Characterization of MgCo2O4 as an electrode for high performance supercapacitors

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Electrochimica Acta 161 (2015) 312–321

Characterization of MgCo2O4 as an electrode for high performancesupercapacitors

Syam G. Krishnan a, M.V. Reddy b,c,*, Midhun Harilal a, Baiju Vidyadharan a,Izan Izwan Misnon a, Mohd Hasbi Ab Rahim a, Jamil Ismail a, Rajan Jose a,*aNanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Kuantan, MalaysiabDepartment of Materials Science & Engineering, National University of Singapore, 117546 SingaporecDepartment of Physics, National university of Singapore 117542, Singapore

A R T I C L E I N F O

Article history:Received 1 December 2014Received in revised form 23 January 2015Accepted 9 February 2015Available online 10 February 2015

Key words:electrochemical energy storagepseudocapacitorsasymmetric capacitorslithium ion battery

A B S T R A C T

Metal cobaltites have promising electrochemical properties for their application as an energy storagemedium. In this paper, usefulness of MgCo2O4 as a supercapacitor electrode is demonstrated andcompared its performance with two other cobaltites, MnCo2O4 and CuCo2O4. The materials aresynthesized using molten salt method and characterized by X-ray diffraction, scanning electronmicroscopy, BET surface area, cyclic voltammetry, galvanostatic charge–discharge cycling, andelectrochemical impedance spectroscopy techniques. The MgCo2O4 electrodes show superior chargestorage properties in 3 M LiOH among a diverse choice of electrolytes. The MgCo2O4 show highertheoretical (�3122 F/g) and practically achieved capacitance (�320 F/g), larger coulombic efficiency, andcycling stability than the other two; therefore, it could be developed as a low-cost energy storagemedium.

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1. Introduction

Rapid technological advancement along with the depletingnatural resources demand smarter production, usage and storageof energy. Supercapacitors are a class of energy storage devicesemploying non-faradic charge accumulation process (electricdouble layer capacitance, EDLC), faradic charge transfer process(pseudocapacitance) or combination of both processes (hybridcapacitance) [1–3]. Carbons (graphene, carbon nanotubes andactivated carbon) are the choice to build EDLC (capacitance range20–50 mF cm�2), transition metal oxides (capacitance up to2000 mF cm�2) and conducting polymers show pseudocapacitance(PC), and layered and hybrid materials combine the two storagemodes [4]. Owing to the larger capacitance, PC materials withdesirable capacitive properties are actively sought. Electrochemi-cal reversibility, availability of an array of oxidation states andhigher electrical conductivity are the properties of a material to beselected as a pseudocapacitor electrode. Many transition metaloxides (TMOs) are proposed as candidates for pseudocapacitiveelectrodes; summary of which are available in recent articles

* Corresponding authors.E-mail addresses: [email protected], [email protected] (M.V. Reddy),

[email protected] (R. Jose).

http://dx.doi.org/10.1016/j.electacta.2015.02.0810013-4686/ã 2015 Elsevier Ltd. All rights reserved.

[5–14]. Among them, compounds of cobalt offer superior perfor-mance than other binary metal oxides although they are expensivedue to its lower abundance in the earth’s crust (<10 ppm).

In recent years, ternary TMOs (TTMOs) are used in electro-chemical application because two metals contribute to redoxreaction. Furthermore, TTMOs’ structural diversity provide oppor-tunities to modify the physical and chemical properties such thatthe capacitance can be tailored [1,3,15]. An added advantage ofsynthesizing cobalt based TTMOs is the reduction in the cost of rarecobalt by partially substituting it with other TMOs [16]. The TTMOssuch as ZnCo2O4 [17,18], CuCo2O4 [19], LiCoO2 [20–22], MnCo2O4

[23,24] are tested as anode materials for lithium ion batteries andsupercapacitors. Table 1 shows the summary of a literature surveyon the electrode characteristics of transition metal cobaltites forsupercapacitor applications. Majority of the activities are centeredon MCo2O4 (M=Cu, Zn, Mn and Ni), possibly because of the hightheoretical capacitance offered by them. Theoretical capacitance ofthese materials are calculated from their redox potentials (SeeSupplementary Information for details of calculation) and com-pared with that of Co3O4 in Fig. 1. We refer Vidyadharan et al. [25]for a brief overview on the capacitive performance of Co3O4

nanostructures. The capacitance so far achieved is also indicated inthe Fig. 1. One would see that the reported materials have slightlylower theoretical capacitance than the parent compound, exceptMnCo2O4; and NiCo2O4 have practically achieved >90% of it. On the

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Table 1Comparison of performance of ternary metal oxides of cobalt as an electrode of supercapacitor reported earlier. The method of preparation is also given in the table. SCS refersto solution combustion synthesis and HTM, hydrothermal method. The 2-electrode in the potential range refers to a working supercapacitor made with the target material asone of the electrodes.

Material Cs (Fg�1) Stability Potential range (V) Ref

CuCo2O4 by SCS 338 (3 M KOH) @1 Ag�1 96%/5000 0.5 [30]LiCoO2 70.17 (1 M LiPF6/EC + DMC) 74.86%/1000 3.0 (2- electrode) [21]LiCoO2 by HTM 58 (LiClO4) @ 2 mA cm�2 85%/1000 1.5 (2- electrode) [22]MnCo2O4.5 porous urichin like nanostructures by HTM 151 (1 M KOH) @ 5 mVs�1 �100%/2100 0.5 [15]MnCo2O4 nanowires by facile HTM 349.8 (1 M KOH) @ 1 Ag�1 �94%/4000 0.45 [43]MnCo2O4 spinel by facile sol-gel method 405 (2 M KOH) @ 5 mA cm�2 �95%/1000 0.4 [44]MnCo2O4 by electroless-electrolytic synthesis 832 (0.5 M NaOH) @ 20 mVs�1 �80%/1000 1.0 (2- electrode) [31]ZnCo2O4 nanotubes by electrospinning 770 (6 M KOH) @10 Ag�1 89.6%/3000 0.5 [45]ZnCo2O4/CNF by co-precipitation 77 (6 M KOH) @2 mA cm�2 Not reported 1.2 (2- electrode) [46]NiCo2O4/Ni submicron particles by sol-gel 217 (1 M KOH) @ 1 mA cm�2 96.3%/600 0.45 [47]NiCo2O4 by thermal decomposition 746 (1 M NaOH) @2 mVs�1 �100%/

10,0000.6 [48]

NiCo2O4 flower-like nanostructures HTM 658 (6 M KOH) @1 Ag�1 �100%/10,000

0.55 [49]

NiCo2O4–graphene composite nanowires by HTM 737 (2 M KOH) @1 Ag�1 94%/3000 0.45 [50]NiCo2O4 nanosheets grown on nickel foam by HTM 1088 (2 M KOH) @ 5 mA cm�2 N.R/2000. 0.55 [51]NiCo2O4 microspheres by microwave assisted heating 1006 (6 M KOH) @1 Ag�1 93.2%/1000 0.45 [52]NiCo2O4 nanorods and nanoflakes by chemical bath synthesis 490 (nanorods) &330 (nanoflakes) (2 M KOH) @ 2 mA cm�2 93%/1000 0.4 [53]NiCo2O4 nano sheets by HTM 999 (2 M KOH) @20 Ag�1 84.6%/3000 0.4 [54]NiCo2O4 by sol-gel 1254 (2 M KOH) @2 Ag�1 70.4%/1000 0.5 [55]NiCo2O4 multiple heirarchical structures by HTM 2623 (3 M KOH) @1 Ag�1 94%/3000 0.5 [56]NiCo2O4 aerogel by sol-gel 1400 (1 M NaOH) @25 mVs�1 �100%/2000 N.R [57]NiCo2O4@ NiCo2O4 nanorodsNiCo2O4 @ NiO nanoflakes

1925 (3 M KOH) @0.5 Ag�1

2210 (3 M KOH) @0.5 Ag�185.4%/2000�100%/2000

0.4 [58]

NiCo2O4 spinel by HTM 1619 (3 M KOH) @ 2 Ag�1 N.R/1000 0.4 [59]

S.G. Krishnan et al. / Electrochimica Acta 161 (2015) 312–321 313

other hand, MgCo2O4, an anode material reported for lithium ionbattery [23], have superior theoretical capacitance than most ofthe MCo2O4 (M=Cu, Zn and Ni) (Fig.1). However, no effort has so farbeen undertaken to evaluate its electrochemical properties forsupercapacitor application.

We have evaluated the supercapacitive performance ofMgCo2O4 and compared its performance with two similarcobaltites, viz. CuCo2O4 and MnCo2O4. The materials weresynthesized by molten salt method (MSM) owing to its potentialto synthesize transition metal oxides [26–28] on a large scale. Theexperimental results reported in this paper show great promise topursue with MgCo2O4.

Fig. 1. Comparison of the supercapacitive performance of cobalt oxide with othercobaltites.

2. Experimental Details

2.1. Synthesis and characterization of MCo2O4

MCo2O4 (M=Mg, Mn, Cu) powders were prepared by mixing 1 MMSO4.5H2O (99%, Sigma Aldrich), 2 M CoSO4.7H2O (98%, Fluka) and0.88 M LiNO3 (99%, Alfa Aesar), 0.12 M LiCl (99%, Merck). The ratioof metal ion to molten salt was 1: 10. For easier synthesis of acrystalline and single phase material, LiNO3 (oxidizing flux) andLiCl (mineralizing agent) were used. The mixture was placed in analumina crucible and then heated at 280 �C (heating rate3 �C min�1) for 3 h in air in a box furnace (Carbolyte, UK). Afterthe mixture was slowly cooled down (cooling rate 3 �C min�1) toroom temperature at 25 �C, it was washed with distilled water toremove excess Li salts and filtered. Afterwards, the remainingpowder was calcined at 70 �C. The calcined sample was furtherheated at 200 �C for 2 h in flowing N2 gas to remove the moisturetraces remained during washing.

Crystal structure and phase of the materials were studied byXRD using Rigaku Miniflex II X-ray diffractometer employing CuKaradiation (l = 1.5406 Å). Gas adsorption behavior and BET surfacearea of the materials were determined using Micrometrics (Tristar,3000) instrument in nitrogen atmosphere. Morphology of thesamples was analyzed using Scanning Electron Microscopy (SEM;JEOL JSM-67500F).

2.2. Electrochemical studies

In a typical procedure, a paste of electrode material wasprepared by mixing and stirring MgCo2O4 (80%), Super P(conductive carbon, Alfa Aesar) (10%), and polyvinylidene fluoride(PVDF) (10%) using N-methyl pyrrolidinone (NMP) as a solvent for24 h. The slurry was coated on ultrasonically cleaned nickel foamsubstrate. The slurry coated nickel foam was dried in an oven at60 �C for 24 h. The dried electrodes were pressed at a pressure of5 ton using a hydraulic press. Similarly electrodes of CuCo2O4 and

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Fig. 2. XRD pattern of a) MgCo2O4 b) MnCo2O4 c) CuCo2O4.

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MnCo2O4 were prepared. The mass loading was �2 mg andgeometrical area of working electrode was �1 cm2. Theelectrochemical properties of the electrodes were studied bycyclic voltammetry (CV), charge discharge cycling (CDC), andelectrochemical impedance spectroscopy (EIS) in three-electrodeconfiguration. Six electrolytes, viz. 1 M LiOH, 3 M LiOH, 3 M KOH,6 M KOH, 1 M K2SO4 and 1 M H2SO4, were tested; among them 3 MLiOH gave the best performance. A potentiostat galvanostat(PGSTAT M101, Metrohm Autolab B.V; The Netherlands) was usedfor electrochemical measurements employing Nova 1.9 software.An Ag/AgCl and a platinum rod were used as the reference andcounter electrodes, respectively.

Fig. 3. SEM images of MCo2O4 (a) Cu, (b) Mn and (

3. Results and discussion

3.1. Structure and morphology of MSM synthesized MCo2O4 (M=Mg,Mn, Cu)

XRD patterns in Fig. 2 reveal the phase information, poly-crystallinity and the cubic structure of MCo2O4. The position andintensity distribution of the XRD are similar thereby indicating thatthe materials are isostructural. Sharp peaks are obtained for (311)plane followed by (4 0 0) and (511) planes. All the peaks fit well tothe reported cubic spinel structure having space group Fd3m : 2.The lattice parameters of MCo2O4 are a = 8.09, 8.108 and 8.101 Å forthe Mg, Mn and Cu analogues, respectively. Small variation in thelattice parameter is due to differences in the ionic radii of Mg, Mnand Cu.

The measured BET surface area of the MCo2O4were �0.45,18.94and 9.81 m2g�1 and their average pore diameters, determinedusing Barett-Joyner-Halenda (BJH) analysis, were �10.8, �15.1 and�9.8 nm for M=Mg, Mn and Cu, respectively. A large difference inBET surface area was observed for the MgCo2O4 sample comparedto the other ones, the reason for which was investigated using SEM.The SEM images of the samples are shown in Fig. 3. Aggregatedspheroidal particle morphology was observed for the Mg and Cuanalogues, whereas the Mn analogue was observed to be flakes.The flake-like structure is assigned to the higher BET surface area ofMnCo2O4. Although both MgCo2O4 and CuCo2O4 have similaraggregate shape their sizes are markedly different. The CuCo2O4

has much smaller particle size (�100 nm) compared to theMgCo2O4 (�700 nm–1 mm), the observed smaller particle size isassigned to the higher BET surface area of the Cu analogue.Although all the materials are synthesized through similarprocedure changes in morphology was observed, which couldbe due to differences in the crystallization behaviors of therespective MCo2O4 systems. Furthermore, MgO is well known for

c) Mg, Bar scale: 1 mm; and insets are 100 nm.

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S.G. Krishnan et al. / Electrochimica Acta 161 (2015) 312–321 315

its hygroscopic behavior, which would reduce the surface area.Nevertheless, reason for different morphology upon similarsynthesis route is beyond the scope of the present article, whichis restricted to the evaluation of the electrochemical properties ofMgCo2O4 for its usefulness as a supercapacitor electrode.

3.2. Electrochemical properties of MCo2O4 (M=Mg, Mn, Cu)

3.2.1. Cyclic VoltammetryTo evaluate the usefulness of MCo2O4 as a supercapacitor

electrode, their redox behavior was studied by CV in six electro-lytes, viz. 1 M LiOH, 3 M LiOH, 3 M KOH, 6 M KOH, 1 M K2SO4 and1 M H2SO4. Among them 1 M K2SO4 and 1 M H2SO4 showed noredox peaks; the CV curves were similar to that of nickel foamsubstrate (See Supplementary Information). Although KOH andLiOH showed redox peaks larger voltammetric currents wereobserved for 3 M LiOH (See Supplementary Information); there-fore, this electrolyte was used for further evaluation of electro-chemical properties. Fig. 4a–c show the CV of the MCo2O4 samplesat varying scan rates at a potential window of �0.6 V in 3 M LiOH.The CV curves show oxidation and reduction peaks during chargingand discharging thereby indicating faradic reaction; the areaenclosed by the CV curve indicates the charge stored. The oxidationand reduction peaks shifted to negative and positive potentialswith scan rate owing to the electrical polarization in the electrode[29]. The redox peaks in CV of Fig. 4(a & c) are attributed to faradic

Fig. 4. CV of MCo2O4 as a function of scan rate for (a) MgCo2O4; (b) MnCo2O4, and (c) C

reaction involving Co4+/Co3+ and M2+/M+ with OH� ions [30]. Theredox peak of the electrode materials can be attributed to thefollowing reactions [3,31–33].

2MgCo2O4 þ 2H2O þ e ����������!Charging

Discharging

2MgO þ 4CoOOH

4CoOOH þ 4H2O þ 2e ����������!Charging

Discharging

4Co OHð Þ2 þ 4 OHð Þ� (1)

2CoOOH þ MnOOH þ e ����������!Charging

Discharging

MnCo2O4 þ H2O þ OH�

MnO2 þ H2O þ e ����������!Charging

Discharging

MnOOH þ OH� (2)

uCo2O4; (d) Variation of specific capacitance of MCo2O4 as a function of scan rates.

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316 S.G. Krishnan et al. / Electrochimica Acta 161 (2015) 312–321

2CuCo2O4 þ 2H2O þ e ����������!Charging

Discharging

Cu2O þ 4CoOOH

4CoOOH þ 4H2O þ 2e ����������!Charging

Discharging

4Co OHð Þ2 þ 4 OHð Þ� (3)

The ratio between the area of anodic (oxidation) and cathodic(reduction) cycles is a measure of electrochemical reversibility,termed as coulombic efficiency (h). The h of MgCo2O4, MnCo2O4

and CuCo2O4 determined from CV curve measured at a scan rate of2 mVs�1 are 93, 91 and 85%, respectively, thereby showingimproved electrochemical reversibility for the MgCo2O4 electrode.The h of MgCo2O4 increased up to 97% for a scan rate of 100 mVs�1

(See Supplementary Information), which would result from thedifference in ion movement at different scan rates. Superiorelectrochemical reversibility would provide capacity retention ofthe supercapacitor for long cycle of operation. The specificcapacitance (CS) of the electrodes were calculated from the CVcurves using the equation

Cs ¼ 1mvðE2 � E1Þ

ZE2

E1

iðEÞdE (4)

where m is the mass of the active material, n is the scan rate, E2� E1gives the potential window, and i(E) is the current at each potential.The CS of the MgCo2O4, MnCo2O4 and CuCo2O4 electrodes at a scanrate of 2 mVs�1 (shown as an inset in corresponding CV curves)were calculated to be 362, 359 and 210 Fg�1, respectively. Fig. 4(d)shows the variation of CS with scan rate. The MgCo2O4 andMnCo2O4 electrodes showed similar CS despite much largerdifference in their BET surface areas (�0.5 and �19 m2g�1,repectively). The CS decreases with increase in scan rate indicatingthat ion diffusion is limited at the surface at higher scan ratethereby dominating EDLC over PC. The MnCo2O4 showed thehighest value at this region (>30 mVs�1) owing to its larger BETsurface area. At higher scan rates (>30 mVs�1) the CS for electrodeswere practically constant owing to the limited movement of ionsonly to the surface of the electrode material. On the other hand, atlower scan rates (<10 mVs�1), the CS is higher due to faradic

Fig. 5. Voltammetric current as a function of square root of scan rate of the MCo2O4

electrodes in 3 M LiOH electrolytes. The symbols are the experimental points andthe solid line is a linear fit.

reaction and allowed maximum utilization of the active material inthe electrode.

In pseudocapacitive materials, the dependence of scan rate (n)with voltammetric current (i) depends on whether the capacitanceoriginates from surface redox reactions or bulk diffusion. Forsurface redox reaction, i / n and for semi-infinite bulk diffusion, i[34]. A straight line for i / ffiffiffi

vp

(Fig. 5) is observed; therefore, bulkdiffusion occurred during the electrochemical reaction. Thisrelationship further indicates that the diffusion of OH� is likelyto control the redox reaction occurring in the electrochemicalprocess. The apparent diffusion coefficient (D) of OH� ion at 25 �C iscalculated in all the materials employing Randles–Sevcik equation[35]

ip ¼ 2:69 � 105 � n3=2 � A �ffiffiffiffiDp� C0 �

ffiffiffinp

(5)

where ip is the peak current, n is the number of electrons involvedin the reaction, A is the surface area of the electrode, D is thediffusion coefficient of the electrode material, C0 is the protonconcentration and n is the scanning rate. The D was calculated fromthe slope of the ip vs

ffiffiffivp

curve as explained elsewhere [36,37]. TheD of MgCo2O4, MnCo2O4 and CuCo2O4 were 2.5 �10�13, 3.3 � 10�14,6.25 �10�14 cm2s�1, respectively. The details of calculation are inthe supplementary information. Obviously, MgCo2O4 supports anorder of magnitude higher D and is expected to be the source of itshigher capcitance. A higher D accelerates the ion transport andslows down the electrode polarization during the charge–discharge process [38]. Therefore, MgCo2O4 posses a fasterelectrode reaction due to quicker ionic transportation owing toits higher ‘D’ than other electrodes.

3.2.2. Galvanostatic charge discharge studiesTo quantify the CS and the rate capability of the samples,

galvanostatic charge-discharge (CD) measurements were per-formed. Fig. 6compares CD curves of the electrodes at a currentdensity of 0.5 Ag�1 in 3 M LiOH with potential window of 0.5 V.Asymmetric shape of the CD curves indicate the faradic behavior ofthe material. Most of the capacitance is generated in the range0.35 to 0.5 V for MgCo2O4 whereas that for MnCo2O4 and CuCo2O4

is 0.32 to 0.5 V. The potential drop between charge and dischargecurves arise due to incomplete faradic reaction and internalresistance in the electrode. Using the ratio of initial potential drop(VIR) to the corresponding discharge current (ID), the internalresistance of the electrode could be calculated. The internalresistances for MCo2O4 in 3 M LiOH were 2.5 (VIR � 2.5 mV),

Fig. 6. Comparison of CD curves of (a) Mg, (b) Mn and (c) Cu cobaltites at a currentdensity of 0.5 Ag�1.

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Fig. 7. Discharge curves of MCo2O4 (a) M=Mg (b) M=Mn (c) M=Cu; the panel (d) compares discharge curves of MCo2O4 at 0.5 Ag�1.

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4.8 (VIR � 4.8 mV), 4.9 V (VIR � 2.5 mV) for M=Mg, Mn, Cu,respectively. The lower internal resistance of MgCo2O4 could bedue to improved electrical conductivity and the CS of the electrodematerial thereby.

Figs. 7a–c show the discharge curves of the electrodes as afunction of current density to calculate the practically available CS.There are three common segments in each discharge curves, viz. (i)a fast initial potential drop followed by (ii) a slow potential decayand (iii) a faster potential drop corresponding to EDLC. The CS fromthe discharge curves can be calculated using the equation

Cs ¼ I � tm � Dv

(6)

where I,t, m and DV are applied current, discharge time, activemass and potential difference, respectively. Fig. 8 represents thevariation of CS as a function of specific current density for MCo2O4.The CS showed an exponential decay with current density for allthe electrodes as observed from the CV analysis. However, CSdetermined from the CD curves showed clearly superior values forthe MgCo2O4, which was only marginal in CV experiments. The CSof MgCo2O4, MnCo2O4, CuCo2O4 from the discharge curve was 321,225 and 133 Fg�1, respectively at a current density of 0.5 Ag�1.While looking at the BHJ analyses (Section 3.1), improved pore sizeand pore volume of MgCo2O4 is expected to contribute to its highercapacitance.

Cycling stability under extreme load of the electrodes iscrucial for practical applications of supercapacitors. Therefore,galvanostatic CD measurements at varying current densities(2 Ag�1, 5 Ag�1, 10 Ag�1) for 2000 cycles, which was equallydivided into four quarters of 500 cycles, were conducted (Fig. 9).The last quarter (1500–2000) repeated the capacitance retentionat 2 Ag�1 after cycling through 5 and 10 Ag�1. Initial decrease ofcapacitance for the first 500 cycles was observed for all thethree electrodes; the capacitance showed a small incrementafter 1000 cycle showing the structural activation and poreopening in the electrodes [39]. At the end of the 2000 cycle,MgCo2O4 and CuCo2O4 exhibited improved capacitance than at500th cycle while MnCo2O4 showed only 80% retention. Thedetails of the cycling stability for the three samples are compiledin the Table 2.

The ratio of discharging (TD) to charging time (TC) from chargedischarge cycle gives the h of the device following the relation

h ¼ TD

Tc� 100 (7)

The h of the materials remained constant for the tested2000 cycles at 93, 91 and 85% for MgCo2O4, MnCo2O4 and CuCo2O4,respectively. The h determined from the CD measurements areconsistent with that evaluated from the CV analysis.

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Fig. 8. Plot showing the variation of capacitance with current density determinedfrom the CD curves. The open squares, filled circles and the arrow marks show theexperimental data and the solid line is a trend line depicting an exponential decayfunction.

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3.2.3. Electrochemical Impedance Spectroscopy StudiesIn order to further evaluate the ion transport kinetics and

electrode conductivity, electrochemical impedance spectroscopy(EIS) measurements were performed in the frequency range

Fig. 9. The variation of specific capacitance with cycle number with variable current denswhere first three cycles carried current densities of 2 Ag�1, 5 Ag�1, 10 Ag�1 and the fina

0.1 MHz to 2 mHz with an ac perturbation of 10 mV. Figs. 10a–cshow the Nyquist plot for all samples in 3 M LiOH at open circuitpotential. The EIS spectrum of a supercapacitor electrode is usuallydivided into three segments following three processes; (i) the bulkresistance of the device (RS), synonymously called equivalent seriesresistance (ESR) at high frequency (>1 kHz); (ii) capacitive effectsat intermediate frequencies (<1 kHz); and (iii) Warburg imped-ance resulting from the frequency dependence of ion diffusion/transport in the electrode - electrolyte interface at the lowfrequencies (<5 Hz). The high frequency offset at the real part (RS)is a combination of the electrolytic resistance, contact resistance atthe interface between the current collector and the active materialand the intrinsic resistance of the active material. The value of RSmeasured from the high frequency offset for sample MgCo2O4 ismuch lower than that of the other two sample (see Table 3) whichdemonstrates the superior electrical conductivity of the sample.The EIS spectrum presented a small semicircle in the high tomedium frequency region. The diameter of the semicircle is ameasure of the kinetic resistance to ion transfer at the solid oxide/liquid electrolyte interface, known as the charge transfer resistanceRCT. The low RCT measured for the sample MgCo2O4 implies lowerresistance to ion movement through the pores of the materialwhich leads to higher utilization of the active material andsubsequently higher capacitance. At intermediate frequencies, the

ity. (a) MgCo2O4 (b) MnCo2O4 (c) CuCo2O4. 2000 cycles was equally divided in to four,l one repeated for 2 Ag�1.

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Table 2Comparison of capacitance retention of MCo2O4 at a current density of 2 Ag�1. Thecapacitance were measured at variable current densities and the table indicates thecapacitance at the end of 500 and 2000 cycle where the measurement was made at2 Ag�1.

MgCo2O4 MnCo2O4 CuCo2O4

Current density (Ag�1) 2 2 2Number of cycles 500 2000 500 2000 500 2000CS (Fg�1) 160 344 288 240 152 176

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Nyquist plot of electrode/electrolyte system shows a straight line;the angle of the line with respect to the real axis determines theorigin of capacitance. The capacitance is EDLC if the angle is 90�

and a deviation from this value indicates capacitance from iondiffusion. The angle of the EIS curve with respect to its real axis ofelectrodes varies from �80 to �60�. The slope of MgCo2O4 is �60�which indicate that the total capacitance arise from ion diffusionand charge accumulation. The diffusive contribution from theother electrodes were relatively lower.

The observed EIS spectrum is fitted using Nova 1.9 software toan equivalent circuit proposed for supercapacitors (Fig. 10(d)); thefitted parameters obtained are summarized in Table 3. The fitted RSand RCT values of the samples are in good agreement with thatdirectly calculated from the Nyquist plot (Table 3). The lower

Fig. 10. Nyquist plot for (a) MgCo2O4 (b) MnCo2O4 (c) CuCo2O4 (d) equivalent circuit acontinuous line is the fitted data. The inset shows expanded high frequency region and

Warburg impedance of MgCo2O4 indicate that the sample offer lessresistance to the diffusion of ions and better charge transportbehaviour, which could be assigned to the observed lower RS andlower resistance of the electrode. The redox process occur due tothe diffusion of ions through the active electrode material leads tothe appearance of constant phase element (CPE) in the equivalentcircuit. The CPE impedance (ZCPE) is given by [4]

ZCPE ¼ 1B jvð Þn (8)

where B and n (0 < n < 1) are frequency independent parameters.For n = 1, the system behaves as a pure capacitor and for n = 0, pureresistor. The ZCPE value for the electrodes are 45.6, 72.6 and 57(mFs)1/n and the n value was found to be 0.91, 0.91 and 0.94 for(M=Mg, Mn and Cu) electrodes respectively indicating that theelectrodes are more capacitive in nature.

Finally, we correlate the results of CD that MgCo2O4 havesuperior power capability compared to the other electrodes usingEIS measurements. As mentioned, the second segment of thefrequency region (<1000 Hz) of EIS curve represents the electri-cally charged electrode-electrolyte interface generating the super-capacitive behavior of the device. This portion of EIS is linearrepresenting time-dependent process of diffusion and accumula-tion of charges at the accessible suface. The charge relaxation time,which is a measure of power density, of the above process can be

t open circuit potential. The solid circle indicates the experimental value and the electrical equivalent of pseudocapacitor electrode showing transport parameters.

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Fig. 11. The variation of the (a) real (C') part of capacitance with frequency and imaginary (C”) part of capacitance with frequency at open circuit potential for (a) MgCo2O4 (b)MnCo2O4 (c) CuCo2O4 using 3 M LiOH as electrolyte.

Table 3Summary of transport parameters form the fitted circuit and determined directly from the spectrum.

Material RS (V) RCT (V) Cdl (mF) ZCPE (mFs)1/n W (mMho)

From intercept Fit From diameter Fit

MgCo2O4 0.71 0.72 0.13 0.14 1.11 45.9 (n = 0.91) 276MnCo2O4 1.32 1.35 0.29 0.38 1.54 72.6 (n = 0.91) 469CuCo2O4 0.88 0.89 0.17 0.18 1.62 57 (n = 0.94) 818

320 S.G. Krishnan et al. / Electrochimica Acta 161 (2015) 312–321

determined by expressing the total capacitance as a combinationas of real (C0) and imaginary (C00) parts [40,41] as.

CðvÞ ¼ C0 ðvÞþ C}ðvÞ (9)

where C0 ðvÞ ¼ �Z0 ðvÞ

vjZðvÞj2 and C}ðvÞ ¼ Z}ðvÞvjZðvÞj2. Fig 11(a) shows the

variation of calculated C0 (v) of the electrodes with frequency. TheC0 of the electrodes determined from EIS is consistent with CVmeasurements. A small variations observed in the Fig. 11(a) thanthat determined from the CV curves could be attributed to factorssuch as chemical and physical heterogenity and the deeply trappedimmobile ions during EIS measurement [42].

For all the three electrodes, the C00 showed a bell-shaped curvesFig 11(b). The t could be calculated from the C00 vs. frequency curveemploying the relation t ¼ 1

f 0[40] by measuring the peak

frequency (fo). The fo determined from the graph were 2.2,1.2 and 2.8 Hz for MgCo2O4, MnCo2O4, CuCo2O4, respectively andtheir corresponding t were 0.46, 0.83 and 0.35 s. i.e., the powercapability of MgCo2O4 is superior to the Mn analogue but inferiorto the Cu analogue. High power capability of the CuCo2O4 couldresult from the improved electrical conductivity of Cu compared tothe other elements. Nevertheless, owing to the superior achievedcapacitance and comparable power capability of MgCo2O4, thematerial offers large potential to be developed as a practicalsupercapacitor electrode.

4. Conclusions

In conclusion, a ternary compound MgCo2O4 offers highertheoretical capacitance than many of the metal cobaltites; anelectrode of which fabricated on nickel foam substrate gavesuperior practical capacitance compared to two similar controlmaterials, viz. MnCo2O4, CuCo2O4 in 3 M LiOH electrolyte. The CVand CD measurements show superior coulombic efficiency for the

MgCo2O4 (�93%) electrode compared to the other electrodes. TheCV measurements show that the MgCo2O4 electrodes also offershigher OH� ion diffusivity in it than the others. Owing to thesefactors, the MgCo2O4 electrodes gave the highest CS despite thelowest BET surface area measured for this material. The resultswere validated using electrochemical impedance spectroscopy andgalvanostatic cycling techniques. The studies show that MgCo2O4

is characterized by low equivalent series resistance and internalresistance; furthermore, MgCo2O4 has lower relaxation time forimproved power capability. The results demonstrated in this papershows huge promise for MgCo2O4 to be developed as highperforming supercapacitive energy storage device.

Acknowlegements

This work is supported by Research and InnovationDepartment of UMP and Malaysian Technological UniversityNetwork (MTUN).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.electacta.2015.02.081.

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