FeNi@N‐Doped Graphene Core–Shell Nanoparticles on Carbon Matrix Coupled with MoS2 Nanosheets as a Competent Electrocatalyst for Efficient Hydrogen Evolution Reaction

Synthesis of noble‐metal‐free electrocatalysts for green hydrogen production is crucial to overcoming the energy demand of modern society. One of the most competitive and alternative noble‐metal‐free electrocatalysts for hydrogen evolution reaction (HER) is molybdenum disulfide (MoS2)‐based composites. Herein, it is shown that MoS2 nanosheets grow on FeNi@N‐doped graphene nanoparticles/N‐doped carbon matrix (FeNi@NG/NCM@MoS2), using the hydrothermal method. FeNi@NG/NCM@MoS2 hybrid displays outstanding HER performance with a low overpotential of 79 mV at 10 mA cm−2, a small Tafel slope of 40.2 mV dec−1, and high durability. First‐principles density functional theory simulations confirm the electron transformation from FeNi alloy to NG surface of FeNi@NG particle and subsequently further transfer to MoS2 nanosheets, which decrease the Gibbs free energy (ΔGH* ≈ −0.08 eV) and local work function for enhanced HER activities. This work highlights the understanding of electron transfer in demonstrating the kinetic reaction of the HER process and offers a new avenue for constructing efficient MoS2‐based electrocatalysts.


Introduction
In the last few decades, human population of the world is rapidly growing that will require a huge amount of energy source for daily life activities. The development of sustainable clean energy technologies is highly essential to meet the requirement of the modern world and simultaneously keep safe our livable environment. Hydrogen is a vital clean energy source and a valuable alternative to fossil fuels for the long-term future. [1] At present, 96% of hydrogen is produced from fossil fuels such as coal gasification and steam methane reformation process. [1,2] However, this process produces some toxic greenhouse gases including carbon dioxide, [2] that seriously affect our environment. Electrochemical water splitting is a promising green approach for clean and renewable hydrogen production and can also overcome environmental challenges. [3,4] Generally, high active electrocatalysts have been used to derive the hydrogen evolution reaction (HER) at low overpotential. The most documented highly electroactive catalyst for HER is Pt-based materials (PBM). [5,6] However, the high cost and scarcity of PBM limit its largescale hydrogen production. Therefore, it is highly desirable to develop a highly active, environmentally friendly, low-cost, and earth-abundant elements-based electrocatalyst for HER. In this connection, some competitive noble-metal-free electrocatalysts have been reported for enhanced HER performance. [7][8][9][10][11][12] Molybdenum disulfide (MoS 2 ) nanosheets are one of the emerging candidates to replace noble-metal-HER catalysts due to their low cost, remarkable performance, and excellent stability. [13][14][15] It is believed that HER active sites in intrinsic MoS 2 nanosheets are mostly located on exposed edges. [13] Because of rich terminating sulfur groups at the edges of monolayer or few-layer of MoS 2 nanosheets, it can display better HER performance compared to that of bulk MoS 2 . [16] However, the MoS 2 nanosheets tend to agglomerate due to inherent stacking by van der Waals forces, which decreases the active sites for HER activity. The synthesis of MoS 2 nanosheets with maximum numbers of exposed active sites and nonagglomeration is an Synthesis of noble-metal-free electrocatalysts for green hydrogen production is crucial to overcoming the energy demand of modern society. One of the most competitive and alternative noble-metal-free electrocatalysts for hydrogen evolution reaction (HER) is molybdenum disulfide (MoS 2 )based composites. Herein, it is shown that MoS 2 nanosheets grow on FeNi@N-doped graphene nanoparticles/N-doped carbon matrix (FeNi@NG/NCM@MoS 2 ), using the hydrothermal method. FeNi@NG/ NCM@MoS 2 hybrid displays outstanding HER performance with a low overpotential of 79 mV at 10 mA cm −2 , a small Tafel slope of 40.2 mV dec −1 , and high durability. First-principles density functional theory simulations confirm the electron transformation from FeNi alloy to NG surface of FeNi@NG particle and subsequently further transfer to MoS 2 nanosheets, which decrease the Gibbs free energy (ΔG H * ≈ −0.08 eV) and local work function for enhanced HER activities. This work highlights the understanding of electron transfer in demonstrating the kinetic reaction of the HER process and offers a new avenue for constructing efficient MoS 2 -based electrocatalysts.
ideal strategy. The hierarchical morphology of MoS 2 nanosheets is a better choice for exposing more active sites on the edges for the enhanced HER performance. [17] Another fundamental problem of MoS 2 nanosheets is their semiconducting nature, [18] which restricts the electron transfer to reach the catalytic active sites and combines with adsorbed H + in an acidic medium to produce hydrogen mole cule. [19,20] So, the poor catalytic activity of observed for MoS 2 nanosheets in the HER process. The introduction of conductive materials into MoS 2 can enhance the electron transfer rate to active edges of MoS 2 and can improve the HER performance. [19,20] Recently, MoS 2 nanosheets are grown on various carbon materials such as graphene, carbon nanotubes, hollow carbon sphere, carbon cloth, etc. for enhanced HER performances. [21][22][23][24] Nevertheless, it is found that the conductivity of graphitic carbons severely damages with growth or deposition of semiconductor materials and oxygen functionalization. Carbon encapsulated metals nanomaterials can provide an opportunity to maintain high electrical conductivity for MoS 2 nanosheets by penetrating electrons from core metal particles to carbon surface and further transferring them to semiconducting MoS 2 nanosheets. [25] Moreover, carbon can be used as a substrate for the growth of MoS 2 as well as protection for metallic core metals nanoparticles in both acidic, [26,27] and alkaline media, [28] in electrochemical HER performance. It is highly desired to develop a method to anchor MoS 2 nanosheets on carbon encapsulated metals nanomaterials for high HER performance.
Here, we reported that MoS 2 nanosheets grow on FeNi@N-doped graphene on/in N-doped carbon matrix (FeNi@NG/NCM@MoS 2 ) by hydrothermal method. The MoS 2 nanosheets grow both FeNi@N-doped graphene (FeNi@NG) and N-doped carbon (NDC) matrix and formed hierarchical morphology with highly exposed active sites for HER activity. The FeNi@NG/NCM@MoS 2 hybrid exhibits outstanding HER performance with a low overpotential of 79 mV at 10 mA cm −2 and a small Tafel slope of 40.2 mV dec −1 . Besides, our electrocatalysts are highly stable and durable in acidic media. Finally, we performed density functional theory (DFT) calculations for our experimentally observed FeNi@NG/NCM@MoS 2 electrocatalyst. The simulated Gibbs free energy, electron difference density, and electronic properties of the FeNi@NG/NCM@MoS 2 model demonstrate and validate the role of electron transfer in the excellent HER electrocatalytic activity.

Synthesis of FeNi@NG/NCM
The FeNi@NG/NCM was prepared according to the previous report with some modifications. [29] Briefly, 0.5 g of Ni(CH 3 COO) 2 ·4H 2 O, 0.6 Fe(NO 3 ) 3 ·9H 2 O, 0.2 g of polyvinylpyrrolidone, and 1 g of urea were ground together. The powder was dissolved in ethanol and kept at room temperature to evaporate the solvent. Then, the reaction mixture was heated at 750 °C in a tube furnace for 45 min under an Ar atmosphere. The sample was collected and treated with 1 m H 2 SO 4 solution for 6 h to remove FeNi without N-doped graphene shells.

Synthesis of FeNi@NG/NCM@MoS 2
The FeNi@NG/NCM@MoS 2 hybrids structure was prepared by the hydrothermal method. 0.1 g of as-prepared FeNi@NG/NCM and 0.1 g of (NH 4 ) 2 MoS 4 precursor were dispersed in water, followed by sonication. The reaction mixture was transferred to a 25 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h. The precipitate was obtained by centrifugation and washed with distilled water and ethanol, respectively. The final product was obtained after drying in the vacuum oven for 20 h.

Synthesis of MoS 2 Nanosheets Sphere
The MoS 2 spheres were also prepared by hydrothermal method. 0.1 g of (NH 4 ) 2 MoS 4 precursor was dissolved in water by sonication and subsequent similar steps were used for the synthesis of FeNi@NG/NCM@MoS 2 nanocomposites.

Characterizations
In order to observe the morphologies and microstructures of the as-prepared products, they were characterized by scanning electron microscope (SEM Hitachi S-4800) and transmission electron microscope (TEM, JEM-2010). The elemental mapping images of the sample were observed by energy-dispersive X-ray (EDX) spectroscopes, attached with TEM. The crystal phase compositions of products were analyzed by an X-ray diffractometer (XRD, Bruker D-8 Advanced diffractometer). Raman spectrometer (a JYHR800, 532 nm laser source) was used to acquire Raman spectra of samples. Brunauer-Emmett-Teller (BET) surface area was measured from the specific surface porosity analyzer (ASAP2020 Mack instruments). The elemental composition and valence state of products were analyzed by X-ray photo electron spectroscopy (XPS, Thermo ESCALAB 250XI system with Al Kα X-ray as an excitation source).

Electrochemical Measurements
All the catalytic measurements for HER performance were analyzed by a three-electrode system on an electrochemical workstation (CHI 760D, Chen Hua Instruments Co. Ltd., Shanghai, China). A glassy carbon electrode was used as a working electrode, while a carbon rod (demission: dimeter = 3 mm, length = 50 mm), and saturated calomel electrode (SCE) were used as a counter, and reference electrode, respectively. Catalyst sample (4 mg) and 5 wt% of Nafion solution (40 µL) were dispersed in ethanol (960 µL) by sonicating to obtain a homogeneous ink. For electrochemical tests, 5 µL of ink was dropped onto a glassy carbon electrode and waited to dry in the air before being used. The cyclic voltammograms (CVs) with a scan rate of 100 mV s −1 were performed in 0.5 m H 2 SO 4 solution for 50 cycles. The linear sweep voltammetry (LSV) curve was measured in the same solution at a scan rate of 5 mV s −1 . For the stability test, continuous 1500 CV cycles were measured at a scan rate of 100 mV s −1 from −0.6 to 0 V versus SCE, and remeasure LSV curve. The chronoamperometric carves were obtained for 10 h to judge the long-term stability of the sample. For the electrochemical active surface area of catalysts, the double-layer capacitances (C dl ) were calculated from CV cycles with a potential window of 0 and 0.1 V versus SCE at different scan rates. Electrochemical impedance spectroscopy (EIS) tests were performed in the frequency range of 100 kHz to 100 MHz with an AC voltage of 5 mV in 0.5 m H 2 SO 4 solution.

Computational Methodology
DFT simulations were carried out for the model of experimentally observed electrocatalysts, using Quantum ATK, [30] and the results were analyzed on VESTA and vnl Version 2019.12. [30] To model the experimentally observed MoS 2 nanosheets grow on FeNi@N-doped graphene/N-doped carbon matrix (FeNi@NG/NCM@MoS 2 ), first, i) a single layer of 5% N-doped C matrix (NCM) was modeled, ii) then a single layer of MoS 2 sheet, iii) followed by 5% nitrogen-doped graphene shell which encapsulated the FeNi nanoparticles (FeNi@NG), iv) and finally these three species were attached on each other and built the FeNi@NG/NCM@MoS 2 hybrid electrocatalyst. All these model structures are shown in Figure S1 (Supporting Information).
The HER activity of FeNi@NG/NCM, MoS 2 , NCM@MoS 2 , and FeNi@NG/NCM@MoS 2 was calculated in the form of Gibbs free energy, using DFT calculations. Subsequently, one H atom was attached to the surfaces of these four species and the systems were allowed to be relaxed. Although the size of the theoretical models was smaller than their experimentally observed species, the essential effect on the electronic structure was already captured by this simple geometry. The generalized gradient approximation with the Perdew-Burke-Ernzerhof exchange-correlation functional and double Zeta Polarized basis set was employed for the structural and energy optimization. In this work, a linear combination of atomic orbitals method was used for Fe, Ni, Mo, S, C, N, and H atoms. [31] A 7 × 7 × 3 Monkhorst-Pack k-point sampling was adopted for structural optimization and 7 × 7 × 7 for the electronic property simulations.
As reported in the previous work, the overall HER mechanism over the surface of the electrocatalyst was evaluated with a three-state diagram, consisting of an initial H + state, an intermediate H* state, and 1/2H 2 as the final product. As is known, the H* free energy of (ΔG H * ) is a key for describing the HER activity of the electrocatalyst. An electrocatalyst with a positive value ΔG H * shows low kinetics of adsorption of hydrogen. While a negative ΔG H * value is responsible for low kinetics of release of a hydrogen molecule. So, the acceptable value of ΔG H * must be zero; i.e., the ΔG H * value of PM-based catalyst such as Pt is near-zero as ΔG H * ≈ 0.09 eV. [32] The ΔG H * can be determined with the help of Equation (1) In Equation (1), ΔE denotes the total energy change which can be calculated from the DFT simulations, ΔZPE and ΔS are the changes in zero-point energy and entropy, respectively. And T is the room temperature (298.15 K). As reported elsewhere, the theoretical working potential is independent of the pH, which is due to the free energies of the elementary reactions varying in the same way as pH. So, the potential determining step remains constant. To simplify the theoretical simulated data we set the pH = 0. The free energy of hydrogen at standard conditions is assumed as the energy of 1/2H 2 . The entropy of the H 2 is taken from the NIST database [33] Here, E (H*) and E (*) are calculated from the DFT which is of the given surface with and without H adsorption, respectively, and E(H 2 ) is the DFT energy of a molecular H 2 in the gas phase. After all, the ΔE H * is the binding energy of adsorbed hydrogen. Therefore, Equation (1) can be rewritten as Equation (3) is the final equation for calculating the free energy of H.

Synthesis and Characterization
Scheme 1 shows the synthesis process of FeNi@NG/NCM@MoS 2 hybrid electrocatalyst The FeNi@NG/NCM was prepared according to the previous report. [29] The FeNi@NG/NCM were dispersed in water and then (NH 4 ) 2 MoS 4 precursor was added to the dispersion. The (NH 4 ) 2 MoS 4 dissociated in the thiomolybdate anions (MoS 4 ) 2− and ammonium cation (NH 4 ) + in hydrothermal conditions. The thiomolybdate anions are adsorbed pyridinic N-doping sites of the FeNi@NG/NCM by electrostatic attraction, [22] and subsequently act as nucleation sites for the growth into MoS 2 . On further reaction, the MoS 2 starts to grow on FeNi@NG/NCM. Most MoS 2 nanosheets vertically grow on FeNi@NG/NCM. This vertical growth might be possibly due to its lamellar structure.
A flaky type structure can be clearly observed in SEM images of FeNi@NG/NCM sample as shown in Figure 1a. The areas of these flaky structures are several square micrometers. On further magnification, the surface of the flaky structure is relatively rough, and some nanoparticles can be seen as shown in Figure 1b. When (NH 4 ) 2 MoS 4 precursor is treated without FeNi@NG/NCM sample in hydrothermal conditions, the sphere-like structures are obtained ( Figure S2a, Supporting Information). Actively, the sphere-like structure is consisted of nanosheets and suggests the agglomeration of MoS 2 ( Figure S2b, Supporting Information).
TEM images of FeNi@NG/NCM sample are shown in Figure 1c, where it can be seen that the flaky structure is of several micrometers. Well-defined nanoparticles can be seen on/in the flaky structure (Figure 1c  After the treatment of FeNi@NG/NCM sample with (NH 4 ) 2 MoS 4 ) precursor, the SEM images show that the surface morphology of the flaky type structure of FeNi@NG/NCM is changed to hierarchical as shown in Figure 2a. Actually, the hierarchical structure denotes that MoS 2 nanosheets grow on FeNi@NG/NCM. The hierarchical sphere-like (shown by red arrows) morphology indicates MoS 2 nanosheets, which are grown on FeNi@NG nanoparticles, while the MoS 2 nanosheets are also observed on NCM (Figure 2b). This suggests that hierarchical vertically MoS 2 nanosheets are grown on N-doped carbon of both FeNi@NG and NCM. From careful observation, it can be observed that these nanosheets are very small, and basal to edges is very high. The edges of most MoS 2 nanosheets are highly exposed. Recently, it is reported that hierarchical morphology MoS 2 and its highly exposed edges are beneficial for enhanced HER performance. [17,34] The MoS 2 nanosheets are scattered on the surface of the NCM in the NCM@MoS 2 composite, which is different from the FeNi@NG/NCM@MoS 2 hybrid ( Figure S4a,b, Supporting Information).
Analysis of TEM images also confirms that the surface morphology of FeNi@NG/NCM is changed. The MoS 2 nanosheets grow on FeNi@NG/NCM can be clearly observed in Figure 2c. The FeNi@NG/NCM sample is more transparent without nanoparticles places, while it becomes darker in the nanoparticles (Figure 2d). MoS 2 nanosheets can be observed on NCM in the NCM@MoS 2 sample as shown in Figure S4c  In addition, the Raman spectral analysis of FeNi@NG/NCM@MoS 2 , NCM@MoS 2 hybrids, and MoS 2 nanosheets, shown in Figure 3b, was also performed. FeNi@NG/NCM@MoS 2 hybrids show intense D-and G-bands peaks at 1346 and 1584 cm −1 , respectively, while the D-and G-bands peaks were observed for NCM@MoS 2 sample at 1356 and 1595 cm −1 , respectively. These peaks confirm the characteristic bands of carbonaceous materials. [35] The intensity ratio (I D /I G ) of FeNi@NG/NCM@MoS 2 and NCM@MoS 2 are about 0.9 and 1.03, respectively, and show relativity graphitization of carbon. The weak peaks are observed at 373 and 401 cm −1 , which can be assigned as in-plane (E 1 2g ) and out-of-plane (A 1 g ) vibration modes of MoS 2 , respectively, [36,37] while these peaks are more intense in the pure MoS 2 nanosheets. The frequency difference (Δk) can be used to estimate the number of MoS 2 layers in nanosheets. Δk of MoS 2 in all samples is about 27 cm −1 and suggests that the number of MoS 2 layers is more than 5. [36,37] The BET results show the specific surface area of MoS 2 ( Figure S7a, Supporting Information), FeNi@NG/NCM@MoS 2 ( Figure S7b, Supporting Information), and NCM@MoS 2 ( Figure S7c, Supporting Information) are 11, 18, and 33.2 cm 3 g −1 , respectively.
The XPS spectrum was acquired for the surface elemental composition and chemical state of the samples. The survey XPS spectra of FeNi@NG/NCM@MoS 2 and pure MoS 2 nanosheets are shown in Figure S8a of the Supporting Information. The FeNi@NG/NCM@MoS 2 composites show peaks for Fe, Ni, Mo, S, C, N, and O elements, while MoS 2 nanosheets contain Mo, S, C, and O elements. The high-resolution XPS spectrum of the Fe 2p region has main peaks at about 707.2 and 720.3 eV, which can be associated with Fe 2p 3/2 and Fe 2p 1/2 bands of zero valance of metallic Fe (Figure 4a). [38,39] The peaks at about 710.3 and 723.7 eV are related to Fe 2p 3/2 and Fe 2p 1/2 signals of oxidized iron and/or satellite of iron. [29,40] The Ni 2p XPS spectrum shows two intense peaks at 853.2 and 871.2 eV, which correspond to Ni 0 2p 3/2 and Ni 0 2p 1/2 of metallic Ni, respectively (Figure 4b). [41,42] The Mo 3d spectrum of FeNi@NG/NCM@MoS 2 hybrid shows four peaks (Figure 4c). The peak at 225.25 eV matches the binding energy of the S 2s band. [43,44] The well-defined peaks at 228 and 231.31 eV correspond to Mo 3d 5/2 and Mo 3d 3/2 bands of the Mo 4+ oxidation state of the MoS 2 compound. [45,46] The weak and broad peaks centered at 234.3 eV are due to oxidation of Mo to Mo 6+ . [46] The Mo 3d 5/2 and Mo 3d 3/2 bands of pure MoS 2 nanosheets peaks at about 228.18 and 231.40 eV show Mo 4+ oxidation state, respectively. The XPS spectrum of the S 2p region of FeNi@NG/NCM@MoS 2 hybrid shows pair of peaks at 162.11 and 160.8 eV of S 2p 1/2 and S 2p 3/2 spin-orbit doublets of S 2− species, respectively (Figure 4d), [43,44,47]   atoms in FeNi@NG/NCM@MoS 2 hybrid compared to that of pure MoS 2 nanosheets suggest the interaction between MoS 2 nanosheets and FeNi@NG/NCM. These results also indicate a relatively high electron density on the Mo and S atoms of MoS 2 nanosheets in FeNi@NG/NCM@MoS 2 hybrid as compared to pure MoS 2 nanosheets. A peak at higher binding energies (168.7 eV) can be related to SO 3 2− and/or SO 4 2− species, possibly due to the surface oxidation of MoS 2 . [43] The high-resolution XPS spectra of C 1s region of FeNi@NG/NCM@MoS 2 hybrid shows peak for CC/CC (284.2 eV) and NC/CO (≈285.5 eV) and CO (≈287.95 eV) ( Figure S8b, Supporting Information). [48,49] The high-intensity peak of CC/CC bonds indicates the presence of graphitic carbon in the composite after the growth of MoS 2 nanosheets. The N 1s spectrum show peak centered at 399.7 eV ( Figure S8c, Supporting Information), [29] and it is very difficult to correctly deconvolute it into different nitrogen species. The reason behind this is the overlapping with the Mo 3p region. However, well-defined peaks for pyridinic, pyrrolic, and graphitic nitrogen have been previously observed in the XPS spectrum of FeNi@NG/NCM. [29]

Electrochemical HER
The HER performance of as-prepared samples and 20% Pt/C were measured with the help of a three-electrode system on an electrochemical workstation in 0.5 m H 2 SO 4 solution. The LSV curves of the catalysts were measured at a scan rate of 5 mV s −1 with iR correction (Figure 5a). As usual, 20% Pt/C catalyst shows the best HER performances and required an overpotential of 35 mV at a current density of 10 mA cm −2 . The MoS 2 , NCM@MoS 2 , and FeNi@NG/NDM samples display poor HER performance with an overpotential of 191, 212, and 365 mV at a current density of 10 mA cm -2 , respectively (Table S1, Supporting Information). Upon growth of MoS 2 nanosheets in FeNi@NG/NCM sample, the overpotential significantly decreased to 79 mV at a current density of 10 mA cm −2 . This HER performance of FeNi@NG/NCM@MoS 2 catalyst is 2.42, 2.79, and 6.42 times that of MoS 2 , NCM@MoS 2 , and FeNi@NG/NCM samples, respectively. Furthermore, a physical mixture (PM) of FeNi@NG/NDM sample and MoS 2 nanosheets with a 1:1 ratio was investigated for HER performance. PM also shows poor HER performance at 325 mV at a current density of 10 mA cm −2 . The FeNi@NG/NCM@MoS 2 catalyst shows better performance than those of other as-prepared catalysts and also the better or comparable performance with reported MoS 2 -based catalysts (Table S2, Supporting Information). This suggests that the enhanced HER performance is due to the synergistic effect between FeNi@NG/NCM and MoS 2 nanosheets.
In order to estimate the HER activities of the investigated samples, the Tafel plot analyses were studied (Figure 5b). The Tafel slopes of catalysts are obtained from LSV carves by fitting them in the Tafel equation. Generally, the smaller value of the Tafel slope of catalysts is believed to be useful for practical application. The  20% Pt/C catalyst displays a small Tafel slope of 29.5 mV dec −1 . Among other tested samples, the FeNi@NG/NCM@MoS 2 catalyst demonstrates the smallest Tafel slope (40.2 mV dec −1 ) and most effective kinetics for hydrogen production. The Tafel slope of MoS 2 (67.4 mV dec −1 ), NCM@MoS 2 (69.2 mV dec −1 ), PM (90.8 mV dec −1 ), and FeNi@NG/NDM (137.4 mV dec −1 ) catalysts are higher and comparatively slower reaction kinetics for HER. To study the rate-determining step of HER in acidic media, the HER mechanism comprises of three basic reactions, Volmer reaction (H 3 O + + e − → H ads + H 2 O, initial discharge step, and Tafel slope ≈120 mV dec −1 ), Heyrovsky reaction (H 3 O + + H ads + e − → H 2 + H 2 O, electrochemical desorption step and Tafel slope ≈40 mV dec −1 ), and the Tafel reaction (H ads + H ads → H 2 , recombination step and Tafel slope ≈ 30 mV dec −1 ). [43,[50][51][52] From Tafel slope values, the rate-limiting step for HER is either Volmer-Heyrovsky or Volmer-Tafel reaction in acidic conditions. The Tafel slope value of FeNi@NG/NCM@MoS 2 hybrid is (40.2 mV dec −1 ), and may follow the Volmer-Heyrovsky rection. Possibly the rate-determining step is an electrochemical desorption step.
The electrochemical double-layer capacitances (C dl ) of the investigated samples were calculated from CVs and their electrochemical surface area is estimated as shown in Figure S10 of the Supporting Information. The C dl value of samples are in the order of FeNi@NG/NCM@MoS 2 (7.2 mF cm −2 ) > MoS 2 (4.9 mF cm −2 ) > NCM@MoS 2 (4.3 mF cm −2 ) > FeNi@NG/NCM (1.2 mF cm −2 ). Comparative analysis of the C dl values of these samples led to indicate that the active sites are larger for FeNi@NG/NCM@MoS 2 hybrid than those of FeNi@NG/NCM and MoS 2 nanosheets. So, the results suggest that the HER catalytic activity is mostly due to MoS 2 nanosheets instead of FeNi@NG/NCM product. Besides, MoS 2 nanosheets in FeNi@NG/NCM@MoS 2 hybrid are highly exposed to HER catalytic activity and easily accessible to electrolytes compared to that of pristine MoS 2 nanosheets.
Furthermore, the EIS of these catalysts was measured as shown in Figure 5c. The Nyquist plots were fitted with an equivalent circuit (inset of Figure 5c)  Another important aspect of an electrocatalyst is stability. So, the catalyst with long-term stability can be used for practical hydrogen production. The stability of FeNi@NG/NCM@MoS 2 sample was tested by performing 1500 continuous CV cycling in 0.5 m H 2 SO 4 solution and comparable with reported composites catalysts (Table S2, Supporting Information). The LSV curves of the catalyst before and after 1500 cycles are shown in Figure 5d. It can be analyzed that only small decay in HER performance especially at lower current density is observed compared to the initial one. This indicts the high stability of the prepared catalyst. Moreover, the same sample was characterized by TEM, HRTEM, and XPS after 1500 HER performance. The flaky structure with hierarchical morphology of FeNi@NG/NCM@MoS 2 sample can still be observed in TEM images after 1500 HER performance ( Figure S11a,b, Supporting Information). The HRTEM image shows that the FeNi@NC nanoparticles and MoS 2 nanosheets are observed in the composite ( Figure S11c, Supporting Information). These results confirm the long-term stability of the prepared catalyst. However, the structure of MoS 2 nanosheets is slightly damaged during the stability test compared to the fresh sample. The XPS survey spectrum also confirms the existence of Mo, S, Fe, Ni, C, N, and O elements in the composite ( Figure S12a, Supporting Information). The peak intensity of Mo 6+ and SO 4 2− species in XPS spectra of the post-HER sample increased which indicates the further oxidation of these elements ( Figure S12b,c, Supporting Information). Furthermore, the XPS spectra confirm the presence of metallic Ni and Fe with zero valence states after 1500 cycles ( Figure S12d,e, Supporting Information), however, the surfaces of these elements are oxidized as can be observed from the oxidation peaks. The XPS spectrum of C is not obviously changed in the post-HER sample and suggests the high stability of carbon materials ( Figure S12f, Supporting Information). Moreover, chronoamperometric analysis was performed to test the long-term stability of FeNi@NG/NCM@MoS 2 sample. It is found that the current density loss was about 12.8% after 10 h, which validates the long-term stability of the catalyst.

First-Principles Electronic Properties
As discussed earlier, first-principles DFT studies were performed to countercheck the enhanced HER activity of FeNi@NG/NCM@MoS 2 . The NG encapsulated FeNi nanoparticles on N-doped carbon matrix interacted with MoS 2 nanosheet (for details see Computational Methodology section, Supporting Information). The free energy for H adsorption (ΔG H* ) was simulated over the surface of the observed FeNi@NG/NCM@MoS 2 , FeNi@NG/NCM, MoS 2 , and NCM@MoS 2 (see Figure S1, Supporting Information). The DFT simulated ΔG H* values of these four species are shown in Figure 6a and Table S3 (Supporting  Information), where the ΔG H* value of FeNi@NG/NCM, NCM@MoS 2 , MoS 2 , and FeNi@NG/NCM@MoS 2 are −0.87, −0.14, −0.11, and −0.08 eV, respectively. Generally, an efficient electrocatalyst must have reasonable ΔG H* to overcome the reaction barriers of the HER process, in the adsorption and desorption stages. Comprehensive analyses of Figure 6 led to conclude that the ΔG H* value of FeNi@NG/NCM@MoS 2 is ideal, and very close to the thermodynamic limit value (vide supra). These simulated Gibbs free energies further confirm and validate the efficient performance of the FeNi@NG/NCM@MoS 2 electrocatalyst for HER. The high activity of FeNi@NG/NCM@MoS 2 can be regarded due to the ideal electrostatic interaction of hydrogen with the surface of MoS 2 of FeNi@NG/NCM ( Figure S13f, Supporting Information). Besides, the ΔG H* value of FeNi@NG/ NCM is very high (−0.87 eV). The reason behind this is the strong adsorption energy (−1.24 eV) of H* that subsequently obstructs the HER process and reduces the performance of the catalyst. From Table S3 of the Supporting Information, it can be analyzed that the hydrogen adsorption energy for FeNi@NG/ NCM, MoS 2 , NCM@MoS 2 , and FeNi@NC/NCM@MoS 2 are −1.24, −0.51, −0.48, and −0.45 eV, respectively. So, the stronger ΔG H* of hydrogen adsorption for FeNi@NG/NCM, MoS 2 , NCM@MoS 2 hinders the hydrogen desorption process which finally reduces the overall HER process. The excellent HER activity of FeNi@NC/NCM@MoS 2 hybrid is due to the interaction of H and S at the surfaces of MoS 2 of the FeNi@NC/ NCM@MoS 2 , where H + is attached. The main reason behind the excellent catalytic activity of FeNi@NC/NCM@MoS 2 is the electron transfer mechanism. The transfer of electrons occurs from FeNi alloy core to NG shell of FeNi@NG nanoparticle and then to MoS 2 nanosheet in FeNi@NG/NCM@MoS 2 . This electron transfer phenomenon can be seen in Figure 6b and Figure S14 (Supporting Information), where the electron density on MoS 2 surface becomes moderately increased and attracts protons, leading to the efficient evolution of hydrogen molecules. Possibly, some electrons may be electron transferred from the FeNi@NG particles to NCM and then to MoS 2 nanosheets. So, FeNi alloy has a key role in this electron transformation and enhances the activity of MoS 2 in FeNi@NG/NCM@MoS 2 . On the other hand, the intercharge transferring phenomenon in FeNi@NG/NCM is at its peak which offers strong attractive forces for proton. So, the excited H* cannot be easily desorbed from the surface of the catalyst which reduces the HER performance. In the case of NCM@MoS 2 , the hydrogen adsorption energy is −0.48 eV, which is still higher than that of FeNi@NC/ NCM@MoS 2 , responsible for lower HER performance as listed in Table S3 of the Supporting Information.
In addition, the efficient HER activity of FeNi@NC/ NCM@MoS 2 may be due to an excellent interaction of FeNi@NC/NCM@MoS 2 nanoparticles with MoS 2 nanosheets. The partial density of the state plot of FeNi@NC/NCM@MoS 2 is shown in Figure 6c, where the bonding orbitals of Ni and Fe have strong hybridization in the region of −10 to 0 eV. While the antibonding orbitals of Fe, Ni, Mo, S, N, and C have mutual orbital overlapping in the energy range of 0 to 10 eV. The improved orbital hybridizations of these bonding and antibonding orbitals are responsible for the excellent charge transferring and overall stability of MoS 2 in FeNi@NC/NCM@MoS 2 . Moreover, the existence of FeNi alloy within the NG shell and its interaction with MoS 2 results in extra features near the Fermi level. So, this charge is transferred from FeNi alloy core to NG shell, and then between FeNi@NC/NCM and MoS 2 sheet. All these charges transferring consequently reduce the Fermi level of FeNi@NC/NCM@MoS 2 . The work function can be determined from the reciprocal of Fermi energy. The work function decreases from 4.40 (FeNi@NG/NCM) to 3.56 eV (FeNi@NC/ NCM@MoS 2 ) as can be analyzed in Table S3 of the Supporting