Looking Far Beyond the Classical Carbon Nanomaterials N-Doping while Preparing Efficient Metal-Free ORR Electrocatalysts

The development of cheap, highly efficient and durable electrocatalysts for the kinetically sluggish oxygen reduction reaction (ORR) represents a challenging issue for the sustainable development of devices at the heart of renewable energy technology i.e. metal-air batteries and fuel cells [1, 2]. Along with recent intensive research efforts in reducing or replacing Pt-based electrocatalysts in ORR, light-heteroelement doped carbon nanomaterials (CNMs) with at least one dimension tailored at the nanometer scale have raised the interest of the catalysis and material science community. In particular, nitrogen-doped CNMs (N-CNMs) have been intensively investigated as metal-free systems in light of their remarkable electrochemical properties in ORR under both alkaline and acidic environment [3, 4]. To date, the nature of the active sites on these complex materials and the comprehension of the underpinning electrochemical mechanisms, remain poorly understood if not highly controversial. This contribution shows an original and highly interdisciplinary approach to the production of tailored N-decorated and catalytically active N-CNMs and provides fundamental insights on their complex structure-reactivity relationship in the activation and electrochemical conversion of dioxygen [5, 6]. The rationale of this study lies on the fine-tuning of the adsorption strength of O2 (substrate) and/or its reduction intermediates (products) to selected N-heterocycle active sites. A systematic screening of variably substituted N-heterocycles has revealed the existence of a rational trend, coherent with the Sabatier principle, between the atomic charge at the ?-carbon atoms (qC?) of the heterocycles and the Eon values measured for the ORR at the respective metal-free systems (Fig. 1). As a result, the first statistical Volcano plot representation based on well-defined N-containing groups has been proposed [7]. It provides a quick identification of the "just right" interaction between the catalytically active site(s) and dioxygen and it paves the way to the rational development of tailored and more catalytically active metal free electrocatalysts for the ORR.