(Invited) Approaches to Enhance Hematite Performance for Photoelectrocatalytic Water Oxidation

Résumé fourni par la source

Photoelectrochemical (PEC) water splitting represents a sustainable and cost-effective route to convert solar energy directly into chemical energy in the form of molecular hydrogen. Hematite (α-Fe2O3) has been targeted as one of the most promising metal oxide photoanodes in PEC configuration due to its natural abundance, effective use of visible light and excellent photo and chemical stability. However, hematite photoanodes still underperform in terms of solar-to-hydrogen efficiency, far below its corresponding theoretical value. The sluggish four-electron-transfer water oxidation reaction is one of the main reasons for the lower efficiency of hematite photoanodes. To facilitate the water oxidation process, surface modification of hematite via decoration co-catalysts has been proposed as a promising strategy to lower the reaction barrier. One of the most efficient electrocatalyst for water oxidation is iridium. Here, I present our work on single-atom iridium on hematite (α-Fe2O3/sIr) and evaluate their role as catalytic site for photoelectrocatalytic water oxidation reaction. The α-Fe2O3/sIr delivered a low onset potential (Figure 2a, determined by butler plots, Figure 2a right) of∼0.82 V vs RHE, which cathodically shifted 0.44 V with respect to bare α-Fe2O3, as well as a significantly improved photocurrent value, particularly at lower potentials. This was related to the faster hole transfer exhibited by α-Fe2O3/sIr, demonstrated by in situ transient absorption spectroscopy. Density functional theory calculations revealed the mechanism for water oxidation using sIr as a catalytic centre to be the preferred pathway as it displayed a lower onset potential than the Fe sites (Figure 2b). Our energy band structure calculations showed that sIr induces mid-gap states with Ir 4d orbitals, which could serve as hole traps, facilitating the hole transfer from α-Fe2O3 to sIr followed by fast water oxidation. The reaction on the sIr site has a significantly lower energy barrier(1.01 eV) than when Fe acts as the active site (1.80 eV). These results provide for the first time a deeper understanding of the interplay between the electronic structure, hole transfer, and depletion in water oxidation mechanisms. If time allows it, I will also summarise additional approaches to promote hematite photoactivity my research group has been exploring over the last few months. Figure 1