Recent advances in mesoporous nanostructured materials and nanohybrids for supercapacitor applications: a review

Abstract

Mesoporous nanomaterials and nanohybrids have grabbed the attention of researchers for supercapacitor applications with their unique structural attributes and enhanced electrochemical properties. Recent developments have focused on optimizing the synthesis process and functionalization to achieve higher specific surface areas, tailored pore structures, and improved conductivity. These advancements are important for regular improvements in power density, cycling stability, and energy density of supercapacitors. Regardless of this, the challenges remain, particularly in the scalability of synthesis processes; the integration of nanohybrids with diverse materials enhances the long-term cycling stability under practical operating conditions. Prospects are promising, with ongoing research directed towards novel material combinations, advanced fabrication techniques, and the development of environmentally sustainable processes. Emerging trends suggest that the integration of technology could further accelerate the design and optimization of mesoporous nanomaterials and nanohybrids for developing future supercapacitors. This review article focuses on the theoretical and fundamental aspects of charge storage mechanisms for supercapacitor applications with respect to mesoporous nanomaterials and nanohybrids.

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1. Introduction

Over the past two decades, nanotechnology has emerged as a rapidly growing field, attracting extensive research interest due to its potential for advancing a wide spectrum of applications. Fields like medicine,¹ materials development,² the electronics industry,³ food science,⁴ molecular engineering,⁵ molecular biology, and organic chemistry⁶ are using this technology due to the properties of these materials. It can be used in many ways to improve the quality of manufactured materials or open horizons for new uses that were difficult to achieve.⁷ One significant application of nanotechnology is in electrical devices, where it is incorporated into the manufacturing processes of solar cells, processors, batteries, and supercapacitors. This incorporation is due to the significant increase in efficiency that nanotechnology provides.³

The nanomaterials are one-dimensional materials with size in the range of 1 to 100 nanometers, and exhibit unique properties that modulate the characteristics and properties of final products as per the requirement of applications (as shown in Fig. 1).⁸ This field of study focuses on understanding these materials and their shapes (morphology) at a very small scale. Fig. 1 further emphasizes this point by illustrating that, the materials as diverse as gold atoms and bacteria can all be considered nanomaterials, depending on their size at the nanoscale. Interestingly, this size range can be further broken down into subcategories, such as nanoparticles (1–100 nm), nanotubes (1–100 nm in diameter), and nanofilms (1–100 nm in thickness).^9,10

Fig. 1 A comparison of common materials and nanoparticles according to the size.⁸ Copyright 2014. Adopted from MDPI.

Nanomaterials can be categorized into various kinds according to their nanoscale dimensions or based on their internal and surface structures at the nanoscale.^7,11,12 Nanoporous materials have pores of 100 nm or smaller, are one of the actegory of nanomaterials. The materials at this level are named based on if they have nano-sized pores or voids in the structure. The materials are divided into two types: microporous and mesoporous materials. The focus will be on mesoporous nanomaterials and nanohybrids in this review to explain their usefulness and importance towards the development of supercapacitors.^13–16 Porous nanomaterials can be organic/inorganic materials in the bulk state with voids or pores in their structure. They are classified and named according to their position and the size of the pores. The location of the pores is an essential factor for determining the applications of these materials, as the pores on the surface are fundamentally different from the pores in the internal structure of the bulk, and they were separated and named open and closed pores. The open pores are on the surface of the bulk; in contrast, the closed pores are in the internal structure of the bulk, and the surface area is free of any pores. These materials can also be identified by its specific characteristics and different applications.¹⁷ In addition to the location of the pores, the material properties are also based on the pore size, as there are different types of pores such as micropores, mesopores and macropores of 0.2–2 nm, 2–50 nm and 50–1000 nm respectively. Fig. 2 briefly explains different methods to synthesize mesoporous nanomaterials. These six methods are mainly used to synthesize nanoporous materials; of course, there are also many other ways to synthesize diverse types of nanomaterials, but the focus is on nanoporous materials in this review. Therefore, these five methods are the most common and widely used.^17–22

Fig. 2 An overview of different methods to synthesize mesoporous nanomaterials.¹⁹ Reproduced with permission from Wiley. Copyright 2020.

Our previous discussion highlighted mesoporous nanomaterials and their potential use in supercapacitors. It is important to recognize that these materials extend beyond singular compositions. Fig. 3 illustrates the concept of mesoporous nanohybrids, which incorporate multiple fused compounds and possess pore sizes ranging from 2 to 50 nanometers. Like conventional nanomaterials, the classification of mesopores within these materials hinges on their dimensional characteristics (as depicted in Fig. 3).²³ Notably, some mesopores consist of single micelles, exhibiting variations in shape, size, and even pore location. This inherent variability translates into diverse material properties and functionalities, ultimately leading to a wider range of applications. Fig. 3 serves as a valuable visual representation. It showcases various mesoporous nanomaterials obtained through a multitude of synthesis methods, and the soft template method in combination with different techniques such as evaporation-induced self-assembly, sol–gel co-assembly, evaporation-induced aggregation, emulsion-induced interfacial assembly, evaporation-driven oriented assembly, and hydrothermal synthesis. The versatility of these synthesis methods is further emphasized in the text, which highlights the utilization of various primary elements. These elements encompass metals, metal oxides, metal sulfides, polymers, carbon, and silicates. Interestingly, the text differentiates between silicate and carbon-based mesopores, classifying them as purely mesoporous, while others are categorized as nano-hybrids based on their composition.^24–27

Fig. 3 Some different dimensions of mesoporous nanoparticles.²³ Reprinted with permission from Springer Nature. Copyright 2019.

The characteristics of mesoporous, hybrid, or normal nanomaterials enable them to expand the horizons of energy conversion and storage applications due to high-density capacity and electrical conductivity, which rely on a large surface area.²⁸ Porous nanomaterials exhibit high surface-to-volume ratios, a crucial property of nanomaterials, which is not limited to porous structures but is prevalent in most nanomaterials. Mesoporous materials, however, have an even greater potential due to their pores, which can be used to tailor their properties based on pore size. Since 1991 there have been numerous research studies and discoveries on supercapacitor concepts. In 1998 mesoporous silica was introduced. To date several approaches have been adopted towards mesoporous nanostructured materials and nanohybrids for supercapacitor applications. Fig. 4 demonstrates a schematic of the roadmap with important years of discoveries of mesoporous nanostructured materials. This review specifically focuses on one of the most significant applications of mesoporous materials: energy conservation and storage in supercapacitors.²⁹Table 1 provides a brief comparison of different mesoporous nanomaterials and nanohybrids, their properties and specific targeted applications.

Fig. 4 A schematic of the roadmap with important years of discoveries of mesoporous nanostructured materials.

Table 1 Different mesoporous nanomaterials and nanohybrids, their properties and specific targeted applications

Type of material	Key properties	Advantages	Typical applications	Selection criteria for target applications	Ref.
Mesoporous silica (e.g., SBA-15 and MCM-41)	Large surface area, easily functionalizable, biocompatible, and adjustable pore size	Chemical versatility and excellent for adsorption	Drug delivery, catalysis, and adsorption	Biocompatibility and high surface area for drug loading or catalyst support	30
Mesoporous carbon	High electrical conductivity, large surface area, and chemical stability	Excellent electrochemical properties	Supercapacitors, batteries, catalysis, and gas storage	Excellent chemical stability and conductivity for energy storage applications	31
Mesoporous metal oxides (e.g., TiO2 and ZnO)	High catalytic activity, tunable electronic properties, and chemical stability	High redox activity and photocatalytic properties	Sensors, catalysis, and photocatalysis	Redox activity and light absorption for sensors or photocatalytic applications	32
Metal–organic frameworks (MOFs)	Porous structure with organic–inorganic hybrid composition	High surface area and tunable chemistry	Gas storage, drug delivery, and catalysis	Pore tunability and high surface area for gas adsorption or separation	33
Carbon-metal oxide nanohybrids	Synergistic properties of both carbon and metal oxides (e.g., MnO2/C)	Enhanced conductivity and stability and redox activity	Supercapacitors, batteries, and sensors	Combination of conductivity and redox activity for energy storage	34
Polymer-metal oxide nanohybrids	High flexibility, stability, and functionality	Mechanical flexibility and conductivity	Flexible electronics, sensors, and drug delivery	Mechanical flexibility and stability for biomedical or flexible electronics	35
Mesoporous zeolites	High surface area, ion-exchange capacity, and thermal stability	Excellent molecular sieves and catalytic activity	Catalysis, gas separation, and ion exchange	Ion-exchange capacity and stability for separation or catalytic processes	36

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This review presents the fundamentals of charge storage mechanisms in supercapacitors by using mesoporous nanomaterials and nanohybrids. It emphasizes electric double-layer capacitance and pseudocapacitance, explaining how mesoporous structures and hybrid compositions enhance these mechanisms. The controlled pore size, huge surface area, and superior conductivity of mesoporous materials facilitate efficient ion transport and charge storage. Additionally, the integration of nanohybrids further improves the electrochemical stability and energy density, making these materials promising for advanced supercapacitor applications. This review on recent developments, challenges, and future prospects of mesoporous nanomaterials and nanohybrids in supercapacitor applications, provides an extensive and more detailed exploration of nanohybrids. This article extends the discussion to recent innovations in nanohybrid composites, addressing challenges in scalability, stability, and energy density. It also provides a forward-looking perspective on future trends and opportunities, making it distinct in its comprehensive analysis of both current technological hurdles and potential advancements in the field.

2. Fundamentals and charge storage mechanisms in supercapacitors

Ultracapacitors are high-capacity charge storage devices, so their capacitance is higher than that of ordinary capacitors (but their voltage values are lower), and the capacitor integrates the ideas of electrolytic capacitors and rechargeable batteries (which are known as the secondary cell). The energy per unit volume stored in supercapacitors is approximately 100 times greater than that in the electrolytic capacitor and battery. Supercapacitors are important for applications which require quick charge/discharge mechanisms in comparison with other energy storage devices.^37–39 The conventional solid dielectric concept has been mostly ignored or modified the fabrication techniques of the supercapacitor. Still, they use dual-layer electrochemical and pseudo-electrochemical capacitance, which gives complete capacitance, with some differences. Fig. 5 shows various supercapacitors and their components.^40–42

Fig. 5 The schematic shows the components of an (a) electrochemical double-layer capacitor, (b) pseudo capacitor and (c) hybrid capacitor.⁴⁰ Copyright 2020. Reprinted with permission from Elsevier.

The materials used in capacitive double-layer electrochemical capacitors (EDLCs) determine the efficiency of supercapacitors. In these pseudo-electrochemical capacitors, carbon and graphene are the materials commonly used as electrodes. Then supercapacitors were developed, and nanoparticles of carbon or graphene were used to enhance the areal surface, which in turn enhanced the efficiency of the capacitor and its capacitance ability by a significant percentage. After that, mesoporous materials were developed to be used as electrodes inside the capacitors, and after testing them, they proved to have high efficiency, good quality and large specific surface area, which helps to increase the faradaic reaction rate on which the pseudo-electrochemical capacitors depend, thus increasing the energy density of the capacitor.^29,43,44 Therefore, many such materials have been evaluated, and as mentioned earlier, mesoporous nanomaterials have different dimensions. For example, 1D mesopores significantly impact capacitor charging and discharging capacities, as they provide higher speed and efficiency than standard nanomaterials, but the capacitance of the capacitor does not increase significantly.²⁹ In contrast, two-dimensional experiments have been conducted by Allah et al., where carbon particles with two-dimensional hexagonal structures were used as electrodes and proved effective in increasing the energy storage capacity in the capacitor, unlike previous reports based on ordinary and carbon-based nanomaterials.^45,46 On the other hand, Wang et al. designed porous carbon spheres in a hierarchical shape consisting of nanochambers in the central part, which also proved highly efficient at enhancing the energy density of the capacitor. Also, it has proven its effectiveness in the long term because the capacitance is not affected. The capacitor was used for 5000 cycles at high current density without any change in its specified capacity.⁴⁷ Here, we get a basic understanding of how effective mesoporous materials are in supercapacitors. Later, the methods for integrating mesoporous structures will be explained in more detail, along with a clearer explanation of how supercapacitors work and the role of both hybrid and standard mesoporous materials in them.

The nature of electrode microstructures and the method of interaction between the electrode and electrolyte are the primary determinants of the energy storage mechanism in supercapacitors (SCs). Despite recent advancements in the effects of electrode microstructures and electrode/electrolyte interface on the conceptual understanding of the storage and performance of the supercapacitors, this content has not been clearly expressed in a wider context.⁴⁸ The classification of supercapacitors according to various electrodes and electrolytes and charge storage processes, and some electrical parameters for the study of supercapacitors are briefly introduced in this section based on different aspects. SCs can be divided into three primary classes based on their physical characteristics and composition: hybrid capacitors (HSCs), electric double-layer capacitors (EDLCs), and pseudocapacitors (PCs), also known as redox SCs.⁴⁹

2.1. Classes of supercapacitors

2.1.1. Electrochemical double-layer capacitors (EDLCs). Due to the near proximity of the electrodes and their surface area, a double layer electrocapacitor has a higher energy density than traditional capacitors. Unlike conventional capacitors, which contain a dielectric medium, EDLCs contain electrolytes (such as Na₂CO₃, H₂SO₄, or KOH).⁵⁰ The thickness of the separator is used to evaluate the value of capacitance of a traditional capacitor. The thickness of the oppositely charged layer obtained at the electrode–electrolyte interface determines the capacitance performance of the EDLC (Fig. 5(a)).⁵¹ The EDLC double layer is substantially thinner than the separator; hence, its capacitance must be much larger than that of the conventional capacitor. In addition to the lower separation distance, the electrode/electrolyte contact has a substantially larger area than that of a normal capacitor, which can boost EDLC capacitance. In this case, applying a voltage across the EDLCs causes a different charge to be received by the electrodes, leading to the transportation of ions to the pores of the electrodes.⁵¹ It is known that EDLCs can exhibit only slight volume fluctuations since electrical charges transfer easily between them. As a result, they exhibit tremendous cycling characteristics. In the 19th century, the concept of EDLCs was initially introduced by Helmholtz during his research on opposing charges on colloidal particle surfaces, as presented in Fig. 6(a). Gouy and Chapman later updated the Helmholtz model to a model of diffusive layers (Fig. 6(b)). The capacitance of the two separated electrodes increased as the distance between them decreased. EDLCs are commonly thought to operate in the same manner as traditional capacitors. Therefore, as the point charge approaches the electrode substrate, a sharp increase in capacitance occurs. This suggests that dipole and ionic movements caused by thermal fluctuations cause ions to be diluted slowly until they are below the concentration of the electrolyte in a medium that forms a thin layer that can be easily dispersed. The capacitance in this model of diffusion is given by using eqn (1):Here, ε₀ represents the vacuum permittivity, ε_r represents the electrolyte permittivity, λ_D represents the Debye length, z represents electron numbers, F is the Faraday constant, Ψ is an electric potential, and T is a measure of the temperature. However, when an electron is very near the surface of the electrode, a large capacitance is produced, which may lead to an overestimation of the potential difference between the two electrodes. Furthermore, Stern also created a model that merged the Gouy–Chapman diffusion layer models and the Helmholtz model to recognize duplet zones of particle diffusion: the outer part known as the diffusive layer and an inner part known as the Stern layer, also known as the compact layer as demonstrated in Fig. 6(c).⁵ The capacitance (C_DL) of a cell can be expressed as:where C_H and C_D are indices that indicate the capacitance of the underlying and diffuse layers, respectively. The EDL is a hypothesis derived from the idea that electrolytes are a dilute solution. However, they are not suitable for use as ionic liquids. These theories are unsuitable for electrolytes that are highly concentrated or ionizing liquids because of the strong short-term Coulomb interactions. Furthermore, it is crucial to remember that the capacitance of EDLCs is determined by the widths of the double layers formed at the intersection of the electrode and electrolyte, which is much less than the thickness of an ion-permeable separator. Because of this, EDLCs can exhibit far greater capacitance than a standard capacitor. Carbonaceous materials are favoured for SC electrodes due to the large surface area, high mechanical and chemical strengths, high conductivity, splendid stable charge–discharge cycles, and economic feasibility.⁵² The electrochemical fingerprints of EDLCs are displayed in Fig. 6(d and e) using galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV), respectively.⁵³ The CV curve exhibits a rectangular-box-type shape, whereas the GCD curve exhibits a symmetric-triangular pattern. EDLCs exhibit both excellent specific capacitance and power density but have low energy density because of the non-faradaic charge storage concept. Recently, efforts have been focused on increasing the energy density of EDLCs, while ensuring that they also have an extremely high-power density and the maximum expected life cycle.

Fig. 6 (a) The Helmholtz model, (b) the Gouy–Chapman model, and (c) the Stern model of charged EDLCs. (d) CV and (e) GCD curves of EDLCs.⁵³ Copyright 2020. Reprinted with permission from Wiley.

2.1.2. Pseudocapacitors (PCs). Conversely, the pseudocapacitor electrodes store electric charges using a reversible faradaic charge transfer process; this involves a quick and reversible electrochemical reaction at the intersection of the electrode and electrolyte (Fig. 5(b)). PCs possess higher energy densities and capacitance performance than EDLCs. However, the power densities of PCs are often lower than those of EDLCs. This is due to the process of a slower increase and decrease of faradaic reaction throughout the charging/discharging process, which causes a shorter cycle life and makes them less mechanically stable. The interaction between the electrodes and electrolyte occurs through a faradaic reaction that involves charge exchange, which relies on voltage. Therefore, pseudocapacitors are voltage-dependent. There are four mechanisms that take place in PCs: (i) redox pseudocapacitance, (ii) intercalation pseudocapacitance, (iii) doping pseudocapacitance, and (iv) underpotential deposition-based pseudocapacitance (Fig. 7(a–d)).⁵⁴ In the case of redox pseudocapacitance, the adsorption of ions is driven by the charge transfer process between the electrode surface and electrolyte. The CV curve in Fig. 7(a) shows the reversible redox pseudocapacitance characteristics of RuO₂ in an acid medium. In the case of intercalation pseudocapacitance, ion intercalation onto bulk electrode materials followed by faradaic charge-transport takes place without phase transformations. However, this is hardly observed, and almost all materials show a mixed storage mechanism. The CV curve response of orthorhombic Nb₂O₅ is demonstrated in Fig. 7(b) which exhibited a blend of capacitive and battery-like behaviours. Electrode materials like metal oxides are used for redox pseudocapacitance, whereas potential metal oxides and metal sulfides are used for intercalation pseudocapacitance. The CV curve in Fig. 7(c) shows doping pseudocapacitance. Because of the reversibility of the charge-discharge kinetics, charge accumulation in underpotential deposition happens when metal ions or hydrogen adsorb into monolayers on noble metal surfaces, which are at a very high-power density. However, the energy density in the underpotential deposition process is low as compared to redox pseudocapacitance and intercalation pseudocapacitance because the range of the operational potential limit is very narrow (0.3–0.6 V).⁵⁴ The CV curve of lead at the gold electrode surface (Au.Pb_ads) in Fig. 7 (d) shows the underpotential deposition-based pseudocapacitance. In consequence, redox pseudocapacitance and intercalated pseudocapacitance processes are preferable to under potential deposition-type charge storage mechanisms for pseudocapacitors. Changing the physical properties of surfaces by adjusting their nanostructures can lead to better performance of PC materials. This is because the diffusion distances decrease, and in some cases, the electronic structure can be changed to control different types of conversions that can occur during processing. Amorphous RuO₂·nH₂O (RuO₂·2H₂O: 1.9 nm = 720 F g⁻¹ Vs⁻¹. RuO₂·2H₂O: 9.9 nm = 200 F g⁻¹) is an example of a pseudocapacitor that is intrinsically dependent on morphology and particle size.⁵⁵ Similarly, another example is orthorhombic Nb₂O₅ (T-phase: 35 nm = 400 F g⁻¹).⁵⁶ However, extrinsically it only depends on the nanostructured surface. The cycling retention and power density of PCs are restricted, and they are hindered by the slow charge–discharge mechanism of PC materials, which causes them to undergo swelling and phase changes during charge–discharge.

Fig. 7 Different charge storage mechanisms and CV graphs for PCs. (a) Redox, (b) intercalation, (c) doping, and (d) underpotential deposition based pseudocapacitance.⁵⁴ Reprinted with permission from the Royal Society of Chemistry. Copyright 2019.

2.1.3. Hybrid supercapacitors (HSCs). The introduction of hybrid supercapacitors has eliminated many of the problems that have been encountered in the past with EDLCs and pseudocapacitors. Hybrid supercapacitors represent a middle ground in between two batteries. HSCs mainly consist of a synthetic combination of an electrode with capacitive carbon and either a lithium-decorated electrode or a pseudocapacitive material (Fig. 5(c)). HSCs are preferred over EDLCs and pseudocapacitors; besides the increased working potential, they exhibit improved cycling stability, and better power density and energy density. HSCs involve both faradaic and nonfaradaic storage mechanisms. In HSCs, the cathode is intercalated with faradaic components, while the anode is intercalated with non-faradaic components, which provides both good energy and power density without disturbing the affordability and cycle stability.⁵³ HSCs work by utilizing the potential space between the electrode materials to increase the overall cell voltage, namely negative and positive electrodes constructed of EDLCs and pseudocapacitive materials, respectively. Based on the combination of electrode materials, HSCs are of three types: (a) battery type (b) asymmetric, and (c) composite hybrid.⁵⁷ In asymmetric and battery-type hybrids, the electrode materials are varied whereas in composite hybrids the electrode materials are similar. In addition, composite hybrid electrodes show high specific capacitance and corrosion stability, including a wider working potential window. CV and GCD properties of battery, asymmetric, and composite hybrid types are shown in Fig. 8. As shown in Fig. 8(a and d), both electrodes display the signature faradaic peaks and charge/discharge plateaus in the CV and GCD characteristics of battery-type supercapacitors. In the asymmetric type, both electrodes show an absolutely rectangular CV curve and triangular GCD curve (Fig. 8(b and e)). The ΔQ/ΔU (C = ΔQ/ΔU) ratio is utilized to calculate the electrochemical performance of the asymmetric type supercapacitor. The capacitive and the battery-type electrodes are mixed to form a composite hybrid supercapacitor. Also, an electrode shows battery-type behaviour (anodic and cathodic peaks) in the composite hybrid supercapacitor-based device. The CV and GCD characteristics of the entire device based on a composite hybrid supercapacitor display high capacitance with apparent variation from the ideal properties of capacitance (Fig. 8(c and f)). The ΔQ/ΔU (C = ΔQ/ΔU) ratio is not suitable to measure the electrochemical properties of the composite hybrid supercapacitor. AC, CNTs, 3D mesoporous carbon, graphite, graphene, and various metal oxide or polymer-based carbon composites have all been described as cathode materials for HSCs. For example, at 90 mA g⁻¹, a simple solvothermal strategy is used to construct HSCs by utilizing an Fe₃O₄ nanoparticle/graphene composite as an electrode material, which produces 1000 mAh g⁻¹ (i.e., a reversible specific capacity).⁵⁸ Lim et al. reported the development of HSCs using AC (MSP-20) and mesoporous Nb₂O₅/carbon architectures as the cathode and anode, respectively, resulting in 90% capacity retention at 1000 mA g⁻¹ over 1000 cycles as well as a high-power density of 18 510 W kg⁻¹ and energy density of 74 Wh kg⁻¹ in an electrolyte of LiPF₆ (1.0 m)/ethyl carbonate and dimethyl carbonate in a 1 : 1 volume ratio.⁵⁹ For the overall betterment of supercapacitors for commercial use, HSCs prevent their self-discharging and enable them to increase their capacity and cycling stability.

Fig. 8 Conventional CV (up) and GCD (bottom) curves for battery (a and d), asymmetric (b and e), and composite hybrid (c and f) type supercapacitors.⁵⁷ Copyright 2018. Reprinted with permission from American Chemical Society.

2.2. Methods for parameter calculations

Supercapacitor cells contain two layers of capacitors that are arranged such that they act as if they are stacked side by side. The capacitance of the entire cell was determined from eqn (3).

1/C = 1/C₁ + 1/C₂ (3)

If all the electrodes are prepared of the same material, C₁ = C₂, and the capacitance of both electrodes is the same, and the total capacitance of the SC is half that of any electrode. These types of SCs are called symmetric SCs. When the capacitances of the two electrodes are not the same, C₁ ≠ C₂, the net capacitance, is considered by the lower-capacitance electrode material side. This configuration is called an asymmetric SC. Several characteristics, including the time constant, power density (p), voltage (V), energy density (E), and cycling stability, are used to calculate the specific capacitance (C_sp) of a supercapacitor.^60,61C_sp can be calculated using eqn (4) and obtained from the CV measurements.

where C_sp is expressed in F g⁻¹, the average charge–discharge process is denoted by Q, the active material (electrode) mass is represented by m, and the potential window is denoted by V (units of V).

By integrating the current response, the C_sp of a supercapacitor inside the voltage area can be evaluated by using the CV curve, as expressed in eqn (5).

where I represents the discharge current, ΔV is the operational discharge potential rate, and ν is the scan rate.

From the GCD measurement, the electrode's C_sp was determined using eqn (5). The discharge duration is proportional to C_sp in such a case for both the two-electrode and three-electrode asymmetric cells.

I, Δt, ΔV, and m stand for the current density (applied), charge/discharge period, potential window, and the active material (electrode) mass, respectively. The C_sp produced from galvanostatic testing in the 2-electrode symmetric cell arrangement can be measured using eqn (6).I, Δt, m, and ΔV correspond to the steady discharge current, discharge time, active material electrode material mass, and potential window, respectively.

Eqn (8) and (9) can be used to compute the energy density (E, Wh kg⁻¹) and power density (P, W kg⁻¹) based on galvanostatic tests.

where Δt is the discharge process time gap and ΔV is the voltage window (V).

In addition, for the HSCs to be in charge equilibrium, the positive and negative electrodes must be in charge equilibrium (q⁺ = q⁻), and m determines the whole charge ‘q’ load in the respective electrode and can be calculated by using eqn (10), where m is the mass of the active material per area (cm²) and E is the potential window of the electrode.⁵⁷

q = C_sp × m × ΔE (10)

To establish the charge balance q⁺ = q⁻, eqn (11) can be used to balance the mass ratio of the two electrodes.

m₊/m₋ = (C_sp− × ΔE₋)/(C_sp+ × ΔE₊) (11)

As a result, the mass proportion between the two active materials (electrodes), m₊/m₋, can be tuned to improve the asymmetric SC performance. Other characteristics, such as iR_drop as well as equivalent series resistance (ESR), can be calculated using eqn (12) and (13).⁶²

ESR = iR_drop/2I (12)

iR_drop = ΔV + bI (13)

where iR_drop and ΔV represent the potential drop and voltage variation between the true voltage and applied voltage at the electrode, respectively, I represents the current loaded (A), and b is twice the ESR value.

Eqn (14) can be utilized to calculate the η% (energy output efficiency) of the electrode.

η(%) = (t_d/t_c) ×100 (14)

Here, t_d is the discharging time and t_c is the charging time.

2.3. Charge storage mechanisms in mesoporous nanomaterials

The performance of electric double-layer capacitors (EDLCs) is significantly influenced by the pore size of electrode materials.^63,64 Research has shown that maintaining high capacitance at high charging rates is difficult when ion desolvation and slow ion transport occur in micropores—especially when the pore size is smaller than that of solvated ions.⁶⁵ This limitation leads to poor rate capability. In contrast, ordered mesoporous structures, with pores larger than solvated ions, allow easier electrolyte infiltration and provide efficient pathways for ion transport, enhancing rate performance. Furthermore, the incorporation of micropores within mesoporous walls increases the specific surface area, creating more active sites for charge storage. However, despite extensive studies, the specific feature that predominantly governs electrode performance remains uncertain.⁶⁶ Factors such as pore size, structure, crystallinity, specific surface area, and pore volume all play critical roles, but their precise contributions are not yet fully understood. To resolve this, systematic investigations—potentially integrating machine learning—are proposed. Ordered mesoporous materials with tunable properties like pore size, wall thickness, and geometry could serve as model systems for such studies.

Various mesoporous materials, including transition metal oxides, carbides, sulfides, composites, and doped mesoporous carbons, have been explored for pseudocapacitive applications.^67,68 These materials exhibit markedly improved capacitance, rate capability, and cycling stability compared to their bulk counterparts. For instance, nitrogen-doped ordered mesoporous few-layer carbons (OMFLC-N) show superior performance compared to undoped mesoporous carbons due to enhanced surface redox activity at nitrogen-related defects and improved mass transport. Vertically aligned mesoporous structures further benefit ion diffusion by minimizing tortuosity, enabling ultrahigh rate capabilities even at scan rates as high as 2000 mVs⁻¹.⁶⁹ Notably, thick MXene-based mesoporous films demonstrate excellent pseudocapacitive behavior that is independent of film thickness, owing to reduced tortuosity and efficient ion insertion pathways. Similarly, mesoporous crystalline films such as iso-oriented α-MoO₃ display significantly higher pseudocapacitive charge storage than their amorphous equivalents. This is attributed to enhanced Li⁺ intercalation within the lattice of nanocrystalline grains, leading to threefold higher capacitive contributions. Collectively, these findings highlight the promise of mesoporous architectures for engineering high-performance supercapacitor electrodes.

3. Mesoporous nanomaterials for supercapacitors

Fortunately, switching from conventional materials to nanostructured matrices/electrodes can help to enhance energy storage technology. Nanostructured carbon materials, including graphene-based materials, aerogels, carbon nanotubes, quantum dots, and nanodiamonds, have garnered significant attention in the context of supercapacitor materials recently. Because of their distinct pore structure suitability and chemical stability, carbon nanotubes (CNTs) have been deemed the best material for use in supercapacitors out of all of them.

3.1. Supercapacitors from 1D, 2D, and 3D mesoporous nanomaterials

3.1.1. Carbon nanomaterials. Carbon-based nanomaterials and nanohybrids are employed in supercapacitor synthesis in large quantities due to low cost and availability in a variety of forms, including foils, mats, fibers, powders, and composites. Additionally, for many reasons, carbon materials are perfect for creating supercapacitor electrode materials; for example-(i) they possess high surface area, (ii) they are low-cost materials, (iii) they are easily available in high abundance on earth and (iv) they are well suited for supercapacitor electrode production technologies.⁷⁰ The capacitance of any material is primarily determined by surface sites available for electrolyte ions. In addition, the surface sites of electrodes vary as per surface functionality, pore size character, structure of pores and distribution and conductivity of ions. Thus, these are the characteristics that influence the electrochemical results of any material for supercapacitor applications.⁷¹ The carbon materials, having a large surface area per unit mass causes high charge availability at the matrix interface which effectively enhances the capacitance of the material.⁷²

Thus, carbon materials, like 1D carbon nanotubes (CNTs), 2D reduced graphene oxide (rGO), and 3D derived carbons (Fig. 9) have received extensive attention for their potential applications as electrode materials. Fig. 9 shows different carbon materials and their properties which can be used for fabricating nanocomposites for supercapacitor applications.

Fig. 9 Carbon materials for supercapacitor applications.

3.1.2. 1D- carbon nanotubes (CNTs). Carbon nanotubes have one-dimensional (1D) structures and diameters as small as a few nanometers and were first discovered by Iijima in 1991.⁷³ A CNT can be viewed as a honeycomb web of carbon hexagons wrapped into a smooth cylindrical tube under the microscope. The strong covalent carbon–carbon bonds along the tube axis result in ultrahigh stiffness and tensile strength, while the sp² hybridization results in amazing electric behavior.⁷⁴ The remarkable qualities of carbon nanotubes (CNTs) are their high surface area, low density, low thickness, and strong chemical stability, which set them apart from most of the conventional bulk or micro/nanomaterials.

Carbon nanotubes are created by the chemical vapour deposition (CVD) process involving catalytic deterioration of a few hydrocarbons, and by carefully controlling distinct parameters, it also becomes possible to create different nanostructures in different conformations that additionally control their crystalline structure.^75,76 Moreover, CNTs possess high packing density; as a result they have lower equivalent series resistance (ESR), and thus electrolyte ions easily enter through the mesoporous network, resulting in superior power performance.⁷⁷ These all-excellent properties make CNTs widely useful in new energy storage systems and make them suitable as electrode materials for supercapacitors.

Furthermore, CNTs have been considered as a potential support for surface active materials due to their open tubular network. Active materials for electrochemical energy devices, such as polymers, transition metal oxides and sulfides can form composites with CNTs and enhance performance by accelerating charge transfer or redox processes.⁷⁸ Wu et al. produced the composite of CNTs for supercapacitor applications by using a Ni catalyst to grow them through walls of the carbonized wood matrix.⁷⁹ They discovered that the capacitance values obtained by this method were comparable to or greater than those of the simple activated wood carbon-based SCs. One-dimensional CNTs possess quick ion transfer with storage in between charge/discharge phases. The capacitance increased to 215.3 F g⁻¹ and surface area/mass increased from 365.5 to 537.9 m² g⁻¹. Zhou et al. reported a novel high-ability electrode material intended for supercapacitors with high mass loadings of MnO₂ put upon the CNT matrix.⁸⁰ The composite MnO₂/CNTs was synthesized utilizing the oxidation of KMnO₄ in a CNT dispersion. Thus, the formed nanocomposite electrodes exhibited a high capacitance value of 2579 mF cm⁻² at rate 1 mA cm⁻² because CNTs provide structural reinforcement for the pseudo-capacitance of the MnO₂ matrix. Hence, all these synergistic enhancements brought about by combining CNTs with active moieties will markedly enhance the prospects of application of supercapacitors and build a new path towards the creation of multifunctional and high-performance supercapacitors. When 1D CNTs are hybridized with metals and graphene, they exhibit enhanced electrochemical characteristics and a higher supercapacitance than when the component materials are used alone. MoSx/GCNT/CP was one such super capacitive electrode material which is directly prepared on carbon paper by using the electrochemical deposition method. The direct growth of the free-standing graphene on CNTs produced the integrated high specific surface area and binder free electrode. The better capacitance of the disclosed electrode material was largely attributed to the high conductivity and high surface area provided by the CNT base.⁸¹ In another report, along with nitrogen doped 1D-CNTs (bamboo shaped) the efficient fusion of different components such as two different electrode materials (N-glbNT-850 and N-glbNT//NiMn₂O₄-graphene) was developed using a simple and scalable method (Fig. 10). The synthesized electrode material was transformed into a flexible supercapacitor film with the incorporation of high concentration (96 wt%) ionic liquid (BMIM and TFSI) and a polymer host methyl cellulose (4 wt%). The supercapacitor was found to have a power density of 3.39 kW kg⁻¹, extended voltage range of 3.4 V, amazing cycling stability of 96.3% up to 10 000 cycles and superior energy density of 96.3 Wh kg⁻¹.⁸²

Fig. 10 (a) Schematic illustration of N-glbNT-750-950 synthesis, (b) SEM and TEM images of N-glbNT-850, (c) pictures of a LED powered using a supercapacitor at different time intervals, flexibility, bending and a stretched supercapacitor film, (d) cyclic voltammetry curves at various scan rates of an ionogel electrode (N-glbNT-850), (e) GCD curves at different current densities and (f) Nyquist plot.⁸² Copyright 2023. Reprinted with permission from Elsevier.

3.1.3. 1D carbon nanotube-transition metal oxide. Another NiCo layered double hydroxide (NiCoCe-LDH/CNT) nanocomposite was synthesized by M. Dinari et al. by applying a simple hydrothermal technique. Co and Ni show good electron conductivity, but structural deformation remains a significant issue when using metal oxides which adversely affects electrochemical behavior of the composite material.⁸³ Here, CNTs act as a structural agent and as a conductor moiety, improving electron conduction and preventing agglomeration of NiCo-LDH nanosheets and it is also helpful in enhancing specific capacitance. The capacitance exhibited by NiCo-LDH is negligible vis-a-vis the NiCo-LDH/CNT nanocomposite. Cerium doping (Ce⁴⁺/Ce³⁺) was further done to enhance surface area along with specific capacitance. As shown by the nitrogen adsorption isotherms given in Fig. 11(c) specific surface area was successfully enhanced after Ce doping, resulting in an overall increase in NiCoCe-LDH/CNT composite capacitance. The resulting nanocomposite exhibited a specific capacitance of around 187.2 F g⁻¹ which was at a rate of 1 A g⁻¹. Further retention of capacitance of composite NiCoCe-LDH/CNT was reported to be 85.6% after 9000 cycles as shown in Fig. 11(e) with coulombic efficiency remaining intact at 100% in aqueous KOH.

Fig. 11 (a and b) FESEM and TEM images of the NiCoCe-LDH@CNT composite, (c) adsorption–desorption isotherms of nitrogen for NiCo-LDH/CNT and 10% Ce–NiCo-LDH/CNT composites, (d) CV graphs for NiCo-LDH, NiCo-LDH/CNT, and Ce–NiCo-LDH/CNT (with different Ce ratios) at 10 mV s⁻¹, and (e) coulombic efficiency and long-term cycling stability of 10% Ce–NiCo-LDH/CNT after 9000 cycles at 1 A g⁻¹.⁸³ Reprinted with permission from American Chemical Society. Copyright 2021.

Hu and group⁸⁴ fabricated NiCoO₂@CNTs/NF by a simple hydrothermal process as shown in Fig. 12. Here, carbon nanotubes (CNTs) were initially assembled onto surface sites of nickel foam at room temperature. CNTs were uniformly covered on Ni foam using the self-assembly method which provided larger surface area essentially required for smooth material growth. As a result of the remarkable synergistic effect between the CNTs and the NiCoO₂ nanosheets, the NiCoO₂@CNTs@NF composite electrode shows improved capacitive properties and stability. Activated carbon supported on Ni foam electrodes and NiCoO₂@CNTs@NF electrodes were also used to build asymmetric supercapacitors (ASCs). With a high specific capacitance of 151 Fg⁻¹ at 5 mA cm⁻², outstanding rate capability (83.8% retention), strong stability (over 90% retention), and a high energy density of 56.0 Wh kg⁻¹, these ASCs exhibit outstanding performance.

Fig. 12 (a) Schematic representation of the preparation of the NiCoO2@CNTs@NF binder-free electrode, (b and c) SEM images of CNTs@NF and intercalated NiCoO2@CNTs@NF, (d and e) TEM images of NiCoO2@CNT@NF, (f) cyclic voltammetry curve comparison of negative and positive electrodes, (f) AC@NF and NiCoO2@CNT@NF, (g) cyclic voltammetry curves of the device at 50 mA cm⁻² current density and varied scan rates, (h) GCD curves at different current densities and (i) cycling stability at 30 mA cm⁻².⁸⁴ Copyright 2021. Reprinted with permission from Elsevier.

Ma et al. investigated electro-deposition of CNT/Ni₃S₂ composites based on surface-sites of nickel foam.⁸⁵ The Ni₃S₂/CNTs hybrid nanoarchitecture demonstrated a huge surface area and an outstanding specific capacitance of 1644 F g⁻¹ at a current density of 1 A g⁻¹, retaining 998 F g⁻¹ at 20 A g⁻¹, according to studies of a constructed electrode for a supercapacitor. Furthermore, in a 3 M KOH electrolyte, the Ni₃S₂/CNTs electrode exhibits exceptional cycling stability, retaining 91.5% of its capacitance after 2000 cycles. The Ni₃S₂/CNTs nanocomposites are emerging as strong contenders for high-performance binder-free supercapacitor electrodes because of these exceptional qualities.

3.1.4. 2D carbon-based moieties-graphene derivatives. Graphene due to its excellent surface and morphological characteristics has been employed as a potential EC electrode material with high specific surface area, good electroconductivity, and low packing density.⁸⁶ In electrode materials for SCs, 2D graphene along with porous derivatives offers significant advantages. Due to graphene's crystalline morphology, it exhibits an interesting electronic structure and mechanical properties. Available charge transport carriers of graphene which are basically electrons, travel in a ballistic (movement) pattern within a crystal lattice plane, and as a result the composite material shows high conductivity.⁸⁷ Graphene has also been found to exhibit agglomeration of graphene sheets, which decreases the total surface area and makes its real performance inferior. Thus, recent studies documented improved supercapacitance ability by templating 2D graphene with other conductive moieties.⁸⁸

Zheng et al.⁸⁹ prepared a nanohybrid made of nitrogen/phosphorus co-doped porous carbon-coated graphene (NPG) employing carbonization followed by activation (with phytic acid) of graphene oxide using ethylenediamine as shown in Fig. 13. Synthesized NPG exhibited a peculiar nanohybrid morphology by utilizing graphene layers packed between porous carbon nanosheets which restricts the agglomeration of 2D graphene layers which results in enhancement of surface area. An improved specific capacitance of about 201 F g⁻¹ at 0.5 A g⁻¹ and greater capacitance retention of 84.6% (in 20 000 cycles) were demonstrated by the hierarchically micro/mesoporous NPG structure.

Fig. 13 (a) Schematic for the preparation process of KNPG and (b) CV curves of prepared electrode materials at a scan rate of 100 mV s⁻¹ in 6 M KOH.⁸⁹ Copyright 2018. Reprinted with permission from American Chemical Society.

Lu et al. created graphene oxide flakes that ranged in size from a few micrometers to nanometers via sonochemical treatment, and they evaluated this material for use in semiconductor electrodes.⁹⁰ After reducing the size of graphene, according to compositional and electrochemical analysis, a distorted aromatic configuration on the edges was observed which also enhanced the sp³-carbon defects. An increased number of edges present in small-scale graphene results in overall capacitance enhancement. A novel (Ni-BTC@GO) composite was synthesized by Chen et al.⁹¹ by a simple hydrothermal method as shown in Fig. 14(a); 1,3,5-benzenetricarboxylic acid (BTC) was utilized as the prime organic based ligand and nickel nitrate as the metal salt, and both make a composite with GO. 3D Ni-BTC@GO nanocomposites possessed an octahedral framework (Fig. 14(b)) and avoided GO aggregation. The overall synergistic effect of GO and Ni-BTC achieved a specific capacitance value of around 1199 F g⁻¹ which was at a rate of 1 A g⁻¹, leading to an enhanced cycling stability value of 84.47% in 5000 cycles at a rate of 10 A g⁻¹.

Fig. 14 (a) Schematic to represent the synthesis mechanism of the Ni-BTC@GO composite, (b) 3D structure of Ni-BTC@GO composites showing an octahedral framework, (c) cyclic voltammetry curves in different voltage windows at 50 mV s⁻¹ scan rate, (d) GCD graphs at different current densities and (e) cycling stability at 10 A g⁻¹ up to 10 000 cycles.⁹¹ Copyright 2023. Reprinted with permission from American Chemical Society.

3.1.5. 3D mesoporous carbons. 3D mesoporous carbon structures with different porous morphologies have been tested for super capacitor material synthesis and exhibited good energy storage abilities.⁹² Meso-porous carbon structures are made utilizing different synthesis processes like chemico-physical activation, catalytic processing, carbonization methods and combining with different conductive materials.⁹³ In another research study carried out by Xiran li et al., nitrogen and sulfur co-doped 3D mesoporous carbon–Co₃Si₂O₅(OH)₄ electrode materials were synthesized for supercapacitor applications.⁹⁴ Porous carbon obtained from bamboo leaves was employed, and the layered framework of silicate led to varied ion transfer pathways. Carbon structures co-doped using nitrogen and sulfur exhibited huge specific capacitance owing to the fastest faradaic interactions. After testing in a 3.0 M potassium hydroxide solution with a three-electrode system, the produced composite demonstrated increased capacitance like the electrode materials, displaying 1600 F g⁻¹ capacitance at an operating current density of 1.0 A g⁻¹. Mesoporous carbon-based materials synthesized from carbon waste from biomass offer a viable way to create devices with higher energy densities for a variety of energy storage uses. A 3D hierarchical porous nanosheet supercapacitor fabricated from Juliflora wood carbon trash was transformed into a treasure by Aravindha Raja Selvaraj et al.⁹⁵ The template free activated carbon nanosheets were produced by a simple pyrolysis method using KOH as the activator. Notable improvements in porosity, morphology, specific surface area, surface functional groups and pore volume were observed by altering the pyrolysis temperature. The obtained supercapacitor material was found to have high surface area (2786–2943 m² g⁻¹), a high energy density of 56.73 Wh kg⁻¹ at 249 W kg⁻¹, an excellent cycling stability of 92.5% up to 6000 cycles and a high specific capacitance of 588 F g⁻¹ at 0.5 A g⁻¹. In another research study Xiaohua Zang et al. reported the synthesis of carbon fiber supported porous graphitic carbon nanosheets.⁹⁶ Synthesis methods involve the use of a freeze-dried mixture of glucose, K₃[Fe(C₂O₄)₃]·H₂O and melamine. The mixture was carbonized in an inert environment of nitrogen gas to obtain the target sample CNS/CF_0.15. The generated sample possesses an extremely large concentration of ion-accessible microspores which facilitated the transport of ions and provided an adequate number of active sites for energy storage. Because of its unique architecture, CNS/CF0.15 has amazing electrochemical characteristics, including a high capacitance of 354.7 F g⁻¹ at 0.5 A g⁻¹, an outstanding rate capability with 71.4% capacitance retention at 30 A g⁻¹, and an exceptional cycle stability of 96.8% after 10 000 cycles. At a power density of 456.7 W kg⁻¹, the symmetric supercapacitor achieves a high energy density of 24.5 Wh kg⁻¹.

Promising results have also been observed when using coal as a precursor for 3D mesoporous carbon in the construction of mesoporous carbon-based supercapacitors. Yan Lv et al. reported one such study (Fig. 15) in which coal oxide fragments and a crosslinking agent PVA are used for the synthesis of PVA/coal hydrogels.⁹⁷ The hydrogel was freeze dried and then calcined to obtain the 3D hierarchical porous carbon aerogels (3D HPCAs). Research indicates that by varying the mass concentration of PVA and coal oxide, the amount of micropores and mesopores can be regulated. With exceptional stability even after 50 000 cycles, a high-rate performance of 187 F g⁻¹ at 20 A g⁻¹, and a specific capacitance up to 260 F g⁻¹ in the 3-electrode system at 1 A g⁻¹, the improved material exhibits extraordinary electrochemical features.

Fig. 15 Schematic representation for the preparation of 3D hierarchical porous coal-based aerogels, (a) CV curves of the prepared electrode at a scan rate of 50 mV s⁻¹, (b) GCD curves of materials with varying PVA amounts at 1 A g⁻¹, (c) CV curves of the HPCAs-0.4-800, (d) GCD curves of the HPCAs-0.4-800, (e) specific capacitance at a current density of 0.5–20 A g⁻¹ of the samples having varying PVA ratios and (f) Nyquist plots of 3D HPCAs.⁹⁷ Copyright 2020. Reprinted with permission from Springer Nature.

For understanding the concept of mesoporous materials for supercapacitor applications, Feili Lai et al. developed a complex micro–mesoporous supercapacitor by using polymer/ZnCl₂, sucrose and sulfuric acid for the carbon precursor in a wide operating voltage range.⁹⁸ This design enhances energy storage efficiency and performance, making it suitable for various applications requiring stable and high-capacity energy output over a broad voltage spectrum. Nitrogen-doped, cross-coupled micro–mesoporous carbon-metal networks (N-STC/M_xO_y) were constructed using urea as a nitrogen source and several metal salts (Fe(NO₃)₃·9H₂O and Co(NO₃)₃·6H₂O). The supercapacitor was found to have an energy density of 114 Wh kg⁻¹ and enhanced gravimetric capacitance of 377 F g⁻¹. Ashan et al.⁹⁹ synthesized 3D hierarchically mesoporous nanostructures consisting of metal oxides i.e. zinc-nickel-cobalt oxide (ZNCO) nanowires (Zn_0.6Ni_0.8Co_1.6O₄). It was constructed by hierarchical aggregation of ZNCO nanoparticles. Furthermore, the particulate nature of ZNCO nanowires allowed deep electrolyte diffusion, traversing reversible charge storage ability at enhanced current densities. When the current density was set to 1 A g⁻¹, the produced ZNCO nanowires demonstrated a high specific capacitance value of 2082.21 F g⁻¹ and great stability across 5000 charge–discharge cycles.

3.1.6. Supercapacitors from low-cost nanomaterials. In recent times, low-cost materials along with their recycling have exhibited great advantages due to the development of clean energy. Energy efficiency has aroused great concern regarding the choice of materials for super-capacitor electrodes. As technology continues to advance, the integration of energy storage performance, waste reutilization, and product efficiency must be further optimized. As a result, the proper procurement and usability of porous carbon is critical for future development of electrode materials.¹⁰⁰ Porous carbon compounds that can be derived directly from biomass have a significant benefit over other forms of electrode materials. All organic stuff, including all living things such as animals, plants, microbes, excrement, and metabolites created by living things, are referred to as biomass.¹⁰¹ The biomass-based porous carbon structures are employed as energy devices.¹⁰⁰ Converting biomass into “nanostructured porous carbon” via various methods is the best way to create sustainable energy for storage and equipment, because porous carbon-based nanomaterials possess very high specific surface area, highly pore-based structure, flexible pore size, very stable chemical and physical properties, and high electrical conductivity along with outstanding cycling stability upon being employed as active materials in electrochemical applications.¹⁰²

Zhang et al.¹⁰³ synthesized an environmentally friendly carbon material utilizing seaweed crushed powder using a simple gas-phase reaction as shown in Fig. 16(a and b). Then an alkali activation (KOH) technique was used to enhance the number of porous sites on the carbon material's surface. The surface of the prepared carbon material (CS-2) possessed a distinctive 3D honeycomb structure as shown in Fig. 16(c), bearing enhanced specific surface area, along with exceptional electrical conductance employing KOH solution. The enhanced specific surface area of the carbon material provided large active moieties in electrolyte solution. The mesoporous moieties turned out to be highly electrochemically active structures of the carbon electrode material, possessing a huge specific capacitance value of around 448.3 F g⁻¹. The used CS-2 also exhibited excellent cycling stability with 91.39% retention in 3000 working cycles.

Fig. 16 (a) Diagrammatic representation of the CS-2 electrode preparation method, (b) TGA and DTG curves of CS-2, (c and d) SEM pictures of CS-2 before and after activation, (e) TEM pictures and (f) HRTEM pictures of CS-2, and the corresponding SAED pattern.¹⁰³ Copyright 2021. Reprinted with permission from American Chemical Society.

Sudhan et al. reported a biomass-derived rice straw based activated carbon material and performed its activation using potassium hydroxide at 600 °C in an argon inert chamber.¹⁰⁴ After KOH activation micropores and mesopores were developed, having an enhanced specific surface area of around 1007 m² g⁻¹ as detected by using the N₂ adsorption isotherm. SEM analysis described disordered pore formation on the carbon surface as shown in Fig. 17. The specific capacitance achieved was 332 F g⁻¹ and capacitance retention was nearly 99% after 5000 cycles.

Fig. 17 FE-SEM pictures of carbon materials made from rice straw (a and b) prior to and (c and d) following KOH activation.¹⁰⁴ Copyright 2017. Reprinted with permission from American Chemical Society.

Araichimani et al. produced feasible SiO₂ based on rice-husk which could be utilized as electrodes for supercapacitors.¹⁰⁵ The mesoporous nanomatrix had enabled the redox reactions which resulted in super-capacitive activity. The specific capacitance of the SiO₂ nanomatrix obtained was 216 F g⁻¹ at an operating current density (CD) of 0.5 A g⁻¹. PANI− BAWDC (boron assisted/doped wood derived carbon) was synthesized by Liu et al.¹⁰⁶ The PANI−BAWDC nanocomposite had 421 F g⁻¹ capacitance at 10 mV s⁻¹. This type of research investigation will strengthen the efforts of researchers to produce more effective super capacitor-based devices.

3.2. Mesoporous transition metal oxides

Mesoporous transition metal oxides (TMOs) are considered as active materials for pseudo-capacitors. The huge specific capacitances are due to fast redox reactions between electrode materials and electrolytes, which results in fast ion and electron transport within the electrode and along the electrode–electrolyte interface.¹⁰⁷

Ruthenium(IV) oxide (RuO₂) happens to be the most sought-after electrode material by virtue of its high specific capacitance amounting to around 700 F g⁻¹, less resistance to charge transfer, and good chemical and thermal stability. RuO₂ is utilized as an electrode, due to the large variation of oxidation states among Ru²⁺, Ru³⁺, and Ru⁴⁺ which mainly contributes to the pseudo-capacitance.¹⁰⁸ Similarly, by virtue of the given theoretical capacity, affordable cost, and variable oxidation states of nickel, NiO also been demonstrated as an important pseudo-capacitor material. Wu et al.¹⁰⁹ used a sol–gel process in conjunction with a supercritical drying technique to form highly porous NiO. They prepared Ni(OH)₂ moieties prior heating to produce NiO which resembled aerogels, having a porosity of 80–90% and a high surface area. The average specific capacity amounted to 125 F g⁻¹. Javed et al.,¹¹⁰ synthesized a two-metal oxide (ZnCO₂O₄) based carbon-cloth composite (ZCO@CC) of nanobelt shape with an electrode by a simple hydrothermal technique as shown in Fig. 18(a) for hybrid supercapacitors. The ZCO nanobelts have equally covered the carbon cloth and formed a hierarchical nanostructure which successfully increases the surface area of the material which is clearly observed from SEM images as shown in Fig. 18(b–d).

Fig. 18 (a)The creation of mesoporous ZCO nanobelt intercalated carbon cloth with post-heating treatment, (b) SEM image of bare-carbon cloth, (c and d) a low-resolution SEM image of ZCO nanobelts on CC and their cross-sectional view, and (e) a high-resolution SEM image of ZCO@CC.¹¹⁰ Copyright 2020. Reprinted with permission from Elsevier.

With a specific capacitance of 1197.14 F g⁻¹ and a current density of 2 A g⁻¹, the ZCO@CC electrode had excellent electrochemical properties. At a current density of 10 A g⁻¹, it demonstrated a good rate-specific capability of around 75.18 percent. Narasimharao et al. in their research showed that the hierarchical interlocking of nanosized metal oxides can result in higher morphological boundaries which can offer huge surface area for significantly larger diffusion of ions. They synthesized mesoporous nickel titanate (NTO) rods having a hexagonal shape with an average diameter of less than one micro meter. The electrode material exhibits a great retention of 91%, an excellent specific capacitance of 542.26 F g⁻¹ and high stability up to 2100 cycles. Research opened a new avenue for investigating nano-hetero-architectures with different materials such as Ni/Mn/Cu-oxides for hybrid supercapacitor electrode materials.¹¹¹

When employed as supercapacitor electrode materials, crystalline metal hydroxides and oxides have often been found to be preferred over amorphous or low-crystalline structures. However, Chengxiang Sun et al. reported a straightforward and scalable method for creating amorphous FeOOH nanoflowers@multi-walled carbon nanotubes (FeOOH Nfs@MWCNTs). These self-assembled amorphous FeOOH nanoflowers on MWCNTs have a distinctive heterostructure with a very low-crystalline character (Fig. 19). Because of its distinct structural characteristics, the material demonstrated an impressive rate performance of 167 F g⁻¹ at 11.4 A g⁻¹ and a specific capacitance of 345 F g⁻¹ at pH 8.¹¹²

Fig. 19 (a) Schematic depiction of the FeOOH @MWCNT composite, (b) SEM image of FeOOH Nfs@MWCNTs, (c) diagrammatic representation of layering of the electrode material and (d) GCD curves of FeOOH Nfs@MWCNTs at different pH values.¹¹² Copyright 2020. Reprinted with permission from American Chemical Society.

Kumar et al.¹¹³ reported a NiCo₂O₄ composite which was like a honeycomb; made on Ni foam using the combustion technique providing a binder free electrode material for the supercapacitor. Heat evolved during combustion strongly influences the complexation tendency of glycine, nickel and cobalt nitrate-based precursors used during synthesis resulted in a NiCo₂O₄ structure on Ni foam. NiCo₂O₄ electrode performance was influenced by amounts of nitrate and glycine used in the process, and the specific capacity of the composite appeared to be 646 F g⁻¹ at a rate of 1 A g⁻¹.

3.3. Metal–organic frameworks (MOFs)

Metal–organic frameworks (MOFs) are highly porous materials composed of metal ions or clusters organised with organic linkers, forming intricate three-dimensional structures. Their unique characterstics, such as large surface area, tunable pore sizes, and adjustable chemical functionality, make MOFs promising materials for energy storage devices, including supercapacitors. For supercapacitor applications, MOFs serve as active materials or templates for the synthesis of hybrid or composite materials. Their high porosity ensures efficient ion diffusion and transport, which is crucial for achieving high capacitance. Additionally, the modular nature of MOFs allows for functionalization with different metal ions and organic linkers, enabling control over the electronic properties. This flexibility can lead to enhanced electrochemical performance, such as higher energy density, better cycling stability, and faster charge–discharge rates.

To understand this in detail Chhetri et al. presented an excellent review on Zeolitic Imidazolate Framework (ZIF)–(ZIF-8 and ZIF-67) intercalated hybrid nanocomposites for supercapacitors.¹¹⁴ ZIFs present a promising opportunity in this area. The porous and hollow design of ZIF-derived metal composites facilitates electrolyte ion diffusion, thereby improving reaction kinetics. When combined with materials like graphene, reduced graphene oxide, MXenes, or carbon nanotubes/rods, their cycling stability is further enhanced. Incorporating conjugated polymers, which enhance electrical conductivity and make ZIF surfaces accessible to electrolytes, has proven to be a more effective approach for boosting the electrochemical performance of ZIFs.

Another research study conducted by Pathak et al., developed the Ti₃C₂T_x MXene-decorated hollow carbon fiber (MX/HCF) with nanorod arrays of vanadium cobalt phosphide (V–CoP@MX/HCF) grown on its surface, and utilized as a positrode for supercapacitors.¹¹⁵ The study shows that V–CoP@MX/HCF outperforms CoP@MX/HCF and CoP@HCF in terms of electrochemical results (1899 F g⁻¹). ZIF-67 placed on electrospun polyacrylonitrile (PAN) fibers is converted into Co-CNT@CNF for the cathodic electrode by a straightforward thermal process. The freestanding Co-CNT@CNF exhibits a specific capacitance of 407 F g⁻¹ and excellent cycling stability.

By using Ti₃C₂T_x, MXene-decorated MOF (MX-5@PCNF)– porous electrospun with carbon nanofibers as an electrode material for supercapacitors was fabricated by the Kisan group as shown in Fig. 20(a).¹¹⁶ The FESEM image of MX-5@PCNF is shown in Fig. 20(b). The prepared MX-5@PCNF exhibits 574 F g⁻¹ specific capacitance at 1 A g⁻¹, with excellent cycling stability and 97% capacitance retention after 10 000 cycles. Additionally, flexible symmetric and asymmetric devices prepared by using MX-5@PCNF (Co₃O₄@NF//MX-5@PCNF) produce high energy densities of 24 Wh kg⁻¹ and 75 Wh kg⁻¹, respectively as shown in Fig. 20(c and d). To understand the key fabrication methods, structure features, properties, and performance metrics of various mesoporous nanomaterials and nanohybrids in supercapacitor applications, Table 2 provides the detailed comparative review studies.

Fig. 20 (a) Schematic diagram for synthesizing MX-5@PCNF, (b) FE-SEM image, (c) CV curve, and (d) GCD curve for MX-5@PCNF.¹¹⁵ Copyright 2023. Reprinted with permission from Royal Society of Chemistry.

Table 2 Comparative study indicating the fabrication-structure–property-performance relationship of various mesoporous nanomaterials and nanohybrids in supercapacitor applications

Material	Fabrication method	Structure	Properties	Performance	Specific capacitance	Ref.
Mesoporous carbon	Template-assisted synthesis	High surface area, interconnected mesopores, and tunable pore size	High electrical conductivity and stable structure	High specific capacitance and good cycle stability	410 F g^−1 to 500 F g^−1	117 and 118
Mesoporous silica	Sol–gel method and etching	Uniform mesopores, large pore volume, and silica framework	Moderate conductivity and chemically stable	Moderate capacitance, improved with conductive additives	506 F g^−1 to 613 F g^−1	119 and 120
Carbon/silica nanohybrids	CVD on mesoporous silica	High surface area, carbon-coated mesopores, and tunable pore structure	Enhanced conductivity and stable under mechanical stress	Improved capacitance over silica alone and good rate capability	326 F g^−1	121
Mesoporous metal oxides	Hydrothermal and sol–gel	High porosity, nanoscale metal oxide particles, and tailored pore diameter	High specific surface area and pseudocapacitive properties	High capacitance and moderate cycle stability	1061 F g^−1	122 and 123
Carbon/metal oxide hybrids	Co-precipitation and in situ hybridization	High conductivity carbon framework and embedded metal oxide nanoparticles	Enhanced conductivity and capacitance, and tunable porosity	High specific capacitance and improved cycle life	1051 F g^−1	124 and 125
Conductive polymer hybrids	Electrochemical polymerization and in situ doping	Mesoporous structure and polymer coating on nanomaterials	High conductivity, flexible, and stable in electrochemical environments	High capacitance and excellent cycle stability	115.8 Fg^−1 and 608.3 Fg^−1	126–128
Carbon nanotube hybrids	Chemical vapor deposition and self-assembly	High aspect ratio, mesoporous network, and aligned or random CNT structures	Excellent conductivity and large surface area	High rate performance and moderate capacitance	269.3 mF cm^−2	129 and 130
Metal organic frameworks (MOF)	Solvothermal synthesis and template etching	Large surface area, tunable mesoporosity, and highly ordered framework	Low conductivity and highly porous	Low capacitance on its own, enhanced with conductive composites	1952 mF cm^−2	131 and 132
MXene-based nanohybrids	Layered synthesis, etching, and hybridization	Layered structure, tunable interlayer distance, and mesoporous architecture when hybridized	High conductivity and chemically tunable surface	High capacitance and excellent rate capability	1190 F g^−1	133–135
Graphene-based hybrids	Sol–gel, hydrothermal, and chemical reduction	High surface area, mesoporous, and few-layered graphene structure	High conductivity, flexible, and stable under mechanical deformation	Very high capacitance, long cycle life, and fast charge/discharge	1700 F g^−1	136 and 137

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4. Mesoporous nanohybrids for supercapacitors

Presently, binary and ternary transition metal oxides show excellent electrical conductance and electrochemical characteristics in comparison to single metal oxides and show potential to be chosen as remarkable superconductors. Ru, Ni, Mn, and their oxides, aerogels, CNTs, etc have been utilized for this purpose.

4.1. Carbon nanomaterials and nanohybrids co-doped with nitrogen/phosphorus

Generally, carbon materials show great potential for supercapacitors, but their energy densities are typically unsatisfactory. These carbon-based materials usually have low capacitance due to which they are not suitable for making energy storage devices.¹³⁸ To overcome these drawbacks, heteroatom doping is an effective technique to improve electrochemical characteristics in carbon-based electrodes,¹³⁹ because the utilization of n-type dopant elements and/or p-type dopant moieties in the carbon structure results in increased capacitance. Nitrogen doping boosts the electrical conductivity of carbon architectures and produces more pseudo-capacitance for supercapacitors.¹⁴⁰ P-type doping facilitates carbon wettability aspects and enhances electrochemical stability of the electrode, along with excellent cyclability characteristics.¹⁴¹

4.1.1. Phosphorus and nitrogen co-doping. The supercapacitor properties of N and P-doped carbon electrodes get enhanced due to the co-doping.¹⁴² Zhang et al.¹⁴³ synthesised N–P co-doped carbon microspheres which showcased enhanced electrochemical properties. Melamine formaldehyde was adopted as a carbon and nitrogen source in this study, while the 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) catalyst was employed as a phosphorus source. Also, because of the N and P co-doping, carbon microspheres showed good cycling stability and a high specific capacitance of 200 F g⁻¹ at a density of 0.5 A g⁻¹. To meet the requirements for high capacitance performance and exceptional rate capability, a hierarchical porous structure with micro or mesopores may be beneficial. Zhao et al.¹⁴⁴ prepared nitrogen/phosphorus co-doped graphene (NPG) (Fig. 21) which not only enhances defects and active sites that are available, but also prevents graphene agglomeration and improves material performance. The as prepared NPG electrode in 1 M ZnSO₄ electrolyte achieved good electrochemical performance. A specific capacitance value of around 210.2 F g⁻¹ was obtained with a current density of 0.5 A g⁻¹ along with 94.6 Wh kg⁻¹ energy per kg at a rate of 449.9 W kg⁻¹ in the 0–1.8 V range.

Fig. 21 (a) Schematic depiction for the preparation of the NPG composite, (b) representation of doping in the composite, (c) electrochemical characteristics of Zn/ZnSO₄//NPG-0.75 ZHSC, (d) GCD curves at 0.5–5 A g⁻¹, (e) Ragone plot, (f) SEM image of the nanocomposite and (g–i) elemental mapping of C, N and P.¹⁴⁴ Copyright 2022. Reprinted with permission from Elsevier.

Another approach by Devarajan, J et al. explained the excellent supercapacitor applications of nitrogen/phosphorous doped carbon nanotubes.¹⁴⁵ The study reveals that N, P/CNT co-doping achieved a good specific capacitance of approximately 359 F g⁻¹ at 0.5 A g⁻¹ current density. Its electrochemical characteristics were further enhanced by constructing a symmetric supercapacitor, which exhibited a specific capacitance of 109 F g⁻¹ at 0.5 A g⁻¹, in addition with a 15 Wh kg⁻¹ energy density and a 250 W kg⁻¹ power density. The device demonstrated an energy efficiency of 93% over 6000 charge–discharge cycles.

Panda and colleagues synthesized P and N doped activated carbon (PCN-x), which has many micropores and is appropriate for gas adsorption and energy storage applications, using a solvothermal technique.¹⁴⁶ In an acidic and alkaline medium, the produced P and N doped porous carbon (PCN-800) operating at 800 °C showed a high specific capacitance (Cs) of 576 F g⁻¹ and 478 F g⁻¹ at 1 A g⁻¹ current, respectively. Additionally, it displayed a notable H₂ storage capacity of 3.3 wt% at 78 K and 1 bar pressure.

A study by Wang et al. demonstrated an N–P doped biomass carbon material (RM@NP) which shows a good surface area of 3768 m² g⁻¹ with a meso- and microporous structure.¹⁴⁷ RM@NP has relative mass fractions of 2.3% and 0.19%, respectively, indicating that it is efficiently doped with phosphorus and nitrogen. The prospects of carbon material-based electrodes for supercapacitor applications were evaluated through electrochemical tests. In a three-probe system, RM@NP achieved a specific capacitance of 461 F g⁻¹ at 1 A g⁻¹ in 6 M KOH, and 345 F g⁻¹ at 9 A g⁻¹, with a capacitance retention rate of 75.0%. In a two-electrode system, RM@NP demonstrated a specific power of 626 W kg⁻¹ when the specific energy was 10.1 Wh kg⁻¹.

4.1.2. Boron co-doping. As discussed in the above section the doping strategy can tune the electrochemical characteristics of the carbon materials. When carbon materials are doped with boron, which induces the p-type conductivity, along with additional heteroatoms like nitrogen and phosphorous it leads to encouraging results for the fabrication of materials which have significant energy storage capabilities. Xiaoyu Ren et al.¹⁴⁸ synthesised a nitrogen/boron-doped carbon composite for supercapacitor electrode materials. MWCNTs along with dopants produced an electrode material which exhibits remarkable electrochemical properties. A high specific surface of 1500.5 m² g⁻¹ and 0.86 cm³ g⁻¹ pore volume helped the material to exhibit a specific capacitance of 432.31 F g⁻¹ and stability of 95% up to 3000 cycles. Ghotbi et al.¹⁴⁹ prepared phosphorus-doped as well as boron-doped porous carbon-based nanostructures (Fig. 22).

Fig. 22 (a) Schematic representation of the synthesis of layered Zn gallate phosphate and (b) Zn gallate, and GCD graph of (c) non-doped (d and e) boron-doped electrodes.¹⁴⁹ Copyright 2021. Reproduced with permission from Elsevier.

Phosphorus, boron doped carbon-based electrodes showcased remarkable capacitance amounting to 323 and 276 F g⁻¹ corresponding to a current density value of 1 A g⁻¹ respectively. Major challenges during the synthesis of the advanced carbon material-based supercapacitor are flexibility, porosity, and large mass density because one property affects the other. To tackle this problem Lina Ma et al. fabricated a high-density porous carbon material by high thermal pyrolysis followed by acid treatment, which has high B, O, N, and P doping (tetratomic-doping). The one-step pyrolysis of B, N/P, O dopants (B₄Na₂O₇·10H₂O and (NH₄)₂HPO₄) and precursors along with rGO at 900 °C under an inert atmosphere of N₂ leads to carbon fibers doped with highly conductive 3D porous heteroatoms. The final B, O, N, P–CNF/RGO/BC electrode materials show an impressive power density (2.08 mWh cm⁻³) and high-power density of 498.4 mWh cm⁻³, excellent capacitance of 118.7 F cm⁻³ and high capacity retention of 99.6% up to 20 000 cycles as shown in Fig. 23.¹⁵⁰Table 3 shows the details of carbon materials doped with phosphorus, nitrogen and boron for supercapacitors.

Fig. 23 (a) Schematic diagram for the synthesis of O, B, N, P–CNF/RGO electrode materials, (b) GCD curves, (c) cycling stability at 50 mA cm⁻² and (d) Nyquist plot and related circuit.¹⁵⁰ Copyright 2020. Reprinted with permission from American Chemical Society.

Table 3 Carbon nanomaterials doped with phosphorus, nitrogen and boron for supercapacitor applications

Sr no.	Sample material	Electrolyte used	Current density (A g^−1)	Specific capacitance (F g^−1)	Ref.
1	P-doped carbon	1 M-KOH	1	322	149
2	Carbon doped with- B, N, P	1 M H2SO4	0.55	519	151
3	Carbon doped with- B, N	6 M-KOH	0.56	330	152
4	N, P-doped carbon	2 M-KOH	1	318	153
5	B-doped carbon	6 M-KOH	0.5	377	154
6	B, N-Doped carbon	1 M-H2SO4	1	423	155
7	N, P- doped carbon	6 M-KOH	1	232	156
8	N, P, S- doped carbon	6 M-KOH	1	474	157
9	N- Doped carbon	6 M-KOH	1	284	158
10	N, S-doped carbon	2 M-KOH	0.5	285	159

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4.2. Transition metal oxides- nanohybrid strategy

Mixed transition metal oxide electrodes have sparked widespread interest because of their superior electrochemical performance due to their multiple oxidation states compared to individual transition metal oxide electrodes. The TMOs exhibit superior electrochemical characteristics when evaluated with carbon-based moieties.¹⁶⁰ Moreover, the mixed transition metal oxides possess much higher capacitance in comparison to the single metal oxides. They possess better electronic conductivity, and show higher pseudo-capacitance, which have attracted much attention. Recently, Marry et al.¹⁶¹ synthesised a NiCo₂O₄@ZnCo₂O₄ binary composite material by using a simple hydrothermal method. It was discovered that the obtained hybrid structure consists of a ZnCo₂O₄ nanosheet base and urchin (NiCo₂O₄) had a higher specific capacity owing to the composite material's synergistic effects viz particle size, specific surface area and pores size, which could facilitate electron conduction. They used this composite for supercapacitor application and the electrode exhibited a maximum capacitance of 1029C g⁻¹ with a current density of 1 A g⁻¹ when utilized with 2 M KOH solution. Later, Marry et al.¹⁶² reported an rGO/NiCo₂O₄@ZnCo₂O₄ ternary composite prepared similarly with a simple hydrothermal method. Both the electrostatic and electrochemical charge storage mechanisms are obtained concurrently in the triple major composite. Thus, the performance is enhanced by rGO with the arrangements of pores and the specific surface area. High surface area makes it easier for the electrode and electrolyte ions to make contact, which leads to a high capacitance value. Thus, the ternary composite rGO/NiCo₂O₄@ZnCo₂O₄ presented a specific capacitance of 1197C g⁻¹ at 1 A g⁻¹. The synthesis methods and properties of metal oxide-based composites are shown in Table 4.

Table 4 Preparation techniques and characteristics of metal oxide intercalated composites as electrode materials for supercapacitors

Sr no.	Material	Preparation method	Electrolyte	Specific capacity (F g^−1)	No of cycles	Retention (%)	Ref.
1	Al2O3–ZnO composite	Solvent-precipitation method	1 M Na2SO4	463.7	5000	96.9	163
2	FeCo2O4/PANI nanocomposite	Sonication method	6 M KOH	658.9	3000	92	164
3	rGO/FeNiS2 composite	Hydrothermal method	2 M KOH	1013	10 000	90	165
4	NiMoO4@NiS2/MoS2 nanowires	Hydrothermal method	6 M KOH	970	5000	65.3	166
5	Fe-intercalated rGO	Solvent precipitation method	2 M KOH	896	8000	85	167
6	CuCo2S4@NiCo(OH)2 composite	Hydrothermal method	3 M KOH	2340	2000	92	168
7	NiCo2S4/NiMoO4 nanohybrid	Hydrothermal method	PVA/KOH solution	2323	10 000	90	169
8	MWCNT/CuCo2O4 composite	Precipitation method	2 M KOH	1053	10 000	93	170
9	ZnO/NiO@MWCNT composite	Hydrothermal method	0.1 M K4[Fe(CN)6]	1988.8	2000	97.2	171
10	NiCo2O4/CeO2 composite	Sol–gel combustion method	2 M KOH	1355	6000	95.3	172

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5. Future perspectives and challenges

Mesoporous nanomaterials and nanohybrids have drawn the attention of researchers due to their exceptional porous architecture which plays a vital role in the development of supercapacitor applications. Optimizing the porosity structure of various mesoporous nanomaterials is essential for maximizing the synergistic benefits of each component. As far as we know, hierarchical porous carbon, designed with micro-, meso-, and macropores, is highly desirable. This structure offers several benefits in addition to a high surface area for charge storage: numerous ion adsorption sites are provided by micropores, whereas mesopores and macropores speed up ion diffusion into interior areas, increasing the pace of electrochemical reactions and producing large power densities. The fabrication of mesoporous materials typically involves processes like calcination or chemical etching, which can create surface defects. These defects can be mitigated through subsequent surface modification, particularly in large-scale production. Once an optimal mesoporous structure with excellent electrochemical performance is achieved, it can serve as an ideal scaffold for constructing hierarchical hybrids by incorporating other materials. This method can significantly improve supercapacitors' performance, resulting in high energy and power densities. The potential of mesoporous nanomaterials and nanohybrids to enhance supercapacitor performance—particularly in striking a balance between high energy and power densities—has drawn a lot of interest. Recent advancements reveal that mesoporous carbons, metal oxides, and conductive polymers exhibit remarkable electrochemical performance, with specific capacitances exceeding 500 F g⁻¹ in some cases. The integration of nanohybrids, combining complementary materials such as carbon and transition metal oxides, further enhances performance by leveraging synergistic effects, improving conductivity and stability. For instance, mesoporous graphene-MnO₂ hybrids have demonstrated capacitances of over 400 F g⁻¹ with excellent cycling stability, attributed to the efficient ion transport within the mesopores. Similarly, mesoporous silica coated with RuO₂ has shown enhanced capacitance retention at high current densities. Emerging research also highlights the role of pseudocapacitive materials like NiCo₂O₄ within mesoporous frameworks, achieving high energy densities of up to 80 Wh kg⁻¹ while retaining fast charge–discharge rates.

Nevertheless, despite their great potential, several obstacles still need to be overcome before these materials can be developed and widely used in supercapacitor technologies. Addressing these challenges will shape the future perspectives of this field. Future research will likely focus on creating more advanced synthesis techniques to produce hierarchical architectures with precise control over pore size and distribution, which will improve performance. Furthermore, investigating novel materials that combine metal oxides, sulfides, or nitrides with the benefits of conventional carbon-based materials is a viable way to increase energy density without compromising power density.

Another area of significant interest is the integration of nanohybrids, which combine different nanomaterials to exploit their complementary properties. For instance, carbon-based materials with excellent electrical conductivity can be hybridized with metal oxides or conducting polymers to enhance capacitance while maintaining fast charge–discharge capabilities. The challenge here is to create a stable interface between different materials without compromising structural integrity or conductivity. To produce well-defined nanohybrids, advanced nanofabrication techniques such as atomic layer deposition, chemical vapor deposition, and self-assembly approaches are anticipated to be used more frequently. Additionally, combining 2D materials like MXenes with mesoporous structures offers a new horizon for high-performance supercapacitors due to MXenes' high conductivity and surface area.

Despite the potential of mesoporous nanomaterials and nanohybrids, scaling up these materials for commercial applications remains a critical challenge. Many current fabrication techniques are complex, expensive, and not easily scalable for mass production. The high cost of raw materials, particularly when incorporating rare or expensive elements like ruthenium in metal oxides, is another barrier to widespread adoption. Future research must concentrate on creating more scalable and affordable production processes, including solution-based methods or green synthesis strategies that make use of cheap, plentiful ingredients. Additionally, improving the stability and longevity of these materials in practical devices is crucial, as supercapacitors must retain their performance over thousands of charge–discharge cycles. The mesoporous nanomaterials and nanohybrids represent a promising avenue for advancing supercapacitor technology, with the potential to significantly enhance energy and power densities. But in order to fully realize their promise in commercial applications, it will be imperative to overcome the obstacles of scalable production, material stability, electrolyte compatibility, and environmental effects. The next generation of high-performance supercapacitors will be unlocked by the integration of cutting-edge materials, creative production processes, and sustainable practices as this field of study develops. Fig. 24, presents the challenges and future perspective of the present discussion.

Fig. 24 Diagrammatic representation of challenges and future perspectives of mesoporous nanostructured materials and nanohybrids for supercapacitor applications.

6. Conclusions

The principles of charge storage mechanisms in supercapacitors employing mesoporous nanomaterials and nanohybrids are highlighted in this review paper, along with its revolutionary potential in energy storage technology. The large surface area and permeable architecture of mesoporous nanomaterials facilitate efficient ion transport and enhanced electrochemical interactions, leveraging both electric double-layer capacitance (EDLC) and pseudocapacitance for superior energy storage. Nanohybrids, by integrating the properties of diverse materials like carbon nanostructures with metal oxides or conducting polymers, offer synergistic benefits that greatly increase supercapacitor performance. The combination of EDLC and faradaic processes in these materials results in higher energy and power densities. Future research efforts should aim at refining synthesis methods, optimizing material interfaces, and ensuring scalable production to translate these advanced materials from the laboratory to real-world applications. As advancements continue, mesoporous nanomaterials and nanohybrids are poised to play a pivotal role in the development of next-generation supercapacitors, addressing the growing need for effective, high-capacity, and reliable energy storage applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data underlying the results and presented in the article are available as part of the article and no additional source data are required. All the figures and tables taken from different articles are used with the copyright permission.

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