Development of microbial fuel cells based on alternative materials including ionic liquids and nanoparticles for bioenergy production

Resumen

Microbial fuel cells (MFCs) are a promising technology that has received increasing attention over recent years since they offer new opportunities for bioenergy production from a broad range of biomass and wastes, including wastewater. Thus, MFCs could help to address major pressing issues such as the current global energy crisis and the availability of clean water for human use. Although significant efforts have been devoted to enhancing the efficiency of this technology for its practical application, the levels of current and power output in these devices are still relatively low. The search for alternative materials and the development of cost effective and low maintenance MFC configurations have been the purpose of most of the previous research in this area. Separator and electrode materials are main components directly related to key processes occurring in MFCs such as mass transfer processes and reaction kinetics, which in turns directly affect the overall MFC performance. This Ph.D thesis focuses on the development of MFCs based on alternative materials such as ionic liquids and nanoparticles for separator and cathode construction. Firstly, a critical overview of the current state of this technology is provided, covering recent MFC advances such as new types of separator, new anode and cathode materials, MFC configurations, the scopes of application for bioenergy production and waste treatment, MFC modeling and future perspectives. Following, it is presented a new membrane-cathode assembly for its use in air-cathode single-chamber MFCs. Ionic liquid membranes offer great prospects for their application in MFCs in order to replace expensive separator materials such as Nafion. The assembly developed here consists of embedding a polymer inclusion membrane based on ionic liquid (methyltrioctylammonium chloride, [MTOA][Cl]) into the cathode material to form a single component that acts simultaneously as cathode and separator. Several step-by-step preparation sequences were studied to optimize its construction. The resulting assemblies were compared with that of a conventional configuration formed by an ionic liquid polymer inclusion membrane and a cathode set up as separate components. The results show that overall power performance and Columbic efficiency can be greatly improved by the embedded configuration developed in this work. The use of ionic liquids is also investigated in cylindrical ceramic-based MFCs. This MFC type offers great advantages such as simple cell construction and low cost. The improvement of the cell performance is explored through the application of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), [EMIM][Tf2N], as part of the ceramic separator and as part of activated carbon-based cathodes. Polarization tests prove that power performance greatly increases by incorporating the ionic liquid as part of the activated layer of the cathode electrode, while the ionic liquid-modified ceramic separator option is limited by the occurrence of the so-called overshoot phenomenon. Finally, nanostructured cobalt spinel oxides doped with copper and nickel are tested as ORR catalysts in single-chamber MFCs and compared with platinum, the standard MFC cathode catalyst that yet has very high cost. These catalysts were previously characterized by TEM, XRD and LSV analyses after being synthesized by thermal decomposition. Spinel oxides are cost competitive and can offer significant ORR catalyst activity depending on their chemical composition and structure. Several Cu:Co and Ni:Co atomic ratios were assessed mainly in terms of power performance, achieving power densities very close to that provided by platinum and showing their potential application in MFC devices.