Síntesis y caracterización de materiales grafénicosaplicación en almacenamiento de energía

Resumen

[EN] The chemical route has been selected in this thesis, among the different methods used for the preparation of graphene materials, because it offers the possibility to prepare a wide range of materials which can be adequate for applications related to energy, environment and chemical processes (catalysis). Additionally, it offers the possibility of an easier scale up. The final aim is to design graphene materials of tailored structure and properties, based on the knowledge of the influence of the parent graphite and processing at the different steps involved in their preparation. This Thesis includes a study of the influence on the characteristics of the graphenic material of: i) the parent graphite, ii) the oxidation method for obtaining graphite oxide (Brodie vs Hummers), iii) the conditions of the exfoliation of the graphite oxide (sonication time), iv) the type of reduction used for obtaining the graphene (chemical vs thermal) and iv) the temperature used in the process of thermal exfoliation/reduction of the graphite oxide. Moreover, some of the graphenic materials prepared have been tested as positive electrode redox flow battery of vanadium. The achieved results show that the crystalline structure of the parent graphite influences the type and distribution of the oxygen functional groups present in the graphene oxide (GO) and the size of the GO sheets. GOs obtained from larger size crystal graphites show a larger sheet size, with hydroxyl and epoxy groups, preferentially located in the basal plane. The GO obtained from smaller crystal size graphite has lower size sheet and the functional groups are located at the edges of sheet with greater amount of carboxyl groups. These structural differences are very important to their behavior during the reduction treatments. The influence of the structure of the graphite oxide (GrO) during the exfoliation step has become obvious by the different times required and its effect on the size of GO sheets. The exfoliation of the Gras obtained from more crystalline graphites requires shorter times and gives better GO yields with larger sheet sizes of the sheets. The study of the temperature effect on the exfoliation/reduction of GrO has evidenced the simplicity and effectiveness of the process and its versatility for producing graphenic materials with different structural characteristics and number of layers, in all cases below 10. The treatment used in the oxidation of graphite has been determinant in the structural characteristics of GrO and consequently in the Gas and in the final graphene. The Brodie's method is less severe than Hummers one, but it generates more stable oxygen functional groups (located at the edges of sheets), which are more difficult to remove and thus the reconstruction of the pristine graphite structure. Testing of the materials produced in vanadium redox flow battery, has evidenced a better performance of the graphene obtained by thermal exfoliation-reduction of Gras. This is explained by a better restoration of the Sp2 C structure which directly affects the electrical conductivity of the material, together with the presence of some residual hydroxyl groups which act as active sides in the redox reactions and favor the contact of the electrode with the electrolyte.