Template-free ultraspray pyrolysis synthesis of N- and Fe-doped carbon microspheres for oxygen reduction electrocatalysis

The overall energy efficiency in fuel cells is deeply affected by the sluggish kinetics of the two electrodes processes. Between them the most kinetically hindered is the oxygen reduction reaction (ORR) at the cathode. Efforts are therefore addressed to develop PGM-free catalysts for ORR. In this work Fe- and N-doped porous carbon electrocatalysts were synthesized in a continuous flow apparatus via a template-free ultrasonic spray pyrolysis (USP) method. Aqueous solutions of sugars, N-compounds and Fe(CH3COO)2 were nebulized and pyrolyzed yielding carbon microspheres, as revealed in Figure 1 by Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB) milling and Energy Dispersive Spectroscopy (EDS). Microspheres initially possess empty cores and a smooth shell. Further 900°C annealing leads to a collapse of this shell, and formation of porous microspheres with high roughness and iron-rich aggregates. X-ray Diffraction (XRD) and Photoelectron Spectroscopy (XPS) were used to investigate bulk and surface chemistry: microspheres were found to undergo graphitization; Fe and Fe3C segregate and become encapsulated within the carbon phase, while the nitrogen present in the precursor solution results in the formation of pyridinic/pyrrolic N-centers. The microspheres were tested as electrocatalysts for the oxygen reduction reaction (ORR) in acidic solution. Polarization curves using a Rotating Disk Electrode (RDE) yielded electrocatalytic behavior, and the number of exchanged electrons, calculated from Koutecky-Levich plots, suggests that direct formation of H2O is the preferred ORR mechanism. The inherent scalability of continuous USP flow method represents a significant advantage compared to alternative synthetic strategies requiring batch processing or surface catalyzed deposition of nanostructured carbon materials.