Interlayer Structure Engineering of MXene‐Based Capacitor‐Type Electrode for Hybrid Micro‐Supercapacitor toward Battery‐Level Energy Density

Abstract

Micro‐supercapacitors are notorious for their low energy densities compared to micro‐batteries. While MXenes have been identified as promising capacitor‐type electrode materials for alternative zinc‐ion hybrid micro‐supercapacitors (ZHMSCs) with higher energy density, their tightly spaced layered structure renders multivalent zinc‐ions with large radii intercalation inefficient. Herein, through insertion of 1D core‐shell conductive BC@PPy nanofibers between MXene nanosheets, an interlayer structure engineering technique for MXene/BC@PPy capacitor‐type electrodes towards ZHMSCs is presented. Owing to simultaneously achieving two objectives: (i) widening the interlayer space and (ii) providing conductive connections between the loose MXene layers, enabled by the conductive BC@PPy nanospacer, the approach effectively enhances both ion and electron transport within the layered MXene structure, significantly increasing the areal capacitance of the MXene/BC@PPy film electrode to 388 mF cm−2, which is a 10‐fold improvement from the pure MXene film electrode. Pairing with CNTs/MnO2 battery‐type electrodes, the obtained ZHMSCs exhibit an areal energy density up to 145.4 μWh cm−2 with an outstanding 95.8% capacity retention after 25000 cycles, which is the highest among recently reported MXene‐based MSCs and approaches the level of micro‐batteries. The interlayer structure engineering demonstrated in the MXene‐based capacitor‐type electrode provides a rational means to achieve battery‐levelenergy density in the ZHMSCs.

MXene-Based Materials for Solar Cell Applications

“…After acid treatment, the MAX phases are exfoliated into 2D M 2 XT x , M 3 X 2 T x , or M 4 X 3 T x MXenes (where T x represents the surface terminations, e.g., OH, O, F, or Cl). These 2D systems have been named as MXenes for two principal reasons: they originate from the MAX phases following the removal of A-element atoms and they are structurally analogous to graphene [92][93][94][95]. Figure 7a demonstrates a typical structure of the MAX phase and their derived 2D MXene family (surface terminations are not shown here).…”

“…The latest exciting development is in the application of MXenes in biosensors, photothermal therapy of cancers, dialysis, neural electrodes, and theranostics [99,100]. Many applications of MXenes are based on their exceptional combinations of properties, including their high mechanical properties and electrical conductivity [93,101,102], hydrophilic features attributable to their functionalized surfaces providing the opportunity to bond with different species [100,103], absorptions of electromagnetic waves [104], and assistance in creating stable colloidal solutions in water [105]. Another area in which MXenes excel in comparison with other nanomaterials is in electromagnetic applications such as printable antennas and electromagnetic interference shielding [106].…”

“…2D MXenes are usually produced from top-down methods by selective etching and exfoliation of their MAX precursor phases possessing strong M-X and weak M-A bonding. Unlike graphene produced by mechanical exfoliation of graphite, 2D MXenes cannot be produced by mechanical exfoliation due to the metallic nature of M-A bonding; however, M-A bonds are more chemically active than the more robust M-X bonds, which provide exfoliation of MAX phases by selective etching of A-element layers [2,93,96].…”

“…The Young's modulus of 330 GPa for a monolayer Ti 3 C 2 T x MXene was measured using atomic force microscopy nano-indentation. Although some other studies have explored the mechanical properties of MXenes, further studies are still demanded [4,93,149]. All pristine MXenes have a metallic behavior similar to their MAX precursors; however, those surface terminations affect their electronic properties as functionalized MXenes have been predicted to be either metals or semiconductors, depending on the containing elements and surface functional groups [150].…”

“…Generally, all pristine MXenes are metallic; however, some display semiconducting behavior upon surface functionalization. Theoretically, the electronic properties of 2D MXenes are divided into two groups of (a) topologically trivial and (b) topologically nontrivial metal/semimetal or semiconductor, in which the strength of spin-orbit coupling (SOC) play a crucial role [93,101,150,151].…”