Performance Analysis of Jatropha Oil-Based Polyurethane Acrylate Gel Polymer Electrolyte for Dye-Sensitized Solar Cells.

Biobased polymers are useful materials in substituting conventional petroleum-derived polymers because of their good properties, ready availability, and abundance in nature. This study reports a new jatropha oil-based gel polymer electrolyte (GPE) for use in dye-sensitized solar cells (DSSCs). The GPE was prepared by mixing jatropha oil-based polyurethane acrylate (PUA) with different concentrations of lithium iodide (LiI). The GPE was characterized by infrared spectroscopy, thermal analysis, lithium nuclear magnetic resonance analysis, electrochemical analysis, and photocurrent conversion efficiency. The highest room-temperature ionic conductivity of 1.88 × 10-4 S cm-1 was obtained at 20 wt % of LiI salt. Additionally, the temperature-dependent ionic conductivity of the GPE exhibited Arrhenius behavior with an activation energy of 0.42 eV and a pre-exponential factor of 1.56 × 103 S cm-1. The electrochemical stability study showed that the PUA GPE was stable up to 2.35 V. The thermal stability of the gel electrolyte showed an improvement after the addition of the salt, suggesting a strong intermolecular interaction between PUA and Li, which leads to polymer-salt complexation, as proven by Fourier transform infrared spectroscopy analysis. A DSSC has been assembled using the optimum ionic conductivity gel electrolyte which indicated 1.2% efficiency under 1 sun condition. Thus, the jatropha oil-based GPE demonstrated favorable properties that make it a promising alternative to petroleum-derived polymer electrolytes in DSSCs.

Introduction

Polymer electrolytes (PEs) have been extensively explored for electrochemical device applications such as fuel cells, batteries, sensors, photochemical solar cells, and supercapacitors.[1] PEs offer numerous advantages over conventional liquid electrolytes in terms of stability, ease of handling, and good safety performance. PEs should possess specific criteria including a high ionic conductivity, or at least comparable to that of liquid electrolytes, as well as thermal, chemical, and electrochemical stability[2] if the materials are to be considered as a practical replacement for the liquid electrolytes.

Solar energy is one of the most promising renewable energy sources among other options available nowadays. One important application of polymer electrolytes is in solar cells. The solar cell or photovoltaic cell is a device that converts light into electrical energy. It is convenient as it does not require any mechanical movement or movable parts to generate electricity. Dye-sensitized solar cells (DSSCs) are the third generation of photovoltaic cells invented in 1991 by a team led by Gratzel.[3] Currently, the photovoltaic market is still dominated by silicon technology despite the expensive raw materials and the high energy-consuming process technologies used. DSSC is emerging as an alternative solar harvester, being of low cost and with a simple preparation procedure.[4,5] Generally, a DSSC is made up of a photoanode, a counter electrode, and an electrolyte in between. The photoanode consists of a dye-coated nanocrystalline semiconductor oxide, typically TiO2. It acts like a roadway for the electrons coursing through the cell. The counter electrode is usually a film of graphite or platinum. An electrolyte containing a redox couple fills the gap between the electrodes. The redox mediator is usually an organic solvent containing a redox system, such as an iodide/triiodide couple.[6,7]

The main challenge in the existing DSSC is the evaporation and leakage of the liquid electrolyte that hinders the long-term performance and practical use of the DSSC. Thus, much attention has been given and thorough investigation has been conducted to tackle this problem, for example, by introducing solid-state or gel polymer electrolytes (GPEs).[8] In contrast with liquid electrolytes, such gel polymers have no or very limited leakage problems.[9,10] Gel electrolytes exhibit higher ionic conductivities compared to the solid polymer electrolyte systems. Various kinds of gel electrolytes have been reported, in which different types of polymer matrixes have been used, such as polyvinyl alcohol,[11] polyacrylonitrile,[12−14] polyacrylamide,[15] poly(methyl methacrylate),[16] and polyethylene oxide.[9,17,18] The interest in utilizing biobased polymers as the host polymer has accelerated in recent years because of the desire to find an alternative to the petroleum-based polymers. It is a well-known fact that fossil fuels will be to run out in the near future. In view of this, the use of easily accessible natural sources could provide a better strategy.

Polyurethane is a versatile material suitable to be used as the host polymer matrix. Typically, polyols used in polyurethane synthesis are made from a chemical intermediate derived from petroleum or natural gas. With the increasing emphasis on environmental issues and the importance of utilizing renewable resources in industrial processes, efforts have been made to produce biobased polyols to replace the conventional counterparts.[19] A variety of renewable feedstock can be used for the production of biobased polyols such as vegetable oils,[20−22] fatty acids,[23] fatty acid methyl esters,[24] crude glycerol,[25] wood,[26,27] crop residues,[28,29] and protein feedstocks.[30] Among all, vegetable oils have proven to be the most attractive source for the preparation of biobased polyols, owing to the fact that they are readily available and abundant in nature. In fact, the conversion of vegetable oils into polyols has been one of the most promising developments transforming the oil industries.

Jatropha oil is a nonedible oil that is extracted by crushing the seeds of the Jatropha curcas plant. It is a small or large shrub plant and is a drought-resistant crop belonging to the Euphorbiaceae family. In Malaysia, jatropha is one of the important crops after oil palm and rubber. Commercial cultivation of jatropha in Malaysia is mainly for biodiesel production.[31] The oil is nonedible because of the presence of the toxic phorbol ester group.[32] Jatropha oil is odorless and colorless when fresh and becomes yellow on standing. As for the chemical composition, the oil contains unsaturated fatty acids which are oleic acid (18:1) and linoleic acid (18:2). Herein, jatropha oil was used to prepare polyols for the production of a polyurethane acrylate (PUA) GPE. The characteristics of the jatropha oil-based GPE were evaluated and its performance in a DSSC application was investigated.

Results and Discussion

Infrared Spectroscopy Analysis

Fourier transform infrared spectroscopy (FTIR) was used to study the interaction and the effect of lithium iodide (LiI) salt concentration on the properties of the gel electrolytes. Figure shows the FTIR spectra for PUA and PUA–LiI gel electrolytes at various LiI salt concentrations. Generally, PUA was made up of a soft segment region of the oil-based polyol and a hard segment region of the urethane linkage and end-capped with the acrylate group. The spectra for the whole system did not show the stretching vibration band at 2270 cm–1 of the isocyanate (−N=C=O) group, which confirmed that all of the isocyanate groups reacted during polymerization.[33] Three important regions of the spectra were considered, including the NH stretching mode (3700–3100 cm–1), carbonyl stretching mode (1750–1600 cm–1), and ether stretching mode (1150–1000 cm–1).[22] These regions were the subject of focus as they correspond to the oxygen and nitrogen atoms in the PUA molecules that could coordinate with the lithium ions from the doping salt.

Figure 1

FTIR spectra of the PUA electrolyte at different LiI weight loadings and the vibration regions of (A) amine, (B) carbonyl, and (C) ether groups.

The corresponding peaks in Figure show the vibration region and the possible coordination sites of Li+ ions at different positions: (A) to the nitrogen atoms of the NH groups, (B) to the carbonyl oxygen, and (C) to the ether oxygen. There were notable peak shifting to a higher wavenumber and changes in the peak intensity for the amine region after the addition of the LiI salt. As for the carbonyl and ether regions, the peak shifted to a lower wavenumber for the PUA–LiI electrolytes. From these observations, it can be deduced that the Li+ ions coordinated with the oxygen atoms (C=O and C–O–C), which resulted in the stretching of the C=O bond toward Li+. The stretching affected the neighboring N–H by restricting the N–H bending and stretching. A similar observation has been reported for PU–LiI and PU–NaI systems.[20] The possible formation of complexes between Li+ ions with the oxygen atoms from the PUA host is schematically shown in Figure a,b. All of these interactions led to a change in the microstructure of the polymer, which was also evident from the thermal characterization.

Figure 2

Possible coordination sites of Li+ ions with (a) urethane group and (b) ether group of PUA.

7Li Nuclear Magnetic Resonance Analysis

The overall lithium nuclear magnetic resonance (NMR) properties of the system were studied by using four selected representative compositions. Figure displays the 7Li NMR spectra for the PUA–LiI GPE at different salt concentrations, and the inset shows the magnified version. Chemical shifts were externally referenced to solid LiCl. The NMR spectra displayed a single resonance peak for all of the samples. As illustrated by the spectra, the addition of LiI significantly shifted the 7Li signal downfield, implying that there was an interaction between LiI and PUA. It was suggested that the interaction occurred between the Li+ ions and the oxygen atoms in the soft segments and the urethane group in the hard segments.[34,35]

Figure 3

7Li NMR of the GPE at (a) 10, (b) 15, (c) 20, and (d) 25% of LiI salt loadings.

Thermal Analysis

Thermal stability is one of the key factors determining the suitability of the GPE application in DSSCs. The thermogravimetric (TG) and its corresponding differential thermogravimetric (DTG) thermograms of pure PUA and PUA–LiI gel electrolytes are depicted in Figure A,B. Related thermal stability data are shown in Table , where Tdmax refers to the temperature at which maximum degradation occurred. Three main processes of degradation can be observed in the thermograms as the gel electrolytes are subjected to continuous heating. The thermal decomposition (Td) stages of pure PUA were observed at the temperature regions of 50–230, 230–350, and 350–530 °C for Td1, Td2, and Td3. Meanwhile, for PUA–LiI gel electrolytes, the decomposition stages were at 50–270, 270–415, and 415–530 °C, respectively. The first weight loss step was attributed to the degradation of the hard segments of the urethane linkage. There are three decomposition mechanisms of the urethane bond that were proposed at this stage, namely (i) degradation of urethane to isocyanate and alcohol, (ii) the formation of primary amine and carbon dioxide, and (iii) the formation of secondary amine and carbon dioxide.[21,33,36,37] The second and the third correspond to the decomposition of the soft segments of PUA which was related to the ester group. Overall, pure PUA showed an early degradation temperature compared to the rest of the systems. Upon the addition of LiI salts, the degradation temperature increased, suggesting there were changes in the dimensional arrangement of the matrix. The DTG peak of Tdmax1, Tdmax2, and Tdmax3 increased in the presence of lithium salts. These changes could be explained by the fact that the lithium salts in ionic form would be capable to disrupt the PUA chain, creating a randomized arrangement. The strong intermolecular interaction between the lithium ions and the PUA chain required more heat to break the bond, which explained the increase in thermal stability.[36]Figure also shows that the residue of the sample slightly increased with the LiI content. The inference indicated an appreciable stable matrix suitable for normal conditions and outdoor applications. The differential scanning calorimetry (DSC) thermograms of pure PUA and PUA–LiI gel electrolytes at different salt loadings are depicted in Figure C, and the Tg values are listed in Table . A decreasing trend of Tg could be observed as the LiI loading increased up to 20 wt % and decreased thereafter. The decrease in Tg resulted in an enhanced mobility of the ions, and subsequently an increase in the ionic conductivity of the gel electrolytes, which was revealed in the impedance study.[38]

Figure 4

Variations of (A) TG, (B) DTG, and (C) DSC thermograms of PUA containing different amounts of LiI concentration.

Table 1

Analysis of TG, DTG, and DSC Thermograms

composition	Tdmax1 (°C)	Tdmax2 (°C)	Tdmax3 (°C)	residue (%)	Tg (°C)
PUA–LiI 0%	110	282	450	5	–19
PUA–LiI 5%	218	364	467	7	–22
PUA–LiI 10%	222	352	466	11	–23
PUA–LiI 15%	217	344	472	11	–23
PUA–LiI 20%	229	339	474	17	–24
PUA–LiI 25%	222	336	472	24	–21
PUA–LiI 30%	224	333	473	22	–21

Impedance Analysis

Impedance spectroscopy analysis was performed to study the conductivity properties of the PUA electrolyte. Figure shows the variation of log ionic conductivity of the GPE as a function of the LiI content at room temperature. Based on the data, it is observed that the conductivity increased by increasing the salt content from 0 to 20 wt %. The conductivity increased four orders of magnitude, from 10–8 for pure PUA to 10–4 for PUA–LiI 20 wt %. The increase in conductivity could be related to the increase in the amount of effective mobile charge carriers. The highest conductivity of 1.88 × 10–4 S cm–1 for PUA–LiI 20 wt % was consistent with the DSC results for which this system had the lowest glass-transition temperature. Theoretically, as Tg decreased, the polymer tended to become more flexible, therefore enhancing the ionic conductivity. Beyond the salt dopant of 20 wt %, the conductivity decreased. At this level, the interionic distance decreased and the ion–ion interactions became progressively significant. It has been deduced that at high salt concentrations, there would be a possibility of occurrence of ion aggregation, resulting in the formation of ion clusters, thereby decreasing the number of effective mobile charge carriers and, simultaneously, the ionic conductivity.[39]

Figure 5

Variation of log conductivity of the PUA–LiI electrolyte at room temperature.

The temperature-dependent ionic conductivity of the highest conducting gel electrolyte (PUA–LiI20) is depicted in Figure by using the Arrhenius model at a temperature range of 303–373 K. The conductivity of the gel electrolyte showed a linear relationship against temperature with a regression line of 0.9936, indicating that the system followed the Arrhenius behavior, which suggests that the conductivity was thermally assisted. An increasing trend of conductivity could be observed from the graph with temperature. As the temperature increased, the free volume increased because of lattice vibration. This promoted ion mobility to favor interchain and intrachain ion hopping, thus increasing the conductivity of the gel electrolyte.[40] The pre-exponential factor (σ0) could be obtained from the y-axis and the plot intercept, whereas the activation energy (Ea) could be calculated from the slope, m, of the plot of log conductivity against 1000/T. The activation energy and pre-exponential factor values were calculated from the graph as 0.42 eV and 1.56 × 103 S cm–1, respectively.

Figure 6

Arrhenius plot for the conductivity of PUA–LiI20 GPE.

Chronoamperometry

The total ionic transference number of the highest conducting gel electrolyte containing 20 wt % LiI salt was determined by the dc polarization method. This technique was used to analyze the mobile species in the electrolyte, which were either ions or electrons.[41]Figure shows the plot of the normalized current against time for the gel electrolyte. It was observed that the current decreased with an increase in time because of the depletion of ionic species and becomes constant in the fully depleted condition. The residual current was due to the electron migration across the electrolyte interfaces. The graph shows the initial total current (IT) falling rapidly in the first 50 s. The ionic (tion) and electronic (telectron) transference numbers of the gel electrolyte were further determined by using 3 and 4. The obtained values of IT and IS were 6.94 × 10–7 and 1.83 × 10–8 A. The tion and telectron values of the PUA gel electrolyte were 0.97 and 0.03, respectively. The ionic transference number was significantly higher than the electronic transference number. Thus, it could be concluded that the charge transport in the gel electrolyte was primarily because of ionic migration, and the electron contribution was negligible in the system. The result was found to be in agreement with the findings reported in previous studies.[42,43]

Figure 7

Polarization current vs time plot for the PUA–LiI20 gel electrolyte.

Electrochemical Stability

The electrochemical stability of electrolytes is a crucial characteristic to be evaluated based on the application point of view in electrochemical devices such as photoelectrochemical cells, energy devices, and batteries. The electrochemical stability window of the PUA–LiI20 gel electrolyte was evaluated by linear sweep voltammetry (LSV), and its corresponding voltammogram is shown in Figure . Initially, a small current was observed through the working electrode from the open-circuit potential until it reached a potential of 2.35 V. After this, the current increased gradually, which could be related to the decomposition of the polymer electrolyte.[44] The PUA–LiI20 gel electrolyte with an electrochemical window of 2.35 V was stable enough for use as the electrolyte material of a DSSC.

Figure 8

LSV of the PUA–LiI20 electrolyte.

Photocurrent Voltage Measurement

The effectiveness of a DSSC is measured by the light-to-electrical energy conversion efficiency. In essence, the electrolyte has a huge influence on the performance of a DSSC. A preliminary study was performed with the fluorine-doped tin oxide (FTO)/TiO2–dye/PUA–LiI/Pt/FTO configuration using three representative gel electrolyte ratios of 10, 20, and 30 wt % LiI. Figure shows the J–V characteristic curves of the DSSC with PUA–LiI GPE systems and the inset figure shows the parameters. Among the three GPE systems, PUA–LiI 20% demonstrated the highest conversion efficiency (1.2%) with a short-circuit current density (Jsc) of 5.54 mA/cm2, open-circuit voltage (Voc) of 0.45 V, and a moderate fill factor (FF) of 0.47. This finding is consistent with the ionic conductivity trend of the gel electrolytes. It can be deduced that the ionic conductivity of the gel electrolyte has a significant influence on the efficiency performance of DSSCs. Table shows the comparison of biobased PU from castor oil, palm oil, and jatropha oil having different iodide salt systems. Each one of the oils has its own unique characteristics and suitable to be used as in the production of PU-based polymer electrolytes. It is worth to mention that their performance is highly dependent on the preparation method of PU, choice of salts, as well as the fabrication method of the DSSC itself. The efficiency obtained in this study is comparatively higher than that of the previous systems. The physical properties of the gel electrolyte give an advantage that improves the overall DSSC performance. This could be explained by the enhancement of the contact among the working electrode, the electrolyte, and the counter electrode. The efficiency is still considered low for commercial applications; nevertheless, the properties could be enhanced with some improvement in the preparation techniques, especially during the formulation of the electrolyte. This includes the addition of (i) secondary polymer matrix, (ii) secondary/ternary salt, (iii) plasticizers, and (iv) fillers into the polymer electrolyte.[9]

Figure 9

Current–voltage characteristics of the PUA–LiI gel electrolyte systems under 1 sun condition.

Table 2

Properties of DSSCs with Biobased PU GPE Having Various Types of Iodide-Based Salts

GPE systems	Jsc (mA cm–2)	Voc (V)	FF	η (%)	ref
castor oil-based PU–NaI–I2	3.60	0.49	0.46	0.80	Ibrahim et al.[45]
palm oil-based PU–MPII–EC–I2	3.30	0.70	0.36	1.00	Su’ait et al.[21]
jatropha oil-based PUA–LiI-I2	5.54	0.45	0.47	1.20	current study

Materials and Methodology

Materials

Jatropha oil was commercially obtained from Biofuel Bionas Sdn. Bhd., Kuala Lumpur, and was used as received. Dimethylformamide (DMF), absolute ethanol (99%), and iodine were supplied by R&M Chemicals. LiI was purchased from Sigma-Aldrich. FTO-coated glasses (3 mm, sheet resistance: 8 Ω/sq), TiO2 nanoparticles (particle size: 20 nm), ruthenium dyes, cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (N719), and platinum paste (Platisol T) were purchased from Solaronix, Switzerland. The FTO glass was ultrasonically cleaned with acetone, ethanol, and deionized water to remove any impurities before use.

Preparation of the PUA-Based GPE

Jatropha-oil based PUA was prepared according to the procedure described in a previous report by the same authors.[46] The prepared PUA was mixed with LiI at different percentage ratios (LiI: 0, 5, 10, 15, 20, 25, and 30%). Initially, a calculated amount of LiI and I2 was dissolved with DMF in a sealed vial at room temperature. Then, PUA was added, and the mixtures were stirred for 3 h at 50 °C. The resulting PUA–LiI GPEs were analyzed in terms of ionic conductivity, electrochemical properties, and thermal stability.

Characterization

Fourier Transform Infrared Spectroscopy

Vibrational spectroscopic studies were conducted using an FTIR spectrometer (PerkinElmer: model 1000 series) instrument equipped with a universal attenuated total reflectance accessory. The analyses were conducted at room temperature. The samples were scanned at 4 cm–1 resolution, and the spectra were recorded in the wavenumber range 4000–280 cm–1.

Lithium Nuclear Magnetic Resonance

7Li NMR measurement was performed by using a JEOL NMR spectrometer (JEOL JNM-ECS400, Japan). The gel electrolytes were dissolved with DMSO-d6 prior to testing. The chemical shifts were reported in ppm, and LiCl was use as the internal reference.

Thermogravimetric Analysis

Thermal stability of the GPEs was examined by thermogravimetric analysis (TGA) using a TG analyzer (TGA-7), PerkinElmer (USA). The experiment was carried out under a nitrogen atmosphere (flow rate: 50 mL/min) from 50 to 600 °C at a constant heating rate of 10 °C/min.

Differential Scanning Calorimetry

DSC measurements were carried out using a DSC Q20, TA Instrument (USA) in the temperature range of −60 to 100 °C at a constant heating rate of 10 °C/min under a nitrogen atmosphere (50 mL/min).

Electrochemical Impedance Spectroscopy

Impedance measurements were carried out by electrochemical impedance spectroscopy using VersaSTAT-4 (Princeton Applied Research, USA). The experiments were conducted in the frequency range of 1 Hz to 1 MHz with an applied ac amplitude signal of 10 mV at room temperature. The value of bulk resistance, Rb, of the gel electrolytes was determined from the Nyquist plots obtained. The conductivity, σ, can be calculated as followswhere t is the thickness of the sample, Rb is the bulk resistance, and A is the electrode–electrolyte contact area. A conductivity–temperature dependence study was conducted in the temperature range 30–100 °C at 5 °C intervals with a constant humidity. The gel electrolytes were heated in a thermostatic oven model Venticell. The temperature-dependent conductivity, σ, is expressed by the following Arrhenius equationwhere σ0, Ea, K, and T are the pre-exponential factor, activation energy, Boltzmann constant, and absolute temperature, respectively.

Chronoamperometry Analysis

The chronoamperometry measurement was performed for the highest conducting GPE by the direct current (dc) polarization technique using VERSASTAT-4 (Princeton Applied Research, USA), under a fixed dc voltage of 10 mV. The transference number (tion) was determined from the current versus time plot. Wagner’s polarization technique was used to evaluate the transference number by using the following equationswhere IT is the total current (ionic and electronic) at the starting time (t = 0) and IS is the current on saturation (electronic current only).[47]

Electrochemical Stability

The electrochemical stability window was measured by linear sweep voltammetry using VersaSTAT-4 (Princeton Applied Research, USA). The measurement was carried out between 0 and 4 V at a scan rate of 10 mV/s.

Preparation of DSSC

A DSSC consisting of a photoanode and a counter electrode, with a gel electrolyte in between, was prepared as shown in Figure . The photoanode was prepared by coating a TiO2 paste on the FTO glass substrate using the doctor-blade technique and sintered at 450 °C. The photoanode was immersed in ruthenium dye, Ruthenizer N719, solution in absolute ethanol (0.3 mM) for 24 h to allow the dye molecules to be absorbed by the surface of the TiO2 layer. After sensitization, the photoanode was washed with absolute ethanol and left to dry at room temperature. The PUA GPE was directly cast onto the dye-absorbed TiO2 electrode. Platinum-coated FTO was used as the counter electrode. It was prepared by the brush painting method of Platisol on the FTO glass and annealing at 450 °C for 10 min. The photoanode and the counter electrode were assembled together in a sandwich-like configuration, with the gel electrolyte placed between the electrodes. The photovoltaic performance of the DSSC was measured under illumination using a Keithley high-voltage source model 237 under air mass (AM) 1.5G radiation at 100 mW/cm2 controlled by a radiometer. All of the experiments were conducted at room temperature.

Figure 10

Assembly procedure of the DSSC.

From the current–voltage characteristic curves, FF and light-to-energy conversion efficiency (η) were calculated using the following equationswhere Pmax is the maximum power output, Jmax and Vmax represent the current density and voltage at maximum power output, respectively, Jsc and Voc correspond to the short-circuit current and open-circuit voltage, and Pin is the power of the incident light.

Conclusions

A biobased GPE created from jatropha oil-based PUA has been successfully prepared. The characteristics of the gel electrolyte were examined by vibrational, thermal, and electrochemical analyses. The FTIR study showed that a complexation took place between the Li+ ions and the nitrogen and oxygen groups from the PUA matrix. This resulted in an increase of thermal stability after the addition of the salt dopant. The effect of the salt concentration on the ionic conductivity of the gel electrolyte was analyzed by electrochemical impedance spectroscopy (EIS). The EIS study showed that the highest ionic conductivity obtained was 20 wt % LiI at room temperature. The temperature-dependent ionic conductivity followed Arrhenius behavior with an activation energy of 0.42 eV. Further, from the chronoamperometry study, the PUA gel electrolyte showed an ionic conductor behavior, and the system was stable up to 2.35 V. A preliminary study of application of a gel electrolyte in a DSSC obtained a photocurrent conversion efficiency of 1.2%. Further study of the DSSC preparation is needed in order to achieve a high conversion efficiency.