Correlating theoretical boundary layer thickness to the power output of a microbial fuel cell with a complex anode geometry operated at varying flow rates

The flow regime in a microbial fuel cell is important for multiple reasons. For example, biofilm thickness and substrate supply are influenced by the velocity of the liquid flow through the chamber and over an anode, which in turn determines power output. This study presents the correlation of the results of computational fluid dynamics simulation and electrochemical data of tubular air diffusion microbial fuel cells. A finite volume analysis approach is used to simulate fluid dynamics in a very complex geometry to calculate velocity profiles around individual fibers of graphite fiber brush. From these profiles, the theoretical boundary layer thicknesses are determined to be between 140 and 240 μm for flow rates between 0.01 and 10 mL min−1. The power output was measured at different external resistances and the average power densities were correlated to the boundary layer thicknesses. The power density reached a maximum of 94.7 ± 12.8 mW m−2 under constant and 81.6 ± 8.4 mW m−2 under shifting polarization at 1 mL min−1 at the lowest boundary layer thickness of 152 μm.