Bifunctional Lead–Iron Oxyfluorides, 3D Cubic PbFeO2f and 2D Layered Pb3Fe2O5F2, Driving Photoelectrochemical and Electrochemical Water-Oxidation

Abstract

Introduction Photoelectrochemical water splitting has attracted attention as a method of generating solar fuel such as hydrogen gas to be useful as a renewable energy carrier and a clean fuel of an internal‐combustion engine. For water oxidation of the half reaction in the water splitting, mixed-anion compounds have appeared as good candidates of the catalyst. In the mixed-anion compounds, a metal center is coordinated by several kind of anionic species. This environment induces their wide variety of physical and chemical properties not to be realized in single anion counterparts. The mixed-anion compounds possess visible-light responsibility and can utilize sunlight efficiently, therefore, which have fascinated photocatalyst researchers. The reason why they absorb visible light is the narrow bandgap with less electronegative anion species than oxygen. However, they are not very stable for photoelectrochemical water-oxidation because photogenerated holes oxidize the anion species except for oxygen in the compounds and cause degradation. Recently, we have reported an oxyfluoride Pb2Ti2O5.4F1.2 as a stable photoanode material that absorbs visible light at wavelengths up to ~500 nm.1 Oxyfluoride containing O2− and F− anions in the same phase is an example of the mixed-anion compounds. The other anion F− is less oxidized than O2−. Thus, the Pb2Ti2O5.4F1.2 photoanode showed essential stability toward the degradation by photogenerated holes exceptionally. However, this is the sole example of a visible-light-responsive oxyfluoride photoanode. Development of a new oxyfluoride photoanode absorbing visible light is important to gain the insight for water oxidation using the oxyfluorides. In the present work, another type of lead-iron oxyfluorides, PbFeO2F and Pb3Fe2O5F2, are applied to water oxidation as not only a photoanode but also an electrocatalyst.2,3 The lead-iron oxyfluoride, PbFeO2F, drives photoelectrochemical water-oxidation in visible light up to ~600 nm. In addition, this oxyfluoride functions as the electrocatalyst for water oxidation.2 In the electrochemical water-oxidation, another Pb3Fe2O5F2 shows higher performance for water oxidation than PbFeO2F. It seems that the crystal structure of two-dimensional layered perovskite Pb3Fe2O5F2 influences the superior performance to three-dimensional cubic perovskite PbFeO2F.3 Results and Discussion PbFeO2F and Pb3Fe2O5F2 powder were synthesized through a solid-state reaction and that in the evacuated glass tube, respectively.2,3 They were deposited on a fluorine-doped tin oxide (FTO) glass substrate by using electrophoresis method. The PbFeO2F or Pb3Fe2O5F2/FTO electrode was subjected to post-necking treatment to deposit TiO2 layer (TiO2/PbFeO2F or Pb3Fe2O5F2/FTO electrode), followed by electrodeposition of cobalt phosphate (Co-Pi) cocatalyst (Co-Pi/TiO2/PbFeO2F or Pb3Fe2O5F2/FTO electrode). Linear-sweep voltammetry for the as-prepared electrodes was conducted under intermittent visible-light irradiation (>400 nm) in 0.1 M K3PO4 solution (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. The Co-Pi cocatalyst, known as a water-oxidation promoter, improved the charge transfer at the electrode/water interface and charge separation in the electrode. Meanwhile, the Co-Pi/TiO2/PbFeO2F/FTO electrode exhibited an anodic photocurrent in visible light with longer wavelengths than ~600 nm. These tendencies were like those of Pb3Fe2O5F2 electrodes though the photocurrent were slightly higher than the PbFeO2F electrodes (Figure 1A, right). On contrary to the photoelectrochemical water-oxidation, a clear difference was observed when using those lead-iron oxyfluorides as an electrocatalyst. Under dark conditions, controlled-potential electrolysis with non-modified PbFeO2F and Pb3Fe2O5F2/FTO electrode was performed at +1.7 V vs. RHE (Figure 1B). The evolved O2 was detected by GC analysis in the measurement. The amount of that was almost one-fourth of electrons flowed during the electrolysis, which means the water oxidation was the major path for the electrochemical reaction. Besides, Pb3Fe2O5F2/FTO electrode showed 8 times higher activity than PbFeO2F. That reason has not been revealed but the difference is likely to be attributed to two-dimensional layered perovskite Pb3Fe2O5F2 and three-dimensional cubic perovskite PbFeO2F. References 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).Mizuochi, K. Izumi, Y. Inaguma and K. Maeda, A bifunctional lead–iron oxyfluoride, PbFeO2F, that functions as a visible-light-responsive photoanode and an electrocatalyst for water oxidation, RSC Adv., 11, 25616 (2021).Mizuochi, K. Oka, Y. Inaguma and K. Maeda, A two-dimensional perovskite oxyfluoride Pb3Fe2O5F2 as a catalyst for electrochemical oxidation of water to oxygen, Sustain. Energy Fuels, 6, 2423 (2022). Figure 1