Binder-Free Heterostructured NiFe₂O₄/NiFe LDH Nanosheet Composite Electrocatalysts for Oxygen Evolution Reactions

Constructing two-dimensional (2D), free-standing, nonprecious, and robust electrocatalysts for oxygen evolution reactions (OERs) is of primary importance in the commercial water-splitting technology. Herein, we have constructed a 2D heterostructured NiFe₂O₄/NiFe layered double hydroxides (LDH) mixed composite on a low-cost stainless-steel mesh substrate using a low-temperature one-step wet chemical synthesis method and have also investigated the effect of starting material concentration on the formation of the NiFe₂O₄/NiFe LDH mixed composite. The as-prepared NiFe₂O₄/NiFe LDH-25 electrocatalyst drives a 100 mA/cm² OER with the lowest reported overpotential of 190 mV and a Tafel slope 21.5 mV/dec and drives a stable 100 mA/cm² OER process in 1 M KOH. These OER activities are superior to that of the state-of-the-art RuO₂ OER electrocatalyst. The excellent OER activity appears to be due to the synergetic effect of NiFe LDHs and NiFe₂O₄. In addition, the vertically aligned heterostructure of the NiFe₂O₄/NiFe LDH composite thin sheets provides a large number of active edge sites, directly attached to the highly conducting substrate, which contributes to improving the electronic conductivity of the electrocatalyst. This work provides valuable insight into the design and one-step synthesis of NiFe₂O₄/NiFe LDH bimetallic mixed oxide and hydroxide composite thin films with enhanced OER activity and stability as well as deciphering the origin of the OER enhancement by metal oxides and metal hydroxides.