Development of capacitive electrodes based on reduced-Graphene Oxide for applications in microbial electrochemical technologies

In recent decades, interest in microbial electrochemical technologies (MET) has drastically grown. MET rely on the ability of some microorganisms to form electroactive biofilms and to catalyze oxidation reactions at the anode and / or reduction reactions at the cathode. These technologies have aroused interest for their incredible versatility and sustainability as they can convert organic matter contained in wastewater and CO2 (inorganic source) contained in gas waste into H2 and high-value chemicals such as biofuels. Depending on their configuration, MET can also be applied for nutrient recovery, microbial electrochemical remediation, seawater desalinization and production of bioenergy. Despite the enormous variety of case studies reported in literature, MET nowadays still suffer from an issue related to scale up due to low power output if compared with modern renewable energy sources such as solar, wind, hydropower, fuel cells and lower removal yields than wastewater treatment systems such as anaerobic digesters (AD). Power output depends on cell voltage, which is negatively affected by ohmic voltage losses and low mass transfer, and current density, which is strictly connected to microbial biomass density attached on the electrode surface. Just for comparison, in AD the active biomass density is at least 30 times higher than in MET. One approach to overcome current and voltage limitations in MET is the use of capacitive, slurry-composed materials as electrodes. In capacitive flowable or capacitive slurry electrodes (CSE), an active material is dispersed in aqueous electrolyte in the form of a slurry and used to form a continuous and conducting framework that enables charge store and ions percolation throughout the thickness of the electrode. In CSE, where high and complex interactions between substrates and microbial biomass can occur in a 3D space, current density and mass transfer are not limited by the amount of microorganisms attached on the electrode surface. Furthermore, the presence of a conductive and capacitive media contributes in the reduction of ohmic losses. Due to remarkable electron-transport, mechanical, electrochemical, thermal and catalytic properties, graphene, in the form of reduced graphene oxide (rGO), holds great promise for potential use as active material in these technologies. This study deals with the process of production, characterization and optimization of rGO for the obtainment of a capacitive slurry which can either fulfill the reactor volume, packing together substrates and microorganisms and store large quantity of charge for applications in MET.