Introduction

Electrochemical storage devices can provide a highly effective answer to the growing energy and environmental crisis that society faces today [1,2,3]. Although domestic energy utilization is high due to the increasing per capita consumption of primary energy resources, the industrial segment accounts for a large proportion of the total produced energy of the world, which prompts assertive energy solutions for this sector [4,5,6]. In this scenario, alternative energy sources such as solar, wind, geothermal, and tide energy offer significant advantages, including reduced greenhouse gas emissions, enhanced energy security, and the potential for long-term sustainability [7]. However, fluctuations are inherent to these energy sources due to their intermittent nature, which demands rapid response energy storage solutions [8]. Thus, supercapacitors can leverage the field due to their fast response and high-power abilities, stabilizing the energy supply by enhancing grid stability and maximizing the integration of renewable energy into our energy systems [9, 10].

Although several studies have been conducted on transition metal oxides (such as NiO, Co3O4, and MnO2) as electrodes for supercapacitors due to their high specific capacitance [10, 11], a research segment has been focusing on rare-earth elements. Such compounds tend to offer stable trivalent ions, in which their unpaired 4f electrons typically are not involved in chemical bondings and could represent unique properties for energy storage [12]. Extensive research focuses on rare-earth-based materials, mainly rare-earth oxides, sulfides, hydroxides, and composites. These materials have acquired significant attention in scientific studies due to their unique properties and potential applications in storage systems [13]. Aiming to introduce novel materials or configurations that could potentially incorporate rare-earth elements or their compounds in supercapacitor technology, recent research has been using nanotechnology tools to obtain shape and size control over the materials [14]. Also, their interaction with well-known ions that present storage applications is important, aiming to raise properties not observed before.

Vanadium (V) is an abundant occurring element in the Earth’s crust [15]. Its ability to exist in multiple valence states results in various compounds in nature [16], which make vanadium-based materials suitable for utilization as electrode materials in supercapacitors. Among several vanadium-based compounds, vanadates (VO43−) can undergo easily reversible redox reactions, which means they can efficiently store and release electrical energy through chemical transformations [17]. Gadolinium (Gd), a member of rare-earth elements, is not commonly used as a material for supercapacitor electrodes itself. However, it finds application as a dopant for transition metal oxides or as a component of composites prepared with electroactive polymers [18,19,20]. Also, gadolinium vanadate (GdVO4) is not extensively explored to prepare supercapacitor electrodes.

Herein, we proposed an innovative preparation of an electrode for electrochemical storage comprised of well-defined nanospheres of GdVO4 prepared by a microwave-assisted hydrothermal (MAH) method. The material was fully characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), Raman, and X-ray diffraction (XRD), giving rise to potential properties that match unprecedented performance in the literature using a Gd-based material. The electrode delivered a high specific capacitance of 1203.75 F g−1 at 1 A g−1. An assembled asymmetric supercapacitor was also performed, reaching 80.63 F g−1 at 1 A g−1 and 130.2 Wh kg−1 of energy density when the power density was 2880.18 W kg−1.

Experimental section

Sample preparation

GdVO4 nanospheres were synthesized using Gd2O3 (99.99%, Sigma-Aldrich), NH4VO3 (99.9%, Sigma-Aldrich), and HNO3 (65%, Synth) as precursors. The solutions were prepared as follows: Firstly, Gd(NO3)3 solution was prepared by dissolving Gd2O3 in concentrated HNO3 solution under magnetic stirring at 60 °C. Separately, 1.0 mmol of the Gd(NO3)3 was added to 45 mL of deionized water at 90 °C under stirring. Similarly, NH4VO3 was dissolved in 45 mL of deionized water at 90 °C under stirring. Then, the two precursors’ solutions were mixed under constant stirring for 30 min. This procedure promoted the co-precipitation of the amorphous GdVO4 in the reactional mixture. This mixture was transferred to a 100-mL Teflon autoclave, reaching 90% of its total volume, allowing the maximum pressure efficiency. The autoclave was finally sealed and placed in the MAH system using 2.45 GHz microwave radiation with a maximum power of 800 W. The reactional mixture was heated at 160 °C under a nominal heating rate of 140°/min (at 800 W) by direct interaction of water molecules with microwave radiation and remained under a constant pressure of 2.5 bar for 32 min. After that, the autoclave was naturally cooled to room temperature. Finally, the solid product was water washed several times until neutral pH; then, it was dried at 80 °C for 12 h.

Measurements and characterization

The nanocrystals were structurally characterized by X-ray diffraction (XRD) using a D/Max-2000PC Rigaku (Japan) diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range from 10° to 80° at a scanning speed of 2°/min in the normal routine. The average crystallite size ‘dDS’ was calculated using the Debye–Scherrer relation [Eq. (1)]:

$$d_{{{\text{DS}}}} = \frac{0.9 \lambda }{{\beta \cos \theta }}$$
(1)

where ‘β’ is the full width at half maxima (FWHM = 1.518 rad) in radians, ‘2θ’ is the diffraction angle in degrees (24.79°), and ‘λ’ is the wavelength (1.543 Å) of the X-rays employed.

Micro-Raman spectroscopy was conducted on a Horiba Jobin–Yvon (Japan) spectrometer equipped with a charge-coupled device (CCD) detector and argon-ion laser (Melles Griot, USA) operating at 514.5 nm and a maximum power of 200 mW. X-ray photoelectron spectroscopy (XPS) was performed using a ScientaOmicron ESCA + spectrometer with a high-performance hemispheric analyzer (EA 125) with monochromatic Al Kα (hν = 1486.6 eV) radiation as the excitation source. The operating pressure in the ultrahigh vacuum chamber (UHV) was 2 × 10–9 mbar during analysis. The survey and high-resolution XPS spectra used energy steps of 50 and 20 eV, respectively. The morphology and sizes of the nanocrystals were observed on a field emission scanning electron microscope (FE-SEM) model Inspect F50 (FEI Company, Hillsboro, OR) operating at 5 kV.

Electrochemical studies

The electrochemical tests were conducted using an Autolab PGSTAT302N potentiostat (Metrohm, The Netherlands) and the NOVA 2.1.4 software for data analysis. A three-electrode electrochemical cell configuration was employed, consisting of an Ag/AgCl reference electrode, a platinum wire counter electrode, and a working electrode comprising a nickel foam coated with the active material (A = 1.5 cm2). To prepare the ink for the working electrode, the following procedure was followed: a weight ratio of 8:1:1 for the active material, carbon black, and a solution of PVDF (Poly(vinylidene fluoride), Sigma-Aldrich) in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) was measured using a Kern 410 analytical balance. Subsequently, this mixture was homogenized for 10 min in an agate mortar. Then, 20 µL of the resulting ink was deposited onto the nickel foam. The electrode was then dried for 4 h at 60 °C and pressed onto a nickel rod.

The electrochemical studies were performed in KOH 2.0 M electrolyte solution. Cyclic voltammetry (CV) was tested at various scan rates: 5, 10, 20, 40, and 80 mV s−1 at a potential range of 0.2–0.5 V. The galvanostatic charge–discharge (GCD) studies were performed at current densities range from 1 to 15 A g−1. To evaluate the stability of the material, a 3100-cycle test was performed with electrochemical impedance spectroscopy studies (EIS) in a range of 0.1–1000 Hz.

The specific capacitance (F g−1) was calculated from the GCD curves using Eq. (2):

$${\text{Cs}} = \frac{I \times \Delta t}{{\Delta V \times m}} {\text{F}}\;{\text{g}}^{ - 1}$$
(2)

where Cs is specific capacitance, I is the current, Δt in (s) is the discharge time, ΔV in voltage represents the potential drop, and m is the mass of the electrode material in mg.

To calculate the Coulombic efficiency (η), Eq. (3) was used [21]:

$$\eta = { }\frac{{t_{{\text{d}}} }}{{t_{{\text{c}}} }}{ } \times { }100{\text{ \% }}$$
(3)

where td is the discharging time and tc is the charging time.

The asymmetric cell was performed in a configuration of two electrodes, one working electrode containing the active material and the other with activated carbon. The preparation of both electrodes followed the same procedure mentioned above. First, the electrochemical tests were performed with the carbon electrode in the three electrodes’ configuration to obtain its current and specific capacitance values. These values were used with those of the active material to calculate the required amount of activated carbon for the electrode to be used in the asymmetric cell tests, following the mass/charge, according to Eq. (4) [21]:

$$\frac{m + }{{m - }} = \frac{Q + }{{Q - }} = \frac{{m \times {\text{Cs}} \times \Delta V}}{{m \times {\text{Cs}} \times \Delta V}}$$
(4)

where m+− and Q+− mean the mass/charge balance, m is the mass (mg), Cs is specific capacitance, and ΔV in voltage represents the potential drop of activated carbon and the active material.

The energy density (ED) was calculated using Eq. (5) [22]:

$${\text{ED}} = \frac{1}{2} \cdot C_{{\text{s}}} \left( {\Delta V} \right)^{2}$$
(5)

where Cs is the specific capacitance (F g−1) obtained in the GCD tests for the asymmetric supercapacitor and ∆V (V) is the potential range between the cathode and anode.

The power density (PD) was obtained with Eq. (6) [22]:

$${\text{PD}} = 3600 \times \frac{{{\text{ED}}}}{\Delta t}$$
(6)

where ∆t (s) is the time of discharge of the device. All the tests were carried out at room temperature.

Results and discussion

Structural and morphological characterizations

Figure 1 shows the XRD pattern of the GdVO4 nanospheres. The material presents a tetragonal system with a zircon-type structure and I41/amd space group, corresponding to the lanthanide vanadates (LnVO4) structure. The reference pattern of pure GdVO4 (JCPDS card No. 17-0260) was perfectly indexed to the experimental phase obtained, and the characteristic peak broadening observed was caused by the nanometric size of the crystallites [23, 24]. The most intense reflection peak observed at 24.9° corresponds to the (200) plane in the tetragonal structure. The average crystallite size ‘dDS’ was estimated to be around 94 nm, similar to the reported ones [24,25,26].

Figure 1
figure 1

XRD diffraction pattern of GdVO4 nanospheres.

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The structural order in the short range for the GdVO4 nanospheres was determined by Raman spectroscopy. GdVO4 crystallizes in the zircon-type structure with space group I41/amd. According to factor group analysis, there are 12 Raman active modes at the center of the Brillouin zone, as stated by the following irreducible representation: Γ = 2A1g + 4B1g + B2g + 5Eg. The modes observed in the region 260–1000 cm−1 are internal stretching and bending vibrations of the VO4 tetrahedra, and the modes observed at 119, 154, and 231 cm−1 are external ones [27]. Figure 2 shows the Raman spectra of the GdVO4 nanospheres excited at 514.5 nm. Intense and defined nine Raman modes are found at 119, 154, 231, 262, 378, 476, 791, 804, and 866 cm−1, which can be assigned to the B1g, Eg, Eg, B2g, A1g, B1g, B1g, Eg, and A1g modes, respectively. They agree with the reported literature and confirm the structural short-range order of the nanospheres [28].

Figure 2
figure 2

Raman spectra of the GdVO4 nanospheres.

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XPS analysis identified the chemical composition, binding energy, and oxidation state of elements on the GdVO4 nanospheres’ surface. The binding energies were obtained by calibrating the spectra through the C 1s peak at 285 eV. Figure 3A shows the survey XPS spectra of the sample, and it identified Gd 4d, V 2p, and O 1s orbitals. The C 1s contamination peak observed can be ascribed to adventitious hydrocarbon from the XPS instrument.

Figure 3
figure 3

XPS spectra of the GdVO4 nanospheres. a survey; b Gd 4d; c V 2p; and d O 1s.

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A high-resolution XPS spectrum of each element was carried out to analyze the materials further. The number of appropriate components was added for each spectrum, and a Shirley-type background was used for the fitting. The Gd 4d high-resolution XPS spectrum is shown in Fig. 3B. The spectrum presents two peaks at binding energy (BE) of 142.1 and 146.9 eV due to Gd 4d5/2 and Gd 4d3/2 orbitals, which can be ascribed to the Gd3+ valence state [29, 30]. The V 2p high-resolution XPS spectrum is shown in Fig. 3c. Except for the vanadium oxides, V metal gives asymmetric peak shapes in the V 2p region. V 2p peak has significantly split spin–orbit components (Δmetal=7.6 eV), and the splitting Δ-value varies with the chemical state (ΔV2O5 = 7.4 eV). Also, FWHM for each spin–orbit component is the same, but for V 2p, the V 2p1/2 component is much broader than the V 2p3/2 peak; as a result, the V 2p1/2 peak is much shorter than expected. The spectrum was fitted with two components at BE of 517.2 and 524.6 eV, corresponding to V 2p3/2 and V 2p1/2,, respectively, which can be attributed to the V5+ oxidation state [30]. Also, there is no multiplet splitting upon deconvolution, as V+5 does not present any unpaired electrons. Consequently, these results attest that V+5 has not decayed, and other species (V+4, V+3, etc.) are absent due to symmetrical peak fit [31]. The O 1s high-resolution XPS spectrum is presented in Fig. 3D, which was deconvoluted into three peaks. The 530 and 531.4 eV peaks correspond to V–O and Gd–O bonds in a tetragonal lattice of GdVO4, respectively. The 533.1 eV peak can be ascribed to hydroxyl groups/surface-adsorbed oxygen [32,33,34]. These findings confirmed the successful formation of GdVO4 nanospheres and their surface elucidation.

FE-SEM images of GdVO4 nanospheres are shown in Fig. 4. The images show regular spherical nanoparticles in the 70–100 nm range. Despite the occurrence of some agglomeration resulting from the sample preparation process, it is evident that the particles exhibit distinct and well-defined spherical morphology. This characteristic is advantageous for our applications, as alternative morphologies could compromise the material’s performance. Moreover, the nanospheres demonstrate a uniform surface, as anticipated, owing to the favorable influence of the pressure and temperature employed in the MAH synthesis procedure, which promotes phase purity.

Figure 4
figure 4

FE-SEM images of the GdVO4 nanospheres with different magnifications: A 50 k and B 150 k.

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Electrochemical studies

CV and GCD tests were performed to evaluate the energy storage properties of the prepared GdVO4 nanopheres (Fig. 5). The CV curves at different scan rates (5–80 mVs−1) are shown in Fig. 5A, demonstrating pseudocapacitive characteristics with distinct redox peaks within 0–0.5 V [35]. It is possible to observe the typical shift on the oxidation peaks for higher potentials with the scan rate increase, followed by reduction peaks shifted to lower potentials. Considering the crystal structure of the material elucidated by XRD and the species revealed by XPS, it is believed that the charge transfer reaction occurs at the surface of the material, which could be represented as:

$${\text{GdVO}}_{4} + x{\text{K}}^{ + } + x{\text{e}}^{ - } {\text{ }} \to {\text{K}}_{x} \left( {{\text{GdVO}}_{4} } \right)$$

where x is the mole fraction of intercalated K+ ions. It is hypothesized that the presence of VO43− components in the crystal contributes more significantly to electron transfer, owing to their high conductivity, possessing the ability to enhance the electrode capacitance substantially. Even though the shape of CV curves is not rectangular at any scan rate, this finding can be ascribed to the electrolyte effect and some possible contributions of the structure shape. In a parallel study, Fend et al. synthesized egg-shaped GdVO4 nanospheres using a 1 M H2SO4 electrolyte. Their research observed no detectable redox reaction, further reinforcing our discussion [28].

Figure 5
figure 5

a CV curves for GdVO4 nanospheres at different scan rates; b redox peak current versus square root of scan rates; c GCD profiles for GdVO4 nanospheres at 1, 2, 5, 10, and 15 A g−1; d specific capacitances versus current densities. All the tests were carried out in a 2.0 M KOH solution.

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The redox peak currents versus the square root of the scan rates were plotted to evaluate the charge transfer behavior, according to Fig. 5B. A well-fitted linear regression with R2 of 0.9985 and 0.9962 for the anodic and cathodic peaks, respectively, shows a linearity that confirms that the electrodes possess the pseudo-capacitor energy storage mechanism [35, 36]. The GCD tests were performed at current densities range from 1 to 20 A g−1 (Fig. 5C). All graphs have a plateau, consistent with the CV curves. The values of specific capacitance, based on the discharge curves and calculated using Eq. (2), were 1203.75 F g−1, 813.33 F g−1, 542.85 F g−1, 314.28 F g−1 and 214.28 F g−1 at the current density of 1 A g−1, 2 A g−1, 5 A g−1, 10 A g−1 and 15 A g−1, respectively. The Coulombic efficiencies were calculated with Eq. (3): 93.15% (1 A g−1), 95.61% (2 A g−1), 97.44% (5 A g−1), 100% (10 A g−1) and 100% (15 A g−1).

Subsequently, Fig. 5D illustrates the relationship between specific capacitance and current density, allowing for an assessment of the rate capability. As anticipated by prior studies, the specific capacitance values decreased as the current density increased [35, 36]. This observation can be attributed to reduced ion diffusion and decreased ion adsorption on the electrode, particularly at the outer surface of the pores. However, ions have sufficient time to access the electrode’s inner and outer surfaces at lower current densities, leading to a higher specific capacitance [21]. The specific capacitance of the GdVO4 nanospheres at 1 A g−1 demonstrates excellent performance compared to previous studies, as indicated in Table 1, where it is possible to notice that the morphology and synthesis method influence the supercapacitor performance.

Table 1 Comparison of specific capacitance (Cs) of GdVO4 nanospheres electrode with previously reported materials
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To further investigate the electrochemical performance of the GdVO4 nanospheres for supercapacitors, cycling stability was evaluated by performing 3000 charge–discharge cycles at 20 A g−1 (Fig. 6). Figure 6A reveals a progressive increase in its specific capacitance during the initial 1000 cycles, reaching 133 F g−1; this phenomenon can be attributed to the activation of the electrode material.[37]. Subsequently, a plateau was attained, succeeded by variations in specific capacitance, which decreased by 20%. Later, in the concluding cycles covering from 2350 to 3100, the system once again achieved a specific capacitance of 133 F g−1.

Figure 6
figure 6

a Specific capacitances versus number of cycles at 12 A g−1 and b EIS analysis in as-prepared material and after 500 charge–discharge cycles of the prepared GdVO4 nanospheres. Tests were performed in 2.0 M KOH solution.

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To assess the resistance of GdVO4 nanospheres during cycling, EIS was conducted on the as-prepared material and after 500 cycles. Figure 6B illustrates the corresponding Nyquist plots, in which a perfect line parallel to the imaginary axis could not be seen, indicating that a constant phase element (CPE) would be needed in the system. Before utilizing the material, we observed that the data tended to have a higher imaginary part, indicating a high capacitive contribution. After the cycling, the slope for this is equivalent to 0.5, suggesting that a mass transport limitation takes place.

To study the practical implementation of GdVO4 nanospheres, an asymmetric supercapacitor cell (ASC) was assembled using it as the anode and activated carbon as the cathode. Upon substituting the values acquired from the prior GCD analysis into Eq. (4) for the three-electrode system, in conjunction with the outcomes from the activated carbon, a mass balance ratio of 10 was computed, leading to the preparation of the new activated carbon.

The results of the electrochemical tests are presented in Fig. 7. The rectangle-like CV curves (Fig. 7A) indicate the contribution of the pseudocapacitance mechanism for energy storage. An operating voltage window of − 0.2 to 1.6 V was obtained. The shape of the CV curves was maintained at high scan rates, showing a high-rate capability [21, 36, 41].

Figure 7
figure 7

a Cyclic voltammetry curves at different scan rates for the hybrid system and b charge–discharge profiles at 1, 2, 5, 8, 10, 15, and 20 A g−1. c Specific capacitances versus current densities. d stability after 3000 cycles at 20 A g−1. Tests were performed in 2.0 M KOH solution.

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From the ASC (Fig. 7B), the specific capacitances were calculated using Eq. (2): 80.63, 70, 50, 40, 37.5, 37.5, and 25 F g−1 at the current densities of 1, 2, 5, 8,10,15, 20 A g−1, respectively. Figure 7C shows that the ASC kept rate performance upon increasing the current density, especially from 8 to 15 A g−1. The Coulomb efficiencies were 95.56% (1 A g−1), 94.92% (2 A g−1), 97.06 (5 A g−1), 94.44 (8 A g−1), and 100.00% (10.15 and 20 A g−1). The ASC led to a high energy density value of 130.2 Wh kg−1 at a power density of 2880.18 W kg−1(at 1 A g−1 in 2.0 M KOH). These are outstanding results compared to the current literature, according to Table 2. The GdVO4 nanospheres/activated carbon was subjected to cycling at 20 A g−1 to evaluate its stability (Fig. 7D). Similar to the three-electrode system, the specific capacitance of this system experiences an initial increase in the first 1000 cycles, peaking at 142 F g−1. Subsequently, it exhibits two plateaus: one at 129 F g−1 spanning cycles 1200 to 1950 and another at 116 F g−1 between cycles 1951 and 3100.

Table 2 Performance of the GdVO4/activated carbon ASC compared with the literature for similar systems
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Conclusions

This study highlights the potential of electrochemical storage electrodes, being a promising solution to address the current energy and environmental challenges. It also introduces a new electrode material (based on GdVO4 nanospheres) synthesized using a microwave-assisted hydrothermal method, which is vital in preparing nickel foam-based electrodes. A thorough characterization of the material revealed its outstanding properties for energy storage applications. The electrode exhibited a high specific capacitance of 1203.75 F g−1 at 1 A g−1 and demonstrated good cycling stability after 500 cycles. Furthermore, an asymmetric supercapacitor constructed using the GdVO4 electrode achieved a specific capacitance of 80.63 F g−1 at 1 A g−1 and an energy density of 130.2 Wh kg−1, with a power density of 2880.18 W kg−1. These findings highlight the potential of rare-earth-based materials, particularly GdVO4, as a valuable electrode for energy storage applications. Overall, this study contributes to understanding rare-earth-based materials and their potential implications in advancing storage technologies.

Data and code availability

Available under request.