Interplay between Activation Processes, Physicochemical Properties and Electrochemical Performance of “Core-Shell” Carbon Nitride Pt-Ni ORR Electrocatalysts Based on Hierarchical Graphene Supports
Authors: Nale Angeloclaudio , Negro Enrico , Bang Yannick , Vezzù Keti , Pagot Gioele , Pace Giuseppe , Polizzi Stefano , Bertoncello Renzo , Di Noto Vito 
Published: February 2020

The sluggishness of the kinetics of the oxygen reduction reaction (ORR) is one of the most relevant phenomena curtailing the operation capability of proton-exchange membrane fuel cells (PEMFCs)[1]. Hence, the development of advanced ORR electrocatalysts (ECs) is one of the main goals of the research in this field. The ECs described here exhibit a “core-shell” morphology: a hierarchical graphene-based support (H-GR) “core” is covered by a carbon nitride “shell” stabilizing the active sites in “coordination nests” [2]. The “core” comprises highly defected graphene nanoplatelets [3] and carbon black nanoparticles; the latter act as spacers and facilitate the charge and mass transport phenomena that take place during the EC operation. The ECs proposed here are characterized by a very low loading of platinum, on the order of ca. 5 wt%, that is the “active metal”; Ni is introduced as the “co-catalysts” to improve the ORR performance [2]. An extensive post-synthesis activation process (A) is applied to the ECs, significantly affecting their chemical composition, structure and morphology. The ORR performance of the “activated” ECs is much improved in comparison with state-of-the-art Pt/C reference ECs. This work is aimed at investigating the effect of different activation parameters on the ORR performance and reaction mechanism, aiming at a robust and reliable upscaling of the preparation process [4].
The proposed ECs are extensively characterized both before and after A to study the complex interplay between the synthetic/activation parameters, the physicochemical properties, and the electrochemical ORR performance both “ex-situ” and in single PEMFC. The bulk chemical composition of the ECs is determined by means of Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and CHNOS microanalyses; the structure is investigated through wide-angle X-ray diffraction (WAXD) and vibrational spectroscopies (e.g., confocal micro-Raman); the surface composition and oxidation states are probed with X-ray photoelectron spectroscopy (XPS); morphology is observed by high-resolution transmission electron microscopy (HR-TEM); the details of the ORR performance and reaction pathway as a function of the pH of the environment are elucidated by cyclic voltammetry with the rotating ring-disk electrode (CV-TF-RRDE). Finally, the ECs are used to fabricate membrane-electrode assemblies (MEAs) that are tested in single PEMFC in operating conditions.

Acknowledgement

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 785219, and from the BIRD 2018 program of UNIPD.

References

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[4] V. Di Noto, E. Negro, K. Vezzù, F. Bertasi, G. Nawn, L. Toncelli, S. Zeggio, F. Bassetto, Patent application PCT/IB2016/055728 (2016).