Large Scale Synthesis of NiCo Layered Double Hydroxides for Superior Asymmetric Electrochemical Capacitor

We report a new environmentally-friendly synthetic strategy for large-scale preparation of 16 nm-ultrathin NiCo based layered double hydroxides (LDH). The Ni50Co50-LDH electrode exhibited excellent specific capacitance of 1537 F g−1 at 0.5 A g−1 and 1181 F g−1 even at current density as high as 10 A g−1, which 50% cobalt doped enhances the electrical conductivity and porous and ultrathin structure is helpful with electrolyte diffusion to improve the material utilization. An asymmetric ultracapacitor was assembled with the N-doped graphitic ordered mesoporous carbon as negative electrode and the NiCo LDH as positive electrode. The device achieves a high energy density of 33.7 Wh kg−1 (at power density of 551 W kg−1) with a 1.5 V operating voltage.


Results
A new synthetic method was used to prepare NiCo-LDHs and Ni(OH) 2 and Co(OH) 2 in a homogeneous ethylene glycol-water system. The ultrathin 2D nanostructure was obtained for these materials. Among them, Ni 50 Co 50 -LDH exhibited excellent electrochemical performance, being listed in Table S1 (Support Information, SI). Therefore, Ni 50 Co 50 -LDH was mainly characterized the structure and perform the electrochemical measurement. The synthetic mechanism was illustrated in Fig. 1. Ammonia was gradually generated by the hydrolysis of urea (Eq. 1). Then NH 3 molecules reacted with Ni and Co metal ions and formed complexes (Eq. 2). Excess amount of NH 3 molecules produced OH − and the nickel and cobalt ions were formed Ni(OH) 6 and Co(OH) 6  Ni(OH) 6 and Co(OH) 6 octahedra nuclei were self-assembled to form the infinite 2D sheets composed of metal cations occupy the centre of octahedra's edge and hydroxide ions at vertexes. These 2D sheets further extended and form the Ni 50 Co 50 -LDH nanosheets. The nanosheets were washed with ethanol and water. It is expected that H 2 O molecules and NO 3 − ions will be retained within the interlayer space of LDH through hydrogen bond. Importantly, 10 gram scale of Ni 50 Co 50 -LDH can be readily prepared by this simple synthetic method (Fig. S1, SI), which holds great promise for mass production.
The morphology of as-prepared samples was characterized by SEM and TEM techniques. SEM images in Fig. S2 showed that ultrathin nanosheets were about a thickness of ~16 nm. TEM, HRTEM images and the selected-area electron diffraction (SAED) patterns of α-Ni(OH) 2 ,α-Co(OH) 2 and Ni 50 Co 50 -LDH were shown in Fig. 2. TEM images confirmed that this method could be used to prepare the ultrathin nanosheets transition metal hydroxides and LDHs (Fig. 2a,c,e). In the HRTEM images, lattice fringes were observed on the nanosheets (Fig. 2b,d,f). At the same time, the SAED patterns collected from the nanosheet also exhibited diffraction rings but vague spots, indicating the crystallinity of these samples is relatively low. In addition, some pore structure was also found, which will be advantageous to the electrolyte diffusion.
The crystal structure of Ni 50 Co 50 -LDH and α-Ni(OH) 2 and α-Co(OH) 2 were further characterized by XRD analysis. As shown in Fig. 3a 31,32 showing typical low-crystalline α-hydroxides with weak diffraction peaks of (003), (006) and (012) planes in the XRD patterns. The low crystallinity is in accordance with the above-mentioned HRTEM and SAED characterization. Figure 3b shows the FTIR spectra of Ni 50 Co 50 -LDH, α-Ni(OH) 2 and α-Co(OH) 2 samples. They have similar IR bands. The signal at 3453 cm −1 is the O-H stretching band, arising from interlayer water molecules and metal-hydroxyl groups. The band centered at 1634 cm −1 can be ascribed to the bending vibration of water. Additionally, the band at 1388 cm −1 can be assigned to the vibration of interlayer CO 3 2− 32,33 . Figure S3a and b show the N 2 adsorption-desorption isotherms and the corresponding Barret-Joyner-Halenda (BJH) pore size distribution of these samples, respectively. The samples presented a type III curve with H1 hysteresis loop at high relative pressure, indicating the presence of macropores and mesopores. The adsorption isotherms became rapidly saturated at low relative pressure, illustrating the low adsorption volume of metal oxides or LDHs. A platform at P / P 0 = 0.20-0.80 originated from the outer superfacial adsorption of nanosheets, contributing the low adsorption volume. In addition, a hysteresis loop at a higher relative pressure (P / P 0 = 0.80-0.99) was obtained. This loop resulted from the macroporous adsorption among the overlap gaps of the nanosheets. It was noted that the desorption branch of LDH showed type IV with H2 hysteresis loop, suggesting the existence of mesoporous structure. The BET surface areas of the Ni 50 Co 50 -LDH, α-Co(OH) 2 and α-Ni(OH) 2 were 80, 97 and 119 m 2 g −1 , respectively, and the average pore size was mainly less than 10 nm. On the other hand, the ion radii (74pm) of Co 2+ is larger than 72 pm of Ni 2+ , thus with cobalt doping, the interlayer distance was widened and facilitate ion transfer. Nitrogen absorption-desorption measurement indicates that the specific surface of LDHs is increased with the increase of cobalt contents and confirms the presence of mesoporous loop (see Fig. S4). Electrochemical measurements were carried out to study the charge storage performance of the Ni 50 Co 50 -LDH samples in 6 mol L −1 KOH electrolyte. Figure 4a shows the CV curves for the LDH electrode at different scan rates. A set of distinct redox peaks were observed between 0.1 V and 0.5 V vs. Hg/HgO, which are consistent with the capacitive behavior reported for Ni(OH) 2 and Co(OH) 2 34,35 . The current intensity increased almost linearly with the increase of scan rate, implying excellent reversibility and rapid charge-discharge response 36,37 .
The mechanisms of electric energy storage for pseudo-capacitor are proposed as follows (Eq. 5-7). The pseudo-capacitance of LDH is attributed from both α-Co(OH) 2 and α-Ni(OH) 2 . Redox reactions of α-Co(OH) 2 contain two steps as shown in Fig. 5a. The electrons were transported among Co 2+ , Co 3+ and, Co 4+ ions with the protons transfer (Eq. 5 and 6). Equation 7 illustrates the charge/discharge mechanism of Ni(OH) 2 .    Figure 4b shows the galvanostatic charge-discharge curves of the LDH electrode at different current densities. As a typical battery material, the LDH showed almost symmetrical charge and discharge curves, indicating fast and good electrochemical reversibility. The specific capacitance of the LDH achieved excellent initial specific capactance of 1537 F g −1 at 0.5 A g −1 and 1181 F g −1 even at current density as high as 10 A g −1 . A 1000-cycle stability curve collected at 2 A g −1 is shown in Fig. 4c. The initial specific capacitance was 1494 F g −1 , the value slowly increased to a maximal value of 1542 around 400 th cycle, which was attributed to the activation of Ni-based electrode materials 38 . The retention of the specific capacitance was 80.3% after 1000 cycles. Electrochemical impedance spectroscopy (EIS) was carried out to evaluate the diffusion of electrolyte ions to porous structure and charge transfer at the interface of LDH (Fig. 4d). The impedance plots exhibited two distinct parts including a semicircle in the high-frequency region and a sloped line in the low-frequency region. The charge transfer resistance (R ct ) was estimated to be ~ 0.8 Ω from the semicircle diameter at the high-frequency. The small R ct could be attributed to the ultrathin nanosheets morphology, which allows efficient charge transfer between the electrolyte and LDH. In addition, the solution resistance (R s ) was estimated to be ~0.48 Ω from the left intersection point of the semi-circle and Z'-axis. The low R ct and R s as well as high specific capacitance support that the Ni 50 Co 50 -LDH is an excellent capacitive electrode material for ultracapacitors.
The specific capacitance and capacitance retention were measured for Ni 50 Co 50 -LDH, α-Co(OH) 2 and α-Ni(OH) 2 using galvanostatic charge/discharge. As shown in Fig. 5c,d, the specific capacitance of LDH is substantially larger than that of α-Ni(OH) 2 and α-Co(OH) 2 (Fig. 5c). The capacitance of α-Ni(OH) 2 rapidly decreased with the increase of current density. However, the capacitance retention of α-Co(OH) 2 at 10 A g −1 is lightly over that at 0.5 A g −1 (see Fig. 5d). Though the specific capacitance of α-Co(OH) 2 is much less than that of pure α-Ni(OH) 2 , 50% cobalt-doped α-Ni(OH) 2 (Ni 50 Co 50 -LDH) visibly exhibits excellent electrochemical performance, involving superior specific capacitance and the capacitance retention to pure α-Ni(OH) 2 . This is also confirmed by the other reseaches. Lang et al. obtained Ni 44 Co 56 oxide nanoflakes with a maximum specific capacitance of 1227 F g −1 at 0.625 A g −1 based on 0.4 V operating potential 39 . When the atom ratio of nickel and cobalt is close to 1:1, these kinds of materials exhibit the superior electrical conductivity 25 . Cobalt was introduced in LDH to improve the conductivity of electrode materials 25 and raise the oxygen overpotential advantageous to widening potential window 18 . Co 2+ were oxidized to form the conductive CoOOH in discharge process, resulting in the increase of conductivity of electrode materials 26 . Due to the cobalt introduced to participate in the electrochemical redox reaction, good conductivity improves the charge transfer and low R ct and R s are helpful with Faradic reaction, resulting in Ni 50 Co 50 -LDH presents high performance in electrochemical energy storage than nickel hydroxide. The comparable CV curves for the LDH and α-Ni(OH) 2 and α-Co(OH) 2 at 100 mV s −1 is shown in Fig. 5b. The serious polarization is shown in CV curve of α-Ni(OH) 2 . The reversibility of LDH is visibly improved due to cobalt doped, which is helpful with the Columbic efficiency and the materials utilization.
The potential window of Ni 50 Co 50 -LDH is relatively small (~0.55 V), which seriously limit its practical application. In order to enlarge the operating voltage window, we fabricated an asymmetric device using GOMC as negative electrode and Ni 50 Co 50 -LDH as positive electrode (denoted as GOMC//Ni 50 Co 50 -LDH), as shown in Fig. 6a. GOMC is a promising negative electrode material that has long cycling stability and 1.0 V operating potential window in alkaline electrolyte 40 . The CV of GOMC presented a typical rectangular shape in agreement with its electric double-layer capacitive behavior.
The designated asymmetric capacitor has an optimal operating voltage of 1.5 V. When the voltage reached 1.6 V, water splitting occurs and the current drastically increased (Fig. 6b and Fig. S5b). For asymmetric UCs, the charges on anode and cathode should be balanced (q+ = q− ). This can be achieved by manipulating the mass loading of active materials on each electrode. According the following equation, the total charge (q) of one electrode stored is depending on the specific capacitance (C), the potential window (Δ E) and the mass of the electrode (m) 41,42 .
Therefore, the ratio of mass loading of negative and positive electrode materials can be calculated by Eq. 9.
Based on the data of the specific capacitances and potential windows of two electrodes, the optimal mass ratio between GOMC and LDH was 5:1.
Cyclic voltammetry and galvanostatic charge/discharge measurements were collected from the asymmetric UC device. As shown in Fig. 6b, the capacitance of GOMC//Ni 50 Co 50 -LDH asymmetric UC increase gradually due to the activation of nickel hydroxide. Figure 6c shows CV curves measured at different scan rates, the large area of different curves clarified superior performance of this device. Galvanostatic charge/discharge curves were conducted at different current densities (Fig. 6d)  Cycling stability is a key factor for evaluating the device performance in practical application. Figure 6f shows the CV curves collected at 200 mV s −1 as a function of number of cycles. The capacitance quickly increased in the first 500 cycles, which is in good agreement with the Ni 50 Co 50 -LDH electrode performance measured in 3-electrode system. Thereafter, the capacitance decrease gradually. The specific capacitance retention rate was 109% after 10000 cycles (Fig. 7a). The asymmetric device exhibits high specific energy density and excellent cycling stability. These excellent electrochemical performances could be attributed to: (i) the ultrathin and porous nature of Ni 50 Co 50 -LDH and (ii) fast charge transfer, rapid mass transport and anti-corrosion of GOMC. Moreover, GOMC//Ni 50 Co 50 -LDH device could successfully power a red light-emitting-diode (LED) with a nominated voltage of 1.5 V for over 6 min after charging with current density of 4 mA cm −2 (Fig. 7b).

Discussions
Based on the above analysis, it could be found the ultrathin Ni 50 Co 50 -LDH has been successfully synthesized by the efficient and low cost strategy. The morphology characterizations showed that ultrathin nanosheets were about a thickness of ~16 nm and the electrochemical results reveal that Ni 50 Co 50 -LDH possesses high specific capacitance. The ultrathin porous nanostructure can not only be beneficial for efficient ion and electron transport but also improve specific surface area to increase active sites for the energy storage. In addition, the excellent conductivity of as-prepared material has demonstrated by the EIS testing which may also attribute to the enhanced capacitance.
In summary, we have demonstrated a scalable and environmentally-friendly strategy for large-scale preparation of ultrathin Ni 50 Co 50 -LDH. The Ni 50 Co 50 -LDH exhibited high pseudo-capacitance and kinetic properties to be used as the cathode materials for electrochemical energy storage. Therefore, we have developed the asymmetric capacitor composed of Ni 50 Co 50 -LDH and GOMC, which exhibits wide operating voltage of 1.5 V, excellent stability (109% capacitance retention after 10000 cycles), high energy density (33.7 Wh kg −1 ) and power density (5452 W kg −1 ). We believe this novel strategy can be extended to prepare other ultrathin 2D capacitive materials for charge storage devices.  then dried at 60 °C. The as-prepared sample was denoted as Ni 50 Co 50 -LDH. The same experimental procedures were also employed to prepare ultrathin Ni(OH) 2 , Ni 79 Co 21 -LDH, Ni 76 Co 24 -LDH, Ni 64 Co 46 -LDH, Ni 35 Co 65 -LDH and Co(OH) 2 by changing the ratio of the nickel and cobalt source.

Methods
Characterization. Powder X-ray diffraction measurements were performed by a MSAL-XD2 X-ray diffractometer (Cu Kα , 36 kV, 20 mA, λ = 1.5406 Å). The morphologies of LDH samples were examined by field-emission scanning electron microscope (SEM) (FSEM, ZEISS Ultra 55) and high resolution transmission electron microscope (TEM) (HRTEM, JEOL JEM-2100F) with an accelerating voltage of 200 kV. The FT-IR spectra were collected by a Nicolet 6700 FT-IR spectrometer. Nitrogen sorption isotherms of samples were collected by a Micromeritics TriStar 3000 Analyzer at 77 K. Elemental analysis was performed by the inductively coupled plasma optical emission spectrometer (Perkin Elmer, optima 2000DV), indicating the Ni/Co atom ratio of LDH.
Electrochemical measurements. Working electrode was fabricated by sandwiching the mixture of active materials (8 mg), carbon black and PTFE (with a mass ratio of 80:15:5) between two pieces of nickel foams. The mass loading of the electrode was measured by the mass difference before and after sandwiching. A nickel foil and an Hg/HgO electrode were used as current collector and reference electrode, respectively. All electrochemical measurements were performed on a CHI660D electrochemical workstation in a standard three electrodes cell at room temperature. Cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) tests were all performed in 6 mol L −1 KOH aqueous solution. EIS analysis was performed at the frequency range of 100 kHz ~0.1 Hz with amplitude of 5 mV. Asymmetric capacitors were fabricated by using N-doped graphitic ordered mesoporous carbon (GOMC) as negative electrode and Ni 50 Co 50 -LDH as positive electrode, and their electrochemical performance was measured in 6 mol L −1 KOH aqueous solution by a 2-electrode cell system.