MXene-Based Heterostructure Nanocomposites for Next-Generation Supercapacitors

Supercapacitors are increasingly becoming a crucial component of energy storage, serving as a bridge between traditional capacitors and rechargeable batteries. They have fast charge/discharge, high power density, and long cycle life, which is appealing in many applications such as portable electronics, electric vehicles, and grid-level energy management. Nonetheless, there is one major obstacle to this, namely, reaching greater energy density without compromising power capability or stability.

Supercapacitor electrode materials MXenes Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides have precipitated immense interest as electrode materials in supercapacitors. Their high rate of conductivity, tunable surface chemistry, and layered morphology make them specifically favorable to high-rate energy storage. However, perfect MXenes do not exist; they have problems with restacking the layers, a small ion-accessible surface area, and oxidation. To address these drawbacks, scientists are progressively resorting to heterostructure nanocomposites based on MXene. Through forming smartly engineered architectures of MXenes and complementary materials, one can enable synergies and bring supercapacitor performance much more in line with practical needs.

 1. What Are MXenes?

MXenes are a broad family of 2D materials typically represented as Mn+1XnTx, with M an early transition metal (Ti, V, Nb, Mo), X being carbon and/or nitrogen, and Tx denoting surface terminations (-O, -OH, -F, -Cl, etc.). They are normally formed by means of selective etching of the A-layer of their 3D MAX phase precursors (Mn+1AXn) with A being an element (such as Al or Si). The etching process eliminates the A element, which is replaced with stacked 2D layers that can be further delaminated to single- or few-layered nanosheets.

In the case of supercapacitors, the MXenes have several inherent benefits:

• Good electrical conductivity, comparable to or even greater than that of most metals.
• Polar terminations are hydrophilic, and the surface can be in close contact with aqueous electrolytes.
• Multiply layered structures whose spacing between layers can be adjusted and in which an ion between the sheets may be present.
• Great density of the sites on the surface that can hold the pseudocapacitive charge.

These qualities enable MXenes to facilitate redox reactions on surfaces as well as double-layer capacitance. Nevertheless, their potential cannot be achieved without the introduction of additional structural and compositional engineering.

2. The rationale behind MXene-based heterostructures is as follows.

MXenes are promising but have several inherent limitations on their use:

• Restacking and aggregation: Interlayer repulsive forces between MXene sheets are prone to a collapse configuration, which decreases the ion transportation routes and available surface area.
• Limited porosity Stacked films may become dense and diffusion-limited, particularly in high mass loading.
• Instability with chemistry: Most MXenes are unstable to oxidation in aqueous or during protracted storage.

The heterostructure nanocomposites based on MXenes are designed to overcome these problems by incorporating MXenes and other functional materials. MXene nanosheets are combined with carbon materials, conducting polymers, metal oxides, chalcogenides, metal-organic frameworks (MOFs), or layered double hydroxides (LDHs) in a heterostructure. The main point is to architect the systems in such a way that the parts are complementary to each other:

• Secondary phases may also serve as spacers or pillars that inhibit MXene restacking.
• Other redox-active materials can add an additional pseudocapacitance.
• Porous networks and hierarchies may also be advantageous to the ion diffusion and accessibility of electrolytes.
• Strong structures can enhance mechanical integrity and long cycle life.

Simply put, heterostructuring converts the MXenes from potent single-crystal exquisite constructs into versatile building blocks in the engineered energy storage frameworks.

3. Large Overviews of MXene-Based Heterostructures of Supercapacitors.

3.1 MXene-Carbon Heterostructures.

Graphene, reduced graphene oxide, carbon nanotubes, and porous carbons are carbon-based materials that are natural partners of MXenes. Carbon is usually characterized by high surface area, chemical stability, and good electrical conductivity, whereas MXenes are characterized by metallic-like transport and rich surface chemistry. In heterostructures of MXene and carbon:

• Carbon structures serve as conductive pads to, and between, layers of MXene and mitigate restacking.
• Hierarchical pore improves electrolyte penetration and ion diffusion micropores, mesopores, and macropores.
• The network of conductive activities is combined to enable rapid transportation of electrons despite being loaded with bulk mass.

They typically have a higher rate of performance, higher accessible capacitance, and mechanical flexibility and can be used not only with traditional coin cells but also with a flexible and wearable supercapacitor.

3.2 Conducting Polymer Composites based on MXene.

Polymers like polyaniline (PANI), polypyrrole (Ppy) and PEDOT-based systems are conductors where charge is stored primarily by fast redox reaction along the conjugated backbones. They are able to provide high pseudocapacitance but are very susceptible to mechanical instability and degradation with repeated cycling.

When integrated with MXenes:

• The polymer can be interposed upon sheets of MXene as a space and ion-conductive bridge.
• MXenes offer very high-level conductivity channels that counteract the conductivity of certain polymers, which is relatively low.
• The MXene scaffold can buffer mechanical stresses the polymer undergoes during the swelling and shrinkage.

Consequently, MXene polymer heterostructures are capable of providing high capacitance values with better rate and cycling performance than polymer-alone electrodes.

3.3 MXene-Metal Oxide and Chalcogenide Heterostructures.

Pseudocapacitive materials, the transition metal oxides and chalcogenides, are particularly studied because they have several oxidation states available. Most of them are however inherently poor electronic conductors thus limiting their rate performance. In heterostructures of MXene and metal oxides or MXene-chalcogenides:

• MXene is a conductor and collector backbone used in minimizing the resistance to charge- transfer by a large margin.
• Coating oxide or chalcogenide nanostructures, such as nanorods, nanosheets, or nanoparticles, on the MXene surface or directly on it, provides a large number of redox-active sites.
• Intimate interfacial contact facilitates fast transportation of electrons and ions at the heterointerface.

Such composites are especially appealing to asymmetric supercapacitors in which one of the electrodes is designed to have high pseudocapacitance and the other electrode to have electrical double-layer storage and long-term stability.

3.4 MXene, MOF, LDH, and other hybrid structures.

Other than the simplest binary systems, MXenes can be employed in combination with more complicated architectures such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and layered double hydroxide (LDHs). These materials are usually characterized by high surface areas, sharp porosity, and tunable chemistry. In such hybrids:

• MOFs or LDHs can be either directly grown on MXene surfaces or assembled in 2D/2D layers.
• The frameworks bring about excessive ion-accessible channels and spots, and MXenes guarantee a network of continuous conductivity.
• Post-treatment (i.e., pyrolysis of MOF) of the MXene can result in doped porous carbons or mixed metal oxides on MXene, enhancing the electrochemically active surface further.

Such multicomponent heterostructures demonstrate the ability of MXenes to provide a flexible platform allowing the creation of hierarchical electrodes to be customized to meet a set of performance objectives.

5. Electrochemical Performance and Advantages.

The MXene-based heterostructure electrodes have also exhibited outstanding electrochemical behavior in different supercapacitor types. Typical advantages include:

Improved specific capacitance: The dual-layered storage on MXene surfaces, together with pseudocapacitance due to secondary materials in most cases, results in a higher capacitance than either.

Improved rate capabilities: although with high-conductivity MXene backbones, pore structures may be charged and discharged fast even when the current densities are high.

• High volumetric capacitance in well-designed films: the well between MXene layers can be retained, yet the openings are ion-exchangeably available, allowing it to be used with high performance in a small volume where compact and on-chip devices are necessary.
• Long cycle life: When subject to controlled mechanical and chemical degradation, the heterostructures made of MXenes can be reused tens of thousands of times for charge-discharge with only a relatively small loss of capacity, by comparison with the existing battery-like materials.
• Mechanical flexibility: The free-standing MXene-based films, fibers, and textiles allow flexible and wearable supercapacitors, thus opening opportunities to include them in smart textiles, wearable electronics, and soft robotics.

6. Difficulties and Future Projection.

Promotional innovations notwithstanding, there are several issues that MXene-based heterostructure supercapacitors have yet to resolve prior to the large-scale commercialization of these innovations:

Stability and oxidation: It is common that MXenes are usually susceptible to oxidation in water and wet air, and in that manner, it may result in a reduction in their electrical properties and electrochemical performance. There is an urgent need to develop protective mechanisms, e.g., encapsulation, controlled storage, or other electrolytes.

Scalable and benign to the environment synthesis: Traditional MXene is frequently based on the dangerous etchants. It is being actively developed, and greener, safer, and more scalable routes are underway, although more effort is needed before they can be put into practice in industry.

Restacking control without density loss: It is a fine balance between too much or too little restacking as well as high volumetric performance. The rational design of spacers, pore formers, and assembly strategies will be important.

Complex interfaces: As heterostructures become more complex (i.e., multicomponent MXene carbon polymer oxide systems), it is difficult to disentangle the contribution of each interface to the overall performance. Data-driven design requires advanced characterization and modeling that includes in situ/operando.

Integration on the device level: Half-cell measurements will not be sufficient anymore, though; symmetric and asymmetric or hybrid devices will be necessary to prove that it works in reality. This involves optimization of current collectors, separators, packaging, and scale-up of electrode fabrication.

In the future, heterostructure nanocomposites using MXenes will have a major contribution in the energy storage systems of the next generation. Their tunable chemistry, structural versatility, and excellent electrical properties provide them a rich palette in which to create supercapacitors capable of fulfilling the challenging needs of the flawless, electrified future.