From NiMoO₄ to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy

Abstract

Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO 4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO 4 ·H 2 O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO 4 ·H 2 O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO 4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.

Oxygen-Plasma-Induced Hetero-Interface NiFe2O4/NiMoO4 Catalyst for Enhanced Electrochemical Oxygen Evolution

“…In Figure S1c,d , the surface of the NiMoO 4 nanowires is covered with small-sized NiFe PBA NPs, indicating the expected process of the iron exchange. Figure 1 c reveals peaks at 357, 828, 872, and 950 cm −1 for the NiFe PBA/NiMoO 4 , which can be assigned to the Mo-O vibration, and this result is consistent with previous reports [ 32 ]. In addition, the NiFe PBA/NiMoO 4 exhibits three peaks around 2100 cm −1 , which can be attributed to -CN [ 48 , 49 ].…”

“…Among these transition metal compounds, NiMoO 4 on Ni foam has been researched as a promising electrocatalyst for its facile synthesis in large quantities. However, its intrinsic activity still remains defective, especially for its deficient active site (only Ni site) [ 29 , 30 , 31 , 32 ].…”

Correction to “From NiMoO4 to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy”

“…This agrees with our X-ray diffraction (XRD) data after molybdenum etching, in which no crystalline phase other than the one of flower-NiMoO 4 could be detected. 2 By HRTEM, they could infer that the formed γ-NiOOH nanorods are built up from nanosheets when the etching and oxidation step occur subsequently and not simultaneously, which confirms our observation of a roughened sheet-like morphology of our nanorods after catalysis. As in our work, the removal of the vibration spectra of the nanorods was detected by time-resolved in situ Raman spectroscopy measured without applied bias.…”

“…They attributed this to a very limited molybdenum leaching rate that was accelerated by higher concentrated KOH or temperature. 2 With the same reasoning and instead considering two different crystal structures, one with more dense/closer packed Ni atoms, it would agree with our detected different molybdenum leaching rates among the different nanostructures, which also possess different crystal structures. This addendum is meant to highlight and acknowledge some recent work we missed in our contribution, with the intention that the additional comments and comparisons made here bring a more complete understanding of the structures and processes present in these systems.…”

A Robust Approach to In Situ Exsolve Highly Dispersed and Stable Electrocatalysts

“…Additionally, the band located at ≈520 cm −1 belongs to the stretching vibration mode of Ni(OH) 2 (Figure 2B). [35,36] Compared to pristine γ-NiOOH, copper doped samples have peaks slightly shifted toward higher wavenumber, which can be attributed to weakening vibration of NiO caused by the doping of copper ions into the γ-NiOOH structure. [37] X-ray absorption fine structure (XAFS) analysis and AIMD calculation show that Cu was doped into the layered crystal structure of the γ-NiOOH with an interstitial mechanism instead of the substitution mechanism observed with perovskites.…”