Electrochemical performance of lanthanum cerium ferrite nanoparticles for supercapacitor applications

Lanthanum cerium ferrite nanoparticles has been synthesized for the first time via hydrothermal and co-precipitation method. The structural and morphological study of the nanoparticles have been examined using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The electrochemical study of J1 and J2 electrodes have been examined using three electrode system in 6 M KOH electrolyte using cyclic voltammetry (CV), galvanostatic charging-discharging (GCD) and electrochemical impendence spectroscopy (EIS). The highest specific capacitance of 1195 F/g has been obtained at a scan rate of 10 mV/s from hydrothermal synthesis nanomaterial electrode (J2) and long cycling life 92.3% retention after 2000th cycles. Furthermore, the energy density and power density of the J2 electrode at a current density of 5 A/g was 59 Wh/kg and 9234 W/kg, respectively. Hence, the fabricated J2 electrode is a favorable candidate for super-capacitor applications.


Introduction
Supercapacitors are highly convenient in such applications where high power density and short charging time is required such as in power emergency actuators in airlines, in hybrid electric vehicles, photographic flashes, electric screwdriver, gondolas and high power devices [1]. Supercapacitors also have played a significant role to stabilize the power supply and voltage in numerous industries where load fluctuation is the major issue [2]. Moreover, flexible supercapacitors are used in both wearable and portable electronic through twisting and folding conditions [3]. A supercapacitor consists of two electrodes, a separator and an electrolyte. It is divided in to two main categories owing to charge storage mechanism; electric double layer capacitors (EDLCs) and pseudocapacitors. EDLCs are stored charge at the electrode/electrolyte interface by physical adsorption/desorption and it depends on the contact surface between electrode and electrolyte. In the start, EDLCs electrodes were made using conducting polymers, metal oxides and lithium ions materials whereas currently researcher are widely using, carbon nanotubes (CNTs), graphene and activated carbon because these materials have many remarkable properties such as lightweight, large surface areas, excellent conductivity and electrochemical stability [4,5]. Faradic reactions (oxidationreduction reactions) are used in pseudocapacitors to store charge nearby the surface and inside the bulk of the electrodes [6]. Pseudocapacitors electrodes are made using conducting polymers, transition metal oxides and transition metal oxides composites. The major drawback of pseudocapacitors is shorter lifetime as compare to EDLCs [7,8]. T. Brousse et al., reported that metal oxide has been studied for use in pseudo-capacitors such as RuO, PbO 2 , Ni(OH) 2 , MnO 2 [9] because these materials contain various corrosion state which make possible the faradic arraign relocate mechanism. In the literature, the authors reported that only the precious metal oxide has shown good stability such as RuO and IrO. But both of these are very expensive metals [10]. By contrast, MnO 2 has been reported high theoretical capacitances in the range of 1100-1300 F/g, less toxic and much low in cost [11,12]. Poor cycling stability of MnO 2 is the major problem for using it as a pseudocapacitor material [13]. The electrolytes used in supercapacitors are mainly divided into three categories solid-state electrolytes, liquid-state electrolytes and gel-state electrolytes. Solid based electrolytes supercapacitors are prevented from leaking, but their interaction between electrolyte and electrode is poor. However, in liquid-state electrolytes the major problem is leakage so that's why commonly the gel-state electrolytes are used during the fabrication of supercapacitors [14,15].
In the literature many authors reported that ferrites are chemically and thermally stable in the range of nanometer scale and displays unexpected behavior both in chemical and physical properties [16]. In many applications cobalt ferrites are most suitable such as gas sensors [17], pigments [18], photocatalysts [19], magnetic resonance imaging [20], and in electrochemical energy storage devices [21,22]. Furthermore, the ferrites properties can be improved by doping/substitution of other metallic elements (i.e., iron, cobalt, cerium, aluminum) in octahedral and tetrahedral sites.
Doping is a noteworthy and more effective technique to well tune the desired properties of semiconductors. The dopant can be improved electrochemical properties of ferrites semiconductors nanomaterial by shifting energy band structure, tuning the morphology, producing additional active sites at the grain boundaries and enhance the surface to volume ratio [23].
For both Li-ion battery and super-capacitor applications spinel ferrites MFe 2 O 4 (M = Co, Ni, Zn, Fe, Cu, etc.) as well as their nano-composites have been used as a potential material [24][25][26]. They are abundant in nature, inexpensive, environment-friendly and stable. A number of researchers have been reported that electrochemical performance of spinel ferrites nanomaterials can be improved with doping of various elements [27][28][29] [32] for super-capacitor electrode.
In this article a new lanthanum cerium ferrite (LaCeFe 2 O 4 ) has been synthesized highly porous nanoparticles through two different synthesis methods involving ''chemical co-precipitation and hydrothermal methods'' as an electrode material for supercapacitor. The surface morphology, structure and elemental composition of the synthesized materials by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX) have been studied. Furthermore, the electrochemical capacitive behavior of lanthanum cerium ferrite (LaCeFe 2 O 4 ) electrodes have been investigated via cyclic voltammetry (CV), galvanostatic charging/discharging (GCD) and electrochemical impendence spectroscopy (EIS) in 6 M KOH electrolyte. The synthesize materials have exhibited a noteworthy specific capacitance (C sp ) and cycling stability as compare to previously reported ferrites materials.

Co-precipitation method for LaCeFe 2 O 4
Initially, three solutions were prepared through adding 0.3 mol of Fe(NO 3 ) 3 Á9H 2 O, 0.05 mol of La(NO 3 ) 3 Á6H 2 O and 0.05 mol of Ce(NO 3 ) 3 Á6H 2 O in 20 ml distilled water respectively. Then, all the solutions were mixed together under constant stirring at room temperature for 15 min to get homogeneous mixture and named A. In the next step, 02 mol of NaOH was dissolved in 25 ml distilled water used as a precipitating agent and added drop wise into solution A with the help of burette under constant stirring at 80°C for 3 h. Brick red precipitates were formed, when whole the NaOH solution was added into solution A. At the end, the formed precipitates were washed six times with ethanol and distilled water and dried in an oven at 200°C for 24 h. In this process, the synthesize material was named as J1. The schematic diagram of co-precipitate method was shown in Fig. 1.

Hydrothermal method for LaCeFe 2 O 4
First, three solutions were prepared through adding 0.3 mol of Fe(NO 3 ) 3 Á9H 2 O, 0.05 mol of La(NO 3 ) 3-6H 2 O and 0.05 mol of Ce(NO 3 ) 3 Á6H 2 O in 20 ml distilled water respectively. Then, all the solutions were mixed together under constant stirring at room temperature for 30 min to get homogeneous mixture and named A. After that, 02 mol of NaOH was dissolved in 25 ml distilled water as a precipitating agent and added drop wise into solution A with the help of burette. When whole the NaOH solution was added into solution A, its color was changed into brick red and named B. In the next step, solution B was shifted to Teflon lined (TL) autoclave and kept in an oven and heating at 140°C for 18 h. At the end, the obtained final product was cool down and six times with ethanol and distilled water and dried in an oven at 200°C for 24 h. In this process, the synthesize material was named as J2 . The schematic diagram of hydrothermal method was shown in Fig. 2.

Morphological and structural characterization
The morphology of the synthesized nanomaterial have been examined using scanning electron microscopy (SEM, LEO 1450 VP) and transmission electron microscopy (TEM, JEOL JEM-2100) operating at an accelerating voltage of 2000 V respectively.
The elemental composition has been examined using an energy-dispersive X-ray spectroscopy (EDX, EAG AN461). The crystal structure of LaCeFe 2 O 4 nanostructures have been examined by means of X-ray powder diffraction (XRD, Rigaku, CuKa, k = 1.5418 Å ).

Electrode fabrication and electrochemical measurements
where I is the current (A), m is the mass of active material (mg), DV is the potential window (V), v is the scan rate (mV/s) and Dt is the discharge time (s). The power density E d (Wh/kg) and energy density P d (W/kg) of the LaCeFe 2 O 4 supercapacitor were calculated by using Eqs. (3) and (4) [48,49], respectively.
where I is the current (A), m is the mass of active material (kg), DV is the potential window (V), and Dt is the discharge time (h).

Results and discussion
The morphology of LaCeFe 2 O 4 nanostructures have been examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both co-precipitation and hydrothermal method synthesized LaCeFe 2 O 4 nanostructures micrographs are shown in Fig. 3a, b. The micrograph of co-precipitate method shows that nanoparticles are formed on the other hand the micrograph of hydrothermal method displays that agglomeration formation of porous nanoparticles is formed. The agglomeration in the nanoparticles is formed may be due to interfacial surface tension [50]. TEM image of the hydrothermal method synthesized LaCeFe 2 O 4 nanoparticles are shown in Fig. 3c. It shows that the uniformly distributed spherical nanoparticles having a diameter in the range of 160-180 nm. This nanoparticles like morphology of LaCeFe 2 O 4 with high surface area might produce additional active sites for more efficient contact between the surface of electrode material and electrolyte ions which can improve the electrochemical process and capacitive ability of the nanostructures. The chemical composition of both J1 and J2 samples have been examined using energydispersive X-ray spectroscopy (EDX) and spectrum is shown in Fig. 3d, e. The weight and atomic percentage of the elements present in JI and J2 nanostructures has shown in the table (inset of Fig. 3d, e). These ratios confirm that the lanthanum and cerium are close to stoichiometry, which is in good agreement with the 1:1 ratios of lanthanum and cerium in ferrites. More interestingly, the EDX spectrum has represented that no impurity element present in both J1&J2 nanostructure sample. X-rays diffraction patterns of synthesized lanthanum cerium ferrite (LaCeFe 2 O 4 ) through co-precipitate and hydrothermal method are shown in Fig. 4. The XRD patterns containing four prominent scattering peaks in the 2 theta range of 20°-80°which corresponds to the crystal planes (111), (200), (220) and (311) demonstrating the formation of cubic lanthanum cerium ferrite structure (space group Fd3m). The XRD patterns are good agreement with JCPDS card  and (81-0792) [51,52]. Furthermore, the XRD patterns (Fig. 5) represent the considerable contraction of the peaks of J2 sample, which is indication of the formation of large crystallites. More interestingly, it can be seen that no extra peaks are observed in both the samples of XRD patterns (J1 and J2) which represents the formation of pure LaCeFe 2-O 4 nanoparticles without calcination. Moreover, the J2 (hydrothermal process preparation) sample peaks have highly sharpness and more intensity as compere to J1 (co-precipitation process preparation) sample which are indicating the well crystalline nature of the J2 sample therefore hydrothermal route synthesized LaCeFe 2 O 4 nanoparticles are more suitable for supercapacitor electrode material.
The as-fabricated LaCeFe 2 O 4 nanoparticles electrochemical capacitive performance has been investigated via CV and GCD tests. The CV curves of J1 and J2 nanoparticles have been tested at a scan rate of 15 mV/s in the potential range of 0.1-0.6 V by using 6 M KOH electrolyte are shown in Fig. 5a. The CV curves of both the samples J1 and J2 indicate the Faradic type capacitive features, for J1 sample two peaks are observed at * 0.44 V and * 0.33 V for the oxidation and the reduction processes respectively, while for J2 sample three peaks are observed in which one peak for oxidation process (* 0.52 V) and two peaks for reduction process (* 0.32 V and * 0.38 V) respectively. There might be two main reasons for the observation of three peaks in J2 sample (1) both lanthanum and cerium have various oxidation states and (2) formation of highly pores and more active sites nanoparticles via hydrothermal method. The specific capacitance (C sp ) of J1 and J2 sample at a scan rate of 15 mV/s were obtained from Eq. (1) are shown in Fig. 6a. In addition, to get more information about the electrochemical behavior of J2 sample, CV tests were recorded at different scan rates in the potential range of 0.1-0.6 V in 6 M KOH electrolyte are shown in Fig. 5b. It could be observed that with the increase of scan rate the area under the curves gradually increases. This represents that scan rates are directly proportional to the CV current, which exhibit the ideal behavior of capacitive [53]. Moreover, the specific capacitance calculated from CV curves from the Eq. (1) is also dependent on the scan rate. The maximum specific capacitance C sp (1195 F/g) was obtained at the lowest scan rate (10 mV/s), while minimum specific capacitance C sp (1020 F/g) was obtained at the highest scan rate (50 mV/s). This could be attributed that more ohmic resistance occur at larger scan rates [54]. Furthermore, Fig. 5b represents the couples of different boarding redox peaks analogous to the redox rare earth elements of La 3? / La 2? and Ce 3? / Ce 2? . The La 3þ =La 2þ system : Ce 3þ =Ce 2þ system : The CV curves (Fig. 5a, b) show the same behavior at different scan rates, which illuminating the reversibility of the redox reactions. The comparative specific capacitance C sp (F/g) of J1 and J2 electrodes at scan rate of 15 mV/s in the potential range of 0.1-0.6 V in 6 M KOH electrolyte are shown in Fig. 6a and the obtained specific capacitance C sp from other different scan rates are shown in Fig. 6b.
All the GCD curves of J2 (LaCeFe 2 O 4 ) electrode show the deviations from linearity owing to the pseudo-capacitive nature which is good agreement with CV results. Figure 7 represent the energy losses   Variation specific capacitance C sp (F/g) vs. different scan rates for J2 sample arising due to equivalent series resistance or internal resistance in the charging and discharging curves are indicated the IR drop at the turning point [56]. Furthermore, the specific capacitance is progressively decrease with increasing discharge current density due to increasing of IR drop.
The specific capacitance C sp (F/g) from GCD curves were calculated at different current densities by using Eq. (2) and are shown in Fig. 8a. Power density (P d ) and energy density (E d ) are two important parameters to investigate the electrochemical performance of supercapacitor electrode materials. The power density P d (W/kg) and energy density E d (Wh/kg) from GCD curves were calculated at different current densities via Eqs. (3) and (4) respectively. Ragone plot obtained from GCD curves at different current densities for J2 electrode are shown in Fig. 8b. More interestingly, the supercapacitor performance parameters such as specific capacitance (C sp ) Power density (P d ) and energy density (E d ) with some other reported ferrite based materials are given in Table 1. It could be noted that J2 electrode is superior to previous reported ferrites.
Cyclic stability is another significant parameter because it is used to determine the durability and efficiency of supercapacitor [59]. The cycling measurements for J2 electrode has been studied via CV tests for 2000 cycles at the scan rate of 10 mV/s and are shown in Fig. 9. The J2 electrode shows 92.3% capacitance retention after 2000th cycles, which indicates the high rate of capability and electrochemical long-term cycling stability.
Electrochemical impedance spectroscopy (EIS) measurement of J1 and J2 electrodes have been tested within open circuit potential in the frequency range of 100 kHz to 10 MHz in 6 M KOH electrolyte and Nyquist plot are shown in Fig. 10. Inset of Fig. 10 shows the fitted Nyquist plot equivalent circuit which consists of Warburg impedance (Wo), solution resistance (R s ) charge transfer resistance (R ct ) and constant phase element (CPE) to examine the capacitive behavior.
High frequency intercept on the real axis shows the solution resistance (R s ) which contains an intrinsic resistance of substrate, contact resistance of active material and ionic resistance of electrolyte, however, semicircular arc represents the charge transfer resistance (R ct ) between electrode and electrolyte interface and a constant phase element (CPE) shows the nonideal double layer capacitance. The Warburg impedance (Wo) indicating the frequency dependence diffusion resistance of ions electrolyte. The fitted  values of EIS spectra for both J1 and J2 electrodes are given in Table 2. Which indicating the J2 electrode have a worthy response as compare to J1 electrode. Hence, J2 electrode is a promising contender for supercapacitors.

Conclusion
LaCeFe 2 O 4 nanoparticles were synthesized using two simple methods ''co-precipitate and hydrothermal process'' and examined as a potential material for supercapacitor electrodes. The structural and electrochemical properties of the LaCeFe 2 O 4 nanoparticles were studied. XRD patterns were conformed to the formation of single-phase ferrite. SEM micrographs was conformed to the formation of porous nanoparticles that enable charge transfer of electrolyte. TEM study was conformed to the uniformly distributed spherical nanoparticles having diameter in the range of 160-180 nm. EDX spectrums were shown the formation of LaCeFe 2 O 4 without any impurity element. The fabricated electrodes of LaCeFe 2 O 4 electrodes were investigated in 6 M KOH electrolyte via CV,GCD and EIS. The electrochemical properties of hydrothermal method synthesis LaCeFe 2 O 4 nanoparticles was shown a good Faradaic behavior, with a high specific capacitance of 1195 F/g   at a scan rate of 10 mV/s. The cyclic stability was studied via CV and the capacitance retention was 92.3% after 2000 cycles. The energy and power density of the LaCeFe 2 O 4 supercapacitor electrode at a current density of 5 A/g was 59 Wh/kg and 9234 W/ kg respectively. The EIS study also confirmed that J2 electrode have a good response as compare to J1 electrode.