2D nanoporous Ni(OH)2 film as an electrode material for high-performance energy storage devices

A two-dimensional (2D) nanoporous Ni(OH)2 film was successfully developed from triethanolamine (TEA) as the alkali source and soft template using a scalable hydrothermal technique. The nanostructured Ni(OH)2 film was flexible and translucent, and could be directly compressed on a current collector. Owing to the uniform well-defined morphology and stable structure, the Ni(OH)2 film binder-free electrode displayed a high specific capacity, exceptional rate capability, and admirable cycle life. The specific capacitance was 453.6 mA h g−1 (1633 F g−1) at 0.5 A g−1. The assembled Ni(OH)2//activated carbon (AC) asymmetric supercapacitor (ASC) device had an energy density of 58.7 W h kg−1 at a power density of 400 W kg−1. These prominent electrochemical properties of Ni(OH)2 were attributed to the high electrical conductivity, high surface area, and unique porous architecture. Free tailoring, binder-free, and direct pressing were the most significant achievements of the Ni(OH)2 film in the development of high-performance energy storage devices.


Introduction
Increasing applications of high-powered electric devices, fossil fuel depletion, and ever-increasing global environmental problems have motivated researchers to develop various high-energy density energy storage devices, mainly comprising batteries and supercapacitors. [1][2][3][4][5][6] As one of the most promising energy storage devices, supercapacitors exhibit fast charge-discharge ability and a long cycle life, are environmentally friendly, and can provide more power density than batteries. [7][8][9] However, the apparent disadvantage of electrochemical supercapacitors is their low energy density. Therefore, pseudocapacitive materials with faradaic charge storage have attracted increasing interest, such as transition metal oxides, 10,11 hydroxides, 12,13 and conducting polymers, 14,15 providing fast reversible faradaic redox reactions. However, pseudocapacitance involving multiple redox reactions oen suffers from low performance and a lack of cycling stability, mainly caused by the low electric conductivity and rapid decay of the electroactive surface area resulting from the instability of the microstructure/morphology upon fast and repeated charging/discharging. 16,17 Therefore, the development of electrode materials with excellent rate capability, cycling stability, and high specic capacitance retention, remains challenging.
Among various electrode materials, transition metal hydroxides (TMH) have been investigated extensively as electrode materials owing to their abundant oxidation states that can produce reversible redox reactions. [18][19][20] Ni(OH) 2 is a promising active material for electrochemical supercapacitors owing to its high theoretical capacity (2082 F g À1 ), excellent faradaic redox reaction, abundant resources, and environmental friendliness. 21,22 Despite these favorable factors, the capacitance performance of various Ni(OH) 2 -based materials is generally far below that of theoretical capacitors and their actual cycling life is usually poor. 23,24 Many strategies have been devised to enhance capacitors and the actual cycling stability of fabricated Ni(OH) 2 materials. For example, many controllable synthetic routes have been proposed for designing hierarchical nano or microstructures and framework structures to improve the contact area of electrodes and provide fast and efficient charge transfer, effectively improving electrode material utilization. 25,26 For example, Xiong et al. 27 reported that Ni(OH) 2 nanosheets grown on nickel foam showed a specic capacitance of 2384.3 F g À1 at 1 A g À1 with a good cycling ability ($75% of initial specic capacity remained aer 3000 cycles). Liang et al. 28 synthesized a Ni(OH) 2 //AC composite by depositing the soluble precursor of Ni(OH) 2 on AC and obtained a specic capacitance of 2949 F g À1 at a scan rate of 20 mV s À1 with excellent rate and cycling performance. Furthermore, Dai et al. 29 synthesized a Ni(OH) 2 nanocrystals grown on graphene sheets as a pseudocapacitor material, which exhibited a high specic capacitance of 1335 F g À1 at 2.8 A g À1 , and a high retention rate of almost 100% aer 2000 cycles. Kundu et al. 30 fabricated sheets with nickel metal (Ni/Ni(OH) 2 ) NPs using a hydrothermal method, which achieved a specic capacitance of 450 F g À1 at 1 mA cm À2 , with a retention rate of up to 90% at 15 mA cm À2 aer 4000 cycles.
Owing to the anisotropic structure, two-dimensional nanosheets can provide shorter diffusion paths for electrons and ions, and possess signicant advantages in various applications. Preparing large-area 2D nanosheets from the direct growth of nanostructures is considered an effective and straightforward method. 31,32 The high-power electronic transport of 2D nanosheets can satisfy fast charging storage requirements. Furthermore, 2D nanomaterials can demonstrate the inherent properties of 2D structures by stacking layerby-layer, which is conducive to the superexibility and high mechanical properties of synchronization. 33 Traditionally, working electrodes for energy storage devices are prepared using slurry coating technology by mixing active material, conductive carbon, and polymer binder in a certain proportion and coating on the collector. [34][35][36] In fact, the binder increases the total resistance and decreases the electrode conductivity, restricts electron transport, and leads to deterioration of the electrochemical performance. 37,38 Conductive carbon occupies a certain electrode volume and hinders the potential performance, with almost no contribution to capacitance. Therefore, constructing electrode materials with a largearea 2D structure that can be embedded directly on the current collector without any binder is worthwhile, and the expected electrode might offer a highly efficient electron-conducting channel. In this work, we rationally constructed translucent 2D Ni(OH) 2 nanosheets with TEA as the alkali source and so template, as electrodes for energy storage devices. The 2D Ni(OH) 2 semitransparent lm was obtained from Ni(OH) 2 nanosheets by ltration. Aer cutting to a suitable size, the Ni(OH) 2 lms could be directly pressed onto Ni foam. Direct contact between active material and collector was able to transport charge efficiently. Compared with the traditional slurry coating method, the direct pressing method was simple to operate, had low equipment requirements, and made industrial batch production easy to realize. The reasonable electrode design tactics would be of great signicance for highperformance energy storage devices.

Synthesis of 2D Ni(OH) 2 lm
Ni(OH) 2 nanosheets were synthesized using a scalable hydrothermal technique without strict synthesis conditions and complex reaction steps. In a typical procedure, NiCl 2 $6H 2 O (2 mmol) was dissolved in H 2 O (30 mL) and TEA (12 mmol) was added slowly and stirred for 20 min. The mixture was then transferred into a 50 mL autoclave and heated at 180 C for 12 h. Aer the reaction, the obtained green gels were collected by ltration and washed with distilled water. The green product was dried with lter paper at 60 C and the Ni(OH) 2 lm was obtained by slowly removing it from the lter paper.

Electrochemical measurements and calculations
The electrode was fabricated by cutting Ni(OH) 2 lm into a suitable size (1 Â 1 cm) and directly pressing onto two nickel foam pieces (1 Â 1 Â 0.15 cm). Aer pressing on the tablet machine at a pressure of 10 MPa for 1 min, the working electrode was obtained and denoted as Ni(OH) 2 -direct pressing. The electrode prepared using the traditional slurry-coating method was denoted as Ni(OH) 2 -slurry-coating. In the three-electrode system, the Ni(OH) 2 electrodes from direct pressing and slurry coating were directly used as the working electrode, Hg/HgO as the reference electrode, and a platinum plate as the auxiliary electrode. The electrochemical properties of the samples were tested by cyclic voltammetry (CV), constant current chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS) at room temperature in a 6 M aqueous KOH electrolyte on an electrochemical workstation (CHI660E, Chenhua, China). EISs were recorded in the range of 10 5 -0.01 Hz with an ac perturbation of 5 mV with an open circuit voltage. The specic capacity (C s ) of the as-obtained electrode was calculated according to the equation C s ¼ IDt/3.6 Â mDV, where C s (mA h g À1 ) is the specic capacity, and I (A), Dt (s), m (g), and DV (V) are the discharge current, time, active material mass, and working potential range, respectively.
To further determine the potential of Ni(OH) 2 lm for practical applications, an optimized ASC was assembled using Ni(OH) 2 lm as the positive electrode and commercial activate carbon (AC) as the negative electrode, with a cellulose lm sandwiched then immersed in 6 M KOH electrolyte. Typically, the mass ratio of negative electrode to positive electrode was set as 3.3 in the ASC device according to the charge balance equation Q + ¼ Q À . The active material masses were 5.3 and 1.6 mg for the negative and positive electrodes, respectively. The overall electroactive mass of the ASC was about 6.9 mg. The specic capacity (C cell ), energy density (E), and power density (P) of ASC were calculated according to the following three equations, respectively: where C cell (mA h g À1 ) is the specic capacity of the assembled device, M (g) is the total mass of active materials on both positive and negative electrodes, V (V) is the voltage window, E (W h kg À1 ) is the energy density, and P (W kg À1 ) is the power density of the ASC.

Characterization of the product and microstructure
In the synthesis process, primary nanoparticles usually have high surface energy and are prone to agglomeration to form larger particles. 39,40 Capping agents can inhibit the aggregation and growth of nanoparticles to a certain extent by coating nanoparticles, which controls the nanoparticle size. 41 In general, nanomaterials can exhibit excellent optical properties owing to their smaller particle size and fewer surface defects. 42 TEA is a weak alkali with good solubility in aqueous solution, and is low-cost and easy access. TEA acts as a capping agent to coordinate with metal ions and delay the precipitation rate, and can act as a mesopore template to produce hierarchical micro/ mesoporous zeolites. 43,44 In the present work, the Ni(OH) 2 lm was synthesized using TEA as the capping agent, template, and alkali source. The synthesis process was divided into two steps: (i) under magnetic stirring, a stable and clear green NiCl 2 homogeneous solution was formed at room temperature, with coordination occurring between nickel ions and excess TEA (Ni 2+ + 2(HOCH 2 CH 2 ) 3 N / [Ni((HOCH 2 CH 2 ) 3 N) 2 ] 2+ ); (ii) with the increase in reaction temperature, the coordination equilibrium was broken and the OH À concentration in solution increased due to TEA electrolysis. Ni(OH) 2 nucleation occurred via [Ni((HOCH 2 CH 2 ) 3 N) 2 ] 2+ + 2OH À / Ni(OH) 2 + 2(HOCH 2 CH 2 ) 3 N, and TEA was adsorbed around the nucleus of Ni(OH) 2 as a capping agent. Therefore, 2D Ni(OH) 2 nanosheets were formed by nucleation on the TEA template. The reaction mechanism showed that TEA coordinated with Ni ions to restrain spontaneous nucleation in solution, then OH À attacked Ni ions to form Ni(OH) 2 nuclei, which grew oriented by TEA template. Therefore, 2D Ni(OH) 2 nanosheets were produced through coordination, direction, and capping by TEA molecules. The obtained green gels were collected by ltration to afford the Ni(OH) 2 lm (Fig. 1).
The morphological and structural characteristics of the 2D Ni(OH) 2 lm were characterized by SEM and TEM. SEM images of the as-prepared Ni(OH) 2 lm, as shown in Fig. 2A and B, exhibited a uniform porous morphology. TEM micrographs showed the detailed microstructure of Ni(OH) 2 ( Fig. 2C and D). The ultrathin Ni(OH) 2 lm was observed to be interconnected and stacked evenly. The HRTEM image (Fig. 2D) showed that the Ni(OH) 2 nanoparticles were well crystallized, with lattice spacings of 0.23 nm and 0.21 nm, corresponding to the (101) and (103) planes of Ni(OH) 2 , respectively. 45 Energy dispersive Xray spectroscopy (EDX) elemental mapping images (Fig. 2E) indicated that elements Ni, C, N, and O were distributed homogeneously over the nanosheets.
The surface chemical state of the Ni(OH) 2 lm was analyzed by XPS, as shown in Fig. 4A. The survey spectra clearly indicated the occurrence of Ni, C, and O elements at their corresponding binding energies in the synthesized Ni(OH) 2 lm. The core-level spectrum of Ni 2p XPS showed two major peaks at 855.9 (Ni 2p 3/ 2 ) and 875.7 eV (Ni 2p 1/2 ), with a spin-energy separation of 17.6 eV, which was characteristic of the Ni(OH) 2 phase (Fig. 4B). 47,48 The high-resolution C1s spectrum (Fig. 4C) was tted to three peaks, which were respectively assigned to C-C backbones (284.

Electrochemical performance of Ni(OH) 2 lm
The electrochemical performances of the Ni(OH) 2 lm was estimated in the electrode employing a three-electrode system in 6 M KOH aqueous solution. Fig. 5A shows a typical comparison between the Ni(OH) 2 -direct-pressing and Ni(OH) 2slurry-coating electrodes at 10 mV s À1 . The CV curves showed that the Ni(OH) 2 -direct-pressing electrode had a better specic capacity than the Ni(OH) 2 -slurry-coating electrode, owing to a higher integral area. Fig. 5B shows the CV curves of the Ni(OH) 2 -direct-pressing electrode at different scan rates in the range of 5-50 mV s À1 . The CV curves were composed of strong redox peaks, indicating that the faradaic capacitance characteristics were controlled by a faradaic redox reaction. All CV curves showed similar anodic and cathodic peaks with increasing scan rates, indicating a good reversibility at the nanostructure interface and excellent rate capability. Furthermore, as the scan rate was increased, the potential difference between the corresponding cathode peak and the anode peak increased due to polarization. The CV curves at 50 mV s À1 showed obvious peaks, indicating that the porous Ni(OH) 2 lm was conducive to rapid reaction. The Faraday process of Ni 2+ /  Ni 3+ accompanied by OH À in the Ni(OH) 2 lm electrode material under alkaline conditions was described as follows: Ni(OH) 2 + OH À 4 NiOOH + H 2 O + e À . 50 The GCD curves, as shown in Fig. 5C, further corroborated the Faraday capacitive behavior of the Ni(OH) 2 -direct-pressing electrode. The discharge curves all showed a wide potential plateau that shortened with increasing current density. From the GCD curves, the calculated specic capacity values were 453.6, 426, 396.7, 354.2, 312.2, and 232.2 mA h g À1 at 0.5, 1, 2, 5, 10, and 20 A g À1 , respectively, and the capacity retention was 51.2% under the operation current density performed 20 times. The cycling performance of the Ni(OH) 2 -direct-pressing and Ni(OH) 2 -slurry-coating electrodes at 10 A g À1 was measured and is shown in Fig. 5D. The cycling test suggested that the Ni(OH) 2direct-pressing electrode possessed a high stability for longterm applications. The initial specic capacity retention rates of the Ni(OH) 2 -slurry-coating and Ni(OH) 2 -direct-pressing electrodes were 71.5% and 85.6%, respectively, aer 2000 cycles of  charge and discharge. The capacity performance of the Ni(OH) 2direct-pressing electrode was clearly better than that of the Ni(OH) 2 -slurry-coating electrode produced using the traditional manufacture method. In the charge-discharge process, the crystal structure of the electrode active material changed, leading to deformation of the material structure. Aer undergoing multiple charge-discharge cycles, the active material easily detached from the current collector, resulting in capacity attenuation. 51 Furthermore, the performance of the Ni(OH) 2direct-pressing electrode was similar to those of Ni(OH) 2 and Nibased nanostructure electrodes recently reported, as summarized in Table S1. † Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) of the as-formed Ni(OH) 2 and Ni(OH) 2 -multiple bending electrodes were performed to compare their electrochemical behavior. Interestingly, the integral area of the CV curves in Fig. 6A increased aer multiple bending, which indicated that the Ni(OH) 2 -multiple bending electrode possessed a much larger capacity. Fig. 6B shows that the Ni(OH) 2 -multiple bending electrode possessed better capacity performance. The calculated specic capacity of the Ni(OH) 2 -multiple bending and Ni(OH) 2 electrodes was 421 and 511 mA h g À1 at a current density of 1 A g À1 , respectively. As shown in Fig. 6C, the Ni(OH) 2 -multiple bending electrode showed superior specic capacities of 522.5, 511, 420.6, 380.6, 322.2, and 284.7 mA h g À1 at 0.5, 1, 2, 5, 10, and 20 A g À1 , respectively. EIS analysis was performed to further investigate the electrochemical behavior of the Ni(OH) 2 -multiple bending electrode. Fig. 6D shows that the superior performance of the Ni(OH) 2 -multiple bending electrode could be attributed to the lower electrochemical impedance aer multiple bending.
Although Ni(OH) 2 lm attached to the Ni foam through direct pressing had a high faradaic capacity, its energy density was limited owing to the narrow voltage range. The ASC device consisting of as-synthesized Ni(OH) 2 lm and commercial AC was assembled to broaden the voltage window. The typical characteristics of the electrochemical double-layer charge storage behaviors of AC were shown by the CV curves with a rectangular shape and linear GCD curves, among others, as shown in Fig. S1. † The CVs of the ASC at different potential windows were investigated with a scan rate of 10 mV s À1 (Fig. 7A). The overall CV curves displayed a pair of broad and indenite redox peaks from faradaic peaks in a two-electrode system. With expansion of the potential window from 1.0 to 1.7 V, the CV area increased signicantly, and more pronounced redox reactions appeared in a wider voltage window, resulting in a higher specic capacity and energy density. Fig. 7B shows the CV curves of the ASC device at various scanning rates, in which the peak currents and CV loop areas increased with increasing scanning rate, while the peak potential varied slightly, suggesting the low overpotential resulting from the high conductivity of the two electrodes. GCD curves of different voltage windows and current densities were investigated to evaluate the charge storage capacitance of the ASC (Fig. 7C). As the voltage window was gradually increased, the discharge time increased markedly, suggesting a dramatically enhanced specic capacity in the wider voltage window. The discharge and charge curves of ASC were almost symmetrical and the inner resistance voltage drops were low at different operation current densities, reecting good electrochemical reversibility and coulombic efficiency. The specic capacity values of the ASC device were calculated to be 45.8, 42.7, 34, 27.8, and 19.1 mA h g À1 at 0.5, 1, 2, 5, and 10 A g À1 , respectively, and the capacity retention was 42% under the operation current density performed 20 times (Fig. 7D). A similar bending test was performed on ASC (Fig. S2 †), resulting in acceptable electrochemical performance degradation in the contrast experiment. The specic capacity values of ASC were calculated to be 37.5, 36.2, 31.9, 25.2, and 17.9 mA h g À1 at 0.5, 1, 2, 5, and 10 A g À1 , respectively, and the capacity retention was 48% under the operation current density performed 20 times (Fig. S3 †), which provided a better rate capability.
Energy density (E cell ) and power density (P cell ) are the key parameters for evaluating the ASC device. The energy and power delivery abilities of the Ni(OH) 2 lm//AC ASC were determined by Ragone plots, as shown in Fig. 8A. The E cell of the Ni(OH) 2 lm//AC ASC reached 58.7 W h kg À1 at a low P cell of 400 W kg À1 ,   and decreased to 24.5 W h kg À1 at a P cell of 8000 W kg À1 . The E cell of the Ni(OH) 2 lm-multiple bending//AC ASC decreased to 48 W h kg À1 at a P cell of 400 W kg À1 , and 22.9 W h kg À1 at a P cell of 8000 W kg À1 . The energy and power delivery abilities of the Ni(OH) 2 lm//AC and Ni(OH) 2 lm-multiple bending//AC ASCs were superior to those of previously reported supercapacitor systems, as listed in Fig. 8A. The cycling stability of the ASCs were further evaluated using 10 000 successive GCDs between 0 to 1.6 V at a current density of 5 A g À1 . The capacity retention rates of the Ni(OH) 2 //AC and Ni(OH) 2 lm-multiple bending//AC ASCs were 91% and 82%, respectively, indicating a high charge transfer efficiency in long-term cycling. Based on the good cycling stability, the Ni(OH) 2 lm//AC ASC reported herein could serve as an efficient, long-lifetime, and high-energydensity storage system with a great potential in practical applications.

Conclusions
In summary, a facile hydrothermal method was applied to fabricate an ultrathin Ni(OH) 2 lm. Large-area 2D transparent and exible nanostructures were formed in a straightforward manner. The Ni(OH) 2 lm was directly pressed on Ni foam as the working electrode without binder or conductive carbon. The Ni(OH) 2 lm electrode showed a high capacity of 453.6 mA h g À1 (1633 F g À1 ) at a current density of 0.5 A g À1 , and a high rate capability owing to its high specic surface area and mesoporous structure. The Ni(OH) 2 -direct-pressing//AC ASC presented an outstanding specic capacitance of 45.8 mA h g À1 (165 F g À1 ) at 0.5 A g À1 . The maximum energy density of the Ni(OH) 2 -lm//AC ASC was 58.7 W h kg À1 at 400 W kg À1 . The assembled ASC showed long-term cycling stability (91% capacity retention ratio aer 10 000 cycles at 5 A g À1 ). These encouraging results conrmed that the 2D Ni(OH) 2 lm is a promising candidate for the development of highperformance energy storage devices.

Conflicts of interest
There are no conicts to declare.