Synthesis of Cubic Ni(OH)2 Nanocages Through Coordinating Etching and Precipitating Route for High-Performance Supercapacitors

Rational design of cage-like structure is an effective method for the improvement of the capacitive performance of transition metal hydroxides. In this work, cubic Ni(OH)2 nanocages (Ni(OH)2 NCs) were constructed through a coordinating etching and precipitating (CEP) route. Ni(OH)2 NCs possess abundant active sites, sufficient diffusion channels, and accelerated electron transfer rate, which are beneficial for electrochemical kinetics. As a positive electrode for supercapacitors, the Ni(OH)2 NCs/Ni foam (NF) electrode presents a high specific capacitance of 539.8 F g−1 at 1 A g−1, which is much larger than that of broken Ni(OH)2 NCs/NF (Ni(OH)2 BNCs/NF, 87.3 F g−1 at 1 A g−1). In addition, the Ni(OH)2 NCs/NF electrode still retains 96.9% of its initial specific capacitance after 2000 cycles. The asymmetric supercapacitor (ASC) devices were assembled using Ni(OH)2 NCs/NF and activated carbon (AC)/NF as positive and negative electrodes, respectively. The ASC exhibits a higher energy density of 23.3 Wh kg−1 at a power density of 800 W kg−1 compared to Ni(OH)2 BNCs/NF (3 Wh kg−1 at 880 W kg−1). These results demonstrate that the Ni(OH)2 NCs/NF electrode presents potential applications in the field of energy storage. The design of cage-like structure paves an effective way to achieve high-performance electrode materials. Electronic supplementary material The online version of this article (10.1186/s11671-019-3096-6) contains supplementary material, which is available to authorized users.


Background
To overcome the challenges of environment pollution and energy crisis, there are significant demands to develop safe, renewable, clean, and high-performance energy storage devices as alternatives to fossil fuels [1,2]. Supercapacitors possess excellent characteristics to meet these issues, such as high-power capability (10-20 times that of batteries), high-rate performance, short charging time, and environment-friendly nature [3,4]. Electric double-layer capacitors (EDLCs) and pseudocapacitors (PCs) are the commonly researched two types of supercapacitors. Thereinto, PCs governed by redox on/near the surface of transition metal oxide/hydroxide electrodes always have higher energy density than EDLCs and have become the hot issues in this field [5][6][7][8][9][10]. As a typical transition metal hydroxide, Ni(OH) 2 was reported as a high-performance electrode material for PCs due to the redox couple of Ni 3+ /Ni 2+ in an alkaline medium [11,12]. Nevertheless, the acquired specific capacitance of Ni(OH) 2 is always much lower than the theoretical value because of the insufficient utilization of electrode materials.
Inspired by kinetics, the capacitive performance of electrode materials can be mediated through the design of microstructure and morphology. Tremendous efforts have been devoted to the synthesis of Ni(OH) 2 electrode materials with unique microstructures to achieve highefficiency storage performance [13,14]. Thereinto, constructing cage-like hollow porous structure was regarded as an effective method to obtain high-performance electrodes. Specifically, a cage-like structure can make full use of the inner and outer surface area and provide enough redox-active sites, leading to enhanced specific capacitance. Furthermore, the porous shell affords amounts of diffusion paths for electrolyte, which is beneficial to the reversibility of the electrode, resulting in excellent cycling stability and high-rate performance. Regarding electron transfer kinetics, the nanosized thin shell refines the transfer route of electrons and accelerates the electron transfer rate [15,16]. Thus, higher capacitive performance of Ni(OH) 2 can be obtained through the design of cage-like hollow porous architecture.
The templated chemical process is the commonly used method to prepare cage-like architectures [17,18]. The final products can accurately duplicate the geometrical shape of the templates and retain a well-defined morphology with narrow size distribution [19,20]. In this work, Ni(OH) 2 NCs were fabricated using cubic Cu 2 O crystals as sacrificial templates through thiosulfate involved coordinating etching and precipitating (CEP) principle. The synthesized Ni(OH) 2 NCs/NF was employed as positive electrode for supercapacitors and Ni(OH) 2 BNCs/NF was introduced as a contrast sample to confirm the structural advantages of cage-like architecture. Ni(OH) 2 NCs/NF displays a high specific capacitance of 539.8 F g −1 at 1 A g −1 , which is much larger than that of Ni(OH) 2 BNCs/NF (87.3 F g −1 at 1 A g −1 ). The asymmetric supercapacitor (ASC) device presents a high energy density of 23.3 Wh kg −1 at 800 W kg −1 , and this value is much larger than that of Ni(OH) 2 BNCs/NF// AC (3 Wh kg −1 at 880 W kg −1 ). The results reveal that Ni(OH) 2 NCs/NF electrode exhibits an attractive prospect in supercapacitors. The way to design cage-like hollow porous architecture is also meaningful in other fields, such as sensors and catalysts.

Preparation of Cu 2 O Templates
Cubic Cu 2 O crystals were synthesized according to our previous report [21]. Fifty milliliters NaOH solution (2 M) was added into the stirred CuCl 2 ·2H 2 O (500 ml, 0.01 M) within 3 min at 55°C. After stirring for 30 min, 50 mL 0.6 M ascorbic acid solution was added dropwise. The final samples were centrifugated after 3 h and dried in a vacuum.

Synthesis of Ni(OH) 2 NCs
400 mg Cu 2 O templates and different dosage of NiCl 2 powers were poured into a 1000-mL beaker containing 400 mL mixed water and alcohol (volume ratio = 1:1). The mass ratio of Cu 2 O templates and NiCl 2 powers is controlled as 5:1, 2.5:1, 1.67:1, and 1.25:1 (corresponding NiCl 2 dosage is 80 mg, 160 mg, 240 mg, and 320 mg, respectively). After ultrasonic treatment for 10 min, 13.2 mg polyvinyl pyrrolidone (PVP) was dispersed into the solution under stirring. After 30 min, 160 mL 1 M Na 2 S 2 O 3 was added dropwise into the solution at room temperature. After 3 h, the final products were collected by centrifugation and dried in an oven. Ni(OH) 2 BNCs were obtained through the ultrasonic treatment of Ni(OH) 2 NCs for 2 h in alcohol (Additional file 1: Figure S1).

Materials Characterizations
The structure and chemical composition of the products were analyzed by X-ray powder diffraction (XRD, Rigaku D/Max-2400) using Cu Kα radiation and ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS, USA). The morphologies of the products were investigated on a Zeiss Gemini 300 field emission scanning electron microscopy (FESEM). Transmission electron microscope (TEM) observations were conducted on a FEI F20 device. The specific surface area and porous feature were measured on a Belsort-max instrument.

Electrochemical Measurements
All the electrochemical measurements were performed on a μIII Autolab workstation in 3 M KOH with Pt foil (1 cm × 1 cm) and Ag/AgCl (saturated KCl) as counter and reference electrodes, respectively. The working electrodes were constructed by the following procedures: first, the electrode materials (Ni(OH) 2 NCs obtained at different reaction time and Ni(OH) 2 BNCs), acetylene black, and polytetrafluoro ethylene (5% PTFE) were mixed together with a mass ratio of 80:15:5 in ethanol. And then, the mixture was coated onto NF (1 cm × 1 cm) and dried in an oven. The loading mass was calculated as 3.4 mg/cm 2 . The electrochemical performance was examined by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance (EIS). The EIS tests were performed between 0.01 and 100 kHz with a perturbation amplitude of 5 mV. The specific capacitance of the electrodes was calculated according to the following equation: where I is the discharge current (A), t is the discharge time (s), ΔV is the potential window (V), m is the total mass (g) of electrode materials. The ASCs were prepared with Ni(OH) 2 NCs (or Ni(OH) 2 BNCs)) and AC as the positive and negative electrodes, respectively. The AC electrode was prepared by coating a mixture of AC and PTFE binder (90:10) on NF (1 cm × 1 cm). Then the two electrodes were assembled together with a separator in 3 M KOH.

Results and Discussions
Characterizations XRD pattern of the prepared Ni(OH) 2 NCs was recorded in Fig. 1a. The observed three strong peaks located at 33.1°, 38.5°, and 60.2°correspond to (100), (101), and (003) crystalline planes of hexagonal β-Ni(OH) 2 (JCPDS no. 14-0117) [22]. XPS measurements were conducted to confirm the chemical composition. Ni, O, and C signals are clearly observed in the survey spectrum, revealing that the sample is mainly composed of Ni and O. As displayed in Fig. 1c, the concentrated signals located at 873.7 eV and 856.1 eV with a separation of 17.6 eV can be attributed to Ni 2p 1/2 and Ni 2p 3/2 of Ni 2+ , respectively [23,24]. The peaks located at 879.9 eV and 861.7 eV are the corresponding satellite signals for Ni 2p 1/2 and Ni 2p 3/2, respectively. As shown in Fig. 1d, the O1s peak located at 531.2 eV presents a typical feature of Ni-O-Ni bond in Ni(OH) 2 [25,26]. On the basis of the above discussions, the as prepared products can be deduced to Ni(OH) 2 phase.
SEM and TEM observations were employed to further confirm the morphology feature of the products. Additional file 1: Figure S2a (Fig. 2b), which is more or less the same as Cu 2 O templates. As can be seen from the inset of Fig. 2b, the surface of Ni(OH) 2 cubes is composed of quantities of fine particles and presents porous characteristic. The TEM image in Fig. 2c exhibits apparent internal cavity, revealing the cage-like feature of Ni(OH) 2 products. As displayed in Fig. 2d, the edge length is of 500 nm, which is consistent with the observation of SEM. Moreover, the shell thickness of Ni(OH) 2 NCs is identified as 50 nm (Fig. 2d). The investigations of SEM and TEM demonstrate the cage-like feature of the products. The cage-like hollow porous structure provides a large surface area and amounts of diffusion paths, which may favor the mass transport process, leading to outstanding capacitive performance.
The optical photographs and TEM images of Ni(OH) 2 NCs were recorded at different reaction times to realize the formation mechanism. As displayed in Fig. 3a, the reaction solution exhibits brick-red color at 5 min, indicating that little reactions occur at the initial stage. Afterwards, the color of the solution gradually becomes lighter. After 3 h, the color of the solution turns into light green, which is the color of the final products. As illustrated in Fig. 3b, the products present partly hollow internal cavity due to the dissolution of Cu 2 O templates at 5 min. Moreover, the etching of internal Cu 2 O preferentially occurred at the corner owing to the adequate diffusion kinetics. The internal Cu 2 O crystals dissolve continuously until it completely disappears at 3 h. The schematic diagram was illustrated in Scheme 1. Generally, the formation mechanism of Ni(OH) 2 NCs is shown below (Eq. (2)): Ni 2+ ions in Eq.  4)).
Equations (3) and (4) are the mechanism for S 2 O 3 2− involved CEP process, which happens in Steps 2 and 3. The detailed kinetics process is similar to the formation of Co(OH) 2 NCs in our published article [27]. The transport of S 2 O 3 2− towards Cu 2 O determines the corrosion rate and the released OH − ions from interior  Fig. 4 shows the N 2 adsorption-desorption isotherm curves of Ni(OH) 2 NCs and Ni(OH) 2 BNCs. The BET surface area of Ni(OH) 2 NCs is 54.7 m 2 /g, which is much larger than that of Ni(OH) 2 BNCs (38.1 m 2 /g). The results indicate that the hollow porous architecture endows Ni(OH) 2 NCs with a larger specific surface area. The pore size distributions (insets of a and b) reveal the mesoporous structure of Ni(OH) 2 NCs and Ni(OH) 2 BNCs. The pore volume of Ni(OH) 2 NCs is calculated as 0.25 cm 3 /g, which is larger than Ni(OH) 2 BNCs (0.19 cm 3 /g). Furthermore, a concentrated pore distribution between 2.7 and 6.1 nm is investigated for Ni(OH) 2 NCs, which is related to the interspace between nanoparticles. However, no obvious concentrated pore distribution is observed for Ni(OH) 2 BNCs, revealing the destruction of ordered diffusion channels. The large surface area and ordered diffusion channels are beneficial for electrochemical kinetics, resulting in excellent capacitive performance.

Electrochemical Performance of Ni(OH) 2 NCs
In order to obtain the best capacitive property, Ni(OH) 2 NCs with different shell thickness were prepared by controlling the dosage of NiCl 2 powders. As shown in Fig. 5, the shell thickness apparently increases from 27.4 to 76.7 nm with the increase of mass ratio from 5:1 to 1.67: 1. However, the shell thickness only slightly increases from 76.7 nm to 79 nm with the further increase of mass ratio to 1.25:1. The results can be attributed to the kinetics difficulty in mass diffusion caused by the hindering of shell. The GCD curves of Ni(OH) 2 NCs obtained with different NiCl 2 dosage were measured and the data were recorded in Fig. 6a. It is clear that the sample with Cu 2 O/NiCl 2 2.5:1 displays the longest discharge time under 4 A/g, indicating the best capacitive performance. This result can be ascribed to the suitable mass transport kinetics derived from the moderate shell thickness. Furthermore, the capacitive performance of Ni(OH) 2 NCs obtained with Cu 2 O/NiCl 2 2.5:1 was contrastively evaluated with Ni(OH) 2 BNCs. As shown in Fig. 6b, significant redox peaks are clearly observed in the CV curves of Ni(OH) 2 NCs and Ni(OH) 2 BNCs, revealing pseudocapacitive characteristic of the two electrodes. The redox process corresponds to the storage mechanism related to Ni(OH) 2 /NiOOH redox couple illustrated in Eq. (5) [28,29].
The CV encapsulated area of Ni(OH) 2 NCs is much larger than that of Ni(OH) 2 BNCs, demonstrating higher Scheme 1 Schematic diagram for the formation process   Fig. 6c. The CV curve still retains a well-defined shape even at a high scan rate of 100 mV/s, demonstrating outstanding rate capability and high electrochemical reversibility. Furthermore, the peak current linearly increases with the square root of scan rates, revealing that bulk diffusion is the dominated factor (Additional file 1: Figure S3). As presented in Fig. 6d, the GCD curves of Ni(OH) 2 NCs show longer discharge time than Ni(OH) 2 BNCs at 1 A g −1 , proving that Ni(OH) 2 NCs exhibit higher specific capacitance than Ni(OH) 2 BNCs. Fig. 6e presents the GCD curves of Ni(OH) 2 NCs at different current densities. The calculated specific capacitances for Ni(OH) 2 NCs are 539.8, 445.5, 409.4, 391.3, 360.2, and 340.7 F g −1 at 1, 2, 3, 4, 5, and 6 A g −1 , respectively (Additional file 1: Figure S4). Those values calculated for Ni(OH) 2 BNCs are 87.3, 77.4, 72.9, 67.8, 64.1, and 60.5 F g −1 at corresponding current density (Additional file 1: Figure S5). The structural advantages for Ni(OH) 2 NCs are illustrated in Fig. 5f. First, the cage-like feature provides quantities of active sites for Faraday reactions. Secondly, the porous thin shell shortens the migration distance of electrons, resulting in high electron transfer rate. Third, the porous shell affords sufficient diffusion channels for electrolyte, improving the utilization rate of Ni(OH) 2 . The cycling stability of Ni(OH) 2 NCs was evaluated by repeating the GCD measurements at 8 A g −1 (Fig. 6 g). It is noticed that the specific capacitance still retains 96.9% of its initial value after 2000 cycles, which is much larger than that of Ni(OH) 2 BNCs (61.5%, Additional file 1: Figure S6). As shown in the inset, the last 10 cycles show little difference compared to the first 10 charge-discharge cycles, revealing excellent stability. The little attenuation of the capacitance can be attributed to the small amount of shedding of Ni(OH) 2 NCs from NF. The internal void and pores in the shell afford enough space for the release of a strain during the cycling process [30].
In order to confirm the advantages of cage-like structure in kinetics, EIS spectra were recorded in Fig. 6h and the equivalent circuit was illustrated in Additional file 1: Figure S7. The equivalent circuit is mainly composed of Rs, Rct, Zw, CPE, and CL. Thereinto, Rs is internal resistance of the electrode system. Rct is the charge transfer resistance related to the radius of semicircle in EIS spectra. Zw is the Warburg impendence corresponding to the slope of EIS in high frequency. Although Ni(OH) 2 NCs/NF electrode has more or less the same Rs value (0.27 Ω) compared to Ni(OH) 2 BNCs/NF (0.25 Ω), Ni(OH) 2 NCs/NF has a much lower Rct (120.8 Ω) than that of Ni(OH) 2 BNCs (976.5 Ω), revealing higher electron transfer rate. The high electron transfer rate can be attributed to enough thin shell of Ni(OH) 2 NCs. Apparently, Ni(OH) 2 NCs/NF electrode presents a much larger slope than Ni(OH) 2 BNCs/NF, demonstrating more straightway diffusion process. The unimpeded diffusion can be ascribed to the ordered channels and porous characteristic of Ni(OH) 2 NCs/NF electrode. On the basis of the above discussions, Ni(OH) 2 NCs/NF electrode possesses significant advantages in electrochemical kinetics compared to Ni(OH) 2 BNCs/NF.

Electrochemical Performance of the ASC device
The ASC device of Ni(OH) 2 NCs/NF//AC was constructed according to Fig. 7a. Ni(OH) 2 NCs/NF electrode and AC were separated by a cellulose paper. As illustrated in Fig. 7b, the CV curve of AC electrode presents nearly rectangular feature, revealing typical EDLC storage mechanism. Moreover, the AC electrode can be cycled within − 1 to 0 V and Ni(OH) 2 NCs/NF electrode can be cycled within 0 to 0.6 V, revealing that the ASC device can afford an operation voltage of 1.6 V. The CV curves displayed in Fig. 7c show a well-defined shape even at high scan rates, implying excellent mass transport kinetics and eminent reversibility. GCD curves of the ASC device at different current densities were shown in Fig. 7d. The energy density and power density of the device were calculated according to Fig. 7d. An energy density of 23.3 Wh Kg −1 is achieved at a power density of 800 W Kg −1 . An energy density of 9.6 Wh Kg −1 is still obtained even at a high power density of 8000 W Kg −1 . The energy density is much larger than that of Ni(OH) 2 BNCs/NF//AC ASC (Additional file 1: Figure S8, 3 Wh Kg −1 at 880 W Kg −1 ). Furthermore, the maximum energy density of the ASC is also larger than those of Ni(OH) 2based materials [31,32]. The cycling stability was estimated by repeating GCD measurements at 4 A g −1 for 2000 cycles. The final specific capacitance still retains 90.1% of its largest value and this value is much larger than that of Ni(OH) 2 BNCs/NF//AC ASC (Additional file 1: Figure  S9, 60%). In addition, the last ten GCD curves are similar to the first ten cycles, exhibiting excellent stability of the ASC device. As shown in Fig. 7f, Ni(OH) 2 NCs still retain the uniform cubic cage-like morphology after 2000 cycles, further demonstrating the excellent cycling stability. The loss of the specific capacitance may be attributed to the small amount of active material dropping from NF.

Conclusions
Overall, Ni(OH) 2 NCs were successfully constructed through a CEP method and used as an electrode for supercapacitors. Ni(OH) 2 NCs present a large specific surface area of 54.7 m 2 /g and a concentrated pore size distribution between 2.7 and 6.1 nm. The thin shell shortens the transfer route and improves the electron transfer rate. As a positive electrode for supercapacitors, Ni(OH) 2 NCs/NF displays a specific capacitance of 539.8 F g −1 at 1 A g −1 , which is much larger than that of Ni(OH) 2 BNCs/NF//AC (87.3 F g −1 at 1 A g −1 ). The specific capacitance still retains about 96.9% of its initial value after 2000 cycles. The ASC of Ni(OH) 2 NCs/NF// AC possesses an energy density of 23.3 Wh Kg −1 at 800 W Kg −1 , which is much larger than that of Ni(OH) 2 BNCs (3 Wh Kg −1 at 880 W Kg −1 ). The results demonstrate that the designed Ni(OH) 2 NCs have potential applications in the field of energy storage.

Additional Files
Additional file 1: Figure S1. SEM image of the Ni(OH) 2 BNCs sample. Figure S2. (a) XRD pattern and (b) SEM image of Cu 2 O templates. Figure  S3. The relationship between peak current and square root of scan rates. Figure S4. The relationship between specific capacitance and chargingdischarging current densities for Ni(OH) 2 NCs/NF. Figure S5. (a) The GCD curves of Ni(OH) 2 BNCs/NF at different current densities; (b) The relationship between specific capacitance and current densities for Ni(OH) 2 BNCs/NF. Figure S6. The cycling stability of Ni(OH) 2 BNCs/NF. Figure S7. The equivalent circuit of EIS. Figure S8. The GCD curves of Ni(OH) 2 BNCs/NF//AC at different current densities. Figure S9