Ultrathin NiCo-MOF Nanosheets for High-Performance Supercapacitor

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Ultrathin NiCo-MOF Nanosheets for Highperformance Supercapacitor Electrodes Yanzhong Wang, Yuexin Liu, Huiqi Wang, Wei Liu, Ying Li, Jinfang Zhang, Hua Hou, and Jinlong Yang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02128 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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ACS Applied Energy Materials

Ultrathin NiCo-MOF Nanosheets for High-performance Supercapacitor Electrodes Yanzhong Wanga*, Yuexin Liua, Huiqi Wanga, Wei Liua, Ying Lia, Jinfang Zhanga, Hua Houa, JinlongYanga,b aSchool

of Materials Science and Engineering, North University of China, Taiyuan 030051, P.R. China

bState

Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China

Abstract Metal organic frameworks (MOFs) have attracted intensive attention for high-performance supercapacitors owing to their large specific surface area and tunable pore structure. Herein, ultrathin NiCo-MOF nanosheets are fabricated by a facile ultrasonication at room temperature, and employed as a supercapacitor electrode material. The unique nanosheet-like structure of NiCo-MOF provides more electroactive sites and shorter pathway of electron transfer and electrolyte diffusion, resulting in the excellent electrochemical performance with a high specific capacitance of 1202.1 F g-1 at 1 A g-1. In addition, an asymmetric supercapacitor of NiCo-MOF//activated carbon was assembled in 2 M KOH electrolyte. It delivers an energy density of 49.4 W h kg-1 at a power density of 562.5 W h kg-1 in a voltage window of 1.5 V. The results demonstrate a new method to fabricate ultrathin MOF

*

Corresponding author: Tel. / Fax: +86-351-3557519. E-mail: [email protected] (Y.Z. Wang). 1

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nanosheets for high-performance supercapacitor electrode materials. Keywords: Metal organic framework; Asymmetric supercapacitor; Ultrathin nanosheets; Pseudocapacitor; NiCo-MOF; Terephthalic acid 1 Introduction The renewable sources, including solar energy, wind energy, and tidal energy, are considered as the promising methods for solving the fossil fuel crisis and greenhouse effect1-2. However, these renewable energies are intermittent, which needs a reliable electrochemical energy storage device for the practical application. Among the different electrochemical energy strorage devices, supercapcitors are most promising owing to their fast charging/discharging, high power density, and long cycle life 3. In general, supercapacitors are classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors according to the charge-storage mechanism. EDLCs possess high power density and long-term stability, but they exhibit much lower energy densities due to the low capacitance of the carbon-based electrodes

4-5.

In contrast,

pseudocapacitors can deliver high energy densities because they utilize fast and reversible redox reactions for energy storage. However, most of pseudocapacitors are subjected to the poor cyclic stability and low power density 5. Thus, extensive research efforts are devoted to improving the energy density and cycling stability of supercapacitors. The asymmetric supercapacitors (ASCs) have been developed and considered as a useful method to increase the energy density of supercapacitors with high power density because they fully utilize the merits of EDLCs and pseudocapacitors. In 2


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general, the ASCs are composed of a battery-type electrode and a electrical double-layer electrode, delivering the high energy density owing to the increase of specific capacitance and the operating voltage window 6-7. Therefore, the battery-type electrode plays a vital role for achieving high-performance ASCs. Nevertheless, the conventional battery-type electrode materials, such as transition metal oxides (sulfides, nitrides, and phosphides)

8-12,

and conducting polymers

13-14,

cannot meet

these requirement owing to their poor electronic conductivity, low specific capacitance, and short-term cycling stability

15.

Therefore, it is crucial to develop a

new material for high-performance supercapacitors. Metal-organic frameworks (MOFs), as an emerging porous crystalline materials

16,

have attracted increasing attentions for wide applications in gas separation 17, catalysis 18,

drug delivery

19,

chemical sensors

20,

and heat storage

21.

MOFs also have drawn

extensive attention as a promising supercapacitor electrode material owing to their high specific surface areas, tunable pore structure, and diverse structures

22-23.

Moreover, MOFs are convenient to prepare using the cheap Fe, Co and Ni salts as the starting materials. As we have known, the transition metals ions of Fe, Co and Ni can be designed and synthesized the different kinds of MOFs with potential pseudocapacitive redox centers

16.

The applications of MOFs for supercapacitor

electrodes generally include two cases: (1) MOFs are employed as a template for fabricating different porous carbons, metal oxide, metal sulfides, metal phosphides, and their composites via a one-step pyrolytic process or ions exchanges MOFs are directly employed as supercapacitor electrode materials 3


24-26;

27-28.

(2) The


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calcination of MOFs for preparing metal-based electrode materials inevitably destroy the framework of MOFs more or less, decreasing the specific surface area and active redox sites. Therefore, the merits of MOFs can be fully exploited by the direct application as supercapacitor electrodes. However, MOFs have low electrical conductivity

and

chemical

stability,

which

inhabits

their

applications

in

supercapacitors. To address the above issues, one method is to introduce conductive polymer (polyaniline, and polypyrrole)

29,

carbon nanotubes

30-31,

and graphene

32-33

into

MOFs. The other strategy is to design and synthesize layered-structure MOFs, which can provide the short channels for electron transport and electrolyte ions diffusion 34. For example, Wei et al. first reported two-dimensional Ni-based MOF, which delivered a high specific capacitance of 1127 F g-1 at 0.5 A g-1 and outstanding cycling stability (the retention of capacitance is over 90% after 3000 cycles) 35. Pan et al. reported that the accordion-like Ni-MOF were synthesized by a hydrothermal method and subsequent ultrasonication in ice-cold water, exhibiting a specific capacitance of 988 F g-1 at 1.4 A g-1 36. Chen's group synthesized a Ni-MOF, and used it as supercapacitor electrodes in KOH and K4Fe(CN)6 mixed electrolyte. The reversible Fe(CN)64-/Fe(CN)63- redox couple in K4Fe(CN)6 resultes in the high specific capacitance of 175 mAh g-1 at 0.1 A g-1, and a high energy density of 55.8 W h kg-1 with a power density of 7000 W kg-1

34.

Wei’s groups synthesized

layered-structure Co-MOF nanosheets via a hydrothermal method, which achieved a high specific capacitance of 2564 F g-1 at 1 A g-1

37.

4


Wang et al. synthesized a

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two-dimensional Co-MOF, delivering the maximum specific capacitance of 2474 F g-1 at 1 A g-1 38. The reports mentioned above confirm that the two-dimensional MOFs are a promising candidate for high-performance supercapacitor electrode materials. However, the reported layered Ni and Co-MOF was generally synthesized at 120 oC via the hydrothermal method. The high temperature would result in the re-stacking of 2D layered structure. Very recently, Tang’s groups synthesized ultrathin NiCo-MOF nanosheets through the ultrasonication method, and used it as electrocatalytic oxygen evolution

39.

The thickness of NiCo-MOF is ~ 3.1 nm, which results in the more

exposed active surface for redox reactions, and provides the short pathway of charge transfer and electrolyte ion diffusion. Thus, the ultrathin nanosheets are expected to have excellent capacitive performance. As we have known, the ultrathin NiCo-MOF nanosheets were not reported for the application as supercapacitor electrodes. Herein, we investigate the ultrathin NiCo-MOF nanosheets for supercapacitor electrode materials. The NiCo-MOF nanosheets were facilely synthesized by a ultrasonication method at room temperature according to the modified Tang’s method 39.

The effect of the atomic ratio of Ni and Co ions in MOFs on the electrochemical

properties was investigated, indicating that the ultrathin NiCo-MOF nanosheets exhibited the maximum specific capacitance of 1202 F g-1 at 1 A g-1 with the atomic ratio of Ni and Co in NiCo-MOF of 1:2. Moreover, the assembled asymmetrical supercapacitor devices exhibit a high energy density of 49.4 W h kg-1 at a power density of 562.5 W h kg-1 . 2 Experimental 5



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2.1 Preparation of NiCo-MOFs The preparation process was modified by the previous method experiment,

0.5

mmol

CoCl2·6H2O,

0.25

mmol

39.

NiCl2·6H2O,

In a typical and

0.75

mmol terephthalic acid（PTA）were dissolved into the mixture solution containing 2 ml ethanol, 2 ml deionized water and 30 ml N, N-Dimethylformamide (DMF). Next, 0.8 ml triethylamine (TEA) was quickly poured into the above solution, and stirred for 5 min. The colloidal suspension was then continuously ultrasonicated for 4 h at room temperature. The products were washed with ethanol for several times via centrifugation, and dried at 60 oC for 10 h in a vacuum oven, denoted as NiCo-MOF. In addition, Ni-MOF and Co-MOF were also prepared by the same experimental process. 2.2 Characterization The microstructures were observed by scanning electron microscopy (SEM, Hitachi SU5000), and transmission electron microscopy (TEM, JSM-2100F). Atomic force microscopy (AFM) was performed with a Dimension FastScan Bio (Bruker). X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance diffractmeter equipped with a Cu Kα (λ = 1.540598 Å) radiation source. The Fourier transform infrared spectroscopy (FT-IR) were measured on a Nicolet 750 Fourier transform infrared spectrometer. The nitrogen adsorption-desorption isotherms were collected on a Micrometitics ASAP 2020 at 77 K. 2.3 Electrochemical measurements The electrochemical characterization of NiCo-MOF were performed in a 6


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three-electrode system with 2 M KOH as the electrolyte. A saturated calomel electrode (SCE) and platinum plate were used as the reference and the counter electrodes, respectively. The working electrode was fabricated by mixing 85 wt.% NiCo-MOF, 10 wt.%carbon black (super P), and 5 wt.% polytetrafluoroethylene (PTFE, 60 wt%). The mixture was pressed onto a nickel foam (1 cm × 1 cm). The cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurement were performed on a SP 200 electrochemical workstation (Bio-logic, France) at the room temperature. 3 Results and discussion The preparation processes for ultrathin NiCo-MOF nanosheets are schematically illustrated in Scheme 1. Briefly, NiCl2·6H2O, CoCl2·6H2O, and PTA were completely dissolved into the mixture of ethanol, water and DMF. After TEA was added into the above solution, the mixture were ultrasonicated for 4 h, producing NiCo-MOF materials. The crystal structure of as-prepared samples were investigated by the XRD measurements, and the results were shown in Fig. 1a. It shows that the XRD patterns of Ni-MOF, Co-MOF and NiCo-MOF are isostructural to the previously reported Ni-based MOFs with the space group of C2/m

28.

In addition, the intensity of peaks

for Ni-MOF is higher than that of NiCo-MOF and Co-MOF, which agrees with the previous report

41.

The transition metal-based MOFs were constructed by

edge-sharing MO6 octahedral parallel chains, and the terephthalate groups bridged the layers. The M-O-M chains along c-axis constructed a conductive network frame, resulting in improving the electrochemical performance 37. 7



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The FT-IR spectra (Fig. 1b) demonstrates that the stretching vibrations of –COO-, OH-, and para-aromatic CH groups were observed in Ni-MOF, Co-MOF, and NiCo-MOF. The intensive peak at 3437.5 cm-1 is assigned to the stretching vibration of OH groups 42. The three peaks at 2926.8, 816.8 and 750.8 cm-1 are characteristic of the para-aromatic C-H stretching bands. Another two intensive peaks at 1575.7 and 1386 cm-1 are attributed to the vas (-COO) and vs (-COO) of terephthalate anions, respectively 37. These results are in agreement well with the previous report 37-38. The microstructure of as-prepared samples were characterized by SEM. Fig. 2a and b show that NiCo-MOF consists of uniform wrinkled nanosheets. In addition, Ni-MOF and Co-MOF also exhibit the similar sheet-like structure (Fig. S1). Fig. 2c shows that the NiCo-MOF only contains Ni, Co, C, and O elements without other impurities, and these elements are uniformly distributed in the NiCo-MOF frame structure (Fig. 2d-g). To further characterize the morphology of NiCo-MOF, TEM images are provided as Fig. 3. The NiCo-MOF are approximately transparent (Fig. 3a), indicating the ultrathin sheet-like structure. The atomic force microscopy (AFM) profile indicates that the thickness of NiCo-MOF nanosheets was 1.74 ~ 3.87 nm (Fig. 3d and e), which is similar to the previous report

39.

Moreover, the NiCo-MOF

nanosheets are flexible and wrinkled, providing the more exposed active surface for more charge storage. Higher resolution TEM image (Fig. 3b) shows that the NiCo-MOF exhibits well-distributed nanoparticles with an average particle size of 2~3 nm. Some identical lattice fringes were uniformly dispersed in the locations marked with red circles (Fig. 3c). The lattice spacing is 0.28 nm, which is assigned to 8


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the (220) plane of Co-MOF (the inserted magnification in Fig. 3c) 43. The nitrogen adsorption-desorption isotherms of Ni-MOF, Co-MOF and NiCo-MOF were shown in Fig. 4a. The specific surface areas of Co-MOF, Ni-MOF and NiCo-MOF are 44.8, 34.9 and 54.6 m2 g, respectively, indicating that the bimetals are beneficial to increasing the specific surface area of NiCo-MOF. The obtained size distributions (Fig. 4b) demonstrate that the pore volume peaks of the samples are in the range of mesopores (2~6 nm), indicating the mesoporous structure. The relatively large surface area and mesoporous structure provide the more electroactive sites and fast channels for electrolyte ions diffusion, which can result in high charge storage and rate capability. The chemical bonding state of NiCo-MOF were characterized using XPS, and the resulsts are shown in Fig. 5. The survey scan spectrum (Fig S2) demonstrates the presences of Ni, Co, O, and C elements in NiCo-MOF, and the element contents are listed in Table S1. It shows that the atomic ratio of Co and Ni is about 1.84, which is close to the mole ratio of Ni and Co salts. In the high-resolution Ni 2p spectrum (Fig. 5a), the fitting peaks at 856.1 and 873.7 eV with a spin-energy separation of 17.6 eV are attributed to Ni 2p3/2 and Ni 2p1/2 spin-orbits, respectively 44. Additionally, the two broad peaks at 861.4 and 880.1 eV are identified as shake-up satellites (‘‘Sat.’’) of Ni 2p3/2 and Ni 2p1/2, respectively. The above results display the characteristic bands of Ni2+ 45. Similarly, Fig. 5b shows that the peaks located at around 781.4 and 797. 2 eV are indexed to Co 2p3/2 and Co 2p1/2, and the other two peaks centered at 785.5 and 797.2 eV correspond to shake-up satellites, which are characteristic band of Co2+. 9



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These results demonstrate that Ni and Co ions are predominantly in the valence state of Ni2+ and Co2+ in the as-synthesized NiCo-MOF nanosheets. The spectrum (Fig. 5c) of C 1s can be divided into three peaks at 284.4, 285.7, and 288.24 eV, which are attributed to the bonds of C-C=C, C=O, and O–C=O, respectively 46. The spectrum of O 1s (Fig. 5d) can be indexed into two peaks, which correspond to the metal–oxygen bonds M–O–M (531.25 eV) in the metal oxides, the oxygen in –OH (533.12 eV)

31.

Therefore, XPS results verify the formation of nickel and cobalt terephthalate phase. The pseudocapacitive performances of Ni-MOF, Co-MOF and NiCo-MOF as electrode materials were investigated by CV, GCD and EIS tests. The CV curves of Ni-MOF, Co-MOF, and NiCo-MOF measured at a scan rate of 10 mV s-1 were shown in Fig. 6a. The different shapes of CV curves indicated the different redox reactions in the Ni-MOF, Co-MOF, and NiCo-MOF. The pair of distinct redox peaks corresponds to the redox reactions of Co2+/Co3+ and Ni2+/Ni3+ transitions with the aid of OHanions

47.

Apparently, NiCo-MOF electrode shows a larger CV areas than other

electrodes, indicating that the synergistic effect of Ni and Co ions leads to a higher specific capacitance. Fig. 6b shows the CV curves of the NiCo-MOF electrode at scan rates from 5 to 70 mV s-1. All CV curves indicate the similar shape. Meanwhile, the peak currents gradually improve with the increase of scan rates. The pair of redox peaks still can be observed at the scan rate up to 70 mV s-1, which indicates excellent kinetic reversibility and rate performance

48.

In addition, the positions of

oxidation and reduction peaks shifted slightly to more positive and negative potentials with increasing the scan rate, which are mainly related to the polarization of 10


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electrodes 49-50. The GCD curves of Ni-MOF, Co-MOF, and NiCo-MOF measured at 1 A g-1 to quantify their specific capacitances, are shown in Fig. 6c. Obviously, the non-linearity in the GCD curves further verifies the pseudocapacitance behavior of Ni-MOF, Co-MOF, and NiCo-MOF, which agrees with the results from CV curves. The NiCo-MOF electrode shows the longer discharge time than that of Ni-MOF and Co-MOF, corresponding to higher specific capacitance. Fig. 6d and Fig. S3 demonstrate the GCD curves of NiCo-MOF at various current densities. The approximately symmetric charge-discharge curves at all current densities demonstrate the outstanding electrochemical reversibility and capacitive performance

48.

The

specific capacitances were calculated from the discharge curves are plotted in Fig. 6e. Obviously, NiCo-MOF electrode exhibits higher specific capacitance than those of the Ni-MOF and Co-MOF electrodes at the same current density. The specific capacitance of NiCo-MOF electrode is 1202.1 F g-1 at 1 A g-1, which is much higher than those of Ni-MOF (840 F g-1) and Co-MOF (650.6 F g-1). Moreover, NiCo-MOF electrode still retains high specific capacitance of 651 F g-1 at 50 A g-1, which is about 54.2 % of the value of capacitance at 1 A g-1, indicating the excellent rate capability. The effect of the atomic ratio of Ni and Co in NiCo-MOF on the electrochemical performance were investigated, and the results are shown in Fig. S4. Clearly, NiCo-MOF (the atomic ratio of Ni and Co is 0.5) exhibit the maximum specific capacitance. Moreover, compared with the previous reports of MOF-based supercapacitor electrode materials in Table 1, the NiCo-MOF exhibits excellent 11



capacitive performance, which was attributed to ultrathin nanosheets structure. The ultrathin NiCo-MOF nanosheets provide abundant electroactive sites for Faradaic redox reaction, and the short pathway of electron transfer and electrolyte ions diffusion. The Nyquist plots of the NiCo-MOF, Ni-MOF, and Co-NOF electrodes are demonstrated in Fig. 6f. The Nyquist plots consist of a quasi-semicircle in the high frequency region and a inclined line in the low-frequency region,which represent the charge-transfer resistance (Rct) at the electrode/electrolyte interface, and the Warburg impedance of electrolyte ion diffusion from the electrolyte to the electrode interface, respectively. The intercept with the x axis at high frequency represents the equivalent series resistance (Rs). The Rs value of NiCo-MOF is 0.25 Ω, which is lower than that of Ni-MOF and Co-MOF (0.42 Ω). All the samples exhibit the small Rct, which indicates a low charge transfer resistance at the interface of electrode and electrolyte owing to the ultrathin nanosheets structure. The cycling stability of NiCo-MOF electrode was tested by GCD at 5 A g-1. As shown in Fig. S5, NiCo-MOF still exhibits the specific capacitacne of 929 F g-1 over the consecutive 5000 cycles, and the capacitance retention is 89.5 % of the initial capacitance (1038 F g-1), denoting the outstanding cycling stability. To futher investigate the practical applications of NiCo-MOF, the asymmetric supercapacitors were fabricated by utilizing the NiCo-MOF electrode and commercially activated carbon (AC) as the positive and negative electrodes, respectively, denoted as NiCo-MOF//AC. The mass ratio of the two electrodes in 12


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ASCs was calculated according to the following equation:

𝑚+ 𝑚

―

𝐶 ― ∆𝑉 ―

= 𝐶 + ∆𝑉 + , where, m is

the electrode mass, C is the specific capacitance, and ΔV is the potential window. According to the specific capacitances of 207.3 F g-1 for activated carbon, and 1202.1 F g-1 for NiCo-MOF at 1 A g-1, and the potential windows of -1~0 V for activated carbon, and 0~0.6 V NiCo-MOF, the mass ratio of NiCo-MOF and AC electrode materials was calculated to be 1:3.5. Herein, the typical mass loading of the asymmetric supercapacitors is about 13.5 mg cm-2 (NiCo-MOF is ~3 mg cm-2 and AC is ~10.5 mg cm-2). Fig. 7a shows the CV curves of NiCo-MOF and AC electrodes at 30 mV s-1, respectively. The results indicate that the stable potential windows of ASCs can be extended to be 1.5 V. Fig. 7b shows the CV curves of NiCo-MOF//AC at scan rates of 5~50 mV s-1. The shape of CV curves clearly reveals the electric double-layer capacitive and Faradaic redox behaviors at all scan rates. As shown in Fig. 7c, the GCD curves of NiCo-MOF//AC were tested at the current densities of 0.5, 1, 3, 5, 7, and 10 A g-1, respectively, and the calculated specific capacitances of the asymmetric supercapacitor are presented in Fig. 7d. The approximately symmetric GCD curves exhibit the excellent columbic efficiency and electrochemical reversibility. The NiCo-MOF//AC device delivers a high specific capacitance of 158.1 F g-1 at 0.5 A g-1, and remains 56.6 F g-1 at 10 A g-1, respectively, indicating the excellent capacitive performance of the NiCo-MOF//AC asymmetric supercapacitor. The energy and power densities are essential to estimate the practical applications of supercapacitors. The Ragone plot of the NiCo-MOF//AC ASCs were was shown in 13



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Fig. 8a. This device delivers a high energy density of 49.4 Wh kg-1 with a power density of 562.5 W kg-1, and still keeps 11.25 Wh kg-1 at a power density of 17.69 kW kg-1. Obviously, the energy density is much higher than that of AC//AC supercapacitors with aqueous electrolyte (

Ultrathin NiCo-MOF Nanosheets for High-Performance Supercapacitor

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