Materials For Solar Fuel Production Research Articles

A Bifunctional Lead–Iron Oxyfluoride, PbFeO2F, That Drives Photoelectrochemical and Electrochemical Water Oxidation

Introduction Photoelectrochemical water splitting has attracted attention as a method of generating solar fuel, such as hydrogen that is expected to be utilize as a renewable energy carrier and clean fuel of an internal‐combustion engine.1 In the photoanode materials reported so far, ordinary metal oxides require ultraviolet light for operation because of their wide bandgaps (>3 eV), and visible-light-responsive metal oxides need an additional electrochemical bias because their conduction-band minimum is a lower position than the H+/H2 reduction potential. Mixed-anion compounds fascinating recent photocatalyst researchers possess visible-light response because of a narrow bandgap with less electronegative anion species.1,2 However, they are not very stable for water oxidation, because photogenerated holes oxidize the anion species except for oxygen in the compounds. Recently, we have reported an oxyfluoride Pb2Ti2O5.4F1.2 as a stable photoanode material that absorbs visible light at wavelengths as long as ~500 nm.3 The VBM of Pb2Ti2O5.4F1.2 is mainly composed by the O 2p orbitals. As the result, the Pb2Ti2O5.4F1.2 photoanode is essentially stable toward self-oxidation by photogenerated holes. However, this is the sole example of a visible-light-responsive oxyfluoride photoanode. Development of a new oxyfluoride photoanode absorbing visible light is important for enabling the further design of visible-light-responsive photoanode materials. In the present work, a lead–iron oxyfluoride PbFeO2F is applied to a photoanode material and electrocatalyst for water-oxidation. PbFeO2F, which can be synthesized by a high-pressure method,4 has been reported to show a yellow colour. Thus, it is expected to function as a visible-light-responsive photoanode material. Moreover, PbFeO2F containing Fe cation is possible to provide catalytic performance for water oxidation, which means that this material may also function as an electrocatalyst. Results and Discussion According to the previously reported method,4 PbFeO2F powder was synthesized. It was deposited on a fluorine-doped tin oxide (FTO) glass substrate by electrophoresis. The PbFeO2F/FTO electrode was subjected to post-necking treatment by immersing in 0.1 M Ti{OCH(CH3)2}4 –isopropanol solution to deposit TiO2 layer (TiO2/PbFeO2F/FTO electrode), followed by electrodeposition of cobalt phosphate (Co-Pi) cocatalyst (Co-Pi/TiO2/PbFeO2F/FTO electrode). Linear-sweep voltammetry of the as-prepared electrodes in 0.1 M K3PO4 solution was conducted under intermittent visible-light irradiation (Figure 1A). The PbFeO2F/FTO electrode exhibited no photocurrent response. By contrast, a slight photocurrent response was observed for the TiO2/PbFeO2F/FTO electrode because of mainly improved interparticle conductivity in the electrode. Furthermore, a clear anodic photocurrent was observed for the Co-Pi/TiO2/PbFeO2F/FTO electrode. Loading the Co-Pi cocatalyst, known as a water-oxidation promoter, improved charge transfer at the electrode/water interface and charge separation from the surface to the bulk. Besides, the Co-Pi/TiO2/PbFeO2F/FTO electrode exhibited an anodic photocurrent under visible-light irradiation with wavelengths as long as ~600 nm. It indicates that PbFeO2F can function as a photoanode material under a wide range of visible-light wavelengths. Controlled-potential electrolysis with the as-prepared PbFeO2F/FTO electrode was operated at +1.7 V vs. RHE under dark conditions (Figure 1B). The evolved O2 was observed by GC analysis in the measurement. It was almost one-fourth the amount of electrons, which flowed during the electrolysis, giving a high Faradaic efficiency of 97%. This result means that the water oxidation was the major path at the PbFeO2F/FTO electrode. In addition, α-Fe2O3/FTO electrode does not work at this potential because of the larger overpotential than the PbFeO2F/FTO electrode for water oxidation. It reveals that PbFeO2F can also function as an electrocatalyst at a relatively low overpotential for water oxidation. References Seo, H. Nishiyama, T. Yamada and K. Domen, Visible-Light-Responsive Photoanodes for Highly Active, Stable Water Oxidation, Angew. Chem. Int. Ed., 57, 8396 (2018).Miyoshi and K. Maeda, Recent Progress in Mixed‐Anion Materials for Solar Fuel Production, Solar RRL, in press.Hirayama, H. Nakata, H. Wakayama, S. Nishioka, T. Kanazawa, R. Kamata, Y. Ebato, K. Kato, H. Kumagai, A. Yamakata, K. Oka and K. Maeda, Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode, J. Am. Chem. Soc., 141, 17158 (2019).Inaguma, J.-M. Greneche, M.-P. Crosnier-Lopez, T. Katsumata, Y. Calage and J.-L. Fourquet, Structure and Mössbauer Studies of F−O Ordering in Antiferromagnetic Perovskite PbFeO2F, Chem. Mater., 17, 1386 (2005). Figure 1

Recent Progress in Mixed‐Anion Materials for Solar Fuel Production

Mixed‐anion materials, consisting of metal cation(s) and more than two anionic species in a single phase, are promising as photocatalysts for water splitting and CO2 fixation using visible light. Oxynitrides, oxysulfides, and oxyhalides that contain d0 transition‐metal or d10 typical‐metal cations have absorption bands in the visible‐light region and band‐edge potentials suitable for these reactions. In general, visible‐light absorption by these materials arises from the p orbitals of anions that are less electronegative than oxygen, forming a valence band above the level of oxygen 2p orbitals. Less electronegative anions in mixed‐anion compounds are essential for visible‐light absorption but are inherently less stable than oxygen, potentially causing oxidative decomposition of the materials during photoreaction. More importantly, mixed‐anion compounds are more difficult to synthesize than single‐anion compounds like oxides, and reducing the density of defects that act as recombination centers for photogenerated charge carriers in the material remains challenging. There is therefore much room for improving the activities of mixed‐anion photocatalysts. Herein, the development of mixed‐anion materials for solar‐to‐fuel conversion over the past 10 years is highlighted with a focus on key milestones in research developments.