Photocatalytic conversion of carbon dioxide into methanol in reverse fuel cells with tungsten oxide and layered double hydroxide photocatalysts for solar fuel generation

Abstract

The phenomena of the photocatalytic oxidation of water and photocatalytic reduction of CO₂ were combined using reverse photofuel cells, in which the two photocatalysts, WO₃ and layered double hydroxide (LDH), were separated by a polymer electrolyte (PE) film. WO₃ was used for the photooxidation of water, whereas LDH, comprising Zn, Cu, and Ga, was used for the photoreduction of CO₂. For this process, photocatalysts pressed on both sides of the PE film were irradiated with UV-visible light through quartz windows and through the space in carbon electrode plates and water-repellent carbon paper for both gas flow and light transmission. 45% of the photocatalyst area was irradiated through the windows. The protons and electrons, which were formed on WO₃ under the flow of helium and moisture, transferred to the LDH via the PE and external circuit, respectively. Methanol was the major product from the LDH under the flow of CO₂ and helium. The observed photoreduction rates of CO₂ to methanol accounted for 68%–100% of photocurrents. This supports the effectiveness of the combined photooxidation and photoreduction mechanism as a viable strategy to selectively produce methanol. In addition, we tested reverse photofuel cell-2, which consisted of a WO₃ film pressed on C paper and LDH film pressed on Cu foil. The photoelectrodes were immersed in acidic solutions of pH 4, with the PE film distinguishing the two compartments. Both the photoelectrodes were completely irradiated by UV-visible light through the quartz windows. Consequently, the photocurrent from the LDH under CO₂ flow to WO₃ under N₂ flow was increased by 2.4–3.4 times in comparison to photofuel cell-1 tested under similar conditions. However, the major product from the LDH was H₂ rather than methanol using photofuel cell-2. The photogenerated electrons in the irradiated area of the photocatalysts were obliged to diffuse laterally to the unirradiated area of photocatalysts in contact with the C papers in photofuel cell-1. This lateral diffusion reduced the photocatalytic conversion rates of CO₂, despite the advantages of photofuel cell-1 in terms of selective formation and easy separation of gas-phase methanol.