ORR Electrocatalysts with a Low Pt Loading Comprising Hierarchical Graphene-Based Supports

Abstract

This report presents a new family of electrocatalysts (ECs) for the oxygen reduction reaction (ORR) that comprises hierarchical graphene-based supports (H-GRs). The ECs, which are intended for application at the cathode of proton-exchange membrane fuel cells (PEMFCs), exhibit a “core-shell” morphology. The “core” is the H-GR support, which is covered by a carbon nitride “shell”. The latter bears “coordination nests”, which stabilize the ORR active sites [1]. The H-GRs consist of two components, namely: (i) high defected graphene nanoplatelets, which are supported on ZnO nanoparticles (NPs) [2]; and (ii) carbon black, that acts as spacer and facilitates the charge and mass transport phenomena that take place during the operation of the ECs. Two groups of ECs are synthesized, I and II. I includes the pristine H-GR support; II comprises H-GR suitably treated to etch the ZnO NPs. Both I and II are characterized by a very low loading of platinum, on the order of ca. 3 wt%, that is the “active metal” in the ORR active sites [1]. Ni and Cu are introduced as “co-catalysts” to improve the ORR performance [1]. All the ECs undergo an extensive post-synthesis activation process (A), that significantly affects their chemical composition, structure and morphology. As a result, a remarkable ORR performance is achieved that is significantly improved in comparison with state-of-the-art Pt/C reference ECs. The proposed ECs are extensively characterized both before and after A to study the complex interplay between the synthetic parameters, the physicochemical properties, and the electrochemical ORR performance both “ex-situ” and in single PEMFC. In this regard, particular attention is dedicated to clarify the role played by the graphene nanoplatelets included in the H-GR support. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and CHNOS microanalyses are adopted to determine the bulk chemical composition of the ECs. The surface composition and oxidation states are probed with X-ray photoelectron spectroscopy (XPS), while the structure of the ECs is investigated through wide-angle X-ray diffraction (WAXD) and vibrational spectroscopies (e.g., confocal micro-Raman). High-resolution transmission electron microscopy (HR-TEM) is used to study the morphology. Cyclic voltammetry with the rotating ring-disk electrode (CV-TF-RRDE) allows the details of the ORR performance and mechanism to be elucidated. Finally, the ECs are used to fabricate membrane-electrode assemblies (MEAs) that are tested in single PEMFC in operating conditions. Acknowledgements This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 696656. The authors thank the Strategic Project “From materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this research.