Tuning the Electronic Structures of Mo-Based Sulfides/Selenides with Biomass-Derived Carbon for Hydrogen Evolution Reaction and Sodium-Ion Batteries

Abstract

A key research focus at present is the exploration and innovation of electrode materials suitable for energy storage and conversion. Molybdenum-based sulfides/selenides (primarily MoS₂ and MoSe₂) have garnered attention in recent years due to their intrinsic two-dimensional structures, which are conducive to ion/electron transfer or insertion/extraction, making them promising candidates in electrocatalytic hydrogen production and sodium-ion battery applications. However, their inherently poor electronic structures have led most research efforts to concentrate on modifications aimed at enhancing their performance in hydrogen evolution reactions (HERs) and sodium-ion batteries (SIBs). Owing to their remarkable chemical inertness, expansive specific surface areas, and tunable pore architectures, carbon-based materials have garnered significant attention in research. The utilization of biomass as a renewable and environmentally sustainable precursor offers considerable benefits, including abundant availability, ecological compatibility, and cost-effectiveness. Consequently, recent scholarly endeavors have concentrated intensively on the synthesis of valuable carbon materials derived from renewable biomass sources. This review addresses the scientific challenges related to the development of electrode materials for HERs and SIBs in electrochemical energy storage and conversion. It delves into the recent focus on the two-dimensional transition-metal chalcogenides, particularly MoS₂ and MoSe₂, and the difficulties encountered in modulating their electronic structures when applied to HERs and SIBs. The review proposes the use of eco-friendly and widely sourced biomass-derived carbon (BMC) as a supporting matrix combined with MoS₂ and MoSe₂ to regulate their structures and enhance their electrocatalytic activity and sodium storage performance. Additionally, it highlights the existing challenges faced by these BMC/MoS₂ and BMC/MoSe₂ composites and offers insights into future developments.

1. Introduction

With the rapid advancement of society and the burgeoning population, our demand for energy is escalating. Currently, 80% of the world’s energy supply is derived from fossil fuels. However, these fuels have limited reserves and are non-renewable. Continued reliance on fossil fuels will significantly constrain societal development. Additionally, the combustion of fossil fuels has led to global warming, altering our living environment [1]. At present, the shortage of non-renewable energy and the deteriorating environment are driving us to seek a new type of energy that is both clean and has a high energy density to replace non-renewable sources [2,3,4,5]. This urgency compels researchers to explore technologies for clean, affordable, efficient, and sustainable energy storage and conversion. The advancement of electrochemical energy storage and conversion technologies is of paramount importance for achieving a sustainable supply of clean energy [6,7,8,9].

Electrochemical devices, such as water-splitting systems and secondary-ion batteries, have emerged as pivotal technologies in recent decades, playing an essential role in global sustainable development initiatives [10,11]. Sodium-ion batteries (SIBs) and hydrogen production through electrocatalytic water splitting have been extensively studied due to their availability and environmental friendliness, making them two primary alternatives [12,13,14]. SIBs offer numerous advantages: (1) Low Cost: Sodium resources are abundant and evenly distributed, with sodium constituting about 2.4% of the Earth’s crust, roughly 370 times more than lithium. The cost of SIBs is about one-twentieth that of lithium-ion batteries. Aluminum foil, used as the current collector, is lightweight and inexpensive, and hard carbon anodes are cheaper than graphite, reducing the weight by about 10% and cost by approximately 8%. (2) High Safety: Sodium has a low redox potential and does not easily form dendrites, preventing fires or explosions during overcharging, over-discharging, or puncturing. (3) Compatibility with Existing Technologies: The energy storage mechanism and manufacturing process of SIBs are very similar to those of lithium-ion batteries, allowing for compatibility with existing lithium battery equipment. (4) Fast Charge–Discharge Capability: Sodium ions de-solvate more easily, and the interfacial reaction kinetics are better, enabling rapid charge and discharge cycles. (5) Excellent Performance in Extreme Temperatures: SIBs maintain over 90% capacity retention at −20 °C and exhibit stable performance at high temperatures. (6) Low-Salt-Concentration Electrolytes: The smaller Stokes diameter of sodium ions allows for lower electrolyte concentrations to achieve the same ionic conductivity. (7) High Energy Density: Sodium does not form an alloy with aluminum, enabling the use of aluminum foil as a current collector, which lowers costs while increasing energy density. These advantages underscore the significant potential of SIBs in the energy storage sector [15,16].

Despite the remarkable strides in SIB technology, there remains a pressing demand for high-performance electrode materials with robust and durable structures for efficient Na⁺ storage. Moreover, the hydrogen evolution reaction (HER) presents a highly promising avenue for efficient energy conversion in electrocatalytic water splitting. When hydrogen releases its stored energy, it generates water as the sole byproduct, making it an exceptionally clean energy source. Notably, hydrogen possesses the highest energy content per unit mass, at 33.3 kWh kg⁻¹ [17]. This makes hydrogen one of the most promising and sustainable energy carriers. However, hydrogen predominantly exists in nature as a part of compounds, necessitating its production for practical use. Traditional hydrogen production methods, such as steam methane reforming and coal gasification, still generate greenhouse gases [18]. In contrast, water electrolysis is a green and straightforward hydrogen production method that addresses the intermittency of renewable energy sources and reduces carbon emissions [19]. Although platinum-based catalysts exhibit excellent performance in the HER, the limited reserves and high cost of precious metals underscore the need to develop Earth-abundant catalysts as alternatives to platinum [20,21,22]. For these energy devices, obtaining high-performance, cost-effective electrode materials has been a significant focus. Therefore, to fully advance clean energy development, it is highly desirable to realize smart devices with multifunctional materials.

Transition-metal chalcogenides (TMDs), primarily transition-metal sulfides and selenides, are considered high-energy-density materials with abundant reserves on Earth and are currently a hot research topic [23]. Due to their easily controllable morphology and high theoretical specific capacity, transition-metal chalcogenides are highly competitive compared to other anode materials and have garnered widespread attention in the application of alkali-metal-ion batteries [24,25]. Metal selenides and metal sulfides, both members of the transition-metal chalcogenide family, exhibit similar properties. Metal selenides display higher electronic conductivity because selenium has a much higher dielectric constant than sulfur. Additionally, the metal–selenium bonds in selenides are weaker than the metal–sulfur bonds, which is beneficial for conversion reactions [26,27].

Two-dimensional TMDs consist of metal atoms sandwiched between two layers of chalcogen atoms, exhibiting a structure analogous to that of graphene. These materials are characterized by strong covalent bonding within the chalcogen layers and weak van der Waals forces between adjacent layers. TMDs encompass sulfides, selenides, and tellurides, following the general chemical formula MX₂, where M represents metal atoms such as tungsten (W), molybdenum (Mo), or nickel (Ni), and X denotes chalcogen atoms, including sulfur (S), selenium (Se), or tellurium (Te) [28,29]. The main methods for synthesizing two-dimensional TMDs [30,31,32] can be summarized as follows: (1) Chemical Vapor Deposition (CVD): A widely used technique where gaseous precursors react on a substrate to form high-quality, large-area TMD thin films such as MoS₂ and MoSe₂. The process allows for precise control over thickness and crystallinity. (2) Liquid-Phase Exfoliation: This method involves exfoliating bulk TMD crystals into monolayers or few-layer nanosheets in a solvent through sonication or chemical means. It is scalable and cost-effective, though it often yields less crystalline products. (3) Hydrothermal/Solvothermal Synthesis: TMDs are synthesized by reacting metal precursors with chalcogen sources in a high-pressure, high-temperature aqueous or organic solvent environment. This method offers control over the morphology and phase, producing nanostructures like nanosheets or nanospheres. (4) Electrochemical Exfoliation: By applying an electric potential to bulk TMD materials in an electrolyte solution, layered TMDs can be exfoliated into ultrathin sheets. This method is efficient and produces high-quality exfoliated layers. (5) Molecular Beam Epitaxy (MBE): This technique involves the deposition of TMDs onto a substrate under ultra-high vacuum using atomic or molecular beams of the constituent elements. It yields high-purity and high-quality films but is more costly and complex. (6) Sulfurization/Selenization of Metal Films: Metal films (e.g., Mo or W) are first deposited on a substrate and then reacted with sulfur or selenium vapors at high temperatures to form TMD layers. This approach offers good control over the material composition and layer thickness. Each of these methods varies in scalability, cost, and the quality of the resulting TMDs, which affects their suitability for different applications, such as energy storage and catalysis.

Among these, MoS₂ is a quintessential TMD from group VI B and is a primary focus in energy storage and conversion research. Since the successful exploration of MoS₂, other TMDs within the same group, such as WS₂, MoSe₂, and WSe₂, have been extensively studied [33,34]. However, telluride members like MoTe₂ and WTe₂ have received relatively little attention. Compared to other Earth-abundant catalysts, two-dimensional TMDs possess several unique characteristics: (1) Increased Surface Area: The two-dimensional layered structure of TMDs enhances the specific surface area of the electrocatalysts, providing abundant active sites for reactions. (2) Enhanced Edge Conductivity: The in-plane conductivity of 2D TMDs is lower than their edge conductivity, facilitating easier electron transport along the edges. (3) Diverse Electronic Structures: The different d orbitals of various transition metals result in diverse electronic structures for TMDs. By tuning the transition-metal atoms, TMDs can exhibit varied electronic structures, leading to different catalytic behaviors [35]. MoS₂ and MoSe₂, as key members of the TMD family, exhibit versatile properties that make them valuable in various fields beyond electrochemical energy storage, such as optoelectronics, sensors, flexible electronics, lubricants, and so on [36]. For example, MoS₂ and MoSe₂ are also utilized in chemical and biological sensors due to their large surface area and high sensitivity to environmental changes. They have been used to detect gases, biomolecules, and other analytes with high sensitivity and selectivity [37]. Additionally, the unique catalytic mechanisms of TMDs in the HER provide multiple research avenues, offering valuable insights for the structural regulation of other novel HER catalysts. As anode materials for SIBs, TMDs feature a sandwiched structure where metal atoms are covalently bonded to adjacent layers of sulfides or selenides, with each layer connected via van der Waals forces. During the insertion of alkali metal ions, the weakening of van der Waals forces allows these ions to easily intercalate into the layered structure [38]. The primary energy storage mechanism for TMDs as SIB electrode materials is the intercalation/conversion reaction, which offers a favorable theoretical capacity for sodium storage [39]. Additionally, transition-metal chalcogenides are cost-effective, possess high theoretical capacities, and come in diverse types, providing significant advantages for use in SIBs. These attributes make TMDs a promising and widely applicable anode material for the future development of SIBs. The synthesis methods for MoS₂ or MoSe₂, two prominent two-dimensional TMDs, usually involve hydrothermal/solvothermal synthesis with metal precursors (e.g., ammonium molybdate) mixed with sulfur or selenium sources (e.g., thiourea or Se powder) in a solvent. This allows for morphology control, producing various nanostructures such as nanosheets, nanospheres, or flower-like structures. In addition, bulk MoS₂ or MoSe₂ can be exfoliated into monolayer or few-layer nanosheets in a solvent using sonication or chemical methods [40].

Although TMD catalysts are cost-effective and exhibit high theoretical catalytic activity and electrochemical stability, their catalytic performance is constrained by two primary factors: first, the number of active sites exposed at the edges is significantly higher than that on the basal planes, resulting in inert basal plane catalytic activity; second, the conventional 2H semiconductor-type TMDs have relatively low electrical conductivity, which impedes the rapid transfer of electrons to the active sites [41]. When applied to SIBs, the major issue with transition-metal chalcogenides is the significant volumetric expansion during intercalation and conversion processes [42,43]. This expansion can lead to structural damage and pulverization of the electrode materials, ultimately resulting in poor electrochemical performance. Additionally, the conductivity of transition-metal selenides is also often inadequate. Therefore, current research focuses on strategies to mitigate volumetric expansion while simultaneously enhancing the material’s conductivity.

This work focuses on MoS₂ and MoSe₂, two layered TMDs that are important for SIBs and HER applications, as shown in Figure 1. The development of these materials’ applications and structural innovations have received a lot of attention in recent years. Their unique physicochemical properties endow them with strong potential as bifunctional electrode materials for energy storage and conversion. However, limitations such as insufficient active sites and suboptimal electronic conductivity hinder their widespread application. This review will cover recent advances in modifying the electronic structure of MoS₂/MoSe₂ using environmentally friendly biomass-derived carbon materials. This approach aims to enhance the suitability of these materials as electrode components for efficient sodium-ion battery charge–discharge cycles and sustained hydrogen evolution. The review will discuss the sources and classifications of biomass-derived carbon (BMC), specific methods for tuning the electronic structure of MoS₂/MoSe₂, and the underlying mechanisms. The insights provided here will contribute to the development and application of BMC/Mo-based sulfides/selenides in SIBs and the HER.

2. Biomass-Derived Carbon

Greenhouse gases are produced when fossil fuels like coal, oil, and natural gas are burned [44]. This is a well-established fact. However, improper disposal of biomass waste also contributes to greenhouse gas emissions. Statistics show that approximately 1 to 1.5 billion tons of biomass waste are generated worldwide each year [45,46]. While a portion is effectively utilized, a significant amount is either landfilled or incinerated. Landfilled biomass waste, through microbial decomposition, not only produces methane or ammonia but also poses a risk of contaminating water sources. Furthermore, the incineration of biomass waste releases large quantities of carbon dioxide and particulate pollutants into the atmosphere. Therefore, the incineration and landfilling of biomass waste are detrimental to achieving environmental protection and the “dual carbon” goals [47,48]. Biomass represents the most abundant carbon source material outside of petrochemical-based materials. In the context of global “carbon neutrality” targets, it becomes crucial to explore how to stabilize and utilize the carbon dioxide absorbed from the atmosphere by plants through photosynthesis in a more enduring form [49]. This makes it an important subject for in-depth research to ensure the effective and sustainable use of biomass. In recent years, biomass-derived carbon materials, which have increasingly garnered attention from both academia and industry, may offer a significant solution to this important issue [50]. According to the International Energy Agency, biomass refers to various organic entities formed through photosynthesis, including all plants, animals, and microorganisms. In the energy sector, the definition of biomass is broader and often encompasses all organic substances with certain energy content, nutritional value, impact toughness, or physiological characteristics [51]. This definition has expanded over time with advancements in technology, scientific progress, and broader applications. Currently, there are three primary approaches to the development of biomass resources: (1) Direct Utilization of Biomass: This involves directly using biomass such as corn, soybean meal, and cottonseed meal as animal feed. (2) Resource Utilization of Secondary Waste: This includes the recycling of secondary waste products, such as the composting of biogas residue. (3) Indirect Conversion of Biomass: This approach involves converting biomass into biomass materials using microbial, physical, or chemical methods. For example, pyrolysis can be used to carbonize dead leaves in an oxygen-free environment to produce carbon materials, which can then be applied as working electrodes in capacitors. The simplicity and low cost of obtaining biomass-derived carbon make it more accessible for development and utilization. Research in recent years has focused on using inexpensive biomass as raw materials to develop efficient biomass carbon conversion and preparation methods. This includes efforts to manufacture high-performance carbon-based products at low cost and to explore their multifunctional applications and underlying reaction mechanisms.

Structural Engineering of BMC

Biomass carbon (BMC) materials primarily refer to the carbohydrates and compounds naturally present in plants and animals, which are transformed into porous solid forms through biomass pyrolysis under low-oxygen or anaerobic conditions. Biomass is characterized by its abundant availability, low cost, and diversity in composition and structure. Converting biomass into nanocarbon materials has become one of the most attractive and effective methods for obtaining electrocatalysts from economically viable, environmentally friendly, and renewable resources [52,53]. BMC materials are specifically activated carbons derived from biomass raw materials through high-temperature carbonization in an inert gas atmosphere; they have a simple elemental composition, where carbon constitutes a substantial proportion. Novel carbon nanomaterials derived from biomass resources have shown immense potential in various catalytic reactions, including electrochemical energy storage batteries, the HER, the oxygen evolution reaction (OER), and CO₂ capture [54,55]. Biomass can be classified into several categories based on its source, as illustrated in Figure 2. These categories mainly include the following: (1) Plant-based Biomass: This includes materials like coconut shells and lignocellulose. (2) Lignocellulosic biomass is commonly encountered in everyday human activities, such as corn stalks, cotton, peanut shells, and tree leaves. It consists of three main components: (1) Animal-based Biomass: Derived from animal sources. (2) Polysaccharide-based Biomass: Comprising various polysaccharides. (3) Protein/Bacterial Biomass: Derived from proteins and bacteria. The surface structures of these biomass precursors can vary significantly due to their different origins. The dimensional properties of biomass carbon materials can impact their efficiency in processes like filtration, battery performance, or catalysis. For instance, materials with nanoscale dimensions often exhibit enhanced electrical conductivity and catalytic activity. Therefore, selecting the appropriate precursor for preparing BMC materials is crucial before initiating the synthesis process.

The most prominent feature of BMC precursors is their porosity, also known as specific surface area. On the one hand, the presence of pores and various functional groups around them facilitates adsorption and provides more active sites, enhancing catalytic and adsorption performance [56]. On the other hand, the overall thermal stability of BMC precursors can affect the material’s performance. Excessive carbonization during thermal treatment can lead to pore blockage and deactivation, reducing the material’s porosity and adversely impacting its adsorption properties. Additionally, the content of elements, especially trace elements, in BMC precursors significantly influences the properties of the final product. Due to the varying electronegativity of elements, they can adsorb free ions within the biomass material [57]. In the application environment of BMC materials, these ions can effectively undergo ion exchange with the surrounding materials or form new dynamic equilibria with environmental cations and anions. This process can enhance mutual adsorption between elements, which is crucial for applications in catalysis and energy storage. By employing various physical and chemical activation methods, as well as soft/hard-templating techniques and aerogel carbonization methods, BMC with precisely engineered structures can be fabricated. For instance, Qiu et al. used ammoniation and glucose and thiophene as precursors to create nitrogen and sulfur co-doped mesoporous carbon using a silica template [58]. Lai et al. used ferrous sulfate, glucose, and melamine to create an in situ sacrificial templating technique [59]. In this system, graphitic carbon nitride (g-C₃N₄) is synthesized as an in situ two-dimensional template via the thermal decomposition of melamine at 600 °C. The sources of iron and carbon are glucose and ferrous sulfate, respectively. At a higher temperature (900 °C), Fe/Fe₃C is encapsulated within graphitic nanocarbon. Using glucose and Prussian Blue (PB) as sources of iron and nitrogen, respectively, Yang et al. immediately pyrolyzed the combination to create nitrogen-doped carbon materials [60]. Iron, carbon, nitrogen gas, different iron carbides (Fe₇C₃, Fe₂C, Fe₃C), and carbon nitrides (CN, C₂N₂⁺, C₃N₂⁺, C₃N₃⁺) are produced during the pyrolysis process of PB. Nitrogen-doped graphite layers grow more easily when iron and iron carbides are present. Additionally, the Fe/Fe₃C@N graphitized carbon nanocapsule structure prevents the agglomeration of carbon nanosheets. Using natural biomass to produce porous carbon is a clean and sustainable approach. Biomass-derived carbon materials inherently contain heteroatoms, making them valuable for obtaining naturally doped structures [61]. In contrast, conventional methods for preparing heteroatom-doped carbon materials often require the introduction of external heteroatom sources, such as melamine, urea, amines, polyaniline, polypyrrole, or polyacrylonitrile [62]. However, these chemicals are inherently toxic and do not meet sustainability requirements. On the other hand, when utilized as precursors for the synthesis of carbon materials, the majority of biomass materials are naturally rich in heteroatoms like phosphorus (P), sulfur (S), and nitrogen (N) and enable in situ doping of these elements. For example, Zhou et al. synthesized sulfur and nitrogen co-doped carbon nanosheets (RN-800) by thermally breaking down peanut roots stimulated with MgCl₂ in a single step [63]. RN-800 demonstrated a tiny Tafel slope of 67.8 mV·dec⁻¹, an onset potential of only 27 mV, and an overpotential of 116 mV at a current density of 10 mA·cm⁻² in HER tests carried out in 0.5 M H₂SO₄. In contrast to nitrogen-doped carbon frameworks, sulfur doping enhanced the charge density around the sulfur and carbon atoms, according to density functional theory (DFT) simulations. This enhancement in charge density is the primary reason for the superior catalytic activity of RN-800.

Additionally, proteins offer distinct advantages in the field of electrocatalysis due to their unique molecular structures, natural templating properties, and rich elemental compositions. These advantages include the following: (1) High Surface Area and Porosity: Proteins are rich in carbon and possess unique molecular structures. After high-temperature pyrolysis, they can form naturally porous carbon materials with high surface areas suitable for electrocatalytic applications. (2) In Situ Nitrogen Doping: Proteins contain nitrogen-rich molecules, such as amino acids, which ensure in situ doping of nitrogen (self-doping). This approach generates additional defects compared to external nitrogen sources, increasing the number of active sites and electron density. For instance, Wang et al. utilized egg-white protein, which was dried and mixed with cobalt nitrate, followed by pyrolysis under a nitrogen atmosphere, to produce Co-N-C materials [64]. Electrochemical tests demonstrated that cobalt–nitrogen doping exhibited superior performance compared to solely nitrogen-doped materials, with a half-wave potential only 74 mV lower than that of commercial platinum–carbon catalysts. Consequently, the unique and controllable structure of biomass-derived carbon materials exerts a strong anchoring effect on MoS₂/MoSe₂. The natural porous structure effectively prevents nanoparticle agglomeration, which improves the electronic cloud distribution and regulates the electronic structure and active sites of MoS₂/MoSe₂. Biomass carbon can donate or accept electrons, altering the charge distribution in MoS₂/MoSe₂. This can lead to changes in their electronic band structure, potentially enhancing their electrical conductivity or modifying their electronic properties for specific applications. When biomass carbon is combined with MoS₂/MoSe₂, it can create interfaces that impact the electronic interactions between the two materials. This can modify the electronic band alignment, influencing the charge transfer efficiency and overall electronic behavior. This enhancement leads to improved ion storage and catalytic activity, thereby enhancing the performance of MoS₂/MoSe₂ in SIBs and the HER.

3. Electronic Modulation of Mo-Based Sulfide/Selenide Materials

Within the category of two-dimensional materials, Mo-based 2D materials stand apart from other dimensional materials in a number of ways as follows: (i) Molybdenum-based 2D materials have shorter ion diffusion lengths, which promote ion and electron diffusion and transport. This accelerates charge carrier dynamics in molybdenum-based electrode materials, improving metal-ion battery rate performance. (ii) Their extensive specific surface area enhances the exposure of active sites on the surface, thereby facilitating more efficient electrochemical reactions and significantly increasing the contact area between molybdenum-based electrode materials and the electrolyte in metal-ion batteries. (iii) Molybdenum-based two-dimensional materials possess adjustable surface properties, such as the ease of dislodging surface atoms from the lattice, leading to the formation of vacancy defects. Additionally, chemical functionalization, heteroatom doping, and expanded interlayer spacing reduce ion transport lengths and expose more reactive sites, collectively augmenting the electrochemical performance of molybdenum-based electrode materials. Clearly, molybdenum-based 2D materials demonstrate superior electrochemical properties, leading to a growing body of research and publications on their applications in electrochemical energy storage in recent years [65,66,67].

Based on the crystal structure of MoX₂ (X = S and Se), MoX₂ can be categorized into several phases: the semiconducting 2H phase with a trigonal prismatic structure, the metallic 1T phase with an octahedral structure, and the 3R phase [68]. In the 2H and 3R phases, Mo-S coordination adopts a trigonal prismatic D_3h structure, whereas in the 1T phase, Mo-S coordination adopts an octahedral Oh structure. The transformation between these phases can occur by rotating one of the chalcogenide planes by 60 degrees around the c-axis, shifting from a trigonal prismatic to an octahedral coordination. Due to the instability of the 1T phase, structural distortions can occur, leading to other metastable polymorphs, including superstructures such as 1T′, 1T′′, and 1T′′′ with varying unit cells [69]. In these distorted structures, Mo-Mo associations occur, such as dimerization in the 1T′ phase and trimerization in the 1T′′ and 1T′′′ phases. The distinct crystal structures and electronic configurations of these metastable polymorphs endow them with intriguing physical properties, which have potential applications in various fields. However, most semiconducting 2H-phase MoX₂ structures exhibit inherently poor electrical conductivity, which is linked to the direct bandgap present in monolayers—1.8 eV for MoS₂ and 1.5 eV for MoSe₂ [70,71]. Because of their low electronic conductivity, the semiconducting 2H-phase MoX₂ structures exhibit poor electrochemical performance for energy conversion and storage applications. On the other hand, when compared to the semiconducting 2H phase, the metallic 1T-phase MoX₂ exhibits far higher electrical conductivity. As an illustration, exfoliated 1T-phase MoS₂ possesses conductivity that is seven orders of magnitude higher than that of 2H-phase MoS₂ [72]. Many approaches, such as the combination of conductive carbon materials (carbon matrices and conductive polymers), alloys, and phase engineering, have been developed to increase the conductivity of MoX₂ [73]. Building on this, introducing BMC into MoX₂ materials is expected to facilitate the optimization of the MoX₂ electronic structure.

Recently studied MXenes are 2D transition-metal carbides or nitrides, known for their high conductivity, mechanical strength, and hydrophilicity. These characteristics make them ideal electrodes for electrochemical reactions, including the HER and SIBs [74,75]. For instance, the layered structure of MXenes facilitates fast ion/electron transfer, leading to efficient hydrogen production. Surface modification, such as functionalization with -OH or -F groups, can enhance their catalytic activity. MXene-based composites with TMDs (MoS₂, MoSe₂) have gained attention [76,77]. These combinations leverage the high conductivity of MXenes with the catalytic or sodium-ion storage capabilities of MXene-MoS₂/MoSe₂ composites. MXene-MoS₂/Se₂ composites can offer more active sites, better ion/electron transfer, and improved stability, leading to enhanced HER performance [78]. For SIBs, these composites can enhance the storage capacity and cycling stability of SIBs by providing better ion diffusion pathways and structural robustness [79]. On the other hand, metal–organic frameworks (MOFs) are known for their large surface areas and tunable pore structures. These features enhance mass transfer and active-site exposure, making MOFs promising candidates in HER catalysis and SIBs [80,81]. By combining MOFs and MoS₂/MoSe₂, the composites exhibit synergistic effects. The MOF’s large surface area increases the exposure of active sites on MoS₂/MoSe₂, while the conductive framework supports fast electron transfer [82]. This enhances the overall HER efficiency and reaction kinetics in SIBs. The integration with MOFs mitigates these issues by providing a flexible framework that buffers volume changes during Na⁺ insertion/extraction, thus enhancing the cycling stability [83].

3.1. BMC/MoS₂ for SIBs and HER

MoS₂ exhibits a diverse range of unique properties, making it a focal point of research across various scientific disciplines. Its applications are particularly well explored in the fields of tribology, energy storage and conversion, and chemical sensing. The MoS₂ crystal is composed of S-Mo-S unit layers held together by van der Waals forces. Each stable unit layer (a monolayer of MoS₂) consists of two sulfur atoms and one molybdenum atom in between, arranged in a hexagonal structure where the Mo and S atoms interact through ionic covalent bonds in a trigonal prismatic configuration. According to DFT calculations, the edge site of MoS₂ exhibits a Gibbs free energy of hydrogen adsorption (∆G_H) that is comparable to that of noble metal platinum (Pt) at approximately 0.08 eV [84]. This indicates its strong potential as a catalyst for the HER [85]. Although the edge sites of MoS₂ exhibit strong catalytic capabilities, MoS₂ itself still suffers from inherent limitations, such as inert basal planes and low electron transfer efficiency, which hinder its potential for further enhancing catalytic performance. As an anode material for SIBs, MoS₂ offers a high theoretical specific capacity (670 mAh g⁻¹) and, due to its layered structure, is particularly well suited for use in these batteries [86,87]. Its application in alkali-metal-ion batteries has been extensively studied. However, several intrinsic drawbacks significantly impact the electrochemical performance of MoS₂: (1) Low Electrical Conductivity: MoS₂ is predominantly a semiconductor, resulting in generally poor conductive properties. (2) Poor Cycling Stability: Unmodified MoS₂ undergoes significant volume changes during repeated sodium-ion insertion and extraction cycles, leading to electrode pulverization and rapid capacity decay in the battery. (3) Aggregation and Restacking Issues: During electrode fabrication, MoS₂ nanosheets tend to aggregate and restack, which reduces the electrode’s specific surface area and decreases the number of active sites, leading to suboptimal electrochemical performance. These challenges limit the effectiveness of MoS₂ in SIBs and hinder its broader application potential.

Carbon nanomaterials, such as one-dimensional carbon nanofibers, carbon nanotubes, and two-dimensional graphene, have been used in conjunction with MoS₂ to obtain high electrocatalytic efficiency because of their great stability, huge specific surface area, and superior conductivity. Research has demonstrated that carbon nanoparticles inhibit MoS₂’s aggregation and restacking during durability tests, in addition to improving the material’s conductivity and active-site exposure [88]. A new class of carbon nanomaterial called zero-dimensional carbon dots (or CDs) has attracted a lot of interest because of its high fluorescence, fast electron transfer rates, and large electron storage capacity. You et al. reported synthesizing biomass-derived carbon dots (BCDs) using chitosan that was recovered from discarded shrimp and crab shells, as shown in Figure 3a [89]. The BCDs-MoS₂ composite was then created via a straightforward hydrothermal process. The TEM images further confirmed the typical stacked nanosheet structure of the BCDs-MoS₂ composite. Additionally, the nanosheets were observed to have an approximate size of 100 nm. In the HRTEM image of BCDs-MoS₂ (Figure 3b), the (002) crystal plane of MoS₂ and the layered structure of BCDs are clearly visible. Moreover, some lattice defects are also observed, which could serve as new catalytic sites for the HER. The BCDs exhibit a spherical morphology with an approximate size of 5 nm, aligning with the expected dimensions of carbon dots, as depicted in Figure 3c and d. The Figure 3e also presents the electrocatalytic performance of BCDs-MoS₂ for the HER, evaluated using linear sweep voltammetry (LSV) in a 0.5 M H₂SO₄ solution. The study demonstrated a substantial enhancement in the catalytic activity of BCDs-MoS₂, with an overpotential of approximately 232 mV and an initial overpotential of 115 mV at a cathodic current density of 10 mA cm⁻². This performance was markedly superior to that of commercial MoS₂ (281.5 mV) and laboratory-synthesized MoS₂ (236.5 mV). Hu and colleagues utilized carbon derived from Saccharomyces cerevisiae yeast cells to support MoS₂, followed by Pt doping to obtain MoS₂@Pt/YC (Figure 3f) [90]. As shown in the figure, MoS₂@Pt/YC maintains the same nanosheet structure as MoS₂@YC. The catalytic activity of MoS₂@Pt/YC surpasses that of Pt/YC, MoS₂@YC, and YC, indicating its superior intrinsic activity (Figure 3g). The long-term stability of MoS₂@Pt/YC was tested in 0.5 M H₂SO₄. As depicted in Figure 3h, the current density of MoS₂@Pt/YC showed no significant change after continuous electrolysis for 24 h at an overpotential of 118 mV. Furthermore, Figure 3i illustrates that after 3000 CV cycles, the polarization curve exhibits almost no variation. M. Selvakumar’s research group developed an HER catalyst by in situ loading MoS₂ onto activated carbon derived from Diospyros Melanoxylon leaves, which were calcined and treated with acid and alkali, achieving a high specific surface area of 1509 m² g⁻¹. This catalyst exhibited a low Tafel slope of −84 mV dec⁻¹ [91]. Similarly, Amir Mahdi Homayounfard and colleagues synthesized AC/MoS₂ using discarded rose petals as a bio-source of activated carbon. This catalyst demonstrated a low overpotential of 136 mV in 0.5 M H₂SO₄ at a current density of 10 mA cm⁻², with a Tafel slope of 72 mV dec⁻¹ [92].

MoS₂ nanocatalysts suffer from issues such as limited electronic conductivity, large band gaps, and inadequate active-site exposure, which cause major performance differences in real-world applications. Additionally, the van der Waals forces prevalent between MoS₂ nanosheets cause material aggregation, often obscuring the active edge sites and impeding mass transport to the active centers. This aggregation suppresses the catalytic activity of MoS₂. By adding carbon supports, the catalyst’s conductivity can be increased, which will improve electron transfer. The catalyst’s surface area can be further increased by the carbon support’s nanostructure, producing more active sites and raising the HER’s activity. Better HER performance can be achieved by combining MoS₂ with carbon supports to create MoS₂-C composites, which greatly boost conductivity, inhibit particle aggregation, and increase the exposure of catalytic active sites. However, further improvements in HER activity are needed. In addition to enhancing conductivity, optimizing MoS₂’s catalytic performance can involve improving the activity of edge sites, increasing or exposing more active sites, and fine-tuning the electronic structure [93].

Elemental doping is a potent strategy for enhancing the HER performance of TMDs by improving their crystal structures and microstructures. To date, various transition metals, such as cobalt (Co), nickel (Ni), and iron (Fe), as well as anions like sulfur (S) and selenium (Se), have been introduced into the lattices of TMDs [94]. Deng et al. discovered that these dopants might alter the MoS₂ lattice’s local electronic structure when heteroatoms were added to the MoS₂ lattice [95]. Heteroatom doping can modify the local electronic environment, increase the number of electrochemically active sites, improve conductivity, and lower the free energy of hydrogen adsorption during the HER, according to Chang et al.’s doping of heteroatoms into layered MoS₂ nanostructures [96]. Meng et al. created a three-dimensional layered mesoporous structure by combining graphene with MoS₂ [97]. According to their research, there are more active sites exposed by the consistent and evenly distributed interaction between graphene and MoS₂. According to Li et al., MoS₂ nanosheets doped with nickel and cobalt can help with the tandem HER stages, which will greatly increase the catalytic activity in alkaline solutions [98].

The inclusion of different dopants in the MoS₂ matrix increased the intrinsic activity of the planar sulfur atoms in the aforementioned tests. However, due to the aggregation and self-accumulation of MoS₂, active sites are easily obscured, and the defects introduced during the doping process significantly reduce stability and conductivity. Research indicates that loading MoS₂ onto materials with large surface areas and high electrical conductivity, coupled with heteroatom doping, can both expose more active sites and improve conductivity. Based on this, Ji et al. employed wet sugarcane bagasse pyrolysis to produce SCBC carbon materials. Through calcination and hydrothermal reactions, they synthesized cobalt-doped nanosheets with varying amounts of cobalt (Figure 4a) [99]. In this process, PEG-400 was used as a binder and template to facilitate the synthesis of nanosheets, resulting in their stacking into flower-like spherical nanoparticles, ultimately forming Co-MoS₂-SCBC materials. Figure 4b illustrates how new diffraction peaks at 32°, 36°, 40°, and 55°, which correspond to the (200), (210), (211), and (311) crystal planes of CoS₂, respectively, arise when cobalt doping increases, especially at doping levels of 0.50 and 0.67. This suggests that cobalt atoms mix with sulfur atoms to generate CoS₂ as the quantity of cobalt doping increases. An essential metric for assessing the catalyst’s level of graphitization is the ID/IG ratio in Raman spectra. The amount of additional SCBC (carbon nanomaterial) has a major impact on the catalyst’s level of graphitization. Figure 4c illustrates how the ID/IG ratio first rises and subsequently falls with an increase in SCBC. At a 0.2 SCBC concentration, the I_D/I_G ratio reaches its maximum value of 0.963. This could be because the orderly transition from an amorphous to a crystalline graphite structure is weakened by the excessive addition of SCBC. Within these two vibrational modes, the D-band is ascribed to the vibrations of carbon atoms in the disordered structure and surface defects, as well as the vibrations of dangling bonds at the edges of graphite planes. The G-band is ascribed to the vibration of sp²-hybridized C–C bonds in the planar hexagonal lattice. By adding SCBC, MoS₂’s electrical conductivity is greatly increased, which improves the circumstances for increased HER activity. Figure 4d displays the comparison of overpotentials for Co-MoS₂-SCBC at a current density of 10 mA cm⁻². It can be observed that Co-MoS₂-0.67-SCBC-0.2 exhibits the lowest overpotential, which is only 62 mV. Wang et al. utilized ginkgo leaves as a biomass-derived carbon source and, by adjusting the amounts of ammonium molybdate tetrahydrate and BMC, synthesized Mo₂C&MoS₂@NSCx heterojunction composites (Figure 4e) [100]. TEM images (Figure 4f) reveal that Mo₂C&MoS₂ nanoparticles are uniformly dispersed on the carbon sheets, with sizes ranging from 5 to 20 nanometers. The XPS spectra (Figure 4g) clearly show the changes in the valence states and the intensity of Mo due to the introduction of MoS₂. Figure 4h presents LSV curves measured at a scan rate of 1 mV s⁻¹. As shown, the commercial Pt/C catalyst demonstrates the expected electrocatalytic performance, with a near-zero onset potential and achieving an overpotential of 35 mV at a standard current density of 10 mA cm⁻². The overpotentials (η₁₀) for Mo₂C&MoS₂@NSCx (x = 1, 2, 3, 4) at a current density of 10 mA cm⁻² are recorded as 331 mV, 226 mV, 209 mV, and 220 mV, respectively. The incorporation of Mo₂C and MoS₂ as active sites substantially enhances the HER activity of the carbon material derived from ginkgo leaves. In Figure 4i, the Tafel slopes for Mo₂C&MoS₂@NSCx (x = 1, 2, 3, 4), MoS₂@NSC₃, and Mo₂C@NC₃ are 250.8 mV dec⁻¹, 110.5 mV dec⁻¹, 85.5 mV dec⁻¹, 111.6 mV dec⁻¹, 129.7 mV dec⁻¹, and 208.6 mV dec⁻¹, respectively. The Tafel slope value for Mo₂C&MoS₂@NSC₃ indicates that its HER activity likely follows the Volmer–Heyrovsky mechanism. It can be seen that the interface quality of MoS₂ plays a crucial role in the efficiency of HER devices. Many reports have enhanced the HER performance of MoS₂ by constructing highly effective heterojunctions, such as CoS₂/MoS₂ [101], MoS₂/Ti₃C₂ [102], multiple metal single atoms/MoS₂ [103], and so on. A high-quality interface ensures minimal resistance and smooth charge transfer. Furthermore, ensuring good crystallinity and clean interfaces maximizes the number of exposed active sites.

Liu et al. synthesized a two-dimensional heterostructure (Figure 5a) comprising hydroxylated nickel oxide (NiOOH) and molybdenum disulfide (MoS₂) nanosheets on catkin-derived mesoporous carbon (C-MC) and used it as a catalyst for the OER and HER. [104]. The TEM image in Figure 5b reveals that MoS₂@NiOOH@C-MC exhibits a layered, petal-like structure. Further magnification reveals the lattice structures of the layered MoS₂ and NiOOH (Figure 5c). The elemental mapping in Figure 5d,e shows the uniform distribution of Ni, Mo, C, N, O, and S, confirming the successful synthesis of the MoS₂@NiOOH@C-MC heterostructure via in situ methods. As a hydrogen evolution catalyst, MoS₂@NiOOH@C-MC demonstrates competitive performance, with an overpotential of 250 mV and a relatively low Tafel slope (Figure 5f,g). MoS₂@NiOOH@C-MC has a high Cdl value (4.3 mF cm⁻²), indicating the presence of more active sites that support its improved electrocatalytic activity (Figure 5h). As seen in Figure 5i, the catalyst shows remarkable stability, holding on to 98.4% of its starting voltage. Additionally, the MoS₂@NiOOH@C-MC catalyst’s LSV polarization curves demonstrate no deterioration even after 1 to 5000 cycles, suggesting that it has exceptional catalytic stability and endurance in alkaline conditions and is a viable option for use in the future.

Because of their large theoretical capacity, two-dimensional MoS₂ sheets are widely employed as anode materials for lithium- and sodium-ion batteries (LIBs and SIBs). However, the rate performance and lifetime of the electrodes are negatively impacted by their low conductivity and notable volume variations during cycling [105]. To address these issues, Liu et al. prepared MoS₂@biohybrid carbon nanofibers (MoS₂@BHCF) by in situ hydrothermal and thermal treatments of palm silk biomass (Figure 6a) [106]. The MoS₂@BHCF composite has thinner and smaller nanosheets developing on the BHCF scaffold, as seen in Figure 6b. Figure 6c shows the optimal architectures of MoS₂, carbon, and MoS₂/C hybrids stacked horizontally. The extraordinarily high formation energy (E_f) of 5.87 eV per MoS₂ implies that this growth mechanism is thermodynamically unfavorable, even though the horizontally developed MoS₂ structures on the carbon surface are thermodynamically stable at 0 K. In contrast, MoS₂ can grow vertically on the carbon surface, minimizing the formation energy (Figure 6d). MoS₂@BHCF, with its optimized electronic structure, shows significantly enhanced performance as an anode material for SIBs, as demonstrated by rate performance tests (Figure 6e). Additionally, Ding’s research group used dehydrated raupo to produce carbonized amorphous micron-sized carbon ribbons (AMCRs) and subsequently loaded MoS₂ onto them (Figure 6f) [107]. They conducted comprehensive electrochemical testing of the MoS₂@AMCRs composite, including cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), cycling stability, and impedance measurements, to validate the enhancement of MoS₂’s sodium storage performance due to the BMC (Figure 6g–j). Luo et al. and Xiao et al. also utilized BMC to modify the structure of MoS₂, achieving enhanced Na⁺ storage performance [108,109]. During the sodium-ion intercalation process, composites with unique micro-/nanostructures can effectively buffer structural volume changes and provide additional energy storage sites, contributing to the stability and enhancement of electrochemical performance. In addition, stable interfaces prevent unwanted reactions that can form solid–electrolyte interphase (SEI) layers or other degradation products. A high-quality interface minimizes side reactions and prolongs battery life. Moreover, during the insertion and extraction of sodium ions, MoS₂ undergoes significant volume changes. A well-designed interface can accommodate expansion and contraction without causing structural damage, improving cycling stability.

3.2. BMC/MoSe₂ for SIBs and HER

In various energy storage or conversion applications, MoSe₂ is often considered an analog of MoS₂ due to their strong similarities. Recent research on MoSe₂ has demonstrated greater potential, positioning it as a viable alternative to commonly used metal sulfides in energy storage and conversion. Compared to MoS₂, MoSe₂ exhibits superior electrical conductivity due to the inherent metallic nature of selenium (with a conductivity of 1 × 10⁻³ S/m, compared to sulfur’s 5 × 10⁻²⁸ S/m) [110,111]. This makes MoSe₂ one of the most metallic TMDs, which is crucial for electrochemical applications where high ionic and electronic conductivities are essential for enhanced performance. Like graphite, MoSe₂ is a typical TMD with layers packed together by van der Waals interactions and a characteristic two-dimensional lattice structure. Therefore, MoSe₂ can fairly replicate graphene or graphite applications [112]. To maximize performance, suitable structural designs should be taken into account based on the application. In the intercalation process of SIBs, the graphite-like structure facilitates the accommodation of sodium ions within the layered architecture of MoSe₂. MoSe₂, a quintessential two-dimensional material, boasts a substantial theoretical capacity (~422 mAh g⁻¹), a significant interlayer spacing (0.62–0.65 nm), and a narrow bandgap (~1.1 eV), which collectively reduce the insertion/extraction resistance of sodium ions and enhance electrochemical reactivity [113,114]. It is recommended to exfoliate individual MoSe₂ sheets from the bulk structure for alternative uses. Compared to bulk materials, two-dimensional layered materials typically exhibit extraordinary physical, chemical, and electronic properties. Experimental studies indicate that the edges of two-dimensional MoSe₂ are the primary sites for hydrogen evolution electrocatalysis, with the basal planes showing significantly lower electrochemical activity, a phenomenon well documented in graphene. Therefore, to tailor MoSe₂ for specific applications, tuning strategies such as defect or strain engineering may be essential. Like MoS₂ and other similar TMDs, MoSe₂ exists in different phases, with the 1T octahedral and 2H trigonal prismatic structures being two critical crystalline forms. The former exhibits metallic properties, while the latter is semiconducting. According to reports, metallic 1T MoSe₂ possesses an electrical conductivity that is 107 times higher than the semiconductor phase [115,116,117]. Nevertheless, metallic 1T MoSe₂ progressively transitions into the 2H semiconductor phase due to thermodynamic instability [118].

The Wang research group performed chemical treatment on Crepis tectorum fluff biomass, utilizing its porous and hydrophilic interface for the in situ growth of MoSe₂, followed by impregnation and doping with Ru elements, resulting in Ru-MoSe₂/CMT, as shown in Figure 7a [119]. Figure 7b depicts the complete tubular cluster morphology of Ru-MoSe₂/CMT, where Ru-MoSe₂ is observed to coat both the inner and outer surfaces of the tubes. MoSe₂ is uniformly grown on the surface of the hollow carbon tube clusters, with slight aggregation occurring in some localized areas. Additionally, Figure 7c reveals a few Ru nanoclusters (highlighted with white circles). The lattice fringes marked with a yellow line in Figure 7c correspond to the (002) plane of MoSe₂ with a d-spacing of 0.67 nanometers. Further magnification from Figure 7d,e1,e2 confirms the coexistence of 2H-MoSe₂ and 1T-MoSe₂ in Ru-MoSe₂/CMT. The presence of 1T-phase MoSe₂ in Ru-MoSe₂/CMT is likely attributed to the addition of Ru nanoclusters, which induce a partial transformation of 2H-phase MoSe₂ to 1T-phase MoSe₂. Studies indicate that, compared to the semiconductor 2H-phase MoSe₂, the metallic 1T-phase MoSe₂ significantly reduces charge transfer resistance, thus demonstrating superior catalytic activity in the HER. The HER activity of Ru-MoSe₂/CMT was also investigated in an acidic medium using a three-electrode system in 0.5 M H₂SO₄. The LSV curves for Ru-MoSe₂/CMT, MoSe₂/CMT, MoSe₂, and Pt/C are displayed in Figure 7e. Pt/C has the fastest current response with increasing cathodic potential when compared to other samples. For MoSe₂/CMT, Ru-MoSe₂/CMT, MoSe₂, and Pt/C, the overpotentials needed to reach a current density of 10 mA cm⁻² are 184, 252, 37, and 109 mV, respectively. Ru-MoSe₂/CMT has a greater overpotential than Pt/C, even at a current density of 100 mA cm⁻². Ru-MoSe₂/CMT has outstanding stability in 0.5 M H₂SO₄, as demonstrated by the constant-potential (CP) curves in Figure 7f, where the potential remains mostly consistent across a 20 h test period. Compared to other literature-reported MoSe₂-based and Ru-related catalysts, Ru-MoSe₂/CMT also demonstrates significant HER catalytic activity in acidic media, as illustrated in Figure 7g.

Su et al. synthesized a MoSe₂/biowaste carbon/carbon nanotube composite (MoSe₂/BC/CNTs) material using biowaste-sustained mangosteen peel through grinding and calcination [120]. Mangosteen peel biowaste was discarded and utilized as a raw material for pyrolysis, which produced BMC, as Figure 8a illustrates. During the hydrothermal process, the BMC matrix promotes the development and dispersion of MoSe₂, preventing severe MoSe₂ particle aggregation. Additionally, the substrate serves as a useful buffer to support MoSe₂’s volume growth during cycling. Conversely, the inclusion of conductive carbon nanotubes and mechanical resilience improve the electron transport capacity and structural stability. According to Figure 8b’s thermal gravimetric measurement, MoSe₂ makes up roughly 70.11% of the composite’s mass. With a pore size of approximately 3.9964 nm and a specific surface area of 289.035 m² g⁻¹, the MoSe₂/BC/CNTs composite is beneficial for the adsorption and storage of Na⁺ ions (Figure 8c). The cycling performance of MoSe₂ in its bare form and MoSe₂/BC/CNT composite at 200 mA g⁻³ is displayed in Figure 8d. With a high initial discharge capacity of 776.8 mA h g⁻¹ and a Coulombic efficiency of 63.8%, the MoSe₂/BC/CNT composite is impressive. While bare MoSe₂ exhibits a lower initial discharge capacity of 415.5 mA h g⁻¹, with a noticeable decline in capacity after 100 cycles, down to 213.4 mA h g⁻¹ by the 250th cycle, the MoSe₂/BC/CNT composite displays high stability (~405.0 mA h g⁻¹) even after 250 cycles. The authors also evaluated how well carbonized mangosteen peel worked in SIBs. The BMC showed a high discharge capacity of 688.6 mA h g⁻¹ in the first cycle. This immediately decreased to 107.8 mA h g⁻¹ in the second cycle, and after 250 cycles, it stabilized at 130.4 mA h g⁻¹. The carbon in biomass primarily acts as a stable substrate for MoSe₂ nucleation in a heterogeneous manner. Figure 8e,f demonstrate that the MoSe₂/BC/CNT composite operates via a pseudocapacitive-dominant Na⁺ diffusion mechanism. Zeng et al. synthesized a multilayer MoSe₂-based nanostructure with N and P co-doped biochar composite material (MoSe₂/NP-C) [121] using waste chlorella as a precursor and adsorbent. As shown in Figure 8g, this composite combines a one-step selenium procedure with biomass adsorption. An anode for a sodium-ion complete cell built with Na₃V₂(PO₄)₃ (NVP) was the synthesized MoSe₂/NP-C-2. The full cell demonstrated exceptional cycling stability at 500 mA g⁻¹, highlighting the potential of MoSe₂/NP-C for applications in sodium-ion battery devices (Figure 8h–j). In summary, we have compiled tables of the literature on BMC/MoS₂ and BMC/MoSe₂ used for the HER and SIBs, as shown in

Table 1. Comparison of various BMC/MoS₂ or BMC/MoSe₂-based nanocomposites for HER.

Material	Overpotential (At 10 mA cm−2)	Electrolyte	Stability (Efficiency)	Refs.
MoS2@NSC3	209 mV	0.5 M H2SO4	15 h	[100]
Co-MoS2-0.67-SCBC-0.2	62 mV	0.5 M H2SO4	12 h (90~95%)	[99]
MoS2@NiOOH@C-MC	250 mV	1 M KOH	48 h	[104]
Ru-MoSe2/CMT	70 mV	1 M KOH	20 h	[119]
BCDs-MoS2	115 mV	0.5 M H2SO4	20 h	[89]
DAC/MoS2		0.5 M H2SO4		[91]
AC/MoS2-F	136 mV	0.5 M H2SO4	24 h	[92]
MoS2@Pt/YC	118 mV	0.5 M H2SO4	24 h	[90]
Co@NCNT/CW	263 mV (500 mA cm−2)	1M KOH	100 h	[122]

and

Table 2. Comparison of various BMC/MoS₂ or BMC/MoSe₂-based nanocomposites for SIBs.

Material	Current Density (A g−1)	Capacity (mAh g−1)	Capacity Retention @ Cycle Number	Refs.
MoS2/TSFC	0.1	243	64% @ 500 cycles	[108]
MoS2@BHCF	2	227	223 mAh g−1 was maintained after 100 cycles at 0.05 A g−1	[106]
MoSe2/BC/CNTs	5	405	83% @ 250 cycles	[120]
MoSe2/NP-C-2	0.5	215		[121]
GC@MoS2@CC	0.1	589	69.7% from 100 to 200 cycles	[109]
MoS2@AMCRs	1	366	83% @ 300 cycles	[107]

. The biomass carbon–MoS₂/MoSe₂ composite likely enhances the performance of SIBs and HER devices in the following ways: Enhanced Electrical Conductivity: Biomass-derived carbon usually has a porous structure and high surface area, which can provide excellent electrical conductivity. This helps in the efficient transport of electrons during the chemical reactions in both SIB and HER processes. (2) Active Sites for Catalytic Reactions: MoS₂ and MoSe₂ are well known for their catalytic properties, particularly in HER applications. The combination with biomass carbon could create more exposed active sites for the reactions, enhancing the catalytic efficiency. (3) Ion Transport and Storage: In SIBs, the composite might facilitate better ion transport and storage due to the layered structure of MoS₂/MoSe₂. The carbon material could act as a flexible matrix, accommodating volume changes during ion intercalation and deintercalation, improving the structural stability of the electrode. (4) Synergistic Effects: The interaction between the carbon, MoS₂, and MoSe₂ can lead to a synergistic effect, where the combined properties of these materials outperform what each material could do individually. For example, the carbon matrix could prevent the aggregation of MoS₂/MoSe₂ layers, maintaining their high surface area and activity throughout the battery’s or HER device’s lifetime.

4. Issues for Future Research on BMC/MoS₂ or BMC/MoSe₂

Biomass-derived carbon (BMC) is widely available, can be simply prepared, and can be processed into a porous, loose structure. This makes it a natural carrier for MoS₂ and MoSe₂, enabling the modulation of their structures, active sites, electronic conductivity, and ionic diffusion, thus enhancing their performance in the HER and SIBs. However, research and applications of such composites still face several challenges that require further investigation:

(1)

Different sources of biomass significantly impact the structure of biomass-derived carbon (BMC). Currently, there is no standardized framework for classifying the various sources of biomass and their characteristic structures. Guidelines for selecting appropriate biomass materials to achieve effective heteroatom doping, increase interlayer spacing, and introduce defects and active sites are still lacking. The ideal shapes and hierarchical porosity cannot be readily designed from biomass precursors. For the preparation of biomass-derived carbon materials, a deeper understanding of the microscopic structures of biomass and waste precursors is essential, along with precise monitoring of the pore sizes and dimensions of carbon materials. When using chemical activators to achieve high surface areas and hierarchical porous structures, controlling the pore geometry, size, and connectivity remains challenging. There is a need to find cheaper and more effective activators. Controlling heteroatom doping is difficult, as the proportion of dopant atoms adhering to the material cannot be precisely regulated. Further research is necessary to establish a synergistic relationship between the surface area, pore size, and surface chemical activity sites or functional groups. Additionally, uniformly loading and dispersing catalysts on biomass-derived carbon substrates presents a challenge. The influence of complex elements present in biomass on the internal electronic structure after thermal treatment also requires investigation through methods such as density functional theory (DFT) calculations.

(2)

Further studies are needed to determine how to precisely control synthesis conditions, such as temperature, atmosphere, and reaction time.

(3)

The impact of charge transfer and enhanced interfacial interactions between BMC and MoS₂ or MoSe₂ on the conductivity of heterostructures requires further evaluation.

(4)

Studying nanostructures requires a suite of advanced characterization techniques, including those to elucidate the shape, size, and location of enclosed pores, in-plane defects, vortex nanodomains and curvature, and the proportion and characteristics of crystalline and amorphous regions, as well as interlayer spacing and the specific surface area. In situ characterization of BMC/MoS₂ or BMC/MoSe₂ during electrocatalytic hydrogen production or sodium-ion storage, along with the corresponding reaction mechanisms, is essential for comprehensive understanding.

(5)

Despite recent efforts to develop biomass-derived carbon composites with exceptional catalytic and electrochemical performance, the mechanisms by which microstructural features influence these properties remain unclear. A major difficulty is establishing structure–performance connections to forecast the catalytic and electrochemical capabilities of materials. Machine learning is being utilized more and more in materials synthesis as computer technologies progress. Computer systems can learn from generated data, analyze, and anticipate based on their programming thanks to machine learning, a field of computer science that uses data-driven approaches. This improves our understanding of the relationship between material structure and performance. In addition to providing fresh perspectives for catalyst selection, design, and catalytic mechanism research, machine learning models may effectively direct the optimization of catalytic performance.

5. Conclusions

The synthesis of BMC materials commonly involves processes such as pyrolytic carbonization, activation, and surface modification. Recent advancements have introduced techniques like microwave technology, electrospinning, chemical vapor deposition, and spray drying into the preparation of BMC materials. When combined with activation and surface modification techniques, these methods have shown promising results in various fields, including energy storage, sensing, pollutant treatment, carbon capture, electrocatalysis, and bioenergy development. The preparation of BMC materials converts unstable organic carbon from biomass into highly stable solid inorganic carbon, thus preventing the release of greenhouse gases and avoiding the environmental pollution risks associated with incineration and landfilling. This process represents a green and environmentally friendly method of carbon sequestration. To create new carbon materials generated from biomass for energy conversion and storage, multidimensional (0D–3D) biomass-based functional carbon materials must be carefully designed and synthesized. One of the main research challenges in multidimensional carbon material engineering is the exact synthesis of BMC with different structures and properties to satisfy different application objectives. To establish structure–performance connections for BMC materials, experimental research must be linked with machine learning approaches, improved characterization methods, particularly in situ characterization, and theoretical calculations. Accurate BMC synthesis will be made easier with the help of this integration. Furthermore, industrial-scale manufacturing rather than laboratory synthesis is required for the creation of carbon compounds based on biomass. The creation of high-performance carbon materials based on biomass requires the development of large-scale, controllable, and efficient synthesis techniques in order to fulfill the increasing needs of energy conversion and storage applications. Currently, enhancing the carbon yield and managing the geometric shape, size, and connection of pores remain major challenges. Future research should concentrate on creating innovative techniques to control pore architectures and increase the production of BMC, as well as comprehending the structural evolution from organic biomass precursors to inorganic carbon materials.

BMC materials are critically linked to catalytic sciences in energy conversion applications, including photocatalysis, electrocatalysis, and photoelectrocatalysis. The important uses of BMC/MoS₂ and BMC/MoSe₂ in energy storage and conversion, notably in the HER and SIBs, are the main topic of this paper. One of the main goals of research in this area is to create carbon catalysts based on biomass that have good performance. Considerations for high activity, selectivity, and durability, low manufacturing costs, and high yield should all be integrated into the design of such catalysts. The development of high-performance non-metal carbon catalysts based on biomass or the replacement of costly noble metals with cheap, plentiful metals to create economically viable biomass-based catalysts are significant areas for future research. Conversely, distributing or fixing MoS₂ or MoSe₂ on BMC solid substrates has become an important avenue for catalytic research. Machine learning algorithms can analyze large datasets to identify promising compositions or synthesis methods for MoS₂ and MoSe₂. This can speed up the discovery of optimal material formulations for enhanced performance in SIBs and the HER. The models can predict the properties and performance of different MoS₂ and MoSe₂ materials based on their structural and compositional features. This can guide experimentalists in selecting the most promising candidates.

Harnessing the synergistic effects between the carbon substrate obtained from biomass and the guest materials requires optimizing both the loading of the guest substances and the interfacial interactions between the host and guest materials. This is especially crucial for the creation of cutting-edge BMC materials. As of right now, there is still a big difference between BMC commercial applications and laboratory research. A difficult objective for their practical use in SIB energy storage devices will be the development of high-performance electrode materials of commercial-grade quality. Today, electronic gadgets are gradually moving from high-performance, single-function, durable devices to versatile, portable devices. Electronic items that are flexible and portable are in greater demand. Thus, it is crucial to create flexible electrode materials for portable energy storage systems.

In summary, the high-value and refined utilization of biomass resources is imperative. Improving the sustainable development of energy will be greatly aided by increasing the effective use of renewable biomass resources. The novelty of biomass carbon-based MoS₂/MoSe₂ lies in leveraging the abundant, renewable, and structurally tunable properties of biomass carbon (BMC) to create a more efficient composite material. Compared to previous works, the integration of BMC with MoS₂/MoSe₂ provides several advantages, including enhanced electron conductivity, improved catalytic activity, and better structural stability. These features result in significantly improved performance in the HER and SIBs. Additionally, the eco-friendly and scalable nature of BMC makes this composite more suitable for sustainable and large-scale energy applications. There are many opportunities in the production of functional carbon products from biomass. In order to prepare functional carbon materials for large-scale preparation and commercial application in the fields of energy conversion and storage, future studies should concentrate on producing functional carbon materials with a variety of dimensions and qualities from sustainable biomass resources. This necessitates the creation of economical, ecologically responsible, and efficient processes for the preparation of BMC materials. In order to further progress the industrialization of energy conversion and storage devices, future efforts should focus on bridging the knowledge gap between laboratory research and industrial application, as well as the cost-effective industrialization of these materials. Additionally, the following recommendations are suggested: (1) Focus direct research attention on the BMC source for the functionalization of biochar materials. (2) Create low-cost, scalable methods to increase the biomass’s conversion efficiency into materials that have been functionalized into biochar. (3) Create appropriate theoretical frameworks for the creation and manufacturing of biochar and its derivatives. (4) To create high-performance products based on biochar, choose substances that have outstanding physicochemical stability and carbon compatibility. (5) Explore scalable strategies for the use of biochar materials and their composites in electrochemical energy devices.