Synergistic Effects of Active Sites Nature and Hydrophilicity on Oxygen Reduction Reaction Activity of Pt-Free Catalysts

Oxygen reduction reaction (ORR) is a fundamental step in many electrochemical applications, e.g. fuel cells and zinc/air batteries. Being kinetically hindered, ORR requires efficient catalysts typically based on Platinum Group Metals. These materials, though very efficient, present some drawbacks such as costs, availability and technological problems. Finding cheaper substitutes with similar or better electrocatalytic properties and stability is a challenge for the scientific community. Effective candidates are based on metal-nitrogen-carbon catalysts, in which metal is usually iron or cobalt. In this work we highlight the importance of the hydrophilicity of the catalyst active sites on oxygen reduction reaction (ORR) through an electrochemical and physico-chemical study on catalysts based on nitrogen-modified carbon doped with different metals (Fe, Cu, and a mixture of them). BET, XRPD, micro-Raman, XPS, SEM, STEM and hydrophilicity measurements were performed. All synthesized catalysts are characterized not only by a porous structure, with porosity distribution centered in mesoporosity range, but also by the presence of carbon nanostructures. In iron-doped materials these nanostructures are bamboo-like structures typical of nitrogen carbon nanotubes, which are better organized, in larger amount and longer than those in the copper-doped material. Electrochemical ORR results highlight that the presence of iron and nitrogen carbon nanotubes is beneficial to the electroactivity of these materials, but also hydrophilicity of the active site is an important parameter affecting electrocatalytic properties. The most active material contains the mixture of Fe and Cu.