Rakesh
Deka
a,
Shashank
Rathi
a and
Shaikh M.
Mobin
*abc
aDepartment of Chemistry, India. E-mail: xray@iiti.ac.in; Tel: +91 731 6603 336
bCenter for Electric Vehicle and Intelligent Transport System (CEVIT), India
cCenter of Advanced Electronics (CAE), Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, India
First published on 10th October 2023
Coordination polymers have attracted much interest in energy-related applications due to their adaptable structures and unique photophysical and chemical properties. In this study, a coordination polymer, Co-CP, was synthesized using a mixed ligand strategy via a slow diffusion technique. Single-crystal X-ray diffraction studies confirmed the characteristic two-dimensional structure of Co-CP, and plate-like morphology was authenticated through SEM images. Co-CP facilitates ion transport and efficient charge transfer processes, making it an ideal active material for supercapacitor applications. The results from the electrochemical studies demonstrate excellent supercapacitor properties for Co-CP, exhibiting a specific capacitance of 1092 F g−1 at 1.5 A g−1 in 7 M NaOH. Furthermore, the kinetic effect of the electrolyte cation was also investigated in a two-electrode asymmetric supercapacitor (ASC) system by preparing three different gels (NaOH-PVA, KOH-PVA, and LiOH-PVA). Similar trends were observed for the ASC device, with the highest energy density of 17 W h Kg−1 at a power density of 1200 W Kg−1 in NaOH-PVA gel. Overall, the results suggest that Co-CP is a promising active material for supercapacitor applications, and the choice of electrolyte cation has a remarkable impact on the electrochemical performance of the device. This study provides valuable insights for the development and optimization of high-performance supercapacitors based on coordination polymers.
Over the past few years, graphene and other 2D materials such as MXenes,21,22 2D covalent organic frameworks (COFs),23 B4C,24 C3N4,25 2D coordination polymers (CPs)26 and layered metal oxides/hydroxides27,28 have been highly explored as supercapacitor materials. Among these materials, CPs have properties like tunable porosity, extensive surface area, and their flexibility to host a number of guest molecules and are attractive materials to be utilized for supercapacitor application. Moreover, their synthetic procedure also involves straightforward reaction pathways. Their properties can be tailored for a specific application by controlling the synthesized CP's size, shape, and composition. Constructing CPs with mixed linkers to produce 2D layered or 3D scaffolds is attractive due to their extensive use in the fields of energy or gas storage, sensing, and catalysis.29–32
The performance of CPs for supercapacitor applications can be enhanced using several strategies, including selecting the electrolyte since cations possessing a high charge density can increase the capacitance. However, the working potential window depends on the choice of electrolyte; the higher the cations’ redox potential, the greater the potential window will be.33,34 So, the choice of electrolyte becomes vital for the supercapacitor application. Many elements need to be examined, including conductivity, stability under the same working circumstances, mobility of the ions, viscosity, etc.35 Each electrolyte has its own strength and weakness. Among liquid electrolytes, which are most often used for energy storage applications, organic and ionic electrolytes possess a wide range of working potential windows, which results in a high energy density value. The low conductivity and highly viscous nature show moderate capacitance and poor performance.36 In contrast, aqueous electrolytes are inexpensive and environmentally safe. They have high ionic conductivity, low viscosity and exhibit good charge propagation with high performance.37,38
Recent studies have focused on exploring the potential of cobalt-based conductive polymers (CPs) for energy storage purposes, highlighting their unique structural and electrical characteristics. Recently, Shao et al.39 prepared nanorod-shaped Co-MOFs synthesized using a solvothermal method, and they demonstrated notable electrochemical properties. They showed a specific capacitance (Cs) of 414.5 F g−1 at 0.5 A g−1 in a 3 M KOH electrolyte, while maintaining an impressive retention rate of 113% for up to 30000 cycles. With an ASC device, it provides an energy density of 12.0 W h Kg−1 accompanied by a power density of 258.1 W Kg−1. In another report, Kang et al.40 described Cs of 726 F g−1 using a Ni-MOF, which exhibited 94.6% cyclic stability over 1000 cycles and displayed an energy density of 16.5 W h Kg−1. Ma et al.41 presented a Ni/Co based nanoscale coordination polymer (nCP) with a Cs of 1160.2 F g−1, and it retained 66% capacitance over 7000 cycles. Hong et al.42 synthesized a Ni–Co/graphene oxide composite via a one-pot solvothermal reaction, which exhibited a capacitance retention rate of 99.6% after 300 cycles and the observed Cs is 447.2 F g−1. Moreover, various studies were performed to understand the effect of electrolytic cations on supercapacitor application to escalate the material's performance. In this regard, Houpt et al.43 synthesized a composite of CNT-ZIF-MoS2 as an electrode material and explored its electrochemical kinetics using LiCl, NaOH, KCl, and KOH electrolytes. The composite exhibits the highest efficiency in KOH electrolytes. Moreover, Kumar et al.44 developed a binary metal sulfide, denoted as CNS0.15, and its efficacy was investigated across three different electrolytes: NaOH, KOH, and LiOH. Among these, it demonstrated superior performance, specifically in the NaOH electrolyte, exhibiting optimal efficiency. In our recent findings, we have observed a similar trend in a nickel-based polymer (Ni-CP),45 where the highest capacitance was observed in NaOH electrolyte compared to other electrolytes.
In this report, we have synthesized a Co-CP employing a slow diffusion method which is well characterized using various techniques, including SC-XRD, PXRD, FT-IR, SEM, etc. The synthesized Co-CP used for the supercapacitor displayed a specific capacitance of 1092 F g−1 at 1.5 A g−1 while maintaining an excellent capacitance retention of 98% even after 6500 GCD cycles. Furthermore, to evaluate the kinetic effect of electrolytic cations, a thorough study was carried out using 1 M lithium hydroxide, sodium hydroxide and potassium hydroxide. The superior efficiency of Co-CP in NaOH electrolyte can be attributed to the synergistic impact of hydrated ionic radii and the ionic radii of cations. The straightforward synthetic approach at room temperature, simple electrode fabrication techniques, and additive and binder-free approach make it ideal for supercapacitor applications. In addition, an asymmetric (ASC) device was constructed, utilizing Co-CP and activated carbon as the positive and negative electrodes, respectively. This combination resulted in an impressive energy density of 17 W h Kg−1 and a power density of 1200 W Kg−1. This study presents a straightforward synthetic route for Co-CP, offering a promising candidate for future energy storage applications.
Empirical formula | C9H13.50Co0.50N1.50O3.50 |
Formula weight | 228.18 |
Temperature | 293(2) K |
Wavelength | 0.71073 Å |
Space group | Monoclinic, P21/m |
a | 10.3390(4) Å |
b | 7.1811(3) Å |
c | 13.8946(6) Å |
α | 90° |
β | 91.287(3)° |
γ | 90° |
Volume | 1031.35(7) Å3 |
Z | 4 |
Density | 1.470 mg m−3 |
Absorption coefficient | 0.877 mm−1 |
F(000) | 478 |
Crystal size | 0.330 × 0.260 × 0.210 mm |
Theta range for data collection | 2.933 to 29.058° |
Limiting indices | −14 ⇐ h ⇐ 13, −8 ⇐ k ⇐ 9, −17 ⇐ l ⇐ 18 |
Reflections collected/unique | 10592/2712 [R(int) = 0.0611] |
Absorption correction | Semi-empirical from equivalents |
Max. and min. transmission | 1.00000 and 0.71696 |
Refinement method | Full-matrix least-squares on F2 |
Data/restraints/parameters | 2712/0/169 |
Goodness-of-fit on F2 | 1.089 |
Final R indices [I > 2sigma(I)] | R 1 = 0.0529, wR2 = 0.1288 |
R indices (all data) | R 1 = 0.0754, wR2 = 0.1432 |
Largest diff. peak and hole | 0.567 and −0.396 e Å−3 |
CCDC No. | 2285196 |
Fig. 2 (a) X-ray diffraction spectrum, (b) Fourier transform-infrared spectrum, (c) TGA spectra, (d) and (e) FE-SEM images, and (f) elemental compositions of Co-CP. |
Moreover, to further confirm the bonding interactions between the metal atom and the ligand moiety, the FT-IR spectrum were recorded within 4000–400 cm−1 (Fig. 2b). The broad band observed at 3073 cm−1 represents the νOH from the associated water molecule. The band observed at 1660 cm−1 and 1373 cm−1 confirms the asymmetric νas(COO−) and symmetric νs(COO−) carboxylate groups.47 The peaks at 1546 cm−1 signify the phenyl ring stretching vibrational mode.48 The distinct peak observed at 780 cm−1 was attributed to the Co–O stretching frequency.49 Moreover, the peaks at 2971, 1280, 1100, and 1050 cm−1 depict the characteristics of ν(C–H), ν(C–N), ν(C–C), and ν(C–N) stretching frequencies, respectively.50
To understand the thermal stability of the material we also performed thermogravimetric analysis (TGA) for Co-CP (Fig. 2c). From Fig. 2c it is observed that the degradation of Co-CP occurs in two major steps, at first, the degradation starts from 100 °C signifying the removal of coordinated methanol molecules. Beyond that the second steps represent the degradation associated DMF molecule in the moiety up to 200 °C. After that gradual degradation occurs representing breakdown of the benzene ring up to a temperature of 350 °C. At last, the complete degradation of the moiety observed in the temperature starting from 450 °C. Therefore, the high thermal stability of the Co-CP signifies its possibility for electrochemical energy storage at high temperatures.
Studying surface morphology is essential to understanding the ion transportation mechanism. In this context, scanning electron microscope (SEM) images were recorded at various magnifications using Au coating and Al as a substrate for Co-CP (Fig. 2d and e). The observed images demonstrate sheet or plate-like morphology. The disoriented structural morphology with uneven edges may favour a more active site for reaction and could give better transmission possibilities for ions for electrochemical reactions.51 The observed average l/d ratio for Co-CP is 1.908, which may induce more open sites for electrolytic ions to take part in electrochemical reactions. Moreover, the excellent results of Co-CP towards the electrochemical supercapacitor application may be attributed due to the redox active Co(II) metal center and high interspaces within the molecules for high transmission possibilities of ions. Later on, energy dispersive spectroscopy (EDS) was also studied to further authenticate the PXRD results. The observed wt% for Co, N, O, and C are 0.6, 9.4, 34.8, and 46.8, respectively (Fig. 2f). Due to the gold (Au) coating and use of aluminium (Al) as a substrate, the peaks for Au and Al were observed in EDS.
In the context of electrochemical supercapacitor applications, surface area and pore size become crucial parameters. To determine these, Brunauer–Emmett–Teller (BET) analysis was conducted at 77 K and the results are presented in Fig. S3 (ESI†). From the crystal structure of Co-CP, it is seen that both the COO− groups of 5-hydroxyisophthalic acid are connected with the metal center and the N atom of the DABCO linker is also occupied within the metal center. Therefore, the functional groups available to interact with the adsorbed N2 molecule are limited. Consequently, the interaction between adsorbate-adsorbent is weak, and adsorbed molecules are clustered around the material's surface.52,53 So, in the case of Co-CP, we observed a type III BET isotherm. The observed BET analysis reveals a type III isotherm pattern, indicating the characteristic behavior of the material. The BET surface area of Co-CP is found to be 169.613 m2 g−1 with a pore size of 3.829 nm. The observed value of the surface area is comparable to various MOF and coordination polymer-based literature with excellent supercapacitor applications.54–56 The observed surface area offers an increased number of sites for electrochemical reactions, enhancing the potential for efficient energy storage. Additionally, the presence of a suitable pore diameter facilitates improved ion transportation, further enhancing the performance of the system.
As Co-CP delivers the maximum performance in 1 M NaOH, therefore the electrolyte concentration of NaOH further varied to 1 M, 3 M, 5 M, and 7 M (Fig. S4, S7, S8, ESI† and Fig. 3). As the concentration of electrolytes increases, the specific capacitance also improves, and 7 M NaOH delivers the highest capacitance values. As the concentration varies from 1 M to 7 M NaOH, a high charge–discharge time is observed, but the electrochemical measurements are challenging beyond this concertation. At higher electrolytic concentrations, the ionic activity reduces, and the hydration of water diminishes, finally minimizes ionic mobility.57 As 7 M NaOH delivered the highest efficiency in the detailed investigation of electrochemical properties of Co-CP through the CV, GCD techniques are included in Fig. 3b and c. From the CV curve in Fig. 3b, with different scan rates from 5 mV s−1 to 100 mV s−1, the distinct peak of the Co2+/Co3+ redox couple is clearly visible. The observed CV curve shows the pseudocapacitive nature of Co-CP. With an increase of scan rates, the redox couple shifts towards more positive and negative regions due to the increase of ionic mobility at high scan rates. A similar trend is also observed when the concertation of electrolytes varied from 1 M to 7 M NaOH. The difference in the peak potential i.e., ΔEp = Epa − Epc are 120, 140, 150, 200, 230, 260, 280, and 300 mV for 5, 10, 20, 30, 40, 50, 70, and 100 mV s−1 (Fig. 3b). This behavior can illustrate better reaction kinetics at the electrode/electrolyte junction due to easy ionic and electronic transportation.45 The reversible OH− intercalation reaction occurring through the faradaic process can provide an explanation for the pseudocapacitive behavior observed in Co-CP.
Co(OH)2(C9H12.50N1.50O2.50)2 + OH˙ ⇌ CoO(OH)(C9H12.50N1.50O2.50)2 + H2O |
Furthermore, the GCD curve was recorded for Co-CP at various current densities from 1.5 A g−1 to 10 A g−1 (7 M NaOH) (Fig. 3c) and specific capacitance was obtained by using eqn (S1) (ESI†) from the GCD curve (Fig. 3d). The GCD pattern exhibits a typical nonlinear behavior representing the pseudocapacitive nature of the material, these characteristics resemble very well with the CV plot. The GCD pattern represents the symmetrical nature for charging and discharging, suggesting higher Coulombic efficiency. The pseudocapacitive material delivers two distinct regions for GCD: (i) the sudden fall in potential represents the internal resistance offered by the solutions, and (ii) the next decline of potential represents the capacitive nature of Co-CP.58Co-CP being the pseudocapacitive material, offered a specific capacitance of 1092 F g−1 at 1.5 A g−1 and this was maintained up to 633 F g−1 at 10 A g−1, suggesting high-rate performance at a high current density. The decrease in the efficiency at high current density indicates that at low current densities, a high-rate performance is experienced because of the ohmic drop, but at high current density, the reaction kinetics become sluggish. At low current density, the electrolytic cation gets enough time to interact with the active site of the material, thus offering a high capacitance value. The highest specific capacitance was obtained at different NaOH concentrations.
Moreover, to find all the resistance parameters of Co-CP in 1 M (NaOH, KOH, and LiOH), the Nyquist plot was recorded within the frequency range of 0.1 to 105 Hz (Fig. 4a). The Nyquist plot is comprised of two distinct regions: (i) semicircular loop at high-frequency region; the intercepts along the x-axis represents the solution resistance (Rs) representing all the resistance offered by the solutions, (ii) a vertical straight line in the low-frequency region, the slope of this line along the imaginary axis (−Z′′) represents the charge transfer resistance.59 Along with this two-resistance parameter, two capacitance parameters are also associated: (i) double layer capacitance (Cdl), representing of storage of charge at electrode/electrolyte junctions, and (ii) pseudo capacitance (Cp), signifying charge storage via a redox reaction. The Warburg impedance (W) in the low-frequency region was also found because of the linear diffusion. In redox reactions, high diffusion rate of electrolyte ions lead to the Warburg impedance.59 The circuit fitting parameters for the Nyquist plots are tabulated in Table S3 (ESI†). The results obtained from CV and GCD analysis resemble well with the Nyquist plots, as Co-CP experiences the highest solutions resistance in 1 M LiOH (11.2 Ω), followed by 1 M KOH (8.6 Ω), and 1 M NaOH (7.45 Ω). The combined influence of hydrated ionic radii and ionic sizes of Li+, K+, and Na+ illustrate well the solution resistance offered by the electrolyte ions. Another interesting parameter for the supercapacitor study is the cyclic stability of the materials. Here, we have studied the cyclic stability of Co-CP in 1 M (NaOH, KOH, and LiOH) up to 6500 cycles through continuous GCD cycles (Fig. 4b). The Co-CP shows excellent cyclic retention of 98%, 97%, and 96% respectively for NaOH, KOH, and LiOH after completion of 6500 GCD cycles. The high cyclic retention indicates the excellent stability of Co-CP throughout the continuous cycles of GCD. After the completion of a few hundred cycles, the retention value goes above 100%; this behavior is experienced due to the increase in the wettability of the material and activation of the material.45,60 During this stage, the electrolyte ions traverse the interior surface in order to access the pores of the active materials.61 As the cycling continues, the surface wettability of the material continues to improve. This enhanced wettability facilitates the ongoing diffusion of electrolyte ions throughout the electrode's microstructure, creating more efficient pathways to reach deeper particles within the material. This enhanced diffusion process contributes to an activation effect that persists throughout the electrochemical cycling. Consequently, this leads to an increase in capacitance as the cycling progresses.62,63 The minute loss of capacitance value may occur due to the continuous intercalation and deintercalation process of electrolyte ions through the electrode surface which causes mechanical stress and reduces the capacitance value. To support the loss of minimum capacitance value after the cyclic stability measurement, the electrochemical active surface area (EASA) is measured before and after the cyclic stability analysis. Furthermore, detailed information regarding EASA calculation is provided in the ESI,† Fig. S12. The observed EASA of Co-CP before the measurement is 0.7 cm2, which is reduced to 0.55 cm2 after 6500 GCD cycles. The decrease in the EASA value favored the slight degradation of the cyclic retention value. Moreover, the morphology of Co-CP was also investigated after the completion of the stability test. It is evident from Fig. S13 (ESI†) that the plate or sheet-like surface morphology is intact even after obtaining the cyclic stability measurements, which suggests the high stability of Co-CP.
S. no. | Material | Electrolyte | Current density (A g−1) | Specific capacitance (F g−1) | Cyclic stability | ASC device | Ref. | |
---|---|---|---|---|---|---|---|---|
Energy density (W h Kg−1) | Power density (W Kg−1) | |||||||
1 | Co,N-doped CP | 6 M KOH | 1 | 330 | 98%, 3000 cycles | 9.1 | 700 | 64 |
2 | {Ni(TPTA)(1-4-bib)}-MOF | 1 M KOH | 0.5 | 227.1 | 90.8%, 2000 cycles | 8.36 | 501.3 | 65 |
3 | Co-MOF derived Co3O4 | 2 M KOH | 1.3 | 226.1 | 89.9%, 20000 cycles | 0.092 mW h cm−3 | 1.34 mW h cm−3 | 66 |
4 | Ni-MOF/rGO | 6 M KOH | 1 | 954 | 80.25%, 4000 cycles | 17.13 | 750 | 67 |
5 | Ni-MOF nanosheets | 1 M KOH | 1 | 1024.4 | 49%, 5000 cycles | 13 | — | 68 |
6 | Co-NTA | 3 M KOH | 1.4 | 395 | 96.5%, 5000 cycles | 4.18 mW h cm−3 | 231.2 mW h cm−3 | 69 |
7 | Ni–Co-MOF | 3 M KOH | 1 | 827.9 | — | 29.1 | 800 | 70 |
8 | Co based film | 1 M LiOH | 0.6 | 206.7 | 98.5%, 1000 cycles | 7.18 | — | 71 |
9 | Zn(tbip) derived porous C | 6 M KOH | 0.5 | 369 | 96%, 2000 cycles | 12.5 | 7200 | 72 |
10 | Ni-CP | 7 M KOH | 3 | 802 | 95% after 5000 cycles | 15 | 1137 | 45 |
11 | Co-MOF | 3 M KOH | 0.5 | 414.5 | 113%, 20000 cycles | 12 | 258 | 47 |
12 | rGO/ZIF-8 | 1 M KOH | 1 | 336 | 96%, 10000 cycles | 11.7 | 500 | 73 |
13 | Ni-BPDC/GO-3 | 6 M KOH | 1 | 630 | 95.7%, 10000 cycles | 16.5 | 250 | 74 |
14 | Ni-MOF | 1 M KOH | 1 | 1024 | 49.1%, 5000 cycles | 14.6 | 400 | 68 |
15 | Ni–C/Ni-BDC | 6 M KOH | 2 | 672 | 57% | 17.8 | 350 | 75 |
16 | Co-CP | 7 M NaOH | 1.5 | 1092 | 98%, 6500 cycles | 17 | 1200 | This Work |
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2285196. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3ya00378g |
This journal is © The Royal Society of Chemistry 2023 |