Ultrafast synthesizing nanoflower-like composites of metal carbides and metal oxyhydroxides towards high-performance supercapacitors

Highlights

• The 3D nanoflower-like F-MXene/CoOOH composites were prepared by an ultrafast method.

• The gravimetric specific capacitance of 955.08 C/g was achieved at 5 mV/s.

• The capacitance retention rate was as high as 84.1% after 100,000 cycles at 1 A/g.

Abstract

Because of their outstanding electrochemical performance and high theoretical specific capacity, metal oxyhydroxides or metal hydroxides have been paid significant attention as electrode materials for supercapacitors. Unfortunately, because of their poor conductivity, the experimental capacity is considerably lower than the theoretically expected value. Similarly, the use of 2D transition metal carbides (MXene) in energy storage electronic devices has also drawn research attention. However, its self-restacking makes it difficult for electrolyte ions to reach the material's active sites. Therefore, in this research work, an ultrafast one-step method for the synthesis of 3D nanoflower-like MXene/metal oxyhydroxide composites has been proposed. Composite materials synergistically combine the high theoretical capacity of metal oxyhydroxides with good electrical conductivity of MXene, resulting in excellent supercapacitive performance. The resulting composite delivered the highest specific capacity of 955.08 C/g at a scan rate of 5 mV/s and its maximum energy density of 32.13 Wh/kg is attained at a power density of 324.97 kW/kg due to synergistic combination of MXene and metal oxyhydroxides. Furthermore, the capacity retention rate at 1 A/g current density is as high as 84.1% after ultralong 100,000 cycles. Due to the excellent performance of synthesized 3D nanoflower-like MXene/metal oxyhydroxide composites, it can be used for electrode materials fabrication with enhanced energy storage properties for supercapacitors to fulfill energy needs.

Introduction

Supercapacitors have gained increasing research interest due to their high power density, broad operating temperature, long cycling life, and high rate capacity compared to rechargeable batteries and conventional capacitors [1], [2], [3], [4], [5], [6], [7]. The performance of supercapacitors is greatly influenced by the morphology of nanostructured electrode materials. Nanomaterials can be categorized to various morphologies ranging from three-dimensional (3D) to zero-dimensional (0D) [8], [9], [10], [11], [12], [13]. However, 2D materials with significant lateral dimensions can provide more electrochemical active sites for the respective applications [14], [15], [16], [17], [18], [19]. 2D nanofillers, such as metal oxyhydroxides or hydroxides (e.g., Co-based oxyhydroxides, Co-based hydroxides, Ni-based hydroxides, Ni/Co-based hydroxides) are fascinating because their simple synthesis routes and negative and positive charges in their layered structures can provide abundant active sites during electrochemical based applications and respective processes [20], [21], [22], [23], [24], [25], [26], [27], [28]. In several studies, it has been reported that Co-based oxyhydroxides/hydroxides possess excellent electrochemical redox behavior in alkaline electrolytes [29]. Recently most of the reported studies, Co-based oxyhydroxides/hydroxides are generally based on bimetallic and trimetallic materials including NiCo, CoMn, and CoNiAl oxyhydroxides/hydroxides [22]. These oxyhydroxides/hydroxides are superior to their unary counterparts, but they are costly and due to their complex structure, can utilize only a smaller number of active sites [30]. It has been proposed that the active site in Co-based oxyhydroxides/hydroxides is an unsaturated CoO_6–x octahedron, which on the ultrathin surface can be highly exposed [24]. So far, only a few studies have reported on Co-based oxyhydroxides to be utilized as electrode material for supercapacitors. Consequently, it is necessary to design a novel pathway for the easy synthesis of monometallic Co-based oxyhydroxides as robust electrode materials. Furthermore, for its effective utilization, it is necessary to improve the conductivity of these materials.

MXenes, are a type of promising 2D material that possess high electrochemical stability, and excellent electrical conductivity [31], [32], [33], [34], [35], [36]. It is especially suitable for composites with other active materials as a matrix in energy storage applications [37,38]. The interfacial interaction of MXenes with metal oxyhydroxides/hydroxides is a credible way to improve electrochemical performance. Composite materials based on MXenes with metal oxyhydroxides/hydroxides can synergistically combine the high theoretical capacity of metal oxyhydroxides/hydroxides with good electrical conductivity of MXene, which can result in excellent power/energy density and outstanding stability thus acting as efficient cathode materials for supercapacitors [37,39]. Traditional synthesis of metal oxyhydroxides/hydroxides like co-precipitation, self-assembly, and hydrothermal methods are always uncontrollable and complex, and it is hard to obtain homogeneous electrode materials [40]. For example, MXene/CoNiAl hydroxides composites prepared by electrostatic self-assembly, the weak force and the uneven distribution of functional groups at the surface restrict their performance [41].

The synthesis of three-dimensional (3D) material seems to be an appropriate way to resolve these issues. In this aspect, 3D porous MXene/metal hydroxides composites were synthesized by the in-suit hydrothermal method [42]. These 3D materials possess various excellent characteristics such as porous structure and high specific surface, therefore, they possess great potential for utilization in various fields. However, high temperature and environmental constraints, restrict effective coupling between MXene and metal oxyhydroxides/hydroxides. The above problem should be solved by designing a simple procedure for the synthesis of 3D porous material based on MXene/metal oxyhydroxide or MXene/metal hydroxide composites that can efficiently work as electrode materials for supercapacitors.

Herein, we proposed an innovative one-step in-situ growth route for CoOOH arrays on Ti₃C₂T_x MXene nanosheets, using 2-methylimidazole ligand and methanol solvent, with ultrashort reaction time (30 min) for the first time that only involves a facile ultrasound chemical coprecipitation of Co²⁺ and 2-Methylimidazole. Unlike previous synthetic routes, this method neither needs any complex synthesis procedure nor any harsh environmental requirement to get a hierarchical structure. The obtained MXene/CoOOH composite possesses 3D porous nanostructures and synergistically combines the best possible properties of its components, the high theoretical capacity of CoOOH superior electrical conductivity of MXene, which leads to excellent electrochemical properties with an excellent specific capacity of 955.08 C/g at the potential sweep rates of 5 mV/s and magnificent cycling stability (retained 84.1% after ultralong 100,000 cycles at 1 A/g). This work will may provide a new avenue for the straightforward fabrication of morphologically controlled electrode materials for high-end supercapacitors applications.