Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution

Abstract

Water oxidation is a critical step in water splitting to make hydrogen fuel. We report the solution synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) electrocatalytic properties of ∼2–3 nm thick films of NiO_x, CoO_x, Ni_yCo_1–yO_x, Ni_0.9Fe_0.1O_x, IrO_x, MnO_x, and FeO_x. The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochemical properties of the oxides. Ni_0.9Fe_0.1O_x was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm^–2 at an overpotential of 336 mV with a Tafel slope of 30 mV dec^–1 with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrO_x control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni_0.9Fe_0.1OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, Ni_yCo_1–yO_x thin films showed decreasing activity relative to the pure NiO_x films with increasing Co content. This finding is explained by the suppressed in situ formation of the active layered oxyhydroxide with increasing Co. The high OER activity and simple synthesis make these Ni-based catalyst thin films useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting or in high-surface-area electrodes for water electrolysis.