A High Power-Density, Mediator-Free, Microfluidic Biophotovoltaic Device for Cyanobacterial Cells

Abstract

Biophotovoltaics has emerged as a promising technology for generating renewable energy because it relies on living organisms as inexpensive, self-repairing, and readily available catalysts to produce electricity from an abundant resource: sunlight. The efficiency of biophotovoltaic cells, however, has remained significantly lower than that achievable through synthetic materials. Here, a platform is devised to harness the large power densities afforded by miniaturized geometries. To this effect, a soft-lithography approach is developed for the fabrication of microfluidic biophotovoltaic devices that do not require membranes or mediators. Synechocystis sp. PCC 6803 cells are injected and allowed to settle on the anode, permitting the physical proximity between cells and electrode required for mediator-free operation. Power densities of above 100 mW m^-2 are demonstrated for a chlorophyll concentration of 100 μM under white light, which is a high value for biophotovoltaic devices without extrinsic supply of additional energy.

Keywords: bioenergy; biophotovoltaic devices; cyanobacteria; microfluidics.

Figure 1

a) Schematic of the device before insertion of the electrodes, seen at an angle through the glass slide. The lithographically defined PDMS pillars retain molten metal due to its surface tension, and the hole provides an opening for insertion of the Pt electrode. b) Model of the full device including platinum cathode and InBiSn anode. c) Schematic representation of the microfluidic biophotovoltaic device in action. Synechocystis cells settled by gravity on the InBiSn electrode deliver electrons to the latter by oxidizing water. On the platinum cathode oxygen and hydrogen ions are supplied with electrons and combine to water, which closes the circuit. d) Top view of the device design. e) True-color image of a device filled with a solution containing Coomassie blue to visualize the 25 μm high channels. f) True-color image of a device immediately after injection of Synechocystis cells at a chlorophyll concentration of around 100 μM. g) True-color image of a device filled with Synechocystis cells that were allowed to settle on the anode during 24 h.

Figure 2

a) Comparison of the voltage output from the same microfluidic device loaded with salt medium only (BG11) or Syenechocystis cells in medium in the dark and with light. The x-axis has been converted to a current density through division of the measured current by the surface of the InSnBi anode, and the error bars show the standard deviations for three consecutive, independent repeats on the same device. Inset: Response of a biophotovoltaic device as well as of an abiotic control under sequential illumination. b) Power density generated by the microfluidic devices filled with salt or cells in dark/illuminated environment.

Figure 3

a,b) Output voltage (filled circles, solid line, blue axis) and available power density (hollow circles, dashed line, green axis) as a function of current from two further abiotically loaded devices (BG11 cell medium supplemented with 0.25 M NaCl).