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.
Highlights
Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels
The synthesis temperature as well as the holding time is in most publications reported, while less information is given on the heating rate that is known to have a strong influence on the formed materials
In this work we perform a detailed study on the stability and role of molybdenum in nickel molybdate hydrate grown on nickel foam for use as oxygen evolution catalyst in alkaline media together with the influence of the heating rate during the synthesis
Summary
Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. Due to the facile and versatile way of synthesizing nickel or cobalt molybdates in different nanostructures and different surface areas, this catalyst has recently been investigated closely as catalyst for OER and HER in alkaline media Those compounds are versatile and can only act as intermediate for the actual catalyst after an additional annealing and reduction step of the starting compound.[7,35,37,41] Metal molybdates have already been synthesized on metal foams or as powders and investigated in a large number of publications.[7,30,45−52,35−38,41−44] the stability of the metal molybdate or the role of molybdenum is not fully clarified.[35,36,38] It is essential to know the active catalytic phase and the role of added elements. Since the catalytic performance of a material can in general be improved by either providing more active sites or improving the single active site, understanding the role of each element is fundamental for further enhancements
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
