Unveiling the Oxygen Evolution Mechanism with FeNi (Hydr)oxide under Neutral Conditions

Large-scale solar-driven water splitting is a way to store energy, but it requires the development of practical and durable oxygen evolution reaction (OER) catalysts. The present paper aims to investigate the mechanism of the OER, local pH, high-valent metal ions, limitations, conversions, and details during the OER in the presence of FeNi foam using in situ surface-enhanced Raman spectroscopy. This research also explores the use of in situ surface-enhanced Raman spectroscopy for detecting species on foam surfaces during the OER. The acidic media around the electrode not only limit the process but also affect the phosphate ion protonation and overall catalysis effectiveness. The study proposes that FeNi hydroxides serve as true catalysts for OER under neutral conditions, rather than FeNi phosphates. However, phosphate species remain crucial for proton transfer and water molecule adsorption. Changes observed in pH at the open-circuit potential suggest new insights concerning the coordination of Ni(II) to phosphate ions under certain conditions. By extrapolating the Tafel plot, the overpotential for the onset of OER was determined to be 470 mV. Furthermore, the overpotentials for current densities of 1 and 5 mA/cm² were 590 and 790 mV, respectively. These findings offer valuable insights into the advancement of the OER catalysts and our understanding of the underlying mechanism for efficient water splitting; both are crucial elements for the purpose of energy storage.