Advanced nanostructured carbon materials for electrochemical energy storage devices: supercapacitors and micro-capacitors

The main objective of this PhD Thesis is the synthesis and characterization of advanced nanostructured carbon materials for energy storage applications. Porous carbon materials have been intensely investigated as electrodes for energy storage applications because of their low-cost, versatility of structure/texture, good conductivity and high cycling life. A relevant electrochemical application of carbon materials is hydrogen storage by electro-reduction of water in alkaline and neutral media. Two activated carbons (ACs), with different porous texture and surface chemistry, were used and characterized by different electrochemical techniques in two electrolytes (6 M KOH and 0.5 M Na2SO4). Comparing the two samples, it was suggested that the surface functionality and porosity of the activated carbons have an important influence in the electrochemical hydrogen storage process. From the galvanostatic charge-discharge (GCD) curves, it was shown that the electrochemical hydrogen storage capacity was higher in basic medium than in neutral medium. The capacity was also higher for the AC with the higher porosity development in both electrolytes. The in situ Raman spectra collected showed that carbon-hydrogen bonds are formed reversibly in both electrolytes during cathodic conditions. It was confirmed that, in both electrolytes, the hydrogenation of carbon atoms was produced more easily for the sample with lower amount of surface oxygen groups. In basic medium, for the two samples, the formation of carbon-hydrogen bonds proceeded at more positive potential with respect to the thermodynamic potential value for hydrogen evolution. Currently, templated carbons have attracted much attention because the combination of both tailored and ordered porous network with a nano-sized structure could result in the development of unique features for their potential applications. The zeolite-templated carbon (ZTC) synthesized in the nanochannels of zeolite Y is a promising candidate as electrode for electric double-layer capacitors because of its unique structure consisting of buckybowl-like nanographenes assembled into a three-dimensional regular network with a well-defined pore size of 1.2 nm and large surface area. The electrochemical behaviour of the ZTC, focusing on both the surface chemistry and structural changes produced under different electrochemical conditions in 1M H2SO4 medium, was studied. The electrochemical quartz crystal microbalance (EQCM) allowed simultaneous monitoring of the voltammetric and gravimetric responses of ZTC. Under electrochemical oxidation conditions, a high anodic current and a net mass increase were recorded, resulting in the increase of the specific capacitance owing to the contribution of the pseudocapacitance, mainly derived from the hydroquinone-quinone redox couple. Under more severe electrochemical conditions, a net mass loss was observed, revealing that electrochemical gasification took place. Surface chemistry, before and after the electrochemical treatments, was analyzed through temperature programmed desorption (TPD) experiments. Both the EQCM and the TPD experimental results obtained suggested that the electrochemical oxidation and gasification of the ZTC took place simultaneously. Furthermore, in situ Raman spectroscopy was used to further characterize the structural changes produced in ZTC under the electrochemical conditions applied, supporting that high potential values produced the electrochemical oxidation and degradation of the carbon material. Furthermore, the recent technological trend towards portable electronic devices has leaded a strong interest in small-scale energy storage devices. Thin film capacitors have great potential to be used as power source in small-scale energy storage devices. In this PhD Thesis, a silica-templated ordered mesoporous carbon thin film was directly synthesized on a graphite current collector using an ordered mesoporous silica thin film as hard-template. The nanostructure of the silica, the composite silica/carbon and the mesoporous carbon thin films was characterized by field emission scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy microanalysis system, transmission electron microscopy and Raman spectroscopy. Silica thin film, which was uniformly deposited onto the graphite plate surface, presented mesopores of around 8 nm perpendicularly disposed to the current collector. Carbon thin film, which almost replicates the nanostructure of the silica thin film, showed mesopores of around 2-3 nm. Electrochemical behaviour of both the mesoporous carbon and the composite silica/carbon thin films was analyzed by cyclic voltammetry and GCD in 1 M H2SO4 solution, demonstrating that the thin films synthesized show exceptional properties in terms of specific capacitance, rate performance and electrochemical stability to be used as electrodes for micro-capacitors. The use of ionic liquid and organic-based electrolytes for supercapacitors is being widely studied in recent years, because they allow increasing the operating voltage with respect to the aqueous electrolytes that leads to an increase of the energy density of the device. In the present PhD Thesis, the electrochemical behaviour of a superporous AC with a tailored porosity (high apparent specific surface area and a high volume of micropores with an average pore size of around 1.4 nm) was analyzed in different non-aqueous electrolytes. The AC showed very high capacitance (higher than 160 F g-1) values in the PYR14 TFSI at different temperatures (20, 40 and 60 oC) as well as in 1M Et4N BF4/PC, 1M PYR14 BF4/PC and 1M PYR14 TFSI/PC. The tailored porosity of the AC made possible to obtain very high capacitance values, making this carbon material a promising candidate to be used as electrode for electrochemical capacitors using non-conventional electrolytes. It was also confirmed that several parameters, such as the ion/pore size ratio, the ion shape, the ion solvation and the conductivity and viscosity of the electrolyte have a strong influence on the electrochemical behaviour of the AC.