Hierarchical “Core-Shell” Electrocatalysts for the Oxygen Reduction Reaction (ORR) based on Graphene “Cores” and Metal Alloy Carbon Nitride “Shells”

The sluggish kinetics of the oxygen reduction reaction (ORR) is one of the most important bottlenecks in the operation of several families of advanced energy conversion and storage devices, such as metal-air batteries and low-temperature fuel cells (e.g., proton-exchange membrane fuel cells, PEMFCs and anion-exchange membrane fuel cells, AEMFCs). Accordingly, the development of efficient ORR electrocatalysts (ECs) capable to address this issue and minimize the high activation ORR overpotentials is one of the most active research areas in this field. It is also to be highlighted that an efficient ORR EC must exhibit several additional features, including: (i) a facile electron transport between the active sites and the external circuit, to minimize the ohmic drops; (ii) a suitable morphology, to ensure that reactants and products are able to easily reach and be removed from the active sites; and (iii) a high durability and a low cost, to comply with the requirements set by the applications. This work describes the features of a new family of ECs for the ORR that are able to meet all of the above requirements. The ECs exhibit a hierarchical “core-shell” morphology; they include a graphene-based nanostructured “core” covered by a carbon nitride “shell” embedding the ORR active sites in carbon- and nitrogen-based “coordination nests”. The nanostructured “core” exploits the unique properties of graphene, with a particular reference to its high electron conductivity and low microporosity. The carbon nitride “shell” plays a crucial role to stabilize the active sites in its “coordination nests”; thus, the resulting ECs exhibit a high durability, significantly improved in comparison with that of state-of-the-art, “reference” Pt/C ECs. The proposed “core-shell” ORR ECs are obtained by customizing a unique and extremely flexible preparation protocol [1-3], that allows to fine-tune the morphology and the chemical composition of the ECs. Two main groups of ORR ECs belonging to this family are considered. The former comprises ECs with a low loading of platinum-group metals (L-PGM), optimized for operation in an acid medium at the cathode of PEMFCs; the latter consists of ECs that do not include platinum (N-PGM), intended for application in the alkaline medium in devices such as AEMFCs and metal-air batteries. This work overviews the synthetic strategies used to obtain the ECs comprising the graphene-based nanostructured “cores” and discusses the complex correlations existing between the preparation parameters, the physicochemical properties and the electrochemical performance. Finally, the most promising avenues and new directions for the research are indicated, with the aim to obtain ECs comprising graphene-based nanostructured “cores” exhibiting an improved ORR performance and durability, and at lower costs, in comparison with state-of-the art “reference” ECs. References [1] V. Di Noto, E. Negro, K. Vezzù et al., The Electrochemical Society Interface, Summer 2015, 59-64 (2015). [2] V. Di Noto, E. Negro, A. Bach Delpeuch et al., patent application 102017000000211 filed on 02 January 2017. [3] V. Di Noto, E. Negro, K. Vezzù et al., patent application 102015000055603 filed on 28 September 2015.