Reaping the catalytic benefits of both surface (NiFe2O4) and underneath (Ni3Fe) layers for the oxygen evolution reaction

Abstract

Surface and interface engineering predominantly determines the overall surface dynamics, and the surface layers can benefit substantially from a chemically different underneath layer. Herein, we demonstrate the surface tuning of nickel ferrite (NiFe₂O₄) with awaruite (Ni₃Fe). The NiFe₂O₄ grows in the form of a crystalline monolayer encompassing bimetallic Ni₃Fe nanocrystals embedded within a polyaniline (PANI) matrix. This is achieved by a simple and highly reproducible chemical approach. Benzyl alcohol (BA) and PANI steer the growth of metallic Ni₃Fe covered with a monolayer of NiFe₂O₄ under mild solvothermal conditions. PANI presumably renders anchoring sites and guides the formation of fine and monodisperse nanocrystals of NiFe₂O₄/Ni₃Fe, while BA acts as a reducing agent and controls the nucleation and the growth of NiFe₂O₄. In the absence of PANI, the formation of large and agglomerated NiFe₂O₄ nanoparticles is observed. The fine interface created between the NiFe₂O₄ monolayer and Ni₃Fe imparts unique and superior surface attributes; Ni₃Fe renders a high electroactive surface area and faster charge transport, while the NiFe₂O₄ monolayer affords high catalytic ability. As a result, the NiFe₂O₄/Ni₃Fe/PANI electrode efficiently catalyzes the oxygen evolution reaction (OER), and requires a substantially lower overpotential than those of NiO/PANI, NiFe₂O₄, and Fe₂O₃/PANI. The Tafel values suggest that the kinetics of the half-reaction at the catalytic interface of NiFe₂O₄/Ni₃Fe/PANI is much faster (52.3 mV dec⁻¹) than that of NiO/PANI (82.5 mV dec⁻¹), NiFe₂O₄ (86.0 mV dec⁻¹) and Fe₂O₃/PANI (108.3 mV dec⁻¹). The turnover frequency of NiFe₂O₄/Ni₃Fe/PANI is 0.27 s⁻¹, 5.4 fold higher than that of NiFe₂O₄ (0.05 s⁻¹). Moreover, the electrochemically active surface area, electrochemical impedance, surface charge transport properties, and active sites of NiFe₂O₄/Ni₃Fe/PANI and NiFe₂O₄ are estimated, compared and correlated to the OER performance. In addition, density functional theory calculations show a decrease in the barrier of the potential-determining step from *O to *OOH, indicating a favorable electronic modulation at the interface of NiFe₂O₄ and Ni₃Fe, which reduces the overpotential requirement.