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. NiMoO₄ has 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 NiMoO₄ nanorods grown on a nickel foam (NF) andcovered with a thin layer of Co₃O₄ deposited 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 Co₃O₄ nanoparticles covering the surface of a single NiMoO₄ nanorod. 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@Co₃O₄ possess 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@Co₃O₄ exceeds the activity of the most efficient catalysts recently published. Furthermore, the catalysts are compared using overpotential at different current densities (Figure 1d). NiMoO₄@Co₃O₄ retains its highest catalytic activity at 10, 50 100, and 200 mA/cm² current 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 2) were measured for hydrous NiMoO₄and NiMoO₄@Co₃O₄ before 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 Mo3d_5/2 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 NiMoO₄ is 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@Co₃O₄ core-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, ¹ Alberto Vomiero^1,4

¹ Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Sweden 
² Istituto di Microelettronica e Microsistemi-CNR (CNR, IMM), Bologna, Italy
³ Istituto di Struttura della Materia-CNR (ISM-CNR), Trieste, Italy
⁴ Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Venezia Mestre, Italy

Contact persons:

 Getachew Solomon, email: getachew.solomon@ltu.se