Doped porous carbon materials for energy storage and production

Tesis doctoral presentada en el Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica de la Universidad de Oviedo, junio de 2017The nitrogen‐doping of carbon materials has been attracting increasing attention in the field of energy storage (for use as electrodes in supercapacitors and Li‐ion batteries) and energy production (for use as electrocatalysts in fuel cells). This is due to the fact that the incorporation of this heteroatom into the carbon framework has a positive effect on a number of properties including electronic conductivity, surface wettability, catalytic activity or resistance to oxidation. As a result, the development of synthesis procedures that allow the precise control and optimization of the textural, structural and chemical properties of nitrogen‐doped carbon materials has attracted considerable interest. These considerations have been the main source of inspiration for the present PhD Thesis which is focused on the design and fabrication of nitrogendoped porous carbon materials for energy storage and production. Firstly, the synthesis of different nitrogen‐doped porous carbon materials with optimized textural and chemical properties for use as supercapacitor electrodes was carried out. Special emphasis was placed on increasing their energy density by means of the following two strategies: a) enhancement of their specific capacitance and b) enlargement of their voltage window. The electrochemical performance of the supercapacitor was improved by a rational design of the materials´ properties. Nitrogen‐doped carbon capsules and microspheres were obtained by means of a nanocasting technique, using pyrrole as N‐containing carbon precursor. These carbon materials were tested as electrodes for supercapacitors in various electrolytes (1 M H2SO4 and 1 M TEABF4/AN) in which they exhibited high specific gravimetric capacitances and provided large energy and power densities. In addition, the carbon microspheres offered a remarkable volumetric capacitance as a result of their high packing density. When these materials were subjected to an additional chemical activation process, a notable increase in their textural properties (i.e. specific surface area and pore volume) was achieved. Thereby, their specific surface areawas increased substantially while their original morphology was retained. On the other hand, a simple procedure based on the direct carbonization of citrate salts was devised for the synthesis of nitrogen‐doped mesoporous carbon materials, which were employed as electrodes in supercapacitors in aqueous (1 M H2SO4) and ionic liquid (EMImTFSI/AN) electrolytes. Finally, a biomass residue with a high nitrogen content (defatted soya flour) was investigated as carbon precursor. From this precursor, highly microporous nitrogen‐doped carbon materials were synthesized and tested as electrodes for supercapacitors in aqueous media (1 M H2SO4 and 1 M Li2SO4). These microporous carbons were obtained by means of hydrothermal carbonization followed by a chemical activation step. The carbon materials exhibited a good capacitive performance in both gravimetric and volumetric terms. Secondly, the synthesis of different nitrogen‐doped porous carbon materials and their use as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells were explored. Initially, these electrocatalysts were synthesized by means of the direct carbonization of citrate salts. By this simple procedure, nitrogen‐doped mesoporous carbon materials were obtained. These materials exhibited an excellent catalytic activity in basic media, comparable to that of a commercial platinum‐based catalyst, and a much greater durability and selectivity against methanol electrooxidation. On the other hand, nitrogendoped carbon microspheres with tunable pore size ranging from micro‐ to mesopores were synthesized by means of a nanocasting technique. In this way, the effect of the pore size distribution upon ORR catalytic activity was assessed. Finally, Fe‐N‐doped mesoporous carbon capsules were studied as electrocatalysts for the oxygen reduction reaction. These carbon materials exhibited a higher catalytic activity than a commercial platinum‐based catalyst in a basic medium, a similar catalytic activity in an acid electrolyte and a better stability and tolerance to methanol than platinum in both of these electrolytes.Peer reviewe