Further Insight into the Conversion of a Ni–Fe Metal–Organic Framework during Water-Oxidation Reaction

Abstract

Metal–organic frameworks (MOFs) are extensively investigated as catalysts in the oxygen-evolution reaction (OER). A Ni–Fe MOF with 2,5-dihydroxy terephthalate as a linker has been claimed to be among the most efficient catalysts for the oxygen-evolution reaction (OER) under alkaline conditions. Herein, the MOF stability under the OER was reinvestigated by electrochemical methods, X-ray diffraction, X-ray absorption spectroscopy, energy-dispersive spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy, nuclear magnetic resonance, operando visible spectroscopy, electrospray ionization mass spectroscopy, and Raman spectroscopy. The peaks corresponding to the carboxylate group are observed at 1420 and 1520 cm–1 using Raman spectroscopy. The peaks disappear after the reaction, suggesting the removal of the carboxylate group. A drop in carbon content but growth in oxygen content after the OER was detected by energy-dispersive spectra. This shows that after the OER, the surface of MOF is oxidized. SEM images also show deep restructures in the surface morphology of this Ni–Fe MOF after the OER. Nuclear magnetic resonance and electrospray ionization mass spectrometry show the decomposition of the linker in alkaline conditions and even in the absence of potential. These experimental data indicate that during the OER, the synthesized MOF transforms to a Fe–Ni-layered double hydroxide, and the formed metal oxide is a candidate for the OER catalysis. Generalization is not true; however, taken together, these findings suggest that the stability of Ni–Fe MOFs under harsh oxidation conditions should be reconsidered.

Intrinsic Lability of NiMoO4to Excel the Oxygen Evolution Reaction

“…Due to a relatively better alkaline stability of NiWO 4 over the analogous NiMoO 4 , the surface oxidation could presumably be less pronounced. 40 In addition to the redox feature in CV recorded between 0.89 and 1.95 V (vs RHE), the Raman data obtained with the electrode after 10 CV cycles also strikingly resembled the fresh NiWO 4 catalyst (Figure 7a primarily NiO(OH) ED /NF, as mentioned above. The polarization curve obtained from the LSV showed a redox peak at around 1.35 V, followed by a sharp increase in the current density to 175 mA cm −2 at 1.62 V (vs RHE) (Figure 8a).…”

“…A detectable increase in the current density in the higher applied potential region, above 1.6 V (vs RHE), indicated that upon redox cycling, there is a generation of more number of catalytically active Ni­(III) sites on the NiWO 4 surface to enhance the OER. Due to a relatively better alkaline stability of NiWO 4 over the analogous NiMoO 4 , the surface oxidation could presumably be less pronounced . In addition to the redox feature in CV recorded between 0.89 and 1.95 V (vs RHE), the Raman data obtained with the electrode after 10 CV cycles also strikingly resembled the fresh NiWO 4 catalyst (Figure a,b).…”

“…These two independent studies only revealed the instability of the in situ grown NiMoO 4 on NF. However, the formation of NiO­(OH) from other nickel-based materials was also identified as a true catalyst. , In contrast, a majority of the literature reports on the OER study with NiMoO 4 did not perceive that NiMoO 4 is a precatalyst, and the formation of the oxidic phase from this precursor catalyst is inherent to deliver a better catalytic activity. ,,− Thereby, the identification of electroactive species generated from the NiMoO 4 needs further in-depth analysis.…”

Candidate for Catalyst during Water-Oxidation Reaction in the Presence of Manganese Compounds, from Nanosized Particles to Impurities: Sleep with One Eye Open

“…Recently, metal–organic frameworks (MOFs) have been reported as efficient catalysts for WOR . Although the WOR mechanism and active sites are enigmas, , MOF decomposition and metal (hydr)­oxide formation during WOR should be carefully investigated. , Deibert et al reported that the decomposition of a Mn 4 O 4 -based MOF results in MnO x formation, as a true catalyst under photochemical WOR conditions . Organic ligands and thermodynamically favorable MnO x formation are a driving force toward MOF decomposition.…”

Water Oxidation in the Presence of a Nickel Coordination Compound: Decomposition Products, Fe Impurity in the Electrolyte, and a Candidate as a Catalyst

“…The details of OER in the presence of Ni coordination compounds have been thoroughly investigated. − During OER in the presence of a Ni coordination compound, one should be attentive to the role of the Ni (hydr)­oxide formed toward OER. However, pure Ni (hydr)­oxide is a poor catalyst for OER, − but when Fe as an additional compound or as an impurity is combined with Ni (hydr)­oxide, it forms one of the most efficient OER catalysts under alkaline conditions. , …”