Sulfur doped carbon nanohorns towards oxygen reduction reaction

Platinum has been proved to be the most effective electrocatalyst for oxygen reduction reaction (ORR) in proton-exchange membrane (PEM) fuel cells; however, it presents some disadvantages such as its scarcity and, consequently, high cost. Recently, the development of electrocatalysts has been directed towards designing low Pt or Pt free materials. Carbon nanomaterials have shown good performance when used as Pt catalyst supports. Moreover, by modifying the properties, carbonaceous materials can be used also as catalysts. An alternative to change the carbon nanomaterial properties is their modification with heteroatoms. In the present work sulfur-doped carbon nanohorns (SCNHs) were synthesized by a modified chemical vapor deposition method (mCVD). Toluene and thiophene were used as respective carbon and carbon‑sulfur sources and ferrocene as catalytic agent. Iron amount was evaluated to determine the optimal synthesis conditions and influence in its properties. Physical and chemical characterization were performed by means of X-ray diffraction, scanning electron microscopy, Raman spectroscopy and high resolution scanning and transmission electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. Additionally, the behavior of the obtained materials towards the ORR was studied by linear sweep voltammetry. Tubular structures with conical and horn-like shapes were obtained where sulfur and carbon formed thiophene type bonds. According to iron amount variation results, reducing Fe concentration improves morphological properties and maintains structural characteristics. Electrochemical results show acceptable activity for ORR, hence SCNHs can be considerate as non-Pt electrocatalyst for fuel cell electrodes.