Review of MXene electrochemical microsupercapacitors

Abstract

The rapid development of miniaturized and wearable electronics has stimulated growing needs for compatible miniaturized energy storage components. Owing to their unlimited lifetime and high-power density, miniaturized electrochemical capacitors (microsupercapacitors) are considered to be an attractive solution for the development of these microelectronics, but they often depend on the choice of electrode materials and fabrication protocols for scalable production. Recently, a new family of two-dimensional transition metal carbides, carbonitride and nitrides (referred to as MXenes) has shown great promise in advanced microsupercapacitors with high energy and power densities. This was achieved thanks to the high pseudocapacitance, metallic conductivity and ease of solution processing of MXene. In this review, recent progress on MXene synthesis, microstructure design, and fabrication strategies of MXene microsupercapacitors are discussed, and their electrochemical performance is summarized. Further, we briefly discuss the technical challenges and future directions.

Introduction

The advent of fifth-generation (5G) communication in the fourth industrial revolution has fostered widespread research interest in the Internet of Things (IoTs) and sensor networks [1]. In these applications, a vast diversity of heterogeneous microsensors can connect and communicate with each other without network latency, disrupting various fields such as environmental monitoring, radio frequency identification, wearable electronics, and personal health care. Given these ubiquitous microsensors are usually discrete nodes that operate in independent wireless manners and sometimes are embedded in the human body, it is highly desirable to develop maintenance-free micro-/nano-scale power modules that are compatible with them.

One promising solution is to develop distributed energy sources that harvest energy from renewable sources such as solar, wind, thermal, or mechanical triggering/vibration [2,3]. To formulate such self-powered power units, an energy storage device is regarded as an essential short-term accumulator that captures charge from energy harvesting devices when microsensors are in stand-by mode and deliver continuous power to these microsensors when they are active. Microbatteries or thin-film batteries that store energy by the chemistry of lithium-ion intercalation are the most popular choice as the miniaturized power source [4], but they suffer from their limited lifetime and low power density, which means periodic maintenance and replacement need to be carried out. Besides, the low power density also limits their use in some applications where a high spike of current is required [5].

As an alternative to batteries, electrochemical capacitors (ECs), also known as supercapacitors, are energy storage devices that store charge by adsorption of electrolyte ions onto the surface of electrode materials or by pseudocapacitive faradaic reactions (between the surface of the electrode material and the ions in the electrolyte). The energy density of ECs is relatively low as compared with batteries, but still can be sufficient to power various electronics for hours (Fig. 1). An electrochemical capacitor has a theoretically unlimited lifetime (>10⁵ cycles) and delivers higher power densities as compared with batteries, which renders them better candidates as the energy storage component for microelectronics.

However, conventional ECs are too large for the microdevices, and the assembly methods of conventional supercapacitors are not compatible with the fabrication techniques in the microelectronic industry; this reality has fostered a great interest in the miniaturization of supercapacitors. The term “microsupercapacitor (MSC)” has been adopted to describe the supercapacitor devices which can be promisingly integrated with microelectronics, and it comes mainly in two flavors: thin-film electrodes with sandwich structure (thickness< 10 ​μm) or arrays of planar microelectrodes with microscale sizes in at least two dimensions [5,6]. The concept of MSC has recently been further extended to fiber-based electrodes with core-shell structure [[7], [8], [9]]. Among these architectures, the in-plane interdigital design offers a multitude of advantages over the others because of its shorter ion diffusion distance, more substantial exposure of the electrode materials to the electrolyte, and the ease of integration with other microelectronics.

Porous carbons with high surface areas have been widely used for the fabrication of the microsupercapacitors, these include carbide-derived carbon [10], onion-like carbon [11], activated carbon [12], photoresist-derived carbon [[13], [14], [15], [16]], carbon nanotubes (CNTs) [[17], [18], [19]], graphene [20,21], and laser scribed graphene [22]. Although these devices benefit from the high conductivity and large surface areas of the electrode material, they also deliver low energy densities as carbon materials store charges only through the adsorption of ions in the electrolyte. In order to overcome the low energy density limitations while maintaining high power densities, pseudocapacitive materials including conducting polymers [[23], [24], [25], [26]], transition metal oxides (RuO_2, MnO₂, MoO₃, Nb₂O₅, VN) [[27], [28], [29], [30], [31], [32], [33]], black phosphorous(BP) [34], Conductive 2D metal-organic frameworks (MOFs) [35] and transitional metal dichalcogenides [36] have been explored for the fabrication of the microsupercapacitor devices. However, the poor electronic conductivity of metal oxides/hydroxides and MOFs may lead to mediocre power and cycling performance.

Recently, a large group of early transition metal carbides, nitride or carbonitride (MXenes) have been identified as a new class of two-dimensional (2D) materials [37]. The general formula of MXene is M_n+1X_nT_x, where M stands for an early transition metal (Ti, Mo, Cr, Nb, V, Sc, Zr, Hf or Ta), X represents carbon or nitrogen, n is usually an integral number between 1 and 3, and T_x is the surface termination such as hydroxyl, oxygen or fluorine. These surface terminations render the hydrophilicity of MXenes and also have a significant influence on their Fermi level density of the states, thereby electronic properties [38,39]. MXenes are synthesized by selectively removing the “A” layer from their parent MAX phases, and “ene” was added to the last to show its 2D nature that is similar to graphene. Since its discovery in 2011 [40], MXenes have been particularly attractive as electrode materials for energy storage applications because of their unique structures including: (1) the inner conductive transition metal carbide layer that enables efficient electron transportation; (2) a transitional metal oxide-like surface that acts as active sites for fast redox reactions. Although there are more than 20 different MXenes that have been synthesized [[41], [42], [43], [44]], most research to date has been focused on titanium carbide (Ti₃C₂) for MSC application due to its ultrahigh conductivity (2.4 ​× ​10⁴ ​S ​cm⁻¹) [45] and volumetric capacitance (1500 ​F/cm³) along with ultra-high rate capability(10 ​V/s) in acidic media [46]. Besides their high volumetric capacitance, MXenes are particularly favorable for MSCs because of their 2D nature, which enhances the mechanical stability and shortens the ion diffusion paths between the positive and negative electrodes [47]. In this review, we present the latest developments in MXene-based microsupercapacitors, including electrode material design, different deposition/patterning techniques, and device architecture. Finally, challenges and perspectives of MXene-based MSCs are discussed.