsh seawater conditions. Overall, this study demonstrates a feasible approach for preparing nitrogen-doped nanoalloys and confirms their superior performance in seawater hydrogen production, providing a novel strategy for the design and synthesis of efficient seawater hydrogen-evolving electrocatalysts based on transition metal alloys.

2. Results and discussion

2.1 Synthesis and characterization of N-doped NiHf nanoalloy

The nitrogen-doped NiHf nanoalloy (N-NiHf@NF) catalyst was in situ grown on the processed nickel foam (NF) substrate using a combination of hydrothermal reaction and ammonia-plasma conversion process (Fig. 1a). Initially, Hf doped β-Ni(OH)₂ (Hf-Ni(OH)₂) was hydrothermally synthesized using Ni(NO₃)₂·6H₂O and HfCl₄ as metal sources. Hf-Ni(OH)₂@NF was characterized by X-ray diffraction (XRD). Fig. S1a† shows excellent agreement between the diffraction patterns of as-synthesized Hf-Ni(OH)₂@NF and the standard reference (JCPDS #14-0117), verifying successful catalyst preparation.²⁹ Scanning electron microscopy (SEM) images (Fig. S1b and c†) reveal the basic morphology of Hf-Ni(OH)₂@NF, which consists of nanosheets assembled into nanoflower-like structures. Then, Hf-Ni(OH)₂@NF was converted to N-NiHf@NF by ammonia-plasma, and for comparison, we also prepared NiHf nanoalloys with different nitrogen doping contents as well as pure NiHf (NiHf@NF) by plasma conversion and synthesized NiHf nanoalloys (NiHf@NF (CVD)) by conventional chemical vapor deposition (CVD).

Fig. 1 Synthesis procedure and physical characterization of N-NiHf@NF. (a) Schematic of the synthetic process. (b) XRD patterns. (c) SEM image, (d) HRTEM image, (e) SAED pattern, (f) HAADF-STEM image and corresponding elemental mappings, and (g) EDS pattern and element content of N-NiHf@NF.

The target catalyst (N-NiHf@NF) was then characterized by XRD. As illustrated in Fig. 1b and S2,† the XRD patterns of N-NiHf@NF exhibit characteristic peaks corresponding to the (200) and (111) crystallographic planes of Ni nanoalloy (JCPDS #04-0850).³⁰ Unlike conventional CVD-derived NiHf@NF (CVD), the plasma enhanced chemical vapor deposition (PECVD) prepared NiHf@NF shows diffraction peaks shifted to lower 2θ due to plasma-induced lattice expansion and increased interplanar spacing. This shift arises from high-energy particle bombardment during plasma processing, which induces lattice expansion and consequently increases the interplanar spacing.^31,32 Moreover, the diffraction peak of N-NiHf@NF after nitrogen doping exhibits a positive 2θ shift, indicating the occurrence of lattice contraction, which is caused by the smaller atomic radius of nitrogen compared to the matrix elements Ni and Hf.³³ XRD analysis revealed that the characteristic peaks of Hf were not detectable in the XRD pattern due to its relatively low content, indicating that the overall crystal structure of the catalyst was predominantly governed by metallic Ni. This observation confirmed that the NiHf nanoalloy structure remained intact during nitrogen doping without forming nitrides or other secondary phases. Furthermore, SEM images (Fig. 1c) clearly showed the presence of surface wrinkles on the nanosheets, which were distinct from the smooth morphology typically observed in LDHs. Meanwhile, TEM reveals the folds of the nanosheets (Fig. S3†). HRTEM images (Fig. 1d) reveal well-defined lattice fringes with spacings of 0.185 nm and 0.211 nm, corresponding to the (200) and (111) crystallographic planes, respectively. These measurements agree perfectly with the selected area electron diffraction (SAED) pattern in Fig. 1e, which displays four distinct diffraction rings indexed to the (111), (200), (220), and (311) planes. The above data match well with the characteristic peaks in the XRD patterns, further confirming that nitrogen doping does not disrupt the crystalline structure of the NiHf nanoalloy. The high-angle annular dark-field (HAADF) image and the corresponding elemental distribution map (Fig. 1f) reveal the uniform distribution of Ni, Hf, and N elements within the N-NiHf@NF material. The energy-dispersive X-ray spectroscopy (EDS) spectrum and the metal atomic content table (Fig. 1g) indicate that Ni has the highest atomic percentage (89.48%), followed by Hf (1.36%) and N (9.16%), indicating their successful alloying.

Subsequently, X-ray photoelectron spectroscopy (XPS) analysis revealed the surface elemental composition and oxidation states of the synthesized materials. The survey spectrum of N-NiHf@NF (Fig. S4a†) confirms the presence of Ni, Hf, N, C, and O. Deconvolution of the high-resolution C 1s spectrum (Fig. S4b†) yields three distinct peaks at binding energies of 284.6, 285.79, and 288.69 eV, corresponding to characteristic carbon species; these peaks correspond to the C–C bond, O–C–O group, and O–C=O functional groups, respectively. XPS data for N-NiHf@NF, NiHf@NF, and NiHf@NF (CVD) were analyzed and are presented in Fig. S5.† In the high-resolution Ni 2p spectrum (Fig. S5a†), the 2p_3/2 and 2p_1/2 peaks of metallic Ni are typically located around 852.6 and 869.8 eV, indicating the presence of Ni⁰. Peaks at 855.7 and 873.5 eV correspond to Ni in the +2 oxidation state.^34,35 In the Hf 4f spectrum of N-NiHf@NF (Fig. S5b†), characteristic peaks of 4f_5/2 and 4f_7/2 are observed at 16.5 and 18.1 eV, indicating that the oxidation state of Hf are 0 and +4, respectively.^36,37 Compared with NiHf@NF and NiHf@NF (CVD), the N-NiHf@NF catalyst exhibits significant electronic structure modifications in its XPS spectra. Specifically, the Ni 2p binding energies shift positively by 0.13 and 0.18 eV, while the Hf 4f binding energies undergo positive shifts of 0.05 and 0.12 eV, respectively. This positive shift in binding energy confirms the successful doping of nitrogen atoms into the nanoalloy. Analysis of the N 1s high-resolution XPS spectrum (Fig. S5c†) shows that the plasma-treated N-NiHf@NF sample exhibits distinct N–M (M = Ni/Hf) characteristic peaks at 397.7 eV, while N–H bond characteristic peaks are observed at 399.6.^38,39 These findings demonstrate that plasma treatment successfully achieves nitrogen doping under mild conditions and effectively modulates the surface chemical state of the NiHf nanoalloy.²⁸ The experimental results demonstrate that nitrogen doping effectively modulates the electronic structure of the NiHf nanoalloy and enhances its electrical conductivity.

Furthermore, X-ray absorption spectroscopy was carried out to unveil the chemical states, local atomic structure, and coordination structure of the N-NiHf@NF nanoalloy (Fig. 2). The normalized Ni K-edge X-ray absorption near-edge structure (XANES) profiles were derived from first-order derivatives (Fig. S6a–c†), and each maximum was used to determine the K-edge positions of Ni in the N-NiHf@NF nanoalloy and reference samples (Ni foil and NiO), respectively.⁴⁰ In the Ni K-edge (Fig. 2a) XANES spectra of N-NiHf@NF nanoalloy, the near-edge absorption position of the Ni K-edge (8334.15 eV) is much closer to that of Ni foil (8333.0 eV) and farther from that of NiO (8336.18 eV), indicating that the valence state of Ni is close to zero and corresponds to metallic Ni.^41,42 The valence fitting (Fig. S6d†) also verifies that the mean valence of nickel in N-NiHf@NF nanoalloy is +0.7. Hf is an L-edge metal, and the intensity of the white line peak corresponds to its valence state.⁴³ In the L₃-edge XANES spectra of Hf (Fig. 2b), N-NiHf@NF and HfO₂ exhibit sharp rising edges at 9565.5 and 9564.7 eV, respectively, which are attributed to the 2p → 5d transitions. The Hf L-edge white line peak of N-NiHf@NF is positioned below that of HfO₂, demonstrating that the valence state of Hf is less than +4. These results confirm the relationship between the absorbed edge energy and the intensity of the white line peak corresponding to the valence state of the metal. In order to further study the local structure (bond length and coordination numbers), the Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) spectrum can be used. In Fig. 2c, the FT-EXAFS spectrum of Ni displays a sharp first coordination shell at 2.08 Å, originating from Ni–Ni scattering, with no discernible second shell scattering.⁴⁴ In comparison with Ni foil (characterized by a strong Ni–Ni coordination peak at 2.08 Å) and NiO nanoparticles (with a Ni–O peak at 2.54 Å), the absence of significant Ni–O interactions in the material is confirmed. This further substantiates that Ni exists in the catalyst in an alloy form.^45–47 In Fig. 2d, N-NiHf@NF exhibits a Hf–N scattering peak at 1.47 Å, which is likely attributed to the scattering from the doped nitrogen atoms. Compared with HfO₂ nanoparticles, a deviation of 0.18 Å is observed, which strongly demonstrates the absence of Hf–O species and provides clearer evidence for the formation of the alloy structure in the catalyst.

Fig. 2 Atomic and coordination structure analysis of N-NiHf@NF. (a and b) Normalized XANES profiles of Ni K-edge (inset: enlarged near-edge absorption curves) and Hf L-edge. (c and d) FT-EXAFS profiles of Ni K-edge and Hf L-edge. (e and f) EXAFS R-space fitting curves of the Ni K-edge and Hf L-edge. (g) WT-EXAFS signals of EXAFS of the Ni K-edge and Hf L-edge for N-NiHf@NF and reference samples. The reference samples are Ni foil, NiO, and HfO₂, respectively.

To ensure the reliability of the data in R-space, further theoretical fitting was conducted. The quantitative R-space fitting results of the FT-EXAFS spectra for all metals are shown in Fig. 2e and f with the corresponding fitting parameters summarized in Table S1.† The reliability of the fitted parameters is verified by the small R-factor. The coordination numbers (CN) of Ni–Ni and Hf–N in N-NiHf@NF are 4.93 and 10.7, respectively. This high coordination number is primarily attributed to pronounced multiple scattering effects. The Hf L-edge EXAFS data, particularly at the L₃-edge (corresponding to the 2p_3/2 → 5d transition), reveals multiple scattering effects with significant clarity. Furthermore, the inherently long-range ordered crystal structure of the alloy material results in strong and well-defined multiple scattering paths.^48,49 We performed high-resolution wavelet transform (WT) analysis on the EXAFS spectra at the Ni K-edge and Hf L-edge in both R-space and k-space.⁵⁰ As shown in Fig. 2g, the WT-EXAFS analysis clearly resolved the characteristic coordination information and bond strengths for each sample. The N-NiHf@NF catalyst exhibited distinct intensity maxima at 2.08 and 1.47 Å⁻¹, which were unambiguously assigned to Ni–Ni and Hf–N bonds, respectively. The local structural informations of Ni and Hf are confirmed through WT, providing a clearer picture of the alloy structure in the catalyst. In summary, synchrotron radiation characterization precisely reveals the microstructure and local electronic properties of the as-prepared N-NiHf@NF nanoalloy.

2.2 HER performance in pure alkaline electrolyte

First, we systematically explored the effect of nitrogen doping content on the HER performance of the alloy catalyst in 1.0 M KOH electrolyte. By varying the NH₃/Ar gas mixture ratio during synthesis, we designed and prepared a series of N-NiHf@NF samples with controlled N-doping levels. XRD patterns (Fig. S7†) confirm the successful synthesis of all samples. Electrochemical measurements (Fig. S8†) reveal that the N-NiHf@NF catalyst prepared under 10% NH₃/Ar exhibits superior HER performance, achieving an overpotential of 30 mV, compared to those synthesized under 5% NH₃/Ar (44 mV) and 20% NH₃/Ar (53 mV). These results demonstrate that while N-doping enhances the HER activity of the nanoalloy catalyst, insufficient or excessive doping negatively impacts performance, likely due to the obstruction of intrinsic active sites. Thus, the 10% NH₃/Ar-derived N-NiHf@NF catalyst was identified as the optimal catalyst with the most favorable N-doping level.

This study identifies optimal seawater electrolysis catalysts through evaluation and comparison of electrochemical performance between nitrogen-doped and undoped NiHf nanoalloy catalysts; we systematically tested the HER activities of bare NF, Pt/C@NF, as-prepared NiHf@NF, NiHf@NF (CVD), and N-NiHf@NF catalysts in 1.0 M KOH electrolyte. The best performing catalyst was subsequently selected for seawater electrolysis applications. As shown in the linear sweep voltammetry (LSV) curve in Fig. 3a, the N-NiHf@NF catalyst demonstrates outstanding HER activity, achieving a remarkably low overpotential of 30 mV at 10 mA cm⁻². Under the same testing conditions, the overpotentials of the NiHf@NF and NiHf@NF (CVD) catalysts were 57 and 196 mV, respectively. Comparative analysis showed that the nitrogen doping strategy can significantly reduce the overpotential of NiHf@NF. Notably, the HER performance of the N-NiHf@NF catalyst is even better than that of the Pt/C@NF catalyst (43 mV); this enhancement is principally ascribed to nitrogen doping induced electronic structure modulation in the alloy, which effectively promotes the reaction kinetics. The Tafel curve depicted in Fig. 3b illustrates that N-NiHf@NF has a Tafel slope of 157 mV dec⁻¹, lower than those of NiHf@NF (249 mV dec⁻¹) and NiHf@NF (CVD) (300 mV dec⁻¹). This demonstrates that N-NiHf@NF exhibits significantly enhanced reaction kinetics. Fig. 3c shows that the double-layer capacitance (C_dl) values of N-NiHf@NF, NiHf@NF, and NiHf@NF (CVD) are 38.26, 37.12, and 6.12 mF cm⁻², respectively. Electrochemical impedance spectroscopy (EIS) of the catalyst (Fig. 3d and Table S2†) indicates that the charge transfer resistance (R_ct) of N-NiHf@NF is 0.94 Ω. A smaller R_ct suggests that N-NiHf@NF has a faster charge transfer rate and superior electrical conductivity during the HER compared to other catalysts.⁵¹ The durability and stability of the N-NiHf@NF catalyst were tested for over 90 h at 10 mA cm⁻² current density. In Fig. 3e, the test results reveal that the overpotential of the catalyst is only slightly attenuated, verifying its exceptional electrochemical stability. In summary, the N-NiHf@NF catalysts demonstrated both high catalytic performance and excellent electrochemical stability for the HER. To ensure the reliability of the experimental data, all tests were rigorously repeated and verified. Triplicate measurements were conducted for each nanoalloy electrode to confirm the accuracy and repeatability of the above performance descriptors. Fig. S9a–d† illustrate that the overpotentials, Tafel slopes, C_dl and EIS of various catalysts have high reproducibility. The high HER activity of N-NiHf@NF is comparable or even superior to that of most active alloy and heteroatom doped catalysts (Table S3†). In the 1.0 M KOH alkaline HER system, N-NiHf@NF with 10% nitrogen doping via plasma treatment shows superior catalytic performance, as evidenced by comprehensive electrochemical analysis.

Fig. 3 The HER activity of N-NiHf@NF, NiHf@NF, NiHf@NF (CVD), blank NF and commercial Pt/C@NF in an H₂-saturated 1.0 M KOH electrolyte. (a) LSV curves, (b) Tafel plots, (c) C_dl, and (d) EIS. (e) Electrochemical stability measurements of N-NiHf@NF at a constant current density of 10 mA cm⁻².

2.3 HER performance and stabilities in alkaline seawater

To evaluate the catalyst's practical applicability for seawater electrolysis, we employed a progressive testing strategy: first introducing the optimized electrode material into a simulated seawater electrolyte (1.0 M KOH + 0.5 M NaCl), followed by transition to real alkaline seawater (1.0 M KOH + seawater). This stepwise approach enables isolation of performance-influencing variables and facilitates identification of the performance degradation mechanisms observed in pure alkaline solutions, including chloride-induced corrosion and local pH variations.⁵² Notably, while divalent cations (Ca²⁺ and Mg²⁺) present in seawater may induce potential catalyst poisoning by occupying active sites, their effects are usually less pronounced than those of chloride attack under comparable conditions. Due to the low concentrations of Ca²⁺ and Mg²⁺, their competitive oxidation is usually neglected, thereby rendering chloride oxidation the predominant competitive reaction in seawater electrolysis systems.²

Subsequently, we investigated and compared the HER catalytic performance of the N-NiHf@NF catalyst in 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + seawater (Fig. 4). Furthermore, we evaluated the HER performance of N-NiHf@NF, NiHf@NF, NiHf@NF (CVD), and bare NF in 1.0 M KOH + 0.5 M NaCl electrolyte, including reproducibility data (Fig. S10†). N-NiHf@NF exhibits the lowest overpotential, indicating superior HER activity under simulated seawater conditions. As shown in Fig. 4a, the N-NiHf@NF catalyst exhibits excellent performance in both electrolyte systems, with overpotentials of 69 and 68 mV required for 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + seawater, respectively, at 10 mA cm⁻² current density. These results suggest that N-NiHf@NF exhibits outstanding catalytic activity, although its HER performance in natural seawater shows a moderate overpotential increase of 38 mV compared to that in 1.0 M KOH electrolyte. While the inherent impurity ions in seawater may influence the electrocatalytic activity, the catalyst maintains remarkable performance under these challenging conditions. As illustrated in Fig. 4b, the Tafel slope in 1.0 M KOH + seawater is 198 mV dec⁻¹, which is lower than that in 1.0 M KOH + 0.5 M NaCl (208 mV dec⁻¹). In both electrolytes, Fig. 4c shows that the C_dl of 1.0 M KOH + seawater is 39.03 mF cm^—2, which is slightly lower than that of 1.0 M KOH + 0.5 M NaCl (45.37 mF cm⁻²). This difference is likely due to metal cations in real seawater obscuring the active sites, resulting in lower C_dl performance in 1.0 M KOH + seawater compared to simulated seawater. The EIS results of this different catalysts are shown in Fig. 4d and Table S4.† The R_ct of the 1.0 M KOH + seawater catalyst is 0.9 Ω, which is smaller than that of 1.0 M KOH + 0.5 M NaCl (1.2 Ω). The tests are repeated in three independent trials of the experiment, and each test data point exhibits good reproducibility (Fig. 4e). Fig. 4f presents the stability performance of the N-NiHf@NF catalyst during the HER in 1.0 M KOH + seawater electrolyte, maintaining a constant current density of 10 mA cm⁻² for 120 h. From the test curve, it is observed that the overpotential of N-NiHf@NF decayed minimally after 120 h. In contrast, the Pt/C@NF electrode demonstrated poor operational durability, with its electrochemical activity decaying substantially within merely a few hours. This indicates that N-NiHf@NF exhibits excellent electrochemical activity and durability in alkaline seawater electrolysis for the HER, with notable corrosion resistance. To demonstrate the HER performance of the prepared catalysts in different real seawater samples (Fig. S11†), seawater was collected simultaneously from four regions in China (Fujian, Hainan, Liaoning, and Shandong) to evaluate the HER catalytic performance with a 1.0 M KOH + seawater volume ratio of 1 : 1. Fig. 4g and S12† reveal that the catalyst achieves HER performance comparable to seawater electrolytes from Fujian (overpotential of 68 mV at 10 mA cm⁻²) across diverse regions, with highly reproducible. This confirms that N-NiHf@NF has universal applicability in seawater from different regions. The HER performance of N-NiHf@NF in seawater from different regions is summarized in Table S5.† Both Table S5 and Fig. S12† demonstrate that the HER performance is optimal in seawater from the Fujian region. The N-NiHf@NF catalyst exhibits outstanding HER activity in 1.0 M KOH + seawater electrolyte, demonstrating performance that rivals or surpasses most reported HER catalysts under comparable conditions (Table S6†).

Fig. 4 The evaluation of HER activity of the N-NiHf@NF catalyst in 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + seawater media. (a) LSVs, (b) Tafel slopes, (c) C_dl and (d) EIS of the N-NiHf@NF catalysts. Error bars representing the (e) overpotentials at 10 mA cm⁻², Tafel slopes, C_dl and EIS. (f) Electrochemical stability measurements of N-NiHf@NF and Pt/C@NF at a constant 10 mA cm⁻². (g) LSVs of 1.0 M KOH + seawater in different regions.

The N-NiHf@NF catalyst is susceptible to Cl⁻ induced corrosion in alkaline seawater, and prolonged stability testing can induce structural and compositional degradation. We further uncover the morphological, structural, and compositional stability of N-NiHf@NF electrode after long-term electrochemical durability tests via TEM, HRTEM, SAED, EDS and XPS techniques. As shown in Fig. 5a, HRTEM characterization of N-NiHf@NF reveals that the (200) and (111) crystalline facets are still present after long-term stability testing in real seawater, consistent with those observed in fresh N-NiHf@NF catalysts (Fig. 1d). The SAED pattern of N-NiHf@NF (Fig. 5b) exhibits four well-defined diffraction rings, which are indexed to the (111), (200), (220), and (311) crystallographic planes. The presence of these low-index crystal faces in the SAED pattern corroborates the HRTEM analysis. EDS analysis (Fig. 5c) confirms that N-NiHf@NF contains Ni, Hf, and N elements. The inserted table shows a decrease in the atomic percentage of N after the stability test, while there is a slight increase in the atomic percentages of Ni and Hf metals. This may be attributed to the dissolution of N on the surface, which exposes the underlying Ni and Hf metals. HAADF imaging and elemental mapping (Fig. S13†) confirm the homogeneous distribution of Ni, Hf, and N species throughout the N-NiHf@NF catalyst structure. These results collectively indicate that N-NiHf@NF maintains good structural stability even after prolonged exposure to seawater.

Fig. 5 Structural and compositional stability characterization of N-NiHf@NF nanoalloy after long-term OER durability measurement. (a) HRTEM image. (b) SAED pattern. (c) EDS pattern and element content. XPS high resolution spectra of (d) Ni 2p, (e) Hf 4f, and (f) N 1s for the N-NiHf@NF electrode before and after the stability testing.

To investigate the compositional stability of the N-NiHf@NF catalyst, XPS comparative analysis was performed on samples before and after electrochemical performance testing. The high-resolution Ni 2p spectra (Fig. 5d) showed a significant decrease in the intensity of the characteristic peaks of Ni⁰ (852.0 eV) after the stability test, while quantitative XPS analysis (Table S7†) showed that the atomic percentage of Ni decreased after stability tests. This phenomenon is mainly attributed to the dissolution of Ni⁰ during the electrolysis process.^53,54 In the Hf 4f spectrum (Fig. 5e), the relative intensities of the 4f_7/2 (16.1 eV) and 4f_5/2 (17.6 eV) peaks remain unchanged, but the overall binding energy is positively shifted by about 0.3 eV. This shift is attributed to redistribution of the surrounding electronic structure caused by the local oxidation of nitrogen, leading to a slight shift in the electronic coordination environment around Hf.³⁶ The changes in the N 1s spectra (Fig. 5f) further confirm this mechanism: the strength of the N–M (M = Ni/Hf, 397.5 eV) bond decreases after the test, the characteristic N–O peak appears at 403.5 eV.²⁸ Meanwhile, the observation in Table S7† shows that there is a slight decrease in the atomic percentage of N after the stability test. This suggests that under alkaline electrolytic conditions, partially doped nitrogen atoms may be converted to gaseous NH₃/N₂ and released, while the other part undergo nitrogen oxidation to form soluble nitrogen-containing anions, accompanied by a shift in their electronic coordination environment.⁵⁵ The incorporation of nitrogen plays a critical role in stabilizing the catalyst structure due to its higher electronegativity compared to Ni and Hf. The strong oxidative tendency of nitrogen facilitates preferential oxidation reactions at nitrogen sites, leading to its oxidative dissolution. This sacrificial oxidation process effectively prevents the degradation of the Ni–Hf matrix, thereby preserving the structural integrity of the catalyst. These findings provide a crucial basis for understanding the catalyst deactivation mechanism and optimizing material design.

2.4 Corrosion-resistance mechanism and durability of N-NiHf@NF

The corrosion resistance mechanism of the materials was systematically investigated using potentiodynamic polarization (PDP) measurements and chronopotentiometric analysis.⁵⁶ These electrochemical techniques allowed comprehensive evaluation of the catalyst's high activity and stability in chloride-containing electrolytes. The corrosion potential (E_corr) and corrosion current density (j_corr) served as critical indicators of the electrode's actual corrosion behavior. Typically, a higher E_corr value corresponds to lower corrosion tendency (indicating better thermodynamic stability), while a lower j_corr value reflects slower corrosion kinetics.⁵⁷ The corrosion-resistant effect of Hf incorporation is demonstrated through comparative analysis of PDP curves between N-NiHf@NF, Pt/C@NF and its Hf-free counterpart N-Ni@NF (Fig. 6a). The corresponding fitting results are shown in Fig. S14† and the derived corrosion parameters are summarized in Fig. 6b. Electrochemical measurements show that N-NiHf@NF exhibits a significant anodic shift in E_corr (0.955 V vs. RHE) compared to N-Ni@NF (0.918 V vs. RHE) and Pt/C@NF (0.929 V vs. RHE), along with a substantially lower j_corr 0.04 mA cm⁻² compared to N-Ni@NF (0.122 mA cm⁻²) and Pt/C@NF (0.27 mA cm⁻²). These results clearly demonstrate that Hf doping effectively enhances the catalyst's corrosion resistance, as evidenced by the anodic shift in E_corr and the substantial reduction in j_corr. Chronopotentiometric tests further confirmed the outstanding stability of N-NiHf@NF, which maintained stable operation for 110 h at 100 mA cm⁻² current density in 1.0 M KOH + seawater significantly outperforming Pt/C@NF (Fig. 6c). This exceptional durability demonstrates the catalyst′s effective resistance to Cl⁻ induced corrosion. Thus, these results clearly demonstrate that the rationally designed N-NiHf@NF electrode possesses significantly enhanced corrosion resistance compared to Pt/C@NF.

Fig. 6 (a) Potentiodynamic polarization curves of N-NiHf@NF, N-Ni@NF and Pt/C@NF. (b) Comparison of E_corr and j_corr on N-NiHf@NF, N-Ni@NF and Pt/C@NF catalysts. (c) Electrochemical stability test of N-NiHf@NF and Pt/C@NF for 110 h at 100 mA cm⁻².

In the field of catalytic materials research, establishing robust structure–activity relationships and proposing plausible mechanisms fundamentally rely on compelling experimental performance comparisons, systematic physicochemical characterization, and consistency with established mechanisms. Previous studies have verified that hydrogen adsorption energy (ΔG_H*) is the dominant factor that governs alkaline HER activity,⁴² whereas the *OH adsorption energy determines the rate of H₂O dissociation to provide available H*. Therefore, the adsorption/desorption of reacting intermediates (*OH and H*) determines the overall HER kinetics and efficiency.⁵⁸ Synchronously balancing the adsorption energy of the reacting intermediates not only accelerates the H₂O dissociation, but also avoids the undue adsorption of H* and *OH to block the active sites, which can promote the efficiency of the alkaline HER. Ni-based alloys incorporate transition metals (e.g., Mo, Fe, and Cr) to modulate the d-band center, mitigating excessively strong Ni–H binding and driving ΔG_H* towards zero.⁵⁹ For seawater electrolysis, the initial step involves cleaving the H₂O molecule (Volmer step). Alloys like Ni-based alloys (with Fe, Mo, and Ru) lower the water dissociation energy barrier, accelerating H* generation. Oxygen-affinity sites on the alloy surface (e.g., Ru) promote H₂O adsorption/dissociation.⁶⁰ Competitive Cl⁻ adsorption can poison active sites. High-valency metals (e.g., Cr, Mo, and Hf) in Ni alloys preferentially adsorb OH⁻, forming a passivation layer that repels Cl⁻. This provides a reasonable explanation for the performance enhancement of N-NiHf@NF alloy, consistent with widely accepted mechanisms in electrocatalysis, particularly for seawater HER.

3. Conclusion

In summary, this work presents an N-NiHf@NF catalyst that simultaneously addresses the multifaceted requirements of cathodic materials for seawater electrolysis, demonstrating both exceptional catalytic activity and remarkable long-term stability. The study reveals that nitrogen doping via plasma bombardment effectively promotes nanoalloy formation while significantly enhancing both the HER performance and structural stability of the catalyst. Moreover, the resulting N-NiHf@NF catalyst effectively resists Cl⁻ corrosion. We have also investigated its electrochemical corrosion resistance mechanism. The N-NiHf@NF catalyst demonstrates outstanding operational stability, maintaining consistent electrochemical performance for over 110 h at a current density of 100 mA cm⁻². This study develops an efficient and corrosion-resistant electrocatalyst that is expected to facilitate the commercialization of seawater electrolysis technology, and introduces a plasma-assisted synthesis strategy that promotes both alloy formation and heteroatom doping. The proposed methodology provides a promising avenue for developing simple yet effective solutions to overcome the challenges associated with seawater electrolysis technology.

Data availability

The data that support the findings of this study are available on request from the corresponding authors.

Author contributions

Y. Y., H. T., and X. C. conceived the idea and supervised the project. R. M., J. H. M., and R. C. developed the synthesis and characterization of nanoalloy electrocatalysts. J. K. L., P. F. G., Y. J., and Z. Y. J. helped in data analysis. X. M., L. Z., and L. L. provided technical support for TEM and XAFS characterization. H. T. and G. X. W. revised the manuscript. All authors have given approval to the final version of this manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (grant no. 52373087 and 52001257), the Natural Science Basic Research Program of Shaanxi (no. 2020JQ-167 and 2024JC-YBMS-131), and the Department of Science and Technology of Hubei Province (2024CSA076).

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Footnote

† Electronic supplementary information (ESI) available: Additional material and electrochemical characterization data (PDF). See DOI: https://doi.org/10.1039/d5ta03735b

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