Cobalt Phosphate Catalysts for the Oxygen Evolution Reaction

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ABSTRACT: The development of high-performance electrocatalysts for the oxygen evolution reaction (OER) is an essential technology to generate clean hydrogen energy from water. The success of this technology mainly relies on the discovery of earth-abundant, highly efficient and stable Co-based catalysts that can replace expensive and scarce noble metals for the water splitting process.

Among these, CoPi-based materials have been found to be promising candidates as alternative electrodes due to their unique Co-site geometries that facilitate the favorable adsorption and oxidation of water molecules. However, the development of a robust Co-based catalyst is still challenging.

In the present work, we report the synthesis of two-dimensional (2D) cobalt phosphate hydrates exhibiting excellent OER activity. The NH4CoPO4*H2O hydrates were characterized by X-ray diffraction, infrared and Raman spectroscopy, and SEM/TEM images. The results showed that the 2D nanosheet structure significantly enhances the electrocatalytic performance of the hydrates by increasing the surface exposure of the cobalt to oxygen molecules and by stabilizing the octahedral coordination geometry of the Co ions.

The crystallographic analysis revealed that the NH4CoPO4*H2O crystals form rosettes with petals reaching sizes of up to 300 nm cross-sections. The structure of the rosettes is governed by a mixture of crystalline cobalt phosphates and hydrogen phosphates with some residual CoCl2. Combined with elemental ratios obtained from EDX and DRX measurements, these findings indicate that the peaks observed in the X-ray diffraction pattern correspond to octahedral and tetrahedral CoO6 octahedra and two monoclinic cobalt hydrogen phosphates from dipyramidal class and rhombohedral CoO10 units from prismatic class respectively.