Abstract
SummaryPhotoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl− ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl− on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.
Highlights
Various photoanodes composed of oxide semiconductors on conducting glass substrate have been widely studied for artificial photosynthesis technology with water oxidation into O2 under simulated solar light and low applied bias in aqueous electrolyte solution (Fujishima and Honda, 1972; Hisatomi et al, 2014; Kang et al, 2015; Sivula and van de Krol, 2016; Roger et al, 2017), whereby the efficiency was determined to be improved by loading with metal oxides (MOx) such as FeOx, NiOx, and CoOx (Zhong et al, 2015; Chemelewski et al, 2014)
Photoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy
O2 and hypochlorous acid (HClO) production in aqueous solution containing ClÀ on photoanodes modified with various metal oxides was investigated
Summary
Various photoanodes composed of oxide semiconductors on conducting glass substrate have been widely studied for artificial photosynthesis technology with water oxidation into O2 under simulated solar light and low applied bias in aqueous electrolyte solution (Fujishima and Honda, 1972; Hisatomi et al, 2014; Kang et al, 2015; Sivula and van de Krol, 2016; Roger et al, 2017), whereby the efficiency was determined to be improved by loading with metal oxides (MOx) such as FeOx, NiOx, and CoOx (Zhong et al, 2015; Chemelewski et al, 2014). In seawater containing ClÀ anions, hypochlorous acid (HClO, neutral form of Cl2) production reaction (abbreviated as CPR) by ClÀ oxidation proceeds concurrently with the OER by water oxidation (Figure 1). We systematically investigated the effect of loading various simple metal oxide species (MOx), including precious metals, on the selectivity toward HClO/O2 production from unbuffered NaCl aqueous solution over a BiVO4 photoanode, which is one of the most popular visible-light responsible photoanodes for efficient water splitting (Hong et al, 2011; Saito et al, 2012; Fuku et al, 2017), and found that MnOx loading could suppress the CPR on the photoanode under a wide range of conditions. We could advocate a new hypothesis why manganese is selected as the element for OER center of natural photosynthesis (Barber., 1998; Yano et al, 2006; Umena et al, 2011). (Figure 1)
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