Boron Nitride Nanostructures (BNNs) Within Metal–Organic Frameworks (MOFs): Electrochemical Platform for Hydrogen Sensing and Storage

Abstract

Boron nitride nanostructures (BNNs), including nanotubes, nanosheets, and nanoribbons, are renowned for their exceptional thermal stability, chemical inertness, mechanical strength, and high surface area, making them suitable for advanced material applications. Metal–organic frameworks (MOFs), characterized by their porous crystalline structures, high surface area, and tunable porosity, have emerged as excellent candidates for gas adsorption and storage applications, particularly in the context of hydrogen. This paper explores the synthesis and properties of BNNs and MOFs, alongside the innovative approach of integrating BNNs within MOFs to create composite materials with synergistic properties. The integration of BNNs into MOFs enhances the overall thermal and chemical stability of the composite while improving hydrogen sensing and storage performance. Various synthesis methods for both BNNs and MOFs are discussed, including chemical vapor deposition, solvothermal synthesis, and in situ growth, with a focus on their scalability and reproducibility. Furthermore, the mechanisms underlying hydrogen sensing and storage are examined, including physisorption, chemisorption, charge transfer, and work function modulation. Electrochemical characterization techniques, such as cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge, are used to analyze the performance of BNN-MOF systems in hydrogen storage and sensing applications. These methods offer insights into the material’s electrochemical behavior and its potential to store hydrogen efficiently. Potential industrial applications of BNN-MOF composites are highlighted, particularly in fuel cells, hydrogen-powered vehicles, safety monitoring in hydrogen production and distribution networks, and energy storage devices. The integration of these materials can contribute significantly to the development of more efficient hydrogen energy systems. Finally, this study outlines key recommendations for future research, which include optimizing synthesis techniques, improving the hydrogen interaction mechanisms, enhancing the stability and durability of BNN-MOF composites, and performing comprehensive economic and environmental assessments. BNN-MOF composites represent a promising direction in the advancement of hydrogen sensing and storage technologies, offering significant potential to support the transition toward sustainable energy systems and hydrogen-based economies.

1. Introduction

Hydrogen, as a clean and renewable energy source, holds significant promise in addressing global energy and environmental challenges. It is considered a key element in the transition to sustainable energy systems due to its high energy density and environmentally friendly combustion, producing only water as a byproduct [1]. Therefore, the development of efficient hydrogen sensing and storage technologies is crucial for advancing clean energy solutions [2,3,4]. Boron nitride nanostructures (BNNs), including nanotubes, nanosheets, and nanoribbons, have garnered considerable attention in recent years due to their exceptional thermal stability, chemical inertness, and unique electronic properties [5,6,7,8]. Lale et al. (2018) conducted a comprehensive review on boron nitride (BN) for hydrogen storage, summarizing over 15 years of experimental and computational efforts [5]. The study explores BN’s potential for molecular hydrogen storage and chemical hydrogen storage, emphasizing its promise for hydrogen storage at room conditions, pending experimental validation. Ortiz et al. (2020) conducted a review on boron nitride-based nanocomposites, emphasizing their synthesis methods, especially in polymer- and ceramic-based matrices [6]. The focus was on exploiting boron nitride’s exceptional properties, including high thermal stability and good mechanical strength, for applications in energy, environment, and health sectors, showcasing their potential in diverse fields. Revabhai et al. (2022) discusses the diverse applications of boron nitride nanostructures, including hexagonal boron nitride (h-BN, Figure 1), boron nitride quantum dots (BNQDs), boron nitride nanosheets (BNNSs), and boron nitride nanotubes (BNNTs), in catalysis, optoelectronics, sensors, electronics, and biomedical research [7]. The review explores their properties and synthesis methods (both top-down and bottom-up approaches) and emphasizes their promising roles in hydrogen storage and sensing applications, providing insights into future prospects in analytical sciences. Hayat et al. (2022) summarized the synthesis methods and applications of boron nitride (BN) and BN-based nanocomposites, emphasizing their role in solving global energy, health, and environmental challenges [8]. These materials, known for their wide band gap and versatile properties, find applications in photocatalysis, pollutant degradation, photovoltaics, drug delivery, and sensors, with discussions on challenges in large-scale manufacturing and potential solutions, providing valuable insights for the development of effective BN-based materials. These contributions make BNNs excellent candidates for various applications, including hydrogen storage and sensing.

The electron clouds around the boron and nitrogen atoms can interact with hydrogen molecules, causing them to stick to the surface of the material through physisorption process. Researchers developed methodologies to explore those unique properties. Moussa et al. (2014) developed a method to produce boron nitride nanoparticles (BN-NPs) using spray pyrolysis, creating stable structures with specific surface areas [9]. These BN-NPs were used to create hollow core-mesoporous shell nanoparticles, which were then used to confine and destabilize ammonia borane, enhancing its dehydrogenation properties. This novel approach enabled the production of a safe and practical hydrogen storage composite material that releases pure hydrogen at low temperatures. Weng et al. (2020) enhanced hydrogen adsorption in porous boron nitride (BN) materials by modulating their chemical states [10]. Carbon and oxygen co-doped BN microsponges demonstrated 2.5–4.7 times higher hydrogen uptake capacity per specific surface area than undoped BN structures, offering a significant advancement in efficient and low-cost hydrogen storage for fuel cell vehicles.

However, leveraging these nanostructures effectively requires integration into a supportive matrix that can enhance their functionality and stability. BNNs, particularly in its hexagonal form (h-BN), exhibits a range of exceptional properties, such as high thermal conductivity, electrical insulation, and chemical stability; however, BN also has certain limitations that can affect its performance in specific applications like hydrogen storage and sensing. BN by itself has a relatively low hydrogen storage capacity compared to other materials. The intrinsic surface area and pore volume of BN are not sufficient to store large amounts of hydrogen. Additionally, BN is chemically inert, which can be a disadvantage for applications requiring interaction with gas molecules for storage or sensing. This chemical inertness can limit its ability to adsorb hydrogen or detect certain analytes effectively. Although BN has a high surface area compared to bulk materials, it is still lower than the highly porous structures of MOFs. A higher surface area is crucial for enhanced gas adsorption. The electronic and structural properties of BN are relatively fixed and cannot be easily tuned for specific applications. This limits its flexibility in designing systems tailored for hydrogen storage or gas sensing.

Metal–organic frameworks (MOFs) are highly advantageous for enhancing boron nitride (BN) in hydrogen storage and sensing applications due to their exceptionally high surface area and porosity. These characteristics are crucial for maximizing hydrogen storage capacity and providing numerous adsorption sites for gasses. Moreover, MOFs offer unparalleled tunability, allowing precise control over pore sizes and shapes during synthesis and the incorporation of various organic linkers and metal ions. This tunability enables the design of MOFs optimized for specific applications, enhancing their adsorption properties and reactivity.

The combination of MOFs and BN results in a synergistic effect, leveraging the strengths of both materials. BN provides structural support and thermal stability, while MOFs enhance adsorption capabilities. This hybrid approach often leads to improved performance compared to using individual components, with increased hydrogen storage capacities and better adsorption–desorption kinetics. Additionally, MOFs’ chemical and thermal stability makes them suitable for practical applications in varying environmental conditions, ensuring durability and efficiency in hydrogen storage and sensing.

Compared to other materials like graphene, carbon nanotubes (CNTs), zeolites, and polymers, metal–organic frameworks (MOFs) stand out due to their high surface area, extensive tunability, and stability. While materials like graphene and carbon nanotubes offer high surface area and conductivity, they lack the pore customization of MOFs. Zeolites and covalent organic frameworks (COFs) provide high porosity but do not match the structural and chemical tunability of MOFs. Polymers and porous organic polymers (POPs), though flexible and functional, generally have lower surface areas. Metal nanoparticles and oxides offer catalytic properties but fall short in terms of surface area and porosity.

Based on the existing literature, the integration of boron nitride nanostructures (BNNS) within metal–organic frameworks (MOFs) represents a cutting-edge approach in the realm of hydrogen storage and sensing materials. Dai et al. (2020) developed a novel method to synthesize spherical superstructures consisting of boron nitride nanosheets (SS-BNNSs) from metal–organic framework nanosheets [11]. These unique SS-BNNSs, preserving their spherical morphology, demonstrated excellent catalytic activity for selective oxidative dehydrogenation of propane. Their work presents a promising synthetic strategy for fabricating 3D spherical superstructures of 2D nanosheets, offering potential applications in catalysis, energy storage, and related fields.

The literature presents a range of strategies for integrating boron nitride nanostructures (BNNS) into metal–organic frameworks (MOFs) to enhance electrochemical detection [12]. While significant research has been conducted on various MOFs and their modifications to optimize electrochemical sensors for point-of-care applications [13,14,15] and address environmental issues [16,17,18], there is a need to emphasize their potential in energy applications [19]. Given the limited publications in this area, this paper aims to fill the gap by focusing specifically on hydrogen capture.

In this study, the development of MOFs functionalized with BNNS for sensing and/or capturing hydrogen is summarized. The promising prospects of BNNS/MOF composites as next-generation materials for hydrogen storage and sensing are highlighted. The synthesis process, which involves the careful incorporation of BNNS into MOF matrices, is outlined, emphasizing the high surface area provided by BNNS and the porous nature of MOFs. The interactions between hydrogen molecules and the composite material are defined, elucidating the mechanisms governing hydrogen adsorption and desorption kinetics. Furthermore, the unique electronic and structural properties presented by BNNS and imparted to the MOF are investigated. This research paves the way for the development of efficient, lightweight, and highly sensitive hydrogen storage systems and sensors, contributing significantly to the advancement of clean energy technologies.

2. Boron Nitride Nanostructures (BNNs) Properties and Synthesis

Hexagonal BNNs are synthetic materials that do not occur naturally and must be synthesized through specialized processes [20]. These nanomaterials come in various dimensions, including zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanotubes and nanorods, and two-dimensional (2D) nanosheets [21].

Each dimensional form of BNNs exhibits remarkable thermal stability, capable of withstanding extreme temperatures exceeding 1000 °C without significant structural degradation [22]. This property makes BNNs highly suitable for applications in high-temperature environments such as aerospace, electronics, and refractory materials [23]. Their chemical inertness further enhances their suitability for harsh environments, as BNNs resist corrosion and do not react with most chemicals, including acids and alkalis. This resistance ensures longevity and reliability in applications requiring exposure to corrosive substances, such as in chemical processing equipment and protective coatings.

BNNs exhibit various solid-state structures, with the hexagonal form being the most stable and resembling graphite, where boron and nitrogen occupy alternating positions. In contrast, the less stable cubic sphalerite structure is similar to diamond [24]. The bonding between boron and nitrogen is predominantly covalent, with strong σ bonds resulting from sp2 orbital hybridization—B-N in BNNs and C-C in graphite [25]. However, unlike graphite’s purely covalent bonds, BNNs display a partially ionic character due to the presence of electron pairs in the sp2 hybridized B–N bonds. Both materials also feature weak van der Waals forces between their layers, which contribute to their anisotropic properties [26]. This unique ionic character in BNNs results in distinct electronic and thermal behaviors when compared to the fully covalent structure of graphite.

Zero-dimensional (0D) BNN nanoparticles exhibit high hardness and can be used in composite materials to enhance their strength [25]. One-dimensional (1D) BNN nanotubes and nanorods provide excellent tensile strength and flexibility, making them suitable for applications in reinforced materials and nanocomposites [25]. Specifically, boron nitride nanotubes (BNNTs) are cylindrical nanostructures that combine high strength, flexibility, and thermal conductivity [27]. Two-dimensional (2D) BNN nanosheets possess exceptional strength and rigidity, rivaling that of diamond, which is advantageous for creating strong, lightweight materials and coatings [25]. These boron nitride nanosheets (BNNSs) are similar to graphene but electrically insulating, making them suitable for various electronic applications. Additionally, boron nitride nanoribbons (BNNRs) are narrow, ribbon-like structures that combine the properties of BNNSs and BNNTs, offering unique electronic and mechanical properties [28]. Moreover, the 2D form has a high surface area, enhancing effectiveness in catalysis and adsorption applications [29]. Despite their varied dimensional properties, all forms of BNNs are outstanding electrical insulators with high dielectric strength, making them ideal for use in electronic and electrical applications where insulation from high voltages is critical.

The studies on boron nitride (BN) nanomaterials reveal diverse synthesis methods and their respective advantages and limitations (

Table 1. Comparison of synthesis methods for BNNs-description.

Synthesis Method	Advantages	Limitations	Typical BNNs Produced	Applications
Ball Milling	Simple, accessible	Reduced crystallinity, potential particle re-agglomeration	Nanosheets, nanoparticles	Composite materials, photocatalysis
Template-Assisted Growth	High control over structure	Complex, requires precise conditions	Nanotubes	Ordered arrays, high-quality nanotubes
Sol–Gel	Flexible, tailored properties	Time-consuming, complex handling	Nanosheets, polysaccharide nanosystems	Electronic films, biomedical applications
Laser Ablation	Precise material control	High cost, requires sophisticated equipment	Films, reduced-size particles	Film fabrication, particle size reduction
Chemical Vapor Deposition (CVD)	High-quality materials, versatility	Requires high temperatures, complex equipment	BNNTs, h-BN films	High-quality films, nanotubes

) [30,31,32]. Ball milling mechanically grinds bulk BN into nanoscale particles, but it may reduce crystallinity and necessitate additional purification. Sarma et al. (2024) [33] demonstrated that cryo-milling effectively produces BN nanosheets with reduced thickness and improved thermal stability compared to traditional methods like sonication. This method, while effective for bulk production, involves complex process conditions and may face scalability issues. Conversely, ball milling is a simpler, more accessible technique explored by Li and Chen (2015) [34], Lyebo et al. (2023) [35], Ranjithkumar et al. (2022) [36], and Ghosh et al. (2023) [37]. Lyebo et al. found that ball milling of hexagonal BN (hBN) enhanced photocatalytic activity by improving light absorption and decreasing electron-hole recombination, although this method can result in reduced crystallinity and increased formation of dislocations. Ghosh et al. reported the successful incorporation of ball-milled hBN into aluminum composites, achieving significant improvements in mechanical properties, although prolonged milling can lead to re-agglomeration of particles.

Template-Assisted Growth uses templates like porous alumina to guide the formation of highly ordered BNNs, though it demands precise control and is complex. Bechelany et al. (2007) [38] utilized a template-assisted polymer thermolysis process to create highly ordered BN nanotube arrays. This method is intricate but effective for producing well-ordered nanotubes with controlled dimensions. Wang et al. (2008) [39] employed ammonia borane and template-aided vapor-phase pyrolysis to produce BN nanotubes with specific textures, a method that provides high-quality nanotubes but requires careful control of temperature and deposition cycles.

The sol–gel process involves creating a colloidal solution that evolves into a gel containing BNNs, providing flexibility but requiring careful handling and time. The techniques, as demonstrated by Yu et al. (2024) [40] and Ozcan and Maltas Cagil (2023) [41], offer advantages in flexibility and tailored properties. Yu et al. developed a flexible, thermally conductive composite film using sol–gel methods, highlighting the ease of integrating BN nanosheets into a matrix for electronic applications. However, sol–gel processes can be time-consuming and may involve complex steps to achieve uniform dispersion and optimal properties. Ozcan and Maltas Cagil focused on pH-sensitive BN-polysaccharide nanosystems for drug delivery, showcasing the sol–gel method’s versatility in biomedical applications, though the release rates and stability in different pH environments may vary.

Laser ablation employs high-power laser pulses to ablate a boron target in a nitrogen atmosphere, allowing for precise material control but with high equipment costs [42,43]. The technique presents advanced methods for BN film fabrication and particle size reduction [44,45]. Haque et al. highlighted the pulsed laser annealing process for producing high-quality cubic BN films, which is precise but requires sophisticated equipment and control over laser parameters. Muneoka et al. demonstrated that femtosecond laser irradiation can effectively reduce BN particle sizes and improve dispersibility in water. This method, while promising for enhancing particle characteristics, can be complex and costly due to the need for high-energy lasers and controlled environments.

Chemical vapor deposition (CVD) reacts boron and nitrogen sources at high temperatures to produce BNNs on a substrate, offering versatility and high-quality results, though it requires sophisticated equipment and high operational temperatures. The reviewed works demonstrate the versatility and effectiveness of CVD methods in synthesizing various boron nitride nanostructures, including BNNTs and h-BN films. Ahmad et al. (2015) and Silva et al. (2017) both emphasize the control over growth parameters that CVD techniques offer, which is crucial for tailoring the size and morphology of BN nanostructures [46,47]. Wang et al. (2022) and Wa et al. (2023) further highlight the importance of temperature and catalyst concentration, as well as flow distribution, in optimizing BNNT production [48,49]. Shi et al. (2010) and Zhao et al. (2023) focus on the synthesis of h-BN films, with Shi et al. achieving continuous thin films and Zhao et al. achieving layer-selective growth through a two-step CVD process [50,51]. Nguyen et al. (2024) contribute to the understanding of the thermokinetics of h-BN growth, which is essential for improving the quality and consistency of CVD-grown h-BN [52]. Yuan et al. (2024) provide a critical analysis of commercially available CVD h-BN, highlighting the need for improved defect density and accurate specifications for reliable device fabrication [53]. Yamamoto et al. (2023) and Brown et al. (2023) demonstrate innovative approaches to synthesizing thick multilayered h-BN and c-BN/diamond heterostructures, respectively, showing the potential for these materials in advanced electronic applications [54,55]. Torres-Castillo et al. (2022) focus on functionalizing BNNTs to improve their integration into polymer matrices, enhancing their practical applicability [56].

Each method is tailored to achieve specific forms and properties of BNNs, influencing their suitability for different applications. While ball milling and sol–gel methods offer relatively simpler and more accessible approaches with practical applications, they come with limitations such as potential particle re-agglomeration and time-consuming processing. Template-based and laser ablation techniques provide higher control over material properties but often require more sophisticated equipment and precise process control. On the other hand, the advancements in CVD techniques for BN nanostructures underscore the method’s potential for producing high-quality materials with tailored properties for a wide range of applications.

3. Metal–Organic Frameworks (MOFs) Characteristics and Synthesis

Metal–organic frameworks (MOFs) are crystalline materials composed of metal ions or clusters coordinated to organic ligands, creating a three-dimensional porous structure (Table 2). MOFs possess several key characteristics that make them highly valuable in various applications [57,58,59]. One of the most notable features of MOFs is their high surface area, which can exceed 1000 m²/g [60]. This extensive surface area is beneficial for gas storage, separation, and catalysis, as it results from the inherently porous nature of the material [61]. Another important characteristic of MOFs is their tunable porosity [62,63]. The pore size and shape within MOFs can be precisely controlled during synthesis by selecting different metal nodes, such as zinc or copper, and organic linkers, like terephthalic acid. This ability to customize the porosity allows MOFs to be tailored for specific applications, enhancing their functionality and efficiency. Stability is also a crucial aspect of MOFs. Many MOFs demonstrate high stability under various environmental conditions, making them suitable for practical applications [64,65]. However, the stability of MOFs can vary, and some may require specific conditions, such as an inert atmosphere or controlled humidity, to maintain their structural integrity.

An example of an MOF that is suitable for incorporating boron nitride nanostructures (BNNs) is ZIF-8 (zeolitic imidazolate framework-8) [66]. ZIF-8 is a promising candidate due to its high surface area and microporosity, which provide ample space for the integration of BNNs. Additionally, ZIF-8 is known for its stability under a wide range of conditions, including high temperatures and varying pH levels, which complements the thermal stability of BNNs. The combination of ZIF-8 and BNNs can enhance applications in hydrogen storage [67] and adsorption [68]. Saeed et al. (2023) [67] utilized ZIF-8 with BNNs to enhance hydrogen storage, emphasizing the high surface area and mechanical stability of ZIF-8. Their work showed that these nanostructures significantly improved molecular hydrogen storage capacity. Similarly, Huang et al. (2024) [68] developed ZIF-8@h-BN nanocomposites for the adsorption of ofloxacin, revealing that ZIF-8 enhances the adsorption capacity of h-BN. They used molecular dynamics simulations to show superior adsorption performance compared to pristine h-BN. Both studies highlight the importance of ZIF-8 in improving storage and adsorption properties due to its structural characteristics. In addition, it can be used for gas separation applications. Guo et al. (2023) [69] explored mixed-matrix membranes (MMMs) using ZIF-8@BNNS, achieving enhanced CO₂ permeability and CO₂/N₂ selectivity. Their work demonstrated that ZIF-8 improves gas separation performance by creating efficient gas transmission pathways, which is crucial for applications requiring selective permeability. Moreover, it can be used in flame retardancy and corrosion protection. Yin et al. (2022) [70] created a ternary hybrid of BN-OH/ZIF-8/PA to enhance flame retardancy in epoxy resins, achieving significant reductions in fire hazard metrics. Huang et al. (2023) [71] designed a smart sol–gel coating incorporating BTA-ZIF-8@BN-OH for corrosion protection, demonstrating improved long-term anticorrosion performance. The integration of ZIF-8 with BNNs provided enhanced thermal and chemical stability, essential for safety applications. This then offering improved mechanical strength and thermal stability.

Doping and functionalizing ZIF-8 with BNNs can also provide additional active sites and thus enhance catalysis. Tian et al. (2022) [72] investigated Cu-functionalized ZIF-8, producing Cu/BN nanocomposites that showed improved light harvesting and electron-hole separation, directly impacting CO₂ photoreduction activity. This functionalization led to a significant enhancement in catalytic performance, demonstrating the importance of doping ZIF-8 with metals to tailor its electronic properties for specific applications. Shang et al. (2023) [73] focused on B/N co-doped hierarchical porous carbon electrodes derived from CNF/BNNS/ZIF-8 nanocomposites. These electrodes exhibited high specific capacitance and excellent cycle stability, highlighting the role of B and N co-doping in improving electrochemical performance. The co-doping strategy is crucial for creating defects and active sites within the carbon matrix, enhancing the material’s overall functionality. Atta et al. (2023) [74] studied the photo-electrocatalytic properties of g-C₃N₄ modified with ZIF-8 and boron doping. They found that this combination significantly enhanced photocatalytic activity and electrical conductivity. Boron doping improved the electrical properties, while ZIF-8 hybridization influenced photocatalytic efficiency, illustrating how dual modifications can synergistically enhance material properties. Enhance electronic, catalytic, and electrochemical properties, providing tailored solutions for specific applications. Habibi et al. (2023) [75] developed a Cu/ZIF-8/BN nanocatalyst for electrocatalysis, achieving high sensitivity in detecting clopidogrel. The copper functionalization provided enhanced electroanalytical performance, underlining the importance of metal doping in improving catalytic properties for sensing applications.

Several types beyond the favored ZIF-8 can be effectively used for incorporating boron nitride nanostructures (BNNs) to enhance their properties. One such MOF is MOF-5, which consists of zinc ions coordinated with terephthalic acid. MOF-5 boasts a high surface area and tunable porosity, making it suitable for gas storage and adsorption applications. Although it has moderate stability, its porous structure can benefit from the integration of BNNs, potentially improving its mechanical and thermal properties. Liu et al. (2019) [76] discuss the unique properties of low-dimensional metal–organic frameworks (LD MOFs) like large surface area and high aspect ratio, which make them ideal for applications in catalysis, energy storage, gas adsorption and separation, and sensing. However, they emphasize the need for further exploration of synthetic principles and dimensional-dependent properties. Unnikrishnan et al. (2021) [77] highlight the potential of MOF-5 in multifunctional polymer nanocomposites, focusing on enhancing interactions between MOFs and polymers to improve mechanical properties and flame retardancy. They also discuss the integration of MOFs with other two-dimensional materials such as graphene and boron nitrides. Lotfi and Saboohi (2014) [78] examine hydrogen adsorption in MOF-5, showing that boron substitution enhances binding energies and hydrogen storage capacity, particularly with Sc-doped, boron-substituted MOFs. This study complements the catalytic applications discussed by Tan et al. (2022) [79] who focus on MOF-derived materials for clean energy conversion and environmental remediation, emphasizing their adjustable pore environment and homogeneous void structure for CO₂ reduction and H₂ evolution. Xinbo et al. (2019) [80] review CO₂ capture and storage using MOFs, highlighting their high specific surface area and tunable pore size. They discuss strategies to improve low-pressure adsorption performance and propose potential routes for the resource utilization of captured CO₂. This aligns with the environmental applications of MOF-derived materials discussed by Tan et al. (2022) [79] and Liu et al. (2019) [76].

Another promising MOF is MIL-101, made of chromium ions and terephthalic acid. Known for its exceptionally high surface area and large pore volume, MIL-101 is highly stable under diverse conditions, including high humidity and temperatures. The large pores of MIL-101 can easily accommodate BNNs, enhancing its capacity for gas storage and catalysis. Atta et al. (2024) [81] explore the photo-electrocatalytic properties of g-C₃N₄ and NH₂-MIL-101 composites, showing that B-g-C₃N₄/NH₂-MIL-101 exhibits the highest ethanol production rate during CO₂ reduction. This research underscores the importance of composite modification for enhanced photoelectrocatalytic activity, similar to the catalytic focus of Tan et al. (2022) [79] on MOF-derived materials. An et al. (2024) [82] address thermal management with MOF-decorated boron nitride/natural rubber composites, which exhibit improved thermal conductivity and dual passive cooling modes. This research complements Unnikrishnan et al. (2021) [77] who discuss MOFs’ integration with other materials for enhanced composite performance.

Additionally, UiO-66, composed of zirconium ions and terephthalic acid, stands out due to its high surface area and remarkable chemical and thermal stability. The tunable porosity of UiO-66 allows for the effective incorporation of BNNs, which can further augment its high-temperature application capabilities. Hatamluyi et al. (2020) [83] develop a novel electrochemical sensor using UiO-66-NH₂ for detecting Oxaliplatin, demonstrating high selectivity and sensitivity due to increased binding sites and specific surface area. This aligns with the theme of UiO-66 enhancing functional performance in diverse applications, as discussed by other authors. Long et al. (2024) [84] present Janus composite films with UiO-66 and BNNSs for high energy storage dielectrics, significantly improving dielectric constant and energy storage density. This study complements Atta et al. (2024) [81], who explore advanced composite materials for energy-related applications.

Meanwhile, HKUST-1, which features copper ions coordinated with benzene-1,3,5-tricarboxylate, offers a highly porous structure and high surface area. Although HKUST-1 has moderate stability and can degrade in moist environments, integrating BNNs can enhance its mechanical properties and stability, making it suitable for gas storage and separation applications. Samui and Begum et al. (2024) [85] discuss the removal of gas molecules using 2D nanomaterials, including HKUST-1, which offer high surface area and adjustable properties for effective gas adsorption. This aligns with the environmental applications discussed by Xinbo et al. (2019) [80] on MOFs. Haroon et al. (2022) [86] investigate the photocatalytic coupling of aryl halides using Cu₃P/hBN derived from HKUST-1, demonstrating efficient C–C coupling under visible light. This complements the catalytic applications discussed by Tan et al. (2022) [79] and Atta et al. (2024) [81] on MOF-derived materials.

Table 2. Summary of key metal–organic frameworks (MOFs) including their characteristics, applications, and references. This table highlights various MOFs used for incorporating boron nitride nanostructures (BNNs) and their respective properties.

Material	Characteristics	Applications	References
ZIF-8	High surface area and microporosity; stability under various conditions.	Hydrogen storage [67], adsorption [68], gas separation [69], flame retardancy [70], corrosion protection [71].	[66,67,68,69,70,71]
MOF-5	Zinc ions with terephthalic acid; high surface area; moderate stability.	Gas storage, adsorption, catalysis [76], polymer nanocomposites [77].	[76,77,78,79,80]
MIL-101	Chromium ions with terephthalic acid; exceptionally high surface area; large pore volume; high stability.	Gas storage, catalysis, photo-electrocatalysis [81], thermal management [82].	[81,82]
UiO-66	Zirconium ions with terephthalic acid; high surface area; chemical and thermal stability.	Electrochemical sensors [83], high energy storage dielectrics [C28].	[83,84]
HKUST-1	Copper ions with benzene-1,3,5-tricarboxylate; highly porous; moderate stability.	Gas storage, separation [85], photocatalysis [86].	[85,86]

Table 2. Summary of key metal–organic frameworks (MOFs) including their characteristics, applications, and references. This table highlights various MOFs used for incorporating boron nitride nanostructures (BNNs) and their respective properties.

Material	Characteristics	Applications	References
ZIF-8	High surface area and microporosity; stability under various conditions.	Hydrogen storage [67], adsorption [68], gas separation [69], flame retardancy [70], corrosion protection [71].	[66,67,68,69,70,71]
MOF-5	Zinc ions with terephthalic acid; high surface area; moderate stability.	Gas storage, adsorption, catalysis [76], polymer nanocomposites [77].	[76,77,78,79,80]
MIL-101	Chromium ions with terephthalic acid; exceptionally high surface area; large pore volume; high stability.	Gas storage, catalysis, photo-electrocatalysis [81], thermal management [82].	[81,82]
UiO-66	Zirconium ions with terephthalic acid; high surface area; chemical and thermal stability.	Electrochemical sensors [83], high energy storage dielectrics [C28].	[83,84]
HKUST-1	Copper ions with benzene-1,3,5-tricarboxylate; highly porous; moderate stability.	Gas storage, separation [85], photocatalysis [86].	[85,86]

Various synthesis techniques offer distinct advantages for the development of metal–organic frameworks (MOFs) and their composites (

Table 3. Overview of synthesis methods for metal–organic frameworks (MOFs) and their composites, including characteristics, applications, and references. This table outlines various techniques used to develop MOFs and their integration with boron nitride nanostructures (BNNs).

Method	Characteristics	Applications	References
Solvothermal Synthesis	Dissolves reactants in solvent, heated in sealed vessel; effective for hydrogen storage [67], flame retardancy [70].	Development of nanostructured materials.	[67,70]
Microwave-Assisted Synthesis	Rapid heating with microwave radiation; improves crystallinity and reduces reaction times.	Enhances material properties [C16].	[72]
Electrochemical Synthesis	Anodic dissolution of metal in presence of organic ligand; straightforward route to MOFs.	Electrocatalytic properties [75].	[75]
Sonochemical Synthesis	Uses ultrasound waves to accelerate mixing and reaction rates; improves dispersion of reactants.	Self-assembly approaches for material creation [70,71].	[70,71]
Mechanochemical Synthesis	Physical grinding of reactants without solvents; used for creating micro/mesoporous carbon electrodes.	Fabrication of heterostructural carbon electrodes [73].	[73]
In situ Growth	Direct formation of MOFs on substrates; precise control over composition and structure.	Development of ZIF-8@BN composites [68,69,71].	[68,69,71]
Self-Assembly	Spontaneous organization of molecules; effective for creating hybrids.	Development of BN-OH/ZIF-8 hybrids [70].	[70]
MOF-Derived Synthesis	MOFs used as precursors; tailored material properties.	Cu-functionalized porous boron nitride [72].	[72]

). Solvothermal synthesis, which involves dissolving reactants in a solvent and heating them in a sealed vessel, has been employed effectively by Saeed et al. (2023) [67] for developing nanostructured materials aimed at hydrogen storage. Similarly, Yin et al. (2022) [70] used solvothermal synthesis to create BN-OH/ZIF-8 hybrids, demonstrating its utility in producing materials for flame retardancy. On the other hand, microwave-assisted synthesis, characterized by rapid heating using microwave radiation, facilitates reduced reaction times and improved crystallinity. Tian et al. (2022) [72] utilized this method to synthesize Cu-functionalized porous boron nitride from a metal–organic framework, showcasing its effectiveness in enhancing material properties.

Electrochemical synthesis, which involves anodic dissolution of a metal in the presence of an organic ligand, offers a straightforward route to MOFs. Habibi et al. (2023) [75] leveraged this technique to integrate copper and ZIF-8 with boron nitride, aiming to enhance electrocatalytic properties. Meanwhile, sonochemical synthesis, which uses ultrasound waves to accelerate mixing and reaction rates, can significantly improve the dispersion of reactants. This method was effectively employed in self-assembly approaches by Yin et al. (2022) [70] and Huang et al. (2023) [71]. Mechanochemical synthesis, involving the physical grinding of reactants without solvents, was used by Shang et al. (2023) [73] to fabricate B/N co-doped micro/mesoporous carbon electrodes for creating similar heterostructural carbon electrodes. In situ growth, allowing for direct formation of MOFs on substrates, was used by Zefang Huang et al. (2024) [68], Huang et al. (2023) [71], and Guo et al. (2023) [69] to develop ZIF-8@BN composites for various applications. Self-assembly, which involves the spontaneous organization of molecules, was employed by Yin et al. (2022) [70] for creating BN-OH/ZIF-8 hybrids. Lastly, MOF-derived synthesis, utilizing MOFs as precursors, was demonstrated by Tian et al. (2022) [72] for obtaining Cu-functionalized porous boron nitride.

The choice of synthesis technique and the specific doping or functionalization strategy are crucial for optimizing the properties of ZIF-8 and BNNs composites for various applications. Techniques such as in situ growth [68,69,71], self-assembly [70], and MOF-derived synthesis [72] enable precise control over the material’s composition and structure, thereby enhancing their functionality. Doping and functionalization further tailor these materials’ electronic, catalytic, and electrochemical properties, making them suitable for advanced applications in hydrogen storage [68,69], adsorption [68], flame retardancy [70], corrosion protection [71], gas separation [69], and catalysis [72]. Each synthesis method offers unique advantages that contribute to the development of high-performance MOF-based composites.

4. Approach to Integrating BNNs Within MOFs and Characterization Through Electrochemical Techniques

Integrating BNNs within MOFs can be achieved through various methods, which can be characterized by electrochemical techniques such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry, and linear sweep voltammetry (LSV), which are also used for assessing electrochemical properties. Figure 2 shows different methods for incorporating BNNs into MOFs: functionalization with organic polymers, integration during synthesis, high-pressure exfoliation, and electrochemical deposition. Each method is characterized using various electrochemical techniques, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry, and linear sweep voltammetry (LSV). These techniques assess properties such as electron transfer efficiency, charge transfer resistance, stability, and catalytic activity. Functionalizing BNNs with organic polymers or other functional groups can significantly enhance their compatibility with MOFs. This process improves the dispersion of BNNs within the MOF matrix, leading to better integration and overall performance. Non-covalent interactions, such as π–π stacking, play a crucial role in forming stable composites [87], which are particularly effective in electrochemical applications. When these functionalized BNNs are incorporated into MOFs, they create more conductive pathways for electron transfer [88,89]. CV then is employed to study the redox behavior of these composites, allowing for the identification of oxidation and reduction peaks that signify the presence of electroactive species and the efficiency of electron transfer within the material [90]. Composites of BNNs and MOFs are created by integrating BNNs with MOF precursors during the synthesis process [91]. Achieving a uniform distribution of BNNs within the MOF structure is essential as it preserves the structural integrity and significantly enhances the electrochemical properties of the resulting composite material. To evaluate these properties, EIS is employed to measure the resistance to electron flow within the composite [92]. When BNNs are effectively incorporated into the MOF matrix, the composite exhibits reduced charge transfer resistance, indicating improved conductive pathways and more efficient ion mobility throughout the material. High-pressure exfoliation and surface modification of BNNs are effective techniques for increasing their surface area and reactivity, making them more compatible for integration with MOFs [93]. This method is particularly advantageous for developing composites with enhanced thermal and electrical properties. Chronoamperometry is used for evaluation of composite [94]. By measuring the current response at a fixed potential, chronoamperometry characterizes the durability and stability of BNNs-MOF composites for long term performance, which is essential for their application in energy storage and other electrochemical devices. Electrochemical deposition to deposit BNNs onto MOFs, forming coatings that significantly enhance the conductivity and electrochemical stability of the MOF [95,96]. To assess the properties of these BNNs-MOF composites, the electrochemical technique of LSV is employed. This approach is particularly useful for evaluating the onset potential and catalytic activity of the material in specific electrochemical reactions, such as hydrogen evolution or oxygen reduction [97].

5. Mechanisms of Hydrogen Sensing and Storage

Hydrogen sensing involves detecting the presence and concentration of hydrogen gas in an environment. Detecting hydrogen within BNNs-MOF composites involves the use of several advanced analytical techniques. Gas Chromatography (GC) [98] offers high sensitivity and quantitative analysis of hydrogen absorption or release. Mass Spectrometry (MS) [99] is particularly effective in detecting trace amounts of hydrogen and analyzing desorption characteristics. At the molecular level, Fourier-Transform Infrared Spectroscopy (FTIR) [100] provides insights into hydrogen interaction and storage. Thermogravimetric Analysis (TGA) [101] evaluates the thermal stability and quantifies hydrogen release across different temperatures. X-ray Photoelectron Spectroscopy (XPS) [102] uncovers surface interactions, revealing chemical bonding and oxidation state changes. Raman Spectroscopy [103] identifies adsorption sites and examines hydrogen interaction within the composite material. Additionally, Hydrogen Permeation Tests [104] focus on the kinetic behavior of hydrogen diffusion, crucial for storage and sensor applications, whereas hydrogen storage in BNN-MOF systems relies on the material’s ability to adsorb and desorb hydrogen efficiently. However, there are several primary mechanisms for hydrogen sensing and storage (Figure 3) [105,106,107,108,109,110].

Physisorption mechanism involves the weak and reversible adsorption of hydrogen molecules onto the surface of materials through van der Waals forces [111]. In this process, the hydrogen molecules remain intact and do not form chemical bonds with the surface. The interaction is purely physical, with the hydrogen molecules being held close to the surface by weak attractive forces. These are weak, non-covalent interactions that arise from induced dipoles within the molecules. Physisorption typically occurs at low temperatures, where the thermal energy is insufficient to overcome these weak interactions, making it a reversible process. This mechanism is crucial for applications where easy desorption of hydrogen is necessary.

Chemisorption mechanism involves the formation of strong covalent bonds between hydrogen and the surface atoms of BNNs or MOFs [112]. This process often begins with the dissociation of hydrogen molecules (H₂) into individual hydrogen atoms, which then form σ bonds with the material’s active sites. Unlike physisorption, chemisorption is typically irreversible due to the strength of these chemical bonds. The adsorption of hydrogen can significantly alter the electronic structure of the material, leading to changes in its electrical or optical properties. Chemisorption is particularly important in applications where stable, strong adsorption of hydrogen is needed.

The charge transfer mechanism involves the redistribution of electrons between the adsorbed hydrogen and the surface of BNNs or MOFs [113]. When hydrogen adsorbs onto the material, it can either donate or accept electrons, depending on the relative electronic states of the hydrogen and the material allowing either n-type or p-type doping. This electron transfer results in a change in the material’s electrical conductivity or resistance, which can be measured to detect the presence of hydrogen. This mechanism is particularly useful in the development of hydrogen sensors, where changes in electrical properties are used as a detection signal.

Work function modulation occurs when the adsorption of hydrogen onto the surface of BNNs or MOFs alters the energy required to remove an electron from the material, known as the work function [114]. The presence of hydrogen can change the distribution of electronic states at the surface. If hydrogen adsorbs and donates electrons to the surface, it could lower the work function. Conversely, if it withdraws electron density, the work function might increase. This change can also be understood as a shift in the Fermi level of the material and can be detected by electronic devices, such as Field-Effect Transistors (FETs), making work function modulation a valuable mechanism for hydrogen sensing. By monitoring changes in the work function, the presence and concentration of hydrogen can be accurately detected.

Optical sensing relies on the changes in the optical properties of BNNs or MOFs upon hydrogen adsorption [115,116]. When hydrogen interacts with the surface, it can alter the electronic structure of the material, leading to changes in properties such as absorbance or fluorescence. These changes can be detected using spectroscopic techniques, such as UV-Vis or fluorescence spectroscopy. By monitoring these optical signals, hydrogen levels can be detected and quantified. Optical sensing is particularly advantageous for applications requiring non-invasive and real-time hydrogen detection.

The spillover effect is a mechanism where hydrogen molecules adsorbed onto a metal catalyst, such as palladium, dissociate into hydrogen atoms [117]. These atoms then migrate or “spill over” onto the surface of nearby materials like boron nitride nanosheets (BNNs) or metal–organic frameworks (MOFs). This migration increases the surface area available for hydrogen storage, thereby enhancing the overall hydrogen storage capacity of the composite material. The spillover effect is particularly valuable in hydrogen storage applications, as it allows for more efficient use of the storage material by leveraging the catalytic properties of metals to increase hydrogen uptake.

Hydrogen bonding and interactions with functional groups on the surface of BNNs and MOFs play a critical role in hydrogen storage [118]. Functional groups, such as hydroxyl (-OH), amine (-NH₂), or carboxyl (-COOH) groups, can form hydrogen bonds with hydrogen molecules, facilitating their adsorption and retention on the surface. By modifying the surface chemistry of BNNs and MOFs to introduce or optimize these functional groups, the storage capacity and reversibility of hydrogen adsorption can be significantly improved. This mechanism is essential for developing materials with enhanced hydrogen storage capabilities as it allows for the fine-tuning of surface interactions to achieve optimal performance.

Table 4. Summary of the key methods, techniques, and mechanisms of hydrogen sensing and storage in BNN/MOF composites along with their respective applications.

Mechanism/Method	Applications	Key Features	References
Gas Chromatography (GC)	Hydrogen sensing	High sensitivity, quantitative analysis of hydrogen absorption or release.	[98]
Mass Spectrometry (MS)	Hydrogen sensing	Effective in detecting trace amounts of hydrogen and analyzing desorption characteristics.	[99]
Fourier-Transform Infrared Spectroscopy (FTIR)	Hydrogen interaction analysis	Provides molecular-level insights into hydrogen interaction and storage.	[100]
Thermogravimetric Analysis (TGA)	Hydrogen sensing and storage	Evaluates thermal stability and quantifies hydrogen release across temperatures.	[101]
X-ray Photoelectron Spectroscopy (XPS)	Hydrogen interaction analysis	Reveals surface interactions, chemical bonding, and oxidation state changes.	[102]
Raman Spectroscopy	Hydrogen sensing	Identifies adsorption sites and examines hydrogen interaction within composite materials.	[103]
Hydrogen Permeation Tests	Hydrogen diffusion analysis	Focuses on the kinetic behavior of hydrogen diffusion for storage and sensing applications.	[104]
Physisorption Mechanism	Reversible hydrogen storage	Involves weak van der Waals interactions, suitable for low-temperature, reversible adsorption.	[111]
Chemisorption Mechanism	Stable hydrogen storage	Forms strong covalent bonds with hydrogen; typically irreversible.	[112]
Charge Transfer Mechanism	Hydrogen sensing	Redistribution of electrons changes electrical properties for detection.	[113]
Work Function Modulation	Hydrogen sensing	Monitors adsorption-induced changes in work function for precise hydrogen detection.	[114]
Optical Sensing Mechanism	Non-invasive, real-time hydrogen detection	Detects changes in absorbance or fluorescence upon hydrogen adsorption.	[115,116]
Spillover Effect	Enhanced hydrogen storage	Hydrogen dissociates on metal catalysts and migrates to material surfaces, increasing storage capacity.	[117]
Hydrogen Bonding with Functional Groups	Improved hydrogen storage capacity	Functional groups (e.g., -OH, -NH2) facilitate adsorption and retention, enhancing storage performance.	[118]

highlights the various mechanisms and analytical techniques employed in the detection and storage of hydrogen using BNN/MOF composites. It categorizes methods based on their applications, key features, and the specific techniques used for evaluation, with corresponding references to the literature.

6. Conclusions and Future Work

BNN-MOF composites have promising industrial applications, particularly in hydrogen storage and sensing. In fuel cells for hydrogen-powered vehicles, they offer lightweight, efficient storage due to their high surface area and tunable porosity. They can also support large-scale energy storage for industries by storing excess renewable energy as hydrogen. In hydrogen sensing, their high sensitivity and fast response times make them ideal for safety monitoring in hydrogen production, distribution, and industrial processes. Additionally, BNN-MOF composites enhance electrochemical devices like batteries and electrolysis systems, improving hydrogen production efficiency. In environmental applications, they help with gas purification and pollution control by capturing harmful emissions. To fully realize the potential of BNN-MOF composites, future research should focus on optimizing synthesis and functionalization to enhance performance. Developing scalable synthesis techniques and studying functional groups that improve hydrogen adsorption will refine material design. Detailed mechanistic studies on hydrogen interactions, such as physisorption and spillover effects, and further exploration of electrochemical behavior will also boost efficiency. Improving stability and durability is essential for long-term applications, while hybrid systems combining BNN-MOFs with other materials could further improve hydrogen storage and sensing capabilities. Tailored research for automotive and industrial sensors is also crucial to meeting industry demands. Environmental and economic assessments, like life cycle and cost–benefit analyses, will help guide sustainable synthesis and market integration. By addressing these research areas, BNN-MOF composites can become a cornerstone in hydrogen storage and sensing technologies, advancing clean energy solutions and supporting the global transition to sustainability.