<i>In situ</i> electrochemical monitoring with an open circuit auxiliary electrode in microbial electrochemical cells treating sediments

A set of six low-cost 3D printed microbial electrochemical cells (MECs), each containing four electrodes and one ceramic membrane were constructed. The electrodes included a polarized working electrode (WE, polarized at +0.2 V <i>vs.</i> Ag/AgCl), a reference electrode (RE) and a counter electrode (CE) typical in three-electrode electrochemical cells and an additional fourth electrode <i>i.e.</i>, auxiliary electrode (AE). The AE was identical to the WE, but it was kept in open circuit. This study was conducted to evaluate the potential of AE as a measurement of the bulk potential and control for the effect of polarization of the WE. Two different salinity levels, <i>i.e.</i>, 12 and 50 mS cm ^-1 were tested in triplicate along with propionate concentrations of 0.1 g L^-1 and 1 g L^-1 to study the oxidation of propionate by the microbial community present in the reactor. Continuous monitoring of the electrode potentials, cyclic voltammetry (CV) at different scan rates and electrochemical impedance spectroscopy (EIS) data were obtained throughout the experiment to compare the results from the AE and the WE. The open-circuit auxiliary electrode (AE) was useful to control the state of the electrolyte and to distinguish changes in the system caused by the continuous polarization. The use of the AE allowed to compare the changes in the EIS diffusion slope caused by the WE polarization. This study showed that AE in MECs helps to understand and predict the electrochemical reactions more precisely.