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 CoOx 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.