molecules

Article
Magnetic CuFe2O4 Nanoparticles with Pseudocapacitive
Properties for Electrical Energy Storage
Wenyu Liang , Wenjuan Yang, Sadman Sakib and Igor Zhitomirsky *

Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S4L7, Canada
* Correspondence: zhitom@mcmaster.ca

Abstract: This investigation is motivated by increasing interest in the development of magnetically

ordered pseudocapacitors (MOPC), which exhibit interesting magnetocapacitive effects. Here, ad-
vanced pseudocapacitive properties of magnetic CuFe2 O4 nanoparticles in negative potential range
are reported, suggesting that CuFe2 O4 is a promising MOPC and advanced negative electrode mate-
rial for supercapacitors. A high capacitance of 2.76 F cm−2 is achieved at a low electrode resistance in
a relatively large potential window of 0.8 V. The cyclic voltammograms and galvanostatic charge–
discharge data show nearly ideal pseudocapacitive behavior. Good electrochemical performance
is achieved at a high active mass loading due to the use of chelating molecules of ammonium salt
of purpuric acid (ASPA) as a co-dispersant for CuFe2 O4 nanoparticles and conductive multiwalled
carbon nanotube (MCNT) additives. The adsorption of ASPA on different materials is linked to
structural features of ASPA, which allows for different interaction and adsorption mechanisms.
The combination of advanced magnetic and pseudocapacitive properties in a negative potential
range in a single MOPC material provides a platform for various effects related to the influence of
pseudocapacitive/magnetic properties on magnetic/pseudocapacitive behavior.

Citation: Liang, W.; Yang, W.; Sakib,

Keywords: spinel; dispersant; copper; iron; oxide; supercapacitor; magnetic
S.; Zhitomirsky, I. Magnetic CuFe2 O4
Nanoparticles with Pseudocapacitive
Properties for Electrical Energy
Storage. Molecules 2022, 27, 5313. 1. Introduction
https://doi.org/10.3390/ The ability to combine advanced electrical and magnetic properties in a single ma-
molecules27165313 terial holds great potential for the development of novel devices based on the control of
Academic Editors: Yucheng Lan and electrical/magnetic properties in magnetic/electric fields. Various materials combining
Jin Jia ferroelectric and magnetic properties [1], also called multiferroics [2], have been developed.
Materials of different types such as perovskites, boracites, hexagonal manganates and
Received: 6 July 2022
materials of the BaMeF4 (Me = Mn, Fe, Co, Ni) and hexagonal BaTiO3 families have been
Accepted: 18 August 2022
investigated [1]. Interesting physical phenomena were observed in such materials, such as
Published: 20 August 2022
linear and nonlinear magnetoelectric effects, anomalies of dielectric/magnetic properties
Publisher’s Note: MDPI stays neutral near magnetic/ferroelectric phase transition temperatures, polarization reversal in a mag-
with regard to jurisdictional claims in netic field and magnetization reversal in an electric field [1]. However, it is challenging to
published maps and institutional affil- achieve a combination of ferroelectric and ferri- or ferromagnetic properties in a single crys-
iations. talline phase at room temperature [1]. High electrical resistivity is important to achieving
ferroelectric polarization in an electric field. Many multiferroic materials exhibit relatively
low resistivity, and investigation into their ferroelectric polarization presents difficulties. A
high dielectric constant is usually observed in soft ferroelectrics at low voltages; an increase
Copyright: © 2022 by the authors.
in applied voltage results in a significant reduction in the dielectric constant. Various
Licensee MDPI, Basel, Switzerland.
multiferroic materials, such as BiFeO3 , exhibit antiferromagnetic properties at room temper-
This article is an open access article
ature, while other materials show weak ferri- or ferromagnetism at low temperatures [1].
distributed under the terms and
conditions of the Creative Commons
Oxide materials offer benefits of higher resistivity; however, room-temperature ferroelec-
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tricity has not been observed in advanced magnetic oxides, such as spinels, garnets or
creativecommons.org/licenses/by/ hexagonal ferrites.
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Molecules 2022, 27, 5313. https://doi.org/10.3390/molecules27165313 https://www.mdpi.com/journal/molecules

Molecules 2022, 27, 5313 2 of 14

Recently, significant interest has been generated in magnetically ordered pseudoca-

pacitors (MOPC) [3], which combine advanced magnetic and electrical charge storage
properties. Pseudocapacitive properties of such materials are related to the redox reactions
of metal ions. The capacitance of pseudocapacitive materials is many orders of magnitude
larger than that of ferroelectric materials [4]. Many pseudocapacitive materials and com-
posites exhibit nearly rectangular cyclic voltammograms and linear chronopotentiometry
dependences, indicating their ideal capacitive behavior [5–7]. In contrast to ferroelectric
materials, low resistance is beneficial for charging supercapacitor materials [8]. The re-
duction in particle size of ferroelectric materials to the nanometric scale usually results
in a drastic reduction in spontaneous polarization and dielectric constant. In contrast, a
significant increase in pseudocapacitive properties is achieved in nanostructured MOPC
materials [5].
Ferrimagnetic spinels [9,10] and hexagonal ferrites [11] show promising electrical
charge storage properties based on redox reactions. Interesting phenomena have been
observed in MOPC materials, such as the enhancement of charge storage properties in
magnetic fields [12–14]. Spinel materials have shown good capacitive properties in various
aqueous electrolytes [15]. High areal capacitance has been reported for Fe3 O4 spinel
electrodes [16] in a Na2 SO4 electrolyte. CoFe2 O4 has been found to be another MOPC
material showing high capacitance in KOH and NaOH electrolytes [17–19]. NiFe2 O4 has
shown good capacitive performance in KOH electrolytes [20].
CuFe2 O4 is a promising MOPC material which exhibits ferrimagnetic properties. The sat-
uration magnetization of this material is influenced by cation distribution in tetrahedral (T) and
octahedral (O) cites [21,22] of the spinel structure [(1 − x)Cu2+ (x)Fe3+ ]T [(x)Cu2+ (2 − x)Fe3+ ]O4 .
It is expected that the reduction of Cu2+ and Fe3+ ions in the negative potential range can
result in pseudocapacitive charge storage properties. It is in this regard that Cu2+ ions can
be reduced [23–25] to Cu+ , whereas Fe3+ ions can be reduced [26–28] to Fe2+ . Electrochemi-
cal reduction of Cu2+ and Fe3+ ions can result in changes in their magnetic moments and
influence superexchange interactions of the ions distributed in (T) and (O) positions of the
crystal structure. This can potentially result in a decrease or increase in total magnetization,
which depends on the magnetization of the individual sublattices. Therefore, the investiga-
tion of capacitive properties of CuFe2 O4 in a negative potential range can potentially result
in interesting phenomena related to the influence of pseudocapacitive/magnetic properties
on magnetic/pseudocapacitive behavior.
Significant advances [29–32] in supercapacitor technology have been achieved by the
development of advanced fabrication methods which allow good utilization of fundamental
material properties using advanced design. Colloidal methods offer many benefits for
the fabrication of supercapacitor electrodes. The success in the application of colloidal
methods is inevitably related to the development of advanced dispersant molecules and
advanced dispersion mechanisms [33,34]. Colloidal strategies have a high potential for
the development of supercapacitor electrodes. However, it is challenging to disperse
ferrimagnetic nanoparticles and prevent their agglomeration due to van der Waals and
magnetic attraction forces.
The goal of this investigation is to fabricate and test pseudocapacitive properties
of CuFe2 O4 in a negative potential range. We investigate the magnetic and capacitive
properties of CuFe2 O4 nanoparticles and demonstrate that high areal capacitance can be
achieved in a negative potential range in a neutral Na2 SO4 electrolyte. The electrodes
show nearly ideal pseudocapacitive behavior. High areal capacitance in a relatively large
potential window is achieved at a low impedance, which is critical for practical applications.
The ability to achieve high areal capacitance in the Na2 SO4 electrolyte in a negative potential
range is promising for the fabrication of advanced anodes for supercapacitor devices.
The experimental results presented in this paper indicate that good capacitive behavior
can be achieved by the development of an advanced colloidal strategy for electrode fabri-
cation. The problem of the strong agglomeration of ferrimagnetic CuFe2 O4 nanoparticles
is addressed using an advanced dispersant which allows for strong tridentate chelating
 The experimental results presented in this paper indicate that good capacitive
ior can be achieved by the development of an advanced colloidal strategy for el
Molecules 2022, 27, 5313
fabrication. The problem of the strong agglomeration of ferrimagnetic CuFe2O4 n
3 of 14
ticles is addressed using an advanced dispersant which allows for strong trident
lating bonding to the metal atoms on the particle surface. We demonstrate that th
dal strategy developed in this investigation is a key factor for achieving the supe
bonding to the metal atoms on the particle surface. We demonstrate that the colloidal
pacitive behavior of CuFe2O4 nanoparticles with a record-high areal capacitance
strategy developed in this investigation is a key factor for achieving the superior capacitive
material. Moreover, the CuFe2O4 electrodes prepared using this strategy are on a p
behavior of CuFe2 O4 nanoparticles with a record-high areal capacitance for this material.
the most promising negative electrodes for asymmetric supercapacitors.
Moreover, the CuFe2 O4 electrodes prepared using this strategy are on a par with the most
promising negative electrodes for asymmetric supercapacitors.
2. Results
2. Results CuFe2O4 is a ferrimagnetic material. Magnetization versus magnetic field d
ence at a temperature
CuFe2 O4 is a ferrimagnetic of 5K
material. (Figure 1A)versus
Magnetization showed magnetic
magnetic hysteresis.
field dependence However, t
teresis was very small at 293K (Figure 1B). Magnetic measurements
at a temperature of 5K (Figure 1A) showed magnetic hysteresis. However, the hysteresis indicated th
was very small magnetic behavior
at 293K (Figure 1B).ofMagnetic
CuFe2O4. measurements
Saturation magnetization
indicated theandferrimagnetic
the coercive field of C
behavior of CuFe decreased
O
2 4 . with
Saturationincreasing temperature
magnetization and the in agreement
coercive field ofwith
CuFe other investigations [35
2 4 decreased
O
with increasing temperature in agreement with other investigations [35,36].

Figure 1. Magnetization (M)

Figure 1. versus magnetic
Magnetization (M)field (H)magnetic
versus for CuFefield
2 O4 at (A)for
(H) 5K and2O
CuFe (B) 293
4 at (A)K.5 K and (B) 293 K

Figure 2 shows TEMFigureimages of the

2 shows TEMparticles
images and
of athe
composite
particlesused
and in this investigation.
a composite used in this in
The size of the particles
tion. Thewas
sizebelow 100
of the nm. Such
particles wasparticles
below were usedSuch
100 nm. for the fabrication
particles wereofused
su- for the
percapacitor electrodes by a colloidal technique using MCNTs as conductive additives.
tion of supercapacitor electrodes by a colloidal technique using MCNTs as con The
masses of MCNTs in the CuFe
additives. O4 -MCNT
The2masses composites
of MCNTs in theCFO-0,
CuFe2CFO-10,
O4-MCNT CFO-20 and CFO-30
composites CFO-0, CFO-1
were 0, 10, 20 and20 and CFO-30 were 0, 10, 20 and 30 wt.%, respectively. It will beand
30 wt.%, respectively. It will be shown below that dispersion the below t
shown
efficient mixing persion
of CuFeandO and MCNT had a tremendous impact on the pseudocapacitive
2 4 the efficient mixing of CuFe2O4 and MCNT had a tremendous impac
properties of the CuFe2 O4 -based electrodes.
pseudocapacitive properties of the CuFe2O4-based electrodes.
It is known that nanoparticles are prone to agglomeration due to their high surface
energy and van der Waals attraction forces. Moreover, the magnetic interactions of the
CuFe2 O4 particles also promote their aggregation. Therefore, it is challenging to achieve a
good dispersion of CuFe2 O4 nanoparticles. Another challenge is the good dispersion of
MCNTs. The co-dispersion of CuFe2 O4 particles and MCNTs is critical for their efficient
mixing and fabricating composite electrodes with high conductivity. In such composites,
well-dispersed MCNTs must provide a conductive path to the CuFe2 O4 particles and
facilitate electrochemical redox reactions. As-received MCNTs formed agglomerates with a
typical size of ~500 µm. The SEM images of such agglomerated MCNTs were presented in
a previous investigation [37].
Molecules 2022, 27, x FOR PEER REVIEW
Molecules 2022, 27, 5313 4 of 14

Figure
Figure 2. 2.
TEMTEM images
images for CuFe
for (A,B) (A,B)2 OCuFe O4 and
4 and 2(C,D) (C,D)Arrows
CFO-20. CFO-20. Arrows
show MCNTs.show MCNTs.
The choice of dispersants plays a crucial role in the nanotechnology of composites. It is
importantIt istoknown that nanoparticles
find a dispersant suitable for theare proneoftoboth
dispersion agglomeration
CuFe2 O4 particlesdue andtoMC-
their high
energy
NTs. and van must
The dispersant der Waals
be stronglyattraction
adsorbedforces.
on CuFeMoreover,
2 O4 particles the magnetic
and MCNT, interaction
because
CuFe2O4 particles
non-adsorbed dispersantalsocan
promote
promote their aggregation.
agglomeration. Therefore,
Therefore, CuFe2 Oit 4 is challenging
particles and to a
MCNTs must be co-dispersed using a co-dispersant. The anchoring
good dispersion of CuFe2O4 nanoparticles. Another challenge is the good dispe groups of dispersants
play a vital role in their adsorption on inorganic particles. Recent studies [38] have shown
MCNTs. The co-dispersion of CuFe2O4 particles and MCNTs is critical for their
that monodentate bonding provides relatively weak adsorption and new types of disper-
mixing
sants haveandbeenfabricating
developed with composite electrodes
bidentate chelating with high
or bridging conductivity.
bonding. In such com
Such dispersants
well-dispersed
showed MCNTsand
superior adsorption must provide
facilitated a conductive
the development path to the
of advanced CuFe2O4 particles
nanocomposites
and film deposition technologies [38]. These studies highlighted
cilitate electrochemical redox reactions. As-received MCNTs formed the advantages of charged
agglomerate
dispersants containing chelating anchoring groups which facilitated dispersant adsorption
typical size of ~500 μm. The SEM images of such agglomerated MCNTs were p
by creating complexes with metal atoms on the particle surface [38–40]. Moreover, it was
in a previous
found investigation
that redox-active dispersants [37].
containing chelating groups could facilitate charge
The choice of dispersants
transfer between oxide particles and conductive plays a crucial
additivesrole in thecollectors
or current nanotechnology
and increaseof comp
is important to find
pseudocapacitance a dispersant
[41]. In suitable
this investigation, for the dispersion
we examined properties of of both
ASPA CuFe2O4 parti
molecules
for the co-dispersion of CuFe O particles and MCNTs. It is
MCNTs. The dispersant must be strongly adsorbed on CuFe2O4 particles and MC
2 4 known that ASPA exhibits
redox-active properties [42] in a negative potential range and forms complexes with Cu, Fe,
cause non-adsorbed dispersant can promote agglomeration. Therefore, CuFe2O4
Ni, Zn, Co and other metals [43]. The complex formation involved a tridentate bonding [44].
Itand
was MCNTs
suggestedmust be co-dispersed
that similar complexes (Figureusing 3) a
canco-dispersant.
be formed with The anchoring
Cu and Fe atoms on groups o
sants
the play
surface of a vital
CuFe O
2 4 role in their
particles. It adsorption
was found that on
the inorganic
adsorbed particles.
negatively Recent
charged ASPA studies [
molecules
shown that allowed for electrostatic
monodentate particle repulsion
bonding providesand the improved
relatively weak suspension
adsorptionstability
and new
of CuFe2 O4 particles. Moreover, a good suspension stability of MCNTs was achieved in the
dispersants have been developed with bidentate chelating or bridging bonding. S
presence of ASPA. It is suggested that ASPA adsorption on MCNTs involved hydrophobic
persants showed
interactions of the side superior adsorption
walls of the MCNTs with andbarbiturate-like
facilitated the development
rings [44] of the ASPA of advance
composites
molecules. and film deposition
The electrostatic co-dispersion of technologies [38]. These
the CuFe2 O4 particles studies
and MCNTs highlighted
facilitated
vantages of charged dispersants containing chelating anchoring groups which fa
dispersant adsorption by creating complexes with metal atoms on the particle sur
40]. Moreover, it was found that redox-active dispersants containing chelating
could facilitate charge transfer between oxide particles and conductive additive
 sorption on MCNTs involved hydrophobic interactions of the side walls of the MCNTs
with barbiturate-like rings [44] of the ASPA molecules. The electrostatic co-dispersion of
the CuFe2O4 particles and MCNTs facilitated their improved mixing. Small ASPA mole-
cules efficiently separated individual MWCNs by breaking original large agglomerates
Molecules 2022, 27, 5313 and the improved contact of CuFe2O4 particles and MCNTs was achieved (Figure 5 of 142C,D).
Previous investigations have shown that small organic dispersants facilitate carbon nano-
tube dispersion by an unzipping mechanism [45]. SEM images of CuFe2O4 electrodes
showed
theirnon-agglomerated
improved mixing. Small nanoparticles (Figure
ASPA molecules 4A). separated
efficiently The composite electrodes
individual MWCNs showedby
MCNTs dispersed
breaking originalbetween CuFe2O4 particles
large agglomerates (Figure 4B–D).
and the improved contactEDS mapping
of CuFe confirmed
2 O4 particles and the
MCNTs
uniform was achieved
distribution of(Figure
CuFe22C,D).
O4 andPrevious
MCNTs investigations have shown
in the composite that small organicInfor-
(Supplementary
dispersants facilitate carbon nanotube dispersion by an unzipping
mation, Figure S1 in Supplementary Material). A good dispersion of MCNTs mechanism [45]. was
SEM a key
images of CuFe2 O4 electrodes showed non-agglomerated nanoparticles (Figure 4A). The
factor for the enhanced contact of CuFe2O4 particles with the conductive MCNT network
composite electrodes showed MCNTs dispersed between CuFe2 O4 particles (Figure 4B–D).
and EDS
enhanced electrode performance. Figure 5 shows XRD patterns of as-received
mapping confirmed the uniform distribution of CuFe2 O4 and MCNTs in the composite
CuFe(Supplementary
2O4 and CFO-0, CFO-10, CFO-20
Information, Figure S1 and CFO-30 composite
in Supplementary electrodes.
Material). The XRD
A good dispersion of data
confirmed that composites contained CuFe O and MCNTs.
MCNTs was a key factor for the enhanced contact of CuFe2 O4 particles with the conductive
2 4

MCNT network and enhanced electrode performance. Figure 5 shows XRD patterns of
as-received CuFe2 O4 and CFO-0, CFO-10, CFO-20 and CFO-30 composite electrodes. The
XRD data confirmed that composites contained CuFe2 O4 and MCNTs.

Molecules 2022, 27, x FOR PEER REVIEW . 6 of 1

Figure 3. Adsorption of ASPA on CuFe2 O4 particles, involving chelation of surface atoms (M = Cu or Fe).
Figure 3. Adsorption of ASPA on CuFe2O4 particles, involving chelation of surface atoms (M = Cu
or Fe).

Figure4.4.SEM
Figure SEM images
images of (A)
of (A) CFO-0,
CFO-0, (B) CFO-10,
(B) CFO-10, (C) CFO-20
(C) CFO-20 and (D)and (D) CFO-30
CFO-30 electrodes.
electrodes.
Molecules 2022, 27, 5313
Figure 4. SEM images of (A) CFO-0, (B) CFO-10, (C) CFO-20 and (D) CFO-306electrodes.
of 14

Figure 5. XRD data for (a) as-received CuFe2 O4 and electrodes: (b) CFO-0, (c) CFO-10, (d) CFO-20
Figure 5. XRD data for (a) as-received CuFe2O4 and electrodes: (b) CFO-0, (c) CFO-10, (d)
and (e) CFO-30.
and (e) CFO-30.
CV studies of the CFO-0 electrodes showed very low currents, indicating poor pseu-
docapacitive
CV studiesproperties (Figure 6A).
of the CFO-0 The low currents
electrodes showed andvery
smalllow
CV areas were observed
currents, indicating p
due to the low electronic conductivity of CFO-0. High electronic conductivity is one of the
docapacitive
key factors properties (Figure
for the efficient 6A). Theof low
charge–discharge currents and
pseudocapacitive smallThe
materials. CV areas were
addition
due to the low electronic
of conductive MCNT and conductivity of CFO-0.
the efficient co-dispersion High
of CuFe 2 Oelectronic
4 and MCNTconductivity
resulted in is o
the capacitive behavior of CFO-10, CFO-20 and CFO-30 electrodes
key factors for the efficient charge–discharge of pseudocapacitive materials. The (Figure 6B–D). CFO-10,
CFO-20 and CFO-30 electrodes showed nearly rectangular-shape CVs, indicating good
of conductive
pseudocapacitiveMCNT andand
behavior thefast
efficient co-dispersionreactions.
kinetic charge–discharge of CuFe 2O4 and MCNT re
The observed in-
the capacitive behavior
crease in current of CFO-10,
with increasing scan rateCFO-20
was anotherand CFO-30
indicator electrodes
of good (Figure 6B–
pseudocapacitive
properties of the electrodes.
10, CFO-20 and CFO-30 electrodes It is suggested that capacitive properties of CFO in
showed nearly rectangular-shape CVs, indicatthe negative
potential range can result from Cu2+ reduction [23–25] to Cu+ , and Fe3+ reduction [26–28]
pseudocapacitive
to Fe2+ . Figure 6E behavior and fastversus
shows capacitance kineticscancharge–discharge reactions.
rate dependencies. CFO-0, CFO-10, The obs
crease in current
CFO-20 and CFO-30 with increasing
electrodes showedscan rate was
capacitances another
of 0.04, indicator
2.75, 2.76 of good
and 2.48 F cm − 2 at a pseud
scan rate of 2 mV s −1 and capacitance retention of 25, 17.8, 25.7 and 19.8% at a scan rate of
tive properties of the electrodes. It is suggested that capacitive properties of CF
100 mV s−1 . CV data indicated that CFO-20 electrodes exhibited the best pseudocapacitive
performance. Impedance spectroscopy data showed high imaginary part of impedance for
CFO-0, which was due to low capacitance (Figure 7A). The high real part of the complex
impedance indicated high electrical resistance.
The addition of MCNT resulted in a significant decrease in the imaginary part of the
impedance and an increase in the slope of the Nyquist plot, which indicated improved
capacitive behavior (Figure 7B). Moreover, a significant reduction in the real part of the
impedance showed reduced electrode resistance. The CFO-20 electrodes showed lower
resistance compared to other electrodes.
 reduction [26–28] to Fe . Figure 6E shows capacitance versus scan rate dependencies.
CFO-0, CFO-10, CFO-20 and CFO-30 electrodes showed capacitances of 0.04, 2.75, 2.76
and 2.48 F cm−2 at a scan rate of 2 mV s−1 and capacitance retention of 25, 17.8, 25.7 and
19.8% at a scan rate of 100 mV s−1. CV data indicated that CFO-20 electrodes exhibited the
best pseudocapacitive performance. Impedance spectroscopy data showed high
Molecules 2022, 27, 5313 imaginary part of impedance for CFO-0, which was due to low capacitance (Figure 7A). 7 of 14
The high real part of the complex impedance indicated high electrical resistance.

Figure
Figure6.6.(A–D)
(A–D)CVs at scan
CVs rates
at scan of (a)
rates of 5,
(a)(b)
5,10
(b)and
10 (c)
and20(c)
mV 20s−1
mV −1 for
fors(A) CFO-0, (B) CFO-10,
(A) CFO-0, (C)
(B) CFO-10,
CFO-20 and (D) CFO-30 electrodes; (E) capacitance versus scan rate for (a) CFO-0, (b) CFO-10,
(C) CFO-20 and (D) CFO-30 electrodes; (E) capacitance versus scan rate for (a) CFO-0, (b) CFO-10, (c)
CFO-20 and (d) CFO-30 electrodes.
(c) CFO-20 and (d) CFO-30 electrodes.

Figure 7. (A,B) Nyquist plots of impedance; (C) real and (D) imaginary part of complex
capacitance derived from the impedance data versus frequency for (a) CFO-0, (b) CFO-10, (c) CFO-
20 and (d) CFO-30 electrodes.

The addition of MCNT resulted in a significant decrease in the imaginary part of the
Figure 7. (A,B) Nyquist plots of impedance; (C) real and (D) imaginary part of complex capacitance
impedance and an increase in the slope of the Nyquist plot, which indicated improved
derived from the impedance data versus frequency for (a) CFO-0, (b) CFO-10, (c) CFO-20 and
capacitive behavior (Figure 7B). Moreover, a significant reduction in the real part of the
(d) CFO-30 electrodes.
impedance showed reduced electrode resistance. The CFO-20 electrodes showed lower
resistance compared
The CFO-0, to other
CFO-10, electrodes.
CFO-20 and CFO-30 electrodes showed capacitances (C 0 ) of 0.01, S
1.11, 1.04, 0.94 F cm−2 , respectively, at a frequency of 10 mHz (Figure 7C). The CFO-20
electrode showed the highest CS 0 at frequencies above 30 mHz. The CFO-10, CFO-20 and
CFO-30 electrodes showed a relaxation-type frequency dispersion of CS 0 . The relaxation
frequencies for CFO-10, CFO-20 and CFO-30 electrodes, corresponding to CS 00 maxima,
were found to be 0.05, 0.11 and 0.094 Hz, respectively (Figure 7D). The highest CS 0 at
frequencies above 30 mHz and the highest relaxation frequency of the CFO-20 electrode
indicated its improved performance at high charge–discharge rates in agreement with CV
data at different scan rates for the same electrode. It should be noted that capacitance
obtained from the CV data in a wide potential range (−0.8–0 V) was influenced by a scan
 and CFO-30 electrodes showed a relaxation-type frequency dispersion of CS’. The relaxa-
tion frequencies for CFO-10, CFO-20 and CFO-30 electrodes, corresponding to CS’’ max-
ima, were found to be 0.05, 0.11 and 0.094 Hz, respectively (Figure 7D). The highest CS’ at
frequencies above 30 mHz and the highest relaxation frequency of the CFO-20 electrode
indicated its improved performance at high charge–discharge rates in agreement with CV
Molecules 2022, 27, 5313
data at different scan rates for the same electrode. It should be noted that capacitance8 ob- of 14

tained from the CV data in a wide potential range (−0.8–0 V) was influenced by a scan
rate, whereas the real part of capacitance derived from the impedance data using a low
rate, whereas
amplitude the real(5
AC voltage part
mV)ofdepended
capacitance onderived from the impedance data using a low
frequency.
amplitude AC voltage
The capacitive (5 mV) of
behavior depended on frequency.
the electrodes was also analyzed using galvanostatic
The capacitive
charge–discharge behavior
data of thecurrent
at different electrodes was also
densities analyzed
(Figure using
8). The galvanostatic
increase charge–
in the MCNT
discharge
content data at
resulted indifferent current
the improved densities
shape of the (Figure 8). The increase
charge–discharge curves,inwhich
the MCNT
were content
nearly
resulted in(Figure
triangular the improved
8A–C) forshape
CFO-20of the charge–discharge
and curves, which
CFO-30. The capacitances, were nearly
obtained at 3 mAtriangu-
cm−2
lar (Figure 8A–C) for CFO-20 and CFO-30. The capacitances, obtained at 3 mA cm −2 were
were 2.03, 1.76 and 1.62 F cm for CFO-10, CFO-20 and CFO-30 electrodes, respectively.
−2

2.03, 1.76 and 1.62 F cm−2 for CFO-10, CFO-20 and CFO-30 electrodes, respectively.

(A–C)galvanostatic
Figure8.8.(A–C)
Figure galvanostaticcharge
chargedischarge
discharge data
data atat current
current densities
densities of of
(a)(a) 3, (b)
3, (b) 5, (c)
5, (c) 7, (d)
7, (d) 10,10,
(e) 20 and (f) 40 mA cm −2 for (A) CFO-10, (B) CFO-20 and (C) CFO-30 electrodes; (D) capacitance
(e) 20 and (f) 40 mA cm for (A) CFO-10, (B) CFO-20 and (C) CFO-30 electrodes; (D) capacitance
−2

versuscurrent
versus currentdensity
densityfor
for(a)
(a)CFO-10,
CFO-10,(b)
(b)CFO-20
CFO-20and
and(c)
(c)CFO-30
CFO-30electrodes.
electrodes.

ThisFigure
This Figure99shows
showscyclic
cyclicstability
stabilitydata
datafor
forthe
theCFO-20
CFO-20electrodes.
electrodes.The Theincrease
increaseinin
capacitance during the first 200 cycles can be attributed to morphology
capacitance during the first 200 cycles can be attributed to morphology changes during changes during
cycling. A similar increase in capacitance during initial cycling due to internal electrode
cycling. A similar increase in capacitance during initial cycling due to internal electrode 9 of 14
Molecules 2022, 27, x FOR PEER REVIEW
microstructure changes
microstructure changes was
wasobserved
observedinin other materials
other [46–49].
materials The capacitance
[46–49]. decreased
The capacitance de-
after about 1000 cycles, the capacitance retention after 3000 cycles was 81%. The
creased after about 1000 cycles, the capacitance retention after 3000 cycles was 81%. The capacitance
decrease can
capacitance be attributed
decrease can betoattributed
the partialtocorrosion of the
the partial surfaceofofthe
corrosion thesurface
electrode
of material.
the elec-
trode material.

Capacitance retention
Figure9.9.Capacitance
Figure retentionfor
forCFO-20
CFO-20electrodes.
electrodes.

Table 1 compares the experimental results of this work with literature data of other
investigations. Capacitive properties of CuFe2O4 were mainly investigated in a positive
potential range [50–54] or relatively narrow negative and positive potential ranges [55].
Molecules 2022, 27, 5313 9 of 14

Table 1 compares the experimental results of this work with literature data of other
investigations. Capacitive properties of CuFe2 O4 were mainly investigated in a positive
potential range [50–54] or relatively narrow negative and positive potential ranges [55].
The charging mechanism in the positive potential range [51] involved the decomposition of
CuFe2 O4 to form individual oxides CuO and Fe2 O3 . It is not clear if a reverse reaction at
room temperature can result in the synthesis of CuFe2 O4 . CV data showed well-defined
redox peaks indicating battery-type behavior [50–54,56] in a relatively narrow potential
window. The galvanostatic charge–discharge curves deviated significantly [50–53] from
the ideal triangular shape of the capacitor materials. CuFe2 O4 electrodes were tested in
KOH [50–52,54,55] and H2 SO4 [53] electrolytes. CuFe2 O4 -based electrodes showed high
resistance, which is detrimental for the development of electrodes and devices with high
power density [50,52,53].

Table 1. Properties of CuFe2 O4 electrodes.

Mass Capacitance Scan Rate or Potential Cyclic Cycle

Electrolyte Reference
Loading or Capacity Current Density Range Stability Number
*1940 Fg−1 0–0.6 V vs.
§ 1 Ag−1 6 M KOH 98% 10,000 [50]
1164 Cg−1 Ag/AgCl
−0.1–+0.5 V vs.
§ *334 Fg−1 0.6 Ag−1 1 M KOH 88% 600 [51]
SCE
−0.1–+0.6 V vs.
§ *189.2 Fg−1 0.5 Ag−1 2 M KOH 84% 1000 [52]
Ag/AgCl
3 0.15–0.75 V vs.
*437.3 Fg−1 0.004 Vs−1 0.5 M H2 SO4 88.6% 2000 [53]
mg cm−2 SCE
0.3 2 –0.25–0.35 V
250.8 Fg−1 3 M KOH >90% 1000 [55]
mg cm−2 mVs−1 vs. Ag/AgCl
40 2.76 F cm−2 2 −0.8–0 V vs. 0.5 M
81% 3000 This work
mg cm−2 (69 Fg−1 ) mVs−1 SCE Na2 SO4
§ not presented in the reference; * battery-type behavior.

In contrast to previous investigations, which reported battery-type behavior in the

positive potential range, in our investigation we observed pseudocapacitive behavior in
the negative potential range. Good capacitive behavior was achieved at a high active mass
of 40 mg cm−2 . Recent studies [5,57] have highlighted the need for the development of
efficient electrodes with active mass loading above 10–20 mg cm−2 for practical applications.
Investigations have also revealed [5] the significant influence of active material mass
loading on mass normalized capacitance, which reduced by 2–3 orders of magnitude with
increasing active mass from several µg cm−2 to the level of 10–20 mg cm−2 . High active
mass loading is important for reducing the contribution of inactive components to the total
mass of the electrodes and devices. However, it is challenging to achieve good material
performance at high active mass. The approach developed in this investigation allowed
for high areal capacitance in a Na2 SO4 electrolyte. In this investigation, MCNTs were
used as conductive additives. It is known that MCNTs exhibit low capacitance [58] due to
their low surface area, compared with activated carbons, graphene and other advanced
carbon materials. Due to the low gravimetric capacitance of pure MCNTs, the reduction
in capacitance with the increasing active mass of the composite material, and the low
MCNT content of the composites (e.g., 10% in CFO-10), the contribution of the double layer
capacitance of MCNTs to total capacitance was negligibly small and the pseudocapacitive
properties of the composites were mainly attributed to the pseudocapacitive properties of
the CuFe2 O4 material.
Despite the use of electrodes with high active mass, the electrode resistance was
significantly lower than the resistance of the CuFe2 O4 electrodes reported in previous
investigations [50,52,53]. The ability to achieve high capacitance at a low resistance in a
Molecules 2022, 27, 5313 10 of 14

relatively large voltage window is promising for the development of devices with enhanced
power–energy characteristics. Comparison with the literature data [5] indicated that the
areal capacitance of the CuFe2 O4 -based electrodes is on par with the best negative electrodes
for operation in an environmentally friendly Na2 SO4 electrolyte. Areal capacitance is an
important parameter for matching negative and positive electrodes and the optimization
of device performance. The pseudocapacitive properties of CuFe2 O4 observed in this
investigation coupled with the advanced magnetic properties of this material make it
a promising MOPC material. It is suggested that the pseudocapacitive properties of
CuFe2 O4 in the negative potential range are related to the reduction in Cu2+ and Fe3+ ions.
The reduction process can result in changes in material magnetization. Therefore, the
results of this work provide a platform for the investigation of phenomena related to the
relationship between magnetic and pseudocapacitive properties of MOPC materials. It
should be noted that the electrodes developed in this investigation cannot be used for the
observation of magnetocapacitive phenomena. One of the key factors for the observation
of pseudocapacitive properties of CuFe2 O4 is related to the use of advanced - Ni foam
current collectors which exhibit high corrosion resistance, high conductivity and high
porosity, and facilitate the fabrication of electrodes with high active mass and low contact
resistance. However, Ni is a ferromagnetic material with relatively high magnetization.
It is magnetized under the influence of an external magnetic field and creates its own
magnetic field. Therefore, the use of magnetic current collectors must be avoided. An
important challenge is the development of special non-magnetic current collectors with
high conductivity, high corrosion resistance and high porosity, which will allow for the
good utilization of pseudocapacitive properties of CuFe2 O4 . Other challenges are related
to Lorentz forces, which can influence the ion diffusion and current response.

3. Materials and Methods

CuFe2 O4 nanopowder (particle size <100 nm), ASPA, Na2 SO4 and polyvinyl butyral-
co-vinyl alcohol-co-vinyl acetate (PVBAA, Mw = 65 kDa) were purchased from Millipore
Sigma (Oakville, ON, Canada). Multiwalled carbon nanotubes (MCNT, diameter 13 nm,
length 1–2 µm) were supplied by Bayer Corp (Leverkusen, Germany.
Suspensions containing CuFe2 O4 , conductive MCNT additives and ASPA as a co-
dispersing agent were prepared under probe sonication for 5 min. The mass of ASPA was
15% of the total mass of CuFe2 O4 and MCNT. The content of individual components, such
as commercial MCNT and commercial CuFe2 O4 , in the composites was varied by their
mixing in a desired ratio. The masses of MCNT in the CuFe2 O4 -MCNT composites CFO-0,
CFO-10, CFO-20 and CFO-30 were 0, 10, 20 and 30 wt.%, respectively. The electrodes
were fabricated by impregnating Ni foam (95% porosity, Vale, Mississauga, ON, Canada)
current collectors with ethanol slurries containing CuFe2 O4 , MCNT and PVBAA as the
binder. The binder content was 3% of the total mass of CuFe2 O4 and MCNT. The thickness,
mass loading and area of the electrodes were 0.38 mm, 40 mg cm−2 and 1 × 1 cm2 ,
respectively. The structures and morphologies of the composites were characterized by
scanning electron microscopy (SEM, JEOL JSM-7000F, Austin, TX, USA), transmission
electron microscopy (TEM, Talos 200X, Thermo Fisher Scientific, Waltham, MA, USA) and X-
ray diffraction analysis (Bruker Smart 6000 X-ray diffractometer, Bruker, Billerica, MA, USA,
CuKα radiation). The magnetic measurements were performed using a Quantum Design
Magnetic Properties Measurement System (MPMS, San Diego, CA, USA,). Electrochemical
impedance spectroscopy (EIS) and cyclic voltammetry (CV) investigations were conducted
using a potentiostat (AMETEK 2273, Berwyn, US). Galvanostatic charge discharge (GCD)
was performed by Biologic AMP 300 (Biologic, Willow Hill, IL, USA). The electrochemical
analysis was performed in a three-electrode setting, with a large surface area Pt gauze and
a saturated calomel electrode (SCE) as the counter and reference electrodes, respectively.
The electrodes were analyzed in a potential range between −0.8 and 0 V in 0.5 M Na2 SO4
aqueous electrolyte. The areal (Cs)- and gravimetric (Cm)-specific capacitances were
calculated from CV, EIS and GCD data, as described in previous investigations [5,59,60].
Molecules 2022, 27, 5313 11 of 14

The capacitance was calculated from the cyclic voltammetry (CV) data:
R t(Umax) R0
∆Q 0 Idt + t(Umax ) Idt
C= = (1)
∆U 2Umax
where ∆Q denotes charge, I denotes current and ∆U denotes the potential range, and from
the chronopotentiometry data:
C = I∆t/∆U (2)
The differential complex capacitance C*(ω) = C0 (ω) − iC00 (ω) was calculated at differ-
ent frequencies (ω) from the complex impedance Z*(ω) = Z0 (ω) + i Z00 (ω) data:

−Z00 (ω)
C0 (ω ) = (3)
ω|Z (ω)|2

Z0 (ω )
C00 (ω) = (4)
ω|Z (ω)|2
Following the goal of this investigation, we analyzed capacitive properties by cyclic
voltammetry, impedance spectroscopy and galvanostatic charge–discharge methods and
demonstrated the good cyclic stability of the CuFe2 O4 electrodes.

4. Conclusions
This investigation revealed the pseudocapacitive properties of CuFe2 O4 . A high areal
capacitance of 2.76 F cm−2 was achieved at a low resistance in a relatively large negative
potential window, which makes CuFe2 O4 a promising negative electrode for the devel-
opment of supercapacitors operating in an environmentally friendly Na2 SO4 electrolyte.
The approach developed in this investigation allowed good material performance at high
active mass loading, which is important for practical applications. It was based on the use
of ASPA as a chelating co-dispersant for CuFe2 O4 and MCNT. The ASPA adsorption on
CuFe2 O4 and MCNT involved different mechanisms which were linked to features of the
ASPA structure. CuFe2 O4 nanoparticles combined magnetic ordering and advanced pseu-
docapacitive properties, making CuFe2 O4 a promising MOPC material. The combination
of advanced magnetic and capacitive properties of CuFe2 O4 in the negative potential range
provides a platform for the investigation of new phenomena related to the influence of
pseudocapacitive/magnetic properties on magnetic/pseudocapacitive behavior.

Supplementary Materials: The following supporting information can be downloaded at: https://
www.mdpi.com/article/10.3390/molecules27165313/s1, Figure S1: SEM image of CFO-20 electrode
and EDS mapping results for the selected area in the SEM image.
Author Contributions: Conceptualization, W.L. and I.Z.; methodology, W.L.; software, S.S.; valida-
tion, W.L., W.Y. and I.Z.; formal analysis, W.L. and I.Z.; investigation, W.L., W.Y. and I.Z.; resources,
I.Z.; data curation, W.L.; writing—original draft preparation, W.L. and I.Z.; writing—review and
editing, W.L. and I.Z.; visualization, S.S.; supervision, I.Z.; project administration, I.Z.; funding
acquisition, I.Z. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Natural Sciences and Engineering Research Council
of Canada, grant number RGPIN-2018-04014 and CRC program. Lory Wenjuan Yang received a
scholarship from the China Scholarship Council.
Data Availability Statement: The data is provided in this paper and Supplementary Material.
Acknowledgments: SEM investigations were performed at the Canadian Centre for Electron Microscopy.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the authors.
Molecules 2022, 27, 5313 12 of 14

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