Efficient water oxidation catalysts are required for the development of water splitting technologies. Herein, we report the synthesis of layered hybrid transition metal phosphonate compounds from metal acetylacetonate precursors and various phosphonic acid in benzyl alcohol. The hybrid particles are formed by inorganic layers of divalent transition metals (e.g. Fe, Co, Ni) in distorted octahedra environments separated by bilayers of the organic group.
These hybrid materials are used as precursors for water splitting electrocatalysts in two ways.
On the one hand, their direct use as anode materials, so as oxygen evolution catalyst, involves their gradual transformation to hydroxide nanosheets during operation. It is found that the hybrid particles template the formation in situ of transition metal hydroxide nanosheets of sizes between 5 and 25 nm and thicknesses between 3 and 10 nm.
X-ray absorption spectroscopy measurements suggest that the hybrid acts also as a template for the local structure of the metal sites in the active catalyst, which remain distorted after the transformation. Optimum electrocatalytic activity is achieved with the hybrid compound with a Fe content of 16 %. The combination of the synergistic effect between Ni and Fe with the structural properties of the hybrid results in an efficient catalyst that generates a current density of 10 mA cm-2 at an overpotential of 240 mV, and also in a stable catalyst that operates continuously at low overpotentials for 160 h.
On the other hand, we report that nickel phosphides can be synthesized through thermal treatment of layered nickel phenyl- (NiPh) or methyl-phosphonates (NiMe) that act as single-source precursors. Ni12P5, Ni12P5-Ni2P and Ni2P nanoparticles with sizes of ca. 15-45 nm coated with a thin shell of carbonaceous material were produced. Thermogravimetric analysis coupled with mass spectrometry (TG-MS) showed that H2, H2O, P2 and –C6H5 are the main compounds formed during the transformation of the precursor under argon, while no hazard phosphorous-containing compounds are created, making this a simple and relatively safe route for fabricating nanostructured transition metal phosphides.
