Enhancing Electrocatalysis: Engineering the Fe–Nx–C Electrocatalyst for Oxygen Reduction Reaction Using Fe-Functionalized Silica Hard Templates

Abstract

The availability of robust and accessible active sites in iron–nitrogen-carbon (Fe–N_x-C) electrocatalysts is essential to optimize the oxygen reduction reaction (ORR), which is the main obstacle in the commercial realization of fuel cells. Herein, a modified hard templating method to develop efficient Fe–N_x-C has been presented that not only ensured the generation of a porous architecture but also helped in the homogeneous distribution of Fe throughout the structure. First, silica nanoparticles (NPs) were grown via the Stöber process and then functionalized atomically with iron through two different types of silane chains, i.e., (3-aminopropyl)triethoxysilane (APTES) and N-(2-Aminoethyl)-3-aminopropyltriethoxysilane (EDTMS). The Fe-functionalized silica simultaneously acting as a sacrificial template as well as an iron source was then impregnated with nicarbazin, which was a carbon and nitrogen precursor. The dried mix was subject to pyrolysis (H1) followed by acid washing to dissolve silica templates, and then, again, it was subjected to another pyrolysis treatment (H2). At each proceeding step, ORR activity in both acidic and alkaline media was improved and the samples obtained at the last stage (i.e., H2) outperformed the other counterparts collected at the initial stages of the fabrication pathway. Eventually, the electrocatalyst developed using EDTMS-type silane attached to silica NPs (E_FeNC_H2) demonstrated the highest onset potentials of 990 mV vs RHE in alkaline media and 862 mV vs RHE in acidic media. Moreover, the lower peroxide yield of E_FeNC_H2 signifying nearly direct 4e^– ORR was attributed to the highest specific surface area (627 m² g^–1) and the optimum combination of active moieties dispersed in the porous carbonaceous framework.