Significantly Enhanced Ultrathin NiCo-based MOF Nanosheet Electrodes Hybrided with Ti₃C₂T_x MXene for High Performance Asymmetric Supercapacitors

Yanzhong Wang ^{1, 2, *}, Yuexin Liu ¹, Chao Wang ^{1, 2, *}, Hu Liu, ^{3, 4}

 Jiaoxia Zhang ^{3, 7}, Jing Lin^{3, *}, Jincheng Fan ^{5, *}, Tao Ding^{6, *}, Jong E. Ryu ^{8, *} and Zhanhu Guo ^3,*

¹ School of Materials Science and Engineering, North University of China, Taiyuan 030051 China

² Advanced energy materials and system institute, North University of China, Taiyuan 030051 China

³ Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 37966 USA

⁴ Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of Education, National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou, China

⁵ College of Materials Science and Engineering, Changsha University of Science and Technology, Changsha 410114, China

⁶ College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China

⁷ School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China

⁸ Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695USA

 *E-mail: wyzletter@nuc.edu.cn (Y.Z. Wang); wangchao_nuc@126.com (C.Wang) or zguo10@utk.edu (Z. Guo)

Abstract

In situ synthesis of NiCo based metal-organic framework (MOF) nanosheets and the exfoliation of Ti₃C₂T_x into isolated nanosheets (MXene), called, NiCo-MOF/Ti₃C₂T_x hybrid nanosheets, are simultaneously achieved by a facile ultrasonic method. This method can effectively avoid the oxidation and restacking of Ti₃C₂T_x nanosheets, and also make them uniformly disperse on the surface of NiCo-MOF. The formed NiCo-MOF/Ti₃C₂T_x hybrid nanosheets achieve a high specific capacitance of 815.2 A g^-1 at 1 A g^-1. The practical asymmetric supercapacitor (ASC) is fabricated using activated carbon and NiCo-MOF/Ti₃C₂T_x hybrid nanosheets. The ASC device achieves an energy density of 39.5 Wh kg^-1 at a power density of 562.5 W kg^-1, and also demonstrates a suitable cycling stability with 82.3 % of capacitance retention after 10000 continuous cycles at 5 A g^-1. The enhanced electrochemical property of NiCo-MOF/Ti₃C₂T_x is attributed to the nanosheet-like and mesoporous structure, high electronic conductivity, and synergistic effect of hybrid electroactive components.

Table of Content

Ultrathin NiCo-MOF/Ti₃C₂T_x demonstrated a capacitance of 815.2 A g^-1 and NiCo-MOF/Ti₃C₂T_x//activated carbon delivered 39.5 Wh kg^-1 at 562.5 W kg^-1.

Keywords

MOF            MXene           Nanosheets          Pseudocapacitor         Ti₃AlC₂           Asymmetrical supercapacitors

1. Introduction

The depletion of fossil fuel, serious environmental pollution and global warming have stimulated extensive explorations of renewable clean energy (solar, wind, and hydrogen energies et al.). Nevertheless, the intermittent nature of renewable energies requires efficient energy storage units to supply continuous and stable energy, including batteries, fuel cells, and supercapacitors. In these various energy storage devices, supercapacitors have attracted tremendous attention owing to their fast charging and discharging, high power densities and long-term stability.¹ In the light of the charge-storage mechanism, supercapacitors generally contain two categories: (1) electric double layer capacitors (EDLCs, based on the charges absorbed at the interface of electrode and electrolyte) possess remarkable cycling stability and power density, but exhibit low specific capacitance; (2) pseudocapacitors (involving fast reversible surface redox faradaic reactions) offer relatively high specific capacitances.² In general, the structures and properties of electrode materials significantly influence the electrochemical performance of supercapacitors such as the specific capacitance, rate capability, and cycle life.³ Currently, carbon-based materials,⁴ transition metal oxides,⁵ conducting polymers,⁶ and their composites,⁷ have been investigated for supercapacitor electrodes. However, these electrode materials have demonstrated their shortcomings for applications in supercapacitors such as low specific capacitances for carbon-based materials, limited electronic conductivity for transition metal oxides, and short cycle life for conducting polymers.⁸ Therefore, many studies have focused on developing new electrode materials with a large specific surface area and high electronic conductivity.

MOFs are formed by strong coordination bonds between metal ions and organic ligands, and their structures and properties can be facilely regulated by choosing different metal centers and ligands.^{9, 10} These merits make them have extensive applications in catalysis, gas storage and separation, sensors, bio-medicine, and energy storage and conversion.^11-13 Especially, the unique structure and properties of MOFs are very suitable for high-performance supercapacitor electrode materials such as the extremely large specific surface area, easily adjustable pore size, and abundant pseudocapacitive redox centers.¹⁰ The MOF-based supercapacitor electrode materials mainly include two categories. One is that MOFs are employed as an excellent precursor to prepare porous carbons,¹⁴ various transition metal compounds (oxides, selenides, sulfides, and phosphides et al.),^15-19 and their composites under different preparation conditions.²⁰ Although these materials generally exhibit outstanding electrochemical performances, the prolonged and multi-step processes inevitably increase the production cost. Furthermore, the thermal or chemical treatments would inevitably destroy the porous structure of MOFs, and thus reducing the accessible surface area and the number of electroactive sites.²¹ The other is that MOFs are directly employed as electrode materials for high-performance supercapacitors. However, the limited electronic conductivity of MOFs becomes a big obstacle in the practical application of supercapacitors.²²

In order to circumvent the issue, one way is to develop highly conductive MOFs by adjusting the structure and components. For instance, Jiao’s and Wei’s groups reported the existence of conductive network in the layer-structured Co-MOF and Ni-MOF that can make electrons transfer rapidly in the MOF framework and significantly improve their electronic conductivity, and thus resulting in the excellent capacitive performance.^23-25 The other strategy is that the conductive substrates such as graphene,^{21, 26-28} CNTs,^{29, 30} polyaniline^{31, 32} and polypyrrole³³ are employed to boost the electronic conductivity of MOFs. For example, Banerjee et al. synthesized Ni-MOF/rGO composites via a hydrothermal method that exhibited a specific capacitance of 758 F g^-1, and the assembled device delivered an energy density of 37.8 Wh kg^-1.³⁴ Wen et al. prepared Ni-MOF/CNT composites with high electronic conductivity, indicating a maximum specific capacitance of 1765 F g^-1.²⁹ Shao et al. in situ form polyaniline chains in the pores of UiO-66 to enhance its electronic conductivity, indicating the remarkable electrochemical performance. ³¹ However, the above conductive substrates only play an important role in improving the electronic conductivity, but have little contribution to the specific capacitance of composites. Moreover, the preparation process of these conductive substrates is complex and costly.

MXenes, as novel 2D transition metal carbides and nitrides, have aroused extraordinary interest in supercapacitors owing to their metallic conductivity, large specific surface area, and high packing density.^{35, 36} Among these different MXenes, Ti₃C₂T_x (T stands for a surface terminating functionality including –O, –OH, and –F), has been widely investigated. For instance, Gogotsi et al. first prepared the accordion-like Ti₃C₂T_x by selectively etching Al elements of Ti₃AlC₂ using HF, which gave a specific capacitance of 130 F g^-1.³⁷ By virtue of the high electronic conductivity and 2D structure of Ti₃C₂T_x nanosheets, they have been widely employed as a conductive agent or template to boost the electronic conductivity of carbon nanomaterials,³⁸ metal oxides,³⁹ conducting polymers,⁴⁰ and MOFs ^{41, 42} for enhancing their electrochemical properties. In the preparation of the above-mentioned composite materials, the isolated Ti₃C₂T_x nanosheets were first prepared by an ultrasonification under Ar atmosphere, and then compounded with other materials, which easily results in the oxidation and restacking of Ti₃C₂T_x nanosheets, ⁴³ and thus losing the advantages of Ti₃C₂T_x.

Herein, the in-situ synthesis of NiCo-MOF nanosheets and the exfoliation of Ti₃C₂T_x were simultaneously achieved by an ultrasonic method, which can effectively avoid the oxidation and restacking of Ti₃C₂T_x nanosheets during the preparation process, and also make isolated Ti₃C₂T_x nanosheets uniformly distribute on the surface of NiCo-MOF. In NiCo-MOF/Ti₃C₂T_x hybrid nanosheets, Ti₃C₂T_x can act as a conductive agent to enhance the electronic conductivity as well as a spacer to effectively prevent the agglomeration of NiCo-MOF. It is well known that 2D nanosheets possess high specific surface area, a large number of electroactive sites, and rapid transfer pathway for ion and electron, which would result in the excellent capacitive performance. Accordingly, the rationally designed NiCo-MOF/Ti₃C₂T_x hybrid nanosheets achieved a high specific capacitance of 815.2 F g^-1 at 1 A g^-1. Moreover, the fabricated NiCo-MOF/Ti₃C₂T_x//AC asymmetric supercapacitors exhibited a high energy density of 39.5 Wh kg^-1 with the power density of 562.5 W kg^-1 and excellent cycling performance (82.3% of capacitance retention after 10000 cycles).

2 Experimental

2.1 Chemicals

NiCl₂·6H₂O, CoCl₂·6H₂O, LiF, p-phthalic acid (PTA, 98%), dimethylformamide (DMF, 99.8%), hydrochloric acid (HCl, 36%), and trimethylamine (TEA, 98%) were purchased from Aladdin, and used without any purification. Ti₃AlC₂ powders (~99.998%) were purchased from Foreshman.

2.2 Synthesis of Ti₃C₂T_x

Typically, 2 g of LiF was slowly dissolved into 40 ml HCl (9 M) under stirring, and then 2 g of Ti₃AlC₂ powders were slowly added into the above solution. The mixture was stirred at 40 °C for 24 h. After etching, the mixture was washed five times by centrifugation with deionized water and absolute alcohol until the pH reached 6. The resultant powders were dried at 60 ^oC for overnight.

2.3 Synthesis of NiCo-MOF/Ti₃C₂T_x hybrid nanosheets

NiCo-MOF nanosheets was prepared as the previous report with some modifications.^{44, 45} In a typical synthesis of NiCo-MOF/ Ti₃C₂T_x hybrids, 5 mg Ti₃C₂T_x was added into the DMF solution containing Co²⁺, Ni²⁺, PTA and TEA. Subsequently, the mixture was ultrasonicated for 8 h under Ar flowing. The resultant powers were washed by a centrifugation with ethanol for 3 times, and dried at 60 ^oC for 12 h. The as-prepared NiCo-MOF/Ti₃C₂T_x hybrid nanosheets were denoted as NiCo-MOF/Ti-5. For comparison, NiCo-MOF/Ti-x (x=2.5 and 10 mg) and NiCo-MOF were synthesized under the same experimental condition.

The detailed characterizations and electrochemical measurements can be referred to electrical supporting information.

3 Results and discussion

3.1 Structures and functionality characterization of the MOF-MXene hybrid nanosheets

The synthetic process of NiCo-MOF/Ti₃C₂T_x hybrid nanosheets is shown in Fig. 1. Typically, Ti₃C₂T_x was synthesized by etching Al layers from Ti₃AlC₂ powders with LiF and HCl solution. The negatively charged groups (e.g. -O, -OH, and -F) on the surface of Ti₃C₂T_x could absorb Ni²⁺ and Co²⁺ ions due to the electrostatic force that coordinate with PTA ligand and in situ form NiCo-MOF nanosheets under ultrasonication. Herein, the ultrasonication can not only exfoliate the accordion-like Ti₃C₂T_x into isolated nanosheets, but also in situ synthesize NiCo-MOF nanosheets, resulting in the tight contact of NiCo-MOF and Ti₃C₂T_x nanosheets. The intimate contact structure is convenient for the electron transfer from NiCo-MOF to Ti₃C₂T_x, thus enhancing the electronic conductivity of NiCo-MOF/Ti₃C₂T_x hybrid nanosheets. Compared with the previously reported method to synthesize MOF/Ti₃C₂T_x composites,^{44, 46} this method is facile and cost-effective to prepare NiCo-MOF/Ti₃C₂T_x hybrid nanosheets.

Fig. 1 Scheme of the synthesis of NiCo-MOF/Ti₃C₂T_x

The microstructures of as-prepared samples were characterized by SEM and TEM. Fig. S1a and b show that the large-area exfoliated Ti₃C₂T_x exhibits thin and crumpled nanosheets, and no accordion-like structures were observed, indicating the complete exfoliation of Ti₃C₂T_x.⁴⁷ In the high resolution TEM (HRTEM) image of Ti₃C₂T_x nanosheets (Fig. S1c), the edge of a typical nanosheet demonstrates a few layers thick with the average interlayer spacing of 1.2 nm. Fig. 2a and b show that the NiCo-MOF/Ti-5 hybrid nanosheets display curved nanosheets, and their random distribution effectively prevents the restacking of NiCo-MOF/Ti-5 hybrid nanosheets. In addition, the hybrid nanosheets have a lateral length of several micrometers with nanoscale thickness. In comparison, without the addition of Ti₃C₂T_x, the NiCo-MOF nanosheets prepared by the same experimental process were aggregated into the bulk materials (Fig. S2a). As shown in Fig. S2b and c, the agglomeration of hybrid nanosheets obviously decreases with Ti₃C₂T_x contents.

The microstructures of NiCo-MOF/Ti-5 hybrid nanosheets were further examined by TEM observations as shown in Fig. 2c. It also confirms the sheet-like morphology of NiCo-MOF/Ti-5 hybrid composites, which were stacked by several layers with different lateral lengths. Fig. 2c and d shows that Ti₃C₂T_x nanosheets with a length of 50~100 nm were evenly embedded on surfaces of NiCo-MOF nanosheets. The size of Ti₃C₂T_x nanosheets in NiCo-MOF/Ti-5 is much smaller than that of pristine Ti₃C₂T_x nanosheets (Fig. S1), which is assigned to the long-term ultrasonication. The HRTEM image of NiCo-MOF/Ti-5 hybrid nanosheets reveals the existence of nanoclusters with a dimeter of 3~5 nm on the NiCo-MOF marked by the red circles, indicating the lattice fringes derived from nickel-cobalt oxide in the inset (Fig. 2e), which is similar to the previous report.^{25, 48} Meanwhile, Fig. 2f indicates the visible lattice fringes with a spacing of about 0.24 nm, which corresponds to the (103) lattice plane of Ti₃C₂T_x.⁴⁹ The dark-field scanning TEM (STEM) image with the associated EDX elemental mapping exhibit the existence of Ti, Co, Ni, C, and O elements, further confirming the successful hybridization of Ti₃C₂T_x with NiCo-MOF (Fig. 2 g-m). The thickness of NiCo-MOF/Ti-5 hybrid nanosheets was examined by atomic force microscopy (AFM). By drawing a line profile across the stacked NiCo-MOF/Ti-5 hybrid nanosheets (Fig. S3), the thickness of hybrid nanosheets was found to be 2.7 ~ 5 nm, which is close to the value of the previous report.³⁵