From NiMoO₄ to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved <i>in Situ</i> and <i>Operando</i> 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. 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 <i>operando</i> 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 <i>in situ</i> and <i>operando</i> 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.