Oxygen vacancy-rich Fe3O4-enriched P-Fe2TiO5 sites generated by Na incorporation and NiP deposition for accelerated electrocatalytic OER

Abstract

We developed an Na-incorporated Fe₂TiO₅-Fe₃O₄ (Na-Fe-FT) catalyst for the first time using a modified sol–gel method; the material was then loaded onto an NiP matrix to create a dual-functional electrode for application in electrocatalytic OER and photocatalytic HER. The Na incorporation into Fe-FT generated functional Ti³⁺ states and increased oxygen vacancy concentrations. Electroless NiP deposition forms Ti–O–P bonds in Fe₂TiO₅ and oxygen vacancy-rich Fe₃O₄ phases on the NiP electrodes. The interconnected network morphology with cavity features and distinct grain boundaries on the Fe-FT catalyst provided an increased electrochemically active surface area and active sites for enhanced water splitting. During EIS analysis at OCP, the optimized Na-Fe-FT/NiP showed a very low R_ct value of 282.2 ohm and a significantly higher ECSA compared with other samples. The Na-Fe-FT catalyst exhibited a lower overpotential of 249 mV to reach 10 mA cm⁻² and a Tafel slope as low as 79 mV dec⁻¹. The higher hydrolytic stability of the Ti–O–P bonds and the ferroelectric character of Fe₂TiO₅ in the presence of higher concentrations of Fe³⁺ and NiFe heterojunctions promoted an OPM mechanism at the phosphorous-incorporated Fe₂TiO₅ phase (P-FT) and OVSM mechanism at the oxygen vacancy-rich Fe₃O₄ phase (O_v-Fe) during the OER. Additionally, the catalyst showed broad UV-visible absorption, enabling efficient photocatalytic hydrogen evolution at a yield of 3.34 mmol cm⁻² over 5 hours, along with a high photocurrent density of 12.8 mA cm⁻² at 1.479 V vs. RHE. These results demonstrate that the Na-incorporated Fe₃O₄–Fe₂TiO₅/NiP electrodes (Na-Fe-FT/NiP) serve as effective catalysts for both photo- and electro-catalytic water splitting under industrially relevant conditions.