Interplay between Nitrogen Concentration, Structure, Morphology, and Electrochemical Performance of PdCoNi "Core-Shell" Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

This report describes the electrochemical behavior of a family of "core-shell" electrocatalysts consisting of a carbon nitride (CN) "shell" matrix and a "core" of conducting carbon nanoparticles (NPs). The CN "shell" matrix embeds PdCoNi alloy NPs and covers homogeneously the carbon "core". The chemical composition of the materials is determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis; the structure is studied by powder X-ray diffraction (powder XRD); the morphology is investigated by high-resolution transmission electron microscopy (HR-TEM). The surface activity and structure are probed by CO stripping. The oxygen reduction reaction (ORR) kinetics, reaction mechanism, and tolerance towards contamination from chloride anions are evaluated by cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) method. The effect of N concentration in the matrix (which forms "coordination nests" for the Pd-based alloy NPs bearing the active sites) on the ORR performance of the electrocatalysts is described. Results show that N atoms: 1) influence the evolution of the structure of the materials during the preparation processes, and 2) interact with alloy NPs, affecting the bifunctional and electronic ORR mechanisms of active sites and the adsorption/desorption processes of oxygen molecules and contaminants. Finally, the best PdCoNi electrocatalyst shows a higher surface activity in the ORR at 0.9 V vs. RHE with respect to the Pt-based reference (388 mu Acm(Pd)(-2) vs. 153 mu Acm(Pt)(-2)).