Three-Dimensional Flower-like Fe, C-Doped-MoS2/Ni3S2 Heterostructures Spheres for Accelerating Electrocatalytic Oxygen and Hydrogen Evolution

Abstract

The exploration of high-efficiency bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has long been challenging. The rational design of a catalyst by constructing heterostructures and a doping element are possibly expected to achieve it. Herein, the utilization of flower-like Fe/C-doped-MoS₂/Ni₃S₂-450 spherical structural materials for electrocatalytic HER and OER is introduced in this study. The carboxyferrocene-incorporated molybdenum sulfide/nickel sulfide (Mo_yS_x/NiS) nanostructures were prepared by solvothermal method. After annealing, the iron and carbon elements derived from ferrocenecarboxylic acid enhanced the electrical transport performance and provided rich electronic sites for HER and OER in alkaline media. Specifically, the optimized flower-like Fe/C-doped-MoS₂/Ni₃S₂-450 exhibited efficient bifunctional performance in alkaline electrolyte, with low overpotentials of 188 and 270 mV required to deliver a current density of 10 mA cm⁻² for HER and OER, respectively. This work provides valuable insights for the rational design of energy storage and conversion materials by the incorporation of transition metal and carbon elements into metal sulfide structures utilizing metallocene.

1. Introduction

Environmental problems, such as air pollution and global warming caused by the overuse of fossil fuel, have long been concerning [1,2,3]. To tackle these environmental problems, attention has been shifted from traditional fuels to efficient and clean energies. H₂ is considered as an ideal clean energy and electrolysis of water to produce hydrogen is a very feasible strategy [4]. The electrochemical overall water splitting process is constituted by the hydrogen evolution reaction (HER) on the cathode side and the oxygen evolution reaction (OER) on the anode side and requires catalysts of outstanding performance promoting the reaction process [5]. Noble metals with promising catalytic properties, such as Pt, RuO₂, and IrO₂, have recently been considered as ideal catalytic materials for HER and OER; both of them correspondingly are used in the anode and cathode reaction as a feasible strategy for overall water splitting [6,7,8]. However, the utilization of these materials in the electrochemical overall water splitting field is always restricted due to the high cost and rarity [8,9]. With the development of functional materials in nanoscale, significant advances have been made in the study of electrocatalysts, but most electrocatalysts exhibit highly efficient catalytic activity only for HER or OER reactions [4,5,10]. Hence, it is significant to design a low-cost bifunctional catalyst with excellent performance in both HER and OER process to achieve high efficiency overall water splitting.

Most bifunctional catalysts were designed by integrating excellent active components of HER and OER [4]. The as-designed catalysts could efficiently urge active sites to favorably bind towards both oxygen-containing and hydrogen intermediates, reducing the kinetic energy barriers [5]. Molybdenum disulfide (MoS₂), a representative HER catalyst, has widely attracted attention from researchers [11,12,13] since its free energy of adsorbed H⁺ is close to Pt [14,15]. However, MoS₂ is not suitable as a catalyst for OER process because of providing intrinsically poor OER activity [5,16]. It is a feasible strategy to introduce OER active components into MoS₂-based materials to prepare bifunctional catalysts [11,17]. Additionally, nickel sulfides [18,19,20] exhibited excellent catalytic performance in the OER process due to their unique physicochemical properties. Bolin Li et al. [21] fabricated in situ N-doped Ni₃S₂ nanowires on Ni-foam by high-temperature pyrolysis, which showed excellent performance in OER process, requiring only 271 mV overpotential to achieve a current density of 10 mA/cm². Highly durable OER performance was demonstrated by 40 h chronopotentiometry and 10,000 cyclic voltammetry (CV) cycle tests. The rational construction of MoS₂/Ni₃S₂ heterostructures has the potential to achieve highly efficient bifunctional electrocatalysts [22,23]. Furthermore, constructing rational heterostructures could be beneficial to fulfill the synergy in heterostructures, exposing more active sites, promoting catalyst performance, and the electronic transfer. In addition, OER activity of catalysts also could be effectively enhanced by properly doping iron [24,25]. However, their applications also are hampered due to intrinsically low electrical conductivity [4,26]. To prepare a highly conductive catalyst, for a powder catalyst, this is usually achieved by building heterostructures with superior conductive materials, including carbon nanotubes and graphene [27]. C elements derived from metal-organic frameworks (MOFs) also could improve conductivity of electrocatalysts, provide the efficient pathways for charge transport, reduce resistance to charge transfer, and downgrade overpotential of reactions [28,29,30].

Recently, methods using ferrocene and ferrocenecarboxylic acid to modify electrocatalysts have attracted considerable attention, such as Ziqian Xue et al. [31], who prepared missing-linker metal-organic (MOF) frameworks through modifying Co-BDC with ferrocenecarboxylic acid (Fc), and thus achieved excellent OER performance with ultra-low overpotential and current density of 241 mV at 100 mA cm⁻², respectively. Pitchai Thangasamy et al. [32] synthesized ferrocene-incorporated cobalt sulfide nanoarchitecture, investigated OER performance, and finally gained rather good results: a Tafel slope of 54.2 mV dec⁻¹ and an overpotential as low as 304 mV to drive a current density of 10 mA/cm². The authors attribute the enhancing performance of OER to the strong synergistic effect of Fe-based compounds derived from ferrocene. Few studies, however, have been reported for lowering overpotential of HER process by using ferrocenecarboxylic acid. Given all this, considering the different roles of Fe and C elements in catalysts, we decided to prepare an efficient bifunctional catalyst by using the strategy of doped Fc.

Herein, we successfully prepared the flower-like Fe/C-doped-MoS₂/Ni₃S₂-450 (Fe, C-MoS₂/Ni₃S₂-450) sphere structures by doping ferrocenecarboxylic acid and demonstrated the significance of ferrocenecarboxylic acid in the MoS₂/Ni₃S₂ heterostructures on the electrochemical HER and OER. High-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis were carried out to further characterize the structure and surface states of flower-like sphere structures. In our study, the Fe, C-MoS₂/Ni₃S₂-450 represents a low overpotential of 188 mV and 270 mV to drive a current density of 10 mA cm⁻² for HER and OER, respectively. In this work, we pioneered the use of ferrocenecarboxylic acid-derived iron-carbon doped metal sulfides to enhance the electrical transport performance and increase electronic site for catalytic reaction. This work provides valuable insights for the rational design of energy storage and conversion materials by incorporation of transition metal and carbon elements into metal sulfide structures utilizing metallocene.

2. Results and Discussion

The synthetic procedure of Fe, C-MoS₂/Ni₃S₂-450 is as follows. First, the α-Ni (OH)₂ nanosheets were prepared on the basis of previous reports [26]. As shown in Figure S1(see Supporting Information), it can be seen that the formation of well-crystallized α-Ni (OH)₂ was corroborated successfully by X-ray diffraction (XRD) pattern (see Figure S1) [33]. The as-prepared Ni(OH)₂ product represents nanosheets with smooth surfaces, as shown in the scanning electron microscopy (SEM) image (see Figure S2). The obtained α-Ni (OH)₂ was then mixed with sodium molybdate dihydrate, Ferrocenecarboxylic acid (Fc), and thiourea in a suitable proportion through ultrasonic treatment and magnetic stirring. The solution was then transferred into a 100 mL Teflon-lined autoclave reactor and heated to 200 ℃ for 24 h. After reaction, the as-cooled products were washed several times and collected. To obtain final Fe, C-MoS₂/Ni₃S₂-450, products were annealed at 450 ℃ in Ar/H₂ (90/10%) environment for 2 h, while the control group was prepared by adjusting the annealing temperatures (400, 450, 500, and 550 ℃).

The existence of NiS phase can be confirmed by the XRD pattern shown in Figure 1, in which the diffraction peaks at 2Theta = 18.4°, 32.2°, 35.6°, 40.4°, 48.8°, 50.1°, 52.6°, and 56.25° corresponding to NiS can be clearly observed. By comparison with Joint Committee on Powder Diffraction Standards ((JCPDS) card (No. 12-0041 NiS) [34], it can be seen that the diffraction angles and different crystal faces of NiS phase are corresponding respectively. However, peaks corresponding to different crystal faces of MoS₂ phase were not detected, suggesting the existence of other Mo compounds. Moreover, it could be likely that the MoS₂ phase was formed before annealing under 450 ℃ for 2 h in Ar/H₂ (90%/10%) environmental atmosphere. As the results shown in Figure 1, three legible peaks are found at the position of 2Theta = 14.3°, 33.5°, 58.3°, which accordingly correspond to (002), (101), (110) crystal planes of MoS₂ phases (JCPDS no. 37-1492-MoS₂) [35]. In addition, other obvious diffraction peaks appear in 2Theta = 21.8°, 31.1°, 37.8°, 44.2°, 49.8°, 50.1°, 55.2°, corresponding to (101), (110), (003), (202), (113), (211), (122) crystal planes of Ni₃S₂ phases (JCPDS no. 44-1418-Ni₃S₂) [21], respectively, indicating the transformation from NiS phase to Ni₃S₂ phase. This might result from the breaking of partial Ni-S bonds, allowing S- and Mo- sites to combine and form MoS₂ due to the annealing process at 450 ℃ for 2 h in Ar/H₂ (90%/10%) environmental atmosphere. According to previous work [5], MoS₂ could exhibit excellent catalytic performance in HER process and edge sites of MoS₂ and sulfur anion vacancies could provide sites for increased HER activity. It is shown that annealing temperature is a key factor affecting the hydrogen evolution performance of the catalyst. In order to investigate the elemental surface distribution and valence state of catalyst, Raman measurement was carried out to measure the surface states of flower-like sphere structures. As shown in Figure 1b, the peaks at 298, 370, and 403 cm⁻¹ are corresponding to E_1g, E¹_2g, and A_1g modes of 2H-MoS₂, while the peak at 315 cm⁻¹ is from the modes of Ni₃S₂ [4]. The peak located at 345 cm⁻¹ belongs to the modes of MoS₂/Ni₃S₂ [22,23]. In addition, three sharp peaks can be observed between 800–1000 cm⁻¹, indicating the molecular structure of Mo₃S₁₃ distributing on edge sites of MoS₂ and the existence of rich under-coordinated Mo-S edge sites in the Fe, C-MoS₂/Ni₃S₂-450.

As shown in the SEM figures (Figure 2a), the reaction products of Ni(OH)₂, Na₂MoO₄·2H2O, Fc, and thiourea display three-dimensional flower-like sphere structures. A large number of nanosheets on the sphere surface can be observed, which may result in the increase of effective active sites and promote performances of the catalyst. Moreover, once the feeding rate of Ni(OH)₂ was increased, the product evolved from flower-like spherical structures to irregular forms or spheroidal particles distributed on the previous flower-like spherical surface. To maintain the preferable nanosheet structure, an optimal Ni(OH)₂/Na₂MoO₄ (the amount of Na₂MoO₄ remained at 2 mmol) feeding ratio of 0.2g/Na₂MoO₄ was adopted for the fabrication. As shown in the SEM images (Figure 2b,c), samples at different magnifications clearly reveal the existence of that original three-dimensional flower-like spherical structure, with ultrathin nanosheets distributed evenly on the surface. This result can be further confirmed by the TEM image shown in Figure 2d–g, in which an irregular sphericity structure of Fe, C-MoS₂/Ni₃S₂-450, with its interior clearly shaded, can be observed, suggesting the whole structure is not in a hollow state. The TEM images in Figure 2e,f further confirm the formation of flower-like Fe, C-MoS₂/Ni₃S₂-450 spheres. In Figure 2g, the lattice fringe of 0.62 nm belongs to the (002) lattice plane of MoS₂ [22], while the lattice distances of 0.28 nm are indexed to the (110) plane of Ni₃S₂, as the (110) plane of Ni₃S₂ and the neighboring (002) plane of MoS₂ constitute the interface of MoS₂/Ni₃S₂ heterostructures [22]. In conclusion, the flower-like spherical structure of Fe, C-MoS₂/Ni₃S₂-450 synthesized in this work has good uniformity in both structure and size.

The results of XPS measurement are shown in Figure 3, where in the Ni 2p is composed of the alike characteristic spin-orbit states of Ni 2p_1/2 and Ni 2p_3/2, which are accordingly corresponding to the strong peaks at 857.0 eV and 874.0 eV in the upper half of Figure 3a, with its two weak satellite peaks located at 862.1 eV and 879.5 eV, respectively. Comparing with the lower half of Figure 3a, there were significant positive shifts for peaks of Ni 2p and obvious changes of chemical valence states. After annealing in reducing atmosphere, it is possible that the Fe doping brings identifiable chemical state variation of Ni compared to Fe, C-Mo_yS_x/NiS. The result above is similar to the previous report [22], and can endow catalysts with more active sites for OER process [25]. For the Mo 3d spectrum in Figure 3b, the two peaks at about 229.6 and 232.8 eV are corresponding to Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2. In addition, the peak located at 236.2 eV indicates the presence of a high oxidation state of Mo⁶⁺, which might be due to the existing Mo⁶⁺ that has not been fully reduced before annealing. The chemical valence of Mo has significant change: the peaks belonging to Mo⁶⁺ decrease noticeably in Fe, C-MoS₂/Ni₃S₂-450 as compared with Fe, C-Mo_yS_x/NiS, illustrating that MoS₂ is generated. The peak at about 226.7 eV originates from S 2s, indicating the existence of the chemical bindings of Mo-S. The S 2p spectrum in Figure 3c deconvolutes into two peaks, which accordingly correspond to S 2p_1/2 (163.5 eV) and S 2p_3/2 (162.4 eV), suggesting the existence Mo-S and Ni-S. The nearby peak located at 168.9 eV belongs to the satellite peak of S 2p.

As shown in Figure 3c, the peak of S 2p_1/2 is belonging to the sulfur with low coordination and has a relationship with sulfur defects, which indicates the possibility of abundant defects in Fe, C-MoS₂/Ni₃S₂-450. It may be significant for HER performances. In Figure 3d, the Fe 2p peaks of Fe, C-MoS₂/Ni₃S₂-450 can be deconvoluted, four peaks at 708.3 eV (Fe²⁺ 2p3/2) and 720.5 eV (Fe²⁺2p1/2), 713.5 eV (Fe³⁺2p3/2) and 726.0eV (Fe³⁺2p1/2), suggesting the Fe²⁺ compounds and Fe³⁺ compounds. However, comparing with the Fe 2p spectrum in the lower half of Figure 3d, we clearly find that the binding energy of Fe 2p got shifted further to the left and the amount of Fe²⁺ species was decreased, suggesting partial original Fe²⁺ transformation into Fe³⁺. To investigate the effect of temperature on chemical shifts of Ni, Mo, S, and Fe, XPS measurements were performed on Fe, C-Mo_yS_x/NiS (Figure 3a–d) for comparison. It can be seen that large shifts of binding energy of Ni and Mo, S, Fe, and the heterostructure relationship of MoS₂ and Ni₃S₂ could be strengthened after annealing [4,22]. In addition, annealing changes the chemical environment of Fe, C-MoS₂/Ni₃S₂-450 and treatment chemical state of Fe and Ni elements and may affect catalytic activity.

The electrocatalytic HER and OER performance of the Fe, C-MoS₂/Ni₃S₂-450 flower-like spherical structures were measured in a three-electrode configuration and alkaline electrolyte (1M KOH, PH = 14). Meanwhile, the performance of the commercial Pt/C, RuO₂, and MoS₂, NiS₂, MoS₂/Ni₃S₂ were also investigated. Figure 4a illustrates the polarization curves of the samples obtained at a scan rate of 5 mV/s (corrected with IR compensation already, unless otherwise noted). It can be seen that Fe, C-MoS₂/Ni₃S₂-450 exhibits superior HER activity compared to those of MoS₂, NiS₂, and MoS₂/Ni₃S₂ as the potential increasing. To achieve a current density of 10 mA/cm², a low overpotential of 188 mV should be affordable for Fe, C-MoS₂/Ni₃S₂-450. However, upon comparison among MoS₂/Ni₃S₂ (315mV), NiS₂ (334 mV), and MoS₂ (363 mV), the advantages of Fe and C for effectively promoting the HER activity of MoS₂/Ni₃S₂ could be found. In addition, samples obtained in different annealing temperatures in atmospheres of Ar/H₂ (90%/10%) were also prepared for comparison. As shown in Figure 4f, the HER performance of the Fe, C-MoS₂/Ni₃S₂-450 were effectively improved when temperature was increased, and reached optimum effect at the temperature of 450 ℃. However, the catalytic reaction process of Fe, C-MoS₂/Ni₃S₂-450 became worse when the annealing temperature exceeded 450 ℃. According to XRD analysis (Figure 1a), it is possible that Ni₃S₂ and MoS₂ phase were formed, and thus the electrocatalytic performance of catalysts were modified. It is a practicable method to enhance catalytic properties in some materials’ systems through doping catalytic materials with transition-metal atoms. Meanwhile, according to previous reports [25], the catalytic performance of electrocatalysts could be increased, through adjusting their coordination valence states and chemical environment by doping a small amount of Fe. Therefore, the Fe, C-MoS₂/Ni₃S₂-450 in this study exhibited much higher HER performance than that of MoS₂/Ni₃S₂, which might result from the introduction of Fe and C elements.

To further investigate the HER activity, the Tafel plots from polarization curves were fitted. As shown in Figure 4b, the Tafel slope of the Fe, C-MoS₂/Ni₃S₂-450 (95 mV/decade) is lower than that of pure MoS₂/Ni₃S₂ (168 mV/decade), MoS₂ (140 mV/decade), and NiS₂ (211 mV/decade), indicating that Fe, C-MoS₂/Ni₃S₂-450 has faster reaction kinetics. According to previous reports [4,22], three principal steps are involved in HER, with different Tafel slopes corresponding to associated reaction steps. The first step is Volmer reaction (a Tafel slope of about 120 mV/decade), the second step is Heyrovsky reaction (a Tafel slope of about 40 mV/decade) or Tafel reaction (a Tafel slope of about 30 mV/decade). According to the Tafel slope of Fe, C-MoS₂/Ni₃S₂-450 (−95 mV/decade), its HER could follow the Volmer−Heyrovsky mechanism (H₂O + H_ads + e^- → H₂ +OH⁻), where H_ads represents the H atom on an active site. Meanwhile, the smaller Tafel slopes stand for faster electrocatalytic kinetics of HER and OER.

In addition, electrochemical impedance spectroscopy (EIS) analysis at −0.25 V (V vs. reversible hydrogen electrode (RHE)) was carried out for the in-depth study of the activity of electrocatalytic kinetics of HER. As shown in Figure 4c, it can be seen that charge-transfer resistance in MoS₂, NiS₂, MoS₂/Ni₃S₂, and Fe, C-MoS₂/Ni₃S₂-450 was illustrated by Nyquist. Among all the samples, the lowest charge transfer resistance (R_ct) occurred in Fe, C-MoS₂/Ni₃S₂-450, suggesting faster electrocatalytic kinetics of HER activities and expediting of the Faradaic process on electrode. Hence, the result further reveals that the charge-transfer process of MoS₂/Ni₃S₂ can be accelerated by doping C element.

Electrochemical stability of the catalysts is taken as standard for judging the HER performance of catalyst. The long-term stability of Fe, C-MoS₂/Ni₃S₂-450 was obtained using chronoamperometry and CV test. As shown in Figure 4d (a constant current density of 10 mA/cm² and time of 12 h), potential of the Fe, C-MoS₂/Ni₃S₂-450 was enhanced to only 25 mV over 12 h at 10 mA/cm². The Fe, C-MoS₂/Ni₃S₂-450 exhibited superior stability of HER with only 47 mV degradation in potential after 3000 CV cycles (see Figure 4e).

It is known that catalytic performance is positively correlated with electrochemically active surface areas (ECSA), and ECSA also shows a positive correlation with the double-lay capacitance (C_dl). It is therefore C_dl was obtained from cyclic voltammetry (CV) curves to further evaluate the HER catalytic performance. Based on CV curves (Figure S3), the C_dl values of the NiS₂ (0.15 mF/cm²), MoS₂ (0.24 mF/cm²), MoS₂/Ni₃S₂ (0.85 mF/cm²), and Fe, C-MoS₂/Ni₃S₂-450 (1.04 mF/cm²) were obtained. Through comparison, it can be seen that Fe, C-MoS₂/Ni₃S₂-450 shows the highest C_dl value, meaning higher ECSA of as-obtained Fe, C-MoS₂/Ni₃S₂-450 because of the Fe and C incorporation. The result reveals that the Fe and C incorporation effectively provide more electroactive catalytic sites that lead to enhanced HER performance.

Since electrochemical water splitting is composed of HER and OER processes, it is very necessary to further investigate the OER performance of Fe, C-MoS₂/Ni₃S₂-450. In the polarization curves of OER (see Figure 5a), the lowest overpotential of 273 mV at a current density of 10 mA/cm² is observed in Fe, C-MoS₂/Ni₃S₂-450 (350 mV for RuO₂, 400 mV for NiS₂, 460 mV for MoS₂, and 355 mV for MoS₂/Ni₃S₂), suggesting the superior OER performance of flower-like MoS₂/Ni₃S₂ spheres were improved by doping Fe and C. According to previous reports [25], Fe element has a unique catalytic ability in OER, and suitable doping of Fe helps the low valence state of Ni⁰ in metal oxide/(oxy)hydroxide materials to shift to more active Ni2⁺ with a high valence state in OER activity and optimize the absorption energies of O and OH binding, causing the OER performance to improve.

Likewise, the Tafel plots from polarization curves were fitted to further investigate OER activities. As shown in Figure 5b, a Tafel slope of only 66 mV/decade was observed in Fe, C-MoS₂/Ni₃S₂-450, which is superior to the RuO₂ (82 mA/decade), the NiS₂ (104 mA/decade), the MoS₂ (194 mA/decade), and MoS₂/Ni₃S₂ (95 mA/decade). This result reveals the enhanced reaction kinetic of Fe, C-MoS₂/Ni₃S₂-450.

In addition, EIS (at 1.48 V (vs. RHE), Figure 5c) and chronoamperometry (a constant current density of 10 mA/cm² and time of 12 h, Figure 5d) tests were carried out to investigate the OER performance of the Fe, C-MoS₂/Ni₃S₂-450. As with the HER activity, the lowest R_ct in Fe, C-MoS₂/Ni₃S₂-450 indicates the faster Faradaic process on the Fe, C-MoS₂/Ni₃S₂-450 heterostructures, resulting in excellent OER performance Fe, C-MoS₂/Ni₃S₂-450. As shown in Figure 4d, the Fe, C-MoS₂/Ni₃S₂-450 maintains a high stability at 10 mA/cm², but the left-hand of the chronoamperometry curve has an upward trend. The increased contact resistance between the catalyst and the electrolyte was due to the bubbles from the OER reaction that could not overflow instantly. However, the bubbles busted and disappeared as they became overgrown. After it test (The stability test was carried out by the i-t curve) at 10 mA/cm², the overpotential of the Fe, C-MoS₂/Ni₃S₂-450 reached 60 mV in Figure 5e. To find the best annealing temperature, different samples were prepared to test OER activity by changing the annealing temperature. The result shown in Figure 5f reveals that the lowest overpotential of OER at 10 mA/cm² occurred in samples annealed at 450 ℃, and thus indicate the best OER performance.

In conclusion, the improved HER and OER performance of the Fe, C-MoS₂/Ni₃S₂-450 originated from the doping C element, which resulted in better electrical transport performance with lower overpotential and rich electronic sites for catalytic reaction after suitable annealing. In addition, the doping of Fe helped to enhance the interfacial effects between MoS₂ and Ni₃S₂, as the Fe caused the low valence state of coordinated metals to shift to a more active high valence, optimized the heterostructure, and enhanced the reaction kinetics of OER.

3. Conclusions

In summary, 3-dimensional flower-like Fe, C-doped MoS₂/Ni₃S₂ heterostructural spheres were successfully synthesized via a hydrothermal reaction with Fc (ferrocenecarboxylic acid) followed by annealing. After annealing, two phases of MoS₂ and Ni₃S₂ phase originating from Mo_yS_x/NiS phase were formed, as further confirmed by XRD patterns. The flower-like Fe, C-doped MoS₂/Ni₃S₂ exhibits high HER and OER performance in alkaline-solution environment, wherein the OER is even superior to those of commercial RuO₂. Long-time stability was also exhibited, along with a low overpotential of 273 mV at a current density of 10 mA/cm² and a Tafel slope as low as 66 mV/decade. Comparing with doping ferrocenecarboxylic acid or ferrocene, our Fe, C-doped catalyst presents a superior HER performance with a very small overpotential of 188 mV at 10 mA/cm². The strong synergistic effect of Fe and C elements derived from Fc with MoS₂/Ni₃S₂ further enhances the transfer of electrons and thus provides a rich active site for catalytic reaction.

4. Experimental Section

4.1. Reagents and Chemicals

Nickel chloride hexahydrate (NiCl₂·6H₂O), hexamethylenetetramine (HMT), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), ferrocenecarboxylic acid (Fc), thiourea, potassium hydroxide (KOH), and commercial 20 wt.% Pt/C and RuO₂ were purchased from Lan Tian Commodities Co. Ltd (Nanning, China). All the reagents were used directly without further purification. The Milli-Q water (18.2 MΩ, Millipore) was in all experiments.

4.2. Preparation of Ni(OH)₂ Precursor

The method of preparing Ni(OH)₂ precursor referred to previous work and modified it [26]. An amount of 3 mmol of NiCl₂·6H₂O and 5g HMT were added to a beaker of 100 milliliters of Milli-Q water, stirred to obtain a green homogeneous solution. Then, the green solution was heated at 90 ℃ for 4 h. The green products were washed by centrifugation with Milli-Q water, dried at 70 ℃ in air, and collected.

4.3. Preparation of Fe, C-Mo_yS_x/NiS and Mo_yS_x/NiS

An amount of 0.2 g of pre-fabricated Ni(OH)₂ precursor was dissolved in 20 ml of Milli-Q water with stirring for 5 min. Then, 2 mmol of Na₂MoO₄·2H₂O and 12 mmol of thiourea were added to the previous solution under continuous magnetic stirring for some time. In the meantime, 40 ml of glycol was instilled. After obtaining a homogeneous solution, 0.1 mmol of Fc continued to be added. The solution was treated with ultrasound for 30 min at room temperature and transferred to a 100 mL Teflon-lined autoclave reactor with heating to 200 ℃ for 24 h. After the reaction, the obtained were washed by centrifugation with ethanol, dried at 60 ℃ in air, and collected. The obtained samples were sighted as Fe, C-Mo_yS_x/Ni_yS_x. To prepare the Mo_yS_x/Ni_yS_x, the step of adding 0.1 mmol of Fc was omitted; other steps had no change.

4.4. Preparation of Fe, C-MoS₂/Ni₃S₂ or MoS₂/Ni₃S₂

An amount of 0.1 g of the Fe, C-MoS₂/Ni₃S₂ or the MoS₂/Ni₃S₂ was put in the porcelain boat. Then, the sample was heated to 450 ℃ with a rate of 2 ℃/min for 120 min under Ar/H₂ (90%/10%) atmosphere.

4.5. Preparation of MoS₂

The amounts of 1 mmol of Na₂MoO₄·2H₂O and 6 mmol of thiourea were added to 60 ml of Milli-Q water, with stirring for 15 min, and transferred to a 100 mL Teflon-lined autoclave reactor with heating to 200 ℃ for 24 h. After the reaction, the obtained were washed by centrifugation with ethanol, dried at 60 ℃ in air, and collected.

4.6. Preparation of NiS₂

An amount of 0.1 g of pre-fabricated Ni(OH)₂ precursor was put in the porcelain boat; 0.2 g of sublimed sulfur was put upstream of the porcelain boat. Then, the furnace was heated to 400 ℃ with a rate of 2 ℃/min for 120 min under N₂ atmosphere.

5. Materials Characterization

To ascertain the structures and morphologies, the prepared samples were tested by XRD (Rigaku D/max 2500 V) with a Cu Kα radiation (λ = 0.15406 nm, 40 kV, 40 mA), SEM (Sigma 300, operating at 10 kV), TEM (FEI TEGNAI G2 F30, operating at 200 kV). Raman (Thermo Fischer DXR, λ = 532 nm) and XPS (ESCALAB 250XI) spectra were used to analyze for the surface states.

6. Electrochemical Measurement

All electrochemical performances were measured in a three-electrode system with the electrochemical workstation (CHI660E) at room temperature. During electrochemical measurements of the HER and OER, working electrode (reversible hydrogen electrode (RDE), d = 3 mm, RED MATRIX CHINA LIMITED, 1600 rpm), mercuric oxide electrode (Hg/HgO) as the reference electrode, carbon rod, and Pt foil as the counter electrode for OER and HER, respectively, were used. For the catalyst ink prepared, 5 mg of samples were dispersed in 770 μL of Milli-Q water, 200 μL of ethanol, and 30 μL of Nafion (5 wt.%) via ultrasonication. Then, 5 μL of the catalyst ink was loaded to the surface of the working electrode (a mass of 35 mg/cm²) to test electrochemical performances after drying. The electrochemical performances were evaluated by linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry. N₂ and O₂ were inputted continuously to the 1M KOH solution before the HER and OER measurements were evaluated by linear sweep voltammetry LSV (at a scan rate of 5 mV/s), respectively. In order for a comparison with previous work, all the potentials were calibrated to the reversible hydrogen electrode (RHE) via E _{vs. RHE} = E _{vs. Hg/HgO} + 0.059 × PH + 0.098.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/cryst11040340/s1, Figure S1. XRD patterns of Ni(OH)₂; Figure S2. SEM image of Ni(OH)₂; Figure S3 exhibits EDX mapping images of the Fe, C-MoS2/Ni3S2-450, indicating the the presence of C, S, Fe, Ni and Mo element; Figure S4 EDX analysis images of the Fe, C-MoS2/Ni3S2-450; Figure S5 (a) XPS survey and (b) C 1s spectrum of the Fe, C-MoS2/Ni3S2-450 and Fe, C-MoySx/NiS; Figure S6. (a–d) CV curves of obtained samples in the window of 0.94-1.04 V vs. RHE. (e) estimated Cdl values.

Author Contributions

Conceptualization, X.L., G.L., and S.L.; writing-original draft preparation, X.L., G.L., D.C., and W.C.; experiment, D.C. and T.S.; funding acquisition, N.W., Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Nature Science Foundation, grant number 51972068.

Acknowledgments

This work was supported by the grants from the National Natural Science Foundation (grant NO: 51972068) and Guangxi Key Laboratory of Processing for Non-Ferrous Metals and Featured Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Figures

Figure 1. (a) XRD patterns of Fe, C-MoySx/NiS and Fe, C-MoS2/Ni3S2-450, (b) Raman spectrum of the Fe, C-MoS2/Ni3S2-450.

Figure 2. (a) SEM image of Fe, C-MoySx/NiS, (b,c) Fe, C-MoS2/Ni3S2-450; (d–g) TEM image of Fe, C-MoS2/Ni3S2-450.

Figure 3. XPS spectra of Fe, C-MoS2/Ni3S2-450 and Fe, C-MoySx/NiS: high-resolution XPS analysis of (a) Ni 2p, (b) Mo 3d, (c) S 2p, (d) Fe 2p spectra.

Figure 4. (a) Polarization curves and (b) the corresponding Tafel plots; (c) electrochemical impedance spectroscopy (EIS) analysis of the obtained sample for hydrogen evolution reaction (HER). (d) Chronopotentiometric curve of HER for the Fe, C-MoS2/Ni3S2-450. (e) Linear sweep voltammetry LSV polarization curves of the Fe, C-MoS2/Ni3S2-450 before and after 2000 cycles. (f) LSV polarization curves of samples by different annealing temperatures.

Figure 5. (a) Polarization curves and (b) the corresponding Tafel plots. (c) EIS analysis of the obtained sample for OER. (d) Chronopotentiometric curve of HER for the Fe, C-MoS2/Ni3S2-450 (e) LSV polarization curves of the Fe, C-MoS2/Ni3S2-450 before and after i-t test (The stability test was carried out by the i-t curve) at 10 mA/cm2. (f) LSV polarization curves of samples by different annealing temperatures.