Synthesis of Metal-Free Electrocatalyst Obtained from Different Biomass Sources with High Performance for Oxygen Reduction Reaction in Fuel Cells

Abstract

The development of electrocatalysts with low or zero noble metal content is a key factor in the large-scale commercialization of fuel cells. Platinum and platinum alloys are the most efficient catalysts for the oxygen reduction reaction (ORR), which occurs in the cathode of the fuel cell. Nanocarbon materials doped with nitrogen (ex. carbon nanotubes and graphene) have been recognized as a promising alternative for the development of platinum-free electrocatalysts for the ORR. However, the synthesis of these carbon materials often involves the use of harmful organic reagents, synthesis´s extreme conditions, high costs and often are restricted to small scale production. Recently, the use of biomass as source of carbon and nitrogen has been explored to obtain metal-free electrocatalysts. The results reported in the literature indicate that biomass is a very promising alternative for obtaining of electrocatalysts of low-cost and highly efficient in the ORR. This work describes a two-step method for producing metal-free electrocatalysts from biomass. Two sources of biomass with high nitrogen content in their molecular structure were selected: sargassum and leather. First, each raw material was subjected to pyrolysis treatment at 700 C for 90 minutes under nitrogen, then an activation treatment was given with KOH in nitrogen at 750 C for 90 minutes. The materials were labeled as PAS and PAL for sargassum and leather respectively. The structural properties of the materials were analyzed by Raman spectroscopy and X-ray diffraction. To determine the morphology scanning electron microscope was used. The electrochemical performance was evaluated by rotating disk electrode technique (RDE). Raman spectra showed the D and G bands characteristic of carbon materials. This results indicate that after pyrolysis and activation treatments carbonaceous materials with high disorder in their molecular structure were obtained. BET analysis confirmed high surface areas (up to 2000 m2 g-1) and SEM showed the formation of larger porosity in the surface of the electrocatalysts from biomass. Finally, electrochemical characterization showed high activity for ORR in alkaline media for both electrocatalysts (PAL and PAS). The sample PAL has major current density than platinum electrocatalyst and very close on-set potential (shift 1145 mV). The results obtaining in this work indicate that electrocatalyst from biomass as sargassum and leather are a promising alternative for cathodic reaction in fuel cells.