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Symmetrical Supercapacitor with Graphene-
Supported Iron Hexacyanoferrate Electrodes: Wide
Potential Window and HighEnergyDensity
Lindiomar Borges de Avila Junior ( lindiomarbaj@gmail.com )
Universidade Federal de Santa Catarina
Pablo Cesar Serrano Arambulo
Universidade Federal de Santa Catarina
Luis Torres Quispe
National University of Saint Augustine
Adriana Dantas
Universidade Federal de Santa Catarina
Diogo Pontes Costa
Universidade Federal de Santa Catarina
Edy E. Cuevas Arizaca
Universidad Católica de Santa María Arequipa
Diana Patricia Paredes Chávez
Universidad Católica de Santa María Arequipa
Cesar Danial Valdivia Portugal
Universidad Católica de Santa María Arequipa
Christian Klaus Müller
Faculty of Physical Engineering/Computer Sciences, University of Applied Sciences Zwickau, Zwickau,
Germany
Article
Keywords: Prussian blue, graphene, supercapacitor electrode, high stability, symmetric supercapacitor
Posted Date: January 18th, 2024
DOI: https://doi.org/10.21203/rs.3.rs-3848646/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
Read Full License
Additional Declarations: No competing interests reported.
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Abstract
In this work, iron hexacyanoferrate (FeHCF) particles have been grown onto graphene substrate through
pulsed electrodeposition process. Thus, prepared FeHCF electrode exhibits volumetric capacitance of 88
F cm− 3 (a real capacitance of 26.6 mF cm− 2) and high cycling stability with capacitance retention of
93.7% under deep repeating of 10000 galvanostatic charge-discharge cycles in 1M KCl electrolyte.
Furthermore, two identical FeHCF electrodes were paired up in order to construct a symmetrical
supercapacitor, which delivers a wide potential window of 2 V in 1M KCl electrolyte, and demonstrates
large energy density with an offer of high power density.
1. Introduction
As a result of the depletion of fossil fuels and the subsequent rise in air pollution, there is an urgent
demand for clean and alternative energy sources. Consequently, signicant efforts have been directed
towards harnessing sustainable energy sources, such as solar and wind energy systems. Nonetheless,
the intermittent nature of these renewable energy sources hinders their substantial impact unless the
electricity they generate is eciently stored. Hence, there arises a critical need to employ advanced energy
storage devices to effectively store the energy produced by renewable sources. Supercapacitors, also
referred to as electrochemical capacitors, have garnered signicant interest as innovative energy storage
systems, thanks to their exceptional attributes, such as high-power densities, extended lifespan, and rapid
charging capabilities, setting them apart from conventional batteries [1], [2]. The supercapacitor material
is the core component of supercapacitor electrodes which largely dictates their electrochemical
performance. Variety of electrode materials have been investigated for supercapacitor electrodes,
including carbon materials [3], polymeric materials [4], and metal oxides [5]. In 2008, Chen et al. [6]
achieved the successful synthesis of three distinct types of transition metal hexacyanoferrates (FeHCF,
NiHCF, and CoHCF). These compounds were employed as active electrodes in a supercapacitor operating
with a 1M KNO3 electrolyte. Among the tested materials, FeHCF (425 Fg− 1), NiHCF (574.7 Fg− 1) and
CoHCF (261.56 Fg− 1) electrodes demonstrated higher discharge capacities at a current density of 0.2 Ag-
1. Composites of transition metal hexacyanoferrates with other components have also been used for
supercapacitor electrodes.
There are several concerns that need to be further improved in supercapacitors, for example, to extend the
working voltage, promote the energy/power densities, long life span, lower the construction cost, and their
environmental benign nature [6]. Asymmetric cell conguration is considered an effective way to extend
the voltage window of the cell, where positive electrodes (metal oxide-based) and negative electrodes
(carbon-based) are generally used [7]. Recent studies have indicated that metal oxide (pseudocapacitive
materials) could also be used as the negative electrode in asymmetric supercapacitors [8], [9]. For
example, Hu
et al.
[10] have reported Fe2O3 nanoparticle cluster/reduced graphene oxide (rGO) paper as a
negative electrode which shows improved capacitance in the negative voltage range compared to pristine
rGO paper. In another work, Zhang
et al.
[11] have reported MoO2/MoS2 as a negative electrode which
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shows good electrochemical performance. Nevertheless, designing pseudocapacitive materials as the
negative electrode is not simple and often leads to poor electrochemical performance and instability [12].
In 2014, a supercapacitor based on activated carbon showcased remarkable stability during operation,
maintaining an unexpectedly high cell voltage of 2.2 V [13]. By manipulating the physicochemical
properties of the electrolyte (such as pH or conductivity values) and/or adjusting the surface chemistry of
activated carbons, it becomes feasible to elevate the maximum voltage of aqueous supercapacitors.
These modications directly inuence the over-potential for water splitting [14]. Aqueous electrolyte-
based supercapacitors have garnered attention owing to their favorable attributes, such as relatively high
capacitance, excellent power rates, affordability, and eco-friendliness. However, as they are commonly
operated in highly corrosive electrolytes, there is a risk of corrosion damage to their closures. To enhance
safety, it is preferable to design electrodes suitable for neutral electrolytes [15]. Thus far, only
carbonaceous materials and manganese oxide (MnO2) based electrode materials have shown good
electrochemical performance in neutral media [16], [17]. However, some issues persist with these
materials, e.g. carbon-based electrodes suffer from relatively low capacitance, and MnO2-based
electrodes are restricted in their cycling stability [18].
The chemical forms (PB and analogs) are described by the general formula , due
to their diverse morphologies and easily controllable size, they have received considerable attention in
electrochemical devices, such as electrochemical biosensors [19], [20], Memristors devices [21], [22] and
also for supercapacitors [23]–[26]. The open framework structure of FeHCF has wide channels which
allow the rapid insertion and removal of electrolyte ions through redox reaction between ferrous and ferric
oxidation states in the Fe center [27]–[29]. The rich intercalation chemistry of FeHCF has motivated us to
explore this unique material for supercapacitor applications. In this work, we have grown FeHCF particles
onto a graphene substrate using pulsed electrodeposition for supercapacitor electrodes. This electrode
material shows large volumetric and areal-specic capacitance and ultrahigh cyclic stability in 1M KCl
(pH 5) electrolyte. Furthermore, a symmetrical supercapacitor was constructed by pairing up two identical
electrodes and achieving a high operating voltage of 2 V with high energy density and power density. In
practice, symmetrical supercapacitors are limited to the voltage range of ~ 1.23 V in aqueous electrolytes.
To our knowledge, this is the rst time that a symmetrical supercapacitor based on FeHCF has achieved
2V cell voltage. Additionally, the low-cost and eco-friendly nature of FeHCF makes this material very
attractive for practical application in supercapacitors.
2. Experimental details
First, we prepared graphene lm as a conductive substrate onto polyethylene terephthalate (PET) sheet in
the following way: the suspension of GO (2 mg mL− 1) was mixed with HI (57%) with a volume ratio of
2:0.5. Then the mixture was dropped onto the pre-cleaned PET sheet. After that, the PET sheet was heated
up directly by a hot plate at the temperature of 80°C for 3 hours. The obtained graphene lms were
washed with a copious amount of double distilled water and then ethanol to remove residual iodine. The
prepared substrates were preserved for further electrodeposition of FeHCF. The rGO substrates were
AxMa
y
[
Mb
(
CN
)6
]
z
Page 4/16
electrically contacted by a copper wire and masked to dene the area for the electrodeposition of FeHCF.
Then, it was immersed in the aqueous electrolyte composed of 0.5 mM of FeCl3, 0.5 mM of K3Fe(CN)6, 1
M of KCl, and 0.01 M of HCl. The electrochemical deposition of the PB lms was performed in
potentiostatic mode using an electrochemical workstation (Ivium CompactStat, Eindhoven, Netherlands)
as show in Fig.1. The rGO was used as the working electrode, while the counter and reference electrodes
were a platinum foil and a saturated calomel electrode (SCE), respectively. To electroplate Fe-HFC onto
rGO substrate, pulses of potential were applied, rst a pulse at 0.3 V, followed by pulse at 0 V, -0.3 V and 0
V vs. SCE. All pulses were kept with the same width of 0.1 s. Such a process was done repeatedly 300
times. After this, the samples were washed with water and dried under the stream of nitrogen gas.
The lm morphology was analyzed by eld emission scanning electron microscopy (FEG-SEM, TESCAN
CLARA, Brünn, Czech Republic). The elemental composition was studied with an energy-dispersive x-ray
(EDX) detector (Ultim Max 65 SDD, Oxford Instruments, Wiesbaden, Germany) at 15 kV.
The
crystallographic lm structure was analyzed with a Siemens x-ray diffractometer (D5000, Siemens,
München, Germany) using Bragg-Brentano geometry, a Cu Kα x-ray source (λ = 1.5418 Å), equipped with a
scintillator detector.
3. Results and discussion
The structure analysis of as-prepared FeHCF was done by X-ray diffraction as presented in Fig.2a.
Almost all diffraction peaks were indexed to the face-centered Prussian blue structure according to the
JCPDS Card No. 52-1907, indicating a crystalline cubic structure of FeHCF particles,, with a lattice
constant of a = 10.15 Å (for the 200, 220 and 400 reexes) in agreement with the literature [30]. The peak
at 25° was assigned due to graphene plane stacking diffraction [31]. PB KFe(III)[Fe(II)CN6], or Everitt salt,
is the partially reduced counterpart whereby the Fe center coordinated by C has its oxidation state + 2, due
to the strong eld of the CN ligand. On the other hand, the peripheral cyanide of coordination N, Fe-N, the
Fe atoms are in the + 3 oxidation state and with their tetrahedral holes occupied by alkaline cations,
Fig.2b. In Fig.2c presents a SEM image of electrodeposited FeHCF onto the graphene substrate, one
region showing the FeHCF anchored in the rGO (Fig.2c, left image) and the other with the most
agglomerated particles covering the entire electrode (Fig.2c, right image).
EDX-analysis was performed to determine the layer composition (see Fig.3). From the EDX spectrum a
composition with 42 at% C, 35 at% N, 6 at% O, 12.5 at% Fe, and 4.5 at% Na is obtained. From this
amounts, a mixture of 92% KFe3+[Fe2+(CN)6]▪mH2O and 8% Fe3 + 4[Fe2+(CN)6]3▪mH2O of PB can be
proposed based on the found C/N ratio of ~ 1.2 and a Fe/Na ratio of ~ 2.6. The higher amount of C
compared with N occurs because of carbon from the graphene and surface contaminations.
The electrochemical performance of FeHCF as a supercapacitor electrode was meticulously assessed
through comprehensive cyclic voltammetry (CV) and precise galvanostatic charge-discharge (GCD)
measurements, conducted within a sophisticated three-electrode cell system. In this carefully designed
setup, a highly conductive 1 M KCl electrolyte was employed, while a Pt foil and saturated calomel
Page 5/16
electrode were meticulously chosen as the counter and reference electrodes, respectively, ensuring
accurate and reliable characterization of the FeHCF electrode's behavior. In Fig.4a, a distinct pair of
reversible peaks is evident, corresponding to the redox reactions of FeII/III within FeHCF. These reactions
are accompanied by the insertion of K + ions to maintain charge neutrality [32]. Furthermore, as the scan
rates increase, there is a notable rise in the electrochemical response current. The cyclic voltammetry (CV)
curves exhibit a nearly consistent reversible pattern with slight shifts in peak positions, indicating
excellent electronic conduction within the electrode. The faradaic transition of PB in the presence of K +
ions is proposed as follows [33].
Fe(III)4[Fe(II)(CN)6]3 + 4K+ + 4e− = K4Fe(II)4[Fe(II)(CN)6]3
For further investigation of the capacitive behavior of the FeHCF electrode, a series of galvanostatic
charge-discharge (GCD) measurements were performed at various current densities, as show in Fig.4b.
The discharge curves of the FeHCF electrode displayed a distinct plateau, providing clear evidence of its
pseudocapacitive behavior, which aligns closely with the observations from the cyclic voltammetry
analysis. Areal and volumetric capacitances of the electrode were calculated from discharge curves at
different current densities and results are listed in Table1. At the highest current density of 3.3 A/cm3 the
charge or discharge step was completed in less than 6 s. To eliminate any potential inuence from the
graphene substrate as a working electrode, we conducted charge-discharge measurements on both the
pure graphene substrate and the FeHCF/graphene substrate, under identical conditions. The experiments
were carried out at a current density of 0.33 A/cm3. The results revealed a signicantly lower capacitance
for the pure graphene substrate, as illustrated in Fig.4c. The cyclic stability of the electrode materials is
another important parameter in the selection of supercapacitor electrode. The cycling stability of the
FeHCF was performed under repetitive GCD cycles at the constant current density of 0.33 A cm− 3. The
variation of capacitance with cycle number is shown in Fig.4d. From the start, a gradual decrease in
capacitance was noticed and later a sharp increase in capacitance was observed after 2500 cycles,
which can be explained as the activation process of the electrode material. The long span of the
charging-discharging process may help electrolyte ions intercalate into the open framework structure of
FeHCF nanoparticles, fully utilizing the elective surface area of electrode materials. After repeating 10000
GCD cycles, more than 93% capacitance was retained from its initial one, indicating the ultrahigh stability
of the electrode materials in 1M KCl electrolyte (pH ~ 5). This long-term cyclic stability of FeHCF is
probably due to the strong interaction of FeHCF nanoparticles at the interface of the graphene substrate.
Cycling stability behavior is the foremost metric determining the success of supercapacitor electrode
materials. The inset of Fig.4d shows that the GCD curves taken after 1000, 2000, 3000, 5000, 8000, and
10000 cycles, have no signicant change in the symmetry of charge and discharge curves after
consecutively repeating cycles. Again the electrode material is demonstrating a long-term reliability of the
electrochemical performance. The stability of the electrode materials is also dependent on the choice of
electrolyte. Generally, Prussian blue analogues are known to be more stable in acidic electrolytes than in
neutral and basic media. The above ndings reveal that FeHCF nanoparticles have large specic
capacitance and excellent cycling reliability in KCl electrolyte at ~ pH 5.
Page 6/16
Table 1
Areal and volumetric capacitance of FeHCF at different current densities
Area spec.
current
density
(mA cm− 2)
Area spec.
capacitance
(mF cm− 2)
Volumetric spec. current
density
(A cm− 3)
Area spec.
capacitance
(F cm− 3)
0.02 26.6 0.06 88.0
0.06 17.1 0.19 63.0
0.10 14.0 0.33 46.4
0.16 12.8 0.52 37.2
0.20 8.5 0.66 23.7
0.60 6.3 1.98 15.8
1.00 3.9 3.30 12.3
The symmetrical supercapacitor was mounted as shown in Fig.5a. Before the pairing of two identical
electrodes with the same shape and size, we rst tested FeHCF electrode in negative and positive
potential windows under different potential ranges in a three electrodes cell setup and at a scan rate of
100 mV/s. As shown in Fig.5b, the CV curves of FeHCF electrode exhibit a rectangular shape without any
redox peaks in negative potential region, which is indicating pure capacitive behavior. The rectangular
shape of the pseudocapacitive materials in the negative windows have also been shown in earlier
reported works [34]. The mechanism of rectangular CV curve for metal oxides in the negative windows is
not fully clear. However, to rule out the capacitive behavior of FeHCF electrode in the negative potential
window, we measured the CV of graphene substrate without FeHCF from − 0 to -1V. The CV curve of pure
graphene substrate was smaller than FeHCF; this means that graphene substrate has a lower
capacitance than FeHCF coated graphene substrate in the negative potential window (Fig.5b). Thus, it is
clear that FeHCF the additional capacitance is arising from FeHCF showing typical electrical double-layer
capacitance behavior in negative potential windows with KCl electrolyte. Furthermore, the current leap of
FeHCF electrode at the negative end (~ 1V) of the potential window increases sharply which indicates
hydrogen evaluation in the KCl electrolyte. A sharp increase in current for the FeHCF electrode near to ~
1V, is due to the oxygen evaluation on the electrode.
According to the performance of FeHCF in negative and positive potential windows in three electrode cell
conguration, a symmetrical supercapacitor (two-electrode cell system) was made up by pairing two
identical electrodes of FeHCF separated by lter paper in aqueous 1M KCl electrolyte. The symmetrical
supercapacitor system was operated by cyclic voltammetry over a wide voltage range of 0–2 V at a
constant scan rate of 50 mV/s as shown in Fig.5c. Figure5d shows the CVs of the full cell between 0
and 2V at various scan rates. The capacities of a symmetrical supercapacitor of 49.5 F cm− 3 at a current
Page 7/16
density of 0.16 A cm− 3, and 6.4 F cm− 3 at current density of 3.3 A cm− 3 were recorded from the discharge
curve as shown in Fig.5e. It is worth mentioning that our symmetrical supercapacitor successfully
achieved 2V of the voltage window in two electrode cell congurations. Most of the symmetrical
supercapacitors are limited to voltage range of ~ 1 V [34], [35]. Only recently, Xia et al. [36] developed a
high-voltage symmetric RuO2//RuO2 supercapacitor that gives a cell voltage of up to 1.6 V.
The device prepared herein demonstrates the energy density of 27.5 mWh cm− 3 at the power density of
330 W cm− 3 and the energy density can be maintained at 9.3 mWh cm− 3 with a power density of 12600
W cm− 3 (Fig.5f). This characteristic is notable in that high-power density can be simultaneously
achieved along with high specic energy, thus making these materials very promising for high-energy
storage supercapacitors. The impressive electrochemical performance of the FeHCF electrode material in
the aqueous electrolyte may be ascribed to the following factors: rst the open framework of FeHCF
contains large interstitial sites which can host large amounts of potassium ions that can be transported
within the channels. Second, the high ionic conductivity of the aqueous electrolyte may facilitate the fast-
ionic transport between the electrolyte and the electrode.
4. Conclusion
In summary, iron hexacyanoferrate nanoparticles have been grown onto graphene substrate using pulsed
electrodeposition and used as a supercapacitor electrode. The electrode demonstrates exceptional
electrochemical performance, excelling in both capacitance and stability aspects.This electrode delivers a
high volumetric capacitance of 88 F cm− 3 (areal capacitance of 26.6 mF cm− 2) and maintains the
capacitance at about 93.7% after repeating 10000 galvanostatic charge-discharge cycles in 1M KCl
electrolyte. When two identical electrodes of FeHCF were paired up in the form of a symmetrical
supercapacitor, it achieved a high operating voltage window of 2V. To our knowledge, this is the highest
voltage window reported for symmetrical supercapacitor in aqueous electrolyte, so far. Moreover, the
symmetrical supercapacitor shows an energy density of 27.5 mWh cm− 3 at a power density of 330 W
cm− 3.
Declarations
Acknowledgement
We acknowledge the nancial support of CAPES Govt. of Brazil to carry out this research work, and
Vicerrectorado de Investigación Universidad Católica de Santa María. The authors thanks for funding
Resolution No 29002-R-2022.
Author contributions
LBA contributed to - data acquisition and writing – original draft. PAS was involved in review and editing.
LTQ contributed to review and editing. AD was involved in review and editing. DPC contributed to review
Page 8/16
and editing. EECA was involved in review – editing and funding acquisition. DPPC was involved in review
– editing and funding acquisition. CDVP was involved in review – editing and funding acquisition. CKM
contributed to supervision – review and editing. All authors have commented on previous editions of the
manuscript. All authors read and approved the nal draft.
Conict of interest. The authors declare that they have no known competing interests or conicts of
interest of any kind.
Data availability
The datasets produced or analyzed during the present study are not publicly available, as they also form
a part of an ongoing investigation. However, they are available from the corresponding author upon
reasonable request.
Supplementary information
Not Applicable
Ethical approval
Not applicable
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Figures
Figure 1
Schematic illustration of electrocoating of FeHCF onto graphene substrate.
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Figure 2
Structure and morphology of FeHCF. a) comparison of XRD pattern of FeHCF nanoparticles with
simulated cubic Fe4[Fe(CN)6], b) demonstration of unit cell of cubic FeHCF and insertion/exertion of K+
ion, and c) SEM images of as prepared FeHCF nanoparticles onto graphene substrate.
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Figure 3
EDX analysis of FeHCF measured at 15 keV.
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Figure 4
Electrochemical characterization of FeHCF electrode in negative and positive potential range in three
electrode cell system. a) cyclic voltammogram curves of FeHFC onto graphene substrate at different
potential range in negative and positive region at 100 mV/s scan rates, b) galvanostatic charge-discharge
curves at different current densities, c) difference in charge-discharge curve of bare graphene substrate
and FeHCF/ reduced graphene substrate at 0.33 A cm-3, and d) cycling behavior of FeHCF electrode at
different cycle numbers at 0.66 A cm-3 (inset shows the charge-discharge curve right after taken at
different hundreds of cycles).
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Figure 5
Symmetrical supercapacitor performance. a) schematic illustration of the fabricated symmetrical
supercapacitor device, b) cyclic voltammogram behavior of graphene substrate alone as well as FeHCF
onto graphene substrate in negative and positive potential windows, c) cyclic voltammogram curves of
symmetrical supercapacitor at different voltage windows, d) CV curves of symmetrical supercapacitor at
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different scan rate in a 2V voltage window, e) galvanostatic charge-discharge curves at different current
densities, and f) Ragone plot, energy density versus power density.
