Interfacial Design of a Ta<sub>3</sub>N<sub>5</sub> Thin-Film Photoanode for Highly Stable Oxygen Evolution over a Wide pH Range
Yudai Kawase, Tomohiro Higashi, Keisuke Obata, Yutaka Sasaki, Masao Katayama, Kazunari Domen, Kazuhiro Takanabe
Abstract
Ta3N5-based visible-light-absorbing semitransparent photoanodes for the oxygen evolution reaction (OER) are one of the most promising candidates for the construction of photoelectrochemical (PEC) water-splitting devices with a tandem configuration. Although the PEC properties of Ta3N5 have been evaluated in alkaline aqueous solutions, those in acidic or near-neutral pH aqueous solutions are still unclear. An efficient OER by Ta3N5 in acidic and near-neutral pH aqueous solutions is still challenging because it requires surface modifiers with chemical durability and efficient electrocatalytic properties for the OER. This study focuses on the interfacial design of Ta3N5 semitransparent photoanodes to achieve an efficient and stable OER over a wide pH range. The results of PEC measurements and X-ray photoelectron spectroscopy reveal that self-photooxidation occurs on the bare Ta3N5 surface regardless of the pH values, resulting in photocurrent decrease. A uniform coating of IrOx applied using spin-coating successfully protected Ta3N5, and the IrOx/Ta3N5 photoanodes maintained ∼80% of their initial OER photocurrent even after 4 h. The pH dependence of the photocurrent value of IrOx/Ta3N5 was found to follow that of the electrocatalytic OER rate of IrOx. Furthermore, the onset potential for the OER on IrOx/Ta3N5 was independent of the pH value and was fixed at 0.7 V versus a reversible hydrogen electrode.