Introduction

Caused by fossil fuel-based energy sources, global warming, and environmental pollution problems have brought a few crucial issues, which drives many recent research attempts on exploring new materials for energy storage and energy conversion devices [1, 2]. The development of energy storage devices with high energy and power output, long lifetime, and short charging time is essential for many applications which include electric vehicles, energy storage for wind and large industrial equipment. Supercapacitors, also called electrochemical capacitors or ultracapacitors had attracted much attention in recent decades due to their characteristics of high-power density (> 10 kW kg−1), long-term stabilities (> 105 cycles), high reliability, fast charge/discharge rate, and safe operation mode compared to traditional batteries [3, 4]. Depending on the charge storage processes, supercapacitors can be divided into the electrical double layer capacitors (EDLCs) and pseudocapacitors. EDLCs can store charges from electrostatic charge accumulation at electrode/electrolyte interface. The capacitive behavior of EDLCs mainly depends on the specific surface area and the structure of the electrode materials. Generally, carbon-based materials such as carbon nanotubes, activated carbon, graphene, and recently graphdiyne have been used for EDLCs due to their high surface area and excellent conductivity [5,6,7,8,9]. Pseudocapacitors can store charges due to the reversible faradic reactions involving charge transfer between electrode and electrolyte. Metal oxides, transition metal chalcogenides, and conducting polymers have been employed for pseudocapacitors thanks to their excellent redox activities and multiple available oxidation states [10,11,12]. In recent years, metal-organic frameworks (MOFs) have received much research interest in pseudocapacitors as a result of their high accessible surface area and porosity, tunable pore size, and ordered crystalline structures [13,14,15,16,17,18].

Metal-organic frameworks are a kind of porous materials connected with metal-containing units and organic linkers, and possess large surface area [13]. In recent years, several studies have been focused on MOF-based applications such as gas storage [19,20,21,22], separations [23,24,25,26], and catalysis [27,28,29]. Since metal cations (ions) inside the MOF can offer more active sites for redox behavior towards charge storage phenomena, many metals such as Cu [30, 31], Co [32,33,34], Ni [35,36,37], Fe [38], Cr [39], and Zr [40] have been used for MOFs as supercapacitor electrode materials. Among various MOFs, Co-MOF has received much attention in electrochemical energy storage devices such as lithium-ion batteries and supercapacitors due to its excellent redox performance. Cobalt, as a transition metal, can provide redox-active sites and is suitable to construct MOFs of good electrochemical performance. Furthermore, different oxidation states of cobalt could be involved in surface faradic redox reactions and delivered higher electrochemical activities than other MOF materials. Diaz et al. reported the electrochemical behavior of a Co-based MOF as electrode material firstly [33]. Lee et al. reported a Co-MOF synthesis and the electrode exhibited a specific capacitance of 179.2 F g−1, an energy density of 31.4 Wh kg−1 and a power density of 5.64 kW kg−1 [41]. In some previous works, Co-MOFs showed good capacitive behavior with comparable specific capacitance and good cycling stability [34, 42]. It has been remarked that the electrochemical capacitance performance of the electrodes not only depends on the type of electrode, also it depends on the several factors such as morphology, porosity, size, and composition. The morphology, phase, and porous features of MOF can be controlled with synthesis conditions, including temperature, metal precursor, organic linker, molar concentration, and solvents.

In this paper, we have conducted a systematic study of temperature influence on Co-MOF synthesis using ethanol as solvent. The structure and porosity features of the as-prepared Co-MOF’s were characterized, and their electrochemical performance was evaluated towards an electrode material of supercapacitors. The results suggested that the Co-MOF synthesized at 150 °C showed remarkable electrochemical performance with the specific capacitance of 956 F g−1 at a current density of 0.25 A g−1, respectively. The large specific surface area and micropore structure of Co-MOF/150 lead to superior electrochemical performance towards supercapacitor applications.

Experimental methods

Materials

AR grade of cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O), 1,3,5-tricarboxylic acid (H3BTC), was purchased from Sigma-Aldrich. Ethanol (99.8% assay) and potassium hydroxide (KOH) were purchased from Shanghai Lingfeng Chemical Reagent Co., LTD, China. The purchased chemicals were utilized without additional purification. Milli-Q water with pH 7.2 was used for all the solution preparation.

Preparation of Co-MOF

Different structures of Co-MOF’s were synthesized at various temperatures by the following procedure. In the beginning, 0.9 g of H3BTC and 48 ml of ethanol solution were added together and agitated with a magnetic stirrer for 10 min. Following that, 1.8 g of Co(NO3)2·6H2O was added to the previous solution and continued mixing for an additional 10 min. Later, the mixed solution was shifted into a Teflon-lined stainless steel autoclave (70 ml capacity) and heated at 120 °C for 12 h. After being cooled to room temperature naturally, the obtained purple color precipitate was thoroughly washed and centrifuged with ethanol several times. Finally, the collected sample was placed in a vacuum oven at 60 °C and dried for 24 h. As the prepared sample was marked as Co-MOF/120. The synthesis of Co-MOF/150, Co-MOF/180, and Co-MOF/210 was similar to that illustrated above with different temperatures at 150 °C, 180 °C, and 210 °C, respectively.

Material characterization

Powder X-ray diffraction pattern of Co-MOF samples was obtained by Rigaku Smartlab diffractometer with Cu-Kα radiation (λ = 1.540 Å). The surface morphologies and structures of the samples were observed using scanning electron microscope (Zeiss Merlin). In addition, the morphology of Co-MOF/150 was characterized with transmission electron microscope (FEI Tecnai F30). The textural porosity and specific surface area were analyzed with BET analyzer (Model: ASAP 2020).

Electrochemical measurements

Electrochemical workstation (CHI 660 E) was used for all the electrochemical measurements such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in 3 M KOH electrolyte. Active material (Co-MOF)-coated Ni foam was used as a working electrode with respect to a reference electrode of Ag/AgCl. Platinum (Pt) wire was employed as a counter electrode. The slurry-coated working electrode was fabricated as follows: In brief, active material and carbon black (conductive agent) with a mass ratio of 85:10 was mixed with 5 wt% of Nafion solution to form a homogeneous slurry. Afterward, the slurry was coated on Ni foam electrode (2 cm × 1 cm × 0.1 cm). The total mass of the active material of electrode was 2 mg cm−2 and the total coated surface area was 1 × 1 cm2. Finally, the electrode was allowed to dry at room temperature for 3 h. All the electrochemical studies were carried out under ambient conditions. The EIS spectrum of Co-MOF samples was fitted with ECLab software.

Results and discussion

The morphology of as-prepared Co-MOFs at different temperatures is shown in Fig. 1. From the SEM images, it could be seen that the hierarchical hollow microspheres of Co-MOFs are densely composed of one-dimensional nanorods, and the size of the obtained urchin-like Co-MOF spheres is about 2 μm. It is unusual that some of nanosheets structure has been noticed in Co-MOF/120 sample, and the obtained nanosheets might be the reason to reduce the porous feature of MOF as discussed later in BET analysis. The sheet-like structure disappears in Co-MOF/150, indicating a different nucleation rate due to the higher temperature, which suggests that temperature could play an essential role in the formation of various MOF structures. The hollow structures of Co-MOF and the size of holes in the microspheres were expanded at the temperature 180 °C i.e., almost all the Co-MOF spheres became hollow structures compared with Co-MOF/120 and Co-MOF/150. At very high temperature (210 °C), most of the hollow microspheres disappear, and irregular shapes of MOF have been observed. As the evidence of the SEM analysis, the synthesis temperature plays a crucial role on the structure of Co-MOF concerning initial nucleation rates. TEM analysis further characterized the architectures of Co-MOF nanorod. Uniform sized nanorods was observed from TEM images of Co-MOF/150 sample (Fig. 2a–c), and the length and width of the nanorods are about 100 nm and 40 nm, respectively. It can be seen that the mesoporous of voids between the nanorods, and these voids can improve the electron transport and enhance the active sites for electrochemical reactions.

Fig. 1
figure 1

SEM images of Co-MOF (a, b) Co-MOF/120 (c, d) Co-MOF/150 (e, f) Co-MOF/180, and (g, h) Co-MOF/210

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Fig. 2
figure 2

TEM images of Co-MOF/150 at different magnifications

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The crystallinity of the Co-MOF was investigated by X-ray diffraction (XRD), as shown in Fig. 3. The obtained diffraction peaks agree well with the previously reported Co-MOF [43]. It is noticed that there was no apparent difference in the diffraction peaks among the Co-MOFs except the diffraction intensities. The difference in their intensities was mainly attributed to the presence of water or solvent molecules in MOF pores.

Fig. 3
figure 3

XRD patterns of a Co-MOF/120, b Co-MOF/150, c Co-MOF/180, and d Co-MOF/210

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The textural characteristic such as porosity and specific surface area of as-prepared Co-MOFs were analyzed with BET analyzer. The N2 adsorption-desorption isotherms of Co-MOF samples at 77 K in the relative pressure P/P0 ranging from 0 to 1 are shown in Fig. 4a. The presence of the mesoporous structure in as-prepared Co-MOF samples was validated from type IV hysteresis isotherm loop at the relative pressure between 0.8 and 1.0 [44]. In addition, the mesoporous structure in the Co-MOF hollow spheres was further confirmed with BJH size distribution (Fig. 4b) and the curves show the centered peak in between 20 to 35 nm for all the synthesized Co-MOFs. Since the real pores were not discerned in the nanorods, we assume that the mesoporous behavior may come from the voids among the nanorods (indicated in Fig. 2c) [45]. The textural characteristics of the Co-MOFs are listed in the Table 1. The presence of nanosheets in Co-MOF/120 could reduce the voids between the nanorods, and lower its surface area and porous behavior. The large BET surface area (93.6 m2 g−1) and micropore area (2.60 m2 g−1) of Co-MOF/150 could promote the electrolyte ion migration between the electrode/electrolyte interface and boost the electrochemical redox reactions [46].

Fig. 4
figure 4

a Nitrogen absorption-desorption isotherms of Co-MOF/120, Co-MOF/150, Co-MOF/180, and Co-MOF/210. b Pore size distribution calculated by BJH model

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Table 1 Textural characteristic observed from BET analysis of Co-MOF/120, Co-MOF/150, Co-MOF/180, and Co-MOF/210
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In order to evaluate the electrochemical supercapacitance performance of as-synthesized Co-MOFs electrodes, electrochemical measurements were investigated in 3 M KOH electrolyte from 0 to 0.5 V potential window. The cyclic voltammetry performance of the Co-MOF electrodes is shown in Fig. 5. A couple of redox peaks is observed in the curves, suggesting the pseudocapacitance behavior of Co-MOF electrodes which comes from the faradic redox reactions of various cobalt oxidation states and the possible cobalt conversion involved in the electrochemical reactions can be described by the following expression [47].

$$ \mathrm{Co}{\left(\mathrm{II}\right)}_{\mathrm{s}}+{\mathrm{OH}}^{-}\leftrightarrow \mathrm{Co}\left(\mathrm{II}\right){\left(\mathrm{OH}\right)}_{a\mathrm{d}}+{\mathrm{e}}^{-1} $$
(1)
$$ \mathrm{Co}\left(\mathrm{II}\right){\left(\mathrm{OH}\right)}_{\mathrm{ad}}\leftrightarrow \mathrm{Co}\left(\mathrm{II}\mathrm{I}\right){\left(\mathrm{OH}\right)}_{\mathrm{ad}}+{\mathrm{e}}^{-1} $$
(2)
Fig. 5
figure 5

Cyclic voltammogram (CV) curves of a Co-MOF/120, b Co-MOF/150, c Co-MOF/180, and d Co-MOF/210 at different scan rates. e Comparison of galvanostatic charge-discharge curve of Co-MOF/120, Co-MOF/150, Co-MOF/180, and Co-MOF/210 at 5 mV s−1 (f). Specific capacitance plot of Co-MOF electrodes at different scan rates (i) Co-MOF/120, (ii) Co-MOF/150, (iii) Co-MOF/180, and (iv) Co-MOF/210

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The area of CV curve represents the capacitance behavior of the electrodes. As shown in the Fig. 5e, both the measured current level and the area of the curve from Co-MOF/150 electrode were higher than those of all the other electrodes which indicates the best capacitive behavior. The specific capacitance of the electrodes from CV curves was calculated according to the following formula [48],

$$ C=\frac{1}{m\times v\times \left(V\mathrm{c}-V\mathrm{a}\right)}\times {\int}_{V\mathrm{a}}^{V\mathrm{c}}I(V) dV $$
(3)

where v is the scan rate; m is the mass of the active material of the electrode; Va is the anodic potential; Vc is the cathodic potential; I(V) is the response current density versus potential and V is the potential. As compared in Fig. 5f, a maximum specific capacitance of 1240.4 F g −1 has been attained for the Co-MOF/150 electrode at a scan rate of 5 mV s−1 which is higher compared to other electrodes due to the better ion transport within the mesoporous voids structures [49]. The specific capacitance of electrode decreases with the increase of scan rate, which is due to the insufficient redox reactions at high scan rates.

In addition, the specific capacitance behavior of the electrodes was characterized by galvanostatic charge-discharge (GCD) technique, and the GCD curves of the electrodes are shown in Fig. 6. The non-linear changes in charge and discharge curves (indicated in Fig. 6b) are attributed to the good pseudocapacitance behavior of the electrodes which is consistent with the CV results [50]. The following expression is used for electrodes specific capacitance calculation from GCD measurement [51, 52],

$$ C=\frac{I\times \varDelta t}{\;m\times \varDelta V} $$
(4)

where Δt is the discharge time (s), I is the applied current (A), m is the total mass of the active material of the electrode (g), ΔV is the potential window (V). A maximum specific capacitance of 952.5 F g−1 has been achieved for the Co-MOF/150 electrode at a current density of 0.25 A g−1. With the same current density, however, the other electrodes exhibit a lower specific capacitance of 649.8, 322.1 and 495.6 F g−1 for Co-MOF/120, Co-MOF/180, Co-MOF/210, respectively (Fig. 6f). The larger specific surface area and higher micro-pore area of Co-MOF/150 enhance the electrolyte ion transportation to the electrode and deliver better redox behavior. Table 2 shows a comparison of the current work with previously reported electrodes based on MOF materials. The cyclic stability of the electrodes was investigated by GCD endurance test at 5 A g−1 current density for 2000 cycles (Fig. 7). It can be seen that the specific capacitance retains to be over 85% after 2000 cycles for all the Co-MOF electrodes which suggesting the good electrochemical reversibility during charge/discharge process. The changes in the morphology of Co-MOF/150 electrode was analyzed before and after 2000 cycles and shown in Fig. S1. As the evidence of the figure, the tiny change in the morphology is remarked which suggests that the Co-MOF microspheres undergo considerable aggregation during the long cyclic process.

Fig. 6
figure 6

Galvanostatic charge-discharge profile of a Co-MOF/120, b Co-MOF/150, c Co-MOF/180, and d Co-MOF/210 at different current densities. e Comparison of galvanostatic charge-discharge curve of Co-MOF electrodes at a current density of 0.25 A g−1 (f) specific capacitance plot of Co-MOF/120, Co-MOF/150, Co-MOF/180, and Co-MOF/210 at different current densities

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Table 2 Comparison of specific capacitance of Co-MOF electrodes in this work with previously reported other Co-MOF electrodes
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Fig. 7
figure 7

Specific capacitance retention behavior of Co-MOF electrodes at 5 A g−1 current density

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Electrochemical impedance analysis was investigated to understand the ion transfer phenomenon of the electrode/electrolyte interface. Nyquist plot of the Co-MOF electrodes is shown in Fig. 8. The intercept at the real axis in the high-frequency region corresponds to the solution resistance (Rs) of the electrode/electrolyte interface. [53]. It could be seen that the Rs value of Co-MOF/150 electrode shows a minor value (inset picture of Fig. 8) than others which suggesting better conductivity and better electrolyte ion transportation in the electrode/electrolyte interfaces [54]. The Nyquist plot of Co-MOF/150 electrode before and after 2000 cycles is shown in Fig. S2. As the evidence from the figure, the solution resistance at the high-frequency region is increased after cycles, suggesting that the decreased ion mobilities during the long cyclic process. However, the straight line at lower frequency region is closer to the imaginary axis which indicates that better pseudocapacitive nature and good stability even after cycles. The bode phase angle of Co-MOF the electrodes (Fig. 9) is obtained in the range of 50°–70° at low-frequency region (< 100 Hz) which is attributed to the perfect pseudocapacitance function of electrodes [55]. The higher phase angle of Co-MOF/150 demonstrates better transportation of electrolyte ions and hence achieved greater pseudocapacitance response. The plot of total impedance versus frequency shows three different slopes at different frequency regions. The slope at higher frequency region (105–103 Hz) of all the Co-MOF electrodes exhibited almost zero suggesting that the electrode and electrolyte ions interactions became weaker, and a resistive component is dominant. Whereas, at the low-frequency (< 100 Hz) and the mid-frequency regions (103–102 Hz), the value of the slopes are higher and these can be associated to the greater adsorption of the electrolyte ions onto the electrode surface, thus confirmed the pseudocapacitive nature of Co-MOF electrodes.

Fig. 8
figure 8

Nyquist plot of Co-MOF electrodes from the frequency range of 1 Hz to 105 Hz

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Fig. 9
figure 9

Bode plots of a Co-MOF/120, b Co-MOF/150, c Co-MOF/180, and d Co-MOF/210 electrodes

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Overall, the higher specific capacitance of the Co-MOF/150 electrode can be attributed to the subsequent reasons: (1) the large surface area of the Co-MOF/150 from hollow microspheres guarantees excellent redox-active sites for charge storage, which ends up in high electrochemical performance; (2) a larger micropore area and also the mesoporous voids could reduce the ion diffusion path to the electrode, which often makes sure good rate capability.

Conclusion

In summary, we have synthesized Co-MOF at different temperatures and characterized their performance with various tools. Based on the structure and porous analysis, we have optimized the temperature for obtaining large surface area of Co-MOF. The electrochemical properties of as-prepared Co-MOFs were examined towards electrode material of supercapacitor in 3 M KOH electrolyte. A maximum specific capacitance of 952.5 F g−1 has been obtained for Co-MOF/150 at a current density of 0.25 A g−1. This excellent electrochemical characteristics of Co-MOF/150 may lead to a promising electrode for high-performance supercapacitors.