Rapid synthesis of cobalt manganese phosphate by microwave-assisted hydrothermal method and application as positrode material in supercapatteries | Scientific Reports

Scientific Reports volume 14, Article number: 26550 (2024) Cite this article

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Electrochemical energy storage devices with high specific capacity are of utmost important for the next-generation electronic devices. Supercapatteries (SCs) are highly demanded energy storage devices nowadays as these bridge the low energy supercapacitors and low power batteries. Herein, we report a rapid synthesis of cobalt manganese phosphate (COMAP) by microwave-assisted hydrothermal method and facile fabrication of SCs using electrodes comprising of COMAP as positrode material. The effect of precursor concentration on the microstructure and surface morphology of the COMAP samples are examined initially. Further, the electrochemical performance of COMAP electrodes is studied systematically in 3 M KOH (aqueous) electrolyte. COMAP exhibits excellent charge storage capabilities where type of charge storage mechanism is found to be battery-type based on the calculation obtained from Dunn’s method. The SC electrode fabricated with COMAP synthesized using cobalt: manganese precursor ratio as 80:20 exhibits a highest specific capacity of 191.4 C/g at a scan rate of 1 mV/s. An asymmetric SC (ASC) cell fabricated with COMAP as positrode and activated carbon (AC) as negatrode exhibits a specific capacity of 165.5 C/g at a current density of 1.8 A/g. The COMAP//AC ASC cell exhibits an energy density of 34.1 Wh/kg at a corresponding power density of 1875 W/kg at a current density of 1.8 A/g.

The most critical challenge that humans face in this era is the war between nations. The second and third challenges are energy scarcity and global warming, respectively1. As the population is exploding in the geometric progression where the energy availability is in the arithmetic progression. In this context, energy scarcity become a nightmare to all the nations irrespective of their wealth and ranking. Depletion in the fossil fuels become another crucial situation for the utilization of non-renewable fuels in the future. Sustainable and renewable energy sources such as solar and wind are still in the developing stage and not get much popularity due to their huge cost associated with their installation2,3. Another demerit is their intermittent way of storing the energy whereby the solar cell won’t work at night and wind turbine wont function when there is no wind. Hence energy storage technologies became an urgent requirement to support the renewable energy conversion technologies4,5. Among the various options available, electrochemical energy storage devices such as rechargeable batteries and supercapacitors have achieved great interest in the recent past due to their excellent charge storage capabilities6,7,8,9. Rechargeable batteries are best candidates in terms of their energy density whereas supercapacitors are powerful candidates to supply high power on demand10,11.

Supercapacitors are evolved as contemporary electrochemical energy storage devices that can deliver high power on demand12. These devices are widely used in fork lifts, electric cars like Tesla, etc. where it requires high torque. The main demerit of supercapacitors is the possession of low energy density. It is always difficult to achieve high power and energy density simultaneously. This has triggered the researchers to think about developing such a device that exhibit high energy density and deliver high power simultaneously. This has end up in the development of a novel supercapacitor device known as battery-type supercapacitor where the positive electrode is made of a battery-type electrode-active material12,13,14. These battery-type supercapacitors are commonly termed now-a-days “supercapatteries (SCs)”. This nomenclature is very new in the field of energy storage devices but acquiring acceptance slowly. In a supercapattery, the positive electrode is termed as positrode whereas the negative electrode is termed as negatrode and the parameter that describes the electrochemical performance is the specific capacity (in Coulomb per gram (C/g) unlike in supercapacitor (in Farad per gram (F/g)). Battery-type electrodes possess charge storage in terms of diffusion-controlled mechanisms similar to that in batteries15,16,17. Currently, the technologically advanced modern world is highly dedicated to the improvement in energy storage efficiency of these supercapattery devices. In the present scenario, there is wide investigation on new electrode materials which are capable to store charges by faradaic redox reaction as well as the sensitivity to electrochemical reactions are done. The combination of higher energy density by battery and power density by supercapacitor arise the problem of high potential values. By begins with the capacitive electrodes that delivers redox character of battery electrode, assembled supercapattery device not only improves energy density, but also its power density18.

Electrode-active materials play a crucial role in determining the charge storage performance of either a supercapacitor or a supercapattery18. Based on the charge storage mechanism, the supercapacitor store charge either in terms of electrochemical double layers or by means of pseudocapacitive mechanisms (via reduction-oxidation reactions). In the case of supercapattery electrode, the charge storage possessed by the battery-type electrode is crucial in determining the energy density of the supercapacitor. Transition metal phosphates are excellent battery-type electrode-active materials for charge storage in supercapacitors19,20. The main demerit of developing metal phosphate-based electrode-active materials is their difficulty associated with synthesis. Various methods such as sonochemical method, hydrothermal method, etc. are widely used for the synthesis of these metal phosphates21,22. In order to increase the charge storage, instead of single metal phosphate, double metal phosphates are being used. Double metal phosphates such as nickel cobalt phosphate, nickel manganese phosphate, etc. are reported in the literature with varied morphologies23,24. Among the various bimetallic phosphates, cobalt manganese phosphate (COMAP) is meritorious in terms of facile synthesis routes available, good electrochemical properties, good stabilities, etc.

With the aid of hydrothermal approach for Katkar et al.25 synthesized COMAP thin film on stainless steel substrate using hydrothermal method in order to use it as a binder-free cathode for hybrid supercapacitor. Here the authors performed hydrothermal method at a temperature of 120ºC for 90 min. A monoclinic crystal structure was obtained for this prepared thin film having its morphology change from microflower to nanoflakes in the surface of stainless-steel substrate. The thin film electrode delivers a maximum specific capacity of 571 F/g at a current density of 2.2 A/g in 1 M KOH electrolyte. They assembled an asymmetric supercapacitor cell using this prepared thin film as cathode and reduced graphene oxide as anode. This assembled supercapacitor cell delivers a specific capacitance of 128 F/g at a current density of 1 A/g having energy density of 45.7 Wh/kg and power density of 1.65 kW/kg. Igbal et al.26 prepared a binary composite of COMAP and cobalt manganese sulfide for supercapattery fabrication. Here they prepared COMAP using sonochemical approach, for a duration of 45 min at room temperature, with the on/off pulse ratio as 2:1 and a fixed amplitude of 30%. After preparing the COMAP material, it is kept for drying in muffle furnace at a temperature of 80ºC for 12 h and its blended and calcinated at 400ºC for 4 h. The authors of this work have noticed a flake-like morphology to the as-prepared COMAP. These individual phosphate species are producing an increased current response and mobility rate. In the binary composite prepared with sulfides, the authors noticed a neat surface corresponds to cobalt but a bumpy glowing surface for manganese sulfide. A specific capacity of 537 C/g is achieved for the electrode at a current density of 1 A/g. Using this electrode material, the authors of this work assembled an asymmetric supercapattery (ASC) cell using activated carbon (AC) as the negative electrode. This ASC cell delivers an energy density of 73.5 Wh/kg with a power density of 1020 W/kg. The proposed electrode delivers an efficient supercapattery performance.

Herein, we report the rapid synthesis of fluffy platelet-like COMAP nanostructures via microwave-assisted hydrothermal approach. The synthesis of COMAP nanostructures within a processing period of 12.5 min is not reported in the literature which is the significance of this experimental research work. The conventional hydrothermal method possesses demerits such as non-uniform heating, irregular and non-uniform particle-size and particle-size distribution, extremely long processing time (24 h to 48 h), etc. These demerits are waived off by using microwave-assisted hydrothermal approach. The published works of COMAP are based upon a long duration synthesis route with higher power consumption. This work focus on synthesis in a short-time duration of only 120℃ for 12.5 min. This bulk synthesis in a reduced time period makes the more economic advantages and feasibility for this proposed COMAP is suitable as an electrode-active material for supercapattery fabrication. There is no report based on the microwave-assisted synthesis of COMAP and their application in supercapattery fabrication. To develop efficient electrode materials for supercapattery fabrication in a low-cost method which provides higher performance is a competitive task. In the present work, we introduce COMAP synthesis in a low-cost microwave assisted hydrothermal method to use it as an efficient electrode for supercapattery fabrication. The effect of precursor ratios on the microstructure and surface morphology of COMAP nanostructures are examined by varying the cobalt to manganese precursor ratios. Moreover, the effect of precursor ratios on the electrochemical performances of the COMAP electrodes also examined systematically using various electrochemical characterization tools such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD) measurement.

Cobalt chloride hexahydrate (CoCl2.6H2O), manganese chloride hexahydrate (MnCl2.6H2O), disodium phosphate (Na2HPO4), polyvinylidene difluoride (PVDF), carbon black, and N-methl-2-pyrrolidone (NMP) (C5H9NO; 99%) were purchased from Sigma-Aldrich. Deionized water was used for the microwave hydrothermal synthesis of COMAP nanostructures.

A set of four different COMAP samples were prepared by varying the cobalt precursor to manganese precursor ratio as 80:20, 60:40, 40:60 and 20:80 and labelled as COMAP-82, COMAP-64, COMAP-46 and COMAP-28, respectively. In a typical procedure, 0.1 M of CoCl2.6H2O solution were mixed with 0.1 M MnCl2 .6H2O solution in a total volume of 50 mL (termed as solution 1). Later, 10 mL of 0.1 M Na2HPO4 was prepared separately in deionized water and added dropwise in solution 1 under constant stirring. Further, the solution mixture is subjected to the microwave-assisted hydrothermal procedure at a temperature of 120℃ for 12.5 min at a heating rate of 5ºC per minute. The resultant product was a colloidal solution appeared to be light pink in colour, which was then washed with deionized water and dried in an oven at 60 °C for 24 h. The overall process of synthesizing COMAP using microwave-assisted hydrothermal method is depicted in Fig. 1.

Schematic diagram of synthesizing COMAP by microwave-assisted hydrothermal method.

The crystal structure of the COMAP samples before and after the calcination step were determined using X-ray diffraction (XRD) analysis using an X-ray diffractometer (Bruker D8 Advance) with Cu-kα radiation of wavelength 1.5406 Å. The microstructure and surface morphology of microwave hydrothermally-synthesized COMAP samples were examined using scanning electron microscope (FESEM) imaging and high-resolution transmission electron microscope (HRTEM) imaging. The elemental composition and the distribution of elements in COMAP were analysed using energy dispersive X-ray spectroscope (EDX) connected to the SEM. The mass of samples was measured using a high-precision microbalance having readability of 10 µg.

The positrode for supercapattery was prepared using COMAP coated over nickel foam substrate. In order to achieve electronic conductivity, nickel foam was used as a substrate cum current collector for the electrode. The electrodes for supercapatteries were prepared by slurry-casting method. A slurry was prepared using 75% active-material (COMAP), 10% PVDF, and 15% carbon black. The electrode coating area is fixed as 0.25 cm2 and the mass of COMAP is fixed as 3 mg during the supercapattery electrode fabrication. The slurry was mixed thoroughly with the help of mortar and pestle by adding NMP drop-wise and further drop-casted over the pre-washed nickel foam and dried at 70ºC in a vacuum oven for 24 h. The geometrical area of COMAP electrode was fixed to 0.5 × 0.5 cm2. The mass of electrode-active material (COMAP) was fixed to 3 mg in the electrode. In a similar manner, negatrode was prepared using AC as electrode-active material and followed the same steps as used in the preparation of COMAP electrodes.

The electrochemical performances of COMAP electrodes were examined using CV, EIS, and GCD analyses. The CV analysis was performed within a potential window of 0.0 V to 0.5 V at different scan rates such as 100, 80, 60, 40, 20, 10, 5, 3, and 1 mV/s. The EIS analysis was carried out within a frequency range of 105 to 0.01 Hz. The COMAP electrodes were tested in a three-electrode cell configuration by keeping COMAP electrode as working electrode, platinum wire as counter electrode, and saturated calomel electrode (SCE) as the reference electrode. All the electrochemical characterizations were carried out using 3 M KOH (aqueous) electrolyte. The supercapattery performances of COMAP electrodes was also tested in a two-electrode cell configuration by taking COMAP-82 as positrode and AC as negatrode in 3 M KOH (aqueous) electrolyte.

The crystal structure of different COMAP samples is determined using XRD analysis. Figure 2a shows the XRD spectra of COMAP samples synthesized with different precursor ratios. To show the formation of COMAP and to distinguish it from the individual phases such as cobalt phosphate and manganese phosphate, we have marked all their corresponding peaks in the Fig. 2a with asterisks. The peaks positioned at 9.42º, 10.95º, 13.6º, 27.7º, 35.9º and 55.4º are corresponding to the reflections from the (001), (110), (020), (121), (231), and (501) planes, respectively and agrees with the XRD for COMAP synthesized by different methods reported in the literature26 and this result agrees with JCPDS Card No. 33–432, which confirms the monoclinic crystal structure of cobalt phosphate. From the XRD analysis, it is evident that the formation of other crystalline phases along with COMAP. For example, there exists the appearance of monoclinic manganese phosphate structure (represented with *red colour), which agrees with ICSD Card No. 23-54126. The chemical bonding information in the COMAP samples are examined using FTIR analysis. Figure 2b shows the FTIR spectra of COMAP samples synthesized at different precursor ratios. All the prepared samples possess peaks at 500 cm− 1, 900–1100 cm− 1 and near to 1500 cm− 1. The bending vibration of O-P-O bond appears at the peak position 500 cm− 1 and stretching vibration hold by P-O and P = O bond corresponds to 900–1100 cm− 1 and near to 1500 cm− 1, respectively. All these peaks confirms that samples contained phosphate27.

(a) XRD spectra and (b) FTIR spectra of COMAP-82, COMAP-64, COMAP-46, and COMAP-28.

Surface morphological features of prepared COMAP samples are given in Fig. 3. A fluffy platelet-like morphology can be observed for all the COMAP samples with a variation in their lateral size. This platelet-like morphology is beneficial for ionic diffusion when used them as electrode-active materials in supercapatteries which helps in higher mobility and current26. In COMAP-82 (Fig. 3a, b) exhibits a more platelet-like morphology without any aggregation when compared to that of other samples such as COMAP-64 (Fig. 3c, d), COMAP-46 (Fig. 3e, f), and COMAP-28 (Fig. 3g, h). The electrochemical performance evaluation of the samples shows that COMAP-82 exhibits superior performance (will be discussed at a later stage), hence this sample is taken as the optimized one for the supercapattery fabrication. We have also carried out the HRTEM imaging for the optimized sample, COMAP-82. The HRTEM images of COMAP-82 at different magnifications is given in Fig. 4. The platelet-like morphology and aggregation of many of such individual structures constitute a single platelet (Fig. 4a, b). The polycrystalline nature of COMAP-82 is confirmed from the high-magnification image (Fig. 4c) and selected area electron diffraction (SAED) pattern (Fig. 4d).

Morphology analysis of COMAP samples. SEM images at different magnifications for (a, b) COMAP-82, (c, d) COMAP-64, (e, f) COMAP-46, and (g, h) COMAP-28.

(a-c) HRTEM images at different magnifications and (d) SAED pattern of COMAP-82.

To determine the appearance of elements, present in synthesized COMAP samples, we have performed elemental analysis using EDS spectra and it is shown in Fig. 5. In COMAP-82 sample it contains a weight% of 48.69, 10.24, 37.23 and 3.83% of O, P, Co and Mn with corresponding atomic% of 74.68, 8.11, 15.5 and 1.71%, respectively (Fig. 5a). For COMAP-64 sample, it contains a weight% of 44.68, 13.95, 6.65 and 34.73% of O, P, Co and Mn with corresponding atomic% of 70.64, 11.39, 3.06 and 14.91%, respectively (Fig. 5b). In COMAP-46 sample, it contains a weight% of 27.56, 14.11, 13.51 and 44.82% of O, P, Co and Mn with corresponding atomic% of 54.09, 14.31, 7.72 and 23.88%, respectively (Fig. 5c). For COMAP-28 sample, it contains a weight% of 46.58, 13.58, 25.47 and 14.37% of O, P, Co and Mn with corresponding atomic% of 71.75, 10.81, 11.43 and 6.01%, respectively (Fig. 5d). The elemental mapping images of COMAP samples are given in Fig. 6, which confirms the presence of Co, Mn, P and O in the prepared samples.

Elemental analysis of COMAP samples. SEM-EDX spectra for (a) COMAP-82, (b) COMAP-64, (c) COMAP-46, and (d) COMAP-28. Inset images show the weight% and atomic % of various elements present in the samples.

Elemental mapping analysis of COMAP samples. SEM-EDX mapping images for (a) COMAP-82, (b) COMAP-64, (c) COMAP-46, and (d) COMAP-28 for the elements: (e, i, m, q) cobalt, (f, j, n, r) manganese, (g, k, o, s) phosphor, and (h, l, p, t) oxygen.

The electrochemical performance characteristics hold by synthesized COMAP electrodes are evaluated through CV, GCD and EIS analysis using 3 M KOH electrolyte. The EIS measurement obtained for COMAP electrodes are given in Fig. 7a-d. Figure 7a represents the Nyquist plot obtained for COMAP-82 electrode with inset shows the equivalent circuit model. The obtained Nyquist plot have three regions: semicircle in high-frequency area, Warburg line having slope of 45º in mod-frequency region. A near vertical line to Y-axis in in low-frequency area represents usually an ideal supercapacitor character, but in the present study COMAP-82 electrode shows a shift from this ideal character, hence it is a supercapattery one, not a supercapacitor. From the semi-circle in high frequency region, it is possible to obtain the values of resistances such as bulk solution resistance of electrolyte and ion charge transfer resistance can be obtained. A bulk solution resistance of 0.61 Ω was obtained for COMAP-82 electrode. Figure 7b-d show the Nyquist plot of COMAP-64, COMAP-46 and COMAP-28 electrodes, respectively and they exhibit the similar character as like as COMAP-82 electrode. Here bulk solution resistance of 0.41, 0.58 and 0.54 Ω respectively for the case of COMAP-64, COMAP-46 and COMAP-28. The capacitive character exhibited by all these assembled electrodes are divulged in the low-frequency region, where hydroxide ions accumulation introduced a line parallel to vertical axis. To understand the charge-storage characteristics hold by assembled electrodes CV analysis was performed at a potential window of 0–0.5 V with different scan rates of 100 mV/s, 80 mV/s, 60 mV/s, 40 mV/s, 20 mV/s, 10 mV/s, 5 mV/s, 3 mV/s and 1 mV/s. The elevated potential window in this region delivers higher storage performance and rate capability. The CV curve obtained for COMAP-82 electrode is shown in Fig. 7e. Symmetric character in CV curve obtained for all the scan rates represent efficient charging and discharging capability of prepared electrode. There exists an anodic and cathodic peak with respect to the corresponding region of oxidation and reduction potential values at all the scan rates indicates the appearance of Faradaic reaction. This Faradaic reaction corresponds to battery-type supercapacitor character of prepared electrode. A similar character was observed in the case of other COMAP-64 (Fig. 7f), COMAP-46 (Fig. 7g) and COMAP-28 (Fig. 7h) electrodes also. The capacitive charge storage arises from the intrinsic pseudocapacitive behaviour of Mn3(PO4)2 in this bimetallic phosphate given by the equations,

The asymmetric shape in CV curve is occurred due to the Co atoms partial inclusion into Mn in order to form bimetallic phosphate. Basically, this irregular shape character is due to the highest contribution of pseudocapacitive character by Mn and intercalation/deintercalation of Co surface. The Co metal introduction enables the transition between Co3+ and Co2+ states by reaction kinetics with alkaline metal in KOH electrolyte.

Generally, in the case of bimetallic phosphate, one metal makes a prominent increase in surface area and electrical conductivity to this resultant bimetallic phosphate, while the other metal component induces this supercapattery performance. To distinguish the charge-storage chemistry behind these prepared electrode materials, GCD analysis was done in the voltage range of 0–0.5 V. The GCD curve of these electrode materials have symmetric charge-discharge performance with respect to different current density, representing the efficient reversible charge/discharge characteristics of COMAP electrodes (Fig. 7i-l). The GCD curve shows that the electrode typical battery-type charge storage characteristics with a non-linear relationship to the applied voltage, unlike capacitive-type charge storage. The occurrence of three segments in discharge curve depicts the battery-type electrode material behaviour.

Electrochemical performance evaluations of COMAP electrodes. Nyquist plot obtained for (a) COMAP-82, (b) COMAP-64, (c) COMAP-46, and (d) COMAP-28 (Inset images represent their equivalent circuit); CV curves obtained at different scan rates for (e) COMAP-82, (f) COMAP-64, (g) COMAP-46, and (h) COMAP-28; GCD curves obtained at different current densities for (i) COMAP-82, (j) COMAP-64, (k) COMAP-46, and (l) COMAP-28.

The charge storage character exhibited by these COMAP electrodes possess either a diffusion-controlled or diffusion-limited mechanisms. The gravimetric capacity (Qm, cv (in C/g)) of the COMAP electrode is calculated from the CV curves using the equation,

Where, ‘M’ is the mass of COMAP electrode (here we only taking the mass of COMAP active material), ‘I’ is the current and ‘\(\upsilon\)’ is the scan rate (in mV/s).

Figure 8a shows the mass specific capacity obtained for COMAP-82 electrode with different scan rates. The COMAP-82 electrode exhibits a highest mass specific capacity of 191.4 C/g and 75.6 C/g at the scan rates of 1 and 100 mV/s, respectively. In the case of COMAP-64 (Fig. 8c), this mass specific capacity obtained to be 148.8 C/g and 67.3 C/g at 1 and 100 mV/s respectively. For COMAP-46 (Fig. 8e), the mass specific capacity calculated to be 74.4 C/g and 19.7 C/g respectively. The mass specific capacity calculated for COMAP-28 the mass specific capacity was 79.3 C/g and 28.4 C/g (Fig. 8g) respectively. In addition to the mass specific capacity, area specific capacity is another important criterion for application in portable electronic devices to power it.

Plot of variation in the mass specific capacity at different scan rates for (a) COMAP-82, (c) COMAP-64, (e) COMAP-46, and (g) COMAP-28. Plot of variation in the area specific capacity at different scan rates for (b) COMAP-82, (d) COMAP-64, (f) COMAP-46, and (h) COMAP-28.

For a planar device structure, electrochemical energy device is found to be compact with electrochemical device in consideration and it is not cumbersome. Thus, these assembled categories of supercapattery devices are more preferable in planar electronic device application. The area specific capacity, QA (in C/cm2) of COMAP electrode is calculated from the CV curves using the equation,

Where, ‘A’ is the geometric area of electrode and other variables are discussed in Eq. 4. Figure 8b represents the variation in area specific capacity of COMAP-82 electrode with respect to different scan rates. The maximum area specific capacities of 0.57 C/cm2 and 0.22 C/cm2 are obtained for COMAP-82 electrode at a scan rate of 1 mV/s and 100 mV/s, respectively. In the case of COMAP-64 electrode, the area specific capacity of 0.44 C/cm2 and 0.20 C/cm2 at a scan rate of 1 mV/s and 100 mV/s, respectively (Fig. 8d). For COMAP-46 electrode (Fig. 8f), the calculated values of area specific capacity were 0.22 C/cm2 and 0.05 C/cm2 at the scan rates of 1 and 100 mV/s respectively. In the case of COMAP-28 electrode (Fig. 8h), area specific capacity calculated was 0.23 C/cm2 and 0.08 C/cm2 at a scan rate of 1 mV/s and 100 mV/s, respectively. The optimized stoichiometric ratio of Co: Mn modifies the electronic structure that in turn facilitates the rapid electron/ion transportation pathways. The COMAP nanostructure possesses large amount of active interfacial sites thereby boosting the reactivity as well as the conductivity of electrode, particularly in the case of COMAP-82 electrode and thus it achieved superior electrochemical characteristics.

XPS high-resolution spectra for (a) Co 2p, (b) Mn 2p, (c) P 2p, and (d) O 1s for the COMAP-82 sample.

Since the COMAP-82 sample exhibits superior electrochemical performance when compared to the other 3 samples, it is further analysed to understand its peculiar properties such as surface chemical bonding, surface area, type of charge storage, etc. The surface chemistry and chemical bonding information of the COMAP-82 sample is examined using XPS analysis. High-resolution XPS analysis was introduced to evaluate the Mn/Co influence on surface binding oxidation state of resultant COMAP-82 sample. The high-resolution XPS deconvolution spectra for Co 2p, Mn 2p, P 2p and O 1s are performed. The deconvoluted XPS spectra for Co 2p are shown in Fig. 9a.The Co 2p spectrum is fitted to two prominent binding energy values at 780.9 eV and 797.7 eV corresponds to Co 2p3/2 and Co 2p1/2, respectively25. The deconvoluted spectra for the Mn 2p are given in Fig. 9b, which shows two intense peaks located at 641.4 and 653.3 eV corresponds to Mn 2p3/2 and Mn 2p1/2, respectively25. The satellite peaks are also present in the spectra positioned at 644.4 eV and 655.4 eV corresponding to Mn2+ and Mn3+ state of manganese, respectively25. The deconvoluted XPS spectra for the P2p spectra are depicted in Fig. 9c, which possess a major peak positioned at a binding energy corresponding to 132.7 eV representing the 2p3/2 state, confirms the pentavalent valence state exhibited by phosphate (PO4)3−25. In addition to this, the deconvolution XPS spectra for the O 1s given in Fig. 9d shows three major fitted peaks positioned at 530.0 eV, 530.6 eV and 531.8 eV are corresponding to metal-oxygen, phosphate (P-O) and oxygen in -OH group in COMAP, respectively25.

(a) BET N2 sorption isotherms and (b) BJH N2 sorption pore width distribution curves.

The surface area of an electrode-active material is important in the case of an EDLC as the performance is directly proportional to the active surface area. But in the case of battery-type hybrid electrode, the surface area doesn’t play a significant role in achieving high performance when using them as supercapattery electrodes but the pore-size play a crucial role in determining the performance as the pores decides the penetration of electrolyte-ions through it. The specific surface area and pore-size of COMAP-82 are examined using N2 sorption Brunauer-Emmet-Teller (BET) surface area measurement. The N2 adsorption/desorption isotherms of COMAP-82 sample is depicted in Fig. 10a, which depicts a type IV isotherm character, indicating material with a mesoporous architecture. The specific surface area of COMAP-82 electrode is calculated to be 3.91 m2/g. The pore-size distribution of COMAP-82 during the N2 adsorption and desorption process is given in Fig. 10b. The BJH average pore width of COMAP-82 during the N2 adsorption is found to be 17.8 nm whereas it is 16.6 nm during the N2 desorption process. This clearly shows that the average pore width falls within the mesopore range hence the electrode-active material is said to have a mesoporous architecture. These mesoporous channels facilitate the easy movement of electrolyte-ions through it during the rapid charging/discharging process hence the electrochemical performance of the electrode thus gets enhanced.

In order to investigate the type of charge storage possessed by the COMAP electrode, Dunn’s kinetic method is adopted28. This method helps to distinguish the battery-type and capacitive type charge storage mechanism in the best performing COMAP electrode (which is COMAP-82). Dunn’s kinetic method was used to evaluate the individual mechanism (diffusion-controlled and surface-controlled) that contributes to the net charge storage. It is necessary to illustrate this proposed approach, especially for supercapattery electrodes, because there exist various reaction mechanisms in charge storage and it is mandatory to confirm that the charge storage is battery-type and not capacitive type. A complete study on electrochemical performance exhibited by COMAP-82 electrode can be obtained through plotting of graph connecting scan rate (ʋ) and associated peak current (i) given by Eqs29,

Where ‘a’ and ‘b’ are parameters changing from 0.5 to 1. The value of b is the slope of graph connecting log (i) versus log (ʋ). In accordance to the literature reports, the value of b, ~ 0.5 indicates the redox rection is controlled battery-type character(diffusion-controlled) and if it is ~ 1, the reaction become surface-controlled one30. The plot corresponds to log (i) versus log (ʋ)for anodic and cathodic process are given in Fig. 11a and b. The average value of b obtained to be 0.67, which represents a diffusion-controlled mechanism, hence it is mean that prepared COMAP-82 electrode is a supercapattery one, rather than a perfect capacitor. The contribution to capacitive and diffusion-controlled mechanisms on total charge storage mechanism at different scan rates are given by,

Where, \(k_{1} \upsilon\) and \(k_{1} \upsilon ^{{0.5}}\) correspond to the contribution of current from the capacitive processes and diffusion processes, respectively. In the CV curve (Fig. 11c) at a scan rate if 3 mV/s, it is found that diffusion-controlled contribution is about 96.8% (shaded in orange colour) but the surface-controlled contribution is only about 3.2% (shaded in pink colour). The battery-type (diffusion-controlled contribution) by COMAP-82 electrode was 98.6, 96.8, 94.1, 90.8, 88.3, 88.0, 86.3, 84.3 and 82.1% at a scan rate of 1, 3, 5, 10, 20, 40, 60, 80 and 100 mV/s, respectively (Fig. 11d). The cycling stability of the COMAP-82 supercapattery electrode is performed at a constant scan rate of 40 mV/s for continuous 10,000 cycles and the capacity retention with respect to different cycle numbers is depicted in Fig. 11e. A specific capacity retention of 95.0% is obtained for the COMAP-82 electrode even after completing 10,000 cycles. In order to examine the resistance of the COMAP-82 electrode after the cycling study, EIS analysis is once again implemented. Figure 11f shows the Nyquist plot of COMAP-82 electrode before and after completing the cycling study. From these plots, it can be observed that the resistance of the COMAP-82 electrode is increased upon cycling, which is the reason behind the decrease in capacity after the cycling study.

Dunn’s kinetic method to examine the type of charge storage in COMAP-82 electrode. Plot of, log scan rate v/s log peak current for the (a) anodic peaks, (b) cathodic peaks; (c) CV curve obtained at a constant scan rate of 3 mV/s depicting the charge storage contributions from both diffusion-controlled and surface-controlled reactions; (d) histogram showing the contributions to total charge storage at different scan rates; (e) CV cycling stability test for the COMAP-82 for 10,000 cycles performed at 40 mV/s; (f) Nyquist plots for the COMAP-82 supercapattery electrode before (initial cycle) and after completing 10,000 cycles.

To illustrate the practical application of prepared COMAP-82 electrode we have fabricated an ASC cell using this combination as positrode and AC as negatrode. The AC hold an operating potential window in the range of -1.0 V to 0.0 V and we already optimized the working potential window of COMAP-82 as 0.0 V to 0.5 V by using 3 M KOH (Aqueous) electrolyte (Fig. 12a). Figure 12b represents the CV curve for the assembled COMAP//AC ASC cell in a potential window of 0 to 1.5 V. The CV with respect to different scan rates exhibits a large area and it delivers the efficient electrochemical characteristics of assembled supercapattery. Figure 12c shows the GCD curve of asymmetric supercapattery device and it show a typical charge-discharge of battery-type device with almost same charge-discharge intervals. The Nyquist plot of COMAP//AC ASC cell with equivalent circuit model is given in Fig. 12d. As like as the Nyquist plot of three-electrode arrangement, the asymmetric supercapattery device also possess three distinct regions. The bulk solution resistance of this COMAP//AC ASC cell was obtained to be 3.0 Ω. The mass specific specific capacity (QS) (C/g) and areal capacity (QA) (C/cm2) from GCD curve using the Eqs31,

Where, i is the current density during discharge process, dt is the discharge time. The area capacity QA, cv (in C/cm2) of the COMAP//AC asymmetric supercapattery is calculated from GCD curves using the equation,

The mass specific capacity calculated for this device at different current density is shown in Fig. 13a. The device delivers a mass specific capacity of 165.5 C/g at a current density of 1.8 A/g. The energy density and power density are two major factors for the practical application of this assembled asymmetric supercapattery device. The mass specific energy density (Em) and powder density (Pm) of the COMAP//AC ASC cell is calculated from GCD curves using the equation32,

Where, ‘ΔE’ is the operating potential window.

The COMAP//AC ASC cell exhibits an energy density of 34.1 Wh/kg (Fig. 13b) with a corresponding power density of 1875 W/kg (Fig. 13c) at a current density of 1.8 A/g. A relationship between energy and power density of assembled COMAP//AC ASC cell was given by Ragone plot as shown in Fig. 13d. From this analysis, it is found that prepared ASC cell is a potential candidate for practical application in portable electronics.

We have demonstrated the practical application of asymmetric supercapattery device by lighting up a green light-emitting diode. For this, two similar types of asymmetric supercapattery device are fabricated and connecting in series connection, thus the operating voltage of supercapattery module is 3 V. After charging the device to 3 V, it is connected to green LED and we observed lighting-up of LED having higher intensity as shown in Fig. 13e. From the proposed results of asymmetric supercapattery device it is clear that prepared COMAP electrode is an efficient electrode material for practical application.

(a) CV curves of COMAP-82 positrode and AC negatrode obtained in 3 M KOH (aqueous) electrolyte at different potential windows; (b) CV curves of COMAP//AC ASC cell at different scan rates within a potential window of 0.0 V to 1.5 V in 3 M KOH (aqueous) electrolyte; (c) GCD curves of COMAP//AC ASC cell at different current densities within a potential window of 0.0 V to 1.5 V in 3 M KOH (aqueous) electrolyte; (d) Nyquist plot with an equivalent circuit model (inset) of COMAP//AC ASC cell.

(a) Plot of variation in the mass specific capacity of COMAP//AC ASC cell at different scan rates; (b) Plot of variation in the energy density of COMAP//AC ASC cell at different scan rates; (c) Plot of variation in the power density of COMAP//AC ASC cell at different scan rates; (d) Ragone plot; (e) Digital image of a supercapattery module comprising of two similar COMAP//AC ASC cells connected in series combination and immediately after charging to a potential of 3 V (individual cell provides an operating potential of 1.5 V) lighting-up a green LED (operating voltage of 2.3–2.5 V).

A comparative analysis on different report based on COMAP for supercapacitor/supercapattery applications are given in Table 1. From the present comparison, it is clear that prepared COMAP material act as an efficient electrode for supercapattery fabrication. The proposed low-cost approach facilitates the fabrication of supercapattery device, without introduce any wide range of energy and time consumption. Thus, this method is a suitable one in an industrial scale for the widespread application of supercapattery application based on bimetal phosphates. The proposed synthesis strategy is a facile one which prepare materials with higher scalability, which possess advantageous such as fast heating speed, uniform arrangement in heating system and sensitive reaction over conventional method. Microwave assisted hydrothermal method efficiently prepare particles having narrow distribution and uniform morphology. Thus, microwave assisted hydrothermal method have great potential for ultrafine powder preparation, which promotes a simple, clean and economical route than conventional approaches. It is possible to further improve the performance of COMAP electrodes by the preparation of hybrid/heterostructure with other active materials like carbonaceous compounds, MXenes, conducting polymers, etc.

We have successfully synthesized COMAP by microwave-assisted hydrothermal method in a short processing time of 12.5 min. The variation in the microstructure and surface morphology of the COMAP samples were examined by varying the cobalt to manganese precursor ratio using SEM, SEM-EDX spectral analysis and SEM-EDX mapping analysis. A set of four samples were synthesized, such as COMAP-82, COMAP-64, COMAP-46, and COMAP-28 by varying the cobalt to manganese precursor ratio. Among these samples, COMAP-82 exhibited superior electrochemical performances when compared to others and selected as optimized one to fabricate the ASC cell. The crystal structure of the COMAP samples were determined using XRD analyses and the chemical bonding information was recorded using FTIR analysis and are found to be well matched with the data reported for the COMAP in the literature. The charge storage characteristics of the COMAP electrodes were tested using various electrochemical testing tools such as CV, EIS, and GCD measurement. The SC electrode fabricated with COMAP synthesized using cobalt: manganese precursor ratio as 80:20 exhibits a highest specific capacity of 191.4 C/g at a scan rate of 1 mV/s. An asymmetric SC (ASC) fabricated with COMAP as positrode and activated carbon (AC) as negatrode exhibits a specific capacity of 165.5 C/g at a current density of 1.8 A/g. The COMAP//AC ASC exhibits an energy density of 34.1 Wh/kg at a corresponding power density of 1875 W/kg at a current density of 1.8 A/g.

Data is made available on request.

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This research work was carried out with the financial support from Sunway University Sdn Bhd. under the project entitled “Development of Sustainable Supercapatteries for Next-Generation On-Body Wearables” under Sunway University Internal Grant Scheme 2022 (Grant Code: GRTIN-IGS(02)-GAMRG-03-2022) awarded to Dr. Jayesh Cherusseri.

Research Centre for Nanomaterials and Energy Technology (RCNMET), School of Engineering and Technology, Sunway University, No. 5, Jalan Universiti, Bandar Sunway, 47500, Darul Ehsan, Selangor, Malaysia

Jayesh Cherusseri, A. K. Pandey, MA Zaed & R. Saidur

School of Engineering and Technology, Sunway University, No. 5, Jalan Universiti, Bandar Sunway, 47500, Darul Ehsan, Selangor, Malaysia

Jayesh Cherusseri

Department of Physics, Government College for Women (Affiliated to University of Kerala), Thiruvananthapuram, Kerala, 695014, India

Susmi Anna Thomas

CoE for Energy and Eco-Sustainability Research, Uttaranchal University, Dehradun, India

A. K. Pandey

Department of Physics, Faculty of Science, Centre for Ionics Universiti Malaya, Universiti Malaya, 50603, Kuala Lumpur, Malaysia

N. K. Farhana

School of Engineering, Lancaster University, Lancaster, LA1 4YW, UK

R. Saidur

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Jayesh Cherusseri: Conceptualization, Methodology, Investigation, Data Curation, Validation, Visualization, Writing: Original Manuscript, Writing: Review and Editing, Funding Acquisition, Project Management, Submission of Manuscript. Susmi Anna Thomas: Methodology, Investigation, Data Curation, Writing: Original Manuscript, Writing: Review and Editing. A.K. Pandey: Formal analysis, Funding acquisition, Visualization, Resources, Writing: Original Manuscript; Writing: Review and Editing. MA Zaed: Formal analysis, Visualization. N.K. Farhana: Data curation, Formal analysis, Software, Visualization. R. Saidur: Formal analysis, Project administration, Resources, Writing–review & editing.

Correspondence to Jayesh Cherusseri, A. K. Pandey or R. Saidur.

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Cherusseri, J., Thomas, S.A., Pandey, A.K. et al. Rapid synthesis of cobalt manganese phosphate by microwave-assisted hydrothermal method and application as positrode material in supercapatteries. Sci Rep 14, 26550 (2024). https://doi.org/10.1038/s41598-024-77278-w

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Received: 14 August 2024

Accepted: 21 October 2024

Published: 04 November 2024

DOI: https://doi.org/10.1038/s41598-024-77278-w

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