The development of advanced electrocatalysts (ECs) for the oxygen reduction reaction (ORR) is one of the most active research areas in the field of electrochemical energy conversion and storage (EECS) devices, with a particular reference to ion-exchange membrane fuel cells (IEMFCs). The ORR is a major bottleneck of device operation as it introduces very large overpotentials (no less than 250 mV). Furthermore, ORR ECs for the acidic environment (e.g., those for application in proton-exchange membrane fuel cells, PEMFCs) typically comprise a significant loading of platinum-group metals (PGMs) to achieve a performance and durability level high enough to comply with the requirements of practical devices.[1] This raises serious concerns as PGMs have a very low abundance in Earth’s crust and thus are susceptible to trigger supply bottlenecks.

This work reports a new family of hierarchical ORR ECs (H-ECs) that: (i) are “PGM-free”; (ii) comprise bimetallic Fe-Sn active sites; and (iii) exhibit a structure wherein a support “core” consisting of a blend of nanostructured carbon species is covered by a carbon nitride “shell” stabilizing the active sites in C- and N- “coordination nests”. The H-ECs are obtained by means of a unique synthetic protocol that allows for modulating: (i) the chemical composition of the active sites; (ii) the structure of the material; and (iii) the morphology and porosity features, which play a crucial role to control the transport of reactants and products in the final device and may introduce significant overpotentials.[2],[3] The interplay between the synthetic parameters, the physicochemical properties and the electrochemical performance of the H-ECs is elucidated. Particular emphasis is placed on: (i) studying the impact of the metals on the ORR performance and reaction mechanism, both in an acidic and alkaline environment; and (ii) rationalizing how the features of the nanostructured carbon species in the “core” affect the porosimetric features of the final H-ECs and, in turn, the mass transport properties of the fuel cell (FC) prototypes integrating such H-ECs.

The bulk chemical composition of the H-ECs is determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and CHNS microanalysis. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the chemical composition and the chemical state of the elements on the H-EC surface. Porosimetric features, such as the overall specific surface area and pore size distribution are probed by nitrogen physisorption techniques. Morphology is studied by ultra-high resolution scanning electron microscopy (UHR SEM) and transmission electron microscopy (TEM). The structure of the H-ECs is investigated through wide-angle X-ray diffraction (WAXD). The ORR kinetics and reaction mechanism of the H-ECs both in the acid and the alkaline environment is via the cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) technique. Finally, the H-ECs are adopted to fabricate FC prototypes, which are tested under operating conditions as a function of the partial pressure of oxygen at the cathodic feed.

References

[1] Gröger O., Gasteiger H. A. and Suchsland J.-P. 2015 J. Electrochem. Soc. 162 A2605.

[2] Vezzù K., Delpeuch A. B., Negro E., Polizzi S., Nawn G., BertasiF., Pagot G., Artyushkova K., Atanassov P., Di Noto V. 2016 Electrochim. Acta 222 1778-1791.

[3] Negro E., Delpeuch A. B., Vezzù K., Nawn G., Bertasi F., Ansaldo A., Pellegrini V., Dembinska B., Zoladek S., Miecznikowski K., Rutkowska I. A., Skunik-Nuckowska M., Kulesza P. J., Bonaccorso F., Di Noto V. 2018 Chem. Mater. 30 2651-2659.

Acknowledgements

This work has received funding from: (a) the European Union’s Horizon 2020 research and innovation program under grant agreement 881603; (b) the project ‘Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells’ funded by EIT Raw Materials; and (c) Alkaline membranes and (platinum group metals)-free catalysts enabling innovative, open electrochemical devices for energy storage and conversion AMPERE, FISR 2019 project funded by the Italian Ministry of University and Research.