Current Research in Green and Sustainable Chemistry 4 (2021) 100098

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Current Research in Green and Sustainable Chemistry

journal homepage: www.elsevier.com/journals/
current-research-in-green-and-sustainable-chemistry/2666-0865

Porous carbon from Manihot Esculenta (cassava) peels waste for charge
storage applications
T.E. Amakoromo a, **, O.E. Abumere a, J.A. Amusan a, V. Anye b, A. Bello b, c, *
a
Department of Physics, University of Port Harcourt PMB, 5323, Choba, East-West Rd, Port Harcourt, Nigeria
b
Department of Materials Science and Engineering, African University of Science and Technology (AUST), Abuja, Nigeria
c
Department of Theoretical and Applied Physics, African University of Science and Technology (AUST), Abuja, Nigeria

A R T I C L E I N F O A B S T R A C T

Keywords: The activation of carbon from Manihot Esculenta (cassava) peels as a sustainable biomass precursor was explored
Biomass for application in supercapacitors (SCs); demonstrating a cheap and sustainable environmental friendly source of
Cassava peels carbon materials for potential application as electrode materials for SC. The activation was carried out with three
Activated carbon
different activating agents namely potassium hydroxide (KOH), potassium bicarbonate (KHCO3) and potassium
Microstructures
Supercapacitors
chloride in sodium thiosulphate (KCl–Na2S2O3). The obtained activated carbon (AC) material exhibited SSA up to
828 m2 g
1 for the KOH activation and the electrodes fabricated revealed a specific capacitance (CSP) (300 F g
1
at 0.5 A g
1), excellent rate capability and cycling stability (98% capacitance retention after 5000 cycles) in 6 M
KOH aqueous solution. The study demonstrated an efficient and sustainable approach for high performance SCs
by using biomass as a viable source of the carbon material for SC applications.

1. Introduction resulting to higher energy storage capability than EDLCs [2]. These
materials however, suffer capacity fading at the electrodes due to change
Energy storage systems are in high demand and have become a major in chemical structure during electrochemical cycling [3] contributing to
research focus as the world turns to greener forms of energy to reduce the lower cycle life and power density compared to EDLCs. EDLCs utilize
impact of current energy sources on the climate. With electric vehicles carbon as electrode materials in the form of activated carbon (AC), car-
becoming popular and electronic devices becoming more portable, sus- bon aerogels, carbon nanotubes (CNTs) and graphene. However, AC
tainable and effective energy storage systems are required for these in- presents a more favored source of carbon for the purpose of electrode
novations to be globally adopted. Supercapacitors (SCs) are the most materials due to its large surface area, high porosity, availability and low
promising energy storage devices as they deliver long life span, high cost since it can be easily gotten from both agricultural and forest
power density and good cyclability which commercial batteries lack [1]. biomass [2,4].
SCs are divided into two main categories, the electrochemical double In EDLCs, the energy storage mechanism is based on electrostatic
layer capacitors (EDLC) and pseudocapacitors. In EDLCs, energy is stored adsorption of the ions in the electrolyte/electrode surface interaction.
in the electrode/electrolyte interface. The large surface area of its elec- High capacitance levels are achieved in this system because they adopt
trodes coupled with the porosity of the electrode materials which are highly porous carbon materials known for their large surface areas which
mostly carbon-based, makes the ELDC capable of storing sufficiently allow for good energy storage and high electronic conductivity [5]. The
larger amount of energy in comparison with conventional capacitors. delirious effect of fossils and other forms of carbon on the environment
They also possess high power densities and longer cycle life in compar- and global climate conditions calls for the need to focus on greener,
ison with batteries. Battery type and Pseudocapacitors on the other hand renewable, low cost and sustainable sources of carbon. Biomass, presents
store energy through Faradaic charge transfer reactions, which takes a renewable, abundant and sustainable solution to the table. As the world
place between the electrolyte and the active species on the surface of the shifts to greener sources of energy, the adoption of biomass, is also
electrodes. They are made from conducting polymers or metal oxides, gaining a lot of attention, with over 14% of global energy being produced
giving rise to a redox process similar to what happens in batteries from biomass [6]. Biomass is a feasible and sustainable source of

* Corresponding author. Department of Materials Science and Engineering, African University of Science and Technology (AUST), Abuja, Nigeria.
** Corresponding author.
E-mail addresses: tarila.amakoromo@uniport.edu.ng (T.E. Amakoromo), abello@aust.edu.ng (A. Bello).

https://doi.org/10.1016/j.crgsc.2021.100098
Received 3 February 2021; Received in revised form 31 March 2021; Accepted 3 April 2021
Available online 14 April 2021
2666-0865/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

Scheme 1. For the production of the activated carbon.

materials for energy applications and presently provide ~14% of the compared to the original one [33]. In this study, cassava peel, which is an
total energy consumed and ~ 35% for cooking and heating in third world agricultural waste, is being proposed as source of porous carbon as
countries, especially Africa [7–12]. Bio-inspired materials as electrodes electrode materials in SC applications using three different activating
for SC has become attractive due to the fact that biomass resources are agent such as KOH, KCHO3 and KCl–Na2S2O3. The choice of KCHO3 and
readily available, abundant, naturally renewable, eco-friendly and meet KCl–Na2S2O3 is due to the corrosive and toxic nature of KOH and has
the condition for green and sustainable carbon sources for advancement motivated the used of mild and less corrosive activating agent.
of electrode materials for next cohort of energy storage devices [13,14].
Towards the development of AC as electrodes for SC from biomass ma- 2. Experimental
terials, some progress though slow has been made so far in the area of
research. For example, Seaweed [15], and many ecological biomass 2.1. Synthesis of activated carbon (AC) from cassava peels
material including egg shell [16], wood sawdust [17], pistachio nutshells
[18], cigarette filter [19], sunflower seed shell [20], cypress [21], rice Cassava peels was collected from local farmers in Galadimawa, a sub-
husk [22], hydrochar [23], rubber wood [24] waste American poplar urban region of FCT Abuja, Nigeria. The peels were repeatedly washed to
fruit [25], wood tar [26], grape marcs [27], corncob [28], wheat straw remove all forms of dirt on it. The washed peels were then oven dried at
cellulosic foam [29], among other forms of biomass were investigated as 60  C for 12 h. Approximately 3 g of precursor (cassava peel) was
possible carbon sources for SC applications. All these source of carbon measured, activated with 10 g of potassium chloride (KCl) and 4 g
materials mentioned above are low cost, sustainable and are not harmful Na2S2O3 (sodium thiosulphate) until a uniform mixture as shown in the
to the environment. Agro-industrial products rich in lignocellulose, for scheme below. The mixture was then carbonized under Nitrogen gas (N2)
example cassava petiole (CP) waste, have high potential as sources of flow in a Carbolite tube furnace at 750  C at a ramp rate of 5  C/min and
porous carbon materials, alongside other notable advantages, including left to carbonize for 1 h under the N2 flow, the carbonized material was
ease of acquisition, abundance and availability, renewability and low allowed to cool to room temperature and then consistently washed with
cost. Cassava peels have been comprehensively investigated for appli- distilled water until a pH of 7 was reached. The washed samples were
cations in many industries, utilized in different forms as flakes, flour then dried in an oven at 80  C for 24 h. Similarly, the procedure was
which is highly used in the baking industries for bread and biscuits, repeated with KOH and KHCO3 as the activating agent (Scheme 1).
starch is also been processed from it and used in the textile and other
production industries, and its leaves can also be used as herbs or 2.2. Physico-chemical description
consumed as vegetables. Ismanto et al. [27] prepared high surface area
carbon from cassava peel by chemical activation; and the results revealed The X-ray diffraction (XRD) was carried out to study the structure and
that activation time gives no significant effect on the pore structure of AC the degree of crystallinity of the samples using X-Pert Pro instruments,
produced. However, the pore characteristic of carbon changes signifi- with reflection geometry at 2θ values (10–80 ) with working with a Cu
cantly with impregnation ratio and carbonization temperature, and the Kα radiation source (λ ¼ 0.15418 nm). The Raman spectroscopy analysis
maximum surface area and pore volume were obtained at impregnation was performed using a Jobin Yvon Horiba TX 6400 micro-Raman spec-
ratio 5:2 and carbonization temperature 750  C [30]. Ferromagnetic AC trometer with a power of 15 mW to avoid sample heating. A Zeiss
from cassava (Manihot dulcis) peels activated by iron(III) chloride was Scanning Electron Microscope (SEM/EDS) Zeiss Model; EVO/LS10., was
reported by Dongo et al. The proximate analysis of cassava peel showed used to acquire micrographs of the prepared materials. Nitrogen
that the moisture content, fixed carbon, ash content, and the volatile adsorption/desorption isotherms were measured by a NOVATOUCH
matter were 3.52%, 82.97%, 4.97%, and 8.54%, respectively [31]. In this with a quantachrome Touch-Win software analyzer. All samples were
work, our choice of cassava is due to the fact that it is an agricultural degassed at 100  C for 6 h under high vacuum environments. The specific
produce that is very rich in carbon, popular in Nigeria and other coun- surface area was calculated with the Brunauer-Emmett-Teller (BET)
tries in sub-Saharan Africa; with Nigeria recording yearly production of method from the adsorption branch in the relative pressure range (P=P )
o
up to 53 million metric tons in 2013 [32] and therefore the raw materials
of 0.01–0.5, while the pore size distribution was determined using the
is readily available. The peels pose a major waste management challenge,
Barrett Joyner-Halenda (BJH) method.
as it is a source of agricultural waste across the country, therefore recy-
cling, as AC will be of great benefit. Furthermore, scientific reports on the
2.3. Electrochemical measurements
usage of AC derived from cassava peels as electrode material for elec-
trochemical applications are few. For example, Cassava peels were used
Device electrodes were fabricated using graphite foil (GF) as the
as the precursor for AC-based electrodes for SCs and the surface of the AC
current collectors. In a typical procedure, the active material in the form
was further treated with different concentration of oxidative chemical
of a paste was coated onto the GFs. The as-synthesized carbons from the
agents, such as H2SO4, HNO3, and H2O2 solutions. The presence of
cassava were used as the active material and were made into a paste by
oxygen-containing groups increased the polarity and hydrophilicity of
adding carbon black (CB) additive and polyvinylidene fluoride (PVDF)
AC improved the performance of the electrode. As a result, the CSP of the
binder in a mass ratio of 8:1:1 with few drops of 1-methyl-2-pyrrolidi-
HNO3 modified AC-electrode reached 264.08 F g
1, an increase of 72.6%
none (NMP). The coated electrodes were vacuum dried at 70  C

2
T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

CSP ¼ IΔt=mΔU (2)

where ΔU is the voltage, I is the current (A), v is the sweep rate (mV s
1),
m is the mass of the material, Δt is the discharge time.

3. Results

Fig. 1 depicts the XRD patterns for all the prepared AC samples. The
activated carbon (AC) samples of produced showed similar XRD pattern
with an amorphous signature (i.e. broad diffraction peaks) at a 2-theta
angle of 24–25 and 42–44 corresponding to the (002) and (100)
crystallographic planes of a carbon material.
The acquired representative Raman data of the carbonized from all
three samples KHCO3, KOH and KCl–Na2S2O3 is presented in Fig. 2a. All
three samples display the prominent D band at ~1357 cm
1 1359 cm
1,
1345 cm
1 and a G band at 1582 cm
1 1595 cm
1, 1590 cm
1 for
KHCO3, KOH and KCl–Na2S2O3 respectively. These peaks corresponds to
the vibrational modes of sp2-carbon material [35,36]. The G band arises
from the in-plane vibration stretching of the C–C bond in graphitic ma-
terials and is common to all sp2 carbon materials [37]. The D-peak, is due
Fig. 1. XRD of the produce activated carbon materials. to the breathing modes of sp2 rings activated through a dual resonance
effect in the presence of defects [38]. The presence of the D band leads to
overnight. Using a two-electrode test configuration, electrochemical tests the assumption that the textural properties of our AC material consists
were performed on a BIO-LOGIC (BCS-805) potentiostat operating on the highly disordered structure. SEM micrographs shown in Fig. 2b–d shows
BT-Lab software. Electrochemical measurements carried out include the formation of interconnected porous morphology with dimensions in
galvanostatic charge/discharge (GCD), cyclic voltammetry (CV) and the range of several microns and was analyzed by SEM/EDS as stated
electrochemical impedance spectroscopy (EIS). EIS measurements were earlier. The produced carbon materials exhibited irregular three
carried out at within the frequency of 100 kHz to 0.01 Hz. CSP values dimensional network suggesting that the micrographs morphology could
were estimated thus [34];: be from the volatile organics and swell expansion as reported by Xie et al.
Z [39] and Wang et al. [40]. The three carbon samples exhibited rough
CSP ¼ IdU=vmΔU (1) surfaces with good porosity that could be suitable for the insertion/-
deinsertion ions during the electrochemical process. Samples from
KHCO3 activation shown in Fig. 2a exhibited huge irregular pores similar
And from the GCD curves with equation (2) [34]:
to a honeycomb structure, which is comparable to samples, obtained

Fig. 2. Raman spectrum of (a) carbon from KHCO3, KOH and KCl–Na2S2O3 and (b–d) SEM micrographs of the carbon obtained from KHCO3, KOH and KCl–Na2S2O3,
inset to the figure shows the EDX pattern showing the elemental composition.

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T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

Table 1
EDS elemental composition of carbon from activation with KHCO3, KCl–Na2S2O3, and KOH.
Sample C Na Si Mo K Nb S Ca

KHCO3 92.01 – 0.32 – – 6.64 – 1.04

KCl–Na2S2O3 23.81 0.92 13.68 – 11.92 – 35.68 14
KOH 87.19 – 0.5 7.61 – – 0.01 4.68

Fig. 3. (a–c) Gas adsorption/desorption isotherms of the carbons from KHCO3, KOH, KCl–Na2S2O3 and at different carbonization temperature. Inset to the figure
shows the BJH pore size distribution (d) Bar chart presenting the BET surface area for the three samples.

from the activation with KCl–Na2S2O3, as shown in Fig. 2c. This is a 3.1. Texture analysis
consequence of the presence of the KCl, which has the ability to deter-
mine the macroscopic structure of the carbon materials. However, the The surface area and porosity of the samples, were studied using the
carbon obtained from the KOH activation in Fig. 2c show a densely N2 adsorption-desorption isothermal analysis and the results are pre-
packed fluffy hollow structure compared with the other two samples. The sented in in Fig. 3 with BJH pore size distribution shown as inset to the
inset to Fig. 2c-d shows the EDS (Energy dispersive spectroscopy) and the figures. The shape of adsorption/desorption isotherm suggests the pres-
elemental composition of samples confirm the presence of carbon ma- ence of mainly micropores and the reversibility is direct indication of
terials as shown in Table 1. From the table sample activated with good pore connectivity network within the samples. All the samples
KCl–Na2S2O3, exhibited the least carbon content with a large amount of exhibit typical type IV adsorption-desorption isothermal with an obvious
Sulphur possibly due to incomplete reaction during the activation hysteresis loop. Although the hysteresis loops could feature the Type H4
process. loop which is generally associated with narrow slit-like pores, in this
case, however The KCl–Na2S2O3 shows a very small amount of adsorbed
gas, when compared to the other two samples suggesting that there was
only a small number of pores in the sample. All samples show adsorption-
desorption curves with a sharp rise at relatively low pressure and exhibit
Table 2
hysteresis loop at P=P > 0.45, indicating the coexistence of micropore
BET surface area and porous textural data obtained for carbon materials after o
activation of the cassava peels at 800  C. and mesopore structures. The specific surface areas (SSA) measured by
Sample KHCO3 KCl–Na2S2O3 KOH BET model are presented in Fig. 3 (d) in the form bar chart with the KOH
activated sample with a SSA of 828 m2 g
1, followed by the sample
BET (m2 g¡1) 586 272 828
micropore volume (cm3/g) 0.4017 0.1760 0.3981
activated with KHCO3 with SSA of 586 m2 g
1, while KCl–Na2S2O3
Pore diameter (nm) 2.67 2.64 2.66 showed the least SSA of 272 m2 g
1. To reveal the pore size distribution

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T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

Table 3
Comparison between different biomass materials.
Source of electrode material Activation agent SBET (m2 g
1) Electrolyte Potential (V) Capacitance (F g
1) Ref.
a
Waste tires KOH 1625 1 M H2SO4 0.75 135 [42]
H3PO4 563 6 M KOH 1 106a [43]
Durian shell H3PO4 1 M Na2SO4 1 93.1 [44]
Pomelo peel – 2105 1 M NaNO3 1.7 43.5b [45]
Banana Fibers ZnCl2 & KOH 1097 1 M Na2SO4 1 74b [46]
Cassava peel KOH þ Physical Activation (CO2) 1352 0.5 M H2SO4 1 153 [30]
Recycled jute KOH 1527 3 M KOH 1 185 [47]
Sunflower seed shell KOH 2584 3 M KOH 0.9 311b [20]
Petals Physical (CO2) 509 0.5 M KCl 1 154 [48]
Commercial AC (coconut shells) 779.8 6 M KOH 1 94 [49]
Rubber Seed-Shells KOH 620 6 M KOH 63.2 [50]
Pine Cone KOH 1515 1 M Na2SO4 2 137 [51]
Citrate salts KOH 1600 1 M H2SO4 1.2 200–240 [52]
EMImTFSI/AN 3 100–130
Cypress KOH 1283 1 M H2SO4 1 190a [21]
Sugar cane bagasse ZnCl2 1788 1 M H2SO4 1 300b [53]
Cassava peels KOH, KCl–Na2S2O3 and KHCO3 828 6 M KOH 1.0 96 This work-
272 1.1 80
586 1.2 300b
a
Three electrode measurements.
b
Two electrode measurements.

Fig. 4. Cyclic Voltammogram of (a) KHCO3 activated (b) KOH activated (c) KCl–Na2S2O3 activated at scan rates ranging from 5 mV s
1 to 100 mV s
1 in 6 M KOH
electrolyte (d) Comparative CV at 50 mV s
1 for the three activating agent.

(PSD) of the obtained samples, BJH was used to fit the isothermal curves charge storage capability of these electrode materials when compared to
to give the pore size distribution information. As shown in the figures, all the sample has a low micropore volume (see Table 3).
the activated samples exhibit peaks corresponding to micropores and
mesopores, respectively, indicating the coexistence of both pores. Table 2 3.2. Electrochemical measurements
below summarizes the gas sorption properties of the produced carbon
materials. In addition, micropores are used for charge/ion storage and a The charge storage capability of the AC from the three activating
high volume of micropores in the samples is expected to increase the agent namely KOH, KCl–Na2S2O3 and KHCO3 as the electrode was

5
T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

Fig. 5. Galvanostatic charge discharge (GCD) of (a) KHCO3 activated (b) KOH activated (c) KCl–Na2S2O3 activated at specific current ranging from 0.25 A g
1 to 2.0
A g
1 in 6 M KOH electrolyte (d) Comparative CV at 50 mV s
1 for the three activating agents.

characterized in alkaline 6 M KOH electrolyte to determine the optimum

electrochemical performance of the carbon materials. The electro-
chemical results obtained for the three sets of devices fabricated are
presented in Fig. 4. Fig. 4a present an ideal CV shape for the device
fabricated with electrode materials from KHCO3 activation process, with
the device displaying a voltage of 1.2 V and still maintained the rectan-
gular shape as the scan rate from 5 to 100 mVs
1. Fig. 4b shows a highly
resistive CV shape from 0 to 1.0 V with rectangular CV shape for the
device activated with KOH and the shape changes as the scan rate in-
creases from 5 to 100 mV s
1. Fig. 4c present the CV of the device pre-
pared with electrode from the activation with the KCl–Na2S2O3. The
device could withstand a voltage of 1.1 V when compared with the device
from the KOH activation. Although the shape becomes resistive (oval
shape) as the scan rate increases from 5 to 100 mV s
1.
The comparative CV at a scan rate of 50 mV s
1 between the devices is
presented in Fig. 4d and it shows that the device fabricated with the
electrode material from KHCO3 presented the best result in terms of
shape and capacitance. This we attribute to the moderate surface area
and the pore volume presented in Table 1, which has the ability to hold
more charges when, compared with the other two samples. Similarly the
presence of hydrogen in the activating agent could improve the con-
ductivity and result to the better electrochemical results obtained. Fig. 6. Showing the columbic efficiency for the three devices.
Furthermore, the constant current GCD measurement was made to
confirm the EDL behavior of the AC electrodes and the results is pre- functional groups present in the sample. The CSP based on equation (2)
sented in Fig. 5. Fig. 5 shows the almost linear symmetrical triangular were 300 F g
1, 96 F g
1 and 80 F g
1, at 0.5 A g
1 respectively for
GCD curves at various specific currents for the three device at different KHCO3, KOH, and KCl–Na2SO3, which are in accordance with that ob-
voltages as presented in Fig. 5a, b, and c, indicating a high electro- tained from the CV curves. These results are higher especially for the
chemical reversibility and good accessibility of ions at the electrode KHCO3 and superior to values reported for cassava based and many
interface with electrolyte, confirming the EDLCs charge storage mecha- biomasses derived carbons in the literature and are summarized in
nism of the AC electrode and that the overall capacitance of the electrode Table 2. The excellent electrochemical result is attributed to the extended
combined the EDL capacitance and the Faradaic contributions from the potential window of 1.2 V, Moderate SSA and high micropore volume
which responsible for trapping the ions from the electrolyte. Lastly, the

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T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

Fig. 7. EIS characteristics of the cell: of (a) KHCO3 activation (b) KOH activation (c) KCl–Na2S2O3 activation, and (d) the comparison of the three devices.

morphology of the material give rise to easier accessibility to ions from collector/active material interfacial impedances occurs which also
the electrolyte. This is understood by easy of ion mobility that is closely contribute to the appearance of the semicircle. The ESR is accountable for
linked to the ionic conductivity of the electrolyte leading to quick ion the dissipation of accumulated energy, and high values of the ESR re-
transport and shorter diffusion pathways [41]. This also indicate the duces the entire power and energy efficiency of the device. This high
KHCO3 is the best activating agent for this material. value is due to presence of dopant groups, the hybrid nature of the
Cycling life is fundamental to the evaluation of SCs electrodes. electrodes that guarantees a high voltage window and depends on the
Therefore the AC electrode was subjected to a constant current GCD ionic concentration and conductivity of the electrolyte as stated earlier.
condition for 5000 cycles at a constant current in order to evaluate its The intercepts of the curves with the real impedance axis is the equiva-
stability, and the result is presented in Fig. 6. The Figure shows the lent series resistance (ESR) which includes the resistance of the electro-
Coulombic efficiency as a function of the current, surprisingly the elec- lyte solution and the contact resistance at the interface of active
trode exhibit no capacitance loss rather all sample show a constant effi- material/current collector. The ESR was ~0.9 Ω for the KOH activated
ciency over the period of 5000 cycles. The Figure displays excellent device, 2.0 Ω for the KCl–Na2S2O3 activated device and 0.7 Ω for the
performance of the electrode showing that there is no decay to the KHCO3 activated device, respectively as shown by Fig. 6d that compares
capacitive performance of the electrode suggesting that no significant the three fabricated devices. Indicating a low diffusion resistance for the
change to the microstructural properties of the sample. device fabricated with AC activated with KHCO3. The circuit diagram
As stated earlier EIS is an important parameter for investigating, the obtained from the fitting of impedance data is presented in Fig. 7(a–c) as
electrical conductivity of electrodes The Nyquist plot, which gives in- inset, with all circuits showing similar components. In the equivalent
formation on the resistive behavior of the electrode, as shown in circuit, Rs represent the solution resistance and is connected in series
Figures below. EIS was also used to characterize the AC electrode, and the with the either Q which represent the double layer capacitance related to
Nyquist plot is presented in Fig. 7. Fig. 7(a–c) present the Nyquist plot of the porous electrode. The RCT and W are due to the presence of the
the electrode shows a semicircle at the high-frequency domain as seen by semicircle and the 45 line in the high frequency region. The low fre-
the diameter of the and a vertical line at low-frequency indicating a near- quency region is associated with the mass capacitance (CL) and the
ideal capacitive behavior. The semicircle at the high frequency region is leakage current resistance (RL) connected in parallel.
possibly due to non-uniformity of the current distribution on the elec- As stated earlier impedance spectroscopy is a very powerful tool for
trode giving rise to the variation of the capacitance involved in the analysis of electrochemical devices. Fig. 8a presents the phase angle
impedance plot, and similar plots have been reported for many carbon- change with frequency. A phase angle ~ -80  for the KHCO3 activated
based materials [54,55]. The origin of the semicircle could also attrib- cassava, 79 for the KOH activated cassava and 75 for the KCl–Na2S2O3
uted to the presence of functional dopants on the surface of the carbon was obtained for the electrode which defines an almost ideal capacitive
materials that may contribute to charge transfer process which arise from response for all the devices that were fabricated. We measured the
an electron or ion transfer from one phase (e.g. electrode) to another (e.g. impedance of the electrode materials in the frequency range of 0.01–105
liquid). Furthermore, at the interface between the current Hz AC signal amplitude. The complex plane plots are presented in Fig. 8

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T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

Fig. 8. (a) Bode angle as a function of frequency for the three devices, (b–d) complex capacitance as a function of frequency for (b) KHCO3 activated, (c) KOH
activated and (d) KCl–Na2S2O3 activated.

b-d for all three electrode materials showing the real and imaginary part measure of how fast the cell could be fully charged and discharged
of the capacitance as a function of the frequency. The complex capaci- reversibly. This rapid frequency response of 14 s could be attributed to
tance were calculated using the equations below. the fairly large surface area with a relative amount of mesopores which
enhances the ion transport rate of the device [56].
C ¼
1=ðωZ}Þ (3)
4. Conclusions
0
CðωÞ ¼ C ðωÞ
jC}ðωÞ
(3a)
We have demonstrated the production of high surface area carbon
from cassava peel biomass waste materials. Cassava peels are abundant
0
Z}ðωÞ and renewable; demonstrating a cheap and sustainable environmental
C ðωÞ ¼ (3b)
ωjZðωÞj2 friendly source of carbon materials for SC electrode materials. Three
different activating agent were explore and the sample activated with the
0
Z ðωÞ KHCO3 displayed SSA of 828 m2 g
1 and electrodes fabricated from the
C}ðωÞ ¼ (3c) AC displayed a CSP of 300 F g
1 at 0.5 A g
1 in 6 M KOH. As demon-
ωjZðωÞj2
strated, cassava peels are low cost source of biomass derived carbons by a
0
where Z is the complex impedance written as ZðωÞ ¼ Z ðωÞ þ jZ}ðωÞ, simple, green and environmentally benign process, could be advanta-
0 geous and viable source of the carbon material for SC applications if fully
ω ¼ 2π f , Z and Z} represents the real and imaginary parts of the Nyquist
0 explored.
plot respectively. C ðωÞrepresents the real capacitance represents the
deliverable capacitance, while C}ðωÞrepresents the energy loss [56].
CRediT authorship contribution statement
Based on this, the real capacitances has been calculated to be 0.4 F, 0.3 F
and 0.3 F for KOH activated cassava, KCl–Na2S2O3 activated cassava,
T.E. Amakoromo: Conceptualized idea for research, Responsible for
KHCO3 activated cassava (intercept on the vertical axis of Figures and
data collection/design, Lead author, writing up of article, wrote first
this represent the deliverable capacitance. The imaginary capacitance
draft, co-developed and executed research. O.E. Abumere: and. J.A.
gives an estimate of the time constant which is a characteristic frequency
Amusan: Material contribution, results analysis and proof reading of the
(f0 ) that corresponds to peak of imaginary capacitance is about 0.05 Hz,
manuscript. V. Anye: and. A. Bello: results analysis and proof reading of
0.07 Hz and 0.07 Hz for KOH activated cassava, KCl–Na2S2O3 activated
the manuscript.
cassava, KHCO3 activated cassava and this frequency represent the point
where the capacitive and resistive impedances are the same [56]. The
corresponding time constants are 20 s, 14 s and 14s respectively and is a

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T.E. Amakoromo et al. Current Research in Green and Sustainable Chemistry 4 (2021) 100098

Declaration of competing interest J. Alloys Compd. 854 (2021) 157207, https://doi.org/10.1016/

j.jallcom.2020.157207.
[28] M. Kigozi, R. Kali, A. Bello, B. Padya, G.M. Kalu-Uka, J. Wasswa, et al., Modified
The authors declare that they have no known competing financial activation process for supercapacitor electrode materials from african maize cob,
interests or personal relationships that could have appeared to influence Materials (Basel) 13 (2020) 5412, https://doi.org/10.3390/ma13235412.
the work reported in this paper. [29] G. Gou, F. Huang, M. Jiang, J. Li, Z. Zhou, Hierarchical porous carbon electrode
materials for supercapacitor developed from wheat straw cellulosic foam, Renew.
Energy 149 (2020) 208–216, https://doi.org/10.1016/j.renene.2019.11.150.
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