Artificial photosynthesis: photoanodes based on polyquinoid dyes onto mesoporous tin oxide surface

Dye-sensitized photoelectrochemical cells represent an appealing solution for artificial photosynthesis, aimed at the conversion of solar light into fuels or commodity chemicals. Extensive efforts have been directed towards the development of photoelectrodes combining semiconductor materials and organic dyes; the use of molecular components allows to tune the absorption and redox properties of the material. Recently, we have reported the use of a class of pentacyclic quinoid organic dyes (KuQuinone) chemisorbed onto semiconducting tin oxide as photoanodes for water oxidation. In this work, we investigate the effect of the SnO2 semiconductor thickness and morphology and of the dye-anchoring group on the photoelectrochemical performance of the electrodes. The optimized materials are mesoporous SnO2 layers with 2.5 μm film thickness combined with a KuQuinone dye with a 3-carboxylpropyl-anchoring chain: these electrodes achieve light-harvesting efficiency of 93% at the maximum absorption wavelength of 533 nm, and photocurrent density J up to 350 μA/cm2 in the photoelectrochemical oxidation of ascorbate, although with a limited incident photon-to-current efficiency of 0.075%. Calculations based on the density functional theory (DFT) support the role of the reduced species of the KuQuinone dye via a proton-coupled electron transfer as the competent species involved in the electron transfer to the tin oxide semiconductor. Finally, a preliminary investigation of the photoelectrodes towards benzyl alcohol oxidation is presented, achieving photocurrent density up to 90 μA/cm2 in acetonitrile in the presence of N-hydroxysuccinimide and pyridine as redox mediator and base, respectively. These results support the possibility of using molecular-based materials in synthetic photoelectrochemistry. Graphic abstract

Photoelectrochemical Oxidation in Ambient Conditions Using Earth-Abundant Hematite Anode: A Green Route for the Synthesis of Biobased Polymer Building Blocks

This study demonstrates the use of a photoelectrochemical device comprising earth-abundant hematite photoanode for the oxidation of 5-hydroxymethylfurfural (5-HMF), a versatile bio-based platform chemical, under ambient conditions in the presence of an electron mediator. The results obtained in this study showed that the hematite photoanode, upon doping with fluorine, can oxidize water even at lower pH (4.5 and 9.0). For 5-HMF oxidation, three different pH conditions were investigated, and complete oxidation to 2,5-furandicarboxylic acid (FDCA) via 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was achieved at pH above 12. At lower pH, the oxidation followed another route via 2,5-diformylfuran (DFF), yielding 5-formyl-2-furancarboxylic acid (FFCA) as the main product. Using the oxidized intermediates as substrates showed DFF to be most efficiently oxidized to FDCA. We also show that, at pH 4.5, the addition of the laccase enzyme promoted the oxidation of 5-HMF to FFCA.

Inversion of the Photogalvanic Effect of Conductive Polymers by Porphyrin Dopants

Conductive polymers are widely used as active and auxiliary materials for organic photovoltaic cells due to their easily tunable properties, high electronic conductivity, and light absorption. Several conductive polymers show the cathodic photogalvanic effect in pristine state. Recently, photoelectrochemical oxygen reduction has been demonstrated for nickel complexes of Salen-type ligands. Herein, we report an unexpected inversion of the photogalvanic effect caused by doping of the NiSalen polymers with anionic porphyrins. The observed effect was studied by means of UV-Vis spectroscopy, cyclic voltammetry and chopped light chronoamperometry. While pristine NiSalens exhibit cathodic photopolarization, doping with porphyrins inverts the polarization. As a result, photoelectrochemical oxidation of the ascorbate proceeds smoothly on the NiSalen electrode doped with zinc porphyrins. The highest photocurrents were observed on NiSalen polymer with o-phenylene imine bridge, doped with anionic zinc porphyrin. Assuming this, porphyrin serves both as a catalytic center for the oxidation of ascorbate and an internal electron donor, facilitating the photoinduced charge transport and anodic depolarization.

Photoelectrochemical Water Oxidation by Cobalt Cytochrome C Integrated-ATO Photoanode

Here, we report the immobilization of Co-protoporphyrin IX (Co-PPIX) substituted cytochrome c (Co-cyt c) on Antimony-doped Tin Oxide (ATO) as a catalyst for photoelectrochemical oxidation of water. Under visible light irradiation (λ > 450 nm), the ATO-Co-cyt c photoanode displays ~6-fold enhancement in photocurrent density relative to ATO-Co-PPIX at 0.25 V vs. RHE at pH 5.0. The light-induced water oxidation activity of the system was demonstrated by detecting evolved stoichiometric oxygen by gas chromatography, and incident photon to current efficiency was measured as 4.1% at 450 nm. The faradaic efficiency for the generated oxygen was 97%, with a 671 turnover number (TON) for oxygen. The current density had a slow decay over the course of 6 h of constant irradiation and applied potential, which exhibits the robustness of catalyst-ATO interaction.

Photoelectrochemical Oxidation Assisted Catalyst-Referred Etching for SiC (0001) Surface

In a previous study, we developed an abrasive-free polishing method named catalyst-referred etching (CARE) and used it for the planarization of silicon carbide (SiC) (0001). In this method, Si atoms at step edges are preferentially removed through a catalytically assisted hydrolysis reaction to obtain an atomically smooth and crystallographically well-ordered surface. However, the removal rate is low (< nm/h) and needs to be improved. In this study, we proposed an ultraviolet (UV) light assisted CARE method. In this method, UV light is irradiated onto a SiC surface to generate holes and oxidize the surface. The oxidized area, consisting of SiO2, can be quickly removed to form a nano-pit owing to the higher removal rate of SiO2 compared to that of SiC. The periphery of the nano-pits works as a reaction site, leading to a higher removal rate. To enhance the oxidation rate and form nano-pits, we applied electrochemical bias to the SiC substrate. However, the removal rate did not improve significantly when the bias voltage was higher than 3.0 V. This is because the electrochemical potential of Pt increased with the anodic potential of SiC, which oxidized the Pt surface and degraded the catalyst capability. To avoid this issue, we modified the catalytic pad, where an in-situ refreshment of the Pt surface is possible. As a result, the removal rate increased up to 200 nm/h at a bias of 7.0 V, which is 100 times higher than that of the CARE without UV irradiation. The proposed method is expected to contribute to the enhancement in the productivity and quality of next-generation SiC substrates.

Room-temperature conversion of the photoelectrochemical oxidation of methane into electricity at nanostructured TiO2

The room-temperature operation of a methane-based photo-fuel cell is demonstrated for the first time. This is achieved using a TiO2 nanotube arrays photoanode which shows effective oxidation of methane.

Unveiling the contribution of singlet oxygen in the photoelectrochemical oxidation of benzyl alcohol

Photoelectrochemical approaches are finding their use for the transformation of organic molecules beyond water oxidation and CO2 reduction. Among the different reactions that can be performed, the selective oxidation of...

Direct photoelectrochemical oxidation of hydroxymethylfurfural on tungsten trioxide photoanodes

An important target reaction for solar-powered biomass valorization is the conversion of 2,5-hydroxymethylfurfural (HMF) into key monomers for polyester production.