Tailoring Carbon Nanotube N‑Dopants while Designing Metal-Free Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Medium Giulia Tuci, † Claudio Zafferoni, ‡ Primiano D’Ambrosio, † Stefano Caporali, ‡ Matteo Ceppatelli, †,§ Andrea Rossin, † Theodoros Tsoufis, † Massimo Innocenti,* ,†,‡ and Giuliano Giambastiani* ,† † Institute of Chemistry of OrganoMetallic Compounds, ICCOM-CNR and INSTM Consortium, 50019 Sesto F.no, Florence, Italy ‡ Department of Chemistry, University of Florence, 50019 Sesto F.no, Florence, Italy § LENS - European Laboratory for Nonlinear Spectroscopy, 50019 Sesto F.no, Italy * S Supporting Information ABSTRACT: A straightforward, energy- and atom-saving process to the production of tailored N-doped and catalytically active metal-free carbon nanostructures, has been set up. Our ex situ approach to the N-decoration of the carbon nanotube sidewalls contributes to elucidate the complex structure-reactivity relationship of N-doped carbon nanomaterials in oxygen reduction reactions, providing fundamental insights on the nature of the N-active sites as well as on the role of neighboring carbons. KEYWORDS: ex situ N-doping, doped carbon nanotubes, aryl-diazonium salt chemistry, electrocatalysts, oxygen reduction reaction (ORR) I ntensive 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. 1 Typically, the ORR can proceed either through a four-electron process to combine oxygen with electrons and protons into water as the final product or through a less efficient two-step, two-electron pathway involving the formation of the hydroperoxide ions as intermediates. 1 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. 2 It is generally accepted that N-doped carbon nanostructures can favor the surface O 2 chemisorption/activation improving their catalytic performance in the ORRs remarkably. 2a,3 Although a relatively high number of N-doped carbon nanostructures showing catalytic activity in ORRs have been prepared by the in situ CVD approach, 2 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. The latter imply a number of important issues whose achievement may represent a real breakthrough in the development of novel nanostructured, metal-free catalysts. Indeed, in addition to leading to a fine-tuning of the chemical identity of the N- dopants, an ex situ approach better matches the requirements for energy- and atom-saving processes than the classical CVD approach. In addition, N-dopants are entirely present at the nanotube surface, where the catalytic process takes place. Finally, an ex situ approach can contribute to answering the widely debated question related to the intrinsic ability of different N-containing groups, randomly embedded in the sp 2 CNT network, at promoting ORRs. 4 Although the real nature of the active sites in N-doped carbon nanomaterials still remains unclear, it is generally accepted that pyridine and pyrrole nitrogen atoms contribute differently to the ORR, the former playing a key role in promoting the process. 4 In this regard, a puzzling question arises: What is the effect of the neighboring atomic environ- ment on the ability of pyridine nitrogen atoms to promote ORR? To answer this question, we took advantage of the well consolidated aryldiazonium salt (Tour) functionalization protocol 5 as a convenient synthetic methodology for the ex situ N-doping of MWCNTs with pyridine- and pyrrole- containing dangling groups (Scheme 1). As shown in Scheme 1, 4-aminopyridine (1), 9-amino- acridine (3), and 3-aminocarbazole (5) are selected as N- containing candidates, 3 and 5 being selected as mimics of pyridine and pyrrole frameworks, respectively, embedded in a conjugated Csp 2 network. All reactions proceed smoothly under mild conditions, providing the expected functionalized samples 2, 4, and 6. Careful workup procedures and parallel blank tests (carried out in the absence of the isopentylnitrite reagent) have been used to rule out any possible reagent Received: May 22, 2013 Revised: July 17, 2013 Letter pubs.acs.org/acscatalysis © XXXX American Chemical Society 2108 dx.doi.org/10.1021/cs400379h | ACS Catal. 2013, 3, 2108-2111