Espumas de carbono con propiedades avanzadas derivadas de carbones bituminosos

Resumen

[EN] Carbon and graphite foams are carbon materials with excellent technological characteristics whose properties can vary within a wide range depending on the precursor and the method of synthesis. Coals offer an interesting economical alternative for carbon foam manufacture compared to other graphitic precursors, such us pitches or polymers. The aim of this work is the synthesis of coal based carbon foams with advanced properties that could expand its range of applications beyond the usual thermal or mechanical ones. For this purpose, this thesis focuses on the preparation of carbon foams with a high degree of structural order, as well as the design of foams with a macro-microporous hierarchical structure. In the first part of the thesis, the effect of precursor coal on the degree of structural order of the graphitic foams was initially evaluated. Subsequently, carbon foams with high structural order were synthesized by boron doping. The doped foams were characterized comprehensively in order to determine the effect of the graphitization temperature (2400-2800 °C), the boron loading (0.75-10%) and its nature (inorganic: B2O3, B4C; organic: C5H5N:BH3) on the crystal parameters and the degree of intercalation in the graphitic structure (final concentration and distribution). Thermogravimetric oxidation tests revealed an increase in the temperature of ignition of the doped foams, as the dopant concentration increases. This improvement in oxidation resistance is due to the higher degree of structural order reached in the doped foams, as well as the formation of a boron oxide film during the test. Specifically, when adding 10% of boron oxide, the resulting foam showed to be thermally resistant above 900 °C, value higher than that achieved when a coat of aluminum metaphosphate was employed. However, at low boron loadings, the substitutional boron can play a catalytic role on the thermal oxidation process. On the other hand, graphitic boron doped foams were electrochemically evaluated in lithium ion batteries. In some cases, the reversible capacity values achieved were very similar to those exhibited by commercial graphites, synthesized at a higher temperature and from higher cost precursors. In addition, the findings from this study contribute to improve our knowledge about the influence of boron on the insertion of Li+ ions in the structure of doped carbonaceous materials. The results revealed that, for optimum electrochemical performance, it is necessary to find a compromise between the degree of graphitization and the amounts of substitutional boron and boron carbide present in the doped foams. In the second part of the thesis, a novel method for development of micropores in the carbon foams was proposed, based on the use of chemical activating agents (KOH, ZnCl2) during the foaming step, and followed by a carbonization between 500 and 800 ºC. The activated foams exhibited a macroporous structure, with pores between 100 and 10 µm, and microporosity comprised mainly of pores less than 1 nm in size. Subsequently, the CO2 adsorption capacity at 25 ºC of some activated foams was evaluated by volumetric analysis. The foams activated with KOH have shown a greater ability to adsorb CO2, compared to ZnCl2 activated foams, due to its higher volume of narrow micropores (< 0.8 nm) and its stronger basic character. Also, these foams showed values similar to those of two commercial activated carbons, together with an excellent cyclability and selectivity to N2.