Porosity-adjustable MXene film with transverse and longitudinal ion channels for flexible supercapacitors

https://doi.org/10.1016/j.micromeso.2021.111389Get rights and content

Highlights

  • The porous structure can be easily adjusted in the MXene film.

  • Both transverse and longitudinal channels are constructed in the MXene film.

  • The supercapacitor has a good flexibility and a high areal specific capacitance.

Abstract

Ti3C2Tx (a typical MXene) films can be used as free-standing electrodes due to their ultrahigh electrical conductivity, well flexibility and high electrochemical activity. However, inevitable self-restacking issue of Ti3C2Tx nanosheets largely decreases exposed area and impedes the ion transport in Ti3C2Tx film. Herein, hierarchically porous carbon (HPC) introduced into Ti3C2Tx film can not only act as the pillar for adjacent Ti3C2Tx nanosheets to prevent their typical self-restacking and further accelerate rapid ion transport in the transverse direction, but also ensure rapid ion transport in the longitudinal direction by introducing abundant macro/mesopores. By easily changing the amount of HPC (0–60%), the porosity of the film is well controlled, with adjustable specific surface area (SSA) from 8 m2 g−1 to 755 m2 g−1. The prepared quasi-solid-state supercapacitor fabricated by 60% HPC shows a good stability after different bending angles, high capacitance of 211 mF cm−2, energy density of 4.68 μW h cm−2 at 19.91 μW cm−2, and high capacitance retention of 86% after 10,000 charging-discharging cycles. This work will pave a facile route to alleviate the self-restacking phenomenon of Ti3C2Tx film by constructing transverse and longitudinal channels for rapid ion transport.

Introduction

With the flourishing development of smart electronic products (e.g. intelligent robots, folding mobile phones, and folding computers) and tremendous energy requirements, energy storage devices demand more portable and flexible [1,2]. Supercapacitors (SCs) with ultrahigh power density, long cycle life, and rapid charging-discharging capability, have become one of promising energy storage devices [3,4]. However, as the core component of commercial SCs, the electrodes are generally stiff. Moreover, conducting materials, binder, and current collector are necessary for the preparation of these electrodes, which brings in a low ratio of active materials. Hence, such above-mentioned electrodes cannot meet the flexibility and energy density requirements of smart electronic products in limited space. Therefore, it is necessary to develop flexible, free-standing, and high-performance electrodes for SCs [5,6].

Recently, textiles and cellulose papers were used as the substrate materials to prepare flexible electrodes [7,8]. However, these materials are insulated, which is disadvantageous for rapid electron delivery. Thus, these flexible substrate materials should be compensated with conductivity (e.g. carbonization [9] or introducing conductive polymers [10]). However, carbonization leads to the increase in rigidity of the electrodes, and introducing conducting polymers will result in a decrease in the proportion of active materials. Furthermore, to achieve desirable energy density, electrochemically active materials should be also loaded, indicating a multi-step preparation process with a low efficiency.

In contrast to textile and cellulose papers, 2D graphene is electrically conductive and can be directly prepared into flexible and free-standing electrodes for SCs [11]. However, graphene generally needs to be converted to graphene oxide first through a complex flow path for developing the hybrids. MXene (M: transition metal, X: C/N), a series of 2D transition metal carbides or nitrides, is discovered in 2011 [ [[12], [13], [14], [15]]]. Compared to 2D graphene, MXene has plenty of metallic bonds and ionic bonds, thus providing abundant opportunity for hybrid materials. Moreover, due to plenty of oxygen-containing functional groups on MXene nanosheets surface, MXene is easy to be vacuum-assisted filtered into a flexible film. Additionally, due to their ultrahigh conductivity, hydrophilic nature, and well electrochemical activity, MXene flexible films have become promising candidates of free-standing electrodes for SCs with high capacitance [16,17]. However, the typical self-restacking issue of MXene films resulting from the interactions induced by van der Waals and hydrogen bonds between adjacent MXene nanosheets will impede the electrolyte ion transport and storage, and further decrease the utilization of ion adsorption/desorption sites of MXene [14]. Therefore, many works donated great efforts to solve it, such as creating pores in MXene [18], introducing intercalation materials [19] or inducing vertical arrangement of MXene nanosheets [20]. Particularly, creating pores in MXene is a simple and effective method. Some of the reported strategies constructed electrolyte ion transport channels by chemical etching on MXene nanosheets directly to produce porous structure [[21], [22], [23]]. However, the SSA of the obtained materials (93.6 m2 g−1) still needs to be enhanced. Additionally, these pores (10–100 nm) are produced by the oxidation of metal atom, thus the size of the pores is greatly related to the oxidation degree. Besides, MXene is easily oxidized, making it difficult to control the oxidation degree, thus the pore size of electrode materials is difficult to adjust. Alternatively, cellulose nanofiber (CNF) or carbon nanotube (CNT) are introduced into MXene films to form pores by the randomly weaving, while such a process is difficult to control [19, [24], [25], [26]]. Besides, it has been reported that micropores can enhance the capacitive capability by providing electrolyte ion adsorption/desorption sites. Meanwhile, mesopores improve rate capability by creating high-speed pathway for rapid ion transport, and macropores shorten ion transport distance by serving as ion buffer reservoirs [27,28]. Therefore, it is still of great significance to develop a simple method for the preparation of flexible and free-standing Ti3C2Tx-based electrodes with adjustable pore structure for excellent electrochemical performance.

Herein, as demonstrated in Fig. 1, a facile vacuum-assisted filtration is employed to prepare a Ti3C2Tx/CNF/HPC hybrid film with a developed pore structure, high SSA, and large specific capacitance by innovatively introducing HPC into Ti3C2Tx. HPC with adjustable porous structures, high chemical stability, and tunable SSA acts as the pillar to impede the self-restacking of Ti3C2Tx nanosheets, accelerating the rapid transport of electrolyte ion in the transverse direction. Besides, HPC provides plenty of micropores for electrolyte ion adsorption/desorption and macropores for rapid electrolyte ion transport in the longitudinal direction [[29], [30], [31], [32]] Additionally, owing to these outstanding properties, the optimized film can be used as a high-performance free-standing electrode with an outstanding rate capability and high areal specific capacitance. The prepared SC shows a good flexibility, high areal energy density, and high capacitance retention. This work paves a facile route to alleviate the self-restacking phenomenon of Ti3C2Tx film by constructing electrode with rapid ion transport channels in both transverse and longitudinal directions for practical applications.

Section snippets

Materials

MAX (Ti3AlC2) were purchased from Forsman Technology (Beijing) Co., Ltd. Potassium hydroxide (KOH, 99%) was obtained from Aladdin Reagent Co., Ltd. Hydrochloric acid (HCl, 35–38%) and lithium fluoride (LiF, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd and Sigma Aldrich (Shanghai) Trading Co., Ltd, respectively. The CNF was purchased from Junada (Qingdao) Technology Co., Ltd. Pulping black liquor was provided by Shandong Bohui Paper Industry Co. Ltd as the precursor of lignin.

Structure design for MHPCs

Figure S3a shows the XRD patterns of Ti3AlC2 and Ti3C2Tx. Obviously, after HF-etching, the diffraction peak at 39° corresponding to the (104) plane in Ti3AlC2 disappears. Additionally, the shift of the (002) peak from 8.9° in Ti3AlC2 to 6.3° in Ti3C2Tx confirms the elimination of Al after etching and expansion of sheets’ d-spacing. HR-TEM image of Ti3C2Tx (Fig. S3b) shows two-dimensional layered structure with ultrathin thickness (concluded by its high transparency), indicating the successful

Conclusion

In summary, a hybrid MHPC with well flexibility, high SSA (755 m2 g−1), and large specific capacitance was prepared by innovatively introducing HPC into Ti3C2Tx by a facile vacuum-assisted filtration. The HPC introduced into the electrode can act as the pillar to effectively separate Ti3C2Tx nanosheets, thus creating channels for the rapid electrolyte ion transport in the transverse direction. Additionally, HPC can serve as the pores donor to provide abundant macropores for rapid electrolyte

CRediT authorship contribution statement

Daotong Zhang: Methodology, Software, Data curation, Writing – original draft, Investigation. Min Luo: Writing – review & editing. Kai Yang: Writing – review & editing. Pei Yang: Writing – review & editing. Chaozheng Liu: Writing – review & editing. Weimin Chen: Resources, Supervision, Writing – review & editing. Xiaoyan Zhou: Resources, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by The Natural Science Foundation of the Jiangsu Higher Education Institutions of China [20KJB220008]; Postgraduate Research &Practice Innovation Program of Jiangsu Province [KYCX20_0863]; Program for 333 Talents Project in Jiangsu Province [Grant No. BRA2016381]; The start-up funds for scientific research at the Nanjing Forestry University [163020126]; and The Advanced Analysis and Testing Center of Nanjing Forestry University.

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