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Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International
journal homepage: www.elsevier.com/locate/ceramint
Evaluation of supercapacitive and magnetic properties of Fe3O4 nanoparticles electrochemically doped with dysprosium cations: Development of
a novel iron-based electrode
⁎
Mustafa Aghazadeha, , Mohammad Reza Ganjalib,c
a
b
c
Materials and Nuclear Research School, Nuclear Science and Technology Research Institute (NSTRI), P.O. Box 14395-834, Tehran, Iran
Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran
Biosensor Research Center, Endocrinology and Metabolism Molecular-Cellular Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
Iron oxide
Nanopowder
Dy3+ doping
Electrosynthesis
Electrochemical performance
In this research, a novel one-pot fabrication platform was developed for the preparation of Dy3+-doped iron
oxide nanoparticles (Dy-IONPs). In the procedure, Dy-IONPs are electro-deposited from an additive-free aqueous
mixed solution of iron(III) nitrate, iron(II) chloride and dysprosium chloride salts through applying a current
density of 10 mA cm–2 for 30 min. The analytical data obtained from X-ray diffraction (XRD), field emission
electron microscopy (FE-SEM) and energy-dispersive X-ray (EDX) confirmed the deposited Dy-IONPs to be
composed of magnetite nanoparticles (size≈20 nm) containing about 10 wt% of Dy3+ cations as the doping
agent. The electrochemical data obtained through galvanostatic charge-discharge (GCD) tests showed that DyIONPs provide specific capacitances values of as high as 202 and 111 F g−1at the discharge loads of 0.5 and
5 A g−1, respectively, and reveal capacity retentions of 93.9% and 77.2% after 2000 GCD cycling. These could
be held as proof that the electro-synthesized Dy3+-doped Fe3O4 NPs are suitable candidates for use in supercapacitors. Furthermore, the results of vibrating sample magnetometer (VSM) measurements indicated better
superparamagnetic behavior of the Dy-IONPs (Mr = 0.34 emu g–1 and HCi = 6.25 G) as opposed to pure IONPs
(Mr = 0.95 emu g–1 and HCi = 14.62 G), which originates from their lower Mr and Hci values. Based on the
results, the proposed electro-synthesis method offers a facile procedure for the preparation of high- performance
metal-ion-doped IONPs.
1. Introduction
Supercapacitors or electrochemical capacitors (SCs or ECS) are
classified as electric double layer capacitors (EDLCs) and pseudocapacitors. In principle, EDLCs electrostatically store the charge on the interface of their high surface area carbon electrodes, e.g., activated
carbon (AC), carbon nanotubes (CNTs) and graphene, which is the
reason for characterizes the performance of rapid charge storage and
limited specific capacitance [1]. Pseudocapacitors mainly store charge
through reversible redox reactions and are generally composed of nanomaterials like CuO [2], Co3O4 [3–5], MoO2 [6], V2O5 [7], NiO [8–11],
MnO2 [12–19], Co(OH)2 [20–28], Ni(OH)2 [29–33], Fe2O3 [34–36] and
Fe3O4 [37–40]. This class of SCs could deliver high specific capacitance
values, but suffer poor cycling ability. Among these materials, magnetite (Fe3O4) is an interesting electrode material due its environment
friendliness, natural abundance, low cost and variable oxidation states
[41]. It has been found that the low electrical conductivity of iron oxide
⁎
is a major obstacle for its use as electrode material in ECs [42,43]. To
overcome this issue, several measures have been taken are based on the
enhancing conductivity of iron oxide electrodes through mixing iron
oxide with carbon nanotubes and graphene [44–47], doping with metal
ions [48], and also fabricating novel nanostructures [49–56]. Among
them, metal ion doping has not been thoroughly investigated. In this
paper, we report a novel platform for the preparation of metal ion
(Dy3+) doped iron oxide nanoparticles (Dy-IONPs) through the
cathodic electrodeposition procedure, and about a 20% improvement
was observed in the supercapacitive capability of IONPs as a result of
the doping. This platform is based on the well-known cathodic electrosynthesis (CE) method. According to the method, the nanostructured
oxides/hydroxides could be easily prepared through OH– electro-generation on the cathode surface [57]. It has been reported that the fine
nanoparticles of cobalt and nickel hydroxides, manganese and nickel
oxides could be fabricated through a one-pot electro-synthesis procedure [19,31,58,59]. Yet, this facile fabrication process has not been
Corresponding author.
E-mail address: maghazadeh@aeoi.org.ir (M. Aghazadeh).
http://dx.doi.org/10.1016/j.ceramint.2017.09.206
Received 2 September 2017; Received in revised form 24 September 2017; Accepted 25 September 2017
0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Aghazadeh, M., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.206
Ceramics International xxx (xxxx) xxx–xxx
M. Aghazadeh, M.R. Ganjali
applied for the preparation IONPs until now. It is worth noting that our
team has very recently reported the one-pot electrosynthesis of IONPs
through cathodic electrosynthesis (CE) [60–63]. Here, we applied a
modified CE strategy for the synthesis of Dy3+ doped IONPs, and to the
best of our knowledge, the electro-synthesis of metal ion doped
Fe3O4NPs has not been reported so far. The prepared Dy-IONPs were
characterized by XRD, FE-SEM, VSM, cyclic voltammetry (CV) and
galvanostatic charge-discharge (GCD) techniques, and the results confirmed proper magnetic and charge storage behaviors of the product.
2. Experimental procedure
2.1. Materials
Ferrous chloride tetrahydrate (FeCl2·4H2O, 99.5%), ferric nitrate
nonahydrate
(Fe(NO3)3·9H2O,
99.9%),
dysprosium
chloride
(DyCl3·6H2O 98.99%) and polyvinylidene fluoride (PVDF, (CH2CF2)n)
were procured from Sigma Aldrich. All materials were used as received,
without any purification.
Fig. 2. XRD patterns of un-doped and Dy3+ doped iron oxide NPs.
2.3. Characterization analyses
The morphological observations of the prepared powders were obtained 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 product samples were assessed in the range of −20000 to 20000 Oe
at room temperature, using vibrational sample magnetometer (VSM,
Meghnatis Daghigh Kavir Co., Iran).
2.2. Electrosynthesis of Dy3+ doped Fe3O4 NPs
In this work, the cathodic electrosynthesis (CE) platform previously
reported for the fabrication of polymer-coated iron oxide nanoparticles
(IOPNs) [63–66], was adjusted for the cathodic electro-deposition of
Dy3+ doped IONPs. A schematic graph of the synthesis set-up presented
in Fig. 1. The electrochemical cell included a steel cathode centered
between two parallel graphite anodes, as seen in Fig. 1.
The deposition solution was prepared by dissolving iron(III) nitrate
(2 g), iron(II) chloride (1 g) and dysprosium chloride (0.3 g) in 1 l of deionized water. The electrodeposition experiments were performed using
an electrochemical workstation system (Potentiostat/Galvanostat,
Model: NCF-PGS 2012, Iran) while applying a current at a density of
10 mA cm–2. The deposition time and bath temperature were 30 min
and 25 °C, respectively. After each deposition, the cathode was removed
from deposition bath and repeatedly rinsed with deionized water. Next,
the deposited black film was scraped form the steel and subjected to
separation and purification steps, as illustrated in Fig. 1;(i) the obtained
wet powder was dispersed in deionized water and centrifuged at
6000 rpm for 20 min to removal of free anions, as indicated in Fig. 1,(ii)
the deposit was then separated from water using a magnet, dried at
70 °C for 1 h, and (iii) the resulting black dry powder was labeled as DyIONPs, and used in energy storage experiments.
2.4. Electrochemical tests
Cyclic voltammetry (CV), galvanostatic charging/discharging (GCD)
and electrochemical impedance spectroscopy (EIS) were used for the
electrochemical characterization of the prepared samples. The tests
were performed using an electrochemical station (AUTOLAB®, Eco
Chemie, PGSTAT 30) and a three-electrode set-up containing a (1M)
aqueous electrolyte solution of Na2SO3. The three-electrode set-up was
composed of a working electrode (Dy3+ doped Fe3O4 nanoparticles
paste electrode), an Ag/AgCl reference electrode (saturated with 1M
KCl), and a counter electrode (platinum wire). The working electrode
(WE) was fabricated through the well-known paste procedure [11,13];
according to which, the prepared black Dy-IONPs powder was physically mixed with acetylene black (> 99.9%) and conducting graphite
(at rations of 75:10:10), and the mixture was properly homogenized.
Then, 5 wt% of polyvinylidene fluoride (PVDF), dissolved in n-methylFig. 1. The schematic graph of the cathodic electrodeposition of Dy3+-doped IONPs. The inset presents (i) the electrochemical and (ii) the chemical
steps of the deposition procedure.
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Fig. 3. (a, b) FE-SEM, (c,d) TEM images of un-doped, and (b, c)
Dy3+ doped IONPs, and (e) EDS data for Dy-IONPs sample.
2-pyrrolidone (NMP), was added to the mixture. After partially evaporating the NMP content of the mixture, the resulting paste was
pressed onto Ni foam (surface area of 1 cm2) under 10 MPa. The
resulting electrode was dried for 5 min at about 150 °C in an oven and
eventually the electrode was used as working electrode in the electrochemical tests. The mass of Dy-IONPs powder loaded onto the Ni foam
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M. Aghazadeh, M.R. Ganjali
line, and θ is the diffraction angle of the XRD pattern. From the diffraction line-width of the (311) peak, the average crystallite sizes of the
prepared undoped IONPs and Dy-IONPs were calculated to be 7.1 nm
and 13.1 nm, respectively.
It was reported that the cathodic deposition of metal hydroxides/
oxides can be illustrated as below [57,64]:
Electrochemical step:
2H2O + 2e–→ 2OH– + H2
(1)
Chemical step:
Fe
2+
+ (2-x)Fe3+ + xDy3+ + 5OH–→Fe(II)Fe(III)
(1-x)DyxO4 + 1/2H2O
(2)
As a result of the above reaction, Dy3+ cations enter the Fe3O4
crystal structure through occupation of some sites normally taken by
Fe3+ cations. Hence, Dy3+-doped Fe3O4 is formed on the cathode
surface.
Fig. 3(a,b) present the FE-SEM images of the powders prepared
through the CE platform. Both electrodeposited powders have particle
morphologies and the particle sizes in the range of 10–20 nm. TEM
images of the undoped IONPs and doped Ho-IONPs are also shown in
Fig. 3c and d., uniform particle morphologies are observed for both
IONPs. The undoped IONPs have rather smaller size than doped IONPs,
as clearly seen in Fig. 3c and d. From TEM images, it was measured that
IONPs and Dy-IONPs to have average particle size of 8 nm and 12 nm,
respectively. The elemental analyses of the prepared nanoparticles were
performed through energy-dispersive X-ray (EDX), and the results are
given in Fig. 3e. According to this data, the electro-deposited Dy-IONPs
contained the iron, dysprosium and oxygen elements at the weight
percentages of 62.9%, 9.21% and 27.89%, respectively. Considering the
fact that the Dy3+ cations are located in the sites of Fe3+ in the Fe3O4
crystal structure, these values are very close to the those of magnetite
(i.e. iron(72.36%wt) and oxygen (27.64%wt)).
The magnetic hysteresis loops for the Dy-IONPs and IONPs are
presented in Fig. 4. The data on the naked IONPs has been adapted from
previous work [67,68]. The VSM profiles exhibited no hysteresis and
have S forms, as seen in Fig. 4. This proves the superparamagnetic
performance of both powders. Table 1 lists the magnetic data of the
prepared naked, and Dy3+-doped Fe3O4 powders, which includes saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (HCi).
For the Dy-IONPs powder, the magnetic data are Ms =
46.93 emu g–1, Mr = 0.34 emu g–1 and HCi = 6.25 G. These values
confirm the superparamagnetic nature of the electrosynthesized DyIONPs. Furthermore, these magnetic data are comparable with those of
un-doped IONPs reported elsewhere [67,68], which electro-synthesized
using the same platform. For pure IONPs the values were Ms =
72.96 emu g–1, Mr = 0.95 emu g–1and HCi = 14.61 G [67,68]. Comparing the data listed in Table 1 revealed that Dy3+-doped Fe3O4
powder exhibited lower Mr and Hci values compared with un-doped
Fe3O4 powder, which implicates their improved superparamagnetic
performances. Notably, the Ms value of the doped powder was smaller
than that of un-doped powder, which is due to the low magnetic nature
of dysprosium vs. iron.
Also, the Dy-IONPs exhibit better superparamagnetic characteristics
(i.e. higher Ms and lower Mr and Hci values) as compared with those
reported in the literature (i.e. Ms = 31.3 emu g–1 and Hci = 85.7 G for
Sm3+ doped Fe3O4 powder [69], Ms = 23.6 emu g–1 and Hci = 74.3 G
Fig. 4. Hysteresis loops for un-doped and Dy3+ doped iron oxide nanoparticles.
was about 2.7 mg. CVs were recorded in a 1M Na2SO3 electrolyte in the
potential range of −1.0 to + 0.1 V vs. Ag/AgCl using the working
electrode. The CVs were recorded at the potential sweeps of 2, 5, 10, 20,
50 and 100 mV s–1.
GCD curves were recorded at the different current loads of 0.5, 1, 2,
3, 5, 7 and 10 A g–1 within a potential range of −1.0–0 V vs. Ag/AgCl.
The EIS was conducted in the frequency range between 100 kHz and
0.01 Hz while applying 5 mV at open-circuit potential.
3. Results and discussion
3.1. Structural and morphological characterizations
Fig. 2 shows the XRD patterns of un-doped and Dy3+-doped electrosynthesized Fe3O4 powders. All observed diffraction peaks in both XRD
patterns could be readily referred to pure cubic spinel phase of iron
oxide (JCPDS 01–074–1910). No extra peaks which could be attributed
to hematite and dysprosium oxide/hydroxide were observed in the XRD
pattern, indicating the purity of the electro-synthesized Dy-IONPs.
These observations implicate that the magnetite structure is formed on
the steel surface at the applied CE conditions. Furthermore, the formation of Dy-IONPs during the CE process implicated that Dy3+ cations
play the same role as the Fe3+ cations on the cathode surface. Notably,
small shifts in all peak positions were observed for the Dy3+-doped
powder as compared with the un-doped one. These changes are due to
the substitution of Fe3+ cations in the magnetite lattice with the larger
Dy3+ ions [69–72]. Notably, Fe3O4 has a cubic inverse spinel ferrite
structure with only Fe3+ cations in the octahedral sites and one Fe2+
ion and Fe3+ cations in the two tetrahedral sites [41,42]. From the XRD
pattern of the Dy3+-doped Fe3O4 powder in Fig. 2, two important observations can be made, namely that (i) no specific distortion is seen for
the Dy3+-doped Fe3O4 powder, and (ii) this pattern very well matches
the reference pattern of magnetite.
The average crystallite size (D) of the Dy-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
Table 1
Magnetic data of the un-doped and Dy3+ doped iron oxide nanoparticles.
Sample name
Ms(emu/g)
Coercivity (Hci)G
Positive (Hci) G
Negative (Hci) G
Negative Mr(emu/g)
Positive Mr(emu/g)
Retentivity Mr(emu/g)
Un-doped IONPs [65]
Dy3+ doped IONPs
72.96
46.93
14.6
6.25
−41.87
−29.23
−12.66
−16.21
0.83
0.88
2.73
1.56
0.95
0.34
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Fig. 5. BET and BJH pore size distribution curves for (a, b) the undoped and (b) Dy3+-doped IONPs powders.
for Eu3+ doped Fe3O4 powder [70], Ms = 32.9 and 28.9 emu g–1 at 100
and 300 K for Gd3+ doped Fe3O4 powder [71], Ms = 53.2 emu g–1 for
Cu2+ doped Fe3O4 NPs [72], and Ms = 61.5 emu g–1 for Mn2+ doped
Fe3O4 NPs [72]). So, it was concluded that Dy3+-doping improved the
superparamagnetic behavior of the magnetite nanoparticles.
Surface area is one of key factors determining the supercapacitive
performance of the any SC materials. Hence, N2 adsorption/desorption
profiles of both undoped and Dy-doped powders were measured and
presented in Fig. 5. The N2 profiles of both powders have typical type II
form with a type H3 hysteretic loop, indicating the presence of microporous natures according to the IUPAC classification [15]. The plots of
the pore-size distribution (Fig. 4b) were obtained using the desorption
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Fig. 6. (a) CVs of the Dy-IONP-based working electrode at the various scan rates, (b) CV profiles for the un-doped and Dy3+-doped IONPs at the scan rate of 5 mV/s, (c) CVs of the
undoped IONP- at the various scan rates and (d) the calculated specific capacitances for both electrodes vs. scan rate.
Fig. 7. (a) GCD profiles of Dy-IONP electrodes and (b) the calculated SCs at the different current loads of 0.25–10 A g–1.
3.2. Electrochemical evaluation
branch of BET curves using the Barrett–Joyner–Halenda (BJH) method.
The mean pore diameter and the surface area were estimated to be
1.06 nm and 71.5 m2 g–1 (for undoped IONPs) and 1.04 nm and 81.
2 m2 g–1 (for Dy-doped IONPs).
3.2.1. Cyclic voltammetry
The charge storage abilities of the working electrodes (WE)
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However, it is seen that the Dy3+ doped IONPs electrode deliver greater
total anodic and cathodic charges, and hence have better capacitance.
The Cs values of both working electrodes were calculated from their
CV profiles (Fig. 6a and c) using Eq. (5) [43]:
Table 2
The electrochemical capacitance values reported for nano-structured iron oxide.
Fe3O4 structure
type
Electrolyte
Nanocrystal
Nanoparticles
Nanowires
Nanoparticles
Pyrrole treated
nanowires
Nanosheets
Pristine nanospheres
Mn2+ doped
Microspheres
Spheroidal
nanoassembles
Nanoparticles
Particle
Thin film
Octadecahedrons
Sm3+doped
nanoparticles
Dy3+doped
nanoparticles
un-doped
nanoparticles
Current
density (A/
g)
Specific
capacitance (F/
g)
Ref.
Cs (F g−1) =
3M KOH
1M Na2SO3
0.1M Na2SO3
0.1M Na2SO3
0.1M Na2SO3
1MA
0.4
0.4
0.4
0.4
185
207.7
106
12
190
[37]
[39]
[45]
[45]
[45]
1M Na2SO3
1M Na2SO3
1M KOH
0.42
1
1
83
157.64
268.4
[46]
[47]
[48]
1M Na2SO4
0.5
48
[52]
6M
3M
1M
1M
1M
1
0.1
0.5
0.6
0.5
160
~ 120
118.2
207
[53]
[54]
[55]
[56]
[68]
1M Na2SO3
0.25
228
1M Na2SO3
0.25
188
KOH
KOH
Na2SO3
Na2SO3
Na2SO3
Q
,Q =
m ʋΔV
∫V
Va
I (V ) dV
(5)
a
–1
where Cs is the specific capacitance of the WE (F g ), Q is the total
charge, ΔV is the potential sweep range, m is the mass of IONPs powders (g), v is the sweep rate (V s–1) and I(V) is the current response
during the potential scan. Next, the Cs values were plotted vs. the scan
rates, as shown in Fig. 6d. The calculations revealed that the Dy3+doped electrodes exhibited Cs values as high as 218, 190, 168, 136, 109,
95 and 79 F g–1 at the scan rates of 2, 5, 10, 20, 50, 75 and 100 mV s–1,
respectively. Also, it was found that the un-doped IONPs are enable to
deliver Cs 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 [65]. Comparing
these Cs values revealed that the Dy3+-doped Fe3O4 NPs provide up to
20% larger Cs values, as opposed to those of un-doped NPs. In other
word, it can be understood that the supercapacitive performance of iron
oxide is increased through doping Dy3+. Furthermore, the SC data
confirmed the proper charge storage abilities of the electro-synthesized
Dy3+-doped IONPs for use in supercapacitors.
This
work
This
work
3.2.2. Charge-discharge tests
Galvanostatic charge-discharge (GCD) profiles of Dy3+-doped
IONPs were recorded at current loads of 0.25, 0.5, 1, 2, 3, 5 and
10 A g–1 and are illustrated in Fig. 7a.
These GCD profiles exhibit two different parts including, (i) a
symmetric triangular section at potential range of V < − 0.4 V vs. Ag/
AgCl, and (ii) a nonlinear part at the range of V ≥ − 0.4 V vs. Ag/AgCl.
The pure EDLC behavior of IONPs electrodes is confirmed by the first
part and the second part implicates the faradic performance arising
from the redox reactions of the sulfide ion. The Cs values were calculated using Eq. (6) [11], and the data is presented in Fig. 7b:
fabricated using the un-doped and Dy3+-doped Fe3O4 nano-powders
were evaluate using cyclic voltammetry and the obtained results were
compared with each other. Fig. 6a illustrates the CV profiles obtained
using the WE in the potential range of −1.0 to + 0.2 V vs. Ag/AgCl
using scan rates of 2–100 mV s–1. The shapes of the CV curves clearly
reveal the pseudo-capacitive characteristics of the Dy-IONPs, which is
different from the electric double-layer capacitance. According to the
literature, a combination of both EDLC and faradic reactions has been
reported for the capacitance behavior of iron oxide electrodes in the
Na2SO3 electrolyte [40–43]. The Faradic performance of iron oxide
results from the reduction/oxidation of adsorbed SO32– anions on the
iron oxide surface [40,41]:
Cs (F g −1) =
I ×Δt
m ×∆V
(6)
–1
2SO32‒ + 3H2O + 4e– ↔ S2O32‒ + 6OH–
(3)
S2O32‒ + 3H2O + 8e– ↔ 2S– + 6OH–
(4)
where Cs is the capacitance of fabricated iron oxide electrode (F g ),
ΔV is the discharge potential range, m is the mass of Dy-IONPs powder
(g), I is the discharge current load (A), and Δt is the time of a discharge
cycle. The calculations revealed that the electrodes fabricated from
Dy3+ doped IONPs are capable of delivering Cs values of 228 F g−1,
202 F g−1, 181 F g−1, 156 F g−1, 133 F g−1, 111 F g−1, 92 F g−1 and
74 F g−1 at the respective discharge loads of 0.25, 0.5, 1, 2, 3, 5, 7 and
Fig. 6b shows the CVs of both un-doped and doped IONPs obtained
at the scan rate of 5 mV/s. For both electrodes, some small peaks are
observed on CVs as a result of the above-mentioned redox reactions.
Fig. 8. (a) Calculated specific capacity values and (b) capacity retentions during 2000 GCD cycling at the discharge loads of 0.5 and 5 A g–1.
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electrodes were recorded in the frequency range of 1–105 Hz, and the
resulting Nyquist plots are presented in Fig. 9. The intercept on the xaxis of the Nyquist plots (Fig. 9) in the low-frequency region represents
the electronic resistance of the electrochemical system, also known as
the equivalent series resistance (ESR), denoted as RS. For un-doped and
Dy3+-doped electrodes, the RS values were determined to be 3.06 Ω
and 2.83 Ω, respectively. Furthermore, both plots exhibit a nearly
vertical line in the low frequency region representing the diffusion of
ions to the interface of electrode/electrolyte and the double layer capacitive behavior of the IONPs electrodes. It is well-known that a
straight line close to 90◦ in Nyquist plot implicates low ions diffusion
resistance and a pure EDL capacitive behavior [49]. From the enlarged
zone in the inset of Fig. 9, it can be observed that Dy-IONPs electrode
has a relatively better straight line shape (closer to 45◦) as compared
with that of un-doped electrode, indicating its better EDL behavior. At
the high-frequency range, Nyquist plots of both electrodes are deviated
from the linear shape, which show the change in the capacitive behavior of both electrodes (i.e. changing from EDL to faradic behavior).
Generally, the deviation from the ideal EDL behavior is attributed to the
presence of resistance with a Warburg impedance element, which is
denoted by W, and low charge transfer resistance (RCT) [36,50]. The
values of RCT were measured to be 3.29 Ω and 2.99 Ω for undoped and
Dy-doped electrodes (inset in Fig. 9), indicating the low charge transfer
resistance of doped electrode. This low RCT value may result from the
change in crystal structure of magnetite as a result of Dy3+ introducing.
Furthermore, for Dy-IONPs, this deviation is rather more as compared
with the un-doped electrode and hence, it is expected that the doped
iron oxide exhibits a better pseudocapacitive performance, as confirmed by CV and GCD results.
Fig. 9. The Nyquist plots of the fabricated IONP and Dy-IONP electrodes using the suggested equivalent circuit model.
10 A g−1. These are close to those calculated based on the CVs (Fig. 6b),
confirming the excellent super-capacitive behavior for the electro-synthesized Dy3+-doped Fe3O4 nanoparticles. Furthermore, the Cs values
of the electrodes are comparable with the values reported for iron
oxide, reported in the literate. Table 2 provides a list of the specific
capacitance values observed for nanostructured iron oxide electrodes
until now.
From the Cs values listed in Table 2, it is found that the supercapacitive performance of the iron oxide electrode could be enhanced
through metal ion doping (e.g. Mn [48], Sm [68] and Dy [this work]).
Furthermore, Cs value of Dy-IONPs is higher than those reported for
pure iron oxide electrodes.
The fabricated working electrode was further cycled (2000 cycles)
at the current loads of 0.5 and 5 A g−1 in a 1M Na2SO3 electrolyte. The
values of Cs and capacity retentions for the fabricated Dy-IONPs
powder were calculated during cycling. Fig. 8a and b represent the Cs
values and their retentions vs. cycle number, respectively. It was found
that the Cs value of Dy3+-doped Fe3O4 powder was reduced from
202 F g−1 to 189 F g−1, after 2000 GCD cycles at a discharging current
of 0.5 A g−1 (Fig. 8a). This exhibits a capacity retention of about
93.9%, as can be seen in Fig. 8b. Also, the fabricated Dy3+-doped NPs
can deliver Cs value as high as 86 F g−1 after 2000 GCD cycles at a
current load of 5 A g−1, which shows the electrodes as having capacity
retentions of 77.2% under these discharge conditions (Fig. 8b). These
data confirmed the proper charge storage ability of the Dy3+-doped
Fe3O4 powder.
The corresponding specific energy and power densities of the fabricated iron oxide working electrode were obtained using Eqs. (7) and
(8), respectively [50]:
E (Wh kg –1) = 0.5[Cs (ΔV )2]/3.6
(7)
P (W kg –1) = (E/t)*3600
(8)
4. Conclusion
In summary, a novel and easy electrochemical procedure was developed for the fabrication of Dy3+-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 Dy3+ content of the
electrosynthesized iron oxide products. Galvanostatic charge-discharge
of the fabricated electrode materials indicated that the Dy3+-doped
Fe3O4 nanoparticles are capable of delivering specific capacitance and
specific capacitance values of 228 F g−1, 202 F g−1, 181 F g−1,
156 F g−1, 133 F g−1, 111 F g−1, 92 F g−1and 74 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 the magnetite nanoparticles is enhanced thought Dy3+-doping.
Conflict of interest
The authors declare that there is no conflict of interest.
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