Facile synthesis of a three-dimensional Ln-MOF@FCNT composite for the fabrication of a symmetric supercapacitor device with ultra-high energy density: overcoming the energy storage barrier

Abstract

In order to quench the thirst for efficient energy storage devices, a novel praseodymium-based state-of-the-art three-dimensional metal–organic framework (MOF), {[Pr(pdc)₂]Me₂NH₂}_n (YK-1), has been synthesized by using a simple solvothermal method employing a readily available ligand. YK-1 was characterised by single-crystal XRD and crystallographic analysis. The electrochemical measurements of YK-1 show that it exhibits a specific capacitance of 363.5 F g⁻¹ at a current density of 1.5 A g⁻¹ with 83.8% retention after 5000 cycles. In order to enhance its electrochemical performance for practical application, two composites of YK-1 with graphene oxide (GO) and functionalised multi-walled carbon nanotubes (FCNTs), namely YK-1@GO and YK-1@FCNT, were fabricated by employing a facile ultrasonication technique. The as-synthesized MOF and the composites were characterized by PXRD, FTIR, SEM, and TEM techniques. YK-1@GO and YK-1@FCNT offer enhanced specific capacitances of 488.2 F g⁻¹ and 730.2 F g⁻¹ at the same current density with 93.8% and 97.7% capacity retention after 5000 cycles, respectively (at 16 A g⁻¹). Fascinated by the outstanding results shown by YK-1@FCNT, a symmetric supercapacitor device (SSC) based on it was fabricated. The assembled SSC achieved a remarkable energy density (87.6 W h kg⁻¹) and power density (750.2 W kg⁻¹) at a current density of 1 A g⁻¹, along with very good cycling stability of 91.4% even after 5000 GCD cycles. The SSC device was able to power up several LED lights and even operated a DC brushless fan for a significant amount of time. To the best of our knowledge, the assembled SSC device exhibits the highest energy density among the MOF composite-based SSCs reported so far.

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1. Introduction

The ever-rocketing global energy demand has exhausted fossil fuels (non-renewable sources of energy) in every possible way, and the demand will continue to grow by many fold in the near future.¹ In addition, the widespread use of fossil fuels also causes carbon accretion in the natural cycle,² which has exacerbated environmental pollution. In order to balance the global environmental energy crisis, much effort has been put into developing renewable energy sources (which include solar energy, biofuels, tidal energy, wind energy, and geothermal energy).^3–7 Most renewable sources of energy, with the exception of biofuels, are supplied as electricity (electric power). There is hence an immense need for a highly efficient system for electrochemical storage, such as batteries, fuel cells, and electrochemical supercapacitors.⁸ For a while now, Li-ion batteries have been developed and employed for this objective. Modern modes of communication and transportation have been completely revolutionized by lithium-ion batteries (LIBs).^10,11 These batteries contribute to the improvement of technologies stretching from small electric devices to electric cars and renewable energy production and systems to store them.⁹ However, since batteries rely on redox reactions, the ions entering and leaving the lattices of electrode materials exhibit slow kinetics. As a result, the charging and discharging currents are restricted. In addition to this, during charging/discharging cycles, electrode materials undergo phase variations that pose a safety risk. For example, Li–S batteries experience Li dendrite formation at the Li/electrolyte interface during Li plating/stripping. Due to the enormous volume expansion that occurs when silicon anodes are lithiated, LIBs using silicon as the anode suffer from a severe disadvantage. These run the risk of causing a short circuit and an explosion. Batteries are therefore not appropriate for use in cases demanding long usage lives and large charge/discharge currents, particularly in power plants, because of their short cycles and comparatively limited rate capability.^12,13

Supercapacitors (SCs), novel and more efficient storage devices, have gained substantial attention recently in order to meet the cycle life and specific power requirements. SCs are a kind of energy storage device that can amalgamate the specific energy of a battery with the excellent specific power of an electrolytic capacitor, making them an excellent alternative to a battery.^14,15 Because of their quick storage capacity (i.e., very low discharge time: 1 to 10 s for SC vs. 10 to 60 min for lithium-ion batteries),¹⁶ efficient cycle life (more than 30 000 h for SCs whereas greater than 500 h for batteries), low maintenance cost and experiencing no memory effect, SCs, in the last decade, have garnered a lot of interest.¹⁷ In contrast to batteries, which can only be used between −20 and 60 °C,^16,18 SCs can work at temperatures ranging from −40 to 100 °C.¹⁹ Moreover, SCs are compact as compared to batteries and thus nowadays are used in compact devices like laptops, smart watches, camera flashes, GPS units, etc.

SCs can be categorised into two different types based on their mechanism for storing charge: pseudo-capacitors (PCs) and electric double-layer capacitors (EDLCs).²⁰ PCs use electrochemical oxidation–reduction reactions^21,22 that take place on the active material's surface^23,24 to store energy, unlike EDLCs, which store charge on the active material's inner surface based on carbon.²⁵ The only limitation of SCs compared to an electrochemical battery lies in its low energy density, which is higher in the latter. This is the main reason that supercapacitors still have not replaced batteries. This limitation can be overcome by designing suitable electrodes.

The performance of supercapacitors is considerably improved by the materials used for their electrodes. Metal oxides/hydroxides, carbonaceous materials and conductive polymers are the most common types of electrode materials,^26,27 but they have some drawbacks, including mass production, low specific capacitance and consequently low energy density in the case of carbonaceous materials, poor cycling stability in the case of metal oxides/hydroxides, and structural instability along with rapid capacitance deterioration in the case of conductive polymers.^28,29 These drawbacks limit the practical applications of these materials. Thus, the development of electrode materials with high power density along with high energy density, low cost and durability remains an immense challenge and thus, as a result, has drawn significant attention for the advancement of high-performance SCs that have the competence to boost diffusion kinetics and offer a high accessible surface area.

Metal–organic frameworks (MOFs) and the composites based on them have demonstrated outstanding performance as electrode materials for energy storage applications in order to get around these limitations.^24,30,31 Owing to their immense porosity, large surface area, robust structure, good thermal stability, versatile functionalities, and redox reaction-based assimilated metal centres, MOFs have experienced unparalleled growth in a wide range of applications, including storage and separation of gases, sensors, catalysis,^32–39 and electrochemical energy storage/generation.^40,41

Due to these characteristics, MOFs have shown favourable outcomes when employed as an electrode base material. MOFs offer more active sites and speed up the transport of ions among electrodes and electrolytes when employed directly as electrode materials that rely on incorporating pseudocapacitive redox centres.⁴² For SC applications, a number of MOF and MOF-derived materials, including Ni,⁴³ Co,⁴⁴ Cd,⁴⁵ Zn,⁴⁶ Mn,⁴⁷ and bimetallic compounds like Zn/Co,⁴⁸etc., have been explored to date. In spite of this, the inadequate electrical conductivity and low stability in the course of the charge/discharge process due to the poor electrolyte flexibility of MOFs and MOF-derived materials have been regarded as major limitations in SCs.⁴⁹ Therefore, using bare MOFs directly as an electrode material does not serve the purpose of an ideal supercapacitor electrode, which should not only show a high power density, but also exhibit a high energy density, so the supercapacitor's limitation of low energy density as compared to batteries can be overcome. In this context, the fabrication of MOFs with CNTs, graphene, carbon black, and other materials has proved to be an excellent approach for enhancing energy storage efficacy.

Wen et al. fabricated Ni-MOF/CNT composites that, due to the interaction between Ni-MOF's specific structure and CNT's high conductivity, demonstrated spectacular supercapacitor performance (energy density of 36.6 W h kg⁻¹).⁵⁰

Shagufi et al. synthesized a MOF based on Cu and fabricated its composite with CNTs. The Cu MOF shows a very low specific capacitance, whereas its composite with CNTs exhibits a much greater specific capacitance of 380 F g⁻¹ at a current density of 1.6 A g⁻¹. The result clearly demonstrates that the fabrication of Cu-MOF with CNTs increases its electrochemical energy storage efficiency as the resulting material possesses the properties of both the parent materials.⁵¹ Saraf and co-workers recently examined the impact of rGO on a Cu-MOF and depicted that the specific capacitance of Cu-MOF was significantly improved by the accumulation of rGO.⁵² Furthermore, Anoop et al. also synthesised a Cu-MOF and fabricated it with rGO. The resultant composite shows a specific capacitance of 462 F g⁻¹ at a current density of 0.8 A g⁻¹, much greater than that of rGO, which is 256 F g⁻¹, and Cu-MOF which is 4.1 F g⁻¹ at the same current density, suggesting that the composite material offers much higher energy density than Cu-MOF alone.⁵³ To put it briefly, the addition of a carbon-based material to a MOF improves the pseudocapacitors’ stability while significantly improving the device's electrochemical performance.

In this perspective, MOFs encompassing electroactive lanthanide ions are intriguing due to their diverse coordination chemistry and excellent framework stability.⁵⁴ Compared to transition metal-based MOFs and their composites, lanthanide-based MOFs and their composites as supercapacitor electrode materials have barely been explored.

Ghosh et al. recently synthesized three MOFs based on lanthanides (MOF-Nd, MOF-Pr, MOF-Ce) and used them as electrodes for supercapacitor applications.⁵⁵ MOF-Nd, MOF-Pr, and MOF-Ce exhibited the specific capacitance of 360, 399 and 572 F g⁻¹, respectively, at a current density of 1 A g⁻¹. Wang and co-workers reported various cerium(III) MOF-based composites (Ce–MOF/CNT and Ce–MOF/GO) and examined their electrochemical behaviour.⁵⁶

In this paper, we have synthesized a novel lanthanide-based MOF YK-1 by employing a solvothermal method using 2,5 pdc (H₂pdc) as a ligand. The large ionic radius of praseodymium forms large pores within the MOF structure which provide more surface area for electrolyte ions to interact with, leading to higher capacitance. In addition, Pr has multiple oxidation states (+3, +4), allowing multiple electron transfers and potentially higher capacitance. Moreover, Pr is also cost-effective. Thus, Pr being used as a metal centre offers a good balance between performance and cost.⁵⁵ Furthermore, we have synthesized two composites based on YK-1viz, YK-1@GO and YK-1@FCNT by employing a simple ultrasonication technique. YK-1, YK-1@GO and YK-1@FCNT were all fully characterized by employing different techniques.

The electrochemical properties of YK-1, YK-1@GO, and YK-1@FCNT are assessed by employing cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) studies, and among them, YK-1@FCNT shows the maximum specific capacitance.

The specific capacitance of YK-1, YK-1@GO, and YK-1@FCNT at a current density of 1.5 A g⁻¹ was found to be 363.5, 488.2, and 730.2 F g⁻¹. The YK-1@ GO composite offers higher specific capacitance than YK-1. This is because oxygen functional groups in GO contribute to pseudocapacitance.⁵⁶ Furthermore, GO also provides significant support to avoid MOF agglomeration.⁵³ Moreover, YK-1@FCNT shows the highest supercapacitance value because FCNT surfaces facilitate the even distribution of YK-1, which, during the course of the charge–discharge process, shortens the electron transport pathway. Additionally, the FCNTs’ strong electronic conductivity increased the transportation of electrons from active materials to the current collector. Moreover, because of the carboxyl functional group on the FCNT surface, the wettability of the electrode material improves drastically, providing ions in the electrolyte a more accessible electrode surface and thus enhancing the ionic conductivity at electrode–electrolyte interfaces due to which the composite of YK-1 with FCNTs shows the best electrochemical performance. Furthermore, at a current density of 16 A g⁻¹, YK-1, YK-1@GO and YK-1@FCNT showed 83.8%, 93.8%, and 97.7% retention of the specific capacity after 5000 charge–discharge cycles, respectively.

Due to the highest specific capacitance and high retention capacity of YK-1@FCNT, a device based on it was fabricated, and its performance for different LED lights was evaluated. The supercapacitor device assembled using YK-1@FCNT shows a high energy density of 87.6 W h kg⁻¹ at a power density of 750.2 W kg⁻¹.

2. Experimental section

2.1. Materials

The reagents and materials utilised in this work were all of analytical grade and were used directly out of the package without any purification. Purchases were made from Sigma-Aldrich Chemical Co., India, for pyridine-2,5-dicarboxylic acid (H₂pdc), Pr(NO₃)₃6H₂O, graphite powder, sulfuric acid, potassium permanganate, hydrogen peroxide, multi-walled carbon nanotubes (MWCNTs, 92%), N,N-dimethylformamide (DMF, ≥99.9%), K₂Cr₂O₇, nitric acid (HNO₃) and ethanol. Deionized water (18.2 MF cm) was employed through the entire course of the synthesis.

2.2 Physical methods

FTIR spectral studies were performed on a PerkinElmer model Spectrum GX spectrophotometer under the range of 4000–400 cm⁻¹. The melting point was ascertained by means of the open capillary method. Additionally, the elemental analysis was acquired from the CDRI's Micro-Analytical Laboratory in Lucknow, India. The PXRD patterns were determined using a “Miniflexll” X-ray diffractometer with Cu-K radiation.^57,58 With a 20 °C min⁻¹ heating rate and a temperature range of 40 °C to 800 °C, the thermal behaviour of the MOF was examined using a Shimadzu TGA-50H instrument. SEM was performed on a JEOL JSM-6510LV, Japan, and TEM on a TECHNAI 200 kV (Fei, Electron Optics) equipped with a 35 mm photography system and digital imaging system, to study the morphologies of the synthesized materials.

2.3 Single crystal X-ray structure

Crystal data of YK-1 were collected by employing a Bruker SMART APEX CCD diffractometer operating at 100 K using graphite monochrome and Mo-Kα radiation (λ = 0.71073 Å).^59,60 SAINT software was employed for integration and reduction of data.⁶¹YK-1's space group was investigated using XPREP,⁶² and empirical absorption correction was performed using SADABS.⁶³ With the help of the OLEX-2 software application,⁶⁴ the structure was refined by utilising least-squares techniques on F². Moreover, anisotropic displacement parameters were used to refine the non-hydrogen atoms. The CCDC reference number for YK-1 is 2303152.†Table 1 provides an overview of the crystallographic data and the structure refinement of YK-1.

Table 1 Crystal data and refinement parameters for YK-1

Compound	YK-1
Empirical formula	C16H13N3O8Pr
Formula weight	516.20
Temperature/K	100(2)
Crystal system	Orthorhombic
Crystal colour	Light green
Space group	Fddd
a/Å	17.865(4)
b/Å	18.094(4)
c/Å	27.261(8)
α/°	90
β/°	90
γ/°	90
Volume/Å^3	8812(4)
Z	16
ρ calc/g cm^−3	1.5562
μ/mm^−1	2.253
F(000)	4048.7
Crystal size/mm^3	0.41 × 0.27 × 0.14
Radiation	Mo Kα (λ = 0.71073)
2θ range for data collection/°	5.52–51
Index ranges	−23 ≤ h ≤ 23, −24 ≤ k ≤ 24, −36 ≤ l ≤ 36
No. of reflections collected	44 153
No. of independent reflections	2062 [Rint = 0.1427, Rsigma = 0.0486]
Data/restraints/parameters	2062/0/118
Goodness-of-fit on F^2	1.131
Final R indexes [I ≥ 2σ (I)]	R 1 = 0.0322, wR2 = 0.0821
Final R indexes [all data]	R 1 = 0.0457, wR2 = 0.1000
Largest diff. peak/hole/e Å^−3	0.93/−1.43

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2.4. Synthesis

2.4.1. Synthesis of {[Pr(pdc)₂]Me₂NH₂}_n. YK-1 was synthesized by the reaction of H₂pdc (0.2 mmol, 0.033 g), Pr(NO₃)₃·6H₂O (0.1 mmol, 0.043 g), DMF (5 mL), and triethylamine (2–3 drops) in a Teflon-lined autoclave and was subject to heating at 120 °C for 72 h, as illustrated in Scheme 1.

Scheme 1 Synthesis of YK-1.

The solution thus obtained was subsequently cooled at a rate of 20 °C h⁻¹ to room temperature. After complete cooling, light green crystals of YK-1 appropriate for X-ray analysis were obtained. Yield: 51%, m.p. 310 °C. Elemental analysis (%): C = 37.35; H = 2.08; N = 8.33; calc. for C₁₆H₁₃N₃O₈Pr: C = 37.22; H = 2.52; N = 8.14. IR (KBr cm⁻¹): 3418 (b), 3010 (w), 2935 (w), 2778 (m), 2486 (m), 1608 (s), 1340 (m), 506 (s), 416 (w).

2.4.2. Synthesis of GO. GO was synthesized by employing a modified Hummers’ method using graphite powder, potassium permanganate, sulfuric acid, and hydrogen peroxide.⁶⁵ NaNO₃ (2 g) and graphite powder (4 g) were thoroughly mixed and slowly added to conc. H₂SO₄ (170 mL) over an ice bath. Accordingly, KMnO₄ (12 g) was added dropwise to the as-synthesized mixture while being stirred magnetically and maintaining the temperature below 20 °C. A brownish-coloured viscous liquid was produced upon removal of the ice bath, and the mixture was continually agitated for the following 18 hours. Furthermore, DI water (220 mL) was used to dilute this viscous mixture while keeping the temperature under 50 °C. Following dilution, the obtained mixture was treated with 30% H₂O₂ (10 mL), which caused the mixture to turn brilliant yellow along with the formation of bubbles, signifying reaction completion. Furthermore, it was subjected to stirring for the following 4 hours. To eliminate any impurity ions, the as-synthesized mixture was then thoroughly filtered and washed with a 10% HCl solution, DI water, and ethanol. The product thus obtained, i.e., graphite oxide, was dried.⁶⁶ Later, to transform graphite oxide into graphene oxide, 300 mg of graphite oxide was first dissolved in water and then subjected to sonication for an hour.

2.4.3. Modification of CNTs. By subjecting pristine CNTs to acid treatment, they were functionalized with –COOH groups. In short, a particular quantity of MWCNT was added to 4 M HNO₃ and then sonicated for around 40 min by employing an ultrasonication bath. The obtained suspension was then stirred vigorously with the help of a magnetic stirrer at 50 °C to produce a black solid-like material that was then rinsed with double distilled water and allowed to dry for 24 hours in an oven at 80 °C.^67,68

2.4.4. Synthesis of YK-1@GO and YK-1@FCNT. YK-1@GO was synthesized ex situ using the ultrasonication method. YK-1 and GO in equal quantities were first ground by using a mortar and pestle to get a fine powder and then dispersed in a 2 : 1 (v/v) solution of ethanol and water. The mixture thus obtained was first stirred magnetically for around 15 minutes to produce a homogeneous solution and then vigorously sonicated for about an hour. The obtained solution was then poured into a glass tube and sealed. The glass tube was heated at 333 K for 24 h in an oven. Following heating, the glass tube was allowed to cool to ambient temperature. The precipitate, so obtained, was washed three times with ethanol and three times with distilled water and then again dried in an oven at around 343 K for 14 h to produce a fine, dark black powder of YK-1@GO (Scheme 2).^52,53

YK-1@FCNT was also synthesized by using the above procedure, except that FCNT was used instead of GO (Scheme 2).⁵¹

Scheme 2 Synthesis of YK-1@GO and YK-1@FCNT.

2.5. Electrochemical measurements

The electrochemical performance of the as-synthesized materials (YK-1, YK-1@GO and YK-1@FCNT) as electrode materials for electrochemical supercapacitors was assessed using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) studies. The experiments were conducted at room temperature in a 3 M KOH electrolyte solution using an electrochemical workstation (Metrohm, Multi Autolab M204, serial no. MAC90273) using NOVA software (version 2.1.5). In the three-electrode system setup, platinum wire and Ag/AgCl were used as the counter and reference electrodes, respectively. To prepare the working electrode, a homogeneous slurry was deposited onto 1 cm² area of a 1 cm × 3 cm piece of pre-cleaned Ni-foam. The active material was approximately 2 mg on 1 cm² area of Ni-foam in all three electrodes. The slurry consisted of the as-synthesized materials (80%), poly(vinylidene) fluoride (PVDF) (10%), and acetylene black (10%), in N-methyl-2-pyrrolidone (NMP). After deposition, the electrodes were dried for 12 h at 70 °C and then pressed at a pressure of 10 MPa. The electrodes were immersed in 3 M KOH electrolyte for 12 h before being connected to the three-electrode system.

3. Results and discussion

3.1. Crystal description and topology of {[Pr(pdc)₂]Me₂NH₂}_n (YK-1)

The result of single-crystal X-ray diffraction shows that YK-1 crystallises in an orthorhombic system with the Fddd space group. Table 1 summarises crystal data with refinement parameters, and selected bond lengths and bond angles are provided in Table S1.† The monomeric unit of YK-1 contains one Pr(III) ion, two pdc²⁻ ligands and one free dimethyl ammonium cation. Thus the complex's chemical formula was written as {[Pr(pdc)₂]Me₂NH₂}_n. As illustrated in Fig. 1a, the Pr(III) ion was eight coordinated and bound to eight oxygen and two nitrogen atoms. The four oxygen atoms (O1 and O2) were provided by four different pdc²⁻ ligands, whereas the two oxygen atoms (O4) and two nitrogen atoms (N1) were provided by two pdc²⁻ ligands. Thus, the Pr(III) exhibits distorted square antiprismatic geometry with the surrounding atoms (Fig. 1b). The important bond angles are obtained as O1–Pr1–N1 = 83°, O2–Pr1–N1 = 72.24°, O4–Pr1–N1 = 62.51°, O4′–Pr1′–N1′ = 54.72°, O1–Pr1–O2 = 78.88°, O1′–Pr1′–O2′ = 73.03, O4–Pr1–O4 = 93.24°. The bond lengths between Pr1–O1 (2.382 Å), Pr1–O2 (2.483 Å), Pr1–O4 (2.432 Å) and Pr1–N1 (2.649 Å) are all within the normal range for Pr(III) ion. In general, the pdc²⁻ ligand exhibits various coordination modes (Fig. S1†), whereas in YK-1 it shows only one coordination mode in which the oxygen atoms of one carboxylate group bridges in μ₂–η¹:η¹ mode, whereas the N-atom of pyridine and the O-atom of the second carboxylate group adopt the μ₁–η¹:η¹ chelating mode⁶⁹ (Fig. S2†). Consequently, the entire pdc²⁻ ligand serves as a μ₃-bridge to bind three distinct Pr(III) atoms.

Fig. 1 (a) Monomeric unit, (b) metal polyhedrons, (c) secondary building unit (SBU), (d) two-dimensional sheet, and (e) three-dimensional structure of YK-1.

The secondary building units (SBUs) are shown in Fig. 1c. Additionally an infinitely extended two-dimensional sheet under the connection of the organic linker was formed by SBUs (Fig. 1d). As shown in Fig. 1e, the adjacent layers were supported by pdc²⁻ to create a three-dimensional framework. The ball and stick model of YK-1 along the a, b and c axes is shown in Fig. S3–S5.† The spacefill model and ORTEP view of YK-1 are illustrated in Fig. S6 and S7,† respectively.

The topological method was employed in order to gain a better insight into the nature of YK-1's structure⁷⁰ that shows scu-3,6-I41/amd and dia; 4/6/c1; sqc6 networks along with various SBUs as shown in Fig. 2.

Fig. 2 (a) Standard representation of coordination compounds and valence-bonded MOFs having the topology scu-3,6-I41/amd with SBUs: C₇H₃NO₄, Pr, (b) cluster all nodes representation of valence-bonded compounds (RINGS > 6) having topology scu-3,6-I41/amd with SBUs: C₁₂H₆N₂O₄Pr, C, and (c) cluster all nodes representation of valence-bonded compounds (RINGS > 8) having the topology dia; 4/6/c1; sqc6 with SBUs: C₂₈H₁₂N₄O₁₆Pr₂.

3.2. FTIR, PXRD, TGA, and BET analyses

FTIR spectra of YK-1, the YK-1@GO composite, and the YK-1@FCNT composite were recorded in order to investigate the functional groups present in each sample, as demonstrated in Fig. 3a. In the case of YK-1, at 3418 cm⁻¹ a broad band is observed, which can be attributed to the ν(C–H) pyridyl ring stretching vibration. The weak band observed at 2935 cm⁻¹ and the broad band at 2778 cm⁻¹ correspond to ν(C–H) of Me₂NH₂⁺ and symmetric ν(C–H) stretching. The non-appearance of the characteristic band at around 1700 cm⁻¹, which is attributed to –COOH groups, implies that all carboxyl groups in YK-1 undergo complete deprotonation when they react with metal ions.⁷¹ For YK-1, the two strong bands observed at 1608 cm⁻¹ and 1340 cm⁻¹ correspond to the stretching vibrations of asymmetric and symmetric carboxyl groups, respectively.⁷² The absorption band observed at 765 cm⁻¹ is due to the δ(O–C–O) vibrations of pdc²⁻.⁷³ The absorption band at 506 cm⁻¹ is due to the ν(Pr–N) vibration, and the weak band observed at 416 cm⁻¹ corresponds to ν(Pr–O) vibration.⁷⁴ The structural characteristics of YK-1, as determined by its infrared spectra, are in accordance with the structural characteristics determined by SCXRD.

Fig. 3 (a) The FT-IR and (b) PXRD spectra of GO, YK-1, YK-1@GO, FCNT, and YK-1@FCNT.

The FTIR spectrum of GO featured absorption peaks at 3340 cm⁻¹ corresponding to –OH group stretching vibrations. Furthermore, peaks at 1730 cm⁻¹ and 1619 cm⁻¹ correspond to the C=O stretching vibration of the carboxyl group and C=C skeletal vibrations of the graphene planes. The peak observed at 1080 cm⁻¹ is due to the C–O–C stretching vibration. The FTIR spectrum of the YK-1@GO composite shows the characteristic peaks of both YK-1 and GO, e.g. at 3400 cm⁻¹ (–OH group stretching vibrations), 1080 cm⁻¹ (C–O–C stretching vibrations), 765 cm⁻¹ (O–C–O vibrations of the pdc²⁻), and 506 cm⁻¹ (Pr–N vibrations), confirming that GO has successfully grown on the surface of YK-1.

The typical peak of FCNTs observed at around 3480 cm⁻¹ corresponds to the ν(O–H) peak of the carboxylic acid groups. Moreover, the moderate absorption bands exhibited by FCNTs at 1300 cm⁻¹ and 1550 cm⁻¹, correspondingly, are due to the symmetric and asymmetric stretching modes of the carboxylate (–COO⁻) group. The peaks at around 1046 cm⁻¹ and 3010 cm⁻¹ were attributed to the carboxylic acid group's C–O symmetric stretching vibration and C–H bending stretching.

After the FCNTs had been grown on the surface of YK-1, the surface chemistry of the YK-1@FCNT composite was discovered to comprise the characteristic peaks from both YK-1 and FCNTs. For example, YK-1@FCNT exhibits a peak at 1046 cm⁻¹, which was due to the C–O symmetric stretching vibration of the carboxylic acid group of FCNTs and at 506 cm⁻¹, which corresponds to the ν(Pr–N) vibration of YK-1. However, the change in peak intensities can be attributed to changes in the environment due to composite formation. In contrast, the major peak of FCNTs at ∼3480 cm⁻¹, which corresponds to the ν(O–H) peak of the carboxylic acid groups gets weakened, showing that it participates in chemical bonding between the functionalities of parent materials. These findings are in good accord with the XRD findings, serving as a sign of efficient adornment of the Co-MOF over FCNT platforms to build a hybrid composite.⁷⁵

In order to assess the bulk purity, the PXRD patterns of YK-1 were recorded under ambient conditions. The phase purity of YK-1 was confirmed as the recorded PXRD data were identical to the simulated data (Fig. S8†). Freshly synthesised GO's PXRD pattern is shown in Fig. 3b. The prepared GO sheet displays a very strong peak at 2θ = 10.2°, which is well in line with the literature.^76,77 The diffraction patterns of the YK-1@GO composite are similar to that observed for YK-1, which indicates that the structure of YK-1 is preserved. The peak at 10.2° is slightly reduced for the composite, suggesting that the GO component is affected by YK-1 in the composite.

XRD analyses were carried out to analyse the crystalline nature of the prepared YK-1@FCNT, and the findings are shown in Fig. 3b. Remarkably, the FCNTs showed two peaks at 26.10° and 44°, which correspond to the graphite reflection planes (002) and (100) (Fig. 3b).⁷⁸ The PXRD analysis of the YK-1@FCNT composite exhibited high crystallinity, as all of the peaks’ intensities and locations correspond to those of the characteristic patterns of the parent material. The diffraction signals of all the parent materials, i.e., YK-1 and FCNT, can be easily indexed, implying that the YK-1@FCNT was synthesized successfully.^79,80 The PXRD results of YK-1@GO and YK-1@FCNT exhibit some left and right shifts, which indicates that both the GO and FCNTs are affected by YK-1 in the composite, thus confirming the formation of the composite material. It is a common phenomenon that the peaks get shifted in the composite material due to the interaction of individual materials, and hence, we observed the slight shifting of peaks in YK-1@GO and YK-1@FCNT.⁸¹

Moreover, TGA was performed at 30–800 °C in a nitrogen environment to evaluate the thermal stability of YK-1. The weight loss tends to occur in two steps in YK-1, as shown in Fig. S9.† The first decomposition was observed in the range of 140–360 °C, and the second one was observed at 360–556 °C due to the loss of the dimethyl ammonium cation and pdc²⁻ ligand, respectively.⁶⁹ Thus, YK-1 was stable up to 360 °C and then began to degrade at elevated temperatures producing metal oxides above 650 °C.¹⁰¹

Brunauer–Emmett–Teller (BET) adsorption/desorption measurements were performed to examine the specific surface area of the samples. The results display type-IV isotherm profiles for YK-1, YK-1@GO, and YK-1@FCNT. The surface areas, as measured by the BET method, are found to be 120.3, 182, and 380 m² g⁻¹ for YK-1, YK-1@GO, and YK-1@FCNT, respectively, whereas their pore width obtained by the Barrett–Joyner–Halenda (BJH) method lies in the range of 3–4 nm, which confirms the presence of a mesoporous structure. Moreover, the pore volumes of YK-1, YK-1@GO, and YK-1@FCNT are 0.590, 0.582, and 0.546 ccg⁻¹, respectively. Fig. S10† shows the BET and BJH plots of YK-1, YK-1@GO, and YK-1@FCNT.

3.3. Morphological characterization by SEM and TEM

The morphological characteristics of YK-1, YK-1@GO, and YK-1@FCNT have been examined by SEM and TEM, and are shown in Fig. 4a–j. The SEM micrograph of YK-1 (Fig. 4a and b) shows distorted octahedral-shaped particles. The SEM images of GO and FCNT are depicted in Fig. S11.† Furthermore, in the case of the YK-1@GO composite, the SEM images (Fig. 4c and d) show the presence of both YK-1 distorted octahedral crystals as well as the wavy GO sheets, demonstrating that the ultrasonication-aided synthesis can hasten the dispersion of the involved constituents in the composites formed.⁸¹ Additionally, the TEM images of YK-1@GO substantiate its formation because both the YK-1 particles and GO sheets are present, which are in good accordance with the SEM results. Moreover, the incorporation of FCNTs with YK-1 underwent some notable morphological alterations in the YK-1@FCNT composite, as illustrated in its SEM images (Fig. 4e and f), which explicitly demonstrate the immobilization of FCNTs on the surface of YK-1. The morphological characteristics of the YK-1@FCNT composite revealed distorted octahedral-shaped particles with FCNTs embellished across its surface in a pattern that resembles grape bunches. The substantial variation results from the addition of oxygen-containing functional groups to FCNTs, which enhances their interaction with the YK-1.⁶⁸ It is clearly evident from the TEM images of the YK-1@FCNT composite that YK-1 is well-dispersed across the whole surface of FCNTs (Fig. 4i and j). It is significant to mention that FCNTs (used in composite preparation), even after being subjected to acid treatment, preserved the excellent tube structure of pristine CNTs.

Fig. 4 SEM images of (a and b) YK-1, (c and d) YK-1@GO, (e and f) YK-1@FCNT; TEM images of (g and h) YK-1@GO, (i and j) YK-1@FCNT.

3.4. Electrochemical characterization

The electrochemical characterization was carried out by CV in an electrolyte made with 3 M KOH. CV analysis was performed at different scan rates, ranging from 10 mV s⁻¹ to 100 mV s⁻¹, with a potential window defined between −0.9 V and 0.2 V for the investigation of the redox behaviour and capacitive characteristics of the electrode materials. GCD analysis, on the other hand, was conducted at various current densities, which ranged from 1.5 A g⁻¹ to 15 A g⁻¹, within the same potential window (−0.9 V to 0.2 V) for investigating the charge storage capacity and rate capability of the electrode materials.

In a three-electrode system, the specific capacitance from CV and GCD was calculated using eqn (1) and (2), respectively. On the other hand, the specific capacitance of the fabricated device from CV and GCD was calculated using eqn (3) and (4), respectively. The energy density and power density were calculated from the CV and GCD curves according to eqn (5) and (6), respectively.^82–85

where C_s (F g⁻¹) denotes the specific capacitance, I (A) denotes the charge/discharge current, t_D (s) is the discharging time, m (g) denotes the mass of active materials, M (g) is the total mass of active materials on both the electrodes of the SSC device, Δv (V) represents a potential window, ν is the scan rate, E (W h kg⁻¹) is the energy density and P (W kg⁻¹) denotes the power density, respectively.

The CV curves of YK-1, YK-1@GO, and YK-1@FCNT at various scan rates are displayed in Fig. 5(a, b and c). The observed deviation from a rectangular shape in the CV curves is commonly associated with the presence of pseudo-capacitive behaviour in the electrode materials.⁸⁶ However, when the scan rate is increased in the CV experiments, the area enclosed by the CV curve also increases. This indicates that more charge is being stored or exchanged during each cycle, leading to a larger electrochemical response. However, despite the increased area, the overall shape of the CV curve remains relatively unchanged. The fact that with increasing scan rate (as indicated in Fig. 5d), the capacitance decreases slowly suggests that the electrodes exhibit good rate capability. In other words, they can undergo rapid charging and discharging processes without significantly losing their electrochemical performance. This behaviour is desirable for many energy storage applications, as it allows for efficient charge/discharge cycles even under high scan rates or fast charging conditions.

Fig. 5 CV curves at different scan rates of (a) YK-1, (b) YK-1@GO, (c) YK-1@FCNT and (d) corresponding specific capacitance values of YK-1, YK-1@GO and YK-1@FCNT at particular scan rates.

The specific capacitance (calculated by eqn (1)) of all three electrodes at various scan rates is given in Table 2. The specific capacitance of YK-1 is found to be 318 F g⁻¹ at 10 mV s⁻¹, which is noticeably high for a large potential window (ΔV = 1.1 V). Moreover, YK-1@GO and YK-1@FCNT show an extremely high specific capacitance of 524.8 F g⁻¹ and 666.3 F g⁻¹ at 10 mV s⁻¹, respectively. The decrease in specific capacitance with increasing scan rates, as illustrated in Fig. 5d, can be attributed to two main conditions related to the resistance and kinetic energy of ions. At lower scan rates, the ions have more time to interact with the electrode surface. This extended interaction time allows for better access of ions to the electrode surface, facilitating their adsorption or intercalation into the electrode material. As a result, more charge can be stored, leading to a higher specific capacitance.⁸³

Table 2 Specific capacitance of YK-1, YK-1@GO and YK-1@FCNT against different scan rates

S. no.	Scan rate (mV s^−1)	Specific capacitance (F g^−1) (YK-1)	Specific capacitance (Fg^−1) (YK-1@GO)	Specific capacitance (F g^−1) (YK-1@FCNT)
1.	10	318	524.8	666.3
2.	20	261.8	403.8	617.8
3.	30	231.7	362.4	582.8
4.	40	210.2	329.1	550.9
5.	50	190.1	284.4	523.4
6.	75	164.3	255.3	464.7
7.	100	139.9	222.2	417.1

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However, as the scan rate increases, the time available for ions to interact with the electrode surface is reduced. The kinetic energy of the ions is increased due to the faster scan rate, resulting in shorter interaction times with the electrode surface. This limited interaction time hinders the effective adsorption or intercalation of ions, leading to reduced charge storage and, consequently, a lower specific capacitance.

Moreover, the increased kinetic energy of ions at higher scan rates also contributes to higher resistance. The faster movement of ions in the electrolyte solution encounters more resistance, impeding their efficient transport to the electrode surface. This increased resistance further reduces the effective charge storage capacity and, consequently, the specific capacitance. Therefore, the combined effects of reduced interaction time and increased resistance at higher GCD curves of YK-1, YK-1@GO and YK-1@FCNT at different current densities of 1.5, 3, 5, 10 and 15 A g⁻¹ are displayed in Fig. 6(a, b and c). It can be observed that GCD curves are not standard triangles and exhibit plateaus, which confirms the presence of pseudocapacitive behaviour in the electrodes. The specific capacitance (calculated using eqn (2)) of YK-1, YK-1@GO and YK-1@FCNT is given in Table 3. The maximum specific capacitance of YK-1, YK-1@GO and YK-1@FCNT was 363.5 F g⁻¹, 488.2 F g⁻¹ and 730.2 F g⁻¹ at the current density of 1.5 A g⁻¹. The specific capacitance of YK-1 decreases to 58.6 F g⁻¹ at 15 A g⁻¹, while YK-1@GO and YK-1@FCNT show fairly good capacitance of 130.9 F g⁻¹ and 238.6 F g⁻¹. The maximum energy density of YK-1, YK-1@GO and YK-1@FCNT was found to be 61.1 W h kg⁻¹, 82.1 W h kg⁻¹ and 122.7 W h kg⁻¹ at 1.5 A g⁻¹ respectively, whereas the power densities of YK-1, YK-1@GO and YK-1@FCNT were found to be 825.1 W kg⁻¹, 825.6 W kg⁻¹ and 824.9 W kg⁻¹ at 1.5 A g⁻¹, respectively. The maximum power density of YK-1, YK-1@GO, and YK-1@FCNT was found to be 8204.7 W kg⁻¹, 8250 W kg⁻¹, and 8249.1 W kg⁻¹ at 15 A g⁻¹, respectively. The results show that the specific capacitance decreases as the current density increases from 1.5 A g⁻¹ to 15 A g⁻¹ as indicated by Fig. 6d.

Fig. 6 GCD curves at different current densities of (a) YK-1, (b) YK-1@GO, (c) YK-1@FCNT and (d) corresponding specific capacitance values of YK-1, YK-1@GO and YK-1@FCNT at particular current densities.

Table 3 Specific capacitance of YK-1, YK-1@GO and YK-1@FCNT against different current densities

S. no.	Current density (A g^−1)	Specific capacitance (F g^−1) (YK-1)	Specific capacitance (F g^−1) (YK-1@GO)	Specific capacitance (F g^−1) (YK-1@FCNT)
1.	1.5	363.5	488.2	730.2
2.	3	224.7	319.4	525.5
3.	5	176.4	217.7	420.5
4.	10	96.4	164.5	311.8
5.	15	58.6	130.9	238.6

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Typically, at high current densities, ions primarily interact with the outer surface of the electrode material, whereas at lower densities, they can access both sides of pores in the structure. This difference affects the specific capacitance. In materials with small pores, ion access to inner surfaces is limited, particularly at high currents. This restriction hampers efficient ion adsorption or intercalation, reducing charge storage capacity and specific capacitance. Factors limiting access include pore size constraints and increased electrostatic repulsion within confined spaces. Specific capacitance is influenced by surface area accessibility; materials with larger pores or well-connected networks offer better access, enhancing the capacitance.^87,88 Moreover, as discussed in the BET section, the specific surface area of YK-1@FCNT and YK-1@GO was found to be 3.16 and 1.51 times greater than that of YK-1 along with a pore size in the range of 3–4 nm, confirming a mesoporous structure. Thus, the highest specific capacitance values of YK-1@FCNT can be attributed to increased specific surface area which provides a larger number of accessible pores.

Furthermore, the Nyquist plot of YK-1, YK-1@GO and YK-1@FCNT electrodes is given in Fig. 7 in order to evaluate the electrochemical performance. Typically, when examining the Nyquist plot of a supercapacitor electrode, three distinct regions become apparent based on the frequency range. The value where the plot intercepts in the high-frequency domain is directly linked to the equivalent series resistance (R_s), which is a result of the combined resistance from the electrolyte, electrode, and electrical contacts.⁸⁹ Meanwhile, the diameter of the semi-circle observed within the mid-frequency range represents the charge-transfer resistance (R_ct).⁸⁹ In this intermediate range, a linear trend surpassing 45° indicates the presence of the Warburg impedance, which signifies the diffusion of charge carriers within the electrode material.⁸⁹ Electrodes exhibiting lower R_s values, smaller semicircles (i.e., R_ct), and steeper inclines associated with Warburg impedance are presumed to possess superior electrochemical energy storage capabilities. In light of the above discussion, YK-1@FCNT was found to be the best electrode material, followed by YK-1@GO and YK-1, respectively.

Fig. 7 Nyquist plots of YK-1, YK-1@GO and YK-1@FCNT.

The cycling stability is a critical parameter for the success and practical application of supercapacitors. Cycling stability refers to the ability of a supercapacitor to maintain its electrochemical performance over repeated charge/discharge cycles without significant degradation. This parameter is crucial because energy storage devices, including supercapacitors, are often subjected to numerous charge and discharge cycles throughout their operational lifetime. During each cycle, ions are reversibly adsorbed or intercalated at the electrode surfaces, contributing to the energy storage and subsequent release. However, repeated cycling can lead to several degradation mechanisms, including electrode material degradation, electrolyte decomposition, and changes in the structure or morphology of the electrodes. By improving the cycling stability of supercapacitors, it is possible to enhance their overall performance, reliability, and lifespan, making them more suitable for various energy storage applications, including renewable energy systems, electric vehicles, and portable electronics.

Furthermore, in order to assess the cycling stability and durability of the YK-1, YK-1@GO, and YK-1@FCNT electrodes, cycling stability analysis was performed at a current density of 16 A g⁻¹ for consecutive cycles up to 5000. The GCD curves of the first and last cycles of YK-1, YK-1@GO and YK-1@FCNT electrodes are shown in Fig. 8. The YK-1 electrode retains 83.8% of its SCE after 5000 cycles. On the other hand, YK-1@GO and YK-1@FCNT electrodes retain 93.8% and 97.7% of their SCE after 5000 cycles, respectively.

Fig. 8 Cycling stability curve and GCD curves of the 1^st cycle and 5000^th cycle at the current density of 16 A g⁻¹ of (a) YK-1, (b) YK-1@GO and (c) YK-1@FCNT.

These results suggest that YK-1, as a pristine MOF electrode material, exhibits much favourable characteristics for SC applications, including high SCE and good cycling stability. However, the performance of YK-1 can be further enhanced by synthesizing composites with GO and FCNTs. The synthesis of composites, such as YK-1@GO and YK-1@FCNT, involves combining YK-1 with these carbon-based materials. This approach leverages the unique properties of both YK-1 and the carbon nanomaterials to create electrode materials with improved performance.

GO offers a huge surface area, excellent chemical stability, and good electrical conductivity. Incorporating GO into the YK-1 composite can enhance the overall surface area and offer additional active sites for charge storage. This leads to an increase in specific capacitance compared to YK-1 alone. FCNTs, on the other hand, are functionalized carbon nanotubes that can offer improved electrical conductivity, enhanced charge transport, and increased structural stability. Introducing FCNTs into the YK-1 composite can further improve the overall conductivity of the electrode material, facilitating faster charge transfer kinetics and higher energy storage capabilities.

By combining YK-1 with GO and FCNTs in the composites, the resulting electrode materials can achieve exceptional stability and high specific capacitance. The synergistic effects of YK-1, GO, and FCNTs contribute to improved charge storage capacity, enhanced cycling stability, and efficient charge transfer processes. These findings highlight the potential of YK-1-based composites with GO and FCNTs for the fabrication of highly stable supercapacitor electrodes with significantly improved performance. In Table 4, the performance of state-of-the-art reported MOFs in recent years is summarized and compared with this study. The utilization of such composites can lead to the development of advanced supercapacitor systems with enhanced energy storage capabilities and extended cycle life, making them promising candidates for various energy storage applications.

Table 4 Comparison of specific capacitance of YK-1, YK-1@GO and YK-1@FCNT with Hitherto reported MOFs/MOF derivatives

MOFs/MOF derivatives	Synthesis conditions	Potential window (V)	Electrolytes	Specific capacitance (F g^−1)	Current density/scan rates (A g^−1)	Ref.
Cu-MOF	Stirring	0 to 0.5	1 M LiOH	1274	1	90
Cu-MOF	Slow diffusion	−0.5 to 0.7	1 M Na2SO4	85	1.6	52
Cu-MOF/rGO	Ultrasonication	−0.5 to 0.7	1 M Na2SO4	685.33	1.6	52
Cu-MOF	Solvothermal, 120 °C (12 h)	0 to 0.5	6 M KOH	1148	0.5	91
Cu-MOF/rGO	Ultrasonication	−0.1 to 1	0.5 M Na2SO4	385	1	92
IIT-1/CNT	Solvothermal at RT	0 to 0.8	1 M Na2SO4	380	1.6	51
CuMOF/rGO	Ultrasonication at RT	−1 to 0.2	1 M Na2SO4	462	0.8	53
Ni-MOF	Sonication	0 to 0.6	6 M KOH	280	1	93
Ni/Co-MOF	Sonication	0 to 0.6	6 M KOH	650	1	93
Co-MOF-rGO	Sonication	0 to 0.6	6 M KOH	430	1	93
Ni/Co-MOF-rGO	Sonication	0 to 0.6	6 M KOH	860	1	93
Ni/Co MOF	Solvothermal, 120 °C (1 h)	0 to 0.5	1 M LiOH	530	0.5	94
Zn/Ni MOF	Solvothermal, 100 °C (8 h)	0 to 0.35	6 M KOH	1620	0.25	49
rGO−Ni-doped MOF	Sonication	0 to 0.5	1 M KOH	758	50 mA g^−1	95
Crystalline UiO-66	Solvothermal, 90 °C	−0.1 to 0.5	6 M KOH	452	10 mV s^−1	96
Amorphous UiO-66	Solvothermal, 90 °C (1 month)	−0.1 to 0.5	6 M KOH	920	10 mV s^−1	96
MOX-Fe	Hydrothermal, 120 °C (24 h)	−0.2 to 0.3	6 M KOH	600	1	97
Zn-MOF	Solvothermal, 120 °C	0 to 1.6	1 M Li2SO4	23	2.5 mA g^−1	98
Zn-MOF/PANi	In situ chemical oxidative polymerization	−0.2 to 0.8	1 M H2SO4	477	1	99
(p)-CoS2@CNT	Heating, 400 °C	0 to 0.4	2 M KOH	839, 825	5 mV s^−1, 0.5	100
Ce-MOF	Sonication/stirring	−0.2 to 0.4	3 M KOH	94.8	1	56
Ce-MOF	Sonication/stirring	−0.2 to 0.4	3 M KOH + 0.2 M K3Fe(CN)6	1116.3	1	56
Ce-MOF/GO	Sonication/stirring	−0.2 to 0.4	3 M KOH + 0.2 M K3Fe(CN)6	2221.2	1	56
Ce-MOF/GO	Sonication/stirring	−0.2 to 0.4	3 M KOH	233.8	1	56
Ce-MOF/CNT	Sonication/stirring	−0.2 to 0.4	3 M KOH	129.6	1	56
Ce-MOF/CNT	Sonication/stirring	−0.2 to 0.4	3 M KOH + 0.2 M K3Fe(CN)6	1367	1	56
Ce-MOF	Solvothermal, 90 °C (48 h)	0.0 to 0.8	1 M TEABF4 in CH3CN	572	1	55
Pr-MOF	Solvothermal, 90 °C (48 h)	0.0 to 0.8	1 M TEABF4 in CH3CN	399	1	55
Nd-MOF	Solvothermal, 90 °C (48 h)	0.0 to 0.8	1 M TEABF4 in CH3CN	360	1	55
Ce-H2L MOF	Sonication/hydrothermal, 130 °C (96 h)	0 to 0.45	2 M KOH	1389	1	101
Sm-H2L MOF	Sonication/hydrothermal, 160 °C (72 h)	0 to 0.45	2 M KOH	791	1	101
Eu-H2L MOF	Sonication/hydrothermal, 160 °C (72 h)	0 to 0.45	2 M KOH	560	1	101
Eu-MOF	Hydrothermal, 150 °C (48 h)	−0.8 to 0.5	6 M KOH	468	1	82
Nd-MOF/GO	Sonication/stirring	−0.3 to 0.3	3 M KOH	633.5	0.3	102
Tb-MOF	Hydrothermal, 150 °C (48 h)	−1.3 to −0.4	6 M KOH	510	1	103
YK-1	Solvothermal, 120 °C (72 h)	−0.9 to 0.2	3 M KOH	363.5	1.5	This work
YK-1@GO	Solvothermal, 120 °C (72 h)	−0.9 to 0.2	3 M KOH	488.2	1.5	This work
YK-1@FCNT	Solvothermal, 120 °C (72 h)	−0.9 to 0.2	3 M KOH	730.2	1.5	This work

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3.5 Symmetric device fabrication and its electrochemical evaluation

Due to the highest specific capacitance, energy density, power density and excellent cycling stability of YK-1@FCNT, a prototype symmetric supercapacitor (SSC) device based on it was fabricated. The homogeneous slurry was prepared by taking YK-1@FCNT (80%), acetylene black (10%), and PVDF (10%) in NMP. The slurry so obtained was coated on two stainless steel (SS) plates (current collector) of size 25 mm × 25 mm × 0.5 mm with the help of a brush and then kept for drying in a hot air oven at 70 °C for 12 h. The total mass of active material on one electrode was approximately 20 mg (i.e., 3.2 mg YK-1@FCNT per cm² of the current collector). The fabricated electrodes are shown in Fig. 9a. Filter paper was employed as a separator, which was then soaked in a 3 M KOH solution. The separator was then placed between the two prepared anode and cathode electrodes, and the assembly was packed with the help of tape, as shown in Fig. 9(b and c).

Fig. 9 Digital photos of the (a) prepared electrodes and separator and (b and c) the fabricated SSC device.

The performance of the as-fabricated SSC device was evaluated by CV and GCD in the potential range of 0.0 V to 1.5 V (Fig. 10a–d). The CV curves obtained at different scan rates spanning from 5 mV s⁻¹ to 100 mV s⁻¹ are shown in Fig. 10a. The slight deviation from the non-rectangular CV curve is due to the pseudocapacitive nature of YK-1@FCNT as observed in the three-electrode system. The specific capacitance of the SSC device was found to be 249.9 F g⁻¹ at a scan rate of 5 mV s⁻¹, which corresponds to a very high energy density of 78.1 W h kg⁻¹, indicating the remarkably high performance of the fabricated SSC device. The variation of specific capacitance with respect to scan rate is shown in Fig. 10c, and corresponding numerical values are given in Table 5. The performance of the SSC device was also evaluated by GCD. The GCD curves at different current densities and corresponding specific capacitance values are shown in Fig. 10b and d, respectively. The SSC device shows a very high specific capacitance of 280.2 F g⁻¹ at a current density of 1 A g⁻¹, which corresponds to the energy density and power density of 87.6 W h kg⁻¹ and 750.2 W kg⁻¹, respectively. At the same time, the SSC device shows a promising specific capacitance of 93.3 F g⁻¹ at a current density of 10 A g⁻¹, which corresponds to the energy density and power density of 29.2 W h kg⁻¹ and 7508.6 W kg⁻¹, respectively. The specific capacitance values, corresponding energy density values and power density values are listed in Table 5. The cycling stability of the SSC device was evaluated for 5000 GCD cycles at a current density of 10 A g⁻¹, as shown in Fig. 10e. The SSC device retains 91.4% of its specific capacitance after 5000 continuous GCD cycles which shows real-world applicability of the fabricated SSC device. The performance of the YK-1@FCNT based-SSC device is compared to various MOF/MOF composite-based SSC/ASC devices in Table 6. Moreover, the Ragone plots of the results obtained for the YK-1@FCNT based-SSC device in this study and its comparison with other studies listed in Table 6 are shown in Fig. 11.

Fig. 10 (a) CV curves at various scan rates, (b) GCD curves at different current densities, (c) specific capacitance vs. scan rate curve, (d) specific capacitance vs. current density curve and (e) cycling stability; inset first and last GCD curve of fabricated SSC device.

Fig. 11 (a) Ragone plot for YK-1@FCNT based-SSC device and (b) Ragone plot for comparison of the YK-1@FCNT based-SSC device with other studies in listed in Table 6.

Table 5 The performance of YK-1@FCNT based-SSC device obtained by CV and GCD analyses

CV			GCD
Scan rate (mV s^−1)	Specific capacitance (F g^−1)	Energy density (W h kg^−1)	Current density (A g^−1)	Specific capacitance (F g^−1)	Energy density (W h kg^−1)	Power density (W kg^−1)
5	249.9	78.1	1	280.2	87.6	750.2
10	227.3	71.0	2	229.3	71.7	1500.7
25	163.5	51.1	3	209.6	65.5	2250.0
50	112.4	35.1	4	180.8	56.5	3000.0
75	101.9	31.8	5	153.3	47.9	3748.7
100	86.5	27.0	10	93.3	29.2	7508.6

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Table 6 Comparison of YK-1@FCNT based-SSC device with other MOF material-based SSC/ASC devices

S. no.	Materials	Device-type	Specific capacitance (F g^−1)	Energy density (W h kg^−1)	Power density (W kg^−1)	Cycling stability	Ref.
1	Ni–Zn MOF	SSC	88.66 at 1.1 A g^−1	33.25	900	90.05% (2000 cycles) at 8 A g^−1	104
2.	Cu-MOF	SSC	205.2 at 0.5 A g^−1	18.2	825	87% (10 000 cycles) at 10 A g^−1	105
3.	Ti-MOF	SSC	122.4 at 0.5 A g^−1	10.8	803	79% (10 000 cycles) at 10 A g^−1	105
4.	Zr-MOF	SSC	161.6 at 0.5 A g^−1	14.3	811	83% (10 000 cycles) at 10 A g^−1	105
5.	NiO@Ni-MOF	ASC	144 at 1 A g^−1	39.2	700	94% (3000 cycles) at 10 A g^−1	106
6.	CCA-Co@MOF	ASC	129 at 0.5 A g^−1	45.9	431.7	91% (5000 cycles) at 2 A g^−1	107
7.	Ni-MOF	ASC	147 at 1 A g^−1	45.6	750	85% (10 000 cycles) at 10 A g^−1	108
8.	Mo/Ni-MOF	ASC	165 at 1 A g^−1	59	802.1	90% (20 000 cycles) at 5 A g^−1	109
9.	Co-MOF	ASC	110.0 at 0.5 A g^−1	34.4	375.3	120.5% (5000 cycles) at 6 A g^−1	110
10.	MOF-CNT	ASC	166.5 at 1 A g^−1	23.6	501.5	79.2% (10 000 cycles) at 10 A g^−1	111
11.	Cu-MOF/rGO	SSC	152.79 at 0.5 A g^−1	30.56	600	90.07% (10 000 cycles) at 10 A g^−1	112
12.	YK-1@FCNT	SSC	280.2 at 1 A g^−1	87.6	750.2	91.4% (5000 cycles) at 10 A g^−1	This work

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Moreover, the real-time performance of the SSC device was tested by charging the device with the help of a DC-regulated PSU at 5 V DC for 15 seconds (Fig. 12a). After 15 seconds of charging, the SSC device stored a voltage of 1.28 V when measured with the help of a multi-meter as shown in Fig. 12b (it should be noted that the device was only partially charged in 15 seconds to store 1.28 V).

Fig. 12 (a) Charging of the fabricated SSC device at 5VDC using a DC regulated power supply, (b) voltage stored after charging at 5VDC for 15 seconds; discharging of the charged SSC device using (c and d) 5 mm green LED of 1.8 V, (e and f) 5 mm red LED of 1.8 V, (g and h) 5 mm blue LED of 1.8 V after 45 seconds of charging, and (i) powering a DC Brushless fan with operating voltage 3.5–5.5 V DC using three SSC devices connected in series after charging for 100 seconds.

Furthermore, three LEDs of 1.8 V, viz., green, red, and blue, were connected to a single SSC device to evaluate its practical application. Fig. 12(c–h) clearly shows that the device was successfully able to light up 5 mm green, red, and blue LEDs after a charging cycle of 45 seconds. Finally, three SSC devices were connected in a series configuration and charged for 100 seconds (Fig. 12i). After charging, a DC brushless fan (with operating voltage 3.5–5.5 V DC) was powered for a few seconds (a video is included with the ESI†) which proves that the fabricated SSC device can deliver high current.

4. Conclusion

In summary, we have successfully synthesized a novel lanthanide-based MOF, YK-1, by using a solvothermal approach and fabricated it with GO and FCNTs to form YK-1@GO and YK-1@FCNT composites by adopting a simple ultrasonication technique. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) analyses were carried out to study their electrochemical behaviour. The results show that in a three-electrode system, YK-1@FCNT exhibited the maximum specific capacitance value of 730.2 F g⁻¹ at a current density of 1.5 A g⁻¹, whereas YK-1@GO and YK-1 at the same current density exhibit a specific capacitance of 488.2 F g⁻¹ and 363.5 F g⁻¹, respectively. The maximum energy density of YK-1, YK-1@GO and YK-1@FCNT was found to be 61.1 W h kg⁻¹, 82.1 W h kg⁻¹ and 122.7 W h kg⁻¹ at 1.5 A g⁻¹, respectively. The maximum power density of YK-1, YK-1@GO, and YK-1@FCNT was found to be 8204.7 W kg⁻¹, 8250 W kg⁻¹, and 8249.1 W kg⁻¹ at 15 A g⁻¹, respectively. The YK-1 electrode retains 83.8% of its specific capacitance after 5000 cycles. On the other hand, YK-1@GO and YK-1@FCNT electrodes retain 93.8% and 97.7% of their specific capacitance after 5000 cycles, respectively (at 16 A g⁻¹). Due to the superior performance of YK-1@FCNT, a prototype symmetric supercapacitor (SSC) device based on it was assembled. The SSC device showed a remarkably high specific capacitance (280.2 F g⁻¹), energy density (87.6 W h kg⁻¹) and power density (750.2 W kg⁻¹) at a current density of 1 A g⁻¹ along with very good cycling stability of 91.4% after 5000 GCD cycles. The practicality of the fabricated SSC device was tested by different means, and a single device was able to light green, red, and blue LEDs of 1.8 V after charging for 45 seconds. Finally, a DC brushless fan with an operating voltage of 3.5–5.5 V was powered by three SSC devices connected in series after charging for 100 seconds. These results strongly support that YK-1@FCNT can be a promising and unique composite material of a lanthanide-based MOF and FCNTs for the fabrication of commercial supercapacitors having very high energy density and good cycling stability. Indeed, being a novel material, both YK-1 and YK-1@GO have great potential as well for supercapacitor application due to their high specific capacitance and energy density along with good cycling stability as obtained from three-electrode system studies. Their performance can be tested in a two-electrode system for real-time application in the future.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors acknowledge the Chairperson, Department of Chemistry, Aligarh Muslim University, Aligarh, India. M. Z. acknowledges the Prime Minister's Research Fellowship (PMRF). M. S. and M. Y. K. acknowledge the financial assistance in the form of the Core Research Grant from DST-SERB, New Delhi.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2303152. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt00136b

‡ These authors contributed equally.

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