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CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS
VOL. 41, 2014
A publication of
The Italian Association
of Chemical Engineering
www.aidic.it/cet
Guest Editors: Simonetta Palmas, Michele Mascia, Annalisa Vacca
Copyright © 2014, AIDIC Servizi S.r.l.,
ISBN 978-88-95608-32-7; ISSN 2283-9216
Effect of Different Green Cellulosic Matrices on the
Performance of Polymeric Dye-Sensitized Solar Cells
Federico Bellaa,b, Annalisa Chiapponeb, Jijeesh R. Naira, Giuseppina Meligrana,
Claudio Gerbaldi*a,b
a
Department of Applied Science and Technology – DISAT, Politecnico di Torino, , Corso Duca degli Abruzzi 24, 10129
Torino, Italy
b
Center for Space Human Robotics @Polito, Istituto Italiano di Tecnologia, Corso Trento, 21, 10129 Torino, Italy
claudio.gerbaldi@polito.it
Carboxymethyl cellulose (CMC) and microfibrillated cellulose (MFC) are here employed as bio-sourced
fillers in quasi-solid electrolytes for polymeric dye-sensitized solar cells (DSSCs). High and durable
photovoltaic performances are obtained, due the influence of the different cellulosic fillers on the cell
parameters which are here compared and discussed. The present findings open up new intriguing
prospects in the design of efficient energy conversion devices with eco-friendly/natural additives.
1. Introduction
Solar energy is a resource of primary importance, and the development of technologies for its conversion
is the core topic of third millennium scientific research (Ampelli et al., 2013; Ravaghi-Ardebili et al., 2013).
Being an efficient alternative to the conventional silicon based solar cells, dye-sensitized solar cells
(DSSCs) have been widely considered as a promising practical candidate (Grätzel et al., 2003). One of the
main targets for this technology to intrude into the large-scale global market is to improve its long-term
stability (Lee et al., 2014), which is at present unsatisfactory due to the evaporation of solvents, poor
hermetic sealing and gas permeability (Odobel et al., 2012). In this regard, the replacement of the
commonly used liquid electrolyte, prone to leakage and evaporation in real applications, with a solid/quasi-
solid one, that can ensure high performance and prolonged durability, is a viable option and a stimulating
challenge. In this respect, gels (Bella et al., 2013a), polymer membranes (Bella et al., 2013b), salt-doped
solid polymers (Su'ait et al., 2014), p-type direct band-gap semiconductors (Li et al., 2012) and polymeric
hole conductors (Bailie et al., 2014) have been widely investigated as valid alternatives.
By trapping a suitable liquid electrolyte in the soft cages of a host polymer matrix, gels and polymer
electrolyte membranes can ensure high ionic conductivity and long-term stability. Nevertheless, besides
obtaining high-level long term performance, it is also necessary to design products, processes and
production systems that can be easily scaled-up, which comply with the main canons of green-chemistry
for a sustainable development (Kim et al., 2013). In particular, when considering the components to be
used in energy conversion and storage devices electrolytes, bio-based materials deserve to be primarily
considered as they are readily available, easy to be functionalized and biodegradable (Gao et al., 2014).
Among them, natural cellulose and its derivatives stand out, and their application in flexible Li-ion batteries
(Chiappone et al., 2013), supercapacitors (Wang et al., 2012) and hydrogen production (Ruppert et al.,
2014) is currently under thorough investigation by the scientific community.
In this work, we compare the characteristics of two different cellulosic materials as fillers in quasi-solid
electrolytes for DSSC: carboxymethyl cellulose (CMC) and microfibrillated cellulose (MFC). CMC is an
anionic polysaccharide obtained by the modification of cellulose with sodium hydroxide and
monochloroacetic acid. It is a low cost, biocompatible and edible product, which has been recently
introduced in the field of energy storage and conversion as biopolymer electrolyte for rechargeable proton
DOI: 10.3303/CET1441036
Please cite this article as: Bella F., Chiappone A., Nair J., Meligrana G., Gerbaldi C., 2014, Novel cellulose-based composite polymer
electrolytes for green, efficient and durable energy conversion and storage devices, Chemical Engineering Transactions, 41, 211-216
DOI: 10.3303/CET1441036
211
battery (Samsudin et al., 2014), substrate for thermal-energy storage (Jayaprakash et al., 2011) and
binder for Li-ion batteries graphite anodes (Lee et al., 2005). MFC is obtained by a hydrolysis-free
homogenization process; it may be envisaged as a material composed of cellulose greatly expanded in
surface area and consisting of aggregates of natural cellulose microfibrils (diameter ≈ 20-60 nm, length
from 1 to several micrometres) presenting a web-like structure (Siró et al., 2010). Due to this unique
feature, MFC exhibits both the crystalline and the amorphous domains of cellulose, with a percentage of
crystallinity that ranges between 50 and 70%. In the field of energy technologies, MFC has been proposed
as separator (Xu et al., 2014) and reinforcing agent (Chiappone et al., 2011) for Li-ion battery polymer
electrolytes. In this paper, CMC- and MFC-based polymeric DSSCs were fabricated and characterized,
and outstanding photovoltaic performance and noticeable long-term stability were obtained.
2. Materials and methods
2.1 Materials
Carboxymethyl cellulose sodium salt (CMC, average Mw: 250000), bisphenol A ethoxylate dimethacrylate
(BEMA, average Mn = 1700), poly(ethylene glycol) methyl ether methacrylate (PEGMA, average Mn =
475), polyethylene oxide (PEO, average Mw: 400000), acetonitrile (CH3CN), sodium iodide (NaI), 1-methyl-
3-propylimidazolium iodide (MPII), 4-tert-butylpyridine (TBP) and iodine (I2) were purchased from Sigma-
Aldrich. MFC particles were prepared as described elsewhere (Chiappone et al., 2013) from a sulphite
bleached spruce pulp (supplied by Domsjo); a MFC aqueous suspension (2 wt%) was used in the present
work. 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173, free radical photoinitiator) was obtained
from Ciba Specialty Chemicals.
Conducting glass plates (FTO glass, Fluorine doped Tin Oxide over-layer, sheet resistance 7 Ω sq-1,
purchased from Solaronix) were cut into 22 cm2 pieces and used as substrate for both the deposition of a
thin porous layer of TiO2 from a paste (DSL 18NR-AO, Dyesol) and the fabrication of platinized counter-
electrodes. Sensitizing dye cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II)
bis(tetrabutylammonium), i.e. N719 - Ruthenizer 535 bis-TBA, was purchased from Solaronix.
2.2 Preparation of the CMC-based gel-polymer electrolyte
A homogeneous liquid electrolyte solution was prepared by thoroughly mixing NaI, I2, MPII and TBP in
acetonitrile (see Bella et al. 2013a). To produce the gel-polymer electrolyte, the liquid solution was mixed
with specific amounts of PEO and CMC. In preliminary experiments, a chemometric design of experiments
(DoE) approach was adopted to determine the best gel-polymer electrolyte composition. The formulation
used in this work is the following: 55 wt% liquid electrolyte, 45 wt% solid components (9 wt% CMC in
PEO). This system was kept under constant stirring for 2 h at 50 °C, until a homogeneous highly viscous
jelly-like mixture was obtained.
2.3 Preparation of the MFC-based polymer electrolyte membrane
Differently from the physical gel-polymer electrolyte preparation, the fabrication of the MFC-based polymer
electrolyte membrane was performed through photoinduced polymerization. It is a fast, cost effective,
easily up-scalable and environmentally friendly process, that does not involve the use of any solvents and
catalysts (Bella et al., 2013c). A BEMA:PEGMA (30:70 weight ratio) reactive mixture was prepared, along
with 2 wt% of photoinitiator. A MFC suspension was added to the reactive formulation in order to obtain a
composite containing 30 wt% of filler. The liquid mixture was coated on a polypropylene (PP) substrate
and left 24 h in an oven at 70 °C to assure the water evaporation. Subsequently, it was UV-cured for 3 min
under N2 flux by using a medium vapour pressure Hg lamp (Helios Italquartz), having a irradiation intensity
on the surface of 30 mW cm-2. The resulting self-standing films were peeled off from the PP substrate and
treated in high vacuum at 80 °C overnight, to completely remove traces of moisture and impurities.
Polymer membranes having a final thickness of 100 ± 5 µm (measured with a Mitutoyo series 547
thickness gauge equipped with an ABSOLUTE Digimatic Indicator model ID-C112XBS, with a resolution of
±1µm and a max measuring force of 1.5 N) were obtained.
2.4 Fabrication and characterization of quasi-solid DSSCs
As regards the photoanode preparation, FTO covered glasses were rinsed with acetone and ethanol in an
ultrasonic bath for 10 min. Then, traces of solvents were removed by flash evaporation at 500 °C on a
hotplate. A TiO2 paste layer with a circular shape (Ø = 1 cm) was deposited twice onto FTO by screen
printing technique (AT-25PA, Atma Champ Ent. Corp.) and dried at 100 °C for 10 min on a hot plate. A
sintering process (525 °C, 30 min) led to obtain a mesoporous TiO2 film with an average thickness of 12
µm, measured by profilometry (P.10 KLA-Tencor Profiler). The sensitization of the photoelectrodes was
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performed by soaking in a 0.4 mM N719 dye solution in ethanol for 16 h at ambient temperature, followed
by washing in ethanol to remove the unadsorbed dye.
As for the counter electrodes, 6 nm Pt thin films were deposited by sputtering (Q150T ES, Quorum
Technologies Ltd) onto FTO glasses, previously cleaned with the same rinsing method described above.
CMC-based DSSCs were assembled by spreading with a spatula the gel-polymer electrolyte over the
sensitized photoanode surface. Then, the two electrodes were clamped together, and kept at a distance of
70 μm by means of a layer of inert tape.
As regards the MFC-based polymer electrolyte membranes, they were activated by rapid soaking into a
liquid electrolyte (0.45 M NaI, 0.056 M I2, 0.55 M TBP), and then placed between the two electrodes.
A cyanoacrylate based glue was used for the final sealing of both kinds of cells.
The active area of the cells was 0.78 cm2 and the photovoltaic measurements were performed with a 0.22
cm2 rigid black mask. I-V electrical characterizations under AM1.5G illumination (100 mW cm-2) were
carried out using a class A solar simulator (91195A, Newport) and a Keithley 2440 source measure unit.
The photoelectric performances (i.e., fill factor FF and light-to-electricity energy conversion efficiency η)
were calculated by the following equations:
(1)
(2)
where Voc is the open-circuit potential (V), Jsc the short-circuit current density (mA cm
-2), Pin the incident
light power density (mW cm-2), Vmax (V) and Jmax (mA cm
-2) are the potential and the current density in the
J-V curves, respectively, at the point of maximum power output. I-V measurements were also performed at
light intensities in the range of 0.4-1 sun, by using neutral density filters.
3. Results and discussion
3.1 Gel-polymer electrolyte and polymer electrolyte membrane
Two different quasi-solid electrolytes are investigated in this work, with a dual aim. First of all, we would
like to demonstrate the possibility of using green, cellulose-based resources for the preparation of
sustainable photovoltaic devices. The second aim is to present the characteristics of two different
electrolyte systems, one in the form of membrane, the other in the form of gel.
The CMC-based gel-polymer electrolyte was dense, translucent, not flowing; it could stay stable for long
time even in horizontal line and it did not droop when overturned, as shown in Figure 1(A-B). The
polymeric and cellulosic matrices were homogeneously mixed, and no phase separation occurred even
after a prolonged storage.
Concerning the MFC-based polymer electrolyte membrane, the photoinduced polymerization process led
to a rapid light-driven curing of a macromolecular network which helped in maintaining a homogeneous
dispersion of the filler. As can be seen in Figure 1C, the composite membrane appeared highly
homogeneous, and the interpenetration between the polymeric matrix and network of fibres was excellent,
especially considering that no functionalization of MFC was carried out. After activation in the
iodide/triiodide-based liquid electrolyte, the membrane remained free-standing, easy to handle, non-tacky
and extremely flexible, as clearly visible in Figure 1D.
Figure 1: Aspect of the quasi-solid electrolytes under study. A) CMC-based gel-polymer electrolyte on a
spatula; B) CMC-based gel-polymer electrolyte being overturned for 10 min; C) MFC-based polymer
membrane; D) activated MFC-based polymer electrolyte membrane.
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3.2 Photovoltaic characterization of quasi-solid DSSCs
The two cellulose-based quasi-solid electrolytes were sandwiched between cell electrodes and their I-V
characteristics were measured under 1 sun irradiation. The photovoltaic parameters are listed in Table 1;
the values of the corresponding liquid cell (i.e., a DSSC assembled with the same standard electrodes and
filled with the same liquid electrolyte used for the activation of the quasi-solid electrolytes) are also
reported for comparison purposes. Furthermore, gels and membranes were also prepared in the absence
of the cellulosic matrix, in order to unravel its effective role.
As regards CMC-based gel-polymer electrolyte, a light-to-electricity conversion efficiency of 5.18 % (Jsc =
10.03 mA cm-2, Voc = 0.75 V, FF = 0.69) was measured. The performance of this bio-based cell is much
better than those reported in the literature for other types of biopolymers, such as agarose (Singh et al.,
2013) and κ-carrageenan (Mobarak et al., 2012).
Concerning the MFC-based polymer electrolyte membrane, a remarkable efficiency of 7.03 % (Jsc = 15.20
mA cm-2, Voc = 0.76 V, FF = 0.61) was obtained. To the best of our knowledge, this is the best
performance ever measured for a green polymer electrolyte-based DSSC under 1 sun irradiation.
Furthermore, the efficiency values is even higher than that of the standard liquid cell (η = 6.92 %).
Table 1: Photovoltaic parameters of DSSCs assembled with CMC-based or CMC-free gel-polymer
electrolytes and MFC-based or MFC-free polymer electrolyte membranes. Values for the corresponding
liquid cell used as reference are also reported.
Electrolyte Jsc (mA cm
-2) Voc (V) FF η (%)
CMC-based gel 10.03 0.75 0.69 5.18
CMC-free gel 10.32 0.70 0.71 4.39
MFC-based membrane 15.20 0.76 0.61 7.03
MFC-free membrane 11.70 0.65 0.59 4.42
Liquid 16.30 0.69 0.60 6.92
A detailed analysis of the photovoltaic parameters listed in Table 1 allows a comparison between the
different electrolytes presented in this work.
As regards Jsc, the introduction of CMC did not cause any relevant variation in the photocurrent density
values. On the contrary, Jsc noticeably increased from 11.7 to 15.2 mA cm
-2 when MFC was introduced in
the polymer electrolyte membrane. This was ascribed to the partial crystalline structure of the MFC
network of fibers, thus resulting in a multiplanar light scattering grating. As a consequence, photons which
passed unharmed through the photoanode were recovered and reflected back to the N719 dye molecules,
thus increasing the sensitizer excitation and, as a consequence, the charge photo-injection. By means of
diffused reflectance measurements, we found that the MFC-based polymer electrolyte had a scattering
value equal to 76%, at the wavelength of 530 nm (absorption peak of the N719 dye).The corresponding
value for the MFC-free sample was ~ 10%. Regardless of the type of quasi-solid electrolyte, Table 1
shows that the highest Jsc was measured for the liquid cell: this is a consequence of the fact that the
polymer matrix partially inhibits the ionic diffusion between the two electrodes.
As regards Voc values, it was observed that both CMC and MFC markedly improved the cell photovoltage,
which is given by the difference between the Fermi level of TiO2 and the redox potential of the electrolyte.
Cellulosic materials can establish hydrogen bonds with the TiO2 active material particles in the
photoelectrode, thus hindering the charge recombination between I3
- ions and electrons in the conduction
band of the semiconductor. The reduced recombination phenomena increases the recombination
resistance at the TiO2/electrolyte interface, thus resulting in improved Voc values.
The ameliorated recombination resistance at the TiO2/electrolyte interface in the presence of cellulosic
matrices also enhanced FF values. It was also observed that FF values of gel-polymer electrolytes-based
DSSCs were higher than the polymer electrolyte membrane-based ones. This is due to the fact that gels,
being spreadable and deformable under pressure, can more deeply penetrate the TiO2 mesoporous
structure, thus contacting a higher portion of dye molecules to be regenerated. On the contrary,
membranes are self-standing and their penetration into the electrode structure is less effective.
The effect of different light intensities on the photovoltaic performance of the different quasi-solid systems
under study was then investigated. As shown in Figure 2, Jsc values of both cells increased with an
increased irradiation intensity, while Voc remained almost constant. The CMC-based gel DSSC
demonstrated a 6.98 % light-to-electricity conversion efficiency under the illumination of 40 mW cm-2
(Figure 2A), thus showing excellent performances even under conditions of not optimal lighting (which
usually occur in real outdoor applications). Under the same irradiation, the MFC-based DSSC showed a
8.25 % efficiency (Figure 2B), which is an outstanding value for a quasi-solid device. The inset graphs of
214
Figure 2 also show the linear dependence of Jsc on the light intensity from 0.4 to 1 sun, indicating that
neither the I-/I3
- transportation nor electron transfer at the Pt/electrolyte interface represent the rate-
determining step.
Figure 2: J-V curves of DSSCs assembled with CMC-based gel-polymer electrolyte (A) and MFC-based
polymer electrolyte membrane (B), measured under different light intensities. Inset graphs: Jsc values
plotted as a function of the different light intensity.
3.3 Aging test of quasi-solid DSSCs
In order to assess the long-term stability of the fabricated DSSCs, a 250 h aging tests was carried out at
the temperature of 60 °C under dark. As shown in Figure 3, both the quasi-solid cells showed an improved
efficiency in the initial 100 h of ageing, due to the achievement of the optimum filling of the electrolytes into
the mesoporous photoanode. After 250 h, the CMC-based cell showed an efficiency equal to the 98% of
the initial value, while the MFC-based device performance was even higher than the initial value. Such
excellent results, especially if compared with the 24 % efficiency decrease of the standard liquid cell, are
definitely impressive in view of the real-world application of bio-based polymeric DSSCs.
Figure 3: Normalized light-to-electricity conversion efficiencies versus conservation time at 60 °C of
DSSCs assembled with the CMC-based gel-polymer electrolyte and the MFC-based polymer electrolyte
membrane. Curve for the corresponding liquid cell is also reported.
4. Conclusions
We herewith presented the smart efficacy of cellulosic matrices in cumulatively increasing the
photocurrent, photovoltage and long-term stability of polymeric DSSC devices. This is even more relevant
considering that cellulose derivatives are green bio-sourced materials which can be easily incorporated in
various polymeric systems obtained via different polymerization processes as demonstrated in this work.
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