We report the synthesis of citrate stabilized iron oxide (C-Fe₃O₄) spherical nanoparticles for supercapacitor electrodes. The citrate functional group present in the surface of the Fe₃O₄ nanoparticles effectively controls the morphology and the surface area of the nanostructures. The C-Fe₃O₄ electrodes exhibited a battery-like energy storage properties with a maximum specific capacity of 146 Cg^-1 (242 Fg^-1) which is much higher than the specific capacity of citrate free Fe₃O₄ electrode (62 Cg^-1; 112 Fg^-1). Moreover, the C-Fe₃O₄ electrode showed better cyclic stability (75%) than the citrate free Fe₃O₄ electrode (~35%) after 1000 charge/discharge cycles.

Keywords: Ion oxide, Nanoparticle, Supercapacitor 

Transitional metal oxides (TMOs) are the potential pseudocapacitive electrode materials as they have multiple valence states of the metal ions that could enable a fast-faradaic redox reaction near to the surface region. In the recent past, a variety of TMOs such as RuO₂, MnO₂, Co₃O₄, NiO, Fe₃O₄ was studied for their capacitive performance [1-3]. Among all these transition metal oxides, iron oxides exhibit considerable attractions due to their low toxicity, natural abundance, low cost, environmental friendliness and rich redox chemistry and having multiple applications due to their unique structural, electrical and magnetic properties [4, 5]. Generally, iron oxides are promising negative electrode materials for supercapacitors owing to its excellent electrochemical performance in the negative potential window with a high theoretical capacitance [6]. However, few researchers have been reported Fe₃O₄ as a positive electrode material and displayed considerable electrochemical performances which are even comparable or higher than the well-known TMOs like Co₃O₄, MnO₂, CuO etc., [7, 9].
In general, the metal oxides nanoparticle synthesized through conventional chemical techniques suffers from the agglomeration of nanoparticles, which significantly leads to the formation of large-sized particles with poor surface area and porosity [10]. In order to overcome this issue, the Fe₃O₄ surface should be stabilized with citric acid via the coordination bond which can be acted as a promising catalyst for the formation of pyrimidine derivative compound. Further, high surface area of CA anchored Fe₃O₄ also plays a significant role to determine their catalytic performance [11]. As the electrochemical performance of the supercapacitor electrode highly depends upon the surface area and porous nature of the electroactive materials, it is important to control the size and modifying the porosity of the nanoparticles to provide large electroactive surface area for the effective charge-storage process.
Considering these crucial electrochemical factors, we synthesized water dispersible, citrate stabilized Fe₃O₄ nanoparticles and utilized for the fabrication of supercapacitor electrode. The citrate stabilization considerably controls the particle size, surface area and porosity of the Fe₃O₄ nanoparticles and the fabricated electrode exhibited an excellent battery-like charge storage property in the positive potential window.

The water-dispersible citrate capped iron oxide (C-Fe₃O₄) was prepared according to the synthesis procedure reported in our previous work [12]. In brief, FeCl₃·6H₂O (0.016 mol) and FeCl₂·4H₂O (0.008 mol) were dissolved in 80 ml deionized (DI) water and reflexed for 30 min at 70 °C under argon atmosphere. Subsequently, the 20 mL of 28% ammonia solution was added slowly to the mixture until the formation of a black turbid solution. Then, 4 mL of 2.6 M citric acid solution was added to the reaction mixture and the temperature was raised to 90 °C under refluxing condition and maintained for 60 min. Finally, the mixture was cool down to the room temperature and the C-Fe₃O₄ nanoparticles were centrifuged, washed repeatedly and dispersed in DI water. The citrate free Fe₃O₄ nanoparticles were synthesized by the heat treatment of the C-Fe₃O₄ samples at 300 °C for 30 min per the TG curve (Fig. 1(a)). The FTIR spectra of the samples confirm the successful removal of citrate functional groups such as OH stretching (3,439 cm^-1), C=O stretching (1,602 cm^-1), symmetric stretching of COO^- (1,400 cm^-1) groups from the surface of the C-Fe₃O₄ nanoparticles [13, 14] (Fig. 1(b)).
The structural properties of the synthesized samples were identified by powder X-ray diffraction (XRD, Rigaku, Ultima IV) using Ni filtered Cu Kα radiation (λ = 1.5418 Å) operated at 40 kV and 30 mA in the 2θ range 20°- 80°. Morphological study of the sample was performed utilizing transmission electron microscopy (TEM) (JEOL (Japan) model JEM-2100F). The nitrogen adsorption/desorption measurement was carried out using a Microtrac, BELsorp-mini II surface analyzer employing the volumetric method. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) technique and the pore size distribution was estimated from the desorption branch of the isotherm by the Barrett-Joyner-Halenda (BJH) method. Attenuated total reflectance - Fourier transform Infrared Spectroscopy (ATR-FTIR) measurement was performed using ATR-FTIR spectrometer (Smiths Detection).
The supercapacitor electrodes were fabricated by mixing the active materials (C-Fe₃O₄ or Fe₃O₄) (75 wt%), acetylene black (20 wt%) and polyvinylidene fluoride (5 wt%) in N-methyl-2-pyrrolidone. The obtained paste was coated over a nickel foam substrate of exposed geometric area 1×1 cm² and dried at 100 °C for 12 h in a vacuum oven. The mass of the active material present in the electrodes was determined ~3.5 mg. Electrochemical measurements such as cyclic voltammetry, galvanostatic charge/discharge test and impedance spectroscopy were performed at room temperature (~25 °C) using a ZIVE-SP2 (Korea) electrochemical workstation. The measurements were carried out in a three-electrode setup consisting of a working electrode of Ni foam coated with the active material, a platinum counter electrode and a Hg/HgO reference electrode in 6M KOH aqueous electrolyte. The electrochemical impedance spectra (EIS) of the electrodes were measured in the frequency range of 10 mHz - 100 kHz at equilibrium open circuit potential 0 V with an AC perturbation of 5 mV in 6 M KOH electrolyte.
The specific capacity (Q_s) value of the electrodes was calculated using eq. (1).
 

 
where, I is the constant discharge current (A), Dt is the discharge time (s) and m is the mass of the active material (g) in the electrodes.

The crystal structures of synthesized samples were studied using XRD analysis as shown in Fig. 2(a). The XRD patterns show strong and broad diffraction peaks representing the formation of crystalline Fe₃O₄ nanostructures and the corresponding peaks are indexed to a face-centered cubic lattice of Fe₃O₄ (JCPDS Card No. 19-0629). From the diffraction patterns citrate free Fe₃O₄ exhibited slightly high intensity than the C-Fe₃O₄ representing the crystallinity of the sample increased on heat-treatment. The nitrogen adsorption/desorption isotherms of the samples are shown in Fig. 2(b) and the inset show the pore size distribution of the samples. The adsorption/desorption shows the samples exhibit type-IV hysteresis loop, that is characteristic of mesoporous materials. The BET surface area of the C-Fe₃O₄ sample is calculated to be 131.07 m²g^-1 and it decreased to 67.39 m²g^-1 after the heat-treatment of the sample. These decrease in BET surface area is mainly attributed to the removal of surface functionalities to form more agglomerated nanostructure. Similarly, the total pore volume of the sample also decreases from 0.281 to 0.224 cm³ g^-1 for Fe₃O₄ sample and the estimated average pore diameter of C-Fe₃O₄ and Fe₃O₄ samples are 9 nm and 13 nm respectively, which confirms the mesoporous distribution of the synthesized nanostructures. Thus, these samples can offer considerable attaching area between active materials and the electrolyte for better electrochemical performance [15, 16].
The surface morphology of the C-Fe₃O₄ and Fe₃O₄ samples were examined by TEM analysis. Fig. 3(a) shows the TEM image of C-Fe₃O₄ and it displays well-defined spherical morphology with slightly interconnected nanoparticles of size ranges from 10-20 nm. Moreover, the sample depicts well-dispersed nature without much agglomerations of the nanoparticles. But the TEM image (Fig. 3(c)) of the heat-treated sample (Fe₃O₄) displays highly agglomerated spherical nanoparticles with few slightly big sized nanoparticles. Thus, the subsequent heat treatment and removal of surface citrate groups from the sample highly influence the size of nanoparticles. The corresponding selected area electron diffraction (SAED) pattern of C-Fe₃O₄ and Fe₃O₄ samples are shown in Fig. 3(b) and (d) respectively. As absorbed in XRD, the SAED pattern of the heat-treated sample is more crystalline than the citrate stabilized Fe₃O₄ sample.
Electrochemical capacitance performances of the C-Fe₃O₄ and Fe₃O₄ electrodes were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) test and electrochemical impedance spectroscopy (EIS) in 6M KOH electrolyte. Fig. 4(a) shows the CVs of the bare Ni-foam, C-Fe₃O₄ and Fe₃O₄ electrodes (~3.5 mg) measured at 10 mVs^-1 scan rates. The CV of bare Ni-foam exhibited distinct pair of redox peaks originated from the Faradaic redox reaction of Ni elemental species in an alkaline electrolyte and the similar pair of redox peaks are exhibited in the C-Fe₃O₄ and Fe₃O₄ electrodes. However, the citrate free Fe₃O₄ electrode displays a secondary reduction peak with comparatively broad oxidation peak than the Ni-foam, which corresponds to the redox reaction of Fe(II)/Fe(III) redox couple of the electrode material. The C-Fe₃O₄ electrode shows a significantly larger redox peak area represents the overlapping the two peaks observed in Fe₃O₄ electrode, moreover, the electrode exhibited high background current revealing superior capacitive performance than the other two electrodes. Moreover, these irregular shape of the CVs with redox peaks correspond the non-capacitive faradic or battery-like redox characteristic of the electrodes [17, 18]. This enhancement in C-Fe₃O₄ electrode over Fe₃O₄ is mainly due to the improved surface area and pore density of the sample due to the stabilization of citrate functional groups on the surface. Further, the CVs of the electrodes measured for various scan rates are displayed in Fig. 4(b) and (c). From these CVs, C-Fe₃O₄ electrode possesses higher background current and redox peak intensity for all the measured scan rates. Moreover, the electrodes nearly retain its shapes even at higher scan rates representing better supercapacitor electrode materials.
The GCD curves of the C-Fe₃O₄ and Fe₃O₄ electrodes at 1 Ag^-1 specific current is shown in Fig. 4(d) and the curves display a nonlinear shape, which reveals the non-capacitive faradic reaction that typically exhibited in the supercapattery electrode materials due to the battery-type charge storage mechanism [19, 20]. Similar to CVs, the GCD curve shows a prolonged charge/discharge time for C-Fe₃O₄ electrode than Fe₃O₄ electrode representing better capacitive performance. The specific capacity (Q_s) values of the electrodes were calculated and the C-Fe₃O₄ electrode shows maximum specific capacity 146 Cg^-1 (242 Fg^-1) at a specific current 1 A g^-1, which is much higher than the Fe₃O₄ electrode (62 Cg^-1; 112 Fg^-1).
Fig. 5(a) and (b) shows the GCD curves of C-Fe₃O₄ and Fe₃O₄ electrodes measured for various specific currents and Fig. 5(c) displays the variation of specific capacity of the electrodes for various specific currents. From this, the C-Fe₃O₄ electrode demonstrates higher specific capacity values for all the measured specific currents with relatively higher discharge time than the Fe₃O₄ electrode. Moreover, the specific capacity of the electrodes decreases with the increase in specific currents due to the increasing IR-drop and the limited involvement of active material in a redox reaction concerning the increase in specific currents [21]. Fig. 5(d) shows the cyclic stability of the C-Fe₃O₄ and Fe₃O₄ electrodes for 1,000 charge/discharge cycles at 10 Ag^-1. The C-Fe₃O₄ electrode shows comparable cyclic stability with ~75% of the specific capacity retention after 1,000 cycles. But, the Fe₃O₄ electrode displays only ~35% specific capacity retention representing nearly poor stability of the electrode.
The Nyquist plots of the electrodes are shown in Fig. 6(a) and the inset shows the magnified portion of the high-frequency region of the plots. The Nyquist plots were fitted using Zview software with an equivalent circuit (inset of Fig. 6(a). The circuit consists of a bulk resistance (R_s) [11, 22, 23] and a parallel combination of resistance R_ct and capacitance (C_dl) represents the charge-transfer resistance and electric double layer capacitance contribution of the electrodes. The slight inclined low-frequency slope was fitted with the Warburg element (W_s) corresponding the diffusion control process of the electrodes. Additional constant phase elements CPE₁ included in the circuit due to the non-ideal capacitive behavior of the electrodes and it represents the energy storage process originated from the redox reaction [24]. The best-fitted results of the electrodes are presented in Table.1. From these, the R_s value of the electrodes found to be nearly similar, but the charge transfers resistance R_ct value of C-Fe₃O₄ electrode lower than the Fe₃O₄ electrode. This is mainly attributed to high mesoporosity of the electrode materials may account these better charge-transfer process than the Fe₃O₄ electrode [25]. Fig. 6(b) shows the corresponding Bode phase plots of electrodes with low frequency (100 mHz) phase angle of -76.5 and 55°. The existence of high phase angle at low frequency (approaches slightly close to the ideal capacitor, -90°) for C-Fe₃O₄ electrode denotes the better charge storage properties of the electrode than the citrate free Fe₃O₄ electrodes [26]. Table 1

In summary, the synthesized citrate stabilized iron oxide nanoparticles possess a controlled size, mesoporosity and high surface area than the citrate free Fe₃O₄ nanoparticles. These improved properties demonstrate the C-Fe₃O₄ nanoparticles as a promising supercapacitor electrode material with a battery like a charge storage behavior and excellent electrochemical performances.

The authors R. Manikandan and B.C. Kim acknowledges the Creative Materials Discovery Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future (NRF-2015M3D1A1069710); and the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2014R1A6A1030419), Republic of Korea.