Microfluidic Microbial Fuel Cells for Microstructure Interrogations
The breakdown of organic substances to retrieve energy is a naturally occurring process in nature. Catabolic microorganisms contain enzymes capable of accelerating the disintegration of simple sugars and alcohols to produce separated charge in the form of electrons and protons as byproducts that can be harvested extracellularly through an electrochemical cell to produce electrical energy directly. Bioelectrochemical energy is then an appealing green alternative to other power sources. However, a number of fundamental questions must be addressed if the technology is to become economically feasible. Power densities are low, hence the electron flow through the system: bacteria-electrode connectivity, the volumetric limit of catalyst loading, and the rate-limiting step in the system must be understood and optimized. This project investigated the miniaturization of microbial fuel cells to explore the scaling of the biocatalysis and generate a platform to study fundamental microstructure effects. Ultra-micro-electrodes for single cell studies were developed within a microfluidic configuration to quantify these issues and provide insight on the output capacity of microbial fuel cells as well as commercial feasibility as power sources for electronic devices.
Several devices were investigated in this work. The first prototype consisted of a gold array anode on a silicon dioxide passivation layer that intended to imitate yet simplify the complexity of a 3D carbon structure on a 2D plane. Using Geobacter sulfurreducens, an organism believed to utilize direct electron transfer to electrodes, the 1 mm2 electrode demonstrated a maximum current density of 1.4 μA and 120 nW of power after 10 days. In addition, the transient current-voltage responses were analyzed over the bacterial colonization period. The results indicated that over a 6-day period, the bacteria increased the capacitance of the cell 5-orders-of-magnitude and decreased the resistance by 3X over the bare electrode. Furthermore, over short experimental scales (hours), the RC constant was maintained but capacitance and resistance were inversely related. As the capacitance result coincides with expected biomass increase over the incubation period, it may be possible for an electrical spectroscopy (impedance) non-invasive technique to be developed to estimate biomass on the electrode. Similarly, the R and C relationship over short experimental scales could be explored further to provide insight on biolm morphology. Lastly, fluorescence and SEM microscopy were used to observe the biofilm development and demonstrated that, rather than growing at even density, the bacteria nucleated at points on the electrode, and dendritically divided, until joining to form the "dense" biofilm. In addition, viable microorganisms undergoing cell division were found dozens of microns from electrode surfaces without visible pili connections.
To investigate single-cell catalysis or microstructure effects, a sub-micro-liter microfluidic single-channel MFC with an embedded reference electrode and solid-state nal electron acceptor was developed. The system allowed for parallel (16) working ultra-micro-electrodes and was microscopy compatible. With Geobacter sulfurreducens, the semiconducting ITO electrodes demonstrated forward bias behavior and suitability for anodic characterization. The first prototype demonstrated, with 179 cells on the electrode, a per cell contribution of 223 fA at +400 mV (vs. SHE). The second prototype with a 7 μm diameter electrode produced a current density of 3.9 pA/μm2 (3.9 A/m2) at +200 mV (vs. SHE) and a signal-to-noise ratio (SNR) of 4.9 when inoculated at a seeding density of 109 cells/mL. However, diluting the sample by 10x produced an SNR of 0.5, suggesting that obtaining single cell electron transfer rates to an electrode over short experimental time scales may not be possible with the system as tested. Nevertheless, the platform allows microstructure characterization and multiplexing within a single microfluidic chamber.