Electrochimica Acta 226 (2017) 113–120
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Porous nickel-cobalt layered double hydroxide nanoﬂake array derived from ZIF-L-Co nanoﬂake array for battery-type electrodes with enhanced energy storage performance Jingcheng Zhanga,b , Kesong Xiaob , Tianci Zhangb , Gang Qianb , Yang Wangb , Yi Fenga,b,* a b
School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, People’s Republic of China Instrumental Analysis Center, Hefei University of Technology, Hefei, Anhui, 230009, People’s Republic of China
A R T I C L E I N F O
Article history: Received 24 October 2016 Received in revised form 31 December 2016 Accepted 31 December 2016 Available online 3 January 2017 Keywords: nickel-cobalt layered double hydroxide nanoﬂake array (Ni-Co LDH-NFA) leaf-like Co-containing zeolitic imidazolate framworks (ZIF-L-Co) ZIF-L-Co nanoﬂake array (ZIF-NFA) battery-type electrodes
A B S T R A C T
In view of the complicated synthesis procedures of metallic layered double hydroxides (LDHs) in general preparation, we here report a facile route for synthesizing porous nickel-cobalt layered double hydroxide nanoﬂake array (Ni-Co LDH-NFA) on nickel foam on the base of our previously reported leaf-like Cocontaining zeolitic imidazolate framworks (ZIF-L-Co). The ZIF-L-Co nanoﬂake array (ZIF-NFA) is ﬁrst grown on nickel foam, which serves as a sacriﬁcial template to synthesize Ni-Co LDH-NFA when it reacts with nickel nitrate at room temperature. The as-prepared Ni-Co LDH-NFA could be directly used as battery-type electrodes without polymer binder. Due to the highly ordered layered crystal structure of LDH and the well-deﬁned porous nanostructure of nanoﬂake array, such Ni-Co LDH-NFA electrode exhibits outstanding speciﬁc capacity of 894 C g1 at a current density of 2 A g1. In addition, an assembled asymmetric supercapacitor device also exhibits excellent speciﬁc energy density of 48.6 Wh kg1 at a speciﬁc power density of 1700 W kg1. Even at a high power density of 17 kW kg1, the device could still remain an energy density of 18.5 Wh kg1. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Metallic layered double hydroxides (LDHs) with a common formula of [M2+1-x M3+x(OH)2]x+[(An)x/nmH2O]x (M2+, M3+ can be substituted by the bivalent and trivalent metal cations respectively, and An is exchangeable anion of valence n, x = M3+/(M2+ + M3+)) have widely applied in various ﬁelds including separation , catalysis , ﬁre retardant additives [3,4], drug delivery hosts  and electrochemistry [6,7]. Speciﬁcally, owing to the relatively low cost, high redox activity, and environmentally friendly nature, some reported LDHs such as Ni-Co LDH and Ni-Mn LDH could serve as promising supercapacitor electrodes, showing excellent electrochemical properties for energy storage [7–11]. However, the syntheses of LDHs by traditional methods such as co-precipitation , hydrothermal method  and urea decomposition-homogeneous precipitation , are always uncontrollable and complicated. In order to obtain homogeneous and crystalline precipitates, in the traditional methods it usually
* Corresponding author at: School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, People’s Republic of China. E-mail address: [email protected]
(Y. Feng). http://dx.doi.org/10.1016/j.electacta.2016.12.195 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
demands thermal reaction and additional alkali sources or oxidants to produce OH1 ions or trivalent cations. Taking the syntheses of Mg-Al and Zn-Al LDH nanocomposites reported for example , MgCl2 or ZnCl2 was ﬁrstly mixed with AlCl3 in deionized water, then the pH value was adjusted to be 10 by gradually adding NaOH, which caused the metal co-precipitation (i.e., LDH material), subsequently, the mixture was transferred into a bottle and heated to 80 C for 24 h, the precipitates were ﬁnally washed with deionized water until no chloride was left, and dried at 65 C. Due to the special framework and structural ﬂexibility, Zeolitic imidazolate framework (ZIF) materials as a very important subclass of Metal-organic frameworks (MOFs) have attracted increasing attentions, which has been employed for numerous applications including gas storage [15,16], molecular separation [17,18], catalysts [19,20], and energy ﬁelds [9,21,22]. Beneﬁting from high thermal stability and chemical robustness, Co-containing ZIF-67 could be used as an ideal template for metal oxides/ hydroxides with good activity in energy ﬁelds [9,21,22]. For examples, Co3O4 hollow dodecahedrons with complex interiors through direct pyrolysis of ZIF-67 presented outstanding performances as the anodic material of lithium-ion batteries (LIBs), and it delivered an high reversible capacity of 1550 mA h g1 . In
J. Zhang et al. / Electrochimica Acta 226 (2017) 113–120
addition, for Co3O4/NiCo2O4 double-shelled nanocages starting from ZIF-67 templates after hydrolysis reaction and post-annealing treatment, they were used as electrodes for pseudocapacitors and exhibited a high speciﬁc capacitance of 972 F g1 at a current density of 5 A g1 . Very interestingly, Ni-Co LDH nanocages with superior pseudocapacitance property can be obtained by using ZIF-67 nanocrystals as templates without any subsidiary conditions, only simply nickel nitrate methanol solution is introduced at room temperature , this method offers a feasible way of preparing LDHs. However, these reported ZIF-derived metal oxides/hydroxides almost present particle-like shapes. Since in the preparation process of electrodes, the involved nonconductive polymer binders (PVDF, PTFE, etc.), always cause low-efﬁciency ion transport. On the other hand, the agglomeration of particles also results in low surface area, decreasing active sites for charge storage. Therefore, it is especially favorable to construct ordered ZIF-derived LDH array materials on a conductive substrate. In our previous work, a novel Co-containing Zeolitic imidazolate framework with leaf-like morphology (ZIF-L-Co) is synthesized in aqueous media . Herein, we have ﬁrstly synthesized a highly ordered ZIF-L-Co nanoﬂake array (ZIF-NFA) on a nickel foam (Scheme 1, step I). Subsequently, after immersing the as-perpared ZIF-NFA into nickel nitrate methanol solution for 24 h, nickel-cobalt layered double hydroxide nanoﬂake array (NiCo LDH-NFA) with plenty of nanopores inside can be prepared ﬁnally (Scheme 1, step II). The whole reactions proceed at room temperature without any other auxiliary conditions. In addition, the porous Ni-Co LDH-NFA exhibits excellent electrochemical properties stemming from the highly ordered layered crystal structure of LDH and the well-deﬁned porous nanostructure of nanoﬂake array. More importantly, the asymmetric supercapacitor device assembled with Ni-Co LDH-NFA and active carbon (AC) also shows very high power density and energy density. Our study provides a novel and facile strategy to obtain a binder-free Ni-Co LDH battery-type electrodes for energy storage applications, such strategy is believed to extend to other bimetallic LDH syntheses, and the as-obtained ZIF-NFA and Ni-Co LDH-NFA are expected for other potential applications such as catalyst, sensor and gas separation ﬁelds.
solution at a ultrasonic bath for 25 min to remove the oxide layer on the surface, washed with ethanol and deionized water for several times, and then dried to constant weight in a vacuum oven at 50 C. The synthesis procedure of ZIF-L-Co was described in detail in our previous work . For the ZIF-L-Co ﬁlm preparation, ﬁrstly 11.64 g cobalt nitrate hexahydrate ((Co (NO3)2)6H2O) and 3.28 g 2-methylimidazole (Hmim) were dissolved in 100 mL deionized water respectively. Subsequently, the Ni foam was dipped into 16 ml of Hmim aqueous solution for few minutes, then accordingly 2 ml of cobalt nitrate aqueous solution was added with agitation for 10 min and the reaction was kept for 2 hours at room temperature. Finally, ZIF-NFA could be orderly aligned on Ni foam, and dried at 50 C after washing with ethanol and deionized water. 2.2. Syntheses of Co LDH-NFA and Ni-Co LDH-NFA Typically, appropriate cobalt nitrate hexahydrate ((Co(NO3)2) 6H2O) and nickel nitrate hexahydrate ((Ni(NO3)2)6H2O) were dissolved in 100 mL methanol to obtain 200 mM solution respectively. Co LDH-NFA and Ni-Co LDH-NFA were converted by immersing ZIF-NFA into the prepared two solutions for 24 h at room temperature alternatively. Then the Ni foam with LDH was washed with ethanol twice, and dried at 50 C for 4 h. 2.3. Materials characterization The crystal phases were determined by a Cu Ka radiation powder X-ray diffractometer (XRD, Rigaku D/MAX 2500 VL). The morphology of products were examined by ﬁeld-emission scanning electron microscopy (FESEM, Hitachi SU8020) and high resolution transmission electron microscopy (HRTEM, JEOL JEM 2100). The X-ray photoelectron spectrum (XPS) was recorded on a ESCALAB250Xi spectrometer (Thermo Fisher) with its energy analyzer working in the pass energy mode at 50 eV, and the Mg Ka line was used as the excitation source. The binding energy reference was conducted at 284.8 eV for the C1s peak arising from surface hydrocarbons. N2 adsorption/desorption isotherm and the BET surface area were measured using a Quantachrome Autosorb IQ instrument with liquid nitrogen at 77 K.
2. Experimental section 2.4. Electrochemical measurements 2.1. Synthesis of ZIF-NFA In a typical synthesis, ﬁrstly Ni foam (2 cm * 2 cm in rectangular shape) (surface mass density: 380 g m2; pore density: 110 ppi; thickness: 1.5 mm) was placed in 3 M HCl
The electrochemical properties were investigated by cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) in 2 M KOH solution by using an Electrochemical workstation (Autolab PGSTAT 302N). The
Scheme 1. Schematic representation of the synthesis route from ZIF-NFA to Ni-Co LDH-NFA.
J. Zhang et al. / Electrochimica Acta 226 (2017) 113–120
nickel foam loaded with electroactive materials (4 cm2 in area) was directly used as the working electrode. The KCl saturated Ag/ AgCl was used as the reference electrode, and Pt rod was used as the counter electrode. EIS measurements were tested in the frequency range from 0.1 Hz to 100 kHz at open circuit potential with an ac perturbation of 0.1 V. The speciﬁc capacitance (Cs, C g1) was calculated by using Eq. (1) (CV plots) and Eq. (2) (galvanostatic charge/discharge curves): R Vc IðVÞdV ð1Þ Cs ¼ Va mv 1
Where Cs is the area capacitance (C g ), m is the mass of the active materials (g), n is the scan rate of CV curves (V s1), and represents the potential window (V). Cs ¼
Where Cs is the speciﬁc capacitance (C g1), I is the discharge current (A), m is the mass of the active materials (g), DV is the potential drop excluding the IR drop (V) and Dt is the total discharge time (s). The asymmetric supercapacitor was assembled and measured in a two-electrode system. A piece of Ni-Co LDH-NFA (1 cm * 1 cm) was cut and directly pressed onto another Ni foam (1 cm * 3 cm) as the positive electrode. For the preparation of negative electrode, 90 wt% of commercial active carbon and 10 wt% PVDF was mixed to be a slurry together with NMP, the mixed slurry was then cast a 1 cm2 of area on a Ni foam (1 cm * 3 cm), dried at 80 C for 12 h and pressed under 10 MPa pressure. The two electrodes and a cellulose separator permeable to ion transport were placed into a test ﬁxture consisting of two stainless steel plates, with 6 M KOH aqueous solution as the electrolyte. All the electrochemical measurements were carried out with an Autolab PGSTAT 302N electrochemical workstation. The speciﬁc capacitance (C, F g1),
speciﬁc energy density (SE, Wh kg1) and speciﬁc power density (SP, W kg1) were calculated from chronopotentiometric curves using following Eqs. (3)–(5), respectively. C ¼ mI0DDtV
Where C is the speciﬁc capacitance (F g1), I is the discharge current (A), m' is the total mass of active materials in the positive and negative electrodes (g), Dt is the discharge time (s) and DV is the window potential during the discharge process. V SE ¼ CD 7:2
Where SE is the speciﬁc energy density (Wh kg1), C is the speciﬁc capacitance (F g1), DV is the window potential during the discharge process. SE SP ¼ 3600 Dt
Where SP is the speciﬁc power density (W kg1), SE is the speciﬁc energy density (Wh kg1) and Dt is the discharge time (s). 3. Results and discussion After immersing the obtained ZIF-NFA into Co(NO3)2/Ni(NO3)2 methanol solution for 24 h at room temperature, the ﬁnal Co LDHNFA and Ni-Co LDH-NFA are obtained. The crystallographic structure of ZIF-NFA, Co LDH-NFA and Ni-Co LDH-NFA at each step are examined by powder X-ray diffraction (XRD). The XRD pattern of ZIF-NFA shown in Fig. 1a matches well with the ZIF-L-Co pattern reported in our previous work . And the diffraction peaks of (003), (006), (012), and (110) crystal planes of a typical LDH material are clearly observed (Fig. 1a), which conﬁrms the identical layered LDH structures of the as-obtained two ﬁlms [9,24,25]. It is obvious to see that the full width at half maxima of Ni-Co LDH-NFA is wider than that of Co LDH-NFA, indicating the
Fig. 1. XRD spectra of ZIF-NFA, Co LDH-NFA and Ni-Co LDH-NFA (a), XPS spectra of Co LDH-NFA (b) and Ni-Co LDH-NFA (c,d).
J. Zhang et al. / Electrochimica Acta 226 (2017) 113–120
grain size of Ni-Co LDH is smaller than that of Co LDH according to the Scherrer equation [26,27]. Typical X-ray photoelectron spectroscopy (XPS) spectra of Co LDH-NFA and Ni-Co LDH-NFA are also conducted in Fig. 1b–d. The Co 2p XPS spectra of Co LDHNFA and Ni-Co LDH-NFA are shown in Fig. 1b and Fig. 1d respectively. Both in these two Co 2p XPS spectra, the spin-orbit splitting of Co 2p1/2 (781.7 eV) and Co 2p3/2 (796.9 eV) locates at 15.2 eV, and the intensities of the two Co 2p3/2 satellite lines (indicated as“Sat”) are very low (Fig. 1b, 1d), it reveals the coexistence of Co2+ and Co3+ in Co LDH-NFA and Ni-Co LDH-NFA [8,9,28,29]. In Ni 2p XPS spectrum of Ni-Co LDH-NFA (Fig. 1c), two obvious satellites adjacent to two spin-orbit doublets at 873.3 eV and 855.6 eV can be classiﬁed as Ni 2p1/2 and Ni 2p3/2 signals of Ni2+ [30,31]. These results suggest that the as-obtained ﬁlms on Ni foam are metallic layered double hydroxides. Fig. S1 shows a photograph of pristine Ni foam, ZIF-NFA, Co LDH-NFA and Ni-Co LDH-NFA, respectively. ZIF-L-Co is uniformly grown on the porous Ni foam and looks homogeneously dark blue. Additionally, the homogeneous green color of Co LDH-NFA and NiCo LDH-NFA indicates the strongly adhesion of LDHs to the Ni foam substrate even after immersing for 24 h in methanol solution, implying the in-situ synthesized LDH on Ni foam can be directly used as battery-type electrode for energy storage applications. The ﬁeld-emission scanning electron microscopy (FE-SEM) images of these as-obtained products are shown in Fig. 2. In typically ordered ZIF-NFA (Fig. 2a), average thickness of each nanoﬂake is about
200 nm in a high-magniﬁcation observation (Fig. 2b). When immersing ZIF-NFA into Co(NO3)2/Ni(NO3)2 methanol solution for 24 h, Co LDH-NFA and Ni-Co LDH-NFA present highly ordered nanoﬂake array without any impurity (Fig. 2c and e). During the reactions, the hydrolysis of Ni2+/Co2+ ions produces H+ protons, which gradually etches the ZIF-NFA and breaks the coordinate bonds; meanwhile, some released Co2+ ions are oxidized to Co3+ by dissolved O2 and NO3 ions in the solution, and the Co3+ ions coprecipitate with Ni2+/Co2+ to form Ni-Co/Co LDH [9,22,32]. The high-magniﬁcation SEM images (Fig. 2d and f) further show the clear observation of nanoﬂake array architectures. Ni-Co LDH-NFA consists of many thin nanoﬂakes (thickness: 100 nm) incorporation with some nanoparticles (100 nm) on the top (Fig. 2d). In contrast, Co LDH-NFA is composed of larger and thicker nanoﬂakes (thickness: 500 nm, Fig. 2f). To further study the component and structure of Ni-Co LDHNFA and Co LDH-NFA, the corresponding sample powders are stripped and collected from Ni foam for the transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) analyses, through a strong ultrasonic treatment. The TEM images and the mapping images of elemental distribution for Ni-Co LDH nanoﬂake and Co LDH nanoﬂake are shown in Fig. 3. It is worth noting that the nanoﬂakes in Ni-Co LDH-NFA are nanoporous with approximate 10 nm of pore size, while the nanoﬂake in Co LDH-NFA is solid, and no pore could be observed (Fig. 3a, c). The N2 adsorption/ desorption isotherms are measured to investigated the structure
Fig. 2. FE-SEM image of ZIF-NFA (a, b), Ni-Co LDH-NFA (c,d) and Co LDH-NFA (e,f).
J. Zhang et al. / Electrochimica Acta 226 (2017) 113–120
Fig. 3. TEM image of Ni-Co LDH nanoﬂake (a), elementary mapping of Ni-Co LDH nanoﬂake (b), TEM image of Co LDH nanoﬂake (c) and elementary mapping of Co LDH nanoﬂake (d); the insets in a and c show electron diffraction patterns of Ni-Co LDH nanoﬂake and Co LDH nanoﬂake respectively.
information of Ni-Co LDH nanoﬂake and Co LDH nanoﬂake (Fig. S2), the BET surface area of Ni-Co LDH nanoﬂke was determined to be 16.3 m2 g1 much higher than the value of Co LDH nanoﬂake (5.3 m2 g1), agreeing with TEM results about the nanoporous structure of Ni-Co LDH nanoﬂakes. As nickel nitrate endows with low pH value that leads to high hydrolysis rate, and yields more nucleation positions, resulting in nanoﬂakes with lots of nanopores inside . Moreover, the nanoparticles of Ni-Co LDHNFA (Fig. 2d) may arise from the severe reaction because Ni-Co LDH-NFA nanoﬂakes on the top facilitate better reaction interfaces to react with nickel nitrate methanol solution. In contrast, the nanoﬂakes of Co LDH-NFA are thicker and non-porous, the possible reason is Co2+ has a stronger afﬁnity to the Co-containing template. Therefore, Co2+ is easier to deposit on the ZIF-NFA templates, while the less nucleation sites enable sparser nanoﬂakes yielded . These FE-SEM and TEM results indicate that the grain size of Ni-Co LDH nanoﬂake is smaller than that of Co LDH nanoﬂake, in agreement with the result of XRD discussed above. The selected area electron diffraction (SAED) patterns of Ni-Co LDH nanoﬂake and Co LDH nanoﬂake (inset in Fig. 3a and c) exhibit similar and bright diffraction rings, indicating the poly-crystalline structures. Furthermore, the uniform distributions of Ni, Co and O atoms for Ni-Co LDH nanoﬂake and Co LDH nanoﬂake are also determined by elementary mappings (Fig. 3b and d), no Ni element has been detected in the Co LDH nanoﬂake. To explore the potential applications of porous Ni-Co LDH-NFA and Co LDH-NFA, the electrochemical properties of electrodes have been measured by a three-electrode system in 2 M KOH aqueous solution. Fig. 4a shows the cyclic voltammetry (CV) curves of Co LDH-NFA electrode and Ni-Co LDH-NFA electrode at a scan rate of 20 mV s1. Two pairs of redox peaks in the two CV plots, corresponding to the typical battery-type behaviors of Co2+/Co3+ or Ni2+/Ni3+ redox couples . Due to the considerable proportion of Ni2+ in Ni-Co LDH-NFA, its oxidation peak and reduction peak respectively shift to more positive and negative potential compared with unitary Co LDH-NFA [8,9]. In addition, obviously Ni-Co LDH-NFA electrode has higher speciﬁc capacity than unitary Co LDH-NFA electrode in terms of the much larger closed area of CV curves (Fig. 4a). On the one hand, Ni-Co LDH-NFA consists of much thinner nanoﬂakes with numerous nanopores inside, it shows much larger BET surface area that offers more active sites for charge storage and also enhances ion transport. On the other hand, the incorporation of Ni atoms into crystal lattice of Ni-Co LDH-NFA greatly improves the electron transfer within active materials, as it can be conﬁrmed by the decreasing equivalent series resistance of
electrochemical impedance spectrum (EIS) (Fig. S3). Fig. 4b and Fig. S4 show a series of CV curves for Ni-Co LDH-NFA and Co LDHNFA electrodes at various scan rates. The oxidation peak shifts to a more positive potential and the reduction peak shifts to a more negative potential with the increase of scan rate, respectively. The galvanostatic charge-discharge (GCD) plots of Ni-Co LDH-NFA electrode at different current densities display a typical batterytype behavior (Fig. 4c), the calculated speciﬁc capacity values of asobtained electrodes derived from the GCD plots (Fig. 4c and Fig. S5) are also collected in Fig. 4d. It can be seen that Ni-Co LDH-NFA exhibits signiﬁcantly higher capacity than Co LDH-NFA, it delivers a maximum speciﬁc capacity of 894 C g1 at current density of 2 A g1, and a decreased capacity of 684 C g1 at 40 A g1 (Fig. 4d), which still remains 76.5% of initial capacity at a high current density. In contrast, the maximum speciﬁc capacitance of Co LDHNFA electrode is only 274 C g1 at current density of 2 A g1, and it decreases to 160 C g1 (41.6% remaining at 40 A g1), indicating the better rate capability of Ni-Co LDH-NFA electrode. Moreover, the speciﬁc capacitance (894 C g1 at 2 A g1) of Ni-Co LDH-NFA electrode was much higher in comparison with most of previously reported nickel-cobalt oxide/hydroxide composite, such as ZIF-67derived Ni-Co LDH (602 C g1 at 1 A g1) , ZIF-67-derived Co3O4/ NiCo2O4 double-shell nanocage (408 C g1 at 5 A g1) , NiCo2O4 nanosheet (810 C g1 at 1 A g1) , carbon [email protected]
core-shell structure (292 C g1 at 1 A g1)  and ZIF-67-derived Co3O4 nanotube array (580 C g1 at 1.18 A g1) . The outstanding energy storage performance of our Ni-Co LDH-NFA electrode can be attributed to three main factors: ﬁrst, our prepared Ni-Co LDH-NFA electrode is bind-free, in which the porous nanoﬂake array is directly grown on conductive Ni foam without any nonconductive polymer, as the introduced polymer binder usually worsens ion diffusion and causes high interfacial resistance; second, the layered crystal structure of LDH phase possesses enlarged interlayer spacing that facilitates desirable ions diffusion within active materials ; third, the highly ordered porous nanoﬂake array avoids the possible agglomeration problem of particles, it considerably shortens the ion diffusion path. In order to further demonstrate the practical application of NiCo LDH-NFA electrode in the energy storage performance, an asymmetric supercapacitor device is assembled by using Ni-Co LDH-NFA as anode and the commercial active carbon (AC) as cathode. To achieve the maximum capacity of the fabricated asymmetric supercapacitor device (Ni-Co LDH-NFA//AC), the mass ratio of Ni-Co LDH-NFA to AC is calculated to be 0.224 in terms of the CV results of Ni-Co LDH-NFA and AC in three-electrode system
J. Zhang et al. / Electrochimica Acta 226 (2017) 113–120
Fig. 4. The CV curves of Co LDH-NFA electrode and Ni-Co LDH-NFA electrode at a scan rate of 20 mV s1 (a); the CV curves of Ni-Co LDH-NFA electrode different scan rate of 2, 5, 10, 20, 30, 40 and 50 mV s1 (b); the GCD curves of Ni-Co LDH-NFA electrode at different current density of 2, 5, 8, 10, 15, 20 and 40 A g1 (c); calculated capacity values of NiCo LDH-NFA electrode and Co LDH-NFA electrode from GCD curves at different current density (d).
(Fig. S6). The potential voltage window of the asymmetric supercapacitor is optimized from 1.1 V to 1.9 V, and a series of CV curves for various potential windows at the scan rate of 5 mV s1 are shown in Fig. S7. An obvious oxygen evolution
reaction-induced irreversible process is observed within 1.5–1.9 V, the optimum potential voltage window of the asymmetric supercapacitor device is thus considered as 0–1.7 V. Fig. 5a shows the CV curves at different scan rates, the enclosed area of CV curve
Fig. 5. The CV curves of the assembled asymmetric supercapacitor device Ni-Co LDH-NFA//AC at different scan rate ranging from 0–1.7 V (a); the GCD plots of Ni-Co LDHNFA//AC at different current density (b); ragone plots of the asymmetric supercapacitor comparing with other asymmetric supercapacitors reported recently (c); cycling stability of Ni-Co LDH-NFA//AC measured at a current density of 5 A g1 (d).
J. Zhang et al. / Electrochimica Acta 226 (2017) 113–120
increases with the increasing scan rate that is similar to the observations of the three-electrode system (Fig. 4b). Fig. S8 shows the calculated speciﬁc capacity of Ni-Co LDH-NFA//AC from its galvanostatic charge-discharge curves (Fig. 5b), the maximum speciﬁc capacitance of assembled device reaches 121.2 F g1 at the current density of 2 A g1. Based on these special capacity values from Fig. S7, the energy densities of the Ni-Co LDH-NFA//AC can be further calculated to be 18.5, 20.3, 23.7, 32.3, 35.6 and 48.6 Wh kg1 at power densities of 17000, 12750, 8500, 4250, 3400, and 1700 W kg1, respectively (Fig. 5c). The energy and power density values of as-obtained Ni-Co LDH-NFA//AC are tremendously superior to many other reported Ni/Co-containing oxide- or hydroxide-based asymmetric supercapacitors (Fig. 5c) [11,36– 38], which can be attributed to our prepared Ni-Co LDH-NFA electrode. In addition, the stability of Ni-Co LDH-NFA//AC is also measured by repeating GCD cycles at the current density of 5 A g1 (Fig. 5d), it is found that the assembled supercapacitor achieves capacitance retention of 82% after 3000 GCD cycles, indicating excellent stability of the Ni-Co LDH-NFA//AC device.
4. Conclusions In summary, we have successfully fabricated high orderly ZIFNFA on nickel foam for the ﬁrst time, on the base of our previously reported ZIF-L-Co. A highly ordered porous Ni-Co LDH-NFA on Ni foam is subsequently prepared by using the prepared ZIF-NFA as template when it reacts with the Ni(NO3)2 methanol solution at room temperature. Due to the highly ordered layered crystal structure and well-deﬁned porous nanostructure, such Ni-Co LDHNFA electrode exhibits much higher speciﬁc capacity (894 C g1 at 2 A g1) than most other reported Ni/Co-containing oxide- or hydroxide-based electrodes. Moreover, the assembled Ni-Co LDHNFA//AC cell asymmetric supercapacitor shows very high speciﬁc energy density and power density (48.6 Wh kg1 at a speciﬁc power density of 1700 W kg1). Our work has proposed a novel and facile route to synthesize a porous binder-free Ni-Co LDH-NFA battery-type electrode with enhanced energy storage performance. In addition, this strategy can be applied to preparations of other bimetallic LDHs arrays, and the obtained materials including ZIF-NFA and Ni-Co LDH-NFA can be also used for other potential applications such as catalyst, gas separation, energy conversion and storage ﬁelds.
  
Acknowledgments This work was ﬁnancially supported by National Natural Science Foundation of China (Nos.51571078).
Appendix A. Supplementary data 
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.12.195.
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