Figure 1. Schematic diagrams of the assembled microfluidic biological solar cell. (a) top view and (b) bottom view

A MICROFLUIDIC BIOLOGICAL SOLAR CELL GENERATING HIGH POWER DENSITY

Xuejian Wei, Weiyang Yang and Seokheun Choi * Bioelectronics & Microsystems Laboratory, Department of Electrical & Computer Engineering,

State University of New York at Binghamton, NY, USA

ABSTRACT We developed a novel microfluidic biological solar cell which harvests light energy and delivers

electricity. The biological solar cell used the photosynthetic and respiratory activities of the cyanobacterium Spirulina Platensis to generate a maximum power density of 4.76 W/cm2 in a 90 L microfluidic reactor, which is the highest power density among all miniaturized biological solar cells. The power density was significantly increased by (i) using carbon-based anodic material to promote bacterial adhesion and (ii) constructing a sandwich arrangement of the anode/proton exchange membrane (PEM)/air-cathode to minimize the inter-electrode distance and significantly reduce the internal resistance. KEYWORDS: Biological solar cells, photosynthesis, respiration, cyanobacteria, microfluidics

INTRODUCTION

Biological solar cells are an emerging technology designed to harness Earth’s most abundant and promising energy source (solar irradiation) and self-sustainably produce electricity both day and night [1]. During the day, light energy absorbed in photosynthetic reactions splits water to produce organic substrates (oxygen, electrons); in dark conditions, the device produces electrons via bacterial respiratory metabolism of the substrates generated through photosynthesis [2]. Research into microliter-sized microfluidic biological solar cells contributes essential knowledge about the photosynthetic electron transfer process and the interplay between miniature device architectures and microorganisms that occur in a smaller group of microorganisms, with excellent control over the microenvironment [3-5]. The combination makes microfluidic biological solar cells a versatile platform not only for fundamental bio-solar energy studies but also for sustainable micropower sources. Despite their potential, today’s miniature biological solar cells demonstrate persistent performance limitations. In this work, we created a novel miniature biological solar cell having a high power density.

EXPERIMENTAL Figure 1 shows schematic diagrams of the microfluidic biological solar cell. The device consisted of five different functional layers (Figure 2); (i) a top poly(methyl methacrylate) (PMMA) layer which is transparent for solar energy capturing, (ii) a PMMA microfluidic chamber layer, (iii) a rubber gasket, (iv) an anode/proton exchange membrane (PEM)/air-cathode sandwich layer, and (v) a bottom PMMA layer. All layers were carefully aligned and clamped with 10 small screws. Two tubes for the inlet and outlet were plugged into the holes to form a fluidic channel. The assembled device was sterilized with 70% ethanol and ultraviolet light for 24 hours. Unlike other conventional two-chambered devices, the device utilized the air-cathode to allow freely available oxygen to act as an electron acceptor by the installation of the catalyst side of the air-cathode facing toward the chamber while

2050978-0-9798064-8-3/µTAS 2015/$20©15CBMS-0001 19th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 25-29, 2015, Gyeongju, KOREA

Figure 2. Schematic diagram of the individual lay-ers for the device

Figure 4. Photo-images of (a) the fully assembled bio-logical solar cell with (b) photosynthetic bacterial bio-film, and (c) the bottom view of the device

Figure 3. Schematic diagram of the test set-up. During operation at 12h/12h light/dark cycles, the voltage dif-ferences between anode and cathode were recorded.

the opposite side was exposed to air. Cultures of Spirulina Platensis were cultivated at 25 °C, using 1047 marine medium with 20 g/L of vitamin B12 under continuous illumination. The microfluidic channel allowed for anolyte introduction to the device. First, the chamber was filled with the cyanobacteria-suspended anolyte. Then, the medium was continuously injected by a syringe pump with a flow rate of 1μL/min. The whole operation was done at a 12 hour light/dark intervals under 4 fluorescence light lamps. We measured the potentials between the anodes and the cathodes with a data acquisition system, and recorded the readings every 1 min via customized LabView interface (Figure 3).

RESULTS AND DISCUSSION

The bacterial cells were aggregated and deposited on the anode to form a thin green biofilm (Figure 4). Biofilm cells can utilize the endogenous exoelectrogenic properties of the biofilm to transfer the electrons to the electrode in the absence of exogenous redox mediators. Before long, the gradual increase in current was monitored under the operational conditions of 12h/12h light/dark consecutive cycles. The biological solar cell produced 4.76 W/cm2 of the maximum power density in the dark, which was higher than the 3.6 W/cm2 produced in the light (Figure 5). The power decrease during the illumination is primarily due to the negative impact of photosynthetically evolved oxygen which can compete with the extracellular electron transfer. This is the highest recorded power density among all micro-sized biological solar cells [4, 5]. The significant power enhancement was achieved by using a novel device structure and carbon-based material as an electrode. The single-chambered device configuration was formed by sandwiching the PEM between the anode and air-cathode to significantly decrease proton travel distance. Moreover, the carbon-based anode provided a better natural habitat for the bacterial cells and promoted the bacterial adhesion since the carbon material contains functional organic groups favoring cell vitality as well as easy handling [6]. In addition, the biological solar cell miniaturization inherently produced favorable conditions for increasing power density by reducing internal resistance and improving mass transport, since the small-scale devices are more energy dense than larger units [7].

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Figure 5. Polarization curve and output power measured as a function of current (a) under dark and (b) light conditions.

CONCLUSION

We developed a biological solar cell in a micro-sized chamber. Using a new device architecture, a single-chambered device was constructed in a way that the bacterial cell attachment and the device per-formance can be enhanced. This presented approach could result in barrier-transcending advancements in miniature biological solar cell that could facilitate higher power densities and energy efficiency with self-sustainability, releasing the biological solar cell technology from its restriction to research settings, and translating it to practical, real-world applications. ACKNOWLEDGEMENTS

We would like to thank the Nanofabrication Lab at SUNY-Binghamton for providing the fabrication facilities and the Binghamton University Research Foundation (ICG/TAE) for funding this work.

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extra-cellular electron transport by cyanobacteria,” Biochem. Syst. Trans., 40, 1302-1307, 2012. [3] M. Chiao, K. B. Lam, and L. Lin, “Micromachined microbial and photosynthetic fuel cells,” J. Mi-

croelectromech. Syst., 16, 2547, 2006. [4] H. Lee, and S. Choi, “A micro-sized biosolar cell for self-sustaining power generation,” Lab Chip, 15,

391-398, 2015. [5] S. Choi, “Microscale microbial fuel cells: advances and challenges,” Biosensors & Bioelectronics, 69,

8-25, 2015. [6] H. Richter, K. McCarthy, K.P. Nevin, J.P. Johnson, V.M. Rotello, and D.R. Lovley, "Electricity gen-

eration by Geobacter Sulfurreducens attached to gold electrodes," Langmuir, 24, 4376-4379, 2008 [7] I. Ieropoulos, J. Greenman, and C. Melhuish, “Improved energy output levels from small-scale micro-

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CONTACT *Seokheun Choi; phone: +1-607-777-5913; sechoi@binghamton.edu

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