(M=Mg, Mn) with Co ions, chemical stability and

Electrochimica Acta 243 (2017) 119–128

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Critical influence of reduced graphene oxide mediated binding of M (M = Mg, Mn) with Co ions, chemical stability and charge storability enhancements of spinal-type hierarchical MCo2O4 nanostructures Syam G. Krishnana , Midhun Harilala , Asfand Yarb , Bincy Lathakumary Vijayana , John Ojur Dennisb , Mashitah Mohd. Yusoffa , Rajan Josea,* a b

Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Kuantan, Malaysia Department of Fundamental and Applied Sciences, Universiti Teknologi Petronas, Bandar Seri Iskander, Perak, Malaysia

A R T I C L E I N F O

Article history: Received 22 March 2017 Received in revised form 29 April 2017 Accepted 10 May 2017 Available online 11 May 2017 Keywords: Renewable Energy Asymmetric supercapacitors Batteries Pseudocapacitors

A B S T R A C T

This paper reports that addition of reduced graphene oxide (rGO) in MgCo2O4 improves the binding of Mg with Co thereby minimizing magnesium dissolution in aqueous alkaline electrolytes and the resulting MgCo2O4/rGO electrodes offered impressive improvements in charge storage properties. An isostructural high performing material, MnCo2O4, is used as a benchmark material in this work. The Mg analogues stored >30% more charges than the Mn-analogues in the 3 M LiOH electrolyte despite the former's lower BET surface area; rGO modification further increased charge storage by >60% than the unmodified analogues. Electrochemical measurements show that a larger surface fraction of the Mg analogue is electrochemically active, irrespective of whether or not rGO is present, which arise from, typically for MgCo2O4/rGO, lower internal resistance, lower Warburg impedance, and lower charge transfer resistance than the other electrodes. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Three parameters are often used to rate portable charge (energy) storage devices: (i) the amount of storable charge per unit area or mass of the electrode– known as energy density (ED); (ii) the rate at which charge can be stored and discharged – known as power density (PD); and (iii) the number of cycles through which the device can be recharged without much loss in ED– known as the cycling stability. It is highly desirable for these charge storage devices to have a smaller charging time, i.e., higher PD and ED and longer cycling stability. Lithium ion batteries, which are the current commercial choice, have higher ED (100 – 200 Wh kg1) but lower PD ( 20 nm, Table 2) are peaks in MgCo2O4 and MgCo2O4/rGO could be attributed to the lower than the solvated size of the Li+ ion (38.2 nm) [43]; therefore, faradic reaction involving Co4+/Co3+ and Mg2+ with OH ions as in difference in Q is not arising from the difference in pore size. reaction (1) To address this discrepancy, the quantity of electrochemically For MnCo2O4 electrode, the cathodic/anodic peaks can be active sites in the electrodes accessed by the solvated ions (a) in the attributed to the following electrochemical reaction [40] electrolyte, which varies with scan rates, was determined by modifying the reactions in (1) and (2) as Charging MnOOH þ OH 2CoOOH þ MnOOH þ e @Disharging MnCo2 O4 þ H2 O þ OH MnO2 þ H2 O þ e @Charging Discharging MnCo2 O4 þ 3aOH @oxidation reduction aMnOOH þ 2aCoOOH ð2Þ þ ð1 aÞMnCo2 O4 þ ae ð4Þ Generally, a cathodic peak (c1) arises in an electrochemical reaction due to the reduction process of the electrode material with solvated electrolyte ion. The cathodic peaks in MnCo2O4 MgCo2 O4 þ 3aOH @oxidation reduction aMgOH þ 2aCoOOH electrodes can be explained from the reaction (2) based on the þ ð1 aÞMgCo2 O4 þ ae ð5Þ MnCo2O4/MnOOH, MnO2/MnOOH redox pairs in 3 M LiOH. From and ‘a’ can be calculated using the equation, equation (1), the cathodic peak arising from MnCo2O4 and MnCo2O4/rGO can be attributed to the faradic reaction involving Q m DV ð6Þ a¼ Co2+/Co3+ and Mn2+/Mn3+ [41]. FZ The ratio of anodic area to the cathodic area from CV curves where m is the molecular weight, DV is the redox potential, F is the measures the coulombic efficiency (h) of the electrodes, further Faraday's constant and Z is the oxidation state of the electrode measuring the electrochemical reversibility of all the electrodes. material. The ‘a’ obtained for MgCo2O4, MnCo2O4, MgCo2O4/rGO MgCo2O4/rGO electrode possessed higher h (95%), followed by and MnCo2O4/rGO at 2 mV s1 were 11, 6, 17 and 13%, MnCo2O4/rGO (94%), MgCo2O4 (91%) and MnCo2O4 (86%). The respectively. i.e., despite the lower surface area Mg analogues have superior h of MgCo2O4/rGO suggests its improved electrochemical C 1s

Fig. 5. A cartoon showing the role of rGO in stabilizing the surface Mg atom.

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S.G. Krishnan et al. / Electrochimica Acta 243 (2017) 119–128

Fig. 6. Nitrogen adsorption de-adsorption isotherm of (a) MgCo2O4 (b) MnCo2O4 (c) MgCo2O4/rGO and (d) MnCo2O4/rGO. Inset of each isotherm is the BJH isotherms of corresponding samples.

Table 2 Surface properties of the electrodes determined from the gas adsorption techniques. Material

Surface area (m2 g1)

Total pore volume (cm3 g1)

Average pore diameter (nm)

Micropore area (m2 g1)

MgCo2O4 MnCo2O4 MgCo2O4/rGO MnCo2O4/rGO

50 60 55 62

0.43 0.34 0.40 0.30

30 24.5 28 20.4

11.3 9.6 6.3 7.2

a large fraction of electrochemically active sites which in turn increased when the electrodes are modified with rGO. Note that the Q obtained by MgCo2O4, MnCo2O4, MgCo2O4/rGO and MnCo2O4/rGO electrodes are 12%, 7%, 19% and 13% of its theoretical capacity suggesting a larger utilization of surface area of MgCo2O4/rGO electrode thereby resulting in superior Q. The variation of ‘a’ with scan rate is compiled in supplementary information S3. The ‘a’ decreased with increase in scan rate for all electrodes thereby validating the decrease in Q with increase in scan rate. 3.2.2. Galvanostatic charge-discharge cycling Galvanostatic charge-discharge cycling (GCD) studies were performed to further compare the practical Q, internal resistance and long term cyclability of the electrodes. The potential window for GCD measurements was kept at 0.5 V for all the electrodes. The GCD measurements of the electrodes at 2.5 mA cm2 were shown in Fig. 9a; the discharge curves as a function of current density is shown in the Figure S4 of the Supplementary Information. The shape of GCD curves for all the electrodes were asymmetric consisting three voltage plateau for MgCo2O4/rGO and MnCo2O4/ rGO while two voltage plateau for MgCo2O4 and MnCo2O4. These

plateau in the range of 0 to 0.55 V uphold the redox peaks and battery type charge storage property as determined in CV measurements. The potential drop between the charging and discharging events at the maximum potential point refers to the total electrical resistance of the electrodes, the potential drop for the electrodes were shown in Table 3. Table 3 shows that the rGO modification, owing to the high conductivity of rGO, reduced the potential drop thereby increasing their electrical conductivity. The internal resistance of the electrodes, determined from the ratio of potential drop (DV) to change in discharge current (DI), of MgCo2O4/rGO is lower (0.26 V) followed by MnCo2O4 (0.30 V), MgCo2O4(0.57 V) and MnCo2O4/rGO (0.71 V); i.e., the Mg-analogues consistently showed lower potential drop and internal resistance thereby corroborating the CV results. The practically achievable Q of the electrodes was calculated from the discharge curves using the equation Q¼

It m

ð8Þ

where I, t and m, were applied current, discharge time and active mass, respectively [42]. The Q of MgCo2O4/rGO was determined to be 158 mA h g1 (570 F g1) which was superior to MnCo2O4/


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Fig. 7. FESEM micrographs of (a) MgCo2O4 (b) MnCo2O4 (c) MgCo2O4/rGO and (d) MnCo2O4/rGO. All the micrographs are with a resolution of 1 mm and the corresponding insets are with resolution of 100 nm.

rGO (122 mA h g1; 440 F g1) at 2.5 mA cm2. The Q of MgCo2O4 and MnCo2O4 was 98.6 (355 F g1) and 63 mA h g1 (227 F g1), respectively at 2.5 mA cm2. Fig. 9b shows the variation of Q as a function of current density of all the four electrodes; the results show that the increase in electrical conductivity through rGO modification is reflected only in increasing the charge storability not the rate capability. The rate capabilities slightly lowered after rGO modification. This is surprising because higher electrode conductivities are expected to improve the rate capability also. A possible cause of this effect is the lowering of the average pore diameter upon rGO modification, which reduces the number of pores accessible to the solvated ions. The total Q is contributed by battery type charge storage and EDLC; the range of voltage (onset voltage) of battery type charge storage for MgCo2O4 and MnCo2O4 were 0.3 – 0.5 V. The battery type charge storage onset voltage increased to 0.2 – 0.5 V for both MgCo2O4/rGO and MnCo2O4/rGO electrodes. Therefore, the contribution of battery type charge storage in the total capacitance of MgCo2O4/rGO was increased to 88% than the 75% of MgCo2O4 electrode. Similarly, the battery type charge storage of MnCo2O4 was determined to be 73% and that of MnCo2O4/rGO to be 84%. This signifies that the

Fig. 8. (a) Comparison of CV curves of all the electrodes, inset is the variation of Q with different scan rates (b) Variation of anodic peak current from CV with the square root of scan rate.

addition of rGO increased the electrical conductivity thereby increasing the charge-transfer process. The superior Q of MgCo2O4/

Fig. 9. (a) Comparison of GCD curves of all the electrodes at 1 A g1, (b) comparison of discharge profile of all the electrodes with increasing current densities.

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Table 3 Potential drop of all the electrodes at different current densities. Current density (mA cm2)

Potential drop (mV) MgCo2O4

MgCo2O4/rGO

MnCo2O4

MnCo2O4/rGO

2.5 5 12.5 25 37.5

2.6 9 23 45.5 66.1

1.1 8.8 11.1 18.2 36

3.4 10 27.5 55 82

1.9 11 26.4 51.8 76.9

rGO was owing to the increased onset voltage, improved active surface area as well as its active pore volume. The electrodes with high cycle life are preferred for commercial supercapacitors for long term use. For the practical applicability of these electrodes, cyclic stability under extreme load were performed (Fig. 10) at 25 mA cm2. The Q of MgCo2O4 (Fig. 10a) remained at 100% (78 mA h g1) for 500 cycles. However, a monotonous increase in charge up to 114% of its initial value has been observed upon further cycling until 2100 cycles, which could be attributed to (i) the structural activation and pore opening of the electrode [44] and (ii) dissolution of Mg atoms in the electrolyte and exposure of more electrochemically active Co atoms (as will be shown subsequently). Finally, the electrodes retained 104% of its initial value at the end of 5000 cycles. The advantage of rGO modification on MgCo2O4 can be observed on the stable cycling curves of MgCo2O4/rGO electrodes, which supports the second assignment that the dissolution of Mg atoms are responsible for the inferior stability of the pure MgCo2O4 electrode. The Q of MgCo2O4/rGO electrodes initially increased from 100% (131 mA h g1) to 104% at the end of 700 cycles owing to structural activation of electrode pores [44]. One could observe that this specific charge was retained until the end of 5000 cycles thereby proving the improvement in the stability of MgCo2O4 electrode by rGO modification. This stability improvement can be attributed to the improved binding of Mg ions by rGO modification as shown in schematics of Fig. 5. Fig. 10 (b) show the improvement in the cycling stability of MnCo2O4 electrode with rGO addition. The Q of MnCo2O4 dropped from 100% (56 mA h g1) to 94% at the end of 700 cycles. The Q further increased to 100% at the end of 1200 cycles and again decreased to 97% at the end of 1600 cycles. On further cycling, the specific charge increased to 106% of its initial value. The fluctuations in specific charge storage of MnCo2O4 electrode is clearly observed (Fig. 10b), and finally the decreased to 95% of its initial value at the end of 5000 cycle. The decrease in Q of MnCo2O4 is similar to observed for previous reports on supercapacitors and lithium ion batteries [11,14] and could be attributed to the dissolution of Mn atoms in the aqueous electrolyte as shown subsequently in this article. Improvement

in the specific charge from the initial 100% (100 mA h g1) to 103% of its initial value at the end of 5000 cycles was observed for MgCo2O4/rGO electrodes. Furthermore, the stability of MnCo2O4/ rGO electrode was improved as its retained 103% of initial capacitance from 1400 cycle to 5000 cycle. The reason for its improved stability was analyzed using ICPMS studies. The electrolyte (3 M LiOH) after stability test is analyzed to determine the presence of any metal ions owing to the dissolution of electrode material (Supplementary information, S5). It was determined that 52 ppm Mg ions and 2 ppb Co ions were present in LiOH solution for MgCo2O4 electrode suggesting the dissolving of Mg ions in the electrolyte. Similarly, for MnCo2O4 electrode, the Mn content in electrolyte after 3000 cycles were 28 ppb and Co content were 2 ppb indicating the dissolution of metal ions during long term cycling. There was almost no Mg content determined in the electrolyte for MgCo2O4/rGO electrode indicating proper binding of electrode material owing to the rGO modification (within the limitations of the ICPMS measurements). Also, the dissolution of Mn was decreased to 12 ppb after 5000 cycles for MnCo2O4/rGO electrodes thereby proving the relevance of rGO modification. This lowering of dissolution could be attributed to the improved binding of M ions with Co ions as observed from the XPS spectra. Furthermore, the chemical stability achieved by rGO modification could be attributed to the consistent Q after 5000 cycles for both electrodes. The h can also be determined from the GCD curves from the ratio of charging to discharging times, which was 94, 92, 96 and 95%, at the end of 5000 cycles for MgCo2O4, MnCo2O4, MnCo2O4/rGO, MgCo2O4/ rGO electrodes, respectively. Although all electrodes demonstrated practically acceptable cycling performance improved Q retention and higher h of MgCo2O4/rGO proved its superiority over the other electrodes. 3.2.3. Electrochemical Impedance Spectroscopy (EIS) The charge kinetics parameters at the electrode – electrolyte interface were determined by fitting the experimental EIS spectra of the electrodes with a standard Randles circuit (Fig. 11) using NOVA software. The equivalent circuit consists of a series and

Fig. 10. GCD cycling stability curves for (a) MgCo2O4 (b) MnCo2O4 (c) MgCo2O4/rGO and (d) MnCo2O4/rGO electrodes for 3000 cycles.


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(214), and lower charge transfer resistance (0.36) than the other electrodes.

Acknowledgements This work has been supported by Research and Innovation department of UMP (http://ump.edu.my) through RDU 1503100 and GRS 150342; and Universiti Teknologi PETRONAS under the YUTP-FRG grant (Cost Centre 0153AA-E43). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j. plantsci.2004.08.011(doi:10.1016/j.plantsci.2004.08.011). Fig. 11. Comparison of Nyquist plot of all the electrodes; inset show the lower frequency plot. Circuit fitting for all the electrodes are shown in bottom inset.

parallel combination of total electrode resistance (Rs), charge transfer resistance at the electrode – electrolyte interface (RCT), electric double layer capacitance (Cd), Warburg impedance (ZW) and a constant phase element (CPE) representing the charge dispersion owing to the surface irregularities. The fitted parameters are in Table 4. For ideal charge storage behavior, the electrode should have lower ZW which offer less resistance to the diffusion of ions which in turn lower RS and RCT values of electrodes, which can be observed upon rGO modification. This significant reduction in the above parameters upon rGO modification accounts improvements in charge storage in respective electrodes. The CPE index impedance (n) that determines whether or not the electrode is resistive or capacitive was 0.9; the n = 1 for purely capacitive and n = 0 for purely resistive electrodes. 4. Conclusions In conclusion, we have shown that rGO addition in MCo2O4 (M = Mg, Mn; (MCo2O4/rGO) improves the binding of M ions with Co which has profound influence on the chemical stability of MgCo2O4. This bonding lowered the Mg dissolution in aqueous alkaline electrolytes thereby significantly improving the electrochemical cycling stability. Furthermore, the rGO lowered the total resistance of the electrodes which was beneficial in improving the electrochemically active fraction in the electrode which in turn increased the charge storage capability of the electrodes. The MgCo2O4 showed a discharge capacity of 98.6 mA h g1 (355 F g1) at a current density of 2.5 mA cm2 which is >30% higher than the MnCo2O4 (63 mA h g1; 227 F g1) at similar conditions in the 3 M LiOH electrolyte. The rGO modification further increased charge storage capacity by >60% than the unmodified analogues; the best discharge capacity is (158 mA h g1; 570 F g1) for the MgCo2O4/rGO electrodes. Electrochemical measurements show that 5% of MgCo2O4/rGO electrode surface is more electrochemically active than the MgCo2O electrode. The improved charge storage of the MgCo2O4/rGO has been attributed to its lower internal resistance (0.80 V), lower Warburg impedance Table 4 Characteristic resistances measured from Nyquist plot. Electrode

RS (V)

RCT (V)

Cd (mF)

ZCPE (mFs)1/n

n

W (mMho)

MgCo2O4 MgCo2O4/rGO MnCo2O4 MnCo2O4/rGO

1.15 0.80 1.23 0.88

0.36 0.26 0.50 0.33

1.42 1.04 1.64 1.32

59.2 38.3 73.4 50.4

0.91 0.94 0.90 0.93

272 214 389 262

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