Non-noble electrocatalysts for O2 reduction:: How does heat treatment affect their activity and structure?: Part II.: Structural changes observed by electron microscopy, Raman, and mass spectroscopy

In part I of this paper, a model was proposed to (i) describe the gasification of a spherical carbon black particle by ammonia at high temperature and (ii) explain the activity for the O-2 electroreduction of catalysts based on Fe and carbon heat-treated in NH3. In this second part, such catalysts, heat-treated for various times in ammonia, are characterized by electron microscopy, Raman spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Electron microscopy shows an average particle diameter of 38 nm for the pristine furnace black, decreasing to 28 nm after the carbon powder loses 90% of its mass. This agrees with model prediction. Raman spectroscopy shows that the experimental width at half-maximum of both the graphitic peak (similar to 1595-1600 cm(-1)) and the disorder and graphitic-edge peak (similar to 1345-1355 cm(-1)) constantly decreases with increasing weight loss. It is found that the width at half-maximum of these peaks correlates with the fraction of disordered carbon phase calculated by the model. Last, changes of the ToF-SIMS total and Fe+ signals upon gasification of the carbon black also support the model.