Bifunctional electrocatalysts based on N-Doped graphene and modified with Ti and Co for unitized regenerative fuel cells

Abstract

The increasing demand of energy that humanity had suffer in the last times, added to the global warming expanse associated to the fossil fuels used as main energy source, have made that the humanity has to afford one of its highest challenges: the search for new sources of sustainable energy. Currently, the use of electrochemical energy conversion systems like fuel cells (FC) and electrolysers (PEM) have been raised as good candidate to solve this problem. These kinds of devices are so important than some government agencies have proposed some strategies in order to develop the use of green H2 as alternative for the generation of electrical energy. In this sense, some strategies like the pickups stabilised by the European Union as “European Green Deal” or “EU Hydrogen Strategy” remarks the importance of contribute to the improvement of these technologies in order to reach a more sustainable future. To accomplish these targets, low temperature unitized regenerative fuel cells (URFC) are a promising technology for storing energy via hydrogen. A URFC consists of a single device capable of working as a FC and WE, in such a way that only one of the modes is operational in time. With these devices is possible to generate hydrogen by the electrolysis of water supplied by renewable energies, its storage and its use directly in the URFC to obtain electricity when necessary. However, development of these technologies has its main limitation in the oxygen electrode reactions, which are the oxygen reduction (ORR) and evolution (OER) reactions, whose high overpotential in both reactions (1.23 V vs RHE) make necessary the use of catalysts based on noble metals which increase significantly the cost of manufacture. To solve this, in the last decade the efforts have been focused in the use of carbon-based materials as catalysts combined with early transition metals, creating a new range of materials named composites, whose high conductivity, availability and stability make them good candidates to substitute actual noble metal-based catalysts as Pt or Ir. For this reason, the aim of this doctoral thesis is the design and optimization of a synthesis method which allow the obtention of composites prepared by combining nitrogen doped graphene with Ti and Co nanoparticles. This work has been focused in the optimization of some parameters as the precursor mixture preparation, in order to obtain different N-doped graphene structures, and the pyrolysis temperature and time for the thermal reduction step.