MXene composite with Ni/Co sulfide for enhanced hydrogen evolution reaction

Abstract

To produce clean and green hydrogen energy from electrolytic water, better electrocatalytic materials need to be developed. Herein, we reported the development of a Ti₃C₂ MXene@Ni/Co sulfide hybrid material for the use in hydrogen evolution reaction (HER) process, which is effective and stable. The shape of metal sulfide and its interactions with Ti₃C₂ MXene caused electronic coupling. This increased both the exposed metal sulfide's active edge sites and the charge-transfer speed. When compared with pure MXene and Ti₃C₂ MXene@NiS nanostructure, Ti₃C₂ MXene@Co₄S₃ exhibited outstanding electrocatalytic activity and stability in the HER process. The unique properties of the skeletal structure and the robust interface interaction between Co₄S₃ and MXene are responsible for the superior performance of Ti₃C₂ MXene@Co₄S₃ in the HER process. Our research offers a practical method to synthesize specialized nanostructures for electrocatalysis, electronics, and other applications.

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1. Introduction

The need to develop clean and green energy is driven by increasing concerns about the environment related to the unrestrained use of fossil fuels and energy problems caused by population growth. The development of future green energy and the environmental-friendly generation of hydrogen as a source of energy could both be achieved through the breakdown of water, powered by renewable energy.^1–3 A hydrogen or oxygen evolution reaction (HER or OER) can occur during the water decomposition process, and Ir, Ru, and Pt catalysts are commonly used for both such reactions. When it comes to the widespread industrialization of water electrolysis, the use of expensive precious metals is not a feasible option.^4–6 Consequently, it is necessary to use catalysts that are extremely effective, reliable, and affordable. Nitrides,⁷ transition metal oxides,⁸ phosphides,⁹ sulfides,¹⁰ and selenides¹¹ are among the non-precious metallic materials that have been developed to act as effective electrocatalysts. The enhanced electrocatalyst design is anticipated to benefit from the use of transition metal dichalcogenides (TMDs)⁶ which have been identified as an important series of electrocatalysts with high conductivities, which can be synthesized via simple processing methods. Nickel sulfide,¹² cobalt sulfide,¹³ molybdenum sulfide,¹⁴ and iron sulfide¹⁵ are among the non-precious metal electrocatalysts based on layered TMDs currently being studied concerning their application in general water decomposition. The distinctive structural characteristics of TMD catalysts, such as their structural disorder, capacity for doping with foreign metals, and morphological control, are responsible for their enhanced electrocatalytic activity in the HER and OER processes.¹⁶ Due to their affordability, high activity, and simple synthesis process, nickel- and cobalt-based compounds have drawn the most attention.¹⁷ For the HER and OER, for instance, a Ni–FeS₂ lattice on Ni foam results in improved catalytic activity.¹⁸

The electrocatalytic activity of TMDs can be enhanced by adding a conductive material to them, which leads to variations in the ΔGH* and the electrical conductivity, as determined using density functional theory (DFT) calculations.^16,19 In this regard, a variety of catalyst supports, such as reduced graphene oxide,²⁰ multi-walled carbon nanotubes,²¹ and other carbon-based materials,²² have been investigated. However, the majority of these materials exhibit relatively low electrochemical activity and are hydrophobic, which precludes their use in aquatic conditions. Therefore, it is necessary to identify highly conductive, hydrophilic, and electrocatalytically active catalyst supports to facilitate the design of novel materials with excellent catalytic efficiency.

Research interest has recently been focused on MXenes that are a novel family of metal carbides produced by selectively etching MAX phases. Ti₃C₂ MXene is regarded as the most valuable member of the MXene family. Owing to its unique properties, two-dimensional (2D) Ti₃C₂ MXene has been used in a variety of applications, including supercapacitors, batteries, water-splitting processes, and biomedical applications.^23–26 Ti₃C₂ Mxene has thin atomic layer, high electrical conductivity (9880 S cm⁻¹), hydrophilicity, abundance of active sites, and excellent mechanical properties.²⁷ When it comes to fine-tuning the overall catalytic water-splitting reaction, 2D Ti₃C₂ MXene is an excellent option for conductive catalytic assistance. For instance, due to the increased proportion of exposed MoS₂ edges and the lower ΔGH* values, the MoS₂ growth on Ti₃C₂ MXene results in improved electrocatalytic achievement.²⁸ Therefore, to provide better electrocatalytic materials for the use in HER process, we developed a novel method to produce a synergistic hybrid heterostructured material by fusing the structural characteristics of Ti₃C₂ MXene with the directed growth of NiS and Co₄S₃. With the findings of previous studies,²⁹ we predicted that using MXene to form a hybrid structure composed of Ti₃C₂ MXene@NiS and Ti₃C₂ MXene@Co₄S₃ would enhance the HER activities of both NiS and Co₄S₃.

In this work, we synthesized the NiS and Co₄S₃ on Ti₃C₂ MXene surfaces via hydrothermal synthesis procedures that are easily repeatable. When compared with Ti₃C₂ MXene, the produced Ti₃C₂ MXene@NiS and Ti₃C₂ MXene@Co₄S₃ catalysts exhibited greater catalytic activity about the HER. The overpotentials of the Ti₃C₂ MXene@NiS and Ti₃C₂ MXene@Co₄S₃ catalysts were investigated for the HER. Moreover, fine-tuning the morphology and structure of NiS and Co₄S₃ on the surface of Ti₃C₂ MXene can affect the electron coupling between both NiS and Ti₃C₂ MXene and Co₄S₃ and Ti₃C₂ MXene, thereby enhancing the electron transfer, which leads to an increase in the intrinsic HER activities of the catalysts. The approach used here to enhance a catalyst for the HER offers an opportunity for the future production of excellent TMD catalysts for electrochemical applications.

2. Experimental section

2.1. Materials

The Ti₃AlC₂ (98% powder) used in this experiment was purchased from Aladdin (South Korea). The hydrofluoric acid (HF), ethanol (C₂H₆O), paraffin, nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), and cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) were purchased from Sigma–Aldrich (South Korea). The thiourea (CSN₂H₄) was supplied by Daejung Chemicals (South Korea). The analytic grade of each chemical reagent allowed for use without additional purification.

2.2. Synthesis of multilayer Ti₃C₂ MXene

Typically, 50 mL of 40 wt% HF aqueous solution was gradually filled with 1 g of Ti₃AlC₂ powder and allowed to stand for 48 hours at room temperature (RT). The final product was then continuously washed with deionized water until the pH was neutralized. After collecting it, the produced precipitate (Ti₃C₂ MXene) was dried in a vacuum oven for 10 hours.

2.3. Synthesis of Ti₃C₂ MXene@NiS and Ti₃C₂ MXene@Co₄S₃ hybrid structures

A simple hydrothermal approach was used to synthesize the Ti₃C₂ MXene@NiS and Ti₃C₂ MXene@NiS composites. First, 0.29 g of Ni(NO₃)₂·6H₂O was dissolved in 40 mL of deionized water, and the mixture was agitated for 20 min at RT. Additionally, 10 mL of ethanol was mixed with 0.1 g of Ti₃C₂ MXene before being ultrasonically treated for 30 minutes. The above-prepared nickel solution was then quickly mixed with the Ti₃C₂ MXene solution for 20 minutes. The (0.2 g) thiourea was then added and stirred for the following 20 minutes at RT. The Teflon liner was then filled with this solution, which was then kept at 200 °C for 12 hours in a box furnace. After being washed with ethanol and vacuum-dried at 60 °C for 8 hours, Ti₃C₂ MXene@NiS was obtained. In addition, Ti₃C₂ MXene@Co₄S₃ was synthesized in the same way as Ti₃C₂ MXene@NiS, except that Co(NO₃)₂·6H₂O (0.29 g) was used in place of Ni(NO₃)₂·6H₂O (0.29 g).

2.4. Characterizations

The morphologies and microstructures of the synthesized samples were analyzed by using a field-emission scanning electron microscope (FE-SEM). Transmission electron microscope (TEM) was used to study the intrinsic properties of the prepared material. The crystal structure of the obtained product was characterized by X-ray diffraction (XRD) with a Cu Ka X-ray radiation diffractometer. The surface elements were analyzed by X-ray photoelectron spectroscopy (XPS). Inductively coupled plasma mass spectrometry (ICP-MS) analysis was also used to study elemental composition in the prepared materials.

2.5. Preparation of working electrodes using synthesized materials and commercial 20% Pt/C

The synthesized Ti₃C₂ MXene@NiS (2.5 mg), Ti₃C₂ MXene@Co₄S₃ (2.5 mg), and Ti₃C₂ MXene (2.5 mg), 20% Pt/C, and 5% wt% Nafion (0.1 mL) were ultrasonically mixed into the solution containing deionized water (0.1 mL) and ethanol (0.3 mL). The working electrode was then engineered by drop-casting the 50 μL of the mixed ink onto a piece of carbon paper that was 1 cm × 1 cm in size and air-dried before being subjected to electrochemical experiments.

2.6. Electrochemical measurements

The working electrode was carbon paper supported with all the catalyst materials, whereas the counter and reference electrodes were Pt wire and Hg/HgO, respectively. For comparison, the commercial 20% Pt/C catalyst was also measured. The reversible hydrogen electrode (RHE) was used to convert the observed potentials in this study using the formula E(RHE) = E(SCE) + 0.140 + 0.0591 pH. HER activity of the obtained catalysts was assessed using linear sweep voltammetry (LSV) measurements with a scan rate of 5 mV s⁻¹ and 1 M KOH electrolyte. Without iR compensation, all the potentials in LSV curves were shown. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a frequency range from 100 kHz to 0.01 Hz at the bias voltage of 5 mV. The chronopotential curve was used to evaluate the durability of the material for 10 hours at a constant current density of 10 mA cm⁻².

3. Results and discussion

3.1. Electrocatalyst synthesis and characterization

There are several factors to consider, including crystal phase, morphology, size and shape distributions, surface chemistry, and crystallinity, when synthesizing nanomaterials for electrocatalytic applications.³⁰ It is important to examine the benefits and drawbacks of the many synthesis methods currently in use, before choosing which approach is suitable for a given application. For instance, the wet-chemical approach produced phases having clear boundaries, but with larger particle sizes.³¹ While gas phase methods may create nanosized particles with narrow size distributions, they also need the selection of suitable precursors which may not always be readily available or inexpensive.³² Furthermore, hydrothermal techniques are more effective for controlling the nano-morphology by modifying reaction parameters such as temperature, pressure, etc., even if they frequently result in introducing defects due to the compaction effects brought on by high temperatures.³³ To produce the necessary electrocatalysts, an essential hydrothermal approach was employed in this study that carefully takes into account each of the characteristics indicated above. MXenes, 2D layered nanomaterials of transition metal carbide, have shown outstanding research output since it was originally published in 2011.³⁴ Current research focuses on the materials based on transition metals that are developed on MXene surfaces. We used a hydrothermal approach to produce the metal sulfide (NiS and Co₄S₃) grown on the MXene surface. The MAX phase was first etched in HF solution to prepare the MXene, and after that, the MXene as obtained was mixed with the NiS and Co₄S₃ precursors. The solution mixture was then put into a stainless steel autoclave with a Teflon liner so that a hydrothermal reaction took place in a box furnace for 12 hours at 200 °C. Additionally, water and ethanol were used to wash the Ti₃C₂ MXene@NiS and Ti₃C₂ MXene@Co₄S₃ precipitates, followed by drying at 60 °C in a vacuum oven. The synthesis procedure of metal sulfide with Ti₃C₂ MXene hybrid structure as the HER electrocatalyst is schematically illustrated in Scheme 1. First, FE-SEM analysis was used to examine the morphology and intrinsic structure of the hybrid Ti₃C₂ MXene, Ti₃C₂ MXene@NiS, and Ti₃C₂ MXene@Co₄S₃ structures. The bulk MAX thick layered structure was HF etched for 48 h and then, Ti₃C₂ MXene nanosheets were collected following washing, as seen in the FE-SEM images in Fig. 1(a) and (b). The prepared MXene has a large surface area to support enormous NiS and Co₄S₃ nanoparticles. The FE-SEM images revealed an exfoliated, thin, multilayered accordion-like structure due to the homogeneous intercalation of NiS and Co₄S₃ between the Ti₃C₂ MXene layers. As shown in Fig. 1(c) and (d), NiS was grown vertically on the surface of Ti₃C₂ MXene in the form of nanorods. The existence and consistent distribution of Ni, Ti, C, and S elements in the MXene@NiS hybrid structure are shown in Fig. S1(a) (ESI†). Furthermore, Co₄S₃ particles are uniformly dispersed on the surface of Ti₃C₂ MXene in the honeycomb-like structure of the Ti₃C₂ MXene@Co₄S₃ hybrid, as seen in Fig. 1(e) and (f). Co, Ti, C, and S elements are present and are distributed uniformly throughout the Ti₃C₂ MXene@Co₄S₃ hybrid structure, as shown in Fig. S1(b) (ESI†).

Scheme 1 Schematic illustration for the synthesis process of Ti₃C₂ Mxene composite with Ni/Co sulfide.

Fig. 1 FE-SEM images of the synthesized (a) and (b) Ti₃C₂ MXene, (c) and (d) Ti₃C₂ MXene@NiS, and (e) end (f) Ti₃C₂ MXene@Co₄S₃ materials at low and high magnifications, showing layered nanosheet-like morphology.

Moreover, TEM and high-resolution (HR)-TEM analyses were performed to distinguish the internal morphological properties of the prepared Ti₃C₂ MXene@Co₄S₃ material. The TEM image of Ti₃C₂ MXene@Co₄S₃ nanosheets, which is displayed in Fig. 2(a), revealed the presence of Co₄S₃ particles and their conjugation in ultrathin nanosheets of Ti₃C₂ MXene. This observation provides additional evidence for the accumulation of Co₄S₃ particles on the Ti₃C₂ MXene sheets in the composite material. Also, the Ti₃C₂ MXene@Co₄S₃ nanosheet structure had a significant porous nature. The synthesized material with several nanosheets can deliver more opportunities for ion transport between electrode and electrolyte, which is more favorable for electrocatalytic applications. In addition, in the inverse fast Fourier transform (FFT) image (highlighted in yellow and white colors in the inset of Fig. 2(b)), which is extracted from the HR-TEM image, the edge of the Ti₃C₂ MXene@Co₄S₃ signifies the high crystalline behavior. Using the inverse FFT image (white color), the lattice spacing was determined to be 0.29 nm which corresponds to the Co₄S₃ (100) crystalline plane. Similarly, on the other hand, the lattice spacing was noticed at 0.3 nm which is related to the Ti₃C₂ MXene (022) crystalline plane (yellow color). The obtained results are well in agreement with the FE-SEM images as shown in Fig. 1(e) and (f).

Fig. 2 (a) TEM and (b) HR-TEM images of the Ti₃C₂ MXene@Co₄S₃ composite.

The crystal structure is also an important parameter for the analysis of the material's phase purity, which can be characterized by using an X-ray diffractometer. As shown in Fig. 3(a), Ti₃C₂ MXene@NiS exhibited the XRD peaks at 2θ = 18.4°, 30.3°, 32.2°, 35.8°, 37.6°, 40.6°, 49.2°, 50.3°, 52.8°, 56.4°, and 57.5° that correspond to the (110), (101), (300), (021), (220), (211), (131), (410), (401), (321), and (330) planes of the rhombohedral phase with space group R3m of NiS (JCPDS no. 003-0760). As shown in Fig. 3(b), XRD was also used to analyze the Ti₃C₂ MXene@Co₄S₃ sample. The 2θ peaks could be observed at 30.5°, 35.3°, 46.7°, and 54.2° corresponding to the (100), (101), (102), and (110) planes of the hexagonal phase with space group P63/mmc of Co₄S₃ (JCPDS no. 002-1458). As shown in Fig. S2 (ESI†), all the obtained major diffraction peaks of Ti₃C₂ Mxene agree well with CCDC no. 96-153-2228. As a result, some of the major diffraction patterns of the prepared Ti₃C₂ MXene catalysts were matched with those of the individual NiS (JCPDS no. 003-0760) and Co₄S₃ (JCPDS no. 002-1458) catalysts, which verifies the successful formation of Ti₃C₂ Mxene@Ni/Co sulfide composites.

Fig. 3 XRD patterns of the (a) Ti₃C₂ MXene@NiS and (b) Ti₃C₂ MXene@Co₄S₃ materials.

In addition, the chemical composition of the synthesized catalysts was examined by X-ray photoelectron spectroscopy (XPS). The XPS survey scan spectrum (Fig. S3(a), ESI†) verified the presence of Ni, Ti, C, and S elements in the prepared Ti₃C₂ MXene@NiS composite. In the Ni 2p spectrum of Fig. 4(a), the Ni 2p_3/2 and Ni 2p_1/2 peaks were located at the energies of 852.9, 855.7, and 870.3 eV, respectively. Ni exists in two states: the metallic state at 852.9 eV and the oxidation state of Ni²⁺ at 855.7 eV, respectively. Additionally, two satellite peaks at 860.3 and 873.8 eV were noticed.^35,36 The Ni–S bond is represented by the peak at 161.2 eV in the S 2p spectrum, while the S²⁻ peak is represented by the peak at 162.8 eV for NiS. The partial S oxidation resulted in the appearance of a satellite peak at 168.2 eV (see Fig. 4(b)).³⁷ The charge transfer between MXene and nickel sulfide was also demonstrated by the Ti 2p XPS of Ti₃C₂ MXene@NiS. The 2p_3/2 orbits of Ti³⁺ and TiO, respectively, should be associated with the peaks for Ti 2p (see Fig. 4(c)) at 456.3 and 458.5 eV. Additionally, other peaks located at 464.1 eV can be attributed to Ti–C 2p_1/2 and Ti–O 2p_1/2.^38,39 The C–Ti, C–Ti–C, C–C, and C–OH bonds are represented by the four peaks at 281.7, 284.1, 285.7, and 287.9 eV in the high-resolution C 1s XPS spectrum of MXene@NiS (see Fig. 4(d)).⁴⁰

Fig. 4 High-resolution XPS core-level spectra of the Ti₃C₂ MXene@NiS: (a) Ni 2p spectrum, (b) S 2p spectrum, (c) Ti 2p spectrum, and (d) C 1s spectrum.

Furthermore, the Ti₃C₂ MXene@Co₄S₃ was also examined by XPS to determine the chemical balancing condition. Fig. S3(b) (ESI†) shows the XPS survey scan spectrum of the Ti₃C₂ MXene@Co₄S₃ material, confirming the existence of Co, Ti, C, and S elements. In Fig. 5(a), the Co 2p spectrum, the Co 2p_3/2, and Co 2p_1/2 peaks were located at 778.2 and 793.5 eV, respectively. The metallic state of Co and the oxidation state of Co²⁺ are denoted, respectively, by the Co 2p_3/2 peak energies of 778.2 and 779.7 eV.^41,42 The Co–S bond can be identified by the peak at 161.7 eV in the S 2p spectrum, while the S²⁻ peak is indicated by the peak at 163.3 eV for the Co₄S₃. The partial S oxidation resulted in the appearance of a satellite peak at 168.2 eV (see Fig. 5(b)).⁴³ The charge transfer between MXene and cobalt sulfide was also revealed by the Ti 2p XPS of Ti₃C₂ MXene@Co₄S₃. The 2p_3/2 orbits of Ti³⁺ and TiO, respectively, should be associated with the peaks for Ti 2p (Fig. 5(c)) at 455.1 and 458.1 eV. Additionally, TiC 2p_1/2 and TiO 2p_1/2 are responsible for another peak at 463.7 eV.^38,39 The four peaks at 281.2, 284.1, 285.5, and 288.1 eV, which correspond to the C–Ti, C–Ti–C, C–C, and C–OH bonds, may be fitted into the high-resolution C 1s XPS spectra of MXene@Co₄S₃ (see Fig. 5(d)).⁴⁰ It was determined that the Ti₃C₂ MXene was effectively decorated with metal sulfide. These XPS data show that the strong covalent networks between NiS/Co₄S₃ and Ti₃C₂ MXene modified the electronic structures of Ti, Co, and Ni and that the heterogeneous interfacial electronic coupling might enhance HER activity.

Fig. 5 High-resolution XPS core-level spectra of the Ti₃C₂ MXene@Co₄S₃: (a) Co 2p spectrum, (b) S 2p spectrum, (c) Ti 2p spectrum, and (d) C 1s spectrum.

3.2. Electrocatalytic performance for HER

The LSV measurements of the prepared samples were carried out with a standard three-electrode setup in an alkaline electrolyte (1 M KOH) to assess the HER activities of the electrocatalysts as obtained. All of the synthesized materials utilized as electrocatalysts for HER were coated on carbon paper as a substrate. Furthermore, under the same conditions, the electrocatalytic performances of Ti₃C₂ MXene@NiS, Ti₃C₂ MXene@Co₄S₃, Ti₃C₂ MXene, and 20% Pt/C were examined for comparison. Poor electrocatalytic HER behavior was seen with Ti₃C₂ MXene, as illustrated in Fig. 6(a), which is consistent with the ref. 44. As shown in Fig. 6(b), the Ti₃C₂ MXene@Co₄S₃ had a lower overpotential of 142 mV than the Ti₃C₂ MXene@NiS (173 mV) and Ti₃C₂ MXene (270 mV) to achieve a current density of 10 mA cm⁻², exhibiting excellent electrocatalytic HER activity. On the other hand, the increased HER activity for the Ti₃C₂ MXene@Co₄S₃ may be attributed to a uniform distribution of the cobalt sulfide particles throughout the MXene surface, resulting in large number of active sites. In contrast, the inherent activity of Co₄S₃ may be enhanced by the charge transfer caused by Ti₃C₂ MXene. To analyze the electrocatalytic HER kinetics of the catalysts, the Tafel slopes were further computed from the LSV curves (Fig. 6(c)). The Ti₃C₂ MXene@Co₄S₃ had a lowered Tafel slope of 126 mV dec⁻¹, as expected, compared to the Ti₃C₂ MXene@NiS (157 mV dec⁻¹) and Ti₃C₂ MXene (176 mV dec⁻¹), showing fast charger-transfer performance. Moreover, the charge-transfer kinetics were also investigated using EIS measurements (Fig. 6(d)). The Nyquist plots of the designed Ti₃C₂ Mxene-based catalysts were fitted using Zview software, as shown in the inset of Fig. 6(d). The corresponding equivalent circuit consists of R₁, R₂, CPE, and W₁ parameters which represent an equivalent series resistance, charge-transfer resistance (R_ct), constant phase element, and Warburg element, respectively. In comparison to the Ti₃C₂ MXene, the MXene@NiS and Ti₃C₂ MXene@Co₄S₃ were found to have the lowest R_ct value, which is in good agreement with the findings of the Tafel slopes. These findings show that the multifunctional Ti₃C₂ MXene support and Co₄S₃ shape can boost intrinsic activity, expose the active site more, and speed up charge transfer. The electrochemical surface area of the electrocatalysts was analyzed using the double-layer capacitance (C_dl) to demonstrate the increase in HER activity. To determine the value of C_dl, the cyclic voltammetry (CV) tests were performed in the non-faradaic area (Fig. S4(a,c,e), ESI†). The C_dl of Ti₃C₂ MXene@Co₄S₃ was 68 μF cm⁻², as seen in Fig. S4(f) (ESI†), which is higher than those of Ti₃C₂ MXene@NiS (56 μF cm⁻² in Fig. S4(d), ESI†) and Ti₃C₂ MXene (17 μF cm⁻² in Fig. S4(b), ESI†). The larger active surface area of Ti₃C₂ MXene@Co₄S₃ is evident from the higher C_dl, which leads to more active sites being exposed and suitable contact with electrolytes.

Fig. 6 (a) LSV curves for all the catalyst samples measured at 5 mV s⁻¹ in 1 M KOH, (b) overpotential data taken at 10 mA cm⁻², and (c) kinetic behavior using Tafel slope curves of 20% Pt/C, Ti₃C₂ MXene, Ti₃C₂ MXene@NiS, and Ti₃C₂ MXene@Co₄S₃. (d) EIS plots of the Ti₃C₂ MXene, Ti₃C₂ MXene@NiS, and Ti₃C₂ MXene@Co₄S₃. The inset of (d) shows an equivalent circuit diagram.

The chronopotentiometry (CP) test, an additional important measurement of a noteworthy electrocatalyst, was then used to evaluate the electrocatalytic stability of the material. The CP measurement in Fig. 7(a) was first carried out with a constant current density of 10 mA cm⁻². Moreover, the long-term durability test for the Ti₃C₂ MXene@NiS and Ti₃C₂ MXene catalysts was also investigated and both materials showed good stability performance (Fig. S5 and S6, ESI†). It is remarkable to note that minimal degradation was observed during continuous operation over time. This suggests that Ti₃C₂ MXene-based catalysts have good conductivity and quick charge transfer during the HER process. The current density remained unchanged during nearly a 10-hour test, indicating the considerable HER durability of Ti₃C₂ MXene@Co₄S₃.^45–48 To harness the overpotential, we also evaluated LSV in Fig. 7(b) after the CP test, which indicates that Ti₃C₂ MXene@Co₄S₃ exhibits the least amount of overpotential loss. To determine the elemental composition, before and after the electrolytic test, the Ti₃C₂@Co₄S₃ catalyst was analyzed by the ICP-MS technique. Before the test, the Ti₃C₂@Co₄S₃ catalyst unveiled a concentration of 2.254 × 10⁴ ppb with a relative standard deviation (RSD) of 1.17%, and cobalt had a concentration of 1.731 × 10⁴ ppb with an RSD of 1.02%. Meanwhile, after the test, the Ti₃C₂@Co₄S₃ catalyst showed a concentration of 2.367 × 10³ ppb with an RSD of 0.32%, and cobalt exhibited a concentration of 4.427 × 10³ ppb with an RSD of 0.77%. The obtained results confirmed potential alterations in metal concentrations (Co/Ti), which might reflect the formation of Ti₃C₂ MXene@Co₄S₃ composites. Notably, the consistent mass/molar ratios and low variability suggest minimal metal sulfide leaching.

Fig. 7 (a) Long-term stability test result by the CP technique up to 10 h and (b) LVS curves before and after the 10 h CP test for the Ti₃C₂ MXene@Co₄S₃ sample, showing minimal loss of the potential.

4. Conclusions

In summary, this study involves the synthesis of the multifunctional Ti₃C₂ MXene-supported metal sulfide (NiS and Co₄S₃) directed structures. As a result of its considerable surface area, robust conductivity, and quick charge transport, the resultant Ti₃C₂ MXene@Co₄S₃ catalyst displayed impressive HER activity and exceptional stability over time. Significantly, the shape of Co₄S₃ produces active areas that are easier to reach, and the intriguing Ti₃C₂ MXene support effectively controls the electrical structure. The experimental results, which revealed the 142 mV overpotential to obtain 10 mA cm⁻² HER activity, showed the strong electronic interaction between Ti₃C₂ MXene and Co₄S₃ as a prediction. From these results, the Ti₃C₂ MXene@Co₄S₃ is expected to be a promising material for application in the HER process. This research provides a new method of producing metal sulfide-based composites with efficient MXene catalysts and clarifies how surface electronic coupling might increase the inherent activity of catalysts.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2018R1A6A1A03025708).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm00095a

‡ These authors equally contributed to this work.

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