INTRODUCTION
 Hydrogen (H2), with its high-energy density and zero-emission properties, holds great promise as a sustainable energy carrier (1–4). Seawater electrolysis, as a promising alternative for sustainable hydrogen production, faces challenges due to the expensive and scarce platinum catalyst, as well as the need for additional purification processes to address the complex nature of seawater, resulting in higher production costs (5–7). Compared to the widely adopted practice of hydrogen production through electrolysis of alkaline freshwater, the advancement of hydrogen production from seawater has been relatively sluggish (8). The pH of natural seawater is typically around 8.0 and reduced ionic mobility in seawater leads to a diminished pH dependence of the hydrogen evolution reaction (HER) (9, 10), necessitating higher applied voltages to attain the desired efficiency in hydrogen production (11). Furthermore, the complex composition of seawater and the insoluble deposits formed by alkaline earth metal ions such as Ca2+, Mg2+ during electrolysis make seawater HER challenging (1, 12). More fatally, Cl- has a high affinity for the metal active sites of the catalysts as they have strong depassivation and penetrating properties (13, 14), leading to severe degradation of the catalyst and hindering direct seawater electrolysis for hydrogen production (15–17). Although certain catalysts demonstrated activity at low current densities but offer low hydrogen production rates (18, 19), far below the high-current hydrogen production conditions required for industrial water electrolysis.
 To address this bottleneck issue, various strategies have been developed, including selective active site engineering (20, 21), three-dimensional structure design (22, 23), and carbon layer protection (24, 25). Recently, Guo et al. (26) proposed an innovative strategy, wherein they enhanced the kinetic processes by generating an in situ localized alkaline environment, achieving excellent seawater catalytic performance at 60°C. Metal nitrides (MNs) have inherent chemical stability, electrical conductivity, and excellent catalytic activity, and their surfaces undergo in situ transformation to form highly catalytic active hydroxides, while the nitride core remains stable (27, 28). For example, NiMoN@NiFeN prepared by Yu et al. (29) required an overpotential of 160 mV to reach a current density of 500 mA cm-2 and stable operation for 48 hours, which is attributed to the amorphous NiFeOOH layer formed on the nitride surface. Regrettably, the performance of current MN-based electrocatalysts is still suboptimal and it is urgent to manipulate them for efficient seawater splitting.
 In this work, we present a protective strategy by introducing a V2O3 layer by in situ reduction into a catalyst with low loading of Pt and Ni3N. The V2O3 layer acts as an “armor” during electrolysis, markedly reducing the adsorption of Cl- and alkaline earth cations (Ca2+ and Mg2+) from seawater and preventing corrosion of the active sites on the electrode. Benefiting from the powerful protection mechanism, the assembled Pt-Ni3N@V2O3 catalyst exhibits remarkable HER activity in alkalized natural seawater and maintains its performance for at least 500 hours at industrial-grade current density, surpassing the performance of other reported electrocatalysts.
 RESULTS
 Synthesis and characterization
 The hydrothermal method was used to prepare hydroxide precursors in nanoflower-shaped structures, as illustrated in figs. S1 and S2. Subsequently, calcination in NH3 gas was used to obtain a composite of Ni3N and V2O3, resulting in the formation of numerous rough porous structures on the catalyst surface. Pt-Ni3N@V2O3/NF (nickel foam noted as NF) was constructed by placing the Ni3N@V2O3/NF precursor in an aqueous solution of H2PtCl6 at room temperature. On the basis of previous reports, we speculate on the reduction mechanism and route (30–32). Specifically, V2O3, as a strong reducing agent, first forms (V2O3)x(OH-)y(s) with H2O molecules (chemical eq. S1), and then Pt4+ ions in solution adsorb onto the surface of the (V2O3)x(OH-)y(s). In situ reduction then occurs with the assistance of H2O molecules to form Pt nanoparticles (chemical eq. S2). Theoretical calculations also confirm that Pt nanoparticles preferentially form on the V2O3 surface rather than the Ni3N surface (fig. S2).
 The powder x-ray diffraction (XRD) patterns, as illustrated in Fig. 1A and fig. S3, confirm the presence of a composite phase comprising Ni3N, V2O3, and Pt. These results demonstrate the successful preparation of Pt-Ni3N@V2O3/NF. The hydroxide precursors prepared by the hydrothermal method exhibited nanoflower-like structures with low crystallinity, as shown in figs. S4 and S5. After ammonolysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that Ni3N@V2O3/NF exhibit interconnected porous structures (fig. S6) with lattice stripes of 0.203 and 0.183 nm corresponding well to the (111) and (024) facets of Ni3N and V2O3 (fig. S7), respectively. After the incorporation of Pt, SEM and TEM analyses reveal that Pt-Ni3N@V2O3/NF exhibits a granular morphology, together with a rough surface (Fig. 1, B and C). Pt nanoparticles, formed in situ, have an average particle size of 4.41 nm. The high-resolution TEM (HRTEM) shows that amorphous/low crystallinity V2O3 covers the outer surface of Pt-Ni3N@V2O3/NF (Fig. 1D), with lattice stripes of 0.203 and 0.226 nm corresponding to the (111) facets of Ni3N and the (111) facets of Pt (Fig. 1, E to G), respectively. As demonstrated in fig. S8, the particle size of Pt-Ni3N@V2O3/NF increases with longer impregnation time, and this observation is also supported by the electrochemical properties. In Fig. 1 (H and I), high-angle annular dark-field scanning TEM (HAADF-STEM) images, energy-dispersive spectroscopy (EDS) elemental mapping, and EDS line scan collectively suggest that the internal larger nanoparticles comprise Ni3N, while V2O3 is distributed in the surrounding region. In addition, uniformly anchored, small-sized Pt nanoparticles are found on the surface of the structure, forming a dual-active site. The content of Pt is determined by energy dispersive x-ray spectroscopy to be approximately 6.61% (fig. S9 and table S1), which is consistent with the results of inductively coupled plasma–optical emission spectrometry (ICP-OES) analysis (6.94%). It is worth noting that the reduction of Pt also occurs on the Ni3N surface in the absence of the V2O3 in situ reduction layer (fig. S3C), but with considerable agglomeration (figs. S10 and S11), which is undoubtedly detrimental to catalytic performance. The V2O3 serves as both a potent in situ reducing agent and stabilizer. Its rough surface offers numerous anchor sites and exerts steric hindrance effects, influencing the nucleation and growth kinetics of Pt nanoparticles while impeding their migration and aggregation (30).To address this bottleneck issue, various strategies have been developed, including selective active site engineering (20, 21), three-dimensional structure design (22, 23), and carbon layer protection (24, 25). Recently, Guo et al. (26) proposed an innovative strategy, wherein they enhanced the kinetic processes by generating an in situ localized alkaline environment, achieving excellent seawater catalytic performance at 60°C. Metal nitrides (MNs) have inherent chemical stability, electrical conductivity, and excellent catalytic activity, and their surfaces undergo in situ transformation to form highly catalytic active hydroxides, while the nitride core remains stable (27, 28). For example, NiMoN@NiFeN prepared by Yu et al. (29) required an overpotential of 160 mV to reach a current density of 500 mA cm-2 and stable operation for 48 hours, which is attributed to the amorphous NiFeOOH layer formed on the nitride surface. Regrettably, the performance of current MN-based electrocatalysts is still suboptimal and it is urgent to manipulate them for efficient seawater splitting.
 In this work, we present a protective strategy by introducing a V2O3 layer by in situ reduction into a catalyst with low loading of Pt and Ni3N. The V2O3 layer acts as an “armor” during electrolysis, markedly reducing the adsorption of Cl- and alkaline earth cations (Ca2+ and Mg2+) from seawater and preventing corrosion of the active sites on the electrode. Benefiting from the powerful protection mechanism, the assembled Pt-Ni3N@V2O3 catalyst exhibits remarkable HER activity in alkalized natural seawater and maintains its performance for at least 500 hours at industrial-grade current density, surpassing the performance of other reported electrocatalysts