Hierarchical "Core-Shell" Pt-Ni ORR Electrocatalysts Based on Graphene "Cores" and Carbon Nitride "Shells"
Authors: Negro Enrico , Bach Delpeuch Antoine , Vezzù Keti , Bertasi Federico , Nawn Graeme , Pagot Gioele , Bang Yannick , Bonaccorso Francesco , Di Noto Vito 
Published: February 2020

One of the main bottlenecks in the operation of proton-exchange membrane fuel cells (PEMFCs) is the sluggish kinetics of the oxygen reduction reaction (ORR). The latter gives rise to large overpotentials, which degrade significantly the energy conversion efficiency of the device [1]. Our work addresses this issue by the development of innovative ORR electrocatalysts (ECs) characterized by an improved turnover frequency in comparison with state-of-the-art materials [2]. The proposed ECs comprise a hierarchical support “core” based on graphene flakes, which is covered by a carbon nitride (CN) “shell” embedding the ORR active sites. The hierarchical support “core”, which typically comprises both graphene flakes and a carbon black spacer, is devised to facilitate the charge and mass transport phenomena by: (i) reaping the benefits of graphene (e.g., a very high electron mobility, up to 200000 cm2·V-1·sec-1, and specific surface area, up to ca. 2630 m2·g-1); and (ii) fine-tuning the morphology of the final ECs [3-6].
The proposed ECs are obtained by the optimization of the preparation protocol devised in our research group [7]. In particular, the physicochemical properties and the morphology of the final materials are modulated by a post-synthesis activation step carried out by electrochemical de-alloying. The final ECs bear bimetallic ORR active sites comprising Pt as the “active metal” and Ni as the “co-catalyst” [8]. Preliminary results clearly evidence that this approach is capable to yield hierarchical ECs exhibiting a very promising performance in the ORR despite a very low loading of Pt. In detail, the best EC exhibits an ORR onset potential ca. 30 mV higher with respect to that of the Pt/C reference (see Figure).
The chemical composition of the ECs is determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis. The structure is elucidated by wide-angle X-ray diffraction (WAXD) and vibrational spectroscopies (e.g., confocal micro-Raman); the morphology is probed by, both conventional and high-resolution, scanning electron microscopy (SEM) and transmission electron microscopy (TEM); the “ex-situ” electrochemical performance and ORR reaction mechanism are gauged by cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE). Finally, the ECs are implemented at the cathode of single fuel cell prototypes which are tested in operating conditions.

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Figure 1