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.