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Journal of Inorganic and Organometallic Polymers and Materials (2020) 30:4792–4802https://doi.org/10.1007/s10904-020-01593-8

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Energy Storage Applications of Cobalt and Manganese Metal–Organic Frameworks

S. Sangeetha1 · G. Krishnamurthy1 · Sabine Foro2 · Kalyan Raj3

Received: 6 April 2020 / Accepted: 19 May 2020 / Published online: 2 June 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020

AbstractThis work highlights the electrochemical properties of as-synthesized cobalt and manganese metal–organic frameworks. The electrochemical redox behavior of Co-MOF and Mn-MOF electrode was investigated in 0.1 M KOH solution by cyclic vol-tammetry. The rectangular CV curve obtained by the Mn-MOF electrode implies the pseudocapacitor act from the surface redox reaction of Mn. Well separated oxidation and reduction peaks achieved by the Co-MOF electrode implied the battery like behavior. For battery, the charge is represented as the capacity and the calculated capacity for the Co-MOF was 1064.3 mhAg−1 at 0.1 Ag−1. For pseudocapacitor, capacitance should be used, and achieved capacitance by Mn-MOF electrode was 483 Fg−1 at constant current density 1 Ag−1. With this delineation in mind, it should be clear that Co-MOF is suitable for battery purpose and Mn-MOF is suitable for pseudocapacitor application.

Keywords Co-MOF · Mn-MOF · Crystal structure · Psuedopercapacitor · Battery · Electrochemical charge–discharge · Cyclic stability

1 Introduction

Metal–organic frameworks are the materials exhibiting a wide range of properties due to their large cage-like cavi-ties, three-dimensional network, and presence of active central metal ions, etc. These MOFs are the fascinating materials falling in the micro to mesoporous range with large total surface area. Hence, these materials can be employed for the applications in electrochemical energy storage systems such as, lithium-ion batteries, superca-pacitors, including fuel cells, solar cells and also in cataly-sis, sensors, sewing, and separation, etc. [1–11]. In the past few years, the energy-related and worldwide envi-ronmental related problems are consistently increasing. As a consequence, a series of research towards clean and

reliable energy has been implemented to meet the increas-ing demands for electrical energy storage devices [12–20]. Due to porous nature of MOFs they got serious attention towards energy storage materials and various techniques are being developed to improve the capacitance/capacity of MOFs. Energy storage materials are mainly classified into batteries and psuedocapacitors. There are increasing numbers of new electrode materials that display electro-chemical characteristics that are neither purely capacitive nor purely Faradaic [21]. Some pseudocapacitive materi-als which behave as battery-like current–voltage relation-ship, it may not be useful as a capacitor or as a battery because in many cases, they reduce speed and power [22]. Hence in order find the electrode nature, recent papers quantitatively discuss the differences between true elec-trochemical capacitors, pseudocapacitors, and batter-ies  [4]. The significant difference between battery and pseudocapacitive materials are redox nature and charging/discharging behavior. Any material which shows cyclic voltammograms containing intense separated oxidative and reductive peaks are categorized as battery material. The peak current (i) response of a battery-type electrode will be proportional to the square root of the scanning rate (i~v1/2). The corresponding time scales for batteries, capable of storing orders of magnitude more charge, and

* G. Krishnamurthy drgkmurthy.bub@gmail.com

1 Department of Studies in Chemistry, Bangalore University, Jnanabharathi Campus, Bangalore 560056, India

2 Institute of Materials Science, Darmstadt University of Technology, Alarich-Weiss-Str. 2, 64287 Darmstadt, Germany

3 B.M.S College of Engineering, Bull Temple Road, Basavanagudi, Bangalore 560019, India

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are measured in minutes or hours. Batteries show expo-nential discharge provides steady power to the end. For batteries, the charge is converted into capacity (Eq. 1, mAhg−1) [10, 21, 22]. On the other hand, materials with a psedocapacitor-like response will show rectangular vol-tammograms and linear voltage responses during constant-current discharging. A capacitor-type material will show a linear current response dependency on the scan rate (i~v). If the electrode material falls into the supercapacitor/pseu-docapacitor category, it is necessary to convert the charge into capacitance (C in Farads) [10, 21, 23]. Hence with this delineation in mind, pseudocapacitor behavior of Mn-MOF and battery behavior of Co-MOF have been studied.

Recent studies revealed the attractive 3D framework and excellent catalytic properties of cobalt and manga-nese based metal–organic frameworks, [24–27] which were synthesized by aromatic ligands. But MOFs with aliphatic ligands as a linker and its applications are rarely found in the literature survey. Hence we have reported the stable, porous, and crystalline MOFs synthesized with formic acid as a co-ordinate linker by solvothermal route. Both the crystals have shown different space group, co-ordination mode and electrode behavior, since they were synthesized at similar environmental conditions. Co-MOF has asymmetric co-ordination mode with octahedral geom-etry and Mn-MOF framework has symmetric co-ordination mode. Both the MOFs were made into electrodes and elec-trochemical behavior has been studied.

2 Experimental

2.1 Materials and Methods

Cobalt nitrate and Manganese nitrate were purchased from Sigma-Aldrich and formic acid was purchased from Electrochem Pvt. Ltd. Mumbai (India). Other rea-gents were purchased from Merck Pvt Ltd. All solvents and reagents were used without further purification. Sin-gle crystals were collected on an Xcalibur, Nova diffrac-tometer at 298 K. Field emission scanning electron micros-copy (FESEM) images were collected using a Supra 55 field emission SEM system (Zeiss). The ambient pressure gas sorption isotherms were measured by using NOVA-1000  V  er.3.70  adsorption  equipment.  Electrochemi-cal analyses were made using Autolab potentiostat. The elemental composition was obtained on an Inca 2400 Series II EDX instrument. The thermal gravimetric analy-sis was carried out with an SDT Q600 V20.9 Build 20 ther-mal analyzer from room temperature to 1000 °C in a nitro-gen atmosphere with a heating rate of 10 °C min−1.

2.2 Synthesis of Co‑MOF

The Co-MOF was prepared by the solvothermal method. Co(NO3)2.6H2O (0.06 mmol, 1.2 g) was dissolved in 20 mL of ethanol (solution 1). The 2.76 g of formic acid and 5 drops of trimethylamine were slowly added to solution 1 and the resultant homogeneous solution was then stirred for 5 min and heated in Teflon-lined stainless steel bomb for about 9 h at 100 °C. The obtained solution was kept for crystallization and the green colored crystals were washed with ethanol and dried at room temperature. The obtained crystals were subjected to ATRIR, SEM, SXRD, TG, and BET analysis.

2.3 Synthesis of Mn‑MOF

The 2.76  g of formic acid and trimethylamine were slowly added to the Mn(NO3)2·6H2O (1.2 g, 0.06 mmol) solu-tion. The resulting homogeneous mixture was stirred for 5 min and heated in Teflon-lined stainless steel bomb for about 9 h at 100 °C. The obtained solution was kept for crystallization over 7 days. The crystals obtained were collected by slow evapora-tion of solvent. The crystals were washed with ethanol and dried at room temperature.

2.4 Electrode Preparation and Electrochemical Characterization

The MOF electrode (working electrode) for the electrochemi-cal studies was prepared using a mechanically grounded mixture of prepared MOF (60%), carbon black (20%) and polytetrafluoroethylene (20%). The obtained slurry was then covered onto nickel mesh conductor (1 cm × 1 cm) and dried at room temperature for about 24 h [7]. Subsequently, it was pressed and obtained as the final electrode material. All the electrochemical experiments were conducted on AUTO LAB PGSTAT302N electrochemical work station with a conven-tional three-electrode system consisting of MOF electrode as a working electrode, platinum electrode as auxiliary (counter) electrode and saturated calomel (saturated KCl) as reference electrode. The electrochemical performance of the individual electrode was conducted using a three-electrode assembly in 0.1 M KOH electrolyte [7, 28]. Specific capacity was calcu-lated based on Eq. 1 and specific capacitance was calculated based on Eq. 2.

where C is specific capacity (mhAg−1), Cs (Fg−1) is the spe-cific capacitance, I (mA) is the constant discharge current,

(1)C =Q

M=

I × Δt

3600M

(2)Cs =I × t

mV

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Δt (s) is the discharge time and m (g) is the mass of the active materials. V is the change in potential.

3 Result and Discussion

3.1 Crystal Structure of  C2H6CoO6

The structure refinement and crystallographic data are pre-sented in Table 1. The CCDC deposited number of aliphatic Co-MOF is 1956497. Single crystal X-ray diffraction analy-sis reveals that Co-MOF belongs to the monoclinic system with space group P21/c and designed by 3-dimensional

twofold interpenetrating frameworks. As shown in Fig. 1 Co(II) ion is placed at the crystallographic inversion center and shown asymmetric co-ordination. The symmetric unit contains one Co1 atom and six formate anions. Asymmet-ric unit contains one Co2, four water molecules and two formate anions. Each Co1 is six-coordinated by six oxygen atoms from six formate ions and each Co2 atom is four co-ordinated by four oxygens from four water molecules and two from formate ions. The formate ligand acts as a bridging linker that connects two Co atoms, giving to the 3D frame-work depicted in Fig. 1. The each Co1 is surrounded by four Co1 and two Co2 in trans arrangement (Fig. 1d). Interest-ingly, the chain crystallizes in both right- and left-handed

Table 1 Crystal data and structure refinement Empirical formula C2H6CoO6 C5H9MnO6

Crystal system Monoclinic OrthorhombicSpace group P21/c Pnmaa = 8.684(1) Å α = 90° a = 8.4006(5) Å α = 90°b = 7.165(1) Å β = 97.40° b = 11.7108(7) Å β = 90°c = 9.294(1) Å γ = 90° c = 8.1082(5) Å γ = 90°Z 4 4Density (calculated) 2.143 Mg/m3 1.832 Mg/m3

F(000) 372 448Crystal size 0.280 × 0.160 × 0.120

mm30.280 × 0.240 × 0.200 mm3

Completeness to theta = 25.242° 99.9% 100.0%Data/restraints/parameters 1053/4/97 763/3/61

Fig. 1 a Co-ordination geometry of the Co-MOF (Co: blue, C: grey, O: red, H: yellow), b 3D packing framework and c, d The view along the c-crystallographic axis (Color figure online)

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fashions (Fig. 1d) and exhibits a two-fold interpenetrating framework. The bond lengths of C(1)–O(1), C(2)–O(3), O(2)3–Co(1)–O(2), and O(1)–C(1)–O(4) are 126.0(2) 1.243(2), 180 and 126 Å, respectively.

3.2 Crystal Structure of  C5H9MnO6

The structure refinement and crystallographic data are pre-sented in Table 1. The deposited CCDC number of Mn-MOF is 1960365. The Mn-MOF crystallizes in the orthorhom-bic pnma space group. As shown in Fig. 2, Mn(II) ion is placed at the crystallographic inversion center and manga-nese is surrounded by symmetric co-ordination environment. The each unit contains one Mn ion and six formate anions (Fig. 2a). Meanwhile each Mn ion is located in a six coordi-nated environment with six O atoms from six formate ani-ons. The bond lengths of Mn–O, O–Mn–O, and O–C––O are 2.0945–2.1169, 122.83–121.17, and 125.4 Å, respec-tively. The neighboring Mn ions are bridged by carboxylate

groups, which are further connected to another unit and form 3D networks. The 3D interpenetrating topology is shown in Fig. 2c, d.

3.3 Field Emission Scanning Electron Microscope and Elemental Analysis

Field emission scanning electron microscopy was used to characterize the morphology of synthesized MOFs. Fig-ure 3a shows the SEM micrograph of Co-MOF sample. A square shaped thin plates are arrangement one above the other in the morphology. Energy dispersion X-ray spectrum data mapping (Fig. 3b) shows the percentage composition and exhibit signals of carbon Kα (0.4 keV), oxygen Kα (0.5 keV) and Cobalt Kα (1.0 keV). SEM image of Mn-MOF has given in Fig. 3c, which shows the nano sized rod like crystals with pointing heads. Energy dispersion X-ray analy-sis mapping (Fig. 3d) shows the percentage composition and

Fig. 2 (a, b) Co-ordination geometry of the Mn-MOF (Mn: purple, C: grey, O: red, H: yellow), c 3D packing framework and d The view along the c-crystallographic axis (Color figure online)

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exhibit signals of carbon Kα (0.4 keV), oxygen Kα (0.5 keV) and Manganese Kα (1.0 keV).

3.4 BET Surface Area Measurements

Figure 4a and b are the N2 adsorption/desorption iso-therms of the calcinated Co-MOF and Mn-MOF, respec-tively. The surface areas estimated using the BET method for Co-MOF and Mn-MOF were 70.369 m2g−1 and 44.809 m2g−1, respectively. The average pore diameter was found to be 3.733 nm for Co-MOF and 3.715 nm for Mn-MOF. The recorded isotherms are combination of IUPAC’s [29] types III and IV isotherms obtained for Co-MOF and Mn-MOF respectively. Hysteresis loops provide information related to the shape, connectivity and distribution of mesopores [30, 31]. Therefore, the synthesized MOFs possessed micropores and mesopores [30]. Mn-MOF has shown large hysteresis loop than Co-MOF. In this loop, the Mn-MOF samples exhibited a plateau at the desorp-tion branch which (see arrow 1) surpassed the plateau recorded for the adsorption branch of the loop. Such

behavior is the characteristic of a hindered evaporation process and is usually interpreted as if narrow connec-tions between pores block the transit of the evaporated probe molecule through the porous network of the mate-rial [32].

3.5 Thermal Gravimetric Analysis

Figure 5a has shown degradation mode of Co-MOF was determined by TG analysis. The TG curves of Co-MOF usually refer to two stages of mass losses within differ-ent temperature ranges. The first step of degradation cor-responds to the loss of coordinated water molecules about 35%, which usually occurs in the range of 150–180 °C [33, 34]. Therefore coordinated water molecules are confirmed from TG analysis. The second stage occurs at 200–350 °C with a weight loss about 35%, which can be attributed to the co-ordinate formic acid ligands. It is suggested that the synthesized Co-MOF is stable up to 150 °C. The degra-dation mode of Mn-MOF has shown in Fig. 5b, one step

Fig. 3 a SEM micrograph of Co-MOF, b is EDX pattern of Co-MOF showing elemental composition, c SEM micrograph of Mn-MOF d is EDX pattern of Mn-MOF showing elemental composition

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Fig. 4 a BET surface area measurement and b Pore size distribution pattern of Co-MOF, c BET surface area measurement and d Pore size distri-bution pattern of Mn-MOF

Fig. 5 (a, b) TG analysis of Co-MOF and Mn-MOF respectively

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degradation corresponds to the loss of co-ordinate formic acid linker and after this temperature without any degrada-tion a straight line has seen due to formation of MnO2.

4 Electrochemical Studies

4.1 Cyclic Voltammetry (CV) Studies

Preliminary CV experiments were performed to study the redox behavior of the MOFs prepared. The electrode per-formance was observed at different scan rates between 10 to 100 mV using a three-electrode set up in 0.1 M KOH. The recorded CV (Fig. 6a) for Co-MOF electrode shows the couple of redox peaks between − 1.0 to 1.0 V, which is resulted from the surface redox reactions and these are faradaic in nature [35, 36]. The two redox couples in both cathodic and anodic scan, which is a common behavior as in the reported typical CV of Co electrode [37–41]. Cyclic voltammogram contains the intense and clearly separated oxidative and reductive peaks, which generally due to the electrochemical feature of the battery [4–7]. The corresponding log current versus log potential cali-bration plot in Fig. 6b shows that the Co-MOF electrode displays a good linear relationship (R2 = 0.92). The anodic peak at around 0.25 V and cathodic peak at about 0.15 V (Fig. 6b) are corresponding to different oxidation state of cobalt in Co-MOF. The redox current increases with the increasing scan rate from 10 to 100 m V/s and are proportional to the square root of the scan rate, which

indicates that diffusion-controlled type of electrochemi-cal processes [42].

4.1.1 Charge–Discharge Studies of Co‑MOF

The chronopotentiometry charge–discharge curves at con-stant current obtained from Co-MOF electrode has shown in Fig. 7a. The calculated specific capacities are 373, 429, 641 and 1064 mAhg−1 respectively, for the current densi-ties 0.7, 0.4, 0.2 and 0.1 Ag−1 [Fig. 7c]. These results reveal that the electrode at lower current 0.1 Ag−1 in 0.1 M KOH electrolyte exhibits a superior rate of specific capacity 1064 mAhg−1. The current density is increased from 0.1 to 0.7 Ag−1, which lead to decrease in its specific capacity, which is due to decrease of effective interaction between electrolyte ions and electrode at a large current density [43, 44]. The chronopotentiometry charge–discharge measurement implies that the Co-MOF electrode bring high specific capacity. This should be assigned to the following factors. Firstly, the layered structural features of Co-MOF electrode facilitate adequate penetration of electrolyte onto the surface of active materials in the electrode, resulting in outstanding contact between active materials in the electrode and electrolyte ions and also influence on the efficiency of Faradic reaction, which is the main origin of the high capacity [37]. Secondly, Co-MOF can utilize the redox reaction of the central metal ion in energy storage [45].Third one is the large surface area and high porosity of Co-MOF is beneficial to the permea-tion of electrolytes, endowing the Co-MOF with sufficient electrochemical reactive sites [46]. In addition, to test the

Fig. 6 a CVs of Co-MOF electrode at 50 mVs−1 b CVs of Co-MOF electrode at different scan rate (υ) and Inset: plot of oxidation peak current versus υ1/2

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effect of temperature we have studied the chronopotenti-ometry charge–discharge curves of the Co-MOF electrode at different temperatures (Fig. 7b). The calculated specific capacities (Fig. 7d) of the electrode from their discharge curves are 1064.3, 848.9, 432.2, and 200.6 mAhg−1 for 28, 35, 40, and 45 ℃, respectively. These results reveal that the electrode at lower temperature 28℃ exhibits a superior rate of specific capacity 1064 mAhg−1. The reduced capacity at higher temperature is due to resistance of electrode. As the temperature is increased, the metal ions vibrate more, and more collisions with the electrons occur, hence the resist-ance has increases [7].

Fig. 7 a The charge–discharge curves of the Co–MOF electrode at different current densities, b The charge–discharge curves of the Co-MOF electrode at different temperature, c Specific capacitance of the

Co-MOF electrode versus current densities and d Specific capaci-tance of the Co-MOF electrode versus temperature

Fig. 8 CVs of Mn-MOF electrode at different scan rate from 10 to 100 mV s−1

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4.2 Cyclic Voltammetry (CV) Studies of Mn‑MOF

The charge–discharge performance of Mn-MOF electrode was observed at different scan rates between 10 to 120 mV using a three-electrode set up in solution of 0.1 M KOH. As shown in the Fig. 8a the rectangular CV curve reveals that the pseudocapacitor act resulted from the surface redox reac-tions of Mn-MOF electrode [47]. The anodic peak at around 0.24 V and cathodic peak at about 0.15 V are corresponding to different oxidation state of manganese in Mn-MOF. The anodic and cathodic peak potential difference was found to be 0.09, which shows the reversibility of pseudocapacitor Mn-MOF electrode [21].

4.2.1 Charge–Discharge Studies of Mn‑MOF

The chronopotential charge–discharge performance of Mn-MOF electrode has shown in the Fig. 9.

The calculated specific capacitances are 30, 320, 395, and 437 Fg−1, respectively for the current densities 5, 3.5, 2, and 1 Ag−1 (Fig. 9a). These results reveal that the electrode at lower current density 1 Ag−1, exhibits a superior value of specific capacitance about 437 Fg−1. The Fig. 9b shows the plot of specific capacitance verses current density. The Fig. 9c, and d are the number of repeated charge–discharge cycles at 1 and 5 Ag−1, respectively. These results implied the greater cyclic stability of the Mn-MOF electrode without significant loss of capacity (4% at 1 Ag−1 and 7% at 5 Ag−1).

5 Conclusions

The synthesized Co and Mn MOFs were well character-ized and electrochemical behavior was studied. Co-MOF shows monoclinic system with P21/c space group and Mn-MOF shows orthorhombic with Pnma space group. The

Fig. 9 a The charge–discharge curves of the Mn-MOF electrode at different current densities, b Specific capacitance of the Mn-MOF electrode versus current densities, c, d Cycling performance and stability at 1 Ag−1 and 5 Ag−1 respectively

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as-synthesized Co-MOF has exhibited well-behaved redox events and shows battery behavior. The specific capacity of Co-MOF electrode was 1064 mAhg−1 achieved at 0.1 Ag−1.The Mn-MOF electrode has exhibited well-behaved redox events and shows typical pseudocapacitor behav-ior. The specific capacitance of Mn-MOF was found to be 437 Fg−1 at 1 Ag−1. The comparison data has been summa-rized (Table 2), which implies that these Co-MOF and Mn-MOFs have exhibited are significantly efficient than several reported materials and also these are more promising ones for energy storage applications.

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Table 2 The comparison of specific capacitance values of Co-MOF and Mn-MOF electrodes with reported MOFs

Electrode material Specific capacitance Current References

Mn-BDC 177 Fg−1 0.5 Ag−1 [47]MIL-100 34 Fg−1 0.15 Ag−1 [48]Ce-BTC 502 Fg−1 0.2 Ag−1 [49]Co-MOF 206.76 Fg−1 0.6 Ag−1 [50]Ni BDC 1127 Fg−1 0.5 Ag−1 [51]Zn-BTC derived NPC 134 Fg−1 – [52]IRMOF-3 239 Fg−1 [53]N-doped ZIF-8 285.8 Fg−1 0.1 Ag−1 [54]MOF-199 750 Fg−1 [55]Mn-MOF 437 Fg−1 1 Ag−1 This workNi-MOF 123.4 mAhg−1 1 Ag−1 [56]Co3V2O8 501 mAhg−1 1 Ag−1 [57]Co(L)MOF/RGO 206 mAhg−1 0.5 Ag−1 [58]Zn-MOF 589.28 mAhg−1 1 Ag−1 [7]Co-MOF 1064.3 mAhg−1 0.1 Ag−1 This work

4802 Journal of Inorganic and Organometallic Polymers and Materials (2020) 30:4792–4802

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