Regular Article
Synthesis of NiGa2S4-rGO on nickel foam as advanced electrode for flexible solid-state supercapacitor with superior energy density

https://doi.org/10.1016/j.jcis.2018.09.100Get rights and content

Abstract

Pseudocapacitive electrode materials employed in supercapacitors may bring in high energy density (ED) and specific capacitance (Csc), which are critical for their practical applications. Accordingly, logical design of advanced electrode materials is highly demanded to progress high-performance supercapacitors. Here, for the first time, we suggest a straightforward route for the synthesis of NiGa2S4-rGO as an advanced cathode material supported on nickel foam (NF) for employed in flexible solid-state asymmetric supercapacitors (FSASCs). Due to an abundant ratio of active sites and large surface area of the NiGa2S4-rGO advanced material, the as-prepared NiGa2S4-rGO/NF electrode illustrates considerable electrochemical properties including remarkable specific capacitance (Csc) of 2124.34 F g−1 with excellent rate capability of 73%, and exceptional durability, which are better than NiGa2S4/NF and previously reported transition metal sulfides (TMSs). Furthermore, for the first time a pseudocapacitive advanced anode material of FeSe2-rGO have been successfully fabricated on a nickel foam (NF) substrate by a facile strategy. Element Selenium as the favorable element was offered into the Fe for enhancement and adjustment of the anode material electrochemical performance. The FeSe2-rGO/NF advanced anode electrode presents satisfactory electrochemical properties containing an exceptional specific capacitance (Csc) of 432.40 F g−1, significant rate performance of 57.84% and superior durability, which are better than FeSe2/NF electrode and previously studied Fe-based anode material. Considering the remarkable electrochemical performance of the as-prepared pseudocapacitive advanced electrode materials, a FSASC based on the NiGa2S4-rGO/NF as the cathode electrode and FeSe2-rGO/NF as the anode electrode was assembled. The FSASC device delivers superior Csc of 341.20 F g−1, outstanding energy density (ED) of 121.31 W h kg−1, remarkable cycle stability (only 7.30% damage after 5000 charge/discharge (CD) cycles).

Graphical abstract

A facile route is illustrated for the synthesis of NiGa2S4-rGO advanced cathode electrode material with superior electrochemical performance for flexible solid-state asymmetric supercapacitors.

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Introduction

With persistent advent of applications in areas of electronic devices, flexible and weightless systems have been most considered. The crucial challenge is to fabricate flexible electrode with high ED, high PD and superior cycle stability with suitable electrolytes. Accordingly, the request for flexible energy storage devices will develop remarkably [1]. Therefore, supercapacitors are favorable energy storage candidates due to their superior ED than ordinary capacitors, good PD than batteries, superb cycle life and quick charge-discharge rates [2], [3]. Traditional supercapacitors reveal inappropriate shapes that represents several significant disadvantages for their employment in feasible applications. Therefore, flexible solid-state supercapacitors (FSSCs) have appeared as a recent type of energy storage devices. The substantial benefit of FSSCs over ordinary supercapacitors is the usage of a gel electrolyte and flexible electrodes which can be collected in narrow designs of each form and size, therewith enhancing their applications in flexible devices [4]. Lately, many good works on FSSCs with different materials are accessible. Shen et al. [5] offered a Co3O4@NiCo2O4 by a hydrothermal route with the superior Csc and ED of 9.12 F cm−2 and 75.6 W h kg−1, respectively. Li et al. [6] introduced a Ni-Co/LDH using hydrothermal method with the superb Csc of 2380 F g−1 and ultrahigh ED of 141.03. Kim et al. [7] prepared NiCo2S4 via decoration process with the outstanding Csc of 199.74 F cm−3 and good ED of 5.33 mW h cm−3. Zhao et al. [8] studied a Ni(OH)2/Mn2O3 via a hydrothermal route with the remarkable Csc of 2274.4 F g−1 and good ED of 41.6 W h kg−1. Hu et al. [9] prepared a ZnCo1.5(OH)4.5Cl0.5·0.45H2O by in-situ growth on nickel foam with the outstanding Csc of 3946.5 F g−1 and prominent ED of 114.8 W h kg−1. Zhang et al. [10] fabricated Ni-Mn/LDH by self-assembled process with the high Csc of 1379 F g−1 and excellent ED of 131.17 W h kg−1. Lately, transition metal sulfides (TMSs) have appeared as the notable options for supercapacitors. Their unique chemical and physical features (e.g., superior electrical conductivity, thermal durability than metal oxides), also the good redox chemistry which belongs to their good Csc, make them distinguished from other materials. Wang et al. [11] designed a Co3S4-NiS via an ion-exchange with the high Csc of 1810 mF cm−2 and good ED of 6.44 W h m−2. Sathish et al. [12] studied a CoS using solvothermal method with the Csc of 310 F g−1 and good ED of 5.3 W h kg−1. Wang et al. [13] fabricated CNTs@Ni–Co–S by CVD process with the remarkable Csc of 222 mA h g−1 and excellent ED of 46.5 W h kg−1. Niu et al. [14] prepared a NiCo2S4@Ni3V2O8 by a hydrothermal method with the good Csc of 512 C g−1 and high ED of 42.7 W h kg−1. The NiGa2S4, an advanced of TMSs which is scarcely explored for FSASC. Most remarkably, the NiGa2S4 have indicated much better conductivity and electrochemical performance than NiGa2O4 due to their little band gaps [15]. Nevertheless, to better improve electrochemical properties of NiGa2S4, graphene was employed. Designing the NiGa2S4-rGO, considerably enhance electrochemical properties of NiGa2S4 and presenting a supreme conductivity, wide specific surface area and improved the penetration of the ions, and more active sites. Accordingly, a facile route to prepare a new NiGa2S4-rGO with superb electrochemical properties is extremely favorable. In spite of considerable improvement in the construction of cathode materials, the electrochemical properties of the anode materials should always be contemplated to advance the potential of the FSASC device. Extra attempts need to be carried out to create modern types of anode materials. In recent years, Fe-based materials have been employed as anode for FSASC but FeSe2 is seldom investigated for FSASC. FeSe2 is an appropriate option as negative electrode for FSASCs because of its acceptable electrochemical properties of its ions, owning a suitable operation window and the comparative good electrical conductivity [16]. In order to better improve the electrochemical behavior of FeSe2, graphene was added and FeSe2-rGO was fabricated on nickel foam. Although Yang et al. [17] were able to fabricated the FeSe2 as an anode for FSASCs but FeSe2-rGO anode has not been explored so far for FSASCs.

In light of the above ideas, a NiGa2S4-rGO/NF advanced cathode was fabricated by this study. The NiGa2S4-rGO/NF represents a prominent Csc of 2124.34 F g−1 with significant rate capability. The unified NiGa2S4-rGO/NF architecture equips tiny diffusion directions and abundant sites for ions of electrolyte, and provides adequate mass transfer paths for electrode kinetics. Furthermore, a FeSe2-rGO/NF anode with outstanding Csc of 432.40 and better rate capability than FeSe2/NF was also constructed. Element selenium as the favorable ions was successfully offered than other anode elements for regulation and increment of the anode material properties. The both NiGa2S4-rGO/NF and FeSe2-rGO/NF connected with FSASCs can provide outstanding Csc and significant ED, which are essential for their practical applications. Considering the acceptable properties of the as-obtained pseudocapacitive materials, a FSASC based on the NiGa2S4-rGO/NF advanced cathode electrode and FeSe2-rGO/NF advanced anode electrode was constructed, which illustrates outstanding performances with an outstanding Csc. Nevertheless, a FSASC was constructed by employing the NiGa2S4-rGO cathode and FeSe2-rGO anode on a nickel foam, which depicts acceptable performances with a tremendous flexibility, superb ED, and long-term durability. This FSASC with a new electrode material can provide facile route in design of new type of FSASC systems for flexible electronics.

Section snippets

Chemicals and materials

All chemicals were bought from Sigma-Aldrich and Merck companies. All chemical employed in this study containing: Hydrogen peroxide (H2O2), Poly(vinyl alcohol) (PVA) powder, Hydrochloric acid (HCl), Powdered graphite, Potassium permanganate (KMnO4), Sodium hydroxide (NaOH), Sodium nitrate (NaNO3), Sulfuric acid (H2SO4), Selenium powder, Thiourea, Gallium(III) nitrate trihydrate (Ga(NO3)3·3H2O), Ammonia-water (NH4OH), Hydrazine (N2H4), Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), Iron(II)

Characterizations

After construction of NiGa2S4/NF and NiGa2S4-rGO/NF electrodes, the crystal structure of the as-constructed NiGa2S4/NF and NiGa2S4-rGO/NF was described by XRD, as revealed in Fig. 1a. Apparently, for NiGa2S4/NF, the pattern of XRD affirming the construction of a NiGa2S4 on NF which is in desirable adaptability with the NiGa2S4 introduced (JCPDS Card No. 77-1851) [22]. Furthermore, three sharp peaks were observed which are related to NF substrate [23]. Additionally, in the XRD of the NiGa2S4

Conclusion

For the first time, we have successfully developed a NiGa2S4-rGO/NF advanced cathode and FeSe2-rGO/NF advanced anode electrodes for flexible solid-state asymmetric supercapacitors (FSASCs) using facile route. The NiGa2S4-rGO/NF electrode represents remarkable electrochemical performance containing fast kinetics, low internal resistance, good reversibility, and a significant specific capacitance of 2124.34 F g−1 with a superior rate capability of 73% after increasing the current density by a

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