Engineering of Microbial Fuel Cells technology: Materials, Modelling and Architecture

A Microbial fuel cell (MFC) is a bio-electrochemical reactor, able to convert chemical energy, contained in organic substrate, in electrical energy, thanks to the metabolic activity of microorganisms. Firstly, a fluid-dynamic modelling of different Microbial Fuel Cell configurations to study trajectories and concentration profile of the liquid containing the organic substrate during operation of the device was developed. The study of the device was joined with the study and the synthesis of carbon based aerogels to be used as new electrode materials, both for the anode and the cathode. The aim of the modelling was to understand what happen, from a fluid-dynamic point of view, inside the cell during operation. It was based on the application of equations from fluid-dynamics in order to study both the particle trajectories (using Navier-Stokes equations) and diffusion of substrate inside the reactor (using Fick’s laws). Three different MFC architecture were investigated, starting from a circular shape. To increase the area of the reactor interested by flux exchange with respect to the one in the circular configuration, a new a squared MFC, with a non-alignment of the inlet and the outlet was proposed. Starting from results obtained during the simulation for the squared reactor, to accommodate the flux distribution, a further improvement in architecture was introduced: a drop-shape MFC, with a percentage of fluid area exchanged, higher than 96%. Another possibility to improve MFC performances, is the optimization of materials used as electrodes. To be an efficient electrode, a material must satisfy some important condition: biocompatibility, good electrically conductivity, resistance to electrolytic solutions and high surface area together with high porosity to allow the formation of the biofilm. Carbon based aerogels can satisfy all these properties. Organic aerogels were synthetized following a green approach, starting from marine polysaccharides, like agar and starch and then transformed in carbon based, thanks to a thermal process. The synthesis procedure is the sol-gel technique, followed by a drying process that can extract the liquid part of the gel, leaving the solid structure, without collapse the material. Synthetized materials were analyzed both structurally and morphologically in order to understand if porosity, surface area and chemical composition were appropriate. To enhance some of these properties, a post synthesis treatment was performed: the surface of the aerogel was treated with a KOH solution in order to enlarge pores and increase the porosity of the overall material. The optimized aerogel was tested, as anode, into the square shape MFC and compared with commercial carbon material having the same function. Due to their high surface area, high porosity and good interaction with microorganisms, aerogels presented better performances of commercial materials if used as anode in MFC. Considering, instead, the addition of amino acids as nitrogen source to the previous material, it allowed the used of polysaccharide aerogel, as cathode electrode able to catalyze the oxygen reduction reaction (ORR). They were tested in MFC, compared with the most used catalyst material in literature, that is platinum. Another alternative to platinum in the catalysis of the ORR, is represented by the metal oxide aerogels. In this work, aerogels based on MnxOy were tested. The synthesis of this material is similar to the previous one, with the difference of the addition of the manganese oxide directly between initial precursors. Through the thermal process, the organic part of the material is burned, leaving an oxide structure that is active from a catalytic point of view. After the morphological, structural and chemical analysis of the sample, the catalytic activity of the material was tested, as in the previous case, using the Rotating Ring Disk Electrode (RRDE) technique, in order to investigate its catalytic properties.