Hybrid Organic-Inorganic Materials as Precursors for Water Splitting Electrocatalysts
Abstract： 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 octahedral 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. NiP, NiP-NiP and NiP nanoparticles with sizes of ca. 15-45 nm coated with a thin shell of carbonaceous material wereproduced. Thermogravimetric analysis coupled with mass spectrometry (TG-MS) showed that H, HO, P and –CH 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. The H most likely reacts with the –PO groups of the precursor to form HO and P, and the latter subsequently reacts with the metal to produce the phosphide. The nickel phosphides nanoparticles efficiently catalyze the hydrogen evolution reaction, with NiP showing the best performance and generating a current density of 10 mA cm^-2 at an overpotential of 87 mV and exhibiting long-term stability. CoP and CoP NPs were also synthesized following this method. All in all, these approaches may be utilized to explore the rich metal-phosphonate chemistry for fabricating a large variety of nanostructured materials for electrochemical energy conversion and storage applications. Biosketch： Nicola Pinna studied physical chemistry at the Université Pierre et Marie Curie (Paris). He received his Ph.D. in 2001, and in 2002, he moved to the Fritz Haber Institute of the Max Planck Society (Berlin). In 2003, he joined the Max Planck Institute of Colloids and Interfaces (Potsdam). In 2005, he moved to the Martin Luther University, Halle-Wittenberg, as an Assistant Professor of Inorganic Chemistry. From March 2006 to June 2012 he was researcher at the Department of Chemistry and CICECO of the University of Aveiro and from September 2009 to June 2012 he was also Assistant Professor at the school of chemical and biological engineering Seoul National University in the framework of the world class university project founded by the National Research Foundation of Korea. In July 2012 he joined the Department of Chemistry of the Humboldt University in Berlin. In 2011 he was ranked among the top 100 materials scientists of the past decade by impact. His research activity is focused on the development of novel routes to nanostructured materials, their characterization, and the study of their physical properties.