Ex situ N-doped Carbon Nanotubes as Metal-free Electrocatalysts for Oxygen Reduction Reaction

Intensive research efforts have been devoted in the last few years to the development of efficient and inexpensive alternatives to precious-metal based electrocatalysts (typically based on Pt) for the Oxygen Reduction Reaction (ORR) in fuel cells cathodes. On this ground, nitrogen-doped carbon nanomaterials have recently emerged as candidates at promoting this reaction efficiently. While a relatively high number of in-situ prepared N-doped carbon nanostructures showing catalytic activity have been reported to date, much less work has been done for the synthesis of covalently N-decorated carbon nanomaterials via milder and easily tunable ex-situ functionalization approaches. Taking advantage from consolidated organic functionalization protocols, a series of N-decorated multi-walled carbon nanotubes (N-MWCNTs) bearing well defined N-containing groups have been prepared and completely characterized (TEM, TGA, XPS). Their catalytic activity in the ORR has been evaluated by cyclic voltammetry and directly compared with Glassy Carbon and bare Pt electrodes. Noteworthy, selected samples have shown from good to excellent catalytic performance in terms of onset-potential (Eon) and number of exchanged electrons (nE=-1). Remarkably, our ex-situ approach leads to a precise control of the "chemical identity" of the N-dopant that is of fundamental importance for a deep understanding on the role of the different N-containing groups on the final performance of these metal-free catalysts.