Abstract
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 BiVO(4) photoanode in 0.1 M HNO(3) (pH 1). This was achieved by using Nb(2)O(5) as a protection layer. While Nb(2)O(5) 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 BiVO(4) at pH 1. After stabilizing BiVO(4) with a Nb(2)O(5) protection layer, we added Co(2+) 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 Nb(2)O(5). When we performed the POER using unprotected BiVO(4) with Co((aq))(2+) under the same condition, although POER was enhanced, the enhancement could not be sustained due to the chemical dissolution of BiVO(4). After the POER, we found that a Co(3+)-containing OER catalyst was deposited on the bare BiVO(4) surface. This result suggested that the use of Co(2+) ions as a homogeneous catalyst was possible due to the inertness of the Nb(2)O(5) surface toward the adsorption or deposition of Co ions. This study enabling stable POER of BiVO(4) in 0.1 M HNO(3) using the combination of a Nb(2)O(5) protection layer and Co((aq))(2+) as a homogeneous OER catalyst provides promising possibilities for acidic POER and OER.