Periodic polarization of electroactive biofilms increases current density and charge carriers concentration while modifying biofilm structure

Abstract

Anodic electroactive biofilms (EABs) need to overcome low current densities for applications such as microbial fuel cells or biosensors. EABs can store charge in self-produced redox proteins when temporarily left in open circuit, and discharge them once the electrode is appropriately repolarized, thus behaving as pseudocapacitors. Here we investigated the effect of such periodic polarization on the intrinsic nature of the EABs during their entire growth (i.e. starting from inoculation and for 10 days) on glassy carbon electrodes. An optimal periodic polarization (half-period of 10 s) greatly increased the maximum steady-state current density delivered by the Geobacter-dominated EABs (up to 1.10 ± 0.02 mA cm−2, n = 3 electrodes) when compared to continuously polarized EABs (0.41 ± 0.04 mA cm−2); and increased the amount of electric charges produced per hour by 69 ± 17% even taking into account the half-periods of open circuit. This enhancement was highly correlated with a substantial increase in charge carriers concentration (10.6 ± 0.5 mMe- vs. 2.9 ± 0.6 mMe-), allowing higher charge storage capacity and higher electron mobility across the EABs. Our results suggest that appropriate periodic polarizations may upregulate the expression of heme-containing redox proteins associated with the matrix, such as c-type cytochromes. The EABs grown under periodic polarization presented mushroom-like structures on their top layers, while EABs grown under continuous polarization were flat.