Synthesis and Catalytic Performance of Mo2C/MoS2 Composite Heterojunction Catalysts

Hydrogen, as a clean, safe, and efficient energy carrier, is one of the hot energy sources that have attracted much attention. Mo2C, due to the introduction of C atoms, makes the atomic spacing of the Mo lattice decrease and changes the width of the d-band, which makes the electronic properties of Mo2C similar to that of Pt noble metals, exhibiting excellent electrochemical hydrogen precipitation performance. MoS2, due to its special crystal structure and tunable electronic structure, has been widely studied. In this paper, Mo2C nanoparticles were prepared by high-temperature carbonization, and then two-dimensional layered MoS2 were be loaded on Mo2C nanoparticles by the hydrothermal method to synthesize Mo2C/MoS2 composite catalysts. Their electrochemical hydrogen precipitation (HER) performance under acidic conditions was tested. The above catalysts were also characterized by modern material testing methods such as XRD, SEM, TEM, and XPS. The results showed that the composite catalysts exhibited the most excellent electrochemical hydrogen precipitation performance at Mo2C/MoS2-3, with the lowest overpotential at a current density of 10 mA cm−2, Tafel slope, and electrochemical impedance. At the same time, the electrochemically active area was dramatically enhanced, with good stability under prolonged testing. The catalytic activity was significantly improved compared with that of Mo2C and MoS2. The characterization and experimental results indicate that the heterogeneous structure of Mo2C and MoS2 formed a built-in electric field between the two, which accelerated the electron transfer efficiency and provided more active sites. The Mo2C/MoS2 composite catalyst is a low-cost, easy-to-prepare, and high-efficiency electrochemical hydrogen precipitation catalyst, providing a new idea for developing green and clean energy.


Introduction
With the continuous development of society, humanity's demand for energy continues to expand, and the excessive use of traditional fossil fuels has led to environmental problems and the emergence of the crisis of energy scarcity.Here, there is the search for a new type of clean energy to eliminate the dependence on the limited fossil fuels for the sustainable development of humanity, where social stability and progress of the problems faced have an urgent need to be resolved [1].
Hydrogen has emerged as a paragon of clean energy carriers, embodying a prodigious potential for high energy yield per mass (142 MJ kg −1 ), dwarfing even the venerable fossil fuels, bolstering its prominence at the pinnacles of energy research.The gamut of prevalent hydrogen fabrication techniques encompasses photocatalytic [2,3] and electrolytic hydrogen generation [4][5][6], biomass enzymatic degradation [7,8], and conventional fossilfuel-based hydrogen extraction [9,10].For example, Marjeta et al. [11] synthesized efficient photocatalysts by epitaxial growth of SrTiO 3 on Bi 4 Ti 3 O 12 substrates.The multilayer composite catalysts CoS 2 /MoS 2 and Bi 2 S 2 /MoS 2 prepared by Yang et al. [12] exhibit high Volmer H + + e − → H * (acid) Volmer Heyrovsky Heyrovsky Tafel In HER, the Tafel slope is usually used to determine the mechanism of cathodic H 2 generation: since the HER reaction necessarily includes the process of electron transfer and desorption, when the Tafel slope is around 120 mV dec −1 , the decisive step of HER is the Volmer reaction, and the electron transfer rate is slow, so the reaction rate can be accelerated by increasing the active sites of the catalyst at this time.When the Tafel slope is around 40 mV dec −1 , the decisive step of HER is the Heyrovsky reaction, the desorption process is slow, and the reaction rate can be accelerated by changing the physical properties of the catalyst (specific surface area, roughness, etc.).When the Tafel slope is around 30 mV dec −1 , the electron transfer and desorption rate on the electrode surface are very fast, and then the reaction decisive step is the compound desorption process of H*, i.e., the Tafel reaction, which can be accelerated by increasing the active area of the catalyst as well.
Platinum on carbon (Pt/C) has historically reigned as the superlative catalyst for electrochemical hydrogen production [17], acclaimed for its exceptional efficacy.However, the prohibitive cost of platinum, coupled with its scant availability and vulnerability to catalyst poisoning, invites the imperative of conceiving economical, facilely synthesized, robust, safe, and innocuous catalytic surrogates.Amidst this pursuit, an array of transition metal derivatives, including oxides, carbides, and sulfides, have emerged as contenders exhibiting laudable electrochemical propensities.Liu et al. [18] doped Ru single atoms into CoP nanoparticles and then loaded this catalyst on carbon dots (CDs) to form Ru-CoP/CDs composites.The CDs, which were made from citric acid and ethylenediamine, could regulate the electrocatalytic reaction sites and electronic structure of Ru-CoP/CDs by inhibiting the growth and agglomeration of Ru-CoP, resulting in excellent electrocatalytic performance of the catalysts.In 2021, Park et al. [19] improved the catalytic activity by introducing Ni to form a Ni-Mo binary alloy nitride (Ni 2 Mo 3 N) to weaken the Mo-H bonding and thus improve the catalytic activity, which enabled the catalyst to exhibit excellent catalytic performance.Notably, molybdenum carbide (Mo 2 C) has garnered significant interest as a material of meritorious distinction among electrocatalysts.
Molybdenum carbide [20], a venerated constituent within the pantheon of transition metal carbides, manifests an engorged metallic framework and an augmented interatomic chasm among Mo atoms.This structural transformation mitigates orbital congruency among neighboring metal atoms.Insights from Density Functional Theory (DFT) intimations denote that the electron conjugation across the d orbitals of Mo and the s and p orbitals of carbon engenders a marked augmentation of the d-band in metallic Mo [21].Such alterations consummate an electronic schema and catalytic comportment mirroring that of the noble metals partaking of Group VIII, thus ensuring a pronounced HER acumen [22].
Utilizing the two-dimensional layered material molybdenum disulfide (MoS 2 ), notable for its adjustable electronic properties, robust catalytic stability, and significant natural abundance, MoS 2 emerges as an exceptional electrocatalyst [23].
Extant literature underscores that monolayer molybdenum disulfide, a consummate archetype of transition metal dichalcogenides (TMDCs), has garnered extensive scrutiny owing to its substantial in-plane electron mobility, pronounced current switching ratio, superior photoelectrocatalytic attributes, and robust catalytic hydrogen evolution activity.Of the trio of molybdenum disulfide allotropes (1T, 2H, and 3R), the 2H phase prevails, owing to its augmented stability, crystallizing into a hexagonal lattice with a pronounced planar stratification.Within the MoS 2 architecture, Mo atoms are conjoined with S atoms via ionic linkages, whilst S-S interactions are governed by Van der Waals forces, producing a unique sandwich-like configuration that finds application across industries such as aerospace, lubrication, and energy storage.The 1T-MoS 2 , intrinsically metallic, adopts a tetragonal crystalline assembly, and while it is catalytically active, it is inherently unstable, typically transforming to the more stable 2H phase upon annealing.Conversely, the 3R phase of MoS 2 , which crystallizes under high temperatures within the rhombohedral system, is prone to a phase transition into the 2H-MoS 2 configuration.The preponderance of contemporary research in electrochemical hydrogen evolution orbits around exploring the 2H phase [24,25].
In pursuit of catalytic interfaces conducive to electrochemical hydrogen productionemphasizing potent catalytic efficacy, enduring stability, and economic viability-this investigation involves the synthesis of molybdenum carbide (Mo 2 C) nanoparticles via a high-temperature carbonization route, segueing into the generation of Mo 2 C/MoS 2 composite heterojunction semiconductor catalysts through the integration of two-dimensional MoS 2 onto Mo 2 C nanoparticles using hydrothermal methodology.Employing advanced material characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS), these catalysts were meticulously examined.Their electrochemical hydrogen evolution capability was subsequently evaluated by employing a CHI660E electrochemical workstation.The synthesis revealed that amalgamating Mo 2 C with MoS 2 considerably ameliorates the electronic structure at both the surface and interface of the catalyst.In the composite, n-type Mo 2 C operates as an electron reservoir, augmenting the electrical conductivity within the layered structure and concurrently unveiling additional active sites.Notably, the hydrogen adsorption-free energy (∆G H ) of Mo 2 C is discernibly less negative (−0.65 eV) compared to MoS 2 (2.05 eV), underscoring the enhanced propensity of Mo 2 C to adhere to H + and conduct the reduction reaction with heightened efficiency [26,27].The bifurcation of components also culminates in an optimized electrochemical profile for the catalyst.Electrochemical assays ascertain that the composite catalysts outperform isolated Mo 2 C and MoS 2 nanoparticles in terms of hydrogen evolution.Further optimization of the electronic structure was achieved by fine-tuning the molar ratio of the constituent materials, resulting in an elevation of catalytic performance.In this study, a composite catalyst with a core-shell structure was prepared by innovatively growing MoS 2 encapsulated on the surface of Mo 2 C grains via a facile synthesis method, which exhibited significantly better electrocatalytic performance than that of Mo 2 C and MoS 2 , as well as showing good stability over a long-time test.

Synthesis of Electrocatalysts
Mo 2 C flakes: In this experiment, pure β-Mo 2 C was prepared by the calcination method.Ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 2 O) was used as the molybdenum source, and 1,8-diaminonaphthalene (C 10 H 10 N 2 ) was used as the carbon source.The calcination precursor was prepared by mixing 2 mmol of ammonium molybdate tetrahydrate in 12 mL of deionized water and 3.2 mmol of 1,8-diaminonaphthalene in 10 mL of anhydrous ethanol.The pH of the mixture was adjusted by 1 mol L −1 hydrochloric acid to obtain the precipitation with a molar of 1:2.25 for n (Mo):n (C).The solution was left for 30 min, and then the precipitation was collected and dried at 100 • C for 10 h to obtain the Mo 2 C precursor.The precursor was ground into powder and calcinated in a vacuumed high-temperature tube furnace, which was heated up to 725 • C at a rate of 5 • C min −1 and kept warm for 4 h.After that, it was naturally cooled down to room temperature to obtain β-Mo 2 C nanoparticles.
MoS 2 particles: Ammonium molybdate tetrahydrate was used as the molybdenum source and thiourea (CH 4 N 2 S) as the sulfur source.Typically, 2 mmol ammonium molybdate tetrahydrate and 5.5 mmol thiourea were dissolved in 60 mL of deionized water with stirring to keep a molar ratio of n(Mo): n(S) = 1:4.The black suspension was then transferred into the polytetrafluoroethylene liner of a stainless steel high-temperature hydrothermal reactor.The black suspension was filtered after being heated at 200 • for 8 h to obtain the black solid powder, which was washed three times with 95% ethanol and distilled water and then dried in a constant temperature blast oven at 80 • C for 8 h to obtain the MoS 2 nanoparticles.
Mo 2 C/MoS 2 composite catalyst: Mo 2 C nanoparticles were prepared using the above method.The composite catalysts with different Mo 2 C/MoS 2 molar ratios (0.5, 1, 2, 3, and 4) were prepared by the same hydrothermal method using ammonium molybdate tetrahydrate as the molybdenum source and thiourea as the sulfur source, keeping the content of Mo 2 C unchanged.

Characterization
Powder X-ray diffraction patterns were obtained by a Rigaku SmartLab diffractometer (Tokyo, Japan) using Cu Kα (λ = 0.154178 nm) as a radiation source.The morphology of the samples was measured by scanning electron microscope (FESEM, Ultra-55, Zeiss, Oberkochen, Germany).The samples were pretreated at 105 • C for 12 h before measurement.The electrochemical impedance was measured at the electrochemical workstation CH1660E.X-ray photoelectron spectroscopy was recorded using Kratos AXIS Supra (Shimadzu, Kyoto, Japan).

Electrochemical Test
In this experiment, electrochemical tests were performed using a standard threeelectrode system to evaluate the electrochemical hydrogen precipitation performance of the catalyst.The CHI660E electrochemical workstation was used to load the powder samples to be tested onto a glassy carbon electrode of diameter ϕ = 3 mm with 5 wt% concentration of Nafion solution at 3.5 mg cm −2 .The electrode prepared as described above was used as the working electrode, the C electrode was used as the counter electrode, and Ag/AgCl (saturated KCl-filled) was used as the reference electrode.The LSV curves were obtained by reversible hydrogen electrode potential conversion, and the potential conversion equation for Ag/AgCl was E(vs.RHE) = E(vs.Ag/AgCl) + 0.0592pH + 0.197 (7) The electrolyte was a 0.5 mol L −1 H 2 SO 4 solution and linear voltammetry (LSV) was tested at a sweep rate of 5 mV s −1 .The electrochemical active area (ECSA) of the catalysts was evaluated by testing the double layer capacitance (C dl ), which was subjected to cyclic voltammetry (CV) at sweep rates of 20 mV s −1 , 40 mV s −1 , 60 mV s −1 , 80 mV s −1 , and 100 mV s −1 at potentials ranging from 0.15 to 0.25 V.The catalysts were tested at a sweep rate of 5 mV s −1 .The electrochemical impedance (EIS) of the catalyst was measured in the frequency range of 100,000-0.1 Hz to evaluate the electrochemical hydrogen precipitation performance of the catalyst.The stability of the catalysts was evaluated by recording the changes in the LSV curves before and after 1000 cycles of CV testing and recording the curves for 12 h.EIS was fitted with the program Zview and all experimental results are reproducible.

Characterization of Electrocatalysts
Figure 1 elegantly delineates the X-ray diffraction (XRD) spectra of Mo 2 C; MoS 2 ; and their composite, Mo 2 C/MoS 2 , catalysts.Within these spectra, distinct peaks at 34.4 • , 37.9 • , and 39.5 • are observable, corresponding, respectively, to the (100), (002), and (101) lattice planes of β-Mo 2 C crystals.These peaks are congruent with the salient features of the β-Mo 2 C standard PDF card (PDF#35-0708).The MoS 2 sample, synthesized via hydrothermal methods, exhibited characteristic peaks at 13.5 • and 32.4 • .These are indicative of the (002) and (100) facets of MoS 2 crystals, thus confirming the efficacy of the hydrothermal synthesis in crafting MoS 2 .However, it can be observed that the position of the MoS 2 diffraction peak was low compared to the 2θ value corresponding to the standard peak of 2H-phase MoS 2 , indicating the existence of multiple crystalline phases in the synthesized MoS 2 samples.1T-phase MoS 2 requires lower reaction temperatures and shorter reaction times compared to the 2H-phase MoS 2 , and the position of its (002) crystalline diffraction peaks was around 9 • , which resulted in the leftward diffraction peak position.Nevertheless, a diminution in peak intensity is noted in the XRD pattern of the Mo 2 C/MoS 2 composite due to the 2D layered structure and diminutive grain size of MoS 2 .This nuanced two-dimensional structure manifested less intense diffraction peaks with broader half-peak breadths in pristine MoS 2 samples.In contrast, β-Mo 2 C displayed pronounced, acutely defined peaks due to enlarged grain size and the formation of nanorods through the high-temperature carbonization of molecular clusters.Consequently, in the composite samples, β-Mo 2 C's peaks dwarfed those of MoS 2 in intensity.Yet, vestiges of MoS 2 ′ s diffraction peaks at 13.5 • and 32.4 • remained discernible, substantiating the coexistence of dual-phase Mo 2 C and MoS 2 within the composite.Importantly, the absence of extraneous stray peaks corroborates the sample's compositional purity.
the 2D layered structure and diminutive grain size of MoS2.This nu sional structure manifested less intense diffraction peaks with broader in pristine MoS2 samples.In contrast, β-Mo2C displayed pronounce peaks due to enlarged grain size and the formation of nanorods throug ature carbonization of molecular clusters.Consequently, in the com Mo2C's peaks dwarfed those of MoS2 in intensity.Yet, vestiges of MoS at 13.5° and 32.4° remained discernible, substantiating the coexistence o and MoS2 within the composite.Importantly, the absence of extraneo roborates the sample's compositional purity.These differences indicate a successful heterostructure of the composites, leading to optimized electronic architecture conducive to superior catalytic performance [29].T shifting to lower binding energies is also witnessed in the characteristic C1s peaks at 28 eV for Mo2C/MoS2 and 283.6 eV for Mo2C, as noted in Figure 2c.Lastly, Figure 2d sho cases peaks at 161.6 eV and 160.4 eV for the Mo2C/MoS2-1 samples, corresponding to S2 −2 p1/2 and S2 −2 p3/2 orbitals, respectively.These peaks were discernibly shifted towa lower binding energies compared to the 163.5 eV and 162.2 eV peaks of MoS2, embody the changing electron densities in these subsidized structures.
The specimens underwent meticulous examination utilizing scanning electron m croscopy (SEM), with the corresponding findings delineated in Figure 3. Observatio analysis of Figure 3a reveals that molybdenum carbide (Mo2C) assumed an elongate ell soidal morphology, with each particle approximately 80 nm in diameter, which coalesc The specimens underwent meticulous examination utilizing scanning electron microscopy (SEM), with the corresponding findings delineated in Figure 3. Observational analysis of Figure 3a reveals that molybdenum carbide (Mo 2 C) assumed an elongate ellip-soidal morphology, with each particle approximately 80 nm in diameter, which coalesced to constitute substantial nanorods.These nanorods exhibited a porous exterior, spanning sizes from 400 to 600 nm.Subsequent inspection of Figure 3b unveils a scanning electron micrographic depiction of molybdenum disulfide (MoS 2 ), which displays a composition of stratified nanofloral configurations.The distinctly layered petal-like structure corroborates the dimensional stratification intrinsic to MoS 2 .Upon amalgamation of the two entities, as depicted in Figure 3c, the Mo 2 C phase receded nearly beyond detection; nonetheless, one can discern the presence of nanofloral spherical entities, ranging in size from 600 to 800 nm.This indicates a transformative alteration of MoS 2 ′ s inherent cuboidal morphology post-composite, with the spherical nanofloral architecture potentially unveiling an augmented array of reactive sites.Figure 3d

Electrochemical Results of Electrocatalysts
Figure 5 depicts the electrochemical evaluation results for the various specimens.evidenced by Figure 5a,b, the Mo2C/MoS2 composite catalyst exhibited minimal overp tential at an identical current density.Specifically, this catalyst manifested an overpot tial of merely 278 mV at 10 mA cm −2 current density, thereby signifying its superior h drolytic proficiency for electrochemical hydrogen evolution in contrast to the prist specimen.Concurrently, the Tafel inclination for the Mo2C/MoS2 composite experience considerable descent to 136 mV dec −1 , markedly lower than the 189 mV dec −1 for undilu Mo2C and the 323 mV dec −1 for unadulterated MoS2.This observation implies that composite catalyst substantially diminished the activation energy requisite for the hyd lytic process when paralleled with the pure specimen.Furthermore, Figure 5c unveils electrochemical impedance spectroscopy (EIS) representations for the unmingled Mo MoS2, and the composite catalyst The equivalent circuit diagram and specific values shown in Table 1.It becomes apparent that the composite samples exhibited a reduced radius within the high-frequency region than their unmingled counterparts, a factor t intimates a lower impedance and an accelerated rate of electron transference.This hancement facilitates continuous electron conveyance and chemical reactions, culmin ing in an augmented electrochemical efficacy for hydrogen evolution.
Due to the direct linear relationship between the ECSA of a catalyst and the mag tude of its Cdl, the ECSA computation of materials can be inferred by the measurement their Cdl.As depicted in Figure 5d, the ECSA of the Mo2C/MoS2-1 composite catalys discerned to be 62.4 mF cm −2 , which notably surpasses that of the isolated Mo2C at 4 mF cm −2 and MoS2 at 33.6 mF cm −2 .This observation indicates that the composite catal acquired an enlarged electrochemically active specific surface area post-synthesis, wh resulted in a greater density of active sites.Consequently, this enhancement led to i proved electrochemical hydrogen evolution performance, corroborating the outcom previously illustrated by the polarization curves and Tafel slopes.An excessively hi Tafel slope indicates that the Volmer reaction rate is slow, which can limit the reaction r

Electrochemical Results of Electrocatalysts
Figure 5 depicts the electrochemical evaluation results for the various specimens.As evidenced by Figure 5a,b, the Mo 2 C/MoS 2 composite catalyst exhibited minimal overpotential at an identical current density.Specifically, this catalyst manifested an overpotential of merely 278 mV at 10 mA cm −2 current density, thereby signifying its superior hydrolytic proficiency for electrochemical hydrogen evolution in contrast to the pristine specimen.Concurrently, the Tafel inclination for the Mo 2 C/MoS 2 composite experienced a considerable descent to 136 mV dec −1 , markedly lower than the 189 mV dec −1 for undiluted Mo 2 C and the 323 mV dec −1 for unadulterated MoS 2 .This observation implies that the composite catalyst substantially diminished the activation energy requisite for the hydrolytic process when paralleled with the pure specimen.Furthermore, Figure 5c unveils the electrochemical impedance spectroscopy (EIS) representations for the unmingled Mo 2 C, MoS 2 , and the composite catalyst.The equivalent circuit diagram and specific values are shown in Table 1.It becomes apparent that the composite samples exhibited a reduced arc radius within the high-frequency region than their unmingled counterparts, a factor that intimates a lower impedance and an accelerated rate of electron transference.This enhancement facilitates continuous electron conveyance and chemical reactions, culminating in an augmented electrochemical efficacy for hydrogen evolution.The service stability of the catalysts was tested by cyclic voltammetry and chronoamperometry, and the test results are shown in the following figures: Figure 6a shows the linear voltammetry curves of the Mo2C/MoS2-1 composite catalysts before and after 1000 cycles at a sweep rate of 100 mV s −1 , which shows that the overpotential of the catalysts increased slightly after the cycle, with little change compared with that before the cycle.Figure 6b shows the chronoamperometric current curve of the Mo2C/MoS2-1 composite    The service stability of the catalysts was tested by cyclic voltammetry and chronoa perometry, and the test results are shown in the following figures: Figure 6a shows t linear voltammetry curves of the Mo2C/MoS2-1 composite catalysts before and after 10 cycles at a sweep rate of 100 mV s −1 , which shows that the overpotential of the cataly increased slightly after the cycle, with little change compared with that before the cyc Figure 6b   Due to the direct linear relationship between the ECSA of a catalyst and the magnitude of its C dl , the ECSA computation of materials can be inferred by the measurement of their C dl .As depicted in Figure 5d, the ECSA of the Mo 2 C/MoS 2 -1 composite catalyst is discerned to be 62.4 mF cm −2 , which notably surpasses that of the isolated Mo 2 C at 40.4 mF cm −2 and MoS 2 at 33.6 mF cm −2 .This observation indicates that the composite catalyst acquired an enlarged electrochemically active specific surface area post-synthesis, which resulted in a greater density of active sites.Consequently, this enhancement led to improved electrochemical hydrogen evolution performance, corroborating the outcomes previously illustrated by the polarization curves and Tafel slopes.An excessively high Tafel slope indicates that the Volmer reaction rate is slow, which can limit the reaction rate of the Heyrovsky reaction with the Tafel reaction, further affecting the overall conversion rate of the reaction.The ECSA of the catalyst was significantly increased after the composite of the two, which drastically reduced the Tafel slope of the catalyst and greatly accelerated the rate of electron transfer to produce H* faster.It indicates that the heterogeneous structure can form a high-speed electron transfer channel and dramatically improve the catalytic activity.
The service stability of the catalysts was tested by cyclic voltammetry and chronoamperometry, and the test results are shown in the following figures: Figure 6a shows the linear voltammetry curves of the Mo 2 C/MoS 2 -1 composite catalysts before and after 1000 cycles at a sweep rate of 100 mV s −1 , which shows that the overpotential of the catalysts increased slightly after the cycle, with little change compared with that before the cycle.200), (003), and (110) lattice plan of hexagonal MoS2, respectively.The absence of extraneous peaks underscores the puri of the synthesized Mo2C/MoS2 composited catalysts across the explored molar ratios.A the molar ratio of Mo2C to MoS2 ascended, a concurrent diminution and broadening MoS2 diffraction peaks were observed, signifying the potential of the composite to atte uate interlayer distances and particle size of the lamellar MoS2 structure.This structur alteration renders augmented accessibility to active sites, thereby enhancing the cataly prowess of the material.Notably, at a molar ratio of 4 units of Mo2C to MoS2, the pea representative of MoS2 were not detected, attributed to the insufficiency of MoS2 presen within the mixture.

Characterization of Composite Catalysts with Different Molar Ratios
Upon examining the XPS spectra, it is discernible that the characteristic peaks at 230 eV and 227.4 eV for the Mo2C/MoS2-1 specimen, as well as the analogous peaks for t Mo2C/MoS2-3 specimen depicted in Figure 8a, correspond to the Mo 2+ 3d3/2 and Mo 2+ 3d states of molybdenum, respectively, signifying the presence of Mo2C.Additionally, t peaks observed at 231.9 eV and 228.0 eV for the Mo2C/MoS2-1 sample, along with t peaks at 232.0 eV and 228.0 eV for the Mo2C/MoS2-3 sample, are representative of the Mo 3d3/2 and Mo 2+ 3d5/2 states of molybdenum, ascribed to Mo2C.Furthermore, the peaks 231.9 eV and 228.0 eV about the Mo2C/MoS2-1 specimen and the peaks at 232.0 eV an 228.0 eV relative to the Mo2C/MoS2-3 specimen correspond to the Mo 4+ 3d3/2 and Mo 4+ 3d states, which are indicative of MoS2.In Figure 8b, the peak positioned at 283.0 eV for bo Mo2C/MoS2-1 and Mo2C/MoS2-3 is associated with C1s, denoting Mo2C.Additionally, Fi ure 8c reveals that the characteristic peaks at 161.6 eV and 160.4 eV for the Mo2C/MoS2 sample and the identically characterized peaks for the Mo2C/MoS2-3 sample correspon to the S 2− 2p1/2 and S 2− 2p3/2 states, respectively, indicative of MoS2, whereas the peak 167.6 eV is suggestive of SO4 2− , implicating the partial oxidation of S 2− in atmospheric co ditions.The aforementioned peaks are consistent across the board with no noticeable shi implying that modifications in the molar ratio exert negligible influence on the heter junction between Mo2C and MoS2.This constancy alludes to the relative stability inhere in the heterostructural composition of the Mo2C/MoS2 composite catalyst.Figure 9 delineates the scanning electron microscopy (SEM) representations of the composite catalysts across varying molybdenum carbide to molybdenum disulfide (Mo2C/MoS2) molar ratios.Observations reveal a diminution in MoS2 crystallite dimensions concomitant with an escalation in Mo2C/MoS2 molar proportionality.Precisely, the crystallite dimensions approximate 300 nm for Mo2C/MoS2-0.5;however, this metric contracts to approximately 60-100 nm for the Mo2C/MoS2-3 specimen, with the reduced crystallite size engendering a commensurately augmented specific surface area.Consequently, this engenders a notably superior electrochemical efficacy in hydrogen evolution reactions.Figure 9 delineates the scanning electron microscopy (SEM) representations of the composite catalysts across varying molybdenum carbide to molybdenum disulfide (Mo2C/MoS2) molar ratios.Observations reveal a diminution in MoS2 crystallite dimensions concomitant with an escalation in Mo2C/MoS2 molar proportionality.Precisely, the crystallite dimensions approximate 300 nm for Mo2C/MoS2-0.5;however, this metric contracts to approximately 60-100 nm for the Mo2C/MoS2-3 specimen, with the reduced crystallite size engendering a commensurately augmented specific surface area.Consequently, this engenders a notably superior electrochemical efficacy in hydrogen evolution reactions.fied morphology of MoS2 underwent a transition towards a nanoflower spherical configuration, which is indicative of a considerable morphological transfiguration attributed to the influence of Mo2C.Owing to the layered architecture, the lion's share of catalytically active sites within MoS2 was predominantly localized at the laminae peripheries.Nonetheless, the emergent nanoflower spherical structure precipitated by the composite formation facilitated the generation of additional active cross-sections.In concert with this morphological evolution, the inherent strata manifested numerous intrinsic strain-induced defects, thereby proliferating the density of active sites conducive to catalytic processes.After the integration of the two constituents, the inherently two-dimensional stratified morphology of MoS 2 underwent a transition towards a nanoflower spherical configuration, which is indicative of a considerable morphological transfiguration attributed to the influence of Mo 2 C. Owing to the layered architecture, the lion's share of catalytically active sites within MoS 2 was predominantly localized at the laminae peripheries.Nonetheless, the emergent nanoflower spherical structure precipitated by the composite formation facilitated the generation of additional active cross-sections.In concert with this morphological evolution, the inherent strata manifested numerous intrinsic strain-induced defects, thereby proliferating the density of active sites conducive to catalytic processes.2.

Electrochemical Results of Composite Catalysts with Different Molar Ratios
Materials 2024, 17, x FOR PEER REVIEW 14 of

Electrochemical Results of Composite Catalysts with Different Molar Ratios
Figure 10 delineates the electrocatalytic hydrogen evolution prowess of compos catalysts with varying molar ratios of Mo2C to MoS2, assayed in an aqueous sulfuric ac solution of 0.5 mol L −1 .As discerned from the linear sweep voltammetry profiles illu trated in Figure 10a, the Mo2C/MoS2-0.5 composite mandated the highest overpotent for achieving a current density of 10 mA cm −2 , in stark contrast to the Mo2C/MoS2-3 com posite, which required the least overpotential at the equivalent current density, signifyin its superior electrocatalytic hydrogen evolution capability.Corresponding to these fin ings, Figure 10b presents the electrocatalytic hydrogen evolution attributes of the cataly variants stratified by their molar ratios, congruent with the linear sweep voltammet data: an onset overpotential of 183 mV dec −1 for Mo2C/MoS2-0.5 and 150 mV dec −1 f Mo2C/MoS2-4.This suggests that a molar ratio of Mo2C to MoS2 that is either excessive low or high is detrimental to the electrocatalytic efficacy of the composites.T Mo2C/MoS2-3 composite exhibited the most negligible Tafel gradient, an indicator that th particular formulation can efficaciously mitigate the activation barrier for the electrocat lytic hydrogen evolution process, hence displaying unparalleled performance.Figure 1 exposes the electrochemical impedance spectroscopy (EIS) profiles for the composit across varying molar ratios, echoing the aforementioned empirical data: the Mo2C/MoS 3 composite's Nyquist plots revealed the least pronounced curvature within the high-fr quency electron transport domain, indicative of optimal electrochemical performanc The equivalent circuit diagram and specific values are shown in Table 2.  which can drive the electrons and holes to be transported in the opposite direction, greatly enhancing the charge separation efficiency.For HER, the strong interfacial electric field formed by the PN junction-type heterojunction can significantly increase the d-band center of the catalyst, enhance the adsorption of H*, and contribute to the diffusion of metal ions.A comparison of the results of this work with those of other researchers is shown in  Figure 11 shows the side view of the stability of the Mo2C/MoS2-3 catalysts.It can be seen from Figure 11a that the cyclic voltammetry test was carried out at a sweep rate of 100 mV s −1 , and after cycling for 1000 laps, the LSV curves of the Mo2C/MoS2-3 catalysts showed a relatively small change, and the overpotentials slightly increased at the same current density, which indicated a slight decrease in the catalytic activity of the catalysts.Figure 11b shows that it curve of the Mo2C/MoS2-3 catalyst showed no obvious fluctuation after 12 h of the timed current test at an initial spot level of 223 mV, which exhibited good stability.The above results indicate that the Mo2C/MoS2-3 catalysts have excellent stability in long-time electrochemical reactions.Upon augmentation of the composite ratio, Mo 2 C, serving as an electron donor, endowed MoS 2 with an enhanced concentration of electrons, thereby diminishing its electrochemical impedance.The Mo 2 C/MoS 2 -3 samples, with a surplus of readily available free electrons, demonstrated enhanced electrocatalytic activity and were adept at promoting synergistic electron transfer, which accelerates the reduction of protons to hydrogen.Within the array of composite catalysts with varying composite ratios, the ECSA of the catalysts was similarly deduced through the evaluation of the C dl of the materials under investigation.In Figure 10d, the graphical depictions reveal that the Mo 2 C/MoS 2 -3 composite manifested the highest ECSA, registering a capacitance of 93.8 mF cm −2 .This represents a marked increase of 132% over the pristine Mo 2 C catalyst and an augmentation of 179% in comparison to the unadulterated MoS 2 catalyst.As the composite ratio of Mo 2 C to MoS 2 ascended, there was a discernible upward trajectory in the electrochemical active surface area of the composite catalysts.However, the ECSA of the Mo 2 C/MoS 2 -4 sample exhibited a subtle decline, which can be conjectured to result from an overly elevated Mo 2 C to MoS 2 composite ratio.Such a ratio may diminish the synergetic interaction between the two constituents, predominantly relegating Mo 2 C to serve as the principal component of the catalyst.
Mo 2 C is a narrow-band semiconductor material with a forbidden bandwidth of 1.26 eV, and MoS 2 has a forbidden bandwidth of 2.31 eV, and a type-II heterostructure will be formed after compositing the two.Due to the difference in electrical conductivity between Mo 2 C and MoS 2 , the Mo 2 C/MoS 2 composite catalyst will form a heterostructure similar to a P-N junction at the composite interface [30,31].When the PN junction-type heterojunction is formed, the electrons close to the P-N interface will diffuse to the P region, while the holes close to the P-N interface will diffuse to the N region, leaving the negatively charged P region ions.Due to the high concentration of negatively and positively charged ions assembled at the semiconductor interface, a built-in electric field will be formed, which can drive the electrons and holes to be transported in the opposite direction, greatly enhancing the charge separation efficiency.For HER, the strong interfacial electric field formed by the PN junction-type heterojunction can significantly increase the d-band center of the catalyst, enhance the adsorption of H*, and contribute to the diffusion of metal ions.A comparison of the results of this work with those of other researchers is shown in Table 3 Figure 11 shows the side view of the stability of the Mo 2 C/MoS 2 -3 catalysts.It can be seen from Figure 11a that the cyclic voltammetry test was carried out at a sweep rate of 100 mV s −1 , and after cycling for 1000 laps, the LSV curves of the Mo 2 C/MoS 2 -3 catalysts showed a relatively small change, and the overpotentials slightly increased at the same current density, which indicated a slight decrease in the catalytic activity of the catalysts.

Conclusions
In summation, Mo2C nanoparticles were efficaciously synthesized through a proce dure of high-temperature carbonization, subsequently leading to the creation o Mo2C/MoS2 composite catalysts via the hydrothermal method.Characterization of thes composites utilizing advanced material characterization methodologies, such as X-ray di fraction (XRD), scanning electron microscopy (SEM), transmission electron microscop (TEM), and X-ray photoelectron spectroscopy (XPS), has unequivocally confirmed th successful integration of Mo2C into a heterogenous structural amalgamation with MoS This fusion has substantially curtailed the proclivity of MoS2 to aggregate, thereby engen dering a prolific increase in defects within the MoS2 crystals and unveiling a greater quan tity of catalytically active sites.The potential for further enhancement of the composit catalysts' electronic structure lies in the modulation of the constituent ratios of Mo2C t MoS2, which precipitates enhanced kinetics in the adsorption, desorption, and reductio processes of H + ions.When juxtaposed with the singular Mo2C and MoS2 crystals, it evident that the catalytic efficacy of the composite is markedly superior.Electrochemica assessments, specifically hydrogen evolution reaction (HER) studies, demonstrated tha the Mo2C/MoS2-3 sample heralded an overpotential of merely 224 mV and a Tafel slope o 124 mV dec −1 at a current density of 10 mA cm −2 , showcasing exceptional HER perfo mance.This remarkable performance is ascribable to the heterostructures interlinking th composite components, where Mo2C affords electron conduits by its noble metal-lik characteristics, thereby ameliorating the electrical conductivity of MoS2.Concurrently MoS2's stratified structure has a wealth of crystal defects and an expansive specific surfac area that avails additional active sites germane to the catalyst's function.The empirica data coalesce to indicate that the intrinsic heterogeneous structure of the Mo2C/MoS2 com posite catalyst acts to significantly bolster its catalytic prowess.

Figure 1 .Figure 2 .
Figure 1.XRD pattern of Mo 2 C, MoS 2 , and Mo 2 C/MoS 2 .Analysis of the X-ray photoelectron spectroscopy (XPS) spectra reveals the distinctive peaks at 231.5 eV and 227.9 eV in Figure 2a, which are attributed to the 3d 3/2 and 3d 5/2 orbitals of divalent molybdenum (Mo 2+ ) in Mo 2 C, respectively.Conversely, peaks observed at 235.2 eV and 231.2 eV correspond to the 3d 3/2 and 3d 5/2 orbitals of hexavalent molybdenum (Mo 6+ ) in MoO 3 oxides, signifying oxidative processes affecting the sample surfaces during both the reaction phase and preservation.The Mo 2 C/MoS 2 -1 composite catalysts manifested characteristic peaks at 230.6 eV and 227.4 eV, indicative of the Mo 2+ 3d 3/2 and 3d 5/2 orbitals of the Mo element.These peaks were shifted by 0.53 eV to lower binding energies relative to those of Mo 2 C alone, suggesting an augmented electron cloud density around the composite samples [28].This phenomenon correlates with a decrease in electrochemical impedance and an enhancement in electrocatalytic performance.Figure 2b corroborates the findings from electrochemical characterization.Peaks at 230.6 eV and 227.4 eV correspond to the presence of tetravalent molybdenum (Mo 4+ ) 3d 3/2 and 3d 5/2 orbitals characteristic of MoS 2 .In contrast, peaks at 236.0 eV and 232.2 eV are ascribed to MoO 3 , while those at 233.6 eV and 230.0 eV denote an intermediary oxidation state of molybdenum (Mo ε+ ) with 4 < ε < 6.The intermediate oxidation state, as evidenced by the peaks, indicates that the MoS 2 samples also experienced surface oxidation; notably, the peak at 226.6 eV originated from S2s interference.

Figure 2 .
Figure 2. XPS images of Mo 2 C, MoS 2 , and Mo 2 C/MoS 2 -1 composite catalysts: (a,b) Mo3d; (c) C1s; (d) S2p.In the Mo 2 C/MoS 2 -1 samples, the peaks at 231.8 eV and 228.0 eV are ascribed to MoS 2 components' Mo 4+ 3d 3/2 and 3d 5/2 orbitals.They shifted toward lower binding energy relative to the MoS 2 sample by approximately 1.3 eV.This shift suggests that the electron cloud density around the MoS 2 component increased upon composite formation, with the Mo 2 C component transferring more free electrons to the MoS 2 due to Mo 2 C's role as an electron donor, enriching the electron milieu surrounding MoS 2 .The binding energy discrepancies, denoted by ∆ = 3.2 eV for Mo 2+ spin-split orbitals and ∆ = 3.8 eV for Mo 4+ spin orbitals, differed from those in the Mo 2 C (∆ = 3.6 eV) and MoS 2 (∆ = 3.2 eV) samples, respectively.These differences indicate a successful heterostructure of the composites, leading to an optimized electronic architecture conducive to superior catalytic performance [29].The shifting to lower binding energies is also witnessed in the characteristic C1s peaks at 283.0 eV for Mo 2 C/MoS 2 and 283.6 eV for Mo 2 C, as noted in Figure 2c.Lastly, Figure 2d showcases peaks at 161.6 eV and 160.4 eV for the Mo 2 C/MoS 2 -1 samples, corresponding to the S2 −2 p 1/2 and S2 −2 p 3/2 orbitals, respectively.These peaks were discernibly shifted towards lower binding energies compared to the 163.5 eV and 162.2 eV peaks of MoS 2 , embodying the changing electron densities in these subsidized structures.The specimens underwent meticulous examination utilizing scanning electron microscopy (SEM), with the corresponding findings delineated in Figure3.Observational analysis of Figure3areveals that molybdenum carbide (Mo 2 C) assumed an elongate ellip- provides a vista into the amalgamated growth of Mo 2 C and MoS 2 grains through mutual intercalation.Post-combination morphological transformation yielded predominant agglomerations of spherical Mo 2 C particles, the surfaces of which were adorned with the nano-floral constructions of MoS 2 , substantiating a successful synthesis of Mo 2 C/MoS 2 heterojunction semiconductor catalysts.erials 2024, 17, x FOR PEER REVIEW 8 of entities, as depicted in Figure 3c, the Mo2C phase receded nearly beyond detection; non theless, one can discern the presence of nanofloral spherical entities, ranging in size fro 600 to 800 nm.This indicates a transformative alteration of MoS2′s inherent cuboidal m phology post-composite, with the spherical nanofloral architecture potentially unveili an augmented array of reactive sites.Figure 3d provides a vista into the amalgamat growth of Mo2C and MoS2 grains through mutual intercalation.Post-combination m phological transformation yielded predominant agglomerations of spherical Mo2C pa cles, the surfaces of which were adorned with the nano-floral constructions of MoS2, su stantiating a successful synthesis of Mo2C/MoS2 heterojunction semiconductor catalyst
shows the chronoamperometric current curve of the Mo2C/MoS2-1 compos catalyst with a starting voltage of 297 mV, and the results indicate that the current dens was almost unchanged under the test condition of 12 h acidic electrolyte.The above resu indicate that the Mo2C/MoS2-1 composite catalyst exhibited good stability under lon time endurance tests.

Figure 7
Figure 7 portrays the X-ray diffraction patterns of heterogeneous catalysts compris of variable molar proportions of Mo2C to MoS2.Within these spectrums, one can disce the crystalline signatures of Mo2C at ordinal angles (2θ) of 34.4°, 38.0°, 39.4°, 52.1°, 61.5 69.6°, 72.4°, 74.6°, and 75.5°.These peaks correspond to the facets of the β-Mo2C pol morph (as cataloged in PDF#35-0787) specifically at the (100), (002), (101), (102), (11 (103), (200), (112), and (201) crystallographic planes, respectively, thereby affirming t successful synthesis of Mo2C nanoparticles via thermochemical carbonization.Moreov at diminished Mo2C to MoS2 ratios, diffraction peaks were discerned at 2θ values of 13.5 33.1°, 35.4°, and 57.4°, which are ascribable to the (011), (200), (003), and (110) lattice plan of hexagonal MoS2, respectively.The absence of extraneous peaks underscores the puri of the synthesized Mo2C/MoS2 composited catalysts across the explored molar ratios.A the molar ratio of Mo2C to MoS2 ascended, a concurrent diminution and broadening MoS2 diffraction peaks were observed, signifying the potential of the composite to atte uate interlayer distances and particle size of the lamellar MoS2 structure.This structur alteration renders augmented accessibility to active sites, thereby enhancing the cataly prowess of the material.Notably, at a molar ratio of 4 units of Mo2C to MoS2, the pea representative of MoS2 were not detected, attributed to the insufficiency of MoS2 presen within the mixture.Upon examining the XPS spectra, it is discernible that the characteristic peaks at 230 eV and 227.4 eV for the Mo2C/MoS2-1 specimen, as well as the analogous peaks for t Mo2C/MoS2-3 specimen depicted in Figure8a, correspond to the Mo 2+ 3d3/2 and Mo 2+ 3d states of molybdenum, respectively, signifying the presence of Mo2C.Additionally, t peaks observed at 231.9 eV and 228.0 eV for the Mo2C/MoS2-1 sample, along with t peaks at 232.0 eV and 228.0 eV for the Mo2C/MoS2-3 sample, are representative of the Mo 3d3/2 and Mo 2+ 3d5/2 states of molybdenum, ascribed to Mo2C.Furthermore, the peaks 231.9 eV and 228.0 eV about the Mo2C/MoS2-1 specimen and the peaks at 232.0 eV an 228.0 eV relative to the Mo2C/MoS2-3 specimen correspond to the Mo 4+ 3d3/2 and Mo 4+ 3d states, which are indicative of MoS2.In Figure8b, the peak positioned at 283.0 eV for bo Mo2C/MoS2-1 and Mo2C/MoS2-3 is associated with C1s, denoting Mo2C.Additionally, Fi ure 8c reveals that the characteristic peaks at 161.6 eV and 160.4 eV for the Mo2C/MoS2 sample and the identically characterized peaks for the Mo2C/MoS2-3 sample correspon to the S 2− 2p1/2 and S 2− 2p3/2 states, respectively, indicative of MoS2, whereas the peak 167.6 eV is suggestive of SO4 2− , implicating the partial oxidation of S 2− in atmospheric co ditions.The aforementioned peaks are consistent across the board with no noticeable shi implying that modifications in the molar ratio exert negligible influence on the heter junction between Mo2C and MoS2.This constancy alludes to the relative stability inhere in the heterostructural composition of the Mo2C/MoS2 composite catalyst.

Figure 7 18 Figure 7 .
Figure 7 portrays the X-ray diffraction patterns of heterogeneous catalysts comprised of variable molar proportions of Mo 2 C to MoS 2 .Within these spectrums, one can discern the crystalline signatures of Mo 2 C at ordinal angles (2θ) of 34.4 • , 38.0 • , 39.4 • , 52.1 • , 61.5 • , 69.6 • , 72.4 • , 74.6 • , and 75.5 • .These peaks correspond to the facets of the β-Mo 2 C polymorph (as cataloged in PDF#35-0787) specifically at the (100), (002), (101), (102), (110), (103), (200), (112), and (201) crystallographic planes, respectively, thereby affirming the successful synthesis of Mo 2 C nanoparticles via thermochemical carbonization.Moreover, at diminished Mo 2 C to MoS 2 ratios, diffraction peaks were discerned at 2θ values of 13.5 • , 33.1 • , 35.4 • , and 57.4 • , which are ascribable to the (011), (200), (003), and (110) lattice planes of hexagonal MoS 2 , respectively.The absence of extraneous peaks underscores the purity of the synthesized Mo 2 C/MoS 2 composited catalysts across the explored molar ratios.As the molar ratio of Mo 2 C to MoS 2 ascended, a concurrent diminution and broadening of MoS 2 diffraction peaks were observed, signifying the potential of the composite to attenuate interlayer distances and particle size of the lamellar MoS 2 structure.This structural alteration renders augmented accessibility to active sites, thereby enhancing the catalytic prowess of the material.Notably, at a molar ratio of 4 units of Mo 2 C to MoS 2 , the peaks representative of MoS 2 were not detected, attributed to the insufficiency of MoS 2 presence within the mixture.Upon examining the XPS spectra, it is discernible that the characteristic peaks at 230.6 eV and 227.4 eV for the Mo 2 C/MoS 2 -1 specimen, as well as the analogous peaks for the Mo 2 C/MoS 2 -3 specimen depicted in Figure 8a, correspond to the Mo 2+ 3d 3/2 and Mo 2+ 3d 5/2 states of molybdenum, respectively, signifying the presence of Mo 2 C. Additionally, the peaks observed at 231.9 eV and 228.0 eV for the Mo 2 C/MoS 2 -1 sample, along with the peaks at 232.0 eV and 228.0 eV for the Mo 2 C/MoS 2 -3 sample, are representative of the Mo 4+ 3d 3/2 and Mo 2+ 3d 5/2 states of molybdenum, ascribed to Mo 2 C. Furthermore, the peaks at 231.9 eV and 228.0 eV about the Mo 2 C/MoS 2 -1 specimen and the peaks at 232.0 eV and 228.0 eV relative to the Mo 2 C/MoS 2 -3 specimen correspond to the Mo 4+ 3d 3/2 and Mo 4+ 3d 5/2 states, which are indicative of MoS 2 .In Figure 8b, the peak positioned at 283.0 eV for both Mo 2 C/MoS 2 -1 and Mo 2 C/MoS 2 -3 is associated with C1s, denoting Mo 2 C. Additionally, Figure 8c reveals that the characteristic peaks at 161.6 eV and 160.4 eV for the Mo 2 C/MoS 2 -1 sample and the identically characterized peaks for the Mo 2 C/MoS 2 -3 sample correspond to the S 2− 2p 1/2 and S 2− 2p 3/2 states, respectively, indicative of MoS 2 , whereas the peak at 167.6 eV is suggestive of SO 4 2− , implicating the partial oxidation of S 2−

Figure 9
Figure 9 delineates the scanning electron microscopy (SEM) representations of the composite catalysts across varying molybdenum carbide to molybdenum disulfide (Mo 2 C/MoS 2 ) molar ratios.Observations reveal a diminution in MoS 2 crystallite dimensions concomitant with an escalation in Mo 2 C/MoS 2 molar proportionality.Precisely, the crystallite dimensions approximate 300 nm for Mo 2 C/MoS 2 -0.5; however, this metric contracts to approximately 60-100 nm for the Mo 2 C/MoS 2 -3 specimen, with the reduced crystallite size engendering a

Figure 10
Figure10delineates the electrocatalytic hydrogen evolution prowess of composite catalysts with varying molar ratios of Mo 2 C to MoS 2 , assayed in an aqueous sulfuric acid solution of 0.5 mol L −1 .As discerned from the linear sweep voltammetry profiles illustrated in Figure10a, the Mo 2 C/MoS 2 -0.5 composite mandated the highest overpotential for achieving a current density of 10 mA cm −2 , in stark contrast to the Mo 2 C/MoS 2 -3 composite, which required the least overpotential at the equivalent current density, signifying its superior electrocatalytic hydrogen evolution capability.Corresponding to these findings, Figure10bpresents the electrocatalytic hydrogen evolution attributes of the catalyst variants stratified by their molar ratios, congruent with the linear sweep voltammetry data: an onset overpotential of 183 mV dec −1 for Mo 2 C/MoS 2 -0.5 and 150 mV dec −1 for Mo 2 C/MoS 2 -4.This suggests that a molar ratio of Mo 2 C to MoS 2 that is either excessively low or high is detrimental to the electrocatalytic efficacy of the composites.The Mo 2 C/MoS 2 -3 composite exhibited the most negligible Tafel gradient, an indicator that this particular formulation can efficaciously mitigate the activation barrier for the electrocatalytic hydrogen evolution process, hence displaying unparalleled performance.Figure10cexposes the electrochemical impedance spectroscopy (EIS) profiles for the composites across varying molar ratios, echoing the aforementioned empirical data: the Mo 2 C/MoS 2 -3 composite's Nyquist plots revealed the least pronounced curvature within the high-frequency electron transport domain, indicative of optimal electrochemical performance.The equivalent circuit diagram and specific values are shown in Table2.

Figure 10 .Figure 10 .
Figure 10.Electrochemical performance of Mo2C/MoS2 composite catalysts with different compos ratios: (a) LSV; (b) Tafel slope; (c) EIS; (d) ECSA.Upon augmentation of the composite ratio, Mo2C, serving as an electron donor, e dowed MoS2 with an enhanced concentration of electrons, thereby diminishing its ele trochemical impedance.The Mo2C/MoS2-3 samples, with a surplus of readily availab Figure 11b shows that it curve of the Mo 2 C/MoS 2 -3 catalyst showed no obvious fluctuation after 12 h of the timed current test at an initial spot level of 223 mV, which exhibited good stability.The above results indicate that the Mo 2 C/MoS 2 -3 catalysts have excellent stability in long-time electrochemical reactions.

Table 1 .
Equivalent circuit and impedance fit for electrochemical impedance.

Table 1 .
Equivalent circuit and impedance fit for electrochemical impedance.

Table 1 .
Equivalent circuit and impedance fit for electrochemical impedance.

Table 2 .
EIS values and equivalent circuit diagrams for different composite ratios.

Table 2 .
EIS values and equivalent circuit diagrams for different composite ratios.

Table 3 .
Comparison of the results of this thesis with the experimental results of other researchers.