The synthetic procedure of NiCrS/CN catalysts was schematically depicted in Figure 1. In brief, a porous NiCr microsphere was cathodically electrodeposited, followed by a thermal treatment process using thiourea (CH₄N₂S) to synthesize NiCrS/CN.

The Raman spectra of NiCrS and NiCrS/CN in Figure 2A confirm different characteristic peaks at 1,405.4 and 1,575.6 cm^-1, indicating the formation of C-N overlayers. As depicted in Figure 2B, the diffraction peaks at 31.59°, 38.78°, and 45.31°in spectrum can be well matched with the (200) (211) and (220) crystal planes of NiS₂ (JCPDS No. 11-0099), while the peaks at 34.19° 44.46°and 53.41° can be attributed to the (112) (114) and (300) crystal planes of Cr₂S₃ (JCPDS No. 11-0007). Notably, no carbon diffraction peaks were observed in the X-ray diffraction (XRD) patterns, indicating that the obtained C-N layer possesses a non-crystalline structure and ultrathin thickness. Scanning electron microscope (SEM) images demonstrate that NiCrS/CN possesses a 3D cotton-like architecture with abundant microspheres stacked [Figure 2C-E]. Such a novel structural feature significantly improves mass transfer ability and facilitates bubble detachment under high current densities, thereby enhancing the overall electrocatalytic performance. High-resolution transmission electron microscopy (HRTEM) results in Figure 2F reveal an ultrathin C-N overlayer (~1.5 nm) encapsulated on the NiCrS surface. The observed interplanar spacings of 0.283 and 0.202 nm are ascribed to NiS₂ (200) and Cr₂S₃ (114) planes, respectively. These results collectively demonstrate the distinctive structure of the NiCrS/CN catalyst, featuring an ultrathin C-N overlayer coating over the NiCrS heterostructure surface.

To explore the valence states and chemical compositions, X-ray photoelectron spectroscopy (XPS) analysis of NiCrS/CN was performed. The characteristic peaks at 576.7 and 586.5 eV in the high-resolution Cr 2p spectrum are indexed to the Cr-S bond, as depicted in Figure 3A. In the Ni 2p XPS spectrum [Figure 3B], well-resolved Ni²⁺ and Ni³⁺ species are observed with Ni 2p3/2 peaks at 852.8 and 855.6 eV, suggesting a typical NiS₂ phase. The S 2p XPS spectrum is fitted into two characteristic peaks [Figure 3C]. The characteristic peaks observed at 161.5 and 162.6 eV are assigned to the metal-sulfur bond in NiCrS [Figure 3C]. The C 1s spectrum of Figure 3D displays three components at 284.7, 286.1, and 287.8 eV, ascribed to C-C/C=C bond, C-N/C-S bond, and C=O bond. Moreover, N 1s signals in Figure 3E confirm the existence of pyridine N and pyrrolic N species.

The electrocatalytic performance of the obtained NiCrS/CN electrocatalyst for hydrogen evolution reaction (HER) was explored in a 1.0 M KOH electrolyte. To optimize catalytic activity, the influence of synthesis conditions (thiourea amount, calcination temperature, and calcination time) was systematically investigated. The effect of sulfidation/carbon coating conditions on determining HER activity was first explored in Supplementary Figures 1-3. Among all of the samples, the NiCrS/CN catalyst calcined at 350 °C for 2 h depicted the best HER performance in 1.0 M KOH electrolyte, and the optimal thiourea amount is 0.625 g. Unless otherwise stated, all subsequent electrochemical measurements were carried out under optimized conditions.

As illustrated in Figure 4A and B, the obtained NiCrS/CN catalyst with a takeoff potential of near 0 V can exceptionally derive a HER current density of -10 mA cm^-2 at an overpotential of only 62 mV, 45 mV lower than that observed for NiCrS (107 mV). Notably, at overpotentials of 92 and 106 mV, NiCrS/CN achieves high HER current densities of -25 and -50 mA cm^-2, respectively, which are 3.91 and 4.85 times relative to those of NiCrS (-6.4 and -10.3 mA cm^-2). Strikingly, our NiCrS/CN electrocatalysts even demonstrate Pt-like HER kinetics at a high potential input [Supplementary Figure 4]. The corresponding HER current density at an overpotential of 188 mV could achieve as high as -200 mA/cm² for NiCrS/CN, which is even superior relative to that for the benchmark Pt/C (-156.1 mA/cm²). Tafel plots derived from linear sweep voltammetrys (LSVs) in Figure 4C showed a lower Tafel slope of 62.5 mV dec^-1 for NiCrS/CN compared to NiCrS (87.9 mV dec^-1), suggesting that the C-N layer could efficiently facilitate Volmer-Heyrovsky progress over NiCrS-based electrocatalysts. To further explore the charge-transfer dynamics, electrochemical impedance spectra (EIS) were characterized [Figure 4D]. NiCrS/CN displays lower charge transfer resistance (R_ct) of 26.14 Ω compared to NiCrS (43.36 Ω), indicating rapid charge transfer kinetics between the catalyst and electrolyte via C-N layer coordination. Electrochemical activation surface area (ECSA) displayed a linear correlation with the double-layer capacitance (C_dl). As shown in Supplementary Figures 5-7, NiCrS/CN demonstrates a higher C_dl value of 44.48 mF cm^-2 relative to that of NiCrS (33.32 mF cm^-2), revealing the exposure of more catalytic sites. Operational stability is critical to evaluate the practical application of catalysts. Long-term chronopotentiometry demonstrates that NiCrS/CN possesses robust stability, with only a 9.7% increase in overpotential during 12 h of electrocatalysis [Figure 4E]. In contrast, NiCrS even loses activity within 6 h, indicating the C-N layer may mitigate oxidation-driven degradation by shielding Ni/Cr metallic sites from direct exposure in an alkaline electrolyte, resulting in stable electrolysis of TMS-based electrocatalysts.

Compared to conventional freshwater electrolysis, alkaline seawater splitting could eliminate the intricacy of pretreatment, reduce system complexity, and reduce the operational cost of the electrolyzer for the industrialization of the “green hydrogen economy”. Therefore, we systematically compared HER properties of NiCrS/CN and NiCrS under alkaline seawater conditions. As revealed in Figure 4F, the as-synthesized NiCrS/CN demonstrates extraordinary HER kinetics with an overpotential input of 68 mV at -10 mA cm^-2 in alkaline seawater electrolyte. Strikingly, our NiCrS/CN electrocatalytic system only require an ultralow overpotential input of 143, 164 and 191 mV to afford industrial-relevant current density of -50, -100, and -200 mA cm^-2, which is 12.20, 15.87 and 19.8 times relative to those of NiCrS (-4.1, -6.3 and -10.1 mA/cm²), respectively, at the same overpotential input, due to a fast electron transfer ability as well as enhanced HER kinetic [Supplementary Figures 8-11]. More interestingly, it is noteworthy that NiCrS/CN shows similar HER behaviors in freshwater and seawater, and only a diminished degradation trend (~ 7 mV) in alkaline seawater is observed versus that in 1.0 M KOH electrolyte. Conversely, NiCrS demonstrates lower HER kinetics in alkaline seawater compared with alkaline freshwater [Figure 4G, Supplementary Figures 12 and 13]. To afford current densities of -10 and -50 mA cm^-2, larger overpotentials of 196 and 279 mV are needed in alkaline seawater solution relative to freshwater over the NiCrS system. Such a difference is attributed to C-N, as a protective layer, could compel the blocking species (e.g., bacteria, Cl^-, Mg²⁺ and Ca²⁺ ions in seawater) from surface active sites and achieve a fast HER progress in alkaline seawater media. Overall, by virtue of C-N layer, our NiCrS-based electrocatalytic system demonstrate remarkable catalytic performance in alkaline freshwater and seawater electrolyte, which can compare with other state-of-the-art electrocatalysts such as Ni-SN@C^[35], P-Fe₇S₈@Co₉S₈@Ni₃S₂/NF^[36], RuW/NCN^[37], Ni-SA/NC^[38], MFC-2^[39], NFP@NC^[40], Mo-based elctrocatalysts^[41-43], NiFe-based elctrocatalysts^[44-47], Se-based elctrocatalysts^[48,49], Co-based elctrocatalysts^[50-52], and Co-P-based^[53-55] elctrocatalysts [Figure 4H and Supplementary Table 1].

OER has been identified as a major bottleneck in electrolyzers due to its intrinsic multiple proton-coupled electron transfer process. The sluggish kinetics will require a large theoretical energy barrier associated with a lower hydrogen production efficiency of an electrolyzer. Therefore, we further evaluate the OER performance over NiCrS/CN electrocatalytic systems in a 1.0 M KOH electrolyte [Supplementary Figures 14-16]. As depicted in Supplementary Figures 17-20, the incorporation of a C-N layer can effectively promote OER kinetics of the NiCrS-based electrocatalytic system with an overpotential of 106 mV for 10 mA cm^-2 combined with robust stability in alkaline solution, making it as a promising candidate for water oxidation [Supplementary Table 2]. The polarization results and performance in Figure 5A and Supplementary Figures 21-24 further confirm that minimal degradation is observed in alkaline seawater media over the NiCrS/CN electrocatalytic system rather than NiCrS. Obviously, industrial-scale current densities of 50 and 100 mA cm^-2 could be afforded over the NiCrS/CN electrode at low overpotentials of 162 and 225 mV, respectively, in alkaline seawater solution. Exceptionally, the overpotential of NiCrS/CN in alkaline seawater at as high as 200 mA cm^-2 is still lower than the maximum theoretically overpotential of 480 mV required for initial hypochlorite production, suggesting the NiCrS/CN electrode as a solid barrier to Cl^- corrosion.

Given the excellent HER/OER activities of NiCrS/CN, we explored the overall water splitting performance in alkaline freshwater and seawater electrolyte through a two-electrode configuration, where NiCrS/CN is used as both the cathode and anode. Remarkably, the assembled NiCrS/CN electrolyzer demonstrates outstanding overall water splitting activities in alkaline freshwater and seawater electrolyte. Notably, our NiCrS/CN electrolyzer can produce an industrial current density of 100 mA/cm², with a 50 mV lower energy potential than the NiCrS electrolyzer in alkaline freshwater [Figure 5B]. Even under seawater condition, a highly efficient NiCrS/CN electrolyzer only requires cell voltages of 1.40, 1.49 and 1.55 V to produce current densities of 50, 100 and 150 mA/cm², 2.53, 2.59 and 2.48 times superior to those of NiCrS electrolyzer (19.8, 38.5 and 60.4 mA/cm²) at the same potential input [Figure 5C]. As shown in Figure 5D and Supplementary Figure 25, NiCrS/CN still showed higher ECSA-normalized current densities than NiCrS in both alkaline freshwater and alkaline seawater for overall water splitting. For example, at a potential input of 1.55 V, NiCrS/CN can produce a normalized current density of 0.038 mA/cm²_ECSA in 1 M KOH + seawater, which is 2 times higher than that of NiCrS (0.019 mA/cm²_ECSA). A similar trend is also observed in alkaline freshwater [Supplementary Figure 25]. Supplementary Figure 26 shows a representative GC result over NiCrS/CN for water splitting in alkaline solution. After 1 h of chronoamperometric electrolysis, only hydrogen (H₂) and oxygen (O₂) peaks were detected in the released gas without other impurity gases present, further confirming the high efficiency of NiCrS/CN in electrocatalyzing water splitting into hydrogen production. Furthermore, the assembled NiCrS/CN electrolyzer displays alkali-tolerant ability under extreme conditions (3 M KOH + seawater), indicating a promising application in industrial [Supplementary Figure 27]. Strikingly, the assembled NiCrS/CN configuration demonstrates an outstanding robustness toward electrocatalytic overall water splitting in both freshwater and seawater electrolytes. As revealed in Figure 5E, our NiCrS/CN system can stably maintain a superior 1,200 h electrolysis (50 mA cm^-2@V) without obvious degradation compared to that of the NiCrS electrolyzer in alkaline freshwater solutions. Even when conducted under alkaline seawater conditions, the assembled NiCrS/CN electrolyzer can be stably operated for at least 100 h to achieve industrially relevant hydrogen production from seawater splitting [Figure 5F]. In contrast, the NiCrS system loses its activity after only 39 h of electrocatalysis in harsh conditions. Such robust stability of NiCrS/CN is attributed to the encapsulation of C-N overlayers, which could efficiently anchor S species via strong C-S bonds at the interface. Such a unique configuration benefits in suppressing S species dissolution and leaching into alkaline electrolytes, guaranteeing superior durability. Overall, NiCrS/CN exhibits high-performing electrocatalytic seawater splitting kinetics, which are superior to some Ru-based^[56,57], Co-based^[58,59], NiFe-based^[60,61,62], NiMoN-based^[63-65], and other alkaline water splitting systems^[66,67,68] reported [Figure 5G].

In this work, our NiCrS/CN system demonstrates a striking catalytic performance toward water splitting in both alkaline freshwater and seawater electrolytes. This remarkable enhancement in catalytic kinetics arises from the following features: On the one hand, the C-N layer can induce a strong synergistic effect between metallic active sites and the C-N overlayer. In detail, C-N overlayers could accelerate electron transfer kinetics and withdraw charge to NiCr active sites, which reconfigures a unique charge distribution of metallic active sites with highly efficient spatial electron separation. Therefore, C-N species could configure the orbital distributions (d-band center) of NiCr sites with higher electron states and supply more coordinatively unsaturated sites. Such a unique local electronic configuration could generate a more negative position of the valence band edge near the Fermi level, which endows stronger electron-donating capability of NiCr active sites associated with a lower energy barrier for H₂O molecular dissociation, and thereby optimizes adsorption/desorption capability of intermediated *OH and *H species. Subsequently, the in situ generated H intermediate species could migrate through spillover pathways and desorb from the C-N overlayer to form gaseous H₂, achieving a favorable hydrogen spillover dynamic progress. On the other hand, C-N species could efficiently improve the hydrophilicity of NiCr-based electrocatalysts and enhance the adsorption ability of H₂O molecules on NiCr sites. Also, it is well known that accumulated hydrogen bubbles on the electrode surface could hamper the interaction between the electrolyte and electrode surface, even blocking the catalytic active sites. Therefore, the introduction of C-N overlayers could endow fast kinetics of H₂ molecular release from the catalytic surface, resulting in a robust performance toward electrolyzing hydrogen evolution. More importantly, it is found that the C-N layer can serve as a Lewis base layer to manipulate the alkaline local micro-environment of the NiCr-based electrocatalytic system. Especially, pyridine N species in the C-N layer possess abundant lone-pair electrons that benefit the coordinated dynamics of chlorine on the metallic surface. Such a novel “electron-rich” local configuration could effectively repel Cl^- ions and provide a specific protective layer against Cl^- adsorption. Furthermore, the C-N overlayer could simultaneously stimulate S atom migration via the formation of C-S bonds. The strong C-S interaction could benefit the inhibition of sulfur species leaching into alkaline electrolyte, therefore, achieving a robust electrocatalytic seawater splitting for hydrogen generation in industry.

In summary, we successfully engineered a C-N overlayer encapsulated on NiCrS to endow a high-performance seawater splitting. Thanks to the synergistic effect between the encapsulated C-N overlayers and NiCrS, an optimized local electronic micro-environment has been achieved. Such a unique local electronic configuration could enhance the electron-donating affinity of NiCr active sites and facilitate the reactive kinetics of H₂O adsorption/dissociation, thereby resulting in a thermodynamic balance of intermediate *OH and *H species. More importantly, C-N overlayers are utilized to regulate the chemical state of surficial sites for adsorbed Cl- species, efficiently suppressing Cl corrosion. Meanwhile, the C-N overlayer could simultaneously stimulate S atom migration via the formation of C-S bonds, preventing sulfur species leaching. As a result, the obtained NiCrS/CN electrocatalysts show a striking seawater splitting performance. In 1.0 M KOH electrolyte, the NiCrS/CN catalyst exhibits remarkable HER activity. At a high overpotential input (≥ 180 mV), Pt-like HER kinetics are observed on NiCrS/CN electrocatalysts. Furthermore, the assembled NiCrS/CN cell achieves industrially relevant current densities of 50, 100, and 150 mA/cm² at cell voltages of 1.40, 1.49, and 1.55 V, respectively, demonstrating excellent stability and durability. Notably, the assembled NiCrS/CN electrolyzer demonstrates a robust industrially relevant operational performance and could be operated for at least 1,200 h and 100 h in alkaline freshwater and seawater electrolytes, respectively. This work represents a novel design strategy for developing highly active and robust cost-effective catalysts for large-scale seawater electrolysis.