Investigation on activity and stability of cobalt ferrite NPs in alkaline OER

Transition to green hydrogen production from water electrolysis is a fundamental step in the decarbonization process, but it is currently unviable due to the very sluggish water oxidation half reaction (OER). The biggest limitation to large scale implementation of water electrolysers is given by the fact that the most active electrocatalysts for OER are rare-earth metals. However, many transition metal oxides have been proven to exhibit notable performance in alkaline condition, and cobalt ferrite in the form of nanoparticles (NPs) has been chosen in the present work. A set of nine catalysts ranging from 0% to 100% Co content is tested on glassy carbon support in 1 M NaOH solution. The impact of the cobalt content in the catalysts is studied with cyclic voltammetry at 50 mV/s and by computing the Tafel slope as an indicator of the OER kinetics. Two hours of chrono amperometry at 1.6 V vs. RHE is used to assess the stability of such particles, finding out that all the particles from 33% to 100% Co content present an important drop in the current density, that is exceptionally significant for 50% Co, for which about 90% of the initial current is lost. To understand the reason of such deactivation, four representative compositions are taken into account for before-and-after investigation, focusing on structural and morphological modification. Additionally, bare glassy carbon is studied to explore its behaviour. Through XRD the stability of the crystalline structure is verified, while XPS spectra show no variation in surface composition. Via ICP-OES on the electrolyte, no significant catalyst dissolution is detected, although minor amounts of iron are found susceptible to dissolution/deposition, depending on the catalyst considered. As a result, the physical stability of the catalyst is proven, indicating that the deactivation is due to setup’s limitations. Finally, the onset potential is estimated with DEMS. This study shows that the most active composition is the 90% Co, and that all the spinels are morphologically stable. Future work must focus on improving the setup in both the type of support and the mass transfer limitation relating to oxygen removal.