Análisis de las prestaciones de nuevos materiales para su implementación en sistemas bioelectroquímicos

Abstract

One of the technologies in which most research is being carried out is bioelectrochemical systems (BESs), able to generate electricity or produce high-value-added products depending on the BES used. They also allow wastewater to be cleaned by transforming its organic matter into energy, thanks to the presence of microorganisms that can exchange electrons with a solid electrode. In this thesis, microbial fuel cells (MFC) have been used for electricity production and microbial electrolysis cells (MEC) for H2 production. Single-chamber cells have been used to reduce the energy requirements of the process, as well as to simplify its design and operation. This is, a priori, the most suitable configuration for system scale-up. A synthetic mineral medium was also used to facilitate the study of BES. In this thesis, both types of cells have been studied to optimize all the parameters for their correct operation and possible full-scale implementation. Thus, anodes and cathodes have been studied. For this purpose, different types of cathodes have been evaluated in MFC. This type of cell requires an air-cathode with different gas diffusion layers that allow the uptake of oxygen from the outside to the inside of the cell while preventing leakage from the mineral medium. Thus, it was determined that, by using low external resistances to close the circuit, specifically 10 Ω, the optimum number of gas diffusion layers was two, reaching current densities up to 0.74 mA/cm2. This value was more than 20 % lower than that obtained with the cell with a single gas diffusion layer cathode. However, the GDL-1 cell had too much leakage of mineral medium for its performance to be constant over time. Thus, the fewer the number of gas diffusion layers, the less resistance the cell presented and, thus, generated higher current density at expenses of noticeable mineral medium leakage. In contrast, when using high external resistances, specifically 249 Ω, the current density of different numbers of GDL were similar (almost 0.3 mA/cm2), so the number of GDL did not affect. On the contrary, MECs do not need the presence of an air cathode for their operation, but they do need the presence of a catalyst for H2 generation. For this reason, the material of the cathode, the catalyst to be used, and the deposition technique on the electrode was studied. It was observed that the most effective cathode was the carbon felt, with the Pt@rGO catalyst, synthesized from scratch in this thesis. That combination resulted as the most effective for catalyzing the H+ reduction reaction, obtaining 43 % more current density than commercial inks. On the contrary, if the catalyst intended to be used wants to be different from Pt to save costs, the catalyst with the highest performance corresponded to Ni64Fe18Mo18, where the subscripts correspond to the percentage of that metal contained in the catalyst, which obtained the current density values of more than 1.0 mA/cm2. It was also observed that spray and electrospray Pt deposition techniques obtained higher yields than Dr. Blade, brush painting, or sputtering techniques, because these electrodes had more electroactive area. Finally, for both types of BES, different anode materials were studied, both commercial carbon-based and stainless steel. It was found that the anodes with the highest performance are those with a larger geometric and electroactive area, being these a stainless-steel circular electrode, obtaining values of more than 8 mA and H2 production of 1.2 m3 H2/m3d, a carbon fiber brush, obtaining values of almost 8 mA and H2 production of 1.04 m3 H2/m3d and carbon felt circular electrode, obtaining values of more than 7 mA and H2 production of 0.92 m3 H2/m3d. Palabras clave