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).
The coordination polymer Zn3Na2[FeII(CN)6]2 has an open porous framework that is stable in acidic and neutral aqueous solutions and appears to be an attractive solid for investigation as a material for sodium ion-based batteries.
Several materials have been studied as electrodes for aqueous batteries that use sodium as alkali ion; these include Prussian blue analogue or hexacyanoferrates. The inhibition or disruption on metal−metal charge transfer plays an important role for improving electrochemical stability of the material. The stability improvement is achieved when two external metals are coordinated to N ends in the Na-rich hexacyanoferrates. Additionally, the presence of vacancies in the material is another important factor that influences its stability. In this study, Na x Co 1−y Mn y [Fe(CN) 6 ] has been synthesized at different Mn/Co ratios by precipitation using citrate as a chelating agent to obtain a material without vacancies. Its electrochemical behavior during redox processes and the correlation with the electronic interaction between external metal sites in the framework through the interaction of spins have been studied too. To discuss the effect of the presence of [Fe(CN) 6 ] nvacancies on the electrochemical process, we synthesized a material without citrate for obtaining materials with low ferrocyanide vacancies. The vacancy-free Co 0.55 Mn 0.45 HF versus n-CoMnHF, were compared in this work. These studies reveal that manganese hexacyanoferrate is unstable. The partial substitution of Co by Mn modifies the metals spin ordering and consequently, the interaction between metals coordinated to N in the cyanide linker. Such partial substitution, with a Mn/Co ratio of 1:1 (Co 0.55 Mn 0.45 HF), improves the electrochemical stability and enhances the discharged potential as well. On the other hand, when vacancies are present, the n-CoMnHF compound showed a decrease in its crystallinity as well as in its external metal interaction. Both changes may be due to the presence of coordinated water, which modifies electrochemical performance. A spontaneous hopping from Mn to Fe during oxidation in n-CoMnHF was detected, but this phenomenon was disrupted in Co 0.55 Mn 0.45 HF. Such charge transfer inhibition was associated with the modification of electron delocalization on Fe (LS); which was caused by the external metals; mainly by Co.
This study evaluates the interaction of mesoporous carbon (MC) and nitrogen doped mesoporous carbon (NMC) with Nickel prussian blue analogues (Ni‐PBA) and its effect on electrochemical properties and energy storage. The Raman, IR, Mossbauer and XPS results reveal that MC‐NiPBA composite exhibited a large modification to covalent character of carbon and an increase in defects of carbonaceous material. This latter is associated with the oxidation of carbon sites and reduction of iron in hexacyanoferrate during composite synthesis, which increase the charge subtraction in Fe (Low Spin) through CN ligand, due to sp‐d hybridization between nickel and carbon structure. Electrochemical impedance spectroscopy characterization showed that the interaction between MC and NiPBA decreases the paste resistance and improves the chemical capacitance of the material, whereas, the apparent diffusion coefficient for K‐ion in Ni‐PBA is not affected by the presence of mesoporous carbon. The cyclic voltammetry and galvanostatic characterization confirm the enhancement of MC‐NiPBA capacitance during charge/discharge process due to their synergetic interaction. However, the donor/acceptor characteristics of nitrogen modify the orbital hybridization in doped carbon, inhibiting the carbon‐framework interaction.
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