Abstract
Spirulina is a kind of blue-green algae with good photosynthetic efficiency and might be used for photovoltaic power generation. So this paper used living spirulina as novel photosensitizer to construct spirulina biosolar cell. The results showed that spirulina had the photoelectric conversion effect, and could let the spirulina biosolar cell have 70 μA photocurrent. Meanwhile, adding glucose sucrose or chitosan in the spirulina anode chamber, they could make the maxima current density of the cell greatly increased by 80 μA, 100 μA, and 84 μA, respectively, and the sucrose could improve the maximum power density of the cell to 63 mW/m−2. Phycobiliprotein played an important role in the photosynthesis of spirulina. So in this paper phycobiliprotein was extracted from spirulina to composite with squaraine dye to sensitize nanocrystalline TiO2 photoanode for building dye sensitized solar cell, and the photoelectric properties of the cell also were investigated.
1. Introduction
Many photosensitizers [1–4] have been used to construct photo devices, such as organic dyes, inorganic nanomaterials, and biomass. Solar cells are the main photo devices that achieve the conversion of light energy into electrical energy. In particular, the solar cells based on photosynthesis principle of plants and algae, have great potential for the development and application [5–9]. In 1911, British botanist Potter discovered that microbes could produce current [10]. After the 1980s, electron transfer intermediates were widely used for improving output power of the microbial fuel cell greatly. In 1999, Tatsuo Yagishita et al. studied Synechococcus with glucose to make electricity production [11]. Researchers in University of Massachusetts found that iron reducing bacteria Rhodospirillum could metabolize carbohydrates and produce electricity [12]. US scientist Logan injected new vitality to the research of microbial fuel cell (MFC) by his studies in wastewater treatment and microbiological power generation [13–17]. Spirulina is ancient blue-green algae [18], and the growth and expansion of spirulina are rapid, easy, and convenient due to its cultivation pH 8.6–9.5 and lower demands for the environment [19, 20]. It has the solar energy utilization efficiency up to 18% and the photosynthetic efficiency up to 43% (more than 3 times high to those of advanced plants) due to its phycobiliprotein. So this paper intended to extract phycobiliprotein from spirulina and used it as photosensitizer to sensitize nano-TiO2 to equip the dye sensitized solar cell [21]. Meanwhile, we applied the living spirulina to build the spirulina biosolar cell [22, 23] and investigated the electrical properties of the cell and the influence of carbohydrates on photoelectric performance of the cell.
2. Experimental
2.1. Spirulina Cultivation and Extraction of Phycobiliprotein
Spirulina was cultivated with medium Zarrouk or medium spirulina at 28°C under 3000 lx light and 12 L/12 D photoperiod and was trained for 20 days [24] for the experiment. Phycobiliprotein was extracted from the spirulina by repeated freezing and melting technology under ultrasonic.
2.2. TiO2 Film and Its Sensitization with the Phycobiliprotein and Phycobiliprotein Squaraine
Nanometer TiO2 film was prepared on glass electrode by sol-gel from butyl titanate, diethanolamine, ethyl alcohol, and PEG and was sensitized with the phycobiliprotein or phycobiliprotein squaraine by impregnated method, respectively. The squaraine dye with N-p-carboxybenzyl group was synthesized by the reference [25].
2.3. Construction of Spirulina Biosolar Cell
Spirulina biosolar cell consisted of anode chamber and cathode chamber separated by clapboard, and the clapboard was equipped with proton exchange membrane (Nafion, 14 cm2). Two graphite electrodes in the anode and cathode chamber (about 10 cm2) were placed closely to both sides of the proton exchange membrane, and Ag/AgCl reference electrode in the anode chamber was close to the anode. Spirulina in the culture medium (240 mL) with or without glucose, sucrose, or chitosan was added to the anode chamber, while the culture medium (240 mL) was in the cathode chamber.
2.4. Detection of the Spirulina Biosolar Cell
The spirulina biosolar cell was placed under a xenon light source with the distance of 22 cm to the surface of anolyte. Photoelectric conversion performance of the solar cell was detected at 28°C under xenon lamp by electrochemical workstation.
3. Results and Discussion
3.1. Spirulina and Phycobiliprotein Photosensitizers
Figure 1 showed that the spirulina cultivated was green and spirochete, while the phycobiliprotein extracted from spirulina was blue. The maximum fluorescence emission wavelength of the phycobiliprotein was 650 nm. The phycobiliprotein sensitized the nano-TiO2 film anode and assembled it into the dye sensitized solar cell with KI/I2 acetonitrile solution as electrolyte and graphite as cathode. The results showed that the phycobiliprotein could improve the photovoltage and current of the nano-TiO2 film to 273 mV and 4.2 mA, while using the phycobiliprotein squaraine as the photosensitizer, they could be 407 mV and 5.7 mA.
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3.2. The Spirulina Biosolar Cell and Its Working Principle
Structure and working principle of the spirulina biosolar cell were shown in Figures 2 and 3. Spirulina as electricity generation algae in the anolyte emitted electrons by its photosynthesis and respiration metabolism under light, and the electrons were transferred to the anode. At the same time, the protons were produced, then transferred to the cathode surface after penetrating the proton exchange membrane, and at last the protons reacted with oxygen and electrons to form water.
3.3. Photocurrent of the Spirulina Biosolar Cell
Figure 4 showed changing curves of the current with time for the cells under the dark and light. B1 represented the curve for the cell with 240 mL culture medium in each of the chambers, while S1 for the cell with 240 mL spirulina cultivated for twenty days in anode chamber and 240 mL culture medium in the cathode chamber. There was no light in 900 s. When lighting at the 900 s, the current rose, and fell when the lighting was removed at the 1900 s. B1 indicated that polarization current of the cell increased 19 μA due to the elevated temperature of the solution and the reduction of internal resistance in solution under the illumination. The current of the cell under illumination had largely been increased (S1) by 70 μA due to the electron produced from spirulina by photosynthesis, and after the light was removed, the current decreased a little. Figure 5 showed that the current of the spirulina biosolar cell should be increased by 240 μA under the light for 6 h.
3.4. Influence of Carbohydrates on Photocurrent of the Solar Cell
Figure 6 showed the influence of carbohydrates on photocurrent of the spirulina biosolar cell under the dark and light. B2 indicated the changing of current with time for the cell with 2 g sucrose in 240 mL culture medium anode chamber. S3 showed that the current generated by the spirulina biosolar cell with 2 g sucrose in anode chamber had been increased by 100 μA and indicated that the presence of sucrose could promote the electrons production significantly in photosynthesis of spirulina. After the light was removed, the current of the cell had a slight decrease. S4 and S5 explained that the maxima current only had a little decrease while repeating the experiments due to the slight loss of spirulina. Figure 7 showed that the photocurrent of the spirulina biosolar cell with 3.8 g sucrose in anode chamber was increased 389 μA under the light for 4 h.
The influence of other carbohydrates on photocurrent of the spirulina biosolar cell was investigated. The results were shown in Table 1. The date indicated that glucose and chitosan made the photocurrent of the solar cell increase about 80 μA and 84 μA, respectively. Among them, the photoelectric conversion effect of the spirulina biosolar cell with sucrose was the best and could be improved approximately by 100 μA.
Carbohydrates might improve the photosynthesis rate and photoelectric conversion of the spirulina biosolar cell due to their improving of the activity of hexokinase (an enzyme relating to the cyanobacterial photosynthesis). In the process, the carbohydrates were likely to compensate for lack of a carbon source and conducive to consumption reducing of ATP and NADPH in carbon utilization of the cells. These exogenous carbohydrates provided electrons as the endogenous starch or glucose did and made the electrons flow to PS I. Meanwhile, they improved proton gradient of crossing thylakoid membrane, photosynthetic phosphorylation level of cell, and the efficiency of emitting electrons from spirulina.
4. Conclusions
Phycobiliprotein-squaraine composite had good efficiency for widening and strengthening the UV-Vis light absorption and fluorescence emission of TiO2 film and improving the photovoltage and photocurrent of the DSSC. Living spirulina had photoelectric conversion effect and could transmit the photoelectrons by its photosynthesis out of its cytomembrane in spirulina biosolar cell. Carbohydrates could improve the photocurrent of the solar cell further. Among them, sucrose was the best additive and could let the current and the power density of the spirulina biosolar cell increase by 100 μA and 30 mW/m−2, respectively. Spirulina biosolar cell will have a large potential for practical applications.
Conflict of Interests
The authors declare that they have no direct financial relation with the commercial identities mentioned in this paper that might lead to a conflict of interests for any of them.
Acknowledgments
The authors would like to thank Hebei Natural Science Foundation (no. B2011203231) and the Hebei Key Laboratory of Applied Chemistry Sea Grant (Yanshan University) for providing the financial support for this project.