(General Student Poster Award Winner, 3rd Place) Solar CO2 Reduction to Form Green Syngas Employing a Black Cu3VS4 Photocathode Utilizing a Whole Range of Visible Light
Authors: Nagatsuka Kengo , Yamaguchi Yuichi , Kudo Akihiko 
Published: December 2024

A photoelectrochemical cell as artificial photosynthesis is an attractive system for solar CO2 reduction in H2O to produce a mixture gas of H2 and CO, so called “green syngas.” Development of photocathodes with the response to long wavelength visible light and high selectivity for CO formation is necessary in terms of efficient solar energy conversion. Recently, we reported that a powder-based Cu3VS4 black photocathode was active for solar H2 production utilizing a whole range of visible light.1 This photocathode was a Cu(Ⅰ)-containing metal sulfide, which was a suitable material for solar CO2 reduction.2 Therefore, solar CO2 reduction using a Cu3VS4 photocathode is expected to be achieved by constructing a photoelectrochemical cell. In the present study, we developed a new photoelectrochemical cell employing a black Cu3VS4 photocathode for solar CO2 reduction to produce green syngas.

The black metal sulfide was prepared by a solid-state-reaction (SSR) and a flux method using a mixture of chloride salts (flux).1 The powder-based Cu3VS4 photocathode was fabricated by a particle transfer method contacted with deposited Au as a conductive layer.1,3 A sputtered Au cocatalyst was loaded on the photocathode. Photocathodic properties were measured using a three-electrode system under CO2 atmosphere. The photocathode was irradiated with visible light and simulated sunlight using a 300 W Xe-lamp attached with a long-pass filter and a solar simulator, respectively. The amounts of evolved H2, CO and O2 were determined using an online gas-chromatograph.

Single particle layers with Cu3VS4 (SSR, flux) particles were observed on the photocathodes by SEM images. Large Cu3VS4 (SSR) particles with a size of 5-8 µm contacted with deposited Au layer roughly, while fine contacts with the Au layer were constructed due to small Cu3VS4 (flux) particles with a size of 200-800 nm. The results of SEM-EDS mappings and XPS measurements indicated that the surfaces of Cu3VS4 (SSR, flux) particles were mostly covered with a sputtered Au cocatalyst and a deposited Au layer. Photoelectrochemical CO2 reduction to form a green syngas using the Cu3VS4 photocathodes proceeded under visible light irradiation. During the reaction, green syngas including 24% of CO was successfully obtained using a Cu3VS4 (flux) photocathode. The comparatively high selectivity for CO evolution was due to the 2-type Au functioning as efficient active sites. It was notable that a stability of the photocurrent was drastically improved by employing fine Cu3VS4 (flux) particles instead of large Cu3VS4 (SSR) particles. XRF analysis and SEM images of the photocathode revealed that a large amount of the Cu3VS4 (SSR) particles were peeled off from the Au-deposited substrate after the reaction, while the Cu3VS4 (flux) particles were not. Therefore, the good stability of the photocurrent on the Cu3VS4 (flux) photocathode was due to the good contacts between the photocatalyst particles and the deposited Au layer. The onset of the IPCE agreed well with that of an absorption edge of Cu3VS4, reaching up to 820 nm. In other words, the black photocathode produced a green syngas utilizing a whole range of visible light. Finally, a photoelectrochemical cell consisting of the Cu3VS4 photocathode and a BiVO4 photoanode was constructed. Solar CO2 reduction proceeded applying the bias at 1.0 V between the photocathode and the photoanode. H2 and CO steadily evolved accompanied by O2 evolution from water, reaching unity of the ratio of reacted electrons to holes. These results indicated that solar CO2 reduction to form a green syngas using the Cu3VS4 photocathode was successfully achieved utilizing water as an electron donor.

Thus, it was concluded that a Cu3VS4 photocathode with the high selectivity for CO evolution and the response to long wavelength visible light was successfully developed. Our findings will contribute to development of a photoelectrochemical cell for an efficient solar energy conversion and a sustainable carbon neutrality.

[1] H. Fukai, K. Nagatsuka, Y. Yamaguchi, A. Iwase, A. Kudo, ECS J. Solid State Sci. Technol.
2022, 11, 063002.

[2] S. Yoshino, T. Takayama, Y. Yamaguchi, A. Iwase, A. Kudo, Acc. Chem. Res.
2022, 55, 966.

[3] T. Minegishi, N. Nishimura. J. Kubota, K. Domen, Chem. Sci.
2013, 4, 1120.

Figure 1