Controlling the Photoelectrochemical Water Oxidation Reaction Pathway via Surface Amorphous Overlayer

Hydrogen peroxide (H₂O₂), regarded as a green fuel and powerful oxidant, has represented high attention in the energy and environment field recently. Also, H₂O₂ is attracting attention in that it can be used as an energy carrier since it has an energy density of 2.1 MJ kg^-1, which is comparable to compressed hydrogen (3.5 MJ kg^-1). Over 95% of H₂O₂ is produced by Anthraquinone process, which needs high pressure of hydrogen, requires expensive catalyst, and consumes lots of energy during whole process. Therefore, it is vital to develop an alternative method for H₂O₂ production that is eco-friendly and economical. In this regard, photoelectrochemical (PEC) synthesis of H₂O₂ via 2-electron water oxidation is an alternative route. The water-oxidation process follows two pathways: oxygen evolution reaction (OER) at 1.23 V vs. reversible hydrogen electrode (RHE), and H₂O₂ production reaction at 1.76 V vs. RHE. Unfortunately, the two-electron pathway must compete with the four-electron pathway, which generates O₂. Due to these competitive pathways, the reaction kinetics become sluggish, and intense degradation of electrode occurs. To date, studies have demonstrated that the free energy change of adsorbed OH radicals is a key factor in H₂O₂ evolution. Therefore, it is challenging to design efficient, selective, and stable photoanode materials for efficient H₂O₂ production from H₂O.<br/>For PEC water splitting photoanode materials, semi-conductive metal oxides such as Fe₂O₃, ZnO, BiVO₄, WO₃, SnO₂ and TiO₂ have been widely studied. Among various metal oxides for PEC water oxidation, titanium dioxide (TiO₂) is the most extensively studied and numerous efforts have been conducted to enhance the PEC activity of TiO₂. However, TiO₂ itself is OER favorable because of native defects derived surface oxygen vacancies. Moreover, TiO₂ has an unsatisfactory Faraday efficiency (FE) in water-oxidation reaction (WOR)-mediated H₂O₂ evolution, since TiO₂ has a higher OH adsorption energy compared with BiVO₄, WO₃, and SnO₂, which leads to the undesirable four-electron pathway and O₂ evolution. Since H₂O₂ production by WOR is a competitive reaction with the oxygen evolution reaction (OER), it is crucial and highly challenging to control the reaction pathway toward our goal, H₂O₂.<br/>In this study, we demonstrated the incorporation of amorphous titanyl phosphate (a-TP) overlayer on TiO₂ nanoparticles (TiO₂ NP) is conducive to modulate the WOR pathway and results in highly selective PEC H₂O₂ production. The ultrathin (<i>ca</i>. 2 nm) a-TP was conformally overlaid by <i>in-situ</i> surface reforming via lysozyme-molded mineralization. The a-TP overlayer manipulates the surface adsorption energies for the reaction intermediates to promote the WOR for H₂O₂ production, instead of the competing O₂ evolution reaction. Moreover, the a-TP overlayer conformally passivated the surface trap states, which eventually generated an efficient charge transfer. Consequently, a-TP/TiO₂ shows 3.7-times higher Faraday efficiency (63%) at 1.76 V vs. RHE under 1 sun illumination comparing to bare TiO₂ (17%), which is the highest performance among TiO₂ based catalyst. To sum up, the introduction of the a-TP overlayer is a promising strategy for steering the reaction pathway and achieving efficient solar-to-chemical energy conversion. We believe that introducing an overlayer on photoanodes to manipulate the reaction kinetics provides alternative and environmentally friendly PEC H₂O₂ production method, instead of the conventional anthraquinone process.