NiMoO4@Co3O4 core-shell nanorods: in situ catalyst reconstruction towards high-efficiency oxygen evolution reaction

Various transition metals and their oxides have been explored for oxygen evolution reaction (OER). Among them, nickel molybdate (NiMoO4) nanorods have attracted intensive research interest due to their abundant catalytic sites, high surface area, and ease of synthesis method. NiMoOhas a complex hydrated crystal structure that is composed of a Tetrameric z-shaped unit of Ni octahedra, two NiO6, and two NiO5(OH2) containing coordination water. In our work we designed a core-shell structure consisting of NiMoOnanorods grown on a nickel foam (NF) andcovered with a thin layer of Co3Odeposited via atomic layer deposition (NiMoO4@Co3O4). The scanning electron microscopy images of Figure 1a display in low and high magnification the morphology of the system. The low magnification transmission electron microscopy image of Figure 1b shows the homogeneous layer of sharp-cornered Co3Onanoparticles covering the surface of a single NiMoOnanorod. This core-shell structure turns out to be a highly efficient electrocatalyst for OER. The performances of NiMoO4@Co3O4, hydrous NiMoO4, and bare NF have been characterized by Linear Sweep Voltammetry (Figure 1c). NiMoO4@Co3Opossess a remarkable catalytic activity with an overpotential of 120 mV at a current density of 10 mA/cm2. Hydrous NiMoO4possesses an overpotential of 220 mV at a current density of 10 mA/cm2. The OER performance of NiMoO4@Co3Oexceeds the activity of the most efficient catalysts recently published. Furthermore, the catalysts are compared using overpotential at different current densities (Figure 1d). NiMoO4@Co3Oretains its highest catalytic activity at 10, 50 100, and 200 mA/cmcurrent densities showing an overpotential of 120, 200, 282, and 430 mV, respectively.


Figure 1.  (a) FE-SEM images at low and high magnifications for NiMoO4@Co3O4. (b) Low magnification TEM micrographs for Co3O4-coated NiMoO4. (c) The OER performance of the as-prepared catalyst; polarization curves for NiMoO4@Co3O4, NiMoO4, and NF. (d) Overpotential at different current densities. (Modified Fig. of Adv. Energy Mater. 2021, 11, 2101324).

The origin of this behavior was investigated by multiple experimental techniques, including high-resolution photoemission spectroscopy at the VUV-Photoemission beamline. These techniques consistently reveal the occurrence of an irreversible reconstruction of the catalyst during its activity that determines its highly efficient performances.
The Mo 3d, Ni 3p, and Co 3p levels and the valence band spectra (Figure 2were measured for hydrous NiMoO4and NiMoO4@Co3Obefore and after OER cycles using synchrotron radiation. For both systems, the binding energy of the core level peaks shifts towards lower values after the OER tests. In particular, the Mo3d5/2 component is found at 231.7 eV (0.6 eV decrease). This indicates a variation of the Mo oxidation state from 6+ towards 5+, thus suggesting that Mo becomes more metallic. Similar changes are detected in the valence band spectra. The valence band maximum for NiMoOis observed at 1.2-1.6 eV below the Fermi level (Figure 2b,c). After OER test the valence band maximum rises to 1.0-1.3 eV, again suggesting that the catalyst becomes more conductive during the OER process. A valence band maximum of 0.7 eV is obtained for NiMoO4@Co3O4, which shifts its features by 0.4 eV after the OER test. In-situ Raman analysis was conducted to further characterize and monitor the newly generated phase upon the OER process. The results reveal that the catalyst undergoes irreversible reconstruction during the OER process evidenced by the newly evolved NiOOH Raman peak above 1.4 V, in agreement with the photoemission measurements.
In summary, our results show that catalyst reconstruction plays an important role in promoting and maintaining the high OER activity of NiMoO4@Co3Ocore-shell structures, suggesting that the surface of the catalyst becomes porous and rougher, enabling the easier flow of the electrolyte, which is vital for improving the catalyst performances.

Figure 2.  Photoemission spectroscopy characterization
oft he catalyst. (a) Mo 3d spectra for hydrous NiMoO4, and Mo 3d spectra of NiMoO4@Co3O4 before and after catalysis. (b) Ni 3p spectra for NiMoO4, and Ni 3p and Co 3p spectra for NiMoO4@Co3O4 before and after catalysis. (c) Valence band spectra for NiMoO4 (I), NiMoO4 after OER test (II), NiMoO4@Co3O4(III), and NiMoO4@Co3O4 after OER test (IV). (d) The green spectrum is obtained as a difference between the blue and the pink spectra. All the spectra were acquired using 690 eV p-polarized photons.


This research was conducted by the following research team:

Getachew Solomon 1, Anton Landström 1, Raffaello Mazzaro 2, Matteo Jugovac 3, Paolo Moras 3, Elti Cattaruzza 4, Vittorio Morandi 2, Isabella Concina, Alberto Vomiero1,4


Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Sweden 
Istituto di Microelettronica e Microsistemi-CNR (CNR, IMM), Bologna, Italy
Istituto di Struttura della Materia-CNR (ISM-CNR), Trieste, Italy
Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Venezia Mestre, Italy

Contact persons:

 Getachew Solomon, email:



Getachew Solomon, Anton Landström, Raffaello Mazzaro, Matteo Jugovac, Paolo Moras, Elti Cattaruzza, Vittorio Morandi, Isabella Concina, Alberto Vomiero, “NiMoO4@Co3O4 core-shell nanorods: in situ catalyst reconstruction towards high efficiency oxygen evolution reaction”, Adv. En. Mater. 11, 2101324 (2021)  DOI: 10.1002/aenm.202101324 

Last Updated on Monday, 27 September 2021 15:43