Chemically Functionalized Carbon Nanotubes with Pyridine Groups as Easily Tunable N-decorated Nanomaterials for the Oxygen Reduction Reaction in Alkaline Medium

Efficient and low-cost electrocatalysts capable of facilitating the sluggish cathodic oxygen reduction reaction (ORR) are at the heart of renewable energy technologies based on fuel cells and other electrochemical energy devices. The high cost of the platinum-group metal-based electrocatalysts, together with their scarce reserves in nature, limit their sustainable commercial application in several technological energy-related fields. Looking at effective and efficient alternatives to Pt-based electrocatalysts in ORR, N-doped carbon nanotubes have drawn much attention for their potentiality in this field. Since the first seminal report by Dai and co-workers in 2009, a huge effort has been made to prepare N-CNTs and exploit their catalytic properties. Among the available synthetic methods, Chemical Vapor Deposition (CVD) still remains the most widely used technique, although both the type of N-functional groups and the N-doping level within the materials still remain far from being easily controlled through this synthetic approach. We have recently demonstrated how a simple CNT covalent functionalization with pyridine frameworks embedded in a conjugated (although spatially limited) sp2 carbon network can produce N-decorated nanomaterials featured by remarkable catalytic performance in ORR. Here, we present a full account on the synthesis, and electrocatalytic activity of a series of functionalized MWCNTs with a variety of well defined pyridinic groups. This study offers a clear evidence of the central role played by the pyridine moieties on the electrocatalytic activity of N-CNTs in the ORR. Most importantly, a clear-cut evidence of the catalysts performance (at least in terms of onset potential) in the ORR is provided as a function of the electronic charge density on the N-neighboring carbon atoms and the related N-C? bond polarization. This alternative approach to the N-decoration of carbon nanotubes highlights the importance of the N-chemical surrounding on the catalytic performance of the N-functionalities, offering both an excellent basis to the development of more catalytically active metal-free electrocatalysts for the ORR and an unique model for the in-depth understanding of the underlying mechanism.