Synergistic redox enhancement: silver phosphate augmentation for optimizing magnesium copper phosphate in efficient energy storage devices and oxygen evolution reaction

The implementation of battery-like electrode materials with complicated hollow structures, large surface areas, and excellent redox properties is an attractive strategy to improve the performance of hybrid supercapacitors. The efficiency of a supercapattery is determined by its energy density, rate capabilities, and electrode reliability. In this study, a magnesium copper phosphate nanocomposite (MgCuPO4) was synthesized using a hydrothermal technique, and silver phosphate (Ag3PO4) was decorated on its surface using a sonochemical technique. Morphological analyses demonstrated that Ag3PO4 was closely bound to the surface of amorphous MgCuPO4. The MgCuPO4 nanocomposite electrode showed a 1138 C g−1 capacity at 2 A g−1 with considerably improved capacity retention of 59% at 3.2 A g−1. The increased capacity retention was due to the fast movement of electrons and the presence of an excess of active sites for the diffusion of ions from the porous Ag3PO4 surface. The MgCuPO4–Ag3PO4//AC supercapattery showed 49.4 W h kg−1 energy density at 550 W kg−1 power density and outstanding capacity retention (92% after 5000 cycles). The experimental findings for the oxygen evolution reaction reveal that the initial increase in potential required for MgCuPO4–Ag3PO4 is 142 mV, indicating a clear Tafel slope of 49 mV dec−1.


Introduction
Nowadays, the most commonly used energy resources are fossil fuels, which lead to the emission of hazardous gases into the environment.Moreover, the increasing demand for energy is resulting in the quick depletion of these energy resources.All of these circumstances require renewable energy resources to fulll energy requirements.However, due to the intermittent behavior of wind and solar energy resources, the use of energy storage devices (ESDs) has become more prominent.An ESD balances the demands and supply of electricity.Two similar kinds of electrode are used in electric double-layer capacitors (EDLCs) that deliver quick bursts of energy, long cycle life, and exceptional power density.However, they have low energy density, limiting their ability to meet increasing technological needs. 1,2Lithium-ion batteries have high energy but have a disadvantage in delivering quick bursts of energy, which signicantly lowers their life span compared to an EDLC.Furthermore, if overloaded or overheated, an LIB can explode. 3,4here have been numerous efforts to broaden the EDLC working potential and increase its energy while maintaining its power density.A new asymmetric device known as a supercapattery has recently received a lot of attention because it combines capacitive-graded materials (electrostatic storage) and other battery-graded materials (faradaic storage).RuO 2 , a well-known outstanding pseudocapacitor electrode, demonstrated an exceptional capacitance of 1340 F g −1 .But its high expense and ecotoxicity prevent it from being used commercially.Other oxide-based metals, such as NiO and Co 3 O 4 , show promise, but their electrical resistance is low.As a result, it is critical to identify materials for a supercapattery that are inexpensive but have similar effectiveness to current materials. 5,6he catalysts used in the oxygen electrode for the oxygen evolution reaction (OER) have garnered signicant attention due to their crucial role in these systems.There is an urgent need to develop highly efficient catalysts, free of noble metals, that exhibit superior catalytic activity for the OER.This need arises from the desire to replace expensive and scarce RuO 2 and IrO 2 catalysts. 7cientists are presently focusing on tertiary phosphides as encouraging electrodes in supercapacitors.For example, Lei et al. 8 used a simple hydrothermal process to make a ZnNiCoP-NF nanostructure, which showed 1111 C g −1 capacity at 10 A g −1 .They demonstrated that the fabricated electrochemical design is ideal for energy-storage uses.Yang et al. 9 produced Ni-Co-Mo ternary phosphide formed on carbon cloth bers, which had a specic capacitance of 433 F g −1 at 1 A g −1 current density.
Silver is undoubtedly a viable substitute for carbon-based materials.Moon et al. 10 developed a translucent and exible supercapacitor with silver nanowires coated with gold.2][13] Silver nanoparticles with graphene, metal oxides, carbon nanotubes, and conducting polymers were tried earlier.A matrix of Ag with carbon in a composite form shows no pseudocapacitance.However, due to the increased electrical activity, its EDLC performance improves.The Ag/PANI nanocomposite synthesized by Patil et al. 14 and Tang et al. 15 showed that the presence of Ag in a PANI heterostructure makes the movement of ions faster.The Ag coating on PANI produces high specic energy due to quicker electron transport and electrolyte access via a porous network. 16Copper phosphate (Cu 2 PO 4 ) is thought to be a perfect alternative to conventional metallic phosphates since Cu has signicantly greater conductance.Cu 2 PO 4 exhibits further intriguing dielectric and thermal characteristics.The efficacy of Cu 2 PO 4 in supercapacitors has received little attention.Its electrochemical performance was improved by incorporating metal ions, which increases the conductivity.Thus, additional attempts to enhance the electrical conductivity of such electrode materials are required.The addition of another metal ion with similar properties can signicantly improve the electrical characteristics.Thus, the movement of ions between the two metal phosphates becomes easier and the interfacial impedance of the composite substance can be reduced.Additionally, a large useful surface area is accessible for a redox reaction, increasing the capacity and power density of the supercapacitor. 17erein, precipitation followed by calcination was used to produce silver phosphate nanoparticles (Ag 3 PO 4 NPs) on amorphous MgCuPO 4 .Because of its high electrical conductivity, Ag was selected to improve the electrical conductivity of amorphous MgCuPO 4 .To prevent the recombination of particles, an amorphous MgCuPO 4 and silver complex (Ag(NH 3 ) 2 + ) mixture was made using a sonochemical technique.MgCuPO 4 -Ag 3 PO 4 composites were developed to raise the energy density, reduce resistance, and expand the supercapattery cycle life.The presence of low-E g metal ions in the amorphous structure of MgCuPO 4 is still a novel way to increase the supercapacitive performance.Furthermore, the application of the oxygen evolution reaction (OER) was also examined in the context of MgCuPO 4 -Ag 3 PO 4 composites.The impact of different MgCuPO 4 to Ag 3 PO 4 weight ratios on the overall storage capability was investigated.A comprehensive charge transfer procedure between MgCuPO 4 and solid Ag 3 PO 4 was observed.O).The mixture was stirred continuously for 25 min and transferred into an autoclave and heated at 140 °C.The centrifuge machine (10 000 rpm Model RST-10M) was used to eliminate impurities from the solution.Aer drying at 40 °C the prepared MgCuPO 4 was collected.The addition of NH 3 into a 0.5 M solution of AgNO 3 reduces Ag + .A horn sonicator was used for sonication until a diaminesilver(I) complex was produced.The complex solution of diaminesilver(I) was then combined with 0.02 g of MgCuPO 4 solution and continuously stirred (3-4 h).The mixture was rinsed many times with DIW before drying for 4 h at 40 °C.Finally, the desiccated materials were calcined for 3 h at 300 °C and labeled S 1 .Two more MgCuPO 4 -Ag 3 PO 4 nanocomposites were made for optimization reasons by altering the wt% ratio of as-prepared MgCuPO 4 to 0.03 and 0.05 g and they were designated S 2 and S 3 , respectively.Fig. 1 shows the synthetic process for MgCuPO 4 -Ag 3 PO 4 nanocomposites.Table 1 shows different wt% ratios in three different composites of MgCuPO 4 -Ag 3 PO 4 (S 1 , S 2 , and S 3 ).

2.3.1.
Structural and surface analysis.The crystalline phases of the materials (MgCuPO 4 , S 1 , S 2 , and S 3 ) were analyzed using XRD (D5000, Siemens).Ka radiation with 1.54 Å wavelength and 0.02 s −1 scan rate was used to determine the crystallinity of S 1 , S 2 , and S 3 .FTIR (Thermo Scientic Nicolet IS10 Smart ITR) was used to investigate the purity of the samples (S 1 , S 2 , and S 3 ) which were scanned at 1 cm −1 resolution.X-ray photoelectron spectroscopy (XPS) measurements were taken at binding energies from 0 to 1000 eV, with 1 eV resolution.Single XPS spectra were performed at a resolution of 0.1 eV.Origin Pro 8.1 was used to t the spectra with numerous Gaussian curves.Scanning electron microscopy (SEM) was applied to analyze the morphological characteristics of the MgCuPO 4 -Ag 3 PO 4 nanocomposites.Jenway's 6800 design was used to determine the optical absorbance characteristics over a spectral range of 200 to 800 nm.
2.3.2.Electrochemical studies.The active materials (S 1 , S 2 , and S 3 ) were coated onto nickel foam (NF) and employed as a working electrode in an electrochemical research setup.In a three-cell setup, the counter and reference electrodes were platinum wire and Hg/HgO, respectively.Throughout the electrochemical studies in a typical cell, a 1 M potassium hydroxide (KOH) solution was kept constant.The slurry for the working electrode was prepared by blending 75% active material (MgCuPO 4 -Ag 3 PO 4 ) and 15% carbon black, which were bonded together with the help of PVDF binder (10%).This slurry was mixed continuously for 5-6 h to produce a homogeneous suspension.This suspension was then easily applied to NF.Before starting the experiment, the NF was thoroughly cleaned with HCl, ethanol, acetone, and DIW.The NMP slurry was coated on a 1 × 1 cm 2 area of NF.The total weight of the active material was roughly 5.5 mg.Furthermore, in the supercapattery application, activated carbon (AC) and the best composite were used as the positive and negative electrodes.The electrochemical properties of all of the materials were studied using a workstation CS300 potentiostat.A multichannel Autolab, PGSTAT30 potentiostat was used for the Mott-Schottky (MS) measurements.) and other unwanted impurities were eliminated upon calcination.
The specic surface areas (SSAs) of MgCuPO 4 and the MgCuPO 4 -Ag 3 PO 4 (S 2 ) nanocomposite were calculated using the Brunauer-Emmett-Teller (BET) technique, because S 2 demonstrated the best performance compared to S 1 and S 3 , as described in the following part.The N 2 adsorption-desorption isotherms of MgCuPO 4 and S 2 are shown in Fig. 2(c) and (d).The BET graphs for MgCuPO 4 and S 2 exhibit an IV-type hysteresis loop, showing their porous structure.The porous behavior of MgCuPO 4 and S 2 resulted in decreases in electrolyte ion diffusion time.The SSA of MgCuPO 4 and S 2 were 15.72 and 19.34 m 2 g −1 , respectively.The total pore volumes for MgCuPO 4 and S 2 estimated through BET calculations were 0.04928 and 0.052596 cm 3 g −1 , respectively.The smaller SSA and pore volume of MgCuPO 4 compared to S 2 were due to the absence of Ag 3 PO 4 in the MgCuPO 4 -Ag 3 PO 4 nanocomposite.Furthermore, this may be due to the distribution of Ag 3 PO 4 NPs into MgCuPO 4 . 23ig. 3 ).The bands moved to a lower wavenumber as the amount of Ag 3 PO 4 increased in the MgCuPO 4 -Ag 3 PO 4 nanocomposites. 24The band appearing at 1645 cm −1 belongs to v 1 (A 1 ) H 2 O and those at 3200-3800 cm −1 belong to v 2 (A 1 ) H 2 O. Aer the incorporation of Ag 3 PO 4 (S 1, S 2 , and S 3 ), the band intensities rose considerably, showing that the percentages of phosphorous and water on the MgCuPO 4 -Ag 3 PO 4 surface increased. 25The adsorbed water resulted in increased storage performance due to an enhanced inter-particle path. 26,27.1.3.Optical properties of MgCuPO 4 -AgPO 4 .UV-vis absorption spectroscopy was applied to check the impact of Ag 3 PO 4 on the optical performance of MgCuPO 4 .The spectra are shown in Fig. 3(c).In a deformed octahedral coordinated environment, MgCuPO 4 has an absorption edge at 808 nm and a second absorption edge at 1000 nm, which are due to Cu + and Mg + , respectively.28,29 Because of the decreasing fractions of MgCuPO 4 found in MgCuPO 4 -Ag 3 PO 4 nanocomposites, the absorptivity of MgCuPO 4 progressively declines from S 1 to S 3 .Furthermore, as the proportions of MgCuPO 4 decreased, the absorption edge red shied towards the visible region.The existence of more Ag 3 PO 4 NPs than MgCuPO 4 , which absorbs in the visible region, causes the peaks to redshi.This redshi may also be due to narrowing of the band gap, as evidenced by the altered Kubelka-Munk function of light energy (ahn) 2 versus photon energy (hn), as shown in Fig. 3

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was used to determine the band gap (E g ) values of the samples. 30,31hn The absorption coefficient is denoted by a and the proportionality constant by K. K was determined from K = (2.303× 10 3 )(A)/L, where absorbance is represented by A and optical path by L (1 cm).The E g value was estimated by extrapolating the straight line of the image shown in Fig. 3(d).E g values for S 1 , S 2 , and S 3 and amorphous MgCuPO 4 were 3.9, 3.6, 3.5, and 4.3 eV, respectively.The electrical insulating properties of amorphous MgCuPO 4 were reected in its high E g value.The gradual drop in E g in MgCuPO 4 -AgPO 4 nanocomposites was due to the combined impact between the two elements.The overlapping of energy levels causes a reduction in the band gap according to energy band theory.This indicates that Ag 3 PO 4 were well developed on the MgCuPO 4 surface, exhibiting the combined electronic characteristics.and Mg 0 , respectively. 34The XPS spike for Cu 2p 3/2 located at 932.5 eV correlates to Cu 2+ possibly reacting with a phosphate ion.The peak for Cu 2p 1/2 appeared at 952.5 eV, as indicated in Fig. 4(c). 35The XPS spectrum for P 2p is represented in Fig. 4(d).
The presence of P 2p 1/2 at 135 eV and P 2p 3/2 at 133 eV conrm the presence of phosphorous.The deconvoluted XPS spectrum of P 2p showed that the rst peak at 133 eV accounts for 75-80% of the P 2p signal.This can be ascribed to the oxidation of the MgCuPO 4 surface metal phosphate molecule in PO 4 3− . 36The second spike at 135 eV showed the presence of 10-20% residual metaphosphate. 37S 2 demonstrated the presence of symmetrical spin-orbit components.The XPS spectra for Ag showed two distinct peaks which were separated by 6 eV.The Ag 3d 5/2 peak appeared at 368 eV and Ag 3d 3/2 peak appeared at 374 eV, which corresponds to Ag + , as indicated in Fig. 4(e). 38.1.5.Morphological study of MgCuPO 4 -AgPO 4 .SEM analysis was used to examine the shape of MgCuPO 4 and its three composites with Ag 3 PO 4 (S 1, S 2 , and S 3 ).Fig. 5(a)-(d) depict an uneven form of amorphous MgCuPO 4 and its composites with Ag 3 PO 4 with 40-50 nm particle size.Fig. 5 shows that as the amorphous MgCuPO 4 content of the nanocomposite decreased, more Ag 3 PO 4 NPs were found on the surface of the amorphous MgCuPO 4 .However, for S 1 and S 3 , the recombination of ions could restrict the diffusion of ions.The recombination of ions could be attributed to an overabundance of Ag 3 PO 4 , which restricts the movement of MgCuPO 4 ions. 39,40

Electrochemical results
The synthesized samples (MgCuPO 4 , S 1 , S 2 , and S 3 ) were then electrochemically tested using CV measurements, which were determined in the potential window (PW) of 0-0.6 V at a scan rate of 3-50 mV s −1 , as depicted in Fig. 6.For all samples, faradaic behavior was observed from the presence of redox peaks.This was because of the faster movement of ions within the materials, causing a redox reaction at the surface of the electrodes. 41,42S 2 had the greatest redox current strength at 3 mV s −1 scan rate compared to the other MgCuPO 4 -AgPO 4 nanocomposites.This shows that the addition of Ag 3 PO 4 increased the redox current due to the synergetic impact of MgCuPO 4 and Ag 3 PO 4 .But further addition of Ag 3 PO 4 inuenced the redox potentials, as can be seen from the CV plot of S 3 (Fig. 6(d)).The nding suggested that electrolyte ions engage extensively with the numerous electro-active sites of the materials when the amount of Ag 3 PO 4 increases.With higher loadings of Ag 3 PO 4 in MgCuPO 4 , the electrical transmission route and electrolyte ion diffusion rate of the nanocomposites differ.This indicated that the number of phosphate groups in the MgCuPO 4 -Ag 3 PO 4 composite, as well as metal ion concentrations, affect electron and ion motion. 43Furthermore, the variations in anodic and cathodic potentials can be linked to the

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presence of the redox process. 44The CV curve of MgCuPO 4 changes its shape at 100 mV s −1 (Fig. 6(a)) because the electrolyte ions did not reach the MgCuPO 4 inner sites. 45S 2 had a greater CV area (and thus a greater specic capacity) than MgCuPO 4 at a low scan rate.S 2 also maintained its form at higher scan rates due to its stable behavior. 46,47 For battery-type materials, the most suitable word to determine the storage capability is the specic capacity (C g −1 ).However, some researchers in the literature also use capacitance (F g −1 ) which is not correct.Table 3 shows the values of In eqn (8), Q s indicates the specic capacity, m is the active mass, Dv represents the change in scan rate; the current is denoted by I, and the operating potential by V.The specic capacities calculated for MgCuPO 4 , S 1 , S 2 , and S 3 at various scans are represented in Fig. 7(a).The specic capacities for MgCuPO 4 -Ag 3 PO 4 nanocomposites were higher than that for MgCuPO 4 .The specic capacity for S 2 at 3 mV s −1 was 839 C g −1 , which was higher than that for MgCuPO 4 (335 C g −1 ), S 1 (442 C g −1 ), or S 3 (671 C g −1 ).The specic capacities for all samples (MgCuPO 4 , S 1 , S 2 , and S 3 ) at 3 mV s −1 are represented in Fig. 7(b).When an amount of Ag 3 PO 4 is added into the heterostructure of MgCuPO 4 , the specic capacity increases due to the combined impact of MgCuPO 4 -Ag 3 PO 4 , with a large number of accessible pores and faster movement of ions.A further increase in the amount of Ag 3 PO 4 resulted in a decrease in its capacity.An excess amount of Ag 3 PO 4 also results in blockage of a number of pores.Therefore, the specic capacity for S 3 decreases.The positive and negative current spikes measured through CV readings of S 2 gradually increased with the square root of the scan rate.The value of the I pa /I pc fraction was almost 1.This indicated that the reversible faradaic processes were responsible for charge storage, as shown in Fig. 7(c). 49The linear relation between anodic and cathodic currents was evidence of the battery nature of the MgCuPO 4 -Ag 3 PO 4 (S 2 ) nanocomposite.Fig. 7(d) indicates a straight-line relationship between voltage and log of scan rate, which proves the diffusion-controlled process for S 2 .The chemical equations below can be used to describe redox reactions. 50 The discharge proles of MgCuPO 4 and MgCuPO 4 -Ag 3 PO 4 nanocomposites obtained using the GCD method are shown in Fig. 8(a)-(d).The shape of the GCD curves was non-linear for MgCuPO 4 , S 1 , S 2 , and S 3 because of deep ion interaction, showing that charge storage contribution comes mainly from the redox reactions.At a current density of 2 A g −1 , S 2 had the greatest discharge time compared to the other MgCuPO 4 -AgPO 4 nanocomposite electrodes.At 3.2 A g −1 current, the GCD trajectory of MgCuPO 4 was also relatively shorter than that of S 2 .This showed that the performance of MgCuPO 4 decreases with current density more frequently than the other MgCuPO 4 -Fig.5 (a-d) SEM images for S 1 , S 2 , S 3 , and S 4 composites.

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Ag 3 PO 4 nanocomposites, as illustrated by Fig. 8(a)-(d).The GCD comparison at 2.0 A g −1 for MgCuPO 4 , S 1 , S 2 , and S 3 is represented in Fig. 9(a).As can be seen from Fig. 9(a), S 2 had the largest plateau region compared to the other samples.This may be due to the larger surface area exposed to electrolytic ions, faster transportation of ions, and higher conductivity of S 2 .The specic capacity, Q s , of the electrodes was determined using eqn (11) from the galvanostatic discharge data.
in eqn (11), Q s represents the specic capacity, I is the current, Dt indicates the discharge time, and m is the active mass.Paper Nanoscale Advances increase in the wt% ratio of Ag 3 PO 4 decreases the overall performance.The specic capacities decrease for S 3 to 512 C g −1 at 2 A g −1 and 313.6 C g −1 at 3.2 A g −1 , demonstrating 61% capacity retention.This increase in rate capacity was due to the presence of Ag 3 PO 4 NPs, which decreased the internal impedance of MgCuPO 4 . 51Among all the MgCuPO 4 -Ag 3 PO 4 (S 1 , S 2 , and S 3 ) nanocomposites studied, S 2 had the highest capacity.The increased electrochemical performance for S 2 was due to two conicting effects: raising the quantity of Ag 3 PO 4 increases the conductivity of MgCuPO 4 -Ag 3 PO 4 nanocomposites.However, too many Ag 3 PO 4 nanoparticles degrade the performance of the S 3 electrode due to agglomerations produced, which reduces the contact area between the electrode and electrolytic solution.Another cause was the lack of amorphous MgCuPO 4 in S 3 , as amorphous MgCuPO 4 was responsible for increased active sites for redox reactions. 52Therefore, owing to the structural stability of crystalline Ag 3 PO 4 NPs, the rate capacity of S 2 remains greater than that of MgCuPO 4 .Fig. 9(c) indicates the comparative calculated specic capacities for MgCuPO 4 , S 1 , S 2 , and S 3 at 2.0 A g −1 .
The electrochemical impedance spectra (EIS) of the electrode materials were measured to determine charge and ion transport.The Nyquist graphs of amorphous MgCuPO 4 , S 1 , S 2 , and S 3 are shown in Fig. 9(d).Because of the roughness of the electrode surface, uneven electric eld, and varying electrochemical activity, the form of the EIS plots departs from the semicircular shape. 53The corresponding series resistance was calculated using the contact of the EIS graphs with the real part of the impedance spectrum.At high frequencies, the semicircle width is attributed to charge transfer impedance (R ct ). 54In Fig. 9(d

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Fig. 8 (a-d) The GCD plots for MgCuPO 4 , S 1 , S 2 , and S 3 at different currents. .The slope obtained from the straight line plotted between 1/C 2 and V can be used to evaluate charge carrier density among electrode materials.The charge carrier has a negative relationship with the MS inclination. 55The S 1 , S 2 , and S 3 slopes were lower than those of MgCuPO 4 , implying that the MgCuPO 4 -Ag 3 PO 4 composites had a greater charge carrier density.The S 2 curve had the lowest slope, demonstrating its increased electron density.These ndings suggest that the incorporation of Ag 3 PO 4 into MgCuPO 4 results in a higher density of charge carriers. 56he electron transmission rate of crystalline Ag 3 PO 4 NPs differs from that of diffuse MgCuPO 4 .The number of pores provided by MgCuPO 4 was used to store the electrolyte ions that were able to penetrate the inner surface of MgCuPO 4 .Most of the amorphous MgCuPO 4 surfaces were engaged in the faradaic reactions.Additionally, the electrolyte could not reach internal surfaces at a higher scan rate/current density, thus producing a low specic capacity. 57In contrast, the electrolytic ions interact with the topmost surface layer of Ag 3 PO 4 NPs as well as the inner surface of MgCuPO 4 .Thus, Ag 3 PO 4 adds useful surface area to unutilized amorphous MgCuPO 4 surfaces and maintains a large number of oxidation/reduction reactions at higher currents. 58Despite the fact that amorphous MgCuPO 4 offers a porous structure that helps to store more electrolytic ions, it still had low conductance. 59In this study, the high electrical conductivity of Ag 3 PO 4 NPs xed on the weakly conductive amorphous MgCuPO 4 accelerates the electron transfer mechanism and lowers the impedance of the MgCuPO 4 -Ag 3 PO 4 composites.Furthermore, the Ag 3 PO 4 nanoparticles reduce the distance between electron transport routes and the collector, resulting in increased electrochemical performance. 60

Supercapattery
The broad PW results in increased energy density for SCs.The supercapattery (S 2 //AC) was designed with S 2 and activated carbon (AC).AC due to its permeable and porous structure allows more electrolyte ions to be stored. 61The individual CV graphs were originally measured in a three-cell setup for S 2 and AC to measure the overall voltage of the supercapattery (Fig. 10(a)).The working PW of AC was 0 to −1 V and that of S 2 was 0-0.6 V. Thus, the S 2 //AC supercapattery was stable over 1.6 V.No redox peak appeared at 0-0.5 V, which showed that the dominant storage mechanism was provided by the capacitive nature of AC.At higher potential, >0.6 V, redox peaks emerged, indicating the faradaic reaction of S 2 .Fig. 10(b) shows that the

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shape of the CV graphs for the S 2 //AC supercapattery was not altered at a higher scan (100 mV s −1 ), which proves the high stability of the S 2 //AC device.The power law, which connects current density I with scan rate (v), was used for theoretical study of the electrochemical behavior of the asymmetric device.The charge storage process in a supercapattery is investigated using b-tting.The b-value was calculated using the slope drawn between the log of scan rate and the log of peak current.
The adjustable factors are represented by k and b, as stated in the equations, while v is the scan rate and I is the current.The exponent b is critical in deciding the charge accumulation in a supercapattery.For a b-value of 0-0.5, the dominant charge storage process is diffusion regulated.For a b-value of 0.8-1.0, the dominant charge storage process is adsorption/desorption.For a b-value of 0.5-0.8, the storage process is due to both faradaic and adsorption/desorption processes. 62The b-tting for this supercapattery (S 2 //AC) was between 0.5 and 0.8, as shown in Fig. 10(c).This proves the formation of a supercapattery.
The GCD measurements for the S 2 //AC supercapattery were performed at various densities (1.0 to 2.2 A g −1 ), as depicted in Fig. 10(d).The GCD curves of the S 2 //AC device were nearly symmetrical at all currents, which demonstrates its higher stability. 63Furthermore, the GCD contours were non-linear due to the faradaic response of S 2 .The specic capacity for the S 2 // AC supercapattery was also calculated from eqn (8) and (11).At a current intensity of 1.0 A g −1 , the specic capacity of S 2 //AC was 193 C g −1 , as indicated in Fig. 11(a).The S 2 //AC device can retain 60% of its initial capacity at 2.2 A g −1 current.While the capacity through a CV for the S 2 //AC supercapattery was 183 C g −1 (at 3 mV s −1 ), as indicated in Fig. 11(b).
Fig. 11(c) and (d) represent the stability test for the S 2 //AC supercapattery device against 5000 cycles.The capacity was increased marginally to 110% at the start, owing to the wettability of the electrode surface and slow activation of the electrode. 64Aer 5000 cycles, the capacity declined slowly to 92%, showing that the device was stable.The coulombic efficiency aer 5000 cycles was retained at 98%.The decay in capacity was due to structural degradation of the electrode materials.The nanocomposite agglomeration as well as separation of the electrode material from the electrolyte are also responsible for the decrease in capacity. 65,66ig. 11(e) depicts the Ragone plot of the estimated energy density (E) and power density (P) of the S 2 //AC supercapattery using the following formulae.
where Q denotes specic capacity, V denotes the PW, and t d denotes discharge duration.S 2 //AC provided a high energy density as well as an outstanding power density (49.4 W h kg −1 at 550 W kg −1 and 19.5 W h kg −1 at 6382 W kg −1 ).These results outperformed previous studies, as indicated in Table 4.

Oxygen evolution reaction (OER)
Linear sweep voltammetry (LSV) was employed to evaluate the electrocatalytic performance of MgCuPO 4 , S 1 , S 2 , S 3 , and S 4 composites, as well as the bare stainless steel (SS) substrate.To conduct the investigation, 1 M KOH electrolyte was utilized.The primary objective was to assess their ability to generate a current density of 10 mA cm −2 with minimal overpotential, aiming to match the widely employed anode materials RuO 2 and IrO 2 in various industrial applications.The LSV data revealed distinctive behavior among the tested materials. 67 The MgCuPO 4 composite exhibited an onset overpotential of 340 mV, whereas the S 1 and S 3 composites displayed starting overpotentials of 263 and 221 mV, respectively.In contrast, the S 2 composite showcased an even lower overpotential of 142 mV, further underscoring its electrocatalytic capabilities.Conversely, the bare SS substrate necessitated a higher overpotential of 388 mV to achieve a current density of 10 mA cm −2 (Fig. 12(a)).
The bare SS substrate was included in the assessment as a reference point to evaluate the effectiveness of the composites.The higher overpotential required by the bare SS substrate suggests low electrocatalytic activity and inadequate efficiency in the oxygen evolution reaction (OER).Conversely, the signicantly lower overpotentials observed for the MgCuPO 4 , S 1 , S 2 , and S 3 composites indicate superior performance and potential for diverse applications, particularly in renewable energy conversion and storage devices.
Moreover, Fig. 12(b) illustrates comparable overpotentials at j = 10 mA cm −2 , a typical measure of OER activity.A lower overpotential implies signicantly better performance in the OER. 68,69These ndings highlight the exceptional electrocatalytic capabilities of the MgCuPO 4 , S 1 , S 2 and S 3 composites compared to the bare SS substrate.Their reduced onset overpotentials and improved catalyst utilization suggest the potential to enhance energy storage technologies and enable sustainable energy conversion processes.The effectiveness of the electrochemical processes can be improved by these materials (MgCuPO 4 , S 1 , S 2 , and S 3 ), taking us one step forward to a more environmentally friendly and sustainable energy density.
Tafel plots were used to analyze the reaction kinetics of the MgCuPO 4 , S 1 , S 2 , and S 3 materials, as shown in Fig. 12(c).The Tafel slopes reveal information about the reaction response of the materials.

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The MgCuPO 4 composite exhibits a Tafel slope of 103 mV dec −1 ; the S 1 composite shows a Tafel slope of 95 mV dec −1 ; S 2 shows a Tafel slope of 49 mV dec −1 (see Table 5); and S 3 shows a Tafel slope of 68 mV dec −1 .The reduced Tafel slope of S 2 (117 mV dec −1 ) in comparison to the other composites and bare SS points to quicker reaction rates.As a result, the oxygen evolution process (OER) can occur at greater speeds in the S 2 composite due to its more effective catalytic activity.
The charge transport process inside the MgCuPO 4 , S 1 , S 2 , and S 3 materials, along with the bare SS substrate, was examined using electrochemical impedance spectra (EIS), in conjunction with Tafel studies.Signicant features were discovered from the Nyquist plots, as illustrated in Fig. 12(d).Surprisingly, the semicircle diameter of the S 2 composite was less than those of the MgCuPO 4 , S 1 , S 2 , and S 3 composites.This suggests that the S 2 composite is more conductible.As shown in the insets of Fig. 12(d), analogous circuits were used to analyze the EIS data.These circuits included elements that represented the impedance of the solution (R s ), the inherent resistance (R 1 ) of the catalyst, and the impedance of the electrocatalytic activity (R 2 ).Notably, the electrocatalytic resistance of the S 2 composite

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was determined to be 659 U cm 2 , signicantly lower than the resistances of the MgCuPO 4 (732 U cm 2 ), S 1 composite (788 U cm 2 ) or the S 3 composite (914 U cm 2 ).Table S1 † presents the impedance of MgCuPO 4 , S 1 , S 2 , and S 3 in the oxygen evolution reaction.
To evaluate the long-term electrochemical stability of the MgCuPO 4 , S 1 , S 2 , and S 3 composites, a chronopotentiometric mode was employed.Fig. 12(e) demonstrates a minor potential drop over time, attributed to an improvement in catalytic efficiency.Continuous gas liberation was observed during the stability investigation, indicating the ongoing oxygen evolution process.Fig. 12(f) presents the LSV graphs of S 2 .
The results of the study indicate that the MgCuPO 4 , S 1 , S 2 , and S 3 composites exhibit favorable reaction kinetics and longterm electrochemical stability specically for the oxygen evolution reaction (OER).The lower Tafel slope observed for the S 2 composite suggests faster reaction rates, while the slight reduction in potential during the stability test signies  improved catalytic efficiency.These characteristics are crucial for ensuring the sustained performance and durability of electrochemical devices, underscoring the signicance of these composites in applications related to energy storage and conversion.

Conclusions
The sonochemical and hydrothermal technique followed by calcination treatment effectively synthesizes MgCuPO composites hold great promise as potential candidates for future energy storage devices and further investigation into the oxygen evolution reaction (OER).

Fig. 6 (
a)-(d) illustrate the CV curves for S 2 and the other samples (MgCuPO 4 , S 1 , and S 3 ) that were gradually enhanced with rising scan rates (3-50 mV s −1 ).Fig. S1 † shows the CV curves for MgCuPO 4 , S 1 , and S 3 at an operating potential of 0-0.8 V.
) (inset), the circuit diagram for the tted Nyquist plot is also represented.The semicircle can be seen from the ZCPE and R ct parallel connection in the circuit diagram.According to the EIS diagram, the R s values for S 1 , S 2 , S 3 , and MgCuPO 4 were 0.72, 0.65, 0.81, and 1.05 U, respectively, as shown in Fig. 9(d).The lower R s values of all MgCuPO 4 -AgPO 4 nanocomposites compared to amorphous MgCuPO 4 nanocomposites suggest higher conductance.Moreover, the semicircle diameter of MgCuPO 4 was considerably bigger than that of the MgCuPO 4 -AgPO 4 nanocomposites in the Nyquist plots, which results in greater charge transfer resistance.In all the MgCuPO 4 -Ag 3 PO 4 nanocomposites studied, S 2 had the shortest diameter, indicating the lowest internal resistance.Furthermore, the straight line for S 2 was the sharpest, indicating the lowest ion diffusion resistance.The high-resolution EIS spectra for MgCuPO 4, S 1 , S 2, and S 3 are represented in Fig. 9(e).A potential-dependent capacity test was used to further explore the effect of Ag 3 PO 4 modication on the electrical characteristics of MgCuPO 4 .MS plots were made from the capacitances obtained from the imaginary portion of the impedance.Fig. 9(f) depicts the MS plots for MgCuPO 4 , S 1 , S 2, and S 3 .The dominant charge carriers

Fig. 7
Fig. 7 (a) Specific capacity for MgCuPO 4 , S 1 , S 2 , and S 3 at various scan rates.(b) Specific capacity for MgCuPO 4 , S 1 , S 2 , and S 3 at 3 mV s −1 scan rate.(c) Anodic and cathodic peaks calculated for S 2 .(d) Peak voltage plotted against log of scan rate for S 2 .

Fig. 11 (
Fig. 11 (a and b) Specific capacity for the S 2 //AC supercapattery computed through CV and GCD.(c and d) Charge/discharge and capacity retention for the S 2 //AC supercapattery after 5000 cycles.(e) Energy and power density for the S 2 //AC supercapattery.
Synthesis of MgCuPO 4 -Ag 3 PO 4A hydrothermal and sonochemical technique was used to synthesize MgCuPO 4 -Ag 3 PO 4 composites with various weight ratios of MgCuPO 4 and Ag 3 PO 4 .The as-prepared MgCuPO 4 (set A) was made using the methods described in the literature.

Table 4
Energy vs. power density for the MgCuPO 4 -Ag 3 PO 4 //AC device and comparison with the literature

Table 5
Comparison of Tafel slope and overpotential of this work with previous studies 4and MgCuPO 4 -Ag 3 PO 4 nanocomposites with various MgCuPO 4 -Ag 3 PO 4 content ratios.XRD phase structure analysis veried that the crystalline structure of Ag 3 PO 4 NPs increases with decreasing amorphous MgCuPO 4 content.The FTIR spectra revealed that phosphate and hydroxyl groups were chemisorbed on the MgCuPO 4 -Ag 3 PO 4 surface.The UV-vis spectroscopic analysis revealed that MgCuPO 4 -Ag 3 PO 4 composites were more conductive than MgCuPO 4 .The SEM analysis showed that Ag 3 PO 4 nanoparticles were rmly xed on the amorphous surface of MgCuPO 4 .The MgCuPO 4 -Ag 3 PO 4 nanocompositebased electrode demonstrated increased rate capability from 53% (MgCuPO 4 ) to 59% at 3.2 A g −1 .The rapid electron movement and the large number of active sites given by MgCuPO 4 and Ag 3 PO 4 composites enhanced the rate capacity.Because amorphous MgCuPO 4 has a low electrical conductivity, modifying amorphous MgCuPO 4 with crystalline Ag 3 PO 4 NPs offers a potential method to increase conductivity and rate capability.The constructed S 2 //AC supercapattery had an energy density of 49.4 W h kg −1 at 550 W kg −1 power density.The S 2 //AC supercapattery showed 92% capacity retention aer 5000 cycles.Investigation of the OER application demonstrated that S 2 exhibited the lowest Tafel slope, measuring 49 mV dec −1 .The results obtained from this study suggest that MgCuPO 4 -Ag 3 PO 4