An approach to understanding the electrocatalytic activity enhancement by superexchange interaction toward OER in alkaline media of Ni-Fe LDH

In the present work, the hydrotalcite-like materials known as layered double hydroxides (LDHs) were synthesized. The Ni-Al and Ni-Fe materials with different Ni/Fe ratio were obtained by coprecipitation method at variable pH. The LDH structure was verified by X-ray diffraction, Fourier transform infrared, and Raman spectroscopy. No secondary extra phases were observed for any material. The electronic properties were evaluated by UV-vis spectroscopy, while the magnetic ones were followed by electron paramagnetic resonance (EPR). The results suggested that sample H/Ni-Fe2 (Ni/Fe = 2) has a ferrimagnetic behavior as a result of the combined action of Ni^II-OH-Ni^II, Fe^III-OH-Ni^II, and Fe^III-OH-Fe^III pairs across the layers and ferromagnetic interactions operating between layers. Furthermore, the material H/Ni-Fe1 (Ni/Fe = 1.5) showed a combination of paramagnetic and ferromagnetic interactions which favors a superexchange interaction among metal centers through the OH bridges across the cationic sheets; the superexchange interaction enhances the electrocatalytic activity on the oxygen evolution reaction (OER) in alkaline media. On the other hand, XPS experiments showed that the H/Ni-Fe1 did not exhibit structural changes after electrochemical processes. The activity toward the OER was in the order H/Ni-Fe1 > H/Ni-Fe2 > H/Ni-Al, as was confirmed using in situ linear sweep voltammetry (LSV) coupled with mass spectrometry (differential electrochemical mass spectrometry).