Rational Construction of Honeycomb-like Carbon Network-Encapsulated MoSe2 Nanocrystals as Bifunctional Catalysts for Highly Efficient Water Splitting

The scalable fabrication of cost-efficient bifunctional catalysts with enhanced hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance plays a significant role in overall water splitting in hydrogen production fields. MoSe2 is considered to be one of the most promising candidates because of its low cost and high catalytic activity. Herein, hierarchical nitrogen-doped carbon networks were constructed to enhance the catalytic activity of the MoSe2-based materials by scalable free-drying combined with an in situ selenization strategy. The rationally designed carbonaceous network-encapsulated MoSe2 composite (MoSe2/NC) endows a continuous honeycomb-like structure. When utilized as a bifunctional electrocatalyst for both HER and OER, the MoSe2/NC electrode exhibits excellent electrochemical performance. Significantly, the MoSe2/NC‖MoSe2/NC cells require a mere 1.5 V to reach a current density of 10 mA cm−2 for overall water splitting in 1 M KOH. Ex situ characterizations and electrochemical kinetic analysis reveal that the superior catalytic performance of the MoSe2/NC composite is mainly attributed to fast electron and ion transportation and good structural stability, which is derived from the abundant active sites and excellent structural flexibility of the honeycomb-like carbon network. This work offers a promising pathway to the scalable fabrication of advanced non-noble bifunctional electrodes for highly efficient hydrogen evolution.


Introduction
With the rapid expansion of energy demand and serious environmental pollution, clean energy has attracted much attention in recent years.Hydrogen is regarded as one of the most potentially clean energy sources due to its environmental friendliness and abundant sources [1][2][3][4][5][6].With the advantage of abundant resources, no pollution and nontoxicity, the electrocatalytic pyrolysis of water is the best way to produce hydrogen.However, the hydrogen evolution reaction (HER) during the electrolysis of water usually suffers from several serious drawbacks, such as high energy consumption and low efficiency, which limits its commercial application [7][8][9][10][11].Therefore, it is necessary to develop catalytic materials to facilitate the HER performance during electrolysis of water.Pt-based materials have the best catalytic performance for HER, but the high cost limits their large-scale application [12][13][14][15][16].
Transition metal selenides (TMS) are considered to be one of the best alternatives because of their excellent electrical conductivity, low ionization energy, low hydrogen adsorption barrier, and high electrochemical activity [17][18][19][20].Among them, MoSe 2 has been widely studied by researchers.For example, Setayeshgar et al. prepared MoSe 2 using Na 2 SeO 3 and MoCl 5 as the Se and the Mo sources, respectively [21].Zhao et al. synthesized the non-defective MoSe 2 by a hydrothermal method with an overpotential of 364 mV at 10 mA cm −2 and Tafel slope of 112 mV dec −1 [22].However, the relatively low intrinsic conductivity and structural stability of pure MoSe 2 materials usually lead to their unfavorable catalytic properties.Qian et al. reported the fabrication of Zn-doped MoSe 2 nanosheets synthesized by a one-step hydrothermal method.When tested in 0.5 M H 2 SO 4 , the Zn-doped MoSe 2 had a favorable catalytic performance [23,24].The purpose of full water electrolysis is to evaluate the efficiency and stability of the catalyst in the actual water electrolysis process and its performance in long-term operations.This is crucial to the development of efficient and low-cost water electrolytic hydrogen production technology, especially in promoting the wide application of hydrogen energy as a clean energy source.
Many strategies have been developed to solve the above-mentioned disadvantages of MoSe 2 -based materials, such as doping, carbon coating and constructing porous structures [25][26][27][28].The doped metal cation ions in the MoSe 2 can enhance the electron transportation within the active materials during the HER process.Qian et al. [29] reported the production of Zn-doped MoSe 2 nanosheets synthesized by a one-step hydrothermal method.When tested in 0.5 M H 2 SO 4 , the Zn-doped MoSe 2 exhibited an overpotential of 231 mV at 10 mA cm −2 , lower than that of the undoped pure MoSe 2 .Additionally, carbon matrices have excellent electric conductivity, mechanical flexibility and stable physical and chemical properties.When compositing carbon with pure MoSe 2 , the overall electron conductivity of the composite could be improved.Moreover, the protective carbonaceous layers can avoid direct contact between the MoSe 2 and the electrolyte, therefore alleviating the structural collapse and surface side reactions of active materials.For instance, Xu et al. [30] fabricated core-shell MoSe 2 /C nanospheres by a hydrothermal method.Glucose-derived amorphous carbon could effectively enhance the conductivity and reduce the layers thickness of the MoSe 2 , leading to a low current density of 57.5 mA cm −2 at an overpotential of 200 mV.Liu et al. [31] synthesized flake MoSe 2 /C composites by a simple solid phase method.The carbon acting as a conductive matrix in the MoSe 2 /C composite material can not only facilitate electron transfer within the composites but also improve its structural stability and hinder the aggregation of MoSe 2 nanosheets.Ren et al. [32] reported the fabrication of C-@MoSe 2 composite by a sol-gel method, showing a potential of 170 mV at 10 mA cm −2 and a slope of 72 mV dec −1 .
Heteroatom doping in carbon materials can create many defects, expand the specific surface area, enlarge the active sites and improve the electrical conductivity, which is beneficial to the enhancement of HER performance [33].Furthermore, the rational design of micro/nanostructure such as a porous structure, hierarchical structure or core-shell structure can also improve the specific surface area, shorten the ion and electron transport pathway and facilitate the electrolyte penetration.Importantly, the continuous threedimensional (3D) carbonaceous network ensures the rapid transmission of electrons in all directions to improve the overall electrical conductivity of the composites.Moreover, the 3D conductive network acts as protective matrix that can buffer the volume changes in the inner active materials and guarantee excellent structural integrity of the composite material.Therefore, constructing a continuous 3D carbonaceous network could be a promising method to improve the HER performance of the MoSe 2 -based materials [34].
Herein, the honeycomb-like MoSe 2 /NC composite was prepared by a freeze-drying method followed by an in situ selenization process.The ion and electron conductivity and structural stability of the MoSe 2 /NC have been effectively improved by the continuous honeycomb-like carbonaceous network.Moreover, the nitrogen-doped carbon matrix has numerous defects and a high specific surface area, further increasing the electrochemical performance of the obtained samples.When tested at 1 M KOH, the MoSe 2 /NC delivers overpotentials of 153 mV and 180 mV for HER and OER at 10 mA cm −2 , respectively.Moreover, the MoSe 2 /NC∥MoSe 2 /NC cell requires a potential of only 1.5 V to reach the current density of 10 mA cm −2 for overall water splitting.

Results and Discussion
The unique construction of the honeycomb-like carbon skeleton-encapsulated MoSe 2 / NC was synthesized by a facile freeze-drying method combined with the in situ selenization process.As shown in Figure 1a, the ammonium molybdate, PVP, urea and NaCl were mixed in a one-step freeze-drying process to form a polymer framework precursor.After that, the precursor underwent a facile selenization process at 600 • C in an Ar atmosphere.During this stage, the polymer framework was transformed into an N-doped carbonaceous network with the participation of urea.Significantly, nitrogen atoms replace carbon atoms, creating lattice defects (Figure 1b), whereas the inner ammonium molybdate was in situ converted into a MoSe 2 phase.Furthermore, abundant orderly macropores with honeycomb-like features can be constructed within the composite by just removing the NaCl.The unique honeycomb-like conductive network could be helpful to the ion and electron transportation, electrolyte penetration and overall structural stability, which is beneficial to the performance of overall water splitting.
performance of the obtained samples.When tested at 1 M KOH, the MoSe2/NC delivers overpotentials of 153 mV and 180 mV for HER and OER at 10 mA cm −2 , respectively.Moreover, the MoSe2/NC‖MoSe2/NC cell requires a potential of only 1.5 V to reach the current density of 10 mA cm −2 for overall water splitting.

Results and Discussion
The unique construction of the honeycomb-like carbon skeleton-encapsulated MoSe2/NC was synthesized by a facile freeze-drying method combined with the in situ selenization process.As shown in Figure 1a, the ammonium molybdate, PVP, urea and NaCl were mixed in a one-step freeze-drying process to form a polymer framework precursor.After that, the precursor underwent a facile selenization process at 600 °C in an Ar atmosphere.During this stage, the polymer framework was transformed into an N-doped carbonaceous network with the participation of urea.Significantly, nitrogen atoms replace carbon atoms, creating lattice defects (Figure 1b), whereas the inner ammonium molybdate was in situ converted into a MoSe2 phase.Furthermore, abundant orderly macropores with honeycomb-like features can be constructed within the composite by just removing the NaCl.The unique honeycomb-like conductive network could be helpful to the ion and electron transportation, electrolyte penetration and overall structural stability, which is beneficial to the performance of overall water splitting.The morphologies and interior structures of the MoSe2/NC, the MoSe2/C and the commercial MoSe2 were analyzed by SEM and TEM measurements.According to Figure 2a, the MoSe2/NC shows a uniform interconnected porous structure, and no obvious structural collapse is detected.A higher magnification image (Figure 2b) reveals that the porous structure is constructed by nanoplates with a diameter of around 100-200 nm.Moreover, the nanoplate seems to have a rough surface, which could be attributed to the decomposition of polymer and urea.The MoSe2/C (Figure S1a,b) appears to have structural collapse after the selenide process, demonstrating that urea could be helpful in the improvement of structural stability for the sample during the heat treatment process.Figure S2a,b show the SEM images of the commercial MoSe2 particles with a diameter of around 8 µm.More detailed information of the obtained samples was investigated by TEM measurement.As shown in Figure 2c, the MoSe2/NC composite is constructed by numerous nanoplates which display multi-layered and ultrathin features.Therefore, the porous structure of the MoSe2/NC is mainly due to the orderly stacking of hierarchical nanoplates.Figure 2d shows a high-resolution TEM image of the MoSe2/NC composite.Abundant lattice fringes can be observed in the composite, confirming that the active materials are well embedded into the honeycomb-like carbonaceous network.The magnified TEM image (Figure 2e) further recognizes that the lattice spacing is 0.241 nm, belonging to the (103) space of the standard MoSe2 and demonstrating that the nanocrystalline in the carbon skeleton is The morphologies and interior structures of the MoSe 2 /NC, the MoSe 2 /C and the commercial MoSe 2 were analyzed by SEM and TEM measurements.According to Figure 2a, the MoSe 2 /NC shows a uniform interconnected porous structure, and no obvious structural collapse is detected.A higher magnification image (Figure 2b) reveals that the porous structure is constructed by nanoplates with a diameter of around 100-200 nm.Moreover, the nanoplate seems to have a rough surface, which could be attributed to the decomposition of polymer and urea.The MoSe 2 /C (Figure S1a,b) appears to have structural collapse after the selenide process, demonstrating that urea could be helpful in the improvement of structural stability for the sample during the heat treatment process.Figure S2a,b show the SEM images of the commercial MoSe 2 particles with a diameter of around 8 µm.More detailed information of the obtained samples was investigated by TEM measurement.As shown in Figure 2c, the MoSe 2 /NC composite is constructed by numerous nanoplates which display multi-layered and ultrathin features.Therefore, the porous structure of the MoSe 2 /NC is mainly due to the orderly stacking of hierarchical nanoplates.Figure 2d shows a high-resolution TEM image of the MoSe 2 /NC composite.Abundant lattice fringes can be observed in the composite, confirming that the active materials are well embedded into the honeycomb-like carbonaceous network.The magnified TEM image (Figure 2e) further recognizes that the lattice spacing is 0.241 nm, belonging to the (103) space of the standard MoSe 2 and demonstrating that the nanocrystalline in the carbon skeleton is ascribed to MoSe 2 .Figure 2f shows the selected-area electron diffraction (SAED) patterns of the MoSe 2 /NC, and the result confirms the existence of the MoSe 2 phase.As shown in  Figure 3a shows the XRD patterns of the samples after the selenization process.Fo the MoSe2/NC, obvious diffraction peaks can be detected at around 31.7°, 37.9°, 47.1° and 55.7°, corresponding to the planes of (100), ( 103), ( 105) and (108) for the standard MoSe (JCPDS 29-0914), respectively.No other residual peaks are detected, indicating the high purity of the MoSe2/NC composite, consistent with the TEM result as well.In the absenc of urea, the product (MoSe2/C) shows similar diffraction peaks to the MoSe2/NC, which confirms that the MoSe2 phase can be easily synthesized by the fabrication strategy in ou work.In addition, the commercial MoSe2 was characterized, and the diffraction peaks cor respond well to the standard MoSe2 phase (JCPDS 29-0914).Compared with commercia MoSe2, the MoSe2/NC and the MoSe2/C display broader diffraction peaks and weaker peak intensities.These peak characteristics represent the lower crystallinity of the MoSe2/NC and the MoSe2/C, which could be ascribed to the abundant amorphous carbonaceous ma trix in the composite.Raman measurements were carried out to further investigate th bonding information and properties of the carbon in the obtained samples.As shown in Figure 3b, three samples show similar peaks between 200 cm −1 and 560 cm −1 , attributed t the characteristic peaks of the MoSe2 [26].Moreover, two obvious peaks located at abou 1360.07 cm −1 and 1584.83cm −1 can be detected for both the MoSe2/NC and the MoSe2/C which can be ascribed to the amorphous carbon (D band) and graphitic carbon (G band) respectively [30].The peak intensity ratio of the D band and the G band (ID/IG) can reflec the graphitic degree of the composite.The ID/IG values of the MoSe2/NC and the MoSe2/C are 1.06 and 1.09, respectively, indicating that carbon in the MoSe2/NC and the MoSe2/C is mainly dominated by graphitized carbon.The porous structure of the three sample was further investigated by the N2 adsorption and desorption measurements.The XP spectra of Mo3d, N1s and C1s (Figure S5) demonstrate that carbon and nitrogen have been successfully doped in the MoSe2/NC, which could effectively enhance the catalytic activit of the obtained catalysts.Figure 3c shows the N2 adsorption and desorption isotherm curves of the MoSe2/NC, the MoSe2/C and the MoSe2.The three curves can be assigned t H2 type (Type II Isotherm: S-type isotherm), indicating a mesoporous structure.Based on Figure 3a shows the XRD patterns of the samples after the selenization process.For the MoSe 2 /NC, obvious diffraction peaks can be detected at around 31.7 • , 37.9 • , 47.1 • and 55.7 • , corresponding to the planes of (100), ( 103), ( 105) and (108) for the standard MoSe 2 (JCPDS 29-0914), respectively.No other residual peaks are detected, indicating the high purity of the MoSe 2 /NC composite, consistent with the TEM result as well.In the absence of urea, the product (MoSe 2 /C) shows similar diffraction peaks to the MoSe 2 /NC, which confirms that the MoSe 2 phase can be easily synthesized by the fabrication strategy in our work.In addition, the commercial MoSe 2 was characterized, and the diffraction peaks correspond well to the standard MoSe 2 phase (JCPDS 29-0914).Compared with commercial MoSe 2 , the MoSe 2 /NC and the MoSe 2 /C display broader diffraction peaks and weaker peak intensities.These peak characteristics represent the lower crystallinity of the MoSe 2 /NC and the MoSe 2 /C, which could be ascribed to the abundant amorphous carbonaceous matrix in the composite.Raman measurements were carried out to further investigate the bonding information and properties of the carbon in the obtained samples.As shown in Figure 3b, three samples show similar peaks between 200 cm −1 and 560 cm −1 , attributed to the characteristic peaks of the MoSe 2 [26].Moreover, two obvious peaks located at about 1360.07 cm −1 and 1584.83cm −1 can be detected for both the MoSe 2 /NC and the MoSe 2 /C, which can be ascribed to the amorphous carbon (D band) and graphitic carbon (G band), respectively [30].The peak intensity ratio of the D band and the G band (ID/IG) can reflect the graphitic degree of the composite.The ID/IG values of the MoSe 2 /NC and the MoSe 2 /C are 1.06 and 1.09, respectively, indicating that carbon in the MoSe 2 /NC and the MoSe 2 /C is mainly dominated by graphitized carbon.The porous structure of the three samples was further investigated by the N 2 adsorption and desorption measurements.The XPS spectra of Mo3d, N1s and C1s (Figure S5) demonstrate that carbon and nitrogen have been successfully doped in the MoSe 2 /NC, which could effectively enhance the catalytic activity of the obtained catalysts.Figure 3c shows the N 2 adsorption and desorption isotherm curves of the MoSe 2 /NC, the MoSe 2 /C and the MoSe 2 .The three curves can be assigned to H 2 type (Type II Isotherm: S-type isotherm), indicating a mesoporous structure.Based on the Brunauer-Emmett-Teller (BET) method, the specific surface areas of the MoSe 2 /NC, the MoSe 2 /C, and the commercial MoSe 2 were calculated to be 221.70,89.56 and 3.70 m 2 g −1 , respectively.A large specific area usually has abundant active sites and is beneficial to the improvement of ion and electron diffusion rates and electrolyte penetration.Figure 3d shows the pore size distributions of the samples.The pore size is mainly distributed between 20 and 80 nm, confirming the mesoporous structure of the three samples.
the Brunauer-Emmett-Teller (BET) method, the specific surface areas of the MoSe2/NC the MoSe2/C, and the commercial MoSe2 were calculated to be 221.70,89.56 and 3.70 m 2 g −1 respectively.A large specific area usually has abundant active sites and is beneficial to the improvement of ion and electron diffusion rates and electrolyte penetration.Figure 3d shows the pore size distributions of the samples.The pore size is mainly distributed be tween 20 and 80 nm, confirming the mesoporous structure of the three samples.The obtained samples were coated onto nickel foam to evaluate the HER performance by testing in 1 M KOH aqueous solution.As shown in Figure 4a, the MoSe2/NC electrode displays the best HER performance with an overpotential of 153 mV at 10 mA cm −2 and a low onset overpotential of only 50 mV.In comparison, the MoSe2/C and the commercia MoSe2 show overpotentials of 184 mV and 219 mV at 10 mA cm −2 , respectively, highe than that of the MoSe2/NC.The pure nickel foam without active materials was also tested and exhibits relatively poor electrochemical performance (an overpotential of 284 mV a 10 mA cm −2 ), consistent with previous studies.Tafel plots were constructed from the po larization curves to elucidate the HER mechanism.The most advanced and fastest HER process should be determined by the Tafel reaction process of hydrogen recombination which implies that a smaller Tafel slope dictates a faster HER process.Consistent with the LSV results, the Tafel slopes were also optimized for the samples with the honeycomb like carbon framework.As shown in Figure 4b, the Tafel slopes of the MoSe2/NC and the MoSe2/C were determined to be 75 mV dec −1 and 125 mV dec −1 , respectively, lower than that of the commercial MoSe2 (218 dec −1 ) and the pure metal matrix (262 mV dec −1 ).There fore, the values indicate that the HER process can be inferred as the Volmer−Heyrovsky mechanism on the obtained electrodes [34].The enhanced HER activity of the MoSe2/NC was further evaluated and compared in Figure 4c.The result demonstrates that the MoSe2/NC electrode exhibits the best overpotential and Tafel slope, which could be at tributed to the abundant active sites and stable structure induced by the honeycomb-like carbonaceous network derived from the polymer by freeze-drying and pyrolysis.Mean while, the robust carbon skeleton can prevent the aggregation of the inner MoSe2 nano crystalline during the electrochemical process [32].
To investigate the electrode kinetics during the HER process, the charge transfer re sistance (Rct) at −1.27 V (vs.RHE) of the obtained samples was measured by an electro chemical impedance spectroscopy (EIS) technique.As shown in Figure 4d, all the curve consist of semicircular shapes.The Nyquist plots indicate that the MoSe2/NC has the smallest Rct value (about 5 Ω), whereas those of the MoSe2/C, the commercial MoSe2 and The obtained samples were coated onto nickel foam to evaluate the HER performance by testing in 1 M KOH aqueous solution.As shown in Figure 4a, the MoSe 2 /NC electrode displays the best HER performance with an overpotential of 153 mV at 10 mA cm −2 and a low onset overpotential of only 50 mV.In comparison, the MoSe 2 /C and the commercial MoSe 2 show overpotentials of 184 mV and 219 mV at 10 mA cm −2 , respectively, higher than that of the MoSe 2 /NC.The pure nickel foam without active materials was also tested and exhibits relatively poor electrochemical performance (an overpotential of 284 mV at 10 mA cm −2 ), consistent with previous studies.Tafel plots were constructed from the polarization curves to elucidate the HER mechanism.The most advanced and fastest HER process should be determined by the Tafel reaction process of hydrogen recombination, which implies that a smaller Tafel slope dictates a faster HER process.Consistent with the LSV results, the Tafel slopes were also optimized for the samples with the honeycomb-like carbon framework.As shown in Figure 4b, the Tafel slopes of the MoSe 2 /NC and the MoSe 2 /C were determined to be 75 mV dec −1 and 125 mV dec −1 , respectively, lower than that of the commercial MoSe 2 (218 dec −1 ) and the pure metal matrix (262 mV dec −1 ).Therefore, the values indicate that the HER process can be inferred as the Volmer−Heyrovsky mechanism on the obtained electrodes [34].The enhanced HER activity of the MoSe 2 /NC was further evaluated and compared in Figure 4c.The result demonstrates that the MoSe 2 /NC electrode exhibits the best overpotential and Tafel slope, which could be attributed to the abundant active sites and stable structure induced by the honeycomb-like carbonaceous network derived from the polymer by freeze-drying and pyrolysis.Meanwhile, the robust carbon skeleton can prevent the aggregation of the inner MoSe 2 nanocrystalline during the electrochemical process [32].
To investigate the electrode kinetics during the HER process, the charge transfer resistance (R ct ) at −1.27 V (vs.RHE) of the obtained samples was measured by an electrochemical impedance spectroscopy (EIS) technique.As shown in Figure 4d, all the curves consist of semicircular shapes.The Nyquist plots indicate that the MoSe 2 /NC has the small-est R ct value (about 5 Ω), whereas those of the MoSe 2 /C, the commercial MoSe 2 and the nickel foam are 6, 10 and 27 Ω, respectively.This result demonstrates that the MoSe 2 /NC has the fastest charge transfer and more favorable reaction kinetics for HER catalysis.Furthermore, the specific surface area (ECSA) of the obtained samples was evaluated by CV tests at different scanning rates of 20, 40, 60, 80 and 100 mV s −1 (Figure S3) to reveal the mechanism of the best HER performance for the MoSe 2 /NC electrode.The double-layer capacity (C dl ) (Figure 4e) was calculated based on CV measurements to evaluate the ECSA values of the three molybdenum-based composites.The MoSe 2 /NC possessed a C dl value of 17.9 mF cm −2 , larger than that of the MoSe 2 /C (11.1 mF cm −2 ) and the commercial MoSe 2 (1.57mF cm −2 ).The highest ECSA of the MoSe 2 /NC indicates that the porous carbon framework leads to abundant active sites for the electrochemical reactions.The higher ECSA value of the MoSe 2 /NC compared to the MoSe 2 /C could be ascribed to the extra structural defects caused by the N-doped carbon and uniform honeycomb-like structure, consistent with previous studies [34].The superior electrochemical performance of the MoSe 2 /NC was confirmed by the stability performance test.As shown in Figure 4f, the MoSe 2 /NC showed good stability at 10 mA cm −2 .The LSV curves of this test (seen in the Figure 4f inset) also demonstrate the excellent durability of the MoSe 2 /NC.The LSV curves of the initial and after 30 h tests (inset in Figure 4f) also verify the excellent durability of the MoSe 2 /NC.The improved HER activity of the MoSe 2 /NC may be attributed to its unique honeycomb-like network structure and N-doped carbon, which results in robust structural flexibility and fast ion and electron transportation.The OER catalytic activities of the prepared samples were further evaluated by LSV and EIS measurements in a 1 M KOH solution at a scan rate of 5 mV s −1 .As shown in Figure 5a, the MoSe2/NC delivers an overpotential of only 180 mV at 10 mA cm −2 , much lower than those of the MoSe2/C (255 mV) and the MoSe2 (355 mV).Moreover, the MoSe2/NC possesses the best overpotential even at a high current density of 50 mA cm −2 .In addition, the MoSe2/NC displays the smallest Tafel slope of 76 mV dec −1 among the obtained samples, such as the MoSe2/C (97 mV dec −1 ) and the commercial MoSe2 (154 mV dec −1 ) (Figure 5b). Figure 5c shows the overpotentials and Tafel slopes of the obtained three samples and glassy carbon electrode, demonstrating the superior catalytic activity of the MoSe2/NC toward OER.The measurement of current response vs. the operation time was carried out to investigate the stability of the MoSe2/NC.The fast electron and ion The OER catalytic activities of the prepared samples were further evaluated by LSV and EIS measurements in a 1 M KOH solution at a scan rate of 5 mV s −1 .As shown in Figure 5a, the MoSe 2 /NC delivers an overpotential of only 180 mV at 10 mA cm −2 , much lower than those of the MoSe 2 /C (255 mV) and the MoSe 2 (355 mV).Moreover, the MoSe 2 /NC possesses the best overpotential even at a high current density of 50 mA cm −2 .In addition, the MoSe 2 /NC displays the smallest Tafel slope of 76 mV dec −1 among the obtained samples, such as the MoSe 2 /C (97 mV dec −1 ) and the commercial MoSe 2 (154 mV dec −1 ) (Figure 5b). Figure 5c shows the overpotentials and Tafel slopes of the obtained three samples and glassy carbon electrode, demonstrating the superior catalytic activity of the MoSe 2 /NC toward OER.The measurement of current response vs. the operation time was carried out to investigate the stability of the MoSe 2 /NC.The fast electron and ion transportation and favorable structural integrity derived from the robust honeycomb-like carbonaceous network could enhance the OER catalytic activity of the MoSe 2 /NC.To confirm the above conjecture, EIS curves and stability testing of the samples were carried out.Figure 5d shows Nyquist plots for the MoSe 2 /NC, the MoSe 2 /C and the commercial MoSe 2 in 1 M KOH electrolyte at a potential of 1.64 V vs. RHE.The solution resistance (R s ) and the charge transfer resistance (R ct ) are related to the size of the semicircle in the low-and high-frequency regions.The EIS value of the MoSe 2 /NC is about 3.2 Ω, smaller than that of the MoSe 2 /C (4.5 Ω) and the commercial MoSe 2 (7.8 Ω).As shown in Figure 5e, the electrode still has excellent current density retention after durability testing for 10 h.In addition, the LSV curves of the initial and after long-term stability measurements (inset in Figure 5e) were tested, and the nearly overlapping curves indicate the superior electrochemical performance.The morphology characterization of the MoSe 2 /NC after the stability test (Figure S4a,b) shows that the honeycomb-like structure can be well maintained during the OER process, verifying the robust stability of the porous carbon network.Compared with previous studies, the MoSe 2 /NC obtained in our work exhibits great potential as an advanced catalyst for OER (Figure 5f) [35].The remarkable HER and OER performance of the MoSe2/NC motivates us to further explore its practical performance as both the anode and the cathode for overall water splitting by constructing a two-electrode electrolyzer in alkaline conditions (schematically represented in Figure 6a).Figure 6b shows that the MoSe2/NC‖MoSe2/NC cell requires a potential of 1.5 V to achieve the current density of 10 mA cm −2 during overall water splitting, which is comparable to the commercial Pt/C‖RuO2 cell.Figure 6c further shows the durability performance of the MoSe2/NC at 10 mA cm −2 in a 1 M KOH solution for long-term operation [41].The current density can still be maintained at 9.1 mA cm −2 after 10 h with a retention of 91%, suggesting excellent durability of the MoSe2/NC electrode during overall water splitting after long-term operation.The corresponding SEM images after testing (Figure S6) confirm the considerable structural stability during the catalytic process.As compared with previously developed catalysts, the MoSe2/NC exhibits outstanding electrochemical performance, indicating that it is one of the best potential bifunctional catalysts for overall water splitting (Figure 6d) [42].The remarkable HER and OER performance of the MoSe 2 /NC motivates us to further explore its practical performance as both the anode and the cathode for overall water splitting by constructing a two-electrode electrolyzer in alkaline conditions (schematically represented in Figure 6a).Figure 6b shows that the MoSe 2 /NC∥MoSe 2 /NC cell requires a potential of 1.5 V to achieve the current density of 10 mA cm −2 during overall water splitting, which is comparable to the commercial Pt/C∥RuO 2 cell.Figure 6c further shows the durability performance of the MoSe 2 /NC at 10 mA cm −2 in a 1 M KOH solution for longterm operation [41].The current density can still be maintained at 9.1 mA cm −2 after 10 h with a retention of 91%, suggesting excellent durability of the MoSe 2 /NC electrode during overall water splitting after long-term operation.The corresponding SEM images after testing (Figure S6) confirm the considerable structural stability during the catalytic process.As compared with previously developed catalysts, the MoSe 2 /NC exhibits outstanding electrochemical performance, indicating that it is one of the best potential bifunctional catalysts for overall water splitting (Figure 6d) [42].

Synthesis of the MoSe2/NC Composite
The MoSe2/NC was fabricated by the freeze-drying method combined with an in situ selenization process.Typically, 10 g polyvinylpyrrolidone (PVP, Mw = 1,300,000) was first dissolved in 100 mL deionized water.Then, 5.6 g ammonium molybdate, 5 g urea and 10 g NaCl were also added to the above solution and vigorously stirred for 2 h to obtain a transparent solution.After that, the transparent solution was treated using a freeze-drying method for 24 h.Finally, the obtained precursor (1 g) was mixed with selenium powder (2 g), and then the mixture was annealed at 600 °C for 4 h under an Ar atmosphere with a temperature rate of 3 °C min −1 .After cooling down naturally, the samples were washed with water to remove the NaCl, and the final product was named MoSe2/NC.For comparison purposes, the MoSe2/C was also prepared by a similar method without the addition of urea.The commercial MoSe2 particles were purchased from Sigma-Aldrich (Shanghai, China).

Structural Characterization
X-ray diffraction (XRD) measurements were performed on a D/Max 2700 X-ray diffractometer with Cu-Kα radiation.The structure and morphology of the prepared samples were characterized by field-emission scanning electron microscopy (FESEM FEI Nova Nano SEM 230, Beijing, China) and transmission electron microscopy (TEM, JEOL-JEM-2100F, Shanghai, China).Raman spectroscopy was obtained on a Renishaw 1000.The specific surface area and pore size distribution of the MoSe2/NC were characterized with a surface area detecting instrument by N2 physisorption (ASAP 2020 HD88).

Electrochemical Measurements
All electrochemical tests were performed on a CHI 660e electrochemical workstation using a standard three-electrode test.During the HER test, the active materials and Polyvinylidene fluoride (PVDF) with a mass ratio of 9:1 were mixed and dispersed into N-

Synthesis of the MoSe 2 /NC Composite
The MoSe 2 /NC was fabricated by the freeze-drying method combined with an in situ selenization process.Typically, 10 g polyvinylpyrrolidone (PVP, Mw = 1,300,000) was first dissolved in 100 mL deionized water.Then, 5.6 g ammonium molybdate, 5 g urea and 10 g NaCl were also added to the above solution and vigorously stirred for 2 h to obtain a transparent solution.After that, the transparent solution was treated using a freezedrying method for 24 h.Finally, the obtained precursor (1 g) was mixed with selenium powder (2 g), and then the mixture was annealed at 600 • C for 4 h under an Ar atmosphere with a temperature rate of 3 • C min −1 .After cooling down naturally, the samples were washed with water to remove the NaCl, and the final product was named MoSe 2 /NC.For comparison purposes, the MoSe 2 /C was also prepared by a similar method without the addition of urea.The commercial MoSe 2 particles were purchased from Sigma-Aldrich (Shanghai, China).

Structural Characterization
X-ray diffraction (XRD) measurements were performed on a D/Max 2700 X-ray diffractometer with Cu-Kα radiation.The structure and morphology of the prepared samples were characterized by field-emission scanning electron microscopy (FESEM FEI Nova Nano SEM 230, Beijing, China) and transmission electron microscopy (TEM, JEOL-JEM-2100F, Shanghai, China).Raman spectroscopy was obtained on a Renishaw 1000.The specific surface area and pore size distribution of the MoSe 2 /NC were characterized with a surface area detecting instrument by N 2 physisorption (ASAP 2020 HD88).

Electrochemical Measurements
All electrochemical tests were performed on a CHI 660e electrochemical workstation using a standard three-electrode test.During the HER test, the active materials and Polyvinylidene fluoride (PVDF) with a mass ratio of 9:1 were mixed and dispersed into

Figure 1 .
Figure 1.(a) Schematic illustration of the fabrication process of the honeycomb-like MoSe2/NC composite; (b) The advantages of MoSe2/NC.

Figure 1 .
Figure 1.(a) Schematic illustration of the fabrication process of the honeycomb-like MoSe 2 /NC composite; (b) The advantages of MoSe 2 /NC.

Figure
Figure 2g-k, the elemental mapping results again verify the existence of Mo, Se, C and N elements in the composite.The red and green areas in Figure 2i,k suggest that the MoSe 2 nanocrystalline is uniformly distributed in the honeycomb-like carbon network.

Figure 2 .
Figure 2. The morphologies and interior structures of the obtained samples.(a,b) SEM images.(c TEM images.(d,e) HRTEM images.(f) The selected-area electron diffraction (SAED) patterns.(g-k High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and el emental mapping images of the MoSe2/NC composite.

Figure 2 .
Figure 2. The morphologies and interior structures of the obtained samples.(a,b) SEM images.(c) TEM images.(d,e) HRTEM images.(f) The selected-area electron diffraction (SAED) patterns.(g-k) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images of the MoSe 2 /NC composite.

Molecules 2024 ,
29, x FOR PEER REVIEW 6 of 11MoSe2/NC has the fastest charge transfer and more favorable reaction kinetics for HER catalysis.Furthermore, the specific surface area (ECSA) of the obtained samples was evaluated by CV tests at different scanning rates of 20, 40, 60, 80 and 100 mV s −1 (FigureS3) to reveal the mechanism of the best HER performance for the MoSe2/NC electrode.The double-layer capacity (Cdl) (Figure4e) was calculated based on CV measurements to evaluate the ECSA values of the three molybdenum-based composites.The MoSe2/NC possessed a Cdl value of 17.9 mF cm −2 , larger than that of the MoSe2/C (11.1 mF cm −2 ) and the commercial MoSe2 (1.57mF cm −2 ).The highest ECSA of the MoSe2/NC indicates that the porous carbon framework leads to abundant active sites for the electrochemical reactions.The higher ECSA value of the MoSe2/NC compared to the MoSe2/C could be ascribed to the extra structural defects caused by the N-doped carbon and uniform honeycomb-like structure, consistent with previous studies[34].The superior electrochemical performance of the MoSe2/NC was confirmed by the stability performance test.As shown in Figure4f, the MoSe2/NC showed good stability at 10 mA cm −2 .The LSV curves of this test (seen in the Figure4finset) also demonstrate the excellent durability of the MoSe2/NC.The LSV curves of the initial and after 30 h tests (inset in Figure4f) also verify the excellent durability of the MoSe2/NC.The improved HER activity of the MoSe2/NC may be attributed to its unique honeycomb-like network structure and N-doped carbon, which results in robust structural flexibility and fast ion and electron transportation.

Figure 4 .
Figure 4. Electrochemical properties of the MoSe2/NC, the MoSe2/C, the commercial MoSe2 and the pure nickel foam for HER.(a) LSV curves.(b) Corresponding Tafel plots.(c) Comparison diagram of LSV and Tafel.(d) Nyquist plots.(e) Calculated Cdl of the obtained samples in 1 M KOH aqueous solution.(f) Electrochemical stability of the MoSe2/NC electrode at different current densities for 30 h (Inset: LSV curves of the MoSe2/NC before and after stability measurement).

Figure 4 .
Figure 4. Electrochemical properties of the MoSe 2 /NC, the MoSe 2 /C, the commercial MoSe 2 and the pure nickel foam for HER.(a) LSV curves.(b) Corresponding Tafel plots.(c) Comparison diagram of LSV and Tafel.(d) Nyquist plots.(e) Calculated Cdl of the obtained samples in 1 M KOH aqueous solution.(f) Electrochemical stability of the MoSe 2 /NC electrode at different current densities for 30 h (Inset: LSV curves of the MoSe 2 /NC before and after stability measurement).

Molecules 2024 ,
29,  x FOR PEER REVIEW 7 of 11 transportation and favorable structural integrity derived from the robust honeycomb-like carbonaceous network could enhance the OER catalytic activity of the MoSe2/NC.To confirm the above conjecture, EIS curves and stability testing of the samples were carried out.Figure5dshows Nyquist plots for the MoSe2/NC, the MoSe2/C and the commercial MoSe2 in 1 M KOH electrolyte at a potential of 1.64 V vs. RHE.The solution resistance (Rs) and the charge transfer resistance (Rct) are related to the size of the semicircle in the low-and high-frequency regions.The EIS value of the MoSe2/NC is about 3.2 Ω, smaller than that of the MoSe2/C (4.5 Ω) and the commercial MoSe2 (7.8 Ω).As shown in Figure5e, the electrode still has excellent current density retention after durability testing for 10 h.In addition, the LSV curves of the initial and after long-term stability measurements (inset in Figure5e) were tested, and the nearly overlapping curves indicate the superior electrochemical performance.The morphology characterization of the MoSe2/NC after the stability test (FigureS4a,b) shows that the honeycomb-like structure can be well maintained during the OER process, verifying the robust stability of the porous carbon network.Compared with previous studies, the MoSe2/NC obtained in our work exhibits great potential as an advanced catalyst for OER (Figure5f)[35].

Figure 5 .
Figure 5. Electrochemical properties of the MoSe2/NC, the MoSe2/C and the MoSe2 for OER.(a) LSV curves.(b) Corresponding Tafel plots.(c) Comparison diagram of LSV and Tafel.(d) Nyquist plots of the obtained three samples.(e) Electrochemical stability of the MoSe2/NC after 10 h test at constant point.(Inset: LSV curves of the MoSe2/NC before and after stability tests.)(f) Comparison of the overpotentials between our work and previous studies [36-40].

Figure 5 .
Figure 5. Electrochemical properties of the MoSe 2 /NC, the MoSe 2 /C and the MoSe 2 for OER.(a) LSV curves.(b) Corresponding Tafel plots.(c) Comparison diagram of LSV and Tafel.(d) Nyquist plots of the obtained three samples.(e) Electrochemical stability of the MoSe 2 /NC after 10 h test at constant point.(Inset: LSV curves of the MoSe 2 /NC before and after stability tests.)(f) Comparison of the overpotentials between our work and previous studies [36-40].

Figure 6 .
Figure 6.The electrochemical performance of the MoSe2/NC‖MoSe2/NC cell during overall water splitting.(a) Schematic diagram of the MoSe2/NC‖MoSe2/NC electrolyzer.(b) LSV curves of the MoSe2/NC‖MoSe2/NC cell.(Inset: a camera picture of the electrode during water splitting.)(c) Stability test of the MoSe2/NC‖MoSe2/NC cell.(d) The comparison of the cell voltage for our electrolyzer with previous reports [43-49].

Figure 6 .
Figure 6.The electrochemical performance of the MoSe 2 /NC∥MoSe 2 /NC cell during overall water splitting.(a) Schematic diagram of the MoSe 2 /NC∥MoSe 2 /NC electrolyzer.(b) LSV curves of the MoSe 2 /NC∥MoSe 2 /NC cell.(Inset: a camera picture of the electrode during water splitting.)(c) Stability test of the MoSe 2 /NC∥MoSe 2 /NC cell.(d) The comparison of the cell voltage for our electrolyzer with previous reports [43-49].