An Approach to Understanding the Electrocatalytic Activity Enhancement by Superexchange Interaction toward OER in Alkaline Media of Ni–Fe LDH

Abstract

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 NiII–OH–NiII, FeIII–OH–NiII, and FeIII–OH–FeIII 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).

A First-Principles Study of Oxygen Formation Over NiFe-Layered Double Hydroxides Surface

“…The calculated band gap of NiFe-CO 3 -LDH is 1.12 eV as shown in Figure S5. We found that different anion based solutions did not show an important catalytic effect on OER, which is in agreement with the experiment [42]. The valence band maxima were mainly as a result of the Fe 3d contribution, while the conduction band minima were mainly composed of the Ni 3d, Fe 3d and small O 2p orbitals.…”

“…In 2013, Dai and co-workers synthesized a carbon nanotubes/NiFe-LDHs complex [12], which was found to be highly active for the OER in alkaline solutions and exhibited higher electrocatalytic activity and stability for oxygen evolution than the commercially-available precious metal Ir catalysts. Guzmán-Vargas' group studied the electrocatalytic activity enhancement by superexchange interaction toward OER in an alkaline media of Ni-Fe LDH [42]. The arrangement of the Fe atoms in the lattice may form a specific magnetic structure that favors the superexchange interaction.…”

Conditions for magnetic and electronic properties of ultrathin Ni–Fe hydroxide nanosheets as catalysts: a DFT+U study

“…Overall, the above comparison intuitionally reflects the spontaneous magnetism is a crucially decisive factor for the enhanced electrocatalytic performance of OER/ORR of Ni-Fe LDHs in alkaline media. The present result is not only consistent with the experimental observation [24] but also highlights the concept of magnetic catalysis, that is, only the regulation magnetic coupling is compatible with magnetic catalyst. It was proposed that the Fe atoms, rather than the Ni atoms, are the available sites for reactions in Ni-Fe LDHs [21].…”

“…This again evidences that the Fe 3+ ions rather than the Ni 2+ ions in the LDHs are indeed the active sites, which agrees with the theoretical prediction based on the experiment [21]. And such, the impurity states, contributing to the magnetic coercivity for the nanosheet, instead of the VBM certainly will make a dominant effect on the successive oxidation-reduction reaction; and from another point of view, this verifies the fact that the real Ni2Fe LDHs are in general stable [23,24,31]. Crucially, the calculated adhesion energy of one Cl atom on the Ni2Fe hydroxide nanosheet is −2.70 eV, which is weaker than the dissociation energies of O-H (−5.29 eV per H atom) and Fe(Ni)-OH bonds (−3.02 eV Figure 5 Total and projected DOS and band structures of (a) Ni2Fe hydroxide, and (b) Ni2Fe(OH)6 nanosheets in the ground state, respectively.…”

“…Cheap Ni-Fe LDHs not only possess exciting catalytic properties which make them be used as catalysts for oxidation reactions [9,20−22], but also can be exfoliated to form the unilamellar nanosheet, exhibiting 2D intrinsic magnetization [23]. In particular, a recent experiment revealed that the magnetic interaction be-tween metal centers in Ni-Fe LDHs significantly enhances the electrocatalytic performance of oxygen evolution reaction (OER) in alkaline media [24]. This is a strong indication of the relationship between electrocatalytic activity and magnetic properties of the catalyst, also offering a new strategy for developing catalytic activity of LDHs, that is, by the magnetic field.…”