Fe-carbon nitride 'Core-shell' electrocatalysts for the oxygen reduction reaction

Electrochim. Acta 222, 1778-1791 (2016) In this report, the preparation of Fe-carbon nitride (CN)-based electrocatalysts (ECs) with a 'core-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', embedding Fe species in 'coordination nests'. The latter is formed by the presence of carbon and nitrogen ligands on the surface of the CN matrix, the 'shell'. 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 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/CA '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.