Supercapacitor Performance and Characterization of La Doped V₂O₅ Nanoparticles

Introduction

Transition metal oxides such as Vanadium Pentoxide (V₂O₅), is a promising electrode material for high-performance supercapacitors due to their exceptional electrochemical properties. V₂O₅ exhibits attractive characteristics but further enhancements are sought to maximize its energy storage capabilities [1-3]. It is observed that doping strategies have gained significant attention and hence the dopant Lanthanum (La) has been chosen. La-based compounds such as La (NO₃)₃ are known for their high specific capacitance and high-power density making them suitable for high-potential electrode materials [4]. However, traditional fabrication methods and the use of binders often involve complex procedures which can hinder

Recently, there has been a growing interest in binder-free electrodes which eliminate the need for additional materials that can compromise performance. This method simplifies fabrication and enhances the electrochemical efficiency of supercapacitors. By focusing on binder-free electrode fabrication and optimization of charge-discharge behaviour and enhancement of the overall energy density and power retention of the supercapacitor devices was studied [5,6]. The insights gained from this work have significant implications for the development of high-performance energy storage solutions for various applications including portable electronics, electric vehicles, and renewable energy systems.

Experimental Methods

Synthesis of La doped V₂O₅ nanoparticles

Pure V₂O₅ nanoparticles was synthesised by using the co-precipitation method.  Initially, 5–10 drops of HCl was added to the 1mol solution of V₂O₅ and stirred for 10–15 minutes using magnetic stirrer.  A precipitation was obtained and it was rinsed twice using DI water and then ethanol.  The rinsed yellowish-white precipitation was dried for 24 hours in room temperature to remove the moisture content.   It was annealed at 600 ℃ for 3 hrs.  The synthesised Pure V₂O₅nanoparticles are collected from the filter paper.  The collected samples were then grinding with a mortar for 10 minutes for further study. The same procedure is followed for La doped V₂O₅ at different wt % such as 3%, 5%, and 7%.

Preparation of pure V₂O₅ and La doped V₂O₅ electrodes.

For the preparation of functional electrode, the pure Vanadium pentoxide nanoparticles, NMP, and polyvinylidene fluoride (PVDF) are mixed at a mass ratio of 16:3:1 to form a homogeneous mixture for the electrochemical experiment.  Subsequently, this substance is applied onto a nickel foam substrate to create a functional electrode. The reference auxiliary electrode consisted of a platinum wire and nickel foam (NF) sheet which was used as the working electrode on to which 7 mg V₂O₅ nano powder was coated. The same procedure was repeated for 5% La doped V₂O₅ nanoparticles also.

The performance of V₂O₅ electrodes is assessed at electrochemical workstation by cyclic voltammetry (CV) analysis.

Characterization Techniques

The surface analysis of the synthesized V₂O₅, and La doped V₂O₅ nanoparticle was Carried out by using Field Emission Scanning Electron Microscopy (FE-SEM, JSM 6500F, JEOL, Tokyo, Japan) equipped with Energy Dispersive Spectroscopy (EDS), The Crystal structures and phases of all the samples were analysed using X-ray diffractometer (XRD, Bruker D₂ phaser, Japan, Cu K_α radiation). The composition Studies of the particles and chemical state were done by FTIR Spectroscopy. UV Vis, NIR spectroscopy study were performed by UV Visible spectrophotometer (JASCD 760). An advanced technology was used to capture the microstructure image of the nanoparticle’s materials (FE-SEM). The electrochemical studies were Carried out by CV, GCD and EIS (Impendence) methods.

Result and Disscusion 

 XRD Analysis

Fig. 1 shows the XRD report of both pure V₂O₅ and La doped V₂O₅ nanoparticles. The results from XRD Studies indicates that the major peaks shifted to higher angles due to the addition of dopants at varying concentrations. Measurements were taken from 10° to 80° at a scanning rate of 0.4° min⁻¹. Notably, La doped V₂O₅ nanoparticles showed a more significant shift at a doping concentration of 5% wt. The most prominent diffraction peaks (010) exhibited a slight right shift toward higher angles compared to pure V₂O₅ nanoparticles, confirming an orthorhombic structure that aligns with the standard reference (JCPDS card No. 00-001-0359) for V₂O₅. This shift in the XRD positions of doped vanadium suggests successful incorporation of La into the host structure at the V⁵⁺ sites, which may be reason for an increase in crystallite size (D). The dominant peak of the nanoparticles is noted at an angle 20.23° (2q), corresponding to the Miller indices (0 1 0).

Using the Debye-Scherrer equation:

where D is the average crystallite size, β is the peak width at half maximum, θ is the diffraction angle and λ is the wavelength of Cu K_α radiation.

The average crystallite size increases when doping concentration was increased. This increase can be attributed to particle aggregation. The average crystallite size for pure V₂O₅ is 71.25 nm, while for 5 % of La doped sample it is 90.68 nm. This result indicates a decrease in full width at half maximum and an increase in intensity of the (2 0 0) plane. This may be due to the similarity in ionic radii between La³⁺ (0.65 Å) and V⁵⁺ (0.54 Å) from previous studies reported in the literature [7].

Figure 1: (a)XRD graph for Pure V2O5 and La doped V2O5 nanoparticles and (b)Right shifting of the prominent peaks in the samples Click here to View Figure

FT-IR Spectra analysis

Fig. 2, shows the FTIR spectra of pure V₂O₅ and La doped V₂O₅ are annealed at 600 °C, in the range 532 -840 cm^-1. The absorption band at 432 cm⁻¹ is attributed to the V-O vibrations which may activate V₂O₅ surface [8,9].

The FTIR spectra peaks of pure V₂O₅ and La doped V₂O₅ nanoparticles exhibit a slight shift in peak towards higher wavenumbers (right side) [10]. This shift indicates improved La efficiency. The shift range is between 555 and 680 cm⁻¹. Absorption bands are interface of La doped V₂O₅ confirming the presence of distinct compounds formed during V-O-V absorption above 600 °C.

FTIR spectra of activated La doped V₂O₅ revealed that the observed bands between 532-617 cm^-1 may be attributed to absorption. The peak originated from 555 cm^-1 is due to species cross-linked at the edges of activated V₂O₅ nanoparticles, whereas the peak at 680 cm^-1 arises from species bound to active V-O-V [5]. The broad band spanning in the range 725 to 840 cm^-1 is attributed to the stretching vibration of La surface on activated La doped V₂O₅.

Figure 2:  FT-IR Spectrum analysis for PureV2O5 and La doped V2O5 nanoparticles. Click here to View Figure

The peaks disappeared FT-IR studied indicates the removal of loose absorption species during the calcination. It was conformed with XRD analysis.

UV-visible Analysis

Strong absorption bands are observed in the range of 420 to 440 nm which is corresponding to the charge transfer band of V₂O₅ [11]. Fig. 3(a) shows the UV-visible absorption spectrum of La doped V₂O₅ sample. The absorption band observed between 600 – 800 nm range, indicates that the d-d transition of the vanadyl group. It evidences the presence of V⁴⁺ ions, in the spectrum. The absence of V⁴⁺ ions suggests that most vanadium species are in the oxidized V⁵⁺ form, although the minor presence of V⁴⁺ ions cannot be ruled out [12].

Figure 3(a): UV visible spectroscopy analysis pure V2O5 and La doped V2O5 nanoparticles and absorption of band gap energy level. Click here to View Figure

The absorption spectrum from the Fig. 3(a) it can be observed that shows a slight shift towards longer wavelengths. This may be due to the incorporation of La doped V₂O₅ lattice significantly and it reduces the band gap. Fig. 3(a) Band gap energies for pure V₂O₅ and La-doped V₂O₅ samples (3%, 5%, and 7% La) are compared in the pie chart, the band gap energy derived from the equation of the Tauc plot for experimentally observed energy levels.

Figure 3(b): Band gap energy of pure V2O5 and La doped V2O5 nanoparticles Click here to View Figure

To determine the band gap energy (E_g) of the samples, using the Tauc plot method was given below.

Where E_g is band gap energy, h is Planck’s constant c is velocity of light, λ is wavelength of maximum absorption. For both direct and indirect transitions, n equals to 1/2 or 2 [13].

The plot in Fig. 3 (b) illustrates the Tauc plot analysis, where the rising area represents the calculated band gap values for the samples. The obtained E_g values (3.28- 3.47 eV) are consistent and the average bandgap energy of La doped V₂O₅ nanoparticles is 3.31 eV.

Photoluminescence (PL) Analysis

Figure 4: Emission for PL spectra for pure V2O5 nanoparticles and La doped V2O5 nanoparticles Click here to View Figure

Fig. 4 presents the photoluminescence (PL) spectra of V₂O₅ nanoparticles. The precures of V₂O₅ nanoparticles exhibits two emission bands: one in the UV region attributed to exciton-exciton collision processes and another in the visible region, arising due to electron-hole recombination within the band gap.

In the visible range, a broad peak is observed around 529 nm and 553 nm. Annealing at 600 °C enhances the intensity of visible PL emission which may be due to single-ionized oxygen vacancies (VO⁺) at 529 nm. Doping of V₂O₅ particles with compounds often leads to the loss of oxygen. This increase in PL emission intensity is strongly associated with a significant rise in band gap energy observed in La doped V₂O₅ nanoparticles at 5% of La doping.

SEM Analysis

The FESEM image of pure V₂O₅ and various percentage of La doped V₂O₅ (3 %,5 % and 7 % by Wt. percentage) are shown in Fig .5 (a-d). From the Fig. it is observed that pure V₂O₅ nanoparticles exhibit nanoflake structure with rough and uniform grain boundary whereas La doped V₂O₅ are formed with uneven surfaces and rough edges.

Figure 5: FESEM image of Pure V2O5 and various concentration of La doped V2O5 nanoparticles Click here to View Figure

From the SEM image, average particles size of V₂O₅ nanoparticles is observed between 41-54 nm and for La doped V₂O₅ the average particle size ranges from 46-101 nm. Morphology studies of pure V₂O₅ and La doped V₂O₅ nanoparticles reveal that 5 % of La doped V₂O₅ nanoparticles shows nanoflakes structure. which significantly enhances supercapacitor property when compared to the other samples (pure V₂O₅ ,3 % La and 7 % La doped V₂O₅ nanoparticles).

It is reported in literature [15] that nanoflakes structure (Fig. 5c) play an important role for enhancing supercapacitor property.

Energy Dispersive Analysis X-ray (EDAX) Studies

Elemental nanoparticles analysis of pure V₂O₅ and various percentage of La doped V₂O₅ nanoparticles were carried out using EDAX. EDAX spectra of pure V₂O₅ and La doped V₂O₅ nanoparticles samples are shown in Fig. 6

In the pure V₂O₅ nanoparticles samples, the chemical compositions of V and C are found to be 77.19 % and 18.31 % Wt. and La doped V₂O₅ nanoparticles samples, the chemical compositions La are found to be 4.50 % Wt. respectively.

Figure 6: EDAX spectra of pureV2O5 and La doped V2O5 nanoparticles Click here to View Figure

Electrochemical studies

 Cyclic voltametric (CV)

Electrochemical performance was investigated using a three-electrode system, with a platinum wire as the counter electrode, Ag/AgCl as the reference electrode, V₂O₅ as the working electrode and 1M KOH as the electrolyte. Fig. 7 (a) and (b) shows the cyclic voltametric (CV) graphs for pure and 5% of La doped V₂O₅ nanoparticle electrodes, recorded within a potential window of -0.7 to 0.6 V at different scan rates ranging from 5 to 100 mV/s.

All samples exhibited oxidation and reduction peaks in the CV plots along with capacitive behaviour indicating pseudo-capacitive nature of the material [16]. The specific capacitance of the material from CV curve was calculated using the following formula,

where A is Area under the curve, M is Mass of the active material, K is Scan rate, and V is Potential window

Figure 7: (a) and (b) Cyclic voltammetry Graph of Pure V2O5 and 5% of La doped V2O5 nanoparticles Click here to View Figure

Specific capacitance values arrived from CV curves were 191.72 and 172.96 F/cm² for pure and 5% of La doped V₂O₅ nanoparticles respectively. Notably, V₂O₅ nanoparticles doped with 5% of La exhibit higher capacitance. CV curves for 5% of La doped V₂O₅ nanoparticles at different scan rates were taken and from the result it is observed that the quasi-rectangular shape of the CV curves is influenced by the scan rate, with specific capacitance increasing as the scan rate rises [17]. This is because the enhanced scan rate improves the utilization of the electrode’s effective area resulting in higher specific capacity.

X-ray diffraction (XRD) and CV result indicate that the specific capacitance of the nanoparticles is primarily influenced by two factors. one is the intensity of the (200) planes of the stable V₂O₅ layered structure and the another is the sample’s surface roughness which may be determined by the doping concentration [18]. Nanoparticles synthesized with 5% La doping exhibited significant surface roughness with predominant (010) planes showing comparatively  high specific capacitance thom other La doped V₂O₅ nanoparticles.

Galvanostatic charge-discharge (GCD)

Galvanostatic charge-discharge (GCD) was employed to investigate the charge storage performance of the material and to understand the sustainability of the La doped V₂O₅ nanoparticles for supercapacitor electrodes.

Figure 8: GCD curves are Pure V2O5 and 5 % of La doped V2O5 nanoparticles. Click here to View Figure

GCD measurement are carried out at different current density in the ranging from 0.5 to 6 A/g in three electrode system with the same potential widow using for CV studies. Fig. 8

The specific capacitance of the material from GCD graph was calculated using the relation.

where I is the cathodic and anodic current, ∆t is the discharge time, M is the mass of the active material and ∆V is the difference in the potential window.

The GCD studies shows the highest specific capacitance of La doped V₂O₅ nanoparticle, which could reach 119.96 F/g at the current density of 0.5 A/g. Additionally, the specific capacitance still retains 11.27 F/g when the current density of 6 A/g.

Impendence analysis

Fig. 9 shows electrochemical behaviour of La doped V₂O₅ nanoparticles on electrode using electrochemical impedance spectroscopy (EIS). This may be used  to measure the internal resistance (R_S) and charge transfer resistance (R_CT).

EIS measurements were conducted at of 0.1 to 10 kHz frequency range with an AC amplitude of 10 mV and an equilibrium duration of 1 second. In EIS, there is no semi-circle was observed in the higher frequency range due to the low charge transfer resistance.

Figure 9: Impendence analysisGraph of Pure V2O5 and 5 % of La doped V2O5 nanoparticles current density Click here to View Figure

Cyclic voltammetry (CV) profiles at different scan rates, along with cyclic stability and resistive behaviour assessments indicate the electrode’s potential for supercapacitor applications showing promising charge storage performance [19].

Figure 10: Cyclic stability of pure V2O5 and 5% La doped V2O5 nanoparticles Click here to View Figure

Minimizing the gaps between nanoparticles could enhance the electrode’s charge storage and conversion efficiency. Fig. 10 shows cyclic stability of pure V₂O₅ and La doped V₂O₅ nanoparticles was conducted for 500 cycles. The observed cyclic stability of pure V₂O₅ is 116% and La doped V₂O₅ nanoparticles is 107 % at the current density of 2A/g. Cyclic stability of La doped V₂O₅ nanoparticles is lower than pure V₂O₅ nanoparticles for supercapacitor application properties.

Conclusion

In this work structural, optical, and electrochemical properties of Lanthanum doped Vanadium Pentoxide (V₂O₅) nanoparticles are discussed. XRD analysis of La doped V₂O₅ NPs shows an orthorhombic structure. FT-IR spectroscopy reveals the presence of alky halide groups contributing to the absorption peak in the visible region. UV-Vis spectroscopy showed an optical band gap energy in the range 3.28-3.47 eV which is consistent with the reported values of La-doped V₂O₅. Photoluminescence (PL) emission spectra showed broad peaks in the visible region with enhanced intensity when the NPs were annealed at 600°C for 3 hr. This may be due to an increase in single-ionized oxygen vacancies (VO⁺). Notably, 5% La doped V₂O₅ nanoparticles exhibited a significant rise in oxygen vacancies which correlates the observed nanoflakes structure revealed by FE-SEM analysis. Electrochemical characterization through Cyclic Voltammetry (CV), Galvanostatic Charge-Discharge (GCD), and Electrochemical Impedance Spectroscopy (EIS) showed an improved supercapacitor performance for La-doped V₂O₅ NPs compared to pure V₂O₅. Specifically, 5% La-doped V₂O₅ NPs exhibited the highest specific capacitance of 119.96 F/g at a current density of 0.5 A/g. These findings highlight the potential of La-doped V₂O₅ NPs as a promising electrode material for high-performance supercapacitor applications.

Acknowledgement

The author would like to thank, (Insert university name and Dept. name) for their guidance and support to complete this article.

Funding Sources

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Conflict of Interest

The authors do not have any conflict of interest.

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