Effect of silica leaching treatment during template-assisted synthesis on the performance of FeNC catalysts for oxygen reduction reaction

The development of cost-effective and environmentally sustainable electrocatalysts is crucial for advancing the commercialization of proton exchange membrane fuel cells (PEMFCs). In this work, we investigate the effect of different silica leaching strategies on the synthesis of iron-nitrogen-carbon (FeNC) electrocatalysts for the oxygen reduction reaction (ORR). Three FeNC samples were prepared using SBA-15 mesoporous silica as the template and subjected to varying removal techniques: hydrofluoric acid (HF), sodium hydroxide followed by hydrochloric acid (NaOH+HCl), and an acid-free Teflon-assisted process. Physical-chemical characterization reveals significant differences in the surface area and porosity of the three catalysts. The specific surface areas of FeNC treated with NaOH+HCl and HF are 1352 m²/g and 1403 m²/g, respectively, while the Teflon-treated sample exhibits a much lower value of 732 m²/g. Electrochemical tests using a rotating ring disk electrode (RRDE) apparatus demonstrate superior ORR activity of the FeNC treated with NaOH+HCl, achieving onset potentials of 0.93 VRHE and 0.81 VRHE in alkaline and acidic media, respectively, outperforming the HF- and Teflon-treated counterparts. The HF treatment, while effective in removing silica and metallic impurities, poses environmental and safety challenges. The Teflon-assisted approach, despite its promise as a greener alternative, results in a lower surface area and diminished ORR activity, with onset potentials of 0.89 VRHE and 0.76 VRHE in alkaline and acidic media, respectively. This study highlights the importance of optimizing silica removal methods to balance the catalyst performance, safety and sustainability, with the NaOH+HCl method emerging as the most effective approach for producing high-performance FeNC ORR catalysts for fuel cell applications.