In this study, a new family of electrocatalysts (ECs) for the oxygen reduction reaction (ORR) is presented. The ECs, that exhibit a “core-shell” morphology, include: (i) a hierarchical graphene-based support (H-GR) “core”, that is covered by: (ii) a carbon nitride “shell” stabilizing the active sites in “coordination nests” [1]. The H-GR comprise two main components, namely: (i) ZnO nanoparticles supporting highly defected graphene nanoplatelets [2]; and (ii) carbon black nanoparticles; the latter improve the charge and mass transport phenomena associated to EC operation. The active sites of the ECs do not include platinum-group metals (PGMs); instead, they are based on Fe (playing the role of “active metal”) and also contain Sn as the “co-catalyst” [1]. The synthetic route that is pursued to obtain the proposed ECs consists in the following main steps: (i) preparation of a precursor comprising the H-GR support; and (ii) multi-step pyrolysis process yielding the carbon nitride “shell” and the active sites [1]. In this study, other treatments are introduced in the synthetic route (e.g., chemical etching; electrochemical cycling; and/or further pyrolysis steps), with the purpose of fine-tuning the morphology, pore structure, and chemical composition of the ECs and thus maximize the performance in the ORR.

The ECs obtained in this study undergo an extensive characterization campaign, with the aim of elucidating the details of the interplay between the parameters of the synthetic route, the physicochemical features and the electrochemical performance and durability. In particular, the role of the various components included in the H-GR support (i.e., the highly defected graphene nanoplatelets, the ZnO and the carbon black nanoparticles) is studied accurately, striving to discern the impact of each in the properties of the final ECs. The bulk chemical composition of the samples is determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and CHNOS microanalysis. X-ray photoelectron spectroscopy is adopted to probe the chemical composition and the oxidation states of the elements on the surface of the ECs. The structure of the samples is studied by means of vibrational spectroscopies (e.g., confocal micro-Raman) and wide-angle X-ray diffraction (WAXD). The morphology and pore structure of the samples is investigated by means of high-resolution transmission electron microscopy (HR-TEM) and nitrogen physisorption techniques. The performance, reaction mechanism and durability of the ECs in the ORR is studied through the CV-TF-RRDE technique (cyclic voltammetry with the thin-film rotating ring-disk electrode). Finally, the interplay between the ORR kinetics and the chemical composition of the active sites is clarified by studying the ORR mechanism at different pH values.

Acknowledgements

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 696656. The authors wish to thank the Strategic Project of the University of Padova “Materials for Membrane-Electrode Assemblies to Electric Energy Conversion and Storage Devices (MAESTRA)” for funding. V.D.N. thanks the University Carlo III of Madrid for granting him the “Catedra de Excelentia” (Chair of Excellence).

References

[1] V. Di Noto, E. Negro, K. Vezzù, F. Bertasi, G. Nawn, The Electrochemical Society Interface, Summer 2015, (2015) 59-64.

[2] V. Di Noto, E. Negro, A. Bach Delpeuch, F. Bertasi, G. Pagot, K. Vezzù, Patent application 102017000000211 (2017).