Rapid synthesis of cobalt phosphate microplates for supercapacitors

Abstract

In this study, cobalt phosphate is synthesized using a step hydrothermal approach on a stainless steel substrate. The effect of sodium dodecyl sulfate (SDS) was studied to tune the morphology of the electrode. The structural confirmation of cobalt phosphate was performed using XRD and FT-Raman studies. The SDS concentration influences the microplate's structure were observed through SEM analysis. Further, elemental mapping was carried out using EDS. The optimized electrode shows a specific capacitance of 3.92 F cm⁻² at 20 mA cm⁻². Additionally, an aqueous hybrid device was assembled and tested using the CPS3//AC configuration. The device shows a specific capacitance of 93.96 mF cm⁻² at 5 mA cm⁻². The cyclic stability of the device was tested for up to 5100 cycles, revealing 99.97% capacitive retention. Furthermore, time series analysis is employed to predict and forecast the cyclic stability data. Thus, the synthesized cobalt phosphate electrode can be used in energy storage devices.

---

1. Introduction

The growing need for sustainable and effective energy solutions has attracted considerable attention to the development of high-performance energy storage technologies, such as supercapacitors.¹ Supercapacitors are essential for bridging the gap between batteries and ordinary capacitors because of their high power density, quick charge–discharge rates, and exceptional cycle stability.² Based on their charge storage mechanisms, a variety of electrode materials have been studied: battery-type or prominent redox reactions at a given potential (NiO, Co₃S₄, CoFe₂O₄, and Co₃O₄); pseudocapacitive or surface fast faradaic reactions or multiple redox reactions within a potential range (MnO₂, RuO₂, and Mn₃O₄); and electric double layer or non-faradaic reaction (EDLC; carbon allotropes).^3,4 The distinct electrochemical characteristics, structural stability, and high theoretical capacitance of transition metal phosphates have made them highly desirable as supercapacitor electrode materials.^5–7

In particular, cobalt phosphate [Co₃(PO₄)₂] is an inexpensive and renewable resource with favorable electrochemical characteristics.⁸ The concentration-dependent specific activity of organic surfactants during electrodeposition modifies the deposition morphology. Surfactants exhibit a distinctive capability to self-organize into structures and act as templates during the electrodeposition process. SDS is used to facilitate uniform film formation and adhesion by reducing the surface tension and enhancing the dispersion of cobalt phosphate particles.⁹

There are many advantages to thin film technology, including better conductivity, greater surface area, and the ability to incorporate it into flexible and micro-scale systems.¹⁰ Several techniques, including sol–gel,¹¹ electrochemical deposition,¹² decomposition method,¹³ microwave-assisted method,^14–17 chemical vapor deposition (CVD),¹⁸ and hydrothermal synthesis,^19–22 have been used to synthesize cobalt phosphate electrodes. Iqbal et al.¹² reported the sonochemical synthesis of cobalt phosphate. The highest electrochemical performance was revealed by the sonochemically synthesized cobalt phosphate, which reached a maximum specific capacity of 285 C g⁻¹ at 3 mV s⁻¹ and 221 C g⁻¹ at 4.1 A g⁻¹. At a power density of 425 W kg⁻¹, an asymmetric supercapattery device demonstrated a specific capacity of 147.2 C g⁻¹ and an energy density of 34.8 Wh kg⁻¹. After 10 000 cycles at 8 A g⁻¹, the device maintained 87.2% of its capacity, demonstrating exceptional stability.

The effect of calcination temperature on the structural and electrochemical characteristics of 2D cobalt phosphate is reported by Numan et al.²³ A maximum specific capacity of 352 C g⁻¹ was found for cobalt phosphate calcined at 200 °C in a 1 M KOH electrolyte. A supercapattery made of this material had a power density of 346 W kg⁻¹ and an energy density of 51.95 Wh kg⁻¹. In another work, a bouquet-like Co₃(HPO₄)₂(OH)₂ is hydrothermally synthesized utilizing red phosphorus, proving its suitability as a cathode material for hybrid supercapacitors (HSCs). The prepared electrode demonstrated an outstanding rate capability of 83.6 mAh g⁻¹ at 100 A g⁻¹, good stability of 92% retention after 5000 cycles at 10 A g⁻¹, and a high specific capacity of 119.2 mAh g⁻¹ at 1 A g⁻¹. After 10 000 cycles at 3 A g⁻¹, a constructed HSC device with porous carbon as the anode and Co₃(HPO₄)₂(OH)₂ as the cathode reached an impressive power density of 33.75 kW kg⁻¹ and a high energy density of 44.6 Wh kg⁻¹. The device also retained 91.8% of its capacity.²⁴

The hydrothermal process is one of the best ways to synthesize cobalt phosphate electrodes for supercapacitors. Katkar et al.²⁵ hydrothermally produced microflower-shaped hydrous cobalt phosphate films with urea-controlled morphology, which showed a high specific capacitance of 800 F g⁻¹. These films were used to fabricate both solid-state and aqueous asymmetric supercapacitors. In comparison to the solid-state device (70 F g⁻¹, 24.91 Wh kg⁻¹), the aqueous device performed better (163 F g⁻¹, 58.12 Wh kg⁻¹). By powering the LEDs, the solid-state gadget demonstrated exceptional stability and usefulness. Previously, we demonstrated the hydrothermal synthesis of cobalt phosphate-based nanoflakes on nickel foam. In 1 M KOH, the cobalt phosphate hydrate exhibited a specific capacitance of 1173.33 F g⁻¹ at 30 mA cm⁻² with 78% retention after 5000 cycles. In this report, it was observed that uniform growth with negligible internal resistance could be obtained using the hydrothermal method.²⁶ Additionally, it offers a regulated setting for the synthesis of superior electrodes, which is essential for attaining the intended electrochemical characteristics and improving the overall performance of supercapacitors.^27–29

In this study, we present the synthesis of cobalt phosphate using a hydrothermal route on stainless steel (SS) substrates. SDS was utilized as a surfactant to improve the adhesion of the electrodes by reducing the surface tension, which enhances the quality of the deposited electrodes. The structural, morphological, and electrochemical characteristics of the synthesized SDS-modified cobalt phosphate (CPS) electrodes were thoroughly examined. To evaluate the suitability of cobalt phosphate electrodes for supercapacitor applications, their electrochemical performance was assessed using galvanostatic charge–discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results indicate that the cobalt phosphate electrode possesses an enhanced capacitive performance.

2. Experimental section

The CPS electrodes were synthesized using a one-step hydrothermal method, with all chemicals sourced from Sigma Aldrich (purity ∼99.0%). Initially, 0.1 M cobalt nitrate [Co (NO₃)₂·6H₂O] and 0.1 M urea were dissolved in 20 ml of double-distilled water (DDW). Separately, a solution of 0.01 g of sodium dodecyl sulfate (SDS) in 10 ml of DDW was sonicated for 10 minutes and then added to the cobalt nitrate and urea solution. Subsequently, 10 ml of 0.1 M ammonium dihydrogen phosphate (ADP) in DDW was introduced into this solution. The solution was stirred for 5 min to ensure complete dissolution of the salts. The precleaned SS substrates were then immersed vertically in the chemical baths, which were placed in a hydrothermal autoclave at 90 °C for 90 min. After this process, violet-red colored cobalt phosphate electrodes were deposited on the SS substrates. These electrodes were rinsed 2–3 times with DDW and dried at 60 °C for 2 hours. The electrodes were coded as CPS1, CPS2, CPS3, CPS4, and CPS5 for the subsequent variation in SDS as 0.01, 0.02, 0.03, 0.04, and 0.05 gm, respectively. A schematic illustration of the synthesis process is shown in Fig. 1.

Fig. 1 Schematic route for the synthesis of the cobalt phosphate electrode on the SS substrate.

3. Results and discussion

3.1. X-ray diffraction (XRD) technique

The structure of CPS electrodes was scrutinized using an XRD analyzer. XRD data provide valuable insights into structural parameters, such as the crystal and phase integrity of the prepared electrodes. Fig. 2(a) depicts the XRD pattern of all CPS electrodes. The XRD patterns reveal well-defined diffraction peaks appearing at 2θ values of 11.32, 13.22, 27.11, and 55.04°, which correspond to the (110), (020), (031), and (501) planes, respectively. Furthermore, the XRD patterns of all CPS (CPS1, CPS2, CPS3, CPS4, and CPS5) were well matched with ICDD card no. 00-033-0432. The enlarged view of the hkl plane (020) is shown in Fig. S1. The planes corresponded to a monoclinic phase with a 12/m space group. The robust crystalline structure of CPS is advantageous for enhancing the cycle life of the electrode and ensuring stability during electrochemical processes with minimal susceptibility to degradation.

Fig. 2 (a) XRD patterns of all the CPS electrodes. (b) Raman spectra of the CPS3 electrode.

3.2. FT-Raman spectroscopy

FT-Raman analysis was carried out for the composition study of the SDS-assisted CPS electrodes. The plots are displayed in Fig. 2(b). It shows phosphate-related signals at 440 cm⁻¹, 550 cm⁻¹, and 914–1060 cm⁻¹ assigned to symmetric and asymmetric (PO₄)³⁻ (ν₂ and ν₄) active modes, H₂PO₄ and aromatic rings of (PO₄)³⁻, respectively.^30,31 The torsional and external modes of vibrations at 133 cm⁻¹ are due to lattice vibration. The stretching at 160–230 cm⁻¹ is assigned to P–O–P bending vibrations. Additionally, the Raman peaks at 230–280 cm⁻¹ are metal–oxygen modes.³² Similar work has been reported by M. Baril et al.³¹ The synthesized KCoHP₂O₇·2H₂O microstructure reveals POP formation. In addition, the phosphate groups occurred at 1082 and 1016 cm⁻¹. B. Qiao et al.³³ reported the Raman analysis of amorphous cobalt phosphate nanocubes synthesized using a one-step ion-exchange method. They reported the presence of various phosphate modes at 270, 878, 937, 1009, and 1057 cm⁻¹ by Co–O, P–O–P, (PO₄)³⁻ symmetric and asymmetric modes, respectively. The literature supports the above discussion of the Raman signal of CPS material.^34–37

3.3. Scanning electron microscope (SEM)

The morphological evaluation of the SDS-modified CPS electrodes was analyzed using SEM. The SEM analysis shows a plate-shaped morphology.³⁸ As shown in Fig. 3(a)–(e), CPS microplates are evenly distributed on the SS substrate. The SDS-modified CPS microstructures showed significant improvements in morphology compared to the unmodified electrodes.³⁹ Anionic SDS appears to promote the formation of well-defined microstructures, controlling overgrowth and potentially maximizing the material's overall performance toward supercapacitor application. The additives in the anionic surfactant-modified solution led to the formation of heterogeneous nuclei. As demonstrated in the SEM images, some microplate-crystals are broken likely due to increased agitation speed promoting collisions between forming particles and resulting in fractured nuclei.⁴⁰ The EDS mapping of the CPS3 electrode is shown in Fig. 4. The presence of Co, P, and O supports the cobalt phosphate phase. The elemental composition is illustrated in Table S1.

Fig. 3 (a)–(e) SEM images at different magnifications with possible growth illustrations for all the CPS electrodes.

Fig. 4 EDS spectra with elemental mapping of the CPS electrode.

The formation of heterogeneous nuclei can be controlled by adjusting precursor/surfactant concentration, reaction time/temperature, etc., which avoids many broken crystals. As the SDS concentration increased, the microplates were transformed from a thin plate to a rod-like structure. CPS3 showed an intermediate structure, with a microplate with tampered corners. The possible schematic of the CPS3 microstructure is schematically presented for better understanding. Compared to CPS4 and CPS5, CPS3 shows less agglomeration. The surfactant-modified CPS3 exhibited a more uniform and fine distribution of microplates, which is favorable for enhancing the electrode surface area and facilitating efficient charge transfer during electrochemical analysis.

3.4. Electrochemical analysis

The electrochemistry was recorded using a potentiostat in a three-electrode system in an aqueous medium of 1 M KOH. During measurements, the CPS is used as a working electrode along with Hg/HgO as a reference electrode. Here, the counter electrode is a platinum wire. The active mass loading is ∼3 to 4 mg cm⁻². All electrochemical measurements were carried out at room temperature.

The electrochemical analysis tool CV is performed to estimate the nature and capacitive performance of CPS (CPS1, CPS2, CPS3, CPS4, and CPS5) microplate electrodes. Fig. 5(a) demonstrates the CVs of all CPS (CPS1, CPS2, CPS3, CPS4, and CPS5) electrodes in a 1 M aqueous KOH electrolyte at a fixed scan rate of 10 mV s⁻¹. As a result of the Co²⁺/Co³⁺ redox reaction, the CV curves exhibit a pair of redox peaks. All CV plots display slight redox peaks that distinguish them from the typical rectangular shape observed in conventional EDLCs, explaining their pseudocapacitive nature in all CPS electrodes within a fixed significant −0.2 to 0.5 V potential window. In all CPS electrodes, CPS3 has a larger area under the CV curve, indicating that it has superior performance. To study the effect of scan rate variation, CV curves for all CPS electrodes were recorded at different scan rates (10–100 mV s⁻¹), as shown in Fig. S2. It was observed that with the scan rate, the CV area also increased. This proves that the scan rate is proportional to the current density. Furthermore, as the scan rate increases, the redox peak position shifts towards higher potentials, which is attributed to the electrode's polarization effect. The CPS3 electrode exhibits superior electrochemical reversibility and high-rate performance compared to the other CPS electrodes. This enhancement in electrochemical activities is attributed to the optimal concentration of anionic SDS within the precursors. The charge storage mechanism in alkaline media at the interface of all CPS electrodes can be described as follows, where cobalt changes from +2 to +3:²⁶

Co₃(PO₄)₂ + OH⁻ ↔ CO₃(PO₄)₂OH + 3e⁻ (1)

CO₂P₂O₇ + 2OH⁻ ↔ CO₂P₂O₇(OH⁻)₂ + 2e⁻ (2)

Fig. 5 (a–b) CV and (c–d) GCD of the cobalt phosphate electrode on the SS substrate.

GCD is a reliable and effective technique for assessing the charge storage capacity of materials. The GCD was recorded for all CPS (CPS1, CPS2, CPS3, CPS4, and CPS5) electrodes at a current density of 20 mA cm⁻² in 1 M KOH as an electrolyte. Fig. 5(c) demonstrates a GCD curve of all CPS microplates within a voltage ranging from −0.2 to 0.45 V. The GCD potential window was reduced to avoid the supersaturation of electrolyte ions during testing. Each GCD curve shows a non-triangular GCD plot, providing evidence of a reversible redox reaction occurring at the electrode/electrolyte interface. The supercapacitive measurements were calculated using the following equations:

where C denotes the specific capacitance (F g⁻¹), C_sp represents the specific capacity (mAh g⁻¹), I_d denotes the current density (mA cm⁻²), t_d represents the discharge duration (s), m represents the deposited mass, and ΔV denotes the operating potential window. From the GCD plots, CPS3 shows the maximum charging/discharge time over the remaining CPS electrodes, as shown in Table 1.

Fig. 6 (a) Comparison of computed specific capacitance and capacity at various current densities. (b) Ragone plot. (c) Nyquist plot. (d) Radar plot for all the CPS electrodes.

Table 1 Comparative table showing the supercapacitive attributes of the synthesized electrodes

Sample code	Specific capacitance (F cm^−2)	Specific capacity (µAh cm^−2)	Energy density (µWh cm^−2)	Power density (mW cm^−2)	Coulombic efficiency (%)	R s (Ω)	R ct (Ω)
CPS1	2.59	324.76	73.07	4.28	84.90	0.96	43.82
CPS2	2.99	374.05	84.16	4.36	80.38	1.29	25.73
CPS3	3.92	490.66	110.39	4.63	71.34	1.24	10.15
CPS4	3.37	421.43	94.82	3.98	82.74	1.18	51.07
CPS5	1.94	243.34	54.75	4.24	83.32	0.92	48.32

---

The obtained specific capacitance of the CPS electrode was 3.92 F cm⁻² at 20 mA cm⁻², respectively, as presented in Table 2.

Table 2 Table showing the computed specific capacitance/capacity and rate capability of the CPS3 electrode

Electrode	Current density (Id) (mA cm^−2)	Specific capacitance (F cm^−2)	Specific capacity (µAh cm^−2)	Rate capability (%)
CPS3	20	3.92	490.66	100
	25	3.69	461.84	75.30
	30	3.59	449.48	61.07
	35	3.50	438.40	51.05
	40	3.28	410.88	41.87

---

As shown in Fig. 5(d) and Fig. S4, to examine the rate capability, the GCD of CPS3 was recorded at various current densities (20, 25, 30, 35, and 40 mA cm⁻²), demonstrating the excellent rate capability of the CPS3 electrode. The optimized CPS electrode shows better performance compared with the reported literature, as shown in Table 3.

Table 3 Literature survey of the cobalt phosphate-based electrodes

Sr. no.	Material	Method	Morphology	Specific capacitance	Stability	Ref.
1	Cobalt phosphate	Ion-exchange method	Nanocubes	539.2 F g^−1 at 1 A g^−1	89.1% after 3000 cycles	33
3	Cobalt phosphate	Chemical bath deposition	Nanospheres	678 F g^−1 at 10 mV s^−1	90.46% after 10 000 cycles	41
4	Cobalt phosphate	Microwave-assisted hydrothermal	Nanosheets	164.52 C g^−1 at 1 A g^−1	101% after 1000 cycles	42
5	Cobalt phosphate	Microwave-assisted hydrothermal	—	272.74 C g^−1 at 1 A g^−1	98.55% after 3000 cycles	43
6	Cobalt phosphate	Co-precipitation method	3D flower	680 F g^−1 at 1.0 A g^−1	89.3% after 10 000 cycles	8
7	Cobalt phosphate	Hydrothermal	Nanobelt	1766 F g^−1 at 5 mV s^−1	93% after 4000 cycles	44
8	Cobalt phosphate	Sonochemical	Nanoflakes	221.4 C g^−1 at 4.1 A g^−1	—	12
9	Cobalt phosphate	Hydrothermal	Bouquet-like	119.2 mAh g^−1 at 1 A g^−1	92% after 5000 cycles	24
10	Cobalt phosphate	SILAR	Nanoparticles	1147 F g^−1 at 1 mA cm^−2	—	45
11	Co3P2O8·8H2O	Chemical precipitation	Nanoparticles	446 F g^−1 at 0.5 A g^−1	100% after 1000 cycles	46
12	Cobalt phosphate	Hydrothermal	Microplate	3.92 F cm ^−2 (1189.86 F g ^−1 ) at 20 mA cm ^−2	—	This work

---

This is proved by decreasing charging/discharging times with increasing current density. Fig. S3 demonstrates the GCD graphs of all the remaining CPS electrodes. From the GCD curves, CPS revealed the extrinsic pseudocapacitive behavior of CPS microplates, analogous to battery-grade materials.¹² Further, energy and power density calculations showed that CPS3 depicted the maximum values as 110.39 µWh cm⁻² at 4.63 mW cm⁻², as presented in Tables 4 and 5. Such a high energy density in the half-cell reaction motivated the fabrication of the real-time device. Overall, the results indicate the ability of the CPS3 electrode to efficiently store and release charge, making it suitable for high-power applications.

Table 4 Comparative table showing the supercapacitive attributes of the fabricated CPS3//AC device

Electrode	Current density (Id) (mA cm^−2)	Specific capacitance (mF cm^−2)	Specific capacity (µAh cm^−2)	Energy density (µWh cm^−2)	Power density (mW cm^−2)
CPS3//AC	5	93.96	36.54	25.58	3.30
	10	57.14	22.22	15.55	6.60
	15	31.13	12.10	8.47	9.90
	20	26.46	10.29	7.20	13.20

---

Table 5 Literature survey of the cobalt phosphate-based devices

Sr. no.	Device name	Specific capacitance	Energy density	Power density	Stability	Ref.
1.	AC//Co3P2O8·8H2O	55 F g^−1 at 0.5 A g^−1	11.9 Wh kg^−1	3590 W kg^−1	—	47
2.	Co3(PO4)2//Co3(PO4)2	165 F g^−1 at 0.5 A g^−1	52.8 Wh kg^−1	756 W kg^−1	96.1% after 2000 cycles	47
3.	Co11(HPO3)8(OH)6//AC	65.3 C g^−1 at 1 mA cm^−2	14.5 Wh kg^−1	799.4 W kg^−1	83.5% after 7800 cycles	48
4.	(S-CP4//PVA-KOH//rGO)	77 F g^−1 at 1.5 mA cm^−2	27.37 Wh kg^−1	1500 W kg^−1	94% after 5000 cycles	45
5.	Co3(PO4)2//AC	174 C g^−1 at 0.4 mA cm^−2	35.5 Wh kg^−1	293.9 W kg^−1	93% after 3000 cycles	49
6.	Co 3 (PO 4 ) 2 //AC	93.96 mF cm ^−2 at 5 mA cm ^−2	25.58 µWh cm ^−2	3.30 mW cm ^−2	99.97% after 5100 cycles	This work

---

Another important metric to study charge storage kinetics is the EIS technique. EIS evaluation plays a vital role in assessing the exact electrode conductivity. By subjecting low-amplitude AC signals across a spectrum of frequencies, EIS analysis provides important insights into impedance characteristics, containing capacitive and resistive parameters at room temperature. EIS helps interpret components, such as the ion diffusion process, electrode/electrolyte interface kinetics, and overall electrode material integrity.⁵⁰ Therefore, EIS analysis of all CPS electrodes is carried out over a frequency ranging from 0.01 Hz to 100 kHz (Fig. 6(c)).

All CPS electrodes showed a tiny real axis intercept (straight line) and an infinitesimal semicircle in the low imaginary impedance region. This may be interpreted with the help of series resistance (R_s) and charge transfer resistance (R_ct).⁵¹ Here, the key factor is that the diameter of the semicircle is directly proportional to the CPS/KOH (electrode/electrolyte) interface contact. All the CPS electrodes show a smaller semicircle, proving low resistance and faster charge transfer capacity. A straight line at a higher impedance (low-frequency) explains Warburg impedance (Z_w), which comes from the easy diffusion of OH⁻ ions of electrolytes within CPS electrodes. They may result in dependence on impedance on the square root of the frequency.

Overall, electrochemical assessments of the CPS3 electrode provided a strong effect of structural and morphological properties on electrochemical performance in the electrode system. The strong skeleton of the microplates provided a large exposed surface area and conductive properties. Hence, to further study its ability for practical purposes, we checked the electrochemical performance in the two-electrode device system. An aqueous state asymmetric device was fabricated using CPS3 as the working electrode and activated charcoal as the counter electrode. These two electrodes were immersed in 1 M KOH and separated using thin porous filter paper. The device is named CPS3//AC. The CPS3//AC device was tested for CV, GCD and stability through GCD cycles. Before these tests, the working voltage was stabilized using different voltage ranges at a 100 mV s⁻¹ scan rate. This variation in voltage is shown in Fig. 7(a). CPS3//AC showed a maximum voltage of 1.6 V in the positive region. After selecting the appropriate maximum voltage, we carried out scan rate variations, as depicted in Fig. 7(b).

Fig. 7 (a) CV at various potential windows. (b) CV at various scan rates. (c) GCD at various current densities. (d) Nyquist plot of the CPS3//AC device.

The GCD showed non-linearity in the charging and discharging profiles. Additionally, there is a negligible voltage drop in the GCD, indicating lower internal resistance. The highest areal capacitance was calculated and observed to be 93.96 mF cm⁻² at 5 mA cm⁻². Further, CPS3//AC depicted the highest areal energy density of 25.58 µWh cm⁻² at 3.30 mW cm⁻² power density. Similar to a three-electrode study, GCD cycles were employed to check device stability during consecutive charging and discharging. Fig. 8 shows the obtained stability plot of the CPS3//AC over 5100 cycles. The highest retention of 99.97% was obtained after 5100 cycles.

Fig. 8 (a) Actual, predicted and forecasted cyclic stability study. (b) ACF. (c) PACF plots of the CPS3//AC device.

TSA analysis is a useful tool for gaining insights into the forecasting of the cyclic stability data. To find out stationarity, the Augmented Dickey-Fuller (ADF) method is employed, which confirms the stationarity of the data (p-value: −40.1133 (0.0001)). Furthermore, Holt–Winters exponential smoothing is utilized for predicting and forecasting, which reveals the alpha (α) and beta (β) parameter values to be 0.0091 and 0.0124, respectively. From these, the equations for level (l) and trend (r) can be written as follows:⁵²

l_t = 0.0091y_t + 0.9909(l_t−1 + r_t−1), (7)

r_t = 0.0124(l_t − l_t−1) + 0.9876r_t−1. (8)

The actual and predicted data using TSA are found to be nearly equal, which is also confirmed using the mean squared error (MSE) (8.3395 × 10⁻⁵). The TSA is employed to forecast the data up to 400 cycles. The autocorrelation function (ACF) and partial autocorrelation function (PACF) plots are used to find out the correlation. Fig. 8(b) and (c) illustrates the ACF and PACF plots for the data, which show that the forecasted data are within the acceptable range with zero correlation. Hence, the utilization of TSA is crucial for predicting the long-term performance of electrodes.

To check the conductivity of the device, EIS spectra were taken and fitted with the equivalent circuit. The values of R_s are found to be ∼2.1 Ω, which signifies the higher conductivity of the device, as shown in Fig. 7(d). Overall, CPS3 showed consistent results in a two-electrode device system due to its higher conductivity, improved surface structure, strong bonding with the substrate, and binder-free nature.

4. Conclusion

The effect of SDS on the synthesis route of cobalt phosphate was successfully studied. The XRD and Raman studies confirm the formation of the cobalt phosphate phase. The surfactant SDS changes the microplates to microrods as its concentration changes, which is useful for tailoring the morphology observed through SEM. In a three-electrode configuration, the optimized electrode shows a specific capacitance of 3.92 F cm⁻² at a current density of 20 mA cm⁻². Furthermore, the hybrid aqueous device is assembled with activated carbon, which shows a specific capacitance of 93.96 mF cm⁻² at 5 mA cm⁻². The material retains up to 99.97% of capacitance after 5100 cycles. Further, Holt–Winters exponential smoothing was utilized to predict and forecast the cyclic stability of the device using time series analysis.

Author contributions

Satyajeet S. Patil: writing – original draft, data curation, validation, visualization, and conceptualization. Annapurna T. Paroji: software, formal analysis, and methodology. Akhilesh P. Patil: software, writing – review & editing, and formal analysis. Rahul S. Redekar: software, writing – review & editing, and formal analysis. Minaj M. Faras: writing – review & editing, validation, formal analysis, writing – review & editing, and funding acquisition. Shweta M. Pawar: writing – review & editing, and data curation, Nilesh L. Tarwal: resources, validation, formal analysis, writing – review & editing, and funding acquisition. Pramod S. Patil: supervision, validation, and visualization.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj02665b.

Acknowledgements

The authors are thankful to the Physics Instrumentation Facility Centre (PIFC) and Common Facility Centre (CFC) for providing various characterizations at Shivaji University, Kolhapur, M. S., India. The authors acknowledge the Computer Center, Shivaji University, Kolhapur, M.S., India, for providing computational facilities. RSR is thankful to DST for providing the INSPIRE fellowship (DST/INSPIRE Fellowship/2019/IF190812). MMF would like to acknowledge Mahatma Jyotiba Phule Research Fellowship (MJPRF-2021), Government of Maharashtra, India, for providing financial assistance. NLT is thankful to DST-SERB for providing financial assistance through the Teachers Associateship for Research Excellence (TARE) scheme (TAR/2021/000307).

References

1. A. Afif, S. M. H. Rahman, A. Tasfiah Azad, J. Zaini, M. A. Islan and A. K. Azad, Advanced materials and technologies for hybrid supercapacitors for energy storage – A review, J. Energy Storage, 2019, 25, 100852.
2. R. T. Yadlapalli, R. R. Alla, R. Kandipati and A. Kotapati, Super capacitors for energy storage: Progress, applications and challenges, J. Energy Storage, 2022, 49, 104194.
3. K. V. Sankar, S. C. Lee, Y. Seo, C. Ray, S. Liu, A. Kundu and S. C. Jun, Binder-free cobalt phosphate one-dimensional nanograsses as ultrahigh-performance cathode material for hybrid supercapacitor applications, J. Power Sources, 2018, 373, 211–219.
4. S. Mahadik, S. Surendran, J. Y. Kim, D. Lee, J. Park, T.-H. Kim, H.-Y. Jung and U. Sim, Recent progress in the development of carbon-based materials in lead–carbon batteries, Carbon Neutralization, 2023, 2(4), 510–524.
5. H. Zhao and Z. Y. Yuan, Insights into transition metal phosphate materials for efficient electrocatalysis, ChemCatChem, 2020, 12(15), 3797–3810.
6. D.-K. Lee, J. Lim, J. Park, D. Kim, S. Surendran, G. Janani, J. Y. Kim and U. Sim, Design of Experiments (DoE)-Based Optimization of Synthetic Processes in Nickel Phosphides for High-Performance Electrochemical Application, Mater. Trans., 2022, 63(10), 1345–1350.
7. H. Kim, S. Surendran, Y. Chae, H. Y. Lee, T.-Y. An, H. S. Han, W. Park, J. K. Kim and U. Sim, Fabrication of an ingenious metallic asymmetric supercapacitor by the integration of anodic iron oxide and cathodic nickel phosphide, Appl. Surf. Sci., 2020, 511, 145424.
8. V. Vinothkumar, C. Koventhan, S.-M. Chen, M. Abinaya, G. Kesavan and N. Sengottuvelan, Preparation of three dimensional flower-like cobalt phosphate as dual functional electrocatalyst for flavonoids sensing and supercapacitor applications, Ceram. Int., 2021, 47(21), 29688–29706.
9. A. Inamdar, S. Mujawar, V. Ganesan and P. Patil, Surfactant-mediated growth of nanostructured zinc oxide thin films via electrodeposition and their photoelectrochemical performance, Nanotechnology, 2008, 19(32), 325706.
10. A. Nyabadza, M. Vázquez, S. Coyle, B. Fitzpatrick and D. Brabazon, Review of materials and fabrication methods for flexible nano and micro-scale physical and chemical property sensors, Appl. Sci., 2021, 11(18), 8563.
11. S. Habib, Sol–gel Synthesis and Electrochemical Studies of Zinc-doped Lithium Cobalt Phosphates for Water Electrocatalysis, Master's thesis, Quaid I Azam University, 2022.
12. M. Z. Iqbal, J. Khan, S. Siddique, A. M. Afzal and S. Aftab, Optimizing electrochemical performance of sonochemically and hydrothermally synthesized cobalt phosphate for supercapattery devices, Int. J. Hydrogen Energy, 2021, 46(29), 15807–15819.
13. Z. Yao, G. Wang, Y. Shi, Y. Zhao, J. Jiang, Y. Zhang and H. Wang, One-step synthesis of nickel and cobalt phosphide nanomaterials via decomposition of hexamethylenetetramine-containing precursors, Dalton Trans., 2015, 44(31), 14122–14129.
14. P. Arunachalam, M. N. Shaddad, A. S. Alamoudi, M. A. Ghanem and A. M. Al-Mayouf, Microwave-assisted synthesis of Co3 (PO4) 2 nanospheres for electrocatalytic oxidation of methanol in alkaline media, Catalysts, 2017, 7(4), 119.
15. X. Hu, R. Li, S. Zhao and Y. Xing, Microwave-assisted preparation of flower-like cobalt phosphate and its application as a new heterogeneous Fenton–like catalyst, Appl. Surf. Sci., 2017, 396, 1393–1402.
16. M. N. Mustafa, M. A. A. M. Abdah, A. Numan, Y. Sulaiman, R. Walvekar and M. Khalid, Microwave-assisted fabrication and optimization of nickel cobalt phosphate for high-performance electrochromic supercapacitors, J. Energy Storage, 2023, 73, 108935.
17. G. Zhou, W. Wang, G. Gu, Y. Li and Y. Liu, Microwave assisted synthesis of cobalt phosphate nanoparticles and their antiproliferation against human lung cancer cells and primary osteoblasts in vitro, Int. J. Chem., 2011, 3(4), 127.
18. B. Schwenzer, K. M. Roth, J. R. Gomm, M. Murr and D. E. Morse, Kinetically controlled vapor-diffusion synthesis of novel nanostructured metal hydroxide and phosphate films using no organic reagents, J. Mater. Chem., 2006, 16(4), 401–407.
19. W.-K. Chang, C.-S. Wur, S.-L. Wang and R.-K. Chiang, Hydrothermal synthesis and characterization of two cobalt phosphates based on double-four-ring units with fluorine-occlusion and phosphite-substitution modifications, Inorg. Chem., 2006, 45(17), 6622–6627.
20. P. Feng, X. Bu and G. D. Stucky, Hydrothermal syntheses and structural characterization of zeolite analogue compounds based on cobalt phosphate, Nature, 1997, 388(6644), 735–741.
21. P. K. Katkar, S. J. Marje, S. B. Kale, A. C. Lokhande, C. D. Lokhande and U. M. Patil, Synthesis of hydrous cobalt phosphate electro-catalysts by a facile hydrothermal method for enhanced oxygen evolution reaction: effect of urea variation, CrystEngComm, 2019, 21(5), 884–893.
22. P. K. Katkar, S. J. Marje, V. G. Parale, C. D. Lokhande, J. L. Gunjakar, H.-H. Park and U. M. Patil, Fabrication of a high-performance hybrid supercapacitor based on hydrothermally synthesized highly stable cobalt manganese phosphate thin films, Langmuir, 2021, 37(17), 5260–5274.
23. A. Numan, J. Iqbal, P. Jagadish, S. G. Krishnan, M. Khalid, M. Pannipara and S. Wageh, Tailoring crystallinity of 2D cobalt phosphate to introduce pseudocapacitive behavior, J. Energy Storage, 2022, 54, 105371.
24. P. Lu, Y. Chen, R. Zhou, C. Guo, X. Liu, F. Yang and Y. Zhu, Novel bouquet-like cobalt phosphate as an ultrahigh-rate and durable battery-type cathode material for hybrid supercapacitors, Sci. China Mater., 2022, 65(6), 1503–1511.
25. P. K. Katkar, S. J. Marje, S. S. Pujari, S. A. Khalate, A. C. Lokhande and U. M. Patil, Enhanced energy density of all-solid-state asymmetric supercapacitors based on morphologically tuned hydrous cobalt phosphate electrode as cathode material, ACS Sustainable Chem. Eng., 2019, 7(13), 11205–11218.
26. S. S. Patil and P. S. Patil, Exploring the potential of binder-free synthesized cobalt-based phosphate hydrate and pyrophosphate electrodes for supercapacitors, J. Energy Storage, 2023, 71, 108155.
27. J. Li, L. Zhang, P. Yang and X. Cheng, Morphological evolution of Co phosphate and its electrochemical and photocatalytic performance, CrystEngComm, 2018, 20(43), 6982–6988.
28. R. S. Redekar, S. S. Patil, P. S. Patil and N. L. Tarwal, Manganese cobalt oxide-polythiophene composite for asymmetric supercapacitor, Chem. Eng. J., 2025, 503, 158209.
29. B. Pandit and B. R. Sankapal, Cerium Selenide Nanopebble/Multiwalled Carbon Nanotube Composite Electrodes for Solid-State Symmetric Supercapacitors, ACS Appl. Nano Mater., 2022, 5(2), 3007–3017.
30. M. Prekajski, M. Mirković, B. Todorović, A. Matković, M. Marinović-Cincović, J. Luković and B. Matović, Ouzo effect—New simple nanoemulsion method for synthesis of strontium hydroxyapatite nanospheres, J. Eur. Ceram. Soc., 2016, 36(5), 1293–1298.
31. M. Baril, H. Assaaoudi and I. S. Butler, Pressure-tuning Raman microspectroscopic study of cobalt(II), manganese(II), zinc(II) and magnesium(II) pyrophosphate dihydrates, J. Mol. Struct., 2005, 751(1), 168–171.
32. N. Padmanathan, H. Shao and K. M. Razeeb, Multifunctional nickel phosphate nano/microflakes 3D electrode for electrochemical energy storage, nonenzymatic glucose, and sweat pH sensors, ACS Appl. Mater. Interfaces, 2018, 10(10), 8599–8610.
33. B. Qiao, X. Li, Y. Zhou, X. Chen, Z. An, Y. Chen and P. Chen, Amorphous Cobalt Phosphate Porous Braced-Frame Nanocubes as Highly Efficient Supercapacitor Electrodes, Cryst. Growth Des., 2022, 22(12), 7452–7460.
34. H. Shao, N. Padmanathan, D. McNulty, C. O′Dwyer and K. M. Razeeb, Supercapattery based on binder-free Co₃(PO₄)₂·8H₂O multilayer nano/microflakes on nickel foam, ACS Appl. Mater. Interfaces, 2016, 8(42), 28592–28598.
35. T. E. Bakare, M. N. Pillay and W. E. van Zyl, A supercapacitor electrode formed from amorphous Co₃(PO₄)₂ and the normal spinel Co^IICo^III2O₄, J. Solid State Chem., 2021, 302, 122422.
36. M. Shimizu, Y. Tsushima and S. Arai, Electrochemical Na-Insertion/Extraction Property of Ni-Coated Black Phosphorus Prepared by an Electroless Deposition Method, ACS Omega, 2017, 2(8), 4306–4315.
37. J. Qi, Y. P. Lin, D. Chen, T. Zhou, W. Zhang and R. Cao, Autologous Cobalt Phosphates with Modulated Coordination Sites for Electrocatalytic Water Oxidation, Angew. Chem., 2020, 132(23), 9002–9006.
38. A. G. Reiss, I. Gavrieli and J. Ganor, The Morphology of Gypsum Precipitated Under Hyper-saline Conditions – Preliminary Results from Dead Sea - Red Sea Mixtures, Procedia Earth Planet. Sci., 2017, 17, 376–379.
39. T. M. Chagwedera, J. Chivavava and A. E. Lewis, Gypsum Seeding to Prevent Scaling, Crystals, 2022, 12(3), 342.
40. G. Li, H. Xiao, L. Liang, X. He and N. Qi, Influence of cationic surfactants on the growth of gypsum crystals, Open Phys., 2023, 21, 1.
41. M. N. Ganth, S. Prabahar, R. T. Karunakaran, S. Nivetha and S. Dhinesh, Facile synthesis of cobalt phosphate electrode material for enhanced electrochemical supercapacitor applications, Ionics, 2023, 29(8), 3261–3271.
42. E. S. Agudosi, J. E. Goh, M. Khalid, K. Ramam and F. Sanhueza, Microwave-assisted hydrothermal synthesis and characterisation of cobalt phosphate nanosheets as electrode material for high-performance supercapacitors, Energy Storage, 2024, 6(5), e680.
43. N. M. Saidi, A. Khairudin, M. N. Mustafa, F. S. Omar, O. Gerard, Y. S. Tan, M. Khalid and A. Numan, Highly amorphous cobalt phosphate synthesized via microwave route as a superior positive electrode material for asymmetric supercapatteries, J. Energy Storage, 2024, 97, 112647.
44. S. K. Shinde, M. B. Jalak, S. S. Karade, S. Majumder, M. S. Tamboli, N. T. N. Truong, N. C. Maile, D.-Y. Kim, A. D. Jagadale and H. M. Yadav, A Novel Synthesized 1D Nanobelt-like Cobalt Phosphate Electrode Material for Excellent Supercapacitor Applications, Materials, 2022, 15(22), 8235.
45. V. V. Patil, S. S. Pujari, S. B. Bhosale, S. S. Kumbhar, V. G. Parale, J. L. Gunjakar, H.-H. Park, C. D. Lokhande, M. G. Mali, D. S. Mhamane and U. M. Patil, Hydrous and Amorphous Cobalt Phosphate Thin-Film Electrodes Synthesized by the SILAR Method for High-Performing Flexible Hybrid Energy Storage Devices, Energy Fuels, 2022, 36(20), 12791–12806.
46. J.-J. Li, M.-C. Liu, L.-B. Kong, M. Shi, W. Han and L. Kang, Facile synthesis of Co₃P₂O₈·8H₂O for high-performance electrochemical energy storage, Mater. Lett., 2015, 161, 404–407.
47. H. Mao, F. Zhang, X. Liu, J. Qiu, B. Li and Z. Jin, Synthesis of cobalt phosphate nanoflakes for high-performance flexible symmetric supercapacitors, J. Mater. Sci.: Mater. Electron., 2018, 29(19), 16721–16729.
48. Y. Tian, X. Lian, Y. Wu, W. Guo and S. Wang, The morphology controlled growth of Co₁₁(HPO₃)₈(OH)₆ on nickel foams for quasi-solid-state supercapacitor applications, CrystEngComm, 2020, 22(31), 5218–5225.
49. N. Duraisamy, N. Arshid, K. Kandiah, J. Iqbal, P. Arunachalam, G. Dhanaraj, K. Ramesh and S. Ramesh, Development of asymmetric device using Co₃(PO₄)₂ as a positive electrode for energy storage application, J. Mater. Sci.: Mater. Electron., 2019, 30(8), 7435–7446.
50. S. S. Raut, O. Bisen and B. R. Sankapal, Synthesis of interconnected needle-like Bi₂O₃ using successive ionic layer adsorption and reaction towards supercapacitor application, Ionics, 2017, 23(7), 1831–1837.
51. H. Pang, S. Wang, W. Shao, S. Zhao, B. Yan, X. Li, S. Li, J. Chen and W. Du, Few-layered CoHPO₄·3H₂O ultrathin nanosheets for high performance of electrode materials for supercapacitors, Nanoscale, 2013, 5(13), 5752.
52. S. S. Patil, A. P. Patil, R. S. Redekar, S. M. Pawar, N. L. Tarwal and P. S. Patil, Study of 2D layered nickel pyrophosphate using 3D Bode mapping and stability forecasting for supercapacitors, Colloids Surf., A, 2025, 707, 135829.

---