Effect of the molar concentration ratio of copper cobalt phosphate in supercapacitor application

Abstract

Balancing the content ratio of materials in an application is challenging. A small mistake could lead to current leakage, unintended harm, reduced longevity, and various other possible concerns. This study aims to evaluate the significance of copper cobalt phosphate (CCP) by utilising them in particular applications with appropriate ratios to enhance their effectiveness. The optimal concentration of Cu and Co is crucial for stability, reducing deterioration, and ensuring a longer cycle life. Low concentrations lead to insufficient ion availability and high concentrations reduce effective charge carriers. The synthesised composite displayed unique morphologies, such as nano-cubical rods and flower-like nanoflakes, indicating its diverse structural characteristics. These features contribute to enhanced electrochemical performance by providing a large number of active sites. Electrochemical characterisation was performed using a three-electrode system. Specifically, CCP at a specific concentration exhibited optimal performance, demonstrating a specific capacitance of 134.5 F g⁻¹ at a current density of 1 mA cm⁻² and a power density of 1.3 kW kg⁻¹. Furthermore, it showcased an energy density of 15.79 W h kg⁻¹ with better stability. The flower-like CCP//rGO solid hybrid supercapacitor (HSc) device exhibits a capacitance of 52.5 F g⁻¹ at a scan rate of 10 mV s⁻¹ within a 1.4 V potential window, demonstrating excellent cycling stability with 77.66% retention after 2000 cycles. These findings underscore the potential of copper cobalt phosphate nanocomposites as concentration variation yields promising electrode materials for supercapacitor applications, owing to their superior electrochemical performance.

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1. Introduction

Currently, the world's increasing population and consumption heavily rely on fossil fuels as the primary source for producing electrical energy. However, the use of fossil fuels has a detrimental impact on both the environment and the economy. As a result, the world is shifting towards renewable energy resources such as solar, wind, hydraulic energy, etc.^1,2 to reduce environmental pollution and produce cost-effective electric energy. Recently, there has been a significant demand for non-exhaustive renewable energy sources.³ Energy storage devices are now a crucial component of renewable energy.^4,5 Therefore, researchers and manufacturers are interested in creating renewable energy storage devices with high performance. Energy storage devices are classified into four systems: batteries, capacitors, fuel cells, and supercapacitors (SCs).^6,7 Batteries store electrical energy in chemical form and release energy by converting chemical energy through redox reactions.⁸ Batteries and fuel cells both produce electrical energy in similar ways.⁹ However, due to the occurrence of chemical processes, batteries produce high energy densities but low power densities.¹⁰ These days, SCs are promising materials that can be used in many energy storage systems due to their superior power density and storage qualities, which include higher capacitance, environmental friendliness, and adjustable working temperature.¹¹ In addition, SCs have special qualities, are simple to use, require little maintenance, and offer excellent energy storage performance at a reasonable price with long-term stability.^12,13 Electrochemical storage systems (SCs) are categorized into three main types: electric double-layer capacitors (EDLCs), pseudo-capacitors, and hybrid supercapacitors.^14,15 EDLCs store charges electrostatically or non-radically,^16–18 while pseudo-capacitors use metal oxides and conducting polymers for high electrochemical performance.^19,20 Hybrid supercapacitors store charges both electrostatically and electrochemically, with symmetric or asymmetric configurations depending on material assembly,^21–23 each offering distinct charge storage mechanisms. However, optimising their performance requires innovative electrode materials with superior structural and electrochemical properties.

In the past few years, enormous reserves of transition metal phosphates have been discovered, and the synthesis and application of transition metal phosphates with optical potential, electrical, magnetic, catalytic, and other properties have been hot issues in recent years.^24–26 TMPs are inexpensive and simple to prepare. Furthermore, transition metal phosphates show increased electrochemical activity, which suggests that they could replace noble metals.²⁷ Besides, transition metal phosphates have been created for the photocatalytic degradation of water-soluble organic dyes because of their stability and lack of toxicity.²⁸ Due to their unique advantages, such as providing numerous active sites for reactions and enhancing interfacial charge transport by minimizing diffusion path lengths within the structure,²⁹ metal phosphates exhibit significant potential. The strong P–O covalent bonds confer high chemical stability, making these materials structurally robust. As a result, metal phosphates with diverse structural morphologies play a crucial role in various device applications. Consequently, cobalt-based materials (e.g., cobalt oxide, cobalt phosphate) exhibit pseudocapacitive behavior due to their ability to undergo reversible redox reactions. And copper enhances the electrochemical activity of composite materials, particularly when combined with cobalt.

The combination of cobalt and copper in materials like cobalt copper phosphate (CCP) introduces multiple oxidation states, enabling a broader range of redox reactions for charge storage.³⁰

Researchers have utilized various metal phosphates in this regard with different content ratios. Meshal Alzaid and colleagues produced Cu–CoMnP by an easy sonochemical method, demonstrating 340 C g⁻¹ at a current density of 0.5 A g⁻¹ with copper and cobalt (50/50) at the same molar concentration.⁴² Here, a balanced concentration enhances ion mobility. Several researchers have documented the findings displayed in Table 1 for improved capacitive behavior using various deposition methods.

Table 1 Recent research on cobalt copper phosphate for supercapacitor application with a substrate and different chemical methods

Sample	Specific capacitance (Fg^−1)	Method	Energy density (W h kg^−1)	Power density (kW kg^−1)	Stability	Ref.
Co3(PO4)2/20 mg GF//ppAC	149	Co-precipitation	52	847 at 1 A g^−1	80%	31
					10 000
Zn0.50Co00.50Mn(PO4)2	1704.21 at 1.2 A g^−1	Sonochemical	45.45	4250 at 5 A g^−1	93% after 1500 cycles	32
Cobalt manganese phosphate	128 at 1 A g^−1	Hydrothermal	45.7	1650	84% after 6000 cycles	33
Hydrous nickel phosphate thin film	1700 (0.5 mA cm^−2)	SILAR	40.37	1689	96.5% after 5000 cycles	34
Ni1.38Co1.62(PO4)2	1116 at 0.5 A g^−1	Chemical bath deposition	47	468 at 0.5 A g^−1	83.7% after 4000 cycles	35
Ni2P/Co2(P2O7)//activated carbon	4052 at 1 A g^−1	Solvothermal	57.7	800	82.8% after 5500 cycles	36
Cobalt phosphate (Co2P2O7)	266 at 5 mV s^−1	Hydrothermal method	83.16	9350	84% for 5000 cycles	37
Nickel–cobalt phosphate	2228 at 1.5 A g^−1	Potentiostatic electrodeposition	65.7	2200	89%	38
85% Co3(PO4)2 and 15% rGO	593.2 at 2.0 A g^−1	Sonochemical approach	68.06	7650	98.03% after 2000 GCD cycles	39
NiCo–P/CNT/CW	1657.3 at 10 mA cm^−2	Electrochemical deposition	12.1	3950	92.4% after 10 000 cycles	40
MXene/NiCoP	490.8 at 1 A g^−1	Electrodeposition	21.13	2250	94.12% after 5000 cycles	41
CuCoP	134.5 at 0.2 A g^−1	Hydrothermal	15.79	1300	88.78% after 3000 cycles

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Thus, achieving a suitable equilibrium of content in an application is a challenging endeavor. A minor error can result in current leakage, undesired damage, diminished lifespan, and other potential issues. In the present analysis, a one-step hydrothermal technique was used to create transition-metal phosphate-based electrode materials with a flower-like structure using different content ratios. The concentration of these elements can influence the electrochemical properties of the material, such as conductivity, specific surface area, and active sites. The micromorphology increases the ion transport rate and electrode–electrolyte contact surface. The use of cobalt copper phosphate as a cathode material leverages the pseudocapacitive redox properties of cobalt and the electrical conductivity of copper. SEM and TEM reveal a nanoflower morphology with high porosity, critical for enhancing ion diffusion. The optimal Cu and Co ratio of sample S4 improves electrochemical activity. The CCP electrode has a high specific capacity and good cycling stability. The study reveals that varying the concentrations of these materials can impact key performance metrics of supercapacitors.

2. Experimental details

2.1 Chemicals

Chemicals including cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O), and potassium dihydrogen orthophosphate (KH₂OPO₄) were purchased from SDFCL, and urea (NH₂·CO·NH₂) was purchased from Molychem. Graphite powder (100 microns) was purchased from SDFCL. The substrate used was stainless steel (SS) 304 grade, with dimensions of 4 cm × 1 cm, and thickness was 0.05 mm. Distilled water was used for all experimental steps, including different electrochemical methods.

2.2 Fabrication of flower-like CCP

This involves direct growth of the CuCo(PO₄) named CCP nanomaterial on an SS substrate. Usually, before working with SS substrates, it is necessary to polish them well with fine-grade paper, then rinse them multiple times using acetone and double-distilled water (DDW). Following this, SS underwent ultrasonic cleaning with DDW and acetone for a duration of 20 minutes prior to synthesis. First, a solution containing 6 mM cobalt nitrate hexahydrate and 4 mM copper nitrate trihydrate was made in 25 ml of distilled water and stirred continuously for 30 minutes at 400 rpm. Further, 5.0 mM urea was added to the same solution to obtain a completely transparent solution. Also, 3.0 mM potassium dihydrogen orthophosphate was added to 25 ml of distilled water and a transparent solution was observed. Then the whole solution was added to the main beaker and stirred for half an hour. After that, the mixture was poured into a Teflon autoclave and SS plates were arranged diagonally inside, followed by a 12 hour hydrothermal treatment at 120 °C. The same procedure was applied, but with different concentrations of Co : Cu in 0 : 10, 2 : 8, 4 : 6, 6 : 4, 8 : 2, and 10 : 0 content ratios named S1, S2, S3, S4, S5, and S6 shown in Fig. 1. Here, the optimal material was rinsed with distilled water after which the gathered product was allowed to air dry at room temperature. To enhance the active sites, all of the resultant films underwent a 2 hour annealing process at 300 °C. Furthermore, the samples were investigated for structural and electrochemical analysis.

Fig. 1 Synthesis of CCP samples using a hydrothermal method.

2.3 Characterization

The study used various techniques to analyze the structure and chemical composition of a CCP nanomaterial. X-ray diffraction (XRD) with a CuKα source, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) (JEOL IT3000 at 20 kV), transmission electron microscopy (TEM), selected area electron diffraction (SAED) with an FEI Tecnai G2 F20 X-TWIN, X-ray photoelectron spectroscopy (XPS), and electrochemical (EC) measurements were used to confirm the material's properties. In EC measurement, the synthesized CCP was used as the working electrode, Ag/AgCl as a reference electrode, and a platinum (Pt) wire as a counter electrode.

3. Results and discussion

3.1 Structural properties

XRD patterns were recorded to study the crystallographic nanostructure, formation of the material, and orientation of the plane for all prepared samples^43,44. Fig. 2a shows the XRD patterns of the CCP electrodes indicating a prominent diffraction peak. The resulting scattering angle orientations of copper phosphate of sample S1 at 2θ angles were 15.2, 18.0, 23.4, 30.4, 33.7, 37.0, 44.6, 56.7, and 63.4 with Miller indices (100), (110), (−102), (021), (121), (−122), (014), (−402), and (314) respectively. These data were compared with standard data (X-Pert High Scorecard 96-100-8248)⁴⁵ shown in Fig. 2a. Furthermore, data for the cobalt phosphate of sample S6 was compared with X-Pert High Scorecard 96-152-9154, confirming the prepared sample's existence. The XRD pattern revealed distinct peaks corresponding to the crystallographic planes of the two phases present in the binary composition shown in Fig. 2b for sample S4. The positions and intensities of these peaks were compared to standard reference patterns of X-Pert High Scorecard 01-082-1304, which confirmed the presence of copper and cobalt phosphate in the nanocomposite of sample S4. The crystallite size for all samples was calculated using the Scherrer formula at lattice parameter (−122) and Bragg's angle of 37.0°. The calculated crystallite size is 44.6, 51.4, 56.1, 59.8, 60.9, and 67.8 nm for S1, S2, S3, S4, S5, and S6 respectively. In this case, it is evident that as the amount of cobalt increases and the amount of copper decreases in the CCP electrode, the size of the crystallites also increases due to the disparity in atomic radius between the two elements. In addition, the peak intensity at various 2θ angles decreases with increasing concentration of Co indicating Co–Cu aggregation.^46,47

Fig. 2 (a) XRD of the different molar concentrations. (b) Elaborated XRD image of S1 (pure copper), S6 (pure cobalt), and S4 (CCP) in the range 33°–37°.

3.2 FTIR spectroscopy

FTIR spectroscopy was utilized for functional group analysis, revealing variations in peak strength on SS-based CCP composition at the 400–2000 cm⁻¹ wavenumber (Fig. 3).⁴⁸ The CCP electrode reveals several characteristic absorption bands corresponding to various vibrational modes. The strong bands observed at 1020 cm⁻¹ and 1056 cm⁻¹ in samples S1 and S6, respectively, are attributed to the asymmetric stretching vibrations denoting the phosphate group (*P–O) bonding shown in Fig. 3.⁴⁹ Bands approximately in sample S6 at 936 cm⁻¹ and sample S1 at 950 cm⁻¹ are ascribed to the symmetric stretching mode of the phosphate group (*P–O).⁵⁰ The vibrations in the region of 650–600 cm⁻¹ are linked with the bending vibrations of the P–O linkage (+P–O) in S1 and S6. The presence of Cu–O and Co–O bonds is indicated in the region of 600–400 cm⁻¹.^51,52 These bands provide evidence of the successful incorporation of copper and cobalt into the phosphate structure. The O–H bending vibrations ranging from 1850–2000 cm⁻¹ are assigned to water molecules.⁵³ The distinct vibrational bands corresponding to phosphate groups and metal–oxygen bonds indicate a well-defined phase composition, crucial for consistent electrochemical performance. The presence of hydroxyl groups suggests potential surface modifications, which can influence the interaction with electrolytes in supercapacitor applications. Understanding these structural features is essential for optimizing the material for supercapacitor applications, where efficient energy storage and stability are paramount.

Fig. 3 FTIR spectra of S1 to S6 CCP samples where *P–O and +P–O indicate stretching and bending vibrations of the phosphate group.

3.3 Scanning electron microscopy (SEM)

SEM was used to analyze the surface morphology of the material, with characteristic images displayed in Fig. 4 for the as-synthesized CCP samples. The highly agglomerated nano-bundle of CCP was seen to have a rectangular rod-like morphology in the SEM micrographs of S1 and S2 ^54,55. However, the SEM analysis shows micron-sized flower-like morphology made by a nano-cubical rod of the as-prepared CCP having a size of 5 μm at 10 000× magnification for S3 & S5 samples. More horizontal flakes or platelets are grown in the S4 sample, where cubical rods are transformed into tightly packed asymmetrical nanoflowers. The surface morphology of all samples is made up of densely packed nanoflakes that resemble 3D morphology advantageous for effective electron transport and provide better stability than others.⁵⁶ At 10 000× magnification, the rough surface of the flower-petal is seen which is favorable for supercapacitor application. Additionally, the S5 sample showed morphology resembling a nanosphere. However, when copper content tends to zero, all the flowers suddenly collapse into petals separately, as seen in S6 .⁵⁷ This indicates that Cu causes certain particles to form elongated clusters consisting of smaller petals grouped into larger agglomerates enhancing the material's active surface area for supercapacitor application. As seen in Fig. 5a–e, the EDS mapping of SEM reveals a uniform dispersion of Co, Cu, and P in the composite material, confirming the successful preparation method for CCP composite materials.

Fig. 4 SEM images of different molar concentrations for all the samples.

Fig. 5 (a) Composition of S4 (b) cobalt, (c) copper and (d) phosphate. (e) EDS mapping and elemental distribution for the S4 optimized sample.

3.4 XPS analysis

XPS was used to analyze the chemical bonding of CCP. The survey spectra (Fig. 6a) confirm the presence of Co, Cu, O, and P elements, aligning with the earlier findings from EDS analysis. Fig. 6 illustrates the high-resolution XPS spectra for Co 2p, Cu 2p, P 2p, and O 1s, respectively. In Fig. 6b, the binding energies of Co 2p show two peaks at 783 eV and 797 eV, corresponding to Co 2p_3/2 and Co 2p_1/2, respectively.^58,59 The binding energies for Cu 2p_3/2 are observed at 933.7 eV and 955.5 eV (Fig. 6c). The two distinctive peaks of P 2p are displayed in Fig. 6d at binding energies of 133.4 and 134.4 eV, which correlate to the phosphorus atoms' 2p_3/2 and 2p_1/2 core levels in the phosphate group. The high-resolution peak obtained at 285 eV of C 1s is due to metal phosphate bonding and surface-absorbed oxygen linked to the peak at 531.8 respectively. Oxygen vacancy which functions as donor defects and promotes electron hopping between Cu²⁺, Cu⁺, Co³⁺, and Co²⁺ ions is responsible for this increased electrical conductivity (Fig. 6d). Thus, XPS analysis verified the successful synthesis of CCP, with the results showing strong consistency with the EDS and XRD analyses.

Fig. 6 XPS analysis of the S4 sample: (a) survey of CCP, (b) Co 2p scan, (c) Cu 2p scan and (d) P 2p scan.

3.5 TEM analysis

Further, to study the surface morphology and microstructure of resultant S4 electrodes, the active materials were scraped from the stainless steel (SS) current collector for TEM analysis shown in Fig. 7a–d. Fig. 7a and b clearly showed the spike rod composite which is consistent with SEM sample S4 at 10 000× magnification and the particle size was also smaller. Fig. 7c and d indicate a polycrystalline nature with the apparent lattice fringes of 2.2 nm corresponding to the CCP (300) crystal face. Fig. 7c presents the SAED patterns, which are indexed to a monoclinic CCP structure, consistent with the XRD results based on the Xpert high score data (01-082-1304). Here, the boundaries of fringes show distortion caused by lattice mismatch with binary metals mixed with the phosphate group.

Fig. 7 Sample S4 data (a and b), TEM images at various magnifications, (c) SAED pattern, and (d) line pattern.

4. Electrochemical analysis

4.1 CV analysis

CV was used to observe the electrochemical performance of a CCP electrode in a three-electrode cell assembly, at 10 to 100 mV s⁻¹ sweep rate in 1 M KOH with a suitable potential window (–0.4 to 0.8 V) shown in Fig. 8a. It is observed that increasing the scan rate increases the area under the curve with a slight shift in anodic and cathodic peaks indicating a diffusion-controlled reaction including faradaic behavior. Additionally, when Co was incorporated in Cu in different proportions (S2 to S5), the redox peaks of Co (Co²⁺/Co³⁺) shift to lower potentials indicating easier redox transitions and improved catalytic performance.

Fig. 8 (a) Cyclic voltammetry of CCP with varying concentration, (b) CV curves of S1 to S6 CCP samples at 100 mV s⁻¹ and (c) capacitance vs. scan rate graph based on CV data.

Here, the specific capacitance (F g⁻¹) at any particular sweep rate is determined by eqn (1).

where C_s, ν, Δv and m correspond to the specific capacitance, potential sweep rate, potential window and the active mass on the electrode material. Meanwhile, ∫I dv, the integrated part, is the active area under the CV graph. The S4 electrode has greater area under the CV curve in comparison with the other electrodes. The mass loading of CCP was determined gravimetrically and ranged from 0.5 to 1 mg cm⁻². Square electrodes with a 1 cm² active area were prepared. Electrochemical testing was conducted in a three-electrode configuration, using 1 M KOH as the electrolyte, with a Pt wire as the counter electrode, Ag/AgCl as the reference electrode and the prepared material as the working electrode. The mass of the active material was determined by weighing the substrate before and after deposition using an analytical balance. Further, the specific capacitance of electrodes S1, S2, S3, S4, S5, and S6 was calculated at 100 mV s⁻¹, respectively shown in Fig. 8b. The best electrochemical performance compared to other electrodes with a specific capacitance of 498.1 F g⁻¹ was found in sample S4 at a 10 mV s⁻¹ scan rate. As shown in Fig. 8c, the specific capacitance decreases as the sweep rate increases. The S4 electrode shows the maximum specific capacitance for all sweep rates indicating that its sites are more active for ion diffusion.

4.2 GCD study

The electrochemical performance was investigated using GCD at 0.2, 0.4, 0.6, 0.8, and 1 mA cm⁻² current densities within the potential range −0.2 V to 0.45 V in 1 M KOH electrolyte shown in Fig. 9a. All of the samples indicate asymmetric charge–discharge curves resulting in a rapid faradaic reaction, which is responsible for the charge-storage phenomenon suitable for supercapacitors. Fig. 9b displays the behavior of all CCP samples at a current density of 1 mA cm⁻², where the S4 sample exhibits a slight, sharp initial IR drop in voltage during both the charging and discharging phases. This is attributed to ohmic losses across the internal resistance (R_i), also known as the cell's equivalent series resistance (ESR).

Fig. 9 (a) GCD profiles of all CCP electrodes at different current densities, (b) GCD at 1 mA cm⁻², (c) Ragone plot of all samples with energy and power densities and (d) efficiency of all samples.

Moreover, Q_s (C g⁻¹) and C_s (F g⁻¹) from GCD characteristic curves are measured using eqn (2)

where V, w, I_d, and T_d stand for the electrodes' respective potential window, active electrode weight, discharge current density, and discharge time. The specific capacity (C g⁻¹) and specific capacitance (F g⁻¹) are represented by Q_s and C_s based on the GCD properties of the material. Fig. 9c displays a Ragone plot of the energy density versus power density graph. The S4 electrode has high energy density than others indicating that it can store a lot of energy in a small amount of mass. The efficiency of the S4 sample is superior to that of other samples at a variety of current densities shown in Fig. 9d. The result speculates the significance of the content ratio.

4.3 EIS investigation

EIS is one of the most crucial instruments for analyzing the electrodes' circuitry parameters. EIS fitting software (such as ZsimpWin, Nova, or EC-Lab) was used to fit the experimental data to the proposed equivalent circuit model. The diffusive and capacitive behaviors of the CCP electrodes were examined in 1 M KOH electrolyte with a frequency range of 0.1 to 1 MHz and the associated Nyquist plots in a 1 : 1 aspect ratio for the x–y axis is shown in Fig. 10. Every sample's Nyquist plot shows a diagonal line in the low-frequency zone and a slight semicircle region in the high and medium-frequency zones. The high-frequency intercept on the real axis (Z′) yielded the equivalent series resistance (R_s), which is connected to the electrolyte resistance and the contact resistance at the electrode/electrolyte interface. The evaluated circuitry parameters are depicted in Table 2. When compared to other samples, S4 exhibits the highest conductivity, as seen in Fig. 10. At higher AC frequencies, all samples create an inductor (L). Inductive behavior was found in all samples as a result of charge buildup on the electrode surface. The charge transfer resistance (R_ct) is responsible for the semi-circle diameter in the high-frequency region, while the Warburg resistance (Z_w) is linked to the diffusion of electrolyte ions in the electrode material and is represented by the slope of the linear curve in the low-frequency region. The low value of R_ct in the S4 electrode disseminates the fast charge transfer rate with better ion transport resulting in a greater diffusion coefficient (D_c) shown in Table 2. All electrodes demonstrate capacitive behavior featuring no noticeable semicircle and a nearly parallel increase of the impedance along the imaginary axis.⁶⁰ The flower-like nanoflakes have high electrical conductivity, which improves rate performance and helps to minimize the equivalent series resistance of CCP. Deriving the equivalent circuit helps in quantifying the resistive and capacitive properties of the material. The effectiveness of Faraday reactions is further increased by the smaller particle size, which offers a quick pathway for the intercalation or delamination of electrolyte ions.⁶¹ This outcome is in line with the GCD and CV findings.

Fig. 10 Experimental Nyquist plots of CCP samples; inset shows the equivalent circuit diagram.

Table 2 Impedance circuitry simulated values of CCP samples

Sample	L (H)	R s (Ω)	C dl (F)	R ct (Ω)	Q (CPE) (Ω)	R 1 (Ω)	C dl (F)	R 2 (Ω)	W (Ω)	D c (cm^2 s^−1)
S1	1.65 × 10^−6	5.932	5.00 × 10^1	40.98	3.05 × 10^−5	1.05 × 10^4	3.96 × 10^−5	0.01	3.22 × 10^−5	1.12 × 10^−11
S2	1.42 × 10^−6	5.64	3.81 × 10^−5	4.241	1.86 × 10^−4	4.405	4.14 × 10^−6	1170	2.39 × 10^−4	8.53 × 10^−11
S3	1.51 × 10^−6	6.294	4.07 × 10^−5	3.902	4.01 × 10^−4	267.7	3.07 × 10^−5	458.6	2.7 × 10^−4	13.9 × 10^−11
S4	1.67 × 10^−6	5.314	3.92 × 10^−5	2.745	2.24 × 10^−4	580.6	1.34 × 10^−5	1255	1.8 × 10^−4	12.5 × 10^−10
S5	1.73 × 10^−6	6.55	3.19 × 10^−5	0.277	1.95 × 10^−4	2781	8.48 × 10^−4	2211	11.9 × 10^−4	1.39 × 10^−10
S6	1.65 × 10^−6	3.456	4.81 × 10^−5	3.373	1.67 × 10^−4	6768	6.05 × 10^−4	1544	3.71 × 10^−4	5.53 × 10^−11

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4.4 Electrochemical stability

Electrochemical stability is a critical parameter for evaluating the performance of CCP as an electrode material in supercapacitors or other energy storage devices. It refers to the ability of the material to maintain its structural integrity, performance, and capacity over prolonged electrochemical cycling and under various electrochemical conditions. Copper can provide good electrical conductivity, while cobalt often contributes to better redox stability. The stability is assessed by measuring the specific capacitance over many cycles. Further, we have checked the stability of the S4 electrode using GCD tests up to 3000 cycles with 88.78% capacitive retention at 10 mA cm⁻² current density in 1 M KOH electrolyte shown in Fig. 11. It can be seen that the ESR is almost constant after 3000 cycles, indicating higher ion diffusion and high mobility. In the S4 sample, copper incorporation can improve the structural integrity of cobalt phosphate by strengthening the material and reducing degradation during electrochemical cycling. This makes the material more stable and durable, allowing it to maintain high performance over a longer period.

Fig. 11 Stability study of the S4 sample using GCD at 10 mA cm⁻² current density for 3000 cycles in 1 M KOH electrolyte.

5. Solid hybrid supercapacitor (HSc) device

Reduced graphene oxide (rGO) and optimized S4 flower-like CCP nanomaterial electrodes were used as an anode and cathode to produce the solid HSc device, with the flexible polyvinyl alcohol (PVA) serving as a separator. The potential windows of the optimized S4 flower-like CCP (–0.4 to 0.8 V) sample and rGO (–1.0 to 0 V) are presented in Fig. 12a, and the maximum working potential window of the hybrid device was chosen to be between 0 and 1.6 V. The influence of sweep rate on the CV curves is displayed in Fig. 12b, which also reveals the pseudocapacitive and diffusion-controlled characteristics of the device. The HSc operates at different potentials between 1.4 and 1.5 V, as shown in Fig. 12c. At a 10 mV s⁻¹ sweep rate, the maximum SC value is 52.5 F g⁻¹. The GCD of the HSc device is displayed in Fig. 12d at different current densities between 2.0 and 1.5 mA cm⁻². In this case, it was noted that the device discharged quicker than it took to charge. This suggests that notable device performance was attained even with the reduced current density. The HSc has a maximum ED of 11.92 W h kg⁻¹ and a maximum PD of 0.4 kW kg⁻¹. The decrease in specific capacitance, energy density, and power density in the device compared to the pure CCP material arises from the combined effects of increased internal resistance, reduced surface area accessibility, electrolyte penetration issues, device configuration limitations, and potential material degradation. These factors hinder the full utilization of the pure material's electrochemical properties when integrated into a complete supercapacitor device.

Fig. 12 (a) rGO and CCP CV curves at 100 mV s⁻¹, (b) solid HSc device CV curves at different scan rates, (c) solid HSc device CV curves at different potentials, (d) solid HSc device GCD curves at varied current densities, and (e and f) charged solid HSc device connected to an LED glowing at different times.

Additionally, the LED light was illuminated utilizing the SAsHSc device for up to 10 minutes, demonstrating greater and comparable performance than the previous findings.⁵²Fig. 12e and f display the LED lighting images in real time. Additionally, the device's charging and discharging performance was examined at 100 mV s⁻¹ sweep rates inside the 1.4 V potential window for a maximum of 2000 cycles. After that, the area under the CV curves gently decreases with a capacitive retention of 77.66% for up to 2000 cycles.

6. Conclusions

This study underscores the importance of copper cobalt phosphate (CCP) in enhancing supercapacitor performance by optimizing the content ratios of its components. Utilizing a simple and cost-effective hydrothermal method, hierarchical CCP nanoflakes were successfully synthesized for application in low-cost supercapacitors. Various characterization techniques, including thin-film XRD, SEM, FTIR, EDS, and XPS, confirmed the formation and phase purity of the synthesized CCP nanoflakes. Specifically, cubical rod-like flowers with unique morphologies were produced by adjusting the concentrations of copper and cobalt. These concentrations play a critical role in tuning the electrode material's structure and performance, where an optimal Cu/Co ratio leads to elongated nanorods forming a distinctive flower-like nanoflake morphology, attributed to the excessive surface energy during formation. Electrochemical tests revealed that CCP exhibits pseudocapacitive behavior. After 3000 cycles, the CCP electrode achieved a specific capacitance of 134.5 F g⁻¹ at a current density of 0.2 A g⁻¹ in a 1 M KOH electrolyte. Additionally, the electrode demonstrated a maximum energy density of 15.79 W h kg⁻¹ and a power density of 1.3 kW kg⁻¹, while retaining 88.78% of its cyclic stability. In the solid hybrid supercapacitor (HSc) configuration, the flower-like CCP//rGO device delivered a capacitance of 52.5 F g⁻¹ at 10 mV s⁻¹ in a 1.4 V potential window, achieving a maximum energy density of 11.92 W h kg⁻¹ and a power density of 0.4 kW kg⁻¹. The study highlights that varying the copper and cobalt concentrations is an effective strategy to optimize redox reactions, thereby enhancing energy storage capacity. Increasing cobalt content while reducing copper content results in a well-defined 3D nanoflake structure, facilitated by phosphate linkages. This structural refinement improves key performance parameters of supercapacitors, as demonstrated by the enhanced energy density and power density. Moreover, the synergy between copper and cobalt ions within the phosphate framework boosts electrocatalytic activity by balancing redox-active sites and improving electron transfer. This balance not only makes the CCP material more suitable for energy storage but also opens avenues for its use in applications like water splitting.

Data availability

Data will be made available on request.

Author contributions

Conceptualization, methodology, investigation, data curation, and manuscript writing were performed by Mr Pratik S. Sutar. Formal analysis, validation, and writing – original draft preparation were performed by Ms Puja R. Deshmukh. Data collection and formal analysis were performed by Mr Amar L. Jadhav. Writing – review & editing, and supervision were performed by Prof. Anamika V. Kadam. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by DST-SERB, New Delhi, with major sponsoring of the project [grant number (EEQ/2021/000633)] and The Institute of Science, Fort, Mumbai, MH, India. The authors are thankful to the Department of Science and Technology, India, under the DST-FIST (SR/FST/PSI173/2012) program. A part of reported work (characterization) was carried out at the IITBNF, IITB under INUP-i2i which is sponsered by Meity, Government of India.

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