Microwave Assisted Synthesis of Porous NiCo2O4 Microspheres: Application as High Performance Asymmetric and Symmetric Supercapacitors with Large Areal Capacitance

Large areal capacitance is essentially required to integrate the energy storage devices at the microscale electronic appliances. Energy storage devices based on metal oxides are mostly fabricated with low mass loading per unit area which demonstrated low areal capacitance. It is still a challenge to fabricate supercapacitor devices of porous metal oxides with large areal capacitance. Herein we report microwave method followed by a pyrolysis of the as-prepared precursor is used to synthesize porous nickel cobaltite microspheres. Porous NiCo2O4 microspheres are capable to deliver large areal capacitance due to their high specific surface area and small crystallite size. The facile strategy is successfully demonstrated to fabricate aqueous-based asymmetric & symmetric supercapacitor devices of porous NiCo2O4 microspheres with high mass loading of electroactive materials. The asymmetric & symmetric devices exhibit maximum areal capacitance and energy density of 380 mF cm−2 & 19.1 Wh Kg−1 and 194 mF cm−2 & 4.5 Wh Kg−1 (based on total mass loading of 6.25 & 6.0 mg) respectively at current density of 1 mA cm−2. The successful fabrication of symmetric device also indicates that NiCo2O4 can also be used as the negative electrode material for futuristic asymmetric devices.


Calculation of areal capacitance, specific capacitance, full cell capacitance, Coulombic efficiency, energy density and power density
It is important to understand the relation between specific capacitance and cell capacitance. Because in few reported published papers they have used directly the specific capacitance C s instead of using full cell capacitance C, which gives high values of energy densities. So for better understanding we have elaborated the whole calculation procedure in detail. The specific capacitance is the capacitance per unit mass of one electrode as shown in equation (S1). 1,2 (S1) Where C is the measured full cell capacitance of two electrode cell and M is the total mass of both positive and negative electrode. The multiplier 4 adjust the capacitance of cell and the combined mass of two electrode to the capacitance and mass of one electrode. 1,2 1. Three electrode configuration: The specific capacitance was calculated from CV curves using equation (S2) . 3 (S2) where , f(mV s -1 ), and m(g -1 ) are the area under the curve of CV loop, scan rate, potential window and mass of active material in the working electrode respectively.

Two electrode asymmetric cell configuration
The areal and specific capacitances were calculated from CV and CP curves using (S3) where , f(mV s -1 ), , A(cm 2 ) and (g -1 ) are the area under the curve of CV loop, scan rate, potential window, area of electrode, total mass loading of both electrodes respectively.
where I(mA), , A(cm 2 ), and g -1 ) are the constant discharge current, discharge time, area of electrode, discharging voltage after IR drop, mass loading of positive and negative electrode respectively.

Two electrode symmetric cell configuration
The areal and specific capacitances were calculated from CV and CP curves using equation (S 7 & S8) and (S9 & S10) respectively. 3,6 (S7) where , f(mV s -1 ), , A(cm 2 ) and m(g -1 ) are the area under the curve of CV loop, scan rate, potential window, area of electrode and mass of active material on one electrode respectively. The mass loading on one electrode where I(mA), , A(cm 2 ), and m(g -1 ) are the constant discharge current, discharge time after IR drop, area of electrode, discharging voltage after IR drop and mass of active material on one electrode respectively. The multiplying factor of 2 was used in calculating the specific capacitance because the series capacitance was formed in two electrode symmetric devices. 7,8

Calculation of Coulombic efficiency, energy density and maximum power density
Coulombic efficiency, energy density ( and maximum power density ( of both devices were calculated from the CP curves according to equation (S11, S12 & S13) respectively. 2,5,8 (S11) where and discharge time after IR drop and charge time respectively.
By using equation (S1), the full cell capacitance per unit total mass is given by where U (V) is the maximum voltage attained during charge, is the internal resistance which is determined from the voltage drop at the beginning of each discharge, while the represents the voltage drop during discharge cycle which corresponds to IR drop, I(mA) is the constant discharge current and M(Kg) is the total mass of both positive and negative electrode. Figure S1. XRD pattern of as-synthesized precursor All observed peaks can be well indexed to Co(OH) 2 and 3Ni(OH) 2 .2H 2 O according to JCPDS card no.02-0925 and 022-0444 respectively. Figure S2.TGA curve of as-prepared precursor TGA of as-prepared precursor was carried out from room temperature to 600°C in a flowing nitrogen environment as shown in Figure S2. It is evident from Figure S2 that the precursor undergoes multisep weight loss in the temperature range from RT to 300°C. In the first step, weight loss (1.25%) below 200°C can be associated with the removal of adsorbed water and intercalated water molecule. 9 The major weight loss (22.64%) which starts at 245°C and finishes at 300°C can be ascribed to the decomposition and dehydroxylation of hydroxide of nickel and cobalt to form a new stable phase. 10 At temperature greater than 300°C, there is no significant weight loss which is an indication of formation of thermally stable phase.   microspheres as shown in Figure S5. It is evident from the survey spectrum that porous NiCo 2 O 4 microspheres mainly consists of nickel, cobalt, oxygen, and carbon (as reference) species as shown in Figure S5 (a), with no any other species are detected, which is in accordance with EDS analysis. The Ni 2p spectrum is best fitted using Gaussian's method which shows the presence of two spin-orbit doublets characteristic of Ni 2+ and Ni 3+ ,and two shakeup satellites (marked as satellite) as shown in Figure S5 (b oxidation states of cobalt respectively as shown in Figure S5(c). 14-16 Figure S5  In Figure S6 (a), the isotherm belongs to Langmuir type IV characteristics and exhibits an obvious hysteresis loop in the range 0.7-1.0 P/P0 which indicates the presence of mesoporous structure. 19,20 The specific surface area calculated from N 2 adsorption-desorption isotherm using Brunauer-Emmett-Teller (BET) method is 119.68 m 2 g -1 . The high specific surface area will increase the contact area at electrolyte/electrode interface which will provide abundant active sites for Faradaic reaction during electrochemical reaction. The pore volume of assynthesized material is 0.46647 cm 3 g -1 as shown in Figure S6 (b). The large pore volume can serve as a reservoir for ions and also greatly enhance the diffusion kinetics within the electrode material. Figure S6 Figure S6 (c). The narrow and ordered distribution of pores which centres at 8.7 nm corresponds to optimum pore size for excellent electrochemical application. 21,22 The mesoporous porosity as indicated by the pore size distribution will provide the fast diffusion of ions and electron to the electrode material. Thus as-prepared porous microspheres having high specific surface area, large pore volume and narrow pore distribution could be the excellent electrode material for energy storage device.