Conductive hydrogels with 2D/2D β-NiS/Ti₃C₂T_x heterostructure for high-performance supercapacitor electrode materials

Abstract

To obtain a flexible supercapacitor with excellent capacitance, good mechanical properties while maintaining self-healing ability, β-NiS/Ti₃C₂T_x with 2D/2D heterostructure was synthesized by hydrothermal reaction and introduced as filler to the polymer hydrogel PAA/CS(PC) network. Through mechanical and self-healing performance tests, it was found that the hydrogel electrode had high tensile properties with tensile strain up to 566% and excellent self-healing properties with efficiency as high as 84%. The electrode not only exhibits a specific capacitance of 10.92 F/g at a current density of 6 mA/g, but also still shows an excellent stability of 87% at a high current density of 20 mA/g for 2000 cycles. This novel hydrogel facilitates the further development of multifunctional, smart and flexible electronics.

Introduction

Electrically conductive hydrogels (ECHs) are recognized as one of the potential materials for the design and construction of flexible supercapacitors. But the existing conductive hydrogels exhibit poor mechanical properties and low specific capacitance, which still cannot meet the application requirements of flexible stretchable devices. At present, the research focus is mainly on the preparation of hydrogels with high conductivity and stability by adding inorganic substances or conductive polymer, and also have better mechanical strength, so that it has more practical application value. Carbon based nanomaterials and conductive polymer are introduced into the hydrogel matrix as conductive fillers to create conductive hydrogels [1,2], which are conductive by the directional movement of free electrons, are electrically conductive. The resulting hydrogel composites have excellent conductivity, while retaining the unique physical chemistry properties of hydrogel. Wang [3] prepared PANI/PAAm hydrogel electrode with 137.4 mF/cm² of area specific capacitance and 130% of tensile strain. The hydrogel electrode can be effectively recycled without loss of electrochemical properties. Jin [4] obtained PAni-PAAm-GOCS conductive hydrogels with elongation and conductivity of 360% and 2.88 S/m, respectively, by changing the ratio of phytic acid/HCl. Tang [5] developed a stretchable CNT@MnO₂/PAAm hydrogel electrode based on a novel Na₂SO₄-aPUA/PAAm hydrogel electrolyte, whose specific capacitance reaches 478.6 mF/cm². After 3000 charge-discharge cycles, the capacitor retention rate is still kept at 91.5%.

As an important class of transition metal sulfide electrode materials, NiS has attracted widespread attention from researchers due to its high theoretical capacity, low cost, and environmental friendliness [6]. Compared with NiO, NiS has a higher conductivity, higher theoretical capacity, and better cycling stability. Moreover, since sulfur has a lower electronegativity than oxygen, the chemical bond between sulfur and nickel in NiS is weaker than that of NiO, which makes the conversion reaction of nickel sulfide in electrochemistry easier than nickel oxide. The important reason is that NiS has no direct chemical bonding between adjacent atomic layers and can combine with different 2D transition metal sulfide materials to form van der Waals heterostructures that are not limited by lattice matching. The interfacial effect of the heterostructure gives the electrode material better electronic conductivity, mechanical strength, and new ion diffusion paths, which is beneficial to enhance the ion storage capacitance, multiplicative performance and cycling stability of the electrode material. Niraj Kumar [7] synthesized self-assembled NiS–SnS heterostructured electrode materials by two steps to increase the active sites and properties using the combined action of the bimetallic matrix. NiS–SnS exhibits high capacitance of 1653 F/g and excellent cycling stability. Wu [8] has made great contributions to lithium storage by utilizing the pine dendritic β-NiS@Ni₃S₂ 3D heterostructures as a conductive collector to uniformly grow mesoporous NiO nanosheets on the backbone of β-NiS@Ni₃S₂ substrate.

Since the first report in 2011, two-dimensional transition metal carbides/nitrides (MXene) were researched widespread due to their unique physical and chemical properties [9], including energy storage [10], optoelectronics, biomedical and environmental applications [11]. Ti₃C₂T_x MXene exhibits a graphene-like layered structure with a large specific surface area, excellent electrical conductivity, good chemical stability and ionic conductivity, and contains tunable hydrophilic groups such as –OH, =O, and –F. The Ti site at the end of the surface leads to a stronger redox activity than conventional carbon materials, where = O also contributes additional pseudocapacitance, making it a promising class of supercapacitor electrode materials [12]. However, MXene is prone to agglomeration, oxidation and low capacity, so how to enhance the electrochemical properties of MXene is a hot topic of current research. Wang [13] prepared a NiMoO₄/Ti₃C₂T_x heterostructure, resulting in a larger surface area and more active sites in the NiMoO₄/Ti₃C₂T_x electrode, which shortens the migration distance of the electrolyte and accelerates the ion diffusion. The 1T-MoS₂/Ti₃C₂T_x heterostructure prepared by Wang [14] using microwave hydrothermal method has a specific capacitance of 386.7 F/g at 1 A/g The electrochemical properties of MXene were considerably improved. Zhang [15] used the Ni–Fe oxide nanocube to construct the heterostructure not only to enlarge the MXene layer space but also to promote the diffusion of electrolyte and enhance its electrochemical activity. The prepared composite film exhibited an outstanding capacitance of 1038.43 mF/cm² at a current density of 0.5 mA/cm². Thus, the semiconductor and Ti₃C₂T_x MXene heterostructures act through near-surface ion absorption as well as other storage energy for fast reversible Faraday reactions [16], generating more active sites and leading to multiple redox activities [17], excellent ionic conductivity [18], and short diffusion paths at the boundaries of the heterostructured materials.

It is worth noting that during the application, flexible electronic devices are inevitably subject to mechanical damage, resulting in crack, scratch and even disconnections on the surface. These structural damages may cause flexible devices to lose their normal functions [19]. Hence, it is still essential to develop conductive hydrogels with excellent mechanical properties with maintaining self-repair capability so as to solve the durability of devices.

In this paper, 2D/2D β-NiS/Ti₃C₂T_x heterostructured conductive fillers were successfully synthesized by hydrothermal method. On the one hand, β-NiS and Ti₃C₂T_x nanosheets can generate abundant interfaces and form abundant active sites, obtaining high charge transfer capability and excellent electrochemical activity. Moreover, β-NiS nanoparticles can effectively inhibit the aggregation of Ti₃C₂T_x nanosheets and improve their stability. This resulted in a high specific capacitance of 10.92 F/g with the capacitance retention as 87% for the hydrogel flexible electrode, even at a high current density of 20 mA/g. On the other hand, with the introduction of multiple hydrogen bonds, the elongation and self-healing properties of the hydrogels were considerably improved.