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Clearing Up Discrepancies in 2D and 3D Nickel Molybdate Hydrate Structures
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. Université Paris-Saclay. (Hammarström)ORCID iD: 0000-0002-8696-0496
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.ORCID iD: 0000-0001-9834-3164
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.ORCID iD: 0000-0003-2790-116x
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2024 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 63, no 5Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2024. Vol. 63, no 5
Keywords [en]
nickel molybdate hydrate; nanorods, nanosheets layered nickel molybdate, α-NiMoO4, molybdenum leaching, Raman spectroscopy
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-476769DOI: 10.1021/acs.inorgchem.3c03261ISI: 001158182800001PubMedID: 38242537OAI: oai:DiVA.org:uu-476769DiVA, id: diva2:1668253
Funder
EU, Horizon 2020, 765376Available from: 2022-06-13 Created: 2022-06-13 Last updated: 2024-03-01Bibliographically approved
In thesis
1. Potential Electrocatalysts for Water Splitting Devices: A Journey Through the Opportunities and Challenges of Catalyst Classes
Open this publication in new window or tab >>Potential Electrocatalysts for Water Splitting Devices: A Journey Through the Opportunities and Challenges of Catalyst Classes
2022 (English)Doctoral thesis, comprehensive summary (Other academic) [Artistic work]
Abstract [en]

In this thesis work, different classes of catalysts and their suitability for integration into an electrolyzer cell has been investigated.

Ruthenium based molecular catalysts have shown high activities and stabilities towards water oxidation in neutral pH. Especially the oligomeric catalysts exhibited a superior performance. The electrical conductivity of the electrode and the low loading of catalyst might impose limitations on reaching high current densities at reasonable potentials.

Among the tested transition metal single atom catalysts, synthesized by pyrolyzing transition metal doped ZIF-8 structures, cobalt has shown the highest activity towards hydrogen evolution and a stable behaviour in acidic pH. The enhanced stability of single atomic sites compared to the corresponding nanoparticles was proposed. However, also for this class of catalyst, the low number of active sites seems to present a difficulty need to be overcome.

With the novel method presented to fabricate a membrane electrode assembly, the usage of commonly used expensive membranes could possibly be avoided.

Both nickel molybdate hydrate nanoparticle shapes have been proposed to transform in an electrochemical activation step into γ-NiOOH as active phase for the oxygen evolution reaction in alkaline pH. With the removal of molybdenum, a high electrochemical surface area with a large number of exposed nickel sites was indicated to be the origin behind the high catalytic activity of the nanoparticles. Molybdenum was suggested to only serve as structure and pore forming agent. Preliminary results indicated a higher activity for the rod structure towards the oxygen evolution reaction. An essential outcome is that it is uncertain if rods can be isolated synthesized on a nickel foam and hence the absence of the sheet structure should be verified, which could be done for example by selective molybdenum leaching combined with Raman spectroscopy. Furthermore, the two nanostructures are fundamentally different materials and characterized by various techniques.

Among all different classes of catalysts investigated, the nanoparticle catalysts seem to be the most promising for a successful integration in a large scale electrolyzer cell for widespread use.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2022. p. 109
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2165
Keywords
Electrocatalysis, Electrolyzer cell, HER catalyst, OER catalyst, Water splitting
National Category
Chemical Sciences
Research subject
Chemistry
Identifiers
urn:nbn:se:uu:diva-475092 (URN)978-91-513-1547-8 (ISBN)
Public defence
2022-09-12, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
EU, Horizon 2020, 765376
Available from: 2022-08-22 Created: 2022-06-24 Last updated: 2022-10-06Bibliographically approved

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Dürr, Robin N.Maltoni, PierfrancescoGhorai, SagarStröm, PetterAraujo, RafaelEdvinsson, Tomas

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