Desarrollo de nuevas capas de difusión de gases (GDLS) para pilas de combustible tipo PEM
[SPA] El objetivo principal de esta tesis doctoral es desarrollar un método de fabricación de capas de difusión de gases (GDLs) sostenibles medioambiental y económicamente, así como la metodología para la caracterización de las propiedades de dichas GDLs. Para ello, se emplearon diversas técnicas para medir las propiedades físicas de una GDL (conductividad eléctrica, conductividad térmica, hidrofobicidad, permeabilidad al aire, porosidad). También se comprobó su efecto en el rendimiento de una pila de combustible tipo PEM (PEMFC) de cátodo abierto. Para ello, se desarrolla también una metodología de montaje de celda de combustible, comprobando el efecto del pretratamiento de una membrana de intercambio protónico en el número de moléculas de agua. Se comparan distintos procesos de deposición del catalizador (siendo el electrospray, él que mejores resultados otorga) y se estudian diferentes geometrías de canal en el plato bipolar. Comparamos los efectos que proporcionan las distintas concentraciones de fibra vegetal en la fabricación de una GDL, obteniendo los mejores resultados con un 40% de fibra vegetal. Siguiendo la misma línea, se utilizó fibra vegetal de esparto en lugar de fibra vegetal de algodón, obteniendo mejores resultados. Se estudió cómo afecta la temperatura de tratamiento térmico a la conductividad eléctrica mediante procesos de carbonización/grafitización, siendo crítica la temperatura de los 750ºC en la que la conductividad se dispara. Empleando la técnica de difracción de rayos y espectroscopia infrarroja, se observó que a partir de los 300ºC de tratamiento térmico había cambios notables que podían explicarse mediante la degradación de la celulosa. [ENG] The main purpose of this PhD thesis is to develop a method for the fabrication of environmentally and economically sustainable gas diffusion layers (GDLs), as well as the methodology for the characterisation of the properties of such GDLs. To this end, various techniques were used to measure the physical properties of a GDL (electrical conductivity, thermal conductivity, hydrophobicity, air permeability, porosity). Their effect on the performance of an open-cathode PEM fuel cell (PEMFC) was also tested. For this purpose, a fuel cell assembly methodology is also developed, testing the effect of a proton Exchange membrane pre-treatment on the number of water molecules. Different catalyst deposition processes are compared (electrospray being the one that gives the best results) and different channel geometries in the bipolar plate are studied. We compared the effects provided by different concentrations of plant fibre in the manufacture of a GDL, obtaining the best results with 40% plant fibre. Along the same lines, esparto plant fibre was used instead of cotton plant fibre, obtaining better results. The effect of heat treatment temperature on electrical conductivity was studied using carbonisation/graphitisation processes, with the temperature of 750ºC being critical, where conductivity skyrockets. Using the technique of ray diffraction and infrared spectroscopy, it was observed that from 300ºC of heat treatment there were noticeable changes that could be explained by cellulose degradation. The hydrophobic effect caused by the addition of graphene to a porous microlayer (MPL) was also demonstrated, giving key advantages, especially when used together with other water repellents such as Teflon, in an open-cathode PEMFC at temperatures above 60ºC. In addition, the chemical stability of the fabricated GDLs was demonstrated to be good. For this purpose, they were introduced in 0.5M sulphuric acid (H2SO4) for 2 months, simulating an intensive use of a PEMFC operating in a slightly acidic medium. After treatment with H2SO4, slight differences were observed on a micrometric scale, with the amount of coating on the carbon fibres being lower. The hydrophobicity of the GDL decreased by 5% to 11%, resulting in a loss of maximum power point performance of less than 4%. Its effects are noticeable only at 0.45 A/cm2 and above. In all other properties, as well as infrared spectroscopy and X-ray diffraction studies, no noticeable differences were observed.
Fabricación, Capas de difusión (GDL), Pilas de combustible, Fibra vegetal, Energía
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