Hydrodynamic interventions and measurement protocols to quantify and mitigate power overshoot in microbial fuel cells using microfluidics

Abstract

Power overshoot in microbial fuel cells (MFCs) is an indicator of poor performance and its appearance in power density curves will hinder the determination of maximum power densities in continuously operating systems. In this work, a microfluidic approach was applied to an underperforming MFC containing a monoculture Geobacter sulfurreducens electroactive biofilm (EAB) to study power overshoot under idealized conditions. We developed an approach to quantify the degree of power overshoot while certain flow-based interventions were applied, notably shear erosion of the EAB outer layer. Two approaches to acclimation were also attempted. In the first, the MFC was acclimated to high currents before a standard polarization test. This eliminated the remaining overshoot behaviour and returned maximum power densities to pre-overshoot levels, though maximum current density remained low. In the second, a flow-assisted “long-hold polarization test” enabled full acclimation at each applied external resistance. Despite the underperforming state of the MFC, this method resulted in stable power and current density measurements that exceeded those made on the well-performing MFC using the standard polarization test. We conclude that slower electron transfer kinetics in the underperforming MFC can provoke overshoot, but a properly designed experiment that acquired polarization measurements using long-term acclimation at each external resistance overcame this problem.