Assessing the Transformations of Supported Nanocatalysts Used in the Oxygen Evolution Reaction: A Case Study Using NiFe2O4 Nanoparticles Supported on Textured Ni Electrodes

Nanomaterials composed of nickel-iron oxyhydroxides (NiFeO x H y ) are used broadly as oxygen evolution reaction (OER) catalysts in electrolysis systems. These materials undergo complex phase transformations and electrochemical aging that are concurrent with the production of molecular oxygen. It is, therefore, of interest to understand the evolution and migration of the key active species at the interface between the nanocatalyst and the support under the potentials applied for the OER. In this work, nickel ferrite (NiFe2O4) nanoparticles (NPs) were embedded as surface inclusions onto OER-active supports to assess the resulting electrocatalytic phase transformations as a function of potential cycling. The electrochemically activated electrodes retained the supported NPs with a distinct NiFe2O4 phase and demonstrated higher sustained current densities at the OER relevant potentials in comparison to the as-prepared electrodes. Improved performance was attributed to an increase in the electrochemically active surface area and a decrease to the equivalent series resistance. A sufficient period of electrochemical activation was demonstrated after 5 h of electrochemical aging. In contrast after 10 h of electrochemical aging, the samples exhibited a preferential leaching of Ni2+ species from the NiFe2O4 NPs that resulted in a distinct iron oxide (Fe2O3) phase. Cross-sectional analyses using high spatial resolution electron microscopy techniques and electron energy loss spectroscopy (EELS) were used to characterize the NP-to-support interactions and the transformations of these materials. This study highlights the influences of mechanistic degradation on OER activity and provides a detailed characterization of the stability of immobilized NPs embedded onto conductive supports, which is of interest to a wide range of applications in electrolysis.