Tailoring Carbon Nanotube N-Dopants while Designing Metal-Free Electrocatalysts for the Oxygen Reduction Reaction

Intensive research efforts have been devoted in the past few years to the development of efficient, durable, and inexpensive alternatives to precious-metal-based electrocatalysts (typically containing Pt and its alloys) for the oxygen reduction reaction (ORR) in fuel cell (FC) cathodes. On this basis, nitrogen-doped 1D and 2D carbon nanomaterials (occasionally combined with non-noble metal nanoparticles) have recently emerged as valuable candidates capable of promoting this reaction efficiently. Although a relatively high number of N-doped carbon nanostructures showing catalytic activity in ORR have been prepared by an in-situ CVD approach, much less work has been done for the obtainment of catalytically active N-decorated carbon nanomaterials using milder and easily tunable ex-situ (exohedral) organic functionalization techniques. In this contribution we present a straightforward approach to the production of tailored N-doped and catalytically active carbon nanostructures, ultimately providing fundamental insights on the complex structure-reactivity relationships of N-doped carbon nanomaterials. Compared with the classical in-situ approach, the presented method lists a number of remarkable advantages: (1) mild reaction conditions required to N-decorate the CNTs surface (energy-saving process); (2) easy tailoring of the N-containing functional groups and (3) their full exposure (atom-saving) to the nanomaterial outer side, where the catalytic process takes place; (4) absolutely remarkable electrocatalytic activity and long-term stability of selected N-doped samples for ORR in basic medium. Owing to the established versatility of the employed CNT functionalization protocol, the methodology developed in this study can be conveniently applied to the ex-situ heterodecoration of different 1D and 2D nanocarbon materials with light elements for the production of metal-free catalysts.