An Open Gate for High-Density Metal Ions in N-Doped Carbon Networks: Powering Fe-N-C Catalyst Efficiency in the Oxygen Reduction Reaction

Non-noble metal catalysts for the oxygen reduction reaction (ORR) showing catalytic activity comparable or even superior to that of the benchmark Pt/C are highly attractive systems for the development of a mature fuel cell technology. Fe-N-C moieties exhibit an optimal performance in the ORR although a synthetic strategy for their production still remains a challenging matter of catalysis and materials science. Herein, an original and general protocol for the preparation of high-density and discrete Fe-N-C-based single-atom catalysts has been proposed starting from cheap and food-grade raw components. The rational combination of chelating citrate ions with the ancillary monodentate thiocyanate (SCN-) ligand has established an "open gate"for water-soluble iron ions to be accommodated in the form of Fe-N-C moieties within the final C-N networks. Although recent findings in the field of electrocatalysis have pointed out the often beneficial synergistic action between isolated and metallic iron species or iron carbides and their protecting C-N shells, the poor selectivity on the nature of the final Fe species in N-doped C-networks remains a matter of debate and does not shed light on the effective nature of the active species in the process. The highly metal-loaded catalysts in the form of highly dispersed Fe-N-C moieties prepared with the synthetic protocol described in the paper have been tested as electrocatalysts in the ORR, showing electrocatalytic performance under an alkaline environment that ranks among the highest reported so far for related Fe single-atom catalysts (Fe-SACs) of the state-of-the-art.