Introduction

The increasing population of the world demands more and more energy. About 80% of the energy demand we require is fulfilled by traditional fossil fuels. However, a variety of air pollutants which are detrimental to both the atmosphere and public health are emitted from the combustion of fossil fuels. This encourages scientists to search for new sources of energy which are efficient and environment-friendly. Hydrogen can replace our traditional fossil fuels as it has a high energy density and zero emissions. At a large scale, hydrogen can be produced through electrochemical water splitting or hydrogen evolution reaction (HER). A high-performance, stable, and cost-effective electrocatalyst is a must for this reaction. Platinum (Pt) exhibits the highest HER activity to date; however, its expensiveness and low abundance hinder its large-scale uses. A variety of electrocatalysts has been investigated, which shows significant electrochemical performance towards HER [1,2,3,4,5,6].

Many transition metal–based electrocatalysts have been investigated for HER, including nitrides, oxides, sulfides, and carbides, due to their outstanding electrochemical performance [7, 8]. As a promising material for HER, transition metal carbides (TMCs) have received a lot of interest. Among TMCs, molybdenum carbide is one of the most studied compounds because of its platinum-like electronic structure and analogous electrochemical activity towards HER. Between the two phases of molybdenum carbide (Mo2C and MoC), Mo2C shows higher electrochemical activity than MoC [9]. They show outstanding stability and electrochemical activity in both acidic and basic mediums [10]. The main drawback of Mo2C is its low surface area due to high-temperature synthesis. Therefore, people tried to conjugate Mo2C with carbon, nickel, graphene, etc., to enhance the surface area, and hence, the electrochemical performance of Mo2C improves [11,12,13]. Others tried to prepare Mo2C based composites/heterostructures which show high electrocatalytic activity and high stability. However, the mass production of Mo2C at a large scale is still a challenge.

Recently, Mo2C-based composites have been synthesized by many groups, which show excellent electrochemical performance. Mir and Pandey demonstrated that the incorporation of Mo2C nanoparticles with carbon enhances the electrochemical activity and stability [14]. Pant et al. prepared Ni/MoC-decorated carbon fibers which showed high electrochemical activity with a specific capacitance of 312 F/g at 1 A/g [15]. Jingyu Chen and co-workers reported molybdenum carbide@ nickel foam electrodes synthesized via hydrothermal and calcination process. It showed a Tafel slope of 76 mV/dec and an overpotential of 42 mV to achieve a current density of 10 mA/cm2 [16]. Further, Wang et al. synthesized Mo2C-MoC heterostructures synthesized via hydrothermal followed by the annealing method. The prepared heterostructure showed a Tafel slope of only 54.3 mV/dec [17]. Therefore, it can be concluded that the conjugation of molybdenum carbide with other compounds is an effective strategy to improve the electrochemical performance of the material. In short, the reported methods for the synthesis of Mo2C-based composites are typically complex and expensive, which limit large-scale production. Also, to the best of our knowledge, Mo2C/MoC/C composite has not been synthesized yet.

In this work, we have synthesized Mo2C/MoC/C composite through a high-temperature carburization-reduction route in a single step. Magnesium (Mg) was used as a reducing agent during the reaction. The XRD analysis revealed the formation of Mo2C, MoC, and C phases within the sample. The synthesized sample shows high electrocatalytic activity towards HER with a Tafel slope of 125.5 mV/dec. It shows long cycle stability of up to 5000 cyclic voltammetry (CV) cycles.

Experimental synthesis

Synthesis

In the typical synthesis, 1.23 g of ammonium molybdate tetrahydrate ((NH4)6Mo7O24 · 4H2O), 1.40 g of hexamethylenetetramine C6H12N4, and 1 g of magnesium metal powder (Mg) have been used as molybdenum source, carbon source, and reducing agent, respectively. These materials were mixed in an agate mortar and transferred to a specially designed stainless steel autoclave. The autoclave was then sealed and placed in a pot furnace. The temperature was raised to 800 °C from room temperature at a heating rate of 5 °C/min for 12 h. The autoclave was allowed to cool to room temperature, and the black powder was collected. The obtained powder was then washed with dilute HCl and distilled water several times. The final product was obtained after drying it at 80 °C in a hot air oven.

Characterization

The X-ray diffraction measurements were done using PANalytical X-Pert-Pro diffractometer with CuKα radiation (λ = 1.5406 Å). The morphological and microstructural investigations were done through field emission scanning electron microscopy (FESEM; JEOL JSM 5600) and transmission electron microscopy (TEM; JEOL 2100F) at 15 kV and 200 kV.

The electrochemical measurements were obtained through a three-electrode cell assembly of Biologic EC lab instrument (SP-300). Reversible hydrogen electrode and platinum electrode were used as reference and counter electrodes. To prepare the working electrode, 1.5 mg of the prepared sample was dissolved in 250 µl of ethanol and sonicated for 30 min. Twenty microliters of the uniformly distributed particles of the sample was dropped on the top of the glassy carbon electrode having a surface area of 0.07 cm2. All three electrodes were dipped in a 0.5 M H2SO4 electrolyte solution. Before recoding any measurements, IR correction has been done using biologic EC lab software (ZIR program). The ZIR technique experimentally determines the solution resistance (Ru) of 38 Ω and compensates (80%) for it in real time. After initial stabilization of activities, linear sweep voltammetry (LSV) curves were measured at a scan rate of 5 mV/s. To check the stability of the sample, cyclic voltammetry (CV) measurements were done at a scan rate of 100 mV/s for 5000 cycles within a potential window of 0.1 to 0.4 V. LSV curves were recorded before and after 5000 CV cycles.

Result and discussion

XRD

The XRD analysis has been done to investigate the structure and phase formation of the synthesized sample. As shown in Fig. 1, the XRD pattern of the sample confirms the formation of Mo2C (ICDD card no. 00–035–0787), MoC (ICDD card no. 03–065–6664, 01–089–4305), and C (ICDD card no. 01–089–8487) phases within the powder. The volume percent (vol%) of Mo2C, MoC, and C is about 73.34, 19.59, and 7.07, respectively. The composition of the composite has been estimated using Rietveld refinement of the XRD pattern, and the graphical representation is shown in Fig. S1 (Supplementary Information). It revealed the composition of the composite to be 76.3% of Mo2C, 15.1% of MoC, and 8.6% of C. It means the amount of Mo2C within the sample is higher than MoC and C. However, C has the least amount.

Fig. 1
figure 1

The X-ray diffraction pattern of Mo2C/MoC/C composite

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At high temperatures, AHM decomposes to MoO3, NH3, and H2O according to Eq. 2.

$$(NH_4)_6Mo_7O_{24}.4H_2O \stackrel{\Delta }{\to } 7MoO_3 + 6NH_3 + 7H_2O$$
(1)

MoO3 reduces to Mo with the help of Mg, according to Eq. 2 [18]. Also, Mg reacts with oxygen present in the autoclave to form MgO and hence limits the transformation of Mo2C and MoC to their oxides.

$$Mo{O}_{3}+3Mg=Mo+3MgO$$
(2)

Furthermore, Mo reacts with carbon (C) to form Mo2C and MoC. The synthesis mechanism and phase formation of molybdenum carbides using AHM and hexamethylenetetramine as precursors were explained by Upadhyay and Pandey [9]. The crystallite size of the sample has been calculated using the Debye–Scherrer equation [19]. The crystallite size of the composite has been calculated to be equal to 31.6 nm.

FESEM and TEM

The FESEM images of the prepared Mo2C/MoC/C composites are shown in Fig. 2a, b at different magnifications. These micrographs indicate the formation of highly agglomerated carbide structures. Some of the smaller particles show spheroidal structures which are below 1 µm in size. The porous structure of the samples can be observed in Fig. 2b. Further, the TEM micrograph of the sample suggests that the Mo2C and MoC particles are covered by highly conductive carbon layers (Fig. 2c). These layers promote structural stability and facilitate the fast transfer of electrons and ions. The average particle size calculated from TEM is 33.4 nm. The selected area electron diffraction (SAED) pattern of the sample is shown in Fig. 2d. It shows many diffraction rings consisting of distinct spots that are indexed to ((200), (101)) Mo2C, and (311) MoC. These results are consistent with XRD analysis.

Fig. 2
figure 2

a, b FESEM images, c TEM micrograph, and d SAED pattern of Mo2C/MoC/C composite

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Electrochemical studies

HER activity

To investigate the HER activity of the as-synthesized sample, linear sweep voltammetry (LSV) measurements were done at a scan rate of 5 mV/s, as shown in Fig. 3a. At a potential of −400 mV, the sample provides a current density of about −20.16 mA cm−2. The prepared Mo2C/MoC/C composite exhibited an onset overpotential of ∼102 mV. Also, it showed an overpotential of −336 mV to reach a current density of 10 mA cm−2. For an efficient electrocatalyst, stability is a crucial factor that has been tested using CV measurements for 5000 cycles at a scan rate of 100 mV/s. The LSV curve after 5000 CV cycles is shown in Fig. 3a. Here, it can be observed that there is a negligible difference between LSV curves before and after 5000 cycles which indicates the long-term stability of the sample in an acidic medium. Such high stability can be attributed to the structure and crystal phase stability of β-Mo2C along with the protective action of carbon [14]. The Tafel plots are fitted with the Tafel equation (η = a + b log |j|) to further analyze the HER reaction mechanism, where j is the current density and b is the Tafel slope. As shown in Fig. 3b, the prepared Mo2C/MoC/C electrocatalyst shows a Tafel slope of 125.5 mV/dec, suggesting that the prepared electrocatalyst might be based on the Volmer − Heyrovsky mechanism [20]. In our previous work, we have synthesized Mo2C and MoC nanoparticles using the same synthesis procedure as adopted in the present work. Mo2C and MoC showed a Tafel slope of 129.7 and 266 mV dec−1, respectively, which are higher than Mo2C/MoC/C composite (125.5 mV dec−1) [9]. Therefore, these results indicate that the prepared Mo2C/MoC/C composite showed improved electrochemical activity. Table 1 shows the comparison of water splitting performances of Mo2C-based electrocatalysts.

Fig. 3
figure 3

a LSV curves before and after 5000 CV cycles and b Tafel plot of Mo2C/MoC/C nanocomposite

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Table 1 Comparison of HER performances of Mo2C-based nanostructures
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Conclusion

In summary, we have synthesized Mo2C/MoC/C composite through a high-temperature carburization-reduction route. Magnesium was used as a reducing agent during the synthesis process. The XRD results show the presence of Mo2C, MoC, and C phases within the prepared powder. The Mo2C/MoC/C composite shows a Tafel slope of 125.5 mV/dec with long-term cyclic stability up to 5000 CV cycles. Such high electrochemical performance can be attributed to the synergistic effects between Mo2C, MoC, and C and the high conductivity environment provided by carbon. The presence of carbon within the composite is responsible for such high cyclic stability. The above process for synthesizing a noble metal-free catalyst can be an efficient and environmentally safe way to synthesize Mo2C/MoC/C composite.