BIBECHANA
Vol. 20, No. 3, December 2023, 205–212

ISSN 2091-0762 (Print), 2382-5340 (Online)
Journal homepage: http://nepjol.info/index.php/BIBECHANA

Publisher:Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University)Biratnagar

Characterization of carbon derived from candle by
flame-soot method for counter electrodes of

dye-sensitized solar cells

Prakash Joshi1,∗, Umesh Lawaju2, Anupam K.C3

Mim Nakarmi4, Ramani Pradhan5

1Physics Department, Bhaktapur Multiple Campus, Tribhuvan University, Bhaktapur, Nepal
2Department of Physics, Patan Multiple Campus, Tribhuvan University, Lalitpur, Nepal

3Texas State University, San Marcos, Texas, USA
4Dept. of Physics, Brooklyn College and the Graduate Center of the City

University of New York,Brooklyn, NY 11210, USA
5Dept. of Applied Sciences and Chemical Engineering, Pulchowk Campus,

Institute of Engineering, Tribhuvan University, Lalitpur, Nepal
∗Corresponding author. Email: prakash.joshi@bkmc.tu.edu.np

Abstract
Candle soot, carbon samples prepared by flame-soot method, was characterized and investi-
gated for its catalytic ability for the reduction of tri-iodide ions aiming to substitute expensive
platinum based electrode used in dye-sensitized solar cells (DSCs). The Energy Dispersive
X-ray Spectroscopy of the candle soot samples revealed that the soot contains 96% carbon.
Similarly, Scanning Electron Microscopy images show that the candle soot consists of inter-
connected carbon nanoparticles of size ∼50 nm. Furthermore, X-ray Diffraction and Raman
spectroscopy showed that the candle soot consists of disordered and ordered graphitic carbons
in a comparable proportion. The catalytic ability of the candle soot was compared with that of
platinum by Electrochemical Impedance Spectroscopy of the symmetrical electrochemical cells.
The charge transfer resistance (Rct) at the candle soot-electrolyte interface was observed to be
∼4.42 Ω cm2 compared to ∼ 5.04 Ω cm2 that of the platinum-electrolyte interface. The can-
dle soot was prepared by a simple method using low-cost material; hence, it can be a low-cost
and efficient counter electrode material alternate to the platinum used in counter electrodes
of DSCs.

Keywords
Candle soot, Flame-soot method, Counter electrode, Catalyst, Dye-sensitize solar cells, Charge trans-
fer resistance.

Article information
Manuscript received: July 17, 2023; Accepted: August 26, 2023
DOI https://doi.org/10.3126/bibechana.v20i3.58041
This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons.
org/licenses/by-nc/4.0/

205

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prakash.joshi@bkmc.tu.edu.np
https://doi.org/10.3126/bibechana.v20i3.58041
https://creativecommons.org/licenses/by-nc/4.0/
https://creativecommons.org/licenses/by-nc/4.0/


P. Joshi et al./ BIBECHANA 20 (2023) 205-212 206

1 Introduction

Carbonaceous materials are chemically stable, read-
ily available, and low-cost [1] and these materials
have been adopted in the preparation of counter
electrodes of dye-sensitized solar cells (DSCs). The
DSCs, developed by O’Ragen and Gratzel in 1991,
are a relatively new type of solar cells and these
photovoltaic devices can be low-cost compared to
silicon based solar cells [2, 3]. The major com-
ponents of a DSC are photoelectrode, liquid elec-
trolyte, and counter electrode. Photoelectrode is
the front electrode while the counter electrode (CE)
is the back electrode, and the liquid electrolyte is
enclosed in the gap between the two electrodes.
Both the photoelectrode and CE use a transparent
and conducting Fluorine-doped tin oxide (FTO)-
glass substrate as a base. For the photoelectrode,
the FTO-glass substrate is coated with a few mi-
crons thick film of mesoporous nanocrystalline Ti-
tanium dioxide (TiO2) particles. Then, a mono-
layer of light sensitive dye molecules is chemically
adsorbed on the TiO2 film. For the CE, the glass
substrate is generally coated with a platinum.

When the sunlight is incident at the photoelec-
trode, the dye molecules absorb the solar photons
and inject their loosely bound electrons into the
conduction band of the TiO2. After diffusion of
the electrons in the TiO2 film, they arrive at the
conducting part of the photoelectrode. The pho-
toelectrode is connected to the CE via an external
circuit with an electric load. Thus, the electrons at
the photoelectrode pass through the electric load
in the external circuit and generate electric power.
Then the electrons arrive at the CE [3,4]. In order
to enable the oxidized dye molecules to emit pho-
toelectrons continuously (by absorbing the incident
flux of solar photons), the dye molecules should be
regenerated. The iodide ions in the electrolyte do-
nate electrons to the dye molecules and the dye
molecules are regenerated. After donating the elec-
trons, the iodide ions are oxidized to tri-iodide ions,
and they diffuse toward the CE which is generally
coated with a thin film of platinum. The photo-
electrons, arrived at the CE, are transferred to the
diffused tri-iodide ions and the circulation of the
electrons in the solar cell is completed. The tri-
iodide ions after gaining the electrons are reduced
to iodide ions [3–7].

The rate of reduction of tri-iodide ions into io-
dide ions is one of the factors that influence the
light to electricity conversion efficiency of solar cells.
Swift transfer of the tri-iodide ions into iodide ions
results high efficiency of the solar cells. The plat-
inum coated on the CE enhances the rate of elec-
tron transfer [6]. However, the use of platinum
in DSCs has some issues like high cost of plat-
inum and its instability in the electrolyte used in

DSCs [6]. Hence, researchers have proposed various
types of carbon as alternative catalyst to replace
platinum [3–12]. Out of the carbonaceous materi-
als, carbon nanotubes [5], carbon nanofibers [3,7,8],
graphene [9,10] etc. have exhibited comparable cat-
alytic ability for the reduction of the tri-iodide ions
as the platinum. However, the synthesis process of
these carbons is not simple. Consequently, these
types of carbonaceous materials are still expensive.

In 2019, Joshi, Lawaju, and B.K. reported the
composite of mustard oil lampblack and printer
toner as a low-cost and novel counter electrode
material. The light to electricity conversion effi-
ciency of the composite based solar cells was 3.20%
compared to 4.18% of the platinum based refer-
ence solar cells [4]. Similarly, Lawaju and Joshi
used soybean oil lamp black/printer toner compos-
ite as another counter electrode material and re-
ported an efficiency of 2.58% compared to 3.67%
of the platinum based DSCs [11]. Our previous
research has demonstrated that lampblack based
CEs can serve as a catalyst for the reduction of
tri-iodide ions although the catalytic ability of the
CEs was not as efficient as that of the platinum
based CEs. Herein, we have proposed low-cost and
efficient CEs prepared with candle soot for the re-
duction of tri-iodide ions in DSCs. We prepared
carbon samples derived from commercially avail-
able candle using flame-soot method and charac-
terized the candle soot by using X-rays Diffraction
(XRD), Raman spectroscopy, Scanning Electron
Microscopy-Energy Dispersive X-ray Spectroscopy
(SEM-EDS) for exploring structural and elemental
compositions.

In order to evaluate the catalytic ability for the
reduction of tri-iodide ions, impedance spectra can
be obtained by using either a working DSC or a
dummy cell [13]. The structure of a dummy cell is
simpler than that of a working DSC as it is fabri-
cated without dye-sensitized TiO2 film. A dummy
cell is prepared by assembling two electrodes coated
with catalyst. The electrodes are separated with
a spacer and the gap between them is filled with
an electrolyte [3,11,13]. A dummy cell fabricated
with two identical electrodes (known as symmet-
rical electrochemcal cell) is generally employed in
EIS [3, 11, 14]. The value of the charge transfer re-
sistance, Rct at the catalyst-electrolyte interface of
such cell indicates the ability of the catalyst for the
reduction of tri-iodide ions. Smaller value of Rct
at the catalyst-electrolyte interface implies better
ablitity for the tri-iodide reduction [3,13] and hence
high efficiency. In this research, EIS of symmetri-
cal dummy cells were carried out to evaluate the
catalytic ability of the candle soot by measuring
the Rct. The electrodes of the symmetrical elec-
trochemical cells were prepared by coating a film of
candle soot and value of the Rct at the interface of



P. Joshi et al./ BIBECHANA 20 (2023) 205-212 207

the candle soot based film and electrolyte was de-
termined. The value of the Rct at the candle soot-
electrolyte interface was ∼4.42 Ω cm2 compared to
∼5.04 Ω cm2 at the platinum-electrolyte interface.

2 Experimental

2.1 Sample preparation

Candles were bought at a grocery shop in Bhakta-
pur and carbon (soot) was prepared by flame-soot
method (low-cost, traditional and simple method
for the preparation of carbon). We have been using
this method of preparing carbon samples for appli-
cation in supercapacitors and dye-sensitized solar
cells. The procedure of preparing the candle soot is
similar to that described by Joshi et al., [4]. The ex-
perimental setup for the preparation of candle soot
is shown in Figure 1. The flame of the candle lamp
was obstructed by a porcelain bowl, Figure 1(a).
The candle soot, deposited on the inner surface of
the bowl, was collected for characterizations. A
photograph of the collected candle soot sample is
shown in Figure 1(b).

2.2 Characterization of candle soot for
surface morphology and elemental
composition

The elemental and structural composition of the
carbon samples were investigated using SEM-
EDS, XRD, and Raman spectroscopy. The SEM
and SEM-EDS were carried out using FEI Helios
Nanolab 400 Scanning Electron Microscope. Pow-
der diffractometer (D2 PHASER, Bruker) was used
for the XRD of the sample. Figure 2 shows the

SEM images with different magnifications of the
candle soot based carbon film prepared with car-
boxymethyl cellulose (CMC) as a binder. Figure
2(a) and Figure 2(b) are SEM images with ∼6500X
and ∼20000X magnifications, respectively. These
images show porous and rough surface of the film
formed by interconnected carbon particles of a few
tens of nanometers; particles of size ∼50 nm were
dominant ones in the film. Figure 3(a) is the EDS
spectrum and the insets show the scanned area
with elemental composition of the sample. The
candle soot comprises ∼96% of carbon and ∼4%
(atomic) oxygen. Figure 3(b) and Figure 3(c) de-
pict the mapping of the elements in the film. Fig-
ure 4 is the XRD spectrum of the candle soot. The
two broad peaks centered at 2 of ∼250 and ∼420
imply that the candle soot contains mainly amor-
phous form of graphitic carbon [3,15]. We also per-
formed Raman spectroscopy of the samples. Fig-
ure 5 shows Raman spectra with two prominent
peaks centered at wave number ∼1361 cm−1 and
∼1579 cm−1 which are known as D-band and G-
band, respectively. The D-band is attributed to
disoriented or defect riched carbonaceous material
while the G-band arises due to ordered or graphitic
carbon [3, 15, 16]. The findings of the Raman anal-
ysis is analogous to those from the XRD that the
candle soot comprises both disordered and ordered
forms of carbon. The portions of disordered car-
bon and ordered carbon (graphitic crystallites) in
the candle soot were calculated by Lorentzian line
shapes fitting of the Raman spectrum [17]. The
content of disordered carbon was ∼47% compared
to ∼53% of the ordered carbon and the value of
ID/IG was ∼0.9.

Figure 1: Preparation of carbon soot (a) experimental setup and (b) sample of candle soot.



P. Joshi et al./ BIBECHANA 20 (2023) 205-212 208

Figure 2: SEM images of candle soot based film with magnification of (a) ∼ 6500X and (b)∼20000X.

Figure 3: SEM images of candle soot based film with magnification of (a) ∼ 6500X and (b)∼20000X.

Figure 4: XRD spectrum of the candle soot. Figure 5: Raman spectrum of the candle soot.

3 Results and Discussion

The catalytic ability for the reduction of tri-iodide
ions of candle soot was investigated by EIS of
the electrochemical symmetrical cells prepared with
two identical electrodes coated with the film of the

candle soot.

A schematic diagram of the electrochemical cell
is shown in Figure 6. In order to prepare the car-
bon based electrodes, first of all, the paste of candle
soot was prepared by mixing ∼0.1 g of candle soot
with ∼1.5 ml of aqueous solution of CMC (∼2%



P. Joshi et al./ BIBECHANA 20 (2023) 205-212 209

concentration). The mixture was stirred and left
in an ambient environment overnight. The mix-
ture was centrifuged to remove excessive binder so-
lution and doctorbladed onto clean FTO-glass sub-
strates. The carbon film, coated on the FTO, was
dried at a temperature of ∼80 0C for 12 hours. Two
identical candle soot-based electrodes, prepared as
described above, were assembled with a sealant
and a liquid electrolyte containing iodide/tri-iodide
ions (Iodolyte AN-50 purchased from Solaronix,
Switzerland) was injected into the symmetrical cell.
To compare the catalytic ability of the candle soot-
based film, symmetrical electrochemical cells with
platinum coated electrodes were also prepared. The
EIS of the electrochemical cells were performed us-
ing Interface 1010E (Gamry Instruments, USA) in
potentiostatic mode with 10 mV AC signals of fre-
quencies ranging from 0.1 Hz to 100 KHz.

Figure 8 shows their equivalent circuits used to
fit the Nyquist plots [14,18–20]. Figure 8(a) is the
conventional equivalent circuit [14,20] in which Rs is
the series resistance. The Rs is mainly attributed
to the resistance of the electrodes and electrolyte
of the symmetrical cell. Rct is the charge transfer
resistance at electrode-electrolyte interface. Sim-
ilarly, ZN is Nernst diffusion impedance and this
deals with the impedance that arises from the dif-
fusion of ions in the liquid electrolyte. CPE is
a constant phase element, and this is related to
the double layer capacitor formed at the electrode-
electrolyte interface [3,11,14,18–20]. The equiva-
lent circuit shown in Figure 8(a) was used to fit
the Nyquist plot of the platinum based symmetri-
cal cell. For better fitting of the Nyquist plot of the
carbon soot based symmetrical cell, the equivalent
circuit [18] shown in Figure 8(b) was used. This
equivalent circuit was proposed by Kwon et al., to
fit Nyquist plots of porous carbon electrodes. In
this circuit Cdl is the double layer capacitor which
is equivalent to CPE of the conventional equiva-
lent circuit, Figure 8(a). In addition to the circuit
elements mentioned above, electron transport re-
sistance (Rtms) and trap capacitance (Ctrap) were
introduced in the new equivalent circuit [18]. Rtms

and Ctrap arise because of the carbon layer [18]. Ac-
cording to Kwon et al., carbon film is rich in super-
ficial defects which exist mostly at grain boundaries
and near carbon surface. Electrons are trapped in
such regions and Ctrap is formed [18]. The val-
ues of the parameters like Rtms, Ctrap, Rs, Rct

of symmetrical cells were obtained by curve fitting
of the Nyquist plots using Gamry Echem Analyst
(Gamry Instrument, USA). The values of Rtms and
Ctrap were ∼0.79 Ωand∼3.4 µF, respectively. For
the symmetrical cells with platinum electrodes, the
values of these parameters are assumed to be neg-
ligibly small as platinum is highly conducting and
the thickness of platinum used at the electrode is

ultrathin compared to the carbon layer [18].
Rs of the symmetrical cells with platinum and

candle soot were ∼24.17 Ω and ∼25.75 Ω, respec-
tively, and these values are nearly equal. In a
DSC, series resistance of the solar cell can affect
its power conversion efficiency. An increase in se-
ries resistance of the solar cell decreases fill factor
(FF), and low FF results lower efficiency of the so-
lar cell [3]. The series resistance of the solar cell
is mainly the sum of the ohmic resistances con-
tributed from its components–photoelectrode, elec-
trolyte, and counter electrode. As the values of the
ohmic resistances of the platinum based symmetri-
cal cell and candle soot based symmetrical cell are
almost equal, it can be concluded that replacing a
platinum with a candle soot in a DSC will not af-
fect the power conversion efficiency of the solar cell.
Along with the similarity of the Rs values of the
symmetrical cells, their charge transfer resistance,
Rct, were also nearly equal. The values of the Rctof
the symmetrical cells with platinum and candle soot
were ∼7.08 Ω (5.04 Ωcm2) and 6.21 Ω (4.42 Ωcm2),
respectively. The Rct at the catalyst-electrolyte in-
terface of the symmetrical cells tells us effectiveness
of the catalyst in the reduction of tri-iodide ions
into iodide ions in the electrolyte. Smaller value
of Rct implies a faster rate of the reduction of the
tri-iodide ions [3, 11, 14, 19, 20]. Hauch and Georg,
in 2001, mentioned that the value of the Rct of the
catalyst should not be more than 10 Ωcm2 for the
fabrication of a good DSC [14]. In this research,
Rct of the candle soot based electrodes was lower
than 10 Ωcm2. Hence, the candle soot based elec-
trodes can be used as counter electrodes of DSCs.
Moreover, the comparable value of Rct of the can-
dle soot and platinum based cells indicate that the
catalytic ability of candle soot based electrode for
the reduction of tri-iodide ions was also comparable
to that of the platinum based electrode.

The catalytic ability of the carbon based counter
electrode depends upon its surface morphology and
elemental composition. The SEM images shown in
Figure 2 revealed rough and porous surface texture
of the film. This yields a larger surface area and
provides a large electrolyte-catalyst interface, which
can be a merit for the fast reduction of tri-iodide
ions. Also, the concentration of carbon in the can-
dle soot is very high (∼96%) which could be another
reason of low Rctvalue of the candle soot. Simi-
larly, Raman spectroscopy revealed the existence of
disordered and ordered crystalline graphitic carbon
in almost same proportion in the candle soot sam-
ples. Ordered crystalline graphitic carbon yields
high electric conductivity. On the other hand, the
presence of disorded carbon or defect rich carbon in
the candle soots might have contributed in the fast
reduction of tri-iodide ions. Graphitic crystallites
exhibit a structural arrangement comprising basal



P. Joshi et al./ BIBECHANA 20 (2023) 205-212 210

planes, which are parallel to the layers of graphite,
and edge planes, oriented perpendicular to the basal
planes [21–23]. Notably, the electron transfer ki-
netics at the edge planes is more faster than that
in the basal planes [21, 22]. Generally, graphitic
particles of small size contain more edge planes
than those of large particles. Veerappan et al.,
adopted submicron (700 - 900 nm) sized colloidal
graphite (CG) and several micron sized graphite
particles (AG20) for fabrication of counter elec-
trodes of dye-sensitized solar cells. They reported

that the smaller sized graphite particles (CG) have
more edge planes than the larger ones. The sym-
metrical cells based on CG yielded lower Rct than
the symmetrical cell based on AG20 [22]. The can-
dle soot in this research has a particle size of ∼50
nm and the portion of the defective carbon in the
candle soot is significantly high. So, it can be ex-
pected that the candle soot contains plenty of edge
planes which contributed in fast reduction of tri-
iodide ions.

Figure 6: Schematic of electrochemical symmetrical cells. Modified from ref. [14]

Figure 7: Nyquist plots of electrochemical symmetrical cells with (a) platinum coated electrodes and (b)
candle soot coated electrodes.

Figure 8: quivalent circuits used to fit Nyquist plots of electrochemical symmetrical cells with (a) plat-
inum coated electrodes modified from ref. [14] and (b) candle soot coated electrodes modified from ref.
[18].



P. Joshi et al./ BIBECHANA 20 (2023) 205-212 211

4 Conclusion

Candle soot was investigated aiming to use it
as a catalyst to replace expensive platinum used
in DSCs. The candle soot consists of carbon
nanoparticles comprising ordered and disordered
carbon with a high concentration of carbon (∼96%,
Atomic). The EIS of the symmetrical electrochem-
ical cells with the candle soot based electrodes
showed that their series resistance and charge trans-
fer resistance are nearly equal to those of the plat-
inum based symmetrical cells. Candle soot can be
prepared with low-cost precursor and using simple
techniques. So, this carbonaceous material can be
a low-cost and efficient counter electrode material
alternate to expensive platinum used in DSCs.

Acknowledgements

The authors are thankful to Associate Professor
Sudarshana Shakya, Bhaktapur Multiple Campus
(BMC), Surakshya Ghamosu (BMC), Anita Bhadel
(BMC), and Shared Research Operation (SRO) fa-
cility for providing opportunity to use the SEM at
Texas State University, San Marcos, Texas. 9510

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https://doi.org/10.1039/B413177K

	Introduction
	Experimental
	Sample preparation
	Characterization of candle soot for surface morphology and elemental composition

	Results and Discussion
	Conclusion