Proton-Coupled Electron Transfer Reduction of a Quinone by an Oxide-Bound Riboflavin Derivative

The redox properties of a surface-bound phosphate flavin derivative (flavin mononucleotide, FMN) have been investigated on planar-FTO and <i>nano</i>ITO electrodes under acidic conditions in 1:1 CH₃CN/H₂O (V:V). On FTO, reversible 2e^–/2H⁺ reduction of FTO|-FMN to FTO|-FMNH₂ occurs with the pH and scan rate dependence expected for a 2e^–/2H⁺ surface-bound couple. The addition of tetramethylbenzoquinone (Me₄Q) results in rapid electrocatalyzed reduction to the hydroquinone by a pathway first order in quinone and first order in acid with <i>k</i>_H = (2.6 ± 0.2) × 10⁶ M^–1 s^–1. Electrocatalytic reduction of the quinone also occurs on derivatized, high surface area <i>nano</i>ITO electrodes with evidence for competitive rate-limiting diffusion of the quinone into the mesoporous nanostructure

Light-Driven Water Splitting with a Molecular Electroassembly-Based Core/Shell Photoanode

An electrochemical procedure for preparing chromophore-catalyst assemblies on oxide electrode surfaces by reductive vinyl coupling is described. On core/shell SnO₂/TiO₂ nanoparticle oxide films, excitation of the assembly with 1 sun (100 mW cm^–2) illumination in 0.1 M H₂PO₄^–/HPO₄^2– at pH 7 with an applied bias of 0.4 V versus SCE leads to water splitting in a DSPEC with a Pt cathode. Over a 5 min photolysis period, the core/shell photoanode produced O₂ with a faradaic efficiency of 22%. Instability of the surface bound chromophore in its oxidized state in the phosphate buffer leads to a gradual decrease in photocurrent and to the relatively modest faradaic efficiencies

Inner Layer Control of Performance in a Dye-Sensitized Photoelectrosynthesis Cell

Interfacial charge transfer and core-shell structures play important roles in dye-sensitized photoelectrosynthesis cells (DSPEC) for water splitting into H2 and O2. An important element in the design of the photoanode in these devices is a core/shell structure which controls local electron transfer dynamics. Here, we introduce a new element, an internal layer of Al2O3 lying between the Sb:SnO2/TiO2 layers in a core/shell electrode which can improve photocurrents by up to 300%. In these structures, the results of photocurrent, transient absorption, and linear scan voltammetry measurements point to an important role for the Al2O3 layer in controlling internal electron transfer within the core/shell structure

Electron Transfer Mediator Effects in the Oxidative Activation of a Ruthenium Dicarboxylate Water Oxidation Catalyst

The mechanism of electrocatalytic water oxidation by the water oxidation catalyst, ruthenium 2,2′-bipyridine-6,6′-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru­(bda)­(isoq)₂], <b>1</b>, at metal oxide electrodes has been investigated. At indium-doped tin oxide (ITO), diminished catalytic currents and increased overpotentials are observed compared to glassy carbon (GC). At pH 7.2 in 0.5 M NaClO₄, catalytic activity is enhanced by the addition of [Ru­(bpy)₃]²⁺ (bpy = bipyridine) as a redox mediator. Enhanced catalytic rates are also observed at ITO electrodes derivatized with the surface-bound phosphonic acid derivative [Ru­(4,4′-(PO₃H₂)₂bpy)­(bpy)₂]²⁺, <b>RuP</b>²⁺. Controlled potential electrolysis with measurement of O₂ at ITO with and without surface-bound RuP²⁺ confirm that water oxidation catalysis occurs. Remarkable rate enhancements are observed with added acetate and phosphate, consistent with an important mechanistic role for atom-proton transfer (APT) in the rate-limiting step as described previously at GC electrodes

Light-Driven Water Splitting by a Covalently Linked Ruthenium-Based Chromophore–Catalyst Assembly

The preparation and characterization of new Ru­(II) polypyridyl-based chromophore–catalyst assemblies, [(4,4′-PO₃H₂-bpy)₂Ru­(4-Mebpy-4′-epic)­Ru­(bda)­(pic)]²⁺ (<b>1</b>, bpy = 2,2′-bipyridine; 4-Mebpy-4′-epic = 4-(4-methylbipyridin-4′-yl-ethyl)-pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate; pic = 4-picoline), and [(bpy)₂Ru­(4-Mebpy-4′-epic)­Ru­(bda)­(pic)]²⁺ (<b>1</b>′) are described, as is the application of <b>1</b> in a dye-sensitized photoelectrosynthesis cell (DSPEC) for solar water splitting. On SnO₂/TiO₂ core–shell electrodes in a DSPEC configuration with a Pt cathode, the chromophore–catalyst assembly undergoes light-driven water oxidation at pH 5.7 in a 0.1 M acetate buffer, 0.5 M in NaClO₄. With illumination by a 100 mW cm^–2 white light source, photocurrents of ∼0.85 mA cm^–2 were observed after 30 s under a 0.1 V vs Ag/AgCl applied bias with a faradaic efficiency for O₂ production of 74% measured over a 5 min illumination period

A Dye-Sensitized Photoelectrochemical Tandem Cell for Light Driven Hydrogen Production from Water

Tandem junction photoelectrochemical water-splitting devices, whereby two light absorbing electrodes targeting separate portions of the solar spectrum generate the voltage required to convert water to oxygen and hydrogen, enable much higher possible efficiencies than single absorber systems. We report here on the development of a tandem system consisting of a dye-sensitized photoelectrochemical cell (DSPEC) wired in series with a dye-sensitized solar cell (DSC). The DSPEC photoanode incorporates a tris­(bipyridine)­ruthenium­(II)-type chromophore and molecular ruthenium based water oxidation catalyst. The DSPEC was tested with two more-red absorbing DSC variations, one utilizing N719 dye with an I₃^–/I^– redox mediator solution and the other D35 dye with a tris­(bipyridine)­cobalt ([Co­(bpy)₃]^3+/2+) based mediator. The tandem configuration consisting of the DSPEC and D35/[Co­(bpy)₃]^3+/2+ based DSC gave the best overall performance and demonstrated the production of H₂ from H₂O with the only energy input from simulated solar illumination

Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore–Catalyst Assembly Anchored to TiO₂

The ligand 5-PO₃H₂-2,2′:5′,2″-terthiophene-5-trpy, <b>T3</b> (trpy = 2,2′:6′,2″-terpyridine), was prepared and studied in aqueous solutions along with its metal complex assembly [Ru­(<b>T3</b>)­(bpy)­(OH₂)]²⁺ (<b>T3</b>-Ru-OH₂, bpy = 2,2′-bipyridine). <b>T3</b> contains a phosphonic acid group for anchoring to a TiO₂ photoanode under aqueous conditions, a terthiophene fragment for light absorption and electron injection into TiO₂, and a terminal trpy ligand for the construction of assemblies comprising a molecular oxidation catalyst. At a TiO₂ photoanode, <b>T3</b> displays efficient injection at pH 4.35 as evidenced by the high photocurrents (∼350 uA/cm²) arising from hydroquinone oxidation. Addition of [Ru­(bpy)­(OTf)]­[OTf]₂ (bpy = 2,2′-bipyridine, OTf^– = triflate) to <b>T3</b> at the free trpy ligand forms the molecular assembly, <b>T3</b>-Ru-OH₂, with the oxidative catalyst fragment: [Ru­(trpy)­(bpy)­(OH₂)]²⁺. The new assembly, <b>T3</b>-Ru-OH₂, was used to perform efficient light-driven oxidation of phenol (230 μA/cm²) and benzyl alcohol (25 μA/cm²) in a dye-sensitized photoelectrosynthesis cell

Unassisted Uranyl Photoreduction and Separation in a Donor–Acceptor Covalent Organic Framework

The donor–acceptor covalent organic framework (COF) TTT–DTDA (TTT = thieno­[3,2-b]­thiophene-2,5-dicarbaldehyde and DTDA = 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)­trianiline) was prepared and found to have long-lived excited states (>100 ms) characterized by transient absorption spectroscopy. These excited-state lifetimes were sufficient to perform the direct photoreduction of uranium at ppm concentration levels. The photoreduction of soluble uranyl species to insoluble reduced uranium products is an attractive separation for uranium, typically accomplished with sacrificial reagents and protective gases. In the case of TTT–DTDA, illumination in aqueous solutions containing only uranyl ions produced crystalline uranyl peroxide species ([UO2(O2)]) at the COF that were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, and infrared spectroscopy. The maximum absorption capacity of TTT–DTDA was found to be 123 mg U/g COF at pH 5 after 10 h of illumination in solutions devoid of sacrificial reagents or protective gases. The TTT–DTDA COF was recyclable and maintained high selectivity for uranium in competing ion experiments, which are necessary requirements for a practical uranium extraction strategy based on photochemical uranium reduction

All-in-One Derivatized Tandem p⁺n‑Silicon–SnO₂/TiO₂ Water Splitting Photoelectrochemical Cell

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 μm thick SnO₂ films with an outer 4.3 nm shell of TiO₂ added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and p⁺n type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru­(bda)­(4-O­(CH₂)₃PO₃H₂)-pyr)₂], <b>1</b>, (pyr = pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate), and chromophore, [Ru­(4,4′-PO₃H₂-bpy) (bpy)₂]²⁺, <b>RuP</b>²⁺, (bpy = 2,2′-bipyridine), which form 2:1 <b>RuP</b>²⁺/<b>1</b> assemblies on the surface. At pH 5.7 in 0.1 M acetate buffer, these electrodes with a fluorine-doped tin oxide (FTO) back contact under ∼1 sun illumination (100 mW/cm²; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm² with an 88% Faradaic efficiency (FE) for O₂ production at an applied bias of 600 mV versus RHE (ACS Energy Lett., 2016, 1, 231−236). The SnO₂/TiO₂–chromophore–catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO₂ (Ti/<i>a-</i>TiO₂) coating as an interconnect. In the integrated electrode, p⁺n-Si–Ti/<i>a</i>-TiO₂–SnO₂/TiO₂|-2<b>RuP</b>²⁺/<b>1</b>, the p⁺n-Si junction provided about 350 mV in added potential to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate buffer, bias-free photocurrents approaching 100 μA/cm² were obtained for water splitting, 2H₂O → 2H₂ + O₂. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode