English

• Electrocatalytic water splitting provides an efficient and sustainable way to produce high-purity H₂ [1–3]. For a practical application, the catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) should be coupled in an electrolyzer to drive the water splitting [4–6]. There is an intensive need to develop low-cost HER and OER catalysts for replacing state-of-the-art noble metal-based catalysts [7,8]. The integration of different HER and OER catalysts can bring inconvenience in the material preparation and the use of the cell [9]. The two electrodes may interfere with each other due to dissolution and redeposition during the catalytic reaction [10–12]. The usual bifunctional catalysts can only give good activity in one half of the reaction, and thus show only moderate overall water splitting performance [13–15], although they can overcome the above problems. Thus, designing a good HER and OER catalyst with minimal mutual interference is important to achieve efficient overall water splitting. Transition metal interstitial compounds (TMICs) from inserting C, N, and P atoms into interstitial sites of the metal are a class of promising catalytic materials. Generally, the early TMICs (ETMICs) (Mo_xC, Mo_xN, etc.) stand out owing to their noble metal-like electronic structure, high conductivity, and low free energy of H adsorption [16,17]. Zhou et al. in-situ synthesized molybdenum carbide (MoC) quantum dots onto N-doped porous carbon, which is conducive to optimizing the adsorption energy of atomic hydrogen to greatly increase the HER electrocatalytic activity [18]. The late TMICs (Co_xN, etc.) are suitable as OER catalysts [19,20]. In the alkaline electrocatalytic oxidation process, the surface of late TMICs is easily oxidized, forming hydroxide layers through a rapid surface reconstruction on the nitride core. The activity of hydroxide for OER and the good electrical conductivity of TMICs synergistically play a role in enhancing catalytic activity [21,22]. Chen’s group reported metallic Co₄N porous nanowire arrays, demonstrate the active phases are the metallic Co₄N core inside with a thin cobalt oxides/hydroxides shell during the OER process, consequently enhancing the intrinsic catalytic activity of OER [23]. As known, the construction of heterojunctions can produce a more favorable interface by electron transfer for the adsorption/activation of active species, thus enabling to boost the intrinsic activity [24,25]. In particular, heterojunctions composed of early transition metals (ETMs) and late transition metals (LTMs) are the most promising for water splitting due to the “3d electron complementary effect” [26]. The NiMo/NiMoO_x, Ni₃N/VN, and CoS_x@Cu₂MoS₄ have been demonstrated to exhibit improved electrocatalytic activities for HER or OER [27–29]. Therefore, based on the important influence of anion regulation on catalytic activity, it is advantageous to design transition metal synergistic heterostructures that can provide more active sites. This strategy can lead to the development of highly active and easily coupled catalysts for OER and HER, thereby enhancing the overall electrochemical water decomposition activity.

  Here, we have reported the synthesis of two 1D Co-Mo based heterojunctions for coupled HER and OER, respectively, by a pyrolysis of Co-Mo based nanowires (NWs) under different atmospheres. The selection of 1D Co-Mo based nanowires as precursor is based on their potential for large-scale preparation and for producing 1D TMICs with favorable conductivity. The MoC–Co/NWs from a pyrolysis under N₂ atmospheres delivers good activity for HER, affording a current density of 10 mA/cm² (η₁₀) with a low overpotential of 39 mV. On the other hand, the CoMoN–CoN NWs obtained under NH₃ atmospheres showed enhanced OER performance with a η₁₀ of 260 mV. Density functional theory calculations show that the heterojunction construction optimizes the adsorption free energy of hydrogen, thereby essentially enhancing HER activity. The work function proved that the electron transfer in MoC/Co (CoMoN/CoN) heterojunction can introduce electron redistribution which improved its HER (OER) activity consequently. The two catalysts can be easily coupled to assemble a two-electrode cell with a solar-to-hydrogen efficiency of 12.3% at 1.54 V [30].

  Scheme 1 illustrates a process for synthesizing MoC–Co and CoMoN–CoN heterojunctions nanowires. Firstly, the 1D Co-Mo based nanowires were prepared by a coordination process of 2-methylimidazole (2-MIM) and cobalt ions regulated by molybdate ions (Fig. S1 in Supporting information). However, without the regulation of MoO₄²-, the coordination of 2-MIM with Co²⁺ can only form polyhedral structures ZIF-67. X-ray diffraction (XRD) analyses show the slightly different diffraction pattern of the Co-Mo nanowires from the ZIF-67 precursor (Fig. S2 in Supporting information), indicating that Mo successfully introduced the ZIF-67. The formation of the one-dimensional structure may be due to the MoO₄^2- ions restricting the coordination of Co²⁺ and 2-MIM under the conditions of slow drip addition [31,32]. The droplet addition has an important effect on the morphology of the precursor, and the nanowires with controllable diameters and lengths can be obtained by changing the droplet acceleration rate, while adding directly obtained aggregated irregular morphology (Fig. S1). Then the precursors were subjected to a pyrolysis under N₂ and NH₃ atmosphere, respectively, to form MoC–Co NWs and CoMoN–CoN NWs [27].

  Scheme 1

  

  Scheme 1.  Illustration of the synthetic procedures for MoC–Co Heterojunction NWs and CoMoN–CoN Heterojunction NWs.

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  The XRD pattern of MoC–Co NWs resultant catalysts was revealed in Fig. 1a. These peaks at 44.2, 51.5° and 75.9° were associated with metallic Co (PDF #15–0806), while the peaks at 36.4°, 42.3°, and 61.4° can be indexed to the (111), (220), and (311) planes of MoC (PDF #89–2868), demonstrating that the nanowire precursors were transformed into MoC–Co composites by pyrolysis. The scanning electron microscopy (SEM) images (Fig. 1b) clearly revealed that the as-formed MoC–Co sample still exhibited uniform 1D nanowire morphology with an average length of about 500 nm. As a contrast, the pyrolyzed ZIF-67 (Co-NC) has an irregular polyhedral morphology (Fig. S3 in Supporting information), and the XRD pattern shows only peaks corresponding to metal Co (Fig. S4 in Supporting information). The transmission electron microscopy (TEM) images (Figs. 1c and d) further demonstrated the nanowires structure of the MoC–Co sample. Meanwhile, many small size nanoparticles with sizes of 3–5 nm can be observed on the nanowire.

  Figure 1

  

  Figure 1.  The (a) XRD image, (b) SEM images (c, d) TEM images, (e, f) HRTEM images, (g) STEM and EDS images of MoC–Co NWs. High-resolution XPS spectra of (h) Co 2p, and (i) Mo 3d of MoC–Co NWs.

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  The presence of such small size nanoparticles facilitates the exposure of more active sites. The high-resolution TEM (HRTEM) images (Figs. 1e and f) show a well-defined lattice fringe with an interplanar spacing of 0.20 nm adjacent to the small-size nanoparticles, corresponding to the (111) plane of Co. The crystal plane spacing of 0.25 nm corresponded to the (111) crystal plane of MoC. The red interface implies the formation of MoC–Co heterojunction. The intimate interfaces formed by the intersecting Co and MoC planes were discerned, which is favorable for promoting charge transfer between the Co nanoparticle clusters and MoC to provide low hydrogen adsorption energy sites at the interface, enhancing the hydrogen evolution performance of the catalyst. The scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectrometer (EDS) elemental mapping images of MoC–Co NWs (Fig. 1g) illustrate the uniform distribution of Mo, Co, and C elements throughout the nanowires. The above characterizations verified the successful synthesis of MoC–Co heterostructure NWs catalysts. To gain further insights into the chemical composition and valence states of MoC–Co NWs, X-ray photoelectron spectroscopy (XPS) measurements were carried out. The survey spectrum (Fig. S5 in Supporting information) shows the presence of Mo 3d, C 1s, N 1s, O 1s, and Co 2p peaks for MoC–Co NWs. In the C 1s spectrum (Fig. S6a in Supporting information), the peak at 284.2 eV was attributed to C-Mo bonds, corresponding to the MoC component in MoC–Co NWs. The peak at 284.9 eV was ascribed to C–C/C=C bonds. The peak at 286.2 eV was assigned to C–N bonds, which together with the Graphitic-N peak at 400.8 eV in the N 1s spectrum (Fig. S6b in Supporting information), demonstrated the existence of nitrogen-doped carbon. Nitrogen-doped carbon mainly stems from the pyrolysis of 2-MIM, and the doped N atoms can introduce more defects in the carbon framework, further enhancing its conductivity and catalytic performance [33]. Finally, the peak at 288.6 eV corresponded to C=O bonds, likely due to slight surface oxidation [34]. In the fitted high-resolution spectra of N 1s (Fig. S6b), the peaks at 397.4 eV and 398.6 eV were attributed to pyrrolic-N and pyridinic-N, respectively [35]. For the fine-scanned Co 2p spectrum shown in Fig. 1h, the peaks located at 778.7 eV and 793.2 eV corresponded to metallic Co⁰, which were assigned to the pyrolyzed Co. The peaks around 780.3 eV and 796.1 eV were indexed to Co²⁺, along with the peaks at 781.8 eV and 797.5 eV are ascribed to Co³⁺ [36]. The presence of Co²⁺ and Co³⁺ indicated slight surface oxidation of the sample [37]. The other two peaks corresponded to the satellite peaks. With regard to Mo 3d regions (Fig. 1i), the peaks at 230.0 eV and 233.4 eV were assigned to Mo²⁺, consistent with MoC in the XRD patterns. The Peaks at 231.8 eV and 234.0 eV correspond to Mo⁴⁺, while peaks at 232.5 eV and 235.6 eV were attributed to Mo⁶⁺ [38], which was associated with the partial surface oxidation of the catalysts upon air exposure [39,40]. All of the above tests show the formation of Co-Mo based TMICs with C as the anion.

  The CoMoN–CoN heterostructure nanowire structures were obtained by pyrolysis of the precursor to change the anion to N in ammonia atmosphere. The XRD pattern in Fig. 2a confirms the diffraction peaks of CoMoN–CoN NWs matched well with standard CoMo₄N₅ (PDF #65–8957) and Co_5.47N (PDF #41–0943), respectively. No discernible impurities were found by XRD. It can be seen from the SEM images that after high-temperature nitridation, the samples can basically maintain the nanowire structure with slight agglomeration (Fig. 2b). In contrast, the XRD image of nitrided ZIF-67 precursors (CoN–NC) reveals diffraction peaks of Co_5.47N (Fig. S7 in Supporting information), which have a morphology of agglomerated nanoparticles in the SEM image (Fig. S8 in Supporting information). The TEM images of CoMoN–CoN nanowires (Fig. 2c) also show a slight aggregation. The lattice where Co_5.47N (111) and CoMo₄N₅ (101) are in close contact can be clearly observed in the HRTEM images (Figs. 2d and e), showing the formation of the CoMoN–CoN heterojunction. Furthermore, STEM and the corresponding elemental mappings indicate Mo, Co, and N elements distributed uniformly throughout the whole nanowire heterostructures (Fig. 2f). All these observations confirm the successful preparation of well lattice-matched heterointerfaces. The presence of heterogeneous interfaces facilitates optimized electron distribution between the components, inducing synergistic effects to enhance the OER activity of the catalyst. For the CoMoN–CoN heterojunction nanowires, the high-resolution N 1s XPS spectrum in Fig. 2g shows peaks at 398.0, 399.2, and 400.2 eV corresponding to pyrrolic-N, pyridinic-N, and graphitic-N, while the peak at 396.8 eV is ascribed to metal-N bonds [41], matching the Mo-N and Co-N bonds in Co_5.47N and CoMo₄N₅. It is worth mentioning that in the C 1s XPS spectrum (Fig. S9 in Supporting information), peaks at 286.4 eV attributed to C–N bonds, which proved the existence of nitrogen-doped carbon nanoframe. In the Co 2p spectrum (Fig. 2h), the peaks at 780.6 and 796.6 eV are assigned to Co-N bonds in Co_5.47N and CoMo₄N₅, whereas peaks at 781.8 and 797.3 eV originate from Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2 due to slight surface oxidation upon air exposure [42,43]. For CoMoN–CoN NWs, the Mo 3d spectrum (Fig. 2i) shows three doublet peaks corresponding to the Mo³⁺ (230.2/233.7 eV), Mo⁴⁺ (231.9/235.2 eV) and Mo⁶⁺ (232.7/235.9 eV) [44]. The peaks at Mo⁴⁺ and Mo⁶⁺ are attributed to oxidized Mo species due to the minor surface oxidation, while the Mo³⁺ peaks assigned to the Mo species in Mo-N recognized [45]. The above analyses show the successful formation of Co-Mo based carbides and nitrides.

  Figure 2

  

  Figure 2.  The (a) XRD image, (b) SEM images (c) TEM images, (d, e) HRTEM images, (f) STEM and EDS images of CoMoN–CoN NWs. High-resolution XPS spectra of (g) N 1s, (h) Co 2p, and (i) Mo 3d for CoMoN–CoN NWs.

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  The HER performance of the as-prepared MoC–Co heterostructures nanowires was first evaluated in alkaline solution (1 mol/L KOH) using a typical three-electrode system. Pt/C, Co-NC, and MoC–Co nanowires prepared at different nitridation temperatures were also tested for comparison. As shown in Figs. 3a and b, the optimal MoC–Co NWs exhibit lower overpotential (39 mV) at 10 mA/cm² than Co-NC (125 mV) prepared without molybdate, proving that the construction of heterojunction was advantageous for HER. The effect of carbonization temperature on the electrocatalytic activity of the catalyst was investigated. At the same current density of 10 mA/cm², the MoC–Co NWs prepared by pyrolysis at 550 ℃ had the smallest overpotential, outperforming MoC–Co NWs-4 (74 mV) and MoC–Co NWs-6 (54 mV). For practical industrial applications, the MoC–Co NWs under an extremely large current density of 200 mA/cm² only require an overpotential of 248 mV, and surpass commercial Pt/C at current densities above 201 mA/cm² (overpotential >249 mV). The performance of electrocatalysts at large current densities is critical. The HER kinetics of the MoC–Co catalysts were analyzed by deriving the Tafel slopes from LSV (Fig. 3c). The MoC–Co NWs exhibited an ultra-low Tafel slope of 40.5 mV/dec, only slightly higher than 20.8 mV/dec for Pt/C. This implies sufficient proton adsorption on the catalyst surface, with the Volmer-Heyrovsky mechanism governing the HER. Hence, the outstanding reaction kinetics of MoC–Co NWs are corroborated. Additionally, the excellent HER kinetics of MoC–Co NWs were further validated by electrochemical impedance spectroscopy (EIS), as manifested in the fitted Nyquist plots (Fig. S10 in Supporting information). MoC–Co NWs displayed an R_ct of 1.0 Ω, much lower than 18.0 Ω for Co-NC, suggesting that an ultrafast interface electronic transmission contributes to the enhanced conductivity via electronic interaction effects at the interface. The double layer capacitance (C_dl) was derived from CV curves at different scan rates in the 0.15–0.25 V vs. RHE region (Fig. S11 in Supporting information) as shown in Fig. 3d. MoC–Co NWs exhibited the highest C_dl of 7.99 mF/cm², indicating that the constructed heterojunction was conducive to exposure of the active site. To validate this rationale, the turnover frequency (TOF) was examined to further evaluate the intrinsic activity (Fig. S12 in Supporting information). At an overpotential of 100 mV, MoC–Co NWs displayed a TOF of 0.35 s^-1, which is dramatically larger than that of Co-CN (0.031 s^-1), indicating its excellent intrinsic activity. Considering the cost requirements for practical electro-catalytic hydrogen evolution, stability is another important criterion for evaluating HER electrocatalysts. As shown in Fig. 3e, the polarization curves of MoC–Co NWs after 3000 CV cycles remain nearly identical profiles. The SEM image after CV testing (Fig. S13 in Supporting information) also verified well-retained nanowire morphology. Furthermore, the I-t curve clearly shows that the MoC–Co NWs heterostructure could continuously drive a current density of around 10 mA/cm² with very little attenuation within 24 h (inset of Fig. 3e). Finally, multipotential step chronopotentiometry (CP) curves were measured for MoC–Co NWs from 10 mA/cm² to 100 mA/cm² (Fig. 3f). With the current density incrementally increased, the overpotential steadily stepped up accordingly, indicative of superior mass transport properties. When the current density returned to 10 mA/cm² at the end, only 5% overpotential loss occurred compared to the start. Collectively, Fig. 3g further validates the catalytic performance of MoC–Co NWs by benchmarking against other recently reported alkaline hydrogen evolution electrocatalysts (references listed in Table S1 in Supporting information). Evidently, the above results demonstrate the electron transfer in the heterojunction to improve the HER activity.

  Figure 3

  

  Figure 3.  Electrochemical characterization of the catalysts: (a) Polarization curves, (b) overpotential at 10 mA/cm² and 200 mA/cm², (c) Tafel slopes, (d) double layer capacitance, (e) polarization curves of MoC–Co NWs before and after 3000 CV cycles, inset: 24 h I-t curve, (f) CP curves, and (g) benchmarking against other reported Mo, Co-based catalysts.

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  To gain insights into the representative roles of hetero-structures in determining the outstanding HER performance of MoC–Co NWs, density functional theory (DFT) calculations were carried out. Based on structural characterizations and experimental results, three theoretical models were constructed including pure Co metal, MoC, and MoC–Co heterojunctions (Fig. 4a). The electron density map of MoC–Co is shown in Fig. 4b, where the charge density variations indicate strong electronic interactions at the interface between MoC and Co with accumulation of electrons. The influences of MoC–Co heterojunctions on hydrogen evolution were further verified by comparing the absolute value of ΔG_H* (|ΔG_H*|) at different sites (Fig. 4c). On pure Co metal, site 1 has a |ΔG_H*| of 0.72 eV, suggesting difficulty in H* desorption and the lowest activity among all sites, unsuitable for HER. In the MoC–Co model, the Co site has a markedly decreased |ΔG_H*| of 0.59 eV, indicating the MoC–Co synergistic effects facilitate enhanced HER performance. Importantly, the Mo site in MoC–Co possesses the smallest |ΔG_H*| of 0.16 eV which is much smaller than the 0.45 eV on the Mo site in MoC, confirming the Mo point at the MoC–Co interfaces are efficient active sites for hydrogen evolution. Furthermore, the continuous density of states (DOS) distribution near the Fermi level (Fig. 4d) indicates the MoC–Co is metallic, facilitating high electronic conductivity [46]. The above computations elucidate that the MoC–Co NWs effectively modulates the electronic structure and ΔG_H* via synergy between early and late transition metals, thereby substantially enhancing the HER catalysis, consistent with the electrochemical results.

  Figure 4

  

  Figure 4.  (a) Models of MoC, Co, and MoC–Co heterojunction, (b) differential charge densities of MoC–Co heterojunction, blue and yellow contours represent electron accumulation and depletion, respectively. (c) Free-energy diagram of ΔG_H*, and (d) DOS of MoC, Co, and MoC–Co heterojunction.

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  In addition, CoMoN–CoN heterostructures NWs were constructed by anion regulation of Co-Mo based transition metal interstitial compounds, and its OER electrocatalytic activities were also evaluated using a three-electrode configuration in a 1 mol/L KOH solution. As depicted in Figs. 5a and b, CoMoN–CoN NWs required an overpotential of only 260 mV to reach 10 mA/cm² with a Tafel slope of 91.3 mV/dec, outperforming RuO₂ (383.2 mV, 160.1 mV/dec), CoN–NC (331 mV, 132.3 mV/dec) and many Co-Mo based OER catalysts (Table S2 in Supporting information). In addition, the results show that a suitable nitriding temperature is critical for generating OER catalysts with excellent performance.

  Figure 5

  

  Figure 5.  Electrochemical characterization of the catalysts: (a) Polarization curves, (b) Tafel slopes, (c) EIS for OER, (d) work function (WF) drawings of MoC–Co NWs and CoMoN–CoN NWs. (e) Overall water splitting polarization curve, and (f) I-t curve. (g) Collected H₂ and O₂ vol over time, (h) H₂ and O₂ collection setup, inset: H₂ and O₂ vol after 40 min, (i) solar powered electrolysis water device.

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  EIS tests shown in Fig. 5c also evidenced lower electrochemical impedance and thus higher charge transfer efficiency for CoMoN–CoN NWs. Additionally, C_dl values were derived from the CV scans between 0.1–0.2 V vs. RHE to determine the electrochemically active surface area (ECSA) (Fig. S14 in Supporting information). The highest C_dl value of CoMoN–CoN NWs (103.9 mF/cm²) among all catalysts implies the most abundant active site loading on its surface for OER (Fig. S15 in Supporting information). In addition, as shown in Fig. S16 (Supporting information), the OER activity of CoMoN–CoN NWs is significantly better than that of the prepared carbide catalyst (MoC–Co NWs), and the carbide catalyst has better HER activity. Therefore, it is of great significance to regulate HER and OER activities of transition metal interstitial compound heterojunctions by anion-modulation strategy to achieve efficient overall water decomposition. To further reveal the structure-performance correlation, the Scanning Kelvin Probe (SKP) technique was performed to determine the work function. It was proved that there is a Mott-Schottky effect between the MoC/Co (CoMoN/CoN) heterojunction which improved its HER (OER) activity consequently in Fig. S17 (Supporting information) [47]. Furthermore, in Fig. 5d, compared with the 5.63 eV work function of CoMoN–CoN NWs, the smaller value (5.58 eV) of MoC–Co NWs improves its ability to donate electrons. As the HER is highly dependent on the ability of the electrocatalyst to donate electrons, the HER on the MoC–Co NWs surface is boosted, which explains why the MoC–Co NWs has an optimized HER activity than CoMoN–CoN NWs [48].

  Capitalizing on the excellent electrocatalytic performance of MoC–Co NWs and CoMoN–CoN NWs, a MoC–Co NWs//CoMoN–CoN NWs two-electrode system was assembled for overall water splitting. The catalyst will dissolve and redeposition during overall water splitting reaction, and the different metal types of the two electrodes will interfere with each other. The Co-Mo based transition metal interstitial compounds constructed by adjusting anions in this paper do not have the above problems. As shown in Fig. 5e, the electrolyzer delivered a current density of 10 mA/cm² for overall water electrolysis when the applied voltage reached 1.54 V. This performance is superior to that of Pt/C//RuO₂. Moreover, no obvious degradation was observed during the 24 h stability test at 10 mA/cm² in Fig. 5f, indicative of favorable overall stability. Additionally, an H-cell device depicted in Fig. 5h was constructed to determine the Faraday efficiency of the overall water splitting system, where the anode and cathode were separated by a Nafion membrane. The actual volumes of generated hydrogen and oxygen were collected by a drainage method (Fig. S18 in Supporting information). As displayed in Fig. 5g, the recorded H₂ and O₂ vol over time were used to calculate an H₂:O₂ volumetric ratio of 2:0.997, closely matching the theoretical value. This signifies that the Faraday efficiencies (FE) of both electrodes in the MoC–Co NWs//CoMoN–CoN NWs system reached 100%. In light of the overall performance, an integrated water splitting electrolyzer powered by a small solar cell panel was assembled for the MoC–Co NWs//CoMoN–CoN NWs two-electrode system (Fig. 5i). Prominent bubbling was observed on both electrodes at an applied voltage of 1.542 V with this device.

  In conclusion, we create superior and easily coupled HER and OER catalysts for overall water splitting by anion modulation of prepared Co-Mo nanowire precursors. The MoC–Co nanowires delivered HER at 10 mA/cm² with an ultralow overpotential of 39 mV and a small Tafel slope of 40.5 mV/dec, indicating outstanding HER catalytic activity and kinetics. DFT calculations elucidated that Co doping can effectively modulate the electronic structure of MoC, optimize hydrogen adsorption free energy, thereby enhancing the intrinsic HER activity of MoC–Co nanowires. On the other hand, CoMoN–CoN nanowires also exhibited excellent OER activity, affording 10 mA/cm² at a low overpotential of 260 mV. Assembling the MoC–Co and CoMoN–CoN nanowires into a two-electrode system realized solar-driven water splitting with efficiency of 12.3% by 1.54 V. This work paves a new avenue to explore the rational design of efficient and easily coupled HER and OER catalysts for water splitting system.

  Declaration of competing 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.

  CRediT authorship contribution statement

  Gen Zhang: Writing – original draft, Data curation. Ying Gu: Writing – review & editing, Methodology. Lin Li: Visualization. Fuli Ma: Supervision. Dan Yue: Software. Xiaoguang Zhou: Writing – review & editing, Methodology. Chungui Tian: Writing – review & editing.

  Acknowledgments

  We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (No. 91961111), the Natural Science Foundation of Heilongjiang Province (No. ZD2021B003), and Fundamental Research Funds for the Central Universities (No. 2572022BU05).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110110.

  
  
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  Scheme 1  Illustration of the synthetic procedures for MoC–Co Heterojunction NWs and CoMoN–CoN Heterojunction NWs.

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  Figure 1  The (a) XRD image, (b) SEM images (c, d) TEM images, (e, f) HRTEM images, (g) STEM and EDS images of MoC–Co NWs. High-resolution XPS spectra of (h) Co 2p, and (i) Mo 3d of MoC–Co NWs.

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  Figure 2  The (a) XRD image, (b) SEM images (c) TEM images, (d, e) HRTEM images, (f) STEM and EDS images of CoMoN–CoN NWs. High-resolution XPS spectra of (g) N 1s, (h) Co 2p, and (i) Mo 3d for CoMoN–CoN NWs.

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  Figure 3  Electrochemical characterization of the catalysts: (a) Polarization curves, (b) overpotential at 10 mA/cm² and 200 mA/cm², (c) Tafel slopes, (d) double layer capacitance, (e) polarization curves of MoC–Co NWs before and after 3000 CV cycles, inset: 24 h I-t curve, (f) CP curves, and (g) benchmarking against other reported Mo, Co-based catalysts.

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  Figure 4  (a) Models of MoC, Co, and MoC–Co heterojunction, (b) differential charge densities of MoC–Co heterojunction, blue and yellow contours represent electron accumulation and depletion, respectively. (c) Free-energy diagram of ΔG_H*, and (d) DOS of MoC, Co, and MoC–Co heterojunction.

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  Figure 5  Electrochemical characterization of the catalysts: (a) Polarization curves, (b) Tafel slopes, (c) EIS for OER, (d) work function (WF) drawings of MoC–Co NWs and CoMoN–CoN NWs. (e) Overall water splitting polarization curve, and (f) I-t curve. (g) Collected H₂ and O₂ vol over time, (h) H₂ and O₂ collection setup, inset: H₂ and O₂ vol after 40 min, (i) solar powered electrolysis water device.

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