Synergistic Germanium-Decorated h-BN/MoS₂ Heterostructure Nanosheets: An Advanced Electrocatalyst for Energy Storage Applications

Abstract

Increasing concerns about the vulnerability of the world’s energy supply and the necessity to implement sustainable technologies have prompted researchers to develop high-performance electrocatalysts that are affordable and efficient for converting and storing renewable energy. This article reports a facile approach to fabricating two-dimensional (2D) Ge-decorated h-BN/MoS₂ heterostructure nanosheets by self-assembly for multiple electrochemical applications such as supercapacitor and hydrogen evolution reactions. The organization of the physical and chemical links between the germanium modulations on the heterostructure of boron nitride/molybdenum sulphide (Ge/h-BN/MoS₂) were facilitated to generate more active sites. Furthermore, the asymmetric supercapacitor of Ge-decorated h-BN/MoS₂ amplified the capacitance to 558.53 F g⁻¹ at 1 A g⁻¹ current density and 159.19 F g⁻¹ at 10 A g⁻¹, in addition to a retention rate of 85.69% after 2000 cycles. Moreover, the Ge-decorated h-BN/MoS₂ catalyst realized a low over-potential value, with an RHE of 0.57 (HER) at 5 mA/cm², a Tafel value of ∼204 mV/dec, and long-term electrolysis stability of 10 h. This work may open the door for further investigations on metal-decorated heterostructures, which have a significant potential for both supercapacitor and water-splitting applications.

1. Introduction

Due to the quick depletion of non-renewable energy resources, high energy demand and environmental pollution have been addressed by focusing on stimulating interest in sustainable and renewable energy. The alarm about environmental pollution and high energy demand means that green energy and high-performance, eco-friendly energy production/storage systems are immediately required in energy technology engineering [1,2,3]. Electrocatalytic approaches are one of the most appealing and efficient storage technologies for producing high-pure fuels, high energy density, and sustainable energy sources to meet the demands of energy consumption. Therefore, the development of effective and reliable electrocatalysts is mandatory to speed the electrocatalytic (especially HER and OER) processes to create highly pure oxygen/hydrogen effectively [4]. On the cathode side of the electrodes, a 2e− transfer assists the hydrogen evolution reaction (HER), and on the anode side, a 4e− transfer assists the oxygen evolution reaction (OER). Both sides have a pair of half-cell reactions throughout the water splitting process.

Commercial noble metal Pt hybrids/alloys for HER and Ir/Ru oxides/alloys for OER electrocatalysts currently exhibit benchmark electrocatalytic properties (long-term stability and low overpotentials) and are widely regarded as exceptional electrocatalysts [5]. Recently, several bimetallic nanohybrids based on Mo and Ni sulphide and other materials, such as Ni-MoS2 [6], NiS/MoS₂ [7], and MoS2/NiS₂ [8], have demonstrated significant promise as cost-effective electrode materials for water splitting due to their prudently tuned synergistic effects and favourable interface architecture. He et al. [9] created hydrothermal graphene/metallic MoS₂ nanostructure heterostructures for a hydrogen assessment catalyst, while MoS₂/WS₂ heterostructures were constructed by Kim et al. [10] for increased hydrogen assessment methods. As a result, a great deal of work has gone into developing efficient electrocatalysts composed of materials that are devoid of noble metals [11]. However, most of them are concerned only with HER or OER electrocatalytic activity. Developing efficient electrocatalysts that enhance both HER and OER for total water splitting has remained a major issue thus far. The development of earth-abundant materials that allow effective conversion efficiency and storage is a critical problem for realizing such green energy systems. However, as far as we know, there is not enough research on Ge/h-BN/MoS₂ nanohybrids for energy conversion and storage.

Owing to their distinct physical and chemical characteristics, metal decorations containing heterostructures have sparked widespread interest as a class of appealing earth-abundant materials for supercapacitor applications. Research communities are increasingly considering non-noble-metal-based electrocatalysts such as transition-metal chalcogenides, transition-metal oxides, hydroxides, carbides, nitrides, and phosphates, which are less expensive and highly efficient [12,13,14]. The naturally abundant MoS₂, composed of trigonal prismatic Mo and S molecular lamellae, possesses semiconducting properties that make it a highly efficient and cost-effective material for electrolysis. By increasing the thickness of the molybdenum sulphide from a single sheet, it may be possible to enhance the bandgap from 1.3 eV to 1.9 eV [15].

Catalytic support, lubrication, self-clean coatings, electronic devices, and other applications of h-BN, commonly known as white graphene, have all been proven. Unlike graphene, h-BN has a wide bandgap (6.0 eV), which makes it unsuitable for photocatalytic applications. Chemical functionalization and doping of heteroatoms can change the electrical structure of h-BN. Hybrid materials and nanocomposites based on h-BN have recently been shown to be attractive prospects for electrochemical applications [16]. However, to achieve the same level of tunability in three dimensions as traditional semiconductors (e.g., Ge, Si, etc.), additional research into interface control and a material assay of homogenous 2D materials is required. The decoration of 2D materials and 3D semiconductors with and without dangling bonds on the surface has been investigated from the standpoint of the perfect preparation of three-dimensional materials by weak Van der Waals contact. Meanwhile, boron nitride, a 2D material with a high bandgap and a smooth surface, has been successfully used to create several Van der Waals heterostructures, such as graphene/h-BN, MoS₂/h-BN, phosphorene/h-BN, and Bi/Sb (111) [17]. Binary composites have also been reported in several places. Furthermore, it has been shown that binary rGO composites have outstanding electrochemical characteristics. Jiao et al. [18] found that a MoS₂/rGO composite produced using the hydrothermal method shows good electrochemical characteristics with about 95% of capacitance retention. At a current density of 0.2 A g⁻¹, E. Iyyamperumal et al. [19]. found that vertically oriented BCN nanotubes have a 321 F g⁻¹ specific capacitance. Because MoS₂ and Ge can store lithium ions, the overall electrode energy density is improved up to three times over graphite, with 1700 mAh/cm³ by volumetric capacity [20]. MoS₂/rGO demonstrate exceptional electrochemical characteristics, with more than 1200 F g⁻¹ specific capacitance, according to the report of Zhao et al. [21]. A MoS₂/graphene layer-controlled aerogel composite at a 100 mA g⁻¹ current density, with a specific capacitance of much more than 1000 mA g⁻¹, was also reported to have a reversible capacity [22]. The goal of our current research is to develop a simple chemical approach to evaluate the electrochemical characteristics of a Ge/h-BN/MoS₂ heterostructure. The structure of the h-BN/MoS₂ material effectively addresses the concern regarding the enormous volume variation that occurs throughout the germanium process. In another way, the rational hybridization of h-BN/MoS₂ and Ge is mutually advantageous in addressing their respective inherent electrochemical flaws. The multi-element synergized h-BN/MoS₂ nanosheets provide excellent supercapacitor and electrocatalytic performance in addition to long-term electrolysis stability. In a two-electrode-cell configuration, Ge/h-BN/MoS₂ was employed for the study for uses like supercapacitors and water splitting.

2. Materials and Methods

All the compounds employed in this study were AR-grade components that were not purified further, such as oleylamine (OLA, C₁₈H₃₇N, 70%) and germanium (−100 mesh, ≥99.9). Hexagonal boron nitride (h-BN) micro powder and molybdenum sulphide (MoS₂) (98% purity) were purchased from SRL products. N-methyl pyrrolidone (NMP, C₅H₉NO, Merck) was used without additional distillation.

2.1. Synthesis of h-BN/MoS₂ Heterostructures

To prepare the h-BN/NMP dispersion, 10 mg of h-BN was dispersed in 100 mL of NMP solvent using sonication for 1 h. After that, 20 mg of bulk MoS₂ was added to the h-BN/NMP solutions and thoroughly mixed. The resulting solutions were ultrasonically combined for 30 min, agitated for 24 h at room temperature, and centrifuged at 4000 rpm for 30 min. The top liquid was eliminated, and the solid precipitation, h-BN/MoS₂ heterostructures, was obtained.

2.2. Synthesis of Ge-Decorated h-BN/MoS₂ Heterostructures

Toluene and oleylamine (15 mL) were added after Ge and h-BN/MoS₂ were mixed in a 2:1 weight ratio (30 mL). After vigorously agitating the solution for 12 h at room temperature, the Ge/h-BN/MoS₂ composites were produced by centrifuging 5 mL toluene and 35 mL ethanol three times at 8000 rpm for 10 min. Furthermore, heat of 350 °C for 2 h was used to remove excess oleylamine ligands on the surface of the Ge nanoparticles in an atmosphere-free furnace, which can overcome the poor electrical conductivity. After annealing, the Ge/h-BN/MoS₂ nanohybrids were taken out and used in the tests to determine exactly what their structures and performance were (Figure 1).

3. Characterization

The phase of the Ge/BN/MoS₂ catalyst was identified using X-ray diffraction (XRD, Cu kα using X-Pert pro-PANalytic) with λ = 1.54178 Å. The surface functional groups and bonding nature of h-BN/MoS₂ and Ge/h-BN/MoS₂ were determined using FTIR (JASCO 460 PLUS, Pfungstadt, Germany) in the range of 400 to 4000 cm⁻¹. Transmission electron microscopy (TEM, JEOL, Akishima, Tokyo, Japan) with a 200 kV accelerating voltage was used to characterize the surface morphology and physiochemical structure of Ge/h-BN/MoS₂. The configuration details on the surface of the synthesized Ge/BN/MoS₂ material were examined using an ESCALab250 X-ray photoelectron spectrometer (XPS). The allowed sample size depends on the instrument configuration, but commonly, samples on the order of ∼1 × 1 cm² and less than 1 cm high can be analysed in most systems.

All the electrocatalysis studies for OER/HER using Ge/h-BN/MoS₂ electrocatalysts were displayed on an electrochemical measurement performed (CH Instruments Inc., Bee Cave, TX, USA) using the standard. All electrochemical measurements were carried out using a standard three-electrode electrochemical workstation (K-Lyte 1.3 potentiostat) with MoS₂ coated glassy carbon (GC), platinum rod, and Ag/AgCl electrode used as the working electrode, the counter electrode, and the reference electrode, respectively. According to the equation below, all potentials were rectified versus reversible hydrogen electrodes (RHE). In 0.5 M H₂SO₄, ERHE = ESCE + 1.01 V. Typically, 0.004 g of 1 mL of mixed (700 µL of water and 300 µL of ethanol) as-sonicated catalyst was added, and then 100 µL Nafion (10 wt%) was added. After 20 min of ultrasonication, an ink solution was obtained. Desiccated ink catalysts were dropped onto a glassy carbon electrode (GCE, catalyst loading 0.4 mg cm²) and dried in a 50 °C oven. Linear sweep voltammetry (LSV) and double-layer capacitances (Cdl) were performed at various sweep rates and frequency ranges of 105 to 0.1 Hz, while electrochemical impedance spectroscopy (EIS) was also performed. Chronoamperometric I versus t curve tests were also performed to find out how stable the Ge/h-BN/MoS₂/GCE system was after it was fixed.

Electrode Preparation for Energy Storage Applications

The prepared electrodes were analysed for supercapacitor application in order to determine the best material. For the preparation of the working electrode, carbon black, PVDF, and the active material were combined in a ratio of 10:10:80. These components were ground using a mortar and pestle, and NMP was added drop-wise to form a homogenous slurry. The slurry was coated with 11 cm of Ni foam and dried for 12 h at 90 °C in a hot-air oven. The dried electrode was used in a conventional three-electrode system consisting of a working electrode (active material), Pt (counter), and Ag/AgCl (reference) with 2 M of potassium hydroxide (KOH) as an electrolyte. The CHI60 electrochemical workstation was used to investigate the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) of the electrode. The capacitance from the CV graph was calculated using the formula given below [23]:

$$C_{sp}=\frac{\int IdV}{2mv\Delta V}$$

(1)

Here, current is I, the scan rate is v, the active material mass is m, and the potential window is V. The following equation was used to compute the specific capacitance of the electrode materials from a GCD plot.

$$C_{sp}=\frac{I\Delta t}{\Delta V\times m}$$

(2)

Here, I denotes current, t denotes discharge duration, V denotes potential window, and m denotes material mass (g).

4. Results and Discussion

4.1. Physicochemical Characterizations of Ge/h-BN/MoS₂

The crystalline features of h-BN/MoS₂ and Ge-modulated h-BN/MoS₂ were evaluated with XRD analysis. Figure 2a shows the XRD pattern of taken h-BN power that exhibits diffraction peaks at 26.6°, 41.8°, and 43.90°, which correspond to the (002), (100), and (101) [JCPDS: 34-0421] planes that are indexed. The MoS₂ shows crystalline peaks at 14.2°, 32.5°, 33.5°, 35.8°, 39.5°, 49.0°, 58.40°, and 60.1°, corresponding to the (002), (100), (101), (102), (103), (105), (110), and (008) planes, and suggests the layered hexagonal structure of MoS₂ according to the file number 03-065-1951. Two types of diffraction peaks can be seen in the XRD pattern of the h-BN/MoS₂ hybrid. The extra peaks are closely matched with the lattice structures of the MoS₂ sheets in addition to the typical diffraction properties of the h-BN sheets. The preparation of Ge/h-BN/MoS₂ leads to the diffraction peaks that emerged at 27.2°, 45.3°, 53.7°, 65.8°, and 72.8° being indexed as (111), (220), (311), (400), and (331). This clearly shows that the Ge nanocomposites form themselves with h-BN and MoS₂. FTIR was also employed to characterise the chemical structure of h-BN/MoS₂ and Ge/h-BN/MoS₂ and to determine the surface functional groups and bonding nature, as shown in Figure 2b.

The two peaks at 1369 cm⁻¹ and 870 cm⁻¹ for h-BN/MoS₂ are allocated to the boron-nitrogen-boron (B-N-B) bending mode and the boron-nitrogen (B-N) in-plane stretching mode, respectively, while the hydroxyl groups that are adsorbed on the MoS₂ nanosheets are assigned to 3439 cm⁻¹ and 1692 cm⁻¹. The bending mode is attributed to the positively charged hydroxyl present on the MoS₂ [24,25]. The creation of a Ge-N bond is evident in the absorption bands at 791 cm⁻¹, clearly confirming the interaction between h-BN/MoS₂ and Ge. From the above FTIR results, it can be said that Ge-, h-BN-, and MoS₂-based nanocomposites have been achieved.

XPS survey spectral analysis was carried out to confirm the presence of boron (B), nitrogen (N), molybdenum (Mo), sulphur (S), and germanium (Ge) elements in the obtained Ge/h-BN/MoS₂, as shown in Figure 3.

To understand the bonding pattern, high-resolution spectra of each individual element were examined. XPS spectra of the boron core-level emission (B 1s) and nitrogen core-level emission (N 1s) (Figure 3a,b) revealed the network of B-N bonding in h-BN, which were centred at binding energies (BEs) of 188.89 and 397.2 eV, respectively. The molybdenum (Mo 3d) core-level emission spectra corresponding to h-BN/MoS₂ result in well-resolved peaks at BEs of 230.9 and 234.4 eV, indicating spin-orbit components Mo 3d3/2 and Mo 3d3/2, respectively, in Figure 3c [26]. The S 2p spectra (Figure 3d) of MoS₂ nanosheets in the S²⁻ oxidation state exhibit a peak at 162.4 eV and broaden toward higher BEs. Because each MoS₂ sheet has a low grafting density and a restricted number of lamellae, the S 2p spectra produced a weak signal that could not be resolved in S 2p2/3 and 2p1/2 orbits-based peaks [27,28]. Ge 3d exhibits a large peak below 31.8 eV in its XPS spectra (Figure 3e), which can be attributed to Ge.

The direct development of Ge on nanosized MoS₂ sheets over platelets of h-BN is confirmed by the XPS spectra of the distinctive components.

HRTEM was used to explore the nanostructure characteristics and surface morphology of h-BN/MoS₂ and Ge/h-BN/MoS₂ hybrid materials. The MoS₂ nanosheets were consistently formed across the h-BN nanosheets in the h-BN/MoS₂ materials, as shown in Figure 4a,b (surface morphology), Figure 4c (lattice pattern), and Figure 4d (SAED pattern). Each MoS₂ nanosheet is made up of molecular lamellae, with greater resolution imaging and lattice fringes in the inset. This shows that the sheets have crystallized significantly. The 2D structure grew with an average particle size of 50–55 nm.

Similarly, the germanium-incorporated MoS₂ nanosheets were also formed across the h-BN nanosheets in the Ge/h-BN/MoS₂ hybrid materials, as shown in Figure 4e,f (surface morphology), Figure 4g (lattice pattern), and Figure 4h (SAED pattern). MoS₂ initially formed a small 2D nucleus and then expanded into a large 2D crystal on h-BN while accumulating germanium crystals, according to the Frank–Van der Merwe mechanism. The h-BN/MoS₂ and Ge/h-BN/MoS₂ nanocomposites also have diffraction rings and a bright spot in their SAED patterns. This shows that the nanocomposites are polycrystalline.

4.2. Supercapacitor Performances

The energy storage capability of the prepared electrodes was analysed by subjecting them to CV and GCD techniques. Figure 5 shows the CV curve obtained for h-BN, MoS₂, h-BN/MoS₂, and Ge/h-BN/MoS₂ at scan rates in the range of 10 and 100 mV s⁻¹ within a −0.2 to 0.6 V potential window. Pseudocapacitive behaviour is evident from the CV graphs, as they depict oxidation and reduction peaks. The active material stability was determined with the diffusion of KOH into the active material. At a lower scan rate, the KOH electrolyte can move around more easily. This means that the supercapacitor has a higher capacitance at a lower scan rate than at a higher rate.

It is evident that the capacitance of h-BN ranges from 765.44 F g⁻¹ to 317.68 F g⁻¹ for scan rates in the range of 10 and 100 mV s⁻¹, whereas MoS₂ delivers a capacitance of 572.06 F g⁻¹ at 10 mV s⁻¹, which is lower than h-BN. The composite h-BN and MoS₂ (i.e., h-BN/MoS₂) delivered capacitance as high as 814.85 F g⁻¹. It is evident from Figure 5c that the h-BN/MoS₂ shows excellent reversibility of the h-BN/MoS₂ electrode, as there is no apparent deviation in the characteristic CV loops. From the CV plots of h-BN (Figure 5a), MoS₂ (Figure 5b), and h-BN/MoS₂ (Figure 5c), the area encircled by the loop appears to improve on incorporating MoS₂ into h-BN. The capacitance of h-BN/MoS₂ decreases in the order 814.85, 733.84, 680.13, 639.07, 600.38, and 478.38 F g⁻¹ for scan rates in the series of 10, 20, 40, 60, 80, and 100 mV s⁻¹. The supercapacitor system is seen to deliver a remarkable specific capacitance of up to 927.78 F g⁻¹ at a 10 mV s⁻¹ scan rate, due to the introduction of Ge onto the h-BN/MoS₂ composite (Figure 5d). Even at a lower scan rate, 10 mV s⁻¹, the capacitance delivered is satisfactory, at 546.26 F g⁻¹. Herein, with an increase in the scan rate, capacitance increases as follows: 927.78, 821.84, 753.69, 698.37, 651.78, and 546.26 F g⁻¹.

The distorted triangular peaks in the charge/discharge profiles of h-BN and MoS₂ within the potential window of 0.4 V indicate the electrode shows pseudocapacitance. The formation of an h-BN/MoS₂ composite enhanced the capacitance. The h-BN/MoS₂ GCD curve (Figure 5g) shows that the capacitance increases significantly from 431.9 F g⁻¹ to 470.956 F g⁻¹ due to the addition of MoS₂. The alignment of MoS₂ sheets among the h-BN sheets improves the surface area and augments the accessibility of electrolytes into the active site, thereby delivering better performance. The addition of Ge to the h-BN/MoS₂ composite increases capacitance to 558.53 F g⁻¹ at 1 A g⁻¹ current density and to 159.19 F g⁻¹ at 10 A g⁻¹ current density, as shown in Figure 5h. The synergic effect of favourable defective sites along with the conductive Ge matrix not only exhibits OER electrocatalytic distinction, but also delivers exceptional capacitance. The consistency in the shape of the GCD curves of the active materials under study reveals the stability of the electrode material after a continuous charge–discharge cycle. Stability is an important factor for real-life applications of supercapacitor electrodes. Both h-BN/MoS₂ and Ge/h-BN/MoS₂ underwent 2000 charge/discharge cycles (Figure 5i). Ge/h-BN/MoS₂ kept 85.69% of its original capacity, while h-BN/MoS₂ kept 72.09%. Ge/h-BN/MoS₂ is more stable because it is a good electrical conductor, has a larger surface area, and has a low charge transfer resistance. When compared with several of the reported materials and their manufacturing, as shown in

Table 1. Comparison of specific capacitance and capacitance retention.

S. No	Samples	Cs (Fg−1)	Retention	Cycle Number	References
1	Boron and nitrogen co-doped porous carbon	504 (1 Ag−1)	91	10,000	[29]
2	MoS2 nanodots	122 (1 Ag−1)	86	1000	[30]
3	MoS2/rGO@PANI	160 (1 Ag−1)	-	-	[31]
4	MoS2 nanostructures/PEDOT	244 (1 Ag−1)	92	9000	[32]
5	Boron/nitrogen co-doped carbon	330 (0.5 Ag−1)	89	5000	[33]
6	MoS2-rGO/PEDOTNP	298 (1 Ag−1)	93	10,000	[2]
7	PANI/Nitrogen-doped carbon composites	276 (0.2 Ag−1)	80	5000	[34]
8	H-NrGO/PANI	510 (1 Ag−1)	74	2000	[35]
9	rGO/PANI nanofiber	442 (0.3 Ag−1)	84	2000	[36]
10	Polyaniline/boron-doped graphene nanohybrid	406 (0.5 Ag−1)	83	5000	[37]
11	BCN/ MoS2-11	283 (1 Ag−1)	-	-	[1]
12	Ge/BN/MoS2	558 (1 Ag−1)	85	2000	This work

, the Ge/h-BN/MoS₂ material can be an efficient electrode material and demonstrates superior electrochemical behaviour for supercapacitor applications [29,30,31,32,33,34,35,36,37].

4.3. Electrocatalytic Examinations of Ge/h-BN/MoS₂/GCE

The electrocatalytic excellence of Ge/h-BN/MoS₂ modified on GCE was assessed using a standard three-electrode system containing alkaline 0.5 M H₂SO₄. the OER performances of h-BN and MoS₂ samples were also compared in the control, as shown in Figure 6a. The h-BN, MoS₂, h-BN/MoS₂, and Ge/h-BN/MoS₂ exhibited the overall potential of (η₁₀) 0.54, 0.56, 0.62, and 0.67. An anodic peak of oxidation at around a potential of 1.7 V (versus RHE) was observed on Ge/h-BN/MoS₂; this could be attributable to the amorphous metal hydroxide layer intermediate stage of development. Two primary features in the LSV polarization data for Ge/h-BN/MoS₂ and h-BN/MoS₂ nanostructures were obtained, which reveal a rise in the polarization signals associated with the germanium/molybdenum ion oxidation located after 1.7 V, as shown in Figure 6a. Moreover, the oxidation current starts to rise quickly after the potential value of 1.7 V versus RHE along with the little bubble appearance on the surface of the Ge/h-BN/MoS₂ modified carbon electrode, which reflects the proceeding of oxygen evolution. By measuring the LSV polarization potential values with respect to the theoretical oxygen evolution potential, the overpotential (η₁₀) of Ge/h-BN/MoS₂ was calculated with a reliable overpotential of 0.67 mV at 5 mA/cm².The measured overpotential values of Ge/h-BN/MoS₂ were identified to be higher than the germanium-free h-BN/MoS₂ nanohybrid, i.e., 0.62 mV. Strikingly, this overpotential decrease in the presence of germanium could be ascribed to the effect of synergistic between the revealed active sites, with a lot of defects supported by Ge conductive support (Ge/h-BN/MoS₂ nanohybrid). Furthermore, the obtained OER results highlight that the Ge/h-BN/MoS₂ nanohybrid shows remarkable oxygen bubble desorption performance. Overall, the findings support Ge/h-BN/MoS₂ kinetics and electrocatalytic activity for OER. Furthermore, the OER stability (Figure 6c) and HER stability (Figure 6f) of the chronoamperometry tests were used to examine the Ge/h-BN/MoS₂ electrode during a 10 h period. In the OER study, there is no noticeable change in the initial current densities and potential in these tests, indicating consistent conductivity, mass transport, and durability. The h-BN, MoS₂, h-BN/MoS₂, and Ge/h-BN/MoS₂ exhibit a current density of 5.5, 7.8, 12.8, and 17.1 mA/cm², respectively. Similarly, the EIS data (Figure 6b) obtained in the Nyquist graphs display much fewer semicircles corresponding to Ge/h-BN/MoS₂ compared with h-BN/MoS₂, indicating lower resistance. The Ge/h-BN/MoS₂ catalyst’s greater conductivity is attributable to the Ge conductive framework, making it easier for electrical charges to move and reducing the chance of Ohmic losses [38].

The HER electrocatalytic performance of the Ge/BN/MoS₂ nanohybrid was examined using LSV curves at a scan rate of 5 mVs⁻¹ between 0.5 and 0.1 V versus RHE in an Ar-saturated aqueous 0.5 M H₂SO₄ solution, respectively. For comparison, pure h-BN and MoS₂ nanosheets, as well as Ge/h-BN/MoS₂ nanohybrids, were tested for HER performance. For a blank control, pure boron nitride sheets with no catalyst loading were evaluated under the same circumstances. The polarization curves produced by graphing the current density (J) versus the applied potential (V) in 0.5 M H₂SO₄ are shown in Figure 6d. The current densities of h-BN, MoS₂, h-BN/MoS₂, and Ge/h-BN/MoS₂ are −2.2, −2.9, −4.2, and 13.5 mA/cm², respectively. The Ge/h-BN/MoS₂ nanohybrid electrode showed an overall potential at −13.5 mA/cm² and low overpotential (η₁₀) of 0.57 mV to reach “J” cathodic 5 mA/cm². Similarly, pristine BN and MoS₂ exhibited lower HER electrocatalytic activity, with overall potentials of −2.2 and −2.9 mV, respectively. Over the studied potential range, the blank boron nitride was shown to have minimal HER activity. Similarly, the Tafel plot demonstrates the electrocatalytic process implicated in the HER (overpotential versus log j). The Tafel slopes of the Ge/h-BN/MoS₂ nanohybrid, nanostructured h-BN/MoS₂, MoS₂, and BN were calculated as 204, 203, 671, and 220 mV dec⁻¹, respectively (Figure 6e).

5. Conclusions

In summary, the Ge/h-BN/MoS₂ heterostructure was successfully synthesized using a simple chemical method. Efficient interface design, defect engineering, phase modification, and activated edges are beneficial for boosting the performance of materials in electrochemical energy conversion and storage. The present work justifies the report on the facile approach to fabricating two-dimensional (2D) Ge-decorated h-BN/MoS₂ heterostructure nanosheets by self-assembly. The physicochemical link between germanium modulation on heterostructure boron nitride/molybdenum sulphide (Ge/h-BN/MoS₂) and oxygen/hydrogen evolution reaction (HER/OER) is better organised when there are more active sites. Moreover, the Ge-decorated h-BN/MoS₂ catalyst realised an overall potential value with RHE of 17.1 (OER) and −13.5 (HER) at 5 mA/cm², Tafel value of ∼204 mV/dec (HER), and long-term electrolysis stability of 10 h. The CV curve shows remarkable specific capacitance up to 927.78 F g⁻¹ at a scan rate of 10 mV s⁻¹, due to the introduction of Ge onto the h-BN/MoS₂ composite. Even at a lower scan rate, 10 mV s⁻¹, the capacitance delivered is satisfactory, at 546.26 F g⁻¹. The GCD curve shows the addition of Ge to the h-BN/MoS₂ composite amplified the capacitance to 558.53 F g⁻¹ at 1 A g⁻¹ current density and 159.19 F g⁻¹ at 10 A g⁻¹. With a capacitance retention ratio of 85.69% over 2000 cycles, the electrode has outstanding cyclic stability. The results presented in this work show that building the electrode material into a stable structure improves its electrochemical performance significantly. Overall, these results show that a 2D germanium ion on heterostructure, ultrathin h-BN/MoS₂ electrodes is good for electrochemical devices that convert and store energy.