Interconnected NiS nanosheets supported by nickel foam: Soaking fabrication and supercapacitors application
Graphical abstract
Introduction
Supercapacitors (SCs) have attracted much attention due to the unique properties including low maintenance cost, long lifespan, high power density, and fast charge/discharge process [1], [2], [3], [4], [5]. In addition, as power sources, the SCs hold great potential in applications requiring fast bursts of energy or as back-up power sources in electric vehicles. On the basis of energy storage mechanism, supercapacitors (SCs) can be classified into two kinds: double-layer capacitance and pseudocapacitance [6]. The double-layer capacitances store energy via accumulating charge at the interface between electrode and electrolyte. Carbon-based materials, with high specific surface area, thermal stability and conductivity, are a class of typical materials employed in current commercial double-layer capacitances [7], [8], [9]. Pseudocapacitors, storing energy by reversible redox reactions on electrode materials, usually exhibit a higher value of specific capacitance (Cm) than double-layer capacitances due to the presence of Faradaic process. Metal oxides, metal sulfides and polymers are the representative materials used as electrode materials of pseudocapacitors [10], [11].
It is known that the properties of electrode material are determined by the structure. To achieve feasible structure for the electrodes, extensive attention has been fixed on the preparation of low-dimensional nanosized materials (e.g. nanowires, nanorods, nanobelts and nanosheets) due to their unique physical, chemical and optical properties [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Meanwhile, nanosized metal sulfides (e.g. CoS, SnS, NiS, MoS and CuS) have shown prominence as semiconductor materials because of the special optical, magnetic, electrochemical and catalytic properties [22], [23]. The metal sulfides have displayed a wide range of applications, such as in electrochemical capacitors [24], lithium battery cathode materials [25], solar cells [26], catalytic hydrodesulfurization reactions [27] and photoconductive material [28]. In particular, the superior characteristics of metal sulfides make them suitable for pseudocapacitance electrode material. When the metal sulfide contacts the hydroxide ions in the electrolyte, the following reversible redox reaction occurs: MS + OH− ↔ MSOH + e− (M: Co, Ni, etc.) [29]. There reversible reaction facilitates charge storage and the conduction of the electrons and hydroxide ions in the electrode material.
Among numerous metal sulfides, nickel sulfides have been studied extensively, due to the superior performance, facile synthesis and morphological diversity [30]. The character of nanosized materials is closely linked with their structures. Therefore, much research has been fixed on synthesizing nanosized nickel sulfides with novel morphology, exploring the growth mechanism and regulating their size and shape [31], [32], [33], [34], [35]. Currently, a variety of nanosized nickel sulfides with different shapes have been synthesized successively: Zheng et al. prepared a hierarchical flower-like β-NiS via solvothermal synthesis [31]. Electrochemical testing of the flower-like β-NiS electrode exhibited impressive electrochemical performances and high specific capacitance. Zhang et al. synthesized Ni3S2 nanorod@Ni(OH)2 nanosheet core–shell nanostructures using a one-step hydrothermal reaction [32]. Detailed electrochemical characterization showed that the electrode material has high specific capacitance. Nowadays, there are other methods to preparing nanosized nickel sulfide simply and efficiently, which makes the application of nickel sulfide develop widely [36], [37], [38].
In this paper, nickel sulfide nanosheets array grown on the surface of nickel foam (named as NiS-NF) were synthetized via a simple soaking method. The nickel hydroxide grown onto the surface of nickel foam (named as Ni(OH)2-NF) was used as the precursor. By ultrasonic assist conditions, Ni(OH)2-NF is converted to NiS-NF. It is worth noting that this method is effective and the product is neat and orderly. When employed as electrode, the nickel sulfide nanosheets show excellent electrochemical performance and good cycle stability. To evaluate the capacitive performance of the composite in practical application, an asymmetric supercapacitor (named as NiS-NF//AC) was fabricated using the NiS-NF as positive material and active carbon as negative material. The asymmetric supercapacitor exhibited high specific capacitance and outstanding cycle stability. Therefore, the NiS-NF electrode can be a great promising electrode for practical applications in supercapacitors.
Section snippets
Materials
Sodium hydrosulfide hydrate (NaHS·xH2O) and Hydrochloric acid (HCl) were purchased from Aladdin Ltd. (Shanghai, China). The other chemicals were purchased from Tianjin Yaohua Chemical Reagents Co (Tianjin, China). All the chemicals were used as received without further purification. Nickel foam was bought from Changsha Lyrun New Material Co. Ltd. (Changsha, China). The water used throughout all experiments was purified through a Milli-Q system.
Growing Ni(OH)2 nanosheets array onto Ni Foam (Ni(OH)2-NF)
Ni(OH)2 array was grown onto Ni foam by a wet
Reaction mechanism
The sulfide reaction is explained by the solubility product constant (Ksp) and schematic (Fig. 2). In general, materials with lower Ksp values are more stable than those with higher Ksp values. As a result, the latter are prone to conversion into the former in specific solution (with the same cations or anions) [40]. As known, the solubility product constant (Ksp) values for Ni(OH)2 and NiS is 5.5 × 10−16 and 1.3 × 10−25, respectively [29]. The Ksp values of NiS are significantly lower than those
Conclusion
In summary, nickel sulfide nanosheets array grown onto nickel foam were successfully prepared through a simple soaking method under ultrasonic assisted conditions. As precursor, the nickel hydroxide naonsheets on the surface of nickel foam can be directly converted into the nickel sulfide nanosheets via an anion exchange reaction in NaHS aqueous solution. This interlaced structure of nanosheets favors the conduction of electrons; therefore, the electrode materials demonstrate excellent
Conflict of interest
The authors declared that they have no conflicts of interest to other work.
Acknowledgments
This work was supported by National Natural Science Foundation of China (21353003), Heilongjiang Province Natural Science Funds for Distinguished Young Scholar (JC201404), Special Innovation Talents of Harbin Science and Technology for Distinguished Young Scholar (2014RFYXJ005), Fundamental Research Funds of the Central University (HEUCFZ), Natural Science Foundation of Heilongjiang Province (B201404), Program of International S&T Cooperation special project (2013DFA50480), Special Innovation
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