
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12571-6
10.1016/j.heliyon.2024.e36540
e36540
Research Article
Enhanced electrochemical performance with exceptional capacitive retention in Ce–Co MOFs/Ti3C2Tx nanocomposite for advanced supercapacitor applications
Siddiqui Rabia a
Rani Malika dr.malikarani@wum.edu.pk
a⁎
Shah Aqeel Ahmed b
Siddique Sadaf c
Ibrahim Akram d
a Department of Physics, The Women University, Multan, 66000, Pakistan
b Wet Chemistry Laboratory, Department of Metallurgical Engineering, NED University of Engineering and Technology, Karachi, 75270, Pakistan
c Department of Chemistry, Pakistan Institute of Engineering and Applied Sciences (PIEAS), 45650, Islamabad, Pakistan
d Department of Physics, College of Science, King Khalid University, Abha, 61413, Saudi Arabia
⁎ Corresponding author. dr.malikarani@wum.edu.pk
20 8 2024
15 9 2024
20 8 2024
10 17 e3654018 5 2024
4 8 2024
19 8 2024
© 2024 Published by Elsevier Ltd.
2024

https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
This study introduces a high-performance Ce–Co MOFs/Ti3C2Tx nanocomposite, synthesized via hydrothermal methods, designed to advance supercapacitor technology. The integration of Ce–Co metal-organic frameworks (MOFs) with Ti3C2Tx (Mxene) yields a composite that exhibits superior electrochemical properties. Structural analyses, including X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), confirm the successful formation of the composite, featuring well-defined rod-like Ce–Co MOFs and layered Ti3C2Tx sheets.

Electrochemical evaluation highlights the exceptional performance of the Ce–Co MOFs/Ti3C2Tx nanocomposite, achieving a specific capacitance of 483.3 Fg⁻1 at 10 mVs⁻1, a notable enhancement over the 200 Fg⁻1 of Ce–Co MOFs. It also delivers a high energy density of 78.48 Whkg⁻1 compared to 19 Whkg⁻1 for Ce–Co MOFs. Remarkably, the nanocomposite shows outstanding cyclic stability with a capacitance retention of 109 % after 4000 cycles and electrochemical surface area (ECSA) of 845 cm2, coupled with a reduced charge transfer resistance (Rct) of 2.601 Ω and an equivalent series resistance (ESR) of 0.8 Ω. These findings demonstrate that the Ce–Co MOFs/Ti3C2Tx nanocomposite is a groundbreaking material, offering enhanced energy storage, conductivity, and durability, positioning it as a leading candidate for next-generation supercapacitors.

Keywords

Hydrothermal method
Metal organic frameworks (MOFs)
Specific capacitance
Cyclic stability
Electrochemical surface area
Equivalent series resistance (ESR)
Energy density
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pmc1 Introduction

Supercapacitors are emerging as promising next-generation energy storage devices due to their long cyclic stability, high power/energy density, and eco-friendly nature [[1], [2], [3]]. They offer significant advantages over dielectric capacitors, fuel cells, and rechargeable batteries for high performance energy storage applications [4,5]. Researchers are constantly exploring new materials and designs to enhance their performance [6,7].

However, common electrode materials like carbon, transition metal oxides, and conducting polymers often fail to meet the required standards due to low capacitance, high cost, and poor stability [8]. Thus, developing new materials with higher energy densities, better rate capabilities, and longer cycling lifetimes is essential.

Various studies have enhanced supercapacitor performance through novel material synthesis and composites, like Nickel and cobalt oxide electrodes on willow catkin-derived activated carbon achieved up to 800.9 F g−1 [9], with excellent cyclic stability. Copper/chromium MOF derivatives showed 535.1 F g−1 [10], maintaining 90.3 % capacitance after 5000 cycles. Willow catkin-derived carbons reached 105 F g−1 [11], retaining 89.23 % capacitance after 1000 cycles. Alumina from recycled aluminum cans achieved 1297 F g−1 [12], with 92.7 % retention after 5000 cycles. Integration of cobalt hydroxide on graphene oxide/nickel foam achieved 2688 F g−1 [13], demonstrating excellent stability. CuO nanostructures synthesized via chemical deposition showed 184.58 F g−1 [14], and MXene/nickel hydroxide composites achieved up to 675 F g−1 [15], with exceptional cyclic stability.

However, Metal-organic frameworks (MOFs) are considered promising for supercapacitor electrodes due to their large surface areas, adjustable pore sizes, and redox-active metal centers [16]. Transition metal-based MOFs have been explored for various applications, including catalysis and energy storage, showing significant potential [[17], [18], [19]]. But rare earth elements (RE) offer unique properties such as strong oxygen affinity and diverse coordination chemistry, making them attractive for enhancing MOFs performance [20,21]. Despite, their potential, the use of RE-based MOFs, particularly in combination with 2D materials for energy storage, is under investigation.

Two-dimensional (2D) materials like titanium carbide (Ti3C2Tx or MXene), have shown great promise due to their large electrochemical active surface area, superior conductivity, and reversible surface redox reactions [[22], [23], [24], [25], [26]]. Integrating MOFs with MXene can potentially enhance supercapacitor performance by combining the benefits of both materials, though this combination remains unstudied [27].

In this study, we present a novel composite of Ce–Co MOFs and Ti3C2Tx for supercapacitors, aiming to overcome current limitations and improve energy storage capabilities [28]. Ce–Co MOFs selected for their structural stability and high surface area, while Ti3C2Tx was chosen for its excellent conductivity and surface redox properties [[29], [30], [31], [32], [33]].This combination is expected to provide significant improvements in specific capacitance and cyclic stability.

We systematically evaluate the electrochemical performance of this composite, including specific capacitance and cycling stability. Our results show a cyclic stability of 109 %, the highest reported among similar materials. To highlight the advantages of our work, we compared the performance of Ce–Co MOFs/Ti3C2Tx with well-known MOFs like Ni3(HITP)2, Co-MOF, Ni/Co-MOF, Cu3(HHTP)2, V-MOF, Cr-MOF, Mn-MOF, and Fe-MOF [[34], [35], [36], [37], [38], [39], [40], [41]] in Table 6. Our research demonstrates the synergistic effects of combining Ce–Co MOFs with Ti3C2Tx, leading to enhanced energy storage capabilities.

The novelty of this research lies in the innovative integration of Ce–Co MOFs with Ti3C2Tx, which has not been reported before. The Ce–Co MOFs provide structural stability and high surface area, essential for efficient charge storage and ion diffusion, while Ti3C2Tx enhances the conductivity and electrochemical performance of composite material. This study contributes to the understanding of advanced supercapacitor materials, offering new insights into their individual and synergistic effects.

We have chosen specific ratios of Ce, Co, and Ti3C2Tx based on their individual electrochemical properties and previous literature, ensuring an optimal balance between stability, surface area, and conductivity [21,25]. The rationale behind these ratios is supported by the enhanced performance observed in our electrochemical analysis, which aligns with findings in related studies on MOFs and MXenes for supercapacitor applications. This strategic combination and its successful implementation demonstrate the potential of our approach in advancing supercapacitor technology.

2 Experimental detail

2.1 Materials

To synthesize the required sample, cerium (III) nitrate hexahydrate (Ce(NO3)3 · 6H2O) [99.9 % pure], Cobalt (II) nitrate hexahydrate Co(NO3)2 · 6H2O) [98 % pure], Trimesic Acid (C6H3(CO2H)3) [95 % pure], deionized (DI) water [99.9 % pure], and ethanol absolute (CH3CH2OH) [99.8 % pure], all purchased from Sigma Aldrich are used. Mxene (Ti3C2Tx) was synthesized from MAX (Ti3AlC2) powder using a similar method as mentioned in our previous research paper [42,43].

2.2 Synthesis of Ce–Co MOFs/Mxene

The Ce–Co MOFs and their composite with Mxene (Ti3C2Tx) were synthesized using a hydrothermal method. For the synthesis, Ti3C2Tx (0.001 g) was dissolved in a solution of deionized water and ethanol (3:1 v/v), stirred for 10 min, and then sonicated in an ice bath for 1 h (Solution 1). In a separate beaker, Cerium nitrate hexa-hydrate (0.5 g), Cobalt nitrate hexahydrate (0.5 g), and trimesic acid (1 g) were added to a mixture of deionized water and ethanol (3:1 v/v) and stirred for 30 min (Solution 2). Solutions 1 and 2 were then combined, stirred for 15 min, and transferred to a Teflon-lined autoclave. The mixture was heated in an oven at 200 °C for 18 h. After cooling to room temperature, grey-colored precipitates were obtained. These precipitates were washed and centrifuged multiple times with a mixture of ethanol and deionized water to ensure purity and maintain a pH of approximately 7. The washed precipitates were then dried at 60 °C in an oven for 12 h. The resulting dried powder was used for further characterization. It is noteworthy that the identical procedure was followed for the synthesis of Ce–Co MOFs, except without the addition of Ti3C2Tx.

3 Results & discussion

3.1 X-ray diffraction (XRD) analysis

The X-ray diffraction (XRD) patterns for Ce-MOFs, Co-MOFs, Ti₃C₂Tₓ, Ce–Co MOFs and Ce–Co MOFs/Ti₃C₂Tₓ, naocomposite shown in Fig. 1, were analyzed to evaluate phase composition and interactions. The XRD patterns provide a clear distinction between the phases present in Ce–Co MOFs, Ti₃C₂Tₓ, and their composite. The data, summarized in Table 1, highlight 2θ angles and corresponding peak intensities for each sample. The XRD analysis of the Ce–Co MOFs/Ti₃C₂Tₓ composite reveals significant variations in peak intensities compared to the individual phases, as summarized in Table 1. The peak at 39.75° 2θ, with a relative intensity of 100.00 % in Co-MOF, decreases to 75.85 % in the composite, indicating possible overlap or interaction with Ti₃C₂Tₓ. In contrast, the peak at 67.78° 2θ for Ti₃C₂Tₓ, which maintains a relative intensity of 100.00 % in the composite, suggests a strong presence of Ti₃C₂Tₓ. Intensity of the Ce-MOF peak at 31.88° 2θ increases from 8.50 % to 14.25 % in the composite. This increase signifies a notable presence of Ce-MOF, despite being less dominant than Ti₃C₂Tₓ.Fig. 1 XRD pattern of Co-MOFs, Ce-MOFs, Mxene (Ti3C2Tx), Ce–Co MOFs and Ce–Co MOFs/Ti3C2.

Fig. 1

Table 1 XRD peak summary for Ce-MOF, Co-MOF, Ce–Co MOFs, and Ce–Co MOFs/Ti₃C₂Tₓ composite.

Table 1Sample	2θ°	Peak Intensity (a.u)	Relative Intensity (%)	Crystalline Phase	JCPDS Card Number	
Ce-MOFs	31.88	30.89	8.50	Ce-MOFs	01-089-8430	
37.87	363.18	100.00	
39.73	63.97	17.61	
57.28	217.47	59.88	
69.61	27.14	7.47	
Co-MOFs	6.48	13.03	15.49	Co-MOFs	00-015-0806	
18.79	41.30	49.10	
24.12	51.13	60.78	
39.75	84.12	100.00	
56.71	23.99	28.52	
67.64	48.31	57.43	
Ce–Co MOFs	6.80	78.33	90.05	Ce–Co MOFs	01-089-8430,
00-015-0806	
14.05	39.39	45.28	
18.91	33.22	38.18	
22.51	14.46	16.62	
32.18	15.33	17.62	
33.64	17.34	19.93	
39.73	86.99	100.00	
67.68	16.76	19.27	
Ce–Co MOFs/Ti₃C₂Tₓ	31.88	28.77	14.25	Ce-MOFs	01-089-8430	
33.30	8.56	9.05	Co-MOFs	00-015-0806	
39.74	71.77	75.85	Ti₃C₂Tₓ	01-085-0655	
60.07	11.90	12.58	Ti₃C₂Tₓ	01-085-0655	
67.78	94.62	100.00	Co-MOFs	00-015-0806	

To accurately identify overlapping peaks and confirm phase presence, several methods were used. For distinct peak identification, unique 2θ angles for each phase facilitated their identification, even with peak overlap. Also, XRD patterns were compared with JCPDS card references to match observed peaks with known phases. By comparing the XRD pattern of composite with those of the individual components, changes in peak positions and intensities indicated phase interactions and preserved structures.

Overall, the XRD data confirms the successful integration of Ce-MOF, Co-MOF, and Ti₃C₂Tₓ in the composite, with observable changes in peak intensities reflecting phase interactions and the integrity of each component.

To calculate the structural parameters, i.e., lattice constant, lattice strain, and dislocation density, data derived from XRD analysis is used using x-pert highscore software and summarized in Table 2 [44,45]. Given that the materials are described as having a hexagonal structure according to JCPDS cards.(1) 1d2=43(h2+hk+k2a2)+l2c2

Where, “a” is the lattice constant, “c” is the lattice constant perpendicular to the plane, “h,k and l” are the Miller indices.Table 2 Lattice Constant, Lattice Strain and Dislocation density for Co-MOFs, Ce-MOFs, Ce–Co MOFs, Ce–Co MOFs/Ti3C2Tx.

Table 2Material	Lattice Constant	Lattice Strain (ε)	Dislocation Density (δ)
( × 10⁻³ nm⁻2)	
Co-MOF	a = 21.061 Å	0.00121	1.34	
c = 15.542 Å	
Ce-MOF	a = 23.131 Å	0.00142	1.79	
c = 16.610 Å	
Ce–Co MOF	a = 20.197 Å	0.00162	2.14	
c = 13.543 Å	
Ce–Co MOFs/Ti₃C₂Tₓ	a = 19.892 Å	0.00160	3.01	
c = 12.186 Å	

The lattice strain can be calculated using the Williamson-Hall method, which is based on the broadening of the XRD peaks.(2) Δdd=B4tanθ+Kλdcosθ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐

Where, Δdd is the relative strain, “B” is the peak broadening due to size and strain, “θ” is Bragg angle, “K” is constant (0.94 for hexagonal shape), “λ” is the X-ray wavelength.

The dislocation density “ρ” can be estimated using the following formula:(3) ρ=1d2‐‐‐‐‐‐‐‐‐‐‐‐‐‐

Where, “D” is the average crystallite size, which can be derived from the Scherrer equation:(4) D=KλBcosθ‐‐‐‐‐‐‐‐‐‐‐‐‐‐

3.2 Scanning Electron Microscopy (SEM) analysis

SEM micrographs in Fig. 2(a and b), show distinct rod-like structures within the Ce–Co MOFs. These structures indicate well-defined crystalline formations, which align with the expected morphology of metal-organic frameworks. The rod-like shapes suggest that the Ce–Co MOFs have formed elongated structures, attributed to the specific crystal growth habits under the given synthesis conditions. This observation confirms the successful synthesis of Ce–Co MOFs with the expected morphology, consistent with previous studies [46,47].Fig. 2 SEM micrograph of (a, b) Ce–Co MOFs, (c,d) Ce–Co MOFs/Ti3C2Tx nanocomposite, (e) Mxene (Ti3C2Tx).

Fig. 2

In Fig. 2(c and d), the SEM micrographs depict the Ce–Co MOFs/Ti3C2Tx nanocomposite reveal the presence of delaminated sheets of Ti3C2Tx [Fig. 2(e)] [48]. This suggests that the Ti3C2Tx layers in the nanocomposite have undergone exfoliation or delamination, resulting in thin, two-dimensional sheets dispersed within the MOFs matrix. The delaminated Ti3C2Tx sheets indicate a high degree of interaction and integration between the MOFs and Ti3C2Tx components. This integration can enhance the properties of composite, making it a promising material for applications in catalysis and energy storage.

Therefore, the SEM micrographs provide visual evidence of well-defined rod-like structures in the Ce–Co MOFs and delaminated Ti3C2Tx sheets in the Ce–Co MOFs/Ti3C2Tx nanocomposite. These morphological features align with the expected characteristics of each component, underscoring the successful synthesis and integration of these materials. This opens up opportunities for tailored applications based on their unique structural properties.

3.3 Energy dispersive X-ray spectroscopy (EDS)

EDS is a fundamental analytical technique used to determine the elemental composition of Ce–Co MOFs, Ti3C2Tx, and their composite, providing a detailed understanding of the chemical makeup of material.

Fig. 3 (a) shows the EDS analysis of the Ce–Co MOFs, revealing a high Carbon (C) content at 42.92 wt% and Oxygen (O) content at 43.86 wt%. The Presence of Cobalt (Co) at 0.50 wt% and Cerium (Ce) at 12.72 wt% confirms the organic ligands (Trimesic acid) and metal oxide components characteristic of MOFs.Fig. 3 EDS analysis of (a) Ce–Co MOFs, (b) Mxene (Ti3C2Tx), (c) Ce–Co MOFs/Ti3C2Tx.

Fig. 3

Fig. 3(b) presents the EDS analysis of Ti3C2Tx, synthesized from Max phase using HF. It shows a dominant Titanium (Ti) presence at 70.92 wt% and 44.75 at%, along with Carbon (C) at 6.68 wt% (16.8 at%), Oxygen (O) at 9.13 wt% (17.24 at%), and other elements. These findings align with the expected composition of Ti3C2Tx, confirming its successful synthesis.

Fig. 3(c) illustrates the EDS analysis of the Ce–Co MOFs/Ti3C2Tx nanocomposite synthesized hydrothermally. It shows Carbon (C) at 20.88 wt% (46.62 at%), Oxygen (O) at 23.14 wt% (38.79 at%), Titanium (Ti) at 4 wt% (2.24 at%), Cerium (Ce) at 48.9 wt% (9.36 at%), and Cobalt (Co) at 1.36 wt% (0.62 at%). These results indicate the successful incorporation of Ti3C2Tx into the composite structure.

The EDS profile of composite demonstrates the coexistence of Ti3C2Tx and MOFs components, providing insights into its enhanced properties and potential applications in advanced research areas such as in catalysis, energy storage, and sensors.

3.4 Fourier transformation infrared spectroscopy (FTIR)

The FTIR spectrum of the Ce–Co MOF/Mxene composite provides valuable insights into its chemical composition. In Fig. 4(a), the FTIR spectrum of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx shows several common transmittance peaks. At 755.76 cm−1, there are bending vibration of metal-oxygen (M − O) bonds i.e., Ce–O and Co–O bonds in cerium and cobalt containing species. The peak at 1110.28 cm−1 reflects the C–O stretching vibration in carboxylate groups of trimesic acid. At 1367.53 cm−, the spectrum indicates C–H bending in the aromatic ring of trimesic acid, while the peak at 1442.95 cm−1 shows the C–H bending in aliphatic groups. The peak at 1558.5 cm−1 display C=O stretching vibration in the carboxylate groups of trimesic acid, and the peak at 1607.17 cm−1corresponds to the C=C stretching vibration in the aromatic ring of trimesic acid. A broad peak from 3000 to 3600 cm−1 indicates O–H stretching vibrations in hydroxyl groups. These peaks are consistent in both materials, indicating common structural features or molecular groups shared between Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx, with trimesic acid as a linker and cerium nitrate, cobalt nitrate, and Ti3C2Tx as precursors. Additionally, the Ce–Co MOFs/Ti3C2Tx spectrum shows a peak at 1664.9 cm−1, suggesting the presence of C=O stretching in carbonyl groups, possibly from adsorbed water or residual solvent.Fig. 4 (a) FTIR spectrum of Ce–Co MOFs, Ce–Co MOFs/Ti3C2Tx, (b) Ti3C2Tx (Mxene).

Fig. 4

In Fig. 4(b), the FTIR spectrum of Ti3C2Tx displays distinctive peaks. A broad region from 600 to 1200 cm−1 indicates Ti–O–Ti bending vibrations, while a small peak at 1206.9 cm−1corresponds to Ti–O stretching vibrations. A small peak at 1362.7 cm−1, corresponds to C–F stretching vibrations, confirming the incorporation of fluorine from the HF treatment. A sharp peak at 1579.84 cm−1, matches the C=C stretching vibrations in aromatic rings. These Ti–O–Ti bending and Ti–O stretching vibrations confirm the successful synthesis of Ti3C2Tx from Max using HF.

FTIR analysis confirms the successful synthesis of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx, revealing their shared molecular features and structural characteristics. The distinct peaks in Mxene based composite further demonstrate its potential in enhancing the properties of the composites for advanced applications.

3.5 Electrochemical analysis

The electrochemical properties of the synthesized materials were investigated using an electrochemical workstation (CHI-760-E model,CH-Instruments, Texas) equipped with a three-electrode setup. Working electrode comprised of a glassy carbon (GC) electrode having an area of 7.07 mm2. Prior to use, GC electrode underwent ultrasonic treatment for 30 min and was then coated with a catalyst of 0.002 g of nanocomposite dispersed in solution of Ethanol and Nafion (in a ratio of 500 μL: 25 μL) to create a uniform ink. A precise volume of this ink was deposited onto the GC electrode surface using a technique like drop-casting. The ink was evenly spread over the electrode. The electrode was dried to allow the solvent to evaporate, leaving a uniform layer of the active material. The mass loading was then calculated by dividing the total mass of the active material by the electrode area, i.e.,Massloading=0.002g0.0707cm2=0.0005gcm‐2

The activated mass used in the calculations was carefully measured and disclosed, amounting to 0.002 g of the nanocomposite. This value was consistently used to calculate specific capacitance, energy density, and power density. By carefully controlling the volume of catalytic ink deposited and knowing the concentration of the active material in the ink, the desired mass loading on the electrode surface can be achieved. The accuracy of this measurement is crucial for consistent and reproducible electrochemical characterization.

An Ag/AgCl was utilized as a reference electrode, and the platinum wire as a counter electrode. All electrodes were submerged in an electrolyte solution of 1M KOH, chosen for its high ionic conductivity, stability, and compatibility with the electrode materials. The use of 1 M KOH as the electrolyte was rationalized based on its ability to provide a stable and conductive environment, which is essential for the optimal performance of supercapacitors.

For the analysis of electrochemical performance, cyclic voltammetry (CV), galvanostatic charge-dischrage (GCD) measurements and electrochemical impedance spectroscopy (EIS), were employed. Specific capacitance (Cs) in Fg−1 was determined by Equations. (1), (2) for CV and GCD curves, respectively:(5) Cs=1msΔV∫VaVcI(V)dv‐‐‐‐‐‐‐‐‐‐‐‐

(6) Cs=I×Δtm×ΔV‐‐‐‐‐‐‐‐‐‐‐‐

In these equations, the integral area of the CV curve is represented by “∫dV”, mass loading of active material in grams against the glassy carbon electrode is represented by “m”, potential window is represented by “V”, and the scan rate in millivolts per second is symbolized by “ms”.

In equation (2), the discharge current in amperes (A) is represented by “I”, and discharge time duration in seconds (s) is denoted by “Δt”.

EIS investigation was conducted over a frequency range from 1 Hz to 105 Hz, with an AC voltage amplitude of 10 mV. Additionally to assess the improved performance of supercapacitors, the energy density in units of Wh kg−1 and power density in units of W kg−1 were calculated utilizing equations (3), (4), respectively [49].(7) E=17.2Cs(ΔV)2‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐

(8) P=3600EΔt‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐

In these equations, “Cs” and “ΔV” denotes the specific capacitance and voltage window in units of Fg−1, and V, respectively, whereas “Δt” represents the discharge time in seconds obtained from GCD profile.(a) Cyclic Volatmmetry (CV) Measurements

Cyclic voltammetry (CV) measurements provide crucial insights into the capacitive behavior of the Ce–Co MOFs in Fig. 5(a). The optimized potential window for the electrochemical measurements was determined to be −0.3 V–0.3 V vs. Ag/AgCl. This potential window was selected to maximize the capacitance while ensuring the stability of the electrode materials. The determination of this window involved preliminary cyclic voltammetry (CV) scans to identify the potential limits beyond which degradation of the electrode materials occurs. Specific capacitance measurements reveal that Ce–Co MOFs exhibit a value of at a scan rate of 200 Fg-1 at a scan rate of 10 mVs-1, indicating a moderate energy storage capacity and suggesting their potential utility in energy storage applications. In contrast, the Ce–Co MOFs/Ti3C2Tx nanocomposite in Fig. 5(b), shows a significant specific capacitance, reaching 488.3Fg-1 at the same scan rate. This considerable enhancement reflects the synergistic effects of combining Ce–Co MOFs and Ti3C2Tx, which contributes to superior capacitive performance.Fig. 5 CV profile of (a) Ce–Co MOFs, (b) Ce–Co MOFs/Ti3C2Tx (c) CV profile at 2 mVs−1 highlighted with redox peaks.

Fig. 5

In Fig. 5(c), the CV profile of Ce–Co MOFs/Ti3C2Tx nanocomposite displays distinct redox peaks labeled "a" and "b". These redox peaks, are seen at approximately −0.009 V and 0.16 V versus Ag/AgCl, indicating additional faradaic reactions due to the presence of MXene [[50], [51], [52]]. Enhanced electrical conductivity and large surface area of Ti3C2Tx (MXene) facilitate more efficient charge transfer and ion exchange, further boosting the specific capacitance of nanocomposite. This effect is clearly demonstrated in the variation of specific capacitance with scan rate, as shown in Fig. 6 of Table 3.Fig. 6 Graphical representation of Specific Capacitance of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx at various scan rates.

Fig. 6

Table 3 Specific Capacitance of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx at different scan rates using Cyclic voltammetric profiles.

Table 3Specific Capacitance (Fg−1) using CV measurements	
Scan Rate	10 mVs−1	20 mVs−1	30 mVs−1	50 mVs−1	100 mVs−1	
Ce–Co MOFs	200	140	120	101.3	79.3	
Ce–Co MOFs/Ti3C2Tx	488.3	417.5	394.4	363.3	351.6	

The observed enhancements in specific capacitance, attributed to the synergistic effects and additional faradaic reactions facilitated by MXene, highlight the potential of the nanocomposite for high-performance supercapacitor applications. These findings pave the way for development of energy storage devices with improved efficiency and capacity, addressing the growing demand for advanced energy storage solutions.

Furthermore, the electrochemical active surface area (ECSA) measurements of the electrodes are assessed across scan rates, ranging from 10 mVs-1 to 100 mVs-1, using Equation (11) [53,54].(9) ECSA=CdlCs‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐

Where, ECSA is defined as the ratio of the double-layer capacitance of the electrodes (Cdl) to the specific capacitance of the KOH electrolyte (Cs).

Analysis of Cdl involves the slope of the linear fit curve, depicting the relationship between scan rate (mVs−1) and current (A), as illustrated in Fig. 7. Meanwhile, Cs denotes the specific capacitance characteristic of the KOH electrolyte. ECSA measurements, calculated using Equation (5), reveal that Ce–Co MOFs/Ti3C2Tx nanocomposite exhibits an ECSA of 845 cm2, significantly higher than the 106 cm2 for Ce–Co MOFs. Remarkably, the electrode exhibiting the highest ECSA also showcased the highest Cs value, as determined through comprehensive cyclic voltammetry (CV) analysis as in Table 3. These findings illustrate the potential of Ce–Co MOFs/Ti3C2Tx nanocomposite for advanced supercapacitor applications, offering both increased efficiency and capacity for high performance energy storage devices.(b) Galvanostatic Charge Discharge (GCD) Measurements

Fig. 7 Ecsa profile of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx.

Fig. 7

The GCD measurements of the Ce–Co MOFs, shown in Fig. 8(a), reveal their electrochemical properties. At a current density of 0.1 Ag-1, the specific capacitance is measured as 105 Fg-1. The charge and discharge curves exhibit a linear profile, indicating efficient charge storage and release processes, which is typical of supercapacitor behavior and confirms the suitability of material for energy storage.Fig. 8 GCD measurements of (a) Ce–Co MOFs, (b) Ce-CoMOFs/Mxene, (c) Galvanostatic charge and discharge profiles at 0.1 Ag-1 highlighted with corresponding plateaus for Ce-CoMOFs/Mxene.

Fig. 8

Incorporation of Ti3C2Tx (MXene) into the Ce–Co MOFs matrix significantly enhances the specific capacitance of the resulting nanocomposite, as illustrated Fig. 8(b). At the same current density of 0.1 Ag-1, the Ce–Co MOFs/Ti3C2Tx nanocomposite demonstrates a specific capacitance of 443.5 Fg-1, a substantial improvement over Ce–Co MOFs alone. This enhancement highlights the synergistic effect achieved by combining Ce–Co MOFs with Ti3C2Tx. Fig. 8(c), further illustrates the charge and discharge profiles of the nanocomposite, which exhibit non-linear behavior with distinct plateaus. These plateaus suggest redox processes during ion intercalation and de-intercalation within the electrode material, aligning with the redox peaks identified in the CV measurements in Fig. 5(c). This non-linear charge-discharge curves and plateaus indicate efficient ion transport and storage within the electrode material, contributing to the high specific capacitance and rapid energy storage capabilities of the Ce–Co MOFs/Ti3C2Tx nanocomposite.

Fig. 9, corresponding to Table 4, visually represents the specific capacitance of the Ce–Co MOFs/Ti3C2Tx nanocomposite at various current densities. The superior performance of the nanocomposite, demonstrated by its enhanced specific capacitance and distinct charge-discharge profiles, highlights its potential for high-performance supercapacitor applications. These findings provide valuable insight into designing and developing advanced energy storage materials with improved efficiency and capacity, meeting the increasing demand for sustainable energy technologies.Fig. 9 Graphical representation of Specific Capacitance of Ce–Co MOFs and Ce–Co MOFs/Mxene at various Current Densities.

Fig. 9

Table 4 Specific Capacitance of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx at different current densities.

Table 4Specific Capacitance (Fg−1) using GCD measurements	
Sample	0.1 Ag-1	0.3 Ag-1	0.5 Ag-1	0.7 Ag-1	1Ag-1	5 Ag-1	
Ce–Co MOFs	112.76	89.7	69.5	14.8	6.3	4.1	
Ce–Co MOFs/Ti3C2Tx	443.5	185.8	130.7	126.7	101.6	78.3	

Fig. 10, The energy density measurement of Ce–Co MOFs revealed a value of 19 Wh/kg, indicating moderate energy storage capabilities. This suggests that the Ce–Co MOFs alone possess some potential for energy storage applications, although at a moderate level. However, this energy density may not fully meet the rigorous requirements of high-performance supercapacitors, particularly those demanding rapid charge-discharge cycles and high energy density.Fig. 10 Graphical representation of Energy Density and Power Density of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx at different current densities.

Fig. 10

On the other hand, the incorporation of Ti3C2Tx (MXene) into the Ce–Co MOFs matrix resulted in a remarkable enhancement in energy density for the nanocomposite. The Ce–Co MOFs/Ti3C2Tx nanocomposite revealed a significantly enhanced energy density of 78.48 Whkg−1, showcasing a substantial improvement over the energy storage capacity of Ce–Co MOFs alone. This notable rise in energy density can be primarily attributed to the enhanced specific capacitance achieved by the addition of MXene. The unique properties of MXene, like, its high electrical conductivity and enormous surface area, contribute to the efficient storage and release of electrical energy within the nanocomposite structure.(c) Cyclic Stability

Fig. 11 demonstrates the outstanding cycling stability and capacitance maintenance rate of Ce–Co MOFs/Ti3C2Tx nanocomposite, as highlighted in Table 6. This represents a significant advancement in the field of Ce-organic compounds. The exceptional cycling stability of the Ce–Co MOFs/Ti3C2Tx nanocomposite can be attributed to the synergistic properties of Ce–Co MOFs and Ti3C2Tx. The nanocomposite structure likely offers a stable framework for ion cycling during charge and discharge processes, preventing degradation of the material over extended cycles. Ti3C2Tx (MXene), known for its high electrical conductivity and mechanical strength, plays a crucial role in enhancing the overall stability of the nanocomposite. The MXene component may act as a protective layer, shielding the Ce–Co MOFs from structural degradation and maintaining the integrity of the electrode material [55].The interactions between Ce–Co MOFs and Ti3C2Tx at the nanoscale level likely create stable interfaces and pathways for efficient ion transport, reducing the likelihood of electrode degradation [56]. The remarkable 109 % capacitance retention rate indicates that the Ce–Co MOFs/Ti3C2Tx nanocomposite not only maintains but exceeds its initial capacitance over cycling. This suggests that the nanocomposite can undergo repeated charge and discharge cycles with minimal loss in capacitance, highlighting its potential for long-term and durable energy storage applications [57].(d) Electrochemical Impedence Spectroscopy (EIS)

Fig. 11 Cyclic stability Performance of Ce–Co MOFs/Ti3C2Tx nanocomposite @10 mV/s.

Fig. 11

The Electrochemical Impedance Spectroscopy (EIS) parameters for Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx nanocomposites are detailed in Table 5, with Nyquist plots depicted in Fig. 12. For Ce–Co MOFs, the Equivalent Series Resistance (Rs) is 1.3 Ω, indicating moderate internal resistance that affects ion transport. The Charge Transfer Resistance (Rct) is 3.59 Ω, reflecting slower charge transfer kinetics, which can impact performance. The Series Resistance (RL) is 1.93 Ω, showing significant additional resistance within the circuit. The Warburg Impedance (W) is 0.00013 Ω, suggesting relatively efficient ion diffusion. The Faradaic Capacitance (Cf) is 2.3 μF, indicating a limited contribution from faradaic reactions.Table 5 EIS parameters of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx nanocomposites.

Table 5Sample	Rs (Ω)	Rct (Ω)	RL (Ω)	W (Ω)	Cf (μF)	
Ce–Co MOFs	1.3	3.59	1.93	0.00013	2.3	
Ce–Co MOFs/Ti3C2Tx	0.8	2.601	1.03	0.00001	4.6	

Table 6 Comparison of this work with various materials for supercapacitor application in 3-electrode configuration.

Table 6Electrode Material	Electrolyte	Capacitance (Fg−1)	Capacitance Retention %	Ref.	
CeO2@ZIF-8	3M KOH	424 Fg-1	90 % retention over 5000 cycles	[58]	
Ni–Co MOFs	6 M KOH	108.5 C g−1	84.3 % retention after 10000 cycles	[59]	
Fe–CeO2	6 M KOH	148 C g−1	74.5 % retention after 5000 cycles	[60]	
Ni/Zn BTC MOFs	3 M KOH	263.8 mAh g−1	95.5 % retention over 5000 cycles	[61]	
Ce(COOH)3	3M KOH	140 F g−1	107.9 % retention after 60000 cycles	[62]	
Ce-doped α-cobalt hydroxide	2M KOH	415 C g−1	73 % even after 2000 cycles	[63]	
Ti3C2Tx/Co3O4	6 M KOH	240 F g−1	–	[64]	
Co-PC@MX-CNF	3 M KOH	426.7 F g−1	90.36 %	[65]	
Ce–Co MOFs/Ti3C2Tx	1 M KOH	443.5 F g−1	109 % retention over 4000 cycles	Present Work	

Fig. 12 Nyquist plot (a) Ce–Co MOFs, (b) Ce–Co MOFs/Ti3C2Tx.

Fig. 12

In contrast, Ce–Co MOFs/Ti3C2Tx demonstrates improved performance with a lower Rs of 0.8 Ω, which signifies better ionic conductivity. The Rct is reduced to 2.601 Ω, suggesting enhanced charge transfer efficiency. The RL is also lower at 1.03 Ω, reflecting improved circuit efficiency. The Warburg Impedance is significantly lower at 0.00001 Ω, indicating better ion diffusion. The Cf is higher at 4.6 μF, demonstrating an increased contribution from faradaic reactions to the overall capacitance. Overall, the Ce–Co MOFs/Ti3C2Tx nanocomposite exhibits superior electrochemical performance compared to Ce–Co MOFs, with lower resistance values, improved ion diffusion, and higher faradaic capacitance.

The structural and electrochemical performance analysis of Ce–Co MOFs and Ce–Co MOFs/Ti3C2Tx reveals notable improvements due to the incorporation of Ti3C2Tx. Ce–Co MOFs/Ti3C2Tx exhibits reduced lattice constants compared to Co-MOF and Ce-MOF, indicating structural adjustments, with consistent lattice strain suggesting similar impacts from compositional changes. The higher dislocation density in Ce–Co MOFs/Ti3C2Tx (3.01 × 10⁻³ nm⁻2) suggests increased structural defects, which may enhance electrochemical activity by providing additional active sites.

Electrochemical tests show that Ce–Co MOFs/Ti3C2Tx delivers significantly higher specific capacitance than Ce–Co MOFs across all scan rates in cyclic voltammetry (CV), indicating superior faradaic reaction contributions and energy storage capability. Similarly, galvanostatic charge-discharge (GCD) measurements reveal better performance at various current densities, demonstrating the material's effectiveness under high-power conditions.

Electrochemical impedance spectroscopy (EIS) further highlights the advantages of Ce–Co MOFs/Ti3C2Tx,with lower equivalent series resistance (0.8 Ω) and charge transfer resistance (2.601 Ω) compared to Ce–Co MOFs. This reduction in resistance indicates enhanced ionic and charge transfer conductivities. Additionally, the higher faradaic capacitance (4.6 μF) of Ce–Co MOFs/Ti3C2Tx reflects improved charge storage capabilities.

In comparison to other materials in Table 6, Ce–Co MOFs/Ti3C2Tx achieves a specific capacitance of 443.5 F g⁻1 and an impressive capacitance retention of 109 % over 4000 cycles, positioning it as a highly effective material for supercapacitor applications.

4 Conclusion

The incorporation of Ti3C2Tx into Ce–Co MOFs has markedly improved the structural and electrochemical properties of the composite material. The reduced lattice constants and increased dislocation density observed in Ce–Co MOFs/Ti3C2Tx suggest beneficial structural changes that enhance electrochemical performance. Specifically, the Ce–Co MOFs/Ti3C2Tx composite demonstrates significantly higher specific capacitance and improved energy storage capabilities compared to Ce–Co MOFs, as evidenced by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements.

The electrochemical impedance spectroscopy (EIS) results reveal that Ce–Co MOFs/Ti3C2Tx exhibits lower equivalent series resistance and charge transfer resistance, indicating superior ionic and charge transfer conductivity. This, coupled with a higher faradaic capacitance, underscores the enhanced charge storage performance of the composite. When compared to other materials in the literature, Ce–Co MOFs/Ti3C2Tx shows exceptional specific capacitance and remarkable capacitance retention, highlighting its effectiveness and stability as an advanced material for supercapacitor applications. Overall, the Ce–Co MOFs/Ti3C2Tx composite proves to be a highly promising candidate for next-generation energy storage devices due to its superior electrochemical performance and durability.

Data availability statement

All data generated or analyzed during this study is included in this article. No additional data are available.

CRediT authorship contribution statement

Rabia Siddiqui: Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Malika Rani: Writing – review & editing, Validation, Resources, Project administration, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. Aqeel Ahmed Shah: Software, Resources, Investigation. Sadaf Siddique: Methodology, Investigation. Akram Ibrahim: Software, Resources, Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We are thankful for The Women University, Multan and NRPU grant from the 10.13039/501100010221 Higher Education Commission of Pakistan (HEC), Pakistan that accommodates this project. We acknowledge, IST-Islamabad, and NILOPE for providing facility of SEM and EDS analysis for our samples.
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