Enhancing the Supercapacitive and Superparamagnetic Performances of Iron Oxide Nanoparticles through Yttrium Cations Electrochemical Doping

A one-pot electrosynthesis platform is reported for fabrication of Y3+ doped iron oxide nanoparticles (Y-IONPs). In this procedure, Y-IONPs are electro-deposited from an additive-free aqueous solution of iron(III) nitrate, iron(II) chloride and yttrium chloride. The analysis data provided by X-ray diffraction (XRD), field emission electron microscopy (FE-SEM) and energy-dispersive X-ray (EDX) confirmed that the deposited Y-IONPs sample is composed of magnetite nanoparticles (size≈20nm) doped with about 10wt% Y3+ cations. The performance of the prepared Y-IONPs as supercapacitor electrode material was studied using cyclic voltammetry (CV) and galvanostat charge-discharge (GCD) tests. The obtained electrochemical data showed that Y-IONPs provide SCs as high as 190.3 and 138.9 F g−1 at the discharge loads of 0.25 and 1 A g−1, respectively, and capacity retentions of 95.9% and 88.5% after 2000 GCD cycling. Furthermore, the results of vibrating sample magnetometer measurements confirmed better superparamagnetic behavior of Y-IONPs (Mr=0.32 emu g-1 and HCi= 6.31 G) as compared with pure IONPs (Mr=0.95 emu g-1 and HCi= 14.62 G) resulting from their lower Mr and Hci values. Based on the obtained results, the developed electro-synthesis method was introduced as a facile procedure for the preparation of high performance metal ion doped magnetite nanoparticles.


Introduction
Supercapacitors (SCs) have received great attentions among various energy storage devices both in academic and practical applications.They show high power densities and can be fully discharged or charged in seconds, which are suitable for large instantaneous current densities.SCs performance is highly dependent on the specific surface area and conductivity of the electrode materials 1 .The active material of electrode is a determining element of SCs, which dedicate their electrochemical performance.Hence, it is quite understandable that developing efficient and costeffective electroactive materials be main challenge in the improvement of performance of supercapacitors.Hence, extensive researches have been focused to find proper electrode materials for SCs.Pseudocapacitors mainly store charges from reversible redox reactions and generally composed of transition metal oxides/hydroxides like tin oxide 2 , titanium oxide 3 , cobalt oxide 4,5 , nickel oxide [6][7][8] , vanadium oxide 9 , zinc oxide 10 , manganese oxides [11][12][13][14][15] , cobalt hydroxides [16][17][18][19][20] , nickel hydroxides [21][22][23][24][25] and iron oxides [22][23][24][25][26][27][28][29][30][31][32] .This type of SCs could deliver high specific capacitance, but present poor cycle stability.Among metal oxides, magnetite (Fe 3 O 4 ) is the interested candidates as a result of its environmental friendliness, natural abundance, low cost and variable oxidation states 33 .It was found that low electrical conductivity of iron oxide is major obstacle for use as electrode material in ECs 34,35 .For refine this issue, three solutions of novel nano-structures fabricating [36][37][38][39][40][41][42][43] , metal ions doping 44 , and also combination with conductive carbon-based nanomaterials [45][46][47] have been established.Reviewing the results of these works indicates that performance of iron oxide electrode is improved due to enhancing its conductivity and redox activity.Notably, metal ion doping strategy has been rarely investigated.In this paper, we report a novel platform for the preparation of metal ion (Y 3+ ) doped iron oxide nanoparticles (Y-IONPs) through cathodic electrodeposition procedure, and also about 15% improvement in the supercapacitive capability of IONPs as a result of Y 3+ doping.This platform is based on the well-known cathodic electrosynthesis (CE) method.In this method, nanostructured metal oxides/hydroxide could be easily prepared through OH -electro-generation on the cathode surface [48][49][50] .However, this method has not been applied for the preparation IONPs until now.It is worth noting that we very recently reported one-pot electrosynthesis of naked and polymer coated IONPs through cathodic electrosynthesis (CE) method [51][52][53] .Here, we applied a CE strategy for the synthesis of Y 3+ doped IONPs.To the best of our knowledge, electrochemical synthesis of metal doped Fe 3 O 4 NPs has not been reported until now.The prepared Y-IONPs were characterized by XRD, IR, FE-SEM, VSM, cyclic voltammetry (CV) and galvanostat charge-discharge (GCD) techniques.The results of these analyses confirmed the proper magnetic and charge storage behavior of the prepared Y 3+ doped iron oxide NPs.

Electrosynthesis of Y 3+ doped Fe 3 O 4 NPs
The cathodic electrosynthesis (CE) platform previously reported for the fabrication of naked and coated magnetite nanoparticles (MNPs) 51,53 , was here modified for the electrosynthesis of Y 3+ doped IONPs.A schematic view of the preparation route is provided in Fig. 1.The electrosynthesis set-up was composed of a (316 L, 5cm×5cm×0.5mm)steel cathode centered between two parallel graphite anodes, as shown in Fig. 1.The electrolyte solution was prepared by mixing 2g iron(III) nitrate, 1g iron(II) chloride and 0.3g yttrium chloride in 1 liter aqueous solution.The electrodeposition runs were conducted on an electrochemical workstation system (Potentiostat/Galvanostat, Model: NCF-PGS 2012, Iran) with applying dc current density of 10mAcm -2 .The deposition time and bath temperature were 30 min and 25º C, respectively.
After each deposition run, the cathode was bring out from solution and rinsed several times with deionized H 2 O.Then, the deposited black film was scraped form the steel and subjected to separation and purification steps, as noted in Fig. 1;(i) the obtained wet powder was dispersed in deionized water and centrifuged at 6000rpm for 20min to removal of free anions, as indicated in Fig. 1,(ii) the deposit was then separated from water solution by a magnet, dried at 70º C for 1h, and (iii) the resulting black dry powder was named Y-IONPs, and used for further evaluations.

Characterization analyses
The SEM images of the prepared powder were provided through field-emission scanning electron microscopy (FE-SEM, Mira 3-XMU with accelerating voltage of 100 kV).The crystal structure of the prepared powder was determined by X-ray diffraction (XRD, Phillips PW-1800) using a Co Kα radiation.The magnetic properties of the prepared Y 3+ doped IONPs were assessed in the range of −20000 to 20000 Oe at room temperature using vibrational sample magnetometer (VSM, model: Lake Shore, USA).reference electrode (saturated with 1 M KCl), and a counter electrode (platinum wire).The working electrode (WE) was fabricated through the well-known paste procedure 34,37 ; First, the prepared black Y-IONPs powder was physically mixed with acetylene black (>99.9%) and conducting graphite (with rations of 75:10:10), and the mixture was homogenized properly.Then, 5%wt polyvinylidene fluoride (PVDF) dissolved inN-Methyl-2-pyrrolidone (NMP) was added into the mixture.After partially evaporating the NMP content of the mixture, the resulting paste was pressed at 10 MPa onto Ni foam (surface area of 1cm 2 ).The resulting electrode was dried for 5 min at about 150 °C in oven.In final, the fabricated electrode was used as working electrode in the electrochemical tests.The mass loading of Y-IONPs powder onto the Ni foam was about 2.5 mg.The CVs of the fabricated working electrode were recorded in a 1M Na 2 SO 3 electrolyte in the potential range of -1.0 to+0.1 V vs. Ag/AgCl.The CV profiles were recorded at the potential sweeps of 2, 5, 10, 20, 50 and 100 mV s -1 .The GCD curves were recorded at the different current loads of 0.25, 0.5, 1, 2, 3 and 5 A g -1 within a potential range of -1.0 to -0.35V vs. Ag/AgCl.The EIS was conducted in the frequency range between 100 KHz and 0.01 Hz with applying 5 mV at open-circuit potential.The purity of the prepared samples was resulted from absence of extra XRD peak in their patterns.Generally, a cubic inverse spinel ferrite structure is reported for crystal structure of Fe 3 O 4 , where Fe 3+ cations are only located in the octahedral sites, but the tetrahedral sites are occupied by both of Fe 2+ and Fe 3+ cations 25,29 .It is worth noting that, compared to reflections of undoped sample, small shifts in all reflections of doped sample were observed in Fig 2 .These changes are due to the larger ionic radius of Y 3+ cations toward Fe 3+ cations (i.e.92 pm vs. 60pm) [54][55][56][57] .From these results, it is stated that Y 3+ cations are located in some octahedral and/or tetrahedral sites owing to the Fe 3+ cations in the magnetite crystal structure.Hence, our deposition product has Y 3+ doped Fe 3 O 4 crystal structure.This means that Y 3+ cations reacted like Fe 3+ cations during CE process.And the following mechanism could be written for the formation of Y-IONPs 51,52 :

Structural and morphological characterizations
First, the OH -ions are generated on the cathode surface through electrochemical step, which shown as step (i ) in the inset of Fig. 1: Then, the metal cations i.e.Fe 2+ , Fe 3+ and Y 3+ cations are then chemically reacted with the OH -generated on the cathode to form iron oxide, which shown as step (ii ) in the inset of Fig. 1: Chemical step: (2) In final, the Y 3+ doped Fe 3 O 4 deposited on the cathode and growth during the time of deposition.The average crystallite size (D) of the Y-IONPs was calculated using the Debye-Scherrer equation, D=0.9λ/βcos(θ), where λ is the X-ray wavelength, β is the full width at half maximum of the diffraction line, and θ is the diffraction angle of the XRD pattern.From the diffraction line-width of (311) peak, the average crystallite sizes of the prepared undoped and Y 3+ doped IONPs were calculated to be 7.2 and 12.1nm.
Figs. 3a-d present FE-SEM images of the electrosynthesized iron oxides.Notably, for better comparison, the FE-SEM images of undoped IONPs have been provided from Refs 52,53 .It is clearly seen that the electrodeposited sample has particle morphology and the particle size in the range of 10-20nm.The elemental analysis of the prepared nanoparticles was provided through energy-dispersive X-ray (EDX), which is presented in Fig. 3e data clearly proved the formation of Fe 3 O 4 NPs doped with ~10% Y 3+ through our developed CE strategy.The magnetic hysteresis loops for the prepared Sm-IONPs and IONPs (provided from our previous works 52, 53) are shown in Fig. 4. No hysteresis is seen in the VSM profiles and the curves have S like form, as seen in Fig. 4.These observations implicated that the prepared Y-IONPs have superparamagnetic behavior.The magnetic data of the prepared Y-IONPs are listed in Table 1.
For Y-IONPs, the magnetic data i.e. saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (H Ci )are observed to be; Ms=43.64 emu g -1 , Mr=0.32 emu g -1 and H Ci = 6.31G.These data confirmed the superparamagnetic nature of the electrosynthesized Y-IONPs.Also, our Y-IONPs exhibit better superparamagnetic characteristics i.e. higher Ms and lower Mr and H Ci values as compared with those reported in the literature i.e.Sm 3+ , Eu 3+ , Gd 3+ , Cu 2+ and Mn 2+ doped IONPs [54][55][56][57] .Furthermore, these magnetic data are comparable with those of undoped IONPs electro-synthesized at a similar electrochemical condition in our previous works.The hysteresis behavior of pure IONPs have been previously studied by the authors, and the reported data are Ms=72.96emu g -1 , Mr=0.95 emu g -1 and H Ci =14.61 G 51,53 .The Y 3+

Cyclic voltammetry
Cyclic voltammetry was used to evaluate the supercapacitive performance of the working electrode (WE) fabricated from the electrosynthesized Y 3+ doped IONPs and comparison with undoped ones.Fig. 5a presents the CV curves of the prepared working electrode within the potential range of -1.0 to +0.2V vs. Ag/AgCl with applying the scan rates of 2-100 mV s -1 .The shapes of the CV curves clearly reveal the pseudocapacitive characteristics of the Y-IONPs, which is different from the electric double-layer capacitance.In the literature, a combination of both EDLC and pseudocapacitance involving the reduction/oxidation of specifically adsorbed SO 3 2-anions on the iron oxide surface has been reported for the capacitance behavior of pure Fe 3 O 4 electrode in the Na 2 SO 3 solution [39][40][41] , which are seen by small peaks on the CV curve.For Y-IONPs electrode, there are some small peaks i.e. humps on CVs (Fig. 4a) as a result of redox reactions of SO 3 2-anions attached onto the surface of Y 3+ doped Fe 3 O 4 nanoparticles 33,35 : For comparison, a CV data of the working electrode fabricated from the undoped IONPs is also provided, and compared with Y 3+ doped Fe 3 O 4 nanoparticles.Fig. 5b shows the CVs of both undoped and doped IONPs at the scan rate of 5mV/s.It is seen that the Y 3+ doped IONPs electrode deliver greater anodic and cathodic currents, and hence would have larger SC values.
The SC values of the both working electrodes were calculated from their CV profiles by integrating the area under the current-potential curves using Eq. ( 5) 18 : where C is the capacitance of prepared Y-IONPs powder (F g -1 ), Q is the total charge, ΔV is the potential window, m is the mass of Y-IONPs powder (g), v is the scan rate (V s -1 ) and I(V) is the current response during the potential scan.Then, the SCs were plotted vs. scan rate, as shown in Fig. 5c.The calculations revealed that the Y 3+ doped Fe 3 O 4 NPs are capable to give SC values as high as 198.7, 174.3, 152.5, 127.8, 99.9, 84.2 and 75.1 F g -1 at the scan rates of 2, 5, 10, 20, 50, 75 and 100 mV s -1 , respectively.Also, it has been reported that the undoped NPs are enable to deliver SC values of 181,159, 140, 112, 92, 83 and 68 F g-1 at the scan rates of 2, 5,10, 20, 50 and 100 mV s -1 , respectively [51][52][53] .Comparing these SC values revealed that the Y 3+ doped Fe 3 O 4 NPs provide up to 10% larger SC values as compared with those of undoped NPs.In other word, it can be said that the supercapacitive performance of iron oxide NPs is increased by Y 3+ doping into the structure of magnetite.Furthermore, the SC data confirmed the proper charge storage abilities of the electro-synthesized Y 3+ doped IONPs for use in supercapacitors.The electrochemical behavior of Y 3+ doped Fe 3 O 4 NPs was also investigated by GCD and EIS measurements, which are discussed below.
These GCD profiles are very similar to those reported for iron oxide electrode in Na 2 SO 3 electrolyte [28][29][30][31][32][33][34][35] , and relative symmetric triangular form at the applied potential range.The form of GCD profiles shows the pseudocapacitance performance of the electrode due to the faradic reactions (Eqs.3 and 4).The SCs were calculated using Eq. ( 6) 20 , and the data is presented in Fig. 6b: where C is the capacitance of prepared Y-IONPs powder (F g -1 ), Q is the total charge, ΔV is the potential window, m is the mass of Y-IONPs powder (g), v is the scan rate (V s -1 ) and I is the applied current load (A) and Δt is the time of a discharge cycle.Notably, some IR drop is also seen in all GCD profiles.The calculations revealed that the Y 3+ doped IONPs are capable of delivering SC values of 190.3, 159.4,138.9, 117.5, 96.7, 82, 66.8 and 55.9 F g −1 at the discharging   These values are close to those calculated based on the CVs (Fig. 5b), confirming the excellent super-capacitive behavior for the electro-synthesized Y 3+ doped Fe 3 O 4 nanoparticles.Furthermore, the charge storage ability of our prepared Y-IONPs sample is comparable with the reported SC data for the nanostructured Fe 3 O 4 electrodes in the literature, which listed in Table 2.
Comparing the SC values listed in Table 2 revealed that the supercapacitive performance of our prepared Y-IONPs is higher than those reported for pure iron oxide electrodes, and hence the enhancement of charge storage ability of iron oxide through yttrium cations doping is confirmed.
The fabricated working electrode was further cycled (2000 cycles) at the current loads of 0.25 and 1 A g −1 in 1M Na 2 SO 3 electrolyte.The SC values and capacity retentions of the fabricated Y-IONPs were calculated during cycling.Figs.7a and b represent the SCs and SC retentions vs. cycle number, respectively.It was found that the SC value of Y 3+ doped Fe 3 O 4 NPs is reduced from 190.3 F g −1 to 182.5 F g −1 after 2000 GCD cycling at a discharging current of 0.25 A g −1 (Fig. 7a), which exhibited about 95.9% capacity retention, as seen in Fig. 7b.Also, the fabricated Y 3+ doped NPs enable to deliver specific capacitance as high as 122.9 F g −1 after 2000 GCD cycles at a current of 1 A g −1 , which showed that the electrode had capacity retention of 88.5%   at this discharging rate (Fig. 7b).These data confirmed the proper charge storage ability of Y 3+ doped Fe 3 O 4 nanoparticles.
The energy and power densities of the fabricated working electrode were also calculated using Eqs.(7 and 8) 27 : (7) (8)   where E, C, ΔV, P and Δt are the specific energy, specific capacitance, potential window, specific power and discharge time, respectively.It was found that our electrode provides energy density and power density as high as 27.7 Wh/g and 8.02W/kg, respectively.These electrochemical data provides the proper supercapacitive performance of the electro-synthesized Y 3+ doped Fe 3 O 4 nanoparticles.

Conclusion
In summary, a novel and easy electrochemical procedure was developed for the fabrication of Y 3+ doped magnetite nanoparticles.The XRD, FE-SEM and EDS analyses proved the magnetite phase, fine particle morphology with 20 nm in size and 10%wt Y 3+ content of the electrosynthesized iron oxide products.Galvanostat charge-discharging the fabricated electrode materials indicated that the Y 3+ doped Fe 3 O 4 nanoparticles are capable to deliver specific capacitances as high as with delivering a specific capacitance values of 190.3, 159.4,138.9, 117.5, 96.7, 82, 66.8 and 55.9 F g −1 at the discharging loads of 0.25, 0.5, 1, 2, 3, 5, 7 and 10 A g −1 , respectively.It was found that the supercapacitive ability of magnetite nanoparticles is increased up to 20% thought Y 3+ doping.

Fig. 1 .
Fig. 1.Schematic graph of the cathodic electrodeposition of Y 3+ doped IONPs.The inset presents (i) electrochemical and (ii) chemical steps of deposition procedure

Fig. 2
Fig. 2 shows the XRD patterns of electro-synthesized undoped and Y 3+ doped Fe 3 O 4 powders.All the observed diffraction peaks in the XRD pattern could be readily referred to the pure cubic phase [space group: Fd3m (227)] of Fe 3 O 4 with cell constant a = 8.389 Å (JCPDS 01-074-1910).The purity of the prepared samples was resulted from absence of extra XRD peak in their patterns.Generally, a cubic inverse spinel ferrite structure is reported for crystal

Fig. 5 .
Fig. 5. (a) CVs of the fabricated Y-IONPs working electrode at the various scan rates, (b) (c) CV Yofiles for undped and Y3+doped MNPs at the scan rate of 5mV/s, and (c) the calculated specific capacitances for both electrodes vs. scan rate.

Fig. 6 .
Fig. 6.(a) GCD Yofiles of Y-IONPs electrode and (b) is calculated SCs at the different current loads of 0.25 to 5 A g -1

Ta b l e 2 .
E l e c t r o c h e m i c a l c a p a c i t a n c e v a l u e s r e p o r t e d f o r F e 3 O 4 b a s e d w o r k i n g e l e c t
. In this data, it is seen that the electrodeposited Y-IONPs have the Fe, Y and O atoms with the weight percentages of 62.06%wt, 9.78%wt and 28.16%wt, respectively.With considering the fact that Y 3+ cations plays a same role of Fe 3+ cations in the CE process, these values are matched with the weight percentages of Fe(72.36%wt) and O (27.64%wt) in the Fe 3 O 4 chemical formula.These Fig. 2. XRD patterns of undoped and Y 3+ doped Fe 3 O 4 NPs