Hierarchical graphene-based low-loading Pt “core-shell” ORR electrocatalysts for proton exchange membrane fuel cells

In this report we propose innovative ECs for the oxygen reduction reaction (ORR) that include a low loading of platinum-group metals (L-PGM). These L-PGM ECs are meant for application at the cathode of a proton exchange membrane fuel cell (PEMFC) and are characterized by a “core-shell” morphology. The “core” is covered by a carbon nitride “shell” that provides “coordination nests” embedding the ORR active sites [1]. The latter are based on a very low loading of Pt (ca. 3 wt% of the EC) and also include Ni and Cu as “co-catalysts” [1]. In the proposed ECs the “core” exhibits a hierarchical nanostructure including: (i) Cu nanoparticles supporting graphene nanoplatelets; and (ii) optionally, carbon black nanoparticles, to modulate the mass and charge transfer processes involved in the operation of the ECs. The proposed L-PGM ECs are prepared customizing the synthetic protocol devised in our research group [1]. The L-PGM ECs exhibit an improved ORR performance, both in terms of specific and mass activity, in comparison with conventional state-of-the-art ECs. However, this result is reached only if a suitable post-synthesis activation process (Act) is carried out on the L-PGM ECs. Accordingly, a detailed study of the physicochemical properties of the proposed L-PGM ECs is performed both before and after Act. The chemical composition is evaluated by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis. The thermal stability is gauged by high-resolution thermogravimetry. The structure is probed by vibrational spectroscopies (e.g., confocal micro-Raman) and wide-angle X-ray diffraction (WAXD). The morphology is evaluated by high-resolution transmission electron microscopy (HR-TEM). Finally, the electrochemical performance and the ORR mechanism are investigated by cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE). Results allow to elucidate the complex interplay existent between the parameters of the synthetic protocol, Act and the ORR, shedding light on the oxygen reduction reaction mechanism and on how to maximize the activity of this very promising family of hierarchical nanostructured L-PGM graphene-based “core-shell” ECs. References [1] V. Di Noto, E. Negro, K. Vezzù, F. Bertasi, G. Nawn, The Electrochemical Society Interface, Summer, 2015 (2015) 59-64. [2] S. Sharma, B. G. Pollet, J. Power Sources, 208 (2012) 96-119.