Photoelectrochemical CO2 Reduction to Formate over Hybrid System of CdS Photoanode and Formate Dehydrogenase Under Visible Light Irradiation
Authors: Higashi Masanobu , Toyodome Takumi , Tanaka Itsuki , Yoshida Tomoko , Amao Yutaka 
Published: July 2022

1. Introduction
Photocatalytic and photoelectrochemical (PEC) CO2 reductions using semiconductor materials under solar light have attracted significant attention. The development of CO2-reduction systems operating under visible-light irradiation is essential for the practical use of solar energy because visible light takes up almost half of the solar radiation spectrum. Most metal sulfides, such as CdS, have suitable conduction band levels for CO2 reduction and narrow bandgaps, enabling visible-light absorption. However, most metal sulfides are unstable in aqueous solution under photoirradiation due to the occurrence of self-oxidative deactivation (photocorrosion) by photogenerated holes (e.g., CdS + 2h+ → Cd2+ + S). CO2 reduction in an aqueous solution competes with H2 evolution because the CO2 reduction potential is more negative compared to the water reduction potential. Therefore, the catalysts that can selectively reduce CO2 have been utilized. Enzymes are biocatalysts that only act on specific substrates. For example, formate dehydrogenase (FDH) can reduce CO2 to formate with 100% selectivity by accepting electrons from coenzyme NADH. We have recently reported PEC reduction of CO2 to formate using a sacrificial reagent-free system consisting of a TiO2 photoanode and FDH.1 However, this system cannot utilize visible light owing to the wide bandgap of TiO2. In this study, we attempted to fabricate stable CdS photoanode and construct a hybrid system consisting of the CdS photoanode, NADH, and FDH for the reduction of CO2 to formate under visible-light irradiation.
2. Experimental
The CdS photoanode was fabricated by the chemical bath deposition (CBD) method according to a previous report.2 The CdS electrode was calcined in N2 flow at 200, 300, and 400 °C for 30 min. The electrochemical cell used for photocurrent measurements comprised a prepared electrode, a counter electrode (Pt wire), an Ag/AgCl reference electrode, and a borate buffer solution (pH 8) containing K4[Fe(CN)6]•3H2O. The electrodes were irradiated using a 300 W Xe lamp fitted with an L-42 cut-off filter. CO2 reduction was carried out using two-electrode system (counter electrode: carbon paper) in phosphate buffer solution (pH 7) under CO2 bubbling. Rh complex [Cp*Rh(bpy)(H2O)]2+,3 NAD+, and FDH was added in the counter side.
3. Results and discussion
The stabilities of the photocurrents of the CdS electrodes were evaluated at a fixed potential (–0.5 V vs. Ag/AgCl) under continuous visible light irradiation. As shown in Fig. 1a, for CdS electrodes with and without calcination at 200 and 300 °C, the photocurrent densities gradually decreased with the start of light irradiation. After the reaction, large cubic particles with a particle size of approximately 100–300 nm, reflecting the crystal structure of K2Cd[Fe(CN)6], were observed on the electrodes. XRD analysis also confirmed the formation of K2Cd[Fe(CN)6]. As previously reported, K2Cd[Fe(CN)6] forms spontaneously on the CdS surface during photoirradiation by reaction of dissolved Cd2+ cations via photocorrosion and [Fe(CN)6]4− anions in the solution.4 Therefore, this gradual decrease in photocurrent density was mainly due to the photocorrosion of CdS. In contrast, calcination at 400 °C positively influenced the CdS electrode, gradually increasing the photocurrent density. The number of electrons passing through the outer circuit for over 1 h (19.7 C, corresponding to 205 μmol) exceeded the number of moles of CdS (approximately 22 μmol) in the electrode. Although large K2Cd[Fe(CN)6] particles were observed on the CdS electrode after the reaction, the amount was significantly lower than that observed on the other electrodes. Instead, fine particles of K2Cd[Fe(CN)6] (particle size ~10 nm) were densely formed on the CdS surface. Therefore, fine K2Cd[Fe(CN)6] particles generated on the surface effectively scavenged photogenerated holes in CdS and enabled the oxidation of [Fe(CN)6]4− to [Fe(CN)6]3−.4

Reduction of NAD+ to NADH using [Cp*Rh(bpy)(H2O)]2+ as an electron mediator was examined in two-electrode system consisting of the CdS photoanode (calcined at 400 °C) and a carbon paper counter electrode with no applied bias (0 V). Relatively stable photocurrent (0.2 mA cm–2) was observed for 60 min. UV-vis spectroscopy and liquid chromatography analyses revealed that the amount of 1,4-NADH, which functions as a coenzyme, was increased with progression of reaction by reduction of NAD+ over the carbon electrode accepting electrons from CdS photoanode. The PEC reduction of CO2 to formate was attempted by introducing FDH in the system. As shown in Fig. 1b, the amount of produced formate was increased with the increasing irradiation time and reached 1285 nmol, which exceeded that of FDH (64 nmol).

Ishibashi, M. Higashi, S. Ikeda, Y. Amao, ChemCatChem, 2019, 11, 6227.

Ikeda, T. et al, ChemSusChem, 2011, 4, 262.

Ruppert, et al, Tetrahedron Lett., 1987, 28, 6583.

Shirakawa, M. Higashi, O. Tomita, R. Abe, Sustain. Energy Fuels, 2017, 1, 1065.

Figure 1