Metal-free Electrocatalysts from Tailored ex-situ N-doped Carbon Nanotubes for Oxygen Reduction Reactions

Intensive research efforts have been devoted in the last few years to the development of efficient, durable and inexpensive alternatives to precious-metal based electrocatalysts (typically based on Pt and its alloys) for the Oxygen Reduction Reaction (ORR) in fuel cells (FC) cathodes. On this ground, nitrogen-doped 1D and 2D carbon nanomaterials (occasionally in combination with non-noble metal nanoparticles) have recently emerged as valuable candidates capable of promoting this reaction efficiently. It is generally accepted that N-doped carbon nanostructures can favor the surface O2 chemisorption/activation improving the catalytic performance of these metal-free species in the ORRs, remarkably. While a relatively high number of in-situ prepared N-doped carbonnanostructures 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 exsitu functionalization approaches. 2. Results and discussion Taking advantage from consolidated organic functionalization protocols, a series of Ndecorated 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 Oxygen Reduction Reaction (ORR) has been evaluated by cyclic voltammetry; all activity trends have been directly compared with both Glassy Carbon and bare Pt electrodes. Noteworthy, selected samples have shown from good to excellent catalytic performance in terms of both onset-potential (Eon) and number of exchanged electrons (nE=-1). Remarkably, our ex-situ approach leads to a precise control of both surface functionalization loading and "chemical identity" of the N-dopant. This latter aspect is of fundamental importance for a deep understanding of the role of the different N-containing groups on the final performance of these metal-free catalysts; it also contributes to answer on a widely debated question on the ability of differently N-decorated carbon nanostructures to promote ORRs, efficiently.