Graphene-Supported Fe, Co, Ni Carbon Nitride Electrocatalysts for the ORR in Alkaline Environment

The development of advanced energy conversion systems characterized by a high efficiency and a low environmental impact is one of the main targets of research [1]. As of today, important efforts are devoted to low-temperature fuel cells (FCs) mounting an acid electrolyte, typically a proton-conducting membrane (e.g., Nafion®) and using electrocatalysts (ECs) with a significant loading of platinum-group metals (PGMs). In conventional low-temperature FCs, most of the PGM loading is typically concentrated at the FC cathode to promote the poor kinetics of the oxygen reduction reaction (ORR), one of the major bottlenecks in FC operation. Very recently, viable OH--conducting membranes were designed [2], opening the possibility to devise efficient anion-exchange membrane fuel cells (AEMFCs). In these systems the ORR takes place in an alkaline environment and so it is possible to adopt “Pt-free” ORR. In this work, new “Pt-free” ORR ECs are reported; the materials comprise a graphene support “core”, which is covered by a carbon nitride “shell” coordinating the ORR active sites [3]. The proposed materials reap the benefits offered by graphene, including: (i) a high electrical conductivity, minimizing the ohmic losses; and (ii) a low microporosity, to facilitate the mass transport of reactants and products. The carbon nitride “shell” coordinates the bimetallic active sites, which include: (i) a 3d-“active metal” (i.e., Fe, Co, Ni), which bestows most of the ORR performance; and (ii) an oxophilic “co-catalyst” (Sn), which stabilizes the “active metal” and improves the ORR kinetics with a bifunctional mechanism. The chemical composition, the thermal stability, the surface chemical composition, the morphology, the structure and the ORR performance and mechanism are studied and clarified. The results proved very promising, clearly showing the potential of this family of “Pt-free”, “core-shell” graphene-supported ECs for application at the cathode of AEMFCs. In particular, cyclic voltammetry (CV-TF-RRDE) measurements in an alkaline environment demonstrated that the best material exhibits an ORR overpotential ca.70 mV higher with respect to a 10 wt.% Pt/C reference. References [1] F. Bonaccorso, L. Colombo, et al., Science, 347, (2015) 1246501. [2] G. A. Giffin, S. Lavina, G. Pace, V. Di Noto, J. Phys. Chem. C, 116, (2012) 23965. [3] V. Di Noto, E. Negro, K. Vezzù, F. Bertasi, G. Nawn, The Electrochemical Society Interface, Summer 2015, (2015) 63.