Introduction

In the context of the massive reduction in fossil fuels and the increase in greenhouse gas emissions, developing low-cost, clean, and environmentally friendly energy technologies is crucial. Considerable efforts have been made to renewable energy production using biological, solar, hydroelectric, and ocean energy. The lack of an efficient energy storage system to store this renewable energy is impeding energy research. Supercapacitors (SCs) are highly sought after because of their rapid charge/discharge rate, superior specific capacitance, high power density, high reliability, long cycle life, and rapid release of a huge current [1,2,3,4]. SCs are promising electrochemical energy storage devices extensively used to charge electronic devices and electric cars. Considerable research has been conducted to enhance the performance of lightweight, flexible supercapacitors to fulfill the increasing need for wearable and portable electronics [5]. A typical SC consists of components such as a current collector, separator, electrode material, and electrolyte [6].

The electrode is a critical component of SC devices; hence, the choice of electrode material significantly impacts its performance and properties. Typical SC electrodes must have thermal stability, high specific surface area (SSA), corrosion resistance, excellent electrical conductivity, adequate chemical stability, and favorable surface wettability [7]. In addition, they should be cost-effective and environmentally friendly. Furthermore, their capacity to facilitate faradic charge transfer is vital for enhancing capacitance performance [8]. Materials based on nanostructured carbon, transition metal oxides/hydroxides, conducting polymers, and nanocomposites were successfully employed as active electrode materials for SCs [9,10,11]. Transition metal (TM)-based compounds are the most common redox-active materials used as electrode materials for SCs, especially Ni, Fe, Co, Ti, Mo, V and Nb in the form of oxides and hydroxides [12, 13]. Because of their exceptional specific capacitance and high conductivity, transition metal oxides may produce large amounts of energy and electricity. Many nanostructured TiO2 materials and their composites are used for SC applications since they are non-toxic, environmentally safe, abundant, and cost-effective. TiO2 has a low specific capacity (160 mA hg− 1 in the actual capacitance, half its theoretical capacity of 335 mA h− 1), low ionic diffusivity, and weak conductivity [14]. Sandwiched between reduced CNT, GO, and other conductive polymers/templates, TiO2 has been demonstrated as an efficient electrode material for supercapacitor applications. The composite forms of TiO2 have excellent conductivity, an approachable interface, and long-lasting chemical stability. Strategies like designing a porous structure and elemental doping are receiving significant attention to improve electrochemical performance further. Porous TiO2 nanoparticles can be fabricated by sol-gel methods, hydrothermal synthesis, ion implantation, and physical vapor deposition, as seen in the previous reports [15,16,17,18]. Therefore, as starting points for the synthesis of different functional materials, metal-organic frameworks, or MOFs, have attracted much interest lately [19, 20]. MOFs are porous materials composed of metal ions or clusters connected by organic ligands.

MOFs can be directly used in supercapacitors since the metal ions present in MOFs have redox activity and can rapidly react with electrolyte ions to store energy. Porous carbon, metal sulfide, and oxides derived from MOFs have been successively utilized as electrode materials in supercapacitor applications, emerging as prominent research focus areas [21,22,23]. The resulting derivatives effectively retain critical features of the original MOF precursors, such as a high specific surface area, substantial pore volume, and distinctive skeleton structure. Among the diverse range of MOF-derived materials, MOF-derived titanium oxide (TiO2) materials offer advantages such as controlled morphology, high surface area, and tunable properties [24, 25]. Doping in titanium dioxide modifies its electronic, optical, and catalytic properties, expanding its range of applications. Doping TiO2 with transition metal ions such as Fe, Co, Ni, and Cu can alter the optical and catalytic properties. For example, transition metal doping can extend the absorption edge of TiO2 to visible light, improving its photocatalytic activity. It can also introduce energy levels within the bandgap, facilitating charge separation and improving the efficiency of photocatalytic reactions [26, 27]. Particularly, Fe doping in TiO2 is crucial for asymmetric supercapacitors as it can enhance the material’s conductivity, leading to improved charge and discharge rates. Additionally, it can increase the specific capacitance of TiO2, enhancing its energy storage capacity [28]. The presence of Fe can also facilitate redox reactions vital for charge storage, contributing to improved electrochemical performance and stability by optimizing asymmetric supercapacitor performance [29].

Thus, our study aims to synthesize porous structured transition metal doped TiO2 nanocrystals through the MOF route and its application potential as electrode material for SC applications. The TiO2 and the doped TiO2 were fabricated through a facile MIL-125 MOF route. The study’s innovation is found in the straightforward fabrication method, which does not require complex equipment, harmful chemicals, or solvents, making it an environmentally friendly process. Moreover, the research stands out for using the MOF route to create different doped TiO2 structures with a porous nature. This approach is relatively unexplored in the development of electrode materials for supercapacitors. The synthesized materials were analyzed by employing various characterization techniques, encompassing structural phase determination (utilizing X-ray diffraction and Raman spectroscopy), surface morphology determination (via high-resolution transmission electron microscopy), chemical state (by X-ray photoelectron spectroscopy), surface area measurement (using BET analysis), and evaluation of electrochemical properties. The improved capacitive performance revealed by the Fe-doped TiO2 electrode can be attributed to the nanoscale architecture facilitated through Metal-Organic Framework (MOF) templating, wherein electrochemically active sites promote efficient electron and ion transport across the structure and thus improved conductivity. These nanostructures would additionally provide short ion diffusion pathways for efficient ion transport.

Experimental work

Chemicals

Titanium (IV) butoxide (97%), N, N-dimethyl formamide (99.8%) Iron (II) chloride hexahydrate (99.99%), Terephthalic acid (98%), and absolute ethanol (98%) were procured from Sigma-Aldrich. All the reagents were used without further filtration, and the synthesis was carried out using deionized water. The solvothermal method was conducted in a BAOSHISHAN 100 ml autoclave with a PTFE liner.

Synthesis of PT and FeT electrode materials via MOF route

The Fe-doped MOF-derived TiO2 was prepared using the procedure described in previous studies. The comprehensive synthesis method is as follows. The MOF-derived TiO2 was synthesized using the solvothermal method in a 100 ml Teflon-lined autoclave. A mixture of 6 ml methanol and 3 g terephthalic acid,1 ml titanium butoxide, and 54 ml N, N-dimethyl methanamide were added dropwise with sustained stirring for 10 min. The reaction mixture was then heat treated at 150 °C for 24 h. The obtained reaction mixture was then washed using methanol, dried in the oven at 80 °C for 12 h, and then annealed at 400 °C for 5 h (at a rate of 2 °C/min). The same procedure was repeated to synthesize doped TiO2 material by adding 50 mg of FeCl2 to the latter reaction mixture. The synthesized materials were successfully designated as PT to represent undoped titanium dioxide and FeT to label titanium dioxide doped with a 50% concentration of iron (Fe), and they were methodically preserved for successive analytical characterizations.

Materials characterization

The synthesized electrode material underwent characterization at ambient temperature through various techniques. Powder X-ray diffraction (XRD) using a Panalytical-Empyrean X-ray diffractometer verified the phase, employing Cu Kα radiation and scanning 10 to 80° diffraction angles. The FEX-u confocal microscope Raman/PL system measured Raman spectra in the wavenumber of 50–800 cm-1 with an excitation wavelength of 532 nm. FESEM and HR-TEM analyses were conducted using the 200KV FEI Tecnai G2 F20 instrument. For TEM, the materials were uniformly dispersed in isopropanol, drop-cast onto a copper grid, and allowed to dry overnight. BET measurements were performed using AUTOSORB-1Q-MP-XR standard adsorption equipment with 99.995% pure N2 gas. An Omicron Nanotechnology X-ray photoelectron spectrometer (XPS) with Al Kα radiation determined surface composition and chemical states. The electrochemical analysis utilized a CHI 660E electrochemical analyzer. Chronopotentiometry (CP/GCD), cyclic voltammetry (CV), and electrochemical impedance analysis provided measurements within a potential window of 0 to 0.6 V. CV characteristics were measured at sweep rates ranging from 1 to 300 mV− 1. Capacitance was calculated using CV and CP-charge/discharge curves. EIS analyses were performed in the frequency range of 100 kHz to 1 Hz.

Fabrication of working electrode material and electrochemical characterizations

The active material, PT, and FeT-coated nickel plate (working electrode) were used for electrochemical characterization. Before characterizing, a 1 × 1 cm2 piece of cut nickel plate was washed for 20 min with 3 mM of HCl using ultrasonication. This method can be used to get rid of the NiO layer on the surface of the nickel plate. The nickel plate was washed with deionized H2O, ethanol, and acetone and then dried in a hot air oven at 70 °C for 6 h. The working electrode was made the same way as described in the earlier literature [30]. The material was mixed with polyvinylidene fluoride (PVF, Sigma-Aldrich) and activated charcoal in a weight ratio of 8:1:1. (Sigma-Aldrich). A few drops of 1-methyl-2-pyrrolidinone were then added to make the mixture thicker (Sigma-Aldrich). The paste was put on the nickel plate so that it was 0.5 mm thick. The nickel (Ni) plate was left out in the air to dry. Then, for 6 h, it was heated to 80 °C. The weight of the electrode material used was found to be between 0.3 mg and 0.5 mg.

The following electrodes were employed to investigate the electrochemical characteristics of synthesized materials. Electrochemical measurements were carried out utilizing a three-electrode system within a CHI 760E workstation comprising a working electrode made of synthesized material, a platinum counter electrode, and a silver and silver chloride reference electrode. A 2 M potassium hydroxide (KOH) solution was used as the electrolytic medium. The following calculations are employed for the electrochemical characterizations.:

  1. i.

    Galvanometric charge-discharge (GCD) was done at 0–0.6 V potentials and current densities of 2–9 Ag-1. The discharge curves of GCD were used to figure out the specific capacitance (Cs, in Fg-1) of the electrode for the MOF-derived PT and FeT electrode materials that were made in a 3-electrode system,

$$Cs = I.\Delta t/\;m\,\;\left( {\Delta V} \right)$$
(1)

where I = discharge current in A, t = discharge time in s, m = total mass in g, and ΔV = potential difference in V.

  1. ii.

    Under an open-circuit potential, electrochemical impedance spectroscopy (EIS) tests were done between 1 Hz and 100 kHz with a 5-mV amplitude of AC perturbation (OCP). Different electrochemical properties, such as series resistance (Rs), charge-transfer resistance (Rct), constant phase element (CDL), and pseudocapacitance (CF), were estimated by fitting the EIS data with the right equivalent circuit.2 through the Metal-Organic Framework (MOF) route

The supplemental information includes a detailed procedure for symmetrically assembling electrodes in a 2-electrode system.

Results and discussion

illustrates the schematic representation of the synthesis method employed for the production of both pristine and Fe-doped TiO2 through the Metal-Organic Framework (MOF) route

Scheme 1
scheme 1

Synthesis of TiO2 and Fe-doped TiO2 via MOF route

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Structural phase analysis

The crystal phase formation of PT and FeT samples and its analysis was carried out using XRD characterization technique. Figure 1(a) depicts the XRD graph of PT and FeT materials recorded at room temperature.

Fig. 1
figure 1

(a) XRD pattern of the synthesized PT and FeT samples. (b) Williamson-Hall plot of the synthesized PT and FeT samples

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The diffraction peaks obtained for the prepared samples correspond to planes indexed in Fig. 1 and matched with the JCPDS No. 021-1272, demonstrating the formation of anatase TiO2 with a tetragonal structure [27]. The sharpness of the peak depicts the crystalline nature of the samples devoid of any impurity peaks. The width of a maximum XRD Bragg’s peak was used to calculate the typical average grain sizes, Davg using the Scherrer formula,

$${D_{avg}} = {\rm{ }}0.9\lambda /\beta cos\theta$$
(2)

Where λ indicates the X-ray wavelength (1.5405 Å), Davg indicates the average grain size, β indicates the full width at half maximum, and θ is the diffraction angle. The Davg of the synthesized PT and FeT electrode materials were calculated, and the value was around 35 nm and 43 nm respectively. This value is comparable to those reported for TiO2 material [31]. From the recorded XRD pattern, it is evident that the width of the diffraction peaks and peak intensity decreased after doping with Fe ion in TiO2, which is associated with the lattice constants and further analyzed by the Williamson-Hall method. The strain and grain size have been calculated using a Williamson-Hall equation as given below,

$$\beta cos\theta {\rm{ }} = {\rm{ }}0.9\lambda /{\rm{ }}{D_{avg}} + {\rm{ }}4\varepsilon sin\theta$$
(3)

Where ε represents the lattice strain, Fig. 1(b) depicts the Williamson-Hall graphs by plotting 4 sinθ vs. cosθ for all significant peaks and using linear best fitting [32]. The slope of the graph represents lattice strain, whereas the reciprocal of the intercept represents the mean grain size. The computed grain sizes of 32 and 38 nm and strains of 6.5 and 6.9 (line-2 m-4 × 10-3) are in good agreement with the values published for TiO2 material. Using grain sizes, the length of dislocation lines per unit volume i.e. dislocation density of the crystal lattice, can be calculated using the following equation,

$$\delta {\rm{ }} = {\rm{ }}1/{\left[ {{D_{avg}}} \right]^2}$$
(4)

The obtained δ (line/m2 × 1015) values are 9.76 and 6.92. The synthesized PT and FeT electrode materials confirm the single-phase formation corresponding to X-ray diffraction analysis.

Raman spectroscopy studies

Raman spectroscopy has been carried out to confirm the informations obtained by XRD analysis and to understand the doping-induced structural changes in the material. Figure 2 shows the Raman spectra of PT and FeT materials. The intense peak at 144 cm− 1 and an intensity peak at 643 cm− 1 denote the symmetric stretching vibrations of O-Ti-O bonds (Eg mode) [33, 34]. Furthermore, symmetric bending of O–Ti–O bonds (B1g) was observed at 399 cm− 1, and anti-symmetric bending vibrations of the O–Ti–O bond (A1g, or B1g) were detected at 518 cm− 1. The Fe-doped samples demonstrated approximately the same bands as the PT sample, indicating that the Fe ions were placed perfectly into the crystal structure, replacing some Ti 4+ ions. Thus, it is concluded that the pure anatase phase of the TiO2 material is preserved after doping with Fe, which coincides with the XRD data [35].

Fig. 2
figure 2

Raman graph for the synthesized PT and FeT samples over the 50–800 cm–1 range

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Morphological analysis

The elemental analysis and FESEM micrographs of the as-prepared MOF-derived PT and FeT electrode material are presented in Fig. S1(a-d). The EDS spectra show the existence of Ti, Fe, and O elements, as shown in Fig. S1(a, c). Within the limits of our detector, no further elements were found. This indicates the purity of PT and FeT samples. The undoped PT samples (Fig. S1. (b)) have disc-like structures with sizes ranging from 100 to 400 nm. The surface was found to be rough with porosity due to interconnected crystals. After Fe doping (Fig.S1. (d)), a highly porous cuboid structure was found with increased particle size ranging from 1 to 3 μm.

Further morphological information of the MOF-derived PT and FeT samples was understood by HRTEM analysis Fig. 3).The upper panel Fig. 3(a-c) and lower panel Fig. 3(d-f) in the TEM image shows the MOF-derived PT and FeT samples respectively. The low magnification images in Fig. 3a and d confirm that the morphology varies as the dopant is introduced to the MOF-derived TiO2. From the high magnification images, (Fig. 3b and e), the crystallinity of the NPs is confirmed with d spacing of 3.55 and 3.56 Å. This further indicates the formation of the anatase phase and that the results are in good agreement with the XRD and Raman spectra. The selected area diffraction (SAED) pattern (Fig. 3c, and 3f) shows small bright rings that indicate the polycrystalline nature of the TiO2 particles [36].

Fig. 3
figure 3

HRTEM pictures of the synthesized PT and FeT materials showing the morphological variation (a) and (d); d-spacing (b) and (e) corresponds to the plane (101) of anatase phase TiO2 and SAED pattern (c) and (f)

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X-ray photoelectron spectroscopy (XPS) studies

The effect of the substitution of Fe iron into TiO2 and impurities in the synthesized PT and FeT samples were further confirmed using XPS studies by measuring the survey spectrum. In contrast, the degree of oxygen deficiency, atomic bonding configuration, and oxidation number of each element were analyzed by core level XPS spectra. Figure 4a shows the XPS survey spectrum of PT as well as FeT samples, respectively. The excited photoelectron lines of Ti 2p, Fe 2p, and O 1s are examined within the 0-1350 eV limit. It reveals the existence of Ti, Fe, and O elements in the samples [37]. Additionally, the appearance of carbon, C element is assigned to the carbon double-sided tape used to fix the samples. No other elements have been detected in the PT and FeT samples. Hence, this confirms the integrity of the produced materials, which is further supported by EDS analysis.

Core level Ti 2p Spectra

Figure 4(b, d) displays the high-resolution core-level XPS spectra of Ti 2p in the synthesized PT and FeT samples. It displays two peaks with a binding energy of 458 eV and 464 eV, corresponding to the photoelectron peaks of Ti 2p3/2 and Ti 2p1/2, respectively [38]. Compared to PT, Ti 2p deconvoluted peaks of FeT were slightly shifted to higher energy from 458.3 eV to 458.9 eV [39]. The energy spin-orbital splitting between the binding energies of Ti 2p3/2 and Ti 2p1/2 is 5.7 eV for both samples, which is characteristic of Ti with a + 4 oxidation state in anatase TiO2 with a strong bonding between Ti and O atoms [40]. It can be noticed that binding energies remain the same after Fe doping in the XPS spectra of Ti 2p. This implies that doping does not affect the chemistry of the Ti atoms in the TiO2 samples.

Core level fe 2p spectra

The high-resolution XPS spectra of the Fe 2p peak for the FeT sample are displayed in Fig. 4e. The Fe 2p core level spectrum demonstrates the existence of two characteristics peaks Fe 2p3/2 and 2p1/2 at binding energies of 709.9 eV and 723.4 eV which corresponds to the exchange interaction between the outermost 3d electrons and the remaining 3s electrons of the atom [41]. The spin-orbital splitting energy of 13.5 eV denotes the existence of multiple oxidation states of Fe ions. Additionally, two satellite peaks, S1 (715.1 eV) and S2 (735.2 eV) are observed in the Fe 2p peak [42]. With the existence of huge background in Fe 2p, the analysis of the oxidation of Fe is quite challenging. Upon careful examination, the Fe2p3/2 peak can be resolved into two fine peaks at binding energies of 704.7 and 709.9 eV and Fe 2p1/2 into 723.4 eV and 729.6 eV peaks. This corresponds to the oxidation state of Fe3+ and Fe2+ ions in the FeT sample [43]. This suggested that an oxidation state (Fe3+ and Fe2+) of iron ions within the FeT sample existed in the sample.

Core level O 1s spectra

The high-resolution O 1s core level spectra for PT and FeT samples are shown in Fig. 4(c, d). The existence of asymmetry and widening in the O 1s spectra in both samples can be attributed to oxygen atoms linked to metal ions. The difference in binding energy between FeT O1s and Ti 2p3/2 peaks in 71.2, which is very close to the assigned binding energy of TiO2 (71.5 eV) then TiO (73.4 eV) and Ti2O3 (75 eV). This confirms the formation of TiO2 without any other suboxides [44]. The O1s core level can be deconvoluted into three distinct peaks at 529.4, 531.1, and 532.1 eV for PT and 529, 530.1, and 531.5 eV for the FeT sample. This corresponds to the crystal lattice oxygen, ionically bonded to an oxygen atom and covalently bonded to two metal atoms like Fe-O-Fe or Ti-O-Ti or Ti-O-Fe that creates a TiO2 crystal system [45]. Thus, it is confirmed that the Fe dopant incorporation does not affect the anatase TiO2 structure.

Fig. 4
figure 4

Top-XPS survey spectrum (a), high-resolution spectra of Ti 2p (b) and O 1s (c) of PT sample and Bottom- XPS survey spectrum (a), core level spectra of Ti 2p (d), Fe 2p (e) and O 1s (f) of FeT sample respectively

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Surface area analysis

The N2 adsorption-desorption isotherm graph at room temperature for the synthesized MOF-derived PT and FeT samples is revealed in Fig. S2 (a b). The measured isotherms fall into the H3 category of type IV adsorption-desorption isotherms according to the International Union of Pure and Applied Chemistry (IUPAC) standard. The hysteresis loop found in the wide range P/P0 = 0.2 to 1 confirms the presence of mesopores in both samples [46]. The specific surface area calculated from the volume of absorbate for both materials using the Brunauer-Emmett-Teller (BET) method was 156 for PT and 181 m2g− 1 for FeT. The narrow pore size distribution of the prepared MOF-derived PT and FeT materials is depicted in Fig. S2 b. From Fig. S2 (a, b) most of the pores lie in the range of 5–70 nm for PT and 10–100 nm in diameter. The average pore radius and volume of both materials are calculated, and they are found to be 19.1 nm and 0.25 cm3g− 1 for PT and 22 nm and 0.33 cm3g− 1 for FeT, respectively. Typically, specific pore volume and surface area can significantly enhance the electrode’s specific capacitance by facilitating electrolyte ion insertion and exertion inside the electrode material [47].

Electrochemical measurements

Cyclic voltammetry (CV) analysis

To investigate the effect of the morphology and Fe doping on the electrochemical properties of TiO2 electrodes, a cyclic voltammogram (CV) was recorded over the potential range between 0 and 0.6 V vs. Ag/AgCl in the 2 M of aqueous electrolyte KOH solution (Fig. 5.)Fig. S3 shows the experimental setup for the electrochemical performance of PT and FeT samples. Figure 5a shows the CV plot of FeT and PT (Insert Fig. 5a) at 1 mV− 1. Additionally, the PT and FeT samples’ reaction rate kinetics were verified by running the CV at various sweep rates from 1 to 100 mVs− 1 and 1-300 mVs− 1, respectively (Fig. 5(b, c)). The electrochemical redox reaction mechanism for TiO2 electrode resulting from the surface or subsurface intercalation/deintercalation cation processes in the alkaline electrolyte is represented by the following reactions, [48]

$${\left( {Ti{O_2}} \right)_{surface}} + {\rm{ }}{A^ + }\;\left[ {{e^ - }\neg } \right]{\rm{ }} \to {\rm{ }}{\left( {Ti{O_2}^ - {M^ - }} \right)_{surface}}$$
(5)

Where A is K+ in the electrolyte.

Fig. 5
figure 5

Electrochemical performance of the synthesized MOF-derived PT and FeT samples. (a) CV plot at 1 mVs− 1, (b) PT-CV curves at different sweep rates from 1-100 mV s− 1, (c) FeT-CV curves at various sweep rates from 1-300 mVs− 1

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Charge/discharge response and specific capacitance

Galvanostatic charge-discharge measurements were presented to understand further the supercapacitive performance of the synthesized MOF-derived PT and FeT samples. Figure 6 demonstrates the GCD profiles obtained at different current densities for PT and FeT samples over a potential of 0–0.6 V. Figure 6 clearly shows that the GCD profiles of all fabricated electrodes at different current densities are almost linear, demonstrating the behavior of the electrodes as double-layer charge storage behavior throughout the electrochemical reaction. The FeT electrode has a longer discharge time than PT, indicating a higher energy storage capacity, providing excellent conductive channels for electrolyte ions to penetrate the FeT electrode resulting in a maximum specific capacitance of 925 Fg− 1 at 3 Ag− 1. The decrease in specific capacitance of the electrodes is caused by the electrolyte ions not having enough time to enter the active sites at higher current densities. Also, comparison of the Specific capacitance of synthesized PT and FeT materials with previously reported literature was shown in Table 1. Furthermore, the energy density and power density at the maximum specific capacitance were calculated using standard equations and found to be 46.2 Whkg− 1 and 900 Wkg− 1, respectively [49].

Fig. 6
figure 6

Electrochemical performance GCD investigation of (a) PT and (b) FeT samples at the current densities of 2 to 9 Ag− 1

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The long-term cycling test was analyzed from the galvanostatic charge/discharge technique in 2 M aqueous KOH and shown in Fig. (S4). The areal capacitances of PT and FeT electrodes remain stable within these limits. The electrode retains 60% of the initial SC after 2500 cycles for the PT sample and 75% after 2500 cycles for the FeT sample, which indicates its excellent cycling ability for Fe doped TiO2 sample [50].

Electrochemical impedance spectroscopy (EIS) analysis

Figure 7(a, b) depicts a Nyquist graph for PT and FeT samples based on an EIS analysis of three electrodes’ electrochemical conductivity as a frequency function. The Fig. 7(a, b) illustrates the typical impedance curve at high magnification. The modified electrode’s Nyquist plot contains a semicircle at higher frequencies and a straight line at lower frequencies.

Table 1 Comparison of the specific capacitance of synthesized PT and FeT materials with previously reported literature
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The higher frequency region’s semicircle represents the oxide–electrolyte interface’s resistance. There is resistance contribution from the Faradaic redox process in this region. Warburg impedance, which measures the diffusive resistance of OH ions in the electrode material, is characterized by the lower frequency area. The Nyquist graph reveals a linear line along the y-axis, attributed to Faradaic behavior because of the reversible Faradaic redox reaction [54]. The intercept on the real axis at high frequency represents the equivalent series resistance (ESR), related to the ionic and electronic charge transfer resistance, intrinsic charge transfer resistance, and diffusion, in addition to the contact resistances of active material–current collector. This parameter is important in supercapacitor analysis, specifically within the frequency range where the impedance spectrum intersects the real axis. Samples of PT and FeT have ESR values of 2.5 and 2, respectively. This result shows that PT and FeT electrodes are electrochemically stable and redox processes are feasible [55].

Fig. 7
figure 7

Nyquist graph over the frequency for (a) PT and (b) FeT samples

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Electrochemical performing of symmetric 2-electrode assembly

A two-electrode assembly comprised of PT and FeT electrode materials was created to test the actual performance of the capacitor condition. Figure 8 illustrates the electrochemical characteristic findings of the manufactured symmetrical capacitor. Figure 8a, b shows the CV pattern for the symmetric cell using a 2 M KOH electrolyte that was tested against different sweep speeds in the potential range of 0–1.2 V. The CV plot demonstrates the high-rate capacity of the electro-active material, which shows a gradual increase in the current density as the scan rate increases. The CV graph’s shape can be observed at slower sweep rates, demonstrating the electrode material’s faradic capability. The specific capacitance (SC) of the 2-electrode system was determined via the formula,

$$SC = 4C/q$$
(6)

Where q is the mass of the electrodes and C represents the cell-calculated capacitance. With a sweep rate of 10 mV− 1, the synthesized FeT electrode materials produced capacitance value of 65 Fg− 1 it higher than PT sample. Since the layout of the electrochemical cell significantly impacts the capacitance of PT and FeT electrode materials, the capacitance of the 3-electrode system is greater than that of the 2-electrode system. Figure 8b, c shows the GCD curve obtained for current densities from 1 to 5 Ag− 1. The optimum capacitive behavior of the material is indicated by the charging and discharging curve being drawn as a straight line. The electrochemical impedance spectroscopy shows the low resistive properties of the materials. (Fig. 8e) Because of their lower diffusive resistance, PT and FeT electrode materials show up as a straight line in the low-frequency zone. The series resistance (Rs) 0.36 Ω and charge transfer resistance (Rct) 32.1 Ω for FeT and Rs 0.14 Ω and Rct 78.32 Ω for PT which clearly delivers the charge transfer resistance is very low in FeT sample. Compared to the three-electrode cell, the capacitance values of the symmetric two-electrode assembly cell are lower due to the cell’s design. Figure 8f shows the results of an evaluation of the cycling performance of PT and FeT electrode materials in KOH electrolyte at 5 Ag− 1. The prepared electrodes showed promising retention of 52% and 67% (Fig. 8g) of the original capacitance maintained on 6000 charge-discharge cycles. The reduced cyclic stability of the supercapacitor device fabricated on a nickel plate can be attributed to material degradation, electrochemical instability, and issues at the electrode-electrolyte interface. The self-discharge study was conducted for both samples for 600 s which delivers the FeT device have longer self-discharge properties compared to PT. The PT, FeT device reached 0.1 V and 0.4 V at 200 s, respectively.

Fig. 8
figure 8

Electrochemical properties of a symmetrical 2-electrode PT/FeT cell. (a, b) CV curve with 1–10 mVs− 1, (c, d) GCD curve with 1–5 Ag− 1, (e) EIS graph (inset: equivalent circuit), (f) capacitance retention, and (g) self-discharge study

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Conclusion

In conclusion, this work paves the way for further advancements in the design and synthesis of electrode materials for supercapacitors, with a focus on sustainable and environmentally friendly approaches. The PT and FeT samples were synthesized by a facile and novel MIL-125 MOF route. XRD, Raman, and TEM analysis confirmed the anatase crystal structure for the synthesized doped and undoped samples. The careful examination of samples using FESEM revealed noticeable morphological variations directly resulting from the inclusion of dopant. Raman and XPS spectroscopic studies obtained evidence of oxygen vacancies and trap states. When applied for supercapacitor electrode material, Fe-doped TiO2 demonstrated enhanced capacitive performance attributed to nanoscale architecture facilitated through MOF templating, facilitating promising electron and ion transport. Electrochemical characterizations, comprising cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy, confirm the superior electrochemical performance of Fe-doped TiO2, with a maximum specific capacitance of 925 Fg− 1 at 3 Ag− 1. The study underscores the potential of MOF-derived Fe-doped TiO2 for developing efficient, cost-effective electrodes for flexible asymmetric supercapacitor systems.