Nickel cobaltite nanograss grown around porous carbon nanotube-wrapped stainless steel wire mesh as a flexible electrode for high-performance supercapacitor application
Graphical abstract
Nickel cobaltite nanograss with bimodal pore size distribution is grown around the carbon nanotube-wrapped stainless steel wire mesh as a high capacitance and stable electrode for high-performance and flexible supercapacitors.
Introduction
Recently, much effort has been made aiming to explore the high-performance supercapacitors due to the high power demand of the new electrical systems [1]. Traditional electric double-layer capacitors (EDLCs) store charges in electric double layer at the interface between electrode and electrolyte. Electrochemical supercapacitors (ECs) not only store charge like the EDLCs, but they also exhibit faradaic reactions between electrode and electrolyte in the appropriate potential window. Through redox faradaic reactions, ECs can store much more charge than traditional EDLCs. Some materials like carbon-based materials, metal oxides, metal sulfides, and conducting polymers have turned out to be highly perspective active electrode materials for ECs [1], [2], [3], [4]. Among these candidate materials, nickel and cobalt-based oxides such as nickel cobaltite (NiCo2O4) are promising for application in alkaline ECs due to their high specific capacitance, superior stability, and good corrosive resistance in the alkaline electrolyte [5], [6], [7], [8].
Nickel cobaltites with spinel structure exhibit better electrochemical activity and electrical conductivity than the pure nickel and cobalt oxides, making them suitable for application in ECs with alkaline media [5], [7]. To achieve both high energy density and power density, it is generally essential that the electrode materials exhibit good electrical conductivity, huge surface area, and applicable pore size distribution [9], [10], [11], [12]. Micropores may provide huge surface area to the electrode material, but narrow micropores cannot be fully accessed by the electrolyte. The use of mesoporous nickel cobaltite materials circumvents the difficulty in electrolyte penetration and spreading in ultrafine pores [9], [13]. Nanostructured nickel cobaltites display superior capacitive performance than the bulk ones, primarily resulting from their large surface area and suitable pore size for facilitating the transport of electrolyte ions [14]. Nickel cobaltites with various types of configuration such as nanosheet [15], [16], [17], [18], [19], [20], [21], [22], [23], nanoplate/nanoflake [24], [25], [26], [27], [28], [29], nanoflower [30], [31], nanotube/nanorod [32], [33], [34], [35], [36], nanowire [37], [38], [39], [40], [41], [42], and urchin-like nanostructures [43], [44], [45], [46] have been demonstrated to have a strong positive impact on their capacitive performance. Thus, shape control of nickel cobaltite nanomaterials plays the key role in the success of the supercapacitor technology [40], [47], [48]. In addition, the electrical conductivity of electrode contributed from the active material and supporting substrate (current collector) also acts a critical factor in determining the internal resistance of ECs and the utilization of active material. There are several strategies for increasing electrical conductivity of electrode, the most straightforward being the composite electrode introduced by adding the carbon materials that have high electrical conductivity such as activated carbon [49], [50], carbon nanotube (CNT) [51], [52] and graphene [53], [54], [55]. Nickel cobaltites grown on highly conductive and porous Ni foam and graphene foam also reveal an enhanced supercapacitive performance [56], [57].
In this work, an innovative electrode substrate featuring CNT-wrapped stainless steel (SS) wire mesh is explored for supporting the nickel cobaltite nanograss as a high-performance electrode for ECs. SS wire mesh has the benefits of low cost, high flexibility, good resistance to corrosion, and easy production. Scheme 1 illustrates the formation of nickel cobaltite nanograss around SS wire mesh and CNT-wrapped SS wire mesh. Three-dimensional (3D) porous CNT film with interconnected networks could be homogeneously wrapped around the SS wire mesh by electrophoretic deposition (EPD). Nickel cobaltite nanograss with individual nanorods is grown around the CNT-wrapped SS wires by hydrothermal reactions. Highly conductive and porous CNT film provides conductive networks for fast transport of electron and accommodates large amounts of electrolyte for facile transport of electrolyte. Nickel cobaltite nanograss with bimodal pore size distribution (narrow and wide mesopores) provides large capacitance through the redox faradaic reactions between individual nanorods and ions. Thus, the nickel cobaltite nanograss with CNT-wrapped SS wire mesh electrode is expected to exhibit high specific capacitance, good rate performance, and long cycle life.
Section snippets
Experimental
Type 304 SS sheet and wire mesh (with a spacing size of 0.15 mm) were employed as the substrate to support the active materials. SS substrates (2 cm × 2 cm) were rinsed with acetone and then vigorously washed with de-ionized water. Multiwalled CNTs with an outer diameter of 20–40 nm and a length of 0.5–200 μm were purchased from ECHO Chemical Co. Ltd. (Taiwan). The raw CNT powder was etched with boiling nitric acid solution (15 M) under reflux for 1 h. The CNTs were then extensively washed with
Results and discussion
Fig. 1a shows the SEM photograph of nickel cobaltite nanograss grown on SS sheet after heat treatment. During hydrothermal synthesis, the hydroxyl ions are produced from the hydrolysis of urea, which simultaneously react with nickel and cobalt ions to form nickel-cobalt hydroxide. The emerging nickel-cobalt hydroxide nuclei undergo adsorption and aggregation processes on the SS surface. SS surface has numerous active sites available for the nucleation and subsequent growth of nickel-cobalt
Conclusions
Uniform nickel cobaltite nanograss composed of nanorods is prepared by a simple hydrothermal method. The nanograss exhibits distinct bimodal pore size distribution, that is, small (<5 nm) and large mesopores (>20 nm). Large mesopores result from the space between nanorods, while small mesopores come from the individual porous nickel cobaltite nanorods. The electrode featuring nanograss structure exhibits large surface area for easy access of the electrolyte ions and good strain accommodation
Acknowledgments
The authors acknowledge financial support from the Ministry of Science and Technology, Taiwan (Grant No: MOST 103-2221-E-151-057-MY3).
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2020, Energy Storage MaterialsCitation Excerpt :Therefore, the behavior verified in Fig. 25 indicates that the orientation and distribution of water dipoles change as a function of the polarization, thus altering the value of the local relative permittivity (εr) in the compact region of the electrical double-layer [189]. It must be highlighted that the electrodes must be preconditioned for ≈300 s at the fixed (rest) potential in the double-layer region [1–382] (or voltage in the case of symmetric coin cells) until a negligible background current is obtained in order to get a quasi-equilibrium potential for the Faradaic process at t = 0 [127]. In addition, the verification that the transient currents are independent on the stirring of the solution present in the three-electrode cell indicates that the diffusion of ions during the SSRR (see eq. (17)) proceeds into the hydrated oxide sub-layers (e.g., gel-layers) [127,362,363].