Increasing power output and isopropanol biodegradation by trickle-bed microbial fuel cells by optimizing air relative humidity and oxygen flux at cathode

Abstract

The cathode air flow rate and relative humidity (RH) of a trickle-bed microbial fuel cell (TB-MFC) were optimized using the response surface methodology to obtain a high oxygen flux and a low electrical resistance at the cathode and thereby promote the oxygen reduction reaction. Under optimal air flow rate (2.20 L/min) and RH (20%) conditions, the TB-MFC system exhibited the highest oxygen flux (3.08%/cm2‧min) and the lowest resistance (5.33 Ω); its regression model had an R2 > 0.95 and the desirability coefficient was 0.78. These results demonstrate that the optimized air flow rate and RH support high oxygen flux and low resistance at the cathode. Using biochar combined with conductive carbon black (CCB) as the cathodic catalyst and optimal cathodic operating conditions, yielded a mineralization efficiency of isopropanol of 96.9% and a power density of 555 mW/m3 at an empty bed residence time and isopropanol influx concentration of 60 s and 300 ppmv, respectively. These results suggest that isopropanol can be effectively mineralized directly into CO2 and H2O by a TB-MFC, reducing the by-product accumulation, and thus increasing the power generation of the TB-MFC. Optimizing the air flow rate and RH of the biochar/CCB-based cathode in the TB-MFC increases the oxygen flux and reduces the electrical resistance of the cathode of TB-MFC, which can thus be used to treat exhaust gases that contain organics.