Structural identification of CN@NiCoS heterostructure electrocatalystCN@NiCoS was synthesized via hydrothermal-sulfidation/carbon-coating process, while CN@NiS, CN@CoS and NiCoS were prepared for comparison using a similar method (see Methods section). The X-ray diffraction (XRD) pattern (Fig. 1a) of CN@NiCoS on Ni foam (NF) electrode shows characteristic diffraction peaks of Ni3S2 (JCPDS No. 73-0698) and Co9S8 (JCPDS No. 73-1442), along with diffractions signals from the Ni foam substrate. Corresponding Raman spectra (Fig. 1b) of the CN@NiS and CN@NiCoS further confirm the presence of CN overlayers with distinct D and G bands at 1367.8 and 1593.4 cm−1 17, respectively. The absence of prominent carbon diffraction in XRD thus reflects its amorphous nature and ultrathin thickness. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of CN@NiCoS (Fig. 1c) depict an ultrathin CN overlayer approximately 1 ~ 1.5 nm encapsulating the outer surface of the NiCoS particle. High-resolution TEM (HRTEM) images (Fig. 1d) clearly resolve a Ni3S2/Co9S8 heterostructure within the NiCoS nanoparticle, with interplanar spacing of 0.237 and 0.248 nm corresponding to Ni3S2 (003) and Co9S8 (400) planes. These results collectively elucidate the unique structure of the CN@NiCoS catalyst, featuring an ultrathin CN shell encapsulating Ni3S2/Co9S8 heterostructure, which is distinct notably from CN@NiS (single Ni3S2 phase), CN@CoS (single Co9S8 phase) and NiCoS (without the C layer), as shown in Fig. 1e and Supplementary Fig. 1 and 3).Fig. 1: Structural characterization of CN@NiCoS heterostructure electrocatalyst.a XRD profile of CN@NiCoS/NF. b Raman spectra of NiCoS (blue), CN@NiS (orange) and CN@NiCoS (red). c–e HRTEM image of CN@NiCoS (d is the enlarged area of the image in c). High-resolution XPS spectra of CN@NiS and CN@NiCoS (f) Ni 2p, (g) S 2p, (h) C 1 s.The surface compositions and electronic structures of CN@NiCoS and CN@NiS catalysts were investigated further using X-ray photoelectron spectroscopy (XPS), as depicted in Fig. 1f–h. In the Ni 2 P XPS spectrum of pristine CN@NiCoS (Fig. 1f), distinct Ni2+ and Ni3+ components are clearly identified with characteristic Ni 2p3/2 peak at 855.9 and 861.7 eV, respectively, indicative of a typical Ni3S2 phase18,19. Moreover, the Co 2p XPS profiles (Supplementary Fig. 2a) show deconvoluted peaks at 797.6 and 781 eV, accompanied by satellite peaks at 778.7 and 795.3 eV, demonstrate the presence of the Co2+ and Co3+ species in Co9S8 phase20,21. The characteristic peaks of S 2p3/2 and S 2p1/2 located at 161.7 and 162.8 eV can be ascribed to metal sulfur (M-S) bonds (Fig. 1g)22,23 in the Ni3S2 and Co9S8 phase. Notably, compared to CN@NiS, the upshift of Ni 2p along with the opposite downshift of S 2p in CN@NiCoS strongly infers electron transfer from Ni sites to adjacent S atoms, likely due to the formation of Ni3S2/Co9S8 heterojunction as observed in Fig. 1c and Supplementary 1c. The charge distribution at the Ni3S2/Co9S8 interface can significantly promote HER performance due to fast electron transfer24. The C 1 s spectra (Fig. 1h) reveals three distinct carbon species at 284.7, 285.7 and 288.5 eV, attributed to C-C/C = C, C-N/C-S and C = O species, respectively25. Furthermore, intensified N 1 s signals in the CN@NiCoS (Supplementary Fig. 2b)) further demonstrate the presence of doped nitrogen in the CN overlayer.HER performance in alkaline water and seawaterThe HER performance of various TM catalysts (Ni foam, NiCoLDH, NiCoS, CN@NiS, CN@CoS and CN@NiCoS) was evaluated using linear scan voltammogram (LSV) in 1 M KOH alkaline media. As shown in Fig. 2a and Supplementary Fig. 4a, the CN@NiCoS catalyst exhibits a remarkably low overpotential of only 4.6 mV to deliver a current density of 10 mA cm–2, which is far lower than that of NiCoS (37 mV), CN@CoS (58.9 mV), CN@NiS (59 mV), NiCoLDH (173.9 mV) and NF (198.8 mV). Furthermore, the CN@NiCoS catalyst also delivers low overpotential at higher current density (83.9 mV at η100, 173.6 mV at η500 and 236 mV at η1000). Tafel slope analysis (Fig. 2b and Supplementary Fig. 4b) shows that the CN@NiCoS catalyst achieves a lower Tafel slope (37.9 mV dec–1) compared to NiCoS (52.8 mV dec–1), CN@CoS (53.8 mV dec–1), CN@NiS (102.4 mV dec–1), NiCoLDH (91.2 mV dec–1) and NF (117.4 mV dec–1), indicating the dominant Volmer-Heyrovsky mechanism in the HER process.Fig. 2: Electrochemical performance of CN@NiCoS catalyst in alkaline water.HER performances of Ni foam (gray), NiCoLDH/NF (green), NiCoS/NF (blue), CN@NiS/NF (orange) and CN@NiCoS/NF (red) electrodes (electrode area: 1 cm × 1 cm) in 1 M KOH solution. a LSV curves with iR-corrected. b Tafel plots. c Plots of current density difference against scan rates. d Galvanostatic measurement at j = −100 mA cm−2.To further elucidate the electron transfer dynamics, electrochemical impedance spectra (EIS) measurement was conducted at stationary potential (Supplementary Figs. 6a, 4c). The CN@NiCoS catalyst exhibits the smallest charge transfer resistance (Rct) among all studied catalysts, implying faster charge transfer kinetics at the catalyst/electrolyte interface. The electrochemical activation surface area (ECSA) correlated linearly with the double-layer capacitance Cdl (Fig. 2c and Supplementary Fig. 4d). The CN@NiCoS catalyst exhibits a larger Cdl value (108.6 mF cm–2) compared to NiCoS (84.8 mF cm–2), CN@NiS (24.2 mF cm–2), CN@CoS (10.6 mF cm–2), NiCoLDH (2.88 mF cm–2) and NF (0.7 mF cm–2), unveiling exposure of more electrochemically active sites. To reveal the intrinsic HER activities, the activation energy (Ea) of studied electrocatalysts were compared (Supplementary Fig. 6b). According to Arrhenius equation, the Ea of CN@NiCoS (5.7 kJ mol–1) is lower compared to NiCoS (28.97 kJ mol–1), CN@NiS (21.12 kJ mol–1), NiCoLDH (36.4 kJ mol–1) and NF (11.88 kJ mol–1). Notably, the CN overlayer alone exhibits almost no HER activity (Supplementary Fig. 7). The enhancement of HER activity of NiCoS due to CN encapsulation strongly suggests the promotional effect of CN overlayer that modulates the electronic structure of NiCoS heterojunction for enhanced HER activity.Long-term stability is a prerequisite for the feasibility of practical application. We evaluated different electrocatalysts at a high current density of 100 mA cm−2 (Fig. 2d). Apparently, Ni foam and NiCoLDH require significantly higher overpotentials to maintain a constant current density. In comparison, CN@NiS exhibits a stable smooth polarization curve but with higher overpotential, while NiCoS catalyst initially shows low potential but rapidly deteriorates, resulting in unsatisfactory HER stability. Remarkably, the CN@NiCoS catalyst achieves long-term stability at 100 mA cm−2 over 1000 h in 1 M KOH electrolyte, with no distinct degradation and lower overpotential compared to other reported catalysts in the literatures7,24,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49, by comparing activity (e.g. A-CoB/Mxene ((η10 = 15 mV)48, Ni3Sn2-NiSn2Ox (η10 = 14 mV)28, and stability (e.g. GDY/MoO3 (120 h; 100 mA cm−2)37, MoC-Mo2C (1000 h; 30 mA cm−2)38 and C-Co-MoS2 (240 h; 100 mA cm−2)45) (Supplementary Fig. 8 and Tables 1, 2). The existence of CN layer improves catalyst stability, while the Ni3S2/Co9S8 heterostructure delivers high intrinsic activity through rapid electron transfer, which breaks the activity-stability trade-offs of conventional TMSs electrocatalysts.Furthermore, we systematically assessed the HER activity of CN@NiCoS catalyst in alkaline seawater electrolyte to demonstrate its excellent durability against the impurity ion. LSV curves (Fig. 3a) reveal that the CN@NiCoS requires only 8 mV overpotential to deliver 10 mA cm–2 current density in alkaline seawater media. Additionally, the CN@NiCoS catalyst exhibits a considerably low Tafel slope of 39.4 mV dec–1 (1 M KOH) and 68.9 mV dec–1 (1 M KOH + seawater), as shown in Fig. 3b. Notably, even at high current densities of 100, 500 and 1000 mA cm–2, CN@NiCoS maintains low overpotential of 79.8, 193.2 and 281 mV, respectively, in the corresponding alkaline seawater electrolyte (Fig. 3c), highlighting the promotional role of bifunctional Ni3S2/Co9S8 interface and CN overlayer in improving the HER activity of alkaline seawater. We observed that the intrinsic HER activity in alkaline seawater has an attenuated tendency compared to that in 1 M KOH electrolyte, likely due to the ion impurities50. Meanwhile, the CN@NiCoS catalyst also presents excellent stability in alkaline seawater, maintaining performance for over 400 and 200 h at 100 and 1000 mA cm−2 (Fig. 3d, e), reflecting the protective role of CN overlayers against electrolyte impurities and the great potentiality of CN@NiCoS for industrial seawater electrolysis. Supplementary Fig. 9 further demonstrates the reproducibility of the CN@NiCoS catalyst, in either 1 M KOH or 1 M KOH + seawater solutions. To further demonstrate the potential of the CN@NiCoS catalyst in practical operating conditions, we evaluated its electrochemical performance under various solvent environments and pH conditions (Supplementary Figs. 10, 11)51,52,53, and achieved the optimal HER activity in a 1 M KOH solution.Fig. 3: Electrochemical performance of CN@NiCoS catalyst in seawater.HER activity in 1 M KOH (red) and 1 M KOH + seawater (green). a HER polarization curves. b Tafel plots. c HER overpotentials at j = 10, 100, 500 and 1000 mA cm−2, respectively. Durability test in different electrolyte in 100 mA cm−2 (d) and in 1 A cm−2 (e).Dynamic sulfur migration of CN@NiCoS under working conditionTo unveil the surface reconstruction of catalytic active sites, state-of-the-art ex situ/in situ characterizations were employed to monitor the structure evolution of CN@NiCoS catalysts during the HER process. According to XRD and TEM results (Supplementary Fig. 12a, b), the bulk phase composition of the CN@NiCoS catalyst remain unchanged before and after HER process, suggesting the structural stability of heterojunction during reaction. This stability can be further supported by XPS survey spectra (Supplementary Fig. 12c), without noticeable changes in the Ni 2p, Co 2p, N 1s and C 1s spectra, while the S 2p spectrum shows a significant enhancement after HER process. As shown in Fig. 4a, unlike the pristine CN@NiCoS, the intensity of the S-O peaks at 168.8 eV increases significantly after HER, indicating more S incorporation into the electrode surface due to enhanced surface sensitivity of XPS techniques. The increased peak intensities of -C-SOx and C-N/C-S peaks in the C 1s spectrum after HER (Fig. 4b) further validate the migration of S atoms from the Ni3S2/Co9S8 heterojunction to the CN overlayer, forming a C-S bond54 during HER process. Meanwhile, the Ni 2p3/2 and Co 2p3/2 peaks of the CN@NiCoS catalyst post-HER (Supplementary Fig. 13) also shift to lower binding energy (BE) of 855.7 eV and 782.3 eV respectively, while the S 2p3/2 signals (Fig. 4a) distinctly shift towards a higher BE of 161.8 eV. These opposite shifts can be ascribed to the reduction of metal centers and the formation of S vacancies (Vs) on CN@NiCoS during the outbound migration of S atoms into the CN overlayers55. The generation of Vs on CN@NiCoS was further verified by the electron paramagnetic resonance (EPR) spectra (Fig. 4c), showing an enhanced EPR signal at g = 2.000 after HER process56,57. Similarly, the HAADF-STEM image of CN@NiCoS after HER (Fig. 4d) also reveals trapped S atoms on the outer CN overlayer, evident from enhanced Z-contrast57,58. The atomic arrangement of Co9S8 from the view of the (331) plane is illustrated in Fig. 4e. The green circles emphasize the crystal surface of Co9S8 (331) without deformation, while the red circles display the crystal surface of Co9S8 (331) is slightly deformed, as a characteristic feature indued by S vacancies in TMSs59. Additionally, the Ni3S2 (−111) also exhibits noticeable deformation with similar atomic rearrangement. Especially at the Ni3S2/Co9S8 interfacial region, the dim features of defected sites/missing atoms are clearly resolved as highlighted with dashed boxes, suggesting highly defective Ni3S2/Co9S8 interface (Fig. 4f and Supplementary Fig. 14) compared to the fresh sample before HER (Fig. 1d). This reflects the preferential formation of S vacancies at the Ni3S2/Co9S8 interface, leading to a more disordered lattice and missing atoms (dim features) at the interface60. Collectively, these findings provide compelling evidence for the migration of S atoms from the NiCoS surface to encapsulated CN overlayers, leaving S vacancies at the NiCoS heterointerface and S dopants in the CN overlayers through the forming covalent C-S bonds.Fig. 4: Identification of Sulfur migration at CN@NiCoS interface.S 2p (a) and C 1 s (b) XPS spectra of CN@NiCoS and NiCoS before and after 100 h stability test in 1 M KOH solution. c The electron paramagnetic resonance (EPR) of CN@NiCoS before and after HER stability. HAADF-STEM images of CN@NiCoS (d–f) after HER stability test. The green, yellow and red circles represent Co, S atoms and S vacancy respectively. In situ Raman spectra of CN@NiCoS (g) and CN@NiS (h) at different potential.To further verify the entrapment of sulfur within the CN layer, we measured the sulfur content in the electrolyte and on the surface of the NiCoS and CN@NiCoS catalyst after HER. The S 2p XPS spectrum (Fig. 4a) reveals that the CN@NiCoS exhibits a more distinct S-O component compared to NiCoS after HER, indicating a higher content of surface S species on the CN@NiCoS electrode due to CN encapsulation. Additionally, ICP-OES analysis further reveals a significant increase in S content in the electrolyte of NiCoS after HER, being eighteen times higher than that of CN@NiCoS (Supplementary Table 3), suggesting the irreversible sulfur leaching from NiCoS into the electrolyte during HER. This substantial loss of S atoms into the electrolyte may lead to severe collapse/deformation of the NiCoS structure after HER (Supplementary Fig. 15). Accordingly, we can thus conclude that the formation of Ni3S2/Co9S8 heterojunction in CN@NiCoS stimulates the S migration, which is subsequently captured by CN overlayers through the forming of C-S bonds. The as-formed S/CN@NiCoS-Vs pairing sites promote efficient charge transfer at the heterojunction, thereby retaining S migration into the electrolyte and improving both activity and long-term stability of the catalyst. Conversely, the NiCoS catalyst without a carbon protection layer continuously loses sulfur into the electrolyte during reaction, eventually leading to deactivation.The dynamic migration and trapping of S atoms on CN@NiCoS and CN@NiS during HER process were further observed by in situ Raman spectroscopy (experimental setup detailed in Supplementary Fig. 16). With the applied potential decreases to −0.5 V, the intensities of C-S and S = O bands of CN@NiCoS (Fig. 4g) at 739.4 and 1068.4 cm–1 simultaneously intensify compared to the open circuit potential (OCP)61. In contrast, the weak C-S and S = O bands on CN@NiS (Fig. 4h) manifests that the Ni3S2/Co9S8 heterostructure in CN@NiCoS is more favorable to stimulating sulfur migration. The strong C-S interaction further implies the inhibition of sulfur leaching into the electrolyte due to the encapsulation of CN overlayers, that improves long-term stability. The C = C bands of CN@NiCoS also shows a significant redshift of 15 cm−1, greater that of CN@NiS (9 cm−1), suggesting weakened C = C vibrations due to sulfur incorporation into the CN overlayers. Meanwhile, we also notice a slightly decreasing intensity of the C = N peak (Fig. 4g) along with the appearance of metal-N peak62 in CN@NiCoS after HER (Supplementary Fig. 17), indicating the sulfur incorporation may weaken the C = N interaction and enhance metal-N interaction with electron deficient Ni2+/Co2+ sites, strengthening the interfacial structures of CN@NiCoS towards the long-term stability.Mechanism for stability and activity enhancement during long-term HERTo understand surface reconstruction induced by S migration and its effect on electrocatalytic activity, we calculated the formation energy of S vacancies and migration pathways with corresponding energy barrier in various sulfides. The formation energy of S vacancies in the Ni3S2/Co9S8 heterostructure (−0.84 eV) is more negative compared to Ni3S2 (−0.20 eV) and Co9S8 (−0.51 eV) phases, indicating that S vacancy formation in the NiCoS heterostructure is thermodynamically favored over Ni3S2 and Co9S8 phases alone63,64. This observation is consistent with our STEM analysis, showing defective Ni3S2/Co9S8 interface on CN@NiCoS after HER (Fig. 4f and Supplementary Fig. 14). To investigate the S migration mechanism, we explored multiple pathways and corresponding energy barrier for Ni3S2, Co9S8 and NiCoS (Fig. 5b, c). The Ni3S2/Co9S8 interface exhibits the lowest migration energy barrier, manifesting more favorable sulfur migration in the NiCoS heterojunction. As depicted in Fig. 5b, the process begins with the activation and cleavage of the metal-S bond (S2), followed by the adsorption of free S atoms at the metal site (S3). These adsorbed S atoms then occupy adjacent defect sites, creating an S vacancy at the original position (S4), likely representing a transition state with the highest energy and an imaginary frequency (Supplementary Fig. 18). Subsequently, these atoms migrate across NiCoS (S5 and S6) and are ultimately trapped by the CN shell through the formation of an S-C bond (S7). Throughout this process, we infer that lattice mismatch/rearrangement at the Ni3S2/Co9S8 interface favors S vacancies formation (Figs. 4d–f, 5a) and thus facilitates the S migration along the heterojunction.Fig. 5: Density Functional Theory (DFT) calculations for CN@NiCoS heterostructure.a The formation energy of S vacancy for Ni3S2, Co9S8, NiCoS heterojuction. b Energy barrier of sulfur migration for CN@NiCoS heterostructure. c Corresponding sulfur migration trajectory in CN@NiCoS. Charge density difference of NiCoS (d) and CN@NiCoS (e). The isosurface value is 0.01 e/ Å3, where green and red contours represent the electron accumulation and loss, respectively. f The density of states (DOS) of CN@NiCoS and S/NC@NiCoS-Vs. Reaction free-energy diagram of Ni3S2 (gray), Co9S8 (orange), NiCoS (blue), NiCoS-Vs (green)and S/NC@NiCoS-Vs (red), for Water dissociation (g), and Hydrogen evolution (h).Figure 5d, e illustrates the charge density difference, revealing electron accumulation along the NiCoS interface and CN overlayer. The electron enrichment along the NiCoS interface and CN overlayer can promote the H* and H2O* adsorption, serving as catalytically active sites for HER42. This can be observed in Fig. 5f–h and Supplementary Fig. 19 that the NiCoS heterostructure possess stronger H2O adsorption ability (−0.79 eV), H2O dissociation energy barrier (0.57 eV), optimal hydrogen adsorption free energy (−0.14 eV) and higher density of states (DOS) intensity compared to Ni3S2 and Co9S8. We further explore the impact of Vs and S doping on HER activity by DFT calculations. By incorporating S vacancies, the NiCoS-Vs possess lower energy barrier (Fig. 5g, h), higher d-band center tower Fermi level and fast reaction kinetics process (Supplementary Fig. 20) compared to NiCoS alone. This result indicates that the presence of Vs in NiCoS heterostructures can regulate the orbital distributions (d-band center) with higher electron states near Fermi level and introduce more coordinatively unsaturated sites65,66, thereby adjusting H* adsorption and accelerating the HER kinetic process. Furthermore, the S and N doped C layer (S/N-C) displayed more optimized ΔGH2O and ΔGH* values compared to N-C (Supplementary Fig. 21e, f), which aligns with experimental observations that S/N-C displays rapid HER dynamic with lower overpotential, Tafel slope and charge transfer impedance (Supplementary Fig. 21a–c). This finding reveals that the formation of S-doped C after sulfur migration further improve hydrogen evolution activity due to the redistribution of the electronic structure.The dynamic trapping of sulfur atoms by the CN overlayer not only ensures the long-term stability of catalyst, but also enhances its electrocatalytic activity. Comparing with pristine CN@NiCoS, the promotional role of S/NC@NiCoS-Vs was further elucidated by DFT calculations. Figure 5f illustrates that S/NC@NiCoS-Vs exhibits significantly higher DOS intensity and d-band center close to Fermi energy level than CN@NiCoS. Moreover, S/NC@NiCoS-Vs demonstrates stronger H2O adsorption (−1.68 eV), optimal H2O dissociation (0.26 eV) and H adsorption energy (−0.08 eV) compared to CN@NiCoS (Fig. 5g, h), indicating accelerated kinetics for water dissociation and hydrogen evolution facilitated by S-Vs pair formation. The role of S-Vs pair may be attributed to the creation of localized states by S vacancies, which induces a strong d-d orbital overlap (Ni 3d and Co 3d). Additionally, S-doped C activates additional d electron states near the Fermi level, promoting charge transfer and H* adsorption for enhanced HER activity. Furthermore, the as-formed C-S bridge bond, thus providing pathways for more available electrons migration, to participate in HER reaction and achieve high activity and stability alkaline HER67.
