Annealing modification of MXene films with mechanically strong structures and high electrochemical performance for supercapacitor applications

Highlights

• The strategy of annealing modification was performed on Ti₃C₂T_x MXene films.

• Preferable gravimetric/volumetric capacitance was obtained at 650 °C for 1 h.

• The modified Ti₃C₂T_x MXene film showed a high tensile strength of 32 MPa.

• The modified electrode exhibited good capacitance of 1590 F cm⁻³ with 442 F g⁻¹.

• The assembled supercapacitor demonstrated a volumetric energy density of 15.2 Wh L⁻¹.

Abstract

Two-dimensional transition metal carbides and nitrides (MXene) have shown outstanding performances in electrochemical energy storage, but their investigations of both mechanical and electrochemical performance for structural power systems are lack of being discussed. In this work, modified MXene films with mechanically strong structures and high electrochemical performance are obtained via directly annealing strategies. Due to the beneficial chemical composition on the surface and the structural variation under appropriate annealing treatment conditions, the modified Ti₃C₂T_x MXene electrode can deliver an ultrahigh volumetric capacitance of 1590 F cm⁻³ with a gravimetric value of 442 F g⁻¹ at 0.5 A g⁻¹, a good rate capability of 1030 F cm⁻³ at 20 A g⁻¹, a cycling stability with a retention of 95.4% after 5000 cycles in the acid electrolyte and a high tensile strength of 32 MPa, demonstrating its great potential as a multifunctional electrode for structural energy and power systems. The influences of different electrolytes on the intercalation-induced electrochemical process of modified MXene films were also probed. Furthermore, the assembled symmetric supercapacitor with the modified Ti₃C₂T_x MXene film demonstrates a favorable volumetric energy density of 15.2 Wh L⁻¹ and a power density of 204.8 W L⁻¹.

Introduction

The greatly increased demands for structural energy and power systems have prompted significant interest in the development of multifunctional electrodes, which could simultaneously provide electrochemical energy storage and structural functionalities [1,2]. From the electrochemical aspect, the use of fast surface redox for energy storage with pseudocapacitive mechanisms can potentially enable devices that provide much higher energy densities than that of electrical double-layer capacitors, in which the energy capacity is due to the electrosorption of ions on porous carbon electrodes. To date, various pseudocapacitive materials have been widely investigated, such as transition metal oxides [3], metal sulfides [4] and conductive polymers [5]. Among them, two dimensional pseudocapacitive materials are highly attractive for manipulation for use in structural energy and power systems due to their easy processability, enabling fabrication of flexible and freestanding electrodes for energy storage devices.

As a new but quickly expanding family of two-dimensional materials, MXenes have captured extensive attention in recent years and are usually fabricated by etching metal atom layers from ternary transition metal carbides (MAX phases), where M stands for an early transition metal (Ti, V, Cr, Ta, Nb, Zr, Mo), A is a group IIIA or IVA element (Al, Ga, In, Tl, Si, Ge, Sn, Pb) and X represents C and/or N elements [[6], [7], [8], [9]]. The etching treatment usually brings about mixed oxygen and fluorine terminal groups on the MXene surfaces, offering a broad variety of chemistries [10]. With a general formula of M_n+1X_nT_x (n = 1, 2, 3), in which T_x signifies the abundance of surface-terminating moieties (-O, –F and –OH), MXene-based materials have been reported to exhibit great potential applications in broad areas, especially in the energy storage field [11], benefiting from the main composition of a metallically conductive carbide core with a terminated transition metal on the surface that can undergo redox reactions. Even though, densely packed MXene electrodes with high ion-accessible surface areas and low ion transport resistance are crucial to the realization of high-density electrochemical capacitive energy storage but have proved to be very challenging to produce. Several approaches, including the introduction of functional guest materials to fabricate hybrid electrodes and engineer interpenetrating networks with open or porous architectures, have been probed to enhance the electronic conductivity or ionic kinetics. For example, carbon materials such as carbon nanotubes [12] and holey graphene sheets [13], the metallic oxide of MnO₂ [14], and polymers such as polypyrrole [15] could be actively hybridized with MXene films to restrain the self-stacking of MXene nanosheets. The sacrificial template of Fe(OH)₃ [16] and polymer spheres [17] between MXene layers could lead to interconnected nanoporous MXene films with promoted efficient ion transport. Although amazing performances, such as ultrahigh capacitance and rate capability, could be achieved, these strategies sometimes inevitably compromise the volumetric electrochemical properties. Additionally, the inclusion of guest materials or modification may potentially weaken the mechanical properties, which is apt to be neglected but quite vital for structural energy and power systems. Therefore, new strategies to engineer favorable structures of MXene-based electrodes with comprehensive consideration of all aspects, including the gravimetric and volumetric electrochemical properties as well as good mechanical performance, are highly desirable [18].

As the electrochemical performance of MXenes is predominantly attributed to the nature of the surface terminal groups according to a combination of density functional theory calculations and experiments [19], controllable surface modification has been verified to be a powerful method to maximize the potential of MXenes in energy storage. Gogotsi and coworkers illustrated that the capacitance of Ti₃C₂T_x is predominantly attributed to changes in the Ti oxidation state, i.e., the pseudocapacitance [20]. Therefore, a lower terminal group concentration on the surface of an MXene could lead to more Ti atoms participating in redox reactions. Recently, several studies revealed that the surface and structural properties of MXenes could be modified by annealing treatment alone or combined with an alkalinization process [21]. For instance, Dong et al. explored the electrochemical properties of Ti₃C₂T_x with intercalated carbon chains after 600–1000 °C treatment in an Ar atmosphere, and the modified electrode with HEX⁺ intercalation reached a gravimetric capacitance of 364.3 F g⁻¹ at 800 °C, higher than that of pure Ti₃C₂T_x [22]. However, this method requires complex ion exchange procedure for ion intercalation along with a tedious washing process. Alshareef et al. investigated the effects of the postetch annealing gas composition on the structural and electrochemical properties of Ti₂CT_x MXene electrodes and demonstrated that a significant improvement in the supercapacitor performance could be obtained upon heat treatment in Ar, N₂ and N₂/H₂ ambient [23]. Jiang and Liu's group directly treated Ti₃C₂T_x films with alkalization and calcination to explore the changes in electrochemical properties [24,25] and an ultrahigh volumetric capacitance of 1805 F cm⁻³ with a gravimetric value of 475 F g⁻¹ could be achieved [25]. Nevertheless, a thorough study of the influence of annealing conditions on both the electrochemical and mechanical properties of MXene films with in-depth insight into the mechanisms is lack of being discussed.

Herein, Ti₃C₂T_x MXene films were directly treated in an Ar atmosphere under different annealing conditions. We report for the first time the influence of annealing treatment on the morphologies, structures and especially the electrochemical and mechanical properties of Ti₃C₂T_x MXene films. Due to the beneficial chemical composition on the surface and the structural variation under appropriate annealing treatment conditions, the modified Ti₃C₂T_x MXene film displays a preferable combination of gravimetric/volumetric electrochemical properties and mechanical properties, which is favorable for multifunctional energy and power systems.