Mitigation of the Anodic Shift of the Onset Potential in a TiO ₂ /α-Fe ₂ O ₃ Photoanode by Inserting a Nickel Ferrite (NiFe ₂ O ₄ ) Blocking Layer

It is widely recognized that the insertion of TiO₂ as a blocking layer into a photoanode in a PEC (photoelectrochemical) system can retard the electron-hole back recombination. However, it has been also reported that it can induce an anodic shift in the onset potential in the current density-voltage curve. In this study, we introduced a nickel ferrite (NiFe₂O₄) layer as an additional layer to offset the adverse effect of the TiO₂ layer by preventing Ti diffusion from TiO₂. We demonstrated that the α-Fe₂O₃ photoanode in which both NiFe₂O₄ and TiO₂ layers were inserted not only maintained the high current density but also exhibited a cathodic shift in the onset potential, compared to that of α-Fe₂O₃ with the TiO₂ layer. Furthermore, by depositing an additional NiFeO _<i>x</i> passivation layer, we utilized the combined benefits of the NiFe₂O₄ underlayer and the passivation layer, ultimately achieving a cathodic shift of 0.12 V_RHE. Using various electrochemical analyses and time-resolved spectroscopic techniques, we investigated the origin of the improvement in the performance of the photoanode by inserting a NiFe₂O₄ layer.