New Transition Metal-CN ORR Electrocatalysts with a ' Core-Shell ' Structure

Abstract

The oxygen reduction reaction (ORR) plays a crucial role in the operation of a number of advanced energy conversion devices, including fuel cells and metal-air batteries. In general, the ORR kinetics is very sluggish, giving rise to very large losses; suitable electrocatalysts are needed to achieve a performance level compatible with applications [1]. Thus, the development of improved ORR electrocatalysts is a major objective of the research. Very recently, new families of energy conversion devices were developed adopting alkaline electrolytes. Typical examples of such systems include anion-exchange membrane fuel cells (AEMFCs) and lithium-air batteries. All these devices have a considerable potential to achieve a high performance level even without the application of ORR electrocatalysts based on platinum-group metals (PGMs), which are very expensive and prone to give rise to supply bottlenecks [2]. Indeed, in an alkaline medium, “outer shell” processes are heavily involved in the ORR mechanism; consequently, the direct adsorption of oxygen on active sites based on PGMs is not necessary [3]. As a result, in an alkaline medium the ORR can be effectively promoted by active sites based on non-noble, first-row transition metals including Fe, Co, Mn and Ni [3, 4]. In this work, a new family of ORR electrocatalysts meant for the alkaline environment is devised, taking advantage of an innovative preparation protocol [5]. The electrocatalysts consist of a carbon nitride matrix coordinating active sites based on first-row transition metals; the carbon nitride matrix further coats graphite nanoparticles characterized by a good electron conductivity, which act as the support and improve the dispersion of the active sites giving so rise to a “core-shell” morphology [6]. The chemical composition of the electrocatalysts is investigated by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis; the morphology is characterized by high-resolution scanning electron microscopy (HR-SEM) and high-resolution transmission electron microscopy (HR-TEM); the surface structure and surface chemical composition is elucidated by nitrogen physisorption techniques and X-ray photoelectron spectroscopy, respectively. The structure of the electrocatalysts is studied by powder X-ray diffraction and vibrational spectroscopies. Finally, the performance of the electrocatalysts in the ORR and the reaction mechanism are determined by cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) technique (see Figure 1).