Supercritical ethanol deposition of Ni(OH)2 nanosheets on carbon cloth for flexible solid-state asymmetric supercapacitor electrode

doi:10.1016/j.supflu.2020.104774

The Journal of Supercritical Fluids

Volume 159, 1 May 2020, 104774

https://doi.org/10.1016/j.supflu.2020.104774 Get rights and content

Highlights

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Supercritical ethanol was used for depositing Ni(OH)₂ NSs arrays on carbon cloth.
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Hierarchical nanostructured Ni(OH)₂ was obtained due to the supercritical fluids.
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Numerous open channels and passages are generated between the Ni(OH)₂ NSs.
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Ni(OH)₂ NSs arrays/ECC electrode exhibits superior electrochemical properties.
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Flexible solid-state device shows high electrochemical performance.

Abstract

Ni(OH)₂ nanosheets (NSs) were grown onto an electrochemically activated carbon cloth (ECC) with a self-supported microstructure by using efficient ethanol assisted supercritical deposition method. Benefited from the structural features of the well-developed porous architecture with effective channels for electrolyte ions diffusion and accessible redox sites, the nano-heterostructure brings a high utilization of electroactive material and thereby good electrochemical performance. The specific capacity of 918 mC cm⁻² at 2 mA cm⁻² with good rate performance (68.4 % retention at 20 mA cm⁻²) was achieved for the obtained Ni(OH)₂ NSs/ECC flexible electrode. The assembled flexible solid-state device with a 1.8 V operating potential displays an energy density of 8.3 mW h cm^-3 at 20 mW cm^-3. Additionally, the as-fabricated device reveals cyclic stability with 87 % capacity retention (10,000 cycles at 50 mA cm⁻²). These results show the importance of supercritical fluid deposition as a potential method for supercapacitors electrode.

Graphical abstract

Introduction

Recently, wearable and portable electronic devices have aroused a strong concern, which increases the demands for flexible and green power sources [[1], [2], [3], [4]]. As an efficient charge storage system, supercapacitors (SCs) have caused intense attention in the past few decades by the virtues of rapid charge/discharge rates, ultra-long cycle life, and high safety [[5], [6], [7], [8], [9]]. Particularly, flexible solid-state SCs retain the intrinsic properties of surface dominated revisable redox reactions; meanwhile, good mechanical stability can be achieved [[10], [11], [12], [13], [14]].

Adopting asymmetric supercapacitors (ASCs) is a good pathway to heighten the specific energy density. ASCs usually consist of two kinds of electrodes with a broad potential window, which is an electric double-layer capacitive electrode as the negative terminal and a pseudocapacitive electrode as the positive terminal. The pseudocapacitive electrode not only contributes more capacitance but also expands the operating voltage, and both can contribute to the overall energy density of the integrated device.

A large variety of carbon materials have been investigated as flexible solid-state SCs electrode materials in recent years, such as graphene [15], nanotubes [16], and carbon fibers [17]. However, the pursuit of high energy density and low cost is still a challenge for the research of flexible solid-state SCs.

Transition metal oxides [[18], [19], [20]], hydroxides [[21], [22], [23], [24]], and sulfide [25,26] as pseudocapacitive materials have been thoroughly researched over the past years. Among various metal hydroxides, Ni(OH)₂ is regarded as a good pseudocapacitive material owing to its low cost, large specific capacity, and abundant natural reserve [22,27]. It is noteworthy that the pseudocapacitive materials are usually not directly used as electrodes for solid-state flexible ASCs, they must compound with flexible conductive substrates, such as carbon cloth [[28], [29], [30]], graphene [31,32], and metal substrates [33]. Carbon cloth-based flexible scaffold with the characteristics of good mechanical strength, high electrical conductivity, lightweight, and low cost has been widely utilized for flexible energy storage devices [1,3]. Consequently, combining Ni(OH)₂ with carbon cloth is an efficient pathway for obtaining a high-performance flexible electrode. However, the direct deposition of metal hydroxide on the carbon cloth via the hydrothermal method suffers from irregular morphology with a relatively large geometry. These non-preferable aspects can lead to low utilization of active materials and low specific capacity of the integrated electrode [22,34,35]. Moreover, porous nano-heterostructured materials possess good electrochemical properties, thanks to the large specific surface areas and improved contact at electrode-electrolyte interfaces. In this regard, numerous efforts have been made to exploiting effective synthesis methods for constructing functional materials. Nevertheless, the progress has been achieved, the design of mesoporous materials with open architectures remains a challenge.

Compared to conventional solvents, the distinct properties of supercritical fluids are their gas-like diffusivity, near-zero surface tension, and good solvent capability. They can deliver reactant to the complex, porous surface with uniform surface coverage. In addition, the good wettability of supercritical fluids ensures good contact between the reactant and substrate [[36], [37], [38]]. Herein, we report a simple ethanol-assisted supercritical deposition method for decorating Ni(OH)₂ nanosheets (NSs) on electrochemically activated carbon cloth (ECC) scaffold as electrode materials for supercapacitors with a highly open porous structure. The ECC substrate with a well-known porous surface not only provides nucleation sites for the uniform growth of Ni(OH)₂ NSs but also immensely boosts the specific capacitance when used directly as a negative electrode. The synthesized Ni(OH)₂ NSs flexible electrode showed superior specific capacity and rate performance. Furthermore, high energy density, good mechanical stability and cyclic stability of the assembled solid-state flexible Ni(OH)₂ NSs//ECC ASCs could be achieved.

Section snippets

Preparation of ECC

Prior to deposition, the commercial carbon cloth was subjected to an electrochemical activation process as follows: the as-received carbon cloth (Ce Tech Co., Ltd., WOS 1009) was thoroughly rinsed with ethanol, acetone and deionized H₂O for 15 min, respectively. After drying, the resulting substrate underwent an activation reaction in a mixed solution containing H₂SO₄ and HNO₃ (1:1 in volume) at 3 V potential for 10 min. Afterward, the activated substrate denoted as ECC was placed in deionized H

Results and discussion

The electrochemically activated carbon fibers show a well-defined porous feature as shown in Fig. 1a, which proves that the oxidation process could generate a large number of pores. Such a distinct porous structure can supply numerous nucleation sites for the uniform deposition of Ni(OH)₂ NSs [3]. In the meantime, the activation process with the acidic solution also brought functional groups on the surfaces of carbon cloth. These generated functional groups were verified by the characterization

Conclusions

In summary, hierarchical nanostructured Ni(OH)₂ NSs arrays were directly deposition on ECC via a one-step ethanol assisted supercritical deposition method and employed as a high-property electrode for flexible solid-state asymmetric supercapacitor. The binder free electrode exhibits a 3D networks like-structure assembled of 2D NSs arrays that grown in a vertical fashion along the y-axis. Numerous open channels and passages are generated between the neighboring NSs which are conducive to

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.

Acknowledgments

This work was supported by National key research and development program (Grant No.2016YFB0901600) and NSFC (Grant No. 21303162 and Grant No. 11604295).

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Supercritical ethanol deposition of Ni(OH)2 nanosheets on carbon cloth for flexible solid-state asymmetric supercapacitor electrode

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of ECC

Results and discussion

Conclusions

Declaration of Competing Interest

Acknowledgments

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