Fe-carbon nitride Core-shell ectrocatalysts for the oxygen reduction reaction

In this report, the preparation of Fe-carbon nitride (CN)-based electrocatalysts (ECs) with acore-shell morphology for the oxygen reduction reaction (ORR) is described. The ECs consist of spherical XC-72R carbon nanoparticles, the cores, that are covered by a CN matrix, the shell, that embeds Fe species in coordination nests. The latter consist of hollow cavities in the CN matrix, whose internal surface is covered by N-and C-ligands able to stabilize alloy nanoparticles or active sites. Two families of CN-based ECs are prepared, which are grouped on the basis of the concentration of N atoms in the CN shell. Each group comprises of both a pristine and an activated EC; the latter is obtained from the pristine EC by a suitable series of treatments (A) devised to improve the ORR performance. The chemical composition of the CN-based ECs is determined by Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and microanalysis. The thermal stability under both inert and oxidizing atmospheres is gauged by High-Resolution Thermogravimetric Analysis (HR-TGA). The structure is probed by powder X-ray diffraction, and the morphology is inspected by Scanning Electron Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM). The surface area of the CN-based ECs is determined by nitrogen physisorption techniques, and the surface composition is probed by X-ray Photoelectron Spectroscopy (XPS). The electrochemical performance and reaction mechanism of the CN-based ECs in the ORR is investigated in both acid and alkaline environments by cyclic voltammetry with the Thin-Film Rotating Ring-Disk Electrode setup (CV-TF-RRDE). The influence of the preparation parameters and of the treatments on the physicochemical properties, the ORR performance, and reaction mechanism is studied in detail. In the alkaline environment the FeFe2-CNl 900/ C-A core- shell EC shows a remarkable ORR onset potential of 0.908 V vs. RHE which, with respect to the value of 0.946 V vs. RHE of the Pt/C ref., classifies the proposed materials as very promising Platinum Group Metal-free ECs for the ORR. (C) 2016 Elsevier Ltd. All rights reserved.