Charge and mass transfer impedance through different growth phases of anode-respiring biofilm

Abstract

Electrochemically active biofilms (EABs) respire in a unique form in which they utilize solid external materials as terminal electron acceptors for their metabolism. When microorganisms use anodes of microbial fuel cells (MFCs) or microbial electrolysis cells (MECs) and so on for respiration, they are called “anode-respiring biofilm (ARB)”. There were complex bio-electrochemical and mass transfer processes as ARBs grew in bioreactors. Currently, very little real time data is available characterizing charge and mass transfer impedance during ARB formation. In the present study, ARB was cultivated in a three-electrode bioreactor for about one month. We measured charge and mass impedance to find out the limiting factors for power output through different phases of ARB growth. Electrochemical impedance spectroscopy (EIS) was also used to investigate the charge and mass transfer impedance, while Cyclic Voltammetry (CV) was used to measure the electrochemical activities. CV and EIS were all measured at five different times (80, 280, 430, 460 and 500 h) during ARB formation. Current density curve shows that power output of ABR increased as biofilm grew and it goes up from 0.35 A m - 2 at 280 h to 9.41 A m - 2 at 500 h. CVs also illustrated that the electron transfer rate of ABR increased greatly over time. We can observe that the limiting current density from CVs has a similar trend with the current curve of ARB which demonstrates that the current generation ability of ABR increases during biofilm formation. Impedance value of anode-respiring biofilm displays that ohmic resistance is around 25 Ω showing little change. However, charge transfer impedance goes a downward trend. They are 12500, 2346, 996, 520 and 132 Ω respectively at these five different time which drops sharply with biofilm growing. The reason is that the microorganisms inside ARB increases significantly especially operating after 460 h, which increases the substrate utilization rate and electron transfer rate of ABR greatly. However, mass transfer impedance shows a upward trend over time. We found that charge transfer resistance accounts for 99.8% and 72.9% respectively, at 80 and 460 h. But it keeps dropping to just around 33.1% at 500 h, which demonstrates that charge transfer process is the limiting factor of current production at the beginning of operation. But it keeps dropping to just around 33.1% at 500 h proving that mass transfer process gradually plays an important role in power output of ARB when biofilm is mature. Therefore different methods to be used for reducing the charge or mass transfer resistance during ARB formation in bio-electrochemical system is of great importance to improve its electrochemical performance.