Chemically Functionalized Carbon Nanotubes with Pyridine Groups as Easily Tunable N-Decorated Nanomaterials for the Oxygen Reduction Reaction

Efficient and low-cost electrocatalysts capable of fostering 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; anyway, both content (%) and type of N-doping in the final material still remain far from being easily controlled through this synthetic approach. We have recently demonstrated how a simple CNT covalent functionalization with pyridine-containing frameworks gives N-decorated nanomaterials featured by remarkable catalytic ORR performance. Our study offers a clear evidence of the central role played by the pyridine moieties on the electrocatalytic activity of N-CNTs in the ORR. A clear-cut evidence of the catalysts performance (in terms of process kinetics) 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. Overall, our alternative approach to the N-decoration of carbon nanostructures highlights the importance of the N-chemical surrounding on the ultimate catalyst performance while offering an excellent basis to the development of more catalytically active metal-free electrocatalysts for the ORR as well as an unique model for the in-depth understanding of the underlying mechanism.