Alkaline Electrochemical Water Oxidation by NiFe-based Metal Oxide Catalysts

The oxygen evolution reaction (OER) is the ideal anodic reaction for a photoelectrochemical cell that converts solar energy into chemical fuels; however it is currently limited by catalyst efficiency and cost. In order to overcome these limitations, much effort has been devoted to the discovery of more efficient earth-abundant metal oxide electrocatalysts, particularly mixed-metals oxides, as well as understanding their mechanisms. This thesis addresses both of these challenges.;An O2-sensitive fluorescence-based screening assay was developed and arrays of ternary oxide electrocatalysts were examined. Analysis of the intensity of the resulting fluorescence signal allowed for the identification of compositions with high activity. Compositions exhibiting the highest activities were validated through Tafel analyses, and good qualitative agreement was observed between screening results and electrochemical measurements. While Fe is known to improve the activity of Ni oxide catalysts, the incorporation of a third metal into Ni-Fe binary oxides was often observed to further enhance OER activity, with Ni-Fe-Al amorphous oxide showing the highest activity during our initial screening.;The previously observed instability led to the investigation of a well-defined NiFeAlO4 inverse spinel oxide as a water oxidation electrocatalyst, where structural analyses confirmed the substitution of Al for Fe lattice sites. A comparison of electrochemical activity against compositionally and structurally relevant oxides, including the known OER catalysts NiO, NiFe 9:1, and NiFe2O4 established NiFeAlO4 as a superior electrocatalyst with no Al leaching, and cyclic voltammograms of the oxides indicated that the electron-withdrawing M+3 ions in the inverse spinels make Ni+2 sites more difficult to oxidize. Furthermore, it was observed that neither of the bimetallic spinel oxides (NiFe2O 4 & NiAl2O4) outperformed NiFeAlO4, suggesting a unique synergistic effect between all three metal sites that influences the OER rate determining step.;In addition to catalyst discovery, we also pursued mechanistic studies to better understand the dramatic activity enhancement that results when Fe is added to various Ni oxide electrocatalysts. Operando Mossbaurer spectroscopic studies of a 3:1 Ni:Fe layered oxide and anhydrous Fe oxide electrocatalyst was performed. Catalyst materials were prepared by a hydrothermal precipitation method that enabled growth of the oxide catalysts directly on a carbon paper electrode, thereby enhancing charge transport and mechanical stability needed for the operando studies. Fe+4 species are evident in the NiFe-oxide catalyst during steady-state water oxidation, accounting for up to 21% of the total Fe in the catalyst. No Fe+4 is detected under any conditions in the Fe-oxide catalyst. The lifetime of the observed Fe +4 species suggests they do not participate directly in water oxidation; however, their presence has important implications for the promoting effect of Fe in NiFe-oxide electrocatalysts.