(Invited) Electrocatalysts for the Oxygen Reduction Reaction and the Hydrogen Evolution Reaction Based on “Core-Shell”, “PGM-Free” Carbon Nitride Systems

Abstract

The key electrochemical processes at the heart of the “hydrogen economy” involve: (i) the recombination of hydrogen and oxygen to form water in fuel cells (FCs); and (ii) the splitting of water to yield hydrogen and oxygen in water electrolyzers (ELs). Depending on the particular device taken under consideration, such processes typically occur either in a highly acidic environment (e.g., in proton-exchange membrane fuel cells, PEMFCs, and in proton-exchange membrane electrolyzers, PEMELs) or in strongly alkaline conditions (e.g., in anion-exchange membrane fuel cells, AEMFCs; and in anion-exchange membrane electrolyzers, AEMELs). In both cases, such processes require suitable electrocatalysts (ECs) to minimize overpotentials and achieve a performance and durability level compatible with applications. State-of-the-art ECs for both FCs and ELs typically suffer from important drawbacks such as: (i) high overpotentials; (ii) a poor durability; and (iii) a high loading of strategic elements, with a particular reference to platinum-group metals (PGMs); correspondingly, significant concerns are raised due to potential supply bottlenecks.In this work, a new family of “PGM-free” ECs is proposed, especially targeted to promote the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER) in either/both the acidic and the alkaline environment. The ECs are obtained by means of a multi-step synthetic process [1]. In the first step a precursor is prepared by coupling a carbon black support with a hybrid inorganic-organic macromolecular system consisting of an organic binder coordinating: (i) a first-row transition metal (e.g., Fe) meant to act as the “catalyst”; and (ii) Sn, playing the role of “co-catalyst”. Subsequently the precursor undergoes a multi-step pyrolysis process followed by suitable chemical treatments meant to remove labile species and reaction byproducts, yielding the final EC. Optionally, the EC may be coupled with an additional aliquot of the hybrid-inorganic precursor. In this case, further pyrolysis/chemical treatment steps are carried out to obtain the final product.The final “PGM-free” ECs proposed herein exhibit a “core-shell” morphology. The carbon black “core” is covered by a carbon nitride “shell” stabilizing the active sites based on the “catalyst” and the “co-catalyst” into C- and N- “coordination nests”. The ECs undergo an extensive physicochemical characterization, meant to determine: (i) the chemical composition, both in the bulk and on the surface (adopting analytical techniques such as ICP-AES, CHNS microanalysis, EDX and NAP-XPS); (ii) the morphology (by means of UHR-SEM); (iii) the porosimetric features (by nitrogen and water physisorption studies); and (iv) the structure (e.g., through wide-angle X-ray diffraction). The kinetics and reaction mechanism of the proposed ECs in the ORR and HER are determined by advanced electroanalytical techniques, with a particular reference to cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE). The synthetic parameters are finally correlated with the results of the physicochemical and the electrochemical characterization, shedding light on the most promising avenues to guide the preparation of further ECs exhibiting improved performance and durability, suitable for implementation in next-generation FCs and ELs.