Enhanced Structural Stability of NickelâCobalt Hydroxide via Intrinsic

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Enhanced structural stability of nickel-cobalt hydroxide via intrinsic pillar effect of metaborate for high-power and long-life supercapacitor electrodes Yuanzhen Chen, Wei Kong Pang, Haihua Bai, Tengfei Zhou, Yong-Ning Liu, Sai Li, and Zaiping Guo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04427 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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Enhanced structural stability of nickel-cobalt hydroxide via intrinsic pillar effect of metaborate for high-power and long-life supercapacitor electrodes Yuanzhen Chen1, 2, Wei Kong Pang2, 3, Haihua Bai1, Tengfei Zhou2, Yongning Liu1, *, Sai Li4 & Zaiping Guo2, *

1

The State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China.

2

Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2500, Australia. 3

Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirawee DC, NSW 2232, Australia.

4

School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, PR China.

1 Environment ACS Paragon Plus

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Abstract

Layered α-Ni(OH)2 and its derivative bimetallic hydroxides (e.g. α-(Ni/Co)(OH)2) have attracted much attention due to their high specific capacitance, although their insufficient cycling stability has blocked their wide application in various technologies. In this work, we demonstrate that the cycling performance of α-(Ni/Co)(OH)2 can be obviously enhanced via the intrinsic pillar effect of metaborate. Combining the high porosity feature of the metaborate stabilized α-(Ni/Co)(OH)2 and the improved electronic conductivity offered by graphene substrate, the average capacitance fading rate of the metaborate stabilized α-(Ni/Co)(OH)2 is only ~ 0.0017% per cycle within 10,000 cycles at the current density of 5 A g−1. The rate performance is excellent over a wide temperature range from -20 to 40 °C. We believe that the enhancements should mainly be ascribed to the excellent structural stability offered by the metaborate pillars, and the detailed mechanism is discussed.

Keywords: pillar effect, bimetallic hydroxides, metaborate, graphene, supercapacitor

Main text Advanced materials with excellent electrochemical performance are urgently needed to satisfy the continuing growth of highly efficient energy storage devices. Electrochemical double-layer capacitors (EDLC) with high power density (up to 10 kW kg−1) and excellent cycling performance (super-long cycle life of over 100,000 cycles) have become one of the major power sources for


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portable electronic devices, although their relatively low energy density (< 10 Wh kg−1) hinders their wider application.1-3 In this context, pseudo-capacitive materials, such as α-Ni(OH)2, have attracted scientific attention for use in supercapacitors, given their high specific capacitance. α-Ni(OH)2 is usually unstable in strong alkaline electrolyte, however, and is gradually transformed into β-Ni(OH)2 through the process of α-Ni(OH)2 → γ-NiOOH → β-Ni(OH)2 or α-Ni(OH)2

β-Ni(OH)2, resulting from the loss of crystal water and corruption of the layered structure.4, 5

Research results show that the intercalated crystal water expands the Ni(OH)2 interplanar spacing

to ~ 0.78 nm, which crucially provides the transport channels for ions to accomplish the full transformation of Ni(OH)2 → NiOOH and achieve high capacitance.6 To further increase the capacitance, the strategy of incorporating foreign anions (e.g. nitrate, chloride, sulphate, acetate, etc.) into the interlayer region of α-Ni(OH)2 was employed.7 Research results show that the interplanar spacing has indeed been expanded, and these intercalated foreign anions do not occupy hydroxide sites due to the mechanical stress induced by size mismatch, but sit within the interlayer region.8, 9 It should be noted, however, that the capacitance is not directly proportional to the interlayer spacing. Lee et al.7 reported that the interlayer spacing of α-Ni(OH)2 with SO42intercalation was larger than that of α-Ni(OH)2 intercalated by Cl-, but the former delivered a lower specific capacitance because the stronger bonding between SO42- and the Ni(OH)2 layers prevents ion exchange. In addition, the cycling performance (with a high capacitance fading rate of ~ 0.02 % of initial specific capacitance per cycle within 500 cycles) was not greatly improved, possibly due to the dissolution of anions into the electrolyte during the charge-discharge process. Similar


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results were also reported by Hu et al.10 In order to improve the cycling stability, partially substituting other transition metal ions (for instance Co2+, etc.) into α-Ni(OH)2 to form bimetallic complexes was investigated and led to increased cycle life due to the synergetic effect.11-13 Moreover, the low accessible surface areas and limited electronic conductivity have hindered the usage of transition metal compounds as electrodes for supercapacitors.14 The low accessible surface areas can be overcome by morphological control, such as by forming porous structures to increase the surface area, shorten the ionic diffusion paths, and alleviate the strain generated during the ion insertion/de-insertion process.15,

16, 17

On the other hand, incorporating active

materials with conductive materials (e.g. graphene) can effectively improve the supercapacitor performance.18-20 In this work, in addition to the high electronic conductivity offered by graphene and the high specific surface area (SSA) contributed by the porous nanosheet morphology, the intrinsic pillar effect of metaborate is introduced to enhances the structural stability of Ni-Co hydroxide. Herein, the metaborate pillars intrinsically exist in the interlayers of Ni-Co hydroxide and have intensive bonding with the Ni-Co hydroxide layers, which is different from the case of previously reported foreign pillar groups (e.g. nitrate, chloride, sulphate, acetate, etc.). The foreign pillar groups are normally intercalated into the interlayers of Ni or Co hydroxide via ionic exchange and usually have only weak bonding with the hydroxide layers.7, 10 In contrast, the intrinsic metaborate pillars are strongly bonded with the hydroxide layers, leading to excellent structural stability, so that the active material presents superior cycling performance.


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To determine the phases that exist in the products, X-ray diffraction (XRD) was conducted, and the results are shown in Figure 1a. The broad reflection peaks observed in the patterns indicate the poor crystallinity of the porous metaborate stabilized α-(Ni/Co)(OH)2/ graphene (PMNC/G-x, x = 0, 1, 2, and 3, representing graphene loading of 0, 10, 20, and 30 mg, respectively) samples, with peak positions at 12.5, 34.0, and 60.5°, corresponding to the d-spacing of 7.08, 2.64, and 1.53 Å, respectively. Delmas et al.21 and Rajamathi et al.22 have reported that the interlayer spacing of α-Ni(OH)2·xH2O is one third of the lattice parameter c, and therefore, the reflections at 12.5, 34.0, and 60.5° are respectively correlated with the (003), (101), and (110) planes of α-Ni(OH)2·xH2O. The interlayer spacing between two α-Ni(OH)2 layers is typically ≥ 7.8 Å, and could be ~ 7.6 Å if the water molecules are closed-packed with the hydroxide ions6. In our work, as derived from the d-spacing of the (003) planes, the interlayer spacing for the samples is ~ 7.08 Å, which is ~ 9.2% smaller than the previously reported values (≥ 7.8 Å).7 This significantly smaller spacing between the Ni(OH)2 layers possibly implies less intercalated water molecules. The reflection peak at 26°, which can be observed in the patterns of the PMNC/G-x samples, but is absent in the PMNC, is assigned to graphene. To investigate the components of the active material, the PMNC sample was annealed at 750 °C for 2 h, and the Rietveld-fit profile of the XRD pattern is shown in Figure 1b. The Rietveld analysis confirms that the annealed sample contains a major M3(BO3)2 (M = Ni and Co) phase with orthorhombic symmetry and Pnma space group, and a minor NiO phase with the rock-salt


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Figure 1. . (a) XRD patterns of PMNC and PMNC/G-x (with x representing different graphene loadings) composites. (b) XRD pattern of PMNC sample after annealing at 750 °C for 2 hours. XPS spectra of (c) B, (d) Ni, (e) Co, and (f) C for PMNC/G-2. (g) Raman spectra of graphene and PMNC/G-2. (h) The proposed layered structure of Ni0.5Co0.5(BO2)y(OH)2-y·xH2O with B-O “pillar” to stabilize the metal hydroxide layered structure.


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structure and Fm-3m space group, in a weight ratio of ~ 5:1. Given that Ni3(BO3)2 and Co3(BO3)2 have different lattice parameters, only one M3(BO3)2 phase with lattice parameters a = 5.445(1), b = 8.406(1), and c = 4.504(1) Å is exclusively observed, which deviates from the standard Ni3(BO3)2 and Co3(BO3)2 values, indicating that M3(BO3)2 is a solid-solution of Ni and Co compounds. From this, it is not difficult to deduce that the Ni and Co in the original active material exist in an α-(Ni/Co)(OH)2·xH2O solid-solution. To investigate the various chemical states of the bonded elements, X-ray photoelectron spectroscopy (XPS) spectra of PMNC/G-2 were obtained. The survey spectrum shows that the sample consists of B, C, O, Co, and Ni (Figure S1a in the Supporting Information). As can be seen from Figure 1c, the peak with the binding energy of 191.6 eV for the B 1s level can be assigned to the BO2- group,23 not BO33- with high binding energy (192.8 eV) ,24, 25 indicating that the “B-O” exists as BO2- in the initial samples. The formation of BO33- mentioned in the XRD analysis (Figure 1b) is ascribed to the dehydration reaction between BO2- and OH- during the annealing process. The XPS spectrum of Ni 2p3/2 (Figure 1d) shows one major peak at 855.3 eV and one satellite peak at 861.4 eV, which correspond to the energy level of Ni2+.18, 25 Similarly, the spectrum of Co 2p3/2 (Figure 1e) also displays one major peak at 780.9 eV and one satellite peak at 785.4 eV, respectively, which are assigned to Co2+.26, 27 The atomic ratio of Ni: Co: B was estimated from the integrated areas to be ~ 1.03: 1: 1.02, in which the atomic ratio of Ni: Co shows great consistency with the pre-set stoichiometric ratio of 1:1. To supplement the XPS results, Fourier transform


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infrared spectroscopy (FTIR) (Figure S1b) was conducted on the PMNC sample and confirmed the presence of B-O bonding, water molecules, and M-OH (M = Co, Ni). Figure 1f shows the XPS spectrum of C 1s, in which the binding energy at 284.5 eV reflects the C-C bonds of graphene, and the peaks at 286.5, 287.7, and 288.8 eV correspond to C-OH, C=O, and O=C-OH groups respectively,28,

29

suggesting the presence of oxygen groups that could

enhance the adhesion between PMNC and graphene.27 To further investigate the possible bonding between graphene and PMNC, the Raman spectra of graphene and PMNC/G-2 were compared, and the results are shown in Figure 1g. The intensity ratio of the D to the G band for carbon (ID/IG) in PMNC/G-2 (0.26) is significantly higher than that for the bare graphene (0.09), indicating that more structural defects are present on the surfaces of the composite and that adhesion exists between the graphene substrate and the PMNC active material.28, 30 In addition, PMNC/G-2 also shows two peaks at 460 and 1020 cm-1, which respectively correspond to the lattice mode and the 2nd order lattice mode of alpha Ni-Co hydroxide,8 as is consistent with the results of the XRD tests. In addition, thermogravimetric analysis (TGA) was used to investigate the thermal behavior of the initial metaborate stabilized α-(Ni/Co)(OH)2 (MNC) sample. As shown in Figure S2, the weight loss of 7.5% before 135 ˚C is assigned to the loss of adsorbed water. In the temperature range from 135 to 226 ˚C, the weight loss of 3.6% is ascribed to the loss of intercalated water molecules. This weight loss is smaller than that of the reported α-Ni(OH)2,31,

32

which is

attributed to the partial occupation of intercalated water sites by the metaborate pillars. In the


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broad temperature range from 226 to 740 °C, the weight loss of 12 % should be attributed to the decomposition of Ni0.5Co0.5(BO2)y(OH)2-y. Based on the above measurement results, we herein propose the structure of Ni0.5Co0.5(BO2)y(OH)2-y·xH2O, as shown in Figure 1h, in which the layers of (Ni0.5Co0.5)(OH)2 are quasi-aligned in parallel, interstratified by water molecules, and more importantly, connected by “B-O” pillar (possibly side-to-side-sharing BO4 tetrahedral networks). Constrained by the strong B-O bonds, in addition to the hydrogen bonds or van der Waals forces between the water molecules and the hydroxide ions, the stability of the layered structure is greatly enhanced, which is critically important for raising capacitance retention during the charge-discharge process. The nanosheet morphology of the as-prepared samples was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure S3 shows SEM images of ultrathin PMNC and PMNC/G-x nanosheets. With increasing graphene loading, the PMNC grows on the surface of the graphene, as shown in Figure S3d. TEM observations of the initial metaborate stabilised α-(Ni/Co)(OH)2 (MNC) clearly reveal a smooth nanosheet morphology without pores (Figure 2a), while after being annealed at 150 °C for 3 h, many pores with an average diameter of ~ 2.5 nm are found to be homogeneously distributed on the surface (Figure 2b, c, e). A (003) crystal surface with interlayer spacing of 7.15 Å was also observed, as shown in Figure 2d, which is very close to that from the XRD result (7.08 Å). The high-angle annular dark field (HAADF) scanning TEM (STEM) observations of PMNC (Figure 2e) clearly show that many pores with an average diameter of ~ 2.5 nm were formed on the PMNC nanosheets. In


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Figure 2. . (a) TEM image of initial MNC. (b) TEM image of porous MNC (PMNC) annealed at 150 °C. (c) High resolution TEM (HRTEM) image of PMNC with uniform nanometer pores. (d) HRTEM image of (003) crystal surface in PMNC, with the inset showing that its interlayer spacing is about 0.715 nm. (e) HAADF STEM image of PMNC. (f) Mapping images of B, O, Co, and Ni corresponding to (e). (g) TEM image of PMNC/G-2 with many pores. (h) HRTEM image of PMNC/G-2, showing that the porous MNC grows on the surface of graphene. (i) TEM image of ultrathin PMNC/G-2. (j), (k), and (l) HRTEM images of PMNC/G-2 corresponding to regions I, II, and III, respectively, in the TEM image (i), demonstrating that the active material directly grows on the surface of the graphene.


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addition, an α-(Ni0.5Co0.5)(OH)2 sample without the introduction of metaborate was prepared by another precipitation method33, and it also shows a porous structure after heat treatment at 150 °C (Figure S4), indicating that the formation of pores should be ascribed to the water deintercalation. The element mapping images (Figure 2f) confirm that PMNC consists of B, O, Co, and Ni. Additionally, the uniform distribution of Co and Ni indicates that they mutually dope and grow into nanosheets. For a typical PMNC/G sample, the TEM observations show that the PMNC adheres to the graphene surfaces (see Figure 2g-l). Here, there are obvious lattice fringes of graphene with interplanar spacing of 0.34 nm and of PMNC with interplanar spacing of 0.25 nm. Brunauer-Emmett-Teller (BET) tests were conducted to examine the porous nature of the samples, as shown in Figure S5. The SSA value was 156, 210, and 217 cm2 g-1 for the initial MNC, the PMNC, and the PMNC/G-2, respectively. The Barrett-Joyner-Halenda (BJH) analysis showed that all the samples exhibit a broad pore size distribution from 5-100 nm. PMNC and PMNC/G-2 display an extra-small pore size of ~ 2.5 nm, which represents the pores on the PMNC nanosheets and is consistent with the high resolution TEM (HRTEM) observations. The compact adhesion of porous MNC on the graphene surface, consistent with the XPS and Raman spectroscopic analyses, is significantly important for raising the electronic conductivity of the MNC particles. Cyclic voltammetry (CV) is generally used to characterize the capacitive behavior of an electrode material. Figure 3a shows the CV curves of the initial MNC, PMNC, and PMNC/G-x composites measured at the scanning rate of 5 mV s−1 in 6 M KOH aqueous solution. A pair of strong anodic and cathodic peaks, located at ~ 0.35 and ~ 0.15 V (vs. Hg/HgO), is observed in all


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the CV curves, indicating that the capacitance characteristics are mainly determined by Faradic redox reactions. Notably, compared with the sample (initial MNC) without pores, which shows a quasi-triangular CV curve over the range of 0.45-0.5 V (inset of Figure 3a), those samples with abundant pores (PMNC and PMNC/G-x) show rectangular CV curves, which is similar to those for electric double layer capacitors24, 25, suggesting that the pores also contribute capacitance. Figure 3b shows the charge-discharge curves (0–0.4 V vs. Hg/HgO) of PMNC/G-2 measured at different current densities using a three-electrode system. The results show that the charge and discharge plateaus are at ~0.3 and ~0.25 V (vs. Hg/HgO) electrode, respectively. The specific capacitance of our as-prepared samples at different current densities in 6 M KOH is shown in Figure 3c. It can be seen that the specific capacitance of the initial MNC electrode is always higher than that of the α-Ni0.5Co0.5(OH)2 electrode, indicating that the metaborate pillars can efficiently enhance capacitance by providing smooth and stable pathways for ionic transportation. A high specific capacitance of 1668 F g−1 can be obtained at 0.5 A g-1 for the initial MNC electrode, which is comparable to previously reported values.19, 34, 35 When the current density is increased to 40 A g-1, the specific capacitance of the initial MNC electrode is 854 F g-1, which is ~ 51% of the value at 0.5 A g-1. When pores are introduced, the specific capacitance is increased to 1762 and 1127 F g-1 at 0.5 and 40 A g-1, respectively, clearly showing the better rate capability of PMNC, with this improvement being attributed to the porous structure with its increased reaction sites and enhanced ionic transportation. When different masses of graphene were added into the samples (PMNC/G-x), there was a marginally positive effect on their specific capacitances, but a great impact on the rate


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Figure 3. (a) CV curves of initial MNC, PMNC, and PMNC/G-x composites measured at a scanning rate of 5 mV s-1, with the inset showing the magnified CV curves in the potential range of 0.4-0.5 V (vs. Hg/HgO). (b) Charge and discharge curves of PMNC/G-2 at different current densities from 1 to 40 A g-1. (c) Specific capacitance at different current densities for the samples, with the inset showing the specific capacitance of the composites with different graphene loading tested at 10 A g-1. (d) Nyquist plots of α-Ni0.5Co0.5(OH)2·xH2O, the initial MNC, PMNC, and PMNC/G-2 tested in the three-electrode system, with the inset showing an enlargement of the indicated range. (e) Cycling performance of PMNC//AC and PMNC/G-2//AC asymmetric supercapacitors, where AC is activated carbon, tested at 5 A g-1 for 10,000 cycles (inset: charge and discharge curves of PMNC//AC and PMNC/G-2//AC supercapacitors).


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performance. When 1.3 wt.% graphene was added, the rate performance was not greatly improved, but when the graphene loading increased to 3.0 and 4.4 wt.%, the capacitance at 40 A g-1 was ~ 1520 F g-1, which is 84% of the capacitance (1809 F g-1) at 0.5 A g-1, indicating the excellent rate capability and the positive contribution of graphene in the PMNC/G-x composites. Such excellent rate performance is better than most of the reported values.36-39 Herein, considering the optimum graphene addition (inset of Figure 3c) and the electrochemical performance, PMNC/G-2 shows the best rate performance and highest specific capacitance. Therefore, the following analyses are mainly focused on the PMNC/G-2. In this work, the excellent rate performance should be closely related to efficient ion migration and high conductivity. Therefore, we performed electrochemical impedance spectroscopy (EIS) tests from 0.01 Hz to 100 kHz using a three-electrode configuration. Usually, the intersection of the curve with the real axis reflects the equivalent series resistance (Rs) at high frequency, which is contributed by the resistances of both the electrolyte and the electrode,40 and the diameter of the semicircle is attributed to the charge transfer resistance (Rct) at the electrode material/electrolyte interface,37 while the linear region corresponds to the Warburg diffusion process (W), reflecting the ion diffusion into the electrode materials.40 From the Nyquist plots shown in Figure 3d, the equivalent series resistance of the PMNC/G-2 sample is 0.21 Ω, much smaller than those of the other three samples, implying the important role of graphene in promoting conductivity. In addition, the Rct values of PMNC and PMNC/G-2 were estimated to be ~0.54 and 0.31 Ω from the semicircle diameter at high frequency, while those of the initial MNC and α-Ni0.5Co0.5(OH)2 are


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too large to be included in the same figure. The small Rct could be attributed to the ultrathin nanosheets and porous morphology, which allows efficient charge transfer between the electrolyte and electrode. Furthermore, based on the linear fitting equation (Zʹ - ω-1/2, where Zʹ is the real part of the impedance and ω is the angular frequency),41 as shown in Figure S6, the calculated OHionic diffusion coefficient of the initial MNC (7.85 × 10-11 cm2 s-1) is apparently ~14 times that of α-Ni0.5Co0.5(OH)2 electrode (5.51 × 10-12 cm2 s-1), revealing that the OH- ions show much better diffusion in the initial MNC than in α-Ni0.5Co0.5(OH)2,42, 43 which should be attributed to the intrinsic pillar effect of the metaborate towards smooth ion diffusion. Furthermore, PMNC and PMNC/G-2 show higher OH- ionic diffusion coefficients of 1.33 × 10-10 and 5.95 × 10-10 cm2 s-1 (~108 times the value for α-Ni0.5Co0.5(OH)2), respectively, implying that the pillar effect of metaborate and the porous structure could synergistically enhance the OH- diffusion. The low Rs and Rct, as well as the high ionic diffusion coefficient, support the proposition that PMNC/G-2 is an excellent capacitive electrode material for supercapacitors. The high capacitance due to the redox character of pseudocapacitor electrode materials and the fast ion-transport property of the electric double-layer storage in porous carbon have led to the successful fabrication of asymmetric capacitors in other works, where they were used as the positive and negative electrodes, respectively.19,

44

In this work, in order to investigate the

performance of supercapacitor devices, PMNC/G-2 and PMNC were employed as the positive materials in asymmetric capacitors, while commercial activated carbon (AC) was used for the


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negative electrode. Herein, AC with a SSA value of 2000 m2 g-1 showed a specific capacitance of 210 F g-1 at 1 A g-1 along with good rate performance (Figure S7). As a long cycling life is an important requirement for supercapacitor applications, a cycling-life test was carried out on the PMNC//AC and PMNC/G-2//AC asymmetric supercapacitors in the voltage window of 0-1.4 V at a current density of 5 A g-1 for 10,000 cycles. Figure 3e shows the specific capacitance (calculated by the total mass of the positive and negative electrode materials) of the asymmetric capacitor as a function of the cycle number. Both supercapacitors show close specific capacitance at the beginning, namely, 118 and 122 F g−1 for PMNC//AC and PMNC/G-2//AC, respectively. After 10,000 cycles, the former one only maintains 64% of its initial specific capacitance, while the latter one still shows high capacitance retention of 83%, corresponding to a capacitance fading rate of 0.0017% per cycle. Such a low capacitance fading rate is far less, by 1−2 orders of magnitude, than other reported values (Figure S8 and Table S1). Herein, it should be stated that the intrinsic pillar effect of the metaborate plays an important role in the structural stability. Comparison with the cycling performances of the initial MNC and α-Ni0.5Co0.5(OH)2 strongly supports this point (Figure S9). Therefore, the stabilized structure is the precondition for the ultra-long cycling performance, and then the graphene further enhances the capacitance retention. With a view to application of the supercapacitor as a power source, evaluation of the electrochemical performance was conducted at different environmental temperatures. Figure S10a shows the CV curves of the PMNC/G-2//AC asymmetric capacitor tested at the scanning rate of 5


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mV s-1 at different temperatures from -20 to 40 °C. It is clearly seen that the supercapacitor has a high polarization potential at low temperature (0 and -20 °C), which is attributed to the large ionic diffusion resistance. This is supported by the high Rct value (radius of arc in intermediate frequency region), as shown in the EIS spectrum in Figure S10b. In addition, the corresponding specific capacitances (Figure S10c) were calculated according to the charge-discharge curves collected at different temperatures (Figure S11), and the results show that at high temperature, it is possible to store and release high capacity at low current density, while at high current density, the capacitor displays higher specific capacitance retention at room temperature than at 40, 0, and -20 °C. Even so, as a whole, the PMNC/G-2//AC supercapacitor can produce good electrochemical performance within a broad temperature window. The power density and energy density are generally used as important parameters to characterize the electrochemical performance of a supercapacitor. Based on the charge-discharge data for the as-fabricated asymmetric supercapacitor, the relationship between energy density and power density (based on the total mass of positive and negative electrode materials) was constructed, as shown in Figure 4a. The PMNC/G-2//AC supercapacitor displayed a high energy density of 41 Wh kg-1 at a power density of 216 W kg-1. Even at a high power density of 4200 W kg-1, the device still has an energy density of 29.3 Wh kg-1. The energy and power densities of the supercapacitor are much higher than those of previously reported pseudocapacitor electrode materials, such as nickel or cobalt compounds,45-51 Ni-Co binary compounds,52, 53 and carbon materials used in electric double layer capacitors.54-58 Furthermore, as seen from Figure S10d, the PMNC/G-2//AC


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Figure 4. (a) Comparison of specific energy density vs. power density (based on the total mass of positive and negative electrode materials) for the asymmetric supercapacitor (PMNC/G-2//AC), other asymmetric pseudocapacitors in the literature using redox materials and AC as positive or negative electrode, 45, 46, 48, 49, 52, 53 respectively, and electric double layer capacitors using carbon materials (AC and graphene) as electrodes in aqueous electrolyte.19, 54-58 (b) Device based on three asymmetric capacitors keeps the XJTU logo lighted for 60 minutes. (c) Schematic illustration of point conduction mode of PMNC and the enhanced conductive net enabled by graphene in PMNC/G-2.


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supercapacitor even yields higher energy density when tested at low temperature (0 and -20 °C) than other carbon materials (< 10 Wh kg-1) tested at room temperature.54-58 To facilitate application of the PMNC/G, we connected three asymmetric capacitors in series to light a XJTU logo consisting of 27 blue light emitting diodes (LEDs; driving voltage 2.5 V) for at least 60 minutes (Figure 4b). Additionally, we demonstrated that this assembly could also drive an DC motor with a nominal voltage of 3 V (Supporting Information, Movie S1), indicating the high power and energy densities of the PMNC/G-2//AC. The analysis discussed above clearly indicates that the enhancements in the capacitance, rate, and cycling performances are closely related to the high structural stability provided by the intrinsic pillar effect of the metaborate and the enhanced conductivity offered by graphene. Herein, we propose an underlying mechanism, as illustrated in Figure 4c. The metaborate pillars stabilize the layered structure and provide smooth 2-dimensional diffusion pathways for OH- along the a/b axes, ensuring a long cycle life, overwhelmingly better than for the α-Ni0.5Co0.5(OH)2 without the pillar effect of metaborate (Figure S8). The ultrathin and porous morphology offers a larger SSA to increase the density of electrochemical reaction sites and enhance the reaction kinetics. The good adhesion with graphene further provides excellent electronic conductivity to the PMNC nanosheets. In summary, the pillar effect of the metaborate groups, the porous structure, and the presence of the graphene have obviously enhanced the stability of the layered structure of the initial MNC, as well as the transport of ions and electrons, resulting in high rate performance, long-term cycling


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performance, and high energy density at both low and high temperatures. Finally, this work suggests that the PMNC/G composite is a promising positive material for high performance supercapacitors.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: Details on materials synthesis and characterization methods, the IR spectrum of MNC, SEM images of PMNC/G-x samples, TEM images of the contrastive material (α-(Ni0.5Co0.5)(OH)2), BET test results on PMNC/G-2, the diffusion coefficient calculation for OH- ion, related electrochemical performance of an asymmetric supercapacitor tested at different temperatures, and comparison of the cycling performance with other published works. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors Email: [email protected] (Z.P. Guo), [email protected] (Y.N. Liu).

Author Contributions Y.C., Y.L., and Z.G. conceived and designed the experiments. Y.C. and H.B. carried out supercapacitor fabrication and electrochemical measurements. Y.C. conducted materials synthesis. Y.C. and S.L. performed the LED demonstration. Y.C., W.K.P., T.Z., Y.L., and Z.G.


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analyzed the data and co-wrote the paper. All authors discussed the results and commented on the manuscript.

Notes

The authors declare no competing financial interests.

Funding Sources This work was supported by the China Postdoctoral Science Foundation (No. 2012M521760), the National Natural Science Foundation of China (No. 51602246), the Fundamental Research Funds for the Central Universities (No. xjj2014052).

ACKNOWLEDGMENTS We are grateful to Dr. Dongqi Shi, Dr. Yuhai Dou, and Dr. Gilberto Casillas-Garcia for materials characterization. We thank Ms. Tania Sliver for the language polishing. Y. C. is supported by the China Scholarship Council. REFERENCES (1) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; Su, D.; Stach, E. A.; Ruoff, R. S. Science 2011, 332, 1537-1541. (2) Yan, J.; Liu, J.; Fan, Z.; Wei, T.; Zhang, L. Carbon 2012, 50, 2179-2188.


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