Enabling Solar Water Oxidation by BiVO₄ in Strongly Acidic Solutions

The oxygen evolution reaction (OER) is paired with various electrochemical and photoelectrochemical reduction reactions used for fuel and chemical production. As there is a strong interest in performing many of these reduction reactions in strongly acidic solutions to increase the reaction rate, efficiency, or selectivity, there is also a great interest in enabling efficient and stable OER in strongly acidic solutions. In this study, we report stable photoelectrochemical OER (POER) of a BiVO₄ photoanode in 0.1 M HNO₃ (pH 1). This was achieved by using Nb₂O₅ as a protection layer. While Nb₂O₅ was rarely used as a protection layer for photoelectrodes in the past, we show its excellent capability to suppress both the chemical and photoelectrochemical dissolution of BiVO₄ at pH 1. After stabilizing BiVO₄ with a Nb₂O₅ protection layer, we added Co²⁺ ions to the electrolyte as an OER catalyst to enhance the POER. We found that Co_(aq)²⁺ can serve as a homogeneous OER catalyst without being deposited as a CoO_x solid catalyst on Nb₂O₅. When we performed the POER using unprotected BiVO₄ with Co_(aq)²⁺ under the same condition, although POER was enhanced, the enhancement could not be sustained due to the chemical dissolution of BiVO₄. After the POER, we found that a Co³⁺-containing OER catalyst was deposited on the bare BiVO₄ surface. This result suggested that the use of Co²⁺ ions as a homogeneous catalyst was possible due to the inertness of the Nb₂O₅ surface toward the adsorption or deposition of Co ions. This study enabling stable POER of BiVO₄ in 0.1 M HNO₃ using the combination of a Nb₂O₅ protection layer and Co_(aq)²⁺ as a homogeneous OER catalyst provides promising possibilities for acidic POER and OER.