Cobalt and Calcium - based metal-organic frameworks (Co-MOF and Ca-MOF) [Co (tpa) (mi) and Ca (tpa)] (tpa = terephthalic acid and mi = methyl imidazole) was synthesized through solvothermal method. Structural characterization revealed that metal centers are equally occupied by Co2+ and Ca2+ ion. Supercapacitive behaviour of synthesized Co-MOF and Ca-MOF were evaluated using cyclic voltammetry (CV), galvano static charge/discharge, and electrochemical impedance spectroscopy (EIS) measurements in 6 M KOH as electrolyte. The Co and Ca-MOFs exhibited outstanding specific capacitance of 1726 and 185Fg− 1 at a discharge current density of 1 Ag− 1, good rate capability, and 97.4% capacitance retention after 3000 cycles. To the best of our knowledge, Co-MOF and Ca-MOFs are used in supercapacitor and its encouraging properties indicate that Co/Ca-MOF can be promising candidate as an advanced electrode material for supercapacitors.
Research Article
Cobalt and Calcium-based Metal-Organic Frameworks (MOFs) as an Energy and Advanced Electrode material for Supercapacitors
https://doi.org/10.21203/rs.3.rs-2251146/v1
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Ca-MOF
Co-MOF
Electrochemical study
cyclic voltammetry
Supercapacitor
MOFs are called polymers of coordination which have gained more attention in the field of electrochemical energy storage systems, including fuel cells, lithium ion batteries, super capacitors, solar cells, etc., thanks to their large surface area, pore volume and metal centres incorporated in adjustable size [1–3]. MOF has recently been used as direct electrode materials for super capacitors. These electrochemical capacitors have received great attention among different energy storage devices like batteries, fuel cells and conventional capacitors because of their high power density, fast charging-discharging rate, and exceptionally long cycle life [4]. Super capacitors appeared among the conventional capacitors and batteries [5]. Supported by the charging storage mechanism, two types of electric double capacitors and pseudo capacitors are supercapacitors [6]. Nevertheless, the instability of most MOFs or MOF-derived electrode material during the charge-discharge cycle is widely considered as a major limitation for their use in super capacitors [7–8] improved performance from the Co-MOF can be realized well than that of other super capacitor hybrid material [9–10]. Supercapacitor requires high mobility of electrons and ions, and materials such as Co(OH)2 Nano-flakes are known to be promising candidates [11] for this. MOF materials also displayed carbonization which was to be used as materials for the supercapacitor candidate [12]. Recently, Lee etal.,[13] reported that Co-MOF-71 can be used directly as a porosity aiding electrochemically driven ion exchange for supercapacitor applications. Here, this case is revisited to explore the reasons for the unexpected reactivity. Co (bdc) (DMF)] (Co-MOF-71, bdc = 1,4-benzenedicarboxylate, DMF = N,N-dimethylformamide) was first synthesized in 2005 by Yaghi et al., [14]. A pseudo-capacitive property was found in an aqueous LiOH electrolyte by CV studies up to 206.76 Fg− 1 for the specific capacitance with mild degradation over 1000 charge–discharge cycles. This value is positive and close to the specific capacitance values recorded for Co (OH)2 or CoO(OH) in magnitude [15–16]. Most MOFs are transition metals assisted (Co, Cu, and Cr) that are toxic to humans, and therefore to the environment [17]. Due to its excellent biocompatibility and comparatively high availability on Earth, calcium is not to extent and cost-efficient metal. Many MOFs based at Calcium have shown promising properties. For example, CaSDB MOF (SDB: 4, 4'-sulfonyldibenzoic acid) has a high CO2/N2 adsorption selectivity and an excellent Xenon absorption capacity, whereas MOF-1201 and MOF-1203 calcium frameworks are ready to encapsulate cis-1, 3-dichloropropene agricultural fumigant [18–19].
2.1. Materials
All chemicals used in this research work, such as cobalt nitrate hexahydrate (Co (NO3)2•6H2O), calcium nitrate hexahydrate (Ca(NO3)2•6H2O), Terephthalic acid (Tpa), methyl imidazole (MI) and N, N- dimethylformamide (DMF), were analytical grade and obtained from SDFCL (SD fine chemistry limited). Methylimidazole and chloroform were reagent grade chemicals obtained from Spectrochem Private limited. All these chemicals were used without further purification as received.
2.2 Synthesis Procedure
2.2.1 Synthesis of Co-MOF
The mixture of solution of Co(NO3)2·6H2O (1.0 gm) and terephthalic acid (0.70gm) which were prepared separately in 15 ml of DMF with mild stirring followed by the addition of about 0.5 ml of 1-methylimidazole to get a clear solution was taken in to Teflon-lined stainless steel autoclave and heated in an oven at 120°C for 72 h. Then the reaction system was allowed for cooling to room temperature. The obtained white coloured crystalline material was filtered and then washed repeatedly with fresh DMF and chloroform. Finally, the product was dried at room temperature for 24 h (Figure.6.1).
2.2.2 Synthesis of Ca-MOF
Dissolving the solution of Ca (NO3)2.6H2O (1.0 gm) and terephthalic acid (0.70gm) in 15 ml of DMF with moderate stirring was prepared to obtain a clear solution. The resulting solutions were moved to an autoclave made of Teflon-lined stainless steel and heated for 72 h in an oven at 120°C. It then allowed the autoclave to cool down to room temperature. The white colored crystalline material obtained was purified and then repeatedly washed with DMF and chloroform solvents. Eventually the stock has been dried for about 24 h at room temperature. The following can be given the reaction scheme (Fig. 1).
2.3. Characterization Techniques
The above products were characterized by vibrational properties of the functional groups were studied by the Shimadzu 8400S FTIR spectrophotometer with IR grade KBr pellets in the range of 400–4000 cm− 1. Powder X- ray diffraction (PXRD) to obtain information on phase structure, purity, nature (crystalline or amorphous), etc. A Bruker D8 Advance X- CuKα radiation diffractometer was used and the µ = 0.1541874 nm. The FE-SEM images were taken on the Zeiss FESEM instrument. The pore size, the pore volume and the specific material surface area were calculated using the data obtained by Nova1000 BET instrument. Thermo gravimetric analysis (TGA) on a Perkin-Elmer thermal analyzer was carried out in the N2 atmosphere at a heating rate of 20°C min− 1.The crystal data was collected at the Xcalibur, Eos and Nova diffractometer. During data collection, the crystal was maintained at 293(2) K. The structure was solved using Intrinsic Phasing with the ShelXT structure solution program using Olex2 and refined using the ShelXL refining package to CGLS minimization.
2.4. Electrochemical measurements
All electrochemical tests have been performed at the CHI 660E electrochemical workstation. The working electrode was prepared with a known quantity of active material (80 wt% or ~ 2 mg) was prepared with graphite powder (15wt %) and polytetrafluoroethylene (PTFE) binder (5wt %) along with an ethanol. This mixture was consequently hand pressed on nickel mesh from all sides. Copper wire was used as the current collector. This electrode was left for drying at room temperature for about 24 h. The cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy were conducted under atmospheric conditions in 6M KOH electrolyte solution. The saturated calomel electrode (SCE), platinum electrode and prepared Co-MOF and Ca-MOF electrodes were used as the reference, counter and working electrodes respectively.
3.1 Crystal structure determination of Co-MOF
There are two types of cobalt ions with different coordination environments in octahedral geometry in the presence of DMF molecules. Crystal Data for C16H16CoN4O4 (M = 387.26 g/mol):crystal type; triclinic, space group: P-1 (no.2), crystal parameters; a = 7.3256(7) Å, b = 9.0006(9) Å, c = 13.7874(14) Å, α = 82.0240(10)°, β = 78.2670(10)°, γ = 72.3450(10)°, V = 845.25(15) Å3, Z = 2, T = 293(2) K, µ(MoKα) = 1.044 mm-1, Dcalc = 1.522 g/cm3, 7277 reflections measured (3.028° ≤ 2Θ ≤ 53.992°), 3633 unique (Rint = 0.0193, Rsigma = 0.0297) which were used in all calculations. The final R1 was 0.0433 (I > 2σ (I)) and wR2 was 0.1116 (all data) (Table 1).
| Identification code | Ca-MOF | Co-MOF |
|---|---|---|
| Empirical formula | C8H10O7Ca | C16H16CoN4O4 |
| Formula weight | 258.24 | 387.26 |
| Temperature/K | 293(2) | 293(2) |
| Crystal system | monoclinic | triclinic |
| Space group | P21/c | P-1 |
| a/Å | 7.1063(8) | 7.3256(7) |
| b/Å | 21.653(3) | 9.0006(9) |
| c/Å | 6.5929(6) | 13.7874(14) |
| α/° | 90 | 82.0240(10) |
| β/° | 92.24(1) | 78.2670(10) |
| γ/° | 90 | 72.3450(10) |
| Volume/Å3 | 1013.7(2) | 845.25(15) |
| Z | 4 | 2 |
| ρcalcg/cm3 | 1.692 | 1.522 |
| µ/mm1 | 0.638 | 1.044 |
| F(000) | 536.0 | 398.0 |
| Crystal size/mm3 | 0.24 × 0.08 × 0.08 | 0.350 × 0.100 × 0.080 |
| Radiation | MoKα (λ = 0.71073) | MoKα (λ = 0.71073) |
| 2Θ range for data collection/° | 6.038 to 50.694 | 3.028 to 53.992 |
| Index ranges | -8 ≤ h ≤ 4, -24 ≤ k ≤ 26, -7 ≤ l ≤ 7 | -9 ≤ h ≤ 9, -11 ≤ k ≤ 11, -17 ≤ l ≤ 17 |
| Reflections collected | 3633 | 7277 |
| Independent reflections | 1855 [Rint = 0.0385, Rsigma = 0.0699] | 3633 [Rint = 0.0193, Rsigma = 0.0297] |
| Data/restraints/parameters | 1855/6/163 | 3633/0/226 |
| Goodness-of-fit on F2 | 0.941 | 1.070 |
| Final R indexes [I > = 2σ (I)] | R1 = 0.0439, wR2 = 0.0870 | R1 = 0.0433, wR2 = 0.1075 |
| Final R indexes [all data] | R1 = 0.0840, wR2 = 0.0993 | R1 = 0.0509, wR2 = 0.1116 |
| Largest diff. peak/hole / e Å−3 | 0.60/-0.29 | 0.46/-0.25 |
In Co-MOF, Co ion is connected by two BDC and imidazole ligands and each group connects two monomer units to form a 1D frame works (Fig. 2.a), specified together with x, y and z axis to form 1D, 2D and 3D channels with a diameter of Å12 (Fig. 2.b, 2.c and 2.d).
3.2 Crystal structure determination of Ca-MOF
Crystal Data for C8H10O7Ca (M = 258.24 g/mol) crystal type: monoclinic; space group; P21/c (no. 14); unirt parameters; a = 7.1063(8) Å, b = 21.653(3) Å, c = 6.5929(6) Å, β = 92.24(1)°, V = 1013.7(2) Å3, Z = 4, T = 293(2) K, µ(MoKα) = 0.638 mm− 1, Dcalc = 1.692 g/cm3, 3633, reflections measured (6.038° ≤ 2Θ ≤ 50.694°), 1855 unique (Rint = 0.0385, Rsigma = 0.0699), which were used in all calculations. The final R1 was 0.0439 (I > 2σ (I)) and wR2 was 0.0993 (all data) (Table 1).
In Ca-MOF context, Ca ion is connected by one BDC ligand and each group connects two monomer units to form a 1D frame works (Fig. 3a), specified together with x, y and z axis to form 1D, 2D and 3D channels with a diameter of Å12 (Fig. 3b), 3c) and 3d).
3.3 Bond lengths and bond angles of Ca-MOF and Co-MOF
Table 2 shows the Ca-MOF and Co-MOF bond lengths. Ca-O bond lengths are 2.364(5) and 2.080(14) Å. Ca-C bond lengths are 2.414(5) and 1.777(16) Å. Table 3 shows the Co-O bond angles are 103.5 (6) and 165.2 (14). Co-C bond lengths are 135.8(14) and 138.81(17) Å. Finally, these 3D sheets are connected by carboxylate groups of bdc and Mi ligands are coordinated to Ca and Co(II) cation in a mono dentate manner, forming 3D frameworks
| Atom | Atom | Length/Å | Atom | Atom | Length/Å | |
|---|---|---|---|---|---|---|
| C(1) | C(2) | 1.389(4) | Co(1) | O(3) | 2.006(2) | |
| C(2) | C(3) | 1.381(5) | Co(1) | N(1) | 2.056(2) | |
| C(3) | C(4) | 1.386(4) | O(1) | C(1) | 1.275(3) | |
| C(4) | C(5) | 1.386(4) | O(2) | C(1) | 1.236(3) | |
| C(4) | C(8) | 1.508(5) | C(1) | C(2) | 1.503(3) | |
| C(5) | C(6) | 1.375(5) | C(2) | C(3) | 1.387(3) | |
| C(7) | Ca(1)1 | 2.870(3) | C(4) | C(3)1 | 1.385(3) | |
| C(8) | O(3) | 1.263(4) | O(3) | C(5) | 1.252(4) | |
| O(1) | Ca(1)1 | 2.526(2) | C(6) | C(7) | 1.373(4) | |
| O(2) | Ca(1)1 | 2.485(2) | C(6) | C(8) | 1.382(4) | |
| O(2) | Ca(1)2 | 2.364(2) | N(1) | C(9) | 1.306(4) | |
| O(5) | Ca(1) | 2.369(3) | N(1) | C(11) | 1.367(4) | |
| O(6) | Ca(1) | 2.387(3) | C(9) | N(2) | 1.338(4) | |
| O(7) | Ca(1)1 | 2.521(3) | N(2) | C(10) | 1.340(4) | |
| O(7) | Ca(1) | 2.633(3) | N(2) | C(12) | 1.473(4) | |
| Ca(1) | C(7)3 | 2.871(3) | C(13) | N(4) | 1.327(3) | |
| Ca(1) | O(1)3 | 2.526(2) | N(4) | C(14) | 1.350(4) | |
| Ca(1) | O(2)3 | 2.485(2) | N(4) | C(16) | 1.470(4) | |
| Ca(1) | Ca(1)3 | 3.639(6) | C(14) | C(15) | 1.352(4) |
| Atom | Atom | Atom | Angle/˚ | Atom | Atom | Atom | Angle/˚ | |
|---|---|---|---|---|---|---|---|---|
| C(2) | C(1) | C(6) | 118.6(3) | O(3) | Co(1) | O(1) | 100.01(9) | |
| C(5) | C(6) | C(1) | 120.6(3) | O(3) | Co(1) | N(3) | 128.4(10) | |
| C(1) | C(7) | Ca(1)1 | 171.6(2) | O(1) | Co(1) | N(1) | 100.0(8) | |
| O(1) | C(7) | C(1) | 119.8(3) | N(3) | Co(1) | N(1) | 101.4(8) | |
| O(1) | C(7) | Ca(1)1 | 61.5(16) | C(1) | O(1) | Co(1) | 100.2(15) | |
| O(2) | C(7) | C(1) | 119.5(3) | O(2) | C(1) | O(1) | 122.2(2) | |
| C(7) | O(1) | Ca(1)1 | 92.5(18) | C(5) | O(3) | Co(1) | 105.2(2) | |
| Ca(1) | O(1) | Ca(1)1 | 96.4(8) | C(7) | C(6) | C(8) | 119.6(2) | |
| C(7) | O(2) | Ca(1)1 | 94.3(18) | C(7) | C(6) | C(5) | 120.9(3) | |
| C(7) | O(2) | Ca(1)2 | 166.6(2) | C(9) | N(1) | C(11) | 105.3(2) | |
| Ca(1)2 | O(2) | Ca(1)1 | 97.2(8) | C(9) | N(1) | Co(1) | 127.3(19) | |
| Ca(1)1 | O(7) | Ca(1) | 89.8(8) | C(11) | N(1) | Co(1) | 127.1(2) | |
| O(2)4 | Ca(1) | O(7) | 145.9(8) | N(1) | C(9) | N(2) | 111.4(3) | |
| O(2)3 | Ca(1) | O(7) | 65.1(8) | C(9) | N(2) | C(10) | 107.1(3) | |
| O(2)4 | Ca(1) | O(7)3 | 68.6(8) | C(9) | N(2) | C(12) | 124.6(3) | |
| O(2)3 | Ca(1) | Ca(1)3 | 90.7(6) | C(15) | N(3) | Co(1) | 131.7(19) | |
| O(2)4 | Ca(1) | Ca(1)1 | 166.6(6) | N(3) | C(13) | N(4) | 112.5(2) | |
| O(5) | Ca(1) | O(7) | 77.9(8) | C(15) | C(14) | N(4) | 107.0(3) | |
| O(7)3 | Ca(1) | C(7)3 | 79.9(9) | C(14) | C(15) | N(3) | 109.2(3) |
3.4 FTIR studies
The Co-MOF FTIR spectrum is shown in the Fig. 4. The aromatic C-H stretching vibration of 1,4-benzene dicarboxylic acid can be assigned to a mild absorption group at 3125 cm− 1. The 1675 and 1557 cm− 1 bands are administered as metabolites of C = O stretching vibration arising from the 1,4-benzene dicarboxylic acid, and methyl imidazole C-C aromatic ring skeletal vibration, respectively. Around 1345 cm− 1 the solid band corresponds to the stretching pulse of the C-O. In addition, the asymmetric and symmetric stretching vibrations O-C = O and also the C-O stretching vibration of unreacted 1,4-benzene dicarboxylic acid and reacted acid variety will be assigned to the absorption bands within the area 813 to 1095 cm− 1. The 659 to 749 cm− 1 band are allocated to aromatic ring vibrations in and out of plane bending [20].
The Ca-MOF FTIR spectrum is shown in Fig. 4. The broad absorption band based at 3250 − 3000 cm− 1 is due to the presence of hydrogen-bonding links between frameworks. Two high peaks of 1513 cm− 1and 1369 cm− 1could be ascribed to the TPA's stretching vibrations of mass (O-C = O) and ms (O-C-O). Bending movements of m(C-H) occurred at 829 and 744 cm− 1 (phenyl) indicates the existence of Ca-O bonds. The C = O change of carboxylate vibrations shows that the bonding with the metal Ca+ 2 is formed centrally with the ligand terephthalic acid.
3.5 PXRD studies
The sharp peaks in the PXRD pattern (Fig. 5), which shows the material's crystalline existence. The major metallic peaks at 2θ levels, 10.34, 13.74, 19.67, 21.65 and 27.65, indicate metal coordination to form the complex through the ligands.
Figure 5 shows the Co-MOF PXRD sequence, which shows the presence of the prominent metallic peak at 2θ values at 10.43, 18.48, 20.97, 30.55 and 31.84 indicates metal and ligand coordination to form a complex. A pattern of a lot of noise and extra peaks suggests the phase structure and purity of the obtained products. It means the resulting phase is strongly crystalline and solid. Such results confirm the Co-MOF's particulate size and also its excellent crystalline composition.
3.6 FESEM
Figure 6 displays the Co-MOF FESEM images at various magnifications (1, 2, 20µm, and 200 nm). The images show the shoe soles like cluster close to structures. Unlike slabs they also appear with rough and broken surfaces.
Though Ca-MOF, the particles are free and agglomerated particles with probably the same phase (as shown by XRD results). Figure 6 displays the FESEM images of Ca-MOF in different magnifications (1 and 200 nm).
3.7 EDX
EDX analysis was performed in order to identify the elements present in the synthesized Co-MOF and Ca-MOF. The Ca-MOF and Co-MOF EDX spectrum (Fig. 7) shows elemental peaks of C, O and Co and Ca with a weight ratio. Thus the EDX analysis confirms the basic composition of the Ca-MOF and Co-MOF (Table 4).
| Name of the compounds | Percentage Composition | |||
|---|---|---|---|---|
| Cobalt | Carbon | Oxygen | Total% | |
| Co-MOF | 26.15 | 58.28 | 15.57 | 100 |
| Ca-MOF | Percentage Composition | |||
| Calcium | Carbon | Oxygen | Total% | |
| 27.18 | 37.61 | 35.21 | 100 | |
3.8 BET surface area measurement
N2 adsorption was measured for the pore volume and surface area of Co-MOF using a NOVA-1000 BET surface area analyser instrument. During 4 hours products were degassed at 180°C. Co-MOF surface area was calculated using the non-local functional density theory (NLDFT). For the substances, the pore textural properties, the surface area of the BET, the total pore width, the average pore diameter of the adsorption, and the Langmuir specific surface area were measured. Figure 8 shows the Co-MOF isotherms of N2 adsorption–desorption. The isothermic sorption obtained with nitrogen gas indicates type IV isotherm with no hysteresis indicating the existence of the microporous complex. The BET surface area obtained, 109.8 m2g-1 has provided the adsorption potential of our compounds with the adsorbed volume = 5.27 m2g-1 and the micropore volume = 0.01391 cc / g corresponding to the average pore diameter = 50.619Å.
Figure 8 shows the N2 adsorption–desorption isotherms of Ca-MOF. The adsorption isotherms obtained with nitrogen gas signify type IV isotherm with no hysteresis, which indicates the microporous structure of the material. Obtained BET surface area is about 3.4262m2/g, which represents the adsorption capacity of our compounds. The adsorption capacity reaches (the amount adsorbed) the value of about 6.575m2/g-1 and the average pore diameter of about 39.17Å, which corresponds to micropore volume, 0.1055 cc/g.
3.9 TGA analysis
Figure 9 shows the Thermogravimetric curve for the compound measured at a heating rate of 20°C/ min without pre-treatment under an N2 atmosphere. The Co-MOF indicates three points for weight loss. The first weight loss between the temperature range of 100°C and 210°C was observed, the second weight loss between 250°C-450°C was observed, and the third weight loss was observed between 450°C-550°C. The first weight loss (20%) between the 250°C-450°C region was observed, which could be due to the loss of free lattice molecule DMF, and also. The second weight loss of around 14%was observed between 250°C-450° C and 35% weight loss observed between 450°C-550°C can be due to the elimination of the phenyl ring and carboxylate group. No such weight loss was found till 600°C after this. At the start all solvent molecules had vanished. The framework breaks down, leading to the end product being amorphous material. In the temperature range up to 150°C the Co-MOF is stable.
Figure 9 shows that the material is stable up to 200°C the weight loss at two points. First weight loss was observed between the temperature 200–250°C and the second weight loss was observed in the range 450–600°C. The first weight loss of about 8% in the range 200–250°C could be attributed to the loss of free DMF molecule. The second weight loss of about 40% was observed in the range of temperature 450–600°C, which could be due to the cleavage of the phenyl ring and the carboxylate group.
4.1 Preparation of working electrodes
A paste of the sample of Co-MOF or Ca-MOF with (80wt% or ~ 4mg) was prepared with graphite powder (15wt %) and PTFE binder (5wt %) along with a drop or two of ethanol. This mixture was consequently pressed on nickel mesh from all sides. Copper wire was used as the current collector. This electrode was left for drying at room temperature for about 24h. The galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and impedance studies were made using 6M KOH solution on electrochemical (CHI 608E) instrument. The saturated calomel electrode (SCE), platinum (foil) electrode and prepared Co-MOF and Ca-MOF electrodes separately were used as the reference, counter and working electrodes respectively [22].
4.2 Cyclic Voltammetry studies of Co-MOF
The redox peaks observed in CV curves for Co-MOF electrode, were related to that of pseudo capacitive properties. The sharp redox peaks of Co-MOF can be ascribed to the faradic reactions of the various oxidation states of Co and the reaction paths (Eqs. 1 and 2) can be explained as follows [23].
The electrochemical behaviour of the Co-MOF was studied by CV in the potential range of 0.0 to 0.4 V at different scan rates (10 to 100mVs− 1). The specific capacitance values were calculated using the Eq. 3. [24].
Figure 10a) shows the CV curves of Co-MOF electrode. All the CV curves in the Figures show a couple of redox peaks, which indicate the pseudo capacitive behaviour of the electrodes from the surface redox reactions [25]. The anodic and cathodic peaks at about 0.31 V and 0.13 V might be due to the conversion of different cobalt oxidation states like Co(II) to Co(III) and then Co(IV) in Co-MOF. The oxidation and reduction peaks of the electrodes have shifted to the positive and negative directions at higher scan rates, which could be attributed to the internal resistance of the electrodes. Also, the K+ ions can easily diffuse into the available free spaces of electrode at lower scan rates, causing to efficient redox reactions [26]. The Co-MOF electrode shows a significant enhancement in redox peak current when compared to BCPE. This indicates a very good electro catalytic activity of Co-MOF.
Figure 10b) shows the CV curves of prepared Ca-MOF electrode. All the CV curves show a couple of redox peaks. The anodic and cathodic peaks at about 0.39 V and 0.11 V might be due to the conversion of different cobalt oxidation states like Ca(I) and Ca(II) in Ca-MOF. The Ca-MOF electrode shows a significant enhancement in redox peak current when compared to BCPE indicating a good electro catalytic nature of Ca-MOF [27]. The following redox reactions (3 and 4). Figure 10c) and d) are the Relationships between the graph of anodic peak current (Ipa) versus square root of scan rate (v1/2) was plotted in the Co-MOF and Ca-MOF electrodes in a 6M KOH solution at different scan rates between 10 to 100 mVs− 1
4.3 Galvanostatic charge-discharge studies
The GCD curves at different current densities with Co-MOF are shown in Figure. 6.22. The elevation in the charge-discharge curves of all electrodes is consistent with the CV results, indicating the pseudo capacitance behaviour due to the Faradic processes. The nonlinear shape of GCD curves, credited to the faradic reactions of Co2+/Co3+associated with OH−, validates the CV results [28].
Figure 6.22 shows the charge-discharge curves of the electrodes at various current densities of 6M KOH electrolytic solution. It can be noticed that all the charge-discharge curves are of the non-triangle nature. It shows levels at a potential for 0.5 V in the discharge curves, signifying the presence of pseudo capacitance nature due to the faradic redox reaction to Co+ 2/Co+ 3 with electrolytes. The specific capacitance is 1726, 244, 210, 86 and 30 Fg− 1 respectively with current densities (1, 2, 3, 4 and 5Ag− 1).
The specific capacitance of electrode materials is estimated at discharge curves by the Eq. 3.8 Chap. 3 [29]. Additionally, when the current density is increased to some point, the particular specific capacitance of the electrode is diminished because of the fewer interfaces between electrolyte ions and electrode at superior current densities. Especially at the lesser current density, the electrolyte ions can simply penetrate into all available places on the electrode surface, which leads to efficient insertion reaction [30–32].
Figure 6.23 exhibits the charge-discharge curves of Ca-MOF electrode. The GCD study was carried out in the potential range of 0.0 to 0.5 V at various current densities (1, 2, 3, 4 and 5 Ag− 1) to evaluate the cyclic stability and specific capacitance of Ca-MOF electrode. We found that the Specific capacitance varied from 185, 127, 121, 110 and 102 Fg− 1 with an increase in current density for the current densities 1, 2, 3, 4 and 5 Ag− 1 respectively [33].
4.4 Energy density and power density of Co-MOF and Ca-MOF
The energy density values of Co-MOF are 215.75, 30.50, 26.25, 10.75 and 3.75 (wh/kg− 1) and power density values are 0.062, 0.125, 0.187, 0.250 and 0.312 (w/kg− 1) respectively with discharge time. The energy density and power density of Ca-MOF electrode are 23.12, 15.87 15.18, 13.75 and 12.81 (Wh/kg− 1) and power density values are 0.062, 0.124, 0.186, 0.260 and 0.321 (W/kg− 1) respectively. Figure 6.23 and 6.24 shows the energy density and power density of Co- MOF and Ca-MOF in 6M KOH electrolytes, and it is calculated by the Eq. 3.9 and 3.10 [34].
4.5 EIS
EIS measurements were made at a frequency range of 100 kHz to 0.01 Hz with a disturbance potential of 5 mVs− 1 as shown in the Fig. 12. The Co-MOF semicircles in the high frequency range are smaller than the Ca-MOF semicircles. A smaller semi circle indicates a small charge-transfer resistance (Rct) [35], the straight lines in the low frequency region shows an ideal electrode capacitive behaviour, while the calculated Rct for Co-MOF and Ca-MOF are 6.3 and 70.4 Ω respectively. The interception of the real axis in the high frequency range of Nyquist plots indicates the equivalent series resistance (ESR), the combined series resistance of electrolytes, electrodes, electrolytes and electrodes. The Co-MOF and Ca-MOF ESRs are 3.012 and 20.88Ω respectively (Table 5).
Table.5 Impedance data of Co-MOF and Ca-MOF electrodes in a 6M KOH solution at 5 mVs-1 scan rate
|
Electrodes |
Rs/Ω
|
Rct/Ω |
Cdl/F(1)
|
W
|
|
Co-MOF |
3.012 |
6.349 |
2.804 × 10-7
|
0.001186 |
|
Ca-MOF |
20.88 |
70.04 |
1.751 × 10-6
|
0.001952 |
The solvthermal methods was successfully employed to synthesize Co-MOF and Ca-MOF. The success of these methods lies in obtaining the pure and crystalline products when compared to the high temperature and time consuming conventional methods. Both Co-MOF and Ca-MOF are BDC derivatives and they exhibit many potential energy storage based applications. The Co-MOF electrode shows a highest specific capacitance 1726 Fg-1 compared to Ca-MOF 185 Fg-1 at a current density of 1Ag-1. Moreover, Co-MOF is the better and suitable candiadate for the super capacitor application it delivers high energy and power density and excellent rate performance with long cycle life. This research work may give way for new class of MOFs with improved electro catalytic behaviour. These results would inspire the researchers to explore MOFs and the researcher on outcomes will inspire us to search for more MOF-based materials for energy and application.
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No competing interests reported.