MEMS Microbial Fuel Cells and Photosynthetic Electrochemical Cells

In the MEMS microbial fuel cell, as the yeast metabolized glucose, electrons were liberated within the organism. These electrons were captured by a fuel cell fabricated from Si and glass using micromachining technologies. In this fuel cell, glucose and live yeast were mixed in solution with a diffusional redox mediator that “siphoned” those electrons from the yeast cells and donated them to the anode. The electrons traveled through an external load to the cathode, where they were passed to an electron acceptor agent (ferricyanide). Experimentally, the μMFC generated a peak open circuit voltage of 450 mV, peak current density of 15 μA/cm^2, and corresponding power density of 2.3 nW/cm^2.

The first MEMS photosynthetic electrochemical cell harnessed Anabaena to liberate electrons from water under illumination and captured those electrons in a micromachined fuel cell similar to the μPFC. Experimentally, under the illumination of a desk lamp 60 W bulb, the μPEC1 produced 400 mV peak open circuit voltage, 2.0 μA/cmcurrent density, and 0.04 nW/cm power density. There was one unintuitive feature of the 2 2μPEC1—it was capable of sustaining electrical output in the dark comparable to output in the light, even when photosynthesis did not contribute electrons in the dark. It did this by reverting to a microbial fuel cell in the dark, metabolizing the glucose that it had generated for itself.

In the second version of our photosynthetic electrochemical cell (μPEC2), the MEMS design, microfabrication, and assembly were improved. Moreover, instead of utilizing live cultures of photosynthetic bacteria, we isolated from spinach just the photosynthetic sub-cellular organelles called thylakoids. By harnessing just thylakoids, the complexities of whole, live photosynthetic bacteria were reduced. Under illumination intensity of 2000 μmol photons/m^2/s (approximately the solar intensity of a cloudless spring day), the μPEC2 generated a peak 480 mV open circuit voltage, 1.0 μA/cm^2, and 5.4 pW/cm^2.

To increase the energy conversion efficiency of these MEMS fuel cells, we proposed immobilizing the thylakoids and electron mediators directly onto the electrode using “bioelectrocatalytic” self-assembled monolayers (bio-SAMs). Immobilization of thylakoids onto bio-SAMs of cystamine and pyrroloquinoline quinone (PQQ) was demonstrated for the anode. When such bio-SAMs are optimized, we expect marked efficiency gains for MEMS microbial fuel cells and photosynthetic electrochemical cells, such that they could rival more conventional power technologies.