Impacts of Temperature on the Stability of Tropical Plant Pigments as Sensitizers for Dye Sensitized Solar Cells

2.1. Plant Materials

Brightly red/purple colored plant parts (flowers, fruits, tubers, and leaves) were harvested to determine the presence of anthocyanin (Table 1).

2.2. Anthocyanin Extraction

The anthocyanin extractions of the above plant parts were made following the procedure described by Rodriguez-Soana and Wrolstad [14]. 5 g of each freshly collected plant samples was used to extract the anthocyanin pigments. The pigments were initially extracted using 150 mL of 70% ethanol (w/v%) and stored overnight at 4°C. On the following day, the extraction was mixed thoroughly by using a magnetic stirrer for two hours under air-conditioned room temperature (25°C). The extraction was filtered using Whatman's ashless 110 mm filter paper to remove any solid residues. Subsequently, the extracts were centrifuged at 4500 rpm using a Denley BS400 (UK) centrifuge machine for five minutes to separate all residues. Lastly the supernatant of the ethanolic extracts was gently mixed with equal volumes of petroleum ether to separate polar and nonpolar pigments. The final ethanolic extract was assumed to carry only the polar anthocyanin pigments. This component was carefully poured to a 10 mL glass bottle, tightly stoppered and wrapped in aluminum foil to avoid exposure to light and treatments for different temperature regimes.

2.3. Plant Screening for Anthocyanin Pigments

Screening of separated anthocyanin pigments was done by measuring their absorbance spectra using UV-vis spectrophotometer (Shimadzu UV-1800, Japan). Before the commencement of absorbance measurements, each of the samples was treated with 45 μL of concentrated HCl [15]. This acidification process converts anthocyanin derivatives to anthocyanidin that gives absorption spectra in the region of 490–550 nm [11, 15, 16]. Plant extracts that showed higher absorption spectra were selected for further investigations to evaluate the impacts of varying temperature regimes. All measurements were done in three replicates per sample.

2.4. Determination of Anthocyanin Content

To finalize the sample selection for DSSCs, those extracts that showed the highest UV-vis absorbance reading were chosen, and their anthocyanin contents were determined following the pH differential method described by Giusti and Wrolstad [11]. The results were expressed as micrograms per gram fresh weight.

Anthocyanin content was calculated according to the following equation:(1)Anthocyaninpigmentcontent=A×MW×DF×103ε×L,where A = (A_520nm–A_700nm) pH 1.0 − (A_{520 nm}–A_{700 nm}) pH 4.5, MW (Molecular Weight) = 449.2 g/mol for cyanidin-3-glucoside, DF = Dilution factor, ε = 26900 L mol⁻¹ cm⁻¹, 10³ is the factor for converting g to mg, and L is the assumed path length in cm.

Aliquots of plant extracts were brought to pH 1 and 4.5 and allowed to equilibrate for one hour. The absorbance of each equilibrated solution was then measured at 520 nm (λ_max) and 700 nm for haze correction. Spectroscopic absorbance readings were repeated against 70% ethanol as the reference. All measurements were done in three replicates per sample.

The MW used in this formula corresponds to the predominant anthocyanin in the sample. In some cases, predominant anthocyanin in a material may be known and could be different from cyanidin-3-glucoside. However, throughout the years, there has been a lack of uniformity in the values of absorptivity of purified anthocyanin, mainly due to difficulties of obtaining pure crystalline anthocyanin in adequate quantities [11, 17]. Since there is a huge variety of anthocyanin spread in nature, it has been suggested that if the major anthocyanin is unknown, it can be expressed as cyanidin-3-glucoside because that is the most abundant anthocyanin in nature [11, 12, 17–20].

2.5. Impacts of Storage Temperature on Anthocyanin Stability

The anthocyanin extracts of M. malabathricum, H. rosa-sinensis, and C. variegatum were stored in a tightly stoppered glass bottle fully covered with aluminum foil to avoid exposure to light. Extracts were stored at three different storage temperatures, namely, 4°C, −20°C, and 25°C, to evaluate the stability during storage. In order to determine the anthocyanin contents, the spectroscopic absorbance of the extracts were initially determined for three consecutive days followed by weekly measurements over a period of four months from September 2012 to January 2013.

2.6. Degradation Rate of Anthocyanin during Storage

The first-order reaction constant rate (k) and half-life (t_1/2) were calculated using the following equation [21]:(2)ln⁡(CtCo)=−k×t,t1/2=ln⁡(0.5)×k−1,where C_o is the initial monomeric anthocyanin content and C_t is the monomeric anthocyanin content after t minute storage at a given temperature.

2.7. Photovoltaic Test of DSSC

The preparations of TiO₂ anode are described elsewhere [22]. The anodes were dipped in the dye extract for overnight at room temperature (25°C) and dried out [15]. The cell was assembled using Dyesol's Test Cell Assembly Machine with the Surlyn (50 μm, Dyesol). The electrolyte solution containing tetrabutylammonium iodide (TBAI; 0.5 M)/I₂ (0.05 M), acetonitrile, and ethylene carbonate (6 : 4, v/v) [16] was introduced through a predrilled hole in platinum counter electrode. The cell was kept under irradiation of about 3-4 h for light soaking.

Finally I-V characteristic of the DSSC was measured under 1 sun level (DYESOL Solar Simulator LP-156B). The effective irradiated area of solar cell was 0.25 cm². The performance of DSSC sensitized with anthocyanin pigments extracted from M. malabathricum, H. rosa-sinensis, and C. variegatum was evaluated by short circuit current (J_sc), open circuit voltage (V_oc), fill factor (ff), and energy conversion efficiency (η).

The absorbance spectra of the dye adsorbed on TiO₂ electrodes were also measured. Before the commencement of absorbance measurements, each of the TiO₂ electrodes were dipped in the dye extract overnight at room temperature (25°C) and air dried.

3.1. Plant Selection for DSSCs

As shown in Figure 1, the maximum absorbance of anthocyanin varied significantly in different species. Jacaranda obtusifolia, Licuala orbicularis, Spinacia oleracea, and Durantaerecta flower extracts showed no absorbance at 520 nm; hence it can be concluded that they do not possess anthocyanin. Among the rest, 17 other plant extracts showed maximum absorbance of 0.1 or lower and therefore they were not selected to further investigations. On the other hand, the remaining sample extracts showed absorbance maxima greater than 0.1. However, only three species, each representing fruit, flower, and leaves (Melastoma malabathricum, Hibiscus rosa-sinensis, and Codiaeum variegatum), which showed that highest absorbance maxima were selected for further investigations.

3.2. Determination of Anthocyanin Content of Selected Plant Extracts for the Evaluation of DSSCs

Table 2 showed that among the samples investigated after preliminary screening, the highest anthocyanin concentration was found to be in the fruit pulp of M. malabathricum (8.43 mg L⁻¹), followed by H. rosa-sinensis (4.63 mg L⁻¹) then C. variegatum (2.22 mg L⁻¹).

3.3. The Absorbance Spectrum

All three extracts showed prominent peaks at 490–550 nm after the extracts were acidified with HCl (Figure 2(a)). This result indicated and proved once again that more anthocyanidin presents in the extracts [11, 15, 16].

On the other hand, Figure 2(b) showed that M. malabathricum extract exhibited the best absorbance after being adsorbed into the TiO₂ electrode. This extract also gave the best efficiencies in DSSCs, while C. variegatum in TiO₂ gave the second best absorbance, followed by H. rosa-sinensis. The absorbance results of the dye adsorbed TiO₂ electrodes were consistent with I-V characteristics data.

3.4. The Effect of Storage Temperature on Anthocyanin Stability

The storage temperature had a strong influence on the degradation of anthocyanins extracted from all three extracts (see Figure 3 and Table 3).

The most distinctive pattern that was found in all three species was that anthocyanin pigments decreased progressively when stored at 25°C over a three-month period. However the stability of all three pigments was relatively high when the temperature was maintained at 4°C.

The degradation rates are represented by the half-life values; the higher the number, the more stable the anthocyanin extract. Result showed significantly greater stability of anthocyanin in all three species when they were stored at 4°C, and storage at 25°C resulted in much faster degradation. The highest half-life periods for anthocyanin in M. malabathricum, H. rosa-sinensis, and C. variegatum were 540.77, 571.19, and 352.86 days at 4°C, respectively, and it decreased rapidly to 110.71, 219.74, and 254.25 days at 25°C over a period of three months.

Similar results were reported by Janna et al. [23], who also studied the stability of Melastoma malabathricum and found that the suitable storage condition for anthocyanin pigment is acidic solution in dark and low temperature (4°C). The result of this investigation was also consistent with other similar studies where they found that anthocyanin pigments degrade faster as the temperature increases to 25°C and the stability is maintained at low temperatures (i.e., 4°C) [12, 21, 23].

A previous study on the anthocyanin degradation in black carrot showed that the t_1/2 value in shalgam drinks maintained at 4 and 25°C were 34 and 11 weeks, respectively [24]. A similar study also found that the t_1/2 value of monomeric anthocyanin of black carrot showed a distinct difference of 71.8 and 18.7 weeks, respectively, when maintained at 4 and 20°C, respectively [21]. Our investigation showed that frozen anthocyanin extracts maintained at −20°C also ensure a good stability over a period of three months; however, the best storage temperature was still 4°C.

3.5. The Efficiency of Natural Dye

The current-voltage characteristics of the DSSCs sensitized with the anthocyanin pigment extracted from M. malabathricum fruit pulp, H. rosa-sinensis flowers, and C. variegatum leaves are shown in Figure 4. The conversion efficiencies (η) of DSSCs were 1.16, 0.16, and 1.08%, respectively (Table 4). The highest effciency was obtained from DSSC sensitized with M. malabathricum fruit pulp extract with the open curcuit voltage (V_oc = 0.383 V), short curcuit current density (I_sc = 6.17 mA/cm²), and fill factor (ff = 0.44).

Natural pigments extracted from fruits and vegetables such as chlorophyll and anthocyanins have been extensively investigated as sensitizers for DSSCs. By far, the best performance reported was obtained from beet roots with an efficiency of 2.71% [25, 26].

Other studies include Punicagranatum, Hibiscus sabdariffa, pomegranate juice, wild Silicon prickly pear (Opuntia vulgaris), Rhoeospathacea, Mangosteen pericarp, red turnip, Ficus reusa, and Hibiscus surattensis with conversion efficiencies of 1.86, 1.6, 1.5, 2.06, 1.49, 1.17, 1.70, 1.18, and 1.14%, respectively [6, 7, 27–31].

Our study has shown that extract from M. malabathricum yielded the highest efficiency, 1.16%. The result is encouraging and the methods employed to maintain its stability is extremely promising. High efficiency obtained in the fruit pulps of M. malabathricum can be attributed to the carbonyl and hydroxyl groups of anthocyanin molecules present [3, 6, 7, 25]. This ability favours photoelectric conversion as it allows effective binding with the surface of TiO₂ porous film. Further improvements in refinement of extraction and application methods will no doubt increase the efficiency of this dye in DSSCs.