Electrochemical Edge: MXenes Empowering Next-Gen Batteries and Hydrogen Evolution

Introduction

MAX phases and their derivatives, known as MXenes, have emerged as groundbreaking discoveries in advanced materials, showcasing vast potential in energy applications. These materials boast a unique combination of properties, encompassing high electrical conductivity, mechanical strength, and chemical stability, thus rendering them ideal candidates for energy storage and conversion technologies (Figure 1). Thanks to their superior performance and versatility, MXenes have been instrumental in driving significant advancements in battery technology and catalytic processes. Their attributes include enhanced battery capacity and cycle stability alongside customizable surface functionalities tailored for specific applications.

Read more about

• MXenes Structure and Discovery
• Properties of MXenes
• MXenes Battery Superchargers
• MXenes Hydrogen Boosters
• Conclusion
• References

MXenes Structure and Discovery

MAX Phases

MAX phases, first discovered in the 1960s, are named for their unique composition, consisting of an early transition metal (M), an A-group element (A), and either carbon or nitrogen (X).¹ These materials are characterized by a layered hexagonal structure, where 'n' layers of M-X are interleaved with a single layer of A, represented by the general formula M𝑛+1_n+1AX_n (with n = 1, 2, or 3).

MXenes

MXenes are derived from MAX phases through a selective etching process, typically involving the removal of the A-group element using hydrofluoric acid or other etchants.² This process results in two-dimensional (2D) materials with the general formula M𝑛+1_n+1X_nT_x, where T represents surface terminations (such as -OH, -O, or -F) that occur during synthesis.

MXenes inherit the metallic conductivity and layered structure from their MAX phase precursors while gaining additional functional properties due to their surface chemistry and high surface area. These unique characteristics make MXenes particularly attractive for advanced battery technologies and catalytic processes, positioning them as essential materials in pursuing sustainable energy solutions (Figure 1).

Properties of MXenes

MXenes exhibit a remarkable array of properties, making them suitable for various applications:

• Electrical Conductivity: MXenes maintain high electrical conductivity due to their metallic bonding, making them ideal for electrode materials in batteries and supercapacitors.³
• Mechanical Strength: Despite their 2D nature, MXenes are mechanically robust, offering high tensile strength and flexibility.⁴
• Chemical Stability: MXenes exhibit good chemical stability, particularly in aqueous environments, which is advantageous for energy applications.⁵
• Surface Functionalization: The presence of surface terminations enables tuning of MXenes' properties, enhancing their electrochemical performance and catalytic activity.⁶

MXenes Battery Superchargers

MXenes are utilized in various types of rechargeable batteries, including lithium-ion batteries (LIBs), alkali-ion (e.g., Na⁺, K⁺) storage batteries, multivalent-ion (e.g., Mg²⁺, Zn²⁺, and Al³⁺) storage batteries, and metal batteries. The specific MXene materials used in these applications include Ti₃C₂T_x, V₂CT_x, Mo₂CT_x, Ti₂CT_x, Ti₃CNT_x, Cr₂TiC₂T_x, W_1.33CT_x, Mo_1.33CT_x, Nb_1.33CT_x, and V_(2-x)CT_x, among others. ²  MXenes are used in different components of batteries such as anodes, cathodes, separators and even as conductive additives in composite electrodes, making them versatile and highly valuable for energy storage research. Mxenes can play a role in modifying the separator to prevent dendrite growth, extending battery-life.⁷

Lithium-Ion Batteries (LIBs)

In LIBs, MXenes, particularly Ti₃C₂T_x, have demonstrated high capacity, fast charging rates, and excellent cycle stability.² The large and tunable interlayer spaces facilitates rapid lithium-ion intercalation/de-intercalation, while the high conductivity ensures efficient electron transport. As documented by Gogotsi and Anasori, it was observed MXene as an anode in lithium-ion batteries (LIBs) can deliver at a rate of C/25, a capacity of 225 mAh g^-1. Additionally, MXenes have also been explored as cathodes, enhancing the overall conductivity and performance of the electrode materials (e.g., Ti₃C₂T_x).

Sodium-Ion Batteries (SIBs)

Based on current research progress, several MXenes have shown promising performance as anode materials in sodium-ion batteries (SIBs) ². Some of the best-performing MXenes include Ti₃C₂T_x, Ti₂C_0.5N_0.5T_x, Nb₂CT_x, 3D Ti₃C₂T_x, 3D V₂CT_x, and 3D Mo₂CT_x. These MXenes have demonstrated high specific capacities, excellent rate performance, and good long-term cycling stability, making them suitable candidates for application in SIBs. The benefits of using MXenes in SIBs include high electrical conductivity, ultrathin 2D structure providing a large specific surface area, ion transport ability, structural stability with large layer spacing, surface functionalization, and the ability to form composites, all of which contribute to improved capacity properties, rate performance, and cycle stability in sodium-ion batteries.

Zinc-Ion Batteries (ZIBs)

In ZIBs, MXenes serve as both cathode and anode materials. The presence of functional groups such as -OH on MXene surfaces enhances zinc ion adsorption and diffusion, leading to high performance in aqueous ZIBs. Studies have shown that V₂CT_x MXenes can achieve high specific capacities and long cycle life.⁸ Specifically, V₂CT_x MXenes used as cathodes exhibit excellent reversibility and capacity retention over extended cycling.

MXenes Hydrogen Boosters

MXenes have also emerged as efficient HER catalyst, a critical reaction for sustainable hydrogen production. Their high electrical conductivity and tunable surface chemistry enable effective catalysis at low overpotentials. Mxenes have shown remarkable HER activity and stability, offering a cost-effective, abundant, and tunable alternative to Pt-based catalysts. Tailoring MXenes to have specific surface functional groups, such as hydroxyl and oxygen terminations, significantly enhances their catalytic performance by improving conductivity and hydrophilicity, making them promising candidates for efficient hydrogen production. ⁹

Conclusion

The advent of MAX phases and MXenes has ushered in a new era of material science, with significant implications for energy applications. The unique combination of properties such as high conductivity, mechanical strength, and surface functionality makes MXenes particularly attractive for advanced battery technologies and catalytic processes like HER. As research continues, MXenes are expected to play a pivotal role in the development of next-generation energy storage and conversion systems. By incorporating MXenes into their research, scientists can leverage superior performance characteristics and versatility, enabling breakthroughs in energy technology and innovation. Their versatility in battery applications, ranging from anode and cathode materials to conductive additives, alongside their efficiency in catalytic processes, positions MXenes as key materials in the pursuit of sustainable energy solutions.