One-pot green synthesis of hierarchically porous ZnCr₂O₄ spinel for enhanced charge storage

Abstract

The increasing worldwide energy crisis and the pressing necessity to have sustainable energy management systems have heightened studies in the development of advanced energy storage systems. The novelty of this work is the development of a one-pot green hydrothermal route to prepare hierarchically porous ZnCr₂O₄ using renewable phytochemical constituents to assist and control the formation of the material for sustainable energy-storage applications. The enhanced electrochemical performance originates from the synergistic effect of nanoscale crystallinity, porous morphology, and fast ion/electron transport kinetics. Structural analysis through XRD validates the creation of pure cubic spinel ZnCr₂O₄ with an average crystallite size of about 20 nm indicating a well-defined crystallinity. Raman spectroscopy also confirms the integrity of the structure by characteristic vibrational modes of the spinel lattice. The analysis of the SEM indicates that it has a hierarchically porous and agglomerated nanostructure, which is advantageous to the electrochemical activity, and the distribution of Zn, Cr, and O is homogenized without any impurities, as EDX and elemental mapping show. Electrochemical analysis shows that there is high pseudocapacitive behavior with mixed charge storage behavior through analysis of CV. GCD results show that the specific capacitance of GCD is high, 233.8 F g⁻¹ at 0.8 A g⁻¹, and that it has reasonable rate capability at elevated current densities. EIS data show that there is a low charge transfer resistance (R_ct = 2.53 Ω) and increased ion diffusion coefficient (∼9.11 × 10⁻⁹ cm² s⁻¹) which demonstrates good electrochemical kinetics. The synergistic effect of nanoscale crystallinity, porous morphology, and green synthesis approach is credited to the improved performance. The present paper has identified the promise of ZnCr₂O₄ as an efficient and sustainable electrode material with great potential in future applications in next-generation energy storage systems and hybrid device architectures.

Lemon-peel-extract-assisted hydrothermal synthesis produces hierarchical porous ZnCr₂O₄ nanostructures with a specific capacitance of 233.8 F g⁻¹ for supercapacitor applications.

1. Introduction

The energy crisis has become acute in the world due to the rapid depletion of fossil fuel reserves, the rising consumption of energy in the world and the issue of environment.¹ Traditional energy sources do not only lead to greenhouse gas emissions but also it faces limitations to sustainability.² The move to more renewable energy sources including solar and wind energy has proved to be a promising solution but due to the intermittency of these sources, energy storage technologies have to be efficient. In this regard, high energy storage mechanisms are needed to overcome the energy generation and consumption gap.³ Consequently, the challenge of coming up with high-performance, sustainable and low cost energy storage materials has become a research imperative. To overcome these challenges, energy storage devices, such as batteries and supercapacitors have been thoroughly investigated.⁴

In this context, advanced electrochemical energy-storage technologies play a key role in renewable-energy integration and development of sustainable energy system; Mohammadi Moradian et al. pointed out that lithium-based batteries have become the central player in renewable-energy system and properties such as safety, degradation and long-term performance are challenging.⁵ Ali et al. showed that the electrode performance in rechargeable Zn-ion batteries is heavily influenced by the electronic conductivity, diffusion pathways, structural stability, redox activity, and surface/interface properties.⁶ These consideration highlight the need to develop sustainable materials for rechargeable electrode to provide efficient charge transport and stable electrochemical performance. Faradaic redox reactions in batteries are usually associated with a high energy density, whereas supercapacitors have a high power density and long cycle life due to rapid charge/discharge reactions. Battery-type materials, pseudocapacitors or electric double-layer capacitors (EDLCs) are considered to be the broad categories of energy storage systems based on their charge storage mechanisms.⁷ Hybrid systems combining capacitive and faradaic behavior have attracted considerable interest amongst these since they can combine the benefits of both batteries and supercapacitors.⁸

Transition metal oxides have been studied extensively as electrode materials in energy storage by virtue of their multi oxidation states and extensive redox chemistry.^9,10 Other materials, including MnO₂, NiO, Co₃O₄ and Fe-based oxides have also been shown to exhibit excellent electrochemical behavior, but they are generally characterized by low electrical conductivity, structural instability and low rate capability.¹¹ Different systems of transition metals based on Zr have been extensively investigated and shown to perform well in terms of electrochemical characteristics to be used in energy storage. Ibrahim A. Nikola et al. reported Zr-MOF/GO nanocomposite with 2520 F g⁻¹ and increased cycling stability.¹² Yuhan Cui et al. prepared GO/MXene-supported NiZrP with a high capacitance and retention of 83.3% after 10 000 cycles.¹³ M. Dhivya Angelin et al. fabricated Zr-doped ZnO with 518 F g⁻¹ and 94% stability.¹⁴ The Pr-doped ZrO₂ composites produced by M. M. Moharam et al. had better capacitance of 933.53 mF cm⁻¹.¹⁵ MOF-derived ZrO₂/C composite with 241.5 F g⁻¹ (∼97) retention was developed by Vishal Shrivastav et al.¹⁶ Spinel type ZnCr₂O₄, in this case, has been considered a possible candidate due to its stable cubic structure, excellent chemical stability, and more than one redox-active site. ZnCr₂O₄ electrodes have a superior electrochemical activity because of the synergistic effect of Zn²⁺ and Cr³⁺. However, there are still constraints like medium conductivity and resistance to ion diffusion. To address these issues, several methods such as elemental doping, heterostructure creation as well as controlled synthesis methods have been used in order to customize its structural and electrochemical characteristics.^17,18

In recent years, green synthesis methods received significant interest because they are environmentally friendly, cost-effective, and do not use toxic substances. Plant-extract-assisted synthesis is one of them, and it has shown to be very effective in fabricating nanoparticles.¹⁹ Lemon peel extract which contains bioactive compounds including citric acid, flavonoid, and polyphenols is a reducing and stabilizing agent in the synthesis.²⁰ It can be seen that lemon peel extract is used to provide naturally occurring source of citric acid, flavonoids and polyphenols which act as a suitable source of coordinating metal ions and aid in its stabilization during the course of precursor preparation, thus decreasing the use of synthetic complexing or stabilizing additives for synthesis, which is chemically functional, renewable, low cost, and available. Green and sustainable synthesis methods with citrus-derived precursors have shown a great potential in the fabrication of advanced materials and electrochemical uses. Kheireddine Mehdi et al. synthesized ZnO nanoparticles by the use of lemon leaf extract and obtained higher capacitance of up to 365 F g⁻¹ and better performance of sensing.²¹ N. Dhivya et al. created lemon peel extract-based Ag₃PO₄/Ag/TiO₂ nanocomposites, and achieved 281.54 F g⁻¹ with ∼97.9 percentage stability up to 10 000 cycles.²² Hosein Banna Motejadded Emrooz et al. synthesized meso-microporous carbon using lemon waste, which resulted in a high surface area (∼2110 m² g⁻¹) and capacitance (254 F g⁻¹) and good cycling performance.²³ Hanaa M. Abuzeid et al. emphasized that plant-based extracts are reducing and stabilizing agents, which allow producing metal oxides in an eco-friendly manner with an improved electrochemical and functional capacity.²⁴ Furthermore, the presence of these organic molecules has the effect of improving superior dispersion and disaggregation, and as a consequence of this, the electrochemical activity of the products that result from disaggregation is also enhanced.²⁵

The novelty of this work is that the developed ZnCr₂O₄ is hierarchically porous, obtained by a one-pot green hydrothermal strategy with a renewable natural citrus fruit extract as a multifunctional synthesis media, which does not require any synthetic additive or complex processing route to achieve the nanostructural crystallinity and the hierarchically porous morphology, and is found to be effective in the charge-storage behavior with a specific capacitance of 233.8 F g⁻¹ at 0.8 A g⁻¹ as well as the improved ion-transfer kinetics. Therefore, in this research, a structure–property relationship between green synthesis chemistry, morphological evolution, and electrochemical performance are established and it may provide a sustainable pathway for the development of the ZnCr₂O₄ electrode.

2. Experimental section

2.1. Materials required

The metal precursors were zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, ≥Sigma-Aldrich) and chromium nitrate nonahydrate (Cr(NO₃)₃·9H₂O, ≥Sigma-Aldrich). Sodium hydroxide (NaOH, ≥98%, Merck) was used as pH controller. Fresh lemon peels were obtained in a local market and used as a natural reducing and stabilizing agent. All experimental procedures were done using deionized (DI) water without additional purification.

2.2. Preparation of lemon peel extract

Lemon peels were carefully washed and dried in the air, and then in the oven at 60 °C until all the moisture was removed. The peels were dried and then ground into a fine powder. The powder (∼ 19 g) was dissolved in 100 mL of DI water and stirred by heating it at 80 °C and stirring it continuously. This mixture was allowed to cool and filtered to give a clear lemon peel extract which was utilized as a green complexing and stabilizing media.²⁶

2.3. Synthesis of ZnCr₂O₄

To synthesize, 0.3 M aqueous solutions of Zn(NO₃)₂·6H₂O (8.92 g) and 0.6 M Cr(NO₃)₃·9H₂O (24.009 g) were prepared in the 100 ml lemon peel extract. It was stirred with a magnet and allowed to mix for 30 min and then dropwise NaOH was added to stabilize the pH (∼10). The homogeneous mixture was placed in a Teflon-lined autoclave and heated at 180 °C to a period of 12 h. The precipitate was centrifuged, rinsed in DI water and ethanol, dried at 80 °C, grind, to powder form and annealed at 500 °C during 3 h (ref. 27) (Fig. 1).

Fig. 1. Synthesis of ZnCr₂O₄ using lemon peel extract.

It is believed that the citric acid and the oxygen-containing functional groups of flavonoids and polyphenols in lemon peel extract interact with the Zn²⁺ and Cr³⁺ ions, by complexation/coordination, during the synthesis and regulate the local availability of the metal ions, thus moderating the nucleation and the subsequent growth of the particles during hydrothermal treatment. The organic molecules may also adsorb at the growing surfaces of the particles and suppresses uncontrolled growth and aggregation; the organic molecules then are removed by subsequent annealing, which promotes the crystallization of the cubic ZnCr₂O₄ spinel phase.

2.4. Material characterizations

X-ray diffraction (XRD, PANalytical X'Pert PRO) was used to analyze the crystal structure with the Cu Kalpha radiation (λ = 1.5406 Å). The Raman spectroscopy was done in Renishaw in Via Raman Microscope.²⁸ Surface morphology was observed with the help of scanning electron microscopy (SEM, JEOL JSM-7600F). These methods verified the purity of the phase, vibrational characteristics, and microstructural characteristics of the synthesized ZnCr₂O₄.²⁹

2.5. Electrode fabrication

The working electrode was prepared by dissolving active ZnCr₂O₄, conductive carbon black and polyvinylidene fluoride (PVDF) binder in weight ratio of 80 : 10 : 10 using N-methyl-2-pyrrolidone (NMP) as solvent. A slurry was applied evenly to nickel foam (1 × 1 cm²) and the sample was dried at 80 °C 12 h. This electrode was then subjected to moderate pressure in order to increase the adhesion and contact between the electrodes and conduct electrochemical measurements³⁰ (Fig. 2).

Fig. 2. Electrode fabrication of ZnCr₂O₄.

2.6. Electrochemical measurement

The electrochemical performance was evaluated using a three-electrode system including ZnCr₂O₄-coated nickel foam as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode in a 1M KOH electrolyte. The CHI660E Electrochemical Workstation was utilised to conduct cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). The capacitive characteristics and charge transport properties were assessed by measurements conducted within an optimised potential window.³¹

3. Result and discussion

3.1. X-ray diffraction (XRD)

XRD is an efficient method that is employed to ascertain the structure of crystalline materials, purity of phases, and the lattice parameters. Fig. 3a and b shows the XRD of the synthesized sample of ZnCr₂O₄. The diffraction peaks observed at 2θ values around ∼30.2°, 35.5°, 37.2°, 43.3°, 47.5°, 53.7°, 57.2°, 62.8°, 66.4°, 71.2°, 73.5°, and 75.8° are indexed to the (220), (311), (222), (400), (331), (422), (511), (440), (531), (620), (533), and (622) crystallographic planes, respectively. These diffraction planes are matched with the usual pattern of ZnCr₂O₄ indicating that the cubic spinel structure with space group Fd3̄m (227) was formed as expected from JCPDS card no. 22-1107.³² The peaks are clear and well defined indicating crystallinity and purity of the phases without the presence of any impurity phase.³³ Analysis of the crystallite size and structural defects was further analyzed with the help of the Debye–Scherrer equation and dislocation line density (DLD). The mean size of the crystallites was estimated at around 20 nm, which showed that the ZnCr₂O₄ formed was in the form of nanocrystallites. Average crystallite size (D) was determined from the Debye–Scherrer equation:³⁴

1

Fig. 3. (a) XRD pattern of ZnCr₂O₄ (b) calculated parameters of ZnCr₂O₄.

In the above equation, K is the shape factor (0.9), λ is the wavelength of Cu Kα radiation (1.5406 Å), β is the full width at half maximum (FWHM) of the diffraction peak in radians and θ is the Bragg diffraction angle. The crystallite size was determined from the most intense (311) diffraction peak and found to be about 20 nm on average. In addition, the dislocation line density (δ) was determined by:³⁴

2

where D is the crystallite size. The resulting dislocation density was found to be 2.51 × 10¹⁵ lines per m² which corresponds to the moderate level of structural defects in the lattice of ZnCr₂O₄. The crystallite size is relatively small and thus it gives a greater surface to volume ratio which are advantageous in electrochemical applications to increase the number of active sites to store the charge.³⁵ Moreover, the dislocation line density (2.51 × 10¹⁵ lines per m²) is calculated, which means that there are lattice defects that may lead to the diffusion of the ions and increase the electrochemical reaction dynamics. The negative correlation between the size of crystallites and DLD confirm that it's a moderate level of defects which is good for the energy storage job. In addition, to determine the interplanar spacing (d-spacing), the XRD peaks were used to determine the interplanar spacing, which was found on average to be 2.47 Å. Effective structure of spinel was confirmed from this d-spacing data which was similar to the typical lattice parameters of cubic ZnCr₂O₄. In order to control the electrochemical behavior, the correct interplanar distance is an important factor which determines the direction of the ion transport. The d-spacing obtained reveals that there are good ionic diffusion paths in the crystal lattice which is important in providing fast charge storage and redox reaction in supercapacitor or battery like application.³⁶

3.2. Raman spectroscopy

Vibrational spectroscopies, of which Raman spectroscopy is a part, are based on inelastic scattering of monochromatic light and provide insights into the lattice dynamics, bonding environment and structural symmetry of materials. Raman spectrum of ZnCr₂O₄ (Fig. 4) has characteristic peaks which are very well defined with the spinel structure intrinsic vibrational modes at the positions of 178, 300, 360, 502 and 650 cm⁻¹. The Raman spectrum of ZnCr₂O₄ (Fig. 4) shows five well-defined characteristic vibrational bands at about 178, 300, 360, 502 and 650 cm⁻¹, which are in agreement with the formation of cubic spinel structure. Raman intensity is affected by various experimental factors such as laser power, acquisition time, crystallite orientation and the scattering efficiency of the sample and it is not necessarily a sign of structural disorder. For the cubic spinel structure, ZnCr₂O₄, five Raman-active modes (A_1g + E_g + 3F_2g) are expected the peak at ∼650 cm⁻¹ is assigned to the A_1g mode that corresponds to the symmetric stretching vibration of oxygen atoms around the tetrahedral Zn²⁺ sites, whereas the bands at ∼502 and ∼360 cm⁻¹ are attributed to F_2g modes associated with metal–oxygen bending and stretching vibrations, respectively. The lower-frequency peaks at ∼300 and ∼178 cm⁻¹ are assigned to the E_g and F_2g lattice vibrational modes related to oxygen displacement and translational motion of spinel framework, respectively. The clear separation of the characteristic modes confirms the preservation of the cubic spinel symmetry.²⁷

Fig. 4. Raman spectrum of ZnCr₂O₄ indicating the vibrational characteristics of the cubic spinel structure.

3.3. Scanning electron microscopy (SEM)

SEM is a surface microscopy that employs a high-energy electron beam to scan morphology, particle size, and microstructure with a high spatial resolution. The SEM images of ZnCr₂O₄ shown in Fig. 5a–c indicate that it has a highly agglomerated, yet hierarchically structured morphology which consists of densely packed nanograins that form micro-clustered assemblies. At low magnification (Fig. 5a), aggregated structures of the material take the shape of caulifower, whereas intermediate magnification (Fig. 5b) between the granular domains reflects the existence of high interaction between particles and their growth. At more magnification (Fig. 5c), the surface can be clearly seen to consist of fine nanocrystallites, indicating the nanoscale nature in agreement with the XRD. This hierarchical structure increases the surface roughness and offers large quantities of electrochemically active locations.³⁵ Moreover, the 3D SEM surface profile (Fig. 5d) shows a high degree of height difference and topography irregularity, which proves the existence of porous channels and topography irregularity. This porosity is very important in the penetration of electrolytes and the effective diffusion of ions during charge–discharge. The particle size distribution (Fig. 5e) with a Gaussian fit shows an average size of the particle is approximately 28.3 µm which shows uniform aggregation behavior. In general, the porous nanostructure enables high ion and electron transfer rates, as well as good electrode–electrolyte contact, thus, ZnCr₂O₄ is a promising material in high-performance electrochemical energy storage applications.³⁷Fig. 5f presents the adsorption–desorption isotherm and pore-size distribution of the investigated electrode material. The BET specific surface area is 44.3 m² g⁻¹, with a pore volume of 0.082 cm³ g⁻¹. The average pore diameter of 7.4 nm indicates a mesoporous contribution to the pore structure.³⁸ The separation between the adsorption and desorption branches and increased uptake at high relative pressures are consistent with pore filling. This porous structure may improve electrolyte penetration and increase the accessibility of electrochemically active sites. These textural characteristics can facilitate ion transport and support charge storage during electrochemical cycling.³⁸

Fig. 5. (a–c) SEM morphology of ZnCr₂O₄ (d) 3D surface topology (e) particle size distribution (f) N₂ adsorption–desorption isotherm of ZnCr₂O₄.

3.4. Transmission electron microscopy (TEM)

The TEM images of synthesized nanostructures of ZnCr₂O₄ are shown in Fig. 6a–d while the HR-TEM images of the nanocrystals are displayed in Fig. 7a–c. The SEM is an advanced characterization technique which can be used to study the lattice structure, crystallinity, particle size and internal morphology of the nanomaterials at the nanoscale level. TEM is very crucial for materials used in supercapacitors because it can offer direct evidence of the dispersion of nanoparticles, the porosity and crystallographic ordering, which play important roles in electron transport and ion diffusion, and accessibility of the electrolyte during electrochemical charge–discharge processes. The TEM images in Fig. 6a and b show that the ZnCr₂O₄ is composed of densely-packed and interconnected nanoparticles, which exhibit a near-spherical shape and exhibit some degree of agglomeration, creating a porous nanostructured network that is favorable for the acceleration of ion migration and the improvement of electroactive surface area. The nanosized particles leads to shorter diffusion path for electrolyte ions that can enhance the capacitance behavior and rate capability. The HR-TEM images in Fig. 6c and d indicate the synthesized ZnCr₂O₄ is highly crystalline with clearly defined and continuous lattice fringes. The distinct lattice structure suggests good electronic conduction and structural stability, critical for long-term cyclic performance in supercapacitor applications. Overall, the TEM results showed that the synthesized ZnCr₂O₄ has good nanoscale structural properties which are suitable for use in high-performance energy storage devices.

Fig. 6. (a) TEM image at 500 nm, (b) TEM image at 300 nm, (c) HR-TEM image at 5 nm displaying crystalline nanoparticle boundaries, and (d) HR-TEM image at 1 nm.

Fig. 7. EDX spectrums of ZnCr₂O₄: (a) elemental spectrum (b) SEM image at 50 µm and (c–e) elemental mapping of Zn, Cr and O.

3.5. Energy-dispersive X-ray spectroscopy (EDX)

Energy-dispersive X-ray spectroscopy (EDX) is an elemental analysis method in association with SEM that determines and measures the chemical composition using characteristic X-ray emissions.³⁹ The EDX spectrum of ZnCr₂O₄ (Fig. 7a) provides a clear indication of the presence of Zn, Cr and O with no additional impurity peaks, which is a clear indication of high phase purity of the material synthesized. The measured percentages of weights (∼19.70% Zn, 49.53% Cr and 30.33% O) are consistent with the stoichiometry of the spinel ZnCr₂O₄ structure. The SEM image on the scale of 50 µm (Fig. 7b) depicts the identical aggregated and granular morphology, which guarantees the compositional analysis is representative of the bulk sample. Moreover, the images of the elemental mapping (Fig. 6c–e) show that the distribution of Zn, Cr and O on the whole surface is uniform, which proves the uniform dispersion of elements. Zn mapping (Fig. 7c) indicates no localized clustering but a uniform distribution, and Cr (Fig. 7d) indicates the same, which shows a uniform distribution, indicating that it has been fully incorporated into the spinel lattice. The formation of metaloxygen bonding in the structure is further confirmed with the use of oxygen mapping (Fig. 7e). The lack of phase separation or elemental segregation is an indication of the efficiency of the green hydrothermal synthesis route. This homogenous distribution of elements is essential in providing uniform electrochemical activity, increased conductivity pathways and redox stability, which make ZnCr₂O₄ suitable.⁴⁰

3.6. Galvanostatic charge–discharge (GCD)

GCD is an electrochemical method to assess the capacity of charge storage, reversibility and rate capability under constant current. The GCD profiles of ZnCr₂O₄ (Fig. 8a) portray quasi-symmetric charge–discharge curves with a little bit of non-linearity, suggesting a mixture of electric double-layer capacitance and faradaic redox action. The discharge time varies systematically with the current density (0.82 A g⁻¹) with the characteristic rate-dependent behavior as a result of restricted ion diffusion at higher currents. The higher discharge time at lower current density (0.8 A g⁻¹) indicates an increased ability to store charge. Fig. 8b shows that the calculated specific capacitance (F g⁻¹) varies between 233.8 and 50.3 F g⁻¹ (0.8 to 2 A g⁻¹) respectively, under current density increases proving good but anticipated rate performance (Table 1). This is due to the fact that there is a lack of exploitation of active material at high current densities.⁴¹ In addition, the Ragone plot (Fig. 8c) shows how the energy density relates to the power density, which proves the energy-power trade-off phenomenon. The ZnCr₂O₄ electrode provides a maximum energy density of about 31.8 Wh kg⁻¹ at lower power density (about 800 W k⁻¹), and 6.8 Wh kg⁻¹ at higher power density (about 1850 W kg⁻¹). This behaviour indicates that, even though the energy density decreases with increasing power density, the material exhibits good performance at high power rates. The various GCD parameters can be inferred that ZnCr₂O₄ is an efficient charge carrier, also the internal resistance and reversible electrochemical behavior suggests that it can be used as a promising electrode material for energy storage.⁴²

Fig. 8. (a) GCD curves of ZnCr₂O₄ (b) capacitance of ZnCr₂O₄ (c) Ragone plot of ZnCr₂O₄.

Table 1. Specific capacitance of ZnCr₂O₄ at 0.8–2 A g⁻¹ current densities.

Current density (A g−1)	Specific capacitance (F g−1)
0.8	233.8
1.0	163.4
1.2	115.3
1.4	97.7
1.6	83.6
1.8	53.6
2.0	50.3

Table 2 summarizes the specific capacitance obtained in the present work (233.8 F g⁻¹ at 0.8 A g⁻¹) which is moderate compared with highly engineered zinc chromate-based electrodes that are supported with conductive materials or have additional electroactive materials, thus, the significance of the present work is not to achieve the highest specific capacitance but to achieve an electrochemically active single-phase ZnCr₂O₄ through a plant-extract-assisted green hydrothermal route, and further electrode engineering may be performed to increase the capacitance.

Table 2. Comparison of present ZnCr₂O₄ study with previous reported ZnCr₂O₄ based electrodes.

Electrode material	Synthesis/modification	Specific capacitance	Key feature	Ref.
ZnCr2O4@CC	Hydrothermal, carbon-cloth supported	374 F g−1 at 1 A g−1	Binder-free supported electrode	43
ZnCr2O4/MnO2	Sonication/composite	1175.35 F g−1	MnO2-containing composite electrode	44
γ-Irradiated ZnCr2O4	Sol–gel auto-combustion + γ-irradiation	79 F g−1	Irradiation-modified ZnCr2O4	45
ZnCr 2 O 4 (present work)	Lemon-peel-assisted green hydrothermal	233.8 F g− 1 at 0.8 A g− 1	Plant-extract-assisted single-phase ZnCr 2 O 4	This work

The specific capacitance of the present ZnCr₂O₄ electrode (233.8 F g⁻¹ at 0.8 A g⁻¹) is not the highest possible for some highly engineered materials based on ZnCr₂O₄, but, considering the sustainability of the synthesis route, the present approach demonstrates a useful balance between electrochemical performance and greener material processing, rather than aim at the highest absolute capacitance. However, further optimization of surface area, porosity, conductivity and composite formation would be necessary to achieve better practical energy-storage performance.

3.7. Cyclic voltammetry (CV)

CV is an electrochemical method to investigate the charge-storage process, redox activity, reversibility and reaction kinetics of electrode materials by measuring current response versus applied potential. Fig. 9a shows the CV curves of ZnCr₂O₄ at 5–50 mV s⁻¹ and show two wide anodic/cathodic humps, indicating that charge storage is controlled by faradaic pseudocapacitive reactions on the surface and not ideal electric double-layers behaviour. As scan rate increases, the covered CV area and the response of the current is increased gradually and the redox characteristics can still be distinguished, which is evidence of the good electrochemical reversibility and rapid reaction kinetics. The small change in peak at high scan rates is due to polarization and internal resistance. The b-value of the relationship:⁴⁶

i = av^b 3

Fig. 9. Cyclic voltammetry (CV) analysis of ZnCr₂O₄: (a) CV curves at different scan rates (b) b-value, (c and d) kinetic analysis, and (e) capacitive vs. diffusion contribution.

Calculated values of b = 0.56 (Fig. 9b) indicating that faradaic contribution with partial capacitive control is dominated by diffusion-assisted charge-storage. The redox-driven nature is further supported by Fig. 9c which shows the evolution of current response over the potential window. The linearity of i/v^1/2versus v^1/2 in Fig. 9d is also in line with the Dunn analysis, which shows the presence of capacitive and diffusion-controlled processes.⁴⁷Fig. 9e indicates that diffusion-controlled contribution is reduced to 34.8% instead of 62.8% with an increment of scan rate of 5 to 50 mV s⁻¹, but the capacitive contribution increases to 65.1% rather than 37.1%. This trend proves that low scan rates allow deeper active sites to be penetrated by the electrolyte ions and high scan rates make charge storage more surface-dominated. The overall results of the CV indicate that ZnCr₂O₄ has positive redox action, reasonable rate capacity, and mixed charge-storage features which can be applied in electrochemical energy-storage.⁴³

The kinetics of ZnCr₂O₄ reaction in 1M KOH is related to the reversible OH⁻ ion adsorption/desorption and redox conversion of the transition-metal active centers at the electrode/electrolyte interface. The general pseudocapacitive reaction can be expressed as follows:ZnCr₂O₄ + OH⁻ + H₂O ⇆ ZnCr₂O₄(OH) + e⁻

More specifically, the electrochemical response is largely governed by chromium-based surface redox transitions:Cr³⁺ + OH⁻ ⇆ Cr⁴⁺OOH + e⁻and possible surface-assisted zinc participation can be written as:Zn²⁺ + OH⁻ ⇆ ZnOOH + e⁻

The current response follows the power–law relation:

i = av^b 3

with i being maximum current, v being scan rate, a and b being adjustable constants, b = 0.5 represents diffusion control, and b = 1 represents capacitive control. The disaggregation of current contributions is explained by the following equation of Dunn:⁴⁷

(V) = k₁v + k₂v^1/2 4

In which k₁v denotes the capacitive contribution and k₂v^1/2 denotes its diffusion-controlled component. Therefore, the electrochemical kinetics of ZnCr₂O₄ is dictated by the coupled surface redox reactions and ion-diffusion, thus the mixed battery-type/capacitative behavior in KOH electrolyte.

3.8. Electrochemical impedance spectroscopy (EIS)

EIS is a frequency-dependent methodology, which is applied to assess charge-transfer behavior, ion diffusion, interfacial resistance as well as overall electrode kinetics in electrochemical systems. The Fig. 10a shows the Nyquist plots of ZnCr₂O₄ before and after the electrochemical test, and the extracted fitting parameters are accompanied in Fig. 10(b).⁴⁵ ZSim software was used to fit the spectra with the equivalent circuit model of R(CR)(CR)W as illustrated in the inset, where R_s is solution resistance, C is interfacial capacitance, R is charge-transfer resistance, and W is the Warburg diffusion element, representing ion diffusion at the electrode/electrolyte interface.^48,49 The value of the intercept on the real axis in the high-frequency region provides the series resistance (R_s) that declines to 1.21 Ω after cycling compared to 1.57 Ω pre cycling, reflecting a better electrolyte/electrode contact and reduced ohmic loss. The depressed semicircle in the intermediate-frequency region is a reflection of the charge-transfer resistance (R_ct) that also drops to 2.53 Ω after testing, which confirms increased faradaic reaction kinetics and improved interfacial electron transfer.⁵⁰

Fig. 10. (a and b) ZnCr₂O₄: (a) Nyquist plots of (EIS) before and after cycling (b) fitted equivalent circuit to extract the parameters.

The inclined tail in the low-frequency regime is related to the Warburg impedance, which displays the diffusion characteristics of the ions of the electrolyte in the porous structure of ZnCr₂O₄. The diffusion coefficient of potassium-ions (D_k⁺) is calculated to have increased by a significant amount of 4.47 × 10⁻⁹ to 9.11 × 10⁻⁹ cm² s⁻¹ after electrochemical cycling indicating that the electrode becomes more accessible to ion transport upon activation. This is enhanced by electrolyte wetting, uncovering more electroactive sites and stabilization of conductive pathways during cycling.⁵¹ The electrode/electrolyte double-layer response is linked to the first element in the circuit, and the second CR element to faradaic charge-transfer processes at the active surface. Comprehensively, the reduced R_s, lower R_ct, and improved D_k⁺ all indicate that ZnCr₂O₄ has good charge-transfer properties and ion-diffusion kinetics, which are very useful in performing effective electrochemical energy-storage.⁵²

4. Conclusions

Overall, a summary of the results is that ZnCr₂O₄ nanostructures could be obtained through a green hydrothermal technique by the use of lemon peel extract as a natural stabilizing and reducing agent, which proved to be an environmentally friendly and sustainable method. Structural analysis was used to verify the existence of pure cubic spinel phase (Fd3̄m) with nanoscale crystallite size (∼20 nm), and Raman measurements were used to confirm the integrity of the lattice. Morphological analysis showed a hierarchically porous and agglomerated structure, which enables plenty of electroactive sites and enables transport of ions easily. The homogeneous distribution of Zn, Cr, and O was confirmed by the elemental analysis with no impurities. The electrochemical studies showed a high pseudocapacitive behaviour with a high specific capacitance of 233.8 F g⁻¹ at 0.8 A g⁻¹ and a low charge transfer resistance (∼2.53 Ω). These results clearly show that there is a close relationship between green synthesis, structural characteristics and electrochemical performance. This contribution opens up new opportunities in creating sustainable electrode materials, and the future perspective in single-phase hybrid energy storage systems and scalable green technology.

Conflicts of interest

All authors have read and approved the manuscript for publication in this journal, and the authors declare that they have no conflict of interest of any kind.

Data availability

The data supporting the findings of this manuscript are available from the corresponding author upon reasonable request.