Sulfur-Doped Carbon Aerogel as a Metal-Free Oxygen Reduction Catalyst

Resorcinol-formaldehyde-based carbon aerogels were modified by treatment with H2S at 650 and 800 degrees C and used as catalysts for the oxygen reduction reaction (ORR) in an alkaline medium. On the material treated at high temperature, the electron transfer number was close to 4e(-) and the onset potential for the ORR was shifted to a more positive value. An excellent tolerance to methanol crossover was also detected. The treatment resulted in sulfur doping in the form of thermally stable thiophenic compounds, which increased the surface hydrophobicity. This sulfur leads to a positive charge on the carbon atoms, which are the reaction sites. The hydrophobicity of the surface enhances the withdrawal of O-2 from the electrolyte and its adsorption on the surface. The specific micro-/mesoporosity contributes to a high dispersion of sulfur heteroatoms on the surface and to the accessibility of the surface sites to the electrolyte and thus to oxygen dissolved in water.