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Characterization of MgCo2O4 as an electrode for high performance supercapacitors
Electrochimica Acta 161 (2015) 312–321
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Characterization of MgCo2O4 as an electrode for high performance supercapacitors 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, * a b c
Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Kuantan, Malaysia Department of Materials Science & Engineering, National University of Singapore, 117546 Singapore Department of Physics, National university of Singapore 117542, Singapore
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 December 2014 Received in revised form 23 January 2015 Accepted 9 February 2015 Available online 10 February 2015
Metal cobaltites have promising electrochemical properties for their application as an energy storage medium. In this paper, usefulness of MgCo2O4 as a supercapacitor electrode is demonstrated and compared its performance with two other cobaltites, MnCo2O4 and CuCo2O4. The materials are synthesized using molten salt method and characterized by X-ray diffraction, scanning electron microscopy, BET surface area, cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical impedance spectroscopy techniques. The MgCo2O4 electrodes show superior charge storage properties in 3 M LiOH among a diverse choice of electrolytes. The MgCo2O4 show higher theoretical (3122 F/g) and practically achieved capacitance (320 F/g), larger coulombic efficiency, and cycling stability than the other two; therefore, it could be developed as a low-cost energy storage medium. ã 2015 Elsevier Ltd. All rights reserved.
Key words: electrochemical energy storage pseudocapacitors asymmetric capacitors lithium ion battery
1. Introduction Rapid technological advancement along with the depleting natural resources demand smarter production, usage and storage of energy. Supercapacitors are a class of energy storage devices employing non-faradic charge accumulation process (electric double layer capacitance, EDLC), faradic charge transfer process (pseudocapacitance) or combination of both processes (hybrid capacitance) [1–3]. Carbons (graphene, carbon nanotubes and activated carbon) are the choice to build EDLC (capacitance range 20–50 mF cm2), transition metal oxides (capacitance up to 2000 mF cm2) and conducting polymers show pseudocapacitance (PC), and layered and hybrid materials combine the two storage modes [4]. Owing to the larger capacitance, PC materials with desirable capacitive properties are actively sought. Electrochemical reversibility, availability of an array of oxidation states and higher electrical conductivity are the properties of a material to be selected as a pseudocapacitor electrode. Many transition metal oxides (TMOs) are proposed as candidates for pseudocapacitive electrodes; 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.2015.02.081 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
[5–14]. Among them, compounds of cobalt offer superior performance than other binary metal oxides although they are expensive due to its lower abundance in the earth’s crust (<10 ppm). In recent years, ternary TMOs (TTMOs) are used in electrochemical application because two metals contribute to redox reaction. Furthermore, TTMOs’ structural diversity provide opportunities to modify the physical and chemical properties such that the capacitance can be tailored [1,3,15]. An added advantage of synthesizing cobalt based TTMOs is the reduction in the cost of rare cobalt by partially substituting it with other TMOs [16]. The TTMOs such as ZnCo2O4 [17,18], CuCo2O4 [19] , LiCoO2 [20–22], MnCo2O4 [23,24] are tested as anode materials for lithium ion batteries and supercapacitors. Table 1 shows the summary of a literature survey on the electrode characteristics of transition metal cobaltites for supercapacitor applications. Majority of the activities are centered on MCo2O4 (M=Cu, Zn, Mn and Ni), possibly because of the high theoretical capacitance offered by them. Theoretical capacitance of these materials are calculated from their redox potentials (See Supplementary Information for details of calculation) and compared 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 in the Fig. 1. One would see that the reported materials have slightly lower theoretical capacitance than the parent compound, except MnCo2O4; and NiCo2O4 have practically achieved >90% of it. On the
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Table 1 Comparison 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 refers to solution combustion synthesis and HTM, hydrothermal method. The 2-electrode in the potential range refers to a working supercapacitor made with the target material as one of the electrodes. Material
Cs (Fg1)
Stability
Potential range (V) Ref
CuCo2O4 by SCS LiCoO2 LiCoO2 by HTM MnCo2O4.5 porous urichin like nanostructures by HTM MnCo2O4 nanowires by facile HTM MnCo2O4 spinel by facile sol-gel method MnCo2O4 by electroless-electrolytic synthesis ZnCo2O4 nanotubes by electrospinning ZnCo2O4/CNF by co-precipitation NiCo2O4/Ni submicron particles by sol-gel NiCo2O4 by thermal decomposition
338 (3 M KOH) @1 Ag1 70.17 (1 M LiPF6/EC + DMC) 58 (LiClO4) @ 2 mA cm2 151 (1 M KOH) @ 5 mVs1 349.8 (1 M KOH) @ 1 Ag1 405 (2 M KOH) @ 5 mA cm2 832 (0.5 M NaOH) @ 20 mVs1 770 (6 M KOH) @10 Ag1 77 (6 M KOH) @2 mA cm2 217 (1 M KOH) @ 1 mA cm2 746 (1 M NaOH) @2 mVs1
0.5 3.0 (2- electrode) 1.5 (2- electrode) 0.5 0.45 0.4 1.0 (2- electrode) 0.5 1.2 (2- electrode) 0.45 0.6
[30] [21] [22] [15] [43] [44] [31] [45] [46] [47] [48]
NiCo2O4 flower-like nanostructures HTM
658 (6 M KOH) @1 Ag1
0.55
[49]
NiCo2O4–graphene composite nanowires by HTM NiCo2O4 nanosheets grown on nickel foam by HTM NiCo2O4 microspheres by microwave assisted heating NiCo2O4 nanorods and nanoflakes by chemical bath synthesis NiCo2O4 nano sheets by HTM NiCo2O4 by sol-gel NiCo2O4 multiple heirarchical structures by HTM NiCo2O4 aerogel by sol-gel NiCo2O4@ NiCo2O4 nanorods NiCo2O4 @ NiO nanoflakes NiCo2O4 spinel by HTM
737 (2 M KOH) @1 Ag1 1088 (2 M KOH) @ 5 mA cm2 1006 (6 M KOH) @1 Ag1 490 (nanorods) &330 (nanoflakes) (2 M KOH) @ 2 mA cm2 999 (2 M KOH) @20 Ag1 1254 (2 M KOH) @2 Ag1 2623 (3 M KOH) @1 Ag1 1400 (1 M NaOH) @25 mVs1 1925 (3 M KOH) @0.5 Ag1 2210 (3 M KOH) @0.5 Ag1 1619 (3 M KOH) @ 2 Ag1
96%/5000 74.86%/1000 85%/1000 100%/2100 94%/4000 95%/1000 80%/1000 89.6%/3000 Not reported 96.3%/600 100%/ 10,000 100%/ 10,000 94%/3000 N.R/2000. 93.2%/1000 93%/1000 84.6%/3000 70.4%/1000 94%/3000 100%/2000 85.4%/2000 100%/2000 N.R/1000
0.45 0.55 0.45 0.4 0.4 0.5 0.5 N.R 0.4
[50] [51] [52] [53] [54] [55] [56] [57] [58]
0.4
[59]
other hand, MgCo2O4, an anode material reported for lithium ion battery [23], have superior theoretical capacitance than most of the MCo2O4 (M=Cu, Zn and Ni) (Fig. 1). However, no effort has so far been undertaken to evaluate its electrochemical properties for supercapacitor application. We have evaluated the supercapacitive performance of MgCo2O4 and compared its performance with two similar cobaltites, viz. CuCo2O4 and MnCo2O4. The materials were synthesized by molten salt method (MSM) owing to its potential to synthesize transition metal oxides [26–28] on a large scale. The experimental results reported in this paper show great promise to pursue with MgCo2O4.
2. Experimental Details 2.1. Synthesis and characterization of MCo2O4 MCo2O4 (M=Mg, Mn, Cu) powders were prepared by mixing 1 M MSO4.5H2O (99%, Sigma Aldrich), 2 M CoSO4.7H2O (98%, Fluka) and 0.88 M LiNO3 (99%, Alfa Aesar), 0.12 M LiCl (99%, Merck). The ratio of metal ion to molten salt was 1: 10. For easier synthesis of a crystalline and single phase material, LiNO3 (oxidizing flux) and LiCl (mineralizing agent) were used. The mixture was placed in an alumina crucible and then heated at 280 C (heating rate 3 C min1) for 3 h in air in a box furnace (Carbolyte, UK). After the mixture was slowly cooled down (cooling rate 3 C min1) to room temperature at 25 C, it was washed with distilled water to remove excess Li salts and filtered. Afterwards, the remaining powder was calcined at 70 C. The calcined sample was further heated at 200 C for 2 h in flowing N2 gas to remove the moisture traces remained during washing. Crystal structure and phase of the materials were studied by XRD using Rigaku Miniflex II X-ray diffractometer employing CuKa radiation (l = 1.5406 Å). Gas adsorption behavior and BET surface area of the materials were determined using Micrometrics (Tristar, 3000) instrument in nitrogen atmosphere. Morphology of the samples was analyzed using Scanning Electron Microscopy (SEM; JEOL JSM-67500F). 2.2. Electrochemical studies
Fig. 1. Comparison of the supercapacitive performance of cobalt oxide with other cobaltites.
In a typical procedure, a paste of electrode material was prepared 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 for 24 h. The slurry was coated on ultrasonically cleaned nickel foam substrate. The slurry coated nickel foam was dried in an oven at 60 C for 24 h. The dried electrodes were pressed at a pressure of 5 ton using a hydraulic press. Similarly electrodes of CuCo2O4 and
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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, polycrystallinity and the cubic structure of MCo2O4. The position and intensity distribution of the XRD are similar thereby indicating that the materials are isostructural. Sharp peaks are obtained for (3 11) plane followed by (4 0 0) and (5 11) planes. All the peaks fit well to
Fig. 2. XRD pattern of a) MgCo2O4 b) MnCo2O4 c) CuCo2O4.
MnCo2O4 were prepared. The mass loading was 2 mg and geometrical area of working electrode was 1 cm2. The electrochemical properties of the electrodes were studied by cyclic voltammetry (CV), charge discharge cycling (CDC), and electrochemical impedance spectroscopy (EIS) in three-electrode configuration. 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 M LiOH gave the best performance. A potentiostat galvanostat (PGSTAT M101, Metrohm Autolab B.V; The Netherlands) was used for electrochemical measurements employing Nova 1.9 software. An Ag/AgCl and a platinum rod were used as the reference and counter electrodes, respectively.
the reported cubic spinel structure having space group Fd3m : 2. The lattice parameters of MCo2O4 are a = 8.09, 8.108 and 8.101 Å for the Mg, Mn and Cu analogues, respectively. Small variation in the lattice parameter is due to differences in the ionic radii of Mg, Mn and Cu. The measured BET surface area of the MCo2O4 were 0.45, 18.94 and 9.81 m2 g1 and their average pore diameters, determined using Barett-Joyner-Halenda (BJH) analysis, were 10.8, 15.1 and 9.8 nm for M=Mg, Mn and Cu, respectively. A large difference in BET surface area was observed for the MgCo2O4 sample compared to the other ones, the reason for which was investigated using SEM. The SEM images of the samples are shown in Fig. 3. Aggregated spheroidal particle morphology was observed for the Mg and Cu analogues, whereas the Mn analogue was observed to be flakes. The flake-like structure is assigned to the higher BET surface area of MnCo2O4. Although both MgCo2O4 and CuCo2O4 have similar aggregate shape their sizes are markedly different. The CuCo2O4 has much smaller particle size (100 nm) compared to the MgCo2O4 (700 nm–1 mm), the observed smaller particle size is assigned to the higher BET surface area of the Cu analogue. Although all the materials are synthesized through similar procedure changes in morphology was observed, which could be due to differences in the crystallization behaviors of the respective MCo2O4 systems. Furthermore, MgO is well known for
Fig. 3. SEM images of MCo2O4 (a) Cu, (b) Mn and (c) Mg, Bar scale: 1 mm; and insets are 100 nm.
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its hygroscopic behavior, which would reduce the surface area. Nevertheless, reason for different morphology upon similar synthesis route is beyond the scope of the present article, which is restricted to the evaluation of the electrochemical properties of MgCo2O4 for its usefulness as a supercapacitor electrode.
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reaction involving Co4+/Co3+ and M2+/M+ with OH ions [30]. The redox peak of the electrode materials can be attributed to the following reactions [3,31–33]. Charging
2MgO þ 4CoOOH 2MgCo2 O4 þ 2H2 O þ e ! Discharging
3.2. Electrochemical properties of MCo2O4 (M=Mg, Mn, Cu) 3.2.1. Cyclic Voltammetry To evaluate the usefulness of MCo2O4 as a supercapacitor electrode, their redox behavior was studied by CV in six electrolytes, viz. 1 M LiOH, 3 M LiOH, 3 M KOH, 6 M KOH, 1 M K2SO4 and 1 M H2SO4. Among them 1 M K2SO4 and 1 M H2SO4 showed no redox peaks; the CV curves were similar to that of nickel foam substrate (See Supplementary Information). Although KOH and LiOH showed redox peaks larger voltammetric currents were observed for 3 M LiOH (See Supplementary Information); therefore, this electrolyte was used for further evaluation of electrochemical properties. Fig. 4a–c show the CV of the MCo2O4 samples at varying scan rates at a potential window of 0.6 V in 3 M LiOH. The CV curves show oxidation and reduction peaks during charging and discharging thereby indicating faradic reaction; the area enclosed by the CV curve indicates the charge stored. The oxidation and reduction peaks shifted to negative and positive potentials with 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
Charging
4CoðOHÞ þ 4ðOHÞ 4CoOOH þ 4H2 O þ 2e ! 2
(1)
Discharging
Charging
MnCo2 O4 þ H2 O þ OH 2CoOOH þ MnOOH þ e ! Discharging
Charging
MnOOH þ OH MnO2 þ H2 O þ e !
(2)
Discharging
Fig. 4. CV of MCo2O4 as a function of scan rate for (a) MgCo2O4; (b) MnCo2O4, and (c) CuCo2O4; (d) Variation of specific capacitance of MCo2O4 as a function of scan rates.
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reaction and allowed maximum utilization of the active material in the electrode.
Charging
Cu2 O þ 4CoOOH 2CuCo2 O4 þ 2H2 O þ e ! Discharging
Charging
4CoðOHÞ þ 4ðOHÞ 4CoOOH þ 4H2 O þ 2e ! 2
(3)
Discharging
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 of 2 mVs1 are 93, 91 and 85%, respectively, thereby showing improved electrochemical reversibility for the MgCo2O4 electrode. The h of MgCo2O4 increased up to 97% for a scan rate of 100 mVs1 (See Supplementary Information), which would result from the difference in ion movement at different scan rates. Superior electrochemical reversibility would provide capacity retention of the supercapacitor for long cycle of operation. The specific capacitance (CS) of the electrodes were calculated from the CV curves using the equation 1 Cs ¼ mvðE2 E1 Þ
ZE2 iðEÞdE
(4)
E1
where m is the mass of the active material, n is the scan rate, E2 E1 gives the potential window, and i(E) is the current at each potential. The CS of the MgCo2O4, MnCo2O4 and CuCo2O4 electrodes at a scan rate of 2 mVs1 (shown as an inset in corresponding CV curves) were calculated to be 362, 359 and 210 Fg1, respectively. Fig. 4(d) shows the variation of CS with scan rate. The MgCo2O4 and MnCo2O4 electrodes showed similar CS despite much larger difference in their BET surface areas (0.5 and 19 m2 g1, repectively). The CS decreases with increase in scan rate indicating that ion diffusion is limited at the surface at higher scan rate thereby dominating EDLC over PC. The MnCo2O4 showed the highest value at this region (>30 mVs1) owing to its larger BET surface area. At higher scan rates (>30 mVs1) the CS for electrodes were practically constant owing to the limited movement of ions only to the surface of the electrode material. On the other hand, at lower scan rates (<10 mVs1), 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 and the solid line is a linear fit.
In pseudocapacitive materials, the dependence of scan rate (n) with voltammetric current (i) depends on whether the capacitance originates from surface redox reactions or bulk diffusion. For surface redox reaction, i / n and for semi-infinite bulk diffusion, i pffiffiffi [34]. A straight line for i / v (Fig. 5) is observed; therefore, bulk diffusion occurred during the electrochemical reaction. This relationship further indicates that the diffusion of OH is likely to control the redox reaction occurring in the electrochemical process. The apparent diffusion coefficient (D) of OH ion at 25 C is calculated in all the materials employing Randles–Sevcik equation [35] pffiffiffiffi pffiffiffi ip ¼ 2:69 105 n3=2 A D C0 n (5) where ip is the peak current, n is the number of electrons involved in the reaction, A is the surface area of the electrode, D is the diffusion coefficient of the electrode material, C0 is the proton concentration and n is the scanning rate. The D was calculated from pffiffiffi the slope of the ip vs v curve as explained elsewhere [36,37]. The D of MgCo2O4, MnCo2O4 and CuCo2O4 were 2.5 1013, 3.3 1014, 6.25 1014 cm2s1, respectively. The details of calculation are in the supplementary information. Obviously, MgCo2O4 supports an order of magnitude higher D and is expected to be the source of its higher capcitance. A higher D accelerates the ion transport and slows down the electrode polarization during the charge– discharge process [38]. Therefore, MgCo2O4 posses a faster electrode reaction due to quicker ionic transportation owing to its higher ‘D’ than other electrodes. 3.2.2. Galvanostatic charge discharge studies To quantify the CS and the rate capability of the samples, galvanostatic charge-discharge (CD) measurements were performed. Fig. 6compares CD curves of the electrodes at a current density of 0.5 Ag1 in 3 M LiOH with potential window of 0.5 V. Asymmetric shape of the CD curves indicate the faradic behavior of the material. Most of the capacitance is generated in the range 0.35 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 discharge curves arise due to incomplete faradic reaction and internal resistance in the electrode. Using the ratio of initial potential drop (VIR) to the corresponding discharge current (ID), the internal resistance of the electrode could be calculated. The internal resistances 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 current density of 0.5 Ag1.
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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 Ag1.
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 be due to improved electrical conductivity and the CS of the electrode material thereby. Figs. 7a–c show the discharge curves of the electrodes as a function 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 decay and (iii) a faster potential drop corresponding to EDLC. The CS from the discharge curves can be calculated using the equation Cs ¼
It m Dv
(6)
where I,t, m and DV are applied current, discharge time, active mass and potential difference, respectively. Fig. 8 represents the variation of CS as a function of specific current density for MCo2O4. The CS showed an exponential decay with current density for all the electrodes as observed from the CV analysis. However, CS determined from the CD curves showed clearly superior values for the MgCo2O4, which was only marginal in CV experiments. The CS of MgCo2O4, MnCo2O4, CuCo2O4 from the discharge curve was 321, 225 and 133 Fg1, respectively at a current density of 0.5 Ag1. While looking at the BHJ analyses (Section 3.1), improved pore size and pore volume of MgCo2O4 is expected to contribute to its higher capacitance.
Cycling stability under extreme load of the electrodes is crucial for practical applications of supercapacitors. Therefore, galvanostatic CD measurements at varying current densities (2 Ag1, 5 Ag1, 10 Ag1) for 2000 cycles, which was equally divided into four quarters of 500 cycles, were conducted (Fig. 9). The last quarter (1500–2000) repeated the capacitance retention at 2 Ag1 after cycling through 5 and 10 Ag1. Initial decrease of capacitance for the first 500 cycles was observed for all the three electrodes; the capacitance showed a small increment after 1000 cycle showing the structural activation and pore opening in the electrodes [39]. At the end of the 2000 cycle, MgCo2O4 and CuCo2O4 exhibited improved capacitance than at 500th cycle while MnCo2O4 showed only 80% retention. The details of the cycling stability for the three samples are compiled in the Table 2. The ratio of discharging (TD) to charging time (TC) from charge discharge cycle gives the h of the device following the relation
h¼
TD 100 Tc
(7)
The h of the materials remained constant for the tested 2000 cycles at 93, 91 and 85% for MgCo2O4, MnCo2O4 and CuCo2O4, respectively. The h determined from the CD measurements are consistent with that evaluated from the CV analysis.
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Fig. 8. Plot showing the variation of capacitance with current density determined from the CD curves. The open squares, filled circles and the arrow marks show the experimental data and the solid line is a trend line depicting an exponential decay function.
3.2.3. Electrochemical Impedance Spectroscopy Studies In order to further evaluate the ion transport kinetics and electrode conductivity, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range
0.1 MHz to 2 mHz with an ac perturbation of 10 mV. Figs. 10a–c show the Nyquist plot for all samples in 3 M LiOH at open circuit potential. The EIS spectrum of a supercapacitor electrode is usually divided into three segments following three processes; (i) the bulk resistance of the device (RS), synonymously called equivalent series resistance (ESR) at high frequency (>1 kHz); (ii) capacitive effects at intermediate frequencies (<1 kHz); and (iii) Warburg impedance resulting from the frequency dependence of ion diffusion/ transport in the electrode - electrolyte interface at the low frequencies (<5 Hz). The high frequency offset at the real part (RS) is a combination of the electrolytic resistance, contact resistance at the interface between the current collector and the active material and the intrinsic resistance of the active material. The value of RS measured from the high frequency offset for sample MgCo2O4 is much lower than that of the other two sample (see Table 3) which demonstrates the superior electrical conductivity of the sample. The EIS spectrum presented a small semicircle in the high to medium frequency region. The diameter of the semicircle is a measure of the kinetic resistance to ion transfer at the solid oxide/ liquid electrolyte interface, known as the charge transfer resistance RCT. The low RCT measured for the sample MgCo2O4 implies lower resistance to ion movement through the pores of the material which leads to higher utilization of the active material and subsequently higher capacitance. At intermediate frequencies, the
Fig. 9. The variation of specific capacitance with cycle number with variable current density. (a) MgCo2O4 (b) MnCo2O4 (c) CuCo2O4. 2000 cycles was equally divided in to four, where first three cycles carried current densities of 2 Ag1, 5 Ag1, 10 Ag1 and the final one repeated for 2 Ag1.
S.G. Krishnan et al. / Electrochimica Acta 161 (2015) 312–321 Table 2 Comparison of capacitance retention of MCo2O4 at a current density of 2 Ag1. The capacitance were measured at variable current densities and the table indicates the capacitance at the end of 500 and 2000 cycle where the measurement was made at 2 Ag1.
Current density (Ag1) Number of cycles CS (Fg1)
MgCo2O4
MnCo2O4
CuCo2O4
2 500 160
2 500 288
2 500 152
2000 344
2000 240
2000 176
Nyquist plot of electrode/electrolyte system shows a straight line; the angle of the line with respect to the real axis determines the origin of capacitance. The capacitance is EDLC if the angle is 90 and a deviation from this value indicates capacitance from ion diffusion. The angle of the EIS curve with respect to its real axis of electrodes varies from 80 to 60 . The slope of MgCo2O4 is 60 which indicate that the total capacitance arise from ion diffusion and charge accumulation. The diffusive contribution from the other electrodes were relatively lower. The observed EIS spectrum is fitted using Nova 1.9 software to an equivalent circuit proposed for supercapacitors (Fig. 10(d)); the fitted parameters obtained are summarized in Table 3. The fitted RS and RCT values of the samples are in good agreement with that directly calculated from the Nyquist plot (Table 3). The lower
319
Warburg impedance of MgCo2O4 indicate that the sample offer less resistance to the diffusion of ions and better charge transport behaviour, which could be assigned to the observed lower RS and lower resistance of the electrode. The redox process occur due to the diffusion of ions through the active electrode material leads to the appearance of constant phase element (CPE) in the equivalent circuit. The CPE impedance (ZCPE) is given by [4] ZCPE ¼
1 n BðjvÞ
(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, pure resistor. 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 the electrodes are more capacitive in nature. Finally, we correlate the results of CD that MgCo2O4 have superior power capability compared to the other electrodes using EIS measurements. As mentioned, the second segment of the frequency region (<1000 Hz) of EIS curve represents the electrically charged electrode-electrolyte interface generating the supercapacitive behavior of the device. This portion of EIS is linear representing time-dependent process of diffusion and accumulation of charges at the accessible suface. The charge relaxation time, which is a measure of power density, of the above process can be
Fig. 10. Nyquist plot for (a) MgCo2O4 (b) MnCo2O4 (c) CuCo2O4 (d) equivalent circuit at open circuit potential. The solid circle indicates the experimental value and the continuous line is the fitted data. The inset shows expanded high frequency region and 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 3 Summary of transport parameters form the fitted circuit and determined directly from the spectrum. RS (V)
Material
MgCo2O4 MnCo2O4 CuCo2O4
RCT (V)
From intercept
Fit
From diameter
Fit
0.71 1.32 0.88
0.72 1.35 0.89
0.13 0.29 0.17
0.14 0.38 0.18
determined by expressing the total capacitance as a combination as of real (C0 ) and imaginary (C00 ) parts [40,41] as. 0
CðvÞ ¼ C ðvÞþ C} ðvÞ 0
where C ðvÞ ¼
(9)
0
Z ðvÞ and C}ðvÞ ¼ Z ðvÞ 2 . Fig 11(a) shows the vjZðvÞj2 vjZðvÞj }
variation of calculated C0 (v) of the electrodes with frequency. The C0 of the electrodes determined from EIS is consistent with CV measurements. A small variations observed in the Fig. 11(a) than that determined from the CV curves could be attributed to factors such as chemical and physical heterogenity and the deeply trapped immobile ions during EIS measurement [42]. For all the three electrodes, the C00 showed a bell-shaped curves Fig 11(b). The t could be calculated from the C00 vs. frequency curve employing the relation t ¼ f1 [40] by measuring the peak 0
frequency (fo). The fo determined from the graph were 2.2, 1.2 and 2.8 Hz for MgCo2O4, MnCo2O4, CuCo2O4, respectively and their corresponding t were 0.46, 0.83 and 0.35 s. i.e., the power capability of MgCo2O4 is superior to the Mn analogue but inferior to the Cu analogue. High power capability of the CuCo2O4 could result from the improved electrical conductivity of Cu compared to the other elements. Nevertheless, owing to the superior achieved capacitance and comparable power capability of MgCo2O4, the material offers large potential to be developed as a practical supercapacitor electrode.
Cdl (mF)
ZCPE (mFs)1/n
W (mMho)
1.11 1.54 1.62
45.9 (n = 0.91) 72.6 (n = 0.91) 57 (n = 0.94)
276 469 818
MgCo2O4 (93%) electrode compared to the other electrodes. The CV measurements show that the MgCo2O4 electrodes also offers higher OH ion diffusivity in it than the others. Owing to these factors, the MgCo2O4 electrodes gave the highest CS despite the lowest BET surface area measured for this material. The results were validated using electrochemical impedance spectroscopy and galvanostatic cycling techniques. The studies show that MgCo2O4 is characterized by low equivalent series resistance and internal resistance; furthermore, MgCo2O4 has lower relaxation time for improved power capability. The results demonstrated in this paper shows huge promise for MgCo2O4 to be developed as high performing supercapacitive energy storage device. Acknowlegements This work is supported by Research and Innovation Department of UMP and Malaysian Technological University Network (MTUN). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.02.081. References
4. Conclusions In conclusion, a ternary compound MgCo2O4 offers higher theoretical capacitance than many of the metal cobaltites; an electrode of which fabricated on nickel foam substrate gave superior practical capacitance compared to two similar control materials, viz. MnCo2O4, CuCo2O4 in 3 M LiOH electrolyte. The CV and CD measurements show superior coulombic efficiency for the
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Characterization of MgCo2O4 as an electrode for high performance supercapacitors 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, * a b c
Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Kuantan, Malaysia Department of Materials Science & Engineering, National University of Singapore, 117546 Singapore Department of Physics, National university of Singapore 117542, Singapore
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 December 2014 Received in revised form 23 January 2015 Accepted 9 February 2015 Available online 10 February 2015
Metal cobaltites have promising electrochemical properties for their application as an energy storage medium. In this paper, usefulness of MgCo2O4 as a supercapacitor electrode is demonstrated and compared its performance with two other cobaltites, MnCo2O4 and CuCo2O4. The materials are synthesized using molten salt method and characterized by X-ray diffraction, scanning electron microscopy, BET surface area, cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical impedance spectroscopy techniques. The MgCo2O4 electrodes show superior charge storage properties in 3 M LiOH among a diverse choice of electrolytes. The MgCo2O4 show higher theoretical (3122 F/g) and practically achieved capacitance (320 F/g), larger coulombic efficiency, and cycling stability than the other two; therefore, it could be developed as a low-cost energy storage medium. ã 2015 Elsevier Ltd. All rights reserved.
Key words: electrochemical energy storage pseudocapacitors asymmetric capacitors lithium ion battery
1. Introduction Rapid technological advancement along with the depleting natural resources demand smarter production, usage and storage of energy. Supercapacitors are a class of energy storage devices employing non-faradic charge accumulation process (electric double layer capacitance, EDLC), faradic charge transfer process (pseudocapacitance) or combination of both processes (hybrid capacitance) [1–3]. Carbons (graphene, carbon nanotubes and activated carbon) are the choice to build EDLC (capacitance range 20–50 mF cm2), transition metal oxides (capacitance up to 2000 mF cm2) and conducting polymers show pseudocapacitance (PC), and layered and hybrid materials combine the two storage modes [4]. Owing to the larger capacitance, PC materials with desirable capacitive properties are actively sought. Electrochemical reversibility, availability of an array of oxidation states and higher electrical conductivity are the properties of a material to be selected as a pseudocapacitor electrode. Many transition metal oxides (TMOs) are proposed as candidates for pseudocapacitive electrodes; 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.2015.02.081 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
[5–14]. Among them, compounds of cobalt offer superior performance than other binary metal oxides although they are expensive due to its lower abundance in the earth’s crust (<10 ppm). In recent years, ternary TMOs (TTMOs) are used in electrochemical application because two metals contribute to redox reaction. Furthermore, TTMOs’ structural diversity provide opportunities to modify the physical and chemical properties such that the capacitance can be tailored [1,3,15]. An added advantage of synthesizing cobalt based TTMOs is the reduction in the cost of rare cobalt by partially substituting it with other TMOs [16]. The TTMOs such as ZnCo2O4 [17,18], CuCo2O4 [19] , LiCoO2 [20–22], MnCo2O4 [23,24] are tested as anode materials for lithium ion batteries and supercapacitors. Table 1 shows the summary of a literature survey on the electrode characteristics of transition metal cobaltites for supercapacitor applications. Majority of the activities are centered on MCo2O4 (M=Cu, Zn, Mn and Ni), possibly because of the high theoretical capacitance offered by them. Theoretical capacitance of these materials are calculated from their redox potentials (See Supplementary Information for details of calculation) and compared 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 in the Fig. 1. One would see that the reported materials have slightly lower theoretical capacitance than the parent compound, except MnCo2O4; and NiCo2O4 have practically achieved >90% of it. On the
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Table 1 Comparison 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 refers to solution combustion synthesis and HTM, hydrothermal method. The 2-electrode in the potential range refers to a working supercapacitor made with the target material as one of the electrodes. Material
Cs (Fg1)
Stability
Potential range (V) Ref
CuCo2O4 by SCS LiCoO2 LiCoO2 by HTM MnCo2O4.5 porous urichin like nanostructures by HTM MnCo2O4 nanowires by facile HTM MnCo2O4 spinel by facile sol-gel method MnCo2O4 by electroless-electrolytic synthesis ZnCo2O4 nanotubes by electrospinning ZnCo2O4/CNF by co-precipitation NiCo2O4/Ni submicron particles by sol-gel NiCo2O4 by thermal decomposition
338 (3 M KOH) @1 Ag1 70.17 (1 M LiPF6/EC + DMC) 58 (LiClO4) @ 2 mA cm2 151 (1 M KOH) @ 5 mVs1 349.8 (1 M KOH) @ 1 Ag1 405 (2 M KOH) @ 5 mA cm2 832 (0.5 M NaOH) @ 20 mVs1 770 (6 M KOH) @10 Ag1 77 (6 M KOH) @2 mA cm2 217 (1 M KOH) @ 1 mA cm2 746 (1 M NaOH) @2 mVs1
0.5 3.0 (2- electrode) 1.5 (2- electrode) 0.5 0.45 0.4 1.0 (2- electrode) 0.5 1.2 (2- electrode) 0.45 0.6
[30] [21] [22] [15] [43] [44] [31] [45] [46] [47] [48]
NiCo2O4 flower-like nanostructures HTM
658 (6 M KOH) @1 Ag1
0.55
[49]
NiCo2O4–graphene composite nanowires by HTM NiCo2O4 nanosheets grown on nickel foam by HTM NiCo2O4 microspheres by microwave assisted heating NiCo2O4 nanorods and nanoflakes by chemical bath synthesis NiCo2O4 nano sheets by HTM NiCo2O4 by sol-gel NiCo2O4 multiple heirarchical structures by HTM NiCo2O4 aerogel by sol-gel NiCo2O4@ NiCo2O4 nanorods NiCo2O4 @ NiO nanoflakes NiCo2O4 spinel by HTM
737 (2 M KOH) @1 Ag1 1088 (2 M KOH) @ 5 mA cm2 1006 (6 M KOH) @1 Ag1 490 (nanorods) &330 (nanoflakes) (2 M KOH) @ 2 mA cm2 999 (2 M KOH) @20 Ag1 1254 (2 M KOH) @2 Ag1 2623 (3 M KOH) @1 Ag1 1400 (1 M NaOH) @25 mVs1 1925 (3 M KOH) @0.5 Ag1 2210 (3 M KOH) @0.5 Ag1 1619 (3 M KOH) @ 2 Ag1
96%/5000 74.86%/1000 85%/1000 100%/2100 94%/4000 95%/1000 80%/1000 89.6%/3000 Not reported 96.3%/600 100%/ 10,000 100%/ 10,000 94%/3000 N.R/2000. 93.2%/1000 93%/1000 84.6%/3000 70.4%/1000 94%/3000 100%/2000 85.4%/2000 100%/2000 N.R/1000
0.45 0.55 0.45 0.4 0.4 0.5 0.5 N.R 0.4
[50] [51] [52] [53] [54] [55] [56] [57] [58]
0.4
[59]
other hand, MgCo2O4, an anode material reported for lithium ion battery [23], have superior theoretical capacitance than most of the MCo2O4 (M=Cu, Zn and Ni) (Fig. 1). However, no effort has so far been undertaken to evaluate its electrochemical properties for supercapacitor application. We have evaluated the supercapacitive performance of MgCo2O4 and compared its performance with two similar cobaltites, viz. CuCo2O4 and MnCo2O4. The materials were synthesized by molten salt method (MSM) owing to its potential to synthesize transition metal oxides [26–28] on a large scale. The experimental results reported in this paper show great promise to pursue with MgCo2O4.
2. Experimental Details 2.1. Synthesis and characterization of MCo2O4 MCo2O4 (M=Mg, Mn, Cu) powders were prepared by mixing 1 M MSO4.5H2O (99%, Sigma Aldrich), 2 M CoSO4.7H2O (98%, Fluka) and 0.88 M LiNO3 (99%, Alfa Aesar), 0.12 M LiCl (99%, Merck). The ratio of metal ion to molten salt was 1: 10. For easier synthesis of a crystalline and single phase material, LiNO3 (oxidizing flux) and LiCl (mineralizing agent) were used. The mixture was placed in an alumina crucible and then heated at 280 C (heating rate 3 C min1) for 3 h in air in a box furnace (Carbolyte, UK). After the mixture was slowly cooled down (cooling rate 3 C min1) to room temperature at 25 C, it was washed with distilled water to remove excess Li salts and filtered. Afterwards, the remaining powder was calcined at 70 C. The calcined sample was further heated at 200 C for 2 h in flowing N2 gas to remove the moisture traces remained during washing. Crystal structure and phase of the materials were studied by XRD using Rigaku Miniflex II X-ray diffractometer employing CuKa radiation (l = 1.5406 Å). Gas adsorption behavior and BET surface area of the materials were determined using Micrometrics (Tristar, 3000) instrument in nitrogen atmosphere. Morphology of the samples was analyzed using Scanning Electron Microscopy (SEM; JEOL JSM-67500F). 2.2. Electrochemical studies
Fig. 1. Comparison of the supercapacitive performance of cobalt oxide with other cobaltites.
In a typical procedure, a paste of electrode material was prepared 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 for 24 h. The slurry was coated on ultrasonically cleaned nickel foam substrate. The slurry coated nickel foam was dried in an oven at 60 C for 24 h. The dried electrodes were pressed at a pressure of 5 ton using a hydraulic press. Similarly electrodes of CuCo2O4 and
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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, polycrystallinity and the cubic structure of MCo2O4. The position and intensity distribution of the XRD are similar thereby indicating that the materials are isostructural. Sharp peaks are obtained for (3 11) plane followed by (4 0 0) and (5 11) planes. All the peaks fit well to
Fig. 2. XRD pattern of a) MgCo2O4 b) MnCo2O4 c) CuCo2O4.
MnCo2O4 were prepared. The mass loading was 2 mg and geometrical area of working electrode was 1 cm2. The electrochemical properties of the electrodes were studied by cyclic voltammetry (CV), charge discharge cycling (CDC), and electrochemical impedance spectroscopy (EIS) in three-electrode configuration. 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 M LiOH gave the best performance. A potentiostat galvanostat (PGSTAT M101, Metrohm Autolab B.V; The Netherlands) was used for electrochemical measurements employing Nova 1.9 software. An Ag/AgCl and a platinum rod were used as the reference and counter electrodes, respectively.
the reported cubic spinel structure having space group Fd3m : 2. The lattice parameters of MCo2O4 are a = 8.09, 8.108 and 8.101 Å for the Mg, Mn and Cu analogues, respectively. Small variation in the lattice parameter is due to differences in the ionic radii of Mg, Mn and Cu. The measured BET surface area of the MCo2O4 were 0.45, 18.94 and 9.81 m2 g1 and their average pore diameters, determined using Barett-Joyner-Halenda (BJH) analysis, were 10.8, 15.1 and 9.8 nm for M=Mg, Mn and Cu, respectively. A large difference in BET surface area was observed for the MgCo2O4 sample compared to the other ones, the reason for which was investigated using SEM. The SEM images of the samples are shown in Fig. 3. Aggregated spheroidal particle morphology was observed for the Mg and Cu analogues, whereas the Mn analogue was observed to be flakes. The flake-like structure is assigned to the higher BET surface area of MnCo2O4. Although both MgCo2O4 and CuCo2O4 have similar aggregate shape their sizes are markedly different. The CuCo2O4 has much smaller particle size (100 nm) compared to the MgCo2O4 (700 nm–1 mm), the observed smaller particle size is assigned to the higher BET surface area of the Cu analogue. Although all the materials are synthesized through similar procedure changes in morphology was observed, which could be due to differences in the crystallization behaviors of the respective MCo2O4 systems. Furthermore, MgO is well known for
Fig. 3. SEM images of MCo2O4 (a) Cu, (b) Mn and (c) Mg, Bar scale: 1 mm; and insets are 100 nm.
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its hygroscopic behavior, which would reduce the surface area. Nevertheless, reason for different morphology upon similar synthesis route is beyond the scope of the present article, which is restricted to the evaluation of the electrochemical properties of MgCo2O4 for its usefulness as a supercapacitor electrode.
315
reaction involving Co4+/Co3+ and M2+/M+ with OH ions [30]. The redox peak of the electrode materials can be attributed to the following reactions [3,31–33]. Charging
2MgO þ 4CoOOH 2MgCo2 O4 þ 2H2 O þ e ! Discharging
3.2. Electrochemical properties of MCo2O4 (M=Mg, Mn, Cu) 3.2.1. Cyclic Voltammetry To evaluate the usefulness of MCo2O4 as a supercapacitor electrode, their redox behavior was studied by CV in six electrolytes, viz. 1 M LiOH, 3 M LiOH, 3 M KOH, 6 M KOH, 1 M K2SO4 and 1 M H2SO4. Among them 1 M K2SO4 and 1 M H2SO4 showed no redox peaks; the CV curves were similar to that of nickel foam substrate (See Supplementary Information). Although KOH and LiOH showed redox peaks larger voltammetric currents were observed for 3 M LiOH (See Supplementary Information); therefore, this electrolyte was used for further evaluation of electrochemical properties. Fig. 4a–c show the CV of the MCo2O4 samples at varying scan rates at a potential window of 0.6 V in 3 M LiOH. The CV curves show oxidation and reduction peaks during charging and discharging thereby indicating faradic reaction; the area enclosed by the CV curve indicates the charge stored. The oxidation and reduction peaks shifted to negative and positive potentials with 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
Charging
4CoðOHÞ þ 4ðOHÞ 4CoOOH þ 4H2 O þ 2e ! 2
(1)
Discharging
Charging
MnCo2 O4 þ H2 O þ OH 2CoOOH þ MnOOH þ e ! Discharging
Charging
MnOOH þ OH MnO2 þ H2 O þ e !
(2)
Discharging
Fig. 4. CV of MCo2O4 as a function of scan rate for (a) MgCo2O4; (b) MnCo2O4, and (c) CuCo2O4; (d) Variation of specific capacitance of MCo2O4 as a function of scan rates.
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reaction and allowed maximum utilization of the active material in the electrode.
Charging
Cu2 O þ 4CoOOH 2CuCo2 O4 þ 2H2 O þ e ! Discharging
Charging
4CoðOHÞ þ 4ðOHÞ 4CoOOH þ 4H2 O þ 2e ! 2
(3)
Discharging
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 of 2 mVs1 are 93, 91 and 85%, respectively, thereby showing improved electrochemical reversibility for the MgCo2O4 electrode. The h of MgCo2O4 increased up to 97% for a scan rate of 100 mVs1 (See Supplementary Information), which would result from the difference in ion movement at different scan rates. Superior electrochemical reversibility would provide capacity retention of the supercapacitor for long cycle of operation. The specific capacitance (CS) of the electrodes were calculated from the CV curves using the equation 1 Cs ¼ mvðE2 E1 Þ
ZE2 iðEÞdE
(4)
E1
where m is the mass of the active material, n is the scan rate, E2 E1 gives the potential window, and i(E) is the current at each potential. The CS of the MgCo2O4, MnCo2O4 and CuCo2O4 electrodes at a scan rate of 2 mVs1 (shown as an inset in corresponding CV curves) were calculated to be 362, 359 and 210 Fg1, respectively. Fig. 4(d) shows the variation of CS with scan rate. The MgCo2O4 and MnCo2O4 electrodes showed similar CS despite much larger difference in their BET surface areas (0.5 and 19 m2 g1, repectively). The CS decreases with increase in scan rate indicating that ion diffusion is limited at the surface at higher scan rate thereby dominating EDLC over PC. The MnCo2O4 showed the highest value at this region (>30 mVs1) owing to its larger BET surface area. At higher scan rates (>30 mVs1) the CS for electrodes were practically constant owing to the limited movement of ions only to the surface of the electrode material. On the other hand, at lower scan rates (<10 mVs1), 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 and the solid line is a linear fit.
In pseudocapacitive materials, the dependence of scan rate (n) with voltammetric current (i) depends on whether the capacitance originates from surface redox reactions or bulk diffusion. For surface redox reaction, i / n and for semi-infinite bulk diffusion, i pffiffiffi [34]. A straight line for i / v (Fig. 5) is observed; therefore, bulk diffusion occurred during the electrochemical reaction. This relationship further indicates that the diffusion of OH is likely to control the redox reaction occurring in the electrochemical process. The apparent diffusion coefficient (D) of OH ion at 25 C is calculated in all the materials employing Randles–Sevcik equation [35] pffiffiffiffi pffiffiffi ip ¼ 2:69 105 n3=2 A D C0 n (5) where ip is the peak current, n is the number of electrons involved in the reaction, A is the surface area of the electrode, D is the diffusion coefficient of the electrode material, C0 is the proton concentration and n is the scanning rate. The D was calculated from pffiffiffi the slope of the ip vs v curve as explained elsewhere [36,37]. The D of MgCo2O4, MnCo2O4 and CuCo2O4 were 2.5 1013, 3.3 1014, 6.25 1014 cm2s1, respectively. The details of calculation are in the supplementary information. Obviously, MgCo2O4 supports an order of magnitude higher D and is expected to be the source of its higher capcitance. A higher D accelerates the ion transport and slows down the electrode polarization during the charge– discharge process [38]. Therefore, MgCo2O4 posses a faster electrode reaction due to quicker ionic transportation owing to its higher ‘D’ than other electrodes. 3.2.2. Galvanostatic charge discharge studies To quantify the CS and the rate capability of the samples, galvanostatic charge-discharge (CD) measurements were performed. Fig. 6compares CD curves of the electrodes at a current density of 0.5 Ag1 in 3 M LiOH with potential window of 0.5 V. Asymmetric shape of the CD curves indicate the faradic behavior of the material. Most of the capacitance is generated in the range 0.35 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 discharge curves arise due to incomplete faradic reaction and internal resistance in the electrode. Using the ratio of initial potential drop (VIR) to the corresponding discharge current (ID), the internal resistance of the electrode could be calculated. The internal resistances 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 current density of 0.5 Ag1.
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317
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 Ag1.
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 be due to improved electrical conductivity and the CS of the electrode material thereby. Figs. 7a–c show the discharge curves of the electrodes as a function 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 decay and (iii) a faster potential drop corresponding to EDLC. The CS from the discharge curves can be calculated using the equation Cs ¼
It m Dv
(6)
where I,t, m and DV are applied current, discharge time, active mass and potential difference, respectively. Fig. 8 represents the variation of CS as a function of specific current density for MCo2O4. The CS showed an exponential decay with current density for all the electrodes as observed from the CV analysis. However, CS determined from the CD curves showed clearly superior values for the MgCo2O4, which was only marginal in CV experiments. The CS of MgCo2O4, MnCo2O4, CuCo2O4 from the discharge curve was 321, 225 and 133 Fg1, respectively at a current density of 0.5 Ag1. While looking at the BHJ analyses (Section 3.1), improved pore size and pore volume of MgCo2O4 is expected to contribute to its higher capacitance.
Cycling stability under extreme load of the electrodes is crucial for practical applications of supercapacitors. Therefore, galvanostatic CD measurements at varying current densities (2 Ag1, 5 Ag1, 10 Ag1) for 2000 cycles, which was equally divided into four quarters of 500 cycles, were conducted (Fig. 9). The last quarter (1500–2000) repeated the capacitance retention at 2 Ag1 after cycling through 5 and 10 Ag1. Initial decrease of capacitance for the first 500 cycles was observed for all the three electrodes; the capacitance showed a small increment after 1000 cycle showing the structural activation and pore opening in the electrodes [39]. At the end of the 2000 cycle, MgCo2O4 and CuCo2O4 exhibited improved capacitance than at 500th cycle while MnCo2O4 showed only 80% retention. The details of the cycling stability for the three samples are compiled in the Table 2. The ratio of discharging (TD) to charging time (TC) from charge discharge cycle gives the h of the device following the relation
h¼
TD 100 Tc
(7)
The h of the materials remained constant for the tested 2000 cycles at 93, 91 and 85% for MgCo2O4, MnCo2O4 and CuCo2O4, respectively. The h determined from the CD measurements are consistent with that evaluated from the CV analysis.
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Fig. 8. Plot showing the variation of capacitance with current density determined from the CD curves. The open squares, filled circles and the arrow marks show the experimental data and the solid line is a trend line depicting an exponential decay function.
3.2.3. Electrochemical Impedance Spectroscopy Studies In order to further evaluate the ion transport kinetics and electrode conductivity, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range
0.1 MHz to 2 mHz with an ac perturbation of 10 mV. Figs. 10a–c show the Nyquist plot for all samples in 3 M LiOH at open circuit potential. The EIS spectrum of a supercapacitor electrode is usually divided into three segments following three processes; (i) the bulk resistance of the device (RS), synonymously called equivalent series resistance (ESR) at high frequency (>1 kHz); (ii) capacitive effects at intermediate frequencies (<1 kHz); and (iii) Warburg impedance resulting from the frequency dependence of ion diffusion/ transport in the electrode - electrolyte interface at the low frequencies (<5 Hz). The high frequency offset at the real part (RS) is a combination of the electrolytic resistance, contact resistance at the interface between the current collector and the active material and the intrinsic resistance of the active material. The value of RS measured from the high frequency offset for sample MgCo2O4 is much lower than that of the other two sample (see Table 3) which demonstrates the superior electrical conductivity of the sample. The EIS spectrum presented a small semicircle in the high to medium frequency region. The diameter of the semicircle is a measure of the kinetic resistance to ion transfer at the solid oxide/ liquid electrolyte interface, known as the charge transfer resistance RCT. The low RCT measured for the sample MgCo2O4 implies lower resistance to ion movement through the pores of the material which leads to higher utilization of the active material and subsequently higher capacitance. At intermediate frequencies, the
Fig. 9. The variation of specific capacitance with cycle number with variable current density. (a) MgCo2O4 (b) MnCo2O4 (c) CuCo2O4. 2000 cycles was equally divided in to four, where first three cycles carried current densities of 2 Ag1, 5 Ag1, 10 Ag1 and the final one repeated for 2 Ag1.
S.G. Krishnan et al. / Electrochimica Acta 161 (2015) 312–321 Table 2 Comparison of capacitance retention of MCo2O4 at a current density of 2 Ag1. The capacitance were measured at variable current densities and the table indicates the capacitance at the end of 500 and 2000 cycle where the measurement was made at 2 Ag1.
Current density (Ag1) Number of cycles CS (Fg1)
MgCo2O4
MnCo2O4
CuCo2O4
2 500 160
2 500 288
2 500 152
2000 344
2000 240
2000 176
Nyquist plot of electrode/electrolyte system shows a straight line; the angle of the line with respect to the real axis determines the origin of capacitance. The capacitance is EDLC if the angle is 90 and a deviation from this value indicates capacitance from ion diffusion. The angle of the EIS curve with respect to its real axis of electrodes varies from 80 to 60 . The slope of MgCo2O4 is 60 which indicate that the total capacitance arise from ion diffusion and charge accumulation. The diffusive contribution from the other electrodes were relatively lower. The observed EIS spectrum is fitted using Nova 1.9 software to an equivalent circuit proposed for supercapacitors (Fig. 10(d)); the fitted parameters obtained are summarized in Table 3. The fitted RS and RCT values of the samples are in good agreement with that directly calculated from the Nyquist plot (Table 3). The lower
319
Warburg impedance of MgCo2O4 indicate that the sample offer less resistance to the diffusion of ions and better charge transport behaviour, which could be assigned to the observed lower RS and lower resistance of the electrode. The redox process occur due to the diffusion of ions through the active electrode material leads to the appearance of constant phase element (CPE) in the equivalent circuit. The CPE impedance (ZCPE) is given by [4] ZCPE ¼
1 n BðjvÞ
(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, pure resistor. 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 the electrodes are more capacitive in nature. Finally, we correlate the results of CD that MgCo2O4 have superior power capability compared to the other electrodes using EIS measurements. As mentioned, the second segment of the frequency region (<1000 Hz) of EIS curve represents the electrically charged electrode-electrolyte interface generating the supercapacitive behavior of the device. This portion of EIS is linear representing time-dependent process of diffusion and accumulation of charges at the accessible suface. The charge relaxation time, which is a measure of power density, of the above process can be
Fig. 10. Nyquist plot for (a) MgCo2O4 (b) MnCo2O4 (c) CuCo2O4 (d) equivalent circuit at open circuit potential. The solid circle indicates the experimental value and the continuous line is the fitted data. The inset shows expanded high frequency region and 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 3 Summary of transport parameters form the fitted circuit and determined directly from the spectrum. RS (V)
Material
MgCo2O4 MnCo2O4 CuCo2O4
RCT (V)
From intercept
Fit
From diameter
Fit
0.71 1.32 0.88
0.72 1.35 0.89
0.13 0.29 0.17
0.14 0.38 0.18
determined by expressing the total capacitance as a combination as of real (C0 ) and imaginary (C00 ) parts [40,41] as. 0
CðvÞ ¼ C ðvÞþ C} ðvÞ 0
where C ðvÞ ¼
(9)
0
Z ðvÞ and C}ðvÞ ¼ Z ðvÞ 2 . Fig 11(a) shows the vjZðvÞj2 vjZðvÞj }
variation of calculated C0 (v) of the electrodes with frequency. The C0 of the electrodes determined from EIS is consistent with CV measurements. A small variations observed in the Fig. 11(a) than that determined from the CV curves could be attributed to factors such as chemical and physical heterogenity and the deeply trapped immobile ions during EIS measurement [42]. For all the three electrodes, the C00 showed a bell-shaped curves Fig 11(b). The t could be calculated from the C00 vs. frequency curve employing the relation t ¼ f1 [40] by measuring the peak 0
frequency (fo). The fo determined from the graph were 2.2, 1.2 and 2.8 Hz for MgCo2O4, MnCo2O4, CuCo2O4, respectively and their corresponding t were 0.46, 0.83 and 0.35 s. i.e., the power capability of MgCo2O4 is superior to the Mn analogue but inferior to the Cu analogue. High power capability of the CuCo2O4 could result from the improved electrical conductivity of Cu compared to the other elements. Nevertheless, owing to the superior achieved capacitance and comparable power capability of MgCo2O4, the material offers large potential to be developed as a practical supercapacitor electrode.
Cdl (mF)
ZCPE (mFs)1/n
W (mMho)
1.11 1.54 1.62
45.9 (n = 0.91) 72.6 (n = 0.91) 57 (n = 0.94)
276 469 818
MgCo2O4 (93%) electrode compared to the other electrodes. The CV measurements show that the MgCo2O4 electrodes also offers higher OH ion diffusivity in it than the others. Owing to these factors, the MgCo2O4 electrodes gave the highest CS despite the lowest BET surface area measured for this material. The results were validated using electrochemical impedance spectroscopy and galvanostatic cycling techniques. The studies show that MgCo2O4 is characterized by low equivalent series resistance and internal resistance; furthermore, MgCo2O4 has lower relaxation time for improved power capability. The results demonstrated in this paper shows huge promise for MgCo2O4 to be developed as high performing supercapacitive energy storage device. Acknowlegements This work is supported by Research and Innovation Department of UMP and Malaysian Technological University Network (MTUN). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.02.081. References
4. Conclusions In conclusion, a ternary compound MgCo2O4 offers higher theoretical capacitance than many of the metal cobaltites; an electrode of which fabricated on nickel foam substrate gave superior practical capacitance compared to two similar control materials, viz. MnCo2O4, CuCo2O4 in 3 M LiOH electrolyte. The CV and CD measurements show superior coulombic efficiency for the
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