Ex-situ Decoration of Carbon Nanotubes with Pyridine-based Frameworks; an Easily Tunable Approach Towards Efficient N-doped Metal-Free Catalysts for the Oxygen Reduction Reaction

Efficient and low-cost electrocatalysts for the oxygen reduction reaction (ORR) are at the heart of renewable energy technologies. Indeed, the high cost of the platinum-group metal-based electrocatalysts, together with the limited reserves of these noble metals in nature, constitutes the major drawback to their sustainable commercial application in several technological energy-related fields. Following the recent intensive research efforts in reducing or replacing Pt-based electrocatalysts in ORR, light-heteroelement doped 1D carbon nanotubes, particularly those containing nitrogen (N-CNTs), have drawn much attention for their potentiality in this sector. Since the first seminal report by Dai and co-workers in 2009, a huge work has been carried out to prepare N-CNTs and to fully exploit their catalytic properties. Among the available synthetic methods, Chemical Vapor Deposition (CVD) still remains the most effective and widely used technique to produce N-doped CNTs; anyway, both the N-doping level and the type of N-containing functional group distribution within the final material still remain far from being easily controlled through this synthetic approach. One recent study by us has demonstrated how a simple CNT covalent decoration with pyridine frameworks embedded in a conjugated (although spatially limited) sp2 carbon network, can translates into systems featured by remarkable catalytic performance in ORR, both in terms of onset potentials (Eon) and diffusion-limited current density (J) values. Here, we present a full account on the synthesis, and electrocatalytic activity of a series of ex-situ N-doped MWCNTs using a variety of well defined pyridinic frameworks. This study offers an unambiguous evidence of the central role played by the pyridine moieties on the electrocatalytic activity of N-decorated MWCNTs in the ORR. Most importantly, a clear-cut evidence of the catalysts efficiency in the ORR as a function of the C-N bond polarization (descriptor) is given. As a result, the first volcano plot based on this descriptor and associated to the catalytic performance in ORR of totally metal-free systems will be presented.