In situ deposited cobalt-magnesium selenates as an advanced electrode for electrochemical energy storage

Manchi Nagaraju,S.Chandra Sekhar,Bhimanaboina Ramulu,Shaik Junied Arbaz,Jae Su Yu

Department of Electronics and Information Convergence Engineering,Kyung Hee University,1732 Deogyeong-aero,Gihung-gu,Yongin-si,Gyeonggi-do 17104,Republic of Korea

Abstract Currently,bimetallic selenates have attracted much attention as a prominent electrode composite material for supercapacitors owing to their higher redox chemistry and superior electrical conductivity.Herein,we synthesized cobalt-magnesium selenates(CoSeO3-MgSeO4,CMS)via a facile hydrothermal process,followed by selenization.At first cobalt-magnesium oxide(Co2.32Mg0.68O4,CMO)was in situ prepared by a one-pot hydrothermal method.An investigation on the morphological change was performed by synthesizing the same CMO samples at different growth times by keeping the temperature constant.The CMO electrode designed for 8 h of growth time(CMO-8 h)with an attractive morphology showed a higher areal capacity of 101.7μAh cm-2(at 3 mAcm-2)than the other CMO electrodes prepared for 6 and 10 h.Further exalted performance was achieved by the selenization of the CMO-8 h sample to form the CMS material.At 3 mA cm-2,the resulted CMS exhibited nearly three times higher capacity,i.e.,385.4μAh cm-2,than the CMO-8 h electrode.Additionally,an asymmetric cell fabricated with CMS as a positive electrode also revealed good energy storage performance.Within the applied voltage between 0 and 1.5 V,the asymmetric cell demonstrated maximum energy density of 0.159 mWh cm-2(18.6 Wh kg-1)and maximum power density of 18.47 mW cm-2(1938 W kg-1),respectively.Thus,novel magnesium-based metal selenates can act as an efficien electrode for energy storage.? 2022 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Co2.32Mg0.68O4;CoSeO3-MgSeO4;In situ formation;Energy storage;Asymmetric cell.

1.Introduction

Transition metal oxides(TMOs)have been broadly promoted in diverse fields especially in energy storage research owing to their several advantages of high redox-active sites,electrochemical stability,good electrical conductivity,and natural benignity[1-5].Compared to single TMOs,binary TMOs are more prominent to attain the exalted electrochemical performance by potentially exploiting the exclusive properties from respective solitary metal oxides[6,7].Up to now,various TMOs,such as NiCo2O4,NiO,Co3O4,MgO,MgCo2O4,etc.,have been studied in different research field[8-14].Among these TMOs,magnesium oxide has attracted considerable interest due to its good electrical conductivity,cheap cost,high rigidity,and eco-friendliness[15-18].On the other hand,cobalt oxide has been used as a prominent electrode material in battery,water splitting,and supercapacitor(SC)applications because of its high electrochemical activity,multi-oxidation states,and affordable cost[19,20].Therefore,combining the above two metal oxides into a composite form may enhance the electrochemical energy storage performance by utilizing the synergistic effects of respective magnesium and cobalt oxides[21,22].Some previous literature also suggested that the cobalt magnesium oxides can be considered as a promising electrode of SCs[23,24].

However,the compromised electrical conductivity of TMOs impedes the acquisition of high energy storage performance.Recently,metal chalcogenides have gained dominant interest as an electrode candidate due to the better electrical conductivity and higher electrochemical response compared to TMOs[25-31].Particularly,selenium has a considerable electrical conductivity of 1×10-3S m-1,which is greater than that of sulfur(5×10-28S m-1)[32,33].Moreover,the substitution of oxygen with selenium can alter the crystal structure properties.Consequently,the metal selenide-based electrode materials can demonstrate superior electrochemical properties.On the other side,active material synthesis is also one of the crucial parameters.Among the versatile methods,hydrothermal synthesis is one of the popular techniques to synthesize active materials.Moreover,it is cost-effective and user-friendly,and can produce a high yield.The hydrothermal method also has other advantageous features like direct deposition of the active material,uniform volumetric heating,alteration of reaction parameters such as temperature/pressure,producing homogeneous and versatile morphologies.Besides the synthesis methods,the morphology of active material also has a significan impact on the electrochemical properties.Up to now,versatile morphologies such as nanofl wers,nanospheres,nanowires(NWs),nanorods,etc.have been designed and investigated for their effect on electrochemical performance.Particularly,sheet-like and wirelike morphologies have received considerable interest because they provide high surface area and swift charge carriage.However,preparing the active materials with the combination of these nanostructures would further enhance the charge storage performance owing to their respective structural properties.

Based on the above discussion,we synthesized cobaltmagnesium selenate(CoSeO3-MgSeO4,CMS)with hybrid morphology,i.e.,nanosheets(NSs)and NWsviathe hydrothermal method,followed by a selenization process.So far,as we know,this is the firs report on the fabrication of CMS active material with the hybrid morphology in the absence of binders,and the investigation of its electrochemical behavior was performed in both three-and two-electrode systems.Firstly,cobalt-magnesium oxide(Co2.32Mg0.68O4,CMO)was directly deposited on the nickel form(NF)through a singlestep hydrothermal process without the use of binders.The effect of growth time on the morphology change was systematically investigated by synthesizing the CMO samples at the growth times of 6,8,and 10 h Among these,the CMO electrode obtained at 8 h(CMO-8 h)exhibited NSs and NWs which offer numerous redox-active sites.Benefitte from redox activity and hybrid morphology,the CMO-8 h electrode demonstrated a higher electrochemical response than the other electrodes.To further enhance the redox property,the CMO-8 h sample was selenized in an inert medium.The resulting cobalt-magnesium selenate electrode,i.e.,CoSeO3-MgSeO4(CMS),showed an improved redox response and also delivered a higher areal capacity than the CMO-8 h electrode.Furthermore,the practicability of the CMS electrode as the cathode was also explored by constructing the asymmetric cell.The asymmetric cell also demonstrated noteworthy energy storage performance as well as real-time operations.

2.Experimental procedure

All the raw materials and chemicals used in the preparation process are provided in the Section-I of the Supporting Information(SI).

2.1.Preparation of cobalt-magnesium oxide(CMO)

Initially,the NF was cleaned by using 1 M hydrochloric acid(HCl)for 10 min and subsequently washed with deionized water(DIW).Afterward,200 mg of cobalt(II)nitrate hexahydrate(Co(NO3)2·6H2O),150 mg of magnesium(II)nitrate hexahydrate(Mg(NO3)2·6H2O),180 mg of urea(CO(NH2)2),and 200 mg of ammonium fluorid(NH4F)were dispersed in 60 mL of DIW.Later,NF was placed into the prepared solution and the complete system was then transferred to the autoclave.After the hydrothermal process at 150 °C for 8 h,the CMO grown NF was rinsed with DIW and desiccated in an oven at 100°C for about 3 h.The weight of CMO loaded on NF is noted to be 3.5 mg cm-2.Also,to study the influenc of reaction time on the surface morphology,various CMO materials were fabricated with two separate reaction times at 6 and 10 h.The CMO samples prepared at 6,8,and 10 h are named CMO-6 h,CMO-8 h,and CMO-10 h,respectively.

2.2.Synthesis of cobalt-magnesium selenate(CMS)

For selenization,the CMO-8 h grown NF and selenium(Se)powder were placed in two different quartz boats.Later,these quartz vessels were placed adjacent to each other at in the tubular furnace at downstream and upstream,respectively.Subsequently,the furnace was heated up to 400 °C for 2 h.Lastly,the product was received after reaching the room temperature normally.The loaded mass of the material was found to be～5.5 mg cm-2.

3.Results and discussion

Fig.1.Schematic diagram for the preparation of the(a,b)CMO-8 h material with hybrid morphology and(c,d)CMS material.FE-SEM images of the(e)CMO-8 h and(f)CMS materials.

Fig.1 represents the growth mechanism of the CMO and CMS materials via a hydrothermal method,followed by selenization.Firstly,the growth solution was made with Mg(NO3)2·6H2O,Co(NO3)2·6H2O,CH4N2O,and NH4F in 60 mL of DIW(Fig.1(a)(i)).Later,the cleaned NF was inserted into the above mixture and transferred to the autoclave liner(Fig.1(a)(ii)).Under the hydrothermal conditions,the Mg2+and Co2+ions react with CH4N2O surfactant and structure-directing agent NH4F,to form CMO nuclei(Fig.1(b)).Subsequently,all these nuclei were in situ grown on the NF in the form of nanosheet arrays(NSAs)as well as nanowires(NWs).Next,the selenization process was carried out in a tubular furnace in the ambiance of nitrogen gas,as shown in Fig.1(c).The CMS material with mixed morphology was obtained as illustrated in Fig.1(d).The surface morphologies of CMO-8 h and CMS were observed by using a field-emissio scanning electron microscope(FESEM)shown in Fig.1(e)and(f).The FE-SEM image of the CMO-8 h sample at low magnificatio is presented in Fig.1(e)(i).The NF substrate was completely covered by the CMO nanostructures as seen in Fig.1(e)(ii).The highmaginificatio FE-SEM images in Fig.1(e)(iii)revealed the vertical growth of CMO NSAs on the surface of the NF substrate.Moreover,several NWs anchored like fencing on these NSAs can also be observed.These new mixed nanostructures could provide many active sites to execute redox reactions.The FE-SEM images of the CMS material are presented in Fig.1(f).The low-maginificatio FE-SEM images in Fig.1(f)(i)and(ii)represented no significan change in the morphology even after the selenization.But,the magnifie FE-SEM image in Fig.1(f)(iii)disclosed the change in the NSAs into nanoparticles(NPs).Besides,the NWs were cut into relatively smaller pieces,which is likely ensued during the selenization.The FE-SEM images of the CMO-6 h and CMO-10 h materials are shown in Fig.S1(a)and S2(a)of the SI.The energy-dispersive X-ray(EDX)spectroscopy analysis was performed to check the elements in the CMS sample.The obtained EDX results in Fig.2(a)showed Mg,Co,O,and Se peaks,respectively.The elemental mapping pictures in Fig.2(b)(i-iii)confirme that all the above elements are evenly distributed on the resulted morphology.To study the phase of prepared materials,X-ray diffraction(XRD)was employed.From Fig.2(c),the peaks at the 2θvalues of 44.4°,51.7°,and 76.3° in both spectra correspond to NF substrate.The peaks noticed at 2θvalues of 18.9°,31.7°,38.3°,38.3°,59.0°,and 73.7° in the orange-colored XRD pattern are well coincident with the crystalline planes of(111),(220),(222),(331),(511),and(620)of the Co2.32Mg0.68O4phase(JCPDS#01-081-0669),respectively.The other peaks at the 2θvalues of 30.0°,50.2°,and 57.9° in the green-colored XRD pattern are related to the crystalline plane of(121),(222),and(301)of MgSeO4phase(JCPDS #00-017-0845),respectively.The remaining peaks revealed the CoSeO3phase(JCPDS #00-031-0341).The oxidation states in the CMS sample were investigated by X-ray photoelectron spectroscopy(XPS).The XPS full survey scan spectrum in Fig.2(d)displayed the peaks of Mg 1s,Co 2p,O 1s,and Se 3d,which are in well agreement with the EDX results.Fig.2(e)displays the highresolution(HR)XPS spectrum of the Mg 1s.From this figure it can be observed that one peak was located at 1305.8 eV,signifying the divalent state of the Mg element[34].The HR XPS spectrum of the Co 2p is presented in Fig.2(f).The Co 2p3/2peak is fitte into dual peaks at 781.2 and 797.0 eV,and the Co 2p1/2state is also split into dual peaks at 784.7 and 800.6 eV.This spin-orbit splitting of Co 2p suggests Co3+and Co2+states,respectively[35].The other two peaks located at 804.2 and 788.2 eV are separated into two satellite peaks of Co 2p3/2and Co 2p1/2,respectively.The HR XPS spectrum of the O 1s is presented in Fig.2(g).Two peaks noted at the binding energies of 531.4 and 532.5 eV signify the Se-O-M bond and hydroxide(M-OH)bond,respectively[36].The HR XPS spectrum of the Se 3d consists of two peaks at 50.6 and 51.7 eV which are the distinctive peaks of Se 3d5/2and Se 3d3/2,respectively(shown in Fig.2(h))[37,38].All the above obtained results together revealed that the synthesized material is CoSeO3-MgSeO4.

Fig.2.(a)EDX spectrum and(b)elemental mapping images of the CMS material.(c)XRD patterns of the CMO-8 h(orange)and CMS(green)materials.(d)XPS survey scan spectrum and HR XPS spectra of(e)Mg 1s,(f)Co 2p,(g)O 1s,and(h)Se 3d elements.

The electrochemical properties such as cyclic voltammetry(CV),galvanostatic charge-discharge(GCD),and electrochemical impedance spectroscopy(EIS)of the synthesized electrodes were evaluated in the three-electrode set-up.All of the prepared electrodes were used as a working,Ag/AgCl as a reference and platinum as a counter electrode.Fig.3(a)depicts the compared CV profile of the CMO-6 h,CMO-8 h,CMO-10 h,MO-8 h,and CO-8 h electrodes recorded at a fi ed sweep rate of 20 mV s-1in the potential range 0-0.5 V.A pair of redox peaks were observed in almost all the CV profiles indicating typical battery-type charge storage process involved in all the CMO-based electrodes and solitary CO-8 h electrode.Since the magnesium element does not execute any redox reactions,no significan anodic/cathodic peaks were noticed in the CV profil of the MO-8 h electrode.In contrast,the cobalt element with high redox activity can perform reversible redox reactions.However,the magnesium element improves the electrical conductivity of the CMO composite due to its superior conductivity to the cobalt element.Among all the electrodes,the CMO-8 h electrode showed a higher CV area and superior current response by exploiting synergistic characteristics of CMO material as well as mixed morphology.In addition,the GCD curves of the CMO-6 h,CMO-8 h,CMO-10 h,MO-8 h,and CO-8 h electrodes were also tested at a fi ed current value of 3 mA cm-2.A non-linear GCD curve(Fig.3(b))additionally confirme the battery-behavior charge storage features of all the electrodes except the MO-8 h electrode.The obtained outcomes are reliable with the CV results.Moreover,the areal(CA)and specifi capacity(CS)values of all the above electrodes were estimated using Eqs.(S1)and(S2)of the SI.As shown in Fig.3(c),the CMO-8 h electrode showed a higher CA(Cs)value of 101.7μAh cm-2(48.4 mAh g-1),whereas the CMO-6 h,CMO-10 h,MO-8 h,and CO-8 h electrodes showed the areal capacity values of 86.4μAh cm-2(34.5 mAh g-1),62.5μAh cm-2(29.7 mAh g-1),4.1μAh cm-2(3.4 mAh g-1),and 55μAh cm-2(18.3 mAh g-1),respectively.Thus,the CMO-8 h electrode was taken as an optimum electrode.The attained superior performance of the CMO-8 h electrode is ascribed to the following merits.This hybrid morphology of the CMO-8 h electrode provides several merits such as NWs which can offer a large surface area,enabling plenty of charge accommodation that can enhance the redox process.Besides,these NWs act as the electron superhighways to supply the generated charge promptly to an external load.On the other hand,the NSs may further promote the surface area of the entire material,which boosts the redox process rate by offering numerous redox sites.The hierarchical connection of these NSs also boosts the charge transportation.The nanosized gaps among these NSs permit the electrolyte ions to diffuse,followed by the stimulation of the entire active material matter.Exploiting multiple structural merits of the hybrid morphology,the CMO-8 h electrode demonstrated a superior capacity to the CMO-6 h electrode.In the case of the CMO-10 h electrode,all the NWs disappeared and only NSs were observed due to the long growth time.This results in the decrease of electroactive sites,which leads to diminished redox reactions,followed by less charge storage.As a result,the CMO-10 h electrode showed lower capacity than the CMO-8 h electrode,whereas the CMO-6 h electrode has less active material mass than the CMO-8 h electrode,resulting in the less capacity.Owing to the lack of other respective transition metal species as well as hybrid morphology in solitary CO-8 h and MO-8 h electrodes,they also showed lower electrochemical response compared to the CMO-8 h electrode.

The CV profile of the CMO-8 h electrode tested at various sweep rates of 5-30 mV s-1are shown in Fig.3(d).On the other hand,the GCD profile of the CMO-8 h electrode were collected at 3 to 20 mA cm-2,as displayed in Fig.3(e).From these two figures the redox peaks in CV profile and nonlinear behavior in GCD curves are noticeable,which reveals excellent redox reversibility and descent rate-capability of the CMO-8 h electrode,respectively.In the CV curve obtained at 10 mV s-1,a small inverted peak aroused within the potential window of 0.31-0.33 V can be seen.This may be caused due to the Iviumstat instrument error.As can be observed from the CV curves,the current response is about to increase rapidly at around 0.3 V owing to the redox reactions of active material.The current range in the Iviumstat instrument will be changed due to this abrupt change in the current value,causing this small peak.The CA(CS)values of the CMO-8 h electrode at the current densities of 3,5,7,10,15,and 20 mA cm-2were 101.7μAh cm-2(48.4 mAh g-1),88.3μAh cm-2(42 mAh g-1),77.4μAh cm-2(36.8 mAh g-1),65μAh cm-2(30.9 mAh g-1),59.9μAh cm-2(28.5 mAh g-1),and 49.1 μAh cm-2(23.4 mAh g-1),respectively(Fig.3(f)).These areal/specifi capacity values of the CMO-8 h electrode are higher than the other electrodes at all the current densities.The electrochemical performances of the CMO-6 h and CMO-10 h electrodes are shown in Fig.S1(b)&(c)and S2(b)&(c)of the SI,respectively.

Fig.3.Comparative(a)CV curves,(b)GCD curves,and(c)areal/specifi capacity values of the CMO-6 h and CMO-8 h,CMO-10 h,CO-8 h,and MO-8 h electrodes.(d)CV and(e)GCD curves of the CMO-8 h electrode.(f)Comparison of areal and specifi capacity values of the CMO-6 h,CMO-8 h,and CMO-10 h CO-8 h,and MO-8 h electrodes.

To further achieve the enhanced electrochemical performance,the CMS electrode was fabricated via the selenization process of the CMO-8 h electrode.The electrochemical properties of both the CMO-8 h and CMS electrodes were compared at a fi ed sweep rate of 5 mV s-1(Fig.4(a)).It is apparent from these CV profile that the CMS electrode depicted a higher CV response.Besides,the CMS electrode exhibited higher charging and discharging time than another CMO-8 h electrode(Fig.4(b)).To observe the change in the electrical conductivity of the CMS electrode after the selenization process,the electrochemical impedance spectroscopy(EIS)test was performed on the CMO-8 h and CMS electrodes.It is well known that the firs intercept of the EIS curve with the x-axis represents the bulk solution resistance(Rs)which is composed of the electrolyte resistance,the internal resistance of active material,and the contact resistance between the current collector and active material.As shown in Fig.4(c),the Rsvalue of the CMO-8 h electrode was noted to be 1.5Ω.However,this value of the CMS electrode was decreased to～1.34Ω,which means that its internal resistance is decreased due to the selenization process.In other meaning,the electrical conductivity of the CMS electrode is improved.Moreover,the charge transfer resistance(Rct)value of the CMS electrode was observed to be lower,i.e.,～1Ωthan that of the CMO-8 h electrode(～1.7Ω),signifying the improved charge transportation rate.Therefore,these EIS results verify that the CMS electrode has relatively higher electrical conductivity than that of the CMO-8 h electrode.The CV profile of the CMS electrode measured at various sweep rates from 5 to 30 mV s-1revealed good redox response and excellent electrochemical reversibility as presented in Fig.4(d).The GCD curves of the CMS electrode measured at various current values(3-20 mA cm-2)also represented good reversibility and rate capability.The GCD curves look nonlinear and thus,they are identical even at high current densities,which reveals an enhanced rate capability as presented in Fig.4(e).The influenc of bare NF in the capacity contribution was also investigated by comparing its electrochemical performance with the CMO-8 h and CMS electrodes.As shown in the CV and GCD curves of Fig.4(a)and(b),these two plots revealed the negligible electrochemical response of bare NF.Differently,the CMO-8 h and CMS electrodes delivered a higher current response in CV and larger chargedischarge times in GCD analysis than those of the bare NF.This performance in both the analyses is solely attributed to the deposited CMO-8 h and CMS active materials on the NF surface.The CAand CSvalues of the CMS electrode calculated from 3 to 20 mA cm-2are plotted in Fig.4(f).At 3 mA cm-2,the CMS electrode exhibited a higher CA(CS)value of 385.4μAh cm-2(54.3 mAh g-1),which is nearly three times larger than that of the CMO-8 h electrode(101.7μAh cm-2(48.4 mAh g-1)).Additionally,the CMS electrode retained the CA(CS)value of 313.4μAh cm-2(40.7 mAh g-1)even at 20 mA cm-2with a notable rate capability of 81.3%.The achieved CAvalues of the CMS electrode were substantially higher than those of the CMO-8 h electrode at all the measured current values.Fig.4(g)shows the cycling constancy results of the CMS electrode tested at a fi ed current of 10 mA cm-2.After 3000 cycles,the CMS electrode exhibited good constancy with notable retention of 86.3%.The EIS test for the CMS electrode was conducted before and after the cycling test,and the obtained Nyquist plots are in the insert of Fig.4(g).The Rsvalues of the CMS electrode before and after the cycling measurements were noticed as 1.35Ωand 1.57Ω.Meanwhile,charge transfer resistance(Rct)was observed to be～0.25Ωafter the cycling test.Besides the cycling stability,the mechanical stability of the prepared CMO-8 h and CMS electrodes along with bare NF was also investigated.Fig.S5(a)of the SI shows the photographic images of the bare NF,CMO-8 h/NF,and CMS/NF samples.To verify the mechanical stability,all these samples were pressed up to 10 MPa with the help of a hydraulic press.No cracks or any other form of damage in all the above three samples,especially from the CMO-8 h and CMS samples,can be observed from Fig.S5(b)of the SI.Moreover,the bending test was also carried out on all these samples by bending at different angles as presented in Fig.S5(c)of the SI.However,no fracture or peeling of active material from the CMO-8 h and CMS samples is noticed.These two experiments signifie good stability of the NF substrate even after preparing the CMO and CMS materials on its surface by the hydrothermal and selenization processes.

Fig.4.Comparative(a)CV curves,(b)GCD curves,and(c)EIS plots of the CMO-8 h and CMS electrodes.(d)CV and(e)GCD curves of the CMS electrode.(f)Areal and specifi capacities of the CMO-8 h and CMS electrodes.(g)Cycling capability of the CMS electrode.Inset in(g)shows the EIS plots of the CMS electrode before and after the cycling test.

Table 1Comparison of energy and power densities of the CMS/AC asymmetric cell with previously published reports.

A pouch-type asymmetric cell was constructed using the CMS/NF as a positive electrode and the activated carbon coated NF(AC/NF)as a negative electrode,as depicted in Fig.5(a).The AC material is one of the crucial components in an asymmetric cell.The AC material is more often used as a negative electrode candidate in the construction of asymmetric/hybrid cell due to several features.The AC material stores and dissipates the charges rapidly by the means of physical adsorption and desorption,respectively.As a result,high power density can be attained.Moreover,the AC material usually possesses high surface area and high porosity property which favor large charge accommodation and easy electrolyte diffusion.Owing to the stable surface chemistry and high mechanical rigidity,the AC material demonstrates high-rate performance as well as long life span.Besides,the AC material is inexpensive due to its abundance in nature.All these crucial aspects make the AC material a suitable negative electrode candidate in the construction of asymmetrical cell.Therefore,the AC material was chosen as the negative electrode in the present work.The fabrication process of the AC/NF electrode and its electrochemical properties are described in Section-II of the SI.At first the active areas of both the electrodes were placed facing each other.A Whatman filte paper was then inserted in between them to act as a separator,which prevents the short circuit and provides the mobility to the electrolyte ions.The nickel tabs that serve as connecting terminals were attached to both the electrodes.The entire setup was placed in a non-conductive aluminum pouch and 1 M KOH aqueous electrolyte solution was added into the pouch.This pouch was then packed properly using a hot sealer.Before fabricating the pouch-type asymmetric cell,the mass balancing of CMS/NF and AC/NF electrodes was done using Eq.(S5)of the SI to determine the optimum mass of the negative(AC)electrode material,which was about 2.7 mg cm-2.This mass was then coated on the NF to make a negative electrode,i.e.,AC/NF.

The CV profile of CMS and AC coated NF electrodes recorded at the sweep rate of 5 mV s-1are plotted together in Fig.5(b)to check the operating voltage range of the asymmetric cell.To prove this declaration,the CV curves of the asymmetric cell were measured in different applied voltages from 0 to 0.8,0-1.0,0-1.2,0-1.4,0-1.5,and 0-1.6 V(at 20 mV s-1)as exhibited in Fig.5(c).The asymmetric cell showed a stable CV outline up to 1.5 V without any deviations.However,a small deviation is noticed in the applied voltage of 0-1.6 V.Additionally,the GCD profile of the asymmetric cell obtained at various applied voltage windows ranging from 0 to 0.8 V to 0-1.6 V(at 3 mA cm-2)are shown in Fig.5(d).As CV results,the GCD profile obtained at a higher applied voltage window of 0-1.6 V also showed some deviations in the charge curve.Therefore,the optimum working voltage range of the asymmetric cell was fi ed to be 0-1.5 V.The CV profile of the asymmetric cell were obtained at different scan rates of 5 to 100 mV s-1within 0-1.5 V,as shown in Fig.5(e).All the CV profile revealed electric double-layered capacitive and faradaic behaviors,representing the well-balancing of both charges.The non-linear GCD curves obtained from 3 to 20 mA cm-2within 0-1.5 V,also confirme well mass balancing and good redox reversibility.At 3,5,7,10,15,and 20 mA cm-2,the asymmetric cell delivered the CA(CS)values of 213.8μAh cm-2(29.6 mAh g-1),213.1μAh cm-2(29.2 mAh g-1),212μAh cm-2(29 mAh g-1),197.7μAh cm-2(27.7 mAh g-1),181μAh cm-2(24.8 mAh g-1),and 171μAh cm-2(24.7 mAh g-1),respectively(Fig.5(g)).The crucial factors of the asymmetric cell are energy density(Ed)and power density(Pd)which are evaluated using Eqs.(S3)and(S4)of the SI,respectively.The resulted values of these parameters are plotted in the Ragone diagram(Fig.5(h)).The asymmetric cell delivered higher Edof 0.159 mW h cm-2and a maximum Pdof 18.47 mW cm-2.The energy and power densities of the CMS//AC asymmetric cell are compared to that of several previously reported works as presented in Table 1.Besides,a cycling test for the asymmetric cell was tested at 10 mA cm-2.As demonstrated in Fig.5(i),the asymmetric cell showed decent cycling constancy with good retention of 62.5% even after 3000 cycles.The EIS graphs of the asymmetric cell recorded before and after the cycling study are presented in the inset of Fig.5(i).The Rctvalues before and after the cycling test were noticed as 1.88Ωand 1.95Ω,while the Rsvalues are noted to be 0.26Ωand 0.32Ω,respectively.Moreover,the real-time ability of the asymmetric cell was tested by powering different electronic devices.The photographic image of two pouch-type asymmetric cells connected in series is shown in Fig.5(j)(i).Fig.5(j)(ii)represents the green light-emitting diode(LED)before and after connecting to the asymmetric cell set-up.As shown in Fig.5(j)(iii)and(iv),the blue and green LEDs were lit up with high intensity when they were connected to the asymmetric cell set-up.Fig.5(j)(v)and(vi)shows the photographic images of a rotating toy fan before and after the connection with the asymmetric cell set-up.

Fig.5.(a)Schematic diagram for the construction of the asymmetric cell.(b)CV curves of the AC and CMS electrodes at their respective potential windows.(c)CV and(d)GCD curves of the asymmetric cell in different potential windows.(e)CV and(f)GCD curves of the asymmetric cell.(g)Areal and specifi capacity values and(h)Ragone plot of the asymmetric cell.(i)Cycling measurement of the asymmetric cell.Inset in(i)represents the EIS plots of the asymmetric cell before and after the cycling measurement.(j)Practicability of the asymmetric cell.

4.Conclusion

In summary,we successfully synthesized the CMO-8 h and CMS electrodes with the hybrid morphology on NF substrate using a facile hydrothermal approach,followed by selenization processes.The influenc of reaction time on the morphology was investigated in detail.Among all the electrodes,a CMO-8 h electrode showed a higher CAvalue of 101.7 μAh cm-2at 3 mAcm-2.Moreover,to improve the electrochemical properties,the CMO-8 h sample was selenized to form CMS in a tube furnace in an inert gas atmosphere.The CMS sample exhibited NPs as well as NWs,which can act a vital role to get improved electrochemical performance by providing good surface area and excess active sites.Moreover,the EIS results confirme superior electrical conductivity of the CMS electrode to the CMO-8 h electrode.As a result,at 3 mA cm-2,the CMS electrode showed an improved CAvalue of 385.4μAh cm-2,which is nearly three times higher than that of the CMO-8 h electrode.The CMS also showed good cycling constancy with retention of 86.3%(after 3000 cycles).Afterward,an asymmetric cell was fabricated with AC-coated NF and CMS electrodes.The fabricated asymmetric cell showed steady operation within the applied voltage of 0-1.5 V and demonstrated a high CAvalue of 213.8μAh cm-2.Moreover,an asymmetric cell exhibited high Edand Pdvalues(0.159 mWh cm-2and 18.47 mW cm-2).Realtime properties for an asymmetric cell were also tested by powering different electronic appliances.

Conflic of Interests

The authors declare that they have no known competing financia interests.

Acknowledgments

This work was supported by the National Research Foundation of Korea(NRF)grant funded by the Korean government(MSIP)(No.2017H1D8A2031138 and No.2018R1A6A1A03025708).

Supplementary materials

Supplementary material associated withthis article can be found,in the online version,at doi:10.1016/j.jma.2021.12.012.