ABSTRACT
When electrocatalysts are prepared, modification of the morphology is a common strategy to enhance their electrocatalytic performance. In this work, we have examined and characterized nanorods (3D) and nanosheets (2D) of nickel molybdate hydrates, which previously have been treated as the same material with just a variation in morphology. We thoroughly investigated the materials and report that they contain fundamentally different compounds with different crystal structures, chemical compositions, and chemical stabilities. The 3D nanorod structure exhibits the chemical formula NiMoO4·0.6H2O and crystallizes in a triclinic system, whereas the 2D nanosheet structures can be rationalized with Ni3MoO5-0.5x(OH)x·(2.3 - 0.5x)H2O, with a mixed valence of both Ni and Mo, which enables a layered crystal structure. The difference in structure and composition is supported by X-ray photoelectron spectroscopy, ion beam analysis, thermogravimetric analysis, X-ray diffraction, electron diffraction, infrared spectroscopy, Raman spectroscopy, and magnetic measurements. The previously proposed crystal structure for the nickel molybdate hydrate nanorods from the literature needs to be reconsidered and is here refined by ab initio molecular dynamics on a quantum mechanical level using density functional theory calculations to reproduce the experimental findings. Because the material is frequently studied as an electrocatalyst or catalyst precursor and both structures can appear in the same synthesis, a clear distinction between the two compounds is necessary to assess the underlying structure-to-function relationship and targeted electrocatalytic properties.
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. NiMoO4 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, NiMoO4·H2O 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 NiMoO4·H2O. 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 NiMoO4 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.
